by Tsui Hon-chung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1993. / Includes bibliographical references (leaves 74-77). / Abstract --- p.i / Acknowledgements --- p.ii / Biography --- p.iii / Abbreviations --- p.v-vi / Chapter 1. --- Introduction / Chapter 1.1 --- Styryl-lactones from Piper genus --- p.1-2 / Chapter 1.2 --- Styryl-lactones from Goniothalamus genus --- p.2-15 / Chapter 1.3 --- Biosynthetic pathways --- p.15-17 / Chapter 2. --- Results and discussion / Chapter 2.1 --- Goniofufurone: Synthesis and absolute configuration --- p.18-28 / Chapter 2.2 --- Synthesis of (+)-goniofufurone --- p.28-34 / Chapter 2.3 --- Synthesis of (+)-goniobutenolide A and (-)-goniobutenolide B --- p.34-37 / Chapter 2.4 --- Synthesis of (+)-goniopypyrone --- p.37-39 / Chapter 2.5 --- Synthesis of (+)-altholactone --- p.39-41 / Chapter 2.6 --- Synthesis of (+)-goniotriol and (+)-7-acetylgoniotriol --- p.41-44 / Chapter 3. --- Conclusions --- p.45-47 / Chapter 4. --- Experimental --- p.48-73 / Chapter 5. --- References --- p.74-77 / Chapter 6. --- List of spectra --- p.78 / "1H-NMR spectra of compounds 14，16, 20，21b, 22，26 and 27" --- p.79-85

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Plakortide E (85), which is isolated from the Jamaican marine sponge Plakortis halichondrioides, contains a five-membered peroxide ring, with the oxygen atoms are linked to tertiary C4 and C6 centers.34,57 In this thesis, the total synthesis of plakortide E (85) is described. A novel palladium-catalyzed approach towards 1,2-dioxolanes has been developed. A lipase-catalyzed kinetic resolution was employed to provide optically pure 1,2-dioxolane central cores. Coupling of the central cores and side chains was achieved by a Negishi reaction. All four isomeric structures of plakortide E methyl ester, namely, 86a-d were synthesized, and one of these molecules, 86d proved to be natural plakortide E methyl ester on the basis of 1H, 13C NMR spectra and specific rotation. With plakortide E methyl ester (4S,6R, 10R)-(-)- cis-(86d) and its other three isomers in hand, we successfully converted them into plakortone B (3S,4S,6 R,10R)-(87a), and its isomers ent-87a, 87b and ent-87b via an intramolecular oxa-Michael addition/lactonization cascade reaction. Saponification converted 1,2-dioxolane 86d into plakortide E (85a) whose absolute configuration (4S,6 R,10R) was confirmed by comparison of spectral and physical data with those previously reported.35b / Sun, Xiaoyu. / Adviser: Henry N. C. Wong. / Source: Dissertation Abstracts International, Volume: 73-06, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 183-195). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Graduation date: 1995

published_or_final_version / abstract / toc / Chemistry / Doctoral / Doctor of Philosophy

L'oxydation de Baeyer-Villiger (BV) par voie enzymatique est une méthode efficace pour obtenir de lactones sous forme énantiomériquement pure. Les Baeyer-Villiger Monooxygénases (BVMO) sont ainsi capables d'oxyder de nombreux substrats avec une stéréospécificité remarquable.Nous avons recherché de nouvelles enzymes dans le génome de deux souches appartenant au genre Acinetobacter, A. baylyi ADP1 et A. baumannii AYE. Six gènes ont été clonés dans E. coli. Leur profil de substrat a été étudié en utilisant des cellules entières de ce microorganisme recombinant comme biocatalyseur. Quatre enzymes ont montré une spécificité de substrat similaire, avec une préférence pour les petites cétones cycliques et pour les substituants aryliques. Une de ces enzymes a permis le Dédoublement Cinétique Parallèle Régiodivergent d'une bicyclohepténone et la désymétrisation de cyclobutanones benzyliques avec, dans chaque cas, une énantiosélectivité intéressante car conduisant à des énantiomères rarement obtenus par réaction de BV enzymatique.Dans une seconde partie, des Dédoublements Cinétiques Dynamiques, associant réaction de BV enzymatique et racémisation in situ ont été réalisés avec des cellules entières d'E. coli produisant la Cyclohexanone Monooxygenase (CHMO) issue d'A. calcoaceticus. La racémisation de cyclohexanones α-substituées, habituellement difficilement racémisables, a été assurée par l'emploi de solutions tampons à base de sels de phosphate ou de glycine. Les -caprolactones correspondantes ont été isolées sous forme d'esters méthyliques hydroxylés quasi énantiopurs avec des rendements compris entre 70 et 80%. / Enzyme-mediated Baeyer-Villiger oxidation is nowadays largely recognized as an efficient method to obtain highly optically active lactones. An increasing number of Baeyer-Villiger Monooxygenases from various sources has been found to oxidize a large range of substrates with a