Synthesis of Two Tetra- and Four Pentasaccharide Fragments of Shigella flexneri Serotypes 3a and X O-Antigens from a Common Tetrasaccharide Intermediate

Relying on trichloroacetimidate chemistry, six tetra- and pentasaccharide fragments of the {2)-[alpha-D-Glcp-(1 -> 3)]-(alpha-L-Rhap-(1 -> 2)-alpha-L-Rhap-(1 -> 3)-[Ac -> 2]-alpha-L-Rhap-(1 -> 3)-beta-D-GlcpNAc-(1 ->}(n) ((E)AB(Ac)CD)(n) polymer were synthesized as their propyl glycosides by use of a common fully protected (E)AB(Ac)C intermediate (9). Tetrasaccharide 9 derived from the condensation of an EA donor and a BAcC acceptor. Partial and full deprotection gave free tetrasaccharides (E)AB(Ac)C and (E)ABC, respectively. Alternatively, 9 was converted into a trichloroacetimidate donor, which provided linear pentasaccharides (E)AB(Ac)CD and (E)ABCD, following a reaction with a D acceptor and subsequent partial or total deprotection, respectively. Additionally, the selective removal of the 2(A)-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)AB(Ac)C 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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)