Iron-catalyzed divergent synthesis of coumarin fused N-heterocycles via 6π-electrocyclization and isoxazole ring-opening

We have developed a facile, iron-catalyzed one-pot strategy for the regioselective synthesis of chromeno[4,3-b]quinolin-6-one derivatives. This transformation proceeds via a three-component tandem annulation involving 4-hydroxycoumarin, aldehydes, and anilines, through a one-pot imine formation followed by a 6π-electrocyclization under acidic conditions. Furthermore, we extended this methodology to a three-component annulation of 4-hydroxycoumarin with ammonium acetate and isoxazole derivatives, proceeding via iron-catalyzed Michael-type addition of isoxazoles, followed by ring opening and subsequent cyclization towards synthesis of chromeno[4,3-b]pyridine. The photophysical properties of the resulting fused heterocycles showed moderate to good fluorescence quantum yields.