Synthesis of alanine-based colorimetric sensors and enantioselective recognition of aspartate and malate anions

Article abstract of DOI:10.1039/c1ob05135k

Two chiral colorimetric sensors (1,2) were synthesized and characterized by spectroscopic techniques and their enantioselective recognition of chiral dicarboxylic anions (d/l-aspartate and d/l-malate) was examined by UV-vis and ¹H NMR spectrosc

Full text of DOI:10.1039/c1ob05135k

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PAPER
Synthesis of alanine-based colorimetric sensors and enantioselective
recognition of aspartate and malate anions†
Wei-Chi Lin, Yu-Ping Tseng, Chi-Yung Lin and Yao-Pin Yen*
Received 25th January 2011, Accepted 10th May 2011
DOI: 10.1039/c1ob05135k
Two chiral colorimetric sensors (1,2) were synthesized and characterized by spectroscopic techniques
and their enantioselective recognition of chiral dicarboxylic anions (D/L-aspartate and D/L-malate) was
examined by UV-vis and 1H NMR spectroscopy. Interaction of the receptors 1 and 2 with the
enantiomers of aspartate or malate caused different color changes, and they act as optical
chemosensors for the recognition of D-aspartate vs. L-aspartate and D-malate vs. L-malate. Receptor 1
exhibits high enantioselective binding for aspartate anions [KA(D)/KA(L) = 12.15].
in order to detect malpractice in the food industry.8 The chiral
Introduction
discrimination of malic acid enantiomers has been achieved
through CPL,9a HPLC,9b,c electrophoresis,9d,c NMR,9f absorption
spectroscopy,9g and fluorescence.9h Although there has been a
lot of effort reported on the chiral recognition of malic acid,
the selective color change for enantiomers of malic acid is still
rare.10
The need for new synthetic chiral receptors for the molecu-
lar recognition of biologically important anions has prompted
research on new chiral receptors with selective recognition of
anions.1 The development of a chiral artificial receptor could find
applications in the development of pharmaceuticals, enantiose-
lective sensors, catalysts, enzyme models, and other molecular
devices.2 Many artificial receptors of different kinds have been
synthesized for selective anion recognition, which include chiral
macrocyclic, acyclic polyamine, chiral calixarene, and chiral
cyclodextrin derivatives.3 The main objective for the molecular
design of a chemosensor is to achieve specific molecular recogni-
tion and transfer of the recognition event into a signal. The has
been intense interest in naked eye-based chemosensors which can
detect and discriminate enantiomers.4
Commonly L-aspartic acid is found in nature, while recent
research suggests that D-aspartate is an essential intriguing
molecule found within the neuron system of animals ranging from
mollusks to vertebrates.5 High concentrations of L-aspartate have
also been observed in the early-onset dementia of the Alzheimer
type.6 Hence, considerable attention has been paid to the synthesis
of enantioselective receptors for aspartate anions. On the other
hand, malic acid usually exists in nature in only one enantiomeric
form, L-malic acid. The synthetic mixture of malic acid is used
as an acidulant in pharmaceutics, but the abnormal levels of
its concentration leads to some diseases.7 On the other hand,
D-malic acid is found in appreciable concentration only in the
metabolism of some micro-organisms. Therefore, some efforts
have been developed to distinguish between malate enantiomers
Herein, we report the preparation of new chiral chromogenic
receptors 1 and 2 and their utility in the enantioselective colori-
metric discrimination between certain enantiomers (D/L-aspartate
and D/L-malate) (Chart 1). The differentiation of the enantiomers
is very difficult because of their similar chemical and physical
properties. Hence, in this work, the anthraquinone chromogenic
unit was chosen as a scaffold to link the chiral barrier (L-alanine
group) and thiourea as the binding site (Scheme 1). Mostly,
alanine is employed as a chiral source in building the desired
chiral molecule.11 Here, the enantioselective recognition ability for
1
chiral dicarboxylate anions has been investigated by H NMR
and UV-vis spectroscopy. Different color changes were observed
when receptor 1 interacts with the enantiomers of aspartate, which
illustrates that 1 has a good enantioselective recognition ability for
aspartate anions.
