Organocatalytic Asymmetric Sulfa-Michael Addition of Thiols to α,β-Unsaturated Hexafluoroisopropyl Esters: Expeditious Access to (R)-Thiazesim

Organocatalytic Strategy for a Formal 1,6-Conjugate Hydroxylation

Thanks to the versatile vinylogous hexafluoro iso-propyl acrylate molecular platforms, a regio- and enantioselective sulfa-Michael reaction was achieved upon organocatalytic conditions, allowing the subsequent amidation, one-pot oxidation, and Mislow-Bravermann-Evans [2,3]-sigmatropic rearrangement along with a 1,3-chirality transfer, leading eventually to a formal asymmetric 1,6-hydroxylation sequence.

Eco-Friendly Functionalization of Ynals with Thiols under Mild Conditions

A new eco-friendly method for the synthesis of mono- and multifunctional organosulfur compounds, based on the process between ynals and thiols, catalyzed by bulky N-heterocyclic carbene (NHC), was designed and optimized. The proposed organocatalytic approach allows the straightforward formation of a broad range of thioesters and sulfenyl-substituted aldehydes in yields above 86%, in mild and metal-free conditions. In this study, thirty-six sulfur-based derivatives were obtained and characterized by spectroscopic methods.

Electrochemically promoted thio-Michael addition of N-substituted maleimides to thiols in an aqueous medium

A stable and practical electrochemical method was developed to promote the thio-Michael addition of N-substituted maleimides to various thiols in an aqueous medium. This protocol was found to be excellent in terms of facile scale-up, oxidant- and catalyst-free conditions, broad substrate scopes, good functional group tolerance, and easily available substrates. Notably, a plausible reaction mechanism was derived from the results of a series of control experiments and CV studies, which indicated that a radical pathway might speed up the thio-Michael addition under constant current.

Metal-Free Enantioselective 1,4-Addition of Diarylphosphine Oxides to α,β-Unsaturated Carboxylic Esters

The catalytic asymmetric conjugate addition of phosphorus nucleophiles to unsaturated compounds, catalyzed by metallic or nonmetallic catalysts, has been extensively developed. However, the enantioselective transformations involving α,β-unsaturated carboxylic esters for constructing chiral c-p bonds have been rarely reported, particularly in metal-free processes. In this study, we present a novel metal-free methodology for enantioselective 1,4-addition of diarylphosphine oxides to α,β-unsaturated carboxylic esters using classical chiral oxazaborolidine catalysts. Remarkably high yields and enantioselectivities were obtained for most of the products. Furthermore, these valuable chiral phosphorus esters serve as crucial intermediates that can be transformed into various derivatives including amides, acids, and alcohols in a single step.

Electronic Effects in a Green Protocol for (Hetero)Aryl-S Coupling

Aryl and heteroaryl iodides have been efficiently converted into the corresponding thioacetates in cyclopentyl methyl ether (CPME), a green solvent, under Cu catalysis. The chemoselectivity of the reaction is mainly controlled by electronic factors, enabling the conversion of both electron-rich and electron-deficient substrates into the corresponding thioacetates in good to excellent yields. The products can be easily deprotected to the corresponding thiolates to carry out additional synthetic transformations in situ. Surprisingly, despite CPME's relatively low dielectric constant, the reaction rate significantly increased when conducted under microwave irradiation conditions. This synthetic methodology exhibits a remarkable tolerance to functional groups, mild reaction conditions, and a wide substrate scope, utilizing a safe and inexpensive CuI pre-catalyst in the green solvent CPME. A non-aqueous workup allowing for the complete recovery of both catalyst and solvent makes this approach an environmentally sustainable protocol for C(sp2) sulfur functionalization. Additionally, the reaction shows selective cross-coupling with iodides in competition with chlorides and bromides, allowing its use in multistep syntheses. To demonstrate the potential of this methodology, it was applied to the high-yield synthesis of a photochromic dithienylethene, where a selective synthesis had not been reported before.

