Absolute Asymmetric Photocyclization of Isopropylbenzophenone Derivatives Using a Cocrystal Approach Involving Single-Crystal-to-Single-Crystal Transformation

Absolute asymmetric photocyclization of isopropylbenzophenone derivatives was achieved by means
of a cocrystal approach. Three chiral salt crystals formed by carboxylic acid derivatives with achiral
amines could be prepared by spontaneous crystallization. In the M-crystal of 4-(2,5-diisopropylbenzoyl)benzoic acid with 2,4-dichlorobenzylamine, a twofold helical arrangement occurs in a
counterclockwise direction to generate the crystal chirality. Conversely, the clockwise helix exists
alone in the P-crystal. Irradiation of the M-crystal at >290 nm caused highly enantioselective
Norrish type II cyclization to give the (R,R)-cyclopentenol, (R)-cyclobutenol, and (R)-hydrol in a
6:3:1 molar ratio, resulting in successful absolute asymmetric synthesis, while irradiation at around
350 nm afforded the (R,R)-cyclopentenol as the sole product. The reaction proceeded via single-crystal-to-single-crystal transformation, and therefore the reaction path producing the (R,R)-cyclopentenol could be traced by X-ray crystallographic analysis before and after irradiation.