Tandem beta-Fragmentation-Hydrogen Abstraction Reaction of Alkoxy Radicals in Steroidal Systems

A number of tertiary or hemiacetalic steroidal alcohols, 2-hydroxy-3,4-dinor-2,3-secocholestan-5-one 2,5-hemiacetal (1), 2beta,3beta-dihydro-3'H-cyclopropa[2,3]-cholestan-5alpha-ol (7), 3beta-phenyl-5alpha-hydroxycholestan-2-one (10), 5alpha-hydroxycholestan-3-one (15), 3beta,5alpha-dihydroxycholestan-7-one 3-acetate (21), and 4,4-dimethyl-19-hydroxy-5alpha-cholestan-3-one 19,3-hemiacetal (27) have been prepared in order to test a new tandem beta-fragmentation-hydrogen abstraction reaction. The alkoxy radicals generated by irradiation of these alcohols with visible light in the presence of (diacetoxyiodo)benzene, iodine, and molecular oxygen undergo a beta-fragmentation reaction followed by peroxidation of the C-radical formed. The peroxy radical reacts further with iodine to give an alkoxy radical and iodoxyl radical (IO(*)). The new alkoxy radical can produce intramolecular functionalization of suitably positioned carbons by hydrogen abstraction through a six-membered cyclic transition state to give tetrahydrofurans or can be intramolecularly trapped by carbonyl groups suitably disposed so as to generate a new alkoxy radical that can afford gamma-lactones by beta-fragmentation.