Tandem au-catalyzed 3,3-rearrangement-[2 + 2] cycloadditions of propargylic esters: expeditious access to highly functionalized 2,3-indoline-fused cyclobutanes

Gold-Catalyzed Lactone Synthesis: Advancements and Insights

Lactones represent a class of fundamental structural motifs ubiquitous in nature, holding significance across diverse scientific domains such as pharmaceuticals, natural products, drug discovery, and industry. Despite their simplicity, the synthesis of lactones has garnered considerable interest due to their pivotal roles. Gold, traditionally regarded as a noble metal, has emerged as an efficient catalyst, challenging conventional perceptions. The utilization of gold in lactone synthesis has captivated researchers, leading to the development of numerous effective methodologies. Motivated by this, we present a comprehensive compilation of reports on the gold-catalyzed synthesis of lactones, encompassing literature till date.

Rhodium(III)-Catalyzed Intramolecular Cyclization and Sequential Aromatization of Ynamides with Propargyl Esters: Access to 2,5-Dihydropyrroles and Pyrroles

Disclosed herein is a rhodium(III)-catalyzed intramolecular cyclization of ynamides with propargyl esters. A variety of highly functionalized 2,5-dihydropyrroles were obtained in moderate to good yields with high E/Z selectivities. Subsequent oxidation of the products gave valuable pyrrole derivatives. Additionally, scale-up reactions and late-stage derivatizations highlight the potential synthetic utility of this methodology.

Tandem Isomerization/α,β-Site-Selective and Enantioselective Addition Reactions of N-(3-Butynoyl)-3,5-dimethylpyrazole Induced by Chiral π-Cu(II) Catalysts

Allenes are important building blocks, and derivatization of products via cycloadditions of allenes could become a powerful strategy for constructing carbocyclic and heterocyclic rings. However, the development of catalytic site-selective and enantioselective cycloaddition reactions of allenes still presents significant challenges. Here, we report chiral π-Cu(II)-complex-catalyzed isomerization of N-(3-butynoyl)-3,5-dimethyl-1H-pyrazole to generate N-allenoylpyrazole in situ and subsequent α,β-site-selective and enantioselective [3 + 2], [4 + 2], or [2 + 2] cycloaddition or conjugate addition reactions. The asymmetric environment created by the intramolecular π-Cu(II) interactions provides the corresponding adducts in moderate to high yield with excellent enantioselectivity. To the best of our knowledge, this is the first successful method for chiral-Lewis-acid-catalyzed tandem isomerization/α,β-site-selective and enantioselective cycloaddition or conjugate addition reactions of latent non-γ-substituted allenoyl derivative.

Regiodivergent and Stereoselective Synthesis of Highly Substituted 1,3-Dienes via Arylative Acyloxy Migration of Propargyl Esters

We report the first catalyst-controlled regiodivergent method that enables the synthesis of structurally diverse 1,2,3,4-tetrasubstituted conjugated dienes with excellent regio- and stereochemical outcomes from the same set of readily available propargyl esters and diaryliodonium salts. In this diene chemistry, the in situ generated, highly electrophilic aryl-CuIII complex serves not only as a π-Lewis acid catalyst for alkyne activation/acyloxy migration but also as an aryl electrophile equivalent. The competitive arylative 1,2- and 1,3-acyloxy migration patterns are exquisitely dictated by Cu and Au/Cu relay catalyses, respectively, providing a modular and attractive approach to traditionally inaccessible tetrasubstituted 1,3-dienes in a regiodivergent manner. Finally, the synthetic utility of this method is demonstrated by further synthetic derivatization of 1,3-dienes into an array of useful compounds.

Gold-Catalyzed Anti-Markovnikov Oxidation of Au-Allenylidene to Generate Alkylidene Ketene

It remains a long-standing challenge to directly convert alkynes to carboxylic derivatives. Herein, a unexpectedly anti-Markovnikov oxidation of a unique Au-allenylidene pathway instead of a traditional α-oxo gold carbene routine is disclosed for in situ formation and transformation of highly unsaturated alkylidene ketenes, which are subsequently trapped by broad nucleophiles such as alcohols, phenols, water, amines, and sulfoximines to easily access α,β-unsaturated drugs and natural product derivatives by a multicomponent reaction. Based on this scenario, polyacrylate and polyacrylamide are efficiently afforded by corresponding multicomponent polymerization.

