13C-NMR SPECTROSCOPY OF FATTY ACIDS AND DERIVATIVES
Epoxides and Acyclic Ethers
1,2-Epoxides
Much basic information on epoxy acids is available in the paper by Bascetta and Gunstone. Chemical shifts are given for the cis and trans 3,4- to 17,18-epoxyoctadecanoates, for seven cis-epoxy octadecenoates, for three 1,2-epoxyalkanes, and four epoxy cycloalkanes. In general the epoxide carbon atoms have chemical shifts of around 56.9 (cis) and 58.5 ppm (trans), but these may appear as two different signals through the long-range influence of a carboxyl group or the omega methyl function. The effect of the epoxide function on the chemical shifts or nearby carbon atoms has been given the values shown in Table 1 for the approximate chemical shifts and the difference from sp2 carbon atoms not influenced by other functional groups (29.3 ppm):
Table 1 |
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| α (ppm) | β (ppm) | γ (ppm) | |
|---|---|---|---|
| Cis | 27.6 (-1.71) | 26.4 (-2.93) | 28.9 (-0.38) |
| Trans | 31.9 (+2.57) | 25.8 (-3.50) | 29.1 (-0.23) |
The values for α carbon atoms are significantly larger than the corresponding values for olefinic compounds (< 0.1) and help to identify the epoxide function.
Table 2 |
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| A | B | C | D | |
|---|---|---|---|---|
| 1 | 173.69 | 173.84 | 173.81 | 174.2 |
| 2 | 33.63 | 33.82 | 33.75 | 34.1 |
| 3 | 24.54 | 24.68 | 24.63 | 24.9 |
| 4 | 28.85 | 28.98 | 28.80 | * |
| 5 | 29.18 | 28.98 | 28.80 | * |
| 6 | 28.85 | 28.98 | 28.80 | * |
| 7 | 26.25 | 25.80 | 29.20 | 26.5 |
| 8 | 27.48 | 31.89 | 25.99 | * |
| 9 | 56.67 | 58.44 | 132.19 | 56.3, 57.2 |
| 10 | 56.67 | 58.54 | 123.78 | 54.5, 55.3 |
| 11 | 27.48 | 31.89 | 25.99 | 35.0, 35.3 |
| 12 | 26.25 | 25.80 | 56.79 | 70.0, 70.8 |
| 13 | 28.85 | 28.98 | 56.16 | 37.5, 37.9 |
| 14 | 29.18 | 29.26 | 27.49 | 25.6 |
| 15 | 29.18 | 29.26 | 27.11 | * |
| 16 | 31.51 | 31.63 | 31.46 | 31.9 |
| 17 | 22.29 | 22.42 | 22.27 | 22.6 |
| 18 | 13.66 | 13.81 | 13.63 | 14.2 |
| * also unassigned peaks at 28.0, 28.1, 29.0, 29.2, 29.3 This material will be a mixture of two diastereoisomeric products, hence the double signals obtained for some of the carbon atoms. | ||||
| A, methyl cis-9,10-epoxystearate [Bascetta]. | ||||
| B, methyl trans-9,10-epoxystearate [Bascetta]. | ||||
| C, methyl cis-12,13-epoxyoleate (vernolate) and other epoxyoctadecenoates [Gunstone]. | ||||
| D, methyl cis-9,10-epoxy-12-hydroxystearate (from ricinoleate) [Lie Ken Jie - the paper includes information on the 12-oxo and 13-hydroxy derivatives]. | ||||
Alaiz et al. have given some chemical shifts for the three monoepoxides produced from ethyl linoleate and Frykman et al. provide incompletely assigned shifts for cis 5,6-epoxyeicosanoic acid.
From a study of epoxidised oils, Gunstone obtained data for mono-, di-, and triepoxystearates. These were based on the products from palm super olein rich in oleic acid, soybean oil rich in linoleic acid, and linseed oil rich in linolenic acid. The di- and tri-epoxides exist in several stereochemical forms, each with its own chemical shifts. For example one 9,10;12,13-diepoxide has epoxide shifts at 57.19, 57.13, 56.71, and 56.64 ppm and the other stereoisomer at 56.99, 56.93, 54.33, and 54.17.
The cis-epoxide functions at 9,10 from oleate, 12,13 from linoleate, and 15,16 from linolenate have different influences on the ω1-ω3 chemical shifts as set out in the following Table (Gunstone).
