Photochemical reactions of selected expoxides (1975)
AuthorsHii, G. S. C.show all
The synthesis and photolysis of 3S(JR)-3,4-epoxy-1-phenyl-1-butanone (1), 2S,3S(2R,3R)- and 2S,3R(2R,3S)- 3,4-epoxy-2-methyl-1-phenyl-1-butanones (2) and (3), 3S(3R)-3,4-epoxy-2,2-dimethyl-1-phenyl-1-butarione (4), oxiranyl benzoate (25) and 2,)-epoxy-3-methyl-1-butanol benzoate (28) are described and the structures of 1S,2S(1R,2R)- and 1R,2S(1S,2R)- 2-methyl-1-phenyl-3-buten-1-ols (9) and (10), and 1S,2R,3R(1R, 2S, 3S)-, 1S, 2R, 3S(1R, 2S. 3R)-, 1R, 2R, 3S(1S, 2S, 3R)- and 1R,2R,3R(1S,2S,3S)- 3,4-epoxy-2-methyl-1-phenyl-1-butanols (12), (13), (14) and (15) unambiguously determined. Photolysis of a solution of 3S(3R)-3,4-epoxy-1-phenyl-1-butanone (1) in benzene gave the isomeric 1S,2S,3S(1R,2R,3R)- and 1R,2S,3S(1S,2R,3R)- 2,3-epoxy- 1-phenyl-1-cyclobutanols (30) and (31). The formation of these products is rationalised by a mechanism involving γ-hydrogen abstraction by the excited carbonyl moiety and closure of the resulting 1,4-biradical. Fragmentation of the 1,4-biradical is a less important process and only a trace of acetophenone could be detected. Photolysis of 2S,3S(2R,3R)- and 2S,3R(2R,3S)- 3,4-epoxy-2-methyl-1-phenyl-1-butanone (2) and (3) gave 1R,2S,3S,4S(1S,2R,3R,4R)- and 1R,2R,3R,4S(1S,2S,3S,4R)- 2,3-epoxy-4-methyl-1-phenyl-1-cyclobutanol (36) and (35) respectively. The stereospecificity of these reactions is rationalised. Photolysis of 3S(3R)-3, 4-epoxy-2, 2-dimethyl- 1-phenyl-1-butanone (4) gave 3-methyl-2-buten-1-ol benzoate (29), a product resulting from initial α-cleavage. Only a trace of product resulting from γ-hydrogen abstraction could be isolated. Oxiranyl benzoate (25), the C(2) oxygen analogue of 3,4-epoxy- 1-phenyl-1-butanone (1), and 2 ,3-epoxy-3-methyl-1-butanol benzoate (28) were photochemically unreactive, but ground state addition of hydrochloric acid to oxiranyl benzoate (25) was observed when the compound was photolysed with 1849 - 2537 Å ultraviolet lamps in dichloromethane. The photolysis of 4α,5-epoxy-5α- and 4β,5-epoxy-5β-cholest-2-enes (51) and (52) in acetone to give the skeletal rearrangement products A-nor-B-homo- 5(β- and 5α-cholest-2-en-6-ones (53) and (55) respectively are described. The mechanism proposed for the formation of these products involves initial C(4) - O bond cleavage followed by stereospecific C(5) - C(10) bond migration to C(4).