3- quinoxalinone, tautomerism

Alkyl-Alkylidene Tautomerism. Some 2- or 3-(substituted alkyl)quinoxalines, like 3-ethoxycarbonylmethyl-2(177)-quinoxalinone (133), have long been known to exist in equilibrium with their (substituted methylene) tautomers, for example 3-ethoxycarbonylmethylene-3,4-dihydro-2( 1 /7)-quinoxalinone (133a).The effects of solvent change, protonation, and the like on such tautomeric systems have been examined as well as the kinetics thereof. In... 

Although there can be no real doubt that the potentially tautomeric 2(177)-quinoxalinones (e.g., 1), and 2,3 (17/,477)-quinoxalinediones normally exist as such,... 

Most tautomeric quinoxalinones have been made either by primary synthesis (see Chapter 1) or by direct or indirect hydrolysis of halogenoquinoxalines (see Section 3.2.2). The remaining preparative routes are illustrated in the following classified examples. 

The important conversion of tautomeric quinoxalinones into halogenoqui-noxalines has been discussed in Section 3.1.1. Other reactions are covered in the subsections that follow. 

It seems likely that direct alkylation of a tautomeric quinoxalinone invariably gives a mixture of O- and A-alkylated products in which the more soluble... 

Note These are only a few typical examples of the complexes formed by tautomeric quinoxalinones. 

Methoxycarbonylmethyl-2(l//)-quinoxalinone (220) with p-chlorobenzene-diazonium chloride gave a product, initially formulated as 3-[a-(p-chloro-phenylazo)-a-methoxycarbonylmethylene]-3,4-dihydro-2(l//)-quinoxalinone (221) but later as the tautomeric 3-[a-(p-chlorophenyUiydrazono)-a-methoxy-carbonylmethyl]-2(l//)-quinoxalinone (222) (HCl, H2O, AcOH, <5°C, 10 min, then 95°C, 30 min 90%). ° Such tautomerisms have been studied in some detail.(See also Section 6.6.)... 

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