Pyrrole-containing hybrids as potential anticancer agents: An insight into current developments and structure-activity relationships

Cancer poses a significant threat to human health. Therefore, it is urgent to develop potent anti-cancer drugs with excellent inhibitory activity and no toxic side effects. Pyrrole and its derivatives are privileged heterocyclic compounds with significant diverse pharmacological effects. These compounds can target various aspects of cancer cells and have been applied in clinical settings or are undergoing clinical trials. As a result, pyrrole has emerged as a promising drug scaffold and has been further probed to get novel entities for the treatment of cancer. This article reviews recent research progress on anti-cancer drugs containing pyrrole. It focuses on the mechanism of action, biological activity, and structure-activity relationships of pyrrole derivatives, aiming to assist in designing and synthesizing innovative pyrrole-based anti-cancer compounds.

Kinetic resolution of 1-(1-alkynyl)cyclopropyl ketones via gold-catalyzed divergent (4 + 4) cycloadditions: stereoselective access to furan fused eight-membered heterocycles

Chiral eight-membered heterocycles comprise a diverse array of natural products and bioactive compounds, yet accessing them poses significant challenges. Here we report a gold-catalyzed stereoselective (4 + 4) cycloaddition as a reliable and divergent strategy, enabling readily accessible precursors (anthranils and ortho-quinone methides) to be intercepted by in situ generated gold-furyl 1,4-dipoles, delivering previously inaccessible chiral furan/pyrrole-containing eight-membered heterocycles with good results (56 examples, all >20 : 1 dr, up to 99% ee). Moreover, we achieve a remarkably efficient kinetic resolution (KR) process (s factor up to 747). The scale-up synthesis and diversified transformations of cycloadducts highlight the synthetic potential of this protocol. Computational calculations provide an in-depth understanding of the stereoselective cycloaddition process.

Elucidation of Chalkophomycin Biosynthesis Reveals N-Hydroxypyrrole-Forming Enzymes

Reactive functional groups, such as N-nitrosamines, impart unique bioactivities to the natural products in which they are found. Recent work has illuminated enzymatic N-nitrosation reactions in microbial natural product biosynthesis, motivating interest in discovering additional metabolites constructed using such reactivity. Here, we use a genome mining approach to identify over 400 cryptic biosynthetic gene clusters (BGCs) encoding homologues of the N-nitrosating biosynthetic enzyme SznF, including the BGC for chalkophomycin, a CuII-binding metabolite that contains a C-type diazeniumdiolate and N-hydroxypyrrole. Characterizing chalkophomycin biosynthetic enzymes reveals previously unknown enzymes responsible for N-hydroxypyrrole biosynthesis, including the first prolyl-N-hydroxylase, and a key step in the assembly of the diazeniumdiolate-containing amino acid graminine. Discovery of this pathway enriches our understanding of the biosynthetic logic employed in constructing unusual heteroatom-heteroatom bond-containing functional groups, enabling future efforts in natural product discovery and biocatalysis.

Cp*Ir(III) complexes catalyzed solvent-free synthesis of quinolines, pyrroles and pyridines via an ADC strategy

A trio of Ir(III) complexes that are held together by a picolinamidato moiety were created. In our earlier research, we demonstrated the catalytic activity of the complexes for producing alpha-alkylated ketones from a ketone or secondary alcohol with a primary alcohol in the presence of a catalytic amount of a Cp*Ir(III) catalyst and tBuOK in toluene at 110 °C using the hydrogen-borrowing technique. Earlier many research groups had synthesized quinoline, pyrrole, and pyridine derivatives using 2-amino alcohol and ketone or secondary alcohol derivatives as starting materials, but in all those cases the reaction conditions are not suitable in terms of green synthesis like more catalyst loading, base loading, long reaction time, and high temperature. In addition, most of the reactions contain phosphine a hazardous by-product, along with the catalyst. Keeping in mind these shortcomings, we tried to expand the use of our catalysts after achieving an excellent result in our previous work, and we were successful in producing quinoline, pyrrole, and pyridine derivatives through acceptor-less dehydrogenative coupling (ADC) procedures at 90-110 °C under neat/solvent-free conditions and achieved good to exceptional yields of those nitrogen-containing heterocycles. This methodology is attractive because it is environmentally benign and allows for the "green" synthesis of nitrogen-containing heterocycles. All that is required is a modest quantity of catalyst and base, and the by-products are merely H2O and H2.

Mechanosynthesis of Pyrrole-2-carboxylic Acids via Copper-Catalyzed Spiroannulation/Ring-Opening Aromatization of 4-Arylidene Isoxazol-5-ones with Enamino Esters

An efficient and practical tandem reaction of 4-arylidene isoxazol-5-ones with enamino esters catalyzed by an inexpensive copper salt has been established in a ball mill. This innovative approach yields a diverse array of structurally novel pyrrole-2-carboxylic acids, showing excellent tolerance toward different functional groups. By integrating spiroannulation and ring-opening aromatization processes, this protocol introduces a facile and cost-effective strategy for synthesizing highly functionalized pyrrole derivatives.

Evaluating Pigments as a Biosignature: Abiotic/Prebiotic Synthesis of Pigments and Pigment Mimics in Planetary Environments

Pigments serve a multitude of functions in biology including light harvesting for photosynthesis, radiation protection, membrane support, and defense. The ubiquity of pigments-especially within extremophiles found in high-radiation, high-salinity, and dry environments-and their detectability via mission-ready techniques have elevated these molecules as promising targets in the search for evidence of life elsewhere. Moreover, the detection of pigments has been proposed as a "smoking gun" for extraterrestrial life as it has been suggested that these molecules cannot be generated abiotically. However, while pigments may hold promise as a biosignature, current understanding of their possible prebiotic origins remains understudied and uncertain. Better understanding of the abiotic synthesis of pigments is critical for evaluating the biogenicity of any pigment detected during missions, including by the Mars Perseverance rover or from returned samples. Compounding this uncertainty is the broad definition of pigment as it includes any compound capable of absorbing visible light and by itself does not specify a particular chemical motif. While not experimentally verified, there are promising prebiotic routes for generating pigments including hemes, chlorophylls, and carotenoids. Herein, we review the biochemistry of pigments, the inherent assumptions made when searching for these molecules in the field, their abiotic synthesis in industry and prebiotic reactions, prebiotically relevant molecules that can mimic their spectral signatures, and implications/recommendations for future work.

