Liquid chromatographic resolution of mexiletine and its analogs on crown ether-based chiral stationary phases

Article abstract of DOI:10.1002/chir.22318

Mexiletine, an effective class IB antiarrhythmic agent, and its analogs were resolved on three different crown ether-based chiral stationary phases (CSPs), one (CSP 1) of which is based on (+)-(18-crown-6)-2,3,11,12- tetracarboxylic acid and the other two (CSP 2 and CSP 3) are based on (3,3'-diphenyl-1,1'-binaphthyl)-20-crown-6. Mexiletine was resolved with a resolution (R_S) of greater than 1.00 on CSP 1 and CSP 3 containing residual silanol group-protecting n-octyl groups on the silica surface, but with a resolution (R_S) of less than 1.00 on CSP 2. The chromatographic behaviors for the resolution of mexiletine analogs containing a substituted phenyl group at the chiral center on the three CSPs were quite dependent on the phenoxy group of analytes. Namely, mexiletine analogs containing 2,6-dimethylphenoxy, 3,4-dimethylphenoxy, 3-methylphenoxy, 4-methylphenoxy, and a simple phenoxy group were resolved very well on the three CSPs even though the chiral recognition efficiencies vary with the CSPs. However, mexiletine analogs containing 2-methylphenoxy group were not resolved at all or only slightly resolved. Among the three CSPs, CSP 3 was found to show the highest chiral recognition efficiencies for the resolution of mexiletine and its analogs, especially in terms of resolution (R_S). Chirality 26:272-278, 2014. 2014 Wiley Periodicals, Inc.

Full text of DOI:10.1002/chir.22318

CHIRALITY 26:272–278 (2014)
Liquid Chromatographic Resolution of Mexiletine and Its Analogs on
Crown Ether-Based Chiral Stationary Phases
*
KAB BONG JIN, HEE EUN KIM, AND MYUNG HO HYUN
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan, Republic of Korea
ABSTRACT
Mexiletine, an effective class IB antiarrhythmic agent, and its analogs were resolved
on three different crown ether-based chiral stationary phases (CSPs), one (CSP 1) of which is based
on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid and the other two (CSP 2 and CSP 3) are based on
(3,3’-diphenyl-1,1’-binaphthyl)-20-crown-6. Mexiletine was resolved with a resolution (RS) of greater
than 1.00 on CSP 1 and CSP 3 containing residual silanol group-protecting n-octyl groups on the sil-
ica surface, but with a resolution (RS) of less than 1.00 on CSP 2. The chromatographic behaviors for
the resolution of mexiletine analogs containing a substituted phenyl group at the chiral center on the
three CSPs were quite dependent on the phenoxy group of analytes. Namely, mexiletine analogs
containing 2,6-dimethylphenoxy, 3,4-dimethylphenoxy, 3-methylphenoxy, 4-methylphenoxy, and a
simple phenoxy group were resolved very well on the three CSPs even though the chiral recognition
efficiencies vary with the CSPs. However, mexiletine analogs containing 2-methylphenoxy group
were not resolved at all or only slightly resolved. Among the three CSPs, CSP 3 was found to show
the highest chiral recognition efficiencies for the resolution of mexiletine and its analogs, especially
in terms of resolution (RS). Chirality 26:272–278, 2014. © 2014 Wiley Periodicals, Inc.
KEY WORDS: chiral stationary phase; enantiomer separation; liquid chromatography; mexiletine;
mexiletine analogs
INTRODUCTION
primary amino compounds.10–15 CSP 1 was also very success-
ful in the resolution of secondary amines.16–20 In addition,
CSP 2 and CSP 3 (Fig. 1) based on (3,3’-diphenyl-1,1’-
binaphthyl)-20-crown-6 were very successful in the resolution
of α- and β-amino acids, fluoroquinolone compounds, amino
alcohols, and various primary amino compounds.21–27 How-
ever, study of the liquid chromatographic resolution of
mexiletine and its analogs on crown ether-based CSPs is
quite rare. Only CSP 2 was used for the resolution of racemic
mexiletine during the evaluation of the CSP for the resolution
of various primary amino compounds, but the baseline sepa-
ration was not obtained.22 In this study, we prepared various
mexiletine analogs by changing the methyl group at the chi-
ral center of mexiletine to the substituted phenyl group and
by varying the 2,6-dimethylphenoxy group of mexiletine to
another substituted phenoxy group. Racemic mexiletine and
its analogs thus prepared were resolved on CSP 1, CSP 2,
and CSP 3 and the chromatographic behaviors for the resolu-
tion of racemic mexiletine and its analogs on the three CSPs
were compared.
