Chemistry of Azulenequinones and Their Analogues

Azulenequinones have been attracting great interest in recent years. After a brief introduction concerning an earlier study of azulenequinones we review recently developed areas of azulenequinone chemistry, which became possible by the discovery of a facile one-pot synthesis of azulenequinones, or their bromo derivatives, by means of the bromine oxidation of various azulene derivatives in an aqueous solvent. Various functionalized, and some annulated, azulenes are used as substrates to determine the scope and limitations of this method. The bromoazulenequinones can be used as synthons, from which various azulenequinones can be derived by nucleophilic displacement. In a hope to encourage other chemists to develop new azulenequinone chemistry, we herein describe the process of the discovery, starting from the original motivation. In addition, we touch on remarkable and mysterious products whose structures remain undetermined and our speculation concerning their reaction paths. Some physical and chemical properties of azulenequinones, together with those of azulenequinone methides and diazoazulenequinones, are also described. This is the first complete review of the field of newly developing azulenequinones.