Cobalt(II)-Catalyzed Synthesis of γ-Diketones from Aryl Alkenes and Its Utilization in the Synthesis of Various Heterocyclic Compounds

An earth-abundant Co(II) salt-catalyzed mild and affordable synthetic route has been developed for the synthesis of industrially relevant 1,4-dicarbonyl compounds (or γ-diketones) via oxidative coupling between aryl alkenes and ketones (both cyclic and acyclic) using TBHP and DBU as the oxidant and base, respectively. 1,4-Dicarbonyl compounds are known to be synthesized using expensive metal catalysts, dual catalysts, or low-cost metal complexes combined with an additive or ligand template, which further needs to be synthesized. Herein, we report the synthesis of 1,4-dicarbonyl compounds using cobalt(II) acetate as a catalyst without any expensive co-catalyst or ligand templates. This methodology has a broad substrate scope with significant yields and good functional group tolerance. Generation of unsymmetrical 1,4-dicarbonyls at room temperature and its versatile synthetic expansion to produce synthetically and biologically valuable heterocyclic compounds are salient features of this novel methodology. In addition, various controlled experiments such as primary kinetic isotope effect study, Hammett analysis with variation of the nature of the substituents on the styrene ring, and theoretical calculations (density functional theory) unravel the mechanistic intricacies involved in this new, simple, and atom-economic methodology.

Design, Synthesis, Biological Evaluation and in Silico Studies of Curcumin Pyrrole Conjugates

Curcumin conjugated heterocyclic compounds are potent candidates with drug likeness against various bacterial pathogens. A set of curcumin-based pyrrole conjugates (CPs) were synthesized and characterized by FT-IR, 1H and 13C NMR and HR-MS techniques. The results of free radical scavenging activity of the synthesized CPs, evaluated by FRAP and CUPRAC assays, showed the potency of these compounds as effective antioxidants. CP3 exhibits the highest antioxidant activity amongst the CPs. The bactericidal efficacy of CPs was screened against ESKAP bacterial pathogens, and CPs were found to possess better antibacterial property than curcumin, specifically against staphylococcus aureus bacteria. In addition, serum albumin (BSA and HSA) binding interaction of these CPs were determined by UV-visible and fluorescence spectrophotometric techniques. In-silico molecular docking study was performed to determine the binding patterns of molecular targets against Staphylococcus aureus tyrosyl tRNA synthetase, and serum albumin proteins. The structure-activity relationship showed that the presence of multiple phenolic hydroxyl groups, and electron withdrawing groups on the structure of CP molecule, enhances its antioxidant and antibacterial activity, respectively.

Elucidation of chalkophomycin biosynthesis reveals N-hydroxypyrrole-forming enzymes

Reactive functional groups, such as N-nitrosamines, impart unique bioactivities to the natural products in which they are found. Recent work has illuminated enzymatic N-nitrosation reactions in microbial natural product biosynthesis, motivating an interest in discovering additional metabolites constructed using such reactivity. Here, we use a genome mining approach to identify over 400 cryptic biosynthetic gene clusters (BGCs) encoding homologs of the N-nitrosating biosynthetic enzyme SznF, including the BGC for chalkophomycin, a CuII-binding metabolite that contains a C-type diazeniumdiolate and N-hydroxypyrrole. Characterizing chalkophomycin biosynthetic enzymes reveals previously unknown enzymes responsible for N-hydroxypyrrole biosynthesis, including the first prolyl-N-hydroxylase, and a key step in assembly of the diazeniumdiolate-containing amino acid graminine. Discovery of this pathway enriches our understanding of the biosynthetic logic employed in constructing unusual heteroatom-heteroatom bond-containing functional groups, enabling future efforts in natural product discovery and biocatalysis.

Reaction Pattern and Mechanistic Aspects of Iodine and Iodine-Based Reagents in Selenylation of Aliphatic, Aromatic, and (Hetero)Cyclic Systems

Organoselenium compounds have been the subject of extensive research since the discovery of the biologically active compound ebselen. Ebselen has recently been found to show activity against the main protease of the virus responsible for COVID-19. Other organoselenium compounds are also well-known for their diverse biological activities, with such compounds exhibiting interesting physical properties relevant to the fields of electronics, materials, and polymer chemistry. In addition, the incorporation of selenium into various organic molecules has garnered significant attention due to the potential of selenium to enhance the biological activity of these molecules, particularly in conjunction with bioactive heterocycles. Iodine and iodine-based reagents play a prominent role in the synthesis of organoselenium compounds, being valued for their cost-effectiveness, non-toxicity, and ease of handling. These reagents efficiently selenylate a broad range of organic substrates, encompassing alkenes, alkynes, and cyclic, aromatic, and heterocyclic molecules. They serve as catalysts, additives, inducers, and oxidizing agents, facilitating the introduction of different functional groups at alternate positions in the molecules, thereby allowing for regioselective and stereoselective approaches. Specific iodine reagents and their combinations can be tailored to follow the desired reaction pathways. Here, we present a comprehensive review of the progress in the selenylation of organic molecules using iodine reagents over the past decade, with a focus on reaction patterns, solvent effects, heating, microwave, and ultrasonic conditions. Detailed discussions on mechanistic aspects, such as electrophilic, nucleophilic, radical, electrochemical, and ring expansion reactions via selenylation, multiselenylation, and difunctionalization, are included. The review also highlights the formation of various cyclic, heterocyclic, and heteroarenes resulting from the in situ generation of selenium intermediates, encompassing cyclic ketones, cyclic ethers, cyclic lactones, selenophenes, chromones, pyrazolines, pyrrolidines, piperidines, indolines, oxazolines, isooxazolines, lactones, dihydrofurans, and isoxazolidines. To enhance the reader's interest, the review is structured into different sections covering the selenylation of aliphatic sp2/sp carbon and cyclic sp2 carbon, and then is further subdivided into various heterocyclic molecules.

