Cyclohexadienone, and

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... 

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... 

For all QM complexes, the ring signals in both and 13C JH NMR spectra indicate the lack of aromaticity. The carbonyl group appears in the 13C JH NMR spectrum in the range of 18418-20116ppm, which is the region observed for 2,5-cyclohexadienones and quinones.38... 

It is seen from Table 7 that all these carbenes in solution react nonstereospeci-fically, indicating triplet character. For some carbenes such as CF3CH, PhCH, (CH3)sSiC(C02C2H5), 2,6-di-t-butyl-carbena-cyclohexadienone and 4,4-dimethyl-carbena-cyclohexadienone, the degree of non-stereospecifity is quite small. Other carbenes usually contain 7t-bonds, aromatic rings or heavy atoms as functional groups. 

Bassan et al. (117) studied the intermolecular tautomerization of 2,4-cyclohexadienone and the associated barriers using one... 

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

Fluorinated phenols generally undergo oxidation to cyclohexadienone and benzoquinone derivatives, and to the products of ring rearrangement and ring degradation. 

Fluonnatedphenols are usually oxidized to cyclohexadienone and benzoqui-none derivatives and to products of ring rearrangement and ring degradation Phenols with a tnfluoromethyl group at the nng are hydroxylated by potassium persulfate [57], whereas chloric acid causes oxidation to quinone, mg contraction, and incorporation of chlorine in the product [58] (equation 53)... 

Ramamurthy et al. have examined enantioselective and diastereoselective ODPM rearrangements of cyclohexadienone and naphthalenone derivatives within MY zeolites, where M is an alkali ion [38, 39]. For example, in Scheme 4.27, in NaY in the presence of several chiral inductors such as ephedrine, pseudoephedrine, and camphorquinone-3-oxime, an enantioselective ODPM rearrangement of 65 and 66 took place to afford 68 in 30% ee yield. In the frame of the chiral auxiliary approach, the compound 67 linked to (S)-ephedrine was irradiated within NaY to give 68 in moderate diastereoselective excess (de) (59%). Interestingly, the reaction media of KY, RbY, and CsY reverse the diastereoselectivities. The cation-dependent diastereo-meric switch has been discussed with respect to the N- or O-functional group in 67 [39]. Recently, Arumugan reported that the irradiation of naphthalenones 65 linked chiral auxiliaries (R3 = COO(—)-Ment or (S)-NHCH(Me)Ph) in Li+ or Na + Nafion resulted in chiral products ( 14% de) [40],... 

Sivasubramanian, K., Kaanumalle, L.S., Uppili, S., and Ramamurthy, V. (2007) Value of zeolites in asymmetric induction during photocydization of pyridones, cyclohexadienones and naphthalenones. Organic S. Biomolecular Chemistry, 5 (10), 1569-1576. 

Cyclohexadienones and quinone methides from phenolic antioxidants and UV absorbers... 

Conjugate addition to conjugated dienones. Marshall and co-workers have studied the reaction of the reagent wdth cyclohexadienones and conclude that stcric factors account for the ratio of 1,4- to 1,6-addition. The last two examples illustrate the virtual absence of angular methylation. 

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

Elaborate mechanistic schemes have been suggested for the principal rearrangements of cyclohexenone, 2,5-cyclohexadienone, and bicyclo-hexenone systems induced by w - tt excitation which are compatible with the experimental data outlined above. In essence, these mechanisms are based on the common concept that the complicated structural changes are initiated in an electronically excited state. For the appreciably complex ketones considered, reaction initiation in a vibrationally excited ground state produced by adiabatic ir n demotion is expected to be readily suppressed in solution by collisional deactivation. It has been pointed out that by this general concept the rearrangements provide a decay path for electronically excited states which allows transfer of minimal amounts of enei to the environment in each step. 

Other halogenated ketones for which photolysis studies have been reported are 3-chloro-2-butanone and 4-chloro-2-butanone , perfluorocyclobutanone , di-chloromethyl and trichloromethyl cyclohexadienones , and bromodiketones . 

Birch reduction of aromatic ethers is well known to afford alicyclic compounds such as cyclohexadienes and cyclohexenones, from which a number of natural products have been synthesized. Oxidation of phenols also affords alicyclic cyclohexadienones and masked quinones in addition to C—C and/or C—O coupled products. All of them are regarded as promising synthetic intermediates for a variety of bioactive compounds including natural products. However, in contrast to Birch reduction, systematic reviews on phenolic oxidation have not hitherto appeared from the viewpoint of synthetic organic chemistry, particularly natural products synthesis. In the case of phenolic oxidation, difficulties involving radical polymerization should be overcome. This chapter demonstrates that phenolic oxidation is satisfactorily used as a key step for the synthesis of bioactive compounds and their building blocks. 

As a synthetic method, the reaction of aryllead triacetates with phenols is useful for the production of two structural types the 6-aryl-2,4-cyclohexadienones and the sterically hindered 2,6-diarylphenols substituted with alkyl groups on the C-3 to C-5 positions. The steric compression in the putative aryloxylead intermediate seems to play a decisive role in the success of the ligand coupling step. This is particularly true for the arylation of hindered phenols with the more hindered aryllead reagents, which takes place at room temperature. In contrast to the reactions of phenols with arylbismuth reagents (chapter 6), 0-arylation products are exceptional and generally obtained in very small yields. 

Ethylenic groups are present in units 47,48, and 51 and also in cyclohexadienone and quinoid units (Figure 7.6). It is difficult to determine the total number of ethylenic groups. Reported estimates suggest that there are 0.05-0.1 ethylenic groups/phenyl-propane units in softwood lignin [117]. 

Fig. 6. Benzoquinones, cyclohexadienones and phenols mixture in UV and electrochemical detection...

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

A new route to (a,a,-difluorobenzyl)phosphonic esters which does not use the DAST reagent, employs 4,4-dimethoxy-2,5-cyclohexadienone and its reaction with diethyl (lithiodifluoromethyl)-phosphonate (Scheme 7). ... 

The photorearrangement of chiral cyclohexadienones and cyclo-hexenones has been covered in other reviews and is not examined here [173]. However, it is now well established that the formation of lumiketones from rigid enones and dienones (226, 228, and 231), or from monocyclic, chiral cyclohexenones having C-4 as the only asymmetric center, occurs with more than 99% selectivity [174] (Scheme 36). 

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