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

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.