Kinetic studies of the ring-contraction from 1a to 1b under a protective N2 atmosphere revealed that this process is a zero-order reaction, typical for a photocyclization reaction (see SI for details). The lightinduced isomerization of 1a to 1b follows the expected Woodward-Hoffmann rules and proceeds in a disrotatory manner. Different solvents do not have an obvious influence on the yields (Table and).