Using a Diketone or Related Synthon

Diketones, like diacetyl and related synthons, react readily with o-phenylene-diamines or related reduced substrates to afford quinoxalines. Only when both synthon and substrate are unsymmetric are two isomers formed, and this simation has been largely avoided in recent literature. The following classified examples illustrate many of the possibilities available from such syntheses. 

2-Benzenediamine (155) and m,m -dimethylbenzil (156) gave 2,3-di-m-tolyl-quinoxaline (157) (EtOH, reflux, 2 h 93%) similarly, 1,2-diaminocyclohexane and , t -dimethoxybenzil gave 2,3-bis(p-methoxyphenyl)-5,6,7,8-tetrahydro-quinoxaline (158), via oxidation of the unisolated 4a,5,6,7,8,8a-hexahydro analogue (MeOH, reflux, 1 h then crude product, S, 140°C, min 36%). °° 

4-Chloro-3-nitro-1,2-benzenediamine gave 6-chloro-2,3-dimethyl-5-nitroqui-noxaline (159) (Ac2, EtOH, 60°C, 20 min 84%) ° analogs likewise.  

2- Benzenediamine gave 2,3-bis[(triisopropylsilyl)ethynyl]quinoxaline (163, R=SiPr ) [Pi3SiC=CC(=0)C(=0)C=CSiPr3, activated molecular sieve, PhMe, 80°C, 20 min 95%) or 2,3-bis(phenylethynyl)quinoxaline (163, 

4-Nitro-l,2-benzenediamine (165) and l-methyl-2-phenylglyoxal (166) gave a mixture of 2-methyl-7-nitro- (167) and 2-methyl-6-nitro-3-phenylquinoxaline (168) (MeOH, reflux, min 70%), subsequently separated chromatogra- 

2-Benzenediamine gave ethyl 3-methyl-2-quinixalinecarboxylate (160) [AcC(=0)C02Et (made in situ), TsOH, PhH, reflux, 2 h 74%]119 or N-methyl-3-p-nitrophenyl-A-phenyl-2-quinoxalinecarboxamide (161) [BzC(=0)-CONMePh, AcOH, reflux, 30 min 76%] 582 

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