Amidofluorene-appended lower rim 1,3-diconjugate of calix[4]arene: Synthesis, characterization and highly selective sensor for Cu2+

Article abstract of DOI:10.3762/bjoc.12.163

Functionalization of calix[4]arene with amidofluorene moieties at the lower rim led to formation of the 1,3-diconjugate of calix[4]arene L as a novel fluorescent chemosensor for Cu²⁺. The receptor molecule L exhibited a pronounced selectivity towards Cu²⁺ over other mono and divalent ions. The formation of the complex between L and Cu²⁺ was evaluated by absorption, fluorescence and ¹H NMR spectroscopy. The sensor L showed a remarkable color change from colorless to purple and a fluorescence quenching only upon interaction with Cu²⁺. The 1:1 stoichiometry of the obtained complex has been determined by Job's plot. The association constant determined by fluorescence titration was found to be 1.8 × 106 M^-1. The sensor showed a linear response toward Cu²⁺ in the concentration range from 1 to 10 μM with a detection limit of 9.6 × 10^-8 M.

Full text of DOI:10.3762/bjoc.12.163

Amidofluorene-appended lower rim 1,3-diconjugate of
calix[4]arene: synthesis, characterization and
highly selective sensor for Cu2+
Rahman Hosseinzadeh*1, Mohammad Nemati1, Reza Zadmard2
and Maryam Mohadjerani3
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 1749–1757.
1Department of Organic Chemistry, Faculty of Chemistry, University of
Mazandaran, Babolsar, Iran, 2Chemistry and Chemical Engineering
Research Center of Iran (CCERCI), Tehran, Iran and 3Department of
Molecular and Cell Biology, Faculty of Basic Science, University of
Mazandaran, Babolsar, Iran
doi:10.3762/bjoc.12.163
Received: 03 May 2016
Accepted: 14 July 2016
Published: 04 August 2016
Email:
Associate Editor: N. Sewald
Rahman Hosseinzadeh* - r.hosseinzadeh@umz.ac.ir
© 2016 Hosseinzadeh et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
calix[4]arene; chemosensor; copper ions; fluorene; fluorescence
Abstract
Functionalization of calix[4]arene with amidofluorene moieties at the lower rim led to formation of the 1,3-diconjugate of
calix[4]arene L as a novel fluorescent chemosensor for Cu2+. The receptor molecule L exhibited a pronounced selectivity towards
Cu2+ over other mono and divalent ions. The formation of the complex between L and Cu2+ was evaluated by absorption, fluores-
cence and 1H NMR spectroscopy. The sensor L showed a remarkable color change from colorless to purple and a fluorescence
quenching only upon interaction with Cu2+. The 1:1 stoichiometry of the obtained complex has been determined by Job’s plot. The
association constant determined by fluorescence titration was found to be 1.8 × 106 M−1. The sensor showed a linear response
toward Cu2+ in the concentration range from 1 to 10 µM with a detection limit of 9.6 × 10−8 M.
Introduction
Owing to the substantial role of fluorescent chemosensors in tyrosinase, etc. [5-9]. However, excess amounts of Cu2+ are
biological, environmental, and chemical processes, their design hazardous, due to generate reactive oxygen species (ROS) that
and synthesis, especially for detection of metal ions has at- disturbs cellular metabolism [10,11]. On the other hand, its defi-
tracted great attention in supramolecular chemistry [1-4]. ciency may cause haematological and neurological diseases
Copper is one of the crucial biological elements, which presents [12]. Therefore, the selective and sensitive measurement of
as catalytic cofactor for a variety of metallo-enzymes such as Cu2+ ions – either as significant environmental pollutant or an
superoxide dismutase, cytochrome c oxidase, lysyl oxidase and essential trace element in human body – is a great challenge and
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the development of synthetic receptors based on organic ligands 2 with 9H-fluoren-2-amine (4, obtained from nitration of fluo-
for the detection of Cu2+ seems to be necessary [13]. Accord- rene and reduction of the resulting 2-nitrofluorene, (3)) in the
ingly, during the last decades, synthesis and modification of presence of N,N-dicyclohexylcarbodiimide (DCC) in dichloro-
chemosensors based on supramolecular structures, especially methane at room temperature gave the desired receptor L in
calixarene derivatives, has been an appealing field for research 81% yield (Scheme 2). All the compounds including receptor L
[14]. Calixarene has been considered as an effective molecular has been characterized by various spectral techniques such as
scaffold in the improvement of fluorescent and chromogenic 1H, 13C NMR and high resolution mass spectrometry (HRMS)
sensors, especially for metal-ion recognition [14,15]. (see Supporting Information File 1, Figures S1–S11).
