On irradiation in solution with ultraviolet (UV) light, cis-stilbene undergoes reversible photocyclization to give trans-4a,4b-dihydrophenanthrene, an intermediate that can be trapped oxidatively with hydrogen acceptors such as iodine, oxygen, or tetracyanoethylene to give phenanthrene in high yield. This type of photoreaction occurs with a wide range of substituted stilbenes and related molecules, including various polycyclic and heterocyclic analogs; in addition, certain systems with a single heteroatom (nitrogen, oxygen, or sulfur) in place of the central π bond undergo photocyclization, creating a new five-membered heterocyclic ring. In some of these photocyclizing systems, the transient dihydroaromatic intermediates are trapped without added oxidants by the elimination of suitable leaving groups or by various hydrogen shifts.
Photocyclization is the preferred method for the synthesis of many different polynuclear aromatic systems.
The discovery and early development of stilbene photocyclization has been surveyed, and several other general reviews have appeared.