Quinoxaline-2,3-diones tautomerism

Ultraviolet and infrared spectroscopy indicate that quinoxaline-2,3-dione type structures are preferred to tlie tautomeric 3-hydroxy-quinoxalin-2 One or 2,3-dihydroxyquinoxaline forms. The light absorption properties (UV) of quinoxaline-2,3-dione have been compared with those of its NN -, ON-, and OO -dimethyl derivatives (79, 80, and 81), and also its N- and 0-monomethyl derivatives (43 and 82). The parent dicarbonyl compound and its mono- and di-A -methyl derivatives show very strong carbonyl absorption near to 1690 cm split into two peaks. 

Quinoxalin-2(l//)-ones are in tautomeric equilibrium with quinoxalin-2-ols, but physical measurements (UV spectroscopy) indicate that they exist predominantly as cyclic amides rather than as hydroxy compounds. IR studies have also shown that quinoxalin-2(lE/ )-ones have carbonyl absorptions in the region 1690-I660cm and the tautomeric equilibrium is shifted towards the amide form. IR spectroscopy also indicates that quinoxaline-2,3-dione-type structures are preferred to the tautomeric hydroxy forms. - Analogously, 2-sulf-anylquinoxaline exists in solution as quinoxaline-2(l//)-thione. 3-Acylmethyl-substituted... 

F. Tautomerism of Quinoxalin-2-ones and Quinoxaline-2,3-diones AND OF 5- and 6-Hydroxyquinoxalines... 

Quinoxaline-2,3-diones 62a are known to exist as such rather than as hydroxyquinoxalinones 62b or as dihydroxyquinoxalines 62c from infrared and ultraviolet measmements. A mixture of the dimethyl derivatives of the three possible tautomeric forms is obtained on treatment of a suspension of quinoxaline-2,3-dione in methanol with ethereal diazomethane. ... 

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