Enyne [4+4] Cycloaddition/Oxidation: Ring Contraction via Cyclopropanones and Their Anionic Ring‐Opening Reactions

Abstract

Irradiation of a 1,3-enyne tethered to a 2-pyridone, in the presence of oxygen, leads to formation of a seven-membered ring product, an overall [4+4-1] reaction. This transformation involves two unstable intermediates and a sequence of unusual reactions. An initial [4+4] photocycloaddition of the enyne with the pyridone yields a 1,2,5-cyclooctatriene. Photooxidation of this triene forms a cyclopropanone and subsequent photoextrusion of carbon monoxide gives the observed 1,4-cycloheptadiene product. The first-formed cyclooctatriene and the cyclopropanone could be observed and characterized spectroscopically. The cyclopropanone underwent CO extrusion both photochemically and thermally to give the cycloheptadiene product. Addition of fluoride or acetylide to the most stable cyclopropanone occurred chemoselectively at the two different silicon groups rather than the carbonyl group. The resulting cyclopropanone ring openings gave unsaturated aldehydes.

Cyclization Reactions of Oxyallyl Cation. A Method for Cyclopentane Ring Formation

“…Some side reactions have been observed, as well. These include the Favorskii rearrangement (Scheme , example 1), cheletropic elimination of carbon monoxide (example 2), formation of trifluoroethoxy acetals from the starting mesyloxyketone, and the unexpected intramolecular attack of an oxygen nucleophile to the oxyallyl cation with the tetrahydrofuran ring closure (example 3). The Favorskii rearrangement and the acetal formation can be suppressed by performing the reaction in toluene in the presence of lithium perchlorate.…”