Abstract The reaction between 3, 4-diaminopyridine and α-methylsulfinylacetophenone in benzene containing acetic acid under reflux gave 2-phenylpyrido [3, 4-b] pyrazine. In this way, 2-(4-methoxyphenyl) pyrido [3, 4-b]-pyrazine and 2-(3, 4-dimethoxyphenyl) pyrido [3, 4- b] pyrazine were obtained by condensation of 3, 4-diaminopyridine with the corresponding α- methylsulfinylacetophenones.