Comparison of odour thresholds and odour qualities of the enantiomers of 4‐mercapto‐2‐alkanones and 4‐acetylthio‐2‐alkanones

4‐Mercapto‐2‐alkanones constitute a class of naturally occurring polyfunctional thiols; 4‐mercapto‐2‐heptanone, for example, has been reported in cooked red bell pepper. The objective of this study was to determine the impact of the chain length on the odour thresholds and the odour qualities of the enantiomers of these chiral volatiles. 4‐Acetylthio‐2‐pentanone and 4‐acetylthio‐2‐hexanone and the corresponding 4‐mercapto‐2‐alkanones were synthesized by the addition of thioacetic acid to 3‐alkene‐2‐ones and subsequent lipase‐catalysed hydrolysis. The absolute configurations and the gas chromatography (GC) elution order of the enantiomers on a chiral stationary phase were determined on the basis of¹H NMR anisotropy effects in combination with lipase‐catalysed enantioselective hydrolysis. These compounds were included into a homologous series of 4‐mercapto‐2‐alkanones and 4‐acetylthio‐2‐alkanones exhibiting alkyl carbon chain lengths from 5 to 10. The odour thresholds of the enantiomers were compared using capillary gas chromatography‐olfactometry (GC‐O). The odour thresholds of the mercapto‐compounds were consistently lower than those of the corresponding acetylthio‐compounds. Both series of compounds exhibited minima of the odour thresholds at alkyl carbon chain lengths of 7 and 8, respectively. Odour thresholds of the (S)‐enantiomers of 4‐mercapto‐2‐hexanone and 4‐acetylthio‐2‐hexanone were significantly lower than those of the corresponding (R)‐enantiomers. The (S)‐enantiomers of the mercapto‐compounds had more fruity and pleasant notes than the (R)‐enantiomers. Although the number of judges was limited, the comparative data demonstrate the impact of the chain length on odour thresholds and odour qualities of members of a homologous series and underline the role of the configurations for the sensory properties of chiral volatiles.