An improvement in syntheses of mercaptans. VIA-Thiolesters

Photocatalytic O- to S-Rearrangement of Tertiary Cyclopropanols

Despite the importance of heteroatom-substituted cyclopropane derivatives in drug design and organic synthesis, cyclopropanethiols remain critically underexplored. Inspired by the wide use of the Newman-Kwart rearrangement to access valuable thiophenols from phenol feedstocks, we report the development of a photocatalytic approach for efficient ambient temperature aliphatic O- to S-rearrangement on tertiary cyclopropanol derivatives. After demonstrating that a range of cyclopropanethiols-that are difficult to access by other methods-can be obtained with this strategy, we show that these rearranged products can be easily hydrolyzed and further derivatized. We conclude this study with mechanistic findings that enabled an initial extension of this approach toward other classes of aliphatic alcohols.

New formation mechanism of allylic trithiocarbonates from sodium O-(2-Alkenyl) dithiocarbonates via sequential pericyclic reactions: Density functional theory study

Density functional theory (DFT) calculations at the B3LYP/6-31G(d) level demonstrated that sodium O-(3-phenylallyl) dithiocarbonate undergoes [3,3]-sigmatropic rearrangement to sodium S-(1-phenylallyl) dithiocarbonate, which then isomerizes to the more thermodynamically stable sodium S-(3-phenylallyl) dithiocarbonate. The calculations also showed that sodium 2-alkenyl trithiocarbonates and their esters are more labile towards the allylic rearrangement.

Identification of novel aroma-active thiols in pan-roasted white sesame seeds

Screening for aroma-active compounds in an aroma distillate obtained from freshly pan-roasted sesame seeds by aroma extract dilution analysis revealed 32 odorants in the FD factor range of 2-2048, 29 of which could be identified. The highest FD factors were found for the coffee-like smelling 2-furfurylthiol, the caramel-like smelling 4-hydroxy-2,5-dimethyl-3(2H)-furanone, the coffee-like smelling 2-thenylthiol (thiophen-2-yl-methylthiol), and the clove-like smelling 2-methoxy-4-vinylphenol. In addition, 9 odor-active thiols with sulfurous, meaty, and/or catty, black-currant-like odors were identified for the first time in roasted sesame seeds. Among them, 2-methyl-1-propene-1-thiol, (Z)-3-methyl-1-butene-1-thiol, (E)-3-methyl-1-butene-1-thiol, (Z)-2-methyl-1-butene-1-thiol, (E)-2-methyl-1-butene-1-thiol, and 4-mercapto-3-hexanone were previously unknown as food constituents. Their structures were confirmed by comparing their mass spectra and retention indices as well as their sensory properties with those of synthesized reference compounds. The relatively unstable 1-alkene-1-thiols represent a new class of food odorants and are suggested as the key contributors to the characteristic, but quickly vanishing, aroma of freshly ground roasted sesame seeds.

[Cascade reactions of unsaturated xanthates and related reactions: computer-assisted molecular design and analysis of reaction mechanisms]

Chugaev reaction provides access to olefins from alcohols without rearrangement through pyrolysis of O-alkyl S-alkyl dithiocarbonates (xanthates) via cis elimination. Xanthates having no beta-hydrogens undergo thione-to-thiol rearrangement to give S,S-dialkyl dithiocarbonates (thiolcarbonates). Based on these backgrounds, we intended to use the dithiolcarbonates as precursors of thiols. Xanthates could be converted to the corresponding dithiolcarbonates by Lewis-acid catalysis. Pyridine N-oxides and 4-dimethylaminopyridine also catalyzed the rearrangement reaction. On the other hand, O-(2-alkenyl) S-alkyl dithiocarbonates undergo [3,3]-sigmatropic rearrangement to give the allylically rearranged S-(2-alkenyl) S-alkyl dithiocarbonates. The homoallylic xanthates gave the corresponding dithiolcarbonates on heating with phenols. Pyrolysis of allylic dithiolcarbonates caused retro-ene type reaction to give the allylic sulfides in which allylic rearrangement also occurs. Coupling of these pericyclic reactions with intramolecular Diels-Alder reaction affords a one-pot synthetic method for the construction of hydroisobenzothiophenes. The mechanisms of the cascade and related reactions are demonstrated by location of the transition states using molecular orbital calculation method at various levels of theory involving density functional theory (DFT).

Hydrolysis and aminolysis of alkyl xanthate esters and cellulose analogues

The hydrolysis and aminolysis of a series of S-substituted O-alkylxanthate esters was studied in 20% v/v aqueous methanol at 35°C. The pH-rate profiles of the hydrolyses showed water and hydroxide-ion-catalyzed reactions. The reaction of 2,4-dinitrophenyl cellulose xanthate (CelXDNP) and p-nitrobenzyl cellulose xanthate (CelXNB) with polyalanine and lysozyme produced a covalent bond between the polypeptide and the cellulose matrix, as shown by solid-state 13C NMR. However, the nature of the bonding could not be identified. The reaction of nucleophiles (H2O, OH-, RNH2) and xanthic esters was consistent with an addition-elimination mechanism through a tetrahedral intermediate. Brønsted plots against the pKa of the nucleophile (βnu) or the nucleofuge of the substrate (βlg) were used to characterize the rate-determining step. The pKa values of the nucleophiles ranged between -1.74 and 15.74, and for the nucleofuges, they were in the range of 10.50-0.92. For nucleophiles with pKa values up to about 10, βlg was 0.10-0.15, and βnu changed from 0.48 to 0.35 for the strongest electron-withdrawing nucleofuge. It was concluded that the water-catalyzed hydrolyses, and also aminolyses with moderately basic amines, occur with rate-determining formation of the tetrahedral intermediate. For strong bases such as hydroxide ion, the disappearance of the intermediate becomes the slowest step. The reaction of cellulose xanthic esters with external nucleophiles as hydroxide ion and amines shows simple first-order kinetics and is slower than alkyl or sugar xanthates, probably due to the diffusion effect through the tight cybotactic region of cellulose. Key words: hydrolysis, aminolysis, alkyl xanthic esters, cellulose xanthic esters, sugar xanthic esters.