HPLC separation of 2-aryloxycarboxylic acid enantiomers on chiral stationary phases

Article abstract of DOI:10.1007/s11172-021-3165-8

The possibility for separating enantiomers of a number of practically significant 2-aryloxycarboxylic acids was studied by normal- and reversed-phase HPLC on popular chiral stationary phases. The best separation parameters were achieved on the chiral phases with the polysaccharide base Chiralcel OD-H and Chiralpack AD under the normal-phase HPLC conditions. The (S)- and (R)-enantiomers of 2-(1-naphthyloxy)- and 2-(2-iodophenoxy)propionic acids with enantiomeric excess ee >99% were isolated using preparative chiral HPLC.

Full text of DOI:10.1007/s11172-021-3165-8

Russian Chemical Bulletin, International Edition, Vol. 70, No. 5, pp. 900—907, May, 2021
900
HPLC separation of 2-aryloxycarboxylic acid enantiomers
on chiral stationary phases
A. A. Tumashov,a,b S. A. Vakarov,a L. Sh. Sadretdinova,a E. N. Chulakov,a G. L. Levit,a
V. P. Krasnov,a,c and V. N. Charushina,c
aPostovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences,
22/20 ul. Sof´i Kovalevskoi, 620108 Ekaterinburg, Russian Federation.
Fax: +7 (343) 374 1189. E-mail: tumashov@ios.uran.ru
bInstitute of Natural Sciences and Mathematics, Ural Federal University,
19 ul. Mira, 620002 Ekaterinburg, Russian Federation
cInstitute of Chemical Engineering, Ural Federal University,
19 ul. Mira, 620002 Ekaterinburg, Russian Federation
The possibility for separating enantiomers of a number of practically significant 2-aryloxy-
carboxylic acids was studied by normal- and reversed-phase HPLC on popular chiral stationary
phases. The best separation parameters were achieved on the chiral phases with the polysaccharide
base Chiralcel OD-H and Chiralpack AD under the normal-phase HPLC conditions. The
(S)- and (R)-enantiomers of 2-(1-naphthyloxy)- and 2-(2-iodophenoxy)propionic acids with
enantiomeric excess ee >99% were isolated using preparative chiral HPLC.
Key words: 2-aryloxycarboxylic acids, enantiomers, high performance liquid chromatography,
chiral stationary phases, separation selectivity.
2-Aryloxy-substituted carboxylic acids and their de-
interest, on the frequently used CSPs in normal- and re-
versed-phase HPLC and also to isolate some of these
acids using preparative HPLC.
rivatives are of significant practical interest, since they
are chiral resolving agents, precursors and struc-
tural analogs of a number of agrochemicals, physic-
ologically active compounds, chiral catalysts, and re-
agents.1—4 It is known that the biological activity of opti-
cal isomers of 2-aryloxy acids is substantially different5—7
and, hence, the development of methods for control
of enantiomeric purity of compounds of this class is an
urgent task.
Experimental
(RS)-2-Phenoxypropionic acid (1a),10 (RS)-2-(2,6-dichloro-
phenoxy)propionic acid (1b),11 (RS)-2-(4-chloro-2-methyl-
phenoxy)propionic acid (1c),12 (RS)-2-(2-iodophenoxy)propi-
onic acid (1d),11 (RS)-2-(4-fluorophenoxy)propionic acid (1e),11
(RS)-2-(4-nitrophenoxy)propionic acid (1f),13 (RS)-2-me-
sitylpropionic acid (1g),14 (RS)-2-(4-methoxyphenoxy)propi-
onic acid (1h),15 (RS)-2-(4-acetylphenoxy)propionic acid (1i),11
(RS)-2-benzyloxypropionic acid (2a),16 (RS)-2-(1-naphthyloxy)
propionic acid (2b)17, (RS)-2-phenoxy-3-propionic acid (2c),10
(RS)-2-methoxy-2-phenylacetic acid (2d),18 (RS)-2-phen-
ylthiopropionic acid (2e),11 (RS)-2-phenoxyisovaleric acid (2f),19
and individual (S)-enantiomers of acids 1a—g, 1i, and 2e
were synthesized using known procedures. Distilled МеОН
and specially prepared water (Khromatek water purifica-
tion system, Russia) of type 2 according to GOST 52501 with
a specific resistance of 18.2 mOhm cm–1 were used for HPLC
analysis. Other commercially available reagents were used as
received.