Calix[4]arene derivatives having different binding centers such
as nitrogen, oxygen and sulfur sites for recognition of metal General procedure for UV–vis experiments
ions can be readily synthesized [14,16,17]. A number of The evaluation of cation-ligand interaction was performed with
calix[4]arene-based fluorescence sensors for copper ions have UV–vis spectroscopy. The UV–vis measurement of receptor L
been reported in the literature [18-27], including calix[4]arene in CH3CN exhibited three absorption peaks at 282, 290, and
bearing anthraceneisoxazolymethyl [3], quinolone [28], 314 nm. Furthermore, the variation of UV–vis spectra was
5-nitrosalicylaldehyde [29], 3-alkoxy-2-naphthoic acid [30], monitored by adding 10 equivalents of different metal ions as
coumarine [31] and benzothiazole [10,15] groups.
perchlorate salts (Cu2+, Hg2+, Pb2+, Zn2+,Co2+, Ni2+, Cd2+,
Ag+, Ba2+, K+, Na+ and Li+).
In continuation of our studies on developing novel chemosen-
sors containing fluorenyl moieties as fluorogenic group [32-34], Unlike Cu2+ ions, the addition of 10 equiv of other metal ions to
we report here the design and synthesis of a new fluorene- the 1.0 × 10−5 M solutions of L resulted in no significant
appended 1,3-diconjugate of calix[4]arene (L), which possess changes in the absorption spectra of ligand L. However, addi-
distal amide groups on the lower rim of the conical framework tion of Cu2+ to L resulted in a blue shift of the absorption band
to recognize Cu2+ with high sensitivity and specificity in the area of 280–290 nm (see Supporting Information File 1,
(Scheme 1).
Figure S12). Furthermore, a weak and broad absorption band
from 600–800 nm was also observed (Figure 1). As shown in
Figure 2, there was an evident color change from colorless to
purple, which could be observed by the naked eye.
The selectivity of receptor L towards Cu2+ was precisely
assessed by UV–vis spectroscopy during titration with different
concentrations of Cu2+ from 0 to 100 equivalents in CH3CN. In
the absence of Cu2+ ions, the ligand L exhibited three absorp-
tion bands at ≈282, ≈290 and ≈314 nm (Figure 3). Upon addi-
tion of Cu2+, the absorption band at 282 nm showed a blue shift
along with an increase in peak intensity. The two observed isos-
bestic points at 295 and 318 nm upon addition of Cu2+ revealed
formation of a stable complex between L and the copper ion.
Moreover, upon addition of Cu2+, at higher concentrations of L
a broad signal appeared at the area of 600–800 nm which can be
related to a d→d transition (see Supporting Information File 1,
Figure S13). This result is in accordance with those reported in
literature [10] and indicates that the L binds copper as Cu2+.
Scheme 1: Fluorene appended 1,3-diconjugate of calix[4]arene.
Results and Discussion
Synthesis
The synthesis of 5,11,17,23-tetra-tert-butyl-25,27-di[(9H- Fluorescence titrations of L with metal ions
fluoren-2-yl)aminocarbonylmethoxy]-26,28-dihydroxy- Fluorescence spectroscopy was applied to investigate the recog-
calix[4]arene (L) is depicted in Scheme 1. The synthesis began nition properties of chemosensor L toward various metal ions in
with the reaction of p-tert-butylcalix[4]arene and ethyl bromo- MeCN. In the absence of metal ions, the receptor L represents
acetate in the presence of potassium carbonate in acetonitrile. fluorescence emission at ≈365 nm when excited at 280 nm.
Ester hydrolysis of compound 1 afforded the 5,11,17,23-tetra- Upon addition 10 equiv of Cu2+ to the MeCN solution of sensor
tert-butyl-25,27-di(hydroxycarbonylmethoxy)-26-28-di- L, a remarkable fluorescence quenching was observed
hydroxy calix[4]arene (2) in high yield. Coupling of this diacid (Figure 4). In order to explore the selectivity of sensor L, simi-
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Scheme 2: Synthesis route of fluorene-appended amido-linked 1,3-diconjugate of calix[4]arene L.
