NMR chemical shift reagents in structural determination of lipid derivatives: II. Methyl petroselinate and methyl oleate

Whentris(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octaneodionato)europium (III)—Eu(fod)₃—forms a complex with a sufficiently basic functional group in a donor molecule, the change in the magnetic environment of protons near the coordination site causes their nuclear magnetic resonance (NMR) signals to shift to different positions. Consequently Eu(fod)₃ and other compounds that similarly affect NMR signals have been designated chemical shift reagents (csr). Because of their ability to shift proton signals, csr substantially increase the amount of structural information that can be obtained from NMR spectroscopy, frequently converting complicated splitting patterns into first-order spectra. Some generally useful experimental and interpretive csr techniques are described here using methyl petroselinate and methyl oleate as examples. Csr studies of methyl petroselinate reveal that the position of the double bond is at C-6, and that there is no chain substitution or branching before C-9. Csr studies of methyl oleate reveal that the position of the double bond is at or beyond C-9, and that there is no chain substitution or branching before C-6. Some suggestions are presented for expanding the amount of structural information that can be obtained by csr studies of unsaturated lipid derivatives. Presented in part at AOCS Meeting, Atlantic City, October, 1971.     

Saturated keto esters from lesquerolic,dimorphecolic, and densipolic acids

Summary The naturally occurring, unsaturated, hydroxy fatty esters, methyl lesquerolate (methyl 14-hydroxy-cis-11-eicosenoate), methyl dimorphecolate (methyl 9-hydroxy-trans, trans-10,12-octadecadienoate), and methyl densipolate (methyl 12-hydroxy-cis,cis-9,15-octadecadienoate) have been converted to the corresponding saturated keto esters by tow routes. The unsaturated esters were subjected to a hydrogenation-dehydrogenation reaction in the presence of Raney nickel or their saturated derivatives were dehydrogenated by copper chromite catalysis. Yields of the keto esters are 65–82% in the nickel-catalyzed reactions, and 71–94% by copper chromite-catalyzed dehydrogenation. In the hydrogenation-dehydrogenation system the order of reactivity is: methyl lesquerolate>methyl dimorphecolate>methyl densipolate. Relationships between structure and reactivity of these compounds, methyl 12-hydroxystearate, and methyl ricinoleate are discussed. W. Utiliz. Res. Dev. Div., ARS, USDA.     

Preparation and properties ofgem-dichlorocyclopropane derivatives of long-chain fatty esters

Methyl oleate (18∶1) and linoleate (18∶2) were readily transformed to the correspondinggem-dichlorocyclopropane derivatives in high yield, using triethylbenzylammonium chloride as the phase-transfer catalyst in the presence of aqueous NaOH and CHCl₃. Reaction of dichlorocarbene with methyl 12-hydroxystearate furnished methyl 12-chlorostearate (49%) and 12-O-formylstearate (19%). The hydroxy group in methyl ricinoleate was protected (O-tetrahydropyran-2′-yl) prior to dichlorocyclopropanation of the ethylenic bond. Removal of the protecting group allowed the hydroxy group to be converted to a chloride,O-acetyl, azido orO-formyl function. Treatment of methyl ricinoleate with thionyl chloride, followed by the reaction with dichlorocarbene gave the corresponding 12-chloro-dichlorocyclopropane derivative. The dichlorocyclopropane derivative of oleic acid was transformed to a C₁₉ allenic fatty acid when treated witht-butyl lithium. However, the remaining dichlorocyclopropane derivatives, containing an additional functional group in the alkyl chain, failed to yield the corresponding allenic derivatives. All derivatives were characterized by a combination of spectroscopic and chromatographic techniques, including infrared,¹H nuclear magnetic resonance (NMR), and¹³C NMR spectroscopy.     

