Facile and stereoselective synthesis of (Z)-15-octadecenoic acid and (Z)-16-nonadecenoic acid: Monounsaturated omega-3 fatty acids

Article abstract of DOI:10.1007/s11745-009-3378-3

Facile syntheses of the monounsaturated omega-3 fatty acids, (Z)-15-octadecenoic acid and (Z)-16-nonadecenoic acid, are presented. Commercially available hydroxy fatty acids were esterified and oxidised, followed by the Wittig reaction to introduce the omega-3 olefinic bond; hydrolysis yielded the omega-3 fatty acids in high purity. An examination of different reaction conditions for the Wittig step found that THF as solvent and coupling temperatures of -78 °C gave optimal stereoselectivity, affording the omega-3 olefins in Z:E ratios 97:3. The syntheses have overall yields of ~43%, and utilise straightforward, robust chemistry, that may be readily scaled up and reproduced. Also presented is a method for accurately determining the double bond geometry and isomeric purity of the fatty acid products using ¹H-¹³C-HSQC NMR and GC-MS, respectively.

Full text of DOI:10.1007/s11745-009-3378-3

Lipids (2010) 45:159–165
DOI 10.1007/s11745-009-3378-3
ORIGINAL ARTICLE
Facile and Stereoselective Synthesis of (Z)-15-Octadecenoic Acid
and (Z)-16-Nonadecenoic Acid: Monounsaturated Omega-3 Fatty
Acids
Tristan Rawling
Michael Murray
•
Colin C. Duke
•
Pei H. Cui
•
Received: 29 October 2009 / Accepted: 25 November 2009 / Published online: 13 January 2010
Ó AOCS 2010
Abstract Facile syntheses of the monounsaturated
omega-3 fatty acids, (Z)-15-octadecenoic acid and (Z)-16-
nonadecenoic acid, are presented. Commercially available
hydroxy fatty acids were esterified and oxidised, followed
by the Wittig reaction to introduce the omega-3 olefinic
bond; hydrolysis yielded the omega-3 fatty acids in high
purity. An examination of different reaction conditions for
the Wittig step found that THF as solvent and coupling
temperatures of -78 °C gave optimal stereoselectivity,
affording the omega-3 olefins in Z:E ratios C97:3. The
syntheses have overall yields of *43%, and utilise
straightforward, robust chemistry, that may be readily
scaled up and reproduced. Also presented is a method
for accurately determining the double bond geometry
and isomeric purity of the fatty acid products using
PCC
Pyridinium chlorochromate
Polyunsaturated fatty acid
Tetrahydrofuran
PUFA
THF
Introduction
Omega-3 polyunsaturated fatty acids (PUFAs) have been
the subject of intense recent interest because their benefi-
cial actions in cells and in vivo are biologically distinct
from those of their omega-6 counterparts. Thus, omega-3
fatty acids possess anti-inflammatory and anti-cancer
effects, and have also been shown to inhibit platelet
aggregation, promote neuronal cell survival, increase
membrane fluidity and to modulate the expression of genes
that regulate lipid metabolism [1]. Populations with a high
intake of omega-6 PUFAs exhibit an increased risk of
breast, colon and prostate cancer, compared to those with a
high consumption of omega-3-containing oils [2–5].
Moreover, in vivo and in vitro studies have shown that
omega-6 PUFAs stimulate carcinogenesis and tumour
growth and metastasis, while omega-3 PUFAs repress
tumourigenesis [2, 6, 7]. PUFAs undergo biotransformation
to eicosanoid derivatives catalysed by cyclooxygenase,
lipoxygenase and cytochrome P450 enzymes; these eico-
sanoids mediate many of the cellular effects of PUFAs
1
13
H– C-HSQC NMR and GC–MS, respectively.
