Esters chiral, photodeconjugation

Buffer catalysis has been applied to induce chiral induction by enantio-selective protonation remarkable enantiomeric excess was achieved in the photodeconjugation of a,/3-unsaturated ketones and esters by using chiral catalysts for the ketonization of photoenols in aprotic solvents.29... 

The percentage of ee does not depend on the conversion yield and the aminoalcohols can be recovered unchanged in the photodeconjugation of esters and lactones. A study of the dependence of the enantioselectivity of the photodeconjugation of esters in nonpolar solvents as a function of concentration of the chiral inductor (Fig. 2), indicated that very small amounts of the inductor are needed. 

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. 

Figure 4. Temperature effect of the enantiomeric excess of the photodeconjugation of esters 2b in the presence of chiral /3-aminoalcohols. (1) Inductor 7a (2) inductor 7b (3) inductor (-)-5c (4) inductor (-)-5b (5) inductor 7c.

For synthetic purposes, 2b only gives satisfactory selectivities, and disappointing results have been obtained with amides [49], a-aminoesters [24s], and a-bromoesters [49]. A more general access to chiral deconjugated esters is needed and diastereoselective photodeconjugations might be the solution to this problem. 

With chiral molecules, there is no need for external chiral inductors to induce a diastereoselection. For most esters made from conjugated acids and chiral alcohols, the diastereoisomeric excess (de) of the photodeconjugation process, carried out in apolar solvents containing catalytic quantities of alcohols (and/or amines), remains low. This results from the low steric interactions developed by the alkoxy group during the protonation step of the dienol, as already indicated [32]. However, if protonation of photo-dienols from one enantioface is sterically hindered, as can be observed for isobornyl derivatives 8c and 8d, values of de up to 95% can be attained [50] (Table 4 and Scheme 6). 

Piva O, Mortezaei R, Henin F, Muzart J, Pete JP. Highly enantioselective photodeconjugation of a,p-unsaturated esters, origin of the chiral discrimination. J. Am. Chem. Soc. 1990 112 9263-9272. 

Because the synthesis of chiral fluoro compounds is of great interest, due to their biological properties, the asymmetric photodeconjugation of a-fluoro-a,P-unsaturated esters 46 has been explored. Despite the high acidity of the hydrogen on the C2 atom, the diastereoselective isomerization to the P.y-unsat-urated isomers is effective with levels of induction for 47 similar to those for non-fluorinated substrates (Scheme 25). 

Mortezaei, R., Henin, F, Muzart, J., and Pete, J. R, Enantioselective photodeconjugation of a,P-unsaturated esters in the presence of catalytic amounts of a chiral-inducing entity. Tetrahedron Lett., 26, 6079, 1985. 

Bargiggia, F. and Piva, O., Diastereoselective photodeconjugation of chiral a,(i-unsaturated esters. Tetrahedron Asymm., 12,1389, 2001. 

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