Thermal decomposition of individual positional isomers of methyl linolenate hydroperoxides,hydroperoxy cyclic peroxides and dihydroperoxides

The methyl esters of 4 individual positional isomers of hydroperoxides, 2 positional isomers of hydroperoxy cyclic peroxides and a 9,16-dihydroperoxide were prepared by autoxidation of methyl linolenate and separated by preparative high pressure liquid chromatography. Isolated hydroperoxide isomers were thermally decomposed and the resulting volatile components analyzed. Each hydroperoxide or hydroperoxy cyclic peroxide isomer yielded characteristic volatile products. The major volatiles from each acyclic hydroperoxide corresponded with those predicted to arise by carboncarbon scission on either side of the corresponding alkoxy radical intermediate and little evidence was found of isomerization between the various positional isomers occurring during the process. A similar mechanism would account for the volatile products obtained from the cyclic peroxides. 2,3-Pentanedione was a significant odor contributor arising from the 13,15-epidioxy-16-hydroperoxide isomer although it was only a minor decomposition product.     

Proportion of geometrical hydroperoxide isomers generated by radical oxidation of methyl linoleate in homogeneous solution and in aqueous emulsion

The proportion of geometrical hydroperoxide isomers generated by aerobic oxidation of methyl linoleate (18∶2 Me) in either aqueous emulsion consisting of Tris-HCl buffer (pH 7.4) or in a homogeneous dichloromethane solution was determined to understand the mechanism of lipid oxidation in different reaction systems. Four geometrical isomers were generated after oxidation of 18∶2 Me in dichloromethane: methyl 13-hydroperoxy-cis-9,trans-11-octadecadienoate, methyl 13-hydroperoxy-trans-9,trans-11-octadecadienoate, methyl 9-hydroperoxy-trans-10,cis-12-octadecadienoate, and methyl 9-hydroperoxy-trans-10,trans-12-octadecadienoate in the ratios of 1∶4∶1∶4, respectively. The ratios between each isomer did not change until the peroxide value (PV) increased to 58 meq/kg. Oxidation of 18∶2 Me in aqueous emulsion yielded the same geometrical isomers of hydroperoxide. However, the ratios were different: 3∶2∶3∶2 until the PV increased to 110 meq/kg. Predominant (60%) formation of trans,trans hydroperoxide isomers was obtained in the oxidation of a mixture of 18∶2 Me and methyl laurate (12∶0 Me). These results are interpreted to reflect the importance of the concentration of hydrogen atom-donating equivalents to the kinetic preference for different products. The high effective concentration of hydrogen donors in the oxidation of 18∶2 Me in emulsions favored the formation of the less stable cis,trans isomers. The lower concentration of hydrogen donor in the dichloromethane solution effectively slowed hydrogen donation and led to the strong preference for the more stable trans,trans isomers. This interpretation was further tested by preparing emulsions of 18∶2 Me and 12∶0 Me to dilute concentration of hydrogen-donating species using the nonhydrogen-donating 12∶0 Me. Consistent with the proposed hypothesis, the proportion of trans,trans isomers increased as a result of 12∶0 Me addition.     

Hydrogenolysis and hydrocracking of the carbon-oxygen bond. 2. Thermal cleavage of the carbon-oxygen bond in guaiacol

Low-rank coals and their precursors contain, in addition to aromatic hydroxy groups, aromatic methoxy groups. In the present work a model compound, guaiacol, is used for the study of the behaviour of the carbon-oxygen bonds under thermolytic conditions. The thermolysis of guaiacol is studied in tetralin, naphthalene and without solvent under hydrogen or nitrogen pressure at 578–618 K. The compound is homolytically converted by first-order kinetics. The major product is pyrocatechol. Phenol, o-cresol, methyl catechols and methyl guaiacols are also formed. When tetralin is present it reacts in a molar ratio of 1:4 with guaiacol to form naphthalene. The source of hydrogen when tetralin is not present is guaiacol itself because molecular hydrogen does not participate in the reaction. The kinetics and reaction mechanism are discussed.     

