Acids with conjugated unsaturation

Conjugated Dienes

The conjugated dienes attracting most attention are the so-called conjugated linoleic acids (CLA). These occur at low levels in the meat and milk fats of ruminant animals where they are produced by enzymic modification of linoleic acid in the rumen. They may also be produced by 9-desaturation of octadec-11-enoic acid. These acids show important physiological effects and are reported to inhibit tumour growth. Alkali-isomerization of linoleic acid also gives a mixture of 9c,11t and 10t,12c dienes with some other isomers as minor components. See our web page on the occurrence of conjugated fatty acids.

By whatever method CLA is produced, it is usually a complex mixture of positional isomers and of stereoisomers, and full analysis requires gas chromatography, silver ion chromatography, and mass spectrometry of appropriate derivatives. ¹³C-NMR has also been used to identify these mixtures and appropriate compounds have had to be synthesised or isolated. The best results are given by Lie Ken Jie et al. (1997) for all the 9,11 diene isomers and by Davis et al. (1999) for selected 8,10-, 9,11-, 10,12- and 11,13-isomers. Chemical shifts are given in Tables 1 and 2.

Table 1. Chemical shifts (ppm) for the four isomeric 9,11-18:2 acids (Lie Ken Jie et al., 1997)
	9c,11t	9t,11c	9t,11t	9c,11c
1	174.32	174.34	174.22	174.27
2	34.10	34.10	34.09	34.10
3	24.95	24.95	24.98	24.97
4	29.06	28.97	29.04	29.14
5	-	-	-	-
6	-	-	-	-
8	27.66	32.06	32.61	27.46
9	129.89	134.51	132.16a	131.87c
10	128.71	125.72	130.37b	123.58d
11	125.58	128.57	130.51b	123.72d
12	134.76	130.17	132.43a	132.14c
13	32.92	27.72	32.68	27.54
14	29.41	29.73	29.40	29.68
15	28.95	28.97	28.97	29.04
16	31.77	31.77	31.82	31.81
17	22.65	22.65	22.68	22.69
18	14.12	14.12	14.13	14.13
abcd: These pairs may be interchanged.

Table 2. Chemical shifts (ppm) for isomeric 18:2 methyl esters (Davis et al., 1999).
	9c,11c	9t,11t	9c,11t	11c,13t	8t,10c	10t,12c
1	179.98	179.86	180.09	180.09	180.05	180.09
2	34.06	34.00	34.05	34.06	34.03	34.06
3	24.68	24.67	24.67	24.69	24.63	24.67
4	-	-	29.03	-	-	29.06
5	-	-	29.12	-	-	-
6	-	-	29.03	-	-	-
7	29.57	29.36*	29.66	-	32.78	-
8	27.52	32.56	27.65	-	124.32	29.39
9	131.87	132.21	129.89	29.73	125.83	32.87
10	123.72	130.46	128.73	27.69	128.54	134.58
11	123.55	130.30	125.58	130.04	130.27	125.70
12	132.19	132.53	134.80	128.66	27.71	128.60
13	27.43	32.63	32.90	125.65	29.76	130.16
14	29.64	29.42*	29.40	134.66	-	27.68
16	31.76	31.77	31.76	31.59	31.87	31.50
17	22.64	22.63	22.63	22.29	22.68	22.57
18	14.09	14.10	14.10	13.95	14.12	14.06

Enynes

Reports are available for the chemical shifts for all carbon atoms in trans-octadec-11-en-9-ynoic acid and esters (also known as ximenynic or santalbic acid) and other acids/esters containing conjugated enyne systems (Table 3).

