Co(thd)2: a superior catalyst for aerobic epoxidation and hydroperoxysilylation of unactivated alkenes: application to the synthesis of spiro-1,2,4-trioxanes

Diversification of Pharmaceutical Manufacturing Processes: Taking the Plunge into the Non-PGM Catalyst Pool

Recent global events have led to the cost of platinum group metals (PGMs) reaching unprecedented heights. Many chemical companies are therefore starting to seriously consider and evaluate if and where they can substitute PGMs for non-PGMs in their catalytic processes. This review covers recent highly relevant applications of non-PGM catalysts in the modern pharmaceutical industry. By highlighting these selected successful examples of non-PGM-catalyzed processes from the literature, we hope to emphasize the enormous potential of non-PGM catalysis and inspire further development within this field to enable this technology to progress toward manufacturing processes. We also present some historical contexts and review the perceived advantages and challenges of implementing non-PGM catalysts in the pharmaceutical manufacturing environment.

Recent advances in the synthesis and antimalarial activity of 1,2,4-trioxanes

The increasing resistance of various malarial parasite strains to drugs has made the production of a new, rapid-acting, and efficient antimalarial drug more necessary, as the demand for such drugs is growing rapidly. As a major global health concern, various methods have been implemented to address the problem of drug resistance, including the hybrid drug concept, combination therapy, the development of analogues of existing medicines, and the use of drug resistance reversal agents. Artemisinin and its derivatives are currently used against multidrug- resistant P. falciparum species. However, due to its natural origin, its use has been limited by its scarcity in natural resources. As a result, finding a substitute becomes more crucial, and the peroxide group in artemisinin, responsible for the drugs biological action in the form of 1,2,4-trioxane, may hold the key to resolving this issue. The literature suggests that 1,2,4-trioxanes have the potential to become an alternative to current malaria drugs, as highlighted in this review. This is why 1,2,4-trioxanes and their derivatives have been synthesized on a large scale worldwide, as they have shown promising antimalarial activity in vivo and in vitro against Plasmodium species. Consequently, the search for a more convenient, environment friendly, sustainable, efficient, and effective synthetic pathway for the synthesis of 1,2,4-trioxanes continues. The aim of this work is to provide a comprehensive analysis of the synthesis and mechanism of action of 1,2,4-trioxanes. This systematic review highlights the most recent summaries of derivatives of 1,2,4-trioxane compounds and dimers with potential antimalarial activity from January 1988 to 2023.

Unified Total Syntheses of Benzenoid Cephalotane-Type Norditerpenoids: Cephanolides and Ceforalides

Detailed herein are our synthetic studies toward the preparation of the C₁₈- and C₁₉-benzenoid cephalotane-type norditerpenoids. Guided by chemical network analysis, the core structure of this natural product family was constructed in a concise manner using an iterative cross-coupling, followed by a formal inverse-electron-demand [4 + 2] cycloaddition. Initial efforts to functionalize an alkene group in the [4 + 2] cycloadduct using a Mukaiyama hydration and a subsequent olefination led to the complete C₁₈-carbon framework. While effective, this approach proved lengthy and prompted the development of a direct alkene difunctionalization that relies on borocupration to advance the cycloadduct to the natural products. Late-stage peripheral C-H functionalization facilitated access to all of the known cephanolides in 6-10 steps as well as five recently isolated ceforalides in 8-13 steps.

Enantioselective Total Synthesis of 10-Desoxy Analogue of a Previously Reported Natural Peroxyguaidiol

Described herein is an enantioselective synthesis of an analogue of a previously reported guaiane endoperoxide isolated from aerial parts of Croton arboreous. The polycyclic framework of the target structure was constructed with the C-7 stereogenic center derived from L-(-)-carvone and other stereogenic centers installed via substrate chirality-induced asymmetric reactions, starting with the synthesis of the seven-membered ring through regioselective enolization of carvone, ring-expansion, and installation of a conjugated C═C bond. Further functionalization was then achieved through regioselective enolization, triflation, and installation of an isopropenyl group. During the synthesis, some exceptions to the well-known rules of "thermodynamic control" and "kinetic control" in enolization of asymmetric cyclic ketones were observed. In construction of the bridged five-membered and endoperoxy rings, a peroxycarbenium [3 + 2] cycloaddition reaction with alkenes was carried out with several alkenes-silyl-gem-dihydroperoxides of different relative configurations. However, no expected [3 + 2] products were observed. Finally, the five-membered ring was smoothly installed through an intramolecular Darzens reaction, and the peroxy functionality was introduced via a carbon-centered radical-mediated reaction with triplet oxygen, followed by an intramolecular etherification under acidic conditions. Comparison of the ¹H and ¹³C NMR spectra of the synthetic analogue and the natural product revealed that the latter was definitely not an endoperoxide.

