Straightforward sequential and one-pot synthesis of a pentasaccharide repeating unit corresponding to the cell wall O-antigen of Shigella boydii type 18

Synthesis of a pentasaccharide repeating unit corresponding to the cell of wall O-antigen Shigella boydii type 18 has been achieved by sequential as well as iterative glycosylations in one-pot. Use of p-methoxybenzyl group (PMB) as an in situ removable protecting group allowed obtaining the desired pentasaccharide derivative in a generalized glycosylation condition and in one-pot condition. Synthesis of a beta-L-rhamnosidic linkage present in the molecule has been successfully achieved using l-rhamnosyl thioglycoside donor having a picoloyl group at remote C-3 position influencing beta selectivity in the glycosylation. A combination of N-iodosuccinimide (NIS) and perchloric acid supported over silica (HClO₄–SiO₂) has been used as thiophilic glycosylation promoter in all glycosylation reactions. TEMPO mediated selective oxidation of the primary hydroxyl group has been carried out at the late stage of the synthetic strategy.