Synthesis of novel push-pull-type solvatochromic 2′-deoxyguanosine derivatives with longer wavelength emission

Article abstract of DOI:10.1016/j.tetlet.2010.03.012

We synthesized novel push-pull-type fluorescent guanosine derivatives, ^CNG and ^AcG containing 1,6- and 2,7-disubstituted pyrene chromophores. 1,6-Disubstituted pyrene derivatives, 1,6-^CNG (3b) and 1,6-^AcG (3c),

Full text of DOI:10.1016/j.tetlet.2010.03.012

Tetrahedron Letters 51 (2010) 2606–2609
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Tetrahedron Letters
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Synthesis of novel push–pull-type solvatochromic 20-deoxyguanosine
derivatives with longer wavelength emission
*
*
Yoshio Saito , Azusa Suzuki, Kazutoshi Imai, Nobukatsu Nemoto, Isao Saito
Department of Chemical Biology and Applied Chemistry, School of Engineering, Nihon University, Koriyama, Fukushima 963-8642, Japan
a r t i c l e i n f o
a b s t r a c t
We synthesized novel push–pull-type fluorescent guanosine derivatives, CNG and AcG containing 1,6- and
Article history:
2,7-disubstituted pyrene chromophores. 1,6-Disubstituted pyrene derivatives, 1,6-CNG (3b) and 1,6-Ac
G
Received 16 February 2010
Revised 1 March 2010
Accepted 4 March 2010
Available online 10 March 2010
(3c), exhibited highly solvatochromic fluorescence emission at longer wavelength (ꢀ540 nm). The envi-
ronmentally sensitive fluorescent deoxyguanosines such as 3b and 3c can be used as powerful tools for
structural studies of nucleic acids and molecular diagnostics.
Ó 2010 Elsevier Ltd. All rights reserved.
Fluorescent nucleosides are widely used as reporter probes for
investigating chemical, biochemical, and biological phenomena,1
and numerous fluorescent nucleosides have been reported.2
Among these, modified fluorescent guanosines are of special
importance due to their unique properties of forming higher
ordered self-assembled structures by base pairing.3 The design of
environmentally sensitive fluorescent guanosine derivatives is,
therefore, very important not only as a DNA reporter probe or fluo-
rescence sensor but also as a fluorescent building block in supra-
molecular chemistry. In our continuous studies directed toward
the development of practically useful fluorescent nucleosides, we
have reported various types of pyrene-labeled fluorescent nucleo-
sides such as base-discriminating fluorescent nucleoside PyU,4
photochromic vinylpyrene-substituted guanosine 1,5 and pyrene-
substituted 8-ethynyl guanosine 2.6 While these pyrene-contain-
ing nucleosides are useful as fluorescent reporter molecules,7 there
is still great demand for further development of solvatochromic
guanosine derivatives possessing stronger fluorescence emission
at a longer wavelength greater than 500 nm. We now wish to
report an intriguing concept for the molecular design of novel
push–pull-type solvatofluorochromic guanosine derivatives 3 and
4 that emit strong fluorescence at ca. 540 nm. In order to construct
an intramolecular donor–acceptor system, an electron-withdraw-
ing aromatic group (acceptor) and C8 of guanosine (donor) are
directly attached to pyrene chromophore via triple bonds. We
report herein the synthesis and photophysical properties of novel
push–pull-type fluorescent guanosines which contain 1,6- and
2,7-disubstituted pyrene chromophores (Fig. 1).
The synthetic route of pyrene-labeled 20-deoxyguanosines,
1,6-HG (3a), 1,6-CNG (3b) and 1,6-AcG (3c), is outlined in Scheme
Pushdonor
acceptor
EWG
O
O
O
N
N
N
N
NH
NH2
NH
N
N
NH
NH2
N
N
N
NH2
Pull
HO
HO
HO
O
O
O
OH
OH
push-pull-type guanosine
Figure 1. Structure of pyrene-labeled fluorescent guanosine derivatives.
OH
1
2
3 and 4
* Corresponding authors.