Department of Applied Chemistry, Providence University, 200 Chungchi
Road, Sha-Lu, Taichung Hsien 433, Taiwan. E-mail: ypyen@pu.edu.tw;
Fax: +886-4-26328001 ext. 15218; Tel: +886-4-26327554
† Electronic supplementary information (ESI) available: Color changes
for titration of 1 with hydroxide. 2D COSY spectrum of sensors 1–2 and
NMR titration data of receptors 1–2 with D/L aspartate and D/L-malate.
See DOI: 10.1039/c1ob05135k
Chart 1
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Scheme 1 Reagents and conditions: (i) ethylenediamine, 50 ◦C, 1 h, 37%; (ii) N-Boc-L-alanine, CDI (1,1¢-carbonyldiimidazole), rt, 48 h; (iii) H2SO4,
CH2Cl2, 6 h; (iv) p-nitronaphthyl isothiocyanate, THF, reflux, 72 h; (v) p-nitrophenyl isothiocyanate, THF, reflux, 72 h.
Results and discussion
The preparation of chiral receptors 1 and 2 is depicted in
Scheme 1. A synthetic intermediate, 1,4-bis(2-aminoethylamino)-
anthracene-9,10-dione (3) was prepared from 9,10-dihydroxy-
2,3-dihydroanthracene-1,4-dione.12 Reaction of 3 with N-boc-L-
alanine in the presence of CDI (1,1¢-carbonyldiimidazole) gave
the intermediate 4, which was then treated with H2SO4 to
afford the compound 5. Subsequently, reaction of 5 with p-
nitronaphthyl isothiocyante or p-nitrophenyl isothiocyanate gave
the corresponding receptors 1 and 2, respectively, in good yields.
All of these compounds were characterized by IR, 1H NMR, 13
NMR and HRMS.
C
Fig. 1 A series of spectra taken over the course of the titration of 5 ¥ 10-5
M DMSO/H2O (4/1, v/v) in 1 with a standard solution of D-aspartate
at 25 ◦C. The titration profile (insert) indicates the formation of a 1 : 1
complex.
Anion binding studies
The colorimetric enantioselective sensing ability of the receptors
1 and 2 with D- and L-aspartate anions in the solvent system
DMSO/H2O (4/1, v/v) was monitored by UV-vis absorption and
naked eye experiments. The anions were added as tetrabutylam-
monium salts to the DMSO/H2O (4/1, v/v) solutions of the
receptors 1 and 2 (5 ¥ 10-5 M). The receptor 1 showed three
absorption bands in DMSO/H2O (4/1, v/v) at 384, 595, and
639 nm, respectively. The interaction of receptor 1 with D-aspartate
anion was investigated in detail through the UV-vis spectroscopic
titration, and the spectral behavior was observed (Fig. 1a). Upon
addition of D-aspartate to receptor 1 in DMSO/H2O (4/1, v/v),
the intensities of the absorption peaks at 384, 595, and 639 nm
gradually decreased, while a new absorption band at 520 nm
evolved (Fig. 1a).
Fig. 2 Color changes of complex 1 upon addition of various anions in
DMSO/H2O (4/1, v/v): (a) 1 only; (b) 1 + 2.0 equiv. of L-aspartate; (c)
1 + 2.0 equiv. of D-aspartate.
D-aspartate anion concentration, a sigmoidal curve was obtained,
which is shown in the inset Fig. 1b. To corroborate the 1 : 1 ratio
between 1 and D-aspartate, Job’s plot analysis was also executed.
The measured absorbance variation (DA = Aabs - Ai) reaches a
maximum when the molar fraction of ([1]/[1] + [D-aspartate]) is
0.5, confirming the 1 : 1 stoichiometry (Fig. 1c).
In the naked eye experiments, a color change of sky blue to
dark magenta was observed for receptor 1 (Fig. 2c). Two clear
isosbestic points at 429 and 559 nm indicate that there is a balance
between the complex and host–guest in solution.13 By plotting the
changes in the absorbance intensity of 1 at 520 nm as a function of
In studies of anion recognition, it is of primary importance of
discriminate whether the process is a hydrogen bond complexation
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Table 1 Ka values, correlation coefficients (R) and enantioselectivities
(KD/KL) of receptors 1 and 2 with anions
or a proton transfer of a Brønsted-type acid–base equilibrium.
