Synthesis of a tetrasaccharide analog corresponding to the repeating unit of the O-polysaccharide of Salmonella enterica O59: Unexpected stereo outcome in glycosylation

Convergent synthesis of a tetrasaccharide analog corresponding to the repeating unit of the O-polysaccharide of Salmonella enterica O59 is presented. A thioglycoside disaccharide donor was prepared by the glycosylation of two thioglycosides by tuning their relative reactivity. An unexpected stereochemical outcome was observed in a glycosylation using an ethyl 2-O-acetyl-3-O-benzyl-4, 6-O-bensylidene-thio-galactoside donor, where the alpha-galactoside was formed in spite of the presence of the 2-O-acetyl participating group.