Photoinduced molecular transformations. Part 130. Novel stereospecific photorearrangement and stereospecific addition of methanol in steroidal α,β-unsaturated cyclic ketone oximes

Abstract

Irradiation of 5α-cholest-1-en-3-one oxime or its 4,4-dimethyl derivative in a protic or an aprotic solvent gave 4′α,5′-dihydro-4-nor-5α-cholest-1-eno[2,1-c]isoxazole or the corresponding 3,3-dimethyl derivative arising from an unprecedented photorearrangement. The molecular structure of the former was established by X-ray crystallographic analysis. 1β-Methoxy-5α-cholestan-3-one oximes or their 4,4-dimethyl derivatives, arising from an unprecedented photoaddition of methanol to the double bond of the enone oximes, were the accompanying products in both of these photoreactions and no lactams were formed. Deuterium-labelling studies on the photoreactions of 5α-cholest-1-en-3-one oxime and its trideuteriated derivative established that a deuteron or a proton is stereospecifically introduced at the 2α-position of the Steroidal oxime in this photorearrangement. A pathway which involves an unprecedented stereospecific addition of a proton or a deuteron to the photogenerated, twisted double bond of the oximes from the rear side, followed by fragmentation of the resulting carbocation and an intramolecular 1,3-dipolar addition of the nitrite oxide intermediate, is proposed regarding the formation of the isoxazole from the excited oximes. Deuterium-labelling studies also established that the addition of methanol to the double bond of the oximes is stereospecific too. On the basis of the labelling study it is concluded that the mechanism for the photoaddition of the methanol involves a stereospecific syn addition of the methanol to the photogenerated, twisted, ground-state double bond of the oximes from the rear side.