3(5)-Amino-5(3)-hydrazinopyrazole dihydrochloride (8) reacts with pentane-2,4-dione (9a) to afford 2-(3′,5′-dimethylpyrazol-1′-yl)-5,7-dimethylpyrazolo[1,5-a]pyrimidine (10a) instead of the 3-(3′,5′-dimethylpyrazol-1′-yl)-4,6-dimethyl-1H-pyrazolo[3,4-b]pyridine isomer (11). Similarly, the reaction of 8 with phenyl-1,3-butanedione (9b) resulted into the formation of 2-(3′-methyl-5′-phenylpyrazol-1′-yl)-5-methyl-7-phenylpyrazolo[1,5-a]pyrimidine (10b) out of the four possible regioisomers. The structure of the reaction products 10a and b was established by analysis of high-resolution 1H NMR spectra. Complete spectral analysis has been achieved utilizing (1H–13C) HMQC as well as (1H–13C) and (1H–15N) HMBC experiments.