Synthesis of branched tri- to pentasaccharides representative of fragments of Shigella flexneri serotypes 3a and/or X O-antigens

Fragments of the {2)-[α- d-Glc p-(1→3)]-α- l-Rha p-(1→2)-α- l-Rha p-(1→3)-[Ac→2]-α- l-Rha p-(1→3)-β- d-Glc pNAc-(1→} n ( (E)AB AcCD ) n polymer were synthesized. D(E)A, CD(E)A, AcCD(E)A were obtained according to a linear strategy, whereas BCD(E)A and B AcCD(E)A were derived from the condensation of appropriate BC and D(E)A building blocks. Oligosaccharides were synthesized as their propyl glycoside, relying on (i) the efficient trichloroacetimidate chemistry, (ii) a common EA allyl glycoside, and (iii) a 2 -trichloroacetamido- d-glucopyranose precursor to residue D. Final Pd/C-mediated deprotection, run under a high pressure of hydrogen, ensured O-acetyl stability. All targets are parts of the O-antigen of Shigella flexneri 3a, a prevalent serotype. Non- O-acetylated oligosaccharides are shared by the S. flexneri serotype X O-antigen. [Display omitted]