DEVELOPMENT OF CHIRALITY SENSORS FOR THE DETERMINATION OF ABSOLUTE STEREOCHEMISTRY OF CHIRAL MOLECULES VIA ECCD By Mercy Anyika A DISSERTATION Submitted to Michigan State University In partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Chemistry 2012 ABSTRACT DEVELOPMENT OF CHIRALITY SENSORS FOR THE DETERMINATION OF ABSOLUTE STEREOCHEMISTRY OF CHIRAL MOLECULES VIA ECCD By Mercy Anyika A quality of molecules that causes them to be non-superimposable on their mirror images is known as Chirality. Because of the effects that chirality can have on the chemical and pharmacological properties of molecules, there is an increased interest in developing methods for use in assigning the absolute stereochemistry of chiral molecules. This thesis will detail some of the work we have accomplished using exciton coupled circular dichroism to probe chirality in organic molecules. This dissertation focuses on two parts. The first part introduces the concept of exciton coupled circular dichroism (Chapter I) and introduces the electronically tuned porphyrin tweezer TPFP, which has enhanced sensitivity for chirality sensing (Chapter II). We were able to employ analugues of this tweezer to develop working mnemonics for assigning the absolute stereochemistry of chiral hydroxyl ketones and sulfoxides, two important classes of functional groups commonly encountered as building block in the synthesis of complex molecules. In using tweezers in the study of absolute stereochemistry of molecules, in order to study molecules with only one site of attachment, the molecules are first derivatized with an achiral carrier molecule to provide the second requisite site of attachment. The second part of this dissertation focuses on addressing this group of molecules and the rationale about how we designed and synthesized the MAPOL host molecule that would allow for the assignment of absolute stereochemistry of mono-coordinating molecules, without requiring derivatization with carrier molecules (Chapter III). Lastly, chapter IV will describe the successful application of MAPOL host as a chirality reporter for a number of chiral molecules including mono amines, carboxylic acids and alcohols which would otherwise require derivatization in order to employ conventional methods. Dedicated to my beloved family for their love and support. ! iv! ACKNOWLEDGEMENTS I would like to first express my deepest gratitude to my advisor, Professor Babak Borhan, for his guidance, continuing encouragement and patience during my graduate career. Professor Borhan’s mentorship and criticism helped me improve as a research scientist and will continue to benefit me during my whole life. I am thankful to my committee members, Professor William Wulff, Professor James Jackson and Professor Milton Smith, for their help and suggestions during my graduate work. I would like to thank all the members of the Borhan group for their invaluable help and friendship during the past five years. I especially would like to thank the people who worked with me on the CD project: Dr. Chrysoula Vasileiou, Dr. Marina Tanasova, Dr. Xiaoyong Li and Carmin Burrell. The academic discussions we had always drove me to think hard and critically. I especially want to thank Carmin for patiently reading through my entire thesis, understanding what it was I was trying to say, and giving suggestions. I look forward to returning the favor soon. I would like to thank Robert Acho, a very talented and patient undergraduate student who worked with me on the MAPOL project. He was the best undergraduate student I could ever ask for. I also wish to thank everyone who sat in room 524 at one time or the other: Xiaoyong Li, Marina Tanasova, Carmin Burrell, Calvin Grant and Bardia Soltanzadeh: thank you for allowing me the freedom to play my loud music when I needed some stress relief. I especially thank Dr. Wenjing Wang for being a good friend from when we first joined the lab, and willingly coming ! v! along to dance even when she felt embarrassed to do so. I also wish to thank Roozbeh Yousefi for introducing me to salsa dancing. I will really miss those Saturday nights of dancing. A special group of people deserve special acknowledgements. They were there when I needed a friend to laugh with, or a shoulder to cry on. Luis Mori Quiroz, Camille Watson, Carmin Burrell and Calvin Grant. Thank you for the hugs, laughter and for your prayers. Last but not least, I want to express my gratitude to my mum and sisters Lois, Zet and Sarah for being very understanding and supporting my carrer. Even though you are far away, you are always in my heart. ! ! vi! TABLE OF CONTENTS List of Tables ...................................................................................................................................x List of Figures .............................................................................................................................. xiii List of Schemes ..............................................................................................................................xx Key to Symbols and Abbreviations ............................................................................................ xxii Chapter 1 Introduction to ECCD ......................................................................................................................1 1.1 Chirality ...................................................................................................................1 1.1.2 Methods for Assigning Absolute Stereochemistry ..................................................6 1.2 Optical Rotatory Dispersion (ORD) and Circular Dichroism (CD) Spectroscopy ............................................................................................................7 1.2.1 A Brief Introduction to Circular Dichrometers (Spectropolarimeter) ...................15 1.3 Exciton Coupled Circular Dichroism (ECCD) ......................................................16 1.3.1 Theoretical Background of ECCD .........................................................................16 1.3.2 The Quantum Mechanics Explanation of ECCD ...................................................23 1.3.3 ECCD Method in Determining Absolute Stereochemistry of Chiral Molecules ....................................................................................................26 1.4 Alternative Chromophoric Hosts in ECCD Studies...............................................29 1.4.1 Use of Zinc Porphyrins as Chromophoric Hosts ...................................................33 1.5 Research aim ..........................................................................................................39 References ............................................................................................................................41 Chapter 2 Use of Zn-TPFP Tweezers for Assignment of Absolute Stereochemistry of Compounds ....................................................................................................................................51 2.1 Development of Electron Deficient Porphyrin Tweezers ......................................51 2.2 Determination of Absolute Stereochemistry for Chiral Hydroxy Ketones………56 2.2.1 Background ............................................................................................................56 2.2.2 Synthesis of Chiral Hydroxy Ketones....................................................................60 2.2.3 ECCD Studies of Chiral Hydroxy Ketones Using Zn-TPFP Tweezer ..................61 2.2.3.1 Binding Affinity of Zn-TPFP Tweezer to Hydroxy Ketones ................................61 2.3 Development of Electron Deficient Porphyrin Tweezers ......................................51 2.2 Determination of Absolute Stereochemistry for Chiral Sulfoxides .......................77 2.3.1 Background ............................................................................................................77 ! vii! 2.3.2 Conventional Methods for Assigning Absolute Stereochemistry of Sulfoxides ..............................................................................................................81 2.3.3 ECCD Studies of Chiral Sulfoxides Using Zn-TPFP Tweezer .............................85 2.3.3.1 Binding affinity of Zn-TPFP Tweezer to Sulfoxides.............................................85 2.3.3.2 Probing the Zinc-Sulfoxide Binding ......................................................................88 Experimental Procedures .....................................................................................................96 References ..........................................................................................................................155 Chapter 3 Determination of Absolute Stereochemistry for Compounds with One Site of Attachment ...................................................................................................................................162 3.1 Backgound ...........................................................................................................165 3.1.1 Design of MAPOL ...............................................................................................165 3.1.2 Synthesis of MAPOL ...........................................................................................174 3.1.2A First Generation Approach to MAPOL via Suzuki Coupling ..............................178 3.1.2B Second Generation Approach to MAPOL via Suzuki and Oxidative Coupling...............................................................................................................185 3.1.2C Third Generation Approach to MAPOL via the 2+2 Synthesis ...........................193 3.1.2D Fourth Generation Approach to MAPOL via Lindsey Type Condensation……198 3.1.3 Investigating the Type of Interaction and Nature of Complex Formed Between MAPOL and Amines ............................................................................201 Experimental Procedures ...................................................................................................212 References ..........................................................................................................................219 Chapter 4 Determination of Absolute Stereochemistry of α-Chiral Amines ...............................................227 4.1 Background ..........................................................................................................227 4.1.1 Conventional Methods for Assigning Absolute Stereochemistry of α-Chiral Amines ..................................................................................................228 4.2 ECCD Studies of Chiral Mono Amines Using MAPOL .....................................232 4.2.1 Determination of Chirality for Primary Amines ..................................................236 4.3 Determination of Chirality for Secondary Amines ..............................................243 4.3.1 Conventional Methods for Assigning Absolute Stereochemistry of αα-Chiral Secondary Amines ................................................................................243 4.3.2 ECCD Studies of Secondary Amines Using MAPOL .........................................246 4.4 Chiral Aziridnes ...................................................................................................248 4.4.1 ECCD Studies of Chiral Aziridines Using MAPOL ...........................................249 Experimental Procedures ...................................................................................................252 4.5 Determination of Absolute Stereochemistry of Chiral Carboxylic Acids ....................................................................................................................253 4.5.1 α-Chiral Carboxylic Acids...................................................................................255 4.5.2 Use of MAPOL for Absolute Stereochemical Determination of α-Chiral Carboxylic Acids...................................................................................260 4.5.4 Use of MAPOL for Absolute Stereochemical Determination of ! viii! β-Chiral Carboxylic Acids ...................................................................................272 4.4 Chiral Cyanohydrins ............................................................................................277 4.4.1 Conventional Methods for Assigning Absolute Stereochemistry of Cyanohydrins .......................................................................................................278 4.6.2 Use of MAPOL for Absolute Stereochemical Determination of Cyanohydrins .......................................................................................................282 Experimental Procedures ...................................................................................................286 References ..........................................................................................................................306 ! ix! LIST OF TABLES Table 1-1 Definition of exciton chirality for a binary system ................................................23 Table 2-1 ECCD data of chiral diamines and derivatized carboxylic acids ...........................54 Table 2-2 ECCD data for chiral hydroxy ketones in hexane ..................................................71 Table 2-3 ECCD data for chiral sulfoxy alcohols in hexane ..................................................92 Table 2-4 Crystal data and structure refinement for bb58_0m .............................................107 Table 2-5 Atomic coordinates for bb58_0m .........................................................................109 Table 2-6 Bond lengths and angles for bb58_0m .................................................................110 Table 2-7 Anisotropic displacement parameters for bb58_0m .............................................112 Table 2-8 Hydrogen coordinates for bb58_0m .....................................................................113 Table 2-9 Torsion angles for bb58_0m.................................................................................114 Table 2-10 Hydrogen bonds for bb58_0m ..............................................................................114 Table 2-11 Crystal data and structure refinement for bb64_0m .............................................116 Table 2-12 Atomic coordinates for bb64_0m .........................................................................118 Table 2-13 Bond lengths and angles for bb64_0m .................................................................119 Table 2-14 Anisotropic displacement parameters for bb64_0m .............................................122 Table 2-15 Hydrogen coordinates for bb64_0m .....................................................................123 Table 2-16 Torsion angles for bb64_0m.................................................................................124 Table 2-17 Hydrogen bonds for bb64_0m ..............................................................................125 Table 2-18 Crystal data and structure refinement for bb48_0m .............................................127 Table 2-19 Atomic coordinates for bb48_0m .........................................................................129 Table 2-20 Bond lengths and angles for bb48_0m .................................................................130 ! x! Table 2-21 Anisotropic displacement parameters for bb48_0m .............................................132 Table 2-22 Hydrogen coordinates for bb48_0m .....................................................................133 Table 2-23 Torsion angles for bb48_0m.................................................................................134 Table 2-24 Hydrogen bonds for bb48_0m ..............................................................................134 Table 2-25 Crystal data and structure refinement for bb73_0m .............................................136 Table 2-26 Atomic coordinates for bb73_0m .........................................................................138 Table 2-27 Bond lengths and angles for bb73_0m .................................................................139 Table 2-28 Anisotropic displacement parameters for bb73_0m .............................................142 Table 2-29 Hydrogen coordinates for bb73_0m .....................................................................143 Table 2-30 Torsion angles for bb73_0m.................................................................................144 Table 2-31 Hydrogen bonds for bb73_0m ..............................................................................144 Table 2-32 Crystal data and structure refinement for bb42_0m .............................................146 Table 2-33 Atomic coordinates for bb42_0m .........................................................................148 Table 2-34 Bond lengths and angles for bb42_0m .................................................................149 Table 2-35 Anisotropic displacement parameters for bb42_0m .............................................151 Table 2-36 Hydrogen coordinates for bb42_0m .....................................................................152 Table 2-37 Torsion angles for bb42_0m.................................................................................153 Table 2-38 Hydrogen bonds for bb42_0m ..............................................................................153 Table 3-1 CD spectral data for hydrogen bonded amino alcohol complexes in DCM at room temperature .............................................................................................170 Table 3-2 Conditions tried for Suzuki cross coupling reactions ..........................................180 Table 3-3 Attempts to synthesize 18.....................................................................................183 Table 3-4 Oxidative coupling conditions for the synthesis of MAPOL ...............................187 Table 3-5 UV-vis data for newly synthesized porphyrins ....................................................196 ! xi! Table 4-1 ECCD data for chiral amines bound to MAPOL..................................................238 Table 4-2 ECCD data for chiral amines bound to MAPOL and Zn-MAPOL ......................241 Table 4-3 ECCD data for chiral secondary amines bound to MAPOL ................................248 Table 4-4 ECCD data for chiral aziridines bound to MAPOL .............................................250 Table 4-5 ECCD data for α-chiral carboxylic acids bound to Zn-MAPOL .........................267 Table 4-6 ECCD data for β- and γ-chiral carboxylic acids bound to Zn-MAPOL ...............274 Table 4-7 ECCD data for cyanohydrins bound to Zn-MAPOL............................................282 ! xii! LIST OF FIGURES Figure 1-1 Representation of chiral molecules ..........................................................................1 Figure 1-2 Enantiomeric forms of 3-mercaptohexanal and carvone .........................................2 Figure 1-3 Structures of (R)- and (S)-Thalidomide ...................................................................3 Figure 1-4 Examples of some chiral molecules and their mirror images ..................................4 Figure 1-5 Light as an electromagnetic radiation ......................................................................7 Figure 1-6 Linearly and circularly polarized light ....................................................................8 Figure 1-7 Circularly polarized light passing through a chiral medium ..................................10 Figure 1-8 Positive and negative Cotton effects ......................................................................12 Figure 1-9 Electron redistribution upon light excitation for a molecule .................................13 Figure 1-10 The right hand rule to determine the direction of magnetic transition dipole moment .......................................................................................................14 Figure 1-11 Schematic representation of a CD spectropolarimeter ...........................................15 Figure 1-12 Exciton coupled circular dichroism of steroidal 2,3-bisbenzoate ..........................17 Figure 1-13 Splitting of the excited states of isolated chromophores i and j by exciton interaction ..................................................................................................18 Figure 1-14 The UV-vis spectrum and ECCCD spectrum upon through space interaction of two degenerate chromophores .........................................................19 Figure 1-15 The expected ECCD spectrum of dibenzoate and its rationalization .....................21 Figure 1-16 A qualitative explanation of ECCD .......................................................................22 Figure 1-17 Dibenzoate chirality method – Distance effect ......................................................25 Figure 1-18 ECCD active abscisic acid .....................................................................................27 Figure 1-19 Application of the ECCD method in absolute configurational assignment of natural products .................................................................................................28 ! xiii! Figure 1-20 The induced chirality of biphenol upon binding to chiral amine ...........................30 Figure 1-21 Determination of stereochemistry of chiral alcohols by ECCD.............................31 Figure 1-22 Derivatization methods for analysis of absolute configurations with various chromophores ............................................................................................32 Figure 1-23 Structure and schematic representation of a porphyrin tweezer ............................35 Figure 1-24 Zn-porphyrin tweezer for stereochemical determination of chiral lysine methyl ester ............................................................................................................36 Figure 1-25 Determination of chirality for derivatized α-carboxylic acid ................................38 Figure 1-26 Inoue’s porphyrin tweezer for stereochemical determination ................................39 Figure 2-1 Proposed strategies for improving porphyrin sensitivity .......................................52 Figure 2-2 Electronically tuned tweezers for ECCD studies ...................................................53 Figure 2-3 Assignment of absolute configuration of acyclic chiral diamines using ECCD .....................................................................................................................55 Figure 2-4 The hydroxy ketone moiety in natural products ....................................................56 Figure 2-5 Use of hydroxyl ketone in the synthesis of Taxol’s ring D....................................56 Figure 2-6 Titration of Zn-TPFP ester with hydroxy ketone, and the non-linear least square fit.................................................................................................................62 Figure 2-7 Titration of Zn-TPFP porphyrin tweezer with 1,12-hydroxy ketone and the non-linear least square fit .......................................................................................63 Figure 2-8 A series of porphyrin tweezers of varying linker lengths ......................................64 Figure 2-9 Proposed binding of hydroxy ketones with Zn-TPFP tweezer. .............................65 Figure 2-10 Utaka’s porphyrin tweezer. ....................................................................................66 Figure 2-11 Colquhoun’s tweezer systems showing chain folding and multiple binding to different polyimide triplet sequences .................................................................67 Figure 2-12 Overlay of ECCD signals obtained for 1,3- and 1,8-hydroxy ketones ..................69 Figure 2-13 Proposed complexation between tweezer and hydroxy ketones ............................72 ! xiv! Figure 2-14 Proposed conformation of the complex formed between Zn-TPFP tweezer and (S)-(+)-3-hydroxy-2,2-dimethylcyclohexananone .............................73 Figure 2-15 Synthetic and naturally occurring compounds containing a sulfoxide moiety ....................................................................................................................75 Figure 2-16 Geometric comparison of a sulfoxide to a carbonyl and tertiary phosphine oxide.......................................................................................................................76 Figure 2-17 Two resonance structures of a sulfoxide bond .......................................................76 Figure 2-18 Conversion of alliin to ajoene via allicin ...............................................................78 Figure 2-19 Asymmetric synthesis of sulfoxides by oxidation of sulfides................................79 Figure 2-20 Solvation model for the interaction of (R)-2,2,2-trifluoro-phenylethanol with both (R) and (S) configurations of aryl methyl sulfoxides.............................81 Figure 2-21 Formation of N-(methoxyphenylacetyl) sulfoximines from sulfoxides .................82 Figure 2-22 Titration of Zn-TPFP porphyrin with phenylmethyl sulfoxide, and the non-linear least square fit .......................................................................................85 Figure 2-23 Proposed binding of a sulfoxy alcohol to Zn-TPFP tweezer .................................87 Figure 2-24 Sketch of the periodic table showing the preference of O- and S-bonding sulfoxide complexes as suggested by IR spectroscopy..........................................89 Figure 2-25 IR titration of methyl phenyl sulfoxide with TPFP porphyrin ...............................90 Figure 2-26 Proposed working mnemonic for sulfoxy alcohols................................................94 Figure 2-27 Obtained ECCD spectra of 3 (S,S), and 3-ent (R,S) respectively ..........................95 Figure 3-1 Nomenclature for assigning atropisomers. Chirality in biaryl compounds……..167 Figure 3-2 The induction of chirality of various biphenols upon binding to chiral diamines by hydrogen bonding ............................................................................168 Figure 3-3 Formation of 1:1 complex between diamine 2a and biphenol 1a at room temperature, and ternary complex 3a’ at low temperature with excess amine…169 Figure 3-4 Point to axial chirality transfer from amino alcohols to 2,2’-biphenylbridged bis(free base porphyrin) 4 via hydrogen bonding ...................................170 Figure 3-5 Macroscopic expression of the chirality of an amino alcohol and mono amine ! xv! by a double amplification mechanism in liquid crystalline media via hydrogen bonding ................................................................................................172 Figure 3-6 Proposed host systems for stereochemical determination of chiral molecules .............................................................................................................173 Figure 3-7 The McDonald synthesis of porphyrins ...............................................................174 Figure 3-8 Natural products Korupensamine A and Hippadine ............................................179 Figure 3-9 Oxidative coupling of 2-naphthol in the synthesis of BINOL .............................188 Figure 3-10 Phenolic coupling in the preparation of racemic molecules VANOL and VAPOL ................................................................................................................192 Figure 3-11 Dimeric porphyrins synthesized by Ogoshi .........................................................195 Figure 3-12 Important porphyrins obtained along the way .....................................................200 Figure 3-13 Potential sites for hydrogen-bonding on porphyrin unit ......................................201 Figure 3-14 UV-Vis curves of MAPOL and Ishii’s host .........................................................203 Figure 3-15 Interaction between quinone and porphyrin via multiple hydrogen bonds ..........204 Figure 3-16 1 Figure 3-17 A4-TPP porphyrin ................................................................................................206 Figure 3-18 UV-Vis titration of Zn-MAPOL 44 solution .......................................................208 Figure 3-19 ECCD spectrum obtained upon binding of 44 with a chiral diamine ..................208 Figure 3-20 Job’s continuous plot, and the non-linear least square fit ....................................210 Figure 4-1 Molecules containing the amine functionality .....................................................227 Figure 4-2 Inuoe’s porphyrin tweezer for stereochemical determination ..............................230 Figure 4-3 Mosher’s model for correlating the configuration with the 1HNMR shifts and the expected signs of ΔδSR .............................................................................232 Figure 4-4 Perturbation of an equilibrium system by introduction of a chiral guest with point chirality, creating diastereomers………………….....................................233 ! H-NMR titrations of MAPOL and TPP porphyrin with isopropyl ethylamine ............................................................................................................205 xvi! Figure 4-5 P and M conformers of MAPOL..........................................................................233 Figure 4-6 Proposed complexation of chiral amine with P- and M-helicities of MAPOL ...............................................................................................................234 Figure 4-7 Newman projections of a chiral amine complexed with P- and M-helicities of MAPOL, and the predicted ECCD signs from the favored conformations .........236 Figure 4-8 Proposed binding model for (S)-cyclohexyl ethylamine to MAPOL...................239 Figure 4-9 MAPOL and analogues of MAPOL used for ECCD studies ...............................240 Figure 4-10 CD signals of amines A) 5R, B) 5S, and C) 8S with Zn-MAPOL 58 and MAPOL 9 ............................................................................................................242 Figure 4-11 The syn and anti conformers of the (R)-MTPA amide of (R)-2methylpiperidine in equilibrium. .........................................................................243 Figure 4-12 Syn and anti rotamers for the (S)- and (R)-MTPA amides of (R)-2methylpiperidine ..................................................................................................244 Figure 4-13 Proposed binding model for (R)-benzhydrylpyrrolidine to MAPOL...................247 Figure 4-14 Examples of some common building blocks containing a carboxylic acid functionality .........................................................................................................255 Figure 4-15 Nicolaou’s use of a carboxylic acid in the total synthesis of swinholide A…….256 Figure 4-16 Derivatization of an α- chiral carboxylic acid for absolute stereochemical assignment using Mosher analysis .......................................................................257 Figure 4-17 Some common CDA’s .........................................................................................258 Figure 4-18 Derivatized carboxylic acids with Nakanishi’s and Borhan’s carriers for use in CD spectroscopy ..............................................................................................259 Figure 4-19 Two carriers yielding different ECCD signals .....................................................260 Figure 4-20 At equilibrium, P and M helices exist in equal amounts......................................261 Figure 4-21 Newman projections for triodal system with each enantiomer of PBA bound....................................................................................................................261 Figure 4-22 Proposed complexation and stereodifferentiation of carboxylic acids with MAPOL ...............................................................................................................262 ! xvii! Figure 4-23 MAPOL 9, MAPOL analogue 44 and Zn-MAPOL 58 ........................................263 Figure 4-24 Proposed binding of carboxylic acid and amine to MAPOL via hydrogen bonding ................................................................................................264 Figure 4-25 α-chiral carboxylic acid involved in both metal coordination and hydrogen-bonding with M-helix of Zn-MAPOL .................................................265 Figure 4-26 Three host porphyrins used by Ogoshi investigated the binding of α-amino acid esters .............................................................................................................266 Figure 4-27 Host for carbohydrate recognition .......................................................................266 Figure 4-28 Proposed binding model for chiral carboxylic acids to Zn-MAPOL ...................268 Figure 4-29 Proposed binding of substrates with aromatic groups at chiral center. In both, the models involving π-π interactions between the aromatic group and porphyrin result in predicted sign that are in agreement with the obtained results ...................................................................................................................269 Figure 4-30 Proposed binding model for 29(S) to Zn-MAPOL ..............................................271 Figure 4-31 Proposed binding model for acid 32 to Zn-MAPOL ...........................................272 Figure 4-32 1-arylethylamines used by Hoye as reagents .......................................................273 Figure 4-33 Conformational model for the determination of the sign of Δδ ...........................274 Figure 4-34 TBP porphyrin tweezer used for remote chirality sensing ...................................275 Figure 4-35 Proposed binding model for β-chiral carboxylic acids with Zn-MAPOL ...........276 Figure 4-36 Possible rotomers around the C-C=O bond .........................................................271 Figure 4-37 Possible rotomers of γ-chiral carboxylic acids .....................................................272 Figure 4-38 Proposed binding model for β-carboxylic acid to Zn-MAPOL by a two-point coordination to zinc and biphenol unit.................................................................281 Figure 4-39 NMR representative conformer of A) the (R)-MPA ester of an aldehyde cyanohydrin and B) the (S)-MPA ester................................................................284 Figure 4-40 Riguera’s assignment of keto cyanohydrins using 1H-NMR analysis .................284 Figure 4-41 Proposed model showing ternary complexes of A) (R)- and B) (S)cyanohydrin with (S)-mandelate-DMAPH+ ion pair............................................285 ! xviii! Figure 4-42 CD spectrum obtained for (R)-mandelonitrile with TPFP tweezer ......................286 Figure 4-43 Two possible Zn-cyanohydrin coordinations .......................................................287 Figure 4-44 UV-Vis titration of A, MAPOL solution (1 µM in hexane) with pchloromandelonitrile ............................................................................................288 Figure 4-45 Derivatization of a chiral alcohol with R- and S-MTPA acid (and chlorides) for Mosher ester analysis .....................................................................................292 Figure 4-46 Proposed complexation of chiral alcohols with P and M helices of MAPOL .....293 Figure 4-47 UV-Vis titration of nitro-MAPOL solution (1 µM in hexane) with (S)-hexan2-ol (0.01 M in DCM). Insert: obtained ECCD spectrum of (S)-hexan-2-ol with nitro-MAPOL, in hexane solvent at 0 °C…………………………………295 Figure 4-48 Proposed host molecules for assigning absolute stereochemistry of chiral alcohols…………………………………………………………………………296 ! xix! LIST OF SCHEMES Scheme 2-1 Use of chiral hydroxyl ketone in the synthesis of (S)-Ipsenol ...............................56 Scheme 2-2 Yamamoto’s synthesis of α-hydroxy ketones using nitrosobenzene.....................57 Scheme 2-3 Assignment of absolute stereochemistry of Streptoketol A using Mosher ester analysis ..........................................................................................................57 Scheme 2-4 Assignment of absolute stereochemistry of α-cyclic hydroxy ketones .................58 Scheme 2-5 Synthesis of chiral hydroxyl ketones via Jacobsen epoxidation..………………..59 Scheme 2-6 Representative scheme for synthesis of chiral sulfoxy alcohols ............................83 Scheme 3-1A Retrosythetic analysis I. .......................................................................................176 Scheme 3-1B Retrosynthetic analysis II.....................................................................................176 Scheme 3-1C Retrosynthetic analysis III ...................................................................................177 Scheme 3-1D Retrosynthetic analysis IV ...................................................................................177 Scheme 3-2 Synthesis of DTMPX and DTMPD, employing Suzuki cross coupling reaction .................................................................................................................180 Scheme 3-3 Synthesis of several poprhyrin-containing molecules via Suzuki coupling reaction .................................................................................................................181 Scheme 3-4 Synthesis of bis-iodo biphenyl 16 ........................................................................182 Scheme 3-5 Synthesis of 41. and Planned elaboration of 41 to 18 and 19 ..............................186 Scheme 3-6 Synthesis of porphyrin 43 ....................................................................................189 Scheme 3-7 Synthesis of catalyst.............................................................................................189 Scheme 3-8 Fischer’s synthesis of porphyrins.........................................................................193 Scheme 3-9 Synthesis of hemin 54 via deuteroporphyrin 53 ..................................................194 Scheme 3-10 MacDonald’s 2+2 porphyrin synthesis ................................................................194 Scheme 3-11 Ogoshi’s route to porphyrin B .............................................................................196 ! xx! Scheme 3-12 Attempts to synthesize MAPOL using Ogoshi’s method ....................................197 Scheme 3-14 Ogoshi’s synthesis of 63 ......................................................................................198 Scheme 3-15 Successful synthesis of MAPOL .........................................................................199 Scheme 3-16 Successful synthesis of MAPOL .........................................................................199 Scheme 3-17 Synthesis of Diazoxide BPDZ-44, using (S)-2-amino-3-methylbutane ..............228 Scheme 4-1 Successful synthesis of MAPOL .........................................................................199 Scheme 4-2 Complex formation between carrier/monoamine conjugate and porphyrin tweezer .................................................................................................................229 Scheme 4-3 Complex formation between carrier/monoamine conjugate and porphyrin tweezer .................................................................................................................245 Scheme 4-4 Possible transformations of cyanohydrins ...........................................................282 Scheme 4-5 Synthesis of nitro-MAPOL 68 .............................................................................294 ! xxi! Key to Symbols and Abbreviations α angle of rotation [α] specific rotation Å angstrom A CD amplitude ACN acetonitrile AcOH acetic acid Ar aromatic BF3•OEt2 boron trifluoride diethyl ether BINOL 1,1’-Bi-2-naphthol Bn benzyl BnBr benzyl bromide BnCl benzyl chloride CD circular dichroism CE Cotton effect p-chloranil tetrachloro-1,4-benzoquinone cm centimeter d doublet DCM dichloromethane DDQ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone DET diethyl tartrate DFT density functional theory DIPT diisobutylaluminum hydride ! xxii! DMAP 4-diaminopyridine DMF N,N-dimethylformamide ee enantiomeric excess Et3N triethylamine ε molar absorption coefficient ECCD Exciton Coupled Circular Dichroism Et2O diethyl ether EtOAc ethyl acetate eq equivalents g gram(s) h hour(s) HKR hydrolytic kinetic resolution HPLC high pressure liquid chromatography HRMS high resolution mass spectrometry Hz hertz iPr isopropyl IR infrared J NMR coupling constant Kassoc association constant LAH lithium aluminum hydride m magnetic dipole transition moment m multiplet ! xxiii! mCPBA 3-chloroperoxybenzoic acid MeOH methanol min minute mg milligram MHz megahertz M molar µM micromolar MS mass spectrometry m/z mass to charge ratio n refractive index NaOH sodium hydroxide nm nanometer NMR nuclear magnetic resonance ORD optical rotatory dispersion PCC pyridinium chlorochromate Ph phenyl PMB para-methoxybenzyl q quartet R rotational strength s singlet SAD Sharpless asymmetric dihydroxylation SAE Sharpless asymmetric epoxidation rt room temperature ! xxiv! t triplet TBAF tetrabutylammonium fluoride TBS t-butyldimethylsilyl TFA trifluoroacetic acid THF tetrahydrofuran TPP-tz 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin tweezer UV-vis ultraviolet-visible spectroscopy Zn-TPP zinc tetraphenylporphyrin Zn-TPP-tweezer zinc 5-(4-carboxyphenyl)-10,15,20- triphenylporphyrin tweezer Zn(OAc)2 zinc acetate ! xxv! Chapter 1 Introduction to ECCD 1-1 Chirality Man’s fascination with asymmetric objects goes back to the 1800’s. With the discovery 1 of the existence of two forms of tartrate by Louis Pasteur in 1847, the art of exploring dissymmetry in molecules became a molecular science. This fascination grew into an intriguing 2,3 problem to be solved when in 1874, Van’t Hoff and LeBel suggested that molecules possess a three-dimensional structure that may result in dissymmetry. The introduction of the word 4 “chiral” by Lord Kelvin, forty years after Pasteur’s discovery represents a milestone in the history of absolute configuration, and marked the beginning of a new era in the determination of 5 absolute stereochemistry of chiral molecules. The word “chiral” originates from the Greek word for hand (cheir) and introduces the concept of “handedness” in reference to a pair of nonsuperimposable mirror images. Figure 1-1. Representative chiral molecules: 3methylbutan-2-ol (A) and Taxol® (B). 
 1
 Examples of chiral molecules can range from simple molecules like 3-methylbutan-2-ol that possesses point chirality, to large natural products with multiple chiral centers, such as Taxol® (Figure 1-1). The two mirror images of the molecule are referred to as “enantiomers” and, despite their identity in chemical composition, they may be of different abundance and possess different physical and/or biological properties. For instance, living organisms use exclusively one chiral form of a molecule (homochirality). Virtually all active forms of amino acids are of the L-form (D-serine being a notable exception), while most biologically relevant sugars are of the D-form. Enzymes being chiral, often distinguish between two enantiomers, and preferentially bind to a single enantiomer since it can fit inside an enzyme’s binding cavity, while the other cannot. Changes in chirality affect chemical and biological properties of some chiral molecules. For instance, while (S)-mercaptohexanal (Figure 1-2) has a pleasant, fruity odor, its R enantiomer has a pungent sulfur smell. Similarly, the S enantiomer of Carvone has the odor of caraway seeds, while its (R) enantiomer has the odor of spearmint. SH O SH O H (3S)-3-mercaptohexanal H (3R)-3-mercaptohexanal O O (R) carvone (S) carvone Figure 1-2. Enantiomeric forms of 3-mercapto hexanal and carvone. 2
 Chiral organic molecules are of extreme interest to the pharmaceutical, agrochemical and other industries. 19 Drugs generally work by interacting with receptors on cell surfaces, or enzymes within cells. Receptors have a specific three-dimensional structure which will allow only the isomer that fits precisely to bind preferentially while the other has little or no activity. Considering that the two enantiomers of a natural product or synthetic drug may have different physiological properties, the resolution and clinical testing of both enantiomers is a prerequisite 19 to the development of the optimum drug. Most of the pharmaceuticals sold commercially contain one or more chiral centers. Drugs obtained from natural sources or those prepared from natural materials are produced as pure enantiomers, because chiral compounds from nature usually occur as the pure form of one of the enantiomers. However, until recently (1970s) most chiral drugs produced synthetically from achiral starting materials were produced and sold in their racemic form, even though their therapeutic effect was mainly due to only one isomer. In most cases, the unwanted isomer was physiologically inactive and did not cause any serious side effects. However, that was not always the case. One tragic example of historical note is thalidomide. O O N O N NH O NH O O O O (S)-Thalidomide (R)-Thalidomide Figure 1-3. Structures of (R)- and (S)-Thalidomide. Thalidomide was developed and sold from 1957 - 1961 in almost 50 countries. It was mainly prescribed to pregnant women as an antiemetic to treat morning sickness and as a sleep 3
 aid. As shown in Figure 1-3, thalidomide contains a stereocenter and so exists in its two 4 enantiomeric forms. Tragically, while the (S)-isomer had the desired anti-nausea effects, the (R)-form was teratogenic and caused fetal abnormalities, such as severely deformed limbs. Chirality is not only becoming an important issue in the pharmaceutical industry, but also 19 in the agrochemical and other industries as well. As a result, there is a need for simple and effective ways for the determination of absolute configuration. A B Me H Et CO2H H Me HO2C Et Me H 2-methylbutyric acid Me Me C H H C Me H allene D C O2N HO2C HO2C O2 N CO2H NO2 NO2 CO2H NH2 H2N H H E Figure 1-4. Examples of some chiral molecules and their mirror images. 
 The variety of chiral molecules is complemented by different types of chirality. Point chirality arises when four different substituents are attached to a tetrahedral carbon making the carbon a chiral center (Figure 1-4A). The letters R (from Latin “rectus” meaning right) and S 4
 (from Latin “sinister” meaning left) are used to indicate the configuration (arrangement of 6 groups) on the chiral center, based on priorities set in place by Cahn, Ingold and Prelog. The classic point chirality is represented by 2-methylbutyric acid (Figure 1A). As shown, (S)-2methylbutyric acid is not superimposable on its mirror image (R)-2-methylbutanoic acid. This non-superimposability of mirror images is the only necessary requirement for chirality. However, molecules do not need to have a chiral center to be chiral. Figure 1-4 A-C shows the various forms of chirality. 7 Figure 1-4B shows an allene where the overall molecule is not planar, but rather the two π bonds are perpendicular to each other causing the molecule to lack any plane of symmetry when both ends are unsymmetrically substituted. Figure 1-4C shows a biphenyl molecule, bearing large groups on each side of the C-C single bond. This molecule is chiral because of restricted rotation around the single bond, hence it is “locked” into a chiral conformation since steric hindrance between the ortho-substituents prevents bond rotation. This restriction of rotation is the source of geometric isomerism (atropisomerism). Restricted rotation can also be found in spiranes, which are compounds that have two rings with a common carbon atom, as shown in Figure 1-4D. Because of this, the rings are perpendicular to each other giving rise to 8 9 axial chirality. Figure 1-4E shows a classic example of an inherently chiral chromophore, hexahelicene. Despite the fact that this molecule does not have any asymmetric carbons, it is not planar, but rather lacks a plane of symmetry as it traces a helix when one side of the molecule lies above the other due to crowding of the rings. The defined helix can be either right-handed (+)-hexahelicene, or left-handed (-)-hexahelicene and therefore, non-superimposable on its mirror image. 10 5
 1-1.2 Methods for Assigning Absolute Configuration Due to the role of chirality in physical, biochemical and physiological properties of chiral 1 molecules, there is an increasing interest in determining molecular chirality, and the need for simple and effective ways for the determination of absolute configuration is more prominent than ever. There are several methods that are used to establish the absolute stereochemistry of chiral 5 molecules including: chemical correlation with known chiral compounds, NMR Mosher ester method, 13 as well as X-ray crystallography. NMR-based configurational assignment of chiral compounds has been the most widely 13 used approach to determining chirality. The method originated by Mosher and co-workers involves derivatization of a chiral alcohol with two enantiomers of a chiral acid, bearing an 1 aromatic group at the chiral center. The H-NMR spectra of the two obtained diastereomers are RS then compared, and the anisotropic shielding (Δδ ) for the protons neighboring the chiral center are measured. The scope of the Mosher ester method has been extended further to amines and 5 carboxylic acids, however, discrepancies with conformational stability of diastereomers, and the resulting misinterpretation of the data may introduce error in the analysis. In addition, a small limitation is posed by the need for chemical derivatization, that requires multi-milligram quantities of material. X-ray crystallography is the most unambiguous method for absolute stereochemical determination. Although X-ray crystallography provides full stereochemical analysis of chiral compounds in a single experiment, several shortcomings associated with the method, such as the need to obtain an X-ray quality crystal, the need for multi milligram amounts of the compound of 6
 interest, and the requirement of a heavy atom in order to obtain absolute stereochemistry make this method less desirable. These limitations necessitate the development of a general and more accessible protocol that allows for easy stereochemical determination. 1-2 Optical Rotatory Dispersion (ORD) and Circular Dichroism (CD) spectroscopy λ E M Direction of Propagation Figure
1­5. Light as electromagnetic radiation. For interpretation of the references to color in this and all other figures, the reader is referred to the electronic version of this dissertation. 
 Chiroptical methods based on optical rotatory dispersion (ORD) and circular dichroism (CD) spectroscopy have been employed for the absolute stereochemical determination of chirality. The advantage of chiroptical analysis is the high sensitivity, generally requiring only µM concentrations of compound. Optical Rotatory Dispersion and Circular Dichroism are based on the ability of chiral molecules to interact with polarized light. Polarized light is a transverse wave, consisting of both an electric 7
 and magnetic component, which oscillate perpendicular to one another in the direction of the 18 propagation of light, thus forming a right handed coordinate system. 18 is defined by the direction of its electric field vector. The polarization of light (Figure 1-5) Ordinary light sources such as the sun or a light bulb are unpolarized, since light waves propagate in all directions. On the other hand, when unpolarized light passes through a polarizing filter, only the light waves with oscillation parallel to the direction of the filter pass through. E L Polarizing
 R filter light 
 linearly
 polarized
 light left
and
right
 circularly
polarized
 light
 Figure 1-6. Linearly and circularly polarized light. The light passing through the filter is now aligned in one plane of oscillation, and is defined as linearly polarized light. Linearly polarized light only oscillates in one specific direction, where the electric field (E) remains constant in magnitude, but traces out a helix as a function of time, 20 and can be resolved into two circularly polarized light beams: a left circular component (L) and a right circular component (R) as shown in Figure 1-6. The two basic phenomena of the interaction of light and matter, absorption (or extinction (ε)) and a decrease in velocity (c), are caused by the interaction of the electric (E) vector of the propagating wave with the electrons of the component atoms. This interaction has two effects: 8
 reducing the velocity of propagation (also called retarding the light) and decreasing the amplitude of the E vector. Reducing the velocity of propagation is called refraction and is described by the index of refraction, n (first described by Fresnel in 1825), and decreasing the amplitude of the vector E is called absorption and is described by the molar absorption coefficient, ε. For most substances, simple refraction and absorption are the only detectable results of such an interaction even if the light is polarized. However, when polarized light interacts with chiral matter, its properties such as intensity (amplitude), polarization, velocity (c) or refraction (n), wavelength (λ), etc may change. 21 For instance, when plane polarized light passes through an achiral or racemic compound, refraction or absorbance of the left and right circularly polarized light is equally affected. On the other hand, if it passes through an optically active medium (Figure 1-7) the refraction (equation 1-1) 18 and absorbance of either one of the circularly polarized components is altered to a greater extent as compared to the other component. 21 x left and right circularly polarized light light
propagation y chiral
sample z right
circularly
polarized
light Figure
1­7. Circularly polarized light passing through a chiral medium, which absorbs the left circularly polarized component more than the right. 9
 Δn = nL - nR ≠ 0 (1-1) Δε = εL - εR ≠ 0 (1-2) Symbols nL and nR are the refractive indices for left and right circularly polarized light, respectively. Because the left and right circularly polarized light travels through the optically active medium at different velocities, i.e. they are characterized by the different refraction indexes n, the two components are no longer in phase, and the resultant vector is rotated by an angle α relative to the original plane of polarization. The angle α, reflecting this rotation of the vector is called an optical rotation. Optical rotation is the oldest of chiroptical methods that is used as a criterion for enantiomeric purity only if optical rotation data of the compound had been previously identified. When specific optical rotation [α] is plotted as a function of wavelength, an Optical Rotatory Dispersion (ORD) spectrum is generated, which can be used for absolute 34 stereochemical determinations. The specific optical rotation can be calculated from the 18 observed angle of rotation α, as expressed by equation 1-3 where α is the angle of rotation -1 (degree units), c is the mass-based concentration of the sample (g mL ), and l is the path length (decimeters). [α] = α/cl (1-3) The difference in refractive indices for left and right circularly polarized light is related to the 18 angle of rotation, α, and is represented by equation 1-4: α = (nL – nR) 1800 l / λ0 10
 (1-4) where α is the angle of rotation, in degrees, nL and nR are the refractive indices for left and right circularly polarized light, l is the path length in decimeters, and λ0 is the wavelength in vacuum of the light beams, in centimeters. Because ORD is only based on the difference in the refractive indices, and all chiral molecules exhibit a molecular refraction at almost any wavelength of irradiation, theoretically, ORD can be detected over all wavelengths. Typically, the sodium D-line (589 nm) is used to detect and quantitate optical activity. In contrast to ORD, Circular Dichroism (CD) is based on the difference in absorption between the left and the right circularly-polarized components of a circularly-polarized light (equation 1-2). 18 Due to nonequivalent absorption, the two circularly polarized components are not only out of phase, but also of unequal amplitude. Consequently, the resulting electric field vector E (Figure 1-6) does not oscillate along a straight line but rotates along an ellipsoid path and produces elliptically polarized light. Both ORD and CD effects are manifestations of the same phenomenon and are also referred to as the Cotton Effect (CE), in honor of French physicist Aimé Cotton who first observed both phenomena. CD is an absorptive process and therefore, it can be detected for chromophore-containing molecules in the vicinity of 18 an absorption band, and plots Δε vs. wavelength. The molar amplitude A of an ORD can be related to the intensity of the CD curve, Δε, by equation 1-5: A ≈ 40.28Δε 22 (1-5) The shape and appearance of a CD curve is similar to that of the ordinary UV-vis absorption curve of the electronic transition to which it corresponds. The only difference is that, unlike the ordinary UV-vis absorption curves, CD curves may be positive or negative as shown in Figure 1- 11
 8, depending on the absorptive properties of the chiral media and, subsequently, the outcome of Equation 1-2. + + 0 Δε 0 - nm Δε nm A. positive CD - B. negative CD Figure 1-8. Positive (A) and negative (B) Cotton effects. 
 The sign of CD depends on the direction of a momentary dipole called electric dipole transition moment (edtm) µ, resulting from the excitation of the chromophore from its ground state to an excited state. The direction of µ is the same as the direction in which the electrons are pushed during the transition. In an achiral molecule (Figure 1-9A), the net electron redistribution upon interaction with the light is always planar, and the net µ is nulled (Figure 1-9A). In a chiral molecule (Figure 1-9B) 23 the electron rearrangement is always helical and the electronic transition causes the charge displacement, which generates the electron dipole transition moment. If the helix of electron motion is right-handed then a positive CD is observed, and vice versa, i.e. if the helix of electron motion is left-handed then a negative CD is observed. 12
 A B coronene µ=0 hexahelicene m net µ net m Figure 1-9. Electron redistribution upon light excitation for a transition of A achiral and B chiral molecule. The rotation of the electric charge creates a magnetic field, the strength and direction of which may be described by the magnetic transition dipole moment denoted by the vector r m 5 (Figure 1-9). The direction of a magnetic transition moment, can be determined by application 24 of the “right hand rule” (Figure 1-10 ) to the rotation of the charge (circular electric current). Instructively, the outstretched thumb points to the direction of the magnetic transition dipole 25 moment when the right hand fingers are curved in the direction of electron flow. 13
 m The fact that left-handed circularly polarized light induces a right-handed transition follows from the interaction of the electric vector, and the r µ electron flow component of the magnetic field, referred to as r m rotational strength, R. The rotational strength, R, electron flow which is a theoretical parameter representing the sign and strength of a CD Cotton effect (CE), is given by the scalar product of the electric and magnetic m transition moments (Equation 1-6): 5 Figure 1-10. The right hand rule to determine the direction of magnetic dipole transition moment (m). r r R = µ . m = µmcos β (1-6) Where µ and m are the electric and magnetic transition dipole moments, respectively, and β is the angle between the two transition moments. The sign of the CE is positive when the angle is acute (0 < β < 90°) or in the limiting case, parallel, and it is negative when the angle is obtuse (90° < β < 180°) or in the limiting case, antiparallel. Dextrorotation results when R > 0, together with positive CD, and levorotation is generated when R < 0, together with negative CD curve. There is no CE when the electric and magnetic transition dipole moments are perpendicular to each other. 14
 1-2.1 A brief introduction to Circular Dichrometers (spectropolarimeter) Circular dichrometers are used to record both ORD and CD spectroscopy. The essential features of a spectropolarimeter are shown in Figure 1-11. modulator light polarizer light source linearly polarized light sample circularly polarized light CD spectrum Δε nm Lock-in amplifier photomultiplier tube Figure 1-11. Schematic representation of a CD spectropolarimeter. 
 A xenon lamp is typically used as the source of light. This light passes through a monochromator consisting of a series of crystal prisms to produce linearly polarized light. In the CD spectropolarimeter, the optical system is comprised of two monochromators (a double monochromator), which helps in reducing stray light. The linearly polarized light is then modulated into left and right circularly polarized light. The modulator consists of a thin crystalline plate known as a wave plate. When linearly polarized light is incident on a wave plate at 45°, the light is divided into two equal electric field components, one of which is retarded by a quarter wavelength by the plate. This throws the two components 90° out of phase with each other such that upon emerging, one is always maximum, while the other is always zero and vice versa. The effect is to produce circularly polarized light. The 90° phase shift is produced by a 15
 precise thickness d of the birefringent crystal, which, because of the 90° shift, is referred to as a quarter wave plate. This then passes through the sample chamber. The light transmitted through the sample is measured by a photomultiplier tube, which produces a current whose magnitude depends on the number of incident photons. This current is then detected by a lock-in amplifier and recorded. (Figure 1-11) Most CD spectropolarimeters measure differential absorbance, ΔA, between the left and right circularly polarized light, which can then be converted to Δε based on the Beer-Lambert law, using Equation 1-7. 5 ΔA = Δε c l (1-7) Where ΔA is the difference in absorbance and Δε is the difference in molar extinction -1 -1 coefficients (M L ). Since Optical Rotatory Dispersion (ORD) and Circular Dichroism (CD) are manifestations of chiral substances and are not observed for achiral compounds or racemic mixtures, they can be used to detect and quantitate optical activity. However, ORD and CD by themselves do not allow the configuration of a given product to be defined. 1-3 Exciton Coupled Circular Dichroism (ECCD) 1-3-1 Theoretical background of ECCD Excitation of a chiral system with a single chromophore results in ORD or CD. For chiral systems containing two or more chromophores, excitation with polarized light induces a throughspace interaction referred to as the Exciton Coupled Circular Dichroism (ECCD) and results in a bisignate CD curve - ECCD spectrum. In contrast to ORD and CD, the ECCD method is a nonempirical approach to establishing the absolute configuration of chiral compounds first 34 discovered by Harada and Nakanishi. 16
 st 1 Cotton effect O O Δε O nm O O O H nd 2 Cotton effect syn Positive ECCD spectrum Figure 1-12. Exciton Coupled Circular Dichroism (ECCD) of steroidal 2,3bisbenzoate. ECCD is based on the through space exciton coupling between two or more chirally oriented non-conjugated chromophores. The coupling of the chromophores’ electric transition dipole moment leads to the observed bisignate CD spectrum and the nonempirical determination of their orientation. Figure 1-12 shows the exciton coupling between two benzoates in steroidal 2,3-bisbenzoate. 26 Due to the oscillation of the main electric transition dipole, excitation of a bis- chromophoric system yields two sets of the through space interaction of edtm: an in-phase or an 31 out-of-phase interaction. These interactions cause the energy level of the excited state (exciton) to split into two states: the out-of-phase stabilizing dipole-dipole interaction (low energy interaction, α-state) and the in-phase destabilizing dipole-dipole interaction (high energy interaction, β-state) (Figure 1-13). The difference in the λmax of the two UV-vis peaks is due to the energy gap, 2vij, and is called the Davydov splitting, 29 after the Russian physicist who developed the theory of exciton coupling in the electronic spectra of molecular crystals. 17
 β 2νij Excited state α energy Ground state Chromophore i Exciton system Chromophore j local excitation delocalized local excitation excitation Figure 1-13. Splitting of the excited states of isolated chromophores i and j by exciton interaction. The energy gap 2νij is referred to as the Davydov splitting. The two split transitions lead to two absorptions that are differentiated by the absorption wavelength: the out-of-phase transition is detected at higher wavelength, while the in-phase transition, being higher in energy, appears at lower wavelength (Figure 1-14A, dotted lines). The magnitude of 2vij depends on the nature of chromophores. For degenerate chromophores, the difference 2vij is relatively small, and the wavelength of the two transitions may be relatively similar. Alternatively, for different chromophores the energy gap 2vij between the two energy 29
 levels is substantial and the two transitions might be observed separately. Davydov splitting in the UV-Vis and the CD spectra. 18
 Figure 1-14 depicts Δλ Davydov splitting A Δλ B A λ (nm) λ (nm) Figure 1-14. The UV-vis spectrum (A) and ECCD spectrum (B) upon through space interaction of two degenerate chromophores. The two observed Cotton effects are shown using dashed lines, while the observed, summation curves are in solid lines. In case of the plain polarized light (UV-vis spectrum), the interaction of the two excited degenerate chromophores is detected as two component spectra of the same sign, which usually appear as a single absorption with double intensity (Figure 1-13B, solid line), consisting of two transition representing the α and β exciton states (Figure 1-13B, dotted line). In case of circularly polarized light, depending upon whether the edtms are in phase (symmetric) or out of phase (anti symmetric), a Cotton effect of different sign is produced (Figure 1-13C, dotted lines). As a result, a spectrum with two peaks (bisignate curve), one positive and one negative CD couplets is generated. This phenomenon is known as Exciton Coupled Circular Dichroism, leading to a bisignate spectrum (Figure 1-13B, solid line). The difference in λmax of the two peaks is 2vij. An ECCD spectrum may be positive (+) or negative (-). In a positive ECCD spectrum, the positive CD appears at higher wavelength (lower energy), followed by the negative CD at 19
 lower wavelength (higher energy). In the case of a negative ECCD spectrum, the negative CD appears at higher wavelength (lower energy), followed by the positive CD at lower wavelength (higher energy). The overall sign of ECCD generated from a bis-chromophoric exciton coupling depends on the angular arrangement of electric transition dipole moment (etdm). There are two possible orientations of electric transition dipole moments (or chromophores): clockwise or counterclockwise. Positive ECCD is observed for the clockwise orientation, and negative ECCD for the counterclockwise orientation. For example, a clockwise orientation of the chromophores in Figure 1-12 results in a positive ECCD spectrum. The relationship between helical orientation of chromophores and observed ECCD can be established based on the direction of net charge oscillation in the two coupled systems, or based on the interaction between the electric and magnetic moments of chromophores. Figure 1-15 depicts analysis of exciton coupling interaction of two chromophores or, more precisely, two etms’ of the two chromophores that are pre-set in a clockwise helical orientation relativeto one another. The helix associated with the charge rotation generated from the edtm transition can be visualized by placing the partial edtm in a cylinder, aligned along the axis of the “total” edtm, r µ (Figure 1-15). The individual edtm µ of an electric transition, for each chromophore, couples to the other in phase (symmetric) or out of phase (asymmetric), Figure 1-15 i and ii respectively. In the case i where the two electric transition dipole moments couple in phase, the “total” electric transition dipole moments are oriented along the chromophoric C2 axis, and in case B, where they couple out of phase, the “total” electric transition moments are oriented perpendicular to the chromophoric C2 axis. As shown in Figure 1-15 (i), the symmetric coupling results in a counterclockwise helical movement, therefore, according to the right hand rule, the magnetic transition dipole moment (m) is anti-parallel to μ, leading 20
 (i) High Energy Counterclockwise (ii) Low Energy Clockwise Δε nm µ µ nd 2 m Negative Cotton effect m Positive 1 Cotton effect st Positive ECCD spectrum Figure 1-15. The expected ECCD spectrum of dibenzoate and its rationalization. to a negative Cotton effect (negative CD band). (Figure 1-15 i) As expected, the out of phase interaction causes the opposite effect leading to parallel m and µ vectors (Figure 1-15 ii) leading r r to a positive Cotton effect (positive CD band). According to Equation 1-6, (R = µ . m = µmcos β) the sign of a Cotton effect depends exclusively on the angle between the electric and the magnetic transition dipole moment. Therefore, the symmetric coupling of the edtm will result in a negative peak (β = 180°, cosβ = -1), while the anti-symmetric will result in a positive one (β = 0°, cosβ = 1), both of equal magnitude. The positions of these two Cotton effects relative to each other (which defines whether the overall ECCD spectrum is positive or negative) depends on the relative energies of the two occurring transitions. The in phase coupling is of higher energy because of the repulsion between like charges, and so the corresponding negative Cotton effect will appear at a shorter wavelength. Because nomenclature dictates that the 21
 st bisignate ECCD curve be named after the lower energy 1 Cotton effect, the spectrum of the dibenzoate discussed above (Figures 1-15) will be referred to as a positive ECCD curve. A B O O (i) O (ii) O O O H syn Figure 1-16. A qualitative explanation of ECCD. A. Structures of benzoate and dibenzoate esters. edtm’s are shown in red; B. Possible in phase (i) and out of phase (ii) interactions of the edtm’s of the degenerate chromophores (shown in blue). Exciton Coupled Circular Dichroism can be further explained by examining its 25 application to a specific molecule, bearing two identical benzoate groups shown in Figure 1-16, where benzoate esters of a vicinal cyclohexanediol are interacting. Benzoate esters have a strong UV-Vis absorption band at around 230 nm, arising from the π → π* transition of the conjugated aromatic ring with the carbonyl group. 30 The large electric dipole transition moment µ of each benzoate group is oriented collinearly with the long axis of the molecule, almost parallel to the direction of the C-O bond, and oscillates in both directions (Figure 1-16A). Determination of absolute configuration means determination of the absolute sense of chirality between the C(2)-O and C(3)-O bonds, by looking down the C-C bonds from front to back, the orientation of the two 22
 benzoate groups is set in a clockwise manner, which leads to positive chirality. The absolute sense of twist stays the same regardless of whether it is viewed from C(3) to C(2) or vice versa. 1.3-2 The Quantum Mechanics Explanation of ECCD The ECCD theory can also be explained by Quantum Mechanical considerations. As mentioned previously, when a molecule contains two identical chromophores, as a result of their through space interactions, excitation is delocalized between the two chromophores, and the excited 31 state is split into two α and β states (Figure 1-13). r 1 r r R" ,# = ± $% 0 Ri j • (µioa & µ joa ) 2 1-8 Based on theoretical calculations on the binary system, the rotational strength, R, which represents the sign and the strength of a CD Cotton effect, can be defined by Equation 1-8,26 ! where the positive and negative signs correspond to α- and β- state, respectively, Rij is the distance vector between two chromophores, µioa and µjoa are the etdm of excitations, and σo is the excitation number of transition from 0 to α state. Table 1-1. Definition of Exciton Chirality for a Binary System.