Figure 1: Absorption spectra of L (1.0 × 10−5 M) and its complexes with different metals (10 equiv) in MeCN.
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Figure 2: Color changes of receptor L upon addition of 10 equiv of various metal ions as their perchlorate salts.
Figure 3: Influence of the addition of increasing amounts (0–100 equiv) of Cu2+ on the absorption spectra of L (1.0 × 10−5 M) in MeCN.
lar experiments were carried out in the presence of other per-
chlorate salts of Cu2+, Hg2+, Pb2+, Zn2+, Co2+, Ni2+, Cd2+,
Ag+, Ba2+, K+, Na+ and Li+. The fluorescence spectra showed
almost no obvious change relative to the free ligand L
(Figure 4).
Titration of L with Cu2+ resulted in a gradual quenching of the
fluorescence emission at the 365 nm band as a function of in-
creasing Cu2+ concentration. The fluorescence intensity spec-
trum showed a gradual decrease up to 5 equiv of Cu2+. With
higher Cu2+ concentrations the fluorescence quenched com-
pletely (Figure 5a). The stoichiometry of the complex was
found to be 1:1 between L and Cu2+ based on the Job’s plot and
Figure 4: Fluorescence spectra of L (1.0 × 10−5 M) in MeCN upon ad-
dition of different metal ions (10 equiv) with an excitation at 280 nm.
its association constant was calculated using the Benesi–Hilde-
brand equation [35] to be 1.8 × 106 M−1 (see Supporting Infor-
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Figure 5: (a) Fluorescence spectra (1.0 × 10−5 M) of L recorded upon the addition of copper ion (0–5 equiv) in MeCN; excitation at 280 nm and emis-
sion at 365 nm. (b) Calibration curve of fluorescence intensity of L with Cu2+ ion concentrations.
mation File 1, Figures S14 and S15). From these results it can Competitive metal ion titrations
be concluded that the Cu2+ ion has a preferential interaction In order to examine the competitive recognition of Cu2+ by
with receptor L. It can be concluded from the results displayed sensor L, the effect of Cu2+ was studied in the presence of other
in Figure 5b that the fluorescence quenching intensity at 365 nm metal ions. The results showed that the fluorescence emission
has a linear relationship with Cu2+ concentration. The fluores- intensity of [L + Mn+] is altered in the presence of Cu2+ ions
cent response of L toward Cu2+ has a linear dynamic range (Figure 6). It is noteworthy that complex [L + Mn+] exhibited
from 1 to 10 µM with a detection limit of 9.6 × 10−8 M (see no considerable fluorescence variations in comparison with L,
Supporting Information File 1, Figure S16).
and addition of Cu2+ ions to the corresponding solutions
Figure 6: Fluorescence intensity of L (1.0 × 10−5 M) upon the addition of 10 equiv Cu2+ in the presence of 10 equiv of other metal ions (as perchlo-
rate salts) in MeCN.
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resulted in fluorescence quenching. Therefore, L can recognize were first broadened and then finally disappeared. Furthermore,
Cu2+ even in the presence of other metal ions in acetonitrile the signals of Ar–CH2 (Hd, He) and –O–CH2 (Hf) porotons,
solution.
which are close to the binding site of receptor L were affected
and downfield shifted by the complexation of L with Cu2+. A
detailed analysis of the 1H NMR spectra reveals the significant
1H NMR titration of L with Cu2+
In order to understand the mode of complexation of L with changes of almost all the other proton signals in the ligand. For
Cu2+, 1H NMR titrations were carried out (Figure 7). Upon the instance, the peak of aromatic ring hydrogen atoms (Hc,
initial addition of Cu2+ into the CD3CN solution of L, signals resonated at 7.35 ppm) was converted to two distinct peaks at
related to OH phenolic groups (Hb) and amidic protons (Ha) 7.20 and 6.95 ppm (Hc1, Hc2). In addition, signals belonging to
Figure 7: 1 H NMR (400 MHz, CD3CN) spectra of L upon addition of (a) 0.0 equiv, (b) 0.2 equiv, (c) 0.4 equiv, (d) 0.6 equiv, (e) 0.8 equiv, (f) 1.0
equiv, (g) 1.2 equiv and (h) 1.5 equiv of Cu2+.
