Acetylenic and Allenic Acids and Esters
Chemical shifts for acetylenic and allenic acids and esters will be discussed under the following headings:
- Shifts of the unsaturated carbon atoms and the influence of the carboxyl group on these
- The effect of the unsaturated system on nearby carbon atoms
- Systems more complex than those containing only one unsaturated centre
A monoacetylenic acid where the unsaturated system is a long way from the carboxyl group has a characteristic shift of 80.19 ppm. This, however, frequently appears as two signals because of the differing effect of the carboxyl group on the two sp carbon atoms. Appropriate numerical values have been cited by Gunstone et al. (1976) and Bus et al. (1976, 1977) (Table 1 below), and these values have been interpreted by Knothe and Bagby (1995) as rational functions.
Knothe and Bagby (1995) expressed the chemical shifts and the difference between them as rational functions, and gave the following equations for the acetylenic acids and their glycerol esters where R1 and R2 are the chemical shifts of the two sp carbons and ‘s’ is the numerical value of the difference between the two chemical shifts.
|Acids||R1 = 80.45 – (30/u2)||R2 = 79.80 + (50/u2)||s = (80/u2) – 0.65|
|Glycerol esters||R1 = 80.40 – (40/u2)||R2 = 79.90 + (40/u2)||s = (80/u2) – 0.50|
The effect of the triple bond on nearby carbon atoms is larger than that exerted by sp2 carbon atoms and values given by Gunstone et al. (1976) and by Bus et al. (1976, 1977) are collected in Table 2 (below). The shifts for acetylenic and propargylic carbon atoms are easily distinguished from those for olefinic and allylic carbon atoms, and the effect on the beta, gamma, and delta carbon atoms should also be apparent.
More complex systems containing acetylenic groups have also been reported, including those with two acetylenic groups, those with acetylenic and olefinic groups – both conjugated and non-conjugated, and those with hydroxy and other functional groups. Further information on acetylenic compounds is available in the papers of Gosalbo et al. (1993; monoacetylenic compounds used as intermediates in the synthesis of cyclopropane esters), Spitzer et al. (1997) (papers on acetylenic and allenic compounds), Rooney and Capon (1998) and Lie Ken Jie et al. (1991, 1997) (Table 3 below).
Lie Ken Jie et al. (1992) have reported the 13C-NMR spectra of fifteen C18 methyl esters containing the allenic group (-CH=C=CH-). These unsaturated carbon atoms have chemical shifts of approximately 91, 204, and 91 ppm but the unit has a much smaller effect on the adjacent carbon atoms (~29.1) than olefinic (cis 27.2, trans 32.6) or acetylenic groups (18.8). Typical information is given in Table 4 (below), and full details are available in the original paper.
|Table 1. Chemical shift (ppm) for sp carbon atoms in monoacetylenic acids CH3(CH2)mC≡C(CH2)nCOOH compared to the value of 80.19, and the difference between the two shifts|
|The explanation of above table is based on chemical shifts for stearolic acid (9a 18:1). The chemical shifts for C9 and C10 at 80.04 and 80.34 differ from 80.19 by –0.15 and +0.15 and the difference between the two values is 0.30.|
|Table 2. Influence of the acetylenic unit on the chemical shifts of nearby carbon atoms|
|Gunstone et al. (1976)||-10.96||-0.56||-0.84||-0.53||-0.16||-0.11|
|Bus et al. (1976, 1977)
|Table 3. Chemical shifts for sp2 carbon atoms between two triple bonds in diynoic acids.|
|Number of CH2 groups between sp carbon atoms||Chemical shifts of CH2 groups|
|Table 4. Chemical shifts for selected C18 allenic acids|
|* Ester of C14 acid.
# These values could be exchanged.
- Bus, J., Sies, I. and Lie Ken Jie, M.S.F. 13C-NMR of methyl, methylene and carbonyl carbon atoms of methyl alkenoates and alkynoates. Chem. Phys. Lipids, 17, 501-518 (1976).
- Bus, J., Sies, I. and Lie Ken Jie, M.S.F. 13C-NMR of double and triple bond carbon atoms of unsaturated fatty acid methyl esters. Chem. Phys. Lipids, 18, 130-144 (1977).
- Gosalbo, L., Barrot, M., Fabrias, G., Arsequell, G. and Camps, F. Synthesis of deuterated cyclopropane fatty esters structurally related to palmitic and myristic acids. Lipids, 28, 1125-1130 (1993).
- Gunstone, F.D., Pollard, M.R., Scrimgeour, C.M., Gilman, N.W. and Holland, B.C. 13C Nuclear magnetic resonance of acetylenic fatty acids. Chem. Phys. Lipids, 17, 1-13 (1976).
- Knothe, G. and Bagby, M.O. 13C-NMR spectroscopy of unsaturated long-chain compounds: an evaluation of the unsaturated carbon signals as rational functions. J. Chem. Soc. Perkin Trans. 2, 615-620 (1995).
- Knothe, G., Lie Ken Jie, M.S.F., Lam, C.C. and Bagby, M.O. Evaluation of the 13C-NMR signals of the unsaturated carbon atoms of triacylglycerols. Chem. Phys. Lipids, 77, 187-191 (1995).
- Lie Ken Jie, M.S.F. and Lam, C.C. 13C-NMR studies of polyunsaturated triacylglycerols of type AAA and mixed triacylglycerols containing saturated, acetylenic, and ethylenic acyl groups. Chem. Phys. Lipids, 78, 1-13 (1995).
- Lie Ken Jie, M.S.F., Lam, C.C. and Yan, B.F.Y. Carbon-13 nuclear magnetic resonance studies on some synthetic acetylenic glycerol triesters. J. Chem Res., 141, 1023-1045 (1993).
- Lie Ken Jie, M.S.F. and Wong, C.F. Synthesis and NMR properties of positional isomers of methyl allenic fatty esters. Chem. Phys. Lipids, 61, 243-254 (1992).
- Lie Ken Jie, M.S.F., Cheung, Y.K., Chau, S.H. and Yan, B.F.Y. 13C-NMR spectra of positional isomers of long-chain conjugated diacetylenic fatty esters. Chem. Phys. Lipids, 60, 179-188 (1991).
- Lie Ken Jie, M.S.F., Pasha, M.K. and Alam, M.S. Oxidation reactions of acetylenic fatty esters with selenium dioxide/tertbutyl hydroperoxide. Lipids, 32, 1119-1123 (1997).
- Rooney, F. and Capon, R.J. Callyspongynes A and B: new polyacetylenic lipids from a Southern Australian marine sponge, Callyspongia sp. Lipids, 33, 639-642 (1998).
- Spitzer, V., Tomberg, W., Hartman, R. and Aicholz, R. Analysis of the seed oil of Heisteria silvanii (Olacaceae) – a rich source of a novel C18 acetylenic acid. Lipids, 32, 1189-1200 (1997).
- Spitzer, V., Tomberg, W. and Pohlentz, G. Structure analysis of an allene-containing estolide tetraester triglyceride in the seed oil of Sebastiana commersoniana (Euphorbiaceae). Lipids, 32, 549-557 (1997).