3-Hydroxybenzo[g]quinolones: dyes with red-shifted absorption and highly resolved dual emission

Article abstract of DOI:10.1016/j.tetlet.2009.06.024

New derivatives of 3-hydroxyquinolone (3HQ) with a fused benzene ring (3-hydroxybenzo[g]quinolones) have been synthesized. They display a remarkable red shift of their absorption spectrum in comparison with other 3HQ analogs allowing their excitation by c

Full text of DOI:10.1016/j.tetlet.2009.06.024

Tetrahedron Letters 50 (2009) 4714–4719
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
3-Hydroxybenzo[g]quinolones: dyes with red-shifted absorption and highly
resolved dual emission
Mykhailo D. Bilokin a, , Volodymyr V. Shvadchak a,b, Dmytro A. Yushchenko a,b, Andrey S. Klymchenko b,
*
Guy Duportail b, Yves Mely b, Vasyl G. Pivovarenko a,b,
*
a Department of Chemistry, National Taras Shevchenko University of Kyiv, 01601 Kyiv, Ukraine
b Laboratoire Biophotonique et Pharmacologie, UMR 7213 du CNRS, Université de Strasbourg, Faculté de Pharmacie, 74 route du Rhin, 67401 Illkirch, France
a r t i c l e i n f o
a b s t r a c t
Article history:
New derivatives of 3-hydroxyquinolone (3HQ) with a fused benzene ring (3-hydroxybenzo[g]quinolones)
have been synthesized. They display a remarkable red shift of their absorption spectrum in comparison
with other 3HQ analogs allowing their excitation by common He/Cd and Ar–ion lasers. As a result of their
irreversible excited-state intramolecular proton transfer (ESIPT) reaction, they display a dual fluorescence
in a series of solvents of varying polarities, starting from toluene to methanol. The dual emission of these
dyes correlates well with solvent H-bond basicity, which is connected with the effect of this solvent prop-
erty on the kinetics of the ESIPT reaction. In addition to their red-shifted absorption and fluorescence,
these new derivatives show a larger separation of their two emission bands and a more appropriate range
of their intensity ratio than the previously synthesized 3HQs. These properties allow an improved ratio-
metric evaluation of the local H-bond basicity of unknown environments, which will favor future appli-
cations of the new dyes in polymer and biological sciences.
Received 16 April 2009
Revised 31 May 2009
Accepted 3 June 2009
Available online 9 June 2009
Keywords:
Fluorescent sensors
Ratiometric probes
Intramolecular proton transfer
ESIPT
3-Hydroxyquinolones
Solvent basicity
Ó 2009 Elsevier Ltd. All rights reserved.
Hydrogen-bond acceptor ability
1. Introduction
However, despite their advantages over common single-band
probes, 3HFs exhibit relatively low photostability and quantum
Fluorescence methods are powerful tools for investigating bio-
molecular interactions.1 There is notably a strong demand for the
development of new fluorescent probes that could sense the elec-
tric fields as well as the H-bond donor and H-bond acceptor abili-
ties in protein binding sites. In this respect, probes with a dual
emission (two-channel emitters) present the strong advantage
over intensiometric probes (single-channel emitters) to give a
ratiometric response independent of the probe concentration. Ex-
cited state intramolecular proton transfer (ESIPT)2–4 is a very effec-
tive approach for the design of probes with dual fluorescence. The
ESIPT reaction leads to the formation of two tautomers (normal N*
and tautomeric T* forms) in the excited state of the probe, which
exhibit highly separated emission bands. The most interesting rep-
resentatives of ESIPT probes are 3-hydroxyflavones and their deriv-
atives (3HFs).5,6 They have been shown to be effective tools for
investigating the polarity6–8, H-bond donor ability,8,9 electronic
polarizability8, and electrostatic effects in different media10,11
including model lipid membranes,12–15 cell membranes15–17, and
proteins.18,19 Moreover, these dyes were shown to be useful to
determine the nature and concentration of cations and anions.20–22
yields in water that limit their applications. As a consequence,
the development of new dual-fluorescence probes with improved
fluorescent properties is strongly required.
