Selective recognition of Zn2+ by salicylaldimine appended triazole-linked di-derivatives of calix[4]arene by enhanced fluorescence emission in aqueous-organic solutions: role of terminal -CH2OH moietieefs in conjunction with the imin

Article abstract of DOI:10.1016/j.tetlet.2009.03.126

A series of lower rim 1,3-di-derivatives possessing Schiff's base cores were synthesized using triazole unit as linker moiety by further introducing butyl (L₂), one -CH₂OH (L₃) and two -CH₂OH (L₄)-con

Full text of DOI:10.1016/j.tetlet.2009.03.126

Tetrahedron Letters 50 (2009) 2730–2734
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Selective recognition of Zn2+ by salicylaldimine appended triazole-linked
di-derivatives of calix[4]arene by enhanced fluorescence emission
in aqueous-organic solutions: role of terminal –CH2OH moieties in conjunction
with the imine in recognition q
*
Rakesh K. Pathak, Sk. Md. Ibrahim, Chebrolu P. Rao
Bioinorganic Laboratory, Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400 076, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of lower rim 1,3-di-derivatives possessing Schiff’s base cores were synthesized using triazole unit
as linker moiety by further introducing butyl (L2), one –CH2OH (L3) and two –CH2OH (L4)-containing moi-
eties, respectively, in order to bring additional support for ion binding. Based on fluorescence and absorp-
tion spectroscopies it has been shown that Zn2+ could be selectively recognized by the Schiff’s base core
and not by the triazole core among the ten metal ions studied both in methanol and in aqueous solutions
of methanol and acetonitrile, wherein the –CH2OH moieties augment the fluorescence response by pro-
viding additional coordinations to the Zn2+. Thus L4 exhibited a fluorescence enhancement of ꢀ65, ꢀ48
and ꢀ25-fold in methanol, aqueous solutions of methanol and acetonitrile, with minimum detection lim-
its of 174, 313 and 320 ppb, respectively. Both the excitation and emission wavelengths fall in visible
region.
Received 20 November 2008
Revised 16 March 2009
Accepted 18 March 2009
Available online 21 March 2009
Keywords:
Triazole-linked Schiff’s base appended 1,3-
di-derivatives of calix[4]arene
Selective recognition of Zn2+
Fluorescence enhancement
Augmentation of fluorescence response by –
CH2OH
Ó 2009 Elsevier Ltd. All rights reserved.
Zn2+ is one of the most abundant transition metal ions present in
living cells, including those from human, by exhibiting functional as
well as structural roles.1 Cellular studies2a showed that many mam-
malian organs including brain2b,c, pancreas2d,e and prostate2f accu-
mulate pools of labile Zn2+ which has been shown by imaging. A
Zn2+ metabolic disorder is closely associated with several neurolog-
ical diseases.3 To expound the biological role of Zn2+ which is spec-
troscopically and magnetically silent, it is necessary to have a direct
recognition method. Different families of fluorescent Zn2+ probes
have been reported in the literature on the basis of small as well
as large supramolecular systems.4–6 Supramolecular systems con-
taining calixarenes have received much attention as basic molecu-
lar scaffolds for building appropriate binding core suitable for ion
recognition as well as for host guest chemistry.7 There are a number
of reports available for selective recognition of Zn2+ by other
systems5,6, but calix[4]arene-based examples are rather limited.8
Recently, we reported calix[4]arene based Zn2+ and Hg2+ chemosen-
sors.9 In the present Letter, we report the synthesis, characteriza-
tion and fluorescence recognition properties of a triazole-linked
calix[4]arene derivatives of salicylaldimines towards Zn2+. Such
Schiff’s base conjugates have been developed owing to their good
binding capacity towards metal ions by exhibiting appropriate
changes in their absorption and emission properties. A series of re-
lated derivatives, namely, L1, L2, L3 and L4, have been synthesized in
order to demonstrate the binding ability of the terminal –CH2OH
groups in conjunction with the imine moiety towards metal ions
in general and Zn2+ in particular. L1, not possessing Schiff’s base
core, has been used as a control molecule in these studies.
