Synthesis of Pyrazolo[3,4‐d]pyrimidin‐4‐ones Catalyzed by Br?nsted‐acidic Ionic Liquids as Highly Efficient and Reusable Catalysts

A convenient and efficient method for the synthesis of pyrazolo[3,4‐ d ]pyrimidin‐4‐ones via heterocyclization reaction of 5‐amino‐1 H ‐pyrazole‐4‐carboxamides with triethyl orthoesters using two Br?nsted‐acidic ionic liquids, 3‐methyl‐1‐(4‐sulfonic acid)butylimidazolium hydrogen sulfate [MIM + (CH 2 ) 4 SO 3 H][HSO 4 − ] or N ‐(4‐sulfonic acid)butyl triethylammonium hydrogen sulfate [Et 3 N + (CH 2 ) 4 SO 3 H][HSO 4 − ], as efficient homogeneous catalysts under solvent‐free conditions is described.