Recent advances in the Willgerodt-Kindler reaction

Visible-Light-Driven Multicomponent Reactions for the Versatile Synthesis of Thioamides by Radical Thiocarbamoylation

Thioamides are widely used structures in pharmaceuticals and agrochemicals, as well as important synthons for the construction of sulfur-containing heterocycles. This report presents a series of visible-light-driven multicomponent reactions of amines, carbon disulfide, and olefins for the mild and versatile synthesis of linear thioamides and cyclic thiolactams. The use of inexpensive and readily available carbon disulfide as the thiocarbonyl source in a radical pathway enables the facile assembly of structurally diverse amine moieties with non-nucleophilic carbon-based reaction partners. Radical thiocarbamoylative cyclization provides a practical protocol that complements traditional approaches to thiolactams relying on deoxythionation. Mechanistic studies reveal that direct photoexcitation of in situ formed dithiocarbamate anions as well as versatile photoinduced electron transfer with diverse electron acceptors are key to the reactions.

Chemoselective Thioacylation of Amines Enabled by Synergistic Defluorinative Coupling

A mild and chemoselective method for the thioacylation of amines, including amino acids and peptides, using gem-difluoroalkenes and sulfide, is reported. The distinguishing of the different nucleophilic sites (S-site and diverse N-sites) by the chemoselective C-F bond functionalization of gem-difluoroalkenes enables the unique synergistic defluorinative coupling reaction. This reaction features mild conditions, is operationally simple, efficient, and gram-scalable, tolerates various functional groups, and is activator-free and without racemization. Thioamide moieties were incorporated site-specifically into bioactive compounds. The proposed mechanism is illustrated by a DFT calculation.

Direct Synthesis of α-Ketothioamide Derivatives through a One-Pot Reaction of Sulfur Ylides, Nitrosobenzenes, and Thioacetic Acid

A one-pot approach has been developed for the synthesis of α-ketothioamide derivatives from sulfur ylides, nitrosobenzenes, and thioacetic acid. This protocol is carried out under mild reaction conditions in generally moderate to excellent yields without any precious catalysts, affording the derivatives with structural diversity. Additionally, a possible mechanism for this chemical transformation is proposed.

Direct Transamidation of Thioamides with Amines via Acetophenone-Promoted Enamine Catalysis under Metal-Free Conditions

Herein, we developed a highly selective, efficient, and simple method for direct transamidation of thioamides with amines, promoted by commercially available acetophenone under metal-/solvent-free conditions. The reaction tolerated a wide range of functional groups and substrates, including single- or double-thioamides, benzylamines, or alkyl/cycloalkyl-substituted aliphatic amines. The present protocol can be applied to gram-scale in good yields. In addition, the Pt-/Ni-complexes of double-transamidation products were obtained in good to excellent yields. The investigation of photophysical properties indicated that the fluorescence spectra of Pt-complexes showed an emission band centered at 550-750 nm and exhibited red fluorescence when irradiated by a UV lamp (365 nm).

Contemporary Applications of Thioamides and Methods for Their Synthesis

Thioamides are naturally occurring isosteres of amide bonds in which the chalcogen atom of the carbonyl is changed from oxygen to sulfur. This substitution gives rise to altered nucleophilicity and hydrogen bonding properties with importance for both chemical reactivity and non-covalent interactions. As such, thioamides have been introduced into biologically active compounds to achieve improved target affinity and/or stability towards hydrolytic enzymes but have also been applied as probes of protein and peptide folding and dynamics. Recently, a series of new methods have been developed for the synthesis of thioamides as well as their utilization in peptide chemistry. Further, novel strategies for the incorporation of thioamides into proteins have been developed, enabling both structural and functional studies to be performed. In this Review, we highlight the recent developments in the preparation of thioamides and their applications for peptide modification and study of protein function.

