Multi-component solvent-free versus stepwise solvent mediated reactions: Regiospecific formation of 6-trifluoromethyl and 4-trifluoromethyl-1H-pyrazolo[3,4-b]pyridines

A simple and efficient regiospecific synthesis of 6-trifluoromethyl-1H-pyrazolo[3,4-b]pyridines has been achieved in excellent yields via a three-component reaction between 2-hydrazinobenzothiazoles, α-cyanoacetophenones and trifluoromethyl-β-diketones under solvent-free conditions. This method was found to be more convenient than the classical stepwise solvent mediated process, which furnished not only the opposite regioisomer i.e. 4-trifluoromethyl-1H-pyrazolo[3,4-b]pyridines (50–60%) but also an undesired amide, 5-acetylamino-1-(benzothiazol-2′-yl)-3-phenyl-1H-pyrazole as a side product. The structure of the two regioisomers was established with certainty on the basis of rigorous analysis of 1H, 13C, 19F-NMR spectral data and (1H13C) gs-HMQC, (1H13C) gs-HMBC experiments.