Diastereo- and Enantioselective Gold(I)-Catalyzed Intermolecular Tandem Cyclization/[3+3]Cycloadditions of 2-(1-Alkynyl)-2-alken-1-ones with Nitrones

Recent transformations of vinylidene cyclopropanes (VDCPs)

Methodological studies on the transformations of vinylidene cyclopropanes (VDCPs) have been substantially developed in the past few decades, and significant progress has been achieved in visible light-mediated, non-metal-related, and transition metal-catalyzed processes. In particular, when reactive functional groups are introduced into the backbone of VDCPs, a variety of cascade or sequential transformations can take place to produce more complex cyclic or polycyclic compounds. This review describes the recent advancements in this field.

Cascade Reaction of Enyne-Amides with Sulfur-Ylides for the Synthesis of Indole-Tethered 5-Oxaspiro[2.4]hept-6-ene Derivatives

An unexpected cascade reaction of enyne-amides with sulfur-ylides has been developed. This cascade reaction involves cycloisomerization, dearomatic cyclopropanation, ring-opening rearomatization, and subsequent cyclopropanation, differing from the common [2 + n] cyclization of enyne-amides. A variety of (spirocyclopropane)dihydrofuran derivatives have been efficiently and conveniently synthesized in a single vessel, exhibiting excellent diastereoselectivity and good functional group tolerance.

Asymmetric [3 + 3] cycloaddition of cinnamaldehyde-derived N-aryl nitrones with 2-indolemethanols enabled by chiral phosphoric acid

We described a chiral phosphoric acid (CPA) catalyzed asymmetric [3 + 3] cycloaddition of cinnamaldehyde-derived N-aryl nitrones with 2-indolylmethanols to prepare various indole-fused 1,2-oxazines in high yields (up to 96%) with excellent enantioselectivity (>99% ee). Control experiments indicate that hydrogen bonding plays important roles in controlling the enantioselectivity of products. This strategy provides an efficient pathway to construct enantioenriched indole-fused 1,2-oxazines from N-aryl nitrones with 2-indolylmethanols.

Enantioselective Au(I)-Catalyzed Cycloisomerization/Addition of Oxygen Nucleophiles to 2-Alkynylenones by the Tethered Counterion-Directed Catalysis Strategy

The tethered counterion-directed catalysis (TCDC) strategy enables the Au(I)-catalyzed highly enantioselective synthesis of bicyclic furan derivatives via a reaction sequence combining the cycloisomerization of 2-alkynyl enones and the addition of nucleophiles. A large range of oxygenated nucleophiles, such as water, alcohols, carboxylic acids, and peroxides, have successfully been used as nucleophiles, delivering the chiral furane derivatives in high enantioselectivities (mostly above 90% enantiomeric excess). The CPAphosAuCl complexes were used with catalytic loadings as low as 0.2 mol % in most cases, in combination with silver carbonate as the chloride abstractor.

Stereoselective Double Spirocyclization of 2-Benzyl-3-alkynyl Chromone with Nitrone via Gold-Catalyzed Cascade Reactions

A novel, highly stereoselective gold-catalyzed spirocyclization of 2-benzyl-3-alkynyl chromone with nitrone is described. This cascade reaction involves gold-catalyzed cycloisomerization, nitrone-olefin [3 + 2]-annulation, alkene oxidation, and rearrangement for the formation of spirocyclic products. Interestingly, the isoxazolidine ring generated from [3 + 2]-annulation donates oxygen to alkene to generate a new pyran-3(4H)-one and azetidine ring for dispiro-benzofuran formation upon heating. This work demonstrates the one-pot, gold-catalyzed, multiple-step reaction, and the reaction temperature directly affects the formation of spirocyclic products.

