Construction of Benzene Rings by Copper-Catalyzed Cycloaddition Reactions of Oximes and Maleimides: An Access to Fused Phthalimides

Multicomponent reactions (MCRs) yielding medicinally relevant rings: a recent update and chemical space analysis of the scaffolds

In this review we have compiled multicomponent reactions (MCRs) that produce cyclic structures. We have covered articles reported since 2019 to showcase the recent advances in this area. In contrast to other available reviews on this topic, we focus specifically on MCRs with strong prospects in medicinal chemistry. Consequently, the reactions operating in a single-pot and yielding novel rings or new substitution patterns under mild conditions are highlighted. Moreover, MCRs that do not require special reagents or catalysts and yield diverse products from commercially available building blocks are reviewed. The synthetic schemes, substrate scope, and other key aspects such as regio- and stereoselectivity are discussed for each MCR. Using cheminformatic tools, we have also attempted to characterize the chemical space of the scaffolds obtained from these MCRs. We show that the MCR scaffolds are novel, more complex, and globular in shape compared to the approved drugs and clinical candidates. Thus, our review represents a step towards identifying and characterizing the novel ring space that can be accessed efficiently through MCRs in a short timeframe.

Recent advances in the transformation of maleimides via annulation

Over the past five years, maleimide scaffolds have gained considerable attention in organic synthesis for their role in forming cyclized molecules through annulation and C-H activation. As versatile and reactive coupling agents, maleimides have enabled the efficient synthesis of various cyclized products, including annulation, benzannulation, cycloaddition, and spirocyclization, with applications in medicinal chemistry, drug discovery, and materials science. Despite the extensive study of maleimide chemistry, certain reactions-such as cycloaddition-based annulation, photoannulation, and electrochemical transformations-remain underexplored despite their promising potential in the pharmaceutical and chemical industries. Recent advancements, such as photocatalysis and electrochemical methods, have expanded the utility of maleimides, providing more sustainable and selective approaches for synthesizing complex molecules. This review compiles research published between 2019 and 2024, highlighting the substrate scope, reaction diversity, and industrial relevance of maleimide-based annulation strategies. Additionally, we discuss emerging trends and future directions in maleimide chemistry, exploring opportunities for novel reaction pathways and broader applications in synthetic biology and materials science.

Benzannulation with Et3N as 1,3-Diene Variants

With triethylamine as a 1,3-diene variant, a simple and practical process for the synthesis of phthalimides has been developed from readily available maleimide. The transformation can be performed in the absence of a metal catalyst with high levels of functional group tolerance. Various phthalimide compounds were constructed in moderate to good yields under mild conditions. Mechanism research indicates that oxygen and acid also play crucial roles in this reaction.

Oxime Esters: Flexible Building Blocks for Heterocycle Formation

Oxime esters as the applicable building blocks, internal oxidizing agents, and directing groups in the synthesis of -, S-, and O-containing heterocycle scaffolds have gained great attention in the last decade. This review provides an overview of recent advances in the cyclization of oxime esters with various functional group reagents under transition metal and transition metal-free catalyzed conditions. Moreover, the mechanistic aspects of these protocols are explained in detail.

Skeletal Transformation of Oxindoles into Quinolinones Enabled by Synergistic Copper/Iminium Catalysis

We describe a synergistic Cu/secondary amine catalysis for skeletal transformation of an oxindole core into a quinolinone skeleton, which generates several structurally new pyridine-fused quinolinones. The synergistic reactions allow expansion of a five-membered lactam ring by radical cation-triggered C-C bond cleavage and enable a further intramolecular cyclization with the aim to construct totally distinct core skeletons.

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.

Copper-catalyzed [4+2] oxidative annulation of α,β-unsaturated ketoxime acetates with ethyl trifluoropyruvate

A novel route for tandem C-N/C-O formation via copper-catalyzed [4+2] oxidative annulation of α,β-unsaturated ketoxime acetates with ethyl trifluoropyruvate to synthesize valuable trifluoromethyl-containing 2H-1,3-oxazines in moderate to good yields is developed. This procedure represents the first [4+2] oxidative annulation of oxime derivatives with activated CO bonds and provides an alternative route towards functionalized 2H-1,3-oxazines.

