Reactive 1,2,5-cyclooctatrienes, formed by photocycloaddition of 2-pyridones with enynes, are stabilized by steric shielding, slowing or preventing an otherwise facile [2 + 2]-dimerization reaction. Diisopropylsilyl ether-tethered reactants paired with an alkene substituent (R) produce allenes that are stable (R = TMS) or that isomerize to 1,3-dienes by hydrogen migration (R = alkyl). Under acidic conditions, hydrolysis of the photoproduct's silyl ether can lead to a [3,3]-sigmatropic rearrangement.
Intramolecular [4 + 4] photoreaction of 2-pyrones with a 1,3-enyne yields an unstable 1,2,5-cyclooctatriene product. Without a C4 pyrone substituent, 1,3-hydrogen migration converts the allene to a 1,3-diene, with a skeleton related to dactylol. With methoxy substitution, Cope rearrangement yields a nine-membered ring fused to a cyclobutane. Both structures were confirmed by X-ray crystallography. The Cope rearrangement is apparently reversible, reforming the allene which undergoes a proton shift to the more stable 1,3-diene product.
a b s t r a c tBenzene is unreactive as a photochemistry partner with 2-pyridone, however benzene substitution can allow for an efficient [4+4] (a k a, 'para') photocycloaddition. Productive substituents include alkoxy, cyano, ester, and trifluoromethyl, suggesting that inductively electron-withdrawing groups are sufficient, however a single fluorine substituent does not result in cycloaddition. When the product of this cycloaddition contains a conjugated system (e.g., an unsaturated ester) a secondary photocycloaddition can follow, forming triply bridged all-syn-[3]ladderane products.
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.
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