Flavouring and odorant thiols from renewable natural resources by InIII-catalysed hydrothioacetylation and lipase-catalysed solvolysis

Studies on analogs of DAMASCENOLIDETM: Part 4. Synthesis and odor evaluation of sulfur-containing analogs of DAMASCENOLIDETM

DAMASCENOLIDETM [1, 4-(4-methylpent-3-en-1-yl)furan-2(5H)-one], which is isolated from damask rose, is a useful aroma compound with a citrus-like odor. We have previously reported on the synthesis and odor properties of 34 analogs of 1 as part of our new aroma compound development project. In order to develop better aroma compounds and to gather more information on structure-odor relationships, 6 novel sulfur-containing analogs of 1 were synthesized. Odor evaluation revealed that their odors differed significantly from those of the corresponding sulfur-free compounds. The introduction of a sulfur atom does not necessarily result in a sulfur-like odor. In particular, the 2(5H)-thiophenone analogs gave waxy, oily, and lactone-like odors that are uncharacteristic of sulfur-containing compounds. In many synthesized analogs, the introduction of a sulfur atom led to an increase in odor intensity, as expected.

Microbiological versus Chemical Reductive Sulfidation: An Experimental and Theoretical Study

Microbiological reductive sulfidation (RS) has rarely been documented, although it represents an efficient strategy for thiol formation. In this work, we reported on the sulfate-respiring bacterium Desulfovibrio sp.86 that has previously demonstrated RS activity toward the pesticide chlordecone. The purpose of this study was to assess its substrate versatility using a set of 28 carbonyls, to compare with chemical RS and to rationalize the observed trends using a dual experimental and theoretical approach. The chemical RS generally proceeds in two steps (S/O exchange using a sulfur donor like P₄S₁₀, reduction of the thione intermediate). Intriguingly, chlordecone was found to be converted into chlordecthiol following the first step. Hence, we designed a protocol and applied it to the 28 substrates to assess their propensity to be directly converted into thiols with the P₄S₁₀ treatment alone. Finally, we performed density functional theory calculations on these carbonyls and their thiocarbonyl derivatives to build a set of structural, electronic, and thermodynamic parameters. The results showed that chemical and microbiological RS probably involved two distinct mechanisms. Chemically, we observed that several carbonyls, possessing electron-withdrawing groups and/or aromatic rings, were directly transformed into thiols in the presence of P₄S₁₀. The correlation obtained with the electron affinity of the thiones led us to conclude that a probable single-electron reductive transfer occurred during the first step. We also found that Desulfovibrio sp.86 transformed a variety of aldehydes and ketones, without ever detecting thiones. No significant correlation was observed with the calculated parameters, but a relationship between aldehyde RS biotransformation and bacterial growth was observed. Differences in selectivity with chemical RS open the way for further applications in organic synthesis.

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.

Efficient synthesis of organic thioacetates in water

Thioacetates as precursors of thiols are interesting starting points for synthesizing other organosulfur compounds. Herein, we propose a simple, efficient and fast method to obtain organic thioacetates using water as a solvent. Taking into account the great attention that has been paid toward environmentally friendly synthetic procedures in the past decades, we prove the role and the strength of the thioacetate anion as a nucleophile for nucleophilic displacement reactions in an aqueous medium. The reactions were carried out under pH control, to prevent the decomposition of the mesylate starting materials, using potassium carbonate as a safe and mild base. A simple work up allows products to be obtained with excellent yield and acceptable purity.