Initially a genuine H-bond complex was formed and further the
formation of deprotonated L-, which can be ascribed to a “frozen”
proton release from the donor (the acid) to the acceptor (the base)
(eqn (1)) and the more advanced proton release process: 14
Receptor
Anionc
Ka/Ma
Rb
KD/KL
12.15
7.89
1
D-aspartate
L-aspartate
D-malate
(6.27 0.03) ¥ 103
(5.16 0.02) ¥ 102
(5.03 0.05) ¥ 103
(6.38 0.04) ¥ 102
(2.58 0.04) ¥ 103
(4.62 0.03) ¥102
(2.42 0.06) ¥ 103
(7.83 0.03) ¥102
0.9964
0.9922
0.9962
0.9931
0.9952
0.9927
0.9986
0.9952
LH + M2- ꢀ L- + HM-
(1)
L-malate
2
D-aspartate
L-aspartate
D-malate
5.59
The band that develops at 520 nm might be due to the
deprotonated receptor L- (1 = LH), which was confirmed by the
Brønsted acid–base reaction by adding 1.0 equivalent of strong
base [n-Bu4N]OH (cf . SI-1, see the ESI†). The spectral behavior
revealed that deprotonation of the NH fragment by D-aspartate
is responsible for the drastic color change, as a result of a charge
transfer interaction between the nitrogen atom of the thiourea
unit and the electron deficient 4-nitronaphthyl moiety. Such a
deprotonation is mainly related to the intrinsic acidity of LH15
and the stability of HM- in solution.16 The deprotonation of
receptor 1 with D-aspartate was corroborated by1H NMR titration
experiments carried out in DMSO-d6/H2O (4/1, v/v) (Fig. 3).
It was found that the proton signal of N–H1 (d = 10.21 ppm),
which is closer to the 4-nitronaphthyl group (signals of N–H
protons were assigned with respect to the 2D NOESY spectrum
of 1) (cf . SI-2a, see the ESI†), broadened significantly early
in the titration and underwent downfield shifts with increasing
D-aspartate concentration. The N–H1 peak disappeared after
3.10
L-malate
a The data were calculated from UV-vis titration in DMSO/H2O (4/1,
v/v). b The data values for R were obtained by the results of nonlinear
curve fitting. c The anions were used as their tetrabutylammonium salts.
addition of 1.0 equivalent of D-aspartate, whilst a new signal
was observed at d = 12.34 ppm. This suggest that there might
be the formation of a [HM]- (M = D-aspartate anion) species.
The monodeprotonation is also signalled by the significant upfield
shift of the proton of the naphthyl group (C–HK). Such an effect
could be derived from the through-bond propagation onto the
naphthyl framework of the electronic charge generated on N–H
deprotonation. When D-aspartate was added, the thiourea (N–H2)
signal was found to undergo downfield shift. The results implied
that the complex formation of the receptor 1 with D-aspartate
leads to the monodeprotonation of the receptor. In evidence
for this supposition, the intermolecular N ◊ ◊ ◊ H ◊ ◊ ◊ O hydrogen
bond distances were calculated at the HF/6-31G(d) level using
ab initio calculations (Fig. 4). This could be explained that four
protons of thiourea were directed toward anion ligands, but each
hydrogen-bond distance is different as shown in SI-Table 1 (see
the ESI†). Among them, the proton (H1) that is connected to
the 4-nitronaphthyl group has a much shorter distance to the
carboxylic group than a typical hydrogen-bond distance, which
17
˚
ranges between 1.86 and 2.16 A.
These two mechanisms (hydrogen bond complex or proton
transfer) could also be explained by the 1H NMR dilution
experiment.18 If the process is a deprotonation, the equilibrium
is independent of the concentration of host and guest because the
equilibrium constant is dimensionless. On the other hand, if the
binding process is a hydrogen bonding interaction, the equilibrium
is shifted to the dissociation direction. The results revealed that
the C–H proton of the naphthyl group did not undergo any
significant chemical shift to the direction of dissociation by
Fig. 3 1H NMR (400 Hz) spectra of sensor 1 (10 mM) in solution
(DMSO-d6/H2O = 4/1, v/v) upon addition of various quantities of
D-aspartate: (a) 0 eq.; (b) 0.2 eq.; (c) 1.0 eq.
Fig. 4 Optimized geometries from ab initio HF/6-31G(D) calculations.
