Regioselective One-Pot Benzoylation of Triol and Tetraol Arrays in Carbohydrates.

Protection of 2,3,4- O-unprotected α-galacto- and α-fucopyranosides with BzCN and DMAP/DIPEA as the base afforded directly and regioselectively the 3- O-unprotected derivatives. The rationale for these studies was to take advantage of the eventual cooperativity of the "cyanide effect" and "the alkoxy group mediated diol effect". This way, even the totally unprotected α-galactopyranosides could be regioselectively transformed into the corresponding 2,4,6- O-protected derivatives. The great utility of these building blocks was demonstrated in efficient trisaccharide syntheses.