Development of a library of 6-arylcoumarins as candidate fluorescent sensors

Article abstract of DOI:10.1021/ol070142z

A bromocoumarin scaffold (1) was reacted wtth various boronic acid derivatives (2a-l) to afford a library of 6-arylcoumarins (3a-1). This library was found to contain candidate fluorescent sensors for peptidase activity and for nitric oxide.

Full text of DOI:10.1021/ol070142z

ORGANIC
LETTERS
2007
Vol. 9, No. 7
1315-1318
Development of a Library of
6-Arylcoumarins as Candidate
Fluorescent Sensors
Tomoya Hirano,* Kenichi Hiromoto, and Hiroyuki Kagechika*
School of Biomedical Science, Institute of Biomaterials and Bioengineering,
Tokyo Medical and Dental UniVersity, 2-3-10 Kanda-Surugadai, Chiyoda-ku,
Tokyo 101-0062, Japan
hiraomc@tmd.ac.jp; kageomc@tmd.ac.jp
Received January 19, 2007
ABSTRACT
A bromocoumarin scaffold (1) was reacted with various boronic acid derivatives (2a−l) to afford a library of 6-arylcoumarins (3a−l). This library
was found to contain candidate fluorescent sensors for peptidase activity and for nitric oxide.
Functional fluorescent molecules are useful in many fields
of scientific research, including analytical chemistry or cell
biology. For example, fluorescent molecules, whose fluo-
rescence properties can be changed by binding to or reacting
with ions, small molecules, or enzymes, enable us to estimate
the concentration or activity of the targets.1,2 Although some
theoretical approaches to the design of such sensors have
been reported,2,3 most currently available sensors were
developed empirically. Combinatorial synthesis of libraries
of fluorescent molecules is an effective approach.4 Chang
et al. have developed libraries of styryl dyes bearing DNA-
sensitive fluorescent sensors5 and â-amyloid sensors,6 and
Finney et al. developed a fluorescent Hg2+ sensor via
construction of a library.7
Coumarins are of interest because of their pharmacological
activity8 and photophysical properties, and they have been
applied as laser dyes9 and fluorophores for fluorescent
sensors.10 Bauerle et al.11 and Wang et al.12 reported coumarin
libraries with diversity at the 3-position via Suzuki-Miyaura,
(5) Lee, J. W.; Jung. M.; Rosania, G. R.; Chang, Y.-T. Chem. Commun.
2003, 1852-1853.
(6) Li, Q.; Lee, J.-S.; Ha, C.; Park, C. B.; Yang, G.; Gan, W. B.; Chang,
Y.-T. Angew. Chem., Int. Ed. 2004, 43, 6331-6335.
(7) Mello, J. V.; Finney, N. S. J. Am. Chem. Soc. 2005, 127, 10124-
10125.
(8) (a) Romines, K. R.; Morris, J. K.; Howe, W. J.; Tomich, P. K.; Horng,
M.-M.; Chong, K.-T.; Hinshaw, R. R.; Anderson, D. J.; Strohbach, J. W.;
Turner, S. R.; Mizak, S. A. J. Med. Chem. 1996, 39, 4125-4130. (b) Raad,
I.; Terreux, R.; Richomme, P.; Matera, E.-L.; Dumontet, C.; Raynaud, J.;
Guilet, D. Bioorg. Med. Chem. 2006, 14, 6979-6987.
(9) Koefod, R. S.; Mann, K. R. Inorg. Chem. 1989, 28, 2285-2290.
(10) (a) Komatsu, H.; Miki, T.; Citterio, D.; Kubota, T.; Chindo, Y.;
Kitamura, Y.; Oka, K.; Suzuki, K. J. Am. Chem. Soc. 2005, 127, 10798-
10799. (b) Setsukinai, K.; Urano, Y.; Kikuchi, K.; Higuchi, T.; Nagano, T.
J. Chem. Soc., Perkin Trans. 2 2000, 2453-2457.
(1) Lackowitz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.;
Springer Science: New York, 2006.
(2) de Silva, A. P.; Gunaratne, H. Q.; Gunnlaugsson, T.; Huxley, A. J.
M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97,
1515-1566.
(3) (a) Miura, T.; Urano, Y.; Tanaka, K.; Nagano, T.; Ohkubo, K.;
Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 8666-8671. (b) Urano, Y.;
Kamiya, M.; Kanda, K.; Ueno, T.; Hirose, K.; Nagano, T. J. Am. Chem.
Soc. 2005, 127, 4888-4894.
(4) (a) Finney, N. S. Curr. Opi. Chem. Biol. 2006, 10, 238-245. (b)
Zhu, Q.; Yoon, H.-S.; Parikh, P. B.; Chang, Y.-T.; Yao, S. Q. Tetrahedron
Lett. 2002, 43, 5083-5086. (c) Cao, H.; Chang, V.; Hernandez, R.; Heagy,
M. D. J. Org. Chem. 2005, 70, 4929-4934.
(11) Schiedel, M.-S.; Briehen, C. A.; Bauerle, P. Angew. Chem., Int.
Ed. 2001, 40, 4677-4680.
(12) Silvakumar, K.; Xie, F.; Cash, B. M.; Long, S.; Barnhill, H. N.;
Wang, Q. Org. Lett. 2004, 6, 4603-4606.
10.1021/ol070142z CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/28/2007

