Alkene ozonolysis

Water mediated redox-neutral cleavage of arylalkenes via photoredox catalysis

Cleavage of carbon-carbon bonds remains a challenging task in organic synthesis. Traditional methods for splitting Csp2=Csp2 bonds into two halves typically involve non-redox (metathesis) or oxidative (ozonolysis) mechanisms, limiting their synthetic potential. Disproportionative deconstruction of alkenes, which yields one reduced and one oxidized fragment, remains an unexplored area. In this study, we introduce a redox-neutral approach for deleting a Csp2 carbon unit from substituted arylalkenes, resulting in the formation of an arene (reduction) and a carbonyl product (oxidation). This transformation is believed to proceed through a mechanistic sequence involving visible-light-promoted anti-Markovnikov hydration, followed by photoredox cleavage of Csp3-Csp3 bond in the alcohol intermediate. A crucial consideration in this design is addressing the compatibility between the highly reactive oxy radical species in the latter step and the required hydrogen-atom-transfer (HAT) reagent for both steps. We found that ethyl thioglycolate serves as the optimal hydrogen-atom shuttle, offering remarkable chemoselectivity among multiple potential HAT events in this transformation. By using D2O, we successfully prepared dideuteromethylated (-CD2H) arenes with good heavy atom enrichment. This work presents a redox-neutral alternative for alkene deconstruction, with considerable potential in late-stage modification of complex molecules.

Multiple photofluorochromic luminogens via catalyst-free alkene oxidative cleavage photoreaction for dynamic 4D codes encryption

Controllable photofluorochromic systems with high contrast and multicolor in both solutions and solid states are ideal candidates for the development of dynamic artificial intelligence. However, it is still challenging to realize multiple photochromism within one single molecule, not to mention good controllability. Herein, we report an aggregation-induced emission luminogen TPE-2MO2NT that undergoes oxidation cleavage upon light irradiation and is accompanied by tunable multicolor emission from orange to blue with time-dependence. The photocleavage mechanism revealed that the self-generation of reactive oxidants driving the catalyst-free oxidative cleavage process. A comprehensive analysis of TPE-2MO2NT and other comparative molecules demonstrates that the TPE-2MO2NT molecular scaffold can be easily modified and extended. Further, the multicolor microenvironmental controllability of TPE-2MO2NT photoreaction within polymer matrices enables the fabrication of dynamic fluorescence images and 4D information codes, providing strategies for advanced controllable information encryption.

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).

Development of a Strategy for the Total Synthesis of Aspidosperma Alkaloids via the Cyclobutenone-Based PET-Initiated Cationic Radical-Driven [2+2]/Retro-Mannich Reaction

A novel strategy for the synthesis of Aspidosperma alkaloids has been achieved via a photoredox-initiated [2+2]/retro-Mannich reaction of tryptamine-substituted enaminones as a key step. The developed chemistry has been applied to the construction of the core tetracycle of Aspidosperma alkaloids (±)-aspidospermidine and (±)-limaspermidine.

Electrochemical Generation of Peroxy Radicals and Subsequent Peroxidation of 1,3-Dicarbonyls in an Undivided Cell

The generation of peroxy radicals from hydroperoxides with subsequent selective peroxidation of 1,3-dicarbonyls in an undivided electrochemical cell under constant current conditions is reported. The method provides a variety of peroxy-containing barbituric acids and 4-hydroxy-2(5H)-furanones with yields of up to 74%. Only the combination of anodic and cathodic processes provides efficient peroxidation by generating a set of alkoxy and peroxy radicals. NaNO3 acts as both an electrolyte and a redox mediator of radical reactions.