B(C6F5)3-Catalyzed Dehydrogenation of Pyrrolidines to Form Pyrroles

Pyrroles are important N-heterocycles found in medicines and materials. The formation of pyrroles from widely accessible pyrrolidines is a potentially attractive strategy but is an underdeveloped approach due to the sensitivity of pyrroles to the oxidative conditions required to achieve such a transformation. Herein, we report a catalytic approach that employs commercially available B(C6F5)3 in an operationally simple procedure that allows pyrrolidines to serve as direct synthons for pyrroles. Mechanistic studies have revealed insights into borane-catalyzed dehydrogenative processes.

Mn(II) Promoted Divergent-Convergent Domino Reaction Giving Dinuclear Tetrasubstituted Pyrrole Complex

Domino reaction of benzo[d]thiazole-2-methylamine (S1) has been developed in the presence of MnCl2 ⋅ 4H2O, leading to tetrasubstituted pyrrole coordinated dinuclear Mn(II) complex 1 ([MnClP]2, P-=2,3,4,5-tetrakis(benzo[d]thiazol-2-yl)pyrrol-1-ide). The reaction process has been studied by assigning a series of intermediates based on time-dependent mass spectrometry, control experiments, crystallography, and density functional theory (DFT) theoretical calculation. A plausible mechanism involving an unprecedented divergent-convergent domino sequence has been proposed. Compound S1 could be activated by MnCl2 ⋅ 4H2O via coordination, which divergently produces two intermediates imine II (1-(benzo[d]thiazol-2-yl)-N-(benzo[d]thiazol-2-ylmethyl)methanimine) and alkene C (1,2-bis(benzo[d]thiazol-2-yl)ethene) through oxidative self-condensation and free radical coupling followed by elimination, respectively. They could then react with each other convergently via formal [3+2] cycloaddition to give deprotonated tetrasubstituted pyrrole coordinated intermediate [MnClP] after aromatization. Dimerization of [MnClP] produces the final product 1. Three C-C bonds and one C-N bond are formed through this six-step domino sequence. The corresponding organic skeleton (HP: 2,2',2'',2'''-(1H-pyrrole-2,3,4,5-tetrayl)tetrakis(benzo[d]thiazole)) has been obtained from 1 and shows a higher fluorescent quantum yield (52 %) than the reported 3,4-diphenyl substituted analogue 2,2'-(3,4-diphenyl-1H-pyrrole-2,5-diyl)bis(benzo[d]thiazole) (DPB) (42 %).

Chiral Au(III) chelates exhibit unique NCI-60 cytotoxicity profiles and interactions with human serum albumin

Au(III) bis(pyrrolide-imine) chelates are emerging as a class of versatile, efficacious metallodrug candidates. Here, we synthesised two enantiopure chiral ligands H2L1 and H2L2 (tetradentate cyclohexane-1,2-diamine-bridged bis(pyrrole-imine) derivatives). Metallation of the ligands with Au(III) afforded the chiral cationic complexes AuL1 and AuL2. The in vitro cytotoxicities of AuL1 and AuL2 determined in the NCI-60 single-dose drug screen were 56.5% and 89.1%, respectively. AuL1 was subsequently selected for a five-dose NCI-60 screen, attaining GI50, IC50, and LC50 values of 4.7, 9.3 and 39.8 μM, respectively. Hierarchical cluster analysis of the NCI-60 data indicated that the profile for AuL1 was similar to that of vinblastine sulfate, a microtubule-targeting vinca alkaloid. Reactions of AuL1 with glutathione (GSH) in vitro confirmed its susceptibility to reduction, Au(III) → Au(I), by intracellular thiols. Because human serum albumin (HSA) is responsible for transporting clinically deployed and investigational drugs, we studied the uptake of AuL1 and AuL2 by HSA to delineate how chirality impacts their protein-binding affinity. Steady-state fluorescence quenching data acquired on the native protein and data from site-specific probes showed that the compounds bind at sites close enough to Trp-214 (subdomain IIA) of HSA to quench the fluorophore. The bimolecular quenching rate constants, Kq, were ca. 102 times higher than the maximum diffusion-controlled collision constant of a biomolecule in water (1010 M-1 s-1), confirming that static fluorescence quenching was the dominant mechanism. The Stern-Volmer constants, KSV, were ∼104 M-1 at 37 °C, while the affinity constants, Ka (37 °C), measured ∼2.1 × 104 M-1 (AuL1) and ∼1.2 × 104 M-1 (AuL2) for enthalpy-driven ligand uptake targeting Sudlow's site I. Although far- and near-UV CD spectroscopy indicated that both complexes minimally perturb the secondary and tertiary structure of HSA, substantial shifts in the CD spectra were recorded for both protein-bound ligands. This study highlights the role of chirality in determining the cytotoxicity profiles and protein binding behaviour of enantiomeric Au(III) chelates.

Trifluoromethyl-β-dicarbonyls as Versatile Synthons in Synthesis of Heterocycles

Trifluoromethyl group relishes a privileged position in the realm of medicinal chemistry because its incorporation into organic molecules often enhances the bioactivity by altering pharmacological profile of molecule. Trifluoromethyl-β-dicarbonyls have emerged as pivotal building blocks in synthetic organic chemistry due to their facile accessibility, stability and remarkable versatility. Owing to presence of nucleophilic and electrophilic sites, they offer multifunctional sites for the reaction. This review covers a meticulous exploration of their multifaceted role, encompassing an in-depth analysis of mechanism, extensive scope, limitations and wide-ranging applications in diverse organic synthesis, covering the literature from the 21st century. This comprehensive review encapsulates the applications of trifluoromethyl-β-dicarbonyls and their synthetic equivalents as precursors of complex and diverse heterocyclic scaffolds, fused heterocycles and spirocyclic compounds having medicinal and material importance. Their potent synthetic utility in cyclocondensation reactions with binucleophiles, cycloaddition reactions, C-C bond formations, asymmetric multicomponent reactions using classical/solvent-free/catalytic synthesis have been presented. Influence of unsymmetrical trifluoromethyl-β-diketones on regioselectivity of transformation is also reviewed. This review will benefit the synthetic and pharmaceutical communities to explore trifluoromethyl-β-dicarbonyls as trifluoromethyl building blocks for fabrication of heterocyclic scaffolds having implementation into drug discovery programs in the imminent future.

