Synthesis of (S)-ricinoleic acid and its methyl ester with the participation of ionic liquid

Article abstract of DOI:10.1016/j.chemphyslip.2014.06.005

(R)-Ricinoleic acid methyl ester obtained from commercial castor oil was transformed in a three-step procedure into its S-enantiomer in overall 36% yield using ionic liquid (1-butyl-3-methylimidazolium acetate) in the key step process. The developed procedure provides easy access to (S)-ricinoleic acid and its methyl ester of over 95% enantiomeric excess. Optical rotations of the newly obtained compounds as well as their chromatographic and spectral characteristics are provided and discussed in the context of enantiopurity both of the substrate material and the final products.

Full text of DOI:10.1016/j.chemphyslip.2014.06.005

Chemistry and Physics of Lipids 183 (2014) 137–141
Contents lists available at ScienceDirect
Chemistry and Physics of Lipids
journal homepage: www.elsevier.com/locate/chemphyslip
Synthesis of (S)-ricinoleic acid and its methyl ester with the
participation of ionic liquid
*
Józef Kula , Radoslaw Bonikowski, Malgorzata Szewczyk, Kornelia Ciolak
Institute of General Food Chemistry, Lodz University of Technology, Lodz 90-924, Poland
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 6 May 2014
Received in revised form 11 June 2014
Accepted 19 June 2014
Available online 20 June 2014
(R)-Ricinoleic acid methyl ester obtained from commercial castor oil was transformed in a three-step
procedure into its S-enantiomer in overall 36% yield using ionic liquid (1-butyl-3-methylimidazolium
acetate) in the key step process. The developed procedure provides easy access to (S)-ricinoleic acid and
its methyl ester of over 95% enantiomeric excess. Optical rotations of the newly obtained compounds as
well as their chromatographic and spectral characteristics are provided and discussed in the context of
enantiopurity both of the substrate material and the final products.
Keywords:
ã 2014 Published by Elsevier Ireland Ltd.
Ricinoleic acid
(S)-Ricinoleic acid
Enantiopurity
Ionic liquids
Castor oil
1. Introduction
the organisation of lipid membranes (Jenske et al., 2008). Taking
into consideration the industrial importance of R-configured
(R)-Ricinoleic acid or (Z)-(R)-12-hydroxyoctadec-9-enoic acid is
a major constituent of castor oil from Ricinus communis seeds
(Euphorbiaceae family) and comprises up to 87–92% of all fatty
acids present in the oil (Broch-Jensen et al., 1997; Pradhan et al.,
2012). Methyl (R)-ricinoleate is produced by transesterification of
castor oil with methanol by chemical or enzymatic catalysis
(Knothe et al., 2012; Baron et al., 2014; Goswami et al., 2013).
Ricinoleic acid (castor oil) is widely exploited for industrial
utilisation (Mutlu and Meier, 2010). This unusual hydroxy fatty
acid has an R-configured carbon atom and an OH group in
homoallylic position, which are the reasons for its multi-
directional chemical and biochemical transformations (Behr
et al., 2012; Kula et al., 2000; Braga and Belo, 2014). Its S-
configured enantiomer or (Z)-(S)-12-hydroxyoctadec-9-enoic acid
has not been reported, to the best of our knowledge, to occur in the
plant kingdom, although there are several works describing the
presence of ricinoleic acid (configuration not reported) in a family
other than the large spurge family (Euphorbiaceae) (Parveen and
Rauf, 2008; Katagi et al., 2011; Hosamani and Katagi, 2008). It is
common knowledge that the biological activity of the chiral
compound strongly depends on the configuration of its molecule.
