A CATALYTIC BORYLATION-BASED APPROACH TO AUTOLYTIMYCIN AND ALLIED STUDIES By Luis Sanchez A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Chemistry 2011 ABSTRACT A CATALYTIC BORYLATION-BASED APPROACH TO AUTOLYTIMYCIN AND ALLIED STUDIES By Luis Sanchez Since its discovery in 1999, Ir-catalyzed C–H activation/borylation has established itself as a potent tool for the construction of aromatic and heteroaromatic building blocks. Given their bench stability and versatility in chemical transformations, the use of boronates is synthetically advantageous. Catalysis by iridium features a regioselectiviy that is primarily directed by sterics, as opposed to electronics, complementing electrophilic aromatic substitution and functional group-directed metalation. Synthetic utilization of this characteristic allows access to structural moieties that are, in fact, relatively inaccessible via traditional routes. This last decade has been prolific with work on improvements, adaptations, and various applications of such borylations reported by many research groups. Herein, a number of synthetic aspects of this chemistry are discussed. Halogen tolerance, for instance, allows for the preparation of multifunctional cross-coupling partners as useful building blocks. This feature is showcased in the diversity-oriented route to the COX-2 inhibitor DuP 697 and a series of analogs. The special regioselectivity attained on heteroaromatics has been utilized toward the development of a novel convergent route to the TMC-95 core. For this purpose, preparation of a key intermediate was made possible via a neutral and mild Ir-catalyzed deborylation method, developed in our laboratories, that allows the preparation of heteroaromatics with unusually placed boronate groups via diborylation/monodeborylation. In addition, the same gentle deborylation procedure was adapted for the preparation of deuterium- labeled compounds in a site-selective manner, emphasizing the importance of the broad functional group tolerance of Ir-catalyzed C–H activation/borylation. Perhaps the most attractive of its characteristics is that, given the cleanliness with which it proceeds, Ir-catalyzed C–H activation/borylation is amenable to one-pot transformations. This concept is highlighted in our synthetic approach to the Hsp90 inhibitor autolytimycin. Taking advantage of both the regioselectivity and halogen tolerance, our method enabled a highly convergent construction of the full carbon network of this natural product. We provide an illustration of a C–H activation/borylation-based three-step one-pot process, which is 3 2 complemented by a B-alkyl sp –sp Suzuki coupling on a highly inactivated aromatic chloride, an unprecedented transformation in complex molecule synthesis. The late-stage applicability of these boron-based methods lead to the preparation of highly elaborated 5-alkyl-3-amidophenols, motifs that are present in many natural products and that are difficult to obtain via conventional methods. To Luis, Gloria, and Elsa. iv ACKNOWLEDGEMENTS I would like to start by thanking Professor Robert E. Maleczka, Jr. for being a truly supportive mentor. Being knowledgeable, approachable, serious about student learning, and concerned about his students’ future makes him an excellent advisor. I owe him tremendous gratitude for giving me the opportunity to conduct research with independence during the years in his lab, making my own decisions and my own mistakes, and for being always helpful and understanding. I also want to express my appreciation to my guidance committee members: To our collaborator, Professor Milton R. Smith, III, who was essentially my co-advisor during this time, for the absolutely rich research experience that I had for being part of the boron group. I am as well particularly grateful to Professor Babak Borhan who, besides being an extraordinary teacher and a good friend, was always there to answer all my doubts, not only the research-related ones. Finally, I want to thank Professor William D. Wulff, for serving as my second reader. It has been a privilege to interact with such a knowledgeable professor, not only for academic matters, but also while having informal conversations sharing food and drinks. Also, sharing a project with (now) Professor Feng Shi and (now) Dr. Monica Norberg was an amazing experience and I am truly thankful to them; to Monica for being a splendid lab partner and to Feng for instructing me on the perils of multi-step synthesis, among various synthetic aspects, right at the beginning. I too want to acknowledge people that make research possible at the chemistry building: Dan Holmes and Kermit Johnson (NMR facility), Richard Staples (X-ray crystallography), Professor v Dan Jones, Beverly Chamberlain, and Lijun Chen (Mass spectrometry facility), for their outstanding work and willingness to help. As well, I would like to thank past and present members of both the Maleczka and Smith groups for being terrific lab mates and taking the time to explain reactions, lab techniques, proper use of equipment, and essentially sharing the knowledge. For the same reason, I am thankful to Suzi Miller and Paul Herrinton, from Boropharm. This continual learning process called a PhD was more enjoyable with great people around. I am extremely grateful to my undergraduate advisor, Professor Helena Maruenda; an energetic researcher, a strict advisor, and a great person, whose influence has transcended all these years. From her, I learned that responsibility is the key to success. I want to specially thank a few good friends of mine, Aman Desai, Yves Coello, Vanesa Mendez, Angela Carrillo, Adriana Murillo, Fernando Geu, Rosario Amado, and Luis Mori, for valuable science-related discussions and for the not-science-at-all ones too. I also want to express my gratitude to my parents who, given the failure of my first “reaction setup” (which involved shampoo, tap water, a tangerine peel, and darkness), got me a fancy chemistry set a few years later when it was sufficiently safe for my age. For encouraging me to follow my curiosity, giving me freedom to choose my own path, and working hard so I can pursue my dreams, I owe them endless appreciation. Finally, I want to thank the wonderful person I share my life with, my wife Aman Kulshrestha, for her permanent motivation and for continuous discussion on recent papers, mechanisms, and spectroscopy. I know that I will remember my PhD years with nostalgia. I think I should have started a collection of those reaction mechanisms written on paper napkins. vi TABLE OF CONTENTS LIST OF TABLES.....................................................................................................................ix LIST OF FIGURES ....................................................................................................................x LIST OF SCHEMES..................................................................................................................xi LIST OF SYMBOLS AND ABBREVIATIONS....................................................................xviii Chapter 1 Introduction ............................................................................................................1 1.1. Ir-catalyzed aromatic C–H activation/borylation ............................................1 1.2. Features of Ir-catalyzed C–H activation and their synthetic value...................3 1.3. Applications of Ir-catalyzed C–H activation/borylation ..................................5 Chapter 2 Synthesis of the COX-2 inhibitor DuP 697 and related studies................................9 2.1. Synthesis of DuP 697 and analogs via C–H activation/borylation–Suzuki coupling.........................................................................................................9 2.2. Access to uncommon borylated regioisomers and selective deuteration via C–H activation/borylation–deborylation....................................................... 20 2.3. Second-generation approach to DuP 697 via C–H activation/borylation– deborylation–Suzuki coupling...................................................................... 25 Chapter 3 Model studies for the synthesis of the TMC-95 core ............................................. 27 3.1. Target choice and significance ..................................................................... 27 3.2. General analysis of the reported syntheses of TMC-95 compounds .............. 28 3.3. Our synthetic approach to the TMC-95 core................................................. 38 3.4. Results and discussion ................................................................................. 42 Chapter 4 A synthetic approach to autolytimycin .................................................................. 47 4.1. Target choice and significance ..................................................................... 47 4.2. Autolytimycin and the geldanamycin family (C15-ansamycins) ................... 51 4.3. General analysis of the reported total syntheses of C15-ansamycins............. 53 4.4. Our synthetic approach to autolytimycin ...................................................... 64 Chapter 5 A synthetic approach to autolytimycin: Synthesis of Suzuki partners .................... 70 5.1. Preparation of the aryl partner...................................................................... 70 5.1.1. Synthesis of the amide portion ........................................................... 70 5.1.2. C–H activation/borylation/amidation/oxidation .................................. 73 5.2. Synthesis of the alkyl partner ....................................................................... 75 5.2.1. Installation of the C12 and C14 stereocenters..................................... 76 5.2.2. Asymmetric crotylation and final steps............................................... 78 Chapter 6 A synthetic approach to autolytimycin: B-alkyl sp –sp Suzuki coupling.............. 81 3 vii 2 6.1. Literature precedence................................................................................... 82 6.1.1. The Suzuki cross-coupling................................................................. 82 6.1.2. B-alkyl Suzuki coupling..................................................................... 84 6.1.3. Aryl chlorides as electrophiles in B-alkyl Suzuki coupling................. 85 3 2 6.1.4. B-alkyl sp –sp couplings in complex molecule synthesis ................. 90 6.2. Preliminary results ....................................................................................... 93 3 2 6.3. Evolution to a successful B-alkyl sp –sp Suzuki coupling .......................... 99 Chapter 7 A synthetic approach to autolytimycin: Ring-closing metathesis ......................... 116 7.1. Literature precedence................................................................................. 117 7.2. Results and discussion ............................................................................... 128 7.2.1. Preliminary results ........................................................................... 128 7.2.2. Relay RCM ...................................................................................... 139 7.2.3. Suppressing isomerization................................................................ 144 7.2.4. Removal of the C2–C3 double bond................................................. 150 7.3. Final remarks ............................................................................................. 155 Chapter 8 Summary and conclusions .................................................................................. 157 8.1. Synthesis of the COX-2 inhibitor DuP 697 and analogs ............................. 157 8.2. Model Studies for the synthesis of the TMC-95 core.................................. 159 8.3. Mild site-selective deuteration of arenes..................................................... 162 8.4. A synthetic approach to autolytimycin ....................................................... 163 Chapter 9 Experimental details and characterization data.................................................... 168 9.1. General considerations............................................................................... 168 9.2. Experimental details for Sections 2.1 and 2.3: DuP 697 and analogs .......... 170 9.3. Experimental details for Section 2.2: Deborylation .................................... 185 9.4. Experimental details for Chapter 3: TMC-95 core...................................... 190 9.5. Experimental details for Chapter 5: Autolytimycin, Suzuki partners........... 198 9.6. Experimental details for Chapter 6: Autolytimycin, Suzuki coupling.......... 234 9.7. Experimental details for Chapter 7: RCM and final steps ........................... 247 Bibliography ........................................................................................................................... 272 viii LIST OF TABLES Table 1. Deuterodeborylation of various borylated arenes. ........................................................ 23 Table 2. Optimization study for the preparation of N-Boc 7-pinacolboryl-L-tryptophan methyl ester via Ir-catalyzed deborylation................................................................ 42 Table 3. Evaluated methods for the generation of the BBN adduct from and alkyl iodide in Suzuki coupling studies. ........................................................................................ 100 Table 4. Suzuki coupling results on model substrates using Method A for generation of the BBN adduct. .......................................................................................................... 101 Table 5. Suzuki coupling results on the fully elaborated substrates using Method A for generation of the BBN adduct................................................................................ 102 Table 6. Suzuki coupling results on model substrates using Method B for generation of the BBN adduct. .......................................................................................................... 103 Table 7. Suzuki coupling results on model substrates using Method C for generation of the BBN adduct. .......................................................................................................... 106 Table 8. Suzuki coupling results on the fully elaborated substrates using Method C for generation of the BBN adduct................................................................................ 107 Table 9. Suzuki coupling results on the fully elaborated substrates at high concentration. ....... 109 Table 10. Concentration effect in a B-alkyl Suzuki coupling observed in Kigoshi’s total synthesis of haterumalide NA. ............................................................................... 112 Table 11. Summary of synthesized RCM substrates and evaluated metathesis conditions........ 134 Table 12. Metathesis reactions using RRCM substrates........................................................... 143 Table 13. RCM results using catalyst cis-Caz-1....................................................................... 148 ix LIST OF FIGURES Figure 1. DuP 697 and three basic analogs. ............................................................................... 10 Figure 2. TMC-95 compounds and their structural units............................................................ 27 Figure 3. The Hsp90 inhibitor autolytimycin and its 1,3,5-trisubstituted aromatic core.............. 47 Figure 4. Autolytimycin, reblastatin, and the geldanamycin family. .......................................... 51 Figure 5. Geldanamycin and its analogs in clinical trials. .......................................................... 52 Figure 6. Selected examples of ligands that allow coupling reactions with aryl chlorides. ......... 86 Figure 7. Model amidochlorophenol substrates used to screen Suzuki coupling conditions in our synthetic route to autolytimycin. .................................................................... 95 Figure 8. Metal–alkylidene complexes commonly used as metathesis catalysts. ...................... 119 Figure 9. Literature examples of complex molecules containing a trisubstituted olefin formed via Ru-catalyzed RCM in total synthesis.................................................... 123 Figure 10. Recently developed metathesis catalysts................................................................. 130 x LIST OF SCHEMES Scheme 1. Example of a C–H activation/borylation catalyzed by (Ind)Ir(COD)-dppe..................1 Scheme 2. Proposed catalytic cycle of Ir-catalyzed C–H activation/borylation. ..........................2 Scheme 3. Synthetic utility of borylated arenes. ..........................................................................4 Scheme 4. Gaunt’s synthesis of (±)-rhazinicine. ..........................................................................5 Scheme 5. Shibasaki and Kanai’s total synthesis of SM-130686..................................................6 Scheme 6. Sarpong’s synthesis of complanadine A. ....................................................................7 Scheme 7. C–H activation/borylation–Suzuki coupling-based synthesis of DuP 697. ..................9 Scheme 8. Preparation of various types of DuP 697 analogs...................................................... 11 Scheme 9. Examples of 3-arylations in the preparation of DuP 697 analogs. ............................. 12 Scheme 10. C–H activation/borylation-based preparation of triflate Suzuki partners. ................ 13 Scheme 11. 2-Arylations in the preparation of DuP 697 analogs via Suzuki coupling under “wet” conditions. ..................................................................................................... 13 Scheme 12. 2-Arylation in the synthesis of DuP 697 analogs via Suzuki coupling under “dry” conditions....................................................................................................... 14 Scheme 13. Bromination step in the preparation of DuP 697 analogs ........................................ 15 Scheme 14. Bromination of diarylated thiophenes at unusual positions in the preparation of DuP 697 analogs...................................................................................................... 16 Scheme 15. Preparation of 5-unbrominated DuP 697 analogs.................................................... 17 Scheme 16. Original linear synthetic route to DuP 697.............................................................. 18 Scheme 17. DuPont’s improved route toward DuP 697. ............................................................ 19 Scheme 18. Diborylation/monodeborylation of thiophenes and indoles. .................................... 20 Scheme 19. Preparation of a 7-monoborylated tryptophan derivative........................................ 21 Scheme 20. Second-generation synthesis of DuP 697................................................................ 25 Scheme 21. Preparation of a DuP 697 analog via the second-generation approach..................... 26 xi Scheme 22. Access to the β,γ-diol moiety in the total syntheses of TMC-95 compounds........... 28 Scheme 23. Danishefsky’s preparation of the oxindole core from D-serine (middle) and Williams’ modification from a derivative of L-serine (bottom)................................. 29 Scheme 24. A Direct comparison between Danishefsky’s and Williams’ syntheses of TMC95 compounds.......................................................................................................... 30 Scheme 25. Inoue and Hirama’s preparation of the oxindole core from D-serine........................ 31 Scheme 26. Installation of the β,γ-diol moiety via epoxidation in Inoue and Hirama’s total synthesis of TMC-95 A............................................................................................ 32 Scheme 27. Preparation of a “simplified” TMC-95 analog by Moroder. .................................... 34 Scheme 28. Synthesis of Moroder’s second-generation TMC-95 analog via a ring-closing Suzuki coupling....................................................................................................... 35 Scheme 29. Enzymatic preparation of 7-bromo-L-tryptophan. ................................................... 36 Scheme 30. Preparation of Vidal’s strained TMC-95 analogs. ................................................... 36 Scheme 31. Non-enzymatic preparation of a 7-bromo-L-tryptophan derivative.......................... 37 Scheme 32. Possible facial selective oxidation for the synthesis of TMC-95 compounds........... 38 Scheme 33. Our synthetic plan toward TMC-95 compounds. .................................................... 39 Scheme 34. Access to 7-borylated indoles via diborylation/monodeborylation. ......................... 41 Scheme 35. Preparation of a 7-pinacolboryl-L-tryptophan derivative......................................... 43 Scheme 36. BiCl3-mediated deprotection of a 7-pinacolboryl-L-tryptophan derivative. ............. 44 Scheme 37. Preparation of the tyrosine unit for the synthesis of the TMC-95 core..................... 45 Scheme 38. Preparation of a tyrosine–asparagine didpeptide for the synthesis of the TMC95 core..................................................................................................................... 45 Scheme 39. Preparation of a model tripeptide for the synthesis of the TMC-95 core.................. 46 Scheme 40. One-pot C–H activation/borylation/amidation/oxidation......................................... 48 Scheme 41. Smith’s synthetic route to trienomycins A and F. ................................................... 49 Scheme 42. Preparation of a 5-alkyl-3-aminophenol from 3,5-dinitrobenzoic acid. ................... 49 xii Scheme 43. Preparation of a 5-alkyl-3-aminophenol from 3,5-dihydroxybenzoic acid............... 50 Scheme 44. General approach to C15-oxygenated C15-ansamycins. ......................................... 54 Scheme 45. Late stage attachment of aryl and alkyl fragments in the syntheses of herbimycin A and macbecin I. ................................................................................. 55 Scheme 46. Attachment of aryl and alkyl fragments in recent syntheses of herbimycin A and macbecin I......................................................................................................... 56 Scheme 47. First steps in Baker’s first total synthesis of macbecin I.......................................... 57 Scheme 48. First steps in Panek’s total syntheses of macbecin I and herbimycin A. .................. 58 Scheme 49. Andrus’ total synthesis of geldanamycin. ............................................................... 59 Scheme 50. Panek’s synthesis of the left-hand portion of reblastatin. ........................................ 60 Scheme 51. Completion of the synthesis of reblastatin by Panek. .............................................. 61 Scheme 52. First steps in Panek’s total synthesis of geldanamycin. ........................................... 62 Scheme 53. Final steps in Panek’s total synthesis of geldanamycin. .......................................... 62 Scheme 54. Panek’s route in the total synthesis of autolytimycin. ............................................. 63 Scheme 55. Our synthetic approach to autolytimycin. ............................................................... 65 Scheme 56. Observed side reactions during Ir-catalyzed C–H activation/borylation/amidation/oxidation................................................................ 66 Scheme 57. One-pot Ir-catalyzed C–H activation/borylation/amidation/oxidation. .................... 67 Scheme 58. Attempts to replace the chloro with a bromo substituent in amidation/oxidation model studies........................................................................................................... 67 Scheme 59. Flexible access to the stereochemistry on the building blocks for the construction of autolytimycin................................................................................... 68 Scheme 60. Preparation for 3,4-isopropylidene-L-threonic acid methyl ester. ............................ 70 Scheme 61. Installation of the 1,1-disubstituted olefin moiety in construction of the amide portion of autolytimycin. ......................................................................................... 71 Scheme 62. Preparation of the amide portion in our route to autolytimycin. .............................. 72 xiii Scheme 63. One-pot C–H activation/borylation/amidation/oxidation applied in our synthetic approach to autolytimycin......................................................................... 73 Scheme 64. Preparation of protected amidochlorophenols in our route to autolytimycin............ 74 Scheme 65. Optimized preparation of PMB-protected 3-bromo-5-chlorophenol........................ 75 Scheme 66. Two stages in the preparation of alkyl partner in our route to autolytimycin. .......... 76 Scheme 67. Installation of the C12 and C14 stereocenters of autolytimycin............................... 76 Scheme 68. Protective group manipulations and recycling step in the synthesis of autolytimycin’s alkyl half. ....................................................................................... 77 Scheme 69. Preparation of the crotylation substrate in the synthesis of autolytimycin. .............. 78 Scheme 70. Asymmetric crotylation in our synthetic route to autolytimycin.............................. 79 Scheme 71. Final steps in the preparation of the alkyl partner in our synthetic approach to autolytimycin........................................................................................................... 79 Scheme 72. Preparation of a fully elaborated alkyl bromide to be tested in the generation of the BBN adduct in our synthetic approach to autolytimycin. .................................... 80 Scheme 73. Preparation of a modified alkyl partner in our synthetic approach to autolytimycin........................................................................................................... 80 Scheme 74. The Suzuki coupling step in our synthetic approach to autolytimycin. .................... 81 Scheme 75. The Suzuki coupling catalytic cycle. ...................................................................... 82 Scheme 76. Potential β-hydride elimination in the Suzuki coupling catalytic cycle involving alkylboron compounds. ............................................................................ 84 Scheme 77. Methods for the preparation of alkylboron Suzuki partners..................................... 85 Scheme 78. Suzuki couplings on aryl chlorides using DavePhos. .............................................. 87 Scheme 79. Suzuki couplings on aryl chlorides using JohnPhos. ............................................... 88 Scheme 80. Suzuki couplings of aryl chlorides and alkylboronic acids using QPhos. ................ 88 Scheme 81. B-alkyl Suzuki couplings of aromatic chlorides using the NHC ligand IPr.............. 89 Scheme 82. Example of a B-alkyl Suzuki coupling with an aryl chloride using PEPPSI. ........... 90 xiv Scheme 83. A B-alkyl Suzuki coupling on a vinyl iodide featured in Paquette’s synthetic studies toward altohyrtin A. ..................................................................................... 91 Scheme 84. B-alkyl Suzuki coupling step in Marshall’s synthesis of discodermolide................. 92 Scheme 85. First productive real-system Suzuki coupling in our synthetic approach to autolytimycin........................................................................................................... 94 Scheme 86. Modest results obtained with a model substrate in a preliminary screening............. 95 Scheme 87. Successful Suzuki coupling reactions on model amidochlorophenols. .................... 96 Scheme 88. Attempts to prepare isolable alkylborons from our fully elaborated iodide.............. 97 Scheme 89. Successful Suzuki coupling on a model substrate via generation of the BBN adduct from a Grignard reagent................................................................................ 98 Scheme 90. Example of a set of partially optimized conditions for the B-alkyl Suzuki coupling in our synthetic route to autolytimycin..................................................... 104 Scheme 91. Optimized Suzuki coupling conditions used in our route to autolytimycin. ........... 110 Scheme 92. Concentration effect in a B-alkyl Suzuki coupling observed by the Organ group using the highly active Pd-PEPPSI system. ............................................................ 111 Scheme 93. Reduction of a nitro group under Suzuki coupling conditions with an alkylBBN species. ......................................................................................................... 113 Scheme 94. Dialkyl-BBN “ate” complexes as sources of hydride............................................ 114 Scheme 95. Reduction of a diene under Suzuki coupling conditions with an alkyl-BBN.......... 114 Scheme 96. The ring-closing metathesis step in our synthetic approach to autolytimycin. ....... 116 Scheme 97. Schematic illustration of a general metathesis and a ring-closing metathesis reactions. ............................................................................................................... 117 Scheme 98. Chauvin’s mechanism, proposed in 1971, for the catalyzed olefin metathesis involving metal alkylidene and metallacyclobutane intermediates.......................... 118 Scheme 99. Mechanism of the metathesis of a symmetrical cis olefin to its trans isomer......... 120 Scheme 100. Substrate-controlled E/Z selectivity in the key RCM step in Nicolaou's total synthesis of coleophomones B and C. .................................................................... 124 Scheme 101. Late-stage RCM in the total synthesis of amphidinolide Y. ................................ 125 xv Scheme 102. Late-stage formation of a trisubstituted olefin via RCM and its isomerization in Smith's total synthesis of kendomycin................................................................ 127 Scheme 103. Attempted RCM in Andrus’ total synthesis of geldanamycin.............................. 128 Scheme 104. Earliest RCM results in our synthetic approach to autolytimycin. ....................... 129 Scheme 105. Selective PMB deprotection of a secondary alcohol in the presence of a PMBprotected phenol. ................................................................................................... 132 Scheme 106. Preparation of carbamate-containing RCM substrates......................................... 133 Scheme 107. Proposed mechanism for the six-carbon chain loss from RCM substrates. .......... 135 Scheme 108. Example of a successful RCM in a congested system featured in a formal synthesis of eleutherobin........................................................................................ 136 Scheme 109. Preparation and evaluation of demethylated RCM substrates. ............................. 137 Scheme 110. Proposed facilitation of ring-closing metathesis via ring size reduction. ............. 138 Scheme 111. Preparation and evaluation of a ring-reduced RCM substrate.............................. 139 Scheme 112. Schematic representation of an RRCM and its corresponding RCM. .................. 140 Scheme 113. RRCM-based strategy to avoid the undesired six-carbon chain loss from the RCM substrates. .................................................................................................... 141 Scheme 114. Installation of a metathesis relay chain via an ozonolysis/Wittig olefination sequence and the preparation of the RRCM substrates. .......................................... 142 Scheme 115. Isomerization-RCM sequence observed in the synthesis of serpendione. ............ 145 Scheme 116. Reported formation of a ruthenium hydride decomposition product from an NHC-containing ruthenium methylidene complex. ................................................ 146 Scheme 117. Catalyst cis-Caz-1 and its trans form.................................................................. 147 Scheme 118. RCM under aqueous conditions using the nonioinic amphiphile PTS. ................ 149 Scheme 119. Proposed saturation of the C2–C3 double bond of the RCM substrate. ............... 150 Scheme 120. Installation of the C2–C3 saturation on the amide portion. ................................. 151 Scheme 121. Preparation of the 2,3-saturated aryl chloride Suzuki substrate. .......................... 152 xvi Scheme 122. Preparation of the 2,3-saturated RCM substrate. ................................................. 153 Scheme 123. RCM with 2,3-saturated RCM substrate. ............................................................ 154 Scheme 124. Preparation of "ring-opened autolytimycin"........................................................ 155 Scheme 125. Our original diversity-oriented route to DuP 697 and an alternative.................... 157 Scheme 126. Access to building blocks used in the preparation of DuP 697 analogs. .............. 158 Scheme 127. Our synthetic plan toward the TMC-95 family. .................................................. 159 Scheme 128. Preparation of a triptophan-based building block for the synthesis of the TMC-95 core. ........................................................................................................ 160 Scheme 129. Preparation of a model tripeptide for the synthesis of the TMC-95 core.............. 161 Scheme 130. Mild site-selective deuterium-labeling of arenes................................................. 162 Scheme 131. Summarized preparation of advances intermediates in our synthetic approach to autolytimycin..................................................................................................... 163 Scheme 132. C–H activation/borylation/amidation/oxidation in our route to autolytimycin. .... 164 3 2 Scheme 133. B-alkyl sp –sp Suzuki coupling used in our route to autolytimycin................... 165 Scheme 134. Summary of RCM attempts in our approach to autolytimycin............................. 166 xvii LIST OF SYMBOLS AND ABBREVIATIONS Å angstrom [α] specific rotation δ chemical shift µW microwave Ac acetyl APCI atmospheric pressure chemical ionization aq aqueous Ar aryl atm atmospheres B2Pin2 bis(pinacolato)diboron BBN 9-borabicyclo[3.3.1]nonane Bn benzyl Boc tert-butyloxycarbonyl BOM (benzyloxy)methyl BOPCl bis(2-oxo-3-oxazolidinyl)phosphonic chloride BPin pinacolboryl BPS tert-butyldiphenylsilyl br broad (spectral peak) brsm based on recovered starting material Bu butyl Bz benzoyl cat. Catalytic xviii Cbz benzyloxycarbonyl CI chemical ionization COD 1,5-cyclooctadiene Cy cyclohexyl d doublet (spectral peak), day (reaction time) DavePhos 2-dicyclohexylphosphino-2'-(N,N-dimethylamino)biphenyl dba dibenzylideneacetone DBU 1,8-diazabicyclo[5.4.0]undec-7-ene DCC dicyclohexylcarbodiimide DCM dichloromethane DDQ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DHQ)2PHAL hydroquinine 1,4-phthalazinediyl diether (DHQD)2PHAL hydroquinidine 1,4-phthalazinediyl diether DIAD diisopropyl azodicarboxylate DIBAL diisobutylaluminium hydride DIEA diisopropylethylamine DIPT diisopropyl tartrate DMAP 4-dimethylaminopyridine DMDO dimethyldioxirane DME 1,2-dimethoxyethane DMF dimethylformamide dmpe 1,2-bis(dimethylphosphino)ethane DMPM (3,4-dimethoxyphenyl)methyl xix DMSO dimethyl sulfoxide dppf 1,1'-bis(diphenylphosphino) ferrocene dr diastereomeric ratio dtbpy 4,4'-di-tert-butyl-2,2'-bipyridine EAS electrophilic aromatic substitution EDC, EDCI 3-(ethyliminomethyleneamino)-N,N-dimethylpropanamine hydrochloride ee enantiomeric excess EI electron ionization ESI electrospray ionization Et ethyl eV electron volt EVE ethyl vinyl ether FAB fast atom bombardment (an ionization technique) GC gas chromatography H2IMes 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene HMDS hexamethyldisilazide HMPA hexamethylphosphoramide HOAt 1-hydroxy-7-azabenzotriazole HOBt hydroxybenzotriazole HRMS high-resolution mass spectrometry IMes 1,3-dimesitylimidazol-2-ylidene Ind indenyl xx iPr isopropyl IPr 1,3-diisopropylimidazol-2-ylidene JohnPhos (2-biphenyl)di-tert-butylphosphine lut lutidine mCPBA m-chloroperbenzoic acid Me methyl Mes mesityl MHz megahertz MOM methoxymethyl mp melting point ms molecular sieves MS mass spectrometry mtbe methyl tert-butyl ether nBu n-butyl NMR nuclear magnetic resonance OAc acetate ON overnight PEPPSI pyridine-enhanced precatalyst preparation, stabilization, and initiation Ph phenyl PinB pinacolboryl PMB p-methoxybenzyl PMP p-methoxyphenyl xxi ppm parts per million PPTS pyridinium p-toluenesulfonate PTS polyoxyethanyl α-tocopheryl sebacate py, pyr pyridine, pyridyl PyBOP benzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate q quartet (spectral peak) QPhos 1,2,3,4,5-pentaphenyl-1'-(di-tert-butylphosphino)ferrocene R substituent RCM ring-closing metathesis RRCM relay ring-closing metathesis RuPhos 2-dicyclohexylphosphino-2',6'-diisopropoxybiphenyl rt room temperature s singlet (spectral peak) SAR Structure-activity relationship sBu sec-butyl SPhos 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl t triplet (spectral peak) tAm tert-amyl, tert-pentyl TBAF tetrabutylammonium fluoride TBAI tetrabutylammonium iodide TBDPS tert-butyldiphenylsilyl TBS tert-butyldimethylsilyl xxii TEA triethylamine TES triethylsilyl tBu tert-butyl THF tetrahydrofuran THP 2-tetrahydropyranyl TIPS triisopropylsilyl TLC thin layer chromatography TMEDA tetramethylethylenediamine TMS trimethylsilyl TOF time of flight Trt triphenylmethyl, trityl Ts tosyl TSE trimethylsilylethyl UV ultraviolet XantPhos 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene xs excess xxiii Chapter 1. Introduction 1.1. Ir-catalyzed aromatic C–H activation/borylation 1 2 C–H activation/borylation is a fairly recent and developing type of C–H activation in which the thermodynamically favored conversion of a C−H bond into a C−B bond is effected by virtue 3 of a transition metal. In general, Re, Rh, Ru, Ir, and Pd have been utilized for selective C–H activation at aliphatic, benzylic, or aromatic positions. MeO2C Cl 2 mol% (Ind)Ir(COD) 2 mol% dppe HBPin 1 MeO2C Cl + cyclohexane, 100 ºC 95% H2 BPin Scheme 1. Example of a C–H activation/borylation catalyzed by (Ind)Ir(COD)-dppe. 4 In 1999, the Smith group demonstrated that aromatic C–H activation/borylation could be catalyzed by iridium species. Since then, there has been a significant advancement around this 5 6 transformation, with important reports from the Smith, Hartwig, Ishiyama, and Miyaura 7 groups and others, which has resulted in the development of a well established method of high synthetic value. 5c Smith introduced the use of (Ind)Ir(COD) as precatalyst in combination with a bisphosphine ligand (usually dppe or dmpe) in borylation reactions with pinacolborane (HBPin) (Scheme 1). This represented a significant advance in terms of catalyst activity, turnover numbers, and product cleanliness. Subsequent improvement by Hartwig, Ishiyama, and Miyaura showed that the precatalyst [Ir(OMe)COD]2 in combination with the bipyridyl ligand dtbpy, using 1 bispinacolatodiboron (B2Pin2) as boron source, could effect borylation reactions at room temperature. 6c bisphosphine L L = or bipyridyl Ir precatalyst + HBPin + L Ir L E = H or BPin L Ir L IrV IrIII R L L E E L Ir L H–E E significant L Ir BPin proton-transfer L H character R E BPin active catalyst E E E L Ir BPin H L or IrIII/IrV cycle R E L Ir BPin L H R E E L Ir BPin L H H E L L BPin E Ir H R E–BPin Scheme 2. Proposed catalytic cycle of Ir-catalyzed C–H activation/borylation. The mechanism of the Ir-catalyzed C–H activation/borylation has been proposed to proceed via a III V Ir /Ir catalytic cycle depicted in Scheme 2, 5c,6g where the active Ir III catalyst is formed in situ I from an Ir precatalyst and the boron source in the presence of the biphosphine or bipyridyl 5l ligand. As reported by Smith, experimental and theoretical data suggest that significant proton 2 transfer character exists in the C–H activation transition state. After such C–H activation, the V resulting Ir intermediate undergoes reductive elimination generating a C–B bond. Installation of boron functionalities via C–H activation is a remarkable synthetic breakthrough. 8 Arylboronic esters and acids are versatile compounds heavily used in Pd- and Cu-catalyzed cross-coupling reactions leading to not only C–C but also C–N and C–O bond formations. 10 Other transformations include Rh-catalyzed additions 11 and Petasis reactions 9 (see also Scheme 3). Synthesis of these compounds is usually carried out from Grignard or lithium species generated via ortho-metallation or from halide-containing precursors via Miyaura coupling. 9 Direct borylation of arenes provides access to these synthetically valuable compounds without depending on the accessibility of the corresponding halides or organometallics. 1.2. Features of Ir-catalyzed C–H activation and their synthetic value. Ir-catalyzed C–H activation/borylation possesses synthetically attractive attributes that have made it emerge as a versatile tool for organic chemistry. First of all, it displays a regioselectiviy that is mainly directed by sterics, as opposed to electronics, complementing electrophilic aromatic substitution and functional group-directed metalation chemistries. Thus, borylation of aromatic substrates typically occurs at the least hindered position as displayed in the example shown in Scheme 1, which makes C–H activation/borylation an excellent tool for the construction of 1,3,5-trisubstituted arenes. 5-7 The large number of aromatic and heteroaromatic substrates studied by our group and others have demonstrated the broad functional-group tolerance of Ir-catalyzed C–H activation/borylation. Compounds containing amines, amides, esters, ethers, nitriles and halides 3 have uneventfully been borylated, which suggests that this chemistry could even be employed in late-stages in complex molecule synthesis. Most importantly, halogen tolerance expands the synthetic utility of borylated arenes. A halidecontaining borylated compounds can be readily employed in an amidation, 5h amination, 5f Sonogashira coupling, or a C–S forming reaction at the halide position (Scheme 3), moreover the resulting boron-containing product can then be further manipulated. R' Suzuki coupling X R BPin amination/ amidation C–H activation /borylation NR'R" R OH BPin Sonogashira coupling oxidation BPin X R R D deuteration X R X R R' BPin C–S bond formation R SR' Y halogenation R BF3K B(OH)2 X Y = Cl, Br, CN X R C–O, C–N bond formation X R (N,O) X R Scheme 3. Synthetic utility of borylated arenes. Also, C–H activation/borylation proceeds with remarkable cleanliness, which makes it amenable to one-pot transformations. In fact, borylations have been successfully joined with subsequent reactions resulting in the rapid preparation of compounds difficult to access by other means. Some examples include one-pot borylation/oxidation, 5h three-step borylation-amidation-oxidation. 4 5d borylation-amination, 5f and even the 1.3. Applications of Ir-catalyzed C–H activation/borylation Ir-catalyzed C–H activation/borylation has been featured in a number of recent reports of complex molecule synthesis. For instance, Gaunt’s total synthesis of (±)-rhazinicine, 12 started with the borylation of a Boc-protected pyrrole, employing the economical (though less active) precatalyst [Ir(Cl)COD]2 under microwave irradiation (Scheme 4). The pyrrole protecting group sterically directed the borylation to the C3 position, position in the absence of Boc. 6d 5k which would otherwise occur at the C2 The TMS group, on the other hand, was rather used to control the selectivity in the late-stage ring-forming Pd-catalyzed C–H alkenylation. The resulting borylated compound was subsequently subjected to Suzuki coupling to install the rhazinicine biaryl system in a one-pot manner with excellent yield to two steps. 1 equiv B2Pin2 2 mol% [Ir(Cl)COD]2 4 mol% dtbpy N TMS Boc BPin O2N O2N I N hexane, µw 100 ºC, 1 h Boc TMS TSEO O O2N N K3PO4(aq) 2 mol% Pd(OAc)2 4 mol% SPhos nBuOH, 100 ºC 78% (one pot) TMS HN O N Me N TMS O O (±)-rhazinicine Scheme 4. Gaunt’s synthesis of (±)-rhazinicine. 5 Boc Shibasaki and Kanai 13 also utilized Ir-catalyzed C–H activation/borylation in their recent synthesis of SM-130686 (Scheme 5). Interestingly, the borylation substrate, prepared in four steps from a commercially available arylboronic acid, already contained a pinacolboryl group. The resulting diborylated arene was subjected to hydroxylamine-promoted oxidation, 14 which afforded a monooxidation of the most sterically available boron group with excellent yield and selectivity. B2Pin2 5 mol% [Ir(OMe)COD]2 10 mol% dtbpy CF3 BPin NBoc2 THF, 80 ºC 97% CF3 BPin PinB NBoc2 BPin O TfO EtOH 87% N BOM O O N BOM TfO Cl BPin NBoc2 HO Cl F3C HO CF3 CF3 NH2OH·HCl NaOH Cl F3C HO O H2 N N O SM-130686 NEt2·HCl Scheme 5. Shibasaki and Kanai’s total synthesis of SM-130686. In the most recent illustration, Sarpong’s total synthesis of complanadine A, 15 borylation took place, remarkably, at a late stage in the synthesis (Scheme 6). Thus, the triflate group of an advanced intermediate (101) was removed under Pd-catalyzed reducing conditions to afford 102. Pyridine 102 was then subjected to Ir-catalyzed borylation to selectively afford a 3-borylated intermediate 103. Most interestingly, this borylation product was used in a Suzuki coupling with the original advanced triflate 101, followed by Boc deprotection to afford the desired natural product complanadine A in an outstandingly clever manner. 6 Me 0.75 equiv B2Pin2 4 mol% [Ir(OMe)COD]2 8 mol% dtbpy N THF, 80 ºC 75% N Boc Me N BPin N Boc 102 103 1) 101 12.5 mol% PdCl2dppf, K3PO4 2) 6 N HCl, 70 ºC 42% (two steps) 5 mol% Pd(OAc)2 5 mol% dppf HCO2NH4, EtN3 DMF, 60 ºC, 90% Me Me N N OTf N Boc NH H N N 101 104 complanadine A H H Me Scheme 6. Sarpong’s synthesis of complanadine A. These three applications of Ir-catalyzed borylation in the preparation complex molecules are great illustrations of its synthetic value, exploiting in all cases its remarkable regioselectivity. Besides, Gaunt also exemplified the amenability of Ir-catalyzed borylation to one-pot transformations, whereas Sarpong provided an excellent instance of its late-stage applicability. Even though there are examples of the application of Ir-catalyzed C–H activation/borylation in the construction of complex molecules, reports of this sort are still rare. The three examples shown are, in fact, recent. Given the synthetic potential discussed herein and the considerable work on improvements and adaptations of this chemistry carried out in the last decade, anticipate that diverse applications will be reported in the near future. 7 5-7 we In this thesis, four main projects will be discussed, which highlight different synthetic attributes of Ir-catalyzed C–H activation/borylation. For starters, halogen tolerance is featured in our diversity-oriented route to the COX-2 inhibitor DuP 697 and a series of analogs (Chapter 2), which we considered is just one demonstration of the exploitability of building blocks that are easily accessible via C–H activation/borylation. A gentle Ir-catalyzed deborylation method developed in our laboratories has permitted the preparation of heteroaromatics diborylation/monodeborylation. 16 with uncommonly placed boronate groups via A particular tryptophan-based building block prepared in this manner has been utilized in model studies toward the development of a novel convergent route to the TMC-95 core (Chapter 3). Furthermore, the same mild deborylation procedure was tailored for the synthesis of siteselective deuterium-labeled compounds from borylated arenes (Section 2.2). Considering that 17 site-specific deuteration methods usually require halogen/organometallic functionalities, the broad functional-group tolerance of Ir-catalyzed C–H activation/borylation is certainly valuable in the application of this method. Finally, we aimed at evaluating the efficiency of our C–H activation/borylation/ amidation/oxidation method in complex molecule synthesis, which turned out to be successful. Our efforts until now, have provided an unprecedented rapid assembly of the full carbon network of the Hsp90 inhibitor autolytimycin (Chapters 4–7). 8 Chapter 2. Synthesis of the COX-2 inhibitor DuP 697 and related studies 2.1. Synthesis of DuP 697 and analogs via C–H activation/borylation–Suzuki coupling DuP 697 (201) is one of the first members of a series of 2,3-diarylthiophenes reported to selectively inhibit the cycloxygenase-2 (COX-2) enzyme. 18 COX-2 plays a key role in prostaglandin biosynthesis in inflammatory cells and in the central nervous system so its inhibition results in analgesic and anti-inflammatory activity. 19 Scheme 7 shows our diversity-oriented route to DuP 697. Halogen tolerance, one of the most valuable features of Ir-catalyzed C–H borylation, makes possible the construction of 204, a building block that contains both halogen and boron functionalities. Preparation of 204 and other synthetically useful thiophene-based scaffolds via C–H activation was reported recently by our group. 5i 1.5 equiv HBPin, 1.5 mol% [Ir(OMe)(COD)]2 3.0 mol% dtbpy 1.2 equiv LDA 3.0 equiv TMSCl S Cl 202 THF, !78 °C " rt 73% TMS S Cl 203 BPin TMS heptane, rt, 42 h 93% TMS S 204 Cl 1) 2 mol % PdCl2·dppf·CH2Cl2 Br 1.5 equiv K3PO4·nH2O 2) 1 mol % Pd2dba3 (HO)2B 4 mol % XPhos 2 equiv K3PO4 206 Cl 204 SO2Me BPin S SO2Me 205 , 87% Br F , 85% S F DuP 697 (201) 3) 1.0 equiv NBS, CH3CN, rt, 87 % Scheme 7. C–H activation/borylation–Suzuki coupling-based synthesis of DuP 697. 9 Given the excellent chemoselectivity achievable in Suzuki couplings, it is possible to attach two different aromatic groups to the thiophene core. Coupling of 204 to an aromatic bromide (205) proceeds under standard conditions while the chloro substituent remains as a spectator. Then, a successive coupling with 206 at the less reactive chloro-substituted position can be carried out under conditions developed by Buchwald, 20 thus furnishing a diarylated thiophene. The TMS group, which was used as a steric director for the C–H activation/borylation step (203 to 204), can be easily replaced with a bromo substituent via electrophilic aromatic ipso-substitution as a final step. Preparation of DuP 697 (201) via this route was performed by Dr. Venkata Kallepalli, 16 who demonstrated a remarkable 42% overall yield for 5 steps from commercially available and inexpensive 2-chlorothiophene (202). SO2Me SO2Me CF3 Br Br S F S 208 DuP 697 (201) F N CF3 CN N CF3 Br Br S F 207 S 209 CF3 Figure 1. DuP 697 and three basic analogs. 10 This route is characterized by its ability to generate a diverse array of analogs. 