Gold(I)-Catalysed Cyclisation of Alkynoic Acids: Towards an Efficient and Eco-Friendly Synthesis of γ-, δ- and ϵ-Lactones

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.

Improved Rigidin-Inspired Antiproliferative Agents with Modifications on the 7-Deazahypoxanthine C7/C8 Ring Systems

To improve their aqueous solubility characteristics, water-solubilizing groups were added to some antiproliferative, rigidin-inspired 7-deazahypoxanthine frameworks after molecular modeling seemed to indicate that structural modifications on the C7 and/or C8 phenyl groups would be beneficial. To this end, two sets of 7-deazahypoxanthines were synthesized by way of a multicomponent reaction approach. It was subsequently determined that their antiproliferative activity against HeLa cells was retained for those derivatives with a glycol ether at the 4'-position of the C8 aryl ring system, while also significantly improving their solubility behavior. The best of these compounds were the equipotent 6-[4-(2-ethoxyethoxy)benzoyl]-2-(pent-4-yn-1-yl)-5-phenyl-1,7-dihydro-4H-pyrrolo[2,3-d]pyrimidin-4-one 33 and 6-[4-(2-ethoxyethoxy)benzoyl]-5-(3-fluorophenyl)-2-(pent-4-yn-1-yl)-1,7-dihydro-4H-pyrrolo[2,3-d]pyrimidin-4-one 59. Similarly to the parent 1, the new derivatives were also potent inhibitors of tubulin assembly. In treated HeLa cells, live cell confocal microscopy demonstrated their impact on microtubulin dynamics and spindle morphology, which is the upstream trigger of mitotic delay and cell death.

Dinuclear NHC-gold(I)-thiolato and -alkynyl complexes: synthesis, anticancer activity, and catalytic activity in lactonization reactions

A series of novel dinuclear NHC-gold-thiolato and -alkynyl complexes bearing aromatic linkers were successfully synthesized by an efficient and simple synthetic route. The catalytic activity of these complexes was tested in a lactonization reaction. The reaction proceeds in high efficiency, in short reaction time and under mild conditions, and is complementary to existing methods. Furthermore, the digold(I)-thiolato derivatives exhibit remarkable cytotoxicity towards several cancer cell lines.

Gold-Zinc Cooperative Catalysis for Seven-exo-dig Hydrocarboxylation of Internal Alkynes

Seven-exo-dig hydrocarboxylation of nonactivated internal alkynes with conformationally flexible linker chains was achieved with cooperative gold-zinc catalysts composed of an imidazo[1,5-a]pyridinylidene ligand including a bipyridine coordination site at the C5 position. A proximity effect of the gold and zinc sites was essential for their high catalytic activity, in which the internal alkyne activated by the cationic gold species was attacked by the carboxylic acid deprotonated by the basic zinc site. Using a gold(I)-complex with a bulky aromatic N-substituent, 2,6-dibenzhydryl-4-methylphenyl group, for the NHC ligand facilitated seven-membered ring formation while minimizing intermolecular hydrocarboxylation as an undesired side reaction. The reaction mechanism was investigated by quantum chemical calculations.

Improved synthetic routes to N-heterocyclic carbene-metal-diketonato complexes of gold and copper

In our search for simple synthetic routes to N-heterocyclic carbene (NHC)-metal complexes and their derivatives, we herein report an operationally simple, expedient and scalable method to obtain the widely used NHC-metal-diketonates. The reported complexes are synthesized for the first time under mild, aerobic conditions and in excellent yields in a sustainable manner. The protocol is general with regards to the anionic co-ligand and the ancillary carbene ligands. The spectroscopic and crystallographic characterization of the complexes reveal a bidentate binding mode of the diketonate ligand to copper while the gold-congener is C-bound. Finally, the reported Au complex was shown to be an efficient pre-catalyst for the hydrocarboxylation of alkynes.

Hydrogen bonding-enabled gold catalysis: ligand effects in gold-catalyzed cycloisomerizations in hexafluoroisopropanol (HFIP)

Gold catalysis has witnessed immense evolution in recent years, yet it still requires the use of activators to render the common [AuCl(L)] complexes catalytically active. Herein, the H-bonding donor properties of hexafluoroisopropanol (HFIP) are utilized for Au-Cl bond activation and the ancillary ligand and counteranion effects on cycloisomerization reactions are showcased in HFIP as solvent.

