Recent advances in the syntheses of pyrroles

Base-Mediated Synthesis of Polysubstituted Pyrroles from N-Allyl Ketimines and Alkynes: Interplay of Carbanions

Terminal (het)arylacetylenes react (KOBut/DMSO, 60 °C, 1 h) with N-allyl ketimines to afford 2-(het)aryl-4-(het)arylmetyl-5-ethylpyrroles in up to 71% yield as a result of the interaction of acetylenic and azadienic carbanions with C=N and C≡C bonds. This new reaction opens a one-pot access to synthetically and pharmaceutically prospective compounds.

Ligand Assisted Co(II)-Catalyzed Multicomponent Synthesis of Substituted Pyrroles and Pyridines

Herein, we describe a sustainable Co(II)-catalyzed synthesis of pyrroles and pyridines. Using a Co(II)-catalyst [CoII 2(La)2Cl2] (1 a) bearing redox-active 2-(phenyldiazenyl)-1,10-phenanthroline) (La) scaffold, various substituted pyrroles and pyridines were synthesized in good yields, taking alcohol as one of the primary feedstock. Pyrroles were synthesized by the equimolar reaction of 2-amino and secondary alcohols. A series of 2,4,6-substituted symmetrical pyridines were prepared via a three-component reaction of NH4OAc with 1 : 2.2 molar primary and secondary alcohols, respectively. Unsymmetrically substituted 2,4,6-trisubstituted, 2,4,5,6-tetrasubstituted, and 2,3,4,5,6-pentasubstituted pyridines were achieved via a multi-component coupling reaction of alcohols and NH4OAc. Catalyst 1 a showed encouraging results during the gram-scale synthesis of these N-heterocycles. Mechanistic investigation revealed synergistic involvement of cobalt metal and the ligand during the catalytic reactions.

Rhodium/selenium dual catalysis for accessing 2-aminopyrroles from N-sulfonyl-1,2,3-triazoles

Herein, we report a novel rhodium/selenium dual catalytic process for the synthesis of 2-aminopyrroles from N-sulfonyl-1,2,3-triazoles. The proposed cooperative catalytic mechanism involves Rh(II)-catalyzed formation of Rh-azavinyl carbene from triazole, followed by selenium-catalyzed generation of ylide, which subsequently undergoes annulation with another Rh-azavinyl carbene. The simple and mild dual catalytic strategy accommodates a variety of electron-withdrawing and electron-donating functional groups, affording various 2-aminopyrrole derivatives in moderate to good yields.

Catalytic Asymmetric De Novo Synthesis of Chiral Pyrroles Through Desymmetrizing Oxidative [3+2]-Cycloaddition and the Van Leusen Reaction

Central chirality in heteroarene derivatives arising from unsymmetrically substituted heteroarene rings is an intriguing but underexplored topic. Herein, we reported the first catalytic enantioselective de novo construction of centrally chiral pyrroles through desymmetrizing oxidative [3+2]-cycloaddition by employing silver catalysis. This judicious desymmetrization can produce at least four continuous stereogenic centers without creating any additional stereocenter. Furthermore, to introduce a more diverse set of substituents, we developed the first catalytic asymmetric Van Leusen reaction with α-substituted TosMIC for the synthesis of centrally chiral pyrroles. A wide range of polycyclic 2-substituted, 3,4-fused pyrroles were obtained in high yields and with good to high enantioselectivities. This report includes the elaboration of methanobenzo[f]isoindole to synthetically challenging building block chiral isoindole compounds, which are synthesized enantioselectively for the first time.

Highly Diastereoselective Synthesis of 2,3-Dihydropyrroles via Formal [4 + 1] Annulation Reaction of α,β-Unsaturated Imines and in Situ Generated Pyridinium Ylide

We report a cascade approach for the synthesis of 2,3-dihydropyrroles derivatives via a formal [4 + 1] annulation reaction of α,β-unsaturated imines with in situ generated pyridinium ylides. Importantly, this protocol is compatible with diverse substituted imines as well as pyridinium ylides, constructing 2,3-dihydropyrroles with excellent yield and selectivity. Thereafter, the Merrifield resin-supported pyridinium ylide as a potential C1 synthon was also employed in our strategy and reused several times, resulting in products with excellent yield and diastereoselectivity.

