Stereoelectronic Control in the Ozone‐Free Synthesis of Ozonides

Interrupted Dance of Five Heteroatoms: Reinventing Ozonolysis to Make Geminal Alkoxyhydroperoxides from C═N Bonds

Four heteroatoms dance in the cascade of four pericyclic reactions initiated by ozonolysis of C═N bonds. Switching from imines to semicarbazones introduces the fifth heteroatom that slows this dance, delays reaching the thermodynamically favorable escape path, and allows efficient interception of carbonyl oxides (Criegee intermediates, CIs) by an external nucleophile. The new three-component reaction of alcohols, ozone, and oximes/semicarbazones greatly facilitates synthetic access to monoperoxyacetals (alkoxyhydroperoxides).

Bridged 1,2,4-Trioxolanes: SnCl4-Catalyzed Synthesis and an In Vitro Study against S. mansoni

A synthesis of bridged 1,2,4-trioxolanes (bridged ozonides) from 1,5-diketones and hydrogen peroxide catalyzed by SnCl₄ was developed. It was shown that the ratio of target ozonides can be affected by the application of SnCl₄ as a catalyst and varying the solvent. A wide range of bridged 1,2,4-trioxolanes (ozonides) was obtained in yields from 50 to 84%. The ozonide cycle was moderately resistant to the reduction of the ester group near the peroxide cycle to alcohol with LiAlH₄. The bridged ozonides were evaluated for their antischistosomal activity. These ozonides exhibited a very high activity against newly transformed schistosomula and adult Schistosoma mansoni.

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.

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 β,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.

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.

Stereoelectronic power of oxygen in control of chemical reactivity: the anomeric effect is not alone

Although carbon is the central element of organic chemistry, oxygen is the central element of stereoelectronic control in organic chemistry. Generally, a molecule with a C-O bond has both a strong donor (a lone pair) and a strong acceptor (e.g., a σ*_C-O orbital), a combination that provides opportunities to influence chemical transformations at both ends of the electron demand spectrum. Oxygen is a stereoelectronic chameleon that adapts to the varying situations in radical, cationic, anionic, and metal-mediated transformations. Arguably, the most historically important stereoelectronic effect is the anomeric effect (AE), i.e., the axial preference of acceptor groups at the anomeric position of sugars. Although AE is generally attributed to hyperconjugative interactions of σ-acceptors with a lone pair at oxygen (negative hyperconjugation), recent literature reports suggested alternative explanations. In this context, it is timely to evaluate the fundamental connections between the AE and a broad variety of O-functional groups. Such connections illustrate the general role of hyperconjugation with oxygen lone pairs in reactivity. Lessons from the AE can be used as the conceptual framework for organizing disjointed observations into a logical body of knowledge. In contrast, neglect of hyperconjugation can be deeply misleading as it removes the stereoelectronic cornerstone on which, as we show in this review, the chemistry of organic oxygen functionalities is largely based. As negative hyperconjugation releases the "underutilized" stereoelectronic power of unshared electrons (the lone pairs) for the stabilization of a developing positive charge, the role of orbital interactions increases when the electronic demand is high and molecules distort from their equilibrium geometries. From this perspective, hyperconjugative anomeric interactions play a unique role in guiding reaction design. In this manuscript, we discuss the reactivity of organic O-functionalities, outline variations in the possible hyperconjugative patterns, and showcase the vast implications of AE for the structure and reactivity. On our journey through a variety of O-containing organic functional groups, from textbook to exotic, we will illustrate how this knowledge can predict chemical reactivity and unlock new useful synthetic transformations.

Criegee Intermediates Beyond Ozonolysis: Synthetic and Mechanistic Insights

After more than 70 years since their discovery, Criegee intermediates (CIs) are back at the forefront of modern chemistry of short-lived reactive intermediates. They play an important role in the mechanistic context of chemical synthesis, total synthesis, pharmaceuticals, and, most importantly, climate-controlling aerosol formation as well as atmospheric chemistry. This Minireview summarizes key aspects of CIs (from the mechanism of formation, for example, by ozonolysis of alkenes and photolysis methods employing diiodo and diazo compounds, to their electronic structures and chemical reactivity), highlights the most recent findings and some landmark results of gas-phase kinetics, and detection/measurements. The recent progress in synthetic and mechanistic studies in the chemistry of CIs provides a guide to illustrate the possibilities for further investigations in this exciting field.

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 pK_a units decrease in basicity in comparison with a typical dialkyl amine are attributed to the strong hyperconjugative n_N→σ*_C-O interaction with the two antiperiplanar C-O bonds. Due to the weakness of the complementary n_O→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, β,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 n_O→σ_C-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.

