Directing-Group-Enabled Cycloaddition of Azides and Alkynes toward Functionalized Triazoles

Late-Stage Functionalization Strategies of 1,2,3-Triazoles: A Post-Click Approach in Organic Synthesis

The 1,2,3-triazole scaffolds are an important class of biologically privileged heterocyclic compounds with several key applications in chemistry, biology, medicine, agriculture, and material science. The "postclick" functionalization of 1,2,3-triazoles may emerge as a promising tactic for the construction of molecular architectures of therapeutics and is considered to be a growing area of investigation. This interest extends beyond the regioselective Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) method that involves the trapping of Cu(I)-triazole with suitable precursors. In this Perspective, we highlight the growing impact of postclick strategies in organic synthesis required for the late-stage functionalization of 1,2,3-triazoles with a hope that this emerging concept may provide ample opportunities in modern organic synthesis of notable applications in medicinal chemistry, biology, and materials science.

Direct organocatalytic chemoselective synthesis of pharmaceutically active benzothiazole/benzoxazole-triazoles

Benzothiazole and benzoxazole heterocyclic ring-containing 1,4,5-trisubstituted-1,2,3-triazoles are well known for their wide range of applications in pharmaceutical and medicinal chemistry, but their high-yielding metal-free selective synthesis has always remained challenging as no comprehensive simple protocol has been outlined to date. Owing to their structural and medicinal importance, herein, we synthesized various benzothiazole and benzoxazole heterocyclic ring-containing 1,4,5-trisubstituted-1,2,3-triazoles in high to excellent yields with chemo-/regioselectivity from the library of benzothiazole/benzoxazole-ketones and aryl/alkyl-azides through an enolate-mediated organocatalytic azide-ketone [3 + 2]-cycloaddition under ambient conditions in a few hours. The commercial availability or quick synthesis of the starting materials and catalysts, a diverse substrate scope, chemo-/regioselectivity, quick synthesis of pharmaceutically active known compounds and their analogues, and numerous medicinal applications of functionalized benzothiazole/benzoxazole-triazoles are the key attractions of this metal-free organo-click reaction.

Enzyme-Catalyzed Regioselective Synthesis of 4-Hetero-Functionalized 1,5-Disubstituted 1,2,3-Triazoles

Enzyme-catalyzed novel protocols for the regioselective construction of fully substituted 1,2,3-triazoles by employing 2-azido-1,3,5-triazine (ADT) as a 1,3-dipole for the cycloaddition reaction with the activated alkene in an aqueous medium have been developed. Various 4-heterosubstituted-1,2,3-triazoles were readily assembled in good to excellent yields with high regioselectivity. This reaction also features wide substrate scope, strong functional group tolerance, gram-scale synthesis, and an environmentally friendly process.

Nitrous oxide as diazo transfer reagent

Nitrous oxide, commonly known as "laughing gas", is formed as a by-product in several industrial processes. It is also readily available by thermal decomposition of ammonium nitrate. Traditionally, the chemical valorization of N2O is achieved via oxidation chemistry, where N2O acts as a selective oxygen atom transfer reagent. Recent results have shown that N2O can also function as an efficient diazo transfer reagent. Synthetically useful methods for synthesizing triazenes, N-heterocycles, and azo- or diazo compounds were developed. This review article summarizes significant advancements in this emerging field.

CuI/DMAP-Catalyzed Oxidative Alkynylation of 7-Azaindoles: Synthetic Scope and Mechanistic Studies

An efficient and practical method for the N-alkynylation of 7-azaindoles has been established by using CuI/DMAP catalytic system at room temperature and in open air. This simple protocol has been successfully employed in the synthesis of a wide range of N-alkynylated 7-azaindoles with good yields. Also, this approach is well-suited for large-scale N-alkynylation reactions. The designed N-alkynylated 7-azaindoles were further subjected to Cu-/Ir-catalyzed alkyne-azide cycloaddition (CuAAC/IrAAC) or "click" reaction for the rapid synthesis of 1,4-/1,5 disubstituted 1,2,3-triazole decorated 7-azaindoles. A mechanistic study based on density functional theory (DFT) calculations and ultraviolet-visible (UV) spectroscopic studies revealed that the CuI and DMAP combination formed a [CuII (DMAP)2 I2 ] species, which acts as an active catalyst. The DFT method was used to assess the energetic viability of an organometallic in the C-N bond formation pathway originating from the [CuII (DMAP)2 I2 ] complex. We expect that the newly designed Cu/DMAP/alkyne system will offer valuable insights into the field of Cu-catalyzed transformations.