Catalytic [4+2]- and [4+4]-cycloaddition using furan-fused cyclobutanone as a privileged C4 synthon

Cycloaddition reactions play a pivotal role in synthetic chemistry for the direct assembly of cyclic architectures. However, hurdles remain for extending the C4 synthon to construct diverse heterocycles via programmable [4+n]-cycloaddition. Here we report an atom-economic and modular intermolecular cycloaddition using furan-fused cyclobutanones (FCBs) as a versatile C4 synthon. In contrast to the well-documented cycloaddition of benzocyclobutenones, this is a complementary version using FCB as a C4 reagent. It involves a C-C bond activation and cycloaddition sequence, including a Rh-catalyzed enantioselective [4 + 2]-cycloaddition with imines and an Au-catalyzed diastereoselective [4 + 4]-cycloaddition with anthranils. The obtained furan-fused lactams, which are pivotal motifs that present in many natural products, bioactive molecules, and materials, are inaccessible or difficult to prepare by other methods. Preliminary antitumor activity study indicates that 6e and 6 f exhibit high anticancer potency against colon cancer cells (HCT-116, IC50 = 0.50 ± 0.05 μM) and esophageal squamous cell carcinoma cells (KYSE-520, IC50 =  0.89 ± 0.13 μM), respectively.

Rhodium-Catalyzed Asymmetric Hydrogenation and Transfer Hydrogenation of 1,3-Dipolar Nitrones

Owing to their distinctive 1,3-dipolar structure, the catalytic asymmetric hydrogenation of nitrones to hydroxylamines has been a formidable and longstanding challenge, characterized by intricate enantiocontrol and susceptibility to N-O bond cleavage. In this study, the asymmetric hydrogenation and transfer hydrogenation of nitrones were accomplished with a tethered TsDPEN-derived cyclopentadienyl rhodium(III) catalyst (TsDPEN: p-toluenesulfonyl-1,2-diphenylethylene-1,2-diamine), the reaction proceeds via a novel 7-membered cyclic transition state, producing chiral hydroxylamines with up to 99 % yield and >99 % ee. The practical viability of this methodology was underscored by gram-scale catalytic reactions and subsequent transformations. Furthermore, mechanistic investigations and DFT calculations were also conducted to elucidate the origin of enantioselectivity.

Elucidating the chemical dynamics of the elementary reactions of the 1-propynyl radical (CH3CC; X2A1) with 2-methylpropene ((CH3)2CCH2; X1A1)

Exploiting the crossed molecular beam technique, we studied the reaction of the 1-propynyl radical (CH3CC; X2A1) with 2-methylpropene (isobutylene; (CH3)2CCH2; X1A1) at a collision energy of 38 ± 3 kJ mol-1. The experimental results along with ab initio and statistical calculations revealed that the reaction has no entrance barrier and proceeds via indirect scattering dynamics involving C7H11 intermediates with lifetimes longer than their rotation period(s). The reaction is initiated by the addition of the 1-propynyl radical with its radical center to the π-electron density at the C1 and/or C2 position in 2-methylpropene. Further, the C7H11 intermediate formed from the C1 addition either emits atomic hydrogen or undergoes isomerization via [1,2-H] shift from the CH3 or CH2 group prior to atomic hydrogen loss preferentially leading to 1,2,4-trimethylvinylacetylene (2-methylhex-2-en-4-yne) as the dominant product. The molecular structures of the collisional complexes promote hydrogen atom loss channels. RRKM results show that hydrogen elimination channels dominate in this reaction, with a branching ratio exceeding 70%. Since the reaction of the 1-propynyl radical with 2-methylpropene has no entrance barrier, is exoergic, and all transition states involved are located below the energy of the separated reactants, bimolecular collisions are feasible to form trimethylsubstituted 1,3-enyne (p1) via a single collision event even at temperatures as low as 10 K prevailing in cold molecular clouds such as G+0.693. The formation of trimethylsubstituted vinylacetylene could serve as the starting point of fundamental molecular mass growth processes leading to di- and trimethylsubstituted naphthalenes via the HAVA mechanism.