Cobalt-Catalyzed Cyclization of Unsaturated N-Acyl Sulfonamides: a Diverted Mukaiyama Hydration Reaction

The cycloisomerization of β-, γ-, and δ-unsaturated N-acyl sulfonamides to N-sulfonyl lactams and imidates is reported. This transformation is effected in the presence of a Co^III(salen) catalyst using t-BuOOH or air as the oxidant. The method shows good functional group tolerance (alkyl, aryl, heteroaryl, ether, N-Boc) and furnishes an underexplored class of cyclic building blocks. The strong solvent dependence of the transformation is investigated, and the synthetic versatility of the N-sulfonyl imidate product class is highlighted.

Aerobic Oxidation Reactivity of Well-Defined Cobalt(II) and Cobalt(III) Aminophenol Complexes

This paper describes the synthesis and reactivity studies of three cobalt complexes bearing aminophenol-derived ligands without nitrogen substitution: Co^II(^tBu2APH)₂(^tBu2AP)₂ (1), Co^III₂(^tBu2APH)₂(^tBu2AP)₂(μ-^tBu2BAP)₂ (2), and Co^III(^tBu2AP)₃ (3), where ^tBu2APH = 2-amino-4,6-di-tert-butylphenol, ^tBu2AP = 2-amino-4,6-di-tert-butylphenolate, and μ-^tBu2BAP = bridging 2-amido-4,6-di-tert-butylphenolate. Stoichiometric reactivity studies of these well-defined complexes demonstrate the catalytic competency of both cobalt(II) and cobalt(III) complexes in the aerobic oxidative cyclization of ^tBu2APH with tert-butylisonitrile. Reactions with O₂ reveal the aerobic oxidation of the cobalt(II) complex 1 to generate the cobalt(III) species 2 and 3. UV-visible time-course studies and electron paramagnetic resonance spectroscopy indicate that this oxidation proceeds through a ligand-based radical intermediate. These studies represent the first example of well-defined cobalt aminophenol complexes that participate in catalytic aerobic oxidation reactions and highlight a key role for a ligand radical in the oxidation sequence.

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 of Non-symmetrical Dispiro-1,2,4,5-Tetraoxanes and Dispiro-1,2,4-Trioxanes Catalyzed by Silica Sulfuric Acid

A novel protocol for the preparation of non-symmetrical 1,2,4,5-tetraoxanes and 1,2,4-trioxanes, promoted by the heterogeneous silica sulfuric acid (SSA) catalyst, is reported. Different ketones react under mild conditions with gem-dihydroperoxides or peroxysilyl alcohols/β-hydroperoxy alcohols to generate the corresponding endoperoxides in good yields. Our mechanistic proposal, assisted by molecular orbital calculations, at the ωB97XD/def2-TZVPP/PCM(DCM)//B3LYP/6-31G(d) level of theory, enhances the role of SSA in the cyclocondensation step. This novel procedure differs from previously reported methods by using readily available and inexpensive reagents, with recyclable properties, thereby establishing a valid alternative approach for the synthesis of new biologically active endoperoxides.

Hydroperoxidations of Alkenes using Cobalt Picolinate Catalysts

Hydroperoxides were synthesized in one step from various alkenes using Co(pic)2 as the catalyst with molecular oxygen and tetramethyldisiloxane (TMDSO). The hydration product could be obtained using a modified catalyst, Co(3-mepic)2, with molecular oxygen and phenylsilane. Formation of hydroperoxides occurred through a rapid Co-O bond metathesis of a peroxycobalt compound with isopropanol.

