Eur. J. Org. Chem. 2008: 5526–5542
Relying on trichloroacetimidate chemistry, six tetra- and pentasaccharide fragments of the {2)-[α-D-Glcp-(13)]-α-L-Rhap-(12)-α-L-Rhap-(13)-[Ac2]-α-L-Rhap-(13)-β-DGlcpNAc-(1}n ((E)ABAcCD)n polymer were synthesized as their propyl glycosides by use of a common fully protected (E)ABAcC intermediate (9). Tetrasaccharide 9 derived from the condensation of an EA donor and a BAcC acceptor. Partial and full deprotection gave free tetrasaccharides (E)ABAcC and (E)ABC, respectively. Alternatively, 9 was converted into a trichloroacetimidate donor, which provided linear pentasaccharides (E)ABAcCD and (E)ABCD, following a reaction with a D acceptor and subsequent partial or total deprotection, respectively. Additionally, the selective removal of the 2A-levulinoyl protecting group in 9 allowed for chain elongation at this position. The glycosylation of the resulting acceptor with a D donor, and subsequent partial or total deprotection, gave branched pentasaccharides D(E)ABAcC and D(E)ABC. 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.