Zinc-Catalyzed Synthesis of Functionalized Furans and Triarylmethanes from Enynones and Alcohols or Azoles: Dual XH Bond Activation by Zinc

Copper-Catalyzed Directed Hydroindolation/Annulation Sequence of Alkynes with Indoles via Copper Carbenes

Described herein is an efficient copper-catalyzed tandem alkyne indolylcupration-initiated 1,2-indole migration/6π-electrocyclic reaction of allene-ynamides with indoles by the in situ-generated metal carbenes. This method allows the efficient synthesis of valuable indole-fused spirobenzo[f]indole-cyclohexanes with high regio- and stereoselectivity. In addition, this reaction affords rapid access to the functionalized spirobenzo[f]indole-cyclohexanes in the absence of indoles by a presumable 5-exo-dig cyclization/Friedel-Crafts alkylation via copper-containing all-carbon 1,4-dipoles.

Synthesis of Highly Substituted Furans via Intermolecular Enynone-Aldehyde Cross-Coupling/Cyclization Catalyzed by N-Heterocyclic Carbenes

A new strategy for facile access to multifunctionalized furans via N-heterocyclic carbene-catalyzed cross-coupling/cyclization of ynenones with aldehydes has been explored. This protocol features readily obtainable starting materials, mild and metal-free conditions, broad substrate scope, good functional group tolerance, excellent yields, and easy scale-up. Synthetic utility of the protocol has been further corroborated through functionalization of complex substrates and postmodifications of the product.

Access to 2-Alkenyl-furans via a Cascade of Pd-Catalyzed Cyclization/Coupling Followed by Oxidative Aromatization with DDQ

An unprecedented DDQ-mediated oxidative aromatization of 2-bezylidene-dihydrofurans yielding 2-alkenyl-furans is disclosed. Integration of this transformation with a prior Pd-catalyzed reaction of α-propargylic-β-ketoesters and (hetero)aryl halides into a one-pot cascade process opens a direct modular route to highly substituted 2-vinyl-furans. Experimental and computational studies reveal that the crucial step of the oxidative-aromatization involves facile hydride transfer from the dihydrofuran ring to the O-center of DDQ.

Controllable Access to Diazo-functionalized 2-Methylene-2,3-dihydrofurans and Diazo-functionalized Furans from Enynones and Diazo Carbonyl Compounds

Using enynones and diazo carbonyl compounds as identical starting materials, methods for chemoselective and regioselective constructs of diazo-functionalized 2-methylene-2,3-dihydrofurans and diazo-functionalized trisubstituted furans have been established in a AgSbF6/DBU/DCE/0 °C system and a AgSbF6/DBU/Et2O·BF3/DCE/0 °C system, respectively. A Lewis acid and organic base cocontrolled reaction for the synthesis of diazo-functionalized trisubstituted furans is infrequent. For diazo-functionalized 2-methylene-2,3-dihydrofuran synthesis, the reaction possesses excellent diastereoselectivity and Z-selectivity. On the basis of Rh2(OAc)4-mediated unique decomposition of diazo-functionalized 2-methylene-2,3-dihydrofurans, an application to diastereoselective construction of a 5-methylene-4,7-dihydro-5H-furo[2,3-c]pyran frame has been achieved for the first time.

Simmons-Smith Cyclopropanation: A Multifaceted Synthetic Protocol toward the Synthesis of Natural Products and Drugs: A Review

Simmons-Smith cyclopropanation is a widely used reaction in organic synthesis for stereospecific conversion of alkenes into cyclopropane. The utility of this reaction can be realized by the fact that the cyclopropane motif is a privileged synthetic intermediate and a core structural unit of many biologically active natural compounds such as terpenoids, alkaloids, nucleosides, amino acids, fatty acids, polyketides and drugs. The modified form of Simmons-Smith cyclopropanation involves the employment of Et2Zn and CH2I2 (Furukawa reagent) toward the total synthesis of a variety of structurally complex natural products that possess broad range of biological activities including anticancer, antimicrobial and antiviral activities. This review aims to provide an intriguing glimpse of the Furukawa-modified Simmons-Smith cyclopropanation, within the year range of 2005 to 2022.

Photocatalytic synthesis of tetra-substituted furans promoted by carbon dioxide

A CO2-promoted transition metal-free photocatalytic synthesis of tetra-substituted furan derivatives from 1,3-diketones as the only starting material.

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.