Organoselenium Compounds: Chemistry and Applications in Organic Synthesis

Selenium, originally described as a toxin, turns out to be a crucial trace element for life that appears as selenocysteine and its dimer, selenocystine. From the point of view of drug developments, selenium-containing drugs are isosteres of sulfur and oxygen with the advantage that the presence of the selenium atom confers antioxidant properties and high lipophilicity, which would increase cell membrane permeation leading to better oral bioavailability. In this article, we have focused on the relevant features of the selenium atom, above all, the corresponding synthetic approaches to access a variety of organoselenium molecules along with the proposed reaction mechanisms. The preparation and biological properties of selenosugars, including selenoglycosides, selenonucleosides, selenopeptides, and other selenium-containing compounds will be treated. We have attempted to condense the most important aspects and interesting examples of the chemistry of selenium into a single article.

Late-Stage Molecular Editing Enabled by Ketone Chain-Walking Isomerization

Herein, a method for the isomerization of ketones in a manner akin to the chain-walking reaction of alkenes is described. Widely available and inexpensive pyrrolidine and elemental sulfur are deployed as catalysts to achieve this reversible transformation. Key to the utility of this approach was the elucidation of a stereochemical model to determine the thermodynamically favored product of the reaction and the kinetic selectivity observed. With the distinct selectivity profile of our ketone chain-walking process, the isomerization of various steroids was demonstrated to rapidly access novel steroids with "unnatural" oxidation patterns.

On the way to potential antifungal compounds: synthesis and in vitro activity of 2-benzofuranylacetic acid amides

Crop losses caused by microbial infections are a significant global issue, especially in tropical regions. The development of novel antimicrobial agents, particularly antifungal agents, has been explored from various perspectives, including chemical synthesis. However, conventional approaches typically involve synthesizing new and potent compounds on a small scale (a few milligrams), making the scale-up of the reaction a major challenge. In this manuscript, we present a method for the synthesis of new and active (against Fusarium oxysporum) benzofuranyl acetic acid amides. Our strategy allows us to synthesize the key precursor on the gram scale, enabling the production of sufficient quantities of other active compounds within short timeframes for conducting biological studies. All the reactions used in this manuscript are recognized by their industrial application.

Construction of Thioamide Peptides from Chiral Amino Acids

Thioamide peptides were synthesized in a straightforward one-pot process via the linkage of diverse natural amino acids in the presence of thiolphosphonate and trichlorosilane, wherein carbonyl groups were replaced with thiono compounds with minimal racemization. Experimental and computational mechanistic studies demonstrated that the trichlorosilane enables the activation of carboxylic acids via intense interactions with the Si-O bond, followed by coupling of the carboxylic acids with thiolphosphonate to obtain the key intermediate S-acyl dithiophosphate. Silyl-activated quadrangular metathesis transition states afforded the thioamide peptides. The potential applications of these thioamide peptides were further highlighted via late-stage linkages of diverse natural products and pharmaceutical drugs and the thioamide moiety.

Towards More Practical Methods for the Chemical Synthesis of Thioamides Using Sulfuration Agents: A Decade Update

Compounds possessing a thioamide function play a crucial role in organic synthesis, serving as key building blocks. They are also important in the pharmaceutical chemistry and drug design, owing to their ability to mimic the amide function in biomolecules while retaining or developing biological activity. From the synthetic viewpoint, several methods have been developed for preparing thioamides using sulfuration agents. The purpose of this review is to give an update of the last decade of contributions focusing on the formation of thioamides employing different sulfur sources. When appropriate, the cleanness and practicality of the new methods are highlighted.

Three-component selective synthesis of phenothiazines and bis-phenothiazines under metal-free conditions

An iodine-containing reagent promoted three-component method for the selective synthesis of phenothiazines and bis-phenothiazines has been developed. The present protocol starts from simple and easily available cyclohexanones, elemental sulfur, and inorganic ammonium salts, selectively producing phenothiazines and bis-phenothiazines in satisfactory yields under aerobic conditions. This method has the advantages of simple and readily available starting materials and metal-free conditions, affording a facile and practical approach for the preparation of phenothiazines and bis-phenothiazines.

