Cross-conjugated cyclohexadienones photochemical

Photochemical cyclohex-2-enoneH> bicyclo (3.1.0) hexan-2-one rearrangement irradiation of testosterone acetate, 322 Photochemical rearrangements of cross-conjugated cyclohexadienones and their photoisomers... 

Photochemical oxacarbene formation, 307 Photochemical rearrangements of cross-conjugated cyclohexadienones, 330 Photochemical rearrangements of enol esters and enol lactones, 339... 

The initially formed cyclopropyl ketones from the photolysis of cross-conjugated cyclohexadienones are also photolabile. Usually the photochemical rearrangements of cyclopropyl ketones involve cleavage of the cyclopropyl bond which forms part of the cyclopentenone ring followed by either (a) a substituent migration or (b) rearrangement through a spiro intermediate. While the literature in this area is too voluminous to review in... 

The rearrangement of cross-conjugated cyclohexadienones to bicyclo[3.1.0]-hex-3-en-2-ones was one of the first photochemical reactions to be thoroughly studied for mechanistic and synthetic purposes 344). The examples outlined below are the conversion of the lactone a-santonin to lumisantonin 345) (3.29)... 

The photochemical rearrangements of cross-conjugated cyclohexadienones in general, and of 4,4-diphenylcyclohexadienone (81) in particular, have been intensively studied.115 When 81 is irradiated in dioxane-water, first 6,6-di-phenylbicyclo[3.1.0]hex-3-one-2 (82) is obtained which, on further irradiation, forms 83, 84, and 85. The primary photorearrangement product, 82, can also be obtained by photosensitization of 81, but not by irradiation of 81 in piperylene. Therefore 82 is formed from the lowest triplet of 81. The subsequent rearrange-... 

Schuster, D. I. Mechanisms of photochemical transformations of cross-conjugated cyclohexadienones, Acc. Chem. Res. 1978,11, 65-73. 

A more detailed study of this process has sought to establish the scope of the process. Thus Schultz and his coworkers have reported that irradiation of the cyclohexadienone (269) affords the adduct (270). This process can be quantitative using extended photolysis times but if the reaction is stopped after 14 h a 1 1 mixture of (270) and (271) is obtained. Other examples of the process are reported. The irradiation of a series of quinone monoacetals (272) in acetic acid has provided a flexible high yield route to substituted cyclopentenones. Such a rearrangement path is typical of the processes encountered in the photochemical rearrangement of cross conjugated cyclohexadienones. Some examples of the utility of this system are shown in Scheme 9. - ... 

Both the dienones (280) are photochemically reactive and can be converted into the enones (281). These are also photoreactive and subsequent irradiation, or over irradiation of the initial solution yields the norbornenones (282). The formation of the enones (281) is thousht to arise via the zwitterionic intermediate (283) analogously with the rearrangement of the cross conjugated cyclohexadienones reported above. The norbornenones (282) are formed by a 1,3-migration within the enone (281 ). ... 

Ill. Blay, G., Photochemical Rearrangements of 6/6 and 6/5 Fused Cross conjugated Cyclohexadienones. In Horspool, W. M., Lenci, F. (eds), CRC Handbook of Organic... 

Photochemical Rearrangements of 6/6- and 6/5-Fused Cross-Conjugated Cyclohexadienones... 

This topic has been excellently reviewed in the previous edition of this handbook. Caine, D., Photochemical rearrangements of 6/6-and 6/5-fused cross-conjugated cyclohexadienones, in Handbook of Organic Photochemistry, Horspool, W.M. Ed., C.R.C. Press, Boca Raton, FL, 1995,701. 

Caine, D., Alejande, A.M., Ming, K., and Powers, W.J., III, Photochemical rearrangements ofbicyclic 6/5-fused cross-conjugated cyclohexadienones and related compounds, /. Org. Chem., 37, 706, 1972. 

Caine, D., Deutsch, H., Chao, S.T., Van Derveer, D.G., and Bertrand, J.A., Photochemical conversion of methoxy-substituted 6/6-fused cross-conjugated cyclohexadienones into isomeric tricyclode-cenones,/. Org. Chem., 43, 1114,1978. 

Kropp, P.J., Photochemical rearrangements of cross-conjugated cyclohexadienones. V. A model for the santonin series, /. Org. Chem., 29, 3110,1964. 

Caine, D., Kotian, P.L., and McGuiness, M.D., Synthesis and photochemical rearrangements of bicyclic cross-conjugated cyclohexadienones containing 5-oxy substituents, J. Org. Chem., 56,6307, 1991. 

The mechanistic photochemistry of cross-conjugated 2,5-cyclohexadienones has been comprehensively examined (Scheme 21). The photochemical isomerization of the... 

Cross-conjugated Dienones.- The photochemical rearrangement of a series of cyclohexadienones (244 - 247) has been studied by Schultz and his coworkers. The reactions encountered are typical of such compounds and involve rearrangement to a bicyclic species. In the first example, the irradiation of (244), the bicyclic species could not be isolated or detected since rapid ring opening was presumed to take place affording... 

Cross-conjugated Dienones.- The isomers (254) and (255) axe obtained from the direct irradiation of the cyclohexadienone (256) in benzene. Prolonged irradiation of the mixture results in the photoisomerization of (254) into (255). The irradiation of (255), however, does not bring about isomerization. The study of enantiomerically pure (256) was also carried out and from this it was found that the starting material isomerized into a mixture of enantiomers. The photochemical rearrangement of the cyclohexadienone (257) in methanol occurs in a stereospecific fashion and yields the adduct (258) whose structure was proven by X-ray crystallographic analysis. Cyclohexadienone (259) is photochemically reactive and can be converted in two steps into the adduct (260) as shown in Scheme 8. This process represents the intramolecular trapping of the zwitterionic intermediate (261). ... 

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