V‑Shaped Bis-Coumarins: Synthesis and Optical Properties

A highly efficient procedure for the synthesis of bis-coumarins fused at the pyranone ring has been developed. The electron-rich phenols reacted with esters of coumarin-3-carboxylic acids, leading to substituted chromeno­[3,4-<i>c</i>]­chromene-6,7-diones. The reaction is catalyzed by both Lewis acids and 4-dimethylaminopyridine. The most probable mechanistic pathway involves Lewis acid catalyzed or DMAP catalyzed transesterification, followed by intramolecular conjugate addition of α,β-unsaturated esters to phenols and subsequent oxidation of the initially formed intermediate. The reaction is compatible with various functionalities such as NO₂, Br, and OMe. Not only benzene derivatives but also dihydroxynaphthalenes are reactive in this reaction, and the structure of the product can be controlled by adjusting the reaction conditions. Furthermore, a double addition is possible, leading to a horseshoe-shaped system comprised of seven conjugated rings. Compounds with four structurally unique skeletons have been obtained and have been shown to strongly absorb in the violet, blue, and/or green regions of the visible spectrum. Most of them display strong greenish yellow fluorescence, which can be modulated by both structural changes and the character of the solvents. Again, introduction of an electron-donating group in the chromeno­[3,4-<i>c</i>]­chromene-6,7-diones caused a significant red shift in both the absorption and emission maxima, and the effect became especially noteworthy in the case of amino substituents