Cross-conjugated cyclohexadienones rearrangements

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)... 

As just described, Zimmerman has reported one instance of a dienone rearrangement which definitely does not fit Chapman s general picture. Schuster has provided two reports410,411 of cross-conjugated cyclohexadienones which eliminate radical species. With the trichloro-methyl-substituted ketone 34, both cleavage to the cresol and rearrangement to lumiproduct are quenched by dienes.411 Stern-Volmer quenching plots indicate that the rate at which the excited triplet reacts exceeds 10° sec"1 for both reactions.412... 

C. Rearrangements of Cross-Conjugated Cyclohexadienones and Their Photoisomers... 

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-... 

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... 

As one would expect, conjugated ketones are mostly poor hydrogen abstractors. This is a fortunate circumstance, because it leaves room for a variety of reactions that are quite useful from the preparative point of view. These include rearrangements [24], addition reactions and 2+2 cycloaddition with alkenes. An example of rearrangement of a cross-conjugated cyclohexadienone is shown above. This gives a cyclopropylcyclopentenone that opens up in the reaction medium (see Scheme 2.13). 

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. 

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. 

Schultz, A.G., Photorearrangement reactions of cross-conjugated cyclohexadienones, in CRC Handbook of Organic Photochemistry and Photobiology, Horspool, W.M. and Song, P.-S., Eds., CRC Press, Boca Raton, FL, 1995, 685 (b) Zimmerman, H.E. and Lynch, D.C., Rapidly rearranging... 

In the monocyclic series, the 2,4-cyclohexadienones rearrange to photoproducts of potential synthetic value. However, for efficient 1,2-acyl migration, only a relatively small number of substrates are suitable. These must be highly substituted, for example (48). Whereas on direct excitation in methanol, cleavage to the isomeric ketenes (49) (4> 2 0.42) predominates, the remarkably stereoselective 1,2-acyl shift to the bicyclohexenone (50) is found either in trifluoroethanol or when the dienone is adsorbed on silica gel. The conversion to (50) is followed by a reversible phototransformation to the cross-conjugated dienone (51) and accompanied by aromatization to (52) to a minor extent. Such reactivity has also been verified for tetra- and penta-methylated 2,4-cyclohexadienones. The only photoreaction of the hexamethylated homolog, on the other hand, is ketene formation. ° ... 

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