Keywords Omega-3 Á Fatty acid Á Wittig reaction Á
Stereoselectivity
Abbreviations
DCM
DMF
Dichloromethane
N,N-dimethylformamide
Fatty acid methyl ester
FAME
NaN(TMS)2 Sodium bis(trimethylsilyl)amide
T. Rawling (&) Á P. H. Cui Á M. Murray
Pharmacogenomics and Drug Development Group,
Faculty of Pharmacy A15, University of Sydney,
Sydney, NSW 2006, Australia
[
8, 9]. Because omega-3 and omega-6 PUFAs give rise to
parallel series of eicosanoids with differing potency, the
omega-3 olefinic bond is strongly implicated in the
beneficial properties of the parent omega-3 PUFAs.
Most studies of the biological effects of omega-3 fatty
acids have focussed on naturally occurring PUFAs, such as
a-linolenic acid and eicosapentaenoic acid. However, the
e-mail: trawling@pharm.usyd.edu.au
C. C. Duke
Pharmaceutical Chemistry, Faculty of Pharmacy,
University of Sydney, Sydney, NSW 2006, Australia
1
23

1
60
Lipids (2010) 45:159–165
OH
OH
TLC was carried out on Merck silica gel 60 F TLC plates.
2
54
1
13
H-NMR (400 MHz), C-NMR (100.5 MHz), and H– C-
1
13
O
O
1
2
HSQC-NMR spectra were recorded using a Varian 400-MR
instrument. Spectra were referenced internally to NMR
1
13
solvent (CDCl ; H d 7.26 ppm, C d 77.10 ppm). High
3
Resolution Mass Spectra (HRMS) were obtained by the
Mass Spectrometry Unit at the School of Chemistry, The
University of Sydney, on a Bruker 7T Fourier Transform Ion
Cyclotron Resonance Mass Spectrometer. Melting points
were measured on a Stuart SMP10 melting point apparatus.
GC–MS was performed on a PolarisQ GC–MS–MS ion
trap mass spectrometer coupled to a Trace GC, which was
fitted with a ZB Wax column (30 m 9 0.25 mm 9
0.25 lm). Automated injections were performed on an
AS2000 Autosampler. The injection port temperature was
190 °C and 1.0-lL injections were made in splitless mode,
with the split valve opened after 1 min. Helium was used
as carrier gas, at a flow rate of 1 mL/min. The GC oven
temperature was programmed as follows: initial tempera-
ture of 100 °C for 2.5 min, then ramped at 5 °C/min to
280 °C, and held for 6.5 min giving a total run time of
45 min. Eluted compounds were subjected to methane
chemical ionisation and detection was made in full scan
mode scanning the MS from 100 to 600 amu in 2 ls.
Compounds 1 and 2 were converted to their methyl esters
using acetyl chloride/methanol prior to GC–MS analysis.
Fig. 1 The (Z)-monounsaturated omega-3 fatty acids prepared in this
work
assignment of particular cellular actions to the omega-3
bond is confounded by possible interference from other
double bonds in these molecules. Indeed, with the excep-
tion of the omega-3 double bond, the olefinic bonds in
eicosapentaenoic acid are analogous to those in the major
omega-6 PUFA arachidonic acid. Surprisingly, there has
been little work done to date on the synthesis of mono-
unsaturated omega-3 fatty acids, which may serve as
probes to clarify some of the biological properties of the
omega-3 double bond.
Syntheses of the monounsaturated omega-3 fatty acid
(
Z)-15-octadecenoic acid (1) (Fig. 1) have been reported.
