Synthesis of chlorodeoxy trisaccharides related to the Shigella flexneri Y polysaccharide

Chloromethoxylation of di-O-acetyl-L-rhamnal has been employed as a convenient route to methyl 3,4-di-O-acetyl-2-chloro-2-deoxy-alpha-L-rhamnopyranoside 7, which was converted into an ethyl thioglycoside 10, suitable for use as a glycosyl donor. It and a disaccharide analogue 20 were effective donors in N-iodosuccinimide-triflic acid promoted glycosylations of a 2-acetamido-2-deoxy-D-glucopyranoside acceptor 22. Chlorodeoxy saccharides have previously been shown to be suitable derivatives to probe oligosaccharide-protein interactions, and this synthetic strategy provided three monochlorodeoxy trisaccharide congeners of the native Shigella flexneri epitope 2, alpha-L-Rhap-(1-->3)-alpha-L-Rhap-(1-->3)-beta-D-GlcNAcp+ ++-(1-->O)-Me.