Synthesis of unstrained Criegee intermediates: inverse α-effect and other protective stereoelectronic forces can stop Baeyer-Villiger rearrangement of γ-hydroperoxy-γ-peroxylactones

How far can we push the limits in removing stereoelectronic protection from an unstable intermediate? We address this question by exploring the interplay between the primary and secondary stereoelectronic effects in the Baeyer-Villiger (BV) rearrangement by experimental and computational studies of γ-OR-substituted γ-peroxylactones, the previously elusive non-strained Criegee intermediates (CI). These new cyclic peroxides were synthesized by the peroxidation of γ-ketoesters followed by in situ cyclization using a BF₃·Et₂O/H₂O₂ system. Although the primary effect (alignment of the migrating C-R_m bond with the breaking O-O bond) is active in the 6-membered ring, weakening of the secondary effect (donation from the OR lone pair to the breaking C-R_m bond) provides sufficient kinetic stabilization to allow the formation and isolation of stable γ-hydroperoxy-γ-peroxylactones with a methyl-substituent in the C6-position. Furthermore, supplementary protection is also provided by reactant stabilization originating from two new stereoelectronic factors, both identified and quantified for the first time in the present work. First, an unexpected boat preference in the γ-hydroperoxy-γ-peroxylactones weakens the primary stereoelectronic effects and introduces a ∼2 kcal mol^-1 Curtin-Hammett penalty for reacquiring the more reactive chair conformation. Second, activation of the secondary stereoelectronic effect in the TS comes with a ∼2-3 kcal mol^-1 penalty for giving up the exo-anomeric stabilization in the 6-membered Criegee intermediate. Together, the three new stereoelectronic factors (inverse α-effect, misalignment of reacting bonds in the boat conformation, and the exo-anomeric effect) illustrate the richness of stereoelectronic patterns in peroxide chemistry and provide experimentally significant kinetic stabilization to this new class of bisperoxides. Furthermore, mild reduction of γ-hydroperoxy-γ-peroxylactone with Ph₃P produced an isolable γ-hydroxy-γ-peroxylactone, the first example of a structurally unencumbered CI where neither the primary nor the secondary stereoelectronic effect are impeded. Although this compound is relatively unstable, it does not undergo the BV reaction and instead follows a new mode of reactivity for the CI - a ring-opening process.

Mechanism of photochromic transformations and photodegradation of an asymmetrical 2,3-diarylcyclopentenone

A mechanistic study of the photochromic properties and photodegradation processes of an asymmetrical diarylcyclopentenone bearing thiophene and benzothiophene units using stationary photolysis, nanosecond laser flash photolysis and time-resolved luminescence was performed. It was found that the light-induced reversible isomerization of (3-(2,5-dimethyltiophen-3-il)-2-(2-methyl-1-benzylthiophen-3-il)cyclopent-2-en-1-one, compound 1) from open to closed form is a common photochromic transformation inherent to diarylethenes, while the photodegradation process proceeds in two ways. The first is a formal 1,2-dyotropic rearrangement, proceeding without the participation of oxygen. The second is the oxygen-dependent mechanism involving the excitation of the open form 1A into the triplet state, quenching of the latter by dissolved oxygen, and oxidation of the initial compound by singlet oxygen.

Testing the limits of radical-anionic CH-amination: a 10-million-fold decrease in basicity opens a new path to hydroxyisoindolines via a mixed C-N/C-O-forming cascade

An intramolecular C(sp3)-H amidation proceeds in the presence of t-BuOK, molecular oxygen, and DMF. This transformation is initiated by the deprotonation of an acidic N-H bond and selective radical activation of a benzylic C-H bond towards hydrogen atom transfer (HAT). Cyclization of this radical-anion intermediate en route to a two-centered/three-electron (2c,3e) C-N bond removes electron density from nitrogen. As this electronegative element resists such an "oxidation", making nitrogen more electron rich is key to overcoming this problem. This work dramatically expands the range of N-anions that can participate in this process by using amides instead of anilines. The resulting 107-fold decrease in the N-component basicity (and nucleophilicity) doubles the activation barrier for C-N bond formation and makes this process nearly thermoneutral. Remarkably, this reaction also converts a weak reductant into a much stronger reductant. Such "reductant upconversion" allows mild oxidants like molecular oxygen to complete the first part of the cascade. In contrast, the second stage of NH/CH activation forms a highly stabilized radical-anion intermediate incapable of undergoing electron transfer to oxygen. Because the oxidation is unfavored, an alternative reaction path opens via coupling between the radical anion intermediate and either superoxide or hydroperoxide radical. The hydroperoxide intermediate transforms into the final hydroxyisoindoline products under basic conditions. The use of TEMPO as an additive was found to activate less reactive amides. The combination of experimental and computational data outlines a conceptually new mechanism for conversion of unprotected amides into hydroxyisoindolines proceeding as a sequence of C-H amidation and C-H oxidation.