Design, synthesis and computational study of new benzofuran hybrids as dual PI3K/VEGFR2 inhibitors targeting cancer

Design and synthesis of a new series of benzofuran derivatives has been performed. ¹H-NMR, ¹³C-NMR, elemental analysis, and IR were used to confirm the structures of the produced compounds. Hepatocellular carcinoma (HePG2), mammary gland breast cancer (MCF-7), epithelioid carcinoma cervical cancer (Hela), and human prostate cancer are used to test anticancer activity (PC3). In compared to DOX (4.17-8.87 µM), Compound 8 demonstrated the highest activity against HePG and PC3 cell lines, with an IC₅₀ range of 11-17 µM. Compound 8 inhibited PI3K and VEGFR-2 with IC₅₀ values of 2.21 and 68 nM, respectively, compared to 6.18 nM for compound LY294002 and 31.2 nM for compound sorafenib as PI3K and VEGFR-2 reference inhibitors, selectively. The molecular docking and binding affinity of the generated compounds were estimated and studied computationally utilizing molecular operating environment software as a PI3K and VEGFR-2 inhibitor (MOE). In conclusion, compound 8 exhibited significant action against hepatocellular and cervical cancer cell lines. Mechanistic study showed that it had a dual inhibitory effect against PI3K and VEGFR-2.

Preparation of thioamides from alkyl bromides, nitriles, and hydrogen sulfide through a thio-Ritter-type reaction

A novel thio-Ritter-type reaction of alkyl bromides, nitriles, and hydrogen sulfide has been explored, providing a straightforward approach toward functionally important thioamides. This transformation features a broad substrate scope, operational simplicity, use of available feedstock chemicals, and late-stage functionalizations of bioactive molecules. The reaction mechanism is also proposed.

Green and Sustainable Visible Light-Mediated Formation of Amide Bonds: An Emerging Niche in Organic Chemistry

Amide bond is one of the most fascinating functional groups in nature due to its stability, conformational diversity, high bond polarity, and abundance in numerous natural products and drug candidates,...

Thioamide synthesis via copper-catalyzed C–H activation of 1,2,3-thiadiazoles enabled by slow release and capture of thioketenes

A copper-catalyzed coupling of 1,2,3-thiadiazoles with various amines under base-free conditions was developed as a robust protocol for the synthesis of thioamide derivatives via C–H activation/Cu coordination strategy.

Green Chemistry in the Synthesis of Pharmaceuticals

The principles of green chemistry (GC) can be comprehensively implemented in green synthesis of pharmaceuticals by choosing no solvents or green solvents (preferably water), alternative reaction media, and consideration of one-pot synthesis, multicomponent reactions (MCRs), continuous processing, and process intensification approaches for atom economy and final waste reduction. The GC's execution in green synthesis can be performed using a holistic design of the active pharmaceutical ingredient's (API) life cycle, minimizing hazards and pollution, and capitalizing the resource efficiency in the synthesis technique. Thus, the presented review accounts for the comprehensive exploration of GC's principles and metrics, an appropriate implication of those ideas in each step of the reaction schemes, from raw material to an intermediate to the final product's synthesis, and the final execution of the synthesis into scalable industry-based production. For real-life examples, we have discussed the synthesis of a series of established generic pharmaceuticals, starting with the raw materials, and the intermediates of the corresponding pharmaceuticals. Researchers and industries have thoughtfully instigated a green synthesis process to control the atom economy and waste reduction to protect the environment. We have extensively discussed significant reactions relevant for green synthesis, one-pot cascade synthesis, MCRs, continuous processing, and process intensification, which may contribute to the future of green and sustainable synthesis of APIs.

