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

• Published in: Angewandte Chemie International Edition 2017-01, Vol.56 (1), p.319-323
• Main Authors: Kulyk, Svitlana, Khatri, Buddha B., Sieburth, Scott McN
• Format: Article
• Language: English
• Subjects: Aldehydes; allenes; Carbon monoxide; Carbonyl compounds; Carbonyl groups; Carbonyls; Contraction; Cycloaddition; cyclopropanone; electrocyclic reactions; enynes; Extrusion; Fluorides; Intermediates; Irradiation; Oxidation; Photooxidation
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201608684

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. Addition, subtraction, evolution: Higher‐order photocycloaddition of an enyne and a diene (in the form of a pyridone) yields an eight‐membered ring containing an allene moiety. When air is introduced into the photoreaction, photooxidation and photoextrusion of CO comprises an overall [4+4−1] reaction process. Nucleophiles add to the silicon atoms flanking the keto group, rearranging the cyclopropanone to an enal.