A colorimetric and fluorescent turn-on chemosensor operative in aqueous media for Zn2+ based on a multifunctionalized spirobenzopyran derivative

Article abstract of DOI:10.1039/c004871b

By conjugating spiropyran chloride 8 with 2-amino-N-(quinolin-8-yl) acetamide (5), multifunctionalized spirobenzopyran derivative SPQN was designed and synthesized as a water soluble colorimetric and fluorescent turn-on chemosensor for Zn²⁺. In 50% aqueous ethanol buffer solution, SPQN displayed a selective chelation fluorescence enhancement (16-fold) at 650 nm and visible color change (from colorless to red) with Zn²⁺ among the metal ions examined. In addition, as the third channel to display the metal binding characteristics of SPQN, operating on an efficient FRET process between the quinoline and the merocyanine moiety of the sensor, ratiometric determination of Zn²⁺ can be realized.

Full text of DOI:10.1039/c004871b

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
A colorimetric and fluorescent turn-on chemosensor operative in aqueous
2
+
media for Zn based on a multifunctionalized spirobenzopyran derivative†
Jian-Fa Zhu, Han Yuan, Wing-Hong Chan* and Albert W. M. Lee
Received 6th April 2010, Accepted 4th June 2010
First published as an Advance Article on the web 12th July 2010
DOI: 10.1039/c004871b
By conjugating spiropyran chloride 8 with 2-amino-N-(quinolin-8-yl)acetamide (5), multifunctionalized
spirobenzopyran derivative SPQN was designed and synthesized as a water soluble colorimetric and
2
+
fluorescent turn-on chemosensor for Zn . In 50% aqueous ethanol buffer solution, SPQN displayed a
selective chelation fluorescence enhancement (16-fold) at 650 nm and visible color change (from
2
+
colorless to red) with Zn among the metal ions examined. In addition, as the third channel to display
the metal binding characteristics of SPQN, operating on an efficient FRET process between the
2
+
quinoline and the merocyanine moiety of the sensor, ratiometric determination of Zn can be realized.
Introduction
reversible heterolytic cleavage of the spiro C–O bond followed by
cis–trans isomerization, which generates a metastable merocyanine
The design and synthesis of metal chemosensors with high
selectivity and sensitivity is an active field of supramolecular
7
(
ME). By appending an additional receptive site in position
8¢ on the benzopyran moiety, the isomerization of SP to ME
1
chemistry. Particular attention has been focused on targeting
could be modulated by specific metal ions. The metal ion-
mediated ring opening of SP to ME is characterized by its color
change from colorless to red, recognized readily by the naked-
eye. Spiropyran-based chemosensory systems for alkali, alkaline
heavy and/or transition metal (HTM) ions for their detection
in the cell environment. The zinc(II) ion is the second most
abundant transition metal essential for the human body with a
concentration ranging from sub-nM to 0.3 mM. The detection
and imaging of Zn in biological samples are of significant interest
owing to its unique role in physiological functions. Numerous
scientific endeavours have been engaged in the development of
2
3
8
earth, and lanthanide metals have been reported. We have found
2
+
that by superseding the commonly used nitro- group in the C6¢-
position with a tert-butyl group, a more photostationary closed SP
form can be resulted. Such a photostationary SP derivative is an
ideal molecular platform for the design of molecular/ion sensing
probes. In that context, we have developed novel spiropyran-
based chemosensors 1 and 2 for the colorimetric and fluorescent
4
fluorescent chemosensors for the in vitro and in vivo detection
2
+
5
2+
of Zn . However, some of the reported Zn sensors have poor
water solubility, and are subject to the interference of other metal
ions. Furthermore, the majority of existing sensors are based on
fluorophores such as anthracene, fluorescein and quinoline which
2+
9
detection of Cu (Scheme 1). To continue our interest in the
sensor development, herein we present a new colorimetric and
fluorescent turn-on chemosensor 3 that responds to zinc ions in
aqueous solutions.
6
5c–e
emit at wavelengths of <550 nm. , Their use in cell environments
may suffer from the interference of autofluorescence. Therefore,
better designed and improved performance fluorescent sensors are
still in great demand.