Metal-Free Aminohalogenation of Quinones With Alkylamines and NXS at Room Temperature

A simple and practical strategy for intermolecular aminohalogenation of quinone with alkyl amines and NXS was developed, in which haloamines generated in situ were employed as bifunctional reagents. The reaction system is reliable, efficient and wide in substrate range, which is suitable for the two-fold aminochlorination of 1, 4-benzoquinones, large-scale reaction and late-stage modification of pharmaceuticals.

Deconstructive Insertion of Oximes into Coumarins: Modular Synthesis of Dihydrobenzofuran-Fused Pyridones

In the presence of a copper catalyst, a series of oximes undergo deconstructive insertion into coumarins to afford structurally interesting dihydrobenzofuran-fused pyridones in moderate to good yields with good functional group compatibility. The reaction likely involves a radical relay annulation, leading to the ring opening of the lactone moiety of the coumarins, and simultaneous formation of three new bonds. The investigation of photoluminescent properties reveals that several obtained compounds may have potential as fluorescent materials.

Divergent construction of 3-(indol-2-yl)succinimide/maleimide and fused benzodiazepine skeletons from 2-(1H-indol-1-yl)anilines and maleimides

Divergent construction of 3-(indol-2-yl)succinimide/maleimide and indoyl/pyrrolyl fused benzodiazepine skeletons from 2-(1H-indol-1-yl)anilines and maleimides is presented.

Iminyl radical-triggered relay annulation for the construction of bridged aza-tetracycles bearing four contiguous stereogenic centers

Bridged tetracyclic nitrogen scaffolds are found in numerous biologically active molecules and medicinally relevant structures. Traditional methods usually require tedious reaction steps, and/or the use of structurally specific starting materials. We report an unprecedented, iminyl radical-triggered relay annulation from oxime-derived peresters and azadienes, which shows good substrate scope and functional group compatibility, and can deliver various bridged aza-tetracyclic compounds with complex molecular topology and four contiguous stereogenic centers (dr > 19 : 1) in a single operation. This transformation represents the first example of trifunctionalization of iminyl radicals through simultaneous formation of one C–N and two C–C bonds. DFT calculation studies were conducted to obtain an in-depth insight into the reaction pathways, which revealed that the reactions involved an interesting 1,6-hydrogen atom transfer process.

A novel radical relay annulation is realized for the construction of various bridged aza-tetracyclic compounds with complex molecular topology and four contiguous stereogenic centers (dr > 19 : 1) in a single operation.

Synthesis of Highly Substituted Biaryls by the Construction of a Benzene Ring via In Situ Formed Acetals

Herein, we present an interesting method for the construction of a benzene ring using propargylic alcohols and 1,3-dicarbonyls, which involves three new C-C bond formations via cascade alkylation, formylation, annulation, and aromatization to make substituted biaryls. This one-pot Brønsted acid-promoted protocol utilizes the unique reactivity of the acetal formed under the reaction conditions. Alkynyl methyl ketones could be employed instead of 1,3-dicarbonyls as they are converted to 1,3-dicarbonyls by hydration under the reaction conditions.

Synthesis of Succinimide Spiro-Fused Sultams from the Reaction of N-(Phenylsulfonyl)acetamides with Maleimides via C(sp2)-H Activation

Presented herein is an effective preparation of succinimide spiro-fused sultams through the coupling reaction of N-(phenylsulfonyl)acetamides with maleimides. It is deduced that this reaction should proceed through a cascade process including Rh(III)-catalyzed C(sp²)-H bond cleavage of N-(phenylsulfonyl)acetamide, maleimide double bond insertion into the C-Rh bond, β-hydride elimination, reductive elimination, and intramolecular aza-Michael addition. Notably, this cascade procedure features simultaneous annulation and spirocyclization through traceless fusion of the directing group into target product by using air as an economical oxidant to assist the regeneration of the active Rh(III) catalyst. This new method has several advantages including readily accessible starting materials with broad scope, significantly reduced synthetic steps, redox-neutral conditions, high atom-economy, and sustainability.

Haloamines as Bifunctional Reagents for Oxidative Aminohalogenation of Maleimides

An unprecedented copper-catalyzed oxidative aminohalogenation of electron-deficient maleimides with secondary amines and NXS (X = Cl, Br, I) was developed, in which the N-X bonds generated in situ were used as difunctionalized reagents. The distinctive features of this multicomponent reaction include a simple green catalytic system, a spectral substrate range, and the late-stage modification of drug molecules. Most importantly, this umpolung radical cascade strategy exploits the in situ formation of N-iodoamines that enable efficient alkene aminoiodination.

