Application of L-proline derivatives as chiral shift reagents for enantiomeric recognition of carboxylic acids

Article abstract of DOI:10.1002/chir.20948

Four proline-derived chiral receptors 5-8 were readily synthesized starting from L-proline. The enantiomeric recognition ability of chiral receptors was examined with a series of carboxylic acids by ¹H NMR spectroscopy. The molar ratio and the

Full text of DOI:10.1002/chir.20948

CHIRALITY 23:463–471 (2011)
Application of L-Proline Derivatives as Chiral Shift Reagents for
Enantiomeric Recognition of Carboxylic Acids
HAYRIYE NEVIN NAZIROGLU,1 MUSTAFA DURMAZ,2 SELAHATTIN BOZKURT,2 AND ABDULKADIR SIRIT
2
*
1Department of Chemistry, Karamanoglu Mehmetbey University, 70100 Karaman, Turkey
2Department of Chemistry, Selc¸uk University, 42099 Konya, Turkey
ABSTRACT
Four proline-derived chiral receptors 5–8 were readily synthesized starting
from L-proline. The enantiomeric recognition ability of chiral receptors was examined with a se-
ries of carboxylic acids by 1H NMR spectroscopy. The molar ratio and the association constants
of the chiral compounds with each of the enantiomers of guest molecules were determined by
using Job plots and a nonlinear least-squares fitting method, respectively. The Job plots indicate
that the hosts form 1:1 instantaneous complexes with all guests. The receptors exhibited differ-
ent chiral recognition abilities toward the enantiomers of racemic guests. Among the chiral
receptors used in this study, prolinamide 6 was found to be the best chiral shift reagent and is
effective for the determination of the enantiomeric excess of chiral carboxylic acids. Chirality
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23:463–471, 2011.
2011 Wiley-Liss, Inc.
KEY WORDS: chiral recognition; L-proline; carboxylic acid; NMR titration; enantiomeric excess
INTRODUCTION
activities. The investigation of the chiral recognition of car-
boxylic acids by artificial receptors was of critical importance
in the preparation, separation, and analysis of enantiomeri-
cally pure carboxylic acids and disclosing the mechanism of
interaction of the carboxylic acids with biological systems.
Therefore, a variety of chiral shift reagents, such as chiral
amines,19–24 amino alcohols,25 Tro¨ger’s Base,26 macrocyclic
amines and amides,27–34 crown ethers,35–37 and calixar-
enes,38–43 have been described in the literature to determine
the enantiomeric purity and understand the basis of the
mechanism of host–guest complexations. To date, however,
there are only a few examples of proline-derived receptors44,45
for the enantiomeric recognition of carboxylic acids.
In continuation of our work on the synthesis of chiral
receptors based on the calix[4]arene platform equipped with
various functionalities including crown46 and azacrown
ethers,47 amides,48 Schiff bases,49,50 and quaternary ammo-
nium salts51 and their catalytic activities and enantiomeric
recognition properties toward chiral amines, carboxylic
acids, and amino acid derivatives, we herein report the syn-
thesis of L-proline-based chiral receptors and their recogni-
tion abilities for carboxylic acids by 1H NMR spectroscopy.
Determination of enantiomeric composition is of great im-
portance due to the role of chirality in drug discovery, food
industry, pesticide chemistry, biological, and separation sci-
ences. Natural living systems are composed of chiral biologi-
cal materials and they interact with each stereoisomer of a ra-
cemic drug, respectively, as well as metabolize each enan-
tiomer by way of a separate pathway to produce different
pharmacological activities. Therefore, one stereoisomer may
produce the desired therapeutic activities, whereas the other
may be inactive or produce harmful effects.