Spiropyrans (SPs) are photochromic compounds that can
isomerize in response to UV/vis irradiation as a result of a
Results and discussion
On the outset of our investigation, to confer the synthetic
host with selective binding affinity to transition metal ions, we
envisioned that multifunctional ligands could be appended onto
the spiropyran scaffold through the C8¢ carbon. In addition to the
phenoxy group generated from the ring opening reaction, the C8¢
pendant of the host should incorporate three additional ligating
Department of Chemistry, Hong Kong Baptist University, Kowloon Tong,
Hong Kong SAR, China. E-mail: whchan@hkbu.edu.hk; Fax: + 852 3411
7
348; Tel: +852 3411 7076
†
Electronic supplementary information (ESI) available: Supplementary
data (Figs. S1–S23). See DOI: 10.1039/c004871b
Scheme 1 Metal ion-mediated ring opening of spiropyran.
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Scheme 2 Synthesis of SPQN.
sites so as to bind metal ions with four coordinating ligands. To be
a viable probe operative in aqueous medium, the binding energy
of the probe and the metal ion must be comparable to that of
the hydration energy of the metal. Inspired by a recent work by
In addition to bearing multifunctional ligating sites for bind-
ing metals, SPQN comprises two non-interacting fluorophoric
systems. We anticipate that the binding characteristics and photo-
physical properties of SPQN would be quite interesting.
5d
2+
Zhang et al. for the development of a fluorescent Zn sensor,
We have previously established that only Cu can effectively
we envision that commercially available 8-aminoquinoline would
provide us with a semi-rigid molecular platform to introduce multi-
functional ligating sites favoring the binding on zinc metal. Thus,
treatment of 8-aminoquinoline with 2-bromoacetyl bromide in the
presence of triethyl amine gave the corresponding amide 3 in 96%
induce the ring opening of colorless spiropyrans 1 and 2 to the
corresponding red colored merocyanine open form. This unique
property was exploited to develop a naked-eye detection system
for copper. By synthesizing a structurally more elaborate SPQN,
we intended to tune the metal binding property of the probe to
respond to other metal ion(s). Screening tests revealed that, upon
yield. Amide 3 underwent smooth substitution reaction with NaN
3
2
+
2+
affording azide 4, which was then reduced by triphenylphosphine
to give rise to 2-amino-N-(quinol-8-yl)-acetamide (5) in 72%
yield. It is noteworthy that 5, possessing three nitrogen ligating
groups, is a good candidate to be appended on the spiropyran
scaffold. The requisite spiropyran precursor 8 was assembled from
addition of common metal ions, only Cu and Zn can turn the
colorless SPQN probe to red color in aqueous ethanol solution. To
evaluate its potential as a “naked-eye” zinc detection probe, SPQN
was subjected to UV-vis titration with increasing concentrations of
2
+
zinc. As shown in Fig. 1, upon addition of up to 2 equiv. of Zn to
a 50% aqueous ethanol buffer solution of SPQN, the absorbance
at 230 and 350 nm decreased with concomitant formation of new
peaks at 240, 380 and 540 nm, which induced a color change from
colorless to red. Clear isosbestic points at 250, 290 and 360 nm were
observed, which indicate the formation of only one visible active
zinc complex with the probe. Furthermore, a linear dependence of
2
-hydroxy-3-hydroxymethyl-5-tert-butylbenzaldehyde (6) in two
simple synthetic operations as shown in Scheme 2. Condensation
reaction between 6 and N-methyl-2,3,3-trimethylindolenine in
ethanol gave a good yield of spiropyran derivative 7, which
underwent a fast nucleophilic substitution reaction with thionyl
chloride to afford the corresponding chloride 8. The crude labile
2
+
8, without purification, was allowed to conjugate with quinoline
the absorbance at 520 nm as a function of Zn concentration was
2
+
derivative 5 to give rise to multifunctional spiropyran-based
fluorescent sensor SPQN in 36% yield. The purity of SPQN was
observed. 10 mM of Zn can induce an 11-fold enhancement of the
absorbance at 520 nm. Similarly, the spectrophotometric titration
1
13
2+
fully confirmed by H and C NMR, and HRMS analysis.
of SPQN with Cu was also conducted (Fig. S1, ESI†). On the
3
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11
formation of protonated phenolic compound. The fluorescence
of the complex increased abruptly from pH 4.0 to 6.0, then reached
a plateau of a maximum under the physiological pH window (Fig.
