Base- and Co(II)-catalyzed ring-opening reactions of perhydrooxireno[2,3-d][1,2]dioxines: an efficient route to 4-hydroxy-2,3-epoxy-ketones

Desymmetrization and Kinetic Resolution of Endoperoxides Using a Bifunctional Organocatalyst

Bifunctional thiourea/amine organocatalysts have been used for the desymmetrization of meso-endoperoxides using the Kornblum-DeLaMare reaction, giving 4-hydroxyketones in 78-98% yields with ≤98:2 enantioselectivity. The influence of the catalyst structure, solvent, and temperature was examined. The most promising catalyst was applied to the kinetic resolution of racemic endoperoxides to give enantioenriched materials (≤99:1 er).

Revisiting Bromohexitols as a Novel Class of Microenvironment-Activated Prodrugs for Cancer Therapy

Bromohexitols represent a potent class of DNA-alkylating carbohydrate chemotherapeutics that has been largely ignored over the last decades due to safety concerns. The limited structure-activity relationship data available reveals significant changes in cytotoxicity with even subtle changes in stereochemistry. However, no attempts have been made to improve the therapeutic window by rational drug design or by using a prodrug approach to exploit differences between tumour physiology and healthy tissue, such as acidic extracellular pH and hypoxia. Herein, we report the photochemical synthesis of highly substituted endoperoxides as key precursors for dibromohexitol derivatives and investigate their use as microenvironment-activated prodrugs for targeting cancer cells. One endoperoxide was identified to have a marked increased activity under hypoxic and low pH conditions, indicating that endoperoxides may serve as microenvironment-activated prodrugs.

Synthesis and Study of the Antimalarial Cardamom Peroxide

A full account of our previously disclosed synthesis of the monoterpene dimer cardamom peroxide is reported. Inspired by hypotheses regarding the potential biosynthetic origins of this natural product, several unproductive routes are also reported. The chemical reactivity of this structurally unique metabolite in the presence of iron(II) sources is also reported as is its antimalarial activity against Plasmodium falciparum clinical isolates from several Cambodian provinces.

Rearrangements of organic peroxides and related processes

This review is the first to collate and summarize main data on named and unnamed rearrangement reactions of peroxides. It should be noted, that in the chemistry of peroxides two types of processes are considered under the term rearrangements. These are conventional rearrangements occurring with the retention of the molecular weight and transformations of one of the peroxide moieties after O-O-bond cleavage. Detailed information about the Baeyer-Villiger, Criegee, Hock, Kornblum-DeLaMare, Dakin, Elbs, Schenck, Smith, Wieland, and Story reactions is given. Unnamed rearrangements of organic peroxides and related processes are also analyzed. The rearrangements and related processes of important natural and synthetic peroxides are discussed separately.

Oxidation of alkylaromatics to aromatic ketones catalyzed by metalloporphyrins under the special temperature control method

The aerobic oxidation of alkylaromatics to aromatic ketones catalyzed by metalloporphyrins under the special temperature control method was systematically investigated. Three novel and key points were found to have significant functions in this process, that is, the special temperature control method (initiation at higher temperature and reaction at lower temperature), the synergistic effect of the composite catalysts comprising cobalt and manganese porphyrins, and the amount of catalysts in the reaction. Subsequently, the effects of substitutes on alkylaromatics were also explored under the same conditions. Results showed that alkylaromatic conversions gradually increased from 8.5% to 54.3% with the para-substituents shifting from the electron-donating group to the electron-withdrawing group (i.e., methoxy < hydro < bromo < acyl < nitro). A possible mechanism for this reaction was also proposed.

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.

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).

Artemisinin and a series of novel endoperoxide antimalarials exert early effects on digestive vacuole morphology

Artermisinin and its derivatives are now the mainstays of antimalarial treatment; however, their mechanism of action is only poorly understood. We report on the synthesis of a novel series of epoxy-endoperoxides that can be prepared in high yields from simple starting materials. Endoperoxides that are disubstituted with alkyl or benzyl side chains show efficient inhibition of the growth of both chloroquine-sensitive and -resistant strains of Plasmodium falciparum. A trans-epoxide with respect to the peroxide linkage increases the activity compared to that of its cis-epoxy counterpart or the parent endoperoxide. The novel endoperoxides do not show a strong interaction with artemisinin. We have compared the mechanism of action of the novel endoperoxides with that of artemisinin. Electron microscopy reveals that the novel endoperoxides cause the early accumulation of endocytic vesicles, while artemisinin causes the disruption of the digestive vacuole membrane. At longer incubation times artemisinin causes extensive loss of organellar structures, while the novel endoperoxides cause myelin body formation as well as the accumulation of endocytic vesicles. An early event following endoperoxide treatment is the redistribution of the pH-sensitive probe LysoSensor Blue from the digestive vacuole to punctate structures. By contrast, neither artemisinin nor the novel endoperoxides caused alterations in the morphology of the endoplasmic reticulum nor showed antagonistic antimalarial activity when they were used with thapsigargin. Analysis of rhodamine 123 uptake by P. falciparum suggests that disruption of the mitochondrial membrane potential occurs as a downstream effect rather than as an initiator of parasite killing. The data suggest that the digestive vacuole is an important initial site of endoperoxide antimalarial activity.