Synthesis and Physical Properties of Potential Biolubricants based on Ricinoleic Acid

The desire to replace petroleum-based materials with environmentally friendly and sustainable alternatives has stimulated the development of vegetable oil-based materials as biolubricants. Our studies have focused on molecules that might be produced by biosynthesis of genetically altered oilseed plants with limited post-harvest modification. Various ricinoleate and 12-hydroxystearate esters and estolides were synthesized and their melting points and viscosities were documented. The antifriction and antiwear properties of some esters were evaluated with a microtribometer. The purities of all the products were >98–99% by gas chromatography. Some of these compounds showed melting points, viscosities, and lubricity suitable for uses as biolubricants. Various ricinoleate esters acylated at the 12 positions with short-chain acids were particularly promising.     

The GLC and TLC resolution of diastereoisomeric polyhydroxystearates and assignment of configurations

Trifluoroacetate (TFA) derivatives of methyl 12-hydroxystearate, methyl ricinoleate, five positional isomers of methylthreo- anderythro-dihydroxystearate, four diastereoisomeric methyl 9,10–12-trihydroxystearates, and four racemic diastereoisomeric methyl 9,10–12,13-tetrahydroxystearates were prepared and analyzed by gas-liquid chromatography (GLC). The isomericthreo- anderythro-dihydroxystearates that had not previously been resolved by GLC were separated. Excellent resolution of the diastereoisomeric pairs of methylthreo- anderythro-9,10–12-triand and methylerythro, erythro- andthreo, threo-9,10–12,13-tetrahydroxystearates was obtained by GLC of their TFA derivatives. Analyses of these high-molecular-weight compounds were carried out on polar and nonpolar packed columns used routinely for methyl ester analysis. The various methyl mono-, di-, tri-, and tetrahydroxysterate esters were also analyzed by thin-layer chromatography (TLC) on Silica Gel G adsorbent layers and on Silica Gel G impregnated with sodium arsenite. Relative and absolute configurations were assigned to the various diastereoisomeric tri- and tetrahydroxysterates based on information obtained from GLC, TLC, synthetic ratios, and molecular-models. A micro hydroxylation method that gives quantitative yields ofthreo- anderythro-dihydroxy acids from various concentrations of C₁₈ monoene geometrical isomers was developed. Subsequent GLC analysis of the isomeric methyl dihydroxy TFA derivatives allows the quantitative determination of double-bond configuration on small samples without expensive or specialized equipment. Presented at the AOCS Meeting, Los Angeles, April 1966. under appointment from Oak Ridge Associated Universities. An operating unit of the Oak Ridge Associated Universities, under contract with the US Atomic Energy Commission.     

Conversion of methyl ricinoleate to methyl 12-ketostearate with Raney nickel

A convenient laboratory preparation of methyl 12-ketostearate is described. Methyl ricinoleate is converted to methyl 12-ketostearate in 70–75% yield by Raney nickel. The type and quantity of Raney nickel have a marked influence on the yield as well as on the time and temp required for the conversion. The reaction is not a direct isomerization as previously assumed but appears to be a two-step process. Methyl ricinoleate is hydrogenated rapidly to methyl 12-hydroxystearate which is then dehydrogenated slowly to the product. Hydrogenolysis of the alcohol function is a competing reaction which is minimized by the proper choice of reaction conditions. A laboratory of the W. Utiliz. Res. & Dev. Div., ARS, USDA.     

Analyses of lipids and oxidation products by partition chromatography: Hydroxy fatty acids and esters