2
+
-
S2, ESI†). At a higher pH, Zn may be complexed by OH to
2
+
reduce the free Zn , which in turn reduces its coordination with
the spiropyran. Therefore, subsequent metal binding studies were
carried out in HEPES buffer solution at pH = 7.4.
2
+
The kinetics of Zn –SPQN complex formation were found to
be highly dependent on the water content of the solvent system
(
Fig. 3). In pure ethanol, a stable and steady fluorescent reading
2
+
was observed within one minute after one equiv. of Zn was
introduced into the sensor solution. Increasing the water content
of the aqueous ethanol solution from 20% to 80%, not only did the
time required for reaching the steady fluorescent reading extend
from 2.5 to 6 min, but also the intensity of the fluorescence of the
complex was suppressed by as much as 50%. To have a compromise
on high sensitivity, short response time and practical applications
of the metal sensing probe, in subsequent investigation, we chose
50% aqueous ethanol as the optimum solvent system.
2
+
Fig. 1 UV-vis spectra of SPQN (10 mM) upon the titration of Zn (0–2
equiv.) in buffer solution (50 mM HEPES, 50% ethanol, pH = 7.4). Inset:
2+
calibration curve A520 nm as a function of Zn concentration.
basis of non-linear fitting of the titration curve of a 1 : 1 binding
model (vide infra), the association constant of Zn –SPQN and
Cu –SPQN complexes were found to be 6.18 ¥ 10 and 3.76 ¥
2
+
2
+
6
7
-1
To fully assess the viability of using SPQN as a fluorescent
1
0 M , respectively.
2
+
chemosensor for Zn , SPQN (10 mM) was titrated with increasing
Due to the intrinsic high sensitivity associated with fluorescent
2
+
concentrations of Zn . Fig. 4(a) shows that the maximum
detection, fluorescent sensors, particularly “off-on” sensors, are
desirable and very much sought after sensing devices. SPQN,
showing very weak fluorescent emission at 650 nm upon excitation
at 515 nm, met the basic requirement to confer the probe with a
low fluorescence “off” state. The fluorescent intensity of SPQN
at 650 nm was dramatically enhanced upon addition of 1 equiv.
fluorescence emission of the probe at 650 nm gradually increased
2
+
2+
of Zn . Zn -mediated opening of the spiropyan ring of SPQN
followed by formation of a tight complex with the proximate
nitrogen ligands could enhance the rigidity of the complex. The
observed dramatic fluorescence enhancement of 15.8-fold could be
a result of the combination effect of internal charge transfer and
10
chelation-enhanced fluorescence (CHEF). It is noteworthy that
2
+
other metal ions, except Pb , did not induce any enhancement on
2
+
the fluorescence of SPQN (Fig. 2). Furthermore, the Zn sensing
ability of SPQN at different pH values was examined. At a pH
below 4.0, the fluorescence of SPQN is very weak, presumably due
to the protonation of the phenoxy and the amino group leading to
a weak coordination ability for Zn . Under acidic conditions, the
solution of SPQN turns to yellow color which is indicative of the
Fig. 3 Fluorescent emission intensity of SPQN (10 mM) at 650 nm (lex
2+
2
+
= 515 nm) in response to the addition of Zn (1 equiv.) as a function of
time in ethanol with different contents of water.
Fig. 2 Fluorescence spectra (lex = 515 nm) of SPQN (10 mM) in the
2+
2+
2+
2+
2+
presence of various metal ions (1 equiv. of Mg , Ca , Ni , Cd , Co ,
Fig. 4 Fluorescence spectra (lex = 515 nm) of 10 mM SPQN upon the
2+
2+
+
+
+
+
2+
2+
Hg , Pb , Li , Ag , Na , K , Zn ) in buffer solution (50 mM HEPES,
titration of Zn (0–5.0 equiv.) in buffer solution (50 mM, HEPES, 50%
2+
5
0% ethanol, pH = 7.4). Inset: visible emission (irradiated by 365 nm light)
ethanol, pH = 7.4). Inset: fluorescence intensity ratio as a function of Zn
2+
observed from SPQN in the absence and presence of Zn (1 equiv.).
concentration.
