Cyclic Fatty Acids

Fatty acids containing a carbocyclic unit are most commonly cyclopropene or cyclopentene compounds (see our web page on natural cyclic fatty acids). Eicosanoids such as the prostaglandins are also cyclopentane derivatives but these are not included here, and acids with heterocyclic units are discussed elsewhere (see epoxy acids).

Cycopropenyl Fatty Acids

The best known cyclopropene acids are malvalic and sterculic which are present in high levels in sterculia oils and at lower levels in kapok seed oil (around 12%) and in cottonseed oil (about 1%). The highly reactive cyclopropene acids are destroyed during refining and hydrogenation of the oils. They have attracted interest because they inhibit the biological desaturation of stearic to oleic acid. 2-Hydroxysterculic acid, which has also been identified, is probably an intermediate in the bioconversion of sterculic to malvalic acid by α-oxidation.

NMR data for the cyclopropene acids are presented in two Tables. Table 1 contains data for sterculic and malvalic acids and for natural and synthetic compound closely related to these. The compounds in the Table 2 are synthetic compounds with 14-16 or 24-26 carbon atoms. The cyclopropene unit itself has a methylene signal (7.0-7.5 ppm) and two olefinic signals. The latter differ by a value that is high when the cyclopropene unit is close to the carboxyl group and diminishes as it moves away. Figures in the two Tables show differences of 3.6 (Δ4), 2.5 (Δ5), 0.5 (Δ8), and 0.3 (Δ9). The cyclopropene unit also has a marked influence on the chemical shifts of nearby carbon atoms. Pollardhas given shielding parameters for the cyclopropene ring of -3.73 (α), -2.38 (β), -0.5 (γ), and -0.3 (Δ). These are larger than the corresponding parameters for the olefinic group.

Table 1. Chemical shifts (ppm) for sterculic, malvalic, and related esters
	A	B	C	D	E
CH2	7.247			7.46	7.43
1	173.233	172.819	173.204		175.93
2	33.875	34.036	33.878	33.93	70.49
3	24.696	24.731	24.660	24.78	34.44
4	28.925	28.889	28.773	27.02	24.77
5	28.925	28.935	28.871	25.75
6	29.069	29.080	27.196	108.88
7	27.192	27.205	25.788	110.20	27.33
8	25.836	25.836	109.092	26.12	26.02
9	109.193	109.175	109.556	27.48	109.23
10	109.459	109.474	25.902		109.52
11	25.897	25.897	27.255		26.10
12	27.250	27.250	29.275		27.45
13	29.274	29.274	29.243
14	29.265	29.265	29.243
15/ω-4	29.154	29.154	31.754	32.04	31.94
16/ω-3	31.747	31.747	22.561	22.77	22.74
17/ω-2	22.550	22.550	13.993	14.11	14.18
18/ω-1	13.979	13.979
A, sterculic glycerol ester (α-chain) [Howarth and Vlahov, 1996]; B, sterculic glycerol ester (β-chain where chemical shifts differ from those in the α-chain) [Howarth and Vlahov, 1996]; C, malvalic glycerol ester (α-chain) [Howarth and Vlahov, 1996]; D, 6,7 isomer of sterculic acid [Pollard, private communication]; E, 2-OH sterculic ester [Spitzer, 1991] corrected).

