Stereospecific beta-L-Rhamnopyranosylation through an S(N)i-Type Mechanism by Using Organoboron Reagents

Stereospecific beta-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 beta-L-rhamnopyranosides (beta-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 C-13 kinetic isotope effect (KIE) measurements and DFT calculations were consistent with a concerted S(N)i mechanism with an exploded transition state. In addition, the present glycosylation method was applied successfully to the synthesis of a trisaccharide, alpha-L-Rhap-(1,2)-beta-L-Rhap-(1,4)-Glcp, derived from Streptococcus pneumoniae serotypes 7B, 7C, and 7D.