Scale-up of Sodium Persulfate Mediated, Nitroxide Catalyzed Oxidative Functionalization Reactions

BACKGROUND

Oxidation is a valuable tool in preparative organic chemistry. Oxoammonium salts and nitroxides have proven valuable as reagents and catalysts in this endeavor.

OBJECTIVE

The objective of this study is to scale up the oxidative amidation, ester formation, and nitrile formation using nitroxide as an organocatalyst.

METHODS

Oxidative functionalization reactions were scaled from the 1 mmol to the 1 mole level. Sodium persulfate was used as the primary oxidant, and a nitroxide was employed as a catalyst. The products of the reactions were isolated in analytically pure form by extraction with no need for column chromatography.

RESULTS

The oxidative amidation and esterification of aldehydes can be scaled up from 1 mmol to 1 mole effectively, with comparable product yields being obtained at each increment. This work shows that conditions developed on a small scale can be transferred to a larger scale without reoptimization. The oxidative functionalization of aldehydes to prepare nitriles is not amenable to direct scale-up due to the concomitant formation of significant quantities of the corresponding carboxylic acid, thereby compromising the product yield.

CONCLUSION

Two of the three oxidative transformations studied here can be scaled up successfully from the 1 mmol to the 1 mole level.

One-pot synthesis of thioethers from indoles and p-quinone methides using thiourea as a sulfur source

Thiourea is an inexpensive and user friendly sulfur reagent that acts as a sulfur source. A simple and efficient protocol has been developed to access thioethers by reacting indoles with p-quinone methides using thiourea as the sulfur source. In our experiments, the reaction apparently proceeded through an S-(3-indolyl)isothiuronium iodide intermediate and subsequent generation of indolethiol that attacked the 1,6 position of p-quinone methides to give desired thioethers in good to excellent yields.

Metal-free thiolation of sulfonyl hydrazone with thiophenol: synthesis of 4-thio-chroman and diarylmethyl thioethers

A simple, efficient, and transition metal-free approach was developed for accessing 4-thio-substituted chroman and diarylmethyl thioethers from sulfonyl hydrazones. This protocol provides straightforward access to a class of diarylmethane derivatives with good to excellent yields. This operationally simple protocol exhibited good tolerance for labile functional groups, providing biologically relevant chemical libraries. This safe and feasible route is applicable to the large-scale synthesis of 4-thio-substituted chromans, which are of great synthetic interest because of their further reaction potential.

Progress on the Cu-Catalyzed 1,4-Conjugate Addition to Thiochromones

Carbon-carbon bond formation is one of the most important tools in synthetic organic chemists' toolbox. It is a fundamental transformation that allows synthetic chemists to synthesize the carbon framework of complex molecules from inexpensive simple starting materials. Among the many synthetic methodologies developed for the construction of carbon-carbon bonds, organocopper reagents are one of the most reliable organometallic reagents for this purpose. The versatility of organocuprate reagents or the reactions catalyzed by organocopper reagents were demonstrated by their applications in a variety of synthetic transformations including the 1,4-conjugate addition reactions. Sulfur-containing heterocyclic compounds are a much less studied area compared to oxygen-containing heterocycles but have gained more and more attention in recent years due to their rich biological activities and widespread applications in pharmaceuticals, agrochemicals, and material science. This paper will provide a brief review on recent progress on the synthesis of an important class of sulfur-heterocycles-2-alkylthiochroman-4-ones and thioflavanones via the conjugate additions of Grignard reagents to thiochromones catalyzed by copper catalysts. Recent progress on the synthesis of 2-substituted thiochroman-4-ones via alkynylation and alkenylation of thiochromones will also be covered in this review.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Organocatalytic Enantioselective Sulfa-Michael Additions to α,β-Unsaturated Diazoketones

Enantioselective sulfa-Michael additions to α,β unsaturated diazocarbonyl compounds have been developed. Quinine-derived squaramide was found to be the best catalyst to promote C-S bond formation in a highly stereoselective fashion for alkyl and aryl thiols. The easy-to-follow protocol allowed the preparation of 22 examples in enantiomeric ratios up to 97:3 and reaction yields up to 94%. The mechanism and origins of enantioselectivity were determined through density functional theory (DFT) calculations.

