Rh(III)-catalyzed [4 + 1] annulation of 1-arylindazolones with alkynyl cyclobutanols: access to indazolo[1,2-a]indazolones

A convenient and efficient synthesis of structurally diverse indazolo[1,2-a]indazolones via a Rh(III)-catalyzed [4 + 1] annulation of 1-arylindazolones with alkynyl cyclobutanols has been achieved by combining C-H and C-C bond cleavage. This cascade reaction features readily available starting materials, good functional group tolerance, broad substrate scope, and excellent atom-economy.

Photocatalytic Proficiency of Cinnoline Moiety for Cross-Coupling Reactions: A Two in One Photocatalyst

Herein, we have developed a new class of organic photocatalysts that can mimic transition metals for several oxidative and reductive organic cross-coupling transformations. Due to its wide potential window in both the oxidation and reduction ranges, cinnoline exhibits dual catalytic activity under visible light illumination, acting as both a photoreductant and photooxidant.

Construction of Diversified Penta-Spiro-Heterocyclic and Fused-Heterocyclic Frameworks with Potent Antitumor Activity

Multiple-spiro/fused-heterocyclic frameworks containing indazolone are structurally unique and represent a class of potentially dominant skeletons. In this work, we successfully fulfilled Rh(III)-catalyst mediated substrate- and pH- controlled strategies to construct four novel types of complicated penta-spiro/fused-heterocyclic frameworks via C-H activation/[4+1] and [4+2] annulation cascades. This method had mild reaction conditions, a broad scope of substrates, moderate to good yields, and valuable applications, which could realize for the first time the generation of the novel di-spiro-heterocyclic and multiple fused-heterocyclic products with unique structures. More importantly, novel spiro[cyclohexane-indazolo[1,2-a]indazole] scaffold constructed by this method exhibited potent antitumor activity against a variety of refractory solid tumors and hematological malignancies in vitro. Overall, our work provided new insights into the construction of complex and diverse multiple spiro/fused-heterocyclic systems and offered novel valuable lead compounds for the discovery of antitumor drugs.

Rh(III)-Catalyzed C-H Functionalization/Annulation of 1-Arylindazolones: Divergent Synthesis of Fused Indazolones and Allyl Indazolones

Rh(III)-catalyzed C-H/N-H annulation and C-H allylation of phenylindazolones have been realized by employing 5-methylene-1,3-dioxan-2-one and 4-vinyl-1,3-dioxolan-2-one as scalable cross-coupling partners, delivering functionalized indazolone fused heterocycles and branched and linear allyl indazolones respectively in moderate to high yield. These divergent synthesis protocols showcase mild conditions, broad substrate scope, and high functional-group compatibility. In addition, scale-up synthesis and preliminary mechanistic exploratory were also accomplished.

Tandem Transformation of Indazolones to Quinazolinones through Pd-Catalyzed Carbene Insertion into an N-N Bond

Serendipitous and expedite transformation of 1-aryl- and 2-aryl-1,2-dihydro-3H-indazol-3-ones to 1,2-di(hetero)aryl- and 2,3-di(hetero)aryl-2,3-dihydroquinazolin-4(1H)-ones, respectively, was achieved in high efficiency by reacting them with aldehydic N-tosylhydrazones. The protocol proceeded through a cascade process involving base-mediated Pd-carbenoid generation by the decomposition of N-tosylhydrazones, nucleophilic attack of indazolone on the Pd-carbenoid complex, and intramolecular ring expansion via N-N bond cleavage. The utility of the strategy is demonstrated toward the synthesis of bioactive NPS 53574, a calcium receptor antagonist.

Rhodium-Catalyzed Annulations and Heck Coupling/Aza-Michael Addition for the Synthesis of Benzothiadiazinoisoquinoline 6,6-Dioxides and Benzothiadiazinoisoindole 5,5-Dioxides, Respectively

A new and efficient protocol has been demonstrated for the synthesis of benzothiadiazinoisoquinoline 6,6-dioxides and benzothiadiazinoisoindole 5,5-dioxides in good to excellent yields. These compounds are formed through a sequential Rh(III)-catalyzed C-H cyclization of dihydrophenylbenzothiadiazine 1,1-dioxides with alkynes and oxidative Heck coupling/aza-Michael addition of dihydrophenylbenzothiadiazine 1,1-dioxides with acrylates, respectively.