Mexiletine, 1-(2,6-dimethylphenoxy)propan-2-amine, (Fig. 1)
has been known to be an effective class IB antiarrhythmic
agent and approved as a racemic form for the treatment of
ventricular arrhythmias.1,2 Between the two enantiomers
of mexiletine, (À)-(R)-enantiomer was reported to posses
greater antiarrhythmic properties than the opposite enantio-
mer.3 In addition, the two enantiomers of mexiletine analogs
were reported to show different activity in producing a tonic
block or as blockers of voltage-gated Na+ channels and, con-
sequently, the enantioselective synthesis of mexiletine ana-
logs has attracted much attention.4,5 In this instance, the
exact determination of the enantiomeric composition or en-
antiomeric purity of mexiletine and its analogs are quite
important.
For the determination of the enantiomeric composition or
purity of mexiletine and its analogs, the liquid chromato-
graphic method utilizing chiral stationary phases (CSPs)
has been applied. For example, Chiralcel OD, OD-R, and
OD-H (cellulose tris-3,5-dimethylphenylcarbamate) columns
from Daicel Chemical Industries (Tokyo, Japan) were suc-
cessfully used for the liquid chromatographic determination
of the enantiomeric composition or purity of mexiletine and
its analogs.4,5 Chiralcel OD-H and OJ-H (cellulose tris-4-
methyl-benzoate) columns were also used for the resolution
of N-acyl derivatives of mexiletine.6,7
EXPERIMENTAL
A high-performance liquid chromatography (HPLC) system consisting of
a Waters model 515 HPLC pump (Milford, MA), a Rheodyne model 7725i
injector (Rohnert Park, CA) with a 20-μl sample loop, a Waters 2487 detec-
tor, and a YoungLin Autochro data module (Software: YoungLin Autochro-
WIN 2.0 plus) was used for the liquid chromatography. The chiral column
Mexiletine and its analogs contain a primary amino group
at their chiral center. Racemic compounds containing a pri-
mary amino group have been successfully resolved on crown
ether-based CSPs.8,9 In this instance, mexiletine and its ana-
logs are expected to be resolved on crown ether-based CSPs.
Among various crown ether-based CSPs, CSP 1 (Fig. 1)
based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was
very successful in the resolution of α-, β-, and γ-amino acids,
fluoroquinolone compounds, amino alcohols and various
© 2014 Wiley Periodicals, Inc.
Contract grant sponsor: National Research Foundation of Korea; Contract
grant number: NRF-2013R1A2A2A04014576.
*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 15 January 2014; Accepted 05 February 2014
DOI: 10.1002/chir.22318
Published online 27 March 2014 in Wiley Online Library
(wileyonlinelibrary.com).

