1.4- Cyclohexadienones

Linearly conjugated cyclohexadienones usually photorearrange with ring fusion to a cis diene-ketene. The reaction is reversible, so that in the absence of a nucleophile little change is observed. A good example of this type of transformation is the formation of photosantonic acid  

In the presence of cyclohexyl amine the photolysis of the dienone (58) gives the cyclohexyl amide expected from dienone ring fission and reaction of cyclohexyl amine with the ketene. However, in the presence of weaker nucleophiles the ring opened product is not trapped and instead there is a relatively slow formation of phenols  

The acetoxyl migration is thought to go through intermediates like (59) and (60)  

Heavily substituted o-dienones, such as 2,3,4,5,6,6-hexamethyl and 2,3,4,6,6-pentamethyl cyclohexadienone, upon irradiation give not the ketene but a substituted bicyclic compound of the following type (62) 

The use of trifluoroethanol as solvent or absorption of the dienone on silica gel promotes the photoconversion of dienones into bicyclic ketenes/ For the photolysis of (63) it has been shown by low-temperature infrared and ultraviolet spectroscopy that the initial photolysis gives a ketene which can be efficiently trapped by cyclohexylamine or, in the absence of a good nucleophile, thermally rearranges by a (,4 +, 2,) allowed process to a bicyclic ketone (64)  

---

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... 

---