Enantio- and E-selective Carbonyl–Ene Cyclization of 5-Substituted 5-Hexenals Catalyzed by Tris(pentafluorophenyl)borane-Assisted Chiral Phosphoric Acids

The enantio- and E-selective carbonyl-ene cyclization (6-endo-trig) of 5-substituted 5-hexanals has been achieved using a new Lewis-acid-assisted chiral Brønsted acid (LBA), i.e., a tris(pentafluorophenyl)borane-chiral phosphoric acid complex. The high enantioselectivity and E-selectivity are induced through bicyclic transition states within the confined LBA via a proposed stepwise pathway including a covalent substrate-LBA intermediate.