Thermal isomerisation of methyl linoleate hydroperoxides. Evidence of molecular oxygen as a leaving group in a radical rearrangement

Chan, Henry; Levett, Gordon; Matthew, Jennifer A.

doi:10.1039/c39780000756

Cited by 52 publications

(20 citation statements)

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“…The peroxyl radicals thus produced are unstable at high temperature, and radical elimination of the peroxyl radical results in the release of 02 as a leaving group to form a carbon-centered radical (Porter & Wujek, 1984). The formation of the carbon-centered radical is substantiated by the finding that the isomerization of MeLOOH is accompanied by the exchange of the oxygen atoms of the hydroperoxy group with atmospheric oxygen (Chan et al, 1978). The alkyl radical produced then reacts with an a-tocopheroxyl radical to form 6-0-alkyl-a-tocopherols (8) (Yamauchi et al, 1995b).…”

Section: Reaction Of A-tocopherol With Methyl Linoleate Hydroperoxidesmentioning

confidence: 61%

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Vitamin E: Mechanism of Its Antioxidant Activity.

Yamauchi

1997

FSTI

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The antioxidant activity of vitamin E (a-tocopherol) during the peroxidation of unsaturated lipids has been reviewed based on its reaction products. Free-radical scavenging reactions of a-tocopherol take place via the a-tocopheroxyl radical as an intermediate. If a suitable free radical is present, a non-radical product can be formed from the coupling of the free radical with the a-tocopheroxyl radical. The reaction products of a-tocopherol with lipid-peroxyl radicals are 8a-(lipid-dioxy)-a-tocopherones which are hydrolyzed to a-tocopherylquinone. If the supply of oxygen is insufficient, a-tocopherol can trap the carbon-centered radicals of lipids to form 6-0-(lipidalkyl)-a-tocopherols. On the other hand, the dimer and trimer of a-tocopherol is formed by the bimolecular self-reaction of the a-tocopheroxyl radical in a reaction mixture containing a large amount of a-tocopherol. The other product-forming pathway yields isomeric epoxy-a-tocopherylquinones and their precursors, epoxyhydroperoxy-a-tocopherones, but the mechanism of this pathway remains unknown. The reaction products of other vitamin E compounds (7-and 6-tocopherols) during lipid peroxidation are almost the same as those formed from the a-tocopherol. The tocopheroxyl radicals of 7-and 6-tocopherols prefer to react with each other to form dimeric products that are still effective as antioxidants.

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Section: Reaction Of A-tocopherol With Methyl Linoleate Hydroperoxidesmentioning

confidence: 61%

“…This suggests that the carbon-centered radicals trapped by a-tocopheroxyl radicals at high temperature are produced by ~-scission of methyl linoleate-peroxyl radicals (Chan et al, 1978;Porter et al, 1981). …”

Section: A-tocopherol As the Free Radical Scavengermentioning

confidence: 99%

Vitamin E: Mechanism of Its Antioxidant Activity.

Yamauchi

1997

FSTI

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“…The reversibility of oxygen addition has been established by oxygen scrambling in studies on hydroperoxide rearrangements with isotopically labeled hydroperoxides or molecular oxygen and has led to the conclusion that the rearrangement of conjugated diene hydroperoxides proceeds via -fragmentation (Chan et al 1978(Chan et al , 1979Porter et al 1980). As in the autoxidation of methyl oleate, the rate of the -fragmentation depends on the geometry of the pentadienyl radical generated in the process.…”

Section: Figure 10mentioning

confidence: 99%

“…The peroxidation reaction is reversible, based on oxygen scrambling studies with methyl linoleate hydroperoxides by Chan et al (1978Chan et al ( , 1979. In Fig.…”

Section: Figure 16mentioning

confidence: 99%

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Autoxidation of Conjugated Linoleic Acid Methyl Ester in the Presence of α‐Tocopherol: The Hydroperoxide Pathway

