Chiral differentiation of novel isoxazoline derivatives on clicked thioether and triazole bridged cyclodextrin chiral stationary phases

Article abstract of DOI:10.1039/c4ra03476g

Isoxazoline derivatives have been disclosed in the art as having acaricidal and insecticidal activity and as potential precursors for the syntheses of natural products. This work first demonstrates the chiral resolution of isoxazoline derivatives that had not been studied before on native cyclodextrin (CD) chiral stationary phases (CSPs). Two structurally well-defined CSPs based on native CD were prepared via different click procedures and applied for the enantioseparation of isoxazolines. Most of the studied isoxazolines were found to be well resolved (R_s > 1.5) under reversed phase mode, especially 4NPh-OPr, which exhibits the best enantioselectivity and resolution (α = 2.22; R_s = 4.16). Optimal resolutions were achieved by evaluating the influences of mobile phase composition, substitution moieties and CSP linkages on the separation. This contribution verifies that excellent enantioseparation of isoxazolines can be accomplished on smartly designed native CD-CSP, which provides a facile and economic way to obtain enantiopure isoxazoline derivatives.

Full text of DOI:10.1039/c4ra03476g

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PAPER
Chiral differentiation of novel isoxazoline
derivatives on “clicked” thioether and triazole
bridged cyclodextrin chiral stationary phases†
Cite this: RSC Adv., 2014, 4, 30492
Xiaobin Yao, Yan Gong, Rehangu Mamuti, Wenwen Xing, Hao Zheng,
*
*
Xiangyang Tang and Yong Wang
Isoxazoline derivatives have been disclosed in the art as having acaricidal and insecticidal activity and as
potential precursors for the syntheses of natural products. This work first demonstrates the chiral
resolution of isoxazoline derivatives that had not been studied before on native cyclodextrin (CD) chiral
stationary phases (CSPs). Two structurally well-defined CSPs based on native CD were prepared via
different click procedures and applied for the enantioseparation of isoxazolines. Most of the studied
isoxazolines were found to be well resolved (Rs > 1.5) under reversed phase mode, especially 4NPh-OPr,
which exhibits the best enantioselectivity and resolution (a ¼ 2.22; Rs ¼ 4.16). Optimal resolutions were
achieved by evaluating the influences of mobile phase composition, substitution moieties and CSP
linkages on the separation. This contribution verifies that excellent enantioseparation of isoxazolines can
be accomplished on smartly designed native CD-CSP, which provides a facile and economic way to
obtain enantiopure isoxazoline derivatives.
Received 17th April 2014
Accepted 20th June 2014
DOI: 10.1039/c4ra03476g
www.rsc.org/advances
the synthesis of a variety of novel isoxazolines via a 1,3-dipolar
cycloaddition reaction between mono-substituted alkenes and
nitrile oxides.15
1. Introduction
Compounds incorporating heterocyclic rings continue to
attract considerable interest owing to their wide range of
interesting biological activities. Amongst them, ve-membered
heterocyclic compounds occupy a conspicuous place in the
eld of natural and synthetic organic chemistry. Isoxazolines,
as a class of ve-membered heterocyclic compounds, have
found wide applications as agrochemical agents and phar-
maceuticals. Such compounds possess various biological
activities such as insecticidal, antibacterial, antibiotic, anti-
tumour, antifungal, antimicrobial, in vitro anti-tuberculosis
activity and anti-inammatory activities.1–9 In addition, iso-
xazoline derivatives have been recognized as important inter-
mediates for the synthesis of alkaloids and other natural
products.10,11
As is known, the bioactivity of chiral compounds is closely
related to its stereochemistry. Isoxazoline enantiomers in
agrochemicals can have diverse effects on plants and insects
and could bring about negative effects to the environment
and be toxic to human health. Hence, it is necessary to obtain
optically pure isoxazolines to make best use of their perfor-
mance and minimize potential harm.16–19 Since the catalytic
asymmetric synthesis is tedious and difficult, most isoxazo-
lines are generated in laboratories with equal amounts of
enantiomers in achiral environments. As a result, it is highly
desirable to build efficient and cost effective enantiosepara-
tion approaches for obtaining the enantiopure isomers from
the abundant isoxazoline racemates.
