Photodeconjugation enantioselective

Accordingly, apolar solvents, in particular n-hexane and dichloromethane, were found to be convenient for these enantioselective photodeconjugation reactions. 

The amide (38a) is photochemically inert on irradiation in ether. The related compound (38b) is, however, photochemically reactive and undergoes fission by a Norrish Type II process to yield a mixture of products.The results of a study of the enantioselective photodeconjugation reactions of the lactones (39) have been published. The behaviour of the ketones (40) and (41) in the isotropic and the two solid phases of heneicosane (CcxH ) has been evaluated. The influence of the various phases on the ratio of elimination to cyclization products of the ketones was discussed. The modification of the photoreactivity of ketones (42) in cyclodextrin has been... 

Henin, F., Mortezaei, R., Muzart, J., Pete, J.-P, and Piva, O., Enantioselective photodeconjugation of conjugated esters and lactones in the presence of ephedrine. Tetrahedron, 45, 6171, 1989,... 

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]. 

To minimize this troublesome process, the concentration of the intermediate has to be kept as low as possible by means of catalytic tautomer-ization of the dienol. To make the chiral discrimination as great as possible, a strong interaction between the prochiral intermediate and the chiral environment is needed. When the chiral inductor is introduced into the starting molecule itself, diastereoselective photodeconjugations can be observed. If an external chiral inductor is used, enantioselective protonations are obtained. This approach, which may be catalytic if the chiral auxiliary is not consumed in the process, can take advantage of the acidic property of enols [27] and of the possibility of an acid- or base-catalyzed tautomer-ization of enols [28]. 

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. 

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

Synthetic applications of the diastereoselective photodeconjugation of unsaturated esters 13 and 17 were explored and compared with the corresponding enantioselective approach. Highly enantioselective syntheses... 

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. 

Asymmetric Photodeconjugation Principle Enantioselective Photodeconjugation of a,P-Unsaturated Esters and Lactones Diastereoselective Photodeconjugations. .. 70-7... 

While the concept of enantioselective photodeconjugation is of great interest, the process is efficient in relatively few cases and cannot be appHed to the total synthesis of natural products. The diastereoselective reaction has been also examined in parallel to the enantioselective one. High levels of induction (up to 88%) were first achieved using Hehnchen s derivatives as chiral alkoxy moieties (Scheme 20, entry a) " unfortunately, these chiral alcohols are expensive or require numerous steps to be prepared. 

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. 

This chapter covers organocatalytic processes where the enantio-determining step involves a proton transfer. Most of the organocatalytic processes outlined herein share a key step in common, i.e. the enantioselective protonation of an enolate or enol intermediate species obtained in situ from various precursors. The main organocatalytic approaches reported in the literature may be classified according to the nature of these precursors (Scheme 3.1). Special emphasis will be giveu to decarboxylation of malonates, addition of protic nucleophiles (NuH) to keteues or to a,P-unsaturated carbonyl compounds. We will also focus on tautomerisation of enols formed in situ via photodeconjugation of a,P-unsaturated esters and on the protonation of silyl enolates. Finally, a last section will be devoted to other miscellaneous substrates. 

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