Recent Advances in Functionalization of Pyrroles and their Translational Potential

Palladium-Catalyzed Solvent-Controlled Divergent C2/C5 Site-Selective Alkynylation of Pyrrole Derivatives

Among the known aromatic N-heterocycles, pyrroles are significant and versatile privileged components in pharmacologically relevant molecules. Herein, we demonstrate a protocol for the selective construction of alkynylated pyrroles in a diversity-oriented fashion through divergent C2/C5 site-selective alkynylation of pyrrole derivatives by employing a palladium catalyst with two different solvent systems. In the presence of 1,4-dioxane, the C2-alkynylation process via chelation-assisted palladation is favored. However, the use of a dimethylformamide/dimethyl sulfoxide (DMF/DMSO) solvent system could override the chelation effect of a weak ester-directing group, favoring the C5-alkynylation process via electrophilic palladation. Late-stage modification of biologically relevant scaffolds and further derivatization of the products highlighted the potential utility and importance of the solvent-controlled regiodivergence.

Recent advances in the electrochemical functionalization of N-heterocycles

Nitrogen-containing heterocyclic cores are of immense importance due to their high abundance in naturally occurring or synthetic molecules having wide applications in different fields of basic and applied sciences. The functionalities introduced in an N-heterocyclic core play an important role in regulating the physiochemical behavior of the particular N-heterocycles to alter their chemical and biological reactivity. Suitably functionalized N-heterocycles demonstrate their widespread applications in pharmaceuticals, agronomy, materials sciences, synthetic chemistry, pigments, etc. During the last decade, electrochemistry has emerged as a sustainable alternative to conventional synthetic approaches by minimizing reagent uses and chemical waste. Synthetic chemists have extensively utilized the tool to functionalize N-heterocycles. This is evidenced by the appearance of more than a hundred methods on the topic over recent years, signifying the importance of the synthetic area. This review is focused on the accumulation of synthetic methods based on the electrochemical functionalization of N-heterocycles developed over the recent decade. Literature reports on the C-/N-H-functionalization and functional modifications of N-heterocycles that are accessible through the available search engines are included in the review. Relevant mechanistic details in support of the reported reactions are discussed to present a clear picture of the reaction pathways. The review aims to provide a clear picture of the possible pathways of electron transfer, the electrochemical behavior of different N-heterocyclic cores, functionalization reagents, and the chemical processes that occur during the electrochemical functionalization/modification of N-heterocycles.

Enantioselective synthesis of α-(3-pyrrolyl)methanamines with an aza-tetrasubstituted center under metal-free conditions

Construction of a chiral methanamine unit at the C3 position of pyrrole is highly desirable; nevertheless, it remains challenging due to its intrinsic electronic properties. Herein, we present an operationally straightforward and direct asymmetric approach for accessing α-(3-pyrrolyl)methanamines under benign organocatalytic conditions for the first time. The one-pot transformation proceeds smoothly through an amine-catalyzed direct Mannich reaction of succinaldehyde with various endo-cyclic imines, followed by a Paal-Knorr cyclization with a primary amine. Several N-H/alkyl/Ar α-(3-pyrrolyl)methanamines with an aza-tetrasubstituted center have been synthesized with good yields and excellent enantioselectivity.

Divergent synthesis of pyrrolizine derivatives through C-H bond functionalization of pyrroles

Presented herein is the synthesis of diversely functionalized pyrrolizines from the reaction of N-alkoxycarbamoyl pyrroles with CF3-ynones. The formation of the product is based on a C-H bond activation-initiated cascade process including N-alkoxycarbamoyl group-directed alkenylation of the pyrrole scaffold followed by simultaneous intramolecular nucleophilic addition along with cleavage and transfer of the directing group. By taking advantage of the rich chemistry of the transferred alkoxycarbamoyl moiety, the products could be transformed into a series of structurally and biologically interesting pyrrolizine derivatives. To our knowledge, this is the first example in which the N-alkoxycarbamoyl unit acted as a transferable and transformable directing group for the divergent synthesis of pyrrolizines.

