Copper-Catalyzed C–O Bond Formation: An Efficient One-Pot Highly Regioselective Synthesis of Furans from (2-Furyl)Carbene Complexes

Direct access to furan and cyclopropane derivatives via palladium-catalyzed C-H activation/alkene insertion/annulation

A practical and effective palladium-catalyzed C-H activation/alkene insertion/annulation has been reported for the synthesis of furans and cyclopropanes from cyclic 1,3-diketones or 1,3-indandione and diverse alkenes, resulting in moderate to good yields. This protocol demonstrates excellent selectivity and is well-compatible with a wide range of alkene substrates, exhibiting exceptional regioselectivities, high efficiency, and good functional group tolerance.

Silver(I)-Catalyzed Tandem Reaction of Enynones and 4-Alkenyl Isoxazoles: Synthesis of 2-(Furan-2-yl)-1,2-dihydropyridines

Silver(I)-catalyzed tandem reaction of enynones with 4-alkenyl isoxazoles provides access to 2-(furan-2-yl)-1,2-dihydropyridines. No competitive cyclopropanation of alkenes and O-H insertion via (2-furyl)carbene complexes were observed. The cascade reaction proceeds via the formation of (2-furyl)metal carbene intermediate, the N-O bond cleavage of 4-alkenyl isoxazoles/rearrangement, subsequent 6π electrocyclic reaction, and [1,5] H-shift. The successive construction of both 1,2-dihydropyridine skeleton and furan frame has been achieved in the one-pot reaction. A broad range of readily available enynones and 4-alkenyl isoxazoles are suitable to this protocol; however, when R³ is the alkyl group such as n-Bu and Me, a complicated mixture was generated without the desired products. In addition, in the case of R⁴ = bulky group such as R₃^'SiOCH₂, the reaction gave an in situ oxo-product of (2-furyl)silver carbene. An atom-economic strategy for the synthesis of 2-(furan-2-yl)-1,2-dihydropyridines has been established.

Synthesis of N-, O-, and S-heterocycles from aryl/alkyl alkynyl aldehydes

In the field of heterocyclic synthesis, alkynyl aldehydes serve as privileged reagents for cyclization reactions with other organic compounds to construct a broad spectrum of N-, O-, and S-heterocycles. Due to the immense application of heterocyclic molecules in pharmaceuticals, natural products, and material chemistry, the synthesis of such scaffolds has received wide attention. The transformations occurred under metal-catalyzed, metal-free-promoted, and visible-light-mediated systems. The present review article highlights the progress made in this field over the past two decades.

Synthesis of Furans via Rhodium(III)-Catalyzed Cyclization of Acrylic Acids with α-Diazocarbonyl Compounds

An efficient protocol for the synthesis of furans through Rh(III)-catalyzed vinyl C-H activation from acrylic acids and α-diazocarbonyl compounds has been developed. The reaction features broad functional group tolerance and affords a series of furans in moderate to good yields. Moreover, no additives such as copper or silver salts are required. Some control experiments are performed to give insight into the mechanism of this cascade transformation and the decarbonylation process is involved in the formation of the furan product.

Copper(I)-Catalyzed Direct Oxidative Annulation of 1,3-Dicarbonyl Compounds with Maleimides: Access to Polysubstituted Dihydrofuran Derivatives

An efficient annulation method for the synthesis of polysubstituted dihydrofurans from 1,3-dicarbonyl compounds and maleimides is described. The reactions can afford furo[2,3-c]pyrrole derivatives with satisfactory yields. The developed strategy realizes the direct oxidative double C(sp³)-H functionalization in the presence of copper(I) salts and 2-(tert-butylperoxy)-2-methylpropane. Meanwhile, this protocol features a mild reaction condition and simple catalytic system. A reaction mechanism involving a single electron oxidation is also proposed.

A Predictive Chemistry DFT Study of the N2O Functionalization for the Preparation of Triazolopyridine and Triazoloquinoline Scaffolds

The whole reaction mechanism of the functionalization of N2O for the synthesis of triazolopyridine and triazoloquinoline scaffolds has been unveiled by means of DFT calculations. The rate determining step of...

Ready Access to Densely Substituted Furans Using Tsuji-Wacker-Type Cyclization

A competent method for the construction of highly substituted furans catalyzed by Pd(II) and Cu(II) chloride has been developed. The method provides easy access to di-, tri-, and tetrasubstituted furans from corresponding diols with relatively mild conditions in a unified strategy. The developed method has been successfully tested with more than 25 substrates, which resulted in furans of multiple substitution patterns with up to 84% isolated yields.

Preparation of 4-(Nitromethyl)furan Derivatives and Their Application in the Syntheses of Bis(furan-2-yl)oximes

An efficient method for the preparation of tetrasubstituted furans, which contains a nitromethyl group at the 4-position, has been developed. The applications of 4-(nitromethyl)furans on the synthesis of highly functionalized bis(furyl)oxime were explored for the first time.