HPLC on chiral stationary phases (CSPs) is the most
popular method for analysis of the enantiomeric compo-
sition of 2-aryloxy acids. Theoretical aspects of chromato-
graphic separation of optical isomers of compounds of this
class evoke special interest. The influence of the structures
of the stationary phases Chiralcel OD and Chiralpak AD,
as well as the structures and characters of substituents of
the analytes (derivatives of 2-aryloxy acids) on the separa-
tion efficiency of the enantiomers, was systematically
studied.8 Specific features of separation of a series of
substituted 2-aryloxypropionic acids and their esters and
amides were studied in brush-type chiral columns (R,R)-
β-GEM and (3S,4R)-Whelk O1.9
1H and 13С NMR spectra were recorded on a Bruker Avance
500 instrument (operating frequency 500 MHz) relative to SiMe4
(internal standard) in CDCl3 at ~20 °С. Melting points of the
compounds were measured on a Stuart SMP3 instrument
(Barloworld Scientific, Great Britain). Elemental analysis was
The purpose of this work is to study the conditions of
chromatographic separation of enantiomers of 2-aryloxy-
carboxylic acids and their structural analogs (2-alkyloxy-
and 2-arylthiocarboxylic acids), which are of practical
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 900—907, May, 2021.
1066-5285/21/7005-0900 © 2021 Springer Science+Business Media LLC

HPLC enantioseparation of aryloxycarboxylic acids
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
901
carried out on a Perkin-Elmer 2400 CHNS-O automated
analyzer, series II (Perkin-Elmer Instruments, USA). Optical
rotations were determined on a Perkin-Elmer М341 polarimeter
(Perkin-Elmer Instruments, USA). Specific rotation and con-
centration of solutions are expressed in deg mL g–1 dm–1 and
g (100 mL)–1, respectively.
where K1 and K2 are the retention factors of the first and second
enantiomers, respectively. Resolution of the stereoisomer peaks
(RS) was calculated as follows:
RS = 2(τ2 – τ1)/(W1 + W2),
where τ1 and τ2 are the retention times of the first and second
enantiomers (min), and W1 and W2 are the peak widths of the
first and second enantiomers at the base (min).
HPLC study of acids 1a—i and 2a—f. HPLC analysis was
carried out on a Knauer Smartline-1000 chromatograph (Knauer,
Germany) equipped with columns (250×4.6 mm) packed with
the Chiralcel OD-H, 5 μm (Daicel Corp., Japan) or Chiralpak
АD, 5 μm (Daicel Corp., Japan) sorbents and on an Agilent-1100
chromatograph (Agilent Technologies, USA) equipped with
columns (250×4.6 mm) packed with the (S,S)-Whelk-O1, 5 μm
(Regis Technologies Inc., USA) or Kromasil 5-Cellucoat RP,
5 μm (ЕКА Chemicals AB, Sweden) sorbents. The sample volume
was 10 μL, and the column temperature was 24 °С. The elution
rate for the Chiralcel OD-H and Chiralpak АD columns was
1 mL min–1, and that for the (S,S)-Whelk-O1 and Kromasil
5-Cellucoat RP columns was 0.8 mL min–1. The eluent compo-
sitions and retention times (τR) are presented in Table 1.
Detection was carried out at a wavelength of 220 nm using
a Smartline-2550 spectrophotometric detector (Knauer, Germany)
and a G-1315B diode array detector (Agilent Technologies,
USA). Detection at the wavelength 430 nm was carried out using
a Chiralyser polarimetric detector (IBZ Messtechnik GMBH,
Germany).
Preparative enantioseparation of 2-aryloxypropionic acids
1d and 2b was performed on a Shimadzu LC-20 Prominence
chromatograph (Shimadzu, Japan) equipped with a column
(250×20 mm) packed with the Chiralcel OD-H sorbent (5 μm)
(Daicel Corp., Japan). Detection was carried out at λ = 220 nm,
and the eluent flow rate was 10 mL min–1
.
Preparative separation of racemic 2-(2-iodophenoxy)propi-
onic acid (1d). Racemic acid 1d (420 mg) was dissolved in PriOH
(14 mL). The resulting solution was injected by portions into the
preparative chromatograph (volume of the injected sample 1 mL,
hexane—PriOH—TFA (40 : 1 : 0.004) mobile phase). The frac-
tions containing enantiomers (S)-1d (τ1 = 12.5—21.5 min) and
(R)-1d (τ2 = 25.4—34.9 min) were collected and dried under
reduced pressure. The fractions containing a mixture of enan-
tiomers were collected separately and evaporated, and the chro-
matographic separation was repeated. The residues were com-
bined and recrystallized from a hexane—EtOAc (20 : 1) mixture.
