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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 1and 2in hydroorganic medium gives the 5,10-dihydro compounds 6and 7; 1and 2present a single 2e−polarographic wave, in contrast to phenazine which shows two monoelectronic waves. The catalytic hydrogenation of 1and 2gives 6and 7and does not involve the pyridinic ring as in the case of pyridopyrazines. AlLiH4does not react with 2but 1is reduced into the 1,2-dihydro derivative 8. The behavior of 1and 2is 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). NaBH4reacts 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 1to 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.

The electrochemical reduction of quinoxalino[2,3-b]quinoxaline gives fluoflavine (2) the structure of which is discussed. The electrochemical reduction of fluoflavine in acidic medium leads to a hexahydroquinoxalino[2,3-b]quinoxaline. A reduction mechanism is proposed based on cyclic voltammetry results and preparative electrolysis. The results obtained in the case of quinoxalino[2,3-b]quinoxaline are used to rationalize the results obtained in the case of pyrazino[2,3-b]pyrazines and pyrazino[2,3-b]quinoxalines.

Nitration of styryl ketones with nitrogen peroxide leads to the corresponding α-nitroethylenic ketones in good yield. However, the presence of methoxy groups involves a nitration on the aromatic nucleus.LiAlH4reduction of α-nitrostyrylketones yields the corresponding racemic erythro- and threo-aminoalcohols; after acylation and cyclisation a dihydro-quinoline or a 2-oxazoline is obtained.Electrochemical reduction of these nitrostyryl-ketones selectively provides the α-ketoximes, which are themselves reduced electrolytically to α-aminoketones. These latter compounds dimerise in neutral medium to dihydro-2,5-pyrazines which aromatise to the corresponding pyrazines. In acid medium an α-ketol is obtained.A mechanism for the reduction is proposed implicating an enaminol intermediate.

The electrochemical reduction of compounds of the typehas been investigated in hydroorganic medium between pH 0 and 7. For 3(R1 = R2 = R3 = C6H5), the 3,4,5-triphenylisoxazole is obtained at pH 1. 3in neutral medium, 1(R1 = CH3, R2 = R3 = C6H5) and 2(R1 = C2H5, R2 = R3 = C6H5) lead to 4,5-dihydroxy-2-isoxazoline (only one of such compounds is described in the literature). The structure of these compounds has been determined by the usual spectroscopic methods (ir, nmr, mass spectrometry) and by radiocrystallography. A mechanism which accounts for the whole set of our results is proposed: it involves an intermediate α,β-nitrosoethylenic compound which cyclizes into a 4,5-dihydroxy-2-isoxazoline or gives an 1,3-diketone oxime upon reduction; this last compound cyclizes into an isoxazole.

The hydrogenation of 2,3-dimethylpyrazino[2,3-b]quinoxaline 1and 2-phenylpyrazino[2,3-b]quinoxaline 2leads to the corresponding 5,10-dihydroderivatives 3band 4b. LiAlH4reduction of 2, of 2,3-dimethyl-6,7-diphenylpyrazino[2,3-b]pyrazine 8and 2,6,7-triphenylpyrazino[2,3-b]pyrazine 9furnishes the corresponding 1,2,3,4-tetrahydroderivatives 7, 10, and 11. NaBH4reduction of 2leads to a mixture of 4band 7. In hydroorganic medium 1and 2are electrochemically reduced to 3band 4bwith which they form a redox system.

The pyrazolo[1,5-a]pyrimidine-7-amines are electrochemically reduced in hydroorganic medium at low pH into the corresponding 4,5-dihydro compounds. A mechanism is proposed featuring the intermediacy of the parent pyrazolo[1,5-a]pyrimidine itself. The 4,5-dihydroderivatives can be rearomatized, thus offering a facile synthetic route to the pyrazolo[1,5-a]pyrimidines.

The structure of fluorindine is established by nmr as the 5,14-dihydroquinoxalino[2,3-b]phenazine. The catalytic hydrogenation of 2,3-di(p-methoxyphenyl)pyrazino[2,3-b]phenazine 2aleads to the 6,11-dihydro derivative 4a. The electrochemical reduction in an hydroorganic medium furnishes 4aand then the 1,4,6,11-tetrahydro derivative 8a. In dry DMSO the voltammogram shows four monoelectronic reversible systems corresponding to the successive formation of a radical anion, dianion, radical trianion, and tetraanion. Thus 2aappears as a new example of the very restricted class of compounds leading to tetraanions upon electrochemical reduction. The catalytic hydrogenation of 2,7-diphenylpyrazino[2,3-g]quinoxaline 1aor the reaction of LiAlH4with 1,2,7,8-tetramethylpyrazino[2,3-g]quinoxaline 1bleads to 1,2,3,4-tetrahydro compounds. The electrochemical reduction of 1aand 1bin hydroorganic medium leads successively to 1,4-dihydro and then to 1,4,6,9-tetrahydro compounds which undergo a further rearrangement. In dry DMSO 1aand 1bbehave differently from 2a: one only observes two reversible monoelectronic systems.

A conformational study of 1,4-diacetyl-1,4-dihydro-6,7-dimethylquinoxaline is carried out by low temperature nmr in CD2Cl2. Important acetyl rotation induced shifts have been observed for the periand orthoprotons. These results, together with low temperature nOe experiments, permit the determination of the structures and conformations of four diacetyl derivatives of dihydropyrido[3,4-e] as-triazines and of one diacetyl derivative of a dihydropyrido[4,3-e] as-triazine.

The electrochemical reduction of fluoflavine 2gives compound 9to which we had previously assigned a hexahydroquinoxalino[2,3-b]quinoxaline structure. A crystallographic study of a diacetylated derivative of 9shows that 9is in fact 2-(o-aminoanilinomethyl)benzimidazole. This result permits us to propose a detailed reduction mechanism for 2and its diacetyl derivative 25.