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LEWIS ACID MEDIATED IN SITU GENERATION OF MUNCHNONES AIMED AT
DIVERSITY ORIENTED SYNTHESIS OF NITROGEN CONTAINING
DIHYDROHETEROCYCLES
By

Satyamaheshwar Peddibhotla

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Department of Chemistry

2004

ABSTRACT
LEWIS ACID MEDIATED IN SITU GENERATION OF MUNCHNONES
AIMED AT DIVERSITY ORIENTED SYNTHESIS OF NITROGEN
CONTAINING DIHYDROHETEROCYCLES
By

Satyamaheshwar Peddibhotla

This dissertation describes the development of Lewis acid mediated/catalyzed
generation of mesionic 5-oxazolones (mfinchnones) from amino acid derived
azlactones. Milnchnones are highly reactive cyclic azomethine ylides, a class of 1,
3-dipoles used extensively for generation of five-membered nitrogen containing
heterocycles. Our mild method allows for highly stereoselective cycloadditions
with different dipolarophiles. Furthermore, it allows isolation of
dihydroheterocyclic scaffolds without problems of decarboxylation, aromatization
and isomerizations normally encountered in cycloadditions of traditionally used
N-alkylated mitnchnones.

The generation of diverse small molecular scaffolds with potential biological
activity is an ongoing theme in our group. Five-membered nitrogen containing
rings are at the core of many natural products and prospective libraries of drug
discovery programs. On account of the structural rigidity and the three
dimensional disposition of functional groups, dihydroheterocyclic five-membered
rings that we have accessed are perfectly suited for the aim of drug discovery.
They have a different shape profile from aromatic heterocycles generally
encountered in drug discovery programs and as such might interact with entirely

new therapeutic targets. Chapter I lays a strong foundation for the need of such

molecules. Chapter II includes a survey of azlactones as a viable template for
diversity oriented transformations. However, stereochemical diversity has not
been efficiently achieved from azlactones. Stereochemical diversity is extremely
important in screening for new ligands which can display functional groups for
interactions with macromolecules that govern life as well as disease. Our
proposed route to dihydroheterocycles from milnchnones is discussed in the
context of other stereoselective methods that employ lewis acids in l, 3-dipolar
cycloadditions of closely related nitrogen ylides.

Discovery of mild in situ method of generation of mtinchnones (cyclic
azomethine ylides) from azlactones using Lewis acids and their use for
stereoselective synthesis of nitrogen containing dihydroheterocycles; 2-
imidazolines and Al-pyrrolines are described in Chapter III and IV respectively.
The scaffolds are obtained with high diastereoselectivity (stereochemical) and are
amenable to diverse substitution (appendage diversity) and in many ways
complement other methods towards these scaffolds. The dihydroheterocyclic
scafi‘olds offer unexplored territory in terms of their structure (3-D disposition of
functional groups) and their function (biological activity and catalysis). Chapter V
describes selective and enhanced toxicity of the combination of imidazolines with
chemotherapeutic drugs (camptothecin and cis-platin) in cancer cells. The
mechanism and potential of this novel effect is being studied by other researchers
in the group. The imidazoline scaffolds can catalyze a novel stereoselective
reaction of ketenes with N-acylimines. We have also explored their use as analogs

of amino alcohol ligands for diethyl zinc additions to electrophiles.

To

Vasudha

iv

ACKNOWLEDGEMENTS

This dissertation would be incomplete without the mention of all those
who contributed to my successful graduate career. Being his first student to
graduate, both J etze and I have gone through times of learning and teaching. Jetze
gave me a lot of independence to explore my own ideas, through out my graduate
career in this group. His confidence in me has helped me achieve goals that might
have seemed out of reach at that point of time. Whether I was lucky for his career
or he for mine is a topic that is feverishly discussed from time to time. Time
probably will be a better judge.

I would like to thank Profs. Borhan, Wulff, and Dellapena for serving on
my committee and their advise on several occasions. In fact, Babak (Dr. Borhan)
is more of a fiend than a professor to me. I admire his ability to be able to ask
questions ranging from specific to the more philosophical ones. I have great
respect for Prof. Wulff, especially his composure and how he deals with people
irrespective of who they are. Despite of his busy schedule Prof. Dellapenna has
been extremely kind, accommodating and understanding. We had a very
productive association while working on the biosynthesis of vitamin E. I want to
thank Prof Hart for a summer fellowship in recognition of my graduate work. I
also want to thank Prof. Maleczka for help with different things at different times
and mainly for CEM 852.

All the past and present group members have been good friends. .I in and
Jayakurnar were good work colleagues. I enjoyed their company working late

hours in the lab. They kept me in good humor through good and bad times.

I especially want to thank Dr. Rui Huang for his expertise on X-ray
crystallography. The confidence with which we could talk about the
stereochemical issues throughout this work relied on the X-ray structures that Dr.
Huang solved. Dr. Long Le, Dr. Kermit Johnson and Dr. Daniel Holmes have
been helpful with the NMR spectroscopy. Nancy Lavrik, was ever helpful and
kind soul and you can only hope to find more people like her through life.

I would like to thank Sundar, Radha, RG, Pops, Malini, Manish, Param
Edith, Mapitso, Ben and Chryssoula; friends who enriched my experience with
help, support and encouragements to difi‘erent degrees and at different times
during the last few years.

I am grateful for the support of my parents and siblings, who have been
crucial to my success. Their love and faith has helped me overcome all hurdles
and pursue my goals tirelessly.

Finally, I am grateful to god that I have Vasudha as the love of my life.
Those long hours of lab work were made easy in her company. I am really
fortunate that I can discuss chemistry and biology with Vasudha with the same
ease as personal issues. She is a great critic as well as someone who encourages
and helps all the way to the end. I can only hope that I have learnt from her
“Never say I give up” attitude towards work and everything she does. I am really
grateful to her for proof reading and help with formatting of this dissertation and

the defense.

vi

TABLE OF CONTENTS
(Images in this dissertation are presented in color)

List of Tables ....................................................................... xi
List of Schemes .................................................................... xii
List of Figures ..................................................................... xv
Key to Symbols and Abbreviations ............................................. xix
CHAPTER 1
Small Heterocyclic Scaffolds and Drug Discovery ............................. l
A. Drug discovery process ................................................ 1
B. A small molecule revolution .......................................... 2
C. Chemical genetics brings small molecules center stage .......... 4
D. Target-oriented synthesis (TOS) (Trial and Error method). . .7
E. Combinatorial chemistry (Trial and Selection method) ........... 9
F. Number game (HTS) vs. quality of libraries ....................... 12
G. Diversity deficit in combinatorial libraries vs. natural
products ...................................................................................... 16
H. Rescuing Combinational Chemistry ................................. 19
1. Chemical diversity and biological space ............................. 22
J. Diversity oriented synthesis(DOS) ............................................. 25
1) Appendage Diversity ........................................... 28
2) Stereochemical Diversity ...................................... 28
3) Skeletal Diversity ............................................... 29
i. Differentiation ........................................... 29
ii. folding ................................................... 39
4) Integrated Forward-Synthetic Analysis for Generating
Complexity and Diversity ................................................. 42
L. Small molecule heterocyclic scaffolds .................................... 43
M. Conformational constraint/rigid scaffolds ................................... 45
N. 1, 3-dipolar cycloadditions and confonnationally
constrained heterocycles ............................................................... 45
0. References ..................................................................................... 48
CHAPTER II
Azlactones to novel dihydroheterocyclic scaffolds ....................................... 56
A. Introduction ................................................................................... 56
1) Easily accessible starting materials .................................... 57
2) Pluripotency ....................................................................... 59
3) Skeletal diversity through azlactones ................................ 61
i. Differentiation/ Intermolecular reactions ................ 61
ii. Folding / Intramolecular reactions ........................ 63

vii

B. 1, 3-Dipolar cycloadditions and stereochemical diversity ............. 66

C. Lack of stereochemical diversity in milnchnone cycloadditions...67
D. Potential of Lewis acid mediated generation of mtlnchnones &

synthesis of novel dihydroheterocyclic scaffolds ......................... 70
E. Lewis acid mediated cycloadditions of nitrile ylides &
azomethine ylides .......................................................................... 74
F. Nitrile ylides and Lewis acids ........................................................ 75
1) Synthesis of 2-oxazolines ................................................... 75
2) Synthesis of Az-pyrrolines .................................................. 76
3) Synthesis of 2-imidazolines ................................................ 77
G. Azomethine ylides and Lewis acids .............................................. 78
1) Synthesis of 2-imidazolines ................................................ 78
2) Synthesis of pyrrolidines .................................................... 79
3) Synthesis of dihydroheterocycles ....................................... 83
H. Current work .................................................................................. 84
1. Experimental Section ...................................................................... 86
J. References ....................................................................................... 93
CHAPTER III
Silicon mediated diastereoselective multi-component synthesis of
imidazolines ..................................................................................................... 99
A. Biological Importance of 2-Imidazolines and Imidazoles .............. 99
B. Chemical significance of imidazolines ........................................... 103
C. Lewis acid mediated synthesis of imidazolines .............................. 104
D. Lewis acids for in situ generation of milnchnones and
Cycloadditions ................................................................................ 108
E. Possible mechanism for synthesis of imidazoline ........................... 112
F. Diversity .......................................................................................... 1 16
G. Proposed origin of Diastereoselectivity .......................................... 119
H. Synthetic methods for chiral imidazolines ...................................... 121
1) Resolution of enantiomers ................................................... 121
2) Use of Chiral auxillaries ...................................................... 123
i. Chiral acid chlorides ................................................. 123
ii. Chiral amino acid derived R1 .................................. 124
iii. Chiral amines as auxillaries .................................... 125
1. Synthesis of diamino acids and other scaffolds ............................... 127
1) 2-unsubstituted imidazolines ............................................... 128
2) 2-trifluoromethylimidazolines ............................................. 128
3) 2-amino-5-oxazolones ......................................................... 129
J. Ring opening of 2-phenyl imidazolines ........................................... 130
K. Synthesis of Imidazoles from 2-imidazoline-4-carboxylic acids...132
L. Experimental Procedure .................................................................. 134
M. References ...................................................................................... 160

viii

CHAPTER IV
Silver acetate catalyzed exo-selective synthesis of Al-pyrrolines and derived

pyrrolidines ..................................................................................................... 165
A. Biological and chemical significance of A'pyrrolines .................. 165
B. Access to Al-pyrrolines using mttnchnone-alkene

Cycloadditions .............................................................................. 166
C. TMSCI (Lewis acid) mediated mild protocol to A'-pyrroline-5-

carboxylates .................................................................................. 169
D. Screening of additional Lewis acid for synthesis of

AI -pyrrolines ................................................................................. 1 73
E. Diversity in AgOAc catalyzed synthesis of Al-pyrroline-S-

carboxylates .................................................................................. 175
F. Probable origin of diastereoselectivity ......................................... 179
G. Expanding the diversity of A'pyrroline templates ........................ 182
H. Synthesis of pyrrolidines and DOS .............................................. 183
1. Importance of diastereoselective diversity-generating

pyrrolidine syntheses .................................................................... 184
J. Extension of diastereocontrol and synthesis of exo pyrrolidines..187
K. Enantiocontrol in AI-pyrroline synthesis ...................................... 190
L. Experimental Section .................................................................... 191
M. References .................................................................................... 206

CHAPTER V

Applications of Imidazoline Scaffolds in Therapy and Organocatalysis ....... 211
A. Sensitization of Tumor Cells towards Chemotherapy ................. 211

1) Enhancing the activity of Camptothecin by Novel

Imidazolines ..................................................................... 21 1
2) Chemotherapy and chemoresistance — camptothecin

& cis-platin ....................................................................... 212
3) NF-KB, the master key to cell growth,

differentiation & apoptosis ............................................... 213
4) NF -KB as a target for drug design .................................... 215
5) Imidazoles, kinase inhibition and NF «B ........................ 216
6) From Imidazoles to Imidazolines ..................................... 218
7) Sensitization of leukemia cells toward CPT .................... 218

i. Imidazolines and Camptothecin ................................. 218

ii. Inhibition of camptothecin induced NF-KB -DNA

binding by imidazoline l ............................................ 220

iii. Effect of imidazoline l on I-xBa phosphorylation ..... 223

8) Significance ...................................................................... 225

ix

B. Catalytic Asymmetric Synthesis of 4, 5-dihydro-1, 3-oxazin
-6-ones. Precursors to 0., B-dialkyl-B-amino acids using

imidazoline-4-amides as novel organocatalysts .................................. 227
1) Introduction to B-amino acids ...................................... 227
2) Nucleophilic catalysts and stereocontrol in reaction
of ketenes and imines ................................................... 228
3) Imidazolines as nucleophillic catalysts ........................ 231
4) Preparation of chiral imidazoline-4-amides
catalysts ........................................................................ 234
5) Three component synthesis of 4, 5-dihydro-1, 3-
oxazin-6-ones ............................................................... 235
6) Problems ....................................................................... 239
7) Modifications ............................................................... 240
8) Three component to a two-component
synthesis of dihydroxozinones ..................................... 241
C. Chiral 4-hydroxymethyl Imidazolines as ligands for diethyl
zinc addition ......................................................................................... 244
1) Introduction .................................................................. 244
2) Imidazolines as N-heterocyclic carbine ligands ........... 244
3) Imidazolines as nitrogen ligands .................................. 245
i. 2-hydroxymethylimidazolines ligands for
diethyl zinc additions to aldehydes ............................. 248
ii. Our 4-hydroxymethylimidazolines as ligands
for diethylzinc additions ............................................. 250
D. Experimental Section ..................................................................... 255
E. References ..................................................................................... 271
APPENDIX .................................................................................................... 281

Efficient two-step synthesis of methylphytylbenzo quinones:
precursor intermediates in the biosynthesis of vitamin E .................... 282

LIST OF TABLES
(Images in this dissertation are presented in color)

Table III-1. Variation of R3 and R4 groups .................................................... 118
TableIII-2. Diversity of various groups on the imidazoline scaffold ............. 119
Table IV-l. TMSCI mediated synthesis of Al-pyrroline ................................ 172
Table IV~2. Lewis acid screen for synthesis of A'pyrrolines ......................... 174
Table IV-3. Cycloaddition of various alkenes ................................................ 176
Table IV-4. Variation of substituents at C2 and C5 in Al-pyrrolines ............... 179

Table V-l . Synthesis of dihydroxazinones using imidazoline based
catalysts. ......................................................................................................... 237

Table V- 2. Chiral 4-hydroxymethy imidazoline ligands in diethyl
zinc additions .................................................................................................. 254

xi

LIST OF SCHEMES
(Images in this dissertation are presented in color)

Scheme [-1. Skeletal diversity from isoquinoline intermediate on solid phase....33
Scheme [-2. Squaric acid as a versatile precursor to various compound shapes..34

Scheme I-3. Allenes and transition metals: a diverging approach to

Heterocycles ........................................................................................................ 35
Scheme [-4. A combinatorial scaffold approach based upon a multi-component
reaction ................................................................................................................. 36
Scheme [-5. Diversity-oriented synthesis of azaspirocycles ................................ 37

Scheme 1-6. Cross-conjugated trienes for diene-transmissive cycloadditions ..... 37

Scheme [-7. Skeletal diversity via a branched pathway ....................................... 38
Scheme I-8. A skeletal diversity generating folding process ............................... 41
Scheme 1-9. Generation of skeletal complexity and diversity ............................. 43
Scheme 11-] . Synthesis of dihydroheterocycles from milnchnones ..................... 71

Scheme II-2. Lewis acid mediated insitu generation of mtlnchnones from
azlactones l, 3 dipolar cycloaddition with dipolarophile .................................... 85

Scheme III-1. Addition of the tosylimines leads to loss of C02 and PhSOzH
to gain aromaticity ............................................................................................... 1

Scheme III-2. Staudinger reaction imines and ketenes generated from

Azlactones ........................................................................................................... 106
Scheme III-3. Lewis acid mediated generation of mttnchnone

& cycloadditions ................................................................................................. 107
Scheme III-4. Lewis acid screen for the synthesis of Imidazolines ................... 112

Scheme III-5. Nitrilium ion intermediate is not involved in the formation
of lrnidazolines .................................................................................................... 113

Scheme III-6. Generation of azomethine ylides by silatropy .............................. 114

Scheme III-7. Proposed mechanism for the formation of imidazolines via the

xii

1, 3 dipolar cycloadditions .................................................................................. 115

Scheme III-8. Origin of Diastereoselectivity ...................................................... 120

Scheme III-9. Resolution of racemic imidazolines using (-) Menthol ................ 122
Scheme III-10. Synthesis of racemic imidazoline III-2-2, and (-) S, S- III-2-l

and (+) R. R- [11-2-1 ............................................................................................ 124
Scheme III-11. Asymmetric induction by chiral acid chloride ........................... 125

Scheme III-12. Amino acid at R. position as a chiral template .......................... 125

Scheme III- 13. Use of chiral sulfinimine in milnchnone cycloaddition ............ 127

Scheme III-14. Extension of methodology for other templates .......................... 128

Scheme III-15. Synthesis of 2-aminoazlactones and 2-aminoimidazolines ........ 130
Scheme III-16. Tautomers of 2-arninoazlactones ............................................... 130
Scheme III-17. Reductive ring opening of imidazolines to diamino acids ......... 131

Scheme III-18. Synthesis of imidazoles from imidazolines ................................ 133
Scheme IV-l . Lewis acid mediated mild protocol to Al-pyrroline-

5-carboxylates. .................................................................................................... 169
Scheme IV-2. Isomerization of cis A'-pyrroline to trans Al-pyrroline .............. 178

Scheme IV-3.(a) proposed syn-dipole orientation for acyclic azomethine
ylides derived from imines of a-amino acid esters ............................................. 180

Scheme IV-3. (b) munchnbne (cyclic azomethine ylide) locked as an

anti-dipole ........................................................................................................... 180
Scheme IV-4. Ring opening and isomerization of cycloadducts ....................... 182
Scheme IV-S. Ring opening with different nucleophiles ................................... 183
Scheme IV-6. Extension of diastereocontrol to all four centers of

Al-pyrroline ........................................................................................................ l 88
Scheme IV-7. Asymmetric synthesis A'-pyrroline ............................................ 19]

Scheme V-l. Multicomponent synthesis of imidazolines ................................. 232

xiii

Scheme V-2. Failed attempt to B-lactams using racemic imidazoline-4-

carboxylates ........................................................................................................ 233
Scheme V-3. B-lactams from N-tosyl-a-iminoester using racemic

imidazoline catalyst .......................................................................................... 234
Scheme V-4. Synthesis of chiral imidazoline-4-amide catalysts ..................... 234
Scheme V-S. Proposed mechanism for catalytic synthesis of

Dihydrooxazinones ........................................................................................... 239
Scheme V-6. Synthesis of 2-aryl dihydrooxazinones from N-acylimines ....... 242

Scheme V-7. Synthesis of chiral 4-hydroxymethyl imidazolines from
racemic imidazoline-4-carboxylic acid (III-la) ............................................. 252

xiv

LIST OF FIGURES
(Images in this dissertation are presented in color)

Figure Ma. The drug discovery process ........................................................ 2
Figure I-lb. The drug discovery funnel .......................................................... 3
Figure I-2. Chemical Genetics ........................................................................ 4
Figure I-3a. Target Oriented Synthesis (TOS) ............................................... 8
Figure I-3b. Representation of a single target in chemical Space ................... 8
Figure I-4. High Throughput Screening (HTS) ............................................. 13
Figure I-5a. New drugs approved by FDA vs. R&D spending in billions ..... 15

Figure I-Sb. Number of US Food and Drug Administration (FDA) new
drug approvals (N DA, blue), new molecular entities (N ME, red) and NME
with significant improvements over existing products (green), between the

years 1998 and 2002 ....................................................................................... 15
Figure I-6. Comparison of diversity ............................................................... 18
Figure [-7. Goal in medicinal and combinatorial chemistry.- .......................... 18
Figure I-8. Matching of biological and chemical structure spaces ................. 23
Figure I-9. Structural diversity is not identical to biological diversity ........... 23

Figure I-10. Structural Similarity is not identical with biological similarity..24

Figure [-1 1. Comparison of TOS, medicinal chemistry and DOS ................. 26
Figure I-12. Two general approaches for planning pathways to generate
skeletal diversity ............................................................................................. 30
Figure I-13. Nature’s Synthesis of Secondary Metabolites ............................ 31
Figure I-l4. Flexible core structure libraries vary functionalities as well as
orientation of display ...................................................................................... 32
Figure 11-1. Isomeric Oxazolones and structures ............................................ 56
Figure 11-2. Cyclodehydration of N-acyl-a-aminoacids to azlactones ............ 57

XV

Figure 11-3. Syntheses of milnchnones from basic building blocks ................ 59
Figure 11-4. Diverse reactive sites and intermediates accessible from
Azlactones ...................................................................................................... 60

Figure 11-5. Cyclocondensation reactions of azlactones to generate diverse
heterocycles .................................................................................................... 61

Figure 11-6. Cycloadditions of mitnchnones to generate diverse heterocyclic

skeletons. ...................................................................................................... 62
Figure 11-7. Alkenes as intramolecular diversity elemnts (6 elements) ......... 64
Figure 11- 8. Alkynes as intramolecular diversity elemnts (0’ elements) ........ 65

Figure 11- 9. Alkenes as intramolecular diversity elements in condensations

(0 elements) ................................................................................................... 66
Figure II-lO. Synthesis of pyrroles from mfinchnones ................................... 68
Figure 11-1 1. Synthesis of imidazoles from mfinchnones ............................... 69
Figure II-12. Synthesis of bicyclic scaffolds fi'om cycloaddition of
isomfinchnones ............................................................................................... 72
Figure 11-13. Synthesis of isoxazolidines as mimic of transcription

regulators ........................................................................................................ 74
Figure II-14. Synthesis of oxazolines from oxazoles ..................................... 76
Figure II-15. Synthesis of Az-pyrrolines from isocyanoacetates .................... 77
Figure 11-16. Synthesis of 2-imidazolines from isocyanoacetates .................. 78
Figure 11-17. Synthesis of 2-imidazolines from azomethine ylides ................ 79
Figure 11-18. Synthesis of pyrrolidines from a—iminoesters ............................ 80

Figure II-l9. Synthesis of pyrrolidines from aziridines and electron-rich

alkenes ............................................................................................................. 81
Figure II-20. Synthesis of dihydroheterocycles from soft enolization of
imidates ............................................................................................................ 83
Figure III-1. Biological activity of molecules with 2-imidazoline nucleus ..... 100

xvi

Figure III-2 (a & b). p53 bound to MDM2; key residues for binding ............. 102

Figure III-2 (c) .Overlay of p53 residues and nutlin ...................................... 102
Figure III-2 (d) The bromo phenyl moiety of nutlin deep in the groove of
MDM2 ............................................................................................................ 102
Figure III-2 (e) Representative structure of a nutlin ....................................... 102
Figure III-3. Generation of imidazolium salt through Pd-catalysis ................ 108
Figure III-4. (a) 1H spectra for a representative B-lactam (III-A) and
imidazoline (III-l-l) ....................................................................................... 109
Figure III-4. (b) '3 C spectra for a representative B-lactam (III-A) and
imidazoline (III-l-l) ....................................................................................... 110
Figure III-4. (c)X-ray crystal structure of III-l-land 111-1 -7 .......................... 111
Figure III-5. Soft enolization vs. 1, 2-Prototropy ........................................... 115
Figure III-6. Chem3D models for minimized structures of 2, 4-dimethyl
azlactone, 2-Methyl-4-phenyl azlactone, 4-methyl 2-phenyl azlactone .......... 121
Figure III-7. X-Ray crystal structure of (+) (R, R) - [11-2-1 ............................ 123

Figure III-8. Use of chiral sulphinimes in azomethine ylide cycloadditions...126

Figure III-9. Synthesis of vicinal diamine from imidazolines ......................... 128
Figure III-10. Biological activity of molecules with imidazole nucleus ......... 132
Figure IV-l. Biologically active pyrrolines and pyrrolidines .......................... 165
Figure IV-2. Dipolar cycloaddition reactions of N-alkylated mtlnchnones ..... 167
Figure IV-3. Primary cycloadducts isolated fiom intramolecular

Cycloadditions ................................................................................................. 168
Figure IV-4. Primary cycloadducts isolated from intermolecular

cycloadditions .................................................................................................. 168
Figure IV-S. Azomethine generation by N-silylation ...................................... 173
Figure IV- 6. X-ray crystal structure of IV-l-6 ............................................... 177

xvii

Figure IV -7. Calculated electrostatic isopotential surfaces for exo- and

endo- approach of dicyanocyclobutene ............................................. 181
Figure IV- 8. Synthesis of pyrrolidines from a-iminoesters .................... 185
Figure IV-9. Exo selectivity tuned using bulky phosphine ligands ................ 187
Figure IV-lOa. X-ray crystal structure of IV-l-S, IV-1-10 ............................ 189
Figure IV-lOb. X-ray crystal structure of IV-l-13 ......................................... 190
Figure V-l. General NF -KB activation pathway in eukaryotic cells .............. 214

Figure V-2. Inhibition of NF-KB might play a key role in the pathways
involved in inflammation, diabetes and cancer ............................................. 217

Figure V-3. Imidazoline that showed maximum potentiation of apoptosis....218

Figure V-4. Sensitization of CEM cells to camptothecin by imidazoline 1,
measured over 48 hours .................................................................................. 220

Figure V-5. Inhibition of CPT activated NF -I<B binding by imidazoline 1
using EMSA ................................................................................................... 222

Figure V- 6. Relative amount of phosphorylated I-KB (at ser-32 & ser-36)..224

Figure V-7. Decrease in the tumor size in RIF -1 tumors ............................... 225
Figure V-8. Planar chiral heterocycles for enantio-selective additions to
ketenes ............................................................................................................ 229
Figure V-9. Mechanism for stereocontrol by a nucleophilic catalyst ............ 230
Figure V-IO. Use of Cinchona alkaloid derivative as a multifunctional
nucleophilic catalyst ....................................................................................... 231
Figure V-l 1. X-ray structure showing trans geometry of

dihydrooxazinone ........................................................................................... 236
Figure V-12. PHIM and PHOX ligands in solid state ................................... 246

Figure V- 13. 2-hydroxymethylimidazoline ligands for diethyl zinc
addition ........................................................................................................... 249

Figure V- 14. X-ray structures of chiral 4-hydroxymethylimidazolines
(R, R)-V-1-l7 and (S, S) -V-l-l7 as ViewerLite 3D models .......................... 253

xviii

ADMET
ALLN
ATF

KEY TO SYMBOLS AND ABBREVIATIONS

Administration, distribution, metabolism, elimination, toxicity
Acetyl-L-leucinyl-L-leuciny1-L-nor1eucinal
Activating transcription factor

CPT /CPT-11 Camptothecin

CDDP
CSA
DBU
DCC
DMAP
DCM
DNA
DHFR
DOS

5

cc
EDCI

eq.
FAP

8

HTS
HRMS
h

IR
I-KB
IKK

J

LAH
LDA
M
MHz
m/z
MAPK
MCR
MCSL
MEKK- 1
MS
MSA
nM
NBS
NHC
NMR
NF «B
PDTC

cis- Diamminodichloro Platinum(II) (Cis-Platin)
camphor sulfonic acid

1, 5-Diazabicyclo[5.4.0]undec-5-ene
Dicyclohexylcarbodiimide
4-(dimethylamino)pyridine
Dichloromethane

Deoxy ribo nucleic acid
Dihydrofolate receptor

Diversity oriented synthesis
chemical shift (parts per million)
Enantiomeric excess

Ethyl dimethylaminopropyl carbodiimide
Equivalent

bis-ferrocenyl amide phosphine
gram

High throughput screen

High resolution mass spectrometry
hour

Infra Red

Inhibitor of KB

I-KB kinase

coupling constant

Lithium aluminium hydride
lithium diisopropylamide
Molar(concentration)

Mega Hertz

mass to charge ratio
Mitogen-Activated Protein Kinase
Multi-component reaction
Multiple core structure libraries
MAPKinase Kinase

Mass Spectrometry

methyl sulfonic acid

nano molar

N-bromosuccinamide
N-Heterocyclic carbene

nuclear magnetic resonance
Nuclear factor-KB
pyrrolinedithiocarbamate

xix

PHOX
PHIM
PPTS

Phosphino-oxazolines
Phosphino-imidazoline
Pyridinium p-toluenesulphonic acid

Proteasome multi-catalytic protease unit in cells

QUINAP
RIF - 1
RNA

R

S

SMPPI
Tat/T AR
TEA
TMSCI
TNF-o.
TOS

uM

1 -[2-(diphenylphosphino)- l -naphthalenyl]- Isoquinoline
Radiation induced fibrosarcoma

Ribonucleic acid

rectus(Cahn-Ingold-Prelog system)
sinister(Cahn-Ingold-Prelog system)

molecule inhibitors of protein-protein interactions
viral transactivation protein

Triethyl amine

Trimethylsilyl chloride

Tumor Necrosis Factor-a

Target oriented synthesis

micro molar

CHAPTER I
SMALL HETEROCYCLIC SCAFFOLDS AND DRUG DISCOVERY.

A. Drug discovery process

One of the major goals in chemical and biological research is the identification of
high affinity and specific ligands to receptors and enzymes. The identification of
such ligands is a fundamental step in the development of molecular probes for the
study of receptor and enzyme function, as well as for the eventual development of
therapeutic agents or drugs.I Drug discovery is the process whereby compounds
with activity against a specified target or function are identified, evaluated, and
optimized for clinical applications} The initial step in drug discovery (Figure I-l)
involves the identification and validation of a target, generally a gene product, for
which pharmacological modulation of target activity is predicted to produce a
desired effect. Typical goals include enzyme inhibitors, receptor agonists or
antagonists, and transporter inhibitors or activators. Target identification and
validation may involve gene/protein expression profiling, phenotype analysis in
cell culture and transgenic mouse models, and evaluation of humans with gene
deletion/mutation. Follow-up preclinical evaluation includes analysis in animal
models of compound efficacy and pharmacology (ADMET: administration,
distribution, metabolism, elimination, toxicity) and studies of specificity and drug
interactions. Initial drug discovery thus requires a robust assay of target activity
and a collection of compounds for testing. "Hits" from initial screening are
evaluated on the basis of many criteria, including but not limited to potency,

specificity, toxicity, pharmacology, biophannaceutical properties, and efficacy in

 

identification
validation
assay

bioinformatics
genomics
proteomics

 

assay
HTS

WWW
library
SAR

Medicinal
chemistry

in vitro
cellular
mechanism

 

 

Pharmaco-
kinetics
invivo
ToxISafety

Drug discovery * 0 -
x I

Figure I-la. The drug discovery process. Adapted from www.signalpharm.com/
discovery _platforms.html.

Pre-cinical
development

 

 

   

animal models, to select lead compound(s) for optimization by synthetic
chemistry and more extensive preclinical evaluation in animal models. These
preclinical data form the basis to carry out clinical trials.3

B. A small molecule revolution

In a classical manner, lead compounds were derived from the extraction of natural

products from plants, animals, insects, or microorganisms. Following positive

responses from several fractions, the next step is identification of the chemical

entities responsible for the biological activity.

/& govt

 

I /. (4 Assays\ /lnventory~-.)
Molecular \
Biology Biologcal
\\ 50an / Optimization
1 /
/

 

Figure I-lb. The drug discovery funnel. Adapted from www.akosgmbh.de/
Drug_ discovery _process.html
Synthesis is then undertaken to obtain specific compound(s) in large quantities or
to obtain simpler analogues that may exhibit similar biological response. These
traditional approaches are time consuming and very expensive and the inability to
identify multiple high-quality leads that are novel, tractable, and efficiently
Optimizable was a key bottleneck in the drug discovery environment.4 Modern
drug discovery has hence seen a paradigm shift to screening small molecule
libraries for their ability to bind to a preselected protein target. Small molecules
can exert powerful effects on the functions of macromolecules that comprise

living systems. This remarkable ability makes them useful, both as research tools

for understanding life processes and as pharmacologic agents for promoting and
restoring health. As compared to peptides and natural products, small-molecules
posses numerous advantages in the drug discovery process such as easier
synthesis, higher metabolic stability and bioavailability, easier access to
analogues with increased potency, greater solubility and support for structure—
activityI relationship studies.5

C. Chemical genetics brings small molecules center stage

Chemlcal genetics

 

 

 

 

 

 

 

 

 

 

 

 

 

b) Reverse
- Protein
plate with yeast cells
one compound
per well
Find Ilgand for
protein of Interest
0 olo 0 ol I i '°°"'““
0 [Cl 0 010
0 fig; 010
0 0| 0 OIO
\N/
select compound
based on phenotype 0O
11

 

.. ‘f .
I saw

[Identify proteln

0c Assay for phenotype

 

Figure I-2. Chemical Genetics. Adapted from Ref. 10.

Classic FORWARD-GENETICS is a powerful method for identifying genes that
regulate biological processes in lower organisms. It is difficult, however, to use a
genetic approach in mammals because of their slow rate of reproduction, large
physical size, and large, diploid genome.6 A second limitation of the genetic
approach is that most mutations are not conditional — they cannot be turned on or
off at will. Even when conditional mutations can be found (for example,
temperature-sensitive mutations or mutant alleles under the control of an
inducible promoter), expressing the mutant allele usually entails causing an
environmental stress to the organism, such as a temperature shift, which itself can
have marked consequences7 and may confound the interpretation of results. This
drawback prevents the functions of essential genes from being investigated
because organisms with mutations in these genes are not viable. In addition,
organisms that carry constitutive mutations may have time to compensate for their
effects by up regulating related genes, which can obscure the initial effects of a
mutation.

In a chemical-genetic approach, exogenous ligands (small molecules) are likewise
used to alter the function of a single gene product within this complex cellular
context. Chemical-genetic screens use a three-step procedure (Figure I-2) that
entails: first, assembling a set of ligands (molecules capable of altering protein
function) as mutation equivalents; second, doing a high-throughput screen for
ligands that result in a phenotype or affect a biological process of interest (such as
cell division, programmed cell death or embryonic pattern formation) and last,

identifying protein targets of these active ligands. The advantages are as follows:

e The chemical-genetic approach is conditional and readily applicable to
cells derived from complex organisms such as vertebrates. This is because a
molecule can be added or removed at any time, enabling a kinetic analysis of the
consequences of changes in protein function in vivo.

0 This permits an analysis of the cellular or physiological events that occur
immediately afier altering the activity of a specific protein.

0 In higher organisms, such as mice and humans, the approach can be used
for 'genetic-like' screens in cells and tissues of these organisms, complementing
genetic-screening methods.

0 Not only protein-binding reagents, but other exogenous ligands such as
RNA-binding antisense reagents and DNA-binding reagents 8' 9 can be similarly
used. '0

0 Finding small molecules or mutations that affect a Specific pathway and
identifying the cellular target of the small molecule or the molecular sequence of
the mutant gene can shed light on the pathway.'1

0 From the perspective of drug discovery, the “Chemical Genetics”
approach offers the means for the simultaneous identification of proteins that can
serve as targets for therapeutic intervention (“therapeutic target validation”) and
small molecules that can modulate the functions of these therapeutic targets
(“chemical target validation”). The structures of the small-molecule modulators
provide leads for the drug discovery process, where, for example,

pharmacokinetic and pharmacodynamic properties can be optimized.

However the macromolecules that carry out the many functions required for life
have enormous structural diversity, and this suggests that complementary levels
of structural diversity will be needed in collections of candidate small molecules
in order to find specific modulators for each of those functions. With the
completion of the drafi human genome sequence and genomics and proteomics
research providing rapid information on the discovery of novel genes and
proteins, there is a pressing need to study protein functions in a timely fashion.
This has put small molecules on center stage and more efficient and systematic
methods to identify specific molecular probes are in ever increasing demand. In
particular, “natural product like” libraries are in great demand, complementing the
available natural products that are utilized for modulating (i. e., activating or
inactivating) protein function(s).12 Synthetic organic chemists have used three
different approaches to gain access to these compounds; target oriented synthesis,
combinational chemistry (very often referred as ‘combichem’) and diversity
oriented synthesis.

D. Target-oriented synthesis (T OS) (Trial and Error method)

The first approach uses target-oriented synthesis (TOS) and relies primarily on
nature to discover small-molecules with useful, macromolecule-perturbing
properties. Natural compounds can be identified in screens of extract mixtures,
isolated, and then structurally characterized by using a variety of spectroscopic
techniques (Figure I-3a). Once such a structure has been identified, it and related
analogs can become a target for chemical synthesis. In universities, the targets are

often natural products, whereas in pharmaceutical companies, the targets are

drugs or libraries of drug candidates. The aim of the synthesis effort in TOS is to
access a precise region of chemical space, which is ofien defined by a complex
natural product known to have a useful function (Figure I-3b). TOS as well as
medicinal and combinatorial chemistries ( for over 40 years ago) have been
advanced by the development of a general planning strategy known as retro-
synthetic analysis, in which a complex target is transformed into a sequence of
progressively simpler structures by formally performing chemical reactions in the
reverse-synthetic direction.

Target oriented synthesis : Convergent

Complexity generating reactions 0 90
Fragment coupling reactions >8 —"’ O
Retro-synthetic Anubwis El —>CEI
(a)
‘ descriptor 2

 

Ades/ptofi descriptor 1

(b)
Figure [-3. (a) Target Oriented Synthesis (TOS); (b) Representation of a single
target in chemical space. Adapted from Ref. 52
Prior to this, strategic solutions to the problems of synthesizing different target
structures were developed on a case-by-case basis. Retro-synthetic concepts and
thinking depend on the existence of a defined target structure. Synthesis pathways

in TOS are linear and convergent, and they are planned in the reverse-synthetic

direction by using retro-synthetic planning, which aims to move in the direction

of complex to simple. This problem-solving technique involves the recognition of
key structural elements in reaction products, rather than reaction substrates, that
code for synthetic transformations. Repetitive application of this process allows a
synthetic chemist to start with a structurally complex target and find a structurally
simple compound that can be used to start a synthesis. The basic subunit of retro-
synthetic planning is ‘to transform’, that is, the theoretical transformation of a
product into a substrate by formally performing a chemical reaction in the
reverse-synthetic direction. To make use of a transform in retro-synthetic analysis
one must first identify the corresponding “retron”, that is, the enabling structural
subunit (“keying element”) that permits its application, in the chemical target.
Structurally simplifying transforms are critical in TOS when devising a retro-
synthesis for a complex target structure, and iterative application of these
transforms can lead to a plan for an efficient synthesis. Reactions that join
together two different building blocks, called fragment-coupling reactions, are of
great importance. Reactions that generate structural complexity stereoselectively
(e.g. oxy-Cope and Diels-Alder reactions) are also of considerable value and are
used widely in target-oriented organic synthesis. TOS does not share the aim of

accessing diversity.

E. Combinatorial chemistry (Trial and Selection method)

Peptide libraries constituted the first modular arrays of functionality to be
explored in a prospecting fashion, with the vision that peptide leads could be
transformed into more pharmacologically active molecules. Solid phase peptide

synthesis was first introduced to overcome the technical challenge of performing

iterative couplings to yield long polypeptide chains. The nascent polypeptide
chain is immobilized in this method, most commonly to spherical polystyrene
beads, allowing coupling reagents to be added in high molar excess and by
products (including the unused reagents) to be removed simply by washing the
insoluble beads. This led to the generation of large combinatorial libraries of
peptides'3 and oligonucleotides, '4 which were then screened against a receptor or
enzyme to identify high-affinity ligands or potent inhibitors, respectively. These

and the oligomeric systems that followed them, such as peptoids, '5

' '7 were assembled readily, even robotically, on

polycarbamates '6 and 'azatides
solid-phase and their input materials were available in great variety.18 While these
studies clearly demonstrated the power of library synthesis and screening
strategies, peptides and oligonucleotides generally have poor oral activities and
rapid in viva clearing times, and therefore their utility as pharmacological probes
or therapeutic agents was often limited.'9 Moreover, the formidable challenge of
turning a flexible ligand with modest affinity into a potent lead has limited their
relevance. Although solid phase peptide synthesis was only recently adapted to
non-peptidic small molecules (<600-700 molecular weight), which have favorable

2° simplification of the purification of synthetic

pharmacokinetic properties,
intermediates on the solid phase method has led to an increase in synthetic
productivity. Again taking a lead from solid phase peptide (and oligonucleotide)
synthesis, 2‘ solid phase syntheses have been performed in parallel '5’ 22 i. e.,
similar reactions are performed, but the structures of the building blocks in key

fi'agment-coupling steps are varied. Solid phase, parallel synthesis is an example

10

of what is commonly referred to as combinatorial synthesis and is most
commonly used by medicinal chemists in pharmaceutical companies and
universities to synthesize a focused library of related compounds sharing
structural features necessary for binding to a preselected protein target, allowing
the general principles of retro-synthetic analysis to be applied readily. A second
variation of solid phase synthesis, one that extends it beyond a mere purification
technique, can provide a staggering increase in the ability of organic synthesis to
produce collections of small molecules. This potential was realized originally in
peptide synthesis with the invention of the split-and-pool (split-pool) strategy of
synthesis.23 The strategy has more recently also been used in organic synthesis,
resulting in structurally complex and diverse libraries of synthetic small
molecules.24 In this method, a collection of beads is split into reaction vessels that
subsequently each receive a unique set of reagents, for example, one of a
collection of building blocks. Cycles of pooling, resplitting, and further chemistry
then result in large collections of compounds that are spatially segregated on
unique beads. Split-pool synthesis is referred to as the “one bead-one
compound” approach, and it is analogous to genetic recombination. Encoding
methods, which are analogous to the genetic code, have been developed that
record the chemical history of the synthetic compounds, allowing the structures of
compounds selected in screens to be inferred.25

Due to the extensive time period required for the identification of drug-like
candidates, combinatorial chemistry is now firmly integrated in the drug

discovery enterprise for both lead identification and lead optimization.26 By

11

applying combinatorial chemistry, it is possible to obtain large sets of compounds
over short periods of time, by applying parallel high-throughput synthesis. " 4’ 27
The source of the starting or lead compounds can vary and may include a natural
product, a known drug, or a rationally designed structure developed from a
mechanistic hypothesis and/or a crystal structure of a macromolecule of interest
(focused libraries) or simply exhibiting some intuitively appealing combination
(prospecting libraries) of molecular weight, polarity, conformational constraint,
novelty and so on.'8 Libraries of compounds that are deemed at the outset to be
more 'drug-like' are more favored, comprising known pharmacological motifs.
Library synthesis can be accomplished efficiently using solid-phase synthesis to
append different sets of building blocks to a common molecular skeleton which
facilitates binding to a preselected protein target.22 Retro-synthetic planning is
used in this context to devise pathways to the target structure that permit the
addition of diverse sets of building blocks during the actual synthesis. If this
common skeleton contains multiple reactive sites with potential for orthogonal

23' 28 can be used

functionalization, the powerful technique of split-pool synthesis
to access all possible combinations of building blocks (namely, the complete
matrix) efficiently.

F. Number game (HTS) vs. quality of libraries

Since the human genome sequence has been solved, the pharmaceutical industry
has more than 3500 potential drug targets while previously only about 400 targets

were pursued.29 The advent of genomic sciences, rapid DNA sequencing,

combinatorial chemistry, cell-based assays, and automated high-throughput

12

screening (HTS) has led to a "new" concept of drug discovery (Figure I-4). In this
new concept, the critical discourse between chemists and biologists and the
quality of scientific reasoning are sometimes replaced by the magic of large
numbers. Large numbers of hypothetical targets are incorporated into in vitro or
cell-based assays and exposed to large numbers of compounds representing
numerous variations on a few chemical themes or, more recently, fewer variations

on a greater number of themes in high-throughput configurations.

STOCK PLATE

 

ADD
COMPOUND

INCUBATE
CELLS

   

DETECT CELLULAR CHANGE OF
MICROPLATE WITH INTERESTPHENOTYPE
CELLS

 

ANTIBODY
CYTOBLOT

ASSAY MICROSCOPY

 

REPORTER ASSAY
Figure 1-4. High Throughput Screening (HTS).Adapted from Ref. 10
It was hoped that this experimental design would be suitable to identify many

substances, which can modify the targets in question. Many such "hits"--

compounds that elicit a positive response in a particular assay--would then give
rise to more leads, i.e., compounds that continue to Show the initial positive
response in more complex models (cells, animals) in a dose-dependent manner.

Eventually, the number of compounds also would increase. Data points generated
by large screening programs at a pharmaceutical company have amounted to
roughly 200,000 at the beginning of the 19905. Data points are screening results
describing the effect of one compound at one concentration in a particular test.
This Figure I- rose to 5 to 6 million at the middle of the decade and is presently
approaching or even passing the 50-million mark.33 The goal for such an
endeavor, in part, was to obviate the need for a rational approach completely. A
large enough library with the right HTS assay promised the ultimate brute-force
method for obtaining bioactive compounds without biologists and organic
chemists getting in the way.30 However, the pharmaceutical industry is currently
facing a major challenge (Figure 1- 5a) As measured by the number of new
compounds entering the market place, the top 50 companies of the pharmaceutical
industry collectively have not improved their productivity during the 19905.3|
Two-thirds of the prescription drugs approved by the US Food and Drug
Administration (FDA) between 1989 and 2002 were modified versions of existing
medicines or identical to drugs already on the market (Figure I- 5b). Over the last
five years the number of ’new molecular entities' (NME) approved for use as
drugs by the FDA has steadily decreased. In 2002, only 17 NME were approved,
and of those only 7 were classified as being significant improvements over

existing products.” 32’ 33

14

 

 

1996 1997 1998 1999 2000 2001

 

- Total R&D spent + New Drug Approvals]

 

 

 

 

Figure I-Sa. New drugs approved by FDA vs. R&D spending in billions. Adapted
from www.mindfull .o GE/2004/Dru -I dust -FaIIS' Peter Landers /W all
Street Journal 24 Feb 2004.

 

100
90
80 .

7o .

60

50

4o

30

 

1998 1999 2000 2001 2002

 

 

 

Figure I-5b. Number of US Food and Drug Administration (FDA) new drug
approvals (NDA, blue), new molecular entities (NME, red) and NME with
significant improvements over existing products (green), between the years 1998
and 2002. Adapted from Ref.29

 

G. Diversity deficit in combinatorial libraries vs. natural products

Although the reasons underlying this apparent lack of productivity remain
unclear, it may in part reflect significant deficiencies in the types of chemical
structures generated using combinatorial approaches. Combinatorial chemistry
allows for the synthesis of vast numbers of compounds; indeed, millions of
compounds are realizable by one chemist on their own in a few weeks using split-
pool combinatorial techniques.34 However, the limitation is that the compounds
produced have limited structural diversity.

Traditionally, natural products have been a major source of new drugs, and many
successful drugs were originally synthesized to mimic the action of molecules
found in nature.35 One of the unquestionable hallmarks of natural products is their
exquisite level of three-dimensional sophistication. The diversity of ring
skeletons, and the way in which they present functional groups topologically, and
often provide highly specific biological activities. This follows from the
proposition that essentially all natural products have some receptor binding
capacity.36 Natural molecules, however, differ substantially from synthetic ones
(Figure 1-6). These differences appear to be amplified when the products of
combinatorial synthesis are considered. Although an important aim in
combinatorial chemistry is to generate highly diverse libraries, the need for speed
and automation introduces new structural idiosyncrasies into the method.

Since Lipinski first published his "rule of five",37 describing statistically the
optimal combination of properties of drug-like molecules, additional methods for

identifying distinguishing features between drugs and other organic molecules

16

have been sought. Approaches have included the characterization of molecular
frameworks and substituents, 38 the statistical analysis of different drug databases,
39 the application of artificial neural networks to differentiate drugs from reagents,
4° and the introduction of a drug-like index to rank compounds. 4' The differences
between three different compound classes, natural products, molecules from
combinatorial synthesis, and drug molecules, were investigated (Figure I-6). The
major structural differences between natural and combinatorial compounds
originate mainly from properties introduced to make combinatorial synthesis
more efficient. These include the number of chiral centers, the prevalence of
aromatic rings, the introduction of complex ring systems, and the degree of the
saturation of the molecule as well as the number and ratios of different
heteroatoms. Natural products generally have high binding affinities for specific
receptor systems and their biological action is ofien highly selective. In view of
these facts, it is interesting to consider that the search for the replacement of
natural compounds with synthetic ones is usually based on exactly these kinds of
'unfavorable' modifications. Given the stereospecificity of most biological targets,
it is likely that many non-stereospecific synthetic analogues, created, for example
by the introduction of aromatic rings, represent suboptimal compromises,
especially in terms of selectivity. This situation appears to be amplified in the case
of combinatorially synthesized compounds. In addition, the greater flexibility of
combinatorial products is likely to have detrimental entropic consequences for the
binding of these compounds; it may also adversely affect their ability to induce

conformational changes in the receptor required for biological function.

