Convergent chemical synthesis of the pentasaccharide repeating unit of the O-antigen from E. coli O158

Abstract

Synthesis of the pentasaccharide repeating unit of the O-antigen from E. coli O158 through a convergent [3 + 2] strategy is reported. Synthesis of the crucial β-ManNAc moiety was achieved from a β-Glc derivative through inversion of the 2-position by an azide nucleophile in excellent yield. The non-reducing end disaccharide α-D-Gal-(1→3)-β-D-GlcNAc was formed through activation of the thioglycoside Gal donor in a chemoselective manner. A late stage TEMPO-mediated oxidation was used to install the desired uronic acid moiety. The required glycosylations were accomplished through activation of thioglycosides using NIS in the presence of H₂SO₄–silica with good to excellent yields and stereoselectivity.