Enantiopure hydroxy fatty acids, including the only available (R)-
ricinoleic acid, are desirable, for instance, to study their impact on
ricinoleic acid, its availability, wide range of application and our
experience in the chemical transformation of this acid (Kula et al.,
2000, 1996), we decided to attempt access to its S-configured
enantiomer. In 1982 McGhie et al. (McGhie et al., 1982) tried to
make an inversion of the configuration in (R)-ricinoleic acid to its
optical antipode via the Mitsunobu reaction. However, the authors
did not provide data on the reaction yield or on the purity of the
products obtained. Moreover, the reagents used therein were
hazardous and environmentally hostile, and separation of the
product from two by-products, hydrazinedicarboxylate and
phosphane oxide, is usually problematic (Dembinski, 2004; But
and Toy 2006); hence the conclusion that inversion of chiral
alcohol via the Mitsunobu reaction may not be suitable for large-
scale preparations.
There are a number of papers on the SN2 reaction of secondary
alcohol sulfonates (mesylates) with the alkali metal salts of
carboxylic acids, however, such a substitution has several draw-
backs, especially if it were to be used on a larger scale, which was
discussed in a recent work (Shi et al., 2010). The same authors also
present an interesting protocol for the inversion of secondary
alcohols using the complex of R3N–RCOOH as a nucleophile
providing excellent yields and enantiomer excess for several
alcohols. The reaction is carried out in toluene at elevated
temperature (60–110 ꢀC), and 2–9 equivalents of the tunable
amine-acid complex are necessary. We tried to apply this
procedure to transform methyl (R)-ricinoleate into its optical
antipode. The best result under the conditions tried (TEA–AcOH
* Corresponding author. Tel.: +48 426313418.
E-mail address: jozef.kula@p.lodz.pl (J. Kula).
http://dx.doi.org/10.1016/j.chemphyslip.2014.06.005
0009-3084/ã 2014 Published by Elsevier Ireland Ltd.

138
J. Kula et al. / Chemistry and Physics of Lipids 183 (2014) 137–141
3:6 equiv., 70 ꢀC, 15 h) allowed us to obtain the inverted product in
ca. 25% yield (R-1–S-1). However, the optical purity of the sample,
which was specially purified by column chromatography (99.6%,
(t), 29.05 (t, 22.51 (t), 2 ꢂ CH2), 28.98 (t), 27.38 (t), 25.00 (t), 24.88
(t), 14.00 (q).
GC) was unsatisfactory, showing [
compared to the optical rotation of the substrate ester, [
a
]D = ꢁ2.40 (c 8, CHCl3) as
2.2. Synthesis of (Z)-(S)-12-acetoxy-octadec-9-enoic acid methyl ester
(3)
a
]D = 3.03
(vide infra). This may indicate that this homoallylic alcohol is very
sensitive to the reaction conditions.
Recently, we turned our attention to the green protocol for the
nucleophilic substitution reaction of sulfonate esters by recyclable
ionic liquids as published by Yajun Liu (Liu et al., 2012) to develop
the transformation of (R)-ricinoleic acid into its S-enantiomer.
Crude mesylate 2 (42 g, 99 mmol) and 1-butyl-3-methylimida-
zolium acetate (20.2 g, 102 mmol) were stirred at 40 ꢀC for 48 h.
Then water (80 mL) was added and the product was extracted with
hexane (3 ꢂ100 mL), washed with water (100 mL) and dried over
anhydrous MgSO4. The solvent was evaporated to obtain a brown
liquid (34 g) which was preliminarily purified by flash chromatog-
raphy (silica gel, hexane/acetone 90:10) to get rid of the unreacted
2. Materials and methods
mesylate (monitored by TLC and 1H NMR) and
a little of
Methyl (R)-ricinoleate R-1 was prepared by common trans-
esterificarion of commercial castor oil with methanol in the
presence of sodium methoxide to deliver a product (b.p. 173–
177 ꢀC/1.5 Torr) of 92% purity (GC). Triethylamine, methanesulfonyl
chloride and ion liquid, 1-butyl-3-methylimidazolium acetate
(BMIM-OAc), were purchased from Sigma–Aldrich.
