Selective 4,6-O-benzylidene formation of methyl α-D-mannopyranoside using 2,6-dimethylbenzaldehyde

While methyl α-d-glucopyranosides and α-d-galactopyranosides selectively form 4,6-O-benzylidenes when reacted with excess benzaldehyde in the presence of acid catalyst methyl α-d-mannopyranosides does not exhibit the same selectivity because of the cis-arrangement of the C2 and C3 hydroxyl groups. The selectivity for the 4,6-O-benzylidene is restored by using 2,6-dimethylbenzaldehyde instead of benzaldehyde. In addition the excess 2,6-dimethylbenzaldehyde is easily recovered from the reaction by extraction with petroleum ether and can be reused without further purification. The 2,6-dimethylbenzylidene exhibits properties similar to the unsubstituted benzylidene with regard to chemical synthesis.