Analysis of fat acid oxidation products by countercurrent distribution methods. V. Low-temperature decomposition of methyl linoleate hydroperoxide

Methylcis, trans diene conjugated linoleate hydroperoxide isolated by counterenrrent distritbution from 4°C, auatoxidation of methyl linoleate was stored in atmospheres of oxygen and of nitrogen at 4°C. in darkenss. Besides manometric changes, infrared and ultraviolet characteristics, peroxide value, diene conjugation, and molecular weights were followed on samples removed at various periods of storage up to 53 days. These same analyses were obtained on fractions obtained by counter-current distributions. Evidence for the reaction that occurs on storage in oxygen may be summarized thus: 1 mole oxygen absorbed by linoleate hydroperoxides destroys 1 molecis, trans diene conjugation, 1/2 mole peroxide group, and 1 mole linoleate hydroperoxide; dimers of varying polarities, scission acids, and isolatedrans bonds are formed. Since to volume changes were observed in the nitrogen storage of methyl linoleate hydroperoxide, changes in chemical and physical characteristics can only be related to time of storage. Storage in nitrogen at 4°C, destroys diene conjugation, peroxides, and linoleate hydroperoxide and produces dimers of varying polaritics, seission neids, and isolatedrans bonds. Destruction of diene conjugation was one-fourth as rapid in a nitrogen atmosphere as in oxygen. While differences in reactions and products were observed between oxygen and nitrogen storage, particularly in rates and in countereurrent distribution patterns, the similarity of products from oxygen and nitrogen storage is remarkable. One methyl linoleate hydroperoxide is formed regardless of storage atmosphere, dimirization and attendant destruction of double bonds and peroxides proceed. This is a laboratory of the Northern Utilization Research and Development Division, Agricultural Research Service, U.S. Department of Agriculture.     

Gas-liquid chromatography of the positional isomers of methyl nonynoate

A mixture of all the positional isomers of methyl nonynoate is poorly resolved by gasliquid chromatography on packed polar (diethylene glycol succinate) and packed nonpolar (Apiezon L) columns. Better resolution is obtained on capillary columns, with the polar liquid phase giving baseline separations between all the isomers except 9:T4 and 9:T5. The nonynoic esters are eluted later than methyl nonanoate on either liquid phase. The isomers with the triple bond near the center of the molecule come off first, and the elution time increases as the triple bond moves toward either end of the chain. Methyl 8-nonynoate, the only isomer with a terminal triple bond, does not follow this pattern. Presented at the AOCS meeting in Cincinnati, 1965.     

Analysis of the geometric isomers of methyl linoleate by gas chromatography

The four geometric isomers of methyl linoleate have been quantitatively determined by gas chromatography on Apiezon L and DEGS polyester capillary columns. Three peaks were eluted from the Apiezon L column: (a) the 9-cis, 12-cis isomer; (b) the 9-cis, 12-trans isomer; and (c) the 9-trans, 12-cis and 9-trans, 12-trans isomers combined. The DEGS polyester column also resolved three peaks: (a) the 9-trans, 12-trans isomer; (b) the 9-cis, 12-cis and 9-cis, 12-trans isomers combined; and (c) the 9-trans, 12-cis isomer. Since the separation of isomers was different on each column, the content of each of the four isomers could be determined from the combined results. Quantitative results agreed closely with the per centtrans bonds as determined by infrared analysis. Presented at the fall meeting of the American Oil Chemists' Society, Chicago, Illinois, October 30-November 1, 1961. Supported in part by a grant from Cargill, Ineorporated.     