 Qualitative Definition Quantitative Definition Cotton effects Positive Chirality Rij • (µioa " µjoa)Vij > 0 positive first and negative second Cotton effects Negative Chirality Rij • (µioa " µjoa)Vij < 0 negative first and positive second Cotton effects ! 
 ! 23
 If Equation 1-8 is positive (+) the observed ECCD spectrum is positive, and if it is negative (-) then the observed ECCD spectrum is negative. From the equation above, the sign of 32 the bisignate curve completely depends on the spatial orientation of the two chromophores. As 33 shown in Table 1-1, if the electric transition dipole moments of the two chromophores from front to back constitutes a clockwise orientation, then according to Equation 1-8 above, a st nd positive bisignate spectrum will be observed, which refers to a positive 1 and negative 2 Cotton effects, while an opposite but otherwise identical spectrum is produced by the counterclockwise orientation. 34,35 Therefore, the sign of an ECCD spectrum for a chiral molecule can be predicted as long as the spatial orientation of two chromophores is known. Moreover, the sign of a bisignate ECCD spectrum can enable the determination of the absolute orientation of two chromophores in space in a non-empirical manner. st nd The difference in Δε between the 1 and 2 Cotton effects is called the amplitude, A, of the 36 ECCD couplet (Figure 1-14). This amplitude depends on several factors: a. Molar absorption coefficient (ε) of the interacting chromophores. The amplitude is 2 proportional to ε . Therefore, in order to have increased sensitivity with ECCD methods, chromophores with strong absorptions are preferred. These highly active chromophores enable CD measurements to be performed at micro-molar concentrations, which make it an extremely useful property when only limited amount of chiral compound is available. b. Interchromophoric distance (R). The amplitude is inversely proportional to the distance 37,38 R, between the interacting chromophores. To achieve enhanced interaction, the coupling chromophores should be oriented close to each other in space. This tendency is 24
 29 exemplified by a series of dibenzoates as shown by Nakanishi, (Figure 1-17) where remote dibenzoates generally exhibit weaker amplitudes. N O O O H N O O O O O N 1,4- dibenzoate 1,6- dibenzoate +89.4 +27.5 O H O H N O O H N N 1,8- dibenzoate -26.4 Figure 1-17. Dibenzoate chirality method – Distance effect. Although the 1,8-dibenzoate has an interchromophoric distance of 12.8 Å, a relatively strong ECCD is still observed. Generally, a distance of about 13 Å is enough to observe ECCD for most organic molecules. However, strong chromophores such as porphyrins (Soret band λmax at 415 -1 -1 nm, ε = 350,000 M cm ) are known to couple at distances of up to 50 Å. 25
 39-41 c. Projection angle between the interacting chromophores. The A value is maximal at a chromophoric projection angle of around 70°. There is no exciton coupling when the chromophores are either parallel or 180° to each other. d. Number of interacting chromophores (X, Y, Z). The A value of the system is the summation of the A values of each pair of interacting chromophores, i.e. the principle of pair-wise additivity holds in systems comprising three or more chromophores: Atotal = Axy+Axz+Ayz. This observation has been proven by experiments and theoretical calculations. 42-44 Exciton coupling is not limited to degenerate chromophores, and can occur between different chromophores if the UV-Vis absorption λmax values for the interacting chromophores are relatively close. In systems containing two different chromophores, two opposite Cotton effects appear, slightly red- and blue-shifted from the respective maxima of the interacting chromophores. 26 The interacting chromophores do not have to be within the same molecule, as long as they are in close proximity in space. In particular, supramolecular compounds, like 45 stacking of anthocyanins 41 and porphyrin containing brevetoxins have been reported to give rise to exciton coupled CD bands. 1.3-3 ECCD method in determining absolute stereochemistry of chiral molecules The solid correlation between the sign of the ECCD spectrum and the absolute orientation of chromophores, signifies the non-empirical character of the ECCD method. Considering the dependence of ECCD on the presence of chromophores, ECCD spectroscopy can be applied to 26
 the characterization of a wide variety of chromophore-containing molecules. Alternatively, in the absence of a chromophore, chemical modification with appropriate chromophoric groups can provide a conjugate that is amenable for ECCD analysis. As a result, non-empirical chirality determination of substrates of chemical and biological interest has been achieved.29,
 46‐49 Figure 1-18 is an example of determining the absolute stereochemistry of (+)-abscisic acid by the ECCD method. 50,51 Abscisic acid, a plant hormone, contains two non-conjugated chromophores, an enone and a dienoic acid, separated by the only chiral center, a tertiary alcohol. HOOC OH O COOH O OH Figure 1-18. ECCD active abscisic acid. The enone and the dienoic acid moieties interact through space, resulting in a positive ECCD couplet. Based on the sign of the spectrum, the two chromophores must be oriented in a clockwise manner, which directly indicates the β-configuration of the hydroxyl group. 27
 A R OAc HO O O O-glcNAc O Pavoninin-4 --------------------------------------------------------------------------------------------------------------------O O O O O B O i-Pr O Periplanone B reduction O O O OH O OBz benzoyl protection i-Pr i-Pr Figure 1-19. Application of the ECCD method in absolute configurational assignment of natural products. Figure 1-19 shows two examples where introduction of a benzoyl group via hydroxyl derivatization allowed for ECCD detection, as a result of the coupling of Bz with a preexisting chromophore in the molecule. In A, the configuration of the 15-glcNAc group in Pavoninin-4, which is a shark repellant, was determined according to the observed ECCD couplet between the 52 enone and the introduced bromobenzoate moiety. Figure 1-19B depicts stereochemical determination of the potent cockroach sex excitant periplanone B, containing only one chromophore (the diene). The absolute stereochemistry of the 28
 macrocycle was determined after reduction of the carbonyl and derivatization of the resulting secondary alcohol with a chromophore. The observed negative ECCD established the counterclockwise correlation of the secondary hydroxyl group with respect to the diene in the favored conformation of the macrolactone. Subsequently, the orientation of the epoxide and the 53 i-Pr were defined. 1-4 Alternative Chromophoric Hosts in ECCD Studies As mentioned earlier, the ECCD method requires the presence of at least two interacting chromophores in the molecule. This requirement limits the use of ECCD method with inherently achromophoric molecules that also lack a point of derivatization. In order to address this issue, chromophoric receptors have been designed to act as hosts for non-chromophoric, chiral molecules. Once the chiral substrate binds to the host receptor, either covalently or noncovalently, it will induce the host to adopt a preferred chiral conformation, which can then be observed as an ECCD couplet. This chiral induction was first demonstrated on dyes bound to 54 polypeptides in their helical conformations. This binding induced a helical orientation within the chromophoric molecule producing an ECCD-active species. The process for transmission of configurational information comprises at least two elementary processes: (1) complex formation between the chiral molecule (guest) and the chromophoric receptor (host) and (2) some dynamic 55,56 processes associated with it, usually a conformational change of interacting molecules that can induce the signal. If the interactions between the two molecules are strong enough this will lead to efficient transmission of information. In order to simplify the observed ECCD signal, the chromophoric hosts used for ECCD studies are either achiral or in the form of racemates. When 29
 an achiral host is used, it can adopt the dictated chirality of the chiral substrate that binds to it through chiral induction. This induction leads to the formation of a preferred helical conformer, the orientation of which can be determined by ECCD. In the case of a chiral host, preferential binding of the chiral substrate with one of the enantiomers of the ECCD active host yields an induced ECCD spectrum. Br HO OH Br O2N + Br HO OH Br O2N H N NO2 R NH HN R H H Br O O Br tBu tBu N H O2N NO2 NO2 Figure 1-20. The induced chirality of biphenol upon binding to chiral amine. Binding between the host and guest molecules can either be covalent or non-covalent, depending on the complex being formed. One example of induced chirality on a prochiral host, 57 upon interaction with a chiral guest, is shown in Figure 1-20. The biphenol shown is an ECCD active molecule (atropisomeric molecule), however, the racemic mixture of the two enantiomers is ECCD silent. When the chiral trans-1,2-cyclohexane diamine is added to the racemic mixture of the biphenyl, a matched complex with one enantiomer of biphenol is formed as shown due to the hydrogen bonding between the phenols and the amine groups. 58-60 Formation of an optically active complex is observed as an ECCD spectrum, the sign of which is dictated by the absolute stereochemistry of the chiral diamine. 57 This is an example of non-covalent complex formation. In a similar manner, covalent 30
 61 bonding has been used to determine the absolute stereochemistry of chiral secondary alcohols. As shown in Figure 1-21, the chromophoric reagent, 3-cyanocarbonyl-3’-methoxycarbonyl-2,2’binapthalene, can be esterified with chiral secondary alcohols and the resultant complex exhibits induced ECCD as a result of favoring one atropisomer, due to restricted rotation about the C-C phenyl bonds. The chirality has been transferred from the chiral alcohols to the binaphthylene system by minimizing the steric interaction between the substituents on the chiral alcohols with the methyl ester group in the binaphthylene. COOMe COCN COOMe COOR* R*OH COOMe COCN DMAP, CH3CN Figure 1-21. Determination of stereochemistry of chiral alcohols by ECCD. 31
 A. B. C. OH O R NH2 Br Base OH N N N O R Cu(ClO4)2 NH4SCN X OCu N N N O R S-Configuration X N O Cu N N O R R-Configuration Figure 1-22. Derivatization methods for analysis of absolute configurations with various chromophores. 32
 In recent years, several methods for the determination of absolute stereochemistry of difunctional amines by the ECCD method have been developed, after derivatization of the functional groups to introduce the required chromophores. Figure 1-22 shows a few examples of chromophores that have been applied. In A, Gawronski et. al. 62 used pthalimide to study the absolute stereochemistry of diamines. Their study also included the use of benzoate as the chromophore for hydroxyl and carboxyl groups to include the study of absolute stereochemistry of amino acids and amino alcohols. Lo et al. 63 have used 7-diethylaminocoumarin-3-carboxylate shown in B for amines, hydroxyl and carboxyl functional groups. 64-66 Canary and coworkers have published a number of systems, one of which is shown in C, where they make use of quinoline chromophores for the formation of chiral propellers. In these systems, a tripodal system bearing a chiral group on one of the arms is synthesized. Upon complexation of Cu(II) or Zn(II), one of the two possible propeller conformations (P or M) is formed, depending on the chirality of the original molecule, giving rise to the corresponding characteristic ECCD couplet. 1-4.1 Use of Zinc Porphyrins as Chromophoric Hosts Throughout the literature, there is a predominance of porphyrin-based systems as chromophoric hosts for application in the ECCD method of stereochemical determination. The study of porphyrins has received increased interest in recent years since they have been utilized 67 for the development of various projects, both of chemical and biochemical interest. 33
 Especially in the case of chromophoric receptors for the determination of stereochemistry using ECCD, the 68 unique features of these highly conjugated rings have made them extremely attractive targets: (1) Their planar structures provide a well-defined binding pocket that is accentuated by substitutions on the ring. There are many sites that can be derivatized, such as the meso and βpositions, the central metal and the inner nitrogen atoms. By varying the substituents on the periphery of porphyrins, the solubility of the porphyrin containing compounds can be easily modified, for use in both polar and non-polar solvents. -1 (2) Porphyrins have a large extinction coefficient of around 400,000 M -1 cm ; the amplitude (A) of the Cotton effects depend on the extinction coefficient of the chromophores and hence the intense absorption of the porphyrins greatly enhance the sensitivity of CD. (3) Absorption maximum of porphyrins rests around 418 nm for the main absorption band (Soret band), located in a region of the spectrum far red shifted than most chromophores that likely preexist in the system under investigation (typically carbonyls and olefins). This prevents the unwanted interaction between the introduced chromophore and the chiral substrate that can potentially complicate the spectral analysis. (4) Metal incorporation into the porphyrin ring can be easily achieved, providing a coordination center for the binding of the chiral molecule. Metalloporphyrins, such as zinc porphyrins and magnesium porphyrins, can provide extra stereodifferentiation with their Lewis acidic binding sites. Therefore, porphyrins have been recognized as the ideal chromophores for detecting subtle changes in their close environment, including chirality induction. (5) Porphyrins have efficient electric transition dipole moments that can couple over a distance of ~50Å. 26 34
 TPP Porphyrins have been widely utilized in stereochemical studies by CD. These include 40,69,70 absolute configurational assignments, stereochemical differentiations of sugars and amino acids,71,72 and interactions with bio-macromolecules. 73,74 Porphyrins, such as the alkyl connected bis-metalloporphyrins shown in Figure 1-23, can be used as chromophoric host systems for stereochemical determination of chiral compounds because of their ability to bind to the chiral compounds at the metal centers and report the chirality of bound guests based on Exciton Coupled Circular Dichroic Spectroscopy (ECCD). 67,75-78 N Zn N N N N Zn N O N Zn N Zn or O O O Zn O Zn O O O Figure 1-23. Structure and schematic representation of a porphyrin tweezer. For instance, upon complexation with a chiral L-lysine methyl ester (through Zn-N coordination, Figure 1-24) the two porphyrins of the achiral tweezer adopt a helical orientation induced by the helicity of the bound diamine. The two possible ECCD active conformations are clockwise ((+)helicity) or counterclockwise ((-)-helicity), yielding a positive or a negative ECCD, respectively. It is now accepted that induction of helicity directly correlates with the difference in the relative 35
 sizes of the substituents, defined by their A-values. Viewing the system depicted in Figure 1-24, the bound diamine bears two substituents at the chiral center that are not involved in binding and, therefore, create an asymmetric environment in the tweezer binding pocket. Those are the ester group and the hydrogen. According to the A-values, hydrogen is considered as the small group, while the ester group (COOMe) plays the role of the large substituent. The porphyrin closest to the chiral center (‘stereo-differentiating” porphyrin) faces the most steric interaction that dictates the final helical disposition of the two porphyrins. Steric interaction within the complex is expected to be alleviated upon positioning the stereodifferentiating porphyrin away from the larger substituent towards the smaller one. This discrimination generates the “chiral screw” that is detected by the CD spectrometer. Thus, the presented (S)-diamine differentiation between the small “H” and large “COOMe” is expected to induce a counterclockwise helical twist between the two porphyrins. Indeed, the ZnTPP-tweezer complex with (S)-lysine methyl ester exhibits a negative ECCD. The alternative, clockwise helical twist, in which porphyrin positions closer to the larger substituent, can be considered disfavored, relative to the counterclockwise twist. Direct correlation between the experimentally-observed sign of ECCD and the A-values of the substituents at the chiral center has been established in screening of a variety of chiral substrates. The role of A-values in stereo-differentiations allows for a direct translation of the observed ECCD sign into the appropriate special distribution of the substituents at the chiral center and, therefore to establishing the absolute chirality of the bound guest. Such direct correlation defines porphyrin tweezer-based analysis as a non-empirical and highly reliable and appealing tool for the absolute stereochemical determination of other classes of chiral substrates. Use of porphyrin tweezers, however, is limited by the requirement of dual coordination. Therefore, compounds that have one site of attachment such as chiral primary and secondary 36
 amines, alcohols and carboxylic acids cannot be directly used in the stereo-determination with the porphyrin tweezer because the single site of attachment means coordination to only one metalloporphyrin. This does not lead to the formation of a sandwiched host-guest complex, where the chiral substrate is locked inside the tweezer. Consequently, the relative orientation of the two porphyrin chromophores as well as the electric dipole transition moments will be random due to the free rotation around the pentylene linker resulting in unpredictable or zero CD signal. In order to solve this problem, these molecules have been derivatized with “carriers” which offer the required extra binding sites (usually nitrogen-containing functionality) for ditopic complexation. (Figure 1-25) Various carriers designed for binding and analysis of molecules, such as alcohols, monoamines, carboxylic acids, allow for the systematic analysis of these substrates and introduction of working mnemonics correlating the observed ECCD sign with the ECCD active conformation (i.e. the spatial orientation of the substituents at the chiral center) of the bound guest. Zn O Zn O O Porphyrin tweezer L H N H2N O P1 L P2 M P2 S O Zn O N H NH2 Zn S M O O O O Figure 1-25. Determination of chirality for derivatized α-carboxylic acid. 37
 P1 L While the carrier methodology enables wide use of tweezers as chirality sensors, direct complexation of chiral molecules (without derivatization) is one of our primary goals. In pursuit of this goal, Inoue and co-workers designed an octaethyl substituted porphyrin tweezer linked by an ethylene linker (Figure 1-26) for use with mono alcohols and amines for ECCD 81,82 Upon binding with this tweezer, the steric interaction between the chiral measurements. center and the ethyl groups at the 3,7 positions of the non-bound porphyrin to slide away generating a right-handed screw for S- substrates and a left-handed screw for R- substrates leading to ECCD signals. However this method requires a large excess of chiral substrates 1,00010,000 equivalents, and the signals for alcohols were weak. X 7 N NM N N Me R H X = OH, NH2 N 3 X 3 7 Me R H 7 3 7 N M N N 3 M = Mg or Zn Right Handed Screw Figure 1-26. Inoue’s porphyrin tweezer for stereochemical determination. 
 The signal strength is often used as a criterion of sensitivity for the receptor molecule. One of our primary goals is to improve the sensitivity of the receptor and to develop more widely applicable chirality sensors. 38
 1-5 Research Aim The use of porphyrin tweezers for assignment of absolute stereochemistry has grown a lot in the last decade. However, a key challenge and, moreover, the aim of this thesis work, is the study of mono coordinating compounds, such as carboxylic acids, primary and secondary amines and alcohols. These compounds cannot be directly used in the stereo-determination with the porphyrin tweezers because the one site of attachment means coordination to only one metalloporphyrin. This does not single to the formation of a helical twist within the complex, hence, they cannot be studied in this way. To date, the lion’s share of research in this area requires derivatization of the compounds with “carrier” molecules in order to introduce a second coordination site. Consequently, the aims of this thesis can be summarized as follows: • Design, synthesis and study of a range of electronically and sterically tuned tweezers to afford useful candidates with enhanced binding affinity as well as sensitivity. With the improved binding affinity, these porphyrin tweezers would be applied for ECCD study of several different classes of oxygen containing compounds as well as remote chirality sensing. • Design, synthesis and evaluation of novel host molecules that will allow for the absolute stereochemical assignment of compounds containing one coordination site without the need for derivatization. The outcome of this research will provide chemists facile and reliable methods for nonempirical assignment of chirality for a series of important organic molecules at the microscale devoid of any chemical derivatization, which is a challenging task for conventional methods. This study will open a broad pathway for absolute stereochemical determination using the exciton coupled circular dichroism protocol. 39
 REFERENCES 40
 References: 1. Pasteur, L. “The Assymmetry of Natural Occuring Compounds” American Book Company, New York, 1901. 2. Allenmark, S.; Gawronski, J. "Determination of Absolute Configuration - An Overview Related to This Special Issue" Chirality 2008, 20, 606-608. 3. Stephens, P. J.; Devlin, F. J.; Pan, J. J. "The Determination of the Absolute Configurations of Chiral Molecules Using Vibrational Circular Dichroism (VCD) Spectroscopy" Chirality 2008, 20, 643-663. 4. Thomson, W. "Kelvin's Baltimore Lectures and Modern Theoretical Physics. Historical and Philosophical Perspectives" MIT Press 1987. 5. Eliel, E. L. W., S. W. "Stereochemistry of Organic Compounds" Wiley: New York 1994. 6. Bruice, P. Y. "Organic Chemistry 3rd ed." Prentice-Hall, Inc.: Upper Saddle Rivevr, New Jersey 2001. 7. Rossi, R.; Diversi, P. "Synthesis, Absolute Configuration, and Optical Purity of Chiral Allenes" Synthesis-Stuttgart 1973, 25-36. 8. Oki, M. "In Topics in Stereochemistry (Eds.: Allinger, N. L.; Eliel, E. L.; Wilen, S. W. " Interscience Publication, New York 1983. 9. Laarhoven, W. H.; Prinsen, W. J. C. "Carbohelicenes and Heterohelicenes" Topics in Current Chemistry 1984, 125, 63-130. 10. Prinsen, W. J. C.; Laarhoven, W. H. "Determination of the Enantiomeric Excess of Hexahelicene and its Methyl-Substituted Derivatives by High-Performance LiquidChromatography" Journal of Chromatography 1987, 393, 377-390. 11. Bada, J. L. "Meteoritics - Extraterrestrial Handedness?" Science 1997, 275, 942-943. 41
 12. Bonner, W. A.; Kavasmaneck, P. R. "Asymmetric Adsorption of dl-alanine Hydrochloride by Quartz" Journal of Organic Chemistry 1976, 41, 2225-2226. 13. Bonner, W. A.; Kavasmaneck, P. R.; Martin, F. S.; Flores, J. J. "Asymmetric Adsorption by Quartz - Model for Prebiotic Origin of Optical-Activity" Origins of Life and Evolution of the Biosphere 1975, 6, 367-376. 14. Cronin, J. R.; Pizzarello, S. "Enantiomeric Excesses in Meteoritic Amino Acids" Science 1997, 275, 951-955. 15. Dale, J. A.; Mosher, H. S. "Nuclear Magnetic Resonance Enantiomer Reagents Configurational Correlations via Nuclear Magnetic Resonance Chemical Shifts of Diastereomeric Mandelate, o-Methylmandelate, and Alpha-Methoxy-AlphaTrifluoromethylphenylacetate (mtpa) esters" Journal of the American Chemical Society 1973, 95, 512-519. 16. Feringa, B. L.; van Delden, R. A. "Absolute Asymmetric Synthesis: The Origin, Control, and Amplification of Chirality" Angewandte Chemie-International Edition 1999, 38, 3419-3438. 17. Freire, F.; Seco, J. M.; Quinoa, E.; Riguera, R. "The Assignment of the Absolute Configuration of 1,2-diols by Low-Temperature NMR of a Single MPA Derivative" Organic Letters 2005, 7, 4855-4858. 18. Hazen, R. M.; Filley, T. R.; Goodfriend, G. A. "Selective Adsorption of L- and D-Amino Acids on Calcite: Implications for Biochemical Homochirality" Proceedings of the National Academy of Sciences of the United States of America 2001, 98, 5487-5490. 19. Kavasmaneck, P. R.; Bonner, W. A. "Adsorption of Amino-Acid Derivatives by d-Quartz and l-Quartz" Journal of the American Chemical Society 1977, 99, 44-50. 20. Lambert, J. B. S., J. F.; Lightner, D. A.; Cooks,R. G. "Organic Structural Spectroscopy" Prentice Hall, Upper Saddle River 1998. 21. 22. Stinson, S. C. "Chiral chemistry" Chemical & Engineering News 2001, 79, 45-+. Solomons, T. W. "Organic Chemistry" John Wiley & Sons Inc., New York 1978. 42
 23. Velluz, L. L., M.; Grosjean, M. "Optical Circular Dichroism" Academic Press, New York 1965. 24. Mislow, K.; Glass, M. A. W.; Obrien, R. E.; Rutkin, P.; Djerassi, C.; Weiss, J.; Steinber.Dh "Configuration Conformation and Rotatory Dispersion of Optically Active Biaryls" Journal of the American Chemical Society 1962, 84, 1455-&. 25. Snatzke, G. "Semi-Empirical Rules in Circular Dichroism of Natural Products" Pure and Applied Chemistry 1979, 51, 769-785. 26. Vasileiou, C. PhD Thesis 2006. 27. Snatzke, G. "Circular Dichroism and Absolute Conformation - Application of Qualitative mo Theory to Chiroptical Phenomena" Angewandte Chemie-International Edition in English 1979, 18, 363-377. 28. Harada, N. N., K. "Circular Dichroic Spectroscopy: Exciton Coupling in Organic Stereochemistry" University Science Books: Mill Valley 1983. 29. Kuhn, W. "The Physical Significance of Optical Rotatory Power" Transactions of the Faraday Society 1930, 26, 0293-0307. 30. Kirkwood, J. G. "On the Theory of Optical Rotatory Power" Journal of Chemical Physics 1937, 5, 479-491. 31. Nakanishi, K. "Circular Dichroism Principles and Applications" 1994. 32. Wiesler, W. T.; Vazquez, J. T.; Nakanishi, K. "Pairwise Additivity in Exciton Coupled cd Curves of Multichromophoric Systems" Journal of the American Chemical Society 1987, 109, 5586-5592. 33. Chen, S. M. L.; Harada, N.; Nakanishi. K. "Long-Range Effect in Exciton Chirality Method" Journal of the American Chemical Society 1974, 96, 7352-7354. 43
 34. Harada, N.; Nakanishi, K. "Exciton Chirality Method and its Application to Configurational and Conformational Studies of Natural Products" Accounts of Chemical Research 1972, 5, 257-&. 35. Harada, N.; Chen, S. L.; Nakanishi, K. "Quantitative Definition of Exciton Chirality and Distant Effect in Exciton Chirality Method" Journal of the American Chemical Society 1975, 97, 5345-5352. 36. Hug, W.; Wagniere, G. "Optical Activity of Chromophores of Symmetry C2" Tetrahedron 1972, 28, 1241-&. 37. Wagniere, G.; Hug, W. "Polarization and Sign of Long Wavelength Cotton Effects in Chromophores of Symmetry C2" Tetrahedron Letters 1970, 4765-&. 38. Weckerle, B.; Schreier, P.; Humpf, H. U. "A New one-step Strategy for the Stereochemical Assignment of Acyclic 2-and 3-Sulfanyl-1-Alkanols Using the CD Exciton Chirality Method" Journal of Organic Chemistry 2001, 66, 8160-8164. 39. Oancea, S.; Formaggio, F.; Campestrini, S.; Broxterman, Q. B.; Kaptein, B.; Toniolo, C. "Distance Dependency of Exciton Coupled Circular Dichroism Using Turn and Helical Peptide Spacers" Biopolymers 2003, 72, 105-115. 40. Pescitelli, G.; Gabriel, S.; Wang, Y. K.; Fleischhauer, J.; Woody, R. W.; Berova, N. "Theoretical Analysis of the Porphyrin - Porphyrin Exciton Interaction in Circular Dichroism Spectra of Dimeric Tetraarylporphyrins" Journal of the American Chemical Society 2003, 125, 7613-7628. 41 Matile, S.; Berova, N.; Nakanishi, K. "Intramolecular Porphyrin pi, pi-Stacking: Absolute Configurational Assignment of Acyclic Compounds with Single Chiral Centers by Exciton Coupled Circular Dichroism" Enantiomer 1996, 1, 1-12. 42. Matile, S.; Berova, N.; Nakanishi, K. "Exciton Coupled Circular Dichroic Studies of Self-Assembled Brevetoxin - Porphyrin Conjugates in Lipid Bilayers and Polar Solvents" Chemistry & Biology 1996, 3, 379-392. 43. Matile, S.; Berova, N.; Nakanishi, K.; Fleischhauer, J.; Woody, R. W. "Structural Studies by Exciton Coupled Circular Dichroism Over a Large Distance: Porphyrin Derivatives of 44
 Steroids, Dimeric Steroids, and Brevetoxin B" Journal of the American Chemical Society 1996, 118, 5198-5206. 44. Dong, J. G.; Akritopoulou-Zanze, I.; Guo, J. S.; Berova, N.; Nakanishi, K.; Harada, N. "Theoretical Calculation of Circular Dichroic Exciton-Split Spectra in Presence of Three Interacting 2-Naphthoate Chromophores" Enantiomer 1997, 2, 397-409. 45. Lichtenthaler, F. W.; Sakakibara, T.; Oeser, E. "Sugar Enolones .6. "Tetra-o-Benzoyl-2Halohexopyranosyl Halides - Preparation, Assignment of Configuration, and Hydrolysis to Enolones" Carbohydrate Research 1977, 59, 47-61. 46. Liu, H. W.; Nakanishi, K. "A Micro Method for Determining the Branching Points in Oligosaccharides Based on Circular Dichroism" Journal of the American Chemical Society 1981, 103, 7005-7006. 47. Goto, T.; Kondo, T. "Structure and Molecular Stacking of Anthocyanins - Flower Color Variation" Angewandte Chemie-International Edition in English 1991, 30, 17-33. 48. Rele, D.; Zhao, N.; Nakanishi, K.; Berova, N. "Acyclic 1,2-/1,3-Mixed Pentols. Synthesis and General Trends in Bichromophoric Exciton Coupled Circular Dichroic Spectra" Tetrahedron 1996, 52, 2759-2776. 49. Wiesler, W. T.; Nakanishi, K. "Relative and Absolute Configurational Assignments of Acyclic Polyols by Circular Dichroism .1. Rationale for a Simple Procedure Based on the Exciton Chirality Method" Journal of the American Chemical Society 1989, 111, 92059213. 50. Zhao, N.; Berova, N.; Nakanishi, K.; Rohmer, M.; Mougenot, P.; Jurgens, U. J. "Structures of two Bacteriohopanoids with Acyclic Pentol Side-Chains From the Cyanobacterium Nostoc PCC 6720" Tetrahedron 1996, 52, 2777-2788. 51. Gimple, O.; Schreier, P.; Humpf, H. U. "A new Exciton Coupled Circular Dichroism Method for Assigning the Absolute Configuration in Acyclic Alpha- and Beta-Hydroxy Carboxylic Acids" Tetrahedron-Asymmetry 1997, 8, 11-14. 45
 52. Harada, N. "Absolute Configuration of (+)-Trans-Abscisic Acid as Determined by a Quantitative Application of Exciton Chirality method" Journal of the American Chemical Society 1973, 95, 240-242. 53. Koreeda, M.; Weiss, G.; Nakanish, K. "Absolute Configuration of Natural (+)-Abscisic Acid" Journal of the American Chemical Society 1973, 95, 239-240. 54. Tachibana, K.; Sakaitani, M.; Nakanishi, K. "Pavoninins, Shark-Repelling and Ichthyotoxic Steroid n-Acetylglucosaminides From the Defense Secretion of the Sole Paradachirus-Pavoninus (Soleidae)" Tetrahedron 1985, 41, 1027-1037. 55. Adams, M. A.; Nakanishi, K.; Still, W. C.; Arnold, E. V.; Clardy, J.; Persoons, C. J. "Sex-Pheromone of the American Cockroach - Absolute Configuration of Periplanone-B" Journal of the American Chemical Society 1979, 101, 2495-2498. 56. Blout, E. R.; Stryer, L. "Anomalous Optical Rotatory Dispersion of Dye - Polypeptide Complexes" Proceedings of the National Academy of Sciences of the United States of America 1959, 45, 1591-1593. 57. Beer, P. D.; Rothin, A. S. "A new Allosteric Bis Crown Ether Ligand That Displays Negative Binding Co-operativity of the Diquat Dication by the Complexation of a Transition-Metal Guest" Journal of the Chemical Society-Chemical Communications 1988, 52-54. 58. Mizutani, T.; Yagi, S.; Honmaru, A.; Ogoshi, H. "Interconversion Between Point Chirality and Helical Chirality Driven by Shape-Sensitive Interactions" Journal of the American Chemical Society 1996, 118, 5318-5319. 59. Mizutani, T.; Takagi, H.; Hara, O.; Horiguchi, T.; Ogoshi, H. "Axial Chirality Induction in Flexible Biphenols by Hydrogen Bonding and Steric Interactions" Tetrahedron Letters 1997, 38, 1991-1994. 60. Bell, C. L.; Barrow, G. M. "Evidence for a Low-Lying 2nd Potential Minimum in Hydrogen-Bonded systems" Journal of Chemical Physics 1959, 31, 1158-1163. 61. Hanessian, S.; Simard, M.; Roelens, S. "Molecular Recognition and Self-Assembly by Non-Amidic Hydrogen-Bonding - An Exceptional Assembler of Neutral and Charged 46
 Supramolecular Structures" Journal of the American Chemical Society 1995, 117, 76307645. 62. Kramer, R.; Lang, R.; Brzezinski, B.; Zundel, G. "Proton-Transfer in Intramolecular Hydrogen-Bonds with Large Proton Polarizability in 1-Piperidinecarboxylic Acids Temperature, Solvent and Concentration-Dependence" Journal of the Chemical SocietyFaraday Transactions 1990, 86, 627-630. 63. Hosoi, S.; Kamiya, M.; Ohta, T. "Novel Development of Exciton-Coupled Circular Dichroism Based on Induced Axial Chirality" Organic Letters 2001, 3, 3659-3662. 64. Skowronek, P.; Gawronski, J. "A Simple Circular Dichroism Method for the Determination of the Absolute Configuration of Allylic Amines" Tetrahedron Letters 2000, 41, 2975-2977. 65. Lo, L. Y., C.; Tsai, C. "A CD Exciton Chirality Method for Determination of the Absolute Configuration of threo-beta-Aryl-beta-hydroxy-alpha-aminoAcid Derivatives" J. Org. Chem. 2002, 4, 1368 - 1371. 66. Canary, J. W.; Allen, C. S.; Castagnetto, J. M.; Chiu, Y. H.; Toscano, P. J.; Wang, Y. H. "Solid State and Solution Characterization of Chiral, Conformationally Mobile Tripodal Ligands" Inorganic Chemistry 1998, 37, 6255-6262. 67. Canary, J. W.; Allen, C. S.; Castagnetto, J. M.; Wang, Y. H. "Conformationally Driven, Propeller-Like Chirality in Labile Coordination-Complexes" Journal of the American Chemical Society 1995, 117, 8484-8485. 68. Holmes, A. E.; Zahn, S.; Canary, J. W. "Synthesis and Circular Dichroism Studies of N,N-bis(2-quinolylmethyl)amino Acid Cu(II) Complexes: Determination of Absolute Configuration and Enantiomeric Excess by the Exciton Coupling Method" Chirality 2002, 14, 471-477. 69. Huang, X. F.; Nakanishi, K.; Berova, N. "Porphyrins and Metalloporphyrins: Versatile Circular Dichroic Reporter Groups for Structural Studies" Chirality 2000, 12, 237-255. 70. Ogoshi, H.; Mizutani, T. "Multifunctional and Chiral Porphyrins: Model Receptors for Chiral Recognition" Accounts of Chemical Research 1998, 31, 81-89. 47
 71. Arimori, S.; Takeuchi, M.; Shinkai, S. "Sugar-Controlled Aggregate Formation in Boronic Acid-Appended Porphyrin Amphiphiles" Journal of the American Chemical Society 1996, 118, 245-246. 72. Matile, S.; Berova, N.; Nakanishi, K.; Novkova, S.; Philipova, I.; Blagoev, B. "Porphyrins - Powerful Chromophores for Structural Studies by Exciton-Coupled Circular-Dichroism" Journal of the American Chemical Society 1995, 117, 7021-7022. 73. Crossley, M. J.; Hambley, T. W.; Mackay, L. G.; Try, A. C.; Walton, R. "Porphyrin Analogs of Trogers Base - Large Chiral Cavities with a Bimetallic Binding-Site" Journal of the Chemical Society-Chemical Communications 1995, 1077-1079. 74. Crossley, M. J.; Mackay, L. G.; Try, A. C. "Enantioselective Recognition of Histidine and Lysine Esters by Porphyrin Chiral Clefts and Detection of Amino-Acid Conformations in the Bound-State" Journal of the Chemical Society-Chemical Communications 1995, 1925-1927. 75. Foster, N.; Singhal, A. K.; Smith, M. W.; Marcos, N. G.; Schray, K. J. "Interactions of Porphyrins and Tranfser-RNA" Biochimica Et Biophysica Acta 1988, 950, 118-131. 76. Suenaga, H.; Arimori, S.; Shinkai, S. "Sugar-Controlled Association Dissociation Equilibria Between DNA and Boronic Acid-Appended Porphyrin" Journal of the Chemical Society-Perkin Transactions 2 1996, 607-612. 77. Huang, X. F.; Borhan, B.; Rickman, B. H.; Nakanishi, K.; Berova, N. "Zinc Porphyrin Tweezer in Host-Guest Complexation: Determination of Absolute Configurations of Primary Monoamines by Circular Dichroism" Chemistry-a European Journal 2000, 6, 216-224. 78. Huang, X. F.; Fujioka, N.; Pescitelli, G.; Koehn, F. E.; Williamson, R. T.; Nakanishi, K.; Berova, N. "Absolute Configurational Assignments of Secondary Amines by CDSensitive Dimeric Zinc Porphyrin Host" Journal of the American Chemical Society 2002, 124, 10320-10335. 79. Huang, X. F.; Rickman, B. H.; Borhan, B.; Berova, N.; Nakanishi, K. "Zinc Porphyrin Tweezer in Host-Guest Complexation: Determination of absolute Configurations of 48
 Diamines, Amino Acids, and Amino Alcohols by Circular Dichroism" Journal of the American Chemical Society 1998, 120, 6185-6186. 80. Proni, G.; Pescitelli, G.; Huang, X. F.; Quraishi, N. Q.; Nakanishi,K.;Berova, N. "Configurational Assignment of Alpha-Chiral Carboxylic Acids by Complexation to Dimeric Zn-porphyrin: Host-guest Structure, Chiral Recognition and Circular Dichroism" Chemical Communications 2002, 1590-1591. 81. Yang, Q.; Olmsted, C.; Borham, B. "Absolute Stereochemical Determination of Chiral Carboxylic Acids" Organic Letters 2002, 4, 3423-3426. 82. Proni, G.; Pescitelli, G.; Huang, X. F.; Nakanishi, K.; Berova, N. "Magnesium tetraarylporphyrin tweezer: A CD-Sensitive Host for Absolute Configurational Assignments of Alpha-Chiral Carboxylic Acids" Journal of the American Chemical Society 2003, 125, 12914-12927. 83. Borovkov, V. V.; Lintuluoto, J. M.; Inoue, Y. "Supramolecular Chirogenesis in Zinc Porphyrins: Mechanism, Role of Guest Structure, and Application for the Absolute Configuration Determination" Journal of the American Chemical Society 2001, 123, 2979-2989. 84. Borovkov, V. V.; Yamamoto, N.; Lintuluoto, J. M.; Tanaka, T.; Inoue, Y. "Supramolecular Chirality Induction in bis(Zinc Porphyrin) by Amino Acid Derivatives: Rationalization and Applications of the Ligand Bulkiness Effect" Chirality 2001, 13, 329-335. 85. Kano, K.; Fukuda, K.; Wakami, H.; Nishiyabu, R.; Pasternack, R. F. "Factors Influencing Self-Aggregation Tendencies of Cationic Porphyrins in Aqueous Solution" Journal of the American Chemical Society 2000, 122, 7494-7502. 86. Kano, K.; Minamizono, H.; Kitae, T.; Negi, S. "Self-Aggregation of Cationic Porphyrins in Water. Can pi-pi Stacking Interaction Overcome Electrostatic Repulsive Force?" Journal of Physical Chemistry A 1997, 101, 6118-6124. 87. Koti, A. S. R.; Periasamy, N. "Cyanine Induced Aggregation in Meso-Tetrakis(4sulfonatophenyl)Porphyrin Anions" Journal of Materials Chemistry 2002, 12, 23122317. 49
 88. Kubat, P.; Lang, K.; Prochakova, K.; Anzenbacher, P. "Self-Aggregates of Cationic Meso-Tetratolylporphyrins in Aqueous Solutions" Langmuir 2003, 19, 422-428. 89. Wang, Y. T.; Jin, W. J. "H-Aggregation of Cationic Palladium-Porphyrin as a Function of Anionic Surfactant Studied Using Phosphorescence, Absorption and RLS Spectra" Spectrochimica Acta Part a-Molecular and Biomolecular Spectroscopy 2008, 70, 871877. 90. Yu, W.; Li, Z. S.; Wang, T. Y.; Liu, M. H. "Aggregation and Supramolecular Chirality of Achiral Amphiphilic Metalloporphyrins" Journal of Colloid and Interface Science 2008, 326, 460-464. 91. 92. Tanasova, M.; Vasileiou, C.; Olumolade, O. O.; Borhan, B., "Enhancement of Exciton Coupled Circular Dichroism with Sterically Encumbered Bis-Porphyrin Tweezers" Chirality 2009, 21, (3), 374-382. Li, X. PhD Thesis 2009. 
