Cyclohexadienones, photochemical rearrangement

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... 

Cyclohexadienones also undergo photochemical rearrangements, but the products are different, generally involving ring opening. ... 

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 photochemical rearrangements of 2,4-cyclohexadienones are of three types (1) ring fission to a cis-diene-ketene ( ) expulsion of a heteroatom from the 6-position with concomitant aromatization of the... 

The photochemical rearrangements of 2,4,6-trimethyl-6-acetoxy-2,4-cyclohexadienone (Formula 105) are of special interest. Irradiation of Formula 105 in ether containing a good nucleophile such as cyclohexyl amine gives the expected a, -unsaturated amide (Formula 106) (48). In sharp contrast to expectation, irradiation of Formula 105 in ether con-... 

Cyclobutenones. Cyclobutenones and cyclobutenediones undergo photochemical rearrangement to unsaturated acids in a manner analogous to that of 6,6-disubstituted-2,4-cyclohexadienones. Irradiation of Formula 133 in ether saturated with water gives Formula 134 (49). In... 

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-... 

CTOSs-conjngated Dienones.—Photochemical rearrangement of dienones of the type shown in (227) is a well known process and leads to bicyclo[3.1.0]hexenones. These compounds are also photoreactive and ring-open to give zwitterions [e.g., (228)]. Schultz et al." have used this rearrangement process and have successfully trapjjed the zwitterion (228) [from (227)] by an intramolecular thermal reaction with the azide group of the side-chain to afford the adduct (229). The cyclohexadienone (230) is photochemically rear-... 

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... 

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. - ... 

In the l960s, Kropp showed that fused bicyclic dienones structurally related to santonin could potentially serve as synthetically useful precursors to either spirocyclic skeletons or hydroazulenones. One of these cases, 91, was successfully used by Marshall and Johnson as the starting point in an elegant synthesis of the spirocyclic sesquiterpene )6-vetivone (Scheme 23). More recently, a variety of bicyclic cyclohexadienones have been studied by Caine and coworkers. For example, bicyclic dienone 92 could be photochemically rearranged to the oxygenated bicyclo[4.3.0]nonenone system 93, along with other rearrangement products, via acetic acid solvolysis of the cyclopropyl ketone intermediate. ... 

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). ... 

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... 

The first photochemical reaction of a cyclohexadienone chromophore in santonin was noted. a-Santonin undergoes photochemical rearrangement in an non-nucleophilic media to give the cyclopropyl ketone lumisantonin, which is itself photochemically labile and yields the linearly conjugated dienone mazdasantonin. 

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

The presence of other rings fused to the cyclohexadienone chromophore may lead to failure of the photochemical rearrangement, probably as a consequence of steric factors preventing P,P -bonding in the excited states. This is the case with dienones 71, 72 and 73. ... 

Rropp, P.J., Photochemical rearrangement of cyclohexadienones and related compounds, Org. 

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. 

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)... 

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