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.     

Deuteration of methylcis-9,cis-15-octadecadienoate with nickel catalyst

Methylcis-9,cis-15-octadecadienoate was partially deuterated with nickel catalyst, and the product was separated into saturate, monoene and diene fractions. Monoenes were separated intotrans andcis fractions, and dienes intotrans,trans, cis,trans andcis,cis fractions. Monoene isomers with double bonds at the 9 and 15 positions predominated in bothcis- andtrans-monoene fractions. Considerable amounts of isomers with double bonds situated on either side of the original 9 and 15 positions were found in thetrans-monoene fraction. Diene was extensively isomerized to positional and geometrical isomers, and deuterium was incorporated into these isomers. Double bond migration was greatest intrans,trans-dienes and smallest incis,cis-dienes. The amount of deuterium in the dienes was proportional to the extent of isomerization experienced by the dienes. ARS, USDA.     

Selective homogeneous hydrogenation of triunsaturated fats catalyzed by tricarbonyl chromium complexes

The need for a selective catalyst to hydrogenate linolenate in soybean oil has prompted our continuing study of various model triunsaturated fats. Hydrogenation of methylβ-eleostearate (methyltrans,trans,trans-9,11,13-octadecatrienoate) with Cr(CO)₃ complexes yielded diene products expected from 1,4-addition (trans-9,cis-12- andcis-10,trans-13-octadecadienoates). Withα-eleostearate (cis,trans,trans-9,11,13-octadecatrienoate), stereoselective 1,4-reduction of thetrans,trans-diene portion yielded linoleate (cis,cis-9,12-octadecadienoate). However,cis,trans-1,4-dienes were also formed from the apparent isomerization ofα- toβ-eleostearate. Hydrogenation of methyl linolenate (methylcis,cis,cis-9,12,15-octadecatrienoate) produced a mixture of isomeric dienes and monoenes attributed to conjugation occurring as an intermediate step. The hydrogenation ofα-eleostearin in tung oil was more stereoselective in forming thecis,cis-diene than the corresponding methyl ester. Hydrogenation of linseed oil yielded a mixture of dienes and monoenes containing 7%trans unsaturation. We have suggested how the mechanism of stereoselective hydrogenation with Cr(CO)₃ catalysts can be applied to the problem of selective hydrogenation of linolenate in soybean oil. No. Market. Nutr. Res. Div., ARS, USDA.     

Concurrent oxidation of accumulated hydroperoxides in the autoxidation of methyl linoleate

Summary 1. Kinetic studies showed that concurrent oxidation of preformed hydroperoxides may be expected to take place at all stages of the autoxidation of methyl linoleate. The rate of oxidation relative to the rate of autoxidation of unoxidized ester is determined chiefly by the extent of the accumulation of hydroperoxides. 2. Infrared spectral analysis of hydroperoxides oxidized to various degrees indicated thattrans, trans diene conjugation and isolatedtrans double bonds produced in the autoxidation of methyl linoleate are related to the concurrent oxidation of the accumulated hydroperoxides. 3. The low absorptivity observed for diene conjugation, compared to that which may be expected for the exclusive production ofcis, trans diene conjugated hydroperoxide isomers during the autoxidation of methyl linoleate is attributed to the concurrent oxidation of accumulated hydroperoxides. 4. The effect of antioxidants in giving a well-defined induction period in the oxidation of hydroperoxides isolated from autoxidized methyl linoleate indicated that the oxidation proceeds by a chain reaction. 5. The primary reaction products of the oxidation of hydroperoxides isolated from autoxidized methyl linoleate were found to be polymers formed in a sequence of reaction involving the diene conjugation. 6. Studies on the autoxidation of methylcis-9,trans-11-linoleate showed thatcis, trans isomerization of the conjugated diene took place with the concurrent production of isolatedtrans double bonds and loss of diene conjugation. Hormel Institute publication no. 138. Presented before the American Oil Chemists’ Society, Philadelphia, Pa., Oct. 10–12, 1955. This work was supported by a grant from the Hormel Foundation.     

