The Eschenmoser sulfide contraction method and its application in the synthesis of natural products

Autocatalysis in Eschenmoser Coupling Reactions

The Eschenmoser coupling reaction (ECR) of thioamides with electrophiles is believed to proceed via thiirane intermediates. However, little is known about converting the intermediates into ECR products. Previous mechanistic studies involved external thiophiles to remove the sulfur atom from the intermediates. In this work, an ECR proceeding without any thiophilic agent or base is studied by electrospray ionization-mass spectrometry. ESI-MS enables the detection of the so-far elusive polysulfide species Sn , with n ranging from 2 to 16 sulfur atoms, proposed to be the key species leading to product formation. Integrating observations from ion mobility spectrometry, ion spectroscopy, and reaction monitoring via flow chemistry coupled with mass spectrometry provides a comprehensive understanding of the reaction mechanism and uncovers the autocatalytic nature of the ECR reaction.

Eschenmoser coupling reactions starting from primary thioamides. When do they work and when not?

Reactions of thiobenzamide or thioacetamide with 4-bromo-1,1-dimethyl-1,4-dihydroisoquinoline-3(2H)-one, 4-bromoisoquinoline-1,3(2H,4H)-dione and two α-bromo(phenyl)acetamides were examined under various conditions (base, solvent, thiophile, temperature) and structure/medium features that influence product distribution (Eschenmoser coupling reaction, Hantzsch thiazole synthesis and elimination to nitriles) were identified. The key factor that enables the successful Eschenmoser coupling reaction involves the optimum balance in acidity of nitrogen and carbon atoms of the intermediary α-thioiminium salts.

Coupling of acceptor-substituted diazo compounds and tertiary thioamides: synthesis of enamino carbonyl compounds and their pharmacological evaluation

This paper describes the synthesis of enamino carbonyl compounds by the copper(i)-catalyzed coupling of acceptor-substituted diazo compounds and tertiary thioamides. We plan to use this method to synthesize indolizidine (−)-237D analogs to find α6-selective antismoking agents. Therefore, we also performed in silico α6-nAchRs binding studies of selected products. Compounds with low root-mean-square deviation values showed more favorable binding free energies. We also report preliminary pharmacokinetic data on indolizidine (−)-237D and found it to have weak activity at CYP3A4. In addition, as enamino carbonyl compounds are also known for antimicrobial properties, we screened previously reported and new enamino carbonyl compounds for antibacterial, antimicrobial, and antifungal properties. Eleven compounds showed significant antimicrobial activities.

This paper describes the synthesis of enamino carbonyl compounds by the copper(i)-catalyzed coupling of acceptor-substituted diazo compounds and tertiary thioamides.

Silica gel and microwave-promoted synthesis of dihydropyrrolizines and tetrahydroindolizines from enaminones

A wide range of N-(ethoxycarbonylmethyl)enaminones, prepared by the Eschenmoser sulfide contraction between N-(ethoxycarbonylmethyl)pyrrolidine-2-thione and various bromomethyl aryl and heteroaryl ketones, underwent cyclization in the presence of silica gel to give ethyl 6-(hetero)aryl-2,3-dihydro-1H-pyrrolizine-5-carboxylates within minutes upon microwave heating in xylene at 150 °C. Instead of functioning as a nucleophile, the enaminone acted as an electrophile at its carbonyl group during the cyclization. Yields of the bicyclic products were generally above 75%. The analogous microwave-assisted reaction to produce ethyl 2-aryl-5,6,7,8-tetrahydroindolizine-3-carboxylates from (E)-ethyl 2-[2-(2-oxo-2-arylethylidene)piperidin-1-yl]acetates failed in nonpolar solvents, but occurred in ethanol at lower temperature and microwave power, although requiring much longer time. A possible mechanism for the cyclization is presented, and further functionalization of the newly created pyrrole ring in the dihydropyrrolizine core is described.

Synthesis of the Kinase Inhibitors Nintedanib, Hesperadin, and Their Analogues Using the Eschenmoser Coupling Reaction

A novel synthetic approach involving an Eschenmoser coupling reaction of substituted 3-bromooxindoles (H, 6-Cl, 6-COOMe, 5-NO₂) with two substituted thiobenzanilides in dimethylformamide or acetonitrile was used for the synthesis of eight kinase inhibitors including Nintedanib and Hesperadin in yields exceeding 76%. Starting compounds for the synthesis are also easily available in good yields. 3-Bromooxindoles were prepared either from corresponding isatins using a three-step synthesis in an average overall yield of 65% or by direct bromination of oxindoles (yield of 65-86%). Starting N-(4-piperidin-1-ylmethyl-phenyl)-thiobenzamide was prepared by thionation of the corresponding benzanilide in an 86% yield and N-methyl-N-(4-thiobenzoylaminophenyl)-2-(4-methylpiperazin-1-yl)acetamide was prepared by thioacylation of the corresponding aniline with methyl dithiobenzoate in an 86% yield.

