Crystal-to-crystal diastereoselective transformation of 2,4,6-triisopropyl-4'-(S)-phenylalaninocarbonylbenzophenone methyl ester

Dissymmetry of the photoproduct was induced by using a chiral substituent, (S)-methylphenylalanine, in the title compound {N-4-(2,4,6-triisopropylbenzoyl)benzoyl] (S)-phenylalanine methyl ester (I)}. On irradiation with light from a 250 W ultra-high-pressure Hg lamp for 7 h through a long-pass filter, the photoreaction in a crystal was 100% complete without the loss of crystallinity. The crystal structures (I), before, and (II) {N-[4-(7-hydroxy 3,5-diisopropyl-8,8-dimethylbicyclo[4.2.0]octa-1,3,5-trie-n- 7-yl)benzoyl]-(S)-phenylalanine methyl ester}, after photocyclization, have been determined by X-ray diffraction. For comparison, a crystal structure analysis has also been carried out for the photoproduct (III) of the 3'-COOMe derivative after recrystallization (methyl 3-(7-hydroxy-3,5-diisopropyl-8,8-dimethylbicyclo[4.2.0]-octa-1,3,5-trien-7-yl)benzoate). The dihedral angle between the central carbonyl plane and the triisopropylphenyl ring deviates from 90 degrees by 10 (1)degrees in (I), which makes an imbalance in the intramolecular O(carbonyl)... H(methine) distances of the isopropyl groups at positions 2 and 6. The crystal structure of (II) indicates that the nearer methine H was predominantly abstracted by the carbonyl O atom in the reaction. The absolute configuration around the asymmetric C atom in the cyclobutenol ring of the product is S.