Photochemische Reaktionen. 68. Mitteilung. Die Photoisomerisierung von α,β‐ungesättigten γ,δ‐Epoxyketonen: 9α, 10α‐ und 9β, 10β‐Oxido‐3‐oxo‐17β‐acetoxy‐Δ 4 ‐östren

Irradiation in the n →π* absorption band of the α,β‐unsaturated γ,δ‐epoxyketone 5 in ethanol at −65° exclusively afforded the rearranged ene‐dione 13 , whereas at + 24° under otherwise unchanged reaction conditions or upon triplet sensitization with Michler 's ketone and with acetophenone at + 24° essentially identical mixtures of 13 (major product), 14 , and 15 were obtained. Selective π→π* excitation of 5 at −78° and + 24° led to similar product patterns. The 9β,10β‐epimeric epoxyketone 7 selectively isomerized to 14 and 15 at + 24° and n → π* or π → π* excitation. Neither the epoxyketones 5 and 7 nor the photoproducts 13–15 were photochemically interconverted. In separate photolyses each of the latter gave the double bond isomers 16 , 18 , and 19 , respectively. Cleavage of 13 to the dienone aldehyde 17 competed with the double bond shift ( → 16 ) when photolyzed in alcoholic solvents instead of benzene. The selective transformations 5 → 13 (at −65° and n → π* excitation) and 7 → 14 + 15 are attributed to stereoelectronic factors facilitating the skeletal rearrangements of the diradicals 53 and 55 , the likely primary photoproducts resulting from epoxide cleavage in the triplet‐excited compounds 5 and 7 , via the transition states 54 , 56 , and 57 . The loss of selectivity in product formation from 5 at higher temperature and n → π* excitation or triplet sensitization is explicable in terms of radical dissociation into 58 and 59 increasingly participating at the secondary thermal transformations of 53 . The similar effect of π → π* excitation even at −78° indicates that some of the π,π* singlet energy may become available as thermal activation energy. It is further suggested that the considerably lesser ring strain in 14 and 15 , as compared with 13 , is responsible that selectivity in product formation from 7 is maintained also at +24° and at π → π* excitation.