RESOLUTION OF MEXILETINE AND ITS ANALOGS
273
temperature was maintained at 20°C by using a JEIO TECH VTRC-620
cooling circulator (Daejeon, Korea). Chiral columns packed with CSP 1,
CSP 2, and CSP 3 were available from prior studies.10,21,23
Mexiletine was available as its HCl salt from Sigma-Aldrich Korea
(Seoul, Korea). All mexiletine analogs (Fig. 2) used in this study were
prepared as shown in Scheme 1. As a representative example, the
detailed synthetic procedure for the preparation of mexiletine analog,
2-(2,6-dimethylphenoxy)-1-phenylethanamine (4a), is described. 2-Bromo-
1-phenylethanone (1 g, 5 mmole), 2,6-dimethylphenol (0.92g, 7.5 mmole)
and potassium carbonate (1.04 g, 7.5 mmole) were dissolved in 10mL of
dimethylformamide (DMF). After stirring the mixture for 24h at room tem-
perature under an argon atmosphere, water (30mL) was added and the
whole aqueous solution was extracted with diethyl ether three times. The
organic solution was dried over anhydrous MgSO4 and then the solution
was concentrated using a rotary evaporator. The residue was purified by col-
umn chromatography on silica gel (hexane/ethyl acetate, 5/1, v/v) to afford
intermediate A (Scheme 1, X = H, R1 = R2 = CH3, R3 = R4 = R5 = H) (0.80g,
66%). 1H NMR (CDCl3, ppm) δ: 7.98-7.95(m, 2H), 7.64-7.58(m, 1H), 7.52-
7.46(m, 2H), 7.06-6.94(m, 3H), 5.12(s, 2H), 2.31(s, 6H). A solution of the
intermediate A (Scheme 1, X = H, R1 = R2 = CH3, R3 = R4 = R5 = H) thus
prepared (0.2 g, 0.83 mmole), ammonium acetate (CH3COONH4, 0.8g,
10.4 mmole) and sodium cyanoborohydride (NaBH3CN, 0.2 g, 3.1 mmol)
in 25mL of methanol was heated to reflux for 48h. After cooling, the reac-
tion mixture was acidified by adding concentrated HCl slowly until the pH
became lower than 2. Methanol was removed using a rotary evaporator
and then water was added to the residue. The resulting solution was
extracted with diethyl ether twice. The aqueous layer was made basic by
adding KOH pellets until the pH became higher than 10. The basic solution
was extracted with diethyl ether (20 mL) twice. The organic solution was
dried over anhydrous MgSO4 and then evaporated to afford the mexiletine
analog, 2-(2,6-dimethylphenoxy)-1-phenylethanamine (4a) (0.16g, 80%).
Fig. 1. Structures of mexiletine, CSP 1, CSP 2, and CSP 3.
Fig. 2. Structures of mexiletine analogs.
Scheme 1. Synthetic route for the preparation of mexiletine analogs.
Chirality DOI 10.1002/chir

274
JIN ET AL.
1H NMR (CDCl3, ppm) δ: 7.47-7.42(d, 2H), 7.39-7.27(m, 4H), 7.01-6.98
(d, 1H), 6.94-6.88(q, 1H), 4.48-4.43(q, 1H), 3.87-3.78(m, 2H), 2.26(s, 6H),
1.90(s, 2H). The structures of all other analytes (4b-f, 5a-f, 6a-f, 6aa, 6ab,
and 7a-f) prepared as shown in Scheme 1 were identified by 1H NMR spec-
tra. Injection samples were prepared by dissolving each analyte in methanol
at a concentration of 1.0 mg/mL and an injection size was typically 2.0 μl.
phenoxy)-1-(substituted phenyl)ethanones were treated with
ammonium acetate and sodium cyanoborohydride in metha-
nol to afford 26 analogs of mexiletine.
In total, 27 analytes including mexiletine and its analogs
were resolved on CSP 1, CSP 2, and CSP 3 and the resolu-
tion results are summarized in Table 1. In order to find the
appropriate mobile phase conditions, various mobile phases
were tested and finally we found that methanol or acetonitrile
in water containing sulfuric acid or perchloric acid as an
acidic modifier are appropriate as mobile phases. For the
resolution of mexiletine on CSP 1, 80% methanol in water
containing sulfuric acid (10 mM) was found to be the best mo-
bile phase condition. The acidic modifier added to the mobile
phase has been known to be used to protonate the primary
amino group of analytes. The two enantiomers of resulting
primary ammonium ion (R-NH+3) of analytes have been known
to induce the enantioselective complexation inside the cavity
of the crown ether ring of the CSP.8,9 For the resolution
of mexiletine analogs 4a-4f, 5a-5f, 6aa, 6ab, and 7a-7f on
CSP 1, 80% methanol in water containing perchloric acid
(10 mM) was found to be the most widely applicable as a
mobile phase. However, mexiletine analogs 6a-6f were not
resolved at all with the use of 80% methanol in water
containing perchloric acid (10 mM) as a mobile phase and
even under other various mobile phase conditions.