In order to demonstrate the selective recognition of Zn2+, a ser-
ies of salicylaldimine appended triazole-linked calix[4]arene di-
derivatives (namely, L2, L3 and L4) have been synthesized (SI 01)
by going through a number of steps10 as shown in Scheme 1 and
the binding cores of the conjugates have been marked in Figure
1. A control molecule that possesses only triazole moiety (L1) has
also been synthesized (SI 01). All the molecules were characterized
by various spectral techniques such as 1H and 13C NMR, ESI MS and
elemental analysis (SI 01, SI 02) and were found to be in cone con-
formation based on NMR spectral data. These conjugates have two
different metal ion binding cores, namely, one with the preorga-
nized Schiff base N2O4 and the other with the lower rim pheno-
lic–OH plus triazole nitrogens. Thus L2, L3 and L4 exhibit two
binding cores, whereas L1 exhibits only one (Fig. 1) as expected.
Recognition of metal ions, namely, Mg2+, Ca2+, Fe2+, Co2+, Ni2+
,
Cu2+, Zn2+, Cd2+, Hg2+ and Pb2+, by these conjugates has been stud-
ied by fluorescence and absorption spectroscopies (SI 03) besides
addressing the role of the solvent. In methanol, L4 exhibited a weak
q
We wish to dedicate this paper to Professor C.N.R. Rao on his 75th birthday.
* Corresponding author. Tel.: +91 22 2576 7162; fax: +91 22 2572 3480.
E-mail addresses: cprao@iitb.ac.in, cprao@chem.iitb.ac.in (C.P. Rao).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.126

R. K. Pathak et al. / Tetrahedron Letters 50 (2009) 2730–2734
2731
R2
R3
R2
R3
R1
R1
O
N
O
N
HO
OH
HO
OH
R
R1
H
R2
H
R3
(L2) t-butyl
(L3) t-butyl
CH2CH2CH3
CH2OH
H
H
(L4) t-butyl CH3
CH2OH CH2OH
N
N
N
N
N
N
N
N
N
N
N
N
OH
OH
O
OH
R
O
OH
OH
OH
OH
R
OH
R
O
OHOH
O
O
O
(c)
(b)
(a)
R
1
R
2
R R
3
R
R
R
R
R
R
R
R
R
Scheme 1. Synthesis of lower rim calix[4]arene-1,3-di-derivatives through triazole link: (a) propargyl bromide, K2CO3, acetone, reflux; (b) 4-azidomethyl salicylaldehyde;
CuSO4Á5H2O and sodium ascorbate in dichloromethane/water (1:1), rt, 12 h; (c) appropriate amine, methanol or ethanol, rt, 4 h. R = tert-butyl.
OH
OH
N
OH
OH
OH
OH
OH
OH
OH
N
N
N
N
N
OH
OH
OH
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
O
OH
O
OH
OHOH
O
O
O
OH
O
OH
OH
OH
O
O
R
L3
R
R
L4
R
R
R
R
R
R
R
R
R
R
R
R
R
L1
L2
Figure 1. Schematic representation of the structures of L1, L2, L3 and L4. Binding cores are shown by encircling.
emission at ꢀ440 nm when excited at 320 nm or at 390 nm. When
base.9a Based on the fluorescence data, the association constant for
L4 with Zn2+ was found to be 37700 500 using Benesi–Hilderand
equation. The minimum concentration of Zn2+ that can be detected
by L4 was found to be 174 ppb in methanol (SI 05). On the other
hand, L4 exhibited almost no change or little quenching in fluores-
cence intensity in the presence of other metal ions (Fig. 2b). To fur-
ther explore the selectivity of L4 towards Zn2+ over Cd2+, two types
of competitive metal ion titrations were carried out in methanol
solution. While in one, it is the Zn2+ bound L4 that was titrated with
Cd2+,namely, {L+10 equiv Zn2+} versus Cd2+, in the second case it
L4 is titrated with M2+ in CH3OH, a progressive fluorescence
enhancement was observed with Zn2+ and the enhancement
reaches 65 5-fold at saturation that occurs at ꢀ5 equiv (Fig. 2)
owing to the chelation followed by prevention of the photoelectron
transfer. Zn2+ titrations have been repeated by exciting the solu-
tions at 390 nm and measuring the fluorescence spectra, and found
an enhancement of >50 fold (SI 04). The number of fold of fluores-
cence enhancement observed in the present case is much higher
than that observed with a calixarene-based naphthalidene Schiff’s
900
80
60
40
20
0
a
b
750
600
450
300
150
0
0
1
2
3
4
5
350 400 450 500 550 600
[Mn+]/[L]
Wavelength (nm)
Figure 2. Fluorescence titration data for L4 with metal ions in methanol (kex = 320 nm) (a) spectral traces obtained during the titration of L4 with Zn2+; (b) relative
fluorescence intensity ratio (I/I0) as a function of [M2+]/[L4] mole ratio for different metal ions; symbols correspond to j = Fe2+; N = Co2+; . = Ni2+; J = Cu2+, I = Zn2+; ꢀ = Cd2+
s = Hg2+; h = Ca2+; d = Mg2+ and H = Pb2+
;
.

