Synthesis of<i>β</i>-D-Ribofuranosyl-(1→3)-<i>α</i>-L-rhamnopyranosyl-(1→3)-L-rhamnopyranose by in situ Activating Glycosylation Using 1-OH Sugar Derivative and Me<sub>3</sub>SiBr–CoBr<sub>2</sub>–Bu<sub>4</sub>NBr–Molecular Sieves 4A System
β-d-Ribofuranosyl-(1→3)-α-l-rhamnopyranosyl-(1→3)-l-rhamnopyranose, the trisaccharide repeating unit of the C. freundii O28,1c O-specific polysaccharide, was synthesized using in situ activating glycosylation of the 1-OH sugar derivatives and a reagent mixture of trimethylsilyl bromide, cobalt(II) bromide, tetrabutylammonium bromide, and molecular sieves 4A. Regioselective tritylation was useful for synthesizing the 3-OH derivatives of methyl, allyl, and benzyl α-l-rhamnosides.
Novel and convenient dehydrative glycosidations of 1-hydroxy glycosyl donors have been developed. Several O-benzylated 1-hydroxy sugars are effectively cross-coupled with a variety of alcohols to give the corresponding O-glycosides in high yields with good α-stereoselectivities by using the heteropoly acid, H4SiW12O40.