Enones, photodeconjugation

Since the early observations of photodeconjugation of conjugated aliphatic carbonyl and carboxyl derivatives [19], the reaction has been generalized to unsaturated esters [20], lactones [21], acids [22], amides [20n], aldehydes [23], aliphatic enones [24], and medium-ring unsaturated enones [25]. 

Photodeconjugation of steroidal enones, which does not follow the preceding scheme, involves a complex mechanism which has not been completely elucidated [26]. Although this reaction does not pass through a dienol intermediate, it is not analyzed in this chapter. 

To be very enantioselective, this reaction has to meet several important requirements. First, photoenols have to be produced as either pure Z or E stereoisomers to allow enantioselective photodeconjugation. Even so, protonation of the Z or stereoisomers from the same, rear side, for example, would produce opposite enantiomers and a low enantiomeric excess (ee) would result (Scheme 3). Fortunately, photoenolization of aliphatic enones is only possible from the Z isomer excited in its singlet state, and the excited molecule has to adopt an s-cis conformation to place the excited carbonyl and the y-H close enough to allow y-H abstraction. Consequently, the enol is formed in a unique configuration. All these observations have led several groups to propose a concerted process involving a 1,5 antarafacial sigmatropic shift for the formation of photodienols [16]. 

Enantioselective photodeconjugation occurs with lactones, esters, and conjugated enones [33]. In principle, as soon as a prochiral photodienol can be produced, an enantioselective protonation is expected in the presence of a chiral -aminoalcohol. However, the corresponding acids and amides are unsuitable starting materials for enantioselective photodeconjugation. 

Scheme 4. Unsaturated lactones, esters, enones and -aminoalcohols considered in enantioselective photodeconjugation reactions.

In conclusion, photodeconjugation of enones or carboxylic acid derivatives is a mild process by which to prepare 3,Y-unsaturated carbonyl and carboxylic derivatives in good yields. These substrates represent interesting frameworks useful for the synthesis of natural products. The asymmetric version of the process also allows the formation of new chiral centers under very mild conditions and with high selectivities. 

Pete, J. R, Photodeconjugation of unsaturated enones and acid derivatives, in CRC Handbook of Photochemistry and Photobiology, Horspool, W. M. and Song, P.-S., Eds., CRC Press, Boca Raton, 1994, chap. 49. 

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