2732
R. K. Pathak et al. / Tetrahedron Letters 50 (2009) 2730–2734
a
b
c
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.20
0.15
0.10
0.05
0.00
2.0
402nm
348nm
305nm
268nm
1.6
1.2
0.8
0.4
0.0 0.5 1.0 1.5 2.0 2.5 3.0
200 250 300 350 400 450
Wavelength (nm)
0.0
0.2
0.6
0.8
1.0
0.4n
2+
[Zn ]/[L ]
m
4
Figure 3. Absorption titration data for L4 with Zn2+ in methanol: (a) spectral traces obtained during the titration; (b) absorbance versus [Zn2+]/[L4]; (c) Job’s plot of nm verses
*
A
nm where nm is mole fraction of the metal ion added and A is absorbance.
was the reverse type of titration, namely, {L+20 equiv Cd2+} by
Zn2+. Based on the two titrations, it was noticed that Cd2+ has neg-
ligible affect on Zn2+ sensing by L4 as can be seen from the fluores-
cence intensity ratio plots (SI 06).
moieties resulted in binding and recognition of Zn2+ and Cd2+ with
no selectivity to either, which is attributable to the orientation of
the two pyrene moieties8c in presence of the metal ion. Compari-
son of these two cases with the present case, namely, L1, suggests
that the orientation of the triazole portions is not well suited for
Zn2+ binding. Hence, the Zn2+ goes to the Schiff’s base binding core
rather than to the triazole core.
Binding of Zn2+ with L4 in methanol has been further shown by
the changes observed in the bands, namely, 269, 305, 348 and
405 nm, in their absorption spectra (Fig. 3a and b) that arise from
the imine in conjugation with the phenolic moieties. The isosbestic
points observed at 261, 295, 326 and 382 nm were indicative of the
transition between the unbound and Zn2+ complexed species.
These results are clearly suggestive of the binding of Zn2+ at the
Schiff’s base core. Stoichiometry of the complex formed between
L4 and Zn2+ was found to be 1:1 based on Job’s plot obtained using
the absorption data (Fig. 3c). The 1:1 complexation has also been
further confirmed by carrying out 1H NMR titration of L4 with
Zn2+ which found that the intensity of the Schiff’s base sal–OH
and the –CH2OH protons decrease by 50% and 25%, respectively,
indicating the disappearance of one proton each from sal–OH
In order to further reconfirm the binding of Zn2+ at the Schiff’s
base core, fluorescence studies were carried out with L3 and L2
(Fig. 1). While L3 has one –CH2OH function, L2 does not, but both
of these contain Schiff’s base core. Similar studies carried out with
these molecules exhibited a fluorescence enhancement of ꢀ25 and
ꢀ10-fold, respectively, for L3 and L2 when titrated with Zn2+ in
CH3OH (SI, 08). Comparison of the fluorescence enhancement fold
observed with L4, L3 and L2 towards Zn2+ (Fig. 4a) clearly suggested
the involvement of –CH2OH in binding Zn2+ at the Schiff’s base
core. Even the absorption titration indicated changes commensu-
rate with the number of –CH2OH groups available, wherein such
changes follow an order, namely, L4 > L3 > L2 (Figs. 3a and 4b–d).
