Thio-oligosaccharides of sialic acid – synthesis of an alpha(2-3)sialyl galactoside via a gulofuranose/galactopyranose approach

A new approach to the synthesis of thio-oligosaccharides containing the N-acetylneuraminic acid-α(2→3)-galactopyranose linkage is described. 3-O-(Trifluoromethylsulfonyl)gulofuranose derivative 5 can be converted into the α-2,3-sialyl-3-thiogalactofuranose derivative 8 in good yield. Partial deprotection of the thiodisaccharide provides an α/β mixture of both galactofuranose and galactopyranose isomers, but this mixture can be transformed efficiently into the desired pyranose-ring form to allow further elaboration into other glycosides via trichloroacetimidate donor 21. This strategy avoids introducing sulfur into 3-O-(trifluoromethylsulfonyl)gulopyranose derivatives, which can be prone to elimination side reactions.