Two-Component versus Three-Component Condensations in the Race between Hydrazide, Triketone, and Hydrogen Peroxide-How do All Six Reactive Centers Cooperate to Incorporate the Most Diverse Set of Heteroatomic Bridges in a Tricyclic Frame?

Relief of stereoelectronic frustration drives the acid-catalyzed three-component condensation of ß,δ'-triketones with hydrazides and H2O2 to the direction where both nucleophiles and all three electrophilic carbons are involved in the formation of a tricyclic sp3-rich ring system that includes four heteroatoms. The otherwise inaccessible tricyclic N-substituted aminoperoxides are prepared rapidly and selectively from relatively simple substrates in good to high yields.

Inverse α-Effect as the Ariadne's Thread on the Way to Tricyclic Aminoperoxides: Avoiding Thermodynamic Traps in the Labyrinth of Possibilities.

Stable tricyclic aminoperoxides can be selectively assembled via a catalyst-free three-component condensation of ß,δ'-triketones, H2O2, and an NH-group source such as aqueous ammonia or ammonium salts. This procedure is scalable and can produce gram quantities of tricyclic heterocycles, containing peroxide, nitrogen, and oxygen cycles in one molecule. Amazingly, such complex tricyclic molecules are selectively formed despite the multitude of alternative reaction routes, via equilibration of peroxide, hemiaminal, monoperoxyacetal, and peroxyhemiaminal functionalities! The reaction is initiated by the "stereoelectronic frustration" of H2O2 and combines elements of thermodynamic and kinetic control with a variety of mono-, bi-, and tricyclic structures evolving under the conditions of thermodynamic control until they reach a kinetic wall created by the inverse α-effect, that is, the stereoelectronic penalty for the formation of peroxycarbenium ions and related transition states. Under these conditions, the reaction stops before reaching the most thermodynamically stable products at a stage where three different heterocycles are assembled and fused at the acyclic precursor frame.

Marriage of Peroxides and Nitrogen Heterocycles: Selective Three-Component Assembly, Peroxide-Preserving Rearrangement, and Stereoelectronic Source of Unusual Stability of Bridged Azaozonides.

Stable bridged azaozonides can be selectively assembled via a catalyst-free three-component condensation of 1,5-diketones, hydrogen peroxide, and an NH-group source such as aqueous ammonia or ammonium salts. This procedure is scalable and can produce gram quantities of bicyclic stereochemically rich heterocycles. The new azaozonides are thermally stable and can be stored at room temperature for several months without decomposition and for at least 1 year at -10 °C. The chemical stability of azaozonides was explored for their subsequent selective transformations including the first example of an aminoperoxide rearrangement that preserves the peroxide group. The amino group in aminoperoxides has remarkably low nucleophilicity and does not participate in the usual amine alkylation and acylation reactions. These observations and the 15 pKa units decrease in basicity in comparison with a typical dialkyl amine are attributed to the strong hyperconjugative nN→σ*C-O interaction with the two antiperiplanar C-O bonds. Due to the weakness of the complementary nO→σ*C-N donation from the peroxide oxygens (a consequence of "inverse α-effect"), this interaction depletes electron density from the NH moiety, protects it from oxidation, and makes it similar in properties to an amide.

How to Build Rigid Oxygen-Rich Tricyclic Heterocycles from Triketones and Hydrogen Peroxide: Control of Dynamic Covalent Chemistry with Inverse α-Effect.

We describe an efficient one-pot procedure that "folds" acyclic triketones into structurally complex, pharmaceutically relevant tricyclic systems that combine high oxygen content with unusual stability. In particular, ß,γ'-triketones are converted into three-dimensional polycyclic peroxides in the presence of H2O2 under acid catalysis. These transformations are fueled by stereoelectronic frustration of H2O2, the parent peroxide, where the lone pairs of oxygen are not involved in strongly stabilizing orbital interactions. Computational analysis reveals how this frustration is relieved in the tricyclic peroxide products, where strongly stabilizing anomeric nO→σC-O* interactions are activated. The calculated potential energy surfaces for these transformations combine labile, dynamically formed cationic species with deeply stabilized intermediate structures that correspond to the introduction of one, two, or three peroxide moieties. Paradoxically, as the thermodynamic stability of the peroxide products increases along this reaction cascade, the kinetic barriers for their formation increase as well. This feature of the reaction potential energy surface, which allows separation of mono- and bis-peroxide tricyclic products, also explains why formation of the most stable tris-peroxide is the least kinetically viable and is not observed experimentally. Such unique behavior can be explained through the "inverse α-effect", a new stereoelectronic phenomenon with many conceptual implications for the development of organic functional group chemistry.

