We have developed a procedure for the direct synthesis of 2-arylazulenes. The 2-arylazulenes are formed in moderate to excellent yields by [8+2] cycloaddition of 2H-cyclohepta[b]furan-2-one derivatives with aryl-substituted silyl enol ethers.
In this paper,w ed escribe an efficient and atomeconomicals ynthesis of highly functionalized pyrroles, pentafulvenes, and pyrrolopyridinesb y[ 2 + +2] cycloaddition-retroelectrocyclization of N-substituted propargylamines with tetracyanoethylene,f ollowed by the treatment of the resultingt etracyanobutadiened erivatives with silica gel. In this reaction, silica gel plays an important role to promote the intramolecular cyclization to afford the heterocyclic products from the tetracyanobutadiene intermediates. The products were obtaineds electively depending on the substituent on the nitrogen atom of the starting propargylamines.Scheme1.Our previous study and planneds trategy to access highly functionalized pyrrole derivatives.[a] Prof.
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.
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