On a separate note, cyclodextrins (CDs) are naturally occur-
ring cyclic oligosaccharides that consist of several (6, 7, 8)
glucose units and are one of the most commonly used economic
chiral selectors for enantioseparation due to their ability to
form ‘host–guest’ inclusion with a large variety of chiral
compounds. Since Fujimura successfully synthesized CD-CSPs
based on amino linkages for the rst time in 1983, they have
received great interest in the area of separation. Chemists are
constantly searching for new synthetic methods for the prepa-
ration of stable CD-CSPs. Numbers of CD-CSPs with various
linkages have been developed, such as ether linkages by
Amstrong,20 urea linkages by Ng21,22 and triazole linkages by
Liang and Ng.23–27
Small pharmaceutical compounds with substituted isoxazo-
line rings such as some phenylisoxazolines exhibit high activity
against gram-positive pathogens.12 A series of 3-substituted-5-
pyridinyl-isoxazolines can be used as protein tyrosine phos-
phatase 1B inhibitors.13,14 Researchers keep dedicating great
efforts to the development of novel isoxazoline derivatives.
More and more products based on an isoxazole nucleus have
been designed and synthesized. Our previous study described
Department of Chemistry, School of Science, Tianjin University, Tianjin 300072,
China. E-mail: wangyongtju@tju.edu.cn; txy@tju.edu.cn; Tel: +86 22 2740 3475
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra03476g
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In recent years, “click chemistry” has attracted more and
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more interest owing to its advantages such as insensitivity to
water and oxygen as well as the mild reaction conditions.
Typical representations are the Cu(I) catalytic 1,3-dipolar
cycloaddition reaction and radical-based thiol–ene reaction,
which have been widely used in the preparation of various
stationary phases.28–30 Great importance was placed on triazole-
linked CD-CSPs that were prepared by 1,3-dipolar cycloaddition
reaction owing to their simple synthesis, controllable structure,
good stability and powerful resolving ability. Our group has
employed a thiol–ene click reaction for constructing a novel
cationic native CD-CSP for enhanced separation of acidic chiral
compounds in HPLC.31 Both the click reactions provide facile
and economic approaches to obtain structurally well-dened
CD-CSPs.
In this work, we prepared two native CD-CSPs with triazole
and thioether linkages via the above described click procedures
and performed the enantioseparation of 26 novel isoxazoline
enantiomer pairs such as 3-aryl-5-phenyl-isoxazolines (Ar-Ph),
3-aryl-5-(pyridin-4-yl)-isoxazolines (Ar-Py) and 3-aryl-5-(2-oxo-
pyrrolidin-1-yl)-isoxazolines (Ar-POr) by reversed phase high
performance liquid chromatography (HPLC). To the best of our
knowledge, this is the rst report on chiral resolution of iso-
xazoline enantiomers using native CD-CSPs, which is expected
to provide a facile and economic way to obtain enantiopure
isoxazoline derivatives at both the analytical and preparative
levels.
Fig. 1 Structures of isoxazoline derivatives.
2. Experimental
2.1 Chemicals and materials
For CSP preparation, azobisisobutyronitrile (AIBN) was
purchased from Tianjin Chemical Reagents (Tianjin, China), 3-
mercaptopropyltrimethoxysilane and 1-allylimidazole were
purchased from Energy-Chemical (Shanghai, China), and
mono-6A-deoxy-(p-tolylsulfonyl)-b-cyclodextrin (TsO-CD) was
synthesized according to the reported procedure.32 Anhydrous
N,N-dimethylformamide (DMF) and toluene were provided by
Heowns (Tianjin, China). Kromasil spherical silica gel (5 mm,
Fig. 2 Structures of triazole and thioether bridged CD-CSPs.
˚
100 A) was obtained from Eka Chemicals (Bohus, Sweden).
For chromatographic experiments, HPLC-grade methanol
(MeOH), acetonitrile (ACN), triethylamine (TEA) and acetic acid
were provided by Guangfu chemical reagents (Tianjin, China).