A liquid-partition chromatographic procedure was used to separate hydroxy fatty acids, their methyl esters, and reduced fatty ester hydroperoxides. Mixtures of methyl stearate, mono- and dihydroxystearate, and mixtures of the corresponding free fatty acids were easily separated. Chromatographic determinations for ricinoleate in castor oils compared favorably with the chemical and infrared analyses. The chromatographic procedure was used to separate hydroxy fatty acids inDimorphotheca andStrophanthus seed oils. The methyl ester of dimorphecolic acid, the principal hydroxy fatty ester ofDimorphotheca oil, behaved like reduced methyl linoleate hydroperoxide and showed a polarity intermediate between methyl 12-hydroxystearate and methyl 9,10-dihydroxystearate. The 9-hydroxy-12-octadecenoic ester ofStrophanthus oil had a larger retention volume than methyl ous hydroxy fatty esters isolated chromatographically. The diene content of the reduced hydroperoxides agrees well with values reported in the literature (1,5,16). The diene content of the chromatographed methyl dimorphecolate is higher than reported by Smithet al. (20) for their preparations but agrees well with the value reported by Chipault and Hawkins (6) for puretrans-trans conjugated methyl linoleate. The extinction coefficient of methyl 12-hydroxystearate at 2.8 μ is higher than that reported for ricinoleate and the absorption band is much sharper. Because of these two conditions no association of the hydroxyl groups is indicated. These results also confirm the purity of the hydroxy fatty esters obtained by LPC. This method has been a valuable adjunct to the study of various oxygen-containing fatty acid and esters and was used to characterize the hydroxy esters obtained from the hydrogenation of methyl linolenate hydroperoxides (9). This work offers a basis for the development of analytical methods to determine the hydroxy and other polar acid content of fatty glycerides and their derivatives.     

Determination of Oxidation of Methyl Ricinoleates

Ricinoleate esters have desirable properties as biolubricants, but their oxidative stability has been questioned. To systematically study the stability of ricinoleate ester and its derivative, methyl esters of castor oil were prepared and methyl ricinoleate was isolated by solvent partitioning. Methyl 12‐acetyl ricinoleate was synthesized from the methyl ricinoleate by interesterification with excess methyl acetate and was then purified by solvent partitioning. The rates of oxidation of methyl linoleate, methyl oleate and the two ricinoleate esters were measured by oxidation of lipid dispersed on glass beads under three temperatures (40–80 °C). The relative amounts of the unoxidized methyl esters were determined periodically by gas chromatography, and the peroxide value of the oils was also determined. The oxidation rates were determined as the peroxide value increase rate, as well as the ester disappearance rate, and the stability of the various esters was compared. Overall, methyl ricinoleate was much more oxidatively stable than methyl oleate at mildly elevated temperatures, and the acetylation of the hydroxyl group on the 12th carbon decreased the stability.     

Analysis of the dark-colored impurities in sulfonated fatty acid methyl ester

A fractionally distilled C₁₄−C₁₆ fatty acid methyl ester, derived from palm oil, was sulfonated with gaseous SO₃ in a falling film reactor to form an α-sulfo fatty acid methyl ester (α-SF; unbleached and unneutralized form). The included dark-colored impurities were then separated from α-SF as a diethyl ether-insoluble matter. After purification by thin-layer chromatography, the colored species were analyzed by ion-exchange chromatography, gel-permeation chromatography, and nuclear magnetic resonance spectrometry. These data suggested that the colored species were polysulfonated compounds with conjugated double bonds. Minor components in the raw fatty acid methyl ester, found by gas chromatography/mass spectrometry, were spiked into the purified methyl palmitate and then sulfonated. The unsaturated methyl ester and hydroxy ester showed the worst color results. The methyl oleate and methyl 12-hydroxystearate were then sulfonated and analyzed. Deep black products were obtained, which showed the same properties as the colored species in α-SF. It was concluded that low levels of unsaturated fatty acid methyl esters and hydroxy esters in the fatty acid methyl ester are the main causes of the coloring.     

Preparation of Sebacic Acid via Alkali Fusion of Castor Oil and its Several Derivatives

