Construction of Pyridazine Analogues via Rhodium-mediated C-H Activation

Catalytic System-Controlled [4 + 1] and [4 + 2] Annulation for the Construction of Pyrazolo[1,2-a]indazoles and Pyrazolo[1,2-a]cinnolines from Pyrazolidinones and Diazo Indandiones

Pyrazolo[1,2-a]indazoles and pyrazolo[1,2-a]cinnolines can be constructed selectively via catalytic system-controlled C(sp2)-H activation/[4 + 1] cyclization and C(sp2)-H activation/[4 + 2] cyclization reaction, utilizing pyrazolidinones and diazo indandiones. Employing two different Rh(III) catalysts, we successfully synthesized two distinct types of nitrogen-containing heterocyclic compounds, pyrazolo[1,2-a]indazoles and pyrazolo[1,2-a]cinnolines, respectively. Moreover, an inexpensive Ru(II) catalyst could also effectively generate pyrazolo[1,2-a]cinnolines with moderate to good yields. Overall, these methodologies offer advantages such as regioselectivity, broad substrate scope, operational simplicity, and product availability.

Nitrogen-bridgehead compounds: overview, synthesis, and outlook on applications

The nitrogen-bridgehead is a common structural motif present in a multitude of natural products. As many of these abundant compounds exhibit biological activities, e.g. against cancer or bacteria, these derivatives are of high interest. While natural products are often associated with problematic characteristics, such as elaborate separation processes, high molecular complexity and limited room for derivatization, purely synthetic approaches can overcome these challenges. Many synthetic procedures have been reported for preparation of artificial nitrogen bridgehead compounds, however, to our surprise only a fraction of these has been tested for their bioactivity. This review is therefore meant to give an overview of existing synthetic methods that provide scaffolds containing bridgehead nitrogen atoms, covering the period from 2000 to 2023. Reviews which cover subunits of this topic are referenced as well.

Electrochemistry-enabled Ir-catalyzed C-H/N-N bond activation facilitates [3+2] annulation of phenidones with propiolates

A mild and efficient [3+2] annulation of phenidones with propiolates has been developed to access N-substituted indole alkylamides, enabled by merging electrochemistry with iridium catalysis using an undivided cell at room temperature. The mechanistic studies have confirmed that the electrochemically mediated catalytic cycle of IrI-IrIII-IrV exhibits enhanced efficiency, mild reaction conditions, and unconventional selectivity.

Rh(III)- or Ru(II)-Catalyzed C-H Annulation with Vinylene Carbonate and an Unexpected Aerobic Oxidation/Deprotection Cascade to Yield Cinnolin-4(1H)-ones

Transition metal-catalyzed C-H annulation reactions have been extensively utilized for the synthesis of cinnolines, especially the N-protected ones; however, none of them can yield cinnolin-4(1H)-ones, a significant class of bioactive skeletons. Herein, we disclose one-pot access to cinnolin-4(1H)-ones through Rh(III)- or Ru(II)-catalyzed C-H activation/annulation of N-aryl cyclic hydrazides with vinylene carbonate, followed by an O2/K2CO3-promoted aerobic oxidation/deprotection cascade. The π-conjugation of the directing groups plays a crucial role in facilitating this transformation. Notably, seven-membered enolic Rh species IMC is characterized via electrospray ionization mass spectroscopy for the first time, which, along with systematic control experiments, provides compelling evidence for the mechanistic pathway encompassing alkenyl insertion, β-oxygen elimination, protonation, and dehydration.

Pyrazolidinone-Aided Ru(II)-Catalyzed Regioselective C-H Annulation with Allenes

Regioselective annulation of allenes via C-H activation represents an elegant synthetic approach toward the construction of valuable scaffolds. Considering the importance of allenes, herein we developed an unprecedented Ru(II)-catalyzed highly regioselective redox-neutral C-H activation/(4 + 1)-annulation of 1-arylpyrazolidinones employing allenyl acetates to access pyrazolo[1,2-a]indazol-1-one derivatives. Additionally, allenyl cyclic carbonates, which were never tested in C-H activation, were utilized to construct a similar class of heterocycles having a pendent alcohol functionality. Notably, double C-H functionalization was achieved by a simple modification of reaction conditions. The synthetic significance of this methodology is underscored by late-stage modification of natural products, broad substrate scope, gram-scale synthesis, and postfunctionalizations.

