2.5- Cyclohexadienone opening

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

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. 

The alkylation procedure can be used to prepare a wide variety of 2-substituted-2,4-cyclohexadienones, 2,3 5i 6 which are useful starting materials. The compounds can serve either as dienes or dienophiles7 in the Diels-Alder reaction and can be opened photochemically to yield substituted A3,5-hexadienoic acids.8... 

In 1968, Qriffiths and Hatt established that although dienylketene (282) (generated as the only photochemical product from cyclohexadienone 281) cyclizes thermally to (281), other thermal cyclizations compete, such as the foimaticxi of the bicyclo[3.1.0]hexenone (283) or solvent (amines or alcohols) addition product (284).The thermal formadm of bicyclo[3.1.0]hexenones (283) from ketenes (282) can be (diotochemically reversed. Thus 2,4-cyclohexadienones (287) can be obtained from 2,S-cyclohexa-dienones (285).The stereospecificity of the thermal ring opening of the bicyclo[3.1.0]hexenones to dienylketenes has been established. 

Studies of the low-temperature photochemistry of umbellone (288) established a dual pathway for its conversion to thymol (291). Besides the opening of (288) to ketene (289), its direct conversion to the cyclohexadienone (290) was also postulated. However, this reaction depends on the substitution pattern, and in the case of lumisantonin (292) only traces of cyclohexenone (294) were detected in the photolysis... 

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... 

Further work by Schuster and his coworkers has shown that the dienone (257) yields, as well as the many products already reported in earlier publications, the new enone (258). The formation of this species arises from the intermediate (259). The cyclohexadienone (260) affords the bicyclic compound (261) on irradiation in methanol. This is formed by the trapping of a zwitterion by methanol. Secondary photolysis affords the cyclohexenone (262) by ring opening of the adduct (261). ... 

Photochemical ring opening of linearly conjugated cyclohexadienones affords dienylketenes (145), which react in one of the following ways recycli-zation to the original or to a stereoisomeric cyclohexadienone, formation of bicyclo[3.l.0]hexenones (146), or addition of a protic nucleophile to yield substituted hexadienecarboxylic acids (147) (Quinkert et al., 1979). 

A detailed study of the photochemical reactivity auid synthetic usefulness of the o-quinolacetates (251) has been reported. Quinkert and his coworkers have lodged a patent dealing with the formation of macrolides from the photochemical ring opening of the cyclohexadienones (252). ... 

A walk rearrangement of hydroxybicyclohexenyl cation 69 is involved in the acid catalyzed isomerization of bicyclohexanone 68 to cyclohexadienone 70. The reversible nature of this walk rearrangement has been demonstrated by the equilibration 69-4-CD3s=t 69-5-CD3, which precedes the ring opening to the protonated cyclohexadienone derivative (Figure 12) (109). 

De Mayo and his coworkers262-264 have reported the photochemical isomerization of the sultones shown in Scheme 26. These compounds ring open in a manner analogous to linearly conjugated cyclohexadienones affording a sulphene intermediate. This intermedi-... 

The photochemistry of 2,4-cyclohexadienones was to prove fruitful [67]. A ring-opening takes place, which was believed to furnish a dieneketene that, under the right conditions, adds protic nucleophiles. As the photoeducts are easily accessible from phenols, these phenols may hence be converted in this manner into open-chain compounds. This structural alteration proceeds in high chemical yield and should lend itself to synthetic application. For ap-... 

A previously described procedure, reported to be efficient for dioxolanes and noncyclic acetals, has been generalized for both cyclic and acyclic acetals of ketones and aldehydes, and it has been successfully applied to the opening of 1,3-dioxanes to enol ethers (eq 81). However, aromatization has occurred for 4-(A -benzoylamino)-4-phenyl-2,5-cyclohexadienone dimethyl ketal (eq 82). This method has also allowed the development of 1-methylcyclopropyl ether as a protecting group for the hydroxyl moiety which is prepared regioselectively from terminal acetonides. ... 

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