Unlocking the Potential of Pyrrole: Recent Advances in New Pyrrole-Containing Compounds with Antibacterial Potential

Nitrogen heterocycles are valuable structural elements in the molecules of antibacterial drugs approved and used to treat bacterial infections. Pyrrole is a five-atom heterocycle found in many natural compounds with biological activity, including antibacterial activity. Numerous compounds are being develop based on the pyrrole heterocycle as new potential antibacterial drugs. Due to the phenomenon of antibacterial resistance, there is a continuous need to create new effective antibacterials. In the scientific literature, we have identified the most relevant studies that aim to develop new compounds, such as pyrrole derivatives, that are proven to have antibacterial activity. Nature is an endless reservoir of inspiration for designing new compounds based on the structure of pyrrole heterocycles such as calcimycin, lynamycins, marinopyrroles, nargenicines, phallusialides, and others. However, many other synthetic compounds based on the pyrrole heterocycle have been developed and can be optimized in the future. The identified compounds were classified according to the type of chemical structure. The chemical structure-activity relationships, mechanisms of action, and antibacterial effectiveness of the most valuable compounds were highlighted. This review highlights scientific progress in designing new pyrrole-containing compounds and provides examples of lead compounds that can be successfully optimized further.

Cascade [3 + 2] Annulation of 1,3-Dicarbonyl Compounds and Ethyl Secondary Amines for Pyrrole Synthesis via Poly C(sp3)-H Bond Functionalization

The synthesis of polyfunctionalized pyrroles via the cascade reactions of 1,3-dicarbonyl compounds and two molecules of ethyl secondary amines has been realized via simple iodine catalysis in the presence of Dess-Martin periodinane (DMP). The formation of the target pyrrole products involves the formation of one new C-C and two new C-N bonds via the major functionalization of six C(sp3)-H bonds, presenting a highly novel and efficient synthetic protocol toward pyrrole scaffolds.

Synthesis of chiral systems featuring the pyrrole unit: a review

Synthetic strategies towards pyrroles within chiral frameworks are summarised, focussing on reports published 2010-2023. The synthesis of pyrroles featuring substituents bearing chiral centres are summarised, as are those whereby pyrroles are located within axially chiral systems courtesy of restricted bond rotation.

Design, synthesis and Anti-PVY activity of planar chiral thiourea derivatives incorporated with [2.2]Paracyclophane

BACKGROUND

Potato virus Y (PVY) is a prominent representative of plant viruses. It can inflict severe damage upon Solanaceae plants, leading to global dissemination and substantial economic losses. To discover new antiviral agents, a class of planar chiral thiourea molecules through the key step of N-heterocyclic carbene-catalyzed nitrile formation reaction was synthesized with excellent optical purities for antiviral evaluations against plant virus PVY.

RESULTS

The absolute configurations of the planar chiral compounds exhibited obvious distinctions in the anti-PVY activities. Notability, compound (S)-4u exhibited remarkable curative activities against PVY, with a half maximal effective concentration (EC50) of 349.3 μg mL-1, which was lower than that of the ningnanmycin (NNM) (EC50 = 400.8 μg mL-1). Additionally, The EC50 value for the protective effects of (S)-4u was 146.2 μg mL-1, which was superior to that of NNM (276.4 μg mL-1). Furthermore, the mechanism-of-action of enantiomers of planar chiral compound 4u was investigated through molecular docking, defensive enzyme activity tests and chlorophyll content tests.

CONCLUSION

Biological mechanism studies have demonstrated that the configuration of planar chiral target compounds plays a crucial role in the molecular interaction with PVY-CP, enhancing the activity of defense enzymes and affecting chlorophyll content. The current study has provided significant insights into the roles played by planar chiralities in plant protection against viruses. This paves the way for the development of novel green pesticides bearing planar chiralities with excellent optical purities. © 2024 Society of Chemical Industry.

Divergent Synthesis of Unsymmetrical Bis-heteroaryl Ketones via Base-Promoted Cascade Reactions of 1,2-Alkynedione-Derived N-Propargylic β-Enaminones

Herein we disclose a transition-metal-free, one-pot two-step strategy for the synthesis of unsymmetrical bis-heteroaryl ketones. N-propargylic β-enaminones generated by the Michael addition of propargylamines onto heteroaryl 1,2-alkynediones have been utilized as synthetic equivalents of pyridine or pyrrole scaffolds. The use of alcohol as a solvent resulted in the formation of 2-alkoxylated pyridine scaffold, whereas the use of DMSO promoted the formation of a pyrrole motif.

