Preparation of two new diasteromeric chiral stationary phases based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid and (R)- or (S)-1-(1-naphthyl)ethylamine and chiral tethering group effect on the chiral recognition

Article abstract of DOI:10.3390/molecules21081051

Two new diastereomeric chiral stationary phases (CSPs) based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid as a chiral tethering group and a Π-basic chiral unit such as (R)-1-(1-naphthyl) ethylamine (CSP 1) or (S)-1-(1-naphthyl)ethylamine (CSP 2) were prepared. The two CSPs were applied to the enantiomeric separation of N-(3,5-dinitrobenzoyl)-1-phenylalkylamines and N-(3,5-dinitrobenzoyl)-α-amino acid derivatives using 20% isopropyl alcohol in hexane as a normal mobile phase. To elucidate the effect of the two chiral units on the chiral recognition, the chiral recognition abilities of the two CSPs were compared with each other and with that of a CSP (CSP 3) based on (R)-1-(1-naphthyl)ethylamine. From the chromatographic chiral recognition results, (R)-1-(1-naphthyl)ethylamine and (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid constituting CSP 1 were concluded to show a cooperative ("matched") effect on the chiral recognition while (S)-1-(1-naphthyl)ethylamine and (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid constituting CSP 2 were concluded to show an uncooperative ("mismatched") effect on the chiral recognition. From these results, it was concluded that (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid can be successfully used as a chiral tethering group for the preparation of new CSPs.

Full text of DOI:10.3390/molecules21081051

molecules
Article
Preparation of Two New Diasteromeric Chiral
Stationary Phases Based on
(+)-(18-Crown-6)-2,3,11,12-tetracarboxylic Acid and
(R)- or (S)-1-(1-Naphthyl)ethylamine and Chiral
Tethering Group Effect on the Chiral Recognition
Rajalingam Agneeswari, Ji Yeong Sung, Eun Sol Jo, Hee Young Jeon, Vellaiappillai Tamilavan
and Myung Ho Hyun *
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University,
Pusan 609-735, Korea; agneeswarichem@gmail.com (R.A.); jysung@kbsi.re.kr (J.Y.S.);
dmsthf4834@hanmail.net (E.S.J.); heeyoung0906@naver.com (H.Y.J.); tamilavan4811@gmail.com (V.T.)
* Correspondence: mhhyun@pusan.ac.kr; Tel.: +82-51-510-2245
Academic Editor: Yoshio Okamoto
Received: 1 July 2016; Accepted: 9 August 2016; Published: 12 August 2016
Abstract: Two new diastereomeric chiral stationary phases (CSPs) based on (+)-(18-crown-6)-2,3,11,12-
tetracarboxylic acid as a chiral tethering group and a
ethylamine (CSP ) or (S)-1-(1-naphthyl)ethylamine (CSP
were applied to the enantiomeric separation of N-(3,5-dinitrobenzoyl)-1-phenylalkylamines and
N-(3,5-dinitrobenzoyl)- -amino acid derivatives using 20% isopropyl alcohol in hexane as a normal
Π
-basic chiral unit such as (R)-1-(1-naphthyl)
1
2
) were prepared. The two CSPs
α
mobile phase. To elucidate the effect of the two chiral units on the chiral recognition, the chiral
recognition abilities of the two CSPs were compared with each other and with that of a CSP
(CSP
results, (R)-1-(1-naphthyl)ethylamine and (+)
CSP were concluded to show a cooperative (“matched”) effect on the chiral recognition
while (S)-1-(1-naphthyl)ethylamine and (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid constituting
CSP were concluded to show an uncooperative (“mismatched”) effect on the chiral recognition.
3) based on (R)-1-(1-naphthyl)ethylamine. From the chromatographic chiral recognition
−
(18-crown-6)-2,3,11,12-tetracarboxylic acid constituting
1
2
From these results, it was concluded that (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid can be
successfully used as a chiral tethering group for the preparation of new CSPs.
Keywords: chiral stationary phase; chiral tethering group; enantiomer separation; liquid chromatography
1. Introduction
Separating enantiomers on liquid chromatographic chiral stationary phases (CSPs) has been
known to be quite effective as an analytical method for the determination of the enantiomeric
composition of chiral compounds and as a preparative method for the separation of enantiomers [
1–5].
