SummaryThe irradiation of 17~-hydroxy-2-oxa-androst-4-en-3-one (1) yields a cyclopropane derivative 2, which is the result of a rearrangement, formally analogous to the 'type A rearrangement' of the enones. Two other products, the dihydroxy compound 5 and the dimer 6, have also been isolated (Scheme I).The photochemical behaviour of the unsaturated lactones has been less widely studied than that of the corresponding ketones. Usually, these lactones dimerise by a well known and documented process [2]. More recently, other reactions like deconjugation [3] or a-fragmentation to ketenes [4] have been described. The three reaction types are formally analogous to the situation described in the case of ketones. Nevertheless, to our knowledge, the rearrangement, well known in the case of cyclic enones [5] ('type A rearrangement', i.e. 3 + 4 [6]) has not been observed in the lactone series.To study this possible photorearrangement of a lactone, compound 1 [7] was selected as model compound.UV.-irradiation of 1 in t-butyl alcohol, in identical conditions as used in the case of testosterone, yields a mixture of the cyclopropane derivative 22) (10,3%), starting material 1 (36,1%), a dihydroxy compound 53) (3,2%) and dimeric material The structural relationship between 2 and 4 has been demonstrated by an independent synthesis of 2. As starting material the cyclopropane derivative 8, obtained 63) (27,2%).