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the fluorene moieties and tert-butyl groups, which are quite far
away from binding site, were also affected by complex forma-
tion. For example, the peaks of the tert-butyl groups of receptor
L resonated at 1.29 (Hh1) and 1.22 ppm (Hh2) turned to one
signal at 1.20 ppm during the titration.
Table 1: Comparison of lower rim 1,3-diconjugate derivatives of
calix[4]arene with chemosensor L.
Ligand
Ka (M)
LOD (M−1)
benzothiazole [15]
2-picolyl [36]
18893 ± 1200
30221 ± 1600
1.58 × 104
403 (ppb)
196 ppb
–
arylisoxazole [3]
benzimidazole [10]
fluorene (this work)
All these observations might be attributed to the changes in
electron density, the anisotropy effect and the changes in the
conformation of receptor L during the complex formation. The
fact that receptor L shows a sensitive recognition affinity to
7.24 × 109
96 ppb
96 ppb
1.8 × 106
copper ions and a much more obvious shift of 1H NMR peaks fluorescent probe for Cu2+ detection. The sensitivity and selec-
in the presence of Cu2+ is actually not surprising because of a tivity of chemosensor L toward Cu2+ has been investigated by
relatively rigid binding pocket of receptor L providing four sites spectroscopic methods. This sensor has a high affinity
of NH and OH which allows a suitable complex formation with (Ka = 1.8 × 106 M−1) for copper ions. The competitive metal-
a copper ion (Scheme 3).
ion titrations by fluorescence spectroscopy showed that Cu2+
can be selectively detected in the presence of other common
metal ions. It is noteworthy to mention that the chemosensor L
can be used as a “naked eye” indicator for Cu2+. Furthermore,
fluorescence quenching of L upon addition of Cu2+ showed a
detection limit of about 9.6 × 10−8 mol/L in the concentration
range from 1 to 10 µM. This result demonstrated the possibility
of quantitative detection of low levels of Cu2+ in MeCN by
using this chemosensor.
Experimental
Instruments and reagents
1H and 13C NMR spectra were recorded at 400 and 100 MHz,
respectively, on a Bruker Avance III 400 MHz spectrometer.
CDCl3 and CD3CN were used as NMR solvents and TMS as
internal standard. High-resolution mass spectra (HRMS) were
recorded on a Qstar ESI-q-TOF mass spectrometer (Applied
Biosystems, Darmstadt, Germany). SPEKOL 2000 Analytik
Jena spectrometer were used for recording of UV–vis spectra in
acetonitrile as solvent. Fluorescence spectra were recorded on a
Scheme 3: A proposed binding mode between L and Cu2+.
To show the high selectivity of chemosensor L for copper ions, JASCO FP-6500 spectrophotometer. All chemicals used in this
the association constant and limit of detection (LOD) were paper including metal salts were purchased from Merck
compared with those reported for other related chemosensors Company and were used without further purifications. Column
(Table 1). It should be emphasized that due to different condi- chromatography was performed on silica gel (200–400 mesh).
tions applied to measure these values, we could not compare As starting material, p-tert-butylcalix[4]arene was synthesized
properly the performance of chemosensor L with those re- according to the reported literature procedure [37].
ported in literature. However, the associating constants of
ligands having two moieties at the lower rim of a calix[4]arene 5,11,17,23-Tetra-tert-butyl-25,27-di(ethoxycarbonyl-
scaffold were compared with the present chemosensor. The methoxy)-26,28-dihydroxycalix[4]arene (1) [38]: A mixture
results indicated that the synthesized chemosensor L has a of p-tert-butylcalix[4]arene (2.0 g, 3.08 mmol) and potassium
lower LOD and a higher Ka in comparison with other carbonate (1.0 g, 7.3 mmol) in dry acetonitrile (50 mL) was
chemosensors, which shows a stable and selective complex- stirred. Ethyl bromoacetate (1 mL, 9 mmol) was added. The
ation with Cu2+.
mixture was heated under reflux for 24 h. It was then filtered
and the residue was washed with dichloromethane (2 × 50 mL).
The solvents were removed from the filtrate under vacuum.
