抄録
The asymmetric cationic cyclization reaction is the key reaction in the biosynthesis of the natural products such as terpenoids and steroids, and recognized as one of the important research issues in synthetic organic chemistry. During the course of our recent studies regarding the reactions of an α-silyl cation, which is presumed to be a short-lived reactive species, we found that an acidic treatment of an α-alkenyl-α-hydroxysilane afforded the intramolecular Friedel-Crafts type cyclization product via a generation of the putative a-silyl cation. In this report, our recent efforts toward the chirality transferring Friedel-Crafts cyclization of an optically active α-alkenyl-α-hydroxysilane and its silyl ether are described. The treatment of the optically active α-alkenyl-α-hydroxysilane 11a with TMSOTf (1 equiv) underwent the cyclization reaction at the γ carbon attached to the silicon in a highly regioselective manner to give the vinylsilane-containing cyclized product 12. Despite the cationic reaction conditions, the chirality of the starting lla was significantly transferred to the product 12. The corresponding tent-butyl substituted analog 14 underwent the cyclization reaction to give the racemic product 15. These results indicated that the silyl group played a crucial role in the chirality transfer of the present cyclization. After several attempts, the use of a catalytic amount of TMSOTf or TfOH was found to be quite effective for the chirality transfer to produce the cyclized product 12 with a small loss of their starting optical purity. The catalytic reaction pathway of the present intramolecular Friedel-Crafts cyclization is underway.
