The reaction of Meldrum's acid with α-dicarbonyl monohydrazones: a new synthesis of pyridazin-3-ones

Abstract

Treatment of the monohydrazones of α-dicarbonyl compounds with Meldrum's acid gives the condensation products (13)–(19) under standard conditions. Reaction of pyruvaldehyde 2-phenylhydrazone or 2-t-butylhydrazone with Meldrum's acid at 80 °C gives the corresponding 2-substituted-2,3-dihydro-6-methyl-3-oxopyridazine-4-carboxylic acids (29) and (30). Cyclisation of the other N-monosubstituted condensation products (13), (14), (16), (18), and (19) to 3-oxopyridazine-4-carboxylic acids (31)–(35) can be effected under basic conditions. Gas-phase pyrolysis of the N-aryl derivatives (13)–(16) at 550 °C and 10^–2 Torr gives N-arylpyridazin-3-ones (38)–(41) while the N-t-butyl derivatives (18) and (19) at 750 °C and 10^–2 Torr give N-unsubstituted pyridazin-3-ones (44) and (45) by additional loss of 2-methylpropene. 2-t-Butylpyridazin-3-ones (42) and (43) can be isolated from the same precursors under milder conditions (500 °C and 10^–2 Torr). Pyrolysis of the N,N-dimethyl derivative (17) gave only polymeric material.