Synthesis and fluorescence properties of 2-aryl-3-hydroxyquinolones, a new class of dyes displaying dual fluorescence

Article abstract of DOI:10.1016/j.tetlet.2005.11.160

A series of 2-aryl-3-hydroxyquinolones (3HQs) with different electron donating aryl substituents at the position 2 were synthesized. Their absorption and fluorescence properties were studied in solvents of medium and high polarity. Almost all the synthesi

Full text of DOI:10.1016/j.tetlet.2005.11.160

Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 905–908
Synthesis and fluorescence properties of
2-aryl-3-hydroxyquinolones, a new class of dyes
displaying dual fluorescence
a
a
Dmytro A. Yushchenko,a,b, Mykhailo D. BilokinÕ, Oleksandr V. Pyvovarenko,
*
b
b
a,b
´
Guy Duportail, Yves Mely and Vasyl G. Pivovarenko
aDepartment of Chemistry, Kyiv National Taras Shevchenko University, 01033 Kyiv, Ukraine
bPhotophysique des Interactions Biomole´culaires, UMR 7175-LC1 du CNRS, Institut Gilbert Laustriat, Faculte´ de Pharmacie,
Universite´ Louis Pasteur, 67401 Illkirch, France
Received 25 October 2005; revised 27 November 2005; accepted 30 November 2005
Available online 20 December 2005
Abstract—A series of 2-aryl-3-hydroxyquinolones (3HQs) with different electron donating aryl substituents at the position 2 were
synthesized. Their absorption and fluorescence properties were studied in solvents of medium and high polarity. Almost all the syn-
thesized 3HQs display dual fluorescence in the tested solvents, in line with an excited state intramolecular proton transfer reaction.
For N-methyl substituted compounds, the intensity ratio of the two emission bands was found to be exquisitely sensitive to solvent
polarity, with a two orders of magnitude change from toluene to dimethylsulfoxide. Consequently, these compounds appear as pro-
spective polarity fluorescent labels for proteins and nucleic acids.
Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction
with dual fluorescence.4–7 ESIPT results in the forma-
tion of two tautomeric forms in the excited state of the
probe. Due to their different photophysical properties,
these tautomeric forms exhibit largely separated emis-
sion bands on the wavelength scale. Moreover, these
two forms also show different sensitivities to their envi-
ronment, so that the ratio of the intensities of the two
emission bands can be used as a sensitive mean to char-
acterize the probe environment.8 The most interesting
and characterized representatives of this class of probes
are the 3-hydroxyflavone derivatives (3HFs) that have
been shown to be highly effective tools for investigating
the polarity,4–9 hydration,10 electronic polarization8 and
electrostatic effects in different media11–14 including
microheterogeneous systems and proteins.15–18
Due to its exquisite sensitivity, fluorescence is one of the
most important techniques for investigating molecular
events in biological systems. However, this technique
strongly relies on the availability of fluorescent probes
with optimal properties. Of particular interest are the
dual fluorescence probes that exhibit two well separated
emission bands.1 In this case, the ratio of the intensities
of the two bands can be used as a signal. This ratiomet-
ric response constitutes a strong advantage of the dual
fluorescent dyes over the intensiometric response of
the common single-band fluorescent probes, since the
ratiometric response does not depend on the probe con-
centration. This advantage is especially of interest in cel-
lular and tissular studies where the local concentrations
of the dyes cannot be controlled. Excited state intra-
molecular proton transfer (ESIPT) reaction2,3 is one of
the most effective principles used in the design of probes
However, despite their significant advantages compared
to common single-band probes, 3HFs present some
drawbacks notably in respect with their limited photo-
stability and quantum yield that limit their applications.
As a consequence, the development of new dual fluores-
cence probes with improved fluorescent properties is
strongly required. In this respect, 2-aryl-3-hydroxyquino-
lones (3HQs), which are structural analogs of 3HFs may
constitute potential interesting candidates.
Keywords: 3-Hydroxyquinolones; Absorption; Fluorescence probes;
Polarity sensors.
*
Corresponding author. Tel.: +33 390 244262; fax: +33 390 244313;
e-mail: ydmytro@yahoo.com
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.11.160

