Synthetic and mechanistic studies in carbohydrate chemistry
Yule, Kerr Carmichael
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The aim of the work described in this thesis was to synthesise a sugar having the ring oxygen replaced by sulphur. 5-Deoxy-5-thio-D-xylopyranose and a number of its derivatives were successfully synthesised. This led to the study of the reactivity of these compounds and their oxygen analogues in some typical carbohydrate reactions. The results of these experiments provided further evidence concerning the mechanisms of certain carbohydrate reactions. Part I : A discussion of the physical and chemical properties of analogous oxygen and sulphur compounds. This discussion was of interest in connection with the comparison of the reactivity of sulphur sugars and their oxygen analogues, and particularly, because a comprehensive survey of this type is not available in the literature. Part II : 5-Deoxy-5-thio-D-xylopyranose and its derivatives. Section I of this part described the preparation and properties of 5-heoxy-5-thio-D~xylopyranose. Evidence, based on UV and IR spectroscopy, N.M.R. studies, chemical reactions, and mutarotation studies, is presented as proof of the existence of the sulphur ring. The mutarotation reaction of 5-deoxy-5-thio-D-xylopyranose and its oxygen analogue was carried out in two buffered solutions of different pH and the results compared. The production of 2-thiophenaldehyde and 2-furfuraldehyde from 5-deoxy-5-thio-Dxylopyranose and its oxygen analogue respectively was studied. Section II describes the preparation, reactions, and methanolysis of 2,3,4-tri-0-acetyl-5-deoxy-5-thio-α-Dxylopyranosyl 1-bromide. The reactions included the Koenigs- Knorr reaction and the reaction of the acetobromide with silver acetate. The results of the methanolysis of the sulphur sugar acetohalide and its oxygen analogue were in agreement with the accepted unimolecular mechanism, and the fact that the oxygen-ring sugar reacts forty times faster than its sulphur analogue is satisfactorarily explained. Section III describes the preparation and hydrolysis of methyl 5-deoxy-5-thio-α- and β-D-xylopyranosides. The kinetic results of the hydrolyses of these methyl glycosides and of methyl β-D-xylopyranoside enabled us to lend support to one of two proposed mechanisms for the acid hydrolysis of glycosides.