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1
2+
Fig. 5 Partial H NMR spectra (400 MHz) of SPQN (5 mM) in CD
3
CN-d
3
: (a) free SPQN; (b) SPQN + 0.1 equiv. of Zn ; (c) SPQN + 0.2 equiv. of
2+
2+
2+
Zn ; (d) SPQN + 0.3 equiv. of Zn ; (e) SPQN + 0.4 equiv. of Zn .
2
+
to almost 16-fold when 1 equiv. of Zn was added, implicating
spiropyran moiety triggered by the metal ion, the two doublets at
d5.78 and 6.99, ascribed to the vinyl protons of the dihydropyran
ring of SPQN, gradually disappeared during the titration. When
2
+
the formation of a tight complex between SPQN and Zn .
The fluorescence intensity increased linearly (linear dependency
coefficient R = 0.997) with the concentration of Zn up to a
molar ratio Zn /SPQN of 1 : 1, which was confirmed by Job’s
plot (Fig S3, ESI†). The corresponding binding constant of Zn –
2
2+
2+
more than 0.4 equiv. of Zn was added into the sensor solution,
2
+
due to extensive line broadening, the resolution of the resultant
2
+
1
H NMR spectra became deteriorated.
SPQN estimated from non-linear fitting of the titration curve was
To rule out the possibility that the fluorescence enhancement
6
-1
13
found to be 7.23 ¥ 10 M , which is in good agreement with that
observed is not due to a chemical reaction (i.e. chemodosimeter),
2
+
2+
obtained from UV-visible titration (vide supra). Although Pb can
also induce fluorescence enhancement of SPQN at 650 nm, on the
basis of non-linear fitting of the titration curve (Fig. S4, ESI†), the
the reversible binding of Zn and the sensor must be established.
The reversibility experiment revealed that the 16-fold fluorescence
enhancement of SPQN caused by the addition of 1 equiv. of zinc
ions can be removed completely by adding 1 equiv. of EDTA. The
same extent of fluorescence enhancement can be recovered when
2
+
corresponding binding constant of the Pb –SPQN complex was
6
-1
found to be smaller (i.e. 3.70 ¥ 10 M ).
To evaluate the binding mode of SPQN–Zn , H NMR titra-
2
+
1
2+
an additional one equiv. of Zn was introduced (Fig. S5, ESI†).
2
+
tion was carried out. Upon gradual addition of 0.4 equiv. of Zn
to the CD CN-d solution of SPQN, a large upfield shift of the
amide proton from d11.15 to 10.10 was observed, indicating the
Convincing evidence of the 1 : 1 binding mode of the complex
2
+
3
3
of SPQN and Zn was obtained by the MALDI-TOF HRMS
spectroscopic data. When the complex was subjected to mass
spectral measurement, a clear peak of m/z 609.2233 corresponding
12
involvement of the carbonyl oxygen in complexing the metal ion.
+
At the same time, the C2 hydrogen of the quinoline moiety at d8.80
to [M + Zn - H] was observed (Fig. S6, ESI†). On the basis of
experienced a substantial upfield shift (8.55 ppm). In contrast,
the combined spectroscopic information, the binding mode of
SPQN–Zn is proposed in Fig. 6.
2
+
the two singlets assigned to the N–H (2.22 ppm) and the CH
adjacent to the amide group (3.22 ppm), respectively, underwent a
2
The selectivities of SPQN to various metal ions were examined
2
+
2+
slight downfield shift to 2.32 and 3.35 ppm, respectively, upon Zn
systematically. Only Zn can induce a dramatic fluorescence
2
+
coordination due to the shielding from the coordinated secondary
nitrogen and carbonyl oxygen. All these observations suggested
the direct interaction between the ligating groups of the probe
enhancement of SPQN, while Pb is the only second metal ion
which also responds to the probe, but to a much less extent (vide
2
+
supra). Common competing metal ions in Zn determination,
2
+
2+
2+
2+
and Zn (Fig. 5). Consistent with the ring opening event of the
such as Mg , Ca and Cd , did not respond to SPQN. The
3
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2+
Fig. 6 Proposed binding mode of SPQN–Zn complex.
2
+
titration of sensor SPQN with Zn in the presence of potential
between the two fluorophoric systems of SPQN, a fluorescence
2
+
competing metal ions was further determined. The results revealed
that even Pb did not show any interference effect on Zn
detection. On the other hand, Cu was found to be a quencher to
the sensing probe.
