2-Acetyl-3-alkylpyrazines

Direct oxidation of ethylpyrazines to the corresponding acetylpyrazines may also be carried out in favourable circumstances using hot chromic acid (75JOC1178). Treatment of 2-ethyl-3-alkylpyrazines with chromic acid yields the corresponding 2-acetyl-3-alkyl-pyrazines in yields of 50-70%. In the absence of the 3-alkyl substituent the yields fall dramatically to less than 10%. Acetylpyrazines are more generally prepared by the inverse addition of a Grignard reagent to a cyanopyrazine. 

Other furans included methyl- 2-furfural [4], 2-acetylfuran [5], 2-propionylfuran [6], l-(2-furanyl)-2-propanone [7] and l-(-2-furanyl)-3-butanone [8]. Several alkylpyrazines were also found, with the 2,5-dimethyl [9] and 2-methy 1-5-ethyl [10] the most abundant derivatives, and some 2-acyl-l-methylpyrroles of which only the formyl [11] and acetyl [12] derivatives were conclusively identified. 

Some 2 -dialkylpyrazines have been oxidized in one step with sodium dichromate in acetic acid, in good yields, to the corresponding 2-acyl-3-alkylpyrazines (678). Thus 2-ethyl-3-methylpyrazine gave 2-acetyl-3-methylpyrazine, 23-diethylpyrazine gave 2-acetyl-3-ethylpyrazine, and 2-ethyl-3,5(or 3,6)-dimethylpyrazine gave 2-acetyl-3,5(or 3,6)-dimethylpyrazine (678). 

Despite the acknowledged importance of these furan- and thiophenethiols and their disulfides in meat-like flavors, it is only recently that compounds of this type have been reported in meat itself (8-10). Grosch and co-workers (11) have suggested that the 2-methyl-3-furanthiol and its disulfide are more important in the aroma of boiled meat than in roast beef where alkylpyrazines, 2-acetyl-2-thiazoline and 2,5-dimethyl-4-hydroxy-3(2 -fiiranone make greater contributions. 

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