Competitive hydrogenation rates of isomeric methyl octadecadienoates

Determination of the relative reaction rates of isomeric methyl octadecadienoates is possible by competitive reduction of a mixture containing an inactive diene and a radioactively labeled isomer. The hydrogenation rate of methylcis-9,cis-12-octadecadienoate with platinum and nickel catalysts is compared to the hydrogenation rate of each of several isomers of methyl octadecadienoate, and the relative rate of the competitive hydrogenations is calculated by a digital computer. Methylcis-9,cis-12 linoleate is reduced the most rapidly of all the dienes studied. The relative rates of the positional isomers tend to decrease with the increasing number of methylene groups between the double bonds, except when one of the double bonds is in the more reactive 15 position. Comparison of the geometric isomers shows thattrans,trans diene is hydrogenated at a slower rate thancis,cis linoleate.

     

Stereospecific synthesis ofcis andtrans fatty esters

Theerythro andthreo isomers of methyl 9,10-dihydroxyoctadecanoate and thethreo isomer of methyl 12,13-dihydroxy-cis-9-octadecenoate were converted into methylcis- andtrans-9-octadecenoate and methylcis-9,rans-12-octadecadienoate, respectively, by reaction of the dihydroxy ester with triethyl orthoformate to give the 2-ethoxy-1,3-dioxolane which was thermally decomposed to the unsaturated ester.     

Preparation of 9,15-octadecadienoate isomers1

Linolenic acid was reduced with hydrazine to produce a mixture containing a max of dienoic acids. After methylation this mixture was separated into trienoic, dienoic, monoenoic, and saturated esters by countercurrent distribution (CCD) with acetonitrile and hexane. The dienoic ester was further fractionated by CCD with methanolic silver nitrate and hexane to separate purecis,cis-9,15-octadecadienoate and the equimixture ofcis,cis-9,12- and 12,15-octadecadienoates. Following isomerization of thecis,cis-9,15-octadecadienoate with selenium, the geometric isomers were fractionated by CCD with methanolic silver nitrate and hexane. Puretrans,trans and purecis,cis isomers were isolated, as well as an unresolved mixture ofcis,trans andtrans,cis isomers. The characteristics of these isomers and related compounds are compared as determined by CCD, IR absorption, and capillary gas-liquid chromatography (GLC). Presented at the AOCS meeting in Minneapolis, 1963. A laboratory of the No. Utiliz. Res. and Dev. Div., ARS, USDA.     

Hydrolysis of linoleate geometric isomers byGeotrichum candidum lipase

Lipase (EC 3.1.1.3) from the microorganismGeotrichum candidum preferentially hydrolyzescis-9 18∶1 andcis,cis-9,12 18∶2 from triacylglycerols, largely ignoring all other positional isomers ofcis 18∶1 as well astrans-9 18∶1. To obtain additional information about the specificity of the enzyme, two triacylglycerols were prepared and utilized as substrates. The lipase hydrolyzed 85%cis,cis-9,12 18∶2 and 15%trans,trans-9,12 18∶2 from the triacylglycerol, containing ca. 50% of each acid. From the triacylglycerol containing 46.3%cis,trans-9,12 18∶2 and 53.7%trans,cis-9,12 18∶2, 44.8 and 55.2% of the two acids were hydrolyzed. Therefore the enzyme discriminated against thetrans,trans isomer but not between thecis,trans andtrans,cis isomers. Scientific contribution No. 535, Agricultural Experiment Station, University of Connecticut. ARS, USDA.     

Gas chromatographic analysis of diels-alder adducts of geometrical and positional isomers of conjugated linoleic acid

