Development of novel positively charged single isomer cyclodextrins and applications in enatiomeric separation and chiral synthesis 1

DEVELOPMENT OF NOVEL POSITIVELY CHARGED SINGLE-ISOMER CYCLODEXTRINS AND APPLICATIONS IN ENANTIOMERIC SEPARATION AND CHIRAL SYNTHESIS TANG WEI-HUA M.. The overall objective of this re

DEVELOPMENT OF NOVEL POSITIVELY CHARGED

SINGLE-ISOMER CYCLODEXTRINS AND

APPLICATIONS IN ENANTIOMERIC SEPARATION

AND CHIRAL SYNTHESIS

TANG WEI-HUA

(M Sc.)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE

2005

Acknowledgements

I would like to express my sincere gratitude to my supervisor, Associate Professor Ng, Siu Choon for his constant guidance, encouragements and beneficial discussions throughout this project I am also indebted to Professor Chan Sze On, Hardy for his kind help in following up my graduation affairs

Special mention should be made to the contribution of Dr Ong Teng-Teng and Dr I M Wayan, who offer great effort in our collaboration on this research project presented in this thesis Their encouragements and suggestions help me very much in my research career My acknowledgement also goes to all peers in Functional Polymer Laboratory:

Mr Lai Xiang-hua, Mr Lee Teck-Chia, Ms Poon Yin-Fun and Dr Zhang Wei-Guang for their support and useful discussions; Dr Liu Shou-Ping, Mr Zou Yong, Mr Zhang Sheng, Xia Hai-Bing and Cheng Da-Ming for their advices and friendship

Last but not least, I express my gratitude to National University of Singapore for the award of research scholarship and Institute of Chemical and Engineering Science Ltd for the financial support to carry out the research work reported herein

Finally, I would thank my wife, Ms Tang Jian for her continuous support, encouragement and great efforts throughout my Ph.D study

Table of Contents

Acknowledgements i

Table of contents ii

Summary vi

List of Publications viii

Abbreviations and Symbols ix

List of single-isomer positively charged CDs x

Chapter 1 Introduction ………1

1 1 Chiral separation: need and perspective 1

1 2 Technques for chiral separation… 5

1 3 History and development of capillary electrophoresis… 6

1 3 1 Principle of capillary electrophoresis 8

1 3 2 Modes of electrophoresis 10

1 4 Chiral selectors in capillary electrophoresis… 11

1 5 Cyclodextrins: the chiral selector for CE… 15

1 5 1 Natural ocurring cyclodextrins 15

1 5 2 Neutral cyclodextrins 18

1 5 3 Charged cyclodextrins 19

1 6 Research objectives and scope 22

1 7 References 25

Chapter 2 Synthesis of positively charged single-isomer cyclodextrins 33

2 1 Approached for cyclodextrins 33

2 2 Synthesis of single-isomer positively charged cyclodextrins 37

2 2 1 Synthesis of quaternary alkylimidazolium single-isomer β-cyclodextrins…… 37

2 2 2 Synthesis of momo-alkyammonium single-isomer β-cyclodextrins 38

2 2 2 Synthesis of mono-(6 A -amino-6 A -deoxy)-CD hydrochlorides 39

2 3 Summary 40

2 4 References 41

Chapter 3 Alkylimidazolium single-isomer β-cyclydextrins as chiral selectors in capillary electrophoresis 43

3 1 Introduction 43

3 2 Experimental 45

3 2 1 Chemicals 45

3 2 2 Apparatus and CE Procedures 46

3 2 3 Calulations 46

3 2 4 Determination of binding constants, apparent complex selectivity and theoretical optimum selector concentration 47

3 3 Results and discussions 49

Part I Chiral recognition ability of BIMCDCl as chiral selector 49

3 3 1 Influence of different chiral selectors on enantioseparation 49

3 3 2 Effect of pH on enantioseparation 51

3 3 3 Effect of BIMCDCl concentration on enantioseparation 52

3 3 4 Effect of temperature on enantioseparation 56

3 3 5 Effect of organic modifier enantioseparation 59

Part II Comparison of chiral recognition ability of quaternary alkylimidazolium β-CDs as chiral selectors 62

