Heteropolyacid supported on amine-functionalized halloysite nano clay as an efficient catalyst for the synthesis of pyrazolopyranopyrimidines via four-component domino reaction

An efficient heterogeneous hybrid catalyst was developed by functionalization of halloysite clay nanotubes by γ-aminopropyltriethoxysilane and then immobilization of a Keggin type heteropolyacid, phosphotungstic acid. This hybrid catalyst was characterized by SEM/EDX, FTIR and XRD and its catalytic activity for the synthesis of pyrazolopyranopyrimidine derivatives via four-component domino reaction of barbituric acid, hydrazine hydrate, ethyl acetoacetate and benzaldehyde was investigated. The results indicated that the hybrid system can promote the reaction to afford the desired products in high yields and short reaction times. The superior catalytic activity of this system was confirmed when compared with those reported, previously. Moreover, this novel heterogeneous catalyst was found to be easily separable and recyclable. It was re-used at least three times with negligible loss of activity. Significantly, this protocol can be extended to nonconventional green heating sources such as microwave and ultrasonic irradiations