1.2- Dihydroquinoxalines preparation

The fusion of a benzene ring to pyrazine results in a considerable increase in the resistance to reduction and it is usually difficult to reduce quinoxalines beyond the tetrahydroquinoxa-line state (91). Two possible dihydroquinoxalines, viz. the 1,2- (92) and the 1,4- (93), are known, and 1,4-dihydroquinoxaline appears to be appreciably more stable than 1,4-dihydropyrazine (63JOC2488). Electrochemical reduction appears to follow a course anzdogous to the reduction of pyrazine, giving the 1,4-dihydro derivative which isomerizes to the 1,2- or 3,4-dihydroquinoxaline before subsequent reduction to 1,2,3,4-tetra-hydroquinoxaline (91). Quinoxaline itself is reduced directly to (91) with LiAlH4 and direct synthesis of (91) is also possible. Tetrahydroquinoxalines in which the benzenoid ring is reduced are well known but these are usually prepared from cyclohexane derivatives (Scheme 30). 

Benzopiperazinones 200 were prepared by Erb et al. via the Ugi four-component reaction followed by palladium-catalyzed intramolecular Al-arylation [61]. XPhos was used as a supporting ligand to afford the 3,4-dihydroquinoxalin-3-ones 200 (Scheme 36). Microwave irradiation was found to be determinant on the reaction efficiency. 

Also included in Expt 8.44, is the preparation of 2,3-diphenylquinoxaline (114) and 3-benzyl-2-oxo-l,2-dihydroquinoxaline (115) from o-phenylenediamine and benzil or phenylpyruvic acid respectively. 

Monoketo analogues of 8-aza-l,4-dihydroquinoxaline-2,3-dione and tautomers have been prepared (2). 

N-(Phenyl-l-amino-4-arsinic acid)-glycine-amide, (HO)20As. C8Hj(NHg).NH.CH2.CO.NH2, is obtained when 3 4-diaminophenyl-arsinie acid in alkaline solution is treated with chloracetamide. It begins to darken at 215° C. and melts with decomposition at 234° to 241° C. If about 25 per cent, more alkali is used in the preparation, 3-amino-6-arsino-l 2-dihydroquinoxaline results ... 

This melts at 226° C., forms an ammonium salt, decomposing at 200° C., and a benzoyl derivative, M.pt. 284° C. In the preparation of the glycine-amide, longer heating, or recrystallisation from an excess of alkali, causes it to change to the quinoxaline. If the latter in sodium hydroxide solution is allowed to stand for several days at 30° C. with an excess of ethylene oxide, the mixture being occasionally shaken, S-hydroiryethyl-amino-6-arsino-l i-dihydroquinoxaline is fomred. 

Reduction of quinoxaline with sodium in THF at 20° yields a deep-purple solution from which 1,4-dihydroquinoxaline is isolated. Reduction with either sodium in refluxing alcohol or lithium aluminum hydride in ether gives 1,2,3,4-tetrahydroquinoxaline. Hydrogenation of quinoxaline over a 5% rhodium-on-alumina catalyst at 100° and 136 atm or over freshly prepared Raney nickel W-6 under similar conditions gives meso-(cis)-decahydroquinoxaline. ° However hydrogenation of quinoxaline over a palladium-on-charcoal catalyst at 180° and 50 atm gives dl-(/rans)-decahydroquinoxaline." ... 

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. 

Preparation of 3-chloro-2-((2-chloroquinolin-3-yl) methylene)hydrazono)-l,2-dihydroquinoxaline... 

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