Simple and efficient protocol for synthesis of pyrido[1,2-a]pyrimidin-4-one derivatives over solid heteropolyacid catalysts

Abstract

Aluminium exchanged tungstophosphoric acid salts with Keggin structure (Al_xH_3−xPW₁₂O₄₀) were prepared by simple ion exchange method. The prepared heteropolyacid salts were characterized by various techniques such as elemental analysis, XRD, FTIR, UV-vis, N₂-physisorption, NH₃-TPD and FTIR-pyridine adsorption. All the prepared catalysts possess both Brønsted and Lewis acid sites, however increase of surface area and Lewis acid sites was observed with increase of Al content. The catalysts have been applied as heterogeneous catalysts to synthesize pyrido[1,2-a]pyrimidines under mild reaction conditions. A variety of biologically active pyrido[1,2-a]pyrimidines were obtained in best yields ever (above 90%) by using a simple, eco-friendly and efficient protocol catalyzed by Al_xH_3−xPW₁₂O₄₀ catalysts. Al₃PW₁₂O₄₀ catalyst showed highest activity due to introduction of more number of Lewis acid sites to parent H₃PW₁₂O₄₀ by complete exchange of H⁺ ions by Al³⁺ ions.