Concise synthesis of di- and trisaccharides related to the O-antigens from Shigella flexneri serotypes 6 and 6a, based on late stage mono-O-acetylation and/or site-selective oxidation

Introducing Oxo-Phenylacetyl (OPAc) as a Protecting Group for Carbohydrates

A series of oxo-phenylacetyl (OPAc)-protected saccharides, with divergent base sensitivity profiles against benzoyl (Bz) and acetyl (Ac) were synthesized, and KHSO₅/AcCl in methanol was identified as an easy, mild, selective, and efficient deprotecting reagent for their removal in the perspective of carbohydrate synthesis. Timely monitoring of AcCl reagent was supportive in both sequential and simultaneous deprotecting of OPAc, Bz, and Ac. The salient feature of our method is the orthogonal stability against different groups, its ease to generate different valuable acceptors using designed monosaccharides, and use of OPAc as a glycosyl donar.

Detailed Investigation of the Immunodominant Role of O-Antigen Stoichiometric O-Acetylation as Revealed by Chemical Synthesis, Immunochemistry, Solution Conformation and STD-NMR Spectroscopy for Shigella flexneri 3a

Shigella flexneri 3a causes bacillary dysentery. Its O-antigen has the {2)-[α-d-Glcp-(1→3)]-α-l-Rhap-(1→2)-α-l-Rhap-(1→3)-[Ac→2]-α-l-Rhap-(1→3)-[Ac→6]≈40 % -β-d-GlcpNAc-(1→} ([(E)ABAc CAc D]) repeating unit, and the non-O-acetylated equivalent defines S. flexneri X. Propyl hepta-, octa-, and decasaccharides sharing the (E')A'BAc CD(E)A sequence, and their non-O-acetylated analogues were synthesized from a fully protected BAc CD(E)A allyl glycoside. The stepwise introduction of orthogonally protected mono- and disaccharide imidate donors was followed by a two-step deprotection process. Monoclonal antibody binding to twenty-six S. flexneri types 3a and X di- to decasaccharides was studied by an inhibition enzyme-linked immunosorbent assay (ELISA) and STD-NMR spectroscopy. Epitope mapping revealed that the 2C -acetate dominated the recognition by monoclonal IgG and IgM antibodies and that the BAc CD segment was essential for binding. The glucosyl side chain contributed to a lesser extent, albeit increasingly with the chain length. Moreover, tr-NOESY analysis also showed interaction but did not reveal any meaningful conformational change upon antibody binding.

Synthesis of the zwitterionic repeating unit of the O-antigen from Shigella sonnei and chain elongation at both ends

Shigella sonnei O-antigen features a zwitterionic disaccharide repeat encompassing two rare monosaccharides. The synthesis of the AB repeat and of trisaccharides ABA' and B'AB, which validates chain elongation at either end, is reported. All targets were synthesized using a postglycosylation oxidation strategy in combination with imidate chemistry. Precursors to residue A were obtained from L-glucose. The AAT (B) donor and acceptor were obtained from D-glucosamine. A one-step Pd(OH)2/C-mediated deprotection provided the propyl glycoside targets.

Linear synthesis of the branched pentasaccharide repeats of O-antigens from Shigella flexneri 1a and 1b demonstrating the major steric hindrance associated with type-specific glucosylation

Shigella flexneri serotypes 1b and 1a are Gram-negative enteroinvasive bacteria causing shigellosis in humans.