Efficient synthesis of a sialic acid alpha(2-->3)galactose building block and its application to the synthesis of ganglioside GM3.

Glycosylation of various galactose derivatives with O-acetylated sialic acid N-phenyltrifluoroacetimidate as the donor was investigated. Efficient alpha(2,3)sialylation of galactose, with up to 73% yield and 8.4:1 stereoselectivity, was realized when 2,3,4-unprotected galactose derivatives and TBSOTf were used as acceptors and promoter, respectively. Sialylation of 2-(trimethylsilyl)ethyl 6-O-tert-butyldiphenylsilyl-beta-D-galactopyranoside (3f) gave the best result, and the resultant Neu5Ac alpha(2-->3)Gal disaccharide was successfully used in the synthesis of ganglioside GM3.