Procedures for the preparation of compounds
According to the previously published procedure [8] 4-hydroxycoumarin (0.324 g, 2 mmol) was dissolved in hot ethanol (6 mL), the appropriate aldehyde (1 mmol) was added and the reaction mixture was refluxed for 24 h. After cooling to room temperature, the solid was filtered out and crystallized to give the product benzylidene-bis-(4-hydroxycoumarin) derivatives 5a-h and 3-(6-oxo-(1H)-benzopyrano[4,3-b]benzopyran-7-yl)-4-hydroxycoumarin derivatives 6a-d (Scheme 1).