Total synthesis of zwitterionic PS A2 oligosaccharides from Bacteroides fragilis

Zwitterionic polysaccharides have unique structures and can elicit T cell-dependent immune responses. To unravel their molecular mode of action and develop potential alternatives for carrier proteins in carbohydrate-based vaccines, we here describe a total synthesis of zwitterionic PS A2 oligosaccharides from Bacteroides fragilis. The pentasaccharide and decasaccharide bearing one or two repeating units were efficiently synthesized via convergent glycosylation strategy. The ingenious protecting groups have enabled the stereoselectivity of glycosylation. A unique synthetic approach was developed to introduce the chiral 3-hydroxybutanoic acid side chain on mannoheptose, which proved very challenging to form in this skeleton.