During the last several decades, quite effective CSPs have been developed by utilizing various chiral
selectors. For example, polysaccharide derivatives [
cyclofructans [11 12], -acidic or -basic aromatic chiral compounds [13
chiral ligand exchange materials [17 18] and chiral crown ethers [19
6
,
7
], cyclodextrins [
8
], macrocyclic antibiotics [
9
,
,
10],
16],
,
Π
Π
,
14], cinchona alkaloids [15
,
–
21] have been widely used as
chiral selectors for the development of CSPs. Those chiral selectors are based on an assemblage of
repeating or non-repeating chiral subunits playing together for chiral recognition or based on a single
chiral unit. In addition, diastereomeric chiral selectors consisting of two chiral units have been reported
to show the “matched” effect on the chiral recognition when the two chiral units are cooperative and,
consequently, enhance the chiral recognition or the “mismatched” effect on the chiral recognition
Molecules 2016, 21, 1051; doi:10.3390/molecules21081051
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Molecules 2016, 21, 1051
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when the two chiral units are uncooperative and, consequently, diminish the chiral recognition,
according to the stereochemistry of the two chiral units. One of the two chiral units has usually been
used as a chiral tethering group when the diastereomeric chiral selectors were bonded to silica gel.
Even though the term of the “matched/mismatched” effect has been originally used to rationalize the
different ratios of two diastereomers synthesized in the reaction of an enantiomerically pure substrate
with an enantiomerically pure reagent [22], the concept of the “matched/mismatched” effect has
also been successfully used to rationalize the chiral recognition behaviors of diastereomeric CSPs.
For example, when the two diastereomeric
Π-basic aromatic chiral compounds consisting of (R)- or
(S)-1-(1-naphthyl)ethylamine and (S)-naproxen were bonded to silica gel through the (S)-naproxen
tethering group, the two chiral units were found to show the “matched” or “mismatched” effect on the
chiral recognition according to the stereochemistry of the 1-(1-naphthyl)ethylamine unit [23]. Similarly,
when diastereomeric chiral crown ethers incorporating (R)-3,3’-diphenyl-1,1’-binaphthyl and (R,R)- or
(S,S)-tartaric acid unit as chiral barriers were bonded to silica gel through the (R,R)- or (S,S)-tartaric
acid unit as a chiral tethering group, the resulting two diastereomeric CSPs were also found to show
the “matched” or “mismatched” effect on the chiral recognition according to the stereochemistry of
the tartaric acid unit [24,25]. However, CSPs based on diastereomers consisting of a Π-basic aromatic
chiral unit and chiral crown ether have not been reported yet.
In this study, two new diastereomeric CSPs (CSP
1
and CSP
2, Figure 1) based on
two chiral units such as (R)- or (S)-1-(1-naphthyl)ethylamine as a
Π
-basic aromatic chiral unit and
(+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid as a chiral tethering group were prepared and applied
to the enantiomeric separation of some racemic chiral compounds under the normal mobile phase
condition. The comparison of the chromatographic chiral recognition results of the two CSPs with
each other and with those of the CSP based on (R)-1-(1-naphthyl)ethylamine as a single chiral unit
(CSP 3, Figure 1) is expected to elucidate the “matched” or “mismatched” effect of the two chiral
units consisting of the two new diastereomeric CSPs on the chiral recognition. Through this study,
we hope to demonstrate that (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid can be used as a useful
chiral tethering group for the preparation of effective CSPs.
Si
Si
Si
Si
Si
NH
O
HN
NH
O
HN
O
O
O
O
O
O
O
O
O
O
O
HN
O
O
O
O
HN
H3C
O
HN
H3C
O
H3C
O
NH
CH3
NH
CH3
O
CSP 1
2
CSP 3
CSP
Figure 1. Structures of CSP 1, CSP 2 and CSP 3.
2. Results and Discussion
Previously, (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid has been used as a very successful
chiral selector for the preparation of chiral crown ether–based CSPs [19 21]. However,
–
(+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid has not been used as a chiral tethering group for
the preparation of diastereomeric CSPs. In this study, (+)-(18-crown-6)-2,3,11,12-tetracarboxylic
acid was used as a chiral tethering group for the preparation of diastereomeric CSPs such as
CSP
1
and CSP
2. Scheme 1 shows how (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was used
as a chiral tethering group for the preparation of CSP
1
and CSP In addition, Scheme 1
2.

Molecules 2016, 21, 1051
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shows the synthetic procedure for the preparation of CSP
(+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid was converted to its dianhydride,
with acetyl chloride via the known procedure [26]. Then the dianhydride,
3
.
To prepare CSP
1
and CSP
, by treating it
, was treated with
(R)-1-(1-naphthyl)ethylamine or (S)-1-(1-naphthyl)ethylamine in the presence of triethylamine in
methylene chloride to afford syn-diamide compound or . The syn-diamide structure of compound
or is believed to stem from the face-selective syn-opening of the dianhydride in the presence
of triethylamine [27 28]. Compound or , then, was treated with 3-aminopropyltriethoxysilane
in the presence of a coupling agent, 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ),
in methylene chloride to afford triethoxysilyl group–containing compound or . Finally, compound
or was bonded to silica gel to afford CSP or CSP CSP was prepared starting from
2,
1
1
2
3
2
3
,
2
3
4
5
4
5
1
2.
