Computer-guided design of a Cu(II) receptor and sensor

Article abstract of DOI:10.1039/b610887c

The computer program CAVEAT was used to design a linker structure to connect four imidazole 4-carboxamide groups in a distorted square planar arrangement optimal for binding Cu(ii) ions. The resulting macrocyclic structure was prepared by the synthesis of

Full text of DOI:10.1039/b610887c

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/njc | New Journal of Chemistry
Computer-guided design of a Cu(II) receptor and sensor
Chen Lin and Dale G. Drueckhammer*
Received (in St Louis, MO, USA) 1st August 2006, Accepted 21st August 2006
First published as an Advance Article on the web 14th September 2006
DOI: 10.1039/b610887c
The computer program CAVEAT was used to design a linker structure to connect four imidazole
4-carboxamide groups in a distorted square planar arrangement optimal for binding Cu(II) ions.
The resulting macrocyclic structure was prepared by the synthesis of a protected monomer
followed by amide coupling reactions to form the cyclic tetramer. Binding to Cu(II) ion was
evaluated using the inherent fluorescence of the receptor and its quenching upon binding to Cu(II)
ion. Much higher concentrations of Zn(II), Co(II), and Ni(II) ions had little or no effect on the
fluorescence and did not significantly compete with the fluorescence response to Cu(II) ions.
3-dimensional chemical structures to identify structures having
Introduction
bonds matching a defined pair of vectors.24,25 Reported here is
the computer-assisted design of a Cu2+ receptor, using
CAVEAT to identify a linker structure to connect individual
ligand groups in the orientation that matches a computational
model of the complex with unlinked ligands. Also reported is
the synthesis of the receptor based on this design as well as
initial studies of Cu2+ affinity and selectivity and the utility of
this receptor as a fluorescence-based Cu2+ sensor.
The design and synthesis of specific ligands and receptors for
metal ions is a field of longstanding and continuing interest.
This work is of importance for a variety of reasons including
the development of sensors for the detection and quantitation
of metal ions in biological systems and environmental sam-
ples.1–4 Success with a Ca2+ sensor and its application to
studies of cell calcium demonstrates the value of sensors for
biologically important metal ions.5 Metal receptors have also
been designed as models or mimics for metalloproteins.6 Metal
ion receptors have been based on a variety of structural motifs
including synthetic acyclic and macrocyclic polydentate
ligands as well as synthetic peptides.7–12 The Cu2+ ion is
somewhat unique in its preference for distorted square planar
coordination and this geometry is observed in a number of
interesting Cu2+ binding proteins.13,14 This preference offers
the potential for development of selective receptors for Cu2+
based on ligand groups organized in a distorted square planar
arrangement.15 A number of Cu2+ sensors have been recently
developed based on both new and old Cu2+-binding
motifs.10,11,16–18
Results and discussion
Design
The design of the Cu(II) receptor was based on the premise that
a distorted square planar arrangement of imidazole ligands
could exhibit selectivity for Cu2+ over other metal ions, and
that CAVEAT could be used to design a connector moiety to
organize the imidazole groups into such an arrangement. The
design process is illustrated in Fig. 1. The complex of Cu2+
with four modified imidazole amide moieties 1, analogous to
type 2 Cu(II) centers in proteins13 was optimized using PM3
calculations. The initial distorted square planar structure was
not precisely symmetrical, with the near planar N–Cu–N
angles ranging from 94.45–95.381. The structure was re-opti-
mized in PM3 with the ‘‘in plane’’ N–Cu–N angles constrained
at 951. The resulting highly symmetrical structure 2 had Cu–N
bond lengths of 1.93 A, in fair agreement with the average
distance of 1.98 A observed in Cu–imidazole complexes.26 The
symmetry assures that one appropriately designed linker
structure can be used for linking each consecutive imidazole
pair in a tetrameric structure. To design the linker structure,
the amide N-methyl and imidazole N-methyl bonds of adja-
cent ligands of 2 (represented by the bonds in bold in the upper
right portion of structure 1) were used to define the vectors for
CAVEAT. A CAVEAT search was conducted of the TRIAD
and ILLIAD databases, both being collections of computer-
generated structures developed specifically for use with
CAVEAT.24 Several possible structures to serve as linkers
between ligand moieties were thereby identified. Of these,
structure 3 from ILLIAD was chosen for development, in
which the C–Ha and C–Hb bonds match the vectors defined by
Recent structural and computational analysis have illu-
strated that most complexes between metal ions and multi-
dentate ligands are far from optimal when one considers all of
the bond angles and dihedral angles, in addition to the more
obvious issues of coordination geometry and bond lengths
about the metal ion.15,19 While metal-binding proteins might
be expected to achieve the ideal metal ligand structure, if this
has been the direction of evolutionary pressure, more sophis-
ticated design approaches are clearly needed for the design of
more optimal man-made multidentate ligands that approxi-
mate the ideal metal coordination environments. Recently
computer-based design approaches to this objective have be-
gun to be pursued.20 We have recently begun a research
program in the application of the computer program
CAVEAT to the design of receptors for molecule and ion
recognition.21–23 CAVEAT searches electronic databases of
Department of Chemistry, Stony Brook University, Stony Brook, NY,
11794-3400, USA. E-mail: ddrueckhamme@notes.cc.sunysb.edu; Fax:
631-632-7960; Tel: 631-632-7923
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
New J. Chem., 2006, 30, 1725–1730 | 1725

View Article Online
Scheme 1
Fig. 1 Structures in the design of the macrocyclic Cu(II) receptor 4.