3-Hydroxyquinolones (3HQs, 1) are structural aza-analogs of
the 3HFs that also display a well-resolved dual emission due to
an ESIPT reaction.23–25 These recently introduced dyes present
important advantages over 3HFs, since they are more photostable
and exhibit higher fluorescence quantum yields in aqueous solu-
tions.23 3HQ dyes were shown to be effective in sensing the viscos-
ity of protic solvents26 as well as both the polarity and basicity of
organic media.27,28
O
H
O
O
H
O
N
H
Ar
N
H
Ar
1a-c
2a-c
Ar:
c
a
b
O
S
* Corresponding authors. Tel.: +38 044 239 33 12; fax: +38 044 220 83 91 (V.G.P.).
E-mail address: pvg_org@ukr.net (V.G. Pivovarenko).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.06.024

M. D. Bilokin et al. / Tetrahedron Letters 50 (2009) 4714–4719
4715
ascribed both to faster non-radiative deactivation of the T* form
and a slower ESIPT reaction for the dyes with the fused benzene
ring. A strong effect on the IN*/IT* ratio is also observed on variation
of the 2-Ar group, similarly to those observed for 3HF derivatives.33
In both 1 and 2 series, the IN*/IT* ratio increases significantly from a
to c, while the fluorescence quantum yield decreases. The observed
changes in the IN*/IT* ratio on increase in the electron-donor ability
of the 2-Ar group could be due to both faster non-radiative deacti-
vation of the T* form and a slower ESIPT reaction.
Laser excitation sources are frequently used in biological exper-
iments. To avoid autofluorescence and photo-degradation of the
biological samples, it is important to use an excitation wavelength
longer than 420 nm. Concerning dual-fluorescence probes, only
few 3HF dyes can be excited by laser sources at 442 or 452 nm,29
while none of the reported 3HQs exhibit significant absorbance
at such wavelengths. In this respect, we present herein the synthe-
sis and spectroscopic properties of new dyes of the 3HQ family
(series 2), which possess one additional benzene ring fused by
the border [g] to the quinolone system. These dyes have been de-
signed by considering the principle that the increase in the length
Similarly to the parent analogs, dyes 2a–c show a strong depen-
dence of their dual emission on the solvent properties. In the polar
solvent methanol, presenting high H-bond donor and acceptor
abilities, the IN*/IT* ratio is much larger than in toluene, which is
apolar and unable to form intermolecular H-bonds (Table 1,
Fig. 3). Previously, the IN*/IT* ratio of 3HQs was shown to depend
strongly on the H-bond accepting ability32,33 (basicity)36 of sol-
vents. Therefore, we studied in detail the correlation of the IN*/IT*
ratio of the benzo-analog dye 2c with solvent basicity for an
extended range of solvents (Fig. 4). Remarkably, in contrast to
3-hydroxyflavones8,9 and N-methyl-3HQs,25,27 for which the IN*/IT*
intensity ratio was largely governed by the solvent polarity, the
IN*/IT* ratio of 3HQ-Bf did not show any regular dependency on
the empirical ET(30)30 polarity parameter or the Lippert polarity
or polarizability functions (Fig. 4a–c). Moreover, this ratio did not
show any correlation with other solvent parameters such as the
Hildebrand’s solubility (Fig. 4d),30 or Abraham’s hydrogen bond
donor acidity parameter (Fig. 4e).31 In contrast, the IN*/IT* ratio
was found to correlate well with solvent basicity (log (IN*/
IT*) = À1.35 + 1.71b, r2 = 0.92, Fig. 4f), as for the parent analog
1c.28 Taking into account that the solvent nature does not modify
considerably the fluorescence quantum yield of 2a–c dyes, the ob-
served solvent-dependent variations of the IN*/IT* ratio are mainly
due to changes in the ESIPT rate. The solvatochromic shifts for dyes
2a–c were smaller than for dyes 1a–c. For example, dye 1c, which
was proposed as a basicity sensor28 exhibits a shift of up to 16 nm
in its N*-band position in a set of 20 solvents of different polarities,
while dye 2c exhibits only a 9-nm shift for the same series of sol-
vents (Table 1). This weaker solvatochromism suggests that dyes
2a–c exhibit a lower difference between their S1 and S0 dipole mo-
ments than dyes 1a–c.
Another important feature in the fluorescence spectra is the
high separation between the two emission bands, which is about
120–150 nm (4200–4900 cmÀ1) for all the dyes of the series 2a–
c, and reaches 160 nm (5200 cmÀ1) for 2c in tetrahydrofuran
(Fig. 3). Such a high energy difference between the emission bands
will facilitate the applications of these probes for ratiometric mea-
surements and fluorescence microscopy imaging of biological
samples.
