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

TL;DR

Scientists synthesized a complex sugar unit from a gut bacterium and confirmed its structure using advanced chemical techniques.

Contribution

First total synthesis of a zwitterionic pentasaccharide from Bacteroides fragilis and confirmation of its stereochemistry.

Findings

A stereocontrolled [2 + 3] glycosylation strategy enabled synthesis of the PS A2 pentasaccharide.

NMR analysis confirmed β-ManNAc and β-Hep configurations align with native PS A2.

Arndt–Eistert homologation was used to introduce a 3-hydroxybutanoic acid onto the heptose sugar.

Abstract

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.

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