Dirhodium(ii)-catalysed cycloisomerization of azaenyne: rapid assembly of centrally and axially chiral isoindazole frameworks

Described herein is a dirhodium(ii)-catalyzed asymmetric cycloisomerization reaction of azaenyne through a cap-tether synergistic modulation strategy, which represents the first catalytic asymmetric cycloisomerization of azaenyne. This reaction is highly challenging because of its inherent strong background reaction leading to racemate formation and the high capability of coordination of the nitrogen atom resulting in catalyst deactivation. Varieties of centrally chiral isoindazole derivatives could be prepared in up to 99 : 1 d.r., 99 : 1 er and 99% yield and diverse enantiomerically enriched atropisomers bearing two five-membered heteroaryls have been accessed by using an oxidative central-to-axial chirality transfer strategy. The tethered nitrogen atom incorporated into the starting materials enabled easy late-modifications of the centrally and axially chiral products via C–H functionalizations, which further demonstrated the appealing synthetic utilities of this powerful asymmetric cyclization.

Rh(ii)-catalyzed asymmetric cycloisomerization of azaenyne through a cap-tether synergistic modulation strategy was described. Diverse centrally and axially chiral isoindazoles were prepared and directed C–H late-stage modifications were developed.

Oxymetalation or oxidative cyclization? mechanism of Pd-catalyzed annulation of enynones

Enynones are powerful synthons for constructing furan derivatives in the presence of transition metal catalysts. Unlike the conventional intramolecular nucleophilic attack with the activation of coinage metals, we propose that enynones undergo an oxidative cyclization process with a Pd(0) species. The full catalytic cycle involves oxidative cyclization, isocyanide insertion, and reductive elimination, which was supported by DFT calculations. Geometric and electronic analyses confirmed the oxidative cyclization process, which proceeds via a Pd(ii) intermediate.

Chemo- and Enantioselective Insertion of Furyl Carbene into the N-H Bond of 2-Pyridones

Asymmetric carbene insertion reactions represent one of the most important protocols to construct carbon-heteroatom bonds. The use of donor-acceptor diazo compounds bearing an ester group is however a prerequisite for achieving high enantioselectivity. Herein, we report a chemo- and enantioselective formal N-H insertion of 2-pyridones that has been accomplished for the first time with enynones as the donor-donor carbene precursors. DFT calculations indicate an unprecedented enantioselective 1,4-proton transfer from O to C. The rhodium catalyst provides a chiral pocket in which the steric repulsion and the π-π interaction of the propeller ligand play a critical role in determining the selectivities.

De Novo and Divergent Synthesis of Highly Functionalized Furans by Cascade Reactions of 2-Hydroxy-1,4-diones with Nucleophiles

Herein, a divergent synthesis of a variety of 2α- and 5α-substituted furan derivatives from 2-hydroxy-1,4-diones is reported. By using appropriate substrates and an acid catalyst, the reactions occurred selectively through cyclization/1,6-conjugate addition or cyclization/Friedel-Crafts-type cascade reactions. A broad range of nucleophilic reagents (>10 types for the 1,6-conjugate addition for 5α substitution and >20 types for the Friedel-Crafts-type cascade reaction for 2α substitution), including alcohols, amides, furan, thiophene, pyrrole, indole, phenols, and many others, can successfully participate in the reactions, providing a universal strategy for a diversity-oriented synthesis of α-substituted furan derivatives. Deuteriation experiments and DFT calculations were carried out to support the proposed reaction mechanisms. Antifungal activity experiments revealed that products with an indole or 4-hydroxycoumarin core substituted at the 2α position showed moderate activities against Rhizoctorzia solani and Botrytis cinerea, respectively.

Gold(iii) promoted formation of dihydroquinazolinones: double X-H activation by gold

An efficient 2-furyl gold-carbene promoted synthetic method was developed for the formation of dihydroquinazolinones from enynones by dual insertion of anthranilamides. In this organic transformation a new C-O and two C-N bond formations occurred and dihydroquinazolinones were obtained with a quaternary centre in moderate to very good yields in one-pot synthesis.

Synthesis of Densely Substituted Sulfonylfurans and Dihydrofurans via Cascade Reactions of α-Functionalized Nitroalkenes with β-Ketosulfones

The reaction of β-ketosulfones with different α-functionalized nitroalkenes affords diversely substituted sulfonylfurans and dihydrofurans. Furthermore, β-ketosulfones react with α-bromonitroalkenes and α-hydrazinonitroalkenes via a cascade Michael addition-cyclization protocol to afford nitrodihydrofurans and hydrazinodihydrofurans, respectively, bearing a key sulfonyl group, in excellent yields with a broad substrate scope. Application of these products has been demonstrated by the synthesis of pyrroles and pyrazoles in good yields. The reaction of β-ketosulfones with nitroallylic acetates yields tetrasubstituted sulfonyl furans through a cascade S_N2'-intramolecular Michael reaction, followed by aromatization. The gram-scale synthesis of a representative example of sulfonylfurans was carried out to demonstrate the synthetic efficiency of the methodology.