Electricity Induced Rhodium-Catalyzed Oxidative C-H/N-H Annulation of Alkynes with Arylhydrophthalazinediones

The development of stoichiometric oxidant-free regioselective annulation protocol is a challenging aspect in organic synthesis. Herein, we disclose electricity as a greener oxidant for the C-H/N-H annulation to construct cinnolines using rhodium(III) catalyst under mild conditions. A detailed mechanistic investigation revealed the possibility of both Rh(III/I) and Rh(III/IV) catalytic cycles for the formation of annulated product. Exclusive regioselectivity, diverse substrate scope, and commercially available cheap graphite electrodes are key features of this protocol.

Ruthenium-catalyzed (spiro)annulation of N-aryl-2,3-dihydrophthalazine-1,4-diones with quinones to access pentacyclic spiro-indazolones and fused-cinnolines

Ru(II)-catalyzed strategies were developed for the [4 + 1] and [4 + 2] oxidative coupling between N-aryl-2,3-dihydrophthalazine-1,4-diones and 1,4-benzoquinones, achieving spiro-indazolones and fused-cinnolines, respectively. Mild, aerobic and external oxidant-free conditions, as well as the use of a ruthenium catalyst for such (spiro)annulative strategies with quinones over reported Rh/Ir-catalyts, underline the rewards of the disclosed protocols.

Synthesis of Succinimide-Linked Indazol-3-ols Derived from Maleimides under Rh(III) Catalysis

The structural modification of N-aryl indazolols as tautomers of N-aryl indazolones has been established as a hot topic in pharmaceutics and medicinal chemistry. We herein disclose the rhodium(III)-catalyzed 1,4-addition reaction of maleimides with N-aryl indazol-3-ols, which provides the succinimide-bearing indazol-3-ol scaffolds with complete regioselectivity and a good functional group tolerance. Notably, the versatility of this protocol is demonstrated by the use of drug-molecule-linked and fluorescence-probe-linked maleimides.

Assembly of the Hydroxycinnoline Core via Hydrazide-Assisted Rh(III)-Catalyzed C–H Functionalization and Annulation

The structural modification of phthalazinones and indazolones has emerged as a pivotal topic in catalytic C–H functionalization events. Herein we report the hydrazide-assisted rhodium(III)-catalyzed cross-coupling reactions of N-arylphthalazinones and N-arylindazolones with vinylene carbonate. This method provides direct access to tetracyclic hydroxycinnolines. Complete site-selectivity and functional group compatibility were observed.

A cascade indazolone-directed Ir(iii)- and Rh(iii)-catalyzed C(sp2)–H functionalization/[4 + 2] annulation of 1-arylindazolones with sulfoxonium ylides to access chemically divergent 8H-indazolo [1,2-a]cinnolines

An indazolone-directed chemoselective synthesis of 8H-indazolo [1,2-a]cinnolines has been realized via a cascade Cp*Ir(iii)- and Cp*Rh(iii)-catalyzed C–H activation/cyclization reaction of 1-arylindazolones with sulfoxonium ylides.

Design and synthesis of spirooxindole–pyrrolidines embedded with indole and pyridine heterocycles by multicomponent reaction: anticancer and in silico studies

We report the synthesis of spirooxindole–pyrrolidines tethered with indole and pyridine heterocycles using 1,3-dipolar cycloaddition, and their anticancer activities and molecular docking studies.

Synthesis of spirosuccinimides via annulative cyclization between N-aryl indazolols and maleimides under rhodium(III) catalysis

The rhodium(III)-catalyzed spiroannulation reaction between N-aryl indazol-3-ols and maleimides is described herein. The developed method is showcased by the construction of spirosuccinimides using bioactive molecule-linked and chemical probe-linked maleimides. Combined mechanistic investigations including the determination of an isolable rhodacycle complex aided the elucidation of a plausible reaction mechanism.

