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A Practical Synthesis of Mannosaminyl-β(1→4)-glucosyl-α(1→2)-rhamnose, the Trisaccharide Repeating Unit of a Streptococcus pneumoniae Capsular Polysaccharide

An efficient chemical synthesis is described for the title trisaccharide repeating unit of the capsular polysaccharide of Streptococcus pneumoniae Type 19F. The key intermediate was a well-accessible indirect β-D-mannosaminyl donor, i.e. 2-(benzoyloxyimino)-2-deoxy-α-D-arabino-hexopyranosyl bromide, which underwent glycosidations with 20 : 1 β-selectivities; the benzoyloxyimino group was reduced with essential manno specificity. The ManNAc-β(1→4)-Glc disaccharide, thus obtained in suitably blocked form, was subsequently converted into 1-fluoride, with which a L-rhamnosyl acceptor was glycosylated by an α(1→2) linkage to yield the target trisaccharide in an altogether 6% overall yield for the 13 steps required from D-glucose.