Studies on the koenigs-knorr reaction : Part IV: The effect of participating groups on the stereochemistry of disaccharide formation

Abstract

Partial benzylation of methyl 2- O-benzyl-α- L-fucopyranoside afforded a mixture of methyl 2,3-, and 2,4-di- O-benzyl-α- L-fucopyranoside which were separated by means of their monoacetates. Partial benzylation of methyl α- L-fucopyranoside gave the 2,4-, and 3,4-dibenzyl ethers in the ratio of 3:2, and no 2,3-isomer could be detected in the reaction mixture. The structures of the three dibenzyl ethers were established: (a) by analysis of the n.m.r. spectra of their acetates, and (b) by methylation, removal of benzyl groups by hydrogenolysis, and characterization of the methyl ethers of the methyl glycosides. Acid hydrolysis of these compounds gave the monomethyl ethers of L-fucose, two of which were identical with known compounds, whereas the third, 4- O-methyl- L-fucose, was a new compound. Selective p-nitrobenzoylation of 2,3-, 2,4-, and 3,4-di- O-benzyl- L-fucose, followed by acetylation and treatment with hydrogen bromide in dichloromethane, gave the three possible mono- O-acetyl-di- O-benzyl-α- L-fucopyranosyl bromides, which were condensed with benzyl 2-acetamido-3,4-di- O-acetyl-2-deoxy-α- D-glucopyranoside. The disaccharide derived from the 2- O-acetyl substituted bromide was enriched in β- L-fucopyranoside, whereas the other two bromides gave mainly the α- L-linked anomer. The α-directing influence of the 3- and 4- O-acetyl substituents is not less than the β-directing influence of the 2- O-acetyl group in similar bromides; participation of acyl groups and electronic-steric influences are discussed as possible explanations for the steric course of the reaction.