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-alpha-D-mannopyranosyl-(1----4)-O-alpha-L-rhamnopyranosyl-(1----3)-D- galactose and an intermediate for the preparation of 2-O-glycosyl-3-O-(alpha-D-mannopyranosyl)-D-glucoses.

ABSTRACT

Treatment of 2-O-benzoyl (1) and 2-O-acetyl (5) derivatives of benzyl 4,6-benzylidene-3-O-(2,3,4-tri-O-acetyl-alpha-L-rhamnopyranosyl)-beta-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 beta-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-(2,34,6-di-O-isopropylidene-alpha-D-mannopyranosy l)- alpha-D-glucopyranoside, an intermediate for the synthesis of 2-O-glycosyl-3-O-(alpha-D-mannopyranosyl)-D-glucoses.