Progress in 1,3-dipolar cycloadditions in the recent decade: an update to strategic development towards the arsenal of organic synthesis

New pyrrolo[3,4-d] isoxazolidines hybrid with furan as antitumor agents and multi-target enzyme inhibitors: Synthesis and in silico study

Herein, new Pyrrolo[3,4-d] isoxazolidines hybrid with furan were synthesized by 1,3-dipolar cycloaddition reaction of nitrone 2 with N-substituted maleimides 3a-j. The synthesized compounds were screened in vitro cytotoxic assay against four cancer cell lines namely, HeLa, HEPG-2, HCT-116 and MCF-7 using doxorubicin (DOX) as a reference using MTT assay. The results demonstrated that compounds 4b and 4j exhibited the highest antitumor activity with IC50 =6.22-16.44  μM in comparable to DOX (IC50 = 4.17-5.57μM). The most active hybrids 4b and 4j were further subjected to multi-targeting assays against EGFR, VEGFR-2, and Topo II. They showed good to moderate inhibitory activities. In addition, flow cytometric analysis of 4b and 4j inhibited cell population of MCF-7 cells in the S phase. Compound 4b, and 4j were further evaluated using molecular docking and dynamics simulations (20 ns) and the EGFR, TOPII, or VEGFR-2 receptor protein. All the data sets accurately predict the strongest binding affinity for the selected compounds, as evidenced by the highest free binding energy from MM/GBSA calculations and significant amino acid steric interactions. Furthermore, the RMS/RMSF/Rg/SASA dynamics parameters show the formed complexes demonstrate satisfactory stability. The ADMET properties indicate that the selected new ligands have shown a promising drug-like profile and can be considered potential candidates for future anti-cancer therapies, with perspective validating their anticancer activity by in vitro studies.

Recent advances of asymmetric catalytic transformations of alkylidene Meldrum's acid derivatives

Amongst the electron-deficient alkene derivatives, as molecular platforms widely used in organic synthesis, the readily available alkylidene Meldrum's acid derivatives hold a unique place due to the high electrophilicity and chemical versatility of the 1,3-dioxan-4,6-dione moiety. This short review intends to give an overview of recent advances in the asymmetric catalytic transformation of alkylidene Meldrum's acids into valuable chiral heterocycles, evolving from seminal investigations into C-C bond construction from the alkene moiety, to more recent enantioselective C-N and C-S bond formation, vinylogous processes, and the exploitation of the reactivity of the 1,3-dioxan-4,6-dione moiety to develop efficient annulation sequences.

Quinazoline-Derived Azomethine Imines as Substrates To Access Polycyclic Compounds

Quinazolines are essential structural constituents of many pharmaceuticals and bioactive natural products. Quinazoline-derived azomethine imines (QAIs) have emerged recently as valuable building blocks for the synthesis of various quinazoline derivatives. This Synopsis presents recent advances in (formal) cycloaddition reactions of QAIs for the synthesis of quinazoline-fused 5- to 8-membered heterocycles as well as three-dimensional compounds.

Hetero Cope-Type Hydroamination of Hetero-Allenes with Oximes Having Olefin Moieties Leading to Nitrones, Causing Intramolecular Cycloaddition

Hetero-allenes such as isocyanates, isothiocyanates, and carbodiimides reacted with oxime having olefin moieties in the manner of hetero Cope-type hydroamination to generate N-modified nitrones, which underwent intramolecular cycloaddition to give intramolecular cycloadducts. Among them, the reaction of isocyanates with oximes proceeded at room temperature to provide the corresponding cycloadducts in very high yields. The efficiencies of these sequential cycloadditions were directly compared by competitive reactions. As a result, the order of reactivity to oxime 1a is isocyanate 2a, isothiocyanate 3a, and carbodiimide 6a. Theoretical studies were also conducted.

Copper-Catalyzed Asymmetric [3 + 2] Cycloaddition of N-2,2,2-Trifluoroethylisatin Ketimines to Access Three Classes of Polyfunctionalized Spiro-Pyrrolidine-Oxindole Motifs

A facile copper-catalyzed [3 + 2] cycloaddition of N-2,2,2-trifluoroethylisatin ketimines with various electron-deficient alkenes to access structurally polyfunctionalized spiro-pyrrolidine-oxindole motifs has been developed. Under the catalytic system, the N-2,2,2-trifluoroethylisatin ketimines could be utilized to react with a series of exocyclic alkenes, including 2-acylamino acrylates, 3-methylene-β-lactams, and sterically hindered cycloalkenes represented by cyclobutenone, to obtain a variety of densely functionalized spiro-pyrrolidine frameworks bearing an α-amino acid ester, β-lactam, and cyclobutanone, respectively, in generally good yields with excellent diastereo- and enantioselectivities.

