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-a cetylneuraminic acid-alpha(2 --> 3)-galactopyranose linkage is described. 3 -O-(Trifluoromethylsulfonyl)gulofuranose derivative 5 can be converted into the alpha-2,3-sialyl-3-thiogalactofuranose derivative 8 in good yield. Par tial deprotection of the thiodisaccharide provides an alpha/beta mixture of both galactofuranose and galactopyranose isomers, but this mixture can be transformed efficiently into the desired pyranose-ring form to allow furthe r elaboration into other glycosides via trichloroacetimidate donor 21. This strategy avoids introducing sulfur into 3-O-(trifluoromethylsulfonyl)gulop yranose derivatives, which can be prone to elimination side reactions.