Synthesis of Substituted Furan/Pyrrole-3-carboxamides through a Tandem Nucleopalladation and Isocyanate Insertion

Microwave-Assisted Selective Nucleopalladation-Triggered Cascade Process: Synthesis of Highly Functionalized 3-Chloro-1H-indenes

A nucleopalladation-triggered cascade transformation of internal alkynes bearing an amino nucleophile and an electrophilic enone was investigated under unconventional microwave-assisted conditions. Among the three possible pathways, the chloropalladation-triggered domino process proceeded selectively to furnish 3-chloro-1H-indenes in good to excellent yields. The reactions under microwave irradiation were completed in 30 min, and the conventional heating required 3-5 h for completion. The yields obtained under nonclassical heating using microwave irradiation are marginally higher (71-97%) than those of the conventional heating conditions (67-96%). The mechanism of this domino process involves chloropalladation of alkynes to deliver σ-vinylpalladium intermediates, intramolecular carbopalladation via Heck-type olefin insertion, and protodepalladation steps. The other two competitive intramolecular aminopalladation-initiated cascades via 7-endo-dig or 6-exo-dig cyclizations leading to oxazepine or benzoxazine scaffolds were not observed.

Palladium-catalyzed cyclizative cross coupling of ynone oximes with 2-haloaryl N-acrylamides for isoxazolyl indoline bis-heterocycles

Metal-catalyzed cyclizative cross-coupling reactions have attracted enormous attention due to their unique cascade nature. We demonstrated, herein, a dual-cyclizative coupling of ynone oxime ethers with acrylamides for the synthesis of methylene-linked isoxazolyl 2-oxindoles. The cascade was triggered by a palladium(II)-catalyzed ynone oxime ether cyclization, which underwent a Heck-type coupling intercepted by an aryl iodide insertion. Control experiments were carried out to understand the mechanism.

Regioselective benzannulation of allylic sulfur ylides with ynones: a rapid access to substituted thioanisoles

The efficiency and selectivity of annulation reactions are often difficult to control in the presence of multiple potential reactive centers, like in the case of allylic sulfur ylides (ASY). Here, we describe a novel base mediated [3+3] benzannulation of ASY and readily available alkynones, which accomplishes the regioselective formation of multisubstituted thioanisoles, highly sought after chemical scaffolds. A new reactivity pattern of ASY has been unearthed, where it acted as both a 3C component and sulfur source in benzannulation. Use of a widely available base, operational simplicity and broad substrate scope are the additional salient features of the conversion.

Base-Mediated Cyclopentannulation of Ynones with Amino Crotonates for Regio- and Stereoselective Synthesis of Pentafulvenes and Cyclopenta[c]quinolines

A regio- and stereoselective synthesis of unsymmetrically substituted pentafulvenes is reported via the condensation of readily available ynones and amino crotonates under very mild conditions. The mechanism of this 3 + 2 annulation involved a vinylogous Michael addition followed by an intramolecular enamine aldol condensation. Substrates with o-bromo tether further cyclized to pentannulated hydroquinolines through an isomerization/S_NAr in the same reaction pot at the elevated temperature.

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.

Ni-Catalyzed electrophile driven regioselective arylative cyclization of ortho-functional diaryl acetylenes for the synthesis of pyridine and indene derivatives

A regioselective arylative cyclization of ortho functional diaryl acetylenes for the synthesis of selectively substituted diaryl pyridine and indene derivatives is accomplished through an electrophile driven alkyne polarization.

Base Mediated Tandem Vinylogous Addition and Cyclization of γ-Phosphonyl/Sulfonyl Crotonates and Ynones: Synthesis of Functionalized 2-Pyrones

A general method for highly functionalized 2-pyrones via a base-mediated sequential vinylogous addition and cyclization of γ-phosphonyl/sulfonyl crotonates and ynones are described. An exclusive E-geometry with respect to the newly generated olefin substituent at C3 of pyrone was observed. Imino glyoxalates and glycine imines similarly reacted with ynones to deliver 3-imino pyrones.

AuBr3 -Catalyzed Chemoselective Annulation of Isocyanates with 2H-Azirine

The chemoselective cyclization of isocyanates with 2H-azirine was achieved with AuBr₃ as catalyst. This transfer sets the stage for the synthesis of aromatic oxazole-ureas in a tandem process. The addition of a catalytic amount of phosphite enhances the process enormously. The reaction can also be performed in a one-pot process using benzoyl azide instead of isocyanate under the same conditions. A detailed study on the role of the phosphite that was applied as an additive revealed that only non-coordinated phosphite can reduce gold(III) and that gold(I) coordinated phosphite is not oxidized. Accompanied by the reduction of gold, HBr is generated in situ, which turned out to be the actual promotor in combination with the remaining AuBr₃ . The positive effect of acid can be explained by a strong N-Au coordination, which tends to break more easily in the presence of small amount of protic acid in the reaction solution.

