Liquid chromatographic direct resolution of flecainide and its analogs on a chiral stationary phase based on (+)-(18-Crown-6)-2,3,11,12-tetracarboxylic acid

Article abstract of DOI:10.1002/chir.20818

Flecainide, an antiarrythmic agent, and its analogs were resolved on a high performance liquid chromatographic chiral stationary phase (CSP) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid with the use of a mobile phase consisting of methanol-acetonitrile-trifluoroacetic acid-triethylamine (80/20/0.1/0.3, v/v/v/v). The chiral resolution was quite successful, the separation factors (α) and the resolutions (RS) for 20 analytes including flecainide being in the range of 1.19-1.82 and 1.73-6.80, respectively. The ortho-substituent of the benzoyl group of analytes was found to cause decrease in the retention times of analytes probably because of the conformational deformation of analytes originated from the steric hindrance exerted by the ortho-substituent.

Full text of DOI:10.1002/chir.20818

CHIRALITY 22:693–698 (2010)
Liquid Chromatographic Direct Resolution of Flecainide and its
Analogs on a Chiral Stationary Phase Based on (1)-(18-Crown-
6)-2,3,11,12-tetracarboxylic Acid
*
AREUM LEE, HEE JUNG CHOI, AND MYUNG HO HYUN
Department of Chemistry and Chemistry Institute for Functional Materials,
Pusan National University, Busan 609-735, Republic of Korea
ABSTRACT
Flecainide, an antiarrythmic agent, and its analogs were resolved on a
high performance liquid chromatographic chiral stationary phase (CSP) based on (1)-
(18-crown-6)-2,3,11,12-tetracarboxylic acid with the use of a mobile phase consisting of
methanol-acetonitrile-trifluoroacetic acid-triethylamine (80/20/0.1/0.3, v/v/v/v). The
chiral resolution was quite successful, the separation factors (a) and the resolutions
(RS) for 20 analytes including flecainide being in the range of 1.19–1.82 and 1.73–6.80,
respectively. The ortho-substituent of the benzoyl group of analytes was found to cause
decrease in the retention times of analytes probably because of the conformational de-
formation of analytes originated from the steric hindrance exerted by the ortho-substitu-
C
VV
ent. Chirality 22:693–698, 2010.
2009 Wiley-Liss, Inc.
KEY WORDS: chiral stationary phase; chiral resolution; enantiomer separation; flecainide;
liquid chromatography; (1)-18-crown-6)-2,3,11,12-tetracarboxylic acid
INTRODUCTION
pounds.8 Especially CSP 1 (Fig. 1) based on (1)-(18-
crown-6)-2,3,11,12-tetracarboxylic acid (2, Fig. 1) has been
known to be quite effective for the resolution of a-,9 b-,10
and g-amino acids,11 racemic primary amines, amino alco-
hols,12 racemic fluoroquinolone antibacterials13 and di-
and tri-peptides.14 While crown ether-based CSPs have
been known to be useful only for the resolution of racemic
primary amino compounds, CSP 1 was found to be effec-
tive for the resolution of secondary amino compounds. For
Flecainide, N-(2-piperidylmethyl)-2,5-bis(2,2,2-trifluoro-
ethoxy)benzamide, has proven to be an effective antiar-
rythmic agent used as its acetate salt form in the treatment
of cardiac arrythmias.1 Flecainide is a chiral drug contain-
ing one chiral center and consequently it is very important
to develop enantioselective analytical methods of separat-
ing and quantifying the two enantiomers to elucidate their
stereoselective differences in pharmacokinetics and phar-
macodynamics.
example, methoxyphenamine containing
a secondary
amino group and secondary amino alcohols related to b-
blockers have been resolved on CSP 1.15,16 However, race-
mic cyclic secondary amino compounds have not been
resolved on CSP 1. Flecainide is actually a cyclic second-
ary amino compound. Consequently, resolution of flecai-
nide on CSP 1 will be the first example for the resolution
of cyclic secondary amino compound on crown ether-
based CSPs. In this study, we wish to extend the use of
CSP 1 to the resolution of flecainide (3) and its analogs
(4–22) shown in Figure 2. To elucidate the role of the 2,5-
di(2,2,2-trifluoroethoxy)benzoyl group of flecainide in the
chiral recognition, analogs containing various benzoyl
group were prepared and their chromatographic behaviors
on CSP 1 were compared with each other.
