β-Selective O-rhamnosylation with a rhamnosyl trichloroacetimidate that has the 4C1 conformation

A β-selective rhamnosylation reaction was accomplished by using 2-O-benzyl-3-O-tert-butyldimethylsilyl-4-O-tert-butyldiphenylsilyl-α-l-rhamnopyranosyl trichloroacetimidate and a catalytic amount of 9-borabicylco[3.3.1]nonyl trifluoromethanesulfonate. The rhamnosyl donor has the 4C1 ring conformation to change the general high α-selectivity of the rhamnosylation reactions.