Diastereospecific Photocyclization of a Isopropylbenzophenone Derivative in Crystals and the Morphological Changes

Reaction of crystals of 2,4,6-triisopropylbenzophenone derivative with the (S)-phenylethylamide group
caused diastereospecific Norrish type II photocyclization by UV irradiation to give (R,S)-cyclobutenol
as a sole product. In contrast, the solution photolysis gave an almost 1:1 mixture of (R,S)- and (S,S)-cyclobutenol. The specific diastereodifferentiation in the crystalline state is attributed to the smooth
transformation with minimum molecular motion due to the very similar molecular shapes as well as the
2-fold helical arrangements between the reactant crystal and the product (R,S)-cyclobutenol crystal. UV
irradiation of the bulk crystals led to cracking and breaking into small fragments. In contrast, the
microcrystals maintained the single-crystalline morphology in the course of photocyclization, suggesting
the single-crystal-to-single-crystal transformation.