Intramolecular photocycloaddition of anthracene and benzene ring systems

The anthracenes 3a–c, substituted in the 9-position with 3,5-dialkoxybenzyloxymethyl groups exhibit, in diluted solutions, intramolecular [4π+4π] photocycloaddition reactions to the polycyclic compounds 4a–c. The quantitative processes are completely reversible by heating, unless acid-catalyzed cleavages lead to thermally stable mono- and diketones: 4a–c→5a–c→6.