Mechanochemistry for the synthesis of non-classical N-metalated palladium(II) pincer complexes

The peculiarities of cyclopalladation of a series of non-classical pincer-type ligands based on monothiooxalyl amides bearing ancillary N- or S-donor groups in the amide units have been scrutinized both under conditions of conventional solution-based synthesis and in the absence of a solvent according to a solid-phase methodology including mechanochemical activation. Grinding the functionalized monothiooxamides with PdCl₂(NCPh)₂ in a mortar or vibration ball mill is shown to serve as an efficient and green alternative to the synthesis of these complex metal-organic systems in solution that can offer such advantages as the absence of any auxiliary and significant rate and yield enhancement, especially for the challenging ligands. The realization of S,N,N- or S,N,S-monoanionic tridentate coordination in the resulting pincer complexes has been confirmed by multinuclear NMR (including 2D NMR) and IR spectroscopy and, in some cases, X-ray diffraction. The course and outcome of the solid-phase reactions have been studied by a combination of different spectroscopic methods as well as SEM/EDS analysis. The preliminary evaluation of cytotoxic activity against several human cancer cell lines has revealed the high potency of some of the cyclopalladated derivatives obtained, rendering further development of solvent-free synthetic routes to this type of complexes very urgent.

Recent Advances in the Synthesis of Ibuprofen and Naproxen

Herein, we review the recent progress in the synthesis of representative nonsteroidal anti-inflammatory drugs (NSAIDs), ibuprofen and naproxen. Although these drugs were discovered over 50 years ago, novel practical and asymmetric approaches are still being developed for their synthesis. In addition, this endeavor has enabled access to more potent and selective derivatives from the key frameworks of ibuprofen and naproxen. The development of a synthetic route to ibuprofen and naproxen over the last 10 years is summarized, including developing methodologies, finding novel synthetic routes, and applying continuous-flow chemistry.

Strained contacts with the cell membrane may influence ligand affinity to G protein coupled receptors: a case of free fatty acid receptor 1 agonists

A set of 1,3,4-thiadiazole-2-carboxamides bearing a substituted biphenyl in the amide portion was synthesised and tested for agonistic activity towards free fatty acid receptor 1 (FFA1). The observed activity trends were impossible to rationalised based solely on the docking energy scores of Glide SP. On the contrary, when the phospholipid cell membrane bilayer was reconstructed around FFA1, it became apparent that inactive compounds displayed significant strained contacts with the membrane while for active compounds the strain was noticeably lower. These findings justify using the improved docking protocol for modelling GPCR-ligand interactions which uses the crystal structure of the receptor and a reconstructed portion of a cell membrane.

NH4I-promoted oxidative formation of benzothiazoles and thiazoles from arylacetic acids and phenylalanines with elemental sulfur

A NH4I/K3PO4-based catalytic system has been established to enable oxidative formation of thiazole compounds from arylacetic acids and phenylalanines with elemental sulfur. While the three-component reaction of anilines or β-naphthylamines with arylacetic acids and elemental sulfur affords benzo[2,1-d]thiazoles and naphtho[2,1-d]thiazoles, the annulation of phenylalanines with elemental sulfur produces 2-benzyl and 2-benzoylthiazoles. This work well complements previous three-component annulations of benzothiazoles from other coupling partners.