Total Synthesis of (±)-Spiroaxillarone A via a Reversible Sulfa-Michael Addition

A bioinspired strategy is described for the total synthesis of spiroaxillarone A, which exhibited significant antimalarial activity against resistant Plasmodium falciparum (IC₅₀ = 2.32 μM). The key steps include an intermolecular ethanethiol Michael addition, o-quinone Michael addition, and subsequent β-ethanethiol elimination. This synthetic sequence provides a potential biosynthetic pathway of spiroaxillarone A.

Preparation of hexafluoroisopropyl esters by oxidative esterification of aldehydes using sodium persulfate

A simple, metal-free route for the oxidative esterification of aldehydes to yield hexafluoroisopropyl esters is reported. The methodology employs sodium persulfate and a catalytic quantity of a nitroxide and is applicable to aromatic, heteroaromatic, and aliphatic aldehydes.

The synthesis of chiral β-naphthyl-β-sulfanyl ketones via enantioselective sulfa-Michael reaction in the presence of a bifunctional cinchona/sulfonamide organocatalyst

Cinchona alkaloid-derived organocatalysts are widely employed in various asymmetric transformations, yielding products with high enantiopurity. In this respect, a bifunctional quinine-derived sulfonamide organocatalyst was developed to catalyze the asymmetric sulfa-Michael reaction of naphthalene-1-thiol with trans-chalcone derivatives. The target sulfa-Michael adducts were obtained with up to 96% ee under mild conditions and with a low (1 mol %) catalyst loading. Selected enantiomerically enriched sulfa-Michael addition products were subjected to oxidation to obtain the corresponding sulfones.

[Asymmetric Hetero-Michael Additions to α,β-Unsaturated Carboxylic Acids by Multifunctional Boronic Acid Catalysts]

Several direct asymmetric Michael additions to α,β-unsaturated carboxylic acids with integrated catalysts comprising chiral bifunctional thiourea and arylboronic acid were developed. First, the asymmetric aza-Michael addition of hydroxylamine derivatives efficiently afforded a variety of optically active β-amino acid derivatives. Furthermore, upon detailed investigation of the reaction, tetrahedral borate complexes, comprising two carboxylate molecules, were found to serve as reaction intermediates. Based on this observation, a drastic improvement in product enantioselectivity was achieved upon benzoic acid addition. Second, on merely changing the solvent, the asymmetric thia-Michael addition of arylthiols afforded both enantiomers of the adducts, which are important building blocks for biologically active compounds.

Organocatalyzed Sulfa-Michael Addition of Thiophenols on Trisubstituted α-Fluoroacrylates, a Straightforward Access to Chiral Fluorinated Compounds

In this manuscript, a simple and efficient sulfa-Michael addition reaction of aryl thiols to trisubstituted α-fluoro-α,β-unsaturated esters both in racemic and, for the first time, in enantioselective version is reported. The commercially available dimer of cinchona derivatives (DHQ)₂PYR was used as a catalyst. This strategy showed a great tolerance for various substrates and substituents, providing fair to excellent yields, moderate to excellent diastereoselectivities (2:1 to >99:1), and low to good enantioselectivities (2 to 87%). The reaction has been applied to the synthesis of fluorinated analogues of diltiazem and tiazesim, both therapeutic agents.

Recent Advances in Synthesis of Chiral Thioethers

Chiral thioethers is an important class of organosulfur molecules with extensive applications, especially in the field of medicine and organic synthesis. This review discusses the recent progress of synthesis of enantioenriched chiral thioethers and hopes to be helpful for related research in the future. It is summarized from organosulfur compounds-participating organic reaction types, including nucleophilic substitution, cross coupling, sulfa-Michael addition, sulfenylation, asymmetric allylic reaction, asymmetric Doyle-Kirmse reaction, Pummerer-type rearrangement, Smiles rearrangement,^[2,3] Stevens and Sommelet-Hauser rearrangement.