Heck, Sonogashira-Hagihara, or Husigen cycloaddition
reactions; they obtained coumarins with a wide range of
absorbance, fluorescence wavelength, and fluorescence in-
tensity. Here, we report the development of a coumarin
library with diversity at the 6-position via Suzuki-Miyaura
coupling.
Scheme 2. Construction of the 6-Arylcoumarin Library
Compound 113 was expected to be a good scaffold for
construction of a 6-arylcoumarin library. The reaction
conditions were optimized for the reaction of 1 with
unsubstituted phenyl boronic acid pinacol ester 2a (Scheme
1). Competitive debromination and ester-hydrolysis were
Scheme 1
considered likely to reduce the yield of the coupling reaction,
or to complicate purification. Typical reaction conditions,
using Pd(PPh3)4 as a catalyst and sodium carbonate as a base
in THF-H2O, yielded both undesired byproducts, and the
coupling efficiency was relatively low. In an aprotic solvent,
such as DMF or DME, no coupling reaction occurred, but
the addition of one-third volume of ethanol and the use of a
weaker base, CsF, allowed the coupling reaction to proceed,
affording 3a in 50-60% yield, although the debrominated
product was also obtained. We examined other Pd catalysts,
and found that PdCl2(dppf) is effective in this reaction. Thus,
a mixture of 1, 2a (1.5 equiv), PdCl2(dppf) (0.1 equiv), and
CsF (2.5 equiv) in DMF at 60 °C afforded 3a in 55% yield
without formation of the debrominated molecule. In this case,
the addition of ethanol was unnecessary. By increasing the
amounts of 2a to 3.0 equiv, PdCl2(dppf) to 0.2 equiv, and
CsF to 5.0 equiv, the yield of 3a was improved to 72%, and
these conditions were adopted for the coupling reaction with
other boronic acid derivatives.
As shown in Scheme 2, the scaffold 1 was reacted with
12 boronic acid pinacol ester derivatives (2a-l), yielding
the corresponding 6-arylcoumarins, 3a-l. Although the
compounds with an electron-withdrawing group, such as nitro
(2e), cyano (2i), or methoxycarbonyl (2j), were obtained in
relatively low yield (28%, 45%, and 46%, respectively), most
of the reactions proceeded in moderate yield. Some functional
groups on the phenyl group were rather easily transformed,
as observed in the syntheses of 3m, 3n, and 3o.
The fluorescence intensities of these molecules in aceto-
nitrile, methanol, and water (1 mM sodium phosphate buffer
at pH 7.4) were measured (Table 1). The absorption
maximum wavelength of each compound was around 410
nm in acetonitrile, 415 nm in methanol, and 430 nm in water
(Figure 1i-iii; Table S1 in the Supporting Information),
while the fluorescence intensities varied. Compound 3j had
the strongest intensity in each solution, whose quantaum yield
was similar to that of ethyl 7-diethylaminocoumarin-3-
carboxylate (unsubstituted at 6-position), a well-known
fluorophore as labeling or sensing biomolecule. Amino-
substituted 3b or o-diamino-substituted 3n had almost no
(13) Corrie, J. E. T.; Munasinghe, V. R. N.; Rettig, W. J. Heterocycl.
Chem. 2000, 37, 1447-1455.
1316
Org. Lett., Vol. 9, No. 7, 2007

