published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 1996. / Includes bibliographical references (leaf 312-316).

The main objective of this research is to investigate aspects responsible for the dynamics of guest molecules complexed with cyclodextrins (CD)s). We have shown with the use of a variety of photophysical techniques that the complexation. dynamics for guests with CDs are dependent on the structure of the guest molecule. An assortment of photophysical methods that included steady-state fluorescence, UV-Vis absorption, and laser-induced optoacoustic (LIOAS) spectroscopies, in combination with time-resolved techniques such as single photon counting fluorescence and triplet-triplet absorption (T-T abs) spectroscopies were employed to obtain a detailed understanding of the photophysics for fluorenones. From these photophysical investigations, we have demonstrated that several effects such as the nature and position of substituents, and the properties of the microenvironment are responsible for the photophysics observed for these aromatic ketones. The complexation of fluorenone and xanthone with CDs was investigated to obtain more information on how the structure of the guest molecule can affect the complexation dynamics of these host-guest systems. Induced circular dichroism (ICD) and picosecond fluorescence spectroscopy were employed to detail the structural differences observed for the CD complexes of these two ketones. Equilibrium constants were observed to be larger for xanthone with β-CD than with fluorenone. This result suggested that a more favorable complex is formed for xanthone than for fluorenone. However, in the presence of α-CD, fluorenone formed 2:1 host-guest complexes that were not observed for xanthone. These photophysical studies with additional support from theoretical calculations provided useful tools for understanding the structural intricacies of CD host-guest systems. These types of studies will be invaluable to the understanding of dynamics within supramolecular systems. To expand our knowledge on the structure-dynamics relationship that exists for CD complexes, we investigated the complexation dynamics of charged probes with CDs. Two styrene derivatives, trans-anethole (t-Ane) and 4-vinylanisole (4-VA), were chosen as precursors for the radical cations examined in these investigations. Quenching studies have demonstrated that the exit of the radical cations of t-Ane and 4-VA complexed with CD, even in the presence of alcohols, was faster than 20 ns. In addition, complexes with 2:1 host-guest stoichiometries were unsuccessful in reducing the dissociation rate constant of these charged species. From these studies we have shown that a structure-dynamics relationship does exist for CD host-guest systems. With the use of a variety of photophysical techniques and theoretical calculations, we have been able to better evaluate how the photophysics of probe molecules can be explored in the study of host-guest complexation. Small changes in structure have important consequences on the binding efficiencies of these probes to CDs. This information will aid in the understanding of the structure-dynamics relationship that occurs in supramolecular systems. / Graduate

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the Degree of Doctor of Philosophy Submitted June 2016. / The work detailed in this PhD involves two distinct, separate areas of research. The first project involved an investigation into the synthesis, characterisation and metal-chelation of pyrido-imidazo[1,2-a]pyridines in the search for new compounds for the treatment of cancer. Previously in our laboratories at the University of the Witwatersrand a small library of imidazo[1,2-a]pyridines were synthesized that showed significant activity (IC50 values between 6.57-21.98 μM) against Caco-2 and HT-29 colorectal cancer cell lines while showing little cytotoxicity towards white blood cells; significantly less than camptothecin the ‘golden standard’. In an attempt to improve the overall activity of the imidazo[1,2-a]pyridines methodology was developed to coordinate zinc, copper and platinum to the imidazo[1,2-a]pyridines to generate novel metal-imidazo[1,2-a]pyridine complexes. These novel compounds were characterised by NMR spectroscopy (where possible) and Single Crystal X-ray Diffraction (SCXRD) and tested against colorectal (Caco-2 and HT-29), leukemic (K562 and HL-60) and breast (MCF-7 and MDA-MB231) cancer cell lines, where the copper-containing compounds showed the most significant activity. A library of 11 copper complexes screened showed excellent activity in all the cell lines tested, some of which were more active than camptothecin. The second part of this PhD involved a detailed investigation into novel methodology to synthesize xanthones and related diones, for example such as 4a-methoxy-2H-xanthene-2,9(4aH)-dione, discovered in our laboratory. Initially this reaction was reported to be mediated by ceric ammonium nitrate (CAN), however it was determined during the course of this investigation that the novel reaction was actually mediated by ceric ammonium sulfate (CAS) and that different products were isolated depending on which reagent (CAN or CAS) was used. In this thesis, the mechanism of the reaction was probed and it was determined that the electronic nature of the starting benzophenone (e.g. (2-hydroxyphenyl)(2,4,6-trimethoxyphenyl)methanone) plays a crucial role in the outcome of the reaction. In addition to the furnishing of xanthones (e.g. 4-methoxy-9H-xanthen-9-one) and spirofurans (such as 2',6'-dimethoxy-3H-spiro[benzofuran-2,1'-cyclohexa[2,5]diene]-3,4'-dione) using this CAS-mediated reaction, novel biaryl-fused dimers (e.g. (4,4'-dihydroxy-[1,1'-biphenyl]-3,3'-diyl)bis((2,4-dimethoxyphenyl)methanone)) were also isolated and these results were explained by the mechanisms detailed in this work. Reliable methodology was developed using a sodium dithionite-mediated method for the conversion of dione products (such as 4a-methoxy-2H-xanthene-2,9(4aH)-dione) to hydroxy-containing xanthones (such as 7-hydroxy-9H-xanthen-9-one). iv Finally, as a proof of concept, this novel CAS-mediated methodology was extended to the synthesis of the nitrogen-containing derivatives, the acridones, and a single acridone, 10-benzyl-2-methoxyacridin-9(10H)-one, was successfully synthesized using CAS as the reagent, albeit in a low yield. / LG2017

