Synthesis and evaluation of spectroscopic properties of newly synthesized push-pull 6-amino-8-styryl purines

Article abstract of DOI:10.1016/j.dyepig.2014.01.025

New 6-amino-8-styryl purines were synthesized using direct C-H bond functionalization. These push-pull compounds showed strong fluorescence, high quantum yields and a noteworthy fluorosolvatochromism. Deprotected purines 7a-c are promising targets for incorporation into nucleic acids as they are still fluorescent in aqueous media.

Full text of DOI:10.1016/j.dyepig.2014.01.025

Dyes and Pigments 105 (2014) 145e151
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Short communication
Synthesis and evaluation of spectroscopic properties of newly
synthesized pushepull 6-amino-8-styryl purines
,
,b,
Roxane Vabre a b, Michel Legraverend b, Sandrine Piguel a
*
a Univ Paris Sud, Orsay F-91405, France
b UMR 176, Institut Curie/CNRS, Bât. 110-112, Centre Universitaire, 91405 Orsay, France
a r t i c l e i n f o
a b s t r a c t
Article history:
New 6-amino-8-styryl purines were synthesized using direct CeH bond functionalization. These push
epull compounds showed strong fluorescence, high quantum yields and noteworthy fluo-
rosolvatochromism. Deprotected purines 7aec are promising targets for incorporation into nucleic acids
as they are still fluorescent in aqueous media.
Received 29 October 2013
Received in revised form
23 January 2014
Accepted 27 January 2014
Available online 7 February 2014
a
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Styrylpurines
Pushepull
Fluorescence
Solvatochromy
Direct CeH bond functionalization
Heterocyclic chemistry
1. Introduction
the electron donor moiety and a p-conjugated styryl linker on the
position 8, substituted with an electron withdrawing group in the
para position of the phenyl ring. Although a styryl spacer has already
been used in the examination of the fluorescent properties of
caffeine or guanosine respectively bearing an oxo or a free amino
group on position 6, the combination of both an N,N0-substituted
amino group and a styryl group on the purine ring has never been
reported.[11,15e17] Moreover, examination of the fluorescence
properties of these compounds in water was not investigated.
Given that we have recently developed methods for the CeH
bond direct functionalization of azoles [18,19], including purines
[20], we applied these conditions for the synthesis of our molecules
thereby optimizing the number of steps and providing an easy and
rapid access to these potential fluorescent compounds. Herein, we
disclose the synthesis and the investigation of photophysical
properties of new pushepull 6-amino-8-styryl purines in organic
solvents and water. The impact of the different substituents is
discussed.
Over the past decade, fluorescence spectroscopy has gained
considerable attention because of its wide range of applications.
Notably, fluorescent molecules are known for their use in bio-
imaging, sensing, following chemical interactions of biomolecules or
monitoring the delivery of therapeutics [1e3]. The photophysical
properties of nucleosides have been studied lately as the purine
scaffold exhibits promising fluorescent properties [4e10]. Indeed,
nucleoside derivatives find applications in biological imaging of live
cells [11,12], in DNA detection [13,14] or as fluorescent probes [15]. As
such, developing new fluorescent purines fulfilling the standards of
an ideal fluorophore and thus displaying high quantum yield com-
bined with large Stokes shift is of great interest. Indeed, extending
the
p-conjugation usually results in a bathochromic shift of ab-
sorption and emissionwavelengths and an increased quantum yield.
Also, the fluorescent properties can be enhanced using pushepull
structures which skeleton is composed of a conjugated
p-electron
system substituted by an electron withdrawing group and an elec-
tron donor one. In this context, we envisioned the synthesis of new
pushepull purines bearing an amino substituent on the position 6 as
2. Experimental
2.1. General experimental methods
* Corresponding author. UMR 176, Institut Curie/CNRS, Bât. 110-112, Centre
Universitaire, 91405 Orsay, France. Tel.: þ33 01 69 86 71 11.
E-mail address: sandrine.piguel@curie.fr (S. Piguel).
Commercially available reagents and solvents were used
without further purification unless otherwise stated. Yields refer to
http://dx.doi.org/10.1016/j.dyepig.2014.01.025
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

