Synthesis of Novel N-Rich Polycyclic Skeletons Based on Azoles and Pyridines

Current development of β-carboline derived potential antimalarial scaffolds

β-Carboline alkaloids are an eminent class of nitrogen-based natural alkaloids and therapeutic molecules which exert various pharmacological activities through diverse mechanisms. A lot of attention has recently been directed towards this moiety in order to develop effective antimalarial drugs. "Malaria", an acute febrile illness caused by diverse Plasmodium parasites, is a continuing and escalating problem that devastates economically less developed countries by significantly increased morbidity and mortality rates. The mounting parasite resistance towards the antimalarial drugs and augmenting the 'habitat of the insect vector' are creating a catastrophe, indicating an urgent need for new efficacious therapeutics to combat this tropical disease. This article comprehensively encapsulates the clinical and preclinical antimalarial scaffolds comprising β-carboline moiety in their structure. Herein, various classes of natural and semi-synthetic analogues of β-carbolines reported in the last decade (2011-2021) have been extensively studied and illustrated. This review will help the readers to develop an insight into the β-carboline based antimalarials and molecular mechanisms lying behind their mode of action, which is anticipated to be beneficial for the future development of new β-carboline based therapeutics.

Novel Pyrido[2',1':2,3]imidazo[4,5-c]quinoline Derivative Selectively Poisons Leishmania donovani Bisubunit Topoisomerase 1 to Inhibit the Antimony-Resistant Leishmania Infection in Vivo

The unique bisubunit structure of Leishmania donovani topoisomerase 1B (LdTop1) is a potential drug target in the parasites unlike the monomeric Top1 from its human host counterpart. Here, we report the design, synthesis, and validation of a chimeric pyrido[2',1':2,3]imidazo[4,5-c]quinoline derivative (C17) as a novel antileishmanial agent that poisons topoisomerase 1-DNA covalent complexes (LdTop1cc) inside the parasites and inhibits Top1 religation activity both in the drug sensitive and antimony-resistant L. donovani clinical isolates. Importantly, the human Top1 is not sensitive to C17. Further, C17 overcomes the chemical instability of camptothecin (CPT) by generating persistent LdTop1cc-induced DNA breaks inside the parasites even after 12 h of drug removal. Intraperitoneal administration of C17 results in marked reduction of the Leishmania amastigotes from the infected spleen and liver of BALB/c mice. C17 confers a host protective immune-response up-regulating the Th1 cytokines facilitating parasite clearance which can be exploited for treating drug-resistant leishmaniasis.

2-Aminopyridine - an unsung hero in drug discovery

2-Aminopyridine is a simple, low molecular weight and perfectly functionalised moiety known for the synthesis of diverse biological molecules. Many pharmaceutical companies across the globe aim to synthesise low-molecular weight molecules for use as pharmacophores against various biological targets. 2-Aminopyridine can serve as a perfect locomotive in the synthesis and pulling of such molecules towards respective pharmacological goals. The major advantage of this moiety is its simple design, which can be used to produce single products with minimum side reactions. Moreover, the exact weight of synthesised compounds is low, which enables facile identification of toxicity-causing metabolites in drug discovery programmes. This manuscript is a quick review of such pharmacophores derived from 2-aminopyridine.

An expeditious microwave assisted one-pot sequential route to pyrido fused imidazo[4,5-c] quinolines in green media

A novel one-pot sequential route for the synthesis of pyrido[2′,1′:2,3]imidazo[4,5-c]quinolines via microwave-assisted sequential reaction of 2-aminopyridines, 2-bromophenacyl bromides, and aldehydes has been developed in green media.

Isolation and synthesis of cryptosanguinolentine (isocryptolepine), a naturally-occurring bioactive indoloquinoline alkaloid

Cryptosanguinolentine (isocryptolepine) is one of the minor naturally-occurring monomeric indoloquinoline alkaloids, isolated from the West African climbing shrub Cryptolepis sanguinolenta. The natural product displays such a simple and unique skeleton, which chemists became interested in well before it was found in Nature. Because of its structure and biological activity, the natural product has been targeted for synthesis on numerous occasions, employing a wide range of different strategies. Hence, discussed here are aspects related to the isolation of isocryptolepine, as well as the various approaches toward its total synthesis.

