Dopamine-selective potentiometric responses by new ditopic sensory elements based on a hexahomotrioxacalix[3]arene

Article abstract of DOI:10.1016/j.bmcl.2006.10.073

New ditopic sensory elements 2 and 3 for catecholamines based on a hexahomotrioxacalix[3]arene, with a boronic acid substituent appended, were designed and synthesized. As an interesting mode of molecular recognition at membrane surfaces, the host, when incorporated into poly(vinyl chloride) (PVC) liquid membranes, displayed excellent potentiometric selectivity for dopamine over other catecholamines (noradrenaline and adrenaline) and inorganic cations (Na⁺, K⁺, and NH₄⁺).

Full text of DOI:10.1016/j.bmcl.2006.10.073

Bioorganic & Medicinal Chemistry Letters 17 (2007) 767–771
Dopamine-selective potentiometric responses by new ditopic sensory
elements based on a hexahomotrioxacalix[3]arene
Ryosuke Saijo, Saori Tsunekawa, Hiroyuki Murakami, Naohiro Shirai,
Shin-ichi Ikeda and Kazunori Odashima*
Graduate School of Pharmaceutical Sciences, Nagoya City University, Tanabe-dori, Mizuho-ku, Nagoya 467-8603, Japan
Received 7 August 2006; revised 18 October 2006; accepted 25 October 2006
Available online 28 October 2006
Abstract—New ditopic sensory elements 2 and 3 for catecholamines based on a hexahomotrioxacalix[3]arene, with a boronic acid
substituent appended, were designed and synthesized. As an interesting mode of molecular recognition at membrane surfaces, the
host, when incorporated into poly(vinyl chloride) (PVC) liquid membranes, displayed excellent potentiometric selectivity for dopa-
mine over other catecholamines (noradrenaline and adrenaline) and inorganic cations (Na+, K+, and NH4þ).
Ó 2006 Elsevier Ltd. All rights reserved.
Catecholamines (dopamine, noradrenaline, and adrena-
line) are sympathetic neurotransmitters and play impor-
tant roles in health and disease.1 The recognition and
sensing of dopamine in biological media is a particularly
interesting subject. Dopamine is usually detected by
HPLC methods.2 On the other hand, the utilization of
synthetic catecholamine receptors in optical detection
is a challenging target in molecular recognition.3 Such
motifs have also been employed as ion-selective elec-
trodes mounted on solvent polymeric membranes, which
display the potentiometric responses to catecholamines
and biogenic amines.4 We have reported that the hexa-
homotrioxacalix[3]arene 1a,5 when incorporated into a
poly(vinyl chloride) (PVC) liquid membrane, displayed
a high selectivity in membrane potential changes for
dopamine, compared to other catecholamines (nor-
adrenaline and adrenaline) and inorganic cations (Na+
and K+).4a For further development of sensory elements
for catecholamines, new hosts 2 and 3,6a,b based on a
hexahomotrioxacalix[3]arene with a boronic acid substi-
tuent appended, are described. As a ditopic catechol-
amine receptor, these hosts have a particular design, in
that (1) the formation of tripodal hydrogen bonds be-
tween the ethereal and phenolic oxygens of the host
acid form a reversible covalent bond to the catechol por-
tion (Fig. 1).5,7
The synthesis of host 2 is outlined in Scheme 1. The
starting compound, 4, was prepared according to the lit-
erature.8 The cyclization of 4 with methanesulfonic acid
under highly dilute conditions in anhydrous CHCl3 gave
5 in 83% yield. The alkylation of 5 with methyl iodide
gave 6 in 86% yield. The diphenyl ether, 7, was prepared
by the method of Buchwald,9 in which 6 was treated
with 3-(4-hydroxyphenyl)propyl acetate, (CuOTf)2ÆPhH,
2,2,6,6-tetramethyl-3,5-heptanedione (TMHD), Cs2CO3
in toluene under reflux. The acetyl-protected alcohol
was converted into a methylamino group in 4 steps.
