PHOTOCHEMISTRY OF 4-VINYL-2,5-CYCLOHEXADIEN-1-ONES - A REMARKABLE EFFECT OF SUBSTITUTION ON THE TYPE-A AND DIENONE PHENOL PHOTOREARRANGEMENTS

4-Carbomethoxy-4-vinyl-2,5-cyclohexadien-1-ones 6a and 6b were prepared from methyl benzoate and methyl 2-methoxybenzoate in 45% overall yields. Irradiation of 6a in benzene solution at 366 nm gave 2-carbomethoxybicyclo[3 2,3,4.2 6,7.1]-2.6-octadien-9-one (7, 35%), 2-carbomethoxybicyclo[3.3.0 1,5]-2,7-octadien-6-one (8, 8%), and 4-carbomethoxy-3-vinylphenol (9, 39%). The structure of phenol 9 was confirmed by irradiation of 4-(2'-bromoethyl)-2,5-cyclohexadien-1-one 5a, which gave 3-(2'-bromoethyl)-2-carbomethoxyphenol (10, 21 %) and 3-(2'-bromoethyl)-4-carbomethoxyphenol (11, 72%); dehydrobromination of 11 provided a vinylphenol that was identical to that produced from photolysis of 6a. The endo-6-carbomethoxy-6-vinylbicyclohexenone 12a was obtained from 23 and was found not to be involved in the photochemistry of 6a at 366 nm; however, photorearrangement of 17 gave 7 in 80% yield. Irradiation of 6b in benzene solution at 366 nm gave 4-carbomethoxy-3-methoxy-5-vinylphenol (18, 65%) and 4-carbomethoxy-3-methoxy-6-vinylphenol (19, 32%); in the presence of trifluoroacetic acid, 6b gave 2-carbomethoxy-5-methoxy-3-vinylphenol (20, 5%) and 2-carbomethoxy-8-methoxybicyclo[3.3.0 1,5]-2,7-octadien-6-one (21, 44%). Reduction of 21 with NaBH4/CeCI3 in ethanol followed by acid-catalyzed hydrolysis gave 2-carbomethoxybicyclo[3.3.0 1,5]-2,6-octadien-8-one (17, 66%), an intermediate in a previously reported total synthesis of the iridoid monoterpene, (+/-)-verbenalol. Irradiation of 5b gave 6-(2'-bromoethyl)-6-endo-carbomethoxy-4-methoxybicyclo-[3.1.0 1,5]hex-3-en-2-one (22, 70%), and dehydrobromination of 22 provided 6-endo-carbomethoxy-4-methoxy-6-vinylbicyclo[ 3.1.0 1,5]hex-3-en-2-one (23, 63%). It was found that 23 was not involved in the photorearrangement of 6b to phenols 18 and 19. 18 and 19 are suggested to arise by direct dienone-phenol photorearrangement of 6b via an initial di-pi-methane process involving the 4-vinyl substituent and the C(5)-C(6) double bond (Scheme V). Dienone-phenol photorearrangement of 4-(acetoxymethyl)-3-methoxy-4-vinyl-2,5-cyclohexadien-1-one (34) was completely regioselective to give 4-(acetoxymethyl)-3-methoxy-5-vinylphenol in quantitative yield.