Access Type

Dissertation/Thesis

Date of Award

January 2019

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

David Crich

Abstract

Glycosylation, being the key reaction of glycobiology and carbohydrate chemistry, demands robust and stereoselective glycosylation methods. The rational development of such methods necessitates knowledge of the glycosylation reaction mechanism and the factors influencing glycosylation reactions. The work discussed in this dissertation is focused on studying glycosylation reaction mechanisms and exploring the key factors influencing them.

Chapter one reviews the background to the research, starting with a brief introduction of the significance of carbohydrates as structural and energy-storage materials. The significance of cell surface carbohydrates and their roles in communication and related processes are discussed along with examples of their applications in biology and therapeutics. Approaches to the synthesis of carbohydrates, glycosylation reactions, and their mechanisms are then discussed. The stability of glycosyl oxocarbenium ions and the influence of protecting groups on oxocarbenium ions are also discussed. This chapter ends with a short description of neighboring group participation.

Chapter two describes the synthesis of a series of oxabicyclo[4.4.0]decane type compounds as models representing the ideal gg, gt, and tg conformations of the pyranoside side chain, and the derivation of experimental limiting coupling constants from them. The preparation of the model compounds, NMR analysis, triage of some compounds on the grounds of lack of conformational purity, derivation of correction factors, and finally, the application of limiting coupling constants in the calculation of side chain population are described. The absence of negative populations on the application of the limiting coupling constants to a series of literature compounds reveals the reliability of the new limiting data set.

Chapter three discusses the influence of protecting groups on the modulation of side chain conformation and the subsequent impact on the glycosylation reactions. The synthesis of a series of 4-O-derivatives of galacto- and glucopyranosides and 6-O- derivatives of glucopyranosides, their NMR analysis, and calculation of their side chain populations, and their correlation with the anomeric reactivity are discussed. Both aroyl and alkanoyl esters modulate the side chain population and moderate the electron withdrawing effects of the ester, whereas, the 4-O-esters of glucopyranosides change the side chain conformation to support the electron withdrawing effect of the ester. In both the galacto- and glucopyranose 4-O-series, highly electron withdrawing esters, such as trifluoroacetyl and trichloroacetyl esters, modulate the side chain conformation to reinforce the electron withdrawing effect of the esters. The glucopyranose 6-O-series did not show any significant modulatory effects as a function of protecting groups. Overall, this study reveals small but consistent changes in the side chain population, with corresponding small influences on glycosylation reactions.

Chapter four starts with a brief introduction to the biological importance of sialic acid glycosides, their occurrence, and synthesis. Approaches to stereoselective sialidation reactions are discussed, and recent advances in sialic acid chemistry along with manipulation of protecting groups in obtaining better α-selectivity are presented. Sialidation reaction mechanisms and the influence of protecting groups and additives, such as acetonitrile, are also surveyed. This chapter ends with a critical note on the significance of kinetic studies and the identification of intermediates in describing sialidation reaction mechanisms.

Chapter five describes a series of direct kinetic experiments on N-acetyl-oxazolidinone protected α- and β- thiosialyl donors designed to probe the influence of acceptor concentration and acetonitrile concentration in sialidation reactions. The synthesis of the thiosialyl donors, pseudo-first order kinetic experiments with respect to the acceptor, and kinetics experiments varying the acetonitrile percentage at constant donor and acceptor concentrations are reported. Both the α- and β-O-sialidation reactions strongly dependent on the acceptor concentration, revealing SN2 or SN2-like reaction pathways via β-sialyl triflate intermediate. The rate of the α-O-siliadation reaction also depends on the concentration of acetonitrile, indicating the likely participation of acetonitrile in the form of a β-sialyl nitrilium ion, which then undergoes SN2 type reactions giving α-O-sialosides. Overall, this study recommends using the highest possible concentrations of the glycosyl acceptor and donor and employing acetonitrile as a cosolvent in the reaction medium to obtain better α-selectivities.

Chapter six describes the development of a cation clock reaction to probe the kinetics of sialidation reactions as an alternative to direct kinetic experiments. The synthesis of a series of cation clock donors is described. Study of the intramolecular glycosylation (cyclization) reactions reveals an SN2 type mechanism for the cyclization. This chapter also describes a series of competition kinetic experiments conducted by changing the acceptor concentration, revealing a considerable dependence on acceptor concentration in the formation of the α-O-sialosides. Hence, α-O-sialidation reactions follow SN2 type reaction pathways, and increasing the electron withdrawing effect of the protecting groups favors the SN2 end of the reaction to a greater extent. The use of acetonitrile in sialidation reactions led to the isolation and characterization of a β-acetamide, which is strong evidence in support of the participation of acetonitrile in the form of β-sialyl nitrilium ions. Overall, the evidence presented in this study is consistent with the direct kinetic experiments.

The dissertation ends with a conclusion chapter and a complete set of experimental data for the studies discussed.

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