Divergent Synthesis of 1H -Indazoles and 1H -Pyrazoles from Hydrazones via Iodine-Mediated Intramolecular Aryl and sp 3 C-H Amination

Iodine-Mediated δ-Amination of sp3 C-H Bonds

We present a direct δ-amination reaction of sp3 C-H bonds, employing molecular iodine (I2) as the sole oxidant under transition-metal-free conditions. This remote C-H functionalization approach is operationally simple and provides facile, efficient access to pyrrolidines and related heterocyclic derivatives from readily accessible substrates.

Synthesis and Photophysical Properties of 3-Substituted-1H-Indazoles: A Pd-Catalyzed Double C-N Bond Formation Strategy via 1,6-Conjugate Addition

A Pd-catalyzed cascade process for the direct synthesis of 3-substituted-1H-indazole employing p-quinone methide (p-QM) and arylhydrazine through Pd-catalyzed double C-N bond formation via 1,6-conjugate addition is reported. This reaction strategy affords efficient and practical access to synthetically important diverse 3-substituted-1H-indazoles in good yields. The photophysical properties of the synthesized 3-substituted-1H-indazoles are investigated, and some of them showed very good fluorescence properties with quantum yields up to 85%. Also, the synthesized 3-substituted-1H-indazole exhibits an acid-sensitive fluorescence turn-off activity.

Synthesis of Quinazolinone-Fused Tetrahydroisoquinolines and Related Polycyclic Scaffolds by Iodine-Mediated sp3 C-H Amination

An iodine-mediated intramolecular sp³ C-H amination reaction producing quinazolinone-fused polycyclic skeletons from 2-aminobenzamide precursors is reported. This reaction does not use transition metals, has a broad substrate scope, and can be used on a gram scale. Under the optimal reaction conditions, a variety of quinazolinone-fused tetrahydroisoquinolines and derivatives of Rutaecarpine were synthesized from readily accessible compounds. The reaction proceeds well with crude 2-aminobenzamide derivatives, allowing for the synthesis of the products from simple 2-aminobenzoic acids and tetrahydroisoquinolines without purification of the 2-aminobenzamide intermediates. Preliminary biological experiments have identified Cereblon (CRBN) inhibitory activity and relevant anti-myeloma medicinal properties in some of these polycyclic products.

Synthesis of N-phenyl- and N-thiazolyl-1H-indazoles by copper-catalyzed intramolecular N-arylation of ortho-chlorinated arylhydrazones

The broad application of 1H-indazoles has prompted the development of several approaches for the synthesis of such compounds, including metal-free, palladium-, or copper-promoted intramolecular N-arylation of in situ-generated or isolated o-haloarylhydrazones. Such methods mainly start from o-bromo derivatives due to the better yield observed when compared to those obtained from o-chloroarylhydrazones. However, the o-chloroarylaldehydes and o-chloroarylketones used to prepare the arylhydrazones are more commercially available and less expensive than brominated analogs. Seeking to cover a lack in the literature, this work reports a convenient protocol for the synthesis of N-phenyl- and N-thiazolyl-1H-indazoles by copper-catalyzed intramolecular N-arylation of o-chlorinated arylhydrazones. Therefore, a series of seven N-phenyl derivatives and a series of six novel N-thiazolyl derivatives was obtained in 10–70% and 12–35% yield, respectively, after stirring the o-chlorinated arylhydrazones, CuI, KOH, and 1,10-phenantroline for 12–48 hours in DMF at 120 °C. The products were isolated by column chromatography on silica gel. All products were fully characterized by HRMS as well as 1H and 13C NMR spectroscopy. Thus, this approach is valuable for promoting the synthesis of N-phenyl-1H-indazoles in a higher yield than that reported in the literature using copper catalysis and the same substrates. This study also prompted the first reported synthesis of pharmacologically interesting N-thiazolyl derivatives.

