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

Stereoselective Synthesis of trans-1,2,4-Trihydrobenzo[d][1,2]azaphosphinine 3-Oxides from In Situ Formed Phosphenes and Formaldimines

Various trans-1,2,4-trihydrobenzo[d][1,2]azaphosphinine 3-oxides are synthesized highly stereoselectively from in situ generated phosphenes and formaldimines under microwave irradiation. Aryl(diazo)methyl(diaryl)phosphine oxides first undergo the Wolff rearrangement to generate phosphenes. Imines, in situ generated from 1,3,5-triazinanes or paraformaldehyde and primary amines, nucleophilically attack the phosphenes followed by a tandem stereoelectronic effect-controlled intramolecular nucleophilic addition and aromatization to give final trans-1,2,4-trihydrobenzo[d][1,2]azaphosphinine 3-oxides, exhibiting completely different annuloselectivity from linear nonformaldimines.

Synthetic Methods for Azaheterocyclic Phosphonates and Their Biological Activity: An Update 2004-2024

The increasing importance of azaheterocyclic phosphonates in the agrochemical, synthetic, and medicinal field has provoked an intense search in the development of synthetic routes for obtaining novel members of this family of compounds. This updated review covers methodologies established since 2004, focusing on the synthesis of azaheterocyclic phosphonates, of which the phosphonate moiety is directly substituted onto to the azaheterocyclic structure. Emphasizing recent advances, this review classifies newly developed synthetic approaches according to the ring size and providing information on biological activities whenever available. Furthermore, this review summarizes information on various methods for the formation of C-P bonds, examining sustainable approaches such as the Michaelis-Arbuzov reaction, the Michaelis-Becker reaction, the Pudovik reaction, the Hirao coupling, and the Kabachnik-Fields reaction. After analyzing the biological activities and applications of azaheterocyclic phosphonates investigated in recent years, a predominant focus on the evaluation of these compounds as anticancer agents is evident. Furthermore, emerging applications underline the versatility and potential of these compounds, highlighting the need for continued research on synthetic methods to expand this interesting family.

Design and Synthesis of Indazole-Indole Hybrid via tert-Butyl Nitrite Mediated Cascade Diazotization/Isomerization/Cyclization

In this report, a tert-butyl nitrite (TBN)-mediated straightforward metal-free approach has been presented for the synthesis of a diverse range of C-3-substituted indazole-indole hybrids using readily accessible 2-(indolin-3-ylidenemethyl)aniline derivatives. This strategy is proposed to occur via a diazonium salt intermediate that is capable of cascade isomerization and intramolecular C-N bond formation through a 5-endo-dig cyclization to achieve a wide variety of indazole-indole hybrids in good yields.

Reaction of 1,3,5-Triazinanes with Phosphoryl Diazomethanes: Access to 5-Phosphoryl-1,2,3,4-tetrahydropyrimidines

Conversion of 1,3,5-triazinanes into 5-phosporyl-1,2,3,4-tetrahydropyrimidines is achieved efficiently through the microwave-assisted reaction with phosphoryl diazomethanes. Both trialkyl and triaryl 1,3,5-triazinanes were converted by diazomethyldiarylphosphine oxides, dialkyl diazomethylphosphonates, and alkyl diazomethyl(aryl)phosphinates and functionalized simultaneously in good to excellent yields. The reaction is a sequence of 1,3,5-triazinane fragmentation, tandem nucleophilic addition of the generated formaldimines and phosphoryl diazomethanes, and final N,N-acetalization.

Access to Fully Substituted Dihydroindazoles Via Hexadehydro-Diels-Alder/[3 + 2] Cycloaddition

A cascade hexadehydro-Diels-Alder (HDDA)/[3 + 2] cycloaddition reaction between tetrayne and N,N'-cyclic acylhydrazone is described. This strategy allows the efficient construction of fully substituted 2,3-dihydro-1H-indazole scaffolds which have insecticidal activity against the third instar larvae of Mythimna separata.

Advances in the synthesis of nitrogen-containing heterocyclic compounds by in situ benzyne cycloaddition

Nitrogen-containing heterocyclic compounds are prevalent in various natural products, medicines, agrochemicals, and organic functional materials. Among strategies to prepare nitrogen-containing heterocyclic compounds, pathways involving benzyne intermediates are attractive given that they can readily assemble highly diverse heterocyclic compounds in a step-economical manner under transition-metal-free conditions. The synthesis of nitrogen-containing heterocyclic compounds from benzyne intermediates offers an alternative strategy to the conventional metal-catalyzed activation approaches. In the past years, chemists have witnessed the revival of benzyne chemistry, mainly attributed to the wide application of various novel benzyne precursors. The cycloaddition of benzynes is a powerful tool for the synthesis of nitrogen-containing heterocyclic compounds, which can be constructed by [n + 2] cyclization of benzyne intermediates in situ generated from benzyne precursors under mild reaction conditions. This review focuses on the application of cycloaddition reactions involving in situ benzynes in the construction of various nitrogen-containing heterocyclic compounds.

Copper-catalyzed cross coupling reaction of sulfonyl hydrazides with 3-aminoindazoles

A novel Cu-catalyzed radical-radical cross coupling reaction of 3-aminoindazoles with sulfonyl hydrazides has been disclosed, enabling the production of diverse 1,3-substituted aminoindazoles in good yields. This methodology is distinguished by readily available starting materials, wide substrate scope and operational simplicity. In addition, a gram-scale reaction has been well demonstrated.

