The intramolecular 5-exo, 7-endo-dig transition metal-free cyclization sequence of (2-alkynylphenyl) benzyl ethers: synthesis of seven-membered fused benzo[b]furans
Base-promoted cyclization of (2-alkynylphenyl) benzyl ethers was studied in detail. The effects of the solvent, base, temperature, reaction time and amount of base on the efficacy of the cyclization reaction were analyzed and a new base–solvent system (tert-BuOK/DMSO) for effective cyclization of (2-alkynylphenyl) benzyl ethers was reported. The results showed that the cyclization reactions proceeded cleanly and smoothly under mild reaction conditions, employing tert-BuOK as a base, and DMSO as a solvent, at room temperature in a short reaction time (1 h). Under these conditions, a number of different substituted (2-alkynylphenyl) benzyl ethers were cyclized to the corresponding fused heterocycle cyclohepta[b]furans. This one-pot, two-step procedure occurred regioselectively giving only the cyclohepta[b]furans as the unique regioisomers via an initial intramolecular 5-exo-dig mode followed by an intramolecular 7-endo-dig mode. The cyclohepta[b]furan derivatives absorbed in the UV region (300–350 nm range) with molar absorptivity coefficient values attributed to spin and symmetry allowed π–π* electronic transitions. An emission located in the purple region (380–440 nm range), with a Stokes shift between 65–100 nm is probably associated with the charge transfer character of the excited state. The electrochemical analysis (CV) of the cyclohepta[b]furans showed an oxidation and reduction process, probably due to the presence of the selenium atom and π-anion radical species.