Energetic Aspects and Molecular Mechanism of 3-Nitro-substituted 2-Isoxazolines Formation via Nitrile N-Oxide [3+2] Cycloaddition: An MEDT Computational Study

Regioselectivity and the molecular mechanism of the [3+2] cycloaddition reaction between nitro-substituted formonitrile N-oxide 1 and electron-rich alkenes were explored on the basis of the wb97xd/6-311+G(d) (PCM) quantum chemical calculations. It was established that the thermodynamic factors allow for the formation of stable cycloadducts along all considered models. The analysis of the kinetic parameters of the main processes show that all [3+2] cycloadditions should be realized with full regioselectivity. In all cases, the formation of 5-substituted 3-nitro-2-isoxazolidines is clearly preferred. It is interesting that regiodirection is not determined by the local electrophile/nucleophile interactions but by steric effects. From a mechanistic point of view, all considered reactions should be treated as polar, one-step reactions. All attempts to locate the hypothetical zwitterionic intermediates along the cycloaddition paths were, however, not successful.

Synthesis of Bridged Cycloisoxazoline Scaffolds via Rhodium-Catalyzed Coupling of Nitrones with Cyclic Carbonate

Bridged isoxazolidines were synthesized via Rh(III)-catalyzed C-H allylation of α-aryl nitrones with 5-methylene-1,3-dioxan-2-one. The nitrone group serves as a directing group and 1,3-dipole in the C-H activation/[3 + 2] cycloaddition cascade, exhibiting excellent chemo- and stereoselectivity along with good functional group compatibility. The resulting skeletal structure was conveniently modified to produce a range of important chemical frameworks, and the protocol was applied to biologically active molecules.

1,6-Dioxo-2-azaspiro[3.4]oct-2-enes and Related Spirocycles: Heterocycles from [3 + 2] Nitrile Oxide Cycloadditions with 2-Methyleneoxetanes, -Thietanes, and -Azetidines

Isoxazolines and 4-membered heterocycles are significant structural motifs in numerous synthetic intermediates and natural products. [3 + 2] Cycloadditions between enol ethers and nitrile oxides have been well studied; however, nitrile oxide cycloadditions with 4-membered heterocycles to give spiroisoxazolines are unreported. Here, we showcase the regio- and diastereoselective [3 + 2] nitrile oxide cycloadditions of 2-methyleneoxetanes, -azetidines, and -thietanes to give an array of 1,6-dioxo-2-azaspiro[3.4]oct-2-enes and related spirocycles. 2D NMR experiments suggested that most of the observed diastereoselectivities were dictated by steric interactions; however, dipolarophiles with H bonding donors reversed the stereochemical outcome. X-ray crystallography confirmed the structural assignments.

One-Bond-Nucleophilicity and -Electrophilicity Parameters: An Efficient Ordering System for 1,3-Dipolar Cycloadditions

Diazoalkanes are ambiphilic 1,3-dipoles that undergo fast Huisgen cycloadditions with both electron-rich and electron-poor dipolarophiles but react slowly with alkenes of low polarity. Frontier molecular orbital (FMO) theory considering the 3-center-4-electron π-system of the propargyl fragment of diazoalkanes is commonly applied to rationalize these reactivity trends. However, we recently found that a change in the mechanism from cycloadditions to azo couplings takes place due to the existence of a previously overlooked lower-lying unoccupied molecular orbital. We now propose an alternative approach to analyze 1,3-dipolar cycloaddition reactions, which relies on the linear free energy relationship lg k₂(20 °C) = s_N(N + E) (eq 1) with two solvent-dependent parameters (N, s_N) to characterize nucleophiles and one parameter (E) for electrophiles. Rate constants for the cycloadditions of diazoalkanes with dipolarophiles were measured and compared with those calculated for the formation of zwitterions by eq 1. The difference between experimental and predicted Gibbs energies of activation is interpreted as the energy of concert, i.e., the stabilization of the transition states by the concerted formation of two new bonds. By linking the plot of lg k₂ vs N for nucleophilic dipolarophiles with that of lg k₂ vs E for electrophilic dipolarophiles, one obtains V-shaped plots which provide absolute rate constants for the stepwise reactions on the borderlines. These plots furthermore predict relative reactivities of dipolarophiles in concerted, highly asynchronous cycloadditions more precisely than the classical correlations of rate constants with FMO energies or ionization potentials. DFT calculations using the SMD solvent model confirm these interpretations.