Revised Mechanism of Gold-Catalyzed Thioallylation of Propiolates

Gold-catalyzed enantioselective thioallylation of propiolates proved effective in delivering highly enantio-enriched α-allyl-β-thioacrylates. In this work, we report a revised mechanism for this process based on the new mechanistic experiments and kinetic data in the presence of a competitive inhibitor. The employment of thioethers as nucleophiles inevitably involves their competitive binding to the only catalytic site of the Au(I) catalyst, which may inhibit the activity. We developed a modified Hammett plot in the presence of a dummy thioether inhibitor, which revealed a true kinetic profile, excluding the effect of inhibition. A revised mechanism suggested that the conjugate addition of thioethers to the Au(I)-activated alkynes is the turnover-limiting step, and the subsequent [3,3]-rearrangement occurs quickly, suggesting the efficacy of the sulfonium-based approach in accelerating Claisen rearrangement. In addition, the enantioselectivity was suggested to be determined during the sigmatropic rearrangement by discriminating the prochiral olefin faces of the allyl group in the σ-bound Au(I) complex.

Trapping of Arynes with In Situ Generated Aryltrifluoromethylnitrones Enables Access to Trifluoromethylated Benzoxazolines

Herein, we report a rare example of trapping arynes using in situ generated aryltrifluoromethylnitrone to access an important class of trifluoromethylated benzoxazolines. This three-component strategy involves a nitrone formation/[3 + 2] cycloaddition/thermal rearrangement cascade and furnishes trifluoromethylated benzoxazolines in high yields. The scope of the reaction is quite broad with respect to aryltrifluorodiazoethanes, nitrosoarenes, and arynes. The proposed reaction pathway is supported through the isolation and characterization of the key reaction intermediates phenyltrifluoromethylnitrone and benzisoxazoline.

Silver/chiral pyrrolidinopyridine relay catalytic cycloisomerization/(2 + 3) cycloadditions of enynamides to asymmetrically synthesize bispirocyclopentenes as PDE1B inhibitors

Significant progress has been made in asymmetric synthesis through the use of transition metal catalysts combined with Lewis bases. However, the use of a dual catalytic system involving 4-aminopyridine and transition metal has received little attention. Here we show a metal/Lewis base relay catalytic system featuring silver acetate and a modified chiral pyrrolidinopyridine (PPY). It was successfully applied in the cycloisomerization/(2 + 3) cycloaddition reaction of enynamides. Bispirocyclopentene pyrazolone products could be efficiently synthesized in a stereoselective and economical manner (up to >19:1 dr, 99.5:0.5 er). Transformations of the product could access stereodivergent diastereoisomers and densely functionalized polycyclic derivatives. Mechanistic studies illustrated the relay catalytic model and the origin of the uncommon chemoselectivity. In subsequent bioassays, the products containing a privileged drug-like scaffold exhibited isoform-selective phosphodiesterase 1 (PDE1) inhibitory activity in vitro. The optimal lead compound displayed a good therapeutic effect for ameliorating pulmonary fibrosis via inhibiting PDE1 in vivo.

Highly Diastereoselective One-Pot Synthesis of 4,5-Dihydrofuro[2,3-b]azocin-6-one Derivatives through Cyclization/[4+4] Annulation Reactions

A variety of 4,5-dihydrofuro[2,3-b]azocin-6-one derivatives were expediently assembled through Au(I)-catalyzed cyclization and 2-(tert-butyl)-1,1,3,3-tetramethylguanidine (BTMG)-mediated [4+4] annulation reactions of enyne-amides and ynones. The reactions exhibit high efficiency with excellent regio- and diastereoselectivity. A broad spectrum of substrates was utilized. The products with an eight-membered ring might be useful in biological chemistry and medicinal science. Furthermore, the products could be facilely converted into various derivatives.