Klok et al. [10] reported a seven step synthesis starting
from 1,12-dodecanediol, in which hydrogenation of 15-
octadecynoic acid is used to prepare the omega-3 bond. An
alternative eleven step synthesis has been reported by
Gunstone et al. [11], starting from undec-10-enoic acid. In
the present article, we describe a four step synthesis of (Z)-
Chemicals
1
5-octadecenoic acid (1), as well as (Z)-16-nonadecenoic
acid (2), in high isomeric purity. The synthetic procedure
employs straightforward, robust, quick and inexpensive
reactions, and may be readily scaled up. 1 occurs naturally
and has been detected in, but not isolated from, animal
products including beef [12], pork [13], cows milk and
butter [14], and in human blood [15] and human milk
The following were purchased from the indicated com-
mercial suppliers and were used as received: 15-hydroxy-
pentadecanoic acid (97%), 16-hydroxyhexadecanoic acid
(98%), 1 M sodium bis(trimethylsilyl)amide in THF,
n-propyltriphenylphosphonium bromide, anhydrous THF
and acetyl chloride (Aldrich, Castle Hill, NSW, Australia);
pyridinium chlorochromate (PCC) and iodomethane
(Fluka, Castle Hill, NSW, Australia), and potassium
carbonate and sodium hydroxide (Ajax Finechem, Taren
Point, NSW, Australia).
[
16, 17]. As well as being useful probes for studying the
properties of the omega-3 double bond in fatty acids of
naturally and non-naturally occurring chain lengths,
these compounds may permit the convenient synthesis of
metabolites, such as epoxides derived from reactions of the
omega-3 bond. The preparation of corresponding metabo-
lites from polyunsaturated fatty acids leads to complex
mixtures that require extensive purification [18].
General Procedure for the Synthesis of the Methyl
Esters 5 and 6
To a solution of the hydroxy fatty acid (3 or 4, 7.87 mmol)
in acetone (160 mL) was added water (8 mL), potassium
carbonate (23.60 mmol) and iodomethane (39.33 mmol).
The resulting mixture was refluxed for 4 h, and then con-
centrated in vacuo. The residue was dissolved in water
Experimental Procedures
Instrumentation
(60 mL), and the solution was acidified with 1 M HCl. The
Preparative liquid chromatography was performed using
Dry Column Vacuum Chromatography (DCVC) with Merck
Silica Gel 60H (5–25 lm) as the stationary phase. A 70-mm
aqueous phase was extracted with DCM (3 9 60 mL), and
the combined extracts were washed with brine (100 mL),
dried over Na SO , and concentrated in vacuo, affording
2
4
(
ID) column was used, and 50-mL fractions were collected.
methyl esters 5 and 6.
1
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Lipids (2010) 45:159–165
161
Methyl 15-hydroxypentadecanoate (5)
White solid (72%). m.p. 49–50 °C (lit. m.p. 49.8 °C (19)).
CI–MS: m/z (%): 273 ([M ? H] , 6), 255 (39), 241 (29),
nitrogen atmosphere, was added NaN(TMS) (1.0 M in
2
THF, 6.43 mmol). The resulting orange mixture was stirred
for 40 min at room temperature. The mixture was then
cooled to -78 °C, and the aldehyde (7 or 8, 3.21 mmol) in
THF (5 mL) was added dropwise by syringe. Stirring at
-78 °C was continued for 30 min after which the reaction
mixture was allowed to warm to room temperature. After
further stirring for 2 h the reaction was quenched with
?
2
1
23 (95), 205 (100), 163 (14), 149 (39), 135 (39), 121 (42),
1 13
07 (26). The H- and C-NMR spectra of the product
were in good agreement with previously reported data [19].
Methyl 16-hydroxyhexadecanoate (6)
saturated aqueous NH Cl (20 mL), and extracted with
4
DCM (3 9 40 mL). The combined extracts were dried
White solid (70%). m.p. 53–54 °C (lit. m.p. 54.5–56 °C
over Na SO , and the DCM removed in vacuo. The residue
2
4
1 13
20)). The H- and C-NMR, and MS spectra of the product
(
was purified on silica gel by stepwise gradient elution with
dichloromethane/hexane (20:80 to 50:50).
were in good agreement with previously reported data [20].
