Studies on pyrazines. Part 34.1 Synthetic approach, stability and tautomerism of 2,6-dihydroxypyrazines
Demethylation of 2,6-dimethoxy-3,5-diphenylpyrazine with iodotrimethylsilane gives the corresponding 2,6-dihydroxy- and 2-hydroxy-6-methoxy-pyrazines, whilst the 3-methyl-5-phenyl analogue affords only monohydroxy compounds. In contrast, 2,6-dimethoxy-3,5-dimethylpyrazine decomposes completely under the demethylation conditions. Hydrolysis of 2,6-diacetoxypyrazines succeeds only in the formation of 2,6-dihydroxy-3,5-diphenylpyrazine. The stability of 2,6-dihydroxypyrazines is discussed on the basis of observations made in the synthetic approach. In addition, it has been established, on the basis of UV spectral analysis, that the 2-hydroxy-6-methoxypyrazines obtained in our work exist predominantly in the hydroxypyrazine form rather than as 1,2-dihydropyrazin-2-ones.