22 heterocycle- (207) and trifluoromethyl-containing 21 For instance, (208, 209) versions of 201, as the ones shown in Figure 1, are interesting in terms of their potentially enhanced bioactivity. We aimed at introducing as much variation as possible into our analog library and, thus, a series of compounds were prepared following the aforementioned sequence, where 204 was sequentially diarylated at the C3 and C2 positions to form intermediates 210 and, from there, various types of analogs were synthesized as displayed in Scheme 8. BPin TMS S 204 Cl 1) Suzuki with Ar1–Br or Ar1–OTf 2) Suzuki with Ar2–B(OH)2 or Ar2–BPin Ar1 Ar1 TBAF S Ar2 TMS NB S S Ar2 Ar2 Ar1 Br2 S Br Ar2 S Ar1 TBAF S Br 210 Ar1 TMS Ar1 NBS Ar2 Br Br S Ar2 Br Scheme 8. Preparation of various types of DuP 697 analogs. For the installation of the first aryl substituent, conditions to which aromatic chlorides are known 23 to show low or no reactivity were required. We opted for PdCl2·dppf as catalyst since it is less air-sensitive and more efficient than other monodentate or bidentate phosphine ligand-containing systems. 24 Activation of aromatic chlorides by the Pd-dppf catalyst is limited to very few 11 25 specific substrates and there are previous examples in the literature where their unreactivity has been exploited for the preparation of chlorine-containing substances. 26 Scheme 9 shows the preparation of a first set of monoarylated thiophenes with four diverse commercially available bromide partners. While the coupling reaction itself worked flawlessly and chemoselectively, it was found that TMS removal could occur when the reaction was run at temperatures higher than 80 ºC and after prolonged reaction times (> 12 h). Fortunately, full conversion was observed in all the cases usually after 2.5 hours to provide the desired Suzuki products with high yields. BPin TMS S Cl 204 Ar1 1.2 equiv Ar1–Br TMS 2 mol% PdCl2·dppf·CH2Cl2 1.5 equiv K3PO4·nH2O DME (0.5 M), 80 ºC F SO2Me Cl S CO2Et CN N N S N TMS S Cl TMS S Cl TMS S Cl TMS S 211 212 213 214 87 % 78 % 73 % Cl 84 % Scheme 9. Examples of 3-arylations in the preparation of DuP 697 analogs. It is important to mention that the aryl partner used for the 3-arylation could as well be a triflate. In all cases, complete chemoselectivity was maintained. Mr. Hao Li demonstrated that while 21 Suzuki couplings with aryl and heteroaryl triflates required additional time (2–6 h), under these conditions there was no evidence of reaction at the chloro-substituted position in any case. 12 More importantly, the aryl triflates used in those couplings were generated from borylated arenes and directly used in the Suzuki reactions without need of purification (Scheme 10). Ar1–H C–H activation/ borylation Ar1–BPin oxidation [Ar1–OH] TfO2 Ar1–OTf Suzuki No purification needed Scheme 10. C–H activation/borylation-based preparation of triflate Suzuki partners. 27 For the installation of the second aryl substituent, Buchwald’s biaryl ligands provided the right conditions to activate the chloro-substituted position. Among multiple examples found in the literature, two specific procedures, described below, were tested. Ar1 TMS Cl S 1 mol% Pd(OAc)2 2 mol% SPhos 2 equiv K3PO4·nH2O toluene/water (10:1) (0.5 M), 100 ºC F SO Me 2 CF3 TMS TMS S Ar2 CN CF3 TMS S 215 Ar1 1.5 equiv Ar2–BPin CF3 S 216 77 % CF3 82 % Scheme 11. 2-Arylations in the preparation of DuP 697 analogs via Suzuki coupling under “wet” conditions. Under “wet conditions” with Pd(OAc)2, SPhos, hydrated base, and toluene/water as solvent 2 (Scheme 11), reactions were quite rapid (30 min to 1 h) but borylated partners (Ar -BPin) were particularly susceptible to protodeborylation. In some cases (e.g., 219, vide infra), it appeared 13 that the reaction did not proceed to completion because at a certain point there was no more borylated partner available. In instances where protodeborylation was not an issue, this procedure permitted a quick and efficient preparation of diaryl thiophenes such as 215 and 216, among others, 21 a in good yields. Ar1 TMS Cl S 211–214 SO2Me Ar1 1.5 equiv Ar2–BX2 1 mol% Pd2dba3 4 mol% XPhos 2 equiv K3PO4 tBuOH or tAmOH (0.5 M) 80 ºC N TMS Ar2 S CO2Et N S N NMe2 TMS TMS S F TMS S F S NC 217 85% 218 219 82% 78% BX2 = B(OH)2 BX2 = B(OH)2 BX2 = BPin Scheme 12. 2-Arylation in the synthesis of DuP 697 analogs via Suzuki coupling under “dry” conditions. On the other hand, under “dry conditions” with Pd2(dba)3, XPhos, anhydrous base, and a tertiary alcohol as solvent (Scheme 12), efficient but slower (2–6 h) reactions were observed. Nonetheless, the competing protodeborylation was slower too, allowing the full consumption of the chlorothiophene starting material with 1.5 equiv of borylated partner. a Preparation of 215 and 216 was performed by Mr. Nathan Gesmundo. 14 9 Protodeborylation under Suzuki conditions is a well documented issue. In the preparation of 219, the boron-containing partner was fully consumed and its deborylated form was isolated during the purification stage. It appears likely that its tendency toward deborylation is the due to b the presence of the cyano substituent ortho to the BPin group. Ar1 TMS Ar2 S Ar1 1.05 equiv NBS acetonitrile (0.05 M), rt Br Ar2 S 215–218 F CN SO2Me N CF3 Br CF3 Br S CF3 220 N Br S 221 66% 77% CF3 S F 222 76% Scheme 13. Bromination step in the preparation of DuP 697 analogs For the bromination step, in most cases, reaction of the diarylated compounds with 1 equiv of 28 NBS in acetonitrile efficiently gave the desired final bromination products via ipsoc substitution (Scheme 13). However, analogs with unusually placed bromo substituents (Scheme 14) were obtained quite efficiently due to the presence of specific electron-rich positions on the b 2-Alkenyl and 2-formyl arylboronic acids are known to readily undergo protodeborylation under Suzuki coupling conditions. 9 A similar behavior could be expected for a 2-cyano arylboronate. c + 28 Acetonitrile significantly enhances the activity of NBS as a source of Br . 15 aryl substituents more reactive toward electrophilic aromatic substitution than the thiophene 5position. Surprisingly, complete selectivity was observed in all cases and formation of other isomers was not detected. Ar1 TMS S Ar1 1–2 equiv NBS Ar2 acetonitrile, rt or benzene, reflux TMS S 219 Ar2 Br CO2Et NC S CF3 N Br NMe2 TMS TMS S Br NMe2 S NC NC 223 46% unoptimized 225 83% CO2Et NC S CF3 N Br TMS S Br TMS S NMe2 S NC 224 91% 226 Proposed intermediate Scheme 14. Bromination of diarylated thiophenes at unusual positions in the preparation of DuP 697 analogs. Analog 225 presented an interesting case (when compared to 223) since bromination occurred at the least sterically available position and this isomer was found to be the only product in the 1 crude material by H NMR. Moreover dibromination was not observed when excess NBS was employed. While it is reasonable that the single bromination on the dimethylamino-substituted 16 ring would inactivate it toward a second ortho-substitution, survival of the TMS group was an d interesting observation. The selectivity observed in the preparation of 225 could be explained in terms of a directed bromination. We consider that this reaction occurs most likely via intermediate 226, where the N-bromo cation could presumably be stabilized by both sulfur atoms, partially distributing its positive charge through the thiophene ring as well. This would explain why the TMS group survives the reaction conditions. Interestingly, attempts to over-brominate isolated 225 (and 227, vide infra) with NBS or Br2 were unfruitful, which supports the proposed formation of an Nbromo intermediate that inactivates the system toward further substitution. Ar1 TMS Ar2 S 219, 225 Ar1 2 equiv TBAF THF or THF/H2O (4:1) (0.05 M), rt S CO2Et CO2Et NC S N Br S CF3 N Br NMe2 S S Ar2 S NC NMe2 S NC 227 83% 228 80% 229 81% Scheme 15. Preparation of 5-unbrominated DuP 697 analogs. Lastly, given that derivatives of DuP 697 where the bromo substituent is absent are known to be selective for COX-2, d 18c,d 21 we also prepared a series of non-brominated analogs The same was observed for 223 and 224, compounds prepared by Mr. Hao Li. 17 via TBAF- mediated desilylation at C5 (Scheme 15). This transformation worked uneventfully and provided the desired analogs in very good yields. SMe SMe PhF, AlCl3 X O 230 X = OH SOCl2 100 % DMF CS2 31% POCl3 O SMe H CO2H SH O pyridine, TEA Cl F F X = Cl SMe SO2Me Br2 mCPBA S SO2Me 80% S F F 231 CH2Cl2 AcOH < 50% Br S F DuP 697 (201) Scheme 16. Original linear synthetic route to DuP 697. The presented approach permitted the preparation of over 25 analogs, some of which exhibited good activity in a COX-2 screen. 29 Synthesis of these molecules would not have been viable by 18a using the original synthetic route 18d to DuP 697 (Scheme 16) or the second-generation route (Scheme 17). The original approach was rather linear, relied on the availability of appropriate arylated acetic acids (like 230), and involved transformations, such as a Friedel-Crafts acylation, that are not functional group-tolerant. Furthermore, introduction of the aryl substituents occurred right at the beginning of the synthetic sequence and before the thiophene ring was formed, which is far from being SAR-friendly. 18 SMe SMe SMe (HO)2B Br Pd(PPh3)4, Na2CO3, toluene/EtOH 4:1 reflux, 80% S F NBS (HO)2B DCM/AcOH 80% S SMe Same Suzuki conditions 80% SO2Me MeOH/water 75% F 231 SO2Me Br2 oxone S Br S S F CH2Cl2 AcOH < 50% Br S F DuP 697 (201) 232 Scheme 17. DuPont’s improved route toward DuP 697. Shown in Scheme 17 is an iterative bromination/Suzuki coupling-based approach to DuP 697 reported by DuPont. 18d The key step, bromination at the thiophene 2-position (required for the second coupling), intrinsically relies on its being the most nucleophilic center in the structure. However, it is clear that an EAS reaction could as well occur on the aryl substituents. In fact, on 30 the thiophene ring itself, 2- or 5-bromination could take place and this would largely depend on the nature of the aryl substituent. An electron-poor aryl would be unreactive to EAS but would likely direct the bromination to the thiophene 5-position due to sterics. Conversely, an electron rich aryl could direct the substitution to the 2-position but would compete in the bromination reaction. This situation certainly limits the applicability of this route in the preparation of analogs. In this route to DuP 697 (Scheme 17), the authors introduced the electron-poor methanesulfonylphenyl substituent as a methylthiophenyl group (electron-rich, apparently to favor the subsequent 2-thiophene bromination) and performed an oxidation at a 19 1 1 later stage. We found that installation of Ar via Ar –X (X = Br, OTf) (Scheme 8) widely expands the diversity that can be introduced into the system. For instance, the thiazole bromide used in the preparation of 214 (Scheme 9) is commercially available, while the corresponding boron compounds are unknown. Thus, the preparation of DuP 697 (201) and analogs hereby presented clearly serves as an example of the synthetic flexibility that can result from combining Ir-catalyzed C–H activation/borylation and Suzuki cross-couplings. An alternative preparation procedure will be presented in Section 2.3. 2.2. Access to uncommon borylated regioisomers and selective deuteration via C–H activation/borylation–deborylation monoborylation R HBPin Cl S PinB Ir catalyst 202 B2Pin2, Ir catalyst 95% S 233 5-borylated regioisomer diborylation BPin BPin Cl BPin N H 2-borylated via monoborylation R 1.5 mol% [Ir(OMe)(COD)]2 PinB S 234 Cl MeOH/CH2Cl2 (2:1), 55 °C 70–80% monodeborylation S Cl 235 3-borylated regioisomer BPin N H 7-borylated via diborylation/ monodeborylation Scheme 18. Diborylation/monodeborylation of thiophenes and indoles. 5g,5i,6d,6h Heterocycles react under Ir-catalyzed C–H activation/borylation conditions generally faster than arenes and display heteratom-directed selectivities as exemplified by 233 (Scheme 18), prepared by monoborylation of 2-chlorothiophene 202. Subjecting these substrates to 20 stronger borylation conditions, higher temperature, or excess of HBPin/B2Pin2 leads to the formation of diborylated products, such as 234. With molecules like 234 in hand, it was found by Dr. Venkata Kallepalli 16 that catalytic [Ir(OMe)(COD)]2 in methanol/DCM (2:1) could efficiently perform a selective protodeborylation and, most interestingly, that the first pinacolboryl group installed via C–H activation/borylation becomes the first one to be removed in this process. In this manner, monodeborylation of diborylated heterocycles allows access to uncommon boron-containing regioisomers. Of particular interest to us were 3-borylated thiophenes (See Section 2.3) and 7-borylated indoles (See Chapter 3). CO2Me CO2Me 2 equiv B2Pin2 NHBoc 3 mol% [Ir(OMe)(COD)] 2 NHBoc BPin N H 236 6 mol% dtbpy 0.28 equiv HBPin mtbe, rt, 24 h ~70% BPin 237 CO2Me NHBoc BPin N H CO2Me CO2Me 1.5 mol% [Ir(OMe)(COD)]2 NHBoc NHBoc + BPin 237 N H 238 MeOH/CH2Cl2 (2:1) 55 °C, 3h N H 236 55% BPin N H 28% 63.3 : 36.7 in 1H NMR of the crude material Scheme 19. Preparation of a 7-monoborylated tryptophan derivative. For purposes described in Chapter 3, we required an efficient preparation of a 7-monoborylated tryptophan derivative (Scheme 19, for the optimization see Table 2), attainable via deborylation 21 of 237. In order to facilitate the isolation of the desired product, it was preferred to allow the reaction to run for an extended period of time (3 h) to ensure full consumption of 237 even though dideborylation took place. Remaining 237 only complicated the isolation step. After 3 h, the crude material thus obtained consisted of a clean 63:37 mixture of desired product 238 and recovered 236, which was easily separable by filtration through silica gel eluting with dichloromethane. Interestingly, 7% of dideborylation is observed after only 1 hour of reaction (Table 2), while half of the diborylated compound is still intact. Such an easy deborylation at the “less heteroaromatic” 7-position was intriguing, so we wondered if this method could be used for the deborylation of borylated arenes, as opposed to heteroarenes. In this endeavor, we explored the possibility of using deborylation as a method for deuterium labeling. This type of transformation had already been demonstrated by Dr. Feng Shi, 31 who found that crude materials from borylation reactions could be treated with D2O (See Table 1, method A) at high temperature to afford the corresponding deuterated arenes. It was verified that high temperature was important to ensure a fast and complete conversion, so it was unclear if the drastically milder conditions presented above would work at all for this sort of substrates. However, we were glad to confirm that, in fact, subjection of purified borylated compounds to [Ir(OMe)(COD)]2 using CD3OD/DCM (2:1) as solvent (Table 1, method B) at 55 ºC, provided the desired deuterated products in good yield with percentages of deuterium incorporation (determined by GC-MS) higher than 95% in most cases. The required reaction times for each substrate were estimated by running the reaction in a mixture in CD3OD/CDCl3 (2:1) and 1 monitoring the progress by H NMR. When CD3OD/DCM (2:1) was used, reactions were allowed to continue for slightly longer time to ensure full conversion. This modus operandi was 22 preferred since monitoring the reaction by TLC, and thus exposing the reaction mixture to air, was not an option. Preliminary experiments established that solvent degassing was essential for this reaction to work. Furthermore, a number of experiments suggested that the catalyst effectively “died” due to exposure to air and no more conversion was detected. This fact implies that the transformation is truly catalyzed by Ir and that deoxygenation of the solvent is indispensable for the survival of the catalyst. Table 1. Deuterodeborylation of various borylated arenes. method A R1 = H R2 R1 method B R1 = BPin 1) borylationa R2 2) D2O, THF, 150 °C 1.5 mol% [Ir(OMe)(COD)]2 D MeOD + CH2Cl2 (2:1), 55 °C 31 entry Method A borylation deuteration a b conditions time product Method B yield deuteration time yield c c Cl Cl 1 Cl . . 1.8 equiv HBPin, 3.25 h 1h 95% 5.5 h 85% 1.8 equiv HBPin, 3.25 h 1h 89% 19 h (69% conversion) - 1.8 equiv HBPin, 3h 1.5 h 67% 4h 69% 1.5 equiv HBPin, 6h 2h 69% 8h 74% D Cl 2 N Cl . . D . F3C Cl . 3 D . Br NC 4 D . 23 Table 1 (cont’d) 31 Method B Method A entry d 5 product Me2N . borylation a conditions MeO 6 Cl . . D yield 2h 92% 10 h 97% 1h 88% 6h 67% 1.5 h D deuteration time 1.5 equiv HBPin, 12 h . yield 2.0 equiv HBPin, 18 h Cl deuteration b time 96% 0.5 h 89% c c CO2Et 7 . Me. N H D TMS 8 S . . Cl D a 2 mol% (Ind)Ir(cod)-dmpe at 150 °C for entries 1–5; 1.5 mol% [Ir(OMe)(COD)]2, 3 mol% dtbpy at room temperature for entry 6. b 0.5 mL D2O (~11 equiv) and 3–4 mL THF or DME c d were added to borylation crude material. Isolated yields, average of two runs. Deuteration for method A was carried out in the presence of 0.5 equiv Ac2O. Two additional substrates (entries 7 and 8) were tested to demonstrate how deborylation of heteroarenes is intrinsically faster as compared to arenes, which turned out to be true. Interestingly, the pyridine substrate (entry 2) was slow and complete conversion was not achievable under our mild conditions. We have not yet elucidated the mechanism by which this 24 reaction takes place and it is definitely worth taking note that a pyridine, which could potentially act as a ligand on iridium, displays lower reactivity. In this manner, we have proven that C–H activation/borylation/deborylation is an efficient and mild approach to the synthesis of deuterium-labeled compounds. Comparable site-specific 17 deuteration methods usually require halogen/organometallic functionalities, which weakens their functional group-compatibility. 2.3. Second-generation approach to DuP 697 via C–H activation/borylation–deborylation– Suzuki coupling Access to 235 via diborylation/monodeborylation (Scheme 18) was particularly appealing since we envisioned that it could provide an alternative route to DuP 697 (201). In fact, as shown in Scheme 20, building block 235 performed as productively as 204 (Scheme 7) in the two 28 consecutive Suzuki couplings. The use of NBS in acetonitrile proved its efficiency as compared to other traditional bromination conditions (like Br2 in DCM/AcOH in Scheme 17), providing DuP 697 (201) in excellent yield, as in the case of the TMS-substituted substrate. SO2Me BPin S 235 Cl 1) 2 mol% PdCl2·dppf·CH2Cl2 Br 1.5 equiv K3PO4·nH2O 2) 1 mol% Pd2dba3 4 mol% XPhos 2 equiv K3PO4 SO2Me 205 , 82% Br S (HO)2B 206 F , 83% DuP 697 (201) 3) 1.0 equiv NBS, CH3CN, rt, 91% Scheme 20. Second-generation synthesis of DuP 697. 25 F The use of 235 as starting material instead of 204 is, in addition, convenient since its preparation from 202 proceeds via relatively mild and scalable conditions preferable over silylation via deprotonation with LDA (Scheme 7, page 9). CO2Et CO2Et BPin S 235 Cl S S 1) 2 mol % PdCl2·dppf·CH2Cl2 1.5 equiv K3PO4·nH2O N Br N Br , 79% NMe2 S 2) 1 mol% Pd2dba3 4 mol% XPhos 2 equiv K3PO4 NMe2 PinB NC 227 , 84% NC 3) 1.0 equiv NBS, CH3CN, rt, 88% Scheme 21. Preparation of a DuP 697 analog via the second-generation approach. The new route was tested via the synthesis of one analog. We were pleased to observe that the three-step sequence proceeded with outstanding yields. Not surprisingly, bromination took place displaying the same behavior as in TMS-substituted thiophenes, providing in the case of 227, an unusually placed bromo substituent. 26 Chapter 3. Model studies for the synthesis of the TMC-95 core 3.1. Target choice and significance The isolation of TMC-95A−D (Figure 2) from the fermentation broth of Apiospora montagnei Sacc. TC 1093 was reported in 2000, 32 introducing a novel family of fungal metabolites with a distinctive cyclic peptide structure composed of L-tyrosine, L-aspargine, a highly oxidized Ltryptophan (containing a β,γ-diol), (Z)-1-propenylamine, and 3-methyl-2-oxopentanoic units. HO HO R1 R 2 O R1 R3 R4 H OH CH3 H B H OH H CH3 C OH H CH3 H D N H R2 A N ! H " NH O O N H CONH2 NH O O R3 R4 OH H H CH3 O TMC-95 O H2N OH NH2 L-tryptophan HO N H HO O O CONH2 H2N OH L-tyrosine L-asparagine O R3 R4 NH2 HO structural units O Figure 2. TMC-95 compounds and their structural units. There is considerable interest in the TMC-95 family due to their remarkable activity and selectivity as proteasome inhibitors. 33 32b Such a bioactivity the treatment of cancer, among other diseases. 34 makes them promising as agents for Rapidly after their isolation, appeared reports of 27 35 total syntheses of TMC-95 A or mixtures of TMC-95 A and B by three different research groups and a quite a few studies on the preparation of analogs. 36 A substantial number of synthetic steps in those reports are spent in the construction of the indole moiety, as will be discussed in the following section. We hereby present our attempt to exploit Ir-catalyzed C–H activation/borylation as a tool to access these compounds in a more convenient way. 3.2. General analysis of the reported syntheses of TMC-95 compounds Three research groups have reported total syntheses of TMC-95 compounds, describing two 35a-c somewhat similar approaches for the synthesis of the β,γ-diol. Namely, Danishesfky 35d,e Williams 35f,g employed a dihydroxylation reaction whereas Inoue and Hirama and used an epoxidation (Scheme 22). HO HO HO N H O N H NH O O HO CONH2 NH O R3 R4 N H O PG N PG HO N O O O N H R O epoxidation or dihydroxylation O R N H 301 R'O R = I, Br, or O R" PG = Boc or CBz NHR'" Scheme 22. Access to the β,γ-diol moiety in the total syntheses of TMC-95 compounds. Preparation of intermediates 301, however, was tackled in different ways by the three groups. Danishefsky’s route (Scheme 23) started with the preparation of 7-iodooxindole (303) from oiodoaniline, which was then deprotonated with LDA and added to an “unnatural” Garner aldehyde derived from D-serine. The resulting aldol adduct was mesylated to promote 28 elimination and form the α,β-unsaturated amide. Modest levels of selectivity were observed (E/Z 1.3:1) in the elimination, thus requiring a recycling isomerization step. This 11-step process (without counting the recycling step) was improved by Williams, who modified it to start from a derivative of natural L-serine and to construct the double bond via Julia olefination of 7iodoisatin (302), which gave better selectivity (E/Z 5:1) and resulted in a more attractive 7-step process. a) Cl3CCH(OH)2 NH2OH·HCl Na2SO4, H2O 66% I O O N H NH2 b) H2SO4, 70 ºC 88–98% I 7-iodoisatin 302 a) NH2NH2·H2O, 125 ºC b) 6M HCl, 60 ºC (81% two steps) Danishefsky O BocN H2 N O O HO a) 7-iodooxindole, LDA (Z) I2, toluene 120 ºC 35% O N H I 304 Williams O MeO O 4 steps 65% O CbzN S N from L-serine (E) BocN b) MsCl, DCM !70 to !50 ºC 81% (E/Z 1.3:1) "unnatural" Garner aldehyde D-serine HN CBz HO I 7-iodooxindole 303 O 5 steps 69% HO O N H LiHMDS, 7-iodoisatin CbzN DMPU, DMF 79 % (E/Z 5:1) S O O O N H I 305 Scheme 23. Danishefsky’s preparation of the oxindole core from D-serine (middle) and Williams’ modification from a derivative of L-serine (bottom). 29 X HO CO2H NH2 BPin Danishefsky: X = H 5 steps (73%) R1 O CO2Me Williams: X = I 4 steps (76–86%) NHR2 L-tyrosine or L-o-iodotyrosine 306a R1 = Me, R2 = CBz 306b R1 = Bn, R2 = Boc O O 304 or 305 R3 N R3N O 306a or 306b I O N H N H R1O 1) LiOH, THF/H2O 2) EDC, HOAt, THF CO2Me PdCl2dppf·CH2Cl2 K2CO3, DME, 80 ºC OR4 O NHR2 H2 N 307 CONH2 O R3N O R1O N H O HO Danishefsky: OsO4, NMO (DHQD)2PHAL tBuOH/water, rt OR4 HO O H N R1 O Williams: CONH2 OsO4, pyridine 0 ºC NHR2 N H O O N R3 O OR4 O H N CONH2 NHR2 308 309 R1 R2 R3 R4 Danishefsky Me Cbz Boc tBu 75% 85% 88% (5:1 desired/undesired) Williams Bn Boc CBz Bn 90% 98% 87% (single isomer, desired) yields Scheme 24. A Direct comparison between Danishefsky’s and Williams’ syntheses of TMC-95 compounds. Both Danishefsky and Williams used the resulting intermediates 304 and 305 in the formation of the biaryl system via Suzuki coupling (Scheme 24). Borylated partners 306a and 306b were prepared respectively from L-tyrosine and L-o-iodotyrosine in a very similar manner. In fact, at this point the two syntheses become basically identical and the only difference is the protective 30 groups employed. Scheme 24 shows a straight comparison between the two; Suzuki crosscoupling and attachment of the L-asparagine unit proceeded uneventfully in both cases, but a prominent difference was observed in the dihydroxylation step. In Danishefsky’s case, reaction was run at room temperature to drive the reaction to full conversion and Sharpless’ (DHQD)2PHAL 37 was used to improve the selectivity levels, achieving a modest 5:1 ratio of desired and undesired isomers. Williams, on the other hand, reported that dihydroxylation with OsO4 in pyridine allowed the reaction to occur at 0 ºC, which resulted in complete stereoselectivity. From the advanced intermediates 309, a final 1:1 mixture of TMC-95 A and B was obtained by 35a-c Danishefsky 35d,e after 13 additional steps, whereas Williams used 9 steps, both featuring entirely different approaches for the formation of the labile (Z)-1-propenylamine unit (see Figure 2), but those details will not be discussed here. OH 3 steps 64–72% OH O a) DIBAL, toluene, !78 ºC b) Ph3P=CO2Me c) AlMe3, toluene OMe O O Br BocN NH2 D-serine 310 NH2 Br d) Boc2O, DMAP NEt3, DCM, rt 37% (four steps) O Br Br O N Boc N Boc O Pd2dba3·CHCl3 BocN O NEt3, THF/NMP, rt 86% (Z/E > 20:1) 311 Br N Boc 312 Scheme 25. Inoue and Hirama’s preparation of the oxindole core from D-serine. 31 35f,g Conversely, instead of a dihydroxylation reaction, Inoue and Hirama employed a face- selective epoxidation for the installation of the β,γ-diol (vide infra), again on an α,β-unsaturated amide (301 in Scheme 22). As shown in Scheme 25 the required Z (as opposed to E) unsaturated amide 312 was obtained from unnatural D-serine in 8 steps with high selectivity by virtue of a Heck coupling. It is important to mention that the use of exactly the same method was 35a simultaneously attempted by Danishefsky to construct his intermediate (E)-α,β-unsaturated amide 304 with little success. O O N Boc O Br N Boc O O DMDO DCM, rt dr > 19:1 Br 312 N Boc O BF3·OEt2 O O DCM 87% (two steps) N Boc Br 313 314 PMP 1) Mg(ClO)4, MeCN 2) EVE, PPTS, THF 3) BuLi, ICH2CH2I !60 ºC to rt 81% (three steps) 4) tBuOK, H2O Et2O, rt then H2O, TsOH, MeOH, rt N Boc O N HO HO O HO O 1) CbzCl, NEt3 NH2·HOTs DMF, 0 ºC to rt O 2) PMPCH(OMe)2 TsOH, THF 46% (three steps) HO I N H 315 HO I NHCbz O N H 316 Scheme 26. Installation of the β,γ-diol moiety via epoxidation in Inoue and Hirama’s total synthesis of TMC-95 A. Thus, subjection of 312 to reaction with DMDO cleanly afforded the epoxidation product 313, with high levels of selectivity, which was immediately activated by BF3·OEt2 to afford 314 in high yield over two steps. The newly stereoselectively installed oxygen atom at the indole 3- 32 position was originally on the N-Boc protective group. This transformation was out of the ordinary but, as shown in Scheme 26, required additional manipulation and hence more steps in comparison to Danishefsky and Williams’ route. Moreover, the authors exchanged the 7-bromo for an iodo substituent for an effective biaryl coupling but no supporting details were given. 35f,g Subsequent steps with 316 were performed in a similar manner as in the other two reports; specifically, Suzuki coupling, attachment of the L-asparagine unit, and manipulations, completing the total synthesis of TMC-95 A after 20 additional steps. An important contribution of Inoue and Hirama’s synthesis was the installation of the (Z)-1-propenylamine unit from Lallothreonine benzyl ester; this creative idea was later exploited by Williams. 35e Besides the total synthesis work, there have been a considerable number of reports on the preparation of TMC-95 analogs, particularly coming from the Moroder 36a-f group, who has worked on structural modifications based on the information provided by the crystal structure of the 20S proteasome–TMC-95 A complex. 38 Moroder demonstrated that the intricate structure of TMC-95 A could be significantly reduced while maintaining good levels of activity. 36b Scheme 27 shows the preparation of a simplified e 35a analog: First, Suzuki coupling of 7-bromo-L-tryptophan derivative 317 with Danishefsky’s f intermediate 306a afforded biaryl 318 and to it was attached the L-serine unit. It is important to e f See Scheme 29 for the method used to access 7-bromo-L-tryptophan. The authors did not discuss the methyl ester hydrolysis, which is not likely to occur under Suzuki conditions. 33 highlight that a macrolactamization intended for the preparation of a fully peptidic analog (i.e., non-oxidized indole moiety) would not proceed (middle), according to their later reports. O O NH NHBoc Br N H 317 PdCl2·dppf·CH2Cl2 K2CO3, DME, H2O 80% + O MeO N H O OH NHCbz OMe 306a 318 O NHCbz H2N O NH NHBoc BPin MeO 36a,36c OtBu CONH2 EDC, HOBt NH NHBoc O OtBu MeO N H O NH CONH2 1) acid deprotection 2) macrolactamization (amide coupling) NHCbz DMSO, AcOH, HCl 40% (two steps) O MeO N H O NH O O NH2 OH O NH CONH2 H 4 equiv PyBOP 4 equiv HOBt 6 equiv DIEA 40% 3S MeO N H O NHCbz N H NH O O NH NHCbz 319 Scheme 27. Preparation of a “simplified” TMC-95 analog by Moroder. 34 CONH2 39 Oxidation of the indole ring under mild conditions afforded a mixture of isomers and, interestingly, only the 3S diastereomer reacted to form the closed tripeptide 319. From these 3 results, Moroder concluded that an sp center at the indole 3-position is required for the ring closure in this system. 36a Moroder developed an alternative route in which the macrocycle was closed using a Suzuki coupling. 36c Conveniently, the base used in the coupling reaction simultaneously caused 3 epimerization at the sp oxindole 3-position leading to higher yields of the cyclized product 320 with a 3S stereocenter (Scheme 28). More importantly, it was proven that the mild procedure 39 used for the oxidation of the indole ring (vide supra) was completely innocuous to the boronic ester group. RHN NHR HN HN R2 H K2CO3 PdCl2·dppf O DME/H2O, 80 ºC, 5 h O Br PinB MeO N H O N H O MeO 57% O 3S O NHCbz NH O O NH CONH2 NHCbz 320 O R= O N H H N O NH2 O Scheme 28. Synthesis of Moroder’s second-generation TMC-95 analog via a ring-closing Suzuki coupling. In this manner, Moroder made evident that the synthetically challenging β,γ-diol moiety of the TMC-95 core could be evaded. A disadvantage in this process, however, is that 7-bromo-L35 tryptophan (322), required for the preparation of starting materials like 317 (Scheme 27), is to be 40 accessed via a low-yielding enzymatic procedure (Scheme 29). L-serine N H Br CO2H CO2H NH2 HO NH2 pyridoxal-5'-phosphate, !2"2 tryptophan synthase 37 ºC, 48 h, 9% N H Br 321 322 Scheme 29. Enzymatic preparation of 7-bromo-L-tryptophan. On the other hand, an important contribution on the preparation of TMC-95 analogs has been made by the Vidal group. 36g,h Vidal opted for a non-enzymatic preparation (Scheme 31) of a 7- bromo-L-tryptophan derivative (323) in seven steps, albeit with moderate enantioselectivity 41 (87% ee), using Corey’s method for asymmetric enolate alkylation. O O Br 323 N H HN HN I R1 O OMe OMe 2 equiv NiCl2(PPh3)2 2 equiv Zn O 4 equiv PPh3 R2 DMF, 50 ºC HN R1 O N H O O NHBoc NH O R2 NHBoc 324 a b c d R1 = Bn, R2 = Me: 13% R1 = Bn, R2 = iBu: 4% R1 = Me, R2 = iBu: 8% R1 = Me, R2 = Me: 10% Scheme 30. Preparation of Vidal’s strained TMC-95 analogs. 36 O POCl3/DMF MgBr NO2 THF, !40 ºC 46% Br N H Br 0 ºC to rt 89% N H Br Br OH 1) Boc2O, NaH, THF PPh3, Br2 2) NaBH4, MeOH, 0 ºC 84% (two steps) O Br OtBu N N Boc DCM, !5ºC Br O Ph Ph Br H Br N Boc O OtBu N 10 mol% N H Ph Ph AcOH THF/H2O rt, 2 h NH2 75% (three steps) 87% ee Br HCl/MeOH quantitative N OtBu N Boc O N O OMe NH2·HCl CsOH·H2O, DCM !60 ºC to !20 ºC Br N Boc 323 Scheme 31. Non-enzymatic preparation of a 7-bromo-L-tryptophan derivative. Intermediate 323 was used in the preparation of fully peptidic TMC-95 analogs (Scheme 30). 36g Nevertheless, the employed Ni-mediated ring closure method required two equivalents of nickel/zinc catalyst, low yields of cyclized products (4–13%) were obtained and, unfortunately, no information was given on the atroposelectivity of this ring-closure. Vidal’s peptidic analogs 324a–d (Scheme 30) are exceptionally interesting since cyclization to form such a strained system did occur under metal-catalyzed conditions (compared to the 37 attempted macrolactamization in Scheme 27). Possibly, this transformation could be improved via condition screening. 3.3. Our synthetic approach to the TMC-95 core The 3-hydroxyoxindole core (Scheme 32) is a common moiety found in natural products like the maremycinsm, 42 convolutamydines, 43 celogentins 44 and several drug candidates, 45 and as such has been the target of important recently developed methods for asymmetric installation of oxygen at the oxindolic C3 position. 47 mild conditions for the oxidation CeCl3·7H2O/IBX. 47b 46 Recently, there have been reports on the discovery of 47a of oxindoles and indoles (Scheme 32), using DMDO and While these methods do not offer any stereoselectivity, they happen to be interesting to us since it is conceivable that such mild procedures, applied to constrained molecules similar to Vidal’s TMC-95 analog 324 (and maybe Moroder’s 319), could perform the oxidation in a facial selective fashion. R DMDO/acetone or CeCl3·7H2O/IBX HO R O N R' R DMDO acetone O N R' N R' 3-hydroxyoxindole 2 O R1 R OMe facial selective oxidation NH O RO N H O NH HO CONH2 RO NHBoc R1 R N H O 2 O OMe NH O O NH CONH2 NHBoc R1, R2 = OH, H Scheme 32. Possible facial selective oxidation for the synthesis of TMC-95 compounds. 38 Our synthetic plan toward the TMC-95 family is shown in Scheme 33. Provided that a facial selective oxidation works as expected, it would be possible to transform intermediate 326, in 4 functional group manipulations, to structure 325, which is equivalent to one of Williams’ intermediates, 35e three steps away from the final 1:1 mixture of TMC-95 A and B. R1 R HO HO N H O 2 O N H NH O O HO 3 steps CONH2 NH O R3 R4 N H TMC-95 A–D HO Williamsʼ synthesis: 1) Boc deprotection 2) amide coupling TBSO 2 O R1 R R1= H, R2=OH OMe NH O O N H O NH oxidation CONH2 (potentially facial selective) NHBoc R1 R 326 2 O OMe NH2 2 O OMe NH O TBSO CONH2 NH O O 325 Intermediate in HO N H O OH N H O R3, R4 = CH3, H R1 R 2 O NH O O N H O R1, R2 = H, OH 3) methyl ester hydrolysis 4) desilylation R1 R NH 1) amide coupling BPin N H 328 R1, R2 = H, OH + CONH2 2) Suzuki coupling HO O Br TBSO O NH NHBoc NHBoc 327 329 Scheme 33. Our synthetic plan toward TMC-95 compounds. 39 CONH2 We consider that it would be fascinating from a synthetic point of view if the strained intermediate 327 (Scheme 33) could be prepared in a convergent manner via two sequential steps: Amide coupling of 328 and 329, followed by a ring-closing Suzuki coupling. Even though Moroder showed that formation of such strained compounds is not favorable (Scheme 27), g 36g Vidal’s reported low-yielding Ni-promoted cyclizations 36a,36c (superstoichiometric amounts of Ni/Zn were used, see Scheme 30) encouraged us to adopt this approach in order to test the efficiency of newly developed catalyst systems for Suzuki couplings. More explicitly, Buchwald’s biaryl ligands are known to outstandingly favor not only the oxidative addition and reductive elimination steps but also the transmetalation step; 27 since transmetalation is particularly what leads the cyclization per se in our system, we consider that this idea is worth testing. 48 We were especially motivated because of a very recent disclosure 49 of SPhos conditions that avoid racemization in Suzuki couplings involving aminoacids. Another interesting recent report 50 is the total synthesis of complestatin by the Zhu group, which involves a ring closure of a similarly strained system via Suzuki coupling. This closure required 1 equiv of PdCl2·dppf, and it is worthy of note that a comparable closure, in studies toward the synthesis of the same natural 51 product, was reported about a decade ago but the amount of catalyst was not specified. The need of highly active catalysts for Suzuki coupling in our case, as opposed to traditional ones (i.e., PdCl2·dppf, used in all the reported total syntheses of TMC-95 A–B and analogs, see g The atroposelectivity in this ring-closing biaryl formation is also unknown. 40 Section 3.2), is evident. We plan to find suitable conditions for this transformation via high throughput screening of Pd sources, bases, solvents, concentration, and ligands. This study will be carried out at Merck in the following months. If our attempts are unsuccessful, it is plausible that we could still perform the coupling under the same conditions used by Moroder 36c (Scheme 28) after partial oxidation of the indole ring (see Scheme 27). Nonetheless, a facial selective 3-oxidation of the resulting cyclized oxindole using 47a DMDO (see Scheme 32) has fewer chances to succeed; the substrate scope of this method unequivocally suggests that the process depends very much on the enolization of the amide, which is expected to be difficult in the constrained molecule. Returning to our synthetic plan, preparation of dipeptide 329 did not appear to be a demanding task, so we aimed at developing a route as efficient as possible. monoborylation R R HBPin BPin Ir catalyst N H B2Pin2, Ir catalyst N H 2-borylated regioisomer diborylation R R BPin 1.5 mol% [Ir(OMe)(COD)]2 BPin N H MeOH/CH2Cl2 (2:1), 55 °C monodeborylation BPin N H 7-borylated regioisomer Scheme 34. Access to 7-borylated indoles via diborylation/monodeborylation. Finally, preparation of 7-borylated indoles by virtue of Ir-catalyzed diborylation followed by monodeborylation (Scheme 34) has previously been demonstrated by Dr. Venkata Kallepalli. 41 16 Preparation of 7-substituted tryptophans can be complicated (see Scheme 29 and Scheme 31); our method appears as a most efficient approach for the preparation of 328. 3.4. Results and discussion Our work started with a search for an efficient route to intermediate 328, which contains an Ltryptophan-based structure with an additional hydroxy group at the β-position and a pinacolboryl substituent at the 7-position. Table 2. Optimization study for the preparation of N-Boc 7-pinacolboryl-L-tryptophan methyl ester via Ir-catalyzed deborylation. MeO NHBoc 1.5 mol% [Ir(OMe)COD]2 O BPin BPin N H MeOH/CH2Cl2 (2:1) (0.2 M) 55 °C 330 Reaction time NHBoc MeO MeO O NHBoc MeO O O + BPin + N H BPin N H 331 NHBoc N H 332 333 1 Relative amounts in crude material determined by H NMR 330 331 332 333 Isolated yield of 331 1h 51.4 41.5 7.1 - nd 2h 12.3 69.5 18.2 - 56% 3h - 63.3 36.7 - 55% In order to optimize the preparation of this 7-borylated compound, we used the deborylation of N-Boc-2,7-dipinacolboryl-L-tryptophan methyl ester (330), accessible via Ir-catalyzed C–H 1 activation/borylation, as a model (Table 2). Monitoring this reaction by H NMR was not viable since the use of deuterated solvents (CD3OD) would lead to the formation of deuterated 42 compounds. On the other hand, TLC analysis would not provide quantitative information. Carrying out three different runs and stopping the reactions after 1, 2, and 3 hours respectively gave pretty valuable information. The fact that almost identical yields of the desired compound 331 were obtained after 2 or 3 hours was an interesting coincidence. From these numbers, it is likely that this procedure could be further optimized by carrying out the reaction for a specific reaction time in between 2 and 3 hours. However, letting the reaction run for 3 hours, and thus, ensuring that the diborylated starting material was fully consumed, was significantly more convenient since purification on silica gel became considerably easier. 1) 2 equiv B2Pin2 3 mol% [Ir(OMe)(COD)]2 O 6 mol% dtbpy OMe 0.28 equiv HBPin mtbe, rt, 24 h, 70% NHBoc N H 332 2) 1.5 mol% [Ir(OMe)(COD)]2 MeOH/CH2Cl2 (2:1) 55 °C, 3h 55% 331 + 28% 332 O OMe NHBoc BPin N H 331 Scheme 35. Preparation of a 7-pinacolboryl-L-tryptophan derivative. Thus, the overall yield in the process of subjecting of N-Boc-L-tryptophan methyl ester (332) to diborylation, followed by deborylation (Scheme 35) can be calculated to be 54% for two steps based on recovered starting material. Most importantly, given their sufficiently different retention factors, 331 and 332 were easily separated by column chromatography silica gel eluting with dichloromethane. Subsequent Boc deprotection of 331 with BiCl3 52 cleanly provided 334 (Scheme 36). While TFA is the most commonly used reagent for Boc deprotection in peptide chemistry, the use of + tBu scavengers (anisole, thioanisole) is recommended in the presence of tryptophan, 43 methionine, and cysteine residues. While working on the preparation of tryptophan-containing dipeptides and tripeptides without boronate groups, Ms. Fang Yi Shen demonstrated that the use of BiCl3 was more convenient than several TFA-based conditions. Application of the same procedure to the 7-borylated 331 was pleasingly efficient. However, we were unable to reproduce the original report, where catalytic BiCl3 was used. 52 A stoichiometric amount added in portions gave the best results. O O OMe BiCl3 (0.6 + 0.6 equiv) acetonitrile/H2O (50:1) 60 ºC, 1+1 h NHBoc Bpin N H quantitative (used crude in following step) 331 OMe NH2 BPin N H 334 Scheme 36. BiCl3-mediated deprotection of a 7-pinacolboryl-L-tryptophan derivative. Since preparation of model building block 334 containing a BPin group and a free amine was viable, we simultaneously worked on the development of a route to dipeptide 329 (Scheme 33). Thus, repeating reported procedures, 53 L-tyrosine (335) was subjected to bromination, followed by Boc protection. Then, application of a TBS diprotection/monodeprotection procedure 54 previously used on N-Boc-L-tyrosine afforded intermediate 336 in good yields. 44 OH 1) Br2 HBr/AcOH rt, 89% O NH2 HO 335 L-tyrosine OH Br 2) Boc2O tBuOH/H2O TBSO pH 9, rt, 89% 3) TBSCl imidazole then K2CO3 H2O, rt, 70% O NHBoc 336 Scheme 37. Preparation of the tyrosine unit for the synthesis of the TMC-95 core. Attachment of the L-asparagine unit was conveniently done in “one step” via activation of 336 as its hydroxysuccinimide ester followed by direct reaction with commercial L-asparagine monohydrate (Scheme 38). We found this method suitable for our purposes since there was no need to protect L-asparagine as an ester (or to deprotect it afterward) and dipeptide 329 was obtained already containing a free carboxylic acid and being ready for the formation of the tripeptide. HO OH Br O NHBoc TBSO 336 N-hydroxysuccinimide DCC, DME, 0 ºC, ON filtration then L-asparagine NaHCO3 dioxane, water, 1 h 79% HN Br O CONH2 O NHBoc TBSO 329 Scheme 38. Preparation of a tyrosine–asparagine didpeptide for the synthesis of the TMC-95 core. Dipeptide 329 was further coupled with 335 (Scheme 39) in a moderate-yielding reaction to afford our first model tripeptide to be subjected to ring-closing Suzuki coupling. Even though preliminary Suzuki experiments with tripeptide 337 under conditions that are optimized to avoid 45 48b recemization of aminoacids have failed to give the desired cyclization product, we have established a fast access to this advanced Suzuki substrate, in a way that sufficient amounts of this compound can be easily synthesized and used to find suitable coupling conditions via high throughput screening. O O OMe PinB NH2 N H 335 HO HN Br O OMe EDC, HOBt NEt3 THF, 0 ºC to rt CONH2 58% O NHBoc TBSO PinB N H HN Br TBSO 329 HN O CONH2 O NHBoc 337 Scheme 39. Preparation of a model tripeptide for the synthesis of the TMC-95 core. As presented in Section 3.2, preparation of 7-substituted derivatives of L-tryptophan (Scheme 29 and Scheme 31) can be a tedious task. Our rapid access to 335 provides an clear example of the synthetic utility of Ir-catalyzed C–H activation/borylation combined with deborylation and its capability to build unusual building blocks under gentle conditions. Furthermore, we consider that 335 could be useful in the preparation of TMC-95 analogs, which have attracted even more attention than the natural products themselves. 34 46 Chapter 4. A synthetic approach to autolytimycin 4.1. Target choice and significance A synthetically appealing characteristic of Ir-catalyzed C–H activation/borylation is the cleanliness with which it proceeds; for instance, when HBPin is used as the boron source, hydrogen gas is the only byproduct generated. This makes it highly amenable to one-pot transformations, which is an important feature of our synthetic approach to the Hsp90 inhibitor autolytimycin (401). HO HO 1 3 NH O MeO R R' O C–H activation-based transformation HO O MeO 5 NH O X NH2 autolytimycin (401) X' Figure 3. The Hsp90 inhibitor autolytimycin and its 1,3,5-trisubstituted aromatic core. C–H activation/borylation is an excellent tool for the construction of 1,3,5-trisubstituted arenes (see Section 1.2) and exploiting it in one-pot transformations results in a rapid build-up of complexity. One specific important illustration is a recent report by our group on the preparation of 5-substituted 3-amidophenols from 3-substituted aryl halides via a one-pot C–H 5h activation/borylation/amidation/oxidation (Scheme 40). This protocol has demonstrated a wide functional group tolerance and the isolated yields are reasonably high, taking into account that it involves three reactions in one step. We targeted autolytimycin (401) to prove the feasibility of using this method in a complex system. 47 Ir-catalyzed C–H activation/ borylation PinB Pd-catalyzed amidation oxone oxidation PinB Br HO Br NH NH R' X X X HO HO X O HO NH NH Me NC R' O NH Bn O F3C 68% O OtBu MeO2C 82% O 46% Scheme 40. One-pot C–H activation/borylation/amidation/oxidation. The 1,3,5-trisubstituted aromatic core of autolytimycin is especially attractive since the three meta substituents are all ortho/para-directors in electrophilic aromatic substitution, which makes its preparation difficult by traditional methods. Autolytimycin’s 5-alkyl-3-amidophenol moiety is also found in other natural products like the trienomycins and proansamitocyn. 55 Substituted versions are found in a considerable number of natural products biosynthesized from 3-amino-5hydroxybenzoic acid (AHBA). 56 57 Previously reported preparations of natural products and analogs containing the same 1,3,5- trisubstitution have generally involved a sequence of manipulations to build suitable aromatic building blocks from commercially available compounds that already contain a 1,3,5trisubstitution pattern. For instance, in Smith’s unified synthetic route to trienomycins A and F, 57a a total of seven steps (which is a substantial portion of steps in the linear sequence) are spent in manipulating the substituents on the aromatic core (Scheme 41). 48 OH OBPS 15 steps NH HO O O NH2 402 OMe O O HN PhO2S + 17 steps used in the construction of the ansa chain 7 steps (+)-trienomycin A NO2 (+)-trienomycin F HO2C NO2 403 Scheme 41. Smith’s synthetic route to trienomycins A and F. NO2 1) BH3!THF, 60% 2) CBr4, PPh3 THF, 85% OH 1) LiOMe, HMPA, 98% NO2 2) acetic acid, HBr, 79% HO2C HO2C NO2 403 OH PhO2S OTBDPS 1) TBDPSCl, imidazole DMF, 95% NO2 2) H2, Pd/C, MeOH, 100% 3) PhSO2Na, DMF 60 °C, 75% PhO2S NH2 402 Scheme 42. Preparation of a 5-alkyl-3-aminophenol from 3,5-dinitrobenzoic acid. During the preparation of the required aromatic precursor, the electron-donating alkyl and amino substituents are to be temporarily masked as the electron-withdrawing carboxylate and nitro groups. Nitro groups can be displaced by reaction with lithium methoxide in HMPA, 58 which makes 3,5-dinitrobenzoic acid 403 a suitable starting material in this approach (Scheme 42). Reduction of the carboxylate and nitro groups and functional group manipulations, afforded the 49 desired precursor 402 after seven steps. This route has also been used by the Blagg group in the development of a general route to simplified ansamycin antibiotics. 57b Alternatively, 3,5-dihydroxybenzoic acid (404) can also be used as starting material, since one of the hydroxyls can be directly transformed into an amino group by reaction with ammonia at high temperature, taking advantage of the potential ketonic character of resorcinols. 59 This method (Scheme 43) was used in the preparation of the aromatic core of seco-proansamitocin, of Kirschning’s studies on the biosynthesis of maytansine and the ansamitocyns. 57c-f 57c as part While this approach is a more direct way compared to Scheme 42, given that the amino group is installed in a non-oxidized form, it still implies a series of manipulations to transform the carboxylate group into a synthetically adaptable alkyl chain. OH 2) MeOH, AcCl, 75% (2 steps) HO2C OH 1) NH3, NH4Cl, 180 ºC, 40 h OH MeO O 404 OTBDPS 1) Boc2O, THF H2O, NaHCO3 2) TBDPSCl, DCM, MeO imidazole, DMAP NH2 O 405 OTBDPS 1) DIBAL, THF !78 ºC to !30 ºC NHBoc 2) CBr4, PPh3 DCM 85% (4 steps) Br NHBoc 406 Scheme 43. Preparation of a 5-alkyl-3-aminophenol from 3,5-dihydroxybenzoic acid. We envisioned that if our one-pot C–H activation/borylation/amidation/oxidation method, could be combined with a subsequent B-alkyl Suzuki coupling, aided by the halogen tolerance in the Ircatalyzed step, we could provide rapid access to 5-alkyl-3-amidophenols, in a route that would be flexible in terms of the alkyl group to be installed at the 5-position. Morover, if highly 50 elaborated amides are used in this process, simple commercially available dihalogenated arenes could be rapidly elaborated into complex substituted phenols. Particularly important in the case of our target molecule, autolytimycin, its phenol moiety imparts its biological properties, 60 as will be discussed in the following section. 4.2. Autolytimycin and the geldanamycin family (C15-ansamycins) HO O R1 R1 NH 17 15 21 O MeO 17 2 15 R 4 5 HO NH O O R3 O O MeO 21 MeO O 11 4 R4 5 6 NH2 R1 O O NH2 -H -H -OMe geldanamycin (408) -OMe -Me -OMe herbimycin A (409) -H -H -H -OMe herbimycin B -OMe -H -OMe herbimycin C -H -OMe -Me -Me macbecin I (410) -H autolytimycin (401) R4 -H -H R3 -OMe reblastatin (407) R2 -H -OMe R1 -OMe -Me -Me macbecin II (hydroquinone) Figure 4. Autolytimycin, reblastatin, and the geldanamycin family. 61 Autolytimycin was first isolated in 2000 from Streptomyces sp. S6699, and was reported to inhibit the oncostatin M signaling pathway that promotes cartilage degradation, which is known to cause arthritis in mice. It was later named when an isolation from Streptomyces autolyticus JX-47 was reported in 2001. 62 Autolytimycin (401) belongs to a family of biosynthetically related natural compounds (Figure 1) produced by certain species of streptomyces. 51 63 Unlike 401, most of the members of the family contain an additional double bond between carbons 4 and 5, a benzoquinone ring instead of a phenol, and an oxygenated center at carbon 15. This C15 center has served as a disconnection point in all the previously reported total syntheses of these compounds. In most of the cases, two building blocks were attached via nucleophilic attack to an alkyl or aromatic aldehyde. A ring-closing amide formation, as well, has so far been an ever-present final key step (See Section 4.3 for detailed information). Interest in these compounds is in part due to their heat shock protein 90 (Hsp90) inhibitory 64 activity. Hsp90 has been identified as one of the most promising anticancer therapeutic targets and members of the geldanamycin family have been referred to as natural prototypes for Hsp90 inhibition. 