Additional arguments on ESIPT kinetics controlling the dual
emission of 2a–c stem from the time-resolved fluorescence decays
obtained for both N* and T* forms of 2a in a series of solvents. In
line with the theory of ESIPT reactions,34 the fluorescence decays
of both forms can be deconvoluted into fast and slow components.
Moreover, the decay curve corresponding to the T* emission band
contains a raising component with a negative amplitude a1 and a
of the conjugated
p-electron system should lead to a red shift of
the absorption band. Then, to further modulate the spectroscopic
properties we introduced 2-aryl substituents of different length
and electron donor ability. The new dyes exhibit superior absorp-
tion and fluorescence properties compared to their parent 3HQ
analogs.
2. Results and discussion
Synthesis (Scheme 1) and purification of the 3HQ dyes 1–2 as
well as their physical properties and their structural data are de-
scribed in the Experimental section. The absorption and fluores-
cence properties of the new dyes 2a–c (benzo-analogs) were
studied and compared with those of the parent dyes 1 in protic
(methanol, ethanol) and aprotic, (toluene, ethylacetate, dichloro-
methane, tetrahydrofuran, THF, acetonitrile, dimethylformamide,
DMF, dimethylsulfoxide, DMSO) solvents of different polarity.30
Data for representative solvents are given in Table 1 and Fig. 1.
All benzo-analogs 2a–c show a considerable (70–90 nm, or 3700–
5000 cmÀ1) red-shift of the absorption maximum as compared to
1a–c. Remarkably, such red-shifted absorption makes them conve-
nient for excitation by He/Cd and Ar–ion lasers.
This red-shift is probably due to the more extended conjugation
of the benzoquinolone ring in comparison with the quinolone ring.
Moreover, the red-shifted absorption of 2b and 2c as compared to
2a is in line with the stronger electron donating ability of their cor-
responding Ar substituent and probably, with their more planar
structure.28 Comparison of the positions of the absorption maxima
for 2a–c in aprotic solvents (Table 1) shows that absorption of ben-
zoquinolones is practically not sensitive to the solvent polarity.
Accordingly, the blue shifts in the absorption spectra of 2a–c in
protic solvents can be ascribed to the effects of specific hydrogen
bonding.
Benzo-analogs 2a–c exhibit two bands in the emission spectra,
which suggests that they undergo an ESIPT reaction, similarly to
their parent analogs 1a–c. The positions of both N* and T* emission
bands in these dyes are shifted to longer wavelengths as compared
to those of the corresponding parent dyes 1a–c (Table 1, Fig. 2). The
total fluorescence quantum yields for 2a–c series are lower than
for 1a–c and do not depend on the solvent nature. The only excep-
tion is water, where their values are below 1%. For all the studied
solvents, the intensity ratio of the two emission bands, IN*/IT*, in
2a–c dyes is larger than in 1a–c. Taking into account changes in
the quantum yields, these larger values of the IN*/IT* ratio can be
O
O
Br
O
DMF/ K2CO3
NH2
Br2
OH
Ar
PPA
O
O
Ar
5a-c
Ar
Ar
4a-c
N
H
NH2
OH
O
O
3a-c
2a-c
Scheme 1. Synthesis of benzo[g]quinolones 2a–c.

4716
M. D. Bilokin et al. / Tetrahedron Letters 50 (2009) 4714–4719
Table 1
Spectroscopic properties of 3HQs and 3HBQs in solutionsa
Solvent
Ar
kabs (nm)
kN* (nm)
kT* (nm)
IN*/IT*
u (%)
1
2
1
2
1
2
1
2
1
2
1. Toluene
2. EtOAc
3. CH2Cl2
4. CH3CN
5. MeOH
a
b
c
a
b
c
a
b
c
a
b
c
371
379
382
363
377
383
366
377
383
362
375
381
361
377
384
448
461
468
447
459
467
447
459
469
445
458
466
442
452
461
419
432
428
403
431
438
423
434
436
424
433
436
425
417
444
493
477
493
478
476
481
475
474
478
475
476
479
476
481
488
505
524
530
513
522
527
507
516
521
510
518
523
516
508
526
610
620
633
611
619
631
602
609
625
608
613
625
610
607
616
0.01
0.02
0.06
0.03
0.02
0.07
0.02
0.02
0.05
0.02
0.03
0.09
0.14
0.16
0.21
0.04
0.07
0.22
0.11
0.13
0.41
0.07
0.09
0.21
0.12
0.14
0.47
0.75
0.94
1.42
38
34
22
35
37
25
40
50
32
32
31
26
33
30
14
9.7
6.3
4.4
8.3
6.7
4.1
13.2
8.5
5.5
9.0
6.7
4.7
13.4
9.7
7.0
a
b
c
a
kabs is the position of the absorption maximum, kN* and kT* are the positions of the fluorescence maxima of the N* and T* forms, respectively, u is the total fluorescence
quantum yield.
