Photochemistry of cyclic α-hydroxy ketones. III. The nature of the photoproducts from 5-hydroxy-5β-cholest-3-en-6-one and the relationship of these to the photoproducts from analogous saturated 5-hydroxycholestan-6-ones

Abstract

Photolysis of 5-hydroxy-5β-cholest-3-en-6-one (2) in hexanes gave 5-oxo-5,6-seco-cholest-3-en-6-al (13) and [(1R, 4R, 7aR)-1-(1S,5-dimethylhexyl)-7a-methyl-2,3,3a,4,7,7a-hexahydro-1H-inden-4-yl]acetaldehyde (14). Similar irradiation of 13 gave 14 and a small amount of 5-hydroxy-B-nor-5β-cholest-3-ene-6α-carbaldehyde (15). The structures of 13 and 15 were confirmed by independent synthesis: the former from cholesteryl acetate via 3β-acetoxy-6,6-dimethoxy-5,6-seco-cholestan-5-one and the latter from 13 by treatment with methanolic sodium methoxide. Photolysis of 5-deuteroxy-5β-cholest-3-en-6-one (26) in hexanes gave the -CDO analogues of the aldehydes 13, 14, and 15. These observations are interpreted in terms of α-cleavage of the C-5—C-6 bond of 2 to give an acyl allyl diradical, which undergoes O—H hydrogen abstraction to give 13. The occurrence of α-cleavage as the major reaction pathway is attributed to the increased stabilization of the alkyl radical center by the hydroxyl substituent. The preference for O—H rather than C—H abstraction in the diradical is attributed to the planar nature of the allyl radical center. The indenylacetaldehyde 14 is considered to arise via a Norrish type II reaction of 13, while the formation of B-nor aldehyde 15 is ascribed to a "dark" aldol reaction of 13.