(S)-2-(2-Iodophenoxy)propionic acid [(S)-1d]. The yield was
48.9 mg (23%). White crystalline powder, m.p. 134—136 °С,
[α]D20 –2.9 (c 1.0, THF); eе > 99%. Found (%): C, 37.22; H, 3.12.
C9H9IO3. Calculated (%): C, 37.01; H, 3.11. HPLC (Chiralcel
OD-H, hexane—PriOH—TFA (40 : 1 : 0.02)): τ(S)-1d = 12.9 min.
The 1H and 13С NMR spectra are identical to the 1H and
13С NMR spectra of (R)-2-(2-iodophenoxy)propionic acid.20
(R)-2-(2-Iodophenoxy)propionic acid [(R)-1d]. The yield was
38.2 mg (18%). White crystalline powder, m.p. 133—134 °С,
[α]D20 +2.4 (c 0.3, THF) (cf. Ref. 20: [α]D26 +2.4 (c 1.0, THF));
ее > 99%. Found (%): C, 37.24; H, 3.11. C9H9IO3. Calculated
(%): C, 37.01; H, 3.11. HPLC (Chiralcel OD-H, hexane—
For HPLC analysis, a precise weighed sample (about 5 mg)
of compounds 1a—i and 2a—f was dissolved in the mobile phase
(10 mL, data in Table 1), and the solution was stirred and filtered
through a Teflon membrane filter with a pore size of 0.45 μm.
The analysis conditions and results of separation of the enantio-
mers of acids 1a—i and 2a—f are given in Table 1.
The retention factor of enantiomers (K) was calculated by
the equation
K = (τR/τ0) – 1,
where τR is the retention time of the analyte (min), and τ0 is the
elution time of an unretained compound (min). The separation
factor (α) was determined by the equation
PriOH—TFA (40 : 1 : 0.02)): τ(R)-1d = 16.7 min. The 1H and 13
С
NMR spectra are identical to those published previously.20
Preparative separation of racemic 2-(1-naphthyloxy)propi-
onic acid (2b). Racemic acid 2b (250 mg) was dissolved in PriOH
α = K2/K1,
Table 1. Results of separation of the enantiomers of compounds 1a—i and 2a—f on various chiral stationary phases
Com-
pound
HPLC conditions
Eluent (ratio)
τR/min
K1
α
RS
Sorbent
τ1
τ2
1a
Chiralcel OD-H
Chiralpak AD
Kromasil 5-Cellucoat
(S,S)-Whelk-О1
Chiralcel OD-H
Chiralpak AD
(S,S)-Whelk-О1
Chiralcel OD-H
Chiralpak AD
Kromasil 5-Cellucoat
(S,S)-Whelk-О1
hexane—PriOH—TFA (10 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeCN—0.25% AcOH (1 : 3)
6.1а
11.9
22.1 (R)
22.3 (R)
12.5c
0.78
2.26
2.36
2.00
2.06
2.35
5.67
1.14
1.89
0.41
4.39
3.13
1.54
1.46
1.00
1.10
1.17
1.14
1.00
1.59
1.00
1.00
10.18
3.68
7.95
—
1.42
1.21
3.12
—
14.8 (S)b
15.3 (S)
12.5c
MeOH—0.25% AcOH (1 : 1)
1b
1c
hexane—PriOH—TFA (40 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeOH—0.25% AcOH (7 : 3)
12.4a
13.4
13.6a
15.2
26.7 (S)
30.3 (R)
hexane—PriOH—TFA (40 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeCN—0.25% AcOH (3 : 2)
8.6c
8.6c
11.6 (S)d
5.4c
16.0 (R)
5.4c
3.42
—
—
MeOH—0.25% AcOH (3 : 2)
21.8c
21.8c
(to be continued)

Tumashov et al.