Thesis (M.Phil.)--Chinese University of Hong Kong. / Bibliography: leaves 40-43.

Thesis (MSc)--University of Stellenbosch, 2008. / Bibliography. Also available via the Internet.

Anthraquinone-xanthone heterodimeric natural products are a diverse family of polyketides highlighted by a unique bicyclo [3.2.2] ring system which links both anthraquinone and xanthone moieties. Both the connectivity of the unique bicyclic ring system and the oxidation state of the two heterocycles vary among the members of this family of natural products. These molecules have been generally isolated as fungal metabolites but also have shown anticoccidial (xanthoquinodin A:0.02 μg/mL), antibiotic (acremonidins A and C; 32 μM and acremoxanthone, MIC; 12.5 μg/mL), and cytotoxicity against various human cancer cell lines. Both anthraquinone and xanthone heterocycles are derived from the anthraquinone chrysophanol, a common biosynthetic intermediate for polyketide synthesis. To date, there have been no reported synthetic efforts or total syntheses of the anthraquinone-xanthone heterodimeric natural products. Related synthetic examples include complex anthraquinone-xanthone biaryls, anthraquinone dimers, and monomeric xanthone and benzophenone-derived natural products. We describe an initial proposed synthesis wherein we aimed to prepare the unique bicyclo [3.2.2] ring system in a late stage operation of functionalized anthraquinone and xanthone units through a photo-mediated cycloaddition. We achieved synthesis of both an anthraquinone-derived oxanthrone ester fragment and the synthesis of several xanthone related natural products, namely graphisin A, sydowinin B, acremonidin E, and pinselin. Key steps involve aryl anion addition to substituted benzaldehyde derivatives, subsequent methyl ester installation, and dehydrative cyclization. Although we have not yet achieved the desired cycloaddition, we contributed to this area by developing efficient, reliable syntheses of various benzophenone and xanthone natural products. We will also describe an alternative strategy to access the bicyclo [3.2.2] core of these natural products via various proposed rearrangements of an anthraquinone-xanthone biaryl intermediate. Cross-coupling of substituted xanthone and naphthalene fragments established the desired biaryl linkage which was further elaborated to afford anthraquinone-xanthone biaryl structures. Attempts to rearrange these biaryls are ongoing to produce the unique core structure of the parent natural products. Initially discovered as a byproduct of the aforementioned cross-coupling reaction, we have achieved homo-coupling of substituted naphthalene fragments. The resulting naphthalene dimers could be further advanced to a series of novel 2,2'-linked anthraquinone dimers including the natural product chrysotalunin.

