Concise Synthesis of the Hexasaccharide Repeating Unit of the Cell Wall O-Antigen of Enterotoxigenic Escherichia coli O140 Strain

A straightforward synthetic strategy has been developed for the synthesis of the hexasaccharide repeating unit of the cell wall O-antigen of Escherichia coli O140 strain in good yield. Several stereoselective 1,2-cis and 1,2-trans glycosylations have been carried out exploiting the presence of the functional group in the glycosyl donor and selection of reaction solvents. A late-stage oxidation of the primary hydroxyl group to the carboxylic functionality has been carried out by the TEMPO-mediated oxidation in a phase transfer condition. The synthesis of the hexasaccharide was carried out by [2+2+1+1] glycosylations.