the imidazole–methyl and amide–methyl C–N bonds, respec-
tively. Incorporation of this linker structure between consecu-
tive monomeric groups in the copper complex 1 gives the
macrocyclic structure 4. Computational modeling of the Cu2+
complex of 4 further supported this design, including the
absence of significant strain introduced at the points of con-
nection between linker and ligand. The receptor has a fair
degree of flexibility, but modeling indicates it is sufficiently
constrained that tetrahedral coordination to a metal ion would
impart significant strain relative to the designed distorted
square planar coordination.
Scheme 2
method formed 12.31 Suzuki coupling of 9 with 12 formed the
protected monomer unit 13.32,33
The monomer 13 was converted to the cyclic tetramer 4 as
shown in Scheme 3. Separate samples of 13 were deprotected
to form the free carboxylic acid 14 and the free amine 15. 14
was converted to the acid chloride followed by reaction with
15 to form the protected dimer 16.34,35 Separate samples of 16
were deprotected to the free acid 18 and the free amine 19 as
with the monomer 13. The dimer units were again coupled via
the acid chloride to form the protected tetramer 17. The amine
was deprotected with HCl to avoid introduction of trifluoro-
acetic acid, which was difficult to separate from the depro-
tected tetramer and when present caused problems in the
coupling step. The ester was hydrolyzed with sodium hydro-
xide as in the previous deprotections. Attempts to form the
cyclic tetramer via the acid chloride and using several other
coupling agents were unsuccessful. Cyclization was success-
fully accomplished using dichlorotriphenylphosphorane in
dilute (0.4 mM) solution at reflux for three days to form the
desired macrocycle 4. Formation of the macrocycle resulted in
a marked simplification of the 1H NMR spectrum as expected
for this highly symmetrical product. Most notably, the four
closely spaced singlets between 3.26 and 3.33 for the four
N-methyl groups of the protected linear tetramer 17 collapsed
Synthesis
The preparation of the macrocyclic tetramer 4 began with
synthesis of a protected monomer unit. The initial steps in the
synthesis are shown in Scheme 1. The thioether 7 was prepared
by copper-mediated coupling of 3-mercaptobenzyl alcohol 527
with 3-bromoiodobenzene 6.28 Conversion of the benzylic
alcohol to the bromide via the mesylate followed by reaction
with methyl-4-imidazolecarboxylate 8 and base formed the
desired intermediate 9 along with the isomer 10 resulting from
alkylation of the wrong imidazole nitrogen. The isomers were
readily separated by column chromatography. The assignment
of isomers was confirmed by synthesis of 9 by N-alklyation of
a glycine ester with the mesylate derived from 7 followed by
formation of the imidazole ring using Gold’s salt, a known
route for the synthesis of imidazole derivatives.29 However,
this reaction gave a low yield and was difficult to reproduce so
the synthetic route of Scheme 1 was preferred. The remainder
of the synthesis of the protected monomer is shown in Scheme
2. t-Boc protection of p-iodo-N-methylaniline 1130 followed by
conversion of the iodide to the boronic ester by the Miyaura
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
1726 | New J. Chem., 2006, 30, 1725–1730

View Article Online
Fig. 2 Decreasing fluorescence of 4 (initial conc. 32.6 mM) with
increasing [CuCl2] (0, 7.7, 15.3, 23.7, 32.1, 40.4, 48.7, 56.9, 65.0, and
86.9 mM).
than 50% quenching observed at saturation. A plot of fluor-
escence intensity vs. CuCl2 concentration gave a dissociation
constant for the Cu2+–4 complex of 3.2 ꢁ 10ꢂ6 M. This
binding constant in organic solvent is influenced by the
counter ion, as the equivalent titration with copper(II) triflate
gave a linear decrease in fluorescence with increasing copper
salt until one equivalent was reached after which the fluores-
cence was unchanged, with insufficient curvature in the fluor-
escence intensity vs. [Cu2+] plot to accurately deduce a binding
constant. The titration was also performed with Zn(II) triflate,
Ni(II) chloride and Co(II) chloride. Zn2+ and Ni2+ showed no
quenching or other change in the fluorescence spectrum at
concentrations up to 2.2 ꢁ 10ꢂ4 and 4.5 ꢁ 10ꢂ4 M, respec-
tively, while Co2+ gave about 5% quenching at a concentra-
tion of 2.8 ꢁ 10ꢂ4 M. As binding of these metal ions could not
be directly evaluated using fluorescence, selectivity for Cu(II)
ions was assessed by competition. Stepwise addition of Cu(II)
Scheme 3
to a single peak at 3.26 integrating to 12 hydrogens in 4.
