A New Route to Vinyl Glycosides and an Application to a Novel C-Glycoside Synthesis

Abstract

Protected vinyl and 1’-substituted vinyl β-D-glucopyranosides, previously not readily accessible, were efficiently synthesised by the reaction of α-mercurated carbonyl compounds with 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl bromide. Derivatisation of these compounds in the aglycon moiety provided a new route to a number of glucopyranosyl acetals. Attempts to utilise these carbohydrate derivatives as substrates for the preparation of O- or C-glycosides were unsuccessful. The reaction of the α-mercurated carbonyl compounds with S-phenyl 4,6-di-O-acetyl-2,3-dideoxy-1-thio-α-D-erythro-hex-2-enopyranoside was investigated and found to give 2,3-unsaturated vinyl glycosides. In an unprecedented reaction, 2,3-unsaturated C-glycopyranosides were obtained from the Lewis acid catalysed rearrangement of the now available 2,3-unsaturated vinyl glycosides. The mechanism of this rearrangement was investigated and an unreported 1H-n.m.r. spectroscopic feature of the 2,3-unsaturated C-glycosides prepared, which may aid the assignment of anomeric configurations, is discussed. The synthetic utility of the 2,3-unsaturated vinyl glycosides was further demonstrated by the synthesis of C-3 branched-chain glycal derivatives from these precursors by either the Claisen rearrangement or a novel nucleophilic displacement reaction.