Photoinduced molecular transformations. Part 133. New photoinduced deconjugation of steroidal α,β-unsaturated cyclic ketone oxime into the β,γ-unsaturated isomer involving stereospecific proton transfer

Irradiation of 1-methyl-5α-cholest-1-en-3-one oxime or its 4,4-dimethyl derivative in either a protic or an aprotic solvent gave 1-methylene-5α-cholestan-3-one oxime or the corresponding 4,4-dimethyl derivative, arising from an unprecedented photodeconjugation of α,β-enone oximes into the β,γ-isomers. Neither the expected isoxazole derivative (a product in the photoreaction of 5α-cholest-1-en-3-one oxime or its 4,4-dimethyl derivative) nor the unsaturated lactam that arises from a photo-Beckmann rearrangement was formed. Deuterium-labelling studies on the photoreactions of 1-methyl-5α-cholest-1-en-3-one oxime established that either a proton or a deuteron is stereospecifically introduced at the 2α-position of the steroidal oxime in this photodeconjugation. A pathway which involves the sterospecific addition of either a proton or deuteron to the photogenerated, twisted double bond of the oximes from the rear side of the steroidal framework, followed by the loss of a proton or deuteron from the 1-methyl group of the resulting carbocation intermediate, is proposed regarding the formation of β,γ-unsaturated oximes from the excited α,β-unsaturated oximes.