Another interesting observation was the shifting of H-4 of the
imidazole ring to about 6.4 ppm in the imidazole 5-carbox-
amide group, apparently due to substantial loss of conjugation
of the amide carbonyl with the imidazole ring due to twisting
caused by steric effects of the tertiary amide. A slightly smaller
shift was reported though not discussed for a morpholine
amide of imidazole-5-carboxylic acid.36 As predicted from this
analysis 13, 16, 17, and 4 exhibited the diagnostic 0, 1, 3, and 4
upfield shifted imidazole protons respectively, further support-
ing the structure of 4.
chloride to each of the solutions of 4 containing Ni2+, Co2+
,
or Zn2+ resulted in a fluorescence quenching curve that was
indistinguishable from the titrations with no other metal salt
present, with significant quenching observed even at the first
concentration of 7.7 ꢁ 10ꢂ6 M Cu2+. This indicates that
Ni2+, Co2+, and Zn2+ ions do not compete significantly with
binding of Cu2+ and that the selectivity for Cu2+ is at least
100-fold vs. each of the other three metals. The selectivity vs.
Ni2+ is noteworthy as Ni2+ exhibits similar or even greater
affinity than Cu2+ toward some simple ligands such as bipyr-
idine, and selectivity vs. Ni2+ has been an issue with some
Cu2+ sensors.10,37 The aqueous insolubility of 4 limits the
more complete affinity and selectivity analysis of this initial
design.
Binding studies
The macrocycle 4 was studied as a receptor for Cu2+ and
other metal ions using fluorescence. The integral phenyl
biphenyl thioether moiety of the receptor provided a good
fluorophore for observation of Cu(II)-induced fluorescence
quenching. 4 is insoluble in water, so these experiments were
conducted in 1 : 1 methanol : tetrahydrofuran. 4 exhibited a
maximum absorbance at 290 nm and an emission maximum at
357 nm, which were used as the excitation and emission
wavelengths in binding experiments. Fig. 2 shows the fluores-
cence emission spectra for 4 with increasing concentration of
CuCl2. As expected, the paramagnetic Cu(II) ion substantially
quenched the fluorescence of the receptor, with slightly more
Conclusion
A novel computer-guided approach was demonstrated for
designing a multidentate ligand for optimal positioning of
individual ligand groups for metal ion binding. This approach
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
New J. Chem., 2006, 30, 1725–1730 | 1727

View Article Online
may be valuable in designing selective metal ion receptors that
with incorporation of appropriate solubility and fluorescence
signaling properties may be valuable as metal ion sensors. This
work further demonstrates the broad utility of the computer
program CAVEAT in molecular design.
stirred at room temperature overnight under nitrogen. The
solution was diluted with ethyl acetate (100 mL), washed with
water (3 ꢁ 30 mL), dried (MgSO4), and concentrated. The
residue was purified by the column chromatography (silica gel,
hexanes : ethyl acetate 4 : 1 to 2 : 3) to give the pure product 9
(0.8 g, 2.0 mmol, 46%) as a brown oil. dH (300 MHz; CDCl3;
Me4Si) 3.80 (3H, s, OMe), 5.49 (2H, s, CH2), 7.05–7.40 (8H,
m, aromatic), 7.66 (1H, s, imidazole), 7.76 (1H, s, imidazole).
dC (75.6 MHz; CDCl3; Me4Si) 49.34, 51.41, 122.19, 122.89,
126.24, 129.01, 129.75, 129.82, 130.04, 130.88, 132.91, 135.39,
137.64, 137.73, 137.99, 142.06, 160.35.
Experimental
General
PM3 calculations were performed using PC Spartan Pro from
Wavefunction, Inc. Chemicals were obtained from commercial
suppliers and used without further purification. Column chro-
matography was performed on silica gel 60 (40–63 mm). NMR
spectra were obtained on a Varian Gemini-2300 instrument.
(4-Iodophenyl)-methyl-carbamic acid tert-butyl ester. To a
solution of (4-iodophenyl)-methylamine 1125 (0.99 g, 4.25
mmol) in dichloromethane (25 mL) was added triethylamine
(1.2 ml, 0.86 g, 8.5 mmol), DMAP (0.052 g, 0.43 mmol) and di-
tert-butyl dicarbonate (1.40 g, 6.4 mmol). The solution was
stirred overnight at reflux under nitrogen. The solution was
diluted with dichloromethane (40 mL), washed with aq. HCl
(1 N, 2 ꢁ 20 mL), dried (MgSO4) and concentrated. The
residue was purified by column chromatography on silica gel
(hexanes : ethyl acetate 8 : 1) to give (4-iodophenyl)-methyl-
carbamic acid tert-butyl ester (1.18 g, 3.54 mmol, 83%) as a
dark oil. dH (300 MHz; CDCl3; Me4Si) 1.44 (9H, s, OCMe3),
3.23 (3H, s, NMe), 7.00 (2H, d, J 8.7, H2,6), 7.62 (2H, d, J 8.7,
H3,5). dC (75.6 MHz; CDCl3; Me4Si) 28.19, 36.96, 77.42,
89.34, 127.19, 137.44, 143.48, 154.19.