Synthesis of Substituted Phthalimides via Ultrasound-Promoted One-Pot Multicomponent Reaction

In this work, a novel strategy for the straightforward synthesis of substituted phthalimides is described, which includes base-mediated Michael addition/intramolecular cyclization/[1,5]-H shift/cleavage of CS₂/aromatization/nucleophilic acyl substitution reaction of 2-(4-oxo-2-thioxothiazolidin-5-ylidene)acetates and α,α-dicyanoolefines under ultrasound (US) irradiation. Some advantages of this method are as follows: having simple operation, easily accessible starting materials, chemoselective cascade process, synthetically useful yields, and green conditions by utilizing US irradiation as a source of energy and using ethanol as solvent.

Beyond conventional construction of the phthalimide core: a review

This review highlights (2010–2021) different strategies for the construction of the phthalimide core apart from traditional synthetic routes.

Pyrrole synthesis through Cu-catalyzed cascade [3 + 2] spiroannulation/aromatization of oximes with azadienes

We disclose an efficient synthetic protocol for the assembly of poly-substituted pyrroles through cascade [3 + 2] spiroannulation/aromatization of oximes with azadienes.

Copper-catalyzed regioselective [3+2] annulation of malonate-tethered acyl oximes with isatins

A copper-catalyzed regioselective [3+2] annulation of malonate-tethered acyl oximes with isatins was developed, affording valuable 2,3-dihydrooxazole-spirooxindoles in moderate to good yields with excellent diastereoselectivity.

Organocatalyzed Synthesis of Highly Functionalized Phthalimides via Diels-Alder Reaction Employing Two Dienophiles

An efficient and facile protocol for the synthesis of biologically and pharmaceutically important phthalimides is developed by l-proline-catalyzed reaction between two dienophiles of α,β-unsaturated aldehydes and maleimides. The reaction involves an efficient benzannulation that proceeds via a formal [4 + 2] cycloaddition of azadiene intermediates generated in situ from enals and N-substituted maleimides. This protocol provides a variety of functionalized phthalimide derivatives, including a potent COX-2 enzyme inhibitor.

Recent developments in cyclopropene chemistry

As readily accessible strained carbocycles, cyclopropenes show a diverse range of reactivities, and a lot of novel and useful transformations have been developed.

Palladium-Catalyzed Aerobic Benzannulation of Amines, Benzaldehydes, and β-Dicarbonyls

A palladium-catalyzed aerobic multicomponent benzannulation of Hantzsch reactants, i.e., amines, aldehydes, and β-dicarbonyls, has been developed. Hence, a viable entry to highly functionalized anilines has been accessed under solvent-free neat conditions. Mechanistically, the palladium chelating with an imine intermediate was proposed to be the key for this novel carbocyclization.

Amine-catalyzed and functional group-controlled chemo- and regioselective synthesis of multi-functionalized CF3-benzene via a metal-free process

A novel strategy for the synthesis of CF₃-containing multi-substituted benzenes with high chemo- and regioselectivities under metal-free and air-tolerant conditions was established.

Regioselective Formal [4 + 2] Cycloadditions of Enaminones with Diazocarbonyls through RhIII-Catalyzed C-H Bond Functionalization

A regioselective formal [4 + 2] cycloaddition for the assembly of highly functionalized benzene rings was successfully developed. In this reaction, olefinic C-H bond functionalization/cyclization cascade reaction followed by rearomatization led to the desired molecules in one step under mild reaction conditions. This protocol also displays a broad substrate scope and good tolerance to a wide range of functional groups. Additionally, the potential utility for the synthesis of highly conjugated polybenzenes and diversification of natural products was also demonstrated.

Atom-Economic Silver-Catalyzed Difunctionalization of the Isocyano Group with Cyclic Oximes: Towards Pyrimidinediones

An unprecedented silver-catalyzed difunctionalization of the isocyano group with cyclic oximes is described. This method allows efficient and atom-economic assembly of a vast array of structurally novel and interesting pyrimidinediones, and tolerates a range of functionalities. The resulting products can be easily converted into some useful compounds. Furthermore, the method can also be applied for the late-stage modification of a few biologically active molecules.