Brønsted Acid Catalyzed, Conjugate Addition of β‐Dicarbonyls to In Situ Generated ortho ‐Quinone Methides—Enantioselective Synthesis of 4‐Aryl‐4 H ‐Chromenes

We describe herein a catalytic, enantioselective process for the synthesis of 4 H ‐chromenes which are important structural elements of many natural products and biologically active compounds. A sequence comprising a conjugate addition of β‐diketones to in situ generated ortho ‐quinone methides followed by a cyclodehydration reaction furnished 4‐aryl‐4 H ‐chromenes in generally excellent yields and high optical purity. A BINOL‐based chiral phosphoric acid was employed as a Brønsted acid catalyst which converted ortho ‐hydroxy benzhydryl alcohols into hydrogen‐bonded ortho ‐quinone methides and effected the carbon–carbon bond‐forming event with high enantioselectivity.