 
 50
 Chapter 2 Use of Zn-TPFP tweezers for assignment of absolute stereochemistry of compounds 2-1 Development of electron deficient porphyrin tweezers Zn-TPP tweezer has successfully been applied for the evaluation of several groups of 1 molecules. However use of this tweezer is limited in two ways. First it requires the chiral guests to possess dual coordination sites, (for example diamines) and second, oxygen functionalities 2 demonstrated low or no binding to this tweezer. Its use in mono coordinating compounds like monoamines and carboxylic acids has been achieved by use of “carrier” molecules, such as 1,43 diamino benzene, which are used to derivatize the chiral molecules. (Figure 1-25) Because of these shortcomings, we adopted two approaches towards the development of more sensitive porphyrin tweezers. As shown in Figure 2-1, the steric and electronic properties of porphyrins can be easily tuned to afford optimal binding affinity as well as sensitivity. This could greatly increase the use and substrate scope of porphyrin tweezers. Our goal was to tune these properties in order to obtain the most optimal tweezers. One approach is based on the hypothesis that increasing the bulk of the porphyrin host could lead to a larger steric interaction between the host porphyrin and the chiral guest molecule. Consequently, stronger ECCD signals are expected. An extra benefit of this approach is that the bulky porphyrin host may sense subtle size differences of substituents at the chiral center, making it capable of effectively differentiating between substituents of 4 5 similar sizes. Another way to approach the goal of achieving a more sensitive porphyrin tweezer is to increase the binding affinity of the tweezer by either introducing electron- ! 51! withdrawing groups at the para positions of the meso phenyl rings within the porphyrin, or using metal cations with higher affinity for nitrogen and oxygen. A P1 P2 R R R L N H2 Zn SH N Ψ P2 R R R R 2 Zn R L O B O O P1 ECCD Amplitude P1 Ψ - Dihedral angle between chromophores O P2 EWG EWG EWG L Zn L Zn H2N O O O S N H2 Zn Ka NH2 O O O EWG SH N 2 Zn O O Figure 2-1. Proposed strategies for improving porphyrin sensitivity: A) increasing the steric bulk and B) introducing electron withdrawing groups. It was hypothesized that increasing the electrostatic interaction between the metal center of the porphyrin with the bound chiral guest would lead to a stronger binding interaction between the porphyrin and the substrates. The more Lewis acidic tweezer could then be employed towards substrates that bind weakly to zinc TPP. As a result, the complexation equilibrium between the Lewis-acidic metallo tweezer and the guest would be strengthened, and enhanced ECCD amplitudes would be expected. Most importantly, we anticipated that the enhanced binding affinity would enable the direct attachment of non-derivatized amino alcohols and diols, ! 52! which would tremendously expand the applications of porphyrin tweezer methodology. A series of novel electron-rich and electron-deficient porphyrin tweezers were synthesized by Dr. 5 Xiaoyong Li of our group, and employed in ECCD studies (Figure 2-2). The electron-deficient zinc porphyrin tweezers (TPFP, TTFP, and TFP) demonstrated greatly improved sensitivity in absolute stereochemical determination. Of these, the highly electron deficient Zn-TPFP tweezer demonstrated the most improvement in sensitivity, yielding increased amplitudes for most substrates as compared to Zn-TPP tweezer. (Table 2-1) 5 Ar Ar = N N N F Zn-TCP tweezer Ar Zn Ar NC Ar N N Ar N Zn N Ar N F3C H3C Zn-TTFP tweezer F O Zn-TFP tweezer Zn-TMP tweezer F F O O MeO F F Zn-TPFP tweezer O Zn-TMOP tweezer Figure 2-2. Electronically tuned tweezers for ECCD studies. The observed ECCD signs obtained with Zn-TPFP are consistent with the ones obtained 2 with Zn-TPP tweezer (Table 2-1) suggesting the two tweezers follow a similar mechanism for stereochemical differentiation as illustrated in Figure 1-25. Dr. Li then employed this electron ! 53! deficient tweezer for the absolute stereochemical determination of erythro and threo diols, amino 5 6 7 alcohols, and diamines as well as epoxy alcohols, 1,n diols and aziridines. 7 Following the success of Zn-TPFP tweezer with these functional groups, we naturally thought to extend its application to other functional groups that have not succumbed to ECCD studies yet, including hydroxyl ketones, sulfoxides, and cyanohydrins. Each of these will be discussed in more detail. ! 54! 2-2 Determination of Absolute Stereochemistry for Chiral Hydroxy Ketones 2-2-1 Background The value of enantiopure hydroxy ketones lies in their increasing utility both as important building blocks in organic synthesis and as substructures in many biologically important compounds. 8 Hydroxy ketones are particularly attractive intermediates because further elaboration provides an expedient route to highly functionalized and stereochemically complex polyoxygenated backbones. Because of this, they are frequently employed in the synthesis of 9 pharmaceuticals, such as Indinovir, a HIV protease inhibitor. Figure 2-4 shows a small sampling of natural products that contain hydroxyl ketones. H CO2H O MeO O O H H OH MeO Sporotricale R2O HO O O O O OH OMe Solanoeclepin A OH O HO R1O O CO2CH3 Secokotomolide Steroidal glycosides Figure 2-4. The hydroxy ketone moiety in natural products. One example employing a hydroxy ketone in natural product synthesis is shown in the 10 work by List and co-workers (Scheme 2-1). ! They developed an asymmetric synthesis of the 55! bark beetle pheromone (S)-Ipsenol via a 1,3-hydroxy ketone intermediate. This molecule is of interest due to its use in insect traps and is needed in kilogram quantities. O (L)-Proline acetone H O OH OH (S)-Ipsenol 73% ee Scheme 2-1. Use of chiral hydroxyl ketone in the synthesis of (S)-Ipsenol. A more elaborate use of hydroxy ketones in natural product synthesis was shown in the 11 first total synthesis of Taxol by Holton and co-workers. Over a number of steps, the C-ring hydroxy ketone precursor was used as an essential handle for elaboration and construction of the D-ring in the natural product. (Figure 2-5) OTES OH O O OH O C TBSO O H O O O NH O C O OH O H OH O O O D O O Figure 2-5. Use of hydroxy ketone in the synthesis of Taxol’s ring D. Assignment of absolute stereochemistry for hydroxy ketones relies heavily on X-ray 1 crystallography, stereoselective synthesis and H-NMR spectroscopy, particularly the Mosher ! 56! 12 ester analysis. As mentioned in chapter one, the difficulty of growing pure single crystals makes X-ray crystallography much less practical in the case of compounds that are not solid or that are difficult to crystallize. The simplest and most commonly used method of stereoselective synthesis is the asymmetric α-hydroxylation of enolates and enol derivatives. Scheme 2-2 shows an example of a catalytic approach to enantioselective synthesis of α-hydroxy ketones. Yamamoto 13 and co-workers introduced an oxy group at the α-position of ketone enolates using nitrosobenzene, and were able to obtain α-hydroxy ketones in up to 97% ee. OSnBu3 + Ph O N (R)-BINAP•AgOTf 10 mol% THF OSnBu3 Ph CuSO4 (0.3 eq) O N H MeOH O OH Scheme 2-2. Yamamoto’s synthesis of α-hydroxy ketones using nitrosobenzene. Mosher ester analysis has been used for the assignment of hydroxy ketones, where the hydroxy ketones are treated as chiral alcohols and derivatized with a chiral derivatizing agent. For example, Thiericke 14 and co-workers assigned the absolute stereochemistry of Streptoketol A (Scheme 2-3) by employing the Mosher ester protocol using 2-phenylbutanoic acid (PBA) as the chiral derivatizing agent. O H O Et H O Ph CH3 Cy O OH O Ph CH3 (R)-PBA Ph (S)-PBA H O OH OH Et Ph O H Scheme 2-3. Stereochemical assignment of Streptoketol A using Mosher ester analysis. ! 57! CH3 Cy The Mosher ester methodology is not always convenient, especially in cases where limited amount of compound is available. Furthermore, there are few reports in the literature 14b, 15 detailing the absolute stereochemistry determination of hydroxy ketones, and these reports are limited to α-hydroxy ketones and even so, they are not general for application to all α hydroxy ketones. O OH O Br O O DCM, 66% Zn Br O O HO O NH2 Na2CO3, THF, 81% O O N H Br Zn O O O O Br O O H O O Zn-TPP porphyrin tweezer N H Zn O O H N H Zn O O 1:1 host/guest complex Scheme 2-4. Assignment of absolute stereochemistry of cyclic α- hydroxy ketones. Recently, Ishi and co-workers reported the absolute stereochemical assignment of cyclic 16 α- hydroxy ketones. In their report, the hydroxy ketones are derivatized with a (3- hydroxypropylamino)acetyl group to act as a “carrier” molecule in order to provide an “amino alcohol-like or hydroxyconjugate” functionality necessary for binding (Scheme 2-4). While their protocol shows promise it is limited to cyclic α-hydroxy ketones, and like Mosher ester, requires derivatization. ! 58! We aimed to develop a system that would require minimum amounts of material, be nonempirical and require no derivatizations prior to employing the exciton coupled circular dichroism protocol. The desired methodology should be general enough to be applicable to any hydroxy ketone, cyclic or acyclic, and applicable to 1, n hydroxy ketones as well. In order to achieve this, we needed to use a porphyrin tweezer capable of binding efficiently with hydroxy ketones. Prior to the use of Zn-TPFP tweezer for diols and epoxy alcohols, we had not imagined that chiral hydroxy ketones could be bound directly with a porphyrin tweezer; in fact they were considered as substrates with only one site of attachment (the hydroxyl group), and thus we would have resorted to derivatization with carrier. The enhanced Lewis acidity of the metallocenter in the Zn-TPFP tweezer makes it feasible to consider the binding of molecules such as hydroxy ketones with the idea that both the hydroxyl and ketone functionalities will bind to the tweezer. 7 O 7 mCPBA (2.4 equiv.) DCM, r.t., 2 h 93% LAH (4 equiv.) O THF, 0 °C to r.t., 1 h 23% OH OBn H2, Pd/C (10%) EtOAc, r.t., 1 h 69% O O O 7 (S,S)-Co-Salen (0.01 equiv.) AcOH (0.04 equiv.) THF (0.02 mL/mmol), 0 °C then H2O (1.1 equiv.) 22 h, r.t.14% OH OH PCC (2.2 equiv.) DCM, MgSO4, r.t. 80% O BnCl (2 equiv.) 50% KOH(aq) 5 equiv.) Bu4N[HSO4] (0.016 equiv.), toluene/water 86% OBn OH Scheme 2-5. Synthesis of chiral hydroxyl ketones via Jacobsen epoxidation. ! 59! 2-2-2 Synthesis of Chiral Hydroxy Ketones Chiral hydroxyl ketones 3 and 4 were synthesized by asymmetric aldol reaction following known procedure. 17 substrates 7, 8 and 9 were readily accessed in three steps from their chiral diol counterparts that were synthesized by Dr. Li via regioselective ring opening of chiral 7 bisepoxides. The chiral bisepoxides were prepared by the Jacobsen hydrolytic kinetic resolution of racemic bisepoxides. 18 Monobenzylation of the chiral diols, 19 followed by PCC oxidation and finally de-benzylation gave the chiral hydroxy ketones (Scheme 2-5). It is worth noting that these extra manipulations do not erode the ee’s as shown for the case of 9-hydroxydecan-2-one, 7(R). ([α] 20 D = -8.9, (c = 0.50, CHCl3; ee = 92%; lit [α] 20 reduction of 2,5-hexanedione by yeast. 20 D -9.7 ee > 95%)). 4 was obtained from Substrates 6 and 10 were purchased from Sigma Aldrich and used without further purification. 2-2-3 ECCD Studies of Chiral Hydroxy Ketones Using Zn-TPFP Tweezer 2-2.3.1 Binding Affinity of Zn-TPFP Tweezer to Hydroxy Ketones As mentioned above, Zn-TPFP tweezer was successfully used to determine the stereochemistry of oxygen containing functionalities (erythro and threo diols, amino alcohols, 5-6 diamines and epoxy alcohols). The principle advantage of the porphyrin tweezer system is its capacity for non-covalent binding of the chiral guest, thus avoiding the need for chemical derivatizations. Since the tweezer strategy is based on the steric interaction between the substituents at the chiral center and one of the two porphyrins, we believed that hydroxyl ketones would behave much like other systems that contain two sites of attachment with one stereocenter ! 60! such as diamines with one chiral center. 1c Therefore, we predicted that the porphyrins would bind to the hydroxyl and carbonyl functionalities, and the helicity of the bound porphyrin tweezer would depend on the steric differentiation experienced by the porphyrin bound to the hydroxyl group attached to the stereocenter. This porphyrin would bind anti to the largest substituent on the chiral center and stereodifferentiate between the two remaining groups attached to the same chiral carbon. The porphyrin bound to the oxygen atom of the carbonyl (an 2 sp center) does not take part in the stereodifferentiation. Upon steric differentiation at the chiral center, the two porphyrins would adopt a specific helicity. Ketones have not previously been shown to bind with zinc porphyrin tweezers and could be a potential problem. For this approach to work, a strong binding between hydroxyl ketone and tweezer is required so that a large population of the chiral guest would bind and thus increase the observed signal. If weakly binding complexes are formed, it could lead to small energetic differences between a number of complexed conformations leading to inconsistent results in the observed ECCD. We gathered information on the binding affinity for ketones by UV-Vis titration analysis of benzophenone with Zn-TPFP ester (Figure 2-6). The association constant of ketone was found -1 5 -1 5 to be 73 M , about 30 times weaker than that of alcohol Ka = 2170 M . With this in mind, there exists the possibility of the porphyrin tweezer not binding to the hydroxy ketones strongly enough to be suitable for our purposes. ! ! 61! A Free tweezer !!!!!!! 0.6 absorbance 0.5 400,000 equiv. 0.4 0.3 0.2 0.1 0 390 410 430 450 Wavelength, nm 0.35 B ΔA at 426 nm 0.30 0.25 0.20 -1 0.15 Ka = 73 M R2 = 0.998 0.10 0.05 0.00 0 1e+5 2e+5 3e+5 4e+5 5e+5 Equivalent of Guest Figure 2-6. A) Titration of Zn-TPFP ester with phenyl methyl ketone (0400,000 equiv) in hexane. B) The non-linear least square fit of the change in absorption vs. equiv of ligand provides the binding constant.! ! ! 62! O A Free tweezer 9 0.499 0.399 70,000 equiv. absorbance 0.299 0.199 0.099 -0.001 350 370 390 410 430 450 470 Wavelength, nm 0.25 0.20 ΔA at 426 nm B 0.15 Ka = 137,516 M R2 = 0.959 0.10 -1 0.05 0.00 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 Equivalent of Guest Figure 2-7. A) Titration of Zn-TPFP porphyrin tweezer with 1,12-hydroxy ketone (0-70,000 equiv) in hexane. B) The non-linear least square fit of the change in absorption vs. equiv of ligand provides the binding constant.! ! 63! OH Interestingly however, as depicted in figure 2-7, the binding of hydroxy ketones with -1 TPFP tweezer was measured to be 137,516 M in hexane, which is comparable to diols Kassoc. = -1 5 152,000 M in hexane. Based on this Kassoc. value, contrary to our initial concern, we expected hydroxy ketones to bind effectively. PhF5 PhF5 F5Ph N N Zn N O N O F Ph PhF5 5 N N Zn PhF5 N N O O n = 1: Zn C3-TPFP tz - a n = 2: Zn C4-TPFP tz - b n = 3: Zn C5-TPFP tz - c n = 4: Zn C6-TPFP tz - d n = 5: Zn C7-TPFP tz - e n = 6: Zn C8-TPFP tz - f Figure 2-8. A series of porphyrin tweezers of varying linker lengths. Assuming that the chirality in the hydroxyl ketones is transferred to the tweezer upon steric interactions between the substituents at the chiral center and one of the porphyrins, we predicted that during complexation the porphyrins would bind to the hydroxyl and carbonyl functionalities, and the helicity of the porphyrin tweezer would depend on the steric differentiation experienced by the porphyrin bound to the hydroxyl group attached to the stereocenter. This porphyrin would bind anti to the largest substituent at the chiral center and stereodifferentiate between the two remaining groups attached to the same chiral carbon. Upon steric differentiation, the two porphyrins would adopt a specific helicity that we could then relate back to the stereochemistry of the bound substrate. ! 64! A P2 P1 M Zn O H B P1 P2 S O expected positive Zn Zn Zn ECCD spectrum P2 P1 S Zn O H P2 M O Zn P1 Zn Zn expected negative ECCD spectrum Figure 2-9. Proposed binding of hydroxy ketones with Zn-TPFP tweezer. In A) the S- chirality results in a predicted positive ECCD spectrum, while the Rchirality results in a predicted negative ECCD spectrum as shown in B. Figure 2-9 shows a proposed binding model for hydroxy ketones with Zn-TPFP tweezer. In A), (an S-hydroxy ketone), the stereodifferentiation would be between the small substituent in back and the medium substituent in front. To avoid unfavorable steric interactions with the larger medium substituent, the porphyrin P2 would rotate towards the smaller (S) group pointing back, away from the larger medium group. The porphyrin P2 bound to the oxygen atom of the 2 carbonyl (an sp center) does not take part in the stereodifferentiation process. Overall, a clockwise (positive) helicity of P2 relative to P1 is generated, resulting in a predicted positive ECCD spectrum. Likewise, in B) (an R-hydroxy ketone) the stereodifferentiation would be between the small substituent in the front and the medium substituent in back. Porphyrin P2 ! 65! would rotate towards the smaller (S) group in front, away from the larger medium group, leading to the overall counterclockwise (negative) helicity of P2 relative to P1, resulting in an expected negative ECCD spectrum. However, when hydroxy-ketones were subjected to ECCD studies with C5-Zn-TPFP tweezer (e) (Figure 2-8), we failed to observe ECCD. The same was the case with the C3 1 tweezer (c) (Figure 2-8), which has been successfully used for the absolute stereochemical 1 determination of chiral 1,n-glycols. The fact that both these tweezers failed to induce a detectable signal upon substrate binding, led us to investigate the possibility that the linker length could be playing a significant role in chirality amplification, especially given the weaker binding of the ketone to the porphyrin. We set out to find a suitable linker length that would lead to orientation of the porphyrins in such a way that we could obtain amplification of chirality. It has been shown in the literature that linker length can play a crucial role in chirality recognition NH N NH N N HN N HN and transfer. 21 Of particular note are the studies by Utaka and Colquhoun who have each shown that it is O O O HN O NH possible to control the substrates that tweezers can recognize by varying the length and nature of linkers: In Utaka’s study of diamines, Figure 2-10. Utaka’s porphyrin tweezer. 21c with a chiral porphyrin tweezer" linked through a fairly rigid" macrocyclic spacer (Figure 2-10)," they observed that binding of short" chain terminal diamines (2-4" methylenes) is detrimental for the ECCD active conformation and using guests with more than six carbons also diminishes the CD amplitude. ! 66! Therefore, further rigidifying the linker was considered not to be beneficial since it might lead to selective recognition for guests only with certain chain lengths. They found that having a fairly short linker, (C3) greatly diminished the amplitudes of shorter diamines (2-4 methylenes), and slightly reduces that of diamines with more than 6 methylenes. O O O O S O O S O O N O O S O O O O O S O NH O O N O O N O HN O O O O O O S O O N O O O H O S NO O S O O O S O O N O O O N O HN O O S O O N O O O S O NH O O O S O O S O O S O O N O O N O NH HN O O O S O O O N O O N O O O O O S H N O O N O O O O O O O H O S NO O O Figure 2-11. Colquhoun’s tweezer systems (red and pink) showing chain folding and multiple binding to different polyimide triplet sequences (black) by the different tweezer molecules each tweezer molecule binds either to a different monomer sequence. Red tweezer, by hydrogen bonding to the imide carbonyls, and pink tweezer, which lacks hydrogen bonding ability, recognizing different polyimide sequences. In a more recent study by Colquhoun et al., they demonstrated that by changing the nature and length of linker, they could target specific imide site from among many. 21a As shown in Figure 2-11, by removing the hydrogen bonding ability from one tweezer, (pink) they are able to change the polyimide sequence the tweezer is able to recognize. ! 67! These investigations show that the linker plays a crucial role in controlling both the distance and the geometry of the two porphyrins relative to each other, and hence in addition to other factors like binding affinity, plays a crucial role in chirality recognition. We hypothesized that because of the rather weak binding affinity of ketones to zincated porphyrin, the steric environment that increases upon coordination may overcome the binding, and so a longer linker could relax the strain. Upon close investigation, an interesting trend is observed, where it appears that a combination of both the tweezer linker as well as the length of the hydroxy ketone play an important role. Figure 2-12 shows an overlay of tweezers C3-C9 with A) a short hydroxy ketone (1,3) and B) a long hydroxy ketone (1,8). As shown, for the short chiral guests, it was found that none of the porphyrin tweezers yield reproducible ECCD, except C8 tweezer that gave a large positive ECCD curve. Increasing the length to C9 causes this amplitude to drop dramatically (Figure 212A). For the longer chiral guest, while there is no observable ECCD signal with shorter linker tweezers, (C3, C4, and C5) we start to obtain ECCD signal with the longer C6 linker. Importantly, the obtained amplitude increases with increasing linker length, with C9 giving the largest amplitude (Figure 2-12B). An important point is that all obtained ECCD signs are consistently of the same sign meaning that the tweezers follow the same stereodifferentiation mechanism. ! 68! OH O A 3 C8, 20 equiv. C9, 20 equiv. Mol CD 195# 145# 95# 45# -5# -55# -105# 385# 395# 405# 415# 425# 435# 445# Wavelength, nm B O C9, 20 equiv. 325# 7 C8, 20 equiv. Mol CD 175# 25# -125# -275# 395# 405# 415# 425# 435# 445# Wavelength, nm ! Figure 2-12. Overlay of ECCD signals obtained for A) 1,3 hydroxy ketone 3(S) and B) 1,8 hydroxy ketone 7(R) with Zn-TPFP tweezers C3-C8 in hexane at 0°C. For both the short and long tweezers, C-8 give the highest amplitude. ! 69! OH Table 2-2. ECCD data for chiral hydroxy ketones in hexane. ! entry predicted sign hydroxy ketone λ nm, (Δε) A neg 424, -13 413, +15 -28 pos 423, +201 412, -93 +294 pos 423, +78 411, -32 +110 neg 422, -11 411, +21 -32 neg 421, +-41 413, +44 -85 pos 422, +99 414, -13 +112 pos 422, +23 415, -14 +37 pos 422, +48 415, -27 +76 neg 421, -4 413, +7 -111 O 1 2 (R) OH 3 (S) 3 5 (S) OH OH 4 (R) 4 O O 2 O OH 5 6 (S) O OH 6 7 (R) O OH 7 8 (R) 8 OH O 9 (R) O OH O 9 10 (S) OH Tweezer:substrate ratio – 1:20, 1 µM tweezer concentration at 0 °C was used for all measurements. All measurements were done in hexane as solvent. To our delight, prominent bisignate CD signals (ECCD) at the Soret region were observed upon complexation of C8-Zn-TFPF tweezer (f) with all the chiral hydroxy ketones at ! 70! micromolar concentrations, and so with these results in hand, we used the optimized C8 ZnTPFP tweezer for our studies. As shown in Table 2-2, the (R) and (S) assignments of the hydroxy ketones are not an important factor in correlating the chirality and the obtained sign of ECCD. Comparing entry 2 and 3, which are pseudo enantiomers, they both result in a positive ECCD signal. This shows that the size of the group at the chiral center is important. To illustrate this, Figure 2-13 A) shows a proposed binding model for 3(S) and 4(R) that are pseudo enantiomers. The binding interactions occur between the hydroxyl group and the carbonyl oxygen with the zincated porphyrins. It is assumed that the binding of P1 to the hydroxyl group happens first because the hydroxyl group binds more strongly to Zn than the carbonyl oxygen. P1 approaches the hydroxyl group from the side opposite the large group (methyl or iPr) then P2 coordinates to the ketone oxygen lone pair, anti to the sterically demanding P1. P2 faces no steric bias since this carbon is not chiral. P1 then relieves steric strain by rotating away from the larger group (alkyl chain) towards the smaller group (hydrogen) in both cases resulting in the more favored complex in which the two chromophores have a clockwise chiral twist. This orientation of the two porphyrins results in the observed positive ECCD spectra for both entries 2 and 3. On the other hand, Figure 2-13 B) shows a proposed binding model for 5(S). Again, the binding interactions occur between the hydroxyl group and the carbonyl oxygen with the zincated porphyrins. In this case, P1 approaches the hydroxyl group from the side opposite the large group (methyl), then P2 coordinates to the ketone oxygen lone pair, anti to the sterically demanding P1. P1 then relieves steric strain by rotating away from the larger group (alkyl chain) towards the smaller group (hydrogen) resulting in the more favored complex in which the two ! 71! chromophores have a counterclockwise chiral twist. This orientation of the two porphyrins results in the observed positive ECCD spectrum for entry 4. A P2 P1 Zn P1 P2 R H O H O Zn Zn Zn obtained positive ECCD spectrum R = CH3, iPr B P2 P1 P2 Zn H CH3 O O P1 Zn Zn Zn obtained negative ECCD spectrum Figure 2-13. Proposed complexation between tweezer and hydroxy ketones A) 3S and 4R that yield positive ECCD and B) 5S that yields negative ECCD spectra. The nature of acyclic systems can lead to a number of rotamers, making it difficult to predict whether or not the conformation of the complexed guest molecules retains the lowest energy conformation of unbound molecules; since complexation with the large tweezer can result in compensating interactions with an overall effect of the host-guest complex adopting a higher energy conformation. ! 72! P2 Zn OH O Zn Zn P1 H O Zn P2 P1 Zn Zn O P2 obtained negative ECCD spectrum P1 Figure 2-14. Proposed conformation of the complex formed between Zn-TPFP tweezer and (S)-(+)-3-hydroxy-2,2-dimethylcyclohexananone 10(S), resulting in a negative ECCD spectrum (421 nm, -4; 413 nm, 7; A = -11). P1 rotates towards the smallest H group, away from the bulky ring CH2 alkyl group. We investigated (S)-(+)-3-hydroxy-2,2-dimethylcyclohexananone (entry 9), and were delighted to find that the trend is not affected by changing the substrate to a more rigid cyclic hydroxy ketone. (S)-(+)-3-hydroxy-2,2-dimethylcyclohexananone can exist in the two possible chair conformations shown (Figure 2-14). Since there is no free rotation around the bonds, the relative positions of the two binding sites: ketone and hydroxy are fixed. This forces the two porphyrins P1 and P2 to approach from opposite sides to avoid steric clash. This translates to an arrangement where the porphyrins are bound anti to one another. Steric differentiation by P1 leads to the rotation of P1 towards the smaller H group at the chiral center (counterclockwise). Note that P2 is not attached to a chiral center and so has no effect on the stereodifferentiation process. This leads to a negative helicity. The observed ECCD of (S)-(+)-3-hydroxy-2,2-dimethylcyclohexananone bound to C8-TPFP tweezer results in a negative signal in accordance with the proposed model. It is worth noting that this trend is not affected by the length of the alkyl chain connecting the carbonyl to the hydroxy group. The proposed mnemonic is applicable for both short chain ! 73! hydroxy ketones and long chain hydroxy ketones. Also, of important note is the fact that the trend is not affected by the substituents at the chiral center. For example entry 3 that has an isopropyl group at the stereocenter gives the expected positive ECCD sign, and also entry 5 that bears an ethyl group also results in the expected negative ECCD sign. These results show that the system is not limited to only methyl substituents at the chiral center. As mentioned above, ketones have weaker binding affinity with zincated porphyrin than alcohol. This, together with the fixed lone pairs on ketone could be invoking a need for P2 to loop around to find the lone pairs on the ketone oxygen, so the porphyrin linker has to be long enough to wrap around comfortably, and relax the strain within the complex. In the case of the short tweezers, an increase in steric bulk as the two porphyrins approach one another could disfavor complex formation, causing a reduced, or no coordination with the ketone, hence, we see no ECCD signal. Note that this is not the case for diols with the same short tweezer,7 probably because with the higher binding affinity an alcohol has to zinc, the complex formed is strong enough to overcome this steric issue. Comparing Figure 2-12A, C8 and C9 with the 1,3 hydroxy ketone, the amplitude drops dramatically, yet going from C8 to C9 with the 1,8 substrate, there is a dramatic increase in amplitude. Here, the complex will not experience as much steric clash. Considering the importance of hydroxy ketones in organic chemistry, this developed protocol is particularly useful for organic chemists who are seeking to determine the absolute stereochemistry of hydroxy ketones. More importantly, the success of the hydroxy ketone case prompted us to look into the ECCD study of complex molecules that contain hydroxy ketones in their framework such as steroids, which are an important class of molecules. The results of this study will be discussed later. ! 74! 2-3 Determination of Absolute Stereochemistry for Chiral Sulfoxides 2-3-1 Background 2 Exciton Coupled Circular Dichroism has been applied for the determination of absolute stereochemistry of chiral diamines, amino alcohols and amino acids 1a, 22 mono amines and mono alcohols, carboxylic acids, 2-3, 23 1c 6 epoxy alcohols, diols,5 as well as lactams. 24 Notably absent from this list are sulfoxides. H3CO CO2H N N H Esomeprazole F O CH3 OCH3 N Sulindac S O CH3 O NH2 S O O S O OH NH2 Alliin Armodafinil Figure 2-15. Synthetic and naturally occurring compounds containing a Sulfoxide moiety. A sulfoxide is a molecule that contains a sulfinyl attached to two carbon atoms. They are structurally similar to carbonyls, with sulfur replacing the carbonyl carbon. Sulfoxides have some interesting characteristics. First, the bond between the sulfur and oxygen atoms differs ! 75! from the conventional double bond between carbon and oxygen in carbonyls. There has been considerable debate over the nature of the S=O bond, with comparison to other well-known molecules bearing the similar (C=O and P=O) motif (Figure 2-16). O R R1 carbonyl carbon R O S R1 sulfoxide O P R2 R R1 tertiary phosphine oxide Figure 2-16. Geometric comparison of a sulfoxide to a carbonyl and tertiary phosphine oxide. In the carbon analogue, the carbon atom forms a typical p-p π-bond with oxygen. In the sulfoxide and phosphine oxide cases, it is believed that the oxygen contributes electrons from its unshared lone pairs, in the 2p orbital to the empty 3d orbital of the sulfur or phosphorus. i.e. d-p π-bonding. However, there is some debate about the compatibility of the energy level overlap of the 3d orbital with the oxygen 2p orbital. The best representation of a sulfoxide bond is shown in the resonance structures shown in Figure 2-17. R O S R1 R O S R1 Figure 2-17. Resonance structures of a sulfoxide bond. A second interesting and important characteristic of sulfoxides is their ability to be chiral. Chiral centers are mainly associated with tetrahedral carbons. However, with sulfoxides, a lone pair of electrons resides on the sulfur atom, giving it tetrahedral molecular geometry analogous ! 76! 3 to an sp carbon. When sulfur is bound to two different R groups, the sulfur atom becomes a chiral center. The Cahn-Ingold-Prelog priority rules are followed when assigning stereochemistry of chiral sulfoxides, and the lone pair of electrons is assigned the lowest priority. Chiral sulfoxides have, and continue to receive considerable attention in organic and medicinal chemistry. 25 They are present in many synthetic bioactive compounds and pharmaceuticals because they have shown a wide range of biological activities such as regulation 25a, 26 of cholesterol metabolism. Furthermore, enantiopure sulfoxides are commonly employed as valuable intermediates in the synthesis of natural products, 27 as well as ligands for asymmetric synthesis in a wide range of carbon-carbon or carbon-heteroatom bond forming reactions. 25b, 28 Sulfoxides are widely used to modulate pharmacological properties of drugs, 25a, 29 thereby increasing their importance in the pharmaceutical industry. Because of the tetrahedral geometry and anionic character of the oxygen of S=O, sulfoxides are used as mimics of the alkoxide functionality. Such substitution allows for modulating chemical stability and reactivity of a drug, while retaining, and in some cases enhancing, their biological activity. 30 For example, if a target protein recognizes a particular chirality of an alkoxide, it might enjoy enhanced 29a, 30a, 30c recognition of the sulfoxide analogue due to the strong polar nature of the S-O bond. ! 77! O S O alliin OH alliinase NH2 S O S OH S allicin 2-propenesulfenic acid O S O S S allicin S S ajoene Figure 2-18. Conversion of alliin to ajoene via allicin. Just like their synthetic counterparts, naturally occurring sulfoxides are also of importance. For instance, garlic has long been used as a therapeutic remedy. When raw garlic is chopped or crushed, the enzyme alliinase which is usually stored in a separate compartment in the garlic, combines with a compound called alliin, (S-2-propenylcysteine S-oxide) and converts it into the intermediate compound 2-propenesulfenic acid, which immediately condenses to give the antibiotic substance allicin (allyl 2-propenethiosulfinate) (Figure 2-18). Allicin is responsible for the aroma of fresh garlic and has antimicrobial and antifungal properties; it also inhibits lipid synthesis in vitro. Allicin can be transformed into an unsaturated sulfoxide disulfide called ajoene, which has anticlotting (antithrombotic) properties. When an onion bulb is cut or crushed, an odourless substance in the bulb, S-1propenylcysteine S-oxide, is similarly transformed into 1-propenesulfenic acid, CH3CH=CH−S−O−H, which rearranges to (Z)-propanethial S-oxide, CH3CH2CH=S+O−, the tearinducing substance of the onion. Despite their increasing importance, there remain very few reports investigating the determination of chirality for sulfoxides. Moreover, there is no direct method for the assignment of their absolute stereochemistry. ! 78! 2-3-2 Conventional Methods for Assigning Absolute Stereochemistry of Sulfoxides There are a number of developed methods for establishing the absolute stereochemistry 31 of sulfoxides. One such method utilizes enantioselective synthesis, most commonly, 32 enantioselective oxidation of sulfides (Figure 2-19). S H3C CH3 Ti(OiPr)4 diethyl (2S,3S)-tartrate 80% cumene hydroperoxide (H2O), CH2Cl2 O S H3C CH3 S-(-)-isomer Figure 2-19. Asymmetric synthesis of sulfoxides by oxidation of sulfides. Other methods for determining absolute stereochemistry of chiral sulfoxides include chiroptical methods like electron circular dichroism and vibrational circular dichroism. 33 However, these are empirical methods that require computations/calculations of a predicted spectrum in order to compare with the obtained experimental spectrum. X-ray crystallography is of great importance in the assignment of absolute 34 stereochemistry of chiral sulfoxides. The first sulfoxide whose absolute stereochemistry was assigned by the X-ray analysis was (+)-methyl-L-cysteine sulfoxide. 35 This breakthrough then made X-ray analysis of fundamental importance in the early studies of chiral sulfoxides, allowing for the assignment of absolute stereochemistry of some optically active sulfoxides that were then used as reference compounds for assigning the absolute stereochemistry of related 36 chiral sulfoxides. ! Although X-ray crystallography is the most unambiguous and reliable 79! method for determining absolute stereochemistry of molecules, the difficulty of growing pure single crystals makes it much less practical in the case of compounds that are not solid or are difficult to crystallize. For these reasons, spectroscopic methods have been developed as alternative approaches for assigning absolute configuration of optically active sulfoxides. The Mosher ester analysis method, which has been widely used for assigning the absolute stereochemistry of chiral molecules, 12 involves the use of a chiral solvating agent (CSA) for derivatization. For sulfoxides however, the use of Mosher ester analysis is limited to only a few cases.37 The most common modification employs chiral solvating agents for studying chiral sulfoxides such as 9-anthryl-1,1,1-trifluoroethanol, 40 N-(3,5-dinitrobenzoyl)-α-phenylethylamine. 38 α-methoxyphenylacetic acid, 39 and (R)-(-)- These reagents form non-covalent interactions with sulfoxides via hydrogen bonding, inducing chemical shift changes in the protons neighboring the chiral sulfoxide moiety. The optically active sulfoxide gives rise to diastereomeric complexes when interacting with both enantiomers of CSA. The absolute stereochemistry of the sulfoxide is empirically related to the sign of the difference in chemical 1 shift for the same H-NMR signal when comparing the complexes. Therefore, in order to determine the absolute stereochemistry of a sulfoxide, it is necessary to record the NMR spectrum for both diastereomeric adducts with a CSA. In the pioneering work by Pirkle and Beare, 38a they used (-)-(R)-2,2,2-trifluoro-phenylethanol as the CSA for assignment of absolute stereochemistry of alkyl methyl and aryl methyl sulfoxides. ! 80! OH O Ph F3C H H (R,R) O S R CH3 CF3 (-)-(R)-2,2,2-trifluoro- R phenylethanol OH O S CF3 CH3 (-)-(R)-2,2,2-trifluorophenylethanol O Ph F3C H H O S CH 3 R (R,S) Figure 2-20. Solvation model for the interaction of (R)-2,2,2-trifluoro-phenylethanol with both (R) and (S) configurations of aryl methyl sulfoxides. They proposed the solvation model shown in Figure 2-20 to rationalize the observed chemical shifts. In the proposed model, the chiral alcohol and the sulfoxide form a complex by a hydrogen-bond between the hydroxyl hydrogen and sulfoxide sulfur lone pair electrons. The inductive effects of the proximal electron withdrawing CF3 group causes an increase in the acidity of the hydroxyl hydrogen, making it more prone to interact with the basic S=O group. In the solvate (R,R), the methyl group faces the CF3 group, while the R group is syn to the phenyl ring. The opposite is true for the diastereomeric solvate (R,S). Due to the shielding caused by the phenyl group, the methyl NMR signal will shift to higher field for (R,S) than for (R,R). The converse is expected for the protons of the R alkyl moiety, whose resonances will shift to higher field for (R,R) than for (R,S). In this way, the absolute stereochemistry of the sulfoxide can be 1 related to the sign of the difference in chemical shift for the same H-NMR signal when comparing the diastereomeric complexes. While this approach is rather simple and fast, it has some significant drawbacks like the requirement of derivatization of chiral sulfoxide before analysis. Moreover, in order to achieve a reliable correlation, the prevalent conformation of the complexes must be known. Given that these are hydrogen-bonded complexes, the conformation can be difficult to predict. With such labile adducts a fast equilibrium exists between several species and conformations in solution, ! 81! resulting in small (or in some cases nonsystematic) chemical shift differences between the diastereomeric complexes, leading to an inability to predict the most prevalent conformation. In 1999, Yabuuchi and co-workers developed 37 an elegant modification to this methodology. This method involves the conversion of sulfoxides into N-(methoxyphenylacetyl sulfoximines, by amination of the sulfoxide with O-mesitylsulfonylhydroxylamine, which occurs with complete retention of chirality at the sulfur atom. After formation of the N(methoxyphenylacetyl sulfoximines, (R)- and (S)-MPA are introduced at the nitrogen atom (Figure-2-21). This approach leads to the formation of more stable complexes due to the introduction of a covalent bond. However, even though this methodology is more reliable than the previous methods, there is the need for derivatization. Hence, the absolute stereochemistry of chiral sulfoxides remains a challenge. As such, we pursued the development of a microscale, nonempirical and efficient protocol to determine the absolute stereochemistry of sulfoxy alcohols without the need for any derivatization. O R1 O 1. Mes-SO2-O-NH2 S 2. KOH R2 3. MPA N-MPHA S 2+ R1 R2 O S N R1 R2 Figure 2-21. Formation of sulfoximines from sulfoxides. H Ph OMe O N-(methoxyphenylacetyl) As mentioned previously, we have introduced the highly Lewis acidic fluorinated ZnTPFP porphyrin tweezer that showed high binding affinity for hydroxyl and epoxide groups. ! 82! 5-6 Following its successful use for the assignment of hydroxy ketones, we hypothesized that the use of this fluorinated tweezer would also lead to a successful methodology in the assignment of sulfoxy alcohols. Initially, one concern for developing mnemonics was determining if the tweezer would bind with the sulfur or the oxygen lone pairs of the sulfoxide (Figure 2-17). Central to the success of this proposed methodology would be the use of a tweezer that is capable to bind to the alcohol as well as keep a consistent binding mode with the sulfoxide (either the sulfur or oxygen lone pair electrons). 2-3-3 ECCD Studies of Chiral Sulfoxides Using C3-TPFP Tweezer 2-3-3.1 Binding Affinity of Sulfoxides with Zn-TPFP Tweezer Porphyrin tweezer (a) employed in this study was synthesized according to literature 5 procedure substituting 1,3-propanediol for 1,5-pentanediol. SH O O S S reflux, 16h OH chiral S HPLC mCPBA -78°C O S O OH O S OH OH Scheme 2-6. Representative scheme for synthesis of chiral sulfoxy alcohols. ! 83! OH Racemic sulfoxy alcohols were synthesized following basic organic chemistry procedures and the pure enantiomeric forms were obtained by HPLC separation, as shown in Scheme 2-6 for phenyl sulfoxy alcohol. First, the thioether was obtained by refluxing an epoxide with benzene thiol neat. This was then followed by mCPBA oxidation of the thio ether in methylene chloride to give the racemic sulfoxide. Individual enantiomers were then obtained after chiral HPLC separation, and all absolute stereochemistry was confirmed by X-ray crystallography. (See experimental details). The binding constant was determined using UV-vis analysis by titration of Zn TPFP porphyrin with phenylmethyl sulfoxide. The UV spectrum shown in figure 2-22 below shows the change observed in the Soret band absorption upon binding of the sulfoxide. Plotting the equivalents of sulfoxide added as a function of the change in absorption at 426 nm leads to a saturation curve which provides the binding constant of the complex formed upon non-linear least square analysis using Sigma plot 2001 program. Calculations of binding constants follows 5 4 -1 protocols described for diols and was determined as 1.08x10 M in hexane. The value obtained 4 -1 6 is comparable to that of epoxy alcohols 2.88x10 M in hexane suggesting a good binding affinity. ! 84! Free tweezer H3C 0.5 0.4 O S Ph 300 equiv. Abs 0.3 0.2 0.1 0 370 390 410 430 450 Wavelength, nm 0.1 0.08 4 Ka = 1.08x10 M R2 = 0.995 ΔA at 426 nm ! 0.06 -1 0.04 0.02 0 0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 Equivalents of sulfoxy alcohol Figure 2-22. A) Titration of Zn-TPFP porphyrin with phenylmethyl sulfoxide (0-300 equiv) in hexane. B) The non-linear least square fit of the change in absorption vs. equiv of ligand provides the binding constant. ! 85! 2-3-3.2 Probing the Zinc-Sulfoxide Binding As mentioned, sulfoxides can exist in two resonance forms (Figure 2-17). Because of this, sulfoxides are polarized molecules that possess two potential sites for coordination to metals: the lone pair of electrons on sulfur or the negative charge on oxygen. As shown in Figure 2-23, switching the coordination point of the sulfoxide from sulfur to oxygen could result in a switch in the predicted ECCD sign, hence, it is important to understand the sulfoxide binding mode to the Zn in order to correctly interpret the obtained results. Assuming that the porphyrin tweezer binds with the sulfoxy alcohols in a similar mode as is proposed for hydroxy ketones, there are two possible scenarios: One where porphyrin P1 binds to the oxygen of the sulfoxide (Figure 2-23 A) and the other where porphyrin P1, binds to the sulfur atom (Figure 2-23 B). In A the sulfoxide coordinates to the zinc via the oxygen. Here, P1 approaches to the hydroxyl oxygen from the front (same side as small lone pair group) leaving the second porphyrin P2 to approach the oxygen from the back to avoid sterics from the first porphyrin and thereby sliding away from the larger R group, to avoid the unfavorable steric interactions; this results in a predicted positive ECCD curve for the R enantiomer shown. However, if the binding were to occur via the sulfur lone pairs, as in case B, then the predicted ECCD sign would be negative. Here, porphyrin P1 approaches the sulfur from the front as shown causing P2 to approach the alcohol from the back, avoiding bulky P1. P2 slides away from the larger OH group resulting in an expected negative ECCD sign. These conflicting results from the proposed possible binding mnemonics stress the importance of understanding the preferred binding mode of the sulfoxide to the zinc. i.e. S-Zn vs O-Zn binding of sulfoxide. ! 86! P2 A H O P1 P1 Zn Zn S R O Zn P2 Zn predicted positive ECCD B P2 Zn P2 P1 OH P1 Zn R S O Zn Zn predicted negative ECCD Figure 2-23. Proposed binding of a sulfoxy alcohol to Zn-TPFP tweezer. In A) sulfoxide coordinates to porphyrin via oxygen lone pairs yielding a predicted positive ECCD signal. In B) sulfoxide coordinates to zinc via sulfur lone pairs yielding a predicted negative ECCD signal. Several X-ray crystallographic studies of the chemistry of transition metals complexes with sulfoxides, specifically, dimethyl sulfoxide (DMSO), have been done. 41 However, because of the difficulty in obtaining an X-ray quality crystal, in addition to the fact that not all compounds are crystalline, it becomes difficult to use X-ray crystallography, therefore, IR spectroscopy has become very important in determining the metal-sulfoxide bonding. ! 87! DMSO is structurally similar to acetone, with sulfur replacing the carbonyl carbon. The -1 normal absorption of the S=O bond occurs at 1050 cm . This is lower than the C=O frequency, since the SO bond has a larger reduced mass than the CO bond resulting in the frequency shift. Metals can bond to DMSO either through its oxygen or its sulfur lone pairs. If the binding is to the sulfur, the metal donates electrons from its 1t orbitals (the t2g) into an empty 1t orbital on the DMSO ligand, thereby increasing the S--O bond order. Thus, if the metal is bonded to the DMSO at the sulfur, the frequency of the S=O absorption increases. If the bonding is to the oxygen of the DMSO, the metal forms a bond with one of the lone pairs on the oxygen, and thereby withdraws electron density from the oxygen. This favors the second resonance form in Figure 2-17, since the oxygen will "seek" to gain electrons to compensate for the electrons donated to the metal. The net effect is the lowering of the S=O bond order, and the S=O absorption appears at lower frequency. This shift of the S-O stretching frequency to lower values 42 on binding to oxygen and to higher values on binding to sulfur has been well established. Figure 2-24 shows a sketch 41 of the periodic table showing the preference of O- or S- bonding as inferred from IR spectroscopy. A close look at this table reveals that there’s a general preference of O-bonding over S-bonding, based on the Hard/Soft acid/base principle. However, it is important to remember that the hardness and softness of a metal ion can be dramatically modified by the nature of the coordinated ligands. ! 88! H o Li B Na K Sc o Ti o V o Sr Y o Zr o Nb Mo Tc o o La Hf Ta o o o Ac o Cr o Mn Fe o s, o Re o Co o Ni o Cu Zn o o Ru s, o Rh s, o Pd s, o Ag Cd In Sn o Os Ir s, o Pt s Hg Tl Pb o Figure 2-24. Sketch of the periodic table showing the preference of O- and S- bonding sulfoxide complexes as suggested by IR spectroscopy. Attempts to crystallize sulfoxides bound to porphyrin were unsuccessful, and so we turned to IR spectroscopy in order to understand the binding of S=O to zinc. IR spectroscopy analysis of monomeric A4 Zn-TPFP complexed with phenyl methyl sulfoxide was used to gain insight into the binding mode of fluorinated tweezer system with sulfoxide. In this experiment, our main interest was the behavior of the SO stretching frequency, since this should be the most informative with respect to the nature of the zinc-sulfoxide bonding. As shown in figure 2-25, upon coordination of sulfoxide to porphyrin, the peak corresponding to the S=O stretch (1048 -1 cm ) decreased in intensity with increasing equivalents of porphyrin, until complete disappearance at 1 equivalents of porphyrin. Addition of sulfoxide to this saturated complex -1 resulted in the re-appearance of S=O at the same frequency (1048 cm ). (See SI for full experimental details). ! 89! Titration of TPFP phorphyrin with methylphenyl sulfoxide 100 equiv S=O % Transmittance, % T r a n s m i t t a n c e 10 equiv S=O 1.0 equiv 0.5 equiv TPFP Sulfoxide 1600 1400 1400 1200 1200 S=O 1000 1000 800 800 Wavenumbers, cm-1 Wavenumbers Figure 2-25. IR titration of methyl phenyl sulfoxide with TPFP porphyrin. From 0.5 eq to 100 eq. in methylene chloride. The S=O stretch disappears at high porphyrin concentration, and re-appears with addition of excess sulfoxide. Usually, the S-O stretch gives rise to a distinct intense peak in IR. However, due to overlap or mixing with other bands such as C-H rocking, it is usually hard or complicated to identify its location. Moreover, the shift of the S-O stretch can sometimes be small, making it 43 very difficult or impossible to determine. However, there is no precedence in literature for the preference for sulfur coordination by zinc metal, based on this, we expected Zn-porphyrin to coordinate to oxygen. Looking at the IR data, the S=O stretch disappears upon complexation, -1 but, there are several bands where we would expect the S-O stretch to be (800-1000 cm ). We do not however observe the appearance of S=O stretch at higher frequencies that would indicate ! 90! -1 a sulfur coordination. (1120-1170 cm ). With this in mind, we proposed that the porphyrin tweexer preferably binds to the oxygen lone pairs of the sulfoxide. This information was useful for us to derive the binding model and working mnemonic for the absolute stereochemical determination for chiral sulfoxides. Following these observations, we proposed that Zn-TPFP tweezer would bind to the sulfoxy alcohols via the oxygen lone pairs as depicted in Figure 2-23 B, and not via the sulfur atom. With this in mind, Zn-TPFP tweezer was examined for configurational assignment of a variety of sulfoxy alcohols via the Exciton Coupled Circular Dichroism protocol. To our delight, prominent and reproducible bisignate CD signals at the Soret region were observed upon complexation of the tweezer with micromolar concentrations of a number of chiral sulfoxy alcohols (Table 2-3). As shown in Table 2-3, the obtained amplitudes are reasonably large and as expected, the enantiomers give opposite signals. The sulfoxides 2, 3, 4, 5-ent, 6 and 7-ent resulted in negative ECCD spectra, while positive signals were observed for sulfoxides 2ent, 3-ent, 4-ent, 5, 6-ent and 7. It is noteworthy that the observed ECCD signals remain constant regardless of the presence or absence of substituents at the hydroxyl binding site. Figure 2-26 shows a proposed binding conformation that correlates the chirality of the sulfoxy alcohols and the sign of ECCD obtained. Here, one binding interaction occurs between the hydroxyl group and one porphyrin of the tweezer, while the other is between the sulfoxide oxygen and the other porphyrin of the tweezer. It is assumed that the porphyrin bound to the hydroxyl group approaches opposite the largest substituent on the sulfoxy alcohol. Since P2 is bound to the sulfoxide oxygen, invariably, this will be the largest group so that P1 and P2 avoid ! 