Synthesis and structural analysis of 5-cyanodihydropyrazolo[3,4-b]pyridines

Several new 3-aryl-5-cyanopyrazolo[3,4-b]pyridines were easily prepared from 3-amino-5-arylpyrazoles and alpha -cyanochalcones. Structural analysis using NMR solution studies revealed the 2H-tautomers as the preferred tautomer in solution (DMSO-d(6)). X-ray diffraction confirmed the 2H-tautomers as the unique tautomer species in the crystalline state as well. Geometry optimization of 1H and 2H-tautomers at semi-empirical levels (AM1, MINDO/3) were performed, indicating that in all cases the 2H-tautomers are more stable than the corresponding 1H-tautomers.