E-mail address: saitoy@chem.ce.nihon-u.ac.jp (Y. Saito).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.012

Y. Saito et al. / Tetrahedron Letters 51 (2010) 2606–2609
2607
O
N
O
N
O
N
N
N
N
N
a)
N
N
NH
N
NH
N
NH
NH2
Br
Br
Br
7
N
N
a)
b), c)
HO
HO
HO
HO
O
O
O
10
OH
OH
Br
OH
5
7
6
O
N
O
N
N
N
b)
O
N
NH
N
N
N
R
NH
N
N
NH
N
N
HO
N
d)
O
N
N
O
HO
11a (R = CN)
11b (R = Ac)
O
OH
OH
OH
8
O
R
11a
or
11b
N
c)
NH
O
R
N
N
N
NH2
NH
N
N
e)
HO
O
N
N
HO
O
OH
4a (2,7-CNG: R = CN)
4b (2,7-AcG: R = Ac)
OH
9a (R = H)
9b (R = Ac)
9c (R = CN)
R
Scheme 2. Reagents and conditions: (a) 2,7-dibromopyrene, Pd(PPh3)4, CuI, Et3N,
80 °C, 3 h, 45%; (b) 4-ethynylbenzonitrile (for 11a) or 4-ethynylacetophenone (for
11b), Pd(PPh3)4, CuI, Et3N, DMF, 80 °C, 6 h, 71% (11a), 68% (11b); (c) NH4OH, MeOH,
rt, 24 h, 73% (4a), 81% (4b).
O
f)
N
N
NH
9a, 9b, or 9c
N
NH2
HO
O
OH
3a (1,6-HCNG: R = H)
3b (1,6- G: R = CN)
3c (1,6-AcG: R = Ac)
1.2
1
2
0.8
Scheme 1. Reagents and conditions: (a) DMF diethyl acetal, MeOH, 55 °C, 6 h,
quant; (b) trimethylsilylacetylene, Pd(PPh3)4, CuI, Et3N, DMF, 50 °C, 6 h; (c) K2CO3,
MeOH, rt, 4 h, 60% in two steps; (d) 1,6-dibromopyrene, Pd(PPh3)4, CuI, Et3N, DMF,
80 °C, 3 h, 59%; (e) ethynylbenzene (for 9a), 4-ethynylbenzonitrile (for 9b), or 4-
ethynylacetophenone (f or 9c), Pd(PPh3)4, CuI, Et3N, DMF, 80 °C, 5 h, 57% (9a), 78%
(9b), 75% (9c); (f) NH4OH, MeOH, rt, 12 h, 70% (3a), 77% (3b), 78% (3c).
3a
0.6
3b
0.4
3c
0.2
0
400
500
600
700
Wavelength (nm)
1. Sonogashira cross-coupling reaction8 of 8-bromo-20-deoxygua-
nosine derivative 6 with TMS–acetylene followed by deprotection
with K2CO3 yielded compound 7, which was then coupled with
1,6-dibromopyrene9 using Pd(PPh3)4 to afford 8. The third
Sonogashira coupling reaction of 8 with ethynylbenzene, 4-ethy-
nylacetophenone, and 4-ethynylbenzonitrile afforded correspond-
ing coupling products 9a–c, respectively. Deprotection of 9a–c
with 28% aq NH4OH–MeOH afforded 3a–c10, which were then eval-
uated for photochemical properties.
Figure 2. Normalized fluorescence emission spectra of 2, 1,6-HG (3a), 1,6-CNG (3b),
and 1,6-AcG (3c) in THF.
Similar results were obtained with 1,6-AcG (3c) (Fig. 3b). As
expected, these push–pull-type deoxyguanosine derivatives 3b
and 3c showed a remarkable solvatofluorochromic property, as
evident from the fluorescence image shown in Figure 3c.
The photophysical properties of 2,7-disubstituted pyrene deriv-
atives were also investigated. Fluorescence spectra of 2,7-CNG (4a)
and 2,7-AcG (4b) measured in different solvents are shown inFigure
4a and b. While fluorescence intensities of both 2,7-CNG (4a) and
2,7-AcG (4b) are strong in non-polar solvents such as chloroform
and ethyl acetate, very weak fluorescence was observed in polar
solvents such as DMF, acetonitrile, and MeOH. In the case of 2,7-
disubstituted pyrene derivatives, red-shift of fluorescence emis-
sion was not observed by changing solvent polarity, although
1,6-disubstituted pyrene derivatives showed a remarkable red-
shift with increasing solvent polarity.