Computational studies of the CuAAC reaction mechanism with diimine and phosphorus ligands for the synthesis of 1,4-disubstituted 1,2,3-triazoles

The Cu(i)-catalyzed azide-alkyne cycloaddition (CuAAC) reactions with diimine and phosphorus ligands have been studied using DFT calculations in order to understand the effect of the nature of the ligands on the catalytic cycle for the formation of the 1,4-regioisomer.

Different Behavior of 2-Substituted 3-Nitro-2H-chromenes in the Reaction with Stabilized Azomethine Ylides Generated from α-Iminoesters

The AgOAc-catalysed reaction of 3-nitro-2-phenyl-2H-chromenes with stabilized azomethine ylides generated from the imines based on methyl glycinate and arylaldehydes leads to a mixture of endo and endo' isomers of the corresponding chromeno[3,4-c]pyrrolidines in a ratio of 2.0-2.3:1 in 85-93% total yields as a result of a Michael addition/Mannich reaction sequence. In a similar reaction involving 2-trifluoromethyl-3-nitro-2H-chromenes, only endo chromeno[3,4-c]pyrrolidines are formed in 85-94% yields. 3-Nitro-2-(trichloromethyl)-2H-chromenes under the same conditions react with these azomethine ylides to give the corresponding Michael adducts as individual anti-isomers with the cis,trans-configuration of the chromane ring in 40-67% yields. Some 4-CF₃-substituted chromano[3,4-c]pyrrolidines exhibited high cytotoxic activity against HeLa human cervical carcinoma cells.

Asymmetric Umpolung (3+2) Cycloadditions of Iminoesters with α,β-Unsaturated-2-acyl Imidazoles for the Synthesis of Substituted Pyrrolidines

An enantioselective (3+2) cycloaddition reaction of iminoesters occurring with opposite regioselectivity is reported. This reaction provides chiral polysubstituted pyrrolidines with high enantioselectivities (up to 97%), diastereoselectivities (>20:1), and yields (up to 99%). Interestingly, changing the alpha-substituents of the iminoesters from an aryl to an aliphatic (benzyl) group or hydrogen resulted in the formation of normal (3+2) cycloaddition products, also in excellent enantioselectivities.

Recent Advances in Copper-Based Solid Heterogeneous Catalysts for Azide-Alkyne Cycloaddition Reactions

The copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction is considered to be the most representative ligation process within the context of the "click chemistry" concept. This CuAAC reaction, which yields compounds containing a 1,2,3-triazole core, has become relevant in the construction of biologically complex systems, bioconjugation strategies, and supramolecular and material sciences. Although many CuAAC reactions are performed under homogenous conditions, heterogenous copper-based catalytic systems are gaining exponential interest, relying on the easy removal, recovery, and reusability of catalytically copper species. The present review covers the most recently developed copper-containing heterogenous solid catalytic systems that use solid inorganic/organic hybrid supports, and which have been used in promoting CuAAC reactions. Due to the demand for 1,2,3-triazoles as non-classical bioisosteres and as framework-based drugs, the CuAAC reaction promoted by solid heterogenous catalysts has greatly improved the recovery and removal of copper species, usually by simple filtration. In so doing, the solving of the toxicity issue regarding copper particles in compounds of biological interest has been achieved. This protocol is also expected to produce a practical chemical process for accessing such compounds on an industrial scale.

Annulation of phosphole sulfides via [3 + 2] cycloaddition with nitrones

We developed a facile pathway for the construction of the phospholene fused isoxazolidine skeleton through the [3 + 2] cycloaddition reaction of phosphole sulfides.