Multicomponent cyclization with azides to synthesize N-heterocycles

Heterocyclic compounds, both naturally derived and synthetically produced, constitute a wide variety of biologically active and industrially important compounds. The synthesis and application of heterocyclic compounds have garnered significant attention and experienced rapid growth in recent decades. Organic azides, due to their unique properties and distinctive reactivity, have become a convenient chemical tool for achieving a wide range of heterocycles such as triazoles and tetrazoles. Importantly, the field of multicomponent reaction (MCR) chemistry provides a convergent approach to access various N-heterocyclic scaffolds, offering novelty, diversity, and complexity. However, the exploration of MCR pathways to N-heterocyclic compounds remains incomplete. Here, we review the use of multicomponent reactions for the preparation of N-heterocycles. A wide range of reactions based on azides for the synthesis of various types of N-heterocyclic systems have been developed.

Iridium-catalysed thioether-directed regioselective cycloaddition of internal alkynes with azides

We report herein a cationic iridium-catalysed thioether-directed alkyne-azide cycloaddition reaction. Diverse 2-alkynyl phenyl sulfides can undergo cycloaddition with different azides in a regioselective fashion. The reaction features high efficiency, a short reaction time, and a broad substrate scope, providing modular access to complex S-containing triazoles.

Base-mediated ketenimine formation from N-sulfonylthioimidates for the synthesis of 5-amino-1-vinyl/aryl-1,2,3-triazoles

N-Sulfonylthioimidate was converted to ketenimine under basic conditions. The reaction with vinyl/aryl azides was induced to cause dipolar cycloaddition to form 5-amino-1-vinyl/aryl-1,2,3-triazoles. The advantages of this method are high efficiency, structural diversity of products favorable yields and applicability to gram-scale operations.

Construction of Axial Chirality via Click Chemistry: Rh-Catalyzed Enantioselective Synthesis of 1-Triazolyl-2-Naphthylamines

A modular and practical click chemistry for atroposelective synthesis of 1-triazolyl-2-naphthylamines is developed. In this protocol, a variety of aromatic or aliphatic azides, and 1-alkynyl-2-naphthylamines could be assembled into valuable 1-triazlyl-2-naphthylamine scaffolds via a [3 + 2] cycloaddition under Rh-catalysis. This asymmetric click technology features easily accessible starting materials, mild reaction conditions, facile scalability, and good enantioselectivity. The good thermostability of products showcases great applicable potential, and the synthetic transformations further expand the molecular diversity of atropisomers.

Synthesis and Reactivity of a Terminal 1-Alkynyl Triazene

1-Alkynyl triazenes are versatile reagents in synthetic organic chemistry, but the structural diversity of this compound class has so far been limited. Herein, we describe the synthesis of a terminal 1-alkynyl triazene. Subsequent functionalization allows the preparation of 1-alkynyl triazenes with a range of functional groups including esters, alcohols, cyanides, phosphonates, and amides. Furthermore, the terminal 1-alkynyl triazene can be used for the synthesis of di- and triynes and for the preparation of (hetero)aromatic triazenes in metal-catalyzed cyclization reactions.