Synthesis of (Z)-3-[amino(phenyl)methylidene]-1,3-dihydro-2H-indol-2-ones using an Eschenmoser coupling reaction

A highly modular method for the synthesis of (Z)-3-[amino(phenyl/methyl)methylidene]-1,3-dihydro-2H-indol-2-ones starting from easily available 3-bromooxindoles or (2-oxoindolin-3-yl)triflate and thioacetamides or thiobenzamides is described. A series of 49 compounds, several of which have previously been shown to possess significant tyrosin kinase inhibiting activity, was prepared in yields varying mostly from 70 to 97% and always surpassing those obtained by other published methods. The method includes an Eschenmoser coupling reaction, which is very feasible (even without using a thiophile except tertiary amides) and scalable. The (Z)-configuration of all products was confirmed by NMR techniques.

Design, Synthesis, and Molecular Docking of Paracyclophanyl-Thiazole Hybrids as Novel CDK1 Inhibitors and Apoptosis Inducing Anti-Melanoma Agents

Three new series of paracyclophanyl-dihydronaphtho[2,3-d]thiazoles and paracyclophanyl-thiazolium bromides were designed, synthesized, and characterized by their spectroscopic data, along with X-ray analysis. One-dose assay results of anticancer activity indicated that 3a–e had the highest ability to inhibit the proliferation of different cancer cell lines. Moreover, the hybrids 3c–e were selected for five-dose analyses to demonstrate a broad spectrum of antitumor activity without apparent selectivity. Interestingly, series I compounds (Z)-N-substituted-4,9-dihydronaphtho[2,3-d]thiazol-3(2H)-yl)-4′-[2.2]paracyclophanylamide) that are carrying 1,4-dihydronaphthoquinone were more active as antiproliferative agents than their naphthalene-containing congeners (series II: substituted 2-(4′-[2.2]paracyclophanyl)hydrazinyl)-4-(naphth-2-yl)-thiazol-3-ium bromide hybrids) and (series III: 3-(4′-[2.2]paracyclophanyl)amido-2-(cyclopropylamino)-4-(naphth-2-yl)thiazol-3-ium bromide) toward the SK-MEL-5 melanoma cell line. Further antiproliferation investigations of 3c and 3e on the healthy, normal unaffected SK-MEL-5 cell line indicated their relative safety. Compound 3c showed an inhibition of eight isoforms of cyclin-dependent kinases (CDK); however, it exhibited the lowest IC₅₀ of 54.8 nM on CDK1 in comparison to Dinaciclib as a reference. Additionally, compound 3c revealed a remarkable downregulation of phospho-Tyr15 with a level (7.45 pg/mL) close to the reference. 3c mainly showed cell cycle arrest in the pre-G1 and G2/M phases upon analysis of the SK-MEL-5 cell line. The sequential caspase-3 assay for 3c indicated a remarkable overexpression level. Finally, a molecular docking study was adopted to elucidate the binding mode and interactions of the target compounds with CDK1.

Two Approaches for the Synthesis of Fused Dihydropyridines via a 1,6-Electrocyclic Reaction: Fluorescent Properties and Prospects for Application

Reactions of penta-2,4-dienethioamides with acetylenedicarboxylic acid, methyl and ethyl esters, and methyl propiolate were systematically studied, and a number of new 2,3-dihydro-5H-thiazolo[3,2-a]pyridines (DTPs) and 4H,6H-pyrido[2,1-b][1,3]thiazines (PTZs) were prepared. A possible mechanism for a multistep domino transformation is suggested, and the key step is the 1,6-electrocyclic reaction. An additional alternative method for the synthesis of new heterocyclic systems was achieved. Evidence of the electrocyclic mechanism of a key step was collected from the analysis of the spatial structure of the synthesized bicyclic nonaromatic pyridines by X-ray diffraction and quantum chemical calculations, as well as from the thermodynamic quantities. DTPs exhibited yellow fluorescence in solution and yellow to red emissions in the solid state. Biological investigations demonstrated the ability of DTPs to penetrate living and fixed cells and presumably accumulate in lysosomes.