This research demonstrates the gas chromatographic analysis of the 4-methyl-1,2,4-triazoline-3,5-dione (MTAD) adducts derived from standards of cis,trans-9,11-octadecadienoic acid, trans,trans-9,11-octadecadienoic acid, and cis,cis-9,11-octadecadienoic acid. Methyl cis,trans-9,11-octadecadienoate and methyl trans,trans-9,11-octadecadienoate formed Diels-Alder addition products with MTAD to produce adducts with similar mass spectral fragmentation patterns but different retention times determined by gas chromatography/ mass spectrometry. Methyl cis,cis-9,11-octadecadienoate reacted slowly and produced two adducts with similar fragmentation patterns and different retention times. These results were comparable to those reported for an analogous series of conjugated hexadienes. Based on hexadiene reactions, methyl cis,trans-9,11-octadecadienoate produced a trans adduct as a major product while methyl trans,trans-9,11-octadecadienoate formed a cis adduct. Methyl cis,cis-9,11-octadecadienoate reacted slowly under the conditions used leaving mostly unreacted material. Of the adducts observed from this isomer, a major trans adduct and a minor cis adduct were formed.     

The geometric isomers of conjugated octadecadienoates from dehydrated methyl ricinoleate

The dehydration of methyl ricinoleate by heatingin vacuo in the presence of KHSO₄ resulted in the formation of the following conjugated octadecadienoates expressed as a percentage of the final product:cis, trans (trans, cis), 14.3;cis, cis, 11.2;trans, trans, 7.3. The isomers contained the double bonds predominantly in the 9,11 position but the possible presence of traces of 8,10 and other conjugated isomers is not excluded. Using urea “inclusion” fractionation and low temp crystallization from acetone methyl,cis-9,cis-11-octadecadienoate was isolated. The methyl esters of commercially dehydrated castor oil fatty acids on the other hand, contained the following percentages of conjugated octadecadienoate isomers:cis, trans (trans, cis), 20.3;cis, cis, 8.0;trans, trans, 5.4. From these mixtures conc ofcis, trans (trans, cis)- andtrans, trans-octadecadienoates were prepared by fractional distillation and low temp crystallization. It was found that the conjugated octadecadienoates consisted of mixtures of positional isomers with double bonds mainly in the 8,10 and 9,11 positions with lesser amounts in the 7,9 and 10,12 positions.     

cis andtrans Analysis of fatty esters by gas chromatography: Octadecenoate and octadecadienoate isomers

A gas chromatographic method has been developed for quantitative determination of thecis andtrans percentages in octadecenoate and octadecodienoate esters. To separatecis- andtrans-monoene and diene isomers on a packed GC column, the fatty esters were stereospecifically epoxipized with peracetic acid. A simple and quantitative epoxidation procedure allows thecis- andtrans-epoxyoctadecanoates to be analyzed without prior isolation from the reaction mixture. No positional or geometric isomerization of the double bond occurred during epoxidation. Synthetic mixtures containingcis- andtrans-6,-9 and-12 octadecenoate isomers were completely separated intocis andtrans fractionstrans-15-Octadecenoate was the only isomer investigated that partially interfered in the analysis. Diene mixtures containingrans,trans-, cis,trans- andcis,-cis-9,12-octadecadienoates were also successfully analyzed by gas liquid chromatography (GLC) after epoxidation with peracetic acid. Each diene isomer formed two pairs of diepoxy diastereomers, some of which could be separated. Onecis,cis-diepoxyoctadecanoate diastereomer peak overlapped thecis,trans-diepoxyoctadecanoate peaks. The totalcis,-cis-diepoxyoctadecanoate percentages were calculated by using the ratio of the twocis,cis-diepoxyoctadecanoate diastereomers. Other positional octadecadienoate isomers were also epoxidized and analyzed by GLC. The large number of possible octadecadienoate isomers requires that some preeiminary fractionation be made before GC analysis is practical for diene isomers. Presented at the AOCS Meeting, Atlantic, City, October 1971 N. Market. Nutr. Res. Div., ARS, USDA.     

Partial reduction of α-Eleostearic acid with hydrazine

The following products are formed during partial reduction of α-eleostearic acid with hydrazine:cis,trans-9,11-octadecadienoic andtrans,trans-11,13-octadecadienoic acids;cis-9-,trans-11- andtrans-13-octadecenoic acids; and stearic acid. The double bonds are reduced individually in the conjugated triene and also in the conjugated dienes that are formed. However, the reduction is selective since thetrans-11 double bonds in the conjugated triene is reduced only slightly to yield the isolated 9,13-diene. Thetrans double bond of thecis,trans conjugated diene reduces at a faster rate than thecis bond. No differences were observed in the rate of reduction of thecis-9 andtrans-13 bonds in the triene or of the bonds in thetrans,trans conjugated diene. No. Utiliz. Res. & Dev. Div., ARS, USDA.     

