Stereospecific β‐ l ‐Rhamnopyranosylation through an S N i‐Type Mechanism by Using Organoboron Reagents

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 ‐Rha p ) with complete stereoselectivity in moderate to high yields without any further additives under mild conditions. Mechanistic studies of the borinic ester mediated glycosylation using 13 C 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, α‐ l ‐Rha p ‐(1,2)‐β‐ l ‐Rha p ‐(1,4)‐Glc p , derived from Streptococcus pneumoniae serotypes 7B, 7C, and 7D.