Furandione I was reacted with 2,4,6-trichlorophenylhydrazine to give the 1H-pyrazole-3-carboxylic acid 2 together with a new pyridazinone derivative 3. Then 2 was converted into the corresponding ester 5 or amide 6 derivatives, respectively, via reactions of its acid chloride 4 with various alcohols or N-nucleophiles. Nitrile derivative 7 was obtained by dehydration of 6a in a mixture of SOCl2 and DMF. Condensation reactions of 2 with some hydrazines led to the formation of pyrazolo [3,4-d] pyridazinone 8 derivatives.