Reversal of regioselectivity (straight vs. cross ring closure) in the intramolecular [2+2] photocycloaddition of phenanthrene derivatives.

Intramolecular [2+2] photocycloaddition of aromatic chain imides possessing bis-phenanthrene moieties afforded straight and cross ring closure products. The ratio of cycloadducts was dependent on reaction time and temperature, which resulted in a reversal of regioselectivity. The reaction was proved to involve a retro cycloaddition.