1.4- Dihydroquinoxalines

Benzenediamine (185) and benzoin (186) gave a separable mixture of 2,3-diphenyl-1,2-dihydroquinoxaline (187, R = Ph) and 2,3-diphenylquinoxaline (188, R = Ph) (dry mixture, microwave irradiation under reflux, 4 min 21% and 67%, respectively) in contrast, similar treatment with m,m -dichloro-benzoin gave only the aromatized product, 2,3-bis(m-chlorophenyl)quinoxa-line (188, R = C6H4Cl-m) (94%). ... 

A mixture of 2-methyl-3-methylene-7-nitro-4-phenyl-3,4-dihydroquinoxaline (97) and its quaternary perchlorate, 2,3-dimethyl-6-nitro-l-phenylquinoxali-nium perchlorate (98), underwent oxidative coupling to give l,2-bis(3-methyl-6-nitro-1 -phenyl-1,2-dihydroquinoxalin-2-ylidene)ethane (99) [Cu-(OAc)2, AcOH-MeCN, 95°C, 20 min 60%] many such condensations... 

Methylquinoxalinium iodide (217) with diethylamine gave 2-diethylamino-1-methyl-1,2-dihydroquinoxaline (218) (EtaO, 20°C, 5 min 91%) and thence either 5-methyl-l,2,3,4,4a,5,10,10a-octahydropyrazino[2,3-/7]qui-... 

Trimethyl- 1,2-dihydroquinoxaline (254) from 2,3,3-trimethyl-2,3-dihydro-2-quinoxalinamine 1,4-dioxide (255) (Na2S202, H20, MeOH, 20°C, until colorless 6% note additional deamination).245... 

Bromo-3-methyl-2-methylene-1 -phenyl-1,2-dihydroquinoxaline 2-Bromomethyl-6-methyl-3-... 

Ether cleavage of 2,3-dimethoxyquinoxaline to give 2,3( l//,4//)-quinoxalinedione was performed by treatment with iodotrimethylsilane generated in situ from chlorotrimethylsilane and sodium iodide, albeit in 15—40% yields <1999JHC1271 >. 2,3-Dimethoxyquinoxaline reacts with 2 equiv of butyllithium to furnish 2,2-dibutyl-3-methoxy-1,2-dihydroquinoxaline (Equation 22) <1999T5389>. 

Benzene-1,2-diamine undergoes reaction with halomethyl phenyl ketones to give 3-aryl-l,2-di-hydroquinoxalines 1, ° or after oxidation 2-arylquinoxalines 2. ° 1,2-Dihydroquinoxalines can be readily oxidized with different reagents. 

Nitrene can be generated by Pd-catalyzed deoxygenation of organic nitro compounds with CO. Carbonylation of nitroenamine 226, derived from 2-nitroaniline and a-substituted aldehyde, was carried out using Pd(dba)2 and dppb. A mixture of 1,2-dihydroquinoxaline 227 and 3,4-dihydroquinoxaline 228 was obtained via sequential reactions of carbon monoxide insertion, heterocyclization, and A-alkylation followed by dealkylation [115]. 

Detailed study of the electrochemical reduction of quinoxaline in aqueous media has revealed that 1,2,3,4-tetrahydroquinoxaline is formed via a sequence involving 1,4-dihydroquinoxaline and acid-catalyzed rearrangement of the 1,4-dihydro derivative to 1,2-dihydroquinoxaline (Scheme... 

Dihydroquinoxalines are formed in the photochemical reactions of quinoxalines with diethyl ether, tetrahydrofuran, and dioxan in the presence of benzophenone. Thus reaction of 2,3-dimethylquinoxaline and tetrahydrofuran yields the product 1 of 1,2-addition. With 2-t-butyl-quinoxaline a mixture of the diastereoisomers resulting from addition to the 3,4-bond is formed but with quinoxaline itself, 2-substituted quinoxalines are produced. These are thought to arise from autoxidation of the intermediate 1,2-adducts. ... 

N-H stretching. The H NMR spectrum, in deuterated acetone, shows a methylene singlet at 5 4.44 and a broad band at 5 5.3 due to N-H absorption, which disappears on addition of D2O. 3-Aryl-l,2-dihydro-quinoxalines give monoacetyl derivatives on treatment with acetic anhydride and are oxidized to the corresponding quinoxalines with ben-zoquinone, ferric chloride, or m-nitrobenzoic acid. Other 1,2-dihydroquinoxalines have been prepared from o-phenylenediamine as illustrated in Scheme 2. 

Dihydroquinoxalines are obtained by the reaction of benzils with iV-phenyl-o-phenylenediamine thus reaction of the diamine with benzil gives 1,2-dihydro-l,2,3-triphenyl-2-quinoxalinol (6). The quinoxalinol is very readily transformed to the corresponding 2-alkoxyderivatives (7) on treatment with alcohols. 

Irradiation of the diazepine N-oxide 8 gives a mixture of the 1,2-dihydroquinoxaline (10) and the oxadiazocine 11 through an intermediate oxaziridine (9). The dihydroquinoxaline undergoes acid hydrolysis to the quinoxalinone 12. It was subsequently shown that quinoxalinones of type 12 undergo ready reaction with ammonia or primary or secondary amines, in the presence of titanium tetrachloride, to give 3-amino- or... 

Treatment of the methosulfate of 2,3-dimethylquinoxaline (16) with ammonia gives 1,2-dihydro-l,3-dimethyl-2-methylenequinoxaline (17) as an unstable oil reaction with p-chlorophenyldiazonium chloride yields the 1,2-dihydroquinoxaline 18. ... 

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