Regioselective Synthesis of 2,3,4-Trisubstituted Pyrroles via Pd(II)-Catalyzed Three-Component Cascade Reactions of Amines, Alkyne Esters, and Alkenes

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.

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.

Efficient synthesis of pentasubstituted pyrroles via intramolecular C-arylation

Immobilized L-aspartic acid beta-methyl ester (Fmoc-Asp(OMe)-OH) was reacted with 4-nitrobenzenesulfonyl chloride, followed by alkylation with various α-haloketones. The resulting intermediates were treated with potassium trimethylsilanolate, which yielded tetrasubstituted pyrroles after a one-step transformation consisting of sequential C-arylation, aldol condensation and spontaneous aromatization. The discovered synthetic strategy enables fast and simple access to pentasubstituted and functionalized pyrroles from a number of readily available starting materials.

Recent advances in cascade reactions and their mechanistic insights: a concise strategy to synthesize complex natural products and organic scaffolds

The beauty of cascade reactions to bestow us with cumbersome organic scaffolds has made them a cutting-edge area of research. Although the planning of cascades may require intuition, their results can be highly impactful. The development of cascades to provide specific targeted molecules of an appropriate structural and stereochemical framework poses a significant challenge but can serve as one of the most impressive tools in organic synthesis. This review shares a broad interest in compiling cascade transformations towards the construction of polycyclic frameworks, induction of chirality/asymmetry in the protocol, etc. to solve diverse challenges in organic synthesis pursuits, as cascades enable the rapid and efficient construction of complex architectures from simple molecules. The studies highlighted herein manifest the utilization of a range of cascade reactions under various classifications for generating natural product skeletons such as palau'amine, benzosimuline, arcutinine, and others from simple building blocks, with emphasis on breakthroughs and potential for asymmetric synthesis. The exquisite synthetic designs of recently completed total synthesis of natural products with a focus on strategic concerns are also highlighted in this review.

Copper-catalyzed C-H/N-H annulation of enaminones and alkynyl esters for densely substituted pyrrole synthesis

Herein, the copper-catalyzed annulation of enaminones with alkynyl esters for the facile synthesis of different pyrroles with a 2,3,4,5-tetrasubstituted structure has been developed. With Cu(OAc)₂ as the only catalyst, the tunable synthesis of 2-vinyl and 2,3-dicarboxyl-functionalized pyrroles has been achieved by using terminal and internal alkynyl esters, respectively. The synthesis of 2-vinyl pyrroles represents the first example accessing 2-vinyl substituted pyrroles via direct cascade reactions involving vinylation and pyrrole ring formation.

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.

K2S2O8-Mediated Synthesis of Highly Functionalized Pyrroles via Oxidative Self-Dimerization of N-Propargylamines

An efficient methodology has been developed for the synthesis of tetra- and pentasubstituted pyrroles via oxidative self-dimerization of N-propargylamines catalyzed by silver benzoate in the presence of K₂S₂O₈ in good yields. The protocol provides a simple route for the synthesis of both tetra- and pentasubstituted pyrroles with two carbonyl groups in the side chain. The methodology can be extended toward the synthesis of pyrrolo[3,4-d]pyridazine.

Base-Promoted Three-Component Cascade Reaction of α-Hydroxy Ketones, Malonodinitrile, and Alcohols: Direct Access to Tetrasubstituted NH-Pyrroles

A base-promoted three-component cascade reaction of α-hydroxy ketones, malonodinitrile, and alcohols has been developed, providing a direct and efficient route to a range of structurally diverse and synthetically useful 2-alkyloxy-1H-pyrrole-3-carbonitrile derivatives. The reaction involved three different bond (C-C, C-O, and C-N) formations in a single step, and its regioselectivity was depended on the structure of the α-hydroxy ketones employed. The use of easily available starting materials, wide substrate scope, good functional group tolerance, operational simplicity, and high atom economy are attractive features of the new method.

Recent Advancements in Pyrrole Synthesis

This review article features selected examples on the synthesis of functionalized pyrroles that were reported between 2014 and 2019. Pyrrole is an important nitrogen-containing aromatic heterocycle that can be found in numerous compounds of biological and material significance. Given its vast importance, pyrrole continues to be an attractive target for the development of new synthetic reactions. The contents of this article are organized by the starting materials, which can be broadly classified into four different types: substrates bearing π-systems, substrates bearing carbonyl and other polar groups, and substrates bearing heterocyclic motifs. Brief discussions on plausible reaction mechanisms for most transformations are also presented.

