Synthesis and chiral recognition of novel chiral fluorescence receptors bearing 9-anthryl moieties

Article abstract of DOI:10.1016/j.tetasy.2004.12.023

Two chiral fluorescence receptors 1 and 2 have been synthesized, and their structures characterized by IR, ¹H NMR, MS spectra and elemental analysis. The chiral recognition of the receptors was studied by ¹H NMR and fluorescence spectra. The results demonstrate that the receptors and tetrabutylammonium mandelate formed a 1:1 complex. Two receptors exhibit good chiral recognition abilities towards the enantiomers of tetrabutylammonium mandelate.

Full text of DOI:10.1016/j.tetasy.2004.12.023

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 833–839
Synthesis and chiral recognition of novel chiral
fluorescence receptors bearing 9-anthryl moieties
Kuo-Xi Xu, Xiao-Jun Wu, Yong-Bing He,* Shun-Ying Liu,
Guang-Yan Qing and Ling-Zhi Meng
Department of Chemistry, Wuhan University, Wuhan 430072, China
Received 20 October 2004; accepted 9 December 2004
Abstract—Two chiral fluorescence receptors 1 and 2 have been synthesized, and their structures characterized by IR, 1H NMR, MS
spectra and elemental analysis. The chiral recognition of the receptors was studied by 1H NMR and fluorescence spectra. The results
demonstrate that the receptors and tetrabutylammonium mandelate formed a 1:1 complex. Two receptors exhibit good chiral
recognition abilities towards the enantiomers of tetrabutylammonium mandelate.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
fluorescence spectroscopy. The synthetic route to the
receptors is shown in Scheme 1.
Molecular recognition of guests by synthetic hosts have
attracted considerable attention in organic, biological
and medicinal chemistry. The development of chiral
artificial receptors, which have the properties of chiral
recognition and chiral catalysis is especially important,1
because these are fundamental characteristics of bio-
chemical systems and could contribute to the develop-
ment of pharmaceuticals, enantioselective sensors and
enzyme models.
2. Results and discussion
2.1. Synthesis
Chiral fluorescence receptors 1 and 2 were efficiently
synthesized by the reaction of intermediates 4 or 6 and
9-anthraldehyde, and subsequent reduction by NaBH4
(Scheme 1). To avoid a cyclic product, intermediates 4
and 6 were prepared with good yields by the reaction
In the synthesis of chiral receptors, amino acids or pep-
tides can be employed as chiral sources in building the
desired molecules because of their accessibility2 and bio-
logical relevance. A great number of artificial chiral
receptors have been synthesized and studied, which con-
tain the chiral macrocyclic and acyclic polyamines,3
chiral calixarenes,4 and chiral cyclodextrin derivatives.5
However, few examples have been reported dealing with
the synthesis and chiral recognition studies of chiral
fluorescence receptors.4a Herein, we report the synthesis
of two new chiral fluorescence receptors 1 and 2 contain-
ing both an amino acid unit and 9-anthryl groups, and
their enantioselective recognition ability for D- and
L-tetrabutylammonium mandelate by 1H NMR and
1
of 3 or 5 and excess ethylenediamine. The H NMR
spectra exhibited all the expected signals with the desired
integral values and support the molecular structures.
The structures of these compounds were characterized
1
by IR, MS, H NMR spectra and elemental analysis.
2.2. Fluorescence spectra
The fluorescence spectra were recorded from a solution
of receptors 1 or 2 in DMSO in the absence and pre-
sence of D- or L-mandelate. Figures 1 and 2 show the
fluorescence spectra of
a mixture of receptor 1
(1.4 · 10ꢀ5 mol Lꢀ1) with different concentrations of
D- or L-mandelate anion in DMSO. By gradually increas-
ing the concentration of the D- or L-mandelate, the fluo-
rescence emission intensities of receptor 1 at 418 and
441 nm (kex = 370 nm) gradually increased, which indi-
cates complexation happened between receptor 1 and
*
Corresponding author. Fax: +86 27 87647617; e-mail: ybhe@
whu.edu.cn
0957-4166/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.12.023

