Regioselective benzoylation of unprotected β-glycopyranosides with benzoyl cyanide and an amine catalyst - application to saponin synthesis

Protection of totally unprotected beta-d-gluco-, beta-d-quinovo- and beta-d-xylopyranosides with BzCN and Et3N as the catalyst afforded directly and regioselectively 3,6-di-O-benzoylated beta-d-glucopyranosides or 3-O-benzoylated beta-d-quinovo- and beta-d-xylopyranosides, respectively. Furthermore, these trans-trans triol and tetrol systems could be regioselectively transformed into their corresponding 2-O-unprotected derivatives in the presence of BzCN as the benzoylating agent and a catalytic amount of 4-pyrrolidinopyridine as a base in one-pot reactions. Hence, BzCN in the presence of a catalytic amount of an amine base exhibits unique properties as an acylating agent, permitting the regioselective protection of all diol, triol and tetrol stereostructures occurring in the commonly found glycopyranosides. The convenient access and great value of some of the derived building blocks are shown through the concise synthesis of natural and unnatural saponins.