Finally, reductive amination with o-formylphenylboron-
ic acid gave the host 2 in 29% yield from 7.10
The synthesis of host 3 is outlined in Scheme 2. Com-
pound 86c was prepared by a method similar to that used
to prepare 7, with methyl 3-(4-hydroxyphenyl)propano-
ate. After the hydrolysis of 8, condensation with
2-((methylamino)methyl)phenylboronic acid gave the
host 3 in 21% yield from 8.
þ
and the NH3 group of dopamine and (2) the boronic
The complexation ability of hosts 2 and 3, compared
with hosts 1b5 and 8 as references, was monitored by
the 1H NMR spectroscopy in CDCl3/CD3OD using
dopamine hydrochloride (DA) and 2-phenylethylamine
hydrochloride (PEA) as guests. The stability constants
(Ks) of the host–guest complexes are listed in Table 1.
Host 2 displayed strong binding to dopamine, compared
Keywords: Membrane potential; Calixarene; Dopamine; Boronic acid.
*
Corresponding author. Fax: +81 52 836 3462; e-mail addresses:
odashima@phar.nagoya-cu.ac.jp; ikeshin@phar.nagoya-cu.ac.jp
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.10.073

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R. Saijo et al. / Bioorg. Med. Chem. Lett. 17 (2007) 767–771
Boronic Acid / Catechol Ester Formation
B
O
O
O
O
O
Cyclic Array of Ethereal and Phenolic Oxygens
for Tripodal Hydrogen Bonding
N
H
H
H
O
O
O
HO
B
HO
R2
2: R1 = -CH3, R2 =
N CH3
OR1
O
O
O
OR1
R1O
O
HO
(H3C)3C
C(CH3)3
B
3: R1 = -CH3, R2 =
HO
1a: R1 = -(CH2)3CH3, R2 = -C(CH3)3
(cone conformer)
N CH3
O
1b: R1 = -CH3, R2 = -C(CH3)3
O
Figure 1. Schematic representation of the complexation geometry of the host–guest complex between host 2 and dopamine hydrochloride.
Br
C(CH3)3
Br
C(CH3)3
a
O
O
OH
O
O
O
OH
O
OH
O
O
HO
O
(H3C)3C
C(CH3)3
4
5
AcO
Br
O
O
O
O
O
b
c
O
O
O
O
O
(H3C)3C
C(CH3)3
O
O
6
O
(H3C)3C
C(CH3)3
7
N
B
OH
HO
O
O
d
O
O
O
O
O
(H3C)3C
C(CH3)3
2
Scheme 1. Synthesis of host 2. Reagents and conditions: (a) methanesulfonic acid, dry CHCl3, rt, 1 h, 83%; (b) MeI, NaH, DMF, rt, 5 h, 86%;
(c) 3-(4-hydroxyphenyl)propyl acetate, (CuOTf)2ÆPhH, TMHD, Cs2CO3, toluene, reflux, 43 h, 44%; (d) i—KOH, EtOH/H2O, rt, 1.5 h; ii—MsCl,
Et3N, CHCl3, rt, 5.5 h; iii—LiBr, acetone, rt, 13 h; iv—MeNH2, K2CO3, THF/H2O, reflux, 4 h; v—o-formylphenylboronic acid, CHCl3/MeOH,
then NaBH4, rt, 7 h, 29% from 7.

R. Saijo et al. / Bioorg. Med. Chem. Lett. 17 (2007) 767–771
769
H3CO
O
N
O
B
OH
HO
O
O
O
a
b
6
O
O
O
O
O
O
O
O
O
O
O
(H3C)3C
C(CH3)3
(H3C)3C
C(CH3)3
8
3
Scheme 2. Synthesis of host 3. Reagents and conditions: (a) methyl 3-(4-hydroxyphenyl)propanoate, (CuOTf)2ÆPhH, TMHD, Cs2CO3, toluene,
reflux, 40 h, 69%; (b) i—KOH, EtOH/H2O, rt, 1.5 h; ii—2-((methylamino)methyl)phenylboronic acid, BOP, Et3N, THF, rt, 4 h, 21% from 8.