In our design of the metal ion fluorescent sensing probe, we have
conjugated two separate fluorophoric moieties for the construction
of SPQN. Would these two moieties communicate with each other
when SPQN is allowed to interact with zinc metal? To shed light
on this issue, the metal binding properties of quinoline derivative
titration of SPQN against increasing concentrations of Zn was
conducted by exciting the quinoline moiety of SPQN at 326 nm.
In contrast to the control compound 5, as shown in Fig. 9, only
the quenching of fluorescence signal of SPQN at 398 nm was
observed. The red-shifted fluorescence signal centered at 488 nm
was completely erased by the inner filter effect caused by the red
2
+
2+
2
+
9a,14
color of the merocyanine moiety of SPQN.
Concomitantly,
the merocyanine moiety of SPQN, serving as the energy acceptor,
is excited to give a new emissive peak at 650 nm. A clear FRET
5, which can serve as the control compound for sensor SPQN,
must be first investigated. Due to structural similarity between 5
5
d
and the fluorescent zinc sensor AQZ reported by Zhang et al.,
2
+
compound 5, coordinating with Zn , displays similar fluorescence
2
+
responses as those of AQZ. Upon addition of 0.5 equiv. of Zn ,
about 8-fold fluorescence signal enhancement and a 90 nm red-
shift from 398 to 488 nm of the fluorescence emission of 5 were
5d
observed (Fig. 7).
Fig. 8 Fluorescence spectra (lex = 326 nm) of control compound 5 (10
mM) in buffer solution (50 mM, HEPES, 50% ethanol, pH = 7.4) in the
2+
presence of different concentrations of Zn .
Fig. 7 Metal ion selectivity profiles of SPQN (10 mM) in the presence of
various metal ions in buffer solution (50 mM, HEPES, 50% ethanol, pH
=
7.4): (gray bars) fluorescence intensity at 650 nm in the presence of 1
2+
2+
2+
2+
2+
2+
2+
+
+
+
+
2+
equiv. of Zn , Hg , Pb , Ni , Co , Cd , Cu , Ag , Li , Na , K , Ca ,
2+
2+
Mg ; (black bars) fluorescence intensity in the presence of 1 equiv. of Zn
followed by 1 equiv. of Hg , Pb , Ni , Co , Cd , Cu , Ag , Li ; 300
equiv. of Na , K ; 100 equiv. of Ca , Mg .
2+
2+
2+
2+
2+
2+
+
+
+
+
2+
2+
2
+
As Zn metal binding to the pendant chelating groups of
, the intramolecular charge transfer (ICT) from amide side
5
group to the quinoline ring is enhanced and its fluorescent signal
is increased (Fig. 8). It is noteworthy that upon excitation of
the quinoline moiety of SPQN at 326 nm, the emissive peak
at 401 nm overlaps significantly with the major absorption
peak of the merocyanine moiety of SPQN (Fig. S7, ESI†). To
examine whether fluorescence energy transfer (FRET) could occur
Fig. 9 Fluorescence spectra (lex = 326 nm) of SPQN (10 mM) in buffer
solution (50 mM, HEPES, 50% ethanol, pH = 7.4) in the presence
2+
of different concentrations of Zn . Inset: dependence of the ratio of
fluorescence intensities at 650 and 398 nm wavelengths of the probe on
2+
the Zn .
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process from the quinoline moiety to the merocyanine moiety of
SPQN took place.
The FRET process was saturated when 1 equiv. of Zn was
added to the probe. The red-shifted emissive peak of the quinoline
2
+
2
+
moiety of SPQN emerged when more than 1 equiv. of Zn was
introduced (Fig S8, ESI†). On the basis of the efficient FRET
2
+
observed in SPQN, a ratiometric detection of Zn is observed as
2
+
shown in the inset of Fig. 9. When as little as 0.1 equiv. of Zn
was added to the probe, an enhancement of 3.0-fold was observed
for I650/I398
.
Spiropyran derivatives have been used to construct molecular
15
logic circuits of high complexity. The optical output signals of
SPQN in response to metal ions binding can be used to design OR
and INHIBIT (INH) logic gates. The OR gate is one of the basic
logic gates that implement logical disjunction, which results in a
16
high output if one or both inputs to the gate are high. The ab-
sorbance of SPQN (10 mM) at 520 nm (Output 1) can be enhanced
2
+
2+
by Cu or Zn or both, enabling the OR logic function when 1
2
+
2+
equiv. of Cu and 1 equiv. of Zn are taken as inputs (Fig. 10).
