Synthesis of Glycoconjugate Benzothiazoles via Cleavage of Benzotriazole Ring

Transition-Metal-Catalyzed Denitrogenative Annulation to Access High-Valued N-Heterocycles

Over the past few years, the development of efficient methods to construct high-valued N-heterocyclic molecules have received massive attention owing to their extensive application in the areas of medicinal chemistry, drug discovery, natural product synthesis and so on. To access those high-valued N-heterocycles, many methods have been developed. In this context, transition-metal-catalyzed denitrogenative annulation of 1,2,3-triazoles and 1,2,3,4-tetrazoles has appeared as a powerful synthetic tool because it offers a step- and atom-economical route for the preparation of the nitrogen-rich molecules. Compared with the denitrogenative annulation of various 1,2,3-triazole frameworks, annulation of 1,2,3,4-tetrazole remains more challenging due to the inertness of the tetrazole moiety. This Review summarizes the significant achievements made in the field of denitrogenative annulation of various 1,2,3-triazoles and 1,2,3,4-tetrazoles including some pioneering examples in this area of research. We anticipate that this denitrogenative annulation reaction will find broad applications in the pharmaceutical industry, drug discovery and other fields of medicinal chemistry.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Development of Diverse Range of Biologically Relevant Carbohydrate-Containing Molecules: Twenty Years of Our Journey*

There is an increasing demand for significant amount of carbohydrate-containing molecules owing to their complete chemical, biological, and pharmacological investigations to better understand their role in many important biological events. Clinical studies of a wide range of simple carbohydrates or their derivatives, glycohybrids, glycoconjugates, and neoglycoconjugates have been conducted worldwide for the successful treatment of various frontline diseases. Herein, a brief perspective of carbohydrate-based molecular scaffolding and my experience during the last 20 years in the area of synthetic carbohydrate chemistry, mainly for their impact in drug discovery & development, is presented.

Recent Developments on Denitrogenative Functionalization of Benzotriazoles

Benzotriazoles are employed as useful synthons in organic synthesis, and due to their unique structural motif, they are able to undergo denitrogenation during the construction of new bonds. Various methods for the functionalization of benzotriazoles as precursors of ­ortho-amino arenediazoniums have recently been developed that involve transition-metal-catalyzed coupling reactions, mainly via cyclization, borylation, alkenylation, alkylation, carbonylation and the formation of carbon–heteroatom bonds. In this short review, we primarily focus on the recent applications of benzotriazoles in organic chemistry that proceed via a denitrogenative process, and the mechanisms are also discussed.

1 Introduction

2 Common Synthetic Routes Allowing Easy Access to Benzotriazole Derivatives

3 Formation of C–C Bonds

3.1 Cyclization Reactions

3.2 Arylation, Alkenylation, Alkylation and Carbonylation Reactions

4 Carbon–Heteroatom Bond Formation

5 Miscellaneous Denitrogenative Functionalization

6 Conclusions and Future Perspectives

Catalyst- and Additive-Free Method for the Synthesis of 2-Substituted Benzothiazoles from Aromatic Amines, Aliphatic Amines, and Elemental Sulfur

Under catalyst- and additive-free conditions, a novel, convenient, environmentally friendly method was developed for the synthesis of 2-substituted benzothiazoles via the three-component one pot reaction from aromatic amines, aliphatic amines, and elemental sulfur. The reaction achieves double C-S and one C-N bond formations via cleavage of two C-N bonds and multiple C-H bonds. Furthermore, the mechanism research shows that DMSO acts as an oxidant in the cyclization reaction.

Renaissance of Ring-Opening Chemistry of Benzotriazoles: New Wine in an Old Bottle

1,2,3-Benzotriazoles could undergo ring cleavage to form ortho-amino arenediazonium or α-diazo-imine species via a Dimroth-type equilibrium. Historically, the synthetic potential of this unique reactivity had remained underdeveloped. Recently, some new strategies have been developed to effect the ring-opening chemistry of benzotriazoles in more practical manners. A wide range of conceptually novel and synthetically useful reactions have been developed, which enable the access to diverse valuable heterocycles and ortho-amino arene derivatives. As one of the players in this field, our group has also contributed a series of intriguing transition-metal-catalyzed denitrogenative functionalizations of benzotriazoles. In this account, we aim to provide an overview of the ring-opening chemistry of benzotriazoles, with a focus on relevant works published in the past decade. In order to show a whole picture of the research field, some pioneering works in its developing history will also be discussed briefly.

Silicon Industry Waste Polymethylhydrosiloxane-Mediated Benzotriazole Ring Cleavage: A Practical and Green Synthesis of Diverse Benzothiazoles

A green modification has been introduced to the synthesis of benzothiazoles by using polymethylhydrosiloxane (PMHS) for successive steps of benzotriazole ring cleavage and cyclization, an approach which was previously developed in our lab by the use of n-Bu₃SnH. The use of the silicone industry byproduct PMHS makes this protocol a cost-effective and nontoxic one and thus may be considered for the industrial importance.