Crystal structure and Hirshfeld surface analysis of (2E)-1-phenyl-3-(1H-pyrrol-2-yl)propen-1-one

The title com-pound, C13H11NO, adopts an E configuration about the C=C double bond. The pyrrole ring is inclined to the phenyl ring at an angle of 44.94 (8)°. In the crystal, mol-ecules are linked by N-H⋯O hydrogen bonds, forming ribbons parallel to (020) in zigzag C(7) chains along the a axis. These ribbons are connected via C-H⋯π inter-actions, forming a three-dimensional network. No significant π-π inter-actions are observed.

Site-selective chlorination of pyrrolic heterocycles by flavin dependent enzyme PrnC

Halogenation of pyrrole requires strong electrophilic reagents and often leads to undesired polyhalogenated products. Biocatalytic halogenation is a highly attractive approach given its chemoselectivity and benign reaction conditions. While there are several reports of enzymatic phenol and indole halogenation in organic synthesis, corresponding reports on enzymatic pyrrole halogenation have been lacking. Here we describe the in vitro functional and structural characterization of PrnC, a flavin-dependent halogenase that can act on free-standing pyrroles. Computational modeling and site mutagenesis studies identified three key residues in the catalytic pocket. A moderate resolution map using single-particle cryogenic electron microscopy reveals PrnC to be a dimer. This native PrnC can halogenate a library of structurally diverse pyrrolic heterocycles in a site-selective manner and be applied in the chemoenzymatic synthesis of a chlorinated analog of the agrochemical fungicide Fludioxonil.

Ag(I)-Mediated Annulation of 2-(2-Enynyl)pyridines and Propargyl Amines to Access 1-(2H-Pyrrol-3-yl)indolizines

An effective Ag(I)-mediated annulation of 2-(2-enynyl)pyridines and propargyl amines was developed, unexpectedly affording a broad range of functionalized 1-(2H-pyrrol-3-yl)indolizines in moderate to excellent yields. The developed method is characterized by operational simplicity, ready availability of starting materials, high regioselectivity, and broad substrate scope under mild reaction conditions. The Ag(I)-promoted cyclization of 2-(2-enynyl)pyridines and propargyl amines possibly results in the formation of the spiroindolizine, the ring-opening rearrangement of which may give the 1-(2H-pyrrol-3-yl)indolizine. Furthermore, a gram-scale reaction and synthetic transformations are also studied.

Visible Light-Induced Hydroacylation of Benzylidenemalononitriles with Aroyl Chlorides Using Silane as a Hydrogen Donor

A novel photoredox-catalyzed direct hydroacylation of benzylidenemalononitriles is described. In this method, aroyl chlorides are employed as a readily available and affordable source of acyl groups, while commercially available tris(trimethylsilyl)silane acts as both the hydrogen atom donor and electron donor. By eliminating the requirement for complex synthesis of acyl precursors and hydrogen atom-transfer (HAT) reagents, this approach offers a convenient and efficient strategy for the hydroacylation of benzylidenemalononitriles.

Self-Assembly of 4,4'-Linked Dipyrromethanes from Unconventional Reactants─Propargylamines, 2-(Vinyloxy)ethyl Isothiocyanate, and Alkylating Agents under Basic Conditions

Self-assembly of 4,4'-linked dipyrromethanes from 2-(vinyloxy)ethyl isothiocyanate, tertiary propargylamines, and alkylating agents has been discovered. The plausible reaction mechanism, the major stages of which have been confirmed experimentally, includes (1) the lithiation of propargylamine (with n-BuLi); (2) the formation of lithium N-[2-(vinyloxy)ethyl]but-2-ynimidothioate (product of the addition of monolithiated propargylamine to isothiocyanate); (3) isomerization of the latter in the corresponding allenylimidothioate (under the action of the t-BuOK/t-BuOH system); (4) low-temperature (<15 °C) intramolecular cyclization of the latter into potassium N-(5-amino-2-thienyl)-N-[2-(vinyloxy)ethyl]amide; (5) the base-induced cleavage of the C-O bond of the N-[2-(vinyloxy)ethyl] group and removal of vinyloxide-anion leading to acetaldehyde; (6) interaction of acetaldehyde with two molecules of N-(5-amino-2-thienyl)-N-[2-(vinyloxy)ethyl]amide-anion resulting in dithienomethane N-anionic intermediate; (7) recyclization of the latter into dipyrromethane S-anionic intermediate. Final S-alkylation affords synthetically challenging 4,4'-dipyrromethanes in a yield of 22-51%. The whole process is carried out in a single synthetic operation in a very short time (∼10-15 min, excluding alkylation time).

Design, synthesis, and pharmacological evaluation of indazole carboxamides of N-substituted pyrrole derivatives as soybean lipoxygenase inhibitors

In this paper, we attempted to develop a novel class of compounds against lipoxygenase, a key enzyme in the biosynthesis of leukotrienes implicated in a series of inflammatory diseases. Given the absence of appropriate human 5-lipoxygenase crystallographic data, solved soybean lipoxygenase-1 and -3 structures were used as a template to generate an accurate pharmacophore model which was further used for virtual screening purposes. Eight compounds (1-8) have been derived from the in-house library consisting of N-substituted pyrroles conjugated with 5- or 6-indazole moieties through a carboxamide linker. This study led to the discovery of hit molecule 8 bearing a naphthyl group with the IC50 value of 22 μM according to soybean lipoxygenase in vitro assay. Isosteric replacement of naphthyl ring with quinoline moieties and reduction of carbonyl carboxamide group resulted in compounds 9-12 and 13, respectively. Compound 12 demonstrated the most promising enzyme inhibition. In addition, compounds 8 and 12 were found to reduce the carrageenan-induced paw edema in vivo by 52.6 and 49.8%, respectively. In view of the encouraging outcomes concerning their notable in vitro and in vivo anti-inflammatory activities, compounds 8 and 12 could be further optimized for the discovery of novel 5-lipoxygenase inhibitors in future. A structure-based 3D pharmacophore model was used in the virtual screening of in-house library to discover novel potential 5-lipoxygenase inhibitors.