Palladium-catalyzed cascade reactions of enynones and isocyanides: access towards functionalized ketenimine and its application

Palladium-catalyzed cascade reactions of enynones and isocyanides have been disclosed. This strategy allows for the synthesis of structurally unusual furan-incorporated ketenimines, which are stable and can undergo a formal [4+1] annulation with another isocyanide to yield indenone derivatives.

Recent progress on donor and donor-donor carbenes

Donor and donor-donor carbenes are two important kinds of carbenes, which have experienced tremendous growth in the past two decades. This review provides a comprehensive overview of the recent development of donor and donor-donor carbene chemistry. The development of this chemistry offers efficient protocols to construct a wide variety of C-C and C-X bonds in organic synthesis. This review is organized based on the different types of carbene precursors, including diazo compounds, hydrazones, enynones, cycloheptatrienes and cyclopropenes. The typical transformations, the reaction mechanisms, as well as their subsequent applications in the synthesis of complex natural products and bioactive molecules are discussed. Due to the rapidly increasing interest in this area, we believe that this review will provide a timely and comprehensive discussion of recent progress in donor and donor-donor carbene chemistry.

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.

Zinc-Catalyzed Asymmetric Formal [4+3] Annulation of Isoxazoles with Enynol Ethers by 6π Electrocyclization: Stereoselective Access to 2H-Azepines

6π electrocyclization has attracted interest in organic synthesis because of its high stereospecificity and atom economy in the construction of versatile 5-7-membered cycles. However, examples of asymmetric 6π electrocyclization are quite scarce, and have to rely on the use of chiral organocatalysts, and been limited to pentadienyl-anion- and triene-type 6π electrocyclizations. Described herein is a zinc-catalyzed formal [4+3] annulation of isoxazoles with 3-en-1-ynol ethers via 6π electrocyclization, leading to the site-selective synthesis of functionalized 2H-azepines and 4H-azepines in good to excellent yields with broad substrate scope. Moreover, this strategy has also been used to produce chiral 2H-azepines with high enantioselectivities (up to 97:3 e.r.). This protocol not only is the first asymmetric heptatrienyl-cation-type 6π electrocyclization, but also is the first asymmetric reaction of isoxazoles with alkynes and the first asymmetric catalysis based on ynol ethers.

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.

Generation of Donor/Donor Copper Carbenes through Copper-Catalyzed Diyne Cyclization: Enantioselective and Divergent Synthesis of Chiral Polycyclic Pyrroles

The generation of metal carbenes from readily available alkynes represents a significant advance in metal carbene chemistry. However, most of these transformations are based on the use of noble-metal catalysts and successful examples of such an asymmetric version are still very scarce. Here a copper-catalyzed enantioselective cascade cyclization of N-propargyl ynamides is reported, enabling the practical and atom-economical construction of diverse chiral polycyclic pyrroles in generally good to excellent yields with wide substrate scope and excellent enantioselectivities (up to 97:3 e.r.). Importantly, this protocol represents the first copper-catalyzed asymmetric diyne cyclization. Moreover, mechanistic studies revealed that the generation of donor/donor copper carbenes is presumably involved in this 1,5-diyne cyclization, which is distinctively different from the related gold catalysis, and thus it constitutes a novel way for the generation of donor/donor metal carbenes.

Syntheses of Z-Iodovinylfurans and 2-Acyl Furans via Controllable Cyclization of Ynenones

A divergent synthesis of Z-iodovinylfurans and 2-acyl furans promoted by NIS via controllable cyclization of ynenones is reported. The reaction proceeded by sequential 5-exo-dig electrophilic cyclization to intermediate 2-(iodomethylene)-2H-furanium cation D, providing a range of synthetically valuable and useful trisubstituted furan derivatives 2 and 3 in moderate to excellent yields. This approach is metal-free, mild, and atom-economic, with good selectivity and high stereoselectivity.

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.

Zinc-catalyzed regioselective C–P coupling of p-quinol ethers with secondary phosphine oxides to afford 2-phosphinylphenols

A highly regioselective C–P coupling reaction of p-quinol ethers with secondary phosphine oxides is developed as a new synthesis method for 2-phosphinylphenols by using Zn(OTf)₂ as the catalyst.

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.