Synthesis and tautomeric structure of novel 3-arylazo-imidazo[1,2-b]pyrazolo[4,3-d]pyridazines

Treatment of 3-acetyl-5-amino-4-cyano-1-phenylpyrazole with hydrazine hydrate afforded the hydrazone derivative. Reaction of the latter hydrazone with hydrazonoyl chlorides was found to be site selective as it afforded the corresponding formazan and not the amidrazone derivatives. The H-1 NMR spectra of the formazan derivatives indicated that they exist as the bis-hydrazone tautomers. Acid-catalysed cyclisation of these dihydroformazans afforded the respective 9-amino-3-arylazo-6-methyl-8-phenyl-2-substituted-8H-imidazo[1,2-Npyrazolo[4,3-d]pyridazines whose electronic absorption spectra revealed that they exist predominantly in the arylazo tautomeric form. The structures of all compounds prepared were confirmed by spectral and elemental analyses. Also the mechanism of the reactions studied are discussed.