3 3 6 Influence of side chain length of the alkylimidazolium substituents on the enantioseparation ability of single-isomer imidazolium-β-CDs in CE 62

3 3 7 Determination of binding constants of complex formation……… …64

3 4 Conclusions 68

3 5 References 69

Chapter 4 Alkylammonium single-isomer cyclodextrins as chiral selectors in capillary electrophoresis 72

4 1 Introduction 72

4 2 Experimental 74

4 2 1 Chemicals 74

4 2 2 Instrumentation and operation conditions 76

4 3 Results and discussions 77

Part I Chiral recognition ability of ALAMCDCl towards amino acids 77

4 3 1 Effect of ALAMCDCl on the electroosmotic flow 77

4 3 2 Effect of CD concentration on enantioseparation of Dns-amino acids 80

4 3 3 Effect of pH on enantioseparation of Dns-amino acids 82

4 3 4 Separation with short capillary 85

4 3 5 Chiral separation of mixture of Dns-amino acids with ALAMCDCl 86

Part II Chiral recognition ability of mono-alkylammonium cationic β-CDs 88

4 3 6 Influence of BGE pH on the enantioseparation of anionic analytes with PeAMCDCl 88

4 3 7 Enantioseparation of racemic acids with PrAMCDCl, BuAMCDCl and PeAMCDCl……… ……… 89

4 3 8 Representative separation electropherograms 101

4 4 Conclusions 103

4 5 References 104

Chapter 5 Use of mono(6-amino-6-deoxy)-CD as chiral selectors in capillary electrophoresis 106

5 1 Introduction 106

5 2 Experimental 108

5 2 1 Chemicals 108

5 2 2 Equipment and data processing 108

5 2 3 Determination of binding constants, apparent complexation selectivity and theoretical optimum selector concentration 110

5 3 Results and discussions 110

Part I Chiral recognition ability of β-CDNH2 110

5 3 1 Effect of pH and CD concentration on EOF 111

5 3 2 Effect of pH on migration times 112

5 3 3 Effect of pH on enantioseparation of acid analytes ……….114

5 3 4 Effect of β-CDNH2 concentration on enantioseparation 116

5 3 5 Determination of binding constants, apparent complexation selectivity and theoretical optimum selector concentration 119

5 3 6 Representative separation electropherograms 123

Part II Comparison of chiral recognition ability of mono-amino CDs 124

5 3 7 Comparison of the resolution ability between β-CD-NH2 and β-CD-NH3Cl: Effect of BGE pH values 124

5 3 8 Comparison of the resolution ability between β-CD-NH2 and β-CD-NH3Cl: Effect of CD concentration 127

5 3 9 Comparisons of the resolution ability between β-CD-NH3Cl and γ-CD-NH3Cl: Effect of CD type 130

5 4 Conclusions 132

5 5 References 133

Chapter 6 Applicability of positively charged single-isomer cyclodextrin in chiral synthesis 136

6 1 Introduction 136

6 2 Experimental 138

6 2 1 Chemicals and instruments 138

6 2 2 General procedure for the NaBH4 reduction of ketones in the presence of MIMCDOTs 139