Gold-Catalyzed Cyclization of Yndiamides with Isoxazoles via α-Imino Gold Fischer Carbenes

Gold catalysis is an important method for alkyne functionalization. Here we report the gold-catalyzed formal [3+2] aminative cyclization of yndiamides and isoxazoles in a direct synthesis of polysubstituted diaminopyrroles, which are important motifs in drug discovery. Key to this process is the formation, and subsequent cyclization, of an α-imino gold Fischer carbene, which represents a new type of gold carbene intermediate. The reaction proceeds rapidly under mild conditions, with high regioselectivity being achieved by introducing a subtle steric bias between the nitrogen substituents on the yndiamide. DFT calculations revealed that the key to this regioselectivity was the interconversion of isomeric gold keteniminiun ions via a low-barrier π-complex transition state, which establishes a Curtin-Hammett scenario for isoxazole addition. By using benzisoxazoles as substrates, the reaction outcome could be switched to a formal [5+2] cyclization, leading to 1,4-oxazepines.

Dipyrrolonaphthyridinedione - (still) a mysterious cross-conjugated chromophore

Dipyrrolonaphthyridinediones (DPNDs) entered the chemical world in 2016. This cross-conjugated donor-acceptor skeleton can be prepared in two steps from commercially available reagents in overall yield ≈15-20% (5 mmol scale). DPNDs can be easily and regioselectively halogenated which opens an avenue to numerous derivatives as well as to π-expansion. Although certain synthetic limitations exist, the current derivatization possibilities provided impetus for numerous explorations that use DPNDs. Structural modifications enable bathochromic shift of the emission to deep-red region and reaching the optical brightness 30 000 M-1 cm-1. Intense absorption and strong emission of greenish-yellow light attracted the interest which eventually led to the discovery of their strong two-photon absorption, singlet fission in the crystalline phase and triplet sensitization. Dipyrrolonaphthyridinedione-based twistacenes broadened our knowledge on the influence of twisting angle on the fate of the molecule in the excited state. Collectively, these findings highlight the compatibility of DPNDs with various applications within organic optoelectronics.

Synthesis, Hydrolytic Stability and In Vivo Biological Study of Bioconjugates of the Tetrapeptides FELL Containing Pyrrole Moiety

BACKGROUND

Bioconjugates are promising alternatives for the multiple targeting of any disease. Pyrrole heterocycle is well known with many activities and is a building block of a lot of medical drugs. On the other hand, peptides are short molecules with many advantages such as small size, ability to penetrate the cell membrane and bond-specific receptors, vectorizing potential, etc. Thus, hybrid molecules between peptide and pyrrole moiety could be a promising alternative as an anti-pain tool.

METHODS

New bioconjugates with a general formula Pyrrole (α-/β-acid)-FELL-OH (NH2) were synthesized using Fmoc/OtBu peptide synthesis on solid support. HPLC was used to monitor the purity of newly synthesized bioconjugates. Their structures were proven by electrospray ionization mass spectrometry. The Paw Pressure test (Randall-Selitto test) was used to examinate the analgesic activity. Hydrolytic stability of targeted structures was monitored in three model systems with pH 2.0, 7.4 and 9.0, including specific enzymes by means of the HPLC-UV method.

RESULTS

The obtained results reveal that all newly synthesized bioconjugates have analgesic activity according to the used test but free pyrrole acids have the best analgesic activity.

CONCLUSIONS

Although free pyrrole acids showed the best analgesic activity, they are the most unstable for hydrolysis. Combination with peptide structure leads to the hydrolytic stabilization of the bioconjugates, albeit with slightly reduced activity.

Crystal structure and Hirshfeld surface analysis of (2Z)-3-oxo-N-phenyl-2-[(1H-pyrrol-2-yl)methylidene]butanamide monohydrate

In the title compound, C15H14N2O2·H2O, the 1H-pyrrole ring makes a dihedral angle of 59.95 (13)° with the phenyl ring. In the crystal, the mol-ecules are connected by C-H⋯O hydrogen bonds into layers parallel to the (020) plane, while two mol-ecules are connected to the water mol-ecule by two N-H⋯O hydrogen bonds and one mol-ecule by an O-H⋯O hydrogen bond. C-H⋯π and π-π inter-actions further link the mol-ecules into chains extending in the [01] direction and stabilize the mol-ecular packing. According to a Hirshfeld surface study, H⋯H (49.4%), C⋯H/H⋯C (23.2%) and O⋯H/H⋯O (20.0%) inter-actions are the most significant contributors to the crystal packing.

Spectroscopic and Computational pH Study of NiII and PdII Pyrrole-Imine Chelates with Human Serum Albumin

Human serum albumin (HSA) efficiently transports drugs in vivo: most are organic. Therefore, it is important to delineate the binding of small molecules to HSA. Here, for the first time, we show that HSA binding depends not only on the identity of the d8 metal ion, NiII or PdII, of their complexes with bis(pyrrole-imine), H2PrPyrr, but on the pH level as well. Fluorescence quenching data for native and probe-bound HSA showed that sites close to Trp-214 (subdomain IIA) are targeted. The affinity constants, Ka, ranged from ~3.5 × 103 M-1 to ~1 × 106 M-1 at 37 °C, following the order Pd(PrPyrr) > Ni(PrPyrr) at pH levels of 4 and 7; but Ni(PrPyrr) > Pd(PrPyrr) at a pH level of 9. Ligand uptake is enthalpically driven, dependent mainly on London dispersion forces. The induced CD spectra for the protein-bound ligands could be simulated by hybrid QM:MM TD-DFT methods, allowing us to delineate the binding site of the ligands and to prove that the metal chelates neither decompose nor demetallate after uptake by HSA. The transport and delivery of the metal chelates by HSA in vivo is therefore feasible.