902
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
Table 1 (continued)
Com-
pound
HPLC conditions
Eluent (ratio)
τR/min
K1
α
RS
Sorbent
τ1
τ2
1d
Chiralcel OD-H
Chiralpak AD
Kromasil 5-Cellucoat
(S,S)-Whelk-О1
Chiralcel OD-H
Chiralpak AD
Kromasil 5-Cellucoat
Chiralcel OD-H
Chiralpak AD
Kromasil 5-Cellucoat
Chiralcel OD-H
Chiralpak AD
(S,S)-Whelk-О1
Chiralcel OD-H
Chiralpak AD
Kromasil 5-Cellucoat
(S,S)-Whelk-О1
Chiralcel OD-H
Chiralpak AD
Chiralcel OD-H
Chiralpak AD
Kromasil 5-Cellucoat
(S,S)-Whelk-О1
Chiralcel OD-H
Chiralpak AD
Kromasil 5-Cellucoat
(S,S)-Whelk-О1
Chiralcel OD-H
Chiralpak AD
Kromasil 5-Cellucoat
(S,S)-Whelk-О1
Chiralcel OD-H
Chiralpak AD
Kromasil 5-Cellucoat
(S,S)-Whelk-О1
Chiralcel OD-H
Chiralpak AD
Kromasil 5-Cellucoat
(S,S)-Whelk-О1
Chiralcel OD-H
Chiralpak AD
hexane—PriOH—TFA (40 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeCN—0.25% AcOH (2 : 3)
13.0 (S)e
11.1a
16.5 (R)
13.4
2.21
2.00
5.09
2.90
1.50
2.49
0.66
4.00
1.95
3.62
4.85
1.04
5.73
4.74
5.12
4.62
3.78
3.86
6.58
2.56
4.18
2.20
0.87
3.47
3.03
3.94
6.00
5.57
4.63
4.22
4.26
4.13
9.81
0.92
1.98
2.16
2.90
1.47
3.97
1.46
2.32
5.37
1.68
1.38
1.31
1.10
1.00
1.35
1.54
1.00
1.18
1.08
1.00
1.14
1.16
1.09
2.18
1.30
1.05
1.00
1.00
1.47
1.50
1.48
1.11
1.00
2.11
1.13
1.12
1.00
1.63
1.36
1.14
1.00
1.51
1.14
1.09
1.17
1.00
1.11
1.00
1.04
1.94
1.16
1.12
1.00
4.04
1.68
1.25
–
3.72
3.77
—
1.73
0.95
—
24.3 (S)
15.9 (S)
11.4a
26.9 (R)
15.9 (R)
13.8
MeOH—0.25% AcOH (7 : 3)
1e
1f
hexane—PriOH—TFA (40 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeCN—0.25% AcOH (2 : 3)
14.1 (S)d
6.2c
19.5 (R)
6.2c
hexane—PriOH—TFA (20 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeCN—0.25% AcOH (1 : 3)
20.5 (S)f
16.0a
23.4 (R)
17.1
17.4c
17.4c
1g
1h
hexane—PriOH—TFA (60 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeOH—0.25% AcOH (3 : 2)
23.7 (S)d
8.38a
26.4 (R)
9.05
25.8
46.0
1.58
0.40
2.00
12.70
2.40
0.93
—
23.6a
hexane—PriOH—TFA (40 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeCN—0.25% (1 : 4)
23.3a
25.1 (S)f
31.5 (R)
24.3
23.2a
MeOH—0.25% AcOH (1 : 1)
16.8 (S)
20.4c
15.8 (R)
1i
hexane—PriOH—TFA (40 : 1 : 0.02)
hexane—PriOH—TFA (20 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeCN—0.25% AcOH (1 : 3)
20.4c
—
30.7 (S)d
14.2a
43.3 (R)
19.3
3.15
7.34
3.73
2.10
—
2a
20.7a
28.7
12.8a
14.3
MeOH—0.25% AcOH (3 : 2)
7.5c
7.5c
2b
2c
2e
2f
hexane—PriOH—TFA (40 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeCN—0.25% AcOH (1 : 3)
18.1 (S)g
16.1a
33.7 (R)
17.7
11.88
1.09
2.33
—
7.00
3.11
2.90
—
3.90
0.96
1.90
3.68
—
1.12
—
17.8a
19.9
MeOH—0.25% AcOH (3 : 2)
24.6c
24.6c
hexane—PriOH—TFA (40 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeCN—0.25% AcOH (2 : 3)
26.6 (S)b
22.8a
40.9 (R)
29.6
19.8a
22.7
MeOH—0.25% AcOH (3 : 2)
21.5c
21.5c
hexane—PriOH—TFA (40 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeCN—0.25% AcOH (1 : 3)
20.8a
29.4
43.8 (S)h
7.3a
49.2 (R)
8.0
MeOH—0.25% AcOH (2 : 3)
12.8a
15.0
hexane—PriOH—TFA (40 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeCN—0.25% AcOH (1 : 3)
12.8c
12.8c
15.8 (S)d
8.91c
17.1 (R)
8.91c
MeOH—0.25% AcOH (1 : 1)
20.1a
20.8
14.6
0.78
5.88
1.08
1.70
—
15
hexane—PriOH—TFA (40 : 1 : 0.02)
hexane—PriOH—TFA (40 : 1 : 0.02)
MeCN—0.25% AcOH (3 : 7)
9.4a
12.6 (S)b
24.2a
14.1 (R)
26.7
Kromasil 5-Cellucoat
(S,S)-Whelk-О1
MeOH—0.25% AcOH (3 : 2)
10.7c
10.7c
Notes. τ1 and τ2 are the retention times of the first and second eluted enantiomers, respectively; K1 is the retention factor of the peak
of the first isomers; and α is selectively; and RS is resolution.
