Artificial carbohydrate antigens: the synthesis of the tetrasaccharide repeating unit of Shigella flexneri O antigen

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Abstract
The synthesis of a tetrasaccharide glycoside, 8-methoxycarbonyloctyl O-2-acetamido-2-deoxy-.beta.-D-glucopyranosyl-(1 .fwdarw. 2)-O-.alpha.-L-rhamnopyranosyl-(1 .fwdarw. 2)-O-.alpha.-L-rhamnopyranosyl-(1 .fwdarw. 3)-.alpha.-L-rhamnopyranoside, representing the repeating unit of the S. flexneri serogroup Y O antigen is reported, together with that of a component trisaccharide. Both oligosaccharide glycosides are functionalized to permit covalent attachment to immunogenic carriers and solid matrices. A key intermediate used to establish .alpha.-1,2-rhamnopyranoside linkages was 2-O-acetyl-3,4-di-O-benzyl-.alpha.-L-rhamnopyranosyl chloride, which was obtained in quantitative yield from 3,4-di-O-benzyl-1,2-O-(methoxyethylidene)-.beta.-L-rhamnopyranose. Thus 8-methoxycarbonyloctyl 3,4-di-O-benzyl-.alpha.-L-rhamnopyranoside prepared from the rhamnopyranosyl chloride was used in a 2nd Koenigs-Knorr reaction to provide an .alpha.-1,2-linked disaccharide. The disaccharide is in turn glycosylated at the C-2'' position with tri-O-acetyl-2-deoxy-2-phthalimido-D-glucopyranosyl bromide to provide the trisaccharide glycoside, 8-methoxycarbonyloctyl O-2-acetamido-2-deoxy-.beta.-D-glucopyranosyl-(1 .fwdarw. 2)-O-.alpha.-L-rhamnopyranosyl-(1 .fwdarw. 2)-.alpha.-L-rhamnopyranoside. Application of the same reaction sequence to a rhamnopyranoside selectively blocked with benzoate esters at C-2 and C-4 gave the S. flexneri tetrasaccharide repeating unit. The success of this strategy was dependent upon the ability to use selective transesterification conditions, which permit removal of O acetate groups while leaving benzoate esters intact.

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