Carbon-Carbon Bond Cleavage for Late-Stage Functionalization

Late-stage functionalization (LSF) introduces functional group or structural modification at the final stage of the synthesis of natural products, drugs, and complex compounds. It is anticipated that late-stage functionalization would improve drug discovery's effectiveness and efficiency and hasten the creation of various chemical libraries. Consequently, late-stage functionalization of natural products is a productive technique to produce natural product derivatives, which significantly impacts chemical biology and drug development. Carbon-carbon bonds make up the fundamental framework of organic molecules. Compared with the carbon-carbon bond construction, the carbon-carbon bond activation can directly enable molecular editing (deletion, insertion, or modification of atoms or groups of atoms) and provide a more efficient and accurate synthetic strategy. However, the efficient and selective activation of unstrained carbon-carbon bonds is still one of the most challenging projects in organic synthesis. This review encompasses the strategies employed in recent years for carbon-carbon bond cleavage by explicitly focusing on their applicability in late-stage functionalization. This review expands the current discourse on carbon-carbon bond cleavage in late-stage functionalization reactions by providing a comprehensive overview of the selective cleavage of various types of carbon-carbon bonds. This includes C-C(sp), C-C(sp2), and C-C(sp3) single bonds; carbon-carbon double bonds; and carbon-carbon triple bonds, with a focus on catalysis by transition metals or organocatalysts. Additionally, specific topics, such as ring-opening processes involving carbon-carbon bond cleavage in three-, four-, five-, and six-membered rings, are discussed, and exemplar applications of these techniques are showcased in the context of complex bioactive molecules or drug discovery. This review aims to shed light on recent advancements in the field and propose potential avenues for future research in the realm of late-stage carbon-carbon bond functionalization.

Caulobacter segnis Dioxygenase CsO2: A Practical Biocatalyst for Stilbenoid Ozonolysis

Ozonolysis is a useful as well as dangerous reaction for performing alkene cleavage. On the other hand, enzymes are considered a more sustainable and safer alternative. Among them, Caulobacter segnis dioxygenase (CsO2) known so far for its ability to catalyze the coenzyme-free oxidation of vinylguaiacol into vanillin, was selected and its substrate scope evaluated towards diverse natural and synthetic stilbenoids. Under optimized conditions, CsO2 catalyzed the oxidative cleavage of the C=C double bonds of various trans-stilbenes, providing that a hydroxyl moiety was necessary in para-position of the phenyl group (e. g., resveratrol and its derivatives) for the reaction to take place, which was confirmed by modelling studies. The reactions occurred rapidly (0.5-3 h) with high conversions (95-99 %) and without formation of by-products. The resveratrol biotransformation was carried out on 50-mL scale thus confirming the feasibility of the biocatalytic system as a preparative method.

Carbon-to-nitrogen single-atom transmutation of azaarenes

When searching for the ideal molecule to fill a particular functional role (for example, a medicine), the difference between success and failure can often come down to a single atom1. Replacing an aromatic carbon atom with a nitrogen atom would be enabling in the discovery of potential medicines2, but only indirect means exist to make such C-to-N transmutations, typically by parallel synthesis3. Here, we report a transformation that enables the direct conversion of a heteroaromatic carbon atom into a nitrogen atom, turning quinolines into quinazolines. Oxidative restructuring of the parent azaarene gives a ring-opened intermediate bearing electrophilic sites primed for ring reclosure and expulsion of a carbon-based leaving group. Such a 'sticky end' approach subverts existing atom insertion-deletion approaches and as a result avoids skeleton-rotation and substituent-perturbation pitfalls common in stepwise skeletal editing. We show a broad scope of quinolines and related azaarenes, all of which can be converted into the corresponding quinazolines by replacement of the C3 carbon with a nitrogen atom. Mechanistic experiments support the critical role of the activated intermediate and indicate a more general strategy for the development of C-to-N transmutation reactions.

Effect of Solvent on the Rate of Ozonolysis: Development of a Homogeneous Flow Ozonolysis Protocol

Herein, we describe the effect of organic solvents on ozone solubility and the rate of ozonolysis reactions using rapid injection NMR spectroscopy. The tabulated solubility and kinetic data allowed for the design of a homogeneous ozonolysis flow reactor capable of delivering precise quantities of dissolved ozone to various olefin substrates.

