Symmetric and asymmetric photocleavage of the cyclobutane rings in head-to-head coumarin dimers and their lactone-opened derivatives

The photochemical behaviour of fifteen derivatives of head-to-head coumarin dimers has been investigated. The photocleavage of the cyclobutane rings in these compounds in solution occurs on irradiation with light of wavelength 277 ± 10 nm to give two molecules of coumarin derivatives via symmetric fission and/or 2,2′-dihydroxystilbene and fumaric (and maleic) acid derivatives via asymmetric fission. The direction of these photofissions is primarily affected by the basic structure of the coumarin dimer derivative, the photocleavage proceeding exclusively with retention of the ring structure when the derivative has a stable five- or six-membered ring fused to its cyclobutane ring. A secondary influence is the steric repulsion between the substituents attached to the cyclobutane ring.