17

 

 

 

     

.5 Combinatorial
Products compounds

.5” Natural

Figure [-6. Comparison of diversity: the plot of the first two principal
components, obtained from a database containing a) a random selection of
combinatorial compounds (n = 13 506), b) natural products (n = 3287), and c)
drugs (n = 10 968). For clarity, the data points from the three databases are plotted
separately but on the same axes. The Figure [-6 shows that combinatorial
compounds cover a well-defined area in the diversity space given by these
principal components. In contrast, natural products and drugs cover almost all of
this space as well as a much larger additional volume. Drugs and natural products
have approximately the same coverage of this space. Adapted from Ref. 43

descriptor 2

 
  
    

Libraries based
on a defined target

descriptor 3 descriptor 1

Figure [-7. The goal in medicinal and combinatorial chemistry is to synthesize a
collection of analogues (blue spheres) of a target structure (lead) having known or
predicted properties (red sphere). Adapted from Ref. 52

 

As well, the process of producing synthetic analogues radically alters the numbers
and ratios of different atom types, such as nitrogen, oxygen, sulfur, and halogens.
These distributions in turn have a direct impact on the patterns of donors and
acceptors available to complement receptor surface properties.

We can view the production of unique bioactive libraries by plants and organisms
as analogous and complementary to the combinatorial generation of synthetic
libraries, each approach providing access to very different types of lead
compounds.I " 42 However, there is a significant difference between these
processes. The generation of combinatorial libraries has been constrained
primarily by the availability of reagents and suitable reactions. In contrast, the
generation of natural product diversity has occurred not only within the
constraints of available biosynthetic reactions and precursors but also in the
context of biological utility. Most natural products have a function, and the
synthetic routes generating these compounds have coevolved with the
requirements of ligand functionality. This suggests that combinatorial synthesis
must also evolve beyond synthetic feasibility to explicitly address the creation of
compounds with proper biological function.43

H. Rescuing Combinational Chemistry (Combichem)

The question arises how to improve the diversity of synthetic and especially
combinatorial compounds. Some features that are being focused on to rescue

combinational chemistry are as follows:

19

 

 

o Chirality is an essential issue. As chiral separation is rather expensive, the
number of chiral centers could be somewhat increased by using readily available
chiral reagents more often in combinatorial syntheses.

e Attempting to change the ratios of different heteroatoms is another
promising way of increasing diversity in combinatorial products. This will also
impact donor/acceptor properties as well as lipophilicities and hence raise the
diversity of designs.

0 Drug-like filters, such as the Lipinski rules, are very helpful in removing
likely problem molecules. However, overly strict adherence to it can have the
adverse effect of restricting diversity and also reducing similarity to natural
products.

0 A large proportion of natural products is biologically active and has
favorable ADMET properties, despite the fact that they often do not satisfy 'drug-
like' criteria. It is likely that by mimicking certain distribution properties of
natural compounds, a substantially more diverse set of combinatorial products
might be made that could also have greater biological relevance. Analogously to
different 'drug-like' filters, the concept of 'natural-product-like' filters can be
introduced based on the statistical analysis of different properties described
above.

Other methods that might help to bring some of the properties of combinatorial
compounds closer to those of natural products, e. g.

0 application of biocatalysis and biotransformations in the synthesis44a or

using natural product templates in library design, 44” have been suggested.

20

0 Dixon and Villar45 have shown that a protein can bind a set of structurally
diverse molecules with very similar affinities in the nanomolar range, whereas a
number of analogs closely related to one of the good binders display only weak
affinities (>2.5 mM). The design and sampling of compound libraries should,
therefore, be guided not only by structural descriptors, but also by descriptors of
biological activity.

0 In ‘click chemistry’ 46" developed by Sharpless and co-workers, a
biological receptor such as an enzyme is permitted to select the best fitting partial
ligands (ligands that don’t occupy the entire binding site) from a range of
modules that are capable of reacting with one another. After the modules bind to
the site their reactivity towards each other induces them to combine forming a
compound that blocks and thus inhibits the entire catalytic site.

0 Dynamic combinatorial chemistry (DCC) 46” involves a target receptor
which is exposed to a library of potential ligands. Each ligand is formed by
reversible combinations of small building blocks in an equilibrium solution.
Selective binding of one of the ligands to the target lowers the ligands
concentration in solution, causing the equilibrium towards the formation of the
ligand. Thus, the system enhances the amount of strong binding ligand and
discriminates against poorly binding ones.

0 The majority of drug targets cluster into densely populated target families.
Numerous lead compounds from a multi-purpose ‘privileged structure’ libraries
are generated, which can address a variety of targets in a single cluster

irrespective of a therapeutic area."6c For e.g. a single library built around a 5,5-

21

 

 

trans- fused lactarn as a core scaffold generated inhibitors for different serine

proteases ( a single family of proteases) involved in different diseases.46c

I. Chemical diversity and biological Space

Combinatorial chemistry aims to access diversity to some degree, and usually
involves synthesizing analogues of a given target structure i.e. to explore a dense
region of chemical space in proximity to a precise or dense region (occupied by a
lead) known to have useful properties (Figure I-8).47a Since the structural diversity
of the products was only due to the building blocks and starting scaffold the
resulting molecular framework is the same in every case.

If the protein binding sites for potential ligands are defined as a biological
structure space, the task is to find compounds that are “complementary" to this
structure space, that is, to correlate the biological structure space with a chemical
structure space. The real question was neglected: how biologically relevant is the
diversity created in chemistry? Even the most beautiful molecules synthesized
using the most elegant methods are useless if they do not reach a biological target.
Even experts fail to agree on how diversity should be defined (Figure I-9 & 10).
We use the term “diversity” in the sense of structural variety on the basis of
molecular scaffolds and substitution patterns. Despite the sequencing of the
human genome, the biological structure space is largely unknown (Figure I-10).
However, it is clear that it cannot be filled by using the substances that exist
today. In theory, it is conceivable that the entire chemical structure space of all the
possible drug molecules could be synthesized and hence filled. HTS would then

automatically identify the suitable molecules. In practice, this approach is not

22

 

 

 

feasible: theoretical considerations have shown that the universe does not contain
enough atoms to synthesize even one copy of every conceivable molecule."7b The
number of ‘drug-like' molecules possible has been estimated to be astronomic

(l 062 to 10200)_48, 49

 

 

 

 

BIOLOGICAL

l— STRUCTURE

SPACE
—-r-»

 

 

*CHEMICAL
STRUCTURE
SPACE

 

 

 

 

 

 

 

 

 

 

 

MISMATCHED OPTIMAL CHEMICAL gggmzuiicgggmtl
BIOLOGICAL AND DIVERSE LIBRARY SPACES
CHEMICAL SPACES

Figure [-8. Matching of biological & chemical structure spaces. Adapted from
Ref. 47a.

OH
Chloramphenicol b. -

 

'OH N\
\
O ':
QH HN O
HOII'QOH NH2
01L o 3/
\MO‘ ”Jj’c'
N\ O\
Clindamycin CC-Puromycin Erythromycin

Figure I-9. Structural diversity is not identical to biological diversity. The four
antibiotics Chloramphenicol, clindamycin, erythromycin and puromycin are
structurally very diverse. They are macrocyclic, heterocyclic or homocyclic. They
bear different fimctional groups such as nitro, ester, keto, hydroxy, thioether,
tertiary amine, or Chloro. Nevertheless, their modes of action are the same. They
all bind to the bacterial peptidyl transferase. They even have overlapping binding
sites at the enzyme cavity Adapted from Ref. 47c.

23

Ratj adone

 

0 Leptomycin A

HOOC \ \ \ \ Anguinomycin A

 

Kazusamycin
\
a W 0
Pironectin Garnahomolide A
\ \ OH 0503 \ \ \ 9H HNa \
\ \ O \ ,5 \ 0
Ho 9
F ostriecin

 

Ebelactone A

 

Figure I-10. Structural similarity is not identical with biological similarity and
vice versa. A collection of similar natural products exerting a diversity of
biological activities. Members of the highly bioactive family of pentenolide
natural products and relatives. They contain a pentenolide head and a long (C6—
C23), often branched unsaturated lipophilic tail with a spare ornamentation of
hydroxy or keto functionalities: antifungal Ratjadone, the cytostatic Leptomycin B
inhibitor of the nuclear export receptor CRM-l, Anguinomycin A cytotoxic
against transformed cells, antibiotic Kazusamycin, immunosuppressant Pironetin,
cytotoxic Leptofitranin A acting on retinoblastoma protein, antileukemic CI 920,
antifungal Garnahomolide A, gap phase specific inhibitor of the mammalian cell

24

cycle Leptolstatin (formula not shown), tubulin inhibitor Discodermolide
(formula not shown), and enzyme inhibitor Ebelactone.Adapted from Ref. 470.

As a comparison there are approximately 105 1 atoms on earth, so every 'drug-
like' molecule (let alone ones not considered ‘drug-like'); cannot be made and one
must be selective. Fortunately, there is hope; there is more than one ‘answer' to
biological ‘problems’ (e. g. the HMG CoA reductase inhibitors: lovastatin,
pravastatin, simvastatin, fluvastatin, atorvastatin, cerivastatin. . .) so we don't need
to make and screen everything. In fact, biological activity is not a rare chemical
property; the reality is that all small molecules are active biologically in some
way or another, even ethanol. In terms of lead generation it is quality (structural
diversity) and, but not just, quantity (number of compounds) that counts.

J. Diversity Oriented Synthesis (DOS)

Diversity-oriented synthesis aims to pick up where traditional combinatorial
chemistry left ofl... Stu Borman, C&EN Washington.46b

Are the regions of chemistry space defined by natural products and known drugs,
the best or most fertile regions for discovering small molecules that modulate
macromolecular function in useful ways? DOS aims to answer this question. In
contrast to target-oriented synthesis (TOS) and combinatorial chemistry, which
aim to access precise or dense regions of chemistry space, diversity-oriented
synthesis (DOS) populates chemical space broadly with small-molecules having
diverse structures. The synthesis effort in DOS aims to create a broad distribution
of compounds in chemistry space, including currently poorly populated (or even
vacuous) space, and in the future, in space found empirically to correlate best with

desired properties (Figure H l).

25

descriptor 2 descriptor 2

    

 

Libraries based
on a defined

     

6590me 3 descriptor I
C

escriptor 3
A

descriptor I

 

Figure M 1. Comparison of TOS (A), medicinal and combinatorial chemistry
(B), and DOS (C). Each three-dimensional plot is meant to represent the chemical
product or collection of products derived from a single synthesis pathway. Each
axis plots a calculable or measurable property of a small molecule (for example,
molecular weight, solubility). A) The aim in TOS is to synthesize a single target
structure having known or predicted properties (red sphere). B) The goal in
medicinal and combinatorial chemistry is to synthesize a collection of analogues
(blue spheres) of a target structure having known or predicted properties (red
Sphere). C) The aim in DOS is to populate chemistry space broadly with complex
and diverse structures having unknown properties (blue spheres) as a first step in
the small molecule discovery process. In some ways, these three approaches to
synthesizing small-molecules represent points along a continuum. Adapted from
Ref. 52

The structures and functions of natural products suggest that structural complexity
may be positively correlated with macromolecule-perturbing function and
specificity of action. This correlation is particularly striking in small molecules
known to disrupt protein—protein interactions. Hence, in contrast to the relatively
flat molecular skeletons ofien used in medicinal and combinatorial chemistry that
tend to project appendages outward along the perimeter of a circle, The goals of
DOS include the development of pathways leading to the efficient (three- to five-
step) synthesis of collections of small molecules having skeletal and
stereochemical diversity with defined coordinates in chemical space. Ideally,
these pathways also yield more globular or spherical molecular skeletons to which

substituents can be potentially appended site- and stereo-selectively during a post-

screening, optimization stage. The diverse skeletons and stereochemistry ensure

26

that the appendages can be positioned in multiple orientations about the surface of
the molecules. Small molecules used in chemical genetics0 screens can be
synthesized with guidance from compounds known to have biological activity5 I or
with the desire to distribute the products in chemical descriptor space. In the latter
case, it may be advantageous to maximize the representation of different
functional groups and conformations in a screen, since in most cases the nature of
the small-molecule-target interaction can not be foreseen. 1' ’ 5 2
In DOS, where the structural complexity of the individual compounds and the
structural diversity of the overall collection are maximized, synthesis pathways
are branched and divergent, and they are planned in the forward-synthetic
direction52 by using forward-synthetic analysis. Forward-synthetic planning aims
to move in the direction of simple and similar to complex and diverse. Forward-
synthetic planning aimed at accessing these diversity elements relies on the use of
diversity generating processes, which are defined as the transformation of a
collection of relatively similar substrates into a collection of more diverse
products. In an ideal DOS pathway all of the products of one diversity-generating
process are substrates for another, thus making it possible to use split-pool
synthesis to access combinatorially matrices of building blocks, stereochemical
isomers, and even molecular skeletons. To maximize efficiency and to be
compatible with one-bead/one-stock solution technology platforms, synthesis
pathways in DOS should be no more than three to five steps (which leaves no
room for protective- group manipulations). The products of one diversity-

generating process should share some common inherent chemical reactivity. This

27

 

common reactivity serves as a keying element that makes the products collective
substrates for a subsequent diversity-generating process. The goal of achieving
diversity can be simplified by considering three distinct diversity elements:
appendages, stereochemistry, and skeletons;

l) Appendage Diversity (Building Block)

The simplest diversity-generating process is the central feature of combinatorial
chemistry and involves the use of coupling reactions to attach different
appendages to a common molecular skeleton. In forward-synthetic analysis these
are referred to as appending processes. If a molecular skeleton has multiple
reactive sites with potential for orthogonal functionalization, then the technique of
split-pool synthesis can be used to harness the power of combinatorics (a
multiplicative increase in the number of products with an additive increase in the
number of reaction conditions), and thereby generate all possible combinations of
appendages (that is, the complete matrix) efficiently. The core of all the
compounds in a library is same.

2) Stereochemical Diversity

Stereochemical diversity increases the number of relative orientations of potential
macromolecule-interacting elements in small molecules. It can best be achieved
by using stereospecific reactions that proceed with enantio— or
diastereoselectivity. The collective transformation of chiral substrates into
products having increased stereochemical diversity (namely, diastereoselective
diversity-generating processes) requires powerful reagents that can override

substrate bias and deliver diastereomeric products with very high selectivity.” It

28

is conceivable that these types of stereochemical diversity generating
transformations could be carried out with spatially segregated, pooled substrates
under common reaction conditions. Forward-synthetic planning that incorporates
multiple stereochemical diversity-generating processes into a single pathway
should also make it possible to generate stereochemical diversity in a
combinatorial fashion, analogous to the ability of appending processes to generate
building block diversity in a combinatorial manner.

3) Skeletal Diversity

Although efficient pathways leading to families of structurally complex small
molecules are being reported with increasing frequency, 5" pathways leading to
structurally diverse products, with many different skeletal arrays, are rare. The
primary reason for this gap in our planning capabilities is that efficient and
effective “branching" pathways remain elusive. Such pathways require the
identification of a substrate or class of substrates that can be differentially
manipulated, in single transformations, to afford products containing distinct
skeletal arrays of connecting atoms.55 There are, at present, two different
strategies for planning DOS pathways that generate skeletal diversity.

i) Differentiation: The first strategy involves using different reagents to
transform a common pluripotent substrate with the potential for diverse reactivity

into a collection of products having distinct molecular skeletons (Figure I-12).

29

. Reagent - based Substrate - based
differentiation approach folding approach

reagent X
= 0 common reagents

D ...... O [1.330

reagent Z>

 

 

 

Figure I-12. General approaches for planning pathways to generate skeletal
diversity. Adapted from Ref. 11.

To address the diversity of the core scaffold, some approaches involve the use of
reaction-based templatess6 or reaction intermediates57 that can be converted in
subsequent steps into a highly diverse set of structures. This approach is
analogous to the natural process of cell differentiation in which a pluripotent stem
cell is transformed into different cell types on exposure to distinct differentiation
factors. These reagent based skeletal diversity-generating transformations are,
therefore, also referred to as differentiating processes. Nature has made extensive
use of this diversification strategy, for example, enzyme-catalyzed construction of
the various sesquiterpene carbon skeletons of secondary metabolites from one

common precursor, famesyl pyrophosphate (Figure I-13).58

AW... __. e0

caryophylline farnesyl pyrophosphate guaiazul‘ *

l .

'2'

   

   

B—cadinene

Figure I-13. Nature’s Synthesis of Secondary Metabolites. Adapted from Ref. 58.

30

Several versatile molecules that were key intermediates used by traditional

medicinal chemists, for example, Corey's lactone, the Wieland-Miescher ketone,

and the Prelog-Djerassi lactone, could lead to a variety of apparently unrelated

pharmacophoric frameworks. These starting materials are small molecules

equipped with many reactive sites and functionalities, which can undergo both

skeletal rearrangements and functional group interconversions. The use of such a

polymorphic compound as the cornerstone of a library would be of great

advantage since:

Many known synthetic key intermediates lead to medicinally important or
“privileged” scaffolds.

They possess high potential for both structural and functional
diversification.

Their chemical transformations and modifications in solution are well-
documented.

The pharmacological activity of their descending structures is well-
precedented.

Finally, while the compounds generated so far are new and will be tested
for biological activity, it is the unique reactivity profile of each scaffold
and its potential to be differentially functionalized that is deemed to be the

most useful feature of this approach.58

Varying the core scaffold as well as the substituents further increases the

flexibility of small-molecule libraries (Figure I-l4).

31

 

V

E] single core ;0 % En
El flexible core > g) (E Q)

Figure I-14. Flexible core structure libraries vary functionalities as well as
orientation of display. Adapted from Ref. 61.

 

Some prominent examples for reagent based differentiation approach from recent
times are presented below.

1) Schreiber and co-workers report a three-step diversity-oriented synthetic
pathway (Scheme I-l) based on reactive dihydroisoquinoline and dihydropyridine
intermediates, which undergo a variety of transformations to generate skeletally
diverse alkaloid—like compounds. The synthetic strategy presented herein enables
the synthesis of skeletally diverse alkaloid-like compounds in only three steps.
Key to the synthesis is the generation and isolation of reactive dihydropyridines
and dihydroisoquinolines, which are useful substrates for a variety of skeleton-

determining transformations.

32

Pri. IPr
R‘ — MS Si?"
,N. (OPh)
500 um macrobeads I?
100 nmollbead N
R0
N ()Ph
NaCNEH3 3 (I34 )2
Reduction 3+2 cycloaddition Br

F O‘N
GEE/Q Br :j: 1. s dipolar
COzMe c. cycloaddition
/¢YIIUOI‘I I’I TEA

2+2 cycloaddition Ph N
~ ...... ee 0
_ COOM E
\ Bf I e / Bf
RC) N
Br:

Scheme [-1. Skeletal diversity from isoquinoline intermediate on solid phase.
This synthetic sequence leads to twelve distinct Skeletons, all of which contain a
hydroxy group, which is important for the microarray technology used in protein-
binding assays, 59 and all with high purity (80 % purity on average by LC-MS).
The modularity of the synthetic pathway allows for the efficient incorporation of
building blocks at a number of key sites on the skeletons. Efforts are currently
underway to develop a library of alkaloid-like compounds through this synthetic
pathway. 60
2) Armstrong and co-workers have identified the squaric acid derived synthesis of
fused aromatic and heteroaromatic cores (Scheme I-2) as a reaction platform to

generate multiple core structure libraries (MCSLs). For example, quinones,

61b 61c

phenols, naphthylfurans, 6" pyrones, indoles, quinolizin- 4-ones, and

61d,6lc

multicyclic systems containing identical substituents can be generated from

a common precursor. Transformations involving squaric acid derivatives can

33

provide libraries containing two variable sites prior to commitment to a final core
structure. Subsequent manipulation of the differentiated hydroxyls or the use of
substituted nucleophiles readily provides a library with up to six variable sites
stereochemical aspects were neglected. An alkynyl allene has been converted to
heterocycles possessing an u-alkylidene cyclopentenone, a 4-alkylidene
cyclopentenone, or a cross-conjugated triene. Thus, a common intermediate has
been converted to three structurally unique compounds by changing only the

reaction conditions and, therefore, controlling various reaction pathways.62

0
Rtfi‘er
R
o 2 o
R. o 9“ R
“I ~/\
R2lIo /

Scheme I-2. Squaric acid as a versatile precursor to various compound shapes.
Adapted from Ref. 61.
3) By taking advantage of competing reaction pathways available via transition
metal-catalyzed carbocyclization processes, Brummond and co-workers58 have

shown the potential generality of converting a common intermediate to at least

34

three structurally unique and thus unknown hetereocycles (Scheme I-3). While the
compounds generated thus far are new and will be tested for biological activity, it
is the unique reactivity profile of each scaffold (a-alkylidene cyclopentenone, 4-
alkylidene cyclopentenone, and cross-conjugated triene) and its potential to be

differentially functionalized that are deemed to be the most useful feature of this

0 pathway a <:/i pathway C (I
MO(CO)3 \ [Rh(CO)2C|]2
\
I .... D I

toluene. N2

approach.

 

pathway b
[RhtCOhC'lz
PPhg. AgBF4,CO

Geo
Scheme [-3. Allenes and Transition metals: A Diverging Approach to
Heterocycles. Adapted from Ref. 62.

4) Olsson63 reports a conceptually distinct methodology of combinatorial
scaffolding built upon first generating the three necessary pharrnacophore
elements followed by constructing the central core unit as a fourth diversity point.
This fourth diversity point is mainly the diverse spatial arrangement of the
pharrnacophore elements. A practical and efficient Mglz multi-component
synthesis of or, B-enones incorporating three diversity elements was carried out

using inexpensive starting materials. Branching into different cores (fourth

diversity element) was achieved by stepwise cyclization (Scheme I-4).

35

 

o

0 CH0 (:7).

M912 '

+ N
Cl
Cl
1) EtNH2. Et2AlI (mum >9911)
2) KOt-Bu

. .. Q

 

 

   

(antizsyn >9921)

(antizsyn 3:1)
Scheme 14. A combinatorial scaffold approach based upon a multicomponent
reaction. Adapted from Ref. 63.

5) Wipf and co-workers64 have extended the multicomponent condensation of
alkenylzirconocenes with N-diphenylphosphinoyl imines and diiodomethane to
the direct cyclopropanation—rearrangement of readily available alkynyl
phosphinamides (Scheme 1-5). The resulting C-[l-(2-arylallyl) cyclopropyl]
alkylamines provided valuable starting points for the diversity-oriented synthesis
of heterocycles. These novel building blocks were converted into heterocyclic 5-
azaspiro[2.4]heptanes, 5-azaspiro-[2.5]octanes, and 5—azaspiro[2.6]nonanes by
means of selective ring-closing metathesis, epoxide opening, or reductive
amination. The resulting functionalized pyrrolidines, piperidines, and azepines are
easily accessible in 2-3 steps in good overall yields and are of considerable

relevance for chemistry-driven drug discovery.

36

R
fig.
N
// Prom”

/¢ —> R'
n=0,1

immolph, \\ x x--- H OH
R .

n mRI
.3.

Scheme I-S. Diversity-Oriented Synthesis of Azaspirocycles.
6) F allis and co-workers established that the organoindium reagent derived from
S-bromo-l, 3-pentadiene and indium metal reacts with excellent regioselectivity
with a variety of aldehydes and ketones to afford the nonconjugated, (y-
pentadienylation) diene products in respectable yields (Scheme I-6). In addition,
the trienes derived from dehydration of the condensation products afford rapid

entry to complex multicyclic skeletons fi'om tandem [4 + 2] cycloadditions.‘55

 

Scheme 1-6. Cross-Conjugated Trienes for Diene-Transmissive Cycloadditions

37

Schreiber and co-workers‘f”5 have reported a branching DOS pathway that builds
on the report by Fallis and co-workers on the use of consecutive Diels-Alder
reactions. The pathway yielded 29,400 discrete compounds comprising 10 distinct
polycyclic skeletons. The six-step, stereoselective synthesis, which affords
products having a central skeleton with between two and four rings and up to six
stereocenters, was achieved using an inexpensive and accessible, “one bead-one

stock solution” technology platform.

 

 

Scheme I-7. Skeletal diversity via a branched pathway. Adapted from Ref. 66.

38

 

Schreiber has adapted the above triene synthesis and subsequent complexity-
generating reactions to phenolic aldehyde-loaded macrobeads and discovered a
set of dienophiles that react only once with the Fallis-type trienes. The latter
observation provides a branch point to the pathway, where diene products are
formed from a single Diels-Alder cycloaddition, and monoene products are
formed from consecutive Diels-Alder reactions involving either the same or
different dienophiles (Scheme I-7). An important feature of the branched pathway
is that the diastereoselection observed in the original report has been extended to
reaction sequences involving different dienophiles.

In contrast to appending processes, these differentiating processes have not (as of
yet) been used to generate skeletal diversity combinatorially. Doing so will
require the identification of differentiating processes having the products-equals-
substrates relationship, that is, all of the skeletally distinct products of one
differentiating process must share a common chemical reactivity that makes them
all potential substrates for another differentiating process. This type of forward-
synthetic planning is challenging, and will require nonmutually exclusive
approaches to the two, potentially conflicting, goals of maximizing structural
diversity and maintaining common reactivity.

ii) Folding: An alternative synthesis strategy circumvents this potential conflict.
In this case, diverse skeletons of small molecules can be accessed combinatorially
by transforming a collection of substrates having different appendages that pre-
encode skeletal information (called 0 elements) into a collection of products

having distinct molecular skeletons using common reaction conditions (Figure I-

39

12).67 This strategy is analogous to the natural process of protein folding,68 in
which different structural information pre-encoded in primary amino acid
sequences is transformed into structurally diverse macromolecules using a
common folding buffer. Thus, these substrate-based skeletal diversity-generating
transformations are referred to as folding processes in forward-synthetic analysis.
An advantage of this approach is that sets of 0 elements can be identified that act
in combination, that is, a matrix of 6 elements can pre-encode all combinations of
distinct skeletal outcomes. These folding processes can be planned by first
identifying a relatively unreactive core structure that can be transformed into a
more reactive intermediate upon treatment with mild reagents. Distinct skeletal
outcomes can then be pre-encoded into a collection of substrates by attaching to
this common core different appendages (6 elements) having complementary
reactivity with the latent, reactive intermediate. Mild conditions can then be used
to liberate the reactive intermediate and to realize the pre-encoded,
complementary reactivity, thus resulting in the formation of different skeletons.

The aromatic furan ring, for example, is a relatively unreactive core structure that,
upon treatment with a mild oxidant, can be transformed into a more reactive,
electrophilic cis-enedione intermediate.69 As shown in Scheme I-8, by appending
three distinct two-carbon side chains containing two, one, or zero nucleophilic
hydroxy groups to a common furan core, it was possible to transform three
structurally similar substrates into three products having distinct molecular
skeletons through a common set of oxidative and acidic reaction conditions (NBS

and PPTS, respectively).

40

 

Scheme I-8. Skeletal diversity generating folding process. Adapted from Ref. 67.

Furan derivative with a side chain containing two nucleophilic hydroxy groups,
underwent NBS-mediated oxidative ring expansion and subsequent ketalization70
to yield the [3.2.1]bicyclic ketal (I-8a). Alternatively, the Evans aldol product,
with a single hydroxy group on its side chain, underwent oxidative ring expansion
and acid-catalyzed dehydration to yield the alkylidene pyran-3-one (I-8b).
Finally, treatment of furan derivative, with no nucleophilic hydroxy groups on its
two-carbon side chain, under the same reaction conditions resulted in oxidative
Opening of the furan ring followed by olefin isomerization71 to yield the trans-
enedione (I-8c).

The use of this substrate-based approach to generate skeletal diversity
combinatorially (namely, achieving a multiplicative increase in skeletons with an
additive increase in appendages) requires at least two sets of 0 elements that can

be appended at different sites and function in combination to pre-encode a matrix

of distinct skeletal outcomes. In contrast to the one-synthesis/one—skeleton

41

approach (which typically involves forming a single molecular skeleton early in a
synthesis), a folding process can be used to generate new skeletons at the end of a
synthesis pathway. This approach facilitates the generation of functionalized
skeletons that might otherwise be difficult to access, such as those having
building blocks coupled through carbon-carbon bonds at stereogenic quaternary
carbon centers and/or potentially unstable structural elements. Additionally,
0’ elements can be attached to a common molecular skeleton by using appending
processes (similar to the way building blocks are appended in the one-
synthesis/one skeleton approach). The maintenance of structural similarity and,
therefore, common reactivity until late in the synthesis pathway facilitates the
realization of this approach using the split-pool technique.

4) Integrated Forward-Synthetic Analysis for Generating Both Complexity
and Diversity (Simple and Similar to Complex and Diverse)

Achieving high levels of both complexity and diversity in the context of a single
DOS pathway will require integrated forward-synthetic planning. One logical
approach is to incorporate complexity generating reactions into stereochemical

and skeletal diversity-generating processes.

42

OHC
U
Ugi 4CC-IMDA R2.

 

 

    

 

,R NC ; =
“2M 2 R1’ (sequential use of complexity- _.
generatIng reactions) R ,HNOC
EtOOC\/\COOH
\ O
\/\ H ’Rz /—\
,N Mes’N N‘Mes
R1 0 CI.,
I'll“;
Y3P Ph
° 0
\ K‘efo R
"‘12:, (complexity-generating reactions 0 O
in the context of a skeletal diversity R ‘ H R.
O generating folding process) ‘ N H M
beta R3 (R) "’R3
3"» alpha R3 (8) / fig

Scheme I-9. Integrated skeletal complexity and diversity. Adapted from Ref. 72.
Recently, the potential of linking stereochemical control, to the challenging
problem of skeletal diversity has been successfully demonstrated72 by using 0
element where stereochemistry of the G-element and not its constitution controls
the outcome of the pathway selected.

L. Small molecule heterocyclic scaffolds

Heterocyclic compounds are important structural motifs present both in natural
and synthetic compounds. It is estimated that far more than half of the published
chemical literature concerns heterocyclic structures. Small polyfunctionalized
heterocyclic compounds play important roles in the drug discovery process 38" 73
and in isolation and structural identification of biological macromolecules.74
Substituted heterocyclic compounds generally offer a high degree of structural

diversity and have had a substantial impact when used as therapeutic agents.

Indeed, analysis of drugs in late development or on the market shows that 68% of

43

30" Therefore, the cost-effective creation of new and

them are heterocycles.
versatile heterocyclic core structures and the approach to them are vital for many
innovative drug discovery programs. It is hence not surprising that research in the
field of combinatorial synthesis of heterocycles has received special attention.”

One striking structural feature common to cyclic molecules and inherent to
heterocycles, which continues to be exploited to great advantage by the drug
industry, lies in their ability to manifest substituents around a core scaffold in a
defined three-dimensional representation, thereby allowing for far less degrees of
conformational freedom than the corresponding conceivable acyclic structures. In
addition, as a result of the presence of heteroatoms such as O, N, and S,
heterocycles often exhibit altered absorption, distribution, metabolism, and
excretion properties.76 The design of rigid scaffolds which allow the incorporation
of a variety of substituents is an attractive approach for the preparation of
combinatorial libraries, especially rigid molecules that position amino acid
functionality in 3-dimensional mimicry of peptide secondary structure, such as B-
tums." Small heterocycles, in particular, are used as rigid, highly functionalized
molecular scaffolds and are biologically very interesting.78 Although libraries of
peptides and other linear oligoarnides continue to play an important role in

15, 79

bioorganic and medicinal chemistry, more compact, conforrnationally

78, 80

constrained heterocyclic scaffolds hold greater appeal for lead discovery

11, 18, 45

prospecting libraries and represent a greater challenge for synthetic

design.81

44

M. Conformational constraint/rigid scaffolds

Studies on conformationally constrained molecular structures indicated that
rigidification of ligand molecules can potentially influence their biological
activity either by modulating their potencies and selectivities for the target
receptors. From the thermodynamic point of view, a conformationally constrained
molecule with correct bioactive conformation should exhibit enhanced affinity as
the reduction in flexibilities reduces loss of entropy upon binding normally
occurring when a rotationally free compound becomes more restricted upon
alignment with the binding site.82"' 82" In addition, such transformation is also a
powerful tool to illustrate possible bioactive conformation of a flexible parent
molecule. As a result of such transformations, it might improve their overall
pharmacological properties such as the ability to cross blood-brain barrier.
Structurally constrained versions of these molecules might produce interesting
properties as they might display altered pharmacological and pharmacokinetic
properties.” '85
N. l, 3-dipolar cycloadditions and conformationally constrained heterocycles
Compact structures which adopt specific conformations are often more valuable
as leads than linear and very flexible molecules. The goals of DOS concur with
above aim as it includes the development of pathways leading to the efficient
(three- to five-step) synthesis of collections of small molecules having skeletal
and stereochemical diversity with defined coordinates in chemical space. The
development of solid-phase synthetic approaches to small molecules, particularly

those which embrace polyfunctional heterocyclic targets is hence of great interest

45

in DOS.86 Structural constraint can be introduced by cyclization more efficiently
compared to sequential reactions, but reaction sequences are longer if rings are
introduced one step at a time. Reactions that generate multiple linkages in one
transformation are extraordinarily powerful in introducing complexity quickly. As
a consequence, cycloaddition and cyclocondensation reactions, which generate
more than one bond in a single step, are particularly attractive in diversity
oriented synthetic design since a premium is placed on reducing the total number
of transformations, to ensure adequate yield and purity. Cycloaddition and its
intramolecular variants have hence become indispensable for the construction of
mono- and polycyclic systems as a consequence of being able to deliver complex
cyclic molecules with many positions where variation or substitution is possible
from relatively simple and accessible precursors.

Inter- and intramolecular Diels-Alder reaction has been used by itself or in
sequence with MCR’s in atom economical approaches to conformationally
constrained tricyclic nitrogen heterocycles.87 Like the Diels-Alder reaction, 1, 3-
dipolar cycloadditions are fundamental processes in organic chemistry for the
synthesis of diverse heterocyclic compounds. The reaction of azomethine ylides
with various dipolarophiles results in highly substituted five-membered nitrogen
heterocycles. l, 3 - dipolar cycloadditions are reactions of considerable interest in
diversity oriented synthesis, since they can provide access to these polyfunctional
molecules with overall good control of the relative configuration of several
consecutive asymmetric centers under relatively simple and scalable experimental

procedures. In addition to being versatile, it is also an atom economical process.

46

One of the most important classes of l, 3-dipolar cycloadditions involves
azomethine ylides. The recent explosion of interest in azomethine ylides is well
justified when considering their synthetic utility in the construction of
heterocycles and polycyclic systems. Not only does the wealth of ylide generation
protocols permit application to a range of heterocyclic structures, but the advent
of chiral control over reaction products and intramolecular protocols allows for
azomethine ylide cycloadditions to be applied to challenging synthetic problems,
particularly those developed by natural product synthesis. Generation of the ylide,
usually in situ, followed by dipolarophile attack affords the heterocycle with
operational simplicity. A noteworthy example is l, 3-dipolar cycloaddition of
azomethine ylides with electronic-deficient olefins which is of considerable
interest in DOS because its stereospecificity enables stereochemical
diversification of up to four tetrahedral centers on pyrrolidine rings. Achieving
diastereoselectivity and enantioselectivity in this reaction is presently a field of
intense scientific investigation. Although established methods for azomethine
ylide generation have proven to be both general and efficient, new procedures are
constantly being divulged. These new methods are not only for ylide generation,
but are also for new chemical equivalents and by careful selection; the synthetic

chemist has a wealth of technologies available at his disposal.

47

 

10.

ll.

12.

13.

14.

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55

CHAPTER II
AZLACT ONES TO NOVEL DIHYDROHETEROCYCLIC SCAFFOLDS.
Oxazolones represent an immensely versatile family of heterocyclic compound for

fixture exploitation by synthetic chemists... .A. 1. Myersl

A. Introduction

In our quest for small molecule heterocyclic scaffolds with novel biological
activities, we focused on the azlactone template (5-oxazolone) on account of its
versatility as a template for synthesis of other heterocycles. Oxazolones can exist

in five isomeric forms all of which are known (Figure II-1)2.

RI 0 R1 0 R1 0 0 o 0 o
Will/$0 I§2=O \INl’We—Rz III/\fm T§k R1
R2 R1 0 R2 R2

5(4H)-oxazolone 5(2H)-oxazolone 4(5H)-oxazolone 2(3H)-oxazolone 2(5H)-oxazolone
or or or
azlactone pseudoxazolone isoxazolone

Figure II-l. Isomeric Oxazolones and structures.
Azlactones or 2-oxazolin-5-ones (5(4H)—oxazolone) are cyclic esters of N—acyl-OI-

amino acids. In 1883 Plbchl first discovered them by condensation of

benzaldehyde and hippuric acid in acetic anhydride, but it was Erlenmeyer (1993)
who rationalized the structure and christened them as “azlactones”.3 Because he
failed to realize the ease of hydrolysis of saturated azlactones, it was in 1908-1910
that Mohr and co-workers were successful in preparing several of them by using

acetic anhydride as dehydrating agent.3 Azlactones gained prominence as the

intermediates primarily responsible for racemization when N—acyl-a-amino acids

56

or N-protected peptides are coupled with nucleophiles (amino acids, amines,
alcohols, etc.) using DCC or other reagents during peptide synthesis.’ Since then
the chemistry of azlactones has evinced great interest mainly as synthons for
peptide synthesis and other heterocycles.6

Efficient multi-component reactions permit the incorporation of several elements
of diversity in a single step and are ideal for application to the diversity-oriented
synthesis of the libraries of complex molecules.5 For our efforts, to generate
natural product like libraries for chemical genetics, we decided to focus on
azlactones as template/fluid core7 for diversity oriented synthesis of other

heterocycles. The following features of azlactones guided us in our choice;

 

0% 0 0
R/IK‘. NJYO Carboxyl acid activator R 6 "I9

R (OH @NR' R2
R=H alkyl or acyl munchnone

 

ICarboxyl acid activator: (RCO)20. DCC. EDCIJ

 

Figure II-2. Cyclodehydration of N-acyl-a-aminoacids to
azlactones/munchnones.

1) Easily accessible starting materials.

As a source of input materials for the synthesis of azlactones, o-amino acids
occupy a special position. They remain unmatched as a source of diverse starting
materials: densely functionalized, commercially available in protected form, as
pure enantiomers, as the natural as well as a myriad of "unnatur " derivatives.8 A

variety of N-acyl-ot-amino acid can be obtained under Schotten-Baumann

conditions or simple acylation of or-arnino acid esters followed by mild basic

57

hydrolysis.9 Traditionally the cyclodehydration of N-acyl-or-amino acid to
azlactones has been carried out by refluxing in acetic anhydride.lo This method
suffers from side reactions, and it is hard to get rid of the acid impurity which is
detrimental to sensitive azlactones. Milder versions involving trifluoroacetic
anhydride (stoichiometric amounts) or mixed anhydrides have also been used
routinely.ll Although high yields are obtained by using DCC, removal of urea
byproduct completely is not trivial.12 Use of water soluble carbodirnides like
EDCI have gained popularity because of the high purity of final azlactones (which
can be used without further purification for subsequent reactions) and also
because they minimize racemization of optically pure products (Figure II-2).'2
Any discussion of azlactones is incomplete without the tautomeric mesionic 1,3-
oxazolium-S-oxides (mitnchnones), formed during the preparation of azlactones
with acetic anhydride or other acid anhydrides.'3’ " Azlactones can also be
trapped in mesionic form by N-alkylation.ls A new palladium-catalyzed route to
prepare milnchnones directly fiom basic building blocks imine, carbon monoxide,
and acid chloride has been developed recently providing an efficient way to add
diverse substituents to this versatile template (Figure II-3).'6 Use of 1-
isocyanocyclohexene as “universal isocyanide” enables postcondensation
modification of Ugi four-component condensation products via munchnone that
reacts with many external and internal nucleophiles to yield a variety of

products. '7

58

[Pd2(dba)3] (5 "101 %) 0

R2 0 PO—toll 15mol°/ 0
HE"; A +co ‘ ”3‘ °’ - ...—«I.

R, CI 3.5, EtN‘Prz, 65 °C 6) I" R2
Amdtsen's method R1

R1-COOH q
0 R2
RNH2 Ugi )Lc. Lb 0 0
—-—> R1 N \ ———> R1—<\ e
RZCHO ' @N

II NC Armstrong '3 method

Figure II-3. Syntheses of milnchnones from basic building blocks.

 

2) Pluripotency

Azlactones afford a host of diverse products with different reagents (Figure II-4)
and the reaction may involve retention or collapse of the heterocycle itself. The
azlactone is an activated internal ester (lactone) of the corresponding N-acyl-or-
amino acid and as such can undergo nucleophilic attack by host of nucleophiles
including alcohols, amines, carboxylates, etc.2’ '8 It also undergoes reactions of

activated carbonyls like Friedel-Crafis arylationl9 and Wittig olefination.20

59

 

R1

/ Nucleophiles
Y°
/~' 0

:9 Electrophiles

Nucleophiles R2

saturated azlactone

 

R1YO 1.3-dipole
|
&7 GIN e 0

R

A
, . \\ I? R2
dipolarohlles B R = H alkyl or acyl

mu'nchnone
II Nucleophiles 2Y0 0%
)oL l: / Nucleophiles NR Nucleophiles
R1 N SD R2
ketene unsaturated azlactone

Figure II-4. Diverse reactive sites and intermediates accessible from azlactones.

Unsaturated azlactones (prepared by condensation with aldehydes or ketones)
present a uniquely reactive exocyclic double bond for additional reactions with
nucleophiles.18 Monosubstituted azlactones at C4 can react with a variety of
electrophiles due to acidity of C4-H bond. Alkylations, arylation, acylation,
Claisen rearrangement, addition of azine N-oxides, addition of diazonium salts
and Michael additions have been well documented.'8 Azadienes first undergo
electrophilic addition at C4 followed by a nucleophilc attack of nitrogen at C2 of
azlactones to afford 2-pyridones.2' Miinchnones and their a-amide substituted
ketene tautomers, are an important family of substrates for the synthesis of
biologically relevant molecules.22 Miinchnones are cyclic azomethine ylides and
hence versatile 1,3-dipolar addition substrates and have proven key to the

construction of a range of natural products and pharrnacologically relevant

22,23a,b 23c,d

heterocycles, including pyrroles, imidazoles, pyrrolines,23¢ and indole

23f

derivatives. Alternatively, the less stable ketene tautomer can serve as a

precursor to III-amino acid and peptide derivatives, via reaction with alcohols or N-

60

 

terminated peptides?“ and their cycloaddition with imines provides access to

peptide- based [3-1actarns.2‘""c

3) Skeletal diversity through azlactones

 

 

 

 

 

Ar
k”
R,COHN R2 0 4 R4
If
N R4
B-Iactem
Rs
R‘IKIOH 1) EtsN l TMSCI
l <
R1 N’ R2 2) R/—\R‘
4-hydroxypyn‘dine 3
O o ,
. .. M
l R4 R3
\ A
R OCHN OCR‘
‘ R2 3 CHIOR),, Aep, A
PM
/\e
R \ 002m 1) / PPhaBr
1 N R t
. 2 2 N OMe
pyrrolIne ) a

 

 

Rf." R2NHCOR1
\
R3 2 R3.
-pyndone
WZNMN'R‘
R7." IiihHCOR.
A
R3 N Nlllle2
pyrimidinone

1)ArH/AICI3 R'TW/ N

 

 

2) POClg. A R2
oxazole

0

Mail I R3X R11 R
> 3

 

R2
disubstituted azlactone

Figure II-5. Cyclocondensation reactions of azlactones to generate diverse

heterocycles.

Azlactones are extremely versatile and reactive templates/intermediates and are

well-suited for the synthesis of diverse heterocycles by both reagent/reaction

condition based differentiating processes and substrate controlled folding

(intramolecular) procedures.25

i. Differentiation] Intermolecular reactions.

The azlactone template can be diversified into hetrocyclic skeletonsls like

26.27

oxazoles,l9 pyridines,

pyridones,”

61

pyrimidinones,28

pyrazolones,

imidazolones, isoxazolidinones, isoquinolines, tetrazoles, etc by short

cyclocondensation sequences.2J8

 

 

 

 

 

R R
Ph “El/Ce — R.
h [>35 R’O R2 1

Pllozs.IN
Q...”
Ni: CIR kAl’ o
:0) OZT’R“ R76 0
R1/4N\ R2 Z—Ar r

R2 N
of Ph

N'R R4 R3

( R2: cyclic amino acid / \
R . ‘ R
_ R 7 I 2

R4 — s n

Figure II-6. Cycloadditions of mtinchnones to generate diverse heterocyclic
skeletons.

 

 

 

 

 

 

 

 

 

Oxazoles, pyrananones, simple pyrrolines and quartemary amino acids can be
accessed with diverse substituents and such diversity of substituents is hard to be
rivaled by other methods (Figure II-5).2"8 1,3-Dipolar cycloaddition reactions of
N-methylated mesoionic oxazolones or munchnone (Figure II-6), provides a

general route for the syntheses of a host of heterocycles with diverse skeletons,

62

for example pyrroles, pyrrolines, azepines and imidazoles, etc.”"’ 23“ 36 By N-
methylation, the azlactones are locked in the mesoionic form and are extremely
reactive dipoles, which afford the heterocycles in very high yields. Strained rings
like cyclopropenes and benazetes also participate in cycloadditions with
mfinchnones to afford 4-pyridones and indoles respectively.2 Gelmi and co-
workers used vinyl phosphonium salts to overwhelm the effect of polarization of
the vinyl group with the strong interaction of phosphonium group and carbonyl
group.29 Trisubstituted pyrrole can thus be accessed with high regiocontrol.
Gribble and co-workers have shown mfinchnones can react in tandem fashion
with cyclooctadiene to give caged compounds.30 Tandem addition of maleimides
to the mesionic Al-pyrroline after decarboxylation occurs in exo fashion
compared to the first endo addition resulting in complex tricyclic heterocycles.l3
Munchnones derived from cyclic amino acids like proline, can be used for
efficient construction of pyrrolizidine and indolizidine skeletons, which form the
core of several natural products and biologically active compounds.3|

ii. Folding I Intramolecular reactions

The development of effective approaches to bridged heterocyclic ring systems is
an important topic in organic synthesis. Because of the ease of constructing two
rings simultaneously, intramolecular 1,3-dipolar cycloaddition strategies have
emerged as an important tool for the synthesis of structurally complex fused
heterocyclic ring systems.32 Particularly, several bicyclic and polycyclic fused
pyrrolidine ring systems are synthesized by the intramolecular cycloadditions of

stabilized acyclic azomethine ylides with tethered dipolarophiles (0 elements)”

63

Combination of the intramolecular 1,3-dipolar cycloaddition with other reactions
offers a number of interesting synthetic possibilities. In spite of copious literature
dealing with bimolecular cycloaddition reactions of mesionic heterocycles,

intramolecular examples have received only a minimum of attention.

R1
m0
E 1lea 6:? °©f§¥°
OH
\ Na J\CF3
N CF3
n H

Figure II-7. Alkenes as intramolecular diversity elemnts (0 elements).

An attractive feature associated with the internal cycloaddition of mesionic
compounds is the opportunity to control the stereochemistry of the products at
several centers. In systems where the dipole and dipolarophile are linked by
several atoms, the highly ordered transition state can also induce useful
regiochemical control.40 Padwa and co-workers utilized the intramolecular
mfinchnone-alkene cycloadditions to craft a series of caged compounds with
quinoline and isoquinoline cores (Figure II-7).41 B-carbolines skeletons found
frequently in alkaloids were generated by a Pictet-Spengler reaction of an

intramolecular azlactone derived from tryptophan.33 Alkynes tethered to N—acyl-

a-amino acids have been shown to undergo intramolecular cycloadditions when
refluxed in acetic anhydride via the miinchnones to afford benzopyrano[4,3-b]
pyrrole and lH-pyrrolo[l,2-c] thiazole derivativesm’ Martinelli and co-workcrs
have developed a new approach to octahydroindoles via intramolecular

cycloadditions of milnchnones with tethered alkynes (Figure II--8).34

    

ITIOS 0
o Ac20 \
0 up > 0' .2
\ eR2 A N
o". R
0 /
0'
#0 \\ sr—fkb
SYeNfiko Ph
Ph

 

Figure 11- 8. Alkynes as intramolecular diversity elemnts (0 elements).

The approach is simple, convergent, widely applicable and uses both natural and
unnatural amino acids. It also provides quick entry to mitomycin skeleton and
other medicinally important polycyclic heterocyclic compounds. The only
limitation is the substituent on nitrogen needed for formation of munchnone

intermediate.

65

0 NHCOR1

3
\
,2
ire/<0 O
3Q
0

\ o
o
\ R2 A R2 /
N‘~ N\ I
R1 R1
R3
R0 0 R3
o “5%?
0 ”Y N o
\
R1 Y

E

Figure 11- 9. Alkenes as intramolecular diversity elements in
condensations(o elements).

