[Synthesis of beta-L-rhamnoside linked oligosaccharides of lipopolysaccharides from Shigella flexneri serotype 6]

Abstract

The synthesis of the trisaccharide O-beta-L-rhamnopyranosyl-(1 leads to 4)-O-beta-L-rhamnopyranosyl-(1 leads to 2)-L-rhamnopyranose (14) and the tetrasaccharide O-2-acetamido-2-deoxy-beta-D-galactopyranosyl)-(1 leads to 2)-O-[beta-L-rhamnopyranosyl-(1 leads to 4)]-O-beta-L-rhamnopyranosyl-(1 leads to 2)-L-rhamnopyranose (21) is described. The latter structure has been proposed as the repeating unit of the O-specific side-chain of the lipopolysaccharide obtained from Shigella flexneri Serotype 6. The key-intermediate was 4-O-acetyl-2-O-allyl-3-O-benzyl-alpha-D-rhamnopyranosyl bromide, which was first linked to benzyl 3,4-di-O-benzyl-alpha-L-rhamnopyranoside, to give a blocked beta-linked disaccharide. This was O-deacetylated and coupled with 2,3,4-tri-O-benzyl-alpha-L-rhamnopyranosyl bromide at O-4' to afford benzyl O-(2,3,4-tri-O-benzyl-beta-L-rhamnopyranosyl)-(1 leads to 4)-O-(2-O-allyl-3-O-benzyl-beta-L-rhamnopyranosyl)-(1 leads to 2)-3,4-di-O-benzyl-alpha-L-rhamnopyranoside (11), which was deprotected to give 14. Deallylation of 11 and coupling with 6-O-acetyl-2-azido-3,4-di-O-benzyl-2-deoxy-alpha-D-galactopyranosyl bromide led to a protected tetrasaccharide from which 21 was obtained. The method of catalysis by silver silicate was employed to obtain the beta-glycosidic linkage of all monosaccharide units.