The Pictet-Spengler Reaction Updates Its Habits

The Pictet-Spengler reaction (P-S) is one of the most direct, efficient, and variable synthetic method for the construction of privileged pharmacophores such as tetrahydro-isoquinolines (THIQs), tetrahydro-β-carbolines (THBCs), and polyheterocyclic frameworks. In the lustro (five-year period) following its centenary birthday, the P-S reaction did not exit the stage but it came up again on limelight with new features. This review focuses on the interesting results achieved in this period (2011-2015), analyzing the versatility of this reaction. Classic P-S was reported in the total synthesis of complex alkaloids, in combination with chiral catalysts as well as for the generation of libraries of compounds in medicinal chemistry. The P-S has been used also in tandem reactions, with the sequences including ring closing metathesis, isomerization, Michael addition, and Gold- or Brønsted acid-catalyzed N-acyliminium cyclization. Moreover, the combination of P-S reaction with Ugi multicomponent reaction has been exploited for the construction of highly complex polycyclic architectures in few steps and high yields. The P-S reaction has also been successfully employed in solid-phase synthesis, affording products with different structures, including peptidomimetics, synthetic heterocycles, and natural compounds. Finally, the enzymatic version of P-S has been reported for biosynthesis, biotransformations, and bioconjugations.

An efficient synthesis of imidazo[2,1-b][1,3,4]thiadiazol-7-ium hydroxides by a one-pot, three-component reaction in water

An improved synthesis of 2-ethyl-5-(2-hydroxy-4-oxoquinolin-3(4H)-ylidene)-6-aryl-5,6-dihydroimidazo[2,1-b][1,3,4]thiadiazol-7-ium hydroxide derivatives 4a–k via the reaction of aryl glyoxal monohydrates 1a–k, quinoline-2,4-diol 2 and 2-amino-[1,3,4]thiadiazole (3) in the presence of Et3N/sulfamic acid in H2O is described. This green protocol is characterized by the use of the readily available catalyst and reactants, short reaction times, operational simplicity and high yields of products. The structures of all compounds were characterized by 1H NMR, 13C NMR and Fourier-transform infrared (FT-IR) spectral data and microanalyses.

Molecular iodine-catalysed oxidative CO–C(alkyl) bond cleavage of aryl/heteroaryl alkyl ketones: an efficient strategy to access fused polyheterocycles

An efficient molecular iodine-catalysed one-pot strategy has been accomplished for the construction of various fused heterocycles under metal and oxidant free conditions.

Synthesis of [1,3,4]Thiadiazolo[3',2':1,2]imidazo[4,5-c]quinolines including Pictet-Spengler Reaction and Exploration of Their C-2 Reactivity through SNAr

This work reports the design of [1,3,4]thiadiazolo[3',2':1,2]imidazo[4,5-c]quinolines using a Pictet-Spengler reaction. The scope of the reaction was achieved from 6-(2-aminophenyl)imidazo[2,1-b][1,3,4]thiadiazole derivatives and available aldehydes. A wide range of aldehydes were employed to examine the scope of the cyclization. In parallel, a mechanism investigation was realized and showed a hydride transfer which led to a dismutation of the intermediate species. To complete this methodological study, a "sequential" oxidation/S_NAr procedure was performed to achieve C-2 nucleophilic substitution using several amine types.

Solvent- and catalyst-free synthesis of imidazo[1,2-a]pyridines under microwave irradiation

A facile solvent- and catalyst-free method for the synthesis of a series of imidazo[1,2-a]pyridines in good to excellent yields by the condensation of 2-aminopyridines with α-bromoketones under microwave irradiation has been developed. The important features of this method are that it is reasonably fast, very clean, high yielding, simple workup and environmentally benign.

