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

Abstract

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 of 1H 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. Copyright © 2014 John Wiley & Sons, Ltd.

Analytical and Sensory Characterization of the Stereoisomers of 3-Mercaptocycloalkanones and 3-Mercaptocycloalkanols

“…Comparison of the Sensory Data to Those of Acyclic β-Mercaptoalkanones and β-Mercaptoalkanols with the Same Chain Lengths. Except for the previously reported high odor threshold of (S)-4-mercapto-2-hexanone, 11 the odor thresholds of the 3-mercaptocycloalkanones 1 and 2 were in the same order of magnitude as those of the corresponding acyclic 4-mercapto-2-alkanones 11 and 2-mercapto-4-alkanones, 15 and there were no significant differences between the enantiomers (Figure 6A). For the C7 homologue 3, there were also no differences between the enantiomers, but the odor thresholds were one order of magnitude higher than those of 4-mercapto-2-heptanone 11 and 2-mercapto-4-heptanone.…”

“…It is noteworthy that in contrast to the linear β -mercaptoalkanones, the acetylation of the mercapto group of the 3-mercaptocycloalkanones did not result in a significant increase of the odor thresholds. In particular for the chain lengths C5 and C6, the odor thresholds of the cyclic acetyl-derivatives were clearly lower than those of the corresponding acyclic 4-mercapto-2-alkanones and 2-mercapto-4-alkanones (Figure B).…”

“…Among the volatile sulfur-containing constituents occurring in foods, − polyfunctional thiols are attractive flavor compounds with low odor thresholds and interesting odor properties. − A 1,3-oxygen sulfur functionality with a defined arrangement of substituents is considered as an essential structural feature constituting the so-called “tropical olfactophore”. , Several of these polyfunctional thiols are chiral; accordingly, studies on structure–odor relationships in homologous series increasingly take into consideration not only the chain lengths and the positions of the functional groups but also the impact of the configurations on the sensory properties. , An example of systematic investigations of these stereochemical aspects is the comparative assessment of the sensory properties of a homologous series of linear β-mercaptoalkanones and β-mercaptoalkanols. For the different positional isomers, the influence of the type and the spatial arrangements of the substituents at the stereogenic centers has been demonstrated. − …”

Analysis of stereoisomers of chiral drug by mass spectrometry

“…2 The properties of chiral molecules on chemical and physical are basically the same, while individual enantiomers differ in their biological activity, mechanism, and toxicity. 4 Thus, great emphasis has been put on the enantiomeric analysis for the drug safety. 4 Thus, great emphasis has been put on the enantiomeric analysis for the drug safety.…”

“…For example, (8R,8′R,7′S)-lyoniresinol enantiomer is strongly bitter whereas (8R,8′R,7′S)-lyoniresinol is tasteless 3 ; the S-enantiomers of 4-mercapto-2-hexanone and 4acetylthio-2-hexanone have more fruity and pleasant notes than the R-enantiomers. 4 Thus, great emphasis has been put on the enantiomeric analysis for the drug safety. 5,6 There are several methods for chiral analysis, involving high-performance liquid chromatography (HPLC), gas chromatography (GC), supercritical fluid chromatography (SFC), thin-layer chromatography, and capillary electrophoresis (CE).…”