Journal of Molecular Structure, 60 (1980) 43-48 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands 43 ST’RUCT’URE DE’T’ERMINATION OF PYRAZOLONE DERlVATIVES AND THEIR HYDRAZONE TYPE PRECURSORS BY IR AND ‘H NMR SPECTROSCOPY P. SOHAR, P. MATYUS and G. SZILAGYl Institut for Drug Research, H-1325 Budapest, POB 82 (HUNGARY) ABSTRACT The structure of the acylation products formed via solvent participation from pyrazolones was elucidated by IR and 1 - H NMR spectroscopy. Tautomerism of the parent compounds, syn-anti isomers of the 3-hydrazino- pyridazine type intermediates and constitutional isomers of their carbethosyl- ated derivatives were studied. D-uring the synthesis of 1 lype compoutlds - possessing potential - biological activity - spectroscopically intereshng phenomena were found, which are the subject of our paper. These compounds can e.xist in three tautomeric forms: in the aromatic hydroxypyrazole (2) and in the 4,5- (2-1 or 2,5_dihydropyrazole (3) form, respectively. R2Q RlcA OH I 7 O X X 1 2 a : R = Me 5.: R = Et c : R = Ph 3: R = 3,&,5-trimethoxyphenyl e : R = 4-nitrophenyl R Q ‘TN, ‘i’ \O X According to the IR spectra the 3-methyl and 3-ethyl substituted derivatives a and b are present in the 2,5_dihydropyrazole form (3_