AgOTf/I2-Mediated Cyclization/Cross-Coupling/Isomerization of Enynones with Phosphorus Ylides: An Expedient Route to Stereoselective Synthesis of (E)-2-Alkenylfurans

Ag(I)-catalyzed cascade reactions involving enynone cyclization and cross-coupling with phosphorus ylides have been achieved for the first time. Subsequent treatment of the reaction mixture with I₂ afforded the corresponding (E)-α-alkenylfurans in 73-95% yields with excellent stereoselectively. A reasonable mechanism has been proposed.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

Silver(i)-catalyzed tandem reaction of enynones and 4-alkynyl isoxazoles: regioselective synthesis of highly functionalized 4H-furan[3,4-c]pyrroles

This work reports a silver(i)-catalyzed tandem reaction of enynones with 4-alkynyl isoxazoles.

Cu-catalyzed cross-coupling of methyl ketones and pyridin-2-amines for the synthesis of N-(2-pyridyl)-α-ketoamides

An efficient copper-catalyzed strategy for the synthesis of α-ketoamides via cross-coupling of methyl ketones and pyridin-2-amines is described. This transformation has provided a simple process for the formation of C−N and C=O bonds to prepare α-ketoamides, which are important substrates and intermediates for the preparation of fine chemicals. The reaction mechanism is investigated, which suggests that the reaction proceeds via a radical pathway.

Synthesis of 2-alkenylfurans via a Ag(i)-catalyzed tandem cyclization/cross-coupling reaction of enynones with iodonium ylides

A silver(i)-catalyzed tandem cyclization/cross-coupling reaction of enynones with iodonium ylides to construct carbon-carbon double bonds has been developed. The strategy provides a novel method for the synthesis of 2-alkenylfurans. This is the first cross-coupling reaction between metal-carbene complexes and iodonium ylides.

Mechanistic Divergence in the Hydrogenative Synthesis of Furans and Butenolides: Ruthenium Carbenes Formed by gem-Hydrogenation or through Carbophilic Activation of Alkynes

Enynes with a tethered carbonyl substituent are converted into substituted furan derivatives upon hydrogenation using [Cp*RuCl]4 as the catalyst. Paradoxically, this transformation can occur along two distinct pathways, each of which proceeds via discrete pianostool ruthenium carbenes. In the first case, hydrogenation and carbene formation are synchronized ("gem-hydrogenation"), whereas the second pathway comprises carbene formation by carbophilic activation of the triple bond, followed by hydrogenative catalyst recycling. Representative carbene intermediates of either route were characterized by X-ray crystallography; the structural data prove that the attack of the carbonyl group on the electrophilic carbene center follows a Bürgi-Dunitz trajectory.

Access to Polysubstituted (Furyl)methylthioethers via a Base-Promoted S-H Insertion Reaction of Conjugated Enynones

A convenient and applicable approach to the construction of diverse functionalized (2-furyl)methylthioether derivatives via base-promoted S-H insertion of conjugated enynones with thiophenols or thiols has been developed. This reaction features readily available starting materials, high atom economy, broad substrate scope, and versatile operation. Moreover, the synthetic utility of this method has been demonstrated by the efficient synthesis of the CNKSPR1 inhibitor precursor and late-stage functionalization of glutathione.

Gold-carbene assisted formation of tetraarylmethane derivatives: double X-H activation by gold

An efficient gold-carbene promoted generation of tetraarylmethane derivatives from enynones and indoles was accomplished by the formation of a new C-O bond and two C-C bonds. It is significant that (2-furyl) gold-carbene assisted addition of two nucleophiles resulted in the formation of tetraarylmethane derivatives with a quaternary centre in moderate to good yields in one pot.

Ag(I)-Catalyzed Tandem Reaction of Conjugated Ene-yne-ketones in the Presence of PhI(OAc)2 and Triethylamine: Synthesis of 2-Alkenylfurans

Silver-catalyzed tandem cyclization-elimination reactions of conjugated ene-yne-ketones in PhI(OAc)₂/triethylamine system lead to the formation of 2-alkenylfurans. 2-Furylsilver carbene and phenyliodonium ylide are proposed as the key intermediates in these transformations.

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.

Insights into the mechanisms of Cu(i)-catalyzed heterocyclization of α-acyl-α-alkynyl ketene dithioacetals to form 3-cyanofurans: the roles of NH4OAc

NH₄OAc is decomposed into NH₃ and HOAc, and both NH₃ and HOAc as the proton shuttle can prompt catalytic reactions.

Synthesis of 2-acetyl trisubstituted furans via copper-mediated deacylation cleavage of unstrained C(sp3)–C(sp2) bonds

A copper-mediated tandem addition/cyclization/carbon–carbon cleavage reaction for the convenient synthesis of 2-acyl trisubstituted furans has been developed.

Palladium-catalyzed oxidative borylation of conjugated enynones through carbene migratory insertion: synthesis of furyl-substituted alkenylboronates

A new method for the synthesis of furyl-substituted alkenylboronates has been developed by palladium-catalyzed oxidative borylation reaction of conjugated enynones.