A review on synthetic strategy, molecular pharmacology of indazole derivatives, and their future perspective

With different nitrogen-containing heterocyclic moieties, Indazoles earn one of the places among the top investigated molecules in medicinal research. Indazole, an important fused aromatic heterocyclic system containing benzene and pyrazole ring with a chemical formula of C₇ H₆ N₂ , is also called benzopyrazole. Indazoles consist of three tautomeric forms in which 1H-tautomers (indazoles) and 2H-tautomers (isoindazoles) exist in all phases. The tautomerism in indazoles greatly influences synthesis, reactivity, physical and even the biological properties of indazoles. The thermodynamic internal energy calculation of these tautomers points view 1H-indazole as the predominant and stable form over 2H-indazole. The natural source of indazole is limited and exists in alkaloidal nature (i.e., nigellidine, nigeglanine, nigellicine, etc.) found from Nigella plants. Some of the FDA-approved drugs like Axitinib, Entrectinib, Niraparib, Benzydamine, and Granisetron are being used to treat renal cell cancer, non-small cell lung cancer (NSCLC), epithelial ovarian cancer, chronic inflammation, chemotherapy-induced nausea, vomiting, and many more uses. Besides all these advantages regarding its biological activity, the main issue about indazoles is the less abundance in plant sources, and their synthetic derivatives also often face problems with low yield. In this review article, we discuss its chemistry, tautomerism along with their effects, different schematics for the synthesis of indazole derivatives, and their different biological activities.

Synthesis of 1H-indazoles by an electrochemical radical Csp2-H/N-H cyclization of arylhydrazones

The development of efficient and sustainable C-N bond-forming reactions to N-heterocyclic frameworks has been a long-standing interest in organic synthesis. In this work, we develop an electrochemical radical C_sp²-H/N-H cyclization of arylhydrazones to 1H-indazoles. The electrochemical anodic oxidation approach was adopted to synthesize a variety of 1H-indazole derivatives in moderate to good yields. HFIP was not only employed as a solvent or the proton donor, but also can promote the formation of N free radicals. This synthetic methodology is operationally simple, and less expensive electrodes would be suitable for this chemistry.

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.

Synthesis of phenanthridines by I2-mediated sp3C–H amination

An I₂-mediated transition-metal-free sp³C–H amination reaction is established for phenanthridine synthesis in an efficient and scalable manner.

Iodine-Catalyzed Regioselective Oxidative Cyclization of Aldehyde Hydrazones with Electron-Deficient Olefins for the Synthesis of Mefenpyr-Diethyl

A regioselective synthesis of polysubstituted dihydropyrazoles and pyrazoles through an iodine-catalyzed oxidative cyclization strategy of aldehyde hydrazones with electron-deficient olefins is described. The protocol adopts very mild reaction conditions and provides desirable yields. The reaction is supposed to proceed via a cascade C-H functionalization, C-N bond formation, and oxidation sequential processes. The overall simplicity and regioselectivity of the catalytic system make this approach a valuable and step-economical tool to construct a C-C bond for the synthesis of Mefenpyr-Diethyl.

Oxidant-controlled divergent transformations of 3-aminoindazoles for the synthesis of pyrimido[1,2-b]-indazoles and aromatic nitrile-derived dithioacetals

An oxidant-controlled divergent reactivity of 3-aminoindazoles is presented, which enables the [3 + 3] annulation and C–H arylation of ketene dithioacetals in disparate pathways to assemble pyrimido[1,2-b]-indazoles and aromatic nitrile-derived dithioacetals.

I2-Mediated transition-metal-free aromatic C–H amination for the synthesis of benzimidazol-2-ones and related heterocycles

A practical I₂-mediated aromatic C–H amination reaction was established for benzimidazol-2-one synthesis under transition-metal-free conditions. The present reaction can be performed directly from disubstituted amines and isocyanates without the purification of the urea intermediates in a scalable fashion.

Synthesis of imidazo[1,5-a]pyridines via I2-mediated sp3 C–H amination

A transition-metal-free sp³ C–H amination reaction has been developed employing molecular iodine for imidazo[1,5-a]pyridine synthesis.