title: Thermal cyclization of phenylallenes that contain ortho-1,3-dioxolan-2-yl groups: new cascade reactions initiated by 1,5-hydride shifts of acetalic H atoms.
abstract: A series of 2-(1,3-dioxolan-2-yl)phenylallenes that contained a range of substituents (alkyl, aryl, phosphinyl, alkoxycarbonyl, sulfonyl) at the cumulenic C3 position were prepared by using a diverse range of synthetic strategies and converted into their respective 1-(2-hydroxy)-ethoxy-2-substituted naphthalenes by smooth thermal activation in toluene solution. Electron-withdrawing groups at the C3 position accelerated these tandem processes, which consisted of 1) an initial hydride-like [1,5]-H shift of the acetalic H atom onto the central cumulene carbon atom; 2) a subsequent 6π-electrocyclic ring-closure of the resulting reactive ortho-xylylenes; and 3) a final aromatization step with concomitant ring-opening of the 1,3-dioxolane fragment. If the 1,3-dioxolane ring of the starting allenes was replaced by a dimethoxymethyl group, the reactions led to mixtures of two disubstituted naphthalenes, which were formed by the migration of either the acetalic H atom or the methoxy group, with the latter migration occurring to a lesser extent. Two of the final 1,2-disubstituted naphthalenes were converted into their corresponding naphtho-fused dioxaphosphepine or dioxepinone through an intramolecular transesterification reaction. A DFT computational study accounted for the beneficial influence of the 1,3-dioxolane fragment on the carbon atom from which the H-shift took place and also of the electron-withdrawing substituents on the allene terminus. Remarkably, in the processes that contained a sulfonyl substituent, the conrotatory 6π-electrocyclization step was of lower activation energy than the alternative disrotatory mode.
authors.affiliation: Departamento de Química Orgánica, Universidad de Murcia, Facultad de Química, Regional Campus of International Excellence, "Campus Mare Nostrum", Espinardo, 30100 Murcia (Spain). alajarin@um.es.
authors.forename: Mateo
authors.initials: M
authors.lastname: Alajarin
authors.forename: Baltasar
authors.initials: B
authors.lastname: Bonillo
authors.forename: Marta
authors.initials: M
authors.lastname: Marin-Luna
authors.forename: Pilar
authors.initials: P
authors.lastname: Sanchez-Andrada
authors.forename: Angel
authors.initials: A
authors.lastname: Vidal
keywords: allenes; cyclization; density functional calculations; naphthalenes; substituent effects