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.

Unorthodox cascade reaction of arynes and N-nitrosamides leading to indazole scaffolds

An unusual cascade annulation of arynes with N-alkyl-N-nitrosamides is developed by leveraging aryne σ-insertion and C(sp³)-H bond functionalization strategies under transition-metal-free conditions at ambient temperature, offering functionalized indazoles in high yields and regioselectivity. The protocol is scalable and exhibits a broad substrate scope. The reaction mechanism is also studied with DFT calculations.

Diazophosphonates: Effective Surrogates for Diazoalkanes in Pyrazole Synthesis

Diazophosphonates, readily prepared from α-ketophosphonates by oxidation of the corresponding hydrazones in batch or in flow, are useful partners in 1,3-dipolar cycloaddition reactions to alkynes to give N-H pyrazoles, including the first intramolecular examples of such a process. The phosphoryl group imbues a number of desirable properties into the diazo 1,3-dipole. The electron-withdrawing nature of the phosphoryl stabilizes the diazo compound making it easier to handle, whilst the ability of the phosphoryl group to migrate readily in a [1,5]-sigmatropic rearrangement enables its transfer from C to N to aromatize the initial cycloadduct, and hence its facile removal from the final pyrazole product. Overall, the diazophosphonate acts as a surrogate for the much less stable diazoalkane in cycloadditions, with the phosphoryl group playing a vital, but traceless, role. The cycloaddition proceeds more readily with alkynes bearing electron-withdrawing groups, and is regiospecific with asymmetrical alkynes. The potential of diazophosphonates for use in bioorthogonal cycloadditions is demonstrated by their facile addition to strained alkynes.

Diazocarbonyl and Related Compounds in the Synthesis of Azoles

Diazocarbonyl compounds have found numerous applications in many areas of chemistry. Among the most developed fields of diazo chemistry is the preparation of azoles from diazo compounds. This approach represents a useful alternative to more conventional methods of the synthesis of azoles. A comprehensive review on the preparation of various azoles (oxazoles, thiazoles, imidazoles, pyrazoles, triazoles, and tetrazoles) from diazocarbonyl and related compounds is presented for the first time along with discussion of advantages and disadvantages of «diazo» approaches to azoles.

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 indazoles from 2-formylphenylboronic acids

A method for the synthesis of indazoles was developed which involves a copper(ii) acetate catalysed reaction of 2-formylboronic acids with diazadicaboxylates followed by acid or base induced ring closure.

Dipyrazolodioxadiazocines as shelf-stable "ready-to-use" precursors for an in situ generation of enolate-iminium 1,4-dipoles: a straightforward atom-economical approach to pyrazolo[5,1-d][1,3,5]dioxazines

An original, facile and highly efficient method for the in situ generation of cyclic enolate-iminium 1,4-dipoles via unique thermal decomposition of easily available dipyrazolodioxadiazocines has been developed. Various substituted pyrazolo[5,1-d][1,3,5]dioxazines have been prepared in high yields via unusual cycloconversion of dipyrazolodioxadiazocines in the presence of ketones. Furthermore, the developed method is 100% atom economical and proceeds under metal-, catalyst- and solvent-free conditions.

Recent advances in the synthesis of organophosphorus compounds via Kobayashi's aryne precursor: a review

This review systematically summarizes the progress in aryne chemistry for the synthesis of organophosphorus compounds via aryne insertion into the C–P, P–N, P–P, P–O, PP, PN and PS bonds.

Asymmetric acyl-Mannich reaction of isoquinolines with α-(diazomethyl)phosphonate and diazoacetate catalyzed by chiral Brønsted acids

α-Diazo-β-isoquinoline derivatives were obtained in excellent yields and enantioselectivities by asymmetric acyl-Mannich reaction of (diazomethyl)phosphonate or diazoacetate with isoquinolines.

Continuous-Flow Visible Light Organophotocatalysis for Direct Arylation of 2H-Indazoles: Fast Access to Drug Molecules

A continuous-flow homogeneous photocatalytic method has been devised for the direct arylation of 2H-indazoles. This visible-light-promoted approach directly accesses a wide range of structurally diverse C3-arylated scaffolds of biological interest in a fast (1 min), single-step reaction by using eosin Y as an organophotocatalyst. Furthermore, a microreactor technology is also employed for the fast synthesis of liver X receptor inhibitor drugs with very good yields under metal-free conditions, whereas the reported methods required multiple steps and much longer reaction times (18-24 h).

Recent Advances in Indazole-Containing Derivatives: Synthesis and Biological Perspectives

Indazole-containing derivatives represent one of the most important heterocycles in drug molecules. Diversely substituted indazole derivatives bear a variety of functional groups and display versatile biological activities; hence, they have gained considerable attention in the field of medicinal chemistry. This review aims to summarize the recent advances in various methods for the synthesis of indazole derivatives. The current developments in the biological activities of indazole-based compounds are also presented.

Tandem nucleophilic addition–cycloaddition of arynes with α-iminoesters: two concurrent pathways to imidazolidines

The tandem nucleophilic addition-cycloaddition reaction of arynes with α-iminoesters gave functionalized imidazolidine derivatives with high yields, complete regioselectivities, and excellent diastereo- and enantioselectivities.