Application of an Enantioselective Benzoin Cyclization: Synthesis of (+)-Sappanone B

Significance

An efficient N-heterocyclic carbene catalyzed benzoin cyclization has been developed. Rovis’ precatalyst A has been modified by substituting the N-phenyl group with the electron-deficient 3,5-bis(trifluoromethyl)phenyl group to increase the acidity. This enables the use of a weaker base to generate the carbene species and therefore nearly suppresses the formation of undesired aldol product 3. In every case the benzoin cyclization yields were higher with modified precatalyst B than with the Rovis precatalyst A. The enantioselectivities are also higher in almost all cases.