6 3 Results and discussions 140

6 4 Conclusions 146

6 5 References 147

Chapter 7 Experimental 150

7 1 Materials 150

7 1 1 Solvents 150

7 1 2 Chemicals 150

7 2 Instrumentation 151

7 3 Synthesis 152

7 3 1 Mono-(6A-(p-toluenesulfonyl)-6A-deoxy)-CDs 152

7 3 2 Mono-(6 A -azide-6 A -deoxy)-CDs 154

7 3 3 Mono-6A-(1-alkyl-3-imidazolium)-6A-deoxy-β-cyclodextrins 156

7 3 4 Mono-6A-N-alkylammonium-6A-deoxy-β-cyclodextrins 169

7 3 5 Mono-6 A -N-amino-6 A -deoxy-cyclodextrins 174

7 3 6 Mono-6A-N-ammonium-6A-deoxy-cyclodextrin chlorides 175

7 4 Chromatographic Environment 176

7 4 1 Chiral separation by use of Capillary Electrophoresis 176

7 4 2 Chiral separation by use of High Performance Liquid Chromatography 180

7 5 References 181

Chapter 8 Conclusions and suggestions for future work 182

8 1 Summary of conclusions 182

8 2 Suggestions for future work 184

Summary

Chirality is a major concern in modern pharmaceutical industry The increasing demand for enantiopure compounds, especially those of pharmaceutical importance, has led to the rapid development of a variety of stereoselective separation technologies Capillary electrophoresis (CE) has become one of the most important techniques for enantiomeric separation and analysis

The overall objective of this research was to develop a family of mono-substituted, single-isomer positively charged CDs as chiral selectors for enantiomeric separation by

CE and as chiral template for chiral synthesis In achieving the overall objectives, novel methodologies were developed for the preparation of single-isomer, positively charged CDs, including mono-alkylimidazolium β-CDs, mono-alkylammonium β-CDs and mono-amino CDs Systematic studies were conducted to investigate the chiral recognition abilities of these CDs towards anionic and ampholytic analytes Additionally, the applicability of these CDs as chiral templates to mediate chiral synthesis was established using a quaternary alkylimidazolium β-CD for the asymmetric reduction of proachiral ketones

Alkylimidazolium single-isomer cationic CDs provided good resolution to dansyl amino acids The length of the alkyl chain of these CDs appeared to play an important role in their enantioseparation abilities A suitable alkyl chain length should be shorter than 4-C

as observed in this study The recognition abilities of quaternary alkylimidazolium

single-isomer CDs were further investigated by theoretical determination of their complex stability constants towards dansyl amino acids Results showed that higher binding constants were obtained by CDs with an alkyl chain length lower than 4-C, which was consistent with the experimental observations

Alkylammolium single-isomer cationic β-CDs demonstrated excellent chiral recognition abilities to hydroxy, carboxylic acids and modest/limited resolution to dansyl amino acids The great enhancement in resolution is mainly attributed to the introduction of attractive electrostatic interaction between selector and analyte

Similarly, β-CD-NH2/β-CD-NH3Cl provided excellent chiral recognition abilities to carboxylic acids and dansyl amino acids since only the least bulky amino group is present on the narrow rim of these CDs Generally, the chiral recognition ability was dependent on the structure of analytes γ-CD-NH3Cl showed better resolution to dansyl amino acids than β-CD-NH3Cl, which confirmed the “tight-fit” inclusion model

Results from chiral synthesis showed that mono-6A-(1-methyl-3-imidazolium) -6A-deoxy-β-cyclodextrin tosylate (MIMCDOTs) presented enhanced enantioselectivity

for optical alcohols, as indicated by higher enantiomeric excess (ee%) values of most product alcohols

In conclusion, the newly developed single-isomer positively charged CDs are effective in enantiomeric separation of anionic analytes and in mediating the asymmetrical reduction

of prochiral ketones

List of Publications

1 Wei-hua Tang, Teng-Teng Ong, Siu-Choon Ng, A family of single-isomer positively

charged cyclodextrin as chiral selector for capillary electrophoresis: mono-6 A -butylammonium-6 A -deoxy-β-cyclodextrin tosylate, Electrophoresis, 2005, 26,

3125-3133

2 Wei-hua Tang, I M Wayan, Teng-Teng Ong, David J Young, Siu-Choon Ng,

Enantioseparation of dansyl amino acids by a novel permanently positively charged single-isomer cyclodextrin: mono-6-N-allylammonium-6-deoxy-β-cyclodextrin

chloride by capillary electrophoresis, Anal Chim Acta, 2005, 546, 119-125

3 Wei-hua Tang, Teng-Teng Ong, Siu-Choon Ng, Synthesis and application of

single-isomer 6-mono(alkylimidazolium)-β-cyclodextrins as chiral selectors in chiral

capillary electrophoresis, Electrophoresis, 2005, 26, 3839-3848

4 Wei-hua Tang, Siu-Choon Ng, Enantioseparation of acid enantiomers in capillary

electrophoresis using a novel single-isomer positively charged β-cyclodextrin: mono-6 A -N-pentylammonium-6 A -deoxy-β-cyclodextrin chloride, J Chromatogr A,