65 HN N HN O geldanamycin (408) MeO 17-AAG 17-DMAG tanespimycin R 17 NH 21 O O MeO OH alvespimycin O HN retaspimycin (hydroquinone) MeO O NH2 Figure 5. Geldanamycin and its analogs in clinical trials. Geldanamycin, the most studied and accessible member in the list, possesses a center at carbon 17 that is susceptible to nucleophilic attack; three C17-substituted derivatives are currently in 66 clinical trials but unfortunately there are toxicity issues (namely, hepatoxicity) attributed to the benzoquinone core. Autolytimycin, reblastatin, together with few non-natural non-benzoquinone (non-hepatoxic) ansamycins have been produced via genetic engineering; 52 60a-c interestingly, these phenolic analogs show appreciably higher affinity for Hsp90 and it has been proposed that this is due to the absence of both the C21 oxygen and the C4–C5 double bond. 60d We recognized that to provide access to autolytimycin via total synthesis in a flexible manner is highly desirable since it would allow the preparation of analogs that are not reachable through genetic engineering or functionalization of the natural products. 4.3. General analysis of the reported total syntheses of C15-ansamycins Members of the geldanamycin family (Figure 4, page 51) have long been targets of total 63 synthesis. Professor James Panek published a comprehensive review on the biosynthesis, design, and preparation of ansamycin antibiotics, which included a complete inspection of the reported syntheses of these compounds. Very recently, Panek himself reported the first total 67 synthesis of autolytimycin reblastatin (vide infra). applying an elegant, convergent route also used in the synthesis of 68 In all the reported total syntheses of C15-ansamycins (eleven in total), a ring-closing amide formation has been a key final step. Also, as mentioned previously, most of the members of the geldanamycin family possess an oxygenated center at carbon 15, which has served as a disconnection point in all of the cases, including two formal syntheses. Family members that have been synthesized multiple times are herbimycin A and macbecin I (Scheme 44). 69 In such syntheses, attachment of the aromatic moiety to the alkyl chain has always involved a nucleophilic attack to either an alkyl aldehyde or an aromatic carbaldehyde. 53 O MeO O NH MeO 15 macrolactamization, oxidation O O 16 MeO MeO OMe 15 R 14 R MeO NH2 HO 6 MeO O MeO O NH2 OPG alkyl aldehyde R = OMe, herbimycin A (409) R = Me, macbecin I (410) MeO aromatic carbaldehyde MeO NR'2 Li 16 OMe NR'2 O 15 OMe 15 O 14 R" R" Scheme 44. General approach to C15-oxygenated C15-ansamycins. When an alkyl aldehyde is used, nucleophilic attack is expected to give the desired product based 69e,69h on the Felkin-Ahn model. However, in the first total synthesis of herbimycin A, Tatsuta reported that the late-stage attachment of the aromatic ring (Scheme 45) provided 412 in good yield but without selectivity. After separation of diastereomers, the undesired isomer was oxidized and then subjected to asymmetric reduction using lithium aluminum hydride treated ® 70 with a chiral amino alcohol (Chirald ) for recycling purposes. 69c,d In order to achieve selectivity, a slight modification was introduced by Kallmerten in his racemic formal synthesis of macbecin I. Thus, subjection of a 5-membered lactol, instead of the free aldehyde, to a reaction with a similar aryllithium species afforded a 1:18 mixture of diastereomers. Unfortunately, it was the minor isomer that had the desired stereochemistry. 54 MeO MeO Tatsuta (Herbimycin A) NHTrt NHTrt O Br MeO MeO MeO OTBS HO OMe OMe MeO MeO nBuLi, THF, !78 ºC then, SM 80% dr 1:1 MeO 411 OTBS 412 Kallmerten (Macbecin I, racemic, formal) MeO 1) MeO N N BnO MeO HO O OMOM MeO Br MeO sBuLi, THF, !20 ºC to rt 2) KH, MeI, DMF 91% dr 1:18 MeO MeO MeO 413 OMOM 414 MeO MeO Martin (Macbecin I, formal) N N OH TBSO O MeO BnO OTIPS Br MeO tBuLi, TMEDA !20 ºC to rt 92% dr 3.5:1 415 HO MeO TBSO HO MeO OTIPS 416 Scheme 45. Late stage attachment of aryl and alkyl fragments in the syntheses of herbimycin A and macbecin I. 69g,69j In his formal synthesis of macbecin I, Martin carried out a yet another interesting modification. Subjecting a six-membered lactol instead of a five-membered one did the trick to favor the formation of the desired isomer, providing 416 with moderate selectivity (3.5:1) in good yield. 55 MeO MeO Cossy (Herbimycin A) N(allyl)2 N(allyl)2 O MeO MeO MeO OTBS Br OMe nBuLi, ether, !78 ºC then, SM 87% dr 1.6:1 OMe HO MeO MeO MeO OTBS 417 418 MeO MeO Micalizio (Macbecin I) N(allyl)2 N(allyl)2 O Br MeO OMe MeO nBuLi, ether, !78 ºC then, SM 81% dr 2:1 HO MeO OMe MeO 419 420 Scheme 46. Attachment of aryl and alkyl fragments in recent syntheses of herbimycin A and macbecin I. Even though the alkyl aldehyde approach suffers from low selectivity, it has been used in the 69l recent total syntheses of herbimycin A and macbecin I by Cossy and Micalizio 69m respectively (Scheme 46). In both cases, it was demonstrated that the free aldehyde (as opposed to a lactol) can be subjected to nucleophilic attack to provide the desired Felkin-Ahn product in moderate selectivity. Cossy’s synthesis of herbimycin A involved an appealing construction of the ansa chain involving three asymmetric allylmetalation reactions, although in a rather linear fashion. Micalizio’s synthesis of macbecin I, on the other hand, involved a very convergent approach, analogous to Panek’s synthesis of reblastatin (vide infra). In contrast, the aromatic carbaldehyde approach (see Scheme 44) has been utilized resulting in all cases in excellent selectivities. The very first total synthesis of a C15-ansamycin (macbecin I) 56 by Baker 69a,b,71 involved the installation of the aromatic ring and the C14 and C15 stereocenters using an Evan’s asymmetric aldol reaction (Scheme 47). This was, however, just the first step, and it was followed by a rather lengthy step-by-step elongation of the ansa chain. MeO MeO O O Ph OMe Et2BOTf, NEt3 DCM, !78ºC 88% 15 14 HO N OMe 12 steps MeO O NO2 O NO2 N MeO O O N O O N Ph MeO 1) NaHMDS O 2) NSO2Ph Ph HO iPr N 5 steps MeO 3) AcOH 83% O MeO N HO O MeO HO O HO iPr N O HO O Scheme 47. First steps in Baker’s first total synthesis of macbecin I. A more convergent route to macbecin I was later reported by Evans, 69f,69i which involved the installation of the seven stereocenters of the molecules through a series of chiral enolate-based reactions. However, a totally different approach was introduced by Panek in his total synthesis of macbecin I, in which six out of the seven stereocenters were installed by exploiting a chiral crotylsilane method developed in his group. Given the similarities between macbecin I and 69k herbimycin A, the latter was also synthesized by the Panek group several years later, using the same left-hand-side intermediates. Attachment of the remaining sections of the ansa chain, nonetheless, was effected in a linear manner in both cases. 57 14 BnO MeO SiPhMe2 CO2Me 421 NO2 MeO MeO TMSOTf DCM, !78ºC 89% dr 20:1 15 OMe OMe NO2 15 HO OMe 14 12 BnO CO2Me MeO MeO NO2 1) BH3·SMe2, THF 2) H2O2, NaOH 85% dr 8.5:1–11:1 OMe HO 12 HO NO2 5 steps HO O OBn CO2Me OMe MeO Scheme 48. First steps in Panek’s total syntheses of macbecin I and herbimycin A. In 2002, Andrus accomplished the first total synthesis of geldanamycin, 72 which was also the first preparation of a compound in the family that did not contain an oxygenated center at C15 (Scheme 49). Anyway, the same disconnection point was used to install the aromatic core, in a manner analogous to Baker’s and Evans’ syntheses of macbecin I (Scheme 47), but performing an asymmetric enolate benzylation instead of an asymmetric aldol reaction. 58 MeO MeO O N NO2 MeO O 15 Br NO2 MeO O OMe MeO Bn 15 NaHMDS, THF !78 ºC 88% dr 19:1 OMe O 20 steps OMe TBSO Bn N O NO2 MeO MeO O O MeO MeO O NH2 HO MeO BOPCl iPr2NEt OMe 11 steps MeO MeO O MeO HO toluene 85 ºC then 3 steps TBSO MeO NH MeO OTES O MeO O NH2 O O NH MeO O O MeO HO HNO3 AcOH 55% 1:10 NH O MeO O MeO HO + O MeO O O MeO NH2 geldanamycin (408) O NH2 o-quinogeldanamycin (422) Scheme 49. Andrus’ total synthesis of geldanamycin. Andrus’ approach, however, was severely linear and after 41 steps the overall yield was evidently low (0.05%). Furthermore, optimized conditions for the final oxidation step provided a 1:10 mixture of desired (408) and undesired (422) oxidized products. This problem was cleverly solved by Panek in his recent synthesis of geldanamycin (vide infra). 59 In recent years, the Panek group has demonstrated the utility of their chiral crotylsilane method, combined with a deoxygenation step, to provide access to C15-deoxygenated C15-ansamycins. This was first illustrated in the total synthesis of reblastatin 406. 68 The first total synthesis of reblastatin started with the reaction of the same chiral crotylsilane reagent employed in previous syntheses (421, compare to Scheme 48) on an appropriately modified aromatic carbaldehyde, followed by a deoxygenation at the benzylic position and installation of the C12 oxygenated center via hydroboration/oxidation as shown in Scheme 50. An additional eleven steps afforded a fully elaborated left-hand half 423. 14 SiPhMe2 BnO BnO CO2Me 421 Br MeO O BnO BnO CO2Me Br BnO Br MeO 5 steps 2) H2O2, NaOH 62% HO CO2Me Br MeO 6 steps O OBn 12 12 CO2Me BnO MeO 15 Et3SiH, BF3·OEt2 BnO BnO 1) BH3·SMe2 THF 15 Br MeO 60% TMSOTf DCM, !50 ºC 64% 15 Br MeO HO BnO MeO OMOM MeO 423 Scheme 50. Panek’s synthesis of the left-hand portion of reblastatin. 68 Panek’s synthesis of reblastatin introduced a number of improvements with respect to previous synthetic approaches, starting from the fact that this new route was much more convergent. Thus, 60 left-hand half 423, was subjected to hydrozirconation/zincation before being added to a fully elaborated right-hand half aldehyde 424, to provide 425 with excellent selectivity at C7, consistent with a Cram-chelate transition state (Scheme 51). The ring-closure was later effected via a Cu-catalyzed amidation reaction, which was the first example of this sort. Until this point all the previous syntheses had involved a macrolactamization. BnO BnO OEt 1) Cp2ZrHCl toluene, 50 ºC MeO 2) ZnMe2, !65 ºC Br MeO MeO O 1.5 equiv 424 MeO MeO O 3 steps OEt 3) OMOM Br MOMO 7 OH MeO 423 425 55% dr 20:1 O BnO HO O Br H2N MeO MeO MOMO MeO OTBS NH MeO CuI NH HN O MeO 3 steps HO K2CO3 toluene 100 ºC 80% O MeO O NH2 reblastatin (406) Scheme 51. Completion of the synthesis of reblastatin by Panek. The Panek group also reported the second total synthesis of geldanamycin, 73 again by exploiting their chiral crotylsilane method. Since a deoxygenation step is required, their route was modified in order to reduce the number of steps. Thus, a tailored chiral crotylsilane 426 was employed, which provides a pyran ring as product. Hydroboration/oxidation and deoxygenation (in the form of a reductive pyran ring-opening) took place in a similar manner as in the case of reblastatin, to 61 afford the corresponding left-hand half 428 comparatively in a lower number of steps (see Scheme 50). iPrO Br MeO O OTMS 14 EtO2C OiPr 15 iPrO Me2PhSi 426 Br MeO TfOH DCM, !90 ºC 84% dr 4:1 O EtO2C 15 14 OiPr 1) BH3·SMe2, THF then H2O2, NaOH 2) Me3OBF4, 4Å ms proton sponge, DCM 71% (two steps) iPrO iPrO iPrO MeO O EtO2C Br Sc(OTf)3 Et3SiH OiPr Br MeO 2 steps OiPr OiPr 14 DCM 72% 12 Br MeO 12 MeO MeO OH 12 CO2Me O MeO 427 428 Scheme 52. First steps in Panek’s total synthesis of geldanamycin. iPrO O NH MeO iPrO O MeO BnO O MeO NH MeO 1) AlCl3, anisole DCM, !78 ºC to rt O 2) 10% Pd/C EtOAc, air 55% (two steps) O O MeO HO O MeO O NH2 NH2 geldanamycin (408) Scheme 53. Final steps in Panek’s total synthesis of geldanamycin. Moreover, in the very final steps, deprotection of the diisopropyl and benzyl ethers, in the presence of the methyl ethers was accomplished with AlCl3 to generate dihydrogeldanamycin, which was readily oxidized with catalytic palladium on carbon under an air atmosphere to give 62 geldanamycin selectively (Scheme 53). In this manner, Panek provided a solution to the selectivity issues observed in the first total synthesis of geldanamycin (Scheme 49). Very recently, Panek reported the first total synthesis of autolytimycin, 67 which is based on the tailored chiral crotylsilane 426, which in the same publication was also employed in a secondgeneration synthesis of reblastatin. Scheme 54 shows the full retrosynthetic scheme for Panek’s synthesis of autolytimycin, highlighting its convergency. Cu-catalyzed ring-closing amidation HO BnO NH Br O1 MeO 15 12 8 MeO O 15 + R3 O HO 7 O hydrozirconation/ zincation/ aldehyde attack MeO 7 O O MeO NH2 OEt 1 8 crotylation/ manipulation BnO BnO reductive pyran opening/ oxidative cleavage O 15 12 BnO Br Br Br O EtO2C 15 429 + 15 OTMS 12 hydroboration/ HO oxidation dr (3:1) MeO O crotylation-based cyclization EtO2C Me2PhSi 430 Scheme 54. Panek’s route in the total synthesis of autolytimycin. Among all the presented synthetic approaches to the C15-ansamycin compounds, Panek’s approach is indeed superior in terms of convergency and rapid installation of stereochemistry. The use of chiral crotylsilanes has provided access to several members of the geldanamycin family, however, each synthesis starts over with a different aromatic core that eventually 63 undergoes the same transformations. It is clear that an approach that introduces the aromatic moiety in a late stage would be significantly more attractive. On the other hand, the chiral crotylsilanes reagents introduce stereocenters in the molecule that are later destroyed. For instance, in the synthesis of autolytimycin (Scheme 54), intermediate 430 possesses four chiral centers, but after reductive pyran opening and oxidative cleavage only two stereocenters are left in intermediate 429, one of which was installed via hydroboration with moderate selectivity (3:1). 4.4. Our synthetic approach to autolytimycin Our synthetic strategy toward autolytimycin (401) is shown in Scheme 55. The target molecule was split into two halves 431 and 432. A B-alkyl Suzuki coupling 74 reaction appeared as an adequate method for the connection of the alkyl chain to the aromatic core and a ring-closing metathesis would allow the closure of the 19-membered macrocycle. Formation of alkylborane 75 coupling partners such as 431 involves, in most of the cases, a hydroboration reaction presence of a β-stereocenter suggested that preparation from an alkylmetal 76 but the intermediate would be more appropriate, since methods for asymmetric hydroboration have not yet been fully developed. 77 Though 9-BBN derivatives are frequently the reagents of choice, 78 we determined that alkyl partner 431 or any boron-containing alternative could be prepared at the appropriate moment from a stable precursor such as 435. 64 HO Ir-catalyzed C–H activation/borylation/ amidation/oxidation RO NH B-alkyl Suzuki 16 coupling 15 NH B OMe 15 O MeO HO 16 O 1 + MeO OR' 8 MeO Cl O 9 RCM MeO O 432 9 NH2 8 OTIPS 431 autolytimycin (401) Br Cl OR" + OR" 15 H2 N 15 O 1 O MeO OR' + MeO asymmetric crotylboration MeO 8 BR* 2 436 435 OTIPS 434 Scheme 55. Our synthetic approach to autolytimycin. On the other hand, we planned to apply our one-pot C–H activation/borylation/ 5h amidation/oxidation method to 3-bromochlorobenzene and amide 434 to access the necessary Suzuki partner amidochlorophenol 432. Ir-catalyzed C–H activation allows the easy preparation of 1,3,5-trisubstituted arenes since its regioselectivity is driven by sterics, not by electronics and besides being amenable to one-pot transformations, it permits the presence of halogens that can then be used for further functionalization. 65 Ir-catalyzed C–H activation/ borylation PinB Br Pd-catalyzed amidation oxone oxidation PinB Br HO NH NH R' X X X undesired self-coupling under Pd catalysis PinB Br R' O possible oligomerization then amidation then oxidation X O R' HN HO O , etc. X X X X Scheme 56. Observed side reactions during Ir-catalyzed C–H activation/borylation/amidation/oxidation. In the presence of halogens, however, it is necessary to suppress undesired self-Suzuki couplings and oligomerizations that might occur under palladium catalysis during the amidation step (Scheme 56). An opportune way to this is to moderately inactivate the Bpin group toward transmetalation under strictly dry conditions. Dr. Feng Shi 31 determined that 3- bromochlorobenzene is the most appropriate dihalogenated substrate for the three-step process, given the complete inactivity of aryl chlorides to the Pd2(dba)3/XantPhos catalyst system used in amidations. Subjection of 3-bromochlorobenzene to our one-pot protocol with a model α,βunsaturated amide successfully provided chloroamidophenol 433 in high yield over three steps (Scheme 57), which bodes well for the employment of this chemistry in our total synthesis. 66 1) HBPin (Ind)Ir(COD), dmpe 2) H2N O Pd2dba3, XantPhos, Cs2CO3, DME HO NH Br 3) filtration 4) oxone, acetone Cl Cl O 433 66% overall yield Scheme 57. One-pot Ir-catalyzed C–H activation/borylation/amidation/oxidation. Thus, access to an amidochlorophenol is straightforward, however, the subsequent alkyl chain attachment has to be performed on this unactivated aromatic chloride. Although the Suzuki cross-coupling is the third most common C–C bond forming reaction used for the preparation of drug candidates, 79 3 2 sp −sp couplings with aryl chlorides are rarely, if ever, used in the synthesis 31 of elaborate structures. Attempts to obtain the corresponding amidobromophenol, starting from 1,3-dibromobenzene or 3-bromoiodobenzene resulted in significant formation of biaryl products, even when purified borylated compounds were subjected to just amidation/oxidation (Scheme 58). 1) H2N O Pd2dba3, XantPhos, HO Cs2CO3, DME PinB NH X Br 3) filtration 4) oxone, acetone Br X = Br, I + mostly biaryl products O X = Br, ~40% X = I, ~20% Scheme 58. Attempts to replace the chloro with a bromo substituent in amidation/oxidation model studies. 67 27,80 Significant advances in the last decade allow coupling reactions with chloroarenes, which were formerly known to be unreactive substrates. Therefore, despite having an aryl chloride electrophilic partner such as 432, marked by the presence of two electron-rich substituents, we 3 2 opted to take the opportunity to evaluate available methods for sp −sp couplings and test their efficacy in this complex system. O H2N OH MeO HO 7 6 7 HO OTIPS 6 OH O 434 asymmetric crotylation OR" 15 L-threonic acid OR" OR' MeO 11 9 435 O dihydroxylation 12 protection MeO O 11 437 Xc 14 manipulation 12 11 asymmetric allylation product 438 Scheme 59. Flexible access to the stereochemistry on the building blocks for the construction of autolytimycin. Having proposed an approach that introduces the aromatic moiety with reactions that do not create stereogenic centers, we aimed at developing a route to the ansa chain that is flexible from a stereochemical point of view. To install the stereocenters on carbons 6 and 7 we turned to the chiral pool (Scheme 59); L-threonic acid contains two chiral oxygenated centers with the same 81 stereochemistry as 434. Since by exploiting carbohydrate-based synthons 68 we could access variations of 434 with altered stereochemistry, this approach was certainly attractive. For our current target, L-threonic acid can be readily prepared from vitamin C. 82 Intermediate 435, on the other hand, bears four stereocenters that can be established by utilizing three (or only two) key stereo-defining steps (Scheme 4). The stereocenter located on C11 and the pendant chain moiety linked to it can be obtained via asymmetric crotylation of aldehyde 437. The two oxygen atoms connected to carbons 11 and 12 and the stereocenter on C12 can be installed by applying a Sharpless dihydroxylation; 37 the hydroxyl groups delivered on the terminal alkene are distinct and can be differently functionalized. At this stage, the relative position of the C11–C12 double bond and the C14 stereocenter suggests the application of an asymmetric enolate allylation, 83 which lead us to recognize 438 as our initial target. 69 Chapter 5. A synthetic approach to autolytimycin: Synthesis of Suzuki partners One of the main disconnections in our synthetic approach to autolytimycin is a B-alkyl Suzuki coupling involving two fully functionalized halves – an amide-containing aryl chloride and an alkyl-BBN adduct – that together form the complete carbon network of the natural product. The preparation of these two elaborated structures is presented in this chapter, which follows our retrosynthetic plan described in Section 4.6 (page 64). 5.1. Preparation of the aryl partner 5.1.1. Synthesis of the amide portion According to the synthetic route developed by Dr. Feng Shi, 31 preparation of amide 434 started h with the synthesis of 3,4-isopropylidene-L-threonic acid methyl ester (501) in three high82 yielding steps from vitamin C (Scheme 60). In this manner, the two present stereocenters are acquired from the chiral pool, avoiding the need for asymmetric induction in the preparation of this half of the molecule. O H2N OH MeO HO 6 8 OH HO 6 OTIPS HO 8 OH HO 8 O O 434 L-threonic acid HO O vitamin C (a) acetone, AcCl (b) H2O2, K2CO3 (b) H2O, 0 °C ! rt (c) MeI, MeCN, reflux 73% (3 steps) O O O 8 OH OMe 501 Scheme 60. Preparation for 3,4-isopropylidene-L-threonic acid methyl ester. h Ester 501 is commercially available but its preparation in large scale from inexpensive vitamin C was significantly more convenient. 70 In order to transform the ester functionality at C8 into the required 2-propenyl group, 501 was TIPS-protected and afterward subjected to a double methyl Grignard addition, followed by dehydration, furnishing a 1,1-disubstituted olefin. Then, acid-promoted deprotection of the acetonide revealed diol 503 (Scheme 61). Dr. Monica Norberg 84 demonstrated that a series of reactions in the preparation of amide 434 could be performed sequentially using crude materials from the preceding step. In this particular case, the crude tertiary alcohol from the Grignard reaction of 502 can be directly dehydrated and subjected to deprotection, without purification of the intermediates. This modus operandi was successfully applied in multigram scale to afford 503 in 68% yield over three steps. O O O TIPSCl, DMAP O OH OMe 501 (a) MeMgCl, ether, 45 °C, 40 min (b) SOCl2, pyridine 75 min O DMF, 90% O 8 OTIPS OMe (c) TFA/MeOH 1h 68% for 3 steps OH HO 8 OTIPS 503 502 Scheme 61. Installation of the 1,1-disubstituted olefin moiety in construction of the amide portion of autolytimycin. Installation of the (E)-α,β-unsaturated amide moiety involved four specific stages showed in Scheme 62. First, diol 503 was transformed into epoxide 504 via tosylation and treatment with a mild base for ring closure. A high yield was obtained, again, in a two-step process without purification of the primary tosylate intermediate. Second, the carbon chain was extended by 85 epoxide opening with allylmagnesium chloride followed by methylation to provide diene 505. It is important to mention that our choice for this specific (and relatively expensive) methylation procedure was based on our need to avoid a silyl group migration from the C7 protected alcohol 71 to the C6 alkoxide. The same issue determined our preference for the bulky TIPS protecting group. 31 For the third stage, a selective reaction of the monosubstituted olefin in the presence of the 1,1-disubstituted alkene was required. Interestingly, treatment of diene 505 with catalytic i 86 osmium tetroxide or potassium osmate together with NaIO4 and 2,6-lutidine resulted in oxidative cleavage, furnishing aldehyde 506 in very good yield and with outstanding selectivity. 84 Even though electron-rich olefins generally react faster than electron-deficient 87 alkenes in osmylation reactions, a number of exceptions to this rule are found in the literature. OH HO (a) TsCl, TEA CH2Cl2, 0 °C ! rt 15 h (a) O OTIPS (b) K2CO3, MeOH 0 °C, 3 h 89% for 2 steps MgCl THF, 0 °C ! rt, 2 h OTIPS (b) MeOTf, KHMDS toluene, "78 °C ! rt 14 h 92% for two steps 504 503 O (OsO4 or K2OsO4·2H2O) NaIO4, 2,6-lutidine MeO 7 6 dioxane/H2O OTIPS 35 min, 77–80% 505 O O H2N 1 (a) EtO H MeO PPh3 MeO toluene, 110 °C 15 h, 96%, E/Z (15:1) OTIPS (b) NH4Cl, AlMe3 benzene, 50 °C 506 22 h, 82% for E only 8 OTIPS 434 Scheme 62. Preparation of the amide portion in our route to autolytimycin. Finally, the synthesis of amide 434 was completed after a Wittig olefination of aldehyde 506 with i commercially available (carbethoxyethylidene)triphenylphosphorane, which gave Since osmium tetroxide solutions tend to deteriorate, the use of the non-volatile potassium osmate is preferable in terms of reproducibility. 72 inseparable isomers of the corresponding ethyl ester in a 15:1 E/Z. A subsequent Weinreb 88 amidation j afforded the desired (E)-α,β-unsaturated amide in an 82% isolated yield. 5.1.2. C–H activation/borylation/amidation/oxidation With amide 434 in hand, Dr. Feng Shi 31 84 and Dr. Monica Norberg confirmed the applicability of our one-pot C–H activation/borylation/amidation/oxidation method on an elaborated system (Scheme 63), to provide our desired B-alkyl Suzuki aryl chloride partner 508. 2.0 equiv HBPin Br Cl 507 2 mol% Ind(Ir)COD 2 mol% dmpe 150 °C, 5 h BPin HO (a) Cl H2N Br 1.4 equiv Cs2CO3 1 mol% Pd2dba3 3 mol% XantPhos O NH Cl MeO O MeO OTIPS 1.5 equiv 434 DME, 100 °C, 3.5 h (b) filtration (c) 1.5 equiv oxone acetone/water, 40 min (d) 1.0 equiv NaIO4, 1 h OTIPS 508 73% (brsm) from 434 58% from 507 Scheme 63. One-pot C–H activation/borylation/amidation/oxidation applied in our synthetic approach to autolytimycin. Performing the three-step sequence gave very good results when the amide was used in excess; 1.5 equivalents were sufficient to keep the undesired Suzuki self-coupling, which was the major j Reaction performed by Dr. Monica Norberg. 73 k side reaction, within acceptable levels (~6%). 84 Most importantly, the excess of amide 434 was recovered in excellent yield. For reasons discussed in Chapter 6, the B-alkyl Suzuki coupling was studied on a series of protected amidochlorophenol substrates (509–511), which were easily obtained from 508 (Scheme 64). TIPSCl, DMAP, NEt3 RO 509 R = TIPS DCM, rt, 12 h, 95% NH NaH, MOMCl O Cl DMF, 0 ºC ! rt, 90% 510 R = MOM MeO 508 R=H OTIPS PMBCl, K2CO3 511 R = PMB DMF, rt, 12 h, 81% PMBO Br H2N Cl 1 equiv 512 O MeO OTIPS 434 1.4 equiv Cs2CO3 2 mol% Pd2dba3 6 mol% XantPhos DME, 100 °C 36 h, 82% PMBO NH Cl O MeO OTIPS 511 Scheme 64. Preparation of protected amidochlorophenols in our route to autolytimycin. An alternative route to protected amidochlorophenols was established by Dr. Monica Norberg. For instance, 511 could also be prepared in high yield by amidation of PMB-protected bromochlorophenol 512 with our elaborated amide 434. Protected phenol 512 is accessible via a one-pot C–H activation/borylation/oxidation (Scheme 65), so essentially the same steps (borylation, amidation, oxidation, protection) are carried out, but in different order. This new k See section 4.6 for a discussion. 74 route would be preferred for the preparation of protected Suzuki aryl chloride partners since undesired reactions and the use of an excess of amide in the preparation of 508 (Scheme 63) can be avoided. a) HBPin [Ir(OMe)COD]2 dtbpy 60 °C, 6 h Cl Br 507 b) H2O2 EtOH, rt 92% (one-pot) OH OPMB PMBCl, Cs2CO3 Cl Br 513 DMF, 17 h, 88% Cl Br 512 Scheme 65. Optimized preparation of PMB-protected 3-bromo-5-chlorophenol. The late-stage installation of aromatic cores from diverse sources that are not necessarily 1,3,5trisubstituted is valuable in terms of potential synthesis of analogs. There have been reports on the preparation of autolytimycin analogs with few non-natural aromatic cores via mutasynthesis (genetic engineering). 89 Total synthesis would significantly expand the scope of aromatic moieties that could be incorporated. 5.2. Synthesis of the alkyl partner Unlike the amide-containing aryl chloride half of the molecule (508), the alkyl-BBN Suzuki partner 514 did require asymmetric induction in its preparation. The construction of this system with four chiral centers could be divided in two stages: the installation of stereocenters at C12 and C14, and an asymmetric crotylation. 75 asymmetric crotylation B OMe OTrt O dihydroxylation 14 OMOM 12 manipulation O 14 12 manipulation asymmetric allylation product MeO MeO Xc 515 514 516 Scheme 66. Two stages in the preparation of alkyl partner in our route to autolytimycin. 5.2.1. Installation of the C12 and C14 stereocenters 90 For our first stereo-defining step, Evans norephedrine-based enolate precursor 517 was allylated using LiHMDS as a base (Scheme 7), which turned out to be a markedly slow and therefore extremely selective reaction that afforded 516 as a single diasteromer by NMR (>97:3). O O O N Ph 517 LiHMDS, allyl iodide O O O H OTIPS R THF, !78 °C 60 h, 93% single isomer BnOLi, BnOH, THF, 97% + 97% recovered chiral auxiliary (a) ADmix-", NaHCO3, H2O/tBuOH, 0–2 °C 80% (3.1:1) or OsO4, K3Fe(CN)6 K2CO3, NaHCO3, quinuclidine, rt 80% (1:1) (b) TIPSCl, DMAP CH2Cl2, 85 % 14 O O 516, R = N Ph 14 12 519 desired O O H OTIPS 518, R = BnO– epi-519 undesired Scheme 67. Installation of the C12 and C14 stereocenters of autolytimycin. Then, in favor of the subsequent dihydroxylation step, we exploited a strategy previously reported by Carter and coworkers in their synthesis of the ABC ring system of azaspiracid: 91 the chiral auxiliary was replaced by a better leaving benzyloxy group to allow the kinetic-controlled 76 37,92 formation of a five-membered lactone after the asymmetric dihydroxylation. In accordance with their results, separation of the resulting diasteromers was viable after TIPS protection (i.e., 519 and epi-519). While the bicarbonate-buffered medium used to avoid racemization of the C14 stereocenter is not expected to influence the stereoselectivity, 37 it is known that terminal alkenes can be poor substrates for Sharpless dihydroxylations. Optimized asymmetric conditions provided a modest 3.1:1 ratio of desired and undesired diastereomers. Isomer 519 was subjected to reductive lactone opening and selective trityl protection of the resultant primary alcohol to give the desired diprotected triol 520. OTrt O O H OTIPS 519 desired (a) LiBH4, MeOH, THF quantitative (b) TrtCl, DMAP, pyr 81%, 98% brsm 14 12 OTIPS HO 520 (a) p-nitrobenzoic acid PPh3, DIAD THF, 89% (b) LiBH4, MeOH THF, 89% OTrt O O H OTIPS epi-519 undesired (a) LiBH4, MeOH, THF quantitative (b) TrtCl, DMAP, pyr 82%, 98% brsm OTIPS HO epi-520 Scheme 68. Protective group manipulations and recycling step in the synthesis of autolytimycin’s alkyl half. 77 In an attempt to utilize the undesired material, the same steps were applied to epi-519 93 simultaneously (Scheme 68). The resulting epi-520 was esterified under Mitsunobu conditions and treated with lithium borohydride to afford a product spectroscopically identical to 520. This sequence demonstrates a viable way to recycle the undesired isomer. Given that non-asymmetric conditions can provide equal yields of a nearly 1:1 mixture of 519 and epi-519, (Scheme 67) asymmetric conditions can be avoided with only a minor decrease in the overall yield. OTrt 12 OTIPS HO 520 (a) KHMDS, MeOTf toluene, !78 °C, 91% (b) TBAF, THF, 95% OTrt 12 (c) SOCl2, DMSO, NEt3 CH2Cl2, !78 °C " 0 °C quantitative H3CO O 11 521 Scheme 69. Preparation of the crotylation substrate in the synthesis of autolytimycin. 85 Methylation of 520 proceeded without complications and the C11 aldehyde was installed after TIPS deprotection and Swern oxidation, to efficiently provide intermediate 521 (Scheme 69). Again, this three-step process can be run on 520 with no purifications without affecting the final yield. 5.2.2. Asymmetric crotylation and final steps For our last enantio-defining step – an asymmetric crotylation reaction – high selectivity was expected since the desired stereochemistry corresponds to a “match case” according to the antiFelkin behavior of (Z)-crotylmetal reagents. 94 In order to provide large quantities of material and simplify its purification (i.e., avoid separation of diastereomers), we searched for a selective and 95 scalable alternative. Roush’s crotylboration was found to be the most suitable since the chiral component, diisopropyl tartrate, is commercially available, inexpensive, and can be used as 78 received. An optimum outcome was achieved when crystalline 522 was used as a precursor for (S,S)-(Z)-crotylboronate 523. O B O 522 (S,S)-DIPT, brine, ether then MgSO4, 90 % NH CO2iPr O B O OTrt 521 OTrt 523 (1.5 equiv) O MeO CO2iPr OH powdered 4 Å ms toluene !78 °C, 20 h 75% single isomer MeO 524 Scheme 70. Asymmetric crotylation in our synthetic route to autolytimycin. Reaction in toluene under strict anhydrous conditions, making sure that all the crotylation agent was in the form of the tartrate ester, 96 afforded 75% of 524 as a single isomer (>97:3) at 60 mmol scale (~25 g of aldehyde). Reactions at small scale performed equally well with yields in the 70–80% range. OTrt OH (a) MOMCl, DMAP, TBAI, iPr2NEt, THF, reflux, 91% I OMOM (b) PPTS, MeOH, 92% (c) I2, PPh3, imidazole benzene, 1.5 h, 77% MeO Scheme 85 526 MeO 524 525 Scheme 71. Final steps in the preparation of the alkyl partner in our synthetic approach to autolytimycin. 79 Finally, crotylation product 524 was manipulated to provide primary iodide 525 in three steps. We envisioned that 525 would be an appropriate precursor for the alkyl partner BBN adduct 526 to be employed in the following Suzuki reaction. As will be discussed in the following chapter, the coupling step required extensive experimentation and, as part of it, we studied the generation of BBN adduct 526 from alkyl bromide 528 as well. Br OH OMOM CBr4, PPh3 DCM 66% MeO 527 OMOM MeO 528 Scheme 72. Preparation of a fully elaborated alkyl bromide to be tested in the generation of the BBN adduct in our synthetic approach to autolytimycin. At the same time, it was necessary to evaluate the effects of the protective group in both the Suzuki coupling and the ring-closing metathesis steps. A specific modification that was done was the replacement of the MOM protecting group with a PMB (Scheme 73). OTrt OH (a) PMBCl, NaH, TBAI THF, reflux, 87% OPMB (b) PPTS, MeOH, 91% (c) I2, PPh3, imidazole benzene, 1.5 h, 93% MeO I 524 MeO 529 Scheme 73. Preparation of a modified alkyl partner in our synthetic approach to autolytimycin. In conclusion, we have established of synthetic routes to access two fragments of the natural product autolytimycin containing its complete carbon network. Special considerations were taken in the design of the synthetic steps, in a way that could be adapted for the preparation of variations of the molecule with different stereochemistry. 80 3 2 Chapter 6. A synthetic approach to autolytimycin: B-alkyl sp –sp Suzuki coupling Following our retrosynthetic plan for the synthesis of the Hsp90 inhibitor autolytimycin described in Section 4.6 (page 64), the complete carbon network of this natural product is to be assembled in a convergent manner via a B-alkyl Suzuki coupling (Scheme 74) between two fully elaborated halves, whose preparation was presented in Chapter 5. Even though the generation of a BBN adduct from an advanced alkyl iodide (603 → 602) to be employed in a Suzuki coupling is well precedented in complex molecule synthesis, 97 the use of an electron-rich aryl chloride (601) as coupling partner was not. The application of this important C–C bond-forming reaction in our synthetic approach to autolytimycin will be presented in this chapter. I OR MeO 603 lithium/halogen exchange B-OMe-BBN B OMe R'O R'O OR NH NH MeO Cl O O 602 MeO OTIPS Suzuki coupling 601 OR MeO OTIPS MeO 604 Scheme 74. The Suzuki coupling step in our synthetic approach to autolytimycin. 81 6.1. Literature precedence 6.1.1. The Suzuki cross-coupling Cross-coupling reactions have had a tremendous impact in organic synthesis. 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 is characterized by the use of organoboron compounds as nucleophilic partners in these reactions. 98 Given the low cost and toxicity, and bench stability of boron-containing compounds, the Suzuki coupling is usually the preferred method. [Pd0] reductive elimination electrophile (organic halide/triflate) oxidative addition R1–R2 coupling product 9 R1–X R1–[PdII]–R2 X = I, Br, Cl, OTf [B] = boron-containing functionality [Pd] = ligand-stabilized palladium R1–[PdII]–X transmetalation [B]–O(base) [B]–R2 X O(base) nucleophile (organoboron compound) Scheme 75. The Suzuki coupling catalytic cycle. The Suzuki coupling follows the same catalytic cycle (Scheme 75) accepted as general for metalcatalyzed carbon–carbon bond-forming reactions. 99 In this way, a palladium(0) species oxidatively adds to an organic halide or triflate (electrophile), forming a Pd(II) intermediate that undergoes a transmetalation with the organoboron partner (nucleophile) to assemble a Pd(II) 82 species containing both organic partners, which will form the desired coupling product via l reductive elimination, process during which the active palladium(0) catalyst is regenerated. In traditional Suzuki systems, the oxidative addition step has been documented to be ratelimiting and it is known that aryl and vinyl halides that are activated by electron-withdrawing groups are more reactive toward oxidative addition than those with electron-donating groups. 100 Currently it is accepted that, in coupling reactions, the identity of the rate-limiting step depends on the strength of the C–X bond in the organohalide, the reactivity of the organometallic partner in transmetalation, and other factors in a case-dependent manner. 101 Since vinyl and aryl electrophiles easily undergo oxidative addition, the Suzuki coupling has 2 2 conventionally been employed for sp –sp bond-forming processes, as exemplified by its widespread use for the formation of biaryl systems. 102 However, besides aryl and vinyl electrophiles, there are plenty of literature examples where alkynyl, allyl, benzyl, and even alkyl halides and triflates participate in a Suzuki coupling under appropriate reaction conditions. 98b On the other hand, the order of reactivity of electrophiles toward oxidative addition has been established to be (under Pd(0)/PPh3 conditions): 103 I > Br > OTf ≫ Cl Thus, for many years, coupling reactions involved almost exclusively the reactive iodides, bromides, and triflates. However, thanks to the development of highly active catalyst systems l – – It is also possible that the X* ligand on Pd could exchange with a O(base) anion prior to the transmetalation per se. 83 bearing bulky phosphine/carbene ligands (See section 6.1.3), organic chlorides now are considered regular participants in this important reaction. 6.1.2. B-alkyl Suzuki coupling The first examples of cross-coupling reaction using alkylboron compounds, specifically alkylBBN species generated via hydroboration, were reported by Suzuki and Miyaura in 1986. 104 Previously, coupling reactions involving alkyl groups bearing β-hydrogens had been limited due to the predisposition of alkylpalladium complexes to undergo β-hydride elimination (Scheme 76). R1–H [Pd0] reductive elimination R2 R1–[PdII]–H oxidative addition R1–X R2 R1 R1–[PdII ] !-hydride elimination R2 P Fe R1–[PdII]–X P R2 transmetalation [B]–O(base) PdCl2 [B] X O(base) PdCl2·dppf Scheme 76. Potential β-hydride elimination in the Suzuki coupling catalytic cycle involving alkylboron compounds. Due to the steric requirements for β-hydride elimination, 105 its rate can be lowered by the use of excess ligand or by employing PdCl2·dppf as a catalyst, which was successfully used by Hayashi 106 bite angle, for coupling reactions with alkylmagnesiums and alkylzinc reagents. Because of its 107 the bidentate bis(diphenylphosphino)ferrocene ligand (dppf) also induces rapid reductive elimination by enforcing a cis geometry between the two organic ligands (coupling 84 3 2 partners) on the square planar Pd(II). This facilitates the formation the sp –sp bond. Furthermore, the use of bulky ligands has also led to the development of efficient methods for 3 3 sp –sp Suzuki couplings. 108 9-BBN R HB O R O R R [RhCl(PPh3)3] R B 1) tBuLi R I B O O OMe B 2) B-OMe-BBN R TIPS 1) S B R M B 2) heating M = MgBr, Li Scheme 77. Methods for the preparation of alkylboron Suzuki partners. Access to alkylboron compounds to be used in coupling reactions (Scheme 77) usually implies a 75 generation in situ via hydroboration of from alkylmetal 76 species, due to purification 109 difficulties. Alternatively, the more readily isolable alkylboronic acids 74 can also be employed. The B-alkyl Suzuki coupling has been extensively used in the area of total synthesis, as will be discussed in Section 6.1.4. 6.1.3. Aryl chlorides as electrophiles in B-alkyl Suzuki coupling 85 110 and trifluoroborates At present, numerous active catalyst systems have been developed that allow coupling reactions with aryl chlorides. 80 The reactivity of such systems is based on the use of bulky electron-rich ligands that readily allow the formation of monoliganded Pd(0) species (as opposed to diliganded) that are extremely active toward oxidative addition. 27 Shown in Figure 6 is a selection of bulky ligands that specifically includes those that have been shown to promote the 3 2 formation of sp –sp bonds. PCy2 P(tBu)2 P Me2N DavePhos P(tBu)3 JohnPhos Buchwald27 Fu23 Ph Ph RO PCy2 iPr OR PCy2 iPr Ph Ph Fe Ph P iPr R = Me, SPhos R = iPr, RuPhos N XPhos N N IMes Nolan115 N QPhos Hartwig113 N Cl Pd Cl N Cl N Pd-PEPPSI-IPr IPr Fürstner114 Organ116 Figure 6. Selected examples of ligands that allow coupling reactions with aryl chlorides. 86 The first example of a successful coupling of an aromatic chloride with an alkylboron compound, specifically hexyl-BBN generated via hydroboration, was reported by Buchwald in 111 1998 when the highly active biaryl ligand DavePhos was introduced (Scheme 78). Remarkably, DavePhos allowed the reaction of chloroarenes (even inactivated by electrodonating groups) and arylboron compounds to occur at room temperature. This, however, was not the case when an alkyl-BBN was used as the coupling partner, which suggests a lower propensity to transmetalation. 1.5 equiv Cl (HO)2B 2 mol% Pd(OAc)2 3 mol% DavePhos 3 equiv CsF dioxane, rt, 2 h MeO MeO 1.5 equiv B Cl MeO 2 mol% Pd(OAc)2 3 mol% DavePhos 3 equiv CsF dioxane, 50 ºC, 22 h MeO Scheme 78. Suzuki couplings on aryl chlorides using DavePhos. Later, the Buchwald group reported that the dimethylamino group on DavePhos (originally assumed to be a bidentate ligand) was not required for high activity. This observation led to the development of a new highly active ligand named JohnPhos (See Figure 6). 112 Once again, as shown in Scheme 79, it is clear that arylboronic acids are significantly more reactive than alkylBNNs as partners. 87 1.5 equiv Cl (HO)2B OAc 1 mol% Pd(OAc)2 2 mol% JohnPhos 3 equiv KF dioxane, rt, 2 h MeO2C OAc MeO2C 1.5 equiv B Cl 1 mol% Pd(OAc)2 2 mol% JohnPhos 3 equiv KF THF, 65 ºC, 20 h MeO2C MeO2C Scheme 79. Suzuki couplings on aryl chlorides using JohnPhos. On the other hand, the massive ligand QPhos, 113 developed by Hartwig, for C–N, C–O, and C–C bond-forming reactions, has been shown to promote coupling reactions between aryl chlorides with alkyl boronic acids. Yet there are no reports of its use with alkyl-BBNs. From comparing the reaction times needed for various chloride substrates (Scheme 80), it is clear that the electronics of the substituents have a significant impact in the reaction rate, presumably affecting in the oxidative addition step. 1.1–1.5 equiv Cl R (HO)2B 1 mol% Pd(dba)2 2 mol% QPhos K3PO4 toluene, 100 ºC R R = p-CN o-Me o-OMe m-OMe 12 h, 97% 18 h, 88% 18 h, 94% 48 h, 94% Scheme 80. Suzuki couplings of aryl chlorides and alkylboronic acids using QPhos. 88 Phosphines are not the only type of ligands that have been studied in B-alkyl Suzuki coupling with chloroarenes. Fürtsner 114 demonstrated that the N-heterocyclic carbene IPr (see Figure 6), generated in situ from its HCl salt in the presence of base, efficiently effected the coupling reaction of both activated and inactivated aromatic chlorides with alkyl-BBN partners generated via hydroboration. This result followed a previous report by Nolan, 115 where IMes (see Figure 6) was shown to activate aryl chlorides for reaction with arylboronic acids. As displayed in Scheme 81, the reaction rates are highly sensitive to the electronic effects of the ring substituents. B O O MeO MeO Cl 1.5 equiv 2 equiv KOMe 1 mol% Pd(OAc)2 2 mol% IPr·HCl THF, reflux 2 h, 98% OMe OMe identical conditions MeO Cl 14 h, 82% MeO 1.5 equiv Scheme 81. B-alkyl Suzuki couplings of aromatic chlorides using the NHC ligand IPr. The use of N-heterocyclic carbenes has been further improved by the Organ group with the development of the pyridine-enhanced precatalyst preparation, stabilization, and initiation (“PEPPSI”) system. 116 Outstandingly, the use of this precatalyst allows reactions to proceed at room temperature. 89 O O H Cl O + MeO B 4 mol% Pd-PEPPSI-IPr K3PO4·nH2O (1.6 equiv) 7 O H MeO 1,4-dioxane rt, 16 h 1.6 equiv O O 7 Scheme 82. Example of a B-alkyl Suzuki coupling with an aryl chloride using PEPPSI. While there has been an explosive development of bulky ligands, both phosphine- and carbenebased, in the last decade, Buchwald’s biaryl ligands have rapidly become the preferred alternative. Recently, Molander 110b reported that by using parallel experimentation (high throughput screening) with multiple catalyst systems, SPhos and RuPhos generated the highest ratios of products in the coupling reactions of primary alkyltrifluoroborates with aryl chlorides. While there have been a considerable number of subsequent advances, 117 the use of aryl chlorides as partners for B-alkyl Suzuki coupling, especially in complex molecule synthesis, is still rare. 3 2 6.1.4. B-alkyl sp –sp couplings in complex molecule synthesis Due to the difficulty of purifying most alkylboranes, B-alkyl Suzuki couplings generally involve a prior in situ formation of the borane. 74 Literature examples that involve a hydroboration to prepare an alkyl-BBN species to be subjected to Suzuki coupling conditions are a considerable in 90 m number. Specifically in complex molecule synthesis, such reported examples generally involve vinyl iodides, 118 vinyl bromides, 119 aryl bromides, 120 121 and aryl triflates as coupling partners. 77 Methods for asymmetric hydroboration have not yet been fully developed but the present stereochemistry on the molecule can induce face selectivity in this process, resulting in welldefined new stereocenters. This concept is showcased in Paquette’s synthetic studies toward altohyrtin A (Scheme 83) among several other examples. O OMe Me O I O OPMB OTMS OMe DBU Me O benzene 60 ºC 122 OPMB OTMS 605 606 9-BBN, THF O OMe O O B TBSO I OTES Cl 1.5 equiv 607 1) OMe Me O OPMB OTMS TBSO 10 mol% PdCl2·dppf K3PO4, H2O, DMF 2) K2CO3, THF/MeOH 3) TBSOTf, 2,6-lutidine Me OPMB OTES OTBS Cl 608 33% overall Scheme 83. A B-alkyl Suzuki coupling on a vinyl iodide featured in Paquette’s synthetic studies toward altohyrtin A. m The large number of examples found using Reaxys™ and Scifinder™ have been carefully reviewed and only few selected examples are cited here. 91 A notable application of the B-alkyl Suzuki coupling is presented in Marshall’s total synthesis of discodermolide (Scheme 84). 97c This is the first example of the generation of a BBN adduct from an elaborated alkyl iodide using lithium-halogen exchange followed by reaction with B97a OMe-BBN. This concept however was first introduced in 1989 by Williard in his studies on the synthesis of the C10–C19 portion of amphidinolide A. Here a hydroboration would not have offered stereoselectivity and would have been incompatible with the diene moiety in 609. Me Me 1) tBuLi Et2O, !78 ºC Me Me I PMBO 2) B-OMe-BBN THF, !78 ºC to rt OTES B MeO Me PMBO 2.2 equiv 609 Me OTES 610 Me Me O O O O TBS MOM Me PMP I Me OMOM 10 mol% PdCl2·dppf K3PO4(aq) DMF 74% 611 Me Me Me Me Me O O O O TBS MOM Me PMP O OTES Me PMB OMOM 612 Me HO Me Me Me O Me HO Me O OH OCONH2 Me OH discodermolide Scheme 84. B-alkyl Suzuki coupling step in Marshall’s synthesis of discodermolide. 92 97 In general, literature examples that feature non-hydroboration routes to alkyl-BBN coupling partners almost exclusively use alkyllithium-based BBN adducts, generated from alkyl iodides and Suzuki-coupled in a one-pot manner. It is important to mention that partners employed in those coupling reactions are vinyl bromides, 97a,b vinyl iodides, 97c-p and aryl iodides, 97q,r substrates that are all exceedingly more reactive than aryl chlorides. Currently, there are plenty of active catalyst systems that allow coupling reactions with aryl chlorides. For some systems, examples of reactions with alkylboronic acids and alkylboron species (generated via hydroboration) have been reported, as discussed in section 6.1.3. To the best of our knowledge, however, there are no examples of the use of aryl chlorides as partners for B-alkyl Suzuki coupling in complex molecule synthesis. Despite this, it is known that aryl chlorides can be coupled with alkyl-BBN species (generated via hydroboration) and therefore, given the multiple reports on the generation of BBN adducts from elaborated alkyl iodides, 97 we envisaged that our desired reaction would be viable. 6.2. Preliminary results Even though the in situ generation of a BBN adduct 526 from our fully elaborated iodide 525 via lithium-halogen exchange, followed by reaction with B-OMe-BBN, was not found to be n 3 2 problematic the crucial sp –sp coupling turned out to be a difficult step (Scheme 85). Optimized conditions allowed a maximum of 5% yield of the fully elaborated Suzuki product (614), accompanied by a bothersome undesired dechlorination side product (615). n This was demonstrated by Dr. Feng Shi 31 through 93 11 B NMR experiments. HO I NH OMOM O MeO 1.15 equiv 525 MeO OH (a) 2.3 equiv tBuLi, ether, !78 °C (b) 2.8 equiv B-OMe-9-BBN, !78 °C " rt 615 dechlorination byproduct B OMe HO HO OMOM NH NH MeO Cl O O 526 MeO OH 613 3 equiv CsF 5 mol% Pd(OAc)2 MeO 10 mol% SPhos (Pd source and ligand added 4 times, every 3 h) THF, 70 ºC, 21 h, 5% MeO OMOM OH 614 Scheme 85. First productive real-system Suzuki coupling in our synthetic approach to autolytimycin. By screening reactions conditions using model substrates (Figure 7), o it was found that protection of the phenol could have a potential benefit. Under basic conditions (required for efficient transmetalation among other advantages), 123 the phenol could be deprotonated, which would affect its solubility or perhaps make the aromatic ring more electron-rich, thereby lowering its reactivity toward oxidative addition. These statements, however, have not been experimentally verified. o Group 1 model substrates were studied by Dr. Feng Shi. 94 31 OR1 OR1 O Cl O N 2 R Cl R1 = Bn, TIPS, Ac R2 = H, SEM, Boc N H R1 = TIPS, MOM, PMB 616, 617, 618 Group 1 Group 2 Figure 7. Model amidochlorophenol substrates used to screen Suzuki coupling conditions in our synthetic route to autolytimycin. I 1.5 equiv 619 (a) 3.0 equiv tBuLi, ether, !78 °C (b) 3.6 equiv B-OMe-BBN, THF, !78 °C " rt OTIPS OTIPS B O Cl N H 616 OMe 2 equiv K3PO4 nH2O 10 mol% Pd(OAc)2 20 mol% SPhos THF, reflux, 12 h < 30% O N H 620 Scheme 86. Modest results obtained with a model substrate in a preliminary screening. Unfortunately, also a vast number of condition-exploring runs with these protected model substrates (Figure 7) only resulted in little, if any, formation of the desired product. The best conditions found to work (~30% conversion) in a model substrate (Scheme 86) were totally inefficient when applied to the fully elaborated system. Up to this point, our main concern was the low reactivity of these highly unactivated (electron-rich) chloride substrates, however, applying reported procedures for the coupling of aryl chlorides with arylboronic or alkylboronic 95 acids on one of our model substrates afforded excellent yields of the coupled products (Scheme 87). The Pd(dba)2/QPhos-promoted reaction was especially significant. Alkylboronic acids are known to be are less reactive towards transmetalation than arylboronic acids. 