Of course, no change was observed in the absorption spectra of
L1 when titrated with Zn2+ (SI 07). Thus the results of both the fluo-
rescence and absorption studies not only indicated binding zone
for Zn2+ to be the Schiff’s base core but also highlighted the role
of the presence of terminal –CH2OH moiety and the support of
these groups in forming a six-coordinated Zn2+ (as in L4 and L3)
species in place of a four-coordinated one otherwise expected with
L2. Five- and six-coordinated Zn2+ species have already been shown
by crystal structures of simple derivatives possessing such moie-
ties wherein the 5th and 6th coordinations indeed are supported
by the presence of –CH2OH groups.11 Thus, L4 is highly selective to-
wards Zn2+, the sensitivity of these molecules towards Zn2+ follow
an order, namely, L4 ꢁ L3 > L2 based on both the fluorescence and
and –CH2OH moieties which in turn involve in binding to Zn2+
.
In order to cross check whether the Zn2+ is going to the triazole
core or not, fluorescence studies were carried out using a control
molecule, namely, L1, which possesses only the triazole core and
not the Schiff’s base core (Fig. 1) and found no change in the fluo-
rescence intensity even at high equivalents of Zn2+. Even the Zn2+
titration of L1, by measuring absorption spectra, exhibited no isos-
bestic point suggesting a non-binding nature of L1 towards Zn2+
.
Thus, both the fluorescence and absorption spectral studies (SI
07) clearly indicated that triazole core is not congenial for Zn2+
binding, at least under the present conditions. When a benzyl moi-
ety is present on a lower rim calix[4]arene of 1,3-di-derivative sim-
ilar to L1, the derivative seems to bind to Ca2+ at about 10 equiv.6b
.
On the other hand, the presence of pyrene groups in place of benzyl
d
c
a
b 2.5
2.5
70
60
50
40
30
20
10
0
2.5
2.0
1.5
1.0
0.5
2.0
1.5
1.0
0.5
0.0
2.0
1.5
1.0
0.5
250
300
350
400
450
250
300
350
400
450
0
2
4
6
8
10
250 275 300 325 350
Wavelength (nm)
[Zn2+]/[L ]
Wavelength (nm)
Wavelength (nm)
x
Figure 4. (a) Plot of I/I0 as a function of [Zn2+]/[L] mole ratio with all four ligands L1, L2, L3 and L4 in methanol. The symbols correspond to N = L4; s = L3; H = L2 and d = L1.
Absorption spectral traces during the titration with Zn2+: (b) L3; (c) L2; (d) L1.

R. K. Pathak et al. / Tetrahedron Letters 50 (2009) 2730–2734
2733
a
b
c
50
40
30
20
10
0
25
75
20
15
10
5
60
45
30
15
0
0
L2
L3
L4
2+
2+
2+
0
2
4
6
8
10 12 14 16
0
2
4
6
8
10 12
L + Zn
4
L + Zn
3
L + Zn
2
[Zn2+]/[Lx]
[Zn2+]/[Lx]
Figure 5. (a) Plot of I/I0 versus [Zn2+]/[Lx] mole ratio with L2, L3 and L4 in aqueous methanol. (b) Plot of I/I0 versus [Zn2+]/[L] mole ratio with L2, L3 and L4 in aqueous
acetonitrile. The symbols carry same meaning as in Fig. 4a. (c) Histogram showing the fluorescence intensity enhancement fold in presence of Zn2+ in methanol (completely
filled), in aqueous methanol (dashed) and in aqueous acetonitrile (unfilled). Error bars were calculated by repeating these experiments three to six times.
absorption studies. However, L1 is not sensitive towards Zn2+ ow-
ing to the absence of any Schiff’s base core. Even the L2 and L3 have
also been subjected to the titration with other M2+ ions which was
already studied with L4 and found results similar to those observed
with L4, i.e., none other than Zn2+ shows any detectable fluores-
cence change (SI 09).