Photoinduced Decarbonylative Rearrangement of Diazabicyclo[2.2.2]Octenones: A Photochemical Approach of Diazabicyclo[4.1.0]heptene Skeleton from Masked o-Benzoquinone

We report a photoinduced decarbonylative rearrangement of diazabicyclo[2.2.2]octenone in the facile synthesis of a functionalized diazabicyclo[4.1.0]heptene skeleton, a unique derivative of the hydropyridazine type structure which could be found in a variety of biologically active natural products. The scope of functional group compatibility in the photoreaction was examined by taking advantage of the easy access of the heterobicyclo[2.2.2] structure from the Diels-Alder reaction of masked o-benzoquinones. 4-Phenyl-1,2,4-triazoline-3,5-dione served as the dienophile which provided the adjacent N-N unit in hexahydropyridazine-type products of subsequent photorearrangement.

K2CO3-Promoted Formal [3+3]-Cycloaddition of N-Unsubstituted Isatin N,N'-Cyclic Azomethine Imine 1,3-Dipoles with Knoevenagel Adducts

The synthesis of dicyclic spiropyridazine oxoindole derivatives by using [3+3]-cycloaddition of N-unsubstituted isatin N,N'-cyclic azomethine imine 1,3-dipoles was reported. The products bearing two consecutive stereocenters, including spiroquaternary stereocenters in one ring structure, can be effectively obtained in moderate to excellent yields (20-93%) and low to moderate diastereoselectivities (1:9-10:1 dr). The synthesized compounds (>35 examples) were characterized by single-crystal XRD, FTIR, NMR, and mass spectral analysis.

Selective Construction of Spiro or Fused Heterocyclic Scaffolds via One-pot Cascade Reactions of 1-Arylpyrazolidinones with Maleimides

Presented herein is a controllable selective construction of spiro or fused heterocyclic scaffolds through the one-pot cascade reactions of 1-phenylpyrazolidinones with maleimides. To be specific, succinimide spiro pyrazolo[1,2-a]pyrazolones were effectively formed via [4 + 1] spiroannulation of 1-phenylpyrazolidinones with maleimides through simultaneous C(sp²)-H bond activation/functionalization and intramolecular cyclization along with the traceless fusion of the pyrazolidinonyl unit into the final product. In this reaction, air acts as a cost-effective and environmentally sustainable oxidant to assist the regeneration of the Rh(III) catalyst. Alternatively, succinimide-fused pyrazolidinonylcinnolines were formed from the same starting materials through an initial [4 + 1] spiroannulation followed by base-promoted skeleton rearrangement of the in situ formed spiro product without isolation. Notable features of these protocols include easily tunable selectivity, broad substrate scope, cost-effective and sustainable oxidant, excellent atom economy, and facile scalability.

Regioselective Dichotomy in Ru(II)-Catalyzed C-H Annulation of Aryl Pyrazolidinones with 1,3-Diynes

Herein, we present a substrate-controlled regiodivergent strategy for the selective synthesis of C3 or C2-alkynylated indoles via ruthenium-catalyzed [3 + 2]-annulation of readily available pyrazolidinones and 1,3-diynes. Remarkably, C3-alkynylated indoles were obtained in good yields when 1,4-diarylbuta-1,3-diynes were employed as the coupling partners. On the other hand, dialkyl-1,3-diynes led to the selective formation of C2-alkynylated indoles. The key features of the strategy are the operationally simple conditions and external-oxidant-free, broad-scope, and substrate-switchable indole synthesis. Scale-up reactions and further transformations expanded the synthetic utility of the protocol.

Synthesis of Spirocyclopropylpyrazole Derivatives via the Cascade Reaction of Alkylidenecyclopropanes with Pyrazolidinones and Trifluoroethanol

A novel synthesis of spirocyclopropylpyrazole derivatives tethered with a trifluoromethyl unit through an unprecedented cascade reaction of alkylidenecyclopropanes with pyrazolidinones and trifluoroethanol (TFE) has been developed. Mechanistically, the reaction involves...

Rh(iii)-catalyzed simultaneous [3 + 3]/[5 + 1] annulation of 1-arylpyrazolidinones with gem-difluorocyclopropenes leading to fluorinated pyridopyrimidinone derivatives

Presented herein is an efficient and concise synthesis of fluorinated pyridopyrimidinone derivatives through formal [3 + 3]/[5 + 1] annulation of 1-arylpyrazolidinones with gem-difluorocyclopropenes.