Table 1. Stability constants (Ks) for the host–guest complexes in
CDCl3/CD3OD = 80/20 at an ambient temperature (ca. 25 °C) with
TMS as the external standard
The potentiometric response curves for membrane III
and a blank membrane are shown in Figures 2a–b,
respectively. Cationic responses were observed in all
cases due to complexation between the monocationic
guests and the neutral hosts in the membrane boundary
region. The relative magnitudes of membrane potential
Guests
Ks [Mꢀ1
Host 2
]
Host 1b
Host 3
Host 8
DA
PEA
400
1700
3200
<10
480
350
60
480
a
DA, dopamine hydrochloride; PEA, 2-phenylethylamine hydro-
chloride.
with hosts 1b and 8. Such a large improvement was not
observed with host 3. This indicates that N–B bond for-
mation between the boron atom and the neighboring
nitrogen atom of host 2 accelerates the association of
boronic acid with catechol portion of dopamine.11 On
the other hand, the selectivity of host 3 for dopamine
over 2-phenylethylamine was improved, compared with
hosts 1b and 8, because of the steric hindrance effect of
the linker (1b vs 8) and the strong binding between the
boronic acid and catechol (3 vs 8).
We next investigated the potentiometric selectivities of
hosts 1b, 2, 3, and 8 incorporated into PVC matrix
liquid membranes (membranes I, II, III, and IV, respec-
tively). The membrane components were as follows:
o-nitrophenyl octyl ether (NPOE) as a membrane sol-
vent, PVC as a polymer matrix,12 and sodium tetra-
kis[3,5-bis(1,1,1,3,3,3-hexafluoro-2-methoxy-2-propyl)phen-
b
yl]borate (NaHFPB) as
a lipophilic anionic site
[host:NaHFPB = 1:0.30 (molar ratio)]. This mixture
resulted in stable and reproducible membrane potentials.
The electrode cell for the potential measurements was as
follows: Ag j AgCl j satd aq KCl j 1 M CH3COOLi j sam-
ple solution j membrane j 10ꢀ2 M NaCl j AgCl j Ag.
R
CF3
Na+·-B
R =
OCH3
CF3
X
R
NH2
4
R
+
HO
NaHFPB (lipophilic anionic site)
Figure 2. Membrane potential (electromotive forces; EMF) vs con-
centration curves for dopamine (d, DA), noradrenaline (j, NA),
HO
dopamine:
X = R = H
adrenaline (m, AD), 2-phenylethylamine ( , PEA), and inorganic
O(CH2)7CH3
*
noradrenaline: X = OH, R = H
cations (h, Na+; n, K+; s, NH4þ), measured at room temperature
with (a) membrane III incorporated with host 3 and (b) blank
membrane without a host.
adrenaline:
X = OH, R = -CH3
NO2
NPOE (membrane solvent)

770
R. Saijo et al. / Bioorg. Med. Chem. Lett. 17 (2007) 767–771
pot
Table 2. Potentiometric selectivity coefficients ðK Þ for PVC matrix
dopamine in comparison with host 1b, having only a
monotopic binding site. Efforts to improve dopamine
selectivity even further are now in progress.