Oxazinones, pyridines and pyrones are obtained after stepwise intramolecular
cycloadditions (Figure II-9) depending on the 0 element (reactive group that

decides the skeleton of the product) on the C4 carbon of the azlactones.22a

B. l, 3-Dipolar cycloadditions and stereochemical diversity 25’ 35

Stereochemical diversity increases the number of relative orientations of potential
macromolecule-interacting elements in small molecules. It can best be achieved
by using stereospecific reactions that proceed with enantio— or
diastereoselectivity. Since diversity-generating processes involve the
transformation of a collection of substrates into a collection of products, it is
critical that the processes used to generate new stereogenic centers are both
selective and general. Many 1, 3-dipolar cycloadditions have the ability to

generate rings (and functionality derived from the transformations of such rings)

66

containing several contiguous stereocenters in one synthetic operation. The
configurations of these new stereocenters arise from the geometry of the dipole
and dipolarophile as well as the topography (endo or exo) of the cycloaddition.
Additional stereospecificity is possible when the reactive n faces of either of the
cycloaddends are made diastereotopic by the use of chiral auxillaries or chiral
complexes with external agents. Since the diastereoselectivity of 1,3-dipolar
cycloaddition is under a powerful substrate control it is challenging to generate
the opposite diastereomeric product. Double-diastereoselective reagents that can
override the face selectivity of both coupling partners, for example, to achieve exo
versus endo selectivity would be highly valuable and the discovery of these types
of powerful reagents is critical to achieving stereochemical diversity in DOS.
F orward-synthetic planning that incorporates multiple stereochemical diversity
generating processes into a single pathway should also make it possible to
generate stereochemical diversity in a combinatorial fashion, analogous to the
ability of appending processes to generate building-block diversity in a

combinatorial manner.

C. Lack of stereochemical diversity in munchnone cycloadditions

It has been well documented that instead of 1, 3-dipolar cycloaddition azlactones
undergo a Staudinger-type reaction when heated with imines in toluene yielding
B-lactams. The azlactone, which is in equilibrium with valence tautomeric ketene
intermediate, undergoes a [2+2] cycloaddition reaction with the imine (Scheme II-

3). It can be concluded that either mesoionic oxazolium oxide, which has the

67

nitrogen atom unsubstituted, is not in reasonable concentration and/or the imine
nitrogen is not nucleophilic enough. Hence formation of ketene takes
predominance, which reacts readily with the imines to afford the B-lactam.6’ 36' 37
In 1970, Huisgen had reported that a moderate equilibrium concentration of the
mesoionic tautomer is essential for the 1, 3-dipolar cycloaddition leading to
heterocycle formation (Figure II-lO).l3 By N-alkylation or acylation, the

azlactones are locked in the mesoionic form and are extremely reactive dipoles,

which afford the pyrroles and other hetereocycles in very high yields.15

002Me
ll'ne
Ph 0 09 H Ph N 002'“ M9020 002Me
\ / 002Me o -002 / \
,N -———> 002Me ——> Ph R
Me® R1 0 R1 'i‘ 1

Me
Figure II-10. Synthesis of pyrroles from mfinchnones.
The analogous reaction of a munchnone with a nitrile to provide an imidazole is
not viable due to the low reactivity of nitriles. Imidazoles (Figure [H 1) can be
generated by treatment of the mtlnchnones with tosylimines. Addition of the
tosylimines results in an unstable bicyclic adduct that loses carbon dioxide and
benzenesulphinic acid to gain aromaticity.23°""""38 The phenylsulphonyl residue as
leaving group enhances the tendency to aromatize (Figure IL] 1). Milnchnones
with different substituents, at the 2 and 4-positions lead regioselectively to
products in which a bond is always formed between the 2-carbon atom of the
dipole and the imine nitrogen atom. This attack was explained on the basis of a

dipole-dipole interaction between partially negative nitrogen of the imine and the

68

electrophilic C-2 atom of the munchnone. The yield reported for these reactions
are low at least partly due to self-condensation of milnchnones.23d The problem of
self-condensation has been suppressed by using a solid-phase approach towards
imidazoles. Bilodeau et 0123 ° used a standard reductive amination protocol to hook
an amino acid ester onto a aldehyde group on a commercially available resin. This
path of attachment also served to alkylate the nitrogen atom of the azlactone that
was made by subsequent cyclodehydration of the amino acid, which was

condensed with tosylamines to afford the imidazoles in good yields and purities.

 

|
”Y e ”W" Ph " G?" N “2
NH “2 .ofix; __-_<_=°_2, P, If
/
"“369 R,” -PhSOzH I}! R1

Figure II-ll. Synthesis of imidazoles from mfinchnones.
1, 3-dipolar cycloadditions of mfinchnones are simple thermal processes, unaided
by external reagents. The products resulting from cycloadditions of mtlnchnones
with various dipolarophiles are aromatic and carry little or no stereochemical
information. In intermolecular reactions, primary cycloadducts have generally not
been isolated and identified because they readily eliminate carbon dioxide
resulting in aromatization.39 Double bond isomerization (in case of alkenes to A2-
pyrrolines) and multiple cycloadditions have been observed after decarboxylation,
especially in case of addition of alkenes.40 This results in loss of stereogenic
centers from the cycloadditions in the final products. Although, the

decarboxylation seems to be driven by the stability gained by aromatization, it is

69

not a concerted process and the harsh conditions often employed might play a
major role. Maryanoff and co-workers have described one example of the
isolation of a Al-pyrroline-S-carboxylic acid from reaction of l, 2-
dicyanocyclobutene from the intermolecular cycloaddition of a munchnone.“ A
notable exception was reported by Padwa and co-workers who successfully
isolated and characterized the primary adducts from intramolecular cycloadditions
of mfinchnones to terminal alkenes.“ ‘3 In the above reactions, expulsion of
carbon dioxide is presumably impeded by the severe structural constraints in the
intermediate polycycle. They were also able to show that the cycloadditions
occurs with exo stereochemistry for the substituents on alkenes and concluded
that it is reflective of the steric constraints imposed by a unimolecular transition
state. The polycyclic heterocyclic compounds oxazoloquinolines and
oxazoloisoquinoline products are potential precursors to amaryllidaceae alkaloids.
Padwa and co-workers further diversified these heterocyclic systems by addition
of organometallic reagents/reduction with LAH to tricyclic lactols. These lactols
undergo ring opening to benzazepine scaffolds which are valuable
peptidomimetics displaying diverse biological activities. 42’ 43
D. Potential of Lewis acid mediated generation of miinchnones and synthesis
of novel dihydroheterocyclic scaffolds.

We envisioned that coordination of a Lewis acid to nitrogen atom of the
azlactone, would increase the equilibrium concentration of munchnone
intermediate and accelerate the azomethine ylide cycloaddition reaction (Scheme

II-l).44 Moreover if milder protocols could be developed by Lewis acid

70

mediation, then it might be possible to isolate either the bicyclic cycloadduct or
the dihydroheterocycles (Scheme II-l) without any decarboxylation or

45, 46

rearrangement. [3+2] cycloadditions in which heterodipolarophiles are

employed to intercept reactive azomethine ylides represent a useful entry into

45"" In case of milnchnones,

functionalized, protected amino acids.
dihydroheterocycles isolated would be masked a,a-disubstituted (quartemary)
amino acids. Interest in the synthesis of biologically active products containing
quaternary carbon atoms has increased notably over the last decade.48 The
interesting properties of a- and B-amino acids with conformational rigidity, in
particular , the a,[3-disubstituted series have recently attracted the attention of
numerous research groups since the incorporation of these products into peptides
can drastically modify their properties.
.134
HX
R.—<\:I° éj R‘fiNrj Ce L.

R; L’®

®
(OH >9
O

J R2 Y
RF’X R1-<\ 1R2

/N I R3 N

'- R4
'0‘ v = o, N, -CH
Scheme II-l. Synthesis of dihydroheterocycles from mfinchnones.

During their efforts to utilize the l, 3-dipolar cycloadditions in the diversity
oriented synthesis of biomimetic probes, Austin and co-workers49 discovered that

the cycloadditions of vinyl ethers with isomilnchnones proceeds with complete

endo diastereoselectivity. Removal of chiral auxillary through an unusually facile

71

ester aminolysis resulted in efficient construction of bicyclic [2.2.1] scaffold
(Figure II- 12). The hetero-bicyclic [2.2.1] scaffolds allows for the diversity both
in terms of position and type of functionality incorporated and were used to
interact with the HIV-1 Tat/T AR system. RN A-protein interactions constitute a
rapidly growmg area of research, especially because of their prevalent role in
many cellular processes.

0

RKanH o o

R 1) condensation i
+ , ; R1 [was

0 0 2) diazotransfar R2 N2

HOMOR3

0R4
P e 0R3 ‘ RFDVOR‘ R5 R6
Re

0 R0: I
Rh(ll) R14 f0 3 *0
N o

I o O N
.. .1 R1 ‘R2
bicyclic [2.2.1] scaffold

 

 

 

 

Figure II-12. Synthesis of bicyclic scaffolds from cycloaddition of
isomunchnones.
Dihydro— and completely reduced five-membered heterocycles represent an
important addition to the repertoire of small molecules that are being used to
control complex biological processes. Mapp and co-workers50 have described the
design and synthesis of isoxazolidines, the very first examples of synthetic small
molecule heterocycles that activate transcription at levels comparable to those of a
natural activation domain. Transcriptional activators play an essential role in the
regulatory network that controls gene-specific transcription (Figure II—l 3).5 1The

misregulation of this complex event cascade is correlated with a growing number

72

of human diseases,52 and thus interest in developing artificial transcriptional
activators has intensified.53 To identify a minimal fimctional unit for a small
molecule-based activation domain, a series of isoxazolidines containing
functional groups typically found in endogenous activation domains were
designed (Figure II-13). The isoxazolidine scaffold was chosen due to the relative
ease with which diverse functional groups could be incorporated in a
stereocontrolled manner onto the conformationally constrained ring, 50’ 54 thus
displaying those groups in a three-dimensional array. Remarkably, one
isoxazolidine (Figure II-13) was nearly as active as the positive control ATF14
(natural ligand) despite a considerable difference in size (MW 290 versus 1674).
Similar to natural activation domains such as ATF14, a balance of hydrophobicity
and polarity was found to be important for overall potency.” 55 Two factors were
suggested to play a role in the isoxazolidine being nearly as active as ATF14
despite the size difference. As an organic molecule, the isoxazolidine ring is
resistant to proteolytic degradation and thus probably has a longer lifetime. In
addition, the isoxazolidine molecule likely populates conformations more closely

related to the final bound state due to structural constraints imposed by the ring.

73

“0

g , __
MN 0 [3+2] hi— 1) 14,98, N—

OH > A OH

 

 

Dihydrofolate receptor (DHFR) " u transcriptlbn

  

 

 

 

Lax A Lax A

Figure II-l3. Synthesis of isoxazolidines as mimic of transcription regulators.
E. Lewis acid mediated cycloadditions of nitrile ylides and azomethine
ylides.“

Control of the stereo-, regio-, and enantioselectivity of l, 3-dipolar cycloadditions
with Lewis acids is a relatively new area and has been examined with only a
limited number of dipoles. A common difficulty is the competition between the
dipole and the dipolarophile for the external reagent when it is a Lewis acid.
When one tries to activate electrophilic dipolarophiles with Lewis acids, binding
to the 1, 3-dipole can predominate, resulting in a slower reaction. Such strong
binding can also cause decreased reactivity of the l, 3-dipole towards nucleophilic
dipolarophiles. Strong binding of 1, 3-dipoles to a Lewis acid is often not

avoidable because of the electronic structure of the 1, 3-dipoles and the typical

74

prescence of Lewis basic sites. Accordingly, Lewis acidic reagents may combine
with the basic sites on certain I, 3-dipoles, making relatively stable salt-like
complexes. The co-ordinating power of external reagents must be carefully
adjusted in order to be successful in attaining high rate enhancements. The nature
of the ligands on the Lewis acid and the lability and exchangeability of these
ligands are typical issues to be considered.

F. Nitrile ylides and Lewis acids

i. Syntheis of 2-oxazolines

Suga et al reported that 5-alkoxyoxazoles react with aldehydes giving 4,5-cis-2-
oxazoline-4-carboxylates stereoselectively when activated with a stoichiometric
amount of methylaluminium B-binapthoxide as the Lewis acid (Figure II-l4).57
This reaction has recently been extended to a catalytically enantioselective
version using an enantiopure methylaluminium [3-binapthoxide.’8 Although the
actual reacting species were not assigned, 5-alkoxyoxazoles are known to behave
as nitrile ylide 1, 3-dipole equivalents in Lewis acid catalyzed reactions with
aldehydes. Ito et 01 developed new catalyzed asymmetric cycloadditions reactions
of formal nitrile ylides. Thus, methyl a-isocyanoalkanoates were allowed to react
with benzaldehyde or acetaldehyde in the prescence of 0.5-] mol % of a chiral
(aminoalkyl)ferrocenylphosphine-gold(l) complex to give optically active methyl

2-oxazoline-4-carboxylates with high enantoselectivity in quantitative yield.

75

0\

0’ AlMe
CO ‘(COzMe

 

 

N \ 30 mol °/o N
I1 ’ I 3. ' : :
R1/ko OMe RZCHO 111/40 'R2 ”wags" ‘7
Suga's approach:
chHO écone
[Au(o-CBH11NC)3]*BF4' N1
CNCH co Me _ / only trans
2 2 7 k0 R2 97 %ea
N/\/NEt2
l
Fe :3th
Ito's approach

Figure II-l4. Synthesis of oxazolines from oxazoles.

The oxazolines were converted into optically active B-hydroxy-or-amino acid
methyl esters. The selective formation of cis-isomers of 2-oxazoline-4-
carboxylates in suga’s reactions is synthetically complimentry to Ito’s method.59
ii. Synthesis of Az-pyrrolines.

Grigg et. a]. recently extended Ito’s reaction to the Ag(I) catalyzed 1,3-dipolar
cycloadditions of methyl isocyanate with a,B-unsaturated carbonyl compounds."0
The rate of this reaction can be enhanced effectively with a catalytic amount of
silver(I) acetate (1 mol%). Grigg et al proposed a reaction mechanism in which
silver(l) acetate adds to the carbon atom of isocyano group to form an

organometallic intermediate that is subsequently deprotonated to generate a nitrile

76

ylide 1, 3-dipole. After the cycloadditions, double-bond migration results from the

imine-enamine tautomerism to afford Az-pyrrolines (Figure II-15).6o

 

 

 

ROZC C02R
CNCHZCOZMe ca" A90“ ; 2/ S
Roc COR H
Gn'gg's approach major: trans
C e0A0 /=\
H e 9 R020 002R
A 9// Ag
ROZC C02R R02¢ C02R
U , Z/‘j
N COZMe a 002Me

 

 

 

Figure II-15. Synthesis of AZ-pyrrolines from isocyanoacetates.
iii. Synthesis of 2-imidazolines.
Although mechanistically suggested to proceed through aldol reaction, 2-
imidazolines (Figure II-l6) have been synthesized from isocyanoacetate esters
using Lewis acid catalysis. Hayashi and Ito et a] reported a diastereoselective
reaction of N-sulfonylimines with methyl isocyanoacetate catalyzed by An (I)

complexes, which provide efficient access to erythro 2, 3-diamino acids."

77

 

MeZSAuCI N .
CN H M __ / casztrans 90:10
C 2C02 e r (N R2 99 q i
l
Tosyl
«RAND
F! e PPh2
Lin's approach

Figure II-l6. Synthesis of 2-imidazolines from isocyanoacetates.
Lin et al have extended the reaction to enantioselective version using catalytic
chiral ferrocenylphosphine Au(I) complex.62
G. Azomethine ylides and Lewis acids
i. Synthesis of 2-imidazolines.
Viso et al reported the very first examples of a highly diastereoselective 1,3-
dipolar cycloadditions of azomethine ylides derived from or-iminoestcrs (Figure
II-l7) with chiral sulfinimines to produce N-sulfinyl imidazolidines and
transformation into non-symmetrical vicinal diarnines.63 Dipole generation under
standard conditions using LiBr, Et3N, MeCN or AgOAc, DBU, toluene had failed.
Use of LDA allowed for successful cycloadditions with a high degree of endo
stereocontrol. Addition of BF 3.0Et2 results in abrogation of cycloadditions
pathway and the reaction proceeds with opposite diastereoselectivity via a step-

wise mechanism.64

78

Ph
”31> THF. 20h
+ ‘

 

. R '
_ o
N’ ‘0 O 78 to4 C
)l OMe Viso's approach
Ar
8
Ph’:'\‘N ;\ OMe 0 Ph
5 I! II
: Lr-O s. 1 LAH NH
.K 10: t' ——> N4 ) . 2 NH2
5C! r "H 2)TFA N "'R
N:\ Ar 'rrR

0 Ar 002Me H0

endo approach major enantiomer
Figure II-l7. Synthesis of 2-imidazolines from azomethine ylides.
ii. Synthesis of pyrrolidines: The reaction of azomethine ylide l, 3-dipoles with
olefinic dipolarophiles to form highly substituted pyrrolidines is a reaction of
intense research interest.‘55 It has been applied towards synthesis of substituted
prolines, which can serve as catalysts and serve as important motifs in many
biologically active molecules. Among the different versions of this reaction, the
most practical approach has been the interaction between stabilized N-metalated
azomethine ylides derived from a-imino glycinate esters and n—deficient alkenes
(Figure II-18). The reaction proceeds under mild conditions and with a high
degree of diastereocontrol. Silver (1) and Lithium (1) metal cations (usually
stoichiometric amounts) are most commonly used along with an excess of tertiary
amine base. Enantiocontrol has recently been extended to this reaction raising its
stock for the generation of complex heterocyclic molecules. While Jorgensen er
al66 used chiral oxazoline complexes of Zn, Zhang and co-workers65 employed

Ag(I) complex with FAP ligands for above cycloadditions.

79

 

CO Me M60 C
a 2 :21
A _
Ph N/\fl/ ' Ph 002MB

0 fi
‘PerEt. 0 °C.
Eth. -20 °C. AgOAc. 3 mol %

Zn(OTf)2. 10 mol %

°7§<r° Mg

I 1 NH HN

5”. NJ Q)" A. g
(Bu ' 2

tBu Fe PAT 2
tBu-BOX ©\ XY'YFFAP
78 %ee. >95 % yield 60%ee, 90 % yield
J¢rgenson's approach Zhang's approach

COZ‘Bu ‘Buozc,

'_—_'/ .
PhANJWrW" e D
' Ph N "'cone
H

o ‘PerEt, -20 °C.
AgOAc, 10 mol %

O \

/N

CO PPM
I

S-QUINAP
Schreiber's approach

 

Figure II-l8. Synthesis of pyrrolidines from a—iminoesters.
Schreiber and co-workers further improved on Zhang’s protocol by expanding the
substrate scope and increasing the enantioselectivities achieved by employing the
QUINAP ligand.67 An important highlight of this work is that they were able to
generate quartenary stereogenic centers by employing substituted amino acids.

Recently, the normal endo selectivity of the above cycloadditions has been

80

reversed by Komatsu et 01 by a opportune selection of bulky phosphine ligands in
the cycloadditions of N-substituted maleimides and a-imino glycinate.68
Reversing the diastereoselectivity which is under substrate control by using
external reagents (Lewis acids) is highly desirable in a diversity oriented synthesis
program.25 Johnson and co-workers”' document reaction conditions that achieve
the first productive cycloadditions of azomethine ylides obtained from Lewis acid
catalyzed carbon-carbon bond cleavage of aziridines (Figure II-19).

in i"
N

/\ 0M8 A
Ph \ N’fil’ = ph N 002m
H

 

 

O EI3N, 40 °C.
00 6””
Pth
CO
S-QUINAP
Komatsu's approach
OMe
Ph
ifh A Ph t3 0025!
A0023 > meager
Ph 002Et ZnClz, toluene, 23 °C 5 OMe
76“ J d. r. : 1.4:1
0V?“ 0 69 %yiald
G
/g \ C?
o \O'an'"

Johnson '3 approach

Figure II-l9. Synthesis of pyrrolidines from aziridines and electron-rich alkenes.

81

 

The constitution of the cycloadduct (a [4+2] stepwise adduct or 3 [3+2]
cycloadduct) obtained was further shown to be strongly dependent on the identity
of the alkene trap employed. The activation barrier for electrocyclic carbon-
carbon bond cleavage in a prototypical N-aryl-2,3-diester aziridine was measured
by Huisgen to be ca. 29 kcal/mol, which translates to rather forcing conditions in
thermal dipolar cycloadditions.45a Johnson et a! hypothesized that the metal-
coordinated ylide is extremely electron poor and thus belongs to type III in the
Sustrnann classification system of 1, 3-dipolar cycloadditions and that rate
acceleration should be observed with electron rich dipolarophiles due to the
correct HOMO/LUMO match. Accordingly, in presence of catalytic ZnClz acyclic
enol ethers intercept the metal-coordinated ylide via the [3+2] pathway to afford
pyrrolidines in good yields and with modest diastereocontrol. It is important to
note that we have been unable to effectively perform these cycloadditions in the
absence of Lewis acid. These experiments indicate that Lewis acid promotion is
essential and provide additional support for the type III classification of this

family of cycloadditions.

82

iii. Synthesis of dihydroheterocycles.

 

 

 

 

 

H
X=(
oer R
N dipolarophile YN CO: E
\r’ 0 = xfcoza
OEt M902. 5 MOI %. 25 °C
30 _ R
if OEt
/g 9 \Q
051 \ ,M Cl
BO 0 g 2
Johnson '3 approach
dipolarophile product % yield
YN COZEt
PhCHO x COzEt 89
Ph
Ph025\ /N C023
N=\ N cogs: so
Ph PhOZS’ ph
°2N\_ /N CozEt
Ph .- CozEt 64
02" Ph

 

 

 

Figure II-20. Synthesis of dihydroheterocycles from sofi enolization of imidates.

Johnson et a1 45 have investigated N-metalated azomethine ylides formed via “soft

enolization” of (imino)glycine esters as alternatives to ylides generated fi'om the

same compounds in the classic thermal [1, 2]-prototropy manifold which require a

weak base working in concert with a metal salt.45b An evaluation of simple Lewis

acids revealed that N-malonylimidates undergo catalyzed [3+2] cycloadditions

reactions with aldehydes, imines, and activated olefins to form oxazolines,

imidazolines, and pyrrolines, respectively (Figure II-20). The reactions proceed

83

optimally at ambient temperature with the addition of 5 mol % of Mng in
CH3CN. Based on experiments the authors suggest that the Mg Lewis acid
promotes a 1, 2-prototropic shift to give a metal-coordinated azomethine ylide,
rather than ionization and proton transfer to give a nitrile ylide. The use of a
scalemic metal complex to catalyze the cycloadditions further opens up the
possibility of enantioselective variants that are not possible therrnolytically.

H. Current work

With the awareness that five-membered nitrogen containing skeletons make up
the core of several natural products and prospective libraries, the generation of
diverse small molecular scaffolds with potential biological activity is an ongoing
theme in our group. Dihydroheterocyclic scaffolds on account of their structural
rigidity and the three dimensional disposition of functional groups are perfectly
suited for this aim. They have a different shape profile from aromatic heterocycles
generally encountered in drug discovery programs and as such might access
entirely new therapeutic target. There are very few reports in literature involving
short and efficient syntheses of dihydroheterocycles, which would also result in
appendage and stereochemical diversity. During my graduate work, I discovered a
mild in situ method of generation of milnchnones (cyclic azomethine ylides) from
azlactones using Lewis acids.” 1 have applied this condition for establishing new
stereoselective methods to synthesize N-containing dihydroheterocycles; 2-
imidazolines‘m’ and A1-pyrrolines.44c The scaffolds are obtained with high

diastereoselectivity and are amenable to diverse substitution (Scheme 11-2).

84

 

Rs
LA = TMSCI N R3
: R j:
/ ,NVRg 1_<\N C0211
R2

 

 

 

_ _ R4
imine
° 0 Lewis acid 0 o 2-imidazolina
R1__<\ = R1_.<\ Io
N R2 N R2
- LA l LA=AgOAc R4 R
azlactone mllnchnone \ or TMSCI R a 3
4v 1 \ ,cozn
R4Ifl\/R3 N 9R2
alkene A1-pyrrolina

Scheme II-2. Lewis acid mediated insitu generation of mtin'chnones from
azlactones l, 3 dipolar cycloaddition with dipolarophile.

These scaffolds offer unexplored territory in terms of their structure (3 -D
disposition of functional groups) and their function (biological activity and
catalysis). In addition to the novel development of these molecules, we have been
able to establish that the imidazoline scaffolds can catalyze a novel stereoselective
reaction of ketenes with N-acylimines derived from glycine esters. This reaction is
currently under investigation in the group. Most of my work has been published in
peer reviewed reputed journals and in light of the potential therapeutic value of
the compounds as inhibitors of activation of NF-KB and enhancers of
chemotherapeutic potential of anti-cancer drugs; 69 we have been able to obtain
patents for the above work.70 The leads are being now carried through cellular and
animal studies in active collaborations with other researchers and industry. In the
future, enantiocontrol and application of the above mild method of generation of

mtlnchnones to other scaffolds, and natural products will be pursued.

85

1. Experimental procedure

General procedure for synthesis of azlactones”.

0 R2 EDCI. HCI

O O
R /U\ *COOH CHZCIZ N

1” R2

 

A solution of benzoyl amino acid (2 mmol) and EDCI.HC1 (2 mmol) in
dichloromethane (20 mL) was stirred at 0°C for 1h till the starting material
disappeared. The reaction mixture was washed successively with cold (containing
ice) water, aqueous NaHCO3 (10 mL) and water (lOmL). The solution was dried
over anhydrous magnesium sulfate, filtered, and the solvent evaporated to dryness

in vacuo giving the oxazolones as solids or oils.

2-phenyl-4-mcthyl-4H-oxazolin-S-one II-l: Cyclodehydration of N-benzoyl-L-
alanine (2 g, 10 mmol) with EDCI.HC1 (1.92 g, 10 mmol.) gave 1.8lg of IL] in
90 % yield. 'H NMR (300 MHz) (CDCl3): 6 1.6 (3H, d, J = 7.4 Hz), 4.46 (1H, q,
J= 7.5 Hz), 7.47 (3H, m), 8.0 (2H, d); l3C NMR (75 MHz) (CDCl;): 816.1, 60.3,

125.3, 127.2, 128.2, 132, 160.7, 178.2; IR (neat): 1828 cm", 1653 cm".

86

o o
Flt—(\I
N

Ph
ll-2
2, 4-diphenyl—4H-oxazolin-5-one Il-2: Cyclodehydration of N-benzoyl-L-
phenylglycine (2 g, 7.8 mmol) with EDCI.HC1 (1.5 g, 7.8 mmol.) gave 1.3g of II-
2 in 70 % yield. 1H NMR (300 MHz) (CDC13)Z 8 5.5 (1H, s), 7.35-7.65 (8H, m),
8.05-8.15 (2H, m); l3c NMR (75MHz)
(CDC13): 8 176. 4, 162.8, 133.9, 133.2, 129.2, 129.18, 129.1, 129, 128.99, 128.96,

128.9, 128.7, 128.4, 127.1, 126.1, 68.4; IR (neat) 1825 cm", 1655 cm".

0 o
911% NH
N /

Il-3
4-(lH-Indol-3-ylmethyl)-2-phenyl-4H-oxazol-5-one II-3: Cyclodehydration of
N-benzoyl-L-tryptophan (2 g, 6.5 mmol) with EDCI.HC1 (1.24 g, 6.5 mmol.) gave
1.5 g of [LB in 80 % yield. 1H NMR (300 MHz) (CDC13): 8 3.5 (2H, ddd, J, = 5
Hz, J2 = 15 Hz, J = 50 Hz), 3.5 (2H, t, J = 5 Hz), 7.05-8.0 (9H, m), 8.18 (1H,
b); ”CNMR (75MHz) (013013):
8 178.1, 161.7, 135.9, 132.6, 128.6, 127.8, 127.3, 125.8, 123, 122, 119.5, 119.1, 1

11. 1, 109.5, 66.6, 27.2; IR (neat) 3416.4 cm", 1815 cm", 1653 cm".

87

0

o
Ph’QniAFO

/0
ll-4

3-(5-Oxo-2-phenyl-4,5-dihydro-oxazol-4-yl)—propionic acid methyl ester [14:
Cyclodehydration of N-benzoyl-glutamic acid-S-metyl ester (4 g, 15 mmol) with
EDCI.HCl (3.17g, 16.5 mmol.) gave 2.6g of II-4 in 70 % yield. IH NMR (300
MHz) (CDCl3): 8 2.16 (1H, sextet, J = 6.6 Hz), 2.4 (1H, sextet, # 6.5 Hz), 2.4
(3H, t, J = 9 Hz), 3.7 (3H, s), 4.54 ( 1H, t, J = 9 Hz), 7.4-7.6 (6H, m), 8.0 (2H, d, J
= 9.9 Hz), 8.2 (2H, d, J = 9.9 Hz); 13C NMR (75 MHz) (CDCI3) 8 177.8, 172.6,
162.2, 161.9, 134.5, 132.8, 130.4, 129.9, 129.8, 128.8, 128.7, 128.5, 128.2, 127. 8,

125.6, 51.7, 29.6, 26.6; IR (neat) cm", 1828 cm", 1651 cm".

0

Me—(SI

Me
Il-5

2, 4-dimethyl-4H-oxazolin-5—one II-S: Cyclodehydration of N-benzoyl-glutarnic
acid-S-metyl ester (4 g, 30 mmol) with EDCI.HC1 (6.3g, 33 mmol.) gave 2.0g of
[LS in 60 % yield. lH NMR (300 MHz) (CDCl3): 8 1.37 (3H, t, J = 8 Hz), 2.05
(3H, s), 4.0-4.16 (1H, m); ”C NMR (75 MHz) (CDC13) 8 179.2,

162.6, 60.4, 16.4, 15.1; IR (neat) cm", 1826 cm", 1686 cm".

88

o o
Ph/-<\ N:/(p,1

ll-B
2-Benzyl-4-phenyl-4H-oxazol-5-one II-6: Cyclodehydration of N-
phenacetylphenylglycine (2 g, 10 mmol) with EDCI.HC1 (1.9g, 10 mmol.) gave
1.64g of II-6 in 90 % yield. 1H NMR (300 MHz) (CDCl;;): 8 3.96 (2H, s), 5.35
(1H, s), 7.2-7.6 (10H, m); 13C NMR (75 MHz) (CDCll) 8 176.2,165.5,133.0,
132.7, 129.3, 128.9, 128.7, 128.5, 128.4, 128.3, 127.7, 127.4, 126.6, 125.42; IR

(neat) cm", 1829 cm", 1674 cm".

ll-7
4-Methyl-4H-oxazol-5-one Il-7: Cyclodehydration of N-formyl-alanine (2 g, 17
mmol) with EDCI.HC1 (3.3g, 17 mmol.) gave 1.2g of II-7 in 70 % yield. lH NMR
(300 MHz) (CDC13): 8 1.46 (3H, t, J = 7.5 Hz), 4.13 (1H, q, J= 7.5Hz), 7.52 (1H,

s); l3C NMR (75 MHz) (CDC13) 178.2, 152.8, 58.3, 16.2; IR (neat) cm", 1817

O 0
F3C\<
N
ll-8

2-trifluoromethyl-4-Methyl-4H-oxazol-5-one 11-8: Cyclodehydration of N-

cm", 1676 cm".

tn'fluoromethyl alanine (2 g, 10.8 mmol) with EDCI.HC1(2.06g, 10.8 mmol.)

89

gave II-8 in 28 % yield. Partial data: IR (neat) cm", 1840 cm"; Pseudooxazolone:

O O
.../3.1

IR (neat) cm" 1809 cm".

Partial data: 2-Benzyl-4-methyl-4H-oxazol-5-one II-9: Cyclodehydration of N-
phenacetyl alanine (4 g, 19.3 mmol) with EDCI.HC1 (3.71 g, 19.3 mmol) gave
3.1g of II-9 as viscous oil in 82 % yield. 1H NMR (300 MHz) (CDCl;): 8 1.47
(3H, d, J = 6.9 Hz), 4.35-4.5 (2H, m), 4.77 (1H, q, J = 7.5 Hz), 7.38 (2H, t, J = 7.2

Hz), 7.48 (1H, t, J = 7.2 Hz, 7.72 (2H, d, J= 7.8 Hz); IR (neat) cm", 1824 cm",

1676 cm".
")—<° O
O \ I
>—NH N Me
0 1140
Ph/—

Partial data: Benzyl-(S)—1-(4,5-dihydro-4-methyl-5-oxooxazol-2-yl)—2-methyl
propyl carbamate II-10: Cyclodehydration of N-benzoyl-L-Val-alanine (1 g,
3.25 mmol) with EDCI.HC1 (0.63 g, 3.26 mmol.) gave 0.79 g of II-10 as viscous
oil in 84 % yield. 1H NMR (300 MHz) (CDC13): 8 0.77 and 0.79( 3H, 2d, J =4.5
Hz), 0.859 (3H, dd, J, = 2.1 Hz, J; = 6.6 Hz), 1.3 (3H, d, J: 7.5 Hz), 2.03 (1H,
m), 4.065 (1H, q, J = 7.2 Hz), 4.4 (1H, q, J = 6 Hz), 4.97 (2H, m), 5.23 (1H, bs),

7.18 (5H, bs); IR (neat): 1828 cm", 1722 cm", 1672 cm".

90

0 O
643—3 I
N Me

ll-11
Partial data: 2-(4-methoxyphenylH-methyl-4H-oxazolin-S-one II-ll:
Cyclodehydration of N-(4—methoxybenzoyl) alanine (3 g, 13 mmol) with
EDCI.HC1 (2.5 g, 13 mmol) gave 2g of [H] as a pale yellow oil in 75 % yield.
'H NMR (300 MHz) (CDC13): 8 1.547 (3H, d, J = 7.5 Hz), 3.85 (3H, s), 4.402
(1H, q, J= 7.8 Hz), 6.92-6.96 (2H, m), 7.88-7.91 (2H, m); IR (neat): 1824 cm'l,

1653 cm".

Ph—<\
N

ll-1 2

Partial data: 2-pheny1-4-isopropyl-4H-oxazolin-5-one II-12: Cyclodehydration
of N-benzoyl-L-alanine (6 g, 27 mmol) with EDCI.HC1 (5.2 g 27 mmol.) gave
4.93 g of 11-12 as a low melting solid in 90 % yield. 'H NMR (300 MHz)
(CDC13): 8 1.055 (3H, d, J = 6.9 Hz), 1.12 (3H, d, J = 6.9 Hz), 2.3-2.41 (1H, m),
4.29 (1H, d, J: 4.5 Hz), 7.47 (3H, t, J = 7 Hz), 7.56 (3H, t, J = 7.2 Hz), 7.99 (2H,
d, J = 7.8 Hz); 13C NMR (75 MHz) (CDC13): 8 17.18, 18.33, 70.13, 125.38,

127.61, 128.11, 128.47, 132.5, 161.7, 177.13; IR (neat): 1826 cm", 1657 cm".

91

O

O
Ph—QNI/Ph

"4 3

2-phenyl«-4-benzyl—4H-oxazolin-5-one II-l3: Cyclodehydration of N-benzoyl—L-
phenylalanine (2 g, 7.38 mmol) with EDCI.HC1 (1.42 g, 10 mmol.) gave 1.77g of
[L] as a white solid in 95 % yield. 1H NMR (300 MHz) (CDC13): 8 3.17 (1H, dd,
J1= 6.6 Hz, J2 = 13.8 Hz), 3.35 (1H, dd, J, = 5.1 Hz, J2 = 13.8 Hz), 4.67 (1H, dd,
J, = 5.1 Hz, J2 = 6.6 Hz), 7.17-7.25 (5H, m), 7.39-7.45 (2H, m), 7.5-7.55 (1H, m),
7.86-7.9 (2H, m); 13C NMR (75 MHz) (CDC13)Z 8 37.3, 66.5, 125.7, 127.2, 127.8,

128.4, 128.7, 129.6, 132.7, 135.2, 161.7, 177.6; IR (neat): 1823 cm'l, 1648 cm".

92

 

_
0

9°

10.

ll.

12.

13.

14.

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Tetrahedron: Asymmetry 1992, 3, 1127-1130.

Denissova, I., Barriault, L. Tetrahedron 2003,5961), 10105-10146.

(a)McKenzie, K. M.; Austin, D. J .; Department of Chemistry, Yale
University, New Haven, CT, USA.; Abstracts of Papers, 223rd ACS
National Meeting, Orlando, FL, United States, April 7-11, 2002. (b)
Savizky, R. M.; Savinov, S. N.; Nathan, D.; Crothers, D. M.; Austin, D. J.;
Department of Chemistry, Yale University, New Haven, CT, USA.;
Abstracts of Papers, 224th ACS National Meeting, Boston, MA, United
States, August 18-22, 2002. (C) Savizky, R. M.; Savinov, S. N.; Nathan, D.;
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New Haven, CT, USA.; Abstracts of Papers, 226th ACS National Meeting,
New York, NY, United States, September 7-11, 2003. (d) Savinov, S.N.;
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J. Org. Lett. 2002, 4(9), 1419-1422. (1) Savinov, S.N.; Austin, D. J. Chem.
Commun. 1999, 1813—1814.

Minter, A. R.; Brennan, B. B.; Mapp, A. K. J. Am. Chem. Soc. 2004,
126(34), 10504 -10505.

Ptashne, M.; Gann, A. Genes & Signals; Cold Spring Harbor Laboratory:
New York, 2001.

(a) Darnell, J. E. Nat. Rev. Cancer 2002, 2, 740-749. (b) Duncan, 8. A.;

Navas, M. A.; Dufort, D.; Rossant, J .; Stoffel, M. Science 1998, 281, 692-
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(a) Ansari, A. Z.; Mapp, A. K. Curr. Opin. Chem. Biol. 2002, 6, 765-772.
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R. V. Org. Biomol. Chem. 2003, 1, 3257-3260.

(a) Kanemasa, S.; Nishiuchi, M.; Kamimura, A.; Hori, K. J. Am. Chem. Soc.
1994, 116, 2324-2339. (b) Bode, J. W.; Fraefel, N.; Muri, D.; Carreira, E.
M. Angew. Chem, Int. Ed 2001, 40, 2082-2085. (c) Minter, A. R.; Fuller,
A. A.; Mapp, A. K. J. Am. Chem. Soc. 2003, 125, 6846-6847.

(a) Drysdale, C. M.; Duenas, B.; Jackson, B. M.; Reusser, U.; Braus, G. H.;
Hinnebusch, A. G. Mol. Cell. Biol. 1995, 15, 1220-1233. (b) Regier, J. L.;
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(c) Lin, J. Y.; Chen, J. D.; Elenbaas, B.; Levine, A. J. Genes Dev. 1994, 8,
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W.; Ebright, R. H.; Triezenberg, S. J. Nucleic Acids Res. 1998, 26, 4487-
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1,3-Dipolar Cycloaddition Chemistry Towards Heterocycles and Natural
Products, Vol. 59: The Chemistry of Heterocyclic Compounds; Jon Wiley
and Sons, INC., New Jersey, 2002, 757-804.

(a) Suga, H.; Shi, X.; Ibata, T. J. Org. Chem. 1993, 58, 7397. (b) Suga, H.;
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(a) Ito, Y.; Sawamura, M.; Hayashi, T. J. Am. Chem. Soc. 1986, 108, 6405.
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Y.; Sawamura, M.; Shirakawa, K.; Hayashizaki, K.; Hayashi, T.
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Hayashi, T.; Kishi, B.; Soloshonok, V. A.; Uozumi, Y. Tetrahedron Lett.
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Zhou, X-T.; Lin, Y-R.; Dai, L-X. Tetrahedron Asymm. 1999, 10, 855-862.

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2003101969.

98

CHAPTER III
SILICON MEDIATED DIASTEREOSELECTIVE MULTI-COMPONENT
SYNTHESIS OF IMIDAZOLINES.

A. Biological Importance of 2-Imidazolines and Imidazoles

Derivatives of 2-imidazoline and 2-imidazole have attracted substantial interest
due to their interesting biological activities.1 Imidazolines are known to exhibit a
wide range of pharmacological activitiesz, including a-receptor stimulation;
vasodepressor activity; a-adrenergic inhibition; and syrnpathomirnetic,
antihistaminic, histamine-like, and cholinomimetic activity. Imidazoline
derivatives have also been found to exhibit anti-inflammatory], anti-nociceptive,
immuno-modulating, antioxidant activities‘. 4,5-Dihydro-lH-imidazoles (2-
imidazolines) are useful intermediates for the synthesis of molecules with
pharmacological activities such as anti-hypercholesterolemic, anti-diabetic, anti-
hypertensive, or anti-cancer activity.3' 5 Cyclic ureas and cyclic thioureas are
reported to be potent inhibitors of human immunodeficiency virus (HIV) protease
and HIV replication.6 2-imidazolines are also convenient building blocks for the
synthesis of pharrnaceutically relevant molecules such as azapenams,
dioxocyclarns, and diazapinones.7 Also, suitably functionalized 2-imidazolines are
easily converted to 2, 3-diamino acids, which are incorporated in a wide range of
antibiotics and other biologically active compounds.8 Streptolidine lactam9 which
incorporates a y-hydroxy-a, B-diaminoacid moiety, is at the core of streptothricin
antibiotics, which due to their high antibacterial activity have drawn a great deal

of synthetic attention (Figure 111-] ). The structures of imidazolines and imidazoles

99

are analogous to those of oxazolines and oxazoles, or thiazolines, and thiazoles,
respectively. The latter heterocycles are found in numerous macrolactams isolated
from marine sources.lo Imidazolines have been easily converted to imidazoles and

incorporated into macrolactam analogues of bistratamide H without loss of

stereochemical integrity.
H020)
H02C
rodent central nervous system 1" V5110 @"fi'hYPOfQ'YOBMiC
cholecystokinin inhibitor actiwtv In rats
NH

N\ N~R1

HNJLNH
HI 0 . ”
Hot-o OJ
NH R

streptolidine lactam
core of streptothricin antibiotics 59131001" 790091“ 89005818

Figure III-1. Biological activity of molecules with 2-imidazoline nucleus.
Recent studies have also revealed that the imidazoline ring is a required
pharmacophore for certain potent anti-hyperglycemic properties. Imidazoline
derivatives, such as midaglizole, deriglidole, and efaroxan, have been identified as
promising anti-hyperglycemic agents.4 An imidazoline moiety has previously
been incorporated into a known antihypertensive agent, clonidine.‘l During the
past decade, the concept of imidazoline (1) receptors has been developed and
gained consensus.‘2 Findings from different laboratories have shown that they are

widely distributed in different tissues and species and may participate in the

100

regulation of various physiological functions. Moreover, recent evidence suggests
their involvement in diverse pathologies, and therefore a more definite knowledge
of the structure and function of this receptor system could help in the search for
therapeutic agents useful for treating efficaciously a variety of disorders such as
hypertension, diabetes mellitus, gastric ulcer, endogenous depression, and

13

stroke. Imidazoline derivatives have been studied as a2-adrenoceptor or

3b’ I" '4' hnidazoline rings with appropriate

estrogen receptor modulators.
substituents can serve as amino acid replacement in a peptide”, making it less
susceptible to proteolytic cleavage and hence improve the stability and therapeutic
utility peptides.

In fact, the imidazoline scaffold like other rigid five-membered heterocycles can
mimic functions of natural peptides and hence are of great therapeutic value."5
Developing small-molecule inhibitors of non-enzyme protein-protein interactions
has been a difficult task because binding pockets on protein surfaces involved are
shallow and dynamic as compared to enzymes which have well defined and rigid
binding pockets or active sites.17 However, the crystal structure of MDM2 bound
to a peptide from the transactivation domain of p53 has revealed that MDM2
possesses a relatively deep hydrophobic pocket that is filled primarily by three
side chains from the helical region of the peptide.18 The existence of such a well-
defined pocket on the MDM2 molecule raised the expectation that compounds

with low molecular weights could be found that would block the interaction of

MDM2 with p53. One such class was a series of cis-irnidazoline analogs named

101

 

HO
1m
\\/N
, 1° 0

/

~<° Om

Figure III-2 (a&b): p53 bound to MDM2; key residues for binding (c) Overlay
of p53 residues and nutlin (d) The bromo phenyl moiety of nutlin deep in the
groove of MDM2 (e) Representative structure of a nutlin. Adapted from Ref. 18
and 19.

102

Nutlins (Figure 111-2).19 The imidazoline scaffold thus replaces the helical
backbone of the p53 peptide and is able to direct, in a fairly rigid fashion, the
projection of three groups into the pockets normally occupied by Phe”, Trp23, and
Len26 of p53. By binding MDM2 in the p53-binding pocket the cis-imidazolines
activate the p53 pathway in cancer cells, leading to cell cycle arrest, apoptosis,
and growth inhibition of human tumor xenografis in nude mice. This example has
served as a paradigm for the study and development of other small molecule
inhibitors of protein-protein interactions (SMPPI’s).19

B. Chemical significance of imidazolines

The most direct chemical utility of imidazolines has been their conversion to
chiral diamineszo, which are widely preferred group of ligands for asymmetric
synthesis. There is definitely a need for more synthetic methods, which can
provide optically active imidazolines with greater choice of substituents. Hegedus
et al have efficiently reacted optically active imidazolines photochemically with a
chromium carbene complex to obtain azapenams2| in good yield and high
stereoselectivity. C2 Symmetric imidazolines have been shown to induce excellent
to modest acyclic diastereoselectivity to form quartemary benzylic centers”,
which to date remains a synthetic challenge. MacMillan et al have reported a very

23, as catalysts for highly enantioselective

impressive use of irnidazolidinones
indole alkylations and Diels-Alder reactions. This represents an important
example in the fast growing field of organocatalysis. Furthermore, chiral 2-

imidazolines have attracted considerable interest as templates for asymmetric

synthesis 24 and as chiral ligands for asymmetric catalysiszs’ 26 and have found

103

wide application as potent N-heterocyclic carbene ligands in organometallic
catalysis.27 N-Heterocyclic “carbene” ligands derived from imidazole were
recently reported as alternatives for phosphines in organometallic catalysis. The
authors claim that unlike the phosphines, the new imidazole ligands do not
dissociate from the transition metals and hence provide sturdy catalysts.”

C. Lewis acid mediated synthesis of imidazolines

The synthesis of stereodefined imidazolines has been scarcely reported despite
their usefulness. Methodologies are limited by the difficult availability of the
precursors, and for this reason the variety of groups bonded at C4 or C-5 of the
ring is quite limited. Most of the imidazolines described are only 4-suhstitutcd28
or 4, 5-disubstituted with the same group (generally phenyl).29 Only a few
examples of syntheses of 4, 5-disubstituted imidazolidines (containing a range of
groups) have been reported?“ 28¢ 30
As part of our program to develop small molecular weight scaffolds containing a
high degree of diversity, we focused on developing a stereoselective synthesis of
substituted imidazolines. 1, 3-Dipolar cycloaddition reactions utilizing N-
methylated mesoionic oxazolones (or milnchnones) provide a general route for the
syntheses of pyrroles and imidazoles.”33 A moderate equilibrium concentration
of the mesoionic tautomer is essential for the 1, 3-dipolar cycloaddition leading to
heterocycle formation.34 By N-methylation, the azlactones are locked in the
mesoionic form, which is extremely reactive with dipoles and affords the
heterocycles in very high yields. Imidazoles can be generated by treatment of the

mtlnchnones with tosylimines. Addition of the tosylimines results in an unstable

104

bicyclic adduct that loses carbon dioxide and benzenesulphinic acid to gain
aromaticity.32’ 35 The phenylsulphonyl residue as leaving group enhances the
tendency to aromatize (Scheme III-1). Munchnones with different substituents, at
the 2 and 4-positions lead regioselectively to products in which a bond is always
formed between the 2-carbon atom of the dipole and the imine nitrogen atom.
This attack was explained on the basis of a dipole-dipole interaction between
partially negative nitrogen of the imine and the electrophilic C-2 atom of the
munchnone. The yield reported for these reactions are low at least partly due to
self-condensation of mllnchnones.32 The problem of self-condensation can be
suppressed by using a solid-phase approach towards the preparation of
imidazoles.33 The munchnone generated on solid phase was condensed with

tosylamines to afford the imidazoles in good yields and purities.

|
N-SO Ph
P“ o e ’11 2 Ph N CPO?“ N R2
Y / 0 R2 0% .N 3 - 002 ’41
Me ,N ———> U H ——-> ph N R1

Scheme III-1. Addition of the tosylimines leads to loss of CO2 and PhSO2H to
gain aromatrcrty.