Modern Strategies for Carbon Isotope Exchange

In contrast to stable and natural abundant carbon-12, the synthesis of organic molecules with carbon (radio)isotopes must be conceived and optimized in order to navigate through the hurdles of radiochemical requirements, such as high costs of the starting materials, harsh conditions and radioactive waste generation. In addition, it must initiate from the small cohort of available C-labeled building blocks. For long time, multi-step approaches have represented the sole available patterns. On the other side, the development of chemical reactions based on the reversible cleavage of C-C bonds might offer new opportunities and reshape retrosynthetic analysis in radiosynthesis. This review aims to provide a short survey on the recently emerged carbon isotope exchange technologies that provide effective opportunity for late-stage labeling. At present, such strategies have relied on the use of primary and easily accessible radiolabeled C1-building blocks, such as carbon dioxide, carbon monoxide and cyanides, while the activation principles have been based on thermal, photocatalytic, metal-catalyzed and biocatalytic processes.

Capturing primary ozonides for a syn-dihydroxylation of olefins

Ozonolysis is a widely used and practical synthetic technique for the deconstructive oxidation of olefins using ozone. While there are numerous ozonolysis reactions that give a myriad of products and functionalities, almost all of them involve scission at the olefin double bond. Using ozone as a constructive reagent rather than a deconstructive one would open new domains of chemical reactivity and amplify molecular complexity in synthetic methodology. Here we report the use of primary ozonides as preparative synthetic intermediates for a safe and green olefin syn-dihydroxylation reaction. Furthermore, we have demonstrated this method using a continuous flow reactor that virtually eliminates peroxide accumulation and extended these applications towards the synthesis of pharmaceutically relevant small molecules such as guaifenesin, the active ingredient in Mucinex, and a precursor to ponesimod, a drug to treat multiple sclerosis.

Aminodealkenylation: Ozonolysis and copper catalysis convert C(sp3)-C(sp2) bonds to C(sp3)-N bonds

Great efforts have been directed toward alkene π bond amination. In contrast, analogous functionalization of the adjacent C(sp3)-C(sp2) σ bonds is much rarer. Here we report how ozonolysis and copper catalysis under mild reaction conditions enable alkene C(sp3)-C(sp2) σ bond-rupturing cross-coupling reactions for the construction of new C(sp3)-N bonds. We have used this unconventional transformation for late-stage modification of hormones, pharmaceutical reagents, peptides, and nucleosides. Furthermore, we have coupled abundantly available terpenes and terpenoids with nitrogen nucleophiles to access artificial terpenoid alkaloids and complex chiral amines. In addition, we applied a commodity chemical, α-methylstyrene, as a methylation reagent to prepare methylated nucleosides directly from canonical nucleosides in one synthetic step. Our mechanistic investigation implicates an unusual copper ion pair cooperative process.

Degradation of Polyacrylates by One-Pot Sequential Dehydrodecarboxylation and Ozonolysis

We establish a synthetically convenient method to degrade polyacrylate homopolymers. Carboxylic acids are installed along the polymer backbone by partial hydrolysis of the ester side chains, and then, in a one-pot sequential procedure, the carboxylic acids are converted into alkenes and oxidatively cleaved. This process enables the robustness and properties of polyacrylates to be maintained during their usable lifetime. The ability to tune the degree of degradation was demonstrated by varying the carboxylic acid content of the polymers. This method is compatible with a wide range of polymers prepared from vinyl monomers through copolymerization of acrylic acid with different monomers including acrylates, acrylamides, and styrenics.