3
(R)-1-(1-naphthyl)ethylamine. The (R)-1-(1-naphthyl)ethylamine w◦as treated with 4-pentenoyl
chloride in the presence of triethylamine in methylene chloride at 0 C to afford terminal double
bond–containing amide compound 6. Then amide compound 6 was hydrosilylated by treating with
triethoxysilane and chloroplatinic acid hexahydrate. The resulting triethoxysilyl group–containing
compound 7 was bonded to silica gel to afford CSP 3.
O
O
O
O
O
O
O
HOOC
HOOC
O
O
O
O
COOH
COOH
HOOC
O
O
O
O
R
O
O
O
O
a
b
N
H
N
O
H
O
R
COOH
O
O
O
O
O
1
(+)-(18-Crown-6)-2,3,11,12-
tetracarboxylic acid
2
3
R
R = ( )-1-(1-naphthyl)ethyl
S
R = ( )-1-(1-naphthyl)ethyl
O
O
O
O
O
R
H5C2O
1
2
c
CSP
+
N
H
Si
N
H
O
O
O
O
d
H5C2O
OC2H5
OC2H5
H
N
H
N
OC2H5
OC2H5
Si
CSP
R
O
4
R
R = ( )-1-(1-naphthyl)ethyl
5
S
R = ( )-1-(1-naphthyl)ethyl
OC2H5
OC2H5
H5C2O Si
NH2
H3C
O
f
g
e
HN
H3C
3
CSP
O
HN
H3C
6
7
Scheme 1. Preparation of CSP
ethylamine or (S)-1-(1-naphthyl)ethylamine, triethylamine, methylene chloride.
1
, CSP
2
and CSP
3
. (
a
) Acetyl chloride, reflux; (
b
) (R)-1-(1-naphthyl)
0
◦C to reflux;
(c) 3-aminopropyltriethoxysilane, 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ), methylene
chloride, room temperature; (
triethylamine, methylene chloride, 0 C; (
toluene, Dean-Stark trap.
d
) 5
µ
m silica gel, toluene, Dean-Stark trap; (
e
) 4-petenoyl chloride,
◦
f) triethoxysilane, H2PtCl6
·
6H2O and (
g
) 5 m silica gel,
µ
CSP
-acidic N-3,5-dinitrobenzoyl derivatives of 1-phenylalkylamines (PAAs),
-amino acid N-propyl amides (NPAs), and -amino acid N,N-dialkyl amides (NNDAs), shown in
1
and CSP
2
containing
Π
-basic naphthyl groups were applied to the resolution of
Π
α
α
-amino acid esters (AAEs),
α
Figure 2, and the resolution results of the two CSPs were compared with each other.
Table 1 shows the chromatographic results for the resolution of PAAs on CSP
1
and CSP 2
using 20% isopropyl alcohol in hexane as a mobile phase. The two enantiomers were resolved quite

Molecules 2016, 21, 1051
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well on the two CSPs, but CSP
1
was found to show better chiral recognition than CSP
2. When the
length of the alkyl group at the chiral center was increased from methyl to pentyl, the retention factors
and the separation factors decreased on both CSP and CSP . When the length of the alkyl group at
1
2
the chiral center was increased further, the retention factors decreased continuously, but the separation
factors decreased only slightly or remained almost constant. In the case of the enantiomeric separation
of PAA-1, the elution order was determined by injecting enantiomerically known samples prepared
from (R)-1-phenylethylamine, but the elution orders for the enantiomeric separation of other analytes
were not determined because enantiomerically known samples were not available.
R2
O
OR2
R1
O
NH
R1
O
N
O
R
O
O
O
R2
O2N
O2N
O2N
O2N
N
N
H
N
H
N
H
R1
H
NO2
NO2
NO2
NO2
N-(3,5-dinitrobenzoyl)-
α-amino acid esters
(AAEs)
N-(3,5-dinitrobenzoyl)-
α-amino acid N-propyl amides
(NPAs)
N-(3,5-dinitrobenzoyl)-
α-amino acid N,N-dialkyl amides
N-(3,5-dinitrobenzoyl)-
1-phenylalkylamines
(PAAs)
(NNDAs)
Figure 2. Racemic compounds resolved on CSP 1 and CSP 2.
Table 1. Resolution of PAAs shown in Figure 2 on CSP 1 and CSP 2 1.
CSP 1
CSP 2
k1
PAAs
R
k1
α
α
PAA-1
PAA-2
PAA-3
PAA-4
PAA-5
PAA-6
CH3
n-C5H6
n-C10H21
n-C13H27
n-C15H31
n-C17H35
15.98 (S)
12.69
9.90
8.34
8.00
2.19
1.94
1.88
1.86
1.85
1.85
13.78 (R)
12.01
9.66
8.93
8.47
1.95
1.75
1.66
1.66
1.66
1.66
7.58
8.10
1
Mobile phase: 20% isopropyl alcohol in hexane. Flow rate: 1.0 mL/min. Detection: 254 nm UV. k1: Retention
factor for the first eluted enantiomer. In parentheses, the absolute configuration of the first eluted enantiomer is
presented. α: Separation factor.
