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

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.

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.

Sustainable Synthesis of Indole-Substituted Densely Functionalized Pyrrole

A novel chromatography- and catalyst-free methodology has been developed for the synthesis of poly substituted pyrrole in good yields via a multicomponent reaction of arylglyoxal, 1,3-dicarbonyl, indole, and aromatic amine. This strategy provides various advantages such as simple experimental and workup procedures, mild reaction conditions, no added catalyst, use of green solvent, and simple purification procedure of pure product without using column chromatography. This green method offers a simple and highly effective strategy to synthesize a wide range of indole-pyrrole conjugates in a one-pot operation.

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.

Access to thiionized-, selenolized-, and alkylated 5-alkylidene 3-pyrrolin-2-one derivatives via a regioselective oxidative annulation reaction

A metal-free regioselective oxidative annulation reaction of readily available 2,4-pentanediones with primary amines has been described. This protocol provides a divergent strategy for the incorporation of various radical donors into 5-alkylidene 3-pyrrolin-2-one skeletons, producing a variety of thiionized-, selenolized-, and alkylated 5-alkylidene 3-pyrrolin-2-one derivatives. Moreover, the diverse synthetic transformations of the 5-alkylidene 3-pyrrolin-2-one products were also investigated.

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.

Synergistic Visible Light and Pd-Catalyzed C-H Alkylation of 1-Naphthylamines with α-Diazoesters

The combination of visible light irradiation and Pd-catalysis has been practically employed for the C-H alkylation reactions of naphthylamines and α-diazo esters, leading to the synthesis of α-naphthyl functionalized acetates via C-C bond construction under mild reaction conditions and under solvent-free conditions. The light irradiation has been proven to play a pivotal role in the reactions, probably by promoting the generation of active carbene species from α-diazo esters.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Thiazole-5-carbaldehyde Synthesis by Cascade Annulation of Enaminones and KSCN with Dess-Martin Periodinane Reagent

The Dess-Martin periodinane (DMP) reagent-mediated reactions of tertiary enaminones with potassium thiocyanate for the synthesis of thiazole-5-carbaldehydes are developed. The product formation involves cascade hydroxyl thiocyanation of the C═C double bond, intramolecular hydroamination of the C≡N bond, and thiazole annulation by condensation on the ketone carbonyl site, representing novel reaction pathways in the reactions between enaminones and thiocyanate salt. DMP plays dual roles in mediating the free radical thiocyanation and inducing the unconventional selective thiazole-5-carbaldehyde formation by masking the in situ generated formyl group during the reaction process.

Cu(II)-catalyzed [4 + 1] and [4 + 3] annulation reactions: a modular approach to N-aryl/alkyl substituted 2,5-diamidopyrroles and diazepines

A one-pot copper-catalyzed [4 + 1] annulation reaction of primary amines with ynamide-derived buta-1,3-diynes for the synthesis of 2,5-diamido bearing N-aryl/alkyl pyrroles in up to excellent yields has been showcased. A broad range of primary amines having highly reactive functional groups are well tolerated. Notably, sterically demanding aniline and primary aliphatic amines are excellent amine sources. Furthermore, the current protocol may yield structurally unique diazepine derivatives. The scale-up reaction and fruitful chemical elaboration of pyrrole motifs highlight the importance of this reaction.

Research advances in palladium-catalysed intermolecular C-H annulation of aryl halides with various aromatic ring precursors

Transition-metal-catalysed C-H functionalization has emerged as a powerful approach for the transformation of organic molecules due to its high atom and step economy. Palladium-catalysed intermolecular C-H annulation of aryl halides, especially those involving annulation of a five-membered C,C-palladacycle with coupling reagents, have attracted considerable attention in the past decades. This review summarizes the progress on palladium-catalysed intermolecular C-H annulation of aryl halides with various aromatic ring precursors. Mechanistically, five-membered C,C-palladacycles as intermediates are involved in the majority of reactions.

Protecting Group-Dependent Synthesis of Densely Substituted Dihydropyrroles v/s Pyrroles via 5-Exo-trig Cascade Radical Cyclization to Alkynyl Vinylogous Carbamates

Densely substituted dihydropyrroles could be synthesized with excellent diastereoselectivity via 5-exo-trig cascade radical cyclization to alkynyl vinylogous carbamates. N-Alkyl/acyl protected alkynyl vinylogous carbamates upon radical cyclization using thiophenol gave substituted pyrroles as against dihydropyrroles, which were formed with N-sulfonyl protecting groups. This enabled a rare example wherein both dihydropyrrole and pyrrole rings are assembled in the same reaction. This strategy could be used for the synthesis of an unprecedented adjacent polyheterocyclic system having a furan-thiophene-pyrrole motif. When vinylogous carbamate is embedded in the isoindole moiety, a pyridoisoindole derivative was formed with excellent diastereoselectivity, instead of the expected pyrroloisoindole product.

Facile approach to multifunctionalized 5-alkylidene-3-pyrrolin-2-ones via regioselective oxidative cyclization of 2,4-pentanediones with primary amines and sodium sulfinates

Highly functionalized 5-alkylidene-3-pyrrolin-2-ones were efficiently synthesized via a four-component cascade cyclization/sulfonylation reaction between readily available 2,4-pentanediones, primary amines and sodium sulfinates under mild conditions.

BF3-promoted reactions of α-amino acetals with alkynes to 2,5-disubstituted pyrroles

An efficient BF3-promoted cyclization of α-amino acetals with alkynes for the assembly of substituted pyrroles has been developed. This cascade process can be easily achieved through a simple metal-free operation under mild conditions.