91! Table 2-3. ECCD data for chiral sulfoxy alcohols in hexane. ! entry 1 predicted sign sulfoxy alcohol S 2 (S) λ nm, (Δε) A O OH neg 432, -27 419, +30 -53 OH pos 432, +39 421, -31 +70 OH neg 425, -34 414, +36 -70 OH pos 426, +37 414, -43 +80 OH neg 422, +199 414, -13 -25 OH pos 427, +88 414, -82 +170 pos 426, +55 414, -12 +67 neg 426, -39 413, +76 -115 OH neg 423, -65 413, +55 -120 OH pos 424, +70 415, -60 +130 OH pos 421, +27 415, -23 +50 OH neg 425, -10 416, +39 -49 O 2 3 S 2-ent(R) S 3 (S,S) O O S 4 3-ent(R,S) 5 4 (S,R) O S O S 6 4-ent(R,R) 7 5 (S) S 5-ent(R) S O OH O 8 OH O 9 S 6 (S) O 10 6-ent(R) S O 11 7 (R) S O 12 7-ent(S) S Tweezer:substrate ratio – 1:20, 1 µM tweezer concentration at 0 °C was used for all measurements. ! 92! steric clash with each other. In the case of substrates 2, 5, 6 and 7, steric relief of P1 is achieved through rotation/sliding of the porphyrin ring away from the largest substituent on the chiral center. Considering substrate 2 depicted in Figure 2-26 A, P1 slides away from phenyl group, in preference of the smaller lone pair of electrons, generating the energetically favored complex in which the two porphyrins are twisted in a clockwise fashion. Consequently, a positive ECCD spectrum is obtained for the 2R, 6R and 7R substrates. For substrates with two chiral centers, 3 and 4, it is assumed that each porphyrin undergoes independent steric differentiation, keeping in mind that the observed ECCD spectrum is the result of the helical twist of the two porphyrins. This is shown in Figure 2-26 B with the binding of sulfoxy alcohol 3(S,S). We propose that P1 approaches the sulfoxide oxygen opposite to the largest substituent at the chiral center, in this case the phenyl group. This way, the phenyl group is anti to the porphyrin and does not participate in the steric differentiation. The two remaining groups, the lone pair of electrons and the alkyl CH2 chain dictate the rotation of the porphyrin ring. Because lone pair electrons are smaller, P1 will rotate clockwise, away from the CH2 chain. In a similar manner, P2 goes anti to the methyl group when binding to the hydroxyl group. Steric differentiation of the remaining substituents (H and CH2 chain) causes the counterclockwise rotation of P2 away from bulky CH2 chain towards the smaller hydrogen. This causes a clockwise helical twist between P1 and P2 which would predict a negative ECCD spectrum. Figure 2-30 shows the positive and negative ECCD spectra obtained with substrates 2 (R) and 3 (S,S) upon complexation with tweezer. ! 93! P2 H O P1 P1 Zn Zn S Ph O Zn P2 Zn predicted positive ECCD P2 Zn P2 OH CH3 H Ph S O P1 P1 Zn Zn Zn predicted negative ECCD Figure 2-26. Proposed working mnemonic as shown for 2ent(R) and 3(S,S) The sign does not depend on the chain length or the groups on the substrates. It is also independent of the alcohol, with primary, secondary and tertiary centers behaving in a similar manner. This leads us to believe that the stereodifferentiation is governed only by the groups directly attached at the chiral centers and the porphyrin coordinating to the sulfoxide oxygen achieves the stereodifferentiation based on the sizes of the groups attached directly to the sulfoxide. ! 94! S O O S OH 50 OH 50 0 A A 20 -10 -40 -70 -50 375 425 395 475 λ, nm 415 435 λ, nm Figure 2-27. Obtained ECCD spectra of 3 (S,S), and 3-ent (R,S) respectively. In conclusion, we have successfully applied the tweezer methodology to determine the absolute stereochemistry of chiral sulfoxides, using very little substrate (milligram quantities) and without requiring any derivatizations. ! 95! EXPERIMENTAL: Materials and methods Anhydrous CH2Cl2 was dried and redistilled over CaH2. The solvents used for CD measurements were purchased from Aldrich and were spectra grade. All reactions were performed in dried glassware under nitrogen. Column chromatography was performed using 1 SiliCycle silica gel (230-400 mesh). H-NMR and 13 C-NMR spectra were obtained on Varian Inova 300 and 500 MHz instrument and are reported in parts per million (ppm) relative to the solvent resonances (δ), with coupling constants (J) in Hertz (Hz). IR studies were performed on a Nicolet FT-IR 42 instrument. UV/Vis spectra were recorded on a Perkin-Elmer Lambda 40 spectrophotometer, and are reported as λmax [nm]. CD spectra were recorded on a JASCO J-810 spectropolarimeter, equipped with a temperature controller (Neslab 111) for low temperature -1 -1 studies, and were reported as λ[nm] (Δεmax [L mol cm ]). Optical rotations were recorded at 20 °C on a Perkin Elmer 341 Polarimeter (λ = 589 nm, 1 dm cell). HRMS analyses were performed on a Q-TOF Ultima system using electrospray ionization in positive mode. General procedure for CD measurement: Zinc porphyrin tweezer 1 (1 μL of a 1 mM solution in anhydrous CH2Cl2) was added to hexane (1 mL) in a 1.0 cm cell to obtain a 1 μM tweezer 1 solution. The background spectrum was recorded from 350 nm to 550 nm with a scan rate of 100 nm/min at 0 °C. Chiral hydroxy ketone (1 to 20 μL of a 1 mM solution in anhydrous CH2Cl2) was added into the prepared tweezer solution to afford the host/guest complex. The CD spectra were measured immediately ! 96! (minimum of 5 accumulations). The resultant ECCD spectra recorded in millidegrees were normalized based on the tweezer concentration to obtain the molecular CD (Mol CD). Binding affinity measurements of hydroxyl ketone upon complexation with Zn-TPFP-tz 1. The binding affinities for porphyrin tweezer complexes were determined through titration of porphyrin tweezer 1 with corresponding guest. The UV spectrum of the titration of hydroxy ketone 8 shown below (Figure S1) demonstrates the change in the Soret band absorption upon binding of the guest to the host. Plot of the equivalence of the hydroxy ketone added as a function of the change in absorption (426 nm) leads to a saturation curve, which provides the binding constant of complex formed upon non-linear least square analysis. Calculations of binding constants via the latter procedure follows previously published protocols and analysis.5 Investigation of importance of linker length. Synthesis of chiral hydroxy ketones Compound 6 and 10 are commercially available from Aldrich. Hydroxy ketones 2-8 were synthesized from their corresponding chiral diols by monobenzylation, 19 followed by oxidation and deprotection. This procedure does not significantly affect the ee’s. as shown by comparing the [α] 20 D obtained for (R)-9-hydroxydecan-2-one (6R) to the reported literature value ([α] 20 D = -8.5, (c = 0.50, CHCl3; ee= 92% (lit -9.7 >95%)) Typical procedure for synthesis of chiral hydroxy ketones from chiral diols as described for the synthesis of 2: ! 97! 1) Procedure for mono benzylation: Reactions were performed at 50 °C. To a flame-dried 5mL round bottom flask equipped with a stir bar was added diol (200 mg, 1.69 mmol) and freshly distilled toluene (2 mL). 0.74 mL of a 50% w KOH aqueous solution (0.5 equiv. pure KOH) and tetrabutylammonium bisulfate (9.2 mg, 1.6 mmol) were subsequently added to the flask and stirred for 15 min to equilibrate. Then, benzyl chloride (0.38 mL, 3.3 mmol) was added and the mixture stirred for 24 h, monitored by TLC until completion. The organic layer was extracted into CH2Cl2 and solvent removed under reduced pressure. The residue was purified by flash chromatography (20% EtOAc/hexane) to afford the mono protected diol (0.3 g, 91%) as a colorless oil. (2R,9R)-9-(benzyloxy)decan-2-ol 1 H-NMR (CDCl3, 300 MHz) δ 0.95 (t, 3H, J= 7.2 Hz), 1.46 (m, 4H), 1.78 (t, 1H, J = 6.3 Hz), 2.91 (m, 2H), 3.61 (m, 1H), 3.91 (m, 1H). (2R,10R)-10-(benzyloxy)undecan-2-ol 1 H-NMR (CDCl3, 300 MHz) δ 1.16-12.10 (m, 18H), 2.11 (s, 3H), 2.39 (t, 2H, J = 7.5 Hz), 3.44 (m, 1H), 4.48 (dd, 2H, J = 12.9 Hz and 11.7Hz), 7.32-7.23 (m, 5H). (2R,13R)-13-(benzyloxy)tetradecan-2-ol 1 H-NMR (CDCl3, 300 MHz) δ 1.60-1.15 (m, 23H), 3.49-3.44 (m, 1H), 3.78-3.76 (m, 1H), 2.91 (m, 2H), 3.61 (m, 1H), 4.48 (dd, 2H, J = 12.9 Hz and 11.7Hz), 7.33-7.24 (m, 5H); ! 98! 13 C-NMR (CDCl3, 75 MHz) δ 19.8, 23.7, 25.7, 26.0, 26.4, 29.7, 29.82, 29.84, 29.85, 29.88, 29.9, 30.0, 36.9, 39.6, 68.4, 70.5, 70.7, 73.0, 75.1, 127.5, 127.7, 127.8, 128.53, 128.57, 139.4. 2) Procedure for oxidation: To a flame-dried round bottom flask was added PCC (0.61 g, 2.2 eq), MgSO2 (5 times PCC) and freshly distilled CH2Cl2 (enough to just cover the reagents). This was stirred vigorously and then alcohol (0.27 g, 1 eq) dissolved in CH2Cl2 (10 mL) was added in one portion. This was stirred at room temperature under niterogen atmosphere and monitored by TLC. After completion of the reaction, ether (2 times CH2Cl2) was added in and stirred for 2 min, then suction filtered. Solvent was removed under reduced pressure and product purified by flash chromatography (20% EtOAc/hexane) to give desired ketone (0.21 g, 80%) (R)-9-(benzyloxy)decan-2-one 1 H-NMR (CDCl3, 300 MHz) δ 1.16 (d, 3H, J= 6 Hz), 1.59-1.21 (m, 10H), 2.10 (s, 3H), 2.38 (t, 2H, J=7.2 Hz), 3.48 (m, 1H), 4.48 (dd, 2H, J= 11.7 and 23.1 Hz ), 7.32-7.25 (m, 5H); 13 C-NMR (CDCl3, 75 MHz) δ 19.6, 23.8, 25.3, 29.1, 29.4, 29.8, 36.6, 43.8, 70.3, 74.8, 103.9, 127.3, 127.6, 128.3, 139.1. (R)-10-(benzyloxy)undecan-2-one 1 H-NMR (CDCl3, 300 MHz) δ 0.95 (t, 3H, J= 7.2 Hz), 1.46 (m, 4H), 1.78 (t, 1H, J = 6.3 Hz), 2.91 (m, 2H), 3.61 (m, 1H), 3.91 (m, 1H). ! 99! (R)-13-(benzyloxy)tetradecan-2-one 1 H-NMR (CDCl3, 300 MHz) δ 1.16-12.10 (m, 18H), 2.11 (s, 3H), 2.39 (t, 2H, J = 7.5 Hz), 3.44 (m, 1H), 4.48 (dd, 2H, J = 12.9 Hz and 11.7Hz), 7.32-7.23 (m, 5H); 3) Procedure for deprotection: To a flame-dried flask containing ketone dissolved in EtOAc was added Pearlman’s catalyst (10%) and stirred at room temperature until completion of reaction as observed by TLC. After completion, the reaction mixture was filtered by vaccum filtration and solvent removed under reduced pressure. Hydroxy ketone was obtained in quantitative yield after flash chromatography (20% EtOAc/hexane) Note: the additional manipulations (protection, oxidation, deprotection) do not affect the ee values as shown from comparison of [α] 20 D Values to known compounds. (S)-4-hydroxypentan-2-one (2S) 1 H-NMR (CDCl3, 300 MHz) δ 0.95 (t, 3H, J= 7.2 Hz), 1.46 (m, 4H), 1.78 (t, 1H, J = 6.3 Hz), 2.91 (m, 2H), 3.61 (m, 1H), 3.91 (m, 1H). (R)-4-hydroxy-5-methylhexan-2-one (3R) 1 H-NMR (CDCl3, 300 MHz) δ 0.95 (t, 3H, J= 7.2 Hz), 1.46 (m, 4H), 1.78 (t, 1H, J = 6.3 Hz), 2.91 (m, 2H), 3.61 (m, 1H), 3.91 (m, 1H). ! 100! (S)-5-hydroxyhexan-2-one (4S) 1 H-NMR (CDCl3, 300 MHz) δ 0.95 (t, 3H, J= 7.2 Hz), 1.46 (m, 4H), 1.78 (t, 1H, J = 6.3 Hz), 2.91 (m, 2H), 3.61 (m, 1H), 3.91 (m, 1H). (R)-9-hydroxydecan-2-one (6R) [α] 20 D= 1 -8.5, (c = 0.50, CHCl3; ee= 92% (lit -9.7 >95%); H-NMR (CDCl3, 300 MHz) δ 1.15 (d, 3H, J= 6.3 Hz), 1.59-1.23 (m, 10H), 2.1 (s, 3H), 2.39 (t, 2H, J=7.2Hz), 3.77-3.73(m, 1H); 13 C-NMR (CDCl3, 75 MHz) δ 14.5, 18.0, 23.5, 23.7, 25.5, 29.1, 29.3, 29.8, 39.2, 43.7, 68,1, 111.1; (R)-10-hydroxyundecan-2-one (7R) 1 H-NMR (CDCl3, 300 MHz) δ 0.95 (t, 3H, J= 7.2 Hz), 1.46 (m, 4H), 1.78 (t, 1H, J = 6.3 Hz), 2.91 (m, 2H), 3.61 (m, 1H), 3.91 (m, 1H). (R)-13-hydroxytetradecan-2-one (8R) 1 H-NMR (CDCl3, 300 MHz) δ 0.95 (t, 3H, J= 7.2 Hz), 1.46 (m, 4H), 1.78 (t, 1H, J = 6.3 Hz), 2.91 (m, 2H), 3.61 (m, 1H), 3.91 (m, 1H). Synthesis of chiral sulfoxy alcohols: To obtain pure enantiomers, the racemic chiral sulfoxides were separated on chiral HPLC column. ! 101! O S OH To a stirred solution of benzene thiol (1 g, 9.07 mmol) and triethyl amine (2.5 mL, 18 mol) in acetonitrile (26 mL) was added 2-bromoethanol (0.71 mL, 9.98 mmol). The reaction was stirred at room temperature for 24 h, treated with saturated NH4Cl (20 mL) and extracted into ethyl acetate (3x20 mL). The combined organic layers were dried (Na2SO4). Solvent was removed and desired thio-ether obtained by flash chromatography (hex/EtOAc 2:1) as a colorless oil in 75% yield. 1 H NMR (CDCl3, 500 MHz) δ 2.12-2.16 (t, 1H J = 5.7), 3.13-3.17 (t, 2H J = 3.3 Hz), 3.75-3.81 (q, 2H, J = 6.0 Hz), 7.22-7.36 (m, 3H), 7.40-7.44 (m, 2H); 13 CNMR (CDCl3, 75 MHz) δ; This was then treated with MCPBA (1:1 eq) in freshly distilled DCM at room temperature for 4 1 h to give the racemic sulfoxide as a clear oil in 45% yield. H-NMR (CDCl3, 500 MHz) δ 2.813.21 (ddd, 2H J1 = 3.6, J2 = 13.8, J3 = ), 3.37 (br s, 1H), 3.97-4.19 (ddd, 2H, J1 = 2.7, J2 = 11.7, J3 = Hz), 3.97 (s, 1H), 7.57-7.61 (m, 3H), 7.88-7.99 (m, 3H), 8.19 (s, 1Hr); 13 C-NMR (CDCl3, 75 MHz) δ 56.6, 58.9, 123.9, 129.4, 131.2, 143.1. HPLC separation was done using OD column, eluting with 4% IPA/hex, at 0.8 mL/min Retention times: Enantiomer 1: 36 min Enantiomer 2: 44 min O S ! OH 102! 2-naphthalenethiol (1 g, 6.93 mmol) was dissolved in 2,2-dimethyloxirane (0.5 g, 6.93 mmol) and refluxed for 16 h. After this, the reaction mixture was directly poured onto a silica gel column and pruduct eluted with 20% EtOAC/hex as a crystalline white solid in 29% yield. 1 H-NMR (CDCl3, 500 MHz) δ 1.50 (s, 6H), 3.41 (s, 3H), 7.65-7.73 (m, 2H), 7.91 (d, 2H J = 8.5), 7.95 (d, 2H J = 8.0 Hz), 8.08 (t, 2H J = 8.5 Hz), 8.53 (s, 1H); 13 C-NMR (CDCl3, 75 MHz) δ 29.1, 30.9, 68.0, 71.2, 119.8, 124.7, 127.7, 128.2, 128.3, 128.7, 130.0, 133.1, 134.7, 141.1; This was then treated with MCPBA (1:1 eq) in freshly distilled DCM at room temperature for 4 h to give the racemic sulfoxide as a clear oil in 50% yield. 1 H-NMR (CDCl3, 500 MHz) δ 1.36 (s, 3H), 1.64 (s, 3H), 2.80-3.09 (dd, 2H, J1 = 13.5, J2 = Hz), 3.97 (s, 1H), 7.57-7.61 (m, 3H), 7.88-7.99 (m, 3H), 8.19 (s, 1Hr); 13 C-NMR (CDCl3, 75 MHz) δ 29.1, 30.9, 68.0, 71.2, 119.8, 124.7, 127.7, 128.2, 128.3, 128.7, 130.0, 133.1, 134.7, 141.1; HPLC separation was done using OD column, eluting with 5% IPA/hex, at 0.8 mL/min Retention times: Enantiomer 1: 30 min Enantiomer 2: 40 min O S OH Benzenethiol (0.88 mL, 8.62 mmol) was dissolved in (R)-(-)-1,2-epoxypropane (1 mL, 8.62 mmol) and refluxed for 16 h. After this, the reaction mixture was directly poured onto a silica gel column and product eluted with 10% EtOAC/hex as a crystalline white solid in 29% yield. 1 H-NMR (CDCl3, 500 MHz) δ 1.24 (d, 3H J = 6 Hz), 2.15 (br, s 1H), 2.80-2.84 (dd, 1H J1 = 8.5, J2 = 13.5), 3.07-3.11 (dd, 1H, J1 = 3.5, J2 = 13.5Hz), 3.82-3.83 (m, 1H), 7.18-7.21 (m, 1H), 7.26! 103! 7.29 (m, 2H), 7.36-7.38 (m, 2H); 13 C-NMR (CDCl3, 75 MHz) δ 22.1, 43.8, 65.7, 126.9, 129.3, 130.3, 136.4; This was then treated with MCPBA (1:1 eq) in freshly distilled DCM at room temperature for 4h to give the racemic sulfoxide as a clear oil in 40% yield. HPLC separation was done using HPLC column, eluting with 5% IPA/hex, at 1.5 mL/min Retention times: Enantiomer 1: 30 min Enantiomer 2: 40 min O S OH Benzenethiol (1.42 mL, 13.86 mmol) was dissolved in 2,2-dimethyloxirane (1 g, 13.86 mmol) and refluxed for 16 h. After this, the reaction mixture was directly poured onto a silica gel column and product eluted with 10% EtOAC/hex as a crystalline white solid in 59% yield. 1 H-NMR (CDCl3, 300 MHz) δ 1.34 (s, 3H), 1.61 (s, 3H), 2.71-3.02 (dd, 2H, J1 = 7.8, J2 = Hz), 3.89 (s, 1H), 7.49-7.54 (m, 3H), 7.63-7.64 (m, 2H); 13 C-NMR (CDCl3, 75 MHz) δ 28.9, 38.4, 45.8, 70.8, 127.4, 128.8, 129.1, 138.5; This was then treated with MCPBA (1:1 eq) in freshly distilled DCM at room temperature for 2 h to give the racemic sulfoxide as a white solid in 44% yield. 1 H-NMR (CDCl3, 300 MHz) δ 1.34 (s, 3H), 1.61 (s, 3H), 2.71-3.02 (dd, 2H, J1 = 7.8, J2 = Hz), 3.89 (s, 1H), 7.49-7.54 (m, 3H), 7.63-7.64 (m, 2H); 13 C-NMR (CDCl3, 75 MHz) δ 28.9, 38.4, 45.8, 70.8, 127.4, 128.8, 129.1, 138.5. HPLC separation was done using HPLC column, eluting with 5% IPA/hex, at 1.5 mL/min ! 104! Retention times: Enantiomer 1: Enantiomer 2: O S OH Benzenethiol (0.88 mL, 8.62 mmol) was dissolved in (S)-(-)-1,2-Epoxypropane (1mL, 8.62 mmol) and refluxed for 16 h. After this, the reaction mixture was directly poured onto a silica gel column and product eluted with 10% EtOAC/hex as a crystalline white solid in 29% yield. 1 H-NMR (CDCl3, 300 MHz) δ 1.30 (d, 3H J = 10.5 Hz), 2.47 (d, 1H J = 5.5 Hz), 2.84-2.91 (dd, 1H, J1 = 14, J2 = Hz), 3.86-3.91 (m, 1H), 7.25-7.44 (m, 5H); 13 C-NMR (CDCl3, 75 MHz) δ; This was then treated with MCPBA (1:1 eq) in freshly distilled DCM at room temperature for 4h to give the racemic sulfoxide as a colorless oil in 40% yield. 1 H-NMR (CDCl3, 300 MHz) δ 1.31 (s, 3H), 1.45 (s, 3H), 2.54-2.84 (dd, 2H, J1 = 12.9, J2 = Hz), 3.71 (s, 1H), 3.97-4.13 (dd, 2H J1 = 12.9, J2 = ), 3.91 (m, 1H); 13 C-NMR (CDCl3, 75 MHz) δ 28.9, 38.4, 45.8, 70.8, 127.4, 128.8, 129.1, 138.5; HPLC separation was done using HPLC column, eluting with 10% IPA/hex, at 1.5 mL/min Retention times: Enantiomer 1: 14 min Enantiomer 2: 19 min O S ! OH 105! Benzyl mercaptan (1.7 g, 13.86 mmol) was dissolved in 2,2-dimethyloxirane (1 g, 13.86 mmol) and some solid NaOH pellets added in. The mixture was then refluxed for 16h, after which the reaction mixture was directly poured onto a silica gel column and product eluted with 20% EtOAC/hexanes as a crystalline white solid in 89% yield. 1 H-NMR (CDCl3, 300 MHz) δ 1.31 (s, 3H), 1.45 (s, 3H), 2.54-2.84 (dd, 2H, J1 = 12.9, J2 = Hz), 3.71 (s, 1H), 3.97-4.13 (dd, 2H J1 = 12.9, J2 = ), 3.91 (m, 1H); 13 C-NMR (CDCl3, 75 MHz) δ 28.9, 38.4, 45.8, 70.8, 127.4, 128.8, 129.1, 138.5; This was then treated with MCPBA (1:1 eq) in freshly distilled DCM at room temperature for 2 h to give the racemic sulfoxide as a white solid in 74% yield. HPLC separation was done using OD-H column, eluting with 2% IPA/hex, at 1.5 mL/min Retention times: Enantiomer 1: 45 min Enantiomer 2: 60 min ! 106! X-Ray Crystallography Data O S OH !! Table 2-4. Crystal data and structure refinement for bb58_0m. ! ___________________________________________________________________ Identification code bb58_0m Empirical formula C10 H14 O2 S Formula weight 198.27 Temperature 173(2) K Wavelength 1.54178 Å Crystal system Monoclinic Space group P 21 Unit cell dimensions a = 5.6789(2) Å a= 90°. b = 9.9447(3) Å Volume Z Density (calculated) Absorption coefficient F(000) Crystal size Theta range for data collection ! b= 99.182(2)°. c = 9.2621(3) Å 516.37(3) Å3 g = 90°. 2 1.275 Mg/m3 2.512 mm-1 212 0.68 x 0.09 x 0.09 mm3 4.84 to 68.05°. 107! Table 2-4 (cont’d) _______________________________________________________________ ! Index ranges -6<=h<=6, -11<=k<=11, -7<=l<=10 Reflections collected 7022 Independent reflections 1804 [R(int) = 0.0449] Completeness to theta = 68.05° 97.5 % Absorption correction Max. and min. transmission Refinement method Data / restraints / parameters Goodness-of-fit on F2 Final R indices [I>2sigma(I)] R indices (all data) Absolute structure parameter Largest diff. peak and hole ! Semi-empirical from equivalents 0.8148 and 0.2791 Full-matrix least-squares on F2 1804 / 1 / 121 1.063 R1 = 0.0310, wR2 = 0.0749 R1 = 0.0344, wR2 = 0.0766 -0.001(19) 0.164 and -0.225 e.Å-3 108! Table 2-5. Atomic coordinates ( x 104) and equivalent isotropic displacement parameters (Å2x 103) for bb58_0m. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. _______________________________________________________________________ x y z U(eq) _______________________________________________________________________ S(1) 2056(1) 1134(1) 1997(1) 30(1) O(1) 4368(3) 1129(2) 1404(2) 43(1) O(2) -2301(3) 37(2) -138(2) 34(1) C(1) -70(4) 2044(2) 705(2) 28(1) C(2) -1205(3) 1213(3) -625(2) 29(1) C(3) -3064(4) 2105(3) -1535(3) 38(1) C(4) 611(4) 708(2) -1537(3) 36(1) C(5) 2430(4) 2395(2) 3409(3) 29(1) C(6) 4340(4) 3272(3) 3513(3) 38(1) C(7) 4669(5) 4207(3) 4629(3) 50(1) C(8) 3112(5) 4256(3) 5628(3) 47(1) C(9) 1212(5) 3386(3) 5511(3) 46(1) C(10) 852(5) 2438(3) 4403(3) 39(1) ____________________________________________________________________ ! 109! Table 2-6. Bond lengths [Å] and angles [°] for bb58_0m. _____________________________________________________ S(1)-O(1) 1.5014(15) S(1)-C(5) 1.800(2) S(1)-C(1) 1.802(2) O(2)-C(2) 1.431(3) O(2)-H(2) 0.8400 C(1)-C(2) 1.537(3) C(1)-H(1A) 0.9900 C(1)-H(1B) 0.9900 C(2)-C(4) 1.519(3) C(2)-C(3) 1.526(3) C(3)-H(3A) 0.9800 C(3)-H(3B) 0.9800 C(3)-H(3C) 0.9800 C(4)-H(4A) 0.9800 C(4)-H(4B) 0.9800 C(4)-H(4C) 0.9800 C(5)-C(6) 1.383(3) C(5)-C(10) 1.384(3) C(6)-C(7) 1.380(4) C(6)-H(6) 0.9500 C(7)-C(8) 1.379(4) C(7)-H(7) 0.9500 C(8)-C(9) 1.374(4) C(8)-H(8) 0.9500 C(9)-C(10) 1.385(4) C(9)-H(9) 0.9500 C(10)-H(10) 0.9500 O(1)-S(1)-C(5) O(1)-S(1)-C(1) C(5)-S(1)-C(1) C(2)-O(2)-H(2) C(2)-C(1)-S(1) C(2)-C(1)-H(1A) ! 105.02(11) 106.95(10) 97.00(10) 109.5 114.39(15) 108.7 110! Table 2-6 (cont’d) _____________________________________________________ S(1)-C(1)-H(1A) 108.7 C(2)-C(1)-H(1B) 108.7 S(1)-C(1)-H(1B) 108.7 H(1A)-C(1)-H(1B) 107.6 O(2)-C(2)-C(4) 105.9(2) O(2)-C(2)-C(3) 110.57(16) C(4)-C(2)-C(3) 110.86(19) O(2)-C(2)-C(1) 109.54(17) C(4)-C(2)-C(1) 112.88(16) C(3)-C(2)-C(1) 107.2(2) C(2)-C(3)-H(3A) 109.5 C(2)-C(3)-H(3B) 109.5 H(3A)-C(3)-H(3B) 109.5 C(2)-C(3)-H(3C) 109.5 H(3A)-C(3)-H(3C) 109.5 H(3B)-C(3)-H(3C) 109.5 C(2)-C(4)-H(4A) 109.5 C(2)-C(4)-H(4B) 109.5 H(4A)-C(4)-H(4B) 109.5 C(2)-C(4)-H(4C) 109.5 H(4A)-C(4)-H(4C) 109.5 H(4B)-C(4)-H(4C) 109.5 C(6)-C(5)-C(10) 121.2(2) C(6)-C(5)-S(1) 119.36(18) C(10)-C(5)-S(1) 119.41(18) C(7)-C(6)-C(5) 119.2(2) C(7)-C(6)-H(6) 120.4 C(5)-C(6)-H(6) 120.4 C(8)-C(7)-C(6) 120.1(3) C(8)-C(7)-H(7) 119.9 C(6)-C(7)-H(7) 119.9 C(9)-C(8)-C(7) 120.3(3) C(9)-C(8)-H(8) 119.8 C(7)-C(8)-H(8) 119.8 ! 111! Table 2-6 (cont’d) _____________________________________________________________ C(8)-C(9)-C(10) 120.5(3) C(8)-C(9)-H(9) 119.7 C(10)-C(9)-H(9) 119.7 C(5)-C(10)-C(9) 118.7(2) C(5)-C(10)-H(10) 120.7 C(9)-C(10)-H(10) 120.7 _____________________________________________________________ Symmetry transformations used to generate equivalent atoms: Table 2-7. Anisotropic displacement parameters (Å2x 103) for bb58_0m. The anisotropic displacement factor exponent takes the form: -2p2[ h2 a*2U11 + ... + 2 h k a* b* U12 ] _______________________________________________________________________ U11 U22 U33 U23 U13 U12 _______________________________________________________________________ S(1) 36(1) 25(1) 31(1) 0(1) 7(1) 7(1) O(1) 35(1) 57(1) 38(1) -6(1) 8(1) 17(1) O(2) 32(1) 24(1) 47(1) -5(1) 15(1) -4(1) C(1) 26(1) 23(1) 34(1) 1(1) 7(1) 2(1) C(2) 26(1) 24(1) 37(1) -2(1) 7(1) -4(1) C(3) 32(1) 41(1) 39(2) 0(1) 2(1) -3(1) C(4) 34(1) 36(1) 38(2) -7(1) 10(1) -6(1) C(5) 30(1) 28(1) 28(1) 1(1) 3(1) 6(1) C(6) 28(1) 44(2) 44(2) -2(1) 5(1) 0(1) C(7) 37(2) 49(2) 59(2) -9(1) -8(1) -1(1) C(8) 53(2) 46(2) 36(2) -10(1) -10(1) 14(1) C(9) 58(2) 48(2) 34(2) 1(1) 12(1) 16(1) C(10) 45(1) 35(1) 40(2) 3(1) 15(1) 2(1) _______________________________________________________________________ ! 112! Table 2-8. Hydrogen coordinates ( x 104) and isotropic displacement parameters (Å2x 10 3) for bb58_0m. _______________________________________________________________________ x y z U(eq) _______________________________________________________________________ H(2) -3425 266 299 50 H(1A) -1355 2383 1216 33 H(1B) 737 2834 354 33 H(3A) -3827 1606 -2397 56 H(3B) -2287 2909 -1849 56 H(3C) -4273 2371 -945 56 H(4A) 1714 85 -954 53 H(4B) 1506 1472 -1842 53 H(4C) -216 242 -2405 53 H(6) 5411 3233 2824 46 H(7) 5970 4817 4709 60 H(8) 3353 4895 6400 56 H(9) 136 3435 6197 56 H(10) -451 1829 4327 46 _______________________________________________________________________ ! 113! Table 2-9. Torsion angles [°] for bb58_0m. ________________________________________________________________ O(1)-S(1)-C(1)-C(2) 81.01(17) C(5)-S(1)-C(1)-C(2) -170.90(14) S(1)-C(1)-C(2)-O(2) 56.12(19) S(1)-C(1)-C(2)-C(4) -61.6(2) S(1)-C(1)-C(2)-C(3) 176.10(15) O(1)-S(1)-C(5)-C(6) 12.8(2) C(1)-S(1)-C(5)-C(6) -96.9(2) O(1)-S(1)-C(5)-C(10) -164.95(19) C(1)-S(1)-C(5)-C(10) 85.3(2) C(10)-C(5)-C(6)-C(7) 0.0(4) S(1)-C(5)-C(6)-C(7) -177.7(2) C(5)-C(6)-C(7)-C(8) 0.2(4) C(6)-C(7)-C(8)-C(9) -0.6(4) C(7)-C(8)-C(9)-C(10) 0.8(4) C(6)-C(5)-C(10)-C(9) 0.2(4) S(1)-C(5)-C(10)-C(9) 177.93(19) C(8)-C(9)-C(10)-C(5) -0.6(4) ________________________________________________________________ Symmetry transformations used to generate equivalent atoms: Table 2-10. Hydrogen bonds for bb58_0m [Å and °]. _______________________________________________________________________ D-H...A d(D-H) d(H...A) d(D...A) <(DHA) _______________________________________________________________________ O(2)-H(2)...O(1)#1 0.84 1.94 2.768(2) 168.1 _______________________________________________________________________ Symmetry transformations used to generate equivalent atoms: #1 x-1,y,z ! 114! Table 2-10 (cont’d) _____________________________________________________ Space Group P21 Wavelength 1.54178 Flack x .... -0.001 Flack (su) . 0.019 Bijvoet Pairs 834 Coverage ... 95.0 DiffCalcMax. 120.08 Outlier Crit 240.17 Sigma Crit.. 0.25 Select Pairs 599 Number Plus 502 Number Minus 97 Aver. Ratio 1.053 RC ......... 1.016 Normal Prob. Plot Sample Size. 834 Corr. Coeff. 0.996 Intercept .. -0.189 Slope ...... 0.990 Bayesian Statistics Type ....... Gaussian Select Pairs 834 P2(true).... 1.000 P3(true).... 1.000 P3(rac-twin) 0.0E+00 P3(false) .. 0.0E+00 G .......... 1.0168 G (su) ..... 0.0264 ! 115! Table 2-10 (cont’d) _____________________________________________________ Hooft y .... -0.008 Hooft (su) . 0.013 _____________________________________________________ O S OH Table 2-11. Crystal data and structure refinement for bb64_0m. ________________________________________________________________ Identification code bb64_0m Empirical formula C14 H16 O2 S Formula weight 248.33 Temperature 173(2) K Wavelength 1.54178 Å Crystal system Orthorhombic Space group P 21 21 21 Unit cell dimensions a = 5.69360(10) Å a= 90°. b = 10.0348(3) Å Volume Z Density (calculated) Absorption coefficient ! b= 90°. c = 22.1186(5) Å 1263.73(5) Å3 g = 90°. 4 1.305 Mg/m3 2.167 mm-1 116! Table 2-11 (cont’d) _____________________________________________________ F(000) 528 Crystal size 0.33 x 0.11 x 0.08 mm3 Theta range for data collection 4.84 to 67.71°. Index ranges Reflections collected Independent reflections Completeness to theta = 67.71° -6<=h<=6, -12<=k<=12, -15<=l<=26 9193 2276 [R(int) = 0.0616] 99.8 % Absorption correction Max. and min. transmission Refinement method Data / restraints / parameters Goodness-of-fit on F2 Final R indices [I>2sigma(I)] R indices (all data) Absolute structure parameter Largest diff. peak and hole Semi-empirical from equivalents 0.8457 and 0.5367 Full-matrix least-squares on F2 2276 / 0 / 157 1.027 R1 = 0.0359, wR2 = 0.0831 R1 = 0.0397, wR2 = 0.0848 0.02(2) 0.450 and -0.206 e.Å-3 ! 117! Table 2-12. Atomic coordinates ( x 104) and equivalent isotropic displacement parameters (Å2x 103) for bb64_0m. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. _______________________________________________________________________ x y z U(eq) _______________________________________________________________________ S(1) 7037(1) 6755(1) 8222(1) 28(1) O(1) 2938(3) 5661(2) 7364(1) 33(1) O(2) 9513(3) 7026(2) 8026(1) 39(1) C(1) 5125(4) 7596(2) 7693(1) 26(1) C(2) 4366(4) 6732(2) 7154(1) 28(1) C(3) 2971(5) 7623(3) 6725(1) 39(1) C(4) 6432(4) 6084(3) 6828(1) 34(1) C(5) 6472(4) 7834(2) 8850(1) 25(1) C(6) 8057(4) 8783(2) 9015(1) 26(1) C(7) 7614(3) 9590(2) 9531(1) 24(1) C(8) 9202(4) 10583(2) 9730(1) 30(1) C(9) 8775(5) 11289(3) 10243(1) 36(1) C(10) 6726(5) 11058(2) 10583(1) 36(1) C(11) 5119(4) 10132(2) 10398(1) 32(1) C(12) 5514(4) 9374(2) 9866(1) 26(1) C(13) 3902(4) 8389(2) 9669(1) 28(1) C(14) 4352(4) 7624(2) 9175(1) 27(1) ______________________________________________________________________ ! 118! Table 2-13. Bond lengths [Å] and angles [°] for bb64_0m. _____________________________________________________ S(1)-O(2) 1.4994(18) S(1)-C(5) 1.790(2) S(1)-C(1) 1.807(2) O(1)-C(2) 1.425(3) O(1)-H(1) 0.8400 C(1)-C(2) 1.536(3) C(1)-H(1A) 0.9900 C(1)-H(1B) 0.9900 C(2)-C(4) 1.526(3) C(2)-C(3) 1.526(3) C(3)-H(3A) 0.9800 C(3)-H(3B) 0.9800 C(3)-H(3C) 0.9800 C(4)-H(4A) 0.9800 C(4)-H(4B) 0.9800 C(4)-H(4C) 0.9800 C(5)-C(6) 1.362(3) C(5)-C(14) 1.421(3) C(6)-C(7) 1.423(3) C(6)-H(6) 0.9500 C(7)-C(8) 1.416(3) C(7)-C(12) 1.424(3) C(8)-C(9) 1.359(4) C(8)-H(8) C(9)-C(10) C(9)-H(9) C(10)-C(11) C(10)-H(10) C(11)-C(12) C(11)-H(11) C(12)-C(13) C(13)-C(14) C(13)-H(13) ! 0.9500 1.407(4) 0.9500 1.367(4) 0.9500 1.418(3) 0.9500 1.418(3) 1.361(3) 0.9500 119! Table 2-13 (cont’d) _____________________________________________________ C(14)-H(14) 0.9500 O(2)-S(1)-C(5) O(2)-S(1)-C(1) C(5)-S(1)-C(1) C(2)-O(1)-H(1) C(2)-C(1)-S(1) C(2)-C(1)-H(1A) S(1)-C(1)-H(1A) C(2)-C(1)-H(1B) S(1)-C(1)-H(1B) H(1A)-C(1)-H(1B) O(1)-C(2)-C(4) O(1)-C(2)-C(3) C(4)-C(2)-C(3) O(1)-C(2)-C(1) C(4)-C(2)-C(1) C(3)-C(2)-C(1) C(2)-C(3)-H(3A) C(2)-C(3)-H(3B) H(3A)-C(3)-H(3B) C(2)-C(3)-H(3C) H(3A)-C(3)-H(3C) H(3B)-C(3)-H(3C) C(2)-C(4)-H(4A) C(2)-C(4)-H(4B) H(4A)-C(4)-H(4B) C(2)-C(4)-H(4C) H(4A)-C(4)-H(4C) H(4B)-C(4)-H(4C) C(6)-C(5)-C(14) C(6)-C(5)-S(1) C(14)-C(5)-S(1) C(5)-C(6)-C(7) ! 106.50(10) 107.12(11) 96.39(10) 109.5 114.10(15) 108.7 108.7 108.7 108.7 107.6 105.78(19) 110.32(18) 110.95(19) 109.50(17) 112.97(18) 107.34(19) 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 122.1(2) 120.77(16) 117.13(17) 119.7(2) 120! Table 2-13 (cont’d) _____________________________________________________ C(5)-C(6)-H(6) 120.1 C(7)-C(6)-H(6) 120.1 C(8)-C(7)-C(6) 122.5(2) C(8)-C(7)-C(12) 118.8(2) C(6)-C(7)-C(12) 118.7(2) C(9)-C(8)-C(7) 120.8(2) C(9)-C(8)-H(8) 119.6 C(7)-C(8)-H(8) 119.6 C(8)-C(9)-C(10) 120.6(2) C(8)-C(9)-H(9) 119.7 C(10)-C(9)-H(9) 119.7 C(11)-C(10)-C(9) 120.5(2) C(11)-C(10)-H(10) 119.8 C(9)-C(10)-H(10) 119.8 C(10)-C(11)-C(12) 120.5(2) C(10)-C(11)-H(11) 119.8 C(12)-C(11)-H(11) 119.8 C(13)-C(12)-C(11) 121.8(2) C(13)-C(12)-C(7) 119.3(2) C(11)-C(12)-C(7) 118.9(2) C(14)-C(13)-C(12) 121.3(2) C(14)-C(13)-H(13) 119.4 C(12)-C(13)-H(13) 119.4 C(13)-C(14)-C(5) 118.9(2) C(13)-C(14)-H(14) 120.6 C(5)-C(14)-H(14) 120.6 _____________________________________________________________ Symmetry transformations used to generate equivalent atoms: ! 121! Table 2-14. Anisotropic displacement parameters (Å2x 103) for bb64_0m. The anisotropic displacement factor exponent takes the form: -2p2[ h2 a*2U11 + ... + 2 h k a* b* U12 ] _______________________________________________________________________ U11 U22 U33 U23 U13 U12 _______________________________________________________________________ S(1) 26(1) 30(1) 27(1) -4(1) 0(1) 4(1) O(1) 28(1) 24(1) 47(1) -8(1) 9(1) -4(1) O(2) 26(1) 55(1) 37(1) -14(1) 4(1) 6(1) C(1) 27(1) 24(1) 28(1) -2(1) 0(1) 3(1) C(2) 26(1) 28(1) 28(1) -4(1) 3(1) -4(1) C(3) 40(1) 44(1) 32(1) -2(1) -7(1) 0(1) C(4) 30(1) 40(1) 32(1) -10(1) 7(1) -4(1) C(5) 25(1) 28(1) 23(1) 4(1) -2(1) 4(1) C(6) 21(1) 31(1) 25(1) 4(1) 2(1) 2(1) C(7) 22(1) 27(1) 24(1) 3(1) -1(1) 2(1) C(8) 28(1) 31(1) 32(1) 3(1) -4(1) 0(1) C(9) 38(1) 32(1) 38(1) -3(1) -10(1) -1(1) C(10) 42(1) 36(1) 31(1) -10(1) -4(1) 6(1) C(11) 34(1) 35(1) 28(1) 2(1) 5(1) 6(1) C(12) 26(1) 27(1) 24(1) 4(1) -2(1) 4(1) C(13) 22(1) 31(1) 29(1) 5(1) 3(1) 1(1) C(14) 24(1) 29(1) 28(1) 2(1) -1(1) -1(1) _______________________________________________________________________ ! 122! Table 2-15. Hydrogen coordinates (x 104) and isotropic displacement parameters (Å2x 10 3) for bb64_0m. _______________________________________________________________________ x y z U(eq) _______________________________________________________________________ H(1) 1748 5969 7540 50 H(1A) 3704 7900 7911 31 H(1B) 5942 8397 7537 31 H(3A) 2451 7100 6376 58 H(3B) 3968 8359 6588 58 H(3C) 1599 7983 6937 58 H(4A) 7200 5446 7099 51 H(4B) 7561 6772 6708 51 H(4C) 5863 5617 6468 51 H(6) 9452 8906 8786 31 H(8) 10583 10759 9503 36 H(9) 9869 11944 10373 43 H(10) 6458 11549 10944 44 H(11) 3729 9995 10627 39 H(13) 2480 8261 9887 33 H(14) 3267 6960 9049 32 _______________________________________________________________________ ! 123! Table 2-16. Torsion angles [°] for bb64_0m. ________________________________________________________________ O(2)-S(1)-C(1)-C(2) 91.46(17) C(5)-S(1)-C(1)-C(2) -159.05(16) S(1)-C(1)-C(2)-O(1) 65.6(2) S(1)-C(1)-C(2)-C(4) -52.0(2) S(1)-C(1)-C(2)-C(3) -174.65(16) O(2)-S(1)-C(5)-C(6) 7.2(2) C(1)-S(1)-C(5)-C(6) -102.78(19) O(2)-S(1)-C(5)-C(14) -170.76(16) C(1)-S(1)-C(5)-C(14) 79.23(18) C(14)-C(5)-C(6)-C(7) 0.6(3) S(1)-C(5)-C(6)-C(7) -177.27(16) C(5)-C(6)-C(7)-C(8) 179.3(2) C(5)-C(6)-C(7)-C(12) 0.4(3) C(6)-C(7)-C(8)-C(9) -176.7(2) C(12)-C(7)-C(8)-C(9) 2.2(3) C(7)-C(8)-C(9)-C(10) -0.8(4) C(8)-C(9)-C(10)-C(11) -0.9(4) C(9)-C(10)-C(11)-C(12) 1.1(4) C(10)-C(11)-C(12)-C(13) 178.8(2) C(10)-C(11)-C(12)-C(7) 0.3(3) C(8)-C(7)-C(12)-C(13) 179.5(2) C(6)-C(7)-C(12)-C(13) -1.5(3) C(8)-C(7)-C(12)-C(11) -1.9(3) C(6)-C(7)-C(12)-C(11) 177.0(2) C(11)-C(12)-C(13)-C(14) -176.8(2) C(7)-C(12)-C(13)-C(14) 1.7(3) C(12)-C(13)-C(14)-C(5) -0.7(3) C(6)-C(5)-C(14)-C(13) -0.5(3) S(1)-C(5)-C(14)-C(13) 177.48(17) ________________________________________________________________ Symmetry transformations used to generate equivalent atoms: ! 124! Table 2-17. Hydrogen bonds for bb64_0m [Å and °]. _______________________________________________________________________ D-H...A d(D-H) d(H...A) d(D...A) <(DHA) _______________________________________________________________________ O(1)-H(1)...O(2)#1 0.84 1.97 2.797(2) 165.8 _______________________________________________________________________ Symmetry transformations used to generate equivalent atoms: #1 x-1,y,z Space Group P212121 Wavelength 1.54178 Flack x .... 0.02 Flack (su) . 0.02 Bijvoet Pairs 920 Coverage ... 99.4 DiffCalcMax. 240.32 Outlier Crit 480.65 Sigma Crit.. 0.25 Select Pairs 679 Number Plus 560 Number Minus 119 Aver. Ratio 0.955 RC ......... 0.931 Normal Prob. Plot Sample Size. 920 Corr. Coeff. 0.999 Intercept .. -0.071 Slope ...... 0.870 Bayesian Statistics Type ....... Gaussian ! 125! Table 2-17 (cont’d) _____________________________________________________ Select Pairs 920 P2(true).... 1.000 P3(true).... 1.000 P3(rac-twin) 0.0E+00 P3(false) .. 0.0E+00 G .......... 0.9274 G (su) ..... 0.0325 Hooft y .... 0.036 Hooft (su) . 0.016. _______________________________________________________________________ O S OH Table 2-18. Crystal data and structure refinement for bb48_0m. __________________________________________________________________ Identification code bb48_0m Empirical formula C9 H12 O2 S ! 126! Table 2-18 (cont’d) _____________________________________________________ Formula weight 184.25 Temperature 173(2) K Wavelength 1.54178 Å Crystal system Orthorhombic Space group P 21 21 21 Unit cell dimensions a = 5.48730(10) Å a= 90°. b = 8.3275(2) Å b= 90°. g = 90°. Volume c = 20.4046(4) Å 932.40(3) Å3 Z Density (calculated) Absorption coefficient F(000) Crystal size Theta range for data collection 4 1.313 Mg/m3 2.743 mm-1 392 0.18 x 0.08 x 0.08 mm3 4.33 to 67.83°. Index ranges Reflections collected Independent reflections Completeness to theta = 67.83° Absorption correction Max. and min. transmission Refinement method Data / restraints / parameters Goodness-of-fit on F2 Final R indices [I>2sigma(I)] R indices (all data) Absolute structure parameter Largest diff. peak and hole -6<=h<=6, -9<=k<=8, -20<=l<=24 6464 1672 [R(int) = 0.0369] 99.5 % Semi-empirical from equivalents 0.8104 and 0.6380 Full-matrix least-squares on F2 1672 / 0 / 111 1.050 R1 = 0.0306, wR2 = 0.0798 R1 = 0.0322, wR2 = 0.0807 -0.01(2) 0.235 and -0.194 e.Å-3 ! 127! Table 2-19. Atomic coordinates ( x 104) and equivalent isotropic displacement parameters (Å2x 103) for bb48_0m. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. _______________________________________________________________________ x y z U(eq) _______________________________________________________________________ S(1) -215(1) 7942(1) 8713(1) 34(1) O(1) 818(3) 9254(2) 9135(1) 46(1) O(2) -2565(3) 5582(2) 9602(1) 43(1) C(1) 1064(4) 6081(2) 9017(1) 33(1) C(2) 11(4) 5667(2) 9682(1) 36(1) C(3) 1047(5) 4091(3) 9929(1) 46(1) C(4) 1448(3) 8008(2) 7954(1) 28(1) C(5) 3558(4) 8906(3) 7914(1) 35(1) C(6) 4741(4) 9013(3) 7316(1) 40(1) C(7) 3836(4) 8216(3) 6771(1) 40(1) C(8) 1723(4) 7309(3) 6822(1) 38(1) C(9) 509(4) 7213(2) 7418(1) 34(1) _______________________________________________________________________ ! 128! Table 2-20. Bond lengths [Å] and angles [°] for bb48_0m. _____________________________________________________ S(1)-O(1) 1.5018(17) S(1)-C(4) 1.7979(19) S(1)-C(1) 1.811(2) O(2)-C(2) 1.425(3) O(2)-H(2) 0.8400 C(1)-C(2) 1.515(3) C(1)-H(1A) 0.9900 C(1)-H(1B) 0.9900 C(2)-C(3) 1.516(3) C(2)-H(2A) 1.0000 C(3)-H(3A) 0.9800 C(3)-H(3B) 0.9800 C(3)-H(3C) 0.9800 C(4)-C(9) 1.379(3) C(4)-C(5) 1.381(3) C(5)-C(6) 1.384(3) C(5)-H(5) 0.9500 C(6)-C(7) 1.387(3) C(6)-H(6) 0.9500 C(7)-C(8) 1.388(3) C(7)-H(7) 0.9500 C(8)-C(9) 1.387(3) C(8)-H(8) 0.9500 C(9)-H(9) O(1)-S(1)-C(4) O(1)-S(1)-C(1) C(4)-S(1)-C(1) C(2)-O(2)-H(2) C(2)-C(1)-S(1) C(2)-C(1)-H(1A) S(1)-C(1)-H(1A) C(2)-C(1)-H(1B) ! 0.9500 106.24(10) 106.24(9) 97.18(9) 109.5 110.74(15) 109.5 109.5 109.5 129! Table 2-20 (cont’d) _____________________________________________________ S(1)-C(1)-H(1B) 109.5 H(1A)-C(1)-H(1B) 108.1 O(2)-C(2)-C(1) 106.67(16) O(2)-C(2)-C(3) 111.57(18) C(1)-C(2)-C(3) 110.58(19) O(2)-C(2)-H(2A) 109.3 C(1)-C(2)-H(2A) 109.3 C(3)-C(2)-H(2A) 109.3 C(2)-C(3)-H(3A) 109.5 C(2)-C(3)-H(3B) 109.5 H(3A)-C(3)-H(3B) 109.5 C(2)-C(3)-H(3C) 109.5 H(3A)-C(3)-H(3C) 109.5 H(3B)-C(3)-H(3C) 109.5 C(9)-C(4)-C(5) 121.74(19) C(9)-C(4)-S(1) 118.62(15) C(5)-C(4)-S(1) 119.58(15) C(4)-C(5)-C(6) 118.72(19) C(4)-C(5)-H(5) 120.6 C(6)-C(5)-H(5) 120.6 C(5)-C(6)-C(7) 120.5(2) C(5)-C(6)-H(6) 119.7 C(7)-C(6)-H(6) 119.7 C(6)-C(7)-C(8) 119.95(19) C(6)-C(7)-H(7) 120.0 C(8)-C(7)-H(7) 120.0 C(9)-C(8)-C(7) 119.90(19) C(9)-C(8)-H(8) 120.0 C(7)-C(8)-H(8) 120.0 C(4)-C(9)-C(8) 119.18(19) C(4)-C(9)-H(9) 120.4 C(8)-C(9)-H(9) 120.4 _____________________________________________________________ Symmetry transformations used to generate equivalent atoms: ! 130! Table 2-21. Anisotropic displacement parameters (Å2x 103) for bb48_0m. The anisotropic displacement factor exponent takes the form: -2p2[ h2 a*2U11 + ... + 2 h k a* b* U12 ] _______________________________________________________________________ U11 U22 U33 U23 U13 U12 _______________________________________________________________________ S(1) 34(1) 41(1) 26(1) 1(1) 0(1) 9(1) O(1) 72(1) 39(1) 28(1) -3(1) -4(1) 14(1) O(2) 36(1) 63(1) 31(1) 0(1) 5(1) -2(1) C(1) 32(1) 33(1) 34(1) 3(1) 0(1) 0(1) C(2) 40(1) 37(1) 31(1) 1(1) -3(1) -4(1) C(3) 49(1) 42(1) 48(1) 13(1) -7(1) -9(1) C(4) 29(1) 30(1) 26(1) 2(1) -3(1) 4(1) C(5) 33(1) 36(1) 36(1) -4(1) -7(1) 0(1) C(6) 34(1) 40(1) 45(1) 2(1) 3(1) -3(1) C(7) 51(1) 40(1) 30(1) 4(1) 6(1) 6(1) C(8) 52(1) 36(1) 27(1) -2(1) -9(1) 3(1) C(9) 35(1) 34(1) 32(1) 2(1) -6(1) -1(1) _______________________________________________________________________ ! 131! Table 2-22. Hydrogen coordinates ( x 104) and isotropic displacement parameters (Å2x 10 3) for bb48_0m. _______________________________________________________________________ x y z U(eq) _______________________________________________________________________ H(2) -3243 5650 9970 65 H(1A) 706 5203 8704 40 H(1B) 2856 6187 9053 40 H(2A) 415 6541 10000 43 H(3A) 326 3833 10356 70 H(3B) 2819 4187 9975 70 H(3C) 667 3235 9616 70 H(5) 4186 9440 8289 42 H(6) 6184 9637 7279 47 H(7) 4662 8291 6363 49 H(8) 1109 6755 6451 46 H(9) -951 6607 7455 40 _______________________________________________________________________ ! 132! Table 2-23. Torsion angles [°] for bb48_0m. ________________________________________________________________ O(1)-S(1)-C(1)-C(2) 70.27(16) C(4)-S(1)-C(1)-C(2) 179.56(14) S(1)-C(1)-C(2)-O(2) 57.37(19) S(1)-C(1)-C(2)-C(3) 178.86(15) O(1)-S(1)-C(4)-C(9) -164.26(16) C(1)-S(1)-C(4)-C(9) 86.45(18) O(1)-S(1)-C(4)-C(5) 12.82(18) C(1)-S(1)-C(4)-C(5) -96.47(17) C(9)-C(4)-C(5)-C(6) 0.4(3) S(1)-C(4)-C(5)-C(6) -176.54(17) C(4)-C(5)-C(6)-C(7) -0.8(3) C(5)-C(6)-C(7)-C(8) 0.3(3) C(6)-C(7)-C(8)-C(9) 0.6(3) C(5)-C(4)-C(9)-C(8) 0.4(3) S(1)-C(4)-C(9)-C(8) 177.40(15) C(7)-C(8)-C(9)-C(4) -0.9(3) ________________________________________________________________ Symmetry transformations used to generate equivalent atoms: Table 2-24. Hydrogen bonds for bb48_0m [Å and °]. _______________________________________________________________________ D-H...A d(D-H) d(H...A) d(D...A) <(DHA) _______________________________________________________________________ O(2)-H(2)...O(1)#1 0.84 1.90 2.729(2) 169.3 _______________________________________________________________________ Symmetry transformations used to generate equivalent atoms: #1 x-1/2,-y+3/2,-z+2 Space Group P212121 Wavelength 1.54178 Flack x .... -0.01 Flack (su) . 0.02 ! 133! Table 2-24 (cont’d) _____________________________________________________ Bijvoet Pairs 655 Coverage ... 97.3 DiffCalcMax. 159.66 Outlier Crit 319.31 Sigma Crit.. 0.25 Select Pairs 518 Number Plus 423 Number Minus 95 Aver. Ratio 1.019 RC ......... 1.010 Normal Prob. Plot Sample Size. 655 Corr. Coeff. 0.997 Intercept .. 0.204 Slope ...... 1.074 Bayesian Statistics Type ....... Gaussian Select Pairs 655 P2(true).... 1.000 P3(true).... 1.000 P3(rac-twin) 0.0E+00 P3(false) .. 0.0E+00 G .......... 1.0096 G (su) ..... 0.0292 Hooft y .... -0.005 Hooft (su) . 0.015 ! 134! O S OH Table 2-25. Crystal data and structure refinement for BB73_0m. ______________________________________________________________________ Identification code bb73_0m Empirical formula C11 H16 O2 S Formula weight 212.30 Temperature 173(2) K Wavelength 1.54178 Å Crystal system Monoclinic Space group P 21 Unit cell dimensions a = 5.70270(10) Å a= 90°. b = 8.9995(2) Å Volume Z Density (calculated) Absorption coefficient F(000) Crystal size Theta range for data collection Index ranges Reflections collected Independent reflections Completeness to theta = 67.76° ! b= 90.611(2) °. c = 10.7011(2) Å 549.165(19) Å3 g = 90°. 2 1.284 Mg/m3 2.395 mm-1 228 0.28 x 0.15 x 0.05 mm3 4.13 to 67.76°. -6<=h<=6, -10<=k<=10, -12<=l<=12 7577 1796 [R(int) = 0.0583] 95.8 % 135! Table 2-25 (cont’d) ______________________________________________________________________ Absorption correction Semi-empirical from equivalents Max. and min. transmission 0.8957 and 0.5484 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 1796 / 1 / 130 2 Goodness-of-fit on F 1.037 Final R indices [I>2sigma(I)] R1 = 0.0372, wR2 = 0.0904 R indices (all data) R1 = 0.0445, wR2 = 0.0936 Absolute structure parameter 0.03(2) Largest diff. peak and hole 0.196 and -0.167 e.Å-3 ! 136! Table 2-26. Atomic coordinates ( x 104) and equivalent isotropic displacement parameters (Å2x 103) for BB73_0m. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. _______________________________________________________________________ x y z U(eq) _______________________________________________________________________ S(1) 991(1) 7850(1) 5549(1) 27(1) O(1) -1446(3) 7813(4) 5031(2) 50(1) O(2) 4746(4) 8556(2) 3502(2) 33(1) C(1) 2786(5) 6599(3) 4656(3) 27(1) C(2) 3531(5) 7179(4) 3362(3) 27(1) C(3) 5142(5) 5999(4) 2812(3) 32(1) C(4) 1489(5) 7467(3) 2482(3) 32(1) C(5) 926(6) 6662(4) 6917(3) 44(1) C(6) -537(5) 7312(4) 7952(3) 31(1) C(7) -2672(5) 6662(4) 8257(3) 34(1) C(8) -3929(5) 7218(4) 9257(3) 35(1) C(9) -3112(6) 8407(4) 9934(3) 40(1) C(10) -1012(6) 9074(4) 9625(3) 37(1) C(11) 278(5) 8518(4) 8634(3) 35(1) _______________________________________________________________________ ! 137! Table 2-27. Bond lengths [Å] and angles [°] for BB73_0m. _____________________________________________________ S(1)-O(1) 1.491(2) S(1)-C(1) 1.803(3) S(1)-C(5) 1.813(3) O(2)-C(2) 1.427(4) O(2)-H(2) 0.8400 C(1)-C(2) 1.544(4) C(1)-H(1A) 0.9900 C(1)-H(1B) 0.9900 C(2)-C(4) 1.513(4) C(2)-C(3) 1.526(4) C(3)-H(3A) 0.9800 C(3)-H(3B) 0.9800 C(3)-H(3C) 0.9800 C(4)-H(4A) 0.9800 C(4)-H(4B) 0.9800 C(4)-H(4C) 0.9800 C(5)-C(6) 1.511(4) C(5)-H(5A) 0.9900 C(5)-H(5B) 0.9900 C(6)-C(11) 1.386(5) C(6)-C(7) 1.393(4) C(7)-C(8) 1.387(4) C(7)-H(7) 0.9500 C(8)-C(9) C(8)-H(8) C(9)-C(10) C(9)-H(9) C(10)-C(11) C(10)-H(10) C(11)-H(11) O(1)-S(1)-C(1) O(1)-S(1)-C(5) ! 1.371(5) 0.9500 1.383(5) 0.9500 1.390(5) 0.9500 0.9500 108.72(15) 105.06(17) 138! Table 2-25 (cont’d) ______________________________________________________________________ C(1)-S(1)-C(5) 94.36(14) C(2)-O(2)-H(2) 109.5 C(2)-C(1)-S(1) 115.3(2) C(2)-C(1)-H(1A) 108.4 S(1)-C(1)-H(1A) 108.4 C(2)-C(1)-H(1B) 108.4 S(1)-C(1)-H(1B) 108.4 H(1A)-C(1)-H(1B) 107.5 O(2)-C(2)-C(4) 106.6(2) O(2)-C(2)-C(3) 110.6(2) C(4)-C(2)-C(3) 110.0(2) O(2)-C(2)-C(1) 109.8(2) C(4)-C(2)-C(1) 113.5(2) C(3)-C(2)-C(1) 106.4(2) C(2)-C(3)-H(3A) 109.5 C(2)-C(3)-H(3B) 109.5 H(3A)-C(3)-H(3B) 109.5 C(2)-C(3)-H(3C) 109.5 H(3A)-C(3)-H(3C) 109.5 H(3B)-C(3)-H(3C) 109.5 C(2)-C(4)-H(4A) 109.5 C(2)-C(4)-H(4B) 109.5 H(4A)-C(4)-H(4B) 109.5 C(2)-C(4)-H(4C) 109.5 H(4A)-C(4)-H(4C) 109.5 H(4B)-C(4)-H(4C) 109.5 C(6)-C(5)-S(1) 112.3(2) C(6)-C(5)-H(5A) 109.1 S(1)-C(5)-H(5A) 109.1 C(6)-C(5)-H(5B) 109.1 S(1)-C(5)-H(5B) 109.1 H(5A)-C(5)-H(5B) 107.9 C(11)-C(6)-C(7) 119.6(3) C(11)-C(6)-C(5) 120.3(3) ! 139! Table 2-25 (cont’d) ______________________________________________________________________ C(7)-C(6)-C(5) 120.0(3) C(8)-C(7)-C(6) 119.3(3) C(8)-C(7)-H(7) 120.4 C(6)-C(7)-H(7) 120.4 C(9)-C(8)-C(7) 120.9(3) C(9)-C(8)-H(8) 119.5 C(7)-C(8)-H(8) 119.5 C(8)-C(9)-C(10) 120.1(3) C(8)-C(9)-H(9) 119.9 C(10)-C(9)-H(9) 119.9 C(9)-C(10)-C(11) 119.5(3) C(9)-C(10)-H(10) 120.2 C(11)-C(10)-H(10) 120.2 C(6)-C(11)-C(10) 120.5(3) C(6)-C(11)-H(11) 119.8 C(10)-C(11)-H(11) 119.8 _____________________________________________________________ Symmetry transformations used to generate equivalent atoms: ! 140! Table 2-28. Anisotropic displacement parameters (Å2x 103) for BB73_0m. The anisotropic displacement factor exponent takes the form: -2p2[ h2 a*2U11 + ... + 2 h k a* b* U12 ] _______________________________________________________________________ U11 U22 U33 U23 U13 U12 _______________________________________________________________________ S(1) 29(1) 25(1) 27(1) 0(1) 0(1) 2(1) O(1) 32(1) 80(2) 38(1) -10(2) -10(1) 23(1) O(2) 34(1) 29(1) 36(1) 4(1) -3(1) -4(1) C(1) 26(1) 23(2) 33(2) 5(1) 2(1) 5(1) C(2) 27(2) 28(2) 26(2) 0(1) 2(1) 0(1) C(3) 32(2) 34(2) 30(2) 2(1) 4(1) 5(1) C(4) 31(2) 36(2) 30(2) -1(1) -5(1) 2(1) C(5) 49(2) 50(2) 35(2) 12(2) 10(2) 19(2) C(6) 31(2) 34(2) 27(2) 8(1) -2(1) 9(1) C(7) 35(2) 32(2) 35(2) -1(2) -5(1) 0(1) C(8) 27(2) 33(2) 44(2) 6(2) 7(1) 1(1) C(9) 48(2) 39(2) 32(2) 3(2) 10(2) 7(2) C(10) 45(2) 33(2) 34(2) 4(2) -10(2) -1(2) C(11) 25(2) 38(2) 42(2) 12(2) -1(1) -2(1) _______________________________________________________________________ ! 141! Table 2-29. Hydrogen coordinates ( x 104) and isotropic displacement parameters (Å2x 10 3) for BB73_0m. _______________________________________________________________________ x y z U(eq) _______________________________________________________________________ H(2) 5932 8433 3962 50 H(1A) 4218 6370 5151 33 H(1B) 1915 5657 4537 33 H(3A) 5683 6326 1991 48 H(3B) 4280 5062 2723 48 H(3C) 6496 5851 3370 48 H(4A) 502 8257 2823 49 H(4B) 560 6557 2386 49 H(4C) 2082 7774 1665 49 H(5A) 279 5681 6677 53 H(5B) 2548 6508 7229 53 H(7) -3262 5846 7786 41 H(8) -5376 6770 9475 42 H(9) -3991 8774 10618 48 H(10) -454 9906 10086 45 H(11) 1728 8967 8423 42 _______________________________________________________________________ ! 142! Table 2-30. Torsion angles [°] for BB73_0m. ________________________________________________________________ O(1)-S(1)-C(1)-C(2) 76.6(3) C(5)-S(1)-C(1)-C(2) -175.9(2) S(1)-C(1)-C(2)-O(2) 56.9(3) S(1)-C(1)-C(2)-C(4) -62.3(3) S(1)-C(1)-C(2)-C(3) 176.6(2) O(1)-S(1)-C(5)-C(6) -67.9(3) C(1)-S(1)-C(5)-C(6) -178.6(3) S(1)-C(5)-C(6)-C(11) -72.7(4) S(1)-C(5)-C(6)-C(7) 109.9(3) C(11)-C(6)-C(7)-C(8) -1.1(5) C(5)-C(6)-C(7)-C(8) 176.3(3) C(6)-C(7)-C(8)-C(9) 0.8(5) C(7)-C(8)-C(9)-C(10) 0.2(5) C(8)-C(9)-C(10)-C(11) -0.9(5) C(7)-C(6)-C(11)-C(10) 0.5(5) C(5)-C(6)-C(11)-C(10) -177.0(3) C(9)-C(10)-C(11)-C(6) 0.5(5) ________________________________________________________________ Symmetry transformations used to generate equivalent atoms: Table 2-31. Hydrogen bonds for BB73_0m [Å and °]. _______________________________________________________________________ D-H...A d(D-H) d(H...A) d(D...A) <(DHA) _______________________________________________________________________ O(2)-H(2)...O(1)#1 0.84 1.95 2.786(3) 170.9 _______________________________________________________________________ Symmetry transformations used to generate equivalent atoms: #1 x+1,y,z Space Group P21 Wavelength 1.54178 Flack x .... 0.03 Flack (su) . 0.02 ! 143! Table 2-31 (cont’d) ______________________________________________________________________ Bijvoet Pairs 776 Coverage ... 83.7 DiffCalcMax. 152.30 Outlier Crit 304.59 Sigma Crit.. 0.25 Select Pairs 515 Number Plus 425 Number Minus 90 Aver. Ratio 1.105 RC ......... 1.085 Normal Prob. Plot Sample Size. 773 Corr. Coeff. 0.985 Intercept .. 0.181 Slope ...... 1.192 Bayesian Statistics Type ....... Gaussian Select Pairs 773 P2(true).... 1.000 P3(true).... 1.000 P3(rac-twin) 0.0E+00 P3(false) .. 0.0E+00 G .......... 1.0824 G (su) ..... 0.0314 Hooft y .... -0.041 Hooft (su) . ! 0.016 144! O S OH Table 2-32. Crystal data and structure refinement for BB42_0m. ___________________________________________________________________ Identification code bb42_0m Empirical formula C9 H12 O2 S Formula weight 184.25 Temperature 173(2) K Wavelength 1.54178 Å Crystal system Orthorhombic Space group P 21 21 21 Unit cell dimensions a = 5.48550(10) Å a= 90°. b = 8.32920(10) Å Volume Z Density (calculated) Absorption coefficient F(000) Crystal size Theta range for data collection Index ranges ! b= 90°. c = 20.4080(4) Å 932.44(3) Å3 g = 90°. 4 1.312 Mg/m3 2.743 mm-1 392 0.33 x 0.20 x 0.08 mm3 4.33 to 67.86°. -6<=h<=6, -9<=k<=9, -24<=l<=24 145! Table 2-32 (cont’d) ______________________________________________________________________ Reflections collected 6308 Independent reflections 1682 [R(int) = 0.0298] Completeness to theta = 67.86° 99.8 % Absorption correction Semi-empirical from equivalents Max. and min. transmission 0.8104 and 0.4610 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 1682 / 0 / 157 Goodness-of-fit on F2 1.106 Final R indices [I>2sigma(I)] R1 = 0.0263, wR2 = 0.0670 R indices (all data) R1 = 0.0273, wR2 = 0.0676 Absolute structure parameter 0.029(19) Largest diff. peak and hole 0.248 and -0.174 e.Å-3 ! 146! Table 2-33. Atomic coordinates ( x 104) and equivalent isotropic displacement parameters (Å2x 103) for BB42_0m. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. _______________________________________________________________________ x y z U(eq) _______________________________________________________________________ S(1) -216(1) 7058(1) 8713(1) 33(1) O(1) 810(3) 5749(1) 9135(1) 46(1) O(2) -2568(2) 9417(2) 9602(1) 42(1) C(1) 1069(3) 8921(2) 9016(1) 32(1) C(2) 3(3) 9332(2) 9682(1) 35(1) C(3) 1044(4) 10907(3) 9932(1) 47(1) C(4) 1447(3) 6988(2) 7956(1) 28(1) C(5) 510(3) 7785(2) 7416(1) 33(1) C(6) 1731(4) 7697(2) 6822(1) 38(1) C(7) 3836(4) 6785(2) 6772(1) 41(1) C(8) 4743(4) 5992(2) 7316(1) 39(1) C(9) 3555(3) 6092(2) 7915(1) 34(1) _______________________________________________________________________ ! 