unsaturated esters. After solvent evaporation, crude diester 3
(26.8 g), 78% pure (GC), was obtained. A sample of the crude
material (0.8 g) was purified by flash chromatography on silica gel
column using hexane/acetone (95:5) as eluent to afford 97% pure
diester 3 (0.38 g). [
a
]D = ꢁ21.8 (c 2.3, CHCl3). IR (cmꢁ1, neat):
2925.8, 2854.9,1738.1,1239.5,1196.6,1170.8,1022.7, 724.9. 1H NMR
Flash chromatography: silica gel for TLC. GC–MS was performed
with a Trace GC Ultra chromatograph coupled with a DSQII mass
(CDCl3, d ppm): 5.45 (m,1H), 5.34 (m,1H), 4.86 (m,1H), 3.66 (s, 3H),
2.30 (m, 4H), 2.02 (s, 3H), 1.99 (m, 2H), 1.57 (m, 4H), 1.29 (m, 16H),
0.87 (t, J = 6.5 Hz, 3H). GC–MS (EI, 70 eV), m/z): 294 (M+ ꢁ 60), 43
(100%), 262 (5), 207 (5),150 (8),124 (9), 96 (20), 81 (24), 67 (40), 55
(37).
spectrometer (Thermo Scientific) equipped with
a Rxi-1ms
capillary column (60 m long, 0.25 mm inside diameter, 0.25 mm
film thickness), temperature program 50–310 ꢀC at 4 ꢀC/min.
Chiral-GC–MS was performed with a Trace GC Ultra chromato-
graph coupled with an ISQ mass spectrometer (Thermo Scientific)
equipped with an RT-BetaDEX-sm capillary column (30 m long,
0.32 mm inside diameter, 0.25 mm film thickness), temperature
program 50–140 ꢀC at 4 ꢀC/min (held for 47 min), then to 230 ꢀC at
20 ꢀC/min. 1H NMR (250 MHz) and 13C NMR (62.90 MHz) were
recorded using CDCl3 solutions with TMS as internal standard
(Bruker DPX-250 Avance). 13C NMR multiplicity was determined
using DEPT experiments. Purity of the products was confirmed by
both GC and TLC analyses. Optical rotations were measured with an
Autopol IV polarimeter (Rudolph Research) and IR spectra were
obtained with the FT-IR spectrometer Nicolet 6700 (Thermo
Scientific).
2.3. Methanolysis of diester 3
A mixture of crude diester 3 (26 g, 47 mmol) and anhydrous
methanol (100 mL) containing sodium methoxide (0.216 g,
0.004 mmol) was refluxed for 2.5 h, then the methanol was
distilled off (60 mL). Water (50 mL) and concentrated hydrochloric
acid (4 mL) were added to the residue and the product was
extracted with hexane (3 ꢂ 25 mL). The combined extracts were
washed neutral with water and dried over anhydrous MgSO4 to
give, after solvent evaporation, crude hydroxy ester S-1 (22.2 g,
purity 77% by GC), which was subjected to silica gel flash
chromatography (hexane/acetone 94:6) to get 99.6% pure (GC)
hydroxy ester S-1 (12.45 g, 36% total yield based on pure R-1).
2.1. Preparation of (Z)-(R)-12-methanesulfonyloksy-octadec-9-enoic
acid methyl ester (mesylate 2)
[
a
]D = ꢁ2.81 (c 5.0, CHCl3). IR (cmꢁ1, neat): 3422.5, 2925.8, 2854.6,
1740.9, 1197.3, 1172.3, 725.5. 1H NMR (CDCl3,
d ppm): 5.53 (m, 1H),
5.42 (m,1H), 3.66 (s, 3H), 3.60 (m,1H), 2.29 (t, J = 7.5 Hz, 2H), 2.20 (t,
J = 6.5 Hz, 2H), 2.04 (m, 2H),1.51 (M, 3H),1.45 (m, 3H),1.29 (m,15H),
0.87 (t, J = 6.5 Hz, 3H). 13C NMR (CDCl3): 174.24 (s), 133.21 (d),
125.20 (d), 71.42 (d), 51.37 (t), 36.79 (t), 35.30 (t), 34.01 (t), 31.78 (t),
29.51 (t), 29.30 (t), 29.02 (t, 2 ꢂ CH2), 27.31 (t), 25.66 (t), 24.85 (t),
22.56 (t), 14.02 (q). GC–MS (EI, 70 eV), m/z): 294 [M+ ꢁ 18 (3)], 55
(100), 198 (24), 124 (48), 98 (45), 96 (53), 84 (58), 82 (43), 74 (73),
69 (50).