Ultra-pure water was prepared using a Milli-Q water purication
system (Billerica, MA, USA). All the isoxazoline racemic pairs
used were synthesized according to our previously reported
procedure,15 and their structures are shown in Fig. 1.
surface coverages of CD for CSP1 and CSP2 were calculated to be
0.85 mmol mꢀ2 and 0.60 mmol mꢀ2, respectively, according to
the following equation:
ꢀ
ꢁ
mmol
m2
ð%CÞð106Þ
%C
¼
100 ꢀ
ðMrÞ
ðS:AÞðncÞð12:001Þ
ðncÞð12:001Þ
where S.A is the surface area of the silica; nc is the carbon
number; Mr is the molecular weight, and C% is from the
elemental analysis results (ESI†).
The particle size distributions of the prepared CSPs are dis-
played in Fig. S3 (ESI†). The electron microscopy images of bare
silica and the CSPs are illustrated in Fig. S4 (ESI†). Most of the
particle sizes fall in the range from 3 to 7 mm, and the
morphology of silica particles does not change much before and
aer reaction.
2.2 Preparation of CSPs
Triazole-bridged CD-CSP (CSP1) and thiolether-bridged CD-CSP
(CSP2) (Fig. 2) were synthesized according to the methods
described in previous reports.31,32 The detailed synthetic
procedures are included in the ESI.† The linkage of CSP1 can
provide hydrogen bonding sites via oxygen and nitrogen as well
as dipole–dipole interactions; the linkage of CSP2 affords elec-
trostatic force on account of cationic heterocyclic ring. The
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following USP standards: k was calculated using k ¼ (tR ꢀ t0)/t0,
where tR is the retention time of the enantiomers and t0 is the
dead time determined by measuring the rst base-line pertur-
bation. a was calculated using a ¼ k2/k1. Rs was calculated using
the equation: 1.18 ꢁ (t2 ꢀ t1)/(Wh1 + Wh2) where Wh is the half
peak width.
2.3 Column packing
The prepared CSPs were dispersed in MeOH followed by
packing into a stainless-steel column (150 mm ꢁ 4.6 mm I.D.)
using typical slurry-packing technique with MeOH as the
packing solvent. The packing session lasted for 30 minutes at a
constant pressure of 5000 psi. The column efficiency for the
studied analytes was calculated by the USP standard.
3. Results and discussion
2.4 Instruments and chromatographic conditions
3.1 Enantioseparation of isoxazoline derivatives on triazole-
bridged CD-CSP (CSP1)
Chromatographic analyses were performed on a Laballiance
HPLC system with a diode array detection (DAD) system (State Initially, ACN/H2O ¼ 40/60 (v/v) or MeOH/H2O ¼ 50/50 (v/v) was
College, PA, USA). ACN/MeOH and deionized water were used as chosen as the MP, and a part of the analytes could be baseline
mobile phases (MP). Samples were dissolved in MeOH/H2O (v/v separated. By optimizing the separation conditions, most of the
¼ 1 : 1) at a concentration of 1 mg mLꢀ1 and the injection analytes were baseline separated within 40 min. The optimal
volume was set as 1 mL. All MP and samples were ltered separation results are listed in Table 1, and several represen-
through a 0.22 mm membrane before usage. The detection was tative chromatograms are shown in Fig. 3. More chromato-
performed at 220–300 nm. Each solution was injected in trip- grams are included in Fig. S5 (ESI†). The encouraging results
licate, and the average value was used. Calculations for capacity demonstrate that CSP1 afforded powerful resolving ability
factor k, selectivity a and resolution Rs were performed towards the studied isoxazoline derivatives.