General Procedure for the Synthesis of Aldehydes 7 and 8
Methyl (Z)-15-octadecenoate (9)
1
To a suspension of PCC (6.68 mmol) and Celite (1.440 g)
in anhydrous DCM (20 mL) under nitrogen, was slowly
added the methyl ester (5 or 6, 3.93 mmol) in anhydrous
DCM (6 mL). The mixture was stirred for 2 h under
nitrogen, after which diethyl ether (50 mL) was slowly
added. The resulting mixture was stirred for 10 min, and
then filtered over Celite. The Celite was washed with ether
Colorlessoil(80%). H NMR (400 MHz, CDCl ): d5.40–5.28
3
(m, 2H, CHCH), 3.66 (s, 3H, OCH ), 2.30 (t, J = 7.6 Hz,
3 HH
2H, CH COO), 2.08–1.98 (m, 4H, CH CHCHCH ), 1.62
2
2
2
(quin, JHH = 7.6 Hz, 2H, CH CH COO), 1.38–1.20 (m, 20H,
2
2
13
CH ), 0.95 (t, J = 7.6 Hz, 3H, CH3).
HH
C NMR
2
(100.5 MHz, CDCl ): d 174.32 (COO), 131.47 (CH), 129.34
3
(CH), 51.40 (OCH ), 34.10 (CH COO), 29.77, 29.62 (3C),
3
2
(
2 9 20 mL), and the filtrate concentrated in vacuo. The
29.56, 29.53, 29.43, 29.28, 29.24, 29.14, 27.08 (CHCH2),
24.95 (CH CH COO), 20.48 (CHCH ), 14.37 (CH ). CI–MS:
residue was purified on silica gel by stepwise gradient
elution with dichloromethane/hexane (40:60 to 100:0).
2
2
2
3
?
m/z (%): 297 ([M ? H] , 100), 265 (94), 247 (68), 191 (10),
78 (12), 163 (17), 149 (24), 135 (24), 121 (25), 107 (15).
1
Methyl 15-oxopentadecanoate (7)
R = 25.43 min. Z:E ratio = 97.0:3.0. HRMS (APCI) calcu-
t
?
lated for C H O ([M ? H–MeOH] ) 265.25259, found
18 33
White solid (86%). m.p. 39–41 °C. CI–MS: m/z (%): 271
265.25307.
?
[M ? H] , 8), 239 (36), 221 (68), 203 (100), 175 (11),
(
1
61 (21), 147 (39), 133 (40), 121 (46), 107 (28). The
13
Methyl (Z)-16-nonadecenoate (10)
1
H- and C-NMR spectra of the product were in good
agreement with previously reported data [21, 22].
1
Colorless oil (76%). H NMR(400 MHz, CDCl ): d5.40–5.28
3
(m, 2H, CHCH), 3.66 (s, 3H, OCH ), 2.30 (t, J = 7.6 Hz,
3 HH
Methyl 16-oxohexadecanoate (8)
2H, CH COO), 2.06–1.98 (m, 4H, CH CHCHCH ), 1.62
2 2 2
(
quin, JHH = 7.2 Hz, 2H, CH CH COO), 1.38–1.22 (m, 22H,
2 2
1
13
White solid (78%). m.p. 41–43 °C. H NMR (400 MHz,
CH ), 0.95 (t, J = 7.6 Hz, 3H, CH3).
2
C NMR
HH
CDCl ): d 9.76 (t, J = 2.0 Hz, 1H, CHO), 3.67 (s, 3H,
(100.5 MHz, CDCl ): d 174.33 (COO), 131.48 (CH), 129.35
3
HH
3
CH O), 2.42 (td, J = 7.6, 2.0 Hz, 2H, CHOCH ), 2.30
3
(CH), 51.41 (OCH ), 34.11 (CH COO), 29.77, 29.65 (2C),
3
HH
2
2
(
t, JHH = 7.2 Hz, 2H, CH COO), 1.66–1.58 (m, 4H,
2
29.62 (2C), 29.58, 29.54, 29.44, 29.29, 29.24, 29.14, 27.09
CH CH COO and CH CH CHO), 1.36–1.24 (m, 20H,
2
(CHCH ), 24.95 (CH CH COO), 20.49 (CHCH ), 14.38
2
2
2
2
2
2
2
?
CH ). CI–MS: m/z (%): 285 ([M ? H] , 8), 253 (52), 235
?
