Compounds of the 3,4-dihydro-ionone series a s models for the photochemistry of y , 6-and 6 ,~-unsaturated ketones and aldehydes. -S u m m w y . The photochemical behaviour of y, 6-and 6, &-unsaturated carbonyl compounds of the dihydro-ionone series has been studied, with special attention to the investigation of oxetane formation uevszis hydrogen abstraction.UV.-irradiation of the dihydro-P-ionone compounds with structure A (1, 7, 14, 18, 24, 29) led to isomeric ethers with structures B (2, 8, 15, 19, 25, 30), C (3, 9, 16, 20, 26, 31) and D (4, 21, 27), isomeric bicyclic alcohols with structurc E (5, 10, 17, 22, 28), and photoreduction products with structure F (6, 11, 12, 13). Photolysis of dihydro-y-ionone (32) gave a complex mixture containing fragmentation product 35, hydrocarbon 36, P-ambrinol (34), oxctanc 33, as well as dihydro-/-ionone (I) and three of its photoproducts (2, 3, 5). The dihydro-a-ionone compounds 37 and 40 gave mixtures of fragmentation products and the oxetanes 38 and 41. Irradiation of t h e side-chain homologues 42 and 45 yielded 43, which photo-cpclizes t o 44. I n contrast, 3,4-dihydro-3',4'-dchydro-/3-ionone (46) gave merely the isomeric open-chain trienc-ketone 47.The structures assigned to the ethers 2 , 3 , 3 3 , 3 8 and to the alcohols 5,10,13 could be confirmed by chemical reactions and mutual interconversions. The structure of the ether 21 had to be established by X-ray analysis, details of which are described.A novel intramolecular hydrogen transfer is involved in formation of etlicrs B. The photocyclization A + D probably proceeds by addition of the carbonyl-C atom to the double bond ( A + h ) , followed by methyl (1 + 2)-shift (h + i). Process A --j. h may also be involved in formation of compounds of type C and E.
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2)Vgl. M . P. Zink, Diss. ETHZ, Nr. 5498 (197.5); auszugsu-eise vorgctragcn an der Herbstversammlung der Schweizerischen Chemischen Gcsellschaft in Lugano am 19. Oktober 1973 [2].Fur die Numericrung der in dieser Mitteilung bcschriebenen Verbindungen siehc Schema 27.