pattern. While the short-lived species (
have almost the same life-times (1.04 ns, 38% for L2; 1.14 ns, 85%
for L3; 1.18 ns, 75% for L4), their long lived counterparts ( 2) exhibit
s1) of the free receptors
s
increasing life times on going from L2 (4.10 ns, 62%) to L3 (4.32 ns,
15%) to L4 (5.50 ns, 25%) indicating the involvement of the added –
CH2OH group(s) in the latter two cases. When Zn2+ is added to L2,
s1 increases by a factor of two (2.14 ns, 30%), whereas s2 (4.26 ns,
70%) does not change owing to the absence of any –CH2OH moiety
in L2. However, when –CH2OH groups are present, as in L3 and L4
(Fig. 1), the s2 exhibit substantial increase (6.57 ns, 47% in case
of L3; 7.17 ns, 60% in case of L4) though the changes observed in
the s1 were similar to those observed for L2 (namely, 2.51 ns, 53%
in case of L3; 2.48 ns, 40% in case of L4). Thus the fluorescence life
time measurements of these receptors and their Zn2+ bound spe-
cies have clearly shown the involvement of –CH2OH moieties in
the ion recognition.
Among the moieties present in the molecules reported in this
Letter, it has been found that the triazole core is not effective for
binding to metal ions, whereas the Schiff’s base core exhibits high
binding affinity towards the metal ions in general and Zn2+ in par-
ticular by exhibiting a large fluorescence enhancement. The sensi-
tivity towards fluorescence response as well as selectivity of these
molecules towards Zn2+ has been further augmented by the pres-
ence of the terminal –CH2OH groups in a positive manner as sup-
ported by steady state as well as life-time fluorescence studies in
addition to the absorption and 1H NMR data. The number of folds
of fluorescence enhancement was found to be dependent on the
Zn2+ solvation ability of the medium. Maximum sensitivity and
selectivity were observed with L4. The sensitivity of L4 towards
Zn2+ has always been higher in methanol than in aqueous metha-
nol followed by aqueous acetonitrile. Preferential binding of Zn2+
to these molecules, namely, L4 >>> L3 > L2, has been clearly demon-
Since L2, L3 and L4 have shown differential fluorescence
enhancement for Zn2+ in methanol medium, titrations were carried
out even in aqueous methanol (20:80) (SI 10) and in aqueous ace-
tonitrile (1:1) (SI 11). Even the studies of aqueous solutions of
methanol and acetonitrile resulted in a similar trend in the fluores-
cence enhancement for Zn2+ recognition, though the number of
folds itself is less than that observed in methanol, owing to a higher
solvation of the organic and inorganic components in water
(Fig. 5). Thus, Zn2+ can be detected selectively among ten divalent
metal ions studied even in both these aqueous solutions by L4 with
a fluorescence enhancement of about ꢀ48-fold and ꢀ25-fold,
respectively, and with a minimum detection limit of 313 and
320 ppb, respectively, (SI 05). Hence L4 can be a sensor for Zn2+
.
The titration of L4 with Zn2+ has been repeated at 390 nm excita-
tion and similar results were found(SI 12). Absorption spectral
changes observed in the titration of L4, L3 or L2 with Zn2+ in aque-
ous methanol (SI 10) and aqueous acetonitrile (SI 11) were similar
to those observed in methanol solution. In aqueous solutions, the
sensitivity of these towards Zn2+ follows L4 >>> L3 > L2, a trend that
was also observed in methanol.
The Zn2+ recognition has been further supported by measuring
the fluorescence life-times of the species formed by time-corre-
lated single photon count (TCSPC) during the titration (Fig. 6).
The TCSPC data obtained in case of simple receptor molecules,
namely, L2, L3 and L4, as well as their Zn2+ bound forms can be fit-
ted to two species (s1 and
s
2) owing to their bi-exponential decay
a5000
b
c
5000
5000
4000
3000
2000
1000
0
4000
3000
2000
1000
0
4000
3000
2000
1000
0
0
10 20 30 40 50
0
10
20
30
40
50
0
10
20
30
40
50
Time (ns)
Time (ns)
Time (ns)
Figure 6. Fluorescence decay plots as a function of time using different molecules: (a) L4; (b) L3 and (c) L2. In each plot: prompt (black); molecule (blue); molecule+Zn2+ (red).