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dilution. Therefore, it can be clearly demonstrated that this process
is due to deprotonation rather than hydrogen bond complexation
(cf . SI-3, see the ESI†).
The enantioselectivity of receptor 1 towards D-aspartate was
compared with that towards L-aspartate. When receptor 1 was
treated with L-aspartate under the same conditions, similar UV-
vis spectral behavior was observed (Fig. 5a). During the process,
the most pronounced effect is the L-aspartate anion-induced color
change from sky blue color to medium purple color (Fig. 2b). In
order to investigate the contribution of the N–H deprotonation
effects with the anion-induced effects, 1H NMR spectral analyses
were carried out in DMSO-d6/H2O (4/1, v/v). A notable feature
of these titrations is that the proton signals of thiourea underwent
downfield shifts during the complex formation of receptors 1 with
L-aspartate. After the addition of 1.0 equivalent of L-aspartate, the
signal of N–H1 disappeared but a [HM]- (M = L-aspartate anion)
peak was not observed in the range of 11–13 ppm. This result
indicates that the complex is formed through multiple hydrogen
bonds and is inconsistent with a deprotonation process between
the receptor and L-aspartate. This could be explained by the result
Fig.
6
UV-vis spectral change of
1
(5
¥
10-5 M) operated in
DMSO-d6/H2O (4/1, v/v) after the addition of 2.0 equivalents of anions:
(a) 1 only; (b) 1 + L-aspartate; (c) 1 + D-aspartate.
1
of the H NMR dilution experiment. In this, the CH proton of
the naphthyl group underwent downfield shift in the direction
of dissociation by dilution (cf . SI-4, see the ESI†). Besides that,
the ab initio calculations indicate that the process is via hydrogen
bond complexation rather than proton transfer (cf . SI-Table 1, see
the ESI†). From the UV-vis titrations, the binding of L-aspartate
allowed the Job’s plot method (Fig. 5) for the determination
of binding stoichiometry, which was found to be a 1 : 1 host–
guest complexation. The association constant was calculated and
shown in Table 1. Comparison of the UV-vis absorption spectra of
complex 1 upon addition of either D- or L-aspartate anion is shown
in Fig. 6. Apparently, receptor 1 has higher enantioselectivity and
sensitivity recognition for D-aspartate over L-aspartate anions in
DMSO-d6/H2O (4/1, v/v).
Fig. 7 1H NMR spectra change of 1 operated in the solution (DM-
SO-d6/H2O = 4/1) after the addition of 2.0 equivalents of anions: (a) 1
only; (b) 1+ L-aspartate; (c) 1+ D-aspartate.
9.05 ppm. The difference observed in the change of chemical
shift for the enantiomers of aspartate demonstrated that 1 has
a stronger interaction with D-aspartate than with L-aspartate. In
addition, the association constant (Ka) of 1 with D-aspartate is
(6.27 0.03) ¥ 103 (mol L-1)-1, while that of 1 with L-aspartate is
(5.16 0.02) ¥ 102 (mol L-1)-1, and the enantioselectivity [KD/KL]
is 12.15 (Table 1). Since receptor 1 has a unique color change and
higher enantioselectivity for D-aspartate than L-aspartate, it can
act as an optical chemosensor for recognition of D-aspartate vs.
L-aspartate. The change in color and different enantioselectivity
with D- and L-aspartate could be related to the stereochemistry
of the receptor and the structure complementarity with guest.
The possible structure of 1 was calculated by quantum chemical
calculation at the DFT level of theory and the minimum energy
structure was shown in Fig. 4. The receptor 1 might have a
conformation with two functional arms on the 1,4-anthraquinone
unit with a minimum distance between the two naphthyl groups of
Fig. 5 A series of spectra taken over the course of the titration of 5 ¥
10-5 M DMSO/H2O (4/1, v/v) solution of 1 with a standard solution of
L-aspartate at 25 ◦C. The titration profile (insert) indicates the formation
of a 1 : 1 complex.
˚
˚
The interaction of receptor 1 with D- or L-aspartate was also
corroborated by 1H NMR titration experiment. Fig. 7 shows
the spectra of 1 and its complexes with equal amounts of D-
or L-aspartate in DMSO-d6/H2O (4/1, v/v). In the presence of
L-aspartate, the characteristic peaks of N–H1 of 1 disappeared,
while N–H2 had a downfield shift from 8.43 to 8.81 ppm; when
treated with D-aspartate, the N–H2 peak shifted from 8.43 to
13.831 A, while the distance between the two sulfurs is 13.270 A.
Based on this model, a suitable conformation was assumed when
it interacts with 1. The D-aspartate anion could fit more perfectly
into the complex inducing a conformation change in the receptor.
