X-ray structure and photoreactivity of pseudo-gem cinnamophane dicarboxylic acid {bis-4,15-(2'-hydroxycarbonylvinyl)[2.2]paracyclophane}

The pseudo-gem cinnamophane dicarboxylic acid 1 was shown to undergo a stereospecific [2+2] photocycloaddition in the solid slate, generating the truxinic acid derivative 2 in 100% chemical yield; the reaction can proceed to completion because it is not an ''independent site'' solid state reaction. The X-ray structure of 1 reveals that its molecules are associated in dimers formed from four almost linear hydrogen bonds. A salient feature is the intramolecular centre-to-centre distance between the reacting double bonds of approximate to 3.37 Angstrom, the smallest known to date. In contrast to trans-cinnamic acid, the topochemical [2+2] photocycloaddition occurs also in solution, with a quantitative chemical yield and a quantum yield of approximate to 0.55 (in methanol).