Fig. 11 Fluorescence output of SPQN (10 mM) at 650 nm (lex = 520 nm)
2+
2+
in the presence of chemical inputs, Zn (10 mM) and Cu (20 mM), and
the logic table of the INH gate.
Conclusions
To extend the metal ion sensing ability of spiropyran-based
materials, by appending a multi-ligating moiety onto a spiropyran
scaffold, we have developed a “naked-eye” and fluorescence turn-
2
+
2+
on chemosensor SPQN for Zn . Selective detection of Zn has
been realized in aqueous solutions with outstanding sensitivity.
All biologically relevant metal ions and toxic heavy metals such as
2
+
2+
2+
Cd , Pb and Hg did not interfere with the zinc ion detection.
Since the fluorescence signal output of the probe is in the near
IR region (i.e. 650 nm) where the autofluorescence of biological
2
+
samples is minimal, its potential use for in vivo detection of Zn is
implicated. The binding mode of sensor and zinc was established
by the combined UV-vis, fluorescence and NMR method. In
2
+
addition, when SPQN interacts with Zn , a fairly efficient FRET
took place between the quinoline (donor) and the merocyanine
(
acceptor) moieties of SPQN. In essence, SPQN has shown
Fig. 10 Absorbance output of SPQN (10 mM) at 520 nm in the presence
of chemical inputs, Zn (10 mM) and Cu (10 mM), and the logic table of
the OR gate.
2
+
2+
2+
its ability for sensing Zn in aqueous solutions through three
different channels: turning the colorless sensor to red, giving an en-
hanced fluorescence signal at 650 nm and delivering a ratiometric
FRET output at I650/I398. In addition, the color and fluorescence
responses of SPQN upon metal chelation were employed for
developing truth tables for OR and INHIBIT logic gates.
On the other hand, the fluorescence output of SPQN at 650 nm
17
(
Output 2) was utilized to fabricate a two-input INH logic gate.
The output signal of an INH gate is inhibited by one of the active
inputs. As shown in Fig. 11, a basic two-input INHIBIT action
2
+
2+
can be obtained for SPQN (10 mM) with Zn (10 mM) and Cu
20 mM) as inputs. Fluorescence emission enhancement at 650 nm
Experimental
(
2
+
of SPQN is observed only in the presence of 1 equiv. of Zn and
1
General methods
2
+
the absence of Cu , displaying the output as “1”. Under other
circumstances the fluorescence of SPQN is quenched, leading to
output “0”.
The melting point was determined with a MEL-TEMPII melting
point apparatus (uncorrected). H NMR and C NMR spectra
1
13
3
962 | Org. Biomol. Chem., 2010, 8, 3957–3964
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+
were recorded on a VARIAN INOVA or Bruka Advance-III 400
spectrometer (at 400 and 100 MHz, respectively) in CDCl . Low
resolution mass spectra were recorded on a Finnigan MAT SSQ-
10 mass spectrometer while high resolution mass spectra were
obtained on a Bruker Autoflex mass spectrometer (MALDI-TOF)
or electrospray ionization high-resolution mass spectra on an API
Qstar Pulsari mass spectrometer. Fluorescent emission spectra
and UV-vis spectra were collected on a PTI luminescence lifetime
spectrometer and a Cary UV-100 spectrometer, respectively.
Unless specified, all fine chemicals were used as received.
HRMS (MALDI-TOF): m/z calcd for C11
found, 201.0901.
H
11
N
3
O [M ] 201.0896,
3
2
.4 Synthesis of (6-tert-butyl-1¢,3¢,3¢-trimethylspiro[chromene-
,2¢-indoline]-8-yl)methanol (7). To a solution of 5-tert-butyl-
-hydroxy-3-(hydroxymethyl)-benzaldehyde (3 g, 14.4 mmol) in
ethanol (20 ml) was added 1,3,3-trimethyl-2-methyleneindoline
2.5 g, 14.4 mmol). The mixture was stirred at room temperature
for about 12 h. Then the solvent was evaporated in vacuo. The
crude product was purified by column chromatography on silica
gel using petroleum ether–ethyl acetate (10 : 1) as the eluant to
afford 7 as yellow oils (3.9 g, 75% yield), which could turn to
7
2
2
(
2
Synthesis of the precursors of SPQN and SPQN
1
red color after several days in the dark. H NMR (400 MHz,
CDCl
J = 10 Hz), 4.50 (m, 2 H), 2.67 (s, 1 H), 1.17–1.32 (m, 15 H).
