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Non-benzenoid Aromatic Compounds [Chemical Structural Class]

 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.

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