Several methods1 have been developed for the determina-
tion of the enantiomeric composition, including liquid2 and
gas chromatography,3 capillary electrophoresis,4 circular
dichroism,5 absorbance spectrometry,6 infrared transmission
spectrometry,7 X-ray anomalous scattering,8 and NMR spec-
troscopy.9–12
Among them, NMR spectroscopy has been reported to be
a sensitive tool for the determination of enantiomeric purity
and the occurrence of interaction between a chiral receptor
and an analyte in the solution state,13–16 and the NMR experi-
ments has also provided the detailed information regarding
the nature of these interactions and the structure of the dia-
stereomeric complexes involved. NMR spectra of enantiom-
ers are identical in an achiral medium, and usually there is
no difference between the spectra of a racemic mixture and
that of a single isomer. However, the use of chiral solvating
agents (CSAs) can lead to differentiations of the spectra of
enantiomers due to the formation of diastereomeric com-
plexes that are held together by weak intermolecular interac-
tions such as van der Waals forces and/or hydrogen
bonds.17 As soon as there is a large enough chemical shift
nonequivalence to give baseline resolution between the sig-
nals of analogous nuclei in these diastereoisomeric com-
plexes, integration yields a direct measurement of the enan-
tiomeric composition.18
MATERIALS AND METHODS
General
Melting points were determined on an Electrothermal 9100 apparatus
1
in a sealed capillary and are uncorrected. H and 13C NMR spectra were
recorded at room temperature on a Varian 400-MHz spectrometer in
CDCl3. Chemical shifts were reported in ppm. IR spectra were obtained
Contract grant sponsors: Scientific and Technical Research Council of Tur-
key (TUBITAK), Research Foundation of Selc¸uk University (BAP), TUBI-
TAK-BIDEB; Contract grant numbers: 109T167, 09201134.
*Correspondence to: Prof. Dr. Abdulkadir Sirit, Department of Chemistry,
Selc¸uk University, 42099 Meram/Konya, Turkey. E-mail: asirit@selcuk.edu.tr
Received for publication 22 October 2010; Accepted 23 December 2010
DOI: 10.1002/chir.20948
Chiral carboxylic acids are important natural products,
drugs, or key building blocks for the preparation of a wide
range of compounds with biological and pharmacological
Published online 6 April 2011 in Wiley Online Library
(wileyonlinelibrary.com).
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2011 Wiley-Liss, Inc.

464
NAZIROGLU ET AL.
on a Perkin Elmer Spectrum 100 FTIR spectrometer using KBr pellets.
Optical rotations were measured on an Atago AP-100 digital polarimeter.
The HPLC measurements were carried out on Agilent 1100 equipment
connected with a Zorbax RX-C18 column. Elemental analyses were per-
formed using a Leco CHNS-932 analyzer.
Analytical TLC was performed using Merck prepared plates (silica gel
60 F254 on aluminum). Flash chromatography separations were per-
formed on a Merck Silica Gel 60 (230–400 Mesh). All reactions, unless
otherwise noted, were conducted under a nitrogen atmosphere. All start-
ing materials and reagents used were of standard analytical grade from
Fluka, Merck, and Aldrich and used without further purification.
Dichloromethane was dried (CaCl2), distilled from CaH2, and stored
over molecular sieves. Other commercial grade solvents were distilled
and then stored over molecular sieves. The drying agent used was anhy-
drous MgSO4.
General procedure for the synthesis of compounds 3 and 4. N-
carbonylbenzyloxy-(S)-proline (0.49 g, 2 mmol) was dissolved with stir-
ring in dry dichloromethane (25 mL) in a 50-mL round bottom flask
equipped with a dropping funnel. The solution was cooled to 258C, and
dicyclohexylcarbodiimide (DCC, 0.41 g, 2 mmol) was added slowly in
small portions. Appropriate secondary amine (2.3 mmol) was added
dropwise to the reaction mixture. After stirring for 4 h at room tempera-
ture, the solution was diluted to double its volume with dichlorome-
thane. Urea formed was separated by filtration. The solvent was
removed. To remove further amounts of urea, the residue was dissolved
in 10 mL of ethyl acetate and heated to 508C. The remaining urea was fil-
tered off. This procedure was repeated until no more urea was formed.
The solvent was evaporated, and the crude products were purified by
flash chromatography (n-hexane/ethyl acetate, 5:1).
Benzyl (S)-(2)-2-(1-indolinylcarbonyl)-1-pyrrolidinecarboxylate [3]. White
solid. Yield 99%; Mp; 137–138 8C; a2D5 5 232.7 (c 1.08, CHCl3); Rf 5 0.38
(n-hexane/ethyl acetate, 1:1). IR (KBr): 2969, 2883, 1687, 1667, 1655,
Analytical grade carboxylic acids were purchased from Aldrich, Fluka,
or Acros and used without further purification as guest molecules for the
experiments: that is, ibuprofen, N-acetyl-alanine, hydroxyisovaleric acid,
and 2-chloropropionic acid (Fig. 1).