Figure 1. Absorption spectra of benzo[g]quinolones 2a–c in methanol.
decay time corresponding to the fast emission decay component s1
of the N* form (Table 2), evidencing clearly that the T* excited state
originates from the N* excited state through an ESIPT reaction. The
long-lived decay time component s2 contributes greatly to the T*
emission, but only marginally to the N* emission, suggesting that
the ESIPT reaction is close to irreversible on the mechanism.
In the case of dye 2a, the forward ESIPT reaction rate, which is
inversely proportional to s
1, is about 2.5-fold slower than for 1a,32
in line with the conclusion drawn from the comparison of their
steady-state spectra. This indicates differences in the proton-trans-
fer process between 1a and 2a. These differences induced by the
additional benzene ring suggest a lower basicity of the carbonyl
oxygen of 2a and/or a lower acidity of the 3-OH group in the S1
state, which could slow down the ESIPT rate reaction.34
Figure 2. Normalized fluorescence spectra of quinolones 2a (panel a) and 1a (panel
b) in organic solvents. Excitation wavelength was fixed at the absorption maximum
in each case.
The nature of the solvent also affects strongly the ESIPT
kinetics. With increasing solvent basicity in the sequence
3. Conclusion
MeCN?DMF?DMSO, the fast time component
s
1 shows a gradual
Three new fluorescent dyes 2a–c were synthesized on the basis
of the 3-hydroxybenzo[g]-quinolone moiety. They exhibit a red-
shifted absorption band, which allows their excitation by He/Cd
and Ar–ion lasers. They also display a dual fluorescence in a large
set of solvents, from apolar toluene to polar methanol, due to an
irreversible ESIPT reaction. The dual emission of these dyes corre-
lates perfectly with solvent basicity (H-bond accepting ability) due
to the direct effect of this property on the ESIPT kinetics. The red-
shifted absorbance, the large range of the fluorescence intensity ra-
tio IN*/IT*, and the large separation of the two emission bands of
these compounds should improve the ratiometric determination
of the basicity parameter of their environment, and should
increase from 0.12 ns up to 2.21 ns, suggesting that the ESIPT rate
decreases by more than one order of magnitude. This confirms that
the dramatic increase of the IN*/IT* ratio with basicity is connected
with the inhibition of the ESIPT reaction.28 Thus, similarly to other
3HQs,32,33,37 solvents of high H-bond basicity slow down the ESIPT
reaction of 2a probably by formation of an intermolecular H-bond
with its 3-OH group, which breaks the intramolecular H-bond
needed for the ESIPT reaction. As a consequence, the fast compo-
nent s1 may reflect the rate of restoration of the intramolecular
hydrogen bond followed by the fast and irreversible ESIPT
reaction.28

M. D. Bilokin et al. / Tetrahedron Letters 50 (2009) 4714–4719
4717
Figure 3. Normalized fluorescence spectra of 2c in different solvents. Excitation wavelength was fixed at the absorption maximum in each case. Fluorescence of solutions of
2c in toluene, THF, and DMSO under UV illumination.
Figure 4. Dependence of log (IN*/IT*) of dye 2c on the solvent ET30 polarity index (a), polarity (
e
À 1)/(2
e
+ 1) (b), and polarizability (n2 À 1)/(2n2 + 1) (c) functions,
Hildebrand’s solubility parameter dH (d),30 Abraham’s hydrogen bond donor acidity
a
(e) and hydrogen bond acceptor basicity parameter b (f).31 Numbering of solvents (1–5)
is as in Table 1. Additional solvents: 6-ethanol, 7-n-butanol, 8-dioxane, 9-THF, 10-DMF, and 11-DMSO. The IN*/IT* data for methanol and ethanol solutions were corrected (by
a factor of 2 and 1.7, respectively) to compensate for the increased bandwidth of the T* band.
