The principal direction of conventional photolysis of the regioisomeric 2,2-dimethyl-5,5-diphenyland 5,5-dimethyl-2,2-diphenyl-substituted 4-diazodihydrofuran-3(2H)-ones 1a and 1b, respectively, is the Wolff rearrangement, while other photochemical processes, which are giving rise to the formation of CÀH-insertion, 1,2-alkyl-or -aryl-shifts, as well as H-atom-abstraction products occur to a much lower degree (Schemes 2 and 3). The ratio of similar reaction products from both regioisomers 1a and 1b is essentially independent of their structure, and a substantial effect of the relative position of the Ph and diazo group to each other on the yield of CÀH-insertion products does not occur. Based on stereochemical considerations, the Wolff rearrangement of diazodihydrofuran-3(2H)-ones apparently proceeds in a concerted manner, whereas the appearance in the reaction mixture of 1,2-shift and H-atomabstraction products points to the parallel generation during photolysis of singlet and triplet carbenes (Schemes 4 and 5).1. Introduction. -Direct photolysis of cyclic diazocarbonyl compounds usually proceeds with elimination of N 2 and formation of the Wolff rearrangement products (for reviews, see [1 -6]). Thus, light-induced decomposition of diazo ketones A with 2,2,5,5-tetraalkyl substituents at the heterocycle (R ¼ Me, Et, and others) by shortwavelength UV light (l > 210 nm) in dioxane solution, as a rule, gives rise to oxetanecarboxylic acids B or their derivatives (Scheme 1) in yields of up to 90 -95% [7].On the other hand, it was recently established [8] that photolysis of tetraphenylsubstituted diazo ketone A (R ¼ Ph) in THF solution, besides ordinary Wolffrearrangement products C, furnished the adduct D of diazo ketone A with THF (Scheme 1). The latter can be considered as the product of the insertion of the terminal N-atom of the diazo group into the HÀC(2) bond of THF. The revealed transformation of tetraphenyl-substituted diazo ketone A is apparently the first example of the photolysis of diazo ketones which proceeds with the retention of both N-atoms of the diazo group in the reaction products [9] [10].