Among various enantioselective analytical methods, liquid
chromatographic separation of enantiomers on chiral station-
ary phases (CSPs) has been known to be the most accurate,
convenient and economic means for the exact determination
of the enantiomeric composition or the enantiomeric purity
of chiral compounds.2 However, only a few efforts have
been devoted to the development of enantioselective analyti-
cal methods of separating and quantifying the two enantiom-
ers of flecainide on liquid chromatographic CSPs. For exam-
ple, the recoveries of (R)- and (S)-flecainide from animal
plasma and tissues have been quantitatively determined by
HPLC on Chiralpak AD column.3 Flecainide has also been
resolved by supercritical fluid chromatography coupled to
electrospray mass spectrometry on Chiralcel OJ column.4
CSPs based on immobilized proteins were also used in the
separation of the two enantiomers of flecainide by HPLC.5,6
In addition, cation exchange CSPs based on b-amino sul-
fonic acid-terminated dipeptide derivatives were used to sep-
arate the two enantiomers of flecainide by nonaqueous capil-
lary electrochromatography.7
Contract grant sponsor: MOEST; Contract grant number: NCRC Program
(R15-2006-022-03001-0)
*Correspondence to: Myung Ho Hyun, Department of Chemistry, Pusan
National University, Busan 609-735, Republic of Korea.
E-mail: mhhyun@pusan.ac.kr
Received for publication 23 September 2009; Accepted 23 October 2009
DOI: 10.1002/chir.20818
Published online 14 December 2009 in Wiley InterScience
Crown ether-based CSPs have been known to be very
effective for the resolution of racemic primary amino com- (www.interscience.wiley.com).
C
VV
2009 Wiley-Liss, Inc.

694
LEE, CHOI, AND HYUN
Fig. 1. Structures of CSP 1 and (1)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (2).
Fig. 2. The structures of flecainide (3) and its analogs (4-22).
Chirality DOI 10.1002/chir

RESOLUTION OF FLECAINIDE AND ITS ANALOGS
695
TABLE 1. Resolution of flecainide (3) and its analogs (4, 8, 13, and 18) on CSP 1 with the variation of the content of
ethanol (EtOH) or methanol (MeOH) in acetonitrile (ACN) as mobile phase containing trifluoroacetic acid
(TFA)-triethylamine (TEA) of the constant ratio [EtOH or MeOH-ACN-TFA-TEA, x/(100-x)/0.1/0.5, v/v/v/v]a
3
4
8
13
18
Alcohol content (x)
in acetonitrile
k1
a
RS
k1
a
RS
k1
a
RS
k1
a
RS
k1
a
RS
30% EtOH
50% EtOH
80% EtOH
30% MeOH
50% MeOH
80% MeOH
10.26 1.28 2.40 52.79 1.23 1.44 47.19 1.23 1.68 13.77 1.53 2.59 68.95 1.17 1.38
10.02 1.32 2.80 47.10 1.25 1.54 41.94 1.26 1.77 13.61 1.57 2.74 64.28 1.17 1.44
14.87 1.38 3.15 51.93 1.29 1.85 43.99 1.31 2.26 18.51 1.63 3.06 79.95 1.21 1.56
7.52 1.37 3.45 38.02 1.28 2.13 34.66 1.29 2.47 11.52 1.62 3.17 45.55 1.21 1.81
6.37 1.40 3.91 29.08 1.32 2.38 25.85 1.34 2.69 10.11 1.65 3.40 35.09 1.24 2.04
8.46 1.48 4.25 31.44 1.38 2.87 27.50 1.41 2.86 12.76 1.73 3.92 41.10 1.29 2.48
aFlow rate: 1.0 ml/min. Detection: 254 nm UV. Column temperature: 208C. k1: Retention factor of the first eluted enantiomer. a: Separation factor.
RS: resolution.
(a) and the resolution (RS) improved continuously. How-
EXPERIMENTAL
ever, the retention factors (k1) decreased as the ethanol
Chromatography was performed with an HPLC system
content was increased from 30 to 50% and then increased
consisting of a Waters model 510 HPLC pump (Milford,
as the ethanol content was increased from 50 to 80%.
