An unexpected tetracyclic product isolated during the synthesis of biscoumarins catalyzed by [MIM(CH2)4SO3H][HSO4]: Characterization and X-ray crystal structure of 7-(2-hydroxy-4-oxo-4H-chromen-3-yl)-6H,7H-chromeno[4,3-b]chromen-6-one

Under solvent-free conditions and in the presence of 3-methyl-1-(4-sulfonic acid)butylimidazolium hydrogen sulfate [MIM(CH2)4SO3H][HSO4], a Brønsted acidic ionic liquid, the reaction of aromatic aldehydes with 4-hydroxycoumarin has been investigated. A wide range of aromatic aldehydes easily undergoes condensation with 4-hydroxycoumarin to afford biscoumarins with good purity in excellent yields. However, reaction of 2-hydroxybenzaldehyde with 4-hydroxycoumarin gave, not the corresponding biscoumarin, but a tetracyclic compound, 7-(2-hydroxy-4-oxo-4H-chromen-3-yl)-6H,7H-chromeno[4,3-b]chromen-6-one, in high yield.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► A Brønsted acidic ionic liquid was used as catalyst in the synthesis of Biscoumarins. ► Biscoumarins were prepared via the reaction of aryl aldehydes with 4-hydroxycoumarin. ► The reaction of 2-hydroxybenzaldehyde with 4-hydroxycoumarin gave a new compound. ► The new compound was characterized by spectral data and X-ray crystallography.