147! Table 2-34. Bond lengths [Å] and angles [°] for BB42_0m. _____________________________________________________ S(1)-O(1) 1.4991(13) S(1)-C(4) 1.7942(16) S(1)-C(1) 1.8128(18) O(2)-C(2) 1.421(2) O(2)-H(2) 0.88(2) C(1)-C(2) 1.518(2) C(1)-H(1A) 0.96(2) C(1)-H(1B) 0.92(2) C(2)-C(3) 1.519(3) C(2)-H(2A) 0.951(18) C(3)-H(3A) 1.01(2) C(3)-H(3B) 0.92(3) C(3)-H(10) 0.99(3) C(4)-C(9) 1.379(2) C(4)-C(5) 1.386(2) C(5)-C(6) 1.387(2) C(5)-H(5) 0.95(2) C(6)-C(7) 1.386(3) C(6)-H(6) 0.95(2) C(7)-C(8) 1.385(3) C(7)-H(7) 1.01(2) C(8)-C(9) 1.388(3) C(8)-H(8) 0.92(2) C(9)-H(9) O(1)-S(1)-C(4) O(1)-S(1)-C(1) C(4)-S(1)-C(1) C(2)-O(2)-H(2) C(2)-C(1)-S(1) C(2)-C(1)-H(1A) S(1)-C(1)-H(1A) C(2)-C(1)-H(1B) ! 1.03(2) 106.24(8) 106.29(8) 97.14(8) 102.4(17) 110.40(12) 111.5(11) 106.0(12) 110.9(12) 148! Table 2-34 (cont’d) ______________________________________________________________________ S(1)-C(1)-H(1B) 104.9(12) H(1A)-C(1)-H(1B) 113.0(17) O(2)-C(2)-C(1) 106.92(13) O(2)-C(2)-C(3) 111.62(15) C(1)-C(2)-C(3) 110.52(16) O(2)-C(2)-H(2A) 108.6(11) C(1)-C(2)-H(2A) 109.0(10) C(3)-C(2)-H(2A) 110.1(10) C(2)-C(3)-H(3A) 108.8(15) C(2)-C(3)-H(3B) 111.5(14) H(3A)-C(3)-H(3B) 107(2) C(2)-C(3)-H(10) 112.4(14) H(3A)-C(3)-H(10) 104.2(18) H(3B)-C(3)-H(10) 112(2) C(9)-C(4)-C(5) 121.43(16) C(9)-C(4)-S(1) 119.78(12) C(5)-C(4)-S(1) 118.72(13) C(4)-C(5)-C(6) 119.41(16) C(4)-C(5)-H(5) 120.5(11) C(6)-C(5)-H(5) 120.0(11) C(7)-C(6)-C(5) 119.74(15) C(7)-C(6)-H(6) 119.2(12) C(5)-C(6)-H(6) 121.0(12) C(8)-C(7)-C(6) 120.08(16) C(8)-C(7)-H(7) 122.2(12) C(6)-C(7)-H(7) 117.7(12) C(7)-C(8)-C(9) 120.65(17) C(7)-C(8)-H(8) 124.7(13) C(9)-C(8)-H(8) 114.6(13) C(4)-C(9)-C(8) 118.67(16) C(4)-C(9)-H(9) 118.5(11) C(8)-C(9)-H(9) 122.8(11) _____________________________________________________________ Symmetry transformations used to generate equivalent atoms: ! 149! Table 2-35. Anisotropic displacement parameters (Å2x 103) for BB42_0m. The anisotropic displacement factor exponent takes the form: -2p2[ h2 a*2U11 + ... + 2 h k a* b* U12 ] _______________________________________________________________________ U11 U22 U33 U23 U13 U12 _______________________________________________________________________ S(1) 35(1) 40(1) 26(1) -1(1) 0(1) -9(1) O(1) 71(1) 36(1) 29(1) 3(1) -5(1) -14(1) O(2) 37(1) 61(1) 29(1) 0(1) 5(1) 2(1) C(1) 33(1) 31(1) 33(1) -1(1) 0(1) -2(1) C(2) 38(1) 35(1) 31(1) 0(1) -3(1) 5(1) C(3) 50(1) 41(1) 48(1) -11(1) -8(1) 9(1) C(4) 30(1) 27(1) 26(1) 0(1) -3(1) -4(1) C(5) 35(1) 31(1) 32(1) -1(1) -7(1) 1(1) C(6) 52(1) 35(1) 28(1) 3(1) -7(1) -3(1) C(7) 51(1) 40(1) 32(1) -5(1) 6(1) -5(1) C(8) 34(1) 38(1) 45(1) -2(1) 2(1) 2(1) C(9) 33(1) 33(1) 35(1) 3(1) -6(1) -1(1) _______________________________________________________________________ ! 150! Table 2-36. Hydrogen coordinates ( x 104) and isotropic displacement parameters (Å2x 10 3) for BB42_0m. _______________________________________________________________________ x y z U(eq) _______________________________________________________________________ H(2) -3090(50) 9360(30) 10006(12) 52(6) H(1A) 2790(40) 8750(20) 9041(9) 28(5) H(1B) 640(40) 9680(20) 8709(9) 38(5) H(2A) 370(30) 8490(20) 9980(8) 23(4) H(3A) 300(50) 11150(30) 10373(12) 63(7) H(3B) 670(50) 11740(30) 9657(11) 53(6) H(10) 2820(50) 10840(30) 10021(11) 51(6) H(5) -910(40) 8420(20) 7455(9) 28(5) H(6) 1180(40) 8290(20) 6451(10) 36(5) H(7) 4700(40) 6760(30) 6335(11) 53(6) H(8) 6120(50) 5350(30) 7317(10) 46(6) H(9) 4170(40) 5510(20) 8330(9) 39(5) _______________________________________________________________________ ! 151! Table 2-37. Torsion angles [°] for BB42_0m. ________________________________________________________________ O(1)-S(1)-C(1)-C(2) -70.31(14) C(4)-S(1)-C(1)-C(2) -179.59(12) S(1)-C(1)-C(2)-O(2) -57.38(16) S(1)-C(1)-C(2)-C(3) -179.05(13) O(1)-S(1)-C(4)-C(9) -12.90(15) C(1)-S(1)-C(4)-C(9) 96.42(14) O(1)-S(1)-C(4)-C(5) 164.22(13) C(1)-S(1)-C(4)-C(5) -86.46(14) C(9)-C(4)-C(5)-C(6) -0.8(2) S(1)-C(4)-C(5)-C(6) -177.84(12) C(4)-C(5)-C(6)-C(7) 1.5(2) C(5)-C(6)-C(7)-C(8) -1.3(3) C(6)-C(7)-C(8)-C(9) 0.4(3) C(5)-C(4)-C(9)-C(8) -0.2(2) S(1)-C(4)-C(9)-C(8) 176.87(13) C(7)-C(8)-C(9)-C(4) 0.4(3) ________________________________________________________________ Symmetry transformations used to generate equivalent atoms: Table 2-38. Hydrogen bonds for BB42_0m [Å and °]. ____________________________________________________________________________ D-H...A d(D-H) d(H...A) d(D...A) <(DHA) _______________________________________________________________ O(2)-H(2)...O(1)#1 0.88(2) 1.86(2) 2.7311(17) 179(2) ____________________________________________________________________________ Symmetry transformations used to generate equivalent atoms: #1 x-1/2,-y+3/2,-z+2 Space Group P212121 Wavelength 1.54178 Flack x .... 0.029 Flack (su) . 0.019 ! 152! Table 2-38 (cont’d) ______________________________________________________________________ Bijvoet Pairs 662 Coverage ... 98.5 DiffCalcMax. 162.95 Outlier Crit 325.90 Sigma Crit.. 0.25 Select Pairs 551 Number Plus 484 Number Minus 67 Aver. Ratio 0.920 RC ......... 0.892 Normal Prob. Plot Sample Size. 662 Corr. Coeff. 0.999 Intercept .. 0.078 Slope ...... 0.809 Bayesian Statistics Type ....... Gaussian Select Pairs 662 P2(true).... 1.000 P3(true).... 1.000 P3(rac-twin) 0.0E+00 P3(false) .. 0.0E+00 G .......... 0.8925 G (su) ..... 0.0242 Hooft y .... 0.054 Hooft (su). 0.012 ! 153! REFERENCES ! 154! References. 1. (a) Huang, X. F.; Borhan, B.; Rickman, B. H.; Nakanishi, K.; Berova, N., Zinc porphyrin tweezer in host-guest complexation: Determination of absolute configurations of primary monoamines by circular dichroism. Chemistry-a European Journal 2000, 6 (2), 216-224; (b) Huang, X. F.; Nakanishi, K.; Berova, N., Porphyrins and metalloporphyrins: Versatile circular dichroic reporter groups for structural studies. Chirality 2000, 12 (4), 237-255; (c) Huang, X. F.; Rickman, B. H.; Borhan, B.; Berova, N.; Nakanishi, K., Zinc porphyrin tweezer in host-guest complexation: Determination of absolute configurations of diamines, amino acids, and amino alcohols by circular dichroism. Journal of the American Chemical Society 1998, 120 (24), 6185-6186; (d) Huang, X. F.; Borhan, B.; Berova, N.; Nakanishi, K., UV-vis spectral changes in the binding of acyclic diamines with a zinc porphyrin tweezer. Journal of the Indian Chemical Society 1998, 75 (10-12), 725-728. 2. Proni, G.; Pescitelli, G.; Huang, X. F.; Nakanishi, K.; Berova, N., Magnesium tetraarylporphyrin tweezer: A CD-sensitive host for absolute configurational assignments of alpha-chiral carboxylic acids. Journal of the American Chemical Society 2003, 125 (42), 12914-12927. 3. Yang, Q.; Olmsted, C.; Borham, B., Absolute stereochemical determination of chiral carboxylic acids. Organic Letters 2002, 4 (20), 3423-3426. Tanasova, M.; Vasileiou, C.; Olumolade, O. O.; Borhan, B., Enhancement of Exciton Coupled Circular Dichroism with Sterically Encumbered Bis-Porphyrin Tweezers. Chirality 2009, 21 (3), 374-382. 4. 5. Li, X.; Tanasova, M.; Vasileiou, C.; Borhan, B., Fluorinated porphyrin tweezer: A powerful reporter of absolute configuration for erythro and threo diols, amino alcohols, and diamines. Journal of the American Chemical Society 2008, 130 (6), 1885-1893. 6. Li, X.; Borhan, B., Prompt Determination of Absolute Configuration for Epoxy Alcohols via Exciton Chirality Protocol. Journal of the American Chemical Society 2008, 130 (48), 16126-+. 7. Li, X., PhD Thesis. 2009. 8. (a) Oppolzer, W., Asymmetric diels-alder and ene reactions in organic-synthesis. Angewandte Chemie-International Edition in English 1984, 23 (11), 876-889; (b) Fujita, M.; Hiyama, T., Highly diastereocontrolled reduction of ketones by means of hydrosilanes - practical synthesis of optically-active 1,2-diols and 2-amino alcohols of threo or erythro configuration. Journal of the American Chemical Society 1984, 106 (16), 4629-4630; (c) Kingston, D. G. I.; Chaudhary, A. G.; Chordia, M. D.; Gharpure, M.; Gunatilaka, A. A. L.; Higgs, P. I.; Rimoldi, J. M.; Samala, L.; Jagtap, P. G.; Giannakakou, P.; Jiang, Y. Q.; Lin, C. M.; Hamel, E.; Long, B. H.; Fairchild, C. R.; Johnston, K. A., Synthesis and biological evaluation of 2-acyl analogues of paclitaxel ! 155! (Taxol). Journal of Medicinal Chemistry 1998, 41 (19), 3715-3726; (d) Chaudhary, A. G.; Kingston, D. G. I., Synthesis of 10-deacetoxytaxol and 10-deoxytaxotere. Tetrahedron Letters 1993, 34 (31), 4921-4924; (e) Gala, D.; DiBenedetto, D. J.; Clark, J. E.; Murphy, B. L.; Schumacher, D. P.; Steinman, M., Preparations of antifungal Sch 42427/SM 9164: Preparative chromatographic resolution, and total asymmetric synthesis via enzymatic preparation of chiral alpha-hydroxy arylketones. Tetrahedron Letters 1996, 37 (5), 611-614; (f) Dallavalle, S.; Nannei, R.; Merlini, L.; Bava, A.; Nasini, G., A synthetic approach to sporotricale methylether. Synlett 2005, (17), 2676-2678. 9. (a) Hansen, K. B.; Rabbat, P.; Springfield, S. A.; Devine, P. N.; Grabowski, E. J. J.; Reider, P. J., Asymmetric synthesis of cis-aminochromanol. Tetrahedron Letters 2001, 42 (50), 8743-8745; (b) Ghosh, A. K.; Bilcer, G.; Schiltz, G., Syntheses of FDA approved HIV protease inhihitors. Synthesis-Stuttgart 2001, (15), 2203-2229; (c) Davies, I. W.; Taylor, M.; Marcoux, J. F.; Matty, L.; Wu, J.; Hughes, D.; Reider, P. J., Stereoselective hydrogen bromide-promoted hydrogenation of an alpha-hydroxyoxime. Tetrahedron Letters 2000, 41 (42), 8021-8025; (d) Bava, A.; Dallavalle, S.; Fronza, G.; Nasini, G.; de Pava, O. V., Absolute configuration of sporotricale and structure of 6hydroxysporotricale. Journal of Natural Products 2006, 69 (12), 1793-1795. 10. List, B.; Pojarliev, P.; Castello, C., Proline-catalyzed asymmetric aldol reactions between ketones and alpha-unsubstituted aldehydes. Organic Letters 2001, 3 (4), 573-575. 11. Holton, R. A.; Kim, H. B.; Somoza, C.; Liang, F.; Biediger, R. J.; Boatman, P. D.; Shindo, M.; Smith, C. C.; Kim, S. C.; Nadizadeh, H.; Suzuki, Y.; Tao, C. L.; Vu, P.; Tang, S. H.; Zhang, P. S.; Murthi, K. K.; Gentile, L. N.; Liu, J. H., First total synthesis of taxol .2. completion of the c-ring and d-ring. Journal of the American Chemical Society 1994, 116 (4), 1599-1600. 12. Dale, J. A.; Mosher, H. S., Nuclear magnetic-resonance enantiomer reagents configurational correlations via nuclear magnetic-resonance chemical-shifts of diastereomeric mandelate, o-methylmandelate, and alpha-methoxy-alphatrifluoromethylphenylacetate (mtpa) esters. Journal of the American Chemical Society 1973, 95 (2), 512-519. 13. (a) Momiyama, N.; Yamamoto, H., Catalytic enantioselective synthesis of alphaaminooxy and alpha-hydroxy ketone using nitrosobenzene. Journal of the American Chemical Society 2003, 125 (20), 6038-6039; (b) Momiyama, N.; Yamamoto, H., Catalytic enantioselective synthesis of alpha-aminooxy and alpha-hydroxy ketone using nitrosobenzene (vol 105, pg 6038, 2003). Journal of the American Chemical Society 2004, 126 (20), 6498-6498. 14. (a) Tang, Y. Q.; Sattler, I.; Grabley, S.; Feng, X. Z.; Thiericke, R., Streptoketol A and B, new secondary metabolites with DNA-binding properties. Natural Product Letters 2000, 14 (5), 341-348; (b) Yang, C.-H.; Chen, J.-W.; Li, Q.; Peng, K.; Pan, X.-F.; Cui, Y.-X., Determination of absolute configuration of alpha-hydroxy ketones using NMR. Chemical Journal of Chinese Universities-Chinese 2006, 27 (7), 1295-1297. ! 156! 15. Tsuda, Y.; Nunozawa, T.; Nitta, K.; Yamamoto, Y., Utilization of sugars in organicsynthesis .3. stereochemical correlation of dihydroxy-beta-diketone and trihydroxy-betadiketone fungal metabolites, (-)-terredionol and terremutin hydrate, with sugar alcohols the absolute-configuration of (-)-terredionol. Chemical & Pharmaceutical Bulletin 1980, 28 (3), 920-925. 16. Ishii, H.; Chen, Y. H.; Miller, R. A.; Karady, S.; Nakanishi, K.; Berova, N., Chiral recognition of cyclic alpha-hydroxyketones by CD-sensitive zinc tetraphenylporphyrin tweezer. Chirality 2005, 17 (6), 305-315. 17. (a) Evans, D. A.; Dart, M. J.; Duffy, J. L.; Yang, M. G., A stereochemical model for merged 1,2- and 1,3-asymmetric induction in diastereoselective Mukaiyama aldol addition reactions and related processes. Journal of the American Chemical Society 1996, 118 (18), 4322-4343; (b) List, B.; Lerner, R. A.; Barbas, C. F., Proline-catalyzed direct asymmetric aldol reactions. Journal of the American Chemical Society 2000, 122 (10), 2395-2396. 18. (a) Schaus, S. E.; Brandes, B. D.; Larrow, J. F.; Tokunaga, M.; Hansen, K. B.; Gould, A. E.; Furrow, M. E.; Jacobsen, E. N., Highly selective hydrolytic kinetic resolution of terminal epoxides catalyzed by chiral (salen)Co(III) complexes. Practical synthesis of enantioenriched terminal epoxides and 1,2-diols. Journal of the American Chemical Society 2002, 124 (7), 1307-1315; (b) Tokunaga, M.; Larrow, J. F.; Kakiuchi, F.; Jacobsen, E. N., Asymmetric catalysis with water: Efficient kinetic resolution of terminal epoxides by means of catalytic hydrolysis. Science 1997, 277 (5328), 936-938. 19. Delazerda, J.; Barak, G.; Sasson, Y., Selective monoetherification and monoesterification of diols and diacids under phase-transfer conditions. Tetrahedron 1989, 45 (5), 15331536. 20. Xiao, M.; Ye, J.; Zhang, Y.; Huang, Y., Reaction Characteristics of Asymmetric Synthesis of (2S,5S)-2,5-Hexanediol Catalyzed with Baker's Yeast Number 6. Chinese Journal of Chemical Engineering 2009, 17 (3), 493-499. 21. (a) Zhu, Z.; Cardin, C. J.; Gan, Y.; Colquhoun, H. M., Sequence-selective assembly of tweezer molecules on linear templates enables frameshift-reading of sequence information. Nature Chemistry 2010, 2 (8), 653-660; (b) Leblond, J.; Petitjean, A., Molecular Tweezers: Concepts and Applications. Chemphyschem 2011, 12 (6), 10431051; (c) Ema, T.; Misawa, S.; Nemugaki, S.; Sakai, T.; Utaka, M., New optically active diporphyrin having a chiral cyclophane as a spacer. Chemistry Letters 1997, (6), 487488. ! 157! 22. (a) Skowronek, P.; Gawronski, J., A simple circular dichroism method for the determination of the absolute configuration of allylic amines. Tetrahedron Letters 2000, 41 (16), 2975-2977; (b) Kurtan, T.; Nesnas, N.; Li, Y. Q.; Huang, X. F.; Nakanishi, K.; Berova, N., Chiral recognition by CD-sensitive dimeric zinc porphyrin host. 1. Chiroptical protocol for absolute configurational assignments of monoalcohols and primary monoamines. Journal of the American Chemical Society 2001, 123 (25), 59625973. 23. Proni, G.; Pescitelli, G.; Huang, X. F.; Quraishi, N. Q.; Nakanishi, K.; Berova, N., Configurational assignment of alpha-chiral carboxylic acids by complexation to dimeric Zn-porphyrin: host-guest structure, chiral recognition and circular dichroism. Chemical Communications 2002, (15), 1590-1591. 24. Tanasova, M.; Yang, Q.; Olmsted, C. C.; Vasileiou, C.; Li, X.; Anyika, M.; Borhan, B., An Unusual Conformation of alpha-Haloamides Due to Cooperative Binding with Zincated Porphyrins. European Journal of Organic Chemistry 2009, (25), 4242-4253. 25. (a) Pellissier, H., Use of chiral sulfoxides in asymmetric synthesis. Tetrahedron 2006, 62 (24), 5559-5601; (b) Hanquet, G.; Colobert, F.; Lanners, S.; Solladie, G., Recent developments in chiral non-racemic sulfinyl-group chemistry in asymmetric synthesis. Arkivoc 2003, 328-401. 26. (a) Fernandez, I.; Khiar, N., Recent developments in the synthesis and utilization of chiral sulfoxides. Chemical Reviews 2003, 103 (9), 3651-3705; (b) Holland, H. L.; Brown, F. M., Biocatalytic and chemical preparation of all four diastereomers of methionine sulfoxide. Tetrahedron-Asymmetry 1998, 9 (4), 535-538. 27. Solladie, G.; Almario, A.; Dominguez, C., Asymmetric-synthesis of natural-products monitored by chiral sulfoxides. Pure and Applied Chemistry 1994, 66 (10-11), 21592162. 28. Nowaczyk, S.; Alayrac, C.; Reboul, V.; Metzner, P.; Averbuch-Pouchot, M. T., Asymmetric thio-claisen rearrangement induced by an enantiopure alkylsulfinyl group. Unusual preference for a boat transition state in the acyclic series. Journal of Organic Chemistry 2001, 66 (23), 7841-7848. 29. (a) Gurtovenko, A. A.; Anwar, J., Modulating the structure and properties of cell membranes: The molecular mechanism of action of dimethyl sulfoxide. Journal of Physical Chemistry B 2007, 111 (35), 10453-10460; (b) Ternois, J.; Guillen, F.; Piacenza, G.; Rose, S.; Plaquevent, J.-C.; Coquerel, G., Towards a robust and reliable method for the reduction of functionalized sulfoxides. Organic Process Research & Development 2008, 12 (4), 614-617. ! 158! 30. (a) Walker, L.; Walker, M. C.; Parris, C. N.; Masters, J. R. W., Intravesical chemotherapy - combination with dimethyl-sulfoxide does not enhance cyto-toxicity invitro. Urological Research 1988, 16 (4), 329-331; (b) Choi, S.; Sainz, B., Jr.; Corcoran, P.; Uprichard, S.; Jeong, H., Characterization of increased drug metabolism activity in dimethyl sulfoxide (DMSO)-treated Huh7 hepatoma cells. Xenobiotica 2009, 39 (3), 205-217; (c) Yueh, M. F.; Kawahara, M.; Raucy, J., High volume bioassays to assess CYP3A4-mediated drug interactions: Induction and inhibition in a single cell line. Drug Metabolism and Disposition 2005, 33 (1), 38-48. 31. Andersen, K. K., Synthesis of (+)-ethyl para-tolyl sulfoxide from (-)-menthyl (-)-paratoluenesulfinate. Tetrahedron Letters 1962, (3), 93-95. 32. (a) Brunel, J. M.; Diter, P.; Duetsch, M.; Kagan, H. B., Highly enantioselective oxidation of sulfides mediated by a chiral titanium complex. Journal of Organic Chemistry 1995, 60 (24), 8086-8088; (b) Mislow, K.; Green, M. M.; Raban, M., Absolute configurations of sulfoxides by asymmetric oxidation of sulfides. Journal of the American Chemical Society 1965, 87 (12), 2761-&. 33. (a) Aiello, A.; Fattorusso, E.; Imperatore, C.; Luciano, P.; Menna, M.; Vitalone, R., Aplisulfamines, New Sulfoxide-Containing Metabolites from an Aplidium Tunicate: Absolute Stereochemistry at Chiral Sulfur and Carbon Atoms Assigned Through an Original Combination of Spectroscopic and Computational Methods. Marine Drugs 2012, 10 (1), 51-63; (b) Andersen, K. K.; Foley, J. W.; Gaffield, W.; Papaniko.Ne; Perkins, R. I., Optically active sulfoxides . synthesis + rotatory dispersion of some diaryl sulfoxides. Journal of the American Chemical Society 1964, 86 (24), 5637-&; (c) Mislow, K.; Green, M. M.; Laur, P.; Chisholm, D. R., Optical rotatory dispersion and absolute configuration of dialkyl sulfoxides. Journal of the American Chemical Society 1965, 87 (3), 665-&; (d) Drabowicz, J.; Dudzinski, B.; Mikolajczyk, M.; Wang, F.; Dehlavi, A.; Goring, J.; Park, M.; Rizzo, C. J.; Polavarapu, P. L.; Biscarini, P.; Wieczorek, M. W.; Majzner, W. R., Absolute configuration, predominant conformations, and vibrational circular dichroism spectra of enantiomers of n-butyl tert-butyl sulfoxide. Journal of Organic Chemistry 2001, 66 (4), 1122-1129. 34. (a) Mikolajczyk, M.; Midura, W. H.; Grzejszczak, S.; Montanari, F.; Cinquini, M.; Wieczorek, M. W.; Karolakwojciechowska, J., Alpha-phosphoryl sulfoxides .8. stereochemistry of alpha-chlorination of alpha-phosphoryl sulfoxides. Tetrahedron 1994, 50 (27), 8053-8072; (b) Fuller, A. L.; Aitken, R. A.; Ryan, B. M.; Slawin, A. M. Z.; Woollins, J. D., The X-Ray Structures of Sulfoxides. Journal of Chemical Crystallography 2009, 39 (6), 407-415. 35. Hine, R., Crystal structure and molecular configuration of (+)-s-methyl-l-cysteine sulphoxide. Acta Crystallographica 1962, 15 (JUL10), 635-&. 36. (a) Fleischer, E. B.; Green, M.; Mislow, K.; Axelrod, M., Absolute configuration of menthyl arenesulfinates. Journal of the American Chemical Society 1964, 86 (16), 3395&; (b) Hope, H.; Delacamp, U.; Homer, G. D.; Messing, A. W.; Sommer, L. H., ! 159! Stereochemical course of grignard reactions at asymmetric sulfur. Angewandte ChemieInternational Edition 1969, 8 (8), 612-&; (c) Delacamp, U.; Hope, H., Crystal structure and absolute configuration of (+)-methyl para-tolyl sufoxide. Acta Crystallographica Section B-Structural Crystallography and Crystal Chemistry 1970, B 26, 846-&. 37. Yabuuchi, T.; Kusumi, T., NMR spectroscopic determination of the absolute configuration of chiral sulfoxides via N-(methoxyphenylacetyl)sulfoximines. Journal of the American Chemical Society 1999, 121 (45), 10646-10647. 38. (a) Pirkle, W. H.; Beare, S. D., Optically active solvents in nuclear magnetic resonance spectroscopy .9. direct determinations of optical purities and correlations of absolute configurations of alpha-amino acids. Journal of the American Chemical Society 1969, 91 (18), 5150-&; (b) Pirkle, W. H.; Beare, S. D.; Muntz, R. L., Optically active solvents for nuclear magnetic resonance .x. enantiomeric nonequivalence of sulfinamides, sulfinates, sulfites, thiosulfinates, phosphine oxides, and amine oxides. Journal of the American Chemical Society 1969, 91 (16), 4575-&. 39. (a) Buist, P. H.; Marecak, D. M., Stereochemical analysis of a quasisymmetrical dialkyl sulfoxide obtained by a diverted biodehydrogenation reaction. Journal of the American Chemical Society 1991, 113 (15), 5877-5878; (b) Buist, P. H.; Marecak, D. M., Stereochemical analysis of sulfoxides obtained by diverted desaturation. Journal of the American Chemical Society 1992, 114 (13), 5073-5080. 40. Deshmukh, M.; Dunach, E.; Juge, S.; Kagan, H. B., A convenient family of chiral shiftreagents for measurement of enantiomeric excesses of sulfoxides. Tetrahedron Letters 1984, 25 (32), 3467-3470. 41. Calligaris, M.; Carugo, O., Structure and bonding in metal sulfoxide complexes. Coordination Chemistry Reviews 1996, 153, 83-154. 42. (a) Cotton, F. A.; Francis, R., SULFOXIDES AS LIGANDS .1. A preliminary survey of methyl sulfoxide complexes. Journal of the American Chemical Society 1960, 82 (12), 2986-2991; (b) Meek, D. W.; Straub, D. K.; Drago, R. S., Transition metal ion complexes of dimethyl sulfoxide. Journal of the American Chemical Society 1960, 82 (23), 60136016. 43. Drago, R. S.; Meek, D., Infrared spectra of some dimethyl sulfoxide complexes. Journal of Physical Chemistry 1961, 65 (8), 1446-&. ! 160! Chapter 3 Determination of Absolute Stereochemistry for Compounds with One Site of Attachment 3-1 Background 3-1.1 Design of MAPOL As seen in previous chapters, a key challenge facing porphyrin tweezer systems is their use in evaluation of mono coordinating compounds such as carboxylic acids, primary and secondary amines and alcohols. These compounds cannot be directly used in stereodetermination with porphyrin tweezers because porphyrin tweezer systems require the substrates to have two coordination sites in order to form a sandwich structure with the tweezer. Substrates with one binding site can only coordinate to one metalloporphyrin and cannot form the sandwich complex. Therefore, there is no chiral induction into the porphyrin tweezer and no ECCD is observed, hence they cannot be studied in this way. Conventional approaches to circumvent this limitation involve derivatization of the substrates with “carrier” molecules, (usually small molecules containing nitrogen functionalities) 1 in order to provide the requisite second coordination site. This is not always feasible if the substrates are in short supply as in the case of natural products. Furthermore, addition of synthetic steps in order to install the carrier, and then assign the absolute stereochemistry can prove impractical and time consuming. We have been investigating the design of a host system that can be used for the assignment of absolute configuration of compounds with one site of attachment without the need for chemical derivatization in a fast and convenient manner via exciton coupled circular dichroism. Information contained in one molecule can be transmitted to another molecule through intermolecular forces. This is seen in processes such as gene transcription, feedback inhibition 161 and allosteric control. Chiral induction, where a chiral molecule induces chirality in an achiral molecule, is one such process. 1-3 Our group has extensively studied chiral induction in zincated 4 porphyrin tweezers as hosts for chiral guest molecules. In these systems, the chiral information from guest molecules is transmitted to the host tweezer systems upon coordination of the chiral guests to the metallated porphyrin centers; this is ultimately observed as an ECCD signal and enables the assignment of absolute stereochemistry of the bound guests. The information transmission process from the chiral guest to the host comprises of two main processes. First is the complex formation, and second a number of dynamic processes associated with formation of the complex, such as chemical reactions or conformational changes of the interacting molecules. Unlike the porphyrin tweezer systems where the complex formation occurs via metal coordination, complex formation by hydrogen bonding has not been extensively investigated for application in the study of absolute stereochemical assignment using ECCD. With the design of a proper host, it is hypothesized that any molecule capable of hydrogen-bonding could potentially be studied using this system. More importantly, if designed well, mono coordinating compounds could be evaluated without being derivatized. Our foray into the area of host-guest complexation via hydrogen bonding was inspired by ongoing research in this area, specifically, biphenol systems that form hydrogen-bonding complexes with amines. 3b, 5 It is known that 2,2’-biphenyl systems are intrinsically chiral and exist in two conformations, P and M. These two forms differ only by rotation around the central single bond 4 (atropisomers). The term atropisomerism, coined by Kuhn, defines isomerism caused by “freezing” the internal rotation about a single bond in a molecule. 162 Rotation can be hindered by substituents, but this rotation is not impossible and proceeds rapidly 5 at room temperature (Fig 3-1). By convention, a clockwise rotation or right-handed turn of the helix is considered as P helicity (plus) and vice versa. A A A' A A' B' B B' M B' B A' B M counterclockwise: M (minus) A A B' A B' A' B A' P A' B P B' B clockwise: P (plus) Figure 3-1. Nomenclature for assigning atropisomers. Chirality in biaryl compounds. (Priority:A>B). In a racemic mixture, the P and M conformers exist in a 1:1 equilibrating mixture. However, this equilibrium can be disturbed by an external chiral bias, causing one population to be favored over the other as the complex interacts with chiral ligands. 2-3 Binding of these biphenyl systems by hydrogen-bonding to a chiral molecule that possesses an amino or hydroxyl functionality would proceed to favor one helical arrangement over the other. The overpopulation of either P or M helicity can then be detected by CD spectroscopy. One of the initial studies on hydrogen bonding in chiral induction was done by Mizutani and co-workers, who showed that a preferential axial chirality could be induced into flexible biphenols such as 1 upon hydrogen bonding with chiral diamines. 163 3a The steric interactions with chiral trans-1,2-diaminocyclohexane derivatives lead to an excess of one atropisomer which is detectable by CD spectroscopy as Cotton effects. These Cotton effects could be assigned to the biphenol chromophore indicating the presence of axial chirality, i.e. a preferential axial sense of rotation in the biaryl compound, and they were only observable after the addition of the chiral inducer (chiral diamine). R1 OHHO R2 R1 H N R2 + R1 OHHO N H R1 R NH HN R R R R1 R2 R2 H H O O R1 R2 (1R,2R)-2a: R = Np (1R,2R)-2b: R = CH3 (1R,2R)-2c: R = H R2 1a: R1 = Br, R2 = NO2 1b: R1 = H, R2 = NO2 1c: R1 = CH3, R2 = H Figure 3-2. The induction of chirality of various biphenols upon binding to chiral diamines by hydrogen bonding. They further investigated the proton transfer process upon complex formation between biphenols and chiral diamines, 3b and its effect on the chirality transfer and induction process (Figure 3-3). They showed that at room temperature the complex exists in a 1:1 ratio of biphenol to diamine. This is because the degree of proton transfer is small and relatively independent from the amount of amine present. However, at lower temperatures (-80 °C), the complexation of two molecules of excess diamine with biphenol resulted in a higher degree of proton transfer forming a 1:2 biphenol:diamine complex, presumably of type 3a’. Interestingly, both the 1:1 and the 1:2 164 complexes form well-ordered chiral supramolecular structures, accompanied by enhanced chiral induction, as evidenced by the observation of Cotton effects. Br Br NO2 NH HO + OH NH O2N Br O2N H O H O K1 Br NO2 N H HN Br K2 + R'R"NH O2N NO2 H O H O N R' R" N H H HN Br 1a (1R,2R)-2a 3a (at room temp) 3a' (at low temp.) Figure 3-3. Formation of 1:1 complex between diamine 2a and biphenol 1a at room temperature, and ternary complex 3a’ at low temperature with excess amine. Ishii et al. described an elegant potential application of induced atropisomerism for chiroptical probes. 2 (Figure 3-4) They synthesized a 2,2’-biphenyl-bridged bis(free base porphyrin) 4 and employed it as a chirality sensor for chiral amino alcohols. They propose that the host porphyrin system and amino alcohols form a 1:1 complex by hydrogen bonding of the amine and hydroxyl functionalities and the biphenyl unit. Upon stereo differentiation, the chiral information from the stereocenter in the amino alcohols is transferred to the host as a preferential axial twist in the biphenol unit of the host (Figure 3-4). This preferential twist was detected by the appearance of exciton-coupled Cotton effects in the CD curve that were assigned to the -2 intense Soret band of the two porphyrin units. At higher amino alcohol concentrations (10 M), they were able to determine the absolute stereochemistry of the amino alcohols, with R amino alcohols yielding negative ECCD spectra and S amino alcohols yielding positive ECCD spectra. 165 Ph Ph N NH HN N Ph HO NH2 HO O Ph N H N H H N H O H O (R)-phenyl alaninol HO H H M- helicity N H N Ph Ph 4 Figure 3-4. Point to axial chirality transfer from amino alcohols to 2,2’-biphenyl-bridged bis(free base porphyrin) 4 via hydrogen bonding. As mentioned earlier, chirality amplification is of both fundamental and practical importance. It is considered a major factor in the origin of homochirality, triggered from an 6 initial small chiral bias. More importantly however, amplification of chirality is continually 3c, 7 being applied in practical ways from asymmetric synthesis to detection of chiral compounds. One growing application of chirality amplification is in liquid crystalline systems that are widely applied in chiral supramolecular assemblies, smart materials, as well as in the development of liquid crystal displays. 8 Briefly, achiral nematic liquid crystal solvents can become chiral upon doping with small 9 amounts of suitable dopants (usually enantiopure biphenols or biphenyls). Upon formation of a 166 complex between host and dopant, a chiral nematic phase (also known as cholesteric phase) is formed with the transfer of molecular chirality of the dopant to the nematic phase organization. This induced chirality affects the optical as well as structural properties of the liquid crystal superstructure. 9 H2 H N O H O HO supramolecular chirality central chirality axial chirality H H O H O P O O N nematic LC cholesteric LC Figure 3-5. Macroscopic expression of the chirality of an amino alcohol and mono amine by a double amplification mechanism in liquid crystalline media via hydrogen bonding. Eelkema and co-workers reported a modification of this system, enabling the use of racemic biphenols. First, they induce axial chirality to the racemic biphenols using chiral amino alcohols and mono amines. 9a They then use this chiral complex as the dopant to induce supramolecular chirality in the liquid crystals by a double transfer of chirality from the stereogenic centers of amino alcohols or amines (point chirality) via the biphenols (axial 1 chirality) to the liquid crystals (supramolecular chirality). From IR and H-NMR spectroscopic 167 studies, they propose the formation of complexes of the type shown in Figure 3-5, between the amino alcohols or amines with the biphenyl host. In the proposed structure, the aliphatic part of the amino alcohols is situated within the binding pocket of the 3,3’-substituents of the biaryl. The 1:1 stoichiometry was obtained from Job’s plot analysis. 9a, 10 This model became key for us in designing the two proposed systems, MAPOL 9 and its phosphoric acid derivative, MAPHOS 10 (Figure 3-6). We designed these systems as host molecules to exploit hydrogen-bonding interaction, in forming ECCD active supramolecular complexes upon the addition of chiral guest molecules such as amines. We postulated that MAPOL will form complexes with amines via the OH groups. We also envisioned replacing the two hydroxyl groups with a phosphoric acid moiety. Phosphate groups have been widely used in chemistry and bio chemistry, in recognition of sugars, 11 as chiral resolving agents in the NMR- based determination of enantiomeric excess of chiral amines helical polymers. 13 12 as well as induction of chirality in By having the phosphoric acid, MAPHOS would act as a proton donor in hydrogen bonding with chiral substrates. It has been shown that porphyrins are ideal chromophores for ECCD host systems due to their many desirable properties such as high extinction coefficient and far removed red-shifted absorbances that would avoid interference with bound aromatic compounds. 14 Therefore MAPOL and MAPHOS were designed with free base TPP porphyrins at the 2 and 2’ positions then with induction of chirality, these chromophores would interact and be responsible for an observed ECCD. 3c 168 Ph N N H Ph Ph Ph N N H N HN H N N NH N Ph Ph Ph Ph Ph Ph Ph HN H N N NH OHHO Ph Ph 9M Ph Ph Ph H N N N OH HO 9P N N H N Ph Ph N N H N HN O HO P O O NH N Ph Ph Ph 10P H N N Ph Ph N HN O HO P O O NH N Ph 10M Figure 3-6. Proposed host systems for stereochemical determination of chiral molecules: MAPOL 9 and MAPHOS 10. We then expect molecules such as 9 and 10 to act as suitable hosts for detection of chirality by hydrogen bonding using the ECCD protocol. The hydrogen bonding idea is especially ideal, because in theory, any functional group that is capable of hydrogen-bonding could be potentially assigned using this system allowing for ECCD assignment of absolute stereochemistry of small molecules such as monoamines, carboxylic acids, amino acids and alcohols, cyanohydrins, aziridines, sugars and polymers etc. without the need for derivatization. 3-1.2 Synthesis of MAPOL At first glance, the synthesis of MAPOL seemed very straightforward owing to the intrinsic symmetry of the molecule. We envisioned several possible disconnections for the 169 synthesis of MAPOL. There are several methods in the literature for the synthesis of porphyrins. The most commonly used method is McDonald’s condensation procedure, 15 which consists of the acid-catalyzed condensation of pyrrole and aldehyde, followed by oxidation of the resultant porphyrinogen intermediate to give porphyrin (Figure 3-7). R H N H+ + R O R H H R H NH HN NH HN H Oxidation NH N R R R R H HN N R Figure 3-7. The McDonald synthesis of porphyrins. Initially, our main concern was the low yields associated with this route, (usually yields of 7-13% is typical for one condensation). Considering that the synthesis of MAPOL using this protocol would require two such condensation steps, we expected even more dismal yields. With this in mind, we resorted to alternative routes for the synthesis. The possible synthetic strategies we initially formulated for the synthesis of MAPOL are highlighted in Scheme 3-1A to Scheme 3-1D. One important thing to note is that we designed MAPOL to have hydrogen atoms at the meso positions of the porphyrin. However, because of the different reaction conditions encountered during these synthetic routes, the hydrogens were replaced with phenyl rings, to avoid complication in isolation of the unsubstituted porphyrin. It was hypothesized that these phenyl groups would not be directly involved in the stereo differentiation process, so the identity of the meso substituent would not be a major concern for the subsequent ECCD evaluation of small chiral molecules. 170 Suzuki cross coupling Ph NH N BPin CHO N HN N N N Zn Ph Ph Ph Ph N N 12 + Ph N Zn Ph N N OH OH N Br 14 + 13 H N I N H H N N 15 OCH3 OCH3 Ph 11 Suzuki cross coupling 16 OH OH I 17 Scheme 3-1A. Retrosythetic analysis I. The key disconnection being between porphyrins and the biphenyl unit, utilizing a Suzuki cross coupling reaction to construct the target molecule. ! Ph Ph Ph Suzuki cross coupling NH N N HN O NH Ph I O NH N N + Ph Ph Ph N HN N Ph 19 Ph 18 OH OH HN Ph Cu mediated coupling N Ph N H H N Ph 9 N HO NH Ph N Ph N HN Ph 20 Scheme 3-1B. Retrosynthetic analysis II. Disconnection between the biphenyl unit. This is a symmetric disconnection whose forward synthesis could involve Suzuki cross coupling or oxidative coupling reactions. ! 171 BPin MacDonald 2+2 decarboxylation COOH NH N N HN NH N COOH OH N + HN OHC OH OH OH N OHC N N H H N N 22 N H COOH 23 COOH CHO OCH3 + OCH3 21 MacDonald 2+2 decarboxylation EtO2C CHO N H 25 26 Scheme 3-1C. Retrosynthetic analysis III. Disconnection within the porphyrins. The forward synthesis would involve a MacDonald 2+2 condensation reaction. ! Ph N N H Ph CHO Ph N NH Ph Ph N H H O O NH N OH OH OCH3 OCH3 HN Ph CHO 17 26 + CHO 9 H N + 14 15 Scheme 3-1D. Retrosynthetic analysis IV. No key disconnection, instead, the forward synthesis would be a linear synthesis involving condensation of protected biphenyl3,3’ bisaldehyde, benzaldehyde and pyrrole. ! 172 3-1.2-A First generation approach to MAPOL via Suzuki coupling. Scheme 3-1A shows the first generation retrosynthetic plan I. Here, the target molecule 11, can be split into two parts: the porphyrin unit and the biphenol unit. The key step, a Suzuki cross coupling reaction 16 was envisioned to forge the bond between the porphyrins and biphenol 2 2 units. While there are several ways to form the sp C-sp C bond between the two halves, we chose a Suzuki coupling reaction because this reaction has previously been employed in the synthesis of molecules involving porphyrin units. 24 The porphyrin unit could be easily synthesized by a condensation reaction of 14 and 15 followed by bromination to 13, and subsequent conversion to the pinacol borane 12 via standard procedures. The bisiodo-biphenyl 16 could be synthesized in two steps from the corresponding commercially available 2,2’biphenol, following known procedures 25 to obtain the two coupling partners. Cross-coupling reactions have had a tremendous impact in organic synthesis. 17 The 2010 Nobel Prize in Chemistry was awarded to Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki for the discovery and development of palladium catalyzed coupling reactions. The Suzuki coupling 0 reaction was first reported in 1979 by Suzuki and Miyaura as the Pd catalyzed coupling of phenylboronic acid with an aryl bromide to form a series of substituted biaryls. inception, the Suzuki !coupling reaction has enjoyed widespread use 18 Since its throughout organic !synthesis, due to mild reaction conditions and broad substrate compatibility. In fact, new catalyst and method development have broadened the possible applications so that the scope of the reaction partners is not restricted to arenes, but now includes alkyls, alkenyls and alkynyls. In addition, potassium trifluoroborate salts and organoboranes or boronate esters may be used in 173 place of boronic acids. !Furthermore, organotriflates, iodides, and chlorides can be employed in place of the bromide. 16b The Suzuki reaction is an extremely versatile and useful reaction for the assembly of biaryl systems, finding wide application in the areas of natural product synthesis, and drug development within academia and the pharmaceutical industry. Figure 3-8 shows an example of two !natural products, Korupensamine A and Hippadine, synthesized using a Suzuki 2 2 reaction as the key !step in the formation of the sp C-sp C bonds indicated. 19 In the synthesis of hippadine for example, the Suzuki coupling works, when other coupling (Kumada and Negishi) reactions failed to provide the desired product. 20 OH OMe HO ------ ------ N O NH O O OH Korupensamine A Hippadine Figure 3-8. Natural products Korupensamine A and Hippadine synthesized by employing Suzuki coupling reaction as the key step. Suzuki cross-coupling reactions have been widely applied in reactions with porphyrin units, typically proceeding in high yields. 21 One interesting example of Suzuki cross coupling with a porphyrin as one coupling partner was reported by Chng et al. in their synthesis of di- 174 trimesitylporphyrin xanthene (DTMPX) 30 and di-trimesitylporphyrin dibenzofuran (DTMPD) 21e 32 (Scheme 3-2). Mes Mes = N N Zn Mes Br t-Bu t-Bu N N O Br Mes 27 pinacol borane, Pd(PPh3)2Cl2, 1,2-dichloroethane Et3N, 90 °C 29 t-Bu Br N N N Zn N N O Mes N N Zn N N Mes Br O Br HN Mes Mes 40 N Mes O Mes NH Mes 30 di-trimesitylporphyrin xanthene (DTMPX) O Mes 28 N Mes B N N Mes Mes Mes O 21% t-Bu N Zn N N Zn N N Pd(PPh3)2, Ba(OH)2•8H2O 1,2-DME/water, 95 °C Mes Mes Mes 31 Pd(PPh3)2, K3PO4, DMF,100 °C 77% Mes N N N Zn N Mes Mes 32 di-trimesitylporphyrin dibenzofuran (DTMPD) Scheme 3-2. Synthesis of DTMPX and DTMPD, employing Suzuki cross coupling reaction. In their work, they employ the meso-triarylporphyrin 5,10,15-trimesitylporphyrin as the key synthon for the cross coupling approach. By regioselectively brominating 40 with Nbromosuccinimide in chloroform at room temperature followed by zinc insertion with 175 Zn(OAc)2•H2O, they are able to quantitatively obtain the zinc(II)bromoporphyrin 27, which is smoothly converted to boronate 28 in excellent yield by using pinacol borane. This boronate porphyrin then serves as a transmetalating agent for the preparation of DTMPX 30 and DTMPD 32. Another elegant use of Suzuki cross coupling reaction with a porphyrin was demonstrated by Hyslop et al. 21f who employ the boronate porphyrin 12 in the synthesis of several porphyrin- containing supramolecular assemblies 34, 35 and 36 (Scheme 3-3). Ph Ph N Pd(PPh3)4 (0.003 mmol) Ba(OH)2•8H2O (0.24 mmol) N Zn Ph N Ph N Pd(PPh3)4 (0.003 mmol) Ba(OH)2•8H2O (0.24 mmol) B OMe Ph N N Zn N N Ph N H 34 MeO N Ph O Zn N Ph 1-(2',5'-dimethoxyphenyl)8-iodoaphthalene (0.08 mmol) O N Zn N 3,6-dibromocarbazole (0.05 mmol), 80 °C N N N N 35 Ph 12 Pd(PPh3)4 (0.003 mmol) Ba(OH)2•8H2O (0.24 mmol) N N-(tert-butoxycarbonyl)-4-iodoL-phenylalanine (0.08 mmol) N N Zn O N HN Ph OH O O 36 Scheme 3-3. Synthesis of several poprhyrin-containing molecules by Suzuki coupling reaction. Encouraged by the successful use of the Suzuki cross coupling reaction in the synthesis of porphyrin systems, we set out to begin the synthesis of MAPOL 9, by Suzuki reaction. 176 A K2CO3 (2.5 eq.) CH3I (2.5 eq.) acetone, reflux, 16 h, 73% OH OH OCH3 OCH3 1. nBuLi (3 eq.) TMEDA (3 eq.) ether, rt. I OCH3 OCH3 2. I2 (3.6 eq.) -78 °C - rt. 71% I 17 37 16 --------------------------------------------------------------------------------------------------------------------------------------------------B O CHO H H N HN (1 eq.) 14 TFA, CH2Cl2 Ph N Ph CH2Cl2, TFA, 3 h, rt. H 55 °C, 2.5 h NH HN N NH then p-chloranil, 30 min 15 41% 38 10% (xs) 39 Ph Br 1. NBS(1 eq.) CH2Cl2, rt.,1 h, 71% 2. Zn(OAc)2 (excess) CH2Cl2, quant. N N Zn Ph N N HBPin (1 eq.) TEA (1 eq.) dichloroethane, Ph (PPh3)2PdCl2 (10 mol%) 45 min. quant. 43 N N Zn N N O B O Ph 12 Scheme 3-4. A) Synthesis of bis-iodo biphenyl 16. B) Synthesis of pinacol borane porphyrin 12 for use in Suzuki coupling. !As suggested by retrosynthetic analysis Scheme I, the forward synthesis of MAPOL can be divided into two parts: the porphyrin synthesis and the synthesis of the di-iodo biphenyl 16. Synthesis of porphyrin begun with the synthesis of dipyrrylmethane 3822 which was then used in a trifluoroacetic acid catalyzed condensation with benzaldehyde, followed by oxidation with pchloranil to afford 5,15-phenyl-porphyrin 39 in good yield. Treatment of 39 with NBS provided brominated porphyrin 13. Metallation with an excess of Zn(OAc)2 gave quantitative yield of the zincated porphyrin 43, which was then borylated to give pinacol borane porphyrin 12, for use in the Suzuki coupling. 21f Synthesis of the bis-iodo biphenyl 16, begun with bis methylation of 177 2,2’-biphenol followed by ortho-lithiation and subsequent iodination of methylated biphenol 37 in good yields (Scheme 3-4). 23 Reported procedures were followed in the synthesis of both 12 and 16. With the two coupling partners in hand, we set out to carry out the Suzuki coupling reaction. First, Ogoshi’s conditions (Scheme 3-2) utilizing 0.5 mol % of palladium catalyst were employed (Table 3-2 entry 1). The reaction was monitored by mass spectroscopy. The reaction proceeded with very low conversion and yields, and unfortunately, the only products observed were mono-coupled biphenyl with either de-halogenation A, and/or retention of the second iodine B. (The majority of the mass was decomposed starting material). Increasing the catalyst loading to 1 mol % (entry 2) or 10 mol % (entry 3) had no effect on the reaction. We then modified the reaction conditions slightly by changing the base to potassium phosphate and the solvent to DMF. These are the conditions used by Chng et al. that gave them higher yield in their synthesis (Scheme 3-2). 21e However, this resulted in only the de-halogenated product A, still in low yields. Changing the catalyst to the more air-stable Pd(dppf)Cl2 under the original reaction conditions, (entry 4), only resulted in reversing the selectivity in preference to mono-coupled product, without de-iodination B. Again, increasing the catalyst loading to 20 mol % had no effect on the reaction outcome. Attempts to re-submit mono-coupled product B to a second coupling reaction resulted in decomposition of starting material. 178 Table 3-2. Conditions tried for Suzuki cross coupling reaction. I Ph I N N Zn B N N O base (x equiv) Pd cat. (x equiv) OCH3 OCH3 + O 110 ºC, 24 h N2 atmosphere N I Ph 12 OCH3 OCH3 16 OCH3 OCH3 + Ph N H H N N N Ph Conditions Yield % A Yield % B 3 4 3 3 3 3 0 7 4 0 0 Pd(PPh3)4, 0.5 mol% 7 DME/H2O 10:1 Ba(OH)2•8H2O (6 eq.) 2 Pd(PPh3)4, 1 mol% DME/H2O 10:1 Ba(OH)2•8H2O (6 eq.) 3 Pd(PPh3)4, 10 mol% DME/H2O 10:1 Ba(OH) 2•8H2O (6 eq.) 4 Pd(dppf)Cl2,10 mol% DME/H2O 10:1 K2PO4 (12 eq.) 5 Pd(PPh3)4 (0.3 eq.) DMF Ba(OH)2•8H2O 6 Pd(dppf)Cl2 (20 mol%) DME/H2O 10:1 Ba(OH)2•8H2O 7 B + Pd(dppf)Cl2 (10 mol%) decomposition products DME/H2O 10:1 179 N B Ba(OH)2•8H2O (6 eq.) 1 H N Ph A Entry Ph N H All these reagents were de-gassed prior to running reaction in a Schlenk flask. We tried changing other factors like reaction times and concentration, but none of these were successful, leading to decomposition of the starting materials. Table 3-2, only shows the more successful conditions that led to formation of coupling products A and/or B. Due to the low yields of 38 and 39, as well as no observation of desired MAPOL under these reaction conditions, we considered the other synthetic routes. 3-1.2-B Second generation approach to MAPOL via Suzuki and oxidative coupling. Following our second-generation retrosynthetic plan II described in Scheme 3-1B, we thought to assemble MAPOL in a convergent manner. There were two different approaches we could take for the synthesis. Taking advantage of the symmetry of the molecule, the main disconnection in this retrosynthetic approach cuts the biaryl motif in half. An oxidative selfcoupling or a Suzuki coupling reaction could then be used to forge the bond between the two phenyl rings. Oxidative coupling unit 20 could be easily accessed from condensation of commercially available starting materials 14, 15 and 33, while coupling partners 19 and 18, for use in a Suzuki coupling reaction could be obtained from conversion of 41. In a forward sense, the two halves of MAPOL are pieced together by a C-C bond formation ortho to the hydroxyl group. In the first approach (Route A) the methoxy group in 41 could be used as a handle to install the iodine, providing 18, which could then be converted to borane 19. We could then use 18 and 19 in a Suzuki coupling reaction to forge the final bond. The second approach (Route B) calls for an oxidative self-coupling reaction of 20. This route is more desirable because, provided that this coupling reaction works as expected, it would enable the synthesis of MAPOL in one step from the easy to prepare precursor 20. The most desirable aspect of this second generation 180 approach is that both routes incorporate the porphyrin into the phenyl right at the beginning making them convergent routes. Suzuki coupling approach: Route A. Ph A CHO + 14 15 1. TFA, CH2Cl2 rt. 1.5h 2. p-chloranil rt. 2 h, 7% OCH3 + N N CHO H N 33 Zn Ph N Ph N OCH3 41 B Ph Ph N N N HN N Zn Ph Ph N Ph Ph NH N Ph Ph NH N O OCH3 41 I HN Ph N O 18 BPin 19 Scheme 3-5. A. Synthesis of 41. B. Planned elaboration of 41 to 18 and 19. We planned to use a Suzuki coupling reaction between the two fully elaborated halves 18 and 19. These two halves could be prepared from 41. Porphyrin 41 was accessed in 7% yield by a TFA catalyzed condensation of 14, 15 and 33, followed by the in situ oxidation by p-chloranil. Elaboration of 41 is required to obtain the desired Suzuki coupling partners 18 and 19. However, all attempts to convert 41 into iodo porphyrin 18, which would then be further converted to 19 were unsuccessful. The only isolated product was the de-metallated starting material 42. Despite varying the reaction conditions, 42 was the only isolated product. (Table 3-3) 181 Table 3-3. Attempts to synthesize 18. Ph N Ph N Zn Ph N Ph 1.TMEDA (x equiv.) nBuLi. (x equiv.) 25 ºC, 2.5 h 2. I2 (x equiv.) N NH N Ph Ph N -78 ºC to rt. HN OCH3 OCH3 41 entry 1 2 3 4 5 18 I yield (%)a conditions TMEDA (1.5 equiv.) n-BuLi (1.5 equiv.) I2 (1.5 equiv.) de-metallation of S.M. TMEDA (2.0 equiv.) n-BuLi (2.0 equiv.) I2 (2.0 equiv.) de-metallation of S.M. TMEDA (3.0 equiv.) n-BuLi (3.0 equiv.) I2 (3.0 equiv.) de-metallation of S.M. TMEDA (4.0 equiv.) n-BuLi (4.0 equiv.) I2 (4.0 equiv.) de-metallation of S.M. TMEDA (5.0 equiv.) n-BuLi (5.0 equiv.) I2 (5.0 equiv.) de-metallation of S.M. a product determined by high resolution mass spectroscopy Ph NH N Ph Ph N HN OCH3 42 182 After considering the previous disappointing results with the Suzuki coupling reaction, and issues with the iodination step, we changed our approach to use oxidative coupling reaction. Oxidative coupling approach: Route B Oxidative coupling of 2-naphthols is a well-established method for the synthesis of binaphthols. 24 These reactions are usually carried out by treatment of naphthols with a transition metal catalyst in the presence of an oxidant such as oxygen or excess of another transition metal. Frequently employed transition metal catalysts are Fe(III) and Cu(III); although use of Mn(III), Ti(IV), V(V), and Ru(III) have also been reported. 25 CuCl2, (10 mol %), (-)sparteine 20mol % A OH AgCl (1.1 equiv.) MeOH, 20 °C, 72 h, 70 % yield, 3 % ee OH OH (R)-(+)-BINOL CuCl(OH)•TMEDA 1 mol % B OH O2, CH2Cl2, 0 °C, 8.5h 90 % OH OH Figure 3-9. Oxidative coupling of 2-naphthol in the synthesis of BINOL. A) CuCl2/(-)sparteine catalyzed system; B) CuCl(OH)•TMEDA complex as catalyst. This type of chemistry has not been previously employed in the synthesis of porphyrin systems. Additionally, there exists the possibility of formation of multiple coupling products (ortho-ortho coupling, para-para coupling, ortho-para coupling, ortho-ether linkage and paraether linkage). However, from a strategic sense, we wished to employ oxidative coupling reaction in the synthesis of MAPOL mainly because it would allow faster access to MAPOL 183 from phenol 20. First, we synthesized BINOL using this chemistry in order to familiarize ourselves with the chemistry. Upon successful synthesis of BINOL, we then set out to employ these reaction conditions for the synthesis of MAPOL. For all oxidative coupling reactions attempted, both the free base porphyrin 20, as well as zincated porphyrin 43 were used. Porphyrin 20 was obtained by BBr3 de-methylation of 41, (which simultaneously de-metallates it) and subsequent zincation of 20 using an excess of Zn(OAc)2 provided 43 (Scheme 3-6). Ph Ph N N Zn Ph N N BBr3, CH2Cl2 rt. 16 h, Ph Ph 89% Ph Zn(OAc)2 (excess) CH2Cl2, 16 h, Ph Ph quant. HN N NH N OCH3 N N Zn N OH 41 Ph N OH 20 43 Scheme 3-6. Synthesis of porphyrin 43. The first conditions tried utilized CuCl with BnNH2 amine ligand 50 (Table 3-4 entry 1). The reaction was carried out open to air, at room temperature. From mass spectroscopy, the reaction resulted in a single product, which turned out to be the demetallated starting material, 20. Subsequently, the catalyst was changed to CuCl(OH)•TMEDA complex, using O2 as the 26 oxidant. CuCl(OH)•TMEDA complex was synthesized as shown in scheme 3-7. CuCl + TMEDA O2 1h., rt. CuCl(OH)•TMEDA Scheme 3-7. Synthesis of catalyst. 