One-pot gold(I)-catalyzed synthesis of 2-pyridonyl alcohols

A highly efficient method for the synthesis of 2-pyridonyl alcohols via gold(I) catalyst was developed. The effect of methanesulfonic acid on the reaction progression with the gold catalyst was determined. The proposed mechanism involves intramolecular cyclization product formation derived from 5-exo-dig and 6-exo-dig addition of the nitrogen for 2-propargyloxypyridine and 2-(but-3-yn-1-yloxy)pyridine, respectively. The pyridinium salt formed by the gold(I) catalyst and methanesulfonic acid undergoes a rearrangement reaction in a weakly basic medium (5% aq. Na₂CO₃) to form N-alkenyl pyridonyl alcohols. N-alkenyl pyridonyl alcohols can be obtained in moderate to excellent yields using this method.

Construction of Enantioenriched γ,γ-Disubstituted Butenolides Enabled by Chiral Amine and Lewis Acid Cascade Cocatalysis

Herein we report a cascade cocatalysis strategy for the facile construction of chiral γ,γ-disubstituted butenolides. The synthetic manifold employs simple alkynoic acids instead of the preformed silyloxy furans or 5-substituted furan-2(3H)-ones. In situ formed 5-substituted furan-2(3H)-ones by AgNO₃ or Ph₃PAuCl/AgOTf catalyzed cyclization of alkynoic acids can smoothly engage in the subsequent chiral diphenylprolinol TMS-ether catalyzed Michael and Michael-aldol reactions. The cascade process serves as a general approach to chiral quaternary γ,γ-disubstituted butenolides.

Cycloisomerization of π-Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

Since the dawn of millennium, catalytic gold chemistry is at the forefront to set off diverse organic reactions via unique activation of π-bonded molecules. Within this purview, cycloisomerization of heteroatom nucleophiles linked to a π-system is one of the well recognized chemistry for the construction of numerous heterocyclic cores. Though the rudimentary aspects of this transformation are reviewed by several groups in different timeline, a holistic view on regiochemistry of such reactions went largely overlooked. Hence, this account emphasizes the gold catalyzed regioselective cycloisomerization of structurally distinctive π-connected hetero-nucleophiles leading to different heterocycles documented in the last two decades. From an application perspective, this account also highlights those methodologies which find a role in the total synthesis of natural products. Wherever appropriate, mechanistic details and contributing factors for selectivity are also discussed.

Synthesis of N-heterocyclic carbene gold(I) complexes

N-heterocyclic carbene gold(I) chloride and hydroxide complexes are regularly used as synthons to access various oxygen-, nitrogen- or carbon-bound gold complexes. They are also widely employed as efficient catalysts in addition reactions of hydroelements to unsaturated bonds and in several rearrangement and decarboxylation protocols. Here we describe the multigram synthesis of the most common mononuclear N-heterocyclic carbene gold(I) chloride complexes bearing the N,N'-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and N,N'-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene (IPr*) ligands. Their synthesis is achieved through the straightforward and practical weak base approach in a total time of 4-5 h. This straightforward methodology is conducted under air and possesses considerable advantages over alternative routes, such as the use of a sustainable reaction solvent, minimal amounts of a mild base and commercially available or easily obtained starting materials. Additionally, we describe the synthesis of the mononuclear gold(I) hydroxide complex bearing the IPr ligand, using the state-of-the-art method requiring 24 h. Finally, the improved synthesis of the dinuclear gold(I) hydroxide complex [{Au(IPr)}₂(μ-OH)][BF₄] is described (~3 h). All procedures can be performed by researchers with standard training and lead to high yields (76-99%) of microanalytically pure bench-stable materials.

A PEGylated N-heterocyclic carbene-gold(i) complex: an efficient catalyst for cyclization reaction in water

A recyclable, PEGylated N-heterocyclic carbene(NHC)–gold(i) catalyst was synthesized, which exhibited high reactivity and regio-selectivity in the cyclization of alkynoic acids and cascade reaction of tryptamine and alkynoic acids in water.