Lee, Ngan Hon. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references (leaves 120-133). / Abstracts in English and Chinese. / Acknowledgements --- p.IV / Abstract --- p.V / 論文摘要 --- p.VII / Table of Contents --- p.IX / List of Figures --- p.XI / List of Abbreviations --- p.XIII / Chapter 1 Introduction --- p.1 / Chapter 1.1 --- Hepatocellular carcinoma (HCC) --- p.1 / Chapter 1.1.1 --- Risk factors --- p.1 / Chapter 1.1.2 --- Molecular mechanism of HCC --- p.4 / Chapter 1.1.3 --- Treatment of HCC --- p.7 / Chapter 1.2 --- Gambogic acid (GA) - a compound derived from Tradition Chinese Medicine (TCM) --- p.9 / Chapter 1.2.1 --- Traditional Chinese Medicine (TCM) --- p.9 / Chapter 1.2.2 --- Gambogic acid --- p.13 / Chapter 1.3 --- Molecular mechanism of apoptosis --- p.18 / Chapter 1.3.1 --- Overview of apoptosis --- p.18 / Chapter 1.3.2 --- Caspases cascade --- p.18 / Chapter 1.3.3 --- Bcl-2 family --- p.20 / Chapter 1.3.4 --- Mitochondria in apoptosis --- p.23 / Chapter 1.4 --- Apoptosis as a strategy for cancer therapies --- p.26 / Chapter 1.5 --- Aims of study --- p.29 / Chapter Chapter 2 --- Materials and Methods --- p.30 / Chapter 2.1 --- Cell culture and treatment --- p.30 / Chapter 2.1.1 --- Cell lines used --- p.30 / Chapter 2.1.2 --- Gambogic acid (GA) --- p.31 / Chapter 2.1.3 --- Chemicals and reagents --- p.31 / Chapter 2.1.4 --- Preparation of solutions --- p.32 / Chapter 2.1.5 --- Procedures --- p.33 / Chapter 2.2 --- Apoptotic detection --- p.35 / Chapter 2.2.1 --- Chemicals and reagents --- p.35 / Chapter 2.2.2 --- Preparation of solutions --- p.35 / Chapter 2.2.3 --- Procedures --- p.37 / Chapter 2.3 --- Effects of GA on gene expression in HepG2 --- p.41 / Chapter 2.3.1 --- Chemicals and Reagents --- p.41 / Chapter 2.3.2 --- Preparation of solutions --- p.41 / Chapter 2.3.3 --- Procedures --- p.43 / Chapter 2.4 --- Protein expression in GA-induced apoptotic cells --- p.51 / Chapter 2.4.1 --- Chemicals and Reagents --- p.51 / Chapter 2.4.2 --- Preparation of solution --- p.51 / Chapter 2.4.3 --- Procedures --- p.54 / Chapter 2.5 --- Caspase cascade study in GA-induced apoptosis --- p.60 / Chapter 2.5.1 --- Chemicals and reagents --- p.60 / Chapter 2.5.2 --- Procedures --- p.60 / Chapter 2.6 --- Downregulation of mRNA using siRNA vector --- p.62 / Chapter 2.6.1 --- siRNA expression vector --- p.62 / Chapter 2.6.2 --- Chemicals and Reagents --- p.63 / Chapter 2.6.3 --- Preparation of solution --- p.63 / Chapter 2.6.4 --- Procedures --- p.64 / Chapter Chapter 3 --- Results --- p.71 / Chapter 3.1 --- GA induces apoptosis in hepatocellular cells --- p.71 / Chapter 3.2 --- Effects of gene expression in HCC --- p.80 / Chapter 3.3 --- Caspase cascade studies in GA-induced apoptosis --- p.83 / Chapter 3.4 --- Caspase 8 activation in GA-treated cells lead to Bid cleavage --- p.89 / Chapter 3.5 --- GA induces Bax conformational changes and cytochrome c release --- p.95 / Chapter 3.6 --- Levels of protein players involved in apoptosis and cell cycle --- p.101 / Chapter Chapter 4 --- Discussion --- p.106 / References --- p.120

Total syntheses of the neuroregenerative natural products vinaxanthone and xanthofulvin have been accomplished. The synthetic routes to both molecules utilize a highly regioselective furan Diels-Alder cycloaddition - aromatization sequence to furnish the catechol fragment present in both natural products. The pentasubstituted catechol was elaborated to a vinylogous amide which was used twice in both syntheses, exploiting the pseudosymmetry found in vinaxanthone and xanthofulvin. This approach enabled the dimerization of 5,6-dehydropolivione forming vinaxanthone, lending significant evidence to a non-enzymatically driven formation of vinaxanthone in Nature. The total synthesis of vinaxanthone was accomplished in nine steps, the shortest synthesis to date, and an additional route was devised to access a set of analogs for biological study. The first total synthesis of xanthofulvin was accomplished in 18 steps and the convergent nature of the synthetic plan allows for analog synthesis. The sets of vinaxanthone and xanthofulvin analogs will be used to examine their inhibition of Semaphorin3A, a protein which inhibits neuronal regeneration, and is the biological target for both molecules.