Table 3. Chemical shifts for acids/esters containing conjugated enyne systems
	(a)	9a,11c-18:2*			9a,11t-18:2*
		Methyl	Glycerol (α)	Glycerol (β)	Methyl	Glycerol (α)	Glycerol (β)
1	174.32	174.27	173.179	172.771	174.31	173.142	172.774
2	34.10	34.074	33.997	34.142	34.074	33.974	34.138
3	24.85	24.917	24.806	24.840	24.921	24.806	24.837
4	28.81#	29.062	29.016	28.973	29.030	28.978	28.936
5	28.88#		28.893	28.874		28.814	28.842
6	30.07		28.817	28.805		28.783	28.710
7	142.96		28.691	28.691		28.786	28.786
8	109.50	19.542	19.500	19.500	19.329	19.324	19.324
9	92.66	94.277	94.224	94.193	88.547	88.444	88.418
10	84.90	77.561	77.482	77.489	79.287	79.304	79.319
11	109.76	109.30	109.300	109.300	107.76	109.826	109.826
12	144.58	142.67	142.618	142.602	143.45	143.296	143.306
13	33.26	30.040	30.031	30.031	32.980	32.998	32.998
14	28.72#		28.817	28.817		28.814	28.814
15	28.56#		28.691	28.691		28.710	28.710
16	31.74	31.706	31.701	31.701	31.690	31.697	31.697
17	22.67	22.645	22.636	22.636	22.614	22.617	22.617
18	14.16	14.119	14.130	14.130	14.103	13.103	13.103
(a) Heisteric acid 7c,9a,11t-18:3 (Spitzer et al., 1997). *All chemical shifts are given to three decimal places - those with 6 digits are reduced to 5 in this Table (Lie Ken Jie et al., 1997). #These shifts may be interchanged.

Conjugated Trienes and Tetraenes

The common natural acids of this type are C₁₈ compounds with triene unsaturation at the 9,11,13 or 8,10,12 positions, and a 9,11,13,15-tetraene. These acids occur in several stereochemical forms. Chemical shifts for these compounds are listed in Table 4. Stellaheptaenoic acid (4c,7c,9t,11t,13t,16c,19c-22:7), isolated from a marine alga, contains the same conjugated tetra-ene unit as α-parinaric acid.

Newer results for α- and β-eleostearic, punicic, and catalpic acids, collected in Table 5, differ from the older data in Table 3. The newer information is obtained from oils containing these acids and chemical shifts are given for both α and β chains. However, there is poor agreement between these two sets of data. This is illustrated for α-eleostearic acid in Table 6 where olefinic signals and assignments are compared.

Table 4. Chemical shifts (ppm) for natural acids with conjugated triene (Gunstone, 1993) and tetra-ene (Spitzer et al., 1996,1997) systems [trivial names punicic (a), α-eleostearic (b), catalpic (c), β-eleostearic(d), jacaric (e), calendic (f), α-parinaric (g), stellaheptaenoic (h)]
	D-9,11,13			D-8,10,12			D-9,11,13,15	D-4,7,9,11,13, 16,19
	ctc	ctt	ttc	ttt	ctc	ttc	cttc	ccttccc
	a	b	c	d	e	f	g	h
5	nc	nc	nc	nc	28.89	28.85	nc	128.98
6	nc	nc	nc	nc	nc	nc	nc	26.61
7	nc	nc	nc	nc	27.81	32.78	nc	128.69
8	27.89	27.83	32.84	32.80	127.81#	126.21#	27.97	128.49
9	128.96	128.95	126.12#	130.73	128.07#	128.71#	132.83	127.33
10	132.38	132.91	128.79#	130.81	128.86#	130.85#	128.24	133.84
11	127.87	125.98	130.82#	134.19	129.07#	132.04#	128.14	133.86
12	128.02	134.90	131.75#	133.98	132.09#	132.71#	132.88	128.52
13	132.62	131.52	132.77#	130.95	132.57#	134.66#	132.91	129.19
14	128.86	130.73	134.70#	130.60	27.89	27.89	128.21	132.18
15	27.65	32.53	27.58	32.53	nc	nc	128.89	26.61
16	31.94	31.58	31.95	31.58	31.57	31.56	134.52	130.37
17	22.39	22.28	22.38	22.28	22.62	22.62	21.33	129.51
18	14.02	13.95	14.00	13.95	14.10	14.11	14.37	26.61*