Computational Studies of Rubber Ozonation Explain the Effectiveness of 6PPD as an Antidegradant and the Mechanism of Its Quinone Formation

The discovery that the commercial rubber antidegradant 6PPD reacts with ozone (O₃) to produce a highly toxic quinone (6PPDQ) spurred a significant research effort into nontoxic alternatives. This work has been hampered by lack of a detailed understanding of the mechanism of protection that 6PPD affords rubber compounds against ozone. Herein, we report high-level density functional theory studies into early steps of rubber and PPD (p-phenylenediamine) ozonation, identifying key steps that contribute to the antiozonant activity of PPDs. In this, we establish that our density functional theory approach can achieve chemical accuracy for many ozonation reactions, which are notoriously difficult to model. Using adiabatic energy decomposition analysis, we examine and dispel the notion that one-electron charge transfer initiates ozonation in these systems, as is sometimes argued. Instead, we find direct interaction between O₃ and the PPD aromatic ring is kinetically accessible and that this motif is more significant than interactions with PPD nitrogens. The former pathway results in a hydroxylated PPD intermediate, which reacts further with O₃ to afford 6PPD hydroquinone and, ultimately, 6PPDQ. This mechanism directly links the toxicity of 6PPDQ to the antiozonant function of 6PPD. These results have significant implications for development of alternative antiozonants, which are discussed.

Vicinal stereocenters via asymmetric allylic alkylation and Cope rearrangement: a straightforward route to functionally and stereochemically rich heterocycles

An asymmetric allylic alkylation/Cope rearrangement (AAA/[3,3]) capable of stereoselectively constructing vicinal stereocenters has been developed. Strategically integrated 4-methylation on the 3,3-dicyano-1,5-diene controls stereoselectivity and drives Cope rearrangement equilibrium in the forward direction. The AAA/[3,3] sequence rapidly converts abundant achiral and racemic starting materials into valuable (hetero)cycloalkane building blocks bearing significant functional and stereochemical complexity, highlighting the value of (hetero)cyclohexylidenemalononitriles as launching points for complex heterocycle synthesis. On this line, the resulting alkylidenemalononitrile moiety can be readily converted into amides via Hayashi-Lear amidation to ultimately yield amido-piperidines, tropanes, and related scaffolds with 3-5 stereocenters and drug-like functionality.

The Diverse Modes of Oxygen Reactivity in Life & Chemistry

Oxygen is a molecule of utmost importance in our lives. Beside its vital role for the respiration and sustaining of organisms, oxygen is involved in numerous chemical and physical processes. Upon combination of the different forms of molecular oxygen species with various activation modes, substrates, and reaction conditions an extremely wide chemical space can be covered that enables rich applications of diverse oxygenation processes. This Review provides an instructive overview of the individual properties and reactivities of oxygen species and illustrates their importance in nature, everyday life, and in the context of chemical synthesis.

Photoexcited Nitroarenes for the Oxidative Cleavage of Alkenes

The oxidative cleavage of alkenes is an integral process that converts feedstock materials into high-value synthetic intermediates^1,2,3. The most viable method to achieve this in one chemical step is with ozone^4,5,6,7, which however poses technical and safety challenges owing to the explosive nature of ozonolysis products^8,9. Herein, we disclose an alternative approach to achieve oxidative cleavage of alkenes using nitroarenes and purple light irradiation. We demonstrate that photoexcited nitroarenes are effective ozone surrogates that undergo facile radical [3+2] cycloaddition with alkenes. The resulting "N-doped" ozonides are safe to handle and lead to the corresponding carbonyl products under mild hydrolytic conditions. These features have enabled the controlled cleavage of all types of alkenes in the presence of a broad array of commonly used organic functionalities. Furthermore, by harnessing electronic, steric, and mediated polar effects, the structural and functional diversity of nitroarenes has provided a modular platform to obtain site-selectivity in substrates containing more than one alkene.