184 The catalyst loading was gradually increased from 1 mol % to 20 mol %. However, these reactions also resulted in de-metallation of the starting porphyrin when 43 was used. When free porphyrin 20 was used, only unchanged starting material was isolated. In a final effort using CuCl(OH)•TMEDA complex, a stoichiometric amount of catalyst was used. Unfortunately, this did not have a positive effect on the reaction either, providing the same results as before (entries 2, 3, 4 and 5). Next we decided to change the copper salt as well as the amine ligand (entry 6). 27 The reaction of 20 as well as 43, in the presence of 10 mol % of this catalyst gave some promising results, yielding trace amounts of desired product (detected by mass spectroscopy) after heating at 40 °C in dichloroethane for 9 days. The majority of the mass was recovered starting material 20 and so we decided to push the reaction. Attempts to optimize the reaction conditions, like longer reaction times (longer than 9 days), (entry10) using oxygen balloon as opposed to running the reaction open to air, increasing the catalyst loading or increasing temperature led to decomposition of starting material and no desired product. Using freshly prepared CuI and CuCl were also unsuccessful. 185 Table 3-4. Oxidative coupling conditions for the synthesis of MAPOL. Ph Ph OH N catalyst (x mol %) N Ph M Ph Zn M= 2H, 9 M= Zn, 58 CuCl(OH)•TMEDA (10 mol %), CH2Cl2 " CuCl(OH)•TMEDA (20 mol %), CH2Cl2 " CuI (10 mol %) L*( 10 mol %) DCE CuI (10 mol %) L* (10 mol %) air 24 h 20 °C rec. S.M. quant. " " rec. 20 quant. O2 8.5 h 0 °C rec. S.M. quant. " " rec. 20 quant. O2 20 h rt. rec. S.M. quant. " " rec. 20 quant. O2 20 h rt. rec. S.M. quant. " " rec. 20 quant. O2 20 h rt. " " O2 9d 40 °C O2 " yield O2 CuCl(OH)•TMEDA (1 mol %), CH2Cl2 temp O2 " time O2 CuCl(OH)•TMEDA (1 mol %), CH2Cl2 Zn Zn Ph O2 " Zn 2H N N " CuCl2, (1 equiv.) BnNH2 (2 equiv.) MeOH Zn 2H N M oxidant conditions Zn 2H N M= 2H, 20 M= Zn, 43 Zn 2H Ph N N OH HO Ph 2H oxidant (x mol %) M Ph Ph N N N M N >9d 40 °C DCE + N CHO H2N CH3 sodium acetoxy borohydride r.t. 16 h, 92 % H N N L* 186 CH3 rec. S.M. quant. rec. 20 quant. trace decomp. In one final attempt to employ oxidative coupling reaction for the synthesis of MAPOL, we were inspired by an elegant modification reported by Wulff and co-workers (Figure 3-10). In the final step of the synthesis of VANOL and VAPOL, they employ an oxidative phenol coupling of the respective monomers in two ways: first in the small scale synthesis, by dimerization of the starting monomers at 190 °C using air as the oxidant, and the large scale synthesis, by heating the monomers in mineral oil, with the introduction of oxygen by an air 28 flow. OH Ph 190 °C, air, neat 61a: 3-phenyl-1-naphthol VANOL monomer Ph Ph OH OH 62b: (VAPOL) 80-89 % on 0.98 g scale of 61b 61b: 2-phenyl-4-phenanthrol VAPOL monomer OH air flow temp, time mineral oil 62a: (VANOL) 87 % on 0.98 g scale of 61a Ph Ph OH OH Ph 61a: 3-phenyl-1-naphthol VANOL monomer 61b: 2-phenyl-4-phenanthrol VAPOL monomer 62a: VANOL 62b: VAPOL Figure 3-10. Phenolic coupling in the preparation of racemic VANOL and VAPOL. In our case however, this chemistry failed to work, resulting in black tar-like decomposition material. 187 3-1.2-C Third generation approach to MAPOL via the 2+2 synthesis. At this point, it seemed as if our system was not well suited for coupling reactions. So we turned our focus to traditional approaches of porphyrin synthesis, involving the acid-catalyzed condensation of 4 equivalents of pyrrole and 4 equivalents of aldehyde followed by oxidation to provide the porphyrin. We originally had some reservations about this route because of the low yields traditionally obtained. With this in mind, we looked into the MacDonald 2+2 approach for the synthesis of porphyrins. 15 The backbone of Fischer’s classical porphyrin syntheses was the 2+2 synthesis that called for the use of 1-bromo-9-methyldipyrromethenes such as 46 and 47 as intermediates. This coupling reaction involves condensation of the dipyrromethenes in boiling formic acid or in organic melts (succinic, tartaric, etc.) usually requiring temperatures of 200 °C or higher (Scheme 3-8). Not only are these harsh conditions, but also the choice of organic acid to be used is determined by trial and error, based on the temperature required to provide the best yield of porphyrin. CH3 R R R H3C NH HN Br R H3C CH3 NH Δ acid N CH3 N HN CH3 R 46:R = CH2CH3 47: R = CH2CH2CO2H H3C R 48: R = CH2CH3 49: R = CH2CH2CO2H Scheme 3-8. Fischer’s synthesis of porphyrins. Fischer’s most famous example of porphyrin synthesis using this approach is his synthesis of deuteroporphyrin 53, an intermediate that was used in the total synthesis of 188 hemin 54 by Munich (Scheme 3-9), which was pivotal to Fischer’s 1930 Nobel prize award. H CH3 H H3C NH HN R H H3C 50:R = CH3 51: R = CH2Br NH H3C N N Cl N Δ Fe + Br N N HN N Br CH3 CH3 H R CH3 H3C CH3 H3C NH HN H3C CH3 HO2C 52 53 CO2H CO2H HO2C CO2H HO2C 54 Scheme 3-9. Synthesis of hemin 54 via deuteroporphyrin 53. These unfavorably harsh reaction conditions led to development of modifications to Fischer’s synthesis. The MacDonald 2+2 synthesis was one pivotal event in porphyrin 15, 29 synthesis. It involves the condensation of 5,5’-diformylpyrromethanes with 5,5’-dihydro dipyrromethanes under mild acidic catalysis. The intermediate porphodimethene is rapidly oxidized to the porphyrin in air (Scheme 3-10). R2 R1 R3 R4 NH HN OHC 55 + H R1 CHO 1. H+ 2. [O] H R5 R6 56 R4 NH N N HN R5 R5 NH HN R5 R3 R2 R6 R6 57 R6 Scheme 3-10. MacDonald’s 2+2 porphyrin synthesis. 189 R N N Zn N N A B N N Zn N N R OR OR N N N Zn N Zn N N N N 61 C 62 R = n-C6H13 tBu R R tBu tBu tBu N Zn N N N MeO2C tBu N N Zn N N MeO2C tBu tBu tBu tBu tBu tBu tBu 63 Figure 3-11. Dimeric porphyrins synthesized by Ogoshi. This method has since been successfully used in the synthesis of a wide range of meso29-30 substituted trans-porphyrins. Several modifications to this method have been developed, 31 one of which is by Ogoshi and coworkers, where instead of the 5,5’-dihydro dipyrromethane, a 5,5’-bis(hydroxybenzyl)dipyrromethane was coupled with a 5,5’-dihydro dipyrromethane to give unsymmetric 5,15-diphenylporphyrins. 32 Furthermore, these condensation conditions have been 190 33 employed in the synthesis of BINOL/biphenyl-porphyrin systems. Porphyrin 61 shown in Figure 3-11 A is an example of one such system containing BINOL as a chiral spacer. Porphyrin systems 62 and 63 shown in B and C are linked by a biphenyl spacer. The synthesis of 62 outlined in Scheme 3-12, involves the condensation of a bis-aldehyde with pyrrole-2-ester, followed by saponification to provide the tetra-acid bis dipyrromethane. Then a MacDonald type condensation with 5,5’-diformyldi-pyrromethane provides 62 in 88% yield. CHO HOOC 1) EtO2C CHO N H HCl/EtOH 90 % 2) NaOH/EtOH HOOC H N N COOH NH 64 65 R R' R' OHC COOH N N H R R CHO N H (R = n-C H ) 6 13 TsOH, DDQ/CH2Cl2-MeOH 5.5% then Zn(OAc)2/CHCl3 88% R N N Zn N N N N Zn N N 62 (R = n-C6H13) Scheme 3-12. Ogoshi’s route to porphyrin B. ! Our attempts to synthesize MAPOL via this route begun with the synthesis of diformyldipyrromethane 59 by a Vilsmeier-Haack reaction from dipyrrylmethane 38. This was then employed in a condensation reaction with tetra-acid 60, obtained by the condensation of bisaldehyde 59 with methyl 1H-pyrrole-2-carboxylate, followed by saponification of the tetra-ester 191 obtained to provide the tetra-acid bis dipyrromethane 60. Unfortunately, attempts to carry out the condensation with diformyldi-pyrromethane 59 were unsuccessful (Scheme 3-13). We hypothesized that this could be because of the free rotation of the diformyldi-pyrromethane bond, causing it to not adopt the required conformation for reaction. In the case of Ogoshi’s synthesis, their diformyldi-pyrromethane has long alkyl chain substituents on the pyrrole, hindering this rotation. N H N H 38 POCl3, DMF 0 °C 30 min then reflux in CH2Cl2 2 h 33% OHC N H 59 N H CHO COOH NH N CHO OCH3 OCH3 26 CHO 1) MeO2C N H HCl/MeOH 79% 2) NaOH/MeOH reflux, 16 h COOH 59,TsOH, DDQ CH2Cl2-MeOH OCH3 OCH3 N 60 N H COOH 16 h, rt. then DDQ 2 d. COOH Scheme 3-13. Attempts to synthesize MAPOL using Ogoshi’s method. 192 decomposition products 3-1.2-D Fourth generation approach to MAPOL via Lindsey type condensation. MeO2C tBu CHO tBu tBu tBu MeO2C + tBu CHO tBu 1) Zn(OAc)2, propionic acid MeO2C N Zn N N N 110 °C 100 min tBu 2) DDQ CHCl3 air rt. 2 h. CHO MeO2C + tBu N H N N Zn N N tBu tBu tBu tBu tBu tBu 63 Scheme 3-14. Ogoshi’s synthesis of 63. An extension of the 2+2 coupling was reported by Ogoshi and co-workers who prepared 5,15-diaryl-β-octaalkylporphyrins by the 33 dipyrromethanes and aromatic aldehydes. co-condensation of β-alkyl-5,5’-unsubstituted They carried out the reactions in refluxing propionic acid in the presence of zinc acetate, and were able to synthesize several porphyrins (obtained as the zinc complex) with substituted phenyl groups in the meso positions in 15-25% yield (Scheme 3-14). They obtained even higher yields (30-40%) of porphyrins by running the reactions in benzene with catalytic quantities of trifluoroacetic acid, after air oxidation. Scheme 3-15 shows the final successful synthesis of MAPOL. We achieved this following Ogoshi’s procedure for the synthesis of 62 and 63. MAPOL was synthesized in three steps from readily accessible starting materials. Ortho-lithiation and subsequent formylation of 37 yields bisaldehyde 26 in good yields. Condensation of bisaldehyde 26, pyrrole and 193 benzaldehyde in propionic acid catalyzed by Zn(OAc)2 followed by DDQ oxidation yields methylated Zn-MAPOL in 17% yield. Upon de-methylation and demetallation with BBr3 we obtained desired MAPOL 9 in 80% yield. Along with MAPOL 9, all of these synthetic routes also provides Zn-MAPOL 58, methylated Zn-MAPOL 44, and the monomer 41 (Figure 3-12) all of which turned out to be useful for investigating the nature of the complex formed between MAPOL 9 and substrates. O O 1. nBuLi, TMEDA, ether -78 °C - rt 2.5 h 2. DMF, 80 °C 57% 37 CHO 14 CHO H N OCH3 OCH3 15 26 1. Zn(OAc)2•H2O (3.3 equiv.) propionic acid, 110 °C, 1.4 h 2. DDQ, CHCl3, air, 2 h 17% 3. BBr3, CH2Cl2, r.t., 4 h, 80% Ph NH N Ph Ph OH OH Ph CHO N Ph Scheme 3-15. Successful synthesis of MAPOL. N HN N H H N N Ph 9 With MAPOL in hand, a series of UV-vis and ECCD studies were done. UV-vis absorption data (λmax) of the newly synthesized porphyrins were obtained in hexane and methyl cyclohexane, and the molar extinction coefficient (ε) was calculated. These solvents were chosen in order to avoid polar solvents that would disrupt hydrogen bonding. Table 3-4 shows the details of this study. 194 Ph NH N N HN Ph Ph N N Zn N N Ph OCH3 OCH3 Ph N Ph OH OH Ph Ph N H N Ph N N N Zn N N Ph Ph Ph analogue 44 MAPOL 9 Ph N N Zn N N Ph OH OH N H Ph Ph N Zn N N Ph Ph Zn-MAPOL 58 Figure 3-12. Important porphyrins obtained along the way. Table3-5. UV-vis data for newly synthesized porphyrins. hexane methyl cyclohexane λ, nm ε λ, nm ε MAPOL 9 413 420,000 416 298,000 Zn- MAPOL 58 414 81,100 421 89,000 analogue 44 412 519,200 420 370,610 Preliminary CD studies were done using the two enantiomers of cyclohexyl ethylamine in both hexane and methyl cyclohexane. No ECCD was obtained with methyl cyclohexane, hence, all CD studies were conducted in hexane. 195 3-1.3 Investigating the type of interaction and nature of complex formed between MAPOL and amines. Assuming that the chirality of amines is transferred to MAPOL via non-covalent interactions, we need to determine that the porphyrin units play no part in formation of the complex and that the amines bind to MAPOL solely by hydrogen bonding with the biphenol unit. potential hydrogen-bond acceptor potential hydrogen-bond donor Ph Ph NH N Ph N HN Ph Ph Ph NH N N Ph HN Ph Figure 3-13. Potential sites for hydrogen-bonding on porphyrin unit. The porphyrin units have two pyrrole protons as well as two nitrogen atoms that could potentially hydrogen bond with the amine (Figure 3-13). In order to use MAPOL as a host for assignment of absolute stereochemistry of amines, there should be no interference with the 1 binding from the porphyrin units. UV-vis, H-NMR and CD spectroscopy seemed ideal for investigation of the intermolecular interaction between MAPOL and amines. UV-vis spectroscopy is a valuable tool for investigating binding. 34 In metallated porphyrin systems, upon titration of an amine into the porphyrin solution, a red-shift of the porphyrin Soret band (B band) is usually observed. This is because of donation of the lone pair electrons of amine to the metal. 1b In free-base porphyrins however, there is no red-shift of the 196 NH2 0.4 OH OH Absorbance 0.3 + 0.2 0.1 0 350 400 450 λ, nm 2.0 Αbsorbance 1.5 1.0 HO HO 1+(R)-5 + HO NH2 0.5 1 0 280 380 580 480 Wavelength / nm 680 Figure 3-14. UV-Vis curves. A) Titration of MAPOL 9 solution (1 µM) with amine 7 in hexane. B) Ishii’s titration of 4 (25 mM) with amino alcohol in DCM. A lack of 2 red-shift indicates that the amine does not bind to the porphyrin. porphyrin Soret band. If the amines bind to MAPOL via the hydroxyl groups, and have no interaction with the porphyrin units, there should not be any shift in the λmax. But if there is 197 some interaction between amine and porphyrins, then we can expect to observe a change in the λmax. Upon titration of MAPOL with amine 2R in hexane, the porphyrin Soret band did not undergo a shift. (Figure 3-14 A) This was an initial indication that there was no interaction between the porphyrin and the amine, meaning that the amine does not bind to the prophyrin unit. This lack of porphyrin-guest interaction has previously been demonstrated in similar porphyrin systems. One such example was reported by Ishii and co-workers (discussed earlier Section 3-4). They designed a 2,2’-Biphenyl-bridged bis(free base porphyrin) 4 and employed it 2 as a chirality sensor for chiral amino alcohols. Upon UV-vis titration of 4 with amino alcohol, they do not observe a red-shift. Figure 3-14B shows the UV-vis spectrum obtained by Ishii and co-workers upon titration of amino alcohol with free-base porphyrin 4. This result, along with 1 additional H-NMR data led them to the conclusion that the chiral induction obtained in the form of ECCD sign, was as a result of chirality transfer upon hydrogen-bonding of amino alcohols to the host biphenol unit. Hydrogen R O H OMe MeO OH O R O NH MeO N H N OMe bonding complexes involving porphyrins have also been observed by Hayashi and coHO 35 workers. The authors investigated the intermolecular R O R Figure 3-15. Interaction between quinone and porphyrin via multiple hydrogen bonds. interactions involved in molecular recognition between tetraarylporphyrin and ubiquinone analogues. Specifically, they investigated the type of intermolecular interaction between quinone and porphyrin as a model to understand the behavior of protein-ligand binding. 198 Second, additional evidence for the hydrogen-bonding interaction between amine and 1 MAPOL was obtained from H-NMR spectroscopy, where measurements of 9 with several amines (methyl benzylamine, cyclohexyl ethylamine and 3-methylbutan-2-amine) from 1:1 to 1:100 ratios were taken (see experimental details for full details). Using 3-methylbutan-2-amine as an example (Figure 3-16 A), It was observed that the phenolic OH peak of 1 (sharp singlet ~6 ppm) underwent a dramatic shift to 1.9 ppm. Furthermore, the peak corresponding to the amine NH2 protons (broad singlet ~1 ppm) significantly shifts downfield (from 1.06 ppm – 2.56 ppm) upon addition of one equivalent of amine. Additionally, the resonances of the amine protons shift upfield significantly, with the αproton experiencing the greatest shift. For example, for 3-methylbutan-2-amine shown, the αproton shifts from 2.67 ppm in the free amine to 1.00 ppm in the bound complex, presumably as 1 a result of lying within the porphyrin shielding cone. Taking a closer look at the H-NMR spectrum, the pyrrole protons (broad singlet at -2.8 ppm) remain unchanged throughout the titration process. If the amine were to have any interaction with the porphyrins in MAPOL, we would expect the pyrrole protons to change. This was supported by using A4-TPP porphyrin as a control Ph (Figure 3-17). This porphyrin has no hydroxyl groups and also has N HN Ph Ph NH no possible sites for interaction with amine except via the pyrrole N protons, hence, there is no possibility of hydrogen bonding. If the Ph Figure 3-17. A4-TPP porphyrin. amine interacts with the porphyrin, the pyrrole protons of the 1 porphyrin should change. Figure 3-16 B shows the H-NMR spectra obtained from titration of TPP with amine 9S. 199 Several observations are notable. First, the pyrrole protons (broad singlet at -2.8 ppm) remain unchanged throughout the titration process, and second, the amine protons do not undergo any shifts. In addition, unlike the MAPOL titration shown in Figure 3-16 A, where both 1 the host and guest H-NMR peaks undergo some kind of change, in this case, both these protons remain the same. i.e. there is no observable shift for either the amine protons, or the porphyrin protons. These studies suggest that the amines are situated deep within the cleft between the two porphyrins flanking the binding site, and not within the porphyrin itself. Additional confirmation that complex formation between MAPOL and amine occurs via hydrogen bonding and not coordination to the porphyrin, was obtained from CD spectroscopy. MAPOL, being a racemic mixture of P and M atropisomers, was ECCD inactive as expected. However when a chiral amine was added to a solution of MAPOL, the solution became CD active. Moreover, when analogue 44 was used as a control in place of MAPOL 9, the solution, upon addition of chiral mono amines under similar conditions, induced no CD spectra. From UV-vis titration, amine binds to analogue 44 via zinc coordination, as evidenced by a 12 nm redshift of the porphyrin Soret band (Figure 3-18). The source of chirality transfer in MAPOL is the rotation around the C-C single bond between the two phenyls. This rotation is brought about upon binding of the chiral amines with MAPOL. Mono amines cannot induce this twist in analogue 44 because they can only bind to one of the porphyrins. On the other hand, when chiral diamine was used with 44, the complex resulted in a CD signal (Figure 3-19). 200 0.5 0.45 Zn 0.4 NH2 Absorbance 0.35 OCH3 OCH3 0.3 0.25 0.2 + Zn 0.15 0.1 0.05 0 370 390 410 430 450 λ, nm Figure 3-18. UV-Vis titration of Zn-MAPOL 44 solution (1 µM) with amine in hexane. A 12 nm red-shift indicates that the amine binds to the zinc. N N Zn N N Ph NH2 Ph OCH3 OCH3 NH2 Mol CD ! Ph Ph N N Zn N N Ph !!!λ,!nm! Ph 44 Figure 3-19. ECCD spectrum obtained upon binding of 44 with a chiral diamine. 201 1 The structure of the complex formed by MAPOL and amines was studied by H-NMR spectroscopy. A complex stoichiometry of 9•amine in CDCl3 was established at 1:1 by Job’s 1 plot analysis from the H-NMR titration data 10 (Figure 3-20). Job's method of continuous variations is a commonly-used analytical technique for 36 determining the composition of coordination complexes in solution. The method involves measuring an intensive property in a series of solutions of constant total molarity, but of varying host-to-guest ratio. In practice, two equimolar stock solutions, one of the host and the other of the guest, are prepared. A set of working solutions is then obtained by mixing VG mL of the stock guest solution with (VT – VG) mL of the stock host solution, where VT is a fixed total volume and VG is a variable, 0 ≤ 2 ≤ VT. The absorbances of these solutions are then measured at a fixed wavelength, and plotted as a function of mole fraction of guest, (VG/VT), or of host, ((VT – VG)/VT). The position of maximum absorbance on this plot, in relation to the molefraction axis, gives the stoichiometry of the complex. The data for a Job's plot can also be more conveniently obtained from a regular titration procedure. In titrating a host solution with an equimolar guest solution, the total molarity of the host-plus-guest mixture is maintained at a constant value, as the host/guest ratio is varied (additivity of volumes can be assumed for the dilute solutions normally used in these experiments). Thus, the data points from a direct titration actually encompass all of the experimental points corresponding to a Job's plot. ! 202 !!!!!!!A 0.5, 0.015 0.016 Δ δ x mole fraction (porphyrin) 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0 0 0.5 1 1.5 mole fraction of porphyrin porphyrin 0.1 B mole fraction porphyrin porphyrin 0.08 0.06 Kass = 98,652 M R2 = 0.994 0.04 -1 0.02 0 0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 Δ δ, ppm Figure 3-21. A) Job’s continuous plot B) The non-linear least square fit of the chemical shift change vs. mole fraction provides the binding constant. 203 In summary, we developed and synthesized a novel porphyrin host system. This compound demonstrated the ability to form a complex with amines via hydrogen bonding, as shown by both NMR and UV-vis spectroscopy. In addition, a 1:1 stoichiometry of the complex was obtained from a Job’s plot analysis. We propose that the new host molesule could provide a direct approach towards chirality sensing of mono coordinating compounds such as carboxylic acids, mono amines and alcohols, and related ECCD studies of these and other substrates will be discussed in the next chapter. 204 Experimental procedures Anhydrous CH2Cl2 was dried over CaH2 and distilled. The solvents used for CD measurements were purchased and were spectra grade. All reactions were performed in oven or flame dried glassware under nitrogen. Solvents used for synthesis of substrate were dried as follows: THF dried over Sodium, dichloromethane dried over CaH. Column chromatography was performed using SiliCycle silica gel (230-400 mesh). 1H-NMR spectra were obtained on Varian Inova 300 MHz or 500 MHz instrument and are reported in parts per million (ppm) relative to the solvent resonances (δ), with coupling constants (J) in Hertz (Hz). CD spectra were recorded on a JASCO J-810 spectropolarimeter, equipped with a temperature controller (Neslab 111) for low -1 -1 temperature studies, and is reported as λ [nm] (Δεmax [mol cm ]). Synthesis of 16 To 30 mL of TMEDA in 550 mL of freshly distilled ether at 25 °C was added 155 mL nBuLi (1.72 M) dropwise. The solution was stirred for 10 min at this temperature and then 2,2’dimethoxy-biphenyl (19.06 g) was added in. The resultant white suspension was stirred at 25 °C for 2.5 h and then cooled to -78 °C. To this mixture was added iodine (40.69 g) and the reaction maintained at -78 °C for 5 min, after which it was allowed to warm slowly to room temperature and quenched with 500 mL of aqueous Na2SO3 solution. Crude mixture was purified by column 1 chromatography (Silica gel, 10% EtOAc in hexane) to give a pure (16) in 71 % yield. H-NMR (CDCl3, 300 MHz): δ 3.45 (s, 6H), 6.88 (t 2H, J = 7.8), 7.30-7.33 (dd, 2H J1 = 1.2, J2 = 7.5), 205 7.78-7.81 (dd, 2H J1 = 1.5, J2 = 7.8); 13 C-NMR (CDCl3, 75 MHz): δ 60.6, 92.4, 125.6, 131.8, 132.0, 139.3, 157.0; Synthesis of 38 A mixture of paraformaldehyde (1.5 g, 50.0 mmol) and pyrrole (347 mL, 5.0 mol) in a 500 mL round bottom flask was degassed with a stream of argon for 10 min at room temperature. The mixture was then heated at 55 °C for about 10 min under argon until the solution became clear. InCl3 (1.11 g, 5.0 mol) was then added to the mixture, and this was then stirred at 55 °C for 2.5 h the heat source was then removed, and NaOH (6.0 g, 0.15 mol) was added. The mixture was stirred for 1 h and then filtered. The filtrate was concentrated on a rotor evaporator. The recovered pyrrole was distilled for re-use. The crude solid obtained was extracted with a solution of 20% EtOAc in hexane, (5 x 50 mL). After removal of solvent by rotor evaporator, crystallization from methanol/water (4:1) afforded the product as pale white crystals in 45 % 1 yield (3.3 g) H-NMR (CDCl3, 300 MHz): δ 3.96 (s, 2H), 6.01-6.03 (m 2H), 6.12-6.15 (m, 2H), 6.63-6.65 (m, 2H), 7.83 (br, s, 2H); 13 C-NMR (CDCl3, 75 MHz): δ 26.3, 106.3, 108.3, 117.2, 129.0; Synthesis of 39 A flame-dried 500 mL round bottom flask equipped with a magnetic stirring bar was charged with di(1H-pyrrol-2-yl)methane 38 (220 mg, 3.0 mmol), benzaldehyde (154 µL, 3.0 mmol), and freshly distilled methylene chloride (293 mL). The solution was degassed with a stream of argon 206 for 10 min. Trifluoroacetic acid (73.4 µL, 0.97 mmol) was added via syringe and the flask shielded from light with aluminum foil. The solution was stirred for 3 h at room temperature, and then quenched by the addition of p-chloranil (486 mg, 3.0 mmol) and the reaction stirred for a further 30 min, after which it was neutralized with triethyl amine (1.4 mL) and poured directly on a silica gel column packed in hexane. The product was eluted with methylene chloride (350 mL). Evaporation of solvent on a rotor evarporator yielded purple crystals that were washed once with hexane, filtered and dried to give 39 in 10 % yield (125 mg). This compound was used 1 without further purification. H-NMR (CDCl3, 300 MHz): δ -3.12 (s, 2H), 7.81-7.78 (m 4H), 8.28-8.25 (m, 4H), 9.07 (d, 4H J = 4.5 Hz), 9.38 (d, 4H J = 4.5 Hz), 10.30 (s, 2H). Synthesis of 43 Freshly distilled methylene chloride (50 mL) was added to a flame-dried 100 mL round bottom flask equipped with a magnetic stirring bar. 5, 15-diphenyl porphyrin 39 (192 mg, 0.4 mmol), and N-bromosuccinamide (70 mg, 0.4 mmol) were added to the flask at room temperature. The solution was degassed with argon, and left to stir at room temperature for 1 h. Evaporation of solvent on a rotor evaporator yielded crude product as purple solid. The crude mixture was purified by column chromatography (Silica gel, 15% EtOAc in hexane) to give 43 in 71 % yield 1 as purple crystals. H-NMR (CDCl3, 300 MHz): δ -2.98 (s, 2H), 7.75-7.80 (m 5H), 8.18-8.21 (m, 5H), 8.93-8.95 (m, 4H) 9.26 (d, 2H J = 5.1 Hz), 9.72 (d, 2H J = 4.8 Hz), 10.15 (s, 1H). HRMS: calc: 540.0950, 542.0929 exp: 540.9796, 542.9818 Synthesis of 12 1,2-dichloroethane (3.33 mL) was added to a flame-dried 50 mL round bottom flask equipped with a magnetic stirring bar. (5-bromo-10,20-diphenyl porphyrin)zinc II 43 (20 mg, 0.053 207 mmol), pinacolborane (40 µL, 0.053 mmol), triethyl amine (60 µL, 0.053 mmol) and transdichlorobis-palladium II (3 mg, 10 mol %) were added to the flask at room temperature. The solution was degassed with argon and heated to 90 °C. the mixture was stirred at this temperature for until completion as monitored by TLC. Upon complete consumption of starting material, the reaction was quenched with an aqueous solution of KCl (5 mL), washed with water, and dried (Na2SO4). Evaporation of solvent on a rotor evaporator yielded crude product as purple solid. The crude mixture was purified by column chromatography (Silica gel, 100 % 1 DCM) to quantitatively give 12 as purple crystals. H-NMR (CDCl3, 300 MHz): δ 1.86 (s, 12H), 7.79-7.78 (m 6H), 8.24-8.23 (dd, 4H J1 = 0.9 Hz, J2 = 3.9 Hz), 9.06 (d, 1H J = 2.1 Hz), 9.13 (d, 13 1H J = 2.1 Hz), 9.37 (d, 1H J = 2.4 Hz), 9.95 (d, 1H J = 2.4 Hz), 10.24 (s, 1H); C-NMR (CDCl3, 600 MHz): δ 25.3, 85.2, 107.2, 120.3, 126.5, 127.4, 131.7, 132.0, 132.7, 134.5, 142.7, 148.8, 149.8, 150.2, 153.8; HRMS: calc: 650.1832 exp: 651.2375 Synthesis of 41 To a flame-dried 3 L round bottom flask equipped with a magnetic stirring bar was added anisaldehyde (0.855 g, 2.3 mmol), benzaldehyde (2 g, 18.8 mmol), pyrrole (1.68 g, 25 mmol) and freshly distilled methylene chloride (2.5 L). The solution was degassed with a stream of argon for 20 min. BF3•OEt (0.15 mL, 1.25 mmol) was added via syringe and the flask shielded from light with aluminum foil. The solution was stirred for 2 h at room temperature, and then quenched by the addition of p-chloranil (12 g, 0.05 mol) and the reaction stirred for a further 2 h. After removal of most of the solvent on a rotor evaporator, the mixture was washed with a solution of 1 M NaOH (2 x 50 mL. The organic layer was extracted with dichloromethane, and 208 solvents removed on a rotor evarporator to give crude product as purple solid. The crude mixture was purified by column chromatography (Silica gel, 20 % EtOAc in hexane) to give 41 as a 1 purple solid. H-NMR (CDCl3, 300 MHz): δ 3.63 (s, 3H), 7.35 (t 2H J = 8 Hz), 7.76-7.71 (m, 10H), 8.01 (d, 1H J = 6.9 Hz), 8.24-8.22 (m, 6H), 8.95-8.89 (m, 8H). HRMS: calc: 706.1711 exp: 706.2531 Synthesis of 20 HRMS: calc: 630.2420 exp: 631.3031 Synthesis of 26 Freshly distilled ether (143 mL) was added to a flame-dried 100 mL round bottom flask equipped with a magnetic stirring bar, and cooled to -78 °C. To this flask was added 2,2’dimethoxy-biphenyl (5 g, 0.023 mol), nBuLi (28 mL, 0.07 mol) and TMEDA (8 mL, 0.07 mol) dropwise. The reaction mixture was kept at this temperature for 5 min, and then allowed to warm to room temperature. The reaction was stirred at this temperature for 2.5 h, after which it was once again cooled to -78 °C, and DMF (10.8 mL, 0.13 mol) added slowly over a 10 min period. The reaction mixture was allowed to warm to room temperature, and immediately quenched with aqueous NH4Cl solution. The organic layer was extracted with ether, and solvents removed on a rotor evaporator. The crude mixture was purified by column chromatography (Silica gel, 10 % 1 15 % EtOAc in hexane) to give 26 as a white solid. H-NMR (CDCl3, 300 MHz): δ 3.57 (s, 6H), 7.34-7.29 (td 2H J1 = 7.5 Hz J2 = 0.9), 7.64-7.61 (dd, 2H J1 = 7.5 Hz J2 = 2.1 Hz), 7.93-7.90 (dd, 209 13 2H J1 = 7.5 Hz J2 = 1.5 Hz), 10.45 (s, 2H); C-NMR (CDCl3, 75 MHz): δ 63.1, 124.3, 128.7, 129.6, 131.5, 137.5, 160.9, 189.7; HRMS: calc: 706.1711 exp: 706.2531 Synthesis of MAPOL 9 Pyrrole (3.6 mL, 0.052 mol), bisaldehyde 26, benzaldehyde and Zn(OAc)2•2H2O were dissolved in propionic acid (270 mL) in a 3 N flask. The mixture was heated to 110 °C, and maintained at this temperature for 100 min. The reaction mixture was then cooled to room temperature and the solvents removed under high vacuum with successive portions of toluene. DDQ (1 g) in chloroform (200 mL) was added to the residue and stirred at room temperature with air bubbling for 2 h. After removal of the solvent on a rotor evaporator, the crude mixture was purified by 1 column chromatography (Silica gel, 10% - 15 % DCM in hexane) to give pure 9. H-NMR (CDCl3, 300 MHz): δ 3.57 (s, 6H), 7.34-7.29 (td 2H J1 = 7.5 Hz J2 = 0.9), 7.64-7.61 (dd, 2H J1 = 7.5 Hz J2 = 2.1 Hz), 7.93-7.90 (dd, 2H J1 = 7.5 Hz J2 = 1.5 Hz), 10.45 (s, 2H). HRMS: calc: 706.1711 exp: 706.2531 210 REFERENCES 211 References 1. (a) Huang, X. F.; Fujioka, N.; Pescitelli, G.; Koehn, F. E.; Williamson, R. T.; Nakanishi, K.; Berova, N., Absolute configurational assignments of secondary amines by CDsensitive dimeric zinc porphyrin host. Journal of the American Chemical Society 2002, 124 (35), 10320-10335; (b) Kurtan, T.; Nesnas, N.; Li, Y. Q.; Huang, X. F.; Nakanishi, K.; Berova, N., Chiral recognition by CD-sensitive dimeric zinc porphyrin host. 1. Chiroptical protocol for absolute configurational assignments of monoalcohols and primary monoamines. Journal of the American Chemical Society 2001, 123 (25), 59625973; (c) Proni, G.; Pescitelli, G.; Huang, X. F.; Nakanishi, K.; Berova, N., Magnesium tetraarylporphyrin tweezer: A CD-sensitive host for absolute configurational assignments of alpha-chiral carboxylic acids. Journal of the American Chemical Society 2003, 125 (42), 12914-12927. 2. Ishii, Y.; Onda, Y.; Kubo, Y., 2,2 '-Biphenyldiol-bridged bis(free base porphyrin): synthesis and chiroptical probing of asymmetric amino alcohols. Tetrahedron Letters 2006, 47 (47), 8221-8225. 3. (a) Mizutani, T.; Takagi, H.; Hara, O.; Horiguchi, T.; Ogoshi, H., Axial chirality induction in flexible biphenols by hydrogen bonding and steric interactions. Tetrahedron Letters 1997, 38 (11), 1991-1994; (b) Takagi, H.; Mizutani, T.; Horiguchi, T.; Kitagawa, S.; Ogoshi, H., Efficient axial chirality induction in biphenyldiol triggered by protontransferred hydrogen bonding with chiral amine. Organic & Biomolecular Chemistry 2005, 3 (11), 2091-2094; (c) Huang, X. F.; Rickman, B. H.; Borhan, B.; Berova, N.; Nakanishi, K., Zinc porphyrin tweezer in host-guest complexation: Determination of absolute configurations of diamines, amino acids, and amino alcohols by circular dichroism. Journal of the American Chemical Society 1998, 120 (24), 6185-6186. 4. (a) Li, X. Y.; Borhan, B., Prompt Determination of Absolute Configuration for Epoxy Alcohols via Exciton Chirality Protocol. Journal of the American Chemical Society 2008, 130 (48), 16126-+; (b) Li, X. Y.; Tanasova, M.; Vasileiou, C.; Borhan, B., Fluorinated porphyrin tweezer: A powerful reporter of absolute configuration for erythro and threo diols, amino alcohols, and diamines. Journal of the American Chemical Society 2008, 130 (6), 1885-1893; (c) Tanasova, M.; Yang, Q. F.; Olmsted, C. C.; Vasileiou, C.; Li, X. Y.; Anyika, M.; Borhan, B., An Unusual Conformation of alpha-Haloamides Due to Cooperative Binding with Zincated Porphyrins. European Journal of Organic Chemistry 2009, (25), 4242-4253; (d) Yang, Q.; Olmsted, C.; Borham, B., Absolute stereochemical determination of chiral carboxylic acids. Organic Letters 2002, 4 (20), 3423-3426. 5. (a) Bell, C. L.; Barrow, G. M., Evidence for a low-lying 2nd potential minimum in hydrogen-bonded systems. Journal of Chemical Physics 1959, 31 (5), 1158-1163; (b) Gurka, D.; Taft, R. W.; Joris, L.; Schleyer, P. V., Regarding proton transfer in hydrogen- 212 bonded complexes as measured by fluorine nuclear magnetic resonance. Journal of the American Chemical Society 1967, 89 (23), 5957-&; (c) Scott, R.; Depalma, D.; Vinograd.S, Proton-transfer complexes .i. preferential solvation of p-nitrophenol-amine complexes in nonaqueous-solvent mixtures. Journal of Physical Chemistry 1968, 72 (9), 3192-&; (d) Kramer, R.; Lang, R.; Brzezinski, B.; Zundel, G., Proton-transfer in intramolecular hydrogen-bonds with large proton polarizability in 1-piperidinecarboxylic acids - temperature, solvent and concentration-dependence. Journal of the Chemical Society-Faraday Transactions 1990, 86 (4), 627-630; (e) Hanessian, S.; Simard, M.; Roelens, S., Molecular recognition and self-assembly by non-amidic hydrogen-bonding an exceptional assembler of neutral and charged supramolecular structures. Journal of the American Chemical Society 1995, 117 (29), 7630-7645. 6. (a) Feringa, B. L.; van Delden, R. A., Absolute asymmetric synthesis: The origin, control, and amplification of chirality. Angewandte Chemie-International Edition 1999, 38 (23), 3419-3438; (b) Mason, S., BIOMOLECULAR HOMOCHIRALITY. Chemical Society Reviews 1988, 17 (4), 347-359; (c) Avalos, M.; Babiano, R.; Cintas, P.; Jimenez, J. L.; Palacios, J. C.; Barron, L. D., Absolute asymmetric synthesis under physical fields: Facts and fictions. Chemical Reviews 1998, 98 (7), 2391-2404. 7. (a) Morino, K.; Watase, N.; Maeda, K.; Yashima, E., Chiral amplification in macromolecular helicity assisted by noncovalent interaction with achiral amines and memory of the helical chirality. Chemistry-a European Journal 2004, 10 (19), 47034707; (b) Soai, K.; Sato, I.; Shibata, T., Asymmetric autocatalysis and the origin of chiral homogeneity in organic compounds. Chemical Record 2001, 1 (4), 321-332. 8. (a) Yashima, E.; Maeda, K.; Nishimura, T., Detection and amplification of chirality by helical polymers. Chemistry-a European Journal 2004, 10 (1), 42-51; (b) Green, M. M.; Park, J. W.; Sato, T.; Teramoto, A.; Lifson, S.; Selinger, R. L. B.; Selinger, J. V., The macromolecular route to chiral amplification. Angewandte Chemie-International Edition 1999, 38 (21), 3139-3154; (c) Huck, N. P. M.; Jager, W. F.; deLange, B.; Feringa, B. L., Dynamic control and amplification of molecular chirality by circular polarized light. Science 1996, 273 (5282), 1686-1688. 9. (a) Eelkema, R.; Feringa, B. L., Phosphoric acids as amplifiers of molecular chirality in liquid crystalline media. Organic Letters 2006, 8 (7), 1331-1334; (b) Pieraccini, S.; Masiero, S.; Ferrarini, A.; Spada, G. P., Chirality transfer across length-scales in nematic liquid crystals: fundamentals and applications. Chemical Society Reviews 2011, 40 (1), 258-271. 213 10. Eelkema, R.; Feringa, B. L., Macroscopic expression of the chirality of amino alcohols by a double amplification mechanism in liquid crystalline media. Journal of the American Chemical Society 2005, 127 (39), 13480-13481. 11. Hendrix, M.; Alper, P. B.; Priestley, E. S.; Wong, C. H., Hydroxyamines as a new motif for the molecular recognition of phosphodiesters: Implications for aminoglycoside-RNA interactions. Angewandte Chemie-International Edition in English 1997, 36 (1-2), 95-98. 12. Das, G.; Hamilton, A. D., Molecular, recognition of carbohydrates - strong binding of alkyl glycosides by phosphonate derivatives. Journal of the American Chemical Society 1994, 116 (24), 11139-11140. 13. Onouchi, H.; Maeda, K.; Yashima, E., A helical polyelectrolyte induced by specific interactions with biomolecules in water. Journal of the American Chemical Society 2001, 123 (30), 7441-7442. 14. (a) Verdine, G. L.; Nakanishi, K., PARA-DIMETHYLAMINOCINNAMATE, A NEW RED-SHIFTED CHROMOPHORE FOR USE IN THE EXCITON CHIRALITY METHOD - ITS APPLICATION TO MITOMYCIN-C. Journal of the Chemical SocietyChemical Communications 1985, (16), 1093-1095; (b) Berova, N.; Gargiulo, D.; Derguini, F.; Nakanishi, K.; Harada, N., UNIQUE UV-VIS ABSORPTION AND CIRCULAR DICHROIC EXCITON-SPLIT SPECTRA OF A CHIRAL BISCYANINE DYE - ORIGIN AND NATURE. Journal of the American Chemical Society 1993, 115 (11), 4769-4775; (c) Cai, G. L.; Bozhkova, N.; Odingo, J.; Berova, N.; Nakanishi, K., CD EXCITON CHIRALITY METHOD - NEW RED-SHIFTED CHROMOPHORES FOR HYDROXYL-GROUPS. Journal of the American Chemical Society 1993, 115 (16), 7192-7198. 15. Arsenault, G. P.; Bullock, E.; Macdonald, S. F., Pyrromethanes and porphyrins therefrom. Journal of the American Chemical Society 1960, 82 (16), 4384-4389. 16. (a) Miyaura, N.; Suzuki, A., Palladium-catalyzed cross-coupling reactions of organoboron compounds. Chemical Reviews 1995, 95 (7), 2457-2483; (b) Suzuki, A., Recent advances in the cross-coupling reactions of organoboron derivatives with organic electrophiles, 1995-1998. Journal of Organometallic Chemistry 1999, 576 (1-2), 147168. 17. (a) Oconnor, S. J.; Williard, P. G., Strategy for the synthesis of the c10-c19 portion of Amphidinolide-A. Tetrahedron Letters 1989, 30 (35), 4637-4640; (b) Marshall, J. A.; 214 Van Devender, E. A., Synthesis of (-)-deoxypukalide, the enantiomer of a degradation product of the furanocembranolide pukalide. Journal of Organic Chemistry 2001, 66 (24), 8037-8041; (c) Marshall, J. A.; Johns, B. A., Total synthesis of (+)-discodermolide. Journal of Organic Chemistry 1998, 63 (22), 7885-7892; (d) Shirai, T.; Kuranaga, T.; Wright, J. L. C.; Baden, D. G.; Satake, M.; Tachibana, K., Synthesis of a proposed biosynthetic intermediate of a marine cyclic ether brevisamide for study on biosynthesis of marine ladder-frame polyethers. Tetrahedron Letters 2010, 51 (10), 1394-1396; (e) Lepage, O.; Kattnig, E.; Furstner, A., Total synthesis of amphidinolide X. Journal of the American Chemical Society 2004, 126 (49), 15970-15971; (f) Yuan, Y.; Men, H. B.; Lee, C. B., Total synthesis of kendomycin: A macro-C-glycosidation approach. Journal of the American Chemical Society 2004, 126 (45), 14720-14721; (g) Maloney, D. J.; Hecht, S. M., A stereocontrolled synthesis of delta-trans-tocotrienoloic acid. Organic Letters 2005, 7 (19), 4297-4300; (h) Maloney, D. J.; Hecht, S. M., Synthesis of a potent and selective inhibitor of p90 Rsk. Organic Letters 2005, 7 (6), 1097-1099; (i) Maloney, K. M.; Danheiser, R. L., Total synthesis of quinolizidine alkaloid (-)-217A. Application of iminoacetonitrile cycloadditions in organic synthesis. Organic Letters 2005, 7 (14), 31153118; (j) O'Connor, S. J.; Barr, K. J.; Wang, L.; Sorensen, B. K.; Tasker, A. S.; Sham, H.; Ng, S. C.; Cohen, J.; Devine, E.; Cherian, S.; Saeed, B.; Zhang, H. C.; Lee, J. Y.; Warner, R.; Tahir, S.; Kovar, P.; Ewing, P.; Alder, J.; Mitten, M.; Leal, J.; Marsh, K.; Bauch, J.; Hoffman, D. J.; Sebti, S. M.; Rosenberg, S. H., Second-generation peptidomimetic inhibitors of protein farnesyltransferase demonstrating improved cellular potency and significant in vivo efficacy. Journal of Medicinal Chemistry 1999, 42 (18), 3701-3710; (k) Smith, A. B.; Xian, M., Design, synthesis, and biological evaluation of simplified analogues of (+)-discodermolide. additional insights on the importance of the diene, the C(7) hydroxyl, and the lactone. Organic Letters 2005, 7 (23), 5229-5232; (l) Arefolov, A.; Panek, J. S., Crotylsilane reagents in the synthesis of complex polyketide natural products: Total synthesis of (+)-discodermolide. Journal of the American Chemical Society 2005, 127 (15), 5596-5603. 18. Stanforth, S. P., Catalytic cross-coupling reactions in biaryl synthesis. Tetrahedron 1998, 54 (3-4), 263-303. 19. Hoye, T. R.; Chen, M. Z.; Hoang, B.; Mi, L.; Priest, O. P., Total synthesis of michellamines A-C, korupensamines A-D, and ancistrobrevine B. Journal of Organic Chemistry 1999, 64 (19), 7184-7201. 20. Mentzel, U. V.; Tanner, D.; Tonder, J. E., Comparative study of the Kumada, Negishi, Stille, and Suzuki-Miyaura reactions in the synthesis of the indole alkaloids hippadine and pratosine. Journal of Organic Chemistry 2006, 71 (15), 5807-5810. 21. (a) Filatov, M. A.; Guilard, R.; Harvey, P. D., Selective Stepwise Suzuki Cross-Coupling Reaction for the Modelling of Photosynthetic Donor-Acceptor Systems. Organic Letters 215 2010, 12 (1), 196-199; (b) Wang, F.; Matsuda, K.; Rahman, A.; Peng, X. B.; Kimura, T.; Komatsu, N., Simultaneous Discrimination of Handedness and Diameter of SingleWalled Carbon Nanotubes (SWNTs) with Chiral Diporphyrin Nanotweezers Leading to Enrichment of a Single Enantiomer of (6,5)-SWNTs. Journal of the American Chemical Society 2010, 132 (31), 10876-10881; (c) Peng, X. B.; Komatsu, N.; Kimura, T.; Osuka, A., Improved optical enrichment of SWNTs through extraction with chiral nanotweezers of 2,6-pyridylene-bridged diporphyrins. Journal of the American Chemical Society 2007, 129 (51), 15947-15953; (d) Chan, K. S.; Zhou, X. A.; Au, M. T.; Tam, C. Y., Synthesis of beta-aryl substituted porphyrins by palladium-catalyzed suzuki cross-coupling reactions. Tetrahedron 1995, 51 (11), 3129-3136; (e) Chung, L. L.; Chang, C. J.; Nocera, D. G., meso-tetraaryl cofacial bisporphyrins delivered by Suzuki cross-coupling. Journal of Organic Chemistry 2003, 68 (10), 4075-4078; (f) Hyslop, A. G.; Kellett, M. A.; Iovine, P. M.; Therien, M. J., Suzuki porphyrins: New synthons for the fabrication of porphyrin-containing supramolecular assemblies. Journal of the American Chemical Society 1998, 120 (48), 12676-12677. 22. Dogutan, D. K.; Ptaszek, M.; Lindsey, J. S., Direct synthesis of magnesium porphine via 1-formyldipyrromethane. Journal of Organic Chemistry 2007, 72 (13), 5008-5011. 23. Cram, D. J.; Degrandpre, M.; Knobler, C. B.; Trueblood, K. N., Host guest complexation .29. expanded hemispherands. Journal of the American Chemical Society 1984, 106 (11), 3286-3292. 24. (a) Li, X. L.; Yang, J.; Kozlowski, M. C., Enantioselective oxidative biaryl coupling reactions catalyzed by 1,5-diazadecalin metal complexes. Organic Letters 2001, 3 (8), 1137-1140; (b) Hovorka, M.; Scigel, R.; Gunterova, J.; Tichy, M.; Zavada, J., The oxidative cross-coupling of substituted 2-naphthols .1. the scope and limitations. tetrahedron 1992, 48 (43), 9503-9516; (c) smrcina, m.; lorenc, m.; hanus, v.; sedmera, p.; kocovsky, p., synthesis of enantiomerically pure 2,2'-dihydroxy-1,1'-binaphthyl, 2,2'diamino-1,1'-binaphthyl, and 2-amino-2'-hydroxy-1,1'-binaphthyl - comparison of processes operating as diastereoselective crystallization and as 2nd-order asymmetric transformation. Journal of Organic Chemistry 1992, 57 (6), 1917-1920; (d) Wang, H., Recent Advances in Asymmetric Oxidative Coupling of 2-Naphthol and its Derivatives. Chirality 2010, 22 (9), 827-837. 25. (a) Dewar, M. J. S.; Nakaya, T., Oxidative coupling of phenols. Journal of the American Chemical Society 1968, 90 (25), 7134-&; (b) Doussot, J.; Guy, A.; Ferroud, C., Selective synthesis of 1,1 '-binaphthalene derivatives by oxidative coupling with TiCl4. Tetrahedron Letters 2000, 41 (15), 2545-2547; (c) Hwang, D. R.; Chen, C. P.; Uang, B. J., Aerobic catalytic oxidative coupling of 2-naphthols and phenols by VO(acac)(2). Chemical Communications 1999, (13), 1207-1208. 216 26. Noji, M.; Nakajima, M.; Koga, K., A new catalytic-system for aerobic oxidative coupling of 2-naphthol derivatives by the use of cucl-amine complex - a practical synthesis of binaphthol derivatives. Tetrahedron Letters 1994, 35 (43), 7983-7984. 27. Nakajima, M.; Kanayama, K.; Miyoshi, I.; Hashimoto, S., Catalytic asymmetric synthesis of binaphthol derivatives by aerobic oxidative coupling of 3-hydroxy-2-naphthoates with chiral diamine-copper complex. Tetrahedron Letters 1995, 36 (52), 9519-9520. 28. (a) Bao, J. M.; Wulff, W. D.; Dominy, J. B.; Fumo, M. J.; Grant, E. B.; Rob, A. C.; Whitcomb, M. C.; Yeung, S. M.; Ostrander, R. L.; Rheingold, A. L., Synthesis, resolution, and determination of absolute configuration of a vaulted 2,2'-binaphthol and a vaulted 3,3'-biphenanthrol (VAPOL). Journal of the American Chemical Society 1996, 118 (14), 3392-3405; (b) Ding, Z. S.; Osminski, W. E. G.; Ren, H.; Wulff, W. D., Scalable Syntheses of the Vaulted Biaryl Ligands VAPOL and VANOL via the Cycloaddition/Electrocyclization Cascade. Organic Process Research & Development 2011, 15 (5), 1089-1107. 29. Markovac, A.; Macdonal.Sf, SYNTHESES WITH 5-DIBROMOMETHYL- AND 5FORMYLPYRROMETHENES. Canadian Journal of Chemistry 1965, 43 (12), 3364-&. 30. (a) Ema, T.; Kuroda, Y.; Ogoshi, H., Selective syntheses of unsymmetrical mesoarylporphyrins. Tetrahedron Letters 1991, 32 (35), 4529-4532; (b) Lindsey, J. S.; Hsu, H. C.; Schreiman, I. C., Synthesis of tetraphenylporphyrins under very mild conditions. Tetrahedron Letters 1986, 27 (41), 4969-4970; (c) Motmans, F.; Ceulemans, E.; Smeets, S.; Dehaen, W., The use of 4,6-disubstituted pyrimidine-5-aldehydes in the synthesis of meso-tetraarylporphyrins. Tetrahedron Letters 1999, 40 (42), 7545-7548; (d) Wallace, D. M.; Leung, S. H.; Senge, M. O.; Smith, K. M., Rational tetraarylporphyrin syntheses tetraarylporphyrins from the macdonald route. Journal of Organic Chemistry 1993, 58 (25), 7245-7257. 31. (a) Lash, T. D., Porphyrin synthesis by the ''3+1'' approach: New applications for an old methodology. Chemistry-a European Journal 1996, 2 (10), 1197-1200; (b) Bruckner, C.; Posakony, J. J.; Johnson, C. K.; Boyle, R. W.; James, B. R.; Dolphin, D., Novel and improved syntheses of 5,15-diphenylporphyrin and its dipyrrolic precursors. Journal of Porphyrins and Phthalocyanines 1998, 2 (6), 455-465; (c) Montierth, J. M.; Duran, A. G.; Leung, S. H.; Smith, K. M.; Schore, N. E., The polymer-supported MacDonald-type porphyrin synthesis: coupling of two dissimilar dipyrromethanes. Tetrahedron Letters 2000, 41 (39), 7423-7426; (d) Uno, H.; Kitawaki, Y.; Ono, N., Novel preparation of betabeta '-connected porphyrin dimers. Chemical Communications 2002, (2), 116-117. 217 32. Ogoshi, H.; Sugimoto, H.; Nishiguchi, T.; Watanabe, T.; Matsuda, Y.; Yoshida, Z., Syntheses of 5-aryl- and 5,15-diaryl-2,3,7,8,12,13,17,18-octaethylporphines. Chemistry Letters 1978, (1), 29-32. 33. Hayashi, T.; Aya, T.; Nonoguchi, M.; Mizutani, T.; Hisaeda, Y.; Kitagawi, S.; Ogoshi, H., Chiral recognition and chiral sensing using zinc porphyrin dimers. Tetrahedron 2002, 58 (14), 2803-2811. 34. Huang, X. F.; Borhan, B.; Berova, N.; Nakanishi, K., UV-vis spectral changes in the binding of acyclic diamines with a zinc porphyrin tweezer. Journal of the Indian Chemical Society 1998, 75 (10-12), 725-728. 35. (a) Hayashi, T.; Asai, T.; Borgmeier, F. M.; Hokazono, H.; Ogoshi, H., Kinetic and thermodynamic analysis of induced-fit molecular recognition between tetraarylporphyrin and ubiquinone analogues. Chemistry-a European Journal 1998, 4 (7), 1266-1274; (b) Hayashi, T.; Miyahara, T.; Koide, N.; Kato, Y.; Masuda, H.; Ogoshi, H., Molecular recognition of ubiquinone analogues. Specific interaction between quinone and functional porphyrin via multiple hydrogen bonds. Journal of the American Chemical Society 1997, 119 (31), 7281-7290. 36. Derr, P. F.; Stockdale, R. M.; Vosburgh, W. C., Complex ions. II. The stability and activity coefficients of the silver-ammonia ion. Journal of the American Chemical Society 1941, 63, 2670-2674.! ! ! 218 Chapter 4 Determination of Absolute Stereochemistry of α-Chiral Amines 4-1 Background The determination of absolute stereochemistry of chiral mono amines without the requirement of chemical derivatization is a problem that has long eluded chemists. Chiral mono amines play an increasing importance both in synthetic chemistry as important intermediates in the synthesis of biologically important molecules, as well as in the pharmaceutical industry. 1 There are many examples throughout literature where chiral amines are used as auxiliaries for 2 asymmetric catalysis, and in biologically important roles. Me NH HO CO2H H O O NH2 CO2H H Dysiherbaine HO HO HO NH OH OH Nojirimycin HO N O N H N Quinine CH3 Coniine Figure 4-1. Molecules containing the amine functionality. 219 Due to the pivotal roles that enantiomerically pure amines play in a diverse range of pharmaceutical and organic synthesis, there is considerable effort currently underway to develop efficient methods for assigning their absolute stereochemistry easily and efficiently. For example, Scheme 4-1 shows the use of (S)-2-amino-3-methylbutane in a key step as a chiral building block in the key step of the synthesis of Diazoxide BPDZ-44, a tissue selective ATP – sensitive potassium channel opener, that results in inhibition of important physiological processes such as insulin release or muscle tone and contractility. O O O S N 3 N H N + H2N sealed tube, 150 °C N 1,4-dioxane, 2 h SCH3 O S N N N H H BPDZ-44 Scheme 4-1. Synthesis of Diazoxide BPDZ-44, using (S)-2-amino-3-methylbutane. ! The main challenge met in assigning absolute configuration of compounds with one functional group, such as mono amines, is the requirement of derivatization, either with chiral derivatization agents for Mosher analysis protocol, or with achiral carrier molecules for ECCD studies. 