Efficient Synthesis of 1,4-Thiazepanones and 1,4-Thiazepanes as 3D Fragments for Screening Libraries

1,4-Thiazepanes and 1,4-thiazepanones represent seven-membered ring systems with highly 3D character and are currently underrepresented in fragment screening libraries. A nuclear magnetic resonance (NMR) fragment screen identified 1,4-acylthiazepanes as new BET (bromodomain and extraterminal domain) bromodomain ligands; however, an efficient and readily diversified synthesis for library development has not been reported. Here we report a one-pot synthesis using α,β-unsaturated esters and 1,2-amino thiols to form 1,4-thiazepanones as precursors to 1,4-thiazepanes with high 3D character. This reaction proceeds in reasonable time (0.5-3 h) and in good yield and tolerates a broad scope of α,β-unsaturated esters. Several 1,4-thiazepanes were synthesized by a two-step transformation and were characterized as new BET bromodomain ligands using protein-observed 19F NMR. This synthesis should provide ready access to diverse 3D fragments for screening libraries.

A solvent-dependent chirality-switchable thia-Michael addition to α,β-unsaturated carboxylic acids using a chiral multifunctional thiourea catalyst

An asymmetric thia-Michael addition of arylthiols to α,β-unsaturated carboxylic acids using a thiourea catalyst that bears arylboronic acid and tertiary amine moieties is reported.

An asymmetric thia-Michael addition of arylthiols to α,β-unsaturated carboxylic acids using a thiourea catalyst that bears arylboronic acid and tertiary amine moieties is reported. Both enantiomers of the Michael adducts can be obtained in high enantioselectivity and good yield merely by changing the solvent. The origin of the chirality switch in the products was examined in each solvent via spectroscopic analyses.

Conjugate Addition of Grignard Reagents to Thiochromones Catalyzed by Copper Salts: A Unified Approach to Both 2-Alkylthiochroman-4-One and Thioflavanone

Grignard reagents undergo conjugate addition to thiochromones catalyzed by copper salts to afford 2-substituted-thiochroman-4-ones, both 2-alkylthiochroman-4-ones and thioflavanones (2-arylthiochroman-4-ones), in good yields with trimethylsilyl chloride (TMSCl) as an additive. The best yields of 1,4-adducts can be attained with CuCN∙2LiCl as the copper source. Excellent yields of 2-alkyl-substituted thiochroman-4-ones and thioflavanones (2-aryl substituted) are attained with a broad range of Grignard reagents. This approach works well with both alkyl and aromatic Grignard reagents, thus providing a unified synthetic approach to privileged 2-substituted thiochroman-4-ones and a potential valuable precursor for further synthetic applications towards many pharmaceutically active molecules. The use of commercially available and/or readily prepared Grignard reagents will expedite the synthesis of a large library of both 2-alkyl substituted thiochroman-4-ones and thioflavanones for additional synthetic applications.

Rh-Catalyzed Asymmetric Hydrogenation of Unsaturated Medium-Ring NH Lactams: Highly Enantioselective Synthesis of N-Unprotected 2,3-Dihydro-1,5-benzothiazepinones

A straightforward method to prepare 1,5-benzothiazepines was reported. Catalyzed by a Rh/Zhaophos complex, unsaturated cyclic NH lactams with a medium-size ring were hydrogenated smoothly, giving remarkably high enantioselectivities. The sulfur atom in the substrates did not bring an inhibition which was observed with commercially available bisphosphine ligands. This method was successfully applied in the scale-up synthesis of (R)-(-)-thiazesim.

