A Facile Synthesis of the Tetrasaccharide Repeating Unit of the O-Antigen fromCronobacter turicensis

GRAPHICAL ABSTRACTThe para-methoxyphenyl glycoside of a tetrasaccharide repeating unit, β-D-GlcNAc-(1→3)-β-D-GlcNAc-(1→3)-[α-D-Glc-(1→2)]-α-L-Rha-O-(C6H4-p-OMe), of the polysaccharide O-antigen from Cronobacter turicensis was synthesized by an efficiently and linear assembly strategy. Consecutive glycosylation of the 3-O- and 2-O-positions of an L-rhamnopyranoside derivative with free 2,3-hydroxyl groups using a D-glucosaminyl and a D-glucosyl donor, respectively, accomplished a branched trisaccharide rapidly. Selective 3′-O-deacetylation of the trisaccharide followed by glucosaminylation afforded the fully protected tetrasaccharide. Multistep protecting group manipulations of the fully protected tetrasaccharide eventually completed the synthesis of the target molecule.