Photochemische Reaktionen. 65.(vorläufige) Mitteilung. Spezifisch π → π*‐induzierte Photoisomerisierungen von 10‐Dimethoxymethyl‐Δ 1,9 ‐octal‐2‐on. Ein photochemischer Zugang zu [4.4.3]‐12‐Oxapropellan‐Derivaten

Selective n → π* excitation of the α,β‐unsaturated enone 1 in hydrocarbon solvents resulted in a deconjugation reaction to 3 , reminiscent of results previously reported for similar systems [2], whereas the photoreactivity of 1 in alcohol solvents at wavelengths >3400 Å was so small that only product 4 has been identified as yet. Excitation of the π → π* transition of compound 1 at 2537 Å initiated additional phototransformations which could not be effected by irradiation in the first absorption band. The [4.4.3]‐12‐oxapropellane derivative 2 was identified as one of the two new major photo‐isomers. A 6:8 mixture of products 2 and 3 , plus about 1 part of an isomer of still unknown structure (see however, the Addendum), were readily formed in hydrocarbon solvents, and a 1:10 ratio of 2 and the unknown product was obtained in methanol. Abstraction of a methoxyl hydrogen by the ketone oxygen is proposed to account for the primary photochemical step in the cyclization to 2 . A hydrogen‐deuterium isotope effect of 2.7 was observed in a competitive experiment using 1 and 1‐d 6. 34% of one deuterium atom were exchanged for hydrogen when 1‐d 6 was photolyzed to 2‐d 6 in t ‐butyl alcohol, which suggests an intermediate of type a in the pathway 1 → 2 possessing a readily exchangeable proton. Steric considerations would require a strongly distorted, non‐planar excited‐state geometry of the enone group of 1 for the oxygen to approach a methoxyl hydrogen. The transformation 1 → 2 represents a novel reaction type in photochemical processes of conjugated enones which are specifically induced by π → π* excitation only.