Rearrangement of the carbon skeleton in the intramolecular photoadduct of anthracene and benzene rings

The effectivity of optical switching between anthracene derivatives and their intramolecular photocycloadducts is impaired by traces of acid. The systematic treatment of with an increasing excess of formic acid revealed that—apart from the normal enolether cleavage →→—a cleavage with rearrangement of the carbon skeleton can occur: →. The driving force is a stability enhancement of the involved carbenium ions →. A further increased excess of formic acid leads finally to a competitive ether cleavage in the tetrahydrofuran ring →.