75,124 original Suzuki conditions 80 Under the for B-alkyl coupling reactions, several alkylboron compounds were compared; alkyl-BBN substrates were found to be the most active, while alkylboronic acids afforded 0% conversion even when TlOH was used as base. OTIPS B(OH)2 1.5 equiv 5 mol% Pd(OAc)2 10 mol% SPhos 2 equiv K3PO4·nH2O THF/water (10:1) reflux, 12 h 91% N H 616 OTIPS 621 O Cl p O N H 622 1.2 equiv B(OH)2 OTIPS 625 O Cl N H 616 OTIPS O 2 mol% Pd(dba)2 4 mol% QPhos 2 equiv K3PO4 toluene, reflux, 48 h 93% N H 620 Scheme 87. Successful Suzuki coupling reactions on model amidochlorophenols. p Kishi 125 2 2 reported rate-enhanced sp –sp Suzuki couplings in the presence of TlOH, presumably due to a fast hydroxyl-halogen exchange at Pd, prior to transmetalation. 96 Previously, the use of alkylboronic acids in Suzuki couplings usually required toxic thalium 125 hydroxide 126 q or thalium ethoxide as additives, so the high activity achieved with QPhos is remarkable. On the other hand, QPhos conditions have not been reported to work with alkylBBN substrates. It is known that different boron-containing compounds may display completely different behaviors in the transmetalation step. For instance, the best conditions found by the Fu 3 3 108a,b group for sp –sp couplings with alkyl-BBN not work at all for alkylboronic acids. reagents (generated via hydroboration) did 108c With these working conditions in hand, we attempted the preparation of a real-system boronic acids 624 from our fully elaborated primary iodides 525 and 529 with no success due to purification issues (Scheme 88). It is known that boronic acids exist in equilibrium with their trimeric cyclic anhydrides, boroxines, 109 and while simple boronic acids can be recrystallized from water, that was not the case in our system. Attempts to isolate the alkyl species as their 110 trifluoroborates 625 also failed. I OR MeO a) 2.0 equiv tBuLi ether, !78 °C b) (iPrO)3B c) 1M HCl(aq) rt BF3 B(OH)2 OR KFHF MeO OR MeO 624 525, 529 R = MOM, PMB K 625 Suzuki coupling on the crude alkylboronic acid using QPhos crystallization Scheme 88. Attempts to prepare isolable alkylborons from our fully elaborated iodide. q The effect of thallium bases in our system was also evaluated by Dr. Feng Shi.31 97 MgCl 1.5 equiv 626 1.5 equiv B-OMe-9-BBN, THF, rt, 30 min B OTIPS OTIPS OMe O O N H Cl 616 10 mol% Pd(OAc)2 20 mol% SPhos 4 equiv DMSO DMF, 110 ºC, 12 h 68% N H 627 Scheme 89. Successful Suzuki coupling on a model substrate via generation of the BBN adduct from a Grignard reagent. 84 On the other hand, Dr. Monica Norberg reported that generation of the BBN adduct from a Grignard reagent could provide a possible solution, although this reaction had to be performed at least at a 10 mmol scale for efficient formation of the alkylmagnesium species from an alkyl chloride. Previously, BBN adducts generated from Grignard reagents had been effectively used by Fürtsner. 127 Unfortunately, generation of the corresponding alkylmagnesium chloride species from our fully elaborated alkyl partner did not provide satisfactory results. Moreover, significant amounts of dechlorination product were observed, especially in the presence of a base. The best results were achieved when DMSO was used as an additive (Scheme 89). The use of DMSO as a palladium “stabilizer” has been previously reported by Sanford. 123 studies 128 Even though, Soderquist’s have shown the importance of the base in Suzuki couplings for efficient transmetalation, among other benefits, in this case the absence of base was intriguingly 98 84 beneficial (See page 113 for a discussion). Additionally Dr. Monica Norberg demonstrated that solvent degassing was important to ensure that the catalyst is long-lived. 3 2 6.3. Evolution to a successful B-alkyl sp –sp Suzuki coupling 97 Examination of the literature examples of generation of BBN adducts from alkyl iodides showed that we were using the most common method for lithium-halogen exchange/reaction with B-OMe-BBN. Nonetheless, different authors preferred to use modified procedures. Thus, we started an investigation on the effects of the BBN adduct generation method in the Suzuki coupling step (Table 3). Until this point, we had only used Method C, with the corresponding addition of THF after B-OMe-BBN was added. This appears to improve the nucleophilicity of the generated alkyllithium species, presumably by breaking its polymeric aggregates. 129 In our case, however, it appeared that an optimum generation of the BBN adduct did not necessarily translate into a successful Suzuki coupling. With Method A, the alkyl iodide was premixed with B-OMe-BBN before the lithium halogen exchange took place. Interestingly, when this reaction was performed in ether or pentane/ether (3:2), little or no conversion was observed in the Suzuki coupling (Table 4). On the other hand, reaction in THF gave promising results, although the dechlorination product was also formed, even in the absence of base. 99 Table 3. Evaluated methods for the generation of the BBN adduct from and alkyl iodide in Suzuki coupling studies. 2.0 equiv tBuLi (1.7 M in pentane) 1.2–1.5 equiv B-OMe-BBN (1.0 M in THF) I 619 B solvent temperature OMe 628 Description Method A 619 was premixed with B-OMe-BBN in THF, ether, or pentane/ether (3:2) (~0.1 M), then tBuLi was added at −78 ºC and the mixture was stirred at room temperature for ~1 hour. Method B 619, dissolved in ether or pentane/ether (3:2) (~0.1 M), was treated with tBuLi at −78 ºC, the mixture was stirred at room temperature for ~1 hour, B-OMe-BBN was then added at −78 ºC, and finally the mixture was stirred at room temperature for ~2 hours. Method C 619, dissolved in pentane/ether (3:2) or ether (~0.1 M) was treated with tBuLi at −78 ºC, B-OMe-BBN was added immediately, and the mixture was stirred at room temperature for ~1 hour. Optional: additional THF after B-OMe-BBN. 100 Table 4. Suzuki coupling results on model substrates using Method A for generation of the BBN adduct. I ~1.5 equiv 619 Method A (a) ~3.6 equiv B-OMe-9-BBN, solvent, !78 °C (b) ~3.0 equiv tBuLi, !78 °C " rt OR B O N H Cl 629 entry 1 628 OR O (no base) 10 mol% Pd(OAc)2 20 mol% SPhos DMF, reflux, 12 h Solvent used in BBN generation ether or pentane/ether (3:2) OR OMe O + N H N H 630 631 R Result MOM No reaction 2 THF TIPS 15% 630 + 30% 631 3 THF MOM 45% 630 + 20% 631 Since the desired product was obtained in higher yield than the best previous result (Scheme 86), these conditions were applied to the fully elaborated substrates. Although in the model substrates the MOM-protected phenol afforded a higher yield of Suzuki product than the dechlorination product (Table 4, entry 3), in the fully elaborated system, the ratio was reversed (Table 5, entry 1). The same conditions were tested on slightly modified substrates in the presence of a base (Table 5, entries 2 and 3), different runs afforded different yields but the Suzuki/dechlorination ratio was maintained as ~1:2, as it was in most of the cases using Method A for the formation of the BBN adduct. 101 Table 5. Suzuki coupling results on the fully elaborated substrates using Method A for generation of the BBN adduct. I OR' RO MeO NH 1.2 equiv 632 Method A (a) 1.32 equiv B-OMe-9-BBN, ether, !78 °C (b) 2.4 equiv tBuLi, !78 °C " rt O MeO OTIPS 636 B OMe RO + RO OR' NH NH MeO Cl O O 633 MeO OTIPS 10 mol% Pd(OAc)2 20 mol% SPhos DMF, reflux, 12 h OR' MeO OTIPS MeO 635 634 Entry R R' additive Result 1 MOM MOM - ~20% 635 + 46% 636 2 PMB MOM 2.0 equiv K3PO4·nH2O 7% 635 + 15% 636 3 PMB PMB 2.0 equiv K3PO4·nH2O 6–15% 635 + 15–30% 636 In Method B the use of THF was avoided, the lithium-halogen exchange step was allowed to run for extra time, and the reaction with B-OMe-BBN was also given enough time to proceed. Interestingly, dechlorination was never observed in this case (Table 6). 102 Table 6. Suzuki coupling results on model substrates using Method B for generation of the BBN adduct. I ~1.5 equiv 637 Method B (a) ~3.0 equiv tBuLi, !78 °C " rt, 1 h (b) ~3.0 equiv B-OMe-BBN, solvent, !78 °C " rt, 2 h OPMB B O Cl N H OPMB OMe O 638 N H additive 10 mol% Pd(OAc)2 20 mol% SPhos DMF, reflux or 85 ºC, 12 h 618 639 entry Solvent used in the BBN generation Temperature in Suzuki coupling Additive Isolated yield 639 1 ether 110 ºC - 18% 2 ether 100 ºC 4 equiv DMSO 38% 3 ether 85 ºC 4 equiv DMSO 45% 110 ºC 4 equiv DMSO ~5% 85 ºC 4 equiv DMSO and 2 equiv K3PO4·nH2O 45% pentane/ether (3:2) pentane/ether (3:2) 4 5 When the lithium-halogen exchange was performed in ether, encouraging results were obtained. From comparing entries 1 and 2, we observed a positive effect of using DMSO as an additive. Another positive effect was attained by lowering the reaction temperature (entry 3). Recent 130 reports have revealed that DMF can act as a source of hydride in Pd-catalyzed 103 dehalogenations at high temperature (150 ºC), we considered that a lower temperature would be safer, even though dechlorination was not observed in these cases. On the other hand, using pentane/ether (3:2) as a solvent in the lithium-halogen exchange resulted in extremely low conversions. Outstandingly, the yield was substantially increased by adding both DMSO and base as additives, without giving any dechlorination product. I OPMB MeO 1.6 equiv 529 Method B (a) 3.2 equiv tBuLi, ether, !78 °C " rt (b) 3.8 equiv B-OMe-BBN, !78 °C " rt B OMe PMBO PMBO OPMB NH NH MeO Cl O O 640 MeO 2 equiv K3PO4 nH2O 10 mol% Pd(OAc)2 MeO 20 mol% SPhos 4 equiv DMSO DMF, 85 ºC, 15 h 40% conversion, 36% yield 59% recovered 511 MeO OPMB OTIPS 511 OTIPS 642 Scheme 90. Example of a set of partially optimized conditions for the B-alkyl Suzuki coupling in our synthetic route to autolytimycin. From these results, we concluded that lithium halogen exchange in ether, followed by Suzuki coupling at 85 ºC in the presence of DMSO and base would afford the best result. Thus, the Suzuki coupling step was carried out with the fully elaborated substrates under these partially 104 optimized conditions (Scheme 90). At this point, the main problem appeared to be the decomposition of the palladium catalyst to form palladium black, observable as the formation of black precipitate. In the presence of DMSO the decomposition occurred in a matter of hours instead of minutes. The results nonetheless were encouraging since a good yield based on recovered starting material is obtained (90%). At this point, we wondered if an optimized formation of the BBN adduct would improve these results, thus we returned to Method C for the formation of the BBN adduct (Table 7). In Method C, the alkyl iodide, dissolved in ether or pentane/ether(3:2), is treated with tBuLi and B-OMeBBN added rapidly one after the other, followed by the addition of THF. Surprisingly, when THF was not added at the end, the Suzuki coupling step proceeded with very poor conversions (~10%). At the same time, addition of THF when pentane/ether (3:2) was used as solvent in the exchanges, also resulted in low yields (Table 7, entry 1). Conversely, when ether was used as solvent, results were encouraging in the presence of both DMSO and base (entry 2) since dechlorination was not observed. These conditions are very similar to those originally found through screening (Scheme 86), which afforded about 30% of desired product (and no dechlorination) but were inefficient in the fully elaborated substrates. The differences, however, were the use of DMF as a solvent in the Suzuki coupling, the addition of DMSO, and solvent degassing. Surprisingly, elimination of the DMSO (entry 3) did not affect the result and comparable yields were obtained, so the increase of yield is either due to DMF or solvent degassing. 105 Table 7. Suzuki coupling results on model substrates using Method C for generation of the BBN adduct. I 1.6 equiv 637 Method C (a) 3.2 equiv tBuLi, !78 °C (b) 3.8 equiv B-OMe-BBN, THF, !78 °C " rt, 1 h OPMB B O Cl N H OPMB OMe OPMB O 638 N H additive 10 mol% Pd(OAc)2 20 mol% SPhos DMF, 85 ºC, 12 h 618 639 O + N H 641 entry Solvent used in the BBN generation Concentration during BBN generation additive Result 1 pentane/ether (3:2) 0.1 M 2 equiv K3PO4·nH2O 10% 639 2 ether 0.1 M 2 equiv K3PO4·nH2O and 4 equiv DMSO 45% 639 3 ether 0.1 M 2 equiv K3PO4·nH2O 50% 639 2 equiv K3PO4·nH2O Full conversion 65% 639 + 10% 641 4 ether 0.5 M On the other hand, we also recognized that the generation of BBN adducts from Grignard reagents (Scheme 89, which resulted in better conversions) was taking place at a higher concentration (0.5 M) than the lithium-halogen exchange (0.1 M). So we evaluated the effect of increasing the concentration (entry 4), which resulted in substantial enhancement in conversion 106 (from ~50% to full conversion), even though dechlorination was formed in these circumstances. The same conditions were then tested on the fully elaborated substrates (Table 8). Table 8. Suzuki coupling results on the fully elaborated substrates using Method C for generation of the BBN adduct. I OPMB PMBO MeO NH 1.6 equiv 529 Method C (a) 3.2 equiv tBuLi, ether, !78 °C (b) 3.8 equiv B-OMe-BBN, THF, !78 °C " rt O MeO OTIPS 643 B OMe PMBO + PMBO OPMB NH NH MeO Cl O O 640 MeO OTIPS 511 2 equiv K3PO4 nH2O 10 mol% Pd(OAc)2 20 mol% SPhos 4 equiv DMSO DMF, 85 ºC, 12 h MeO OPMB OTIPS MeO entry Concentration during BBN generation Result 1 0.1 M 30% conversion 24% 642 2 0.5 M 642 0% 642 + ~10% 643 Table 8 shows the results of applying Method C on the fully elaborated substrates. When the lithium-halogen exchange was carried out at low concentration (0.1 M) the following Suzuki 107 reaction proceeded with moderate conversion, but a high concentration (0.5 M) in the halogen exchange severely affected the performance in the coupling step with only small amounts of the dechlorination product observed. This outcome somehow contradicts our conclusions from Table 7, but there are apparently considerable differences between our model and fully elaborated primary iodides. In general, these experiments suggested that the generation of the adduct should be better run at low concentrations, whereas the Suzuki coupling itself can be driven to full conversion at a high concentration. We figured that this Catch-22 could be solved by evaporating the solvent with a nitrogen stream prior to the coupling reaction. Then, after addition of DMF (solvent in the Suzuki reaction), the coupling can take place at a concentration ten times higher than in previous cases. This modification was applied on the fully elaborated substrates (Table 9). Surprisingly all our initial attempts resulted in no reaction (entry 1), however addition of Pd(OAc)2 and SPhos as a solution in THF resulted in a remarkable improvement, leading to complete formation of the desired product, which was also isolated in high yield. These intriguing results suggest that the formation of the active pre-catalyst SPhos·Pd(OAc)2 is difficult in the presence of the highly concentrated alkyl-BBN adduct. On the other hand, it is interesting that addition of the catalyst mixture as solutions in DMF and DMSO did not provide good results, in contrast with the use of THF as a solvent. In fact, Pd and ligand premixing in THF has been suggested by the Buchwald group for substrates that give poor or no conversions. 131 A subsequent improvement in this reaction has been the elimination of DMF as solvent to be replaced by only the THF solution of catalyst, which has resulted in amazing results (Scheme 91). 108 Table 9. Suzuki coupling results on the fully elaborated substrates at high concentration. I OPMB MeO 1.6 equiv 529 Method C (a) 3.2 equiv tBuLi, ether, !78 °C (b) 3.8 equiv B-OMe-BBN, THF, !78 °C " rt (c) evaporation of solvent B OMe PMBO PMBO OPMB NH NH MeO Cl O O 640 MeO 511 2 equiv K3PO4 nH2O OTIPS 10 mol% Pd(OAc)2 20 mol% SPhos DMF, 85 ºC, 12 h MeO OPMB OTIPS MeO 642 entry palladium source and ligand addition mode Result 1 As solids 0% conversion, no dechlorination. 2 As a degassed solution in THF Full conversion No dechlorination 83% 643 In summary, our preliminary experiments brought us to the realization that this transformation required unusual tweaking. In fact, concentration turned out to be essential in this transformation; the formation of the BBN adduct performed better at low concentration, whereas the Suzuki coupling itself required high concentration. Evaporation of the solvent with a nitrogen stream prior to the coupling reaction allowed us to meet this requirement, but premixing the 109 palladium and the ligand in solution before being added to the reaction vessel was necessary for catalytic activity. By combining these factors, high-yielding conditions for the coupling reaction were found and applied to the fully elaborated Suzuki partners. I OMOM MeO 1.6 equiv 525 (a) 3.2 equiv tBuLi, ether, !78 °C (b) 3.8 equiv B-OMe-BBN, THF, !78 °C " rt B OMe PMBO PMBO OMOM NH NH MeO Cl O O 526 MeO OTIPS 511 2 equiv K3PO4 nH2O 10 mol% Pd(OAc)2 20 mol% SPhos THF, reflux, 12 h 90% MeO OMOM OTIPS MeO 644 Scheme 91. Optimized Suzuki coupling conditions used in our route to autolytimycin. The remarkable effect of concentration on the reaction efficiency was surprising to us since we did not find literature cases where this matter was discussed. Interestingly, in a recent (2008) 3 paper regarding the application of the Pd-PEPPSI catalyst system (see Figure 6, page 86) on sp – 3 sp Suzuki couplings, the Organ group reported that dilution had a detrimental effect on the yield. 116 Explicitly, in one reaction, changing the concentration from 0.8 M to 0.6 M lowered the yield from 71% to 50%. Additionally, no byproducts were found and starting materials were 110 recovered. This proved that the diminution of yield was not due to undesired reactions but to incomplete conversion. It is essential to mention that it is specified in the supporting information that the employed alkyl-BBN species were generated using solid 9-BBN dimer (as opposed to 9BBN solution in THF), which allows their preparation in high concentration. Nevertheless, it is still not clear which step of the catalytic cycle is affected by concentration. TBSO TBSO 4 mol% Pd-PEPPSI-IPr K3PO4·nH2O (1.6 equiv) Br 1,4-dioxane rt, 16h 645 + MeO MeO B 647 0.8 M 0.6 M 1.6 equiv 646 71% 50% Scheme 92. Concentration effect in a B-alkyl Suzuki coupling observed by the Organ group using the highly active Pd-PEPPSI system. 97n Also, in their recent (2008) total synthesis of haterumalide NA, the Kigoshi group vaguely discussed the effect that concentration might have had in one of their Suzuki couplings involving alkyl-BBN species. Specifically, when the generation of the alkyl partner was effected via hydroboration with solid 9-BBN dimer, the Suzuki coupling afforded quantitative yield, whereas the use of 9-BBN solution (dilute) lead to 0% conversion (Table 10). Interestingly, generation via lithium-halogen exchange followed by treatment with B-OMe-BBN (low concentration too) provided low conversion. It is highly probable that evaporation of the solvent at that stage would have solved the low conversion problem. 111 Table 10. Concentration effect in a B-alkyl Suzuki coupling observed in Kigoshi’s total synthesis of haterumalide NA. O O tBuLi O I O MeO O B-OMe-BBN ODMPM B O ODMPM 648 649 O O O O 9-BBN B O O ODMPM ODMPM 650 651 HO O 649 PdCl2·dppf CsCO3(aq) O O OH O or TMS I O 651 OTHP dioxane, rt O ODMPM TMS AcO OTHP 652 O Cl haterumalide NA starting material Conditions for the generation of the BBN adduct Yield in Suzuki coupling 648 tBuLi, B-OMe-BBN 32% 649 9-BBN solution in THF 0% 649 Solid 9-BBN dimer quantitative It is interesting that while in the majority of the cases involving hydroboration in complex molecule synthesis 9-BBN solutions are used, in all the methods presented in section 6.1.3 (page 85), for the coupling of aryl chlorides and alkyl-BBN species, the starting materials are always 112 prepared using solid 9-BBN dimer. The importance of concentration, especially for substrates of comparatively low reactivity in coupling reactions, is certainly an aspect that has not been discussed in the literature. Another aspect of the reactions to discuss is the dehalogenation side reaction found in some of 9 our experiments. Dehalogenation is a known issue in some specific Suzuki couplings but the high levels observed in several of our runs, imply a source of hydride. 130 A number of reports have shown that DMF can act as a source of hydride for Pd-catalyzed reductions/dehalogenations via formation of dimethylamine and carbon monoxide at high temperature (150 ºC). While our reactions were run at a significantly lower temperature, this could still be one of the possibilities. However, this would not explain why dehalogenation is observed in such high levels when the lithium-halogen exchange and reaction with B-OMe-BBN were performed in THF (Table 4, page 101) or when the BBN adduct was prepared from Grignard reagents. NO2 + TfO Pd(PPh3)4 B K3PO4 dioxane 85 ºC, 5 h NH2 NO2 + 48% 30% Scheme 93. Reduction of a nitro group under Suzuki coupling conditions with an alkyl-BBN species. 103a Interestingly, back in 1993, Suzuki reported the unexpected reduction of a nitro group during a coupling reaction between an alkyl-BBN and an aryl triflate (Scheme 93). This result was explained by suggesting that this transformation could occur via a reduction mechanism similar to the one observed for dialkyl-BBN “ate” complexes. 113 132 Dialkyl-BBN “ate” complexes (obtained via reaction of alkyl-BBN and alkyllithium species) can in fact act as reducing agents 132b (Scheme 94) on carbonyl compounds and tertiary alkyl, benzyl and allyl halides. 132c,d An analogous behavior has not been reported for their monoalkylated counterparts. dialkyl-BBN "ate" complex R H B R B R' alkyl-BBN "ate" complex (base)O H + H R' H B + ? (base)O R' B R' Scheme 94. Dialkyl-BBN “ate” complexes as sources of hydride. OTBDPS B I OTBDPS Cy B 656 OTBDPS PdCl2·dppf AsPh3, Cs2CO3 DMF/THF, 70 ºC 3h 653 Cy O O 654 28% 655 9% OTBDPS PdCl2·dppf AsPh3, Cs2CO3 DMF/THF 60 ºC, 3 h, 25% rt, 18 h, 70% + O 655 Scheme 95. Reduction of a diene under Suzuki coupling conditions with an alkyl-BBN. 133 In addition, De Clerq has also reported the reduction of a diene (Scheme 95) during the Suzuki coupling of a vinyl halide with an alkyl-BBN partner. Interestingly, by employing an alternative bulky borane Cy2BH as a replacement for 9-BBN in the hydroboration reaction, the 114 reduction during the Suzuki coupling was not observed. This specific result indicates that the source of hydride is likely related to the BBN species. We observed dehalogenation during our Suzuki experiments, especially when the formation of the BBN adduct was performed in THF (Table 4, page 101). Organolithium species are notorious for having special behaviors in different solvents, for instance, it is known that tBuLi is a tetramer in hexane, a dimer in ether, and a monomer in THF. 129 It is conceivable that dialkyl- BBN “ate” complexes (Scheme 94) could form under special circumstances during the generation of the BBN adduct, which would later act as a hydride source. On the other hand, it has been recognized that Grignard reagents have a higher tendency to form dialkyl-BBN “ate” complexes. 78,134 135 There are quite a few metal-mediated dehalogenation methods where hydride is provided from nucleophilic sources like sodium borohydride in the presence of a base. 136 This, in some way, correlates with the observation that in some of our experiments the absence of base was favorable to avoid the undesired dechlorination reaction. In this specific case, according to the mechanism shown in Scheme 94, the hydride transfer occurs from the BBN bridgehead hydrogen, which is in agreement with the fact that we did not observed the formation of any β-hydride elimination product, even in cases with high dehalogenation levels. In conclusion, through extensive experimentation we have found a “tweaked” set of conditions that allow for an efficient B-alkyl Suzuki coupling Suzuki coupling on a highly inactivated amidochorophenol, which have been successfully used in our synthetic route to autolytimycin. This result establishes that the 5-alkyl-3-amidophenol moiety, which is found in a number of natural products, could be rapidly assembled by activation/borylation/amidation/oxidation and Suzuki coupling. 115 combining Ir-catalyzed C–H Chapter 7. A synthetic approach to autolytimycin: Ring-closing metathesis Our synthetic route toward the Hsp90 inhibitor autolytimycin (401), illustrated in Section 4.6 (page 64), was designed aiming for high convergency. Having established a step sequence that allows a rapid connection of 5-alkyl-3-amidophenols from fully elaborated blocks (Chapters 5 and 6), ring-closing metathesis (RCM) presented the attractive option of bringing together the 19-membered macrocycle from a just-assembled intermediate containing the complete carbon network (701) of the natural product. HO PMBO NH NH RCM, O O MeO manipulation OR MeO MeO 9 HO 8 8 OR' MeO 701 9 O O NH2 autolytimycin (401) Scheme 96. The ring-closing metathesis step in our synthetic approach to autolytimycin. Even though a compound containing a trisubstituted olefin flanked by five trisubstituted stereogenic centers, like 401, was certainly expected to be a challenging RCM target, throughout the years numerous research groups have worked on improving metathesis reaction on relatively unreactive, sterically encumbered, and selectivity-lacking systems. Great advances in this area 137 have been encouraged by application in complex molecule synthesis and the development still continues. The ring closure of macrocycles by means of a metathesis reaction has been extensively used in total syntheses during the last decades. 138 The present chapter discusses our initial efforts to employ this important reaction in our synthetic approach to autolytimycin. 116 7.1. Literature precedence Metathesis is among those remarkable chemical reactions that have fundamentally transformed the synthesis of complex molecules. 139 The 2005 Nobel Prize in Chemistry was awarded to Yves Chauvin, Robert H. Grubbs, and Richard R. Schrock for the development of the metathesis method in organic synthesis. Olefin metathesis is the most used form of this reaction, which, by definition, involves the redistribution of alkylidene fragments by the scission of carbon–carbon double bonds in alkenes, 140 as depicted in Scheme 97. Among its numerous variations, ring- closing metathesis (RCM) occupies a special place, having established itself as one of the preferred strategies for the preparation of cyclic compounds. R3 R1 + R4 metathesis catalyst 141 R3 R1 + R2 R4 R2 metathesis catalyst + Scheme 97. Schematic illustration of a general metathesis and a ring-closing metathesis reactions. Intuitively, metathesis was thought to occur through a process in which two olefins coordinated to a metal and exchanged alkylidene groups. 142 143 In the early seventies, Chauvin proposed a mechanism that implicated the metal-driven fragmentation of one olefin at a time (a non-pairwise process) which, with further experimental evidence by Grubbs, . 117 144 eventually became the widely Initiation: M CR2 + M CR2 H2C H2C M CR2 CH2 M CR2 H2 C M CR2 CH2 M H2 C M CR2 M H2 C CR2 CH2 M CH2 M H2 C + + CR2 CR2 CH2 Propagation: M CH2 + M CH2 H2C H2C M CH2 CH2 M M M CH2 CH2 M H2 C + H2C M CH2 H2 C H2C M M M CH2 + CH2 degenerate metathesis CH2 CH2 M M H2 C M H2 C M CH2 M H2 C M CH2 + CH2 CH2 + + CH2 degenerate metathesis Scheme 98. Chauvin’s mechanism, proposed in 1971, for the catalyzed olefin metathesis involving metal alkylidene and metallacyclobutane intermediates. 118 145 accepted mechanism for this reaction. In due course, the understanding of the important role of metal alkylidenes led to the progressive development of metal carbene complexes able to 146 catalyze olefin metathesis, with pioneering reports by Schrock 147 Grubbs in 1990 ([Mo]-I) and in 1992 ([Ru]-III). iPr iPr CF3 N F3C Mo Me O Me O F3C CF3 Me Ph Me Cy3P Ru Cl Cy3P 146 [Mo]-I Schrock's catalyst Mes N Cl [Ru]-I Ru Cl Cy3P 150 N Mes Cl [Ru]-II 164 Ru Cl Cy3P Mes N Ru Ru Cl Cy3P 147 N Mes Cl Cl Cy3P [Ru]-VI Grubbs catalyst Cy3P Cl [Ru]-III 2nd generation Grubbs catalyst Cy3P 1st generation [Ru]-IV Cy3P Cl Ru Mes N Cl Ru Cl Cy3P 159 [Ru]-V Mes N Cl Cl O 155, 156 N Mes Cl Ru 158 N Mes Cl O 167 [Ru]-VII 1st generation Hoveyda-Grubbs catalyst 170 [Ru]-VIII 2nd generation Hoveyda-Grubbs catalyst Figure 8. Metal–alkylidene complexes commonly used as metathesis catalysts. 119 While highly active and efficient on a number of substrates, 148 molybdenum catalyst [Mo]-I suffered from limited functional group compatibility and excessive oxygen and moisture sensitivity, which restricted its widespread use. Among the organic synthetic community, ruthenium catalysts promptly became popular and went through astonishing development over the years. 149 In 1995, the benzylidene complex [Ru]-I (“First-generation Grubbs’ catalyst”) was 150 introduced 151 as an improved version of the previously reported catalyst [Ru]-III. Although [Ru]-I seemed slightly less reactive than Schrock’s [Mo]-I, its comparatively significant air and water stability made it persist in the organic synthesis arena. 16-electron complex L Ru Cl 152 14-electron complex PCy3 R L Cl R Ru R Cl R [Ru] R Cl PCy3 R R R R R [Ru] R R [Ru] R R R ruthenacyclobutane intermediate Scheme 99. Mechanism of the metathesis of a symmetrical cis olefin to its trans isomer. 153 At the end of 1998, a report by the Herrmann group demonstrated that replacing the phosphine ligands of [Ru]-I with N-heterocyclic carbenes (NHC) noticeably improved its stability. Taking into account that the actual active metathesis catalyst (a 14-electron species) is 154 formed in situ via a reversible ligand dissociation 120 (Scheme 99), the presence of a labile ligand (i.e., a phosphine) is still desirable. Promptly, many research groups almost simultaneously released a series of reports on the preparation and catalytic evaluation of mixed phosphine/NHCcontaining complexes, all condensed within the year 1999. Of particular significance are the 155 reports from the Nolan 156 and Grubbs groups on the synthesis and applications of [Ru]-VI (See Figure 8), an improved catalyst featuring an IMes ligand. Among others, V, 158 157 complex [Ru]- also bearing an IMes carbene, is a case worthy of note, being an enhanced version of the indenylidene complex [Ru]-IV, 161 misassigned 159 whose structure (result of a rearrangement) but rapidly corrected. 158b 160 In the same manner in which [Ru]-IV was shown to be as efficient as [Ru]-I at catalyzing metathesis reactions, 159 comparable results were found when their IMes-substituted versions, [Ru]-V and [Ru]-VI were put side by side. others focused on the use of unsaturated NHC ligands, 165 highly active was initially 163 164 Grubbs’s 162 Yet while development of the [Ru]-II (“Second-generation Grubbs catalyst”) totally changed the panorama with the incorporation of the saturated NHC ligand H2IMes. This seemingly minor difference (compare [Ru]-VI and [Ru]-II in Figure 8) turned out to be the key to superior efficiency in various metathesis reaction types. 167 Hoveyda group 166 Furthermore, still in 1999, a serendipitous finding by the led to the development of catalyst [Ru]-VII, featuring a modification of enormous relevance for catalyst longevity. 168 The newly introduced 2-isopropoxybenzylidene ligand was able to stabilize the complex in its resting 16-electron state and to readily open a coordination site in the presence of the substrate. 121 169 Unsurprisingly, the development of the corresponding H2IMes-containing catalyst [Ru]-VIII (“Second-generation Hoveyda–Grubbs catalyst”), 170 r phosphine-free and extremely robust, was promptly reported. Even though the history of ruthenium-based catalysts seems to have followed a progressive evolution, it is important to consider that “there is no single best catalyst available, and it is 166 unlikely that such a catalyst will be developed”. In fact, during more than one decade, an astonishing number (several hundred) of NHC-bearing ruthenium catalysts have been designed, developed, and fashioned according to specific needs. 172 of a number of these catalysts 171 Eventually, the commercial availability rapidly encouraged the use of metathesis in the preparation of complex molecules, especially macrocycles, having RCM as a late-stage key synthetic step. In general, taking into account a considerable number of literature reports, the use of ring-closing metathesis for the formation of 5–8-membered rings commonly affords the thermodynamic Z olefins, 173 dominant. r whereas larger rings are usually obtained as E/Z mixtures where the E isomer is 174 About RCM reactions forming 8–10-membered rings, it has been recently said that [Ru]-VII and [Ru]-VIII are known for being reisolable once the metathesis reaction is complete, a valuable feature (especially with high catalyst loadings) attributable to a releasereturn (“boomerang”) mechanism that allows the regeneration of the complexes. recent report, 169 168 A very however, argues that the reisolation is possible due to an incomplete activation of the catalysts and that there is a lack of evidence supportive of a significant contribution of such mechanism at low catalyst loadings. 122 their “outcome depends on still imperfectly understood factors”, 175 presumably steric and conformational effects. E-selective RCM R OH O O CHO O O O O HO OR' 702, coleophomone B 703, mycolactone core Z-selective RCM OH 704, terpestacin 1 Unselective RCM (mixture E/Z) OH O O O O O O HO O CHO 706, floresolide B NH N N H OH 705, coleophomone C O O O HO 707, jasplakinolide Br NH Figure 9. Literature examples of complex molecules containing a trisubstituted olefin formed via Ru-catalyzed RCM in total synthesis. For our purposes, we became interested in RCM examples involving the assembly of larger (>10-membered) rings and the selectivities obtained on such ring closures. Applications of ring137-138 closing metathesis in total synthesis have been reviewed at length and due to the vast number of notable examples, cases involving the formation of disubstitued systems (assembled from two terminal olefinic ends) will not be discussed here. Instances of macrocycles formed via the RCM assembly of trisubstituted olefins are significantly fewer. Furthermore, recent 123 ruthenium-mediated examples (Figure 9) as well as older reports featuring molybdenum 148b,c catalysts appear to be limited to methyl-substituted alkenes. E/Z selectivity, as mentioned, 176 is known to be an issue but rather in a case-dependent even unpredictable fashion. This is clearly illustrated by comparing the apparently rigid 12-membered cyclic natural product floresolide B (706), synthesized via RCM (with lack of selectivity), 177 and the flexible-looking, also 12-membered, mycolactone core (703), reported to be closed via metathesis as a single isomer by two independent research groups. 178 In terms of selectivity, on the other hand, 179 Nicolaou’s total synthesis of coleophomones B and C (702 and 705) is certainly out of the ordinary. This report shows how a subtle difference between two substrates (708 and 710, Scheme 100) allowed for the exclusive preparation of both geometrical isomers of the desired 11-membered ring with remarkable selectivity. O O OMe Op-BrBz 10 mol% [Ru]-II DCM (0.015 M) reflux, 3 h 86% O O OMe (E) 708 OMe O O O Op-BrBz O O 709 OMe Op-BrBz 10 mol% [Ru]-II DCM (0.015 M) reflux, 3 h 80% O (Z) 710 O O Op-BrBz 711 Scheme 100. Substrate-controlled E/Z selectivity in the key RCM step in Nicolaou's total synthesis of coleophomones B and C. 124 A meticulous literature search revealed that formation of trisubstituted olefins in larger macrocycles via RCM is even more infrequent. Among those few, the total synthesis of 180 terpestacin (Figure 9, 704) reported by the Trost group is an important example that shows the application of ring-closing metathesis with complete E selectivity to assemble a product featuring three E olefins within a 15-membered ring. O TBSO TESO O 50 mol% [Ru]-II O O DCM (0.001 M) reflux, 3 d O TBSO TESO 712 O O O 713, ~40 % + Ph O HO HO O O O TBSO TESO O O O O 714, ~40 % amphidinolide Y Scheme 101. Late-stage RCM in the total synthesis of amphidinolide Y. 181 Dai’s total synthesis of amphidinolide Y (Scheme 101) provides a case with a more sterically demanding substrate that actually required an excess of [Ru]-II catalyst in order to favor the formation of the product, 713, which inopportunely was obtained as a 1:1 mixture with compound 714, resulting from the incorporation of the ruthenium catalyst’s alkylidene ligand into the system. The desired 17-membered macrocycle 713 was formed as the E olefin 125 selectively, an outcome commonly supported by the notion that, in large rings, the E isomer tends to be dominant, most likely due to a more favored assembly of the corresponding ruthenacyclobutane intermediate (See Scheme 98 and Scheme 99). Nonetheless, the reported 182 total synthesis of jasplakinolide (Figure 9, 707) provides just the right counterexample, since the RCM step provided the 19-membered in a totally unselective manner. 183 Another important report, the total synthesis of kendomycin by the Smith group (Scheme 102), is surely attention-grabbing for many reasons. First of all, even though the RCM substrate employed (715) was a 2:1 mixture of epimers, the 16-membered ring-closed product was cleanly formed exclusively from one of them. In addition, the RCM reaction afforded the Z olefin selectively as a single product (716), which turned out to be the undesired isomer in this case. The authors evaluated a number of methods to obtain the olefin with the desired geometry. Their final route involved the isomerization of the RCM product via the formation of an epoxide and its corresponding deoxygenation. With all these noteworthy applications reported within the last five years, we were encouraged to examine ring-closing metathesis in our synthetic route toward autolytimycin. It is important to mention that RCM had been previously employed in the synthesis of simplified ansamycins 57b reported by the Blagg group. Although this reaction worked efficiently at forming disubstituted olefins from bare carbon chains without oxygenated centers, similarly productive results are not generally found for the construction of trisubstituted olefin-containing targets in the presence of multiple substituted centers. As shown up to this point, successful ring-closures of such macrocycles via metathesis in the literature basically involve systems that are not as sterically encumbered, comparatively speaking. 126 (Z) (2:1) 10 mol% [Ru]-II O OH OMe TBSO O DCM (0.002 M) reflux, 2 d 57 % OH OMe TBSO TBSO TBSO OMe OMe 715 716 TsO OH O 1) TESOTf, DMAP 2,6-lut, py, 0 ºC, 89 % 2) OsO4, py THF, 0 ºC, 78 % 3) MsCl, py DCM, 0 ºC, 95 % BnNMe3OH O MeOH/THF 0 ºC 84 % TBSO OH OMe TBSO TBSO O OTES OMe TBSO OMe OMe 717 718 (E) (E) WCl6, BuLi THF, 0 ºC ! rt 71 % OH O O OTES OMe TBSO O HO HO TBSO O OMe 719 kendomycin (720) Scheme 102. Late-stage formation of a trisubstituted olefin via RCM and its isomerization in Smith's total synthesis of kendomycin. 72b Andrus’ full paper on the synthesis of geldanamycin briefly describes the evaluation of a tentative RCM-based route (Scheme 103). The authors reported the lack of success in a number 127 of metathesis experiments involving various solvents, concentrations, modes of addition, and additives. However, the report implies that only catalyst [Ru]-VI (See Figure 8, page 119) was studied at the time. In any case, considering especially the current availability of catalysts of much higher activity/stability and the structural differences between geldanamycin and autolytimycin (being the latter presumably more flexible, see Scheme 96), we decided to investigate ring-closing metathesis as our first alternative to assemble the autolytimycin macrocycle. MeO MeO NH MeO MeO NH [Ru]-VI MeO O geldanamycin (408) MeO O MeO TBSO OTBS MeO OTES MeO MeO 721 TES 722 Scheme 103. Attempted RCM in Andrus’ total synthesis of geldanamycin. 7.2. Results and discussion 7.2.1. Preliminary results For our first RCM experiments, the discussed robustness of the second-generation Hoveyda184 Grubbs catalyst ([Ru]-VIII, Figure 8) and current reports on its superior performance made [Ru]-VIII our initial metathesis catalyst of choice. In order to follow the progress of the reaction by NMR, our first experiments were performed in deuterated solvents, highly diluted, and under standard heating (80 ºC). Subjection of Suzuki product 644 (see preparation in Scheme 91) to these metathesis conditions (Scheme 104) gave little evidence of any reaction. Surprisingly, prolonged reaction times led to the exclusive consumption of the terminal olefin with respect to 128 1 the disubstituted one (as determined by H NMR). This observation was initially mistaken as a potential dimerization but was later identified to be the result of an olefin isomerization (vide infra). The selective reaction on the terminal olefin (left-hand side) made us consider that the presence of the bulky TIPS group might have hindered the reaction on the disubstituted alkene. Moreover, protective group-free allylic hydroxyl groups are known to display greatly accelerated rates of reaction with ruthenium alkylidenes. 185 Thus, Suzuki products 644 and 642 were subjected to TBAF-mediated TIPS deprotection to afford the new RCM substrates 723 and 724. PMBO PMBO NH 10 mol% [Ru]-VIII O OR1 MeO NH MeO benzene-d6 2 (0.005 M) OR 80 ºC O + OR1 OR MeO MeO PMBO TBAF THF, 0 ºC 86% 644, R1 = MOM, R2 =TIPS TBAF THF, 0 ºC 67% 642, R1 = PMB, R2 =TIPS NH 723, R1 = MOM, R2 =H 725, R1 = MOM O OPMB 727 724, R1 = PMB, R2 =H 726, R1 = PMB MeO Scheme 104. Earliest RCM results in our synthetic approach to autolytimycin. To our dissapointment, RCM reactions on both substrates, 723 and 724, demonstrated that the olefins in our system displayed a tendency to undergo independent reactions (Scheme 104). While the right-hand side olefin led to the loss of a six-carbon chain, the terminal olefin underwent isomerization. Forcing reaction conditions (20 mol% [Ru]-II, toluene, 110 ºC, 16 h) 129 on substrate 724 later allowed the isolation of the major component, 727, which confirmed both the six-carbon chain loss and the isomerization of the terminal olefin. Furthermore, higher catalyst loadings and temperatures drove an apparent decomposition of our substrate, leading to the formation of complex mixtures. Given this situation, we studied the use of a newly developed catalyst [Ru]-IX, 186 a modified second-generation Hoveyda–Grubbs catalyst, known for being efficient with encumbered olefins for possessing o-toluyl substituents in place of mesityls on the carbene ligand. Unfortunately, in our hands, this catalyst provided similar results and even a 187 faster decomposition. Other modified Hoveyda–Grubbs-based catalysts N Cl N Cl Mes N Ru N Mes Cl Ru Cl Cy3P O 186 [Ru]-IX o-toluyl variant of 2nd generation Hoveyda-Grubbs were not explored. iPr N N iPr Cl Ru Cl Cy3P 188, 189 [Ru]-X NeolystTM M2 190 [Ru]-XI Figure 10. Recently developed metathesis catalysts. In the search for catalysts known to have a higher thermal stability, we turned our attention to [Ru]-X, a commercially available catalyst known as Neolyst™ M2. This compound resulted interesting to us for being the H2IMes-containing version of catalysts [Ru]-IV and [Ru]-V (Figure 8) and therefore, intuitively expected to be significantly active. In fact, [Ru]-X has been said to display an improved “performance, especially in ring-closing metathesis (RCM) of triand tetrasubstituted olefins”. 172b We were surprised, however, to find out that literature reports 188 involving [Ru]-X had been rather scarce until recent years, with reports from the Nolan 130 and 189 Grela groups. We also decided to evaluate [Ru]-V, which was found to be as active as [Ru]-X for some substrates. 188a During the course of our studies, the IPr version of Neolyst™ M2, [Ru]-XI (Figure 10), was reported to be more active in several metathesis reactions. 190 By that time, nevertheless, we had already realized that catalyst activity was not as critical for the success of the RCM in our system as the structural features of the substrate itself as will be discussed in the following pages. In attempts to tune the conformation of the molecule, we evaluated a series of common solvents, 191 toluene, benzene, and dichloromethane, together with uncommon ones, like the fluorinated hexafluorobenzene and octafluorotoluene, and the ionic liquids [bmim]BF4 and [bmim]PF6. 192 These uncommon solvents have been reported to facilitate RCM reaction in difficult instances. 191-192 At the same time, the reactions were performed at temperatures from 40 to 110 ºC and the use of microwave irradiation (reported to have an effect on metathesis reactions) 193 allowed us to carry out the reactions at 150–180 ºC, but not even traces of the desired product were found in any case, while isomerization of the terminal olefin and the loss of a six-carbon chain were always present. Furthermore, under forcing conditions, especially at high temperature and high catalyst loading, the substrate appeared to decompose forming complex mixtures. It is actually known that both first-generation, 194 195 and second-generation ruthenium catalysts can gradually decompose by action of water and oxygen (and alcohols), forming ruthenium hydride species that are selective and effective at promoting alkene double-bond isomerization. Even though the use of degassed solvents and nitrogen sparging (a constant flow of nitrogen gas bubbling through the solution) somehow controlled the decomposition of the reaction mixture, 131 the isomerization of the terminal olefin appeared unavoidable (see Section 7.2.3, page 144, for 196 details). Among many solvents tested in our preliminary reactions, acetic acid proved to be effective at controlling to a certain extent the olefin isomerization, which eventually occurred nonetheless after longer reaction times. PMBO PMBO NH O OPMB MeO MeO NH O 2 equiv DDQ DCM/pH 7 buffer (20:1) rt, 1.5 h OH 81% 724 OH MeO OH MeO 728 Scheme 105. Selective PMB deprotection of a secondary alcohol in the presence of a PMBprotected phenol. With these results in hand, we decided to continue with an evaluation of the effect of the protective groups on the RCM outcome. A summary of the results is presented in Table 11, page 134. Treating our substrate 724 with DDQ, a reagent commonly used for removal of PMB groups, 197 led to the exclusive deprotection of the secondary alcohol, leaving the phenolic one untouched, to provide 728 (Scheme 105). Interestingly, the result was the same even in the presence of a large excess of DDQ. The use of the deprotected substrate 728 in RCM, however, did not stop the isomerization of the olefin, nor favored the formation of the desired ring (Table 11, entry 5), so then we intended to lower the reactivity of the right-hand side olefin by installing a carbamate group (Scheme 106) to presumably avoid the bothersome six-carbon chain loss. All these cases (see Table 11, entries 6, 7, and 9) were apparently unreactive substrates that provided 132 isomerization as the only detectable transformation after long reaction times. Equivalent results were observed both in the presence of a small MOM group (entry 6) and in absence of a protective group (entry 9). PMBO 1) 2 equiv O NH N OR1 MeO 724, R1 = PMB NH O O OR1 MeO 2) K2CO3, MeOH OH 723, R1 = MOM RO DCM, rt, 15 min O MeO CCl3 89% 729 53% 730 OCONH2 MeO 4 equiv ZnOTf xs EtSH DCM, 0 ºC, 87% 10 equiv DDQ 60 equiv NaHCO3 DCM/water (18:1) rt, 4 h, 67% (unoptimized) 729, R = PMB, R1 = MOM 731, R = R1 = H 730, R = PMB, R1 = PMB 732, R = PMB, R1= H Scheme 106. Preparation of carbamate-containing RCM substrates. After some experimentation, we found out that simultaneous deprotection of both the phenolic PMB and the MOM groups was achievable under Lewis-acidic conditions in the presence of a thiol scavenger 198 to give compound 731 (Scheme 106). This product actually corresponded to the ring-opened form of our target molecule autolytimycin. Much to our dismay, the use of 731 as RCM substrate, also failed to provide the desired product (Table 11, entry 7) and isomerization was the only observable reaction. As mentioned, metathesis conditions on ringopened autolytimycin containing a PMB-protected phenol (732, prepared via the selective PMB deprotection) afforded equal results (Table 11, entry 9). 133 Table 11. Summary of synthesized RCM substrates and evaluated metathesis conditions. RO RO NH NH RCM conditions O OR1 MeO MeO 1 O 2 + OR1 toluene, benzene, 2 dichloromethane OR or acetic acid MeO (0.001-0.005 M) 40 ºC – 110 ºC OR MeO Entry R R R Substrate Evaluated catalysts Results 1 PMB MOM TIPS 644 [Ru]-VIII only isomerization 2 PMB PMB TIPS 642 - - 3 PMB MOM H 723 [Ru]-II, V, VIII, X 4 PMB PMB H 724 [Ru]-II, VIII, X 5 PMB H H 728 [Ru]-II, V, IX, X 6 PMB MOM CONH2 729 [Ru]-II only isomerization 7 H H CONH2 731 [Ru]-II, X only isomerization 8 PMB PMB CONH2 730 - - 9 PMB H CONH2 732 [Ru]-II, X only isomerization six-carbon chain loss and isomerization six-carbon chain loss and isomerization six-carbon chain loss and isomerization For reactions with substrates 723, 724, and 728, for which transformations besides isomerization were observed, thorough analyses of crude materials and chromatography fractions by LC-MS were performed in an effort to find traces of the desired 19-membered ring, to no avail. In theory, the terminal olefin (left-hand side) is expected to be more accessible for reaction with ruthenium catalysts but for some reason it seemed to participate exclusively in isomerization. 