Synthesis of α-pyrones via gold-catalyzed cycloisomerization/[2 + 1] cycloaddition/rearrangement of enyne-amides and sulfur ylides

A novel gold-catalyzed cycloisomerization/[2 + 1]cycloaddition/rearrangement of enyne-amides and sulfur ylides is reported. This strategy enables rapid and efficient construction of a series of α-pyrone derivatives.

H-Bonded Counterion-Directed Catalysis: Enantioselective Gold(I)-Catalyzed Addition to 2-Alkynyl Enones as a Case Study

H-bonded counterion-directed catalysis (HCDC) is a strategy wherein a chiral anion that is hydrogen-bonded to the achiral ligand of a metal complex is responsible for enantioinduction. In this article we present the application of H-bonded counterion-directed catalysis to the Au(I)-catalyzed enantioselective tandem cycloisomerization-addition reaction of 2-alkynyl enones. Following the addition of C-, N- or O-centered nucleophiles, bicyclic furans were obtained in moderate to excellent yield and enantioselectivity (28 examples, 59-96 % yield, 62 : 38 to 95 : 5 er). The optimal catalytic system, comprising a phosphinosquaramide Au(I) chloride complex and a BINOL-derived phosphoramidate Ag(I) salt, was selected in a combinatorial fashion from a larger library with the help of high-throughput screening. An enantioselectivity switch of ca. 120 Δee% was observed upon addition of the achiral Au(I) component to the Ag(I) salt.

Gold(i)-catalyzed diastereo- and enantioselective [4 + 3] cycloadditions: construction of functionalized furano-benzoxepins

Highly diastereo- and enantioselective [4 + 3] cycloadditions of 2-(1-alkynyl)-2-alken-1-ones with o-QMs have been realized via a simple chiral gold catalysis, providing facile access to various functionalized furano-benzoxepins.

Catalyst-controlled cycloisomerization/[4+3] cycloaddition sequence to construct 2,3-furan-fused dihydroazepines and 2,3-pyrrole-fused dihydrooxepines

A novel catalyst-controlled cycloisomerization/[4+3]cycloaddition sequence of readily available acyclic enyne-amides and crotonate-derived sulfur ylides is reported. This strategy enables the rapid and efficient construction of a series of bicyclic 2,3-furan-fused...

Indium-Catalysed Transfer Hydrogenation for the Reductive Cyclisation of 2-Alkynyl Enones towards Trisubstituted Furans

Indium tribromide catalysed the transfer hydrogenation from dihydroaromatic compounds, such as the commercially available γ-terpinene, to enones, which resulted in the cyclisation to trisubstituted furan derivatives. The reaction was initiated by a Michael addition of a hydride nucleophile to the enone subunit followed by a Lewis-acid-assisted cyclisation and the formation of a furan-indium intermediate and a Wheland intermediate derived from the dihydroaromatic starting material. The product was formed by protonation from the Wheland complex and replaced the indium tribromide substituent. In addition, a site-specific deuterium labelling of the dihydroaromatic HD surrogates resulted in site specific labelling of the products and gave useful insights into the reaction mechanism by H-D scrambling.

Diastereoselectivity-Switchable Gold-Catalyzed Formal [3+2]-Cycloadditions of N-2,2,2-Trifluoroethylisatin Ketimines with Yne-Enones

The unexpected gold-catalyzed formal [3+2]-cycloaddition of N-2,2,2-trifluoroethylisatin ketimines with 2-(1-Alkynyl)-2-alken-1-ones is reported. Both diastereomers of the corresponding cycloadducts were formed in moderate to excellent yields with excellent diastereoselectivities by switching the catalytic system from mono-gold to gold/silver bimetallic catalytic system. The practicality of this protocol is demonstrated by scale-up reaction and the transformations of the cycloadduct.