Surprisingly, the utilization of oxazolones (or azlactones) has not yet resulted in
an efficient entry into a stereoselective highly diverse class of imidazoline
scaffolds (Refer Chapter 2). It has been well documented that azlactones undergo
a Staudinger-type reaction when heated with imines in toluene yielding B-

lactams.”38 The azlactone, which is in equilibrium with valence tautomeric

105

ketene intermediate, undergoes a [2+2] cycloaddition reaction with the imine
(Scheme III-2). It can be concluded that either mesoionic oxazolium oxide, which
has the nitrogen atom unsubstituted, is not in reasonable concentration and/or the
imine nitrogen is not nucleophilic enough. Hence formation of ketene takes

predominance, which reacts readily with the imines to afford the B-lactam.”38

 

 

11
Ph NH H
’1: Ph 0
ph NA toluene ~l ‘ H
:(O + OPhJNI Ph * J ... Ph/ILN NvPh
reflux ,._~
/ s H
o CH2Ph 0
ll-1 lll-I-‘l O Ill-A Ill-B
Ph
Ph CH Ph
NH 1 2
O p PHCO H N
Ph ’ /\ H30 11 H
\( C + N| PD .......
N—< P11) N6) CH3 Ph
90 CHzPh 0
methyl outward or exo
lite l
Ill-B Ill-A

Scheme III-2. Staudinger reaction imines and ketenes generated from azlactones.
The S-oxazolones (azlactones) in this study were prepared from different N-acyl
a-aminoacids by EDCI-mediated dehydration to provide the pure oxazolones in
high yields.” 4-methyl-2-phenyl-2-oxazolin-5-one (II-1) synthesized from N-
benzoylalanine, when heated with N-benzilidene-benzyl amine in toluene under
N2 atmosphere to reflux, gave B-lactam III-A and the amide III-B.4o The
azlactone is in equilibrium with valence tautomeric ketene intermediate, which
then undergoes a ketene-imine or the Staudinger“1 type reaction to form the B-

lactam (III-A). The amide III-B is obtained by hydrolysis of the uncyclized

106

zwitterionic intermediate (Scheme III-2). Theoretical studies have predicted that
torquo-electronic effects play a dominant role over steric effects in the
cycloaddition of the zwitterionic intermediate to the B-lactam III-1a. 42 The imine
preferably adopts the more stable (E) geometry because of the bulky aromatic
substituents and the methyl group (positive inductive effect) on the ketene adopts
an “outward” or exo position to the direction of attack by the imine. Electron
withdrawing benzamido substituents adopts an endo or “outwar ” position in
accordance with the predicted model for these cycloadditions. A conrotatory
electrocyclic ring closure affords B-lactam with cis orientation for the methyl and

phenyl groups in 30 % yield as reported before.“l

RJ C,0 _>R1/u—NH

fig .1 w"
Toluene R2 ‘R4

 

 

 

heat R3
ISO-50%
o 0
R1‘<\NI
R2 R J’Rt R. R
1 o 3
L» tree . H :I
Lit/G) R2 CO2”

 

 

Scheme III-3. Lewis acid mediated generation of munchnone and cycloaddition
We envisioned that a Lewis acid coordination to nitrogen of the azlactone would
increase the equilibrium concentration of munchnone intermediate and mediate
the azomethine ylide-imine cycloaddition reaction. The choice of the imine was
important as unactivated or electron rich imines have rarely been used with

mllnchnones before (tosylimines lead to enhanced aromatization). Only one

107

example where N-benzylidine-benzylamine (III-I-l) was used as the imine is by
Arndtsen and co-workers.43 They used Pd-catalysis to couple basic building
blocks like an imine, acid chloride and carbon monoxide to generate N-alkylated
munchnone in situ. Mild acidic conditions were suggested to accelerate the
reaction to afford the products as N-alkylated imidazolium salts. The main
shortcoming for this method is that only symmetrical imidazolines can be
obtained as two equivalents of the imine are involved in the synthesis, one for
generation of munchnone and other for cycloadditions. Also, only electron rich
aromatic aldehydes can be used. Electron deficient imines and phosphine ligands,
both stop the cycloadditions completely. One advantage was that the imidazolium

salts were easily isolated from the reaction mixtures by precipitation fiom ethereal

 

solutions.
55 °c
[Pd2(dba)3J (5 mol %) Ph
R2 0 2, 2'-bipyridine (10 mol %) Rt-(aékwla,
)_.___N\ + /U\ + co >
H R1 R3 Cl CH3CN Rz/thco?

Figure III-3. Generation of imidazolium salt through Pd-catalysis.
D. Lewis acids for in situ generation of miinchnones and cycloaddition
With one equivalent of SnCl4, TiCl4, ZnClz, silver triflate only degradation was
observed in the reaction of 4-methyl-2-phenyl-2-oxazolin-5-one (II-1) and N-
benzilidene-benzyl amine (III-I-l) in toluene. Harder lewis acids such as TiCl4
and BF3-OEt2 or protic acids such as camphor sulfonic acid (CSA) and methyl

sulfonic acid (MSA) did not promote any product formation (Scheme III-4). Cu

108

and Al based lewis acids or lanthanides were not encouraging either. Only with
TMSCl, on refluxing in toluene a White precipitate separated from the reaction
mixture in 75 % yield, and was isolated and characterized as the imidazoline—4-
carboxylic acid (III-l-l, Figure III-4).

fl-lactam (in CDClg

N H

,tPh

 

Z

d— s
o CHzPh

III-A

 

 

 

 

Imidazoline (in DMSO-dd
Bn

3.; Ph
Ph—(\ IMe

N cozH

lll-1-1

 

 

 

ITTTT[TYYTIrTTYl'YYYYIYYYTTITYY IIVIIYIYVUfTYIYI

T T Y Y
10 9 8 7 6 5 4 3 2 I

Figure III-4. (a) 1H spectra for a representative B-lactam (III-A) and imidazoline
(III-l-l).

109

B-Iactam: IR (car‘) 1751, 1654.
0

JL

Ph NH

I’r,'I ‘\\ Ph

I

 

i

4

o ‘CHzph

Ill-A

 

MM -lil--l 2i

ww w 'v - w

 

I I I I I I I I I I I I I T I I I I I I I I I I I I I
200 175 150 125 100 75 50 25

Imidazoline: IR (cm'1) broad 3600-2500, 1734, 1610.

 

 

 

 

 

 

3'!
N Ph
Ph—<\ |,.Me

N 00214

lll-‘l-1
1‘ ‘T ‘ 5V. ‘3, “AC. ‘ ,w _.. ,f w “ ,2,in W Z, ___L .7. ,‘,_w ,, lw‘ LQAVV LJWJYP‘T‘ ‘22
I I I I I I I I I I j I I I I I I I I I I I j T I I I I I I I I I I I I I I I

200 175 150 125 100 75 50 25

Figure III-4. (b) 13C spectra for a representative B-lactam (III-A; in C130,) and
imidazoline(III-1-1; in DMSO-dg).

110

   
 

c1151 o: o
0 o

III-1-7

Figure III-4. (c)cht: X-ray crystal structure of III-l-l. Right: X-Ray crystal
structure of III-l-7.
The relative stereochemistry has been established unambiguously by X-ray
crystals structure of III-l-l and III-1-7 (Figure 111-4).
The remaining filtrate is made up of 15-20 % of the B-lactam (Scheme 111-2, 111-
la). Subsequently, the same results were obtained in THF and dichloromethane as
solvents. We were encouraged that TMSCl promotes the reaction of azlactones

and imines to afford the imidazoline-4-carboxylate scaffold in very good yields as

111

a single diastereomer, with trans- relation between C4 and C5 substituents

 

(Scheme III-4).
Ill-H
Ph N.
Ph 0 V 80 F"‘_\
11" LA N
N —-—> Pit—s .
0112012 N
111 C02“
' 111-1-11-
LA 96 yield LA % yleld
“Ch 0 msc1 70
811014 0
ch12 0 1 eq.TMSC|/1 eq. 5th 0
EtzAlCI 0 2 eq. TMSCI I 1 eq. £th 0
CuCl 0 T5801 70
31°Phl3 0 PhasiCl 45
A90" 0 CHgCOCl 55
“3'3 ° CSA 0
ZrlO-Prlt 0 TMSOTf 0

Scheme III-4. Lewis acid screen for the synthesis of imidazolines.

E. Possible mechanism for synthesis of imidazoline

While the complete mechanistic details of this process are still under
investigation, the reaction does not seem to proceed via a ring-opened nitrilium
ion intermediate (Scheme III-5).“ ‘5 A common method to trap nitrilium ions is
by nucleophiles like water or methanol. After addition of TMSCI, the reaction
mixture was refluxed with saturated sodium acetate solution. The azlactone was
obtained intact and not even a trace of O-acetylimidate was observed. The

possibility of a step-wise Michael-type addition (via the formation of the nitrilium

112

ion) and subsequent cycloaddition was investigated by first preparing the silyl
enol ether46 with TMSCl (1.0 equiv) and TEA (1.0 equiv) followed by the

addition of the imine and an additional one equivalent of TMSCl.‘7'5°

R1
0 0 TMSCI 0W5 N=\
R1 \ _IEA_. R:lt—<\ +fi3
~(N CHzclz R
R2 2 TMSCI
R4 R
TMS-N R2 H o N H R3
_ 2
Fit-=87???“ ——» R...- I
R3 cozms N COzH
nitrilium ion

Scheme III-5. Nitrilium ion intermediate is not involved in the formation of
Imidazolines

This resulted merely in isolation of starting materials. Excess of triethylarnine also
halted the reaction suggesting that acidic conditions were required. TMSOTf
(likely O-silylation) also did not result in any product formation. This indicates
the requirement of a nucleophilic counterion to establish possible equilibrium
between O-silation and N-silation and a probable C-silation of the azlactone
(Scheme 111-6). The role of counterion in establishing such equilibrium has been
reported by Wilde in his work with N-acyl munchnones." The generation of
azomethine ylides by “silatropy " has recently also been reported by Komatsu and
co-workers.”'54 The authors report the formation of an azomethine ylide after a 1,
2-silatropic shift of or-silylimines resulting in the formation of pyrrolidines as a

mixture of stereoisomers.

113

A §1M93

MfiSiYNVRZ = GrgWRz
R1 1,2-311atropy R1
ot-silyl imine azomethine ylide
Komatsu's approach
0 OSiMe3
o_§\ M63$iCI O \
__‘ ...
R1/k\ R2 R1/A\N R2 ”0'
Me38iCl 1 M93810“
0
0 M SiCl °
’g-KQ + H01 4:; O SlMea+ HCI
R1 9 '1‘ R2 Ri/Q R2
SiMeg N
N-silyl munchnone
Our approach

Scheme III-6. Generation of azomethine ylides by silatropy
Johnson et al have investigated N-metalated azomethine ylides formed via “soft
enolization” of (imino)glycine esters as alternatives to ylides generated from the
same compounds in the classic thermal [1, 2]-prototropy manifold (Figure III-5),
which require a weak base working in concert with a metal salt.44 In light of these
findings, we proposed that the reaction proceeds via a 1, 3-dipolar cycloaddition
(Scheme III-7). While N-silyation results in reactive munchnone, acidic
conditions, during the reaction activate the imine. This double activation should
result in accelerated 1, 3-dipolar cycloadditions affording the bicyclic
intermediate. NMR studies in CDC13 at elevated temperatures (not shown) indeed
confirm the rapid consumption of azlactone in presence of the dipolarophile. The
azlactone does not show any appreciable change in presence of TMSCl, if not

heated with dipolarophile.

114

Lewisacid

 

 

. R
Mlld base I
sz N YCOZR‘ = R2V$®COQR1
H via 1,2-prototropy
R = H, Metal
General approach
H OMe
RZYNYCOZMG MQCIZ Rng 9 \0
0151 002m: 31:10 \0’ M9012
Johnson's approach

Figure III-5. Sofi enolisation vs. 1, 2-Prototropy

Other silyl chlorides such as triphenyl silyl chloride and triethyl silyl chloride also

provided the product as a single diastereomer, in notably longer reaction times

and lower yields (Scheme 111-4, 45% and 70%, respectively). Additional support

of the proposed mechanism is shown in Scheme III-7. We found that one

equivalent of acetyl chloride resulted in the formation of the imidazoline as well,

albeit in somewhat lower yields (Scheme Ill- 4, 55%)."

o R" ° f
o TMSCI R1 1)
R1‘<\ I = N R R3
N R2 ms 2
- R4 TC?
. H
RI; E" R3 H20 N E R3
Rt-<\ . R ‘— R‘_<(3 "R2
o\ 2 ’
N 00211 ms 602”

 

 

 

 

 

 

 

 

c?

Cl

 

Scheme III-7. Proposed mechanism for the formation of imidazolines via the l, 3
dipolar cycloaddition

115

F. Diversity

The cycloaddition reactions proceeded well with a wide variety of imines at
slightly elevated temperatures (dichloromethane, reflux) to provide the highly
substituted imidazolines in very good yields. Only the trans diastereomers (with
respect to R2 and R3) of the imidazolines were observed in almost all cases, as
determined by NOE experiments and X-ray crystallography (Table 111-1). A range
of structurally diverse imidazolines were prepared and are listed in Table III-l and
-2. We have recently reported this highly diastereoselective multi-component one-
pot synthesis of substituted imidazolines.” 56 These low molecular weight
imidazoline scaffolds contain four point diversity applicable to alkyl, aryl, acyl,
and heterocyclic substitution (Table III-1)

R4 groups/amines: 4-methyl-2-phenylazlactone (II-1) derived from N-benzoyl
alanine was reacted with a host of imines (lII-I-l - III-I-14) to afford
imidazolines in moderate to good yields. Benzyl, aryl and aminoacid ester derived
imines of benzaldehyde could be used with good yields of imidazolines. Electron
deficient 4-fluoroaniline as well as electron rich 4-methoxyaniline could be used
successfully. However N-benzhydryl group is too sterically encumbered to afford
any imidazoline (entry III-l-6, Table III-1).

R3 group/aldehydes: Although aromatic aldehydes generally afforded cleaner
reactions and higher yields, it is probably a reflection on the purity of imines
derived from other aldehydes and not the cycloadditions. Benzaldehyde, p-
anisaldehyde (4-methoxybenzaldehyde) derived imines reacted in very good

yields. Electron deficient ethyl glyoxalate and 2-nitrobenzaldehyde can also be

116

used and afford good yields for imidazolines. Among the heterocyclic aldehydes,
pyridine 4-carboxaldehyde was a good substrate, while with unprotected pyrrole-
2-carboxaldehyde or indole-3-carboxa1dehyde, imidazolines were not obtained at
all. Aliphatic aldehydes could not be used because the imines suffered from
possible enamine formation were not obtained in sufficient purity. We have also
been able to extend the method to 5-unsbstituted imidazolines with the use of
formaldehyde imines formed in situ by Lewis acid mediated cracking of triazines
[data not shown].

R2 group/amino acids: The diversity in R2 substituents result from the use of
various amino acids. Both natural and unnatural amino acids which are suitably
protected can be utilized for the formation of azlactone component of this
cycloadditions. Glycine could not be used because the corresponding 2-phenyl-5-
oxazolone does not enolize under the current conditions. Simple or-alkyl amino
acids; alanine, valine, isoleucine and phenylalanine can be used (Table 111-2) and
their corresponding 2-phenyl-4-a1kyl-5-oxazolones result in excellent yield of
imidazolines with N-benzyllidine-benzylamine (III-I-l). Phenylglycine
(unnatural amino acid), tryptophan, and glutamate (acidic) have also been used
with good results.

R1 group/acyl halides: Phenyl and substituted phenyls, alkyl and benzyl groups
could be introduced. Heterocyclic groups like 2-furyl and 2-pyridy1 groups could
not be used as the corresponding N-acyl-ol-amino acids could not be cyclized to

the azlactones.

117

2‘2"

 

 

R4 NHz
Ph\(° Rcho :Phxz‘ Ph\«N ZR;
TMSCI (1.3 eq. ) COZH
Me DCM ICOZH
azlactone ”flux tren cis
s
1H Ill-1a Ill-1b
96 Yield Ratlo
Entry R3 R‘ a mi:
lll-1-1 Ph Bn 75 >95:5
III-1-2 4-methoxyphenyl Bn 78 >95:5
Ill-1-3 Ph 4-fluorophenyl 74 >95:5
Ill-14 4-pyridinyl Bn 76 >95:5
Ill-1-5 -COzEt 4-fluorophenyl 72 >95:5
111-1-6 Ph benzhydryl 0 —
Ill-1-7 Ph -CH2C02Me 7O >95:5
lll-1-8 Ph -CH(CH3)C02Me 66 >95:5
Ill-1-9 Ph -CHZCH2C02Me 51 >95:5
Ill-1-10 Ph Bn 75 ethyl ester
111-1-11 4-pyridiny1 Bn 76 ethyl ester
Ill-1-12 Ph Bn 79 alcohol
III-143 Ph H 70 >95:5
Ill-144 Ph 4-methoxy phenyl 50 >95:5

Table III-1. Variation of R3 and R4 groups

118

Derivatives: The imidazolines can also be suitably derivatized into esters,
alcohols or deprotected to afford N-unsubstituted imidazolines in excellent yields

(entries III-1-10, III-l-l 1, III-1-12, lII-l-l3, Table III-1).

 

 

13.-NH: 2‘
R1\«OR RacHO :R1\«E‘IZC::H R1\« R3
TMSCI(1.3eq.) N_Z R2
reflux OCOZH
azlactone trans2 053
II Illa lllb
Entry R1 R2 R, R. 11 Y1eld(ll) $6 Yleld Ratlo
(Illa) (atb)
111-1 -1 Ph Me Pb Bn 75 75 >95:5
Ill-2-1 Ph Ph Ph Bn 70 65 >95:5
111-24 Ph Ph 4-pyridiny1 Bn 75 55 >95:5
"1-2-5 Ph Ph £0261 4-fluorophenyl 75 68 >95:5
Ill-34 Ph indolyl-S-methyl Ph Bn 80 68 >95:5
lII-4-1 Ph -CH2CH2002Me Ph Bn 69 60 >95:5
111-5-1 Me Me Ph Bn 60 74 50:50
Ill-6-1 Bn Ph Ph Bn 90 30 75:25
Ill-9.1 Bn Me Ph 8n 76 15 67:33
4-methoxy .
111-11-1 pm“ Me Ph Bn 75 62 >955
lll-12-1 Ph iso-Propyl Ph Bn 90 71 >95:5

Table 111-2. Diversity of various groups on the imidazoline scaffold
G. Proposed origin of Diastereoselectivity
The diastereoselectivity appears to primarily arise from the steric interaction of
the bulky silyl group of the azlactone and the R3 moiety of the imine (Scheme III-

8). This results in the imine approaching from endo type of orientation (with

119

respect to R3) of the racemic azlactone resulting in the trans-isomer as the
major/sole product. The electronic effects of the R. position also appear to play a
significant role in the reactivity of the azlactone as well as the diastereoselectivity

of the reaction.

SK H R2 N. C02H
Major
(”R.;
R1_(N (”KO .\‘R3
\Si :\I\R3\ R2 M. COZH
lnor

Scheme III-8. Origin of diastereoselectivity
Reducing the resonance of the stabilized carbocation of the dipolarophile by
changing R1 from a phenyl to a methyl or benzyl group resulted in a significant
decrease in diastereoselectivity (Table 111-2). With R1 was a benzyl group, there
was significant erosion of diastereoselectivity. But it is clear that substituents R2
still controls the outcome. Bulky phenyl as R2 results in a 3:1 ratio in favor of the
trans-imidazoline, while when R2 is methyl, a 3:2 ratio of diastereomers was
observed. The more reactive and unstable 2, 4-dimethylazlactone provided a 1:1
mixture of the cis- and trans-products. Complete switching of diastereoselectivity
to the cis-imidazoline was seen when the positions of the methyl and phenyl
groups were exchanged at C2 and C; (data not shown). Figure III-6 shows the
Chem 3D minimized structures of azlactones with Me and Ph groups at C2 and C4

positions. It is clear that the steric volume needed by a C2 methyl results in

120

blocking the approach of E-imine required for the trans product. In comparison,
the phenyl group can be rotated out of plane and hence is more directional and

makes C2 less hindered for approach of imine to result in trans (major) product.

 

Figure III-6. Chem3D models for minimized structures of TOP (lefi) :2, 4
dimethylazlactone, TOP(right): 2-Methyl—4-phenyl azlactone, BOTTOM: 4-
methyl 2-pheny1 azlactone

H. Synthetic methods for chiral imidazolines

In contrast with more traditional lewis acids, the application of organosilicon
compounds as lewis acids in selective organic synthesis has a relatively brief
history.57 Chiral silicon lewis acids are rarer to find in literature because of
difficulty associated with their generation. Hence we decided to follow some

conventional methods to get our hands on enantiomerically pure imidazolines.

1) Resolution of Enatiomers.

121

In view of the biological importance of imidazolines, it was important to evaluate
bio-activity of the separate enantiomers (chapter V). In order to expedite this
study the diastereomeric menthyl esters of imidazoline III-l-l were prepared
(Scheme 111-9) and separated by column chromatography. Although the Rf values
for the diastereomers are very close, one of the diastereomers was separated clean
in very small amount from the column and was subjected to hydrolysis with LiOH
to explore the feasibility of this step (data not shown). The hydrolysis did not
undergo as planned with the mild base and use of menthol was abandoned to
screen for a chiral alcohol that would allow a better separation of diastereomers

by column chromatography.

R‘N R‘N
R3 EDCI. HCIIDMAP
R1_<\ REFER: > R‘—<::§; + R‘—<‘ :lR:

COOH fiat-I £0 7:0
O O\Z%Z

 

separate and d e
(1R. ZS. 5R) -(-)-Menthol R: 1 R4 W W
N R3 N 3:
R‘—<‘~ ...R, R.—<\ :La,
0 7:0
H0 H0

Scheme III-9. Resolution of racemic imidazolines using (-) Menthol
The use of (+) R and (-) S-l-phenylethanol resulted in better separation of
diastereomers (chapter V) and racemic imidazoline (III-2-l) was resolved using
(+) R-methyl benzyl alcohol. EDCI.HC1 mediated esterification of the racemic
imidazoline III-Z-l and the auxilliary resulted in a mixture of diastereomers,
which were separated using column chromatography. Hydrolysis of the

diastereomers resulted only under reflux with 2N NaOH to give enantiopure

122

imidazolines (-)III-2-l and (+)IlI-2-l. Compound (+)III-2-l was successfully
crystallized in CHClg/octane and the x-ray structure is shown in Figure III-7.58
Even though we were able to successfully resolve the racemic imidazolines by
converting to diastereomeric esters using chiral alcohols and subsequent
hydrolysis, getting the cycloaddition would be more desirable because of the
application for the imidazolines (refer chapter V).

1) Me

Ph
W Ph Ho’\© PhW Phj
Ph_<NI A N Ph N ,tF’h
‘N , COzH EDCI. DMA'P P'H‘NICOzH + Ph—«NlMCOZH

P“ 2) 2N NaOH,reflux P“ P"
(+/-) Ill-2-1 overall 44% (8,8) lll-2-1 (RR) lll-2-1

 

Scheme III-10. Synthesis of racemic imidazoline III-2-l, (-) S, S- III-Z-l and (+)
R, R- III-2-l.

 

Figure III-7. X-Ray crystal structure of (+) (R, R) - III-2-1.

123

2) Use of Chiral auxillaries

i. Chiral acid chlorides

Acetyl chloride was observed to promote the reaction hence in order to explore
the possibility of asymmetric induction by chiral acid chlorides, 4-methyl-2-
phenyl-Z-oxazolin-S-one (II-l), was heated with N-benzilidene-benzyl amine
(III-I-l) in presence of one equivalent of S(-)-camphanic chloride in
dichloromethane (Scheme III-l l). The imidazoline-4-carboxylic acid product was
converted to the ethyl ester and analyzed on a chiral HPLC column. No

enantiomeric excess was observed (data not shown).

0
Ph 0 Cl Phj
0 1) (1 S)—(-)-Camphanic chloride
[3'1wa + NI) > '3th Ph
N pr.) CHZClz, reflux N -.,’ 0025'
".1 Ill-I-1 2) (00002. EtOH (i) III-1.10

 

Scheme III-l l. Asymmetric induction by chiral acid chloride

ii. Chiral amino acid derived R1

We decided to use an amino acid as a chiral auxillary to influence enantiocontrol
via the azlactone. Coupling of N-cbz-L-valine with L-alanine methyl ester and
subsequent hydrolysis with 2N NaOH resulted in the dipeptide N-cbz-L-val-L-
ala-CO2H. The dipeptide acid was then cyclized with EDCI.HC1 to obtain the
corresponding azlactone (II-l0), which was allowed to react with N-
benzylidinebenzylamine in the usual conditions. One diastereomer has been
isolated from the reaction mixture and characterized, demonstrating that this is a

feasible method. Removal of the chiral auxillary is not possible easily. However,

124

the product is an imidazoline based amino acid and Kelly and co-workers have
used similar strategy to develop enantiopure imidazoline-amino acids and

incorporated them into biologically relevant macrolactams.lo

/\OJL ONIKOO EDCI. HCI Ph/\ /\O/u\ OLOO

Y‘LOH 78 DC%
«40
rah/\NIk
——"n'."s"éi(1§" , Ph°i1r§n
. eq.
DCM. reflux C02”
III-104

Scheme III-12. Amino acid at R. position as a chiral template

iii. Chiral amines as auxillaries

Viso et a! reported the very first examples of a highly diastereoselective 1,3-
dipolar cycloadditions of azomethine ylides derived from a-iminoesters with
chiral sulfinimines59 to produce N-sulfmyl imidazolidines (Figure III-8) and
transformation into non-symmetrical vicinal diamines.49 Dipole generation under
standard conditions using LiBr, Et3N, MeCN or AgOAc, DBU, toluene had failed.
Use of LDA allowed for successful cycloadditions with a high degree of endo

stereocontrol.

125

Ph
”@N’) THF. 20h
+ _

N’S“o 0 -78 to4 °c
)I OMe Viso's appmach
N R
Ph’Ng "\ OMe
o Li'—O
. : , 1 LAH NH
&57 I / —>/©::J\( (P‘P'LNA ) . 2 NHz
N=\Ar MR 2) TFA Ar "'R
COzMe H0
endo approach major en a I'

Figure III-8. Use of chiral sulphinimes in azomethine ylide cycloaddition
Addition of BF3.OEt2 results in abrogation of cycloadditions pathway and the
reaction proceeds with opposite diastereoselectivity via a step-wise mechanism.so
We realized that the above observations by Viso and co-workers would perfectly
suit our purpose of generating additional reagent control on stereochemical
diversity of the imidazoline scaffolds.

The chiral sulfinimines were readily prepared from commercially available chiral
p-toluenesulfinamides using reported procedure.49 Starting from S-(+)-p-toluene
sulfonarnide and benzaldehyde, the imine (III-I-l4) was prepared and used for
cycloaddition with 2-phenyl-4-methylazlactone (II-1). It was surprising, that the
sulfinyl group was lost under the acidic reaction conditions and we isolated N-
unsubstituted imidazoline (III-l-l3) in 30 % yield afler precipitation from
DCM/ether (Scheme III-13). Since, it was not clear whether the chiral auxillary

falls off during or afier the cycloadditions, the optical rotation was measured and

was not very promising.

126

 

Q's". PhCHO Q's "
‘NHZ TI(OEt)4 (4 eq.); ‘N
DCM, reflux '
86 % Ph
S(+)-p-toluenesulfinamide "”43
O
a 8’;

 

W? TMSCI (1. 3 eq.:) :30ng
DCM, reflux

"'1 [a1200=+ 3.0 (c= 5, MeOH) ""143
Scheme III-13. Use of chiral sulfmimine in munchnone cycloaddition.

l-Phenyl ethylamine was also tried as a chiral auxillary“ but in accordance with
the stereochemical model proposed above for these reactions, very poor
conversions were seen. Moreover, azlactone (II-l) resulted in 3:2 mixture of
diastereomers (data not shown) which were difficult to separate. The difficulty of
separation combined with the difficulty in selective removal of the chiral 1-
phenylethyl group in presence of other benzylic sites made us abandon this
approach.

1. Synthesis of diamino acids and other scaffolds

The most direct chemical utility of imidazolines has been their conversion to
chiral diamines,20 which are widely preferred group of ligands for asymmetric
synthesis. Efforts towards exploitation of the in-built diversity for synthesis of
chiral diamines and introduction of heteroatom functional groups directly on the
imidazoline ring to afford cyclic ureas (2-oxoimidazolines), 2-aminoimidazolines
and 2-thioimidazolines have been planned as extension of the methodology we
have developed for imidazolines (Scheme III-l4). This “libraries from libraries”

concept has been frequently in combinatorial chemistry and drugsdesign.62

127

R1HN 2-aminoimidazoline
YICROZHR‘

'34 /
HN
Ph R3
\(ICROZH ”2" .9 C02H \

R2

* Gig-rm 2-oxomidazoline

RCOZH
imidazoline diamino aCId R1S\(N NICRozH 2-mIoimidazoII'ne
R2

Scheme III-l4. Extension of methodology for other templates
Kohn and Jung” have developed a stereospecific route to vicinal diamines from
imidazolines. F ormamidine was obtained by catalytic hydrogenation of cynamide
under acidic conditions with palladium on charcoal. Cyclization to imidazoline by
NaOMe followed by mild alkaline hydrolysis with alcoholic aqueous NaOH

afforded the diamines from starting alkenes in good yields (Figure III-9).

 

a, 3 NHch 1% PdlC Br .3

o—.‘ : ‘\\‘

l \ N35 5; :lHCN—> AcOH-MeOi-I NH
HN=/

 

 

MeOH
; \w III : "HM

NV” 0.25 M “2" NH:
NaOH

Figure III-9. Synthesis of vicinal diamine from imidazolines
l) 2-unsubstituted imidazolines.
N-formyl-alanine was cyclized to 4-methylazlactone (II-7) using EDCI.HCl and

2-unsubstituted imidazoline (111-7-1); (1:1 transzcis, with respect to R2:R3) was

128

made from 4-methylazlactone and N-benzylidinebenzylamine(III-I-l) using the
established cycloaddition conditions. Normal silica-gel chromatography was
ineffective in separating the diastereomers. Even derivatization to ethyl esters did
not yield good separation.

2) 2-trifluoromethylimidazolines.

Woodbum and J ohnson64 have reported that when trifluoroactonitrile is reacted
with ethylene diamine, 2-trifluoromethylimidazoline is obtained. When they tried
to isolate the hydrochloride salt of the imidazoline from a non-anhydrousrous
ether solution, they isolated the diamine which shows the lability of the
trifluoromethyl group to hydrolysis. With this precedent we set out to make 2-
trifluoromethyl imidazolines using our route. N-trifluoroacetylalanine was made
from alanine using reported procedure65 in 90 % yield. Cyclodehydration of N-
trifluoroacetylalanine using EDCI.HC1 afforded the 2-trifluoromethyl-4-methyl-5-
oxazolone (II-8). But the oxazolone is in equilibrium with the pseudooxazolone.
The equlibrium is in favour of the pseudooxazolone.66
3) 2-amino-S-oxazolones.

At the onset, we tried to establish a common method to get to 2-amino-5-
oxazolones with different substituents. The starting materials, the ureidoamino
acids could be made by coupling an amine with isocyanate of an amino acid ester.
Alanine methyl ester gave a volatile isocyanate, which was not practical to use.
Hence the synthesis began with phenyl glycine methyl ester, which when treated
with phosgene67 in a biphasic CH2Cl2/sat.NaI-ICO3 (aq.) system yielded the pure

isocyanate in almost quantitative yields. This isocyanate was then treated with

129

various amines and amides. The ureidoacids were then cyclodehydrated with
EDCI.HCl to obtain the 2-aminoazlactones (Scheme III-15). The azlactones were
condensed with imines in the presence of TMSCl under standard protocol and

results are listed in Table III-1.

 

 

 

Ph COCI; Ph 0 Ph
sat. Mal-1003: 0\\ ‘ )\ R1NH2 JL )\
HzN COzMe DCM ‘N 002M9J v R1HN ” C02Me
0 /\ A 8‘
R HN Ph \ N
EDCI.HCI _ ‘ \E‘Z‘o Ill-:1 Pb R‘HNW g :03 H
T .............. ’
00” Ph TMSCI N Ph 2
. DCM . . . .
2-amlnoazlactone reflux 2-amIn0ImIdazolIne

Scheme III-15. Synthesis of 2-aminoazlactones and 2-aminoimidazolines

R,HN\«O o R1NY0 0
Ph Ph

R1NH2 oxazolone 2-amlnolmldazollnc

all”? 1844 cm-1 not Isolated

O

 

G/KNHZ, 1840 cm-1 not isolated
0

(Dim-l 1844 cm-1 not isolated
0

Scheme III-l6. Tautomers of 2-aminoazlactones
The main problem encountered was isolation of these azlactones, because on
concentration they led to multitude of products. Tautomerism with the
iminooxazolidinones (Scheme III-16) can be expected to make the

aminoazlactones highly unstable and moisture sensitive. It is possible that they

130

hydrolyse to the N-carboxy anhydrousrides and hence do not participate in
cycloadditions.68

J. Ring opening of 2-phenyl imidazolines.

The imidazolines resisted direct reduction with sodium borohydride and with
stronger hydride reducing agents like LAH, the 4-carboxylic acid group was
reduced to the corresponding alcohol. Thus imidazoline III-l-l on reduction with
LAH (0.3 eq) in THF resulted in 90 % yield of alcohol (111-1-12). This suggested
that C2 position in imidazolines is resonance stabilized and not sufficiently
electrophilic. Zhou and co-workers have reported successful ring opening of
simple 2-arylimidazolines to the corresponding diamimes.69 They first alkylated
the iminic nitrogen with methyl iodide to obtain N—methyl-Z-arylimidazolium
iodides, which then underwent a smooth reduction ring opening with sodium

borohydride in ethanol to afford the diamines in high yields (Scheme III-17).

P11 P" Ph
anhydrous N Ph
Ph K co NaBH4 (5 eq.) HN 1’"
Ph~<N O 2 3 ePh‘-<\ O o
\ , BnCl Cl$ N 2 ethanol HN .
N = be ( ‘ 0H 5 OH
= OH nzene so % <
reflux Ph Ph
lll-1-1 90 % Ill-1 -15 m4.“
diamino acid

Scheme III-l7. Reductive ring opening of imidazolines to diamino acids.
In order to have removable protecting group on the diamino acid product we
chose benzylation over methylation. Imidazoline III-l-l, was refluxed with
excess benzyl bromide in dry benzene, however no reaction ensued. After a
couple of trials, we realized that addition of powdered anhydrous potassium

carbonate leads to complete conversion, probably by quenching the acid produced

131

in the reaction. Due to purity issues with commercial benzyl bromide we switched
to benzyl chloride. The N 1, Na-dibenzylimidazolium chloride salt could be
isolated by precipitation from DCM/hexanes mixture in almost 90 % yield. We
had two options we could either hydrolyze or reduce the imidazolium salt.
Hydrolysis with 2N NaOH resulted in ring opening, however the reaction was not
very clean. Hence we opted for reduction of imidazolium salt with sodium
borohydride in ethanol as reported. The diamino acid was isolated in good yield
(60 %) by precipitation using DCM /ether.

K. Synthesis of Imidazoles from 2-imidazoline-4-carboxylic acids

 

\ 002H
Hymenialdisine O _ 0
MAP kinase kinase-1 inhibitor
N \ NH
F leo = 6 "M
N
/ \
\
N \ NH N’
COzH
P-selectin inhibitor
anti—inflammatory
Br
1) p38-Map kinase inhibitor
activates NF-xB
2) Glugagon receptor antagonists
anti-diabetic

Figure III-10. Biological activity of molecules with imidazole nucleus.

132

The imidazole nucleus appears in a number of natural products, including the
amino acid histidine and purines. Nitroimidazoles are well-known anti-bacterial
and anticancer drugs. Recently triaryl imidazoles have been identified as p38
Mitogen-activated protein (MAP) kinase inhibitors. Chang et al 70 have been
successful in optimizing this triaryl imidazole lead to a selective antagonist for
human glucagons receptor, which should lead to improved drugs to control
glucose levels in diabetics. Slee et al71 have reported a novel-series of non-
carbohydrate imidazole-based small molecule selectin inhibitors. They are the
first class of non-carbohydrate selectin antagonists with potent anti-inflammatory
activity. Hymenialdisine was recently71 identified to be a nanomolar inhibitor of
Mitogen-Activated Protein Kinase kinase-l (MEK-l). Activated MEK-l
phosphorylates and activates MAP kinases which can translocate to the nucleus,
and through the phosphorylation of a variety of substrates modulate cytoplasmic

events such as cell-proliferation and differentiation (Figure III-10).

R1 ____,R1 R3
1%:3 C02H sealed tube \«2—42;

El‘ltl'y R‘ R2 R3 R‘ 9‘ Y‘dd

 

lll-‘l-17 Ph Me Ph Bn 23
Ill-148 Ph Me 4-pyridinyl Bn 22

Scheme III-18. Synthesis of imidazoles from imidazolines
In view of the biological importance of 2, 4, 5-trisubstituted imidazoles, we
explored oxidative decarboxylation of 2-imidazoline-4-carboxylic acids. After a

few trials heating the compound in neat acetic anhydride, afforded the 2, 4, 5-

133

trisubstituted imidazoles (Scheme III-l8). These results are unoptimized and it is
likely that instead of neat acetic anhydride, stoichiometric amounts and/or lower
boiling solvents may result in higher yields. We and others have observed that
refluxing imidazolines with excess MnO2 results in higher yields and cleaner
reaction.lo
L. Experimental Procedure

Reactions were carried out in oven-dried glassware under nitrogen atmosphere,
unless otherwise noted. All commercial reagents were used without further
purification. All solvents were reagent grade. THF was freshly distilled from
sodium/benzophenone under nitrogen. Toluene, CH2Cl2, and TMSCl were freshly
distilled from CaHZ under nitrogen. All reactions were magnetically stirred and
monitored by TLC with Analtech 0.25 mm pre-coated silica gel plates. Column
chromatography was carried out on silica gel 60 (230—400 mesh) supplied by EM
Science. Yields refer to chromatographically and spectroscopically pure
compounds unless otherwise stated. Melting points were determined on a Mel-
Temp (Laboratory devices) apparatus with a microscope attachment. Infrared
spectra were recorded on a Nicolet IR/42 spectrometer. 1H and 13C NMR spectra
were recorded on a Varian Gemini-300 spectrometer or a Varian VXR-SOO
spectrometer. Chemical shifts are reported relative to the residue peaks of the
solvent CDC13 (a 7.24 for 'H and 677.0 for ”C) and DMSO-d6 (5 2.49 for 'H and
639.5 for '3 C). HRMS were obtained at the Mass Spectrometry Laboratory of the
University of South Carolina, Department of Chemistry & Biochemistry with a

Micromass VG-7OS mass spectrometer. Gas chromatographyllowresolution mass

134

spectra were recorded on a Hewlet-Packard 5890 Series II gas chromatograph
connected to a TRIO-1 El mass spectrometer. All chemicals were obtained from
Aldrich Chemical Co. and used as received.

General procedure for synthesis of imines (III-I-l - III-I-14).

A solution of aldehyde(1mmol) in benzene/ toluene (20 mL) was refluxed with
the amine ( 1 mmol) for 2h. The solvent was evaporated under vacuum to afford
the imine in good yields. These imines were used as such without further
purification.

Synthesis and Characterization of Imidazolines

Ph

1.
PINE-IP11

002H

\“

Ill-1 -1

d1- (48, 5S)—l-Benzyl-4-methyl 2,5-diphenyl-4,5-dihydro-lH-imidazole- 4-
carboxylic Acid (III-l-l)

A solution of benzaldehyde (0.06 g, 0.57 mmol), benzylamine (0.061 g, 0.57
mmol) in anhydrous CH2C12 (15 mL) was refluxed under nitrogen for 2h. 2-
Phenyl-4-methyl-4H-oxazolin—5-one (0.1 g, 0.57 mmol) and chlorotrimethylsilane
(0.08 g, 0.74 mmol) were added and the mixture was refluxed under nitrogen for
6 h and then stirred overnight at r.t. The solvent was evaporated under vacuum.
The product was precipitated using CH2Cl2/hexanes (1:1); yield: 75% (0.155 g);
white solid; 1H NMR (300MHz) DMSO'd6: 8 1.8 (3H, s), 4.05 (1H, d, J = 15Hz),
4.95 (1H, d, J = 14.8 Hz), 5.05 (1H, s), 7.05 (2H, s), 7.25-7.54 (8H, m), 7.74 (2H,

t, J= 7.2Hz), 7.83 (1H, t, J = 6.9Hz), 8.0 (2H, d, J = 8.4Hz); l3C NMR (75MHz)

13S

01918041,: 8 25.2, 48.8, 70.4, 73.3, 122.3, 127.8, 128.3, 128.5, 128.9, 129.1,
129.3, 129.6, 129.7, 132.3, 133.2, 134, 166.1, 169.5; IR (neat): 3350 cm",
1738cm "; M.P. 185-190 °c; HRMS (E1): m/z calcd. for 02.112.sz [M — H],

369.1603; found, 369.1610.

Ph
Ph 1 OMe
‘8
N 5 002H

lll-1-2
Ill-(4S, SS)-l-Benzyl-5-(4-methoxyphenylH-methyl-Z-phenyl- 4, 5-dihydro-
lH-imidazole-4-carboxylic Acid (III-l-2) A solution of p-anisaldehyde (0.077 g,
0.57 mmol), benzylarnine (0.061 g, 0.57 mmol) in anhydrous CH2Cl2 (15 mL)
was refluxed under nitrogen for 2 h. 2-Phenyl-4 methyl-4H-oxazolin-5-one (0.1 g,
0.57 mmol) and chlorotrimethylsilane (0.08 g, 0.74 mmol) were added and the
mixture was refluxed under nitrogen for 6 h and then stirred overnight at r.t. The
reaction mixture was evaporated to dryness under vacuum. The product was
precipitated using CH2Cl2—hexanes (1:1); yield: 78% (0.180); 1H NMR (300MHz)
DMSO-d;: 5 1.8 (3H, s), 3.8 (3H, s), 3.95 (1H, d, J = 15.3Hz), 4.5 (1H, s), 4.9
(1H, d, J = 15Hz), 6.83-6.92 (4H, m), 7.08-7.19 (3H, m), 7.3-7.4 (3H, dd, J, =
5.1Hz, J2 = 1.8Hz), 7.54-7.62 (2H, t, J = 7.2Hz), 762-7.68 (1H, t, J = 7.2Hz), 7.9
(2H, d, J = 6.9 Hz); l3C NMR (75MHz) DMSO'd6: 8 25.3, 48.8, 55.6, 70.9, 74.1,
115.2, 122.2, 123, 125.5, 127.9, 128.4, 129.2, 129.3, 129.6, 129.9, 132.8, 134.2,
161.1, 166.3, 168.4; IR (neat): 3388 cm", 1738 cm"; M.P. 205—208 °c; HRMS

(El): m/z calcd. for C25H23N2O3 [M — H], 399.1709; found, 399.1717.

136

P11 N Ph
111-1-3”
dl-(4S,5S)—l-(4-FluorophenyI)-4-methyl-2,5-diphenyl-4,5-dihydro- 1H-
imidazole-4 carboxylic acid (III-l-3) A solution of benzaldehyde (0.060 g, 0.57
mmol), 4-fluoroani1ine (0.063 g, 0.57 mmol) in anhydrous CH2C12 (15 mL) was
refluxed under nitrogen for 2 h. 2 Phenyl-4-methyl-4H-oxazolin-5-one (0.1 g,
0.57 mmol) and chlorotrimethylsilane (0.08 g, 0.74 mmol) were added and the
mixture was refluxed under nitrogen for 6 h and then stirred overnight at r.t. The
reaction mixture was evaporated to dryness under vacuum. The product was
precipitated using CH2C12—hexanes (1:1) to give the titled compound; yield: 74%
(0.160 g); white solid; lH NMR (300MHz) DMSO-d;: 6 1.98 (3H, s), 5.98 (1H,
s), 7.05-7.65 (14H, m) 13c NMR (75MHz) DMSO-d;: 8 25.2, 71.2, 77.9, 116.9,
117, 117.1, 117.3, 123, 125.1, 125.3, 129.3, 129.4, 129.6, 130.1, 130.3, 130.4,
130.5, 132.5, 133.3, 134.5, 160.4, 163.7, 165.3, 170.4; IR(neat): 3450 cm", 1744
cm"; M.P. 230—232 °C. HRMS (EI): m/z calcd for C23H13FN2O2 [M — H],

373.1352; found, 373.1359.

137

Ph

W /
Ph\«N \ /

N

Ill-14
dl-(4S,SS)-l-Benzyl-4-methyl-2-phenyl-5-pyridin-4yl-4,5-dihydro-1H-
imidazole-4-carboxylic Acid III-1-4 A solution of pyridin-4-carboxylaldehyde
(0.061 g, 0.57 mmol), benzylamine (0.061 g, 0.57 mmol) in anhydrous CH2C12
(15 mL) was refluxed under nitrogen for 2 h. 2-Phenyl-4-methyl-4H oxazolin-S-
one (0.1 g, 0.57 mmol) and chlorotrimethylsilane (0.08 g, 0.74 mmol) were added
and the mixture was refluxed under nitrogen for 6 h and then stirred overnight at
r.t. The reaction mixture was evaporated to dryness under vacuum. The product
was precipitated using EtOAc / MeOH (4:1); yield: 76% (0.161 8); off-white
solid; 'H NMR (300MHz) DMSO-d;: 8 1.8 (3H, s), 4.24 (1H, d, J = 15.9Hz), 4.9
(1H, d, J = 14.8 Hz), 5.15 (1H, s), 7.0-7.15 (2H, m), 7.25-7.35 (3H, m), 7.4-7.5
(2H, m), 7.7-7.9 (3H, m), 7.95-8.05 (2H, m), 8.6-8.7 (2H, m) 13C NMR (75MHz)
DMSO-d;: 8 25.1, 49.1, 70.6, 71.7, 122.1, 123, 127.9, 128.4, 128.8,129.2, 129.4,
132.8, 133.9, 141.4, 149.8, 166.5, 169.05; IR (neat): 3400 cm", 1746 cm"; MP.
185 190 °C. HRMS (EI): m/z calcd. for C23H20N3O2 [M — H], 370.1556; found,

370.1556.

138

c0211
Ill-15”

thicoza
dl-(4S,5S)-l-(4-Fluorophenyl)-4-methyl-2-phenyl-4,5-dihydro- lH-imidazole—
4,5 dicarboxylic acid S-ethyl Ester III-l-S A solution of ethyl glyoxalate (0.058
g, 0.57 mmol) as a 50% solution in toluene (1.03 gmL”), 4-fluoroaniline (0.063
g, 0.57 mmol) in anhydrous CH2C12 (15 mL) was refluxed under nitrogen for 2 h.
2- Phenyl-4-methyl-4H oxazolin-S-one (0.1 g, 0.57 mmol) and
chlorotrimethylsilane (0.08 g, 0.74 mmol) were added and the mixture was
refluxed under nitrogen for 6 h and then stirred overnight at r.t. The reaction
mixture was evaporated to dryness under vacuum. The product was purified by
column chromatography on silica gel using EtOAc-MeOH (4:1); yield: 72%
(0.152 8); White solid; 1H NMR (300MHz) DMSO-d6: 8 1.2 (3H, t, J = 7.2 Hz),
2.03 (3H, s), 4.9 (2H, dq, J) = 7.2 Hz, J2 = 2.1 Hz), 5.48 (1H, s), 7.1-7.8 (9H, m);
13C NMR (75MHz) DMSO-d6: 8 12.8, 24.2, 62.9, 69.1, 75.1, 116.7, 116.9, 121.8,
129.3, 129.5, 129.6, 129.7, 131.5, 134.4, 162.1, 164.0, 166.2, 169.9; IR (neat):
3450 cm", 1743 cm"; MP. 190— 193 °C. HRMS (El): m/z calcd for C20H13FN204

[M — H], 369.1251; found, 369.1255.

