title: Brønsted acid catalyzed, conjugate addition of β-dicarbonyls to in situ generated ortho-quinone methides--enantioselective synthesis of 4-aryl-4H-chromenes.
abstract: We describe herein a catalytic, enantioselective process for the synthesis of 4H-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-4H-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.
authors.affiliation: Institut für Organische Chemie, Universität Leipzig, Johannisallee 29, 04103 Leipzig (Germany); Permanent Address: Faculty of Chemistry, Jagiellonian University Krakow, Ingardena 3, 30060 Krakow (Poland).
authors.forename: Osama
authors.initials: O
authors.lastname: El-Sepelgy
authors.forename: Stefan
authors.initials: S
authors.lastname: Haseloff
authors.forename: Santosh Kumar
authors.initials: SK
authors.lastname: Alamsetti
authors.forename: Christoph
authors.initials: C
authors.lastname: Schneider
keywords: asymmetric synthesis; benzhydrylic alcohols; chiral phosphoric acids; chromenes; xanthenones