Photochemische Reaktionen. 107. Mitteilung. Zur Photochemie offenkettiger 2,6‐bzw. 2,7‐Dien‐Carbonylverbindungen

The Photochemistry of Open‐Chained 2,6‐ or 2,7‐Dien‐Carbonyl Compounds On 1 n , π*‐excitation (λ > 347 nm) citral (5) and the methyl ketone 10 isomerize to compounds A (7, 19) and B (6, 20) , whereas the phenyl ketone 11 changes into the isomer 24 of type E. Evidence is given that the conversions to A and B may arise from the 3 n , π*‐state of the 2,6‐diene‐carbonyl compounds. On 1 n , π*‐excitation (λ = 254 nm) 5 and 10 yield the isomers A (7, 19) and D (18, 22) , but no products of type B. Furthermore, conversion of 10 to the isomer 21 of type C is observed. Selective 1 n , π*‐excitation (λ = 254 nm) as well as selective 1 n , π*‐excitation (λ > 347 nm) of the 2,7‐diene‐carbonyl compounds 12 and 13 give rise to isomerization to the compounds F (25, 28) , exclusively. The intramolecular [2 + 2]‐photocycloadditions are shown to be triplet processes. UV.‐irradiation (λ > 280 nm) of compounds F (25, 28) furnishes the isomeric products G (26, 29) which photoisomerize to oxetanes of type H (27, 30).