834
K.-X. Xu et al. / Tetrahedron: Asymmetry 16 (2005) 833–839
H
H
COOCH3
COOH
NH2
1. SOCl2/ CH3OH
H2NCH2CH2NH2
CH3OH
Ph
Ph
2. ClCH2COOCH3
CH3CN / K2CO3
HN CH2COOCH3
3
HN
O
O
C
H
HN
Ph
C
NH
C
NH2
H
NH
HN
Ph
1.A / CH3OH
2. NaBH4
HN
NH2
C
NH
HN
O
O
4
1
H
H
1. SOCl2 / CH3OH
H2NCH2CH2NH2
CH3OH
COOCH3
COOH
2. ClCH2COOCH3
CH3CN / K2CO3
N
NH
CH2COOCH3
5
HN
O
O
H
H
C
NH
NH2
NH2
C
NH
NH
1. A/CH3OH
N
N
2. NaBH4
C
C
O
HN
O
NH
6
2
Scheme 1. The synthesis of receptors.
300
200
100
0
200
180
240
220
200
180
160
140
120
100
170
160
150
140
130
120
110
100
100
0
2
4
6
8
10 12
0
2
4
6
8
Equiv. of anions
Equiv. of anions
0
400
500
600
700
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Figure 2. Fluorescence spectra of receptor 1 (1.4 · 10ꢀ5 mol Lꢀ1) with
L-mandelate anion in DMSO. The equivalents of anion are: 0.0, 1.0,
2.0, 3.0, 3.5, 4.0, 4.5, 5.5, 6.5, 7.0, 7.5, 8.5 and 11.0. kex = 370 nm. Inset:
changes of fluorescence intensity of 1 at 418 nm upon addition of
L-mandelate. The line is curve fitting. The correlation coefficient (R) of
non-linear curve fitting is 0.9929.
Figure 1. Fluorescence spectra of receptor 1 (1.4 · 10ꢀ5 mol Lꢀ1) with
D-mandelate anion in DMSO. The equivalents of anion are: 0.0, 0.5,
1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.5, 5.0, 6.0 and 8.0. kex = 370 nm. Inset:
changes of fluorescence intensity of 1 at 418 nm upon addition of
D-mandelate anion. The line is curve fitting. The correlation coefficient
(R) of non-linear curve fitting is 0.9936.
D- or L-mandelate. The phenomenon of fluorescence
intensity increasing upon addition of a guest anion is
similar to the anion-induced fluorescence enhancement
already reported.6 In the absence of an anion, the