Controllable Skeletal and Peripheral Editing of Pyrroles with Vinylcarbenes

The skeletal editing of azaarenes through insertion, deletion, or swapping of single atoms has recently gained considerable momentum in chemical synthesis. Here, we describe a practical skeletal editing strategy using vinylcarbenes in situ generated from trifluoromethyl vinyl N-triftosylhydrazones, leading to the first dearomative skeletal editing of pyrroles through carbon-atom insertion. Furthermore, depending on the used catalyst and substrate, three types of peripheral editing reactions of pyrroles are also disclosed: α- or γ-selective C-H insertion, and [3+2] cycloaddition. These controllable molecular editing reactions provide a powerful platform for accessing medicinally relevant CF3-containing N-heterocyclic frameworks, such as 2,5-dihydropyridines, piperidines, azabicyclo[3.3.0]octadienes, and allylated pyrroles from readily available pyrroles. Mechanistic insights from experiments and density functional theory (DFT) calculations shed light on the origin of substrate- or catalyst-controlled chemo- and regioselectivity as well as the reaction mechanism.

Copper-Mediated Cyclization of Terminal Alkynes with CF3-Imidoyl Sulfoxonium Ylides To Construct 5-Trifluoromethylpyrroles

A copper-mediated [3 + 2] cyclization of CF3-imidoyl sulfoxonium ylides and terminal alkynes has been demonstrated. This work provides a practical approach for assembling 5-trifluoromethylpyrroles with the merits of a broad substrate scope, good functional tolerance, and mild reaction conditions. Control experiments and DFT studies indicate that this reaction may involve the addition of π-bonds of terminal alkynes by copper-carbene radicals and hydrogen migration.

Synthesis, Thermal Stability and Antifungal Evaluation of Two New Pyrrole Esters

To develop new chemicals that are stable at high temperatures with biological activity, a pyrrole intermediate was firstly synthesized using glucosamine hydrochloride as raw materials through cyclization and oxidation. Further, two novel pyrrole ester derivatives were prepared via Steglich esterification from pyrrole intermediate with vanillin and ethyl maltol, respectively. Nuclear magnetic resonance (1 H-NMR, 13 C NMR), infrared spectroscopy (IR) and high resolution mass spectrometry (HRMS) were used to confirm the target compounds. Thermal behavior of the compounds was investigated by thermogravimetry (TG), differential scanning calorimeter (DSC) and the pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) methods. The plausible pyrolytic mechanism was proposed. Additionally, their biological activities against the pathogens Fusarium graminearum, Fusarium oxysporum, Fusarium moniliforme, Phytophthora nicotianae, and Rhizoctonia solani were assessed. These target compounds showed outstanding antifungal activities and the highest inhibitor rates of 62.50 % and 68.75 % against R. solani with EC50 values of 0.0296 and 0.0200 mg mL-1 , respectively. SDHI protein sequence was molecularly docked to identify the binding mechanisms in the active pocket and examine the interactions between both the molecules and the SDHI protein.

One-Carbon Ring Expansion of Indoles and Pyrroles: A Straightforward Access to 3-Fluorinated Quinolines and Pyridines

3-Fluorinated quinolines and pyridines are prevalent pharmacophores, yet their synthesis is often challenging. Herein, we demonstrate that dibromofluoromethane as bromofluorocarbene source enables the one-carbon ring expansion of readily available indoles and pyrroles to structurally diverse 3-fluorinated quinolines and pyridines. This straightforward protocol requires only a short reaction time of ten minutes and can be performed under air atmosphere. Preliminary investigations reveal that this strategy can also be applied to the synthesis of other valuable azines by using different 1,1-dibromoalkanes as bromocarbene sources.

3-Trifluoromethyl Pyrrole Synthesis Based on β-CF3-1,3-Enynamides

A new metal-free method for the rapid, productive, and scalable preparation of 3-trifluoromethyl pyrroles has been developed. It is based on the electrophilic nature of the double bond of β-CF3-1,3-enynamides due to the electron-withdrawing characteristics of the trifluoromethyl groups and the strong nucleophilic nature of alkyl primary amines. Evidence for the highly regioselective 1,4-hydroamination was observed after the isolation and characterization of the allenamide intermediate.