Copper-Catalyzed Intramolecular Annulation of Conjugated Enynones to Substituted 1 H-Indenes and Mechanistic Studies

Herein, a copper-catalyzed intramolecular cascade reaction of conjugated enynones to deliver substituted 1 H-indenes is reported. The inexpensive and less toxic copper salt served as the only catalyst in the transformation, affording the 3-(2-furyl)-substituted 1 H-indenes in good to excellent yields under mild reaction conditions with broad functional group tolerance and making it highly appealing for synthetic organic chemistry. Notably, detailed DFT calculations have been carried out to elucidate that the reaction undergoes a copper-mediated 5- exo-dig cyclization of enynones to afford copper-(2-furyl)-carbene intermediate, followed by diene-carbene cyclization (one step but involving 6π cyclization of Cu-carbene and reductive elimination) and 1,5-hydrogen shift to provide the 1 H-indenes.

Copper-Catalyzed Oxidative Carbon-Carbon and/or Carbon-Heteroatom Bond Formation with O2 or Internal Oxidants

Selective oxidation, a fundamental organic transformation of critical importance, produces value-added products from simple organic molecules. This process is extensively used to incorporate heteroatoms into carbon-based molecules, where high-valent metal salts, hypervalent halogen reagents, and peroxides are widely used as oxidants. Oxidation reactions are extremely challenging because their selectivity is hard to control and/or they form significant quantities of unwanted waste derived from the stoichiometric oxidants. Undoubtedly, the utilization of green oxidants such as molecular oxygen (O₂) or internal oxidants provides tunable oxidation abilities and produces no environmentally hazardous byproducts. Thus, synthetic chemists have devoted increasing attention to the utilization of green oxidants to obtain valuable products. Since the first industrial application of noble metal-catalyzed oxidation, i.e., Pd/Cu/O₂-mediated Wacker oxidation, precious metal-catalyzed organic reactions have undergone significant development in both the laboratory and industry. However, the high cost and considerable toxicity of precious metals compel chemists to explore the catalytic activities of earth-abundant, first-row transition metals. Copper is abundant, easy to utilize, and relatively insensitive to water and air. Controllable access to Cu(0), Cu(I), Cu(II), and Cu(III) oxidation states ensures that copper can be applied as a tunable and multifunctional catalyst. Copper-catalyzed transformations involve single-electron transfer (SET), two-electron processes (TEPs) and even the cooperation of SET and TEPs. More importantly, in Cu/O₂ catalytic systems, ligands, additives, and solvents can tune the oxidation state of copper from Cu(I) to Cu(III). As a result, the development of copper-catalyzed aerobic oxidative reactions is possible and desirable. Progress in these synthetic methods will enable breakthroughs in natural product synthesis, materials science, and bioorganic chemistry. This Account describes our efforts over the last several years to develop copper-catalyzed C-C or C-heteroatom bond formation reactions with oxygen or internal oxidants as the oxidant. We primarily focused on reaction with simple substrates, including cross-couplings, cycloadditions, cyclizations, and condensations. These transformations provide convenient and efficient strategies for constructing multiple bonds, such as C-C/C-O bonds, C-C/C-N bonds, and C-N/C-S bonds, in one pot. Various alkynes, furans, benzofurans, lactones, sulfones, thioethers, and nitrogen-containing heterocyclic compounds were synthesized with high selectivity and atom economy from abundant, commercially available and inexpensive starting materials. These methods were successfully applied to the construction of drug molecules and skeletons of natural products. Additionally, the designed control experiments and serendipitous observations have given us mechanistic insights into copper-catalyzed green oxidation. Uncovering the activity of copper-catalyzed green oxidation involving oxygen and oxime esters has allowed us to extend the scope of those green oxidation reactions. We believe that copper-catalyzed green oxidation transformations can be made even more eco-friendly and economical in the synthesis of valuable compounds.

Palladium-catalyzed carbene/alkyne metathesis with enynones as carbene precursors: synthesis of fused polyheterocycles

An unprecedented palladium-catalyzed novel carbene/alkyne metathesis cascade reaction of alkyne-tethered enynones is described. This reaction affords fused polyheterocycles in moderate to good yields. The transformation begins with Pd-catalyzed 5-exo-dig cyclization of the enynone to form the donor/donor metal carbene, which then undergoes metathesis with the alkyne followed by electrophilic aromatic substitution.

Access to sulfonylated furans or imidazo[1,2-a]pyridines via a metal-free three-component, domino reaction

An efficient three-component reaction for the synthesis of sulfonylated furans or imidazo[1,2-a]pyridines has been developed.

Atom-Economical and Metal-Free Synthesis of Multisubstituted Furans from Intramolecular Aziridine Ring Opening

Multisubstituted furans were prepared from dialkyl 2-(aziridin-2-ylmethylene)malonate and/or 1,3-dione through aziridine ring opening by the internal carbonyl oxygen with the assistance of BF3·OEt2, followed by aromatization. This synthetic method is free from any metal and is atom-economical with all of the atoms in the starting material retained in the final product.