RESULTS AND DISCUSSION
The two enantiomers of mexiletine and its analogs have
been known to show different activity for blocking sodium
currents of skeletal muscle fibers as mentioned above and
the difference in their activity was found to be dependent on
the structures.4 Especially, mexiletine analogs containing a
substituted phenyl group instead of a methyl group at the
chiral center and/or a substituted phenoxy group instead of
2,6-dimethylphenoxy group were found to show different
activity in their two enantiomers.4,5 In this instance, in order
to see the structural characteristics for the resolution of
mexiletine and its analogs on CSP 1, CSP 2, and CSP 3, vari-
ous mexiletine analogs were prepared according to the proce-
dure shown in Scheme 1. By treating 2-bromo-1-(substituted
phenyl)ethanones with various substituted phenols in the
presence of potassium carbonate in DMF, 2-(substituted
phenoxy)-1-(substituted phenyl)ethanones (intermediate A
in Scheme 1) were prepared. The resulting 2-(substituted
TABLE 1. Resolution of racemic mexiletine and its analogs on CSP 1, CSP 2 and CSP 3
CSP 1
CSP 2
CSP 3
Analytes
k1
α
RS
k1
α
RS
k1
α
RS
Mexiletine
4a
4b
4c
4d
4e
4f
5a
5b
5c
5d
5e
5f
6a
6b
6c
6d
6e
6f
6aa
6ab
7a
7b
7c
7d
7e
7f
1.50a
4.72b
4.35b
5.00b
8.13b
5.03b
3.99b
3.80b
4.10b
4.81b
7.60b
3.87b
3.50b
4.77b
5.34b
6.10b
9.80b
5.03b
4.47b
4.09b
4.15b
4.23b
4.41b
5.43b
8.39b
3.79b
4.92b
1.15
1.38
1.44
1.46
1.47
1.41
1.38
1.41
1.29
1.40
1.31
1.47
1.40
1.00
1.00
1.00
1.00
1.00
1.00
1.43
1.45
1.42
1.32
1.42
1.32
1.42
1.46
1.25
1.95
1.97
1.67
4.23
2.25
1.67
1.90
1.55
2.11
3.05
2.63
2.19
3.17c
2.76c
1.97c
2.53c
2.31c
3.04c
2.79c
4.88d
3.75d
5.10d
3.54d
5.62d
5.13d
2.61d
1.80d
3.76d
1.86d
3.97d
3.81d
3.29d
3.13d
3.70d
1.81d
3.85d
2.23d
4.09d
3.86d
1.20
1.66
1.73
1.86
1.70
1.79
1.61
3.49
2.87
3.13
2.92
3.79
3.62
1.00
1.00
1.00
1.00
1.00
1.00
3.32
3.81
2.30
3.07
2.09
2.31
2.58
2.36
0.67
1.43
1.57
1.64
2.51
1.89
1.38
4.43
3.82
4.21
4.90
4.99
4.76
8.65e
13.32e
14.81e
15.85e
10.98e
15.23e
15.92e
9.99d
8.31d
10.51d
7.43d
10.93d
10.51d
6.06d
5.35d
6.57d
17.13c
6.56d
6.36d
7.31d
7.22d
7.87d
4.86d
8.23d
5.31d
8.58d
8.32d
1.15
1.54
1.72
1.88
1.76
1.83
1.64
4.14
3.23
3.66
3.33
4.51
4.25
1.09
1.00
1.00
1.15
1.10
1.10
3.29
2.72
2.26
2.50
2.10
2.04
2.48
2.34
1.33
2.09
1.91
2.08
4.29
2.57
1.74
7.91
6.09
6.99
12.09
9.08
7.76
1.02
1.19
1.17
1.19
12.50
12.31
4.63
4.10
5.13
7.80
6.68
5.35
3.86
3.79
2.26
1.67
2.38
3.12
2.20
2.94
5.40
4.56
2.94
2.98
2.64
3.21
4.24
3.12
Mobile phase:
a80% Methanol in water + sulfuric acid (10 mM).
b80% Methanol in water + perchloric acid (10 mM).
c80% Acetonitrile in water + perchloric acid (10 mM).
d80% Acetonitrile in water + perchloric acid (10 mM) + ammonium acetate (1.0 mM).
e50% Acetonitrile in water + perchloric acid (10 mM) + ammonium acetate (1.0 mM). Flow rate: 0.5 ml/min. Detection: 254 nm UV. Temperature: 20°C. k1: Retention
factor of the first eluted enantiomer. α: Separation factor. RS: Resolution.