1478, 1415, 1360, 1337, 1301, 1128, 731 cm21 1H NMR (CDCl3) ppm: d
;
1.83–2.05 (m, 1H), 2.1–2.24 (m, 1H), 2.72–3.05 (m, 1H), 3.12–3.24 (m,
1H), 3.52–3.64 (m, 1H), 3.68–3.78 (m, 1H), 3.8–3.95 (m, 1H), 4.02–4.12
(m, 1H), 4.38–4.52 (m,1H), 4.62–4.69 (m, 1H), 4.96 (d, 1H, J 5 12 Hz),
5.08–5.12 (m, 1H), 5.20 (d, 1H, J 5 12 Hz), 6.94–7.38 (m, 8H), 8.25 (t,
1H, J 5 9 Hz); 13C NMR (100 MHz, CDCl3, Cbz rotamers): d (ppm):
171.13, 170.77, 155.17, 154.32, 143.24, 143.13, 136.98, 136.43, 131.30,
128.68, 128.49, 128.46, 128.24, 128.14, 128.08, 128.04, 127.84, 127.74,
124.67, 124.15, 124.11, 117.67, 117.57, 67.66, 67.26, 59.20, 58.65, 47.90,
47.74, 47.51, 30.83, 29.84, 28.43, 28.16, 24.58, 24.03; Anal. Calcd for
NMR Host–Guest Titrations
Samples for analysis were obtained by mixing equimolar amounts of
chiral ligands with the guests in CDCl3, making the concentrations of
the ligands (or guests) normally 10 mM. For NMR titrations, the guest
compound was dissolved in an appropriate amount of solvent and the
resulting solution evenly distributed among 10 NMR tubes. The first
NMR tube was sealed without any ligand. The ligand was also dissolved
in the appropriate amount of solvent and added in increasing amounts to
the NMR tubes, so that solutions with the following relative amounts
(equiv) of ligand versus guest compound (concentration was 1.0 3 1022
M) were obtained: 0, 0.20, 0.40, 0.60, 0.80, 1.00, 1.20, 2.00, 3.00, 4.00,
6.00, 10.00. Ka was calculated by a nonlinear least-squares fitting method
for compound 6 from the observed Dd values and the respective host
and guest concentrations.
C21H22N2O3 (350.42): C, 71.98%; H, 6.33%; N, 7.99%. Found: C, 71.94%; H,
6.30%; N, 8.02%.
(S)-Benzyl 2-((S)-2-(diphenylmethyl)pyrrolidine-1-carbonyl)pyrrolidine-
1-carboxylate [4]. White solid. Yield 76%; Mp; 141–1428C; a2D5 5 259.0
(c 1.05, CHCl3); Rf 5 0.57 (n-hexane/ethyl acetate, 1:1). IR (KBr): 2974,
2882, 1697, 1644, 1496, 1415, 1354, 1118, 733 cm21 1H NMR (CDCl3)
;
ppm: d 1.38–1.56 (m, 2H, CH2ꢀꢀ pyrrolidine), 1.64–1.82 (m, 2H, CH2ꢀꢀ
pyrrolidine), 1.84–2.06 (m, 4H, CH2ꢀꢀ pyrrolidine), 3.06–3.44 (m, 2H,
CH2ꢀꢀ pyrrolidine), 3.46–3.78 (m, 2H, CH2ꢀꢀ pyrrolidine), 4.24–4.45 (m,
2H, CH2), 4.96–5.27 (m, 3H, 3CH), 7.18–7.35 (m, 15H, ArH); 13C NMR
(100 MHz, CDCl3, Cbz rotamers): d (ppm): 171.40, 171.20, 142.37,
142.33, 129.47, 129.41, 129.21, 129.14, 128.63, 128.58, 128.34, 128.21,
128.08, 128.05, 127.97, 126.90, 126.88, 126.41, 126.34, 67.30, 67.05, 59.96,
58.49, 57.83, 53.93, 53.72, 47.33, 46.73, 46.57, 46.34, 30.59, 29.62, 28.01,
27.77, 24.29, 24.12, 23.85, 23.63; Anal. Calcd for C30H32N2O3 (468.59): C,
76.90%; H, 6.88%; N, 5.98%. Found: C, 76.85%; H, 6.93%; N, 5.92%.
Evaluation of the Stoichiometric Ratio of the Host–Guest
Complex (Job Plots)
The stoichiometry of the complex between chiral hosts and enantiom-
ers of carboxylic acids was determined by a continuous variation plot
(Job plot).52 Equimolar amounts of host and guest compounds were dis-
solved in CDCl3. These solutions were distributed among nine NMR
tubes, with the molar fractions X of host and guest in the resulting solu-
tions increasing (or decreased) from 0.1 to 0.9 (and vice versa). The
complexation-induced shifts (Dd) were multiplied by X and plotted
against X itself.