When Indolizine Meets Quinoline: Diversity-Oriented Synthesis of New Polyheterocycles and Their Optical Properties

Fluorescence-based technologies play a pivotal role in various biomedical applications. Here we report an efficient route to a new class of fluorophores, indolizino[3,2-c]quinolines, via the oxidative Pictet-Spengler cyclization strategy. The condensation of several 2-methylpyridines with 2-bromo-2'-nitroacetophenone allowed for the rapid assembly of indolizines with a 2-nitrophenyl group at the C2 position. The subsequent reduction of the nitro group under mild conditions followed by oxidative Pictet-Spengler cyclization with various aryl aldehydes in the presence of a catalytic amount of FeCl3 furnished the indolizino[3,2-c]quinolines in good overall yields. We also examined the photophysical properties of this new series of polyheterocyclic compounds. Several indolizino[3,2-c]quinolines were found to have unique and desirable optical properties, suggesting that these compounds may be suitable for use as prospective fluorescent probes in aqueous systems.

Synthesis of Naphtho[1',2':4,5]imidazo[1,2-a]pyridines and Imidazo[5,1,2-cd]indolizines Through Pd-Catalyzed Cycloaromatization of 2-Phenylimidazo[1,2-a]pyridines with Alkynes

In this paper, palladium-catalyzed oxidative cycloaromatization of 2-phenylimidazo[1,2-a]pyridine (PIP) with internal alkyne is studied. From this reaction, two classes of fused N-heterocycle, naphtho[1',2':4,5]imidazo[1,2-a]pyridine (NIP) and imidazo[5,1,2-cd]indolizine (IID), were formed through dehydrogenative coupling featured with cleavage of the C-H bonds located on different moiety of the PIP substrates. Moreover, when 5-methyl-2-phenylimidazo [1,2-a]pyridine or 2-mesitylimidazo[1,2-a]pyridine was used, either NIP or IID could be obtained as an exclusive product with good efficiency. Intriguingly, Pd(II) showed different action mode in promoting this reaction compared with Rh(III) and led to the formation of NIP with reversed regio-selectivity for the reaction of asymmetrical alkyne.

Polycyclic imidazo[1,2-a]pyridine analogs – synthesis via oxidative intramolecular C–H amination and optical properties

Oxidative C–H amination has proved to be an efficient strategy to construct pyrido[2′,1′:2,3]imidazo[4,5-b]indoles and their π-expanded analogs.

Stereoselective one-pot synthesis of β-alkylsulfide enol esters. Base-triggered rearrangement under mild conditions

A stereoselective one-pot procedure was developed to prepare S-substituted (Z)-enol esters through a base-triggered rearrangement. This transition metal-free multicomponent approach can be performed under an air atmosphere at room temperature, tolerates a wide set of chemical functionalities and generally affords high isolated yields. The (Z)-selectivity arises from the [1,4]-S- to O-acyl migration.

Exploiting the Divalent Nature of Isonitriles: a novel Pictet-Spengler Amidination process

An isocyanide-based multicomponent reaction (IMCR) utilized for the rapid assembly of novel, biologically relevant dihydropyrrolo[1,2-a]quinoxalines-amidines is herein presented. Starting from 1-(2-aminophenyl)pyrroles, aldehydes, and isonitriles, the target heterocyclic scaffold is assembled in a one-pot, operationally friendly process. With three points of diversity and formation of three chemical bonds in one step, this strategy proves to be very general. Novel, mild methodology for the generation of amidines from secondary amine anilines and isonitriles is also introduced.

Synthesis of biologically active pyridoimidazole/imidazobenzothiazole annulated polyheterocycles using cyanuric chloride in water

An efficient and mild protocol for rapid access to N-fused polyheterocycles via Pictet–Spengler type 6-endo cyclization by cyanuric chloride in aqueous medium has been developed.

Mild and efficient cyanuric chloride catalyzed Pictet-Spengler reaction

A practical, mild and efficient protocol for the Pictet–Spengler reaction catalyzed by cyanuric chloride (trichloro-1,3,5-triazine, TCT) is described. The 6-endo cyclization of tryptophan/tryptamine and modified Pictet–Spengler substrates with both electron-withdrawing and electron-donating aldehydes was carried out by using a catalytic amount of TCT (10 mol %) in DMSO under a nitrogen atmosphere. TCT catalyzed the Pictet–Spengler reaction involving electron-donating aldehydes in excellent yield. Thus, it has a distinct advantage over the existing methodologies where electron-donating aldehydes failed to undergo 6-endo cyclization. Our methodology provided broad substrate scope and diversity. This is indeed the first report of the use of TCT as a catalyst for the Pictet–Spengler reaction.

