Total Synthesis and Anomeric Configuration Revision of Zwitterionic Polysaccharide A2's Pentasaccharide Repeating Unit from Bacteroides fragilis

Zwitterionic polysaccharides (ZPSs) represent a distinctive class of bacterial glycans that elicit immune responses via a T-cell-dependent pathway, making them promising immunotherapeutic agents. Herein, we present the first total synthesis of a zwitterionic pentasaccharide repeating unit of the reported polysaccharide A2 (PS A2) structure from Bacteroides fragilis, accomplished through a stereocontrolled convergent [2 + 3] glycosylation strategy. The synthetic approach successfully addresses the formidable challenges posed by the complex architecture, which features two rare deoxyamino sugars 2-amino-4-acetamido-2,4,6-trideoxygalactose (AAT) and 3-acetamido-3,6-dideoxyglucose (ADG), and a unique 3-hydroxybutanoic acid-functionalized d-glycero-d-manno-heptose (d,d-Hep). A critical advancement involves the use of the Arndt-Eistert homologation reaction to selectively introduce the ether-linked 3-hydroxybutanoic acid onto the Hep moiety. Additionally, four oligosaccharide variants with distinct anomeric configurations of ManNAc and Hep residues were synthesized to revisit and confirm their stereochemical assignments. The stereoselective construction of challenging 1,2-cis-β-ManNAc and 1,2-cis-β-Hep linkages was achieved via Au-(I)-catalyzed glycosyl ortho-hexynylbenzoate glycosylation and B-(C₆F₅)₃-promoted glycosyl trichloroacetimidate glycosylation, respectively. Comparative NMR analysis revealed that β-ManNAc signals matched the reported data and β-Hep configuration aligned more closely with native PS A2.