Synthesis and NMR study of a linear pentasaccharide fragment of the Shigella flexneri 5a O-specific polysaccharide

A convergent chemical synthesis of the methyl glycoside of the linear epitope α-d-Glcp-(1→3)-α-l-Rhap-(1→3)-α-l-Rhap-(1→3)-β-d-GlcNAcp-(1→2)-α-l-Rhap (EBCDA) corresponding to the ramification of the O-antigen of Shigella flexneri serotype 5a is described. The strategy relies on the preparation of a key EB trichloroacetimidate donor and that of an appropriate CDA trisaccharide acceptor. Trichloroacetimidate chemistry was used for the construction of all glycosidic linkages except that of DA, where a bromide donor was preferred. In depth analysis of the pentasaccharide EBCDA 1H and 13C NMR spectra shows that its conformation approaches that of the corresponding fragment in the native polysaccharide.