Enantioselective Polyene Cyclization Catalyzed by a Chiral Brønsted Acid.

The first enantioselective polyene cyclization initiated by a BINOL-derived chiral N-phosphoramide (NPA) catalyzed protonation of an imine is described. The ion-pair formed between the iminium ion and chiral counter anion of the NPA plays an important role for controlling the stereochemistry of the overall transformation. This strategy offers a highly efficient approach to fused tricyclic frameworks containing three contiguous stereocenters, which are widely found in natural products. In addition, the first catalytic asymmetric total synthesis of (-)-ferruginol was accomplished with an NPA catalyzed enantioselective polyene cyclization, as the key step for the construction of the tricyclic core, with excellent yield and enantioselectivity.