(CH ). CI–MS: m/z (%): 311 ([M ? H] , 97), 279 (100), 261
(68), 191 (12), 177 (15), 163 (18), 149 (23), 135 (28), 121 (27),
2
3
(
(
96), 217 (100), 175 (16), 151 (16), 147 (34), 135 (43), 121
1
3
47), 107 (24). The C-NMR spectrum of the product was
109 (19). R = 26.98 min. Z:E ratio = 98.5:2.5. HRMS
t
?
(APCI) calculated for C H O ([M ? H–MeOH] )
in good agreement with previously reported data for a
crude product [23].
1
9 35
279.26824, found 279.26856.
General Procedure for the Synthesis of Omega-3 Fatty
General Procedure for the Synthesis of Omega-3 Fatty
Acid Methyl Esters 9 and 10
Acids 1 and 2
To a suspension of n-propyltriphenylphosphonium bromide
(
To a solution of the omega-3 fatty acid methyl ester (9 or
10, 1.66 mmol) in ethanol (25 mL) was added 1.5 M
7.07 mmol) in anhydrous THF (10 mL) at 0 °C under a
1
23

1
62
Lipids (2010) 45:159–165
NaOH (12 mL). The mixture was heated to 40 °C for
min to obtain a clear solution. Stirring was continued at
HO
O
5
n
n
3
4
. n = 5
. n = 6
room temperature for 2 h. The volume of the reaction
mixture was reduced to 20 mL by rotary evaporation, and
the solution was adjusted to pH 3 with 1.0 M HCl. The
white solid was collected by filtration, thoroughly washed
with water, and dried under vacuum to yield the omega-3
fatty acids 1 and 2.
OH
a
HO
O
O
5
6
. n = 5 (72%)
. n = 6 (70%)
(
Z)-15-octadecenoic acid (1)
b
White solid (96%). m.p. 41–42 °C (lit. m.p. 40.5–41.5 °C
1
O
O
O
(
11)). H NMR (400 MHz, CDCl ): d 5.43–5.27 (m, 2H,
3
n
CHCH), 2.35 (t, JHH = 7.6 Hz, 2H, CH COO), 2.08–1.98
2
7
8
. n = 5 (86%)
. n = 6 (78%)
(
m, 4H, CH CHCHCH ), 1.63 (quin, J = 7.2 Hz, 2H,
2 2 HH
CH CH COO), 1.38–1.22 (m, 20H, CH ), 0.95 (t, J =
HH
2
2
2
1
3
c
7
.4 Hz, 3H, CH3). C NMR (100.5 MHz, CDCl ): d 179.94
3
(COO), 131.48 (CH), 129.35 (CH), 34.03 (CH COO), 29.77,
2
2
9.62 (3C), 29.57, 29.54, 29.42, 29.29, 29.23, 29.05, 27.09
O
O
n
n
(
(
CHCH ), 24.68 (CH CH COO), 20.49 (CHCH ), 14.38
2
2
2
2
?
CH ). CI–MS (FAME derivative): m/z (%): 297 ([M ? H] ,
9. n = 5 (80%)
3
10. n = 6 (76%)
9
2), 265 (100), 247 (80), 191 (13), 178 (16), 163 (24), 149
(
32), 135 (33), 121 (33), 107 (19). R (FAME deriva-
t
d
tive) = 25.54 min. Z:E ratio = 97.0:3.0. HRMS (ESI) cal-
-
culatedforC H O ([M–H] )281.24751, found281.24832.