Discovery of a Series of Thiazole Derivatives as Novel Inhibitors of Metastatic Cancer Cell Migration and Invasion

Effective inhibitors of cancer cell migration and invasion can potentially lead to clinical applications as therapy to block tumor metastasis, the primary cause of death in cancer patients. To this end we have designed and synthesized a series of thiazole derivatives that showed potent efficacy against cell migration and invasion in metastatic cancer cells. The most effective compound, 5k, was found to have an IC₅₀ value of 176 nM in the dose-dependent transwell migration assays in MDA-MB-231cells. At the dose of 10 μM, 5k also blocked about 80% of migration in HeLa and A549 cells and 60% of invasion of MDA-MB-231 cells. Importantly, the majority of the derivatives exhibited no apparent cytotoxicity in the clonogenic assays. The low to negligible inhibition of cell proliferation is a desirable property of these anti-migration derivatives because they hold promise of low toxicity to healthy cells as potential therapeutic agents. Mechanistic studies analyzing the actin cytoskeleton by microscopy demonstrate that compound 5k substantially reduced cellular f-actin, and prevented localization of fascin to actin-rich membrane protrusions. These results suggest that the anti-migration activity may result from impaired actin structures in protrusions that are necessary to drive migration.

Engineering of indole-based tethered biheterocyclic alkaloid meridianin into β-carboline-derived tetracyclic polyheterocycles via amino functionalization/6-endo cationic π-cyclization

A mild, efficient and versatile method has been developed for the construction of a functionalized natural product, meridianin, and its post conversion to pyrimido-β-carboline by cationic π- cyclization. The strategy involves the introduction of an amino group at the C-5 of the pyrimidine ring and utilizing the nucleophilictiy of the C-2 in the indole ring to facilitate cationic π-cyclization.

CuSO4-glucose for in situ generation of controlled Cu(I)-Cu(II) bicatalysts: multicomponent reaction of heterocyclic azine and aldehyde with alkyne, and cycloisomerization toward synthesis of N-fused imidazoles

The catalytic efficiency of mixed Cu(I)-Cu(II) system in situ generated by partial reduction of CuSO(4) with glucose in ethanol (nonanhydrous) under open air has been explored. With this catalysis, the multicomponent cascade reaction of A(3)-coupling of heterocyclic amidine with aldehyde and alkyne, 5-exo-dig cycloisomerization, and prototropic shift has afforded an efficient and eco-friendly synthesis of therapeutically important versatile N-fused imidazoles. Diverse heterocyclic amidines, several of which are known to be poorly reactive, and aldehydes are compatible in this catalytic process.

Solid-phase synthesis and chemical properties of 2-(2-amino/hydroxyethyl)-1-aryl-3,4-dihydropyrazino[1, 2-b]indazol-2-iums

Amino alcohols, diamines, benzenesulfonyl chlorides, and bromoketones were used to prepare polymer-supported 2-(2-(2-(amino/hydroxyl)ethylamino)ethyl)-3-benzoyl-2H-indazoles. Acid-mediated release yielded 2-((amino/hydroxyl)ethyl)-1-aryl-3,4-dihydropyrazino[1,2-b]indazole-2-iums. In neutral pH, iminiums spontaneously cyclized to complex fused heterocycles 3,6,9,10-tetraazatetracyclo[7.7.0.0(2,6).0(11,16)]hexadeca-11,13,15-trienes, 3-oxa-6,9,10-triazatetracyclo[7.7.0.0(2,6).0(11,16)]hexadeca-11,13,15-trienes, and 3,7,10,11-tetraazatetracyclo[8.7.0.0(2,7).0(12,17)]heptadeca-12,14,16-trienes. Transformations were carried out under mild conditions and tolerated diverse substitution patterns.