Methyl (R)-ricinoleate (R-1) (37.5 g, 110 mmol) was dissolved in
dichloroethane (230 mL), and triethylamine (24.2 g, 239.9 mmol)
was added. The mixture was cooled to ꢁ5 ꢀC and methanesulfonyl
chloride (20.7 g, 180.2 mmol) was dropped carefully during
agitation, maintaining the temperature below +5 ꢀC. Then the
reagents were stirred for 3 h at room temperature and acidified
with hydrochloric acid (230 mL, 2 M HCl). The organic layer was
separated and washed twice with other portions of hydrochloric
acid (2 ꢂ100 mL, 2 M) and then three times with water (3 ꢂ 60 mL).
After the organic solution was dried over anhydrous MgSO4, the
solvent was removed by a rotavapor to give a brown product, which
was preliminarily purified by passing through a silica gel (20 g)
column using hexane/acetone (80:20 v/v) as eluent to furnish
crude mesylate 2 (43 g, 91% yield). A sample of the crude material
(0.8 g) was purified by flash chromatography on silica gel column
using hexane/acetone (90:10) as eluent to afford pure mesylate 2
2.4. Hydrolysis of hydroxy ester S-1 to (S)-ricinoleic acid
A mixture of hydroxy ester S-1 (2 g), methanol (20 mL) and KOH
(2.5 g) dissolved in water (10 mL) was refluxed for 2 h. Then
methanol was distilled off (10 mL) and the mixture was acidified
with concentrated hydrochloric acid (2.5 mL). The product was
extracted with ethyl acetate (3 ꢂ 20 mL), washed with water
(2 ꢂ15 mL) and dried over anhydrous MgSO4. The solvent was
evaporated to obtain (S)-ricinoleic acid (1.98 g, 100% pure by TLC
(0.41 g). [
a
]D = +16.33 (c 5.0, CHCl3). IR (cmꢁ1, neat): 2927, 2855,
ppm): 5.51 (m, 1H), 5.38 (m,
1737, 1172, 905, 725. 1H NMR (CDCl3,
d
and 1H NMR). [
a
]D = ꢁ3.54 (c 2.89, CHCl3). IR (cmꢁ1, neat): 3337.3,
1H), 4.67 (qn J = 6.25 Hz, 1H-12), 3.65 (s, 3H-MeO), 2.98 (s, 3H-
MeS), 2.44 (m, 2H), 2.29 (dd, J = 7.25 and 7.75 Hz 2H), 2.01 (m, 2H),
1.65 (m, 4H), 1.29 (m, 16H), 0.87 (t, J = 6.6 Hz, 3H). 13C NMR (CDCl3):
174.28 (s, C1),133.76 (d, C9),123.01 (d, C10), 83.59 (d, C12), 51.42 (q,
MeO), 38.64 (q, MeS), 34.21 (t), 34.04 (t), 32.50 (t), 31.60 (t), 29.35
2968.7, 2928.1, 2856.8, 1709.3, 1160.5, 1128.0, 950.5, 816.2. 1H NMR
(CDCl3, d ppm): 6.20 (br.m,1H), 5.52 (m,1H), 5.40 (m,1H), 3.62 (qn,
J = 6.25 Hz, 1H), 2.33 (t, J = 7.40 Hz, 2H), 2.21 (t, J = 7.0 Hz, 2H), 2.03
(m, 2H), 1.62 (m, 2H), 1.46 ꢁ1.28 (m, 19H), 0.87 (t, J = 6.75 Hz, 3H).).
