1,2,5,10,11,14-hexaoxadispiro[5.2.5.2]hexadecanes: novel spirofused bis-trioxane peroxides

A set of new bis-spirofused 1,2,4-trioxanes 4a-d was obtained from the reaction of cyclohexane-1,4-dione with allylic hydroperoxides 2a-d, bearing an additional hydroxy group in the homoallylic position, by diastereoselective photooxygenation of allylic alcohols 1a-d and subsequent BF(3)-catalyzed peroxyacetalization with the diketone. From the reaction of a monoprotected cyclohexane-1,4-dione 5 with the allylic hydroperoxide 6 derived from the singlet oxygenation of methyl hydroxytiglate, one monospiro compound was obtained, the 1,2,4-trioxane ketone 7, as well as a mixture of the diastereoisomeric syn- and anti bis-1,2,4-trioxanes 8. The structures of bis-1,2,4-trioxanes were examined theoretically by DFT methods and compared with X-ray structural data in order to evaluate the preferential trioxane ring conformational orientation.     

Triple-layered QSAR studies on substituted 1,2,4-trioxanes as potential antimalarial agents: superiority of the quantitative pharmacophore-based alignment over common substructure-based alignment

This study reports the utilization of three approaches – pharmacophore, CoMFA/CoMSIA and HQSAR studies – to identify the essential structural requirements in 3D chemical space for the modulation of the antimalarial activity of substituted 1,2,4-trioxanes. The superiority of quantitative pharmacophore-based alignment (QuantitativePBA) over global minima energy conformer-based alignment (GMCBA) has been reported in CoMFA and CoMSIA studies. The developed models showed good statistical significance in internal validation (q ², group cross-validation and bootstrapping) and performed very well in predicting the antimalarial activity of test set compounds. Structural features in terms of their steric, electrostatic and hydrophobic interactions in 3D space have been found to be important for the antimalarial activity of substituted 1,2,4-trioxanes. Further, the HQSAR studies based on the same training and test set acted as an additional tool to find the sub-structural fingerprints of substituted 1,2,4-trioxanes for their antimalarial activity. Together, these studies may facilitate the design and discovery of new substituted 1,2,4-trioxanes with potent antimalarial activity.     

CuI/l-proline catalyzed click reaction in glycerol for the synthesis of 1,2,3-triazoles

Despite the apparent simplicity of the copper(I) iodide catalyzed CuAAC reaction, the conversion of the catalytic species, i.e. Cu(I) to thermodynamically more stable Cu(II), via aerial oxidation or disproportionation is a major issue. To stabilize the Cu(I) species, the reaction is ideally carried out under an inert atmosphere in the presence of additives such as alcohols, amines, thiols, and aldehydes. Herein, we report the first CuI catalyzed click reaction without an inert atmosphere by employing the CuI/l-proline system in glycerol. The method showed remarkable stability towards sensitive functional groups such as acetonides and 1,2,4-trioxanes.     

A New Reaction of 1,2-Dioxetanes. Formation of 1,2,4-Trioxanes from Aldehydes

1,2-Dioxetanes bearing different 3-phenoxy substituents add to aldehydes to give 1,2,4-trioxanes in yields of 17–75% depending on the nature of the para-phenyl substituent. Illustrations of this new reaction are described.     

Synthesis of antimalarial 1,2,4-trioxanes via photooxygenation of a chiral allylic alcohol

[reaction: see text] Photooxygenation of the chiral allylic alcohol 4-methyl-3-penten-2-ol (3) in nonpolar solvents and subsequent Lewis acid-catalyzed peroxyacetalization afforded a series of monocyclic and spirobicyclic 1,2,4-trioxanes (5, 6). Two products show significant anti-Malaria activity against Plasmodium falciparum when compared with chloroquine.     

Application of thiol-olefin co-oxygenation methodology to a new synthesis of the 1,2,4-trioxane pharmacophore

[reaction: see text] Thiol-olefin co-oxygenation (TOCO) of substituted allylic alcohols generates alpha-hydroxyperoxides that can be condensed in situ with various ketones to afford a series of functionalized 1,2,4-trioxanes in good yields. Manipulation of the phenylsulfenyl group in 4a allows for convenient modification to the spiro-trioxane substituents, and we describe, for the first time, the preparation of a new class of antimalarial prodrug.     

