6- Substituted quinoxalines reduction

The second example (95a), synthesized by Dalcanale and co-worker181 is asymmetric because of one substituted quinoxaline residue (similar to calix[4]arenes containing one m-substituted phenolic unit). Since the menthyl residue attached to the quinoxaline via an ester function is chiral itself a mixture of the two diastereom-ers was obtained which could be separated by chromatography on silica gel. The subsequent reduction of the ester function in 95a led to enantio-pure (+) and (-) alcohols 95b. The compound 95a was used in host-guest complexation studies182 with benzene, fluorobenzene, 2-fluorotoluene, and isobutane in the gas phase by means of Desorption Chemical Ionization Mass Spectrometry however, studies of chiral recognition have not yet been reported. 

Hydrogen and platinum reduce 6-substituted quinoxalines to the 1,2,3,4-tetrahydro derivatives. A formic acid ruthenium catalyst has been applied to the reduction of quinoxa-iine to afford 1,2,3,4-tetrahydroquinoxaline (2d). ... 

The hydrodimerization is also a useful method in the preparation of heterocycles [7,8], 6>-Bis(yS-bicarbethoxyvinylamino)benzene (III) thus yields by intramolecular reductive coupling 2,3-bis(dicarbethoxymethyl)-l,2,3,4-tetrahydroquinoxaline (IV), which on heating gives quinoxaline [7], as in Eq. (2). Compound IV may be anodically oxidized to the substituted quinoxaline [9],... 

Sodium borohydride-acetic acid treatment of 5- and 6-substituted quinoxalines yields 1,2,3,4-tetrahydroquinoxalines. Alternatively, reduction is effected with hydrogen and a platinum catalyst.The acetic acid-borohydride technique gives excellent yields of 5- and 6-amino-... 

Polarographic reduction of 3-substituted quinoxalin-2-ones gives both... 

In those reactions where the fV-oxide group assists electrophilic or nucleophilic substitution reactions, and is not lost during the reaction, it is readily removed by a variety of reductive procedures and thus facilitates the synthesis of substituted derivatives of pyrazine, quinoxaline and phenazine. 

Attempts to isolate 1,4-dihydroquinoxalinc itself were not successful, but the polarographic behavior of quinoxaline and 6-substituted quin-oxalines in buffered aqueous media suggests that in all cases reduction stops at the 1,4-dihydro stage/ - 2,3-Dimethylquinoxaline and 2-d-araho-tetrahydroxybutylquinoxaline show similar polarographic be-havior, ... 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

The preparation of quinoxaline derivatives carrying a substituent in the benzene ring requires suitably substituted o-phenylenediamines. These have been prepared by reductive cleavage (SnCl2) of appropriately substituted 2,1,3-benzoselenadiazoles (I9).21 Benzo-selenadiazoles, readily prepared from 1,2-diaminobenzenes and selenium dioxide, undergo halogenation at positions 4 and 7 and sulfonation at C-4. 5,6-Dichloro- 2,3-diphenylquinoxaline has been synthesized from benzil and l,2-diamino-3,4-dichlorobenzene, the diamine in turn was obtained from 4,5-dichloro-2,l,3-benzoselenadiazole.22... 

Polarographic studies on pyrazine and methylpyrazines indicate that 1 4-dihydropyrazines are the products of reduction. The reduction of pyrazine itself at the dropping mercury electrode proceeds reversibly. The substitution of methyl groups makes the reduction more difficult with an increased number of methyl groups an increased tendency toward irreversible reduction is noted.102-104 The half-wave reduction potentials for pyrazine, methylpyrazine, 2,6-dimethyl-pyrazine, and tetramethylpyrazine are 2.17, 2.23, 2.28, and 2.50 eV, respectively. Pyrazine is thus more easily reduced than pyridine which has a half-wave potential of 2.76 eV, and less easily reduced than quinoxaline which has a half-wave potential of 1.80 eV.105... 