Regio- and stereoselective (3 + 2)-cycloaddition reactions of nitrones with cyclic allenes

An effective approach to access functionalized 2H-cyclonona(deca)[d]isoxazoles and 15-oxo-3,10-methanobenzo[b][1]azacyclododecines has been developed by the reaction of N-aryl-C,C-bis(methoxycarbonyl)nitrones with cyclonona(deca)-1,2-dienes in a one-pot fashion. The reaction of N-aryl-C-(phenylcarbamoyl)nitrones with these allenes proceeds strictly regioselectively giving the mixtures of diastereomeric isoxazolidines containing a double bond at the C⁴-position of the isoxazolidine ring. The quantum chemical calculations show that the regioselectivity of these reactions is in good agreement with the reactivity indices of the considered compounds.

Diversity-Oriented Synthesis of Spiropyrrolo[1,2-a]isoquinoline Derivatives via Diastereoselective and Regiodivergent Three-Component 1,3-Dipolar Cycloaddition Reactions: In Vitro and in Vivo Evaluation of the Antidiabetic Activity of Rhodanine Analogues

An efficient diastereoselective route is developed to get access to novel spiropyrrolo[1,2-a]isoquinoline-oxindole skeletons by a one-pot three-component [3 + 2] cycloaddition reaction of (Z)-5-arylidene-1,3-thiazolidine-2,4-diones, isatin derivatives, and 1,2,3,4-tetrahydroisoquinoline (THIQ). Interestingly, the regioselectivity of the reaction is both temperature- and solvent-dependent, allowing the synthesis of two regioisomeric endo-dispiropyrrolo[2,1-a]isoquinolineoxindoles in excellent yield. Unprecedentedly, each isomeric dispiropyrrolo[2,1-a]isoquinolineoxindole endured retro-1,3-dipolar cycloaddition/recycloaddition reactions under thermal or catalytic conditions to regenerate the corresponding regioisomeric counterpart. In addition, DFT calculations were performed at the M062X/6-31++g(d,p) level of theory to unravel the origin of the reversal of regioselectivity and endo-stereoselectivity of the title 1,3-dipolar cycloaddition reactions. Upon treatment of Isatin, THIQ with (Z)-4-arylidene-5-thioxo-thiazolidin-2-ones as dipolarophiles, unusual rhodanine analogues were formed, along with smaller amounts of a dispirooxindole-piperazine. The structure and the relative configuration of these N-heterocycles were unambiguously assigned by spectroscopic techniques and confirmed by four single-crystal structures. In vitro and in vivo studies reveal that the novel rhodanine derivatives exert antidiabetic activity. The binding affinity with the active site of the enzyme α-amylase was studied by molecular docking. Furthermore, the bioavailability assessed through virtual ADME parameters (Absorption, Distribution, Metabolism, Elimination pharmacokinetics) and the excellent fit with the Lipinski and Veber rules predict good drug-likeness properties for a bromo-substituted 2-sulfanylidene-1,3-thiazolidin-4-one.

Dynamics in Catalytic Asymmetric Diastereoconvergent (3 + 2) Cycloadditions with Isomerizable Nitrones and α-Keto Ester Enolates

Reaction design in asymmetric catalysis has traditionally been predicated on a structurally robust scaffold in both substrates and catalysts, to reduce the number of possible diastereomeric transition states. Herein, we present the stereochemical dynamics in the Ni(II)-catalyzed diastereoconvergent (3 + 2) cycloadditions of isomerizable nitrile-conjugated nitrones with α-keto ester enolates. Even in the presence of multiple equilibrating species, the catalytic protocol displays a wide substrate scope to access a range of CN-containing building blocks bearing adjacent stereocenters with high enantio- and diastereoselectivities. Our computational investigations suggest that the enantioselectivity is governed in the deprotonation process to form (Z)-Ni-enolates, while the unique syn addition is mainly controlled by weak noncovalent bonding interactions between the nitrone and ligand.