Table 1 Optimal separation results of isoxazolines on CSP1a
Analytes
Ar-Ph
k1
2.18
k2
3.51
a
Rs
N1
Conditions
MDOPh-Ph
Ph-Ph
4ClPh-Ph
4MOPh-Ph
3ClPh-Ph
4NPh-Ph
3FPh-Ph
4MOPh-Py
4ClPh-Py
MDOPh-Py
4MetPh-Py
Ph-Py
3ClPh-Py
4NPh-Py
1.61
1.52
1.46
1.42
1.28
1.25
1.07
1.39
1.25
1.34
1.26
1.29
1.16
1.14
1.07
1.32
1.29
1.22
1.26
1.20
1.21
1.18
1.16
1.15
1.12
1.09
2.22
1.44
1.48
1.43
1.57
1.40
1.37
1.33
1.17
1.15
3.79
3.05
2.78
3.80
2.65
2.36
0.78
2.03
1.56
2.41
1.97
1.85
1.51
1.46
0.66
2.29
1.60
1.69
1.63
1.59
1.87
1.63
1.63
1.48
1.32
0.79
4.16
2.95
2.87
2.86
2.54
2.37
1.99
1.75
1.64
1.40
2163
2122
2092
3822
3668
3642
3479
2594
3450
2618
2706
2348
3056
3564
2720
3176
2523
3266
2822
3341
3764
3621
4008
3707
4107
3263
2157
2478
2391
3027
2246
2157
1967
2159
2952
3020
ACN/H2O ¼ 40/60
1.76
1.95
5.77
6.65
7.05
22.01
1.38
1.54
3.83
3.72
2.54
6.60
11.16
13.30
2.56
1.44
2.64
1.85
2.82
3.43
3.75
4.85
6.01
9.62
3.79
2.41
2.72
2.62
4.03
2.30
2.22
1.85
2.76
10.02
8.37
2.68
2.84
8.17
8.49
8.84
23.63
1.92
1.93
5.11
4.69
3.28
7.69
12.74
14.27
3.37
1.87
3.22
2.33
3.38
4.15
4.43
5.64
6.90
10.75
4.14
5.36
3.92
3.88
5.78
3.62
3.11
2.53
3.66
11.74
9.63
ACN/H2O ¼ 30/70
ACN/H2O ¼ 20/80
ACN/H2O ¼ 40/60
Ar-Py
ACN/H2O ¼ 30/70
ACN/H2O ¼ 25/75
ACN/H2O ¼ 20/80
ACN/H2O ¼ 15/85
ACN/H2O ¼ 30/70
ACN/H2O ¼ 30/70
ACN/H2O ¼ 25/75
3NPh-Py
4MetPh-2Py
4MetPh-OPr
4ClPh-OPr
4MOPh-OPr
MDOPh-OPr
Ph-OPr
4NPh-OPr
3ClPh-OPr
3FPh-OPr
2ClPh-OPr
3NPh-OPr
4NPh-OPr
4MetPh-OPr
4ClPh-OPr
MDOPh-OPr
3ClPh-OPr
4MOPh-OPr
Ph-OPr
Ar-OPr
ACN/H2O ¼ 20/80
ACN/H2O ¼ 15/85
MeOH/H2O ¼ 50/50
Ar-OPr
3NPh-OPr
2ClPh-OPr
3FPh-OPr
MeOH/H2O ¼ 40/60
MeOH/H2O ¼ 30/70
a
Conditions: ow rate ¼ 0.6 mL minꢀ1, 25 ꢂC.
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As shown in Table 2, Ar-Ph and Ar-Py are more strongly
retained than Ar-OPr under same conditions due to the exis-
tence of two aryl moieties, which can both form a t inclusion
with the CD hydrophobic cavity. ACN and MeOH afford much
different effects towards the separation of the three categories.
For Ph-Ph and Ph-Py, especially Ph-Ph, although the retention is
greatly weakened by transfer of MP from MeOH/H2O to ACN/
H2O (k1 from 8.31 to 2.24), the Rs experiences a dramatic
increase from 1.02 to 3.05; while for Ph-OPr, methanol affords
much better separation (Rs ¼ 1.99) than ACN (Rs ¼ 0.28). The
reason could be attributed to the nature of ACN and MeOH.
MeOH is a protic solvent that can form H-bonding with the
analytes and CSP, hence favoring the separation of the more
polar Ph-OPr, while ACN is aprotic and better in resolving the
more hydrophobic Ph-Ph and Ph-Py.
In addition, the effect of the water content in the mobile
phase was studied with some model analytes, and the results
are plotted in Fig. S6 (ESI†). With a decrease in the water
content of MP, the retention time rst shows a signicant
decline and so does the resolution, which is a typical charac-
teristic of the reversed-phase mode (RPLC). A slight increase in
the retention is found at an ACN content of 90%, indicating the
separation mode starts a transition to hydrophilic chromatog-
raphy (HILIC).
