Cross-conjugated cyclohexadienones

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... 

In aqueous acid cross-conjugated cyclohexadienones are principally photoconverted to one or more hydroxy ketones. In the case of a-santonin (1), isophotosantonic lactone (5) is formed in about 50% yield. A series of papers by Kropp and co-workers has aided in understanding this reaction/32-39-411 They have shown that the presence of a 4-methyl group (steroid numbering) results in the preferential formation of the 5-7 fused ring system (isophoto-... 

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 thermal isomerization of a spirocyclic enol ether to the ketone [202] (Eq. 176) is probably a homolytic process. However, it is noted that part of the driving force for the reaction must be the bonding of the ethereal oxygen to a designated donor atom of the cross-conjugated cyclohexadienone moiety. 

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-... 

Another synthesis of a bridged hydrocarbon takes advantage of high electron release from the para-position of phenolate anions, which may be used to transform the phenol moiety into a substituted cross-conjugated cyclohexadienone system (S. Masamune, 1961, 1964). 

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

I. Transformations of Conjugated Cyclohexenones and Their Photoisomers. 81 IT. Transformations of Cross-Conjugated Cyclohexadienones and Their Photoisomers. 85... 

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... 

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