Convergent synthesis of the pentasaccharide repeating unit of the cell wall K116 capsular polysaccharide of Acinetobacter baumannii

Synthesis of the pentasaccharide corresponding to the K116 capsular polysaccharide of Acinetobacter baumannii (A. baumannii) has been achieved using a concise [3 + 2] stereoselective block synthetic strategy. A trisaccharide acceptor was synthesized by sequential stereoselective glycosylations of judiciously functionalized monosaccharide intermediates and a disaccharide was prepared using a thioglycoside derivative as glycosyl acceptor. Beta-linked glycosylations were achieved using thioglycoside as glycosyl donor and a combination of N-iodosuccinimide (NIS) and trifluoromethanesulfonic acid (TfOH) as thiophilic activator. Alpha linked glycosyl linkage was achieved using thioglycoside as donor in the presence of a combination of copper(II) bromide and tetra-n-butylammonium bromide (TBAB) in a mixture of 1,2-dichloroethane and dimethyl formamide (DMF). The yields of the glycosylation reactions were very good with satisfactory stereochemistry at the glycosyl linkages.