3.1.2 Enantioseparation of compounds in the Ar-Ph cate-
gory. Based on the above-mentioned results, ACN/H2O was
chosen as MP for the separation of compounds in the Ar-Ph
category. The separation results under ACN/H2O ¼ 40/60 (v/v)
are depicted in Fig. 4.
When Ar-Ph interacts with CD-CSP, both the aromatic
groups on the 3- and 5-positions can enter the CD cavity rst to
form an inclusion complex, and together with the additional
hydrogen bonding, p–p interaction as well as steric effects, a
three-point interaction model can be well established. As seen
from Fig. 4, MDOPh-Ph affords the strongest retention and best
Fig. 3 Representative chromatograms on CSP1. Conditions: ACN/
H2O ¼ 40/60 (a), ACN/H2O ¼ 30/70 (b), MeOH/H2O ¼ 50/50 (c and d);
flow rate ¼ 0.6 mL minꢀ1; 25 ꢂC.
3.1.1 Effect of MP compositions on enantioseparation for
CSP1. Since organic modier plays key roles in the enantiose-
paration on CD-CSPs, it is necessary to make clear which kind of
organic modiers is more favorable for the enantioseparation
in this study. According to the substituents on the 5-position of
the isoxazoline ring, the analytes can be classied into three
categories: Ar-Ph (phenyl ring on the 5-position), Ar-Py (pyridine
ring on the 5-position) and Ar-OPr (pyrrolidinone ring on the
5-position). To investigate the effect of the type of organic
modier, we chose several model analytes, Ph-Ph, 4MOPh-Ph,
MDOPh-Ph, Ph-Py, 4MOPh-Py, 4MetPh-Py, Ph-OPr, 4ClPh-OPr,
and 4ClPh-OPr (Fig. 1), to conduct the enantioseparation
experiments, and the separation results are shown in Table 2.
Table 2 Effect of MP compositions on CSP1a
Analytes
Ar-Ph
Conditions
Ph-Ph
k1
k2
a
Rs
I
II
I
II
I
II
I
II
I
II
I
II
I
II
I
1.76
8.77
0.90
8.29
2.18
13.25
1.04
3.55
1.38
9.48
1.21
4.61
0.43
1.85
0.46
2.62
0.44
2.30
2.68
1.52
1.09
1.20
1.08
1.61
1.15
1.26
1.13
1.39
1.08
1.25
1.18
1.21
1.37
1.23
1.48
z—
1.57
3.05
1.02
0.73
0.79
3.79
1.31
1.11
1.00
2.03
1.10
1.13
1.40
0.28
1.99
0.41
2.87
z—
2.54
9.59
1.09
8.95
3.51
15.18
1.32
4.01
1.92
10.26
1.51
5.46
0.52
2.53
0.57
3.88
—
4MOPh-Ph
MDOPh-Ph
Ph-Py
Ar-Py
4MOPh-Py
4MetPh-Py
Ph-OPr
Ar-OPr
4ClPh-OPr
4ClPh-OPr
II
I
II
3.62
a
Conditions: ow rate ¼ 0.6 mL minꢀ1, 25 ꢂC; (I) ACN/H2O ¼ 40/60, (II)
MeOH/H2O ¼ 50/50.
Fig. 4 Enantioseparation of Ar-Ph on CSP1. Conditions: flow rate ¼
0.6 mL minꢀ1, temperature ¼ 25 ꢂC.
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separation, beneting from the tight inclusion between the CD
cavity and the enlarged hydrophobic part on the 3-position. It is
interesting to nd that 4ClPh-Ph shows higher selectivity and
resolution than 3ClPh-Ph, which indicates that p-position
substitution by –Cl on the phenyl ring affords better inclusion
formation ability than that at the m-position. The reduced
retention and separation of 4NPh-Ph and 4MOPh-Ph suggests
their loose inclusion with the CD cavity, which may be due to
the steric hindrance effect. It was found that strong electron-
withdrawing moieties like –CF3 signicantly diminish the
separation ascribed to the increased polarity of the phenyl ring,
which is reected by the very poor resolution of 3FPh-Ph.
Further optimization was conducted by reducing the ACN
proportion to enhance the inclusion complexation. By gradually
decreasing the ACN proportion from 50% to 30%, most of the
analytes can be baseline resolved except 3FPh-Ph, which was
only partially separated even when 20% ACN was used (Table 1).
