3- quinoxalin-2-one tautomerism

Tautomeric studies of equilibria between enamino and methylene imino form have been reported for several heterocycles. Tautomerism of 1,2,4-tri-azino[4,3- ]quinoxalin-5-ones was deduced to be solvent dependent the enamino form 170b is predominant in DMSO-dg ( H NMR), whereas 170a is the major tautomer in the solid state (IR in nujol) (90JHC691 95H2057) (cf. Section III,B). 

Quinoxalin-2 one (70) is in mobile tautomeric equilibrium with 2-hydroxyquinoxaline (71), but physical measurements fail to demonstrate the presence of the hydroxy form (see following). 

Quinoxalin-2-ones are in tautomeric equilibrium with 2-hydroxy-quinoxalines, but physical measurements indicate that both in solution and in the solid state they exist as cyclic amides rather than as hydroxy compounds. Thus quinoxalin-2-one and its A -methyl derivative show practically identical ultraviolet absorption and are bases of similar strength. In contrast, the ultraviolet spectra of quinoxalin-2-one and its 0-methyl derivative (2-methoxyquinoxaIine) are dissimilar. The methoxy compound is also a significantly stronger base (Table II). Similar relationships also exist between the ultraviolet absorption and ionization properties of 3-methylquinoxalin-2-one and its N- and 0-methyl derivatives. The infrared spectrum of 3- (p-methoxy-benzyl)quinoxalin-2-one (77) in methylene chloride shows bands at 3375 and 1565 cm" which are absent in the spectrum of the deuterated... 

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... 

The formation of imidazoquinoxalines apparently proceeds through the tautomeric form D formed at the first stage of the reaction of the bases of Schiff C in which nucleophilic attack of atom N4 on the imine atom of carbon occurs with the closure of the imidazoline cycle the aromatization of which when exposed to the DMSO or oxygen of the air leads to the imidazo[l,5-fl]quinoxalin-4-one derivatives. 

It should be pointed out that the formation of spiro-quinoxaline derivative 134 could be due to the Michael addition of hydrazine to the partially positive C(3) atom of the quinoxalin-2(l//)-one 50 in the first stage of the reaction mechanism with the formation of intermediate A capable reversible tautomerization to intermediate B. Then cyclization involves the nucleophilic attack of the amino group on the carbonyl group of the 3-arylacylidene fragment of quinoxalin-2(l//)-one (Scheme 6.53). 

It should be pointed out that the quinoxalin-2-one/quinoxalin-2-ol tautomerism like 166 to H, when R = Aik, is impossible and therefore the isocyanate derivative responsible for the course of the rearrangement in the case of R = H cannot be... 

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