MA), a Rheodyne model 7725i injector (Rohnert Park, CA)
with a 20 ll sample loop, a Waters model 484 detector
(Milford, MA) and a YongLin Autochro Data Module (soft-
When the ethanol content was increased further, the
retention times increased too much to be useful. The rea-
son for the minimum retention factors at 50% ethanol in
ware: YongLin Autochro 2000). The temperature of the
acetonitrile is not clear yet. When ethanol in the mobile
chiral column was set at 208C by using a Julabo F30 Ultra-
phase was changed to methanol, retention factors (k1)
temp 2000 cooling circulator (Seelbach, Germany). Chiral
decreased and the separation factors (a) and the resolu-
column packed with CSP 1 [Chirosil RCA(1); 250 mm 3
tions (RS) improved as shown in Table 1. By changing
4.6 mm I.D.] was available from RS tech (Daejeon, Korea).
ethanol to methanol, the mobile phase polarity is expected
Flecainide (3, Fig. 2) was purchased from Sigma Aldrich.
to increase and consequently retention factors (k1) might
Flecainide analogs (4–22, Fig. 2) were prepared by treat-
ing 2-(aminomethyl)piperidine with an appropriate acid
chloride (1.0 equivalent) in the presence of triethylamine
decrease, but the reason for the improved chiral recogni-
tion efficiency is not clear yet.
The trends of the retention factors (k1), the separation
(1.2 equivalent) in methylene chloride at 08C. The struc-
factors (a) and the resolutions (RS) for the resolution of
tures of flecainide analogs (4–22) thus prepared were iden-
selected analytes on CSP 1 with the variation of the meth-
anol content in acetonitrile were exactly consistent with
those observed with the variation of the ethanol content in
acetonitrile. As an example, the chromatograms for the re-
tified by 1H NMR spectra. Each of flecainide (3) and
its analogs (4–22) was dissolved in methanol (usually
1.0 mg/ml) and then used for the resolution on CSP 1. The
usual injection volume was 2.0 ll.
solution of flecainide on CSP 1 with the variation of metha-
nol content in acetonitrile at the constant ratio of trifluoro-
acetic acid and triethylamine (0.1/0.5. v/v) are presented
in Figure 3. From Table 1 and Figure 3 it is concluded
RESULTS AND DISCUSSION
Secondary amino alcohols related to b-blockers were
reported to be successfully resolved on CSP 1 by using a
mixture of ethanol-acetonitrile-trifluoroacetic acid-triethyl-
amine with the ratio of 20/80/0.1/0.5 (v/v/v/v) as a mo-
bile phase.16 However, the mobile phase utilized for the re-
solution of secondary amino alcohols related to b-blockers
was found not to be the optimal one in the resolution of fle-
cainide (3) and its analogs (4–22) shown in Figure 2 on
CSP 1 and consequently, we tried to find the better mobile
phase condition.
As an effort to find the improved mobile phase condi-
tion, we selected five analytes including flecainide (3, 4,
8, 13, and 18). As a first step we resolved the five
selected analytes on CSP 1 with the variation of the ratio
of ethanol-acetonitrile in mobile phase at the constant ratio
of trifluoroacetic acid-triethylamine (0.1/0.5, v/v). The
Fig. 3. Chromatograms for the resolution of flecainide (3) on CSP 1
with the variation of methanol content in mobile phase consisting of metha-
nol-acetonitrile-trifluoroacetic acid-triethylamine (x/100-x/0.1/0.5, v/v/v/v)
chromatographic resolution results are summarized in
Table 1. As the content of ethanol in acetonitrile was
increased from 30 to 50 and then 80%, the separation factor at 208C. Flow rate: 1.0 ml/min. Detection: 254 nm UV.
Chirality DOI 10.1002/chir

696
LEE, CHOI, AND HYUN
TABLE 2. Resolution of flecainide (3) and its analogs (4, 8, 13, and 18) on CSP 1 with the variation of the ratio
of trifluoroacetic acid (TFA)-triethylamine (TEA) in 80 % methanol in acetonitrile as mobile phase
(MeOH-ACN-TFA-TEA, 80/20/x/y, v/v/v/v)a
3
4
8
13
18
TFA–TEA ratio in
mobile phase
k1
a
RS
k1
a
RS
k1
a
RS
k1
a
RS
k1
a
RS
0.05–0.5 (v/v)
0.1–0.5 (v/v)
0.3–0.5 (v/v)
0.1–0.4 (v/v)
0.1–0.3 (v/v)
5.79 1.35 3.02 21.65 1.30 2.27 19.65 1.31 2.49
8.46 1.48 4.25 31.44 1.38 2.87 27.50 1.41 2.86 12.76 1.73 3.92 41.40 1.29 2.48
8.34 1.39 3.38 23.38 1.32 2.15 20.77 1.34 2.76 9.99 1.56 2.97 35.17 1.25 2.18
10.34 1.49 5.00 36.60 1.40 2.73 32.45 1.40 3.22 16.04 1.74 4.38 50.83 1.32 2.48
15.37 1.57 5.04 48.58 1.43 3.12 42.59 1.45 3.23 22.77 1.82 6.80 69.70 1.34 2.92
9.09 1.54 2.79 27.76 1.23 1.84
aFlow rate: 1.0 ml/min. Detection: 254 nm UV. Column temperature: 208C. k1: Retention factor of the first eluted enantiomer. a: Separation.