Four-Component Ring-Opening Reaction of Pyrroles via C-N Bond Cleavage under Multiple Functions of Elemental Sulfur

We report a four-component ring-opening reaction of pyrroles via C-N bond cleavage. In this process, elemental sulfur is used as the sulfur source of thiazole and thioamide and the reductant of olefin. A series of benzothiazoles functionalized with thiopropionamides at the C2 position were synthesized using this method. A plausible reaction mechanism is proposed based on the concise control experiments.

Enantioselective Direct Synthesis of C3-Hydroxyalkylated Pyrrole via an Amine-Catalyzed Aldol/Paal-Knorr Reaction Sequence

Creating functionality with chirality at position C3 of pyrrole is challenging. An operationally simple organocatalytic method has been developed to generate functionality with a chiral tertiary/quaternary stereocenter at position C3 of pyrrole. The process proceeds through an amine-catalyzed direct aldol reaction of succinaldehyde with various acceptor carbonyls, followed by a Paal-Knorr reaction with a primary amine in the same pot. A series of chiral C3-hydroxyalkylated N-alkyl/Ar/H-pyrroles have been synthesized for the first time with good to high yields and excellent enantioselectivity.

Prediction of Antileishmanial Compounds: General Model, Preparation, and Evaluation of 2-Acylpyrrole Derivatives

In this work, the SOFT.PTML tool has been used to pre-process a ChEMBL dataset of pre-clinical assays of antileishmanial compound candidates. A comparative study of different ML algorithms, such as logistic regression (LOGR), support vector machine (SVM), and random forests (RF), has shown that the IFPTML-LOGR model presents excellent values of specificity and sensitivity (81-98%) in training and validation series. The use of this software has been illustrated with a practical case study focused on a series of 28 derivatives of 2-acylpyrroles 5a,b, obtained through a Pd(II)-catalyzed C-H radical acylation of pyrroles. Their in vitro leishmanicidal activity against visceral (L. donovani) and cutaneous (L. amazonensis) leishmaniasis was evaluated finding that compounds 5bc (IC₅₀ = 30.87 μM, SI > 10.17) and 5bd (IC₅₀ = 16.87 μM, SI > 10.67) were approximately 6-fold more selective than the drug of reference (miltefosine) in in vitro assays against L. amazonensis promastigotes. In addition, most of the compounds showed low cytotoxicity, CC₅₀ > 100 μg/mL in J774 cells. Interestingly, the IFPMTL-LOGR model predicts correctly the relative biological activity of these series of acylpyrroles. A computational high-throughput screening (cHTS) study of 2-acylpyrroles 5a,b has been performed calculating >20,700 activity scores vs a large space of 647 assays involving multiple Leishmania species, cell lines, and potential target proteins. Overall, the study demonstrates that the SOFT.PTML all-in-one strategy is useful to obtain IFPTML models in a friendly interface making the work easier and faster than before. The present work also points to 2-acylpyrroles as new lead compounds worthy of further optimization as antileishmanial hits.

Cooperative NHC and Photoredox Catalysis for the Synthesis of 1,4-Dicarbonyl Compounds via Diacylation of Alkenes

An intermolecular 1,2-diacylation of alkenes is disclosed via cooperative N-heterocyclic carbene and photoredox catalysis under the mediation of PPh₃ and Cs₂CO₃. This protocol provides a practical approach for construction of 1,4-dicarbonyl compounds toward novel diketone and pharmaceutical derivatives. Furthermore, the regioselective dicarbonyl compounds can be synthesized by adding acyl azolium salt. Mechanistic investigations suggest that the process was a critical radical/radical cross coupling of ketyl radicals with benzylic C-radicals.

Catalytic Friedel-Crafts Alkylative Desymmetrization of Cyclohexa-2,5-dienones: Access to Linear and Bridged Polycyclic Pyrroles and 3-Arylpyrroles

A catalytic [3 + 2]-cycloaddition/Friedel-Crafts alkylative desymmetrization strategy has been developed for the stereoselective construction of linear and bridged polycyclic pyrroles from alkynylcyclohexa-2,5-dienones. This strategy was further explored for the synthesis of 3-arylpyrroles under Brønsted acid catalysis. Reaction is highly chemo-, regio-, and stereoselective and is compatible with wide range of functionalized cyclohexa-2,5-dienones/pyrroles (>51 examples, ≤98% yields). Gram-scale synthesis and synthetic utility of the products have also been demonstrated to showcase the robustness of present method.