Table 5. Chemical shifts (ppm) for natural acids with conjugated triene systems (Blaise et al., 1997)
	A/α	A/β	B/α	B/β	C/α	C/β
9	131.724	131.685	134.184	134.142	128.937	128.920
10	128.837	128.818	130.631	130.613	132.426
11	125.986	125.965	130.798	130.782	127.858	127.840
12	132.926	132.906	130.963	130.942	128.025
13	130.640	130.631	130.505		132.688	132.668
14	135.164	135.141	134.444	134.426	-	-
(A) α-Eleostearic, (B) β-eleostearic, (C), punicic. Also, catalpic glycerol esters 134.897 (α), 132.760 (α), 131,973 (β), 131,957 (α), 128,715 (α and β), 126.148 (β), 126.132 (α), 130.755 (β), 130.741 (α). - These chemical shifts are not assigned to individual carbon atoms and it is not clear why 9 values are listed.

Table 6. Chemical shifts (ppm) listed in increasing numerical value for olefinic carbon atoms in α-eleostearic acid according to Tulloch and Blaise
Tulloch and Bergter (1979)		Blaise et al. (1997)
Shift (ppm)	Carbon atom	Shift (ppm)	Carbon atom
125.98	11	125.99	11
128.95	9	128.84	10
130.73	14	130.64	13
131.52	13	131.72	9
134.90	12	135.16	14

References

• Blaise, P., Wolff, R. and Farines, M.  Etude regiospecific de triacyglycerols d'huiles vegetales par clivage chimique et RMN ¹³C haute resolution, Oleagineux Corps gras Lipides, 4, 135-141 (1997).
• Davis, A.L., McNeill, G.P. and Caswell, D.C. Analysis of conjugated linoleic acid isomers by ¹³C-NMR spectroscopy. Chem. Phys. Lipids, 97, 155-165 (1999).
• Gunstone, F.D. High resolution ¹³C-NMR spectroscopy of lipids. In: Advances in Lipid Methodology - Two, pp. 1-68 (ed. W.W. Christie, Oily Press, Dundee) (1993).
• Lie Ken Jie, M.S.F., Pasha, M.K. and Ahmad, F. Ultrasound-assisted synthesis of santalbic acid and a study of triacylglycerol species in Santalum album (Linn.) seed oil. Lipids, 31, 1083-1089 (1996).
• Lie Ken Jie, M.S.F., Pasha, M.K. and Ahmad, F. Synthesis and nuclear magnetic resonance properties of all geometrical isomers of conjugated linoleic acids. Lipids, 32, 1041-1044 (1997).
• Mikhailova, M.V., Bemis, D.L., Wise, M.L., Gerwick, W.H., Norris, J.N. and Jacobs, R.S. Structure and biosynthesis of novel conjugated polyene fatty acids from the marine green alga Anadyomene stellata, Lipids, 30, 583-589 (1995).
• Spitzer, V., Tomberg, W., Hartmann, R. and Aichholz, R. Analysis of the seed oil of Heisteria silvanii (Olacaceae) - a rich source of a novel C₁₈ acetylenic fatty acid, Lipids, 32, 1189-1200 (1997).
• Spitzer, V., Tomberg, W. and Zucolotto, M. Identification of α-parinaric acid in the seed oil of Sebastiana brasiliensis Sprengel (Euphorbiaceae). J. Am. Oil Chem. Soc., 73, 569-573 (1996).
• Tulloch, A.P. and Bergter, L. Analysis of the conjugated triene acid containing oil from Fevillea trilobata by ¹³C nuclear magnetic resonance spectroscopy. Lipids, 14, 996-1002 (1979).