The shapes of the fluorescence spectra of 1,6- and 2,7-disubsti-
tuted derivatives were also different. The sharp emission bands of
2,7-disubstituted derivatives are similar to the emission bands from
the vibrational structures of parent pyrene chromophore. In this
case, the fluorescence wavelength was not changed by changing sol-
vent polarity. On the other hand, 1,6-disubstituted derivatives show
a broad emission band and exhibited a solvent polarity-dependent
fluorescence emission at longer wavelength. For absorption spectra,
2,7-derivatives exhibited an intense absorption band at around
340 nm, whereas the absorption bands of 1,6-derivatives were
observed at around 290 and 420 nm (Table 1, Figs. S3 and S4). These
results clearly indicate that both 1,6- and 2,7-isomers have quite dif-
ferent photophysical properties to each other.
2,7-Disubstituted pyrene derivatives, 2,7-CNG (4a) and 2,7-Ac
G
(4b), were also synthesized according to Scheme 2 through a similar
reaction route. Compound 10 prepared from 2,7-dibromopyrene11
and 7 was coupled with ethynylaryl derivatives using Pd(PPh3)4 to
afford 11a and 11b. After deprotection of DMF acetal group, two
2,7-disubstituted pyrene derivatives, 2,7-CNG (4a) and 2,7-Ac
G
(4b), were obtained. The photophysical properties of all newly
synthesized guanosine derivatives 3a–c and 4a–b were examined.
Initially, we measured the fluorescence spectra of 1,6-disubsti-
tuted pyrene derivatives, 1,6-HG (3a), 1,6-CNG (3b), and 1,6-Ac
G
(3c), in THF. As shown in Figure 2, 1,6-HG (3a) having larger
p
-elec-
tron system than parent 2 emitted strong fluorescence at a longer
wavelength (523 nm) than that of 2. Interestingly, we found that
the introduction of electron-withdrawing substituent such as
cyano (3b) or acetyl (3c) group induced ca. 60 nm red shift than
parent 2. We also measured the fluorescence spectra of these
deoxy Gs in various solvents of different polarities. With excitation
of 1,6-CNG (3b) at 422 nm in chloroform, strong fluorescence emis-
sion at 493 nm was observed (Fig. 3a). Upon excitation of 1,6-CNG
(3b) at 424 nm in THF, we observed strong emission at 542 nm.
In ethyl acetate, medium fluorescence emission was observed at
531 nm. In contrast, the fluorescence emission of 1,6-CNG (3b) in
polar solvents such as acetonitrile, DMF, and MeOH was very weak.

2608
Y. Saito et al. / Tetrahedron Letters 51 (2010) 2606–2609
b 50
50
40
30
20
10
0
a
Chloroform
AcOEt
THF
Chloroform
40
AcOEt
30
THF
DMF
DMF
20
Acetonitrile
MeOH
Acetonitrile
10
MeOH
0
430 480 530 580 630 680
Wavelength (nm)
400 450 500 550 600 650 700
Wavelength (nm)
c
THF
DMF
CHCl3 AcOEt
CH3CN MeOH
Figure 3. Fluorescence spectra of (a) 1,6-CNG (3b) (2.5
l
M) and (b) 1,6-AcG (3c) (2.5
l
M) in various solvents. (c) Fluorescence colors of 1,6-CNG (3b) in different solvents. The
sample solutions were illuminated with a 365 nm transilluminator.
100
60
a
b
Chloroform
AcOEt
THF
Chloroform
50
80
60
40
20
AcOEt
40
THF
30
DMF
DMF
20
Acetonitrile
MeOH
Acetonitrile
10
0
MeOH
0
350 400 450 500 550 600 650
350 400 450 500 550 600 650
Wavelength (nm)
Wavelength (nm)
Figure 4. Fluorescence spectra of (a) 2,7-CNG (4a) (2.5
l
M) and (b) 2,7-AcG (4b) (2.5
lM) in various solvents.