Diastereoselective peroxidation of derivatives of Baylis-Hillman adducts

Cyclic derivatives of Baylis-Hillman adducts were synthesized. Cobalt-catalyzed peroxidation of these cyclic lactones afforded silyl peroxides in diastereomeric ratios ranging from 91:9 to 97:3.

Sterically encumbered β-diketonates and base metal catalysis

Metal coordination complexes of the sterically hindered β-diketonate, 2,6-dimesitylbenzoyl pinacolone (esac), are reported for Co, Ni, Cu, and Zn. All four form ML2-type complexes with typical coordination behavior for late-metal β-diketonates, however the effects on established electrochemistry and reactivity vary somewhat per metal. For example, the striking chemical and electrochemical inertness of CoII(esac)2 to oxidation and disproportionation is atypical. Conversely, the behavior of CuII(esac)2 is rather typical relative to other CuII(β-diketonate)2 complexes. These data suggest a relative disfavoring of certain reaction pathways, and represent an important step in modulating the catalysis of the base metals via sterically hindered β-diketonates.

Cobalt-Catalyzed Intramolecular Silylperoxidation of Unsaturated Diisopropylsilyl Ethers

A cobalt-catalyzed intramolecular silylperoxidation reaction was developed that allows for the conversion of unsaturated diisopropylsilyl ethers to 3-sila-1,2,4-trioxepanes. Reduction of the peroxide unit of the 3-sila-1,2,4-trioxepane yields six-membered ring diisopropylsilylene acetals.

Synthesis of Sterically Hindered β-Diketones via Condensation of Acid Chlorides with Enolates

Bulky β-diketones have rarely exceeded dipivaloylmethane (DPM) in steric demand, largely due to synthetic limitations of the Claisen condensation. This work demonstrates hindered acid chlorides to be selective electrophiles in noncoordinating solvents for condensations with enolates. An improved synthesis of DPM is described (90% yield), and crowded β-diketones featuring bulky o-biphenyl or m-terphenyl fragments were prepared in good to excellent yields. These compounds are anticipated to have a steric profile far greater than that of DPM. General reaction conditions and mechanistic considerations are included.

Cobalt-Catalyzed Oxygenation/Dearomatization of Furans

The dearomatization of aromatic compounds using cobalt(II) acetylacetonate with triplet oxygen and triethylsilane converts furans, benzofurans, pyrroles, and thiophenes to a variety of products, including lactones, silyl peroxides, and ketones.

Advances in the synthesis of acyclic peroxides

This review summarises the many developments in the synthesis of acyclic peroxides, with a particular focus on the past 20 years, and seeks to update organic chemists about these new approaches.

Mn-, Fe-, and Co-Catalyzed Radical Hydrofunctionalizations of Olefins

Cofactor-mimetic aerobic oxidation has conceptually merged with catalysis of syngas reactions to form a wide range of Markovnikov-selective olefin radical hydrofunctionalizations. We cover the development of the field and review contributions to reaction invention, mechanism, and application to complex molecule synthesis. We also provide a mechanistic framework for understanding this compendium of radical reactions.

Synthesis and antimalarial activity of novel bicyclic and tricyclic aza-peroxides

Bicyclic and tricyclic aza-peroxides were synthesized and assessed for theirin vitroandin vivoantimalarial activities againstPlasmodium falciparum(3D7 strain) andPlasmodium yoelii nigeriensisin Swiss mice by an oral route, respectively.

Synthesis of Ethers via Reaction of Carbanions and Monoperoxyacetals

Although transfer of electrophilic alkoxyl ("RO+") from organic peroxides to organometallics offers a complement to traditional methods for etherification, application has been limited by constraints associated with peroxide reactivity and stability. We now demonstrate that readily prepared tetrahydropyranyl monoperoxyacetals react with sp(3) and sp(2) organolithium and organomagnesium reagents to furnish moderate to high yields of ethers. The method is successfully applied to the synthesis of alkyl, alkenyl, aryl, heteroaryl, and cyclopropyl ethers, mixed O,O-acetals, and S,S,O-orthoesters. In contrast to reactions of dialkyl and alkyl/silyl peroxides, the displacements of monoperoxyacetals provide no evidence for alkoxy radical intermediates. At the same time, the high yields observed for transfer of primary, secondary, or tertiary alkoxides, the latter involving attack on neopentyl oxygen, are inconsistent with an SN2 mechanism. Theoretical studies suggest a mechanism involving Lewis acid promoted insertion of organometallics into the O-O bond.