Pajunen

Johansson

Hase

et al. 2008

Lipids

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The autoxidation of conjugated linoleic acid (CLA) is poorly understood in spite of increasing interest in the beneficial biological properties of CLA and growing consumption of CLA-rich foods. In this thesis, the autoxidation reactions of the two major CLA isomers, 9-cis,11-transoctadecadienoic acid and 10-trans,12-cis-octadecadienoic acid, are investigated. The results contribute to an understanding of the early stages of the autoxidation of CLA methyl ester, and provide for the first time a means of producing and separating intact CLA methyl ester hydroperoxides as well as basic knowledge on lipid hydroperoxides and their hydroxy derivatives.Conjugated diene allylic monohydroperoxides were discovered as primary autoxidation products formed during autoxidation of CLA methyl esters in the presence and absence oftocopherol. This established that one of the autoxidation pathways of CLA methyl ester is the hydroperoxide pathway.Hydroperoxides were produced from the two major CLA methyl esters by taking advantage of the effect of -tocopherol to promote hydroperoxide formation. The hydroperoxides were analysed and separated first as methyl hydroxyoctadecadienoates and then as intact hydroperoxides by HPLC. The isolated products were characterized by UV, GC-MS, and NMR techniques. In the presence of a high amount of -tocopherol, the autoxidation of CLA methyl ester yields six kinetically-controlled conjugated diene monohydroperoxides and is diastereoselective in favour of one particular geometric isomer as a pair of enantiomers. The primary autoxidation products produced from the two major CLA isomers include new positional isomers of conjugated diene monohydroperoxides, the 8-, 10-, 12-, and 14-hydroperoxyoctadecadienoates. Furthermore, two of these new positional isomers have an unusual structure for a cis,trans lipid hydroperoxide where the allylic methine carbon is adjacent to the cis instead of the usual trans double bond.The 1 H and 13 C NMR spectra of nine isomeric methyl hydroxyoctadecadienoates and of ten isomeric methyl hydroperoxyoctadecadienoates including the unusual cis,trans hydroperoxides, i.e. Me 8-OOH-9c,11t and Me 14-OOH-10t,12c, were fully assigned with the aid of 2D NMR spectroscopy. The assigned NMR data enabled determination of the effects of the hydroxyl and hydroperoxyl groups on the carbon chemical shifts of CLA isomers, identification of diagnostic signals, and determination of chemical shift differences of the olefinic resonances that may help with the assignment of structure to as yet unknown lipid hydroperoxides either as hydroxy derivatives or as intact hydroperoxides.A mechanism for the hydroperoxide pathway of CLA autoxidation in the presence of a high amount of -tocopherol was proposed based on the characterized primary products, their relative distribution, and theoretical calculations. This is an important step forward in CLA research, where exact mechanisms for the autoxidation of CLA have not been presented before. Knowledge of these hydroperoxide formation steps is of cr...

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Hydrocarbon Oxidation

Hobbs¹

2000

Kirk-Othmer Encyclopedia of Chemical Technology

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Hydrocarbon oxidation has long been a significant source of organic chemicals. Both vapor‐ and liquid‐phase reactions are of interest; they are believed to involve free‐radical chain mechanisms. Modern understanding of these reactions has developed over the last half of the twentieth century. Although sharp disputes are frequent, significant agreement on underlying principles is apparent. Vapor‐phase oxidations can be conveniently divided into low, middle, and high temperature ranges. Fundamental changes in mechanisms occur over these regions. The direct oxidation of methane to methanol, formaldehyde, and/or C 2 products has received intense study, but no viable processes have yet been developed. Most other studies in this area are directed toward fuel utilization and pollution abatement. Liquid‐phase oxidations of aliphatics have declined in importance as sources of bulk chemicals. However, the oxidation of butane to acetic acid, and of isobutane to t ‐butylhydroperoxide (to produce propylene oxide), and of cyclohexane to cyclohexanol and cycohexanone (to produce adipic acid and nylon), remain important industrial processes. Oxidations of alkylaromatics continue to be important industrially. The oxidation of p ‐xylene to terephthalic acid and cumene to phenol are dominant routes to these products. The potential for producing fine chemicals by such oxidations is developing rapidly.

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Thermal isomerisation of methyl linoleate hydroperoxides. Evidence of molecular oxygen as a leaving group in a radical rearrangement

Cited by 52 publications

References 0 publications

Vitamin E: Mechanism of Its Antioxidant Activity.

Vitamin E: Mechanism of Its Antioxidant Activity.

Autoxidation of Conjugated Linoleic Acid Methyl Ester in the Presence of α‐Tocopherol: The Hydroperoxide Pathway

Hydrocarbon Oxidation

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