Regioselective Reduction of 3-Substituted <i>N</i>-Acylpyrazinium Salts toward the Synthesis of 1,2-Dihydropyrazines

The regioselective reduction of 3-substituted <i>N</i>-acylpyrazinium salts with <i>n</i>-Bu₃SnH has been developed for the synthesis of 3-substituted 1,2-dihydropyrazines in yields of 56–94%. Substitution of the pyrazinium salts with electron-donating groups favors the formation of the 1,2-isomers as a result of their better stability over the 1,6-isomers. Under mild acidic conditions, 3-methoxy substituted 1,2-dihydropyrazine was easily hydrolyzed in excellent yield to Δ⁵-2-oxopiperazine