Kinetic study on the anelation of heterocycles. 1 . Quinoxalinone derivatives synthesized by hinsberg reaction

Kinetic studies on the Hinsberg condensation were performed trying to improve yields and achieve regio‐selectivity in the attainment of benzene‐substituted 3‐methylquinoxalin‐2(1 H )‐ones. The course of the reactions between o ‐phenylenediamine ( o ‐PDA) and substituted o ‐PDA with pyruvic acid ( 2a ) or ethyl pyruvate ( 2b ) were followed by uv spectrophotometry at different p H values. The formation of 3‐methylquinoxalin‐2(1 H )‐one ( 6a ) was improved using sulphuric acid‐water mixtures, in which the reaction proceeded by a different mechanism. 3‐Methyl‐7‐methoxyquinoxalin‐2(1 H )‐one ( 7b ) was regioselectively synthesized independently of the p H of the reaction media. Reaction of 2‐amino‐4‐methylamine ( 1c ) with 2a or 2b led to a mixture of 6 and 7‐quinoxalinone isomers, 6c and 7c , while 2‐amino‐4‐nitroaniline ( 1d ) and 2,4‐diaminoaniline ( 1e ) with 2a or 2b did not afford the heterocycle. In every case reactions with 2a were 100–1000 times faster than those with 2b . Mechanisms are proposed trying to account for the experimental results.