2734
R. K. Pathak et al. / Tetrahedron Letters 50 (2009) 2730–2734
Chem. Biol. 2008, 4, 168; (e) Que, E. L.; Domaille, D. W.; Chang, C. J. Chem.
Rev. 2008, 108, 1517; (f) Ikeda, A.; Shinkai, S. Chem. Rev. 1997, 97, 1713; (g)
Rurack, K.; Resch-Genger, U. Chem. Soc. Rev. 2002, 31, 116; (h) de Silva, A.
P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.;
Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515; (i) de Silva, A. P.;
Fox, D. B.; Huxley, A. J. M.; Moody, T. S. Coord. Chem. Rev. 2000, 205, 41; (j)
Rurack, K. Spectrochim. Acta, Part A 2001, 57, 2161; (k) Callan, J. F.; de Silva,
A. P.; Magria, D. C. Tetrahedron 2005, 61, 8551.
strated. The ꢀ48 and ꢀ25-fold fluorescence enhancement ob-
served for L4 upon Zn2+ binding in aqueous solutions of methanol
and acetonitrile, respectively, is considerably high and are indica-
tive of the utility of L4 for Zn2+ recognition even under the condi-
tions of aqueous solutions. This is further augmented by the fact
that the excitation and emission studies were both in the visible
region. Therefore, L4 is selective towards Zn2+ in organic as well
as in aqueous organic solutions.
5. (a) Huang, S.; Clark, R. J.; Zhu, L. Org. Lett. 2007, 9, 4999; (b) Liu, Y.; Zhang, N.;
Chen, Y.; Wang, L.-H. Org. Lett. 2007, 9, 315; (c) Zhang, Y.; Guo, X.; Si, W.; Jia, L.;
Qian, X. Org. Lett. 2008, 10, 473; (d) Jiang, W.; Fu, Q.; Fan, H.; Wang, W. Chem.
Commun. 2008, 259; (e) Gunnlaugsson, T.; Lee, T. C.; Prakesh, R. Org. Biomol.
Chem. 2003, 1, 3265; (f) Wu, D.-Y.; Xie, L.-X.; Zhang, C.-L.; Duan, C.-Y.; Zhao, Y.-
G.; Guo, Z.-J. Dalton trans 2006, 3528; (g) Jiaobing, W.; Xiao, Y.; Zhang, Z.; Qian,
X.; Yanga, Y.; Xu, Q. J. Mater. Chem. 2005, 15, 2836; (h) Mikata, Y.; Wakamatsu,
M.; Kawamura, A.; Yamanaka, N.; Yano, S.; Odani, A.; Morihiro, K.; Tamotsu, S.
Inorg. Chem. 2006, 45, 9262; (i) Nolan, E. M.; Jaworski, J.; Okamoto, K.-I.;
Hayashi, Y.; Sheng, M.; Lippard, S. J. J. Am. Chem. Soc. 2005, 127, 16812; (j)
Nolan, E. M.; Burdette, S. C.; Harvey, J. H.; Hilderbrand, S. A. Inorg. Chem. 2004,
43, 2624; (k) Burdette, S. C.; Frederickson, C. J.; Bu, W.; Lippard, S. J. J. Am. Chem.
Soc. 2003, 125, 1778.
Acknowledgements
CPR acknowledges the financial support from DST, CSIR and
DAE-BRNS. R.K.P. and Sk.M.I. acknowledge CSIR for their
fellowships.
Supplementary data
6. (a) Aoki, S.; Sakurama, K.; Matsuo, N.; Yamada, Y.; Takasawa, R.; Tanuma, S.-i.;
Shiro, M.; Takeda, K.; Kimura, E. Chem. Eur. J. 2006, 12, 9066; (b) Chang, K.-C.;
Su, I.-H.; Leeb, G.-H.; Chung, W.-S. Tetrahedron Lett. 2007, 48, 7274; (c) Chang,
K.-C.; Su, I.-H.; Senthilvelan, A.; Chung, W.-S. Org. Lett. 2007, 9, 3363; (d) Shults,
M. D.; Pearce, D. A.; Imperiali, B. J. Am. Chem. Soc. 2003, 125, 10591; (e) van
Dongen, E. M.; Evers, T. H.; Dekkers, L. M.; Meijer, E. W.; Klomp, L. W. J.; Merkx,
M. J. Am. Chem. Soc. 2007, 129, 3494.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.03.126.
References and notes
7. (a) Kim, J. S.; Quang, D. T. Chem. Rev. 2007, 107, 3780; (b) Wagner, B. D. Curr.
