2.5- Cyclohexadienones photoreactions

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

In spite of the encouraging results obtained in the TBCHD-sensitized photooxygenation of several unsaturated substrates, however, this procedure suffers a severe limitation fundamentally due to the thermally and/or photochemically low stability of the sensitizer. The photoreactions of different cyclohexadienone derivatives have been widely studied by Schuster and co-workers [191-192],... 

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. ° ... 

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-... 

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 ). ... 

Enone Cycloadditions and Rearrangements Photoreactions of Cyclohexadienones and Quinones... 

Photoreactions of 2,5-cyclohexadienones Phenols from 4-bromo-2,5-cyclohexadienones... 

The ketones 56,57, and 58 are some of the monocychc P,y-enones whose photochemical reactions upon sensitized irradiation have been examined. Irradiation of the dienone 56 in ether is reported to give the bicyclic ketone 59 via oxa-di-Ji-methane reaction. Recently, the photoreaction of cyclohexadienone 56 and its derivatives having chiral auxiliaries was examined in alkali ion exchanged Y zeolites. Thus, irradiation of 56 in dry (-)-ephedrine-modified zeoHte gave the ODPM product in 30% ee. Significant... 

Singh, V., Sharma, U., Prasanna, V., and Porinchu, M., PericycKcreaction of cyclohexa-2,4-dienones with cyclohexa-1,3-diene and cycloheptatriene the role of cyclohexadienones as JT and component, Cope rearrangement and photoreaction of the adducts. Tetrahedron, 51,6015, 1995. 

In continuation of our interest in designing new methodology based on the chemistry of cyclohexa-dienone and photoreaction of P,Y-enones, " 2 developed a new stereoselective route to sterpuranes, another class of sesquiterpenes containing a tricyclic framework wherein the cyclobutane ring is fused with a hydrindane unit in a linear fashion. In this context, the tricycHc P,y-enones of type 235 were synthesized and their photochemistry was examined upon direct as weU as sensitized irradiation. The desired tricycHc compound 235, containing a P,y-enone chromophore, was prepared from hydroxyme-thylphenol 232. Oxidation of 232 with sodium metaperiodate generated the cyclohexadienone 233, which... 

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