Photoinduced molecular transformations. Part 149. Stereospecific photoadditions and photorearrangements of the oximes of some steroidal α,β-unsaturated cyclic ketones and their deuterio derivatives

Abstract

The photolysis of several steroidal cyclic α,β-unsaturated ketone oximes as model substrates in methanol showed that these excited oximes react in either one of two ways, depending upon their structural features. Thus, while the excited oximes, such as those of 5α-cholest-4-en-6-one and the 5-en-4-one oxime, having their CC bond at their ring junction, lead to the corresponding enamides (15–40%) arising from a photorearrangement, the excited oximes, having their CC bond at their non-ring junction (such as the oximes of 5α-cholest-2-en-1-one and the 3-en-2-one), principally lead to photoisomerization to form transient trans geometrical isomers from which stereospecific additions of proton or methanol to their trans CC bond take place.

The irradiation of (E)-5α-cholest-2-en-1-one oxime in benzene–acetic acid (94:6) gave 1-aza4a-homo-5a-cholest-3-en-2-one arising from a regioselective photorearrangement, while irradiation of the oxime in methanol gave 3β-methoxy-5α-cholestan-1-one arising from photoaddition in low yield. Thus, organic acid accelerates the photorearrangement to the lactam in competition to the α-fission.

In contrast to these photochemical transformations, ground-state Beckmann rearrangements of (E)-cholest-4-en-6-one oxime and (E)-cholest-5-en-4-one oxime, having their CC bond at their ring junction, under the standard conditions gave the corresponding cyclic enamines. Treatment of (E)-5α-cholest-2-en-1-one oxime under the standard conditions of the Beckmann rearrangement, however, gave an unsaturated nitrite as well as the corresponding enamide. The regioselectivities of the formation of cyclic enamine and enamides in the photo- and nonphoto-rearrangements, as well as the structure–photoproduct correlation of the oximes, are discussed.