139

Me02C
1
N

Ph Ph
)1
1 0021-1

Ill-1 -7

dl-(4S,5S)-l-Methoxycarbonylmethyl-4-methyl-2,5 diphenyl- 4,5-dihydro-1H-
imidazole-4-carboxylic Acid III-l-7 To a well stirred solution of 2-phenyl-4-
methyl-4H-oxazolin-5-one (0.5 g, 2.85 mmol) and TMSCl (0.37 g, 3.42 mmol) in
anhydrous CH2C12 (50 mL) was added a solution of (benzylidene-amino)-acetic
acid methyl ester (0.6 g, 3.42 mmol) in anhydrous CH2C12 (20 mL) and the
mixture was refluxed under nitrogen for 10 h and then stirred overnight at r.t. The
reaction mixture was evaporated to dryness under vacuum. The product was
precipitated using CH2C12— hexanes (1:1); yield: 70% (0.70 g); white solid; lH
NMR (300 MHz) DMSO'd6: 8 1.99 (3H, (1H, d, J = 18.3 Hz), 4.53 (1H, d, J =
18.3 Hz), 5.39 ( 1H, s), 7.47-7.50 (5H, m), 7.74-7.87 (5H, m); 13C NMR (75MHz)
DMSO-d;: 8 24.23, 52.09, 70.83, 75.38, 121.84, 128.26, 128.69, 129.52, 129.75,
131.78, 134.02, 167.59, 168.62, 169.1% mp 215—217 °C (dec.) HRMS (El): m/z
calcd. for C20H21N2O4 [M + H] 352.1501, found, 353.1507.IR(neat): 3468 cm",

1747 cm",

Meoch
Ph N Ph
‘8 f
N CO2H
Ill-1-8’
dl-(4S,SS)-1-(l MethoxycarbonyI-ethyl)-4-methyl-2,5-diphenyl- 4,5-dihydro-

lH—imidazole-4 carboxylic Acid III-l-8: To a well stirred solution of 2-phenyl-

140

4-methyl-4H-oxazolin-5-one (0.25 g, 1.5 mrnol) and TMSCl (0.23 ml, 1.8 mmol)
in anhydrousrous CH2C12 (50 mL) added a solution of 2-(benzylidene-arnino)-
propionic acid methyl ester (0.34 g, 1.8 mmol) in anhydrous CH2C12 (20 mL) and
the mixture was refluxed under nitrogen for 10 h and then stirred overnight at r.t.
The reaction mixture was evaporated to dryness under vacuum. The product was
precipitated using CH2C12- hexanes (1:1); yield: 66% (0.340 g); white solid; 1H
NMR (300MHz) DMSO-d6: 8 1.19 (d, J= 6.9, 3H), 2.06 (s, 3H), 3.38 (s, 3H),
4.89 (q, J = 6.9, 1H0, 5.44 (s, 1H), 7.43-7.46 (5H, m), 7.75-7.85 (5H, m). l3C
NMR (75MHz) DMSO-d;: 8 14.9, 25.6, 52.7, 56.7, 71.9, 72.5, 122.2, 128.8,
128.9, 129.6, 130.0, 134.5, 135.8, 169.2, 169.4, mp 222—226 °C. HRMS (E1): m/z
calcd for C21H23N204 [M + H], 367.1658; found, 367.1642. IR (neat): 3431cm",

1740 cm".

002121

N
th _Z‘C 1;: H
Ill-1:;
dI-(4S,SS)-l-(2-Ethoxycarbonyl-ethyl)-4-methyl-2,5-diphenyl- 4,5 dihydro-
lH-imidazole-4-carboxylic Acid III-l-9: To a well stirred solution of 2-phenyl-
4 dimethyl-4H-oxazolin-5- one (1.0 g, 5.7 mmol) and TMSCl (1 mL, 6.8 mmol)
in anhydrous CH2C12 (80 mL) added a solution of 3-(benzylidene-amino)-
propionic acid ethyl ester (1.4 gm, 6.8 mmol) in anhydrous CH2C12 (60 mL) and
the mixture was refluxed under nitrogen for 10 h and then stirred overnight at r.t.

The reaction mixture was evaporated to dryness under vacuum. The product was

141

precipitated CH2C12-hexanes (1:1); yield: 51%; (1.08 g); '11 NMR (300MHz)
DMSO—d;: 8 1.17 (t, J= 7.5, 3H), 1.9 (s, 3H), 2.47-2.52 (m, 1H), 2.52-2.71 (m,
1H), 3.34-3.39 (m, 1H), 3.40409 (m, 3H), 5.42 (s, 1H), 7.46-7.49 (m, 5H), 7.72-
7.87 (m, 5H). 13C NMR (75MHz) DMSO-d;: 8 13.3, 24.8, 30.6, 41.6, 61.0, 70.9,
73.5, 122.7, 128.9, 129.2, 129.8, 130.1, 132.7, 134.0, 167.3, 169.8, 170.9c mp
218-220 °c (dec.). HRMS (El): m/z calcd for C22H25N204 [M + H], 381.1814;

found, 381.1813. IR (neat): 3481 cm", 1743 cm"

Ph

1
Ph N P11
lll-1- :1

dI-(4S,5S)-l-Benzyl-4-methyI-2,5-diphenyl-4,5-dihydro lH-imidazole- 4-
carboxylic Acid Ethyl Ester III-l-10: To a well-stirred suspension of
imidazoline-4-carboxylic acid III-l-l (0.1 g, 0.27 mmol) in anhydrous CH2C12
(30 mL) at 0°C was added a solution of oxalyl chloride (0.14 g, 1.1 mmol) in
anhydrous CH2C12 (5 mL). A solution of DMF (0.001 mL) in anhydrous CH2C12
(1 mL) was added to the reaction mixture and the mixture was stirred at 0 °C for
another 2 h. The CH2C12 was evaporated under vacuum and the reaction mixture
cooled to 0 °C after which absolute EtOH (20 mL) was added. The solution was
allowed to stir for an additional 1 h. The solvent was evaporated under vacuum
and the reaction mixture diluted with anhydrous CH2C12 (30 mL) and washed with
sat. NaHCO; (10 mL) and H20 (10 mL). The organic layer was dried over
Na2804 and was concentrated under vacuum to yield the crude product, which

was further purified by silica-gel column chromatography (EtOAc); overall yield

142

(2 steps from azlactone): 75% (0.095 g); colorless oil. 1H NMR (300MHz)
DMSO-d;: 8 0.84 (3 H, t, J = 7.2 Hz), 1.57 (3 H, s), 3.60 (2 H, q, J = 7.2 Hz),
3.85 (1 H, d, J= 15.3 Hz), 4.32 (1 H, s), 4.74 (1 H, d,J= 15.3 Hz), 6.98 (2 H, dd,
Jl = 6.9 Hz, J2 = 2.1 Hz), 7.27—7.35 (m, 8 H), 7.49—7.51 (2 H, m), 7.76—7.79 (2 H,
m) 13C NMR (75MHz) DMSO-d;: 8 13.80, 27.13, 49.12, 60.06, 71.31, 127.98,
128.03, 128.12, 128.67, 129.02, 129.11, 130.96, 136.40, 136.80, 166.11,171.78;
HRMS (E1): m/z calcd for C2nH27N202, 399.2073; found, 399.2072 ; IR(neat):

1730 cm", 1595 cm".

Ph

W /
PhKN \ IN

N 00251

111-1 -11
III-(45,55)-1-Benzyl-4-methyl-Z-phenyl-S-pyridin-4-yl—4,5 dihydro- 1H-
imidazole-4-carboxylic Acid Ethyl Ester III-l-ll: To a well-stirred suspension
of dl-(3S,4S)-l-benzyl-4-methyl-2- phenyl-5-pyridin-4yl-4,5-dihydro-1H
imidazole-4-carboxylic acid (III-l-4) (0.1 g, 0.27 mmol) in anhydrous CH2C12
(30 mL) at 0°C added a solution of oxalyl chloride (0.14 g, 1.1 mmol) in
anhydrous CH2C12 (5 mL). A solution of DMF (0.001 mL) in anhydrous CH2C12
(1 mL) was added to the reaction mixture and was stirred at 0 °C for another 2 h.
The CH2C12 was evaporated under vacuum and the reaction mixture cooled to 0
°C after which absolute EtOH (20 mL) was added. The solution was allowed to
stir for an additional 1 h. The solvent was evaporated under vacuum and the

reaction mixture diluted with CH2C12 (30 mL) and washed with sat.NaHCO_-; (10

mL) and H20 (10 mL). The organic layer was dried over Nal-ISO4 and was

143

concentrated under vacuum to yield the crude product, which was further purified
by silica-gel column chromatography (ethyl acetate); overall yield (from
azlactone): 76% (0.97 g); pale yellow oil. 'H NMR (300MHz) DMSO-d;: 8 0.86
(3 H, t, J= 7.2 Hz), 1.57 (3 H, s), 3.64 (2 H, q, J= 7.2 Hz), 3.83 (1 H, d, J= 15.3
Hz), 4.27 (1 H, s), 4.77 (1 H, d, J = 15.3 Hz), 6.97 (2 H, dd, J; = 7.2 Hz, J2 = 2.4
Hz), 7.22—7.54 (6 H, m), 7.31-7.54 (2H, m), 7.78-7.81 (2 H, m), 8.59—8.61 (2 H,
m) l3C NMR (75MHz) DMSO-dsz 8 13.45, 27.13, 49.47, 60.83, 71.87, 77.94,
122.56, 127.79, 127.93, 128.55, 128.70, 130.21, 130.51, 135.82, 146.59, 149.75,
166.02, 171.37; HRMS (E1): m/z calcd for C25H27N3O2, 400.2025; found,

400.2038. IR (neat): 1734 cm", 1597 cm".

Ph \ N 1:11
NJ;
01-1

Ill-1- 2

Ph

Ill-(4S, SS)-l-Benzyl-4-methyl-2,5-diphenyl-4,5-dihydro-lHimidazol- 4-yl)
methanol III-l-12: To a well stirred suspension of LiAlH4 (0.12g, 0.3 mmol) in
anhydrous THF (5 mL) was added a solution of 1-benzyl-4-methyll-2,5-dipheny1-
4,5-ihydro-1H imidazole-4-carboxylic acid (0.1 gm, 0.27 mmol) in anhydrous
THF (5 mL) dropwise at 0 °C, the mixture was stirred at the same temperature for
15 min quenched with ice cold sat. NH4C1 solution (caution: NH4C1 solution kept
at 0 °C for about 30 min.; and should be added with extreme care; highly
exothermic reaction and the reaction mixture should be at 0 °C) followed by 10%
HCl (~10 mL). The reaction mixture was diluted with an excess of EtOAc (100

mL) “’38th With H20 (20 mL) dried over anhydrous Na2SO4, filtered, and the

144

solvent evaporated under reduced pressure to yield the crude product which was
purified by column chromatography (EtOAc); overall yield (2 steps): 79% (0.076
8); viscous oil. 1H NMR (300MHz) DMSO-d;: 8 1.25 (3 H, s), 3.48 (l H, d, J =
12 Hz), 3.56 (1 H, d, J= 11.8 Hz), 3.75 (1 H, d, J=12.9 Hz), 3.87 (s, 1 H), 3.94
(1H, d, J = 12.9 Hz), 7.28—7.54 (m, 13 H), 7.77—7.79 (m, 2 H), 8.06 (1 H, br 5)
'3C NMR (75MHz) DMSO-d;: 8 17.25, 51.67, 61.54, 66.28, 66.93, 127.266,
127.68, 128.26, 128.56, 128.82, 129.06, 131.77, 135.48, 138.03, 139.90, 167.91;
HRMS (FAB): m/z calcd for C24H25N2O [M + H], 357.1888; found, 357.1967. IR(

neat): 3316 cm",

H
PNXNrPh
N 5' C0211

lll-1-13
dl-(4S,SS)-4-Methyl-2,5-diphenyl-4,5-dihydro-lH-imidazole-4- carboxylic
Acid III-l-l3: To a well-stirred suspension of imidazoline-4-carboxylic acid III-
1-1 (0.1 g, 0.27 mmol) and cyclohexene (0.1 mL, 1.25 mmol) in anhydrous THF
(30 mL) was added 10% Pd/C (45 mg, 0.06 mmol). The suspension was refluxed
for 36 h. The reaction mixture cooled to r.t. and EtOH (10 mL) was added. The
mixture was filtered through celite, washed with EtOH, and the filtrate was
evaporated under reduced pressure. The crude product was purified by column
silicagel chromatography (EtOH); overall yield: 71% (0.070 g); White solid; 1H
NMR (300MHz) DMSO-d6: 8 1.76 (s, 3 H), 5.34 (s, 1 H), 7.34—7.36 (br, 5 H),
7.69 (2 H, dd, J= 8.1, 7.2 Hz), 7.81 (1 H, dd, J1= 6.9 Hz, J2 = 7.2 Hz), 8.15 (2 H,

d, J = 8.4 Hz) 13C NMR (75MHz) DMSO-d6: 8 25.32, 55.66, 70.79, 72.57,

145

123.12, 128.24, 128.96, 129.42, 129.67, 130.12, 135.42, 136.24, 164.24, 170.77;
mp 222—224 °C (dec.). HRMS (FAB): m/z calcd for C17H17N202 [M + H],

281.1212; found, 281.1289. IR(neat) : 3300 cm", 1736 cm"

OMe

N
Ph Ph
‘1 f
N '. COzH

III-1-14

dl- (4S, SS)-1-(4-methoxyphenyl)—4-methyI-2,S-diphenyl-4,S-dihydro-1H-
imidazole- 4-carboxylic Acid (III-l-l4)

A solution of benzaldehyde (1.42 g, 13.5 mmol), 4-methoxybenzylarnine (1.4 g,
13.5 mmol) in anhydrous CH2C12 (50 mL) was refluxed under nitrogen for 2h. A
mixture of 2-Phenyl-4-methyl-4H-oxazolin-5-one (2.2 g, 12.5 mmol) and
chlorotrimethylsilane (2.03 g, 18.75 mmol) (premixed and stirred for 2h) in
anhydrous CH2C12 (100 mL) were added and the mixture was refluxed under
nitrogen for 6 h and then stirred overnight at r.t. The solvent was evaporated
under vacuum. The product was precipitated using CH2C12/hexanes (1 :1); yield:
50 % (2.41 g); pale pink solid; 'H NMR (300MHz) DMSO—d;: 8 1.93 (3H, s),
3.59 (3H, s), 5.85 (1H, s), 6.74 (2H, d, J = 9 Hz), 7.15-7.6 (12H, m); ”C NMR
(75MHz) DMSO-d;: 8 24.3, 55.1, 70.1, 77.2, 114.3, 122.3, 127.8, 128.3, 128.5,
128.7, 129.1, 129.5, 132.7, 133.5, 158.8, 164.3, 169.5; IR (neat): 3352 cm",

1735cm'1; LRMS (El): m/z calcd. for C24H22N2O3 [M+], 386.163; found, 386.0.

146

Ph\I
Ph N Ph
P11
lll-2-1

dl-(4S,5S)-l-Benzyl-2,4,5 tfiphenyl-4,5-dihydro-lH-imidazole- 4-carboxylic
Acid III-2-l: A solution of benzaldehyde (0.6 g, 5.7 mmol), benzylamine (0.61 g,
5.7 mmol) in anhydrous CH2C12 (120 mL) was refluxed under nitrogen for 2 h.
2,4-Diphenyl-4H-oxazolin-5-one (1.35 g, 5.7 mmol) and chlorotrimethylsilane
(0.8 g, 7.4 mmol) were added and the mixture was refluxed under nitrogen for 6 h
and then stirred overnight at r.t. The product was purified by silica-gel column
chromatography (EtOH—EtOAc, 1:5); yield: 65% (2.1 g); Off-white solid; 1H
NMR (300MHz) DMSO-d6: 8 3.80 (1 H, d, J = 15.6 Hz), 4.62 (1 H, d, J = 15.6
Hz), 4.98 (1 H, s), 6.58 (2 H, d, J = 8.1 Hz), 7.05-7.65 (16 H, m), 7.90 (2 H, d, J
= 7.2 Hz) 13C NMR (75MHz) DMSO-d;: 8 29.7, 48.3, 75.6, 79.1, 123.1, 125.7,
126.7, 127.3, 127.4, 127.9, 128.1, 128.2, 128.8, 128.9, 129, 129.3, 132.9, 133.8,

136, 143.1, 164.8, 168.1d mp 153— 155 °C. HRMS (El): m/z calcd for C23H23N2

[(M — H) — CO2], 387.1526; found, 387.1539. IR(neat): 3400 cm", 1738 cm'I

:1“ ,-
Ph \/
1

;‘ COzH
Ph
III-24

Ph
N

dl-(4S,5S)- l-Benzyl-2,4-diphenyl-5-pyridin-4-yl-4,5 dihydro- III-imidazole-

4-carboxylic Acid (20) A solution of pyridin-4 carboxylaldehyde (0.61 g, 0.57

147

mmol), benzylamine (0.61 g, 5.7 mmol) in anhydrous CH2C12 (120 mL) was
refluxed under nitrogen for 2 h. 2,4-Dipheny1-4H-oxazolin-5-one (1.35 g, 5.7
mmol) and chlorotrimethylsilane (0.8 g, 7.4 mmol) were added and the mixture
was refluxed under nitrogen for 6 h and then stirred overnight at r.t. The product
was precipitated using CH2C12—Et20; yield 55% (1.35 g); off-white solid. 1H
NMR (300MHz) DMSO-d;: 8 4.00 (1 H, d, J = 15.6 Hz), 5.00 (1 H, d, J = 15.6
Hz), 5.38 (1 H, s), 7.1—7.65 (17 H, m), 8.5 (2 H, d, J = 7.2 Hz) 13C NMR
(75MHz) DMSO-d;: 8 45.2, 66.3, 75.6, 123.7, 126.5, 126.9, 128.5, 128.6, 128.8,
129.2, 129.3, 131.9, 133.5, 134.4, 136.2, 143.4, 149.7, 166.6, 166.9; MS (E1): m/z
calcd for C29H24FN302 [M +H], 434.34186; found, 434.1852. IR(neat): 3400

cm", 1733 cm".

c0211
P11
111-2-5

N
\I
N 3’

dI-(4S,SS)-l—(4 Fluorophenyl)-2,4-diphenyl-4,5-dihydro-lHimidazole- 4,5-
dicarboxylic Acid S-Ethyl Ester III-2-5: A solution of ethyl glyoxalate (0.85 g,
8.3 mmol) as 50% solution in toluene (1.03 g/mL), 4-fluoroaniline (0.93 g, 8.3
mmol) in anhydrous CH2C12 (250 mL) was refluxed under nitrogen for 2 h. 2,4-
Diphenyl- 4H-oxazolin-5-one (2 g, 8.3 mmol) and chlorotrimethylsilane (1.16,

10.8 mmol) were added and the mixture was refluxed under nitrogen for 6 h and

148

then stirred overnight at r.t. The reaction mixture was evaporated to dryness under
vacuum. The product was precipitated using CH2C12-Et20; yield: 68% (2.4 g);
white solid. 1H NMR (300MHz) DMSO—d;: 8 0.84 (3 H, t, J= 7.2 Hz), 3.89 (2 H,
dq, J. = 7.2 Hz, J2 = 3 Hz), 4.73 (1 H, s), 6.70—6.84 (2 H, m), 6.89 (2 H, t, J = 9
Hz), 7.34—7.5 (3 H, m), 7.55 (3 H, t, J = 7.5 Hz), 7.65 (2 H, t, J = 8.1Hz), 7.83 (2
H, dd, J. = 8.1 Hz, J2 = 2.1 Hz), 8.10—8.22 (2 H, m) l3C NMR (75MHz) DMSO-
d6: 8 13.3, 61.1, 66.1, 76.3, 115.3, 115.6, 117.3, 117.4, 125, 126.1, 128.2, 128.6,
133, 134.6, 142, 142.1, 155.7, 162.1, 169, 176.8; HRMS (FAB): m/z calcd for
C25H22FN2O4 [M + H], 433.1486; found: 433.1565. IR (neat): 3331 cm", 1736
cm".

Ph

1.
PhKIPh
N COZH

HN \
111-3-1

dl-(4S,5S)-l-Benzyl-4-(1H-indol-3-ylmethyl)-2,5-diphenyl-4,5- dihydro-lH
imidazole-4-carboxylic Acid III-3-l: A solution of benzaldehyde (0.6 g, 5.7
mmol), benzylamine (0.61 g, 5.7 mmol) in anhydrous CH2C12 (120 mL) was
refluxed under nitrogen for 2 h. 4-(1H-Indol-3-ylmethyl)-2-pheny1-4H-oxazol-5-
one (1.65 g, 5.7 mmol) and chlorotrimethylsilane (0.8 g, 7.4 mmol) were added
and the mixture was refluxed under nitrogen for 6 h and then stirred overnight at
r.t. The product was purified by column chromatography on silica gel EtOH-
EtOAc (1 :5); yield: 68% (3.1 g); offwhite solid; 'H NMR (300MHz) DMSO-d;: 8

3.95 (1 H, d, J: 16.2 Hz), 4.6 (1 H, d, J= 16.2 Hz), 5.25 (1 H, s), 6.1 (2 H, d, J=

149

7.8 Hz), 6.90—7.30 (5 H, m), 7.30—8.00 (15 H, m) l3C NMR (75MHz) DMSO-dgi
8 32.3, 48.5, 70.4, 74.4, 105.8, 111, 119, 121.4, 122.7, 126.6, 126.7, 127.8, 127.9,
128.6, 128.7, 128.9, 129.4, 129.7, 132.3, 132.5, 133.7, 136.5, 166, 169.6; mp
>250 °C (dec.). HRMS (El): m/z calcd for C32H26N302 [M — H], 484.2025; found,

484.2011; IR (neat): 3420 cm", 1741 cm".

Ph

3
Ph N P11

00211

III-44
002Me

dl-(4S,5S)-1-Benzyl-4-(2-methoxycarbonyl-ethyl)—2,5-diphenyl- 4,5-dihydro-
lH-imidazole-4-carboxylic Acid (2r)

A solution of benzaldehyde (0.252 g, 2.4 mmol), benzylamine (0.258 g, 2.4
mmol) in anhydrous CH2C12 (100 mL) was refluxed under nitrogen for 2 h. 3-(5-
Oxo-2-phenyl-4,5- dihydro-oxazol-4-yl)-propionic acid methyl ester II-4 (0.5 g, 2
mmol) and chlorotrimethylsilane (0.282 g, 2.6 mmol) were added and the mixture
was refluxed under nitrogen for 6 h and then stirred overnight at r.t. The reaction
mixture was evaporated to dryness under vacuum. The product was precipitated
using CH2C12/ hexanes (1:1); yield: 60% (0.54 11); white solid. 'H NMR
(300MHz) DMSO'd6: 8 2.05—2.25 (2 H, m), 2.30—2.50 (2 H, m), 3.55 (3 H, s),
4.38 (2 H, ddd, J1 = 4 Hz, J2 = 9 Hz, J3 = 25 Hz), 4.86 (1 H, q, J= 3.3), 7.10-7.60
(12 H, m), 7.70—7.90 (4 H, m) '30 NMR (75MHz) DMSO—d;: 8 27.6, 30.1, 43.3,

51.6, 52.7, 127.1, 127.2, 127.3, 128.2, 128.3, 131.5, 131.6, 133.3, 137.8, 167.5,

150

171.4, 173.6; HRMS (FAB): m/z calcd for C27H27N204 [M + H], 443.1893; found,
443.1971; IR(neat): 1734 cm", 1653 cm".

Ph

W
N Ph
BnAg—ZCOgl-l
Ph
Ill-G-‘I

dI-(4S,5S)-l,2-dibenzyl-4,5 diphenyl-4,S-dihydro—lH-imidazole— 4-carboxylic
Acid (2s) A solution of benzaldehyde (0.6 g, 5.7 mmol), benzylamine (0.61 g, 5.7
mmol) in anhydrous CH2C12 (120 mL) was refluxed under nitrogen for 2 h. 2-
Benzyl-4-phenyl-4H-oxazolin-5-one (1.43 g, 5.7 mmol) and chlorotrimethylsilane
(0.8 g, 7.4 mmol) were added and the mixture was refluxed under nitrogen for 6 h
and then stirred overnight at r.t. The product was a mixture of diastereomers (3:1
ratio); yield: 60%. The above trans-diastereomer was obtained by repeated
precipitation from MeOH—Et20. ‘H NMR (300MHz) DMSO-d;: 8 3.47 (2 H, d, J
= 15.6 Hz), 4.31 (2 H, d, J = 15.6 Hz), 5.80 (l H, s), 6.40—7.40 (20 H, m) l3C
NMR (75MHz) DMSO-d;: 8 30.7, 47.5, 72.1, 100.2, 126.2, 126.6, 126.8, 127.3,
127.7, 127.9, 128.0, 128.5, 128.6, 129.0, 131.4, 132.5, 133.4, 136.4, 164.4, 171.6a
HRMS (FAB): m/z calcd for C30H27N2O2 [M + H] 447.2010; found, 447.2072;

IR(neat): 3350 cm", 1704 cm".

151

Ph

3
N P11

C02H
lIl-9-1a
dl-(4S,5.S')-l,2-Dibenzyl-4-methyl S-phenyl4,5-dihydro-lH-imidazole- 4-
carboxylic Acid III-9-1a and dl-(4S,5R)-1,2 Dibenzyl-4- methyl-5-phenyl-4,5-
dihydro-lH-imidazole-4-carboxylic Acid II-9-lb: A solution of benzaldehyde
(1.13g, 10.58 mmol), benzylamine (1.13g, 10.58 mmol) in anhydrous CH2C12
(250 mL) was refluxed under nitrogen for 2 h. 2-Benzyl-4-methyl-4H oxazolin-S-
one (2g, 10.58 mmol) and chlorotrimethylsilane (1.48 g, 10.58 mmol) were added
and the mixture was refluxed under nitrogen for 6 h and then stirred overnight at
r.t. The reaction mixture was evaporated to dryness under vacuum. The product
cis isomer III-9-lb was precipitated using CH2C12-Et20; yield: 30% (0.6 g);
white solid. The trans-isomer III-9-la was then precipitated out from the mother
liquor. A small amount was then reprecipitated to remove traces of III-9-lb;
yield: 17% (0.15g). III-9-1a: 1H NMR (300MHz) DMSO-d6: 8 1.74 (3 H, s), 3.67
(1 H, d, J= 15.3 Hz), 4.11 (1 H, d, J= 14.7 Hz), 4.38 (1 H, s), 4.46 (1 H, d, J=
14.7 Hz) 4.59 (1 H, d, J = 15.3 Hz), 6.77 (2 H, d, J = 7 Hz), 7.00—7.60 (13 H, m)
13C NMR (75MHz) DMSO-d;: 8 22.1, 32.4, 48.1, 70.6, 71.1, 127.6, 128.3, 128.6,
128.9, 129.1, 129.2, 129.7, 132.2, 132.8, 133.5, 164.8, 175.1; HRMS (FAB): m/z
calcd for C25H25N2O2 [M + H], 385.1898; found, 385.1916; IR (neat): 3350 cm",

1624 cm".

152

Ph

1
Bn/\« Ph

N "“COzH

Ill-94b
111-9-111: 1H NMR (300MHz) DMSO—d;: 8 1.14 (3 H, s), 3.94 (1 H, d, J = 15.6
Hz), 4.24 (2 H, q, J: 8.7 Hz), 4.56 (1 H, d, J= 15 Hz), 5.74 (1 H, s), 6.65 (2 H, d,
J = 7.5 Hz), 7.00—7.40 (13 H, m) l3c NMR (75MHz) DMSO-d;: 8 27.1, 38.7,
48.6, 71.5, 73.5, 127.9, 128.3, 128.5, 128.7, 128.9, 129, 129.3, 129.4, 129.5,
129.8, 120.9, 132.2, 133.5, 134.2, 165.6, 170.6 ; HRMS (FAB): m/z calcd for

C25H25N2O2 [M + H], 385.1898; found, 385.1917; IR (neat): 3350 cm", 1738 cm'

c0211

PhAO/ILNIWN Ph

lll-10-1
Benzyl (S)-1-(1-Benzyl-4-methyl 2, S-diphenyl-4,5-dihydro-IH-imidazolyl- 4-
carboxylate)-2-methylpropyl carbamate (III-10-l): A solution of
benzaldehyde (0.34 g, 3.3 mmol), benzylamine (0.35 g, 3.3 mmol) in anhydrous
CH2C12 (15 mL) was refluxed under nitrogen for 2h. 2-Phenyl-4-methyl-4H-
oxazolin-S-one (l g, 3.3 mmol) and chlorotrimethylsilane (0.53 g, 4.93 mmol)
were added and the mixture was refluxed under nitrogen for 6 h and then stirred
overnight at r.t. The solvent was evaporated under vacuum. The product was
precipitated using CH2C12/hexanes (1:1); yield: 25% (0.411 g); white foam; 1H

NMR (300MHz) DMSO-d;: 8 0.4 (3H, d, J = 6.9 Hz), 0.54 (3H, d, J = 6.9 Hz),

153

1.58 (3H, s), 1.8 (1H, s), 3.94 (1H, m), 3.64 (1H, d, 14.7 Hz), 4.25 (1H, s), 4.59
(1H, d, J =9 Hz), 4.76 (1H, d, J = 15.3 Hz), 4.916 (2H, s), 6.36 (1H, s), 6.96-7.22
(14H, m); 13C NMR (75MHz) DMSO-d6: 8 17.5, 17.9, 18.9, 30.6, 30.9, 43.2,
48.6, 60.5, 66.8, 127.2, 127.3, 127.5, 127.8, 128.0, 128.3, 128.4, 135.8, 137.7,
156.3, 171.1, 171.3, 171.6; HRMS (BI): m/z calcd. for C30H33N3O4 [M +H],

499.6007; found, 5002.; IR (neat): 3302 cm", 1759 cm", 1672 cm".

Ill-11-1 9”
d1- (45', 53).]-Benzyl-4-methyl-2(4-methoxyphenyl), 5-diphenyl-4,5—dihydro-
lH-imidazole- 4-carboxylic Acid (III-ll-l): A solution of benzaldehyde (1.026
g, 9.75 mmol), benzylamine (1.043 g, 9.75 mmol) in anhydrous CH2C12 (15 mL)
was refluxed under nitrogen for 2h. 2-(4-methoxyphenyl)-4-methyl-4H-oxazolin-
S-one (2 g, 9.75 mmol) and chlorotrimethylsilane (1.58 g, 14.63 mmol) were
added and the mixture was refluxed under nitrogen for 6 h and then stirred
overnight at r.t. The solvent was evaporated under vacuum. The product was
precipitated using CH2C12/hexanes (1:1); yield: 62% (2.42 g); white solid (light
sensitive in solution!); 'H NMR (300MHz) DMSO-dgz 8 1.692 (3H, 3), 3.851
(3H, s), 4.11 (1H, d, J = 15.9 Hz), 4.92 (2H, d, J = 15 Hz + s), 6.99-7.02 (2H, m),
7.22 (2H, d, J = 9 Hz), 7.27-7.29 (4H, m), 7.31-7.41 (4H, m), 7.9 (2H, d, J = 8.7
Hz); I3C NMR (75MHz) DMSO—d6: 8 24.9, 48.6, 55.7, 69.7, 72.9, 113.5, 114.8,

127.4, 128.0, 128.2, 128.6, 128.8, 129.4, 131.3, 132.3, 133.1, 163.4, 165.4, 169.3;

154

LRMS (El): m/z calcd. for C25H24N203 [M+], 400.4697; found, 400.6; IR (neat):

3350 cm", 1738cm '1.

Ph

)
Ph N Ph
111-12-1 F
Partial data dl- (4S, 5S)-1-Benzyl-4-isopropyl 2, 5-diphenyl-4,5-dihydro—1H-
imidazole- 4-carboxylic Acid (III-12-l): A solution of benzaldehyde (1.58 g, 15
mmol), benzylamine (1.605 g, 15 mmol) in anhydrous CH2C12 (15 mL) was
refluxed under nitrogen for 2h. 2-Phenyl-4-methyl-4H-oxazolin-5-one (3 g, 15
mmol) and chlorotrimethylsilane (2.43 g, 22.5 mmol) were added and the mixture
was refluxed under nitrogen for 6 h and then stirred overnight at r.t. The solvent
was evaporated under vacuum. The product was precipitated using
CH2C12/hexanes (1:1); yield: 71% (4.25 g); white solid; 'H NMR (500MHz)
DMSO-d6: 6 0.8 (3H, d, J = 7 Hz), 0.93 (3H, d, 7 Hz), 2.35 (1H, q, J = 6.5 Hz),
3.97 (1H, d, J =15 Hz), 4.78 (1H, d, J =15.5 Hz), 5.06 (1H, s), 6.88 (2H, d, J = 7
Hz), 7.265-7.527 (9H, m), 7.75 (1H, t, J =7.5 Hz), 7.847 (1H, t, J =7.5 Hz), 7.95
(2H, d, J = 7 Hz) 13C NMR (125MHz) CD301): 8 16.1, 18.1, 36.4, 42.8, 49.5,

70.5, 78.2, 122.5, 128.9, 129.1, 129.5, 129.54, 129.6, 129.7, 130, 130.2, 130.3,

132.9, 134, 134.6, 134.8, 166.5, 169.6; IR (neat): 3345 cm", l732cm ".

155

Ph

)
Ph N Ph
Gang—Ecozl'l
(Pb Ill-I145

dl- (4S, 5S)-l,3-Dibenzyl-4-methyl-2,5-diphenyl-4,5-dihydro-1H-
imidazolinium- 4-carboxylic Acid. Chloride salt (III-l-lS): In a flame dried
flask under nitrogen atmosphere, imidazoline III-l-l (0.9 g, 2.42 mmol) was
suspended in 200 ml dry benzene. Solid anhydrous K2C03 (4.5 g, 26 mmol) was
weighed in followed by addition of benzyl chloride (0.367 g, 2.9 mmol) was
added and the mixture was refluxed under nitrogen atmosphere overnight. After
all the starting material was consumed, the solvent was removed under vaccum.
The residue was dissolved in dichloromethane and the product imidazolium
chloride was precipitated from dichloromethane/hexanes (1 :1) mixture in 77 %
yield (0.926 g) as white foam; 1H NMR (300MHz) CDC13: 8 1.44 (3H, 8), 3.658
(1H, d, J = 15.6), 4.17 (1H, s), 4.27 (1H, d, J = 12.3 Hz), 4.48 (1H, d, J = 12.3
Hz), 4.56 (1H, d, J = 15.6 Hz), 6.77-6.8 (2H, m), 6.91-6.93 (2H, m), 7.05-7.13
(10H, m) 7.32-7.34 (2H, m), 7.57-7.6 (2H, m); l3C NMR (75MHz) CDC13: 8 26.5,
48.6, 66.0, 72.9, 77.6, 127.4, 127.4, 127.5, 127.7, 127.9, 128.1, 128.4, 129.9,

130.6, 135.3, 136.2, 136.5, 165.9, 171.5; LRMS (EI): m/z calcd. for C31H29N202

[M+], 461.2224; found, 461.2; IR (neat): 3350 cm", 1736 cm ".

156

Ph
HN

I?“
HN CozH

Ph
dl—(2S,38)-2,3-bis(benzylamino)—2-methyl-3-phenylpropanoic acid III-1-16: In
a round bottom flask equipped with a stirrer III-l-lS (0.5g, 1 mmol) was
suspended in 25 mL of MeOH. NaBH4 (0.114 g, 3 mmol) was added in three
portions and the mixture was stirred rapidly overnight. After all the starting
material was consumed, MeOH was evaporated off. The residue was dissolved in
ethyl acetate (50 mL) and extracted with saturated ammonium chloride solution.
The organic layer was dried over anhydrous sodium sulfate, filtered and the
solvent removed under vaccum. The residue was dissolved in dichloromethane
and the product diamino acid was precipitated from ether/dichoromethane mixture
a solid foam in 65 % yield (0.24g); 'H NMR (300MHz) (CDCl; + 2 drops D20): 8
1.19 (3H, s), 2.87 (1H, d, J= 12 Hz), 2.98 (1H, d, J= 12 Hz), 3.72 (1H, d, J=15
Hz), 4.10 (1H, s), 4.57 (1H, d, J= 15 Hz), 6.71-7.6 (15H, m); l3C NMR (75MHz)
CDC13: 8 26.0, 48.8, 67.2, 71.6, 71.9, 127.7, 127.9, 128.0, 128.6, 128.6, 128.8,
130.3, 130.5, 135.9, 136.0, 165.3; LRMS (E1) m/z calcd. for C24H25N20 [ M]+,
357.1; found, 357.3 ; HRMS (El): m/z calcd. for C24H25N20 [M] +, 357.1966;
found, 357.1967. IR (neat): 3182om", 3063 cm", 2924 cm", 2858.9 cm", 1616

cm", 1593.4 cm", 1452 cm °'.

157

9,.
@786. 1

lll-1-17
l-Benzyl-4-methyl-2, S-diphenyl-lH-imidazole III-l-l7: A solution of III-l-l
(0.5 g, 0.1.35 mmol) in acetic anhydride (15 mL) was heated to reflux for 6h. The
acetic anhydride was evaporated to dryness in vacuo. The residue was
chromatographed on silica-gel with 3:2 Diethyl ether / Hexanes to afford 0.1 g of
III-l-l7 in 23 % yield. Partial data: IH NMR (300 MHz) (CDC13)1 8 2.3 (3H, s),
5.18 (1H, s), 6.7 —6.8 (2H, m), 7.1 —7.65 (13H, m); 13c NMR (75 MHz) (CDC13)
8; 13.0, 48.5, 125.9, 127.3, 127.9, 127.9, 128.4, 128.5, 128.7, 128.8, 129.7, 130.1,
130.2, 130.3, 130.6, 131.9, 135.4, 137.6, 147.4; MS(EI): calculated for C23H20N2

(m/z) 324.4 and observed (m/z) 324.

1‘"
<> ,N /
\

N

lll-1-18
4-(3-Benzyl-5-methyl-Z-phenyl-3H-imidazol-4-yl)—pyridine III-l-l8: A
solution of III-l-ll (0.1 g, 0.27 mmol) in acetic anhydride (15 mL) was heated to
reflux for 6h. The acetic anhydride was evaporated to dryness in vacuo. The
residue was chromatographed on silica-gel with ethyl acetate to afford 0.02 g of
III-l-18 in 22 % yield. Partial data: 1H NMR (300 MHz) (CDC13): 8 2.3 (3H, s),

. 5.18 (1H, s), 6.7 —6.8 (2H, m), 7.1 -7.65 (12H, m); 13c: NMR (75 MHz) (CDC13):

158

813.5, 29.6, 50.1, 117.6, 126.5, 127.7, 128.4, 128.7, 128.9, 130.3, 137.1, 137.7,

147.4.

159

10.

11.

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164

CHAPTER IV
SILVER ACETATE CATALYZED Exo-SELECTIVE SYNTHESIS OF A'-
PYRROLINES AND DERIVED PYRROLIDIN ES
A. Biological and chemical significance of A'pyrrolines
A'-pyrrolines can be found in nature as biosynthetic intermediates, and as a part of
pheromones, and alkaloids, steroids, hemes and chlorophylls (Figure IV-l)." 2
Myosmine, amathaspiramide E, dysinosin A, broussonetine U1, solamalidine,
veracintine and lanopylins are some recently isolated biologically active alkaloids
which contain a pyrroline unit.3 Moreover, these scaffolds have been shown to

possess insecticidal, fungicidal, anti-viral, anti-tumor and immunoactivity. 3°’ 4

Cl C02”
H020 M NH;

 

5,, 00,11
Amathaspiramide E Kaitooephalin
Y 0% ’2, 90H 0
H o 0 HO
Spirotryostatin A Lactacystin

Figure IV-l. Biologically active pyrrolines and pyrrolidines
In addition to wide ranging biological activity, A'-pyrrolines are important

synthetic intermediates, as they have three contiguous stereocenters, and one

165

prochiral center as part of a cyclic imine, which is amenable to further synthetic
manipulation such as addition of nucleophiles with stereocontrol." 5 Hence, they
are of immense value for the synthesis of pyrrolidines and complex natural
products containing highly functionalized five-membered N containing rings.“ 7
Nitrones generated from A'-pyrrolines have been employed in [3+2]
cycloadditions to generate polycyclic heterocycles like pyrrolizidines,
indolizidines and aza sugars.8 Synthesis of 01,01-disubstituted amino acids
(especially prolines) and their analogs is a topic of intense synthetic interest
because of their application in the field of peptidomimetics, catalysis and
synthesis of natural products.9 A stereoselective and efficient preparation of a
general 131-pyrroline template would provide rapid access to a range of
biologically active natural products including the myosmines, amathaspirarnides,
lactacystin and kaitocephalin type alkaloids.lo

B. Access to A'-pyrrolines using mfinchnone-alkene cycloaddition

Although, there are some reports11 for the synthesis of AI-pyrrolines cycloaddition
of milnchnones to alkenes has not been fruitful as a synthetic method to access
these compounds.”'3 Extensive work by Huisgen and others has established 1,3-
dipolar cycloaddition reactions of N-alkylated mtlnchnones (Figure IV-2), as a
general route for the syntheses of pyrroles, Al-pyrrolines, azepines and
imidazoles." Synthesis of Al-pyrrolines from intermolecular 1, 3- dipolar
cycloaddition of mtlnchnones to alkenes is fraught with following difficulties. '5

Under the conditions generally used (reflux in acetic anhydride) the reaction

166

cannot be controlled at the primary adduct stage as the decarboxylation of bicyclic

intermediates is facile, leading to cyclic azomethine ylide/Al-pyrroline.“5

R
o 9 "_ 3 E R4 R‘ R3 9
/ —* \
IN 0 R1 N R2
R2 0 R2 R
4:0/
R4 R3 R4 R3
R1 Ng R2 R1 Ne R2
l 1 R4 R
R R 3

l j____/R3 l R1/Z/;j(COZI-l

R4 R 1 R2
R4 R3 R N R4 R
A 184% AZ-pyrroline-Z-earboxylate
R‘ N R2 R3
is R3 R2
PYrrole 7-azabicyclo [2.2.1] heptane

Figure IV-2. Dipolar cycloaddition reactions of N-alkylated mfinchnones
This cyclic azomethine ylide then undergoes the following transformations: 1‘

1. Isomerization of double bond leads to formation of Az-pyrrolines and
partial loss of stereochemistry generated during cycloadditions.

2. The position of double bond in AZ-pyrrolines is determined by the C3 and
C4 substituents and often results in mixture if unsymmetrically substituted.
This results in complex inseparable mixtures.

3. Aromatization to pyrroles and complete loss of stereochemical
information.

4. A second cycloaddition with excess alkene to afford 7-azabicyclo [2.2.1]

heptarle derivative.

167

5. a-olefins (monosubstituted alkenes) have been shown not to participate in
the cycloaddition.
6. Even, if the decarboxylation is prevented when reactions are run at lower

temperatures, isomerization to Az-pyrrolines is inevitable.'7

/

R1
A A020 R1
@N O I‘BflUX N/ O
1329 0 R2 0
l R2<9 0 R2 R1 0
@N A020 k
0 reflux N 0

R1

Figure IV-3. Primary cycloadducts isolated from intramolecular cycloadditions
However, Padwa and co-workers have been successful in isolating and
characterizing primary adducts of intramolecular cycloadditions of milnchnones
to terminal alkenes (Figure IV-3).18 Steric constraints (violation of Bredt’s rule)
on the transition state prevent the elimination of carbon dioxide from these

cycloadducts and also result in the observed stereochemistry.

o 0 CI 0
_<9 _A°1_,
reflux
H

 

Figure IV-4. Primary cycloadducts isolated from intermolecular cycloadditions

168

Turchi and co-workers described the isolation of a Al-pyrroline-S-carboxylic acid
from the intermolecular cycloaddition of 1, 2-dicyanocyclobutene to a munchnone
in refluxing acetic anhydride (Figure IV-4).l5 Decarboxylation and hydride
migration are presumably prevented by structural constraints of the bicyclic
product. The authors were able to decarboxylate the intermediate by thermolysis
in refluxing decalin. Exclusive exo selectivity was observed in the primary
cycloadduct.

c. TMSCI (Lewis acid) mediated mild protocol to Al-pyrroline-S-

 

 

carboxylates
Re
INVR3 N R3
R4 \ : R1—'<\ ICOzH
O O / imine N '3
0 0 R2
TMSCI
R1—<\ I wa—h R,-<\ :E, 2—imidazoline—4-earboxylate
N R . N R4
2 I R2
LA \ KWR" ‘ ‘dRa
azlactone munchnone amene R1 \N -. COZH

D‘-pyrroline-5:28rboxylate
Scheme IV-l. Lewis acid mediated mild protocol to Al-pyrroline-S-carboxylates
We had reported that coordination of a trimethylsilyl chloride to nitrogen atom of
the azlactone, can increase the equilibrium concentration of munchnone
intermediate and accelerate the cycloaddition reaction with imines (Scheme III-
7).'9 Moreover, Lewis acid mediation leads to milder protocol (reflux in
dichloromethane compared to acetic anhydride) and it was possible to isolate the

2-imidazoline-4-carboxy1ic acid without any decarboxylation to imidazole and in

169

excellent yields. [3+2] cycloadditions in which heterodipolarophiles are employed
to intercept reactive azomethine ylides represent a useful entry into
functionalized, protected amino acids.20 In case of mfinchnones,
dihydroheterocycles isolated are masked 01, a-disubstituted (quarternary) amino
acids. In case of munchnone-imine cycloadditions we obtained quarternary
imidazoline-4-carboxylic acids. These scaffolds were formed with high
diastereoselctivity. The scaffolds exhibit four-point diversity, are amenable to
alkyl, aryl, acyl and heteroalkyl substitutions and display novel, interesting and
important biological activity.” 2' Buoyed by our own results and the isolation of
Al-pyrroline-S-carboxylic acid by Maryanoff and co-workers, we decided to apply
the mild TMSCl mediated condition for generation of mttnchnones to
cycloadditions with alkenes.ls

Azlactones were synthesized by cyclodehydration of N-acyl-a-amino acids with
EDCI.HC1.22 2-Phenyl, 4-methyl azlactone (II-1) was refluxed in THF along with
maleic anhydride in presence of 1.5 equivalents of TMSCl overnight. The residue
afier acid-base workup and extraction into ethyl acetate afforded crude product,
which was shown to have pyrroline in substantial amount. We were gratified that
the reaction did indeed occur, but unfortunately the product A'-pyrroline could not
be isolated pure (data not shown). Diethyl fumarate, after same extraction
procedure resulted in a complex mixture of diastereomers of Al-pyrroline-Z-
carboxylic acids and N-benzoylalanine (probably from unreacted azlactone). The
mixture of acids was methylated with diazomethane. Column chromatography of

the mixture resulted in 50 % combined yield of Al-pyrrolines in a 2.5:]

170

diastereomeric ratio (Table IV-l, entry IV-l-2). Reaction of 2,4-dimethyl
azlactone with t-butyl acrylate resulted in only 5 % isolated yield of diastereomer
of Al-pyrroline after methylation and column. Most of the N-benzoylalanine was
recovered from unreacted azlactone (Table IV-l , entry IV-l-3). Methyl acrylate
only resulted in 25 % yield of A'-pyrroline-2-carboxylic acid, which was directly
precipitated from the crude product using a mixture of ether and methanol. It is
evident that the conversion of azlactone to product is not complete in THF for the
reaction times and temperature, which were used. Moreover alpha olefms, which
are reported not to react with mtlnchnones, seem to be poor dipolarophiles. It also

occurred to us that TMSCI might not be the ideal Lewis acid for this reaction.

171

alkene R.

o 9 Rf‘V'“ AR;
R1_‘<\N -—> R1 \N . COzH

R2 TMSCI (1.3 eq.) R
THF, reflux 2

 

 

azlactone Dl-pyrroline-S-carboxylate
entry alkene 96 yield product
Ph
Rh 0 N'
N 0
lV-1-1 0 0 -
Ph
COzEt
lV-1-2 _/ .nCOzEt
E1020 MCOzMe
t-BUOzC
IV-1-3 t-Buozc a %
\= P" \ .nC02Me
N
MeOzc
IV-1-4 MOOZC 27%
E Ph N .nCOZMe

Table IV-l. TMSCl mediated synthesis of A'-pyrroline.

Several manipulations of reaction conditions with respect to solvent, time of
reaction and stoichiometry of alkene used were tried, without any success (data
not shown). This led us to search for a different Lewis acid. Related to our work,
Komatsu and co-workers had described in situ generation of N-silylated
azomethine ylides carrying an alkoxy substituent by treatment of a-silylimidates
with trifluorophenyl silane.23 The resulting azomethine ylide is a nitrile ylide
equivalent as it eliminates the alkoxy group after cycloaddition to alkenes, to

afford A'-pyrrolines in good yields (Figure lV-S). Although the configuration of

172

starting alkene (cis- or trans-) is retained in the cycloadditions, no
diastereoselctivity was observed. This is presumably because of the

interconversion of syn and anti forms of the dipole as is well known for acyclic

7n

 

 

systems.
fi’ho
N
0v 0
OR DCM rt 48" OPhH;
TMS OR -
PhSIF e
_2, AQA (1: 1 endo: exo)
R1 NARZ -TMSF R1 0' R2 cozue
nitrile ylide equivalent M9020 > 2 ,- 2M9
DCM, rt, 48h

Ph N’ Ph
(1 :1 endozexo)

Figure IV-S. Azomethine generation by N-silylation.