4. Experimental
Table 2
Time-resolved fluorescence parameters of 2a in organic solvents
4.1. Materials and methods
Solvent
IN*/IT*
s1a, ns (a1
)
s
2, ns (a2)
All reagents were purchased from Sigma-Aldrich. Solvents for
synthesis were of reagent quality and appropriately dried if neces-
sary. For absorption and fluorescence studies, the solvents were of
spectroscopic grade. Melting points were determined on a ‘VEB
Analytik’, Dresden hostage microscope melting point apparatus,
and were uncorrected. Proton NMR spectra were recorded on a
Varian Mercury—400 MHz spectrometer. Tetramethylsilane (TMS)
was used as an internal standard in CDCl3 or DMSO-d6. Mass spec-
tra were measured with a Mass Spectrometer Mariner System
5155.
*
*
*
*
T
N
T
N
DMSO
DMF
MeCN
MeOH
1.47
0.71
0.12
1.31
2.21 (0.995)
0.83 (0.995)
0.12 (0.998)
0.83 (0.99)
2.28 (À0.46)
0.86 (À0.46)
0.13 (À0.48)
0.81 (À0.49)
6.05 (0.005)
5.06 (0.005)
4.99 (0.002)
5.20 (0.01)
5.24 (0.54)
4.83 (0.54)
5.39 (0.52)
5.30 (0.51)
a
IN*/IT* is the ratio of emission intensities of the N* and T* forms; s1
the short-lived and long-lived decay times, respectively, and a1
,
s2 (ns) are
,
a
2 are their relative
amplitudes.
facilitate their applications for ratiometric measurements and
fluorescence microscopy imaging.
Absorption spectra were recorded on a Cary 4 spectrophotome-
ter (Varian) and emission spectra on a FluoroMax 3.0 (Jobin Yvon,

4718
M. D. Bilokin et al. / Tetrahedron Letters 50 (2009) 4714–4719
Horiba) spectrofluorimeter at room temperature. In case of a struc-
tured emission spectrum, we selected the highest peak to deter-
mine the position of the maximum emission wavelength.
Fluorescence quantum yields u were determined with quinine
sulfate in 0.5 M sulfuric acid (u = 0.577) as a reference,35 and
for 2 with 4’-(N,N-diethylamino)-3-hydroxyflavone in ethanol
(u = 0.51).36 All measurements were carried out in a tempera-
ture-controlled cell at 20 0.1 °C. Time-resolved fluorescence mea-
surements were performed with the time-correlated, single-
photon counting technique using the frequency tripled output of
a Ti-Sapphire laser (Tsunami, Spectra Physics), pumped by a Mille-
nia X laser (Tsunami, Spectra Physics).11 The excitation wavelength
was set at 320 nm. The fluorescence decays were collected at the
magic angle (54.7°) of the emission polarizer. The single-photon
events were detected with a microchannel plate Hamamatsu
R3809U photomultiplier coupled to a Philips 6954 pulse preampli-
fier and recorded on a multichannel analyzer (Ortec 7100) cali-
brated at 25.5 ps/channel. The instrumental response function
was recorded with a polished aluminum reflector, and its full-
width at half-maximum was 50 ps. The time-resolved decays were
analyzed both by the iterative reconvolution method and the Max-
imum Entropy Method (MEM).37 The goodness of the fit was eval-
uated from the v2 values, the plots of the residuals and the
autocorrelation function.
J = 8.1 Hz, 1H, H-8); 7.36 (t, J = 8.8 Hz, 1H, H-7); 7.27 (s, 1H, H-
10), m/z (Int,%): 288 [M+1]+ (100).
1-Benzofuran-2-yl-3-hydroxybenzo[g]quinolin-4(1H)-one
(2c):
mp 291–293 °C,
e
max = 7700 l molÀ1 cmÀ1 (acetonitrile) 1H NMR
(400 MHz, DMSO-d6) d 11.45 (br s, 1H, NH); 8.81 (s, 1H, H-5);
8.45 (s, 1H, H-3’); 8.04 (d, J = 9.4 Hz, 1H, H-6); 7.92 (m, 2H, H-
9,5’); 7.76 (d, J = 8.8 Hz, 1H, H-7’); 7.72 (d, J = 8.5 Hz, 1H, H-4’);
7.51 (d, J = 9.1 Hz, 1H, H-7); 7.42 (m, 2H, H-6’,10); 7.35 (t,
J = 8.6 Hz, 1H, H-8), m/z (Int,%): 328 [M+1]+ (100).
