A microwave-assisted facile regioselective Fischer indole synthesis and antitubercular evaluation of novel 2-aryl-3,4-dihydro-2H-thieno[3,2-b]indoles

Asymmetric Total Syntheses of Strychnos Alkaloids via Selective Fischer Indolization

The complex structures and important biological functions of Strychnos alkaloids have attracted a great deal of attention from synthetic chemists. Herein, we describe the concise asymmetric total syntheses of the Strychnos alkaloids, (-)-dehydrotubifoline, (-)-tubifoline, and (-)-tubifolidine, as well as the formal total synthesis of (-)-strychnine. Our strategy features the construction of the common tetracyclic pyrrolo[2,3-d]carbazole structure using regioselective Fischer indolization on unsymmetrical cyclic ketones and late-stage functionalization for divergent synthesis. We developed a stepwise Fischer indolization featuring selective formation of enol triflate to solve the challenging regioselectivity problem, leading to the common tetracyclic ring skeleton in these Strychnos alkaloids. The regioselectivity of Fischer indolization on unsymmetrical cyclic ketones was studied on the basis of different types of ring systems and supported by density functional theory calculations. Overall, our success in the construction of this tetracyclic ring secured the syntheses of Strychnos alkaloids and may provide a general method for the total syntheses of various alkaloids containing this skeleton.

Synthesis of Medicinally Important Indole Derivatives: A Review

Indoles constitute a widely occurring functional group in nature and are present in an extensive number of bioactive natural products and medicinally important compounds. As a result, exponential increases in the development of novel methods for the formation of indole core along with site-specific indoles have been established. Conventional methods for the synthesis of indoles are getting replaced with green methods involving ionic liquids, water as a solvent, solid acid catalyst, microwave irradiation and the use of nanoparticles under solvent-free conditions. In addition, there are immense applications of the substituted indoles in diverse fields.

A cascade deprotonation/intramolecular aldol reaction of α-carbonyl sulfonium ylides with 2-mercaptoindole-3-carbaldehydes and 2-mercaptobenzaldehydes to access thieno[2,3-b]indoles and benzothiophenes

The first catalyst-free cascade deprotonation/intramolecular aldol reaction of α-carbonyl sulfonium ylides with 2-mercaptoindole-3-carbaldehydes and 2-mercaptobenzaldehydes was developed. A series of thieno[2,3-b]indoles and benzothiophenes were smoothly obtained in high to excellent yields. The salient features of the protocol include catalyst-free conditions, an environment-friendly solvent, broad substrate scope, and large-scale synthesis.

One-pot, three-component Fischer indolisation-N-alkylation for rapid synthesis of 1,2,3-trisubstituted indoles

A one-pot, three-component protocol for the synthesis of 1,2,3-trisubstituted indoles has been developed, based upon a Fischer indolisation-indole N-alkylation sequence. This procedure is very rapid (total reaction time under 30 minutes), operationally straightforward, generally high yielding and draws upon readily available building blocks (aryl hydrazines, ketones, alkyl halides) to generate densely substituted indole products. We have demonstrated the utility of this process in the synthesis of 23 indoles, benzoindoles and tetrahydrocarbazoles bearing varied and useful functionality.

Stereoselective synthesis of dispiro heterocycles by [3 + 2] cycloaddition of azomethine ylides with a thiazolo[3,2-a]indole derivative

A green one pot three component [3 + 2] cycloaddition of a thiazolo[3,2-a]indole derivative (generated by the reaction of thieno[2,3-b]indole-2,3-dione and dimethyl acetylenedicarboxylate) with isatin derived azomethine ylides is reported here. An eco-friendly acetyl choline iodide-ethylene glycol (ACI/EG)-mediated deep eutectic solvent system was adopted for the reaction. Spiropyrrolidine oxindoles incorporating multiple stereocenters were obtained in a highly diastereoselective manner in excellent yields.