146
R. Vabre et al. / Dyes and Pigments 105 (2014) 145e151
isolated and purified products. Reactions were monitored by thin-
layer chromatography carried out on silica gel plates (60F-254)
visualized under UV light. Column chromatography was performed
z (%) : 394.3 (100) [M þ Na]þ. HRMS (ESI) calcd for C21H15ClN5
[(M þ H)þ] 372.1016, found 372.1009.
on silica gel 60, 40e63
m
m. Chemical shifts of 1H NMR and 13C NMR
units) and residual non deuterated solvent
2.2.4. (E)-4-(2-(9-benzyl-6-((4-methoxybenzyl)amino)-9H-purin-
were reported in ppm (
d
8-yl)vinyl)benzonitrile (3a)
was used as internal reference. The following abbreviations were
used to designate the multiplicities : s ¼ singlet, d ¼ doublet,
t ¼ triplet, q ¼ quadruplet, bs ¼ broad singlet, m ¼ multiplet. Mi-
crowave irradiation was performed on CEM Explorer (CEM Corpo-
ration). Temperature measurement of the reaction mixture within
the Discovery series was achieved by an IR sensor. The method was
set with maximum power of 150 W, with maximum pressure of
17 bar and used without powermax. Reaction times refer to the
hold time at the desired set temperature. Reaction cooling was
performed by compressed air after the heating period was over.
UVevis experiments were monitored on a Cary Series UVevis
spectrophotometer (Agilent Technologies). Fluorescence spectra
were recorded on a Cary Eclipse fluorescence spectrophotometer
(Agilent Technologies) at room temperature. Measurements were
performed with solutions of OD < 0.1 to avoid re-absorption of the
emitted light, and data were corrected with a blank and from the
variations of the detector with the emitted wavelength. Fluores-
cence quantum yield were measured according to Williams
comparative method using quinine sulfate in 1 M H2SO4 as refer-
ence. [21] Absorption and fluorescence spectra were recorded for
four solutions of increasing concentrations with an absorbance
comprised between 0.01 and 0.1 to avoid re-absorption phenom-
enon. Electrospray ionization mass spectrometry (ESI-MS) was
performed at the Institut Curie and HRMS were performed at the
Small Molecule Mass Spectrometry platform of IMAGIF (ICSN, Gif-
Sur-Yvette, France).
Yellow solid (67%) : 1H NMR (300 MHz, CDCl3)
d 8.44 (s, 1H), 7.71
(d, J ¼ 15.9 Hz, 1H), 7.62e7.46 (m, 4H), 7.36e7.28 (m, 5H), 7.20 (d,
J ¼ 6.8 Hz, 2H), 7.01 (d, J ¼ 15.8 Hz, 1H), 6.85 (d, J ¼ 8.5 Hz, 2H), 6.35
(bs, 1H), 5.51 (s, 2H), 4.81 (bs, 2H), 3.78 (s, 3H). 13C NMR (75 MHz,
CDCl3)
d 159.1, 154.2, 153.5, 147.3, 140.0, 136.0, 134.3, 132.7, 130.5,
129.2, 128.3, 127.6, 126.8, 119.9, 118.7, 116.6, 114.1, 112.1, 55.4, 45.8,
44.2. MS (ESþ) m/z (%) : 473.3 (100) [M þ H]þ. HRMS (ESI) calcd for
C
29H25N6O [(M þ H)þ] 473.2090, found 473.2094.
2.2.5. (E)-9-benzyl-N-(4-methoxybenzyl)-8-(4-(trifluoromethyl)