Discovery of triazenyl triazoles as Nav1.1 channel blockers for treatment of epilepsy

The voltage-gated sodium (Na_v) channel is one of most important targets for treatment of epilepsy, and rufinamide is an approved third-generation anti-seizure drug as Na_v1.1 channel blocker. Herein, by triazenylation of rufinamide, we reported the triazenyl triazoles as new Na_v1.1 channel blocker for treatment of epilepsy. Through the electrophysiological activity assay, compound 6a and 6e were found to modulate the inactivation voltage of Na_v 1.1 channel with shift of -10.07 mv and -11.28 mV, respectively. In the pentylenetetrazole (PTZ) mouse model, 6a and 6e reduced the seizure level, prolonged seizure latency and improved the survival rate of epileptic mice at an intragastric administration of 50 mg/kg dosage. In addition, 6a also exhibited promising effectiveness in the maximal electroshock (MES) mouse model and possessed moderate pharmacokinetic profiles. These results demonstrated that 6a was a novel Na_v1.1 channel blocker for treatment of epilepsy.

Synthesis of Diethoxy Arylphosphoryl Functionalized, Fully Substituted 5-Triazenyl-1,2,3-triazoles via Chelation-Assisted Interrupted Domino Reaction of ortho-Azidophosphonates with Copper(I) Alkynes

We describe the synthesis of 1,4-disubstituted 5-triazenyl-1,2,3-triazoles bearing diethoxy arylphosphoryl moieties via a domino reaction between ortho-azidophosphonates and premade copper(I) alkynides involving chelation-assisted [3+2] cycloaddition followed by interception of the copper(I) triazolide generated by the azide. A resulting dicopper(I) triazoletriazenide complex has been characterized through X-ray diffraction.

Gold-catalyzed synthesis of oxazoles from alkynyl triazenes and dioxazoles

A gold-catalyzed regioselective [3 + 2] cycloaddition of alkynyl triazenes with 1,2,4-dioxazoles was developed.

Three-Component Reactions of α-CF3 Carbonyls, NaN3, and Amines for the Synthesis of NH-1,2,3-Triazoles

The development of methods for the assembly of 1,2,3-triazoles is an important topic due to the broad applications of this motif in various scientific fields. In this work, we demonstrate that the three-component assembly of α-CF₃ carbonyls, NaN₃, and amines was achieved for the selective construction of a variety of 5-amino NH-1,2,3-triazoles under transition-metal-free and open-air conditions. The method provides a general and operationally simple route to functionalized biologically important molecules including carbohydrates, nucleosides, and peptides and exhibits broad substrate scopes. We further demonstrate that the NH-1,2,3-triazoles can be smoothly converted to the regiospecific N-2 alkylated 1,2,3-triazole products. Mechanistic studies based on experiments and density functional theory calculations showed that this transformation proceeds via defluorination-initiated programmed substitution/cyclization/H-transfer to give the 4,5-difunctionalized captodative NH-1,2,3-triazole product.

A recent overview on the synthesis of 1,4,5-trisubstituted 1,2,3-triazoles

Diverse strategies for the efficient and attractive synthesis of a wide variety of relevant 1,4,5-trisubstituted 1,2,3-triazole molecules are reported. The synthesis of this category of diverse fully functionalized 1,2,3-triazoles has become a necessary and unique research subject in modern synthetic organic key transformations in academia, pharmacy, and industry. The current review aims to cover a wide literature survey of numerous synthetic strategies. Recent reports (2017–2021) in the field of 1,4,5-trisubstituted 1,2,3-triazoles are emphasized in this current review.

Bioorthogonal Azide-Thioalkyne Cycloaddition Catalyzed by Photoactivatable Ruthenium(II) Complexes

Tailored ruthenium sandwich complexes bearing photoresponsive arene ligands can efficiently promote azide-thioalkyne cycloaddition (RuAtAC) when irradiated with UV light. The reactions can be performed in a bioorthogonal manner in aqueous mixtures containing biological components. The strategy can also be applied for the selective modification of biopolymers, such as DNA or peptides. Importantly, this ruthenium-based technology and the standard copper-catalyzed azide-alkyne cycloaddition (CuAAC) proved to be compatible and mutually orthogonal.

A Multicomponent Approach toward Angularly Fused/Linear Bitriazoles: A Cascade Cornforth Rearrangement and Triazolization

A multicomponent reaction of triazoloketones, primary amines, and 4-nitrophenyl azide was developed for the synthesis of hitherto unknown angularly fused/linear bitriazoles. The two-stage mechanism was well proven by the isolation of the intermediate. This sequential reaction consists of Cornforth rearrangement and triazolization, which has also been demonstrated in a one-pot manner.

