2.5- Diacetyl-3,6-dimethylpyrazine

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). 

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... 

Figure 4. Profiles of volatiles for high-quality peanuts, raw and roasted (1) ethanol (2) pentane (3) 2-propanol (4) acetone (5) methylene chloride (6) methyl acetate (7) 2-methylpropanal (8) diacetyl (9) 3-methylbutanal (10) 2-methyl-butanal (11) 2,3-pentanedione (12) S-methylpyrrole (13) toluene (14) hexanal (15) 2-methylpyrazine (16) xylene (17) 2-heptanone (18) 2,5-dimethylpyrazine (19) 2-pentylfuran (20) 2-ethyl-5-methylpyrazine (21) 2-ethyl-3,6-dimethylpyra-zine (22) phenylacetaldehyde.

Pyrazino[2,3 -6 ]pyrazines are most readily prepared by the reaction of 2,3-diaminopyrazines with glyoxal. 2,3-Dimethylpyrazino[2,3-Z> ]pyrazine (292) is prepared from.glyoxal and 2,3-diamino-5,6-dimethylpyrazine (291), and from diacetyl and 2,3-diaminopyrazine (48JA1257). 

Diacetyl-3,6-dibenzylpiperazine-2,5-dione (15) reacted with sulfur in dimethyl-formamide and triethylamine to form, after hydrolytic removal of the acetyl groups, 3-benzyl-6-benzylidenepiperazine-2,5-dione (16) (1068). 2-Chloro-5-hydroxy-3,6-dimethylpyrazine heated with solid potassium hydroxide gave 3-hydroxy-2,5-dimethylpyrazine (312), and 2-chloro-6-hydroxy-3,5-diphenylpyrazine with an excess of methanolic sodium methoxide at 150° formed 2-hydroxy-3,5-diphenylpyrazine (873). Decarboxylation of 2-carboxy-3-hydroxypyrazine gave 2-hydroxypyrazine (420) and in this way 24iydroxy[2- C]pyrazine (823) and 2-hydroxy[l- N]pyrazine (822) have been prepared. 

Halo-substituted diaminopyrazines have been successfully condensed with diethyl oxalate and with diacetyl to give pyrazino[2,3-h]pyrazines. 2,3-Diamino-5,6-dimethylpyrazines have been reacted with a large number of diaryl a-diketones and aryl a-keto aldehydes to give aryl-substituted derivatives of the heterocycle. 

Fig. 5.4. Headspace analysis of aroma substances of white-bread crust, a Capillary gas chromatogram (the arrows mark the positions of the odorants), b FD chromatogram. Odorants 1 methylpropanal, 2 diacetyl, 3 3-methylbutanal, 4 2,3-pentanedione, 5 butyric acid, 6 2-acetyl-1-pyrroline, 7 l-octen-3-one, 8 2-ethyl-3,5-dimethylpyrazine, 9 (E)-2-nonenal (according to Schieberle and Grosch, 1992)...

---