Transition Metal-doped Heteropolyacid Catalysts for the Suitable Multicomponent Synthesis of Monastrol and Bioactive Related Compounds

Background: Molybdophosphoric acid (PMo) is used in heterogeneous catalysis. The incorporation of vanadium in the primary structure and a transition metal in the secondary structure of PMo improves the oxidative and acidity capacity. 4- Dihydropyrimidin-2-(1H)-thiones/-ones (DHPMs) derivatives are relevant because of their biological activities. The development of ecofriendly procedures, such as multicomponent reactions, to prepare DHPMs is an important challenge for organic chemistry. Objective: Study the effect of adding transition metal counterions to 11-molybdo-1- vanadophosphoric acid (PMoV) on its catalytic activity for the synthesis of bioactive DHPMs derivatives. Method: Transition metal-doped heteropolyacid catalysts (PMoV, FePMoV, and CuPMoV) were prepared and characterized by Fourier transform infrared spectroscopy, 31P nuclear magnetic resonance, and diffuse reflectance spectroscopy. The acid properties were determined by potentiometric titration. A series of bioactive DHPMs derivatives, such as monastrol, were synthesized by a one-pot procedure using the catalysts under solvent-free conditions. Results: The incorporation of V, Fe and Cu in the primary or secondary structure of PMo improved the catalytic activity of the materials. The proposed DHPMs synthesis required short reaction time (1 h) and a temperature of 80°C. A correlation between the yields of monastrol and the acid strength or the number of acid sites of the catalysts was observed, namely, FePMoV>CuPMoV>PMoV>PMo. The FePMoV catalyst was easily recycled and reused without appreciable loss of activity. Conclusion: The described method is a simple, clean and environmentally friendly alternative for the preparation of 22 bioactive substituted DHPMs with very good yields and high selectivity.