Preparation, chemical and electrochemical reduction of pyrido[2,3-b]quinoxalines and pyrido[3,4-b]quinoxalines

The reaction of 2,3-diaminopyridine with the dimeric 4,5-dimethylcyclohexa-3,5-dien-1,2-dione gives 7,8-dimethylpyrido[2,3-b]quinoxaline, 1, in good yields; in the same way 3,4-diaminopyridine gives the 7,8-dimethylpyrido[3,4-b]quinoxaline 2. The electrochemical reduction of 1 and 2 in hydroorganic medium gives the 5,10-dihydro compounds 6 and 7; 1 and 2 present a single 2e− polarographic wave, in contrast to phenazine which shows two monoelectronic waves. The catalytic hydrogenation of 1 and 2 gives 6 and 7 and does not involve the pyridinic ring as in the case of pyridopyrazines. AlLiH4 does not react with 2 but 1 is reduced into the 1,2-dihydro derivative 8. The behavior of 1 and 2 is thus different from that of pyridopyrazines (which give the 1,2,3,4-tetrahydro compounds) and from that of phenazine (which gives the 5,10-dihydro derivative). NaBH4 reacts with pyridopyrazines to give, according to the substituents, 1,2- or 5,6-dihydro or 1,2,3,4-tetrahydro derivatives. Methylmagnesium chloride reacts with 1 to give a mixture of 2-methyl-1,2-dihydro, 2,6,7-trimethyl, and 4,6,7-trimethylpyrido[2,3-b]quinoxaline. In the case of 2, 4,6,7-trimethylpyrido[3,4-b]quinoxaline is obtained.