D. Screening of additional Lewis acid for synthesis of 151-pyrrolines

Afier screening a variety of Lewis acids we found that silver (I) acetate
successfully catalyzed the cycloaddition reaction of 2-phenyl-4-methyl azlactone
with N-phenyl maleimide generating the bicyclic Al-pyrroline in excellent yield
without isomerization to the Az-pyrrolines or decarboxylation (Table IV-2).8‘ '3
The Al-pyrroline was isolated as methyl ester by in situ methylation with excess
trimethylsilyl diazomethane. It was surprising to note that neither TMSCl nor
TMSOAc resulted in any cycloadditions product. Both Cu (1) and Cu (11) both did
catalyze the cycloadditions but Cu (I) definitely resulted in cleaner reaction and

higher yields. The role of the counter anion seems to be important to favor

equilibration to the N-metallated munchnone. Komatsu had concluded that the

173

exact number of fluorine ligands were important in their work with N-silylated

azomethine ylides (Figure IV-S).23

I'Dh

‘81:)”

Ph
0 o 1)LA(10mol%) 085%,: o
THF, rt, dark ' -‘
P"_<‘~ = tau—<1,

': COzMe

 

 

2) TMSCHNZ N a
11.1 was

entry LA 96 yleld
1 AgOAc‘ 78
2 CuOAc 5O
3 Cu(OAc)2 30
4 TMSCI' 0
5 M9 Br zEtzO 0
6 mom; 0
7 ch12 o
a LiCl 0

$ No significant pyrrole fomation was detected;
‘TMSCI was used in 1.3 eq.;
# Yb(OTf)3 resulted in polymerization

Table IV-2. Lewis acid screen for synthesis of A'pyrrolines.
Grigg et al have recently reported24 highly efficient synthesis of Azpyrrolines
from glycine derived isocyanate esters and alkenes using silver acetate as Lewis
acid. The use of acetate has been suggested to arise from specific need to have a
mild base like acetate ion to deprotonate a-position of the isocyanate ester to
generate a highly reactive azomethine ylide for the 1, 3-dipolar cycloaddition. The
products isolated, AZ-pyrrolines were obtained by in situ hydride migration of A'-
pyrrolines. Hydride migration was either blocked only by a-substitution of the

isocyanoacetate or by constraining the B-position to the iminic N in the A'-

174

pyrrolines. It is important to note that a mixture of diastereomers was seen, again
highlighting the difficulty of achieving complete diastereocontrol because of
equilibration between syn and anti-dipole geometry."

E. Diversity in AgOAc catalyzed synthesis of Al-pyrroline-S-carboxylates

The cycloaddition reactions proceed well with electron deficient alkenes and 10
mol % silver acetate in THF at room temperature to provide the highly substituted
Al-pyrrolines, ofien in good yields. Only the exo adducts of the A'-pyrrolines
were observed with cis-olefins as determined by NOE experiments and X-ray

crystallography (Figure IV-6 and Table IV-3).

175

1) AgOAc (10 mol %) R4

0 ° THF rt dark '- R
_<\N alkene : Ph—G—

 

 

 

\
R 2) TMSCHN2 N a 002R
azlactone
1
% Y'O'd A -pyrrollne
E
ntry R alkene (azb) a 1)
F11
Ph 0V“
' >=o
— P“ \N , cozm
Me
Iv-1-5
EtOzc
EtO co Et @0023
2 Me 2 2 75
C\=/ Ph \N '. cone
Me
IV-1-6
Etozc
BO 0 co Et doom
3 Ph 2 2 15
E’ P" \N ._ cognac
13h
lV-2-6
61020
5102 doom 002151
4 Me i; 75 (2:1) ph 1:1—(j:
co 25, econe econe
(v.1 .7; IV-1 -7b
M9020 COzMe

5 Me M$020 953121th— ”141
\= ( ) eCOZMe N :on"

lV-1-8a IV-1-8b

Table IV-3. Cycloaddition of various alkenes

176

 

”a? f?" 01221 / ‘2’)
(1 "1 11 J .1112; f ‘23)
(N, 0491.1, a R 01121

x 9
C(45)? ., - ‘3‘” . l: . .
" .C14l 8"“ ... 80121
c? . 61411 f ‘ _
/ 83 . \' :-'-
‘\ I \ ’ - V 7 C111)
crux-t . 9 N C113) 87’ O

2‘, r -
LI'IHIf J42. 01111

Figure IV- 6. X-ray crystal structure of IV-l-6.

Alkenes (R3 and R4 substituents).

Electron deficient cis-alkenes (N-phenylmaleimide and diethyl maleate) as well
as trans alkene (diethyl fumarate) resulted in very high yields for Al-pyrrolines
(entries 1, 2 and 4, Table IV-3). However, trans-diethyl firmarate resulted in 2:1
ratio of exo/endo-diastereomers (entry 4, Table IV-3). Diethylfumarate and
diethylmaleate provided both a 3, 4-trans relationship of the ethoxycarbonyl
groups. This is likely the result of the isomerization (Scheme IV-2) of the cis-
substituted A'-pyrroline to the thermodynamically more stable trans-product

(entries 2 versus 4, Table 1V-3). This is in accordance with previous observation

by other researchers. '4

177

e —

r

 

 

o
EtOZQ EtO Etc
3: AmCCbEl £00251 BMe ECChEt
_.._.‘ =
P" \N a cozue ‘ P" \N , C02Me P“ ‘N , 002Me
Me _ Me _ Me
lv-t-e

Scheme IV-2. Isomerization of cis Al-pyrroline to trans Al-pyrroline.
A main highlight is that, monosubstituted alkenes (methyl acryalate) which were
previously not known to participate in intermolecular cycloadditions of
mllnchnones also afforded very high yield for the A1-pyrroline products.18 The
lack of regioselectivity is consistent with reported literature on intermolecular
cycloadditions of mfinchnones with monosubstituted dipolarophiles.26 Efforts to
introduce alkyl or aryl substituents in 4-position with crotonic or cinnamic esters
were not successful (entries IV-2-5 and IV-l3-5, Table IV-4). Electron rich
alkenes such as vinyl trifluoroacetate (entry IV-3-5, Table IV -4) were also

unreactive in the cycloaddition with mfinchnones in accordance with the

27
Sustrnann classification for these dipoles.

178

 

 

O 1) AgOAc (10 mol %) §=0
R _<\ THF, rt, dark 7‘
1 N * Rl‘acozm
R 2) TMSCHN; N '—
2 R
2
azlactone
en 96 leld
try R1 R2 y
lV-2-5 ph pr; 0
lV-13-5 Ph Bn 67
lV-3-5 Ph 3-indolylmethyl 7O
lV-5-5 Me Me 59
lV-9-5 Bn Me 75

Table IV-4. Variation of substituents at C2 and C5 in Al-pyrrolines.
Amino Acids and acid chlorides (R; and R; substituents).
Very good yields were obtained with benzyl and indolylrnethyl substituted
azlactones (entries IV-13-5 and IV-3-5, Table IV-4). The phenyl glycine derived
azlactone reacted with diethyl maleate to afford the Al-pyrroline in a modest 15%
yield (entry IV-2-5, Table IV-4) and was unreactive towards maleimide. The 2-
position on Al-pyrroline scaffold is also amenable to difl‘erent substituents
including the 2-methy1 and 2-benzyl (entries IV-5-5, and IV-5-5, Table IV-4).
F. Probable origin of diastereoselectivity
Only the exo adducts of the Al-pyrrolines were observed as major products
determined by NOE experiments and X-ray crystallography (Figure IV-6). The

exo-preference is in contrast to the endo-preference observed in the synthesis of
. . . . . 2829-30 . . .
pyrrolldmes from acycllc azomethlne ylldes. A notable exceptlon to thls 15

31
the ligand induced exo-selectivity described by Komatsu. The exo-selectivity

179

observed by Padwa was reported to reflect the influence of steric factors in a

o o o o o I 32
unimolecular transrtton state of an intramolecular cycloaddltlon.

(I) (b) (c)

acyclic azomethine ylide Munchnone Munchnone
syn-dipole anti-dipole anti-dipole
R R
L/ “ R L’Ag"" “ L’Ag"‘ \ \
Ph-l’ g / [3th :00 L pthl ‘.q~l
Ph-“ “0 L F)h‘N” i '/ O
N /‘ , :/ H N-
H H
endol lexo endo %(
Ph Ph Ph
I l I
O N O OVNVO O N O
0MB 2. _:
H'“ .uH H H OH H'“ .nH OH
Ph 2 Ph ‘ -. \ ‘ a \
u R O N R 0 Ph N R 0

Scheme IV-3. (a) proposed syn-dipole orientation for acyclic azomethine ylides
derived from imines of ct-amino acid esters; (b) munchnone (cyclic azomethine
ylide) locked as an anti-dipole. Orientation of alkene is same and differing
outcome from (a) and (b) could be a result of different dipole geometry.

Acyclic azomethine ylides derived from a-iminoesters have been proposed to
adopt a preferred syn orientation in presence of Lewis acids due to chelation with
the carboxyl oxygen. 28‘ 29‘ 30 With silver (I) Lewis acids the inter-conversion to the
anti-orientation is minimum and hence the cycloadditions are highly endo
selective.7n The milnchnones are azomethine ylides locked in anti-orientation.

This could be a possible basis for obtaining the opposite exo diastereoselectivity

in the resulting product for the same orientation of alkene (Scheme IV-3).

180

 

Figure IV-7. Calculated electrostatic isopotential surfaces for exo- and endo-
approach of dicyanocyclobutene

Turchi and co-workers have carried out extensive MO calculations to rationalize
the stereochemical outcome.33 AMI calculations for additions of 1, 2-
dicyanocyclobutene to miinchnones indicated that the cycloaddition is a concerted
but nonsynchronous process. Semi-empirical calculations (FMO) have not been
successful in explaining regiochemical and stereochemical outcome of
cycloadditions of milnchnones. Electronic effects were suggested to play a
dominant role in favouring the exo-product in intermolecular cycloaddition
reaction.33 Inspection of electrostatic isopotential surfaces revealed that the exo
transition state shows attractive stabilizing interaction between the CH-NH-CH
portion of dipole and the nitrile groups. In fact, it was found that the exo-adduct is
also electrostatically favoured over the endo-adduct. This may provide a rationale
for the formation of the exo-cycloadduct with this in situ generated munchnone-
alkene cycloaddition reaction. endo transition state for reaction with munchnone

is proposed to be disfavourcd due to repulsive destabilizing interactions between

181

O-C=O region of munchnone and the carbonyls of N-phenyl maleimide (anti-
orientation of carbonyl dipoles expected to be more stable).25

G. Expanding the diversity of A'pyrroline templates25

During isolation the bicyclic cis Al-pyrroline-S-carboxylate by methylation using
trimethyl diazomethane in protic solvents like methanol, we observed
isomerization and equilibration to the thermodynamically more stable ring opened
trans product. Of the various bases screened to convert IV-l-S to IV—1—9, DMAP
resulted in clean conversion and afforded a 50:50 mixture of IV-l-S and IV-1-9 in
quantitative yields. Treatment of the isolated 3, 4-trans A'- pyrroline methyl ester
(IV-l-9) with DMAP in methanol reestablished the 1:1 ratio of IV -1-5 and IV-l-
9, indicating that the interconversion of IV-l-S and IV-1-9 most likely proceeds

via a common ring opened ketene intermediate (Scheme IV- 4).

 

 

 

 

0 Ph ’ - Ph
N u , '
V }=O DMAP c HN Ph ”“9020 ”1
Ph \ ‘ co Me MeOH ph J§o MN“ Ph \ ‘ °
N 2 .222. h com ~ ......
u . M8
lV-1-5 Me ‘ lV-1-9
o F"
.Ph _ _
M6020 HN “ Ph YNEO
Ago DMAP HN ° 6
Ph \ -—'> wk "" PTO—002m + ”'1'9
N E. COzMe MBOH Ph \ O N E.
Me $3. N a 002m M9
Iv-1-s ‘ Me ‘ lV-1-5

Scheme IV -4. Ring opening and isomerization of cycloadducts.
We were successful in selectively trapping the ketene intermediate with

benzylamine to afford the trans amide in good yield (entry IV-l-10, Scheme IV-

182

5). This selective ring-opening reaction provides an opportunity to expand on the

stereochemical diversity of the scaffolds.

 

Ph
:5 ° 0
0% \th Nucleophile R PNHPh
' ' DMAP '
PhQCOZ’Me THF > Ph \ 'v 002R
N Me N Me

Entry Nucleophile R % Yield

 

Iv-1-s MeOH OMe 50 %
IV-1-10 NHan NHBn 76%

Scheme IV-S. Ring opening with different nucleophiles

Thus, we have developed a highly diastereoselective and efficient synthesis of Al-
pyrroline-S-carboxylic acid scaffolds via a Ag-(I) catalyzed generation of
mtlnchnones and subsequent [3+2] cycloaddition to alkenes. The exo-selectivity
complements the endo-selective cycloaddition of related acyclic azomethine
ylides very well. Diastereocontrol can be extended to the three stereocenters by
appropriate choice of substrates and isolation conditions.

H. Synthesis of pyrrolidines and DOS

As a continuing theme of developing new diversity oriented synthetic methods for
small molecule scaffolds with potential biological activity, we focused on
developing stereoselective synthesis for novel A'-pyrrolines and pyrrolidines
derived from them. Stereochemical diversity is desirable as it increases the
number of relative orientations of potential macromolecule-interacting elements
in small molecules. It can best be achieved by using stereospecific reactions that

proceed with enantio- or diastereoselectivity.

183

The dipolar cycloadditions of an azomethine ylide to an alkene is a very useful
synthetic method for the synthesis of pyrrolidines. Two carbon-carbon bonds are
formed in a single operation, usually with a a high degree of regioselectivity,
while upto four new stereocenters are created, often in a highly stereoselective
manner. Therefore, this method is of considerable interest in DOS because its
stereospecificity enables stereochemical diversification of up to four tetrahedral

33 Achieving diastereoselectivity and

centers on pyrrolidine rings.”
enantionselectivity in this reaction has hence attracted intense scientific
investigation.

I. Importance of diastereoselective diversity-generating pyrrolidine syntheses

3‘ involve the transformation of a collection

Since diversity-generating processes
of substrates into a collection of products, it is critical that the processes used to
generate new stereogenic centers be both selective and general. Greater
stereochemical diversity requires powerful reagents that can override substrate
bias and deliver diastereomeric products with very high selectivity. Double-
diastereoselective reagents that can override the face selectivity of both coupling
partners, for example, to achieve exo versus endo selectivity are highly desirable
and the discovery of these types of powerful reagents is critical to achieving
stereochemical diversity in DOS.34

Although established methods for azomethine ylide generation have proven to be
both general and efficient, new procedures are constantly being divulged. These

new methods are not only for ylide generation, but are also for new chemical

184

equivalents and by careful selection; the synthetic chemist has a wealth of
technologies available at his disposal.

002Me M6020

o ' Ph i3: 002m

 

‘PerEt. 0 °C.
Et3N. -20 °C. AgOAc. 3 mol %
Zn(OTf)2, 10 mol %
0 "v. 0 Q QQ
1 Lo) NH H'N F9
N N ' Ar P
tBu 1Bu F8 PAI’ 2
tBu-BOX @2 XV'YFFAP
78 %ee, >95 % yield 60%. 90 % yield
Jwgensons approach Zhang's approach

COz‘Bu ‘Buozc,

=/
A 0M8 ¥ .
Ph NJ\y0( v ph'Q'I’cozm
H

‘PerEt. -20 °C.
AgOAc, 10 mol %

 

 

S-QUINAP
Schreiber's approach

Figure IV- 8. Synthesis of pyrrolidines from a-iminoesters.
However, most 1, 3-dipolar cycloadditions of azomethine ylides catalyzed by
chiral metal complexes have shown only endo-selectivity. Because it is important
that any of the desired diastereomers of cycloadducts can be synthesized
selectively, a study of methods of achieving exo-selectivity has attracted attention

of researchers in recent times.3 '

185

Among the different versions of this reaction, the most practical approach
has been the interaction between stabilized N-metalated azomethine ylides
derived from a-imino glycinate esters and n-deficient alkenes (Figure IV- 8). The
reaction proceeds under mild conditions and with a high degree of
diastereocontrol. Silver (1) and lithium (1) metal cations (usually stoichiometric
amounts) are most commonly used along with an excess of tertiary amine base.
Enantiocontrol has recently been extended to this reaction raising its stock for the
generation of complex heterocyclic molecules. While Jorgensen et 0129 used chiral
oxazoline complexes of Zn, Zhang and co-workers28 employed Ag (1) complex
with FAP ligands for above cycloadditions. Schreiber and co-workers further
improved on Zhang’s protocol by expanding the substrate scope and increasing
the enantioselectivities achieved by employing the QUINAP ligand.30 An
important highlight of their work is that they were able to generate quartenary

stereogenic centers by employing substituted amino acids.

186

Ph Ifh

N
O O 0y y0
A OMe U A
Ph NW > Ph COzMe

O Et3N, 40 °C. u
CU(0Tf)2. 2 "'0' % exo: >95 °/o

0O 64 %ee
Pth
I i Pth

Komatsu's approach

 

Figure IV-9. Exo selectivity tuned using bulky phosphine ligands
Recently, the normal endo selectivity of the above cycloadditions has been
reversed by Komatsu et 01 by a opportune selection of bulky phosphine ligands in
the cycloadditions of N-substituted maleimides and a-imino glycinate (Figure IV-
9).31 Reversing the diastereoselectivity which is under substrate control by using
external reagents (Lewis acids) is highly desirable in a diversity oriented synthesis
program.34
J. Extension of diastereocontrol and synthesis of exo pyrrolidines
Our ultimate aim to develop stereoselective methodology for Al-pyrrolines was to
diversify these templates to other heterocycles.“ 7’ 8' 9 Foremost among these aims,
was reduction to pyrrolidines. 0t, a-disubstituted a-amino acids like lactacystin
(proteosome inhibitor), kaitocephalin (Glutamate receptor antagonist), dysibetaine
(glutamate agonist) have generated considerable interest due to their biological

activities.'0

187

Unusual regioselection and stereochemical outcome resulted in the reaction of
these Al-pyrroline scaffolds (having competing electrophilic centers) with various
nucleophiles.” By a judicious choice of conditions we are able to extend
diastereocontrol to all four centers of these scaffolds (Scheme IV-6).

O

o
Pia—(I
N R

control of
two stereocenters I AgOAc

 

 

Ph gantrol of
' rec stereocenters
0:3“?0 Nu o 0}»:th
Dcozm DMAP' N" = bcone
P" \N " Ph ‘N ’R
R 50—75 %
R-Ih. lV-1-5 NuIMcOl-l, Iv-1-s
“'3'“ ”'1“ Nu- NH,3n,Iv-1-1o
NaBH4
65 % MeOH control of NaCNBH3
four stereocenters MeOHzAcOH
80 %
Ph 0 O
Nvo ,1; Oi—NHPh O §-NHPh
0“ : Bas NU "I ' NU "
. . CO M ---N'u-% . COZMe . COZNU
Phi“): 2 ° Ph N 'R Ph‘" N ’R
R H H
H
m, IV-1-11 R-Ilo. Nu-Nflan. IV-1-12

Scheme IV-6. Extension of diastereocontrol to all four centers of A'-pyrroline.

Thus, bicyclic Al-pyrroline (exo, 3, 4-cis) on treatment with DMAP and
nucleophiles (MeOH) resulted in ring opened Al-pyrroline IV-l-9 (exo, 3, 4-
trans). Reduction of bicyclic Al-pyrroline with NaBH.. in MeOH resulted in
bicyclic pyrrolidine IV-l-ll(exo, 3, 4-cis, 2, 3-trans). Surprisingly, reduction of

ring opened Al-pyrroline with NaBH4/ MeOH resulted in reduction of the 5-

188

carboxylic acid (appendage diversity; data not shown). Afier generation of
iminium ion in 1:1 acetic acid/MeOH was facile with NaCNBH; and led to
monocyclic pyrrolidine IV-l-12 (exo, 2, 3, 4-all trans) diastereomer. Ring
opening of bicyclic pyrrolidine was attempted under various conditions to obtain
ring opened pyrrolidine (exo, 3, 4-trans, 2, 3-cis) without success. All the above

structures have been corroborated with X-ray crystallographic data (Fig. IV-10).

 

Figure IV-10a. X-ray crystal structure of TOP: IV-l-S, BOTTOM: IV-l-9

189

 

 

Figure IV-10b. X-ray crystal structure of IV-l-12.

K. Enantiocontrol in Al-pyrroline synthesis

Moderate enantiomeric excesses have also been achieved in this silver catalyzed
process using chiral P, N-ligands, namely (S)-QUINAP.30 Thus azlactone IL]
was reacted with N-phenylmaleimide, in presence of 10 mol % AgOAc and S-
QUINAP (preformed complex) in THF at -40 °C for 48h. After methylation,
purification the Al-pyrroline isolated was analyzed by 'H NMR in presence and

absence of Eu (III) shift reagent.

190

Ph '1

1) '1‘

O 0 0% yo
:0 0 U = =‘

AgOAc, 10 mol % ,
0
THF, 40 C, lV-1-5

O \ 25 96 ee
/ N
i i 9th

S-QUINAP
2) TMSCHNZ

Z-V

 

Scheme IV-7. Asymmetric synthesis Al-pyrroline

The enantiomeric excess estimated from the chemical shift was 25 %. Achieving
practically useful enantioselectivity will be actively pursued in near future.

L. Experimental Section

Reactions were carried out in flame-dried glassware under nitrogen or argon
atmosphere, unless otherwise noted. Commercial solvents and reagents were used
as received. Anhydrous solvents were dispensed from a delivery system which
passes the solvents through packed columns (tetrahydrofuran, methylene chloride:
dry neutral alumina). All reactions were magnetically stirred and monitored by
TLC with 0.25 pm pre-coated silica gel plates and column chromatography was
carried out on Silica Gel 60 (230—400mesh) supplied by EM Science. Yields refer
to chromatographically and spectroscopically pure compounds unless otherwise
stated. Melting points were determined on a Mel-Temp (Laboratory devices)
apparatus with a microscope attachment. Infrared spectra were recorded on a
Nicolet IR/42 spectrometer. 'H and ”C NMR spectra were recorded on a Varian

Gemini-300 spectrometer, Varian VXR-SOO spectrometer and Unity+-500

191

spectrometer. Chemical shifts are reported relative to the residue peaks of the
solvent CDC13 (7.24 ppm for 1H and 77.0 ppm for 13C). HRMS were obtained at
the Mass Spectrometry Facility of the Michigan State University with a JEOL
J MS HX-l 10 mass spectrometer. Gas chromatography/ low resolution mass
spectra were recorded on a Hewlet—Packard 5890 Series II gas chromatograph
connected to a TRIO-1 EI mass spectrometer. Combustion analysis was carried
out on Perkin Elmer Series II 2400 CHNS/O Analyzer.

General Procedure

1) Method A.

Synthesis of A'-Pyrroline-2-carboxylic acid methyl ester: A solution of
azlactone (Immol), AgOAc (0.1 mmol) and alkene (2 to 3 mmol) in dry THF (50
mL) was stirred under argon in a flame-dried flask in dark for the requisite
amount of time as monitored by TLC. Excess trimethylsilyl diazomethane was
added and the mixture was stirred for 2 h. Completion of esterification was
monitored by TLC (20 % MeOH / EtOAc). The solvent was removed under
vacuum and residue was chromatographed on silica gel with an ether / hexanes

mixture to isolate A'-pyrroline—2-carboxylic acid methyl esters.

1,3a,4,5,6,6a-hexahydro-l-methyl-4,6-dioxo-3,5-diphenylpyrrolo[3,4-

clpyrrole-l-carboxylate methyl ester (IV-l-S):

Nh12%

>=

H3 .~
‘ H 2-phenyloxazol-5(4H)—one (0.4g, 2.28 mmol),
Me(H1) AgOAc (0.038g, 0.228 mmol) and N-

According to general procedure using 4-methyl-

192

phenylmaleimide (0.474g, 2.74 mol) at room temperature for 48 h and
methylation with excess trimethylsilyl diazomethane, 0.62 g of pyrroline IV-l-S
was obtained (75 % yield) as a white solid after silica gel column chromatography
(45 % ether / hexanes). 1H NMR (500 MHz), CDC13: 8 1.75 (s, 3H), 3.81 (s, 3H),
4.35 (d, J = 9 Hz, 1H), 4.91 (d, J = 9 Hz, 1H), 7.21-7.22 (m, 2H), 7.36-7.37 (m,
1H), 7.41-7.42 (m, 4H), 7.44-7.45 (m, 1H), 8.16-8.18 (m, 2H); NOESYID: (HI -
> H2) 6 %, (H1 -> H3) 1 %, (H2 -> H3) 12 % , (H3 -> H2) 10 % (see X-ray data;
crystallized from dichloromethane loctane); l3C NMR + DEPT ( 125MHz)
CDC13: 22.16 (-CH3), 50.30 (-CH), 53.62 (l—COZCH3), 57.61 (-CH), 82.14
(quatemary C), 126.58, 128.65, 129.13, 129.42, 130.16, 132.17 (aromatic CH),
131.59, 131.66 (aromatic quaternary 1C), 167.08, 171.58, 172.90, 173.73; (IR
(cm'l): 3070, 1780.5, 1718, 1614; M. P. = 186-188 °C. HRMS (FAB): m/z calcd
for C21H13N204 [M + H], 363.1345; found, 363.1348; Anal Calcd. for C
zthsNzO4: C, 69.60; H, 5.01; N, 7.73. Found: C, 69.06; H, 4.98; N, 7.56.

3,4-diethyl 2-methyl 3,4-dihydro-2-methyl-S-phenyl-2H-pyrrole-2,3,4-
tricarboxylate (IV -l-6): According to general
procedure using 4-methyl-2-phenyloxazol-5(4H)-one
(0.2g, 1.14 mmol), AgOAc (0.019g, 0.114 mmol) and

diethyl maleate (0.37ml, 2.28 mol) at room

 

temperature for 48 h and methylation with excess
trimethylsilyl diazomethane, 0.31 g of pyrroline IV-l-6 was obtained (75 % yield)
as a white solid afier silica gel column chromatography (30 % ether / hexanes). lH

NMR (500 MHz), CDC13: 8 1.1 (t, J = 7 Hz, 3H), 1.27-1.29 (m, 3H), 1.48 (s, 3H),

193

3.82 (d, J = 1 Hz, 3H), 4.04-4.12 (m, 2H), 4.16-4.26 (m, 3H), 4.73 (d, J = 6.5 Hz),
7.38 (t, J = 7 Hz, 2H), 7.43 (t, J = 6.5 Hz, 1H), 7.82 (d, J = 8H2, 2H);NOESY1D:
(H1 -> HZ) 1 %; (H2 -> H3) 1 %;(Hl -> H3) 1 % (see X-ray crystallographic
data; crystallized from ether); 13C NMR + DEPT (125 MHz) coca: 13.73 (-
COZCH3) 14.06 (-C02CH3) 20.61 (-CH3), 52.85 (—CO2CH3), 53.94 (~CH),
56.80 (-CH), 61.37 (-CH2), 61.56 (-CH2), 80.51 (quaternary 1C), 128.13, 128.27,
131.12 (aromatic CH), 132.51 (aromatic quaternary 1C), 169.13, 169.89, 170.39,
172.73 (IR (cm'l): 2853, 1738.2, 1622.3; M. P. = 100-103 °C; HRMS (FAB): m/z
calcd for C19H23NO6 [M + H], 362.1604; found, 362.1602; Anal Calcd. For
C19H23N06: C, 63.15; H, 6.41; N, 3.88. Found: C, 62.99; H, 6.37; N, 3.89.

3,4-Diethyl 2-methyl 3,4-dihydro-2,S-diphenyl-2H-pyrrole-2,3,4-

tricarboxylate (IV-2-6) According to general

 

O
EtOZC 2:: procedure using 4-methyl-2-phenyloxazol-5(4H)-
”2"...
PhQ:/"'"C02Et one (0.2g, 0.84 mmol), AgOAc (0.014g, 0.084
N/g‘COzMe .
13h mmol) and dlethyl maleate (0.273ml, 2.28 mol) at

room temperature for 60 h and methylation with
excess trimethylsilyl diazomethane, 0.053 g of pyrroline IV-2-6 was obtained (15
% yield) as a yellow oil after silica gel column chromatography (30 % ether /
hexanes) which solidified in ether as a low melting pale yellow solid. 1H NMR
(500 MHz), CDC13: 8 1.19-1.23 (m, 6H), 3.46 (d, J = 8.5 Hz, 1H), 4.1-4.16 (m,
2H), 4.2-4.3 (m, 5H), 3.53 (d, J = 8.5 Hz, 1H), 7.27-7.31 (m, 1H), 7.32-7.46 (m,
7H), 7.92 -7.8 (m, 2H); NOESYlD: (H1 -> H2) 2 %; 13C NMR + DEPT

(125MHz) CDC13: 8 13.83 (-C02CH3), 14.05 (-C02CH3), 61.33 (-CH2), 63.40

194

(-CH2), 65.90 (-COOCH3), 73.08 (-CH), 89.17 (quaternary 1C), 127.37, 127.67,
128.12, 128.47, 128.55, 131.05 (aromatic CH), 132.65, 141.88 (aromatic
quaternary 1C), 168.85, 169.67, 171.99; (IR (cm'l):2853, 1734.2, 1618.5; M. P. =
58-60 ° C; LRMS (El): m/z calcd for C24stNOe [M —C02], 363.2; found, 363.3.
Anal calcd. for C24stNOs: C, 68.07; H, 5.95; N, 3.31. Found: C, 69.36; H, 6.17;
N, 3.34;
3,4-Diethyl-2-methyl-3,4-dihydro-2-methyl-S-phenyl-ZH-pyrrole-2,3,4

tricarboxylate (IV-1-7b) According to general

3 % procedure using 4-methy1-2-phenyloxazol-5(4H)-
151020.,”3002 t
phdnz 5% one (021;, 1.14 mmol), AgOAc (0.019g, 0.114
\
N R1381? mmol) and diethyl fumarate (0.37 mL, 2.28

mol.) at room temperature for 48 h and methylation with excess trimethylsilyl
diazomethane, 0.216 g (50 % yield) of (IV-1-7a) / 2) was obtained as a white
solid after silica gel column chromatography (30 % ether / hexanes). 0.108 g of
pyrroline (lV-l-7b) was obtained (25 % yield) colourless viscous liquid in later
fractions. 1H NMR (500 MHz), CDC13: 8 1.13 (t, J = 7.2 Hz, 3H), 1.24 (t, J = 7.2
Hz, 3H), 1.86 (s, 3H), 3.52 (d, J = 9 Hz, 1H), 3.62 (s, 3H), 4.08-4.18 (m, 4H),
4.86 (d, J = 9 Hz, 1H), 7.34-7.43 (m, 3H), 7.79-7.82 (m, 2H);NOESY1D: (H2 ->
H1) 5 %; (H2 -> H3) 3 %;(H3 -> H2) 2.5 %; 13C NMR + DEPT (125MHz)
CDC13: 14.11 (-C02CH2CH3), 14.25 (-C02CH2CH3), 25.42 (-CH3), 52.68 (-
COZCH3), 58.51 (-C02CH2CH3), 58.74 (-C02CH2CH3), 61.62 (-CH), 61.90 (-

CH), 81.80 (-quaternary C), 128.03, 128.32, 131.17 (aromatic CH) 132.72

195

(aromatic quaternary C), 170.12, 170.8, 171.0, 171.5; HRMS (FAB): m/z calcd
for C19H23N06 [M+H] = 361.1604 and found [M+H] = 361.1606.
Dimethyl 3,4-dihydro-2-methyl-S-phenyl-2H-pyrrole-2,4-dicarboxylate (IV -

1-8a): According to general procedure using 4-

14 %
methyl-2-phenyloxazol-5(4H)-one (0.2g, 1.14
[.3529 H2a 30 % mmol.), AgOAc (0.019g, 0.114 mmol.) and

Ph \N ancazMe 2 % methyl acrylate (0.294 g, 3.42 mol.) at room
Me (“11

temperature for 60 h, 0.172 g of pyrroline IV-l-

8a was obtained (55% yield) as a yellow viscous material after silica gel column
chromatography (25 % ether / hexanes) lH NMR (500 MHz), CDCl3: 6 1.41 (s,
3H), 3.27 (dd, J1= 17 Hz, J2= 9.75 Hz, 1H), 3.53 (dd, J, = 17.5 Hz, J2= 8.25 Hz,
1H), 3.73 (s, 3H), 3.79 (s, 3H), 3.86 (dd, J1= 9.5 Hz, J; = 8.5 Hz, 1H), 7.38-7.41
(m, 2H), 7.43-7.46 (m, 1H), 7.82 -7.83 (m, 2H); NOESYID: (IN -> H2b) 2 % ,
(H2a -> H2b) 30 % , (H2a -> H3) 14 % (H3 -> H23) 6 %; l3C NMR + DEPT
(125MHz) CDC13: 19.81 (-CH3), 38.19 (-CH2), 48.76 (-CH), 52.11 (—C02CH3),
52.90 (-C02CH3), 81.20 (quaternary 1C), 128.01, 128.49, 131.20 (aromatic CH),
133.32 (aromatic quaternary 1C), 172.31, 172.53, 173.63; (IR (cm'l): 2953.4,
1738, 1612.7; HRMS (FAB): m/z calcd for C15H17NO4 [M + H], 276.1236; found,

276.1235.

196

Dimethyl 3,4-dihydro-2-methyl-5-phenyl-2H-pyrrole-2,3-dicarboxylate (IV-
1-8b): additional fractions afforded 0.142 g of product IV-l-8b (45 % yield) as a
mixture of two stereoisomers. No further attempts to

COzMe

separate the two products were successful. 1H NMR
P“ ‘ co M
N a 2 e

\|

(500 MHz), CDC13: 6 1.71 (s, 3H), 1.72 (s, 3H), 2.19 (d,
J= 14 Hz, 1H), 2.44 (d, J: 14.5 Hz, 1H), 2.75 (d, J= 14 Hz, ll-I), 3.12 (d, J =14
Hz, 1H), 3.72 (s, 3H), 3.74 (s, 3H), 3.78 (s, 3H), 3.79 (s, 3H), 3.93 (s, 1H), 4.36
(s, 1H), 7.36 (t, J = Hz, 3H), 7.43 (t, J = Hz, 3H), 7.81-7.83 (m, 2H), 7.87-7.88
(m, 211); 13C NMR (125MHz) CDC13: 8 24.91, 27.20, 29.67, 48.68, 49.62, 52.60,
53.13, 53.42, 53.72, 86.48, 87.48, 128.03, 128.12, 128.5, 130.99, 131.05, 131.51,

169.0, 170.89, 173.55, 174.45, 174.6, 175.04.

l-benzyl-l,3a,4,5,6,6a-hexahydro-4,6-dioxo-3,5-diphenylpyrrolo[3,4-

c]pyrrolc-l-carboxylate methyl ester (IV-13-5): According to general
procedure using 4-benzyl-2-phenyloxazol-
5(4H)-one (0.2g, 0.8 mmol), AgOAc (0.013g,

0.08 mmol) and N-phenylmaleimide (0.165g,

 

0.956 mol) at room temperature for 60 h, 0.24 g of pyrroline IV-l3-5 was
obtained (68 % yield) as a off-white solid afier silica gel column chromatography
(55 % ether / hexanes). 1H NMR (500 MHz), CDC13: 6 3.09 (d, J = 14 Hz, 1H),

3.73 (s, 3H), 3.97 (d, J= 15 Hz, 1H), 4.29 (d, J= 9 Hz, 1H), 4.88 (d, J= 8.5 Hz,

197

1H), 7.02 (d, J = 7 Hz, 2H), 7.21-7.22 (m, 3H), 7.32-7.34 (m, 2H), 7.36-7.41 (m,
4H), 7.49-7.55 (m, 4H), 8.24-8.25 (m, 2H);NOESY1D: (H1 -> H1) 58 °/o, (H2 ->
H3) 12 % , (H3 -> H2) 16 % ; l3C NMR + DEPT (125 MHz) CDC13: 41.4 (-
PhCH2), 50.74 (-CH), 53.35 (1—C02CH3), 56.87 (-CH), 85.51 (quaternary 1C),
126.64, 127.15, 127.2, 127.35, 128.35, 128.49, 128.71, 129.02, 129.15, 129.22,
129.42, 130.23, 131.21, 131.33, 132.22 (aromatic CH), 131.43, 131.68, 135.58
(aromatic quaternary 1C), 167.11, 171.08, 172.75, 173.19; (IR (cm'l): 2980,
1780.5, 1718.8, 1614.6; M. P. = 165-168 °C. HRMS (FAB): m/z calcd for
C27H22N204 [M + H], 439.1658; found, 439.1655. Anal calcd. for C27H22N204: C,

73.96; H, 5.06; N, 6.39. Found: C, 73.3; H, 5.14; N, 6.40.

l-(lH-indol-3-yl)methyl)-l,3a,4,5,6,6a-hexahydro-4,6-dion-3,5

diphenylpyrrolol3,4-c] pyrrole-l-carboxylate methyl ester (IV-3-5):

According to general procedure using 4-((1H-
indol-3 -y1)methyl)-2-phenyloxazol-5(4H)-one

(0.2g, 0.69 mmol), AgOAc (0.0123, 0.069

 

mmol) and N-phenylmaleimide (0.143 g, 0.826
mol) at room temperature for 60h, 0.23 g of pyrroline IV-3-5 was obtained (70
% yield) as a pale brown solid afier silica gel column chromatography (80 % ether
/ hexanes) and precipitation from dichloromethane / ether mixture. 1H NMR (500
MHz), CDC]; + DMSO-daz 8 3.55 (d, J = 15 Hz, 1H), 3.72 (s, 3H), 3.86 (d, J = 15
Hz, 1H), 4.23 (d, J= 9 Hz, 1H), 4.79 (d, J= 9.5 Hz, 1H), 6.29 (d, J= 8 Hz, 2H),

6.91 (m, 1H), 6.95 (t, J = 7.5 Hz, 1H), 7.0-7.02 (m, 3H), 7.07 (t, J = 7 Hz, 1H),

198

7.38 (t,J=7Hz,2Hz), 7.43 (t,J= 7Hz,1H), 7.55 (d,J= 8 Hz, 1H), 8.l7(d,J=
8 Hz, 2H), 9.03 (broad s, 1H);NOESY1D: (Hl -> H1) 42.5 % , (H1 -> H2) 0.5 %
, (HZ -> H3) 7.5 % , (H3 -> H2) 9 % ; 13C NMR + DEPT (125 MHz) CDC13 +
DMSO-da: 28.86 (-IndolleH2), 48.86 (-CH), 52.74 (1—C02CH3), 56.73 (-CH),
85.35 (quaternary 1C), 110. 66, 118.48, 119.03, 121.07, 123.97, 125.46, 125.55,
127.93, 128.02, 129.57, 131.3 (aromatic CH), 108.05, 130.67, 131.11, 135.27
(aromatic quaternary 1C), 167.08, 171.58, 172.9, 173.73; (IR (cm‘l): 3402,
1747.7, 1709.15, 1614.6; M. P. = 230-232 °C. HRMS (FAB): m/z calcd for
C29H23N3O4 [M + H], 478.1767; found, 478.1768.; Anal Calcd. for C29H23N3O4:

C, 72.94; H, 4.85; N, 8.8. Found: C, 71.4; H, 4.84; N, 8.44.

l,3a,4,5,6,6a-hexahydro-l,3-dimethyl-4,6-dioxo-S-phenylpyrrolo[3,4-c]
pyrrole-l-carboxylate methyl ester (IV-S-S):

According to general procedure using 2,4-
H §=O .
2]?" d1methyloxazol-5(4H)-one (0.2g, 1.77 mmol), AgOAc
2
\ ; OzMe

- (0.029g, 0.177 mmol.) and N-phenylmaleimide (0.37g,
M6011)

2.12 mol) at room temperature for 48 h, 0.31 g of pyrroline IV-S-S was obtained
(59 % yield) as a white solid after silica gel column chromatography (80 °/o ether/
hexanes) and precipitation from dichloromethane / ether mixture. 1H NMR (500
MHz), CDC13: 8 1.65 (s, 3H), 2.3 (s, 3H), 3.8 (s, 3H), 4.17 (d, J = 8.5 Hz, 1H),
4.22 (d, J = 9 Hz, 1H), 7.21-7.23 (m, 2H), 7.4 (d, J = 6.5 Hz, 1H), 7.44-7.47 (m,
2H);NOESY1D: (H1 -> H2) 1 %, (H1 -> H3) 0.4 % (H2 and H3 are coupled and

have very close chemical shifls, hence no value for NOE could be obtained); '3C

199

NMR + DEPT (125 MHz) CDC13: 18.56 (1-CH3), 21.96 (3-CH3), 49.47 (-CH),
53.34 (1—COZCH3), 60.58 (-CH), 82.33 (quaternary 1C), 126.19, 126.3, 128.97,
129.28 (aromatic CH), 131.28 (aromatic quaternary C), 169.0, 171.57, 172.7,
173.61; (IR (cm"): 2955, 1780.5, 1716.8, 1653.2; M. P. = 156-158 °C. HRMS

(FAB): m/z calcd for C16H16N204 [M + H], 301.1188; found, 301.1189.

3-Benzyl-1,3a,4,5,6,6a-hexahydro-l-methyl-4,6-dioxo-5-phenylpyrrolo[3,4-c]
pyrrole-l-carboxylate methyl ester (IV -9-5): According to general procedure

using 2-benzy1-4-methyloxazol-5(4H)-one (0.2g,

3 %
0 Ph 1.057 mmol), AgOAc (0.018g, 0.106 mmol) and N-
/H—32I 0 phenylmaleimide (0.22g, 1.26 mol) at room
H2
ph \N M ((383048 temperature for 48 h, 0.31 g of pyrroline IV-9-5 was
e 1

obtained (75 % yield) as a hydroscopic white solid
foam after silica gel column chromatography (65 % ether / hexanes). 1H NMR
(500 MHz), CDC13: 6 1.62 (s, 3H), 3.79 (s, 3H), 3.93-4.01 (m (d + q), 3H), 4.16
(d, J = 8.5 Hz, 1H), 7.18-7.2 (m, 2H), 7.27 (d, J = 7 Hz, 1H), 7.31-7.41 (m, 5H),
7.44-7.47 (m, 2H);NOESY1D: (H1 -> H2) 2 %, (H2 -> H3) 3 % ((H2 and H; are
coupled and have very close chemical shifts, hence dtflicult to get a value for
NOE)); 13C NMR + DEPT (125 MHz) CDC13: 21.77 (-CH3), 38.47 (PhCH2-),
49.37 (-CH), 53.27 (1—C02CH3), 57.81 (-CH), 82.17 (quaternary C), 126.28,
127.29, 128.95, 128.99, 129.28, 129.51 (aromatic CH), 131.26, 134.76 (aromatic

quaternary 1C), 170.8, 171.65, 172.51, 173.5; (IR (cm'l): 2920, 1780.5, 1738,

200

1722.6, 1643.5; M. P. = 46-48 °C. HRMS (FAB): m/z calcd for C22H20N204 [M +

H], 377.1501; found, 377.1500.

2) Method B) Two step synthesis of Al-Pyrroline-Z-carboxylic methyl esters
using silver acetate catalyst: A solution of azlactone (1.0 mmol), AgOAc (0.1
mmol.) and alkene (2 to 3 mmol) in dry THF (50mL) was stirred under argon in a
flame-dried flask in dark for the requisite amount of time. The crude residue was
dissolved in 9:1 benzene / methanol mixture (50 mL). Assuming quantitative
conversion from azlactone to the A‘-pyrro1ine-2-carboxylic acid, the equivalent
amount of trimethylsilyl diazomethane was added and mixture stirred for 2 h and
completion of methylation monitored by TLC. The solvent was removed under
vacuum and residue was chromatographed on silica with ether / hexanes mixture

to isolate AI-pyrroline-Z-carboxylic acid methyl esters.

l,3a,4,5,6,6a-hexahydro-l-methyl-4,6-dioxo-3,5-diphenylpyrrolo[3,4-
clpyrrole-l-carboxylate methyl ester (IV-l-S): Using 4-methyl-2-

phenyloxazol-5(4H)-one (0.4g, 2.28 mmol),

N h 12 % AgOAc (0.038g, 0.228 mmol) and N-
11321: phenylmaleimide (0.474g, 2.74 mmol) 0.2 g of
Ph \
N M 0(2er 6 % pyrroline IV-l-S was obtained (50 % yield) as a
e 1

white solid after silica gel column chromatography (45 % ether / hexanes) and
precipitation from dichloromethane / ether mixture. Characterization was identical

to method A.

201

Dimethyl 3-(phenylcarbamoyl)-3, 4-dihydro-Z-methyI-S-phenyl-ZH-pyrrole—
2, 4-dicarboxylate (IV-l-9): After all IV-l-S was precipitated, the filtrate on

3 % evaporation afforded 0.15 g of pyrroline IV-l-9 (35 %

.Ph
MeO N _ 1
yleld). H NMR (500 MHz), CDCl;: 6 1.5 (s, 1H),

  

03%

P , ”2 3.69 (s, 3H), 3.99 (s, 3H), 4.11 (d, J= 9.5 Hz, 1H),
7 % 0.919, M

5.09 (d, J = 9.5 Hz, 1H), 7.14 (t, J = 7 Hz, 1H), 7.36
(t, J = 8 Hz, 1H), 7.42 (t, J = 8 Hz, 2H), 7.48 (m, 1H), 7.61 (d, J = 8.5 Hz, 2H),
7.83 (d, J = 8 Hz, 2H), 8.98 (s, 1H);NOESY1D: (H1 -> H2) 3 %, (H1 -> H3) 7 %
(H2 -> H3) 3 % , (H3 -> H2) 3 % (see X-ray data; crystallized from
dichloromethane /octane); 13C NMR + DEPT (125 MHz) CDC13: 21.94 (1-CH3),
52.81 (4-C02CH3), 53.67 (1—C02CH3), 55.17 (-CH), 56.32 (-CH), 80.36
(quaternary 1C), 119.63, 124.39, 126.55, 127.82, 127.97, 128.54, 129.04, 131.34
(aromatic CH), 132.61, 137.78 (aromatic quaternary 1C), 166.26, 169.1, 171.42,
177.04; (IR (cm’l): 3346, 2953, 1738 (broad), 1680.2, 1599.1; M. P. = 155-157 °
C; HRMS (FAB): m/z calcd for C22szNzOs [M + H], 395.1607; found, 395.1608;
Anal Calcd. for C221-122N205: C, 66.99; H, 5.62; N, 7.1. Found: C, 67.16; H, 5.48;

N, 7.13.

3) Synthesis of (IV-l-9) from (IV-l-S) in methanol using DMAP: Compound

(IV -1-5) (0.02g, 0.055 mmol) was suspended in 20 m1 of methanol in a glass vial

202

having a screw cap. DMAP (0.007g, 0.06 mmol) was added and reaction mixture
stirred at room temperature. The reaction was monitored by TLC (l: R; = 0.38 and
1b: Rf = 0.35, 50 % ether / hexanes) and aliquots were taken out and the solvent
was evaporated and the residue dried under vaccum for lHNMR. After
determination of ratio of 1:11 by 1H NMR, the residue was chromatographed as

outlined above in procedure 2.

4) Synthesis of (IV-l-lo) from (IV-l-S): Compound (IV-l-S) (0.05g, 0.138
mmol) was suspended in 20 mL of benzylamine in a flame-dried flask under
nitrogen and DMAP (0.016g, 0.14 mmol) was added and mixture stirred at room
temperature for 36 hours. The mixture was dissolved in 50 mL dichloromethane
and extracted with 10 % HCl solution (3x, 30 mL). The dichloromethane layer
was dried over anhydrous sodium sulphate and evaporated under vacuum. The
residue was chromatographed with 45 % ether / hexanes to afford the product as a

white solid foam (0.057g, 76 % yield).

