Asymmetric synthesis of tertiary thiols and thioethers

Enantioselective addition of thiols to trifluoromethyl ketimines: synthesis of N,S-ketals

Enantioselective addition of thiols to acyclic trifluoromethyl ketimines for the construction of trifluoromethylated N,S-ketals was performed successfully using a bifunctional squaramide catalyst.

Dealkenylative Thiylation of C(sp3)-C(sp2) Bonds

Carbon-carbon bond fragmentations are useful methods for the functionalization of molecules. The value of such cleavage events is maximized when paired with subsequent bond formation. Herein we report a protocol for the cleavage of an alkene C(sp³)-C(sp²) bond, followed by the formation of a new C(sp³)-S bond. This reaction is performed in nonanhydrous solvent and open to the air, employs common starting materials, and can be used to rapidly diversify natural products.

Organocatalytic Remote Stereocontrolled 1,8-Additions of Thiazolones to Propargylic Aza-p-quinone Methides

A remote stereocontrolled 1,8-conjugate addition of thiazolones to propargylic aza-p-quinone methides formed from propargylic alcohols has been developed with the aid of a chiral phosphoric acid, and this represents the first report on organocatalytic stereocontrolled 1,8-addition of propargylic aza-p-quinone methides. Notably, the remote stereocontrolled activation protocol enables the construction of vicinal sulfur-containing quaternary carbon stereocenters and axially chiral tetrasubstituted allenes and promotes the chemistry of chiral phosphoric acids.

Organotitanium Nucleophiles in Asymmetric Cross-Coupling Reaction: Stereoconvergent Synthesis of Chiral α-CF3 Thioethers

Asymmetric Ni-catalyzed cross-coupling reactions have become a very attractive tool for the stereoselective construction of valuable organic chiral materials. While various nucleophiles are used in such transformation, organotitanium(IV) has not been used before. Herein we demonstrate, for the first time, that organotitanium species can serve as efficient coupling partners in asymmetric cross-couplings, which have proven to be beneficial, compared to the commonly used organomagnesium and organozinc counterparts. This principle is exemplified by the first asymmetric catalytic synthesis of CF₃-substituted thioethers via a Ni-catalyzed stereoconvergent cross-coupling reaction. Thioether moieties and their derivatives are common motifs in many biologically active compounds, and their enantioenriched fluorinated analogs should be of great interest in the search for novel drugs and agrichemicals.

Synthesis of β-sulfinyl cyclobutane carboxylic amides via a formal α to β sulphoxide migration process

An original tandem reaction consisting of a thermal elimination-addition process was developed. Highly substituted β-sulfinyl cyclobutane carboxylic acid derivatives were obtained from isomeric α-sulfinyl derivatives in a single operation in good to high yields and with high trans diastereoselectivity.

Convenient diastereoselective synthesis of annulated 3-substituted-(5S*,6S*,Z)-2-(2-(2,4-dinitrophenyl)hydrazono)-5,6-diphenyl-1,3-thiazinan-4-ones

Racemic 2-(2,4-dinitrophenyl)hydrazono)-5,6-diphenyl-1,3-thiazinan-4-ones and (Z)-N'-(2,4-dinitrophenyl)-2,3-diphenylacrylohydrazide were formed during the diastereoselective reaction between 4-substituted 1-(2,4-dinitrophenyl)thiosemicarbazides and 2,3-diphenylcycloprop-2-enone under refluxing ethanol. The structures of the synthesized compounds were confirmed by single-crystal X-ray analyses.

Metal-free photocatalytic thiol–ene/thiol–yne reactions

We report a metal-free photocatalytic approach to the synthesis of glycoconjugates, highlighting the mild nature of the reaction conditions.

Pyridinium 1,4-zwitterionic thiolates as a useful class of sulfur-containing synthons: application to the synthesis of 2,5-dihydro-1,4,5-thiadiazepines

A class of air-stable, odorless, easy-to-handle, and readily available sulfur-containing synthons that react with 1,2-diaza-1,3-dienes to afford 2,5-dihydro-1,4,5-thiadiazepines is described.

2-Azaadamantane N-oxyl (AZADO)/Cu Catalysis Enables Chemoselective Aerobic Oxidation of Alcohols Containing Electron-Rich Divalent Sulfur Functionalities

The chemoselective oxidation of alcohols containing electron-rich sulfur functionalities (e.g., 1,3-dithianes and sulfides) into their corresponding carbonyl compounds with the sulfur groups can sometimes be a demanding task in modern organic chemistry. A reliable method for this transformation, which features azaadamantane-type nitroxyl radical/copper catalysis using ambient air as the terminal oxidant is reported. The superiority of the developed method was demonstrated by comparing it with various conventional alcohol oxidation methods.

Benzyl (R)-2-(Acetylthio)Propanoate: A Promising Sulfur Isoster of (R)-Lactic Acid and Ester Precursors

In this paper, an accessible chiral pool synthesis of benzyl (R)-2-(acetylthio)propanoate (acetylthiolactate), which is less odorous than the methyl or ethyl analogue, was performed through a clean SN2 displacement reaction using available AcSK with tris[2-(2-methoxyethoxy)]ethylamine (TDA-1), starting from commercially available benzyl (S)-lactate in 76%, 94% ee (2 steps). Deprotection of the acetyl group using N,N-dimethylethylenediamine afforded benzyl (R)-2-sulfanylpropanoate in 93% yield with 90% ee. These two sulfur-containing benzyl esters were sufficiently odorless to be purified by column chromatography. Direct HPLC analysis was applied to determine the enantiomeric excess without thiazolidin-4-one derivatizations. A complementary debenzylation of benzyl (R)-2-(acetylthio)propanoate was also performed using HBr/AcOH to afford (R)-2-(acetylthio)propanoic acid without critical racemization in 92% yield with 92% ee.

