Tandem β-fragmentation-hydrogen abstraction reaction of alkoxy radicals in steroidal systems

Abstract

A number of tertiary or hemiacetalic steroidal alcohols, 2-hydroxy-3,4-dinor-2,3-secocholestan-5-one 2,5-hemiacetal (1), 2β,3β-dihydro-3'H-cyclopropa[2,3]-cholestan-5α-ol (7), 3β-phenyl-5α-hydroxycholestan-2-one (10), 5α-hydroxycholestan-3-one (15), 3β,5α-dihydroxycholestan-7-one 3-acetate (21), and 4,4-dimethyl-19-hydroxy-5α-cholestan-3-one 19,3-hemiacetal (27) have been prepared in order to test a new tandem β-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 β-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 γ-lactones by β-fragmentation.