Organo-photoredox-Catalyzed Selective Mono- and Bis-C-H Alkylation of Electron-Rich (Hetero)Arenes

Herein, we disclose a simple strategy for the C-H alkylation of electron-rich (hetero)arenes with alkyl bromides employing visible-light-mediated organo-photocatalytic SET processes. The generality of this method has been evidenced by the inclusion of a variety of alkyl radicals (α-alkyl-carbonyl, benzyl, cyanomethyl) as well as diverse biologically active electron-rich arenes and (hetero)arenes under mild conditions. The extent of alkylation with alkyl bromides was found to be controlled by introducing Zn(OAc)2 as a bromide scavenger, ensuring the blocking of potential bromo-arene byproduct formation under photoredox conditions. In addition, a sequential C-H alkylation strategy for selective bis-alkylation has also been developed via chronological incorporation of different alkyl radical precursors in one pot quite efficiently.

Crystallographic, spectroscopic and thermal studies of 1-(4-bromophenyl)-5-(2,5-dimethyl-1H-pyrrol-1-yl)-3-methyl-1H-pyrazole

The new title pyrrole-pyrazole derivative, C16H16BrN3, was synthesized through a citric acid-catalyzed Paal-Knorr reaction between acetonylacetone and 1-(4-bromophenyl)-3-methyl-1H-pyrazol-5-amine under mild reaction conditions. This synthetic protocol is noteworthy for its utilization of stoichiometric amounts of the reactants, an ecofriendly solvent and a cost-effective, non-toxic and biodegradable organocatalyst. A comprehensive understanding of the molecular structure was gained through spectroscopic, thermal and X-ray crystallographic analyses. The crystal structure is characterized by weak interactions, where only C-H...π connections contribute to the hydrogen-bond contacts. The supramolecular assembly is controlled by dispersion forces. However, the energy frameworks demonstrate that these forces act in three dimensions, providing enough stability, as observed in TGA-DSC (thermogravimetric analysis-differential scanning calorimetry) studies.

Regioselective Synthesis of 3-Trifluoromethylpyrroles by [3 + 2] Cycloaddition of N-Acyl α-Amino Acids and 2-Bromo-3,3,3-trifluoropropene

A mild and concise method for the synthesis of 3-trifluoromethylpyrroles via base-mediated [3 + 2] cycloaddition of N-acyl α-amino acids and 2-bromo-3,3,3-trifluoropropene is described. N-Acyl α-amino acids serve as 1,3-dipole precursors without additional activating agents directly. A high level of regioselectivity was observed, regardless of the electronic nature and size of the substituents on 1,3-dipoles.

Synthesis of N-alkoxycarbonyl Pyrroles from O-Substituted Carbamates: A Synthetically Enabling Pyrrole Protection Strategy

The condensation of readily available O-substituted carbamates with 2,5-dimethoxytetrahydrofuran gives N-alkoxycarbonyl pyrroles in a single step and in good yield. By this method, several common amine protecting groups can be introduced on the pyrrole nitrogen. With the exception of N-Boc, N-alkoxycarbonyl groups have seen only minimal use for protection of the pyrrole nitrogen to date. Here, we show that N-alkoxycarbonyl protection can endow pyrrole with distinct reactivity in comparison with N-sulfonyl protection, for example, in a pyrrole acylation protocol employing carboxylic acids with a sulfonic acid anhydride activator.

Gad S, M. El-Mehasseb I, E. El-Kelany K. Isosterism in pyrrole via azaboroles substitution, a theoretical investigation for electronic structural, stability and aromaticity

This work uses ab-initio CBS-QB3 and density functional theory (B3LYP) to analyze the structure, stability, and aromaticity of all isosteric nitrogen-boron pyrroles. The mono-NB unit substituted group of the isosteric NB pyrrole has four isosteres, whereas the multi-NB unit substituted group has two isosteres. These two groups make up all isosteric NB pyrrole. For structural, energetic, magnetic, and electron delocalization criteria, the results highlight the predominance of the PN3B2 isostere and its greater stability over other conformers. In addition, the global reactivity indices, ESP, HOMO-LUMO, and NBO charges have all been estimated to forecast the active side's electron donation and acceptance. These isosteres are categorized as weak electrophiles and marginal nucleophiles. NB-isosteres have poorer stability, HOMO-LUMO gap, and aromaticity than the parent (pyrrole). In general, NB compounds with more ring sharing are less aromatic than NB molecules with less ring sharing. The current study is anticipated to help in understanding of the chemistry of NB substituted molecules and their experimental identification and characterization.

Scalable Synthesis of Silacyclohexanones and Ready Access to Silicon Building Blocks

A simple and efficient two-step method for the synthesis of silacyclohexanones starting from bis(bromoethylsilanes) using TosMIC is presented. The prepared silacyclohexanones were transformed to nine different heterocycles with silicon incorporation. In addition, the developed methodology was used for the synthesis of a sila analogue of the HDAC6 inhibitor tubastatin A.

Rapid, room-temperature self-organization of polyarylated 1H-pyrroles from acetylenes and nitriles in the KOBut/DMSO system

We have discovered that three molecules of arylacetylene are rapidly (15 min) assembled with one molecule of nitrile at room temperature in the KOBut/DMSO system to afford 2-aryl-3-arylethynyl-4-aryl-5-benzyl-1H-pyrroles in up to 76% yield. We assume that this unprecedented self-organization process involves the cascade addition of acetylenic carbanions, first to the CN, then to the CC and CC bonds of the intermediates, followed by pyrrole ring closure via the intramolecular nucleophilic addition of the NH functional group to the CC bond of the final intermediates.

Azido-alkynylation of alkenes through radical-polar crossover

We report an azido-alkynylation of alkenes allowing a straightforward access to homopropargylic azides by combining hypervalent iodine reagents and alkynyl-trifluoroborate salts. The design of a photocatalytic redox-neutral radical polar crossover process was key to develop this transformation. A variety of homopropargylic azides possessing electron-rich and -poor aryls, heterocycles or ether substituents could be accessed in 34-84% yield. The products are synthetically useful building blocks that could be easily transformed into pyrroles or bioactive amines.