906
D. A. Yushchenko et al. / Tetrahedron Letters 47 (2006) 905–908
Table 1. Synthesized 3HQs and their characteristics26
O
O
5
4
5
4
3
Compd. no. Ar
R
H
Total yield (%) Mp (°C)
3
2
OH
OH
6
7
6
7
61a/24b
276
2'
5'
1
3'
4'
2
N
R
Ar
O
1
1
8
8
6'
2
Me 60a
238
F
3HF
3HQ
3
4
Me 71a
257
224
S
Me 60a/32b
CH3
OMe
N
Indeed, substitution of the oxygen atom of the 3HF
moiety by a nitrogen atom in 3HQs will permit addi-
tional modifications, which may further improve the
dye properties. However, only very limited spectroscopic
data are actually available on 3HQs19 and no attempts
have been done to adjust their spectroscopic properties.
In the present work, we describe the synthesis and
spectroscopic characterization of a series of 3HQs with
different substituents in two key positions of the
molecule, namely on the heterocyclic nitrogen atom
and the aromatic ring at position 2.
5
Me 59a/27b
259
293
301
316
294
6a
6b
7
H
55a
Me 57a
Me 49c
Me 52c,d
N
N
O
8
N
a Prepared by pathway 2 (Scheme 1).
b Prepared by pathway 1 (Scheme 1).
c Obtained by nucleophilic substitution of fluorine (Scheme 2).
d DMF as a source of dimethylamine was used instead of the corre-
sponding amine.
2. Results and discussion
2.1. Synthesis
Since some alkaloids are based on the structure of
3HQs, several methods of 3HQs synthesis have been
developed.19–23 We have selected two of these methods
to synthesize our 3HQ derivatives. First, we used the
Algar–Flynn–Oyamada reaction, which is largely used
in 3HF synthesis. Treatment of 20-aminochalcones by
hydrogen peroxide in basic conditions leads to 3HQs.19
Since substitution of the 40 position of the aryl group by
the strong electron donor dialkylamino group has been
shown to increase the sensitivity of the fluorescence
properties of 3HF to its environment,5,6,8 40-dialkyl-
amino substituted 3HQ compounds (6a,b, 7, 8) have
been synthesized. Since the starting 40-(N,N-dialkyl-
amino)phenyl-2-bromoketones cannot be prepared by
the common procedure of bromination, we prepared
them through their 2,2-dibromide intermediates as
recently described.25 Alternatively, we found that the
40-dialkylamino substituted 3HQs could also be prepared
by substituting the halogen atom in a corresponding
fluorine derivative (Scheme 2). We successfully applied
this procedure to the synthesis of quinolones 7 and 8.
However, as previously shown,24 we found that the syn-
thesis of 3HQ derivatives by this method requires a
more complex procedure than for 3HFs (Scheme 1,
pathway 1).
The second used method consists in the synthesis and
subsequent conversion of phenacyl anthranilates to
3HQ derivatives in polyphosphoric acid (PPA)22,24 or
in the presence of a base.24 This method proved to be
simple and effective for the synthesis of most of our
3HQ derivatives and was thus preferred (Scheme 1,
pathway 2, Table 1).
Taken together, our data indicate that pathway 2
through phenacylanthranilates is the most appropriate
one for 3HQ synthesis. Even dialkylamino derivatives
can be easily obtained with substitution of the fluorine
O
O
O
O
OH
Ar
OH
Ar
H2O2/NaOH
EtOH
H2O2/NaOH
EtOH
O
EtOH
1
2
°
80 C
Ar
Ar
NH
R
NH
R
N
R
N
R
1, 4, 5
O
O
O
OH
Br
O
120 °C
K2CO3
DMF
OH
O
+
Ar
PPA
2 h
N
R
Ar
1 - 5, 6a,b
NH
R
NH
Ar
O
R
Scheme 1.

