Expeditious Synthesis of the Hexasaccharide Repeating Unit of the Capsular Polysaccharide of Streptococcus pneumoniae Type 7A

Abstract The hexasaccharide repeating unit corresponding to the capsular polysaccharide of Streptococcus pneumoniae type 7A has been synthesized in good yield using [3+2+1] block synthetic strategy. The synthetic strategy involved a number of challenging stereoselective glycosylation steps, which include β-selective glycosylation of l-rhamnosyl thioglycoside donor, α-selective glycosylations of 2-azido-2-deoxy-d-glucopyranosyl thioglycoside donor and d-galactopyranosyl donor together with the formation of β-glycoside of d-galactosamine moiety and α-glycoside of l-rhamnosyl moiety. Suitably functionalized thioglycosides have been used as glycosyl donors and a combination of N-iodosuccinimide (NIS) and trimethylsilyl trifluoromethanesulfonate (TMSOTf­) has been used as glycosylation promoter.