Double [2+2] Photocycloaddition: Topochemical Conversion of 4-Methyl-7-Styrylcoumarin Dimorphs into a Strained Cyclophane

2-Pyranone-annulated trans-stilbene, viz., 4-methyl-7-styrylcoumarin (4), crystallizes in two photoreactive plate-
and needle-like crystal modifications, each of which undergoes topochemically controlled stereospecific double [2+2] photocycloaddition to afford a strained tricyclic paracyclophane 6 in an isolated yield determined essentially by the molecular packing of
the precursor modification. That the double photocycloaddition occurs under a strict control of the crystal lattice is demonstrated by
the fact that the intermediary mono-cycloaddition product 5 does not lead to the double dimer upon photolysis either as an amorphous
powder or as a thin film or in the solution state. In the context of polymorphism, the dimorphs of 4 constitute the first examples of
photoreactive crystal modifications that undergo rare double [2+2] photocycloaddition reactions to yield a highly strained and
otherwise inaccessible cyclophane.