Screening and Phenotypical Characterization of Schistosoma mansoni Histone Deacetylase 8 (SmHDAC8) Inhibitors as Multistage Antischistosomal Agents

Schistosomiasis (also known as bilharzia) is a neglected tropical disease caused by platyhelminths of the genus Schistosoma. The disease is endemic in tropical and subtropical areas of the world where water is infested by the intermediate parasite host, the snail. More than 800 million people live in endemic areas and more than 200 million are infected and require treatment. Praziquantel (PZQ) is the drug of choice for schistosomiasis treatment and transmission control being safe and very effective against adult worms of all the clinically relevant Schistosoma species. Unfortunately, it is ineffective on immature, juvenile worms; therefore, it does not prevent reinfection. Moreover, the risk of development and spread of drug resistance because of the widespread use of a single drug in such a large population represents a serious threat. Therefore, research aimed at identifying novel drugs to be used alone or in combination with PZQ are needed. Schistosoma mansoni histone deacetylase 8 (SmHDAC8) is a class I zinc-dependent HDAC, which is abundantly expressed in all stages of its life cycle, thus representing an interesting target for drug discovery. Through virtual screening and phenotypical characterization of selected hits, we discovered two main chemical classes of compounds characterized by the presence of a hydroxamate-based metal binding group coupled to a spiroindoline or a tricyclic thieno[3,2-b]indole core as capping groups. Some of the compounds of both classes were deeply investigated and showed to impair viability of larval, juvenile, and adult schistosomes, to impact egg production in vitro and/or to induce morphological alterations of the adult schistosome reproductive systems. Noteworthy, all of them inhibit the recombinant form of SmHDAC8 enzyme in vitro. Overall, we identified very interesting scaffolds, paving the way to the development of effective antischistosomal agents.

Molecular structure and vibrational spectra of 2-(4-bromophenyl)-3-(4-hydroxyphenyl) 1,3-thiazolidin-4-one and its selenium analogue: Insights using HF and DFT methods

2-(4-Bromophenyl)-3-(4-hydroxyphenyl)-1,3-thiazolidin-4-one and its selenium analogue were studied in the gas phase using HF and DFT methods. The functionals considered were B3LYP, BP86 and M06. The basis set for all the atoms was 6-311++ G(d,p). Molecular parameters such as bond lengths, bond angles, rotational constants, dipole moments, electronic energies, and vibrational parameters namely harmonic vibrational frequencies and relative intensities were computed for these compounds. Atomization energies, HOMO-LUMO gaps and natural charges on the atoms were also calculated. The molecular parameters and the vibrational spectra of sulfur compound are in good agreement with the experimental data. Therefore, the data for the selenium analogue should be helpful in its future characterization.

Synthesis of Thieno[3,2-b]indoles via Halogen Dance and Ligand-Controlled One-Pot Sequential Coupling Reaction

A two-pot synthesis of thieno[3,2-b]indole from 2,5-dibromothiophene is described. A halogen dance of 2,5-dibromothiophene was performed with LDA, and subsequent Negishi coupling was performed with 2-iodoaniline derivatives to provide the corresponding coupling products. The resulting two bromo groups have different reactivities, which were utilized for the one-pot Suzuki-Miyaura coupling/intramolecular Buchwald-Hartwig amination to produce thieno[3,2-b]indole via an assisted tandem catalysis that involved in situ ligand exchange.

Seeking potent anti-tubercular agents: Design, synthesis, anti-tubercular activity and docking study of various ((triazoles/indole)-piperazin-1-yl/1,4-diazepan-1-yl)benzo[d]isoxazole derivatives

A series of thirty eight novel 3-(4-((substituted-1H-1,2,3-triazol-4-yl)methyl)piperazin-1-yl/1,4-diazepan-1-yl)benzo[d]isoxazole and 1-(4-(benzo[d]isoxazol-3-yl)piperazin-1-yl/1,4-diazepan-1-yl)-2-(1H-indol-3-yl)substituted-1-one analogues were synthesised, characterised using various analytical techniques and evaluated for in vitro anti-tubercular activity against Mycobacterium tuberculosis H37Rv strain and two 'wild' strains Spec. 210 and Spec. 192. The titled compounds exhibited minimum inhibitory concentration (MIC) ranging from 6.16 to >200μM. Among the tested compounds, 7i, 7y and 7z exhibited moderate activity (MIC=24.03-29.19μM) and 7j exhibited very good anti-tubercular activity (MIC=6.16μM). Furthermore, 7i, 7j, 7y and 7z were found to be non-toxic against mouse macrophage cell lines when screened for toxicity. All the synthesised compounds were docked to pantothenate synthetase enzyme site to know deferent binding interactions with the receptor.

