Synthetic Routes toward Acidic Pentasaccharide Related to the O-Antigen of E. coli 120 Using One-Pot Sequential Glycosylation Reactions

Concise syntheses of the acidic pentasaccharide, related to the O-antigenic polysaccharide of Escherichia coli 120, as its p-methoxyphenyl glycoside, have been achieved using a one-pot sequential glycosylation technique. The glycosylations have been accomplished either by the activation of the thioglycosides using NIS in the presence of FeCl3 or by a preactivation by Ph2SO, TTBP, Tf2O, and the activation of the trichloroacetimidates using FeCl3 alone or TMSOTf. Most of the intermediate steps are high yielding, and the stereo outcomes of the glycosylation steps were excellent. The syntheses of the targeted pentasaccharide have been performed with both three- and four-component, one-pot sequential glycosylation reactions, and in both cases, the orthogonal glycosylations are carried out utilizing catalytic activity of FeCl3. A late-stage TEMPO-mediated regioselective oxidation has been performed to achieve the required uronic acid motif.