Recyclable fluorous-tag assisted two-directional oligosaccharide synthesis enabled by interrupted Pummerer reaction mediated glycosylation

Herein, we report a novel fluorous-tag assisted two-directional oligosaccharide assembly strategy, which maintained the high coupling efficiency of solution-phase synthesis and featured the advantage of an easy purification process comparable to solid-phase synthesis. A well-designed fluorous tag was decorated on the latent anomeric leaving group in interrupted Pummerer reaction mediated (IPRm) glycosylation. The high efficiency of the in-solution phase glycosylation and the unique affinity of the fluorous tag towards polytetrafluoroethylene (PTFE) particles allowed flexible assembly from the reducing end to the non-reducing end and fast purification by PTFE-assisted filtration. Moreover, the fluorous-tagged latent anomeric leaving group could be activated by oxidation and cleaved by IPRm glycosylation, thus enabling the elongation of the carbohydrate chain from the non-reducing end to the reducing end as well as the recovery of the fluorous tag. The present two-directional synthetic strategy is used to assemble the repeating unit of Streptococcus pneumoniae type 14 capsular polysaccharide.