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.

Automated summary

Due to the unique chemical structure and photophysical properties, azulene derivatives hold great potential for various technical applications, such as molecular switching, sensors, solar cells, and biological activities. Apart from functionalizing existing azulene cores, direct construction of azulenes through condensation with odd-membered carbocycles is another classical method. Unconventional cyclization reactions involving alkynes can also lead to the formation of azulene, pseudoazulene, or azulenone skeletons. This mini review provides a classification of the interesting and practical cyclization reactions based on the function of alkynes, with the aim of attracting attention and promoting the development of nonalternant cyclization reactions of alkynes.