Tetracoordinate Boron Intermediates Enable Unconventional Transformations

ConspectusOrganoboron compounds are a class of multifunctional reagents for the construction of carbon-carbon and carbon-heteroatom bonds in modern synthetic chemistry. The transformations of organoboron compounds are usually carried out through tetracoordinate boron intermediates and mainly include additions to unsaturated bonds, rearrangement reactions, transmetalation reactions, and so on. Although great progress has been achieved in improving tetracoordinate boron intermediates, there are still shortcomings, such as sparse activation modes, a paucity of reaction strategies and difficulties in stereoselective control. In this Account, we mainly discuss our recent advances in the development of unconventional transformations of organoboron compounds based on the design of tetracoordinate boron intermediates, including the following three topics: (1) the construction of C-B bonds; (2) the construction of C-C bonds; (3) the design and application of chiral tetracoordinate boron.The development of new strategies to build C-B bonds is of great interest for chemists. We have developed tandem reactions involving multiple tetracoordinate boron intermediates for the selective borylations of alkynes and the synthesis of stable tetracoordinate boron, including a domino-borylation-protodeboronation (DBP) strategy for selective borylations of alkynes, highly regio-, stereo-, and chemoselective Cu-catalyzed diborylation of β-CF₃-1,3-enynes and cascade B-Cl/C-B cross-metathesis and C-H bond borylation for the synthesis of tetracoordinate triarylboranes. We have also developed novel strategies involving tetracoordinate boron intermediates to form C-C bonds because the formation of C-C bonds is an enduring theme of organic chemistry. We disclosed long distance or multiple migration reactions and novel coupling partners in transmetalation reactions, such as long distance 1,4-migrations of tetracoordinate nitrile oxide boron and nitrilium boron intermediates, multiple migrations of tetracoordinate isocyanide boron intermediate, palladium-catalyzed Suzuki-Miyaura coupling of thioureas or thioamides, copper-catalyzed atroposelective Michael-type addition, and a palladium-catalyzed atroposelective Catellani reaction. Moreover, in terms of stereoselective control of the tetracoordinate boron intermediate, we found that a chiral tricoordinate boron complex could activate water to form a chiral tetracoordinate boron complex with Brønsted acidity, which has been successfully applied with high enantioselectivity to the asymmetric catalytic reduction of challenging indoles.This Account summarizes our recent efforts using unconventional transformations of organoboron compounds for the design of tetracoordinate boron intermediates, which not only achieved the precise construction of a wide range of diverse C-B bonds and C-C bonds but also developed a novel chiral Brønsted acid for the asymmetric catalytic reduction of challenging indoles.

Synthesis of thioamide containing polybenzoxazines by the Willgerodt–Kindler reaction

Benzoxazines with thioamide linkages were successfully prepared by the Willgerodt–Kindler route.

Recent advances in the transition metal-free oxidative dehydrogenative aromatization of cyclohexanones

Cyclohexanone is a simple and widely available raw material that can be obtained from lignin biomass, highlighting its renewable and sustainable features. Cyclohexanone, as an important synthon in organic chemistry, has been demonstrated to be viable for constructing functionalized arenes and benzoheteroarenes, with recent extensive development on transition metal-free oxidative dehydrogenative aromatization. This review focuses on recent research progress on the transition metal-free derivation of cyclohexanones via oxidative dehydrogenative aromatization.

Synthesis of primary N-arylthioglyoxamides from anilines, elemental sulfur and primary C-H bonds in acetophenones

A simple method for coupling of anilines, acetophenones, and elemental sulfur to afford N-arylthioglyoxamides has been developed. Reactions proceeded in the presence of Na₂SO₃ and DMSO, thus eliminating the need for transition metals and external oxidants. Functionalities such as halogen, ester, methylthio, and heterocycle groups were compatible with the conditions. Electron-poor acetophenones sometimes gave isosteric glyoxamides.

Sulfurative coupling of acetophenones and aniline in the presence of Na₂SO₃ and DMSO solvent to afford N-arylthioglyoxamides was developed.

Elemental-Sulfur-Incorporated Cyclizations of Pyrrolidines Leading to Thienopyrroles

We report, herein, the synthesis of thieno[3,2-b]pyrroles from the direct oxidative [4 + 1] cyclization of 2-alkynyl pyrrolidines with elemental sulfur. This transformation likely originates from electrophilic attack at the β-position of pyrrolidine followed by an intramolecular thienannulation to deliver the desired product. Mechanistic investigation suggests that the present reaction involves the formation of dihydrothieno[3,2-b]pyrrole as an intermediate.