Novel benzoic acid derivatives: Synthesis and biological evaluation as multitarget acetylcholinesterase and carbonic anhydrase inhibitors

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by dementia, memory impairment, cognitive dysfunction, and speech impairment. The utility of cholinergic replacement by acetylcholinesterase (AChE) inhibitors in AD treatment has been well documented so far. Recently, studies have also evidenced that human carbonic anhydrases (hCAs) serve as an important target for AD treatment. In this direction, the improvement of new multitarget drugs, which can simultaneously modulate several mechanisms or targets included in the AD pathway, may be a potent strategy to treat AD. In light of these data for understanding and developing AD-related multitarget AChE and hCAs inhibitors, in this study, novel methylene-aminobenzoic acid and tetrahydroisoquinolynyl-benzoic acid derivatives (4a-g and 6a-g) were designed. The synthesized analogs were experimentally validated for their effects by in vitro and direct enzymatic tests. Also, the compounds were subjected to in silico monitoring with Schrödinger Suite software to assign binding affinities of potential derivatives based on Glide XP scoring, molecular mechanics-generalized Born surface area computing, and validation by molecular docking. The results revealed that 6c (1,3-dimethyldihydropyrimidine-2,4-(1H,3H)-dione-substituted, K_I  value of 33.00 ± 0.29 nM), 6e (cyclohexanone-substituted, K_I  value of 18.78 ± 0.09 nM), and 6f (2,2-dimethyl-1,3-dioxan-4-one-substituted, K_I  value of 13.62 ± 0.21 nM) from the benzoic acid derivatives in this series were the most promising derivatives, as they exhibited a good multifunctional inhibition at all experimental levels and in the in silico validation against hCA I, hCA II, and AChE, respectively, for the treatment of AD.

Application of NMR Spectroscopy for the Detection of Equilibrating E-Z Diastereomers

Conjugation can lower the energy barrier for unsaturated C-C bond rotations, resulting in a mixture of equilibrating diastereomers at room temperature. Therefore, methods claiming diastereoselective synthesis of conjugated double bonds often require proof that the observed diastereomeric ratio is not because of the diastereomeric equilibration of the product. Variable-temperature (VT) NMR experiments are commonly used to distinguish between the two possibilities. However, the VT technique requires accessories for the NMR spectrometer and more setup time. Here, we show that the rarely used application of 1-D and 2-D nuclear Overhauser effect spectroscopy experiments for the detection of the equilibrating diastereomers is a convenient alternative to the VT technique.

Demethylative Lactonization Provides a Shortcut to High-Yielding Syntheses of Lamellarins

Modular gram-scale syntheses of the trimethyl ethers of lamellarins G (6) and D (7) were achieved from readily accessible precursors in the highest overall yields reported to date (6, six steps, 82%; 7, seven steps, 86%). A novel demethylative lactonization between an aryl methyl ether and a neighboring carboxylic acid was developed for creating the chromenone unit of the targets to avoid the need for additional protection and deprotection steps. The central pyrrole core was constructed in a late-stage [4 + 1] condensation between ethyl bromoacetate and an enaminone possessing the remaining components of the lamellarin skeleton. Exhaustive demethylation of both permethyl ethers 6 and 7 gave the polyphenolic natural lamellarins A4 (3) and H (5), respectively.

Design, synthesis and cytotoxicity of the antitumor agent 1-azabicycles for chemoresistant glioblastoma cells

Twelve multi-functional pyrrolizidinones, indolizidinones and pyrroliazepinones were prepared from formal aza-[3 + 2] and aza-[3 + 3] cycloadditions of five- to seven-membered heterocyclic enaminones as diverse ambident electrophiles. The antitumor activity of these alkaloid-like compounds was investigated through an initial screening performed on human glioblastoma multiform (GBM) cell lines (GL-15, U251), on murine glioma cells line (C6) and on normal glial cells. Of the compounds tested, the new pyrrolo[1,2a]azepinone, [ethyl (3-oxo-1,2-diphenyl-6,7,8,9-tetrahydro-3H-pyrrolo[1,2a]azepin-9a(5H)-yl)acetate] or (Compound-13) exhibited selective cytotoxic effects on GBM-temozolomide resistant cells. Compound-13 exerted dose-dependent cytotoxic activity by promoting arrest of cells in the G0/G1 phase of the cell cycle in the first 24 h. The apoptotic effect observed was in a time-dependent manner. Anti-migratory effect promoted by the treatment with compound-13 was also observed. Moreover, healthy mixed glial cell cultures from rat brain exhibited no cytotoxicity effect upon exposure to compound-13. Thus, the present study paves the way for the use of compound-13 as novel antitumor scaffold candidate for glioma cell therapy.