Synthesis
[3-(3-Bromo-phenylsulfanyl)-phenyl]-methanol (7). To a solu-
tion of 527 (4.21 g, 0.030 mol) and sodium ethoxide (0.090 mol)
in ethanol (45 mL) was added copper powder (0.2 g, 3.13
mmol) and 3-iodobromobenzene 6 (3.84 ml, 8.51 g, 0.03 mol).
The mixture was stirred at reflux for 36 h under nitrogen. After
cooling to room temperature, the solution was filtered and
concentrated to dryness. The residue was dissolved in ethyl
acetate (100 mL) and washed with water (3 ꢁ 30 mL), dried
(MgSO4), and concentrated. The residue was purified by
column chromatography (silica gel, hexanes : ethyl acetate
8 : 1 to 4 : 1) to give 7 (5.05 g, 17 mmol, 43%) as a brown
solid. dH (300 MHz; CDCl3; Me4Si) 4.67 (2H, s, CH2O),
7.14–7.43 (8H, m, aromatic).
Methyl-[4-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-
phenyl]-carbamic acid tert-butyl ester (12). To a mixture
of bis(pinacolato)diboron (0.34 g, 1.34 mmol), PdCl2
(dppf) ꢃ CH2Cl2 (0.029 g, 35.5 mmol) and potassium acetate
(0.35 g, 3.57 mmol) was added a solution of (4-iodophenyl)-
methyl-carbamic acid tert-butyl ester (0.4 g, 1.2 mmol) in
DMSO (6 mL) under nitrogen. The solution was stirred at
80 1C for 3.5 h under nitrogen. After cooling to room temper-
ature, the solution was diluted with ethyl acetate (40 mL),
washed with water (4 ꢁ 20 mL), dried (MgSO4) and concen-
trated. The resulting residue was purified by column chroma-
tography on silica gel (hexanes : ethyl acetate 8 : 1) to give 12
(0.25 g, 7.51 mmol, 63%) as a yellow solid. dH (300 MHz;
CDCl3; Me4Si) 1.34 (12H, s, CMe2), 1.44 (9H, s, OCMe3), 3.27
(3H, s, NMe), 7.24 (2H, d, J 8.7, H2,6), 7.76 (2H, d, J 8.7,
H3,5).
.Methanesulfonic acid 3-(3-bromo-phenylsulfanyl)-benzyl ester.
To a solution of 7 (6.67 g, 22.6 mmol) and Et3N (6.3 ml, 4.57 g,
45.2 mmol) in dichloromethane (80 mL) was added dropwise a
solution of methanesulfonyl chloride (2.63 ml, 3.89 g, 34.0
mmol) in dichloromethane (20 mL) at 0 1C over 30 min under
nitrogen. The mixture was stirred at 0 1C for 3 h and the
reaction was terminated by the addition of ice-cold water (30
mL). The organic layer was washed with ice-cold water (2 ꢁ 20
mL), dried (MgSO4) and concentrated to give the product (8.32
g, 22.3 mmol, 99%) as a yellow solid, which was used directly in
the next step. dH (300 MHz; CDCl3; Me4Si) 2.94 (3H, s,
MeSO2), 5.19 (2H, s, CH2O), 7.17–7.44 (8H, m, aromatic).
[3-(3-Bromo-phenylsulfanyl)-benzyl]-bromide. To a solution
of methanesulfonic acid 3-(3-bromo-phenylsulfanyl)-benzyl
ester (1.2 g, 3.22 mmol) in DMF (20 mL) was added sodium
bromide (0.40 g, 3.89 mmol). The solution was stirred for 4 h
at room temperature under nitrogen. Then the solution was
diluted with ethyl acetate (80 mL) and water (40 mL). The
organic layer was washed with water (2 ꢁ 30 mL), dried
(MgSO4) and concentrated to give the product (1.14 g, 3.18
mmol, 99%) as a brown oil. dH (300 MHz; CDCl3; Me4Si) 4.44
(2H, s, CH2Br) 7.16-7.45 (8H, m, aromatic).
