Photochemische Reaktionen. 71. Mitteilung. Die Photoisomerisierung eines spirocyclischen α, β‐Epoxyketons

Abstract

AbstractOn direct UV. irradiation and on triplet sensitization with acetophenone the spirocyclic epoxyketone (R)‐(−)‐9 undergoes racemization (Φ313/334 0.014, ΦSens 0.0060) and rearrangement to the enantiomeric spiro‐β‐diketones (R)‐(+)‐14 (Φ313/334 0.068, ΦSens 0.0037) and (S)‐(−)‐14 (Φ313/334 0.024, ΦSens 0.0023). The quantum yield data show that triplet reaction due to intersystem crossing is unimportant on direct irradiation, and they exclude that one common diradical intermediate of type d (Scheme 8) for the three reaction paths is involved in both the singlet and the triplet reaction. The postulate of photolytic CαO epoxide cleavage to intermediates of type d for the rearrangement requires that the rate of rearrangement is greater than the rate of rotation around the CαCβ; bond in a given d, and that the rate difference is greater in singlet‐generated d than in the triplet analogue. Reclosure of diradicals d and/or photolytic CαCβ cleavage to diradical e and reclosure can account for the racemization of 9.The optically active spiro‐β‐diketone 14 was found to racemize also on direct irradiation and on triplet sensitization. Furthermore, both 14 and the isomeric β‐diketone 20, which was obtained by UV. irradiation of the homocyclic epoxyketone 19, photochemically isomerize to the enol lactones 23 and 21, respectively.