4-1-2 Conventional Methods of Assigning Absolute Stereochemistry of α-Chiral Primary Amines Being that chiral primary and secondary amines only have one site of attachment, they cannot be directly applied in the conventional chiroptical methods used for the absolute configurational assignment of chiral diamines using porphyrin tweezers. With only one amino 220 group, the sandwiched host-guest complex, in which the chiral substrate is locked inside the tweezer cannot be formed. Consequently, due to the free rotation around the tweezer linker, the two porphyrin chromophores as well as their electric transition dipole moments will be oriented randomly resulting in unpredictable or no CD signals. R1 NHR H R1 R2 HN N Zn H H Zn R2 R1 NH O O NH2 N R2 O O O O NH2 Zn N Zn Zn porphyrin tweezer COOH carrier NH2 amine derivatized with carrier O O O O conjugate/tweezer complex Scheme 4-2. Complex formation between carrier/monoamine conjugate and porphyrin tweezer. The traditional approach for determining the absolute stereochemistry of α-chiral primary amines using ECCD protocol calls for the monoamine to first be derivatized by a carrier molecule in order to introduce the requisite second binding site (usually nitrogen-containing 4 functionality) (Scheme 4-2). This is not desirable because the substrates are often available in very little amounts and so additional synthetic steps in order to install the carrier and assign the absolute stereochemistry can be time consuming and impractical. A second drawback of this approach is that for each carrier, there’s a need for the development of a new mnemonic to relate the observed sign of the ECCD couplet to the absolute stereochemistry of the guest. 5 In 2001, Inuoe and co-workers designed an octaethyl substituted porphyrin tweezer system with a short ethylene linker, and directly employed it for the assignment of the absolute stereochemistry of monoamines and monoalcohols without the need for derivatization.6 As 221 shown in Figure 4-2, the achiral folded syn conformer of the ethane-bridged porphyrin switches to the corresponding chiral extended anti conformer upon ligand binding. X 7 N N M N N Me R H X = OH, NH2 X 3 7 N N Me R H 3 7 3 7 M N N 3 M = Mg or Zn Right Handed Screw Figure 4-2. Inuoe’s porphyrin tweezer for stereochemical determination. The mechanism of the chiral induction is based on the chiral ligand binding to zinc, and subsequent formation of either right- or left-handed twist, due to steric interactions between the ethyl groups on the porphyrin ring, and the largest substituent on the asymmetric carbon of the ligand. Upon addition of the chiral substrate, it binds to one porphyrin of the tweezer, and the steric interaction between the chiral center and the ethyl groups at 3,7 positions of the non-bound porphyrin drive the non-bound porphyrin to slide away generating a right-handed screw for Ssubstrates and right-handed screw for R-substrates. The major drawback of this method is the requirement of an excess of the chiral molecule, as well as low intensity signals for the alcohols. Perhaps the most commonly used method for assigning absolute stereochemistry of mono 7 amines is the Mosher ester analysis. This method involves the preparation of diastereomeric MTPA (or MPA) amides from the two enantiomers of MTPA and the amine of unknown 222 configuration. The NMR spectra of the two diastereomeric derivatives are then recorded and the differences in the chemical shifts are calculated. Mosher’s proposal of the most relevant conformer in solution is generally accepted. In this proposal, for each diastereomer, the most relevant conformer adopts the s-trans arrangement around its N-C-O bonds and the CF3, carbonyl, NH, and C(1’) methane proton groups are all in the same plane, and the CF3 and the carbonyl units are arranged in a syn-periplanar disposition (Figure 4-3). While this is not the only possible conformation, it represents a model that successfully correlates the known results, and as stated by Mosher, the conformation is not intended to represent the preferred ground state conformation of the molecules and may be an average of many conformations.8 In this proposed model (Figure 4-3), substituent S2 is shielded in the (R)-MTPA amide (Figure 4-3 A). Similarly, substituent S1 is shielded by the phenyl substituent in the (S)-MTPA amide (Figure 4-3 B). For the amine shown, substituent S1 has negative Δδ SR values and substituent S2 has positive values (Figure 4-3 C). In the case where the amine has the opposite configuration, as in case D, the signs for the Δδ SR values of S1 and S2 are expected to switch. While these methods works well for amines and are routinely used, for compounds that are available in short supply (like in the case! of initial structural studies of a newly isolated !natural product) the requirement for derivatization of the amines before analysis as well as the empirical nature of Mosher analysis is a major limitation. For these reasons, it is desirable to develop methods that avert these drawbacks. 223 A Ph OMeH N S1 S2 F3C H O S1 MeO S2 Ph O CF3 (R)-MTPA or (S)-MTPACl B MeO Ph H N S1 S2 F3C H O (S)-MTPA or (R)-MTPACl S2 is shielded H C MTPA N H S1 Ph S2 OMe O CF3 S1 is shielded H ΔδSR > 0 S2 S1 ΔδSR < 0 D MTPA N H SR > 0 S1 Δδ S2 ΔδSR < 0 1 Figure 4-3. (A, B) Mosher’s model for correlating the configuration with the HNMR shifts. SR (C, D) expected signs of Δδ . 4-2 ECCD Studies of Chiral Monoamines Using MAPOL As mentioned in Chapter 3, biphenols and amines form hydrogen bonding complexes in 9 solution. Therefore it was hypothesized that a chiral mono amine would cause stereodifferentiation in the MAPOL host via the hydrogen bonding interaction. The helicity of MAPOL could be determined by ECCD and this would be related back to the absolute configuration of the amine. If our designed system works as expected, the principle advantage would be the analysis of absolute stereochemistry without the pre-requisite derivatization as with the tweezer methodologies or Mosher ester analysis. Atropisomers exist as racemic mixtures of the P and M enantiomers. However, this equilibrium can be perturbed by the introduction of a chiral bias, creating distereomers and causing one confomer to be favored over the other as the complex interacts with the chiral ligands. Our interest in the design of MAPOL is to exploit this phenomenon, while taking advantage of the ability of the porphyrins to couple with each other, 224 leading to observable ECCD. We could then directly relate the sign of the ECCD to the chirality of the bound amines. OH HO HO OH P M Figure 4-5. P and M conformers of MAPOL at equilibrium. MAPOL consists of two porphyrin units and a 2,2’-biphenyl linker. The P and M helicity of the system is dictated by the rotation around the C-C single bond between the phenyl units (Figure 4-5). At equilibrium, there is a racemic mixture of atropisomers, and if undisturbed, the system can rest in this equilibrium state indefinitely. Additionally, when at equilibrium, MAPOL is inherently ECCD inactive. However, upon introduction of a chiral amine to the system, (based on the results discussed in Chapter 3, indicating a 1:1 amine:host complex formation) we postulate that the chirality of the amine will cause a preferential hydrogen bonding to favor one atropisomer of MAPOL. Assuming that sterics are responsible for chirality recognition, during the complexation, the largest group on the asymmetric center will position itself in the least sterically encumbered location (Figure 4-6). For both the P or M atropisomers, there is a large cavity for the largest group to occupy. It is believed that the large group plays no role in stereodiscrimination. Nonetheless, in one helicity of MAPOL, the medium group of the amine will be situated in a more sterically tolerable region, and the smallest group will occupy the most sterically congested area. 225 NHN L M S NHN NH H H O O H L NHN NHN M NHN S NH H H H O O NHN N HN N HN S-amine with M-Helix S-amine with P-Helix favored complexation small group occupies sterically congested region disfavored complexation medium group occupies sterically congested region L NHN NHN M NH S H H H O O L NHN NHN N HN NHN M S HN H H H O O NHN N HN R-amine with P-Helix favored complexation small group occupies sterically congested region R-amine with M-Helix disfavored complexation medium group occupies sterically congested region Figure 4-6. Proposed complexation if chiral amine with P- and MHelicities of MAPOL. However, in the other helicity of MAPOL, the opposite would be true, and there will be a more sterically unfavorable interaction with the medium group. In this way, the amine will bind to the MAPOL in the sterically favored helicity. Since the barrier to rotation about the biphenol C-C single bond is small,10 the equilibrium will shift to create an over-population of the favored helicity. The excess of one helicity over the other will lead to an observed ECCD spectrum as a result of exciton coupling of the porphyrins arranged in a chiral fashion within the sterically preferred complex. Figure 4-7 shows a proposed enantiodiscimination mechanism using a chiral amine. In A, 226 P1 A S S NH2 M L Por (R) H O M Por Por S O O O L H2N L (S) S M O O LH H O S M S Por P2 predicted positive ECCD P-helicity favored B Por M P1 Por L M-helicity disfavored P2 M Por P1 P-helicity disfavored P2 P2 P1 Por O L M-helicity favored predicted negative ECCD Figure 4-7. Newman projections of a chiral amine complexed with P- and M-helicities of MAPOL. Inserts: the predicted ECCD signs from the favored conformation. A) (S)-amine binds favorably with the M-helix, resulting in a predicted negative ECCD. B) (R)-amine preferably binds with the P-helix, yielding a predicted negative ECCD. if S- amine binds with the P-helix of MAPOL, there would be unfavorable steric interactions between the medium group and the phenyl ring of MAPOL, hence the (S)- amine preferably binds with the M-helix. This places the porphyrins in a counterclockwise arrangement that would result in a negative CD sign. On the other hand, the (R)- amine preferably binds with the P-helix, avoiding the unfavorable steric interactions that would occur between the medium group and the phenyl ring in the M-helix of MAPOL. In this complex, the porphyrins are arranged in a clockwise manner, resulting in a predicted positive CD sign. 4-2-2 Determination of Chirality for Primary Amines In light of the above proposal, MAPOL 9 was examined for configurational assignment of a variety of chiral mono amines via the Exciton Coupled Circular Dichroism protocol. To our 227 delight, consistent and prominent bisignate CD signals at the porphyrin Soret region were observed upon addition of micromolar concentrations of a variety of chiral amines to a solution of MAPOL in hexanes. As shown in Table 4-1, a number of alkyl and aryl primary amines were tested. The S amines resulted in negative ECCD spectra while positive signals were observed for the R amines. Furthermore, enantiomeric pairs yielded opposite ECCD spectra (compounds 5R and S, 3R and S and 2R and S). Of particular note is compound 5, where the system is able to register small differences in size based on their A value (methyl-1.74 vs. ethyl-1.79). In addition, the system is tolerant of other potential H-acceptor groups like esters (9S). The correlation between substrate chirality and the sign of ECCD is illustrated in Figure 4-8, as shown for (S)-cyclohexyl ethylamine. For both the P or M helicity of the complex, the large cyclohexyl group is positioned in the least sterically encumbered location, while the location of the medium (CH3) and small (H) groups is dictated by the configuration of the chiral center as illustrated. In A, the medium group (CH3) is situated in a more sterically tolerable region, while the smallest group (H) occupies the most sterically congested area. On the other hand, the P complex would have a sterically unfavorable interaction between the medium (CH3) group and porphyrin. In this way, the more favorable M-helicity is promoted in binding with the S-enantiomer. An over-population of the M helicity leads to an excess of the counterclockwise orientation of the porphyrins and the observed negative ECCD spectrum (Table 4-1, entry 1). 228 Table 4-1. ECCD data for chiral amines bound to MAPOL. amine entry predicted sign MAPOL A λ nm, (Δε) NH2 1 neg 418, -237 409, +257 -494 pos 419, +187 410, -345 +532 neg 7S 429, -15 420, +46 -61 NH2 2 7R NH2 3 8S NH2 4 8R pos 427, +36 409, -12 +48 5 9R pos 418, +153 420, -62 +215 pos 425, +161 412, -177 +338 neg 425, -60 411, +108 -168 pos 423, +41 414, -26 +67 pos 422, +147 414, -59 +206 neg 427, -14 415, +24 -38 neg 428, -30 412, +90 -120 pos 429, +62 412, -121 +183 NH2 6 10R NH2 7 10S NH2 8 11R 9 12R 10 13S NH2 NH2 OCH3 H2N O 11 14S 12 15R NH2 NH2 host:guest ratio – 1:20, 1 µM host concentration in hexane at 0 °C was used for all measurements. 229 A Ph Ph Ph CH3 H HN H N N H HH O O N N H Ph H OH Ph H3C N HN O NH N Ph favored helicity, having the smallest group in the most sterically demanding region M-helicity B Ph Ph N N H Ph H N N Ph H HN Ph CH3 H H H O O N HN NH N H3C O Ph H O H less favorable due to more steric congestion of the medium group P-helicity Figure 4-8. Proposed binding model for (S)-cyclohexyl ethylamine to MAPOL. In A, the amine binds to the M-helix and in B, the amine binds to Phelix. The complex formed in B is sterically less favorable due to steric congestion between porphyrin and methyl group. Additional confirmation that complex formation between MAPOL and amine occurs via hydrogen bonding and not coordination to the porphyrin, was supported by the fact that when analogue 44 was used as a control in place of MAPOL 9, the solution, upon addition of chiral mono amines under similar conditions, induced no CD spectra. 11 Moreover, even though Zn- MAPOL 58 yielded CD signals with primary amines, as shown in Table 4-2, in general, the amplitudes were lower than those obtained with MAPOL 9. 230 Ph NH N N HN Ph Ph N N Zn N N Ph OCH3 OCH3 Ph N Ph N N Zn N N Ph OH OH N H Ph Ph OH OH Ph Ph N H N Ph N Ph MAPOL 9 Ph N N Zn N N Ph analogue 44 Ph N Zn N N Ph Ph Zn-MAPOL 58 Figure 4-9. MAPOL and analogues of MAPOL used for ECCD studies. Additionally, a higher concentration of 58 (2 µM) was required in order to obtain ECCD. Even though most of the ECCD signals observed with 58 were consistent with the predicted ECCD signals for 9, several amines like (4R and 10S and 11R) did not produce consistent results with 58. It is important to note that with amines 5S, 5R, 10S and 11R, the quality of the signal with 9 is improved to that observed with 58 (Figure 4-10). These inconsistent signs and complex spectra are presumably due to competitive binding to the Zn metal in the porphyrin. Also amine coordination to the zinc could lead to crowding of the binding pocket, preventing efficient hydrogen bonding with the host. 231 a b Table 4-2. ECCD data for chiral amines bound to MAPOL and Zn-MAPOL in hexane. amine predicted sign MAPOL A λ nm, (Δε) Zn-MAPOL A λ, nm, (Δε) NH2 neg 418, -237 409, +257 -494 428, -133 419, +98 -231 pos 419, +187 410, -345 +532 428, +136 420, -95 +231 neg 429, -15 420, +46 -61 426, -29 420, +25 -54 3R pos 427, +36 409, -12 +48 426, +32 421, -15 +47 4R pos 418, +153 420, -62 +215 429, -31 420, +32 -63 pos 425, +161 412, -177 +338 427, +12 419, -7 +19 neg 425, -60 411, +108 -168 426, -8 417, +3 -11 pos 423, +41 414, -26 +67 427, +69 421, -12 +81 pos 422, +147 414, -59 +206 420, +117 412, -39 +156 neg 427, -14 415, +24 -38 429, -39 418, +43 -82 neg 428, -30 412, +90 -120 no ECCD pos 429, +62 412, -121 +183 no ECCD 2S NH2 2R NH2 3S NH2 NH2 5R NH2 5S NH2 NH2 6R 7R NH2 8S OCH3 H2N O 10S NH2 11R NH2 a b host:guest ratio – 1:20, 1 µM, 2 µM host concentration of host at 0 °C was used ! 232 A 15 200 10 5(R) 100 5 A A NH2 0 0 -5 -100 -10 380 -200 430 380 λ,!nm! 430 λ,!nm! B 8 150 100 4 A 5(S) 50 A NH2 0 0 -4 -50 -8 -100 400" 380 420" 440" λ,!nm! λ,!nm! C 40 40 8(S) 20 20 A O A OCH3 H2N 430 0 0 -20 -40 380 -20 430 λ,!nm! 380 430 λ,!nm! Figure 4-10. CD signals of amines A) 5R, B) 5S, and C) 8S with Zn-MAPOL 58 and MAPOL 9. All amines show better amplitudes and CD spectra with MAPOL. 233 4-3 Determination of Chirality for Secondary Amines Following the success of MAPOL for assigning absolute configuration of primary amines, secondary amines seemed like a natural extension of the methodology. Secondary amines, like primary amines, are also of great interest due to their roles as anti-tumor agents, 1 antibiotics, as well as plant growth regulators and promoters. In synthetic chemistry, they have been used as chiral auxiliaries for asymmetric catalysis, chiral resolving agents as well as chiral intermediates in the synthesis of bioactive complex molecules. 12 4-3-2 Conventional Methods of Assigning Absolute Stereochemistry of α-Chiral Secondary Amines In a similar manner as for primary amines, the absolute configurational assignment of chiral secondary amines has been achieved by Mosher ester analysis method spectroscopy. 13 as well as CD 14 3 2 5 6 N MeO Ph O CF3 anti N O syn Ph OMe CF3 Figure 4-11. The syn and anti conformers of the (R)-MTPA amide of (R)-2-methylpiperidine in equilibrium. Hoye et al. reported a procedure for the assignment of the absolute configuration of cyclic secondary amines with the stereocenter located at either the α- or β-positions. 13b They accomplished this by using the Mosher ester analysis method, using MTPA as the CDA. Their 234 1 proposed model follows the standard procedures of comparing the H-NMR spectra of the 1 corresponding (R)- and (S)-MTPA amides; however the H-NMR spectra of these MTPA amides indicated the presence of two rotamers (syn and anti) in equilibrium with each other shown in Figure 4-11. Therefore, the rules for correlating the NMR shifts and configuration warrant some 8 more explanation. Recalling, Mosher’s proposed model of the most relevant conformer discussed in Section 4-1-2 above, the CF3, carbonyl, NH, and C(1’)H groups should all be in the same plane. In addition, the CF3 and the carbonyl units should be arranged in a syn-periplanar manner. Because of this, it is important to differentiate the spectra of the rotamers because, only the signals of the rotamer that has the asymmetric carbon of the MTPA placed syn to the asymmetric carbon of the amine should be considered. A 3 2 N OMe Ph CF3 O C Me 3 2 N H 6 OMe Ph O CF3 3 N O Ph OMe CF3 H Me 2 N O O H Ph MeO CF3 CF3 (S)-MTPA amide, anti conformer D 3 2 N 5 N Ph MeO (S)-MTPA amide, syn conformer B Me H 5 6 Ph OMe O CF3 (R)-MTPA amide, syn conformer Me H 5 N MeO Ph O CF3 6 N O MeO Ph CF3 (R)-MTPA amide, anti conformer Figure 4-12. Syn and anti rotamers for the (S)- and (R)-MTPA amides of (R)-2methylpiperidine. 235 For example, in the assignment of (R)-2-methylpiperidine shown in Figure 4-12, only the signals due to the syn rotamer A and B in the (S)- and (R)-MTPA amides are relevant and only the Δδ SR values corresponding to these rotamers are valid for assignment purposes. In contrast, the anti conformers C and D should not be considered. L R HO N Boc O NHBoc M N H H M LH H Zn O Zn O NH2 N H O amine derivatized with carrier carrier O R N O Zn porphyrin tweezer N N H Zn O O L O M R N Zn O O conjugate/tweezer complex Scheme 4-3. Complex between carrier/monoamine conjugate and porphyrin tweezer. A second requirement of this method is that only the axial substituents in the syn rotamers should be compared to obtain the Δδ SR value. This is because the axial substituents are more exposed to the shielding/deshielding effects of the CDA. For example, after identifying the resonances for the syn rotamer in the spectra of the (S)- and (R)-MTPA amides, the signals due to the axial proton at C-3 (Figure 4-12 A and B) should then be used to obtain the Δδ SR value. Here, the axial methyl group is more shielded in the (S)-MTPA amide (Figure 4-12 A) than in the (R)-MTPA amide (Figure 4-12 B), and the H-3 proton is more shielded in the (R)-MTPA 236 amide (Figure 4-12 B) than in the (S)-MTPA amide (Figure 4-12 A). The resulting Δδ SR values are negative for the methyl at C-2 and positive for the proton at C-3. The assignment of absolute configuration of secondary amines has also been studied using ECCD spectroscopy (Scheme 4-3). Here, like the case of primary amines, the secondary amines are first derivatized with a carrier molecule in order to provide the required two sites of coordination. 14 The requirement, by both the ECCD methodology and Mosher ester analysis, for derivatized of the sample before analysis, as well as the empirical nature of the Mosher ester analysis method is a major limitation for these methods. 4-3-3 ECCD studies of secondary amines using MAPOL We envisioned a similar binding and stereo-differentiation mechanism for secondary amines as that of primary amines. Figure 4-13 shows the proposed binding for (R)benzhydrylpyrrolidine, where in both the P or M helicity of the complex, the large group, (in this case, the methine carbon with the two phenyl groups) is positioned in the least sterically encumbered location. In the P complex (Figure 4-13A), the medium group (chain of ring) is situated in the more sterically tolerable region, while the smallest group (H) occupies the most sterically congested area. On the other hand, the M complex would have a more sterically unfavorable interaction between the medium group and porphyrin. In this way, the more favorable P-helicity would be promoted in binding with the R-enantiomer, leading to a predicted positive ECCD spectrum, while M-helicity would be promoted in binding with the S-enantiomer and would result in a predicted negative ECCD spectrum. 237 A Ph Ph Ph Ph Ph Ph N H H N N HH O O N N H Ph N HN N Ph less favorable due to more steric congestion of the medium group (chain on ring) Ph Ph N N H Ph H O NH M-helicity B HPh O Ph Ph H N N Ph Ph N H H H O O N HN NH N H Ph Ph O O H Ph favored helicity, having the smallest group in the most sterically demanding region P-helicity Figure 4-13. Proposed binding model for (R)-benzhydrylpyrrolidine to MAPOL. In A, the amine binds to the M-helix and in B, the amine binds to P-helix. The complex formed in A is sterically less favorable due to steric congestion between porphyrin and chain of ring. To our delight, upon addition of chiral secondary amines to a solution of MAPOL in hexane, prominent CD signals were observed (Table 4-3). However, as can be seen in Table 4-3, the obtained ECCD signs do not match the predicted signs using the same binding model as primary amines. Additionally, no ECCD is observed with substrate 14R. Presumably, a different mechanism for enantiodiscrimination is at play for secondary amines than that of primary 238 amines, and so the working model used for primary amines cannot be used for secondary amines. However, despite this fact, the results were promising, in that amines with opposite stereochemistry yielded opposite CD signals, meaning that whatever the source of enantiodiscrimination is, it is consistent. Therefore, further studies on a larger substrate scope are necessary to investigate trends for MAPOL with secondary amines. Efforts to this end will continue in order to identify the source of enantiodiscrimination that could be used to derive a predictable mnemonic for secondary amines. Table 4-3. ECCD data of chiral secondary amines bound to MAPOL in hexane. . predicted sign amine -61 422, 113 411, -42 +155 pos 13S 419, -14 412, 47 neg 12R A pos N H λ, nm, (Δε) no ECCD NH 14R N H host:guest ratio – 1:20, 1 µM host concentration at 0°C was used for all measurements. 4-4 Chiral aziridines Chiral aziridines are useful building blocks for rapid access to nitrogen-containing molecules and heterocycles that are important to pharmaceutical and biologically related fields. 239 In the past decade, substantial amount of work has focused on the asymmetric synthesis of chiral aziridines 15 as well as their ring-opening reactions. 15a, 16 Unlike the well developed asymmetric epoxidations of olefin double bond which could afford epoxy alcohols in a straightforward way and provide well-recognized mnemonics facilitating the empirical assignment of chirality for products, there is no direct access to chiral aziridines from olefins and hence no simple mnemonic available for its chirality assignment. Comparing the optical rotations of ring opened products with reported values is also frequently used. The empirical nature of both methods is a major limitation and the latter approach also suffers from extra synthetic work with sensitive molecules. Furthermore, the necessary transformations and derivatizations are inefficient and time consuming. A general non-empirical expedient protocol addressing the absolute stereochemistry of chiral aziridines has not emerged. 240 4-4:1 ECCD Studies of Chiral Aziridines Using MAPOL We wish to be employ MAPOL in the absolute stereochemical determination of aziridines. Table 4-4. ECCD data of chiral aziridines bound to MAPOL in hexane. aziridine 24-trans predicted sign A neg 428, -18 409, +21 -39 neg 431, +135 422, -79 +214 pos 426, +28 418, -18 +46 pos NH 429, -9 420, +21 -30 OTBS NH OTBS 25-trans MAPOL λ, nm, (Δε) NH 26-cis O EtO 27-cis NH O EtO host:guest ratio – 1:20, 2 µM host concentration at 0 °C was used for all measurements. ! The aziridines studies behaved similar to the secondary amines, where, as can be seen in Table 4-4, the obtained ECCD signs do not match the predicted signs using the same binding model as primary amines. Further studies on a larger substrate scope are currently underway to investigate any trends and establish working models for aziridines. 241 To conclude, we have developed a novel host system and successfully employed it for the absolute stereochemical assignment of primary amines. In addition, we have developed a working model that correlates the observed CD signs to the chirality of the bound primary amines. Moreover, we have demonstrated that MAPOL could potentially be used for absolute configurational assignment of secondary amines and aziridines. However, we have shown that the models that apply to primary amines cannot be applied to secondary amines and aziridines since they predict results opposite to those obtained experimentally. Because of the importance of this class of compounds, additional studies are currently on-going to develop a working model for these two important classes of functional groups, as well as establish a wider substrate scope. 242 Experimental procedures The solvents used for CD measurements were purchased from Aldrich and were spectra grade. All reactions were performed in oven or flame dried glassware under nitrogen. Solvents used for synthesis of compounds were dried as follows: THF dried over Sodium, dichloromethane dried over CaH2. Column chromatography was performed using SiliCycle silica gel (230-400 mesh). 1 H-NMR spectra were obtained on Varian Inova 300 MHz, 500 MHz or 600 MHz instruments and are reported in parts per million (ppm) relative to the solvent resonances (δ), with coupling constants (J) in Hertz (Hz). IR studies were performed on a Galaxy series FTIR 3000 instrument (Matteson). CD spectra were recorded on a JASCO J-810 spectropolarimeter, equipped with a temperature controller (Neslab 111) for low temperature studies, and is reported as λ [nm] -1 -1 (Δεmax [mol cm ]). Determination of binding constant The stock solution of Zn-porphyrin tweezer (1 mM in hexane) was titrated with guest molecule (10 mM in DCM) at different equivalents and the UV-vis spectra was recorded after each addition. The addition of chiral substrate was continued until no visible change in the spectra was observed. Upon formation of the chiral complex, the Soret band of the porphyrin tweezers underwent red-shifts through an isosbestic point. The change of absorption at certain wavelength as a function of the substrate concentration yields an exponential saturation curve, which can be fitted to the following non-linear least square equation: !"# = ! ∗ [ !" + !" + 1 − !! !" + !" + 1 2 ∗ !" 243 ! − 4! ! !"!! ! ] Kassoc porphyrin chiral tweezer + guest substrate•guest complex Where: K = calculated Kassoc; Δabs at point of saturation = L; Total concentration of porphyrin tweezer = a; Equivalents of chiral substrate added = x (this is the variable); Assume the concentration of tweezer-substrate formed in equilibrium is A, Then the concentration of free tweezer is (a – A); The concentration of free substrate is (x – A); Since !! = Then !! = Thus ! = !"#$%&' !"#!$%&$' !"##$#% ! !!! !!! !!!!!! ! !!!!!! ! !!∗!∗! ! The amount of complex formed, A, could be converted to the absorbance obtained at different concentrations of substrate. When the porphyrin tweezer is saturated with substrate, the amount of absorbance should be the maximum amount of absorbance possible for porphyrin tweezer with concentration of a. And if we assume the absorbance at saturation is L, then the amount of complex formed should be: != !"#$%"!&'( ∗! ! 244 Combining the last two equations, the following equation relates absorbance to the amount of substrate added: s !" + !" + 1 − !! !" + !" + 1 !"# = ! ∗ [ 2 ∗ !" ! − 4 ∗ !" ∗ !"! ] 4-4 Synthesis of Chiral Amines Amine 8 was synthesized from L-Lysine. All other amines used for the study were obtained from commercial sources and used without further purification. 245 4-5 Determination of Absolute Stereochemistry of Chiral Carboxylic Acids HO2C EtO2C OH CO2H (S)-2-Cyclopentylmethylpropanedioic acid 1-ethyl ester (R)-2-Cyclohexyl-2-hydroxy2-phenylacetic acid CO2Me HO2C HO2C CO2Me (1R,2R)-1,2-Cyclopropanedicarboxylic (R)-Methyl 3-methylglutalate acid monomethyl ester Figure 4-14. Examples of some common building blocks containing a carboxylic acid functionality. Carboxylic acids are commonly found in phamaceuticals and natural products. They play an important role in natural product synthesis and drug development. 17 Some of the different types carboxylic acids found in drugs are salicylic acid, fusidic acid, citric acid, ascorbic acid, benzoic acid and lactic acid. Carboxylic acids are also commonly encountered as intermediates in the synthesis of complex natural products 18 (Figure 4-14). For example, Figure 4-15 shows 19 the final stages of the total synthesis of Swinholide A by Nicolaou. In the synthesis, they made use of the building blocks 15 and 16 in a Yamaguchi esterification to obtain 17 which was then elaborated to the target molecule. 246 Me OMe MeO Me PMP Me O O Me O O Me Me OTBS + OTMS OMe O OH O HO Me Me TBSO OH Me O MeO O O Me Me O PMP OH Me 15 OMe Cl 1. 16 O Cl Cl Cl Et3N, toluene, 25 °C, 2 h; 16, 4-DMAP, 110 °C, 24 h 2. Ba(OH)2, MeOH, 25 °C, 96 h OMe Me PMP Me O O Me O OTBS Me Me OMe O OH HO2C Me Swinholide A OH O OH O Me MeO O Me Me TBSO Me O O Me Me O PMP OMe Figure 4-15. Nicolaou’s use of a carboxylic acid in the total synthesis of swinholide A. 247 Chiral carboxylic acids can be divided into two main categories: α-chiral carboxylic acids and carboxylic acids with remote stereocentres, β- and γ-chiral carboxylic acids, each of which will be discussed in more details. 4-5:1 α-Chiral Carboxylic Acids The most commonly used method for assigning the absolute configuration of α-chiral 20 employing Mosher analysis method (and its 5b, 21 has been used, after derivatizing the acids with a carboxylic acids is NMR spectroscopy, modifications). Also the ECCD approach carrier molecule since the porphyrin tweezer method is not directly applicable to compounds with one site of attachment. S O H Ar R + OH O H HO H L1 L2 Ar L1 L2 O R H L1 R H H L2 L1 Ar Ar R H H shielded groups L2 R Figure 4-16. Derivatization of an α- chiral carboxylic acid for absolute stereochemical assignment using Mosher analysis. In the Mosher ester analysis method, the chiral acids are derivatized to either amides or esters, using a chiral derivatizing agent. (CDA) 22 The Mosher analysis method, being empirical, requires the chiral carboxylic acids to be derivatized using both the (R)- and (S)- enantiomers of the CDA. Figure 4-17 shows some of the more commonly used CDA’s. As can be seen, the CDA’s usually contain an aryl group that directs its anisotropic cone selectively towards one of the substituents of the asymmetric center of the chiral acid. 248 HO OEt H O (R)-9-AHA H2N OMe O (R)-PGME CO2Me H Ph (R)-methyl mandelate HO Figure 4-17. Some common CDA’s: ethyl 2-(9-anthryl)-2hydroxyacetate (5, AHA), phenylglycine methyl ester and methyl mandelate. 1 By comparing the H-NMR of the two diastereomeric derivatives, the shielding effect RS values (Δδ ) for the protons neighboring the chiral center are measured. When using derivatizing agents for the stereochemical determination of α-chiral carboxylic acids, the conformation of the chiral conjugate must be well understood as this could lead to misleading 5b, 21 results. From a practical point, selecting the most suitable reagent for a specific substrate is still problematic and often, the NMR signals of the diastereoisomeric derivatives are too close, requiring extensive conformational analysis to interpret results. In addition, the empirical nature of this method is limiting in cases where the chiral acid is available in limited amounts. To use chiroptical methods, derivatization of the carboxylic acids with a carrier molecule is usually required. Nakanishi and co-workers 21 first introduced an ECCD protocol for determining the absolute stereochemistry of α-chiral carboxylic acids with derivatization of the substrates as N-γ-aminopropyl amides followed by complexation to a Zn-porphyrin tweezer host, 5a and later Mg-porphyrin host as well (Figure 4-18 A). Later, Borhan and co-workers 5b extended this method to work with different carriers (Figure 4-18 B). The choice of carrier plays an 249 important role, as rigidity/flexibility of the carrier affects the number of possible conformations upon binding to zinc. 5 Ph Ph N N Zn Ph Ph N N Ph N N Zn N Ph N L M H NH O O O O + O O O O 1:1 host/guest complex M N NH2 H LH derivatized chiral acid Nakanishi's diaminopropane carrier NH2 O M NH2 Zn Zn O O Zn-TPP porphyrin tweezer LH H N M O N H Zn LH derivatized chiral acid Borhan's p-phenylenediamine carrier O NH2 Zn O O O 1:1 host/guest complex Figure 4-18. Derivatized carboxylic acids with Nakanishi’s and Borhan’s carriers, for use in CD spectroscopy. ! For example, while both carriers above showed consistent ECCD signs with their individual proposed mnemonics, the mnemonics were opposite to each other. This was proposed to be because of the complexes adopting different conformations in solution. Figure 4-19 shows 250 an example of (S)-methyl butyric acid derivatized with both carriers. In Nakanishi’s model, the small group (H) is syn to the amide hydrogen, and the large (Et) and medium (Me) groups project towards the porphyrin plane, resulting in a positive ECCD. In Borhan’s model, the large group (Et) is perpendicular to the carbonyl group, and the medium group (Me) is gauche to the amide hydrogen. The predicted negative ECCD is as a result of the stereodifferentiation between the large and small groups on the chiral centre. H2N O O H N H Borhan's carrier Et negative ECCD RHN Me O H H2N H Me N H Nakanishi's carrier O Et positive ECCD RHN H Figure 4-19. Two carriers yielding different ECCD signals. A recent joint effort from the groups of Canary and Anslyn details a method for 23 determining the absolute configuration of α-chiral carboxylic acids as their carboxylates, without requiring derivatization of the acid. This approach involves complexation of an N X N Cu N X Cu N P N N achiral copper (II) tripodal host shown in N N Figure 4-20, with the carboxylate of the carboxylic acids. The host system consists M Figure 4-20. At equilibrium, P and M helices exist in equal amounts. ! 251 of a tri-dentate tripodal ligand with three coordinating arms (chromophores). The host, being propeller-shaped, exists in the two enantiomeric forms, P and M in solution. The ligands occupy four of the five coordination sites of the copper metal, leaving a vacant site for coordination of substrate. M P H H N N Et Et Ph Ph (R)-PBA (S)-PBA N = = N Figure 4-21. Newman projections for triodal system with each enantiomer of PBA bound. An M-propeller gives (+) chirality for the orientation of the quinolone electronic dipole moments. ! The substrates are introduced into the host system as the carboxylate form, and upon coordination to the tripod, one twist predominates generating a detectable CD signal. The magnitude and shape of the signal allowed for the assignment of absolute configuration of the carboxylic acids using the pattern recognition protocol LDA. 4-5:2 Use of MAPOL for absolute stereochemical determination of α-chiral carboxylic acids Following the successful use of MAPOL in assigning the absolute stereochemistry of primary amines, we thought to extend its application to carboxylic acids. Carboxylic acids 252 cannot directly be assigned using ECCD protocol because they contain only one coordination site. However, as shown in Figure 4-22, the acids can potentially participate in hydrogen bonding with MAPOL. Ph M Ph N N H Ph H N N Ph L M N S Ph O O H HH O O NH N Ph NHN L M S NHN O O H H H N HN un-favored complexation medium group occupies sterically congested region favored complexation small group occupies sterically congested region NHN O O H H H O O NHN R-acid with P-Helix R-acid with M-Helix L S NHN HN L NHN M S NHN O O O O H HH O O S-acid with M-Helix S-acid with P-Helix NHN N HN NHN N HN favored complexation small group occupies sterically congested region un-favored complexation medium group occupies sterically congested region Figure 4-22. Proposed complexation and stereodifferentiation of carboxylic acids with MAPOL. MAPOL. We postulate that upon introduction of a carboxylic acid to MAPOL in a 1:1 complex, the acid will preferentially bind to one helicity of MAPOL via hydrogen bonding as shown in Figure 253 4-22. During the complexation, the largest group at the chiral center will position itself in the least sterically encumbered location, which can be achieved in both the P or M helicities of the complex. Nonetheless, in one helicity, the medium group will be situated in a more sterically tolerable region, and the smallest group will occupy the most sterically congested area. However, in the other helicity, there will be a more sterically unfavorable interaction with the medium group. In this way, the more favorable helicity is promoted in binding with one enantiomer of acid, leading to an over-population of the favored helicity, and ultimately to an observed ECCD spectrum as a result of exciton coupling of the porphyrins due to the chiral twist. Ph NH N N HN Ph Ph N N Zn N N Ph OCH3 OCH3 Ph N Ph N N Zn N N Ph OH OH N H Ph Ph OH OH Ph Ph N H N Ph MAPOL 9 N Ph Ph N Zn N N Ph analogue 44 N Ph N Zn N N Ph Ph Zn-MAPOL 58 Figure 4-23. MAPOL 9, MAPOL analogue 44 and Zn-MAPOL 58. When several carboxylic acids were tested with MAPOL 9 however, they were ECCD silent. It was proposed that this silence was due to either an inability of the complex to form, or ECCD signals that were too weak to be observed, after formation of the complex. Even more surprisingly, when they were tested with Zn-MAPOL 58, all of the chiral acids were ECCD active. It is important to note that the CD signals observed with 58 are not from bis-coordination 254 to both zinc atoms. This is evidenced by a lack of ECCD when analogue 44 was used as a control in place of Zn-MAPOL 58. The difference in complexation of MAPOL 9 to amines and carboxylic acids could be explained using acid-base chemistry. In contrast to amines (pKa ~38) that act as the H-acceptor, the carboxylic acids (pKa ~4 – 5) would be the proton donor for MAPOL. For the amines, the complex formed would yield a protonated amine (pKa ~10) while the carboxylic acids would form protonated phenol (pKa ~ -2). Because of this, it would be expected that the hydrogen bonding between MAPOL and carboxylic acids be weaker than the corresponding hydrogen bonding between MAPOL and amines. This weakened hydrogen bonding could explain the lack of ECCD obtained when carboxylic acids were complexed with MAPOL. Ph N N H Ph H N N M Ph N S Ph O O H H H O O Ph Ph L NHN HN Ph NH N Ph Ph proposed complex of α-carboxylic acid with M-helix of MAPOL L Ph NHN M Ph S NH H H H O O NHN N HN Ph proposed complex of amine with M-helix of MAPOL Figure 4-24. Proposed binding of carboxylic acid and amine to MAPOL via hydrogen bonding. The carboxylic acid would act as the H-donor, while the amine would be Hacceptor. In light of the observations made above with MAPOL and the methylated analogue of MAPOL, we revised the original proposed binding model. Because free base MAPOL did not yield any observable ECCD spectrum with the chiral acids, while Zn-MAPOL did result in 255 ECCD active complexes with all chiral acids, we propose that unlike the amines, the carboxylic acids bind to host via a two-point complexation involving both the zinc metal and biphenol unit as shown in Figure 4-25. There are several advantages to having multi- L Ph N Ph M Ph N Zn N N Ph S Ph O O H H H O O point recognition sites on a porphyrin host. In addition to N N Zn N N the rigid framework of the porphyrin, it is suitable for Ph thermodynamic studies on multipoint molecular recognition because the rotational freedom of the guest Figure 4-25. α-chiral carboxylic acid involved in both metal coordination and hydrogenbonding with M-helix of ZnMAPOL. alone can be isolated for analysis. In fact, evidence for two-point complexation has been well documented in literature. 24 A common strategy in designing porphyrin receptors is to use a central metal as a Lewis acid site in order to bind a Lewis base functional group of the guest molecule. However, porphyrin receptors bearing multi-point recognition sites have been exploited for the recognition of amino acids, nucleotides, saccharides and other functional groups. 24a, b 24b In one of the early studies by Ogoshi and co-workers, they reported a receptor having double recognition sites, the Lewis acid site (zinc), and a Lewis base site (quinolone). In their studies, they investigated the binding of α-amino acid esters to porphyrin hosts with double recognition sites: metal coordination and hydroxyl hydrogen bonding (Figure 4-26). 256 hydrogen-bonding OCH3 R coordination O OH HO OH N N Zn N H NH2 Ka N Zn HO bifunctional metalloporphyrin CH3 O H3C O N 1N Zn N N N N 2 Zn N N Figure 4-26. Three host porphyrins used by Ogoshi investigated the binding of αamino acid esters. 1 lacks the hydrogen bond donor site and host 2 lacks the hydrogen bonding site. In a continuation of this work, the authors looked at the recognition of carbohydrates by zinc porphyrins designed to have double recognition sites. 24a They designed the host with quinoline moieties, as the hydrogen acceptor site (Figure 4-27, β-ethyl groups omitted for clarity). With this in mind, we propose that the carboxylic acids RO O H O O O H O H H would bind to Zn-MAPOL, a bifunctional host, via both ZnN N N N Zn N N metal coordination and hydrogen bonding to the biphenol unit. As shown in Figure 4-28, the carbonyl group is proposed to be involved in metal coordination with the Figure 4-27. Host for carbohydrate recognition. porphyrin, and the acidic proton involved in hydrogen 257 bonding with the biphenol unit, as a proton donor. Assuming that sterics are responsible for chirality recognition, during the complexation, the large group would be positioned in the least sterically encumbered location in both the P or M conformers and stereodifferentiation would be between the medium and the small groups. It is believed that the large group plays no role in stereodiscrimination. Nonetheless, in one helicity of Zn-MAPOL, the medium group of the acid will be situated in a more sterically tolerable region, and the smallest group will occupy the most sterically congested area. However, in the other helicity of MAPOL, the opposite would be true, and there will be a more sterically unfavorable interaction with the medium group. In this way, the carboxylic acid would bind to Zn-MAPOL in the sterically favored helicity. The equilibrium will shift to create an excess of the favored helicity, leading to an observed ECCD spectrum as a result of exciton coupling of the porphyrins arranged in a chiral fashion within the sterically preferred complex. Figure 4-28 shows a proposed enantiodiscimination mechanism for an α-chiral carboxylic acid. In A, if the acid binds with the M-helix of the host, there would be unfavorable steric interactions between the medium group and the host, hence it preferably binds with the P-helix. This places the porphyrins in a clockwise arrangement that would result in a positive CD sign. On the other hand, the enantiomer shown in B preferably binds with the M-helix, avoiding the unfavorable steric interactions that would occur between the medium group and the host in the P-helix of Zn-MAPOL. In this complex, the porphyrins are arranged in a counterclockwise manner, resulting in a predicted negative CD sign. 258 P1 A P2 P1 O M O O M S Zn Zn P1 OH O M S P2 OH S O Zn L O L L P1 Zn M-helicity disfavored P2 predicted positive ECCD P-helicity favored P2 B M P1 Zn O OH L M O O O O M S Zn S P2 P1 S L O Zn Zn P2 O L P1 P-helicity disfavored M-helicity favored predicted negative ECCD Figure 4-28. Proposed binding model for chiral carboxylic acids to Zn-MAPOL. In A, the acid binds to the P-helix and in B, preferably to M-helix, resulting in the predicted positive and negative CD signs respectively for the two enantiomers. ! Based on the above proposal, Zn-MAPOL 44, was examined for the configurational assignment of a variety of α-chiral carboxylic acids via the ECCD protocol. To our delight, chiral acids bearing different functionalities gave consistent and prominent bisignate CD signals at the porphyrin Soret region, upon addition of micromolar concentrations to a solution of ZnMAPOL in hexane. As shown in Table 4-5, the signs obtained for entries 1,2 and 3 were consistent with the predicted signs using this proposed model. For substrates bearing an aromatic ring at the chiral center (entries 4 and 5) this model results in predicted signs that are opposite to what is observed. This is proposed to be due to π-π stacking interactions between the aromatic ring and the porphyrin. Such interaction has previously been observed in similar systems. 5b Looking at substrate 32 (entry 5) the original model above would place the largest group, 259 (phenyl) in the most sterically accessible location, and the medium group (methyl) in the more sterically congested location, leading to a predicted positive ECCD, however, because of π-π stacking interactions between the phenyl group and porphyrin, the large phenyl group will be positioned in the more sterically congested area placing medium methyl group in the more sterically accessible location, leading to a predicted negative ECCD sign which is observed (Figure 4-29A). Likewise, with favorable π-π stacking interactions in 31, the large phenyl group would preferentially occupy the more sterically congested area placing medium methoxy group in the more sterically congested location, leading to a predicted negative ECCD sign which is observed (Figure 4-29B).!! O ! Por H O O CH3 OH O Ph O O Ph Por Por A Ph O CH 20 10 0 -10 -20 -30 proposed model Por H 3 400 with π−π interactions Ph OH Por H O O OCH 3 OCH3 O Ph Ph Por Por O O 250 proposed model Por H O OCH A O 420 440 λ, nm! 50 -150 -350 400 3 with π−π interactions 420 440 λ, nm! Figure 4-29. Proposed binding of substrates with aromatic groups at chiral center. In both, the models involving π-π interactions between the aromatic group and porphyrin result in predicted sign that are in agreement with the obtained results. 260 Table 4-5. ECCD data for α-chiral carboxylic acids bound to Zn-MAPOL in hexane. entry 1 28S H (M)Br 29S 3 30S H Ph 31R -113 425, -19 415, +21 -40 pos 425, +40 416, -34 +74 neg 425, -316 417, +185 neg 426, -24 418, +16 O OH (M) H 425, -68 416, +45 neg OH R Et (L) 4 neg OAc CH3 (L) A Br O H (M)AcO λ, nm, (Δε) O OH CH3 (L) 2 predicted sign carboxylic acid OPMB OCH3 OH (L) -501 O (M) OCH3 O 5 32R Ph H (L) OH Ph CH3 (M) -40 host:guest ratio – 1:20, 2 µM host concentration at 0 °C was used for measurements. ! Figure 4-30 shows the correlation between the chirality of the acid and the sign of ECCD obtained as shown for acid 29. For both the P or M helicity of the complex, the large methyl group is situated in the least sterically demanding location, while the location of the medium acetate and small hydrogen groups is dictated by the configuration at the chiral center as illustrated. In the M complex (Figure 4-30A), the medium acetate group is situated in a more sterically tolerable region, while the smallest hydrogen group occupies the most sterically congested area. 261 P2 A Ph OAc H H3C Ph N N Ph O N N Zn Zn O N N H HH N N Ph O O Ph AcO Zn O O H O CH 3 Zn P1 favored helicity, having the smallest group in the most sterically demanding region M-helicity P1 B Ph AcO H3C N N O Zn N Ph N Zn Ph H Ph O H H H O O N AcO Zn N N N O O H O CH3 Ph Zn P2 P-helicity less favorable due to more steric congestion of the medium group Figure 4-30. Proposed binding model for 29(S) to Zn-MAPOL. In A, the acid binds to the Mhelix and in B, it binds to the P-helix. The complex formed in B is less favorable due to steric congestion between porphyrin and the methyl group. On the other hand, in the P complex the methyl group would be positioned in the more sterically unfavorable region. In this way, the more favorable M-helicity is promoted in binding with the S-enantiomer, leading to the observed negative ECCD spectrum (Table 4-5 entry 2). Figure 4-31 shows the proposed binding of 32 with Zn-MAPOL. Here, the favored conformer would place the medium methyl group in the more sterically open location, and the 262 large phenyl group in the more sterically congested location because of favorable π-π stacking interactions. This would lead to a predicted negative ECCD sign. P2 A Ph Ph H C Ph 3 N N Zn N N Ph H Ph O N Zn O H HH O O N Zn O O H Ph N N O CH 3 Zn P1 favored helicity, having the smallest group in the most sterically demanding region M-helicity P1 B Ph H3C Ph O N N Zn N Ph Zn Ph N H Ph O H H H O O N Ph Zn N N N O O H O CH3 Ph Zn P2 P-helicity less favorable due to more steric congestion of the medium group ! Figure! 4)31.! Proposed binding model for acid 32 to Zn-MAPOL in a way that places the aromatic ring next to porphyrin for π-π interactions in both helices. In A, the acid binds to theM-helix and in B it binds to the P-helix. The complex formed in B is less favorable due to steric congestion between porphyrin and the phenyl group.! 263 4-5:3 Remote chirality sensing: A case study of β- and γ-chiral carboxylic acids As an extension of the α-carboxylic acids, we wished to apply Zn-MAPOL to solve the issue of remote chirality sensing. To our knowledge, there are few reports 25 addressing the stereochemical determination of substrates bearing β-stereocenters. Conventionally, the absolute configuration of remote stereocenters has been assigned by 1 H-NMR as well as CD spectroscopy. OCH3 H3C NH2 H3C NH2 H3C NH2 H3C NH2 Figure 4-32. 1-arylethylamines used by Hoye as reagents. 