K.-X. Xu et al. / Tetrahedron: Asymmetry 16 (2005) 833–839
835
photoinduced electron-transfer (PET) process between
100
80
60
40
20
0
64
62
60
58
56
54
52
50
48
the anthracene group and the weak electron-withdraw-
ing amide substituents might result in decreased fluores-
cence intensity. Upon the addition of anions, the
interaction of an anion with a receptor unit could dimin-
ish the PET progress to induce fluorescence retrieval.
Therefore anion-induced fluorescence enhancement
was observed.7 The satisfactory result (the correlation
coefficient is over 0.99) of non-linear curve fitting (fluo-
rescence intensity at 418 nm vs equivalents of mandelate
anion) confirmed that receptor 1 and D- or L-mandelate
anion formed a 1:1 complex (see the top right plot of
Figs. 1 and 2).8 For a complex of 1:1 stoichiometry,
an association constant Kass can be calculated by using
the following equation:8,9
0
2
4
6
8
10 12 14
Equiv. of anions
400
500
600
700
Wavelength (nm)
Figure 3. Fluorescence spectra of receptor 2 (1.8 · 10ꢀ5 mol Lꢀ1) with
D-mandelate in DMSO. The equivalents of anion are: 0, 0.32, 0.4, 0.56,
0.88, 1.28, 1.68, 2.48, 4.08, 8.08 and 12.08. kex = 371 nm. Inset: changes
of fluorescence intensity of 2 at 441 nm upon addition of D-mandelate
anion. The line is curve fitting. The correlation coefficient (R) of non-
linear curve fitting is 0.9938.
X ¼ X0 þ ðXlim ꢀ X0Þ=2C0fCH þ CG þ 1=Kass
2
1=2
ꢀ ½ðCH þ CG þ 1=KassÞ ꢀ 4CHCGꢁ g;
where X represents the fluorescence intensity, and CH
and CG are the corresponding concentrations of host
and anion guest. The association constants (Kass) and
correlation coefficients (R) obtained by a non-linear
least-squares analysis of X versus CH and CG are listed
in Table 1.
140
94
92
120
90
Figures 3 and 4 show the changes in the fluorescence
88
spectra of receptor
2
at
a
concentration of
100
80
60
40
20
0
86
84
82
80
78
1.8 · 10ꢀ5 mol Lꢀ1 or 3.6 · 10ꢀ5 mol Lꢀ1 in DMSO
upon addition of D- or L-mandelate. When D- or L-man-
delate anions were introduced to the solution of 2, the
fluorescence emission intensities of receptor 2 at 417
and 441 nm (kex = 371 nm) also increased. The result
of a non-linear curve fitting (at 441 nm) indicates that
a 1:1 complex was formed between receptor 2 with D-
or L-mandelate anion (see the top right plot of Figs. 3
and 4).8 The association constants (Kass) and correlation
coefficients (R) obtained by a non-linear least-squares
analysis of X versus CH and CG are also listed in
Table 1.
0
5
10
15
20
Equiv. of anions
400
450
500
550
600
Wavelength (nm)
Figure 4. Fluorescence spectra of receptor 2 (3.6 · 10ꢀ5 mol Lꢀ1) with
L-mandelate in DMSO. The equivalents of anion are: 0, 0.16, 0.46,
0.87, 1.2, 1.6, 2.5, 3.2, 4.8, 8.8 and 18.8. kex = 370 nm. Inset: changes of
fluorescence intensity of 2 at 441 nm upon addition of L-mandelate
anion. The line is curve fitting. The correlation coefficient (R) of
non-linear curve fitting is 0.9963.
The data in Table 1 illustrate that the two receptors
can bind D- or L-mandelate anions in the same order:
D-mandelate anion, L-mandelate anion and receptor 2
have more selectivity for recognition of the D-mandelate
anion. This may be due to the relatively greater rigidity
of the structure of receptor 2, which results in a more
selective recognition.
L-tetrabutylammonium mandelate because it can
directly provide structural and dynamic information.10
Studies on the chiral recognition were carried out
on a 300 MHz NMR spectrometer using compounds 1
and 2 as chiral solvating agents.
1
2.3. H NMR study
1H NMR experiments were undertaken to assess the chi-
ral recognition properties between receptor and D- or
Table 1. Association constants (Kass) and correlation coefficients (R) of 1 and 2 with D- or L-mandelate in DMSO
Aniona
Receptor 1
Receptor 2
Kass (Mꢀ1
)
R
Kass (Mꢀ1
)
R
D-Mandelate
L-Mandelate
(8.35 0.36) · 104
0.9936
0.9929
(3.64 0.12) · 105
(4.06 0.06) · 104
0.9938
0.9963
(2.08 0.06) · 104
a Anions were used as their tetrabutylammonium salts.