Photoinduced Oxygen Transfer Using Nitroarenes for the Anaerobic Cleavage of Alkenes

Herein we report the anaerobic cleavage of alkenes into carbonyl compounds using nitroarenes as oxygen transfer reagents under visible light. This approach serves as a safe and practical alternative to mainstream oxidative cleavage protocols, such as ozonolysis and the Lemieux-Johnson reaction. A wide range of alkenes possessing oxidatively sensitive functionalities underwent anaerobic cleavage to generate carbonyl derivatives with high efficiency and regioselectivity. Mechanistic studies support that the transformation occurs via direct photoexcitation of the nitroarene followed by a nonstereospecific radical cycloaddition event with alkenes. This leads to 1,3,2- and 1,4,2-dioxazolidine intermediates that fragment to give the carbonyl products. A combination of radical clock experiments and in situ photoNMR spectroscopy revealed the identities of the key radical species and the putative aryl dioxazolidine intermediates, respectively.

Synthesis of 3-epi-Hypatulin B Featuring a Late-Stage Photo-Oxidation in Flow

A synthesis of 3-epi-hypatulin B, a highly oxygenated and densely functionalized bicyclic scaffold, is reported. The carbon skeleton was prepared by functionalization of a cyclopentanone and an intramolecular Mukaiyama aldol reaction. Highlights include a late-stage photo-oxidation of a methoxyallene to provide an ester group. The problems encountered in the batch process were solved by translation into a flow protocol. Our synthesis highlights the value of flow chemistry to enable challenging late-stage transformations in natural product synthesis.

Surfactant-Assisted Ozonolysis of Alkenes in Water: Mitigation of Frothing Using Coolade as a Low-Foaming Surfactant

Aqueous-phase ozonolysis in the atmosphere is an important process during cloud and fog formation. Water in the atmosphere acts as both a reaction medium and a reductant during the ozonolysis. Inspired by the atmospheric aqueous-phase ozonolysis, we herein report the ozonolysis of alkenes in water assisted by surfactants. Several types of surfactants, including anionic, cationic, and nonionic surfactants, were investigated. Although most surfactants enhanced the solubility of alkenes in water, they also generated excessive foaming during the ozone bubbling, which led to the loss of products. Mitigation of the frothing was accomplished by using Coolade as a nonionic and low-foaming surfactant. Coolade-assisted ozonolysis of alkenes in water provided the desired carbonyl products in good yields and comparable to those achieved in organic solvents. During the ozonolysis reaction, water molecules trapped within the polyethylene glycol region of Coolade were proposed to intercept the Criegee intermediate to provide a hydroxy hydroperoxide intermediate. Decomposition of the hydroxy hydroperoxide led to formation of the carbonyl product without the need for a reductant typically required for the conventional ozonolysis using organic solvents. This study presents Coolade as an effective surfactant to improve the solubility of alkenes while mitigating frothing during the ozonolysis in water.

Ozone-Mediated Amine Oxidation and Beyond: A Solvent-Free, Flow-Chemistry Approach

Ozone is a powerful oxidant, most commonly used for oxidation of alkenes to carbonyls. The synthetic utility of other ozone-mediated reactions is hindered by its high reactivity and propensity to overoxidize organic molecules, including most solvents. This challenge can largely be mitigated by adsorbing both substrate and ozone onto silica gel, providing a solvent-free oxidation method. In this manuscript, a flow-based packed bed reactor approach is described that provides exceptional control of reaction temperature and time to achieve improved control and chemoselectivity over this challenging transformation. A powerful method to oxidize primary amines into nitroalkanes is achieved. Examples of pyridine, C-H bond, and arene oxidations are also demonstrated, confirming the system is generalizable to diverse ozone-mediated processes.

Evaluation of an Ozone Chamber as a Routine Method to Decontaminate Firefighters' PPE