The NH of the thiourea group on receptor 1 might have a strong
base and acid interaction with the oxygen of the CO2- group in D-
aspartate (pKa value of the aspartic acid is 3.86 in H2O).19 Hence,
2
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the enantioselective recognition depends on the steric effect and
the structure complementarity of the receptor with the guest.
The high enantioselectivity of receptor 1 for D-aspartate com-
pared to that for the L-aspartate prompted us to investigate
the enantiomers of malate, D- and L-malate anions. A similar
phenomenon in the UV-vis absorption spectra was observed
in Fig. 8. The absorption spectrum (a) was measured in the
absence of anion. As shown in spectra (b) and (c), the CT
absorption bands appeared at 520 nm and the solution color
changes from sky blue color to purple for L-malate and from
sky blue color to pansy for D-malate, respectively (Fig. 9). It is
apparent that receptor 1 showed a selective color change for D-
malate vs. L-malate. The enantioselectivity of 1 for the recognition
of these anions could also be rationalized on the basis of the
stereochemistry of the receptor and the structure complementarity
with guest. The dramatic color changes could also be attributed
to the occurrence of the deprotonation of the N–H proton of
the thiourea moiety with anion (pKa2 value of malic acid is
From the UV-vis titrations, the Job’s plot method showed the
formation of a 1 : 1 stoichiometry complex of 1 with either D- or L-
malate (cf . SI-7 and SI-8, see the ESI†). The association constants
of 1 with D- or L-malate were calculated as (5.03 0.05) ¥ 103 (mol
L-1)-1 and (6.38 0.04) ¥ 102 (mol L-1)-1, respectively. The D/L
[KD/KL] selectivity was found to be 7.89 for malate (Table 1). To
elucidate the interaction between D- or L-malate with receptor 1, ab
initio calculations of the [1·D-malate] complex and the [1·L-malate]
complex were calculated. These calculations clearly showed that
only the proton (H1) of the D-malate has a much shorter distance
to the dicarboxylic group than a typical H-bond distance (cf . SI-
Table 1, see the ESI†).
The influence of the different substituents of the thiourea group
on the anion binding and sensing property was studied. Receptor
2 was examined by replacing the 4-nitronaphthyl group with
the weaker electron-withdrawing 4-nitrophenyl group. With the
progressive addition of either D- or L-aspartate to receptor 2, the
intensity of the absorption peak at 354 nm gradually decreased
and a new band at 471 nm concomitantly evolved. Two clear
isosbestic points at 390 and 544 nm were observed (Fig. 10a, and
cf . SI-9, see the ESI†). These changes were accompanied by the
color changes from blue to green for D-aspartate and from blue to
cornflower blue for L-aspartate (Fig. 11). The color changes caused
by the introduction of either D- or L-aspartate may be ascribed
to the formation of the hydrogen bond complexes between the
receptor 2 and the aspartate anions. From the 1H NMR titration
experiment, it was found that when receptor 2 formed a complex
with D-aspartate, both the proton signals of N–H1 (10.35 ppm)
and N–H2 (8.18 ppm) underwent downfield shifts to 11.25 and
9.10 ppm, respectively, after increasing the concentration of D-
aspartate (cf . SI-10, see the ESI†). On the other hand, when
receptor 2 forms a complex with L-aspartate, both the proton
1
5.20 in H2O).19 This observation was similarly confirmed by H
NMR titration experiments with D-malate in which the peak of
[HM]- (M = D-malate anion) appeared at 12.34 ppm (cf. SI-5,
see the ESI†). But when L-malate was treated against receptor
1, the peak of N–H1 disappeared but no [HM]- (M = L-malate
anion) peak was observed in the range of 11–13 ppm. This result
implied that receptor 1 might only bind with L-malate through
multiple hydrogen bonds. The binding process of receptor 1 with
L-malate might be explained when a 1H NMR dilution experiment
of 1 with L-malate was performed. The result has demonstrated
that the process is via hydrogen bond interactions rather than a
deprotonation (cf . SI-6, see the ESI†).
