Unusual reaction of N-aroyldihydrocyclopenta-pyrazolidinol with ketenes: formation of 1,3,4-oxadiazoles

1-Benzoyl-5-hydroxy-pyrazoline was prepared and its reactions with ketenes, prepared in situ from the corresponding acid chlorides and the mixed anhydride , were studied. In all cases the 1,3,4-oxadiazoles were isolated. In the case of compounds a second diastereomeric oxadiazole was obtained. In the case of and , an interesting aroyl migration product was isolated. Structural assignments of the derived compounds were established by analysis of their IR, MS and NMR spectra (1H, 13C, COSY, NOESY, HETCOR and COLOC). The proposed reaction mechanism is supported by semi-empirical (AM1) MO calculations.