Further explorations on bridged 1,2,4-trioxanes

Three new bicyclo[3.2.1]-type 1,2,4-trioxanes have been designed and synthesized. One of them demonstrates better tolerance of the intramolecular hemiketals to steric crowding in hydroperoxidation. The other represents a prototype for possible manipulation of the transient radicals generated in cleavage reactions. A new substitution pattern in the bridged system is explored through synthesis of the third molecule. The configurations of all stereogenic centers in the bridged system can be effectively controlled by the chirality of the allyl alcohol as illustrated by the enantioselective synthesis of the fourth molecule. Finally, similar bicyclo[3.3.1]-type 1,2,4-trioxanes are shown very difficult to be synthesized because of the involvement of a conformer with two substituents at axial positions at the same time.     

Singlet oxygen addition to chiral allylic alcohols and subsequent peroxyacetalization with β-naphthaldehyde: synthesis of diastereomerically pure 3-β-naphthyl-substituted 1,2,4-trioxanes

The synthesis of a series of eight β-naphthyl-substituted 1,2,4-trioxanes 3a-h by a sequence of singlet oxygen ene reaction of allylic alcohols 1a-h and Lewis acid catalyzed peroxyacetalization of the allylic hydroperoxides 2a-h with β-naphthaldehyde is reported. The ene reactions were performed by solid-state photooxygenation in dye-crosslinked polystyrene beads and resulted in mixtures of diastereoisomeric hydroperoxides 2. Boron trifluoride catalyzed peroxyacetalization resulted in the formation of 3, as well as the 1,2,4-trioxanes 4 and 5, which were formed via acid catalyzed β-hydroperoxy alcohol cleavage.     

Photooxygenation of 3-aryl-2-cyclohexenols: synthesis of a new series of antimalarial 1,2,4-trioxanes

Using easily accessible 3-aryl-2-cyclohexenols, a photooxygenation route for the preparation of bicyclic 1,2,4-trioxanes is reported. Several of these trioxanes have shown significant antimalarial activity against multidrug resistant Plasmodium yoelii in mice by the oral route.     

Alkylation of manganese(II) tetraphenylporphyrin by a synthetic antimalarial trioxane

The synthesis and the antimalarial activity of a new kind of polycyclic 1,2,4-trioxanes are reported. The alkylation of the heme model MnIITPP by the biologically active (IC 50 = 320 nmol L-1) hemiperketal 2 is presented.     

Interaction of Qinghaosu (Artemisinin) with Cysteine Sulfhydryl Mediated by Traces of Non-Heme Iron

The antimalarial action of 1,2,4-trioxanes such as qinghaosu (QHS) may take place through the mechanism shown schematically: In the presence of cysteine traces of non-heme iron (FeSO₄) may cleave the peroxy bond of QHS rapidly, and the transient carbon-centered radical can attack the sulfur ligand to form a covalent bond.     

Synthesis,Structure, and Antimalarial Activity of Tricyclic 1,2,4-Trioxanes Related to Artemisinin

Two sets of tricyclic 1,2,4-trioxanes containing the ABC ( 10 , 11 ) and ACD ring portions ( 21 , 22 , 32 , 33 , 37 , and 38 ) of artemisinin ( 1 ) were synthesized by successive photo-oxygenation of appropriate enol-ether precursors to 1,2-dioxanes and inter- and intramolecular reaction with a carbonyl compound or oxo-substituted side-chain. The structures of 10 , 21 , and 22 were determined by X-ray analysis. The anti-malarial activity of all trioxanes, except 37 and 38 , was evaluated in vitro against chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum parasites. Trioxanes 11 and 21 were as active as artemisinin ( 1 ). It was found that neither the lactone function nor rings B and D of 1 were essential for activity. A possible pharmacophore for artemisinin-like activity, which embodies a spirocyclopentane group attached to C(3) of 1,2,4-trioxane, was proposed.     

New chemistry of zwitterionic peroxides arising by photooxygenation of enol ethers

The rosebengal-sensitized photooxygenation of 2-methoxynorborn-2-ene(1)in acetaldehyde gave cis-1-carboxaldehyde-3-carbomethoxycyclopentane (31%) and the cis and trans-Me derivatives of the cis-fuscd exo-1,2,4-trioxanes arising by addition of a molecule of oxygen and acetaldehyde to 1 at C3 and C2 respectively (13%) Similar photooxygenation of 2-(methoxymethylidene)adamantane in the presence of acetaldehyde, propionaldehyde and pivalaldehyde gave adamantanone (31–42%), and the cis and trans tricylo[3.3.1.1^3,7]decane-2-spiro-6'-[3-alkyl-5-methoxy-1,2,4-trioxanes] in yields of 32–53% Trioxane formation under similar conditions was experienced for 1,1-di-t-butyl-2-methoxyethene and 2-(methylmercaptomethylidene) adamantane. The results are discussed in terms of an intermediate zwitterionic peroxide which can either close directly to a 1,2-dioxetane or, if aldehyde is present, condense across the CO function giving the 1,2,4-trioxane.     