134 Furthermore, when we take into account the mechanism (illustrated in Scheme 107) by which the bothersome six-carbon chain loss is expected to occur, intermediate 735, for instance, could as well lead to the formation of a 13-membered ring, but no evidence of such ring closure was detected either. A possible explanation for this behavior would be that, when the ruthenium complex installs itself into the terminal olefininc end (left-hand side), it renders a non-productive intermediate, in which the metallic center is not able to encounter the other end. PMBO PMBO NH NH O O OR1 MeO OR1 OH MeO MeO 733 736 [Ru] [Ru] R R' R OMe PMBO R' PMBO OH NH NH O O OR1 MeO [Ru] OR1 OH MeO MeO [Ru] 734 735 Scheme 107. Proposed mechanism for the six-carbon chain loss from RCM substrates. Aware of the possibility that our desired trisubstituted olefin was too sterically hindered to be formed via an RCM, we evaluated the likelihood of forming a disubstituted olefin instead. As mentioned earlier, our desired alkene is flanked by five trisubstituted stereogenic centers, which 135 makes it a relatively uncommon RCM target. We became interested in the reported formal synthesis of eleutherobin, 199 a fairly unique example of the formation of an olefin quite sterically encumbered, rather infrequent in the literature. The productive RCM of 738 (Scheme 108) permitted the formation of a disubstituted olefin flanked by four protected oxygenated centers (739). A fascinating detail in this report is that an epimer of the successful substrate (epi738), subjected to identical RCM conditions, displayed a totally different behavior; even though the expected 10-membered product (epi-739) was formed, it was accompanied by compound 740, whose intricate structure apparently formed via the unanticipated ring-opening metathesis of the present cyclohexene ring. O OPMP MOMO MOMO OPMP H 30 mol% [Ru]-II toluene, reflux 6.5 h H OPiv OPMP H H (E) O H OPiv 739, 64% 738 N OPMP H OMe O OAc 737 eleutherobin MOMO OPMP H 30 mol% [Ru]-II toluene, reflux 6.5 h H OPiv epi-738 OPMP H OH OH (E) OPMP H O OPMP OMOM OPMP MOMO N O + H H OPiv OPiv epi-739, 27 % OPMP 740, 15 % Scheme 108. Example of a successful RCM in a congested system featured in a formal synthesis of eleutherobin. 136 We then envisioned that the removal of the methyl substituent from the right-hand side olefin would make our substrate less demanding in terms of sterics. Thus, demethylated s amidochlorophenol 741 was subjected to B-alkyl Suzuki coupling, which proceeded with high efficiency, and subsequent protective group manipulations to afford the desired demethylated 200 substrates (Scheme 109). During the course of our studies, a report by the Hierseman group presented a case similar to ours, where the formation of a trisubstituted olefin was difficult but the ring was successfully formed after removal of a methyl substituent. Unfortunately, unlike in that report, our demethylated substrates exclusively favored the six-carbon chain loss previously observed in the original RCM substrates (see Table 11). PMBO PMBO 1.6 equiv 529 tBuLi, ether, -78 °C then B-OMe-9-BBN, THF, -78 °C ! rt NH Cl O MeO 741 then SM OTIPS 2 equiv K3PO4 nH2O 10 mol% Pd(OAc)2 20 mol% SPhos THF, reflux, 12 h 86% 742 743 or 744 OR1 MeO R1 = H, R2 = H , 744 O + OR1 MeO OR1 OR2 MeO R1 = PMB, R2 = H , 743 NH toluene-d8 80 ºC O R1= PMB, R2 = TIPS , 742 PMBO 10 mol% [Ru]-II NH TBAF THF, 0 ºC 58% DDQ DCM/pH 7 buffer (20:1) 34% MeO Scheme 109. Preparation and evaluation of demethylated RCM substrates. s Prepared by Dr. Monica Norberg 84 137 Considering that the two olefinic ends appeared to have difficulties to find each other for reaction under metathesis conditions, we developed an approach to evaluate the effect of the ring size on the RCM. Our plan, depicted in Scheme 110, called for the disconnection of the amide bond and the formation of an ester in order to turn our RCM target into a 12-membered ring (747), thus presumably making the encounter of the two olefins more likely. PMBO HO NH NH O O MeO RCM OR1 MeO OR2 HO manipulation MeO MeO OCONH2 401 745 ring size reduction "transamidation", manipulation PMBO PMBO NHBoc Boc NH RCM O OMe O HO O O MeO MeO 746 MeO OH 747 Scheme 110. Proposed facilitation of ring-closing metathesis via ring size reduction. Before embarking into the synthesis of RCM substrate 746, a more accessible compound, 749 t (prepared via TIPS deprotection of 748 ), was used as a simplified model. Triene 749, however, turned out to be a rather tough RCM substrate and actually required an excess of catalyst and t 84 Synthesized by Dr. Monica Norberg 138 high temperature to give any evidence of conversion. As shown in Scheme 111, formation of the desired 12-membered ring was not observable and, depending on the catalyst used, either the terminal or the disubstituted olefin were consumed. This route was abandoned to pursue different alternatives aimed to solve both the isomerization of the terminal olefin and the bothersome sixcarbon chain loss from the RCM substrates. OTrt OTrt OMe O RO O R = H , 749 O ? O O O toluene-d8 110 ºC 5 + 19 h MeO R = TIPS , 748 20 + 30 mol% [Ru]-II OTrt MeO TBAF THF, 0 ºC 64% MeO 750a (complex mixture, isomerization observed) 751 OTrt 10 + 90 mol% [Ru]-X OMe 749 O toluene-d8 110 ºC 10 + 10 h HO O MeO 750b Scheme 111. Preparation and evaluation of a ring-reduced RCM substrate. 7.2.2. Relay RCM Given our initial RCM results, we developed an RRCM-based tactic intended to suppress the undesired six-carbon chain loss from our metathesis substrates. Relay ring-closing metathesis 201 (RRCM) was introduced a few years ago by Hoye as an strategy to drive the ring formation from substrates that perform inneficiently in traditional RCM closures (including the formation of tetrasubstituted olefins) via a directed “metal movement through metathesis sequences” 139 depicted in Scheme 112. This problem-solving approach has been certainly successful and has been since then exploited to resolve a number of metathesis issues encountered especially during the synthesis of complex natural and non-natural compounds. RRCM 202 RCM T R R relay chain [Ru] [Ru] R' R R' T R [Ru] [Ru] T T = CH2, O, OSiR2, C(CO2R)2, C=O R Scheme 112. Schematic representation of an RRCM and its corresponding RCM. The RRCM strategy is typically employed to direct the metal center to install itself on a hard-toaccess, sterically encumbered olefinic end. For our purposes, however, our disubstituted olefin appeared to be reactive enough and what we expected was to actually avoid its reaction with the catalyst as initiation step. Our rationalization called for the installation of a relay chain on the terminal olefin, as illustrated in Scheme 113, in order to direct the catalyst into the formation of intermediate 756, which could then carry out the desired ring closure. 140 PMBO PMBO PMBO NH NH OR O R' O [Ru] MeO NH O OR MeO OR OH OH MeO MeO MeO [Ru] 752 753 754 installation of a relay chain PMBO PMBO PMBO NH R' O OR NH O [Ru] MeO OR MeO OH MeO NH OR OH MeO [Ru] 755 756 O MeO MeO OH 757 Scheme 113. RRCM-based strategy to avoid the undesired six-carbon chain loss from the RCM substrates. Installation of the relay chain in our system was executed in a rather inefficient manner (Scheme 114). An appropriate route toward modified alkyl iodides 759 and 760 would be to perform the ozonolysis/Wittig olefination sequence before the installation of the iodide itself. But with ready access to both 525 and 529 (Scheme 71 and Scheme 73) in hand, we opted for this alternative to accelerate the synthesis of the RRCM substrates and test our hypothesis before trying to improve their preparation. The following steps, the corresponding B-alkyl Suzuki couplings and protecting group manipulations proceeded efficiently for the preparation of our RRCM substrates 763, 764, and 765. 141 Ph3P I I I I O3 OR DCM, !78 ºC then PPh3 rt, ON MeO 529, R=PMB 525, R=MOM 771 OR MeO 758 NaHMDS MeO THF O 0 ºC " RT then SM !78 ºC " RT (yield for two steps, 759, R=PMB, 26% unoptimized) 760, R=MOM, 32% PMBO PMBO 1.6 equiv 759 or 760 tBuLi, ether, !78 °C then B-OMe-9-BBN, THF, !78 °C " rt NH Cl OR O MeO 642 then SM OTIPS 2 equiv K PO nH O 3 4 2 10 mol% Pd(OAc)2 20 mol% SPhos THF, reflux, 12 h NH O OR MeO OR' MeO 89% 761 77% 762 TBAF THF, 0 ºC 82% 2 equiv DDQ DCM/pH 7 buffer (20:1) 59% (impure) 761, R= PMB, R' = TIPS TBAF THF, 0 ºC 62% 762, R= MOM, R' = TIPS 763, R= PMB, R' = H 764, R= H, R' = H 765, R= MOM, R' = H Scheme 114. Installation of a metathesis relay chain via an ozonolysis/Wittig olefination sequence and the preparation of the RRCM substrates. 142 Table 12. Metathesis reactions using RRCM substrates. PMBO PMBO NH NH RCM conditions O OR (0.001-0.005 M) MeO 724, R = PMB 723, R = MOM 728, R = H O MeO OR OH OH MeO MeO A + PMBO NH 763, R= PMB 765, R= MOM 764, R= H O OR 766, R = PMB 767, R = MOM 768, R= H Entry Substrate 1 763 MeO 763 3 765 4 765 764 6 764 MeO Conversion Results 20 mol% [Ru]-II benzene-d6, 80 ºC, 1 h 100% ~1:1 A:B ~2:3 A:B 10 mol% [Ru]-X DCM, 40 ºC, 10 h 10 mol% [Ru]-II benzene-d6, 80 ºC, 1 h 10 mol%[Ru]-II toluene, N2 sparging 80 ºC, 5 h 100% A, 60% isolated 100% ~3:2 A:B 100% Mostly B (TLC) Isomerization and decomposition observed in crude material. +5h 5 B Conditions + 1.5 h 2 OR 10 mol%[Ru]-X DCM, 40 ºC, 12 h 10 mol%[Ru]-X toluene, N2 sparging RT, 50, 80 ºC, 1 + 2 + 5 h 143 100% Mostly A, ~50% isolated 100% Decomposition Mostly B, ~30% isolated (~50% isomerized) PMB-protected RRCM substrate 763 was found to react rapidly under metathesis conditions but in a rather unproductive way (Table 12, entry 1), resulting in the “regeneration” of the original RCM substrate 724 and the formation of the six-carbon chain loss product 766, both likely to be formed via a similar mechanism (see Scheme 107). The use of milder conditions (entry 2) provided the exclusive formation of 724 again and no further reaction was observed even after a long reaction time. Based on these results, we considered the possibility that the ring-closure might be hindered by the presence of the large PMB protective group, which justified the preparation of the MOM-protected (765) and deprotected (764) forms. Nonetheless, the reaction with 765 gave almost identical results (entry 3) and mild reaction conditions on 764 (entry 5) provided the original RCM substrate 728. Attempts to force the ring-closure (entries 4 and 6) favored the 6-carbon chain loss and ultimately the decomposition of the reaction mixture. Interestingly, this decomposition and the isomerization of the terminal olefin were somehow controlled during the first hours of reaction (Table 12, entries 1, 3, and presumably 4). Longer reaction times and higher temperatures eventually led to the formation of isomerized products. Disappointingly, formation of the desired 19-membered product was not detected in any case. Even though our RRCM-based approach was unsuccessful, after these experiments we recognized that, in order to find improved RCM conditions, we needed to facilitate both the analysis of crude mixtures and the purification of products, by avoiding somehow the isomerization of the terminal olefin. 7.2.3. Suppressing isomerization The isomerization of olefins under metathesis conditions is an issue well documented in the literature. 203 Even though the mechanism by which it occurs is not yet totally understood, it has been attributed to metal hydride species, formed via decomposition of the ruthenium catalyst. 144 204 This aspect has also been reported to cause troubles in the synthesis of other complex molecules. Among several examples in the literature, 205 206 the studies toward the synthesis of serpendione (769) provide a pretty attention-grabbing example. Two epimers, 770 and epi-770, independently subjected to identical ring-closing metathesis conditions gave two totally different results (Scheme 115); in one case, the desired 8-membered product 771 with high efficiency, whereas the other one gave a 7-membered ring 772 as the main product, resulting from an apparently highly efficient isomerization, followed by the corresponding ring closure. Interestingly, the authors also reported that using first-generation Grubbs catalyst ([Ru]-I) on epi-770 exclusively afforded a dimer, which after being resubjected to reaction with second-generation Grubbs catalyst ([Ru]-II), gave a mixture or 772 and epi-771, where the 7-membered ring was still the major component. MeO2C H 3 mol% [Ru]-II DCM reflux, 6 h 95 % O 770 MeO2C H 3 mol% [Ru]-II epi-770 MeO2C H H O O 771 8-membered ring benzene reflux, 16 h 60 % O MeO2C H MeO2C H O O 769, serpendione MeO2C H O 772 7-membered ring epi-771, expected 8-membered ring Scheme 115. Isomerization-RCM sequence observed in the synthesis of serpendione. This is yet another fascinating example of dissimilar behaviors of two closely related molecules under RCM conditions and definitely worthy of note is the predisposition of one of them to favor 145 an isomerization prior to the RCM itself. Not surprisingly, it has been recognized that these undesired isomerization reactions can also be of synthetic utility. 207 However, this was not an isolated case. In a recent communication on the synthesis polyanthellin A, 208 a related side formation of an undesired 8-membered ring in place of a 9-membered using second-generation Hoveyda–Grubbs catalyst ([Ru]-VIII) was reported. Even in an older report by Overmann, 209 where [Mo]-I (Schrock’s catalyst) was used, a similar transformation was observed. Mes N N Mes Cl Ru Cl PCy3 0.023 M N Mes N Cl Ru Ru Cl H Mes N Cl benzene 55 ºC, 72 h 773 N Mes + CH3PCy3+Cl- 774 "Ru–H" 46 % Scheme 116. Reported formation of a ruthenium hydride decomposition product from an NHCcontaining ruthenium methylidene complex. As mentioned earlier, it had been already shown that both first-generation, 195 generation 194 and second- ruthenium catalysts can slowly decompose by action of moisture and air, forming ruthenium hydride complexes that are active and selective alkene double-bond isomerization 210 catalysts. A report by the Grubbs group, however, has demonstrated that formation of ruthenium hydride species can occur without external agents other than the PCy3 ligand already present in the complex (Scheme 116). Methylidene complex 773, expected to form in situ during RCM reactions, can decompose thermally to form metal hydride 774 surprisingly at only 55 ºC. 146 Moreover, it was demonstrated that complex 774 indeed displays olefin isomerization activity in catalytic amounts. Very recently, we became interested in a report by the Cazin group on the preparation and evaluation of a new catalyst named “Caz-1”. 211 In this newly reported communication it was stated that replacing the PCy3 phosphine, common to numerous metathesis catalysts (see Figure 8, page 119), with a phosphite ligand permitted the formation of a complex of superior activity and longevity, existent in two isomeric forms (Scheme 117). Interestingly, the isolable-in-pureform isomer of this catalyst was found to be in a rare cis conformation ([Ru]-XII) but both isomers were found to have identical activities upon heating during the metathesis reaction. Given that it had been acknowledged that the dissociated phosphine is involved in the decomposition of the ruthenium catalysts, 210 we rapidly decided to assess if the replacement of the phosphine with a phosphite could also imply some inhibition of the decomposition pathway leading to ruthenium hydride species, which would translate into control of olefin isomerization. Mes N Ru Cl N Mes P(OiPr)3 Mes N heating Cl [Ru]-XII cis-Caz-1 N Mes Cl Ru Cl (iPrO)3P trans-Caz-1 Scheme 117. Catalyst cis-Caz-1 and its trans form. Unfortunately, reaction of our RCM substrate 723 with cis-Caz-1 also resulted in the formation of significant amounts of the isomerization product (See Table 13, entry 1). Nevertheless, these new results show that cis-Caz-1 was in fact more active than any of the catalysts that we had tested previously, leading to complete consumption of the starting material, even at lower 147 temperature (85 ºC), after just two hours. Moreover, the combination of lower temperature (85 203 ºC) and the use of p-benzoquinone (a ruthenium hydride scavenger) as additive allowed an almost complete control over the isomerization of the terminal olefin (Table 13, entries 2 and 3). However, the only metathesis product observed was again 767 and no evidence of the formation of the desired 19-membered ring, even in trace amounts, was found under these improved conditions. Table 13. RCM results using catalyst cis-Caz-1. PMBO PMBO NH O MeO OMOM MeO NH 10 mol% [Ru]-XII (cis-Caz-1) O additive OH toluene-d (0.005 M) 8 MeO time, temperature 723 OMOM OMOM MeO 767 entry time, temperature Additive % conversion % isomerization 1 2 h, 100 ºC None 100% 70% 2 2 h, 100 ºC 100% 35% 3 2 h, 85 ºC 100% < 5% 1 equiv p-benzoquinone 1 equiv p-benzoquinone +10 h, 85 ºC 10% We also attempted RCM reactions in aqueous conditions using the amphiphile PTS (Scheme 212 118), as recently reported by Lipshutz. Even though these reactions did not provide any desired 19-membered macrocycle, this method proved to be certainly remarkable, especially 148 because olefin isomerizarion appeared to be totally eradicated. To the best of our knowledge, there are no examples in the literature on the suppression of olefin isomerization by the use the PTS/water system as solvent. More interestingly, these conditions allowed [Ru]-X (Neolyst TM M2), which usually requires high temperature for activation, to catalyze the reaction at room temperature. RO RO NH NH 20 mol% [Ru]-X or [Ru]-II O 2.5% PTS/water sonication for 30 min OR" to dissolve SM RT, 6h MeO MeO R'O MeO 723, R = PMB, R' = MOM, R" = H O OR' ~80% conversion 767, R = PMB, R' = MOM (with [Ru]-X) 731, R = R' = H, R" = CONH2 no reaction 775, R = R' = H polyoxyethanyl !-tocopheryl O H O n O O 4 O O sebacate PTS Scheme 118. RCM under aqueous conditions using the nonioinic amphiphile PTS. Unfortunately, the α-tocopheryl chain in PTS appeared to be a poor solvent for our polar substrates. Aware of this inconvenience, we opted for suspension of the substrate into the solvent system with the aid of sonication. This technique allowed the reaction of our original RCM 149 substrate 723 (resulting in an fairly efficient six-carbon chain loss at room temperature), but completely failed when the significantly more polar ring-opened autolytimycin (731) was employed. These results call for further experimentation, particularly because of the unexpected prevention of olefin isomerization. 7.2.4. Removal of the C2–C3 double bond Since the six-carbon chain loss from the RCM substrates seemed unavoidable, the exclusion of the C2–C3 double bond appeared to be the most reasonable option. Given the apparent difficulty for ring closure in our system, we envisioned that this would also be a tactic to provide the structure with more flexibility. PMBO HO NH NH 2 O 2 3 O MeO 3 RCM OR1 MeO OR2 HO manipulation MeO MeO 776 OCONH2 401 manipulation, !-elimination 2,3-saturation PMBO PMBO O NH O 2 3 HO OR1 MeO 2 3 NH HO O RCM OR2 MeO OR2 MeO 777 2 3 HO 1 MeO OR 778 Scheme 119. Proposed saturation of the C2–C3 double bond of the RCM substrate. 150 To study the behavior of the RCM substrate in absence of the C2–C3 double bond, we sought a new substrate amenable to a subsequent transformation meant to regenerate the α,β-unsaturated moiety and accessible without drastically altering our original synthetic route. A “formal hydration” of the double bond offered 777 (Scheme 119), or its diastereomer, as promising substrates. We found their syn aldol-like structure attractive in terms of ease of synthesis and the likelihood to productively restore the E α,β-unsaturated system. Thus, we proceeded with the synthesis of amide 782, a “saturated” version of our original amide 434, which also was prepared in two steps from aldehyde 506 (Scheme 62). An aldol reaction on 213 506 using Evans phenylalanine-based enolate precursor 779 provided adduct 780 as a single diastereomer. We purposely opted for an asymmetric aldol reaction to avoid handling diastereomeric mixtures. O Bn N O O Bn O 779 Bu2BOTf, NEt3 O MeO OTIPS 506 N O NH2 O O Me2AlNH2 (781) HO MeO DCM, 0 ºC then SM DCM, -78 ºC 80 % O HO MeO + Bn 87 % OTIPS 780 N H O OTIPS 782 783 Scheme 120. Installation of the C2–C3 saturation on the amide portion. Subsequent substitution of the chiral auxiliary with an amino group afforded 782 in very good yield and conveniently in one step using dimethylaluminum amide (781), 214 which had to be freshly prepared for best results. Even though the preparation of the Weinreb reagent Me2AlNH2 (to be used within two weeks) 215 was operationally tedious, other conditions like the 151 combination of ammonium chloride and trimethylaluminum, 216 which we used in the synthesis of our original amide 434 (Page 72) did not provide satisfactory results. Amide 782 was then subjected to arylation under the same conditions used for the unsaturated substrate (Scheme 64). A decent, unoptimized yield was obtained by stopping the reaction at 84 incomplete conversion (Scheme 121). For this transformation, Dr. Monica Norberg had demonstrated the formation of an undesired oxidation product 785 after long reaction times and an excess of aryl bromide partner. This outcome was consistent with a recent report on the use of alcohols as sources of hydrogen for palladium-catalyzed dehalogenations. 217 The oxidation product, however, was not detected at all when the reaction was stopped after just a few hours. Furthermore, little conversion was observed after the third hour of the reaction, suggesting that the catalyst activity can be rapidly depleted. Taking into account the 77% conversion achieved, it is likely that this transformation could be driven to full conversion in a matter of hours, without the formation of the oxidation product, by adding extra catalyst. OPMB OPMB O H2N HO MeO OPMB O Br Cl 1.0 equiv 512 N H HO MeO Cl 1.4 equiv Cs2CO3 2 mol% Pd2dba3 6 mol% xantphos OTIPS dioxane, 100 ºC 3.5 h (77 % conversion) 782 64 % isolated O Cl + OTIPS 784 N H O MeO OTIPS 785, not observed Scheme 121. Preparation of the 2,3-saturated aryl chloride Suzuki substrate. On the other hand, the subsequent Suzuki coupling was severely affected by the presence of the β-hydroxy substituent. Reaction of the new amidochlorophenol 784, under the very same 152 conditions used on the initial unsaturated substrate, provided a fairly complex mixture after the workup step (filtration through silica gel), contrasting very much with the results of the original 1 reaction (Scheme 91). The presence of peaks corresponding to propionyl groups in the H NMR of the crude material suggests that the substrate might have undergone a retroaldol reaction under the coupling conditions. Such process is likely to have been triggered after reaction of the free βalcohol with the excess of B-OMe-BBN remaining from the generation of the BBN adduct (see Section 6.3, page 98, for details on the Suzuki coupling step). This undesired retroaldol would also imply the in situ formation of species susceptible to further reactions (namely a boron enolate and an aldehyde), not only compromising the efficiency of the desired coupling but also complicating the purification process at the end. Retroaldol reactions were not observed in the presence of base at 100 ºC during the arylation step (Scheme 121), which supports the involvement of boron species in this case. Nonetheless, the current conditions allowed the Suzuki coupling reaction to occur to some extent and after column chromatography provided the desired product, although contaminated, and in poor yield. I 1.6 equiv OPMB PMBO MeO NH Cl O HO MeO 529 3.2 equiv tBuLi, ether, -78 °C then 3.8 equiv B-OMe-9-BBN, THF -78 °C ! rt OTIPS 784 PMBO evaporation, then SM 2 equiv K3PO4 nH2O 15 mol% Pd(OAc)2 30 mol% SPhos THF, reflux, 12 h NH O HO OPMB MeO TBAF, THF, 0 ºC ~30 % (two steps) Scheme 122. Preparation of the 2,3-saturated RCM substrate. 153 OR MeO 786, R = TIPS 787, R= H Even though a protective group on the β-alcohol intended to suppress the reaction with B-OMeBBN is also likely to set the substrate up for β-elimination in the presence of base in boiling THF during the Suzuki step, this option is still to be taken into account for future work. For the time being, the current conditions provided sufficient material to test the behavior of the RCM substrate in absence of the C2–C3 double bond. Impure product 786 was directly subjected to TIPS deprotection which provided the new RCM substrate 787 with a reasonable level of purity. PMBO 10+10 mol% [Ru]-X 2.5% PTS/water sonication, 3 + 3 h rt or 60 ºC NH O HO OPMB MeO 787 MeO or 10+10 mol% [Ru]-XII OH 1 equiv p-benzoquinone toluene-d8 (0.005 M) 85 ºC, 3 + 3 h 100 ºC, 2 h PMBO NH O HO OPMB MeO OH MeO 788 detected in small quantities as only product being formed Scheme 123. RCM with 2,3-saturated RCM substrate. In all cases, much to our dismay, subjection of the new RCM substrate to a series of metathesis reactions, including our best conditions to minimize isomerization (Scheme 123) (see Section 7.2.3), failed to provide the desired 19-membered macrocycle. In the case of the reaction using PTS in water, no reaction was detected most likely due to a solubility problem (see page 149 for details on this solvent system). For the reaction with cis-Caz-1 ([Ru]-XII), the starting material appeared to be unreactive, but addition of extra catalyst, longer reaction times, or higher temperature, led to the isomerization of the terminal olefin as the only detectable reaction. Interestingly, running the same conditions on the original and the new substrates side by side, gave the known six-carbon chain loss on the former, proving the activity of the catalysts. These 154 results suggest that our new RCM substrate still requires some tuning, quite likely the modification of the present stereogenic centers. 7.3. Final remarks Our projected goal to synthesize autolytimycin around its 5-alkyl-3-amidophenol moiety led us to develop a route based on C–H activation and coupling reactions that allows access to advanced intermediates convergently. Since the rapid formation of an intermediate containing the complete carbon network of autolytimycin is achievable, ring-closing metathesis came into sight as a quite convenient final key step to assemble its 19-membered macrocyle. Such transformation, however, turned out to be rather tough and still remains as one elusive synthetic step to overcome in this course. As indicated in Section 7.2, functional group manipulations that would be required to conclude the synthesis after a productive RCM are expected to take place uneventfully, as demonstrated with the preparation of “ring-opened autolytimycin” (Scheme 124, 731). PMBO HO NH O MeO OMOM (a) TBAF, 85% (b) CCl3CONCO, then K2CO3, MeOH, 89% NH O O (c) EtSH, ZnOTf, 87% OH NH2 MeO OTIPS O MeO MeO 644 731 Scheme 124. Preparation of "ring-opened autolytimycin". The unsatisfactory outcomes of a variety of RCM conditions and substrate modifications suggest that perhaps a different macrocyclization tactic must be assessed. Still, its known that the slightest variations on the structure of an RCM substrate can modify the conformational 155 174 organization of the molecule, which is critical for the success of this reaction. Examples in the literature have shown that alike substrates with minor differences in stereochemistry can display completely different behaviors under RCM conditions. For instance, the reaction of only one of 183 the epimers subjected to RCM in the synthesis of kendomycin (Scheme 102), the undesired RCM–ROM–RCM cascade observed with one epimer of the RCM substrate in the formal 199b synthesis of eleutherobin (Scheme 108), or the formation of either an eight- or a seven206 membered ring from two epimers in the synthesis of serpendione (Scheme 115), just to mention a few. Considering the existence of effective synthetic methods for inversion of oxygenated stereocenters, 218 modifications of the stereochemistry are to be taken into account for future work in this project. Furthermore, Section 4.4 (See Scheme 59) discusses how our synthetic route is quite flexible in terms of the stereochemistry of the oxygenated centers in proximity to the olefin, making this alternative worth being explored. 156 Chapter 8. Summary and conclusions 8.1. Synthesis of the COX-2 inhibitor DuP 697 and analogs Ir-catalyzed C–H activation/borylation boasts an impressive set of synthetically useful features. Besides allowing the presence of varied functional groups, its halogen tolerance maximizes one’s ability to construct valuable building blocks. This notion was illustrated in our diversity-oriented route to the COX-2 inhibitor DuP 697 and a series of analogs. original building block alternative building block BPin BPin SO2Me TMS S 204 Cl Cl S 235 1) Suzuki with Ar1–Br (or Ar1–OTf) 2) Suzuki with Ar2–B(OH)2 or Ar2–BPin F Ar1 TMS 802 bromination and/or desilylation DuP 697 and analogs Ar1 S 803 Ar2 Ar1 S 804 Ar2 Br CF3 Ar2 S 801 Br CF3 CN Ar1 Ar2 S F Br N NC CO2Et Ar1 S Ar2 NMe2 N NMe2 S N NC 802 bromination Scheme 125. Our original diversity-oriented route to DuP 697 and an alternative. Building block 204 (Scheme 125) was used in the preparation of over 25 analogs, 21 following a sequence of two Suzuki couplings accompanied by bromination, desilylation, or a combination 157 of both. Our experiments showed that the trimethylsilyl group, used as a steric director during the preparation of 204 (Scheme 126), was not a requirement for selectivity or reactivity in the concluding bromination step, so we envisioned an alternative route. A neutral and mild Ir-catalyzed deborylation method developed by Drs. Venkata Kallepalli and Feng Shi 31 16 in our lab, provided access to an alternative building block, 235, which displayed flawless performance in the sequence of two Suzuki couplings and bromination, to access DuP 697 and one analog. The use of 235 as starting material instead of 204 is clearly an improvement since its preparation from 2-chlorothiophene (202) proceeds via gentle and scalable conditions that are preferable over silylation via deprotonation with LDA. 1.5 equiv HBPin, 1.5 mol% [Ir(OMe)(COD)]2 3 mol% dtbpy 1.2 equiv LDA 3 equiv TMSCl Cl S 202 THF, !78 °C " rt 73% TMS S 203 3 equiv HBPin, 2 mol% [Ir(OMe)(COD)]2 4 mol% dtbpy pentane, rt, 60 h 95% Cl heptane, rt, 42 h 93% BPin TMS 204 BPin 1.5 mol% [Ir(OMe)(COD)] 2 PinB S 234 Cl MeOH/CH2Cl2 (2:1), 55 °C 60% Cl S BPin S Cl 235 Scheme 126. Access to building blocks used in the preparation of DuP 697 analogs. This synthesis of DuP 697 and analogs provided a well structured example of the flexibility that results from joining Ir-catalyzed C–H activation/borylation and Suzuki cross-couplings. Furthermore, the use of diborylation followed by monodeborylation to build special building blocks holds the promise of considerable synthetic utility. 158 8.2. Model Studies for the synthesis of the TMC-95 core The concept of synthetically exploiting heteroaromatics with unusually placed boronate groups (prepared via diborylation/monodeborylation) was extended in our model studies to the synthesis of the TMC-95 core. TMC-95 compounds are selective proteasome inhibitors with strong bioactivity profiles, HO HO 32b which has recently attracted significant interest. R1 R N H O 2 O 2 R1 R N H NH O O NH O R3 R4 O OMe NH O <10 steps CONH2 N H 34 N H O TBSO CONH2 NH NHBoc O 327 TMC-95 A–D R1, R2 = H, OH R1 R R3, R4 = CH3, H strained intermediate 2 O OMe NH2 BPin N H 328 + HO O Br TBSO O NH NHBoc CONH2 329 Scheme 127. Our synthetic plan toward the TMC-95 family. 35 Previously reported total syntheses of TMC-95 compounds have involved a considerable number of steps in the construction of the oxidized indole moiety. We have hereby proposed a synthetic approach to the TMC-95 core (Scheme 127) that aims at avoiding excessive manipulation and providing access to the final natural products conveniently from the natural aminoacid units. 159 36g A particular example in the literature reported the preparation of three purely peptidic analogs of TMC-95, comparable to our intermediate 327, however, the tryptophan portion had to be prepared in a seven-step sequence and very low yields were observed in their Ni-catalyzed cyclization (4–13%) (See section 3.2 for details). We consider, nevertheless, that this was an important precedent since the cyclization to form such a strained system did happen and this is the type of reaction that could be improved via condition screening. 1) Ir-catalyzed diborylation 70% O 2) 1.5 mol% [Ir(OMe)COD]2 OMe MeOH/CH2Cl2 (2:1) 55 °C, 3 h NHBoc 55% + 28% (dideborylated) N H 332 3) BiCl3 (0.6 + 0.6 equiv) acetonitrile/H2O (50:1) 60 ºC, 1+1 h quantitative O OMe NH2 BPin N H 334 Scheme 128. Preparation of a triptophan-based building block for the synthesis of the TMC-95 core. We demonstrated that preparation of 334, our desired tryptophan-based building block containing a boronate group at the C7 position and a free amine, was feasible via diborylation/monodeborylation followed by a mild BiCl3-mediated Boc deprotection procedure (Scheme 128). With 334 in hand, we rapidly developed a route to intermediate 329, leading to a quick construction of a model tripeptide to be subjected to ring-closing Suzuki coupling. Additionally, access to a β-hydroxytryptophan derivative was achievable by employing a method developed by Crich. 219 160 OH 1) Br2 HBr/AcOH rt, 89% O NH2 HO 335 L-tyrosine HO HN Br O NHBoc TBSO N-hydroxysuccinimide DCC, DME, 0 ºC, ON filtration OH Br 2) Boc2O tBuOH/H2O TBSO pH 9, rt, 89% 3) TBSCl imidazole then K2CO3 H2O, rt, 70% O O NHBoc then L-asparagine NaHCO3 dioxane, water, 1h 79% 336 O PinB HN N H CONH2 HN 334, EDC, HOBt, NEt3 THF, 0 ºC to rt 58% 329 Br OMe O CONH2 O NHBoc TBSO 337 Scheme 129. Preparation of a model tripeptide for the synthesis of the TMC-95 core. Preliminary Suzuki coupling experiments with intermediate 337 under standard conditions, 48b however, have failed to give the desired cyclization product. Since we have demonstrated a rapid access to an advanced Suzuki substrate, we plan to find suitable conditions via high throughput screening of palladium sources, bases, solvents, concentration, and ligands. This study will be carried out at Merck in the following months. In this manner, our model studies to the synthesis of the TMC-95 core provide an additional example of the synthetic utility of Ir-catalyzed C–H activation/borylation combined with deborylation and its capability to build unusual building blocks under gentle conditions. We consider that 334 could be significantly useful in the preparation of TMC-95 analogs, which have attracted even more attention than the natural products themselves. 161 34 8.3. Mild site-selective deuteration of arenes In a third application, our mild deborylation procedure, typically used on heteroaromatics, was adapted for the preparation of deuterium-labeled arenes (Scheme 130). While the method proved successful, slower transformations were observed and special care was necessary. Specifically, freeze-pump-thaw degassing and the use of an air-free flask were preferred to avoid any exposure of the reaction mixture to oxygen, presumably to ensure the survival of the catalyst. Also, to guarantee good levels of deuterium incorporation, the starting materials were carefully dried in a vacuum dessicator. R 1.5 mol% [Ir(OMe)(COD)]2 R Ir-catalyzed C–H activation/borylation R CD3OD/CH2Cl2 (2:1), 55 °C BPin H D 67-97 % !94% D incorporation Reaction times: 4–10 h 1.5 h Cl Cl Cl X <0.5 h CO2Et Y TMS S Me D D D Cl N H D X= Cl, Br Y= OMe, NMe2, CF3, CN Scheme 130. Mild site-selective deuterium-labeling of arenes. We demonstrated that C–H activation/borylation followed by deborylation in deuterated methanol at 50–60 ºC, allows for the synthesis of deuterium-labeled compounds in a convenient 17 and gentle manner. Analogous site-specific deuteration methods typically require halogen/organometallic functionalities. In this transformation, the broad functional group tolerance of Ir-catalyzed C–H activation/borylation is certainly valuable. 162 8.4. A synthetic approach to autolytimycin Ir-catalyzed C–H activation provides an elegant tactic for the preparation of 1,3,5-trisubstituted arenes, even highly elaborated. Its halogen tolerance and amenability to one-pot transformations permitted the convergent construction of the full carbon network of the Hsp90 inhibitor autolytimycin. O O N O 15 I H2N 15 Ph O 1 O OMOM 13 steps 9 MeO 11 steps MeO 8 525 OTIPS O 8 OH 434 Scheme 131. Summarized preparation of advances intermediates in our synthetic approach to autolytimycin. Syntheses of advanced intermediates were high-yielding and followed a plan that was flexible from a stereochemical point of view (Scheme 131). The key step in our route to this natural product was an application of a one-pot C–H activation/borylation/ amidation/oxidation, employing our advanced amide partner 434 (Scheme 132). During this three-step process, sufficient halogen differentiation was required to suppress undesired self-Suzuki couplings and oligomerizations, so 3-bromochlorobenzene (507) was the most appropriate dihalogenated substrate for this sequence. Nonetheless, the subsequent attachment of the alkyl chain had to be performed then on the resulting electron-rich, unactivated aromatic chloride. 163 HO Br NH Cl O MeO 507 HO 2.0 equiv HBpin 2 mol% Ind(Ir)COD 2 mol% dmpe 150 °C, 5 h O MeO O NH2 autolytimycin (401) PinB Br (a) H2N Cl 1.4 equiv Cs2CO3 1 mol% Pd2dba3 3 mol% XantPhos O MeO HO NH Cl O MeO OTIPS 1.5 equiv 434 DME, 100 °C, 3.5 h (b) filtration (c) 1.5 equiv oxone acetone/water, 40 min (d) 1.0 equiv NaIO4, 1 h OTIPS 508 73% (brsm) from 434 58% from 507 Scheme 132. C–H activation/borylation/amidation/oxidation in our route to autolytimycin. 3 2 After protection, the advanced amidochlorophenol was subjected to a B-alkyl sp –sp Suzuki coupling (Scheme 133). This transformation was unprecedented in complex molecule synthesis and required special tweaking to work appropriately. In fact, our results in real and model systems resulted mostly in extremely low conversions, dechlorination, and formation of only traces of, if any, desired product in a vast number of condition-exploring runs. While investigating alternatives for the formation of the BBN adduct, our experiments showed that its generation worked better at low concentration, whereas the Suzuki coupling itself required a high concentration. To our delight, evaporation of the solvent with a nitrogen stream prior to the coupling reaction did the trick. Thus, the coupling reaction took place at a 164 concentration about ten times higher than that initially investigated. Substantial changes in terms of conversion were realized in this way. Degassed solvents were indispensable to extending the catalyst life and, in addition, the catalytic efficiency was considerably improved by premixing the palladium source and the ligand in solution before being added to the reaction vessel. By combining these observations, high-yielding conditions for the coupling reaction between 511 and 525, using Pd(OAc)2 and SPhos, were found (Scheme 133). I a) 3.2 equiv tBuLi, ether, !78 °C b) 3.8 equiv B-OMe-9-BBN, THF, !78 °C " rt c) solvent evaporation OMOM MeO 1.6 equiv 525 B OMe PMBO PMBO OMOM NH NH MeO Cl O O 526 MeO OTIPS 511 3 2 equiv K3PO4 nH2O 10 mol% Pd(OAc)2 20 mol% SPhos THF, 85 °C, 12 h 90% MeO OMOM OTIPS MeO 644 2 Scheme 133. B-alkyl sp –sp Suzuki coupling used in our route to autolytimycin. Final functional group manipulations have been carried out uneventfully as shown in the preparation of 731 (ring-opened autolytimycin) (Scheme 124) but, unfortunately, effective conditions for the RCM step have not been found. Ring-closing metathesis has been attempted on a series of substrates (Scheme 134), using 6 different RCM catalysts, numerous solvents (including ionic liquids), and a broad range of temperatures. 165 PMBO PMBO NH NH 3 2 O RCM OR1 MeO 9 MeO R3 OR4 MeO 8 R2 R1 = PMB, MOM, H R2 = H, 2 3 O MeO OR1 3 R 8 OR4 9 desired, not found R3 = Me, H R4 = TIPS, CONH2, H PMBO NH observed products O + + OR1 MeO 2 OMe + OR1 OR4 8 R3 3 MeO 805 unreacted terminal olefin isomerized terminal olefin undesired relay RCM additional explored alternatives PMBO PMBO NHBoc NH 2 O R4O O 3 HO OR1 MeO OMe O MeO OR4 MeO ring reduction 2,3-saturation Scheme 134. Summary of RCM attempts in our approach to autolytimycin. During these RCM reactions, decomposition and olefin isomerization problems were to some extent solved via nitrogen sparging/solvent degassing, use of additives, 166 203 or by performing the reaction under aqueous conditions. 212 Yet success in forming the desired 19-membered macrocycle remains elusive. Perhaps the most notable problem is the inconvenient formation of six-membered ring undesired products (805). Four modifications of the original substrate were presented, C8-demethylation, ring reduction (12-membered ring RCM substrate), attachment of a relay chain (for RRCM), and removal of the C2–C3 double bond. The first two were unsuccessful due to the formation of products 805. Attempts to direct the catalyst to the terminal 201 left-hand-side olefin via RRCM were ineffective at stopping the closure of 805 or at forming the expected product. Removal of the C2–C3 double bond has not favored the formation of the desired macrocycle but additional experiments are to be performed bearing in mind this promising modification. 167 Chapter 9. Experimental details and characterization data 9.1. General considerations Unless otherwise stated, starting materials were subjected to purification before use and yields refer to chromatographically and spectroscopically pure compounds. All reactions were carried out in oven-/flame-dried glassware and under nitrogen atmosphere, with the exception of those performed in unpurified solvents or aqueous conditions. All solvents were reagent grade. Acetone, pyridine, dioxane, and methanol were purchased and used as received. 1,2Dimethoxyethane (DME), diethyl ether, and tetrahydrofuran (THF) were distilled from sodium/benzophenone under nitrogen atmosphere before use. Acetonitrile, triethylamine, dichloromethane, benzene, and toluene were distilled from calcium hydride under nitrogen atmosphere before use. Dimethylformamide (DMF) and dimethylsulfoxide (DMSO) were treated with calcium hydride, distilled, and stored over freshly activated 4Å molecular sieves. Freezepump-thaw method was the preferred technique for solvent degassing. Reactions were monitored by thin layer chromatography on precoated silica gel plates, using UV light or phosphomolybdic acid stain for visualization. Column chromatography was performed on 60 Å silica gel (230–400 1 mesh). NMR spectra were recorded on Varian spectrometers: Inova-300 (300.11 MHz for H and 75.47 MHz for 13 1 C), Varian VXR-500 (499.74 MHz for H and 125.67 MHz for 1 Varian Unity-500-Plus (499.74 MHz for H, 125.67 MHz for 1 Varian Inova-600 (599.89 MHz for H and 150.84 MHz for 13 13 C, and 160.34 MHz for 1 C). H and 13 11 1 1 13 1 C), DMSO-d6 (δ 2.49 for H and 39.5 for 168 C), B), or C chemical shifts (in ppm) were referenced to residual solvent signals: CDCl3 (δ 7.24 for H and 77.0 for CD3OD (δ 3.31 for H and 49.15 for 13 13 13 C), C), and 1 acetone-d6 (δ 2.04 for H and 29.84 for 13 C). 11 B chemical shifts were referenced to neat BF3·Et2O (δ 0.0 ppm) as external standard. Melting points were recorded on a MEL-TEMP ® capillary melting point apparatus and are uncorrected. Optical rotations were measured using a Perkin Elmer 341 polarimeter. Low-resolution mass spectra were acquired using gas chromatography-mass spectrometry (GC-MS) on a HP 5890 series II GC coupled to a VG Trio-1 + mass spectrometer operated in EI mode (70 eV). High-resolution mass spectra were acquired at the MSU Mass Spectrometry facility using a Waters GCT Premier GC/TOF instrument (EI, CI), a JEOL HX-110 double-focusing magnetic sector instrument (FAB), or a Waters QTOF Ultima mass spectrometer (APCI, ESI). Pd2(dba)3, Pd(OAc)2, SPhos, XantPhos, and XPhos were purchased from Strem, dtbpy was purchased from Aldrich and used as received. [Ir(OMe)(COD)]2 was prepared according to literature procedures. 220 For section 9.2: 5- trimethylsilyl-2-chlorothiophene (203) and diarylated thiophenes 215 and 216 were prepared by Mr. Nathan Gesmundo by the procedures described in Scheme 7 and Scheme 9. 235 was prepared according to the reported procedure. 16 Palladium precatalysts Pd2(dba)3, PdCl2·dppf·CH2Cl2, and Pd(OAc)2, were purchased from Strem and used as received. Bromide coupling partners 4-bromo-1-methyl-1H-pyrazole and 2-bromo-4-methylthiazole-5-carboxylate were purchased from Aldrich and used as received. Boron coupling partners 4-(dimethylamino)2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile and 4-fluorophenylboronic acid were obtained from Boropharm and Aldrich respectively and used as received. Commercial Nbromosuccinimide (Strem) was crystallized from water, thoroughly dried in a vacuum desiccator, and stored in a refrigerator protected from moisture and light. For section 9.3: Boronic esters 169 4,4,5,5-tetramethyl-2-(3,4,5-trichlorophenyl)-1,3,2-dioxaborolane, tetramethyl-1,3,2-dioxaborolan-2-yl)pyridine, tetramethyl-1,3,2-dioxaborolane, dioxaborolan-2-yl)aniline, 2,6-Dichloro-4-(4,4,5,5- 2-(3-chloro-5-(trifluoromethyl)phenyl)-4,4,5,5- 3-chloro-N,N-dimethyl-5-(4,4,5,5-tetramethyl-1,3,22-(3-chloro-5-methoxyphenyl)-4,4,5,5 -tetramethyl-1,3,2- dioxaborolane, and ethyl 2-methyl-7-(4,4,5,5-tetramethyl-1,3,2-di-oxaborolan-2-yl)-1H-indole3-carboxylate were obtained form Boropharm and dried in a vacuum dessicator overnight before use. 3-bromo-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile (purified by sublimation) was prepared by Mr. Hao Li and was dried in a vacuum dessicator overnight before use. For sections 9.5 and 9.6: Methylmagnesium iodide 3.0 M solution in THF and allylmagnesium chloride 2.0 M solution in THF were purchased from Aldrich and used as received. n-Butyllithium and t-butyllithium solutions were purchased from Aldrich and standardized by titration with diphenylacetic acid 221 prior to use. For Section 9.7: Metathesis catalysts were purchased from Strem and used as received. 9.2. Experimental details for Sections 2.1 and 2.3: DuP 697 and analogs 1.5 equiv HBPin 1.5 mol% [Ir(OMe)COD]2 3 mol% dtbpy TMS S 203 Preparation of trimethylsilane (204): Cl pentane, 45 ºC, 12 h 95% conversion 88% yield BPin TMS S Cl 204 (5-Chloro-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl) 16 In a glove box, [Ir(OMe)(COD)]2 (0.052 g, 0.079 mmol) and HBPin (1.15 mL, 7.86 mmol) were mixed in an air-free flask, forming a bright yellow transparent solution, and to it was added dtbpy (0.042 g, 0.157 mmol) using pentane (3.5 mL) to rinse the test tube used to weigh. To the resulting dark solution was added 5-trimethylsilyl-2170 chlorothiophene, 203, (1.0 g, 5.24 mmol) using pentane (3.5 mL) to rinse the test tube used to weigh. The air-free flask was sealed and taken out of the box. The reaction mixture was heated at 1 45 °C overnight. After 13 hours, H NMR of sample showed a 95:5 ratio of product to starting material. The reaction mixture was concentrated in vacuo and the resulting residue was filtered through a plug of silica gel, eluting with dichloromethane to afford borylation product 204 (1.46 g, 4.61 mmol, 88% yield) as a white solid; mp 67–68 °C (lit MHz) δ 7.26 (s, 1 H), 1.32 (s, 12 H), 0.26 (s, 9 H); 13 16 1 68–69 °C); H NMR (CDCl3, 500 C NMR (CDCl3, 125 MHz) δ 144.7, 139.4, + 139.4, 83.7, 24.8, −0.2; HRMS (ESI): m/z calculated for C17H34O5Si [M+H] 385.1813, found 385.1811. N BPin 1.2 equiv N N . N Br TMS S 204 Cl 2 mol % PdCl2·dppf·CH2Cl2 2 equiv K3PO4·nH2O DME (0.5 M), 80 ºC 73% TMS S Cl 213 Preparation of 213: In an air-free flask provided with a magnetic stir bar were mixed 204 (200.0 mg, 0.631 mmol), 4-bromo-1-methyl-1H-pyrazole (122.0 mg, 0.758 mmol), K3PO4·nH2O (235 mg, 0.95 mmol), and PdCl2·dppf·CH2Cl2 (10.4 mg, 12.6 µmol). The flask with the solid mixture was purged and refilled with nitrogen several times. Degassed DME (1.3 mL) was added under nitrogen atmosphere and the reaction mixture was heated up to 80 °C. The progress of the reaction was monitored by TLC (4:1 hexanes/EtOAc). Once finished, usually after 2.5 hours, the reaction mixture was filtered through a short plug of silica gel eluting with acetone. The filtrate was concentrated in vacuo and the crude product was purified by column chromatography 171 eluting with hexanes/EtOAc (9:1 → 3:1) to afford 213 (124.9 mg, 0.461 mmol, 73% yield) as a 1 white solid. Rf = 0.3 (hexanes/ethyl acetate 5:1); mp 82–83 ºC; H NMR (500 MHz, CDCl3): δ 7.80 (s, 1 H), 7.79 (s, 1 H), 7.13 (s, 1 H), 3.93 (s, 3 H), 0.29 (s, 9 H); 13 C NMR (125 MHz, CDCl3): δ 138.8, 137.8, 133.2, 133.2, 131.0, 128.0, 115.9, 39.1, –0.4; IR (neat): 3139, 3091, –1 3052, 2957, 2898, 1420, 1385, 1312, 1277, 1120, 984, 839, 804, 756 cm ; HRMS (ESI): m/z + calculated for C11H16N2SiSCl [M+H] 271.0492, found 271.0493. CO2Et 1.2 equiv BPin TMS S 204 Cl S CO2Et . S N N Br 2 mol% PdCl2·dppf·CH2Cl2 2 equiv K3PO4·nH2O DME (0.5 M), 80 ºC 84% TMS S Cl 214 Preparation of 214: In an air-free flask provided with a magnetic stir bar were mixed 204 (200.0 mg, 0.631 mmol), ethyl 2-bromo-4-methylthiazole-5-carboxylate (190.0 mg, 0.758 mmol), K3PO4·nH2O (235 mg, 0.95 mmol), and PdCl2·dppf·CH2Cl2 (10.4 mg, 12.6 µmol). The flask with the solid mixture was purged and refilled with nitrogen several times. Degassed DME (1.3 mL) was added under nitrogen atmosphere and the reaction mixture was heated up to 80 °C. The progress of the reaction was monitored by TLC (5:1 hexanes/EtOAc). Once finished, usually after 2.5 hours, the reaction mixture was filtered through a short plug of silica gel eluting with acetone. The filtrate was concentrated in vacuo and the crude product was purified by column chromatography eluting with hexanes/EtOAc (19:1 → 5:1) to afford 214 (190.5 mg, 0.529 1 mmol, 84% yield) as a pale yellow solid. Rf = 0.54 (5:1 hexanes/EtOAc); mp 63–65 ºC; H NMR 172 (500 MHz, CDCl3) δ 7.75 (s, 1 H), 4.34 (q, J = 7.25 Hz, 2 H), 2.76 (s, 3 H), 1.38 (t, J = 7.25 Hz, 3 H), 0.32 (s, 9 H); 13 C NMR (125 MHz, CDCl3) δ 162.5, 161.6, 159.7, 139.4, 133.6, 133.6, 133.0, 121.7, 61.2, 17.3, 14.4, −0.4; IR (neat): 2988, 2959, 2927, 1715, 1539, 1522, 1371, 1321, –1 1259, 1093, 991, 843, 760 cm ; HRMS (ESI): m/z calculated for C14H19NO2SiS2Cl [M+H] + 360.0315, found 360.0315. N 1.5 equiv N . N (HO)2B N F TMS S 213 Cl 1 mol% Pd2dba3 4 mol% XPhos 2 equiv K3PO4 tBuOH (0.5 M), 80 ºC 82% TMS S F 218 Preparation of 218: To an air-free flask provided with a stir bar were added 213 (31.9 mg, 0.118 mmol), 4-fluorophenylboronic acid (24.7 mg, 0.177 mmol), anhydrous potassium phosphate (49.8 mg, 0.236 mmol), Pd2dba3 (1.1 mg, 1.18 µmol), and XPhos (2.2 mg, 4.7 µmol). The flask with the solid mixture was purged and refilled with nitrogen multiple times. To the flask was added degassed tBuOH (0.25 mL), and the resulting suspension was heated up to 80 °C. The progress of the reaction was monitored by TLC. After completion, usually after 6 hours, the reaction mixture was filtered through a short plug of silica gel eluting with acetone. The filtrate was concentrated in vacuo and the material was subjected to column chromatography eluting with hexanes/EtOAc (3:1 → 2:1) to afford 218 (31.8 mg, 0.096 mmol, 82% yield) as a very thick 1 colorless oil. Rf = 0.21 (hexanes/EtOAc 3:1); H NMR (500 MHz, CDCl3): δ 7.37 (m, 3 H), 7.19 (s, 1 H), 7.15 (s, 1 H), 7.02 (m, 1 H), 3.82 (s, 3 H), 0.33 (s, 9 H); 173 13 C NMR (125 MHz, CDCl3): 1 3 δ (163.4, 161.4) (d, JC–F = 247.7 Hz), 141.1, 138.2, 135.9, (131.0, 131.0) (d, JC–F = 8.7 Hz), 4 2 (130.8, 130.8) (d, JC–F = 3.5 Hz), 130.6, 128.0, 117.5, (115.6, 115.5) (d, JC–F = 22.0 Hz), 38.9, –0.1; IR (neat): 3029, 2955, 2897, 2855, 1601, 1527, 1502, 1448, 1265, 1250, 1222, 1157, –1 + 1018, 983, 843, 821 cm ; HRMS (ESI): m/z calculated for C17H20N2SiSF [M+H] 331.1101, found 331.1101. CO2Et 1.5 equiv PinB S N TMS S 214 Cl CO2Et NMe2 . S N NC 1 mol% Pd(OAc)2 2 mol% XPhos 2 equiv K3PO4 tBuOH (0.5 M), 80 ºC 78% NMe2 TMS S NC 219 Preparation of 219: To an air-free flask provided with a stir bar were added 214 (36.8 mg, 0.102 mmol), 4-(dimethylamino)-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile (22.3 mg, 0.153 mmol), anhydrous potassium phosphate (43 mg, 0.204 mmol), Pd2dba3 (1.0 mg, >1.02 µmol), and XPhos (2.0 mg, >4.08 µmol). The flask with the solid mixture was purged and refilled with nitrogen multiple times. To the flask was added degassed tBuOH (0.2 mL), and the resulting suspension was heated up to 80 °C. The progress of the reaction was monitored by TLC. After completion, usually after 6 hours, the reaction mixture was filtered through a short plug of silica gel eluting with acetone. The filtrate was concentrated in vacuo and the material was subjected to column chromatography eluting with hexanes/EtOAc (5:1 → 2:1) to afford 219 (37.4 mg, 0.080 mmol, 78% yield) as a pale yellow solid. Rf = 0.26 (hexanes/EtOAc 3:1); mp 1 177–178 ºC; H NMR (500 MHz, CDCl3) δ 7.78 (s, 1 H), 7.54 (d, J = 9.0 Hz, 1 H), 6.70 (dd, J = 174 9.0, 3.0 Hz, 1 H), 6.66 (d, J = 3.0 Hz, 1 H), 4.25 (q, J = 7.0 Hz , 2 H), 3.03 (s, 6 H), 2.67 (s, 3 H), 1.30 (t, J = 7.0 Hz, 3 H), 0.36 (s, 9 H); 13 C NMR (125 MHz, CDCl3) δ 163.7, 162.5, 159.6, 152.4, 144.5, 141.5, 137.8, 134.8, 134.6, 134.3, 122.2, 119.0, 113.9, 111.9, 99.1, 61.1, 40.0, 17.5, 14.3, −0.2; IR (neat): 2957, 2926, 2857, 2820, 2214, 1711, 1599, 1554, 1516, 1442, 1372, 1323, –1 1264, 1124, 1093, 1003, 843 cm ; HRMS (ESI): m/z calculated for C23H28N3O2SiS2 [M+H] + 470.1392, found 470.1390. F F CN CF3 TMS 1.05 equiv NBS CF3 acetonitrile (0.05 M), rt 66% S CN Br S CF3 CF3 216 220 Preparation of 220: To a solution of 216 (50.1 mg, 0.103 mmol) in acetonitrile (2.0 mL) was added N-bromosuccinimide (19.2 mg, 0.108 mmol) in one portion and the mixture was stirred at room temperature until TLC confirmed completion, usually after 12 hours. The reaction mixture was filtered through a short plug of silica gel eluting with dichloromethane. The filtrate was concentrated in vacuo and purified by column chromatography eluting with hexanes/EtOAc (19:1 → 5:1) to afford 220 (33.5 mg, 0.068 mmol, 66% yield) as a white solid. Rf = 0.57 1 (hexanes/ethyl acetate 5:1); mp 148–150 ºC; H NMR (500 MHz, CDCl3): δ 7.81 (s, 1H), 7.60 (s, 1 H), 7.55 (dd, J = 8.0, 6.5 Hz, 1H), 7.15 (s, 1 H), 7.07 (dd, J = 8.5, 1.5 Hz, 1H), 7.04 (dd, J = 8.0, 1.5 Hz, 1H); 13 1 C NMR (75 MHz, CDCl3): δ (164.2, 162.1) (d, JC–F = 260.2 Hz), (141.4, 3 4 141.4) (d, JC–F = 8.4 Hz), 138.3, (137.3, 137.3) (d, JC–F = 2.3 Hz), 134.4, 133.9, (133.0, 175 2 1 132.7, 132.4, 132.2) (q, JC–F = 33.8 Hz), 132.4, 129.0 (m), (126.0, 123.8, 121.6, 119.5) (q, JC– 3 3 F = 273.4 Hz), (125.3, 125.2) (d, JC–F = 3.6 Hz), 122.1 (septet, JC–F = 3.4 Hz), (116.8, 116.6) 2 2 (d, JC–F = 20.7 Hz), 114.6, 113.4, (101.0, 100.9) (d, JC–F = 15.4 Hz); IR (neat): 2923, 2853, –1 2237, 1727, 1620, 1354, 1205, 1179, 1129 cm ; HRMS (ESI): m/z calculated for C19H8BrF7NS + [M+H] 493.9444, found 493.9448. SO2Me SO2Me 1.05 equiv NBS CF3 TMS CF3 Br acetonitrile (0.05 M), rt 77% S S CF3 CF3 215 221 Preparation of 221: To a solution of 215 (33.1 mg, 0.063 mmol) in acetonitrile (1.3 mL) was added N-bromosuccinimide (11.8 mg, 0.066 mmol) in one portion and the mixture was stirred at room temperature until TLC confirmed completion, usually after 12 hours. The reaction mixture was filtered through a short plug of silica gel eluting with dichloromethane. The filtrate was concentrated in vacuo and purified by column chromatography eluting with hexanes/EtOAc (5:1) to afford 221 (25.8 mg, 0.049, 77% yield) as a white solid. Rf = 0.21 (hexanes/ethyl acetate 5:1); 1 mp 53–54 ºC; H NMR (500 MHz, CDCl3): δ 7.90 (d, J = 8.75 Hz, 2 H), 7.76 (s, 1 H), 7.55 (s, 2 H), 7.38 (d, J = 8.75 Hz, 2 H), 7.18 (s, 1 H), 3.02 (s, 3 H); 13 C NMR (75 MHz, CDCl3): δ 140.1, 2 139.9, 138.7, 137.6, 134.7, (132.9, 132.5, 132.0, 131.6) (q, JC–F = 34.1 Hz), 132.7, 129.9, 129.0 1 3 (m), (128.2, 124.6, 121.0, 117.3) (q, JC–F = 273.3 Hz), 128.0, 121.7 (septet, JC–F = 3.6 Hz), 176 114.2, 44.5; IR (neat): 3087, 2930, 1599, 1469, 1432, 1375, 1279, 1155, 1023, 957, 898, 866, –1 + 841, 790, 771, 738 cm ; HRMS (ESI): m/z calculated for C19H12BrF6O2S2 [M+H] 528.9366, found 528.9366. N N N N 1.05 equiv NBS TMS S F MeCN, rt, 12 h 76% Br 218 S F 222 Preparation of 222: To a solution of 218 (22.9 mg, 0.069 mmol) in acetonitrile (1.4 mL) was added N-bromosuccinimide (12.3 mg, 0.071 mmol) in one portion and the mixture was stirred at room temperature until TLC confirmed completion, usually after 12 hours. The reaction mixture was filtered through a short plug of silica gel eluting with dichloromethane. The filtrate was concentrated in vacuo and purified by column chromatography eluting with hexanes/EtOAc (5:1 → 2:1) to afford 222 (17.8 mg, 0.053, 76% yield) as a very thick colorless oil. Rf = 0.17 1 (hexanes/ethyl acetate 3:1); H NMR (500 MHz, CDCl3): δ 7.32 (m, 2 H), 7.29 (d, J = 0.5 Hz, 1 H), 7.09 (d, J = 0.5 Hz, 1 H), 7.05 (s, 1 H), 7.03 (m, 2 H), 3.81 (s, 3 H); 13 C NMR (125 MHz, 1 3 CDCl3): δ (163.7, 161.7) (d, JC–F = 247.9 Hz), 138.1, 136.8, 131.4, (131.3, 131.2) (d, JC–F = 4 2 8.4 Hz), 129.8, (129.6, 129.6) (d, JC–F = 3.2 Hz), 128.0, 116.5, (115.9, 115.7) (d, JC–F = 22.0 Hz), 111.1, 39.0; IR (neat): 3100, 3071, 2932, 2855, 1603, 1586, 1533, 1504, 1448, 1417, 1288, –1 1234, 1157, 1100, 1093, 985, 972, 841, 810 cm ; HRMS (ESI): m/z calculated for + C14H11N2FSBr [M+H] 336.9810, found 336.9810. 