2.5- Cyclohexadienone

The reactive excited state of dienone is n - Ji triplet state. The excited cyclohexadienones undergo rearrangement reactions. The rearrangement has been extensively studied for 4, 4- diphenylcyclohexa-2, 5-dienone. 

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. 

Upon photolysis in nucleophilic media, like water or ethanol, mazdazantonin gives the ring cleaved acid or ester derivative while a-santonin when photolysed in water gives isophotosantonic Lactone. 

The photolysis of mentioned dienone support the proposed mechanism. 

Photolysis in aqueous acid media causes the reaction to take a different course. The initially formed cyclopropyl Zwitterion is protonated and then cleavage of the three membered ring gives hydroxyalkene. 

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Certain structural features can make the keto-enol equilibrium more favorable by stabi hzmg the enol form Enolization of 2 4 cyclohexadienone is one such example... 

Like the Diels-Alder reaction discussed in Sections 14.4 and 14.5, the Claisen rearrangement reaction takes place through a pericyclic mechanism in which a concerted reorganization of bonding electrons occurs through a six-membered, cyclic transition state. The 6-allyl-2,4-cyclohexadienone intermediate then isomerizes to o-allylpbenol (Figure 18.1). 

Lithium 2,6 dimethylphenoxide, from 2,6-dimethylphenol, 46,115 methylation of, to form 2,6,6-tri-methyl 2,4 cyclohexadienone, 46, US... 

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

In contrast to 2-cyclohexenones where no a-cleavage is observed this reaction occurs efficiently for 2,4-cyclohexadienones 216,217). Among different synthetic applications the synthesis of dimethylcrocetine (a heptaene dicarb-oxylic acid) may serve as an illustration (2.11) 218),... 

The reaction can be used to prepare hexaethyl-2,4-cyclo-hexadienone, m.p. 44-45°, in 82% yield from hexaethylbenzene and 3,4,6,6-tetramethyl-2,4-cyclohexadienone from durene in over 80% yield. 

Disubstituted 2,4-cyclohexadienones (112) undergo photoinduced electrocyclic ring opening to the transient ketene derivatives 113, which can be trapped by nucleophiles to prepare the corresponding carboxylic acid derivatives (114 equation 44)196 197 j le reaction has been employed successfully for the synthesis of various carboxylic acids, esters and amides. 

In addition to the absorptions attributable to aryloxy radicals. Fig. 1 displays a broad shoulder around 315 nm, much longer-lived, which is assigned to a 2,4-cyclohexadienone. This intermediate decays with a rate of 1.25 0.1 sec in hexane solution, to give 2-hydroxyacetophenone (11) via 1,3-hydrogen shift. The rate of appearance of 11 is coincident with the decay rate of the dienone. 

Cyclohexadienones. The perturbation of the electronic absorption spectra by adsorption on silica gel can have a significant effect on subsequent photochemical reactions. For example, Hart has shown that 2,4-cyclo-hexadienones photochemically degrade cleanly in nonpolar solvents to a ketene (1) but in highly polar solvents or adsorbed on silica gel bicyclic ketone (2) is the predominant product.33 The absorption spectra indicate that in nonpolar solvents the lowest singlet state is the n,n state, from which the formation of the ketene proceeds. This n -n band is obscured by the band in polar media, inversion of the energy levels of the n,n, and the first... 

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

An example of 2,4,6-triphenylpyrylium-3-olate (65 R = R = R = Ph, R = H) reacting as a 1,3-dipole was first provided by Suld and Price who obtained a maleic anhydride adduct (C25HigO5). Subsequently, an extensive study of the cycloadditions of this species has been published by Potts, Elliott, and Sorm. With acetylenic dipolarophiles, compound 65 (R = R = R = Ph, R = H) gives 1 1 adducts that have the general structure 74 and that isomerize to 6-benzoyl-2,4-cyclohexadienones (76) upon thermolysis. This thermal rearrangement (74 -> 76) has been interpreted in terms of an intermediate ketene 75. The 2,3-double bond of adduct 74 (R = Ph) is reduced by catalytic hydrogenation. Potential synthetic value of these cycloadducts (74) is demonstrated by the conversion of compound 74 (R = Ph) to l,2,3,4,6-pentaphenylcyclohepta-I,3,5-triene (79 R= Ph) via the alcohol 78 (Scheme 1). ... 

Cyclohexadienone (keto form not aromatic) Phenol (enol form aromatic) ... 

Allyl phenyl ether 6-Allyl-2,4-cyclohexadienone o-Allylphenol... 

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