Heterocyclic compounds with an active methylene moiety are known to have antiinflamatory, anelgetic and large number of other pharmacological activities. In our previously study we confirm that pyrazine derivatives obtained by the nucleophilic substitution of chlorine atom in the pyrazine ring system with active methylene compound, posses above mentioned activity. Some of new pyrazine C-H and N-H acids also demonstrated antithrombotic and antiplatelet activity. Structural analysis of pyrazine acetonitrile derivatives show pyrazinyl-pyrazylidene tautomerism

PyH D Py^- + H⁺ (K₁) PyH + H⁺ D PyH₂⁺ (K₂)

Because pharmacological properties are often related to acidic-base charcter of compounds it was important to maeasure pKa value for this new class of active biological compound. pKa₁ and pKa₂ value were determined by specroscopic and potentiometric methods. Relationship between tautomeric form, pKa and structure of several new pyrazine CH- and N-H acids are discussed.

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