The chromatographic results for the enantiomeric separation of AAEs shown in Figure 2 are
summarized in Table 2. The elution orders indicated in Table 2 were determined by injecting
enantiomerically known samples. In every case, the (S)-enantiomer was eluted first on CSP
the (R)-enantiomer was eluted first on CSP . These results indicate that the chiral recognition is
determined mainly by the 1-(1-naphthyl)ethylamine component of the CSPs. In the enantiomeric
1, whereas
2
separation of AAEs, CSP
of PAAs.
1 was always better (higher α) than CSP 2, as in the enantiomeric separation
Interestingly, the separation factors (
group of analytes was increased. In the case of AAE-
α
) increased as the alkyl chain length of the ester O-alkyl
~AAE , the separation factors ( ) increased
5
8
α
continuously as the ester alkyl chain length was increased from methyl to n-octyl. When the alkyl
chain length of the ester O-alkyl group of analytes was increased, the retention factors (k1) decreased as
usual. The large retention factors (k1) for the resolution of AAE-
for the resolution of AAE- and AAE- might be rationalized by the non-enantioselective
interaction between the 1-naphthyl group of the stationary phase and the phenyl or benzyl group
of analytes. In the presence of a non-enantioselective interaction between the stationary phase
5 and AAE-9 compared to those
1
3
Π-Π
Π-Π
and analytes, the retention times of the analytes might increase, but the separation factors might
decrease. The retention factor (k1) for AAE-11 was quite large compared to those of other analytes.
The non-enantioselective hydrogen bonding interaction between the 4-hydroxy group of the analyte
and the CSP might be the reason for the large retention factor.

Molecules 2016, 21, 1051
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Table 2. Resolution of AAEs shown in Figure 2 on CSP 1 and CSP 2 1.
CSP 1
CSP 2
R1
R2
AAEs
k1
α
k1
α
AAE-1
AAE-2
AAE-3
AAE-4
AAE-5
AAE-6
AAE-7
AAE-8
AAE-9
AAE-10
CH3
CH3
(CH3)2CH
(CH3)2CH
C6H5
C6H5
C6H5
C6H5
C6H5CH2
C6H5CH2
CH3
C2H5
CH3
C2H5
CH3
C3H7
C4H9
C8H17
CH3
C2H5
C2H5
10.60 (S)
7.47 (S)
8.70 (S)
1.93
1.94
2.32
2.37
1.16
1.23
1.24
1.27
1.50
1.57
1.70
12.70 (R)
8.70 (R)
7.44 (R)
1.55
1.59
1.89
1.98
1.11
1.19
1.20
1.22
1.40
1.47
1.59
6.50 (S)
5.57 (R)
27.95 (S)
16.73 (S)
14.49 (S)
10.12 (S)
19.78 (S)
14.75 (S)
42.36 (S)
30.29 (R)
17.96 (R)
15.36 (R)
12.06 (R)
16.10 (R)
11.94 (R)
35.93 (R)
AAE-11 4-HOC6H5CH2
1
Mobile phase: 20% isopropyl alcohol in hexane. Flow rate: 1.0 mL/min. Detection: 254 nm UV. k1: Retention
factor for the first eluted enantiomer. In parentheses, the absolute configuration of the first eluted enantiomer is
presented. α: Separation factor.
The enantiomeric separation of NPAs shown in Figure 2 on CSP
Table 3. The elution orders for the enantiomeric separation of NPAs on CSP
to those for the enantiomeric separation of AAEs on CSP and CSP . CSP
better than CSP in the enantiomeric separation of NPAs. The retention factor (k1) for the enantiomeric
1
and CSP
and CSP
was also found to be
2
was summarized in
1
2
were identical
1
2
1
2
separation of NPA-6 was also quite large compared to those of other analytes.
Table 3. Resolution of NPAs shown in Figure 2 on CSP 1 and CSP 2 1.
CSP 1
CSP 2
R1
NPAs
k1
α
k1
α
NPA-1
NPA-2
NPA-3
NPA-4
NPA-5
NPA-6
CH3
(CH3)2CH
(CH3)2CH2CH
C6H5
C6H5CH2
4-HOC6H5CH2 18.85 (S)
5.03 (S)
2.75 (S)
2.87 (S)
8.92 (S)
6.33 (S)
1.72
2.11
1.79
1.12
1.62
1.69
5.31 (R)
2.56 (R)
2.62 (R)
8.71 (R)
5.13 (R)
17.49 (R)
1.49
1.71
1.64
1.08
1.47
1.47
1
Mobile phase: 20% isopropyl alcohol in hexane. Flow rate: 1.0 mL/min. Detection: 254 nm UV. k1: Retention
factor for the first eluted enantiomer. In parentheses, the absolute configuration of the first eluted enantiomer is
presented. α: Separation factor.