Conclusion
In this study, the amidoflourene-appended calix[4]arene 1,3- Recrystallization of the resulting solid from dichloromethane/
diconjugated L was synthesized as an efficient colorimetric and methanol gave 5,11,17,23-tetra-tert-butyl-25,27-diethoxycar-
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bonylmethoxy-26,28-dihydroxycalix[4]arene (2.3 g, 90%). 1H argon atmosphere. The reaction was monitored by TLC (sol-
NMR (CDCl3, 400 MHz) δ 7.10 (s, 2H, ArOH), 7.05 (s, 4H, vent EtOAc–hexane 2:3). After completion of the reaction, the
ArH), 6.84 (s, 4H, ArH), 4.75 (s, 4H, O-CH2), 4.48 (d, J = 13.2 resulting mixture was treated with 50 mL of saturated aqueous
Hz, 4H, Ar-CH2-Ar), 4.32 (q, J = 7.2 Hz, 4H, -CH2CH3), 3.35 sodium bicarbonate solution and filtered off. The transparent
(d, J = 13.2 Hz, 4H, Ar-CH2-Ar), 1.35 (t, J = 7.2 Hz, 6H, filtrate was concentrated in vacuum in order to remove the
CH2CH3), 1.29 (s, 18H, -C(CH3)3), 1.01 (s, 18H, C(CH3)3) organic solvent. The residue was filtered off to yield compound
ppm; 13C NMR (CDCl3, 100 MHz) δ 169.28, 150.73, 150.29, 4 (1.67g, 97%) as transparent plates. The crude product was
147.07, 141.47, 132.52, 127.97, 125.76, 125.08, 72.41, 61.27, used directly in the next step. 1H NMR (CDCl3, 400 MHz) δ
33.94, 33.82, 31.88, 31.66, 31.05, 14.17 ppm.
7.66 (d, J = 7.6 Hz, 1H), 7.60 (d, J = 8.0 Hz, 1H), 7.50 (d, J =
7.2 Hz, 1H), 7.35 (td, J = 7.6 and J = 0.8 Hz, 1H), 7.22 (td, J =
5,11,17,23-Tetra-tert-butyl-25,27-di(hydroxycarbonyl- 7.6 and J = 1.2, 1H), 6.9 (br s, 1H), 6.73 (dd, J = 8.0 and J = 2.4
methoxy)-26,28-dihydroxycalix[4]arene (2) [38]: A mixture Hz, 1H), 3.84 (s, 2H), 3.70 (br s, 2H, NH2) ppm; 13C NMR
of 5,11,17,23-tetra-tert-butyl-25,27-di(ethoxycarbonyl-me- (CDCl3, 100 MHz) δ 145.75, 145.16, 142.27, 142.15, 133.01,
thoxy)-26,28-dihydroxycalix[4]arene (1, 1.0 g, 1.2 mmol) and 126.64, 125.09, 124.76, 120.67, 118.6, 113.98, 111.82, 36.83
20% aqueous sodium hydroxide (2 mL, 1.2 mmol) in ethanol ppm.
(50 mL) was refluxed for 24 h. The residue was diluted with
water (50 mL). HCl (3 mol·L−1) was added to the resulting 5,11,17,23-Tetra-tert-butyl-25,27-di[(9H-fluoren-2-
suspension until pH 1 was reached. This suspension was yl)aminocarbonylmethoxy]-26,28-dihydroxycalix[4]arene
washed with chloroform (100 mL) and brine (30 mL). The (L): A mixture of 5,11,17,23-tetra-tert-butyl-25,27-di(hydroxy-
organic layer was dried over MgSO4 and the solvents were re- carbonylmethoxy)-26,28-dihydroxycalix[4]arene (2, 0.50 g,
moved under vacuum to give 5,11,17,23-tetra-tert-butyl-25,27- 0.65 mmol) and N,N’-dicyclohexylcarbodiimide (2.6 mmol)
di(hydroxycarbonylmethoxy)-26,28-dihydroxycalix[4]arene (2, was dissolved in 5 mL dichloromethane and stirred for 10 min
0.9 g, 96%). 1H NMR (CDCl3, 400 MHz) δ 7.09 (s, 4H, ArH) , at room temperature. 9H-Fluoren-2-amine (4, 0.20 g, 1.1 mmol)
6.98 (s, 4H, ArH), 6.9–5.4 (bs, 4H, -OH, -COOH), 4.71 (s, 4H, was added and the mixture was stirred for 24 h at room temper-
O-CH2), 4.17 (d, J = 13.6 Hz, 4H, Ar-CH2-Ar), 3.46 (d, J = ature. Then, the solvent was removed and the residue was dis-
13.6 Hz, 4H, Ar-CH2-Ar), 1.28 (s, 18H, -C(CH3)3), 1.09 (s, solved in chloroform (50 mL) and washed with water (30 mL).
18H, C(CH3)3) ppm; 13C NMR (CDCl3, 100 MHz) δ 170.32, The crude product was purified by silica gel column chromatog-
149.07, 149.01, 148.46, 143.14, 132.51, 127.25, 126.31, 125.68, raphy using petroleum ether–ethylacetate 3:1 as eluent to give L
72.42, 34.18, 33.91, 32.32, 31.58, 31.03 ppm.