Table 3. |
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| Epoxide | ω-1 | ω-2 | ω-3 |
|---|---|---|---|
| 9,10 | 14.12 | 22.68 | 31.88 |
| 9,10;12,13 | 14.00 | 22.58 | 31.68, 31.47 |
| 9,10,12,13;15,16 | 10.61, 10.49 | 22.57 | |
Furanoid Acids/Esters
Furanoid esters occur naturally, they are readily made from ricinoleate, and many have been synthesised (see Lie Ken Jie); their 13C-NMR spectra have been fully reported. The substituted furan ring has characteristic chemical shifts (ppm) and the ring also affects sp2 carbon atoms some distance along the alkyl chain.
The original paper contains data for the 4-7 through 15-18 C18 furanoids but only the shifts for the most common 9,12 methyl ester are cited here:
174.30 ppm (C1), 34.15 (C2), 24.99 (C3), 29.05 (C4), 28.95 (C5), 29.05 (C6), 28.14 (C7), 28.14 (C8),154.58 (C9), 104.93 (C10), 104.93 (C11), 154.77 (C12), 28.14 (C13), 28.14 (C14), 29.05 (C15), 31.68 (C16), 22.62 (C17), 14.07 (C18).
Chemical shifts (ppm) for a methyl substituted furanoid fatty acid are reported by Lie Ken Jie.
Acyclic Ethers
Information on acyclic ethers is limited. Isbell et al. have described products of the type -
CH3(CH2)14CHOR(CH2)3COOR
- resulting from the reaction of the C20 δ-lactone with methanol, butanol, 2-ethylhexanol, decanol, and oleyl alcohol. Chemical shifts are listed but only those adjacent to an oxygen atom are assigned. Singh et al. have reacted methyl vernolate (12,13-epoxyoleate) with mono-, di-, tri-, and tetra-ethylene glycol to give a range of products that contain ether groups in addition to 1,2- or 1,4-epoxides, and they have assigned chemical shifts.
References
- Alaiz, M., Maza, M.P., Zamora, R., Hidalgo, F.J., Millan, F. and Vioque, E. Epoxidation of (Z)-9-(Z)-12-(Z)-15-octadecatrienoate with m-chloroperbenzoic acid. Chem. Phys. Lipids, 49, 221-224 1989,.
- Bascetta, E. and Gunstone, F.D. 13C chemical shifts of long-chain epoxides, alcohols, and hydroperoxides. Chem. Phys. Lipids, 36, 253-261 (1985).
- Frykman, H.B. and Isbell, T.A. Synthesis of 6-hydroxy δ-lactones and 5,6-dihydroxy eicosanoic/docosanoic acids from meadowfoam fatty acids via a lipase-mediated self-epoxidation. J. Am. Oil Chem. Soc., 74, 719-722 (1997).
- Gunstone, F.D. The study of natural epoxy oils and epoxidised vegetable oils by 13C nuclear magnetic resonance spectroscopy. J. Am. Oil Chem. Soc., 70, 1139-1144 (1993).
- Isbell, T.A. and Mund, M.S. Synthesis of secondary ethers derived from meadowfoam oil. J. Am. Oil Chem. Soc., 75, 1021-1029 (1998).
- Lie Ken Jie, M.S.F., Bus, J., Groenewegen, A. and Sies, I. 1H and 13C nuclear magnetic resonance studies of 2,5-disubstituted C18 furanoid ester isomers. J. Chem. Soc. Perkin II, 1275-1278 (1986).
- Lie Ken Jie, M.S.F. and Wong, K.-P. A novel method for the introduction of a methyl group into the furan ring of a 2,5-disubstituted C18 furanoid fatty ester via a malonic acid function. Lipids, 26, 837-842 (1991).
- Lie Ken Jie, M.S.F. and Lam, C.K. Ultrasound-assisted epoxidation of long-chain unsaturated fatty esters. Ultrasonics Sonochemistry, 2, S11-S14 (1995).
- Singh, S. Synthesis of oligoethylene glycol ethers from the seed oil of Vernonia anthelmintica. J. Am. Oil Chem. Soc., 74, 609-611 (1997).
- Singh, S., Mahajan, S. and Kaur, H. Synthesis of tetrahydrofuran ring containing oligoethylene glycol ethers from the seed oil of Vernonia anthelmintica. J. Am. Oil Chem. Soc., 76, 103-107 (1999).
The Scottish Crop Research Institute (MRS Lipid Analysis), Invergowrie, Dundee (DD2 5DA), Scotland
Updated: 24/1/2007 |
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