Base-controlled dearomative [3 + 2] cycloadditions between 3-nitro-indoles and fumaric acid amide esters

The base-controlled dearomative [3 + 2] cycloaddition reaction between 3-nitroindoles and fumaric acid amide esters has been disclosed by using the dearomatization and aromatization strategy. Three kinds of diverse functionalized pyrrolo[2,3-b]indole derivatives were obtained respectively with excellent chemoselectivities and good diastereoselectivities using different bases.

Electrophilic halogenations of propargyl alcohols: paths to α-haloenones, β-haloenones and mixed β,β-dihaloenones

The Meyer–Schuster rearrangement of propargyl alcohols or alkynols leading to α,β-unsaturated carbonyl compounds is well known. Yet, electrophilic halogenations of the same alkynols and their alkoxy, ester and halo derivatives are inconspicuous. This review on the halogenation reactions of propargyl alcohols and derivatives intends to give a perspective from its humble direct halogenation beginning to the present involving metal catalysis. The halogenation products of propargyl alcohols include α-fluoroenones, α-chloroenones, α-bromoenones and α-iodoenones, as well as β-haloenones and symmetrical and mixed β,β-dihaloenones. They are, in essence, tri and tetrasubstituted alkenes carrying halo-functionalization at the α- or β-carbon. This is a potential stepping stone for further construction towards challenging substituted alkenones via Pd-catalysed coupling reactions.

This review highlights the development of α-haloenone, β-haloenone and mixed β,β-dihaloenone formations from propargyl alcohols via direct electrophilic halogenations and metal catalysed-halonium interception rearrangements.

Synthesis of polycyclic spiroindolines via the cascade reaction of 3-(2-isocyanoethyl)indoles

Tandem reactions of the yttrium(iii) catalyzed ring-opening reaction of 2,2'-diester aziridines with 3-(2-isocyanoethyl)indoles and the subsequent Friedel-Crafts/Mannich/desulfonylation were reported. A series of polycyclic spiroindolines containing tetrahydro-β-carbolines were obtained in moderate to excellent yields (56-92%) in one step under mild reaction conditions. A possible catalytic mechanism was also proposed.

Cascade Reaction of Tryptamine-Derived Isocyanides with Nitrile Oxides: Construction of Oxazine Fused-Spiroindoline Derivatives

A one-step cascade reaction of tryptamine-derived isocyanides with in situ generated nitrile oxides for the synthesis of fused spiroindolines was described. The desired products could be efficiently synthesized in moderate to good yields (42-87%). The protocol features mild conditions, wide substrate scope, and high efficiency.

(Z)-γ-Alkylidenebutenolide Synthesis through Au(I)-Catalyzed 1,3-Acyloxy Migration and the Carbonyl-Ene Reaction

A bimetallic protocol has been developed to construct (Z)-γ-alkylidenebutenolide compounds from readily available propargyl α-ketoesters. It involves a gold-catalyzed 1,3-acyloxy migration of propargyl α-ketoesters and a carbonyl-ene cyclization of in situ generated allenyl esters. DFT calculations suggest that the copper salt might play dual roles as both chloride abstractor facilitating the generation of highly active gold catalyst and Lewis acid promoting the stepwise intramolecular carbonyl-ene reaction.

Catalytic Gold Chemistry: From Simple Salts to Complexes for Regioselective C-H Bond Functionalization

Gold coordinated to neutral phosphines (R₃ P), N-heterocyclic carbenes (NHCs) or anionic ligands is catalytically active in functionalizing various C-H bonds with high selectivity. The sterics/electronic nature of the studied C-H bond, oxidation state of gold and stereoelectronic capacity of the coordinated auxiliary ligand are some of the associated selectivity factors in gold-catalyzed C-H bond functionalization reactions. Hence, in this review a comprehensive update about the action of different types of gold catalysts, from simple to sophisticated ones, on C-H bond reactions and their regiochemical outcome is disclosed. This review also highlights the catalytic applications of Au(I)- and Au(III)-species in creating new opportunities for the regio- and site-selective activation of challenging C-H bonds. Finally, it also intends to stress the potential applications in selective C-H bond activation associated with a variety of heterocycles recently described in the literature.

