Regioselective Synthesis of Pyrroles from Alkyne-Isocyanide Click Reactions: An Angle Strain-Induced Bond Migration Approach

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.

An Alkyne-Isocyanide Cycloaddition/Boulton-Katritzky Rearrangement/Ring Expansion Reaction: Access to 9-Deazaguanines

An alkyne-isocyanide [3 + 2] cycloaddition followed by a Boulton-Katritzky rearrangement and a ring expansion is demonstrated. Different from the typical Boulton-Katritzky rearrangement, which forms five-membered ring products, the rearrangement-ring expansion method provides a mild, efficient, and atom-economical access to fused 9-deazaguanine structures in high yields.

Modular Synthesis of Tetrasubstituted Pyrroles via an Annulative Migration Reaction of Allenyl Ketones and p-Toluenesulfonylmethyl Isocyanide

The metal-free cyclization of allenyl ketones and p-toluenesulfonylmethyl isocyanide (TosMIC), promoted by Cs₂CO₃, provides a convenient access to tetrasubstituted pyrroles in which an acyl group undergoes 1,2-migration. This tandem Michael addition/annulative migration synthetic strategy is general and high-yielding for various substituted allenyl ketones. Moreover, a phosphoryl or ester moiety is also a suitable functionality to enable such migration.

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.

Conjugated ynones in catalytic enantioselective reactions

Conjugated ynones are easily accessible feedstock and the existence of an alkyne bond endows ynones with different attractive reactivities, thus making them unique substrates for catalytic asymmetric reactions. Their compatibility under organocatalytic, metal-catalyzed as well as cooperative catalytic conditions has resulted in numerous enantioselective transformations. Importantly, conjugated ynones can act as nucleophiles or electrophiles, and serve as easily accessed synthons for different cyclization pathways. This review summarizes the recent literature examples of the catalytic reactions of conjugated ynones and related compounds such as alkyne conjugated α-ketoesters, and classifies these reaction types alongside mechanistic insights whenever possible. We aim to trigger more intensive research in the future to render the asymmetric transformation of ynones as a common and reliable tool for asymmetric synthesis.

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.

Transition-metal-free and facile synthesis of 3-alkynylpyrrole-2,4-dicarboxylates from methylene isocyanides and propiolaldehyde

A transition-metal-free, facile and efficient method for the synthesis of 3-alkynylpyrrole-2,4-dicarboxylates was developed.

Recent Progress in the Synthesis of Pyrroles

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Pyrrole derivatives are nitrogen-containing heterocyclic compounds and widely distributed in a large number of natural and non-natural compounds. These compounds possess a broad spectrum of biological activities such as anti-infammatory, antiviral, antitumor, antifungal, and antibacterial activities. Besides their biological activity, pyrrole derivatives have also been applied in various areas such as dyes, conducting polymers, organic semiconductors.

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Due to such a wide range of applicability, access to this class of compounds has attracted intensive research interest. Various established synthetic methods such as Paal-Knorr, Huisgen, and Hantzsch have been modified and improved. In addition, numerous novel methods for pyrrole synthesis have been discovered. This review will focus on considerable studies on the synthesis of pyrroles, which date back from 2014.

Silver-Mediated Synthesis of Substituted Benzofuran- and Indole-Pyrroles via Sequential Reaction of ortho-Alkynylaromatics with Methylene Isocyanides

A silver-mediated reaction between 2-ethynyl-3-(1-hydroxyprop-2-yn-1-yl)phenols or 2-ethynyl-3-(1-hydroxyprop-2-yn-1-yl)anilines and methylene isocyanides has been developed. A sequential 5-endo-dig cyclization and [3 + 2] cycloaddition process is proposed. This synthetic strategy is atom- and step-efficient and applicable to a broad scope of substrates, allowing the synthesis of valuable substituted benzofuran- and indole-pyrroles in moderate to high yields.

Three-Component Reactions to Spirocyclic Pyrrolidinonylformimidamides: α-Isocyano Lactams as Two-Atom Unit in Silver-Catalyzed Formal [3 + 2] Cycloaddition Reactions

A facile approach to spirocyclic pyrrolidinonylformimidamides has been developed via three-component reactions of isocyanides, alkenes, and amines. The reaction proceeds through a sequence of two distinct reaction pathways; the base-catalyzed conjugate addition of α-isocyano lactams to electron-deficient alkenes and the Ag(I)-catalyzed amine insertion to the isocyanide moiety. Both reactions display markedly different reaction kinetics, allowing the one-pot three-component reactions to be performed in the presence of respective catalysts, a Brønsted base and a silver salt. The formation of spirocyclic pyrrolidin-2-ones represents an unusual use of α-isocyano lactam as a two-atom unit in a formal [3 + 2] cycloaddition reaction. The successful identification of ways to defy the typical three-atom unit role of α-isocyano carbonyl compounds in the [3 + 2] cycloaddition pathways manifests a rapid assembly of medicinally important spirocyclic scaffolds from readily available starting materials.

Copper-Promoted Regioselective Synthesis of Polysubstituted Pyrroles from Aldehydes, Amines, and Nitroalkenes via 1,2-Phenyl/Alkyl Migration

The facile copper-catalyzed synthesis of polysubstituted pyrroles from aldehydes, amines, and β-nitroalkenes is reported. Remarkably, the use of α-methyl-substituted aldehydes provides efficient access to a series of tetra- and pentasubstituted pyrroles via an overwhelming 1,2-phenyl/alkyl migration. The present methodology is also accessible to non α-substituted aldehydes, yielding the corresponding trisubstituted pyrroles. On the contrary, the use of ketones, in place of aldehydes, does not promote the organic transformation, signifying the necessity of α-substituted aldehydes. The reaction proceeds under mild catalytic conditions with low catalyst loading (0.3-1 mol %), a broad scope, very good functional-group tolerance, and high yields and can be easily scaled up to more than 3 mmol of product, thus highlighting a useful synthetic application of the present catalytic protocol. Based on formal kinetic studies, a possible radical pathway is proposed that involves the formation of an allylic nitrogen radical intermediate, which in turn reacts with the nitroalkene to yield the desired pyrrole framework via a radical 1,2-phenyl or alkyl migration.

Highly efficient synthesis of benzodioxins with a 2-site quaternary carbon structure by secondary amine-catalyzed dual Michael cascade reactions

Employing salicylic acids and substituted 3-butyn-2-ones as the substrates, a morpholine catalyzed tandem dual Michael addition afforded a benzodioxin skeleton.

Substituted Pyrrololactams via Ring Expansion of Spiro-2H-pyrroles from Intermolecular Alkyne-Isocyanide Click Reactions

The facile synthesis of 6- to 8-membered pyrrololactams has been developed using a ring expansion of spiro-2H-pyrroles, the products of intermolecular alkyne-isocyanide click reactions. The key to successful ring expansion of spiro-2H-pyrroles to pyrrololactams is the enforced orbital overlap between the internal alkene and the amide carbonyl group through the conformationally locked bicyclic structures. The newly disclosed α-isocyano lactams, substrates for click reactions, should find their utility in the synthesis of pharmaceutically important heterocyclic compounds.