A new and convenient synthetic way to 2-substituted thieno[2,3-b]indoles

A short and robust approach for the synthesis of 2-(hetero)aryl substituted thieno[2,3-b]indoles from easily available 1-alkylisatins and acetylated (hetero)arenes has been advanced. The two-step procedure includes the “aldol-crotonic” type of condensation of the starting materials, followed by treatment of the intermediate 3-(2-oxo-2-(hetero)arylethylidene)indolin-2-ones with Lawesson’s reagent. The latter process involves two sequential reactions, namely reduction of the C=C ethylidene double bond of the intermediate indolin-2-ones followed by the Paal–Knorr cyclization, thus affording tricyclic thieno[2,3-b]indoles.

An experimental and theoretical study on the regioselective synthesis of a new class of spiropyrrolothiazoles with quinoxaline motifs via a 1,3-dipolar cycloaddition reaction. An evaluation of DFT methods

Evaluation of B3LYP/6-31G(d,p), wB97xD/6-31G(d,p) and M06-2X/6-31G(d,p) methods revealed the importance of π/π interactions in regio- and stereoselectivity of cycloaddition reactions.

Synthesis and structural activity relationship study of antitubercular carboxamides

The unusual structure and chemical composition of the mycobacterial cell wall, the tedious duration of therapy, and resistance developed by the microorganism have made the recurrence of the disease multidrug resistance and extensive or extreme drug resistance. The prevalence of tuberculosis in synergy with HIV/AIDS epidemic augments the risk of developing the disease by 100-fold. The need to synthesize new drugs that will shorten the total duration of effective treatment and/or significantly reduce the dosage taken under DOTS supervision, improve on the treatment of multidrug-resistant tuberculosis which defies the treatment with isoniazid and rifampicin, and provide effective treatment for latent TB infections which is essential for eliminating tuberculosis prompted this review. In this review, we considered the synthesis and structure activity relationship study of carboxamide derivatives with antitubercular potential.

Quinoline and quinolones: promising scaffolds for future antimycobacterial agents

Tuberculosis (TB) is still a major health concern worldwide. The increasing incidences of multi-drug-resistant tuberculosis (MDR-TB) necessitate the development of new anti-TB drugs acting via novel mode of action. The search of newer drugs for TB led to the identification of several quinoline-based antimycobacterial agents against both the drug-sensitive and MDR-TB. These agents have been designed by substituting quinoline scaffold with diverse chemical functionalities as well as by modifying quinoline/quinolone-based antibacterial drugs. Several of quinoline/quinolone derivatives displayed excellent antimycobacterial activity and were found free of cytotoxicity. This review highlights the critical aspects of design and structure-activity relationship of quinoline- and quinolone-based antimycobacterial agents.

Novel 1-(4-substituted benzylidene)-4-(1-(substituted methyl)-2,3-dioxoindolin-5-yl)semicarbazide derivatives for use against Mycobacterium tuberculosis H37Rv (ATCC 27294) and MDR-TB strain

A series of eighteen new 1-(4-substituted benzylidene)-4-(1-(substituted methyl)-2,3-dioxoindolin-5-yl)semicarbazide derivatives were designed, synthesized and characterized by spectral and elemental analyses. The derivatives were screened in vitro for antimicrobial activity against Mycobacterium tuberculosis H37Rv (ATCC 27294) and MDR-TB strains. The activity was expressed as the minimum inhibitory concentration in μg/mL. Among the tested compounds 7j, 7m, 7o and 7q possesses equipotent activity as standard drug Isoniazid against MTB while 7m and 7q exhibited higher activity against MDR-TB strain when compared with both the reference drugs isoniazid and rifampicin. Basic structure activity relationships are presented.

