Photodienols enantioselective protonation

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 5. Enantioselective protonation of photodienols by chiral -aminoalcohols.

Enantioselective protonation of the prochiral photodienol performed by use of an external chiral source of protons (pathway a)... 

Piva, O. and CarameUe, D., Asymmetric protonation of photodienols enantioselective synthesis of (R)-2-methylalkanols, Tetrahedron Asymm., 6, 831, 1995. 

Scheme 3.35 Enantioselective protonation of photodienols oiganocatalyzed by p-amino alcohols...

It was shown previously that photodienols derived from a-substituted conjug) aliphatic esters could be protonated enantioselectively in aprotic solvents an<... 

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]. 

The enantioselectivity observed for the deconjugation of ester and lactones is very sensitive to temperatures. Concentrations of R and S isomers formed are directly proportional to the rates /cr and of protonation of the intermediate photodienol. 

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