a Configurations of stereoisomers were not assigned.
b Configuration assignment was performed earlier.10
с No separation.
d Configuration assignment was performed earlier.14
e Configuration assignment was performed by comparison of optical rotation signs of individual enantiomers with the published data.18
f Configuration assignment was performed earlier.29
g Configuration assignment was performed by comparison of optical rotation signs of individual enantiomers with the published data.19
h Configuration assignment was performed earlier.30

HPLC enantioseparation of aryloxycarboxylic acids
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
903
2b, respectively1,5,22; and acid 1d is the basis of the struc-
tures of the chiral catalysis and reagents (derivatives of
hypervalent iodine).23—25
(10 mL). The resulting solution was injected by portions into the
preparative chromatograph (volume of injected sample 1 mL,
hexane—PriOH—TFA (30 : 1 : 0.02)). The fractions contain-
ing enantiomers (S)-2b (τ1 = 18.5—30.5 min) and (R)-11
(τ2 = 32.0—49.5 min) were collected and dried under reduced
pressure. The obtained residues were recrystallized from a hex-
ane—EtOAc (10 : 1) mixture.
The possibility of efficient separation of the enantio-
mers of the aforementioned acids was studied under the
conditions of normal-phase (NP) HPLC on the Chiralcel
OD-H and Chiralpak AD columns and reversed-phase
(RP) HPLC on the Kromasil 5-Cellucoat RP and
(S,S)-Whelk-O1 columns. The Chiralcel OD-H and
Chiralpak AD CSPs demonstrated good resolution ability
in the separation of various racemic compounds, includ-
ing some 2-aryloxyalkanoic acids.8 Both phases have the
modified polysaccharide basis with the consecutive ar-
rangement of phenylcarbamate groups and methyl sub-
stituents at positions 3 and 5 of the aromatic ring, which
increases the probability of enantiomeric recognition.
Binding of these chiral phases to the analyzed substances
is determined by the difference in volumes of the spiral
channels in the polymeric structures of the cellulose
(OD-H) and amylose (AD) derivatives.26,27 In the case of
the Chiralcel OD-H phase, the structure of the grafted
phase is more linear and rigid, whereas the Chiralpak AD
phase has the compact and broad spiral with the well
pronounced channels. The Kromasil 5-Cellucoat RP phase
is an analog of Chiralcel OD-H, which is designed for the
operation with water-organic eluents in RP HPLC. Unlike
the aforementioned CSPs, the (S,S)-Whelk-O1 phase has
the Pirkle-type chiral selector: 1-(3,5-dinitrobenzamido)-
1,2,3,4-tetrahydrophenanthrene. The advantages of this
chiral selector are the presence of π-acidic 3,5-dinitro-
benzoyl groups, two adjacent to each other stereogenic
centers, the rigid conformation, and the aromatic system,
which can be used as a π-basic site. The oxygen atom of
the carbonyl group of 2-phenoxypropionic acid was found
to be capable of forming a stable hydrogen bond with the
amide hydrogen atom of the CSP.9
(S)-2-(1-Naphthyloxy)propionic acid [(S)-2b]. The yield was
38.2 mg (29%). White crystalline powder, m.p. 129—130 °С,
20
[α]D + 49.6 (c 1.0, EtOH); ее > 99%. Found (%): C, 72.29;
H, 5.49. C13H12O3. Calculated (%): C, 72.21; H, 5.59.