General Pyrrolidine Synthesis via Iridium-Catalyzed Reductive Azomethine Ylide Generation from Tertiary Amides and Lactams

An iridium-catalyzed reductive generation of both stabilized and unstabilized azomethine ylides and their application to functionalized pyrrolidine synthesis via [3 + 2] dipolar cycloaddition reactions is described. Proceeding under mild reaction conditions from both amide and lactam precursors possessing a suitably positioned electron-withdrawing or a trimethylsilyl group, using 1 mol% Vaska’s complex [IrCl(CO)(PPh₃)₂] and tetramethyldisiloxane (TMDS) as a terminal reductant, a broad range of (un)stabilized azomethine ylides were accessible. Subsequent regio- and diastereoselective, inter- and intramolecular dipolar cycloaddition reactions with variously substituted electron-deficient alkenes enabled ready and efficient access to structurally complex pyrrolidine architectures. Density functional theory (DFT) calculations of the dipolar cycloaddition reactions uncovered an intimate balance between asynchronicity and interaction energies of transition structures, which ultimately control the unusual selectivities observed in certain cases.

Cu(I)-Catalyzed asymmetric exo-selective synthesis of substituted pyrrolidines via a 1,3-dipolar cycloaddition reaction

An (R)-DM-BINAP/Cu(CH3CN)4BF4 complex catalyzed exo-selective asymmetric 1,3-dipolar cycloaddition (1,3-DCA) reaction of imino esters with α,β-unsaturated pyrazoleamides has been developed. A series of highly functionalized pyrrolidines with multiple stereogenic centers were obtained with good yields and diastereoselectivities and excellent enantioselectivities (up to 99% ee).

Sc(OTf)3-Catalyzed Formal [3 + 3] Cycloaddition Reaction of Diaziridines and Quinones for the Synthesis of Benzo[e][1,3,4]oxadiazines

A formal [3 + 3] cyclization reaction of diaziridines and quinones has been developed offering 1,3,4-oxadiazinanes in generally high yields (up to 96%). The reaction was catalyzed by Sc(OTf)₃ with a large substrate scope for both diaziridines and quinones. The synergistic activation of 1,3-dipolar diaziridines and the dipolar quinones was found to be essential to enable this reaction.

Ruthenium-Catalyzed Cycloadditions to Form Five-, Six-, and Seven-Membered Rings

Ruthenium-catalyzed cycloadditions to form five-, six-, and seven-membered rings are summarized, including applications in natural product total synthesis. Content is organized by ring size and reaction type. Coverage is limited to processes that involve formation of at least one C-C bond. Processes that are stoichiometric in ruthenium or exploit ruthenium as a Lewis acid (without intervention of organometallic intermediates), ring formations that occur through dehydrogenative condensation-reduction, σ-bond activation-initiated annulations that do not result in net reduction of bond multiplicity, and photochemically promoted ruthenium-catalyzed cycloadditions are not covered.

Engaging Isatins in Multicomponent Reactions (MCRs) - Easy Access to Structural Diversity

Multicomponent reactions (MCRs) are a valuable tool in diversity-oriented synthesis. Its application to privileged structures is gaining relevance in the fields of organic and medicinal chemistry. Isatin, due to its unique reactivity, can undergo different MCRs, affording multiple interesting scaffolds, namely oxindole-derivatives (including spirooxindoles, bis-oxindoles and 3,3-disubstituted oxindoles) and even, under certain conditions, ring-opening reactions occur that leads to other heterocyclic compounds. Over the past few years, new methodologies have been described for the application of this important and easily available starting material in MCRs. In this review, we explore these novelties, displaying them according to the structure of the final products obtained.

Recent advancement in the synthesis of diverse spiro-indeno[1,2-b]quinoxalines: a review

The nitrogen-containing indeno[1,2-b]quinoxaline ring is a privileged structurally fused active system and has notable applications in various fields of chemistry.

Silver(I) Oxide-/DBU-Promoted Synthesis of Dihydrofuran Units through Allenyl Silver Formation

A formal [3+2] cyclization mediated by silver(I) oxide and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) is described herein. Through a broad variety of carbonyl compounds, this system can promote cyclization reactions with high yield (up to 85 %) and diastereoselectivity (up to 95:5) for a straightforward access to complex and congested dihydrofuran derivatives in one step under mild conditions. Based on DFT studies, the proposed mechanism would involve an allenyl silver intermediate.

