Non-sensitized photooxygenation of some steroidal isoxazolidines

UV irradiation of the steroidal isoxazolidines 2a, 2b, and 2c in various solvents in the presence of molecular oxygen, results in oxidative cleavage of the epoxyimino bridge to give several products; the N-unsubstituted isoxazolidine 2a afforded the nitro products 3, 4, and 5 and the azoxy compounds 6 and 7, while the N-methyl and N-acetyl derivatives 2b and 2c produced only the nitro compounds 3 and 5 as sole identifiable material. An explanation of the observed photooxygenation processes involving exciplex formation followed by proton transfer from isaxazolidine to molecular oxygen is presented. © 1991.