2-Hydroxypyrazine bromination

Hydroxypyrazines or 2(l//)-pyrazinones are also subject to electrophilic halogenation. A new result is that 1-benzyl- or 1-phenyl-substituted 2(l//)-pyrazinones undergo bromination with NBS or A -iodosuccinimide (NIS) to give the 5-bromo or iodo-2(l//)-pyrazinones in 66-82% yields <2004TL1885, 2005T3953>. 

The classical route to chloropyrazines is by treatment of a hydroxy-pyrazine with phosphoryl chloride bromopyrazines are similarly prepared by using phosphoryl bromide, phosphorus tribromide, or a mixture of both. Thus, treatment of hydro xypyrazine with phosphoryl chloride gives chloropyrazine in 92% yield,147 and treatment of the hydroxy compound with a mixture of phosphoryl bromide and phosphorus tribromide gives bromopyrazine in 58% yield.266 The use of phosphorus pentahalides frequently leads to substitution products (Scheme 22) for example, when hydroxypyrazine is treated with a mixture of phosphoryl bromide and phosphorus pentabromide both monobromo- and 2,6-dibromopyrazines are formed.287,268 Bromina-tion of hydroxypyrazine with bromine in the presence of small... 

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. 

Bromination of 5-s-butyl-3-hydroxy-2-isobutylpyrazine (deoxyaspergillic acid) in aqueous solution or acetic acid gave 2-bromo-3-s-butyl-5-hydroxy-6-isobutylpyrazine (bromodeoxyaspergjllic acid) (87) bromination of 2-s-buty-3-hydroxy-5-isobutyl-pyrazine in acetic acid gave 2-bromo-6-s-butyl-5-hydroxy-3-isobutylpyrazine (93) and 3-hydroxy-2,5-diphenylpyrazine behaved similarly (282). This procedure with bromine in acetic acid was unsatisfactory when applied to simpler hydroxypyrazines (817), and bromination of 3,6(and 5,6)-disubstituted-2-hydroxypyrazines and 2-... 

Brominations of hydroxypyrazines have been described in Section V.1B(4). 

The diazotization of aminopyrazines has been described in earlier sections. Section V.IH records the preparation of 2-fluoropyrazine from 2-aminopyrazine in fluoroboric acid containing copper powder with sodium nitrite (882, 884) and Section V.ll the preparation of iodopyrazines from some aminopyrazines via isodiazotate salts (30) (887). These salts were assigned the isodiazotate structure, on the basis of their inability to couple with 0-naphthol in alkaline solution (887) and they were characterized by hydrolysis in cold 40% aqueous sulfuric acid to the hydroxypyrazine (887). Section V.I K describes the conversion of aminopyrazines to bromopyrazines (798, 800, 807, 890-892) for example, 2-amino-3-methoxy-carbonylpyrazine with hydrobromic acid, bromine, and sodium nitrite in water gave 2-bromo-3-methoxycarbonylpyrazine (798, 890). The diazotization of aminopyrazines to hydroxypyrazines has been described in Section VI. 1C, to alkoxy-pyrazines in Section V1.3C, and to oxopyrazines in Section V1.9A(5). 2-Amino-pyrazine with isopentyl nitrite in benzene gave 2-phenylpyrazine (45%) and some 2-isopentoxypyrazine and 2,2 -dipyrazinyl amino isomers (1211). 

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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