Straightforward protocol for the efficient synthesis of varied N(1)-acylated (aza)indole 2-/3-alkanoic acids and esters: optimization and scale-up

A library of approximately 40 N(1)-acylated (aza)indole alkanoic esters and acids was prepared employing a microwave-assisted approach. The optimized synthetic route allows for parallel synthesis, variation of the indole substitution pattern and high overall yield. Additionally, the procedure has been scaled up to yield multi-gram amounts of preferred indole compounds, e.g.: 2'-des-methyl indomethacin 2. The reported compounds were designed as biomedical tools for primary and secondary in vitro and in vivo studies at relevant molecular targets.

Synthesis and evaluation of functionalized indoles as antimycobacterial and anticancer agents

A new series of 5-fluoro-N(2)-(cyclohexylidene)-3-phenyl-1H-indole-2-carbohydrazides (6a-6e) and their cyclization products 5-fluoro-N-(3-oxo-1-thia-4-azaspiro [4.5]dec-4-yl)-3-phenyl-1H-indole-2-carboxamides (7a-7e, 8a-8e) have been synthesized and evaluated for in vitro antimycobacterial activity against Mycobacterium tuberculosis H37Rv using the Microplate Alamar Blue Assay (MABA). Compounds showed moderate to good inhibitory activity at 6.25 μg/mL. Among them, 7b, 7d, 8b, and 8d were the most potent analogs with an inhibition range of 91-95 %. Additionally, compounds 6a, 7a, 7e, 8a, and 8e were subjected to the National Cancer Institute's (NCI) in vitro disease-oriented antitumor screening to be evaluated for antitumor activity. 8e, the most potent compound examined, displayed broad spectrum antiproliferative activity with particular selectivity against four leukemia cell lines (CCRF-CEM, HL-60 (TB), K-562, and RPMI-8226) with log (10) GI (50) values between -5.68 and -6.09.

Microwave-assisted synthesis of small molecules targeting the infectious diseases tuberculosis, HIV/AIDS, malaria and hepatitis C

The unique properties of microwave in situ heating offer unparalleled opportunities for medicinal chemists to speed up lead optimisation processes in early drug discovery. The technology is ideal for small-scale discovery chemistry because it allows full reaction control, short reaction times, high safety and rapid feedback. To illustrate these advantages, we herein describe applications and approaches in the synthesis of small molecules to combat four of the most prevalent infectious diseases; tuberculosis, HIV/AIDS, malaria and hepatitis C, using dedicated microwave instrumentation.

N-Cyclo-hexyl-2-(5-fluoro-1H-indol-3-yl)-2-oxoacetamide

In title compound, C(16)H(17)FN(2)O(2), the cyclo-hexane ring adopts a chair conformation.. The crystal packing is stabilized by weak π-π stacking inter-actions [centroid-centroid distance = 3.503 (5) Å] and inter-molecular C-H⋯O, N-H⋯O and N-H⋯F hydrogen-bond inter-actions.

Synthesis and antimycobacterial activity of highly functionalized tetrahydro-4(1H)-pyridinones

A series of 35 2e,3e,6e-triaryltetrahydro-4(1H)-pyridinones, 2e,3e,5e,6e-tetraaryltetrahydro-4(1H)-pyridinones and their N-nitroso and N-cyano analogs have been prepared. All these 35 compounds obtained were screened for their in vitro activity against Mycobacterium tuberculosis H37Rv (MTB). Among them, the N-nitrosopyridinones are found to be more active against MTB than the corresponding N-CN analogs, which, in turn, were slightly more active than NH analogs. In particular, the N-nitroso compounds, 3d, 4b and 4e with halogen-bearing phenyl rings at 2,6-positions showed maximum activity with MIC values of 3.97, 3.11 and 3.11 μM, being more efficacious than the first line anti-TB drugs, ciprofloxacin, ethambutol and pyrazinamide. A general trend has also been discerned in all the three classes of NH, N-CN and N-NO compounds, in each of which those bearing four aryl rings display higher activity than that having three analogously substituted aryl rings disclosing that lipophilicity could be an important factor underlying antimycobacterial activity.

