Convergent synthesis of a pentasaccharide corresponding to the cell wall O-polysaccharide of enteropathogenic Escherichia coli O115

A pentasaccharide corresponding to the cell wall O-polysaccharide of the enteropathogenic Escherichia coli O115 has been synthesized using a convergent block glycosylation strategy in satisfactory yield. The synthetic strategy involves several stereoselective glycosylation steps, in which a remote picolinoyl group at C-3 mediated hydrogen bond acceptor aglycon delivery approach for the beta-linked L-rhamnosylation is worth mentioning. An orthogonal glycosylation approach has also been adopted for the preparation of a thioglycoside disaccharide derivative, which was used in the block synthesis. Perchloric acid supported over silica (HClO4-SiO2) has been used as a solid acid catalyst for the activation of glycosyl trichloroacetimidate derivative as well as in the thioglycoside activations. The n-galacturonic acid moiety in the molecules was incorporated by linking a o-galactose unit in appropriate glycosyl linkage followed by a late stage TEMPO mediated regioselective oxidation of the primary hydroxyl group using diacetoxyiodo benzene (DAIB). All intermediate glycosylations were high yielding with satisfactory stereo outcome. (C) 2020 Elsevier Ltd. All rights reserved.