Catalyst-free Synthesis of pyrazolo[3,4-b]pyrrolo[3,4-d]pyridine Derivatives by Aza-Diels-Alder Reaction

A series of pyrazolo[3,4-b]pyrrolo[3,4-d]pyridine derivatives was synthesized via catalyst-free bi-component and one-pot reactions of 5-aminopyrazoles, DMF-DMA (or TMOF) and N-arylmaleimides in xylene as solvent. This protocol showed advantages of milder reaction conditions, easier work-up, shorter reaction times and good yields, over other approaches tested.