Table 1
Photophysical properties of fluorescent guanosine derivatives and their HOMO and LUMO energy levels
Solventa
Ud
HOMOe (eV)
LUMOe (eV)
DELUMO–HOMO (eV)
e
abs
max
b
fl
max
c
Compound
k
(nm)
k
(nm)
2
CHCl3
THF
CH3CN
387
392
385
455
491
498
0.467
0.243
0.110
ꢁ5.06
ꢁ5.01
ꢁ5.20
ꢁ5.12
ꢁ5.39
ꢁ5.34
ꢁ1.96
3.10
2.82
2.68
2.70
3.19
3.23
1,6-HG (3a)
1,6-CNG (3b)
1,6-AcG (3c)
2,7-CNG (4a)
2,7-AcG (4b)
CHCl3
THF
CH3CN
413
413
410
482
523
536
0.407
0.225
0.091
ꢁ2.19
ꢁ2.52
ꢁ2.42
ꢁ2.20
ꢁ2.11
CHCl3
THF
CH3CN
420
421
416
493
542
554
0.258
0.190
0.014
CHCl3
THF
CH3CN
421
422
416
487
534
549
0.237
0.162
0.015
CHCl3
THF
CH3CN
341
341
339
427
454
424
0.129
0.076
0.008
CHCl3
THF
CH3CN
341
341
339
426
453
424
0.081
0.039
0.002
a
b
c
Solvent is used in the presence of 10% v/v of DMF because of its low solubility.
Absorption spectra were measured at 25 °C using 1 cm path length cell (10 M). See Figures S1–S4.
Fluorescence spectra were measured at 25 °C using l cm path length cell (10 M). See Figures 3, 4, S1 and S2.
) were calculated by using 9,10-diphenylanthracene as a reference according to Ref. 12.
The calculation was conducted for N9-methylated guanine derivatives instead of deoxyribose residue.
l
l
d
e
The fluorescence quantum yields (
U

Y. Saito et al. / Tetrahedron Letters 51 (2010) 2606–2609
2609
We next calculated the HOMO–LUMO energy levels of these gua-
References and notes
nosine derivatives. As shown in Table 1, the energy levels of the
HOMO and LUMO obtained for N9-methylated guanine derivatives
1. (a) Printz, M.; Richert, C. Chemistry 2009, 15, 3390; (b) Greco, N. J.; Sinkeldam,
R. W.; Tor, Y. Org. Lett. 2009, 11, 1115; (c) Srivatsan, S. G.; Tor, Y. J. Am. Chem.
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Ed.; Tetrahedron Symposium in Print, 2007, 63.; (c) Wilson, J. N.; Kool, E. T. Org.
Biomol. Chem. 2006, 4, 4265; (d) Ranasinghe, R. T.; Brown, T. Chem. Commun.
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3. (a) Sessler, J. L.; Sathiosatham, M.; Doerr, K.; Lynch, V.; Abboud, K. A. Angew.
Chem., Int. Ed. 2000, 39, 1300; (b) Manet, I.; Francini, L.; Masiero, S.; Pieraccini,
S.; Spada, G. P.; Gottarelli, G. Helv. Chim. Acta 2001, 84, 2096; (c) Shi, X.;
Mullaugh, K. M.; Fettinger, J. C.; Jiang, Y.; Hofstadler, S. A.; Davis, J. T. J. Am.
Chem. Soc. 2003, 125, 10830; For examples of DNA triplexes, see: (d) Moser, H.
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formation, see: (e) Davis, J. T. Angew. Chem., Int. Ed. 2004, 43, 668. and
references therein.
*
at DFT(B3LYP)/6-31G levels were consistent with experimentally
observed spectroscopic data.13 The fluorescence spectrum of
1,6-HG (3a) is red shifted from that of parent 2, consistent with the
smaller HOMO–LUMO gap by the calculation. 1,6-CNG (3b) and
1,6-AcG (3c), both of which have a larger red shift relative to 1,6-HG
(3a), have much smaller HOMO–LUMO energy gap than that of
1,6-HG (3a). On the other hand, the HOMO–LUMO energy gaps for
2,7-CNG (4a) and 2,7-AcG (4b) are larger than that of parent 2, which
are in agreement with the blue shift of their fluorescence. For N9-
methylated 1,6-CNG, the HOMO and the LUMO orbitals are broadly
distributed over the p
-system, whereas for N9-methylated 2,7-CNG,
4. Okamoto, A.; Kanatani, K.; Saito, I. J. Am. Chem. Soc. 2004, 126, 4820.
5. Saito, Y.; Matsumoto, K.; Takeuchi, Y.; Bag, S. S.; Kodate, S.; Morii, T.; Saito, I.
Tetrahedron Lett. 2009, 50, 1403.
the HOMO and LUMO are localized mainly at the guanine base and
phenyl acetylene linker (Fig. S5). These calculationsstrongly support
the view that only 1,6-disubstituted pyrene-labeled guanosine
derivatives, not the 2,7-disubstituted pyrenes, exhibit longer fluo-
rescence emission than 2.
