Stereospecific β-l-rhamnopyranosylations were conducted using a 1,2-anhydro-l-rhamnopyranose donor and mono-ol or diol acceptors in the presence of a glycosyl-acceptor-derived borinic or boronic ester. Reactions proceeded smoothly to provide the corresponding β-l-rhamnopyranosides (β-l-Rhap) with complete stereoselectivity in moderate to high yields without any further additives under mild conditions. Mechanistic studies of the borinic ester mediated glycosylation using 13C kinetic isotope effect (KIE) measurements and DFT calculations were consistent with a concerted SNi mechanism with an exploded transition state. In addition, the present glycosylation method was applied successfully to the synthesis of a trisaccharide, α-l-Rhap-(1,2)-β-l-Rhap-(1,4)-Glcp, derived from Streptococcus pneumoniae serotypes 7B, 7C, and 7D.
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