Photochemische Reaktionen. 117. Mitteilung [1] Zur Photochemie von 5,6‐Epoxydienen und konjugierten 5,6‐Epoxytrienen

Photochemistry of 5,6‐Epoxydienes and of Conjugated 5,6‐Epoxytrienes On singulet excitation (δ = 254 nm) the 5,6‐epoxydiene 6 and the conjugated 5,6‐epoxytrienes 7 and 8 exclusively give products arising from cleavage of the C, C‐bond of the oxirane ( cf. 6 → 9 , 10 , 11 ; 7 → ( E )‐ 15 , 16 , 17 ; 8 → 18 (A+B) , 19 (A+B) , 20 , 21 ). The dihydrofuran compounds 11 and ( E/Z )‐ 15 are formed by cyclization of a ketonium‐ylide a and d , respectively. Photolysis of a gives the carbene b which yields the cyclopropene 9 , whereas d forms photochemically the carbenes f and g which yield the methano compounds 16 and 17 . The isomeric cyclopropene derivatives 20 and 21 are products of the intermediates h and i , respectively, which are formed by photolysis of the ylide e . The cyclopropene 21 isomerizes by intramolecular cycloadditions to 18 (A+B) and 19 (A+B) . ‐ On triplet excitation (λ⩾LD nm; 280 nm; acetone) 6 undergoes cleavage of the C(5), O‐bond and isomerizes to 12 and 14 . However, 7 is converted by cleavage of the C, C‐bond of the oxirane to yield 15 . On treatment with BF 3 O(C 2 H 5 ) 2 6 gives 14 , whereas 7 yields 22 , and 8 forms 23 and 24 .