Ozone chambers have emerged as an alternative method to decontaminate firefighters' Personal Protective Equipment (PPE) from toxic fire residues. This work evaluated the efficiency of using an ozone chamber to clean firefighters' PPE. This was achieved by studying the degradation of pyrene and 9-methylanthracene polycyclic aromatic hydrocarbons (PAHs). The following experiments were performed: (i) insufflating ozone into PAH solutions (homogeneous setup), and (ii) exposing pieces of PPE impregnated with the PAHs to an ozone atmosphere for up to one hour (heterogeneous setup). The ozonolysis products were assessed by Fourier Transform Infrared Spectroscopy (FTIR), Thin-Layer Chromatography (TLC), and Mass Spectrometry (MS) analysis. In the homogeneous experiments, compounds of a higher molecular weight were produced due to the incorporation of oxygen into the PAH structures. Some of these new compounds included 4-oxapyren-5-one (m/z 220) and phenanthrene-4,5-dicarboxaldehyde (m/z 234) from pyrene; or 9-anthracenecarboxaldehyde (m/z 207) and hydroxy-9,10-anthracenedione (m/z 225) from 9-methylanthracene. In the heterogeneous experiments, a lower oxidation was revealed, since no byproducts were detected using FTIR and TLC, but only using MS. However, in both experiments, significant amounts of the original PAHs were still present even after one hour of ozone treatment. Thus, although some partial chemical degradation was observed, the remaining PAH and the new oxygenated-PAH compounds (equally or more toxic than the initial molecules) alerted us of the risks to firefighters' health when using an ozone chamber as a unique decontamination method. These results do not prove the ozone-advertised efficiency of the ozone chambers for decontaminating (degrading the toxic combustion residues into innocuous compounds) firefighters' PPE.

Dealkenylative Ni-Catalyzed Cross-Coupling Enabled by Tetrazine and Photoexcitation

A new and general method to functionalize the C(sp³)-C(sp²) bond of alkyl and alkene linkages has been developed, leading to the dealkenylative generation of carbon-centered radicals that can be intercepted to undergo Ni-catalyzed C(sp³)-C(sp²) cross-coupling. This one-pot protocol leverages the easily procured alkene feedstocks for organic synthesis with excellent functional group compatibility without the need for a photoredox catalyst.

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.

Poly(ethylene glycol) dimethyl ether mediated oxidative scission of aromatic olefins to carbonyl compounds by molecular oxygen

A simple, and practical oxidative scission of aromatic olefins to carbonyl compounds using O₂ as the sole oxidant with poly(ethylene glycol) dimethyl ether as a benign solvent has been developed. A wide range of monosubstituted, gem-disubstituted, 1,2-disubstituted, trisubstituted and tetrasubstituted aromatic olefins was successfully converted into the corresponding aldehydes and ketones in excellent yields even with gram–scale reaction. Some control experiments were also conducted to support a possible reaction pathway.

A simple and practical O₂ oxidized scission of monosubstituted, gem- and 1,2-disubstituted, trisubstituted and tetrasubstituted aromatic olefins to aldehydes and ketones in PEGDME has been developed.

Process intensification of ozonolysis reactions using dedicated microstructured reactors

A dedicated microreactor system ensures exquisite control over mass and heat transfer for performing very fast ozonolysis.

Highly efficient oxidative cleavage of olefins with O2 under catalyst-, initiator- and additive-free conditions

Without employing any external catalyst, initiator and additives, an efficient and eco-friendly protocol has been developed for the synthesis of carbonyl compound via 1,4-dioxane- promoted oxidation of olefins with atmospheric O₂ as the sole oxidant.

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.

Ozonolysis of Alkynes-A Flexible Route to Alpha-Diketones: Synthesis of AI-2

A mild procedure for the low-temperature conversion of alkynes to diketones has been developed and employed in the synthesis of AI-2.

Study on ozonolysis of asymmetric alkenes with matrix isolation and FT-IR spectroscopy