Photosensitized oxidation of methyl linoleate: Secondary and volatile thermal decomposition products

Studies of photosensitized oxidation of methyl linoleate show that the greater relative concentration of 9- and 13-hydroperoxides than 10- and 12-hydroperoxides is characteristic of singlet oxygenation and not due to either simultaneous autoxidation or type 1 photosensitized oxidation. Cyclization of the internal 10- and 12-hydroperoxides accounts for their lower relative concentrations. Secondary products separated by silicic acid and high pressure liquid chromatography were characterized spectrally (IR, UV,¹H-NMR,¹³C-NMR, GC-MS). Major secondary products included diastereomeric pairs of 13-hydroperoxy-10,12-epidioxy-trans-8-octadecenote (I and III) and 9-hydroperoxy-10,12-epidioxy-trans-13-octadecenoate (II and IV); minor secondary products included hydroperoxy oxy genated and epoxy esters. Thermal decomposition of the hydroperoxy cyclic peroxides produced hexanal and methyl 10-oxo-8-decenoate as major volatiles from I and III and methyl 9-oxo-nonanoate and 2-heptenal from II and IV. Hydroperoxy cyclic peroxides may be important sources of volatile decomposition products of photooxidized fats. Presented at 72nd Annual Meeting of the American Oil Chemists Society, New Orleans, LA, May 1981.     

Nutritive value of methyl linoleate and its thermal decomposition products

Methyl linoleate was heated for 10 hrs. at 300°C, in the absence of air and fractionated by alembic distillation and urea adduct-formation. Intestinal absorptions of the urea adduct-forming monomeric nonadduct-forming monomeric, and dimeric fractions were determined. It was found that dimers were half as well absorbed as the monomers. When fed to rats, dimers were better accepted and exhibited some toxicity symptoms different from the non-adduct-forming monomers. The dimers caused diarrhea, irritability, and loss of hair during the early period of administration. The nonadduct-forming monomers were lethal and produced an increase in liver weight. Both fractions depressed growth. With the technical assistance of Oscar giacomantone and Perla Mordujovich.     

Positional isomers formed during the hydrogenation of methyl linoleate under various conditions

Journal of the American Oil Chemists' Society -     

Preparation of deuterium-labelled methyl linoleate and its geometric isomers from natural seed oils

Multi-gram quantities of deuterium-labelled methyl linoleate (methyl cis-9,cis-12-octadecadienoate) and its geometric isomers are readily synthesized fromCrépis alpina (70–80% cis-9-octadecen-12-ynoic acid) andVernonia galamensis (70–80% 12,13-epoxy-cis-9-octadecenoic acid) seed oils. Methylcis- 9,cis- 12- andtrans- 9,cis- – octadecadienoate-12,13-d₂ were prepared by the Lindlar-catalyzed reduction (with D₂ gas) of methylcis- 9- and trans-9-octadecen-12-ynoates, respectively. Methyltrans- 9- octadecen-12-ynoate was synthesized by thep-toluene-sulfinic acid-catalyzed isomerization of the corresponding cis isomer. Methylcis- 9fiis- 12., trans- 9fiis- 12;cis- 9,trans- 12- andtrans- 9, frans-12-octadecadienoate-d₂, d₄ and d₆ were prepared by the Wittig coupling (with stereochemical control) of the appropriate d₂-, d₄- or de-alkyltriphenyl-phosphonium salt with methyl 12-oxo-cis-9- ortrans- 9- dodecenoate (prepared by the para-periodic acid cleavage of methyl 12,13-dihydroxy-cis-9- or trans-9-octadecenoate). Thecis dihydroxy ester was synthesized fromVernonia galamensis seed oil by acetolysis, saponification and then esterification. Thecis dihydroxy ester was isomerized by nitric acid/sodium nitrite to thetrans form and purified by silver resin chromatography. Isotopic purities ranged from 88% (for the d₆ isomers) to 99% (for the d₂ isomers). The mention of firm names or trade products does not imply that they are endorsed or recommended over other firms or similar products not mentioned.     