836
K.-X. Xu et al. / Tetrahedron: Asymmetry 16 (2005) 833–839
1
Tetrabutylammonium mandelate was chosen as the
probe. Figure 5A shows the 1H NMR spectrum of race-
mic mandelate in CDCl3; only one singlet (d 5.20 ppm)
for the CH proton resonance of racemic mandelate
The H NMR spectra of the receptor 2 and its complex
with equimolar amounts of D-, L- or racemic mandelate
are shown in Figure 6. Addition of receptor 2 to racemic
mandelate in CDCl3 caused remarkable upfield shifts (d
4.68 and 4.77 ppm) of the CH proton of mandelate (Fig.
6B). The methine signal has clearly split into two peaks
(DDd 27 Hz). The interaction of receptor 2 with the
D-enantiomer shows that the CH proton has a larger
upfield shift (Dd 0.52 ppm, Fig. 6C) than the CH proton
of the L-enantiomer (Dd 0.43 ppm, Fig. 6D). It is similar
to receptor 1 in the recognition ability for the enantio-
mers, while receptors 1 and 2 have a strong interaction
to D-mandelate. Receptor 2 revealed the highly enantio-
selective recognition for the enantiomers of the
mandelate.
1
was observed in the absence of the host. The H NMR
spectra of receptor 1 (2 · 10ꢀ3 M) and its complex with
equimolar amounts (2 · 10ꢀ3 M) of D-, L- or racemic
mandelate are shown in Figure 5. Two singlet reso-
nances (d 4.83 and 4.88 ppm) due to the CH proton of
racemic mandelate were observed in the presence of
receptor 1 (Fig. 5C), with their intensity ratio being
about 1:1 and with the separation between the two peaks
being 15 Hz. This indicates that the interactions of
receptor 1 with the D- and L-forms of mandelate are
different, resulting in two singlet resonances for the
racemic CH proton. The CH proton singlets of D-
and L-mandelate were shifted upfield by about 0.37
and 0.32 ppm in the presence of receptor 1 (Fig. 5D
and E), respectively. The larger upfield shift of the CH
proton towards the D-enantiomer reveals that the recep-
tor 1 has a stronger chiral recognition ability than with
the L-enantiomer.
1
The H NMR spectra of receptors 1 and 2 show dra-
matic changes in the presence of a guest. Upon the addi-
tion of an equimolar amount of D-mandelate to a
solution of receptor 1, the peaks of the anthracene frag-
ments are shifted downfield to 8.91 ppm and broadened;
one characteristic peak of amide (NH) is shifted upfield
Figure 5. 1H NMR spectra of
1
and its guest complex at 25 ꢁC ([1] = [guest] = 2.0 · 10ꢀ3 M) in CDCl3 at 300 MHz. (A) Racemic
tetrabutylammonium mandelate; (B) receptor 1; (C) receptor 1 + racemic tetrabutylammonium mandelate; (D) receptor 1 + D-tetrabutylammonium
mandelate; (E) receptor 1 + L-tetrabutylammonium mandelate.

K.-X. Xu et al. / Tetrahedron: Asymmetry 16 (2005) 833–839
837
Figure 6. 1H NMR spectra of 2 and its guest complex ([2] = [guest] = 2.0 · 10ꢀ3 M) at 25 ꢁC in CDCl3 at 300 MHz. (A) Receptor 2; (B) receptor
2 + racemic tetrabutylammonium mandelate; (C) receptor 2 + D-tetrabutylammonium mandelate; (D) receptor 2 + L-tetrabutylammonium
mandelate.
1
from 7.05 to 6.86 ppm (Dd 0.19 ppm), while the other
amide (NH) peak disappeared. Upon addition of an
equimolar amount of L-mandelate to a solution of 1,
the peaks of the anthracene fragments also shifted
downfield and broadened; one amide (NH) peak shifted
upfield from 7.05 to 6.82 ppm (Dd 0.23 ppm), while the
other amide (NH) peak disappeared.
of the receptors was studied by H NMR and fluores-
cence spectra. Receptors 1 and 2 exhibit different chiral
recognition abilities towards the enantiomers of D- and
L-tetrabutylammonium mandelate, and formed a 1:1
complex between the host and guest; receptor 2 has a bet-
ter chiral recognition ability than receptor 1. The recep-
tor steric effect, structural rigidity, hydrogen bond and
p–p stacking between the aromatic groups may be respon-
sible for the enantiomeric recognition of mandelates.
1
The H NMR spectra in the interactions of receptor 2
and D- or L-mandelates show that the peaks of the
anthracene fragments are shifted downfield and broad-
ened, but one amide (NH) proton signal at 6.77 ppm dis-
appeared, while the other amide (NH) peak at 7.49 ppm
was shifted downfield to 8.28 ppm for the D-form and
8.20 ppm for L-form.
4. Experimental
4.1. Materials and methods
Acetonitrile was dried and distilled from CaH2; ethyl-
enediamine and thionyl chloride were distilled before
use. All other commercially available regents were used
without further purification. The anions were used as
their tetrabutylammonium salts. Melting points were
measured on a Reichert 7905 melting point apparatus
(uncorrected). The IR spectra were performed on a
Nicolet 670 FT-IR spectrophotometer. Mass spectra
were recorded on a ZAB-HF-3F mass spectrometer.
Elemental analyses were determined by Perkin–Elmer
204B elemental autoanalyzer. 1H NMR spectra were
recorded on a Varian Mercury VX-300 MHz spectro-
meter. Fluorescence spectra were obtained on a Shi-
madzu RF-5301 spectrometer. 9-Anthraldehyde was
prepared according to the literature method.3c,13
The above results illustrate that the enantioselective
recognition of the receptors for D- or L-mandelates is
through multiple hydrogen bonding interactions11 and
the p–p interaction between anthryl-ring of the host
and phenyl-ring of the guest.12 Receptors 1 and 2 exhibit
good enantioselectivity for D-mandelate, and receptor 2
has better selective recognition because the structure of
receptor 2 has a relatively large rigidity than receptor 1.
3. Conclusion
In summary, two chiral fluorescence receptors 1 and 2
have been synthesized. The enantioselective recognition

