Photochemical Asymmetric Synthesis. Irradiation of Ring and Open‐chain Derivatives of L‐Erythritol 1,4‐Dicinnamate

The two analogs, (−)‐2,3‐di‐O‐methyl‐L‐erythritol 1,4‐di‐trans‐cinnamate, 1, and (−)‐2,3‐O‐isopropylidene‐L‐erythritol 1,4‐di‐trans‐cinnamate, 2, were irradiated in benzene solution. Intramolecular 2 + 2 photocycloaddition occurs in both derivatives and the stereochemistry of the cyclobutane products, after removal of the L‐erythritol, was established in both cases as β (symmetry m)‐ and δ (symmetry 2)‐truxinate. While the asymmetric induction was 85% in the synthesis of (+)‐dimethyl δ‐truxinate from the open‐chain dimethoxy derivative 1, the more rigid isopropylidene ring derivative led to an asymmetric induction of only 6% and afforded the opposite enantioner, (−)‐dimethyl δ‐truxinate, in excess. Computer simulation of the diastereomeric ground state conformations which are believed to lead to the observed products from 1 was carried out to enable better understanding of the factors which determine the asymmetric synthesis. The simulations afford geometries and relative energies of the product precursor conformations. The results are in qualitative agreement with the experimental observations in predicting an excess of one of the two possible δ‐diastereomers. Since the geometry is also obtained by the simulation, it allows one to predict the absolute configuration of the predominant chiral product.