1
8 33 2
O
(
Z)-16-nonadecenoic acid (2)
1
2
. n = 5 (96%)
. n = 6 (94%)
OH
1
White solid (94%). m.p. 52–53 °C. H NMR (400 MHz,
CDCl ): d 5.43–5.28 (m, 2H, CHCH), 2.34 (t, J = 7.6 Hz,
Scheme 1 Synthesis of the model (Z)-monounsaturated omega-3
fatty acids. Reaction conditions: a MeI, K CO , acetone/water, reflux,
4 h; b PCC, DCM, rt, 2 h; c [Ph PPr]Br, NaN(TMS) , THF, -78 °C
to rt, 2.5 h; d NaOH, ethanol/water, rt, 2 h
3
HH
2
3
2
H, CH COO), 2.08–1.96 (m, 4H, CH CHCHCH ), 1.61
2 2 2
3
2
(
quin, JHH = 7.2 Hz, 2H, CH CH COO), 1.38–1.20 (m,
2
2
1
3
2
2H, CH ), 0.95 (t, J = 7.6 Hz, 3H, CH3). C NMR
2 HH
(
(
100.5 MHz, CDCl ): d 179.81 (COO), 131.48 (CH), 129.35
CH), 34.07(CH COO), 29.77,29.65(2C), 29.63(2C), 29.58,
2
3
positioning of the double bond and, through careful control
of the reaction conditions, high stereoselectivity may be
achieved. This allowed for the preparation of fatty acids in
high isomeric purity, without the need for time-consuming
preparative HPLC purification.
2
9.55, 29.43, 29.29, 29.24, 29.07, 27.09 (CHCH ), 24.70
2
(CH CH COO), 20.49 (CHCH ), 14.38 (CH ). CI–MS
2 2 2 3
?
(
FAME derivative): m/z (%): 311 ([M ? H] , 94), 279 (100),
61 (68), 191 (10), 177 (15), 163 (17), 149 (24), 135 (24), 121
27), 109 (15). R (FAME derivative) = 27.24 min. Z:E
2
(
The first step in the synthesis was esterification of
15-hydroxypentadecanoic acid (3) and 16-hydroxyhexa-
decanoic acid (4), which was done to facilitate the purifica-
tion of subsequent compounds by chromatography on silica
gel. Initially, esterification of 3 and 4 was attempted fol-
lowing the Fischer protocol (1 g of 3 or 4, 5 equiv acetyl
t
-
ratio = 98.5:2.5. HRMS (ESI) calculated for ([M–H] )
C H O 295.26316, found 295.26399.
19 35 2
Results and Discussion
1
chloride, 50 mL methanol). H-NMR spectra of the products
Synthesis
isolated after liquid–liquid extraction contained a weak
triplet at 4.05 ppm that was attributed to a methylene ester
group i.e. –COOCH CH –, from the product of intermo-
The synthesis of compounds 1–2 is shown in Scheme 1. An
important step in the synthesis was the Wittig reaction of
aldehydes 7 and 8 with n-propyltriphenylphosphonium
bromide to form the omega-3 double bond. The Wittig
reaction was selected as there is no ambiguity as to the
2
2
lecular esterification of two hydroxyfatty acid molecules.
Additional resonances attributable to this product were not
observed, possibly because they may be obscured by
the stronger resonances of the target products. The
1
23

Lipids (2010) 45:159–165
163
intermolecular esterification products could not be removed
by silica gel chromatography. Subsequent attempts using
more dilute reaction conditions (1 g of 3 or 4 in 200 mL of
methanol), which might reduce the rate of intermolecular
esterification, also failed to yield a pure product. Reaction
conditions were therefore sought under which intermolecu-
lar esterification could not occur. Thus, 3 and 4 were ester-
ified using iodomethane, and the triplet attributed to the
intermolecular esterification product was absent from the
Hydrolysis of 9 and 10, to afford the omega-3 fatty acids
1 and 2 in excellent yields, was achieved using NaOH.
Under the mild reaction conditions employed, pure prod-
ucts were obtained without the need for subsequent chro-
matography. In addition, no change in the Z:E ratio was
observed upon de-esterification, indicating base catalysed
isomerisation [29] of (Z) double bonds did not occur under
the reaction conditions.
1
H-NMR spectra of the isolated products. Oxidation of the
Omega-3 Olefinic Bond Characterisation
thus obtained 5 and 6 with PCC proceeded smoothly,
affording the aldehydes 7 and 8, respectively, in good yield.