Water/Alkali-Catalyzed Reactions of Azides with 2-Cyanothioacetamides. Eco-Friendly Synthesis of Monocyclic and Bicyclic 1,2,3-Thiadiazole-4-carbimidamides and 5-Amino-1,2,3-triazole-4-carbothioamides

The reactions of thioamides with azides in water were studied. It was reliably shown that the reaction of 2-cyanothioacetamides 1 with various types of azides 2 in water in the presence of alkali presents an efficient, general, one-step, atom-economic, and eco-friendly method for the synthesis of 1,2,3-thiadiazol-4-carbimidamides 5 and 1,2,3-triazole-4-carbothioamides 4. This method can be extended to the one-pot reaction of sulfonyl chlorides and 6-chloropyrimidines 2'o with sodium azide, leading to final products in higher yields, that is, avoiding the isolation of unsafe sulfonyl azides. The method was furthermore applied to the reaction of N,N'-bis-(2-cyanothiocarbonyl)pyrazine 1h with sulfonyl azides to afford bicyclic 1,2,3-thiadiazoles 8 and 1,2,3-triazoles 9 connected via a 1,1'-piperazinyl linker. 2-Cyanothioacetamides 1 were also shown to react with aromatic azides in water in the presence of alkali to afford 1-aryl-5-amino-1,2,3-triazole-4-carbothioamides 11. In contrast to aromatic azides and similarly to sulfonyl azides, 6-azidopyrimidine-2,4-diones 2o-q react with cyanothioacetamides to form N-pyrimidin-6-yl-5-dialkylamino-1,2,3-thiadiazole-4-N-l-carbimidamides 12. A mechanism was proposed to rationalize the role of water in changing the reactivity of azides toward 2-cyanothioacetamides.

Copper-Catalyzed Coupling of Thioamides and Donor/Acceptor-Substituted Carbenoids: Synthesis of Enamino Esters and Enaminones

The coupling of thioamides and donor/acceptor-substituted diazocarbonyl compounds is reported for the first time. The present report provides a mild, catalytic method that couples thioamides and donor/acceptor-substituted diazocarbonyl compounds to form enamino esters and enaminones. Unlike traditional methods for the synthesis of enamino esters and enaminones from thioamides, both N-alkyl thioamides and thiocarbamates are suitable substrates in this coupling reaction. Copper(I) bromide catalyzes the reaction and provides enamino esters and enaminones chemo- and diastereoselectively in less time than Rh₂(OAc)₄ or Ru(PPh₃)₃Cl₂ catalysts.

NaHS·nH2O-induced umpolung: the synthesis of 2-acyl-3-aminoindoles from aryl methyl ketones and 2-aminobenzonitriles

An efficient method for constructing 2-acyl-3-aminoindoles from methyl ketones and 2-aminobenzonitriles is described, in which NaHS·nH₂O is used as a novel umpolung reagent for the first time in organic synthesis.

Total synthesis of complex alkaloids by nucleophilic addition to amides

This mini review focuses on the recent progress of total synthesis of complex alkaloids based on the nucleophilic additions toN-alkoxyamides, tertiary amides and secondary amides.

Carbon-Carbon Bond Formation by Reaction of Rhodium Azavinylcarbenes with Secondary Amides: Access to Indigo Analogues from Isatins

A novel protocol for the synthesis of unsymmetrical indigo-like (E)-α-amino enaminones by rhodium-catalyzed transformations of isatins with 1-tosyl-1,2,3-triazoles has been developed. A plausible reaction mechanism is proposed to account for the formation of structurally important unsymmetrical indigo analogues, which represents a new strategy for C-C bond formation based on the transformations of secondary amides and rhodium azavinylcarbenes.

New syntheses of (±)-tashiromine and (±)-epitashiromine via enaminone intermediates

The syntheses of the naturally occurring indolizidine alkaloid (±)-tashiromine and its unnatural epimer (±)-epitashiromine are demonstrated through the use of enaminone chemistry. The impact of various electron-withdrawing substituents at the C-8 position of the indolizidine core on the preparation of the bicyclic system is described.

Valosin-containing Protein is a Target of 5'-l Fuligocandin B and Enhances TRAIL Resistance in Cancer Cells

Fuligocandin B (2) is a novel natural product that can overcome TRAIL resistance. We synthesized enatiomerically pure fuligocandin B (2) and its derivative 5′‐I fuligocandin B (4), and found that the latter had an improved biological activity against the human gastric cancer cell line, AGS. We attached a biotin linker and photoactivatable aryl diazirine group to 5′‐I fuligocandin B (4), and employed a pull‐down assay to identify valosin‐containing protein (VCP/p97), an AAA ATPase, as a 5′‐I fuligocandin B (4) target protein. Knock‐down of VCP by siRNA enhanced sensitivity to TRAIL in AGS cells. In addition, 4 enhanced CHOP and DR5 protein expression, and overall intracellular levels of ubiquitinated protein. These data suggest that endoplasmic reticulum stress caused through VCP inhibition by 4 increases CHOP‐mediated DR5 up‐regulation, which enhances TRAIL‐induced cell death in AGS cells. To the best of our knowledge, this is the first example to show a relationship between VCP and TRAIL‐resistance‐overcoming activity in cancer cells.

Aza-Knoevenagel-type condensation of secondary amides: direct access to N-monosubstituted β,β-difunctionalized enamines

An efficient approach toN-monosubstituted β,β-difunctionalized enamines, a class of versatile building blocks for the synthesis of bioactive compounds, is reported.