(0.58 g, 81%) as white powder. Mp: 241–243 °C; 1H NMR
(CDCl3, 400 MHz) δ 10.12 (s, 2H, NH), 7.98 (s, 2H, ArOH),
2-Nitro-9H-fluorene (3) [39]: 9H-Fluorene (6.0 g, 36.1 mmol) 7.73 (d, J = 7.6 Hz, 2H, ArH), 7.63 (d, J = 0.8 Hz, 2H, ArH),
was dissolved in 100 mL of glacial acetic acid at 60 °C. 15 mL 7.51–7.48 (m, 4H, ArH), 7.41–7.37 (m, 2H, ArH), 7.34–7.3 (m,
of nitric acid (65%) were added dropwise (~10 min) at 60 °C 4H, ArH), 7.19 (s, 4H, ArH), 7.01 (s, 4H, ArH), 4.7 (s, 4H,
upon vigorous stirring. After the addition was completed, the O-CH2), 4.3 (d, J = 13.6 Hz, 4H, Ar-CH2-Ar), 3.57 (d, J = 13.6
resulting mixture was further stirred at 60 °C. The reaction was Hz, 4H, Ar-CH2-Ar), 3.56 (s, 4H, Ph-CH2-Ph), 1.33 (s, 18H,
monitored by TLC (solvent EtOAc–n-heptane 1:9). After ap- C(CH3)3), 1.08 (s, 18H, C(CH3)3) ppm; 13C NMR (CDCl3, 100
pearance of the spot of the dinitro product (≈100 min, Rf MHz) δ 165.19, 149.53, 148.85, 148.26, 143.92, 143.60,
≈25%), the mixture was poured into 600 mL of water. The re- 143.34, 141.52, 137.80, 136.16, 132.17, 127.36, 126.65, 126.40,
sulting crude product was filtered off, washed with water and 126.08, 125.76, 124.87, 119.62, 119.57, 117.43, 115.89, 74.84,
recrystallized from 200 mL acetonitrile to give compound 3 36.85, 34.22, 34.01, 32.14, 31.62, 30.98 ppm. HRMS m/z: [M]+
(7.1 g, 92%) as slightly-yellow needles. 1H NMR (CDCl3, calcd for C74H78N2O6, 1090.5860; found, 1090.4344.
400 MHz) δ 8.42 (s, 1H), 8.31 (d, J = 8.4 Hz, 1H), 7.89–7.87
(m, 2H), 7.64 (d, J = 6.4 Hz, 1H), 7.49–7.43 (m, 2H), 4.02 (s,
Supporting Information
2H) ppm; 13C NMR (CDCl3, 100 MHz) δ 148.09, 146.79,
144.81, 143.92, 139.47, 128.87, 127.43, 125.43, 123.14, 121.34,
Supporting Information File 1
120.49, 119.88, 36.96 ppm.
1H NMR and 13C NMR spectra of compounds 1, 2, 3, 4
and L, HRMS of L, UV–vis and fluorescene titration
spectra of L with Cu2+ ion solutions.
9H-Fluoren-2-amine (4) [40]: A mixture of 2-nitro-9H-fluo-
rene (2.0 g, 9.5 mmol), iron powder (1.0 g, 18.7 mmol), and
NH4Cl (0.75 g, 12.46 mmol) was refluxed in aqueous ethanol
(75 mL of alcohol and 25 mL of water) at 85 °C for 4 h under
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-163-S1.pdf]
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27.Schazmann, B.; Alhashimy, N.; Diamond, D. J. Am. Chem. Soc. 2006,
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Acknowledgements
Financial support from the research council of University of
28.Li, G.-K.; Xu, Z.-X.; Chen, C.-F.; Huang, Z.-T. Chem. Commun. 2008,
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Mazandaran is gratefully acknowledged.
29.Liang, Z.; Liu, Z.; Jiang, L.; Gao, Y. Tetrahedron Lett. 2007, 48,
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