HIGHLY ENANTIOSELECTIVE PHOTODECONJUGATION OF ALPHA,BETA-UNSATURATED ESTERS - ORIGIN OF THE CHIRAL DISCRIMINATION

The photodeconjugation of α-methyl conjugated esters can be highly enantioselective when carried out in CH2Cl2 or hexane in the presence of catalytic amounts of chiral β-amino alcohols. Enantiomeric excesses (ee%) up to 70% are described. A correspondence is established between the configuration of the new asymmetric center of the deconjugated ester and the configuration of the asymmetric carbon linked to the nitrogen of the chiral auxiliary. A model involving a nine-membered-ring transition state is proposed to rationalize the results. The effects of the reaction conditions on the level of enantioselectivity arc examined. Discrimination parameters ΔΔH* and ΔΔS* can be determined from experiments at various temperatures. For a given ester and β-amino alcohol pair, ΔΔS* and ΔΔH* are shown to have the same sign which indicates that the major enantiomer corresponds to the most ordered diastereoisomeric transition state. Among the great variety of chiral β-amino alcohols used in this study, 2-(N-isopropylamino)-1-phenyl-1-propanol appears to be the best choice to induce high ee%. The scope and limits of the enantioselective photodeconjugation and the generality of the model are discussed. © 1990, American Chemical Society. All rights reserved.