Catalyst-free direct regiospecific multicomponent synthesis of C3-functionalized pyrroles

An operationally simple catalyst-free protocol for the direct regiospecific synthesis of β-(C3)-substituted pyrroles has been developed. The enamine intermediate, in situ generated from succinaldehyde and a primary amine, was trapped with activated carbonyls before the Paal-Knorr reaction in a direct multicomponent "just-mix" fashion to furnish pyrroles with overall good yields. Several C3-substituted N-alkyl/aryl/H pyrroles have been produced under open-flask conditions with high atom economy and avoiding protection-deprotection chemistry.

Silver-Catalyzed Asymmetric Desymmetrization of Cyclohexadienones via Van Leusen Pyrrole Synthesis

A catalytic asymmetric desymmetrization of cyclohexadienones was accomplished via Van Leusen pyrrole synthesis in the presence of a chiral silver catalyst. The ready access to chiral-fused pyrrole derivatives is attributed to the identification of a suitable isocyanomethyl sulfone surrogate, NasMIC. The current Ag(I)-chiral phosphino-carboamide ligand system can be extended to the kinetic resolution of racemic cyclohexadienones, capitalizing on the differentiated reactivity of chiral quaternary complexes from AgSbF₆, chiral ligand (L*), cyclohexadienone, and NasMIC.

Microwave-assisted palladium catalysed C-H acylation with aldehydes: synthesis and diversification of 3-acylthiophenes

The use of MW allows the efficient palladium(II)-catalysed C-3 acylation of thiophenes with aldehydes via C(sp²)-H activation for the synthesis of (cyclo)alkyl/aryl thienyl ketones (43 examples). Compared to standard thermal conditions, the use of MW reduces the reaction time (15 to 30 min vs. 1 to 3 hours), leading to improved yields of the ketones (up to 92%). The control of positional selectivity is achieved by 2-pyridinyl and 2-pyrimidyl ortho-directing groups at C-2 of the thiophene scaffold. To show the synthetic applicability, selected ketones were subjected to further transformations, including intramolecular reactions to directly embed the directing group in the core structure of the new molecule.

Regioselective oxidative C–H heptafluoroisopropylation of heteroarenes with heptafluoroisopropyl silver

The regioselective C–H heptafluoroisopropylation of heteroarenes with heptafluoroisopropyl silver provided convenient access to a wide range of CF(CF3)2-containing heteroarenes.

Synthesis of Unsymmetrical Biheteroarenes via Dehydrogenative and Decarboxylative Coupling: a Decade Update

The design and development of robust and efficient methods for installing one heterocycle with another is endowed as a ubiquitous and powerful synthetic strategy to access complex organic biheterocycles in recent days due to their pervasive applications in medicinal as well as material chemistry. This perspective presents an overview on the recent findings and developments for the synthesis of unsymmetrical biheteroarenes via dehydrogenative and decarboxylative couplings with literature coverage mainly extending from 2011 to 2021. For simplification of the readers, the article has been subcategorized based on the catalysts used in the reactions.

Trifluoroethanol as a Unique Additive for the Chemoselective Electrooxidation of Enamines to Access Unsymmetrically Substituted NH-Pyrroles

An electrochemical method for the synthesis of unsymmetrically substituted NH-pyrroles is described. The synthetic strategy comprises a challenging heterocoupling between two structurally diverse enamines via sequential chemoselective oxidation, addition, and cyclization processes. A series of aryl- and alkyl-substituted enamines were effectively cross-coupled from an equimolar mixture to synthesize various unsymmetrical pyrrole derivatives up to 84 % yield. The desired cross-coupling was achieved by tuning the oxidation potential of the enamines by utilizing a "magic effect" of the additive trifluoroethanol (TFE). Additionally, extensive computational studies reveal the unique role of TFE in promoting the heterocoupling process by regulating the activation energies of the reaction steps through H-bonding and C-H⋅⋅⋅π interactions. Importantly, the developed electrochemical protocol was found to be equally efficient for the homocoupling of enamines to form symmetric pyrroles up to 92 % yield.

