Triazoles and Their Derivatives: Chemistry, Synthesis, and Therapeutic Applications

Among the nitrogen-containing heterocyclic compounds, triazoles emerge with superior pharmacological applications. Structurally, there are two types of five-membered triazoles: 1,2,3-triazole and 1,2,4-triazole. Due to the structural characteristics, both 1,2,3- and 1,2,4-triazoles are able to accommodate a broad range of substituents (electrophiles and nucleophiles) around the core structures and pave the way for the construction of diverse novel bioactive molecules. Both the triazoles and their derivatives have significant biological properties including antimicrobial, antiviral, antitubercular, anticancer, anticonvulsant, analgesic, antioxidant, anti-inflammatory, and antidepressant activities. These are also important in organocatalysis, agrochemicals, and materials science. Thus, they have a broad range of therapeutic applications with ever-widening future scope across scientific disciplines. However, adverse events such as hepatotoxicity and hormonal problems lead to a careful revision of the azole family to obtain higher efficacy with minimum side effects. This review focuses on the structural features, synthesis, and notable therapeutic applications of triazoles and related compounds.

1-(4-Nitrophenyl)-1H-1,2,3-Triazole-4-carbaldehyde: Scalable Synthesis and Its Use in the Preparation of 1-Alkyl-4-Formyl-1,2,3-triazoles

1,2,3-Triazole-4-carbaldehydes are useful synthetic intermediates which may play an important role in the discovery of novel applications of the 1,2,3-triazole moiety. In this work, a one-step multigram scale synthesis of 4-formyl-1-(4-nitrophenyl)-1H-1,2,3-triazole (FNPT) as a preferred reagent for the synthesis of 1-alkyl-4-formyltriazoles is described, making use of the commercially available 3-dimethylaminoacrolein and 4-nitrophenyl azide. Next, the earlier reported reaction of FNPT with alkylamines is further explored, and for hexylamine, the one-pot sequential cycloaddition and Cornforth rearrangement is demonstrated. In addition, a useful protocol for the in situ diazotization of 4-nitroaniline is provided. This facilitated the complete hydrolysis of rearranged 4-iminomethyl-1,2,3-triazoles and allowed for the recycling of 4-nitrophenyl azide.

Metal-free syntheses of N-functionalized and NH-1,2,3-triazoles: an update on recent developments

An overview of the latest developments in the metal-free synthesis of non-benzo-fused N-functionalized and NH-1,2,3-triazoles is provided in this feature article. Synthetic studies that appeared from 2016 until August 2020 are organized according to a wide-ranging classification, comprising oxidative and eliminative azide-dipolarophile cycloadditions, diazo transfer reactions and N-tosylhydrazone-mediated syntheses. The newly developed methods constitute a significant contribution to the field of 1,2,3-triazole synthesis in terms of structural variation via either the exploration of novel reactions, or the exploitation of existing methodologies.

A NHC-catalyzed 1,3-dipolar cycloaddition reaction of allyl ketones with azides: direct access to 1,4,5-trisubstituted 1,2,3-triazoles

A NHC-catalyzed 1,3-dipolar cycloaddition reaction of allyl ketones with azides has been developed. This strategy could generate 1,4,5-trisubstituted 1,2,3-triazoles in high yields and regioselectivities in the presence of a 20 mol% N-heterocyclic carbene catalyst. A broad substrate scope of this process is also presented.

A mechanistic insight into the effect of piperidine as an organocatalyst on the [3 + 2] cycloaddition reaction of benzalacetone with phenyl azide from a computational study

Several transition structures (TSs) for catalyst-free [3 + 2] cycloaddition and two plausible mechanistic pathways for the organocatalyzed [3 + 2] cycloaddition (32CA) between benzalacetone and phenyl azide were located by quantum chemistry methods. Calculations were carried out with B3LYP, MPWB1K and M06-2X functionals using 6-31G(d) and 6-311G(d,p) basis sets in gas and solvent phases. The calculated activation barriers imply that the lowest barrier pathway is the catalyzed process producing 3-regioisomers through the iminium intermediate and not through the dienamine route. Electronic displacements along the reaction path have been examined using a topological analysis of the electron-localization function (ELF). ELF topological analyses along the intrinsic reaction coordinates (IRC) of both catalyzed and uncatalyzed 32CA reactions indicated that while the first C1-N1 single bond is formed as a dative bond, the formation of the second C2-N3 bond takes place via a C-to-N coupling between the interacting centers of the reagents. Moreover, the ELF analyses imply that the reaction mechanism is a two-stage one-step process in the presence of a piperidine organocatalyst, while bond formation in an uncatalyzed process is almost synchronous.

Enolate-mediated 1,3-dipolar cycloaddition reactions of allyl ketones with nitrile oxides: direct access to 3,5-disubstituted isoxazolines

TMG-catalyzed [3 + 2] organocatalytic 1,3-dipolar cycloaddition reactions of allyl ketones with in situ generated nitrile oxides have been developed. This strategy could generate 3,5-disubstituted isoxazolines in high yields and regioselectivities.

Enolate-mediated 1,3-dipolar cycloaddition reaction of β-functionalized ketones with nitrile oxides: direct access to 3,4,5-trisubstituted isoxazoles

Enolate-mediated [3 + 2] organocatalytic 1,3-dipolar cycloaddition reactions of β-functionalized ketones with nitrile oxides have been developed. This strategy could generate 3,4,5-trisubstituted isoxazole in high yields and regioselectivities.

Organocatalytic 1,3-dipolar [3+3] cycloaddition reaction of 1,2-cyclohexanedione with azides: direct access to highly substituted 1,2,3-triazoles

Enamine-mediated 1,3-dipolar [3+3] cycloaddition reactions of 1,2-cyclohexanedione with azides have been developed. This strategy could generate highly substituted 1,2,3-triazoles in high yields and high levels of regioselectivities. It is demonstrated that various types of products could be obtained with different organocatalysts and substrates.

Copper immobilized ferromagnetic nanoparticle triazine dendrimer (FMNP@TD–Cu(ii))-catalyzed regioselective synthesis of 1,4-disubstituted 1,2,3-triazoles

The synthesis, characterisation and catalytic activity of FMNP@TD–Cu(ii) for the synthesis of 1,4-disubstituted 1,2,3-triazoles are reported.