177 CO2Et CO2Et S S N NMe2 TMS MeCN, rt, 12 h 83% S N Br 1.05 equiv NBS TMS NMe2 S NC NC 219 225 Preparation of 225: To a solution of 219 (50.0 mg, 0.106 mmol) in acetonitrile (2.1 mL) was added N-bromosuccinimide (18.9 mg, 0.11 mmol) in one portion and the mixture was stirred at room temperature until TLC confirmed completion, usually after 12 hours. The reaction mixture was filtered through a short plug of silica gel eluting with dichloromethane. The filtrate was concentrated in vacuo and purified by column chromatography eluting with hexanes/EtOAc (7:1 to 5:1) to afford 225 (48.7 mg, 0.089 mmol, 83% yield) as white solid. Rf = 0.22 (hexanes/EtOAc 1 5:1); mp 134–136 ºC; H NMR (500 MHz, CDCl3) δ 7.86 (s, 1 H), 7.65 (d, J = 8.5 Hz, 1 H), 7.14 (d, J = 8.5 Hz, 1 H), 4.23 (q, J = 7.3 Hz , 2 H), 2.92 (s, 6 H), 2.64 (s, 3 H), 1.28 (t, J = 7.3 Hz, 3 H), 0.38 (s, 9 H); 13 C NMR (125 MHz, CDCl3) δ 162.9, 162.3, 159.7, 156.8, 143.4, 142.7, 139.8, 134.9, 134.0, 132.9, 122.0, 121.5, 120.4, 116.9, 108.1, 61.0, 43.7, 17.4, 14.2, −0.2; IR –1 (neat): 2954, 2924, 2853, 2224, 1717, 1653, 1578, 1523, 1501, 1456, 1437, 1261 cm ; HRMS + (ESI): m/z calculated for C23H27N3O2SiS2Br [M+H] 548.0497, found 548.0498. 178 CO2Et CO2Et S S N Br TMS NMe2 N Br 2 equiv TBAF THF, RT 81% S NMe2 S NC NC 225 227 Preparation of 227: To a solution of 225 (54.2 mg, 0.099 mmol) in THF (2.0 mL) was added tetrabutylammonium fluoride 1 M in THF (0.2 mL, 0.2 mmol) dropwise at room temperature. The completion of the reaction, usually after 2.5–3 hours, was confirmed by TLC. The reaction mixture was quenched with aqueous saturated sodium bicarbonate (5 mL) and extracted with ethyl acetate (3 × 5 mL); combined organic layers were dried over magnesium sulfate, filtered and concentrated in vacuo. Desilylation product 227 (39.1 mg, 0.082 mmol, 83% yield) as a wax after purification by column chromatography eluting with hexanes/EtOAc (1:1 → 3:1). Rf = 0.4 1 (hexanes/EtOAc 3:1); mp 53–54 ºC; H NMR (500 MHz, CDCl3) δ 7.77 (d, J = 5.25 Hz, 1 H), 7.66 (d, J = 8.5 Hz, 1 H), 7.50 (d, J = 5.25 Hz, 1 H), 7.15 (d, J = 8.5 Hz, 1 H), 4.24 (q, J = 7 Hz, 2 H), 2.93 (s, 1 H), 2.62 (s, 1 H), 1.28 (t, J = 7 Hz, 3 H); 13 C NMR (75 MHz, CDCl3) δ 162.6, 162.3, 159.8, 156.8, 139.4, 138.5, 133.7, 132.9, 127.7, 126.9, 122.1, 121.7, 120.6, 116.9, 108.3, 61.1, 43.7, 17.4, 14.3; IR (neat): 3110, 2926, 2795, 2224, 1714, 1579, 1522, 1485, 1437, 1373, –1 1327, 1263, 1128, 1093 cm ; HRMS (ESI): m/z calculated for C20H19N3O2S2Br [M+H] 476.0102, found 476.0105. 179 + CO2Et CO2Et S S N NMe2 TMS N 2 equiv TBAF NMe2 THF, RT 81% S S NC NC 219 229 Preparation of 229: To a solution of 219 (51 mg, 0.109 mmol) in THF (2.2 mL) was added tetrabutylammonium fluoride 1 M in THF (0.22 mL, 0.22 mmol) dropwise at room temperature. The completion of the reaction, usually after 2.5–3 hours, was confirmed by TLC. The reaction mixture was quenched with aqueous saturated sodium bicarbonate (5 mL) and extracted with ethyl acetate (3 × 5 mL); combined organic layers were dried over magnesium sulfate, filtered and concentrated in vacuo. Desilylation product 229 (35 mg, 0.088 mmol, 81% yield) was obtained as a white solid after purification by column chromatography eluting with 1 hexanes/EtOAc (3:1). Rf = 0.25 (hexanes/EtOAc 3:1); mp 185 ºC; H NMR (500 MHz, CDCl3) δ 7.70 (d, J = 5.0 Hz, 1 H), 7.55 (d, J = 9.0 Hz, 1 H), 7.39 (d, J = 5.0 Hz, 1 H), 6.72 (dd, J = 9.0, 3.0 Hz, 1 H), 6.68 (d, J = 3.0 Hz, 1 H), 4.26 (q, J = 7.5, 2 H), 3.04 (s, 6 H), 2.66 (s, 3 H), 1.30 (t, J = 7.5, 3 H); 13 C NMR (75 MHz, CDCl3) δ 163.4, 162.5, 159.6, 152.4, 139.5, 137.4, 134.6, 133.1, 128.3, 125.8, 122.3, 118.9, 114.1, 112.0, 99.5, 61.1, 40.0, 17.5, 14.3; IR (neat): 3108, 2964, 2928, 2859, 2213, 1716, 1701, 1684, 1653, 1601, 1558, 1522, 1506, 1437, 1373, 1263 cm 1 + ; HRMS (ESI): m/z calculated for C20H20N3O2S2 [M+H] 398.0997, found 398.0997. 180 – SO2Me 1.2 equiv BPin 205 Br S 235 Cl SO2Me 2 mol% PdCl2·dppf·CH2Cl2 1.5 equiv K3PO4·nH2O DME, 80 °C, 2 h 82% S Cl 901 Preparation of 2-chloro-3-(4-(methylsulfonyl)phenyl)thiophene (901): In a flame-dried air-free flask were mixed 235 (151 mg, 0.617 mmol), 1-bromo-4-(methylsulfonyl)benzene, 205, (174 mg, 0.741 mmol), potassium phosphate n-hydrate (230 mg, 0.926 mmol), and PdCl2·dppf (9.04 mg, 0.012 mmol), together with a stir bar. The solid mixture was put under high vacuum and refilled with nitrogen several times. Degassed (freeze-pump-thaw) DME (1.25 mL) was added under nitrogen atmosphere. The reaction mixture was heated up to 80 °C. After 2h, TLC (hexanes/EtOAc 2:1) showed that the reaction was complete. The reaction mixture was passed through a short plug of silica gel eluting with acetone and the filtrate was concentrated and adsorbed into a minimum amount of silica gel. Column chromatography eluting with hexanes/EtOAc (2:1) afforded 901 (138 mg, 0.506 mmol, 82% yield) as an off-white solid. Rf = 1 0.27 (2:1 hexanes/EtOAc); mp 114–115 °C; H NMR (600 MHz, CDCl3): δ 7.99 (d, J = 8.2 Hz, 2 H), 7.75 (d, J = 8.2 Hz, 2 H), 7.20 (d, J = 5.9 Hz, 1 H), 7.06 (d, J = 5.9 Hz, 1 H), 3.08 (s, 3 H); 13 C NMR (150 MHz, CDCl3): δ 139.6, 139.3, 136.2, 129.3, 128.0, 127.6, 126.8, 123.6; IR (neat) –1 2923, 2872, 1737, 1717, 1700, 1685, 1653, 1559, 1540, 1521, 1276, 1136, 1023, 837, 771 cm ; + HRMS (ESI): m/z calculated for C11H10O2S2Cl [M+H] 272.9811, found 272.9813. 181 SO2Me SO2Me (HO)2B 1.5 equiv Cl S 901 F 1 mol% Pd2dba3 4 mol% XPhos 2 equiv anhyd K3PO4 tBuOH, 80 °C, 6 h 83% S F 232 Preparation of 2-(4-fluorophenyl)-3-(4-(methylsulfonyl)phenyl)thiophene (232): To an air-free flask was added 901 (44.4 mg, 0.163 mmol), (4-fluorophenyl)boronic acid, 206, (34.2 mg, 0.244 mmol), anhydrous potassium phosphate (69.1 mg, 0.326 mmol), Pd2dba3 (1.5 mg, >1.628 µmol), and XPhos (3.1 mg, 6.51 µmol), and the solid mixture was purged in vacuo and refilled with nitrogen several times. To the flask was added degassed tBuOH (0.65 mL), and the resulting suspension was heated up to 80 °C for 6 h. The mixture was passed through silica gel eluting with acetone. The filtrate was concentrated and purified by column chromatography eluting with hexanes/EtOAc (2:1) to afford 232 (44.9 mg, 0.135 mmol, 83% yield) as an off-white solid. Rf = 1 0.38 (2:1 hexanes/EtOAc); mp 146–148 °C; H NMR (600 MHz, CDCl3): δ 7.82 (d, J = 8.1 Hz, 2 H), 7.42 (d, J = 8.1 Hz, 2 H), 7.36 (d, J = 5.3 Hz, 1 H), 7.22 (dd, J = 8.7, 5.4 Hz, 1 H), 7.15 (d, J = 5.3 Hz, 1 H), 6.98 (dd, J = 8.7, 8.7 Hz, 1 H), 3.05 (s, 3 H); 1 13 C NMR (150 MHz, CDCl3): δ 3 (163.4, 161.7) (d, JC–F = 248.5 Hz), 142.0, 139.6, 138.7, 136.1, (131.2, 131.1) (d, JC–F = 8.1 4 2 Hz), 131.1, 129.8, (129.79, 129.77) (d, JC–F = 4.0 Hz), 127.5, 125.1, (115.95, 115.81) (d, JC–F = 22.2 Hz), 44.5; IR (neat) 3103, 3081, 3015, 2927, 1936, 1903, 1652, 1595, 1540, 1507, 1439, –1 1283, 1313, 1151, 1118, 1093, 962, 901, 882, 841, 774 cm ; HRMS (ESI): m/z calculated for + [M+H] 333.0419, found 333.0419. 182 SO2Me SO2Me 1 equiv NBS S F acetonitrile rt, 12 h 91% Br 232 S F 201 Preparation of DuP 697 (201): To a solution of 232 (23.2 mg, 0.070 mmol) in acetonitrile (1.5 mL) was added N-bromosuccinimide (13.0 mg, 0.073 mmol) in one portion and the mixture was stirred at room temperature for 12 hours. The reaction mixture was filtered through a short plug of silica gel eluting with dichloromethane. The filtrate was concentrated in vacuo and purified by column chromatography eluting with hexanes/EtOAc (4:1) to afford 201 (26.1 mg, 0.064 mmol, 91% yield) as a white solid. Rf = 0.3 (4:1 hexanes/EtOAc); mp 123–123.5 °C (lit 18a 122–124 1 °C); H NMR (600 MHz, CDCl3): δ 7.82 (d, J = 8.7 Hz, 2 H), 7.37 (d, J = 8.7 Hz, 2 H), 7.16 (m, 2 H), 7.11 (s, 1 H), 6.98 (m, 2 H), 3.05 (s, 3 H); 13 C NMR (150 MHz, CDCl3): δ (163.6, 161.9) 1 3 (d, JC–F = 249.5 Hz), 140.78, 140.77, 139.2, 136.5, 132.2, (131.1, 131.0) (d, JC–F = 8.2 Hz), 4 2 129.7, (128.55, 128.52) (d, JC–F = 3.6 Hz), 127.6, (116.16, 116.01) (d, JC–F = 21.9 Hz), 111.9, + 44.4; HRMS (EI): m/z calculated for C17H12O2S2BrF [M] 409.9446, found 409.9452. CO2Et 1.5 equiv PinB S N Cl S 902 CO2Et NMe2 . S N NC NMe2 1 mol% Pd(OAc)2 2 mol% XPhos 2 equiv K3PO4 tBuOH (0.5 M), 80 ºC 84% S NC 229 Preparation of 229: To an air-free flask provided with a stir bar were added 902 (56.4 mg, 0.196 183 mmol), 4-(dimethylamino)-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile (42.7 mg, 0.294 mmol), anhydrous potassium phosphate (83 mg, 0.392 mmol), Pd2dba3 (2.0 mg, >1.96 µmol), and XPhos (4.0 mg, >7.84 µmol). The flask with the solid mixture was purged and refilled with nitrogen multiple times. To the flask was added degassed tBuOH (0.8 mL), and the resulting suspension was heated up to 80 °C. The progress of the reaction was monitored by TLC. After completion, usually after 6 hours, the reaction mixture was filtered through a short plug of silica gel eluting with acetone. The filtrate was concentrated in vacuo and the material was subjected to column chromatography eluting with hexanes/EtOAc (5:1 → 2:1) to afford 229 (65.1 mg, 0.164 mmol, 84% yield) as a white solid, spectroscopically identical to the compound 1 obtained via desilylation (page 180). Rf = 0.25 (hexanes/ethyl acetate 3:1); mp 183–184 ºC; H NMR (500 MHz, CDCl3): δ 7.70 (d, J = 5.0 Hz, 1 H), 7.55 (d, J = 9.0 Hz, 1 H), 7.39 (d, J = 5.0 Hz, 1 H), 6.72 (dd, J = 9.0, 3.0 Hz, 1 H), 6.68 (d, J = 3.0 Hz, 1 H), 4.26 (q, J = 7.5, 2 H), 3.04 (s, 6 H), 2.66 (s, 3 H), 1.30 (t, J = 7.5, 3 H); 13 C NMR (75 MHz, CDCl3): δ 163.4, 162.5, 159.6, 152.4, 139.5, 137.4, 134.6, 133.1, 128.3, 125.8, 122.3, 118.9, 114.1, 112.0, 99.5, 61.1, 40.0, 17.5, 14.3. CO2Et CO2Et S S N NMe2 MeCN, rt, 12 h 88% S N Br 1.05 equiv NBS NC NMe2 S NC 229 227 Preparation of 227: To a solution of 229 (42.5 mg, 0.107 mmol) in acetonitrile (2.1 mL) was added N-bromosuccinimide (19.2 mg, 0.108 mmol) in one portion and the mixture was stirred at 184 room temperature for 12 hours. The reaction mixture was filtered through a short plug of silica gel eluting with dichloromethane. The filtrate was concentrated in vacuo and purified by column chromatography eluting with hexanes/EtOAc (1:1 → 3:1) to afford 227 (44.9 mg, 0.094 mmol, 88%), spectroscopically identical to the compound prepared via desilylation (page 179). Rf = 0.4 1 (hexanes/EtOAc 3:1); mp 53–54 ºC; H NMR (500 MHz, CDCl3) δ 7.77 (d, J = 5.25 Hz, 1 H), 7.66 (d, J = 8.5 Hz, 1 H), 7.50 (d, J = 5.25 Hz, 1 H), 7.15 (d, J = 8.5 Hz, 1 H), 4.24 (q, J = 7 Hz, 2 H), 2.93 (s, 1 H), 2.62 (s, 1 H), 1.28 (t, J = 7 Hz, 3 H); 13 C NMR (75 MHz, CDCl3) δ 162.6, 162.3, 159.8, 156.8, 139.4, 138.5, 133.7, 132.9, 127.7, 126.9, 122.1, 121.7, 120.6, 116.9, 108.3, 61.1, 43.7, 17.4, 14.3. 9.3. Experimental details for Section 2.2: Deborylation General Procedure A. Deuterodeborylation for preparation of deuterated aromatics from purified boronic esters with CD3OD: In an air-free flask provided with a stir bar, a solution of aryl boronic ester (dried overnight in a vacuum dessicator) in methanol-d4/DCM 2:1 (0.2 M) was freeze-pump-thaw degassed (0.01 mmHg) three times and Ir catalyst (1.5 mol%) was added under nitrogen blanketing. The flask was sealed and heated in an oil bath to 55 °C. Required reaction times for each substrate were estimated by running the reaction in a J. Young tube at a 1 0.1 mmol scale using methanol-d4/CDCl3 2:1 as solvent and monitoring the progress by H NMR. Upon completion, the reaction mixture was concentrated, and the residue was directly subjected to column chromatography eluting with pentane or dichloromethane. 185 Cl Cl Cl Cl BPin 1.5 mol % [Ir(OMe)(COD)]2 Cl CD3OD/DCM (2:1), 55 ºC Cl D 903 904 31 Preparation of 1,2,3-trichlorobenzene-5-d (904 , Table 1, entry 1): General procedure A (page 185) was applied to 4,4,5,5-tetramethyl-2-(3,4,5-trichlorophenyl)-1,3,2-dioxaborolane (903) (61.5 mg, 0.2 mmol) with [Ir(OMe)(COD)]2 (2.0 mg, 1.5 mol%) in CD3OD/DCM (2:1) (1 mL). After 5.5 h, the reaction mixture was concentrated in vacuo. Column chromatography eluting 1 with pentane afforded 904 (30.9 mg, 85% yield) as an off-white solid. mp 53–54 °C. H NMR (500 MHz, CDCl3): δ 7.36 (apparent s); 13 C NMR (125 MHz, CDCl3): δ 134.3, 131.5, 128.6, (127.4, 127.2, 127.0) (t, JC–D = 25 Hz); HRMS (EI): m/z calculated for C6H2DCl3 [M] + 180.9363, found 180.9371. Cl CF3 BPin 1.5 mol % [Ir(OMe)(COD)]2 Cl CF3 CD3OD/DCM (2:1), 55 ºC D 905 906 31 3-Chlorobenzotrifluoride-5-d (906 , Table 1, entry 3): General procedure A (page 185) was applied to 2-(3-chloro-5-(trifluoromethyl)phenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (905) (61.3 mg, 0.2 mmol) with [Ir(OMe)(COD)]2 (2.0 mg, 1.5 mol%) in CD3OD/DCM (2:1) (1 mL). After 4 h, the reaction mixture was concentrated. Column chromatography eluting with pentane 1 afforded 27.5 mg of a colorless oil containing 906 (25.1 mg, 69% yield) and pentane. H NMR (300 MHz, CDCl3): δ 7.60 (apparent s, 1 H), 7.54–7.48 (m, 2 H); 186 13 C NMR (125 MHz, CDCl3): δ 134.9, (132.7, 132.4, 132.2, 131.9) (q, JC–F = 35 Hz), 131.9, (130.1, 129.9, 129.7) (t, JC–D = 25 Hz), (126.6, 124.4, 122.3, 120.1) (q, JC–F = 271 Hz), (125.73, 125.70, 125.66, 125.63) (q, JC– F = 4 Hz), (123.37, 123.34, 123.31, 123.28) (q, JC–F = 3.75 Hz). Br CN 1.5 mol % [Ir(OMe)(COD)]2 Br CD3OD/DCM (2:1), 55 ºC BPin CN D 907 908 31 3-Bromobenzonitrile-5-d (908 , Table 1, entry 4): General procedure A (page 185) was applied to 3-bromo-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile (907) (61.6 mg, 0.2 mmol) with [Ir(OMe)(COD)]2 (2.0 mg, 1.5 mol%) in CD3OD/DCM (2:1) (1 mL). After 8 h, the reaction mixture was concentrated in vacuo. Column chromatography eluting with CH2Cl2 1 afforded 908 (27.2 mg, 74% yield) as an off-white solid. H NMR (500 MHz, CDCl3): δ 7.77 (dd, J = 2.0, 1.5 Hz, 1 H), 7.73 (apparent s, 1 H), 7.58 (apparent s, 1 H); 13 C NMR (125 MHz, CDCl3): δ 136.0, 134.7, 130.5, (130.5, 130.3, 130.1) (t, JC–D = 25 Hz), 122.8, 117.2, 114.2; + HRMS (EI): m/z calculated for C7H3DBrN [M] 181.9590, found 181.9592. Me2N Cl BPin 1.5 mol % [Ir(OMe)(COD)]2 CD3OD/DCM (2:1), 55 ºC 909 Me2N Cl D 910 31 Preparation of 3-Chloro-N,N-dimethylaniline-5-d (910 , Table 1, entry 5): General procedure A (page 185) was applied to 3-chloro-N,N-dimethyl-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2- 187 yl)aniline (909) (56.2 mg, 0.2 mmol) with [Ir(OMe)(COD)]2 (2.0 mg, 1.5 mol%) in CD3OD/DCM (2:1) (1 mL). After 10 h, the reaction mixture was concentrated in vacuo. Column chromatography eluting with (pentane/CH2Cl2 3:1) afforded 910 (30.4 mg, 0.194 mmol, 97% 1 yield) as a colorless oil. H NMR (500 MHz, CDCl3): δ 6.69–6.66 (m, 2 H), 6.58 (apparent s, 1 H), 2.94 (s, 6 H); 13 C NMR (125 MHz, CDCl3): δ 151.4, 134.9, (129.8, 129.6, 129.4) (t, JC–D = 25 Hz), 116.0, 112.2, 110.4, 40.3; HRMS (ESI): m/z calculated for C8H10DClN [M+H] + 157.0637, found 157.0636. MeO Cl BPin 1.5 mol % [Ir(OMe)(COD)]2 CD3OD/DCM (2:1), 55 ºC 911 MeO Cl D 912 31 Preparation of 3-Chloroanisole-5-d (912 , Table 1, entry 6): General procedure A (page 185) was applied to 2-(3-chloro-5-methoxyphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (911) (54.0 mg, 0.2 mmol) with [Ir(OMe)(COD)]2 (2.0 mg, 1.5 mol%) in CD3OD/DCM (2:1) (1 mL). After 6 h, the reaction mixture was concentrated in vacuo. Column chromatography eluting with 1 dichloromethane afforded 912 (19.3 mg, 0.134 mmol, 67% yield) as a colorless oil. H NMR (300 MHz, CDCl3): δ 6.92 (apparent s, 1 H), 6.89 (t, J = 2.4 Hz, 1 H), 6.78 (apparent s, 1 H), 3.78 (s, 3 H); 13 C NMR (125 MHz, CDCl3): δ 160.3, 134.8, (130.1, 129.9, 129.7) (t, JC–D = 25 + Hz), 120.7, 114.3, 112.4, 55.3; HRMS (EI): m/z calculated for C7H6DClO [M] 143.0248, found 143.0255. 188 CO2Et CO2Et 1.5 mol% [Ir(OMe)(COD)]2 Me BPin N H CD3OD/DCM (2:1), 55 ºC 913 Me D N H 914 Preparation of ethyl 2-methylindole-3-carboxylate-7-d (914, Table 1, entry 7): General procedure A (page 185) was applied to ethyl 2-methyl-7-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)-1H-indole-3-carboxylate (913) (65.8, 0.2 mmol) with [Ir(OMe)(COD)]2 (2 mg, 1.5 mol%) with a small modification. Before use, dried 913 was treated with a minimum amount of CD3OD in the reaction air-free flask and the solvent was evaporated in vacuo twice, then CD3OD/DCM (2:1) (1 mL) was added, the solution was degassed, and catalyst was added. After 1.5 h, the reaction mixture was concentrated in vacuo. Column chromatography eluting with dichloromethane afforded 914 (39.1 mg, 0.191 mmol, 96% yield) as an off-white solid. N1 deuterated material was not observed after purification by column. mp 132–133 ºC; H NMR (600 MHz, CDCl3): δ 8.35 (br, 1 H), 8.09 (d, J = 7.4 Hz, 1 H), 7.20 (t, J = 7.4 Hz, 1 H), 7.17 (dd, 2 J = 7.4, 1.6 Hz, 1 H), 4.39 (q, J = 7.3 Hz, 2 H), 2.73 (s, 3 H), 1.43 (t, J = 7.3 Hz, 3 H); H NMR (76.75 MHz, pentane): δ 7.65; 13 C NMR (150 MHz, CDCl3): δ 166.1, 143.9, 134.4, 127.2, 122.2, 121.7, 121.3, 110.2 (t, JC–D = 24 Hz), 104.7, 59.5, 14.6, 14.2; IR (neat): 3301, 2976, –1 2914, 2860, 1662, 1595, 1541, 1456, 1438, 1223, 1183, 1097, 802 cm ; HRMS (EI): m/z + calculated for C12H12DNO2 [M] 204.1009, found 204.1019. 189 TMS TMS S S 1.5 mol % [Ir(OMe)(COD)]2 Cl BPin Cl CD3OD/DCM (2:1), 55 ºC D 204 915 Preparation of 2-chloro-5-(trimethylsilyl)thiophene-3-d (915, Table 1, entry 8): General procedure A (page 185) was applied to (5-chloro-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)thiophen-2-yl)trimethylsilane (204) (0.81 g, 2.56 mmol) with [Ir(OMe)(COD)]2 (25.0 mg, 1.5 mol%) in CD3OD/DCM (2:1) (12.8 mL). After 30 minutes, the reaction mixture was concentrated to about 5 mL, diluted with pentane (10 mL) and filtered through a short plug of silica gel packed with pentane. Fractions containing the product were mixed and carefully 1 concentrated to afford 640 mg of a mixture of product and pentane (ca. 1:2 by H NMR, calculated yield 89%). Further concentration in vacuo of a small portion of the material afforded pure 915 (after significant loss) as a colorless oil, which was used for characterization purposes. 1 2 H NMR (600 MHz, CDCl3): δ 6.98 (s, 1 H), 0.28 (s, 9 H); H NMR (76.75 MHz, pentane): δ 7.52; 13 C NMR (150 MHz, CDCl3): δ 140.2, 134.4, 133.2, 127.2 (t, JC–D = 26 Hz), −0.3; IR –1 (neat): 3064, 2957, 2903, 1407, 1240, 1156, 1031, 999, 840, 760, 697, 674 cm ; HRMS (EI): + m/z calculated for C7H10DSiSCl [M] 191.0102, found 191.0110. 9.4. Experimental details for Chapter 3: TMC-95 core 190 O BPin N H 330 O OMe NHBoc cat. [Ir(OMe)(COD)] 2 BPin MeOH/CH2Cl2 (2:1) 55 °C, 3h 55% 331 + 28% 334 Preparation of 7-borylated L-tryptophan O OMe NHBoc + NHBoc N H N H 331 BPin OMe 332 derivative 331: To an air-free flask containing a u degassed solution of 330 (172.0 mg, 0.302 mmol) in methanol (1.0 mL) and dichloromethane (0.5 mL) was added in one portion [Ir(OMe)COD]2 (3.0 mg, 4.52 µmol). The flask was purged and refilled with nitrogen three times and the resulting mixture was heated in oil bath at 55 °C for 3 hours. The reaction mixture was then concentrated in vacuo and filtered through a short plug of silica gel eluting with dichloromethane to remove the iridium residue. The crude material was adsorbed on a minimum amount of silica gel, dried in vacuo, and purified by column chromatography eluting with dichloromethane to afford N-Boc-L-tryptophan methyl ester, 332 (27.2 mg, 0.085 mmol, 28% recovered borylation substrate) and 7-borylated L-tryptophan derivative 331 (73.3 mg, 0.165 mmol, 55% yield) as a white solid, mp 178–178.5 ºC; Rf (331) = 20 1 0.33 (DCM); [α] D +31.9º (c 1.22, CH2Cl2); H NMR (500 MHz, CDCl3) δ 9.10 (br, 1 H), 7.65 (d, J = 7.9 Hz, 1 H), 7.62 (d, J = 7.1 Hz, 1 H), 7.11 (dd, J = 7.9, 7.1 Hz, 1 H), 7.04 (s, 1 H), 5.03 (d, J = 7.5 Hz, 1 H), 4.61 (dd, J = 7.5, 5.4 Hz, 1 H), 3.65 (s, 3 H), 3.32–3.26 (m, 2 H), 1.41 (s, 9 H), 1.37 (s, 12 H); 13 C NMR (125 MHz, CDCl3) δ 172.7, 155.2, 141.3, 129.5, 126.6, 122.7, 122.3, 122.29, 119.1, 109.6, 83.8, 79.7, 54.2, 52.2, 28.3, 27.9, 25.0; IR (neat) 3447, 3052, 2977, u Material prepared by Ms. Fang Yi Shen 191 –1 2931, 1741, 1714, 1591, 1543, 1506, 1435, 1373, 1329, 1294, 1205, 1166 cm ; HRMS (ESI): + m/z calculated for C23H34BN2O6 [M+H] 445.2510, found 445.2513. OH Br HBr/AcOH rt, 89% O NH2 HO OH Br2 HO O NH2·HBr 916 335 L-tyrosine OH Boc2O Br tBuOH/H2O pH 9, rt, 89% HO OH TBSCl imidazole O NHBoc Br then K2CO3 H2O, rt, 70% 917 Preparation of (S)-3-bromotyrosine hydrobromide 916: 53 O NHBoc TBSO 336 Commercial solution of hydrogen bromide 33% w/v in acetic acid (135 mL, 552 mmol) was added dropwise to a suspension of (S)tyrosine, 335, (50.01 g, 276 mmol) in glacial acetic acid (225 mL) while stirring, followed by a solution of bromine (15.4 mL, 299 mmol) in acetic acid (102 mL), added dropwise using an addition funnel over several hours (overnight). The resulting mixture was stirred at room temperature for at least additional 24 hours. The precipitate was filtered and rinsed twice with glacial acetic acid and then with ether several times. The resulting paste was dried under high vacuum overnight and then in a vacuum dessicator in the presence of NaOH pellets. The product (S)-3-bromotyrosine hydrobromide, 916, (84.09 g, 247 mmol, 89% yield discounting the weight of acetic acid present in the crude material) was obtained as an off-white solid (reported 53 white) , mp (crude) 212–218 ºC (dec) (lit 53 20 mp 210–215 °C, dec); [α] D +2.84° (c 8.84, 1 MeOH); H NMR (500 MHz, CD3OD) δ 7.43 (d, J = 2.0 Hz, 1 H), 7.10 (dd, J = 8.5, 2.0 Hz, 1 H), 6.90 (d, J = 8.5 Hz, 1 H), 4.20 (dd, J = 7.4, 5.8 Hz, 1 H), 3.21 (dd, J = 14.8, 5.8 Hz, 1 H), 192 3.07 (dd, J = 14.8, 7.4 Hz, 1 H); 13 C NMR (125 MHz, CD3OD) δ 171.2, 155.4, 135.2, 130.9, 128.0, 117.9, 111.4, 55.3, 36.2. This crude product was used directly in the following step without further purification. Preparation of (S)-N-Boc-3-bromotyrosine (917): 53 To a suspension of (S)-3-bromotyrosine hydrobromide, 916, (6.00 g, 17.60 mmol) in tBuOH (54 mL) and water (6 mL) was added aqueous NaOH (2 M) in small portions to adjust the pH to 9. Di-tert-butyl dicarbonate (4.61 g, 21.11 mmol) was added and the mixture was stirred at room temperature. After 30 minutes, the pH was readjusted to 9, by addition of aqueous NaOH (2 M), and more di-tert-butyl dicarbonate (2.304 g, 10.56 mmol) was added. The resulting mixture was stirred for additional 30 min at room temperature. The solution was transferred to a separatory funnel and washed with hexanes (2 × 150 mL). The aqueous phase was acidified to pH 2–3 by addition of 10% v/v aqueous HCl and extracted with ethyl acetate (3 × 150 mL). Combined organic phases were washed with water (2 × 400 mL) and brine (400 mL), dried over anhydrous MgSO4, and concentrated in vacuo to afford crude (S)-N-Boc-3-bromotyrosine, 917, (5.62 g, 15.61 mmol, 89% yield) as a 20 1 foam. [α] D +11.54° (c 1.63, MeOH); H NMR (500 MHz, acetone-d6) δ 7.41 (d, J = 1.6 Hz, 1 H), 7.11 (dd, J = 8.0, 1.6 Hz, 1 H), 6.92 (d, J = 8.0 Hz, 1 H), 6.00 (br d, J = 8.0 Hz, 1 H), 4.37 (m, 1 H), 3.11 (dd, J = 14.0, 5.2 Hz, 1 H), 2.92 (dd, J = 14.0, 8.8 Hz, 1 H), 1.37 (s, 9 H); 13 C NMR (125 MHz, acetone-d6) δ 173.3, 156.2, 153.5, 134.6, 131.3, 130.5, 117.1, 110.0, 79.3, + 55.6, 37.0, 28.6; HRMS (ESI): m/z calculated for C14H19NO5Br [M+H] 360.0447, found 360.0447. This crude product was used directly in the following step without further purification. 193 Preparation of (S)-O-TBS-N-Boc-3-bromotyrosine (336): To a solution of (S)-N-Boc-3bromotyrosine, 917, (1.30 g, 3.61 mmol) in DMF (15 mL) were successively added imidazole (0.74 g, 10.83 mmol) and TBSCl (1.20 g, 7.94 mmol). The resulting solution was stirred at room temperature overnight. The reaction mixture was then treated with water (15 mL), stirred for 30 min, and extracted with diethyl ether (3 × 30 mL). Combined ether layers were successively washed with 1N aqueous HCl (20 mL), saturated aqueous NaHCO3 (20 mL), water (20 mL), and brine (20 mL). Once dried over Na2SO4, the organic extract was concentrated in vacuo. The resulting yellowish oil was redissolved in THF (10 mL), treated with potassium carbonate 1 M in water (11 mL, 11 mmol), and stirred at room temperature for 1 hour. The mixture was acidified to pH 3 by addition of 1M aqueous HCl and then extracted with ethyl acetate (3 × 10 mL). The combined ethyl acetate layers were dried with Na2SO4, filtered, and concentrated in vacuo. The residue was purified by flash chromatography eluting with hexanes/EtOAc/HOAc 8:1.9:0.1 to provide (S)-O-TBS-N-Boc-3-bromotyrosine (336) as a slightly yellowish oil that became a foam under high vacuum and hardened upon standing to form a white solid (1.20 g, 2.53 mmol, 70% 20 yield), mp 116–118 ºC; Rf = 0.35 (hexanes/EtOAc/HOAc 8:1.9:0.1); [α] D +14.5° (c 0.54, 1 CH2Cl2); H NMR (500 MHz, CDCl3) δ 7.32 (apparent s, 1 H), 6.97 (dd, J = 8.1, 2.0 Hz, 1 H), 6.77 (d, J = 8.1 Hz, 1 H), 4.95 (d, J = 7.7 Hz, 1 H), 4.52 (m, 1 H), 3.11 (dd, J = 13.7, 4.4 Hz, 1 H), 2.94 (dd, J = 13.7, 6.3 Hz, 1 H), 1.41 (s, 9 H), 1.01 (s, 9 H), 0.21 (s, 6 H); 13 C NMR (125 MHz, CDCl3) δ 175.7, 155.4, 151.7, 134.1, 129.9, 129.1, 120.1, 115.3, 80.5, 54.3, 36.6, 28.3, 25.7, 18.3, 4.2; IR (neat) 3307, 2957, 2930, 2859, 1684, 1654, 1496, 1395, 1366, 1289, 1255, 194 –1 + 1167, 1046 cm ; HRMS (ESI): m/z calculated for C20H33NO5BrSi [M+H] 474.1311, found 474.1313. HO OH Br O NHBoc TBSO 336 N-hydroxysuccinimide DCC, DME, 0ºC, ON filtration O HN Br then L-asparagine·H2O TBSO NaHCO3 dioxane, water, 1h 79% CONH2 O NHBoc 329 Preparation of dipeptide 329: To a stirring solution of 336 (0.996 g, 2.1 mmol) and Nhydroxysuccinimide (0.302 g, 2.63 mmol) in DME (21 mL) at 0 °C was added DCC (0.542 g, 2.63 mmol) in one portion. The containing flask was sealed and the reaction mixture was stirred at 0 ºC overnight. The resulting suspension was filtered and the solid (urea) was rinsed with cold DME (3 × 5 mL). The filtrate together with the rinses was concentrated in vacuo, redisolved in dioxane (9 mL), and cooled to about 10 °C. To this solution was added a solution of L-asparagine (1.67 g, 12.60 mmol) and sodium bicarbonate (1.06 g, 12.60 mmol) in water (6 mL) in small portions. After 1 h of vigorous stirring, most of the dioxane was removed under vacuum and the remaining aqueous phase was acidified to pH 3.5 and extracted three times with EtOAc. The combined extracts were washed with water and brine, dried over MgSO4, and evaporated to yield a white foam that was subjected to flash chromatography eluting with hexanes/EtOAc/HOAc 8:1.9:0.1 to afford 336 (74 mg, 7.5% recovered starting material) and the desired dipeptide 329 (972 mg, 1.651 mmol, 79% yield) as a white solid, mp 147–147.5 ºC; Rf = 20 1 0.21 (hexanes/EtOAc/HOAc 8:1.9:0.1); [α] D +16.5° (c 0.49, CH2Cl2); H NMR (500 MHz, CDCl3) δ 8.01 (br, 1 H), 7.31 (s, 1H), 6.99 (br, 1 H), 6.95 (d, J = 8.0 Hz, 1 H), 6.70 (dd, J = 8.0, 195 3.0 Hz, 1 H), 5.69 (br, 1H), 5.56 (br, 1H), 4.71 (m, 1 H), 4.40 (m, 1H), 3.06 (apparent d, J = 13.5 Hz, 1 H), 2.92–2.69 (m, 3 H), 1.26 (s, 9 H), 0.98 (s, 9 H), 0.16 (s, 6 H); 13 C NMR (125 MHz, CDCl3) δ 175.8, 172.3, 171.9, 156.1, 151.3, 134.2, 131.0, 129.2, 120.0, 115.1, 80.4, 55.6, 50.2, 37.3, 37.0, 28.3, 25.7, 18.3, 4.2; IR (neat) 3449, 3297, 3055, 2957, 2857, 1734, 1669, 1604, –1 1495, 1473, 1437, 1372, 1329, 1292, 1254, 1205, 1167, 1047 cm ; HRMS (ESI): m/z calculated + for C24H39N3O7BrSi [M+H] 588.1741, found 588.1742. O PinB O OMe NHBoc N H BiCl3 acetonitrile/H2O (50:1) 60 ºC, 2 h OMe PinB NH2 N H 331 334 O 334 + HO HN Br TBSO O NHBoc EDC, HOBt NEt3 O CONH2 THF, 0 ºC to rt 58% OMe PinB HN Br TBSO 329 HN N H O CONH2 O NHBoc 337 Preparation of 7-borylated L-tryptophan derivative 334: To a solution of 331 (63.6 mg, 0.143 mmol) in acetonitrile (1.4 mL) and water (28 µL) was added BiCl3 (27.1 mg, 0.086 mmol, 0.6 equiv) in one portion. The containing flask was sealed and placed in an oil bath at 60 ºC. The reaction mixture was stirred for 1 hour and then cooled to room temperature. A second batch of BiCl3 (27.1 mg, 0.086 mmol, 0.6 equiv) was added and the suspension was stirred again at 60 ºC for 1 hour. The reaction was quenched with NaHCO3 and the excess of solids was removed by 196 filtration through a plug of celite eluting with acetonitrile. Concentration of the filtrate in vacuo provided a crude material (ca. 100 mg) consisting basically on Boc-deprotected product (334), 1 acetonitrile, and presumably inorganic salts. H NMR (500 MHz, CDCl3) δ 9.40 (s, 1 H), 7.56 (d, J ≈ 8.0 Hz, 1 H), 7.49 (d, J ≈ 8.0 Hz, 1 H), 7.48 (s, 1 H), 7.06 (br, 2 H), 7.04 (t, J ≈ 8.0 Hz, 1 H), 4.66 (m, 1 H), 3.53 (s, 3 H), 3.50 (m, 2 H), 1.30 (s, 6 H), 1.29 (s, 6 H); 13 C NMR (125 MHz, CDCl3) δ 169.2, 141.5, 129.7, 127.9, 126.1, 125.7, 121.8, 104.8, 84.0, 54.5, 54.2, 26.2, 25.0; + HRMS (ESI): m/z calculated for C18H26BN2O4 [M+H] 345.1986, found 345.1992. This crude material was used directly in the following step without further purification and assuming a quantitative yield. Preparation of tripeptide 337: To a stirred slurry of crude 7-borylated L-tryptophan derivative 334 (assumed to contain 49.3 mg of 334, 0.142 mmol, 1.2 equiv) and dipeptide 329 (70.2 mg, 0.119 mmol) in THF (6 mL) were added EDC (45.8 mg, 0.239 mmol) and HOBT (36.6 mg, 0.239 mmol). The mixture was stirred and cooled to 0 °C under nitrogen atmosphere. Triethylamine (166 µL, 1.194 mmol) was added in one portion via syringe and the mixture was allowed to slowly warm to room temperature and stirred for 24 h. The reaction mixture was concentrated in vacuo, adsorbed onto a minimum amount of silica gel, dried under high vacuum, and directly subjected to column chromatography eluting with ether and then ether/EtOAc (1:1 → 0:1) to afford tripeptide 337 (63.1 mg, 0.069 mmol, 58% yield) as an off-white slightly orange 20 1 solid, mp 131.5–133.5 ºC; Rf = 0.32 (EtOAc); [α] D +20.5° (c 0.21, CH2Cl2); H NMR (500 MHz, CDCl3) δ 9.22 (s, 1H), 7.76 (br d, J = 8.0 Hz, 1 H), 7.64 (d, J = 8.0 Hz, 1 H), 7.59 (d, J = 7.0 Hz, 1 H), 7.46 (br d, J = 6.5 Hz, 1 H), 7.27 (d, J = 2.0 Hz, 1 H), 7.11 (s, 1 H), 7.09 (dd, J = 197 8.0, 7.0 Hz, 1 H), 6.89 (dd, J = 8.5, 2.0 Hz, 1 H), 6.74 (d, J = 8.5 Hz, 1 H), 5.95 (br, 1 H), 5.52 (br, 1 H), 4.96 (br d, J = 7.5 Hz, 1 H), 4.76 (m, 1 H), 4.73 (m, 1 H), 4.26 (m, 1 H), 3.60 (s, 3 H), 3.27 (apparent d, J = 6.0 Hz, 2 H), 2.92 (dd, J = 14.0, 5.0 Hz, 1 H), 2.82 (m, 1 H), 2.79 (dd, J = 16.0, 4.0 Hz, 1 H), 2.46 (dd, J = 16.0, 6.5 Hz, 1 H), 1.37 (s, 9 H), 1.35 (s, 12 H), 1.00 (s, 9 H), 0.20 (s, 6 H); 13 C NMR (125 MHz, CDCl3) δ 173.4, 171.9, 171.4, 170.2, 155.5, 151.5, 141.2, 134.0, 130.5, 129.4, 129.0, 126.2, 123.3, 122.0, 120.1, 119.0, 115.3, 109.0, 83.8, 80.5, 55.6, 53.2, 52.4, 49.6, 36.5, 36.45, 28.2, 27.3, 25.7, 25.0, 18.3, −4.3; IR (neat) 3397, 2956, 2916, 2849, –1 1577, 1540, 1459, 1419, 1355 cm ; HRMS (ESI): m/z calculated for C42H62BN5O10SiBr + [M+H] 914.3542, found 914.3549. 9.5. Experimental details for Chapter 5: Autolytimycin, Suzuki partners O OH O O HO (a) H2O2, K2CO3 (b) H2O, 0 °C ! RT AcCl HO HO O O vitamin C acetone HO O HO O (b) MeI, MeCN, reflux 73% (3 steps) 918 Preparation of 5,6-isopropylidene-L-ascorbic acid (918): O O OH OMe 501 31 To a solution of vitamin C (110.02 g, 0.625 mol) in acetone (625 mL) was added acetyl chloride (10.5 mL, 0.147 mol and the resulting mixture was vigorously stirred at room temperature overnight. The abundant precipitate formed was collected by filtration and washed with ice cold acetone to remove the yellow color. The mother liquor was concentrated to about 200 mL and a second crop of precipitate was collected and washed. The mother liquor was again concentrated to about 100 mL and a third crop of precipitate was collected and washed. Combined solids were dried under high vacuum to afford 198 crude 918 (110.06 g) as a beige solid, which was used directly in the following step without further purification. Preparation of 3,4-isopropylidene-L-threonic acid methyl ester (501): 31 An aqueous (750 mL) solution of 918 (110 g, 0.51 mol) and K2CO3 (141 g, 1.018 mol) was cooled on an ice/water bath and ice-cold aqueous H2O2 30% w/v (200 mL) was added in portions along with ice, keeping the internal temperature below 30 °C. Upon complete addition, the reaction was stirred at room temperature for 15 h. Water was removed by rotary evaporation in a 60 °C water bath. Ethanol was added to help remove the excess of water by coevaporation. Finally, the residue was dried under high vacuum. The resulting dry solid was suspended in acetonitrile (~800 mL) using a mechanical stirrer. The suspension was treated with MeI (50 mL, >0.72 mol) and heated to reflux for 1 day. After being cooled to room temperature, it was concentrated to ~200 mL and CH2Cl2 (300 mL) was added to precipitate KI. The solid was filtered off and washed with CH2Cl2 and the combined filtrates were evaporated. Purification by column chromatography eluting with hexanes/EtOAc (1:1 → 1:1.5) afforded 501 (87.5 g, 73% over 3 steps) as a colorless oil. Rf = 0.5 1 (1:1 hexanes/EtOAc); H NMR (500 MHz, CDCl3) δ 4.36 (dt, J = 6.5, 2.5 Hz, 1 H), 4.10 (dd, J 3 = 8.0, 3.0 Hz, 1 H), 4.06 and 3.98 (d of ABq, J = 7.0 Hz, JAB = 8.5 Hz, 2 H), 3.79 (s, 3 H), 2.93 (apparent d, J = 8.0 Hz, 1 H), 1.39 (s, 3 H), 1.32 (s, 3 H); 110.0, 76.2, 70.3, 65.6, 52.7, 26.1, 25.3. 199 13 C NMR (125 MHz, CDCl3) δ 172.5, O O O TIPSCl DMAP O OH OMe DMF 90% 501 Preparation of O O OTIPS OMe 502 O-TIPS-3,4-isopropylidene-L-threonic acid methyl ester (502): 31,84 Triisopropylsilyl chloride (35.2 mL, 166 mmol) was added dropwise to a well stirred solution of 501 (10.52 g, 55.3 mmol) and DMAP (13.51 g, 111 mmol) in dry DMF (152 mL) at room temperature. After 20 h, TLC (hexanes/ether, 6:1) confirmed the complete consumption of the starting material. To the reaction mixture were added saturated aqueous NaHCO3 (150 mL), ether (300 mL), and enough water to dissolve all solids. The phases were separated and the aqueous layer was extracted three times with ether. Water was added as needed to dissolve solids. Combined organic layers were dried over MgSO4 and concentrated under reduced pressure. Column chromatography eluting with hexanes/ether (6:1) provided 502 (17.21 g, 49.7 20 1 mmol, 90% yield) as clear oil. Rf = 0.30 (6:1 hexanes/ether); [α] D +10.4 (c 1.17, EtOH); H NMR (500 MHz, CDCl3) δ 4.41 (d, J = 5.5 Hz, 1 H), 4.25 (q, J = 6.5 Hz, 1 H), 4.05 and 3.97 (d 3 of ABq, J = 6.0 Hz, JAB = 8.5 Hz, 2 H), 3.69 (s, 3 H), 1.32 (s, 3 H), 1.28 (s, 3 H), 1.12–0.95 (m, 21 H); 13 C NMR (125 MHz, CDCl3) δ 171.5, 109.7, 77.1, 73.2, 65.2, 51.8, 26.2, 25.1, 17.79, 17.76, 12.2. 200 O O O O O MeMgI ether, 45 °C 40 min OTIPS OMe pyridine rt, 1 h OTIPS HO 502 SOCl2 919 O OH O TFA OTIPS MeOH, rt, 1 h 68% for 3 steps 920 Preparation of tertiary alcohol 919: HO OTIPS 503 31,84 Commercial methylmagnesium iodide 3.0 M in THF (53.7 ml, 161 mmol) was diluted with ether (120 mL) in a three-necked round-bottomed flask, equipped with a reflux condenser, and a solution of 502 (17.43 g, 50.3 mmol) in ether (120 mL) was added via cannula at room temperature. Additional ether (40 mL) was used for rinsing. The resulting solution was heated in an oil bath at 45 °C for 40 minutes, cooled down to room temperature, and quenched with water. The resultant slurry was treated with saturated aqueous potassium sodium tartrate solution and vigorously stirred until all solids were dissolved (ca. 30 minutes). The phases were separated, more saturated aqueous potassium sodium tartrate solution was added to the aqueous layer to dissolve the solids, and the white aqueous phase was extracted four times with ether. Combined organic layers were dried over MgSO4 and concentrated in vacuo. The resulting material was used in the following step without further purification. Preparation of 1,1-disubstituted olefin 920: 31,84 A solution of 919 (crude from previous step, assumed to be 17.43 g, 50.3 mmol) in pyridine (400 mL, no purification or drying needed) was cooled to 0 °C by an ice/water bath. To this solution was added SOCl2 (7.42 ml, 102 mmol) 201 dropwise via syringe and the mixture quickly turned yellow-orange. The reaction was then warmed to room temperature and stirred for 75 min. Upon completion, the reaction was quenched with half-saturated aqueous Na2CO3 at 0 °C and diluted with ether. The layers were separated and the aqueous layer was extracted with ether. Combined organic layers were washed with water, followed by 5% (w/v) aqueous CuSO4 until the aqueous phase was no longer purple, and then water before being dried over MgSO4. After concentration in vacuo, the crude material was subjected to the following step without further purification. Preparation of diol 503: 31,84 To a stirred solution of 920 (crude from previous step, assumed to be 16.53 g, 50.3 mmol) in methanol (300 mL, used as received) at 0 °C was added TFA (100 ml, 1295 mmol) dropwise. After 5 minutes at 0 °C, the reaction was stirred at room temperature for 1 h open to air, then saturated aqueous Na2CO3 solution was added until no more gas evolved. The resulting mixture was partitioned between half-saturated aqueous NaHCO3 and ethyl acetate. The layers were separated and the aqueous layer was extracted with ethyl acetate. Combined organic layers were dried over MgSO4 and concentrated. The crude diol was purified by flash silica gel chromatography (hexanes/EtOAc, 3:2) to provide 503 (9.87 g, 34.2 mmol, 20 68% yield for three steps) as clear oil. Rf = 0.5 (3:2 hexanes/EtOAc); [α] D −11.4 (c 2.33, 1 EtOH); H NMR (500 MHz, CDCl3) δ 4.98 (m, 1 H), 4.92 (m, 1 H), 4.19 (d, J = 6.5 Hz, 1 H), 3.65–3.59 (m, 2 H), 3.49 (m, 1 H), ~2.65 (br, 1 H), ~2.05 (br, 1 H), 1.73 (dd, J = 1.5, 1.0 Hz, 3 H), 1.10–1.04 (m, 21 H); 13 C NMR (125 MHz, CDCl3) δ 144.7, 114.4, 77.6, 73.4, 63.0, 18.04, 18.00, 17.6, 12.4. 202 OH HO OTs TsCl, TEA OTIPS K2CO3, MeOH HO CH2Cl2, 0 °C ! rt 15 h 503 OTIPS 0 °C, 3 h 89% for 2 steps 921 Preparation of tosylate 921: 31,84 O OTIPS 504 To a solution of diol 503 (9.85 g, 34.1 mmol) in CH2Cl2 (115 mL) at 0 °C (ice/water bath) was added Et3N (46.1 mL, 328 mmol, ~10 equiv) followed by TsCl (7.26 g, 38.1 mmol). The reaction mixture was left to slowly reach room temperature overnight and quenched with saturated NaHCO3. After dilution with CH2Cl2, the layers were separated and the aqueous phase was extracted with CH2Cl2. Combined organic layers were dried over Na2SO4. After concentration in vacuo, the crude material was immediately subjected to the following step without further purification. Preparation of epoxide 504: 31,84 To a stirred solution of 921 (crude from previous step, assumed to be 15.11 g, 34.1 mmol) in commercial MeOH (171 mL) at 0 °C was added K2CO3 (8.02 g, 58.0 mmol) in one portion. The suspension was stirred at 0 °C for 2 h and poured into halfsaturated aqueous NH4 Cl before being extracted three times with CH2 Cl2. Combined organic layers were dried over Na2SO4 and concentrated in vacuo. Purification by column chromatography (hexanes/CH2Cl2, 1.5:1) afforded 504 (8.26 g, 30.5 mmol, 89% yield for two 20 1 steps) as a colorless oil. Rf = 0.5 (1.5:1 hexanes/CH2Cl2); [α] D −8.6 (c 1.17, EtOH); H NMR (500 MHz, CDCl3) δ 4.98 (m, 1 H), 4.86 (m, 1 H), 3.79 (d, J = 6.5 Hz, 1 H), 3.00 (ddd, J = 6.5, 4.0, 3.0 Hz, 1 H), 2.76 (dd, J = 5.0, 4.0 Hz, 1 H), 2.57 (dd, J = 5.0, 3.0 Hz, 1 H), 1.77 (m, 3 H), 203 1.10–1.02 (m, 21 H); 13 C NMR (125 MHz, CDCl3) δ 144.8, 112.2, 78.6, 56.0, 44.9, 18.7, 17.95, 17.90, 12.3. O MgCl OTIPS HO MeOTf, KHMDS THF 0 °C ! rt, 2 h OTIPS 922 504 Preparation of epoxide-opening product 922: MeO toluene, -78 °C ! rt 14 h 94% for 2 steps OTIPS 505 84 To a stirred solution of 504 (8.25 g, 30.5 mmol) in THF (153 mL) at 0 °C was added allylmagnesium chloride (2.0 M in THF, 45.8 ml, 92 mmol). Upon finished addition the ice bath was removed. After 2h the reaction was quenched with water, then stirred with saturated aqueous Na2CO3 solution until a white slurry was formed, which was diluted with ether. The layers were separated and the aqueous layer was extracted with ether. Combined organic layers were dried over MgSO4 and concentrated in vacuo. The residue was dried in vacuo and used in the following step without further purification. Preparation of diene 505: 84 To a stirred solution of 922 (crude from previous step, assumed to be 9.53 g, 30.5 mmol) in toluene (191 mL) at −78 °C was added KHMDS (0.5 M in toluene, 183 mL, 92 mmol) dropwise via cannula. After 30 minutes, MeOTf (16.8 mL, 153 mmol) was added dropwise. The reaction mixture was left to slowly reach room temperature and quenched after 14 h with saturated aqueous NaHCO3. The phases were separated and the aqueous phase was extracted with ether. Combined organic layers were dried over MgSO4 and concentrated. The crude product was purified by flash silica gel chromatography (hexanes/CH2Cl2, 5:1) to provide 505 (9.34 g, 28.6 mmol, 94% yield for two steps) as clear oil. Rf = 0.30 (hexanes/CH2Cl2, 5:1); 204 20 1 [α] D −11.1 (c 1.17, CH2Cl2); H NMR (500 MHz, CDCl3) δ 5.78 (ddd, J = 17.0, 13.0, 6.5 Hz, 1 H), 5.01–4.86 (m, 4 H), 4.26 (d, J = 6.0 Hz, 1 H), 3.44 (s, 1 H), 3.16 (ddd, J = 9.0, 6.0, 4.0 Hz, 1 H), 2.20 (m, 1 H), 2.07 (m, 1 H), 1.72 (s, 3 H), 1.56 (m, 1H), 1.29 (m, 1 H), 1.10–1.00 (m, 21 H); 13 C NMR (125 MHz, CDCl3) δ 145.2, 138.9, 114.5, 112.8, 83.9, 77.2, 58.9, 30.1, 29.6, 19.0, 18.09, 18.05, 12.5. K2OsO4·2H2O NaIO4, 2,6–lutidine MeO OTIPS dioxane/H2O 35 min, 77% 505 31,84 Preparation of aldehyde 506 O MeO OTIPS 506 : To a vigorously stirring suspension of 505 (709.2 mg, 2.172 mmol), NaIO4 (1.86 g, 8.69 mmol), and 2,6-lutidine (506 µL, 4.34 mmol) in dioxane (3.3 mL) in a cold water bath (10–15ºC) was added a solution of potassium osmate dihydrate (8.00 mg, 0.022 mmol) in water (1.1 mL) dropwise forming a white slurry. The reaction mixture was stirred allowing it to warm to room temperature until TLC eluting with hexanes/DCM (5:1) showed full conversion; in this specific run, after 35 minutes. The reaction mixture was diluted with water and CH2Cl2, the phases were separated and the aqueous layer was extracted with CH2Cl2. Combined organic layers were dried over MgSO4 and concentrated. The crude material was subjected to flash silica gel chromatography (hexanes/EtOAc, 15:1) to provide 506 (549.1 mg, 20 1.671 mmol, 77% yield) as light yellow oil. Rf = 0.30 (hexanes/EtOAc, 9:1); [α] D −22.8 (c 1 1.17, acetone); H NMR (500 MHz, CDCl3) δ 9.69 (t, J = 2.0 Hz, 1H), 4.95 (m, 1 H), 4.90 (m, 1 H), 4.27 (d, J = 6.0 Hz, 1H), 3.38 (s, 3 H), 3.16 ddd, J = 9.0, 6.0, 3.25 Hz, 1 H), 2.45 (m, 2 H), 205 1.87 (m, 1 H), 1.73 (s, 3 H), 1.57 (m, 1 H), 1.09–1.01 (m, 21 H); 13 C NMR (125 MHz, CDCl3) δ 202.6, 144.8, 113.2, 83.8, 76.9, 58.7, 40.6, 23.1, 19.0, 18.1, 18.0, 12.4; HRMS (ESI): m/z + calculated for C18H35O3Si [M+H] 329.2512, found 329.2510. OH OTIPS O Cl O N H TIPSCl, DMAP NEt3 MeO N H Cl MeO DCM, 12 h 95 % OTIPS OTIPS 508 509 Preparation of TIPS-protected amidochlorophenol 509: 31 To a solution of 508 (0.5g, 0.980 mmol) and 4-(dimethylamino)pyridine (0.024 g, 0.196 mmol) in dichloromethane (1.6 mL) were successively added triethylamine (0.152 mL, 1.078 mmol) and triisopropylsilyl chloride (0.208 mL, 0.980 mmol) at 0 °C and the mixture was stirred at room temperature for 12 hours. The reaction was quenched with water, extracted with ethyl acetate, dried over MgSO4, and concentrated in vacuo. The product was purified by flash chromatography eluting with hexanes/ethyl acetate (9:1) affording 509 (0.620 g, 0.931 mmol, 95% yield) initially as a viscous oil that crystallized after few days forming waxy white needles. mp 52–53 °C. Rf = 0.35 (9:1 1 hexanes/ethyl acetate); H NMR (500 MHz, CDCl3): δ 7.27 (br, 1 H), 7.14 (t, J = 2 Hz, 1 H), 7.07 (t, J = 1.5 Hz, 1 H), 6.60 (t, J = 2 Hz, 1 H), 6.34 (m, 1 H), 4.95 (s, 1 H), 4.89 (s, 1 H), 4.31 (d, J = 6 Hz, 1 H), 3.44 (s, 3 H), 3.18 (m, 1 H), 2.27 (m, 2 H), 1.91 (s, 3 H), 1.73 (s, 3 H), 1.67 (m, 1 H), 1.33 (m, 1 H), 1.25 (m, 3 H), 1.10–1.03 (m, 39 H); 206 13 C NMR (62.8 MHz, CDCl3): δ 167.5, 157.2, 144.9, 139.8, 136.9, 134.6, 131.9, 115.9, 112.9, 112.5, 109.7, 84.3, 76.2, 58.7, 29.0, 25.0, 19.2, 18.07, 18.04, 17.9, 12.8, 12.6, 12.4; HRMS (ESI): m/z calculated for + C36H65ClNO4Si2 [M+H] 666.4135, found 666.4141. OH OMOM O Cl O N H 1.4 equiv NaH 1.4 equiv MOMCl MeO DMF, 0 ºC to rt 90% Cl N H MeO OTIPS OTIPS 508 510 Preparation of MOM-protected amidochlorophenol 510: Sodium hydride (30.7 mg, 0.768 mmol) (60% dispersion in mineral oil) was rinsed with pentane under nitrogen atmosphere and the solvent extracted via cannula several times. To the solid was added DMF (0.5 mL) and the resulting suspension cooled down to 0 °C. A solution of 508 (280 mg, 0.549 mmol) in DMF (3.5 mL) was added and the resulting solution stirred at 0 °C for some minutes. To the solution was added chloromethyl methyl ether (58.4 µL, 0.768 mmol) causing the immediate evolution of gas, the solution was allowed to warm to room temperature and quenched with water (1.4 mL). The solution was extracted with ether, dried over MgSO4 and concentrated in vacuo. Absolute ethanol was added and the resulting solution pumped down several times. The crude material was purified by column chromatography eluting with hexanes/ethyl acetate (7:1) to afford 510 (273mg, 0.493 mmol, 90% yield) as a slightly yellowish oil. Rf = 0.3 (7:1 hexanes/ethyl acetate); 1 H NMR (500 MHz, CDCl3): δ 7.33 (br, 1 H), 7.26 (t, J = 2 Hz, 1 H), 7.17 (t, J = 2 Hz, 1 H), 6.78 (t, J = 2 Hz, 1 H), 6.35 (m, 1 H), 5.13 (s, 2 H), 4.95 (s, 1 H), 4.90 (s, 1 H), 4.31 (d, J = 5.5 207 Hz, 1 H) , 3.451 (s, 3 H) , 3.446 (s, 3 H), 3.18 (m, 1 H), 2.29 (m, 2 H), 1.91 (s, 3 H), 1.73 (s, 3 H), 1.67 (m, 1 H), 1.35 (m, 1 H), 1.12-1.00 (m, 21 H); 13 C NMR (62.8 MHz, CDCl3): δ 167.5, 158.2, 144.9, 139.8, 137.1, 135.1, 131.8, 113.5, 112.9, 112.5, 106.0, 94.5, 76.3, 58.8, 56.2, 29.0, 25.1, 19.2, 18.08, 18.04, 12.7, 12.4. OH OPMB O O N H 1.2 equiv PMBCl 4.0 equiv K2CO3 MeO Cl N H DMF, 12 h, 81 % Cl MeO OTIPS OTIPS 508 511 Preparation of PMB-protected amidochlorophenol 511: To a suspension of 508 (460.6 mg, 0.903 mmol) and K2CO3 (499 mg, 3.61 mmol) in DMF (2 mL) was added 4-methoxybenzyl chloride (0.148 mL, 170 mg, 1.083 mmol) dropwise and the resulting mixture was stirred at room temperature. After 12 hours, TLC (hexanes/EtOAc 4:1) confirmed full consumption of the starting material. The reaction was quenched with brine and extracted three times with ethyl acetate. Combined organic layers were dried over MgSO4 and concentrated. The excess of DMF was removed by adding small portions of ethanol and pumping down several times. Column chromatography eluting with hexanes/EtOAC (9:1 → 4:1) afforded PMB-protected chloroamidophenol 511 (461.0 mg, 0.731 mmol, 81% yield) as a white solid. Rf = 0.6 (1:1 20 1 hexanes/ether); mp 93–94 °C; [α] D −8.3 (c = 0.33, CH2Cl2); H NMR (500 MHz, CDCl3) δ 7.36 (s, 1 H), 7.31 (d, J = 8.7 Hz, 2 H), 7.29 (t, J = 2.1 Hz, 1 H), 7.07 (t, J = 1.8 Hz, 1 H), 6.89 (d, J = 8.8 Hz, 2 H), 6.70 (t, J = 2.1 Hz, 1 H), 6.35 (qt, J = 1.3, 7.7 Hz, 1 H), 4.95–4.89 (m, 4 H), 208 4.31, (d, J = 5.6 Hz, 1 H), 3.80 (s, 3 H), 3.45 (s, 3 H), 3.18 (ddd, J = 8.8, 5.7, 2.8 Hz, 1 H), 2.34– 2.21 (m, 1 H), 1.91 (d, J = 1.0 Hz, 3 H), 1.73 (s, 3 H), 1.67 (m, 1 H), 1.36 (m, 1 H), 1.10–1.03 (m, 21 H); 13 C NMR (125 MHz, CDCl3) δ 167.6, 159.9, 159.6, 144.9, 139.9, 137.1, 135.0, 131.8, 129.3, 128.4, 114.0, 113.0, 112.3, 111.4, 104.7, 84.3, 76.3, 70.1, 58.8, 55.3, 29.0, 25.1, 19.2, 18.1, 18.0, 12.7, 12.4; IR (neat): 3407, 2963, 2867, 1649, 1632, 1591, 1547, 1452, 1379, –1 1261, 1169, 1115, 1039, 801 cm ; HRMS (ESI): m/z calculated for C35H53ClNO5Si [M+H] + 630.3382, found 630.3408. OH O CDI NH2 Ph CH2Cl2 87% (1S,2R)-(+)-norephedrine O NH Ph 923 90a Preparation of (4S,5R)-4-methyl-5-phenyl-1,3-oxazolidin-2-one (923 ): To a solution of (1S,2R)-(+)-norephedrine (20.0 g, 132.2 mmol) in CH2Cl2 (420 mL) was added carbonyl diimidazole (23.6 g, 145.6 mmol) in one portion and the resulting mixture was stirred at room temperature for 3 h. The reaction mixture was concentrated and the crude material was directly subjected to column chromatography eluting with hexanes/EtOAc (3:4). Concentration in vacuo afforded 20.4 g (87% yield) of 701 as white powder and colorless crystals. Rf = 0.36 (3:4 20 1 hexanes/EtOAc); mp 116–117 °C; [α] D +169.2° (c 1.0, CHCl3); H NMR (500 MHz, CDCl3) δ 7.45–7.20 (m, 5 H), 6.1 (br, 1 H), 5.73 (d, J = 7.5 Hz, 1 H), 4.23 (dq, J = 7.5, 6.5 Hz, 1 H), 0.83 (d, J = 6.5 Hz, 3H); 13 C NMR (125 MHz, CDCl3) δ 159.5, 134.9, 128.48, 128.45, 126.0, 81.0, 52.4, 17.5. 