Castor oil and its three derivatives including methyl ricinoleate, sodium ricinoleate and ricinoleic acid were used as the raw material for alkali fusion to prepare sebacic acid. The reaction parameters including catalyst, ratio of oleochemicals/NaOH, reaction time and reaction temperature were optimized. It was found that Pb₃O₄ (1%) showed the best catalytic performance, and 553 K was considered as the most suitable reaction temperature. The oleochemicals/NaOH ratios of 15:14, 15:14, 15:12 and 15:14 were determined as the optimal ratio for alkali fusion of castor oil, methyl ricinoleate, sodium ricinoleate and ricinoleic acid, respectively. In addition, the optimal reaction time of alkali fusion of castor oil was 5 hours, and that of its derivatives was 3 hours. The maximum yield in sebacic acid of 68.8%, 77.7%, 80.1%, 78.6% can be obtained by using castor oil, methyl ricinoleate, sodium ricinoleate and ricinoleic acid as the raw material, respectively. High purity of sebacic acid was confirmed by GC and melting point analysis. ICP-OES results illustrated that the content of Pb in sebcic acid was less than 1 mg kg⁻¹. Separating glycerol from castor oil was beneficial for alkali fusion, by which, the yield of sebacic acid was increased of approximately 10%, and the reaction time was reduced from 5 to 3 hours. This study provided guiding significance for the future industrial production of sebacic acid.     

Synthesis ofGEM-dibromocyclopropanoid fatty esters using phenyl (tribromomethyl) mercury

Thegem-dibromocyclopropanation of naturally occurring unsaturated hydroxy fatty esters, methyl ricinoleate (I) and methyl isoricinoleate (II), has been undertaken to provide compounds which might have potential utility. Phenyl(tribromomethyl)mercury reacts only with the carbon-carbon double bond of each substrate, leaving the OH group intact. The product, methyl 9,10-dibromomethylene-12-hydroxyoctadecanoate (III) and methyl 12,13-dibromomethylene-9-hydroxyoctadecanoate (IV), obtained from I and II, respectively, were characterized by elemental, infrared (IR) and nuclear magnetic resonance (NMR) analyses.     

Diffusion coefficients of C18 unsaturated fatty acid methyl esters in supercritical carbon dioxide containing 10% mole fraction ethanol as modifier

To determine the molecular diffusion coefficients of C18 unsaturated fatty acid methyl esters in supercritical carbon dioxide (scCO₂) containing 10 mol% ethanol as a modifier, four methyl esters of C18 fatty acids, i.e., methyl oleate, methyl ricinoleate, methyl linoleate and methyl linolenate were selected as the typical solutes. The diffusion coefficients were measured at temperatures from 313.15 to 333.15 K and pressures from 15 to 27 MPa using the Taylor–Aris chromatographic peak broadening (CPB) technique. The influences of temperature, pressure, density and viscosity of the solvent mixture on the diffusion coefficients were examined. The results show that methyl oleate always diffuses faster than methyl ricinoleate at the same operating condition. Moreover, the D₁₂ values in ethanol-modified scCO₂ decrease with the increase of the number of C-C double bonds in C18-methyl ester, which is consistent with the trend reported in pure scCO₂. The diffusivity data are compared with the estimation of eleven predictive models. The modified Wilke–Chang equation is the best purely predictive model and the free volume model of Dymond with two adjustable parameter gives the least errors with average absolute deviations lower than 2.5%.     

Catalytic dehydrogenation of methyl 12-hydroxystearate to methyl 12-ketostearate

Economically attractive catalytic methods for the conversion of methyl 12-hydroxystearate to methyl 12-ketostearate have been developed on a laboratory scale. With 1.0 wt% copper chromite catalyst, pure methyl 12-hydroxystearate is dehydrogenated at 180–263C to methyl 12-ketostearate in 99% yield and 97% purity. Commercial methyl 12-hydroxystearate required 3% catalyst but gave a 98% conversion to product and 100% crude yield. Presented at the AOCs Meeting in Cincinnati, October, 1965. W. Utiliz. Res. Dev. Div., ARS, USDA.     

Effects of acyl chain hydroxyl groups and increasing unsaturation on the autoxidation of cholesteryl esters in aqueous colloidal suspension

Aeration in aqueous collodial suspension results in oxidation of the sterol moiety of cholesteryl linoleate, linolenate, arachidonate and O-tetrahydropyranylricinoleate. The extent of such oxidation is different for each compound, and the O-tetrahydropyranylricinoleate ester oxidizes relatively slowly. Cholesteryl 12-hydroxystearate and ricinoleate are resistant to this oxidation. Factors that may contribute to autoxidative susceptibility of cholesteryl esters are discussed briefly. Presented at the AOCS Meeting, Atlantic City, October 1971.     

