Photoinduced molecular transformations. Part 109. Conformational dependence of the stereochemistry of photochemical 1,3-acyl shifts of β,γ-unsaturated cyclic ketones: conformation-specific photorearrangements of steroidal β,γ-unsaturated cyclic ketones, 7a-homocholest-5-en-7a-one and 4a-homo-5α-cholest-1-en-4-one

Abstract

Direct irradiation of 7a-homocholest-5-en-7a-one in t-butyl alcohol with Pyrex-filtered light resulted in photorearrangement to 5-vinyl-7-nor-5α-cholestan-6-one and 5,7-cyclo-7a-homo-5α-cholestan-7a-one via 1,3-acyl and oxa-di-π-methane rearrangements. Sensitization and quenching studies indicated that the former product arose from an excited singlet state of the β,γ-unsaturated ketone while the latter arose from an excited triplet state.

Direct irradiation of 4a-homo-5α-cholest-1-en-4-one in t-butyl alcohol with Pyrex-filtered light, however, gave 1β-vinyl-4-nor-5α-cholestan-2-one arising from a [1,3] acyl shift as the only product without any accompanying product arising from the oxa-di-π-methane rearrangement. The populations of boat and chair conformations of the ground state of the above β,γ-unsaturated ketones and several related A-ring ketones were calculated by the empirical force field method. The stereochemistry of the products of 1,3-acyl shift in the present photorearrangements, 5-vinyl-7-nor-5α-cholestan-6-one and 1 β-vinyl-4-nor-5α-cholestan-2-one, was found to depend clearly upon the conformation of the ground state of the starting β,γ-unsaturated steroidal ketones and the ratio of the stereoisomers of the products was found to depend on the ground-state population of the conformers of the starting unsaturated ketones.