3.1.3 Enantioseparation of compounds in the Ar-Py cate-
gory. Similar to the separation of compounds in the Ar-Ph
category, substitute of –NO2 on the phenyl ring strongly
weakens the separation of compounds in the Ar-Py category,
such as 4NPh-Py and 3NPh-Py, as shown in Fig. 5, and the 3,4-
methylenedioxy group on MDOPh-Py is helpful for enhancing
the chiral differentiation. In addition, 4ClPh-Py also gains much
better resolution than 3ClPh-Py. However, the –OMe on the
p-position strongly enhances the chiral recognition of Ar-Py
(4MOPh-Py), which is different from compounds in the Ar-Ph
category. This suggests the slight difference in the inclusion
formation mechanism between the two categories with CD
cavity. The pyridine moiety of Ar-Py may form a H-bond with the
–OH on CD rims, hindering its entry into the CD cavity to some
extent. The different separation results of 4MetPh-Py and
4MetPh-2Py may support this speculation. Further optimization
results have been listed in Table 1. Most of the Ph-Py racemates
can be completely resolved (Rs > 1.5) except 3NPh-Py.
Fig. 6 Enantioseparation of Ar-OPr on CSP1. Conditions: flow rate ¼
0.6 mL minꢀ1, temperature ¼ 25 ꢂC.
3.1.4 Enantioseparation of compounds in the Ar-OPr
category. The enantioseparation of compounds in the Ar-OPr
category was performed with MeOH/H2O as MP according to the
results obtained previously. In order to illustrate the substituent
effect of Ar-OPr, Fig. 6 depicts the separation results of Ar-OPr
with MP of MeOH/H2O ¼ 50/50 (v/v), and the optimized results
have been included in Table 1.
As it is found in Fig. 6, the strongest retention falls on
MDOPh-OPr due to the tight inclusion similar to the separation
of MDOPh-Ph and MDOPh-Py, while the a and Rs of MDOPh-
OPr are moderate. This phenomenon indicates that stronger
inclusion does not necessarily bring about better chiral sepa-
ration. As the pyrrolidinone moiety affords H-bonding, dipole–
dipole interaction sites and steric effects, the binding geometry
would play an important role in the chiral differentiation
process. The p-NO2 phenyl ring of 4NPh-OPr may form the most
favorable binding geometry with the CSP, hence showing the
best enantioselectivity (a ¼ 2.22) and resolution (Rs ¼ 4.16),
while the –NO2 at the m-position decreases the separation
remarkably. By a comparison of 4ClPh-OPr, 3ClPh-OPr, 2ClPh-
OPr and Ph-OPr, it was found that the retention and resolution
follows an order of 4ClPh > 3ClPh > Ph > 2ClPh, indicating that
–Cl on the p- and m-positions favors the separation, while –Cl on
the o-position attenuates the chiral differentiation.
Further study of enantioseparations with ACN/H2O as MP
was conducted. It was found that all the Ar-OPr enantiomers are
better separated with MeOH than ACN (Fig. S1†). This indicates
the different recognition mechanisms of Ar-OPr, Ar-Ph and Ar-Py
on CD CSPs. Protic organic modiers are more suitable for the
enantioseparation of isoxazolines with pyrrolidinone moieties.
3.2 Enantioseparation of isoxazolines on thioether bridged
CD-CSP (CSP2)
CSP2 presents similar recognition behaviors as CSP1.
Compounds in the Ar-Ph and Ar-Py categories can be better
separated in the presence of ACN than MeOH, while the reverse
Fig. 5 Enantioseparation of Ar-Py on CSP1. Conditions: flow rate ¼
0.6 mL minꢀ1, temperature ¼ 25 ꢂC.
