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

The enantio- and <i>E</i>-selective carbonyl-ene cyclization (<i>6-endo-trig</i>) 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 <i>E</i>-selectivity are induced through bicyclic transition states within the confined LBA via a proposed stepwise pathway including a covalent substrate-LBA intermediate.