New pyrrole derivatives as DNA gyrase and 14α-demethylase inhibitors: Design, synthesis, antimicrobial evaluation, and molecular docking

An efficient one-pot reaction utilizing readily available chemical reagents was used to prepare novel 2-amino-1,5-diaryl-1H-pyrrole-3-carbonitrile derivatives and the structures of these compounds were validated by spectroscopic data and elemental analyses. All the synthetic compounds were evaluated for their antimicrobial activities (MZI assay). The tested compounds proved high activities on Staphylococcus aureus (Gram-positive bacteria) and Candida albicans (Pathogenic fungi). However, they did not show any activity on Escherichia coli (Gram-negative bacteria). The most effective compounds in MZI assay 7c, 9a, 9b, 11a, and 11b were selected to determine their MIC on S. aureus and C. albicans. Furthermore, DNA gyrase and 14-α demethylase inhibitory assays were performed to study the inhibitory activities of 7c, 9a, 9b, 11a, and 11b. The results illustrated that compound 9b was the most DNA gyrase inhibitor (IC50 of 0.0236 ± 0.45 µM, which was 1.3- fold higher than gentamicin reference IC50 values of 0.0323 ± 0.81 µM). In addition, compound 9b demonstrated the highest 14-α demethylase inhibitory effect with IC50 of 0.0013 ± 0.02 µM, compared to ketoconazole (IC50 of 0.0008 ± 0.03 µM) and fluconazole (IC50 of 0.00073 ± 0.01 µM), as antifungal reference drugs. Lastly, docking studies were performed to rationalize the dual inhibitory activities of the highly active compounds on both DNA gyrase and 14-α demethylase enzymes.

Copper-catalyzed three-component annulation toward pyrroles via the cleavage of two C-C bonds in 1,3-dicarbonyls

The first copper-catalyzed three-component annulation of α,β-unsaturated ketoximes, 1,3-dicarbonyls and paraformaldehyde has been documented. This novel strategy achieved the two C-C bond cleavage of 1,3-dicarbonyl compounds directly as a single-carbon synthon and provided a new and highly efficient method for the synthesis of 2,3-disubstituted pyrroles in moderate to good yields with broad functional group compatibility.

Bifurcated Rhodaelectro-catalyzed C-H Activation for the Synthesis of Pyrroles and Lactones

While electrocatalyzed alkyne annulations of arenes represent a powerful strategy for the assembly of heteroaromatic motives, electrochemical C(sp2)-H activations of alkenes remain scarce. Herein, a strategy for the rhodaelectro-catalyzed functionalization of enamides is presented for the efficient construction of pyrroles using electricity as a sustainable oxidant. Moreover, the tuning of the solvent system allowed a fascinating switch in chemoselectivity, which is not possible with traditionally used chemical oxidants, giving rise to lactone architectures. The rhoda-electrocatalysis features a broad scope as well as high regio- and chemoselectivities.

Construction of pyrroles, furans and thiophenes via intramolecular cascade desulfonylative/dehydrogenative cyclization of vinylidenecyclopropanes induced by NXS (X = I or Br)

Pyrroles, furans, and thiophenes are important structural motifs in biologically active substances, pharmaceuticals and functional materials. In this paper, we disclose an efficient synthetic strategy for the rapid construction of multisubstituted pyrroles, furans, and thiophenes via NXS mediated desulfonylative/dehydrogenative cyclization of vinylidenecyclopropanes (VDCPs). The advantages of this method include wide substrate range, high efficiency and synthetic usefulness of the heterocyclic products under metal-free and mild conditions. The derivatization of pyrrole products and the preparation of functional molecules successfully demonstrated the synthetic potential of the products as platform molecules. The reaction mechanism has been investigated on the basis of control experiments and DFT calculations.

Zr-Catalyzed Synthesis of Tetrasubstituted 1,3-Diacylpyrroles from N-Acyl α-Aminoaldehydes and 1,3-Dicarbonyls

A Zr-catalyzed synthesis of tetrasubstituted 1,3-diacylpyrroles is reported that employs the direct use of N-acyl α-aminoaldehydes with 1,3-dicarbonyl compounds. The products were formed in up to 88% yield and shown to be hydrolytically and configurationally stable under the reaction conditions (THF/1,4-dioxane and H₂O). The N-acyl α-aminoaldehydes were readily prepared from the corresponding α-amino acids. The reaction tolerates a wide array of substrate types including alkyl-, aryl-, heteroaryl-, and heteroatom-containing groups on the aminoaldehyde side chain. A variety of 1,3-dicarbonyls proved amenable to the reaction along with an aldehyde derived from a l,l-dipeptide, an aldehyde generated in situ, and an N-acylated glucosamine.

Electrochemical Organoselenium Catalysis for the Selective Activation of Alkynes: Easy Access to Carbonyl-pyrroles/oxazoles from N-Propargyl Enamines/Amides

Intramolecular electro-oxidative addition of enamines or amides to nonactivated alkynes was attained to access carbonyl-pyrroles or -oxazoles from N-propargyl derivatives. Organoselenium was employed as the electrocatalyst, which played a crucial role as a π-Lewis acid and selectively activated the alkyne for the successful nucleophilic addition. The synthetic strategy permits a wide range of substrate scope up to 93% yield. Several mechanistic experiments, including the isolation of a selenium-incorporated intermediate adduct, enlighten the electrocatalytic pathway.