Acknowledgments
This work was supported by the ECO-NET and ARCUS program
from the French Ministère des Affaires Etrangères between France,
Ukraine, and Russia. D.A.Y. and V.V.S. are recipients of a high-level
Eiffel Fellowship. V.G.P. was supported by a fellowship from Uni-
versity Louis Pasteur.
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Preparation of 4c. To a solution of 2-acetylbenzofuran (3.20 g,
20 mmol) in 50 ml mixture of dry dioxane and diethyl ether (5:1
in v/v) with 2–4 drops of 48% hydrobromic acid, 3.84 g (24 mmol)
of bromine was slowly added at 0 °C up to the formation of 4c
(TLC-control). The solution was stirred at room temperature for an-
other 30 min. Then the liquor were poured into 50 ml of ice water
to afford white needle crystals of 1-(1-benzofuran-2-yl)-2-bromo-
ethanone 4c (4.16 g, yield 87%), pure enough for the next step.
Preparation of 3c. To a solution of 3-amino-2-naphthoic acid
(2.81 g, 15 mmol) in 20 ml of DMF, 2.48 g (18 mmol) of potassium
carbonate were added. The mixture was heated to 70 °C and stirred
for 1 h. After cooling to room temperature, 3.11 g (13 mmol) of
1-(1-benzofuran-2-yl)-2-bromoethanone were added. Then the
mixture was heated to 50 °C and stirred for 1 h. After that, the mix-
ture was poured into 50 ml of ice water. The filtered precipitate
after washing with water and drying afforded white powder of
2-(1-benzofuran-2-yl)-2-oxoethyl-3-amino-2-naphthoate 4.08 g,
which was recrystallized from ethanol. Yield: 91%.
Preparation of 2c. The 2-(1-benzofuran-2-yl)-2-oxoethyl-3-ami-
no-2-naphthoate (0.52 g, 1.5 mmol) was added to 3.3 g of poly-
phosphoric acid and stirred at 120° for 2 h. After that, the
mixture was poured into 10 g of ice and neutralized by 10% aque-
ous sodium carbonate. Filtered precipitate after washing, drying
and recrystallization from DMF afforded the dark purple needles
of dye 2c, 0.38 g. Yield: 73%.
24. Gao, F.; Johnson, K.; Schlenoff, J. J. Chem. Soc., Perkin Trans. 2 1996, 269–273.
25. Yushchenko, D. A.; Bilokin’, M. D.; Pyvovarenko, O. V.; Duportail, G.; Mély, Y.;
Pivovarenko, V. G. Tetrahedron Lett. 2006, 47, 905–908.
26. Yushchenko, D. A.; Shvadchak, V. V.; Klymchenko, A. S.; Duportail, G.;
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3-Hydroxy-2-phenylbenzo[g]quinolin-4(1H)-one (2a): mp 237–
238 °C,
e
max = 7600 l molÀ1 cmÀ1 (acetonitrile). 1H NMR (400 MHz,
DMSO-d6) d 11.55 (br s, 1H, NH), 8.86 (br s, 1H, OH), 8.79 (s, 1H,
H-5); 8.15 (d, J = 8.9 Hz, 1H, H-6), 7.96–7.85 (m, 4H, ArH, Ar’–H’),
7.61–7.50 (m, 4H, Ar’H), 7.41 (t, J = 8 Hz, 1H, H-8), m/z (Int,%): 294
[M+1]+ (100).
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Pivovarenko, V. G. J. Fluorescence 2009, 19, 545–553.
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3-Hydroxy-2-(2-thienyl)benzo[g]quinolin-4(1H)-one (2b): mp
254–255 °C,
(400 MHz, DMSO-d6) d 11.48 (br s, 1H, NH), 8.75 (s, 1H, H-5),
8.21 (d, J = 8.7 Hz, 1H, H-6), 8.24 (d, J = 8.4 Hz, 1H, H-4’); 7.91 (d,
J = 7.9 Hz, 1H, H-3’); 7.89–7.77 (m, 2H, H-9, H5’); 7.45 (t,
e
max = 7100 l molÀ1 cmÀ1 (acetonitrile) 1H NMR

M. D. Bilokin et al. / Tetrahedron Letters 50 (2009) 4714–4719
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