Artificial carbohydrate antigens: synthesis of rhamnose trisaccharide and disaccharide haptens common to Shigella flexneri O-antigens

Abstract

The rhamnose di- and tri-saccharide glycosides, α-L-Rham(p)-(1 → 3)-α-L-Rham(p) and α-L-Rham(p)-(1 → 2)-α-L-Rham(p)-(1 → 3)-α-L-Rham(p), representing portions of the Shigella flexneri serogroup Y lipopolysaccharide repeating unit, have been synthesised in high yield by a series of Königs–Knorr reactions. The synthetic methods used are designed to provide di- and tri-saccharide artificial antigens, after complete deblocking and covalent attachment to protein. 8-Methoxycarbonyloctyl α-L-rhamnopyranoside was converted into the corresponding 2,4-di-O-benzoate via a 2,3-orthobenzoate. Königs–Knorr reaction between this partially blocked rhamnoside and either 2,3,4-tri-O-acetyl-α-L-rhamnopyranosyl bromide or 2-O-acetyl-3,4-di-O-benzyl-α-L-rhamnopyranosyl chloride gave respectively the fully blocked disaccharide and the disaccharide precursor of the trisaccharide. Selective transesterification of the 2′-O-acetate group in the presence of 2,4-di-O-benzoates provides the disaccharide selectively deblocked at the C-2′ position. Reaction with 2,3,4-tri-O-acetyl-α-L-rhamnopyranosyl bromide gave the fully blocked trisaccharide. Removal of the blocking groups yielded the trisaccharide hapten. The ¹³C and ¹H n.m.r. spectra together with the T₁ parameters for the haptens are consistent with a conformation about the aglyconic-oxygen-to-aglyconic-carbon bond (torsional angle ψ) that approaches the eclipsed rather than the staggered arrangement.