Parabanic acids

Alloxan forms an oxime (1007) which is the same compound, violuric acid, as that formed by nitrosation of barbituric acid likewise, a hydrazone and semicarbazone. Reduction of alloxan gives first alloxantin, usually formulated as (1008), and then dialuric acid (1004 R = OH) the steps are reversible on oxidation. Vigorous oxidation with nitric acid and alkaline hydrolysis both give imidazole derivatives (parabanic acid and alloxanic acid, respectively) and thence aliphatic products. Alloxan and o-phenylenediamine give the benzopteridine, alloxazine (1009) (61MI21300). 

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. 

The ureides hydantoin, parabanic acid, alloxan, barbituric acid, and 4-methyluracil show resonance energies of between 2.3 v.e. and 3.1... 

Fichter and Kern O first reported that uric acid could be electrochemically oxidized. The reaction was studied at a lead oxide electrode but without control of the anode potential. Under such uncontrolled conditions these workers found that in lithium carbonate solution at 40-60 °C a yield of approximately 70% of allantoin was obtained. In sulfuric acid solution a 63% yield of urea was obtained. A complete material balance was not obtained nor were any mechanistic details developed. In 1962 Smith and Elving 2) reported that uric acid gave a voltammetric oxidation peak at a wax-impregnated spectroscopic graphite electrode. Subsequently, Struck and Elving 3> examined the products of this oxidation and reported that in 1 M HOAc complete electrochemical oxidation required about 2.2 electrons per molecule of uric acid. The products formed were 0.25 mole C02,0.25 mole of allantoin or an allantoin precursor, 0.75 mole of urea, 0.3 mole of parabanic acid and 0.30 mole of alloxan per mole of uric acid oxidized. On the basis of these products a scheme was developed whereby uric acid (I, Fig. 1) is oxidized in a primary 2e process to a shortlived dicarbonium ion (Ha, lib, Fig. 1) which, being unstable, under-... 

Fig. 4. Mechanisms for decomposition of uric acid-4,S-diol to allantoin (V), alloxan (VI), urea (VII) parabanic acid (XIV) and COj...

The nature of the species that gives rise to the more negative cathodic peak observed on cyclic voltammetry of uric acid (Fig. 2) is not clear. It must be due to some relatively transient species since the peak is not pronounced at the com-pletation of the electrolysis nor is a large amount of parabanic acid formed,... 

Fig. 11. Mechanism for formation of parabanic acids from the methylated uric acid-4,5-diol derived from theobromine (3,7-dimethylxanthine) and caffeine (1,3,7-trimethylxan-thine). Molar amounts of products are those formed in 1 M HOAc...

B. SECONDARY HYDRATION, REARRANGEMENT AND OXIDATION TO PARABANIC ACID... 

The water must not be boiled during the addition of the crystals or afterwards as this will cause decomposition of the alloxan to carbon dioxide, parabanic acid, and alloxantin. 

Parabanic acid can be prepared by the condensation of urea with diethyl oxalate in an ethanolic solution of sodium ethoxide,2 by reaction of urea with an ethereal solution of oxalyl chloride,3 by oxidizing uric acid with an acid solution of perhydrol,4 or by the action of hot, concentrated nitric acid on uric acid.5 The present method gives better yields than the previously reported methods and is better adapted to larger-scale preparations. 

The expressions given here are valid for the multipole formalism of Hansen and Coppens, as described by Eq. (3.35). With other formalisms, similar expressions are used. Experimental molecular potentials reported in the literature include those of imidazole (Spackman and Stewart 1984, pp. 302-320), phosphorylethanolamine (Swaminathan and Craven 1984), alloxan (Swaminathan and Craven 1985), and parabanic acid (He et al. 1988). 

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