Indium(III) chloride/silica gel catalyzed synthesis of pyrazolo[3,4-b]pyrrolo[3,4-d]pyridines

Abstract

A facile synthesis of pyrazolo[3,4-b]pyrrolo[3,4-d]pyridine was developed through an Aza-Diels–Alder reaction of pyrazoylimines with maleimides in the presence of 20% wt InCl3/silica gel in the sealed tube. Based on the metal catalyst scanning study, silica gel impregnated with indium trichloride was found to be the suitable catalyst to improve the Aza-Diels–Alder cycloaddition. In comparison with Brønsted–Lowery acetic acid condition, the Lewis acidic metal catalyst is more efficient for the Aza-Diels–Alder reaction.