Copper-catalyzed cyclization reaction: synthesis of trifluoromethylated indolinyl ketones

A novel, simple, effective and rapid synthetic method to construct the C-2 trifluoromethylated indolinyl ketones via a copper-catalyzed cyclization reaction between N-alkylaniline and β-(trifluoromethyl)-α,β-unsaturated enones was developed. The results of the control experiments show that the reaction may involve a radical mechanism by a single-electron transfer process. Moreover, a broad substrate scope and good functional groups, high diastereoselectivities (dr, up to >20 : 1) as well as gram-scale synthesis make this approach highly attractive.

Copper-Catalyzed Tandem Cross-Coupling/Thermally Promoted [2 + 2] Cycloaddition of 1,6-Enynes and Diazo Compounds To Assemble Methylenecyclobutane-Fused Ring System

An unprecedented copper-catalyzed tandem reaction of 1,6-enynes with diazo compounds via a cross-coupling/[2 + 2] cycloaddition sequence was reported. A library of methylenecyclobutane-fused ring systems including cyclobuta[b]indolines, cyclobuta[b]benzofuran, benzo[b]cyclobuta[d]thiophene, and bicyclo[3.2.0] structures were obtained in moderate to excellent yields under very mild reaction conditions. The reaction exhibited high proximal-regioselectivity and diastereoselectivity. Moreover, 1,6-allenene has proven to be the key intermediate and proceeds via a thermally promoted [2 + 2] cycloaddition in the absence of copper catalyst.

Divergent Gold Catalysis: Unlocking Molecular Diversity through Catalyst Control

The catalyst-directed divergent synthesis, commonly termed as "divergent catalysis", has emerged as a promising technique as it allows chartering of structurally distinct products from common substrates simply by modulating the catalyst system. In this regard, gold complexes emerged as powerful catalysts as they offer unique reactivity profiles as compared to various other transition metal catalysts, primarily due to their salient electronic and geometrical features. Owing to the tunable soft π-acidic nature, gold catalysts not only evolved as superior contenders for catalyzing the reactions of alkynes, alkenes, and allenes but also, more intriguingly, have been found to provide divergent reaction pathways over other π-acid catalysts such as Ag, Pt, Pd, Rh, Cu, In, Sc, Hg, Zn, etc. The recent past has witnessed a renaissance in such examples wherein, by choosing gold catalysts over other transition metal catalysts or by fine-tuning the ligands, counteranions or oxidation states of the gold catalyst itself, a complete reactivity switch was observed. However, reviews documenting such examples are sporadic; as a result, most of the reports of this kind remained scattered in the literature, thereby hampering further development of this burgeoning field. By conceptualizing the idea of "Divergent Gold Catalysis (DGC)", this review aims to consolidate all such reports and provide a unified approach necessary to pave the way for further advancement of this exciting area. Based on the factors governing the divergence in product formation, an explicit classification of DGC has been provided. To gain a fundamental understanding of the divergence in observed reactivities and selectivities, the review is accompanied by mechanistic insights at appropriate places.

Heteroarene-tethered Functionalized Alkyne Metamorphosis

Heteroarene-tethered functionalized alkynes are multipotent synthons in organic chemistry. This detailed Review described herein offers a thorough discussion of the metamorphosis of heteroarene-tethered functionalized alkynes, an area which has earned much attention over the past decade in the straightforward synthesis of architecturally complex heterocyclic scaffolds in atom and step economic manner. Depending upon the variety of functionalized alkynes, this Review is divided into multiple sections. Amongst the vast array of synthetic transformations covered, dearomatizing spirocyclizations and cascade spirocyclization/rearrangement are of great interest. Synthetic transformations involving the heteroarene-tethered functionalized alkynes with scope, challenges, limitations, mechanism, their application in the total synthesis of natural products and future perceptions are surveyed.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Access to substituted cyclobutenes by tandem [3,3]-sigmatropic rearrangement/[2 + 2] cycloaddition of dipropargylphosphonates under Ag/Co relay catalysis

We present herein an unconventional tandem [3,3]-sigmatropic rearrangement/[2 + 2] cycloaddition of simple dipropargylphosphonates to deliver a range of bicyclic polysubstituted cyclobutenes and cyclobutanes under Ag/Co relay catalysis. An interesting switch from allene-allene to allene-alkyne cycloaddition was observed based on the substitution of the substrates, which further diversified the range of compounds accessible from this practical method. Significantly, preliminary biological screening of these new compounds identified promising candidates as suppressors of cellular proliferation.