General procedure for the synthesis of compounds
5
and 7. LiAlH4 (0.74 g, 2.1 mmol) in dry THF (10 ml) at 208C was
stirred for a few minutes under a nitrogen atmosphere. The mixture
was cooled to 08C, and compound 3 or 4 (0.8 mmol) in dry THF (10
ml) was added dropwise over 30 min. The mixture was heated under
reflux for 4 h and then cooled in an ice bath. Aqueous NaOH (2 M)
was added dropwise until a white precipitate of inorganic salts had
formed. The inorganic salts were removed by filtration and washed
with THF (3 3 20 ml). The filtrate was dried (MgSO4) and concen-
trated under reduced pressure. Crude products were purified by flash
Determination of Enantiomeric Purity of N-Acetyl Alanine
Six samples were containing N-acetyl alanine with 0, 20, 40, 60, 80,
and 100% ee, respectively. All samples were prepared by adding 0.25
equiv of chiral shift reagent 6 in the solutions of N-acetyl alanine (final
concentration was 10 mM in CDCl3), and their spectra were recorded on
400-MHz spectrometer. The results were calculated based on
the integrations of the NMR signals of methyl group of N-acetyl alanine
a
isomers.
chromatography (CHCl3/MeOH, 1:6 for compound
5 and 1:5 for
compound 7).
Syntheses
(S)-1-Methyl-2-[(indolinyl)methyl]pyrrolidine [5]. Brown oil. Yield
76%; a2D5 5 282.0 (c 1.50, EtOH); Rf 5 0.36 (CHCl3/MeOH, 1:1). IR
Compound 3 was synthesized from N-carbonylbenzyloxy-(S)-proline
according to a procedure reported by Kobayashi and Horibe53 for (S)-1-
(N-tert-butoxycarbonylprolyl)indoline. Spectroscopic and physical data
for title compound are consistent with those reported in the literature.54
Compound 5 was prepared in one step from N-Cbz prolinamide 3
according to a procedure for the synthesis of (S)-1-methyl-2-[(dihydroi-
soindol-2-yl)methyl]pyrrolidine reported by Terakado and Oriyama.55
Spectroscopic and physical data are consistent with those reported in
the literature.53 Compound 6 was synthesized from N-protected prolina-
mide 3 by modification of a procedure reported by Alcon et al.,56 and
spectroscopic and physical data are consistent with those reported
data.54
(KBr): 3047, 2943, 2773, 1606, 1488, 1249, 1024, 712 cm21 1H NMR
;
(CDCl3) ppm: d 1.59–1.88 (m,3H), 1.97–2.08 (m, 1H), 2.21 (q, lH J 5
9.1 Hz), 2.37–2.47 (m, 4H), 2.92–3.01 (m, 3H), 3.09 (t, 1 H, J 5 8.3 Hz),
3.22 (dd, 1 H, J 5 4.5, 13.4 Hz), 3.4 (t, 2H, J 5 8.4 Hz), 6.47 (d, 1 H, J
57.9 Hz), 6.62 (t, 1 H, J 57.3 Hz), 6.97–7.23 (m, 2H); 13C NMR (100
MHz, CDCl3): d (ppm): 153.18, 129.7, 127.5, 124.5, 117.5, 106.7, 64.7,
57.9, 54.7, 54.6, 41.6, 30.6, 28.9, 22.6; Anal. Calcd for C14H20N2
(216.33): C, 77.73%; H, 9.32%; N, 12.95%. Found: C, 77.70%; H, 9.31%; N,
12.99%.
(2S,20S)-2-(Diphenylmethyl)-1-[(1-methylpyrrolidin-2-yl)methyl]pyrroli-
dine [7]. Brown oil. Yield 65%; a2D5 5 275.0 (c 1.05, CHCl3); Rf 5 0.42
Chirality DOI 10.1002/chir

APPLICATION OF L-PROLINE DERIVATIVES
465
Scheme 1. Synthesis of L-proline-derived chiral compounds.