Transition-metal-catalyzed C-H bond activation/functionalization and annulation of phthalazinones

Phthalazinones and their higher congeners are commonly prevalent structural motifs that occur in natural products, bioactive molecules, and pharmaceuticals. In the past few decades, transition-metal-catalyzed reactions have received an overwhelming response from organic chemists as challenging organics and heterocycles could be built with ease. Currently, the synthesis of phthalazinones largely depends on transition-metal catalysis, especially by palladium-catalyzed carbonylation. Further, the dominance of transition-metal catalysts was realized from the phthalazinones viewpoint, as nitrogen and oxygen atoms endowed upon them act as directing groups to facilitate diverse C-H activation/functionalization/annulation reactions. This highlight describes the various synthetic methods used to access phthalazinones and functionalize them by reacting with various coupling partners via chelation assistance strategy involving C(sp²)-H/N-H bond activation in the presence of transition-metal (Rh, Ru, Pd, and Ir) catalysts. The mechanisms of sulfonylation, halogenation, acylmethylation, alkylation, and annulation reactions are discussed.

Direct synthesis of indazole derivatives via Rh(III)-catalyzed C-H activation of phthalazinones and allenes

A novel Rh(III)-catalyzed annulation of phthalazinones or pyridazinones with various allenes was developed, leading to the formation of indazole derivatives bearing a quaternary carbon in moderate to good yields. The targeted products were synthesized via sequential C-H activation and olefin insertion, followed by β-hydride elimination and intramolecular cyclization. The synthetic protocol proceeded efficiently with broad functional group tolerance, high atom efficiency and high Z-selectivity. The practicability of this method was proved by synthetic transformation.

Rh(iii)-Catalysed synthesis of cinnolinium and fluoranthenium salts using C-H activation/annulation reactions: organelle specific mitochondrial staining applications

The construction of a novel class of indazolo[2,1-a]cinnolin-7-ium and diazabenzofluoranthenium salts was developed by using Rh(iii)-catalyzed C-H activation/annulation reactions with 2-phenyl-2H-indazole, and internal alkynes, which resulted in structurally important polycyclic heteroaromatic compounds (PHAs). This reaction uses mild reaction conditions and has a high efficiency, low catalyst loading, and wide substrate scope. The overall catalytic process involves C-H activation followed by C-C/C-N bond formation. Furthermore, the synthesised cinnolinium/fluoranthenium salts exhibit potential fluorescence properties and 5i was targeted in particular for specific mitochondrial staining in order to investigate cancer cell lines.

Synthesis of quaternary carbon-centered indolo[1,2-a]quinazolinones and indazolo[1,2-a]indazolones via C-H functionalization

An unprecedented Ru(ii)-catalyzed Csp2-H bond activation and annulation reaction of phenylindazolones with diaryl-substituted alkynes and dialkyl-substituted alkynes provided efficient routes for the construction of all-carbon quaternary-centered indolo[1,2-a]quinazolinones and quaternary carbon-centered indazolo[1,2-a]indazolones, respectively. The indolo[1,2-a]quinazolinones were fomed via Csp2-H activation, alkyne insertion and a 1,2-phenyl shift. Indazolo[1,2-a]indazolones were formed through a cascade reaction via the formation of exocyclic double bonds containing indolo[1,2-a]quinazolinones.

Innovative Synthetic Procedures for Luminogens Showing Aggregation-Induced Emission

As a consequence of their intrinsic advantageous properties, luminogens that show aggregation-induced emission (AIEgens) have received increasing global interest for a wide range of applications. Whereas general synthetic methods towards AIEgens largely rely on tedious procedures and limited reaction types, various innovative synthetic methods have now emerged as complementary, and even alternative, strategies. In this Review, we systematically highlight advancements made in metal-catalyzed functionalization and metal-free-promoted pathways for the construction of AIEgens over the past five years, and briefly illustrate new perspectives in this area. The development of innovative synthetic procedures will enable the facile synthesis of AIEgens with great structural diversity for multifunctional applications.

