What is "oxepin"

n. (context organic compound English) The unsaturated seven-membered heterocycle containing six carbon atoms, one oxygen atom and three double bonds; it is tautomeric with the epoxide benzene oxide

Oxepin is an oxygen-containing heterocycle consisting of a seven-membered ring with three double bonds. It exists as an equilibrium mixture with benzene oxide. The oxepin–benzene oxide equilibrium can easily be shifted to one extreme or the other in compounds with this core depending what substituents are present. This compound is not aromatic.