Synthesis of Pyrazolo[3,4-b]pyridines by Cycloaddition Reactions under Microwave Irradiation

Microwave irradiation induces the cycloaddition of pyrazolylimines with aromatic and aliphatic nitroalkenes to afford pyrazolo[3,4-b]pyridines in 5–20 min. Some of these reactions do not occur under classical heating. Tricyclic heterocycles can be synthesized from cycloalkenes. The reactivity and regiochemistry can be understood in terms of the energy and atomic coefficients of the frontier orbital, except in the case of compound 18. Calculations at the HF/3-21G level predict an asynchronous transition structure for this cycloaddition with regiocontrol determined by Coulombic interaction.