3-{3-[40-(tert-Butoxycarbonyl-methyl-amino)-biphenyl-3-yl-
sulfanyl]-benzyl}-3H-imidazole-4-carboxylic acid methyl ester
(13). To a mixture of 12 (0.86 g, 2.58 mmol), PdCl2
(dppf) ꢃ CH2Cl2 (0.113 g, mmol) and K2CO3 (1.37 g,
6.21 mmol) was added a solution of 9 (0.8 g, 1.98 mmol) in
dimethoxyethane (15 mL) under nitrogen. The solution was
stirred at 80 1C for 24 h under nitrogen. After cooling to room
temperature, the solution was concentrated to dryness. Ethyl
acetate (80 mL) was added and the solution was washed with
water (2 ꢁ 30 mL). The organic layer was dried (MgSO4) and
concentrated. The resulting residue was purified by column
chromatography (silica gel, hexanes : ethyl acetate 4 : 1 to 2 : 3)
to give 13 (0.66 g, 1.25 mmol, 63%) as a brown oil. dH (300
MHz; CDCl3; Me4Si) 1.47 (9H, s, OCMe3), 3.28 (3H, s, NMe),
3.76 (3H, s, CO2Me), 5.46 (2H, s, CH2), 6.95–7.50 (12H, m,
3-[3-(3-Bromo-phenylsulfanyl)-benzyl]-3H-imidazole-4-car-
boxylic acid methyl ester (9). To a solution of [3-(3-bromo-
phenylsulfanyl)-benzyl]-bromide (1.56 g, 4.36 mmol) in DMF
(30 mL) was added methyl-4-imidazolecarboxylate 8 (0.6 g,
4.76 mmol) and K2CO3 (3.01 g, 21.8 mmol). The solution was
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
1728 | New J. Chem., 2006, 30, 1725–1730

View Article Online
aromatic), 7.60 (1H, s, imidazole), 7.74 (1H, s, imidazole). dC
(75.6 MHz; CDCl3; Me4Si) 24.86, 28.15, 36.86, 83.28, 83.56,
124.13, 134.97, 146.32, 154.26.
3-(3-{40-[(3-{3-[40-(tert-Butoxycarbonyl-methyl-amino)-bi-
phenyl-3-ylsulfanyl]-benzyl}-3H-imidazole-4-carbonyl)-methyl-
amino]-biphenyl-3-ylsulfanyl}-benzyl)-3H-imidazole-4-carboxy-
lic acid (18). The procedure for the synthesis of 14 was
followed to give 18 (94%). dH (300 MHz; CDCl3; Me4Si)
1.47 (9H, s, OCMe3), 3.26 (3H, s, NMe), 3.34 (3H, s, NMe),
5.47 (2H, s, CH2), 5.52 (2H, s, CH2), 6.13 (1H, s, imidazole),
6.70–7.75 (27H, m, aromatic, 3H from imidazole).
3-{3-[40-(tert-Butoxycarbonyl-methyl-amino)-biphenyl-3-yl-
sulfanyl]-benzyl}-3H-imidazole-4-carboxylic acid 14. To a solu-
tion of 13 (0.91 g, 1.72 mmol) in methanol (18 mL) was added
a solution of NaOH (0.8 g, 0.02 mol) in water (2 mL). The
solution was stirred at room temperature for 24 h. The
solution was concentrated to dryness and water (20 mL) was
added. Aq. HCl (1 N) was added dropwise until the pH
reached 5. The mixture was extracted with dichloromethane
(3 ꢁ 60 mL). The combined organic layers were dried (MgSO4)
and concentrated to give 14 (0.82 g, 1.60 mmol, 93%) as an
orange oil. dH (300 MHz; CDCl3; Me4Si) 1.47 (9H, s, OCMe3),
3.27 (3H, s, NMe), 5.50 (2H, s, CH2), 7.00–7.50 (12H, m,
aromatic), 7.74 (1H, s, imidazole), 7.78 (1H, s, imidazole).
3-{3-[40-(Methyl-{3-[3-(40-methylamino-biphenyl-3-ylsulfa-
nyl)-benzyl]-3H-imidazole-4-carbonyl}-amino)-biphenyl-3-ylsul-
fanyl]-benzyl}-3H-imidazole-4-carboxylic acid methyl ester
(19). The procedure for the synthesis of 15 was followed to
give 19 (99%). dH (300 MHz; CDCl3; Me4Si) 2.82 (3H, s,
amine NMe), 3.32 (3H, s, amide NMe), 3.77 (3H, s, CO2Me),
5.45 (2H, s, CH2), 5.49 (2H, s, CH2), 6.39 (1H, s, imidazole),
6.60 (2H, d, J 8.7, aromatic ortho to amine N), 6.87 (2H, d, J
8.7, aromatic ortho to amide N), 6.90–7.80 (23H, m, aromatic,
3H from imidazole).
3-[3-(40-Methylamino-biphenyl-3-ylsulfanyl)-benzyl]-3H-imi-
dazole-4-carboxylic acid methyl ester (15). To a solution of 13
(0.92 g, 1.74 mmol) in dichloromethane (6 mL) was added
dropwise trifluoroacetic acid (1.25 mL, 0.85 g, 7.46 mmol).
The solution was stirred for 2 h at room temperature. The
reaction mixture was added dropwise to an aq. saturated
NaHCO3 solution (40 mL). The solution was extracted with
dichloromethane (70 mL). The organic layer was washed with
saturated aq. NaHCO3 (2 ꢁ 30 mL), and brine (20 mL), dried
(MgSO4) and concentrated to give 15 (0.74 g, 1.72 mmol,
99%) as a brown oil. dH (300 MHz; CDCl3; Me4Si) 2.86 (3H, s,
NMe), 3.76 (3H, s, CO2Me), 5.45 (2H, s, CH2), 6.66 (2H, d, J
8.4, aromatic ortho to N), 6.90–7.54 (10H, m, aromatic), 7.60
(1H, s, imidazole), 7.74 (1H, s, imidazole).
