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Photodienols protonation

It was shown previously that photodienols derived from a-substituted conjug) aliphatic esters could be protonated enantioselectively in aprotic solvents an<... [Pg.218]

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]. [Pg.142]

Figure 1. Transition state involved in the protonation of a photodienol in the presence of a /3-aminoalcohol. Figure 1. Transition state involved in the protonation of a photodienol in the presence of a /3-aminoalcohol.
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. [Pg.144]

Figure 3. Transition states involved in the asymmetric protonation of a photodienol induced by ( —)-ephedrine. Figure 3. Transition states involved in the asymmetric protonation of a photodienol induced by ( —)-ephedrine.
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. [Pg.150]

Scheme 5. Enantioselective protonation of photodienols by chiral -aminoalcohols. Scheme 5. Enantioselective protonation of photodienols by chiral -aminoalcohols.
When the starting ester is substituted by a group different from a hydrogen atom, the photodienol produced can be considered as a prochiral species. The selective protonation of one of the two Si or Re faces can then be expected under appropriate conditions to furnish a mixture enriched in one of the two stereoisomers. Two approaches toward this goal have been considered for esters, as depicted on Scheme 16 ... [Pg.1441]

Enantioselective protonation of the prochiral photodienol performed by use of an external chiral source of protons (pathway a)... [Pg.1442]

Awandi, D., Henin, E, Muzart, J., and Pete, J. R, Reversal of diastereoselectivity in protonation of chiral photodienols. Tetrahedron Asymm., 2,1101,1991. [Pg.1451]

Piva, O. and CarameUe, D., Asymmetric protonation of photodienols enantioselective synthesis of (R)-2-methylalkanols, Tetrahedron Asymm., 6, 831, 1995. [Pg.1452]

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

See other pages where Photodienols protonation is mentioned: [Pg.323]    [Pg.323]    [Pg.142]    [Pg.149]    [Pg.150]    [Pg.1451]    [Pg.93]   
See also in sourсe #XX -- [ Pg.143 ]




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