Synthesis of trisaccharides related to the branching region of the O-antigen polysaccharides from Escherichia coli O35 and Salmonella arizona O62

The synthesis of three trisaccharides related to the branching region in th e O-antigen polysaccharides from E. coli O35 and S. arizona O62 is reported , A regioselective beta -glycosylation was performed at O-3 of a methyl alp ha -L-rhamnopyranoside derivative. cis-Glycosylation of an alpha -D-galacto samine derivative at O-2 of the methyl alpha -L-rhamnopyranoside took place in excellent yield (90%) to produce, after deprotection, compound 1, Funct ionalization at C-6 of the alpha -D-galactosamine residue by regioselective TEMPO oxidation resulted in the corresponding uronic acid, which subsequen tly gave 2. Treatment of an activated ester, derived from the uronic acid, with ammonia provided the desired carboxamide, and subsequently 3.