(CHCl3/MeOH, 1:1). IR (KBr): 3372, 1646, 1562, 1470, 1406, 1265, 1054,
RESULTS AND DISCUSSION
Synthesis
782 cm21; 1H NMR (CDCl3) ppm: d 1.11–1.58 (m, 4H, 2CH2ꢀꢀ pyrrolidine),
1.66–1.78 (m, 2H, CH2ꢀꢀ pyrrolidine), 1.96–2.14 (m, 4H, 2CH2ꢀꢀ pyrroli-
dine), 2.20 (s, 3H, CH3), 2.22–2.38 (m, 2H, CH2ꢀꢀ pyrrolidine), 2.82–3.04
(m, 2H, CH2ꢀꢀ pyrrolidine), 3.18–3.24 (m, 2H, 2CH), 3.92 (d, J 5 6.9 Hz,
1H, CH), 7.02–7.28 (m, 10H, ArH); 13C NMR (100 MHz, CDCl3): d (ppm):
144.20, 143.92, 129.35, 129.28, 128.87, 128.68, 128.48 (2C), 128.33, 128.14,
126.29, 126.12, 69.12, 65.05, 61.80, 57.71, 56.86, 55.07, 41.67, 30.15, 29.52,
24.30, 22.92;. Anal. Calcd for C23H30N2 (334.50): C, 82.59%; H, 9.04%; N,
8.37%. Found: C, 82.56%; H, 9.09%; N, 8.35%.
Proline is a secondary cyclic, pyrrolidine-based natural pro-
teinogenic amino acid. Either the proline itself or its deriva-
tives with various functionality are found to be an effective
bifunctional organocatalyst for enantioselective Michael, Man-
nich, and Aldol reactions57 and can also act as chiral selectors
in the preparation, separation, and analysis of enantiomers. It
was expected that proline-based chiral receptors could be
useful for enantiomeric recognition of carboxylic acids. Four
proline-derived chiral receptors 5–8 were readily synthesized
starting from L-proline as shown in Scheme 1. The first step
of the synthesis was the preparation of the N-protected L-pro-
line derived from L-proline,58 which was used as a precursor
of the desired chiral receptors. The Cbz-protected L-proline
was then condensed with indoline or (S)-(2)-2-(diphenylme-
thyl)pyrrolidine by a modification of previously reported pro-
cedure53 in the presence of DCC to yield compounds 3 and 4
in good to excellent (76–99%) yields. The corresponding dia-
mines 5 and 7 were then prepared by the reduction of proli-
namides 3 and 4 using excess LiAlH4 in 65–76% yields.55 N-
protected prolinamides 3 and 4 were also converted into chi-
ral receptors 6 and 8 by catalytic hydrogenation56 with Pd/C
in 75–85% yields.
General procedure for the synthesis of compounds 6 and 8.
A
mixture of 3 or 4 (1.14 mmol), cyclohexene (0.54 ml, 6.84 mmol) and
0.18 g of commercial Pd/C (10%) in 15 ml of EtOH was heated under
reflux for 2 h in argon, cooled, and filtered over celite. The catalyst
was washed with EtOH, and the filtrate and wash liquids were evapo-
rated under reduced pressure. Crude products were purified by flash
chromatography (CHCl3/MeOH, 1:7 for compound
6 and 1:6 for
compound 8).
(S)-2-(N-Indolinyl)pyrrolidine-2-carboxamide [6]. Tan oil. Yield 85%;
a2D5 5 2107.3. (c 1.03, CHCl3); Rf 5 0.27 (CHCl3/MeOH, 1:1). IR (KBr):
1
3296, 2951, 2861, 1649, 1598, 1480, 1461, 1392, 1289, 1263, 752 cm21; H
NMR (CDCl3) ppm: d 1.66–1.75 (m, 3H), 2.09–2.2 (m, 1 H), 2.5 (br, 1H),
2.78–2.89 (m, 1H), 3.12–3.24 (m, 3H), 3.8–3.87 (m, 1H), 3.94–4.05 (m,
1H), 4.06–4.14 (m, 1H), 6.98 (t, J 5 7.0 Hz, 1H), 7.12–7.18 (m, 2H), 8.16
(d, J 5 8.5 Hz, 1H); 13C NMR (100 MHz, CDCl3): d (ppm): 173.01,
143.20, 131.32, 127.83, 124.83, 124.19, 117.37, 60.53, 48.11, 47.57, 30.86,
28.40, 26.82; Anal. Calcd for C13H16N2O (216.28): C, 72.19%; H, 7.46%; N,
12.95%. Found: C, 72.15%; H, 7.49%; N, 12.93%.