Fig. 8 UV-vis spectral change of 1 (5 ¥ 10-5 M) operated in DMSO/H2O
(4/1, v/v) after addition of 2.0 equivalents of anions: (a) 1 only; (b) 1 +
L-malate; (c) 1 + D-malate.
Fig. 10 A series of spectra taken over the course of the titration of a 5 ¥
10-5 M DMSO/H2O (4/1, v/v) solution of 2 with a standard solution of
D-aspartate at 25 ◦C. The titration profile (insert) indicates the formation
of a 1 : 1 complex.
Fig. 9 Color changes of complex 1 upon addition of various anions in
DMSO/H2O (4/1, v/v): (a) 1 only; (b) 1 + 2.0 equiv. of L-malate; (c) 1 +
2.0 equiv. of D-malate.
Fig. 11 Color changes of complex 2 upon addition of various anions in
DMSO/H2O (4/1, v/v): (a) 2 only; (b) 2 + 2.0 equiv. of L-aspartate; (c)
2 + 2.0 equiv. of D-aspartate.
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signals of N–H1 (10.35 ppm) and N–H2 (8.18 ppm) similarly
underwent downfield shifts to 10.91 and 8.80 ppm, respectively,
with increasing concentration of L-aspartate (cf. SI-11, see the
ESI†). The relatively small downfield shifts indicate that the
complex is formed through weak hydrogen bonding interactions
and is inconsistent with a deprotonation process between D- or
L-aspartate and the receptor 2. To further support the supposition
of the hydrogen bonding interactions, the dilution experiments
(cf . SI-12 and SI-13, see the ESI†) and the intermolecular
N ◊ ◊ ◊ H ◊ ◊ ◊ O hydrogen bonded distances were calculated by ab
initio calculation (cf . SI-Table 1, see the ESI†). The association
constant of 2 with D- or L-aspartate is calculated as shown in
Table 1. The data in Table 1 illustrated that the association
constants of 1 are much higher than 2 with aspartate anions. The
results demonstrated that the introduction of the p-nitronaphthyl
group enhances the acidity of the thiourea NH, which provides
an effective intramolecular charge transfer and enhances the
hydrogen bonding ability, resulting in strong anion binding.20 All
the results indicate that the receptor 2 is interacting with D- or
L-aspartate anions through hydrogen bonding interactions. The
Conclusion
In conclusion, two new chiral colorimetric receptors were synthe-
sized. Both receptors show good sensitivity and enantioselectivity
for the discrimination of D/L-aspartate and D/L-malate by dra-
matic color changes in DMSO/H2O (4/1, v/v). Thus, both 1 and
2 can be used as optical chemosensors for recognition of D- vs. L-
aspartate or D- vs. L-malate anions. To the best of our knowledge,
receptor 1 is the first synthetic host that can selectively recognize
either D/L-aspartate or D/L-malate by color changes. Future work
will continue to develop practical colorimetric sensors for the
recognition of other chiral enantiomers.
Experimental
General
The chemical reagents were purchased from Acros or Aldrich
Corporation and utilized as received. All solvents were purified by
standard procedures. Melting points were measured on a Yanaco
MP-S3 melting-point apparatus. The infrared spectra were per-
formed on a Perkin Elmer System 2000 FT-IR spectrophotometer.
UV-Vis spectra were measured on a Cary 300 spectrometer. All
NMR spectra were measured on a Bruker spectrometer at 400 (1H)
and 100 MHz (13C) and Varian Unity Inova-600 spectrometers at
600 (1H) and 150 MHz (13C) with DMSO-d6 as solvent. High-
resolution mass spectra were measured with a Finnigan/Thermo
Quest MAT 95XL instrument.
1
comparison of the H NMR and absorption spectra of complex
2 upon addition of either D- or L-aspartate is shown in the ESI
(Fig. SI-14).† Receptor 2 has a unique color change and higher
enantioselectivity recognition for D-aspartate than L-aspartate,
and can act as an optical chemosensor for recognition of D- vs.
L-aspartate.