HPLC (Chiralcel OD-H, hexane—PriOH—TFA (30 : 1 : 0.02)):
τ(S)-2b = 14.4 min. The 1H and 13С NMR spectra of the isolated
compound are identical to the published data.17
(R)-2-(1-Naphthyloxy)propionic acid [(R)-2b]. The yield was
33.8 mg (27%). White crystalline powder, m.p. 128—129 °С,
20
20
[α]D –47.7 (c 1.0, EtOH) (cf. Ref. 21: [α]D –49.2 (c 2.4,
EtOH)); eе > 99%. Found (%): C, 72.14; H, 5.51. C13H12O3.
Calculated (%): C, 72.21; H, 5.59. HPLC (Chiralcel OD-H,
hexane—PriOH—TFA (30 : 1 : 0.02)): τ(R)-2b = 22.0 min. The
1H and 13С NMR spectra of acid (R)-2b are identical to those
published earlier.17
Results and Discussion
The series of aryloxycarboxylic acids 1a—i and 2a—f
was chosen as an object of the study. Compounds 1a—i
have the same nearest environment of the chiral center
and differ only by substituents in the aromatic ring.
Therefore, the influence of these substituents on specific
features of enantioseparation on different phases can be
evaluated. Unlike them, acids 2a—f have other structures
of substituents R1 and/or R2, which provides an informa-
tion about the presence or absence of a relationship be-
tween the separation parameters and character of substitu-
tion at the chiral center. Note that some of the mentioned
compounds are of practical interest. For example, 2-phen-
oxypropionic acid 1a is applied as a chiral selector; acid
1b is the precursor of Lofexidine drug1; herbicides Meco-
prop and Napropamide are synthesized from acids 1c and
A hexane—PriOH—TFA (40 : 1 : 0.02) system of sol-
vents was predominantly used in this work as the mobile
phase for analysis of racemic acids 1a—i and 2a—f under
the NP HPLC conditions. In some cases, the hexane to
isopropanol ratio was varied to achieve better separation
characteristics or shorten the analysis time, which was
restricted by us to 60 min. Under the RP HPLC condi-
tions, a system consisting of МеСN and a 0.25% aqueous
solution of АсОН in different ratios was applied for the
studies on the Kromasil 5-Cellucoat RP column. For the
(S,S)-Whelk-O1 CSP, МеОН was used instead of aceto-
nitrile. Such a choice of the organic modifier was based
on our experiment on the application of columns of this
type.28 In the case of RP HPLC, the eluent flow rate was
0.8 mL min–1, whereas other analysis conditions were
similar to those for NP HPLC.
Com-
pound
1a
X
Y
Z
Com-
pound
2a
R1
R2
H
Cl
Me
I
H
H
Me
H
H
H
H
Cl
H
F
NO2
Me
OMe
Ac
H
Cl
H
H
H
H
Me
H
PhCH2O
(1-Naphthyl)—O Me
Me
1b
2b
1c
2c
PhO
MeO
PhS
PhO
CH2Ph
1d
2d
Ph
Me
Pri
1e
2e
1f
2f
The optical isomers of 2-phenoxypropionic acids
1a—i, except for compounds 1c and 1i, were separated
with different efficiencies on the Chiralcel OD-H CSP
1g
1h
1i
H

Tumashov et al.
904
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
(Table 1). The best separation selectivity α was observed
for unsubstituted 2-phenoxypropionic acid 1a and its
4-methoxy-substituted derivative 1h. It should be men-
tioned that exceeding the elution time equal to 60 min for
more retained isomer of acids 1a and 1f in a hexane—
PriOH—TFA (40 : 1 : 0.02) system required the application
of a stronger eluent (see Table 1). The presence of one
electron-withdrawing substituent in the ortho- or para-
position of the phenoxyl fragment (compounds 1d—f) did
not prevent the resolution of the peaks of enantiomers to
the base line, and no separation was observed only for
derivative 1i with the acetyl group in the para-position.