209 (EtCO)2O DMAP, NEt3 O O NH O THF 97% Ph O O N Ph 923 517 90b Preparation of Evans enolate precursor 517 : To a flask containing 923 (14.28 g, 80.6 mmol) in THF (30 mL) was added DMAP (0.25 g, 2.0 mmol) and triethylamine (14 mL, 108.0 mmol). The mixture was cooled down on an ice/water bath and propionic anhydride (21 mL, 222.5 mmol) was added dropwise. After 4 h of stirring at room temperature, the reaction mixture was poured onto half-saturated aqueous NaHCO3 (100 mL), stirred for 1 h, and extracted with ethyl acetate. Combined organic extracts were washed with water and brine, and dried over MgSO4. After evaporation of the solvent, the crude material was purified by column chromatography eluting with hexanes/EtOAc (5:1). Concentration under high vacuum, under vigorous magnetic stirring, afforded 18.40 g (97%) of 517 a slightly yellowish oil. Rf = 0.35 (5:1 hexanes/EtOAc); 20 1 [α] D +45.6° (c 1.842, CH2Cl2); H NMR (500 MHz, CDCl3) δ 7.50–7.25 (m, 5 H), 5.68 (d, J 3 = 6.6 Hz, 1 H), 4.77 (qd, J = 7.5, 6.6 Hz, 1 H), 3.01 and 2.93 (q of ABq, J = 7.3 Hz, JAB = 17.8 Hz, 2 H), 1.19 (t, J = 7.3 Hz, 3 H), 0.91 (d, J = 7.5 Hz, 3 H); 13 C NMR (75 MHz, CDCl3) δ 173.8, 153.1, 133.3, 128.7, 125.6, 79.0, 54.7, 29.2, 14.5, 8.3. O O O N Ph O LiHMDS, allyl iodide THF, !78 °C 60 h, 93% 517 O O N Ph 516 210 Preparation of asymmetric allylation product 516: In a vacuum/flame-dried 1L round-bottomed flask, lithium bis(trimethylsilyl)amide 1M solution in hexanes (93 mL, 93 mmol) was diluted with THF (77 mL) and cooled down to −78 °C. After 30 minutes, a solution of Evans enolate precursor 517 (18.9 g, 81 mmol) in THF (170 mL) was added dropwise via cannula from a pearshaped flask, which was rinsed twice using more THF (85 mL). After 45 minutes of stirring, a solution of allyl iodide (11.11 mL, 122 mmol) in THF (85 mL) was added slowly (0.5 mL/min) and the resulting mixture was stirred at −78 °C for 60 h or until the starting material (517) was completely consumed as determined by TLC (5:1 hexanes/EtOAc). The reaction was quenched with saturated aqueous NaHCO3 (~500 mL), extracted with ether (3 × 600 mL), dried over 1 MgSO4, and concentrated. H NMR of crude material showed a clean single diastereomer. The crude was subjected to column chromatography eluting with hexanes/EtOAc 5:1 to afford 516 (20.65 g, 76 mmol, 93% yield) as a white solid after removal of solvents under high vacuum. Rf = 0.47 (4:1 hexanes/EtOAc); mp 65–66 °C (lit 1 83 20 69–70 °C); [α] D +44.8° (c 1.167, CH2Cl2); H NMR (500 MHz, CDCl3) δ 7.42–7.32 (m, 3 H), 7.31–7.26 (m, 2 H), 5.78 (dddd, J = 17.0, 10.0, 7.0, 7.0 Hz, 1 H), 5.64 (d, J = 7.5 Hz, 1 H), 5.02 (m, 2 H), 4.77 (dq, J = 7.5, 6.5 Hz, 1 H), 3.86 (sext, J = 6.5 Hz, 1 H), 2.47 (m, 1 H), 2.20 (m, 1 H), 1.17 (d, J = 6.5, 3 H), 0.84 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 176.3, 152.7, 135.2, 133.4, 128.71, 128.66, 125.6, 117.1, 78.7, 54.8, 37.9, 37.1, 16.5, 14.6. 211 O O O O O BnOLi, BnOH N THF, 0 ºC Ph O + BnO NH Ph 516 518 923 97% 97% 91 Preparation of benzyl ester 518 : To a vacuum/flame-dried 2L round-bottomed flask containing benzyl alcohol (14.93 mL, 144 mmol) in THF (315 mL) at 0 °C was added n-butyllithium 1.6M in hexanes (59.0 mL, 94 mmol) dropwise. 30 minutes later, a solution of allylation product 518 (20.65 g, 76 mmol) in THF (63.0 mL) was added slowly, still at 0 °C. After 30 minutes, TLC (hexanes/EtOAc 3:1) showed that the starting material was fully consumed. The reaction was quenched with saturated aqueous NH4Cl (450 mL), extracted three times with diethyl ether, dried over MgSO4, and concentrated in vacuo. The crude material was purified by flash chromatography eluting with hexanes/EtOAc (19:1 → 1:4) to afford 518 (14.97 g, 73.3 mmol, 97% yield) as a colorless liquid and 923 (12.99 g, 73.3 mmol, 97% recovered chiral auxiliary). Rf 20 1 = 0.45 (19:1 hexanes/EtOAc); [α] D +2.9° (c 1.667, CHCl3); H NMR (500 MHz, CDCl3) δ 7.40–7.26 (m, 5 H), 5.76 (dddd, J = 17.1, 10.3, 7.0, 7.0 Hz, 1 H), 5.14 (s, 2 H), 5.05 (m, 2 H), 2.61 (sextet, J = 7.0 Hz, 1 H), 2.46 (m, 1 H), 2.23 (m, 1 H), 0.91 (d, J = 7.0, 3 H); 13 C NMR (75 MHz, CDCl3) δ 175.9, 136.2, 135.4, 128.5, 128.1, 128.1, 116.9, 66.1, 39.3, 37.8, 16.5. O (a) ADmix-!, NaHCO3, H2O/tBuOH, 0–2 °C 80% (3.1:1) O O H OTIPS O O H OTIPS BnO (b) TIPSCl, DMAP CH2Cl2, 85 % 519 desired 518 212 epi-519 undesired Preparation of lactones 519 and epi-519: 91 To a 3L round-bottomed flask containing a solution of 518 (36.5 g, 179 mmol) in water (890 mL) and tBuOH (890 mL) were sequentially added sodium bicarbonate (NaHCO3, 60.0 g, 715 mmol), (DHQ)2PHAL (1.949 g, 2.502 mmol), potassium ferricyanide (K3Fe(CN)6, 247 g, 750 mmol), and potassium carbonate (K3CO3, 104 g, 750 mmol), and a mechanical stirrer was introduced in the flask. Once the mixture was well stirred, the 3L flask was immersed into an acetone bath (at 0 °C) equipped with a CryoCool chiller. After 30 minutes of stirring at 0–2 °C, potassium osmate dihydrate (K2OsO2(OH)4, 0.369 g, 1.001 mmol) was added in one portion. The progress of the reaction was monitored by TLC (hexanes/EtOAc 3:1) of mini-worked-up samples. After 36 h, when the starting material (516) appeared to be completely consumed as determined by TLC, the reaction was quenched with sodium sulfite (Na2SO3, 376 g) and stirred for 1 h at room temperature. The resulting mixture was extracted with ethyl acetate (3 × 1.7 L), and the combined organic layers were washed with a small amount of brine (~100 mL). In order to recycle the chiral ligand (DHQ)2PHAL, the organic extract was washed with 100 mL of 3% aqueous H2SO4 saturated with K2SO4. As precipitate formed, small amounts of water were added to dissolve the salts. The aqueous layer was separated and another 60 mL of saturated aqueous K2SO4 was used to wash the organic phase. The aqueous extract, containing (DHQ)2PHAL, was stored for use in future reactions. The organic solution (more than 5 L) was dried over MgSO4 and concentrated in 1 vacuo. A ratio 3.1:1 of desired/undesired diastereomers was calculated by H NMR. The crude material was subjected to column chromatography (hexanes/EtOAC 1:19 → 4:3) to provide 213 recovered starting material 518 (1.12 g, 5.5 mmol, 3%) and the mixture of five-membered lactones contaminated with ~30% BnOH. The yield for this step was calculated to be 18.60 g (143 mmol, 80% yield). After drying, the impure mixture of lactones was dissolved in dichloromethane (600 mL) together with DMAP (98 g, 804 mmol) in a 2L round-bottomed flask. Triisopropylsilyl chloride (89 mL, 420 mmol) was added via syringe and the resulting solution was stirred at room temperature for 5 hours, when TLC of a mini-worked-up sample, eluting with EtOAc/hexanes (4:3), confirmed the consumption of the starting primary alcohols. The reaction was quenched with saturated aqueous NaHCO3 (200 mL), extracted with CH2Cl2 (3 × 600 mL), dried over sodium sulfate, and concentrated in vacuo. A sequence of two column chromatographies eluting with hexanes/diethyl ether (9:1 → 2:1), afforded the desired TIPSprotected lactone 519 (32.84 g, 115 mmol, 64.2% yield) and its isomer epi-519 (10.77 g, 37.6 20 1 mmol, 21.0% yield). (519): Rf = 0.51 (6:1 hexanes/EtOAc); [α] D +5.7º (c 3.1, CHCl3); H 3 NMR (500 MHz, CDCl3) δ 4.42 (ddt, J = 10.0, 6.0, 4.0 Hz, 1 H), 3.91 and 3.79 (d of ABq, J = 4.0 Hz, JAB = 11.0 Hz, 2 H), 2.67 (m, 1 H), 2.37 (ddd, J = 12.5, 9.0, 6.0 Hz, 1 H), 1.85 (dt, J = 10.0, 12.0 Hz, 1 H), 1.26 (d, J = 7.5 Hz, 3 H), 1.09–1.02 (m, 21 H); 13 C NMR (125 MHz, CDCl3) δ 179.4, 78.3, 64.3, 35.4, 32.0, 17.90, 17.89, 15.4, 11.9. (epi-519): Rf = 0.58 (6:1 20 1 hexanes/EtOAc); [α] D −15.6° (c 3.0, CHCl3); H NMR (500 MHz, CDCl3) δ 4.51 (dddd, J = 3 9.0, 3.0, 3.0, 3.0 Hz, 1 H), 3.91 and 3.74 (d of ABq, J = 3.5 and 3.0 Hz, JAB = 11.0 Hz, 2 H), 2.80 (tq, J = 9.5, 7.5 Hz, 1 H), 2.43 (ddd, J = 12.5, 9.5, 3.0 Hz, 1 H), 1.93 (dt, J = 12.5, 9.0 Hz, 1 214 H), 1.24 (d, J = 7.5 Hz, 3 H), 1.07–1.01 (m, 21 H); 13 C NMR (125 MHz, CDCl3) δ 180.3, 77.7, 65.3, 34.2, 32.2, 17.91, 17.88, 16.4, 11.9. OH O O H OTIPS LiBH4, MeOH, THF quantitative OTIPS HO epi-924 epi-519 undesired Preparation of monoprotected triol epi-924. To a solution of epi-519 (15.01 g, 52.4 mmol) in THF (260 mL) in a 2L round-bottomed flask connected to a nitrogen line was added MeOH (2.5 mL, 61.8 mmol). This solution was cooled to 0 °C and stirred for 10 minutes before LiBH4 2 M in THF (32.7 mL, 65.5 mmol) was added dropwise. Bubbling started after the first 10 mL was added. The solution was stirred 1 h at 0 °C and 2 h at room temperature. Full consumption of the starting material was confirmed by TLC (hexanes/ether 4:1). The reaction was carefully quenched with saturated aqueous NH4Cl added in portions until no bubbling was observed. Then, water (85 mL) and glycerol (65 mL) were added and the solution was stirred at room temperature overnight. The resulting homogeneous solution was mixed with ethyl acetate (450 mL) and brine (150 mL) in a separatory funnel. The phases were separated and the aqueous layer was extracted three times with ethyl acetate. Combined organic layers were dried over Na2SO4 and concentrated in vacuo. Crude material contained ring-opening product epi-924 (15.22 g, 52.4 mmol, 100% yield), pure enough to be directly used in the next reaction without further 20 1 purification. [α] D +9.6º (c 1.16, EtOH); H NMR (500 MHz, CDCl3) δ 3.83 (m, 1 H), 3.45– 3.80 (2 overlapping d of ABq, can be viewed as 4 dd at 3.65 (dd, J = 9.6, 3.6 Hz, 1 H), 3.53 (dd, J = 10.8, 4.8 Hz, 1 H), 3.49 (dd, J = 10.8, 6.5 Hz, 1 H), and 3.48 (dd, J = 9.6, 8.3 Hz, 1 H)), 2.91 215 (br, 1 H), 2.64 (br, 1 H), 1.90 (m, 1 H), 1.45 (m, 2 H), 1.13–0.98 (m, 21 H), 0.94 (d, J = 7 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 69.3, 67.7, 67.6, 36.5, 32.5, 17.9, 17.0, 11.9; HRMS (ESI): + m/z calculated for C15H35O3Si [M+H] 291.2355, found 291.2359. OH OTrt TrtCl, DMAP, pyr OTIPS 82%, 98% brsm HO epi-924 OTIPS HO epi-520 Preparation of diprotected triol epi-520. A solution of trityl chloride (16.07 g, 57.6 mmol) and DMAP (160 mg, 1.310 mmol) in pyridine (50 mL) was added dropwise via addition funnel to a 1L round-bottomed flask containing crude epi-924 (15.22 g, 52.4 mmol) in pyridine (40 mL) and the resulting mixture was stirred at room temperature for 3.5 days. The reaction was quenched with water (200 mL) and extracted with dichloromethane (5 × 0.2 L). Combined organic layers were repeatedly washed with aqueous CuSO4 (5% w/v) (until the washings were light blue) and brine, dried over MgSO4, and concentrated in vacuo. The crude material was subjected to column chromatography eluting with hexanes/EtOAc (40:1 → 9:1 → 1:1) to afford epi-520 as a colorless oil (22.89 g, 43.0 mmol, 82% yield) and epi-924 (2.42 g, 8.33 mmol, 16% recovered 20 1 starting material). Rf (epi-520) = 0.3 (5:1 hexanes/EtOAc); [α] D +11.5 (c 1.275, EtOH); H NMR (500 MHz, CDCl3) δ 7.42 (m, 6 H), 7.27 (m, 6 H), 7.21 (m, 3 H), 3.66–3.58(m, 2 H), 3.41 3 (m, 1 H), 3.01 and 2.92 (d of ABq, J = 5.6 and 5.6 Hz, JAB = 8.8 Hz, 2 H), 2.53 (s, 1 H), 1.91 (m, 1 H), 1.52 (m, 1 H), 1.31 (m, 1 H), 1.13–0.98 (m, 24 H); 13 C NMR (125 MHz, CDCl3) δ 144.4, 128.7, 127.7, 126.8, 86.2, 70.2, 67.9, 67.7, 37.0, 30.8, 18.4, 18.0, 11.9; IR (neat): 3390, 216 –1 2943, 2866, 1490, 1462, 1449, 1384, 1100, 1068 cm ; HRMS (ESI): m/z calculated for + C34H48O3SiLi [M+Li] 539.3533, found 539.3529. OH O O H OTIPS OTIPS LiBH4, MeOH, THF quantitative HO 924 519 desired Preparation of monoprotected triol 924. To a solution of 519 (22.79 g, 80 mmol) in THF (0.4 L) in a 2L round-bottomed flask connected to a nitrogen line was added MeOH (3.7 mL, 92 mmol). This solution was cooled to 0 °C and stirred for 10 minutes before LiBH4 2 M in THF (50 mL, 100 mmol) was added dropwise. Bubbling started after the first 15 mL was added. The solution was stirred 1 h at 0 °C and 2 h at room temperature. Full consumption of the starting material was confirmed by TLC (hexanes/ether 4:1). The reaction was carefully quenched with saturated aqueous NH4Cl added in portions until no bubbling was observed. Then, water (130 mL) and glycerol (100 mL) were added and the solution was stirred at room temperature overnight. The resulting homogeneous solution was mixed with ethyl acetate (600 mL) and brine (200 mL) in a separatory funnel. The phases were separated and the aqueous layer was extracted three times with ethyl acetate. Combined organic layers were dried over Na2SO4 and concentrated in vacuo. Crude material contained ring-opening product 924 (23.11 g, 80 mmol, 100% yield), pure 1 enough to be directly used in the next reaction without further purification. H NMR (500 MHz, CDCl3) δ 3.76 (m, 1 H), 3.66–3.37 (m, 4 H), 3.16 (s, 2 H), 1.88–1.78 (m, 1 H), 1.39–1.29 (m, 2 217 H), 1.08–1.02 (m, 21 H), 0.90 (d, J = 6.8 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 71.2, 68.7, 67.8, 38.2, 34.6, 18.1, 17.9, 11.9. OH OTrt OTIPS 81%, 98% brsm HO OTIPS TrtCl, DMAP, pyr 924 HO 520 Preparation of diprotected triol 520. A solution of trityl chloride (24.39 g, 88 mmol) and DMAP (250 mg, 2.044 mmol) in pyridine (80 mL) was added dropwise via addition funnel to a 1L round-bottomed flask containing crude 924 (23.11 g, 80 mmol) in pyridine (60 mL) and the resulting mixture was stirred at room temperature for 3.5 days. The reaction was quenched with water (250 mL) and extracted with dichloromethane (5 × 0.25 L). Combined organic layers were repeatedly washed with aqueous CuSO4 (5% w/v) (until the washings were light blue) and brine, dried over MgSO4, and concentrated in vacuo. The crude material was subjected to column chromatography eluting with hexanes/EtOAc (40:1 → 9:1 → 1:1) to afford 520 as a colorless oil (34.19 g, 64.2 mmol, 81% yield) and 924 (3.92 g, 13.49 mmol, 17% recovered starting material). 20 1 Rf (520) = 0.43 (9:1 hexanes/EtOAc); [α] D −12.4 (c 2.17, EtOH); H NMR (500 MHz, CDCl3) δ 7.48–7.45 (m, 6 H), 7.32–7.22 (m, 9 H), 3.78–3.72 (m, 1 H), 3.68 (dd, J = 9.5, 4.0 Hz, 1 H), 3.51 (dd, J = 9.5, 7.5 Hz, 1 H), 3.01–2.95 (m, 2 H), 2.57 (d, J = 3.5 Hz, 1 H), 2.10–2.03 (m, 1 H), 1.59 (ddd, J = 13.5, 9.5, 5.0 Hz, 1 H), 1.23 (ddd, J = 13.5, 9.0, 3.5 Hz, 1 H), 1.13–1.07 (m, 21 H), 1.01 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 144.4, 128.7, 127.7, 126.8, 86.2, 69.7, 69.0, 68.0, 37.2, 30.5, 18.0, 17.2, 11.9; HRMS (ESI): m/z calculated for + C34H48O3SiLi [M+Li] 539.3533, found 539.3528. 218 OTrt OTIPS p-nitrobenzoic acid PPh3, DIAD OTrt OTIPS THF, 89% p-NO2BzO HO 925 epi-520 Preparation of Mitsunobu product p-nitrobenzoate 925. To a well stirred mixture of epi-520 (26.14 g, 49.1 mmol), p-nitrobenzoic acid (32.8 g, 196 mmol), and triphenylphosphine (51.5 g, 196 mmol) in THF (0.5 L) at 0 °C was added diisopropyl azodicarboxylate (38.2 mL, 196 mmol) dropwise via syringe and the resulting solution was stirred at room temperature. After 12 h, full consumption of the starting material was confirmed by TLC (hexanes/EtOAc 20:1). The reaction mixture was concentrated in vacuo, the bright orange residue was redissolved in ether (300 mL) and to it was added petroleum ether (150 mL) in portions. The white precipitate was filtered out and the filtrate was concentrated and dried under high vacuum. The crude material was subjected to column chromatography eluting with hexanes/EtOAc (99:1 → 25:1) to afford 925 (29.67 g, 43.5 mmol, 89% yield) as a yellowish oil that became a waxy solid upon standing. Rf = 0.4 (25:1 20 1 hexanes/EtOAc); mp 53–56 ºC (wax); [α] D −3.9 (c 1.07, pentane); H NMR (500 MHz, CDCl3) δ 8.24 (d, J = 8.9 Hz, 2 H), 8.16 (d, J = 8.9 Hz, 2 H), 7.40 (m, 6 H), 7.24 (m, 6 H), 7.20 (m, 3 H), 5.30 (m, 1 H), 3.83 (apparent d, J = 5.0 Hz, 2 H), 2.96 (apparent d, J = 6.0 Hz, 2 H), 2.00 (ddd, J = 14.0, 9.9, 4.0 Hz, 1 H), 1.81 (m, 1 H), 1.53 (m, 1 H), 1.09–0.93 (m, 24 H); 13 C NMR (125 MHz, CDCl3) δ 164.3, 150.5, 144.3, 136.0, 130.7, 128.7, 127.7, 127.6, 126.8, 123.4, 86.3, 74.7, 74.6, 86.6, 65.1, 34.9, 30.6, 17.9, 17.1, 11.9; HRMS (ESI): m/z calculated for + C41H51NO6SiNa [M+Na] 704.3383, found 704.3380. 219 OTrt OTrt OTIPS LiBH4, MeOH OTIPS THF, 89% p-NO2BzO HO 925 520 Preparation of diprotected triol 520 from nitrobenzoate 925: To a solution of 925 (29.67 g, 43.5 mmol) in THF (220 mL) and methanol (2.2 mL, 54.4 mmol) at 0 °C was added LiBH4 2M in THF (27.2 mL, 54.4 mmol) dropwise. The temperature was kept at 0 °C for 1 h and increased to room temperature for 2 h. The reaction was quenched with saturated aqueous NH4Cl (30 mL), water (30 mL), and glycerol (30 mL), and stirred at room temperature. After 10 h, the reaction mixture was diluted with brine and water, extracted with ethyl acetate, and dried with Na2SO4. Column chromatography eluting with hexanes/dichloromethane (3:1 → 1:2) afforded 520 (20.62 g, 38.7 mmol, 89% yield) as a colorless oil spectroscopically identical to the material obtained by 20 tritylation of 924 (page 218). Rf = 0.3 (5:1 hexanes/EtOAc); [α] D −11.1 (c 1.23, EtOH); + HRMS (ESI): m/z calculated for C34H48O3SiLi [M+Li] 539.3533, found 539.3532. OTrt OTrt OTIPS HO KHMDS, MeOTf toluene, !78 °C 91% 520 OTIPS MeO 926 Preparation of methyl ether 926: A solution of KHMDS (39.7 g, 199 mmol) in toluene (332 mL) (prepared in a glove bag) was added dropwise via cannula to a 2L round-bottomed flask containing a solution of 520 (35.37 g, 66.4 mmol) in toluene (332 mL) at −78 °C. When the addition was finished, the mixture was stirred for 15 additional minutes and methyl 220 trifluoromethanesulfonate (32.9 mL, 299 mmol) was then added dropwise under vigorous stirring. The cold bath was removed and the mixture was stirred at room temperature overnight. The reaction was quenched with saturated aqueous NaHCO3, extracted with ether, dried over MgSO4, and concentrated in vacuo. The crude material was purified by column chromatography eluting with hexanes/dichloromethane (3:1 → 1:1) to afford 926 (33.04 g, 60.4 mmol, 91% 20 yield) as a colorless oil. Rf = 0.3 (2:1 hexanes/dichloromethane); [α] D −8.3 (c 1.25, CH2Cl2); 1 3 H NMR (500 MHz, CDCl3) δ 7.40–7.20 (m, 15 H), 3.75 and 3.59 (d of ABq, J = 5.5 and 5.0 3 Hz, JAB = 10.0 Hz, 2 H), 3.42 (s, 3 H), 3.32–3.27 (m, 1 H), 3.01 and 2.92 (d of ABq, J = 5.5 and 6.5 Hz, JAB = 9.0 Hz, 2 H), 2.06–2.00 (m, 1 H), 1.56 (ddd, J = 13.0, 9.5, 4.5 Hz, 1 H), 1.36 (ddd, J = 13.0, 9.5, 4.0 Hz, 1 H), 1.11–1.07 (m, 21 H), 1.03 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 144.5, 128.8, 127.6, 126.7, 86.1, 80.0, 68.8, 66.1, 58.1, 36.2, 30.5, 18.0, 17.4, 11.9. OTrt OTrt OTIPS TBAF THF, 95% MeO 926 Preparation of primary alcohol 927: OH MeO 927 31 To a solution of 926 (33.04 g, 60.4 mmol) in THF (121 mL) at 0 °C was added TBAF (1.0 M in THF) (91 mL, 91 mmol) dropwise and the resulting mixture was stirred at room temperature for 3 h. The reaction was quenched with saturated aqueous NaHCO3, extracted with ether, dried over MgSO4, and concentrated in vacuo. The 221 product was purified by flash chromatography eluting with hexanes/EtOAc (3:2) to give 927 20 (22.41 g, 57.4 mmol, 95% yield) as a colorless oil. Rf = 0.4 (3:2 hexanes/EtOAc); [α] D +1.9 (c 1 2.17, CHCl3); H NMR (500 MHz, CDCl3) δ 7.43–7.41 (m, 6 H), 7.29–7.25 (m, 6 H), 7.23–7.19 (m, 3 H), 3.64 (ddd, J = 11.0, 6.0, 3.5 Hz, 1 H), 3.41 (m, 1 H), 3.32 (s, 3 H), 3.27 (m, 1 H), 2.95 3 and 2.91 (d of ABq, J = 6.0 Hz, JAB = 9.0 Hz, 2 H), 1.87 (m, 1 H), 1.79 (apparent t, J = 6.25 Hz, 1 H), 1.69 (ddd, J = 13.5, 7.0, 5.5 Hz, 1 H), 1.17 (ddd, J = 13.5, 8.0, 5.5 Hz, 1 H), 1.03 (d, J = 7.0 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 144.3, 128.7, 127.7, 126.8, 86.2, 79.4, 68.4, 64.1, 57.0, 34.9, 30.7, 17.9. OTrt OTrt OH SOCl2, DMSO, NEt3 CH2Cl2, !78 °C " 0 °C quantitative MeO 927 Preparation of aldehyde 521: 31 O H3CO 521 A solution of DMSO (10.12 mL, 143 mmol) in CH2Cl2 (582 mL) was added slowly via syringe to a vacuum/flame-dried 2L round-bottomed flask containing oxalyl chloride (6.24 mL, 71.3 mmol) in CH2Cl2 (291 mL) at −78 °C. The transparent solution was stirred for 10 minutes and a solution of alcohol 927 (23.2 g, 59.4 mmol) in CH2Cl2 (116 mL) was added dropwise. The cloudy mixture was stirred for additional 10 minutes and triethylamine (41.4 mL, 297 mmol) was then added dropwise. After 10 minutes at −78 °C and 1 h at room temperature, the transparent reaction mixture was quenched with saturated aqueous NaHCO3, extracted three times with CH2Cl2, dried over MgSO4, and concentrated in vacuo to afford crude 521 as a yellowish oil containing small amounts of CH2Cl2 and DMSO. Aldehyde 222 1 521 can be used crude (quantitative yield calculated by H NMR) in the following step. Purification by column chromatography eluting with hexanes/EtOAc (5:1 → 1:1) afforded 521 20 (22.5 g, 57.9 mmol, 97% yield) as a colorless oil. Rf = 0.5 (5:1 hexanes/EtOAc); [α] D −39.8 (c 1 1.10, CH2Cl2); H NMR (500 MHz, CDCl3) δ 9.60 (d, J = 2.0 Hz, 1 H), 7.46–7.38 (m, 6 H), 7.31–7.25 (m, 6 H), 7.24–7.19 (m, 3 H), 3.57 (ddd, J = 8.5, 4.0, 2.0 Hz, 1 H), 3.37 (s, 3 H), 2.95 (d, J = 6.0 Hz, 2 H), 2.02 (m, 1 H), 1.74 (ddd, J = 14.0, 8.5, 5.0 Hz, 1 H), 1.40 (ddd, J = 12.0, 9.0, 4.0 Hz, 1 H), 0.98 (d, J = 7.0 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 203.9, 144.3, 128.7, 127.7, 126.9, 86.3, 84.0, 68.2, 58.2, 33.7, 30.2, 17.0; HRMS (ESI): m/z calculated for + C26H28O3Na [M+Na] 411.1936, found 411.1935. (Z)-2-butene (a) KOtBu, nBuLi (b) B(OiPr)3 (c) HCl(aq) OH B OH HO O HN HO EtOAc, 4 Å ms diethanolamine B O NH 522 95 Preparation of (Z)-crotylboronic acid diethanolamine ester (522 ): A 2L three-necked roundbottomed flask, provided with a jacketed addition funnel (without a pressure-equalizing arm) and a stir bar, was vacuum/flame-dried and charged with potassium t-butoxide (50.7 g, 424 mmol) under nitrogen atmosphere. THF (315 mL) was added via cannula and the resulting white suspension was vigorously stirred while the flask was evacuated and refilled with nitrogen 223 several times and cooled down to −78 °C. The outer jacket of the addition funnel was filled with a dry ice/acetone mixture. (Z)-2-butene (25 g, 446 mmol) was cooled down to −78 °C in its metallic container (under nitrogen atmosphere) and transferred into the addition funnel via cannula. After the pre-cooled (Z)-2-butene was added to the suspension, the addition funnel was evacuated and refilled with nitrogen several times. n-Butyllithium 1.6M in hexanes (265 mL, 424 mmol) was then transferred via cannula into the addition funnel and added dropwise (very slowly) to the suspension. Upon complete addition, the bright yellow suspension was allowed to warm to −25 °C for 45 minutes and then cooled back down to −78 °C. The addition funnel was evacuated and refilled with nitrogen several times and its outer jacket was warmed to about −50 °C. Triisopropyl borate (99 mL, 424 mmol) (mp −59 °C) was transferred via cannula into the addition funnel and added dropwise (very slowly) to the reaction mixture. The resulting pale yellow suspension was maintained at −78 °C for 20 minutes and then it was rapidly poured into a separatory funnel containing aqueous HCl 1M saturated with NaCl (850 mL, 850 mmol). The pH of the aqueous phase was adjusted to 1 and the two transparent colorless layers were separated. The aqueous layer was extracted with ethyl acetate (3 × 800 mL) and combined organic layers were treated with diethanolamine (35.7 g, 339 mmol) and stirred over freshly activated 4 Å molecular sieve beads (~100 g) under nitrogen atmosphere for 3 h. Filtration and concentration in vacuo afforded a white powdery solid. Recrystallization from dichloromethane/ether gave colorless crystals. Continuous subjection of the mother liquor to crystallization, filtration, and drying under vacuum afforded crotylation agent precursor 522 (22.35 g, 132 mmol, 39% combined yield) as a white solid. Yields varied from run to run. This solid was stored in the fridge in presence of drierite for several months without any detectable decomposition. mp (plates) 150–151 ºC (lit. 95 1 156–157 ºC); H NMR (500 MHz, CDCl3) δ 5.67 (m, 1 H), 5.31 (m, 224 1 H), 4.05 (br, 1 H), 4.01 (m, 2 H), 3.88 (m, 2 H), 3.19 (m, 2 H), 2.78 (m, 2 H), 1.61 (d, J = 6.5 Hz, 3 H), 1.45 (d, J = 8.5 Hz, 2 H); 11 13 C NMR (75 MHz, CDCl3) δ 132.8, 119.8, 62.9, 51.8, 12.5; B NMR (62.8 MHz, CDCl3) δ 13.1; IR (neat): 3133, 3004, 2860, 1638, 1473, 1355, 1272, –1 + 1210, 1103 cm ; HRMS (ESI): m/z calculated for C8H19BNO3 [M+H3O] 188.1458, found 188.1461. O B O 522 NH (S,S)-DIPT brine, ether then MgSO4 CO2iPr O B O OTrt O MeO 521 CO2iPr OTrt 523 (1.5 equiv) powdered 4 Å ms toluene !78 °C, 20 h 75% single isomer OH MeO 524 31 Preparation of asymmetric crotylation product 524 : To a suspension of (Z)-crotylboronic acid diethanolamine ester (522) (16.70 g, 99 mmol) and (−)-diisopropyl tartrate (23.14 g, 99 mmol) in diethyl ether (165 mL) was added brine (165 mL) and the mixture was stirred until two clear layers formed. After 5 minutes, the phases were separated and the aqueous layer was extracted with diethyl ether (4 × 150 mL). Combined organic extracts were washed with brine (300 mL) and transferred to a vacuum/flame-dried 2L round-bottomed flask containing MgSO4 (50 g) and the suspension was vigorously stirred for 3 hours under nitrogen atmosphere. The solid was filtered off and rinsed with diethyl ether under nitrogen blanketing. The filtrate was directly placed into a vacuum/flame-dried three-necked 2L round-bottomed flask containing freshly 225 activated powdered 4 Å molecular sieves (5.5 g, >60 mg/mmol of crotylation agent). The yield of (S,S)-(Z)-crotylboronate 523 was assumed to be 90% (26.50 g, 89 mmol). The solvent (diethyl ether) was carefully distilled off into a liquid-nitrogen-cooled trap using high vacuum and stirring at room temperature. Portions of dry toluene were added and pumped down under high vacuum several times to ensure that the reagent was dry and that all the active boronate was in the form of the tartrate ester derivative. Dry toluene (~400 mL) was added to the flask via cannula and the resulting suspension was cooled down below −78 °C. Aldehyde 521 (23.00 g, 59.2 mmol) (dried in a vacuum (0.01 torr) desiccator over drierite for 1 day before use) was added dropwise as a solution in toluene (200 mL). The reaction mixture was stirred at −78 °C and the progress of the reaction was monitored by TLC (hexanes/EtOAc 7:1). After 20 h, aqueous sodium hydroxide 2.0 M (89 mL, 178 mmol) (2 equiv with respect to crotylboronate) was added and the mixture was allowed to warm to 0 °C. The mixture was stirred for 1 hour and then filtered trough a pad of celite eluting with diethyl ether. Phases were separated and the aqueous layer was extracted three times with diethyl ether. Combined organic layers were dried 1 over K2CO3, filtered, and concentrated. H NMR of the crude material indicated a conversion of about 90% and confirmed the formation of a single diastereomer. Column chromatography eluting with hexanes/EtOAC (10:1) afforded asymmetric crotylation product 524 (19.74 g, 44.4 mmol, 75% yield) as a colorless oil. This reaction can be performed in small scale with equal 20 results and yields in the 70–80% range. Rf = 0.24 (10:1 hexanes/EtOAc); [α] D −14.4° (c 1.03, 1 CH2Cl2); H NMR (500 MHz, CDCl3) δ 7.46–7.42 (m, 6 H), 7.29–7.25 (m, 6 H), 7.28–7.23 (m, 3 H), 5.62 (ddd, J = 17.0, 10.5, 8.5 Hz, 1 H), 5.03 (dd, J = 17.0, 1.5 Hz, 1 H), 5.00 (dd, J = 10.5, 1.5 Hz, 1 H), 3.59 (m, 1 H), 3.29 (s, 3 H), 3.23 (dt, J = 10.5, 2.7 Hz, 1 H), 2.98 and 2.88 (d of 226 3 ABq, J = 5.5 and 7.0 Hz, JAB = 8.5 Hz, 2 H), 2.22 (m, 1 H), 2.10 (d, J = 2 Hz, 1 H), 2.03–1.95 (m, 1 H), 1.59 (ddd, J = 14.2, 10.5, 3.9 Hz, 1 H), 1.21 (ddd, J = 14.4, 10.0, 2.4 Hz, 1 H), 1.11 (d, J = 6.6 Hz, 3 H), 0.97 (d, J = 6.8 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 144.5, 140.1, 128.8, 127.6, 126.8, 115.2, 86.1, 80.1, 73.4, 69.2, 57.0, 40.4, 31.5, 30.4, 17.2, 17.1; HRMS (ESI): m/z + calculated for C30H36O3Na [M+Na] 467.2562, found 467.2560. MOMCl, DMAP OTrt OTrt TBAI, iPr2NEt OH OMOM THF, reflux MeO 91% MeO 524 928 Preparation of MOM-protected homoallylic alcohol 928: 31 To a solution of 524 (4.09 g, 9.20 mmol) in dichloromethane (77 mL) were sequentially added DMAP (0.112 g, 0.920 mmol), N,N-diisopropylethylamine (9.64 mL, 55.2 mmol), chloromethyl methyl ether (2.096 mL, 27.6 mmol), and tetrabutylammonium iodide (0.340 g, 0.920 mmol) and resulting mixture was stirred at room temperature under nitrogen atmosphere. The progress of the reaction was monitored by TLC (hexanes/EtOAc 8:1) of mini-worked-up samples. After 24 h the reaction was quenched with saturated aqueous NH4 Cl. The two phases were separated, and the aqueous layer was extracted three times with ethyl acetate, dried over Na2SO4 and concentrated in vacuo. The product was purified by flash chromatography eluting with hexanes/EtOAc (19:1 → 8:1) to give 928 (4.08 g, 8.35 mmol, 91% yield) as a colorless oil and recovered starting material 524 (0.123 20 1 g, 0.276 mmol, 3%). Rf (928) = 0.3 (8:1 hexanes/EtOAc); [α] D +14.4° (c 2.1, CH2Cl2); H NMR (500 MHz, CDCl3) δ 7.42–7.38 (m, 6 H), 7.27–7.21 (m, 6 H), 7.20–7.15 (m, 3 H), 5.67 227 (ddd, J = 17.0, 10.5, 8.5 Hz, 1 H), 5.02–4.96 (m, 2 H), 4.78 and 4.59 (ABq, JAB = 6.5 Hz, 2 H), 3.57 (dd, J = 8.5, 2.0 Hz, 1 H), 3.37 (s, 3 H), 3.26 (s, 3 H), 3.27 (overlapped m, 1 H), 2.99 and 3 2.84 (d of ABq, J = 5.5 and 7.5 Hz, JAB = 8.5 Hz, 2 H), 2.25 (m, 1 H), 1.99 (m, 1 H), 1.59 (ddd, J = 14.5, 10.5, 4.0 Hz, 1 H), 1.19 (ddd, J = 14.0, 10.0, 2.0 Hz, 1 H), 1.09 (d, J = 6.5 Hz, 3 H), 0.96 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 144.5, 141.1, 128.8, 127.6, 126.7, 114.8, 97.4, 86.1, 80.7, 79.5, 69.1, 56.9, 56.0, 40.5, 33.3, 30.7, 17.4, 17.2; HRMS (ESI): m/z + calculated for C32H40O4Na [M+Na] 511.2824, found 511.2829. OTrt OH OMOM PPTS MeOH 92% MeO 928 Preparation of primary alcohol 527: OMOM MeO 527 31 To a solution of 928 (4.5 g, 9.21 mmol) in MeOH (65.8 mL) was added PPTS (pyridinium p-toluenesulfonate) (2.3 g, 9.21 mmol) at room temperature. The progress of the reaction was monitored by TLC (1:1 hexanes/EtOAc). After 24 h, the reaction was quenched with saturated aqueous NaHCO3, extracted with ether, dried over MgSO4, and concentrated under vacuum. The crude material was subjected to column chromatography eluting with hexanes/EtOAc (1:1) to give 527 (2.08 g, 8.44 mmol, 92% yield) as 20 1 a colorless oil. Rf = 0.4 (1:1 hexanes/EtOAc); [α] D +9.6 (c 1.75, CHCl3); H NMR (500 MHz, CDCl3) δ 5.66 (ddd, J = 17.0, 10.5, 9.0 Hz, 1 H), 5.03–4.98 (m, 2 H), 4.81 and 4.62 (ABq, JAB = 3 6.6 Hz, 2 H), 3.63 (dd, J = 9.0, 1.5 Hz, 1 H), 3.49 and 3.39 (d of ABq, J = 5.0 and 6.0 Hz, JAB = 228 11.0 Hz, 2 H), 3.39 (s, 3 H), 3.33 (s, 3 H), 3.31 (overlapped m, 1 H), 2.32–2.24 (m, 1 H), 2.23 (br, 1 H), 1.74 (m, 1 H), 1.60 (ddd, J = 15.0, 10.0, 6.5 Hz, 1 H), 1.37 (ddd, J = 15.0, 7.0, 1.5 Hz, 1 H), 1.11 (d, J = 6.5 Hz, 3 H), 0.91 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 140.9, 115.1, 97.3, 82.2, 78.6, 68.5, 56.6, 56.0, 40.9, 33.8, 33.4, 17.7, 17.6; HRMS (ESI): m/z calculated + for C13H27O4 [M+H] 247.1909, found 247.1912. OH OMOM I2, PPh3 imidazole benzene 77% MeO 527 I OMOM MeO 525 Preparation of alkyl iodide 525: To a 25mL pear-shaped flask covered with aluminum foil, containing a solution of 527 (100.7 mg, 0.409 mmol) in benzene (4 mL), was added imidazole (41.7 mg, 0.613 mmol), triphenylphosphine (139 mg, 0.531 mmol), and iodine (135 mg, 0.531 mmol) and the resulting suspension was vigorously stirred at room temperature. After 1 h, the reaction mixture was a homogeneous bright yellow suspension. The reaction was quenched by the addition of saturated aqueous Na2S2O3; the resulting two-phase system was stirred and, after some minutes, two colorless and clear phases formed. The phases were separated and the aqueous layer was extracted with ether. Combined organic layers were washed with brine, dried over Na2SO4 and concentrated in vacuo. The crude material was directly adsorbed in silica gel and subjected to column chromatography eluting with hexanes/EtOAc (10:1) to afford 525 20 (112.1 mg, 0.314 mmol, 77% yield) as a colorless oil. Rf = 0.4 (10:1 hexanes/EtOAc); [α] D 1 −11.3° (c 1.067, CH2Cl2); H NMR (500 MHz, CDCl3) δ 5.66 (ddd, J = 17.0, 10.5, 9.0 Hz, 1 229 H), 5.03 (ddd, J = 17.0, 1.5, 1.0 Hz, 1 H), 5.01–4.98 (m, 1 H), 4.81 and 4.62 (ABq, JAB = 7.0 Hz, 2 H), 3.60 (dd, J = 9.0, 2.0 Hz, 1 H), 3.40 (s, 3 H), 3.30 (s, 3 H), 3.28–3.25 (m, 2 H), 3.14 (dd, J = 9.5, 6.5 Hz, 1 H), 2.27 (m, 1 H), 1.70 (m, 1 H), 1.65 (ddd, J = 14.0, 10.5, 3.5 Hz, 1 H), 1.29 (ddd, J = 14.0, 9.5, 2.0 Hz, 1 H), 1.10 (d, J = 7.0 Hz, 3 H), 0.93 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 140.8, 115.1, 97.4, 80.9, 78.8, 57.0, 56.1, 40.7, 36.1, 31.4, 20.0, 19.1, 17.7; + HRMS (ESI): m/z calculated for C13H26IO3 [M+H] 357.0927, found 357.0929. Br OH OMOM CBr4, PPh3 DCM 66% MeO 527 OMOM MeO 528 Preparation of alkyl bromide 528: To a solution of 527 (48.3 mg, 0.196 mmol) and triphenylphosphine (61.7 mg, 0.235 mmol) in dichloromethane (0.65 mL) was added in portions carbon tetrabromide (68.3 mg, 0.206 mmol) and the solution was stirred at room temperature for 4 h. The reaction mixture was diluted with petroleum ether (0.65 mL), and filtered by suction washing with petroleum ether/ethyl acetate (10:1). The organic solution was evaporated and the crude material purified by column chromatography, eluting with hexanes/ethyl acetate (6:1) to give 528 (40 mg, 0.129 mmol, 66.0% yield) as a colorless oil. Rf = 0.4 (6:1 hexanes/ethyl 1 acetate); H NMR (500 MHz, CDCl3): δ 5.67 (m, 1 H), 5.06-4.96 (m, 2 H), 4.81 (d, J = 6.5 Hz, 1 H), 4.62 (d, J = 6.5 Hz, 1 H), 3.61 (dd, J = 9, 2 Hz, 1 H), 3.44 (m, 1 H), 3.39 (s, 3 H), 3.31 (s, 3 H), 3.30 (m, 2 H), 2.27 (m, 1 H) , 2.03 (m, 1 H), 1.68 (m, 1 H), 1.33 (m, 1 H), 1.11 (d, J = 6.5 230 Hz, 3 H), 0.98 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 141.1, 115.3, 97.7, 81.1, 79.2, 57.2, 56.3, 42.9, 40.1, 34.9, 32.2, 18.4, 17.9. OTrt OH PMBCl, NaH TBAI THF, reflux 87% MeO 524 OTrt OPMB MeO 929 Preparation of 929: To a 0 °C slurry of sodium hydride, suspension in mineral oil 60% w/w, (0.439 g, 10.96 mmol), TBAI (0.270 g, 0.731 mmol) and 524 (3.25 g, 7.31 mmol) in THF (25 mL) was added 4-methoxybenzyl chloride (1.095 mL, 8.04 mmol). The resulting suspension was refluxed at 70 °C for 16 hours. The reaction was quenched with aqueous saturated ammonium chloride (25 mL), extracted with EtOAc (3 × 50 mL), dried over MgSO4, and concentrated under vacuum overnight. The crude material was adsorbed on silica gel and purified by column chromatography eluting with hexanes/ether 9:1 to afford 929 (3.58 g, 6.34 mmol, 87% yield) as a 20 1 colorless oil. Rf = 0.4 (hexanes/ether 9:1); [α] D +12.9° (c 0.317, CH2Cl2); H NMR (500 MHz, CDCl3): δ 7.45–7.41 (m, 6 H), 7.27–7.21 (m, 6 H) , 7.23–7.17 (m, 5 H), 6.81 (d, J = 8.5, 2 H), 5.66 (m, 1 H), 5.00–4.93 (m, 2 H), 4.68 (d, J = 11 Hz, 1 H), 4.43 (d, J = 11 Hz, 1 H), 3.76 (s, 3 H), 3.36 (dd, J = 8.5, 2.5 Hz, 1 H), 3.55 (ddd, J = 10.0, 2.5, 2.5 Hz, 1 H), 3.29 (s, 3 H), 2.98 (dd, J = 9.0, 5.0 Hz, 1 H), 2.85 (dd, J = 8.5, 7.5 Hz, 1 H), 2.25 (m, 1 H), 2.01 (m, 1 H), 1.70 (m, 1 H), 1.22 (m, 1 H), 1.05 (d, J = 6.5 Hz, 3 H), 0.95 (d, J = 7.0 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 159.0, 144.5, 141.5, 131.2, 129.5, 128.8, 127.6, 126.7, 114.6, 113.6, 86.1, 81.7, 81.1, 231 73.7, 69.1, 57.2, 55.2, 40.7, 33.4, 30.7, 17.3, 17.2; HRMS (ESI): m/z calculated for C38H44O4Na + [M+Na] 587.3137, found 587.3137. OTrt OH OPMB PPTS MeOH 91% MeO 929 OPMB MeO 930 Preparation of 930: To a solution of 929 (3.44 g, 6.09 mmol) in MeOH (43.5 mL) was added PPTS (1.531 g, 6.09 mmol) at room temperature in one portion. The reaction was stirred for 24 hours at room temperature, quenched with aqueous saturated NaHCO3, extracted with ether, dried over MgSO4, and concentrated under vacuum. The crude material was purified by column chromatography eluting with hexanes/ethyl acetate (1:1) to give 930 (1.78 g, 5.52 mmol, 91% 20 yield) as a colorless oil. Rf = 0.35 (1:1 hexanes/ethyl acetate); [α] D −2.5° (c 0.317, CH2Cl2); 1 H NMR (500 MHz, CDCl3): δ 7.26 (d, J = 8.5 Hz, 2 H), 6.85 (d, J = 8.5 Hz, 2 H), 5.65 (m, 1 H), 5.00 (m, 1 H), 4.97 (m, 1 H), 4.73 (d, J = 11 Hz, 1 H), 4.47 (d, J = 11 Hz, 1 H), 3.78 (s, 3 H), 3.49 (m, 1 H), 3.43-3.35 (m, 4 H), 3.35 (s, 3 H), 2.4 (t, 6 Hz, 1 H, exchangeable), 2.26 (m, 1 H), 1.775 (m, 1 H), 1.665 (m, 1 H), 1.39 (m, 1 H), 1.07 (d, J = 6.5 Hz, 3 H), 0.91 (d, J = 7 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 159.1, 141.2, 131.0, 129.5, 114.9, 113.7, 82.8, 81.0, 73.8, 68.6, 56.9, 55.3, 41.1, 33.9, 33.5, 17.8, 17.6; HRMS (ESI): m/z calculated for C19H31O4 [M+H] 323.2229, found 323.2222. 232 + OH I2, PPh3 imidazole I OPMB OPMB benzene 93% MeO MeO 930 529 Preparation of 529: To a 25mL pear-shaped flask covered with aluminum foil, containing a solution of 930 (100.7 mg, 0.409 mmol) in benzene (4 mL), was added imidazole (41.7 mg, 0.613 mmol), triphenylphosphine (139 mg, 0.531 mmol), and iodine (135 mg, 0.531 mmol) and the resulting brown suspension was vigorously stirred at room temperature. After 1 h, the reaction mixture was a homogeneous bright yellow suspension. Note: the workup and the purification are to be carried out in the dark. The reaction was quenched by the addition of saturated Na2S2O3; the resulting two-phase system was stirred and, after some minutes, two colorless and clear phases formed. The phases were separated and the aqueous layer was extracted with ether. Combined organic layers were washed with brine, dried over Na2SO4 and concentrated until some precipitate was observed. Petroleum ether (10 mL) was added and the suspension was filtered. The solids were washed with petroleum ether/ether (2:1) and the filtrate was directly adsorbed in silica gel. The product was purified by column chromatography eluting with hexanes/ether (5:1) to afford 529 (782.0 mg, 1.809 mmol, 93% yield) as a colorless oil. Rf = 20 1 0.8 (1:1 hexanes/ether); [α] D +1.5° (c 0.733, CH2Cl2); H NMR (500 MHz, CDCl3): δ 7.27 (d, J = 8.5 Hz, 2 H), 6.85 (d, J = 8.5 Hz, 2 H), 5.66 (m, 1 H), 5.01 (m, 1 H), 4.98 (m, 1 H), 4.72 (d, J = 11 Hz, 1 H), 4.47 (d, J = 11 Hz, 1 H), 3.78 (s, 3 H), 3.40 (dd, J = 8.5, 2.5 Hz, 1 H), 3.34 (s, 3 H), 3.30 (ddd, J = 10, 2.5, 2.5 Hz, 1 H), 3.27 (dd, J = 9.5, 4.0 Hz, 1 H), 3.13 (dd, J = 9.5, 6.5 Hz, 1 H), 2.28 (m, 1 H), 1.78–1.70 (m, 2 H), 1.31 (m, 1 H), 1.08 (d, J = 7 Hz, 3 H), 0.92 (d, J = 233 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 159.1, 141.2, 131.0, 129.6, 114.9, 113.7, 81.4, 81.2, 73.9, 57.2, 55.3, 40.9, 36.3, 31.6, 20.15, 19.0, 17.5; HRMS (ESI): m/z calculated for + C19H29IO3Na [M+Na] 455.1059, found 455.1074. 9.6. Experimental details for Chapter 6: Autolytimycin, Suzuki coupling OH OTIPS O N H Cl TIPSCl, DMAP NEt3 O N H Cl DCM, 12 h 91% 433 616 Preparation of TIPS-protected amidochlorophenol 616: 31 5h To a solution of 433 (1.4g, 6.20 mmol) and DMAP (0.152 g, 1.241 mmol) in dichloromethane (10.5 mL) were successively added triethylamine (0.96 mL, 6.82 mmol) and triisopropylsilyl chloride (1.32 mL, 6.20 mmol) at 0 °C and the mixture was stirred at room temperature for 12 hours. The reaction was quenched with aqueous saturated sodium bicarbonate (10 mL). The layers were separated and the aqueous layer was extracted with ethyl acetate (3 × 10 mL). Combined organic extracts were dried over MgSO4 and concentrated. Column chromatography eluting with 8:1 hexanes/EtOAc afforded TIPS-protected amidochlorophenol 616 (2.157 g, 5.65 mmol, 91% yield) as a white solid; mp 84.5–85.5 °C (lit. 31 1 82–83 °C); Rf = 0.33 (8:1 hexanes/EtOAc). H NMR (500 MHz, CDCl3): δ 7.29 (br, 1 H), 7.14 (t, J = 2.0 Hz, 1 H), 7.09 (t, J = 2.0 Hz, 1 H), 6.60 (t, J = 2.0 Hz, 1 H), 6.48 (m, 1 H), 1.91 (apparent t, J = 1.2 Hz, 3 H), 1.80 (apparent d, J = 7.0 Hz, 3 H), 1.25 (m, 3 H), 1.10–1.07 (m, 18 H); 13 C NMR (125 MHz, CDCl3): δ 167.4, 157.2, 139.8, 134.6, 132.7, 131.7, 115.9, 112.5, 109.7, 17.9, 14.1, 12.6, 12.5; IR (neat): 3301, 2946, 2892, 2868, 1662, 1637, 1599, 234 1538, 1449, 1426, 1386, 1286, 1226, 1182, 1142, 1096, 1075, 1013, 953, 918, 901, 883, 860 cm 1 – + ; HRMS (ESI): m/z calculated for C20H33ClNO2Si [M+H] 382.1964, found 382.1960. OH OMOM O N H Cl 433 1.4 equiv NaH 1.4 equiv MOMCl DMF, 0 ºC to rt 82% O N H Cl 617 Preparation of MOM-protected amidochlorophenol 617: Sodium hydride (12 mg, 0.298 mmol) (60% disersion in mineral oil) was rinsed twice with ~2mL dry pentane under nitrogen. Carefully, most of the solvent was removed via cannula and the rest evaporated under vacuum. To the residual white solid was added DMF (200 µL) and the resulting suspension was cooled to 0 °C. A solution of phenol 433 (48.0 mg, 0.213 mmol) in DMF (150 µL) was added and the resulting solution stirred at 0 °C for 30 minutes. To the solution was added chloromethyl methyl ether (23 µL, 0.298 mmol) causing the immediate evolution of gas. The solution was allowed to warm to room temperature and quenched with water (0.5 mL). The solution was extracted with ether, dried with MgSO4 and concentrated in vacuo. The excess of DMF was removed by adding small portions of absolute ethanol and concentrating the resulting solutions under vacuum three times. Column chromatography eluting with hexanes/EtOAc (7:1) afforded 617 (47.2 mg, 0.175 1 mmol, 82 % yield) as a waxy white solid. mp 51.5–53 °C; Rf = 0.30 (7:1 hexanes/EtOAc). H NMR (500 MHz, CDCl3): δ 7.40 (br, 1 H), 7.27 (t, J = 2.0 Hz, 1 H), 7.17 (t, J = 2.0 Hz, 1 H), 6.77 (t, J = 2.1 Hz, 1 H), 6.48 (qq, J = 1.5, 7.0 Hz, 1 H), 5.12 (s, 2 H), 3.44 (s, 3 H), 1.90 (apparent t, J = 1.5 Hz, 3 H), 1.79 (apparent d, J = 7.0 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 167.5, 158.1, 139.8, 135.0, 132.6, 131.8, 113.5, 112.4, 106.1, 94.5, 56.1, 14.1, 12.5; IR (neat): 235 3307, 3128, 3088, 2955, 2827, 1664, 1636, 1602, 1534, 1457, 1427, 1397, 1317, 1274, 1234, –1 1211, 1154, 1082, 1025, 966, 924, 848 cm ; HRMS (ESI): m/z calculated for C13H17ClNO3 + [M+H] 270.0891, found 270.0898. OH O N H Cl 433 OPMB 1.1 equiv PMBCl 1.2 equiv K2CO3 5 mol% 18-crown-6 6 mol% TBAI acetone, reflux, 14 h 49% (unoptimized) O Cl N H 618 Preparation of PMB-protected amidochlorophenol 618 (Unoptimized. For a better procedure see the preparation of amidochlorophenol 511, page 208): To a suspension of chloroamidophenol 433 (1.24 g, 5.49 mmol), potassium carbonate (0.911 g, 6.59 mmol), 18-crown-6 (0.087 g, 0.330 mmol), and TBAI (0.101 g, 0.275 mmol) in acetone (275 mL) was added 4-methoxybenzyl chloride (0.947 g, 6.04 mmol). The resulting mixture was heated to reflux for 14 h. The reaction mixture was concentrated, redissolved in dichloromethane, washed twice with water, then brine, dried over MgSO4, and concentrated in vacuo. Purification by column chromatography (hexanes/EtOAc 5:1) afforded 618 (0.93 g, 2.69 mmol, 48.9 % yield) as a white solid. mp 98–99 1 °C; H NMR (500 MHz, CDCl3): δ 7.33 (br, 1 H), 7.32 (d, J = 13.5 Hz, 2 H), 7.28 (t, J = 1.7 Hz, 1 H), 7.08 (t, J = 1.7 Hz, 1 H), 6.90 (d, J = 13.5 Hz, 2 H), 6.70 (t, J = 1.7 Hz, 1 H), 6.49 (qq, J = 1.5, 6.5 Hz, 1 H), 4.95 (s, 2 H), 3.80 (s, 3 H), 1.91 (apparent s, 3 H), 1.81 (apparent d, 3 H); 13 C NMR (125 MHz, CDCl3): δ 167.5, 159.9, 159.6, 139.9, 135.0, 132.7, 131.8, 129.3, 128.3, 114.0, 112.4, 111.4, 104.7, 70.1, 55.3, 14.1, 12.5; IR (neat): 3314, 3078, 3004, 2933, 2836, 1661, 1589, 236 –1 1539, 1514, 1456, 1420, 1377, 1269, 1173, 1024, 849, 824 cm ; HRMS (ESI): m/z calculated + for C19H21ClNO3 [M+H] 346.1204, found 346.1199. OTIPS B(OH)2 1.5 equiv OTIPS O O Cl N H 616 5 mol% Pd(OAc)2 10 mol% SPhos 2 equiv K3PO4·nH2O 91% N H 624 Preparation of Suzuki product 624: A 10mL round-bottomed flask provided with a magnetic stirrer was charged with potassium phosphate hydrate (55.7 mg, 0.209 mmol), TIPS-protected amidochlorophenol 616 (40 mg, 0.105 mmol), palladium(II) acetate (1.2 mg, 5.24 µmol), SPhos (4.3 mg, 10.47 µmol), and phenylboronic acid (19.2 mg, 0.157 mmol). The flask was evacuated and refilled with nitrogen (this was repeated 2 additional times). Degassed water (25 µL) and THF (0.24 mL) were added and the reaction mixture was allowed to stir at reflux (70 °C) overnight. After the starting material was consumed as determined by TLC, the reaction solution was cooled to room temperature and then adsorbed onto silica gel directly. The product was isolated by flash chromatography eluting with hexanes/dichloromethane (1:2) to give 624 (40.2 mg, 0.095 mmol, 91%) as white waxy solid after removal of the solvent under high vacuum. Rf = 1 0.35 (1:2 hexanes/dichloromethane). H NMR (500 MHz, CDCl3) δ 7.28 (s, 1 H), 7.05 (t, J = 2Hz, 1 H), 6.87 (s, 1 H), 6.48–6.43 (m, 2 H), 2.49 (t, J = 7.5Hz, 2 H), 1.91 (m, 3 H), 1.78 (d, J = 6.5Hz, 3 H), 1.56 (m, 2 H), 1.23 (m, 7 H), 1.13–1.03 (m, 18 H), 0.84 (t, J = 7.5Hz, 3 H); 13 C NMR (62.8 MHz, CDCl3) δ 167.4, 156.4, 144.7, 138.8, 133.0, 130.9, 115.9, 112.5, 108.9, 35.9, 31.4, 30.9, 22.5, 18.0, 17.7, 14.1, 14.0, 12.7, 12.6. 237 (a) KOtBu, nBuLi (b) B(OiPr)3 OH B OH I (c) HCl(aq) 622 625 Preparation of pentylboronic acid (625): In a round-bottomed flask provided with a magnetic stirring bar, a solution of pentyl iodide (2 mL, 15.25 mmol) in diethyl ether (50.8 mL) was treated with tert-butyllithium 1.7M hexanes (18.0 mL, 30.5 mmol) (added dropwise) at −78 °C. After 20 minutes, triisopropyl borate (3.90 mL, 16.77 mmol) was added dropwise and the mixture was stirred for 20 minutes before being rapidly poured into a separatory funnel containing hydrochloric acid 1M saturated with NaCl (30.5 mL, 30.5 mmol). The aqueous layer was adjusted to pH 1 using HCl 2M, the phases were separated and the aqueous layer was extracted with diethyl ether four times. Combined organic layers were dried over MgSO4 and concentrated under vacuum. To the resulting orange solid was added water (50 mL) and the solution was boiled until it became homogeneous and transparent (slightly yellow). After being slowly cooled down to room temperature the formed solid was filtered out and dried under vacuum until it looked like dry white flakes, affording 1.05 g (9.05 mmol, 59.4% yield) of product. Note: Excess of vacuum makes the product "melt", probably forming oligomers and 1 eliminating water. mp 91–92 °C; H NMR (500 MHz, DMSO-d6) δ 7.31 (s, 2 H), 1.40–1.10 (m, 6 H), 0.85 (t, J = 7.25 Hz, 3 H), 0.56 (t, J = 7.75, 2 H); 23.8, 22.0, 15.4, 13.9; 11 13 C NMR (62.8 MHz, CDCl3) δ 34.4, B NMR (62.8 MHz, CDCl3) δ 33.3. 238 1.2 equiv OTIPS O Cl N H 616 OH B OH OTIPS 625 O 2 mol% Pd(dba)2 4 mol% QPhos 2 equiv K3PO4 toluene, reflux, 48h 93% N H 620 Preparation of Suzuki product 620: Inside the dry box, an air free flask provided with a magnetic stir bar was charged with 616 (50 mg, 0.131 mmol), Pd(dba)2 (1.5 mg, 2.62 µmol), QPhos (3.7 mg, 5.24 µmol), anhydrous potassium phosphate (55.6 mg, 0.262 mmol), and pentylboronic acid, 625, (18.4 mg, 0.158 mmol). Anhydrous toluene (300 µL) was added and the air free flask was sealed and removed from the dry box. The reaction mixture was stirred at 100 °C for 48 hours. After the starting material was consumed as determined by TLC, the reaction solution was cooled to room temperature and then adsorbed onto silica gel directly. The product was isolated by flash chromatography eluting with hexanes/ether (5:1) to give 51 mg (0.122 mmol, 93%) of 620 as white waxy solid after removal of the solvent under high vacuum. Rf = 0.35 (5:1 1 hexanes/ether); H NMR (500 MHz, CDCl3) δ 7.26 (br, 1 H), 7.05 (t, J = 2 Hz, 1 H), 6.87 (s, 1 H), 6.47 (q, J = 6.5 Hz, 1 H), 6.44 (s, 1H), 2.49 (t, J = 7.5 Hz, 2 H), 1.91 (s, 3 H), 1.79 (d, J = 6.5 Hz, 3 H), 1.56 (p, 2 H), 1.32–1.20 (m, 7 H), 1.08 (d, J = 8 Hz, 18 H), 0.86 (t, J = 7 Hz, 3 H); 13 C NMR (62.8 MHz, CDCl3) δ 167.4, 156.4, 144.8, 138.8, 133.0, 130.9, 115.9, 112.5, 108.9, 35.9, 31.4, 30.9, 22.5, 18.0, 14.1, 14.0, 12.7, 12.6. 