PHOTO-POLYMERIZATION IN CHIRAL CRYSTALS .3. TOWARD AN ABSOLUTE ASYMMETRIC SYNTHESIS OF OPTICALLY-ACTIVE DIMERS AND POLYMERS WITH QUANTITATIVE ENANTIOMERIC YIELD

An “absolute” asymmetric synthesis of chiral dimers and polymers of variable optical yield and an asymmetric synthesis with quantitative enantiomeric yield starting from a monomer of low optical purity are described. Irradiation of polycrystalline ethyl 2-cyano-3-(p-sec-butyl-3-'(E)-propenoate)phenyl(E)-propenoate (1) of 60% optical purity or above leads to the formation of chiral dimers and oligomers with quantitative enantiomeric yield. Carefully crystallized samples of 1 of 22-60% optical purity lead to enantiomeric excesses of 90% or above. Racemic 1 crystallizes from the melt either in the form of a stable eutectic mixture or in the form of an isomorphous, chiral, metastable phase of different microcomposition. A method for the characterization of the two phases, based on photodimerization and NMR analysis of the diastereomeric dimers, is described. Irradiation of samples of the chiral metastable phase with one enantiomorphous crystal form in excess leads to an “absolute” asymmetric synthesis with variable optical yield, of both signs. Possible routes to the achievement of an “absolute” asymmetric synthesis with quantitative enantiomeric yield, in systems related to 1, are discussed. © 1979, American Chemical Society. All rights reserved.