6. Okamoto, A.; Ochi, Y.; Saito, I. Chem. Commun. 2005, 1128.
7. (a) Saito, Y.; Bag, S. S.; Kusakabe, Y.; Nagai, C.; Matsumoto, K.; Mizuno, E.;
Kodate, S.; Suzuka, I.; Saito, I. Chem. Commun. 2007, 2133; (b) Saito, Y.;
Miyauchi, Y.; Okamoto, A.; Saito, I. Tetrahedron Lett. 2004, 45, 7827; (c) Saito, Y.;
Shinohara, Y.; Bag, S. S.; Takeuchi, Y.; Matsumoto, K.; Saito, I. Tetrahedron 2009,
65, 934; (d) Matsumoto, K.; Shinohara, Y.; Bag, S. S.; Takeuchi, Y.; Morii, T.;
Saito, Y.; Saito, I. Bioorg. Med. Chem. Lett. 2009, 19, 6392; (e) Okamoto, A.; Saito,
Y.; Saito, I. J. Photochem. Photobiol. C: Photochem. Rev. 2005, 6, 108.
8. Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16, 4467.
9. Grimshaw, J.; Trocha-Grimshaw, J. J. Chem. Soc., Perkin Trans. 1 1972, 1622.
10. Spectroscopic data for 1,6-CNG (3b): 1H NMR (400 MHz, DMSO-d6) d 2.31 (m,
1H), 3.29 (m, 1H), 3.59 (m, 1H), 3.67 (m, 1H), 3.93 (m, 1H), 4.51 (m, 1H), 4.76
(m, 1H), 5.21 (m, 1H), 6.53–6.58 (complex, 3H), 7.93–7.98 (complex, 4H), 8.33–
8.43 (complex, 6H), 8.58 (d, 1H, J = 9.1 Hz), 8.70 (d, 1H, J = 9.0 Hz), 10.84 (br,
1H); 13C NMR (DMSO-d6, 100 MHz) d 37.9, 62.7, 71.7, 84.7, 86.4, 88.5, 92.3,
92.5, 94.8, 111.7, 116.4, 117.6, 118.7, 118.9, 123.7 (ꢂ2), 126,1, 126.5, 126.5,
126.7, 127.7, 129.4, 129.6, 130.0, 130.6, 131.0, 131.7, 131.9, 132.0, 132.1, 132.8
(ꢂ2), 133.1 (ꢂ2), 151.6, 154.5, 156.6; HRMS (ESI) m/z 639.1736 calcd for
C37H24N6O4Na [M+Na]+, found 639.1757.
In summary, we have developed novel push–pull-type pyrene-
containing guanosine derivatives. The fluorescence emission of
these nucleosides, particularly, those of 1,6-CNG (3b) and 1,6-Ac
G
(3c), showed a strong solvent dependency. Such environmentally
sensitive fluorescent deoxyguanosines can be used as a powerful
tool for structural studies of nucleic acids and molecular
diagnostics.
Acknowledgment
This work was supported by a Grant-in-Aid for Scientific re-
search of MEXT, Japanese Government.
These synthesized guanosine derivatives are photostable upon UV irradiation.
11. (a) Lee, H.; Harvey, R. G. J. Org. Chem. 1986, 51, 2847; (b) Connor, D. M.; Allen, S.
D.; Collard, D. M.; Liotta, C. L.; Schiraldi, D. A. J. Org. Chem. 1999, 64, 6888.
12. Quantum yields (U) were calculated according to following reference: Morris,
Supplementary data
J. V.; Mahaney, M. A.; Huber, J. R. J. Phys. Chem. 1976, 80, 969.
13. The theoretical calculations were performed by density functional theory (DFT)
*
calculations at B3LYP/6-31G level using SPARTAN 008 for Windows
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.03.012.
(Wavefunction, Inc.).