Linear tetramer (17). The procedure for the synthesis of 16
was followed, but the product was purified by column chro-
matography on silica gel (ethyl acetate : methanol : Et3N, 19 :
1 : 1) to give 17 (60%) as a brown oil. dH (300 MHz; CDCl3;
Me4Si) 1.46 (9H, s, OCMe3), 3.24 (3H, s, NMe), 3.28 (3H, s,
NMe), 3.29 (3H, s, NMe), 3.31 (3H, s, NMe), 3.77 (3H, s,
CO2Me). 5.44 (2H, s, CH2), 5.49 (6H, s, 3 unresolved CH2),
6.36 (2H, s, unresolved imidazole H), 6.38 (1H, s, imidazole)
6.86–6.91 (6H, m, aromatic ortho to amide N), 6.94–7.74 (47H,
m, aromatic, 7H from imidazole). dC (75.6 MHz; CDCl3;
Me4Si) 28.24, 37.08, 37.81, 37.84, 37.86, 45.27, 49.50, 49.72,
51.38, 80.38, 122.24, 125.00, 125.46, 125.65, 126.05, 126.19,
126.23, 126.28, 127.02, 127.27, 128.12, 128.84, 129.35, 129.39,
129.56, 129.62, 129.68, 129.72, 129.86, 129.93, 130.08, 130.12,
130.17, 130.20, 130.51, 130.54, 130.72, 134.22, 135.27, 135.36,
135.42, 135.46, 136.77, 136.80, 136.83, 136.92, 137.35, 137.81,
137.86, 139.44, 139.49, 139.60, 140.82, 140.86, 141.65, 142.05,
143.33, 143.49, 154.49, 160.35, 160.65.
3-(3-{40-[(3-{3-[40-(tert-Butoxycarbonyl-methyl-amino)-bi-
phenyl-3-ylsulfanyl]-benzyl}-3H-imidazole-4-carbonyl)-methyl-
amino]-biphenyl-3-ylsulfanyl}-benzyl)-3H-imidazole-4-carboxy-
lic acid methyl ester (16). To a solution of 14 (0.82 g, 1.60
mmol) in dichloromethane (10 mL) and toluene (10 mL) with
1 drop of dimethylformamide at 0 1C under nitrogen was
added dropwise oxalyl chloride (1.2 mL, 0.83 g, 6.5 mmol).
The solution was stirred for 1 h at 0 1C under nitrogen and the
reaction mixture was concentrated to dryness. Anhydrous
dichloromethane (5 mL) was added and the resulting solution
was added dropwise to a solution of 15 (0.74 g, 1.72 mmol) in
pyridine (5 mL) at 0 1C under nitrogen. The reaction mixture
was allowed to warm to room temperature over 0.5 h and was
stirred overnight at room temperature under nitrogen. The
mixture was then concentrated to dryness. The resulting
residue was dissolved in dichloromethane (60 mL), washed
with aqueous saturated NaHCO3 (2 ꢁ 20 mL), brine (20 mL),
dried (MgSO4), and concentrated. The resulting residue was
purified by column chromatography on silica gel (hexanes :
ethyl acetate 1 : 4 to ethyl acetate : methanol 18 : 1) to give 16
(0.94 g, 1.0 mmol, 63%) as a brown oil. dH (300 MHz; CDCl3;
Me4Si) 1.45 (9H, s, OCMe3), 3.25 (3H, s, NMe), 3.31 (3H, s,
NMe), 3.76 (3H, s, CO2Me), 5.45 (2H, s, CH2), 5.49 (2H, s,
CH2), 6.36 (1H, s, imidazole), 6.87 (2H, d, J 8.1, aromatic
ortho to amide N), 6.90–7.74 (25H, m, aromatic, 3H from
imidazole).