Synthesis of Dihydroquinolinone Derivatives via the Cascade Reaction of o-Silylaryl Triflates with Pyrazolidinones

Presented herein is a novel synthesis of dihydroquinolinone derivatives through an unprecedented cascade reaction of o-silylaryl triflates with pyrazolidinones. Mechanistically, the formation of the title products is believed to involve a cascade procedure including in situ formation of aryne and its addition with pyrazolidinone followed by N-N bond cleavage and intramolecular C-C bond formation/annulation. Compared with literature methods for the synthesis of dihydroquinolinones, this protocol has advantages such as multistep transformations accomplished in one pot, broad substrate scope, mild reaction conditions, and good tolerance of diverse functional groups. In addition, the products thus obtained demonstrated significant in vitro antiproliferative activity in selected human cancer cell lines.

Catalytic System-Controlled Divergent Reaction Strategies for the Construction of Diversified Spiropyrazolone Skeletons from Pyrazolidinones and Diazopyrazolones

A catalytic system-controlled divergent reaction strategy was here reported to construct four types of intriguing spiroheterocyclic skeletons from simple and readily available starting materials via a precise chemical bond activation/[n+1] annulation cascade. The tetraazaspiroheterocyclic and trizazspiroheterocyclic scaffolds could be independently constructed by a selective N-N bond activation/[n+1] annulation cascade, a C(sp² )-H activation/[4+1] annulation and a novel tandem C(sp² )-H/C(sp³ )-H bond activation/[4+1] annulation strategy, along with a broad scope of substrates, moderate to excellent yields and valuable transformations. More importantly, in these transformations, we are the first time to capture a N-N bond activation and a C(sp³ )-H bond activation of pyrazolidinones under different catalytic system.

Ru(ii)-Catalyzed C–H activation/annulation reactions of N-aryl-pyrazolidinones with sulfoxonium ylides: synthesis of cinnoline-fused pyrazolidinones

The first Ru(ii)-catalyzed cascade C–H activation/annulation reactions of N-aryl-pyrazolidinones with sulfoxonium ylides are reported.

An unusual reaction mode of 1-phenylpyrazolidinones toward diazonaphthalen-2(1H)-ones featuring cascade C(sp2)–H and C(sp3)–H bond cleavage

Herein, a novel synthesis of pyrazolidinone fused 1,3-benzooxazepine derivatives via a formal [4 + 3] annulation reaction of 1-phenylpyrazolidinones with diazonaphthalen-2(1H)-ones is presented.

Selective Synthesis of Pyrazolo[1,2-a]pyrazolones and 2-Acylindoles via Rh(III)-Catalyzed Tunable Redox-Neutral Coupling of 1-Phenylpyrazolidinones with Alkynyl Cyclobutanols

An unprecedented divergent synthesis of pyrazolo[1,2-a]pyrazolones and 2-acylindoles via Rh(III)-catalyzed [4 + 1] or [3 + 2] annulation of 1-phenylpyrazolidinones with alkynyl cyclobutanols through redox-neutral multiple bond activation by using -NH and -OH units as directing groups is presented. Notably, different annulation reactions were selectively achieved by simply adjusting the reaction conditions. With features such as simple procedures, easily accessible substrates, and high regio/chemoselectivity, these methods may find wide applications in related areas.

Iridium(iii)-catalysed annulation of pyrazolidinones with propiolates: a facile route to pyrazolo[1,2-a] indazoles

Iridium(iii)-catalysed C–H bond activation/subsequent [4+1] cyclization for the synthesis of pyrazolo[1,2-a] indazoles has been developed.

Rhodium(iii)-catalyzed tandem annulation reaction to build polycyclic benzothiazine derivatives

An efficient rhodium(iii)-catalyzed C–H bond functionalization to construct polycyclic benzothiazine derivatives has been well developed.

Rhodium-Catalyzed [4 + 1] Cyclization via C-H Activation for the Synthesis of Divergent Heterocycles Bearing a Quaternary Carbon

The development of an efficient approach to construct fused polycyclic systems bearing a quaternary carbon center represents a great challenge to synthetic chemistry. Herein, we report a Rh(III)-catalyzed [4 + 1] annulation of propargyl alcohols with various heterocyclic scaffolds under an air atmosphere. Diverse fused heterocycles containing a quaternary carbon center were obtained in moderate to good yields. Additionally, this method features a high atom-economy, metal oxidant free, simple operation, and compatibility with various functional groups.

Rh(iii)-Catalyzed synthesis of pyrazolo[1,2-a]cinnolines from pyrazolidinones and diazo compounds

Pyrazolo[1,2-a]cinnolines were achieved via a rhodium(iii)-catalyzed redox-neutral annulation procedure, exhibiting good regioselectivity and high functional group tolerability.