A PRACTICAL SYNTHESIS OF NOSAMINYL-BETA(1-]4)-GLUCOSYL-ALPHA(1-]2)-RHAMNOSE THE TRISACCHARIDE REPEATING UNIT OF A STREPTOCOCCUS-PNEUMONIAE CAPSULAR POLYSACCHARIDE

An efficient chemical synthesis is described for the title trisacchari de repeating unit of the capsular polysaccharide of Streptococcus pneu moniae Type 19F. The key intermediate was a well-accessible indirect b eta-D-mannosaminyl donor, i.e. oyloxyimino)-2-deoxy-alpha-D-arabino-he xopyranosyl bromide, which underwent glycosidations with 20:1 beta-sel ectivities; the benzoyloxyimino group was reduced with essential manno specificity. The ManNAc-beta(1-->4)-Glc disaccharide, thus obtained i n suitably blocked form, was subsequently converted into 1-fluoride, w ith which a L-rhamnosyl acceptor was glycosylated by an alpha(1-->2) l inkage to yield the target trisaccharide in an altogether 6% overall y ield for the 13 steps required from D-glucose.