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‐trien‐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° by 10 (1)° 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 ed 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.