Transglucosidation of methyl and ethyl D-glucofuranosides by alcoholysis.

The acid catalyzed ethanolysis of methyl 5-O-methyl-alpha- and -beta-D-glucofuranoside and the analogous methanolysis of ethyl 5-O-methyl-alpha- and -beta-D-glucofuranoside have been investigated. For all four reactions, the primarily formed transglycosylation product was a single glycoside that had the opposite anomeric configuration to the starting material. This strongly indicates that a D-glucose methyl ethyl acetal is first formed and is then ring closed by a nucleophilic attack by HO-4, giving either the starting material or a transglycosylation product with the opposite anomeric configuration. Low percentages of the methyl ethyl acetals and of dimethyl acetals were also observed in the reaction product during the methanolysis reactions.