Thermal Decomposition of Some Acetoxyoctadecenoates and Acetoxyoctadecadienoates

The rates of the thermal decomposition (dehydroacetoxylation) and the activation energies of the following three samples have been determined: A) acetylated methyl ricinoleate; B) methyl ester mixture of vicinally unsaturated acetoxyoctadecenoates prepared by reacting methyl oleate and mercuric acetate in acetic acid; C) methyl ester mixture of vicinally unsaturated acetoxyoctadecadienoates prepared by reacting methyl linoleate and mercuric acetate in acetic acid. The thermal decomposition is shown to be a first-order reaction.     

Preparation and physical properties of some C18 unsaturated fatty esters containingL-amino acid residues and methyl esters ofN-stearoylamino acids

A series of five C₁₈ unsaturated fatty esters (1–5) containing anL-amino acid residue (glycine, alanine, valine, leucine, phenylalanine) was prepared from methyl 12-amino-9-cis-octadecenoate and five methylN-stearoyl-amino acid ester derivatives (6–10) from stearoyl chloride and the sameL-amino acids. The infrared analysis of compounds 1–5 showed characteristic absorption bands at 3300 and 1665 cm⁻¹ for the amino and amido functions, while the amido function in compounds 6–10 gave absorption bands at 3300 and 1680 cm⁻¹. The position of the amido group (peptide linkage) in compounds 1–5 was readily determined by mass spectral analysis.¹H nuclear magnetic resonance (NMR) analysis showed a doublet at 7.15 ppm for the amide proton (NHCO) in compounds 1–5, while in compounds 6–10 the amide proton signal appeared at 6.0 ppm. In¹³C NMR, the amido carbonyl resonance appeared at 171–175 ppm. In compounds 1–5 the effect of the amide function on the ethylene carbon (C-9) gave a signal at 124.1–125.3 ppm, while the remaining ethylenic carbon appeared at 131.9–132.5 ppm. The methine carbon (C-12) of the alkenyl chain was shifted to 48.4–49.8 ppm, and the assignment of the various carbon nuclei in the amino acid residue was readily achieved. In the methylN-stearoylamino acid ester derivatives (6–10), the methine carbon adjacent to the amido system and alpha to the carbomethoxy group appeared between 41.2–57.0 ppm depending on the type of alkyl group present in the amino acid moiety. The phenyl system in compounds 5 and 10 was confirmed by the¹³C signals in the 127–136 ppm range and by the proton signals in the 7.0–7.27 ppm region.     

Analysis of Lignins as Propionate Derivatives by NMR Spectroscopy

Lignin propionates were prepared by treating lignin samples with propionic anhydride in pyridine solution and the derivatives obtained were examined by NMR spectroscopic methods. ¹H NMR spectroscopy of lignin propionates offers a possibility to determine the number of hydroxyl groups based on the methyl proton signal (δ ～ 1.2) as well as the methylene proton signal (δ ～ 2.5) of the propionate groups. It was found to be advantageous to use propionate derivatives for the analysis of hydroxyl groups in lignin products exhibiting signals (from >CH- and -CH₂- groups) that interfere with the acetate group signals. Analysis of ester groups by ¹³C NMR spectroscopy gave practically the same results as those earlier reported for acetate derivatives.     

Fatty acids: Part 26.1 partial synthesis of C18 mono-and dimethyl furanoid fatty esters

Dimethylene interrupted dioxo derivatives of 10, 13-epoxy-11-methylocadeca-10, 12-dienoate (from latex) and 9 (10), 12 (13)-epoxyoctadeca-9 (10), 11 (12)-dienoates (from linoleate) were successfully methylated at the methylene carbons located between the two oxo groups using methyl iodide and KOH in DMSO. The resulting dimethyldioxo derivatives were cyclodehydrated to furnish methyl 10, 13-epoxy-11, 12-dimethyloctadeca-10, 12-dienoate and a mixture of methyl 9(10), 12(13)-epoxy-10(11), 11(12)-dimethyl-octadeca-9(10), 11(12)-dienoate. Similarly, methylation at C-11 of methyl 12-oxo-octadec-as-9-enoate (from methyl ricinoleate) gave methyl 9, 12-epoxy, 11-methyloctadeca-9, 11-dienoate on cyclodehydration. 1 For part 25, see reference 1.     

