PURPOSE: To prepare the titled compound useful as synthetic intermediates of antibacterial agents photosensitive dyes for direct positive photographic emulsion, etc., easily, in high yield, by the thermal decarbonation of 3-carboxy-pyrazolo[1,5-b]piridazines in the presence of copper catalyst.

CONSTITUTION: The compound of formula II can be prepared by reacting the compound of formulaI(R1 is H, alkyl, aryl, aralkyl or heterocyclic residue; R2W R4 are H, alkyl, alkoxy, alkylamino, aryl, halogen, or acylamino; R2 and R3 or R3 and R4 may together form a benzene ring) at 150W280°C in the presence of a catalyst comprising copper or its compound (e.g. cupric oxide, copper powder, copper chromate, etc.). The amount of the catalyst is 0.001W0.5pt.wt. per 1pt.wt. of the compound of formulaI.