13C NMR (CDCl3): 179.49 (s), 133.26 (d), 125.14 (d), 71.62 (d), 26.70

J. Kula et al. / Chemistry and Physics of Lipids 183 (2014) 137–141
139
Scheme 1. Transformation of methyl (R)-ricinoleate into methyl (S)-ricinoleate.
(t), 36.22 (t), 34.01 (t), 31.79 (t), 29.47 (t), 29.30 (t), 28.98 (t,
2 ꢂ CH2), 28.91 (t), 27.29 (t), 25.64 (t), 24.63 (t), 22.58 (t), 14.04 (q).
mesylate was +16.3 (c 5, chloroform). Worth noting is the fact that
the mesylate is thermally unstable, so that it could not be analysed
by GC. Crude mesylate was used for further procedures. In the next
step, mesylate 2 was subjected to a configuration inversion
reaction with ionic liquid, 1-butyl-3-methylimidazolium acetate
(BMIM-OAc) (Liu et al., 2012), to give acetate 3. The desired product
was obtained in moderate to good yields (Table 1) because of the
2.5. Methyl (R)-ricinoleate and (R)-ricinoleic acid
For comparison, pure (99.8%, GC) methyl (R)-ricinoleate R-1 was
isolated from crude (starting) material by flash chromatography
(silica gel, hexane/acetone 96:4). [
a]D = +3.03 (c 3.86, CHCl3). Its IR
tendency of the mesylate to the unwanted b-elimination.
and NMR spectra were in accordance with those reported in
(Borsotti et al., 2001; Negelmann et al., 1997). Hydrolysis of the
ester in the manner announced above delivered (R)-ricinoleic acid:
[a]D = +3.89 (c 3.86, CHCl3), spectra (IR, NMR) which were identical
with those recorded for (S)-ricinoleic acid.
This step, occurring in an SN2 fashion, of the R-1 to S-1
transformation is crucial and is worth being discussed herein.
It could be observed (Table 1) that at a higher reaction
temperature the competitive side reaction toward double unsatu-
rated esters increased dramatically (Table 1, entry 1). On the other
hand, at a lower reaction temperature the yields were better but
the reaction time had to be prolonged significantly. The
heterogeneity of the reagents seemed to be responsible, to some
extent, for the reaction time. We observed that the reaction
mixture remained heterogeneous both at 20 ꢀC (room tempera-
ture) and at 30 ꢀC, while raising the reaction temperature to 70 ꢀC
allowed the reaction mixture to become a clear solution. From the
preparative standpoint, precise separation of diester 3 from the
unreacted mesylate 2 is very important, otherwise in the next step
of the process (alcoholysis), substrate R-1 could be regenerated,
thus reducing the enantiomeric ratio. Therefore, the reaction
product (Table 1, entry 5), after work up was chromatographically
(FC) purified to collect one fraction (free of mesylate) containing
double unsaturated esters and diester 3 (78%, GC) and was called
crude diester 3.
2.6. Ozonolysis of methyl ricinoleates 1
Ozonolysis of crude methyl (R)-ricinoleate (3 g) was carried out
in a manner reported elsewhere (Kula et al., 2000), but the 98%
pure (GC) hydroxy acetal (R-4) was isolated by flash chromatogra-
phy (hexane/ethyl acetate 90:10) in 76% yield. [
a
]D = ꢁ8.95 (c 3,
CHCl3). GC–MS (EI, 70 eV), m/z): 204 (M+, 0), 173 (2), 119 (5), 97 (3),
87 (26), 75 (100), 59 (54), 58 (17), 55 (11), 43 (12). Its IR and NMR
spectra were identical with those published earlier (Kula et al.,
2000). Ozonolysis of crude methyl (S)-ricinoleate (3 g) in the same
manner delivered 98% pure hydroxy acetal S-4. [a]D = 8.33 (c 3.4,
CHCl3). Its MS data matched the above.
