Synthesis of Pyrazolo [3, 4-d] pyridazines from 5-(1-Methylhydrazino)-pyridazines by Means of the Vilsmeier-Haack Reaction

Reaction of 2-substituted 5-(1-methylhydrazino)-3 (2H)-pyridazinones (2a-d) with dimethyl-formamide-phosphorus oxychloride afforded 5-substituted 1, 5-dihydro-1-methyl-4H-pyrazolo [3, 4-d] pyridazin-4-ones (3a-d) in good yields. However, concurrent formation of 5-substituted 2, 5-dihydro-2-(2-substituted 1-chlorovinyl)-4H-pyrazolo [3, 4-d] pyridazin-4-ones (6a-c, 12, 13) (minor products) and the 1-methyl-4H-derivatives (3a-c) (major ones) was observed, when starting with the corresponding 2-substituted 5-(2-acetyl-1-methylhydrazino)-3 (2H)-pyridazinones (5a-c, 9, 10) under similar reaction conditions. A plausible mechanism for the reaction is proposed.