Substituted or 2,3-disubstituted quinoxaline 1,4-dioxide derivatives undergo partial reduction with L-ascorbic acid to afford monoxides. ... 

Polarographic studies have been made on pyrazine and methylpyrazines they indicate that 1,4-dihydropyrazines are produced, and that substitution (by methyl groups) makes the reduction more difficult. The reduction of the parent pyrazine proceeds reversibly (125, 586-588). The experimental half-wave reduction potentials [pyrazine (2.17 eV), methylpyrazine (2.23) 2,6-dimethylpyrazine (2.28), tetramethylpyrazine (2.50), pyridine (2.76), and quinoxaline (1.80)] also revealed that pyrazine was more easily reduced than pyridine but less easily reduced than quanoxaline (589). 

A classical synthesis of alkyl- and arylpyrazines involves dimerization of a-amino carbonyl compounds, which may be produced by many methods such as reduction of a-oximino ketones, aminolysis of a-halogeno ketones (Section 6.0T11.2), oxidation of a-amino alcohols and reduction of a-amino acids . The condensation of 1,2-diamines with a-dicarbonyl compounds is available for the synthesis of particularly quinoxaline derivatives (Section 6.03.11.1). The synthetic methods which rely on self-condensation, however, provide only symmetrically substituted pyra-... 

Polarographic reduction of the parent gave a value for the half-wave potential (E1/2 = -0.85 v) which is intermediate between the values for quinoxaline ( 1/2 =-1.09 v) and pteridine ( 1/2 =-0.52 v). The electrochemical reduction of substituted pyrido[2,3-fc]pyrazines has been the subject of a recent study. Along with other fused pyrazine systems, the... 

This ring system has also been called pyrazolo[2,3-a]quinoxaline and pyrazolo[a]quinoxaline. Little work has however been done on this heterocycle. Catalytic reduction of the o-nitrophenylpyrazole 1 results in ring closure to give the pyrazolo[l,5-a]quinoxaline 2. Sequential decarboxylation, treatment with phosphoryl chloride, and catalytic hydrogenation give the parent heterocycle 3. The unsubstituted compound is reported to have a broad band at 240-245 nm in its ultraviolet spectrum in 95% ethanol. A recent patent describes several 4-substituted amino derivatives of pyrazolo[l,5-a]quinoxaline-3-carboxylic acid as possessing antiinflammatory properties. ... 

Nitration of l-phenylpyrazolo[3,4-i>]quinoxalines occurs in the para-position of the pendent phenyl ring. Reduction of the nitro group followed by diazotization and coupling with, for example, 8-naphthol has given several azo dyes. Sulfonation of the parent heterocycle with the pyridine-sulfur trioxide adduct again provides 3-substituted products. 

Cyclic voltammetry (CV) is useful to probe the electronic effect of pterin and quinoxaline groups on the Mo reduction potential and this method was applied to a series of I MoO(dithiolene) complexes. The results are graphically summarized in Figure 2.16. Within a series of T MoO(dithio-lenes), it is clear that pterin (or quinoxaline) substitution causes a significant shift in the Mo redox potential to more positive values compared to simpler dithiolenes like benzenedithiolate (bdt) or ethanedithiolate (edt). This conclusion seems to contradict the results from EPR and MCD studies of Tp MoO(pterin-dithiolene vs. T MoO(bdt), which, as noted above, failed to reveal any differences among the dithiolene complexes. 

A radical process involving N-acyl cyanamides such as 196 was reported in 2010 (Scheme 5.43) [67]. A quinozaline was prepared from 196 by an initial alkenyl radical attack at the nitrile triple bond to give the radical intermediate 197. Subsequent aromatic substitution led to alkyl radical 198, and further in situ addition/reduction provided the product 200 in reasonable yields (24-69%). By involving an aryl radical in the beginning, the yields of the corresponding quinoxalines were good (71-88%). 

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