NNDAs shown in Figure 2 were also resolved on CSP
summarized in Table 4. CSP was also found to be better than CSP
NNDAs. The elution orders were consistent with those for the enantiomeric separation of AAEs and
NPAs. In general, the enantiomeric separation of NNDAs on CSP and CSP was not much different
from that of corresponding NPAs in terms of the separation factors ( ). However, the retention factors
(k1) for the enantiomeric separation of NNDAs on CSP and CSP were always lower than those for
1
and CSP
2. The resolution results are
1
2
in the enantiomeric separation of
1
2
α
1
2
the enantiomeric separation of corresponding NPAs. These results indicate that the N-propyl amide
N-H hydrogen of NPAs is not involved in the chiral recognition. Instead, the N-propyl amide N-H
hydrogen of NPAs is expected to be involved in the non-enantioselective hydrogen bonding interaction
with the stationary phase.
To investigate the effect of a chiral tethering group such as (+)-(18-crown-6)-2,3,11,12-
tetra-carboxylic acid of CSP
for the enantiomeric separation of some selected analytes on the two CSPs were compared with those
on CSP as shown in Table 5. As an example, a comparison of the chromatograms for the enantiomeric
separation of AAE- and NPA- on CSP , CSP and CSP is presented in Figure 3. The elution orders
on CSP were identical to those on CSP . These results also indicate that the chiral recognition is
determined mainly by the 1-(1-naphthyl)ethylamine component of the CSPs. As shown in Table 5,
1 and CSP 2 on the chiral recognition, the chromatographic results
3
3
2
1
2
3
3
1

Molecules 2016, 21, 1051
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the separation factors (
α
) on CSP
1
were always greater than those on CSP
3, except for the enantiomeric
separation of NNDA-11, but those on CSP
the differences in the separation factors (
CSP and CSP on the chiral recognition are opposite. From these results, it is concluded that the
2
were always lower than those on CSP
3. Even though
α
) are not so great, the effects of the second chiral unit of
1
2
chiral tethering unit of (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid of CSP 1 exerts the cooperative
(“matched”) effect with the 1-(1-naphthyl)ethylamine unit on the chiral recognition, but the chiral
tethering unit of CSP
Interestingly, the retention factors (k1) on CSP
CSP . CSP and CSP contain many additional non-enantioselective hydrogen bonding sites such as
crown ether ring oxygens and additional amide sites compared to CSP 3. These non-enantioselective
hydrogen bonding sites on CSP and CSP can possibly improve the retention of analytes. The density
of the 1-(1-naphthyl)ethylamine component of CSP and CSP calculated from the elemental analysis
was 0.34 and 0.32 mmol/g, respectively, and these values are higher than that (0.25 mmol/g) of CSP
2
exerts the uncooperative (“mismatched”) effect on the chiral recognition.
3
are quite small compared to those on CSP and
1
2
1
2
1
2
1
2
3.
The higher density of the 1-(1-naphthyl)ethylamine component of the CSPs can also enhance the
retention of analytes, but does not seem to always improve the enantioselectivity, as evidenced by the
lower separation factors (α) on CSP 2 compared to those on CSP 3.
Table 4. Resolution of NNDAs shown in Figure 2 on CSP 1 and CSP 2 1.
CSP 1
CSP 2
R1
R2
NNDAs
k1
α
k1
α
NNDA-1
NNDA-2
NNDA-3
NNDA-4
NNDA-5
NNDA-6
NNDA-7
NNDA-8
NNDA-9
NNDA-10
NNDA-11
NNDA-12
NNDA-13
NNDA-14
NNDA-15
NNDA-16
NNDA-17
NNDA-18
CH3
CH3
CH3
CH3
CH2CH3
CH2CH2CH3
CH3
CH2CH3
CH2CH2CH3
CH3
CH2CH3
CH2CH2CH3
CH3
CH2CH3
CH2CH2CH3
CH3
8.72 (S)
3.32 (S)
2.03 (S)
3.43 (S)
2.43 (S)
1.42 (S)
3.74 (S)
2.21 (S)
1.98 (S)
8.88 (S)
5.80 (S)
5.41
6.40 (S)
3.96 (S)
3.63 (S)
15.40 (S)
12.01 (S)
10.89 (S)
1.53
1.61
1.58
1.77
1.84
1.74
1.48
1.62
1.69
1.22
1.18
1.00
1.53
1.87
1.82
1.59
1.93
1.98
7.20 (R)
3.29 (R)
1.98 (R)
2.97 (R)
2.01 (R)
1.22 (R)
3.18 (R)
1.90 (R)
1.85 (R)
8.02 (R)
4.76 (R)
5.36
5.06 (R)
3.03 (R)
3.12 (R)
12.68 (R)
11.02 (R)
10.42 (R)
1.32
1.41
1.41
1.51
1.66
1.61
1.40
1.51
1.62
1.20
1.16
1.00
1.36
1.51
1.64
1.34
1.49
1.62
(CH3)2CH
(CH3)2CH
(CH3)2CH
(CH3)2CH2CH
(CH3)2CH2CH
(CH3)2CH2CH
C6H5
C6H5
C6H5
C6H5CH2
C6H5CH2
C6H5CH2
4-HOC6H5CH2
4-HOC6H5CH2
4-HOC6H5CH2
CH2CH3
CH2CH2CH3
CH3
CH2CH3
CH2CH2CH3
1
Mobile phase: 20% isopropyl alcohol in hexane. Flow rate: 1.0 mL/min. Detection: 254 nm UV. k1: Retention
factor for the first eluted enantiomer. In parentheses, the absolute configuration of the first eluted enantiomer is
presented. α: Separation factor.