The presence of two (or three) substituents in the benzene
ring was accompanied by a decrease in α for acids 1b and
1g or by the complete absence of separation in the case of
acid 1c. In the second group of acids (compounds 2a—f),
the changes in the environment of the chiral center did
not affect the separation of the enantiomers in compounds
2a—d and 2f with a high selectivity, and only the negative
result (see Table 1) was obtained when the phenoxyl oxy-
gen atom is replaced by the sulfur atom (compound 2e).
The experimental data for acid 1a in the hexane—
PriOH—TFA (40 : 1 : 0.02) mobile phase obtained on
Chiralpak AD, unlike Chiralcel OD-H, showed the pos-
sibility of efficient separation of the enantiomers within
an appropriate time (Fig. 1, Table 1). A decrease in the
retention times of the chromatographic peaks of the stereo-
isomers accompanied by a decrease in α in the group of
phenoxy acids 1a—i was also observed for compounds 1d,
1f, and 1g. On the contrary, in the case of derivatives 1b,
1e, and 1h, the retention factor of the peak of the first
eluted isomer (K1) increased, but the improvement of
resolution RS was detected for 2-(4-fluorophenoxy)pro-
pionic acid 1e only. Note that on this CSP no difficulties
appeared for the separation of racemic compounds 1c and
1i, which remained unseparated on the Chiralcel OD-H
column (see Table 1). Among acids 2a—f, an increase in
K1 on the Chiralpak AD column compared to the Chiralcel
OD-H chiral phase was observed for compounds 2a and
2d—f, whereas for derivatives 2b and 2c, on the contrary,
the retention time decreased. In addition, in this group of
acids the separation efficiency decreased for all com-
pounds, except for 2-phenylthiopropionic acid 2e for
which the complete resolution of the peaks of the enan-
tiomers was attained.
The results of the study confirmed the high resolution
ability of both CSPs on the polysaccharide basis, Chiralcel
OD-H and Chiralpak AD, toward studied 2-oxy- and
2-thiosubstituted carboxylic acids under the NP HPLC
conditions. The highest selectivity was achieved when
using the phase with modified cellulose for unsubstituted
2-phenoxypropionic acid 1a, which indicates a fairly easy
penetration to narrow chiral channels of Chiralcel OD-H
and active π—π-interactions with the phenyl rings of the
CSP. According to the data in Table 1, the character, ar-
rangement, and number of substituents in the benzene
ring substantially affect the separation selectivity of com-
pounds 1b—i. For example, the high degree of resolution
of peaks of the enantiomers of 2-(4-methoxyphenoxy)
propionic acid 1h can presumably be explained by the
participation of the oxygen atom of the methoxy group,
along with the oxygen atoms of the carboxyl and phenoxyl
groups, in the dipole-dipole interaction with the carbamate
group of the chiral selector. The presence of the halogen
atom in the ortho- and para-positions in compounds 1d
and 1e or the nitro group in the para-position of the benz-
ene ring (acid 1f) did not impede enantiomeric recognition.
However, in the case of another electron-withdrawing
substituent, viz., acetyl group (compound 1i), this process
did not occur because of the side interaction of the oxygen
atom of the substituent with the silanol groups on the
silica gel surface remained after the surface was covered
with the chiral selector. The separation characteristics
become poorer with increasing the number of substituents
U/V
1.5
14.78
22.08
1.0
2
0.5
1
0
10
15
20
25
τ/min
Fig. 1. Chromatogram of (RS)-2-phenoxypropionic acid 1a. HPCL conditions: Chiralpak AD column, hexane—PriOH—TFA
(40 : 1 : 0.02); 1: UV detector at λ = 220 nm; 2: polarimetric detector at λ = 430 nm; retention times of the (S)- and (R)-enantiomers
τ
(S) = 14.78 min and τ(R) = 22.08 min, respectively.

HPLC enantioseparation of aryloxycarboxylic acids
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
905
in the aryl fragment in acids 1b, 1c, and 1g, which is evi-
dently related to steric hindrances for the penetration of
these molecules into narrow chiral cavities of the cellulose-
based CSP. This is also confirmed by the fact of better separ-
ation of enantiomers of these compounds on the Chiralpak
AD phase with a broader chiral cavity (see Table 1).
