Synthesis and Antifungal in Vitro Evaluation of Pyrazolo[3,4-b]pyridines Derivatives Obtained by Aza-Diels–Alder Reaction and Microwave Irradiation

A series of pyrazolo[3,4-b]pyridines were prepared by a microwave-assisted aza-Diels–Alder reaction between pyrazolylformimidamides 1 and β-nitrostyrenes 2 in toluene as the solvent. This procedure provides a simple one-step and environmentally friendly methodology with good yields for the synthesis of these compounds. All compounds were tested for antifungal activity against two clinically important fungi Candida albicans and Cryptococcus neoformans. Within the compounds of the series bearing a –CH3 group on the carbon C-3 of the azole ring (3a–e), the compound without a substituent on the p′-phenyl ring (3a), showed the best activity against both fungi, followed by the p′-Br-phenyl (3c). Within the compounds of the series bearing a tert-butyl group in the carbon C-3 of the azole ring (3f–j), the non-substituted p′-compound (3f) was the most active one, followed by (3h) (p′-Br substituted) that showed the best activity against both fungi. The remaining compounds of this sub-series (3g, i, j) showed similar moderate activities. The antifungal activity of the compounds of the series was found to be correlated with a higher log P and a lower dipole moment in the more active compounds.