AAE-3 (CSP 1)
NPA-2 (CSP 1)
AAE-3 (CSP 2)
NPA-2 (CSP 2)
AAE-3 (CSP 3)
NPA-2 (CSP 3)
mV
mV
0
10
20
30
40
0
3
6
9
12
15
Time(min)
Time (min)
Figure 3. Comparison of the representative chromatograms for the enantiomeric separation of AAE-
and NPA- on CSP , CSP and CSP . For the chromatographic conditions, see the footnotes to
Tables 2 and 3.
3
2
1
2
3

Molecules 2016, 21, 1051
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Table 5. Comparison of the chromatographic results for the resolution of some selected analytes on
CSP 1, CSP 2 and CSP 3 1.
CSP 1
CSP 2
CSP 3
Analytes
k1
α
k1
α
k1
α
PPA-1
AAE-1
AAE-3
AAE-11
NPA-1
NPA-2
15.98 (S)
10.60 (S)
8.70 (S)
14.75 (S)
5.03 (S)
2.75 (S)
2.87 (S)
3.32 (S)
2.43 (S)
2.21 (S)
5.80 (S)
3.96 (S)
12.01 (S)
2.19
1.93
2.32
1.57
1.72
2.11
1.79
1.61
1.84
1.62
1.18
1.87
1.93
8.53 (R)
12.70 (R)
7.44 (R)
11.94 (R)
5.31 (R)
2.56 (R)
2.62 (R)
3.29 (R)
2.01 (R)
1.90 (R)
4.76 (R)
3.03 (R)
11.02 (R)
1.98
1.55
1.89
1.47
1.49
1.71
1.64
1.41
1.66
1.51
1.16
1.51
1.49
1.90 (S)
2.72 (S)
1.61 (S)
4.36 (S)
1.37 (S)
0.80 (S)
0.65 (S)
1.68 (S)
0.80 (S)
0.78 (S)
1.40 (S)
1.10 (S)
2.46 (S)
2.08
1.76
2.14
1.54
1.51
1.79
1.51
1.58
1.80
1.53
1.27
1.53
1.47
NPA-3
NNDA-2
NNDA-5
NNDA-8
NNDA-11
NNDA-14
NNDA-17
1
Mobile phase: 20% isopropyl alcohol in hexane. Flow rate: 1.0 mL/min. Detection: 254 nm UV. k1: Retention
factor for the first eluted enantiomer. In parentheses, the absolute configuration of the first eluted enantiomer is
presented. α: Separation factor.
3. Materials and Methods
3.1. Materials and Instruments
All necessary reagents and solvents were available from Sigma-Aldrich (St. Louis, MO, USA)
or TCI chemicals and the reagents (Tokyo, Japan) were used without further purification and the
solvents were handled in a moisture-free atmosphere. (+)-(18-Crown-6)-2,3,11,12-tetracarboxylic acid
was obtained from RS tech (Daejeon, Korea). The purification of compounds was performed by using
column chromatography (silica gel, Merck Kieselgel 60, 70–230 mesh ASTM). The nuclear magnetic
resonance (NMR) spectra of the compounds were recorded with Varian Mercury Plus spectrometer
(300 MHz). Liquid chromatography was performed with an HPLC system consisting of a Waters
model 515 HPLC pump (Milford, MA, USA), a Rheodyne model 7725i injector (Rohnert Park, CA,
USA) with a 20 µL sample loop, a YoungLin M720 absorbance UV detector (variable wavelength,
Seoul, Korea) and a YoungLin Autochro data module (Software: YoungLin Autochro-WIN 2.0 plus).
The temperature of the chiral column was controlled by using a Julabo F30 Ultratemp 2000 cooling
circulator (Seelbach, Germany). Racemic and optically active analytes were available from a prior
study or prepared using the similar reported procedure [29,30]. Injection samples were prepared by
dissolving each of racemic and optically active samples in methanol (usually 2.5 mg/mL). The usual
injection volume was 5.0 µL.