Sterically controlled C-H alkenylation of pyrroles and thiophenes

Pd-catalyzed C-H alkenylations targeting the least hindered position of N-alkyl pyrroles and 3-substituted thiophenes, as opposed to electronically controlled approaches, are developed. The steric demand and stable bidentate binding mode of the pyrazolonaphthyridine ligand are key to the success of these sterically controlled alkenylations using oxygen as an oxidant.

C-H activation reactions of nitroarenes: current status and outlook

Ring substitution reactions of nitroarenes remain an under-developed area of organic synthesis, confined to the narrow domains of S_NAr and S_NArH reactions. While searching for alternative methodologies, we took stock of the C-H activation reactions of nitroarenes which unearthed a variety of examples of nitro directed regioselective C-H functionalization reactions such as ortho-arylation, -benzylation/alkylation, and -allylation, oxidative Heck and C-H arylation reactions on (hetero)aromatic rings. A collective account of these reactions is presented in this review to showcase the existing landscape of C-H activation reactions of nitroarenes, to create interest in this field for further development and propagate this strategy as a superior alternative for ring substitution reactions of nitroarenes. The prospect of merging the C-H activation of nitroarenes with C-NO₂ activation, thereby harnessing NO₂ as a transformable multitasking directing group, is also illustrated.

Free Amine, Hydroxyl and Sulfhydryl Directed C-H Functionalization and Annulation: Application to Heterocycle Synthesis

Transition metal-catalyzed direct C-H bond functionalization is recognized as an efficient strategy to assemble heterocyclic frameworks. For this purpose, directing groups (DGs) installation on an organic molecule has remained a widely exploited strategy for the years. The installation of directing groups, especially for the amine, alcohol and thiol containing reactants and their removal after the reaction need additional steps. In this regard, the use of free amine, hydroxyl and sulfhydryl as directing groups in native form is advantageous and in recent times, these transformations have stirred undisputable advancements for applications to heterocycle synthesis. In this review, the aromatic sp² -C attached free amine, hydroxyl and sulfhydryl as native functionalities are shown to be useful for the construction of five to seven-membered N-, O- and S-heterocycles.

Chemical Reactivity and Skin Sensitization Studies on a Series of Chloro- and Fluoropyrroles-A Computational Approach

Global density functional descriptors analysis on a series of chloro- and fluoropyrroles provide vital data concerning their overall biochemical activities. In this study, a comprehensive investigation is presented for a series of chloro- and fluoropyrroles using DFT-based descriptors to elucidate physicochemical properties and their relevance to reactivity, charge transfer, site selectivity, and toxicity. Electrophilicity-based charge transfer (ECT) descriptor reveals the fact that chloro- and fluoropyrroles act as electron donors during their interaction with DNA bases. The local descriptor, namely, multiphilic descriptor conveys the activeness of specific sites in chloro- and fluoropyrroles. Further, Toxicity Prediction Komputer Assisted Technology (TOPKAT) studies on carcinogenicity bioassays using four rodent models provide the interesting fact that chloro- and fluoropyrroles exhibit a strong skin sensitization effect in these species.

K2S2O8 mediated synthesis of 5-aryldipyrromethanes and meso-substituted A4-tetraarylporphyrins

The synthesis of dipyrromethanes from pyrrole and arylglyoxylic acids in the presence of K2S2O8at 90 [Formula: see text]C is reported affording dipyrromethanes in very good yields. Unlike an excess pyrrole traditionally used in dipyrromethane synthesis, the current method uses a stoichiometric amount of pyrrole avoiding any use of Brønsted or Lewis acid. A gram scale synthesis of 5-phenyldipyrromethane is also achieved demonstrating potential scale up of dipyrromethanes using this method feasible. Subsequently, dipyrromethanes were converted to A4-tetraarylporphyrins also in the presence of K2S2O8at 90[Formula: see text]C. A direct synthesis of A4-tetraphenylporphyrin from excess pyrrole and phenylglyoxylic acid in the presence of K2S2O8 at 90[Formula: see text]C is also reported.