Unlike the influence of substitution in the aromatic
ring, such changes as an increase in the distance between
the phenyl fragment and chiral center in acid 2a, replace-
ment of the phenyl residue in substituent R1 by the more
bulky naphthyl (acid 2b) or, on the contrary, by a smaller
methyl fragment (acid 2d), as well as the presence of
benzyl (compound 2c), phenyl (compound 2d), and iso-
propyl (compound 2f) groups as substituent R2, exerted
no substantial effect on the high α value (see Table 1). At
the same time, the replacement of the oxygen atom by the
sulfur atom in acid 2e resulted in the absence of separation,
which indicates a substantial contribution of the dipole-
dipole interaction between the indicated oxygen atom and
NH group of the carbamate fragment of the CSP during
enantiomeric recognition on the Chiralcel OD-H phase.
The authors8 assumed that the lower selectivity achieved
for the most part of the considered racemic acids on the
AD CSP compared to OD-H is probably related to easier
accessibility of its spiral cavity for penetration of molecules
of the analytes and shortening of the chiral recognition
time. The improvement of resolution compared to the
chiral phase Chiralcel OD-H observed in several cases is
probably caused by additional factors of enantiomeric
discrimination such as the formation of a hydrogen bond
between the NH group of the carbamate fragment and
halogen atom of the para-substituent in compounds 1c
and 1e or the oxygen atom of the acetyl group in acid 1i.
The resolution of isomers to the base line in the case of
2-phenylthiopropionic acid 2e indicates a lower signifi-
cance of a hydrogen bond between the ester atom of the
analyte and chiral selector for enantiomeric recognition
on the Сhiralpak AD chiral phase compared to Chiralcel
OD-H.The use of the CSP with modified cellulose
Kromasil 5-Cellucoat RP under the RP HPLC conditions
demonstrated the result similar in high characteristics of
selectivity and resolution on the Chiralcel OD-H phase
under the NP HPLC conditions only for 2-phenoxypro-
pionic acid 1a (α = 1.46, RS = 7.95) (see Table 1).
Similarly, no separation was observed in the case of com-
pounds 1c and 2e. For other considered 2-oxy acids, the
U/V
1.2
a
14.42
1.0
0.8
0.6
0.4
0.2
10
15
20
τ/min
U/mV
800
b
22.02
600
400
200
0
10
15
20
τ/min
Fig. 2. Chromatograms of (a) (S)-2b and (b) (R)-2b enantiomers (ee > 99%) of 2-(1-naphthyloxy)propionic acid; HPLC conditions:
Chiralcel OD-H, hexane—PriOH—TFA (30 : 1 : 0.02), flow rate 1 mL min–1, detection at λ =220 nm.

Tumashov et al.
906
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
use of the sorbent of this type in the reversed-phase vari-
ant of chromatography turned out to be appreciably less
efficient than NP HPLC.
selectivity, except for the replacement of the ester oxygen
atom by the sulfur atom. Individual stereoisomers of acids
1d and 2b with ee > 99% were isolated by preparative
HPLC.
The use of the (S,S)-Whelk-O1 column with the Pirkle-
type CSP in RP HPLC also showed a low efficiency. The
highest separation characteristics on this column were
obtained for 2-(2,6-dichlorophenoxy)propionic acid 1b
(see Table 1). Note that the separation of enantio-
mers to the base line was observed for compounds 1g
and 2d, whereas the separation was incomplete for acid
2e (RS = 0.78), and no satisfactory separation was achieved
in other cases.
The substantial worsening of the separation character-
istics observed under the RP HPLC conditions indicates
factors impeding chiral recognition in water-organic sys-
tems. In our opinion, the most probable reason for this
phenomenon is the dissociation of acetic acid due to which
protons add to the amide nitrogen atom of the CSP with
the formation by the latter of ion pairs with АсО–. As
a result, an obstacle appears for such an important site of
enantiomeric discrimination as generation of hydrogen
bonds between the oxygen atoms of the carbonyl and ester
groups of the separated analyte and the NH group of the
chiral selector. At the same time, it is necessary to create
an acidic medium in the eluent to retain analyzed phenoxy
acids in the nonionized form.
This work was carried out within the framework of the
state assignment of the Postovsky Institute of Organic
Synthesis (Ural Branch of the Russian Academy of
Sciences) (No. AAAA-A19-119011790134-1) using the
equipment of the Center for Joint Use "Spectroscopy and
Analysis of Organic Compounds" of the Postovsky Institute
of Organic Synthesis (Ural Branch of the Russian Academy
of Sciences).
This work does not contain descriptions of studies on
animals or humans.
The authors declare no competing interests.
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Received December 8, 2020;
in revised form March 12, 2021;
accepted March 19, 2021