Autoxidation of linoleic acid and behavior of its hydroperoxides with and without tocopherols

The autoxidation of linoleic acid dispersed in an aqueous media and the effect of α-, γ- and δ-tocopherols were studied. The quantitative analysis of the hydroperoxide isomers (13-cis,trans; 13-trans,trans; 9-trans,cis; 9-trans,trans) by direct high-performance liquid chromatography exhibited a prooxidant activity of α-tocopherol at high concentration (3.8% by weight of linoleic acid). On the other hand, α-tocopherol at lower concentrations (0.38 and 0.038%) and γ- and δ-tocopherols at high concentration (3.8%) were antioxidant. Furthermore, the addition of tocopherols modified the distribution of the geometrical isomers. The formation of thetrans,trans hydroperoxide isomers was completely inhibited by the highest concentration of the three tocopherols independently of their antioxidant or prooxidant activity and only delayed by the lower concentrations of α-tocopherol. The addition of tocopherols to hydroperoxide isomers reduced the decomposition rate of these isomers in the order α-tocopherol < γ-tocopherol < δ-tocopherol for thecis,trans hydroperoxide isomer and α-tocopherol ≪ γ-tocopherol ⋍ δ-tocopherol for thetrans,trans hydroperoxide isomer. With these hydroperoxides, as during linoleic acid autoxidation, α-tocopherol was completely oxidized whatever its initial concentration, while γ-tocopherol underwent partial oxidation and δ-tocopherol was practically unchanged.     

Silver nitrate-high performance liquid chromatography of fatty methyl esters

Long chain fatty methyl esters have been separated by high performance liquid chromatography on the basis of number, position, and geometric configuration of double bonds with a silver nitrate-silicic acid column and benzene solvent. Saturated esters are eluted first, followed by methyl elaidate and then methyl oleate. Geometric isomers of methyl 9,12-octadecadienoate and of methyl 9,15-octadecadienoate are also well separated. Methyl linolenate is retained strongly on the column and its elution has not been observed, but thetrans, trans, trans andtrans, cis, trans isomers are separated.     

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.     

A simple method of preparation of methyl trans-10,cis-12- and cis-9,trans-11-octadecadienoates from methyl linoleate

Pure conjugated isomers of linoleic acid were prepared on a large scale by alkali-isomerization of purified methyl linoleate. The methyl esters of alkali-isomerized linoleic acid contained mainly the methyl cis-9,trans-11- and trans-10,cis-12-octadecadienoates (44 and 47%, respectively). These two isomers were then separated and purified by a series of low-temperature crystallizations from acetone. The isomeric purity obtained for the cis-9,trans-11-octadecadienoate isomer was >90% and that of the trans-10,cis-12-octadecadienoate isomer was 89 to 97%. The isolated yield of the two isomers corresponded to 18 and 25.7%, respectively, of the starting material. The structure of the two isomers was confirmed using partial hydrazine reduction, silver nitrate-thin-layer chromatography of the resulting monoenes and gas chromatography coupled with mass spectrometry of the 4,4-dimethyloxazoline derivatives. Fourier transform infrared spectroscopy of the monoenes gave the confirmation of the geometry of each double bond.     

Separation of saturated,unsaturated, and acetylenic fatty acid isomers by silver resin chromatography

A column packed with silver-saturated ion exchange resin (Amberlyst XN 1010) was found to have lipid separation capabilities superior to Amberlyst XN 1005 and similar to Amberlite XE 284. The separation of unsaturated fatty methyl ester isomers by silver resin chromatography using methanol as the eluting solvent has been extended to mixtures containing polyunsaturate and acetylenic fatty esters. Separations are possible on the basis of both total number of double bonds and the geometric configuration. Mixtures containing saturates, elaidate, oleate, linoleate, and linolenate can be separated, but 10% 1-hexene must be added to the methanol to elute the linolenate. Mixtures containingtrans,trans-;trans,cis-; andcis,cis-octadecadienoate isomers have also been separated, and partial resolution ofcis-9,cis-12- andcis-2,cis-15-octadecadienoate isomers was obtained. Sterolate, a monounsaturated acetylenic fatty ester was eluted at the same time as oleate. Crepenynate (cis-9-octadecen-12-ynoate) can be separated from linoleate but not fromcis,trans-octadecadienoate. Employed at the Northern Regional Research Center under a USDA cooperative education program with Purdue University.     