Epoxidation of methyl linoleate. I. The question of positional selectivity in monoepoxidation

The mixture of compounds resulting from the expoxidation of one of the double bonds of methyl linoleate has been examined to determine possible positional selectivity in moneopoxidation. The analysis, which depended on periodic acid cleavage of saturated epoxides or saturated vicinal glycols, has revealed that the two monoepoxidation products, methyl vernolate and methyl coronarate, are produced in essentially equal amounts. Presented at AOCS Meeting in Cincinnati, October 1965. E. Utiliz. Res. Dev. Div., ARS, USDA.     

Epoxidation of methyl linoleate. II. The two isomers of methyl 9, 10: 12, 13-diepoxystearate

Epoxidation of methyl linoleate results in the formation of two stereoisomeric methyl 9,10:12,13-diepoxystearates. The previously well-known isomer, mp 32C, represents only 60% of the total diepoxides formed. The newly isolated form is liquid at room temperature and is measured incompletely by standard titrimetric oxirane analysis. The isomers differ by the relative positions of their oxirane oxygen atoms. The oxygens of the solid are located on opposite sides and those of the liquid are on the same side of the molecular plane. E. Utiliz. Res. Devel. Div., ARS, USDA.     

Evidence for hydroperoxide formation in the autoxidation of methyl linolenate

Summary 1. The results of chemical, spectral, and distillation analyses on the isolated oxidized fractions of three samples of methyl linolenate autoxidized at 0°C. to peroxide values from 600 to 760 m.e./kg. indicated that about 60% of the products consisted of cis,transconjugated diene methyl octadecatrienoate-monohy-droperoxide. 2. A product was isolated from the reduced peroxide concentrates which consisted of about 90% cis,trans- conjugated diene methyl monohydroxyoctadecatrienoate. 3. Analytical micromolecular distillation analyses showed that the total oxidized material isolated from samples of autoxidized methyl linolenate contained only about 15% polymeric material under the conditions of autoxidation employed in this study. 4. Linolenate was shown to be similar to linoleate in forming a cis,trans-conjugated monomeric monohydroperoxide as a major initial product of autoxidation at 0°C. This work was supported in part by a contract between the Office of Naval Research, Department of the Navy, and the University of Minnesota. It was presented at the spring meeting of the American Oil Chemists’ Society, New Orleans, May, 1953.     

Effect of α-tocopherol and Trolox on the decomposition of methyl linoleate hydroperoxides

To clarify the mechanisms of antioxidant action, the effect of α-tocopherol and its water-soluble carboxylic acid derivative, Trolox, was studied on the decomposition of methyl linoleate hydroperoxides (MeLoOOH). Decomposition rate and the distribution of autoxidation products formed from MeLoOOH were followed by analyzing the volatile and nonvolatile products by static headspace gas chromatography and normal-phase high-performance liquid chromatography, respectively. Both α-tocopherol and Trolox markedly inhibited the decomposition of MeLoOOH in a concentration-dependent way. In the absence of antioxidants, MeLoOOH was completely decomposed after incubation for 48 h at 60°C, and in the presence of equal molar concentration of antioxidants only 6–7% of initial MeLoOOH was decomposed even after 280 h of incubation. MeLoOOH produced 1.2% methyl linoleate hydroxy compounds (MeLoOH) in the presence of α-tocopherol and 3.8% in the presence of Trolox. Both antioxidants inhibited the formation of volatile decomposition products and the formation of ketodiene compounds. The hydroxy compounds may be formed by the reaction of alkoxy radical and hydrogen donating antioxidants. Conversion of MeLoOOH into stable MeLoOH demonstrated that the antioxidants α-tocopherol and Trolox trap alkoxyl radicals by H-donation.     

