The photochemistry of 3-diazopyrazolopyridazine has been investigated. The irradiation of 3-diazo-4,5-diphenylpyrazolo[3,4-c]pyridazine in various solvents forms a carbene intermediate, which transforms into 3-substituted derivatives. For photolysis in the presence of acetylacetone or ethyl acetoacetate the coupling reactions which occur at the methylene group are faster than carbene formation, and can lead to direct cyclisation into condensed 1,2,4-triazines. Photolysis in the presence of diethyl malonate forms an acyclic hydrazone.
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