Azulene is a bicyclic non-benzenoid aromatic hydrocarbon, a structural isomer of naphthalene. Azulene is a bicyclic scaffold rarely applied in medicinal chemistry. Azulene is an underused lipophilic bicycle and should be considered as a valuable complement to the collection of medicinal chemistry scaffolds.
Aromatic hydrocarbons are cyclic, planar compounds that resemble benzene in electronic configuration and chemical behavior. Benzene has the molecular formula C6H6 and is the simplest aromatic hydrocarbon. The carbon atoms in benzene are linked by six equivalent σ bonds and six π bonds.