Quinoxaline 2-amino-, tautomerism

It should be noted that, as all carbon positions in pyrazine are identical, the locant 2- in a monosubstituted derivative is unnecessary. All possible reduced derivatives of pyrazine 1, and several of those of its benzo analogues quinoxaline 2 and phenazine 3, are known. There are four dihydropyrazines, the 1,2-, 2,3-, 1,4-, and 2,5-isomers, two tetrahydropyrazines, the 1,2,3,4- and 1,2,3,6-, and hexahydropyrazine or piperazine, the last of which is omitted in this chapter. The reduced quinoxalines are the 1,2- and 1,4-dihydro compounds and 1,2,3,4-tetrahydroquinoxaline. The only known reduced phenazine is 1,4-dihydrophenazine. Hydroxypyrazine 4 and hydroxyquinoxaline 6 have been shown to exist in the tautomeric amide form by spectral studies, and therefore they are formulated as 2(1//)-pyrazinone 5 and 2(l//)-quinoxalinone, respectively. In contrast, aminopyrazine and aminoquinoxaline exist as described in the amino rather than the imino forms (Figure 1). 

Pteridones behave electrochemically akin to quinoxaline, the pyrazine ring being reduced. In the first, reversible step of 6-methyl-2-amino-4(3//)-pteridone (225 R = H), a 5,8-dihydro derivative (226) is formed, which in a pH-dependent reaction tautomerizes to the more stable 7,8-dihydro compound 227 227 is reduced in acidic and neutral solution to the 5,6,7,8-tetra-hydro derivative 228, which may be oxidized to a quinonoid dihydro compound (229) 227 is formed on tautomerization of 229358-362 [Eq. (126)]. 

Vilsmeier reaction, 4, 1051 Furo[3,2-6]pyrroles MO calculations, 6, 979 synthesis, 4, 1069 6, 1009 Furo[3,4-a]pyrrolo[2,1,5-cd]indolizine nomenclature, 1, 22 Furopyrylium salts, 4, 993-995 Furoquinolines biosynthesis, 4, 992 occurrence, 4, 988 pharmacology, 4, 992 reactions, 4, 988 synthesis, 4, 989 Furo[3,2-c]quinolines, 4, 991 Furo[3,4-fe]quinoxaline, 1,3-diphenyl-synthesis, 4, 993 Furoquinoxalines, 4, 992 Furo[2,3-6]quinoxalines synthesis, 4, 992 Furosemide toxicity, 1, 136 Furospinulosin UV spectra, 4, 587 Furospongin-I mass spectrometry, 4, 583 Furo[3,4-d][l,2,3]triazole, 2,6-dihydro-synthesis, 6, 996 Furo[3,4 -d][ 1,2,3]triazoles synthesis, 6, 996 Furoxan, 4-amino-3-aryl-tautomerism, 6, 404 Furoxan, 4-amino-3-methyl-synthesis, 4, 414 Furoxan, 4-aryl-3-methyl-rearrangement, 6, 408 Furoxan, 3-aryl-4-nitro-synthesis, 6, 414 Furoxan, 4-benzoyl-3-methyl-oxime... 

Arylidenemalononitriles (179) reacted with benzofuroxanes 257 to yield quinoxaline dioxides 258 by ring enlargement. a-Amino-A-heterocycles were treated with ethoxymethylenemalonester to yield tautomeric products 259, 260 and 261. ... 

It should be pointed out that the formation of spiro-quinoxaline derivative 134 could be due to the Michael addition of hydrazine to the partially positive C(3) atom of the quinoxalin-2(l//)-one 50 in the first stage of the reaction mechanism with the formation of intermediate A capable reversible tautomerization to intermediate B. Then cyclization involves the nucleophilic attack of the amino group on the carbonyl group of the 3-arylacylidene fragment of quinoxalin-2(l//)-one (Scheme 6.53). 

---