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Synthesis and Inclusion Studies of Stable The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgementTown of the source. The thesis is to be used for private study or non- commercial research purposes only. Cape Published by the University ofof Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University SYNTHESIS AND INCLUSION STUDIES OF STABLE ALLICIN MIMICS AS NOVEL ANTIMICROBIAL AGENTS BY Nashia StellenboomTown Cape Thesis Presentedof for the Degree of DOCTOR OF PHILOSOPHY In the Department of Chemistry UniversityUNIVERSITY OF CAPE TOWN April 2008 Supervisors Professor Roger Hunter and Professor Mino R. Caira Table of Contents ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Chapter 1 : Introduction 1 1.1 Overview of Garlic 1 1.2 Allicin 2 1.3 Thiosulfinates 12 1.4 The Disulfide Bond 14 1.5 Review of Unsymmetrical Disulfide Synthesis 22 1.6 Cyclodextrins 55 1.7 Aims and Objectives 63 Chapter 2 : Unsymmetrical Disulfides 64 2.1 Unsymmetrical Disulfide Synthesis 64 Chapter 3 : S-Alkyl Arylthiosulfinates 109 Town 3.1 Overview 109 3.2 The Chemistry of Oxidation of Unsymmetrical Disulfides 113 3.3 Electron-Rich systems 118 3.4 Electron-Deficient systems Cape 120 3.5 π-Deficient Heteroaromatic System 121 3.6 Stability Tests of S-alkyl arylthiosulfinatesof 128 3.7 S-p-Methoxyphenyl Alkylthiosulfinates 130 Chapter 4 : Cyclodextrin Inclusion 142 4.1 Overview 142 4.2 Experimental, Methods and Materials 142 4.3 Results andUniversity Discussion 146 4.4 Conclusion 161 Chapter 5 : Summary and Conclusion 162 Chapter 6 : Experimental 163 6.1 General Procedure 163 6.2 Synthesis and Purification of Disulfides 164 6.3 Compounds 166 References 193 Abstract ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ABSTRACT Allicin, a known constituent of garlic, is a potent but unstable antimicrobial agent. Consideration of the underlying features responsible for allicin’s activity, as well as its instability, prompted an investigation into substituted S-aryl alkylthiosulfinates as a class of potential allicin mimics with enhanced stability. Synthesis of the targets also inspired development of a novel unsymmetrical disulfide synthesis. This thesis describes the development of a new methodology for synthesizing unsymmetrical disulfides. The synthesis involves converting a thiol to a sulfenylating agent by 1-chlorobenzotriazole (BtCl) in the presence of 1,2,3-benzotriazole (BtH). Addition of a second thiol affords unsymmetrical disulfides in excellent yields. In addition to being a one-pot methodology, the approach offers attractive environmentally friendly and cost-saving aspects. The methodology proved to be versatile, producing all types Townof unsymmetrical disulfides; aromatic-aliphatic disulfides, aromatic-aromatic disulfides as well as aliphatic-aliphatic disulfides including unsymmetrical cysteine disulfides. A class of unsymmetrical disulfides (alkyl p-methoxyphenylCape disulfides), with an activated aromatic ring, were oxidized to the corresponding thiosulfinates (S-alkyl p-methoxyphenylthiosulfinates) using m-CPBA,of and were established to be markedly more stable than allicin itself with only ~5% structural change after 1 month at room temperature. This was in contrast to the same class of thiosulfinates with deactivated rings, which were found to be unstable. A family of relatively stable thiosulfinates was thus prepared by varying the length and fluorine content of the alkyl chain. Such compounds were subjected to a preliminary antibacterial study, which revealed that the S-p-methoxyphenyl alkylthiosulfinates exhibit activity against Gram-positive (Mycobacterium aurum and Staphylococcus aureus) and Gram-negative (EscherichiaUniversity coli) bacteria, qualifying them as allicin mimics at a preliminary level. Lastly, the thesis describes the preparation and characterization of inclusion complexes formed between selected cyclodextrins and S-aryl alkylthiosulfinates. The complexes were investigated by thermal techniques, IR spectroscopy, powder X-ray diffraction and in three cases, single crystal X-ray diffraction. Such complexes may also offer the included molecules valuable stability and biological characteristics. Acknowledgements ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ACKNOWLEDGEMENTS I would like to thank the following people for their contributions to this thesis: Professor Roger Hunter, to whom I dedicate this thesis, for his excellent supervision and guidance, his patience, support, encouragement and profound motivation throughout this project, and for inspiring my passion for organic chemistry. Professor Mino R Caira for his supportive supervision, kindness and concern throughout this project and for whom I have endless respect and