Absence of stereoelectronic control in the photochemistry of two diastereomeric .beta.,.gamma.-epoxy ketones

Steroids Part 22. Photochemistry of 5,6-epoxy-4,4-dimethylcholestan-3-ones and 5,6-epoxy-17-hydroxy-4,4-dimethylestran-3-ones

“…In particular, there is a significant difference in reactivity between isomers (1) and (2) and the decarbonylation of compound (2) in ether appears to involve an unusual 1,5hydrogen transfer., The absence of products arising from cleavage of the epoxide moiety is interesting since, in general, the radicals (a) (Scheme 1) might be expected4 readily to give oxide radicals (b) especially since stereoelectronic effects have been reported to be unimportant in this type of r e a~t i o n .~ The photoreactions of the l-oxaspiro[2.n]alkan-5-ones (3) and (4) are reported6 to be adequately accounted for by a reaction scheme which is analogous to Scheme 1. spectra of the epoxides (7) and (8) respectively at 6 3.14 ( J 5 Hz) and 3.18 (J 3 Hz) confirmed the assigned stereo~hemistry.~ Photolysis of the 3-keto 5a,6a-epoxide (5) in ether solution in Pyrex apparatus, using a medium-pressure Hg lamp, afforded an extremely complex inseparable mixture. The 6-H doublets in the 'H n.m.r.…”

“…The 6-H doublets in the 'H n.m.r. spectra of the epoxides (7) and (8) respectively at 6 3.14 ( J 5 Hz) and 3.18 (J 3 Hz) confirmed the assigned stereo~hemistry.~ Photolysis of the 3-keto 5a,6a-epoxide (5) in ether solution in Pyrex apparatus, using a medium-pressure Hg lamp, afforded an extremely complex inseparable mixture. The absence of the ) and the deuteriated 4-nor-SP,6P-epoxide (12) ('H, 80%, 2H, 17%, 2Ho 3%).…”

“…In the photolyses in ether, it is presumed that the cleavage of the epoxide linkage in the 3-keto epoxides ( 5), By analogy with the 3-keto 4,4-dimethyl-As-steroids, it may be assumed that the preferred conformation of the A-ring of the 3-keto 5,6-epoxides (5) and (6) is a skew-boat owing to the steric interaction between the 10-methyl group and the 4P-methyl group in the normal chair c~nformation. '~ Thus, in conformation (37 (39) n scission of a radical it is generally recognised that the semioccupied orbital of the radical should be coplanar with the bond being broken for efficient reaction.…”

“…Photolysis of the 3-Keto 5~,6a-Epoxide (5) in Methanol.-A solution of the epoxy ketone (5) (400 mg) in methanol (750 ml) was irradiated for 3 h. Evaporation of the methanol gave an oil (446 mg), p. In one experiment other fractions isolated were 5,6a-epoxy-1 7~-hydroxy-4-methyl-4-methylene-3,4-seco-5~-estran-3-a1 (21) (30 mg, 11%); v,,,. 1640 (CH,=CR,), 1 725 (C=O), and 3 460 cm-' (OH); 6 0.73 (s, 13P-Me), 1.8 (br s, CH,=CMe), 2.76 (d, J 6 Hz, 6-H), 3.64 (t, J 8 Hz, 17-H), 4.86 and 5.05 (br s, CH,=CMe), and 9.72 (br s, CHO) (Found: M + , 318.…”

“…~ The photoreactions of the l-oxaspiro[2.n]alkan-5-ones (3) and (4) are reported6 to be adequately accounted for by a reaction scheme which is analogous to Scheme 1. We report here an investigation of the photoreactions of the 5,6-epoxy-4,4dimethylcholestan-3-ones (5) and (6),' their 19-nor analogues (7) and (8), and 4,4-dimethyl-5x-cholestan-3-one (9)8 in an attempt to evaluate further the proposed general reaction schemes and to elucidate any other important controlling factors. The 3-keto 19-nor-5~,6a-epoxide (7) was prepared by monoperphthalic acid oxidation of the A5-compound (10) and the 5P,6P-epimer (8) was obtained from the latter by successive treatment with N-bromosuccinimide (NBS)-aqueous perchloric acid and potassium acetate.…”