Nucleophilic ring opening studies on Benzyl Anhydroribopyranosides

Abstract

This thesis deals with the ring opening reactions of benzyl 3,4-anhydro-£-D ribopyranoside and benzyl 2,3-anhydro-a-D-ribopyranoside. The thesis consists of four sections, namely, introduction, results, discussion and experimental. The introduction begins with the classification of different sugar epoxides followed by representative examples of ring opening reactions of each class. Reactions of 2,3- and 3,4- anhydrohexopyranosides followed by a detailed survey of alkyl 2,3- and 3,4- anhydroribopyranosides are included to show the versatality of nucleophilic ring opening reactions of epoxides to form simple and rare deoxy sugars. The introduction section concludes with some examples of sugar epoxides as chiral starting materials for the syntheses of biologically active molecules. Earlier work from our laboratory had shown that benzyl 3,4- anhydro-ct-D-ribopyranoside reacted with nucleophiles exclusively at C-3. This result was rationalized based on a repulsive interaction between the entering nucleophile and the electron lone pair on the pyranose oxygen, precluding attack at C-4 and directing it to C-3. To establish the validity of this interpretation, it was thought necessary to study the ring opening reactions of benzyl 3,4-anhydro-£-D-ribopyranoside, wherein the anomeric substituent should now direct opening to C-4 on steric grounds, overriding the effect of the oxygen lone pair.