(ZS, 3S, 4S)-N2,N‘-dibenzyl-3,4-dihydro-2-methyl-Na,5-diphenyl-2H-pyrrole-
2,3,4-tricarboxamide (IV-l-10): 1H NMR (500 MHz), CDC13: 8 1.62 (s, 1H),

3.84 (d, J =10 Hz, 1H), 4.31 (dd, 11 = 4.5 Hz, 12 =

   

Ph
RN ,0, .Ph
1 % 14.5 Hz, 2H), 4.45 (dd, J1 = 4.5 Hz, J2 = 14.5 Hz,
H3». . 0
Ph 2 2H), 4.59 (dd, J1 = 6 Hz, 12 = 15 Hz, 2H), 4.75 (dd,
"Me (H1)
2 % NH 11 = 6.5 Hz, 12 = 14.5 Hz, 2H), 4.84 (d, J = 10 Hz,

P“ 1H), 6.98 (broad s, 1H), 7.15 (t, J = 7 Hz, 11-1), 7.2-

203

7.52 (m, 14H), 7.65 (d, J = 8 Hz, 2H), 7.75 (d, J = 6.5 Hz, 2H), 10.92 (s, 1H);
NOESYID: (H1 -> H2) 1% , (HI -> H3) 2 % (H2 -> H3)1% , (H3 -> H2)1%;
13C NMR + DEPT (125 MHz) CDC13: 22.5 (1-CH3), 43.77 (~CONHCH2Ph),
43.9 (—CONHCH2Ph), 56.01 (-CH), 57.5 (-CH), 78.54 (quaternary 1C), 120.21,
124.55, 127.18, 127.76, 127.82, 127.93, 128.02, 128.2, 128.78, 128.87, 128.95,
129.07, 129.18, 129.24 (aromatic CH), 131.54, 133.13, 137.5, 138.27, 138.37
(aromatic quaternary 1C), 167.93, 170.21, 170.62, 176.75; (IR (cm'l): 3325, 2925,
1676,1660,1649.3, 1599.2; M. P. = 80-83 ° C; HRMS (FAB): m/z calcd for
C34H32N4O3 [M + H], 545.2552; found, 545.2554; Anal Calcd. for C34H32N4O3:

C, 74.98; H, 5.92; N, 10.29. Found: C, 74.16; H, 5.88; N, 10.13.

4) Synthesis of (IV-l-ll) from (IV-l-S): Compound (IV-l-S) (0.1g, 0.275
mmol) was suspended in 20 mL of MeOH in a flame-dried flask under nitrogen

Ph and NaBH4 (0.020 g, 0.55 mmol) was added

N 2.4 % . .
H z 3: and mixture strrred at room temperature for 3
3 = _.-
@1186 I‘D 0 5 0/ hours. After all the starting material had
‘ -. 2 e . 0

H N -
1% “H Meth

reacted, MeOH was evaporated off. The
mixture was dissolved in 50 mL dichloromethane and extracted with 5 % HCl
solution (3x, 30 mL). The dichloromethane layer was dried over anhydrous
sodium sulphate and evaporated under vacuum. The residue was

chromatographed with 80 % ether / hexanes to afford the product as white solid

foam (0.076g, 76 % yield).

204

1H NMR (500 MHz), coca: 8 1.71 (s, 3H), 3.72 (s, 3H), 3.97 (d, J = 15.5 Hz,
1H), 4.19 (d, J: 15.5 Hz, 1H), 5.52(d, J = 10 Hz, 1H), 7.18-7.48 (m, 8H), 7.94-
7.97 (m, 2H);NOESY1D: (H1 -> H2) 0.5 %, (H2 -> H3) 2.4 %, (H3 -> H4) 1 %
; ”C NMR + DEPT (125MHz) CDC13: 22.19 (-CH3), 50.95 (-CH), 53.25 (1—
coch3), 57.84 (-CH), 82.93 (quaternary C), 85.93(-CHPh), 124.49, 127.22,
128.94, 129.03, 129.16, 131.88 (aromatic CH), 131.96, 136.40 (aromatic
quaternary 1C), 171.24, 171.78, 173.51; (IR (cm'l): 3360, 2928.3, 1738.1, 1680.2;

HRMS (FAB+): m/z calcd for C21H20N204 [M + H], 364.3945; found, 364.1413.

4) Synthesis of (IV-l-12) from (IV-1-5): Compound (IV-l-S) (0.05g, 0.091
mmol) was suspended in 20 mL of MeOH/AcOH (1:1) in a flame-dried flask
under nitrogen and NaBHr (0.016g, 0.14 mmol) was added and mixture stirred at
room temperature for 3 hours. After all the starting material had reacted, MeOH
was evaporated off. The mixture was dissolved in 50 mL dichloromethane and
extracted with saturated ammonium chloride (3x, 30 mL). The dichloromethane
layer was dried over anhydrous sodium sulphate and evaporated under vacuum.
The residue was chromatographed with 50 % Ether/hexanes to afford the product
as white solid foam (0.032 g, 66 % yield).

'H NMR (500 MHz), CDC13: 8 1.53 (s, 1H), 3.3.43 (t,
J = 11 Hz, 1H), 3.74 (d, J = 12 Hz, 1H), 4.292-4.631
(complex multiplet , 5H), 6.6 (t, J = 6 Hz, 1H), 7.03

(t, J = 3.5 Hz, 2H), 7.12-7.39 (m, 15H), 7.62 (d, J = 8

 

Hz, 2H), 8.43 (t, J = 5.5 Hz, 1H), 10.334 (broad

205

singlet, 1H); NOESYlD: (H1 -> H3) 4 % , (H1 -> H2) 2 %, (H2 -> H3) 1 %, (H3
-> H2) 1 °/o, (H2 -> H4) 6 % , (H3 -> H4) 2 %; (see X—ray data; crystallized fi'om
dichloromethane /octane); '3 C NMR + DEPT (125 MHz) CDC13: 24.1 (1-CH3),
43.47 (-CONHCH2Ph), 43.73 (—CONHCH2Ph), 55.47 (-CH), 58.4 (-CH), 64.47
(-CHPh), 66.75 (quaternary 1C), 120.12, 124.41, 127.14, 127.35, 127.73, 127.94,
128.72, 129.14, 129.29 (aromatic CH), 137.83, 138. 36,138.38, 138.88 (aromatic

quaternary 1C), 169.0, 170.66, 178.2.

206

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(a) Kleinsasser, N. H.; Wallner, B. C.; Harreus, U. A.; Zwickenpflug, W.;
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(a) Kercher, T.; Livinghouse, T. J. Org. Chem. 1997, 62, 805-812; (b)
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2002, 75, 2477-2480.

210

CHAPTER V
APPLICATIONS OF IMIDAZOLINE SCAFFOLDS IN THERAPY AND
ORGANOCATALYSIS.

A. Sensitization of Tumor Cells towards Chemotherapy.

l) Enhancing the activity of Camptothecin by Novel Imidazolines

Apoptosis or programmed cell death is a cellular mechanism that maintains the
cell number and eliminates damaged or mutated cells." 2 Alterations in apoptotic
pathways can disrupt the delicate balance between cell proliferation and cell death
leading to a variety of diseases?” 4 In many cancers, apoptosis is abnormally
down-regulated, either by the mutation of pro-apoptotic proteins or by the up-
regulation of anti-apoptotic proteins.5 Aberrant apoptosis provides an intrinsic
survival advantage to cancer cells causing growth and expansion of the tumor,
and resistance to pro-apoptotic signals including chemotherapeutic agents." 7
Chemotherapeutic agents may also induce anti-apoptotic factors thereby adding to
this intrinsic resistance to chemotherapy.8 The combination of these anti-apoptotic
mechanisms has resulted in an increased dose intensity of chemotherapeutics,
often without the anticipated improved therapeutic results.9 The search for new
chemotherapeutic strategies has therefore shifted to small molecules that can
selectively induce apoptosis in cancer cells or retard the cellular
chemoresistance.lo Strategies using combinations of inducers of apoptosis and/or
inhibitors of anti-apoptotic factors and traditional chemotherapeutic drugs may

provide an improved alternative to conventional chemotherapy. 'Ob' ”

211

2) Chemotherapy and chemoresistance - camptothecin and cis-platin

The antitumor agent camptothecin (CPT) is an alkaloid isolated from the extracts
of the fruit of Camptotheca acuminata and was identified as a topoisomerase 1
inhibitor.“ ‘3 CPT-11 and several water soluble analogues including topotecan
have successfully passed clinical trials in the United States.14 Camptothecin
exhibits its antitumor activity via the formation of a stable ternary topoisomerase I
- DNA cleavable complex.“ '5 Stabilization of this cleaved DNA complex
initiates an apoptotic signaling pathway, ultimately resulting in cell death.“ '6
Concomitant with the initiation of this apoptotic cell signal, these agents induce
anti-apoptotic signaling pathways, which have compromised their efficacy in the
clinic.” 27" This cellular resistance has been attributed to the activation of anti-
apoptotic signaling pathways mediated by several transcription factors, in

d’ 18’ 2" cDNA microarrays

particular the nuclear transcription factor, NF-icB.8
using all annotated human genes have been used to establish the association
between characteristic gene expression patterns in response to drug treatment by
the topoisomerase poison camptothecin.l9 These studies in HeLa cells provided
striking evidence that administration of camptothecin resulted in up-regulation in
a large number of genes controlled by NF-KB.'9’ NF -1cB mediated anti-apoptotic
response induced by DNA damaging agents may also result from the NF -1<B
mediated initiation of cellular DNA repair mechanisms.20 Topoisomerase
inhibitors are in this context also considered DNA damaging agents, since they

exert their apoptotic mode of action via the stabilization of a ternary DNA-drug-

protein cleavable complex.'6 In addition to the induction of anti-apoptotic gene

212

transcription, the topoisomerase 1 inhibitor camptothecin was found to induce the
NF-1cB mediated activation of proto-oncogenes such as c-Myc and cyclin DI and
indirect deregulation of the retinoblastoma tumor suppressor protein (Rb protein).
'7' ’ 27" 2"22 Cis-platin has also been shown to promote a biphasic activation of
NF-KB in macrophages through a mechanism dependent on I-KB. "d It has also
been shown to persistently activate N-terminal c- Jun Kinase which triggers
induction of apoptosis. However, usage of a MEKKl (common to NF-KB
pathway Figure V- 1) also enables activation of NF-KB, leading to a cross talk
between pro-apoptotic and anti-apoptotic signals. '7‘ Thus, chemotherapeutic
treatment by these agents often fails as a result of a NF-xB-mediated double
stimulus, causing chemoresistance and favoring uncontrolled cell growth.

3) NF-KB, the master key to cell growth, differentiation and apoptosis

The mammalian nuclear transcription factor, NF-KB, is a multi-subunit complex
involved in the regulation of gene transcription, including the regulation of
apoptosis.23‘ 2‘ Five distinct subunits of NF-KB are found in mammalian cells,
which include, NF -1cBl (p105/p50), NF-KBZ (p100/p52), RelA (p65), RelB and c-
Rel.24 These subunits can compose a variety of homo-or heterodimers, which
control the specificity and selectivity of specific DNA control elements.”’ 26 In
most unstimulated mammalian cells, NF-icB exists mainly as a homodimer
(p50/p50) or heterodimer (p50/p65) in the cytoplasm in the form of an inactive
complex with the inhibitory protein I-KB (Figure V-l). Many cellular stimuli
including: antineoplastic agents, 27 viruses (e.g. HIV), cytokines, phorbol esters

and oxidative stress, result in the IKK mediated phosphorylation of I-KB on

213

serines 32 and 36, followed by ubiquitinylation and subsequent degradation by the
26 S proteosome.23 Degradation of I-KB ensures the release of NF-KB.28

Upon release, NF-KB translocates into the nucleus where the subunits bind with

 

     
   

 

 

CHEMOTHERAPEU‘IICS CYTOKINES ‘
(Camptothecin) (TNF-alphn, 21.-2) "

 

 

 

 

 

 

Extracellular

Cytoplasrn

 

 

 

 

 

 

Figure V-l. General NF-KB activation pathway in eukaryotic cells.

214

specific DNA control elements and initiate gene transcription (Figure V- 1).

Prior to DNA binding, additional protein phosphorylation events are required for
optimal and specific gene transcription.29 Antiapoptotic genes such as, T RAF 1 ,
TRAFZ, c-IAPI, c-IAP2, IXAP, and [EX-IL, are directly regulated by NF-ch, and
abrogate the apoptotic signals in response to the chemotherapeutic agents.” 30
4) NF-KB as a target for drug design

Inhibition of the nuclear translocation of NF-ch, blocks this induction of gene
transcription and was found to sensitize tumor cells to chemotherapeutic agents
and enhance their antitumor efficacy.”b’ 2'- 27%; 3‘ Baldwin et al have shown the
control of inducible chemoresistance through inhibition of NF «B using a mutated
form of I-cha, a ‘natural inhibitor’ of NF-ch.2| In another study, Piette et al
showed that the overexpression of Ich-a / mutated I-cha regulated the
cytotoxicity caused by camptothecin.32 These pioneering studies illustrated the
clinical potential of NF-ch inhibitors in combination chemotherapy. There are
numerous natural and synthetic inhibitors of NF-ch reported in the literature,33
which include many antioxidants such as PDTC (pyrrolinedithiocarbamate)34'
kinase inhibitors such as hymenialdisine and analogues?”f SC-514,34‘ inhibitors
of Ich degradation such as the proteosome inhibitors lactacystin34M and PS-
3413“" and the IKK inhibitors such as parthenolide (a sesquiterpene lactone).3""“
Even though many of these agents claim inhibition of the NF-KB, enhancement of
chemotherapeutic effect has been limited. The most successful example of

combination therapy using chemotherapeutic agents with NF-KB inhibitors has

been illustrated by the proteasome inhibitor PS-341 (bortezomib), which is

215

currently in Phase 11 clinical trials in the US.3"" 3“ PS-341 inhibits the nuclear
translocation of NF-ch via the inhibition of the 26 S proteasome-mediated
degradation of IKB (Figure V-l). Since PS-341 exhibits significant cell
cytotoxicity it may be used as a single agent or in combination regiments with

classical anticancer agents providing a more than additive apoptotic response.“"’

37
5) Imidazoles, kinase inhibition and NF-KB

The imidazole nucleus appears in a number of natural products, including the
amino acid histidine and purines. Nitroimidazoles are well-known anti-bacterial
and anticancer drugs. Recently triaryl”, triaryl”: and tetraaryl39b imidazoles have
been identified as p38 mitogen-activated protein (MAP) kinase inhibitors. Chang
et al39 have been successful in optimizing this triaryl imidazole lead to a selective
antagonist for human glucagons receptor, which should lead to improved drugs to
control glucose levels in diabetics. Slee et al 40' have reported a novel-series of
non-carbohydrate imidazole-based small molecule selectin inhibitors. They are
the first class of non-carbohydrate selectin antagonists with potent anti-
inflammatory activity. Inhibition of NF-ch nuclear localization reduces human B-
selectin expression and the systemic inflammatory response.40b Hymenialdisine
was recently 4" identified to be a nanomolar inhibitor of Mitogen-Activated

Protein Kinase kinase-1 (MEKK-l).

216

 

 

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2) Glugagon receptor antagonists P-selectin inhbitor
anti-diabetic anti-inflam'netory

Figure V-2. Inhibition of NF-ch might play a key role in the pathways involved
in inflammation, diabetes and cancer.
Activated MEK-l phosphorylates and activates MAP kinases which can
translocate to the nucleus, and through the phosphorylation of a variety of
substrates modulate cytoplasmic events such as cell-proliferation and
differentiation. Shanna et al showed the inhibition of cytokines such as TNF-a
and IL-2 in Jurkat cells by hymenialdisine analogs, indoleazepines.” They
further showed selective inhibition of checkpoint kinases (involved in cell cycle)

by these molecules as compared to the natural product itself. 4":

217

6) From Imidazoles to Imidazolines

As a part of developing new small molecule scaffolds and then finding their
biological activity, imidazolines were tested for prevention of nuclear
translocation of NF-ch in Jurkat cells. The translocation of NF- ch across the
nuclear membrane was inhibited, indicative of a potential mode of action
upstream of nuclear translocation. As a primary screen, several of the
imidazolines were screened for a possible enhancement of apoptotic response of
camptothecin. Upon screening these compounds we found that they exhibited no
apparent cytotoxicity, however further investigations revealed that the
imidazolines enhanced the level of apoptosis induced by chemotherapeutics.

Further, mechanistic studies were then performed on compound 1 (Figure V-3).

N
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Figure V-3. Imidazoline that showed maximum potentiation of apoptotic events
7) Sensitization of leukemia cells toward camptothecin

i. Imidazolines and Camptothecin.

The imidazolines were evaluated for their ability to enhance the activity of
camptothecin (CPT) in cancer cells by evaluating the level of apoptosis induced
by camptothecin in the presence and absence of compound 1. Induction of
apoptosis is the hallmark of most chemotherapeutic agents, including

camptothecin.'2’ 43 Investigation of the imidazolines as apoptotic modulators,

218

revealed that these agents drastically enhanced the level of apoptosis induced by
camptothecin. The level of apoptotic cell death was measured using a caspase-
based screen. Caspase activation plays a central role in the execution of apoptosis
via the proteolytic cleavage of multiple protein substrates by caspase-3, -6 and —
7.44 The level of induction of apoptosis in cells was quantified using a
commercially available Apo-ONETM (Promega) assay, which takes advantage of
caspase-3/7 activity. Treatment of the CEM leukemia T-cells with compound 1
did not induce significant amounts of apoptosis (tested up to 10 11M for 48 h by
Apo-ONETM and cell count, data not shown), indicating that l exhibits no
significant cell cytotoxicity. The imidazoline l was subsequently screened for its
ability to enhance apoptosis induced by the chemotherapeutics, camptothecin
(CPT)”

Enhancement of CPT-induced apoptosis in CEM cells was first investigated at
concentrations of 510 nM CPT, at which CPT has been reported to cause DNA
aberrations but no significant levels of apoptosis in leukemia cells.45 Figure V-4
illustrates the effect of the imidazoline on CEM cells when incubated with CPT
over a 48 hour time period. Treatment of the cells with compound 1 had no effect
on the level of apoptosis. Treatment of the cells with 10 nM CPT resulted in some
cell death starting after 12 hours of drug treatment (Figure V- 2). Combination
treatment of the non-cytotoxic imidazoline l (10 nM) with CPT (10 nM) resulted
in enhancement in apoptotic cell death after 48 hours (Figure V-4).

Additional experiments were performed to quantify the enhancement of the

apoptotic signal in leukemia cells. Having found a phenotype, the molecular

219

mechanism of the NF -l<B signaling pathway was worked out. The imidazoline
was found to block the nuclear translocation of NF -1cB via the inhibition of I-ch

degradation. ‘2

 

 

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0 10 20 3O 40 50 60
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+No drug +Conpound 1 (10nM)

+CPT(10nM) +CPT(10nM)+Conpound 1(10nM)

 

 

 

 

 

 

 

Figure V-4. Sensitization of CEM cells to camptothecin by inridazoline 1,
measured over 48 hours. Data reported as an average of two independent
experiments (error margins are included) Figure V- 2 illustrates cell death through
apoptosis as a function of time for cells alone (pink squares), imidazoline only
(dark blue triangle), CPT (10 nM) (light blue triangle) and CPT (10 nM) in
presence of imidazoline (10 nM) (red squares). 42

ii. Inhibition of camptothecin induced NF-KB -DNA binding by imidazoline 1
Induction of NF-ch activation can proceed via a wide range of signaling
pathways.48 Thus, inhibition of NF-KB activation can proceed via the mediation of
many different pathways. Modulators of these pathways may therefore act as

general activation inhibitors, whereas others may inhibit specific induction

pathways.33 In order to investigate whether the imidazoline l inhibits the specific

220

of camptothecin induced NF-KB - DNA binding in the presence of the compound
1. CEM cells were pre-treated with various concentrations of imidazoline 1,
followed by exposure to camptothecin (1 uM). Pyrrolidine dithiocarboxylic acid
(PDTC), a non-selective NF-ch inhibitor was used as a positive control. The
nuclei were isolated and treated with the labeled ch consensus sequence. As
illustrated in Figure V- 5, the addition of imidazoline l inhibited camptothecin
induced NF-ch nuclear binding in a dose dependent manner. Control lanes
included: DNA only (lane 1), TNF-a activated NF-ch (lane 2), TNF-a activated
NF-ch treated with a p65 antibody, which provided a supershifi (lane 3) and the
unactivated control (lane 4). Treatment of the cells with camptothecin resulted in
the activation of NF-ch as indicated by the strong band of the NF-ch-DNA
complex (lane 5). Inhibition of DNA binding in the presence of the non-selective
NF-ch inhibitor PDTC resulted in a decrease of binding as anticipated (lane 6). A
similar decrease of camptothecin induced DNA binding upon treatment of
imidazoline 1 (concentrations ranging from 10 11M to 10 nM) is clearly illustrated
in lanes 6-9. Comparison of lane 5 (activated by 1 uM CPT) with lane 7
(activated by 1 11M CPT + 1 uM compound 1) indicates a significant decrease in

NF-ch-DNA complex formation.

22]

DNA ++++++++++++

TNF- a - + + - - - - - - - -
a- p65 - - + - - + - - - - -
CPT - - - - + + + + + + + +
1 - - - - - + + + + +
PDTC - - - - - - + - . . . .

123456789101112

_ I—J I .‘, ...“

uuuhuuuuuuil

Figure V-5. Inhibition of CPT activated NF-KB binding by imidazoline 1 using
EMSA. Lane]: KB consensus oligonucleotide; Lane2: NF-KB consensus oligo +
nuclear extract (TNF-a); Lane3: NF-KB consensus oligo + nuclear extract (TNF-
a)+ p65 antibody Lane4: NF-ch consensus 011 go + nuclear extract(no activation);
Lane 5: NF-ch consensus oligo + nuclear extract (1.0 pM CPT); Lane6: NF-KB
consensus + nuclear extract (1.0 uM CPT)+ antibody p65; Lane 7: NF-KB
consensus oligo + nuclear extract (1.0 11M CPT + 1 pM PDTC); Lane 8: NF-KB
consensus oligo + nuclear extract (1.0 uM CPT + 1.0 11M imidazoline 1); Lane 9:
NF-KB consensus oligo + nuclear extract (1.0 11M CPT + 0.1 11M imidazoline 1):
Lane 10: NF-ch consensus oligo + nuclear extract (1.0 pM CPT + 0.01 11M
imidazoline 1); Lane 11: NF-KB consensus oligo + nuclear extract (1.0 aM CPT +
1.0 nM imidazoline 1); Lane 12: NF-KB consensus oligo + nuclear extract (1.0
uM CPT + 0.1 nM imidazoline 1).

Further, Shanna et al discovered that imidazoline 1 inhibits the translocation of

NF—KB across the nuclear membrane in response to stimulation by

camptothecin.42

222

iii. Effect of imidazoline l on I-KBa phosphorylation

As shown in Figure V- 1, the translocation of NF- KB requires the
phosphorylation and subsequent degradation of its inhibitory protein 146 by the
26 S proteosome resulting in the liberation of NEW.47 Thus the effect of the
imidazoline on I-ch phosphorylation and degradation was investigated using a
pS32 ELISA on cytoplasmic extracts. Cytoplasmic extracts fiom CPT treated
cells with or without a pre-treatrnent with inridazoline were analyzed for
phosphorylation at ser-32 and 36. ALLN, a proteosome inhibitor48 and
parthenolide, an IKK inhibitor34H were used as controls to mark the two steps
involved in I-ch degradation leading to NF-ch translocation and gene
transcription. Parallel to the pS32 ELISA, the total I-ch levels in were measured
to accurately determine the percent phosphorylation. As anticipated, the
proteosome inhibitor, ALLN showed accumulation of phosphorylated I-ch,
whereas the IKK kinase inhibitor indicated a decrease in phosphorylation.
Pretreatment of the cells with imidazoline 1, showed a profile similar to ALLN.
Similar experiments were performed using TNF-o. to activate the leukemia cells.
Both TNF-a and camptothecin are known to activate NF-ch by I-KB degradation
following phosphorylation at ser32 and ser36 in CEM cells though with varying
kinetics.42 These studies indicate that imidazoline 1 inhibits nuclear translocation

of NF-KB via the inhibition of l-KB degradation (Figure V-6).

223

 

 

 

  
      
   
    

 

 

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All values were normalized to maximum accumulation seen in ALLN control. All
experiments were run in duplicate and data reported as the average of the two
independent experiments.

Further, luciferase based reporter assays were used to confirm the modulation of
NF-KB pathway by imidazolines upstream of binding with DNA. 1 11M
imidazoline inhibited NF-ch induced transcription by 52% and 35% for TNF-a
and CPT mediated NF-KB activation signal respectively.42

In an active collaboration with Chemgenics Therapeutics Incorporation, Menlo
Park, CA we studied the effect of these drugs in mouse models. RIF -1 tumor cells
were injected sub-cutaneously and allowed to grow to approximately 100 m3.
Administration of drugs was then started and the number of days that the tumor
took to reach ‘four times’ its original size, were counted (Figure V- 7). Their

observations were found to be in agreement with those done by us on CEM cells

(Figure V-4).

224

CGX—E060 -)(- Untreated Control

 

  
 
    

RIF-l Tumors + Cisplatin
6_ + Camptothecin
’3 + l-SP-4-84
,, g s— j‘ —o— CDDP+l-SP-4-84
§> 4 , A )1 —v—Cr'r+1-sr-4-84
g E
O
as”
:2 r
0 I I I I T I 1

 

 

Figure V-7. Decrease in the tumor size in RIF-1 tumors. Tumor volume (y-axis)
vs. Days (x-axis) to ‘four times’ volume. Chemotherapeutic drug: 8 mg/ 1kg;
Imidazolines: 100 mg/ 1kg.”

8) Significance

Antitumor drugs exert their antitumor activity via the initiation of apoptotic
signaling pathways ultimately resulting in cell death. Concomitant with the
initiation of this apoptotic cell signal, many chemotherapeutic agents induce anti-
apoptotic signaling pathways, which have compromised the efficacy of these
antitumor agents in the clinic. Inhibition of this anti-apoptotic transcription factor
retards this cellular resistance. The chemosensitizing agent 1 was found to
sensitize leukemia cells at nanomolar concentrations to camptothecin and
drastically enhance the level of camptothecin mediated apoptosis. Combination
therapy of CEM cells with camptothecin and the imidazoline 1, provided ~75 fold

enhancement of efficacy after 48 hours. The ability of the imidazoline to enhance

the apoptotic events initiated by camptothecin is postulated to be via the

225

 

inhibition of the NF-KB signaling pathway. These studies indicate that
combination therapy of classical anticancer agents with small molecule NF-ch
inhibitors has the potential to target this chemoresistance and provides a new
strategy to treat chemoresistant tumors. Work towards a detailed molecular
mechanism and the clinical potential of these agents is currently under

investigation in our laboratory.

226

B. Catalytic Asymmetric Synthesis of 4, 5-dihydro-l, 3-oxazin-6-ones.
Precursors to a, ll-dialkyl-B-amino acids using Imidazoline-4-amides as Novel
Organocatalysts

1) Introduction to [l-amino acids.

During the last decade, stereoselective synthesis of B-amino acids has gained
considerable attention due to their structural and important pharmacological

50 They exhibit powerful anti-bacterial properties and are key

properties.
constituents of naturally occurring peptides, terpenes, alkaloids, and macrolides,
and as potential precursors for fl-Iactams.5 " 52 It is well known that the B-amino,
a-hydroxyphenylpropionic acid side chain in taxol53 and related terpenoids is
crucial for their anti-cancer activity. Substituted aspartic acid derivatives are
currently of interest medicinally for use as inhibitors of L-asparagine syntlretase,54
key constituents of several proteins involved in blood-clotting cascade55 and for
their ability to act as non-transportable glutamate transporter blockers.56
Although less abundant than the (1.-amino acids, B-amino acids occur in nature in
both free and bound form in peptides. They tend to be stronger bases and weaker
acids compared to 11.-amino acids. B-amino acids are hence used as a-amino acid
surrogates for the construction of peptides which, because of different skeletal
pattern possess interesting folding patterns and conformational properties and
hence enhanced activity and metabolic stability.57

The main approaches58 available till date for the stereoselective synthesis of 8-

amino acids include Amdt-Eistert homologation of a-amino acids, enzymatic

resolution, alkylation of enolates derived from perhydropyrimidin-4-ones and

227

aspartic acid derived oxazolidinones and butanolides, hydride reduction of chiral
aziridine carboxylate esters and subsequent oxidation of product alcohols”,
addition of enolates or their equivalents to imines, Curtius rearrangement,
Michael addition of nitrogen nucleophiles to 01,13-unsaturated esters or imides with
or without trapping of intermediate enolates with electrophiles at the a-position,
hydrogenation, amino hydroxylation and B-lactam synthesis. Although a plethora
of synthetic methods use chiral auxillaries or substrate control for achieving
stereoselectivity, catalytic asymmetric synthesis of B-amino acids is just coming
of age. Jacobsenboand Miller“ 62 have exploited the sequential azidation/
reduction of a, B-unsaturated carbonyl compounds to this end. A different
approach recently reported by Boger utilizes the Sharpless asymmetric amino
hydroxylation of cinnarnate esters.63
2) Nucleophilic catalysts and stereocontrol in reaction of ketenes and imines

Chemists have long used ketenes as useful intermediates in the synthesis of
organic molecules. Since Staudinger’s original discovery, numerous research
groups have made advances both in the generation and reactions of ketenes.
Catalytic stereoselective transformations of ketenes via [2+2] and [4+2]

cycloadditions are at the forefront of research in present times."4

228

O

 

é': Ts.N 10 mol %, catalyst O at“
JL "' k ’ .R; *
R1 R; H R toluene, rt t . R
1. ,1 3 75-98 % ix R2 3
RzN 80-98 % ee

m

\N e.

2%

Fu’s catalyst

Figure V-8. Planar chiral heterocycles for enantioselective additions to ketenes

Fu and co-workers have used the planar-chiral heterocycles (Figure V-8) as
catalysts for the enantioselective addition of alcohols to ketenes and anhydrides.65
They have also extended the utility of these catalysts in the enantioselective
synthesis of B-lactam derivatives.66 The reaction is mechanically distinct /inverse
from the usual [2+2] cycloaddition or the Staudinger reaction between ketenes

and imines. General mechanism for a nucleophilic catalyst mediated stereocontrol
in fi-lactam synthesis is shown in Figure V-9. Addition of a tertiary amine to a
ketene presumably generates ammonium enolate A. This enolate can react with
an electrophilic imine generating zwitterionic intermediate B. Cyclization of the
intermediate B generates the B-lactam and regenerates the catalyst. Asymmetric
induction is imparted at the intermediate A, where the catalyst blocks one n-face
of the ammonium enolate, thereby setting the or-stereogenic center. The catalyst
further controls the orientation of the approaching electrophile (imine) through

non-bonding interactions, thereby setting the B-stereogenic center.

229

Nu: Nucleophilic catalyst O

,T 11
R RKILR

1R2R 2

G O Na NU&R‘
NUVR R;
B A
Ts}i
H R

Figure V-9. Mechanism for stereocontrol by a nucleophilic catalyst
Lectka67 and co-workers have reported a remarkable catalytic system consisting
(Figure V-10) of a chiral organic base and a Lewis acid. They have used 10 mol
% benzoquuinine (Cinchona alkaloid) as chiral nucleophilc or organocatalyst.
Addition of 10 mol % In (111) triflate resulted in dramatic enhancement of reaction
rates and chemical yields. In fact, the same catalyst performs additional roles of
generating the imine in situ and also ring-opening by alcohols amines and amino

acids to provide B-amino acid derivatives.

230

82‘ O 60 %
O H.N,Bz 10 mol % catalyst Tj MeOH BZ‘NH 0

R\/u\01 + :Etozc“'|\/u\oule

 

 

EtOZCXCI 9'9“)" $90099 EtOZC“. "'R catalyst
|n(OTf)3 R

toluene — 95 % ee
-78 °C to rt t
O 82 N

C ‘N
JL” 'k

R H c0251

 

Lackta's approach

Figure V-10. Use of Cinchona alkaloid derivative as a multifunctional
nucleophilic catalyst

3) Imidazolines as nucleophilic catalysts

68' 69 reported a diastereoselective multi-component synthesis of

We recently
highly functionalized imidazolines (Scheme V-l). A silicon mediated 1, 3 dipolar
cycloaddition of the in situ generated munchnone with an imine resulted in the
formation of highly substituted imidazolines. The imidazolines contain four-point
diversity and two stereocenters and the cycloaddition reaction is applicable to
aryl, alkyl, acyl and heterocyclic substitutions. The most widely used
organocatalysts have a five membered heterocyclic moiety, proline, Kagans’s
prolinol, MacMillian’s imidazolidinone catalyst.70 We realized that above
imidazolines, on account of unique arrangement of groups around a rigid five-
membered ring had a accessible face comprising of the iminic nitrogen and
carboxyl group and a hindered face. Hence they seem to have a pocket or catalytic
core with requisite groups and could possibly be used as organocatalysts. Thus,
the present studies focused on the synthesis of enantiopure imidazoline

derivatives and their use as chiral organic bases / catalysts for the reaction of

ketenes with imines.

231

 

R4-NH2

0 o '3‘
R
1mf R3CHO = R1\«N R3
R2 TMSCI (1.3 eq.)/ CHZCIZ NI COzH
’R2

 

reflux
azlactone trans
(major)
0
0
C(10)
cm)
. "
g 0
Cl2ll
2’ 4t 5)
o _
o ‘ 35’8"“.-
Ct3l o' ‘

   

91‘.
0‘25 :-'* “23’ ms: 0

. "\"lb 0 .
“(9 om:

0" cl.

Scheme V-l. TOP: Multi-component synthesis of imidazolines.
BOTTOM: X-ray structure of III-7-l.

In order to achieve imidazoline catalyzed enantioselective B-lactam synthesis few
reactions were attempted. They have resulted in the formation of the amide
probably formed from the intermediate acyliminium ion (Scheme V-2). It has
been found in the literature that only N-tosyl iminoesters7| (electron deficient)
have been used in the asymmetric synthesis using nucleophilic catalysts. But there
is no report concerning the use of nucleophilic imines in the catalytic synthesis of
B-lactam. Initial attempts to achieve the B-lactam synthesis using N—benzylidene-

benzylamine (III-I-l) in presence of racemic imidazoline-4-carboxylic acid and

232

esters were not successful and resulted in the formation of the corresponding

amide (Scheme V-2).

Ph
\\N Ph
Ph—<\ I
N 002Et

 

O
0 CI to ,-78°Ct rt
T uene ot PhAuJK’Ph

Ph PhANAPh
09
®
PhA hikr Ph
PhH

Scheme V-2. Failed attempt to B-lactams using racemic imidazoline-4-
carboxylates.

N-tosyl imine of ethyl glyoxylate resulted in the formation of trans-B-lactam in
moderate yield, using non nucleophilic base (proton sponge) and racemic ethyl
imidazoline-4-carboxylate. Whereas the synthesis of B-lactam using only DMAP
and TEA resulted in the formation of mixture of cis and trans (1:4) B-lactam in

good yields (Scheme V-3).72

233

 

 

¢ r: Nov

0: S—0 benzene 0: 8:0 NC|3

reflux ACOOEt

N“

9%
Ph

>
N

Ph
he
Cl 5- COOEt Ts‘Ni/(O
‘ Ill-1 -10
m ..

 

O: S: O “
,1. NW; NMe2 5‘0“:
”A...

toluene
-78°C to rt

Scheme V-3. B-lactams from N-tosyl-a-iminoester using racemic imidazoline
catalyst

4) Preparation of chiral imidazoline-4-amides catalysts

NH2

....H
Me
Ph~\

 

Phfi Ph\\N
N Ph single enantiomer N Ph Ph
ph—<\ 1"“ > Ph—(x at} Me + Ph—(\ :91)" H
N N -' '1
COOH EDCI. DMAP 5 Y n’ ”Ye
CH2CI2 O P“ P"
(t) ""14 +I- V-amlde-1

separated by column chromatography
Scheme V-4. Synthesis of chiral imidazoline—4-amide catalysts

Resolution of 1-benzyl-4-methyl-2,5-diphenyl-4,5-dihydro-lH-imidazole-4-
carboxylic acid with R-(+)-1-phenyl-ethylamine using EDCI and DMAP in DCM
resulted in the diastereomeric mixture which was separated by column
chromatography, providing enantiopure amides (Scheme V-4). Similarly S-(-)-1-

phenyl-ethylamine also resulted in the formation of diastereomers separable by

234

column chromatography.72 We decided to use imidazoline 4-amide as catalysts
for following reasons;

i) they could be obtained in enantiomerically pure form.

ii) the amide side chain may form a hydrogen bond with the enolate (zwitterionic
species formed with ketene) and thus stabilize the enolate.

iii) it may enhance the steric block on a n-face of the enolate and achieve high
amount of stereocontrol. The absolute configurations are unknown for the amides
because in the x-ray structure the a-methyl group was missing in the amide side
chain and hence had to be discarded.

5) Three component synthesis of 4, 5-dihydro-l, 3-oxazin-6-ones
(Toluene-4-sulfonylimino)-acetic acid ethyl ester (1 mmol) and phenyl ketene
generated in presence. of proton sponge (1 mmol) and the catalyst (+)-V-amide-l
(30 mol %) were allowed to react at -78°C for 12 h and two products, which could
not be separated by column chromatography were obtained (Entry V-l, Table V-
1). The reaction was carried out over a longer time (-78°C, 36h) (Entry V-4,
Table V-l) and the 4, S-dihydro, 1, 3-oxazin-6-one was formed in moderate yields
after some optimization (by crude spectra) and was purified by column
chromatography. The mass spectra of the product indicated the addition of two
molecules of ketene to (toluene-4-sulfonylimino)-acetic acid ethyl ester. The lH
NMR spectra and '3 C NMR spectra also supported the addition of two molecules
of ketene to (toluene-4-sulfonylimino)-acetic acid ethyl ester. The structure of the

product 2-benzylidene-6-oxo-5-phenyl-3-(toluene-4-sulfonyl)-[l, 3] oxazinane-4-

235

carboxylic acid ethyl ester V-6a was unequivocally established by X-ray

crystallographic data (Figure V-l 1).

 

Figure V-l 1. X-ray structure showing trans geometry of dihydrooxazinone.
The reaction was repeated using 7 equivalents of ketene (Entry V-7, Table V-l)
and 60 mol % of catalyst (Entry V—6, Table V-l) but no significant increase in the
yield was achieved. Use of stoichiometric catalyst led to complex reaction

mixture.72

236

NM°2NM92

 

 

0 0 Ph 0
2eq. Ph\)\ CI 0‘ _ Ph‘E‘ko + If
' \ Ph N,
+ 23M. -78°C, 36h R? ”k R1 T03
1 N R1 W TOS .
eq. TOS’ V N catalyst optically active racemic
Ill-I-15 Ph\« Ph V-6a V-6b
N : XYPh
- 0 R2
Entry R1 R, x Catalyst 3320:":gdn3: % yleld(a) % yleld(b)
v.1 -COzEt Me N (+)-V4mlde-1, 0.3 eq. 1eq. products not separable“
V-2 -COzEt Me N (+)-V-amlde-1, 0.3 eq. 1eq. products not separable‘
V-3 -COzEt Me N (+)-V4mlde-1. 0.3 eq. 1eq. poor yields
V4 -COzEt Me N (+)-V-amlde-1, 0.3 eq. 2.5 eq. 30 % -
V6 -C02Et Me N (-)-V-amlde-1 0.3 eq. 2.5 eq. 30 % -
V-6 -COzEt Me N (+)-V4mlde-1 0.6 eq. 2.5 eq. 37% 10 %
V-7 -C02Et Me N (+)-V4mlde-106 eq. 7 eq. 37% 10 %
V-8 -C02Et Me N (+)-V-amlde-1,1eq. 2.5 eq. messy reaction
V-9 -Ph Me N (+)-V4mlde-1, 0.3 eq. 2.5 eq. 20% 10%
V-10 -Ph Me N (+)-V4mlde-1, 0.6 eq. 2.5 eq. 35% 10%
V-11 -Ph Me O (-)V-ester-2, 0.6 eq. 2.5 eq. 0% 50%
V42 -Ph H N (+I-)V.amlde-2, 0.6 eq. 2.5 eq. 0% 0%
d. toluene, -78 °C, 12h; e. DCM. -78°C, 12h
Ph
Ph W
1 Ph N Ph
\ H N . 0 Ph
N = "Y P" i Y
' 0 R2
0 R2
+I- V-amlde-1 +I- V-ester-z

Table V-l. Synthesis of dihydrooxazinones using imidazoline based catalysts

237

Several reports in the 1970’s indicated the formation of the racemic form of the
above dihydrooxazinone.”77 One report from Furukawa et 0178 described the
asymmetric synthesis of B-amino-a, cl-dimethyl-B-phenylpropionic acid via the
asymmetric synthesis of the dihydrooxazinones by using a chiral imine auxillary.
To our knowledge, this is the first example of a catalytically induced asymmetric
multi-component synthesis of dihydrooxazinones. These dihydrooxazinones can
simply be hydrolyzed to the chiral a,B-substituted,B-amino acids.”

The proposed mechanism for the formation of the dihydrooxazinones is depicted
in the following Scheme V-S.72 Acid chloride or ketene can react with
imidazolines-amide leading to the formation of intermediate A (see also Figure V-
2). It is possible intermediate A, the ammonium enolate is stabilized because of
the hydrogen bond with the amide -NH. Intermediate A on reaction with imine
leads to another intermediate B (only one n-face of enolate blocked by catalyst).
Intermediate B collapses to form B-lactam in cases where other catalysts are used.
In our case steric block (possibly the cl-methyl of amide side chain) prevents C-N
bond rotation and cyclization. Intermediate B hence reacts with another molecule
of ketene and leads to the formation of intermediate C which on intramolecular
ring closure and elimination of catalyst affords the 2-alkylidene-4, 5-dihydro-l,3-
oxazin-6-one (Scheme V-6). The above proposed mechanism seems very likely
as, the dihydrooxazinone was obtained as a single diastereomer and in 70 %

enantiomeric excess.

238

 

 

 

 

 

dihydrooxazinone]
HIPhO
l
HV /2 9
EtOOéh P“ N i
HN Ph H
imidazoline- 4- amide >.....
Ph
P AN Ph”\ Ph
h )tj O N .u“
Ph Ph \
Ph “(9 NH
ph 0 Ph
COOEt WAQQ
Ts—N H

A
we
Ph
H C
Ph"\ ph Tst
9 N . a
C Ph \PQM N H 0008

Scheme V-S. Proposed mechanism for catalytic synthesis of dihydrooxazinones
6) Problems so far72
1. Importance of the 4—amide side chain, need to be ascertained clearly.
2. The preparation of (toluene-4-sulfonylimino)-acetic acid ethyl ester is not
trivial and the imine has to be used immediately because it decomposes very fast.
3. The highest yield obtained is 30 % and this results in very difficult
separation problems. The product has to be subjected to column chromatography

at least three times to get analytically pure material.

239

4. The absolute stereochemistry of catalyst imidazoline-4-amides is not
known as the x-ray structure misses the a-methyl group from the l-phenylethyl
side chain. This observation could not be rationalized by simple reasoning.

5. The enantiomeric excess need to be determined with more confidence.
The racemic dihydrooxazinone is needed for this purpose. To this end, resolution
by aminolytic ring opening with optically active amino acid has been attempted
but without any success.

7) Modifications to methodology

1. (Toluene-4-sulfonylimino)-acetic acid ethyl ester is highly unstable and
the preparation is neither trivial nor reliable in terms of yields or purity. Hence,
we decided to try aromatic aldimines, which are more stable and hence allow
several manipulations of the reaction condition and optimization of product yields
and enantiomeric excess. Under identical conditions as used before, catalyst (+)-
V-amide-l (30 mol %) resulted in 20 % yield with N-benzylidinetosylamine (III-
I-lS) (Entry V-9, Table V-l). Doubling the amount of catalyst increased the
yields to 35 %, indicating that aryl aldimines are less reactive than a-iminoesters.
However, they are a lot easier to prepare and are stable molecules which have
long shelf life. Even the multi-component reaction is much cleaner than with the
tosylimino ester.

2. Replacing the 4-amide side chain with the ester, catalyst V-ester-l (60 mol
%) afforded the B-lactam in 50 % yield. No dihydrooxazinone was observed

(Entry V-ll, Table V-l).

240

3. Racemic benzylamide V-amide-Z did not result in a productive reaction
(Entry V-12, Table V-l).

8) Three component to a two-component synthesis of dihydrooxazinones

Continuing with our pursuit for a more reactive imine than N-benzylidine
tosylamine(llI-I-15) and more stable than the tosyliminoesters, made us focus on
the improved procedure by Lectka and co-workers79 for synthesis of B-amino
esters with Cinchona alkaloid derivatives. They generated N-acylimines by in situ
dehydrohalogenation of N-acyl—a-chloroaminoesters. The imines were reactive
but again unstable; hence preparation of imines in situ was preferred. Kobayashi80
have reported an asymmetric Mannich reaction using N-acylimines generated by
using polymer bound bases. We decided to pursue the use of Kobayashi’s method
for the generation of N-acylimines from N-acyl-a-chlorogylcinate ethyl ester
using polystyrene bound piperidine as the base of choice (Scheme V-6).80

We realized an inherent advantage in using the N-acylimines, not only because
they are they reactive but they present an internal nucleophilc after enolization.

i. In essence, we would have achieved tethering of the second equivalent of
ketene involved in the three component synthesis of dihydrooxazinones.

ii. The internal nucleophilc (enolic —OH) would probably lead to higher
rates of dihydrooxazinone synthesis vs. the background non-catalyzed B-lactam
synthesis.

iii. More diversity can be introduced, because of replacement of second

equivalent of ketene with the acyl group from the imine.

241

iv. The product can be expected to be more reactive than the 2-exo-

substituted dihydrooxazinones that were obtained before.

 

 

 

 

 

 

 

 

Ph N CO Et NH
2
)0" j; : PhYNVCOZEt 0
Ph
. /
k EtO c“ N Ph
”RAG T t 2 v 13
CHZCIZ, -78°C—rt, 4o % _"
(e: Proton sponge ; Nu:
Ph Cat Ph\\N i (nucleophile)
Ph 0
EtOsz/k Ph—<\ :KN NYMG Ph NU
Ph’k o i 0 Ph
a tethered nucleophilc catalyst Etozc NH
TS
B-amino acid and
derivatives

Scheme V-6. Synthesis of 2-aryl dihydrooxazinones from N-acylimines
N-benzoylimino ethyl glycinate was made by dehydrohalogenation of N-benzoyl-
a-chloro ethylglycinate in a separate vessel using polystyrene bound piperidine.
The imine was then transferred to a separate reaction vessel and reacted with
phenylacetyl chloride using (+) V-amide-l and proton sponge, using the same
conditions as before (Scheme V-6). Afier acid-base workup and a short silica-gel
column, the product was precipitated using DCM/ether/hexanes mixture.
Although, both proton NMR and EIMS indicated the proposed dihydrooxazinone,
the material could not be obtained analytically pure. Dihydrooxazinones like V-
13, six membered analogues of azlactones have been reported as intermediates
during Amdt-Eistert reaction of peptides.8| They are extremely short-lived and

prone to polymerization.82

242

Though disappointing, the chemistry is possible and with appropriate substitution
it should be possible to isolate pure 2-aryldihdyrooxazinones. It would be
interesting to see their use as templates for synthesis of heterocycles in

comparison to azlactones.

243

C. Chiral 4-hydroxymethyl imidazolines as ligands for diethylzinc addition.

1) Introduction

The synthesis of organic compounds is ultimately dependent on the formation of
carbon-carbon bonds. It is, therefore, clear that the most expeditious route to
chiral compounds is one in which a carbon-carbon bond and a stereocenter are
formed in a single step with high enantioselectivity.83 The use of catalytic
quantities of metal complexes containing chiral ligands has proven to be a
particularly fruitful approach to asymmetric catalysis.84 Generally, the emphasis
in designing ligands has been on maximizing the difference in steric effects
between the alternative pathways in the enantio discriminating step. However, in
recent times the important role of electronic effects in influencing
enantioselectivity is being recognized and exploited, and some examples of strong
electronic effects have been reported, mostly involving phosphorus ligands. 85

2) Imidazolines as N-heterocyclic carbene ligands

2-Imidazolines have found wide application as potent N-heterocyclic carbene
ligands in organometallic catalysis.86 N-Heterocyclic carbenes (NHCs) are being
used increasingly in organometallic chemistry as neutral two-electron-donor
ligands, commonly replacing phosphines in that role. Some attractive features87 of
NHCs are:

i. The strong o-donating capability and low level of n-acidity of these ligands
provide their metal complexes with electronic properties that are often quite
different from those with phosphines or other traditional neutral ligands.88 This

change can sometimes enhance the reactivity of metal-based catalysts that feature

244

 

NHCs. Examples of this improved reactivity include ruthenium-based olefin
metathesis catalysts89 and palladium-based catalysts for C-C and C-N coupling
reactions.90
ii. The possible diversity in the substituents on the nitrogen atoms, which allows

92‘ 93 and electronic features.

for a wide range of steric, 9' asymmetric,
iii. It is possible to functionalize these nitrogen substituents in such a way as to
make ligands capable of chelation.” 95 Such variability has led to the synthesis of
NHC analogues of many traditional ligands.

iv) In contrast, to the corresponding phosphine complexes, ligand dissociation has
never been observed so that these catalysts normally do not depend on an excess
of ligand.86 These properties make them suitable for chiral modifications.

