Synthesis of beta-D-ribofuranosyl-(1 -> 3)-alpha-L-rhamnopyranosyl-(1 -> 3)-L-rhamnopyranose by in situ activating glycosylation using 1-OH sugar derivative and Me3SiBr-CoBr2-Bu4NBr-molecular sieves 4A system

beta -D-Ribofuranosyl-(1 -->3)-alpha -L-rhamnopyranosyl-(1 -->3)-L-rhamnopy ranose, the trisaccharide repeating unit of the C.freundii O28, 1c O-specif ic polysaccharide, was synthesized using in situ activating glycosylation o f the 1-OH sugar derivatives and a reagent mixture of trimethylsilyl bromid e, cobalt(II) bromide, tetrabutylammonium bromide, and molecular sieves 4A. Regioselective tritylation was useful for synthesizing the 3-OH derivative s of methyl, allyl, and benzyl alpha -L-rhamnosides.