2005, 1091, 152-157

5 Wei-hua Tang, Siu-Choon Ng, Synthesis and application of

mono-6-ammonium-6-deoxy-β-cyclodextrin chloride as chiral selector for capillary

electrophoresis, J Chromatogr A, 2005, 1094, 187-191

6 Wei-hua Tang, Siu-Choon Ng, Enantiomeric separation of 8 hydroxy, 10 carboxylic

and 6 dansyl amino acids by mono(6-amino-6-deoxy)-β-cyclodextrin in capillary

electrophoresis, Anal Chim Acta, 2005, 554, 156-162

7 Wei-hua Tang, Siu Choon Ng, Enantioselective separation in capillary electrophoresis

using a novel mono-6 A -propylammonium salt of β-cyclodextrin as chiral selector, Anal

Chim Acta, 2006, 555, 63-67

8 Wei-hua Tang, Siu-Choon Ng, Asymmetric reduction of acetophenones with NaBH 4 in the presence of mono-6 A -(1-methyl-3-imidazolium)-6 A -deoxy-β-cyclodextrin tosylate, J

Inclus Phenom Macrocycl Chem., 2006, in press

9 I M Wayan,Teng-Teng Ong, Wei-hua Tang, David J Young, Siu-Choon Ng, Synthesis

of ammonium substituted β-cyclodextrins for enantioseparation of anionic analytes,

Tetrahedron Lett., 2005, 46, 1747-1749

10 Wei-hua Tang, Siu-Choon Ng, Determination of apparent complex constants of

alkylimidazolium substituted β-cyclodextrins with dansyl amino acids by capillary electrophoresis, in preparation

Abbreviations and Symbols

CD Cyclodextrin

HPLC High performance liquid chromatography

SFC Supercritical fluid chromatography

CSP Chiral stationary phase

NMR Nuclear Magnetic Resonance

FT-IR Fourier Transform Infrared Spectroscopy

THF Tetrahydrofuran

Ts p-toluenesulfonyl

MeOH Methanol

t0 Migration time of neutral marker for EOF

t1 Migration time of first enantiomer

t2 Migration time of less mobile enantiomer

α Separation selectivity

R s Peak resolution

cplx

α Apparent complexation selectivity

β Normalized EOF mobility

μeo Electroosmotic mobility

μeff Effective mobility

μapp Apparent mobility

c opt Theoretical optimum selector concentration

L Total length of capillary

l Effective capillary length from injection site to detection

window

[α]D Optical rotation

List of single-isomer positively charged CDs

(OH)14

(OH)6

CH3, MIMCDOTs

R1=H,

R1

C2H5, EIMCDOTs

R1=H,

C3H7, PIMCDOTs

R1=H,

C4H9, BIMCDOTs

R1=H,

C6H13, HIMCDOTs

R1=H,

C10H21, DIMCDOTs

R1=H,

CH3, DMIMCDOTs

R1=CH3,

OTs

OTs:

O

O

O

-+

R2=

R2=

R2=

R2=

R2=

R2=

R2=

H

(OH)14

(OH)6

CH3, MIMCDCl

R1=H,

R1

C2H5, EIMCDCl

R1=H,

C3H7, PIMCDCl

R1=H,

C4H9, BIMCDCl

R1=H,

C6H13, HIMCDCl

R1=H,

C10H21, DIMCDCl

R1=H,

CH3, DMIMCDCl

R1=CH3,

Cl -+

R2=

R2=

R2=

R2=

R2=

R2=

R2=

Series I

(O H)14

(O H)6 +

R=C3H7, PrAMCDCl

(OH)n-1

H3 N

(OH)2n

n=6, n=7, n=8,

+-Cl