In order to investigate whether the structurally similar in chiral
dicarboxylates could be differentiated by color change, the binding
of receptor 2 with D- and L-malate was studied. Consistent with
the above results of the titration of D- and L-aspartate, the titration
of 2 with D- and L-malate gave similar phenomena in their UV-
vis absorption spectra (Fig. 12) as well as the same color changes
(cf . SI-15, see the ESI†). It appeared that receptor 2 has the same
propensity; formation of hydrogen bonding interactions occurred
upon addition of increasing concentrations of either D- or L-
malate. This supposition is similarly proved by the 1H NMR
titration experiments (cf . SI-16–18, see the ESI†) and dilution
experiments (cf . SI-19 and SI-20, see the ESI†). The hydrogen-
bond distances were also reflected in the ab initio calculations (cf .
SI-Table 1, see the ESI†). The higher enantioselectivity and the
color change of receptor 2 toward D- or L-malate mean that 2 can
also be used as an optical chemosensor for recognition of D- or
L-malate anions.
1,4-Bis(2-aminoethylamino)anthracene-9,10-dione (3). To
a
suspension of of 9,10-dihydroxy-2,3-dihydroanthracene-1,4-dione
(10.5 g, 42 mmol) in MeOH (100 mL), ethylenediamine (29 mL,
0.42 mol) was slowly added at 5 ◦C. The mixture was stirred under
N2 at 50 ◦C for 1 h, and then left stirring at room temperature
overnight. The solution was evaporated under vacuum and the
residue was subsequently washed with acetonitrile and ether to
give 3 (5.2 g, 37% yield) as a dark blue solid. The spectral data of
3 were consistent with the literature data.12
Di-tert-butyl (2S,2¢S)-1,1¢-(2,2¢-(9,10-dioxo-9,10-dihydroanth-
racene-1,4-diyl) bis(azanediyl)bis(ethane-2,1-diyl)) bis(azanediyl)
bis(1-oxopropane-2,1-diyl)dicarbamate (4). To an ice-cooled so-
lution of N-Boc-L-alanine (0.80 g, 4.2 mmol) in dry CH2Cl2
(25 mL), 1,1¢-carbonyldiimidazole (CDI) (0.82 g 5.0 mmol) was
slowly added. The resulting mixture was stirred for 2 h, then
a solution of 3 (0.65 g, 2.0 mmol) in CH2Cl2 (200 mL) was
slowly added at 0 ◦C, and then it was left stirring at room
temperature for 48 h. The solution was poured into 50 mL of 10%
HCl and extracted with CH2Cl2. The organic layer was washed
with NaHCO3, water, and dried over anhydrous MgSO4. After
filtration, the solvent was evaporated under reduced pressure. The
residue was purified on a flash column of silica gel (CH2Cl2/ethyl
acetate/MeOH = 3.5/0.5/0.1) to give the pure product 4 (0.22 g,
42% yield) as a blue solid. mp: 206–207 ◦C. [a]D25 = -12 [c = 0.008,
DMSO], HRMS(FAB) calcd for C34H46O8N6 [M+] 666.3377; found
666.3371.
4-Nitronaphthalenyl isothiocyanate (6). 4-Nitro-1-naphthyl-
amine (0.56 g, 3.0 mmol), di-2-pyridylthionocarbonate (DPT)
(1.10 g, 4.76 mmol) and 4-(dimethylamino)pyridine (DMAP)
(0.10 g, 0.82 mmol) in CHCl3/CH3CN (3/1, v/v, 20 mL) were
Fig. 12 UV-vis spectral change of 2 (5 ¥ 10-5 M) operated in DMSO/H2O
(4/1, v/v) after addition of 2.0 equivalents of anions: (a) 2 only; (b) 2 +
L-malate; (c) 2 + D-malate.
5552 | Org. Biomol. Chem., 2011, 9, 5547–5553
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combined and stirred at room temperature for 4 h. The resulting
mixture was evaporated and the residue was purified on a flash
column of silica gel (ethyl acetate/hexane =◦1/5) to give 6 (0.47 g,
68% yield) as a yellow solid. mp: 93.0–94.0 C. 1H NMR (CDCl3,
400 MHz) d: 7.44–7.46 (d, 1H, J = 8.0 Hz), 7.70–7.80 (m, 2H),
8.18–8.20 (d, 2H, J = 8.4 Hz), 8.58–8.60 (d, 1H, J = 8.8 Hz).