A; B
liquid membranes (I, III and IV) incorporated with hosts (1b, 3, and 8)
and blank membrane without a hosta
Guests
Membraneb
I
III
IV
Blank
PEA
DA
NA
AD
190
1
100
1
260
1
380
1
References and notes
0.043
c
—
0.0040
0.00033
0.0073
0.012
0.00072
0.070
c
—
0.28
0.55
0.18
0.66
0.0091
1. (a) Whitley, R. J.; Meikle, A. W.; Watts, N. B. In Tietz
Textbook of Clinical Chemistry, 2nd ed.; Burtis, C. A.,
Ashwood, E. R., Eds.; Saunders: Philadelphia, 1994; pp
1739-1764; (b) Neumeyer, J. L.; Booth, R. G. In Principles
of Medicinal Chemistry, 4th ed.; Foye, W. O., Lemke, T.
L., Williams, D. A., Eds.; Lea and Febiger: Philadelphia,
1995; Chapter 13.
þ
NH4
K+
Na+
0.016
0.052
0.0038
0.029
0.099
0.0031
DA, dopamine; NA, noradrenaline; AD, adrenaline; PEA, 2-phenyl-
ethylamine.
a Potentiometric selectivities are given by selectivity coefficients (KApo;Bt Þ,
determined at unbuffered solution with DA as a standard.13
b For the compositions of the membranes, see References and Notes.12
c Could not be estimated because of the large deviation from the
Nernstian slope due to weak complexation.
2. (a) Aymard, G.; Labarthe, B.; Warot, D.; Berlin, I.;
Diquet, B. J. Chromatogr. B 2000, 744, 25; (b) Wang, Y.;
Fice, D. S.; Yeung, P. K. F. J. Pharm. Biomed. Anal. 1999,
21, 519; (c) Nikolajsen, R. P. H.; Hansen, A. M. Anal.
Chim. Acta 2001, 449, 1.
3. (a) Coskun, A.; Akkaya, E. U. Org. Lett. 2004, 6, 3107; (b)
Secor, K. E.; Glass, T. E. Org. Lett. 2004, 6, 3727; (c)
Maue, M.; Schrader, T. Angew. Chem. Int. Ed. 2005, 44,
2265; (d) Kolusheva, S.; Molt, O.; Herm, M.; Schrader, T.;
Jelinek, R. J. Am. Chem. Soc. 2005, 127, 10000.
4. (a) Odashima, K.; Yagi, K.; Tohda, K.; Umezawa, Y.
Bioorg. Med. Chem. Lett. 1999, 9, 2375; (b) Shvedene, N.
V.; Nazarova, I. A.; Formanovsky, A. A.; Otkidach, D. S.;
Pletnev, I. V. Electrochem. Commun. 2002, 4, 978; (c)
Katsu, T.; Ido, K.; Sagara, S.; Tsubaki, K.; Fuji, K.
Electroanalysis 2003, 15, 287; (d) Othman, A. M.; Rizka,
N. M. H.; El-Shahawi, M. S. Anal. Sci. 2004, 20, 651.
5. Araki, K.; Inada, K.; Otsuka, H.; Shinkai, S. Tetrahedron
1993, 49, 9465.
changes are listed in Table 2 as potentiometric selectivity
pot
A;B
coefficients (K Þ.13 Unfortunately, it was not possible
to estimate the potentiometric selectivity coefficients of
membrane II because of severe interference by protons.