Dispiro-1,2,4-trioxane analogues of a prototype dispiro-1,2,4-trioxolane: mechanistic comparators for artemisinin in the context of reaction pathways with iron(II)

Single electron reduction of the 1,2,4-trioxane heterocycle of artemisinin (1) forms primary and secondary carbon-centered radicals. The complex structure of 1 does not lend itself to a satisfactory dissection of the electronic and steric effects that influence the formation and subsequent reaction of these carbon-centered free radicals. To help demarcate these effects, we characterized the reactions of achiral dispiro-1,2,4-trioxolane 4 and dispiro-1,2,4-trioxanes 5-7 with ferrous bromide and 4-oxo-TEMPO. Our results suggest a small preference for attack of Fe(II) on the nonketal peroxide oxygen atom of 1. For 4, but not for 5 and 6, there was a strong preference for attack of Fe(II) on the less hindered peroxide bond oxygen atom. The steric hindrance afforded by a spiroadamantane in a five-membered trioxolane is evidently much greater than that for a corresponding six-membered trioxane. Unlike 1, 5-7 fragment by entropically favored beta-scission pathways forming relatively stable alpha-oxa carbon-centered radicals. These data suggest that formation of either primary or secondary carbon-centered radicals is a necessary but insufficient criterion for antimalarial activity of 1 and synthetic peroxides.     

High-performance liquid chromatography of some novel diastereomeric tricyclic 1,2,4-trioxanes

A series of 4a, 10b-dihydronaphtho-[2,1-e]-1,2,4-trioxanes were examined by h.p.l.c. Compounds existing as a pair of diastereomers are only resolved when hydrogen is a substituent at the C3 position. The separations actually observed may arise from conformational differences rather than simple diastereoisomerism.     

A Vibrational Study of Some 1,2,4-Trioxanes

The vibrational spectra of some 1,2,4-trioxanes present two characteristic bands at 790 and 880 cm^?1. On the basis of ¹⁸O-isotopic substitution and comparison with analogous compounds, these bands have been assigned to coupled C? O and O? O stretching modes of the C? O? O element.     

Synthesis and antimalarial activities of structurally simplified 1,2,4-trioxanes related to artemisinin

Eleven derivatives of the clinically useful, antimalarial, 1,2,4-trioxane artemisinin have been synthesized in only several steps from commercial cyclohexanones. Of these simple, tricyclic 1,2,4-trioxanes, 10 showed considerable in vitro antimalarial activity, with one being as potent as artemisinin. Some structure-activity relationship generalizations are made from this series of artemisinin analogs. Triethylsilyl hydrotrioxide (Et₃SiOOOH), prepared in situ from ozone and triethylsilane, is shown to be a mild, fastacting, and effective dioxetane-forming reagent with vinyl ethers and with a vinyl thioether on relatively small (50–100 mg) scale.     

A study of chiral oxazoline ligands with a 1,2,4-triazine and other six-membered aza-heteroaromatic rings and their application in Cu-catalysed asymmetric nitroaldol reactions

Enantiomerically pure oxazoline ligands with variously substituted 1,2,4-triazine rings have been synthesized using the Pd-catalysed cross-coupling amination of 3-halo-1,2,4-triazines. The catalytic efficiency of the ligands was studied in the asymmetric Henry reaction of nitromethane with several aldehydes. The appropriate β-nitro alcohols were formed in good yields (up to 93%) and with up to 78% ee. The impact of the substitution of the 1,2,4-triazine ring on the nitroaldol reaction is discussed. In order to investigate the influence of the 1,2,4-triazine ring on the catalytic activity of the ligands, ligands where the 1,2,4-triazine ring was replaced by a pyridine, pyrimidine, pyrazine or pyridine N-oxide ring were synthesized and applied to asymmetric nitroaldol reactions.     

Chemistry of 1,2,4-trioxanes relevant to their mechanism of action. Part 1: Reaction with Fe(II) salts

Reactions of 6-(1′-phenylvinyl)-1,2,4-trioxanes 2-5, with FeCl₂·4H₂O, FeBr₂, and a combination of hemin (bovine) and reduced glutathione (GSH) under various conditions have been studied.