Photoinduced molecular transformations. Part 134. Photoinduced stereospecific addition of methanol to 5β-cholest-1-en-3-one oxime and photoinduced deconjugation of its 1-methyl derivative involving stereospecific proton transfer

Abstract

Irradiation of 5β-cholest-1-en-3-one oxime 14 in methanol gave 1α-methoxy-5β-cholestan-3-one oxime 16, arising from the photoaddition of methanol to the double bond of the enone oxime. 1α-Methoxy-5β-cholestan-3-one 17 as well as 10β-methoxy-5(10→1)abeo-1β(H),5β,10α(Me)-cholestan-3-one 15, arising from a skeletal rearrangement of 5β-cholest-1-en-3-one (generated from the oxime), were the accompanying products in this photoreaction. Deuterium-labelling studies confirmed that the formation of 1α-methoxy-5β-cholestan-3-oxime 16 involves a stereospecific syn addition of methanol to the photogenerated, twisted, ground-state double bond of the oximes B from the rear side of the steroidal framework.

Irradiation of 1-methyl-5β-cholest-1-en-3-one oxime 23 in methanol, on the other hand, gave almost exclusively 1-methylene-5β-cholestan-3-one oxime 24, arising from photodeconjugation of the α,β-double bond to the β,γ-position. A deuterium-labelling study established that deuterium is stereospecifically introduced at the 2β-position of 1-methyl-5β-cholest-1-en-3-one oxime 23 in the photodeconjugation in methanol[²H]ol while deuterium is stereospecifically introduced at the 2α-position in the photodeconjugation of its 5α-isomer 6. These results are fully consistent with our previously proposed pathway concerning the photodeconjugation of 1-methyl-5α-cholest-1-en-3-one oxime 6; the stereospecific addition of a proton to the photogenerated, twisted double bond of the oxime D from the rear side of the steroidal framework was followed by the loss of a proton from the 1-methyl group of the resulting carbocation intermediate. Neither the isoxazole derivative, a product in the photoreaction of 5α-cholest-1-en-3-one oxime 1, nor the unsaturated lactam from a photo-Beckmann rearrangement was formed in the photoreactions of either 5β-cholest-1-en-3-one oxime 14 or its 1-methyl derivative 23.