that methanol is better than ethanol as an alcohol compo- and Figure 4, indicating that 0.1% trifluoroacetic acid in
nent in mobile phase and the higher content of methanol the mobile phase is the best condition. When the content
in acetonitrile is desirable for the better separation factors of triethylamine was decreased from 0.5 to 0.4 and then to
(a) and the resolutions (RS), but too much content of 0.3% at the constant content of trifluoroacetic acid (0.1%),
methanol in acetonitrile should not be used to maintain the retention factors, separation factors and resolutions
the retention times short enough to be useful. Overall the improved generally as shown in Table 2 and Figure 5.
optimal content of methanol in mobile phase for the reso- Decreasing the content of triethylamine in mobile phase
lution of flecainide (3) and its analogs (4–22) on CSP 1 further was found to increase the retention times of the
seems to be 80%.
two enantiomers too much. From the chromatographic re-
Variation of the ratio of trifluoroacetic acid-triethylamine solution results summarized in Tables 1 and 2, it is con-
in the mobile phase consisting of 80% methanol in acetoni- cluded that the optimal mobile phase for the resolution of
trile is also expected to affect the chromatographic behav- flecainide and its analogs is the mixture of methanol-aceto-
iors for the resolution of selected analytes on CSP 1. In nitrile-trifluoroacetic acid-triethylamine with the ratio of
Table 2, the chromatographic results for the resolution of 80/20/0.1/0.3 (v/v/v/v).
selected five analytes on CSP 1 with the variation of the ra-
The optimum mobile phase consisting of methanol-ace-
tio of trifluoroacetic acid-triethylamine in 80% methanol in tonitrile-trifluoroacetic acid-triethylamine (80/20/0.1/0.3,
acetonitrile are summarized. The representative chromato- v/v/v/v) was applied to the resolution of all of the 20 ana-
grams for the resolution of flecainide with the variation of lytes shown in Figure 2 and the chromatographic resolu-
the content of trifluoroacetic acid or with the variation of tion results are summarized in Table 3. Elution orders of
the content of triethylamine in mobile phase on CSP 1 are the two enantiomers were not determined because opti-
presented in Figures 4 and 5. When the content of tri- cally active samples were not available. Among the 20 ana-
fluoroacetic acid was decreased from 0.1 to 0.05 % or lytes resolved, flecainide (3) shows the shortest retention
increased from 0.1 to 0.3% at the constant content of trie- time. Analyte 4 containing simple nonsubstituted benzoyl
thylamine (0.5%), the retention factors, the separation fac- group shows the highest retention time among 20 analytes
tors and the resolutions decreased as shown in Table 2 except for analytes 7 and 18 containing 3,5-dinitro or 4-
Fig. 4. Chromatograms for the resolution of flecainide (3) on CSP
1 with the variation of trifluoroacetic acid content in mobile phase consist-
ing of methanol-acetonitrile-trifluoroacetic acid-triethylamine (80/20/x/0.5,
v/v/v/v) at 208C. Flow rate: 1.0 ml/min. Detection: 254 nm UV.
Fig. 5. Chromatograms for the resolution of flecainide (3) on CSP 1 with
the variation of triethylamine content in mobile phase consisting of methanol-
acetonitrile-trifluoroacetic acid-triethylamine (80/20/0.1/x, v/v/v/v) at 208C.
Flow rate: 1.0 ml/min. Detection: 254 nm UV.