Mistletonone, a novel antioxidative diarylheptanoid from the branches and leaves of Viscum coloratum

Mistletonone, a novel diarylheptanoid, was isolated from the branches and leaves of Viscum coloratum (Kom.) Nakai (Loranthaceae). It was identified as 1,7-di-(p-hydroxyphenyl)-5-hydroxyl-cis-2,3-epoxy-1-one on the basis of spectral evidence. The compound showed significant scavenging effects on hydroxyl radicals and superoxide anion radicals in the direct assay using the electron spin resonance (ESR) technique.

Osmium Catalyzed Dihydroxylation of 1,2-Dioxines: A New Entry for Stereoselective Sugar Synthesis

A series of 3,6-substituted 3,6-dihydro-1,2-dioxines were dihydroxylated with osmium tetroxide to furnish 1,2-dioxane-4,5-diols (peroxy diols) in yields ranging from 33% to 98% and with de values not less than 90%. The peroxy diols were then reduced to generate a stereospecific tetraol core with R,R,S,S or "allitol" stereochemistry. The peroxy diols and their acetonide derivatives were also ring-opened with Co(II) salen complexes to give novel hydroxy ketones in 77-100% yield, including the natural sugar psicose. Importantly, preliminary work on the catalytic asymmetric ring-opening of meso-peroxy diols using the Co(II) Jacobsens's catalyst indicates that asymmetric sugar synthesis from 1,2-dioxines is possible.

Novel endoperoxides: synthesis and activity against Candida species

Fifteen new endoperoxides have been synthesised and tested for activity against pathogenic Candida species. These endoperoxides can be prepared in high yields, in one to three steps, from inexpensive starting materials. Despite chemical and structural similarities, their inhibitory activity against Candida growth varied greatly from one endoperoxide to another, and one species to another. This study of susceptibility to endoperoxide compounds presented here may lead to the development of potent new antifungal agents.

1,2-Dioxines Containing Tethered Hydroxyl Functionality as Convenient Precursors for Pyran Syntheses

A new method for the construction of tetrahydropyrans derived from readily available 1,2-dioxines containing a tethered hydroxyl moiety is described. The reaction proceeds via a base-catalyzed rearrangement of the 1,2-dioxines to either the isomeric cis or trans gamma-hydroxy enones followed by intramolecular oxa-Michael addition of the tethered hydroxyl group.

Ring-Opening of Unsymmetrical 1,2-Dioxines Using Cobalt(II) Salen Complexes

The regioselectivity of the metal-catalyzed ring opening of unsymmetrical 1,2-dioxines to cis-gamma-hydroxyenones was investigated using two different Co(II) salen complexes. Regioselectivity was determined by direct examination of the enone ratios and by derivitization with a stabilized phosphorus ylide. The steric influence of the substituents on the 1,2-dioxine was the primary influence on regioselectivity. Temperature played little role; however, solvent and selection of Co(II) complex could be used to mildly influence the outcome of the rearrangement for selected substrates. The origins of the selectivity for the reaction are discussed.

A domino ring-opening/epoxidation of 1,2-dioxines

When allowed to react with alkaline hydrogen peroxide, monocyclic 1,2-dioxines ring-open to their isomeric gamma-hydroxyenone intermediates which are rapidly epoxidized to afford trans-4-hydroxy-2,3-epoxyketones in 21-81% yield. In the case of meso-1,2-dioxines, Co(II) complex catalyzed asymmetric ring-opening of the 1,2-dioxine may be employed to furnish enantioenriched epoxides

Triphenylphosphine-Induced Ring Contraction of 1,2-Dioxines

Triphenylphosphine inserts into the peroxide bond of 1,2-dioxines, initiating ring contraction with loss of triphenylphosphine oxide. This process yields dihydrofuran oxides in 54-97% yield from oxirenyl[2,3-c][1,2]dioxines and dihydrofurans from 3,6-dihydro-1,2-dioxines with inversion of stereochemistry at either the 2 or 5 position in the furan product.