Pyridazine 1-oxides base-catalyzed hydrogen-exchange

In pyridazines, base-catalyzed hydrogendeuterium exchange takes place at positions 4 and 5 more easily than at positions 3 and 6. Pyridazine 1-oxide reacts first at positions 5 and 6 and then at C(3) and C(4). Pyrimidine exchanges most readily at the S-position, next at the 4-position, and least readily at the 2-position. In pyrimidine 1-oxide, the reactivity order is 2>6>4>>5. 1,2,4-Triazines easily undergo base-catalyzed hydrogen exchange at the 2-position. 

In pyridazine, base-catalyzed hydrogen-deuterium exchange takes place at positions 4 and 5 more easily than at positions 3 or 6. Deuteration of pyridazine 1-oxide in NaOD/DiO... 

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