Chirality DOI 10.1002/chir

RESOLUTION OF FLECAINIDE AND ITS ANALOGS
697
TABLE 3. Resolution of flecainide (3) and its analogs
(4–22) on CSP 1 with the use of a mixture of methanol-
acetonitrile-trifluoroacetic acid-triethylamine
However, the interaction between the CSP and analytes
affected by the conformational deformation should be non-
enantioselective because the enantioselectivities denoted
by the separation factors for the resolution of some ana-
lytes containing ortho-substituted benzoyl group are not
reduced, but even improves especially for the resolution of
analytes 3 and 13.
(80/20/0.1/0.3, v/v/v/v) as a mobile phasea
Analytes
k1
k2
a
RS
3
4
5
6
7
8
9
15.37
48.58
30.85
38.42
65.68
42.59
31.14
27.34
36.57
35.58
22.77
46.26
43.71
38.99
44.74
69.70
29.80
30.69
40.04
31.75
24.13
69.47
42.88
50.33
74.88
61.76
41.73
33.90
50.10
46.61
41.44
60.60
57.26
51.08
56.82
93.40
37.25
36.52
48.85
39.69
1.57
1.43
1.39
1.31
1.14
1.45
1.34
1.24
1.37
1.31
1.82
1.31
1.31
1.31
1.27
1.34
1.25
1.19
1.22
1.25
5.04
3.12
3.19
3.15
1.46
3.23
2.73
2.11
2.86
3.01
6.80
2.98
2.88
3.10
2.69
2.92
2.35
1.73
2.38
2.06
Electron donating or withdrawing ability of the substitu-
ent at the benzoyl group of analytes does not produce no-
table effects on the resolution of analytes. For example,
analytes containing electron donating group at the para-
position of benzoyl group (analytes 8 and 11) and analy-
tes containing electron withdrawing group at the para-posi-
tion of benzoyl group (analytes 14–18) do not show sig-
nificant difference in their retention factors, separation fac-
tors and resolutions except for the relatively large
retention factor for the resolution of analyte 18. Resolu-
tion of analyte 21 containing 1-naphthoyl group was not
quite different from others. Interestingly, even analyte 22
containing phenylacetyl group show the chromatographic
resolution behaviors which are quite similar to those for
the resolution of other analytes. Consequently, we
expected that even analytes containing simple aliphatic
acyl group can be resolved on CSP 1 and we concluded
that the two 2,2,2-trifluoroethoxybenzoyl groups of flecai-
nide do not play a significant role in the chiral recognition
even though the exact chiral recognition mechanism is
not clear yet.
10
11
12
13
14
15
16
17
18
19
20
21
22
aFlow rate: 1.0 ml/min. Detection: 254 nm UV. Temperature, 208C; k1,
retention factor of the first eluted enantiomer; k2, retention factor of the
second eluted enantiomer; a, separation factor; RS, resolution.
In summary, in this study, racemic cyclic secondary
nitrobenzoyl group. The polar nitro groups of analytes 7 amino compounds such as flecainide and its analogs were
resolved on CSP 1. A mixture of methanol-acetonitrile-tri-
and 18 are expected to induce nonenantioselective inter-
action with the CSP (for example, hydrogen bonding inter- fluoroacetic acid-triethylamine (80/20/0.1/0.3, v/v/v/v)
action between the nitro group of analytes and the amide was found to be the optimal mobile phase condition.
Under the optimal mobile phase condition, 20 analytes
long retention times. Analytes containing 3,5-disubstituted including flecainide and its analogs were resolved quite
NÀÀH hydrogen of the CSP) and consequently bring about
benzoyl group (analytes 5–7) show relatively large reten- well. The type of the substituent(s) on the benzoyl group
of analytes did not affect the chiral recognition behaviors
of analytes significantly, but the ortho-substituent was
tion factors compared to that of flecainide. Consequently,
the shortest retention time of flecainide can be expected
to stem from the two 2,2,2-trifluoroethoxy group attached found to decrease the retention times of analytes probably
because of the conformational deformation originated
from the steric hindrance exerted by the ortho-substituent.
especially to the 2 and 6 position of the benzoyl group.