Sulphide as a leaving group: highly stereoselective bromination of alkyl phenyl sulphides

A conceptionally novel nucleophilic substitution approach to synthetically important alkyl bromides is presented. Using molecular bromine (Br2), readily available secondary benzyl and tertiary alkyl phenyl sulphides are converted into the corresponding bromides under exceptionally mild, acid- and base-free reaction conditions. This simple transformation allows the isolation of elimination sensitive benzylic β-bromo carbonyl and nitrile compounds in mostly high yields and purities. Remarkably, protic functionalities such as acids and alcohols are tolerated. Enantioenriched benzylic β-sulphido esters, readily prepared by asymmetric sulpha-Michael addition, produce the corresponding inverted bromides with high stereoselectivities, approaching complete enantiospecificity at -40 °C. Significantly, the reported benzylic β-bromo esters can be stored without racemisation for prolonged periods at -20 °C. Their synthetic potential was demonstrated by the one-pot preparation of γ-azido alcohol (S)-5 in 90% ee. NMR studies revealed an initial formation of a sulphide bromine adduct, which in turn is in equilibrium with a postulated dibromosulphurane intermediate that undergoes C-Br bond formation.

Applications of N′-monofunctionalised TsDPEN derivatives in asymmetric catalysis

N′-Monoalkylated or N′-mono(thio)acylated(N-sulfonyl)-1,2-diphenylethylene-1,2-diamine (TsDPEN) derivatives are have found extensive applications in asymmetric catalysis of a wide range of synthetic applications.

Catalytic enantioselective one-pot approach to cis- and trans-2,3-diaryl substituted 1,5-benzothiazepines

The first enantioselective catalytic approach to cis- and trans-2,3-diaryl substituted 1,5-benzothiazepines has been conveniently developed in a one-pot fashion, starting from α,β-unsaturated acyl pyrazoles and 2-aminothiophenol. The organocatalytic two-step sulfa-Michael/lactamization sequence is promoted by a readily available bifunctional thiourea and p-toluenesulfonic acid, respectively. The protocol enables access to both N-unprotected cis- and trans-diastereoisomers in moderate to satisfactory overall yields (up to 84%) and good to excellent ee values (up to 99%). Mechanistic investigations helped to shed light on the regio- and stereoselective outcome of the process.

Bifunctional iminophosphorane catalysed enantioselective sulfa-Michael addition of alkyl thiols to alkenyl benzimidazoles

The first enantioselective sulfa-Michael addition of alkyl thiols to alkenyl benzimidazoles, enabled by a bifunctional iminophosphorane (BIMP) organocatalyst, is described. The iminophosphorane moiety of the catalyst provides the required basicity to deprotonate the thiol nucleophile while the chiral scaffold and H-bond donor control facial selectivity. The reaction is broad in scope with respect to the thiol and benzimidazole reaction partners with the reaction proceeding in up to 98% yield and 96 : 4 er.

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.

Umpolung Reaction of α-Imino Thioesters and the Subsequent C-C Bond Formation with the Unexpected Alkylthio Rearrangement

An umpolung reaction of the α-imino thioester was examined, and we found that α-imino thioesters were more effective substrates for the umpolung N-alkylation than conventional α-imino esters and they gave N-alkylated amino thioesters in high yields under mild reaction conditions in a short time. A new type of C-C bond formation followed by an unexpected rearrangement of the alkylthio group took place with the unsaturated ketones to afford the β-alkylthio-α-amino thioesters in high yields with good diastereoselectivity.

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.

Formal [3 + 4] Annulation of α,β-Unsaturated Acyl Azoliums: Access to Enantioenriched N-H-Free 1,5-Benzothiazepines

An unprecedented formal [3 + 4] annulation of α,β-unsaturated acyl azoliums with 2-aminobenzenethiols has been utilized to synthesize enantioenriched N-H-free 1,5-benzothiazepines, which are recognized as privileged structures in numerous biologically active scaffolds. This protocol offers a rapid and direct pathway to access the target compounds with high enantioselectivities and has been applied in the concise synthesis of chiral drug (R)-thiazesim.