Anal. Chem. 2007, 3, 183.
1. Jiang, P.; Guo, Z. Coord. Chem. Rev. 2004, 248, 205. and references cited therein.
2. (a) Chang, C. J.; Jaworski, J.; Nolan, E. M.; Sheng, M.; Lippard, S. J. Proc. Natl.
Acad. Sci. U.S.A. 2004, 101, 1129; (b) Frederickson, C. J.; Suh, S. W.; Koh, J.-Y.;
Cha, Y. K.; Thompson, R. B.; LaBuda, C. J.; Balaji, R. V.; Cuajungco, M. P. J.
Histochem. Cytochem. 2002, 50, 1659; (c) Danscher, G.; Stoltenberg, M. J.
Histochem. Cytochem. 2005, 53, 141; (d) Zalewski, P. D.; Millard, S. H.; Forbes, I.
J.; Kapaniris, O.; Slavotinek, A.; Betts, W. H.; Ward, A. D.; Lincoln, S. F.;
Mahadevan, I. J. Histochem. Cytochem. 1994, 42, 877; (e) Lukowiak, B.;
Vandewalle, B.; Riachy, R.; Conte, J.-K.; Gmyr, V.; Belaich, S.; Lefebvre, J.;
Pattou, F. J. Histochem. Cytochem. 2001, 49, 519; (f) Sorensen, M. B.; Stoltenberg,
M.; Juhl, S.; Danscher, G.; Ernst, E. Prostate 1997, 31, 125.
3. (a) Cuajungco, M. P.; Lees, G. J. Neurobiol. Dis. 1997, 4, 137; (b) Bush, A. I.; Tanzi,
R. E. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 7317.
4. (a) Kimura, E.; Koike, T. Chem. Soc. Rev. 1998, 27, 179; (b) Lim, N. C.;
Freake, H. C.; Bruckner, C. Chem. Eur. J. 2005, 11, 38; (c) Barrios, A. M. ACS
Chem. Biol. 2006, 1, 67; (d) Domaille, D. W.; Que, E. L.; Chang, C. J. Nat.
8. (a) Cao, Y.-D.; Zheng, Q.-Y.; Chen, C.-F.; Huang, Z.-T. Tetrahedron Lett. 2003, 44,
4751; (b) Bagatin, I. A.; De Souza, E. S.; Ito, A. S.; Toma, H. E. Inorg. Chem.
Commun. 2003, 6, 288; (c) Park, S. Y.; Yoon, J. H.; Hong, C. S.; Souane, R.; Kim, J.
S.; Matthews, S. E.; Vicens, J. J. Org. Chem. 2008, 73, 8212; (d) Unob, F.; Asfari, Z.;
Vicens, J. Tetrahedron Lett. 1998, 39, 2951.
9. (a) Dessingou, J.; Rao, C. P.; Joseph, R. Tetrahedron Lett. 2005, 46, 7967; (b)
Joseph, R.; Ramanujam, B.; Pal, H.; Rao, C. P. Tetrahedron Lett. 2008, 49, 6257; (c)
Joseph, R.; Ramanujam, B.; Acharya, A.; Khutia, A.; Rao, C. P. J. Org. Chem. 2008,
73, 5745.
10. (a) Angyal, S. J.; Morris, P. J.; Tetaz, J. R.; Wilson, J. G. J. Chem. Soc. 1950, 2141;
(b) Cort, A. D.; Mandolini, L.; Pasquini, C.; Schiaffino, L. Org. Biomol. Chem. 2006,
4, 4543; (c) Ayala, V.; Iglesias, M.; Rincon, J. A.; Sànchez, F. J. Catal. 2004, 224,
170.
11. Dey, M.; Rao, C. P.; Saarenketo, P.; Rissanen, K.; Kolehmainen, E. Eur. J. Inorg.
Chem. 2002, 2207.