Access to Chiral Polycyclic 1,4-Dihydropyridines via Organocatalytic Formal [3 + 3] Annulation of 2-(1-Alkynyl)-2-alken-1-ones with 3-Aminobenzofurans

A rational designed tandem reaction of 2-(1-alkynyl)-2-alken-1-ones with 3-aminobenzofurans enabled by a chiral bifunctional catalyst is described, affording biologically significant polycyclic 1,4-dihydropyridines in moderate to good yields (43-82%) with good to excellent enantioselectivities (83-99%). This formal [3 + 3] annulation reaction reveals good practicality when conducted on a gram scale, and the cycloadduct has the capability for further elaborations.

Highly enantioselective tandem cycloisomerization/Diels-Alder reaction of 2-(1-alkynyl)-2-alken-1-ones and enals: dual catalysis with platinum and amines

Herein, we disclosed a highly efficient strategy of enantioselective synthesis of 2,3-furan-fused carbocycles bearing three-contiguous stereocenters. This transformation is catalyzed by dual catalysis of PtCl4/chiral amines via tandem dehydrogenative annulation/Diels-Alder reaction of 2-(1-alkynyl)-2-alken-1-ones and enals. The in situ generation of the furan-based ortho-quinodimethane intermediates and the iminium activation of enals are crucial to this transformation.

Recent Advances in the Development of Chiral Gold Complexes for Catalytic Asymmetric Catalysis

Asymmetric gold catalysis has been rapidly developed in the past ten years. Breakthroughs have been made by rational design and meticulous selection of chiral ligands. This review summarizes newly developed gold-catalyzed enantioselective organic transformations and recent progress in ligand design (since 2016), organized according to different types of chiral ligands, including bisphosphine ligands, monophosphine ligands, phosphite-derived ligands, and N-heterocyclic carbene ligands for asymmetric gold(I) catalysis as well as heterocyclic carbene ligands and oxazoline ligands for asymmetric gold(III) catalysis.

Dinuclear gold catalysis

This review summarizes the recent achievements of dinuclear gold-catalyzed redox coupling, asymmetric catalysis and photocatalysis. The dinuclear gold catalysts show a better catalytic performance than the mononuclear gold catalysts in certain cases.

Yne-Enones Enable Diversity-Oriented Catalytic Cascade Reactions: A Rapid Assembly of Complexity

A small-molecule collection with structural diversity and complexity is a prerequisite to using either drug candidates or chemical probes for drug discovery and chemical-biology investigations, respectively. Over the past 12 years, we have engaged in developing efficient diversity-oriented cascade strategies for the synthesis of topologically diverse skeletons incorporating biologically relevant structural motifs such as O- and N-heterocycles, fused polycycles, and multifunctionalized allenes. In particular, we have highlighted the use of simple, linear, and densely functionalized molecular platforms in these reactions.This account details our efforts in the design of novel molecular platforms for use in metal- and organo-catalyzed cascade reactions, which include 2-(1-alknyl)-2-alken-1-ones (yne-enones) for heterocyclization/cross-coupling cascades, heterocyclization/cycloaddition cascades, nucleophilic addition/cross-coupling cascades, nucleophilic addition/heterocyclization cascades, and so on. Moreover, this Account outlines corresponding mechanistic insights, computational information, and applications of these cascades in the construction of various highly substituted carbo- and heterocycles as well as highly functionalized acyclic compounds, e.g., allenes and dienes. In addition to yne-enones, we evolved the functional groups of our original yne-enones to provide a series of yne-enone variants, which resulted in products with complementary reactivities.The reactivity profile of the yne-enones is defined by the presence of an alkyne moiety and a conjugated enone unit and their mutual through-bond connectivity. Owing to the conceptually rapid development of carbophilic activation, we have identified a series of efficient catalytic systems consisting of metal catalysts, including Pd, Au, and Rh complexes, for diversity-oriented cascade catalysis, allowing various unprecedented reactions to be achieved through different-types of reaction intermediates, including all-carbon metal 1,n-dipoles, furan-based o-quinodimethanes (oQDMs), and allenyl-metal species. In addition to commonly known transition-metal catalytic activity, the Lewis acidity of these complexes is crucial to accomplish the corresponding transformation. In addition, highly enantioselective gold(I)-catalyzed heterocyclization/cycloaddition cascades of yne-enones and their variants were achieved by the application of bisphosphines (e.g., Cn-TunePhos), monophosphines, and our developed "Ming-Phos" as chiral ligands. Importantly, Ming-Phos ligands exhibited excellent performance in gold-catalyzed mechanistically distinct [3 + n]-cycloaddition reactions, in which the chiral sulfinamide moiety is possibly responsible for the interaction with the substrate to control enantioselectivity. Subsequently, we demonstrated that the easily prepared polymer-supported Ming-Phos ligand could be applied for heterogeneously gold(I)-catalyzed asymmetric cycloaddition with good stereocontrol. With metal-free catalysis, the divergent functionalization of yne-enones provides numerous synthetic outlets for structure diversification. For example, yne-enones are particularly attractive for use as precursors of various chiral and achiral heterocycles, such as pyrazoles, isoxazoles, pyrroles, and pyrans, etc.