C NMR (100 MHz, CDCl ) d 149.7, 147.8, 142.7, 136.7, 130.0,
27.7, 126.0, 125.9, 123.1, 121.5, 119.4, 118.0, 107.0, 104.0, 61.8,
51.3, 34.1, 31.5, 29.0, 25.7, 20.3. HRMS (MALDI-TOF): m/z
3
) d 7.00–7.20 (m, 4 H), 6.81–6.87 (m, 2 H), 5.70 (d, 1 H,
2
.1 Synthesis of 2-bromo-N-(quinolin-8-yl)acetamide (3). 8-
Aminoquinoline (0.5 g, 3.46 mmol) was dissolved in dry
dichloromethane (20 ml) and triethylamine (0.35 g, 3.46 mmol)
1
3
3
1
was added into this solution. After the mixture was stirred at
◦
0
C for 10 min, bromoacetyl bromide (0.84 g, 4.16 mmol) was
+
calcd for C24
H
30NO [M + 1] 364.2271, found, 364.2244.
2
introduced dropwise to the stirred solution over a period of 20 min.
The reaction mixture was then stirred at room temperature for
h. Subsequently the solvent was removed, the crude product
was purified by column chromatography on SiO , using petroleum
ether–ethyl acetate (6 : 1) as the eluant to afford 3 as white solids
2
.5 Synthesis of SPQN. To a dichloromethane solution (20
3
◦
ml) of alcohol 7 (270 mg, 0.74 mmol) at 0 C, was added 5
2
drops of SOCl . The mixture was stirred at room temperature
2
for about 5 min. Saturated sodium bicarbonate (20 ml) was added
to the reaction mixture immediately. Then the crude product 8
was extracted with dichloromethane (3 ¥ 20 ml). The combined
organic layers were dried, filtered and evaporated to dryness. To
the acetonitrile solution (30 ml) of crude 8, amide 5 (0.15 g,
◦
1
(
1
5
0.88 g, 96% yield) Mp: >300 C, H NMR (400 MHz, CDCl
3
):
0.72 (1 H, s), 8.86 (1 H, dd, J = 4.0, 1.6 Hz), 8.75 (1 H, dd, J =
.6, 3.6 Hz), 8.18 (1 H, dd, J = 8.0, 3.6 Hz), 7.57 (2 H, m), 7.49 (1
):
64.0, 148.6, 138.7, 136.3, 133.8, 127.9, 127.2, 122.5, 121.8, 116.6,
1
3
H, dd, J = 8.4, 4.0 Hz), 4.14 (2 H, s); C NMR (100 MHz, CDCl
3
1
2
0
.74 mmol) and potassium carbonate (0.31 g, 2.22 mmol) were
+
9.7. HRMS (ESI): m/z calcd for C11
found, 264.9957.
H
10
N
2
OBr [M + 1] 264.9976,
◦
introduced successively. The reaction mixture was stirred at 85 C
for 3 h. Then water was added, and the mixture was extracted
with ethyl acetate (3 ¥ 20 ml) and dried by anhydrous sodium
sulfate. After filtering and removal of the solvent in vacuo, the
2
.2 Synthesis of 2-azido-N-(quinolin-8-yl)acetamide (4).
A
mixture of 3 (0.8 g, 3.0 mmol) and sodium azide (0.4 g, 6 mmol)
was dissolved in DMF (20 ml). The reaction mixture was stirred
overnight at room temperature. Then water was added and the
mixture was extracted repeatedly with ethyl acetate (3 ¥ 30 ml).
The combined organic layers were washed several times with
water and dried by anhydrous sodium sulfate. After filtering and
removal of the solvent in vacuo, the residue was purified by column
chromatography on SiO
6 : 1) as the eluant to afford 4 as white solids (0.62 g, 90% yield).