Insights into the Biological Activities and Substituent Effects of Pyrrole Derivatives: The Chemistry-Biology Connection

Pyrrole, with its versatile heterocyclic ring structure, serves as a valuable template for generating a diverse range of lead compounds with various pharmacophores. Researchers and scientists globally are intrigued by pyrrole and its analogs for their broad pharmacological potential, prompting thorough investigations aimed at advancing human welfare. This comprehensive review delves into the diverse activities exhibited by pyrrole compounds, encompassing their synthesis, reactions, and pharmacological properties alongside their derivatives. In addition to detailing the characteristics of pyrrole and its derivatives within the context of green chemistry, the review also examines microwave-assisted reactions. It provides insights into their chemical structures, natural occurrences, and potential applications across various domains. Furthermore, the article investigates structural alterations of pyrrole compounds and their implications on their functionality, highlighting their versatility as foundational elements for both functional materials and bioactive compounds. The review emphasizes the need for ongoing research and development in the field of pyrrole compounds to discover new activities and benefits.

Enal-azomethine ylides: application in the synthesis of functionalized pyrroles

Rhodium-catalyzed [3 + 2] annulation of diazoenals and N-alkyl imines resulted in N-alkyl-pyrrole-3-carbaldehyde derivatives. The reaction involves thermal 6π-electrocyclization and aromatization of a new class of enal-azomethine ylides (EAYs). The EAYs derived from dihydroisoquinoline and 2H-azirine gave fused-pyrrole and pyridine derivatives, respectively. The synthetic importance of pyrrole products has been demonstrated by one-step synthesis of the biologically relevant pyrrolo[3,2-c]quinoline scaffold as well as pyrrolo[2,1-a]isoquinoline which is a core structure of lamellarin alkaloids.

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.

Skeletal Rearrangement of Oxazole to Azepine and Pyrrole through Dynamic 8π Electrocyclizations

We present a novel approach for the skeletal rearrangement of an oxazole into an azepine and pyrrole through a dynamic electrocyclization process, showing an innovative, unconventional reaction sequence. This method enables precise control of regioselectivity in competitive 6π and 8π electrocyclization reactions, rendering the final products rich in functional groups that can be further developed for the synthesis of nitrogen-containing scaffolds. This is an unprecedented example of the selective synthesis of seven- and five-member heterocycles via dynamic electrocyclization ring opening or closure.

N-substituted pyrrole carboxylic acid derivatives from 3,4-dihydroxyketons

Since the chemical industry is largely dependent on petrol-based feedstocks, new sources are required for a sustainable industry. Conversion of biomass to high-value compounds provides an environmentally friendly and sustainable approach, which might be a potential solution to reduce petrol-based starting materials. This also applies for N-heterocycles, which are a common structural motif in natural products, pharmaceuticals and functional polymers. The synthesis of pyrroles is a well-studied and established process. Nevertheless, most routes described are not in line with the principles of green and sustainable chemistry and employ harsh reaction conditions and harmful solvents. In this study, 3,4-dihydroxyketons are used as excellent platform chemicals for the production of N-substituted pyrrole-2-carboxylic- and pyrrole-2,5-dicarboxylic acids, as they can be prepared from glucose through the intermediate d-glucarate and converted into pyrrolic acid derivatives under mild conditions in water. The scope of this so far unknown reaction was examined using a variety of primary amines and aqueous ammonium chloride leading to pyrrolic acid derivatives with N-substituents like alkane-, alkene-, phenyl- and alcohol-groups with yields up to 20 %. The combination of both, enzymatic conversion and chemical reaction opens up new possibilities for further process development. Therefore, a continuous chemo-enzymatic system was set up by first employing an immobilized enzyme to catalyze the conversion of d-glucarate to the 3,4-dihydroxyketone, which is further converted to the pyrrolic acid derivatives by a chemical step in continuous flow.

Discovery of Novel Chiral Indole Derivatives Containing the Oxazoline Moiety as Potential Antiviral Agents for Plants

Potato virus Y (PVY) is an important plant virus that has spread worldwide, causing significant economic losses. To search for novel structures as potent antiviral agents, a series of chiral indole derivatives containing oxazoline moieties were designed and synthesized and their anti-PVY activities were evaluated. Biological activity tests demonstrated that many chiral compounds exhibited promising anti-PVY activities and that their absolute configurations exhibited obvious distinctions in antiviral bioactivities. Notably, compound (S)-4v displayed excellent curative and protective efficacy against PVY, with EC50 values of 328.6 and 256.1 μg/mL, respectively, which were superior to those of commercial virucide ningnanmycin (NNM, 437.4 and 397.4 μg/mL, respectively). The preliminary antiviral mechanism was investigated to determine the difference in antiviral activity between the two enantiomers of 4v chiral compounds. Molecular docking indicated a stronger binding affinity between the coating proteins of PVY (PVY-CP) and (S)-4v (-6.5 kcal/mol) compared to (R)-4v (-6.2 kcal/mol). Additionally, compound (S)-4v can increase the chlorophyll content and defense-related enzyme activities more effectively than its enantiomer. Therefore, this study provides an important basis for the development of chiral indole derivatives containing oxazoline moieties as novel agricultural chemicals.

