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)‐[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 p NAc‐(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 B Ac C 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)