Gold-Catalyzed Synthetic Strategies towards Four-Carbon Ring Systems

Four carbon ring systems are frequently present in natural products with remarkable biological activities such as terpenoids, alkaloids, and steroids. The development of new strategies for the assembly of these structures in a rapid and efficient manner has attracted the interest of synthetic chemists for a long time. The current research is focused mainly on the development of synthetic methods that can be performed under mild reaction conditions with a high tolerance to functional groups. In recent years, gold complexes have turned into excellent candidates for this aim, owing to their high reactivity, and are thus capable of promoting a wide range of transformations under mild conditions. Their remarkable efficiency has been thoroughly demonstrated in the synthesis of complex organic molecules from simple starting materials. This review summarizes the main synthetic strategies described for gold-catalyzed four-carbon ring formation, as well as their application in the synthesis of natural products.

Metal-Free Synthesis of Indolopyrans and 2,3-Dihydrofurans Based on Tandem Oxidative Cycloaddition

The synthesis of versatile scaffold indolopyrans based on C-C radical-radical cross-coupling under metal-free conditions is described. The reaction involving single electron transfer between coupling partners followed by cage collapse allows highly selective cross-coupling while employing only equimolar amounts of coupling partners. Moreover, the mechanistic manifold was expanded for the functionalization of enamines to give the stereoselective synthesis of 2,3-dihydrofurans. This iodine-mediated oxidative coupling features mild conditions and fast reaction kinetics.

Acyl Migration versus Epoxidation in Gold Catalysis: Facile, Switchable, and Atom-Economic Synthesis of Acylindoles and Quinoline Derivatives

We report a switchable synthesis of acylindoles and quinoline derivatives via gold-catalyzed annulations of anthranils and ynamides. α-Imino gold carbenes, generated in situ from anthranils and an N,O-coordinated gold(III) catalyst, undergo electrophilic attack to the aryl π-bond, followed by unexpected and highly selective 1,4- or 1,3-acyl migrations to form 6-acylindoles or 5-acylindoles. With the (2-biphenyl)di-tert-butylphosphine (JohnPhos) ligand, gold(I) carbenes experienced carbene/carbonyl additions to deliver quinoline oxides. Some of these epoxides are valuable substrates for the preparation of 3-hydroxylquinolines, quinolin-3(4H)-ones, and polycyclic compounds via facile in situ rearrangements. The reaction can be efficiently conducted on a gram scale and the obtained products are valuable substrates for preparing other potentially useful compounds. A computational study explained the unexpected selectivities and the dependency of the reaction pathway on the oxidation state and ligands of gold. With gold(III) the barrier for the formation of the strained oxirane ring is too high; whereas with gold(I) this transition state becomes accessible. Furthermore, energetic barriers to migration of the substituents on the intermediate sigma-complexes support the observed substitution pattern in the final product.

Gold-catalyzed enantioselective functionalization of indoles

This review documents the potential of enantioselective gold catalysis for the functionalization of indoles.

Chiral N-heterocyclic Carbene Gold Complexes: Synthesis and Applications in Catalysis

N-Heterocyclic carbenes have found many applications in modern metal catalysis, due to the formation of stable metal complexes, and organocatalysis. Among a myriad of N-heterocyclic carbene metal complexes, gold complexes have gained a lot of attention due to their unique propensity for the activation of carbon-carbon multiple bonds, allowing many useful transformations of alkynes, allenes, and alkenes, inaccessible by other metal complexes. The present review summarizes synthetic efforts towards the preparation of chiral N-heterocyclic gold(I) complexes exhibiting C2 and C1 symmetry, as well as their applications in enantioselective catalysis. Finally, the emerging area of rare gold(III) complexes and their preliminary usage in asymmetric catalysis is also presented.