Hydrogenation of alkali-conjugated linoleate: Isomeric monoene profiles obtained with nickel,palladium and platinum catalysts

Alkali-conjugated linoleate (cis-9,trans-11- andtrans-10,cis-12-octadecadienoate) was hydrogenated with nickel, palladium and platinum catalysts. Thetrans andcis monoenes formed during reduction were isolated, and their double bond distribution was determined by reductive ozonolysis and gas liquid chromatography. About 44–69% of the monoenes were composed of δ¹⁰ and δ¹¹ trans monoene isomers, whereas the δ⁹ and δ¹² cis monoenes amounted to 20–26%. With nickel catalyst, composition of monoene isomers remained the same, even when the hydrogenation temperature was increased. The monoene isomer profiles between nickel and palladium catalysts were indistinguishable. Isomerization of monoenes with platinum catalyst was suppressed at 80 psi. The position of the double bonds in unreacted conjugated diene was always retained, except with nickel at both temperatures and with platinum at 150 C when a slight migration occurred. Geometrical isomerization totrans,trans-conjugated diene was observed in the unreacted diene with nickel (ca. 15% of diene) at both 100 C and 195 C, and with platinum (ca. 7% of diene) at 150 C. ARS, USDA.     

Linoleic acid isomers in heat treated sunflower oils

Heat treatment of sunflower oil resulted in the formation of linoleic geometrical and positional isomers. These isomers were isolated using a combination of column chromatography, urea fractionation, high performance liquid chromatography (HPLC) on a C18 reverse phase column and silver nitrate thin layer chromatography (TLC). Each component was submitted to hydrazine reduction and the resulting monoenes to AgNO₃-TLC. The resultingcis andtrans fractions were submitted to ozonolysis in BF₃-MeOH in order to determine the position of the ethylenic bonds. The major isomers were thecis, trans andtrans, cis 18∶2 Δ9, 12, thetrans, trans 18∶2 Δ9, 12 and somecis, trans, trans, cis andtrans, trans 18∶2 conjugated dienes. Thecis, trans andtrans, cis conjugated dienes were the Δ9, 11, Δ10, 12, Δ11, 13 and Δ12, 14 while thetrans, trans isomers were the Δ9, 11, Δ10, 12 and Δ11, 13. These C18∶2 isomers also were detected in oils collected from restaurants and market vendors. Presented in part at the AOCS/JOCS annual meeting in Honolulu, Hawaii, in May 1986.     

Hydrogen sulfide adducts of methyltrans,trans- 9,11-octadecadienoate

Hydrogen sulfide was added to methyltrans, trans- 9,11-octadecadienoate in benzene solution at 25 C with ultraviolet radiation. GC-MS and GLC analysis of the reaction product showed the presence of methyl oleate, methyl stearate, geometric isomers of methyl 9,11-octadecadienoate, methyl 9,12-epoxy-octadeca-9, 11-dienoate, an unknown compound with an apparent molecular weight of 306, methyl 8-(2′,5′-hexylthienyl) octanoate, an unidentified sul-fur ] containing C₁₈ ester with an apparent molecular weight of 326, methyl 9,12-epithiostearate, an adduct of methyltrans,trans- 9,11-octadecadienoate and ben-zene [bicyclo (4.4.0)-deca-2,5,7-triene-l-(Ω-carboxy-methyl heptyl)-4 hexyl] and a probable mixture of methyl 9,11-epidithiostearate, methyl 9,12-epidithio-stearate, and methyl 10,12-epidithiostearate.