Sulfur-mediated synthesis of unsymmetrically substituted N-aryl oxalamides by the cascade thioamidation/cyclocondensation and hydrolysis reaction

A facile and straightforward synthesis of unsymmetrically substituted N-aryl oxalamides from 2,2'-biphenyldiamines, 2-chloroacetic acid derivatives, elemental sulfur, and water has been developed. This protocol is distinguished by efficiency in water under metal-free conditions for N-aryl oxalamides bearing a side-chain NH2-group; it can be adapted for scale-up synthesis. The scope and limitations of this transformation have been investigated.

New synthesis of 2-aroylbenzothiazoles via metal-free domino transformations of anilines, acetophenones, and elemental sulfur

A new synthesis of 2-aroylbenzothiazoles via iodine-promoted domino transformations of anilines, acetophenones, and elemental sulfur was demonstrated. The highlights of this tandem synthesis are (1) easily available anilines and acetophenones as feedstock; (2) transition metal-free conditions; (3) inexpensive, nontoxic, easy handling, and abundant elemental sulfur as a building block. This synthetic strategy would complement the existing methods in the synthesis of this important heterocyclic scaffold. To our best knowledge, the formation of 2-aroylbenzothiazoles from simple anilines, acetophenones, and elemental sulfur was not previously reported in the literature.

A new synthesis of 2-aroylbenzothiazoles via iodine-promoted domino transformations of anilines, acetophenones, and elemental sulfur was demonstrated.

Multicomponent reactions in crop protection chemistry

An overview is given of the significance of multicomponent reactions in the synthesis of agrochemicals. The most important applications of multicomponent condensations, such as the Biginelli reaction, Bucherer-Bergs reaction, Hantzsch dihydropyridine synthesis, Kabachnik-Fields reaction, Mannich reaction, Passerini reaction, Petasis reaction, Strecker reaction, Ugi reaction and Willgerodt-Kindler reaction, to the synthesis of herbicidally, fungicidally and insecticidally active compounds are presented. Also the mode of action and biological activity of these multicomponent reaction products are reported.

Synthesis, antitumor activity, and molecular docking study of 2-cyclopentyloxyanisole derivatives: mechanistic study of enzyme inhibition

A series of 24 compounds was synthesised based on a 2-cyclopentyloxyanisole scaffold 3–14 and their in vitro antitumor activity was evaluated. Compounds 4a, 4b, 6b, 7b, 13, and 14 had the most potent antitumor activity (IC₅₀ range: 5.13–17.95 μM), compared to those of the reference drugs celecoxib, afatinib, and doxorubicin. The most active derivatives 4a, 4b, 7b, and 13 were evaluated for their inhibitory activity against COX-2, PDE4B, and TNF-α. Compounds 4a and 13 potently inhibited TNF-α (IC₅₀ values: 2.01 and 6.72 μM, respectively) compared with celecoxib (IC₅₀=6.44 μM). Compounds 4b and 13 potently inhibited COX-2 (IC₅₀ values: 1.08 and 1.88 μM, respectively) comparable to that of celecoxib (IC₅₀=0.68 μM). Compounds 4a, 7b, and 13 inhibited PDE4B (IC₅₀ values: 5.62, 5.65, and 3.98 μM, respectively) compared with the reference drug roflumilast (IC₅₀=1.55 μM). The molecular docking of compounds 4b and 13 with the COX-2 and PDE4B binding pockets was studied.

Highlights

Antitumor activity of new synthesized cyclopentyloxyanisole scaffold was evaluated.

The powerful antitumor 4a, 4b, 6b, 7b & 13 were assessed as COX-2, PDE4B & TNF-α inhibitors.

Compounds 4a, 7b, and 13 exhibited COX-2, PDE4B, and TNF-α inhibition.

Compounds 4b and 13 showed strong interactions at the COX-2 and PDE4B binding pockets.