Chirality DOI 10.1002/chir

RESOLUTION OF MEXILETINE AND ITS ANALOGS
275
For the resolution of mexiletine and its analogs 4a-4f on
CSP 2, 80% acetonitrile in water containing perchloric acid
(10 mM) was used as an appropriate mobile phase. However,
for the resolution of mexiletine analogs 5a-5f, 6a-6f, 6aa,
6ab, and 7a-7f on CSP 2, the retention times were quite long
when 80% acetonitrile in water containing perchloric acid
(10 mM) was used as a mobile phase. For example, the reso-
lution of 5e on CSP 2 with the use of 80% acetonitrile in water
containing perchloric acid (10 mM) was found to take more
than 120 min, as shown in Figure 3. However, the addition
of a small amount of ammonium acetate to the mobile phase
was found to decrease the retention times quite a lot. As
shown in Figure 3, when the content of ammonium acetate
added to the mobile phase was increased from 0 to 0.1 to
0.5 and then to 1.0 mM, the retention times were decreased
continuously and the resolution was completed within
40 min without hurting the separation factor (α) and resolu-
tion (RS) noticeably. The ammonium ion (NH+4) added to the
mobile phase is expected to compete with the primary ammo-
nium ion (R-NH+3) of analyte for the complexation inside the
cavity of the crown ether ring of the CSP and reduce the re-
tention times of analytes.8,9 Consequently, for the resolution
of mexiletine analogs 5a-5f, 6aa, 6ab, and 7a-7f on CSP 2,
80% acetonitrile in water containing perchloric acid (10 mM)
and ammonium acetate (1.0 mM) was successfully used as a
mobile phase. However, mexiletine analogs 6a-6f were not
resolved at all under various mobile phase conditions.
For the resolution of mexiletine and its analogs 4a-4f on
CSP 3, 50% acetonitrile in water containing perchloric acid
(10 mM) and ammonium acetate (1.0 mM) was found to be
quite successful. However, for the resolution of 5a-5f, 6a-6f,
6aa, 6ab, and 7a-7f on CSP 3, the retention times were quite
long when 50% acetonitrile in water containing perchloric acid
(10 mM) and ammonium acetate (1.0 mM) was used as a
mobile phase. For example, the resolution of 5d on CSP 3
was found to take almost 4 h. When 80% acetonitrile in water
containing perchloric acid (10 mM) and ammonium acetate
(1.0 mM) was used as a mobile phase, the resolution of 5d
on CSP 3 was completed within 30 min, as shown in Figure 4.
By increasing the acetonitrile content in the aqueous mobile
phase from 50 to 80%, the mobile phase polarity is expected
to be decreased and, consequently, the lipophilic interaction
Fig. 4. Comparison of the representative chromatograms for the resolution
of 5d on CSP 3 with the use of 80% or 50% acetonitrile in water containing
perchloric acid (10 mM) and ammonium acetate (1 mM) as a mobile phase.
Flow rate: 0.5 ml/min. Detection: 254 nm UV. Temperature: 20°C.
between the CSP and analytes is decreased. In this instance,
the retention times should be decreased. The use of 80%
acetonitrile in water containing perchloric acid (10 mM) and
ammonium acetate (1.0 mM) as a mobile phase was quite
successful for the resolution of 5a-5f, 6aa, 6ab, and 7a-7f on
CSP 3. Mexiletine analogs 6a, 6e, and 6f were also resolved
with the use of 80% acetonitrile in water containing perchloric
acid (10 mM) and ammonium acetate (1.0mM) as a mobile
phase. For the resolution of 6d, the use of 80% acetonitrile in
water containing perchloric acid (10 mM) was found to afford
the baseline resolution. However, mexiletine analogs 6b
and 6c were not resolved at all under various mobile phase
conditions.
