Reactions of C-Acylpyrazines

Irradiation of 2,5-bis(methoxycarbonyl)-3,6-dimethylpyrazine in diethyl ether with a 450-W high-pressure mercury lamp (330 nm) gave two significant photoproducts 2-hydroxy-5-methoxycarbonyl-3,6-dimethylpyrazine and l-(l -ethoxy-ethyl)2,5-dimethoxycarbonyl-3,6-dimethyl-l, 4-dihydropyrazine (74). Similar products were isolated when tetrahydrofuran was used as solvent. Corresponding reactions were observed with 2,5-bis(ethoxycarbonyl)-3,6-dimethylpyrazine, 2,5-diacetyl-3,6-dimethylpyrazine, and 2,5-dibenzoyl-3,6-dimethylpyrazine. The mechanism of these reactions were investigated and the initial stage was found to be 

2-Hydroxy-3-phenacylpyrazine was cyclized with concentrated sulfuric acid at 70-80° to 2-phenylfuro[2,3-h]pyrazine(76, X = 0), and 2-mercapto-3-thiophenacyl-pyrazine with methanolic hydrogen chloride at reflux to 2-phenylthieno[2,3-h]-pyrazine (76, X = S) (1446). 

Typical preparations have been described for 2-(r-hydroxyiminomethyl)pyrazine (138) 2-(4 -dimethylamino-T-hydroxyiminobutyl)pyrazine (1186) 2-(2 -hydroxy-iminopentyl)-6-methylpyrazine (645) 2-(2 -hydroxyimino-3, 3 -dimethylbutyl)-6-methylpyrazine (645) 2-[r-hydroxyimino-2-(pyridin-2 -yl) (hyl]pyrazine (642) 2-[2 -hydroxyimino-2 -(pyridin-2 -yl, pyrazin-2 -yl, or pyrimidin-4 -yl)ethyl]pyrazine (642) 2-[l, 2 -bishydroxyimino-2 -(pyridin-2 -yl)ethyl]pyrazine (642) 2-12 -(2 ,4 -dinitrophenylhydrazono)-3 -methylbutyl]-6-methylpyrazine (645) 2-[ 1-(substituted hydrazono)ethyl]pyrazine (1196) 2-[l -phenyl-1 -(substituted hydrazono)methyl]-pyrazine (1196) 2-(r-thiosemicarbazonoethyl)pyrazine (138) 2-(2 -chloro-l -thiosemicarbazonoethyOpyrazine (138) and 2,5-dimethyl-3,6-bis(r-semicarbazono-ethyl)pyrazine (1196). 

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