Reducing-Agent-Free Convergent Synthesis of Hydroxyimino-Decorated Tetracyclic Fused Cinnolines via RhIII-Catalyzed Annulation Using Nitroolefins

A mild Rh-catalyzed method was developed for the synthesis of hydroxyimino functionalized indazolo[1,2-a]cinnolines and phthalazino[2,3-a]cinnolines by reductive [4 + 2] annulation between 1-arylindazolones and 2-aryl-2,3-dihydrophthalazine-1,4-diones with varied nitroolefins. The targeted oxime decorated tetracyclic fused cinnolines were synthesized via sequential C-H activation/olefin insertion/reduction under reducing-agent-free conditions.

Densely functionalized cinnolines: Controlled microwave-assisted facile one-pot multi-component synthesis and in vitro anticancer activity via apoptosis induction

There is an urging continuous need for novel anti-cancer agents due to persistent chemoresistance. Herein, newly synthesized cinnolines are evaluated for their possible anticancer activities and suggested mechanisms. In the current study, a simple and efficient synthesis of densely functionalized cinnolines has been developed that relied on multi-component reaction of ethyl 5-cyano-4-methyl-1-aryl-6-oxo-1,6-dihydropyridazine-3-carboxylates with aromatic aldehydes and nitromethane in dioxane/pipridine under controlled microwave heating. Selected cinnolines (4a-c, e, h, j-n, q-v) were tested for possible anticancer activity using in vitro one dose assay at National Cancer institute, USA. Only cinnoline 4b stood out as the most potent cinnoline derivative (mean GI%=26.33) with broad-spectrum antitumor activity against the most tested cancer cell lines from all subpanels. The target cinnoline 4b emerged as the most active derivative against both leukemia RPMI-8226 and melanoma LOX IMVI cell lines (GI% = 106.06 and 82.1) respectively, with IC₅₀ values equal to 17.12 ± 1.31 and 12.32 ± 0.75 μg/mL, which are comparable to those of staurosporin; 24.97 ± 1.47 and 8.45 ± 0.42 μg/mL, respectively. Cinnoline 4b influenced cell cycle distribution causing pre-G1 apoptosis and cell growth arrest at G2/M phase. It also induced apoptosis in both cell lines as manifested by significant increase in the percent of annexin V-FITC positive apoptotic cells in leukemia RPMI-8226 cells (from 1.09% to 12.47%) and melanoma LOX IMVI (from 1.32% to 19.05%). In addition, it showed lower expression levels of anti-apoptotic Bcl-2 protein, and higher expression levels of pro-apoptotic proteins; Bax, p53, cytochrome c, caspases 3 and 9. CONCLUSION: Induction of mitochondrial intrinsic pathway of apoptosis is a possible mechanism by which cinnoline 4b may confer its anticancer activity.

Phthalazinone-Assisted C-H Amidation Using Dioxazolones Under Rh(III) Catalysis

The preparation of phthalazinone derivatives is pivotal for their utilization as pharmaceutical agents and other entities. Herein, we report the phthalazinone-assisted carbon-nitrogen bond forming reaction using dioxazolones as robust amidation sources under Rh(III) catalysis. The broad functional group tolerance and complete site-selectivity are observed. Notably, a series of transformations of synthesized compounds into biologically relevant N-heterocycles demonstrates the applicability of the developed methodology.

Ru(II)-Catalyzed C-H Hydroxyalkylation and Mitsunobu Cyclization of N-Aryl Phthalazinones

Ruthenium(II)-catalyzed C(sp²)-H functionalization of N-aryl phthalazinones with a range of aldehydes and activated ketone is described. Initial formation of hydroxyalkylated phthalazinones and subsequent Mitsunobu cyclization provided facile access to biologically relevant indazolophthalazinones. The utility of this method is highlighted by synthetic transformations into a series of potentially bioactive scaffolds.