good to excellent stereospecificity.Firstly, in order to enlarge the scope of these biotransformations, the genome of two strains of the Acinetobacter genus, A.baylyi ADP1 and A.baumannii AYE was explored. Six genes were expressed in E. coli and the substrate profile of each enzyme was studied using whole cell biotransformations. Four enzymes showed close substrate specificity with a preference for small cyclic ketones and for arylic substituents. Interestingly, one enzyme led to a Kinetic Parallel Regiodivergent Resolution of a bicycloheptenone and desymmetrisation of benzylic cyclobutanones in an enantiocomplementary manner when compared to the most of already known enzymes.The second part of this work describes the implementation of Dynamic Kinetic Resolution processes combining enzymatic BV oxidation and in situ racemization of α-substituted cyclohexanones to afford corresponding lactones in more than 50% yield. Cyclohexanone Monooxygenase (CHMO) from another Acinetobacter strain, A. calcoaceticus, was selected and the reactions were carried out with whole cells of producing CHMO E. coli strain. The racemization of α-substituted cyclohexanones, usually slowly racemized under basic conditions, was ensured by the use of containing phosphate salts or glycine buffer solutions. Several corresponding -caprolactones were isolated after methylation as enantiopure hydroxy methyl esters in 70-80% yield.

Glycosidases are carbohydrate processing essential enzymes necessary for the growth and development of all organisms such as intestinal digestion, post-translational processing of glycoproteins and the lysosomal catabolism of glycoconjugates. The function of these glycosidases is limited and studies are still in progress to understand their function at cellular level. In recent years, biological role of carbohydrates has resulted in various carbohydrate-based therapeutics2. These carbohydrates serve as a tool to study the function of glycosidases by inhibiting their active site. The concept of inhibition is yet another approach for the discovery of drugs. Glycosidase inhibitors studied are often sugar analogs and a wide range of such inhibitors are reported in the literature.3, 4 Thiosugars, in particular, have gained new perspectives owing to their electronic, geometric, conformational and flexibility differences, as sulfide moiety being less electronegative and more polarizable than the oxygen counter-part.5 These differences make the thiosugars distinct from their oxygen analogs and hence can mimic the active site of the enzyme. Many molecules are reported to be promising glycosidase inhibitors but are not easily accessible due to difficulties in their synthesis. Hence, the chemical synthesis of thio-analogs of carbohydrates, by synthetic routes, remains a major challenge. To address the complexity of synthesis and to make available new strategies, we envisioned the use of benzyltriethylammonium tetrathiomolybdate [BnEt3N]2MoS4, a versatile and efficient sulfur transfer reagent. Objectives of the study: a. Design novel thiosugars as glycosidase inhibitors. b. Devise strategy for the synthesis of novel thiosugars through a simple, practical approach. c. Evaluate the synthesized molecules as glycosidase and HIV-1 protease inhibitors, in silico. d. Study miscellaneous applications of the novel thiosugar-derived thialactones. The thesis is divided into five sections: Section A entitled “Synthesis of deoxythiosugars and thiosugar-based lactones” is divided into two parts, Part A and Part B. Part A – “An introduction and background on thiosugars and sulfur transfer reagents” has been provided. A brief discussion of sulfur transfer reagents in carbohydrate synthesis and earlier work related to the use of benzyltriethylammonium tetrathiomolybdate, [BnEt3N]2MoS4, as an efficient sulfur transfer reagent have been provided. Part B –“Design of inhibitors of glycosidases and HIV-1 protease” deals with the design of inhibitors of glycosidase and HIV-1 protease. The designed thiosugar molecules exhibit the characteristics of sugars and will act as planar molecules to mimic the active site conformation of a good inhibitor. Synthetic methodologies devised and adopted for the synthesis of constrained sugar-derived thialactones include: (a) Double displacement, (b) Displacement-cum-intramolecular thia-Michael addition, (c) Epoxide ring-opening-cum-intramolecular thia-Michael addition, and (d) Displacement-cum-epoxide ring opening in an intramolecular fashion. In all the above mentioned strategies, sulfur transfer step is the crucial step which was achieved by the use of benzyltriethylammonium tetrathiomolybdate [BnEt3N]2MoS46 as the key reagent. (a) Various