Even though the resolution of mexiletine on CSP 1, CSP 2,
and CSP 3 was not so significant, when the methyl group at
the chiral center of mexiletine was changed to phenyl (4a)
or 4-substituted phenyl (4b, 4c, 4d, 4e, or 4f), the resolution
was improved quite a lot on each of the three CSPs. However,
the effect of the type of the substituent (electron-donating or
electron-accepting substituent) of the phenyl group at the
chiral center of mexiletine analogs on the separation factors
(α) was not so significant, the separation factors (α) for the
resolution of 4a-4f on CSP 1, CSP 2, and CSP 3 being in
the relatively narrow range of 1.38–1.47, 1.61–1.86, and
1.64–1.88, respectively. On the contrary, the resolutions (RS)
for the resolution of 4a-4f on CSP 1, CSP 2, and CSP 3 were
found to be quite dependent on the type of the substituent on
the phenyl group at the chiral center of the mexiletine ana-
logs. The resolutions (RS) for the resolution of 4a-4f on
CSP 1, CSP 2, and CSP 3 were in the range of 1.67–4.23,
1.38–2.57, and 1.74–4.29, respectively. On each of the three
CSPs, 4d containing 4-nitrophenyl group at the chiral center
was found to give the highest resolution (RS) value, but the
reason is not clear yet.
Change of the 2,6-dimethylphenoxy group of analytes 4a-4f
to 3,5-dimethylphenoxy group (5a-5f) or to simple phenoxy
group (7a-7f) does not affect the chiral recognition on CSP 1
significantly, as shown in Table 1. The separation factors and
resolutions for the resolution of 4a-4f on CSP 1 are in the
range of 1.38–1.47 and 1.67–4.23, respectively, but those for
the resolution of 5a-5f and 7a-7f on CSP 1 are in the range
of 1.29–1.47 and 1.55–3.12, respectively. Change of the
Chirality DOI 10.1002/chir
Fig. 3. Chromatograms for the resolution of 5e on CSP 2 with the variation
of the content of ammonium acetate in aqueous mobile phase consisting of
80% acetonitrile in water containing perchloric acid (10 mM). Flow rate:
0.5 ml/min. Detection: 254 nm UV. Temperature: 20°C.

276
JIN ET AL.
Fig. 5. Chromatograms for the resolution of 4d, 5d, 6d, and 7d on (a) CSP 1, (b) CSP 2, and (c) CSP 3. Mobile phase: 80% methanol in water + perchloric acid
(10 mM) for CSP 1, 80% acetonitrile in water + perchloric acid (10 mM) + ammonium acetate (1.0 mM) for CSP 2, and CSP 3. Flow rate: 0.5 ml/min. Detection:
254 nm UV. Temperature: 20°C.
Chirality DOI 10.1002/chir

RESOLUTION OF MEXILETINE AND ITS ANALOGS
277
2,6-dimethylphenoxy group of analyte 4a to 3-methylphenoxy
(6aa) or to 4-methylphenoxy (6ab) also does not affect the chiral
recognition on CSP 1 significantly. However, when the 2,6-
dimethylphenoxy group of analytes 4a-4f was changed to
2-methylphenoxy group (6a-6f), the chiral recognition was
not observed at all on CSP 1. From these results, the chiral
recognition for mexiletine analogs on CSP 1 was concluded
to be not affected significantly by the type of phenoxy group
of analytes except for the 2-methylphenoxy group.
In contrast, the type of phenoxy group of mexiletine
analogs was found to show a significant effect on the chiral
recognition on CSP 2 and CSP 3. As shown in Table 1,
analytes 5a-5f containing the 3,5-dimethylphenoxy group
were resolved greater than analytes 4a-4f containing the
2,6-dimethylphenoxy group on CSP 2 and CSP 3 in terms
of the separation factors and resolutions. Between CSP 2
and CSP 3, the latter was found to show a greater chiral rec-
ognition ability for analytes 5a-5f than the former. Analytes
7a-7f containing a simple phenoxy group were also resolved
better than analytes 4a-4f on CSP 2 and CSP 3, but slightly
worse than analytes 5a-5 f. Analytes 6aa and 6ab containing
a 3-methyl or 4-methylphenoxy group were also resolved better
than analyte 4a on CSP 2 and CSP 3. However, analytes 6a-6f
containing the 2-methylphenoxy group were not resolved at all
on CSP 2. Analytes 6b and 6c were also not resolved at all on
CSP 3 and analytes 6a, 6d-6f were only slightly resolved on
CSP 3. The reason for the nonresolution or poor resolution of
analytes 6a-6f on the three CSPs is not clear.