With 7 and 8 in hand we set out to prepare the (Z)-
omega-3 double bond. Wittig reactions using amide bases
Previously, identification of double bond geometry and
isomeric purity of synthetically prepared, unsaturated fatty
1
acid derivatives has been achieved using H-NMR spec-
1
troscopy [10, 26, 30, 31]. In the H-NMR spectra of 1 and
such as NaN(TMS) are reported to yield exclusively the
2
(
Z)-isomer [24–26]. The reaction was initially performed
following a literature procedure [24] (Table 1, entry 2),
2, as well as their precursors 9 and 10, the olefinic proton
resonances were found to be coincident. Consequently,
1
3
1
however GC–MS and C-NMR analysis of the product
isolated from silica chromatography indicated that the
reaction had not proceeded with absolute (Z) selectivity
H-NMR spectroscopy did not provide reliable data
regarding the double bond geometry or isomeric purity of
these compounds. A similar observation had been made in
the synthesis of (E)-vaccenic acid [32].
(
Z:E = 95.5:4.5). The stereoselectivity of the Wittig reac-
tion is dependent on several factors, including the nature of
the aldehyde and ylide, and the reaction conditions [27,
The double bond configurations in 1, 2, 9, and 10 were
identified using the chemical shifts of the signals arising
from the carbons adjacent to the double bond. Assignment
2
8]. Thus, the influence of the temperature at which the
1
13
ylide and aldehyde were combined and the role of different
solvent systems were tested in attempts to improve the
stereoselectivity (Table 1). It emerged that dimethylform-
amide (DMF) decreased the reaction yield and, consistent
with previous findings, THF was found to give optimum
of these resonances was achieved using H– C-HSQC
NMR spectroscopy, where coupling of these signals to the
proton resonances at 2.02 ppm, which arise from the pro-
tons adjacent to the double bond, was observed. Using this
approach, resonances arising from the carbon atoms adja-
1
3
(
Z) selectivity [27, 28]. Higher reaction temperatures
cent to the double bond in the C-NMR spectra of 1, 2, 9,
and 10 were identified at *27 and 20 ppm. Similar shifts
improved the reaction yield, however a concurrent increase
in formation of the (E) isomer was observed. Thus, to
obtain optimal stereoselectivity, the Wittig reaction was
performed in THF at -78 °C, to afford 9 and 10
1
3
have been reported in the C-NMR spectra of a series of
unbranched omega-3 (Z)-alkenes [33], and the chemical
shifts of these resonances may be considered diagnostic for
the omega-3 (Z) configuration. In the case of the (E) iso-
mers, deshielding shifts these resonances to *32 and
(
Z:E C 97.0:3.0).
2
5 ppm, respectively [33]. Weak resonances at *32 and
5 ppm, attributed to the (E) isomers, were observed in the
C-NMR spectra of 1, 2, 9, and 10.
2
Table 1 Yield and Z:E ratio obtained from the Wittig reaction of 7
a
and n-propyltriphenylphosphonium bromide
1
3
In addition to the configurations of double bonds, the
b
c
d
Solvent
Temperature (°C)
Z:E ratio
Yield (%)
chemical shifts of the adjacent carbons atoms indicate the
position of the double bond. Thus, chemical shifts of *20
and 27 ppm are typical for omega-3 (Z) compounds [33]
but shift significantly when the position of the double bond
is changed. For example, they appear at *29 and 27 ppm
THF ? DMF (12:1) Room Temperature 92.5:7.5
70
65
75
69
69
THF ? DMF (12:1)
0
95.5:4.5
95.5:4.5
97.0:3.0
95.5:4.5
THF
THF
Toluene
a
0
-78
-78
1
3
in the C-NMR spectra of omega-4 (Z) alkenes [33]. The
double bond position is also confirmed by the chemical
shift of the methyl group protons which appear at 0.95 ppm
All reactions performed under conditions analogous to those
reported in the general procedure used to prepare 9 and 10. Only the
solvent system and temperature at which 7 was added to the ylide
were changed
1
in the H-NMR spectra of 1, 2, 9, and 10; these lie within
the 0.93–1.03 ppm range reported to be characteristic of
omega-3 fatty acids [34].