3.2. Synthesis of CSPs
Synthesis of compound
using the reported procedure [26
2
: The stirred solution of compound
1
(0.85 g, 2.10 mmol), which was prepared
◦
,
31], in methylene chloride (50 mL) was cooled to 0 C under an argon
atmosphere. A solution of (R)-1-(1-naphthyl)ethylamine (0.86 g, 5.00 mmol) and triethylamine (0.70 mL,
5.00 mmol) in methylene chloride (20 mL) was added drop by drop. The reaction mixture was slowly
warmed to room temperature and then refluxed for 16 h. Then, the reaction mixture was cooled
to room temperature and washed twice with 2 N HCl. The organic layer was dried over Na2SO4.
The solvent was removed under reduced pressure and solid was washed with hexane to afford
1
compound
2
as an off-white solid. Yield: 1.46 g (91%). H-NMR (300 MHz, CDCl3,
δ
): 1.40–1.60 (m, 6H),
2.80–4.00 (m, 20H), 5.90–6.10 (m, 2H), 7.30–8.10 (m, 14H).

Molecules 2016, 21, 1051
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Synthesis of compound
3: Compound
3
was prepared by using the similar procedure for the synthesis
of compound
2
.
In this reaction, (S)-1-(1-naphthyl)ethylamine (0.86 g, 5.00 mmol) was used
instead of (R)-(+)-1-(1-naphthyl)ethylamine. Yield: 1.40 g (87%). 1H-NMR (300 MHz, CDCl3,
δ):
1.40–1.60 (m, 6H), 2.80–4.00 (m, 20H), 5.90–6.10 (m, 2H), 7.30–8.10 (m, 14H).
Synthesis of compound 4: To a stirred solution of compound 2 (1.40 g, 1.90 mmol) in methylene chloride
(50 mL) was added 2-ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ, 1.90 g, 7.50 mmol)
under an argon atmosphere. After 15 minutes, (3-aminopropyl)triethoxysilane (1.75 mL, 7.50 mmol)
was added and the reaction mixture was stirred for overnight. Then, the solvent was removed
under reduced pressure and the crude material was purified by fast column chromatography
(silica gel, methylene chloride:methanol, 90/10, v/v) to afford light brown sticky mass as
1
compound
4.
Yield: 0.90 g (41.7%). H-NMR (300 MHz, CDCl3,
δ): 0.50–0.70 (m, 4H), 1.18(q, 18H),
1.54–1.76 (m, 14H), 2.60–4.00 (m, 28H), 4.40 (s, 4H), 5.90–6.10 (m, 2H), 7.30–8.00 (m, 14H).
Synthesis of compound
of compound . In this reaction, compound
and (3-aminopropyl)triethoxysilane (1.75 mL, 7.50 mmol) were used. Yield: 0.90 g (41.7%).
1H-NMR (300 MHz, CDCl3,
): 0.50–0.70 (m, 4H), 1.18(q, 18H), 1.54–1.76 (m, 14H), 2.60–4.00 (m, 28H)
4.40 (s, 4H), 5.90–6.10 (m, 2H), 7.30–8.00 (m, 14H).
5: Compound
5
was prepared using the similar procedure for the synthesis
4
3
(1.40 g, 1.90 mmol), EEDQ (1.90 g, 7.50 mmol)
δ
,
Synthesis of CSP
a condenser, and an overhead stirrer was charged with silica gel (2.5 g, kromasil, 5
(150 mL). The mixture was heated to reflux to remove water azeotropically. To the slowly stirred
mixture, a solution of compound (0.90 g, 0.80 mmol) in 10mL toluene was added in one portion.
1
and column packing: A 250 mL two neck flask equipped with a Dean-Stark trap,
µ) and toluene
4
The whole mixture was refluxed for 72 h and filtered, washed successively with methanol, acetone,
ethyl acetate, methylene chloride, hexane and diethyl ether. Finally, the modified silica gel was dried
under vacuum to afford CSP
gel was calculated to be 0.17 mmol/g based on carbon from the elemental analysis (C, 10.24; H, 1.62;
N, 0.60). The modified silica gel was slurred in methanol and packed into 150 4.6 mm stainless steel
HPLC column by using conventional slurry packing method with an Alltech slurry packer.
1 (2.95 g). The loading level of the chiral selector (compound 4) on silica
×
Synthesis of CSP
used for the synthesis and column packing for CSP
was used instead of compound . All other procedures were same. The loading level of the chiral
selector (compound ) on silica gel was calculated to be 0.16 mmol based on the carbon content from
the elemental analysis (C, 10.00; H, 1.74; N, 0.59). The modified silica gel was slurred in methanol and
packed into 150 4.6 mm stainless steel HPLC column by using conventional slurry packing method
with an Alltech slurry packer.
