Reactions of Trienes and Higher Polyenes

The photochemistry of natural products, especially those with polyene and carbonyl chromophores85 and of C H10 hydrocarbons,88 has been reviewed. The sunlight photochemistry of retinyl acetate has been reported.87 

Irradiation of l,3,6-triphenylhexa-l,3,5-triene in the presence of iodine in benzene or cyclohexane solution affords a good yield of 1,2,4-triphenylbenzene. This result is surprising, all the more so since the product is also formed in a thermal reaction of the triene with iodine.88 However, attempts by the authors88 to isolate an intermediate in the photochemical reaction were not successful and it is assumed that the cyclization follows a Cope-type path rather than the more 

Further products have been identified from the irradiation of 7-dehydro-cholesterol [(130), provitamin D3]. In ether or alcohol the main component of the photolysate was assigned structure (131) which results from the photochemical cyclization of the trans-Z-cis-conformation of the triene (132) formed by ringopening of the cholesterol.92 From reaction in ethanol two alcohol-addition products (133) and (134) have been identified. Two other products of the toxisterol type have been assigned structures (135), the difference between them 

An oxa-analogue of the hexatriene-bicyclo[3,l,0]hex-2-ene conversion has been reported from the study of the photolysis of the isochromenes (142). 5 Irradiation (Pyrex filter) in methanol gave two products (143) and (144), the second of which arises by secondary photolysis of the oxabicyclohexene (143). This product most likely arises by [4 + 2]photoaddition of the quinone methide intermediate 

5-hydrogen transfer is encountered in the conversion of the triene (147) into (148). This product is also photolabile. 

Direct irradiation of the benzcycloheptene (199a) gave the two isomeric products (200) and (201) in a temperature- and solvent-independent ratio of 95 5. The influence of substituents was obvious from the fact that the irradiation of (199b) exhibited solvent dependency and the yield of the products (200) and (201) showed an increasing predominance of the enrfo-product. The sensitized irradiation was less efficient. 

Arnold and his co-workers have reported the electron-transfer-induced photodimerization of 1,1-diphenylethylene. This reaction is thought to proceed to the triene (202a) which, in the absence of other reaction paths, undergoes hydrogen migration to afford the product (203). When the reaction is carried out 

Murato, B. Frei, H. R. Wolf, and O. Jeger, Helv. Chim. Acta, 1980, 63, 2221. 

Kobayashi, K. Seto, M. Kato, and T. Miwa, Koen Yoshishu-Hibenzenkei Hokozoku Kagaku 

Toronkai Kozo Yuki Kadaku Toronkai, 12th, 1979, 13 (Chem. Abstr., 1980, 92, 214528). 

---