C-Acylpyrazines

Note No thiations of regular C-acylpyrazines have been reported recently The foregoing example may be viewed as the thiation of an amide that is a more commonly used procedure (see Section 8.4.2). 

Preparations of C-acylpyrazines by primary syntheses have been described in Section II.IB by reduction of a-hydroxyimino carbonyl compounds (226-230). In addition to these preparations, reduction of 3-nitrosopentane-2,4-dione (prepared by nitrosation of pentane-2,4-dione) over Pd/C in an autoclave gave 2,5-diacetyl-3,6-dimethylpyrazine, and 2-nitroso-l-phenylbutane-l,3-dione (from 1 -phenylbutane-1,3-dione) similarly treated gave 2,5-dibenzoyl-3,6-dimethylpyrazine (1064). 

The 1,2-bond is homolytically cleaved by both thermolytic and photolytic means to generate a biradical (17) which in the absence of reactive groups generally forms a 2//-azirine (79AHC(25)147). No direct evidence for the biradical has been presented, but indirect evidence points to such a species. Acylpyrazines have in some instances been isolated, and these would arise by dimerization of the biradical (70JCS(C)1825, 7UCS(C)2644). 

Dichloropyrazine gave 2-chloro-6-[m-methoxy-a, a-(trimethylenedithio)-benzyl]pyrazine (196), the acetal of an acylpyrazine [2-(m-methoxyphenyl)-1,3-dithiane anion (made in situ), 2-Me—THF, -100 — 20°C 14%].1482 2-Chloropyrazine gave several Ni or Pd complexes.566,581 Also other examples.374,882... 

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