Direct Access to Functionalized Azulenes and Pseudoazulenes via Unconventional Alkyne Cyclization Reactions

Owing to the unique chemical structure and photophysical properties of azulene, azulene derivatives show great promise for a wide range of prospective technical applications, such as molecular switching, sensors, solar cells, and biological activities. In addition to functionalization of the existed azulene cores, direct construction of azulenes via condensation with odd-membered carbocycles such as tropolone, 2-oxazulanone, cyclopentadiene, and 6-aminofulvene is another classical method for obtaining azulene derivatives. Although alkynes are widely explored for construction of benzene rings, several unconventional cyclization reactions with alkynes produce azulene, pseudoazulene, or azulenone skeletons. This minireview summarizes these interesting and practical cyclization with a classification based on the function of alkyne in the azulene formation process, aiming at capturing the attention of the scientific community to make efforts in the development of such nonalternant cyclization reactions of alkynes.