Transglucosidation of methyl and ethyl D-glucopyranosides by alcoholysis.

The transglucosidations of methyl 4-O-methyl-alpha- and -beta-D-glucopyranoside in ethanolic camphor-10-sulfonic acid, and of ethyl 4-O-methyl-alpha- and -beta-D-glucopyranoside in methanolic camphor-10-sulfonic acid, have been studied. Samples were removed at intervals and the proportions of the glucosides determined by GC of their acetates. The results show that the anomer with the inverted configuration predominates in the initially formed product (approximately 59-70%). This indicates that all the studied reactions proceed via the same mechanism, involving exocyclic C-O cleavage and formation of a glucopyranosylium ion, but that the eliminated alcohol exerts some steric hindrance, which favors the approach of the other alcohol from the opposite side.