Regioselective reduction of 3-substituted N-acylpyrazinium salts toward the synthesis of 1,2-dihydropyrazines

The regioselective reduction of 3-substituted N-acylpyrazinium salts with n-Bu(3)SnH 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 Δ(5)-2-oxopiperazine.