The new 1H-pyrazole-3-carboylic acid 2, pyridazin-3(2H)-one 3, and their various derivatives were prepared by the reactions of the 4-benzoyl-5-phenyl-2,3-dihydro-2,3-furandione 1 and 2,5-dichlorophenylhydrazine. Pyrazolo[3,4-d]pyridazine 7 was obtained from cyclization of the pyrazole-3-carboxylic acid 2 with,5-dichlorophenylhydrazine. The reaction of 1 and pyrazole-3-carbonitriles 6 gave the new oxazin-4-one 9 derivatives. The structures of compounds were characterized on the basis of elemental analyses, mass, IR, H-1, and C-13 NMR spectra. (c) 2006 Wiley Periodicals, Inc.