Peroxycarbenium-mediated C-C bond formation: applications to the synthesis of hydroperoxides and peroxides

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.

Structural Elucidation of Three 9,11-Seco Tetracyclic Triterpenoids Enables the Structural Revision of Euphorol J

Compounds 1-3, the rare examples of 9,11-seco euphane or lanostane triterpenoids featuring an enol-hemiacetal functionality, were isolated from Euphorbia stracheyi. Their structures were elucidated by a combination of spectroscopic, computational, chemical, and single-crystal X-ray diffraction means, which enables the structure of previously published euphorol J to be revised as 1. 1-3 showed significant cytotoxicities on the breast cancer cell line MDA-MB-468 with IC₅₀ values in the range of 2.9-3.9 μM.

Sc(OTf)3-Catalyzed C-C Bond-Forming Reaction of Cyclic Peroxy Ketals for the Synthesis of Highly Functionalized 1,2-Dioxene Endoperoxides

A new and general Sc(OTf)₃-catalyzed C-C bond-forming reaction of 3-(2-methoxyethoxy)-endoperoxy ketals with silyl ketene acetals, silyl enol ethers, allyltrimethylsilane, and trimethylsilyl cyanide has been developed via the reactive peroxycarbenium ions, affording a wide range of complicated 3,3,6,6-tetrasubstituted 1,2-dioxenes bearing adjacent quaternary carbons and 3-acetyl/allyl/cyano functional groups in good yields at room temperature. Notably, the resultant 1,2-dioxenes are structurally stable, which can be facially transformed into another important 1,2-dioxane endoperoxide under conventional hydrogenation conditions without deconstructing the weak O-O bond.

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.

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.

On the ozonolysis of unsaturated tosylhydrazones as a direct approach to diazocarbonyl compounds

The scope and limitations are described of reacting unsaturated tosylhydrazones with O3 followed by Et3N for the generation of 1,4- and 1,5-diazocarbonyl systems. Tosylhydrazones, from tosylhydrazide condensation with readily available δ- and ε-unsaturated α-ketoesters, led in the former case to a 2-pyrazoline whereas the latter cases led to α-diazo-ε-ketoesters, although a terminal alkene produced a tetrahydropyridazinol. Using the ozonolysis-Et3N strategy, tosylhydrazones from cyclic enones give 2,5- and 2,6-diazoketones with aldehyde or ester functionality at the 1-position; the α-diazoaldehydes prefer the s-trans conformation, with a rotation barrier of 74 kJ mol-1 at 25 °C determined by NMR. This one-pot ozonolysis/Bamford-Stevens chemistry demonstrates both the tolerance of tosylhydrazones to ozone, and the subsequently added amine playing a dual role to directly transform the intermediate tosylhydrazone ozonides into products containing reactive diazo and ketone functionalities; such adducts are of particular value as precursors to cyclic carbonyl ylides for 1,3-dipolar cycloadditions.

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.

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.

Reactions of mono- and bicyclic enol ethers with the I2–hydroperoxide system

Reactions of mono- and bicyclic enol ethers with I₂–H₂O₂, I₂–Bu^tOOH, and I₂–tetrahydropyranyl hydroperoxide systems possessing unique and unpredictable reactivity have been studied.

Re2O7-catalyzed reaction of hemiacetals and aldehydes with O-, S-, and C-nucleophiles

Re(VII) oxides catalyze the acetalization, monoperoxyacetalization, monothioacetalization and allylation of hemiacetals. The reactions, which take place under mild conditions and at low catalyst loadings, can be conducted using hemiacetals, the corresponding O-silyl ethers, and, in some cases, the acetal dimers. Aldehydes react under similar conditions to furnish good yields of dithioacetals. Reactions of hemiacetals with nitrogen nucleophiles are unsuccessful. 1,2-Dioxolan-3-ols (peroxyhemiacetals) undergo Re(VII)-promoted etherification but not allylation. Hydroperoxyacetals (1-alkoxyhydroperoxides) undergo selective exchange of the alkoxide group in the presence of either Re₂O₇ or a Brønsted acid.

Synthesis of alkyl hydroperoxides via alkylation of gem-dihydroperoxides

2-Fold alkylation of 1,1-dihydroperoxides, followed by hydrolysis of the resulting bisperoxyacetals, provides a convenient method for synthesis of primary and secondary alkyl hydroperoxides.

TiCl4-catalyzed synthesis of peroxyacetals from aldehydes

Peroxyacetals 2a–2j were prepared by TiCl4-promoted nucleophilic addition of both tert-butyl hydroperoxide (TBHP) and an alcohol to the corresponding aldehyde. The reaction works well with a variety of aldehydes, but not with ketones. The magnitude of the equilibrium constant for hemiacetal formation plays an important role; a large constant enables high conversion to peroxyacetal.

Total synthesis of plakortide E and biomimetic synthesis of plakortone B

The total synthesis of plakortide E (1a) is reported. A novel palladium-catalyzed approach towards 1,2-dioxolanes as well as an alternative substrate-controlled route leading exclusively to cis-highly substituted 1,2-dioxolanes have been developed. A lipase-catalyzed kinetic resolution was employed to provide optically pure 1,2-dioxolane central cores. Coupling of the central cores and side chains was achieved by a modified Negishi reaction. All four isomeric structures of plakortide E methyl ester, namely, 26a-d were synthesized. One of the structures, 26d, was shown to be identical with the natural plakortide E methyl ester on the basis of (1)H, (13)C NMR spectra and specific rotation comparisons. With the plakortide E methyl ester (4S,6R,10R)-(-)-cis-26d and its other three isomers in hand, we successfully converted them into (3S,4S,6R,10R)-plakortone B (2a), and its isomers ent-2a, 2b and ent-2b via an intramolecular oxa-Michael addition/lactonization cascade reaction. Finally, saponification converted 1,2-dioxolane 26d into plakortide E (1a) whose absolute configuration (4S,6R,10R) was confirmed by comparison of spectral and physical data with those reported.

