title: Synthesis of bridged benzazocines and benzoxocines by a titanium-catalyzed double-reductive umpolung strategy.
abstract: A sequence of two titanium(III)-catalyzed reductive umpolung reactions is reported that allows the rapid construction of benzazo- and benzoxozine building blocks. The first step is a reductive cross-coupling of quinolones or chromones with Michael acceptors. This reaction proceeds with complete syn-selectivity for the quinolone functionalization while the anti-diastereomers are obtained as the major products from chromones. With different reaction conditions, the stereochemical outcome can be altered to afford the syn-chromanone products as well. A subsequent reductive ketyl radical cyclization forges the tricyclic title compounds in good yields. A stereochemical model explaining the observed stereoselectivities is provided and the product configurations were unambiguously verified by X-ray analyses and 2D NMR spectroscopic experiments.
authors.affiliation: Institut für Organische Chemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg (Germany), Fax: (+49) 761-203-8715.
authors.forename: Plamen
authors.initials: P
authors.lastname: Bichovski
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Haas
authors.forename: Daniel
authors.initials: D
authors.lastname: Kratzert
authors.forename: Jan
authors.initials: J
authors.lastname: Streuff
mesh_terms: D002384:Catalysis; D002867:Chromones / Q000737:chemistry; D018360:Crystallography, X-Ray; D003500:Cyclization; D008968:Molecular Conformation; D035964:Oxocins / Q000138:chemical synthesis / Q000737:chemistry*; D013237:Stereoisomerism; D014025:Titanium / Q000737:chemistry*
chemical_list: D002867:Chromones; D035964:Oxocins; D014025:Titanium
keywords: catalysis; cyclization; electron transfer; titanium; umpolung