Alkali isomerization of linoleate isomers: Characterization of products

Geometrical isomers of methyl linoleate were reacted with alakli, and the resulting conjugated isomers were separated intotrans,trans;cis,trans; andcis,cis fractions. The position of double bonds in the various fractions was determined by reductive ozonolysis.trans-9,trans-12-Isomer of linoleate formedtrans,trans- andcis,trans-conjugated dienes, whereascis-9,trans-12- andtrans-9,cis-12-isomers in addition formedcis,cis-conjugated dienes. The formation of the products is in accordance with the theoretical predictions. During conjugationtrans double bonds shifted to form atrans bond preferentially. During conjugation ofcis-9,trans-12- andtrans-9,cis-12-linoleate isomers, thecis double bond shifted preferentially over thetrans double bond. A small amount of diene not conjugated was probably a geometrical and positional isomer of the starting material.     

Thermal reactions of methyl linoleate. II. The structure of aromatic C18 methyl esters

This second report describes the characterization of C₁₈ aromatic esters from the heated linoleate and the independent synthesis of two of them. The esters were isolated by a combination of molecular distillation, urea adduction, column chromatography, and gas chromatography. They were characterized by infrared, ultraviolet, NMR, and mass spectroscopy. The analytical data for the isolated esters were compared with the data for the synthetic esters, methyl 11-(2′-methylphenyl) undecanoate, methyl 7-(2′-pentylphenyl) heptanoate, and methyl 8-(2′-butylphenyl) octanoate. The latter two compounds were found to be components of the aromatic fraction isolated from heated linoleate, and their synthesis is deseribed in detail.     

Prooxident activities of chlorophylls and their decomposition products on the photooxidation of methyl linoleate

Although chlorophylls in oils affect oxidative stabilities, little has been known about the prooxidant activity of the chlorophylls or their decomposition products. To evaluate the prooxidant activities of chlorophyll derivatives, chlorophyll (CHL) A and B, pheophytin (PHY) A and B and pheophorbide (PHO) A and B were added to methyl linoleate (ML). The sample oils were then subjected to photooxidation at O C and the peroxide value and absorbance at 234 nm were measured. Chlorophylls catalyzed oxidation of methyl linoleate at concentrations greater than 2.2×10⁻⁹mol/g ML, and CHL B showed a stronger prooxidant activity, ca. twice that of CHL A under the same light intensity. PHY and PHO exhibited stronger prooxidant activities than CHL, the prooxidant activity of PHO being the strongest among them. Moreover, photosensitive activity was found in PHY as well as in CHL. These results suggest that particular attention should be paid to the decomposition products of CHL that affect the quality of vegetable oils.     

Effect of α-tocopherol on the volatile thermal decomposition products of methyl linoleate hydroperoxides

α-Tocopherol and 1,4-cyclohexadiene were tested for their effect on the thermal decomposition of methyl linoleate hydroperoxide isomers. The volatiles generated by thermolysis in the injector port of a gas chromatograph at 180°C were analyzed by capillary gas chromatography. In the presence of either α-tocopherol or 1,4-cyclohexadiene, which are effective donors of hydrogen by radical abstraction, volatile formation decreased in all tests, and significant shifts were observed in the relative distribution of products in certain hydroperoxide samples. When an isomeric mixture of methyl linoleate hydroperoxides (cis, trans andtrans, trans 9- and 13-hydroperoxides) was decomposed by heat, the presence of α-tocopherol and 1,4-cyclohexadiene caused the relative amounts of pentane and methyl octanoate to decrease and hexanal and methyl 9-oxononanoate to increase. A similar effect of α-tocopherol was observed on the distribution of volatiles formed from a mixture of thetrans,trans 9- and 13-hydroperoxides. This effect of α-tocopherol was, however, insignificant with purecis,trans 13-hydroperoxide of methyl linoleate. The decrease in total volatiles with the hydrogen donor compounds, α-tocopherol and 1,4-cyclohexadiene, indicates a suppression of homolytic β-scission of the hydroperoxides, resulting in a change in relative distribution of volatiles. The increase in hexanal and methyl 9-oxononanoate at the expense of pentane and methyl octanoate in the presence of hydrogen donor compounds supports the presence of a heat-catalyzed heterolytic cleavage (also known as Hock cleavage), which seems to mainly affect thetrans,trans isomers of linoleate hydroperoxides.