The relatively large size of 2,2,2-trifluoroethoxy group
might be responsible for the short retention time. In addi-
tion, the ortho-position of 2,2,2-trifluoroethoxy group
might be responsible for the short retention time. Among
analytes containing para-, meta- or ortho-methylbenzoyl
group (analytes 8–10) and analytes containing para-,
meta- or ortho-methoxybenzoyl group (analytes 11–13),
those containing ortho-methyl or ortho-methoxybenzoyl
group (10 and 13) show the shortest retention times. In
addition, analytes 19 and 20, which contain ortho-substi-
tuted benzoyl group also show relatively short retention
times compared with those of analytes containing para-
substituted benzoyl group (analytes 14–18). From these
results, it is concluded that the ortho-substitution at the
benzoyl group of analytes reduces the retention times of
analytes. Even though the reason is not clear yet, the con-
formational deformation of analytes originated from the
steric hindrance exerted by the ortho-substitution seems
to inhibit the interaction between the CSP and analytes
and consequently the retention times are relatively short.
LITERATURE CITED
1. Holmes B, Heel RC. A preliminary review of its pharmacodynamic
properties and therapeutic efficacy. Drugs 1985;29:1–33.
2. Subramanian G, editor. Chiral separation techniques:
a practical
approach, third completely revised and updated edition. Weinheim:
Wiley-VCH; 2007.
3. Hanada K, Akimoto S, Mitsui K, Hashiguchi M, Ogata H. Quantitative
determination of disopyramide, verapamil and flecainide enantiomers
in rat plasma and tissues by high-performance liquid chromatography.
J Chromatogr B 1998;710:129–135.
4. Garzotti M, Hamdan M. Supercritical fluid chromatography coupled
to electrospray mass spectrometry: a powerful tool for the analysis of
chiral mixtures. J Chromatogr B 2002;770:53–61.
5. Oda Y, Asakawa N, Yoshida Y, Sato T. Direct-injection high-perform-
ance liquid chromatographic analysis of drug enantiomers in plasma
with an avidin column coupled on-line to an ovomucoid column.
J Pharm Sci 1992;1227–1228.
6. Blom Ylva, Heldin E. Direct resolution of racemic drugs using cellu-
lose silica as a chiral stationary phase. J Chromatogr A 1993;653:138–
143.
Chirality DOI 10.1002/chir

698
LEE, CHOI, AND HYUN
7. Hebenstreit D, Bicker W, Lammerrhofer M, Lindner W. Novel enan-
12. Hyun MH, Jin JS, Koo HJ, Lee W. Liquid chromatographic resolution
of racemic amines and amino alcohols on a chiral stationary phase
derived from crown ether. J Chromatogr A 1999;837:75–82.
tioselective strong cation exchangers based on sulfodipeptide selec-
tors: evaluation for enantiomer separation of chiral bases by nonaqu-
eous capillary electrochromatography. Electrophoresis 2004;25:277–
289.
13. Hyun MH, Han SC, Koo HJ, Lee W. Separation of the stereoisomers
of racemic fluoroquinolone antibacterial agents on a crown-ether-
based HPLC chiral stationary phase. Chromatographia 2000;52:473–
476.
8. Choi HJ, Hyun MH. Liquid chromatographic chiral separations by
crown ether-based chiral stationary phases. J Liq Chromatogr Rel
Technol 2007;30:853–875.
14. Conrad U, Chankvetadze B, Scriba GKE. High performance liquid
chromatographic separation of dipeptide and tripeptide enantiomers
using a chiral crown ether stationary phase. J Sep Sci 2005;28:2275–
2281.
9. Hyun MH, Jin JS, Lee W. Liquid chromatographic resolution of race-
mic amino acids and their derivatives on a new chiral stationary phase
based on crown ether. J Chromatogr A 1998;822:155–161.
10. Hyun MH, Cho YJ, Jin JS. Liquid chromatographic direct resolution of
b-amino acids on a chiral crown ether stationary phase. J Sep Sci
2002;25:648–652.
15. Steffeck RJ, Zelechonok Y, Gahm KH. Enantioselective separation of ra-
cemic secondary amines on a chiral crown ether-based liquid chroma-
tography stationary phase. J Chromatogr A 2002;947:301–305.
11. Lee SJ, Cho HS, Choi HJ, Hyun MH. Liquid chromatographic resolu-
tion of vigabatrin and its analogue g-amino acids on chiral stationary
phases based on (1)-(18-crown-6)-2,3,11,12-tetracarboxylic acid.
J Chromatogr A 2008;1188:318–321.
16. Zhang D, Li F, Kim DH, Choi HJ, Hyun MH. Resolution of b-blockers
on chiral stationary phases based on (1)-(18-crown-6)-2,3,11,12-tetra-
carboxylic acid: unusual temperature effect. J Chromatogr A 2005;
1083:89–95.
Chirality DOI 10.1002/chir