Deconjugated enones

The dienol is unstable, and two separate processes have been identified for ketonization. These are a 1,5-sigmatropic shift of hydrogen leading back to the enone and a base-catalyzed proton transfer which leads to the / ,y-enone. The deconjugated enone is formed because of the kinetic preference for reprotonation of the dienolate at the a carbon. Photochemical deconjugation is a synthetically useful way of effecting isomerization of a,) -unsaturated ketones and esters to the j ,y-isomers. 

Scheme 39.10 a-Vinylation of alkenes providing trans-selective p,y-deconjugated enones. 

Conversion of PGA2 to the highly sensitive PGC2 was accomplished by deconjugation of the enone system by formation of the y-extended enolate using rm-alkoxide as base and a-protonation by pH 4 buffer. 

A dienol is also formed via hydrogen abstraction by the excited carbonyl group from a second enone molecule in (4.14). This dienol tautomerizes in C6F6 to give the (3,y-unsaturated ketone selectively, the overall reaction thus being deconjugation of the a,(3-unsaturated ketone415K... 

XIV. The Isomerization of Olefins and Deconjugation of Enones / 360 Experimental procedures / 363... 

Deconjugation of oL, -enones. The photochemical deconjugation of a,P-unsat-urated acids and esters via a dienol is often a useful preparative route to the P,y-unsaturated isomers. The corresponding reaction with a,p-enones is erratic it can result in reversion to the original enone or the trans-isomer. Irradiation in the presence of a mild base such as pyridine (or imidazole) can promote conversion of a,P-enones to p,7-enones. A stronger base such as triethylamine catalyzes reconjugation of the intermediate dienol. Base-catalyzed deconjugation is more efficient in polar solvents (DMF, CH,CN, CH,OH) than in ether or hydrocarbon solvents. Thus under proper conditions several a,p-enones previously considered inert to irradiation can be isomerized to the p,y-isomers. 

Irradiation of the enone (130) leads, via trans-cis isomerization and deconjugation, to (131)/ Subsequent irradiation yields products from 1,3- and... 

Since conjugated enolates react almost exclusively at the a-position, low temperature protonation of a conjugated enolate provides the P,y-unsaturated ketone. This deconjugation protocol can be used to functionalize the y-position of an enone, as shown below. 

The Jorgensen group also applied the parent cinchona alkaloids as catalysts to the aza-Michael addition of hydrazones 8 to cyclic enones 9 [4] and the asymmetric deconjugative Michael reaction of alkylidene cyanoacetates 10 with acrolein (11) [5], However, only a moderate level of enantioselectivity was obtained in both reactions (Scheme 9.4). Of note, for the deconjugative Michael reaction, the delocalized allylic anion 12 could be generated via the deprotonation of 10 by the cinchona base and might attack the electrophilic enal at either the a- or the y-position. However, in this study, only the a-adducts were produced. 

A detailed study of the photoenolization of the ketones (143) and their reketonization has been reported. The enone (144) undergoes deconjugation when subjected to u. v. irradiation. White et al. have observed that the photochemical decoiyugation reaction of the triene (145) yields (146) as a mixture of E)- and i isomers. Deconjugation,... 

The triplet state of the ecdysone (152) is reactive and aRords the products shown in Scheme 5. The formation of the reduction products (153) and (154) is presumed to follow a path where loss of a hydroxyl radical yields the allyl radical (155) which then gives the products (153) and (154) by hydrogen abstraction. The ketone (156) is formed by a 1,2-bond migration and the cyclobutanol (157) arises by secondary irradiation of the diketone (156). The enone (158) shows solvent dependent photochemistry. Thus in ethyl acetate the deconjugated product (159) is formed while in methanol the reduced ketone (160) is produced. ... 

Details of the photoenolisation reactions of some acyclic a,0-unsatuzated ketones have appeared aind competing reketonisation mechanisms used to rationalise the wide variation in the photochemical deconjugation reactivity of enones. A study of the photoenolisation of... 

Deconjugation of acyclic P-halo enones. MejSiCl catalyzes the isomerization of j3-halo a,)3-un aturated ketones to the corresponding 8,7-unsaturated ketones. 

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