Silver(i)/base-promoted propargyl alcohol-controlled regio- or stereoselective synthesis of furan-3-carboxamides and (Z)-enaminones

A novel and facile regioselective synthesis of furan-3-carboxamides by a silver(i)/base-promoted reaction of propargyl alcohol with 3-oxo amides has been demonstrated. This one-pot protocol provides a rapid synthetic approach to diverse trisubstituted furan-3-carboxamides via cascade nucleophilic addition, intramolecular cyclization, elimination, and isomerization reactions. Employing a substituted propargyl alcohol, (Z)-enaminones have been obtained with high stereoselectivities by a Ag2CO3-promoted reaction starting from 3-oxo amides via C-N bond cleavage.

3-Heterosubstituted Benzofurans from Hydroxyphenyl Propargyl Alcohols via ortho-Quinone Methide through a Metal-/Catalyst-Free Conjugate Addition/Oxy-Cyclization

A facile route to 3-sulfonylated and 3-(1-benzotriazolyl)-benzofurans is achieved from readily available o-hydroxyphenyl propargyl alcohols (o-HPPAs) and bench-top sodium sulfonylates and benzotriazoles with no assistance of any reagent or catalyst. Bifunctional o-quinone methides (o-QMs) were the putative reaction intermediates ensued from dehydration of o-HPPA. Our study revealed that the o-QM was so choosy in selection of the nucleophiles for the key Michael addition reaction.

Tunable Synthesis of 2-Ene-1,4-diones, 4-Hydroxycyclopent-2-en-1-ones, and 2-(Furan-3-yl)acetamides via Palladium-Catalyzed Cascade Reactions of Allenols

An efficient and regioselective synthesis of 2-ene-1,4-diones, 4-hydroxycyclopent-2-en-1-ones, or 2-(furan-3-yl)acetamides is successfully realized through palladium-catalyzed one-pot multicomponent reactions of allenols with aryl iodides and carbon monoxide in the presence of tertiary amines. Interestingly, the selectivity depends on the substitution patterns of the allenol substrates. To be specific, from the reaction of allenols with no substituent attached on the internal position of the allenic moiety, 2-ene-1,4-diones or 4-hydroxycyclopent-2-en-1-ones were formed selectively through carbonylation of aryl iodide followed by acylation of allenol with the in situ formed acyl palladium species, β-hydride elimination of the in situ formed allyl palladium complex, and further tautomerization or intramolecular aldol reaction. From the reaction of allenols bearing a substituent at the internal position of the allenic unit, on the other hand, diversely substituted 2-(furan-3-yl)acetamides were formed through a cascade process combining carbonylation of aryl iodide, acylation, and carbonylation of allenol followed by intramolecular condensation and amination by tertiary amine featuring an oxidant-free C-N bond cleavage.

A nickel-catalyzed anti-carbometallative cyclization of alkyne–azides with organoboronic acids: synthesis of 2,3-diarylquinolines

An anti-carbonickelative cyclization via reversible alkenylnickel E/Z isomerization of 2-azido phenyl propargyl alcohols with aryl boronic acids is achieved using Ni(acac)₂ as the catalyst to access 2,3-diaryl quinolines.

Base-promoted C–C bond cleavage for the synthesis of 2,3,4-trisubstituted pyrroles from N-propargyl β-enaminones

The synthesis of 2,3,4-trisubstituted pyrroles via base-promoted C–C bond cleavage reaction of N-propargyl β-enaminones is reported.

Copper-Promoted Regioselective Intermolecular Diamination of Ynamides: Synthesis of Imidazo[1,2-a]pyridines

A facile access to 3-heterosubstituted (3-oxazolidinonyl/indolyl/phenoxy) imidazo[1,2-a]pyridines from readily available 2-aminopyridines and electron-rich (internally activated) alkynes like ynamides/ynamines/ynol ethers is achieved via Cu(OTf)2-mediated intermolecular diamination under aerobic conditions. The reaction is highly regioselective, owing to internal electron bias, and thus led to a single regioisomer with heterosubstitution at C3.

Ni-Catalyzed regio- and stereoselective addition of arylboronic acids to terminal alkynes with a directing group tether

Addition of arylboronic acids to directing group tethered acetylenes in a regio and stereoselective manner using an inexpensive catalytic system is achieved for the first time to access highly sought after allyl/homoallyl alcohol/amine units.

Catalyst free synthesis of α-fluoro-β-hydroxy ketones/α-fluoro-ynols via electrophilic fluorination of tertiary propargyl alcohols using Selectfluor™ (F-TEDA-BF4)

A facile access to α-fluoro-β-hydroxyketones via electrophilic fluorination.

A stereoselective thiocyanate conjugate addition to electron deficient alkynes and concomitant cyclization to N,S-heterocycles

A regio- and stereoselective thiocyanate addition to ynones and the concomitant cyclizations of resultant vinyl thiocyanate to access thio-aza heterocycles are achieved using KSCN in AcOH without the need of any metal catalyst.