Chiral Recognition by 1H NMR of Host–Guest Complexes
(2S,20S)-2-[(N-Diphenylmethyl)pyrrolidinyl]-2-carboxamide [8]. Brown
oil. Yield 75%; a2D5 5 268.0 (c 1.05, CHCl3); Rf 5 0.30 (CHCl3/MeOH,
1H NMR spectroscopy was used to investigate the chiral
recognition ability of compounds 5–8, and the racemic
guests, ibuprofen, N-acetyl-alanine, hydroxyisovaleric acid,
and 2-chloropropionic acid were chosen as probes (Fig. 1).
We observed that the methine, methylene, and methyl pro-
ton signals of the carboxylic acids a–d appear as sharp sin-
glet, doublet, and quartet and do not overlap with the peaks
1:1). IR (KBr): 3285, 1619, 1493, 1389, 1266, 1154, 1031, 730 cm21 1H
.
NMR (CDCl3) ppm: d 1.22–1.29 (m, 2H, CH2ꢀꢀ pyrrolidine), 1.30–1.48
(m, 2H, CH2ꢀꢀ pyrrolidine), 1.52–1.68 (m, 2H, CH2ꢀꢀ pyrrolidine), 1.73–
1.90 (m, 2H, CH2ꢀꢀ pyrrolidine), 2.61–2.67 (br, 1H, NH), 2.90–3.01 (m,
4H, 2CH2ꢀꢀ pyrrolidine), 3.55–3.62 (m, 1H, CH), 4.32–4.38 (d, J 5 6.9
Hz, 1H, CH), 4.96–5.04 (m, 1H, CH), 6.98–7.18 (m, 10H, ArH); 13C NMR
(100 MHz, CDCl3): d (ppm):173.10, 142.23, 141.98, 129.55, 129.34,
129.02, 128.81, 128.66, 128.45, 128.06, 127.16, 126.85, 126.33, 60.18,
59.59, 53.10, 47.62, 46.05, 30.63, 27.69, 26.39, 23.81; Anal. Calcd for
1
of the other proton signals in their H NMR spectra. Hence,
they are ideal probes for this study.
Figure 2 shows the 1H NMR spectra for 2-chloro propionic
acid (10 mM) in the absence and presence of the chiral
C22H26N2O (334.46): C, 79.01%; H, 7.84%; N, 8.38%. Found: C, 79.03%; H,
7.82%; N, 8.43%.
Chirality DOI 10.1002/chir

466
NAZIROGLU ET AL.
Fig. 1. Chemical structures of the guests used. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
receptors 5–8 (10 mM). When a solution of racemic 2-chloro
propionic acid (10 mM in CDCl3) was gradually added to a
10 mM solution of 5–8 in CDCl3 until the ratio reached 1:1,
the methine proton signals of 2-chloro propionic acid was
split into two quartets by 1.2, 4.0, and 2 Hz in the presence of
the chiral discriminating agents 5, 6, and 8, respectively,
with upfield chemical shift. The Dd, chemical shift differen-
ces relative to the original signals in the absence of chiral
receptors, are in the range of 0.020–0.057. The methine pro-
ton signal of 2-chloro propionic acid did not show any split-
ting in the presence of 7.
A similar phenomenon was observed when a solution of ra-
cemic N-acetyl alanine (10 mM in CDCl3) was treated with
an equimolar amount of receptors 5–8 (Fig. 3), the methine
proton signals of N-acetyl alanine separated into two quartets
due to the formation of two instantaneous diastereotopic
complexes between the receptors and the enantiomers of N-
acetyl alanine, respectively, with an upfield shift (from d
4.580 to 4.339 ppm and 4.342 ppm for 5, to 4.325 and 4.330
ppm for 6, and to 4.258 and 4.262 for 7). The methine proton
of (S)-2-chloropropionic acid and N-acetyl L-alanine appeared
at lower field than that of (R)-2-chloropropionic acid and N-
acetyl D-alanine, which was confirmed by an increase of the
methine proton signal of (S)- and D-guest when (S)-2-chloro-
propionic acid and N-acetyl D-alanine were added into the
complex of racemic guests with 5 or 6. In most cases, chem-
ical shift nonequivalences were also observed for the methyl
signals of 2-chloropropionic and N-acetyl alanine in the pres-
ence of 5, 6, and 8 (Figs. 2 and 3).
In the case of the (6)-a-hydroxy isovaleric acid, nonequi-
valence was observed for the methyl proton signals when
chiral receptors 5–8 were used. The methyl proton signals
of racemic hydroxy isovaleric acid appear at 0.928 ppm as a
doublet. They were shifted upfield and split into two doublets
by 4.4, 6, 4.8, and 4.0 Hz in the presence of the chiral agents
5–8, respectively (Fig. 4). The Dd values are in the range of
0.048–0.090 Hz, whereas in the presence of the compound 8,
splitting was observed in the methine proton (1.6 Hz) and
the methyl proton signal.