Construction of Polyfunctionalized 2,4-Dioxa-8-azaspiro[5.5]undec-9-enes and 2,4,8-Triazaspiro[5.5]undec-9-enes via a Domino [2+2+2] Cycloaddition Reaction

The three-component reaction of α,β-unsaturated N-arylaldimines, dialkyl but-2-ynedioates, and 2-arylidene Meldrum acids in DCM at room temperature gave mixtures of cis/trans-11-aryl-7-styryl-2,4-dioxa-8-azaspiro[5.5]undec-9-enes in satisfactory yields. The similar three-component reaction with 2-arylidene-N,N'-dimethylbarbituric acids afforded cis-11-phenyl-7-styryl-2,4,8-triazaspiro[5.5]undec-9-enes as major products. On the other hand, the three-component reaction of N-arylaldimines, dialkyl but-2-ynedioates, and 2-arylidene Meldrum acids or 2-arylidene-N,N'-dimethylbarbituric acids afforded cis/trans-isomeric spirocompounds in satisfactory yields with high diastereoselectivity. This domino [2+2+2] cycloaddition reaction proceeded with sequential nucleophilic addition of N-arylaldimine to an electron-deficient alkyne, Michael addition, and annulation process. The stereochemistry of all cis/trans isomeric spirocompounds was clearly elucidated by the determination of 33 single-crystal structures. The diastereoselectivity of the three-component reaction was correlated by DFT calculations.

Selective Construction of Diverse Polycyclic Spirooxindoles via a Three-Component Reaction of Cyclic Mercapto-Substituted β-Enamino Esters, Isatins, and Cyclic 1,3-Diketones

The three-component reaction of alkyl 2-(benzo[b][1,4]thiazin-3-ylidene)acetates, isatins, and 1,3-indanedione (1,3-cyclopentanedione) in ethanol in the presence of acetic acid conveniently afforded spiro[indeno[1,2-b]phenothiazine-6,3'-indolines] or spiro[cyclopenta[b]phenothiazine-4,3'-indolines] in good yields and with high diastereoselectivity. More interestingly, a similar three-component reaction with 4-hydroxychromen-2-one resulted in the unexpected polycyclic spiro[benzo[b]chromeno[3',4':5,6]pyrano[2,3-e][1,4]thiazine-7,3'-indolines] in satisfactory yields. A plausible reaction mechanism was rationally proposed for formation of different kinds of the spiro compounds, and the stereochemistries of the various spiro compounds were clearly elucidated.

Substituent-controlled chemoselective synthesis of multi-substituted pyridones via a one-pot three-component cascade reaction

A substituent-controlled chemoselective cycloaddition reaction which afforded a variety of 2-pyridones and 4-pyridones in good to excellent yields, respectively, has been developed.

Lanthanide-Catalyzed Tandem Insertion of Secondary Amines with 2-Alkynylbenzonitriles: Synthesis of Aminoisoindoles

A lanthanide-catalyzed intermolecular hydroamination of 2-alkynylbenzonitriles with secondary amines has been disclosed, providing a streamlined access to a range of aminoisoindoles in moderate to excellent yields. The salient features of this reaction include high bond-formation efficiency, mild reaction conditions, 100 % atomic efficiency and good functional group tolerance. This methodology has also been successfully applied to the construction of other nitrogen-containing compounds, such as 5 H-imidazo[2,1-a]isoindoles and isoquinolines. A plausible mechanism for the formation of aminoisoindoles involving initial N-H activation by a lanthanide complex followed by C≡N insertion into a Ln-N bond to form an amidinate lanthanide intermediate, which undergoes the cyclization is proposed.

One-pot regioselective synthesis of 2,4-disubstituted quinolines via copper(ii)-catalyzed cascade annulation

A copper(ii)-catalyzed cascade annulation for the one-pot synthesis of 2,4-disubstituted quinolines has been disclosed.

C(sp3)–H dehydrogenation and C(sp2)–H alkoxy carbonylation of inactivated cyclic amines towards functionalized N-heterocycles

A novel and efficient synthesis of tetrahydropyridine-, dihydropyrrole-, and tetrahydroazepine-3-carboxylates via cascade reactions of cyclic amines with CO and alcohols is presented. To our knowledge, this should be the first example in which functionalized N-heterocycles were prepared through Pd-catalyzed C(sp³)–H dehydrogenation and C(sp²)–H carbonylation of cyclic amines.

Selective synthesis of tetrahydroimidazo[1,2-a]pyridine and pyrrolidine derivatives via a one-pot two-step reaction

In the presence of triethylamine, the addition reaction of substituted α-amino acid alkyl esters with dialkyl but-2-ynedioate afforded active β-enamino esters, which in turn reacted with aromatic aldehydes and malononitrile to give tetrahydroimidazo[1,2-a]pyridine derivatives in moderate yields.