Triazenyl Alkynes as Versatile Building Blocks in Multicomponent Reactions: Diastereoselective Synthesis of β-Amino Amides

Multicomponent reactions (MCRs) are powerful tool for the construction of polyfunctional molecules in an operationally simple and atom-economic manner, and the discovery of novel MCRs requests various building blocks. Herein, triazenyl alkynes were disclosed as versatile building blocks in a multicomponent reaction with carboxylic acids, aldehydes and anilines to furnish β-amino amides with the achievement of high diastereoselectivity and structural diversity. In this process, triazenyl alkynes were bifunctional so that the alkyne moiety acts as C2 fragment and triazene serves as directing group to modulate the transition state thus achieving high diastereoselectivity, in consistence with DFT calculations. Furthermore, the triazenyl group also enables diverse late-stage transformation. This protocol opens a new vision for the discovery of building block and rational design of MCRs.

Chelation-Assisted Interrupted Copper(I)-Catalyzed Azide-Alkyne-Azide Domino Reactions: Synthesis of Fully Substituted 5-Triazenyl-1,2,3-triazoles

We describe the synthesis of 1,4-(disubstituted)-5-triazenyl-1,2,3-triazoles through a ligand-free domino copper(I)-catalyzed azide-alkyne-azide process of chelating aryl azides bearing N-P═O, P═O, and SO₃H groups at the ortho position with a wide variety of acetylenes. DFT calculations reveal that Cu-chelation is a crucial factor in the interception of the CuAAC intermediate by the azide. The crystal structure of the catalytic species has been determined by X-ray diffraction.

Transition-metal-free synthesis of 5-amino-1,2,3-triazoles via nucleophilic addition/cyclization of carbodiimides with diazo compounds

A facile and transition-metal-free synthesis of functionalized 5-amino-1,2,3-triazoles with good functional-group tolerance and regioselectivity via nucleophilic addition/cyclization has been described.

Vinyl and Alkynyl Triazenes: Synthesis, Reactivity, and Applications

Aromatic compounds containing triazenyl groups (N₃ RR') have a profound impact on synthetic organic and medicinal chemistry. In contrast, the chemistry of vinyl and alkynyl triazenes was a largely uncharted territory until recently. The situation has changed over the last five years, and it has become apparent that vinyl and alkynyl triazenes are highly interesting compounds with a unique reactivity. The electron-donating properties of the triazenyl group provide alkynyl triazenes with an ynamide-like reactivity, which can be exploited in reactions of the triple bond. Vinyl triazenes, on the other hand, can be used for electrophilic vinylation reactions. The foundation for this new triazene chemistry are synthetic pathways which allow preparing vinyl and alkynyl triazenes in few steps from readily available starting materials. In this Minireview, we summarize recent developments in this area.

Process-Controlled Regiodivergent Copper-Catalyzed Azide-Alkyne Cycloadditions: Tailor-made Syntheses of 4- and 5-Bromotriazoles from Bromo(phosphoryl)ethyne

We developed a regiodivergent syntheses of 4- and 5-bromo-substituted 1,2,3-triazoles in copper-catalyzed azide-alkyne cycloadditions (CuAACs) by taking advantage of bromo(phosphoryl)ethyne 1 as a bromoethyne equivalent. A one-shot dephosphorylative CuAAC of 1 afforded 4-bromotriazoles, which was transformed into a histone deacetylase 8 (HDAC8)-selective inhibitor, NCC-149. However, the direct CuAAC catalyzed by CuI/Cu(OAc)₂ provided 5-bromo-4-phosphoryltriazoles. The consecutive nucleophilic substitution of the bromo group with thiols followed by MeOK-promoted dephosphorylation gave 5-thio-substituted triazoles.

Greener synthesis of 1,2,3-triazoles using a copper(i)-exchanged magnetically recoverable β-zeolite as catalyst

A novel magnetically recoverable Cu(i)-exchanged β-zeolite catalyst was prepared, characterized and applied for the synthesis of 1,2,3-triazoles via the one-pot three-component reaction.