3) Imidazolines as nitrogen ligands

Nitrogen ligands are widely used in asymmetric catalysis and, while relatively
few examples of electronic tuning have been reported pronounced effects have
been noted.84 Oxazolines and amines are the most widely used chiral nitrogen
ligands, but unfortunately there is no straightforward approach to electronic
tuning of either of these types of donor. Although ligands carrying an imidazoline
unit are structurally very similar to oxazoline ligands, they have not received
much attention.94 4-Substituted 2-imidazolines are potentially useful as catalysts
and ligands, especially since they have the same shape as oxazolines and should
therefore give comparable enantioselectivity. Phosphino-oxazolines 2 (PHOX

ligands) are versatile chiral ligands that have found a wide range of applications

in asymmetric catalysis.95 Cationic iridium complexes derived from these ligands

245

have proven to be highly effective catalysts for the enantioselective hydrogenation
of imines and olefins, including unfunctionalized alkenes.96 Crystal structures of
PHIM (Phosphino-imidazoline) complex and the corresponding PHOX complex,
bearing the same substituents at the phosphorus atom (o-Tol) and the stereogenic
center (tBu), closely resemble each other (Figure V-12). However, the torsion
angles (1:) between the central phenyl ring and the five-membered heterocycle
differ by 20 ° (C-C-C-O angle = -11° vs C-C-C-N angle = -31°). The Ir-P
distances (2.3009(12) in li vs 2.3055(17) A in Ir-2) and the Ir-N distances
(2.080(5) vs. 2.106 (3) A) are almost identical, indicating that the electron

densities on the coordinating nitrogen atoms are similar.94

 

+
. Q31
o-Tok I, l . o-Tok. D .
o-Tol’ ‘2) PF6 o-Tol’ PF“

MN MW

Figure V-12. PHIM and PHOX ligands in solid state. Anions and COD’s not
shown.

246

However there are some important differences between the two ring systems: 84' 94

i. Imidazolines are much stronger bases than oxazolines. As a result they are
stronger donors, which may be an advantage in some reactions.
ii. The additional nitrogen atom provides a handle for tuning the electronic and
conformational properties of the ligand by proper choice of the R3 group.
iii. R3 group could also serve as a linker for attaching the ligand to a solid
support.
iv. Importantly, changes in R3 substituents do not significantly affect the chiral
environment created by the group at the 4-position, i.e. the electronic and steric
properties can be varied independently of each other.
Despite this potential, there are few reports of their use as chiral ligands. Brunner
made a brief reference to their use in Rh-catalyzed hydrosilation reactions97 and
Morimoto showed that imidazolines bearing an additional sulfur donor gave
excellent enantioselectivity in Pd—catalyzed allylations.98 Recently, Davies used
ruthenium complexes of pyridyl imidazolines as Lewis acids.99 Dupont used
biiimidazolones in Pd-catalyzed hydroarylations.loo Claver reported that pyridyl
imidazolines are useful ligands for Pd-catalyzed alkene/CO copolymerizations
and noted some electronic effects'o' Brusacca found that the nature of the 1-
substituent of 2-(phosphinoaryl)imidazoline ligands had a substantial effect on the

Yield and enantioselectivity of an intramolecular Heck reaction.‘02

247

 

 

i. 2-hydroxymethylimidazolines ligands for diethylzinc additions to aldehydes
Substantial effort has been put forth to develop methods to promote the
asymmetric addition of alkyl groups to aldehydes and ketones since 50 years.83
For many years, these investigations were largely unsuccessful due to the highly
reactive nature of the organomagnesium and organolithiurn reagents employed.
The breakthrough did not come until 1984, when Oguni103 discovered that
organozinc reagents added enantioselectively to aldehydes in the presence of
chiral amino alcoholsm' '05 Extensive mechanistic studies, primarily from the
Noyori group with DAIB (Figure V-12),'°6‘“° have greatly contributed to the
present understanding of the mechanism of reaction. The design of catalysts for
the asymmetric addition of organozinc reagents to aldehydes to give chiral
secondary alcohols has become the focus of intensive research in present times.'°4’
'05' ‘08 A large number of catalysts for this reaction have been developed that rely
on either Lewis basic or Lewis acidic for catalyzing the reaction.lll The addition
of diethylzinc to aldehydes has been studied exhaustively and amino alcohol
catalysts are available that give excellent enantioselectivity for a wide range of
aldehydes. While there is no pressing need to develop new catalysts, the
surprising and dramatic electronic effects described above with imidazoline are
deserving of further study in their own right.

Casey et al prepared a series of chiral 2-(hydroxyalkyl)imidazolines from

enantiopure amino alcohols.”ll2 These imidazolines were expected to be similar

to B-aminoalcohols in electronic properties, and were used as catalysts for the

248

addition of diethylzinc to aldehydes (Figure V-13). The study was designed
mainly to observe the effect of ligand electronics on enantioselctivity as this
aspect has rarely been studied so far.84

Et2Zn

O - OH
/U\ 5 % ligand /'\/

 

 

P" H toluene, rt P“
aldehyde

R1 R2 % ee, configuration
Bn CF3 1 5%, R

ipr CF3 62%, S

iPr Me 21%. R

iPr OMe 87%, S

tBu OMe 26%, S

 

 

 

 

 

 

 

\
‘8 0 Zn\ R2 H
”J ”a
\ 1 ,’
HO N .,’R E /Zn Et Ph
. 1 t
1493’” transition state

 

 

Figure V-13. 2-hydroxymethylimidazoline ligands for diethyl zinc addition.

The following trends were observed by Casey et (I!84

i. Diastereomer with C4 substituent on opposite face of carbinol group as
shown in Figure V-l3, was the only active isomer. This indicated that the
stereochemistry at both centers and especially at carbinol center was
important.

ii. The predominant formation of (S)-alcohols from (S)-carbinol ligands was

consistent with the observed trend for amino alcohol catalysts.

249

 

iii. Isopropyl substituent at C4, gave the best result, but further increase in size
to t-butyl was detrimental.

iv. The predominant formation of (S)-alcohol has been explained based on
accepted models as shown in Figure V-13.

v. Most striking feature of the para substituents on l-aryl group (R2), are that
groups with stronger inductive effects gave higher % ee’s.

vi. The electron donating 4-methoxy substituent results in R—alcohol like the
electron withdrawing 4-trifluoromethyl group. This was suggested to be
because of coordination of diethyl zinc, which makes 4-methoxy
sucbstituent to be electron withdrawing.

The results reported above emphasize the importance of the concept of ligand

designs that readily permit ‘orthogonal’ tuning of steric and electronic effects, and

illustrate the potential of imidazolines in asymmetric catalysis, especially as
electronically tunable alternatives to oxazolines.

ii. Our 4-hydroxymethylimidazolines as ligands for diethylzinc additions

To gain quick access to a diversity of ligands, a modular system comprised of

simple components is essential. Utilizing amino acids from the chiral pool allows

for the easy incorporation of Chirality into the ligands and opens the way for a

number of straightforward chemical modifications. Amino acids have been used

as the basis for a variety of catalyst systems, and ligands that are based on proline,
valine, tyrosine, tryptophan, serine, and leucine have been reported.”l We have
recently reported highly diastereoselective multicomponent one-pot synthesis of

substituted imidazolines from amino acid based azlactones.”3‘ ”4 These low

250

molecular weight imidazoline scaffolds contain four point diversity applicable to
alkyl, aryl, acyl, and heterocyclic substitution. In light of the exciting work by
Casey et a1 and the use of modular ligands based on amino acids for diethyl zinc
additions to aldehydes, we decided to explore our chiral 4-hydroxymethyl
imidazolines as ligands.

For this purpose racemic imidazoline-4-carboxylic acids derived from the
cycloadditions were converted to diastereomeric esters using R (+)-l-
phenylethanol. The diastereomeric esters were separated by column
chromatography. Reduction with LAH, afforded the chiral 4-
hydroxymethylimidazolines in excellent yields (Scheme V-7). The absolute
configuration was determined for one pair of enantiomeric 4-hydroxymethyl
imidazolines (Figure V-14).

The 4-hydroxymethyl imidazolines tested proved to be moderately good ligands
for addition of diethyl zinc to benzaldehyde. The following trend could be seen

from the limited number of experiments performed (Table V-2).

i. The S, S-carbinol imidazolines resulted in S-configuration 1-
phenylpropanol.
ii. The 1-(4-methoxyphenyl) substituted ligand showed reverse preference

for configuration of l-phenylpropanol as was seen by Casey and co-
workers.
iii. Use of stoichiometric quantity of additive Lewis acid, Titanium

isopropoxide following the method of Walsh and co-workers (Method B,

251

Table V-2) also resulted in opposite configuration of l-phenylpropanol.

There was no improvement in observed ee values.

QH

RY“? R,""_,

R R2 EDCI. MAHCI
COZH
DCMP
Ill-1a
racemic

 

 

 

 

R4 R
R A . R2 "j; 3
1 N r0 R1’<\N 1 R2
0 \FPh _ ;
H0 1) separated by
+ RRR-ester column chromatography RR-alcohol
60—65 °/o
2) Reduction R4 R3
LAH, THF. 0°C N
55—65 % _ R14” -“R2
H0
SSR-ester SS-aloohol
Ester R1 R2 R3 R4 alcohol
V-1-14 Ph Me Ph 8n V-1-17
v-1-15 Ph Me Ph 4'32";ng v.1-1a
_ _ 4~methoxy _ _
V1116 phenyl Me Ph Bn V1119

Scheme V-7. Synthesis of chiral 4-hydroxymethyl imidazolines from racemic
imidazoline-4-carboxylic acid (III-la)

252

N N
Ph .leh Ph Ph
\« \6
N "'I\ N 5
OH = OH
R, R S, S

 

 

Figure V- 14. X-ray structures of chiral 4-i1yd1ux ,ruetl. y" '4 " (R, R) -V-
1-17 and (S, S) -V-1-17 as ViewerLite 3D models.
iv. Use of chiral amides (of R-l-phenylethylamine), as Seto and co-workers
have successfully employed, did not work in this case.
v. 4-isopropyl ligands could not be obtained as the corresponding esters

could not be separated by column chromatography.

253

 

 

R9 éph R9 Ph
R149: R‘AN ' ““
HO/

 

 

 

EtZZn
0 10 mol °/o ligand 0“ H0
PNJLH M thod AIB Ph
e RR-aloohol SS-aloohol
Ligands

R, a" common Method at yield at on, configuration

Ph Bn SS A 60 29%. S

Ph Bn SS 8 80 0%

Ph Bn RR A 89 27%. R

Ph Bn RR B 95 35%. S“

4-methoxy

Ph phenyl SS A 50 30%. R
4-methoxy
phenyl Bn RR A 65 32%, R

Method A: toluene. it

Method B: Ti(0‘Pr)4 (1.2 eq.), toluene. rt
* only 15 mol °/oTi(O‘Pr)4

Table V- 2. Chiral 4-hydroxymethy imidazoline ligands in diethyl zinc additions.

Although, additional experiments are needed to establish synthetic utility of the

quarternary 4-methyl-4-hydroxymethylimidazolines as ligands for diethyl zinc

additions their performance is not all too disappointing in comparison to the 2-

hydroxymethylimidazolines.84

254

D. Experimental Section

General: Melting points were determined by open capillary method using
Electrothermal Melting Point apparatus (Melt-Temp) and are uncorrected. IR
spectra were recorded on a Nicolet spectrophotometer. lH NMR spectra were
recorded with Varian [(Inova or Gemini) 300MHz] spectrometers. Chemical shifi
values are expressed as 6-(ppm ) and J values are in Hz. Splitting patterns are
indicated as s: singlet, d: doublet, t: triplet, m: multiplet, q: quartet and br: broad
peak. l3C-NMR spectra were also recorded on Varian [(Inova or Gemini)
75.MHz] spectrometers. The symbols q, t, d and s indicate the number of protons
(3, 2, 1, and 0 respectively) attached to each carbon atom, as determined by the
'3C DEPT technique. Mass spectra were recorded on Perkin Elmer mass
spectrometer. Column chromatography was performed on a silica gel (60-120)
mesh. THF/diethyl ether were dried over sodiumbenzopenone ketyl and distilled
under nitrogen. Dichloro methane dried over calcium hydride distilled under
nitrogen. All manipulations were conducted over a nitrogen atmosphere by use of

standard Schlenck techniques.

Synthesis of l-Benzyl-4-methyl-2,5-diphenyl-4,5-dihydro-lH-imidazole-4-
carboxylic acid (l-phenyl-ethylyamide V-amide-l from l-Benzyl-4-methyl-
2,5-diphenyl-4,5-dihydro-lH-imidazoIe-4-carboxylic acid: To a well-stirred
suspension of 1-Benzyl-4-methyl-2,5—diphenyl-4,S-dihydro—1H-imidazole-4-

carboxylic acid III-l-1( 1.0g, 0.27 mmol) in dry methylene chloride (25mL), (R)-

255

(+)-l-Phenyl-ethylamine (0.36g, 29mmol) was added EDCI.HC1 (0.57g,
29mmol), after five minutes added a solution of DMAP (.35 gm, 29 mmol) in
methylene chloride (lOmL) and stirred for 5-6 hrs. The reaction mixtures was
washed with water (2x 10 mL), saturated sodium bicarbonate (20 mL), water
(20mL), 2N HCl (20 mL) and then with water (30mL). The organic layer dried
over sodium sulfate and evaporated under reduced pressure. The crude product
was purified by column silica-gel chromatography using ethyl acetate hexane
mixture (1:1).

(+)-V-amide-l: Yield: (0.26 g, 66.6%). {[a]D= +41 .5°)} 1H NMR (300MHz) : 5
1.02 (d, J = 6.9, 3H), 1.56 (s, 3H), 3.85 (d, J = 15.6, 1H), 4.40 (s, 1H), 4.66(d, J
=15.6 ,lH), 4.72 (t, J= 6.9, 1H), 7.07-7.09 (m, 2 H), 7.17-7.55(m, 16 H), 7.69-
7.73 (m, 2H).; l3C NMR (75MHz): 21.39, 27.56, 48.09, 48.73, 72.66, 126.52,
127.24, 127.71, 127.99, 128.42, 128.57, 128.67, 128.95, 129.01, 129.14, 130.75,
130.82, 137.38, 137.60, 143.29, 165.44, 171.61. IR (neat) 1874 cm-, 1498 cm-;
m/z= 473(M+)

(-)-V-amide-l: (0.24 g, 61%). {[ot]D= -37.7°)} 'H NMR (300 MHz) : 8 1.40 (d, J
= 7.2, 3H), 1.61 (s, 3H), 3.77 (d, J = 15.6, 1H), 4.37 (s, 1H), 4.60(d, J =15.6 ,1H),
4.75 (t, J= 7.5, 1H), 6.922-7.090 (m, 2H), 7.11-7.22 (m, 13H), 7.507-7.529
(m,3H), 7.651-7.682(m, 211).: '3c NMR (75MHz): 21.58, 28.08, 47.97, 48.59,
72.62, 126.66, 126.99, 127.200, 127.69, 127.96, 128.21, 128.51, 128.58, 128.64,
129.13, 129.122, 130.70, 130.83, 137.184, 137.22, 143.28, 165.35, 171.62. IR

(neat) 1872 cm-, 1498 cm-; m/z=325 (M+-CO-NH-CH(CH3)Ph).

256

. Synthesis of l-BenzyI-4-methyl-2,5-diphenyl-4,5~dihydro-lH-imidazole-4-
carboxylic acid benzylamide from 1-Benzyl-4-methyI-2,5—diphenyI-4,5-
dihydro-lH-imidazole-4-carboxylic acid V-12:To a well-stirred suspension of
1 -Benzyl-4-methyl-2,5-diphenyl-4,5-dihydro-1H-imidazole—4-carboxylic acid
(0.5g, 1.34 mmol) in dry methylene chloride (150 mL) at 0 °C, was added
EDCI.HCL (0.31 g, 1.608 mmol), after five minutes DMAP (0.2 g, 1.6 mmol) and
stirred for 20 minutes. Benzyl amine (0.17 g, 1.6 mmol) was then added dropwise.
The reaction mixture was then stirred overnight at room temperature. The reaction
mixtures was washed 2N HCl (2 x 50 mL), saturated sodium bicarbonate (2 x so
mL),, and then with brine (50mL). The organic layer was dried over anhydrous
sodium sulfate and evaporated under reduced pressure to afford the amide product
as a white foam V-12: Yield: (0.5 g, 66.6%). 1H NMR (300MHz), CDC13 : 8
1.53 (s, 3H), 3.77 (d, J= 15.3, 1H), 3.91 (dd, J, = 14.7 Hz, J2= 5.1 Hz, 1H ), 4.21
(dd, J, = 15 Hz, J2= 6 Hz, 1H ), 4.35 (s, 1H), 4.60 (d, J=15.6 ,1H), 6.81-7.64 (m,
20 H).; l3C NMR (75MHz), CDC13: 27.71, 42.77, 48.45, 72.18, 76.57, 126.89,
127.46, 127.7, 127.79, 128.03, 128.26, 128.37, 128.4, 128.73, 128.81, 130.39,
130.46, 136.99, 137.17, 138.28, 165.02, 172.26.; EIMS: m/z= 459.3 (M+)
Synthesis of Z-Benzylidene-6-oxo-5-phenyl-3—(toluene-4-sulfonyl)-
[l,3]oxazinane-4-carboxylic acid ethyl ester from (Toluene-4-sulfonylimino)-
acetic acid ethyl ester v.6 using l-Benzyl-4-methyl-2,5—dipheny1-4,5-dihydro-

1H-imidazole-4-carboxylic acid (l-phenyl—ethylyamide (+)-V-amide-l

257

To a well stirred solution of phenylacetyl chloride (0.3 mL, 2.1 mmol) in dry
DCM (4 mL) at -78°C added proton sponge (0.4 gm, 2.1 mmol), after 5-10 min
the color of the solution changes to pale yellow then added a solution of 1-benzyl-
4-methyl-2,5-dipheny1-4,5-dihydro-1 H-imidazole-4-carboxylic acid (1 -phenyl-
ethyl)-amide (V-amide—l) (0.28 gm, 0.6 mmol or 0.14 gm, 0.3 mmol) in 3 mL of
dry DCM. The reaction allowed to stir for 5—10 min on which the color changes to
deep yellow then added a solution of (toluene-4-sulfonylimino)-acetic acid ethyl
ester (0.25 gm, 1 mmol) in dry DCM (3mL). The reaction mixture allowed to stir
for 36 hrs at —78 °C and then allowed to come to room temperature. The reaction
mixture was passed through a silica gel bed to remove unreacted acid chloride and
washed with ethyl acetate (20 mL). The solution was subjected to column
chrotography using 1:3 ethyl acetate: hexanes to isolate pure 2-benzylidene-6-
oxo-5-phenyl-3-(toluene-4-sulfonyl)-[1, 3]oxazinane-4-carboxylic acid ethyl ester
(V -6a). Further elution with same solvent give mixture of 2-benzylidene-6-oxo-5-
phenyl-3-(toluene-4-sulfonyl)-[1,3]oxazinane-4-carboxylic acid ethyl ester and B-

lactam V-6b, and further pure B-lactam.

o

0 Ph
”KAN
1

002 E1
3:0
11
0 O V-6a

2-Benzylidene-6-oxo-5-phenyl-3-(toluene-4-sulfonyI)—[1,3]oxazinane-4-

carboxylic acid ethyl ester V-6a: Yield: 0.17 gm, 34.7%, 'H NMR (300MHz) :

6 0.98 (t, J = 7.5, 3H), 2.49 (s, 3H), 4.05 (q, J=7.5, 2H), 4.16 (d, J= 11.4, 1H),

258

4.50 (d, J =11.4, 2H), 6.45 (s, 1H), 7.10-7.13 (m, 2H), 7.27-7.41 (m, 8H), 7.60
(dd, # 0.6 and 7.5, 2H), 7.80 (d, J= 8.4, 2H); ”‘0 NMR (300MHz) : 8 13.65,
21.74, 49.72, 61.05, 62.15, 115.34, 127.77, 128.56, 128.67, 128.92, 129.02,
129.34, 129.55, 129.86, 130.27, 130.53, 131.75, 134.12, 136.71, 145.50, 165.66,

168.26. IR (neat); 1794, 1749, 1670, 1369; MS; m/z=491.2

o
OJIPh
”RAB: Ph
:0
|\
0° V-10a

Synthesis of 2-Benzylidene—4, S-diphenyl—3(toluene-4-sulfonyl)-l1,
3]oxazinan-6-one V-10a.

To a well stirred solution of phenylacetyl chloride (0.3 mL, 2.1 mmol) in dry
DCM (4 mL) at -78°C added proton sponge (0.4 gm, 2.1 mmol), after 5-10 min
the color of the solution changes to pale yellow then added a solution of 1-
Benzyl-4-methyl-2,5-diphenyl-4,5-dihydro-1 H-imidazole-4-carboxylic acid (1 -
phenyl-ethyl)-amide (0.28 gm, 0.6 mmol or 0.14 gm, 0.3 mmol) in 3 mL of dry
DCM. The reaction allowed to stir for 5-10 min on which the color changes to
deep yellow then added a solution of N-benzylidine-(Toluene-4-sulfonyl)amine
(0.25 gm, 1 mmol) in dry DCM (3mL). The reaction mixture allowed to stir for 36
hrs at -78 0C and then allowed to come to room temperature. The reaction mixture
was passed through a silica gel bed to remove unreacted acid chloride and washed
with ethyl acetate (20 mL). The solution was subjected to column

chromatography using 1:4 ether : hexanes to isolate pure 2-Benzylidene-4,5-

259

diphenyl-3(toluene-4-su1fonyl)-[1,3]oxazinan-6-one V-10a. Further elution with
same solvent give mixture of 2-Benzylidene-4, 5-diphenyl-3(toluene-4-sulfonyl)-
[1, 3]oxazinan-6-one and B-lactam dl-(3R, 4R)-Diphenyl-1-(toluene-4-sulfonyl)-

azetidin-2-one V-10b, and further pure B-lactam.

(3R, 4S)-2-Benzylidene-4,5-diphenyl-3(toluene-4-sulfony1)—[1,3]oxazinan-6-
one V-10a. Yield: 0.17 g, 35 %, 'H NMR (500MHz) : 6 2.47 (s, 3H), 3.95 (d, J=
11.5 Hz, 1H), 5.05 (d, J =11.0 Hz, 1H), 6.41 (s, 1H), 6.8496867 (dd, 2H, J 1 = 7
Hz, J2 = 1.5 Hz, 2H), 7.09-7.1 (dd, 2H, J 1 = 7 Hz, J2 = 1.5 Hz, 2H), 7.2-7.24 (m,
6H), 7.32-7.36 (t, 2H, J = 8.5 Hz, 3H), 7.38-7.41 (t, 2H, J = 8.5 Hz, 3H), 7.63-
7.64 (d, J =7.5 Hz, 2H), 7.72-7.73 (d, J =7.5 Hz, 2H); I3C NMR (125 MHz) : 6
21.84, 29.86, 55.3, 64.36, 115.46, 126.77, 128.07, 128.53, 128.71, 128.71, 128.82,
128.9, 129.72, 129.79, 130.23, 132.26, 132.29, 135.46, 138.53, 139. 63, 145.19,

166.06; IR (neat); 1738, 1666; EIMS; m/z=495.2.

dI-(3R, 4R)-Diphenyl-l-(toluene-4-sulfonyI)-azetidin-2-one [V-10b]. Yield
0.037 g, 10 %, 'H NMR (300MHz) : 6 2.43 (s, 3H), 4.25 (t, J= 3.3 Hz, 1H), 4.96
(d, J =3 Hz, 1H), 7.03-7.3 (m, 15H), 7.67-7.7 (m, 2H); 13C NMR (75 MHz) : 8
21.68, 64.17, 65.78, 126.5, 127.23, 127.55, 128.07, 128.36, 128.6, 128.93, 129.05,
129.13, 129.66, 129.83, 132.79, 135.66, 135.93, 145.29, 165.37; IR (neat); 1795,;

MS; m/z=377.9.

260

General procedure for diethyl zinc reduction:

0 OH

/lL———>

Ph H ph
Method A: According to general procedure by Soai et a1, a 1M solution of
diethyl zinc in hexane (2.2 mL, 2.2 mmol) was added to a solution of the chiral
ligand (10 mol %) and the aldehydes (1mmol) in toluene (5 mL). The mixture was
stirred under alkylation was complete under argon atmosphere in a sealed flask.
The reaction mixture was diluted with dichloromethane (50 mL) and extracted
with 10 % HCl solution. The organic layer was dried over anhydrous sodium
sulfate, filtered and evaporated to obtain an oily residue. The residue was
chromatographed with 20 % ether/hexanes to obtain the l-phenyl propanol. The
enantiomeric excess was found by comparison to literature values. [Soai, K.;
Yokayama, S.; Hayasaka, T. J. Org. Chem. 1991, 56, 4264.]

Method B: Stoichiometn'c or catalytic Ti-isopropoxide was added was added to a
solution of the chiral ligand (10 mol %) in toluene (5 mL) and stirred for 20
minutes followed by 1M solution of diethyl zinc in hexane (2.2 mL, 2.2 mmol)
the aldehydes (1mmol). The mixture was stirred under alkylation was complete
under argon atmosphere in a sealed flask. The reaction mixture was diluted with
dichloromethane (50 mL) and extracted with 10 % HCl solution. The organic
layer was dried over anhydrous sodium sulfate, filtered and evaporated to obtain
an oily residue. The residue was chromatographed with 20 % ether/hexanes to

obtain the 1-pheny1 propanol. The enantiomeric excess was found by comparison

to literature values [Walsh P. J. Acc. Chem. Res., 36 (10), 739 -749, 2003.]

261

OH
Ph

lH NMR (300MHz), CDCl3: 8 0.89 (t, J = 7.5 Hz, 3H), 1.67-1.85 (m, 2H), 1.97
(broad singlet, 1H), 4.561 (t, J = 6.6 Hz, 1H), 7.23-7.35 (m, 5H); l3C NMR
(75MHz), CDC13: 5 10.07, 31.75, 75.87, 125.92, 127.35, 128. 28, 144.53.
Synthesis of l-Benzyl-4-methyl-2, 5-diphenyl-4, 5-dihydro-lH—imidazole-4-
carboxylic acid l-phenyl-ethyl ester V-l-l4.

A well—stirred suspension of 1-Benzyl-4-methy1-2,5-diphenyl-4,5-dihydro-1H-
imidazole-4-carboxylic acid III-l-l ( 1.0g, 2.7 mmol) in dry methylene chloride
(150mL) was made in a flame dried flask under nitrogen atmosphere and cooled
in an ice-bath to 0 °C. To this mixture was added EDCI.HCl (0.57g, 2.9mmol),
followed by DMAP (0.35 gm, 2.9 mmol) and stirred for 20 minutes. (R)-(+)-l-
phenyl-ethanol (0.72g, 2 eq., 5.8 mmol) was added and mixture stirred overnight
at room temperature. The reaction mixtures was washed, 2N HCl (2 x 50 mL),
saturated sodium bicarbonate (2 x 50 mL), and then with brine (50mL). The
organic layer dried over sodium sulfate and evaporated under reduced pressure.
The crude product was purified by column silica-gel chromatography using 70 %

ether / hexane mixture.

262

(211’. N <82...©

NWO
a?

O .\“
(rm
(-)v-1-14 P"

(-)- (RRR) v-1-14. Yield: (0.34 g, 61 %). [(Ih): -118 °, c = 1.2, CHC13; 'H NMR
(500MHz), coca: 8 1.236 (d, J = 6.5, 3H), 1.62 (s, 3H), 3.83 (d, J = 15.5 Hz,
1H), 4.34 (s, 1H), 4.70 (d, J=15.5 Hz,1H), 5.32 (q, J: 6.5, 1H), 6.94-7.16 (m, 4
H), 7.17-7.28 (m, 11H), 7.48-7.49 (t, J = 3.5 Hz, 3H) 7.76-7.77 (m, 2H).; I3C
NMR (125MHz) coch: 21.8, 26.95, 48.88, 72.81, 73.27, 77.61, 125.87, 127.2,
127.57, 127.7, 127.77, 128.0, 128.27, 128.55, 128.64, 130.08, 131.12, 136.59,

136.64, 141.35, 165.92, 171.08.; EIMS: m/z = 474.1(M+).

3H7,“
(2) | S
N S O

O a“
- - - 08'
(+) V1 14 Ph

(+)- (SSR) v-1-14: (0.38 g, 66 %). [61],,= +113 °, c = 1.2, CHC13; 'H NMR
(500MHz), CDC13: 8: 0.957 (d, J = 6.5, 3H), 1.61 (s, 3H), 3.77 (d, J = 15.5 Hz,
1H), 4.33 (s, 1H), 4.66 (d, J =15.5 Hz,1H), 5.33 (q, J= 6.5, 1H), 6.92-6.94 (m, 2
H), 7.17-7.29 (m, 13H), 7.47-7.48 (m, 3H) 7.73-7.75 (m, 2H).; l3c NMR

(125MHz), CDC13: 21.44, 26.85, 48.83, 72.74, 73.57, 77.58, 126.0, 127.45,

263

127.54, 127.79, 127.9, 128.0, 128.14, 128.37, 128.51, 128.53, 128.62, 130.03,

131.15, 136.52, 137.06, 141.77, 165.94, 170.86.; EIMS: m/z = 474.2 (M+).

Ph\'

Ph\«‘Z.\OH lPh

(-)V-1-17
[(-)(4R, 5R)-1-benzyl-4,5-dihydro-4-methyI-2,5-diphenyl-lH-imidazol-4-yl]
methanol V-l-17: Lithium aluminum hydride (0.04 g, 1.06 mmol) was
suspended in 100 mL of dry THF in a flame dried flask under nitrogen
atmosphere. The suspension was cooled to 0 °C in an ice-bath. (-)«RRR ester V-l-
14 (0.34 g, 0.71 mmol) solution in dry THF (10 mL) was added dropwise. The
mixture was stirred at 0 °C and monitored with TLC for complete disappearance
of the ester spot. The reaction mixture was diluted with ethyl acetate (100 mL)
and organic layer was extracted with 5 % HCl solution (3x, 100 mL). The organic
layer was dried over anhydrous sodium sulfate, filtered and evaporated under
vacuum. The residue was chromatographed with 20 % MeOH/ethyl acetate to
afford the (-) RR alcohol V-l-l7 as colourless oil (0.2 g, 79 % yield) which
solidified on standing under high vacuum.
[a]D= -98 °, c = 1.0, CHC13; lH NMR (500MHz), CDC13: 5: 1.35 (s, 3H), 3.08 (d,
J = 11.7 Hz, 1H), 3.17(d, J = 11.7 Hz, 1H), 3.58 (broad singlet, 1H), 3.89 (d, J =
15.6 Hz, 1H), 4.28 (s, 1H), 4.72 (d, J = 15.3 Hz, 1H), 6.83-6.86 (m, 2H), 7.2-7.72

(m, 13H).

264

MeO

N 0/'\
Ph

V-1 -1 5

4, S-dihydro-l-(4-methoxyphenyl)-4-methyl-2, S-diphenyl-lfl imidazole-4-
carboxylic acid l-phenylethyl ester V-l-15. A well-stirred suspension of 1-
Benzyl-4-methy1-2,5~diphenyl-4,5-dihydro-1H-imidazole-4-carboxylic acid III-l-
1 (1 .0g, 2.6 mmol) in dry methylene chloride (150mL) was made in a flame dried
flask under nitrogen atmosphere and cooled in an ice-bath to 0 °C. To this mixture
was added EDCI.HCl (0.6g, 3.1 mmol), followed by DMAP (0.38 gm, 3.1 mmol)
and stirred for 20 minutes. (R)- (+)-1-phenyl-ethanol (0.72g, 2 eq., 58 mmol) was
added and mixture stirred overnight at room temperature. The reaction mixtures
was washed, 2N HCl (2 x 50 mL), saturated sodium bicarbonate (2 x 50 mL), and
then with brine (50mL). The organic layer dried over sodium sulfate and
evaporated under reduced pressure. The crude product was purified by column

silica-gel chromatography using 50 % ether / hexane mixture.

266

 

OMe

(+)-v-1-15 (8811): (0.2 g, 31%). [a]D= +138 °, c = 1.0, CHC13; 'H NMR
(300MHz), (213013: 8: 1.2 (d, J = 6.3, 3H), 1.785 (s, 3H), 3.63 (s, 3H), 4.73 (s,
1H), 5.28 (q, J= 6.3, 1H), 6.56 (d, J = 9H2, 1H), 6.58-6.66 (m, 2H), 7.02-7.35 (m,
13H), 7.62 (d, J = 9 Hz, 2H); l3C NMR (125MHz), CDC13: 21.44, 26.85, 56.83,
72.74, 73.57, 77.58, 126.0,127.45, 127.54,127.79, 127.9, 128.0, 128.14, 128.37,

128.51, 128.53, 128.62, 130.03, 131.15, 136.52, 137.06, 141.77, 165.94, 170.86.

OMe

N
Ph 8 Ph
(36 t
N (Ska OH

[(+)(4S,SS)-4,5-dihydro-l-(4-methoxyphenylH-methyl-Zfi-diphenyl-1H-
imidazol-4-yl]methanol V-l-18: Lithium aluminum hydride (0.03 g, 0.78 mmol)
was suspended in 100 mL of dry THF in a flame dried flask under nitrogen
atmosphere. The suspension was cooled to 0 °C in an ice-bath. (+)-SSR ester V-l-
15 (0.2 g, 0.52 mmol) solution in dry THF (10 mL) was added dropwise. The

mixture was stirred at 0 °C and monitored with TLC for complete disappearance

267

of the ester spot. The reaction mixture was diluted with ethyl acetate (100 mL)
and organic layer was extracted with 5 % HCl solution (3x, 100 mL). The organic
layer was dried over anhydrous sodium sulfate, filtered and evaporated under
vacuum. The residue was chromatographed with 10 % MeOH/ethyl acetate to
afford the (+)-SS alcohol V-1-18 as colourless oil (0.130 g, 65 % yield).

(+)-SS alcohol V-l-l8 [ct]D= +100 °, c = 1.0, CHC13; 1H NMR (500MHz),
CDC13: 6: 1.51 (s, 3H), 2.97 (broad s, 1H), 3.14 (d, J = 11.1 Hz, 1H), 3.21(d, J =
11.7 Hz, 1H), 3.63 (s, 3H), 4.61 (s, 1H), 6.554 (d, J = 9 Hz, 1H), 6.65 (d, J = 9 Hz,
1H), 7.21-7.35 (m, 8H), 7.52 (d, J = 7.2 Hz, 2H), 13C NMR (125MHz), CDC13: 6
21.7, 64.19, 65.74, 65.8, 77.42, 126.52, 127.08, 127.25, 127.57, 127.75, 128.09,
128.21, 128.37, 128.62, 128.87, 128.95, 129.07, 129.15, 129.53, 129.68, 129.85,

130.48, 130.67, 132.81, 135.67, 135.95, 145.31, 165.39.

Ph
l-benzyl-4,5-dihydro-4-methyl-2, S-diphenyl-lH-imidazole-4-carboxylic acid
l-phenylethyl ester V-11-16: A well-stirred suspension of 1-Benzyl-4-methyl-
2,5-diphenyl-4,5-dihydro-1H-imidazole-4-carboxylic acid (1.0g, 2.5 mmol) in dry
methylene chloride (150mL) was made in a flame dried flask under nitrogen
atmosphere and cooled in an ice-bath to 0 °C. To this mixture was added

EDCI.HCL (0.57g, 3.0 mmol), followed by DMAP (0.36 gm, 3.0 mmol) and

268

 

stirred for 20 minutes. (R)-(+)-1-Phenyl-ethanol (0.67g, 2 eq., 5.5 mmol) was
added and mixture stirred overnight at room temperature. The reaction mixtures
was washed, 2N HCl (2 x 50 mL), saturated sodium bicarbonate (2 x 50 mL), and
then with brine (50mL). The organic layer dried over sodium sulfate and
evaporated under reduced pressure. The crude product was purified by column
silica-gel chromatography using 60 % ether / hexane mixture.

(+)- (SSR) V-11-16: (0.27 g, 46%). [a]D= +130°, c = 1.2, CHC13; lH NMR
(300MHz), CDC13: 6: 1.17 (d, J = 6.3, 3H), 1.55 (s, 3H), 3.78 (d, J = 15.9 Hz,
1H), 3.83 (s, 3H), 4.27 (s, 1H), 4.72 (d, J =15.3 Hz, 1H), 5.23 (q, J= 6.6, 1H),
6.91-6.98 (m, 6H), 7.15-7.28 (m, 12H), 7.68 (d, J = 8.6 Hz, 2H); I3C NMR
(125MHz), CDC13 21.6, 26.74, 48.63, 55.02, 72.46, 72.74, 77.27, 113.66, 122.9,
125.59, 126.94, 127.3, 127.37, 127. 46, 127.7, 127.74, 128.0, 128.3, 129.88,

136.41, 136.46, 141.0, 160.74, 165.46, 171.0.

Ph

Meo W
(2) \
N

N BllPh
.nI|\
1; OH

[(-)(4R,5R)-1-benzyl-4,5-dihydro-2-(4-methoxyphenylH-methyl-S-phenyl-
1H-imidazol-4-yl]methanol V-ll-19: Lithium aluminum hydride (0.03 g, 0.78

mmol) was suspended in 100 mL of dry THF in a flame dried flask under nitrogen

269

atmosphere. The suspension was cooled to O 0C in an ice-bath. (-)-RRRester V-
11-16 (0.28 g, 0.55 mmol) solution in dry THF (10 mL) was added dropwise. The
mixture was stirred at 0 °C and monitored with TLC for complete disappearance
of the ester spot. The reaction mixture was diluted with ethyl acetate (100 mL)
and organic layer was extracted with 5 % HCl solution (3x, 100 mL). The organic
layer was dried over anhydrous sodium sulfate, filtered and evaporated under
vacuum. The residue was chromatographed with 10 % MeOH/ethyl acetate to
afford the (-)-RR alcohol V-ll-19 as colourless oil (0.144 g, 68% yield).

[a]D= +114 0, c = 1.0, CHC13; lH NMR (300MHz), CDC13: 6: 1.317 (s, 3H), 2.98
(d, J = 12.0 Hz, 1H), 3.11(d, J = 11.4 Hz, 1H), 3.42 (broad s, 1H), 3.86 (s, 3H),
3.87 (d, J = 15.0 Hz, 1H), 4.23 (s, 1H), 4.78 (d, J = 15.6 Hz, 1H), 6.83-6.86 (m,
2H), 7.0 (d, J = 8.7 Hz, 2H), 7.22-7.65 (m, 9H), 7.67 (d, J = 9 Hz); l3C NMR
(75MHz), CDC13: 6 25.7, 48.57, 66.84, 71.4, 71.7, 127.37, 127.58, 127.65,

128.18, 128.26, 128.4, 128.46, 130.08, 135.65, 135.76, 164.79.

270

 

E.

10.

11.

12.

13.

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280

APPENDIX

281

EFFICIENT TWO-STEP SYNTHESIS OF METHYLPHYTYLBENZO
QUINONES: PRECURSOR INTERMEDIATES IN THE BIOSYNTI-IESIS
OF VITAMIN E

A. Introduction to Tocopherols.

Tocopherols are a class of lipid-soluble antioxidants synthesized in higher plants
and other oxygenic photosynthetic organisms such as cyanobacteria.I The four
tocopherols synthesized (01, B, 6, and y-tocopherol) differ only in the number and
position of methyl groups on the chroman head group. Tocopherols are important
in limiting lipid peroxidation in a variety of organisms and are an essential
component of mammalian diets as vitamin E (2R,4'R,8'R-[01]-tocopherol).2' 3 The
production of tocopherols for animal nutrition and various industrial purposes has
sparked renewed interest in the synthesis and enzymology of the tocopherol
biosynthetic enzymes.2’ 4'6 Although the tocopherol pathway has been studied for
many years,7 it is only recently that pathway enzymes have been cloned,
providing new opportunities to study the substrate specificity and kinetics of the

reactions. 5' 6' 8' 9

B. Biosynthesis of a-tocopherol.

The pathway leading to the synthesis of til-tocopherol is shown in Figure VI-l.
The committed intermediate in the tocopherol biosynthetic pathway is 2-methyl-
6-phytyl-1,4-benzoquinone (MPBQ), the product of the condensation of the
aromatic compound homogentisate (HGA) and the 20-carbon isoprenoid derived

compound, phytyl pyrophosphate.9’ '0 MPBQ methyltransferase catalyzes the first

282

methylation reaction in tocopherol synthesis and the methylation of the same ring

position of a related compound in plastoquinone synthesis. 1" '2

 

11

0” PP. + co;
,t — o o J
1'10sz 0\ ”/0\ 1.;/OW”
OH ' '

Homogentisate Phytyl pyrophosphate

SAM SAM SAH
V > W H v
OH 3

SAH
OH 2-methyl-6-phytyl-1 .4-benzoquinol
\
OH

cyclisatlon HO
H o H
3 3

 

 

17

11

 

2.3-dimethyl-6-phytyl-1 .4-benzoquinol y-tocopherol
SAUH Ho
> O H
3
tat-tocopherol

Figure VI-l. Biosynthesis of a-tocopherol.

Access to a variety of methylated phytylbenzoquinone precursors and
intermediates is required in order to elucidate the regulation and the activity of
MPBQ/MSBQ methyltransferase. 11 Previously reported methods provide some of
these substrates in relatively low yields and as multiple isomers. '2'” The
development of a short and efficient synthetic method applicable to generating a
wide range of these substrates has therefore become an integral part of elucidation

of the biosynthetic pathway. We developed an efficient two-step synthesis of

283

methylphytylbenzoquinones (MPBQs) to study the kinetics and substrate

specificity of MPBQ methyltransferase.

The earlier reported chemical synthesis of MPBQ involved oxidation of
commercially available methylated phenols to methyl quinones followed by
coupling of the phytyl side chain in the presence of BF3'OE12 resulting in a
mixture of positional isomers. '0 Oxidation of the resulting quinols with AgzO
provided a mixture of isomers, which were separated by repeated thin-layer

chromatography.

Unfortunately, due to the limited availability of the required isophytol side chain,
the generation of wide mixtures of isomers, and corresponding low yielding
reactions, this procedure was quite limited as a routine method for the large scale
preparation of the many different tocopherol precursors. We discovered an
alternative preparation of MPBQ derivatives from the naturally available

tocopherols to provide more efficient access to these biosynthetic precursors.

C. Synthesis of required methylphytlbenzoquinone.

Oxidative cleavage of a chroman ring with cerium sulfate affords the quinone
alcohol. '5 The well established FeCl; oxidation as described by Cohen has also
successfully been used in the conversion of (1.-tocopherol to a-
tocopheroquinones.l6 Cerium sulfate oxidation of 6-tocopherol resulted in a
mixture of products from which the quinone alcohol VI-l was readily isolated in

30% yield at multigram scale. The quinone alcohol VI-l was subjected to

284

dehydration with Burgess reagent under argon atmosphere to afford a mixture of

methylphytqulrinones v1-2 — VI-6 (Scheme v1-1)”:"3

   

 

 

 

HO 06(30412
H H2804(eonc)
3 H20. MeOH '
2 hours. r.t.
30 % Vl-1
EtaN
04590 NYC 0
Burgess reagent +
*7 \
THF. Argon. 2h, rt 0
42 % “'2 :
H
/ 3
Vl-3 natural substrate V14
M: ...MH
3
VI-6

Scheme VI-l. Synthesis of methylphytylbenzoquinone.
The positional and geometric isomers VI-2 - VI-6 were isolated by flash
chromatography, followed by two rounds of HPLC purification using a silica
column with hexane/isopropyl ether (992:8) as the eluant. Compounds VI-2 - VI-
6 were isolated in a relative ratio of 1:2:1.1:1.5:2.2. The positional and geometric

isomers from the dehydration will be further used as competitive inhibitors for

285

evaluating the reaction mechanism and kinetic parameters of the MPBQ

methyltransferase.l l

D. Experimental Section

Synthesis of VI-2 — VI-6: The quinone alcohol VI-l (0.4 g, 9 mmol) was
weighed into a flame dried flask under argon atmosphere and dry THF (50 mL)
was added. To this solution was added Burgess reagent (0.4 g, 1.8 mmol) and the
mixture stirred at room temperature ovemight. The reaction mixture was then
diluted with ether (200 mL) and extracted twice with water and the organic layer
was dried over anhydrous sodium sulfate and the ether layer was evaporated to
obtain a yellow residue. The residue was flash chromatographed on silica-gel (5%
etherzhexane) to obtain the methylphytylbenzoquinone as a mixture of five
isomers (0.16 g, 42%) They were further separated by HPLC. Spectral data: V1-
3 1H NMR (500 MHz, acetone-d6) 6 6.6 ( 1H, td, J.=1.5 Hz, J2=4.5 Hz), 6.46 (1H,
td, J1=1.5 Hz, J2=3.5 Hz), 5.22 (1H, qt, Jl=1.5 Hz, J2=7.5 Hz), 3.14 (2H, d, J=7.5
Hz), 2.06—2.07 (3H, m), 2.02 (2H, d, J=1.5 Hz), 1.66 (3H, s), 1.0—1.6 (18H, m),
0.85—0.9 (12H, m); 13C NMR (125 MHz, acetone-d6) 6187.61, 148.56, 146.07,
139.55, 133.01, 132.04, 118.87, 39.85, 39.41, 37.43, 37.42, 37.31, 36.56, 32.81,
32.66, 28.82, 28.67, 27.99, 27.48, 25.25, 24.8, 24.43, 22.29, 22.2, 19.4, 15.36,
15.15; IR (neat): 1670 cm“', 1600 cm". MS (EI): calculated for C27H4402 [M]+
400.64, found [M]+ 400.33.
Purification of MPBQ and its isomers: The mixture of isomeric

methylphytylbenzoquinones was injected on a ReliaSil Silica, 5 pm, 250><4.6 mm

286

normal phase column (Column Engineering, Ontario, CA) with a guard column
containing the same matrix. Chromatography was performed at 30°C with a
mobile phase of 992:8 (v/v) hexanezisopropyl ether flowing at 1.0 mL/min. For
large-scale purification, a preparative column of the same matrix (Column
Engineering, Ontario, CA) was used. The collected fractions were dried down
under vacuum before a small portion was injected onto the analytical column to
monitor the purity. The natural substrate VI-3 was eluted as the second HPLC

peak.

Enzyme Assay: Assays of MPBQ methyltransferase activity were carried out in a
100 11L reaction containing 10 uL of solubilized E. coli extract (50 and 75 pg
protein) expressing the MPBQ methyltransferase of Synechocystis PCC6803, "
50 mM Tris pH 8.0, 10 mM DTT, 80 11M [l4C-methyl]S-adenosy1methionine (56
mCi/mmol), 40 11M unlabeled S-adenosylmethioine, and 100 11M each of the
individual MPBQ compounds VI-2 — VI-6. After incubation at 30°C for 2 h, the
reactions were stopped by the addition of 200 11L of 0.9% (w:v) NaCl, extracted
with 450 uL of 2:1 (vzv) methanol:chlorofonn, centrifuged, and the organic phase
collected and dried under a stream of nitrogen gas. The resulting residue was
resuspended in hexane and fractionated on 250 1160A KF6 silica gel TLC plates
(Whatman, Clifton, NJ, USA) developed with 3:7 (vzv) diethyletherzpetroleum
ether. The TLC plates were exposed on a Molecular Dynamics low energy
phosphor screen (Amersham) to determine the incorporation of MC into the

products.

287

Interestingly, MPBQ methyltransferase was found to utilize all five MPBQ
isomers VI-2 — VI-6 however the activity with the genuine substrate VI-3 (Figure
VI-2) results in specific activity that is 10—100 times higher than those of its
isomers. The method described in this report has also been successfully applied to
large-scale preparation of phytquuinone precursors of (1.-tocopherol (data not
shown) and is applicable to a wide range of methylphytylbenzoquinone

precursors.

288

 

4o _=52>
2:22:
LQLQCL

30 I

20

10
o m--

o
s!’~%\

 

 

 

e-eseu-suumuse-ceueueueweaum

‘

Efficiency relative to genuine substrate
8

 

i
l
l

VI-2 Vl-3 VI-4 VI-5 Vl-6
HPLC Fractions

Figure VI-2. TOP (left): HPLC trace of VI-2 — VI-6.TOP(right): Efficacy of
isomers VI-2 — VI-6 compared to natural MPBQ as substrates monitored on TLC
using l4C-adenosyl methionine. BOTTOM: Efficacy of isomers VI-2 — VI-6
compared to natural MPBQ as substrates for methyltransferase activity. The
percent efficacy of the isomers was based on the average of three independent
assays.

289

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290

 

  

YYYYYYYYYY

1111111211111111111111

L

06

A

1111

l

    

1‘11