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.

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► An unexpected stereo outcome in the glycosylation was observed.
► 1,2-Cis glycoside formed using glycosyl donor having 2-acetoxy group.
► A [2+2] block glycosylation strategy was adopted.
► Selective glycosylation of thioglycoside was done tuning relative reactivity values.