Guanidinium-based sulfonic acid: an efficient Brønsted acid organocatalyst for the synthesis of fused polycyclic dihydropyridines in water

An efficient and eco-friendly protocol for the synthesis of polycyclic dihydropyridine scaffolds has been developed from readily accessible 4-hydroxycoumarin, aldehydes, and ammonium acetate in the presence of guanidinium-based sulfonic acid as a Bronsted acid organocatalyst. This method provides several advantages, such as simple work-up procedure, environmentally benign, recyclable catalyst, and high yields. In addition, water was chosen as a green solvent.