Table 2. Chemical shifts (ppm) for some synthetic cyclopropene esters
	A	B	C	D	E	F	G
CH2	7.43	7.39	7.54	6.92	6.98	6.99	6.92
1	173.88	174.03	173.51	174.33	174.37	174.23	174.33
2	33.56	33.88	32.20	34.11	34.12	34.12	34.10
3	22.82	24.70	21.98	24.95	24.93	24.96	24.94
4	25.40	26.94	107.61	-	-	-	-
5	108.17	25.68	111.18	-	-	-	-
6	110.65	108.72	25.94	-	-	-	-
7	26.02	110.00	27.35	-	-	-	-
8	27.38	26.03	-	-	-	27.08*	27.38
9	-	27.42	-	-	26.01	26.01	25.99
10	-	-	-	27.37	25.69*	109.22	109.07
11	-	-	-	26.01	109.18	109.33	109.32
12	-	-	-	108.99	109.18	26.01	28.06
13	-	-	-	109.34	25.98*	27.36*	-
ω-3	31.97	31.98	31.98	28.06	27.34	31.62	-
ω-2	22.71	22.73	22.75	20.73	22.42	22.45	20.72
ω-1	14.16	14.12	14.18	13.99	13.83	14.01	13.97
In the following structures, cp represents the cyclopropene ring C3H2
A: CH3(CH2)17cp(CH2)3COOCH3 [Hartman]				E: CH3(CH2)3cp(CH2)9COOCH3 [Gosalbo]
B: CH3(CH2)17cp(CH2)4COOCH3 [Hartman]				F: CH3(CH2)4cp(CH2)8COOCH3 [Gosalbo]
C: CH3(CH2)17cp(CH2)2COOCH3 [Hartman]				G: CH3(CH2)2cp(CH2)8COOCH3 [Gosalbo]
D: CH3(CH2)2cp(CH2)10COOCH3 [Gosalbo]
*These two signals could be interchanged.				Compounds D-G are partly deuterated

Cyclopentenyl Fatty Acids

Cyclopentene acids are present in the seed oils of the Flacourticeae. They range from C₆ to C₂₄ compounds with the structure shown. Some members have a second double bond in the side chain. The most common are hydnocarpic acid (16:1), chaulmoogric (18:1), and gorlic (18:2).

The NMR spectra of the cyclopentene acids lack the ω-1 to ω-3 signals characteristic of most fatty acids and instead show signals with unusual chemical shifts associated with the cyclopentene ring. Also present are signals at around 28, 29, and 36 ppm from the carbon atoms designated α, β, and γ in the above structure (Table 3).

Table 3. Chemical shifts (ppm) for some cyclopentenyl esters
	Chaulmoogric	Hydnocarpic	Gorlic
1	174.40	174.40	174.35
2	34.10	34.10	33.99
3	24.93	24.93	24.55
4	29.12	29.12	29.58
5	29.43	29.43	26.78
6	29.56	9.56	129.01
7	29.62	29.62	130.44
8	29.62	29.62	27.20
9	29.84	-	29.18
10	29.76	-	29.69
γ	27.97*	27.97*	27.94*
β	29.23*	29.23*	29.23*
α	36.15	36.15	36.13
a	45.58	45.58
b	135.45	135.43	135.43
c	129.97	129.94	129.98
d	31.96	31.94	31.96
e	29.87	29.85	29.84
Chaulmoogric acid 18:1; hydnocarpic acid 16:1; gorlic acid 18:2. *These two signals could be interchanged

References

• Blaise, P., Fairnes, M. and Soulier, J. Identification of cyclopentenyl fatty acids by ¹H and ¹³C nuclear magnetic resonance. J. Am. Oil Chem. Soc., 74, 727-730 (1997).
• Gosalbo, L., Barrot, M., Fabrias, G., Aesequell, G. and Camps, F. Synthesis of deuterated cyclopropene esters structurally related to palmitic and myristic acids. Lipids, 28, 1125-1130 (1993).
• Hartmann, S., Minnikin, D.E., Romming, H.-J., Baird, M.S., Ratledge, C. and Wheeler, P.R. Synthesis of methyl 3-(2-octadecylcyclopropen-1-yl)propanoate and methyl 3-(2-octadecylcyclopropen-1-yl)pentanoate and cyclopropane fatty acids as possible inhibitors of mycolic acid biosynthesis. Chem. Phys. Lipids, 71, 99-108 (1994).
• Howarth, O.W. and Vlahov, G. ¹³C Nuclear magnetic resonance study of cyclopropenoid triacylglycerols. Chem. Phys. Lipids, 81, 81-85 (1996).
• Spitzer, V. GC-MS characterisation (chemical ionisation and electron impact modes) of the methyl esters and oxazoline derivatives of cyclopropenoid fatty acids. J. Am. Oil Chem. Soc., 68, 963-69 (1991).