Synthesis of the β-L-Rhamnose-containing Conjugation-ready Pentasaccharide Repeating Unit Corresponding to the Cell Wall O-Antigen of Shigella boydii Type 4

A straightforward synthetic strategy has been developed for the synthesis of the acidic pentasaccharide corresponding to the cell wall O-antigen of Shigella boydii type 4 with an aminoethyl linker at the reducing end providing access for conjugation with a suitable aglycone. The synthesis of the desired pentasaccharide involves stereoselective construction of the β-L-rhamnosidic linkage using picoloyl group containing L-rhamnose thioglycoside donor by the activation with a combination of N-iodosuccinimide (NIS) and triflic acid (TfOH). Stereoselective glycosylation as well as removal of the p-methoxybenzyl (PMB) group in situ was achieved in the same pot. The D-glucuronic acid has been incorporated in the pentasaccharide by stereoselective glycosylation of the D-glucosyl moiety and late-stage TEMPO mediated oxidation of the primary hydroxyl group. The glycosylation steps furnished satisfactory yields of the products.