Analysis and Sensory Evaluation of the Stereoisomers of a Homologous Series (C5-C10) of 4-Mercapto-2-alkanols

PhI(OAc)2-Promoted 1,2-Transfer Reaction between 1,1-Disubstituted Allylic Alcohols and Thiophenols

The PhI(OAc)2-promoted 1,2-transfer reaction between allylic alcohols and thiophenols, conducted in an argon atmosphere, has proven to be effective in producing β-carbonyl sulfides from 1,1-disubstituted allylic alcohols in high yields. This method offers a fast and efficient way to synthesize β-carbonyl sulfides, which are valuable intermediates in organic synthesis. This discussion focuses on the effects of the oxidizer, temperature, and solvent on the reaction. A proposed tentative mechanism for this reaction is also discussed.

Construction of Chemical Libraries of Volatile Compounds by Combinatorial Synthesis of Homologous Mixtures: Alk-4-en-1-ols, Alk-4-enals and Methyl Alk-4-enoates

A compound library of sixty six linear compounds, eleven representatives of six molecular families: (E)- and (Z)-isomers of alk-4-en-1-ols, alk-4-enals, and methyl alk-4-enoates, was prepared by combinatorial syntheses to allow the creation of a mass spectral database directly usable for their identification in GC/MS analyses. We demonstrate here that compound libraries can be prepared by combinatorial syntheses using long linear synthetic sequences, i. e., eight step in the case of 4-enals. The resulting mixtures of homologues are still perfectly exploitable to deliver the requested information such as clean mass spectra and good gas chromatographic retention indices.

Configurations and Sensory Properties of the Stereoisomers of 2,6-Dimethyl-4-propyl-1,3-oxathiane and 2,4-Dimethyl-6-propyl-1,3-oxathiane

The heterocyclic compounds 2,6-dimethyl-4-propyl-1,3-oxathiane 1 and 2,4-dimethyl-6-propyl-1,3-oxathiane 2 were obtained by condensing 4-mercapto-2-heptanol and 2-mercapto-4-heptanol, respectively, with acetaldehyde. For both, separation of the eight stereoisomers was achieved via capillary gas chromatography using heptakis(diethyl-tert-butyldimethylsilyl)-β-cyclodextrin as the chiral stationary phase. Their configurations were assigned by combinations of enzyme-catalyzed kinetic resolutions, HPLC separations, and assessments of NMR data. The odor thresholds and odor qualities of the stereoisomers were determined by capillary gas chromatography/olfactometry. The odor thresholds of the stereoisomers of 2 were generally higher than those of 1. For both oxathianes, the stereoisomers in which all substituents are in equatorial positions showed the highest odor thresholds. Most of the stereoisomers of 1 exhibited pleasant flowery, fruity, or sweet nuances; the stereoisomers of 2 were mainly characterized by descriptors, such as broth, mushroom, or pungent. The data demonstrate the impact of the positions of substituents and their spatial orientations on the sensory properties of 1,3-oxathianes.

Chirality: An Important Phenomenon Regarding Biosynthesis, Perception, and Authenticity of Flavor Compounds

Chirality plays an important role in flavor research. This will be outlined using selected examples from the key areas analysis, authenticity assessment, biogenesis, and odor perception. Developments of analytical techniques, in particular the use of chiral stationary phases in capillary gas chromatography, enabled the determination of naturally occurring configurations of chiral volatiles at trace levels. Thus, knowledge of pathways and enzymes involved in the biogenesis of chiral substances was acquired, and enantioselective analysis has become a prominent tool in the authenticity assessment of flavorings. Increasing information is also available on structure-odor relationships of chiral flavor compounds and the influence of their configurations on odor thresholds and odor properties. A substantial extension of these data sets and a further understanding of the role of chirality in the perception of aroma compounds is expected from studies on the enantioselectivity of odorant receptor systems. Developments in these areas will be put into a historical perspective, recent progress will be emphasized, and data gaps will be described.

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

3-Mercaptocycloalkanones and 3-mercaptocycloalkanols (chain lengths C5-C7) were obtained by addition of thioacetic acid to the respective 2-cycloalken-1-ones and subsequent enzyme-mediated hydrolysis and reduction with LiAlH₄, respectively. The stereoisomers were separated via capillary gas chromatography using chiral stationary phases. Their configurations were determined based on ¹H NMR data and enzyme-catalyzed kinetic resolutions. Odor thresholds and odor qualities were assessed by capillary gas chromatography/olfactometry. Compared to the analogous acyclic 4-mercapto-2-alkanones and 4-mercapto-2-alkanols, the cyclic polyfunctional thiols lacked fruity, tropical notes; the perceived odor properties ranged from cooked, roasted vegetables and meat types to onion-related notes. The odor thresholds of the enantiomers of the 3-mercaptocycloalkanones were mainly impacted by their ring size rather than their configuration. For the 3-mercaptocycloalkanols, the (S)-configuration at the stereogenic center bearing the thiol group and the relative configuration of the second asymmetric center with the hydroxyl group were of importance for low odor thresholds.