25-26 Hoye and co-workers, report the determination of the absolute stereochemistry of β- 1 chiral carboxylic acids by H-NMR spectroscopy. Their approach calls for derivatization of the acids with chiral benzylic amines (Figure 4-32). Analysis of the signs of the chemical shift differences of substituent protons allowed for the determination of absolute configuration. They use the signals of the methyl groups of the amides to assess the effectiveness of these systems as reagents. The authors differentiate two diastereochemical possibilities (syn and anti diastereomers) according to the relative orientation of the benzylic methyl group and the methyl substituent at the C-3, and calculate Δδ (defined as the difference between the chemical shifts in the syn and anti diastereomers) for the β-methyl and other groups. Using this method, the β264 methyl groups are always more deshielded in the syn series than in the anti series, whereas protons in the other β-substituent are more shielded in the syn isomers. (CH3) R1 CH3 O N H R2 (CH3) R1 R2 CH3 O N H syn diastereomer H CH3 N H R2 anti diastereomer (CH3) R1 O R1 is shielded (CH3) R1 R2 O H CH3 N H neither R1 nor R2 is shielded R2 O (CH3)R1 H CH3 N H neither R1 nor R2 is shielded R2 (CH3)R1 O H CH3 N H R2 is shielded Figure 4-33. Conformational model for the determination of the sign of Δδ = δ(syn) - δ(anti) of protons in R1 an R2. They proposed a working model shown in Figure 4-33 based on NMR results and molecular calculations. This model is based on two assumptions: first, the the predominant rotamer around the bond between the benzylic proton is eclipsed with the carbonyl group, and second, one of the two non-hydrogen substituents at C-3 occupies the anti position, relative to the carbonyl group. In the case of anti diastereomer, R1 is preferentially shielded, while in the syn diastereomer, R2 is shielded. Consequently, the Δδ values for protons in R1 are positive and those in R2 are negative. In an extension of this work, the same authors extend the scope of the 25 methodology to include carboxylic acids with substituents other than a methyl group at C-3. 265 The methods employing ECCD utilize substrates that carry either an aromatic moiety 27 or contain a group that is potentially derivatizable by the chromophore; a hydroxyl or amino functionality 28 at the stereocenter, thus reducing the problem to determining the stereochemistry adjacent to the chromophoric site stereocenter, i.e. α-chirality. Determining β-stereocenter in the absence of chromophoric/derivatizable sites or sensing remote chirality in general remains a challenging task, where restricting conformational freedom of distal stereocenters is one of the difficulties to overcome. Our group recently introduced a bulky porphyrin tweezer (Zn-TBP) shown in Figure 434, for the assignment of the absolute stereochemistry of a wide variety of β- and γ-chiral carboxylic acids by ECCD protocol. 44 In order to achieve this, the chiral acids were first derivatized with p-phenylenediamine carrier in order to introduce the requisite two binding sites. H2N N N Zn N N N H N N Zn N N O O O O n ArHNOC H (derivatized carboxylic acids) n = 1: β-carboxylic acids n = 2: γ-carboxylic acids O TBP porphyrin tweezer Figure 4-34. TBP porphyrin tweezer used for remote chirality sensing. 266 H iPr H CH3 major ECCD active conformation Here, the t-butyl substituents on the tweezer provide a better coverage of the tweezer’s binding cavity leading to enhanced steric interaction. 4-5:4 Use of Zn-MAPOL for absolute stereochemical determination of β-chiral carboxylic acids Similar to the α-chiral acids, the β- and γ-chiral carboxylic acids were ECCD silent with MAPOL, however, all acids were ECCD active with Zn-MAPOL. Because of this, it is postulated that they follow a similar binding model and stereodifferentiation as the α-chiral carboxylic acids as shown in Figure 4-35, where the acids coordinate to Zn-MAPOL via a twopoint coordination. P1 Zn A L S S O OH M O O M (R) Por O S Por P2 P2 O O L L P1 P1 H O M Zn P2 M-helicity disfavored predicted positive ECCD P-helicity favored P2 B P1 Por M L O S O O S OH M (S) L O O M O Zn Por P2 Zn S P1 O L P1 P-helicity disfavored P2 M-helicity favored predicted negative ECCD Figure 4-35. Proposed binding model for β-chiral carboxylic acids with Zn-MAPOL. In A, the (R)-acid preferable binds to the M-helix and in B, preferably to the P-helix, resulting in the predicted negative and positive CD signs respectively. 267 It is believed that upon complexation, the large group would be positioned in the least sterically crowded location in both the P or M conformers, hence it is believed that this group does not take part in the stereodiscrimination process. In one helicity of Zn-MAPOL, the medium group of the acid will be situated in a more sterically tolerable region, and the smallest group will occupy the most sterically congested area. However, in the other helicity, the opposite would be true, and there will be a more sterically unfavorable interaction with the medium group. In this way, the chiral acid would bind to Zn-MAPOL in the sterically favored helicity. The equilibrium will shift to create an excess of the favored helicity. As a result of the chiral twist between the biphenyl unit, the two porphyrins will be oriented in a chiral fashion. Coupling of the porphyrins’ electric dipole transition moments leads to an observed ECCD spectrum. P1 A P1 Zn M S O S H O OM P2 OH L O CH2 L unfavored rotomer, having large group eclipsing with adjacent methylene and 3 carbons from anti carbonyl group B predicted positive ECCD Zn P2 P1 Zn L M S O O P1 L H O P2 W OH S M CH2 favored rotomer, having large group anti to adjacent methylene and 3 carbons from ecllipsing carbonyl group hile Zn P2 predicted positive ECCD it is possible to have bond rotation at the C2 – Figure 4-36. Rotomers around the C-C bond. C3 bond, the rotomer in Figure 4-36A ! 268 places the large group in an eclipsing conformation with the methylene that is adjacent. In B, the large group is placed in an eclipsing conformation to the carbonyl group that is three carbons away and is anti to the adjacent methylene. In either case, the expected sign is the same. i.e. positive in this case. However, it is proposed that B is the favored rotomer due to less sterics between the large group on C3 and C2 methylene. The same would hold true for the γ-chiral carboxylic acids as shown in Figure (4-37). A P1 P1 Zn M H CH2 OM S OH L S O P2 O H2C O L predicted positive ECCD Zn unfavored rotomer, having large group eclipsing with the adjacent methylene group B P2 P1 L Zn M OH H2C S O M P1 L H O O CH2 P2 S Zn favored rotomer, having large group anti to adjacent methylene group P2 predicted positive ECCD Figure 4-37. Possible rotomers of γ-chiral carboxylic acids. Zn-MAPOL 44, was examined for the configurational assignment of a variety of β- and γ-chiral carboxylic acids via the ECCD protocol. All the chiral acids tested gave consistent and 269 prominent bisignate CD signals at the porphyrin Soret region, upon addition of micromolar concentrations to a solution of Zn-MAPOL in hexane. As shown in Table 4-6, the signs obtained were consistent with the predicted signs. Figure 4-38, shows the correlation between the substrate chirality and the sign of ECCD obtained as shown for β-carboxylic acid 39. We propose a similar binding model for β- and γcarboxylic acids as that for α-carboxylic acids, invoking the two-point coordination. In the M complex (Figure 4-38A), the medium group (CH3) is situated in a more sterically tolerable region, while the smallest group (H) occupies the most sterically congested area. On the other hand, the P complex would have a more sterically unfavorable interaction between the medium (CH3) group and porphyrin. In this way, the more favorable M-helicity is promoted in binding with the R-enantiomer, leading to the observed negative ECCD spectrum (Table 4-6 entry 3). 270 Table 4-6. ECCD data of β- and γ-chiral carboxylic acids bound to Zn-MAPOL in hex. entry predicted sign λ, nm, (Δε) A neg carboxylic acid 428, -62 418, +55 -117 O 1 37R BnO OH O O 2 38R neg Ph 428, -28 418, +11 -39 neg 425, -53 417, +30 -83 pos 426, +20 420, -12 +32 neg 429, -29 419, +26 -55 neg OH 424, -18 418, +13 -31 pos 425, +12 418, -15 +27 OBn OCH3 O 3 39R OH O 4 40R OBn HO CH3 OCH3 5 41R OH Ph O OBn 6 OH 42S O 7 43R O O OH host:guest ratio – 1:20, 2 µM host concentration at 0 °C was used for measurements. 271 A Ph Ph CH3 N Ph H Ph O N Zn N N Zn O H Ph O H H H O O N N N Zn O CH Zn P2 3 N P1 favored helicity, having the smallest group in the most sterically demanding region M-helicity B P1 Ph N N Zn Ph N Ph N CH3 H Ph O O H H H O O Ph Zn N O Zn N O H N N Ph O CH3 Zn P2 P-helicity less favorable due to more steric congestion of the medium group Figure 4-38. Proposed binding model for β-carboxylic acid to Zn-MAPOL by a two-point coordination to zinc and biphenol unit. ! In summary, we have demonstrated the prompt and simple method for the assignment of absolute configuration for chiral β- and γ- chiral carboxylic acids. We propose a two-point coordination model utilizing metal coordination to the porphyrin as well as hydrogen bonding with Zn-MAPOL. 272 4-6 Chiral cyanohydrins NPhth OH O R R CN R CN CONH2 OH OH R R COOH O OH NH2 R OH OH R R' CN O OR' R H R OSO2R' N R OH R CN F N3 NH2 R R CN CN Scheme 4-4. Possible transformations of cyanohydrins. Chiral cyanohydrins are a common functionality in natural products and are also versatile building blocks in organic synthesis. This is mainly because they are easily synthesized by the addition of cyanide to prochiral carbonyls (aldehydes and ketones) in the presence of chiral catalysts or by enzymatic methods, which can easily be transformed into a range of functional groups (Scheme 4-4). 273 The more valuable cyanohydrins derived from ketones (ketocyanohydrins) have been 9 used as intermediates in the preparation of compounds with a quaternary center. In fact, a literature search returns an overwhelming amount of publications that deal with either the 10 synthesis or use of chiral cyanohydrins in the total synthesis of natural products. Because of their importance, there is a need for an easy, simple to use, and efficient way for the reliable assignment of the absolute stereochemistry of cyanohydrins. 4-6-1 Conventional Methods of Assigning Absolute Stereochemistry of cyanohydrins To date, very few attempts are described in literature towards the determination of absolute stereochemistry of chiral cyanohydrins, all of which rely on NMR spectroscopy, in particular Mosher ester analysis method. 29 The first method applying the use of the Mosher ester analysis method towards chiral cyanohydrins was reported by Riguera’s group. 29c Here, they apply their method to aldocyanohydrins. In their report, they derivatize the cyanohydrins as both the enantiomers of 1 (R)- and (S)-methoxy-2-phenylacetic acid (MPA) and compare both the H and spectra of the corresponding cyanohydrin MPA ester derivatives. 13 13 C-NMR C-NMR is necessary because 1 the nitrile group lacks protons and so cannot be analysed by H-NMR. In this way, they found that when the (S)-MPA was used, the substrate H’s were more shielded than the same H’s in the RS (R)-MPA ester, leading to a positive difference in chemical shifts (Δδ ). For the nitrile group, the CN carbon is more shielded in the (R)-MPA ester derivative than in the (S)-MPA ester RS derivative leading to a negative sign for the difference in chemical shifts (Δδ ) (Figure 4-39). 274 A O MeO B H L O ΔδRS > 0 C (1H-NMR) O MeO O CN H H H (R)-MPA L = iPr L CN H RO L CN ΔδRS < 0 (13C-NMR) (S)-MPA Figure 4-39. NMR representative conformer of A) the (R)-MPA ester of an aldehyde cyanohydrin and B) the (S)-MPA ester. Shielding effect is shown by a curved arrow. C) Model placing in space the L and CN substituents of RS the chiral center according to their Δδ signs. The same authors extended this methodology to the ketone cyanohydrins 29a, 29d and using the same principles, described a method for the determination of absolute stereochemistry of ketone cyanohydrins (Figure 4-40). However while this method worked reliably for most all of their reported aldehyde cyanohydrins, for the ketone cyanohydrins, the α-aryl substituted cyanohydrins showed anomalies from the described model of analysis of the chemical shifts, requiring different sets of protons to be considered for their assignment. A third method of assigning A MeO CN O O H L1 ΔδRS < 0 (R)-MPA derivative CN O B L2 MeO H O L2 absolute stereochemistry of cyanohydrins was reported by L1 ΔδRS > 0 Bharatam group. 30 In this (S)-MPA derivative approach, optically active mandelic Figure 4-40. Riguera’s assignment of keto cyanohydrins 1 using H-NMR analysis. acid in the presence of DMAP is used to create a chiral solvating 275 agent (CSA) for the determination of absolute configuration of cyanohydrins. It should be noted that the magnitude of resolution for ketone cyanohydrins was less compared to aldehyde cyanohydrins but sufficient for the determination of optical purity of these compounds, with resolutions in the order of 0.055-0.112 ppm (16.5-33.6 Hz) for aldo-cyanohydrins and 0.0160.031 ppm (4.8-9.3 Hz) for keto-cyanohydrins on 300 MHz NMR. A B O H H O H O H N O O H H H R C N O H N O H O N R H C N N Figure 4-41. Proposed model showing ternary complexes of A) (R)- and B) (S)-cyanohydrin with (S)-mandelate-DMAPH+ ion pair. The H in complex A and the R group in complex B of cyanohydrin are shielded by the phenyl group of mandelate. ! 1 In this method, the H-NMR spectrum of racemic cyanohydrins in the presence of chiral mandelic acid and DMAP are recorded in CDCl3. Taking mandelonitrile as an example, they first 1 take H-NMR of racemic mandelonitrile and both α-protons of the enantiomers of racemic mandelonitrile appear as well resolved singlets. Next the 1 H-NMR spectrum of chiral mandelonitrile was recorded with either (R) or (S)- mandelic acid to determine whether there is any correlation between their absolute configuration and the NMR chemical shift. 276 Cyanohydrins of R- configuration, showed a positive Δδ of S- configuration showed negative Δδ RS RS value, whereas cyanohydrins RS sign is characteristic for this values. Thus, the Δδ enantiomeric series and could be used for the assignment of absolute configuration. However, pyridine compounds could not be assigned using this protocol, since the basic nitrogen on pyridine alters the nature of complex formation. Therefore, while these two methods have been developed for the determination of absolute stereochemistry of chiral cyanohydrins, we see that there is need for an improved (reliable), easy to use and fast way that does not require derivatization of the cyanohydrins. CN OH 12" CD 8" 10 equiv. 4" 0" -4" -8" 370" 390" 410" 430" 450" Wavelength, nm Figure 4-42. CD spectrum obtained for (R)-mandelonitrile with TPFP tweezer. ! To our knowledge, there is no literature precedence on assigning absolute stereochemistry of cyanohydrins using the ECCD protocol. With the development and successful use of TPFP tweezer with hydroxy ketones and sulfoxides discussed in Chapter 2, we thought to extend the ECCD methodology, using fluorinated TPFP tweezer to cyanohydrins. However, 277 when cyanohydrins were tested with TPFP tweezers of varying lengths (C-3 to C-7) they were CD silent. One substrate, (R)-mandelonitrile, gave a positive result with the long alkyl C-8 tweezer (Figure 4-42). Disappointingly however, none of the other substrates were ECCD active with this tweezer. It is unclear why this is the case. We wished to apply MAPOL in determining the absolute stereochemistry of cyanohydrins. 4-6-2 Use of MAPOL for absolute stereochemical determination of cyanohydrins The UV-vis titration of mandelonitrile with MAPOL and Zn-MAPOL is shown in Figure 4-44. While the changes are small, for Zn-MAPOL, there seems to be more of an interaction with the porphyrin than with free-base MAPOL. While studies are currently ongoing in order to propose a working model, preliminary results with a number of cyanohydrins however, produced inconsistent results upon binding with host MAPOL. As shown in Table 4-7 entries 1 and 2, no ECCD signals were obtained when chiral cyanohydrins were complexed with MAPOL. In changing the host to Zn-MAPOL however, all four cyanohydrins of same absolute stereochemistry resulted in positive ECCD. This led us to believe that the cyanohydrins could be complexing with Zn-MAPOL in a dual fashion, in a similar way to carboxylic acids. In the case of cyanohydrins, there are two binding possibilities. One where the hydroxyl group is involved in metal coordination, and the other possibility where the nitrile group is involved in metal coordination. Additional studies including molecular modeling, NMR and IR spectroscopy will be done in order to investigate which of these two possibilities is preferred. 278 Table 4-7. ECCD data of cyanohydrins bound to Zn-MAPOL in hexane. entry Zn-MAPOL λ, nm, (Δε) A predicted sign cyanohydrin MAPOL A λ, nm, (Δε) CN pos OH 1 419, +40 412, -20 +60 pos 419, +14 412, -13 +27 pos 419, +90 412, -21 +101 417, +72 408, -26 +98 pos 419, +14 412, -11 +25 418, +2 406, -0.5 -2.5 - - - - CN OH 2 H3C CN OH 3 Cl CN OH 4 H3CO ! host:guest ratio – 1:20, 2 µM host concentration at 0 °C was used for all measurements. All tested cyanohydrins were (R)-chirality and all resulted in a positive CD signal. Granted, the tested cyanohydrins represent a limited substrate scope (all being derivatives of mandelic acid), however, this is the first example of cyanohydrins succumbing to analysis via ECCD, without the requirement of derivatization. A larger substrate scope, including alkyl cyanohydrins as well as cyanohydrins derived from ketones, needs to be investigated for a binding model to be proposed for this important class of molecules. 4-7 Synthesis of Chiral Cyanohydrins All cyanohydrins used for the study were obtained from commercial sources and used without further purification. 279 4-8 Determination of Absolute Stereochemistry for Chiral Alcohols 4-8-1 Background Enantiomerically pure alcohols are valuable chiral building blocks for synthetic chemists. They form a versatile class of chiral synthons, since they can be incorporated into the biologically active, or natural product structures directly as esters or ethers. For example, the compounds act as key intermediates in the production of pharmaceuticals, fine chemicals and natural products. Examples of pharmaceuticals with chiral alcohols as intermediates are antihypertensive drugs, calcium and potassium channel blocking drugs, antiarrhythmic agents, β3-receptor agonists, anticholesterol and antiviral drugs. They are important starting materials for the formation of amines, amides, thiols and thioethers. In addition, after transforming the hydroxyl function into a leaving group by way of mesylation, tosylation or triflation, they can be used to form new C–C bonds. 4-8-2 Conventional Methods of Assigning Absolute Stereochemistry of chiral alcohols Like chiral mono amines, chiral alcohols cannot be directly applied in the conventional chiroptical methods used for the absolute configurational assignment of chiral diols using porphyrin tweezers because they contain only one site of attachment (one hydroxyl group) and so they can only coordinate to one metalloporphyrin. Traditionally, the absolute stereochemistry of alcohols has been assigned by using Mosher ester analysis, 7-8, 41 or by using ECCD protocol. 42 The use Mosher ester analysis for the determination of absolute configuration of alcohols was first reported in 1973 by Dale and 7 Mosher. Figure 4-45 shows the use of α-methoxy-α-trifluoromethyl-phenylacetic acid (MTPA). 280 This method involves the coupling of alcohol of interest with each enantiomer of MTPA acid. Alternatively, the acid chlorides can be used. H O HO2C CF3 H Ph OMe CF3 R2 O R2 OH R1 CF3 R1 Ph OMe ClOC R-MTPA ester HO2C CF3 MeO Ph ClOC CF3 MeO Ph Ph OMe H O CF3 R2 O R1 MeO Ph S-MTPA ester Figure 4-45 Derivatization of a chiral alcohol with R- and S-MTPA acid (and chlorides) for Mosher ester analysis. However, caution is needed when using the acid chlorides, because when the MTPA acid gets converted to MTPA acid chloride, there is a switch in the priorities of the groups. In the acid, CF3 >COOH, however, in the MTPA acid chloride, CF3 99 %; HNMR (CDCl3, 300 MHz): δ 0.80 (d, 3H, J = 6.6), 0.94 (d, 6H, J = 6.9), 1.89-2.01 (m, 1H), 2.302.44 (m, 2H), 3.78-3.86 (m, 1H), 4.75 (p, 1H, J = 6.6), 4.89-5.02 (m, 2H), 5.67-5.81 (m, 1H), 7.22-7.39 (m, 5H); 13 C-NMR (CDCl3, 75 MHz): δ 14.4, 19.2, 20.8, 30.2, 33.8, 48.3, 54.9, 78.5, + 116.7, 125.6, 128.6, 133.3, 135.4, 152.8, 175.5; HRMS (EI+) [M+H] : calc’d 302.1756, obs’d 302.1761 (R)-benzyl 2-isopropylpent-4-enoate (38): [α]D 20 = -8.6 (c, 2.1 in DCM); configuration (R); Rf 1 = 0.24 (2 % EtOAc in hexane); H-NMR (CDCl3, 300 MHz): δ 0.90 (d, 3H, J = 6.6), 0.93 (d, 3H, J = 6.6), 1.87 (m, 1H, J = 6.6), 2.24-2.38 (m, 3H), 4.94-5.10 (dd, J1 = 17.2, J2 = 20.7), 5.015.79 (m, 1H), 7.28-7.35 (m, 5H); 13 C-NMR (CDCl3, 75 MHz): δ 20.1, 20.3, 30.2, 33.9, 52.4, 65.8, 116.4, 128.0, 128.2. 128.4, 135.8, 136.1, 174.7; MS (DCI) m/z = 232 289 (R)-benzyl 2-isopropyl-4-oxobutanoate (39): configuration (R); Rf = 0.24 (2 % EtOAc in 1 hexane); H-NMR (CDCl3, 500 MHz): δ 13 C-NMR (CDCl3, 125 MHz): δ 22.4, 23.3, 33.1, 43.1, 48.6, 70.6, 73.1, 75.6, 127.3, 127.5, 128.2, 138.2, 179.1, 202.4; (R)-3-(benzyloxycarbonyl)-4-methylpentanoic acid (40): [α]D 20 1 = -8.0 (c, 0.78 in DCM); H- NMR (CDCl3, 500 MHz): δ 0.89 (d, 3H, J = 7), 0.91 (d, 3H, J = 7), 2.00-2.03 (m, 1H), 2.42-2.48 (m, 1H), 2.75-2.82 (m, 2H), 5.12 (AB q, 2H, J1 = 12, J2 = 3.5), 7.28-7.34 (m, 5H), 8.68 (s, br, 1H); 13 C-NMR (CDCl3, 125 MHz): δ 19.4, 20.0, 30.0, 32.7, 47.2, 66.3, 128.1, 128.4, 132.1, 135.9, 174.1, 177.7; MS (DCI) m/z = 250 33 (2R, 3S)-(3-Phenyl-oxiranyl)-methanol (49): 4Å molecular sieves (6 g) were placed into a one-necked 500 mL round bottom flask and heated on heating mantel at 300 °C under reduced pressure for 10 h. The flask was then cooled under nitrogen atmosphere and dry dichloromethane (200 mL) was added, stirred for 5 min and cooled to –23 °C. To the cold i solution was added L-(+)-diisopropyl tartrate (1.5 mL, 5.6 mmol) followed by Ti(O Pr)4 (1.5 mL, 5.2 mmol). The catalyst was allowed to age for 30 min and t-BuOOH solution in toluene (3.3M, 42 mL) was then added via syringe and the mixture was allowed to stir for another 30 min. Cinnamyl alcohol (3.9 g, 29.1 mmol) was dissolved in dry dichloromethane (20 mL) and slowly added to the main solution over 1 h. The reaction mixture was stirred at –23 °C for 7 h, 290 warmed to –12 °C and quenched with 40% solution of NaOH in brine (16 g NaOH, 40 ml H2O and 2 g NaCl) (40 mL). The cooling bath was removed and after 20 min MgSO4 was added and stirring was continued for 10 min. The reaction mixture was allowed to warm to room temperature and filtered through pad of Celite. The filtrate was dried over Na2SO4 and concentrated under reduced pressure. The crude mixture was purified by column chromatography (Silica gel) using 5% EtOAc in hexane solution as eluent to give epoxyalcohol 49 in 94% (5 g) yield. [α]D 33 20 = -48 (c, 1.0 in CHCl3) [literature value, [α]D 20 = -49 (c, 0.01 in 1 CH2Cl2) ]; H-NMR (CDCl3, 300 MHz): δ 2.17 (dd, 1H, J1 = 5, J2 = 6), 3.26-3.28 (m, 1H), 3.79-3.88 (ddd, 1H, J1 =4, J2 = 8, J3 = 12), 3.97 (d, 1H, J = 2.1), 4.05-4.12 (ddd, J1 = 2, J2 = 5, J3 = 13), 7.26-7.42 (m, 5H); 13 C-NMR (CDCl3, 75 MHz): δ 55.6, 61.2, 62.5, 125.6, 128.2, 128.3, 136.6. 34 ((2S, 3S)-3-phenyloxiran-2-yl)methanol (50): To dry THF (100 mL) into a 250 mL flame dried one necked flask equipped with magnetic stir bar and cooled on ice bath was added NaH (0.48 g, 20 mmol, prior to use, NaH was washed with pentane and dried over nitrogen)), followed by tetrabutylammonium iodide (0.5 g, 1.33 mmol) and BnBr (2.4 g, 14 mmol). To this solution a solution of epoxyalcohol 49 (2 g, 13.3 mmol) in THF (10 mL) was added dropwise. After addition was complete the cooling bath was removed and the mixture was stirred at room temperature overnight (8 h). The reaction was then cooled to 0 °C and quenched with water (10 mL). The mixture was stirred for 5 min and dichloromethane (50 mM) was added. The layers 291 were separated and the aqueous phase was extracted with dichloromethane (2 x 20 mL). The organic extracts were combined, washed with brine (50 mL) and water (50 mL), and dried over NaSO4. The mixture was concentrated under reduced pressure on a rotary evaporator (25 °C, 200 mmHg), and the crude mixture purified by column chromatography (Silica Gel, 2% EtOAc in hexane, Rf = 0.35) to give pure 50 in 95% (3 g) yield. [α]D [literature value [α]D 20 35 20 = −38.9 (c 0.82, CHCl3) 1 = −38.9 (c, 0.82 in CHCl3) ]; H-NMR (CDCl3, 300 MHz): δ 3.24-3.27 (m, 1H), 3.61-3.66 (dd, 1H, J1 = 5.1, J2 = 11.4), 3.80-3.89 (ddd, 1H, J1 = 2.1, J2 = 3.3, J3 = 13.8), 4.64 (d, 2H, J = 2.7), 7.26-7.38 (m, 10 H); 13 C-NMR (CDCl3, 75 MHz): δ 55.8, 61.1, 69.8, 73.3, 125.6, 127.7, 127.8, 128.1, 128.3, 128.4, 136.8, 137.8; HRMS (EI+): C16H16NaO2 (M+Na+): 263.1048; found: 263.1048 35 (2R, 3R)-1-(benzyloxy)-3-phenylhex-5-en-2-ol (51): Epoxide 50 (3 g, 15.6 mmol) was placed into a one-neck round bottom flask (flame dried and cooled under nitrogen), dissolved in dry dichloromethane (100 mL) and cooled with ice bath. To the cold solution was added 2 M solution of allylmagnesium chloride (12 mL, 23.4 mmol) dropwise under nitrogen atmosphere. After compete addition ice bath was removed and reaction mixture was allowed to stir at room temperature for 10 h (overnight). The reaction flask was then cooled on an ice bath and quenched with aqueous saturated solution of NH4Cl (100 mL). After 10 min of stirring organic layer was separated and aqueous phase was extracted with dichloromethane (3 x 25 mL). The organic extracts were combined, dried over Na2SO4 and concentrated on a rotary evaporator. Column 292 chromatography with 5% EtOAc in hexane afforded the alcohol 51 (Rf = 0.25) in 72% (2.6 g) yield. [α]D 20 1 = -6.9 (c, 0.92 in CH2Cl2), H-NMR (CDCl3, 500 MHz): δ 2.19 (s, 1H (OH)), 2.50-2.56 (m, 1H), 2.60-2.66 (m, 1H), 2.87-2.91 (m, 1H), 3.30-3.32 (dd, 1H, J1 = 9.5, J2 = 9.3), 3.49-3.51 (dd, 1H, J1 = 4, J2 = 5.5), 4.09-4.12 (m, 1H), 4.52 (s, 2H), 4.97-5.07 (m, 2H), 5.66-5,75 (m, 1H), 7.26-7.40 (m, 10 H); 13 C-NMR (CDCl3, 125 MHz): δ 36.3, 48.0, 72.2, 72.8, 73.3, 116.3, 126.6, 127.7, 128.2, 128.4, 128.9, 136.5, 137.9, 140.5; HRMS (FAB) C19H23O2 (M+H+): calc’d 283.1698, obs’d 283.1690. ((2R, 3R)-1-(benzyloxy)-2-methoxyhex-5-en-3-yl)benzene (44): Alcohol 51 (2 g, 8.5 mmol) was placed into a round bottom flask (100 mL, flame dried and cooled under nitrogen atmosphere), dissolved in dry THF (60 mL) and cooled on ice bath. To this solution NaH (0.3 g, 12.2 mmol, NaH was washed with pentane and dried over nitrogen) was added, followed by iodomethane (2.4 g, 12.7 mmol). After addition was complete the cooling bath was removed and the mixture was stirred at room temperature overnight (8 h). Reaction was then cooled to 0 °C and quenched with water (10 mL). The mixture was stirred for 5 min and dissolved with dichloromethane (50 mL). Layers were separated with separatory funnel and the aqueous phase was extracted with dichloromethane (2 x 20 ml). Organic extracts were combined, washed with brine (50 mL) and water (50 mL), and dried over NaSO4. Mixture was concentrated under reduced pressure on a rotary evaporator (25 °C, 200 mmHg). The crude mixture was purified by column chromatography (Silica gel, 2% EtOAc in hexane, Rf = 0.32) to give an olefin 52 in 98% 293 (2.05 g) yield. [α]D 20 1 = -5.0 (c, 1.23 in CH2Cl2), H-NMR (CDCl3, 500 MHz): δ 2.52-2.58 (m, 1H), 2.61-2.67 (m, 1H), 2.98-3.02 (m, 1H), 3.33-3.36 (m, 1H), 3.44-3.47 (m, 1H), 3.48 (s, 3H), 3.62-3.66 (q, 1H, J = 4.5), 4.49 (s, 2H), 4.5-76 (d, 1H, J = 10.5), 5.07 (d, 1H, J = 17), 5.70, 5.78 (m, 1H), 7.22-7.40 (m, 10 H); 13 C-NMR (CDCl3, 125 MHz): δ 36.4, 47.4, 59.1, 70.8, 73.3, 82.1, 116.1, 126.3, 127.5, 126.6, 127.9, 128.2, 129.2, 136.9, 138.2, 141.0 (3R, 4R)-5-(Benzyloxy)-4-methoxy-3-phenylpentanoic acid (31): Olefin 52 (1.5 g, 0.5 mmol) was placed in a 100 ml one-neck round bottom flask and dissolved in dry CH2Cl2 (50 mL). The solution was cooled to –78 °C on dry ice-acetone bath and ozone was purged through the solution via glass pipette until the appearance of intense blue color in the reaction solution. The ozone source was then removed and the reaction mixture was flushed with nitrogen until clear colorless solution was observed. To the obtained ozonide at –78 °C was added 50% solution of H2O2 (5 mL) and mixture was allowed to warm and stirred for 2 h at room temperature. The reaction mixture was concentrated and submitted to the column chromatography (Silica, 10% EtOAc in Hexanes) to provide compound (31) 67% yield. [α]D 20 1 = +2.2 (c, 0.98 in CH2Cl2), H- NMR (CDCl3, 300 MHz): δ 2.79-3.01 (d of AB quartet, 2H, J1 = 6, J2 = 34.8, J3 = 65.4), 3.303.46 (d of AB quartet, 2H, J1 = 5.4, J2 = 15, J3 = 47.4), 3.5 (s, 3H), 3.65-3.69 (q, 1H, J = 5.4), 4.48 (s, 2H), 7.26-7.40 (m, 10H); 13 C-NMR (CDCl3, 75 MHz): δ 36.19, 43.14, 59.09, 70.28, 294 73.20, 81.92, 126.56, 127.52, 127.56, 127.59, 127.71, 128.07, 128.26, 128.32, 128.54, 128.74, 129.51, 137.89, 139.49 γ-Chiral carboxylic acids: (S)-Methyl 2-(benzyloxy)propanoate (53): 36 To the solution of (S)-ethyl lactate (5 g, 42 mmol) in dry dichloromethane (150 mL) was added benzylbromide (7.9 g, 45 mmol) and silver oxide (10 g, 43 mmol). The reaction mixture was stirred at room temperature for 24 h and filtered through a pad of Celite. The filtrate was concentrated and the obtained crude oil was purified by column chromatography (eluted with 2% EtOAc in hexane) to afford compound 53 in 64% yield (5.6 g): [α]D 20 = -72.7 (c, 6.6 in CHCl3) [literature value [α] D 20 = -74.5 (c, 2.92 in CHCl3), ee% 36 1 = 99] ; H-NMR (CDCl3, 300 MHz): δ 1.32 (t, 3H, J = 7.6), 1.44 (d, 3H, J = 6.9), 4.05 (q, 1H, J = 7), 4.22 (q, 2H, J = 7), 4 57 (AB quartet, 2H, J1 = 11.7, J2 = 49.2), 7.25-7.33 (m, 5H); 13 C- NMR (CDCl3, 75 MHz): δ 14.2, 19.5, 60.6, 72.2, 74.0, 127.8, 128.0, 128.5. (S)-2-(Benzyloxy)propan-1-ol (54): 37 Ester 53 (2.5 g, 12 mmol) was dissolved in dry THF (100 mL) and cooled on ice bath. To this solution LAH (0.5 g, 13 mmol) was added portionwise and reaction mixture stirred at room temperature for 4 h. Cooled on ice bath reaction mixture was then quenched slowly with water (30 ml), diluted with EtOAc (50 mL) and stirred for 30 min. The layers were separated, the aqueous layer was extracted with EtOAc (20 mL) and the combined organic extracts were dried and concentrated. Purification of the crude mixture with 295 column chromatography (Silica Gel, 10% EtOAc in hexane) afforded the alcohol 54 in 95% yield (1.9 g). [α]D 37 20 = +42.8 (c, 5 in CH2Cl2) [literature value [α]D 20 = +45.86 (c, 6 in 1 CHCl3) ]; H-NMR (CDCl3, 300 MHz): δ 1.5 (d, 3H, J = 6), 2.07 (s, br, 1H (OH)), 3.44-3.60 (m, 2H), 3.62-3.68 (m, 1H), 4.44-4.64 (AB quartet, J1 = 11.7, J2 = 49.2), 7.25-7.33 (m, 5H); 13 C-NMR (CDCl3, 75 MHz): δ 15.1, 66.3, 70.7, 75.5, 127.5, 127.7, 128.4, 138.4. (S)-2-(benzyloxy)propanal (55): Aldehyde was obtained via DMP oxidation of alcohol 54 in dry DCM in 87% yield. [α]D 20 38 = -64 (c, 2 in CHCl3) [literature value [α]D 20 = -66 (c, 6 in 1 CHCl3)]; H-NMR (CDCl3, 300 MHz): δ 1.34 (d, 3H, J = 7), 3.90 (qd, 1H, J1 = 7, J2 = 1.7), 4.60 (d, 1H, J = 11.7), 4.66 (d, 1H, J = 11.7), 7.25-7.33 (m, 5H); 13 C-NMR (CDCl3, 75 MHz): δ 15.3, 71.9, 79.4, 127.9, 128.0, 137.2, 203.4; MS (DCI) m/z: 164.1. (S,E)-Ethyl 4-(benzyloxy)pent-2-enoate (56): A solution of aldehyde 55 (2 g, 13 mmol) and Wittig reagent (4.7 g, 13.1 mmol) were dissolved in dry dichloromethane (150 mL) and stirred at room temperature for 18 h. The solvent was removed under reduced pressure and the remaining mixture was purified with column chromatography (Silica Gel, 10% EtOAc in hexane) to afford an ester 56 as a yellow liquid in 84% yield (2.4 g). [α]D 20 1 = +20.3 (c, 5.1 in CH2Cl2); H-NMR (CDCl3, 300 MHz): δ 1.29 (t, 3H, J = 7), 1.31 (d, 3H, J = 6.6), 4.08-4.12 (m, 1H), 4.38 (q, 2H, J = 7.2), 4.41 (d, 1H, J = 12), 5.46 (d, 1H, J = 12), 5.98-6.03 (dd, 1H, J1 = 1.5, J2 = 15.9), 6.84- 296 6.91 (dd, 1H, J1 = 6, J2 = 15.9), 7 23-7.36 (m, 5H); 13 C-NMR (CDCl3, 75 MHz): δ 14.2, 20.6, 60.5, 70.7, 73.8, 121.3, 126.9, 127.6, 127.7, 128.4. 138.1, 149.2, 166.3; MS (DCI) m/z: 234.1 (S)-Ethyl 4-(benzyloxy)pentanoate (57): To the solution of unsaturated ester 56 (1 g, 4.2 mmol) in ethyl acetate was added Pd/C (10 mg). The reaction mixture was stirred under hydrogen atmosphere for 3 h and was filtered through pad of Celite. The filtrate was concentrated under reduced pressure to give 57 in quantitative yield (no benzyl cleavage was detected). [α]D 20 1 = +21.3 (c, 6.26 in CHCl3); H-NMR (CDCl3, 300 MHz): δ 1.25 (d, 3H, J = 6), 1.27 (t, 3H, J = 7), 1.89 (q, 2H, J = 7.2), 2.43-2.46 (dt, 2H, J1 = 3, J2 = 7.5), 3.60 (m, 1H, J = 6), 4.47 (d, 1H, J = 11.7), 4.62 (d, 1H, J = 11.7), 7.29-7.40 (m, 5H); 13 C-NMR (CDCl3, 75 MHz): δ 14.1, 19.3, 30.3, 31.6, 60.1, 70.3, 73.7, 127.4, 127.6, 128.2, 138.7, 173.6; MS (DCI) m/z: 236. (S)-4-(Benzyloxy)pentanoic acid (36): Ester 57 (0.3 g, 1.27 mmol) was dissolved in THF (5 mL) and 2M NaOH (3 ml) and refluxed overnight. THF was then removed under reduced pressure and the remaining solution acidified with 10% HCl and extracted with DCM (3 x 10 mL). Extracts were combined, dried over Na2SO4 and concentrated to give a crude acid in ~96% yield. [α]D 20 1 = (c, 6.26 in CHCl3); H-NMR (CDCl3, 300 MHz): δ 1.22 (d, 3H, J = 6), 1.84 (q, 2H, J = 7.5), 2.47 (dt, 2H, J1 = 2.1, J2 = 7.5), 3.60 (m, 1H, J = 6), 4.41-4.68 (AB q, 2H, J1 = 11.4, J2 = 46.8), 7.28-7.39 (m, 5H); 13 C-NMR (CDCl3, 75 MHz): δ 19.4, 30.2, 31.3, 70.4, 73.6, 127.5, 127.7, 128.3, 138.5, 179.7. 297 (S)-Methyl 2-methoxy-2-phenylacetate (59):39 To the solution of 58 (3 g, 18 mmol) in dry dichloromethane (50 mL) was added iodomethane (3.6 g, 21.6 mmol) and silver oxide (5 g, 21.6 mmol). The reaction mixture was stirred at room temperature for 24 h and filtered through a pad of Celite. The filtrate was concentrated and the obtained crude oil was purified by column chromatography (eluted with 2% EtOAc in hexane) to afford compound 59 in 77% yield (2.5 g): [α]D 20 1 = (c, DCM); H-NMR (CDCl3, 500 MHz): δ 3.37 (s, 3H), 3.69 (s, 3H), 4.74 (s, 1H), 7.31-7.42 (m, 5H); 13 C-NMR (CDCl3, 125 MHz): δ 52.2, 57.3, 82.5, 127.2, 128.6, 128.7, 136.1, 171.1 (S)-2-Methoxy-2-phenylethanol (60): To the ice cold solution of 59 (2 g, 11 mmol) in dry THF (60 mL) was added LAH (0.5 g, 13.3 mmol) portion-wise and the reaction mixture was stirred at room temperature for 4 h. The mixture was then cooled back to 0 °C and was slowly quenched with cold water. After 30 min Na2SO4 (10 g) was added and mixture was stirred for additional 30 min. The solution was then filtered through a pad of Celite, the filter cake was washed with dichloromethane (2 x 25 mL) and the filtrate was dried with Na2SO4 and concentrated. Flash chromatography (10% EtOAc in hexane) afforded alcohol 60 in 95% yield (1.6g). [α]D 20 = 1 +113.2 (c, 3.57 in DCM); H-NMR (CDCl3, 500 MHz): δ 2.54 (s, 1H (OH)), 3.28 (s, 3H), 3.573.68 (m, 2H), 4.27-4.30 (dd, 1H, J1 = 4, J2 = 8), 7.27-7.36 (m, 5H); MHz): δ 56.8, 67.3, 84.6, 126.8, 128.1, 128.5, 138.3. 298 13 C-NMR (CDCl3, 125 (S)-2-Methoxy-2-phenylacetaldehyde (61): To the solution of alcohol 60 in dry dichloromethane (50 mL) was added DMP and mixture stirred at room temperature under nitrogen atmosphere for 4 h. The mixture was filtered through Celite and concentrated. Column chromatography (Silica Gel, 5% EtOAc in hexane) afforded aldehyde 61 in 87% yield. [α]D 1 (c, DCM); H-NMR (CDCl3, 500 MHz): δ; 13 20 = C-NMR (CDCl3, 125 MHz): (R, E)-Methyl 4-methoxy-4-phenylbut-2-enoate (62): 40 Solution of aldehyde 61 (0.5 g, 3.3 mmol) in dry dichloromethane (50 mL) and ylide (1 g, 3.6 mmol) was stirred at room temperature for 12 h, concentrated and directly transferred into the Silica Gel column. The 1 resulting ester was washed out with 2% EtOAc in hexane in 97% (0.66 g) yield. H-NMR (CDCl3, 500 MHz): δ 1.25 (t, 3H, J = 7.2), 3.31 (s, 3H), 4.16 (q, 2H, J = 7.2), 4.74-4.77 (dd, 1H), 6.02-6.08 (dd, 1H, J1 = 1.2, J2 = 15.9), 6.90-6.97 (dd, 1H, J1 = 5.4, J2 = 15.6), 7.26-7.41 (m, 5H); 13 C-NMR (CDCl3, 125 MHz): 14.1, 20.1, 42.0, 60.3, 120.2, 127.0, 127.6, 128.9, 143.6, 152.9, 167.0; MS (DCI) m/z 206. (R)-Methyl 4-methoxy-4-phenylbutanoate (63): Olefin 62 (0.6 g, 2.9 mmol) was dissolved in methanol and Pd/C was added (12 mg). The reaction mixture was stirred under hydrogen atmosphere for 4 h and filtered through Celite. The filtrate was concentrated to afford ester 63 in quantitative yield. [α]D 20 1 = +48.6 (c, 3.40 in DCM); H-NMR (CDCl3, 500 MHz): δ 1.25 (t, 3H, J = 7.2), 3.31 (s, 3H), 4.16 (q, 2H, J = 7.2), 4.74-4.77 (dd, 1H), 6.02-6.08 (dd, 1H, J1 = 1.2, J2 = 299 15.9), 6.90-6.97 (dd, 1H, J1 = 5.4, J2 = 15.6), 7.26-7.41 (m, 5H); 14.1, 20.1, 42.0, 60.3, 120.2, 127.0, 127.6, 128.9, 143.6, 152.9, 167.0; MS (DCI) m/z 208. 1 (R)-4-Methoxy-4-phenylbutanoic acid (33): H-NMR (CDCl3, 500 MHz): δ 1.97-2.17 (m, 2H), 2.47-2.52 (m, 2H), 3.26 (s, 3H), 4.20-4.25 (dd, 1H, J1 = 5.1, J2 = 7.1), 7.29-7.43 (m, 5H), 11.26 (s, br, 1H (OH)); 13 C-NMR (CDCl3, 125 MHz): δ 30.4, 32.8, 56.7, 82.7, 126.5, 127.8, 128.4, 128.5, 141.3, 179.6 300 REFERENCES 301 References 1. Saidi, M. R.; Azizi, N.; Zali-Boinee, H., A simple one-pot three-component reaction for preparation of secondary amines and amino esters mediated by lithium perchlorate. Tetrahedron 2001, 57 (31), 6829-6832. 2. Fukui, H.; Fukushi, Y., NMR Determinations of the Absolute Configuration of alphaChiral Primary Amines. Organic Letters 2010, 12 (12), 2856-2859. 3. Khelili, S.; de Tullio, P.; Lebrun, P.; Fillet, M.; Antoine, M. H.; Ouedraogo, R.; Dupont, L.; Fontaine, J.; Felekidis, A.; Leclerc, G.; Delarge, J.; Pirotte, B., Preparation and pharmacological evaluation of the R- and S-enantiomers of 3-(2 '-butylamino)-4H- and 3-(3 'methyl-2 '-butylamino)-4H-pyrido 4,3-e -1,2,4-thiadiazine 1,1-dioxide, two tissue selective ATPsensitive potassium channel openers. Bioorganic & Medicinal Chemistry 1999, 7 (8), 1513-1520. 4. (a) Gargiulo, D.; Cai, G. L.; Ikemoto, N.; Bozhkova, N.; Odingo, J.; Berova, N.; Nakanishi, K., CD Exciton chirality method - new chromophores for primary amino-groups. Angewandte Chemie-International Edition in English 1993, 32 (6), 888-891; (b) Huang, X. F.; Borhan, B.; Rickman, B. H.; Nakanishi, K.; Berova, N., Zinc porphyrin tweezer in host-guest complexation: Determination of absolute configurations of primary monoamines by circular dichroism. Chemistry-a European Journal 2000, 6 (2), 216-224; (c) Kurtan, T.; Nesnas, N.; Li, Y. Q.; Huang, X. F.; Nakanishi, K.; Berova, N., Chiral recognition by CD-sensitive dimeric zinc porphyrin host. 1. Chiroptical protocol for absolute configurational assignments of monoalcohols and primary monoamines. Journal of the American Chemical Society 2001, 123 (25), 5962-5973. 5. (a) Proni, G.; Pescitelli, G.; Huang, X. F.; Nakanishi, K.; Berova, N., Magnesium tetraarylporphyrin tweezer: A CD-sensitive host for absolute configurational assignments of alpha-chiral carboxylic acids. Journal of the American Chemical Society 2003, 125 (42), 1291412927; (b) Yang, Q.; Olmsted, C.; Borham, B., Absolute stereochemical determination of chiral carboxylic acids. Organic Letters 2002, 4 (20), 3423-3426. 6. Borovkov, V. V.; Lintuluoto, J. M.; Inoue, Y., Supramolecular chirogenesis in zinc porphyrins: Mechanism, role of guest structure, and application for the absolute configuration determination. Journal of the American Chemical Society 2001, 123 (13), 2979-2989. 7. Dale, J. A.; Mosher, H. S., Nuclear magnetic-resonance enantiomer reagents configurational correlations via nuclear magnetic-resonance chemical-shifts of diastereomeric mandelate, o-methylmandelate, and alpha-methoxy-alpha-trifluoromethylphenylacetate (mtpa) esters. Journal of the American Chemical Society 1973, 95 (2), 512-519. 302 8. Sullivan, G. R.; Dale, J. A.; Mosher, H. S., Correlation of configuration and f-19 chemical-shifts of alpha-methoxy-alpha-trifluoromethylphenylacetate derivatives. Journal of Organic Chemistry 1973, 38 (12), 2143-2147. 9. (a) Scott, R.; Vinograd.S, Proton-transfer complexes .2. role of solvent polarity and specific solvation of p-nitrophenol-amine complexes in aqueous solutions. Journal of Physical Chemistry 1969, 73 (6), 1890-&; (b) Scott, R.; Depalma, D.; Vinograd.S, Proton-transfer complexes .i. preferential solvation of p-nitrophenol-amine complexes in nonaqueous-solvent mixtures. Journal of Physical Chemistry 1968, 72 (9), 3192-&; (c) Hudson, R. A.; Vinograd.Sn; Scott, R. M., Hydrogen-bonded complex-ion-pair equilibria in 3,4-dinitrophenol-amine-aprotic solvent system. Journal of Physical Chemistry 1972, 76 (14), 1989-&; (d) Hanessian, S.; Simard, M.; Roelens, S., Molecular recognition and self-assembly by non-amidic hydrogen-bonding - an exceptional assembler of neutral and charged supramolecular structures. Journal of the American Chemical Society 1995, 117 (29), 7630-7645. 10. (a) Ferrarini, A.; Ferroni, F.; Pieraccini, S.; Rosini, C.; Superchi, S.; Spada, G. P., Central-to-Axial Chirality Transfer Revealed by Liquid Crystals: A Combined Experimental and Computational Approach for the Determination of Absolute Configuration of Carboxylic Acids with an alpha Chirality Centre. Chirality 2011, 23 (9), 736-743; (b) Sahnoun, R.; Koseki, S.; Fujimura, Y., Density functional theoretical study on enantiomerization of 2,2 '-biphenol. Journal of Physical Chemistry A 2006, 110 (7), 2440-2447; (c) Kranz, M.; Clark, T.; Schleyer, P. V., Rotational barriers of 1,1'-binaphthyls - a computational study. Journal of Organic Chemistry 1993, 58 (12), 3317-3325. 11. Ishii, Y.; Onda, Y.; Kubo, Y., 2,2 '-Biphenyldiol-bridged bis(free base porphyrin): synthesis and chiroptical probing of asymmetric amino alcohols. Tetrahedron Letters 2006, 47 (47), 8221-8225. 12. Whitesell, J. K., C2 Symmetry and asymmetric induction. Chemical Reviews 1989, 89 (7), 1581-1590. 13. (a) Hoye, T. R.; Renner, M. K., Applications of MTPA (Mosher) amides of secondary amines: Assignment of absolute configuration in chiral cyclic amines. Journal of Organic Chemistry 1996, 61 (24), 8489-8495; (b) Hoye, T. R.; Renner, M. K., MTPA (Mosher) amides of cyclic secondary amines: Conformational aspects and a useful method for assignment of amine configuration. Journal of Organic Chemistry 1996, 61 (6), 2056-2064. 14. Huang, X. F.; Fujioka, N.; Pescitelli, G.; Koehn, F. E.; Williamson, R. T.; Nakanishi, K.; Berova, N., Absolute configurational assignments of secondary amines by CD-sensitive dimeric zinc porphyrin host. Journal of the American Chemical Society 2002, 124 (35), 10320-10335. 303 15. (a) Tanner, D., Chiral aziridines - their synthesis and use in stereoselective transformations. Angewandte Chemie-International Edition in English 1994, 33 (6), 599-619; (b) Tanner, D.; Andersson, P. G.; Harden, A.; Somfai, P., C-2 Symmetrical bis(aziridines) - a new class of chiral ligands for transition metal-mediated asymmetric-synthesis. Tetrahedron Letters 1994, 35 (26), 4631-4634; (c) Tanner, D.; Birgersson, C.; Gogoll, A.; Luthman, K., On the use of C-2 symmetrical aziridines as chiral auxiliaries. Tetrahedron 1994, 50 (32), 9797-9824; (d) Muller, P.; Fruit, C., Enantioselective catalytic aziridinations and asymmetric nitrene insertions into CH bonds. Chemical Reviews 2003, 103 (8), 2905-2919. 16. Pineschi, M., Asymmetric ring-opening of epoxides and aziridines with carbon nucleophiles. European Journal of Organic Chemistry 2006, (22), 4979-4988. 17. (a) Nicolaou, K. C. S., E. J., Classics in Total Synthesis; . VCH: Weinheim: 1996; (b) Liu, M.; Sibi, M. P., Recent advances in the stereoselective synthesis of beta-amino acids. Tetrahedron 2002, 58 (40), 7991-8035; (c) Kato, D.; Mitsuda, S.; Ohta, H., Microbial deracemization of alpha-substituted carboxylic acids: Substrate specificity and mechanistic investigation. Journal of Organic Chemistry 2003, 68 (19), 7234-7242. 18. (a) Custar, D. W.; Zabawa, T. P.; Hines, J.; Crews, C. M.; Scheidt, K. A., Total Synthesis and Structure-Activity Investigation of the Marine Natural Product Neopeltolide. Journal of the American Chemical Society 2009, 131 (34), 12406-12414; (b) Nicolaou, K. C.; Nold, A. L.; Milburn, R. R.; Schindler, C. S.; Cole, K. P.; Yamaguchi, J., Total synthesis of marinomycins AC and of their monomeric counterparts monomarinomycin A and iso-monomarinomycin A. Journal of the American Chemical Society 2007, 129 (6), 1760-1768. 19. Nicolaou, K. C.; Ajito, K.; Patron, A. P.; Khatuya, H.; Richter, P. K.; Bertinato, P., Total synthesis of swinholide A. Journal of the American Chemical Society 1996, 118 (12), 30593060. 20. (a) Latypov, S. K.; Seco, J. M.; Quinoa, E.; Riguera, R., Conformational structure and dynamics of arylmethoxyacetates - dnmr spectroscopy and aromatic shielding effect. Journal of Organic Chemistry 1995, 60 (3), 504-515; (b) Ferreiro, M. J.; Latypov, S. K.; Quinoa, E.; Riguera, R., Assignment of the absolute configuration of alpha-chiral carboxylic acids by H-1 NMR spectroscopy. Journal of Organic Chemistry 2000, 65 (9), 2658-2666; (c) Seco, J. M.; Quinoa, E.; Riguera, R., A practical guide for the assignment of the absolute configuration of alcohols, amines and carboxylic acids by NMR. Tetrahedron-Asymmetry 2001, 12 (21), 29152925; (d) Ferreiro, M. J.; Latypov, S. K.; Quinoa, E.; Riguera, R., The use of ethyl 2-(9-anthryl)2-hydroxyacetate for assignment of the absolute configuration of carboxylic acids by H-1 NMR. Tetrahedron-Asymmetry 1997, 8 (7), 1015-1018. 304 21. Proni, G.; Pescitelli, G.; Huang, X. F.; Quraishi, N. Q.; Nakanishi, K.; Berova, N., Configurational assignment of alpha-chiral carboxylic acids by complexation to dimeric Znporphyrin: host-guest structure, chiral recognition and circular dichroism. Chemical Communications 2002, (15), 1590-1591. 22. (a) Nagai, Y.; Kusumi, T., New chiral anisotropic reagents for determining the absoluteconfiguration of carboxylic-acids. Tetrahedron Letters 1995, 36 (11), 1853-1856; (b) Yabuuchi, T.; Ooi, T.; Kusumi, T., Application of phenylglycine methyl ester (PGME) to determination of the absolute configuration of carboxylic acids having phenylalkyl group. Chirality 1997, 9 (5-6), 550-555. 23. Joyce, L. A.; Maynor, M. S.; Dragna, J. M.; da Cruz, G. M.; Lynch, V. M.; Canary, J. W.; Anslyn, E. V., A Simple Method for the Determination of Enantiomeric Excess and identity of Chiral Carboxylic Acids. Journal of the American Chemical Society 2011, 133 (34), 1374613752. 24. (a) Mizutani, T.; Kurahashi, T.; Murakami, T.; Matsumi, N.; Ogoshi, H., Molecular recognition of carbohydrates by zinc porphyrins: Lewis acid Lewis base combinations as a dominant factor for their selectivity. Journal of the American Chemical Society 1997, 119 (38), 8991-9001; (b) Mizutani, T.; Ema, T.; Yoshida, T.; Kuroda, Y.; Ogoshi, H., Recognition of alpha-amino-acid esters by zinc porphyrin derivatives via coordination and hydrogen-bonding interactions - evidence for 2-point fixation from thermodynamic and induced circular-dichroism spectroscopic studies. Inorganic Chemistry 1993, 32 (10), 2072-2077; (c) Mizutani, T.; Murakami, T.; Ogoshi, H., Dynamics of molecular recognition of multi-point host-guest complex. Tetrahedron Letters 1996, 37 (30), 5369-5372; (d) Rudkevich, D. M.; Shivanyuk, A. N.; Brzozka, Z.; Verboom, W.; Reinhoudt, D. N., A self-assembled bifunctional receptor. Angewandte Chemie-International Edition in English 1995, 34 (19), 2124-2126. 25. Hoye, T. R.; Hamad, A. S. S.; Koltun, D. O.; Tennakoon, M. A., An NMR method for determination of configuration of beta-substituted carboxylic acids. Tetrahedron Letters 2000, 41 (14), 2289-2293. 26. Hoye, T. R.; Koltun, D. O., An NMR strategy for determination of configuration of remote stereogenic centers: 3-methylcarboxylic acids. Journal of the American Chemical Society 1998, 120 (19), 4638-4643. 27. Yashima, E.; Nimura, T.; Matsushima, T.; Okamoto, Y., Poly((4dihydroxyborophenyl)acetylene) as a novel probe for chirality and structural assignments of 305 various kinds of molecules including carbohydrates and steroids by circular dichroism. Journal of the American Chemical Society 1996, 118 (40), 9800-9801. 28. (a) Gimple, O.; Schreier, P.; Humpf, H. U., A new exciton-coupled circular dichroism method for assigning the absolute configuration in acyclic alpha- and beta-hydroxy carboxylic acids. Tetrahedron-Asymmetry 1997, 8 (1), 11-14; (b) Hor, K.; Gimple, O.; Schreier, P.; Humpf, H. U., Absolute configurational assignment of acyclic hydroxy carboxylic acids: A new strategy in exciton-coupled circular dichroism. Journal of Organic Chemistry 1998, 63 (2), 322-325. 29. (a) Louzao, I.; Garcia, R.; Manuel Seco, J.; Quinoa, E.; Riguera, R., Absolute Configuration of Ketone Cyanohydrins by (1)H NMR: The Special Case of Polar Substituted Tertiary Alcohols. Organic Letters 2009, 11 (1), 53-56; (b) Louzao, I.; Manuel Seco, J.; Quinoa, E.; Riguera, R., (13)C NMR as a general tool for the assignment of absolute configuration. Chemical Communications 2010, 46 (42), 7903-7905; (c) Louzao, I.; Seco, J. M.; Quinoa, E.; Riguera, R., The assignment of absolute configuration of cyanohydrins by NMR. Chemical Communications 2006, (13), 1422-1424; (d) Louzao, I.; Seco, J. M.; Quinoa, E.; Riguera, R., The Use of a Single Derivative in the Configurational Assignment of Ketone Cyanohydrins. European Journal of Organic Chemistry 2010, (34), 6520-6524. 30. (a) Moon, L. S.; Jolly, R. S.; Kasetti, Y.; Bharatam, P. V., A new chiral shift reagent for the determination of enantiomeric excess and absolute configuration in cyanohydrins. Chemical Communications 2009, (9), 1067-1069; (b) Moon, L. S.; Pal, M.; Kasetti, Y.; Bharatam, P. V.; Jolly, R. S., Chiral Solvating Agents for Cyanohydrins and Carboxylic Acids. Journal of Organic Chemistry 2010, 75 (16), 5487-5498. 31. Seebach, D. B., U.; Schnurrenberger, P.; Przybyski, M., High-yielde synthesis of 20-, and 25-membered macropentolide, hexolide, and -heptolide, respectrively, from (R)- or (S)-3hydroxybutanoic acid under Yamaguchi's macrolactonization condirions. Helvetica Chimica Acta 1988, 71 (1), 155-167. 32. Braddock, D. C.; Brown, J. M., Asymmetric synthesis and Lewis acid mediated type II carbonyl ene cyclisations of (R)-2-isopropyl-5-methylhex-5-enal. Tetrahedron-Asymmetry 2000, 11 (17), 3591-3607. 33. Nandy, J. P.; Prabhakaran, E. N.; Kumar, S. K.; Kunwar, A. C.; Iqbal, J., Reverse turn induced pi-facial selectivity during polyaniline-supported cobalt(II) salen catalyzed aerobic epoxidation of N-cinnamoyl L-proline derived peptides. Journal of Organic Chemistry 2003, 68 (5), 1679-1692. 306 34. VidalFerran, A.; Moyano, A.; Pericas, M. A.; Riera, A., Synthesis of a family of finetunable new chiral ligands for catalytic asymmetric synthesis. Ligand optimization through the enantioselective addition of diethylzinc to aldehydes. Journal of Organic Chemistry 1997, 62 (15), 4970-4982. 35. Tanaka, T.; Hiramatsu, K.; Kobayashi, Y.; Ohno, H., Chemo- and stereoselectivity in titanium-mediated regioselective ring-opening reaction of epoxides at the more substituted carbon. Tetrahedron 2005, 61 (28), 6726-6742. 36. Aragozzini, F.; Maconi, E.; Potenza, D.; Scolastico, C., Enantioselective Microbial Reduction of Monoesters of 1,3-Dihydroxypropanone - Synthesis of (S)-1,2-OIsopropylideneglycerol and (R)-1,2-O-Isopropylideneglycerol. Synthesis-Stuttgart 1989, (3), 225-227. 37. Takai, K.; Heathcock, C. H., Acyclic Stereoselection .32. Synthesis and Characterization of the Diastereomeric (4s)-Pentane-1,2,3,4-Tetrols. Journal of Organic Chemistry 1985, 50 (18), 3247-3251. 38. SolladieCavallo, A.; Bonne, F., Synthesis of enantiomerically pure threo 1-alkyl-2benzyloxy-propylamines. Tetrahedron-Asymmetry 1996, 7 (1), 171-180. 39. Annunziata, R.; Cinquini, M.; Cozzi, F.; Gilardi, A.; Cardani, S.; Poli, G.; Scolastico, C., Double Stereoselection in the Aldol-Type Synthesis of Gamma-Methyl and Gamma-Alkoxy Beta-Hydroxy Ketones Mediated by Alpha-Sulfinyl Hydrazones. Journal of the Chemical Society-Perkin Transactions 1 1985, (2), 255-259. 40. Bull, S. D.; Davies, S. G.; Domingez, S. H.; Jones, S.; Price, A. J.; Sellers, T. G. R.; Smith, A. D., Diastereoselective [2,3]-sigmatropic rearrangements of lithium N-benzyl-Oallylhydroxylamides bearing a stereogenic centre adjacent to the migration terminus. Journal of the Chemical Society-Perkin Transactions 1 2002, (19), 2141-2150. 307