Reactions of Methyl Ricinoleate and Methyl Acetylricinoleate with Mercuric Acetate in Hot Acetic Acid

In the reaction of methyl ricinoleate and mercuric acetate in glacial acetic acid at 115°C a acetoxy-acetoxymercuri complex ist formed which decomposes with the progress of the reaction and disappears completely to give non-mercurial products. The latter contain 9-acetoxy-12-hydroxy-10-, 10-acetoxy-12-hydroxy-8- and 12-hydroxy-9-octadecenoates together with a relatively small amount of 10,12-octadecadienoate. The hydroxyl group in these compounds is partially acetylated, and the ethylenic bond is partially trans-isomerized. Methyl ricinoleate as well as methyl acetylricinoleate are less readily acetoxylated as compared with oleate and linoleate, indicating the steric hindrance exerted by the hydroxyl and acetoxyl groups to the reaction of mercuric acetate and the ethylenic bond located near these groups.     

Ultrasound-assisted synthesis of santalbic acid and a study of triacylglycerol species inSantalum album (Linn.) seed oil

Methyl ricinoleate (1) was treated with bromine and the dibromo derivative (2) was reacted with ethanolic KOH under ultrasonic irradiation to give 12-hydroxy-octadec-9-ynoic acid upon acidification with dil. HCl. The latter compound was methylated with BF₃/methanol to give methyl 12-hydroxy-octadec-9-ynoate (3). Compound3 was treated with methanesulfonyl chloride in the presence of triethylamine in CH₂Cl₂ to give methyl 12-mesyloxy-octadec-9-ynoate (4). Reaction of methyl 12-mesyloxy-octadec-9-ynoate with aqueous KOH under ultrasonic irradiation (20 kHz) gave (11E)-octadecen-9-ynoic acid (5, santalbic acid, 40%) and (11Z)-octadecen-9-ynoic acid (6, 60%) on acidification with dil. HCl. These isomers were separated by urea fractionation. The¹³C nuclear magnetic resonance (NMR) spectroscopic properties of the methyl ester and the triacylglycerol (TAG) esters of these enynoic fatty acid isomers were studied. The carbon shifts of the unsaturated carbon nuclei of the methyl ester of theE-isomer were unambiguously assigned as 88.547 (C-9), 79.287 (C-10), 109.760 (C-11), and 143.450 (C-12) ppm while the unsaturated carbon shifts of the (Z)-enynoate isomer appeared at 94.277 (C-9), 77.561 (C-10), 109.297 (C-11), and 142.668 (C-12) ppm. In the¹³C NMR spectral analysis of the TAG molecules of type AAA containing either the (Z)-or (E)-enyne fatty acid, the C-1 to C-6 carbon atoms on the α- and β-acyl positions were differentiated. The unsaturated carbon atoms in the α- and β-acyl chains were also resolved into two signals except that of the C-11 olefinic carbon. Sandal (Santalum album) wood seed oil (a source of santalbic acid) was separated by silica chromatography into three fractions. The least polar fraction (7.2 wt%) contained TAG which had a random distribution of saturated and unsaturated fatty acids, of which oleic acid (69%) was the predominant component. The second fraction (3.8 wt%) contained santalbic acid (58%) and oleic acid (28%) together with some other normal fatty acids. Santalbic acid in this fraction was found in both the α- and β-acyl positions of the glycerol “backbone”. The most polar fraction (89 wt%) consisted of TAG containing santalbic acid only. The distribution of the various fatty acids on the glycerol “backbone” was supported by the results from the¹³C NMR spectroscopic analysis.