Mp: 69–71 C, H NMR (400 Hz, CDCl ): 10.55 (1 H, s), 8.86 (1
residue was purified by column chromatography on SiO
2
using
petroleum ether–ethyl acetate (15 : 1–10 : 1) as the eluant to afford
◦
1
SPQN as yellow solids (0.146 g, 36% yield). Mp: 65–67 C. H
NMR (400 MHz, CDCl
3
): 11.24 (1 H, s), 8.82 (1 H, dd, J = 7.2,
.0 Hz), 8.78 (1 H, dd, J = 4.0, 1.6 Hz), 8.16 (1 H, dd, J = 8.0,
.6 Hz), 7.56 (2 H, m), 7.46 (1 H, dd, J = 8.4, 7.2 Hz), 7.11(1 H,
2
1
d, J = 2.4 Hz), 7.05-6.98 (3 H, m), 6.92 (1 H, d, J = 10.4 Hz),
2
, using petroleum ether–ethyl acetate
6
.76 (1 H, m), 6.43 (1 H, d, J = 7.6 Hz), 5.74 (1 H, d, J = 10
(
◦
1
Hz), 3.85 (1 H, d, J = 13.6 Hz), 3.60 (1 H, d, J = 13.6 Hz), 3.23
3
(
1
2 H, s), 2.68(3 H, s), 1.89 (1 H, s), 1.34 (1 H, s), 1.26 (9 H, s),
H, dd, J = 4.4, 1.6 Hz), 8.76 (1 H, dd, J = 6.0, 1.6 Hz), 8.18 (1
1
3
.20 (3 H, s); C NMR (400 MHz, CDCl ): 170.7, 150.0, 148.5,
3
H, dd, J = 8.0, 1.6 Hz), 7.56 (2 H, m), 7.49 (1 H, dd, J = 8.0, 4.4
1
3
147.7,142.6, 139.0, 136.8, 136.2, 134.4, 130.2, 128.1, 127.6, 127.6,
27.3, 124.2, 122.9, 121.7, 121.5, 121.4, 119.3, 118.3, 118.2, 116.6,
106.8, 104.2, 52.6, 51.0, 49.6, 34.0, 31.5, 29.0, 25.7, 20.3. HRMS
Hz), 4.26 (2 H, s); C (100 Hz, CDCl ): 165.1, 148.7, 138.6, 136.4,
3
1
1
33.5, 128.0, 127.2, 122.5, 121.8, 116.8, 53.5. HRMS (ESI): m/z
calcd for C11
+
H
10
N O [M + 1] 228.0885, found, 228.0896.
5
+
(
MALDI-TOF) m/z calcd for C35
H
38
N
4
O [M ] 546.2989, found
2
2
.3 Synthesis of 2-amino-N-(quinolin-8-yl)acetamide (5). To
546.2979.
the solution of azide 4 (0.50 g, 2.2 mmol) in THF–water (6 : 1)
(
10 ml) was added triphenylphosphine (1.15 g, 4.4 mmol) at room
Acknowledgements
◦
temperature. The reaction mixture was then stirred at 85 C for
2
4 h. The mixture was concentrated, and the residue was purified
by column chromatography on SiO using ethyl acetate–ethanol
5 : 1) as the eluant to afford 5 (0.32 g, 72% yield) as white solids.
The work was supported by a grant from the Research Grant
Council of Hong Kong (HKBU 200407).
2
(
◦
1
Mp: 108–110 C; H NMR (400 MHz, CDCl
3
): 11.30 (1 H, s), 8.87
Notes and references
(
1 H, dd, J = 4.4, 1.6 Hz), 8.84 (1 H, dd, J = 6.8, 2.0 Hz), 8.16 (1 H,
1
For reviews, see: (a) Fluorescent Chemosensors for Ion and Molecular
Recognition, ed. A. W. Czarnik, American Chemical Society, Washing-
ton, DC, 1993; (b) A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson,
T. M. Huxley, C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem.
dd, J = 8.4, 2.0 Hz), 7.56 (2 H, m), 7.46 (1 H, dd, J = 8.4, 4.4 Hz),
1
3
3
.65 (2 H, s), 1.82(2 H, s); C NMR (100 MHz, CDCl
3
): 171.6,
148.6, 138.9, 136.3, 134.2, 128.1, 127.3, 121.8, 121.6, 116.5, 46.1.
This journal is © The Royal Society of Chemistry 2010
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