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Table 3 Optimal separation results of isoxazolines on CSP2a
Analytes
Ar-Ph
k1
1.54
k2
2.01
a
Rs
N1
Conditions
MDOPh-Ph
4ClPh-Ph
Ph-Ph
4MOPh-Ph
4NPh-Ph
3ClPh-Ph
3FPh-Ph
MDOPh-Py
4ClPh-Py
1.30
1.27
1.24
1.17
1.14
1.09
1.05
1.11
1.09
1.09
1.07
1.06
1.02
1.00
1.00
1.10
1.08
1.06
1.06
1.05
1.05
1.04
1.03
1.00
1.00
1.00
1.08
1.09
1.08
1.07
1.07
1.05
1.03
1.00
1.00
1.00
1.99
1.72
1.57
2.19
1.63
1.29
1.00
1.50
1.32
1.17
0.96
0.74
z0
0.00
0.00
1.48
1.16
0.94
0.85
0.81
0.72
0.49
0.42
0.00
0.00
0.00
1.17
1.14
0.99
0.94
0.83
0.68
0.20
0.00
0.00
0.00
2404
2323
2327
4395
4770
5014
—
4243
4352
4435
4494
3648
—
ACN/H2O ¼ 40/60
ACN/H2O ¼ 30/70
1.44
1.40
4.32
4.61
5.41
15.13
8.59
9.21
6.21
10.47
6.98
8.34
5.99
7.07
9.05
11.51
14.29
11.12
14.58
10.31
8.49
15.15
9.96
14.79
6.37
7.92
7.31
6.09
9.55
6.79
9.74
5.10
5.86
7.90
4.14
1.84
1.74
5.07
5.23
5.91
15.94
9.52
10.07
6.75
11.16
7.39
8.48
0.00
0.00
9.92
12.38
15.16
11.76
15.35
10.84
8.82
15.67
0.00
0.00
0.00
8.56
7.94
6.58
10.20
7.23
10.24
5.26
0.00
0.00
0.00
ACN/H2O ¼ 20/80
ACN/H2O ¼ 20/80
Ar-Py
Ph-Py
4MetPh-Py
4MOPh-Py
3ClPh-Py
3NPh-Py
4NPh-Py
—
—
4MetPh-2Py
2ClPh-OPr
4ClPh-OPr
4MOPh-OPr
4MetPh-OPr
4NPh-OPr
Ph-OPr
MDOPh-OPr
3FPh-OPr
3ClPh-OPr
3NPh-OPr
4ClPh-OPr
2ClPh-OPr
4NPh-OPr
4MetPh-OPr
4MOPh-OPr
MDOPh-OPr
Ph-OPr
5222
5159
4934
4828
4891
4158
3269
3106
—
—
—
—
4043
3856
4279
3930
3827
—
Ar-OPr
ACN/H2O ¼ 10/90
Ar-OPr
MeOH/H2O ¼ 30/70
3FPh-OPr
3ClPh-OPr
3NPh-OPr
—
—
—
a
Conditions: ow rate ¼ 0.6 mL minꢀ1, 25 ꢂC. ACN/H2O ¼ 10/90.
situation was found for the separation of compounds in the Ph- Py affords the highest enantioselectivity amongst compounds in
OPr category. By optimizing the separation conditions, most the Ar-Py category. 4ClPh-Py was separated with a selectivity of
analytes of Ar-Ph category were baseline resolved (Table 3), and 1.09 while that of 3ClPh-Py was almost zero. It was also found
typical chromatograms are depicted in Fig. 7. As expected, that no separations were achieved with 3NPh-Py and 4NPh-Py,
MDOPh-Ph affords the best separation due to the t inclusion. which indicates that the –NO2 could decrease the enantiose-
Greater enantioselectivity is achieved for 4ClPh-Ph than for paration under the studied separation conditions. It is inter-
3ClPh-Ph, and 3FPh-Ph shows the poorest separation.
esting to nd that 2ClPh-OPr exhibits better separation than
For compounds in the Ar-Py and Ar-POr categories, only 3ClPh-OPr on CSP2, which is different from the separation
partial separation is achieved. Similar to MDOPh-Ph, MDOPh- results on CSP1. This suggests that CSP linkages may have a
signicant inuence on the separation process.
