The Paternò-Büchi reaction involves the photochemical reaction between a carbonyl compound and an alkene to form an oxetane ring. This reaction was first reported in 1909 by Paternò and Chieffi. Several mechanisms are possible, including those involving a diradical intermediate or photoinduced electron transfer. The reaction shows regioselectivity, site selectivity, and stereoselectivity that depend on factors like the solvent, substituents on the carbonyl and alkene, and temperature. The Paternò-Büchi reaction has been used to synthesize biologically active oxetane-containing compounds and to construct more complex carbocyclic and heterocyclic ring systems.