Enantio- and Diastereoselective Carbonyl-Ene Cyclization–Acetalization Tandem Reaction Catalyzed by Tris(pentafluorophenyl)borane-Assisted Chiral Phosphoric Acids

Highly enantio- and diastereoselective carbonyl-ene cyclization was developed with the use of chiral Lewis acid-assisted chiral Brønsted acid (LBA) catalysts, which were prepared in situ from both sterically demanding tris(pentafluorophenyl)borane and chiral phosphoric acids. Along with the performance of standard carbonyl-ene cyclizations, carbonyl-ene cyclization–acetalization tandem reactions with the use of additional aldehydes were demonstrated with high enantio- and diastereoselectivities. On the basis of mechanistic examinations, a stepwise reaction pathway via tertiary carbocation intermediates involving possible transition states is proposed.