Alkyl Propiolates Participated [3+2] Annulation for the Switchable Synthesis of 1,5‐ and 1,4‐Disubstituted 1,2,3‐Triazoles Containing Ester Side Chain

Recent Progress on Synthesis of Functionalized 1,5-Disubstituted Triazoles

Immediately after the invention of 'Click Chemistry' in 2002, the regioselective 1,2,3- triazole scaffolds resulted from respective organic azides and terminal alkynes under Cu(I) catalysis have been well recognized as the functional heterocyclic core at the centre of modern organic chemistry, medicinal chemistry, and material sciences. This CuAAC reaction has several notable features including excellent regioselectivity, high-to-excellent yields, easy to execute, short reaction time, modular in nature, mild condition, readily available starting materials, etc. Moreover, the resulting regioselective triazoles can serve as amide bond isosteres, a privileged functional group in drug discovery and development. More than hundreds of reviews had been devoted to the 'Click Chemistry' in special reference to 1,4-disubstituted triazoles, while only little efforts were made for an opposite regioisomer i.e., 1,5-disubstituted triazole. Herein, we have presented various classical approaches for an expeditious synthesis of a wide range of biologically relevant 1,5- disubstituted 1,2,3-triazole analogues. The syntheses of such a class of diversly functionalized triazoles have emerged as a crucial investigation in the domain of chemistry and biology. This tutorial review covers the literature assessment on the development of various synthetic protocols for the functionalized 1,5-disubstituted triazoles reported during the last 12 years.

Recent Progress in Catalytic Synthesis of 1,2,3-Triazoles

1,2,3-triazoles represent a functional heterocyclic core that has been at the center of modern organic chemistry since the beginning of click chemistry. Being a versatile framework, such an aromatic ring can be observed in uncountable molecules useful in medicine and photochemistry, just to name a few. This review summarizes the progress achieved in their synthesis from 2015 to today, with particular emphasis on the development of new catalytic and eco-compatible approaches. In doing so, we subdivided the report based on their degree of functionalization and, for each subparagraph, we outlined the role of the catalyst employed.

C-C Bond Cleavage Initiated Cascade Reaction of β-Enaminones: One-Pot Synthesis of 5-Hydroxy-1H-pyrrol-2(5H)-ones

An unprecedented C(CO)-C(Ar) bond cleavage of β-enaminones has been realized under mild and transition-metal-free conditions. The cascade transformation based on this C-C bond cleavage involves 1,3-O/C migration and aerobic hydroxylation and leads to various 5-hydroxy-1H-pyrrol-2(5H)-ones with broad functional group tolerance. The application of this methodology has been showcased by preparing 5-alkoxy-1H-pyrrol-2(5H)-one derivatives and a pyrrolo[2,1-a]isoquinolin-3-one derivative.

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.

Cu-Mediated Expeditious Annulation of Alkyl 3-Aminoacrylates with Aryldiazonium Salts: Access to Alkyl N2-Aryl 1,2,3-Triazole-carboxylates for Druglike Molecular Synthesis

Alkyl N-aryl 1,2,3-triazole-carboxylates are important molecules or intermediates in medicinal chemistry, but the synthesis of N²-aryl counterparts remains elusive. Herein, we describe a Cu-mediated annulation reaction of alkyl 3-aminoacrylates with aryldiazonium salts, both of which are readily available substrates. Furthermore, alkyl 2-aminoacrylates are also viable substrates. Diverse alkyl N²-aryl 1,2,3-triazole-carboxylates and their analogues can be rapidly prepared under mild conditions. Especially, this protocol allows one to access several druglike variants of carbonic anhydrase inhibitors and celecoxib.

In-Water Synthesis of 5-Thiolated 1,2,3-Triazoles from β-Thioenaminones by Diazo Transfer Reaction

The synthesis of 1,2,3-triazoles with a sulfur-based side chain has been accessed with the metal-free annulation reactions of readily available β-thiolated enaminones and tosyl hydrazine. By these reactions with water as the only medium, a broad array of 5-thiolated 1,2,3-triazoles have been synthesized with generally good to excellent yields. Except using TMEDA (N,N,N',N'-tetramethylethylenediamine) as the only base promoter, not any other catalyst or additive is required, thus providing an efficient and environmentally benign method for useful 1,2,3-triazole synthesis.