3. Results and discussion
For analytical purposes a pure sample (97%, GC) of diester S-3
was isolated by FC (silica gel, hexane/acetone 95:5). Its measured
optical rotation was equal to ꢁ21.8 (c 2.3, chloroform), while the
literature data for R-3 obtained by enzymatic resolution of racemic
3.1. Inversion of the configuration by ionic liquid
The starting material, methyl (R)-ricinoleate (R-1), was
produced by transesterification of commercial castor oil with
methanol to deliver methyl ricinoleate of 92% purity (GC). The
hydroxy ester was then quantitatively converted to its methane-
sulfonate (mesylate) 2, the purity of which was monitored by 1H-
and 13C-NMR spectra (Scheme 1).
hydroxyl ester
1 amounts to +23.45 (c 1.06, chloroform)
(Nagarajan, 1999).
3.2. Characteristics of (S)-ricinoleic acid and methyl (S)-ricinoleate
A proton multiplet at 3.63 ppm (12-H) disappeared to shift to
4.75. Also, in 13C-NMR the signal at 71.41 (12-C) disappeared to
become visible at 83.58 ppm. After purification by flash chroma-
tography (FC) on silica gel, the specific optical rotation of the
Transesterification of crude diester 3 was carried out in
methanol in the presence of a catalytic amount of sodium
methoxide to give crude S-1. Pure methyl (S)-ricinoleate (>98%,
GC) was isolated by flash chromatography and its optical rotation
Table
1
Substitution reaction of mesylate with ionic liquid under various conditions.
Entry
[BMIM][OAc] Eqv.
Conversion (%)a
Yield (%)b
Temp. (ꢀC)
Time (h)
1
2
3
4
5
1
1
1.3
1.03
1.03
ꢃ90
ꢃ70
ꢃ75
80
30
63
63
65
68
90
70
70
40
40
0.5
2
2
20
48
90
a
Monitored by 1H-NMR as decay of the CH3-S singlet at 2.98.
Yields were determined by TLC and 1H-NMR.
b

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J. Kula et al. / Chemistry and Physics of Lipids 183 (2014) 137–141
3.3. Optical purity study by GC and GC–MS
Significant differences in the value of the optical rotations
reported for both ricinoleic acid and its methyl ester make it
impossible to obtain clear results as far as optical purity of the
compounds is concerned. Therefore, we tried to find the chiral GC
and HPLC conditions under which the optical resolution of mixed
enantiomers 1 could be observed. Unfortunately, all attempts to
obtain resolution both for methyl ricinoleates and ricinoleic acid
derivatives (trimethylsiloxyricinoleic acid trimethylsilyl ester,
trifluoroacetylricinoleic acid methyl ester, perfluoropropionylrici-
noleic acid methyl ester) failed, thereby frustrating this potential
method, likely due to high symmetry of the molecules. However,
ozonolysis of methyl (S)-ricinoleate (S-1) and methyl (R)-ricino-
leate (R-1) in methanol resulted in the formation of respective
hydroxy acetals S-4 and R-4 (Kula et al., 2000) producing,
consequently, a volatile molecule of high asymmetry (Fig. 1).
The enantiomers of acetals 4 could be effectively separated on a
column with a chiral stationary phase (RT-BetaDEX-sm), and the
results are shown in Fig. 1. Their identity was confirmed by GC–MS
and NMR analyses (Fig. 2).
It seems that there is no reason why during the ozonolysis
process, partial racemisation of the substrate hydroxy esters 1
could take place, and that is why the outcomes based on
enantiomer composition of the acetals can be transferred onto
the optical purity of the starting methyl ricinoleates. Thus,
enantiomer excess (ee) of the title methyl (S)-ricinoleate is
95.6% and this is in good agreement with the value obtained by
polarimetric measurements (vide supra). The same refers to the (S)-
ricinoleic acid reported in this paper. Although ricinoleic acid
isolated from castor oil is generally considered to be optically pure,
according to the best of our knowledge no work has been published
confirming this notion by the chromatographic method. This is
very likely to be the first report indirectly showing high ee (99.2%)
of (R)-ricinoleic acid, which is a popular natural compound. In the
light of the above the first use of newly obtained (S)-ricinoleic acid
was to aid in determining the enantiomeric composition of the
major constituent of commercial castor oil.
