Synthesis of 3-substituted indazoles and benzoisoxazoles via Pd-catalyzed cyclization reactions: application to the synthesis of nigellicine

Recent Strategies in Transition-Metal-Catalyzed Sequential C–H Activation/Annulation for One-Step Construction of Functionalized Indazole Derivatives

Designing new synthetic strategies for indazoles is a prominent topic in contemporary research. The transition-metal-catalyzed C–H activation/annulation sequence has arisen as a favorable tool to construct functionalized indazole derivatives with improved tolerance in medicinal applications, functional flexibility, and structural complexity. In the current review article, we aim to outline and summarize the most common synthetic protocols to use in the synthesis of target indazoles via a transition-metal-catalyzed C–H activation/annulation sequence for the one-step synthesis of functionalized indazole derivatives. We categorized the text according to the metal salts used in the reactions. Some metal salts were used as catalysts, and others may have been used as oxidants and/or for the activation of precatalysts. The roles of some metal salts in the corresponding reaction mechanisms have not been identified. It can be expected that the current synopsis will provide accessible practical guidance to colleagues interested in the subject.

Intramolecular redox cyclization reaction access to cinnolines from 2-nitrobenzyl alcohol and benzylamine via intermediate 2-nitrosobenzaldehyde

An efficient base-promoted synthesis of cinnolines from 2-nitrobenzyl alcohol and benzylamine via an intramolecular redox cyclization reaction. The formation of intermediate 2-nitrosobenzaldehyde plays an important role in this transformation.

Synthetic strategy and structure-activity relationship (SAR) studies of 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1, Lificiguat): a review

Since 1994, YC-1 (Lificiguat, 3-(5′-hydroxymethyl-2′-furyl)-1-benzylindazole) has been synthesized, and many targets for special bioactivities have been explored, such as stimulation of platelet-soluble guanylate cyclase, indirect elevation of platelet cGMP levels, and inhibition of hypoxia-inducible factor-1 (HIF-1) and NF-κB. Recently, Riociguat®, the first soluble guanylate cyclase (sGC) stimulator drug used to treat pulmonary hypertension and pulmonary arterial hypertension, was derived from the YC-1 structure. In this review, we aim to highlight the synthesis and structure–activity relationships in the development of YC-1 analogs and their possible indications.

Since 1994, YC-1 (Lificiguat) has been synthesized, and many targets for special bioactivities have been explored, such as stimulation of platelet-soluble guanylate cyclase, indirect elevation of platelet cGMP levels, and inhibition of HIF-1 and NF-κB.

Synthesis of 1H-Indazoles via Silver(I)-Mediated Intramolecular Oxidative C-H Bond Amination

We described a silver(I)-mediated intramolecular oxidative C-H amination that enables the construction of assorted 1H-indazoles that are widely applicable in medicinal chemistry. The developed amination was found to be efficient for the synthesis of a variety of 3-substituted indazoles that are otherwise difficult to be synthesized by other means of C-H aminations. Preliminary mechanistic studies suggested that the current amination proceeds via single electron transfer (SET) mediated by Ag(I) oxidant.

Expedient synthesis of E-hydrazone esters and 1H-indazole scaffolds through heterogeneous single-atom platinum catalysis

Unprotected E-hydrazone esters are prized building blocks for the preparation of 1H-indazoles and countless other N-containing biologically active molecules. Despite previous advances, efficient and stereoselective synthesis of these compounds remains nontrivial. Here, we show that Pt single atoms anchored on defect-rich CeO₂ nanorods (Pt₁/CeO₂), in conjunction with the alcoholysis of ammonia borane, promotes exceptionally E-selective hydrogenation of α-diazoesters to afford a wide assortment of N-H hydrazone esters with an overall turnover frequency of up to 566 hours^-1 upon reaction completion. The α-diazoester substrates could be generated in situ from readily available carboxylic esters in one-pot hydrogenation reaction. Utility is demonstrated through concise, scalable synthesis of 1H-indazole-derived pharmaceuticals and their ¹⁵N-labeled analogs. The present protocol highlights a key mechanistic nuance wherein simultaneous coordination of a Pt site with the diazo N═N and ester carbonyl motifs plays a central role in controlling stereoselectivity, which is supported by density functional theory calculations.

Direct N-Alkylation and Kemp Elimination Reactions of 1-Sulfonyl-1H-Indazoles

The reactions of 1-sulfonyl-1H-indazoles under basic conditions are discussed, and the direct N-alkylation and Kemp elimination reactions of these compounds are reported. A series of 2-(p-tosylamino)benzonitriles and N-alkyl indazoles were prepared in good yields. Moreover, the 2-(p-tosylamino)benzonitriles could be transformed into a diverse range of important derivatives in a one-pot reaction. This method was successfully applied to the total syntheses of quindolinone and cryptolepinone; quindolinone was prepared in a one-pot reaction from 1-sulfonyl-1H-indazole.

Switchable Synthesis of 3-Substituted 1H-Indazoles and 3,3-Disubstituted 3H-Indazole-3-phosphonates Tuned by Phosphoryl Groups

3-Alkyl/aryl-1H-indazoles and 3-alkyl/aryl-3H-indazole-3-phosphonates were synthesized efficiently through a 1,3-dipolar cycloaddition reaction between α-substituted α-diazomethylphosphonates and arynes under simple reaction conditions. The product distribution was controlled by the phosphoryl group, which acted both as a tuning group and a traceless group in the reaction.

