1.2- Dihydro-3-methylquinoxaline

A systematic study of the electrochemical reduction of 2-methyl-, 2,3-dimethyl-, 2-phenyl-, 2,3-diphenyl-, and 2-methyl-3-phenylquinoxaline in alkaline or neutral media has been made. For example, reduction of 2-methylquinoxaline in either an alkaline or neutral medium gives 1,2-dihydro-3-methylquinoxaline at pH 9 and a higher reduction potential,... 

Catalytic reduction of 2-acetyl-3-methylquinoxaline (29) in ethanol with 1 mole of hydrogen, gives a deep crimson solution, from which red-brown needles of 2-acetyl-l,4-dihydro-3-methylquinoxaline (30) are obtained. Ethanolic solutions of (30) reoxidize on exposure to air to 2-acetyl-3-methylquinoxaline, but the solid dye is stable in air for several days. Similar results are obtained with 2-acetyl-3-phenyl-quinoxaline (31), from the reduction of which a purple-red dye (32) is obtained. ... 

In boiling ethanol, under nitrogen and in the presence of palladized charcoal, 2-acetyl-l,4-dihydro-3-methylquinoxaline (30) undergoes dismutation to give a mixture of 2-acetyl-3-methylquinoxaline, 2-acetyl-l,2,3,4-tetrahydro-3-methylquinoxaline (33), and 2-l -hydroxy-ethyl-3-methylquinoxaline (34), The latter compound is the product of sodium borohydride or Meerwein-Ponndorf reduction of 2-acetyl-3-methylquinoxaline. 

Ethyl 2-methyl-3-oxo-3,4-dihydro-6-quinoxalinecarboxylate Ethyl 3-methyl-2-oxo-l,2-dihydro-6-quinoxalinecarboxylate Ethyl 4-methyl-3-oxo-3,4-dihydro-2-quinoxalinecarboxylate Ethyl 8-methyl-3-oxo-3,4-dihydro-2-quinoxalinecarboxylate 2-Ethyl-3-methylquinoxaline... 

Chloro-5-methoxy-3-methylquinoxaline 2-Chloro-8-methoxy-3-methylquinoxaline 6-Chloro-5-methoxy-3-oxo-3,4-dihydro-2-quinoxalinecarbonitrile 1-oxide... 

Quinoxaline170-172 is reduced to 1,4-dihydroquinoxaline (167) in a reversible reaction. 2-Methylquinoxaline is reduced to 3,4-dihydro-2-methylquinoxaline (168),172 whereas 2-phenylquinoxaline forms l,2-dihydro-2-phenylquinoxaline (169) the 1,4-dihydroquinoxalines may be formed primarily, but are then tautomerized to the more stable dihydroquinoxaline. 2,3-Diphenylquinoxaline is reduced to as-l,2,3,4-tetrahydro-2,3-diphenylquinoxaline, the 1,4-dihydro compound is transformed into the 1,2-dihydro derivative, which is further reducible.172... 

N-oxides of this type are accessible from o-nitroaniline-based syntheses (see earlier section). l,2-Dihydro-2-oxoquinoxaline 4-oxide (64) and its N-methyl derivative (65) condense with compoimds containing an active methylene group in the presence of piperidine, yielding 3-alkylated quinoxalinones (66). The oxide 64, when warmed with aqueous potassium cyanide, affords 3-cyano-l,2-dihydro-2-oxoquinoxaline, whereas oxide 65 gives l,2-dihydro-3-hydroxy-2-oxo-l-methylquinoxaline. ... 

Alkyl and arylmagnesium halides react with 2-methylquinoxaline by addition of one mole of reactant to the 3,4-bond. After hydrolysis the 2-alkyl- or 2-aryl-l,2-dihydro-3-methylquinoxalines (52) are obtained. When ethylmagnesium bromide is used a dimeric by-product (53) is also isolatedReaction of 2,3-dimethylquinoxaline with benzonitrile and lithium amide gives l-amino-l-phenyl-2-(3-methyl-2-quinoxalinyl)-ethylene (54). The mono- and dilithium salts of 2,3-dimethylquinoxaline have been generated from the quinoxaline by reaction with one or two equivalents of lithium diisopropylamide (LiNPr, respectively. These salts have been reacted with a variety of electrophilic reagents such as alkyl halides, aryl ketones, esters, and nitriles. " ... 

Methylquinoxaline reacts with both alkyl and aryl magnesium compounds to give, after hydrolysis, 2-alkyl-, or 2-aryl-l,2-dihydro-3-methylquinoxalines. When the dihydro compound 2, derived from p-tolylmagnesium bromide, is heated in ethanol, isomerization occurs, and... 

A 1,5-dihydro-l-oxo derivative (47) of pyrrolo[l,2-a]quinoxaline has been shown to be the product of a supposed Diels-Alder reaction between 2,3-dimethylquinoxaline (46) and maleic anhydride. 2-Methylquinoxaline and 2-methyl-3-phenylquinoxaline give the corresponding derivatives 48. " The reaction between 2,3-dimethyl quinoxaline and maleic anhydride proceeds in 80% yield when the reactants are fused together. " ... 

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. 

---