A convergent strategy has been developed for the synthesis of the tetrasaccharide repeating unit of the O-antigen from Edwardsiella tarda PCM 1156. Sequential glycosylations of a series of rationally protected monosaccharide intermediates were achieved either by the activation of thioglycosides using N-iodosuccinimide (NIS) in conjunction with H2SO4-silica or by activation of trichloroacetimidate by H2SO4-silica only. All glycosylation reactions resulted in the formation of the desired linkage with absolute stereoselectivity and yielded the required derivatives in good to excellent yields. Both azido and phthalimido groups have been used as the precursor of the desired acetamido group depending on the requirement of 1,2-cis- or 1,2-trans-glycosidic linkage.
Keywords: Acetamido sugars; Bacterial O-antigen; H(2)SO(4)–silica.