The 1H‐pyrazole‐3‐carboxylic acid 2 or its remarkably stable acid chloride 3 can easily be converted into the corresponding ester or amide derivatives 4 or 5, respectively, from reaction with alcohols or N‐nucleophiles. Pyrazolo[3,4‐d]pyridazines 6a,b are obtained from cyclocondensation reactions of the pyrazoles 2 and 3, respectively, with phenylhydrazine or hydrazine hydrate, while 6c is formed in an one‐pot procedure from the furan‐2,3‐dione 1 and hydrazine hydrate.

The furan‐2,3‐dione 1 and the thiosemicarbazones 2 combine with loss of carbon dioxide and water yielding the 1‐methylenaminopyrimidine‐2‐thione derivatives 3 in moderate yields (30–60%). Their molecular skeleton is confirmed with aid of an X‐ray structure determination of 3c. Hydrolysis of 3 leads to the 1‐aminopyrimidine‐2‐thione 5.

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A facile synthesis of the title pyrazolecarboxylic acid (III) and a variety of its reactions including amide formation, esterification and cyclocondensation reactions are described. -(SENER, A.; KASIMOGULLARI, R.; SENER, M. K.; BILDIRICI, I.; AKCAMUR, Y.; J. Heterocycl. Chem. 39 (2002) 5, 869-875; Dep. Chem., Fac. Arts Sci., Yuezuencue Yil Univ., TR-65080 Van, Turk.; Eng.) -F. Santoso 05-132