Synthesis, Reactivity, and Properties of Benz[ a ]azulenes via the [8 + 2] Cycloaddition of 2 H -Cyclohepta[ b ]furan-2-ones with an Enamine.
Taku ShojiAkari YamazakiRyuzi KatohKonomi ShimamuraRina SakaiMasafumi YasunamiTetsuo OkujimaShunji ItoPublished in: The Journal of organic chemistry (2022)
Starting with the reaction of 2 H -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% H 3 PO 4 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.