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.