constrained thialactones synthesized by double displacement strategy using tetrathiomolybdate as the sulfur transfer reagent are shown in Scheme – 1. (b) A number of constrained thialactones were synthesized following nucleophilic displacement-cum-intramolecular thia-Michael addition strategy as shown in Scheme – 2. (c) Synthesis of bicyclic thiolactones was achieved using the strategy of epoxide ring-opening-cum-intramolecular thia-Michael addition. (Scheme – 3) (d) A few bicyclic thialactones were synthesized through displacement-epoxide ring opening-cyclization as shown in Scheme – 4. The methodology was also utilized for the synthesis of thiosugar derivatives and azido-thialactones. (Fig. 1) Figure 1 Synthesis of deoxythiosugars: The bicyclic thialactones (designed as inhibitors) on reduction with borohydride exchange resin (BER) easily furnished the deoxythiosugars (Fig. 2). It is worth mentioning that the synthesis of these thiosugars as reported in the literature involved lengthy procedures whereas the present methodology turns out to be short and concise. Figure 2 Section B entitled “Synthesis of amines, β-amino acids and novel thiosugar-based dehydroamino acids” comprises a brief introduction on the importance of amines, β-amino acids and dehyroamino acids. In this section the effective utilization of benzyltriethylammonium tetrathiomolybdate as a key reagent for reductive transformations and its application in the synthesis of amines, β-amino acids and dehyroamino acids have been presented. A one pot reduction of azides to amines followed by intermolecular aza-Michael addition employing tetrathiomolybdate was achieved to furnish a number of different β-amino esters as shown in Scheme -4: Scheme 4 The study was further extended to the reduction of a few anomeric azides to afford the corresponding anomeric amines and derivatives. (Fig. 3) Figure 3 A one-pot thia-Michael addition-vinyl azide reduction in a tandem fashion employing benzyltriethylammonium tetrathiomolybdate was studied and was shown to be effective for the synthesis of thiosugar derived dehydroamino acid derivatives. (Scheme – 5) Scheme 5 Section C entitled “Molecular docking studies of deoxythiosugar probes” gives an overview of different glycosidases, HIV-1 protease and their inhibitors. This section also deals with a brief introduction on active site conformations of potent inhibitors. In this connection we have studied the crystal conformations of the synthesized molecules whose conformations were the same as that of the existing inhibitors in the active site. (Fig. 4) With this background in silico study of the synthesized deoxythiosugar probes was conducted on human glycosidases: α-mannosidase, α-galactosidase, β-glucosidase and HIV-1 protease, respectively. Figure 4 Molecular docking was carried out using Autodock suite, molecular modeling simulation. Separate docking procedures were employed for the four different receptors. The PDBs representing the four enzyme targets were 2V3D, 3H53, 1X9D and 3I8W for β–glucosidase, α–galactosidase, α–mannosidase and HIV–1 protease respectively. The control compounds used for α–mannosidase were mannostatin and kifunensine. NMB, THK, and BED were the positive controls for HIV–1 protease. Similarly, NBV and cyclophellitol were the controls used for β–glucosidase and NOJ, N–methyl calystegine B2 for α–galactosidase. (Fig. 5) Ligands TGSB68 and TGSB482 had the energy value of –6.49 kcal/mol comparable to that of the average reference value of the positive control, and thus, the potent candidate as identified by molecular docking to HIV-1 protease. (Fig. 6a) The control compounds used for α–mannosidase were mannostatin and kifunensine, which bind with mean binding energy of -9.11 and -5.56. In the case of α–mannosidase, the same compounds TGSB68 and TGSB482 were selected due to comparable energy and a good cluster size with that of positive control. (Fig. 6b) For β– glucosidase, ligands TGSC108 and TGSC236, which had comparable values to that of positive control was identified as the Figure 5 Figure 6 potent candidate. (Fig. 6c) In the case of α–galactosidase, again the ligands TGSB68 and TGSB482 were selected based on binding energies. (Fig. 6d) In conclusion, the concept analogy (deoxy nature, planarity, thiosugar framework, lactone moiety) for the design of inhibitors indeed worked positively. The results are really encouraging. An in vivo study of the synthesized novel thiosugar probes will certainly provide a potent inhibitor. Section D entitled “Research methodology” provides experimental procedures adopted with details of synthesis. Section E entitled “Bibliography” provides the references cited in this work.