Catalyst Development for the Synthesis of Ozonides and Tetraoxanes Under Heterogeneous Conditions: Disclosure of an Unprecedented Class of Fungicides for Agricultural Application

The catalyst H_3+x PMo_12-x ⁺⁶ Mo_x ⁺⁵ O₄₀ supported on SiO₂ was developed for peroxidation of 1,3- and 1,5-diketones with hydrogen peroxide with the formation of bridged 1,2,4,5-tetraoxanes and bridged 1,2,4-trioxolanes (ozonides) with high yield based on isolated products (up to 86 and 90 %, respectively) under heterogeneous conditions. Synthesis of peroxides under heterogeneous conditions is a rare process and represents a challenge for this field of chemistry, because peroxides tend to decompose on the surface of a catalyst . A new class of antifungal agents for crop protection, that is, cyclic peroxides: bridged 1,2,4,5-tetraoxanes and bridged ozonides, was discovered. Some ozonides and tetraoxanes exhibit a very high antifungal activity and are superior to commercial fungicides, such as Triadimefon and Kresoxim-methyl. It is important to note that none of the fungicides used in agricultural chemistry contains a peroxide fragment.

Peroxycarbenium Ions as the "Gatekeepers" in Reaction Design: Assistance from Inverse Alpha-Effect in Three-Component β-Alkoxy-β-peroxylactones Synthesis

Stereoelectronic interactions control reactivity of peroxycarbenium cations, the key intermediates in (per)oxidation chemistry. Computational analysis suggests that alcohol involvement as a third component in the carbonyl/peroxide reactions remained invisible due to the absence of sufficiently deep kinetic traps needed to prevent the escape of mixed alcohol/peroxide products to the more stable bisperoxides. Synthesis of β-alkoxy-β-peroxylactones, a new type of organic peroxides, was accomplished by interrupting a thermodynamically driven peroxidation cascade. The higher energy β-alkoxy-β-peroxylactones do not transform into the more stable bisperoxides due to the stereoelectronically imposed instability of a cyclic peroxycarbenium intermediate as a consequence of amplified inverse alpha-effect. The practical consequence of this fundamental finding is the first three-component cyclization/condensation of β-ketoesters, H₂ O₂ , and alcohols that provides β-alkoxy-β-peroxylactones in 15-80 % yields.

Diastereoselective Synthesis of Triterpenoid 1,2,4-Trioxolanes by Griesbaum Co-ozonolysis

Diastereoselective synthesis of triterpenoid 1,2,4-trioxolanes by Griesbaum co-ozonolysis was shown for the first time. Ozonolysis of 2-methoxyoximes (syn-anti-isomers mixture) of allobetulin or methyl oleanoate with CF₃-ketones resulted in asymmetrical spiro-1,2,4-trioxolanes as mixtures of diastereomers in yields up to 80-85%. The configuration of the spiro-C-2 center of individual ozonides was determined by 2D NMR spectra and X-ray crystallographic analysis. The products of ozonolysis of triterpenoid 3-methoxyoximes were mixtures of regioisomeric N-methoxylactams. Thus, the fundamental differences in the oxidation of homologous triterpenoid 2- or 3-methoxyoximes with ozone have been established. These results may afford a new stage in the development of the Griesbaum method as applied to natural compounds and biologically active peroxides.

Contrasting reactivity of fluorinated 2,6-heptanediones towards amines and ammonia, leading to cyclohexanediones or 2-oxa-6-azabicyclo[2.2.2]octanes and evaluation of their cytotoxicity

Synthesis and cytotoxic studies of cyclohexanediones and 2-oxa-6-azabicyclo[2.2.2]octanes obtained in reaction of fluorinated 2,6-hetanediones with amines and ammonia respectively.

Novel Peroxides as Promising Anticancer Agents with Unexpected Depressed Antimalarial Activity

Twenty six peroxides belonging to bridged 1,2,4,5-tetraoxanes, bridged 1,2,4-trioxolanes (ozonides), and tricyclic monoperoxides were evaluated for their in vitro antimalarial activity against Plasmodium falciparum (3D7) and for their cytotoxic activities against immortalized human normal fibroblast (CCD19Lu), liver (LO₂ ), and lung (BEAS-2B) cell lines as well as human liver (HepG2) and lung (A549) cancer-cell lines. Synthetic ozonides were shown to have the highest cytotoxicity on HepG2 (IC₅₀ =0.19-0.59 μm), and some of these compounds selectively targeted liver cancer (selectivity index values for compounds 13 a and 14 a are 20 and 28, respectively) at levels that, in some cases, were higher than those of paclitaxel, artemisinin, and artesunic acid. In contrast some ozonides showed only moderate antimalarial activity against the chloroquine-sensitive 3D7 strain of P. falciparum (IC₅₀ from 2.76 to 24.2 μm; 12 b, IC₅₀ =2.76 μm; 13 a, IC₅₀ =20.14 μm; 14 a, IC₅₀ =6.32 μm). These results suggest that these derivatives have divergent mechanisms of action against cancer cells and malaria-infected cells. A cyclic voltammetry study of the peroxides was performed, but most of the compounds did not show direct correlation in oxidative capacity-activity. Our findings offer a new source of antimalarial and anticancer agents through structural modification of peroxide compounds.