Expanding the Scope of Palladium-Catalyzed B - N Cross-Coupling Chemistry in Carboranes

Over the past several years, a number of strategies for the functionalization of dicarba-closo-dodecaboranes (carboranes) have emerged. Despite these developments, B - N bond formation on the carborane scaffold remains a challenge due to the propensity of strong nucleophiles to partially deboronate the parent closo-carborane cluster into the corresponding nido form. Here we show that azide, sulfonamide, cyanate, and phosphoramidate nucleophiles can be straightforwardly cross-coupled onto the B(9) vertices of the o- and m-carborane core from readily accessible precursors without significant deboronation by-products, laying the groundwork for further study into the utility and properties of these new B-aminated carborane species. We further showcase select reactivity of the installed functional groups highlighting some unique features stemming from the combination of the electron-donating B(9) position and the large steric profile of the B-connected carborane substituent.

Selective and reversible 1,3-dipolar cycloaddition of 6-aryl-1,5-diazabicyclo[3.1.0]hexanes with 1,3-diphenylprop-2-en-1-ones under microwave irradiation

The first example of the cycloaddition of in situ-generated azomethine imine under microwave conditions is described. The reaction of 6-aryl-1,5-diazabicyclo[3.1.0]hexanes with 1,3-diphenylprop-2-en-1-ones proceeds regio- and stereoselectively giving mostly good yields of the corresponding perhydropyrazolopyrazoles. The products of the reaction undergo cycloreversion under the reaction conditions.

Flow Chemistry for Cycloaddition Reactions

Continuous flow reactors form part of a rapidly growing research area that has changed the way synthetic chemistry is performed not only in academia but also at the industrial level. This Review highlights the most recent advances in cycloaddition reactions performed in flow systems. Cycloadditions are atom-efficient transformations for the synthesis of carbo- and heterocycles, involved in the construction of challenging skeletons of complex molecules. The main advantages of translating these processes into flow include using intensified conditions, safer handling of hazardous reagents and gases, easy tuning of reaction conditions, and straightforward scaling up. These benefits are especially important in cycloadditions such as the copper(I)-catalyzed azide alkyne cycloaddition (CuAAC), Diels-Alder reaction, ozonolysis and [2+2] photocycloadditions. Some of these transformations are key reactions in the industrial synthesis of pharmaceuticals.

Dearomative [3 + 2] cycloaddition reaction of nitrobenzothiophenes with nonstabilized azomethine ylides

A highly diastereoselective dearomative [3 + 2] 1,3-dipolar cycloaddition reaction of nitrobenzothiophenes with an in situ-generated nonstabilized azomethine ylides has been developed. The transformation provides a series of functionalized fused tricyclic benzo[4,5]thieno[2,3-c]pyrroles in good yields (up to 92%) under mild reaction conditions. In addition, a gram-scale experiment and the synthetic transformation of the cycloadduct further highlighted the synthetic utility. The relative configurations of the typical products were clearly confirmed by X-ray crystallography.

Predicting Absolute Rate Constants for Huisgen Reactions of Unsaturated Iminium Ions with Diazoalkanes

The kinetics and stereochemistry of the reactions of iminium ions derived from cinnamaldehydes and MacMillan's imidazolidinones with diphenyldiazomethane and aryldiazomethanes were investigated experimentally and with DFT calculations. The reactions of diphenyldiazomethane with iminium ions derived from MacMillan's second-generation catalysts gave 3-aryl-2,2-diphenylcyclopropanecarbaldehydes with yields >90 % and enantiomeric ratios of ≥90:10. Predominantly 2:1 products were obtained from the corresponding reactions with monoaryldiazomethanes. The measured rate constants are in good agreement with the rate constants derived from the one-center nucleophilicity parameters N and sN of diazomethanes and the one-center electrophilicity parameters E of iminium ions as well as with quantum chemically calculated activation energies.

Catalyst-free Tandem 1,3-Dipolar Cycloaddition/Aldol Condensation: Diastereoselective Construction of the Azatetraquinane Skeleton

The one-pot regioselective and diastereoselective method for the synthesis of 5-(het)aroyl-7-(het)arylhexahydrobenzo[4,5]pentaleno[1,6a-b](thia)pyrrolizine-6,12-diones from accessible 1,5-di(het)arylpent-4-ene-1,3-diones or curcuminoids in 38-98% yield was developed. This reaction proceeds as a sequence of 1,3-dipolar cycloaddition of azomethine ylide generated in situ from ninhydrin and (thia)proline at the C═C bond of corresponding enedione, followed by spontaneous intramolecular aldol condensation and leads to the formation of an azatetraquinane scaffold.