Although the methine proton of the ibuprofen could not be
differentiated by any of the chiral receptors, the methyl pro-
ton signals of racemic ibuprofen, appeared at 0.88 and 1.50
ppm as doublets, were shifted upfield and split into two dou-
blets in the presence of the chiral agents 5 and 6, respec-
tively (Fig. 5). (D)-enantiomer of a-hydroxy isovaleric acid
and (S)-enantiomer of ibuprofen appeared at lower field than
the corresponding (L)- and (R)-enantiomers in the presence
of the chiral hosts 5 and 6. They indicate that the (L)- and
(R)-enantiomers bind more strongly compared with their an-
tipodes with the hosts 5 and 6. The results were confirmed
1
by recording the H NMR spectra of the samples containing
nonracemic guests (acids).
Although the structures of the CSAs 5–7 and 6–8 are sim-
ilar, the chiral discrimination abilities of them toward carbox-
ylic acids are obviously different. As shown in Figures 2–5,
the chemical shift nonequivalences of at least one of the pro-
tons of the selected acids were observed in the presence of
chiral receptors 5–6. From a comparison of the DDd values,
it appears that chiral receptors 5 and 6 bearing a benzene
ring fused to a five-membered nitrogen-containing ring show
a significant discrimination ability probably due to the indo-
line moiety. With respect to the solution conformation of re-
ceptor 6 complexed with an equimolar amount of 2-chloro
propionic acid (Fig. 6), the analysis of their 2D NOESY spec-
tra showed cross-peaks between the methylene protons
(next to the amide nitrogen of the indoline and next to the
amine nitrogen in pyrrolidine ring) in chiral receptor and a-
methyl protons in guest. This implies a U-shape-folded con-
formation in solution.
Furthermore, it is clear that compared with compounds 5
and 7, chiral hosts 6 and 8 containing a carbonyl group next
to the chiral center also exhibited better chiral recognition
abilities toward these mentioned carboxylic acids a–d. This
Fig. 2. 1H NMR spectra of (6)-2-chloro propionic acid in the presence of
the chiral receptors 5–8 (1:1 ratio of 10 mM solution in CDCl3 at 258C).
[Color figure can be viewed in the online issue, which is available at wileyon-
linelibrary.com.]
Chirality DOI 10.1002/chir

APPLICATION OF L-PROLINE DERIVATIVES
467
Fig. 3. 1H NMR spectra of N-acetyl DL-alanine in the presence of the chiral receptors 5–8 (1:1 ratio of 10 mM solution in CDCl3 at 258C). [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
Fig. 4. 1H NMR spectra of a-hydroxy isovaleric acid in the presence of the chiral receptors 5–8 (1:1 ratio of 10 mM solution in CDCl3 at 258C). [Color figure
can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Chirality DOI 10.1002/chir

468
NAZIROGLU ET AL.
Fig. 7. Job plots of 5 with (R)- and (S)-ibuprofen [X 5 molar fraction of
ibuprofen, Dd 5 chemical shift change of the a-methyl proton of (R)- and (S)-
ibuprofen]. (n) With pure (S)- ibuprofen, (l) with pure (R)-ibuprofen.
carbonyl function may act as an additional binding site and
play a crucial role for chiral recognition.
It should be noted that, in many cases, the protons far
from the stereogenic center, such as CH3 in N-acetyl alanine,
a-hydroxy isovaleric, and ibuprofen can also be completely
split in the presence of chiral compounds 5–8. This may be
due to the fact that there is a CH3–p interaction39 between
aromatic groups in hosts (5–8) and methyl protons in
guests (a–d) in addition to the major acid–base attractive
interaction.