Toward Biomass-Based Organic Electronics: Continuous Flow Synthesis and Electropolymerization of N-Substituted Pyrroles

Pyrroles are foundational building blocks in a wide array of disciplines, including chemistry, pharmaceuticals, and materials science. Currently sourced from nonrenewable fossil sources, there is a strive to explore alternative and sustainable synthetic pathways to pyrroles utilizing renewable feedstocks. The utilization of biomass resources presents a compelling solution, particularly given that several key bulk and fine chemicals already originate from biomass. For instance, 2,5-dimethoxytetrahydrofuran and aniline are promising candidates for biomass-based chemical production. In this study, we present an innovative approach for synthesizing N-substituted pyrroles by modifying the Clauson-Kaas protocol, starting from 2,5-dimethoxytetrahydrofuran as the precursor. The developed methodology offers the advantage of producing pyrroles under mild reaction conditions with the potential for catalyst-free reactions depending upon the structural features of the substrate. We devised protocols suitable for both continuous flow and batch reactions, enabling the conversion of a wide range of anilines and sulfonamides into their respective N-substituted pyrroles with good to excellent yields. Moreover, we demonstrate the feasibility of depositing thin films of the corresponding polymers onto electrodes through in situ electropolymerization. This innovative application showcases the potential for sustainable, biomass-based organic electronics, thus, paving the way for environmentally friendly advancements in this field.

I2-catalyzed synthesis of 3-aminopyrrole with homopropargylic amines and nitrosoarenes

The synthesis of 3-aminopyrrole using the amination reagent nitrosoarenes and homopropargylic amines catalyzed by I2 through cyclization and amination has been developed. This protocol features excellent functional group tolerance and mild reaction conditions, yielding 3-aminopyrroles in moderate to good yields without a metal catalyst. This method realizes the construction and amination of the 3-aminopyrroles in which nitrosoarenes serve as the amine source and oxidant.

Functionalized 2,3'-Bipyrroles and Pyrrolo[1,2-c]imidazoles from Acylethynylpyrroles and Tosylmethylisocyanide

An efficient method for the synthesis of pharmaceutically prospective but still rare functionalized 2,3'-bipyrroles (in up to 80% yield) by the cycloaddition of easily available acylethynylpyrroles with tosylmethylisocyanide (TosMIC) has been developed. The reaction proceeds under reflux (1 h) in the KOH/THF system. In the t-BuONa/THF system, TosMIC acts in two directions: along with 2,3'-bipyrroles, the unexpected formation of pyrrolo[1,2-c]imidazoles is also observed (products ratio~1:1).

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.

Metal-Free Catalyzed Oxidation/Decarboxylative [3+2] Cycloaddition Sequences of 3-Formylchromones to Access Pyrroles with Anti-Cancer Activity

An efficient and direct approach to pyrroles was successfully developed by employing 3-formylchromones as decarboxylative coupling partners, and facilitated by microwave irradiation. The protocol utilizes easily accessible feedstocks, a catalytic amount of DBU without any metals, resulting in high efficiency and regioselectivity. Notably, all synthesized products were evaluated against five different cancer cell lines and compound 3l selectively inhibited the proliferation of HCT116 cells with an IC50 value of 10.65 μM.

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.

Latest Advances in Highly Efficient Dye-Based Photoinitiating Systems for Radical Polymerization

Light-activated polymerization is one of the most important and powerful strategies for fabrication of various types of advanced polymer materials. Because of many advantages, such as economy, efficiency, energy saving and being environmentally friendly, etc., photopolymerization is commonly used in different fields of science and technology. Generally, the initiation of polymerization reactions requires not only light energy but also the presence of a suitable photoinitiator (PI) in the photocurable composition. In recent years, dye-based photoinitiating systems have revolutionized and conquered the global market of innovative PIs. Since then, numerous photoinitiators for radical polymerization containing different organic dyes as light absorbers have been proposed. However, despite the large number of initiators designed, this topic is still relevant today. The interest towards dye-based photoinitiating systems continues to gain in importance, which is related to the need for new initiators capable of effectively initiating chain reactions under mild conditions. In this paper we present the most important information about photoinitiated radical polymerization. We describe the main directions for the application of this technique in various areas. Attention is mainly focused on the review of high-performance radical photoinitiators containing different sensitizers. Moreover, we present our latest achievements in the field of modern dye-based photoinitiating systems for the radical polymerization of acrylates.