Fragmentation of chloroperoxides: hypochlorite-mediated dehydration of hydroperoxyacetals to esters

Hypochlorites efficently dehydrate hydroperoxyacetals to furnish the corresponding esters. The reaction, which can be accomplished with stoichometric Ca(OCl)(2) or with catalytic amounts of t-BuOCl, appears to involve formation and heterolytic fragmentation of secondary chloroperoxides, species not previously described in solution chemistry.

Carbenoid insertion into the peroxide bond vs the olefin bond of cyclic peroxides

Herein we report examples of the insertion of a carbenoid into a peroxide linkage. This study reveals that intramolecular insertion of carbenes into the peroxide linkage of 3,6-dihydro-1,2-dioxines is preferred over olefin insertion. The initial scope of the reaction and mechanistic considerations, have been probed. This methodology also generates unusual bicyclic hemiacetals (2) and tricyclic peroxides (3).

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.

Spiro- and dispiro-1,2-dioxolanes: contribution of iron(II)-mediated one-electron vs two-electron reduction to the activity of antimalarial peroxides

Fourteen spiro- and dispiro-1,2-dioxolanes were synthesized by peroxycarbenium ion annulations with alkenes in yields ranging from 30% to 94%. Peroxycarbenium ion precursors included triethylsilyldiperoxyketals and -acetals derived from geminal dihydroperoxides and from a new method employing triethylsilylperoxyketals and -acetals derived from ozonolysis of alkenes. The 1,2-dioxolanes were either inactive or orders of magnitude less potent than the corresponding 1,2,4-trioxolanes or artemisinin against P. falciparum in vitro and P. berghei in vivo. In reactions with iron(II), the predominant reaction course for 1,2-dioxolane 3a was two-electron reduction. In contrast, the corresponding 1,2,4-trioxolane 1 and the 1,2,4-trioxane artemisinin undergo primarily one-electron iron(II)-mediated reductions. The key structural element in the latter peroxides appears to be an oxygen atom attached to one or both of the peroxide-bearing carbon atoms that permits rapid beta-scission reactions (or H shifts) to form primary or secondary carbon-centered radicals rather than further reduction of the initially formed Fe(III) complexed oxy radicals.

The Effect of Iodine on the Peroxidation of Carbonyl Compounds

Peroxidation of ketones and aldehydes with iodine as a catalyst was studied. Ketones reacted with 30% aq hydrogen peroxide in the presence of 10 mol % of iodine to yield gem-dihydroperoxides in acetonitrile and hydroperoxyketals in methanol. The yield of hydroperoxidation of various cyclic ketones was 60-98%, including androstane-3,17-dione, while acyclic ketones were converted with a similar efficiency. Aromatic aldehydes were also converted to gem-dihydroperoxides with hydrogen peroxide and iodine as catalyst in acetonitrile and to hydroperoxyacetal in methanol, while the reactivity of aliphatic ones remained the same as in noncatalyzed reactions. tert-Butylhydroperoxide reacted in a similar manner, giving the corresponding perether derivatives. A study was also made of the relative kinetics of dihydroperoxidation from which the Hammet equation gave a reaction constant (rho) of -2.76, indicating the strong positive charge development in the transition state and the important role of rehybridization in the conversion of hydroperoxyhemiketal to gem-dihydroperoxide. In acetonitrile, the iodine catalyst is apparently able to discriminate between the elimination of a hydroxy, methoxy, and hydroperoxy group and addition of water, methanol, and H2O2 to a carbonyl group.

Synthesis of 1,2-Dioxolanes by Annulation Reactions of Peroxycarbenium Ions with Alkenes

[reaction: see text] The annulation reactions of alkenes with peroxycarbenium ions enable the synthesis of a variety of functionalizable 1,2-dioxolanes. Triethysilyl-protected peroxycarbenium ions proved to be optimal for the annulation reaction. Using this method, plakinic acid analogues can be synthesized in three steps from the corresponding ketone and alkene.

Opening of substituted oxetanes with H(2)O(2) and alkyl hydroperoxides: stereoselective approach to 3-peroxyalcohols and 1,2,4-trioxepanes

[reaction: see text] Lewis acid-catalyzed opening of oxetanes by hydrogen peroxide proceeds regioselectively and with good to moderate stereoselectivity to furnish enantiomerically enriched 3-hydroperoxyalkanols. The corresponding opening using alkyl hydroperoxides furnishes 3-peroxyalkanols. The hydroperoxyalkanols are easily converted into enantiomerically enriched 1,2,4-trioxepanes, building blocks for antimalarials.

Synthesis and notable antimalarial activity of acyclic peroxides, L-(alkyldioxy)-L-(methyldioxy)cyclododecanes

Of several bis(alkyldioxy)alkanes and the related acyclic peroxides prepared in this study, 1,1-bis(methyldioxy)cyclododecane showed the most notable antimalarial activity particularly in vivo (almost a half of that of artemisinin).