Abstract
The behaviors of various 4, 5-di-and 3, 4, 5-trisubstituted 1-phenyl-6 (1H)-pyridazinone against caustic alkali and hydrobromic acid were investigated. From these data, the mechanism of the ring-contraction from pyridazinone to pyrazolone, as shown in Chart 1, was discussed. Action of 1-phenyl-4, 5-dichloro-6 (1H)-pyridazinone with boiling caustic alkali took place the ring-contraction to give unexpected 1-phenyl-3-hydroxy-5-pyrazole-carboxylic acid. The mechanism of this reaction was also assumed as shown in Chart 4. Moreover, the reactivity of chlorines on 1-phenyl-3, 4, 5-trichloro-6 (1H)-pyridazinone was elucidated to increase in the order 4-, 5-, 3-position from the result of its methoxylation reaction.
