Azauracil

The names of these compounds as aza analogs were coined in the same way as those of the 6-aza analogs employing the frequently used numbering of uracil (1). This nomenclature is most often used for the principal aza analogs of pyrimidine bases (e.g., 5-azauracil) it is rarely used for further systematic derivatives. 

According to the triazine nomenclature, 5-azauracil is 2,4-dioxo-l,2,3,4-tetrahydro-l,3,5-triazine (2). The subject index of Chemical Abstracts prefers s-triazine-2,4(lH,3H)-dione. Furthermore, some authors use a name derived from the lactim structure, 2,4-dihydroxy-s-triazine (3). The numbering of the substituents is the same for all these types of nomenclature. 

The common method of preparation of 6-alkyl-2,4-dioxotetrahydro-triazines is the cyclization of acyl-biurets by aqueous hydroxide. Formyl biuret which should by analogy 3deld 5-azauracil had not been known until recently. Its transient formation can be expected during further synthesis of 5-azauracil. Piskala and GuP achieved... 

A determination of the dissociation constants of the compounds reveals that 5-azauracil (pi a = 6.73) is practically of the same acidity as 6-azauracil and considerably more acidic than uracil, A fundamental difference between 5-azauracil, on the one hand, and 6-azauracil and uracil, on the other, lies in the low stability of 5-azauracil toward acid and especially to alkaline hydrolysis. This fact appears to be in agreement with the differences in electron densities of these substances computed by the simple MO-LCAO method. ... 

By a more detailed study of the reaction of 5-azauracil with diazo-methane it was found that this reaction is considerably accelerated by the presence of a small amount of water, methanol, or dimethyl-formamide. It does not proceed appreciably in absolute ether. By... 

Works on the oxidation of uric acid has unequivocally established the triazine structure > ° (9) of oxonic acid. This is further confirmed by the straightforward synthesis described by Piskala and Gut. ° The reaction of biuret (11) with potassium ethyloxalate yielded a potassium salt (24), that with ethyl oxamate, the amide of oxonic acid (25). Both these compounds were converted to 5-azauracil. An analogous reaction with diethyloxalate which should produce an ester of oxonic acid resulted in a mixture of urethane and parabanic acid, however. 

Other derivatives of s-triazine, in particular the 2,4-disubstituted ones, are usually prepared by total synthesis and are therefore not closely linked with the chemistry of 5-azauracil unlike the analogous derivatives of 1,2,4-triazine. 2,4-Dimethoxy-l,3,5-triazine was mentioned earlier (e.g., Section II,A,2,a), the other substances are not related to the present subject. 

SWV has been applied to study electrode reactions of miscellaneous species capable to form insoluble salts with the mercury electrode such as iodide [141,142], dimethoate pesticide [143], sulphide [133,144], arsenic [145,146], cysteine [134, 147,148], glutathione [149], ferron (7-iodo-8-hydroxyquinolin-5-sulphonic acid) [150], 6-propyl-2-thiouracil (PTU) [136], 5-fluorouracil (FU) [151], 5-azauracil (AU) [138], 2-thiouracil (TU) [138], xanthine and xanthosine [152], and seleninm (IV) [153]. Verification of the theory has been performed by experiments at a mercury electrode with sulphide ions [133] and TU [138] for the simple first-order reaction, cystine [134] and AU [138] for the second-order reaction, FU for the first-order reaction with adsorption of the ligand [151], and PTU for the second-order reaction with adsorption of the ligand [137]. Figure 2.90 shows typical cathodic stripping voltammograms of TU and PTU on a mercuiy electrode. The order of the... 

The structures of 5-azacytosine and related compounds are of interest because of their biological importance (see Section 2.20.5.6). l-Methyl-5-azacytosine exists in the amino-oxo form (23). 5-Azauracil (l,3,5-triazine-2,4-dione) is of particular interest. IR spectra indicate that it exists in the dioxo form in the solid, but H NMR studies have been interpreted to show that it exists in the monoenolic form in solution. The spectra showed a non-exchange-able sharp singlet at 8.18 8 (H, 24) (760MR(8)224). Derivatives of 5-azacytosine and 5-azauracil are covalently hydrated. Thus 5-azauridine exists entirely in the crystal form as (25) (76MI22000, p.l39>. 

A determination of ihe dissociation constants of the compounds reveals that 5-azauracil (pifa = 6.73) is practically of the same acidity as 6-azauracil and considerably more acidic than uracil. 

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