Enantioselective photodeconjugation of conjugated esters and lactones

The photodeconjugation of conjugated esters and lactones in the presence of small amounts of chiral catalysts is enantioselective. The enantiomeric excess is very sensitive to the conditions of the reaction. When chiral amino alcohols are used as chiral catalysts a synergism is observed between the effect of the amino and hydroxyl groups on the optical purity of the deconjugated molecules. The chiral discrimination between the two diastereoisomeric transitions states leading to the R and S configuration of the product is caracterized by its t LH# and t A model which explains the results is proposed and some examples of generalisation of these studies are described. Optically active molecules are very important in nature and the preparation of pure enantiomers either by resolution of racemates or more recently by asymmetric synthesis has become a challenge of high practical importance for the organic chemist. New very efficient methodologies such as the Sharpless oxidation allowing the synthesis of almost pure enantiomers have been proposed by organic chemists. However, up to now, photochemistry has not been very successful in the field of enantioselective reactions (1). To induce asymmetry, several photochemical ways have been explored: Circularly polarized light (C.P.L.) as the source of chirality, of great theoretical importance, has been used for the enantioselective synthesis of hexahelicenes. However, the optical purity of the products is very low. When C.P.L. is used for the enantioselective destruction of a racemate, high optical purities can be obtained for high conversion rates (2). Unfortunately, high optical purities require the destruction of most of the starting racemate. The bimolecular photoreaction of a chiral molecule with a non chiral molecule may lead to considerable asymmetric induction. Furthermore, the diastereoisomers can be isolated in pure forms and, after removal of the chiral inductor, high optical yields can be obtained. Among these reactions, photocycloadditions of conjugated chiral molecules to alkenes seems very promising for synthetic purposes (3). In some cases the chiral inductor is consumed during the photochemical step and therefore cannot be easily regenerated. Photochemical reductions of ketones by enantiomerically pure amines have also been studied but the optical purity of the alcohols formed remains low (4). The use of chiral sensitizers (5) or chiral solvents (6) also gave very disappointing results in inducing enantioselectivities. The definition of highly enantioselective or very diastereoselective photoreaction would have important consequences in organic synthesis and we examined different possibilities for increasing the enantioselectivity of photochemical reactions. A first possibility would be to create strong interactions between the chiral inductor and the excited state in the transition state leading to the products. Unfortunately, in most cases the lifetimes of the excited states are not long enough to allow strong bimolecular interactions with a chiral catalyst before the deactivation of the excited state. Some photochemical reactions produce ground state intermediates too unstable to be isolated but having lifetimes far longer than the starting excited states enabling their chemical trapping. In principle, it would be possible to induce the enantioselective transformation of such ground state intermediates if they could develop strong interactions with a chiral agent. To test this idea we turned on the photodeconjugation of aliphatic c, s-unsaturated carbonyl compounds (7) and especially conjugated esters (8) or lactones (9). The photodeconjugation reaction of ct, s-unsaturated esters can be very selective and almost quantitative. At 254 nm, the starting ester absorbs UV light far more strongly than the deconjugated product. In polar media, the reaction can be stopped when all the starting material has disappeared. After elimination of the solvent, the deconjugated ester is usually the only new product.