Anomalous Zemplén deacylation reactions of 2- O-acyl-3- O-alkyl or -3- O-glycosyl derivatives of d-galactose and d-glucose: synthesis of O-α- d-mannopyranosyl-(1→4)- O-α- l-rhamnopyranosyl-(1→3)- d-galactose and an intermediate for the preparation of 2- O-glycosyl-3- O-(α- d-mannopyranosyl)- d-glucoses

Abstract

Treatment of 2- O-benzoyl ( 1) and 2- O-acetyl ( 5) derivatives of benzyl 4,6- O-benzylidene-3- O-(2,3,4-tri- O-acetyl-α- l-rhamnopyranosyl)-β- d-galactopyranoside under Zemplén conditions (catalytic amount of sodium methoxide in methanol) gave partially deacylated disaccharides in which the 2- O-acyl groups were retained. Likewise, a similar result was obtained with the β- l-rhamnopyranosyl analogue ( 3) of 1. This anomalous reaction was used in a synthesis of the title trisaccharide ( 17) and of methyl 4,6- O-benzylidene-3- O-(2,3:4,6-di- O-isopropylidene-α- d-mannopyranosyl)-α- d-glucopyranoside, an intermediate for the synthesis of 2- O-glycosyl-3- O-(α- d-mannopyranosyl)- d-glucoses.