Direct synthesis of 2-arylazulenes by [8+2] cycloaddition of 2H-cyclohepta[b]furan-2-ones with silyl enol ethers

Abstract

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.

Development of Heterocycle-Substituted and Fused Azulenes in the Last Decade (2010–2020)

“…There is only one example of electrophilic substitution reaction by heterocyclic compound at the 2-position of the azulene ring, but in this case, a strong electron-donating group such as a dimethylamino group at the 6-position and protection at the 1,3-positions of the azulene ring are both essential. Recently, the reaction of 2 H -cyclohepta[ b ]furan-2-one derivatives 192 with heterocycle-substituted silyl enol ethers have been reported as a new synthetic method for 2-heteroarylazulenes without using cross-coupling reactions ( Scheme 64 ) [ 102 ]. This method is applied to the synthesis of azulenes with various heterocycles such as 2-(pyridyl-, thienyl-, furyl-, and pyrrolyl)azulenes 193 , although the reaction requires high temperature (190 °C).…”

Synthesis and Halochromic Properties of 1,2,6‐Tri‐ and 1,2,3,6‐Tetra‐aryl Azulenes

“…More recently, some 2-functionalised azulene have also been prepared by several groups. [44][45][46][47][48] A distinct trend was observable, with both absorption and emission spectra being directly correlated to the electronic characteristic of aryl functionalization. The electronic absorption spectra were red-shifted when electronic donating groups were attached at the 2-position, while a corresponding blue-shift was observed with electronic withdrawing groups.…”

“…These differences were largely attributed to the direct stabilization of the azulenium cation by functionalization at the 4,7‐positions. More recently, some 2‐functionalised azulene have also been prepared by several groups [44–48] . A distinct trend was observable, with both absorption and emission spectra being directly correlated to the electronic characteristic of aryl functionalization.…”

Synthesis of thiophene-fused heptalenes by cycloaddition of azulenothiophenes with dimethyl acetylenedicarboxylate

“…In recent years, we have investigated the novel synthetic methods for various azulene derivatives and explored their distinctive reactivity 16 – 25 . In these researches, we have reported the synthesis, optical and electrochemical properties of azulenothiophene derivatives 23 , 26 – 28 that become good precursors for the thiophene-fused heptalenes.…”