Ruthenium Catalyzed C-H Acylmethylation of N-Arylphthalazine-1,4-diones with α-Carbonyl Sulfoxonium Ylides: Highway to Diversely Functionalized Phthalazino-fused Cinnolines

A direct ortho-Csp² -H acylmethylation of 2-aryl-2,3-dihydrophthalazine-1,4-diones with α-carbonyl sulfoxonium ylides is achieved through a Ru^II -catalyzed C-H bond activation process. The protocol featured high functional group tolerance on the two substrates, including aryl-, heteroaryl-, and alkyl-substituted α-carbonyl sulfoxonium ylides. Thereafter, 2-(ortho-acylmethylaryl)-2,3-dihydrophthalazine-1,4-diones were used as potential starting materials for the expeditious synthesis of 6-arylphthalazino[2,3-a]cinnoline-8,13-diones and 5-acyl-5,6-dihydrophthalazino[2,3-a]cinnoline-8,13-diones under Lawesson's reagent and BF₃ ⋅OEt₂ mediated conditions, respectively. Of these, the BF₃ ⋅OEt₂ -mediated cyclization proceeded in DMSO as a solvent and a methylene source via dual C-C and C-N bond formations.

Synthesis and Properties of 6-Aryl-4-azidocinnolines and 6-Aryl-4-(1,2,3-1H-triazol-1-yl)cinnolines

An efficient approach towards the synthesis of 6-aryl-4-azidocinnolines was developed with the aim of exploring the photophysical properties of 6-aryl-4-azidocinnolines and their click reaction products with alkynes, 6-aryl-4-(1,2,3-1H-triazol-1-yl)cinnolines. The synthetic route is based on the Richter-type cyclization of 2-ethynyl-4-aryltriazenes with the formation of 4-bromo-6-arylcinnolines and nucleophilic substitution of a bromine atom with an azide functional group. The developed synthetic approach is tolerant to variations of functional groups on the aryl moiety. The resulting azidocinnolines were found to be reactive in both CuAAC with terminal alkynes and SPAAC with diazacyclononyne, yielding 4-triazolylcinnolines. It was found that 4-azido-6-arylcinnolines possess weak fluorescent properties, while conversion of the azido function into a triazole ring led to complete fluorescence quenching. The lack of fluorescence in triazoles could be explained by the non-planar structure of triazolylcinnolines and a possible photoinduced electron transfer (PET) mechanism. Among the series of 4-triazolylcinnoline derivatives a compound bearing hydroxyalkyl substituent at triazole ring was found to be cytotoxic to HeLa cells.

Selective Synthesis in Microdroplets of 2-Phenyl-2,3-dihydrophthalazine-1,4-dione from Phenyl Hydrazine with Phthalic Anhydride or Phthalic Acid

Pyridazine derivatives are privileged structures because of their potential biological and optical properties. Traditional synthetic methods usually require acid or base as a catalyst under reflux conditions with reaction times ranging from hours to a few days or require microwave assistance to induce the reaction. Herein, this work presents the accelerated synthesis of a pyridazine derivative, 2-phenyl-2,3-dihydrophthalazine-1,4-dione (PDHP), in electrosprayed microdroplets containing an equimolar mixture of phenyl hydrazine and phthalic anhydride or phthalic acid. This reaction occurred on the submillisecond timescale with good yield (over 90 % with the choice of solvent) without using an external catalyst at room temperature. In sharp contrast to the bulk reaction of obtaining a mixture of two products, the reaction in confined microdroplets yields only the important six-membered heterocyclic product PDHP. Results indicated that surface reactions in microdroplets with low pH values cause selectivity, acceleration, and high yields.

Ru(II)/Ir(III)-Catalyzed C-H Bond Activation/Annulation of Cyclic Amides with 1,3-Diketone-2-diazo Compounds: Facile Access to 8H-Isoquinolino[1,2-b]quinazolin-8-ones and Phthalazino[2,3-a]cinnoline-8,13-diones

Efficient access to 8H-isoquinolino[1,2-b]quinazolin-8-ones and phthalazino[2,3-a]cinnoline-8,13-diones through cyclic amide-directed Ru(II)/Ir(III)-catalyzed C-H bond activation, has been developed. Consecutive C-H bond activation, carbene insertion, and condensation annulation processes were realized, affording 8H-isoquinolino[1,2-b]quinazolin-8-one and phthalazino[2,3-a]cinnoline-8,13-dione derivatives in good-to-excellent yields under mild conditions, with H₂O and N₂ being generated as the only byproducts.