O₃ and alkenes are important reactants in the formation of SOA in the atmosphere. The intermediates and reaction mechanism of ozonation of alkene is an important topic in atmospheric chemistry. In this study, the low-temperature matrix isolation was used to capture the intermediates such as Primary ozonides (POZs), Criegee Intermediates (CIs), and Secondary ozonides (SOZs) generated from ozonation of 2-methyl-1-butene (2M1B) and 2-methyl-2-butene (2M2B). The results have been identified by the vacuum infrared spectroscopy and theoretical calculation. Our results show that during the ozonation of asymmetric alkenes, two kinds of CIs and more than two kinds of SOZs were generated due to the different decomposition modes of POZs. The infrared absorption peaks of (CH₃)₂COO and CH₃CH₂C(CH₃)OO for O-O telescopic vibration was determined to be 889 cm^-1 and 913 cm^-1, respectively. Using the merged jet method, it was found that a large amount of HCHO was produced during the ozonation of 2M1B, and glyoxal and methylglyoxal were produced in the ozonation of 2M2B. Our findings highlight the importance of asymmetric alkene ozonolysis reactions in producing CIs, further improving the understanding of the generation of CIs from ozonation of alkenes.

An Economical Route to Lamivudine Featuring a Novel Strategy for Stereospecific Assembly

An economical synthesis of lamivudine was developed by employing a new method to establish the stereochemistry about the heterocyclic oxathiolane ring. Toward this end, an inexpensive and readily accessible lactic acid derivative served the dual purpose of activating the carbohydrate’s anomeric center for N-glycosylation and transferring stereochemical information to the substrate simultaneously. Both enantiomers of the lactic acid derivative are available, and either β-enantiomer in this challenging class of 2′-deoxynucleoside active pharmaceutical ingredients can be formed.

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.

Ozone Oxidation of Silylalkene: Mechanistic Study and Application for the Synthesis of Silacarboxylic Acid Derivatives

The addition-type ozone oxidation of silylalkenes is a highly efficient reaction to provide synthetically versatile α-silylperoxy carbonyl compounds. To gain insight into the reaction mechanism, we performed a computational study, which revealed that the reaction proceeds via [1,2]-Brook rearrangement-type silyl migration of primary ozonide. In sharp contrast to the addition-type reactions, the ozone oxidation of α-alkoxysilylalkenes proceeds in a cleavage-type manner to afford excellent yields of silacarboxylic acid esters via the 1,3-cycloelimination of primary ozonide prior to 1,2-silyl migration.

Nine-Step Stereoselective Synthesis of Islatravir from Deoxyribose

A stereoselective nine-step synthesis of the potent HIV nucleoside reverse transcriptase translocation inhibitor (NRTTI) islatravir (EfdA, MK-8591) from 2-deoxyribose is described. Key findings include a diastereodivergent addition of an acetylide nucleophile to an enolizable ketone, a chemoselective ozonolysis of a terminal olefin and a biocatalytic glycosylation cascade that uses a unique strategy of byproduct precipitation to drive an otherwise-reversible transformation forward.

A click-based modular approach to introduction of peroxides onto molecules and nanostructures

Copper-promoted azide/alkyne cycloadditions (CuAAC) are explored as a tool for modular introduction of peroxides onto molecules and nanomaterials.

Hydrodealkenylative C(sp3)-C(sp2) bond fragmentation

Chemical synthesis typically relies on reactions that generate complexity through elaboration of simple starting materials. Less common are deconstructive strategies toward complexity-particularly those involving carbon-carbon bond scission. Here, we introduce one such transformation: the hydrodealkenylative cleavage of C(sp³)-C(sp²) bonds, conducted below room temperature, using ozone, an iron salt, and a hydrogen atom donor. These reactions are performed in nonanhydrous solvents and open to the air; reach completion within 30 minutes; and deliver their products in high yields, even on decagram scales. We have used this broadly functionality tolerant transformation to produce desirable synthetic intermediates, many of which are optically active, from abundantly available terpenes and terpenoid-derived precursors. We have also applied it in the formal total syntheses of complex molecules.

Stereospecific Overman Rearrangement of Substituted Cyclic Vinyl Bromides: Access to Fully Substituted α-Amino Ketones

A versatile thermal Overman rearrangement of enantioenriched, cyclic allylic alcohols providing tertiary allylic amines has been developed. The vinyl bromide used to control enantioselectivity in a preceding CBS reduction is utilized as a synthetic handle for the preparation of tertiary α-amino ketones and related derivatives in an asymmetric fashion.