Cyclic Synthetic Peroxides Inhibit Growth of Entomopathogenic Fungus Ascosphaera apis without Toxic Effect on Bumblebees.

In recent years, the number of pollinators in the world has significantly decreased. A possible reason for this is the toxic effects of agrochemicals reducing the immunity of insects that leads to their increased susceptibility to pathogens. Ascosphaera apis is a dangerous entomopathogenic fungus, afflicting both honeybees and bumblebees. We investigated fungicide activity of cyclic synthetic peroxides against A. apis isolated from Bombus terrestris L. The peroxides exhibited high mycelium growth inhibition of A. apis up to 94-100% at concentration 30 mg/L. EC50 values were determined for the most active peroxides. Two peroxides showed higher antifungal activity against A. apis than the commercial fungicide Triadimefon. The studied peroxides did not reduce the ability of bumblebees to fly and did not lead to the death of bumblebees. A new field of application for peroxides was disclosed.

Ozone-Free Synthesis of Ozonides: Assembling Bicyclic Structures from 1,5-Diketones and Hydrogen Peroxide.

Reactions of 1,5-diketones with H2O2 open an ozone-free approach to ozonides. Bridged ozonides are formed readily at room temperature in the presence of strong Brønsted or Lewis acids such as H2SO4, p-TsOH, HBF4, or BF3·Et2O. The expected bridged tetraoxanes, the products of double H2O2 addition, were not detected. This procedure is readily scalable to produce gram quantities of the ozonides. Bridged ozonides are stable and can be useful as building blocks for bioconjugation and further synthetic transformations. Although less stabilized by anomeric interactions than bis-peroxides, ozonides have an intrinsic advantage of having only one weak O-O bond. The role of the synergetic framework of anomeric effects in bis-peroxides is to overcome this intrinsic disadvantage. As the computational data have shown, this is only possible when all anomeric effects in bis-peroxides are activated to their fullest degree. Consequently, the cyclization selectivity is determined by the length of the bridge between the two carbonyl groups of the diketone. The generally large thermodynamic preference for the formation of cyclic bis-peroxides disappears when 1,5-diketones are used as the bis-cyclization precursors. Stereoelectronic analysis suggests that the reason for the bis-peroxide absence is the selective deactivation of anomeric effects in a [3.2.2]tetraoxanonane skeleton by a structural distortion imposed on the tetraoxacyclohexane subunit by the three-carbon bridge.

Peroxide derivatives as SARS-CoV-2 entry inhibitors.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the cause of the ongoing coronavirus disease 2019 (COVID-19) pandemic. Host cell invasion is mediated by the interaction of the viral spike protein (S) with human angiotensin-converting enzyme 2 (ACE2) through the receptor-binding domain (RBD). In this work, bio-layer interferometry (BLI) was used to screen a series of fifty-two peroxides, including aminoperoxides and bridged 1,2,4 - trioxolanes (ozonides), with the aim of identifying small molecules that interfere with the RBD-ACE2 interaction. We found that two compounds, compound 21 and 29, exhibit the activity to inhibit RBD-ACE2. They are further demonstrated to inhibit SARS-CoV-2 cell entry, as shown in pseudovirus assay and experiment with authentic SARS-CoV-2. A comprehensive in silico analysis was carried out to study the physicochemical and pharmacokinetic properties, revealing that both compounds have good physicochemical properties as well as good bioavailability. Our results highlight the potential of small molecules targeting RBD inhibitors as potential therapeutic drugs for COVID-19.

Lewis Acids and Heteropoly Acids in the Synthesis of Organic Peroxides.

Organic peroxides are an important class of compounds for organic synthesis, pharmacological chemistry, materials science, and the polymer industry. Here, for the first time, we summarize the main achievements in the synthesis of organic peroxides by the action of Lewis acids and heteropoly acids. This review consists of three parts: (1) metal-based Lewis acids in the synthesis of organic peroxides; (2) the synthesis of organic peroxides promoted by non-metal-based Lewis acids; and (3) the application of heteropoly acids in the synthesis of organic peroxides. The information covered in this review will be useful for specialists in the field of organic synthesis, reactions and processes of oxygen-containing compounds, catalysis, pharmaceuticals, and materials engineering.

Synthesis and in vitro Study of Artemisinin/Synthetic Peroxide-Based Hybrid Compounds against SARS-CoV-2 and Cancer.