b
Temperature at which 7 was added to ylide
c
Z:E ratio determined by GC–MS
The isomeric purity of the olefinic compounds was
determined by GC–MS. Gas chromatograms of 9 and 10,
d
Isolated yield based on the aldehyde
1
23

1
64
Lipids (2010) 45:159–165
and the FAME derivatives of 1 and 2, displayed two peaks
with identical mass spectra. Based on order of elution [35],
the first minor compound to elute was identified as the (E)
isomer, and the second major compound as the (Z) isomer.
That the (Z) isomer is the major constituent of the olefinic
8. Smith WL (1989) The eicosanoids and their biochemical mech-
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9
. Fitzpatrick FA, Murphy RC (1988) Cytochrome P-450 metabo-
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Recl Trav Chim Pays Bas 93:222–224
1. Gunstone FD, Ismail IA (1967) Fatty acids. XIII. Synthesis of all
the cis-n-octadecenonic acids. Chem Phys Lipids 1:209–224
1
3
compounds is supported by the C-NMR data. Using a
Zebron ZB-Wax column and relatively long run times of
1
4
5 min, baseline separation was achieved, and the Z:E ratio
was then calculated from the peak areas. Under the GC
conditions described, the FAME derivatives of 1 and 2 had
retentions times of 25.54 and 27.24 min, respectively, with
those of the corresponding (E) isomers at 25.25 and
12. Leheska JM, Thompson LD, Howe JC, Hentges E, Boyce J,
Brooks JC, Shriver B, Hoover L, Miller MF (2008) Effects of
conventional and grass-feeding systems on the nutrient compo-
sition of beef. J Anim Sci 86:3575–3585
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2
6.94 min, respectively.
1
1
4. Couvreur S, Hurtaud C, Lopez C, Delaby L, Peyraud JL (2006)
The linear relationship between the proportion of fresh grass in
the cow diet, milk fatty acid composition, and butter properties.
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Conclusions
This article presents a convenient synthesis of two (Z)-
monounsaturated omega-3 fatty acids with a high degree of
isomeric purity. The four-step synthetic procedure begin-
ning with the hydroxy fatty acids has an overall yield of
1
211:120–128
1
6. Li J, Fan Y, Zhang Z, Yu H, An Y, Kramer JKG, Deng Z (2009)
Evaluating the trans fatty acid, CLA, PUFA and erucic acid
diversity in human milk from five regions in China. Lipids
*
43%, and may be readily scaled up to prepare gram
quantities. The fatty acids described will be valuable model
compounds for researchers to evaluate the functional
importance of the omega-3 double bond in the cellular
actions of naturally-occurring omega-3 PUFAs.
4
4:257–271
1
1
7. Mosley EE, Wright AL, McGuire MK, McGuire MA (2005)
Trans fatty acids in milk produced by women in the United
States. Am J Clin Nutr 82:1292–1297
8. Cui PH, Zhang WV, Hook J, Tattam BN, Duke CC, Murray M
(
2009) Synthesis and NMR characterization of the methyl esters
of eicosapentaenoic acid monoepoxides. Chem Phys Lipids
59:30–37
Acknowledgments The present work was supported by a grant
from the Australian National Health and Medical Research Council.
The authors wish to acknowledge Bruce Tattam for his expertise and
assistance with the GC–MS analysis.
1
1
2
9. Kamitakahara H, Nakatsubo F (2005) Synthesis of diblock
copolymers with cellulose derivatives. 1. Model study with azi-
doalkyl carboxylic acid and cellobiosylamine derivative. Cellu-
lose 12:209–219
0. Kraft ML, Moore JS (2004) Multitechnique characterization of
fatty acid-modified microgels. Langmuir 20:1111–1119
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