Synthesis of naphth[2,1-a]azulenes via [8 + 2] cycloaddition of 2H-cyclohepta[b]furan-2-ones, and their reactivity, optical, and structural properties.

A concise and efficient synthetic route to naphth[2,1-a]azulenes was developed via [8 + 2] cycloaddition of 2H-cyclohepta[b]furan-2-ones with enamines or a silyl enol ether derived from α-tetralones. A diverse array of functionalized naphth[2,1-a]azulenes was further obtained through functionalization reactions, including electrophilic substitution, homocoupling, and cross-coupling reactions. Comprehensive spectroscopic analyses, including ¹H NMR, UV-Vis, fluorescence spectroscopy, and single-crystal X-ray diffraction, along with NICS calculations, revealed reduced aromaticity of the azulene core and a strong dependence of optical properties on the substitution pattern. Furthermore, naphth[2,1-a]azulenes exhibited pronounced halochromic behaviour and fluorescence in acidic media. Electrochemical studies indicated predominantly irreversible redox behaviour, however, certain donor-acceptor systems displayed quasi-reversible responses attributed to slow electron transfer kinetics.