AbstractThe regioselective synthesis of pyrido[2,3‐b]‐ and [3,4‐b]pyrazine derivativesIIby the Hinsberg reaction is reported starting from 2,3 and 3,4‐diaminopyridine and excess of pyruvic acid or ethyl pyruvate as reactants. Good yields (higher than 90%) were obtained for pyrido[2,3‐b]pyrazine derivatives at room temperature using anhydrous methanol and chloroform as solvents which promote regioselective reactions to give 2‐methylpyrido[2,3‐b]pyrazin‐3(4H)‐one (3a) and 3‐methylpyrido[2,3‐b]pyrazin‐2(1H)‐one (4a) respectively. On the other hand, when the same procedure was applied to 3,4‐diaminopyridine results were not so encouraging for the formation of 2‐methylpyrido[3,4‐b]pyrazin‐3(4H)‐one (3b) and 3‐methylpyrido[3,4‐b]‐pyrazin‐2(1H)one (4b). Kinetic studies were performed in aqueous buffers (pH ‐0.89 to 11.5) and different organic solvents trying to improve yields and achieve regioselectivity. The course of the reactions was followed by uv spectrophotometry being those with ethyl pyruvate 2 to 800 times faster than with pyruvic acid. This investigation involves the kinetics and mechanism of this reaction studying its factibility when π‐deficient substrates are used and its regioselectivity accordi