A Novel PPh3 Mediated One-Pot Method for Synthesis of 3-Aryl or Alkyl 1,2-Benzisoxazoles

A novel, efficient, and facile protocol has been developed for transforming 2-hydroxybenzonitriles and bromides into a range of 3-aryl or alkyl substituted 1,2-benzisoxazoles in good to excellent yields mediated by PPh₃. The electronic and steric effects of bromides on the reaction are discussed. This is the first example to construct a C-C bond and heterocycle in a Barbier-Grignard-type reaction featuring easier recovery of PPh₃ than a metallic catalyst in one step.

Ni and Cu-catalyzed one pot synthesis of unsymmetrical 1,3-di(hetero)aryl-1H-indazoles from hydrazine, o-chloro (hetero)benzophenones, and (hetero)aryl bromides

A new nickel-catalyzed cyclization of ortho-chlorobenzophenone hydrazones, to afford 3-aryl-1H-indazoles, is applied in a one-pot synthesis of 1,3-diaryl-1H-indazoles.

N-Tosylhydrazones: versatile synthons in the construction of cyclic compounds

N-Tosylhydrazones have been extensively explored as versatile building blocks in the construction of various cyclic compounds.

Chemical composition of Nigella sativa Linn: Part 2 Recent advances

The black cumin or Nigella sativa L. seeds have many acclaimed medicinal properties such as bronchodilatory, hypotensive, antibacterial, antifungal, analgesic, anti-inflammatory and immunopotentiating. This review article is an update on the previous article published on Nigella sativa L. in this journal in 1999. It covers the medicinal properties and chemical syntheses of the alkaloids isolated from the seeds of the herb.

Infrared irradiation assisted both the synthesis of ( Z)-(aminomethyl)(aryl)phenylhydrazones via the Mannich coupling reaction and their application to the palladium-catalyzed Heck reaction

The synthesis of arylhydrazones 1a–h assisted with infrared irradiation (IR) under solvent-free conditions is herein reported, and their catalytic action in the palladium-catalyzed and IR-assisted Heck coupling reaction are also evaluated.

Synthesis of benzofulvenes through chemoselective Sonogashira and Barluenga couplings of ortho ethynyl-N-tosylhydrazones and cycloisomerization

Chemoselective synthesis of ortho ethynylhydrazones 7 derived from 2-iodoacetophenone via a Sonogashira coupling is reported for the first time.

Synthesis of indazole motifs and their medicinal importance: an overview

Indazoles is an important class of heterocyclic compounds having a wide range of biological and pharmaceutical applications. There is enormous potential in the synthesis of novel heterocyclic systems to be used as building blocks for the next generation of pharmaceuticals as anti-bacterial, anti-depressant and anti-inflammatory. Fused aromatic 1H and 2H-indazoles are well recognized for anti-hypertensive and anti-cancer properties. The present review focuses on novel routes of their synthesis and various biological activities.

Base-catalyzed synthesis of substituted indazoles under mild, transition-metal-free conditions

Back to basics: A transition-metal-free method developed for the synthesis of indazoles involves an inexpensive catalytic system composed of a diamine and K2CO3. Various (Z)-2-bromoacetophenone tosylhydrazones were converted into indazoles at room temperature in excellent yields (see example; Ts = p-toluenesulfonyl). The yield was improved by photoisomerization with UV light when E/Z isomeric mixtures of the starting material were used.

Solution-phase synthesis of a diverse library of benzisoxazoles utilizing the [3 + 2] cycloaddition of in situ-generated nitrile oxides and arynes

A library of benzisoxazoles has been synthesized by the [3 + 2] cycloaddition of nitrile oxides with arynes and further diversified by acylation/sulfonylation and palladium-catalyzed coupling processes. The eight key intermediate benzisoxazoles have been prepared by the reaction of o-(trimethylsilyl)aryl triflates and chlorooximes in the presence of CsF in good to excellent yields under mild reaction conditions. These building blocks have been used as the key components of a diverse set of 3,5,6-trisubstituted benzisoxazoles.

One-pot synthesis of dihydrobenzisoxazoles from hydroxylamines, acetylenedicarboxylates, and arynes via in situ generation of nitrones

Aryne [3 + 2] cycloaddition with nitrones generated in situ from the addition of hydroxylamines to acetylenedicarboxylates affords moderate to good yields of dihydrobenzisoxazoles. This reaction extends the current scope of aryne cycloaddition to include in situ generated nitrones and produces functionalized dihydrobenzisoxazoles with a quaternary center.

A Robust Protocol for Pd(II)-catalyzed C-3 Arylation of (1H) Indazoles and Pyrazoles: Total Synthesis of Nigellidine Hydrobromide

C3-arylated indazole and pyrazoles are privileged structural motifs in agrochemicals and pharmaceuticals. C-3 C-H arylation of (1H) indazole and pyrazole has been a significant challenge due to the poor reactivity of the C-3 position. Herein, we report a practical Pd(II)/Phen catalyst and conditions for direct C-3 arylation of indazole and pyrazole with ArI or ArBr without using Ag additives as halide scavengers. The use of toluene, chlorobenzene, trifluoromethylbenzene and mesitylene as the solvent was found to be crucial for the selectivity and reactivity. We further demonstrate the robustness of this protocol through the first total synthesis of Nigellidine hydrobromide as well as expedient preparation of heterocycles structurally related to pesticides and drug molecules.

Synthesis of o-(dimethylamino)aryl ketones, acridones, acridinium salts, and 1H-indazoles by the reaction of hydrazones and arynes

A novel, efficient route to biologically and pharmaceutically important o-(dimethylamino)aryl ketones, acridones, acridinium salts, and 1H-indazoles has been developed starting from readily available hydrazones of aldehydes and o-(trimethylsilyl)aryl triflates. The reaction proceeds through arynes under mild conditions, tolerates a wide range of functional groups, and provides the final products in good to excellent yields.