The Photodimerisation of Crystalline 2,5-Dibemzylidenecyclopentanome

The solid state [2+2] photocyeloaddition of 2,5-dibenzylidenecyclopentanone and the photostability of 2,5-dibenzylidene-3(+)methylcyclopentanone are used to show that topochemical reaction may proceed in crystals in which the relative orientation of potentially reactive double bonds deviates from that hitherto generally regarded as necessary for such processes.