13C NMR (CDCl3, 100 MHz) d: 144.4, 139.7, 133.6, 130.4, 129.5,
128.5, 125.9, 124.2, 123.7, 123.4, 121.8. HRMS(FAB) calcd for
C11H6N2S [M+] 230.0150; found 230.0146.
Acknowledgements
We thank the National Science Council of the Republic of China
(Taiwan) for financial support and the National Center for High-
Performance Computation.
References
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(2S,2¢S)-N,N¢-[2,2¢-(9,10-Dioxo-9,10-dihydroanthracene-1,4-
diyl)bis(azanediyl)
bis(ethane-2,1-diyl)]bis{2-[3-(4-nitronaph-
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4.0 mmol) in CH2Cl2 (35 mL) was added into a chilled, well
stirred mixture consisting of 96% H2SO4 (12 mmol) and CH2Cl2
(5.0 mL). The resulting mixture was stirred for 6 h at room
temperature. Then the mixture was extracted with H2O (2 ¥
100 mL). The aqueous phase was made alkaline by addition of
4 M NaOH (9.0 mL, 36.0 mmol), extracted with CH2Cl2 (2 ¥
100 mL), and dried with solid MgSO4 (5.0 g). The combined
organic phase was concentrated to give compound 5 in good yield.
Without further purification, the blue solid and triethylamine
(1.0 mL) were dissolved in dry CH2Cl2 (250 mL), and a solution
of 4-nitronaphthyl isothiocyanate (1.84 g, 8.0 mmol) was added
slowly. The mixture was stirred for 72 h at room temperature.
The resulting precipitate was filtered and washed with CH2Cl2 to
afford th◦e pure product 1 (2.89 g, 78% yield), as a blue solid. mp:
195–196 C. [a]2D5 = +45 [c = 0.002, DMSO], 1H NMR (DMSO-d6,
600 MHz) d: 10.83 (t, J = 6.3 Hz, 2H), 10.20 (s, 2H), 8.48 (d, J =
4.2 Hz, 2H), 8.47 (d, J = 4.2, 2H), 8.45 (s, 2H), 8.32 (d, J = 8.4
Hz, 2H), 8.22 (d, J = 3.6 Hz, 1H), 8.21 (d, J = 3.0 Hz, 1H), 8.18
(d, J = 8.4 Hz, 2H), 8.10 (d, J = 8.4 Hz, 2H), 7.80 (t, J = 7.8 Hz,
2H), 7.75 (d, J = 3.6 Hz, 1H), 7.74 (d, J = 3.0 Hz, 1H), 7.72 (t,
J = 7.8 Hz, 2H), 7.6 (s, 2H), 4.91 (t, J = 6.9 Hz, 2H), 3.64–3.55
(m, 4H), 3.46–3.35 (m, 4H), 1.37 (d, J = 6.6 Hz, 6H). 13C NMR
(DMSO-d6, 100 MHz): 181.3, 181.1, 172.8, 146.4, 142.8, 141.7,
134.3, 132.8, 130.1, 128.9, 127.8, 126.1, 125.7, 125.0, 124.9, 123.7,
123.3, 121.7, 109.3, 53.6, 41.7, 31.8, 19.5 ppm. HRMS(FAB)
calcd for C46H43O8N10S2; [M+ + H] 927.2707; found 927.2706.
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◦
1
blue solid. mp: 191–193 C. [a]2D5 = +37 [c = 0.003, DMSO], H
NMR (DMSO-d6, 400 MHz) d: 10.80 (s, 2H), 10.41 (s, 2H), 8.48
(s, 2H), 8.31 (s, 2H), 8.19 (s, 2H), 8.15 (d, J = 8.8 Hz, 4H), 7.91
(d, J = 8.4 Hz, 4H), 7.73 (s, 2H), 7.55 (s, 2H), 4.83 (s, 2H), 3.57
(d, J = 4.0 Hz, 4H), 3.73 (s, 4H), 1.33 (d, J = 6.4 Hz, 6H). 13C
NMR (DMSO-d6, 100 MHz): 181.3, 176.3, 172.6, 146.7, 146.4,
142.3, 134.3, 132.8, 126.1, 124.9, 124.8, 120.7, 109.3, 53.1, 41.6,
39.2, 19.3 ppm. HRMS (FAB) calculated for C38H38O8N10S2 [M+]
826.2315; found 826.2321.
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The Royal Society of Chemistry 2011
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