The newly emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) cause life-threatening diseases in millions of people worldwide, in particular, in patients with cancer, and there is an urgent need for antiviral agents against this infection. While in vitro activities of artemisinins against SARS-CoV-2 and cancer have recently been demonstrated, no study of artemisinin and/or synthetic peroxide-based hybrid compounds active against both cancer and SARS-CoV-2 has been reported yet. However, the hybrid drug's properties (e. g., activity and/or selectivity) can be improved compared to its parent compounds and effective new agents can be obtained by modification/hybridization of existing drugs or bioactive natural products. In this study, a series of new artesunic acid and synthetic peroxide based new hybrids were synthesized and analyzed in vitro for the first time for their inhibitory activity against SARS-CoV-2 and leukemia cell lines. Several artesunic acid-derived hybrids exerted a similar or stronger potency against K562 leukemia cells (81-83 % inhibition values) than the reference drug doxorubicin (78 % inhibition value) and they were also more efficient than their parent compounds artesunic acid (49.2 % inhibition value) and quinoline derivative (5.5 % inhibition value). Interestingly, the same artesunic acid-quinoline hybrids also show inhibitory activity against SARS-CoV-2 in vitro (EC50 13-19 µm) and no cytotoxic effects on Vero E6 cells (CC50 up to 110 µM). These results provide a valuable basis for design of further artemisinin-derived hybrids to treat both cancer and SARS-CoV-2 infections.

Cascade Assembly of Bridged N-Substituted Azaozonides: The Counterintuitive Role of Nitrogen Source Nucleophilicity.

Counterintuitively, the low basicity of the NH2 group in hydrazides makes them preferred nucleophiles for the synthesis of the N-substituted azaozonides in acid-catalyzed three-component condensation with 1,5-diketones and H2O2. In the case of more basic N sources, e.g., hydrazine and primary amines, such condensation does not occur under these reaction conditions. The method can be applied to a wide range of hydrazides and affords the target bicyclic azaozonides in 27-86% yields.

Antimalarial and Anticancer Activity Evaluation of Bridged Ozonides, Aminoperoxides, and Tetraoxanes.

Bridged aminoperoxides, for the first time, were investigated for the in vitro antimalarial activity against the chloroquine-resistant Plasmodium falciparum strain K1 and for their cytotoxic activities against immortalized human normal liver (LO2) and lung (BEAS-2B) cell lines as well as human liver (HepG2) and lung (A549) cancer cell lines. Aminoperoxides exhibit good cytotoxicity against lung A549 cancer cell line. Synthetic ozonides were shown to have high activity against the chloroquine-resistant P. falciparum. A cyclic voltammetry study of peroxides was performed, and most of the compounds did not show a direct correlation in oxidative capacity-activity. Peroxides were analyzed for ROS production to understand their mechanism of action. However, none of the compounds has an impact on ROS generation, suggesting that ozonides induce apoptosis in HepG2 cells through ROS-independent dysfunction pathway.

Synthesis of cyclic aza-peroxides (microreview).

A summary of approaches developed for the synthesis of stable cyclic aza-peroxides is presented.

Synthetic Peroxides Promote Apoptosis of Cancer Cells by Inhibiting P-Glycoprotein ABCB5.

This article discloses a new horizon for the application of peroxides in medical chemistry. Stable cyclic peroxides are demonstrated to have cytotoxic activity against cancer cells; in addition a mechanism of cytotoxic action is proposed. Synthetic bridged 1,2,4,5-tetraoxanes and ozonides were effective against HepG2 cancer cells and some ozonides selectively targeted liver cancer cells (the selectivity indexes for compounds 11 b and 12 a are 8 and 5, respectively). In some cases, tetraoxanes and ozonides were more selective than paclitaxel, artemisinin, and artesunic acid. Annexin V flow-cytometry analysis revealed that the active ozonides 22 a and 23 a induced cell death of HepG2 by apoptosis. Further study showed that compounds 22 a and 23 a exhibited a strong inhibitory effect on P-glycoprotein (P-gp/ABCB5)-overexpressing HepG2 cancer cells. ABCB5 is a key player in the multidrug-resistant phenotype of liver cancer. Peroxides failed to demonstrate a direct correlation between oxidative potential and their biological activity. To our knowledge this is the first time that peroxide diastereoisomers have been found to show stereospecific antimalarial action against the chloroquine-sensitive 3D7 strain of Plasmodium falciparum. Stereoisomeric ozonide 12 b is 11 times more active than stereoisomeric ozonide 12 a (IC50 =5.81 vs 65.18 µm). Current findings mean that ozonides merit further investigation as potential therapeutic agents for drug-resistant hepatocellular carcinoma.