The stoichiometry of the complexes between the receptors
5–8 and the guests was determined by a continuous varia-
tion plot. The total concentration of the hosts and the guest
Fig. 5. 1H NMR spectra of ibuprofen in the presence of the chiral recep-
tors 5–8 (1:1 ratio of 20 mM solution in CDCl3 at 258C). [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
Fig. 6. 2D NOESY spectra of 6 with (6)-2-chloro propionic acid in CDCl3. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Chirality DOI 10.1002/chir

APPLICATION OF L-PROLINE DERIVATIVES
469
TABLE 1. Association constants Ka (mol/l)21 of 6 with chiral
carboxylic acids
Entry
Proton
Guest
K (3 103) M21
KD/KL or KR/KS
1.31a
1
2
3
4
5
6
7
8
Me
Me
Me
(L)-a
(D)-a
(D)-b
(L)-b
(R)-c
(S)-c
(R)-d
(S)-d
3.2860.48
2.5060.34
2.9760.90
1.3260.63
2.9360.15
2.1760.82
2.6060.54
2.0160.37
2.25
1.35
1.29
Me
a-Me
a-Me
a-Me
a-Me
aKL/KD.
was kept constant (10 mM) in CDCl3, whereas the molar frac-
tion of the guest {[G]/([H]1[G])} was continuously varied.
The Job plots of 5 with (R)- and (S)-ibuprofen and 6 with N-
acetyl-L- and D-alanine are illustrated in Figures 7 and 8,
respectively. Maxima were observed when the molar ratio of
the chiral receptors (5 and 6) and each enantiomer of the
guests ((R)- or (S)-ibuprofen and N-acetyl-L- and D-alanine)
was 1:1 (X 5 0.5), which indicated that host 5 and 6 and the
guests formed 1:1 instantaneous complexes. It is apparent
that the chemical shift changes of (R)-ibuprofen and N-acetyl
L-alanine were greater than that of corresponding enantiomers
(S)-ibuprofen and N-acetyl D-alanine in the presence of com-
pound 5 and 6. Meanwhile, all Job plots recorded for 7 and 8
indicated that 1:1 instantaneous complexes were also formed
with all these enantiomerically pure guests, respectively.
To further evaluate the complexation and discrimination
abilities of proline-based receptors, the titration curves of
compounds 5 and 6 with the enantiomers of ibuprofen or N-
acetyl alanine were plotted, respectively (Figs. 9 and 10). It
was found that the signals of the methyl protons of (R)- and
Fig. 8. Job plots of 6 with N-acetyl-D- and L-alanine [X 5 molar fraction
of N-acetyl alanine, Dd 5 chemical shift change of the methyl proton of
N-acetyl-L- and D-alanine]. (n) With pure N-acetyl-L-alanine, (l) with pure
N-acetyl-D-alanine.
Fig. 9. 1H NMR titration curves of compound
(S)-ibuprofen.
5 with (R)- and
Fig. 10. 1H NMR titration curves of compound 6 with N-acetyl D- and
L-alanine.
Fig. 11. Selected region of the 400 MHz NMR spectra of N-acetyl alanine
of various enantiomeric purities in the presence of 6 (0.25 equiv).
Chirality DOI 10.1002/chir

470
NAZIROGLU ET AL.
Fig. 12. The minimum energy structures of 6 complexed with (a) (R)-2-chloro propionic acid and (b) (S)-2-chloro propionic acid. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
(S)-ibuprofen or N-acetyl D- and L-alanine in the 1H NMR
spectra continuously shifted upfield and reached a limiting
value along with the concentration of compounds 5 and 6
gradually increasing. Moreover, the signals of various pro-
tons of the hosts were shifted downfield. These shifts are
due to specific host–guest complexation and indicated that
the interaction between the host and guest also happened by
multiple hydrogen bonds.
troscopy was used to investigate their complexation and
enantiodiscriminating ability. The diastereomeric complexes
obtained from such complexation give rise to distinguishable
1
signals in H NMR spectra, which can be used for the deter-
mination of the enantiomeric purity and for the enantiomeric
recognition and assignment of the absolute configuration of
the corresponding carboxylic acids. The results indicate that
the multiple hydrogen bonding, CH3-p, cation-p, and p2p
stacking between the aromatic groups may be responsible
for the enantiomeric recognition. Further studies of the pro-
line derived chiral receptors to allow asymmetric catalysis
are now in progress in our laboratory.
The association constants (Ka) of compound 6 with the
enantiomers of carboxylic acids were determined from the ti-
tration curves by a nonlinear least-squares fitting method59,60
(Table 1). From Table 1, it was seen that the (D)- or (R)-enan-
tiomer of carboxylic acids (except a) were more strongly
bound to chiral receptor 6 than the (L)- or (S)-enantiomer.
Finally, we have also demonstrated the practical applicabil-
ity of chiral receptor 6 for the measurement of the ee of car-
boxylic acids, using N-acetyl alanine as a model compound.
Six samples containing N-acetyl alanine with 0, 20, 40, 60, 80,
and 100% ee were prepared and the enantiomeric composi-
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