Development of Conjugate Addition of Lithium Dialkylcuprates to Thiochromones: Synthesis of 2-Alkylthiochroman-4-ones and Additional Synthetic Applications

Lithium dialkylcuprates undergo conjugate addition to thiochromones to afford 2-alkylthiochroman-4-ones in good yields. This approach provide an efficient and general synthetic approach to privileged sulfur-containing structural motifs and valuable precursors for many pharmaceuticals, starting from common substrates-thiochromones. Good yields of 2-alkyl-substituted thiochroman-4-ones are attained with lithium dialkylcuprates, lithium alkylcyanocuprates or substoichiometric amount of copper salts. The use of commercially available inexpensive alkyllithium reagents will expedite the synthesis of a large library of 2-alkyl substituted thiochroman-4-ones for additional synthetic applications.

Chiral syntheses of methyl (R)-2-Sulfanylcarboxylic esters and acids with optical purity determination using HPLC

Accessible chiral syntheses of 3 types of (R)-2-sulfanylcarboxylic esters and acids were performed: (R)-2-sulfanylpropanoic (thiolactic) ester (53%, 98%ee) and acid (39%, 96%ee), (R)-2-sulfanylsucciinic diester (59%, 96%ee), and (R)-2-mandelic ester (78%, 90%ee) and acid (59%, 96%ee). The present practical and robust method involves (i) clean S_N 2 displacement of methanesulfonates of (S)-2-hydroxyesters by using commercially available AcSK with tris(2-[2-methoxyethoxy])ethylamine and (ii) sufficiently mild deacetylation. The optical purity was determined by the corresponding (2R,5R)-trans-thiazolidin-4-one and (2S,5R)-cis-thiazolidin-4-one derivatives based on accurate high-performance liquid chromatography analysis with high-resolution efficiency. Compared with the reported method utilizing AcSCs (generated from AcSH and CsCO₃ ), the present method has several advantages, that is, the use of odorless AcCOSK reagent, reasonable reaction velocity, isolation procedure, and accurate, reliable optical purity determination. The use of accessible AcSK has advantages because of easy-to-handle odorless and hygroscopic solid that can be used in a bench-top procedure. The Ti(OiPr)₄ catalyst promoted smooth trans-cyclo-condensation to afford (2R,5R)-trans-thiazolidin-4-one formation of (R)-2-sulfanylcarboxylic esters with available N-(benzylidene)methylamine under neutral conditions without any racemization, whereas (2S,5R)-cis-thiazollidin-4-ones were obtained via cis-cyclo-condensation and no catalysts. Direct high-performance liquid chromatography analysis of methyl (R)-mandelate was also performed; however, the resolution efficiency was inferior to that of the thaizolidin-4-one derivatizations.

Enantioselective Synthesis of Chiral α-Thio-Quaternary Stereogenic Centers via Phase-Transfer-Catalyzed α-Alkylation of α-Acylthiomalonates

An efficient synthetic method for establishing chiral α-thio-α-quaternary stereogenic center was successfully developed. The enantioselective α-alkylation of α-acylthiomalonates under phase-transfer catalytic conditions [50% aq. KOH, toluene, -20 °C, and (S,S)-3,4,5-trifluorophenyl-NAS bromide] provided the corresponding α-acylthio-α-alkylmalonates in high chemical yields (up to 99%) and high optical yields (up to 98% ee).

Stereoselective organocatalytic sulfa-Michael reactions of aryl substituted α,β-unsaturated N-acyl pyrazoles

A variety of functionalised sulfides has been obtained in good to high stereoselectivity via sulfa-Michael reaction of α,β-unsaturated N-acyl pyrazoles with thiols or thioacetic acid using readily available organocatalysts and working under mild conditions.

Catalytic enantioselective α-sulfenylation of β-ketocarbonyls by chiral primary amines

A simple chiral primary amine catalyst was identified to enable effective catalysis in direct α-sulfenylation of acyclic and cyclic β-ketocarbonyls with good yields and excellent enantioselectivities.

Organocatalytic enantioselective Mannich-type addition of 5H-thiazol-4-ones to isatin-derived imines: access to 3-substituted 3-amino-2-oxindoles featured by vicinal sulfur-containing tetrasubstituted stereocenters

3-Substituted 3-amino-2-oxindoles featuring both 5H-thiazol-4-one and vicinal sulfur-containing tetrasubstituted stereocenter structural motifs were obtained in high yields with excellent enantioselectivities and diastereoselectivities.

Asymmetric Synthesis of Tertiary Alcohols and Thiols via Nonstabilized Tertiary α-Oxy- and α-Thio-Substituted Organolithium Species

Nonstabilized α-O-substituted tertiary organolithium species are difficult to generate, and the α-S-substituted analogues are configurationally unstable. We now report that they can both be generated easily and trapped with a range of electrophiles with high enantioselectivity, providing ready access to a range of enantioenriched tertiary alcohols and thiols. The configurational stability of the α-S-organolithium species was enhanced by using a less coordinating solvent and short reaction times.