Cyclic tetramer (4). To a solution of 17 (0.124 g, 72.0 mmol)
in dioxane (2 mL) was added dropwise a solution of HCl in
dioxane (4 M, 2 mL). The solution was stirred overnight at
room temperature. The reaction was quenched by the addition
of triethylamine (2 mL). The resulting mixture was concen-
trated to dryness and then dissolved in dichloromethane (40
mL). The solution was washed with saturated NaHCO3 (2 ꢁ
20 mL), dried (MgSO4) and concentrated to give the free
amine of 17 (0.117 g, 72.1 mmol, 99%) as a brown oil. dH (300
MHz; CDCl3; Me4Si) 2.83 (3H, s, amine NMe), 3.29–3.33 (9H,
3s, amide NMe), 3.77 (3H, s, OMe), 5.47 (8H, 4s, CH2), 6.36
(3H, 3s, imidazole), 6.60 (2H, d, J 8.7, aromatic ortho to
NHMe), 6.82–6.91 (6H, m, aromatic ortho to amide NMe),
6.92–7.80 (45H, m, aromatic). The amine (0.117 g, 72.1 mmol)
was dissolved in methanol (5 mL) and THF (8 mL), and a
solution of NaOH (52.6 mg, 1.32 mmol) in water (1 mL) was
added. The solution was stirred for 2 days at room tempera-
ture. The mixture was concentrated to dryness and dissolved in
water (15 mL). An aq. solution of HCl (1 N) was added
dropwise until the pH reached 6. The mixture was extracted
with dichloromethane (3 ꢁ 50 mL). The combined organic
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
New J. Chem., 2006, 30, 1725–1730 | 1729

View Article Online
layers were dried (MgSO4) and concentrated to give the fully
deprotected linear tetramer (0.1 g, 62.2 mmol, 86%) as a brown
oil. dH (300 MHz; CDCl3; Me4Si) 2.82 (3H, s, amine NMe),
3.28–3.33 (9H, 3s, amide NMe), 5.44–5.50 (8H, 4s, CH2), 6.26
(1H, s, imidazole), 6.36 (2H, s, imidazole), 6.60 (2H, d, J 8.1),
6.61–7.80 (51 H, m, aromatic). A portion of this product
(0.0475 g, 29.5 mmol) was dissolved in dichloromethane (60
mL) and triphenylphosphine dichloride (0.06 g, 18.0 mmol)
was added. The solution was stirred for 3 days at reflux under
nitrogen. After cooling to room temperature, the solution was
washed with saturated aq. NaHCO3 (20 mL), dried (MgSO4)
and concentrated. The resulting residue was purified by col-
umn chromatography (ethyl acetate : methanol : Et3N, 18 :
1 : 1) to give 4 (0.031 g, 19.5 mmol, 66% for the final step) as a
brown oil. dH (300 MHz; CDCl3; Me4Si) 3.26 (12H, s, NMe),
5.47 (8H, s, CH2), 6.36 (4H, s, imidazole), 6.79 (8H, d, J 8.7,
aromatic ortho to N), 7.00–7.60 (44H, m, aromatic). m/z (ES)
1589.46 (MH+. C96H77N12O4S4 requires 1589.51).
5 (a) R. Y. Tsien, in Fluorescent Chemosensors for Ion and Molecule
Recognition, ed. A. W. Czarnik, American Chemical Society,
Washington, DC, 1993, pp. 130–146; (b) R. Y. Tsien, Chem.
Eng. News, July 18, 1994, 72, 34–44.
6 R. H. Holm and E. I. Solomon, Chem. Rev., 2004, 104, 347–348.
7 G. Xue, P. B. Savage, J. S. Bradshaw, X. X. Zhang and R. M.
Izatt, Adv. Supramol. Chem., 2000, 7, 99–137.
8 A. Ikeda and S. Shinkai, Chem. Rev., 1997, 97, 1713–1734.
9 R. P. Cheng, S. L. Fisher and B. Imperiali, J. Am. Chem. Soc.,
1996, 118, 11349–11356.
10 A. Torrado, G. K. Walkup and B. Imperiali, J. Am. Chem. Soc.,
1998, 120, 609–610.
11 S. Bhattacharya and M. Thomas, Tetrahedron Lett., 2000, 41,
10313–10317.
12 N. Jotterand, D. A. Pearce and B. Imperiali, J. Org. Chem., 2001,
66, 3224–3228.
13 K. A. Koch, M. Marjorette, O. Pena and D. J. Thiele, Chem. Biol.,
1997, 4, 549–560.
14 L. Rulısek and Z. Havlas, J. Am. Chem. Soc., 2000, 122,
´
10428–10439.
15 B. P. Hay and R. D. Hancock, Coord. Chem. Rev., 2001, 212,
61–78.
16 R. Kramer, Angew. Chem., Int. Ed., 1998, 37, 772–773.
17 M. Royzen, Z. Dai and J. W. Canary, J. Am. Chem. Soc., 2005,
127, 1612–1613.
Fluorescence titrations
18 Z. Xu, X. Qian and J. Cui, Org. Lett., 2005, 7, 3029–3032.
19 P. Comba and W. Schiek, Coord. Chem. Rev., 2003, 238, 21–29.
20 B. P. Hay, A. A. Oliferenko, J. Uddin, C. Zhang and T. K. Firman,
J. Am. Chem. Soc., 2005, 127, 17043–17053.
21 W. Yang, H. He and D. G. Drueckhammer, Angew. Chem., Int.
Ed., 2001, 40, 1714–1718.
22 H. Huang and D. G. Drueckhammer, Chem. Commun., 2005,
5196–5198.
23 H. Huang and D. G. Drueckhammer, Chem. Commun., 2006,
2995–2997.
The fluorescence titrations were performed using an Aminco-
Bowman Series2 Spectrometer at 20 1C. The emission spec-
trum was scanned from 325–390 nm with excitation at 290 nm.
