6- Amino-5-benzyl-3-methyl-2 -pyrazinone

Acetoxy-5-benzyl-6-diacetylamino-3-methylpyrazine (31) gave 6-amino-5-bcnzyl-3-methyl-2( I //)-pyrazinone (32) (neat H2NNH2, 20°C, 13 h 67% note additional IV-deacetylation).883 Also other examples.304 809 960,1565 1575... 

Amino-5-azido-2,6-pyrazinedicarbonitrile 3-Amino-5-benzoyl-6-bromo-2-pyrazinecarbonitrile 3-Amino-5-benzoyl-2-pyrazinecarbonitrile 3-Amino-5-benzoyl-2-pyrazinecarboxamide 3-Amino-6-benzylamino-2-pyrazinecarboxylic acid 6-Amino-5-benzyl-3-methyl-2(177)-pyrazinone 3-Amino-6-benzylsulfinyl-2-pyrazinecarboxylic acid 3-Amino-6-benzylthio-... 

Amino-5-benzyl-3-methyl-2(l//)-pyrazinone 210 exists predominantly in the oxo form 210a in aqueous buffered solution (XT 2000), but as the tautomer 210b in dioxane. The equimolar mixture of tautomers was observed in dioxane/aq. buffer = 90 10. The linear dependence of the tautomerization constant on the solvent polarity was established. No imino tautomer 210c was observed (93JOC7542). 

Another useful method for the elucidation of the hydroxypyrazine-pyrazinone tautomerism is UV spectral analysis. The objective structure in solution is easily estimated by comparison with the UV spectra of the proton-fixed compounds of two tautomers, O-methylated (22) and A-methylated derivatives (23), which are prepared by methylation of the hydroxypyrazines or pyrazinones with diazomethane (Scheme 2). For example, 6-amino-5-benzyl-3-methyl-2(177)-pyrazinone (21 R = Me, R = CHzPh, X = NH2) has been shown to predominate over the hydroxy form (20) because of its nearly identical UV spectrum with the corresponding V-methylated derivative (23) <93JOC7542>. In contrast, 6-chloro-2-hydroxypyrazines (20 R, R = Me or Ph, X = Cl) exist in the hydroxy form rather than as the tautomeric amide, which is an exceptional example of predominance of the hydroxy form with parallels in the chloro-pyridinone field <7UCS(C)2977>. 

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