Absolute Configurations and Sensory Properties of the Stereoisomers of a Homologous Series (C6-C10) of 2-Mercapto-4-alkanols

A homologous series (C6-C10) of 2-mercapto-4-alkanols was obtained by the addition of thioacetic acid to the respective alkenones and subsequent reduction with LiAlH₄. Gas chromatographic separation of the stereoisomers was achieved using chiral stationary phases. Their absolute configurations were assigned by the correlation of ¹H NMR data and enzyme-catalyzed kinetic resolutions. Odor thresholds and odor qualities were determined by capillary gas chromatography/olfactometry. Compared to the odor qualities reported for the isomeric 4-mercapto-2-alkanols, the homologous series of 2-mercapto-4-alkanols lacked fruity, tropical notes. There was no consistent correlation between the configurations and the odor qualities. However, the observed odor thresholds indicated the importance of the configuration of the asymmetric center bearing the hydroxyl group and the alkyl substituent. The length of this alkyl chain is a main driver for the odor properties, ranging from pungent, vegetable to earthy, mushroom notes.

Determination of the Absolute Configurations and Sensory Properties of the Enantiomers of a Homologous Series (C6-C10) of 2-Mercapto-4-alkanones

The enantiomers of a homologous series (C6-C10) of 2-mercapto-4-alkanones were obtained by lipase-catalyzed kinetic resolution of the corresponding racemic 2-acetylthio-4-alkanones. Their configurations were assigned via vibrational circular dichroism and ¹H NMR anisotropy based methods. Odor thresholds and odor qualities were determined by capillary gas chromatography-olfactometry using chiral stationary phases. There were minima of the odor thresholds for the chain lengths C7 and C8. Except for chain length C8, the enantiomers of the other homologues showed similar odor thresholds. The odor qualities ranged from pungent (C5) to mushroom (C9 and C10) and were similar to those known for the corresponding 1-alken-3-ones with one less C atom. In contrast to their positional isomers (4-mercapto-2-alkanones), the investigated 2-mercapto-4-alkanones do not meet the requirements of a "tropical olfactophore" (i.e., compounds possessing a 1,3-oxygen-sulfur functionality and specific arrangements of the substituents).

Photo-biocatalytic One-Pot Cascades for the Enantioselective Synthesis of 1,3-Mercaptoalkanol Volatile Sulfur Compounds

The synthesis of enantiomerically pure 1,3-mercaptoalkanol volatile sulfur compounds through a one-pot photo-biocatalytic cascade reaction is described. Two new KRED biocatalysts with opposite enantioselectivity were discovered and proved to be efficient on a wide range of substrates. The one-pot cascade reaction combining photocatalytic thio-Michael addition with biocatalytic ketoreduction in an aqueous medium provides a green and sustainable approach to enantiomerically pure 1,3-mercaptoalkanols in high yields with excellent enantioselectivity.

Distributions of the Stereoisomers of β-Mercaptoheptanones and β-Mercaptoheptanols in Cooked Bell Pepper (Capsicum annuum)

2-Mercapto-4-heptanone, 4-mercapto-2-heptanone, and the corresponding mercaptoalcohols, previously identified in cooked red bell pepper (Capsicum annuum), were used as examples to determine the distributions of stereoisomers of naturally occurring polyfunctional thiols. The thiols were isolated using simultaneous distillation-extraction and enriched by affinity chromatography. Enantioselective analysis was performed via multidimensional gas chromatography. For the studied cultivar California Wonder, the investigation of different batches of cooked red bell pepper revealed consistent ratios of the stereoisomers independent of origin and date of purchase. Quantitative estimations showed that the stereoisomers were present in cooked red bell peppers at concentrations in the range of 0.04-10.2 μg/kg. Lower concentrations were observed in cooked green bell peppers. The change from green to red color was also accompanied by shifts in the proportions of stereoisomers in favor of the (S)-enantiomers of the mercaptoheptanones and of the (4S)-configured stereoisomers of 4-mercapto-2-heptanol.