Figure 1. Absorption and emission spectra of 6-arylcoumarin compounds. Absorption spectra of (i) 3b (dotted line), 3c (solid line), (ii)
3n (dotted line), 3o (solid line), and (iii) 3f (dotted line), 3g (solid line) were measured in 3 µM solution in sodium phosphate buffer (pH
7.4, containing 0.3% DMSO as a cosolvent). Fluorescence spectra for (iv) 3b and 3c, (v) 3n and 3o, and (vi) 3f and 3g excited at 420 nm
in the same solvent are shown.
fluorescence, while the acetamido 3c and triazole 3o
compounds had moderate fluorescence. These data suggested
that 6-phenylcoumarin would be a good fluorophore for
sensors of peptidase activity, which transforms an amido
group to an amino group, and of nitric oxide, which reacts
with an o-diamino group in the presence of oxygen to form
triazole14 (Figure 1iv,v). The transformation of the methoxy
group (3g) to a hydroxyl group (3f) also induced a change
of fluorescence intensity, which suggested that this molecule
might be useful as a fluorescent sensor for dealkylating
enzymes, such as glycosidase (Figure 1vi). Any compound,
except 3o, showed no significant pH-dependency around
neutral condition (pH 6.0-8.0). Compound 3o showed 50%
decrease of the fluorescent intensity at pH 8.0 compared with
that at pH 6.0 (data not shown). This decrease might be
derived from the deprotonation of the triazole ring as shown
in the reported o-diamine-based fluorescent sensors for nitric
oxide, and this pH-dependency might be lost by the monom-
ethylation of one of two amine nitrogens.14
Table 1. Fluorescence Intensity of 6-Arylcoumarinsa
The fluorescent property of 7-aminocoumarin derivatives
can be controlled partially by intramolecular charge transfer
(ICT) and twisted intramolecular charge transfer (TICT).1,15
In our library, the proximity between aryl ring and diethy-
lamine moiety might influence those mechanisms. In addi-
tion, the characteristic of fluorescent intensity is similar to
that seen with fluorescein derivatives or Tokyo Green
derivatives, whose fluorescent properties are controlled by
photoinduced electron transfer (PeT).3 At present, no direct
evidence of such mechanisms has been obtained, and the
detailed mechanistic investigations, such as measurements
compd
CH3CN
CH3OH
water
3a
3b
3c
3d
3e
3f
3g
3h
3i
13.2
<1
29.6
<1
16.3
14.1
23.0
19.3
24.1
63.2
<1
8.8
<1
30.0
<1
15.7
7.8
17.5
6.0
13.6
<1
20.4
<1
11.4
3.7
15.8
2.7
11.2
48.2
<1
18.2
38.2
<1
3j
3l
3m
3n
3o
24.7
<1
21.9
18.6
<1
15.5
24.1
<1
14.0
(14) (a) Kojima, H.; Nakatsubo, N.; Kikuchi, K.; Kawahara, S.; Kirino,
Y.; Nagoshi, H.; Hirata, Y.; Nagano, T. Anal. Chem. 1998, 70, 2446-
2453. (b) Gabe, Y.; Urano, Y.; Kikuchi, K.; Kojima, H.; Nagano, T. J. Am.
Chem. Soc. 2004, 126, 3357-3367. (c) Sasaki, E.; Kojima, H.; Nishimatsu,
H.; Urano, Y.; Kikuchi, K.; Hirata, Y.; Nagano, T. J. Am. Chem. Soc. 2005,
127, 3684-3685.
(15) (a) Jones, G., II; Jackson, W. R.; Halpern, A. M. Chem. Phys. Lett.
1980, 72, 391-395. (b) Dahiya, P.; Kumbhakar, M.; Murkherjee, T.; Pal,
H. Chem. Phys. Lett. 2005, 414, 148-154.
a Fluorescence intensity at the excitation and emission maximum
wavelengths of each compound (3 µM) was measured in acetonitrile,
methanol, and water (1 mM sodium phosphate buffer (pH 7.4)). All solutions
contained 0.3% DMSO as a cosolvent.
Org. Lett., Vol. 9, No. 7, 2007
1317

by single wavelength excitation, time-resolved measurement
of fluorescent decays, and lifetimes, are in progress.
Acknowledgment. T.H. thanks The Mochida Memorial
Foundation for Medical and Pharmaceutical Research for fi-
nancial support. The work described in this paper was par-
tially supported by Grants-in-Aid for Scientific Research from
The Ministry of Education, Science, Sports and Culture,
Japan.
In conclusion, our results indicate that library synthesis
of fluorescent molecules is potentially useful for finding lead
compounds for the development of functional molecules.
Introduction of an aromatic substituent ortho to the 7-di-
ethylamino group on the coumarin ring afforded the unique
fluorescent molecules that can be applied to some sensors.
This strategy may also be applicable to other fluorophores
or the chromophores of long-lifetime fluorescent lanthanide
complexes.16 By utilizing the present approach in combina-
tion with the introduction of diversity at other sites of the
coumarin structure, e.g., at the 3-position,11,12 it should be
possible to construct large libraries of candidate sensors.
Supporting Information Available: All experimental
procedures including synthesis details and identification of
each compound. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL070142Z
(16) Terai, T.; Kikuchi, K.; Iwasawa, S.; Kawabe, T.; Hirata, Y.; Urano,
Y.; Nagano, T. J. Am. Chem. Soc. 2006, 128, 6938-6946.
1318
Org. Lett., Vol. 9, No. 7, 2007