Total Synthesis and Structural Studies of Zwitterionic Bacteroides fragilis Polysaccharide A1 Fragments

Total synthesis of a structurally complex zwitterionic hexasaccharide repeating unit of polysaccharide B from Bacteroides fragilis via one-pot glycosylation

Zwitterionic polysaccharides (ZPSs) present on the surface of a common gut commensal Bacteroides fragilis are endowed with unique immunological properties as they can directly bind to T-cells in the absence of protein conjugation. ZPSs are therefore considered to be potential antigens for the development of totally carbohydrate-based vaccines. Herein, we disclose the first total synthesis of a highly branched phosphorylated zwitterionic capsular polysaccharide repeating unit of Bacteroides fragilis. The hexasaccharide repeating unit bearing six different monosaccharides comprises three 1,2-cis-glycosidic linkages, a challenging 1,2-trans linkage in D-QuipNAc-β-(1→4)-D-Gal motif, and a 2-aminoethyl phosphonate appendage. The synthesis of target ZPS was accomplished utilizing an expeditious, highly stereoselective and convergent (1 + 2 + 2 + 1) one-pot glycosylation strategy. The striking features include efficient synthesis of rare deoxy amino sugars D- and L-quinovosamine, stereoselective installation of three 1,2-cis glycosidic linkages, glycosylation of D-quinovosamine donor with a sterically crowded, poorly reactive 4-OH galactose moiety, as well as late stage phosphorylation.

Dissecting the Conformational Stability of a Glycan Hairpin

Systematic structural studies of model oligopeptides revealed important aspects of protein folding and offered design principles to access non-natural materials. In the same way, the rules that regulate glycan folding could be established by studying synthetic oligosaccharide models. However, their analysis is often limited due to the synthetic and analytical complexity. By utilizing a glycan capable of spontaneously folding into a hairpin conformation as a model system, we investigated the factors that contribute to its conformational stability in aqueous solution. The modular design of the hairpin model featured a trisaccharide turn unit and two β-1,4-oligoglucoside stacking strands that allowed for systematic chemical modifications of the glycan sequence, including the introduction of NMR labels and staples. Nuclear magnetic resonance assisted by molecular dynamics simulations revealed that stereoelectronic effects and multiple glycan-glycan interactions are the major determinants of folding stabilization. Chemical modifications in the glycan primary sequence (e.g., strand elongation) can be employed to fine-tune the rigidity of structural motifs distant from the modification sites. These results could inspire the design of other glycan architectures, with implications in glycobiology and material sciences.

Total Synthesis of a Structurally Complex Tetrasaccharide Repeating Unit of Vibrio cholerae O43

Herein we report the first total synthesis of a densely functionalized tetrasaccharide repeating unit of Vibrio cholerae O43, which contains rare deoxy amino sugars d-quinovosamine and d-viosamine attached with the rare amino acid N-acetyl-l-allothreonine. Synthesis of orthogonally protected rare sugars and unnatural amino acid building blocks, stereoselective construction of three consecutive 1,2-cis glycosidic linkages, amide coupling, and the presence of five nitrogen atoms dispersed over four sugar units as well as the carboxylic acid functionality make the total synthesis a formidable task.