D. A. Yushchenko et al. / Tetrahedron Letters 47 (2006) 905–908
907
tum yields are generally higher than those reported for
corresponding 3HFs.5
O
O
OH
OH
HNAlk2
According to the intensity of the short wavelength band
(N*-band) in the fluorescence spectra, the 3HQ deriva-
tives can be divided into three groups that differ consid-
erably by their fluorescence properties. In the first group
composed of compounds 1 and 6a (with R = H), the
short wavelength band exhibits low intensity and limited
sensitivity to solvent polarity, as could be seen from the
changes of the maxima emission wavelengths and two
band intensity ratios (Fig. 2). The second group com-
prises the N-methyl substituted quinolones 2–5. In this
group, considerable changes of the two band intensity
ratio were observed upon changes in the solvent polarity
(Fig. 3, Table 2). Such peculiar emission of these 3HQs
is very similar to fluorescence that was observed for
some 3HFs and can be explained in the same way.10
As an example, the intensity ratio changes by two orders
of magnitude from the less polar toluene (0.04) to
DMSO (2.04) for 3HQ 2. As a consequence, the com-
pounds of this group may constitute suitable polarity
sensors in a wide range of solvents. Finally, the third
group is composed with the dialkylamino substituted
compounds 6b, 7 and 8. The two emission bands of these
compounds were observed to collapse in protic solvents
(spectra not shown). This undesired effect may be
related to the formation of hydrogen bonded complexes
between the protic solvent molecules and both tauto-
meric forms of the dialkylamino 3HQ derivatives. These
complexes markedly shift the N* band to the red region
N
N
200 °C
2 h
CH3
CH3
NAlk2
F
2
7, 8
Scheme 2.
atom by an amino group on the last step of this
pathway.
2.2. Fluorescence properties
As expected, all the 3HQ compounds exhibit two emis-
sion bands in almost all tested solvents, evidencing the
occurrence of ESIPT for all these compounds. Though
most 3HQ derivatives exhibit similar absorption spectra,
their emission spectra were found to depend on the aro-
matic group in position 2. Indeed, the electron donating
properties of this aromatic group substantially influence
the N*/T* ratio (Fig. 1). A detailed study of this
phenomenon will be published separately. Here, we will
discuss only how the solvent polarity influences the
fluorescence properties of 3HQs.
All the studied 3HQs exhibit similar fluorescence quan-
tum yields in all tested solvents (Table 2). These quan-
Ar:
1.4
1.0
Thiophenyl (3)
Toluene
1.2
p-F-C6H4
(2)
CH3CN
0.8
1.0
0.8
0.6
0.4
0.2
0.0
p-Me-C6H4 (4)
p-MeO-C6H4 (5)
EtOH
0.6
0.4
0.2
0.0
MeOH
DMSO
350 400 450 500 550 600 650 700 750
400
450
500
550
600 650
700
Wavelength, nm
Wavelength, nm
Figure 1. Normalized fluorescence spectra of N-methyl-3HQs 2–5 in
DMF. The two bands correspond to the emission of the normal (N*)
and tautomeric (T*) forms resulting from ESIPT.
Figure 2. Normalized fluorescence spectra of 3HQ 1 (R = H) in
organic solvents.
Table 2. Spectroscopic properties of 3HQ 2 in organic solvents27
Solvent
Absorption kabs (nm) Extinction coefficient
Fluorescence
kN* (nm) kT* (nm)
Intensity ratio IN*/IT* Fluorescence quantum yield (%)a
Toluene
Chloroform 368
Acetonitrile 365
Ethanol
Methanol
DMF
372
11,600
12,500
11,600
12,300
10,600
12,000
11,800
419
418
418
428
420
425
430
524
515
521
519
515
531
529
0.040
0.066
0.11
0.48
0.93
0.79
2.03
26.2
24.0
16.1
13.4
21.7
13.5
31.9
365
365
369
370
DMSO
a As a reference was used quinine sulfate in 1.0 N H2SO4.

908
D. A. Yushchenko et al. / Tetrahedron Letters 47 (2006) 905–908
Supplementary data
1.0
0.8
0.6
0.4
0.2
0.0
Toluene
CH3CN
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2005.11.160.
EtOH
MeOH
DMSO
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Acknowledgements
This work was supported by Association Franc¸aise
contre les Myopathies (Contract #LSHB-CT 2003-
50311480) and Eco-Net Program from French Ministere
´
´
des Affaires Etrangeres. D. Yushchenko is a member of
´
College Doctoral Europeen and was supported by the
25. Diwu, Z.; Beachdel, C.; Klaubert, D. H. Tetrahedron Lett.
1998, 39, 4987–4990.
TRIOH European Program and CNRS. The laboratory
of Drs Andre Mann and Jean Suffert is acknowledged
´
26. Synthetic procedures are described in Supplementary data.
27. Absorption and fluorescence spectra were recorded at
room temperature on a Cary 4 spectrophotometer (Var-
ian) and a FluoroMax 3.0 (Jobin Yvon, Horiba) spectro-
fluorometer, respectively. Excitation wavelength for
emission spectra was 360 nm in all cases.
for providing support for organic synthesis. A special
acknowledgment is addressed to Dr. Andrey S. Klym-
chenko for discussions and advices. We are also
thankful to Professor A. P. Demchenko for the
comments.