In connection with our previous research on the synthesis of azo-substituted five membered heterocycles'") we repon here the synthetic route and the spectral characteristics of 2-aryl substituted-3-acetyl-5-[4-( 1 -phenyl-3,5-dimethyl-4-pyrazoly1azo)-phenyll-1,3,4-oxadiazolines 2a-2h. 4-( 1 -Phenyl-3.5-dimethylpyrazoly1azo)-N2-(substituted benzylidene)benzohydrazides la-lhSe8), obtained from the reaction of 1 -phenyl-3,5-dimethyl-4-(4-hydrazinocarbonylphenylazo)-pyrazole (1)9' with various aldehydes, were reacted with Ac20, but no acetyl derivatives of hydrazones were formed, instead, cyclization occured and compounds 2a-2h were obtained (Scheme).The structures of compounds 2a-2h were confirmed by means of elemental analysis and spectral data. Instead of the UV absorptions of la-lh between 348-352 run, 2a-2h showed maxima between 359-367 run. The NH-absorptions of la-lh occured at about 3200 cm", whereas 2a-2h had
PhCompound 1 ArCHO I no absorptions in this region. Because both the starting compounds la-lh and the cyclization products 2a-2h have carbonyl functionalities the ab sorptions of 2a-2h at 1654-1674 cm-' (compounds 2b and 2c showed an additional C=O absorption at 1769 cm-' due to the acetylation of the OH group of the aromatic ring) could not help in structure elucidation. The 5-substituted-3-acetyloxadiazoline structure was supported by the 'H-NMR spectra of 2a-2h. Signals due to -CONH-N= at about 12 ppm disappeared after cyclization. Furthermore, the Ar-CH=N-protons of la-lh which show a one proton singlet at about 8.10-8.70 ppm region were shifted to the 7.06-7.13 ppm region. Although this proton would be expected at a little bit higher field, the inductive effect of 0 and N and the phenyl ring attached to the same C-atom shift this one proton signal more downfield than expected.The molecular structure of the compounds was further elucidated by EI-MS data. Molecular ions at m/z = 566; 508: 498; 482; 509 and 507 confirmed the molecular weights of 2c-2h, respectively. M' of 2a was not detec-