Ruthenium(ii)-catalyzed synthesis of indazolone-fused cinnolines via C-H coupling with diazo compounds

A robust, efficient and scalable method for the synthesis of 12H-indazolo[2,1-a]cinnolin-12-ones was developed. Significantly, a less developed cationic complex [Ru(p-cymene)(MeCN)3(SbF6)2] was found to be effective for this transformation. In this reaction, a tandem pathway of C-H ruthenation, Ru(ii)-carbene formation, migratory insertion and condensation was involved. The results of a primary mechanistic study suggested that the C-H activation process might follow an electrophilic-type metalation/deprotonation mechanism.

Additive-Driven Rhodium-Catalyzed [4+1]/[4+2] Annulations of N-Arylphthalazine-1,4-dione with α-Diazo Carbonyl Compounds

A Rh(III)-catalyzed strategy involving the [4+1] annulation of 2-arylphthalazine-1,4-diones with α-diazo carbonyl compounds was developed, accessing a series of unprecedented hydroxy-dihydroindazolo-fused phthalazines in good to excellent yields. By varying the additive, phthalazino-fused cinnolines were synthesized under Rh-catalyzed conditions via [4+2] annulation between the same starting materials. Notably, such two strategies showed a good functional group tolerance and high atom efficiency.

Iridium-catalyzed [4 + 2] annulation of 1-arylindazolones with α-diazo carbonyl compounds: access to indazolone-fused cinnolines

An Ir-catalyzed tandem strategy for the synthesis of indazolone-fused cinnolines by [4 + 2] annulation of 1-arylindazolones with α-diazo carbonyl compounds.

Microwave assisted one-pot green synthesis of cinnoline derivatives inside natural sporopollenin microcapsules

We present a green and efficient approach for the synthesis of novel cinnoline derivatives inside natural Lycopodium clavatum sporopollenin (LCS) microcapsules via a one-pot microwave (MW) assisted reaction for the first time. We also propose the concept that the robust micrometre-sized sporopollenin microcapsules can act as MW microreactors. We demonstrate the feasibility of this concept by in situ synthesising 8-hydroxy-7-nitro-6-(3-nitrophenyl)-3-oxo-2-(p-tolyl)-2,3,5,6-tetrahydrocinnoline-4-carbonitrile inside the LCS microcapsules via a microwave (MW) assisted reaction of ethyl 5-cyano-4-methyl-6-oxo-1-(p-tolyl)-1,6-dihydropyridazine-3-carboxylate with 1-nitro-2-phenylethylene in the presence of piperidine as a base at 100 °C for 20 minutes. The LCS microparticles are extensively characterised before and after the MW induced reaction using several techniques. The formation of the cinnoline compound inside the LCS microcapsules is confirmed by laser scanning confocal microscopy (LSCM), X-ray diffraction (XRD) and fourier-transform infrared spectroscopy (FTIR) analyses. Using liquid chromatography-mass spectrometry (LCMS) analyses, we show that the structural integrity of the cinnoline compound, recovered from the cinnoline loaded (cinn-loaded) LCS, is preserved. The pure cinnoline is found to show promising optical properties with two λ_max absorption peaks at 310 and 610 nm. Both the pure cinnoline and cinn-loaded LCS show promising antibacterial activity against Pseudomonas aeruginosa (Gram-negative) and Bacillus cereus (Gram-negative) human pathogenic bacterial strains. The successful MW induced reaction of the prominent cinnoline derivative inside the biocompatible LCS microreactors can open up intriguing applications in materials and pharmaceutical sciences.

We have achieved in situ microwave assisted green syntheses of a novel cinnoline derivative inside natural sporopollenin microreactors.

A ruthenium-catalyzed alkenylation–annulation approach for the synthesis of indazole derivatives via C–H bond activation

A new approach to indazole derivatives through a Ru(ii)-catalyzed C–H activation–annulation reaction, which proceeds via C–C and C–N bond forming reactions, is reported.