New findings in the Richter reaction in series of vicinal alkynylpyrazolyldiazonium salts

When treated with sodium nitrite in hydrochloric or hydrobromic acid, posit ional isomers of vic-alkynylaminopyrazoles were transformed to the pyrazolo pyridazines via cyclization of the corresponding alkynylpyrazolyldiazonium salt. The reaction was found to depend on the position of alkynyl and diazo nium group in pyrazole nuclei: the heterocyclization of 5-alkynylpyrazolyi- 4-diazonium salts underwent at 100-105 degrees C and gave 7-chloro-1H-pyraz olo[4,3-c]pyridazines; the heterocyclization of 4-alkynylpyrazolyl-3-diazon ium salts at 50-60 degrees C gave 6-hydroxy-2H-pyrazolo[3,4-c]pyridazines a s major component and 6-halogeno-2H-pyrazolo [3,dr-c]pyridazines as minor c omponent; the cyclization of 3-alkynylpyrazolyl-4-diazonium salts accompany ing with methyl group migration towards the neighbouring nitrogen atom affo rded 7-chloro-1H-pyrazolo[4,3-c]pyridazines.