In silico and in vitro assessment of anti-Trypanosoma cruzi efficacy, genotoxicity and pharmacokinetics of pentasubstituted pyrrolic Atorvastatin-aminoquinoline hybrid compounds

Atorvastatin (AVA) is a third-generation statin with several pleiotropic effects, considered the last synthetic pharmaceutical blockbuster. Recently, our group described the effects of AVA on DNA damage prevention and against Trypanosoma cruzi infection. In this study, our aim was to evaluate the efficacy, safety, and in silico pharmacokinetic profile of four hybrids of aminoquinolines with AVA 4a-d against T. cruzi using in vitro and in silico models. These synthetic compounds were designed by hybridization of the pentapyrrolic moiety of AVA with the aminoquinolinic unit of chloroquine or primaquine. Pharmacokinetics (ADME) and toxicity parameters were predicted by SwissADME, admetSAR and LAZAR in silico algorithms. The trypanocidal activity of AVA-quinoline hybrids were evaluated in vitro against amastigotes and trypomastigotes of T. cruzi, from Y (Tc II) and Tulahuen (Tc VI) strains. In vitro cardiocytotoxicity was assessed using primary cultures of mouse embryonic cardiac cells and in vitro hepatocytotoxicity on bidimensional and 3D-cultured HepG2 cells. Genotoxicity was evaluated by Ames test and micronucleus assay. Despite the overall good in silico ADMET profile, all tested compounds were predicted to be hepatotoxic. All hybrid derivatives presented high trypanocidal activity, against both trypomastigote and intracellular forms of T. cruzi, presenting EC₅₀'s lower than 1 µM besides superior selectivity than the reference drug, without evidences of cardiotoxicity in vitro. The compounds 4a and 4b presented a time-dependent toxicity in monolayer culture of HepG2 but no detectable toxic effects in their spheroids, opposing to the in silico prediction. We can conclude that the AVA-aminoquinoline hybrids presented a hit profile as antiparasitic agents in synthetic pharmaceutical innovation platforms.

Silver-Catalyzed Domino Reaction of CF3-Substituted N-Allenamides with Primary Amines for the Construction of 2-Amido-5-fluoropyrroles

We report herein a domino reaction to construct 2-amido-5-fluoropyrroles from CF₃-substituted N-allenamides. The in situ generated gem-difluorinated ene-ynamides derived from CF₃-substituted N-allenamides, when subjected to silver catalysis with a primary amine, undergo simultaneous hydroamination of the ynamide moiety followed by a 5-endo-trig addition/β-fluoride elimination sequence, enabling the construction of 2-amido-5-fluoropyrroles. This transformation features excellent functional group compatibility. By employing 2-aminophenols, functionalized benzo-oxazoles were produced.

Modular Synthesis of Tetrasubstituted Pyrroles through a Four-Component Cyclization Strategy Using Ammonium Salt as the Nitrogen Source

A four-component synthesis of tetrasubstituted pyrroles was developed under metal-free conditions. The pyrrole ring was formed in one pot through [2 + 1 + 1 + 1] condensation using ammonium salt as the nitrogen source. In this strategy, 1,4-naphthoquinones and maleimides were used as the versatile C2 fragments to provide substituted benzo[f]isoindole-4,9-diones and pyrrolo[3,4-c]pyrrole-1,3-diones, respectively. This work is highlighted by using ammonium salt as the nitrogen source, readily available starting materials and multibond formation (two C-C and two C-N bonds) in a single operation.

One-Pot Synthesis of Unprotected 2-Acylpyrroles from 1,2,3-Triazoles and 2-Hydroxymethylallyl Carbonates

An efficient, tandem one-pot approach to synthesize multisubstituted 2-acylpyrroles from readily prepared N-tosyl triazoles and 2-hydroxymethylallyl carbonates is reported. The reaction proceeds via Rh(II)-catalyzed O-H insertion, [3,3]-sigmatropic rearrangement, Pd(0)-catalyzed oxidative addition, intramolecular cyclization, DBU-promoted E1cB elimination, double bond isomerization, and aromatization, enabling the disconnection and formation of multiple bonds in one reactor. The approach represents a highly regioselective way to access di-, tri-, and tetra-substituted NH pyrroles with high efficiency.

Pyrrole-based EGFR inhibitors for the treatment of NCSLC: Binding modes and SARs investigations

The treatment of advanced non-small cell lung cancer (NSCLC) has made substantial progress due to the rapid development of small molecule targeted therapy, with dramatically prolonged survival. As an effective drug for the treatment of NSCLC, epidermal growth factor receptor (EGFR) inhibitors are currently experiencing issues like severe adverse events and drug resistance. It is urgent to develop novel types of EGFR inhibitors to overcome the abovementioned limitations. Pyrrole always works well as a probe for the creation of novel medication candidates for hard-to-treat conditions like lung cancer. Although the design, synthesis, and biological assays of pyrrole derivatives have been reported, their inhibitory actions against the receptor tyrosine kinase (RTK) EGFR have not been in-depthly studied. This review highlights the small molecule EGFR inhibitors containing pyrrole heterocyclic pharmacophores in recent years, and the research on their mechanism, biological activity, and structure-activity relationship (SAR).

Modular Synthesis of Highly Substituted 3-Azapyrroles by Rh(II)-Catalyzed N-H Bond Insertion and Cyclodehydration

A modular synthesis of highly substituted 3-azapyrroles has been developed using a three-step sequence comprising copper-catalyzed alkyne-azide cycloaddition (CuAAC), N-H bond insertion, and cyclodehydration. 1-Sulfonyl-1,2,3-triazoles (1-STs) can be accessed from common alkyne and sulfonyl azide building blocks by CuAAC using CuTC. Rhodium(II)-acetate-promoted 1-ST denitrogenation results in highly electrophilic rhodium azavinyl carbenes that, here, underwent insertion into the N-H bond of secondary α-aminoketones to form 1,2-aminoalkenes. These products were cyclized and dehydrated using BF₃·OEt₂ into highly substituted 3-azapyrroles. The three steps (CuAAC, N-H bond insertion, and cyclodehydration) could be telescoped into a one-pot process. The method proved to be highly efficient and tolerated a wide range of substituents.

1-Methylimidazole as an Organic Catalyst for [3+3]-Cyclodimerization of Acylethynylpyrroles to Bis(acylmethylidene)dipyrrolo[1,2-a:1′,2′-d]pyrazines

Acylethynylpyrroles, now readily available by the cross-coupling of pyrroles with acylbromoacetylenes in solid Al2O3 media, in the presence of 1-methylimidazole underwent unprecedentedly easy (40–45 °C) cyclodimerization into bis(acylmethylidene)dipyrrolo[1,2-a:1′,2′-d]pyrazines in up to 51% yield. Some other organic and inorganic basic catalysts can also trigger this cyclodimerization, but less efficiently.