Gold-Catalyzed Addition of Carboxylic Acids to Alkynes and Allenes: Valuable Tools for Organic Synthesis

In this contribution, the application of gold-based catalysts in the hydrofunctionalization reactions of alkynes and allenes with carboxylic acids is comprehensively reviewed. Both intra- and intermolecular processes, leading respectively to lactones and linear unsaturated esters, are covered. In addition, cascade transformations involving the initial cycloisomerization of an alkynoic acid are also discussed.

Au-Cavitands: Size governed arene-alkyne cycloisomerization

With an inwardly directed reactive center and a well-defined binding pocket, Au(I) functionalized resorcin[4]arene cavitands have been shown to catalyze molecular transformations. The reactivity profiles that emerge differ from other Au(I) catalysts. The added constraint of a binding pocket gives rise to the possibility that the substrates might have to fit into the resorcinarene pocket; our hypothesis is that substrates that match the available space have different reaction outcomes than those that do not. Herein we report on the intramolecular cyclization of alkyne-aromatic substrates with variable alkynes and aromatic composition. We see that scaffold size most drastically dictates reactivity, especially when the substrate's features are particularly designed. The results of these experiments add to the veritable goldmine of information about the selectivity in catalysis that cavitands offer.

Metal-Catalyzed Synthesis and Transformations of β-Haloenol Esters

In the last years there has been an increasing interest in the search for protocols to obtain β-haloenol esters in an efficient and selective manner as they are versatile building blocks in synthetic organic chemistry. In this article, metal-catalyzed transformations allowing the access to both acyclic and cyclic (i.e., haloenol lactones) β-haloenol esters are reviewed. Metal-catalyzed reactions in which these molecules participate as substrates are also discussed.

Gold nanoparticles stabilized by PEG-tagged imidazolium salts as recyclable catalysts for the synthesis of propargylamines and the cycloisomerization of γ-alkynoic acids

Water-soluble gold nanoparticles prepared in the presence of PEG-tagged tris-imidazolium bromide, containing Au(0) and Au(i) species, are reusable catalysts.

A.E.Favorskii’s scientific legacy in modern organic chemistry: prototropic acetylene – allene isomerization and the acetylene zipper reaction

Alexei Evgrafovich Favorskii was an outstanding organic chemist who left a great scientific legacy as a result of long time and fruitful work. Most of the theoretically and practically important discoveries of A.E.Favorskii were made in the chemistry of acetylene and its derivatives. Nowadays, the reactions discovered by him, which include acetylene – allene isomerization, the Favorskii and retro-Favorskii reactions, the Favorskii rearrangement and the vinylation reaction, are widely used in industry and in laboratory synthesis. This review summarizes the main scientific achievements of A.E.Favorskii, as well as their development in modern organic chemistry. Much consideration is given to acetylene – allene isomerization as a convenient method for the synthesis of methyl-substituted acetylenes and to the acetylene zipper reaction as a synthetic tool for obtaining terminal acetylenes. The review presents examples of the application of these reactions in modern organic synthesis of complex molecules, including natural compounds and their analogues.

The bibliography includes 266 references.

Ru-Catalyzed cascade reaction of α,ω-alkynoic acids and arylethylamines towards the synthesis of aryl-fused heterocycles

A simple, efficient, and mild method for the preparation of aryl-fused heterocycles has been developed using α,ω-alkynoic acids and arylethylamines.

Quantifying electronic similarities between NHC–gold(i) complexes and their isolobal imidazolium precursors

The σ and π donor/acceptor properties of the carbon–gold bond are manifested by changes in ¹⁹⁷Au spectroscopic data.

Gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes: towards functionalised azoles

Vinyl- and saturated azoles were synthesised through gold(i) catalysed regio- and stereoselective intermolecular hydroamination of internal alkynes and subsequent hydrogenation.

A highly active diradical cobalt(iii) catalyst for the cycloisomerization of alkynoic acids

The first cobalt-catalysed cycloisomerisation of alkynoic acids is reported, thanks to the design of a well-defined diradical cobalt(iii) catalyst, in the absence of any additives. The high efficiency, regioselectivity and chemoselectivity are comparable to those of noble metal-based systems. The unique reactivity might be attributed to second coordination sphere effects.

Gold(i)-catalyzed addition of carboxylic acids to internal alkynes in aqueous medium

A broad scope catalytic system for the intermolecular addition of carboxylic acids to internal alkynes, in water under mild conditions, has been developed.