Gold(i)-catalyzed stereoselective cyclization of 1,3-enyne aldehydes by a 1,3-acyloxy migration/Nazarov cyclization/aldol addition cascade

Stereoselective synthesis of bicyclo[3.3.0]octenones from chiral 1,3-enyne aldehydes bearing propargylic acetates is described. The method is based on a Au(i)-catalyzed domino sequence with concomitant transfer of chirality involving 1,3-acyloxy migration followed by Nazarov cyclization and an unprecedented aldol addition. The method furnishes densely functionalized bicyclic structures in high yields, with up to 97% ee and good diastereoselectivity.

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.

Gold catalysed transformation of 2-arylindoles to terphenyl amines via 3-dienyl indoles and Brønsted acid promoted formation of 2-carboxyindoles to 3-indenylindoles via 3-allenylindoles

Gold-catalysed intramolecular reaction of unprotected/protected 3-dienyl indoles leads to the formation of functionalised terphenylamines with high efficiency; this reaction can also be accomplished in one pot by starting with 2-arylindole and propargylic alcohols. This reaction occurs via 3-dienyl indoles and proceeds likely by gold-catalysed cascade cyclisation, aromatisation and fragmentation of the indole heterocyclic moiety. We have also found that PTSA can work well giving comparable yields, but at least one mole equivalent of PTSA is required. On the other hand, 3-indenylindoles have been synthesised from indole-2-carboxylates (2-carboxyindoles) and propargylic alcohols through Brønsted acid mediation. Many of these indenylindoles exhibit isomerism (probably diastereomers) in the solution state (1H NMR). Two examples of indolo-indenyl acetates have been prepared by reacting the above terphenylanilines with PhI(OAc)2. Key products are characterised by X-ray crystallography.

Stereoselective Construction of Methylenecyclobutane-Fused Indolines through Photosensitized [2+2] Cycloaddition of Allene-Tethered Indole Derivatives

Irradiation of 1-(hexa-4,5-dienoyl)indole derivatives in the presence of an aromatic ketone by a high-pressure mercury lamp through Pyrex glass gave the corresponding cyclized products stereoselectively in high yields. The major part of the products was an all- cis-fused methylenecyclobutane-type compound produced through [2+2] cycloaddition, accompanied by small amounts of alkynes via 1,5-hydrogen transfer of a biradical intermediate. Among a range of aromatic ketones, 3',4'-dimethoxyacetophenone was found to sensitize the substrate quite effectively.

Synthesis of Cyclobutane-Fused Angular Tetracyclic Spiroindolines via Visible-Light-Promoted Intramolecular Dearomatization of Indole Derivatives

An intramolecular dearomatization of indole derivatives based on visible-light-promoted [2+2] cycloaddition was achieved via energy transfer mechanism. The highly strained cyclobutane-fused angular tetracyclic spiroindolines, which were typically unattainable under thermal conditions, could be directly accessed in high yields (up to 99%) with excellent diastereoselectivity (>20:1 dr) under mild conditions. The method was also compatible with diverse functional groups and amenable to flexible transformations. In addition, DFT calculations provided guidance on the rational design of substrates and deep understanding of the reaction pathways. This process constituted a rare example of indole functionalization by exploiting visible-light-induced reactivity at the excited states.

Gold(i)-catalyzed enantioselective synthesis of polycyclic indoline skeletons and enantiomerically enriched β-substituted tryptamine-allenes by kinetic resolution

We have developed a gold(i)-catalyzed highly efficient kinetic resolution of indole–allene derivatives, providing polycyclic indoline skeletons in good yields with high regio- and diastereoselectivities and enantioselectivities.

Dearomatization of Indole via Intramolecular [3 + 2] Cycloaddition: Access to the Pentacyclic Skeleton of Strychons Alkaloids

An efficient method to build various multisubstituted polycyclic indoline-annulated normal to medium-size rings through dearomatization of indole via a tandem 1,2-acyloxy migration/intramolecular [3 + 2] cycloaddition process is described. The pentacyclic skeleton of strychnine could be synthesized via this tandem cycloaddition and a further Mannich reaction. This approach would provide a novel strategy to the synthesis of strychons alkaloids.