Benzotriazole as an Efficient Ligand in Cu-Catalyzed Glaser Reaction

Benzotriazole has been established as an efficient ligand in Cu-catalyzed cross-coupling of terminal alkynes to form 1,3-dialkynes using CuI as the catalyst and K2CO3 as the base at room temperature in an open round-bottom flask. The established protocol has the following notable advantages: simple to handle, easy work-up, mild reaction condition, high substrate scope, requirement of less quantity of ligand and also Cu-catalyst, less expensive, and high reaction yield.

Click inspired synthesis of hexa and octadecavalent peripheral galactosylated glycodendrimers and their possible therapeutic applications

Click inspired glycodendrimers comprising a rigid hexapropargyloxy benzene core with peripheral β-d-galactopyranosidic units were developed and evaluated for their therapeutic potential.

Lewis-Acid-Mediated Benzotriazole Ring Cleavage (BtRC) Strategy for the Synthesis of 2-Aryl Benzoxazoles from N-Acylbenzotriazoles

A Lewis-acid-mediated ring cleavage of acylbenzotriazoles (RCOBt) followed by cyclization to corresponding benzoxazoles was achieved in good to excellent yields. The reaction was found consistent with the milligram to gram scale.

Denitrogenative Suzuki and carbonylative Suzuki coupling reactions of benzotriazoles with boronic acids

Unprecedented palladium-catalyzed denitrogenative Suzuki and carbonylative Suzuki coupling reactions of benzotriazoles with boronic acids have been realized, which afforded structurally diverse ortho-amino-substituted biaryl and biaryl ketone derivatives. The key to this success is due to the development of a rationally designed strategy to achieve the ring opening of benzotriazoles with a synergistic activating-stabilizing effect, which enables the in situ generation of the corresponding ortho-amino-arenediazonium species. The present work opens up a new avenue to utilize benzotriazoles as synthetic equivalents of ortho-amino-arenediazoniums, which otherwise could not be directly accessed by existing synthetic methods.

Cu-Catalyzed Click Reaction in Carbohydrate Chemistry

Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC), popularly known as the "click reaction", serves as the most potent and highly dependable tool for facile construction of simple to complex architectures at the molecular level. Click-knitted threads of two exclusively different molecular entities have created some really interesting structures for more than 15 years with a broad spectrum of applicability, including in the fascinating fields of synthetic chemistry, medicinal science, biochemistry, pharmacology, material science, and catalysis. The unique properties of the carbohydrate moiety and the advantages of highly chemo- and regioselective click chemistry, such as mild reaction conditions, efficient performance with a wide range of solvents, and compatibility with different functionalities, together produce miraculous neoglycoconjugates and neoglycopolymers with various synthetic, biological, and pharmaceutical applications. In this review we highlight the successful advancement of Cu(I)-catalyzed click chemistry in glycoscience and its applications as well as future scope in different streams of applied sciences.

An efficient one-pot synthesis of N,N′-disubstituted ureas and carbamates from N-acylbenzotriazoles

A facile one-pot synthesis of carbamates and N,Nʹ-disubstituted symmetrical ureas from N-acylbenzotriazoles has been devised. It is believed that, the intermediate acyl-azide undergo Curtius rearrangement.

First noscapine glycoconjugates inspired by click chemistry

The first click chemistry-inspired noscapine glycoconjugates have been developed in good to excellent yields to increase the therapeutic efficacy of noscapine.

Metal free synthesis of morpholine fused [5,1-c] triazolyl glycoconjugates via glycosyl azido alcohols

A series of diverse glycosyl 1,2-azido alcohols, obtained from readily available carbohydrates, were converted to structurally varied rare and novel sugar derived morpholine fused [5,1-c]-triazoles via a one-pot strategy.

An unprecedented deoxygenation protocol of benzylic alcohols using bis(1-benzotriazolyl)methanethione

A new method of silane or Bu₃SnH promoted radical deoxygenation of benzylic alcohols using bis(1-benzotriazolyl)methanethione has been developed.

Regioselective facile synthesis of novel isoxazole-linked glycoconjugates

One-pot regioselective synthesis of novel isoxazole-linked glycoconjugates, by the reaction ofin situgenerated glycosyl-β-nitrile oxide and different alkynes, has been devised.

Click chemistry inspired highly facile synthesis of triazolyl ethisterone glycoconjugates

Numerous deoxy-azido sugars 3 were prepared by the reaction of tosyl/bromo sugars with NaN3 in dry DMF under heating condition. The 1,3-dipolar cycloaddition of deoxy-azido sugars 3 with ethisterone 4 to afford regioselective triazole-linked ethisterone glycoconjugates 5 was investigated in the presence of CuI and DIPEA in dichloromethane or CuSO4·5H2O and sodium ascorbate in aqueous medium. All the developed compounds were characterized by spectroscopic analysis (IR, (1)H &(13)C NMR, and MS spectra). Structure of triazolyl ethisterone glycoconjugate 5a has been further confirmed by its Single Crystal X-ray analysis.