Quinoxalinium salts

Substituted pyrido[2,3-(i]pyrazinium salts form cycloadducts with quinones (75HCA2529), similar to those formed by quinoxalinium salts. 

Diphenylquinoxaline and dimethyl acetylenedicarboxylate in methanol/ as in the phenanthridine case, combined to give a yellow pseudomethoxide (147) converted by acid into the corresponding quinoxalinium salt (148). Heating the adduct (147) with ethanolic potassium hydroxide gave 45% 2,3-diphenylquinoxaline, along with... 

The reduction of 1-methyl- or ethyl-quinoxalinium salts with aqueous sodium borohydride formed the l-substituted-l,2,3,4-tetra-hydroquinoxaline.131 The tetrahydroquinoxalines were the sole products and were obtained in good yield. 

Deprotonation of an 8,10-diphenyl-6-oxo-5,6-dihydropyrido[l,2-a] quinoxalinium salt gave the zwitterionic pyrido[l,2-a]quinoxalinium-6-olate [84JHC1609, 84KFZ(6)700], which underwent N-methylation on the action of CF3S03Me (84JHC1609). 

Equilibrium constants K for the formation of dimethoxy adducts 45 (Table III) show that benzo [g] quinoxalinium salt 37f is the most reactive in... 

An important and characteristic difference between reactions of N-alkylquinoxalinium salts with 1,3-dinucleophiles, giving rise to the formation of tetrahydro cycloadducts 46, and the annelation reactions with 2,3-dichloroquinoxaline is that cycloadduct formation is reversible. Due to the reversibility of the diaddition reactions and the ambident character of the 1,3-dinucleophiles used, regio- and stereoisomeric cyclization products can be formed. Indeed, regioisomeric adducts 50 or 51 can be obtained when quinoxalinium salts 37 react with ammonium dithiocarbamates. It depends on the reaction conditions which product is favored. When the reaction was carried out in aprotic solvents, such as dimethyl sulfoxide (DMSO) or chloroform, only thiazolo [4,5-6] quinoxalines 50 could be isolated in good yield. When the same reaction is carried out in an ethanol solution it results in the formation of the regioisomeric thiazoloquinoxalines 51 (Scheme 43) (84KGS680). 

It is interesting to note that, due to the reversible character of the cycloaddition, isomerization favoring the product which is thermodynamically more stable may occur and an appropriate side-chain substituent may participate. For example, the thiazolo [4,5-b] quinoxalines 55, formed in the reaction of quinoxalinium salts with thioamides under kinetically controlled conditions, are able to undergo two different isomerizations (Scheme 45). When compound 55 contains an aryl group at C-2 (R2 = aryl) and this compound is heated in an ethanolic solution, the thiazoloquinoxaline 57 is formed. In the case of R2 = CH3, the methyl group participates in the isomerization process, yielding pyrrolo [2,3-b]quinoxalin-2-thione 58 (Scheme 45) (85KGS396). 

Qualitative data on the cyclizations of pyrazinium 36, quinoxalinium 37, 1,2,4-triazinium 38, and pteridinium 40 salts with CH-active acetamides show that their reactivities follow the same order as has been established for diaddition reactions with simple nucleophiles (Section III,B,1) (86KGS1380). For example, the anilide of acetoacetic acid readily forms cycloadducts with quinoxalinium salts 37, but no cyclization products with pteridinium cations 40. By enhancing the NH-activity of the amide group, the cyclization with pteridinium salts could be achieved. Thus, the N-(pyridin-2-yl)-substituted amide of acetoacetic acid is able to undergo the cyclization reaction with the pteridinium cation 40d to afford pyrrolo [2,3-g] pteridine 62 (Scheme 48) (86KGS420). 

When quinoxaline is healed with ethyl azidoformate, quinoxalinium salt 3 is formed. ... 