Spectra were obtained for the solution of 4 (3.26 ꢁ 10ꢂ5 M,
3.1 mL) in 1 : 1 methanol : tetrahydrofuran and after two
successive additions of 9 mL and six additions of 10 mL of a
2.65 ꢁ 10ꢂ3 M CuCl2 solution and of 10 mL of a 7.04 ꢁ 10ꢂ3
M CuCl2 solution, all in 1 : 1 methanol : tetrahydrofuran. For
data fitting, the fluorescence intensity at 357 nm was used.
NiCl2 solution (17.4 mM) was added in six 5 mL portions
followed by five 10 mL portions to a solution of 4 (3.0 mL, 32.6
mM) in 1 : 1 methanol : tetrahydrofuran. CoCl2 titrations were
performed by addition to a solution of 4 (2.09 ꢁ 10ꢂ5 M, 2.9
mL) of 1.04 ꢁ 10ꢂ3 M CoCl2 solution (6 ꢁ 20 mL, 4 ꢁ 40 mL)
followed by 1.04 ꢁ 10ꢂ2 M CoCl2 solution (2 ꢁ 10 mL, 2 ꢁ 20
mL). Zinc triflate titrations were performed by addition to a
24 G. Lauri and P. A. Bartlett, J. Comput. Aided Mol. Des., 1994, 8,
51–66.
25 (a) P. A. Bartlett, G. T. Shea, S. J. Telfer and S. Waterman, in
Molecular Recognition: Chemical and Biological Problems, ed. S.
M. Roberts, Royal Society of Chemistry, London, 1989, pp.
182–196; (b) P. A. Bartlett, F. A. Etzkorn, T. Guo, G. Lauri, K.
Liu, M. Lipton, B. P. Morgan and G. T. Shea, in Chemistry at the
Frontiers of Medicine, Proceedings of the Robert A. Welch Founda-
tion Conference on Chemical Research XXXV, The Robert A.
Welch Foundation, Houston TX, 1992, pp. 45–68.
26 A. G. Orpen, L. Brammer, F. H. Allen, O. Kennard, D. G. Watson
and R. Taylor, J. Chem. Soc., Dalton Trans., 1989, S1–S83.
27 T. C. Thurber, A. Prince and O. Halpern, J. Heterocycl. Chem.,
1982, 19, 961–965.
solution of 4 (2.09 ꢁ 10ꢂ5 M, 3.0 mL) of 8.6 ꢁ 10ꢂ3
Zn(OTf)2 solution (6 ꢁ 5 mL, 3 ꢁ 10 mL, 20 mL).
M
28 H. D. H. Showalter, M. M. Angelo, E. M. Berman, G. D. Kanter,
D. F. Ortwine, S. G. Ross-Kesten, A. D. Sercel, W. R. Turner, L.
M. Werbel, D. F. Worth, E. F. Elslager, W. R. Leopold and J. L.
Shillis, J. Med. Chem., 1988, 31, 1527–1539.
29 R. Kirchlechner, M. Casutt, U. Heywang and M. W. Schwarz,
Synthesis, 1994, 247–248.
30 D. V. Kosynkin and J. M. Tour, Org. Lett., 2001, 3, 991–992.
31 T. Ishiyama, T. M. Murata and N. Miyaura, J. Org. Chem., 1995,
60, 7508–7510.
Acknowledgements
Financial support from the National Science Foundation
(CHE0213457) is gratefully acknowledged. NMR facilities
were supported by a grant from the National Science Founda-
tion (CHE0131146).
32 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457–2483.
33 F. Firooznia, C. Gude, K. Chan, N. Marcopulos and Y. Satoh,
Tetrahedron Lett., 1999, 40, 213–216.
References
1 (a) A. W. Czarnik, Acc. Chem. Res., 1994, 27, 302–308; (b) A. W.
Czarnik, Chem. Biol., 1995, 2, 423–428.
34 I. Azumaya, T. Okamoto, F. Imabeppu and H. Takayanagi,
Tetrahedron, 2003, 59, 2325–2331.
2 L. Prodi, F. Bolletta, M. Montalti and N. Zaccheroni, Coord.
Chem. Rev., 2000, 205, 59–83.
35 I. Azumaya, T. Okamoto, F. Imabeppu and H. Takayanagi,
Heterocycles, 2003, 60, 1419–1424.
3 K. R. Gee, Z.-L. Zhou, D. TonThat, S. L. Sensi and J. H. Weiss,
Cell Calcium, 2002, 31, 245–251.
36 E. Belgodere, R. Bossio, V. Parrini and R. Pepino, J. Heterocycl.
Chem., 1982, 19, 561–566.
4 A. C. Benniston, A. Harriman, D. J. Lawrie, A. Mayeux, K.
Rafferty and O. D. Russell, Dalton Trans., 2003, 4762–4769.
37 R. M. Smith and A. E. Martell, Critical Stability Constants,
Amines, Plenum Press, New York, 1975, vol. 2, p. 235.
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
1730 | New J. Chem., 2006, 30, 1725–1730