Convergent Synthesis of the Octasaccharide Repeating Unit of the K55 Capsular Polysaccharide of Acinetobacter baumannii BAL_204 Strain

Synthesis of the octasaccharide repeating unit of the K55 capsular polysaccharide of Acinetobacter baumannii BAL_204 strain has been achieved in very good yield using a convergent [5+3] block glycosylation strategy. The pentasaccharide and trisaccharide components were synthesized using sequential stereoselective glycosylations. The p- methoxybenzyl (PMB) group was used as temporary alkyl protecting group, which was removed under the thiophilic glycosylation condition by raising the temperature. A late-stage TEMPO-mediated selective oxidation of primary hydroxyl group into carboxylic acid allowed getting the D-glucuronic acid moiety in the octasaccharide. A combination of N-iodosuccinimide (NIS) and perchloric acid supported over silica (HClO4-SiO2) was used as a thiophilic promoter for the activation of thioglycosides. HClO4-SiO2 was also used as a solid acid activator for glycosyl trichloroacetimidate derivative.