Recent advances in the design and synthesis of heterocycles as anti-tubercular agents

Due to the unusual structure and chemical composition of the mycobacterial cell wall, effective tuberculosis (TB) treatment is difficult, making many antibiotics ineffective and hindering the entry of drugs. With approximately 33% of infection, TB is still the second most deadly infectious disease worldwide. The reasons for this are drug-resistant TB (multidrug resistant and extensively drug resistant), persistent infection (latent TB) and synergism of TB with HIV; furthermore no new chemical entity has emerged in last 40 years. New data available from the recently sequenced genome of the mycobacterium and the application of methods of modern drug design promise much for the fight against this disease. In this review, we present an introduction to TB, followed by an overview of new heterocyclic anti-tubercular moieties published during the last decade.

2-(1-Ethyl-5-meth-oxy-1H-indol-3-yl)-N-isopropyl-2-oxoacetamide

In the title compound, C(16)H(20)N(2)O(3), the crystal packing is stabilized by weak π-π stacking inter-actions [centroid-centroid distances = 3.577 (9) and 3.693 (9) Å] and inter-molecular C-H⋯O and N-H⋯O hydrogen-bond inter-actions. The C atoms of the N-isopropyl group are disordered over two sets of sites with occupancies of 0.61(3) and 0.39(3).

Antimycobacterial activity of novel 1,2,4-oxadiazole-pyranopyridine/chromene hybrids generated by chemoselective 1,3-dipolar cycloadditions of nitrile oxides

The 1,3-dipolar cycloaddition of nitrile oxides generated in situ from benzohydroximinoyl chloride and triethylamine to 2-aminopyranopyridine-3-carbonitriles and 2-aminochromene-3-carbonitriles occurred chemoselectively furnishing novel 1,2,4-oxadiazole-pyranopyridine/chromene hybrid heterocycles in moderate yields. In vitro screening of these compounds against Mycobacterium tuberculosis H37Rv (MTB) disclosed that the 1,2,4-oxadiazole-pyranopyridine hybrids display enhanced activity relative to the 1,2,4-oxadiazole-chromene hybrids. Among the compounds screened, 3-[3-(4-chlorophenyl)-1,2,4-oxadiazol-5-yl]-4-(2,4-dichlorophenyl)-8-[(E)-(2,4-dichlorophenyl)-methylidene]-6-methyl-5,6,7,8-tetrahydro-4H-pyrano[3,2-c]pyridin-2-amine (MIC: 0.31 μM) is 1.2, 15.2 and 24.6 times more active than standard antitubercular drugs, viz. isoniazid, ciprofloxacin and ethambutol, respectively.

A green expedient synthesis of pyridopyrimidine-2-thiones and their antitubercular activity

The pseudo four-component domino reactions of N-substituted-4-piperidones, substituted aromatic aldehydes and thiourea in the presence of solid sodium ethoxide under solvent-free conditions afforded pyridopyrimidine-2-thiones in almost quantitative yields by simply grinding for 1-2 min. at ambient temperature. The synthesized compounds were screened for their in vitro activity against Mycobacterium tuberculosis H37Rv. Among them, (E)-6-benzyl-8-(2,4-dichlorobenzylidene)-4-(2,4-dichlorophenyl)-3,4,5,6,7,8-hexahydropyrido[4,3-d]pyrimidine-2(1H)-thione (MIC 2.8 μM) displays the maximum activity, being 2.7 and 1.7 times more active than the first line antitubercular drugs ethambutol and ciprofloxacin, respectively, and less active than rifampicin and isoniazid, by 28 and 7 times, respectively.