Azulene, which has a speciﬁc structure of ring-fused unsaturated seven-membered and ﬁve-membered rings, is a typical non-benzenoid aromatic compound. Whereas naphthalene or other hydrocarbon compounds are generally colorless, azulene, a structural isomer of naphthalene, is dark blue, and the name comes from the Spanish word azul, meaning "blue". Three practical manufacturing methods have been known so far as follows:The ﬁrst method, which was developed by Ziegler and Hafner, is a reaction of ring-opened pyridinium salts or pyrylium salts with cyclopentadienides. The second one, which was developed by Nozoe and Seto et al. is a reaction of tropone derivatives bearing halogen, methoxy, or tosyloxy groups at the 2 position and active methylene compounds, such as cyanoacetates and malonates, in the presence of base. The last method, which was developed by Yasunami and Takase et al., is a reaction of oxaazulanones with enamines. Tropolone derivatives are also classiﬁed as non-benzenoid aromatic compounds. Hinokitiol, a typical tropolone compound, has been known to exhibit an antibacterial effect. Also colchicine, an alkaloid compound with a tropolone ring, shows strong antitumor effects. Thus, tropolone compounds have possibilities in the pharmaceutical ﬁeld, especially for anticancer drugs.
1)K. Ziegler, K. Hafner, Angew. Chem.1955, 67, 301.
2)T. Nozoe, S. Seto, S. Matsumura, T. Asano, Proc. Japan Acad.1956, 32, 339.