Quinoxalines, dihydro-, tautomerism

Alkyl-Alkylidene Tautomerism. Some 2- or 3-(substituted alkyl)quinoxalines, like 3-ethoxycarbonylmethyl-2(177)-quinoxalinone (133), have long been known to exist in equilibrium with their (substituted methylene) tautomers, for example 3-ethoxycarbonylmethylene-3,4-dihydro-2( 1 /7)-quinoxalinone (133a).The effects of solvent change, protonation, and the like on such tautomeric systems have been examined as well as the kinetics thereof. In... 

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... 

A tautomeric equilibrium between enamine and methylene imine forms has been demonstrated to exist in 2-(3,4-dihydro-3-oxo-2(l//)-quinoxalinylidene)-Al-phenylacetamides and 3,4-dihydro-3-oxo-/V-phenyl-2-quinoxaline acetamides when these are in DMSO solution either in the absence or presence of TFA [95JHC671]. The reduction potentials of some pyrazines and their benzo-fused analogs have been summarized as part of an EPR study of the electron transfer interaction between nitrogen heterocycles and n-B N+BHf [95JOM123]. 

The 1,4-dihydro quinoxaline is formed as the first product in reduction of 2-phenylquinoxaline with chromium(II) ethylenediaminetetraacetate or zinc-amalgam at pH > 7 however, a tautomeric isomerization to the thermodynamically more stable 1,2-dihydro isomer occurs. 

Variable-temperature studies of the NMR and IR absorption properties of compounds such as 3,4-dihydro-3-oxo-2-carbethoxymethyl-quinoxaline (27), which is in tautomeric equilibrium with the enamine 28, show that the enamine is less favored at high temperature. Thus in DMSO at 40°, the tautomeric equilibrium contains 100% of the enamine, but at 140° approximately 50% of both tautomers are present. ... 

Quinoxaline and 2-methylquinoxaline react with diphenylcyclo-propenone (75) to give the products 76, analogous to that obtained from the reaction of 75 with pyrazine (see Chapter XIX). The formulation of the products as 1-hydroxy rather than 1,5-dihydro-1-oj o compounds is discussed in Section II. The thioxo analogue of 75 give the 1-mercapto derivative 77 on reaction with quinoxaline. No spectral details are given to permit discussion of possible tautomerism of this compound to the 1,5-dihydro-1-thioxo derivative. 

A series of reactions pertinent to quinoxaline fused 2,3-dihydro-l.ff-l,2-diazepines (77) has been published <90jhc819, 90JHC2I97). These involve intermolecular displacement of the 5-cyano group of C77) by alkoxides, a reaction that proceeds with spontaneous tautomerism to yield diazepinones (78) (Equation (15)) (see Section 9.04.3.2). When the aryl group, R, bears an ortho hydroxyl moiety, the resultant intramolecular reaction yielded compounds, such as (79) <90JHC2209,9i JHC787). Treatment of (79) with dead leads to dehydrogenation however, attempted oxidation of (78 ... 

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