Thus, the amine linkage of host 2 appears to undergo
protonation at the membrane boundary region. Except
for 2-phenylethylamine (PEA), membrane I, in which
host 1b was incorporated, displayed potentiometric
selectivity for dopamine (DA) over other catechola-
mines and inorganic cations. On the other hand, mem-
brane III, in which host 3 was incorporated, displayed
higher selectivities for DA than membrane I, with host
1b incorporated and membrane IV, with host 8 incorpo-
rated. The host-induced changes in membrane potential
[EMF (Fig. 2a) ꢀ EMF (Fig. 2b)] were ca. +90 mV for
DA and <+20 mV for the other catecholamines and
inorganic guests. The potentiometric selectivity coeffi-
6. (a) Host 2; a white solid: mp 94–96 °C. 1H NMR
(500 MHz, CD3OD) d 1.29 (s, 18H, t-Bu), 2.03 (quin,
J = 7.9 Hz, 2H, ArCH2CH2), 2.56 (s, 3H, NCH3), 2.64 (t,
J = 7.3 Hz, 2H, NCH2CH2), 2.96 (t, J = 7.3 Hz, 2H,
ArCH2CH2), 3.11 (s, 3H, OCH3), 3.27 (s, 6H, OCH3),
4.07 (s, 2H, ArCH2N), 4.42 (s, 4H, ArCH2O), 4.49 (s, 4H,
ArCH2O), 4.50 (s, 4H, ArCH2O), 6.83 (d, J = 8.5 Hz, 2H,
ArH), 6.87 (s, 2H, ArH), 7.09 (d, J = 7.6 Hz, 1H, ArH),
7.14–7.17 (m, 1H, ArH), 7.16 (d, J = 8.5 Hz, 2H, ArH),
7.23 (t, J = 7.6 Hz, 1H, ArH), 7.32 (dd, J = 7.6, 2.7 Hz,
4H, ArH), 7.57 (d, J = 7.6 Hz, 2H, ArH). IR (KBr disk)
cient for DA over noradrenaline (NA) was particularly
¼ 0:0040Þ.
pot
DA;NA
remarkable ðK
mmax 3425 (mOH), 2950, 1470, 1380, 1215, 1080 cmꢀ1
.
The potentiometric selectivity coefficient for DA over
adrenaline (AD) in the membranes I and IV could not
be estimated. On the other hand, it could be estimated
HRMS for C52H67BNO9 [M+H]+: Calcd, 860.4903.
Found, 860.4863.; (b) Host 3; a white solid: mp 109–
1
pot
DA;AD
111 °C. H NMR (500 MHz, CD3OD) d 1.29 (s, 18H, t-
in the membrane III ðK
¼ 0:00033Þ. This result
Bu), 2.66–2.75 (m, 2H, COCH2), 2.83 (s, 3H, NCH3),
2.86–2.96 (m, 2H, ArCH2CH2), 3.11 (s, 3H, OCH3), 3.27
(s, 6H, OCH3), 4.42 (s, 4H, ArCH2O), 4.48 (s, 4H,
ArCH2O), 4.50 (s, 4H, ArCH2O), 4.60 (s, 2H, ArCH2N),
6.84 (d, J = 8.5 Hz, 2H, ArH), 6.87 (s, 2H, ArH), 7.05 (d,
J = 8.2 Hz, 1H, ArH), 7.12 (d, J = 8.2 Hz, 2H, ArH), 7.21
(d, J = 8.2 Hz, 1H, ArH), 7.25–7.26 (m, 1H, ArH), 7.32 (d,
J = 8.5 Hz, 4H, ArH), 7.35–7.29 (m, 1H, ArH). IR (KBr
disk) mmax 3425 (mOH), 2950, 1620 (mC@O), 1490, 1360,
would be due to the increase of the potentiometric re-
sponse of AD caused by the effect of the interaction be-
tween the boronic acid group of host 3 and the catechol
unit. Compared with host 1b and 8, host 3 also displayed
an improved potentiometric selectivity for DA over
PEA. This demonstrates the effect of the ditopic binding
sites, that is, theþethereal and phenolic oxygens of the
host for the NH3 group of dopamine and the boronic
acid substituent of the host for the catechol portion.
However, much stronger binding to DA is necessary,
to potentiometric discriminate between DA and PEA,
because the PEA response of blank membrane is about
400-fold stronger than DA. It should be noted that
selectivity reflects the difference in lipophilicity for each
guest.
1215,
1070 cmꢀ1
.
