Synthesis, Reactivity, and Properties of Benz[a]azulenes via the [8 + 2] Cycloaddition of 2H‑Cyclohepta[b]furan-2-ones with an Enamine

Starting with the reaction of 2H-cyclohepta­[b]­furan-2-ones with an enamine, which was prepared from 4-tert-butylcyclohexanone and pyrrolidine, benz­[a]­azulenes having both formyl and tert-butyl groups were obtained in the three-step sequence. Subsequently, both the formyl and tert-butyl groups were eliminated by heating the benz­[a]­azulene derivatives in 100% H3PO4 to give benz­[a]­azulenes without these substituents in high yields. In terms of product yield, this method is the best one ever reported for the synthesis of the parent benz­[a]­azulene so far. The conversion of the benz­[a]­azulene derivatives with a formyl group into cyclohept­[a]­acenaphthylen-3-one derivatives was also investigated via Knoevenagel condensation with dimethyl malonate, followed by Brønsted acid-mediated intramolecular cyclization. The structural features including the bond alternation in the benz­[a]­azulene derivatives were revealed by NMR studies, NICS calculations, and a single-crystal X-ray structural analysis. The optical and electrochemical properties of a series of benz­[a]­azulene derivatives were evaluated by UV/Vis, fluorescence spectroscopy, and voltammetry experiments. As a result, we found that some benz­[a]­azulene derivatives showed remarkable luminescence in acidic media. In addition, the benz­[a]­azulene derivatives with the electron-withdrawing group and cyclohept­[a]­acenaphthylen-3-one derivative displayed good reversibility in the spectral changes under the electrochemical redox conditions.