Oligosaccharides corresponding to the antigenic determinants of the β-haemolytic Streptococci Group A. Part 2. Synthesis and 2D nuclear magnetic resonance analysis of a branched tetrasaccharide hapten

Abstract

The synthesis of a functionalised linear trisaccharide and its use in the synthesis of a branched tetrasaccharide corresponding to a portion of the cell-wall polysaccharide of the β-haemolytic Streptococci Group A is described. The acetate groups in the disaccharide, allyl 2-O-benzoyl-4-O-benzyl-3-O-(3′,4′,6′-tri-O-acetyl-2′-deoxy-2′-phthalimido-β-D-glucopyranosyl)-α-L-rhamnopyrano-side, were transesterified and the resulting free alcohol groups were protected using 2-(trimethylsilyl)-ethoxymethyl chloride. Deallylation, followed by treatment with N,N-dimethyl(chloromethylene)-ammonium chloride, afforded the disaccharide chloride. Reaction of this glycosyl donor with allyl 2,4-di-O-benzyl-α-L-rhamnopyranoside under Königs–Knorr conditions gave the aforementioned linear trisaccharide, allyl 3-O-(2′-O-benzoyl-4′-O-benzyl-3′-O-{2″-deoxy-2″-phthalimido-3″, 4″, 6″-tris-O-[2-(trimethylsilyl)ethoxymethyl]-β-D-glucopyranosyl}-α-L-rhamnopyranosyl)-2,4-di-O-benzyl-α-L-rhamnopyranoside. Selective removal of the 2′-O-benzoate yielded a functionalised trisaccharide for use in the synthesis of the branched tetrasaccharide. Reaction with the glycosyl donor 2,3,4-tri-O-acetyl-α-L-rhamnopyranosyl bromide under Königs–Knorr conditions then yielded the fully blocked tetrasaccharide. Transesterification, followed by hydrogenolysis, hydrazinolysis, and selective N-acetylation, afforded the pure tetrasaccharide, as its propyl glycoside, for use as a hapten in binding studies and NMR studies.