838
K.-X. Xu et al. / Tetrahedron: Asymmetry 16 (2005) 833–839
4.2. Syntheses
FAB-MS m/z (%): 638 (M++1, 10). Elemental analysis
calcd (%) for C41H43N5O2: C, 77.19; H, 6.81; N, 10.98;
found: C, 77.08; H, 6.95; N, 10.87.
Intermediates 4 and 6: a solution of 3 or 5 (1 mmol) in
methanol (30 mL) was added dropwise to the stirred
solution of ethylenediamine (0.6 g, 10 mmol) in metha-
nol (10 mL). The mixture was stirred for 48 h under
N2 protection at room temperature. The solvent and ex-
cess amine were removed under reduced pressure and
the residue dried in vacuo to give product 4 or 6 as a
mildly hygroscopic solid.
Acknowledgements
We thank the National Natural Science Foundation for
financial support (Grant no. 20372054).
Compound 4: yield 95%; IR (cmꢀ1): 3448, 3287, 1674,
1542, 1441, 687; 1H NMR (CDCl3): d 7.39 (br, 1H,
CONH), 7.25–7.19 (m, 5H, ArH), 6.93 (br, 1H, CONH),
3.41 (dd, J = 4.2, 9.0 Hz, 1H, C*HCH2), 3.26–3.13 (m,
5H, NHCH2CO, PhCH2), 3.07–2.89 (m, 4H,
2CONHCH2), 2.74–2.61 (m, 8H, 2CH2CH2NH2).
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Compound 1: yield: 79%; mp: 222–224 ꢁC; ½aꢁ ¼ ꢀ8:6
D
(c 0.05, CHCl3); IR (KBr/cmꢀ1): 3292, 3050,
1
2926,1661, 1599, 1499, 734; H NMR (CDCl3): d 8.26
(s, 2H, anthryl), 8.15 (d, J = 8.4 Hz, 2H, anthryl), 7.85
(d, J = 8.4 Hz, 4H, anthryl), 7.62 (d, J = 8.4 Hz, 2H,
anthryl), 7.43 (br, 1H, CONH), 7.31–7.10 (m, 13H,
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4H, anthryl–CH2), 4.50–4.37 (m, 2H, –NHCHCO–),
3.95 (dd, J = 4.5 Hz, 2H, NHCH2CO), 3.52–3.32 (m,
6H, PhCH2, 2CONHCH2), 3.26 (br, 1H, CH2NHCH2),
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Compound 2: yield: 64%; mp: 196–198 ꢁC; ½aꢁ ¼ ꢀ5:7
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2923,1649, 1525, 1448, 732; H NMR (CDCl3): d 8.28
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6H, anthryl), 7.90 (d, J = 8.1 Hz, 4H, anthryl), 7.80 (d,
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(br, 1H, CONH), 4.41(s, 2H, anthracene–CH2), 4.38
(s, 2H, anthracene–CH2), 3.22–2.57 (m, 13H, 2CH2CH2,
NCH2CO, Pro-ring NCH2 and CH), 2.46 (s, 2H,
2ArCH2NH, D2O exchangeable), 2.30–2.10 (m, 2H,
Pro-ring CH2), 2.10–1.95 (m, 2H, Pro-ring CH2);

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