3.3 Comparison of CSP1 and CSP2
Although CSP1 and CSP2 are both based on native CD, the
minor difference in their structure may lead to remarkably
distinct recognition abilities. The linkage of CSP1 can provide
strong H-bonding sites, dipole–dipole and p–p interactions,
and CSP2 affords an electrostatic force due to the cationic
heterocyclic ring on the linkage. Assisted by the strong elec-
trostatic attraction, CSP2 has been proven to provide superior
Fig. 7 Representative chromatograms on CSP2.ꢀ1Conditions: ACN/
H2O (v/v) ¼ 30/70; 25 ꢂC; flow rate ¼ 0.6 mL min
.
separation efficiency than CSP1 towards some aryl acidic
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from Sigma Aldrich), was used for a comparison by the sepa-
ration of a group of analytes (Ph-Ph, MDOPh-Ph, Ph-Py, 4ClPh-
Py, Ph-OPr, 4ClPh-OPr) under the same conditions. It is inter-
esting that CYCLOBOND I 2000 only afforded enantiosepara-
tion ability toward 4ClPh-OPr with a selectivity of 1.16 (1.48 for
CSP1 and 1.08 for CSP2) under MeOH/H2O ¼ 50/50, while most
of the above analytes can be well enantioseperated on CSP1 and
CSP2. This result suggests that the well-dened CSP structures
can afford superior separation of the studied isoxazoline
derivatives.
4. Conclusions
For the rst time, “click”-derived thioether and triazole bridged
native cyclodextrin chiral stationary phases were reported to
provide good enantioseparation of isoxazoline derivatives.
Aprotic acetonitrile is favorable for the enantioseparation of low
polar Ar-Ph and Ar-Py, while protic methanol is a better organic
modier for highly polar Ar-OPr. Isoxazoles with electron-rich
groups can be resolved easier than those possessing electron-
decient parts. For most isoxazoline analytes, the separation of
the p-substitution on the 3-phenyl ring has advantages over that
of o- and m-substitutions. In addition, CSP's resolution prop-
erties can be nely tuned by altering the linkages to introduce
necessary effects such as H-bonding, p–p and dipole–dipole
interactions. This work provides an effective and economic
approach for the acquisition of optically pure substituted iso-
xazoline enantiomers.
Fig. 8 Comparison of the two CSPs for enatioseparation. Conditions:
(a) ACN/H2O (v/v) ¼ 40/60, 25 ꢂC; (b) ACN/H2O ¼ 40/60 (CSP1), ACN/
H2O ¼ 20/80 (CSP2), 25 ꢂC; (c) ACN/H2O ¼ 30/70 (CSP1), ACN/H2O ¼
10/90 (CSP2), ꢂC; (d) MeOH/H2O ¼ 50/50 (CSP1), MeOH/H2O ¼ 30/70
(CSP2), 25 ꢂC.
racemates in our previous study. In this work, the comparisons
of the two CSPs for the enatioseparation of isoxazolines with
same MP are plotted in Fig. 8.
It can be seen clearly that CSP1 affords higher enantiose-
lectivity compared to most of the studied samples. On one hand,
the higher surface CD loading of CSP1 may contribute to the
better separation; on the other hand, the abundant interaction
sites on the linkage of CSP1 must play important roles in the
differentiation process aer the formation of an inclusion
complex. As shown in Fig. 8a, CSP1 and CSP2 afford close sepa-
ration ability for compounds in the Ar-Ph category. The decreased
enantioselectivity from MDOPh-Ph to 3FPh-Ph reveals a similar
chiral differentiation mechanism for compounds in the Ar-Ph
category on CSP1 and CSP2. However, for compounds in the Ar-
Py category, CSP1 exhibits a much better separation capability
than CSP2 (Fig. 8b). Hence, it is reasonable for us to believe that
the triazole linkage on CSP1 is more favorable for the separation
process via the supply of H-bonding and dipole–dipole interac-
tions. This view is further conrmed by the poorer separation of
compounds in the Ar-OPr category, which bear more polar pyr-
rolidinone moieties on CSP2 (Fig. 8c). In addition, the differences
in the changing tendencies of the selectivity from 4MOPh-Py to
3NPh-Py (Fig. 8b) and from 4NPh-OPr to 3FPh-OPr (Fig. 8c and d)
suggest different separation mechanisms for compounds in the
Ar-Py and Ar-OPr categories on CSP1 and CSP2.
Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (no. 21205086) and the Tianjin Research Program
of Application Foundation and Advanced Technology
(13JCQNJC05400) is gratefully acknowledged.
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