All of these results indicate that the chiral recognition be-
haviors for the resolution of mexiletine and its analogs on
CSP 1 are somewhat different from those on CSP 2 and
CSP 3. The different chromatographic behaviors on CSP 1,
CSP 2, and CSP 3 are clearly demonstrated by the trends
of the retention times for the resolution of 4d, 5d, 6d, and
7d on the three CSPs shown in Figure 5. The retention times
of the first eluted enantiomers for the resolution of 4d, 5d,
6d, and 7d on CSP 1 with the use of 80% methanol in water
containing perchloric acid (10 mM) as a mobile phase were
only slightly different and were on the order of 5d < 4d < 7d
6d. However, the retention times for the resolution of 4d,
5d, 6d, and 7d on CSP 2 and CSP 3 under an identical mo-
bile phase condition with the use of 80% acetonitrile in water
containing perchloric acid (10 mM) and ammonium acetate
(1 mM) as a mobile phase were very much different and were
on the order of 4d < 6d < 7d < 5d. CSP 2 and CSP 3 should
be quite lipophilic because of the lipophilic 3,3’-diphenyl-1,1’-
binaphthyl group, while CSP 1 is relatively less lipophilic
compared with CSP 2 and CSP 3. In this instance, the lipo-
philic interaction of analytes with CSP 1 is expected to be
not significant and does not seem to be dependent on the lipo-
philicity of analytes significantly. In this instance, the reten-
tion times of analytes 4d, 5d, 6d, and 7d on CSP 1 should
be similar. However, the lipophilic interaction of analytes with
CSP 2 or CSP 3 is expected to be quite significant because of
the high lipophilicity of CSP 2 or CSP 3, the retention times
of the analytes being quite dependent on the lipophilicity of
analytes. Analyte 5d should be more lipophilic than analyte
6d because of the additional 3,5-dimethyl group. In this in-
stance, the lipophilic interaction of analyte 5d with CSP 2
or CSP 3 should be greater than that of analyte 6d with
CSP 2 or CSP 3 and, consequently, analyte 5d was retained
longer than analyte 6d on CSP 2 or CSP 3. CSP 3 should
be more lipophilic than CSP 2 because of the additional
n-octyl groups on the silica surface of the stationary phase.
In this instance, the analytes should be retained longer on
CSP 3 than on CSP 2. Analytes 4d and 6d might be more
lipophilic than analyte 7d because of the additional dimethyl
or methyl group on the phenoxy ring. However, the retention
times of analyte 4d and 6d are shorter than those of analyte
7d on CSP 2 and CSP 3. The 2,6-dimethyl group of analyte
4d and the 2-methyl group of analyte 6d, which are near the
complexation site, might exert some steric hindrance for the
complexation of the primary ammonium ions of the proton-
ated analytes inside the cavity of the crown ether ring of
the stationary phase and, consequently, reduce the retention
times. The steric hindrance might originate from the steric
interaction between the 3,3’-diphenyl-1,1’-binaphthyl of CSP
2 or CSP 3 and the nearby 2,6-dimethyl or 2-methyl group of
analytes. In addition, the steric hindrance should be more
significant with the 2,6-dimethylphenoxy group than with the
2-methylphenoxy group of the analyte. In this instance, the
retention times for analyte 4d should be shorter than for ana-
lyte 6d. However, steric hindrance is not expected with CSP 1
because of the lack of sterically bulky 3,3’-diphenyl-1,1’-
binaphthyl group.
In conclusion, three crown ether-based CSPs based on (+)-
(18-crown-6)-2,3,11,12-tetracarboxylic acid or (3,3’-diphenyl-
1,1’-binaphthyl)-20-crown-6 were quite successful for the
resolution of mexiletine and its analogs. The chiral recognition
efficiencies of the three CSPs for the resolution of mexiletine
analogs were found to be quite dependent on the type of
phenoxy group of analytes. Even though the CSP based on
(3,3’-diphenyl-1,1’-binaphthyl)-20-crown-6 containing residual
silanol group-protecting n-octyl groups on the silica surface
was found to be most effective for the resolution of mexiletine
and its analogsm especially in terms of resolutions (RS), any
one of the three CSPs is concluded to be useful for the determi-
nation of the enantiomeric composition of mexiletine analogs
containing a primary amino group.
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