Copper-Catalyzed Asymmetric Reaction of Alkenyl Diynes with Styrenes by Formal [3 + 2] Cycloaddition via Cu-Containing All-Carbon 1,3-Dipoles: Access to Chiral Pyrrole-Fused Bridged [2.2.1] Skeletons

The generation of metal-containing 1,3-dipoles from metal carbenes represents a significant advance in 1,3-dipolar cycloaddition reactions. However, these transformations have so far been limited to reactions based on diazo compounds or triazoles as precursors. Herein, we disclose a copper-catalyzed enantioselective reaction of alkenyl N-propargyl ynamides with styrene derivatives by formal [3 + 2] cycloaddition via Cu-containing all-carbon 1,3-dipoles, which constitutes a novel way for the generation of metal-containing 1,3-dipoles via metal carbenes. This protocol allows the practical and atom-economical synthesis of valuable chiral pyrrole-fused bridged [2.2.1] skeletons in moderate to good yields (up to 90% yield) with excellent diastereoselectivities (dr > 50/1) and generally excellent enantioselectivities (up to >99% ee).

2-Activated 1,3-enynes in enantioselective synthesis

The rapid enantioselective synthesis of valuable building blocks and pharmaceutically important compounds from easily accessible precursors is one of the major areas of focus in organic chemistry. In this context, 2-activated 1,3-enyne has emerged as a powerful synthon in recent years for the efficient synthesis of enantioenriched furans, allenes, 4-H-pyrans, and 4-isoxazolines, which are privileged scaffolds in bioactive compounds and natural products. This review will cover the history of the development of 2-activated 1,3-enyne in enantioselective synthesis along with the corresponding mechanisms, which may motivate further development in this area to forge more complex and valuable molecules.

Deciphering the Origin of Enantioselectivity on the Cis-Cyclopropanation of Styrene with Enantiopure Di-chloro,Di-gold(I)-SEGPHOS Carbenoids Generated from Propargylic Esters

The stereoselective synthesis of cis-disubstituted cyclopropanes by the Au(I)/PPh₃-catalyzed cycloaddition of propargylic esters and styrene has been studied using density functional theory calculations. The computed mechanistic scheme involves the rate-limiting 1,2-rearrangement of the propargylic ester with the π-coordinated gold complex, followed by the (2 + 1)-cheletropic reaction of styrene with the alkenyl-Au(I) carbene intermediate to afford the cis-disubstituted cyclopropane derivative in a high cis/trans diastereomeric ratio. With a ( R)-di-chloro,di-gold-DTBM-SEGPHOS complex as the catalyst, computations are consistent with a rate-determining (2 + 1)-cheletropic reaction, in which facial discrimination is proposed to result from a combination of subtle steric and electronic effects in the SiRe facial approach transition structure, which favor the formation of the cis-cyclopropane diastereomer of 1 R,2 S absolute configuration, as experimentally observed.