Structure odour-activity relationships of alkylpyrazines

The odour thresholds (i.e. the lowest concentration at which an odour can b e detected) of 80 alkylpyrazines, most of them synthesized, were determined by gas chromatography-olfactometry. Trimethylpyrazine (1) had the lowest t hreshold (50 ng/l air) amongst mono-, di-, tri- and tetramethylpyrazine. Su bstitution of the methyl group in position 2 of 1 by an ethyl group yielded 2-ethyl-3,5-dimethylpyrazine (2) showing a 4500-fold lower odour threshold than 1. The thresholds of 2-ethenyl-3,5-dimethylpyrazine (3), 2,3-diethyl- 5-methylpyrazine (4) and 2-ethenyl-3-ethyl-5-methylpyrazine (5) were as low as that of 2. The threshold of 3-ethenyl-2-ethyl-5-methylpyrazine was 8000 times higher than that of 5, indicating that an ethenyl group was only tol erated in position 2, but not in position 3. A further increase in the odou r threshold was found when the ethenyl or the ethyl group was located in po sition 5. Substitution of the ethyl group of 2 by a (Z)-1-propenyl group in creased the threshold only by a factor of 5, whereas the (E)-isomer and the 2-propenyl group enhanced the threshold by factors of 160 and 27000, respe ctively. Also a propyl, butyl, pentyl, isobutyl or hexyl group in position 2 of 2 was too bulky, and in these molecules the odour threshold was at lea st 2200 times higher than that of 2. The geometrical structure of a theoret ical receptor was obtained by superimposing the minimized structures of pyr azines with low thresholds (Chem-X force-field minimization). Sterically fo rbidden regions in the resulting model were found by superimposing pyrazine s with high thresholds.