2-Pyrazinone, 3-phenyl-, nitration

The simple phenyl-substituted pyrazines do undergo nitration in the phenyl ring for example, 2-phenylpyrazine yields the 4-nitrophenyl derivative (mixed acid), although 5-phenyl-2-pyrazinone forms the 3-nitropyra-zinone under similar conditions (75MI1). However, 2,5-dimethyl-3,6-diphenylpyrazine and its /V,A -dioxide are both reported to be nitrated to give the bis-(3-nitrophenyl) products (55JCS3094). 

As already mentioned, hydroxypyrazines exist in tautomeric equilibria with the corresponding pyrazinones which are normally the predominant species in the equilibria. Some of the reactions of hydroxypyrazines are reminiscent of those of phenols they can, for example, be coupled with diazonium salts and brominated and nitrated in either the ortho or para position to the hydroxyl group. Coupling with diazonium salts occurs in neutral or weakly alkaline solution, but if the reaction is carried out in 1 M sodium hydroxide solution, arylation of the pyrazine ring takes place. From hydroxy-pyrazine and benzenediazonium chloride 47% 2-hydroxy-3-phenyl-and 4% 2-hydroxy-3,6-diphenylpyrazine are obtained. 

Hydroxypyrazines, or rather tautomeric 2(l//)-pyrazinones, undergo electrophilic substitution such as nitration and bromination ortho or para to the hydroxy or oxo substituent under mild conditions <53JA5517,56JA4071, 78JHC665). When phenyldiazonium chloride is added to 2(l//)-pyr-azinones in alkaline solution, phenylation of the nucleus takes place with evolution of nitrogen in the same substitution pattern as above. 

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