Synthetic spirocyclic endoperoxides: new antimalarial scaffolds

Design, synthesis and molecular docking calculation studies led to the identification of novel spirocyclic peroxides with in vitro and in vivo antimalarial activity.

Synthesis of silyl monoperoxyketals by regioselective cobalt-catalyzed peroxidation of silyl enol ethers: application to the synthesis of 1,2-dioxolanes

The cobalt-catalyzed peroxidation of silyl enol ethers with molecular oxygen and triethylsilane provided silyl monoperoxyketals in 54%-96% yield. These compounds serve as precursors to peroxycarbenium ions, which undergo annulations to provide 1,2-dioxolanes.

Synthesis of five- and six-membered cyclic organic peroxides: Key transformations into peroxide ring-retaining products

The present review describes the current status of synthetic five and six-membered cyclic peroxides such as 1,2-dioxolanes, 1,2,4-trioxolanes (ozonides), 1,2-dioxanes, 1,2-dioxenes, 1,2,4-trioxanes, and 1,2,4,5-tetraoxanes. The literature from 2000 onwards is surveyed to provide an update on synthesis of cyclic peroxides. The indicated period of time is, on the whole, characterized by the development of new efficient and scale-up methods for the preparation of these cyclic compounds. It was shown that cyclic peroxides remain unchanged throughout the course of a wide range of fundamental organic reactions. Due to these properties, the molecular structures can be greatly modified to give peroxide ring-retaining products. The chemistry of cyclic peroxides has attracted considerable attention, because these compounds are used in medicine for the design of antimalarial, antihelminthic, and antitumor agents.

Radicals in transition metal catalyzed reactions? transition metal catalyzed radical reactions? - a fruitful interplay anyway: part 2. Radical catalysis by group 8 and 9 elements

This review summarizes the current status of transition metal catalyzed reactions involving radical intermediates in organic chemistry. This part focuses on radical-based methods catalyzed by group 8 and group 9 metal complexes. Reductive and redox-neutral coupling methods catalyzed by low-valent metal complexes as well as catalytic oxidative C-C bond formations are reviewed.

Activation of molecular oxygen and its use in stereoselective tetrahydrofuran-syntheses from delta,epsilon-unsaturated alcohols

Bishomoallylic alcohols (pent-4-en-1-ols) underwent efficient oxidative cyclizations, if treated with O(2) and bis{2,2,2-trifluoromethyl-1-[(1R,4S)-1,7,7-trimethyl-2-(oxo-kappaO)bicyclo[2.2.1]hept-3-yliden]ethanolato-kappaO}cobalt(ii) in solutions of 2-propanol at 60 degrees C. Ring closures occurred diastereoselectively and afforded 2,3-trans- (96% de), 2,4-cis- (approximately 60% de), and 2,5-trans-substituted (>99% de) (phenyl)tetrahydrofur-2-ylmethanols as major components. Formation of bicyclic compounds and a 2,3,4,5-substituted oxolane was feasible as exemplified by syntheses of oxabicyclo[4.3.0]nonylmethanols and a derivative of natural product magnosalicin in 61-72% (90-99% de). The effectiveness of tetrahydrofuran synthesis was critically dependent on (i) solvent, (ii) reaction temperature, (iii) initial cobalt concentration, (iv) chain length between hydroxyl and vinyl groups, and (v) substitution at reacting entities. A sequence is proposed for rationalizing observed selectivities.

Synthesis of spiro-bisperoxyketals

Syntheses of spirocyclic bis-1,2-dioxolanes, bis-1,2-dioxanes, and bis-1,2-dioxepanes are achieved through intramolecular ketalizations of hydroperoxy ketones or intramolecular alkylations of gem-dihydroperoxides. The spiroperoxides have excellent thermal and chemical stability, and several display promising activity against P. falciparum.