2
and column packing: A similar procedure mentioned for the preparation of CSP
1 was
2
. In this case, compound (0.90 g, 0.78 mmol)
5
4
5
×
Synthesis of compound 6: The stirred solution of (R)-1-(1-naphthyl)ethylamine (1.5 g, 8.76 mmol) in
methylene chloride (50 mL) was cooled to 0 ◦C and triethylamine (1.34 g, 9.63 mmol) was added and
then the whole mixture was stirred for 15 min. Then, 4-pentenoyl chloride (1.06 mL, 9.63 mmol) was
added drop by drop and stirred for 30 min. The reaction mixture was washed with sodium bicarbonate
(10%) solution, 1 N HCl and brine. The organic layer was dried over Na2SO4. The solvent was removed
under reduced pressure and solid was washed with hexane to afford compound
6 as a white solid.
1
Yield: 1.8 g (81%). H-NMR (300 MHz, CDCl3,
δ
): 1.67 (d, 3H), 2.24 (t, 2H), 2.38 (q, 2H), 4.98 (t, 2H),
5.52–5.58 (m, 2H), 5.58–6.00 (m, 1H), 7.48–7.52 (m, 4H), 7.79 (d, 1H), 7.86 (d, 1H), 8.08 (d, 1H).
Synthesis of compound
in a flame dried round bottom flask (100 mL) under an argon atmosphere. The mixture was
heated to 80 ◦C with gentle stirring. To the clear solution, H2PtCl6
6H2O (5 mg in 1 mL isopropyl
7: Compound 6 (0.80 g, 3.16 mmol) and triethoxysilane (30 mL) were taken
·

Molecules 2016, 21, 1051
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alcohol) was added drop by drop and the stirring was continued for 2 h. The reaction mixture
was cooled to room temperature and concentrated under reduced pressure. The sticky mass
was directly subjected to fast column chromatography (silica gel, methylene chloride:methanol,
90/10, v/v) to afford light brown sticky mass. Yield: 0.8 g (60%). 1H-NMR (300 MHz, CDCl3,
0.62 (t, 2H), 1.19 (t, 9H),
δ
):
1.35–1.50 (m, 2H), 1.52–1.54 (m, 5H), 2.15 (t, 2H), 3.77 (q, 6H), 5.66 (d, 1H),
5.88–6.00 (m, 1H), 7.40–7.60 (m, 4H), 7.79 (d, 1H), 7.86 (d, 1H), 8.08 (d, 1H).
Synthesis of CSP
used for the synthesis and column packing for CSP
was used. All other procedures were same. The loading level of the chiral selector (compound
3
and column packing: A similar procedure described for the preparation of CSP
1 was
3
. In this case, compound (2.7 g, 1.92 mmol)
7
7
)
on silica gel was calculated to be 0.25 mmol/g based on the carbon content from the elemental
analysis (C, 5.73; H, 1.41; N, 0.22). The modified silica gel was slurred in methanol and packed
into 150 × 4.6 mm stainless steel HPLC column by using conventional slurry packing method with
an Alltech slurry packer.
4. Conclusions
The two chiral units consisting of the two new diastereomeric chiral stationary phases (CSPs)
based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid as a chiral tethering group and a
chiral unit such as (R)-1-(1-naphthyl)ethylamine (CSP ) or (S)-1-(1-naphthyl)ethylamine (CSP
were demonstrated to show the “matched” or “mismatched” effect on the chiral recognition,
depending on the stereochemistry of the -basic chiral unit. For the enantiomeric separation of
N-(3,5-dinitrobenzoyl)-1-phenylalkylamines and N-(3,5-dinitrobenzoyl)- -amino acid derivatives
on the two CSPs, the -basic chiral unit was found to play a significant role as a major chiral
Π-basic
1
2)
Π
α
Π
selector in the chiral recognition, but the tethering (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid
group was found to play an additional role showing the “matched” or “mismatched” effect on the
chiral recognition. From these results it was concluded that (+)-(18-crown-6)-2,3,11,12-tetracarboxylic
acid can be successfully used as a chiral tethering group in the preparation of new effective CSPs.
Acknowledgments: This work was supported by the BK21 Plus program, National Research Foundation
(NRF), Korea.
Author Contributions: R.A. performed the preparation of the chiral stationary phases. J.Y.S., E.S.J. and H.Y.J.
performed the chromatographic data collection. V.T. performed the data analysis. M.H.H. designed and supervised
the study.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
The following abbreviations are used in this manuscript:
CSP
Chiral stationary phase
EEDQ
PAA
AAE
NPA
2-Ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline
N-(3,5-Dinitrobenzoyl)-1-phenylalkylamines
N-(3,5-Dinitrobenzoyl)-α-amino acid esters
N-(3,5-Dinitrobenzoyl)-α-amino acid N-propyl amides
N-(3,5-Dinitrobenzoyl)-α-amino acid N,N-dialkyl amides
NNDA
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from authors.
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2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
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