Gold-catalyzed multicomponent reactions

Multicomponent reactions (MCRs) have emerged as an important branch in organic synthesis for the creation of complex molecular structures. This review is focused on gold-catalyzed MCRs with a special emphasis on the recent developments.

Biotin-Targeted Au(I) Radiosensitizer for Cancer Synergistic Therapy by Intervening with Redox Homeostasis and Inducing Ferroptosis

The search for highly selective sensitizers with a novel mechanism for tumor targeting therapy is of considerable interest. In this work, we have developed a series of new biotin-targeted Au(I) complexes. Through systematic biological evaluation and comparison, biotinylated Au(I) complex 3a containing a triphenylphosphine ligand was screened, as it realized both prominent efficient inhibition and selective cytotoxicity to cancer cells, and the effect was better than that of popularly used auranofin. Meanwhile, complex 3a, as a potent radiosensitizer, enhances anticancer effects in vitro and in vivo and has sensitization selectivity. From the action mechanism study, we provide evidence that complex 3a could intervene in redox homeostasis through targeted binding and strong suppression of thioredoxin reductase (TrxR) and induce the ferroptosis death process, enabling it to sensitize tumor cells to radiotherapy. Thus, complex 3a has enormous potential as an efficient and specific radiosensitizing agent in cancer therapy.

Synthesis of 1H-isoindoliums by electrophile-mediated cascade cyclization/iodination of propargylamine-based 1,6-diynes

A highly regio- and chemoselective synthesis of 1H-isoindoliums through a facile and novel cascade cyclization reaction of propargylamine-based 1,6-diynes under mild conditions has been developed. Different functional groups were compatible under the optimized reaction conditions, giving the corresponding products in up to 94% yields. Upon treatment with a base, the alkyne moiety of 1H-isoindoliums could be further transformed to allenes in excellent yields.

Functionalized quinolizinium-based fluorescent reagents for modification of cysteine-containing peptides and proteins

A series of quinolizinium-based fluorescent reagents were prepared by visible light-mediated gold-catalyzed cis-difunctionalization between quinolinium diazonium salts and electron-deficient alkyne-linked phenylethynyl trimethylsilanes. The electron-deficient alkynyl group of the quinolizinium-based fluorescent reagents underwent nucleophilic addition reaction with the sulfhydryl group on cysteine-containing peptides and proteins. The quinolizinium-based fluorescent reagents were found to function as highly selective reagents for the modification of cysteine-containing peptides and proteins with good to excellent conversions (up to 99%). Moreover, the modified BCArg mutants bearing cationic quinolizinium compounds 1b, 1d, 1e and 1h exhibit comparable activity in enzymatic and cytotoxicity assays to the unmodified one.

New quinolizinium-based fluorescent reagents were made by visible light-mediated gold-catalyzed cis-difunctionalization of quinolinium diazonium salts and trimethylsilyl alkyne derivatives.

Fluorescent Labelling of Glycans with FRET-Based Probes in a Gold(III)-Mediated Three-Component Coupling Reaction

Single-site multifunctionalization of glycans is of importance in biological studies considering its crucial role in mediating biological events and human diseases. In this paper, a novel approach for multifunctional labelling of glycans has been developed featuring the use of fluorescence resonance energy transfer-based (FRET-based) probes for fluorescent labelling of glycans through a gold(III)-mediated three-component coupling reaction. Oxidation of glycans into aldehydes followed by the A³ -coupling reaction with FRET-based probes resulted in the single-site formation of fluorescent propargylamine products. The conversion of labelled glycans can be revealed by ratiometric analysis of the FRET signals. This labelling approach results in multifunctionalization of glycans with high selectivity and conversion between 66 and 69 %.

Exploration of new reaction tools for late-stage functionalization of complex chemicals

Chemistry has always had as a target the conversion of molecules into valuable materials. Nevertheless, the aim of past synthesis has primarily focused on achieving a given transformation, regardless of the environmental impact of the synthetic route. Given the current global situation, the demand for sustainable alternatives has substantially increased. Our group focuses on developing selective chemical transformations that benefit from mild conditions, improved atom economy, and that can make use of renewable feedstocks as starting materials. This account summarizes our work over the past two decades specifically regarding the selective removal, conversion, and addition of functional groups that can, later on, be applied at a late stage for the modification of complex molecules.

A comparative study on atomically precise Au nanoclusters as catalysts for the aldehyde–alkyne–amine (A3) coupling reaction: ligand effects on the nature of the catalysis and efficiency

Atomically precise Au₁₃ nanoclusters stabilized by stibines catalyze the aldehyde–alkyne–amine coupling reaction more efficiently than those stabilized by thiols or phosphines. The nature of the catalytic activity is also different, and may be attributed to the weaker coordinating ability of the stibine ligands.

A stibine-stabilised gold nanocluster which acts as a homogeneous catalyst in the A³ coupling reaction.

Aryl transition metal chemical warheads for protein bioconjugation

The past seven years have witnessed the burgeoning of protein bioconjugation reactions highlighting aryl transition metal reagents as coupling partners. This new bioorthogonal organometallic chemistry, which sets the scene for stoichiometric processes in place of the catalytic procedures that developed in parallel, already enabled the forging of C-S and C-C bonds onto protein substrates, respectively in their native state or equipped with pre-installed non-natural terminal alkene or alkyne appendages. Although not yet applied to proteins, related transformations pointing to the creation of C-N bonds have, in addition, just been disclosed by targeting peptide lysine residues. Central to this research was the selection of ligands attached to the transition metal, in order to confer to metal complexes, not only their stability in aqueous medium, but also the desired chemoselectivity. We summarize here this body of work, which has already put in the limelight elaborated palladium and gold complexes equipped with biologically relevant appendages, such as fluorescent and affinity tags, as well as drug molecules. This research holds much promise, not only for the study of proteins themselves, but also for the design of new protein-based biotherapeutics, such as protein-drug conjugates or constrained analogs resulting from macrocyclisation reactions.