Fig. 1. Ozonolysis product of (S)-ricinoleate methyl ester.
amounted to ꢁ2.81 (c 5, chloroform), while the reported data for
the R-enantiomer (measured in chloroform) are higher and some
are divergent: +3.4 (Borsotti et al., 2001; Negelmann et al., 1997),
+3.33 (Castillo et al., 2008). In this situation, we decided to obtain
methyl R-ricinoleate of maximum high purity (>99.5%, GC) starting
from the castor oil we had at our disposal in order to determine its
optical rotation. Rotation measurements were performed three
days after product isolation and did not change thereafter. The
resulting value, {[a]D = +3.03 (c 6.56, chloroform)} was, however,
much lower than the values given in the literature. Thus, based on
the specific optical rotation of the substrate material that we had at
our disposal, the inversion of configuration of this homoallylic
alcohol R-1 occurred in slightly more than 95% to provide a (Z)-(S)-
12-hydroxy-octadec-9-enoic acid methyl ester (S-1), independent
of the optical purity of the starting material. This result suggests
that the nucleophilic substitution of mesylate 2 by BMIM-OAc may
also proceed, to some extent, by the SN1 mechanism.
Hydrolysis of pure methyl esters R-1 and S-1 by conventional
method resulted in the formation of corresponding hydroxy acids
whose optical rotations measured in chloroform were +3.89 and
ꢁ3.54, respectively. Although the literature data for optical
rotations of R-ricinoleic acid are very divergent: ꢁ1.05 (c 0.69,
CHCl3) (Nagarajan, 1999), +3.9 (c 1.0, CHCl3) (Borsotti et al., 2001),
and +3.8 (c 1.1, chloroform) (Negelmann et al., 1997), our results in
this respect are consistent with those received for methyl
ricinoleates and as referred above.
Fig. 2. Chromatograms (chiral-GC) of hydroxy acetals 4 showing, indirectly, the optical purity of (R)- and (S)-ricinoleic acids and their methyl esters: (a) mixture of
enantiomers R-4 and S-4; (b) hydroxy acetal R-4 obtained by ozonolysis of methyl (R)-ricinoleate; (c) hydroxy acetal S-4 obtained by ozonolysis of methyl (S)-ricinoleate.

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4. Conclusions
In summary, the simple and effective procedure for the
transformation of (R)-ricinoleic acid into its antipode of high
optical purity has been developed. The reaction protocol uses, in
the key step of the process, an ionic liquid as a reagent that may be
regenerated and reused several times (Liu et al., 2012). The
developed protocol provides easy access to (S)-ricinoleic acid and
its methyl ester with a 36% total yield based on pure methyl (R)-
ricinoleate, which is available from commercial castor oil. The
enantiomeric purity of the product is 95.6% ee, which was shown
by chiral GC analysis. This analytical procedure was also
successfully used to determine the enantiopurity of ricinoleic
acid and its methyl ester available from commercial castor oil; it
turned out to be 99.2% ee.
Conflict of interest
Kula, J., Quang, T.B., Sikora, M., 2000. Synthesis of enantiomerically pure volatile
compounds derived from (R)-3-hydroxynonanal. Tetrahedron: Asymmetry 11,
943–950.
Kula, J., Sikora, M., Sadowska, H., Piwowarski, J., 1996. Short synthetic route to the
enantiomerically pure (R)-(+)-y-decalactone. Tetrahedron 52, 11321–11324.
Liu, Y., Xu, Y., Jung, S.H., Chae, J., 2012. A facile and green protocol for nucleophilic
substitution reactions of sulfonate esters by recyclable ionic liquids [bmim][X].
Synlett 23, 2692–2698.
The authors declare that there are no conflicts of interest.
Transparency document
The Transparency document associated with this article can be
found in the online version.
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