239 I 1.5 equiv 637 (a) 3.0 equiv tBuLi, !78 °C " rt, 30 min (b) 3.0 equiv B-OMe-BBN, solvent, !78 °C " rt, 2 h OPMB B O Cl N H 618 OPMB OMe O 638 N H 4 equiv DMSO 10 mol% Pd(OAc)2 20 mol% SPhos DMF, reflux or 85 ºC, 12 h 45% 639 Preparation of Suzuki product 639 under partially optimized conditions (Table 6, entry 3): To a 25 mL pear-shaped flask provided with a magnetic stirrer, containing octyl iodide (66.2 µL, 0.367 mmol) in ether (400 µL) at −78 °C, was added tert-butyllithium 1.7 M in pentane (430 µL, 0.733 mmol) and the mixture was stirred at that temperature for 30 minutes. B-methoxy-9borabicyclo[3.3.1]nonane (370 µL, >0.367 mmol) was added dropwise, followed by THF (400 µL) and the reaction mixture was stirred for 5 minutes more at −78 °C and then at room temperature for 1 h 45 m. DMF (1.6 mL) was added and the resulting clear colorless solution was freeze-pump-thaw degassed (0.01 atm) twice. A mixture of PMB-protected chloroamidophenol 618 (84.5 mg, 0.244 mmol), palladium(II) acetate (5.5 mg, 0.024 mmol), and SPhos (20.06 mg, 0.049 mmol) was added to the flask, followed by DMSO (70 µL, 0.977 mmol). A reflux condenser was attached under nitrogen atmosphere and the resulting mixture was refluxed overnight. After 12 h, the reaction mixture was passed through a small plug of silica gel, eluting with acetone. The resulting yellowish solution (~15 mL) was concentrated in vacuo. Ethanol was added and evaporated in vacuo a few times in order to eliminate the excess of DMF. Column hexanes/ether (2:1) afforded pure Suzuki product 639 (46.5 mg, 0.111 mmol, 240 1 45.4 % yield). Rf = 0.35 (2:1 hexanes/ether); H NMR (500 MHz, CDCl3) δ 7.34 (d, J = 8.5 Hz, 2 H), 7.33 (br, 1 H), 7.26 (apparent s, 1 H), 6.89 (d, J = 8.5 Hz, 2 H), 6.83 (apparent s, 1 H), 6.55 (apparent s, 1 H), 6.48 (apparent q, J = 6.8 Hz, 1 H), 4.96 (s, 2 H), 3.80 (s, 3 H), 2.52 (t, J = 7.5 Hz, 2 H), 1.92 (s, 3 H), 1.80 (apparent d, J = 7.0 Hz, 3 H), 1.56 (p, 2 H), 1.33–1.18 (m, 10 H), 0.86 (t, J = 7 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 167.5, 159.4, 159.3,145.0, 139.0, 132.9, 131.1, 129.3, 129.1, 113.9, 112.3, 111.5, 103.5, 69.8, 67.5, 55.3, 36.1, 31.9, 31.2, 29.5, 29.3, + 29.2, 22.7, 14.1, 12.5; HRMS (ESI): m/z calculated for C27H38NO3 [M+H] 424.2852, found 424.2849. I OMOM MOMO MeO NH 1.6 equiv 525 O (a) 2.0 equiv B-OMe-9-BBN, ether, !78 °C (b) 3.2 equiv tBuLi, !78 °C " rt MeO OTIPS 636 B OMe MOMO MOMO + OMOM NH NH MeO Cl O O 526 MeO 10 mol% Pd(OAc)2 OTIPS 20 mol% SPhos DMF, reflux, 12 h MeO OMOM OTIPS MeO 635 510 Preparation of dechlorination product 636 under non-optimized Suzuki conditions (Table 5, entry 1): To a 10mL pear-shaped flask provided with a magnetic stirrer, containing iodide 525 (67.1 mg, 0.188 mmol) in THF (1.5 mL) at −78 °C under nitrogen atmosphere, was added B241 methoxy-9-borabicyclo[3.3.1]nonane 1M in THF (210 µL, >0.207 mmol). Twenty minutes later, tert-butyllithium 1.7 M in pentane (225 µL, 0.377 mmol) was added dropwise and the reaction mixture was stirred for 5 minutes more at −78 °C and then at room temperature. After 30 minutes, DMF (2.4 mL) was added and the solution was frozen and degassed under vacuum (0.01 atm) twice, forming a clear and colorless solution (solution A). A mixture of MOMprotected phenol 511 (87mg, 0.157 mmol), palladium(II) acetate (3.5 mg, 0.016 mmol), and SPhos (12.9 mg, 0.031 mmol) was prepared in a flame-dried 25mL round-bottomed flask and it was purged and refilled with nitrogen three times. Solution A was then transferred via cannula at room temperature into the round-bottomed flask and the mixture was heated up to 110 °C. The progress of the reaction was monitored by TLC (7:1 hexanes/EtOAC). Complete consumption of the SM was achieved after 9 hours and the reaction mixture was concentrated. Absolute ethanol (~1.5 mL) was added and the suspension was concentrated in vacuo to remove the excess of DMF. This procedure was repeated twice. The black thick residue was redissolved in 1.5 mL of acetone, adsorbed onto ~300 mg of silica gel and directly purified by column chromatography eluting with hexanes/EtOAC 7:1 to afford impure 635 (24.4 mg, <0.033 mmol, <21% yield) (apparently contaminated with BBN-containing species) and pure dechlorination product 636 (37.2 mg, 0.072 mmol, 46% yield). Since better conditions were found (see following procedures), the preparation of this Suzuki product was not optimized. The dechlorination product was used for characterization purposes. Rf(636) = 0.35 (7:1 hexanes/ethyl acetate); 20 1 [α] D −9.3° (c 1.85, CHCl3); H NMR (500 MHz, CDCl3): δ 7.37 (br, 1 H), 7.31 (t, J = 2.5 Hz, 1 H), 7.20 (t, J = 8.0 Hz, 1 H), 7.15 (ddd, J = 8.0, 2.5, 1.0 Hz, 1 H), 6.76 (ddd, J = 8.0, 2.5, 1.0 Hz, 1 H), 6.36 (m, 1 H), 5.16 (s, 2 H), 4.95 (s, 1 H), 4.89 (s, 1 H), 4.31 (d, J = 5.5 Hz, 1 H), 3.46 (s, 3 H), 3.45 (s, 3 H), 3.18 (m, 1 H), 2.35–2.20 (m, 2 H), 1.92 (s, 3 H), 1.73 (s, 3 H), 1.67 (m, 1 242 H), 1.35 (m, 1 H), 1.12–1.00 (m, 21 H); 13 C NMR (125 MHz, CDCl3): δ 167.5, 157.7, 144.9, 139.2, 136.6, 132.0, 129.7, 113.4, 112.9, 112.1, 108.0, 94.5, 84.3, 76.4, 58.8, 56.1, 29.1, 25.0, 19.2, 18.09, 18.05, 12.8, 12.4; IR (neat): 3310, 2942, 2867, 1662, 1603, 1540, 1490, 1446, 1386, –1 1269, 1151, 1084, 1014, 883 cm ; HRMS (ESI): m/z calculated for C29H50NO5Si [M+H] + 520.3453, found 520.3460. I OPMB MeO 1.6 equiv 529 (a) 3.2 equiv tBuLi, ether, !78 °C " rt (b) 3.8 equiv B-OMe-BBN, !78 °C " rt B OMe PMBO PMBO OPMB NH NH MeO Cl O O 640 MeO 2 equiv K3PO4 nH2O 10 mol% Pd(OAc)2 MeO 20 mol% SPhos 4 equiv DMSO DMF, 85 ºC, 15 h 40% conversion, 36% yield 59% recovered 511 MeO OPMB OTIPS 511 OTIPS 642 Preparation of Suzuki product 642 under partially optimized conditions: To a flame-dried 25mL pear-shaped flask provided with a magnetic stirrer, containing iodide 529 (109 mg, 0.252 mmol) in ether (1.6 mL) at −78 °C under dry nitrogen atmosphere, was added dropwise tertbutyllithium 1.7 M in pentane (300 µL, 0.503 mmol) and the resulting mixture was stirred at −78 °C for 5 minutes, at room temperature for 1 hour, and then cooled down again to −78 °C. B- 243 methoxy-9-borabicyclo[3.3.1]nonane 1M in THF (277 µL, 0.277 mmol) was added dropwise and the solution was stirred for 10 minutes more at −78 °C and then at room temperature for 1 hour. DMF (1.2 mL) was added and the solution was frozen and degassed under vacuum (0.01 atm) 3 times (Solution A). A mixture of amide 511 (122 mg, 0.194 mmol) and potassium phosphate hydrate (41.1 mg, 0.194 mmol) was prepared in a flame-dried three-necked 50mL roundbottomed flask provided with a reflux condenser and it was purged and refilled with nitrogen several times. Solution A was transferred via cannula at room temperature and the 25mL pearshaped flask was rinsed with DMF (1.1 mL). The rinse was degassed 3 times as mentioned before and was transferred via cannula to the second flask. A solution of palladium acetate (4.4 mg, 0.019 mmol), SPhos (15.9 mg, 0.039 mmol), in DMSO (1.0 mL) was prepared in a small vial. The solution was degassed three times as mentioned before and added to the main reaction flask and the resulting brown suspension was refluxed at 85 °C for 15 hours. The reaction mixture was filtered though a thin pad of silica gel eluting with ether and ethyl acetate and the yellow filtrate was concentrated under vacuum. Ethanol was added and the solution was pumped down several times in order to get rid of the excess of DMF. NMR shows a conversion of 40% to the Suzuki coupling product and 0% of dehalogenation. Column chromatography eluting with hexanes/EtOAc (7:1) afforded 642 (62 mg, 35.6% yield) as a white solid and 59% of recovered 20 1 511. Rf(642) = 0.3 (7:1 hexanes/ethyl acetate); [α] D −13.7° (c 0.667, CH2Cl2); H NMR (500 MHz, CDCl3): δ 7.38 (t, J = 2 Hz, 1 H), 7.34 (br, 1 H), 7.33 (d, J = 8.5 Hz, 2 H), 7.23 (d, J = 8.5 Hz, 2 H), 6.88 (d, J = 8.5 Hz, 2 H), 6.83 (d, J = 8.5 Hz, 2 H), 6.72 (t, J = 2 Hz, 1 H), 6.56 (t, J = 2 Hz, 1 H), 6.35 (m, 1 H), 5.67 (m, 1 H), 5.02–4.94 (m, 3 H), 4.96 (s, 2 H), 4.89 (t, J = 1.5 Hz, 1 H) , 4.70 (d, J = 11 Hz, 1 H), 4.44 (d, J = 11 Hz, 1 H), 4.035 (d, J = 5.5 Hz Hz, 1 H) , 3.79 (s, 3 H) , 3.76 (s, 3 H), 3.45 (s, 3 H), 3.42–3.35 (m, 2 H), 3.33 (s, 3 H), 3.183 (m, 1 H), 2.63 (m, 1 H), 244 2.35 (m, 4 H), 1.98 (m, 1 H), 1.92 (s, 3 H), 1.76 (m, 1 H), 1.73 (s, 3 H), 1.66 (m, 1 H), 1.36 (m, 1 H), 1.24 (m, 1 H), 1.09–1.01 (m, 24 H), 0.77 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 167.5, 159.4, 159.3, 159.1, 145.0, 143.4, 141.4, 139.0, 136.4, 132.1, 131.2, 129.5, 129.3, 129.2, 114.7, 113.9, 113.7, 113.1, 112.9, 112.2, 103.8, 84.3, 81.6, 81.3, 76.5, 73.8, 69.8, 58.8, 57.25, 55.3, 44.8, 40.8, 36.7, 31.2, 29.1, 25.0, 19.1, 18.8, 18.1, 18.05, 17.4, 12.8, 12.4; HRMS (ESI): + m/z calculated for C54H82NO8Si [M+H] 900.5810, found 900.5804. I OMOM MeO 1.6 equiv 525 (a) 3.2 equiv tBuLi, ether, !78 °C (b) 3.8 equiv B-OMe-9-BBN, THF, !78 °C " rt B OMe PMBO PMBO OMOM NH NH MeO Cl O O 526 MeO 511 2 equiv K3PO4 nH2O OTIPS 10 mol% Pd(OAc)2 20 mol% SPhos THF, reflux, 12 h 90% MeO OMOM OTIPS MeO 644 Preparation Suzuki coupling product 644 under optimized conditions: Before use, iodide 525 was dried in a vacuum desiccator over drierite (0.01 torr) for 1 day. A vacuum/flame-dried 25mL pear-shaped flask, provided with a magnetic stirrer and connected to a nitrogen line, was charged with iodide 525 (226 mg, 0.635 mmol) and ether (2.6 mL) via syringe. The solution was stirred, cooled to −78 °C, and to it was added tert-butyllithium 1.7 M in pentane (750 µL, 1.27 mmol) 245 (solution remains colorless and clear). After 3 minutes, B-methoxy-9-borabicyclo[3.3.1]nonane 1M in THF (1.51 mL, 1.51 mmol) was added (white precipitate forms), followed by THF (2.6 mL) (solution becomes clear/colorless rapidly). The solution was stirred for 10 minutes more at −78 °C (white precipitate forms) and then at room temperature for 20 minutes (solution becomes clear and colorless again). The solvent was blown off under a nitrogen gas stream. After all the solvent was evaporated, THF (1 mL) was added, followed by potassium phosphate hydrate (197 mg, 0.793 mmol) and amide 511 (250 mg, 0.397 mmol). The resulting suspension was freezepump-thaw degassed (0.01 torr) three times. In a small vial, a mixture of palladium(II) acetate (8.9 mg, 0.040 mmol) and SPhos (32.6 mg, 0.079 mmol) was dissolved in THF (1 mL). The vial was connected to the Schlenk line and the solution was freeze-pump-thaw degassed (0.01 torr) 3 times. The solution of catalyst was added quickly to the first mixture via syringe causing a sudden color change to dark orange-brown. A water-cooled condenser was attached under nitrogen atmosphere and the reaction mixture was refluxed for 12 hours. After full conversion was confirmed by TLC, the mixture was diluted with acetone and filtered through a short pad of silica gel eluting with acetone. After concentration of the filtrate, the crude material was purified by column chromatography eluting with hexanes/EtOAc (49:1 → 3:1) to afford Suzuki product 644 (294 mg, 0.357 mmol, 90% yield) as a white solid. When necessary, the product was washed with hexanes to remove any yellow/orange color. Rf = 0.29 (4:1 hexanes/EtOAc); mp 84–85 ºC; 20 1 [α] D +6.2° (c 0.5, CH2Cl2); H NMR (500 MHz, CDCl3) δ 7.35 (s, 1 H), 7.34 (br, 1 H), 7.34 (d, J = 8.6 Hz, 2 H), 6.89 (d, J = 8.6 Hz, 2 H), 6.73 (s, 1 H), 6.55 (s, 1 H), 6.35 (t, J = 7.4 Hz, 1 H), 5.67 (m, 1 H), 5.01 (d, J = 17.8 Hz, 1 H), 4.98 (d, J = 10.8 Hz, 1 H), 4.95 (s, 1 H), 4.95 (s, 2 H), 4.89 (s, 1 H), 4.82 (d, J = 6.8 Hz, 1 H), 4.61 (d, J = 6.8 Hz, 1 H), 4.30 (d, J = 6.0 Hz, 1 H), 3.79 (s, 3 H), 3.60 (dd, J = 8.8, 1.8 Hz, 1 H), 3.45 (s, 3 H), 3.39 (s, 3 H), 3.32 (m, 1 H), 3.30 (s, 3 246 H), 3.18 (ddd, J = 8.8, 5.8, 2.7 Hz, 1 H), 2.65 (dd, J = 13.3, 4.9 Hz, 1 H), 2.37–2.19 (m, 4 H), 1.95 (m, 1 H), 1.92 (s, 3 H), 1.73 (s, 3 H), 1.72–1.62 (m, 2 H), 1.36 (m, 1 H), 1.22 (m, 1 H), 1.11 (d, J = 6.6 Hz, 3 H), 1.09–1.02 (m, 21 H), 0.75 (d, J = 6.7 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 167.5, 159.4, 159.3, 145.0, 143.3, 141.0, 138.9, 136.4, 132.1, 129.3, 129.1, 114.9, 113.9, 113.0, 112.9, 112.3, 103.7, 97.4, 84.3, 80.9, 79.3, 76.5, 69.8, 58.8, 57.0, 56.1, 55.3, 44.7, 40.6, 36.7, 31.2, 29.1, 25.0, 19.1, 18.5, 18.1, 18.0, 17.6, 12.8, 12.4; IR (neat): 3245, 2962, 2924, –1 2866, 1656, 1614, 1557, 1541, 1516, 1458, 1437, 1380, 1260, 1096, 1033, 872, 801 cm ; + HRMS (ESI): m/z calculated for C48H78NO8Si [M+H] 824.5497, found 824.5494. 9.7. Experimental details for Chapter 7: RCM and final steps PMBO PMBO NH NH TBAF O MeO OMOM O MeO OMOM 86% OTIPS MeO OH MeO 644 723 Preparation of RCM substrate 723: To a pear-shaped flask containing a solution of Suzuki coupling product 644 (154.4 mg, 0.187 mmol) in THF (3.5 mL) at 0 °C was added TBAF 1M solution in THF (1.87 mL, 1.87 mmol) dropwise. After 1.5 hours, TLC (hexanes/EtOAc 1:1) showed that the reaction was complete. The reaction was quenched with 4 mL of saturated sodium bicarbonate and extracted with ethyl acetate several times. Combined organic layers were dried over MgSO4 and concentrated. Purification by column chromatography eluting with hexanes/EtOAc (2:1 → 1:2) afforded 723 (108.1 mg, 0.162 mmol, 86% yield) as a clear 247 20 colorless thick oil that formed a white wax upon standing. Rf = 0.4 (1:1 hexanes/EtOAc); [α] D 1 +19.8° (c 1.15, chloroform); H NMR (600 MHz, CDCl3) δ 7.35 (s, 1 H), 7.34 (br, 1 H), 7.33 (d, J = 8.6 Hz, 2 H), 6.89 (d, J = 8.6 Hz, 2 H), 6.74 (s, 1 H), 6.55 (s, 1 H), 6.38 (t, J = 7.4 Hz, 1 H), 5.67 (m, 1 H), 5.03 (s, 1 H), 5.01 (d, J = 17.3 Hz, 1 H), 4.98 (d, J = 10.3 Hz, 1 H), 4.95 (s, 2 H), 4.95 (s, 1 H), 4.81 (d, J = 6.8 Hz, 1 H), 4.61 (d, J = 6.8 Hz, 1 H), 3.97 (dd, J = 6.6, 3.5 Hz, 1 H), 3.79 (s, 3 H), 3.59 (dd, J = 8.6, 1.9 Hz, 1 H), 3.44 (s, 3 H), 3.38 (s, 3 H), 3.34 (m, 1 H), 3.30 (s, 3 H), 3.26 (m, 1 H), 2.65 (dd, J = 13.5, 5.3 Hz, 1 H), 2.53 (OH) (d, J = 3.5 Hz, 1 H), 2.37–2.19 (m, 4 H), 1.95 (m, 1 H), 1.93 (s, 3 H), 1.75 (s, 3 H), 1.72–1.64 (m, 2 H), 1.60 (m, 1 H), 1.22 (m, 1 H), 1.11 (d, J = 7.0 Hz, 3 H), 0.76 (d, J = 6.7 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 167.3, 159.4, 159.3, 144.3, 143.3, 141.0, 138.8, 136.0, 132.2, 129.3, 129.1, 114.9, 113.9, 113.9, 113.1, 112.3, 103.8, 97.4, 81.6, 80.9, 79.3, 77.1, 69.8, 58.3, 57.0, 56.1, 55.3, 44.7, 40.6, 36.6, 31.2, 29.1, 23.9, 18.6, 18.0, 17.5, 12.8; IR (neat): 3437, 3077, 2961, 1663, 1611, 1541, 1516, 1462, 1435, –1 1381, 1294, 1250, 1173, 1088, 1036, 909, 819 cm ; HRMS (ESI): m/z calculated for + C39H58NO8 [M+H] 668.4162, found 668.4162. PMBO PMBO NH NH TBAF O MeO OPMB O MeO OPMB 67% OTIPS MeO OH MeO 642 724 Preparation of RCM substrate 724. To a pear-shaped flask containing a solution of Suzuki coupling product 642 (268.0 mg, 0.298 mmol) in THF (10.9 mL) at 0 °C was added TBAF (1M 248 THF) (3 mL, 3 mmol) dropwise. After 1.5 hours, TLC (hexanes/EtOAc 1:1) showed that the reaction was complete. The reaction was quenched with 10 mL of saturated sodium bicarbonate and extracted with ethyl acetate several times. Combined organic layers were dried over MgSO4 and concentrated. The residue was adsorbed on silica gel and purified by column chromatography eluting with hexanes/ether 1:1 → 2:1 to afford 724 (148.2 mg, 67%) as a 20 1 colorless oil. Rf = 0.3 (3:7 hexanes/ether); [α] D -57.4° (c 0.467, benzene); H NMR (500 MHz, CDCl3): δ 7.38 (t, J = 2 Hz, 1 H), 7.34 (br, 1 H), 7.33 (d, J = 8.5 Hz, 2 H), 7.22 (d, J = 8.5 Hz, 2 H), 6.88 (d, J = 8.5 Hz, 2 H), 6.83 (d, J = 8.5 Hz, 2 H), 6.72 (t, J = 2 Hz, 1 H), 6.56 (t, J = 2 Hz, 1 H), 6.37 (m, 1 H), 5.67 (m, 1 H), 5.04-4.94 (m, 4 H), 4.95 (s, 2H), 4.70 (d, J = 10.5 Hz, 1 H), 4.44 (d, J = 10.5 Hz, 1 H), 3.954 (m, 1 H), 3.79 (s, 3 H) , 3.76 (s, 3 H), 3.46(m, 1 H), 3.43 (s, 3 H), 3.41-3.32 (m, 2 H), 3.32 (s, 3 H), 3.26 (m, 1 H), 2.62 (m, 1 H), 2.52 (d, J = 4 Hz, 1 H, exchangeable), 2.26 (m, 4 H), 1.97 (m, 1 H), 1.92 (s, 3 H), 1.75 (s, 3 H), 1.67 (m, 1 H), 1.6 (m, 1 H), 1.24 (m, 2 H), 1.06 (d, J = 6.5 Hz, 3 H), 0.77 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 167.3, 159.4, 159.3, 159.1, 144.3, 143.4, 141.4, 138.8, 135.9, 132.2, 131.2, 129.5, 129.3, 129.2, 114.7, 113.9, 113.9, 113.7, 113.1, 112.3, 103.9, 81.7, 81.7, 81.3, 73.8, 69.8, 58.3, 57.2, 55.3, 44.8, 40.8, 36.7, 31.2, 29.2, 23.9, 18.8, 18.0, 17.4, 12.8; HRMS (ESI): m/z calculated + for C45H61NO8 [M+H] 744.4475, found 744.4448. 249 PMBO PMBO NH NH 20 mol% [Ru]-II O MeO OPMB OH O toluene (0.001 M) 110 ºC, 16 h MeO OPMB MeO 724 727 Formation of 727 under RCM conditions: To a flame-dried three-necked round-bottomed flask, provided with a condenser, containing a solution of RCM susbstrate 724 (8.4 mg, 0.011 mmol) in toluene (9 mL) under nitrogen atmosphere (SM in the three-necked flask was dried overnight nd in a vacuum dessicator) was added a solution of Grubbs 2 generation catalyst [Ru]-II (1.9 mg, 2.2 µmol) in toluene (2.2 mL) via syringe and the resulting mixture was stirred at 110 °C. After 16 h, the dark green solution was filtered through a short plug of silica gel eluting with 1 dichloromethane and concentrated in vacuo. Analysis of the crude material by H NMR revealed the formation of one main product. Column chromatography eluting with hexanes/EtOAc (19:1 1:1) afforded impure 727 (2.2 mg, 3.66 µmol, 32.4% yield) as a dark green oil. Rf = 0.8 1 (hexanes/EtOAc 1:1); H NMR (500 MHz, CDCl3): δ 7.40 (br, 1 H), 7.37 (apparent s, 1 H), 7.33 (d, J = 8.5 Hz, 2 H), 7.20 (d, J = 8.5 Hz, 2 H), 6.88 (d, J = 8.5 Hz, 2 H), 6.83 (d, J = 8.5 Hz, 2 H), 6.70 (apparent s, 1 H), 6.54 (apparent s, 1 H), 5.74 (apparent s, 1 H), 5.48 (apparent q, J = 6.5 Hz, 1 H), 5.43 (apparent s, 1 H), 4.95 (s, 2 H), 4.39 (d, J = 11.5 Hz, 1 H), 4.14 (d, J = 11.5 Hz, 1 H), 3.79 (s, 3 H), 3.76 (s, 3 H), 3.54 (d, J = 6.0 Hz, 1 H), 3.31 (m, 1 H), 3.27 (s, 3 H), 2.59 and 3 2.27 (d of ABq, J = 5.5 and 3.75 Hz, JAB = 13.0 Hz, 2 H), 2.04 (s, 3 H), ~1.94 (m, 1 H), 1.66 (d, J = 11.5 Hz, 3 H), 1.62 (s, 3 H), ~1.27 (m, 2 H), 0.81 (d, J = 6.5 Hz, 3 H); 250 13 C NMR (125 MHz, CDCl3): δ 166.4, 159.5, 159.3, 158.9, 143.5, 141.1, 138.6, 133.3, 131.0, 129.3, 129.1, 126.0, 124.3, 119.6, 114.0, 113.6, 113.0, 112.6, 103.8, 86.2, 77.8, 69.8, 69.4, 58.1, 55.30, 55.26, 44.7, 38.8, 31.2, 19.0, 18.8, 13.3, 12.1; HRMS (ESI): m/z calculated for C37H48NO6 [M+H] + 602.3482, found 602.3476. PMBO PMBO NH NH O OPMB MeO MeO O 2 equiv DDQ DCM/pH 7 buffer (20:1) rt, 1.5 h OH 81% 724 OH MeO OH MeO 728 Preparation RCM substrate 728: To a solution of 724 (40.0 mg, 0.054 mmol) in dichloromethane (3.5 mL) and commercial pH 7 buffer (Orion) (~170 µL) was added solid DDQ (24.4 mg, 0.108 mmol) and the resulting dark mixture was stirred at room temperature for 1 h and 30 min. The reaction mixture was adsorbed on silica gel and directly purified by column chromatography eluting with ether/EtOAC (99:1 → 1:1) to give 728 (27.1 mg, 0.043 mmol, 81 1 % yield) as a slightly yellowish oil. H NMR (500 MHz, CDCl3): δ 7.36 (br, 1 H), 7.33 (d, J = 8.75 Hz, 2 H), 7.31 (t, J = 2.0 Hz, 1 H), 6.88 (d, J = 8.75 Hz, 2 H), 6.76 (apparent s, 1 H), 6.54 (apparent s, 1 H), 6.38 (apparent t, J = 7.5 Hz, 1 H), 5.60 (m, 1 H), 5.06–4.98 (m, 2 H), 5.03 (apparent s, 1 H), 4.953 (s, 2 H), 4.947 (apparent s, 1 H), 3.97 (m, 1 H), 3.79 (s, 3 H), 3.60 (m, 1 H), 3.43 (s, 3 H), 3.32 (s, 3 H), 3,28–3.23 (m, 2 H), 2.57 (dd, J = 13.5, 5.5 Hz, 1 H), 2.55 (br, 1 H), 2.34–2.19 (m, 4 H), 2.09 (br, 1 H), 1.96 (m, 1 H), 1.93 (s, 3 H), 1.75 (s, 3 H), 1.72–1.57 (m, 3 H), 1.23 (m, 1 H), 1.11 (d, J = 6.5 Hz, 3 H), 0.79 (d, J = 6.5 Hz, 3 H); 251 13 C NMR (125 MHz, CDCl3): δ 167.3, 159.4, 159.2, 144.3, 143.2, 139.9, 138.8, 136.0, 132.2, 129.3, 129.1, 115.3, 113.94, 113.93, 113.1, 112.4, 103.8, 81.6, 80.1, 77.1, 73.3, 69.8, 58.3, 57.0, 55.3, 44.8, 40.4, 34.5, 30.9, 29.1, 23.9, 18.7, 18.0, 17.4, 12.8; HRMS (ESI): m/z calculated for C37H54NO7 + [M+H] 624.3900, found 624.3907. PMBO PMBO NH NH CCl3CONCO, then K2CO3, MeOH O MeO OMOM O MeO OMOM 89% OH MeO O NH2 O MeO 723 729 Preparation of allylic carbamate 729. To a solution of 723 (60 mg, 0.090 mmol) in dichloromethane (11.8 mL) was added trichloroacetyl isocyanate (0.021 mL, 0.180 mmol). The reaction was stirred for 15 minutes and MeOH (14.75 mL) was added followed by potassium carbonate (57.1 mg, 0.413 mmol). The reaction was stirred for 30 minutes (monitored by TLC), before the solvent was evaporated in vacuo. The crude product was subjected to column chromatography eluting with ether to afford 729 (56.7 mg, 0.080 mmol, 89% yield) as a clear 20 1 colorless thick oil. Rf = 0.3 (ether); [α] D +4.0° (c 2.33, chloroform); H NMR (600 MHz, CDCl3) δ 7.59 (br, 1 H), 7.38 (s, 1 H), 7.34 (d, J = 8.6 Hz, 2 H), 6.89 (d, J = 8.6 Hz, 2 H), 6.79 (s, 1 H), 6.54 (s, 1 H), 6.33 (t, J = 7.3 Hz, 1 H), 5.66 (m, 1 H), 5.13 (d, J = 4.5 Hz, 2 H), 5.01 (s, 1 H), 5.00 (d, J = 17.5 Hz, 1 H), 4.98 (d, J = 10.5 Hz, 1 H), 4.97 (s, 1 H), 4.96 (s, 2 H), 4.80 (d, J = 6.6 Hz, 1 H), 4.72 (br, 2 H), 4.61 (d, J = 6.6 Hz, 1 H), 3.79 (s, 3 H), 3.59 (dd, J = 8.7, 2.0 Hz, 1 252 H), 3.42 (s, 3 H), 3.38 (s, 3 H), 3.37–3.31 (m, 2 H), 3.30 (s, 3 H), 2.63 (dd, J = 13.6, 5.4 Hz, 1 H), 2.39–2.22 (m, 4 H), 1.96 (m, 1 H), 1.93 (s, 3 H), 1.78 (s, 3 H), 1.73–1.57 (m, 3 H), 1.21 (m, 1 H), 1.10 (d, J = 6.9 Hz, 3 H), 0.76 (d, J = 6.7 Hz, 3 H); 13 C NMR (125 MHz, CDCl3) δ 167.6, 159.4, 159.2, 156.0, 143.2, 141.2, 141.0, 139.0, 135.5, 132.6, 129.2, 129.1, 114.9, 113.9, 113.4, 113.1, 112.3, 103.8, 97.4, 81.1, 80.8, 79.4, 77.9, 69.7, 58.8, 57.0, 56.0, 55.3, 44.7, 40.5, 36.5, 31.0, 29.2, 24.5, 19.4, 18.5, 17.5, 12.9; IR (neat): 3317, 2921, 2851, 1733, 1718, 1653, 1609, –1 1540, 1516, 1457, 1435, 1379, 1301, 1249, 1134, 1095, 1036 cm ; HRMS (ESI): m/z calculated + for C40H59N2O9 [M+H] 711.4220, found 711.4221. PMBO PMBO NH NH CCl3CONCO, then K2CO3, MeOH O MeO OPMB O 53% OR1 OH MeO MeO O O NH2 MeO 724 10 equiv DDQ 60 equiv NaHCO3 DCM/water (18:1) rt, 4 h, 67% (unoptimized) 730, R1 = PMB 732, R1= H Preparation of allylic carbamate 732: To a solution of 724 (23.7 mg, 0.032 mmol) in dry dichloromethane (5.5 mL) under inert atmosphere was added trichloroacetyl isocyanate (~8 µL, >0.064 mmol) and the resulting solution was stirred for 30 minutes. TLC (1:4 hexanes/ether) of a sample confirmed the complete consumption of the starting material. Methanol (6.6 mL) was added, followed by potassium carbonate (20.3 mg, 0.147 mmol), and the suspension was stirred for 30 minutes, time after which TLC confirmed the complete formation of a single new spot. The reaction mixture was concentrated in vacuo and purified by column chromatography (4:1 253 hexanes/ether) to afford 730 (13.2 mg, 0.017 mmol, 52.7 % yield) as a colorless oil. This material was dissolved in dichloromethane (2 mL), placed in a sealed tube, and treated with water (110 µL) and solid sodium bicarbonate (85 mg, 1.006 mmol). While stirring, a freshly prepared solution of DDQ 0.1 M in CH2Cl2 (950 µL, 0.084 mmol) was added dropwise and after 2 hours, a second batch of DDQ 0.1 M in CH2Cl2 (950 µL, 0.084 mmol) was added. TLC (4:1 diethyl ether/hexanes and 4:1 diethyl ether/EtOAc), used to monitor the progress of the reaction, confirmed the formation of one major product. The reaction mixture was concentrated, adsorbed onto silica gel, and subjected to column chromatography eluting with ether/EtOAC (99:1 → 1:1) to give mono-deprotection product 732 (7.5 mg, 0.011 mmol, 67.1 % yield) as a yellowish oil. Rf 1 = 0.30 (4:1 diethyl ether/hexanes); H NMR (500 MHz, CDCl3): δ 7.61 (br, 1 H), 7.36 (t, J = 2.0 Hz, 1 H), 7.34 (d, J = 8.75 Hz, 2 H), 6.89 (d, J = 8.75 Hz, 2 H), 6.81 (apparent s, 1 H), 6.53 (apparent s, 1 H), 6.33 (apparent t, J = 7.5 Hz, 1 H), 5.60 (m, 1 H), 5.13 (d, J = 4.5 Hz, 1 H), 5.05–4.97 (m, 2 H), 5.01 (apparent s, 1 H), 4.97 (apparent s, 1 H), 4.96 (s, 2 H), 4.74 (br, 2 H exchangeable), 3.79 (s, 3 H), 3.60 (m, 1 H), 3.42 (s, 3 H), 3.35 (m, 1 H), 3.32 (s, 3 H), 3.25 (m, 1 H), 2.63 (dd, J = 13.5, 5.5 Hz, 1 H), 2.36–2.2 (m, 4 H), 2.07 (d, J = 2.5, 1 H exchangeable), 1.96 (m, 1 H), 1.94 (s, 3 H), 1.78 (s, 3 H), 1.73–1.59 (m, 3 H), 1.17 (m, 1 H), 1.11 (d, J = 6.5 Hz, 3 H), 0.80 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 167.6, 159.2, 156.3, 156.0, 143.0, 141.2, 139.9, 139.0, 135.5, 132.7, 129.3, 129.1, 115.3, 113.9, 113.4, 113.1, 112.4, 103.7, 81.1, 80.1, 77.9, 73.3, 69.7, 58.8, 57.0, 55.3, 44.7, 40.4, 34.4, 30.8, 29.1, 24.6, 19.5, 18.7, 17.5, 13.0; + HRMS (ESI): m/z calculated for C38H55N2O8 [M+H] 667.3958, found 667.3959. 254 PMBO HO NH NH EtSH, ZnOTf O MeO OMOM O O O NH2 87% OH NH2 MeO O O MeO MeO 729 731 Preparation of ring-opened autolytimycin (731): Dry zinc triflate (14.0 mg, 0.039 mmol) was rapidly added to a vacuum-flame-dried small vial containing a solution of carbamoylation product 729 (7.0 mg, 9.85 µmol) and ethanethiol (10 µL, 0.135 mmol) in dichloromethane (0.1 mL) at 0 °C. The resulting mixture was stirred and after 5 minutes the cool bath was removed. After additional 15 minutes at room temperature, the suspension was diluted with dichloromethane (~1 mL). Saturated sodium bicarbonate (~1 mL) was added dropwise at 0 °C and the resulting mixture was filtered through celite eluting with dichloromethane. The organic phase was separated with a pipette and the aqueous layer was extracted with more dichloromethane. Combined organic layers were dried over Na2SO4 and concentrated. Purification by column chromatography eluting with ether/EtOAc (9:1) afforded ring-opened autolytimycin, 731 (4.7 mg, 8.6 µmol, 87% yield) as a clear colorless thick oil. Rf = 0.26 (4:1 20 1 diethyl ether/EtOAc); [α] D −18.2° (c 0.39, chloroform); H NMR (500 MHz, CDCl3): δ 7.76 (br, 1 H), 7.56 (s, 1 H), 7.39 (br, 1 H), 6.54 (s, 1 H), 6.42 (s, 1 H), 6.31 (t, J = 7.4 Hz, 1 H), 5.59 (m, 1 H), 5.12 (d, J = 4.4 Hz, 2 H), 5.02 (d, J = 17.3 Hz, 1 H), 5.01 (s, 1 H), 4.99 (d, J = 10.3 Hz, 1 H), 4.96 (s, 1 H), 4.91 (br, 2 H), 3.60 (dd, J = 9.2, 3.1 Hz, 1 H), 3.42 (s, 3 H), 3.36 (m, 1 H), 3.32 (s, 3 H), 3.25 (ddd, J = 10.8, 2.4, 2.4 Hz, 1 H), 2.63 (dd, J = 13.6, 5.4 Hz, 1 H), 2.36–2.18 (m, 4 H), 1.96 (m, 1 H), 1.93 (s, 3 H), 1.77 (s, 3 H), 1.74–1.58 (m, 3 H), 1.23 (br, 1 H), 1.14 (m, 255 1 H), 1.11 (d, J = 6.5 Hz, 3 H), 0.80 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 168.5, 156.9, 156.2, 143.1, 141.2, 139.9, 138.6, 136.0, 132.5, 115.3, 113.3, 112.5, 112.1, 105.1, 81.1, 80.1, 77.7, 73.4, 58.8, 57.0, 44.5, 40.4, 34.2, 30.6, 29.0, 24.6, 19.5, 18.8, 17.5, 13.0; IR (neat): 3338, 2960, 2926, 2869, 2247, 1716, 1661, 1616, 1543, 1498, 1436, 1384, 1315, 1261, 1088, –1 + 911, 864, 838, 733 cm ; HRMS (ESI): m/z calculated for C30H47N2O7 [M+H] 547.3380, found 547.3383. Ph3P I I I OPMB O3 DCM, !78 ºC then PPh3 rt, ON MeO I OPMB 771 NaHMDS THF O 0 ºC " rt then SM –78 ºC " tr 26% ( two steps, unoptimized) MeO 529 OPMB MeO 759 Preparation of relay chain-containing iodide 759: A stirring solution of iodide 529 (0.365 g, 0.844 mmol) in dichloromethane (8.5 mL) at −78 °C (dry ice/acetone bath) was saturated with ozone until a blue color persisted. Once the completion of this step was confirmed by TLC (hexanes/EtOAc 9:1) and still at −78 °C, the solution was purged with nitrogen gas until the color dissipated and triphenylphosphine (0.244 g, 0.929 mmol) was added in one portion. The solution was allowed to slowly warm up to room temperature overnight. The reaction mixture was then concentrated and dried under high vacuum. At the appropriate time, the resulting residue was redissolved in THF (2.7 mL) to be used in the next step, assuming a quantitative yield of the aldehyde. Simultaneously, to a solution of 5-hexenyltriphenylphosphonium 222 iodide (771) (574.0 mg, 1.215 mmol) in THF (1.4 mL) at 0 ºC was slowly added NaHMDS 256 1M in THF (1.1 mL, 1.1 mmol) under inert atmosphere. The resulting red solution was stirred for 2 hours at room temperature and then cooled to −78 °C. The aforementioned solution of crude aldehyde was added dropwise and the reaction mixture was allowed to slowly warm up to room temperature overnight. After 12 hours, the reaction was quenched with aqueous saturated NH4Cl (6 mL) and extracted three times with ether. Combined organic layers were washed with brine, dried over MgSO4, and concentrated. Purification by column chromatography, eluting with hexanes/EtOAc (50:1 → 7:1), afforded iodide 759 (111.7 mg, 0.223 mmol, 26.4 % yield), as a slightly yellowish oil, from the complex crude mixture. This procedure was not optimized. Rf = 1 0.21 (7:1 hexanes/ether); H NMR (500 MHz, CDCl3) δ 7.28 (d, J = 8.3 Hz, 2 H), 6.86 (d, J = 8.3 Hz, 2 H), 5.77 (ddt, J = 17.0, 11.5, 7.0 Hz, 1 H), 5.33 (ddd, J = 11.0, 7.5, 6.5 Hz, 1 H), 5.11 (m, 1 H), 4.98 (apparent d, J = 17.0 Hz, 1 H), 4.93 (apparent d, J = 11.5 Hz, 1 H), 4.77 (d, J = 10.75 Hz, 1 H), 4.46 (d, J = 10.75 Hz, 1 H), 3.78 (s, 3 H), 3.39 (dd, J = 9.0, 2.0 Hz, 1 H), 3.34 (s, 3 H), 3.31–3.26 (m, 2 H), 3.14 (dd, J = 8.5, 7.5 Hz, 1 H), 2.51 (m, 1 H), 2.10–1.92 (m, 4 H), 1.81–1.69 (m, 2 H), 1.49–1.35 (m, 2 H), 1.24 (m, 1 H), 1.02 (d, J = 6.5 Hz, 3 H), 0.92 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 159.1, 138.5, 132.4, 131.1, 129.7, 129.5, 114.7, 113.7, 81.9, 81.6, 74.0, 57.2, 55.3, 36.1, 34.5, 33.4, 31.5, 29.0, 27.2, 20.1, 19.2, 18.6; IR (neat): 3079, 3006, 2960, 2926, 2881, 1615, 1514, 1456, 1377, 1302, 1248, 1173, 1100, 1038, 911, 804 –1 + cm ; HRMS (ESI): m/z calculated for C24H38IO3 [M+H] 501.1866, found 501.1871. 257 Ph3P I I I OMOM O3 DCM, !78 ºC then PPh3 rt, ON MeO 525 I OMOM MeO 771 NaHMDS THF O 0 ºC " rt then SM –78 ºC " rt 32% ( two steps, unoptimized) OMOM MeO 760 Preparation of relay chain-containing iodide 760: A stirring solution of iodide 525 (1.764 g, 4.95 mmol) in dichloromethane (50 mL) at −78 °C (dry ice/acetone bath) was saturated with ozone until a blue color persisted. Once the completion of this step was confirmed by TLC (hexanes/EtOAc 9:1) and still at −78 °C, the solution was purged with nitrogen gas until the color dissipated and triphenylphosphine (1.429 g, 5.45 mmol) was added in one portion. The solution was allowed to slowly warm up to room temperature overnight. The reaction mixture was then concentrated and dried under high vacuum. At the appropriate time, the resulting residue was redissolved in THF (15.5 mL) to be used in the next step, assuming a quantitative yield of the aldehyde. Simultaneously, to a solution of 5-hexenyltriphenylphosphonium 222 iodide (771) (3.36 g, 7.12 mmol) in THF (8 mL) at 0 ºC was slowly added NaHMDS 1M in THF (6.4 mL, 6.4 mmol) under inert atmosphere. The resulting red solution was stirred for 2 hours at room temperature and then cooled to −78 °C. The aforementioned solution of crude aldehyde was added dropwise and the reaction mixture was allowed to slowly warm up to room temperature overnight. After 12 hours, the reaction was quenched with aqueous saturated NH4Cl (60 mL) and extracted three times with ether. Combined organic layers were washed with brine, dried over MgSO4, and concentrated. Purification by column chromatography, eluting with 258 hexanes/EtOAc (20:1 → 4:1), afforded iodide 760 (0.673 g, 1.585 mmol, 32 % yield) as a slightly yellowish oil from the complex crude mixture. This procedure was not optimized. Rf = 1 0.7 (4:1 hexanes/EtOAc); H NMR (500 MHz, CDCl3) δ 5.78 (ddt, J = 17.0, 11.5, 7.0 Hz, 1 H), 5.34 (ddd, J = 11.0, 7.5, 6.5 Hz, 1 H), 5.15 (m, 1 H), 4.99 (apparent d, J = 17.0 Hz, 1 H), 4.93 (apparent d, J = 11.5 Hz, 1 H), 4.83 (d, J = 10.75 Hz, 1 H), 4.62 (d, J = 10.75 Hz, 1 H), 3.58 (dd, J = 8.5, 1.0 Hz, 1 H), 3.41 (s, 3 H), 3.30 (s, 3 H), 3.29 (m, 1 H), 3.22 (m, 1 H), 3.14 (dd, J = 9.5, 6.0 Hz, 1 H), 2.51 (m, 1 H), 2.12–1.93 (m, 4 H), 1.71–1.63 (m, 2 H), 1.50–1.36 (m, 2 H), 1.22 (m, 1 H), 1.05 (d, J = 6.5 Hz, 3 H), 0.91 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 138.5, 132.1, 129.6, 114.7, 97.5, 81.2, 79.3, 56.9, 56.1, 36.0, 34.2, 33.4, 31.2, 29.0, 27.2, 20.0, + 19.3, 18.6; HRMS (ESI): m/z calculated for C18H34IO3 [M+H] 425.1553, found 425.1559. PMBO PMBO 1.6 equiv 759 tBuLi, ether, !78 °C then B-OMe-9-BBN, THF, !78 °C " rt NH Cl O MeO NH O then SM OTIPS 2 equiv K3PO4 nH2O 10 mol% Pd(OAc)2 20 mol% SPhos THF, reflux, 12 h 89% MeO OPMB OTIPS MeO 761 642 Preparation Suzuki product 761: To a flame-dried 25mL pear-shaped flask provided with a magnetic stirrer, containing iodide 759 (165.0 mg, 0.381 mmol) in ether (1.2 mL) at −78 °C under nitrogen atmosphere, was added tert-butyllithium 1.7 M in pentane (450 µL, 0.762 mmol) (solution remains colorless and clear) and few minutes later, B-methoxy-9- borabicyclo[3.3.1]nonane 1M in THF (900 µL, <0.904 mmol) was added in one portion (white precipitate forms), followed by THF (1.2 mL) (solution rapidly becomes clear/colorless). The 259 solution was stirred for 10 minutes more at −78 °C (white precipitate forms) and then at room temperature for 30 minutes (solution becomes clear/colorless). The solvent was blown off using nitrogen gas and a thin needle on the rubber cap. After the solvent was completely evaporated, THF (0.5 mL) was added, followed by potassium phosphate hydrate (118 mg, 0.476 mmol) and PMB-protected chloroamidophenol 642 (150.0 mg, 0.238 mmol). The mixture was freeze-pumpthaw degassed three times. A mixture of palladium(II) acetate (5.7 mg, 0.024 mmol) and SPhos (19.6 mg, 0.048 mmol) was dissolved in THF (1.2 mL). This solution was freeze-pump-thaw degassed three times and added in one portion to the first mixture causing a rapid color change to dark orange/brown. The reaction mixture was heated to reflux for 12 h. Once full conversion was confirmed by TLC (hexanes/EtOAc 9:1), the mixture was diluted with acetone and filtered through a short plug of silica gel eluting with additional acetone. After concentration, the product was purified by column chromatography eluting with hexanes/EtOAC (49:1 → 4:1) to afford Suzuki product 761 (190.9 mg, 0.212 mmol, 89% yield) as a white waxy solid. mp 46–47.5 °C. 1 Rf = 0.3 (5:1 hexanes/EtOAc); H NMR (500 MHz, CDCl3) δ 7.40 (t, J = 2 Hz, 1 H), 7.34 (d, J = 8.5 Hz, 2 H), 7.33 (br, 1 H), 7.23 (d, J = 8.5 Hz, 2 H), 6.88 (d, J = 8.5 Hz, 2 H), 6.83 (d, J = 8.5 Hz, 2 H), 6.71 (t, J = 2 Hz, 1 H), 6.56 (t, J = 2 Hz, 1 H), 6.35 (m, 1 H), 5.77 (ddt, J = 17.0, 11.5, 7.0 Hz, 1 H), 5.31 (ddd, J = 11.0, 7.5, 6.5 Hz, 1 H), 5.17 (m, 1 H), 4.98 (apparent d, J = 17.0 Hz, 1 H), 4.96 (s, 2 H), 4.95 (overlapped s, 1 H), 4.92 (apparent d, J = 11.5 Hz, 1 H), 4.89 (apparent s, 1 H), 4.74 (d, J = 11 Hz, 1 H), 4.43 (d, J = 11 Hz, 1 H), 4.31 (apparent d, J = 5.5 Hz, 1 H), 3.79 (s, 3 H), 3.76 (s, 3 H), 3.73 (m, 1 H), 3.45 (s, 3 H), 3.39 (m, 1 H), 3.33 (s, 3 H), 3.18 (m, 1 H), 2.64 (dd, J = 13.5, 5.0 Hz, 1 H), 2.51 (m, 1 H), 2.35–2.19 (m, 3 H), 2.08–1.95 (m, 4 H), 1.92 (s, 3 H), 1.86–1.77 (m, 2 H), 1.73 (s, 3 H), 1.71–1.56 (m, 2 H), 1.47–1.32 (m, 3 H), 1.08–1.02 (m, 21 H), 1.01 (d, J = 6.5 Hz, 3 H), 0.75 (d, J = 6.5 Hz, 3 H); 260 13 C NMR (125 MHz, CDCl3): δ 167.5, 159.4, 159.3, 159.1, 145.0, 143.4, 138.9, 138.6, 136.4, 132.6, 132.1, 131.2, 129.63, 129.59, 129.3, 129.2, 114.6, 113.9, 113.7, 113.0, 112.9, 112.2, 103.7, 84.3, 82.0, 81.7, 76.5, 73.9, 69.7, 58.8, 57.2, 55.28, 55.25, 44.8, 36.5, 34.5, 33.4, 31.1, 29.1, 28.9, 27.1, 25.0, 19.1, 18.7, 18.6, + 18.08, 18.05, 12.8, 12.4; HRMS (ESI): m/z calculated for C59H90NO8Si [M+H] 968.6436, found 968.6431. PMBO PMBO 1.6 equiv 760 tBuLi, ether, !78 °C then B-OMe-9-BBN, THF, !78 °C " rt NH Cl O MeO NH O MeO OMOM then SM OTIPS 2 equiv K3PO4 nH2O 10 mol% Pd(OAc)2 20 mol% SPhos THF, reflux, 12 h 77% OTIPS MeO 762 642 Preparation of Suzuki product 762: To a flame-dried 25mL pear-shaped flask provided with a magnetic stirrer, containing iodide 760 (151.0 mg, 0.357 mmol) in ether (1.2 mL) at −78 °C under nitrogen atmosphere, was added tert-butyllithium 1.7 M in pentane (420 µL, 0.714 mmol) (solution remains colorless and clear) and 3 minutes later B-methoxy-9- borabicyclo[3.3.1]nonane 1M solution in THF (850 µL, >0.847 mmol) was added (white precipitate forms), followed by THF (1.2 mL) (solution rapidly becomes clear/colorless). The solution was stirred for 10 minutes more at −78 °C (white precipitate forms) and then at room temperature for 1 hour (solution becomes clear/colorless). The solvent was blown off using a stream of nitrogen gas and a thin needle on the rubber cap. After solvent was completely evaporated, THF (0.5 mL) was added, followed by potassium phosphate hydrate (118.0 mg, 0.476 mmol) and PMB-protected chloroamidophenol (150.0 mg, 0.238 mmol). The mixture was freeze-pump-thaw degassed three times. A mixture of palladium(II) acetate (5.7 mg, 0.024 261 mmol) and SPhos (19.6 mg, 0.048 mmol) was dissolved in THF (1.2 mL). This solution was freeze-pump-thaw degassed three times and added in one portion to the first mixture causing a rapid color change to dark orange/brown. The reaction mixture was heated to reflux for 12 h. TLC (9:1 hexanes/EtOAc) revealed almost complete conversion. The mixture was diluted with acetone and filtered through silica gel (wet with acetone) eluting with more acetone. After 1 concentration, a 90% conversion was determined by H NMR. The product was purified by column, eluting with hexanes/EtOAC (49:1 → 4:1) to afford 762 (163.0 mg, 0.183 mmol, 77 % 1 yield) as a waxy solid. mp 50.5–52 °C. Rf = 0.27 (5:1 hexanes/EtOAc); H NMR (500 MHz, CDCl3) δ 7.36–7.32 (m, 4 H), 6.89 (d, J = 8.5 Hz, 2 H), 6.73 (apparent s, 1 H), 6.55 (apparent s, 1 H), 6.35 (m, 1 H), 5.78 (ddt, J = 17.0, 11.5, 7.0 Hz, 1 H), 5.32 (m, 1 H), 5.17 (m, 1 H), 4.98 (apparent d, J = 17.0 Hz, 1 H), 4.95 (s, 2 H), 4.95 (overlapped s, 1 H), 4.94 (apparent d, J = 11.5 Hz, 1 H), 4.89 (apparent s, 1 H), 4.83 (d, J = 11 Hz, 1 H), 4.61 (d, J = 11 Hz, 1 H), 4.31 (apparent d, J = 6.0 Hz, 1 H), 3.79 (s, 3 H), 3.58 (m, 1 H), 3.45 (s, 3 H), 3.39 (s, 3 H), 3.34 (m, 1 H), 3.30 (s, 3 H), 3.19 (m, 1 H), 2.65 (dd, J = 13.5, 5.0 Hz, 1 H), 2.52 (m, 1 H), 2.35–2.19 (m, 3 H), 2.11–1.93 (m, 4 H), 1.92 (s, 3 H), 1.74 (s, 3 H), 1.71–1.56 (m, 3 H), 1.48–1.31 (m, 3 H), 1.14 (m, 1 H), 1.11–0.98 (m, 24 H), 0.73 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 167.5, 159.4, 159.2, 145.0, 143.3, 138.9, 138.6, 136.4, 132.3, 132.1, 129.4, 129.3, 129.1, 114.7, 113.9, 113.0, 112.9, 112.3, 103.6, 97.5, 84.3, 81.2, 79.7, 76.5, 69.7, 58.8, 56.9, 56.1, 55.3, 44.7, 36.5, 34.2, 33.4, 31.1, 29.1, 28.9, 27.2, 25.0, 19.1, 18.6, 18.5, 18.09, 18.05, 12.8, 12.4; HRMS (ESI): + m/z calculated for C53H86NO8Si [M+H] 892.6123, found 892.6125. 262 PMBO PMBO NH NH O O TBAF MeO OMOM MeO OMOM 88% OTIPS MeO OH MeO 762 765 Preparation of RRCM substrate 765: To a pear-shaped flask containing a solution of Suzuki product 762 (68.4 mg, 0.077 mmol) in THF (2.5 mL) at 0 °C was added TBAF 1M in THF (770 µL, 0.770 mmol) dropwise. After 2 h at that temperature, TLC eluting with 3:1 hexanes/EtOAc confirmed the full consumption of the starting material. Still inside the cold bath, the reaction was quenched with 6 mL of cold aqueous saturated NaHCO3 and the mixture was stirred for 5 minutes. The layers were separated and the aqueous phase was extracted with ether three times. Combined organic layers were dried over MgSO4 and concentrated. The crude material was subjected to column chromatography eluting with hexanes/ether (3:7 → 3:17) (Rf = 0.18 → 0.4) to afford 765 (49.6 mg, 0.067 mmol, 88% yield) as a waxy solid after high vacuum. mp 53–55.5 1 °C; Rf = 0.4 (3:17 hexanes/ether); H NMR (500 MHz, CDCl3) δ 7.37–7.32 (m, 4 H), 6.89 (d, J = 8.5 Hz, 2 H), 6.74 (apparent s, 1 H), 6.55 (apparent s, 1 H), 6.38 (m, 1 H), 5.78 (ddt, J = 17.0, 11.5, 7.0 Hz, 1 H), 5.32 (m, 1 H), 5.17 (m, 1 H), 5.03 (apparent s, 1 H), 4.99 (apparent d, J = 17.0 Hz, 1 H), 4.95 (s, 2 H), 4.946 (overlapped s, 1 H), 4.94 (apparent d, J = 11.5 Hz, 1 H), 4.83 (d, J = 11 Hz, 1 H), 4.61 (d, J = 11 Hz, 1 H), 3.97 (m, 1 H), 3.79 (s, 3 H), 3.58 (m, 1 H), 3.45 (s, 3 H), 3.39 (s, 3 H), 3.29 (s, 3 H), 3.30–3.19 (m, 2 H), 2.65 (dd, J = 13.5, 5.0 Hz, 1 H), 2.55–2.48 (m, 2 H), 2.30–2.19 (m, 3 H), 2.11–1.95 (m, 4 H), 1.93 (s, 3 H), 1.75 (s, 3 H), 1.73–1.57 (m, 4 263 H), 1.43 (m, 1H), 1.13 (m, 1 H), 1.05 (d, J = 6.5 Hz, 3 H), 0.73 (d, J = 6.5 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 167.3, 159.4, 159.2, 144.2, 143.3, 138.8, 138.6, 136.0, 132.3, 132.1, 129.4, 129.3, 129.0, 114.7, 114.0, 113.9, 113.0, 112.3, 103.6, 97.5, 81.6, 81.2, 79.6, 77.1, 69.7, 58.3, 57.0, 56.1, 55.3, 44.7, 36.4, 34.2, 33.4, 31.0, 29.1, 28.9, 27.1, 23.9, 18.6, 18.5, 17.9, 12.8; HRMS + (ESI): m/z calculated for C44H66NO8 [M+H] 736.4788, found 736.4792. O MeO O Bn O Bn N O O O 1.2 equiv Bu2BOTf 1.32 equiv NEt3 DCM, 0 ºC, 40 min Bn N Bu2BO N OTIPS O 506 DCM, !78 ºC 80% O O HO MeO OTIPS 779 780 Preparation of aldol product 780: A solution of (S)-4-benzyl-3-propionyloxazolidin-2-one (779) 213 (142 mg, 0.609 mmol, 1 equiv) in dichloromethane (2 mL) was cooled below 0 °C using an ice/water/salt bath. Dibutylboryl triflate 1M solution in dichloromethane (730 µL, 0.730 mmol, 1.2 equiv) was added dropwise, followed by triethylamine (113 µL, 0.804 mmol, 1.32 equiv). The bright yellow solution was stirred at 0 °C, on an ice/water bath, for 40 minutes and then cooled down to −78 °C on an acetone/dry ice bath. Then, a solution of aldehyde 506 (200 mg, 0.609 mmol) in dichloromethane (0.4 mL) was added dropwise. The reaction mixture was stirred at −78 °C for 1.5 h after which it was slowly warmed up to −50 °C and stirred for 1.5 h; then again it was slowly warmed up to 0 °C and stirred for another 1.5 h. At this point, the reaction was carefully quenched with pH 7 buffer and MeOH (1:3) (0.6 mL), followed by 30% 264 H2O2 and MeOH (1:2) (1.8 mL) added dropwise. The cloudy mixture was stirred at 0 °C for 1 h and then warmed up to room temperature. The volatile material was removed in vacuo and the residue was extracted with dichloromethane. Combined organic layers were washed with brine, dried over MgSO4, filtered and concentrated. Column chromatography eluting with hexanes/EtOAc (7:1 → 4:1) afforded 780 (272.1 mg, 0.484 mmol, 80% yield) as a clear 20 1 colorless oil. Rf = 0.11 (7:1 hexanes/ethyl acetate); [α] D +20.7° (c 2.039, CHCl3); H NMR (500 MHz, CDCl3): δ 7.33–7.29 (m, 2 H), 7.28–7.25 (m, 1 H), 7.20–7.17 (m, 2 H), 4.92 (s, 1 H), 4.86 (s, 1 H), 4.67 (dddd, J = 9.5, 7.5, 3.5, 3.0 Hz, 1 H), 4.24 (d, J = 6.25 Hz, 1 H), 4.19 and 4.16 3 (d of ABq, J = 7.5 and 3.0 Hz, JAB = 9.0 Hz, 2 H), 3.88 (m, 1 H), 3.74 (ddd, J = 14.0, 7.0, 3.0 3 Hz, 1 H), 3.44 (s, 3 H), 3.23 and 2.76 (d of ABq, J = 3.5 and 9.5 Hz, JAB = 13.25 Hz, 2 H), 3.17 (ddd, J = 8.5, 6.25, 3.0 Hz, 1 H), 3.07 (br, 1 H), 1.76–1.69 (m, 1 H), 1.71 (s, 3 H), 1.56 (m, 2 H), 1.28–1.20 (m, 1 H), 1.24 (d, J = 7.0 Hz, 3 H), 1.07–1.02 (m, 21 H); 13 C NMR (125 MHz, CDCl3): δ 177.3, 153.0, 145.1, 135.1, 129.4, 128.9, 127.4, 113.1, 84.7, 77.6, 71.9, 66.1, 58.9, 55.1, 42.3, 37.8, 30.2, 26.8, 18.6, 18.1, 18.0, 12.5, 10.7; IR (neat): 3530, 2942, 2869, 1784, 1696, –1 1456, 1385, 1210, 1100, 884, 806 cm ; HRMS (ESI): m/z calculated for C31H51NO6SiNa + [M+Na] 584.3383, found 584.3384. Al NH3 Al NH2 781 265 Preparation of dimethylaluminum amide (781): 215a The glassware set-up for this reaction was first assembled and properly flame/vacuum-dried. A solution consisting of trimethylaluminum 25% in hexane (35 mL) and dry dichloromethane (25 mL) was placed in a 100mL three-necked round-bottomed flask. This flask was equipped with a dry ice condenser and a nitrogen gas inlet and was cooled in a dry ice/acetone bath. In another 100mL three-necked flask equipped with a dry ice condenser and a nitrogen gas inlet, were placed a few pieces of sodium metal. Both flasks were connected through a piece of plastic tubing, using flow controllers. The flask containing sodium was also cooled in the same dry ice-acetone bath and ammonia gas was then introduced. When more than 10 mL of the blue liquid ammonia solution accumulated in the flask, the addition of gas was stopped and the stopcocks between the liquid ammonia flask and the trimethylaluminum solution were opened to allow the dried ammonia gas to slowly distill into the reaction flask. When half the addition of ammonia was complete, the cooling bath was removed. Once the addition was complete, dry ice was no longer added to the condenser the mixture was allowed to stir at room temperature overnight under a nitrogen atmosphere until no more gas evolved. This reagent was immediately stored in the freezer as obtained. According to Weinreb, 223 the solution of 781 made by this procedure has a concentration of approximately 1.2 M and lasts for 2 weeks. O Bn N O HO MeO O NH2 Me2AlNH2 781 87% O O HO MeO OTIPS + Bn N H OTIPS 780 782 266 783 O Preparation of amide 782: To aldol product 780 (800 mg, 1.424 mmol) was added trimethylaluminum amide 1.2 M solution (3 mL, >3.56 mmol). The resulting homogeneous solution was stirred at room temperature for 7 hours. TLC (ether) showed that the reaction was complete. The reaction mixture was poured into a separatory funnel containing 2 mL of ice, 2 mL 1.0 M HCl, and 2 mL of ether. When methane production ceased the layers were separated, and the aqueous layer was extracted with two additional 4mL portions of ether, and two 4mL portions of ethyl acetate. Combined organic layers were washed with brine, dried over MgSO4, filtered, and concentrated. Column chromatography eluting with hexanes/EtOAc (1:1 → 1:2) afforded recovered chiral auxiliary (244.9 mg, 1.381 mmol, 97% yield), Rf = 0.3 (1:1 hexanes/ethyl acetate) and amide 782 (499.1 mg, 1.242 mmol, 87% yield) Rf = 0.08 (1:1 hexanes/ethyl acetate) as a colorless thick oil that solidified upon standing. mp 103–104 ºC; 20 1 [α] D −0.5° (c 1.182, CHCl3); H NMR (500 MHz, CDCl3): δ 6.18 (br, 1 H), 5.25 (br, 1 H), 4.92 (s, 1 H), 4.87 (s, 1 H), 4.29 (d, J = 6.5 Hz, 1 H), 3.78 (m, 1 H), 3.69 (d, J = 3.0 Hz, 1 H, exchangeable), 3.45 (s, 3 H), 3.21 (ddd, J = 8.0, 6.5, 3.0 Hz, 1 H), 2.42 (ddd, J = 8.5, 7.25, 3.0 Hz, 1 H), 1.76–1.69 (m, 1 H), 1.71 (s, 3 H), 1.56–1.50 (m, 2 H), 1.30 (m, 1 H), 1.15 (d, J = 7.0 Hz, 3 H), 1.07–1.01 (m, 21 H); 13 C NMR (125 MHz, CDCl3): δ 178.1, 144.8, 113.3, 84.7, 76.9, 72.2, 58.7, 44.6, 29.5, 26.4, 18.6, 18.1, 18.0, 12.4, 11.6; IR (neat): 3345, 3177, 2926, 2867, 1684, –1 1636, 1458, 1420, 1373, 1253, 1094, 1063, 883, 822 cm ; HRMS (ESI): m/z calculated for + C21H44NO4SiH [M+H] 402.3040, found 402.3042. 267 OPMB OPMB O O Cl H2N HO MeO 782 Br Cl N H HO MeO 512 Cs2CO3 cat. Pd2dba3, xantphos dioxane, 100 ºC, 3.5 h OTIPS (77 % conversion) 64 % (unoptimized) OTIPS 784 84 Preparation of amide 784 : Inside a glovebox, Pd2dba3 (21.0 mg, 0.023 mmol), XantPhos (39.8 mg, 0.069 mmol), anhydrous cesium carbonate (262.0 mg, 0.803 mmol), and 1,4-dioxane (1.15 mL) were premixed and stirred in an air-free flask for about 10 minutes. o-PMBbromochlorophenol 512 (225.0 mg, 0.688 mmol) was added followed by amide 782 (230.3 mg, 0.573 mmol). The flask was sealed, taken out of the glovebox, and heated in an oil bath at 100 1 °C for 3.5 hours. The progress of the reaction was monitored by H NMR analysis of worked-up samples (final yield was affected). No significant change was observed between samples taken after 2.5 hours (72% conversion) and 3.5 hours (77% conversion). The reaction was stopped at this point because of the risk of formation of an oxidation product (see page 151 for details). The mixture was cooled to room temperature, filtered through a short plug of celite eluting with ethyl acetate, and concentrated. The crude material was subjected to column chromatography eluting with hexanes/EtOAc (1:1 → 1:2) to afford amide 784 (239.3 mg, 0.369 mmol, 64% yield discounting the weight of ether present in the isolated material) and amide 782 (42.1 mg, 18% 20 recovered starting material). Rf (784)= 0.27 (3:1 hexanes/EtOAc); [α] D +14.3° (c 0.56, ether); 1 H NMR (500 MHz, CDCl3): δ 8.61 (br, 1 H), 7.31 (d, J = 8.5 Hz, 2 H), 7.28 (t, J = 1.7 Hz, 1 H), 7.04 (t, J = 1.7 Hz, 1 H), 6.89 (d, J = 8.5 Hz, 2 H), 6.67 (t, J = 1.7 Hz, 1 H), 4.94 (s, 2 H), 268 4.87 (apparent s, 1 H), 4.84 (apparent s, 1 H), 4.30 (d, J = 6.7 Hz, 1 H), 4.17 (br, 1 H), 3.81 (m, 1 H), 3.80 (s, 3 H), 3.48 (s, 3 H), 3.24 (td, J = 6.7, 2.9 Hz, 1 H), 2.61 (qd, J = 7.1, 2.9 Hz, 1 H), 1.73 (m, 1 H), 1.70 (s, 3 H), 1.54 (m, 1 H), 1.42 (m, 1 H), 1.18 (d, J = 7.1 Hz, 3 H), 1.11–0.97 (m, 21 H); 13 C NMR (125 MHz, CDCl3): δ 173.4, 159.9, 159.6, 144.5, 140.1, 134.9, 129.3, 128.5, 114.0, 113.7, 112.2, 110.9, 104.6, 84.7, 76.7, 72.8, 70.1, 58.6, 55.3, 46.1, 28.9, 26.2, 18.4, 18.1, 18.0, 12.4, 11.9; IR (neat): 3415, 3310, 2941, 2865, 1669, 1591, 1542, 1516, 1458, 1377, –1 1302, 1251, 1201, 1173, 1011, 1036, 884, 824 cm ; HRMS (ESI): m/z calculated for + C35H55ClNO6Si [M+H] 648.3487, found 648.3490. I 1.6 equiv OPMB PMBO MeO NH Cl O HO MeO 529 3.2 equiv tBuLi, ether, !78 °C then 3.8 equiv B-OMe-9-BBN, THF !78 °C " rt OTIPS 784 PMBO evaporation, then SM 2 equiv K3PO4 nH2O 15 mol% Pd(OAc)2 30 mol% SPhos THF, reflux, 12 h NH O HO OPMB MeO OR MeO TBAF, THF, 0 ºC ~30% (two steps) 786, R = TIPS 787, R= H Preparation of Suzuki product 786: To a flame-dried 10 mL pear-shaped flask, provided with a magnetic stirrer, containing iodide 529 (45.1 mg, 0.104 mmol) (dried in a vacuum desiccator over drierite overnight) in ether (0.4 mL) at –78 °C under nitrogen atmosphere, was added tertbutyllithium 1.7 M in pentane (125 µL, 0.208 mmol) (solution remains colorless and clear). A few minutes later, B-methoxy-9-borabicyclo[3.3.1]nonane 1M in THF (0.25 mL, 0.25 mmol) was added (white precipitate forms) followed by THF (0.35 mL) (solution becomes 269 clear/colorless rapidly). The solution was stirred for 10 minutes more at –78 °C (white precipitae forms) and then at room temperature for 30 minutes (solution becomes clear/colorless again). The solution was transferred via cannula to a small air-free flask containing potassium phosphate hydrate (0.032 g, 0.130 mmol) and aryl chloride partner 784 (0.042 g, 0.065 mmol). The solvent was blown off using nitrogen and a thin needle on the rubber cap. After the solvent was evaporared, THF (200 µL) was added. The resulting mixture was freeze-pump-thaw degassed (0.01 torr) three times. In a small vial, a mixture of palladium(II) acetate (2.2 mg, 9.75 µmol) and SPhos (8.0 mg, 20 µmol) was dissolved in THF (0.4 mL). The vial was connected to the Schlenk line and the solution was freeze-pump-thaw degassed (0.01 torr) three times. The solution of catalyst was added quickly to the first mixture via syringe causing a sudden color change to dark orange-brown. The air-free flask was sealed under nitrogen atmosphere and the reaction mixture was refluxed for 12 hours. Once full conversion was confirmed by TLC (3:1 hexanes/EtOAc), the mixture was diluted with acetone and filtered through a short pad of silica gel eluting with acetone. After concentration of the filtrate, the residue was subjected to column chromatography eluting with hexanes/EtOAc (49:1 → 3:1). Fractions containing the main spot at Rf = 0.29 (4:1 hexanes/EtOAc) were combined and concentrated to afford the desired product 786 contaminated with an unidentified closely related compound in a 4:1 molar ratio. This material (~30 mg) was used in the following step without further purification. Preparation of RCM substrate 787: Approximately half of the impure Suzuki product 786 obtained in the previous step, dissolved in THF (1.1 mL), was treated at 0 ºC with TBAF 1M in THF (325 µL, 0.325 mmol), added dropwise. After 1.5 hours, TLC (1:1 DCM/ether) showed that the SM was fully consumed. The reaction was quenched with 2 mL of saturated sodium bicarbonate and extracted with ethyl acetate several times. Combined organic layers were dried 270 over MgSO4 and concentrated. Column chromatography eluting DCM/ether (2:1 → 1:2) afforded 787 (7.3 mg, 0.0095 mmol, 29.5 % yield) as a colorless thick oil. Rf (787) = 0.17 (1:1 20 1 DCM/ether); [α] D +1.7° (c 0.32, ether); H NMR (500 MHz, CDCl3): δ 8.01 (br, 1 H), 7.33 (d, J = 9.0 Hz, 2 H), 7.32 (t, J = 1.7 Hz, 1 H), 7.23 (d, J = 8.5 Hz, 2 H), 6.88 (d, J = 8.5 Hz, 2 H), 6.83 (d, J = 9.0 Hz, 2 H), 6.71 (t, J = 1.7 Hz, 1 H), 6.55 (t, J = 1.7 Hz, 1 H), 5.66 (m, 1 H), 5.02– 4.95 (m, 2 H), 4.85 (apparent s, 1 H), 4.94 (s, 2 H), 4.90 (apparent s, 1 H), 4.70 (d, J = 10.5 Hz, 1 H), 4.44 (d, J = 10.5 Hz, 2 H), 4.00 (m, 1 H), 3.85 (m, 1 H), 3.79 (s, 3 H), 3.77 (s, 3 H), 3.46 (s, 3 H), 3.41–3.35 (m, 2 H), 3.32 (s, 3 H), 3.32–3.28 (m, 1 H), 2.62 (dd, J = 13.3, 5.0 Hz, 1 H), 2.53– 2.48 (m, 2 H), 2.28–2.22 (m, 2 H), 1.97 (br, 1 H), 1.76 (m, 1 H), 1.72 (s, 3 H), 1.62–1.48 (m, 4 H), 1.25–1.22 (m, 1 H), 1.22 (d, J = 7.5 Hz, 3 H), 1.07 (d, J = 7.5 Hz, 3 H), 0.76 (d, J = 7.1 Hz, 3 H); 13 C NMR (125 MHz, CDCl3): δ 173.7, 159.4, 159.3, 159.0, 144.0, 143.5, 141.3, 138.7, 131.1, 129.6, 129.3, 129.1, 114.7, 114.3, 113.9, 113.7, 113.0, 112.1, 103.8, 82.2, 81.5, 81.3, 77.2, 73.8, 72.8, 69.8, 58.5, 57.2, 55.29, 55.27, 46.1, 40.9, 36.6, 31.2, 30.3, 28.4, 26.8, 18.7, 17.7, 17.5, 11.6; IR (neat): 3311, 2953, 2925, 2869, 2852, 1653, 1615, 1558, 1541, 1516, 1457, 1375, 1249, –1 1172, 1146, 1096, 1036, 822 cm ; HRMS (ESI): m/z calculated for C45H64NO9 [M+H] 762.4581, found 762.4588. 271 + BIBLIOGRAPHY 272 BIBLIOGRAPHY 1. 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