C,O-Chelated BINOL/Gold(III) Complexes: Synthesis and Catalysis with Tunable Product Profiles

Unprecedented stable BINOL/gold(III) complexes, adopting a novel C,O-chelation mode, were synthesized by a modular approach through combination of 1,1'-binaphthalene-2,2'-diols (BINOLs) and cyclometalated gold(III) dichloride complexes [(C^N)AuCl₂ ]. X-ray crystallographic analysis revealed that the bidentate BINOL ligands tautomerized and bonded to the Au^III atom through C,O-chelation to form a five-membered ring instead of the conventional O,O'-chelation giving a seven-membered ring. These gold(III) complexes catalyzed acetalization/cycloisomerization and carboalkoxylation of ortho-alkynylbenzaldehydes with trialkyl orthoformates.

C–C coupling reactions using a gold(iii) phosphorus complex confined within metal–organic framework fibers in aqueous solution

HPG@KCC-1/PPh₂/Au NPs were used for the first time as a catalyst for the C–C cross-coupling between allylarenes or methyl acrylate and benzoxazole, and they showed excellent catalytic activities under green conditions.

Modular synthesis of propargylamine modified cyclodextrins by a gold(iii)-catalyzed three-component coupling reaction

A modular approach for the synthesis of propargylamine modified β-cyclodextrins has been developed through a gold(iii)-catalyzed three-component coupling reaction.

Visual detection of formaldehyde by highly selective fluorophore labeling via gold(III) complex-mediated three-component coupling reaction

A novel method for visual detection of formaldehyde with excellent selectivity via a gold(iii) complex-mediated three-component coupling reaction of resin-linked sterically bulky amines and fluorescent alkynes has been developed.

The fluxional amine gold(III) complex as an excellent catalyst and precursor of biologically active acyclic carbenes

A new amine gold(III) complex [Au(C6F5)2(DPA)]ClO4 with the di-(2-picolyl)amine (DPA) ligand has been synthesised. In the solid state the complex has a chiral amine nitrogen because the ligand coordinates to the gold centre through one nitrogen atom from a pyridine and through the NH moiety, whereas in solution it shows a fluxional behaviour with a rapid exchange between the pyridine sites. This complex can be used as an excellent synton to prepare new gold(III) carbene complexes by the reaction with isocyanide CNR. The resulting gold(III) derivatives have unprecedented bidentate C^N acyclic carbene ligands. All the complexes have been spectroscopically and structurally characterized. Taking advantage of the fluxional behaviour of the amine complex, its catalytic properties have been tested in several reactions with the formation of C-C and C-N bonds. The complex showed excellent activity with total conversion, without the presence of a co-catalyst, and with a catalyst loading as low as 0.1%. These complexes also present biological properties, and cytotoxicity studies have been performed in vitro against three tumour human cell lines, Jurkat (T-cell leukaemia), MiaPaca2 (pancreatic carcinoma) and A549 (lung carcinoma). Some of them showed excellent cytotoxic activity compared with the reference cisplatin.

Transition metal-mediated bioorthogonal protein chemistry in living cells

Considerable attention has been focused on improving the biocompatibility of Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC), a hallmark of bioorthogonal reaction, in living cells. Besides creating copper-free versions of click chemistry such as strain promoted azide-alkyne cycloaddition (SPAAC), a central effort has also been made to develop various Cu(I) ligands that can prevent the cytotoxicity of Cu(I) ions while accelerating the CuAAC reaction. Meanwhile, additional transition metals such as palladium have been explored as alternative sources to promote a bioorthogonal conjugation reaction on cell surface, as well as within an intracellular environment. Furthermore, transition metal mediated chemical conversions beyond conjugation have also been utilized to manipulate protein activity within living systems. We highlight these emerging examples that significantly enriched our protein chemistry toolkit, which will likely expand our view on the definition and applications of bioorthogonal chemistry.

Gold and silver catalysis: from organic transformation to bioconjugation

A summary of gold (including AuNPs, Au(i) and Au(iii) complexes) and silver(i) catalysis and their application in bioconjugation reactions.

Bis-cyclometallated gold(III) complexes as efficient catalysts for synthesis of propargylamines and alkylated indoles

Stable bis-cyclometallated gold(III) complexes were developed as efficient catalysts for organic transformation reactions by using two strategies: (1) construction of distorted square planar gold(III) complexes and (2) dual catalysis by gold(III) complexes and silver salts.

Silver and gold-catalyzed multicomponent reactions

Silver and gold salts and complexes mainly act as soft and carbophilic Lewis acids even if their use as σ-activators has been rarely reported. Recently, transformations involving Au(I)/Au(III)-redox catalytic systems have been reported in the literature. In this review we highlight all these aspects of silver and gold-mediated processes and their application in multicomponent reactions.

N-Terminal α-amino group modification of peptides by an oxime formation-exchange reaction sequence

A site-specific and efficient method for N-terminal modification of peptides using oxone for selective oxidation of N-terminal α-amino groups of peptides to oximes followed by transoximation with O-substituted hydroxylamines has been developed.