admiration. Professor David Gammon and Professor Kelly Chibale for offering advice and helping with mechanistic problems. Town Dr. Clive Oliver, Dr. Paul R Meyers, Dr. Bronwyn Kirby, Dr. Vincent Paul, Dr. Margaret Blackie and Jamy Feng, your kindness is invaluable, thank you for your assistance. My colleagues and friends (past and present) in theCape Hunter/Gammon research group, for their advice, assistance, support, insightful discussions, intriguing conversations and friendship. of My colleagues in the Caira (Supramolecular) research group, for all their assistance. All my past and present colleagues in the Department of Chemistry, particularly those in the Organic Synthesis Laboratories. Pete Roberts, Noel Hendricks, Tommie van der Merwe, Martin Brits, Dr. Marietjie Stander, Piero Benincasa forUniversity their analytical services. The National Research Foundation and the University of Cape Town for financial support. My loving family, for their understanding, encouragement and emotional support. Finally, The Almighty, for giving me the strength to complete this challenging task. Abbreviations ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ LIST OF ABBREVIATIONS aq. aqueous Ar aromatic br. s. broad singlet Boc tert-butyloxycarbamate BtH 1,2,3-benzotriazole BtCl 1-chlorobenzotriazole BuSH Butylthiol t-BuSH tertiary butylthiol BzSH benzylthiol cat. catalytic Cp para-carbon Cm meta-carbon Co ortho-carbon Cs ipso-carbon CD cyclodextrin α-CD α-cyclodextrin Town γ-CD γ-cyclodextrin β-CD β-cyclodextrin d doublet dd doublet of doublets ddd doublet of doublets of doubletsCape dt doublet of triplets DEAD diethyl azodicarboxylateof DIMEB heptakis(2,6-di-O-methyl)-β-CD DSC differential scanning calorimtry eq. equivalents EI electron ionisation EtOAc ethyl acetate g grams GC gas chromatography University hr. hour Hz hertz Hm meta-proton Ho ortho-proton HOMO highest occupied molecular orbital HPLC high pressure liquid chromatography HRMS high resonance mass spectrometry IR infra red spectroscopy J coupling constants LDA lithium diisopropylamide LUMO lowest unoccupied molecular orbital m meta Abbreviations ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ m multiplet m/z mass-to-charge ratio mp melting point mg milligram(s) ml millilitre(s) mmol millimole(s) mol mole(s) Me methyl MeO methoxy MeOH methanol m-CPBA meta-chloroperbenzoic acid NMR nuclear magnetic resonance o orthro p para Pet ether petroleum ether Ph phenyl ppm parts per million PMB-Br para-methoxybenzyl bromide Pr-SH propanethiol Town PrSSPr dipropyl disulfide PXRD powder X-ray diffraction q quartet rt room temperature Cape R1SH thiol 1 R2SH thiol 2 of R1SSR1 homodimer 1 R2SSR2 homodimer 2 R1SSR2 unsymmetrical disulfide s singlet SCXRD single crystal X-ray diffractometry t triplet td triplet of doublets TG Universitythermogravimetry THF tetrahydrofuran TLC thin layer chromatography p-TolSH p-tolylthiol p-TolSBt p-tolylthio-benzotriazole intermediate TRIMEB heptakis(2,3,6-tri-O-methyl)-β-CD w/v weight by volume v/v volume by volume Chapter 1 Introduction ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Chapter 1: Introduction 1.1 Overview of Garlic Garlic, Allium sativum, is a plant belonging to the Liliaceae family and has been used as a medicinal agent for many decades.1 It has generated much interest throughout human history as a food and medicine and is one of the most researched medicinal plants. Its folklore and therapeutic benefits date back about five thousand years to the Middle and Far East, and probably originated in the advanced civilizations of the Indus valley, from where it was imported to China before spreading to Egypt, Greece and throughout the Roman Empire into Europe.2,3,4 In ancient civilizations, garlic was used as a treatment for various ailments including headaches, burns and wounds, colds, worms, stings, bites, tumours, heart disease and ulcers.1,5 In more recent times, notably during the past two decades, garlic research has predominantly focused on the topics of atherosclerosis, cardiovascularTown disease, coronary thrombosis and cancer research.6,7 Mechanistic investigation over the last few years suggests that garlic may either prevent or decrease the occurrence of these major chronic diseases, which are mainly due to the abundance of free radicals, by virtue of its strong antioxidant properties.8 In particular, it has been demonstratedCape that garlic reduces serum cholesterol and triglyceride levels, inhibits platelet aggregation, stimulates immune effector cells and acts on bacteria, viruses and alimentary parasites.of1,4,6 The lengthy list of biological activities and health-promoting effects of garlic is attributed to the presence or generation of sulfur-rich
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