HRMS
for
C54H68BNO10Na
[M+2MeOH–2H2O+Na]+: Calcd, 924.4828. Found,
924.4868.; (c) Host 8; colorless hard gum: 1H NMR
(500 MHz, CDCl3)
d 1.29 (s, 18H, t-Bu), 2.63 (t,
J = 7.6 Hz, 2H, COCH2), 2.92 (t, J = 7.6 Hz, 2H,
ArCH2CH2), 3.08 (s, 3H, OCH3), 3.24 (s, 6H, OCH3),
3.68 (s, 3H, COOCH3), 4.44 (s, 4H, ArCH2O), 4.49 (s, 4H,
ArCH2O), 4.50 (s, 4H, ArCH2O), 6.86 (d, J = 8.5 Hz, 2H,
ArH), 6.94 (s, 2H, ArH), 7.12 (d, J = 8.8 Hz, 2H, ArH),
7.29 (d, J = 2.1 Hz, 4H, ArH). IR (KBr disk) mmax 2950,
In conclusion, host 3 having ditopic binding sites dis-
played particularly high potentiometric selectivities for
1740 (mC@O), 1480, 1215, 1070 cmꢀ1
. HRMS for

R. Saijo et al. / Bioorg. Med. Chem. Lett. 17 (2007) 767–771
771
C45H56O9Na [M+Na]+: Calcd, 763.3817. Found,
763.3801. Anal. Calcd for C45H56O9Æ0. 55CHCl3: C,
67.83; H, 7.07. Found: C, 67.82; H, 7.01.
IV, 0.7:66:33:0.5. A membrane without added host (blank
membrane; NPOE/PVC/NaHFPB = 67:33:0.5 wt%) was
also prepared in a similar manner.
pot
7. Lorand, J. P.; Edwards, J. O. J. Org. Chem. 1959, 24, 769.
8. Tsubaki, K.; Otsubo, T.; Morimoto, T.; Maruoka, H.;
Furukawa, M.; Momose, Y.; Shang, M.; Fuji, K. J. Org.
Chem. 2002, 67, 8151.
9. Marcoux, J.-F.; Doye, S.; Buchwald, S. L. J. Am. Chem.
Soc. 1997, 119, 10539.
10. Gray, C. W., Jr.; Houston, T. A. J. Org. Chem. 2002, 67,
5426.
11. Wulff, G.; Lauer, M.; Bo¨hnke, H. Angew. Chem., Int. Ed.
Engl. 1984, 23, 741.
12. The PVC matrix liquid membranes were prepared accord-
ing to the literature (Ceresa, A.; Pretsch, E. Anal. Chim.
Acta 1999, 395, 41). The membrane compositions [host:
NPOE (membrane solvent) : PVC (polymer matrix):
NaHFPB (anionic site), wt%] were as follows: Membrane
I, 0.8:65:33:0.5. Membrane III, 0.9:66:32:0.5. Membrane
13. Potentiometric selectivity coefficients (K
Þ were deter-
A; B
mined at room temperature (ca. 25 °C) by the separate
solution method (Srinivasan, K.; Rechnitz, G. A. Anal.
Chem. 1969, 41, 1203). For potentiometric selectivity
coefficients, see: (a) CRC Handbook of Ion-Selective
Electrodes: Selectivity Coefficients; Umezawa, Y., Ed.;
CRC Press: Boca Raton: USA, 1990; (b) Umezawa, Y.;
Buhlmann, P.; Umezawa, K.; Tohda, K.; Amemiya, S.
¨
Pure Appl. Chem. 2000, 72, 1852; (c) Umezawa, Y.;
Umezawa, K.; Buhlmann, P.; Hamada, N.; Nakanishi,
¨
J.; Aoki, H.; Sato, M.; Nishimura, Y.; Xiao, K. P. Pure
Apple. Chem. 2002, 74, 923; (d) Umezawa, Y.; Buhl-
¨
mann, P.; Umezawa, K.; Hamada, N. Pure Appl. Chem.
2002, 74, 995; (e) Umezawa, Y. In Encyclopedia of
Supramol. Chem; Atwood, J., Steed, J., Eds.; Marcel
Dekker: USA, 2004; p 747.