Anomalous Zemplen deacylation reactions of alpha- and beta-D-mannopyranoside derivatives

Reaction of mono-, di-, and trisaccharide derivatives of methyl beta -D- an d octyl beta -D-mannopyranosides bearing ester groups at isolated and non-i solated positions on the same molecule, under Zemplen conditions (catalytic amount of sodium methoxide in methanol) gave partially deacylated compound s, in which the O-acyl groups were retained at isolated sites. In the case of one disaccharide, all the benzoyl groups remained intact at the reducing end, while all the acetyl functions were removable from the nonreducing en d. In another case, both isolated ester groups at positions 2 and 4 were re tained at the reducing end. The isolated 2-O-acyl groups on methyl alpha -D -mannopyranoside compounds were more labile than on the corresponding beta -mannosides under the same conditions. The mechanism of the reaction may be different for ester groups at isolated or non-isolated positions. In the l atter case, acyl migration may take place and carry acyl groups into a less hindered position. (C) 2001 Published by Elsevier Science Ltd. All rights reserved.