From the reaction of 2-acetylcyclopentanone with benzoylhydrazine, the cyclopentanopyrazole derivative 4 was obtained as the only product. The structural assignment of this compound and also of the hydroxypyrazoline 3, used as a model compound, was established by analysis of their NMR spectra ( 1 H, 13 C, COSY, NOESY, HETCOR and COLOC).