In the two-step reduction of V,V -dialkylpyrazinium and quinoxalinium salts, the first one-electron reduction leads to a cation radical, which is the most persistent oxidation state [332]. 

Quinoxalinium salts have been prepared from IV-substituted o-phenylenediamines by condensation with either 2-htilogenoketones or... 

Reaction of 3,4-dihydro-3-oxo-2-phenylquinoxaline 1-oxides 8 with acetic anhydride in the presence of perchloric acid at room temperature gives high yields (80-96%) of the yellow to red N-acetoxyquinoxalinium perchlorates 9. The salts (R = H or MeO R = Me) decompose rapidly at room temperature, but the remaining quinoxalinium salts are relatively stable. ... 

Various quinoxalinium compounds have been tested for their antitumour activity. l-(2-Diethylaminoethyl)-3-phenylquinoxalinium chloride hydrochloride (4) and related compounds have been tested for their action on the heart. Other quinoxalinium salts have been evaluated for their antiviral and insecticidal properties. Cyanine dyes incorporating a quinoxalinium chromophore have been synthesized, and quinoxalinium salts have been used to induce the photopolymerization of ethylenic derivatives. ... 

Substituted derivatives of imidazo[4,5-h]quinoxaline have also been obtained. The l-methyl-2-iminoquinoxaline 66 reacts with triethyl orthoformate to give 4-methylimidazo[4,5-i)]quinoxaline (67), and the 4-methyl-2-oxo derivative 68 is produced by reaction of 66 with urea. The bis(secondary amine) 69 on reaction with acetyl chloride in di-methylacetamide affords the imidazo[4,5-fe]quinoxalinium salt 70, °... 

Reaction of 1,3-disubstituted 2-methylimidazo[4,5-b]quinoxalinium salts with aromatic aldehydes gives dyes of structure 79. The aryl group has usually been derived from dimethylaniline, pyrrole, pyrazole, " indole,carbazole," isoxazole, and imidazo fused heterocycles." ... 

Miscellaneous Mn02-mediated Reactions. Treatment of a quinoxalinium salt with various alkenes in the presence of Mn02 has been found to effect a novel 1,3-dipolar cycloaddition reaction to give the corresponding pyrrolo[l,2-a]quinoxalines (eq 65). ... 

The data on the Sn reactions of phosphorus-centered nucleophiles with electro-phihc nitroarenes and azines are scarcely available in the literature [11, 159] Although a number of o -adducts derived from the addition of P-nucleophiles to isoquinoline [140], phthalazine [160], 4,7-phenanthroline [161, 162], A -alkyl-pyrazinium, and quinoxalinium salts [163] have been isolated and identified by NMR and X-ray crystallography, no attempts to convert these o -adducts into the corresponding Sn products have been done. For instance, Af-ethylpyrazinium tetrafluoroborate reacts with dialkyl and diaryl phosphonates under very mild conditicHis (room temperature, acetonitrile) to give stable dialkyl 3-phenyl-5,6-dicyano-l-ethyl-l,2-dihydropyrazin-2-yl phosphonates in good yields (Scheme 50) [163],... 

Unequivocal evidence for the formation of o -adducts has been obtained by X-ray diffraction analysis of those adducts which are stable enough to obtain their single crystals [11]. Indeed, the X-ray crystallography data are available for the anionic trinitrobenzene-methoxide and the Janovsky trinitrobenzene-acetone complexes [11, 201, 202] and for the o -adducts of isoquinoline [203], phthalazine [160], and 4,7-phenanthroline [161, 162] with dialkyl phosphonates. Also the X-ray data have been obtained for the neutral o -adducts resulting from the reactions of iV-methylacridinium ion with N-nucleophiles [204, 205] and for the o -adducts of iV-alkyl-substituted 2,3-dicyanopyrazinium and quinoxalinium salts with 0-, C-and P-nucleophiles [163, 194]. 

T riethy len ediamin e Methine bridging with quinoxalinium salts s. 31, 923... 

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