5-substituted derivative converted pteridine oxide

The cleavage of fused pyrazines represents an important method of synthesis of substituted pyrazines, particularly pyrazinecarboxylic acids. Pyrazine-2,3-dicarboxylic acid is usually prepared by the permanganate oxidation of either quinoxalines or phenazines. The pyrazine ring resembles the pyridine ring in its stability rather than the other diazines, pyridazine and pyrimidine. Fused systems such as pteridines may easily be converted under either acidic or basic conditions into pyrazine derivatives (Scheme 75). 

In contrast, pteridines are excellent substrates for both molybdenum hydroxylases and each enzyme can catalyse oxidation at carbon 2, 4 or 7, although not necessarily in the same molecule [ 10, 204-206 ]. For a comparison of the position of attack by the enzymes, the reader is referred to a recent review by the author [11]. Unfortunately, the sequence of oxidation in the multistep pathways is not always clear and therefore the effects of substitution are difficult to interpret. l-Methylpteridin-4-one (55b), which is apparently oxidized at carbon 2, is converted much more rapidly than the 3-methyl isomer (55a) [206], These derivatives correspond to the 3-methyl and 1-methyl analogues of hypoxanthine, respectively. Nevertheless, it is not possible to make direct comparisons of the //-methyl derivatives with unsubstituted pteridin-4-one (56b) because the site of oxidation may differ [206, 207],... 

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