styryl)-9H-purin-6-amine (3b)
Yellow solid (68%) : 1H NMR (300 MHz, CDCl3)
d 8.45 (s,1H), 7.75
(d, J ¼ 15.9 Hz, 1H), 7.61e7.50 (m, 4H), 7.37e7.29 (m, 5H), 7.22 (d,
J ¼ 6.5 Hz, 2H), 7.01 (d, J ¼ 15.9 Hz, 1H), 6.87 (d, J ¼ 8.6 Hz, 2H), 6.24
(bs, 1H), 5.52 (s, 2H), 4.83 (bs, 2H), 3.80 (s, 3H). 13C NMR (75 MHz,
CDCl3)
d 159.1, 154.2, 153.4, 147.6, 139.1, 136.1, 134.9, 130.6, 129.3,
128.2, 127.3, 126.8, 125.8, 122.2, 119.9, 115.7, 114.1, 55.3, 45.7, 44.3.
MS (ESþ) m/z (%) : 516.5 (80) [M þ H]þ. HRMS (ESI) calcd for
C
29H25F3N5O [(M þ H)þ] 516.2011, found 516.1995.
2.2.6. (E)-4-(2-(9-benzyl-6-(pyrrolidin-1-yl)-9H-purin-8-yl)vinyl)
benzonitrile (3c)
Yellow solid (99%) : 1H NMR (300 MHz, CDCl3)
d 8.38 (s, 1H), 7.73
(d, J ¼ 15.8 Hz, 1H), 7.64e7.51 (m, 4H), 7.36e7.27 (m, 3H), 7.18 (d,
J ¼ 6.5 Hz, 2H), 7.03 (d, J ¼ 15.8 Hz, 1H), 5.52 (s, 2H), 4.28 (bs, 2H),
3.82 (bs, 2H), 2.07 (s, 4H). 13C NMR (75 MHz, CDCl3)
d 153.2, 152.8,
151.6,146.0,140.5,136.3,133.2,132.6,129.1,128.1,127.5,126.7,120.8,
118.8, 117.1, 111.8, 49.2 45.5, 30.2. MS (ESþ) m/z (%) : 407.4 (100)
[M þ H]þ. HRMS (ESI) calcd for C25H23N6 [(M þ H)þ] 407.1984,
found 407.1978.
2.2. Chemical
N9-protected purines 1 and 4 were prepared according to the
literature [22,23]. 8-styrylpurines 2aeb and 5aeb were synthe-
tized through our formerly developed direct alkenylation method
[20]. 6-aminopurines 3aeh and 6aec were obtained following our
previously described procedure under Buchwald-Hartwig-inspired
palladium cross-coupling conditions [24]. Purines 7aec were
deprotected under acidic conditions [25].
2.2.7. (E)-9-benzyl-6-(pyrrolidin-1-yl)-8-(4-(trifluoromethyl)
styryl)-9H-purine (3d)
Yellow solid (84%) : 1H NMR (300 MHz, CDCl3)
d 8.38 (s, 1H), 7.76
(d, J ¼ 15.8 Hz, 1H), 7.62e7.53 (m, 4H), 7.35e7.28 (m, 3H), 7.19 (d,
J ¼ 8.0 Hz, 2H), 7.02 (d, J ¼ 15.8 Hz, 1H), 5.52 (s, 2H), 4.29 (bs, 2H),
3.82 (bs, 2H), 2.07 (s, 4H). 13C NMR (75 MHz, CDCl3)
d 153.1, 152.8,
151.6,146.4,139.6,136.4,133.9, 130.7,130.2,129.1,128.1,127.3, 126.8,
125.8, 120.7, 116.2, 49.1, 45.6, 30.2. MS (ESþ) m/z (%) : 450.5 (100)
[M þ H]þ. HRMS (ESI) calcd for C25H23F3N5 [(M þ H)þ] 450.1906,
found 450.1912.
2.2.1. 9-benzyl-6-chloro-9H-purine (1)
White solid (51%) : 1H NMR (300 MHz, CDCl3) 8.80 (s, 1H), 8.10
(s, 1H), 7.37e7.33 (m, 5H), 5.46 (s, 2H). Spectroscopic data were in
agreement with those reported in the literature [22].
2.2.8. (E)-4-(2-(9-benzyl-6-((2-methoxyethyl)amino)-9H-purin-8-
2.2.2. (E)-9-benzyl-6-chloro-8-(4-(trifluoromethyl)styryl)-9H-
purine (2a)
yl)vinyl)benzonitrile (3e)
Yellow solid (95%) : 1H NMR (300 MHz, CDCl3)
d 8.41 (s, 1H), 7.78
White solid (60%) : 1H NMR (300 MHz, CDCl3)
d 8.74 (s, 1H), 8.16
(d, J ¼ 15.9 Hz, 1H), 7.66e7.51 (m, 4H), 7.36e7.29 (m, 3H), 7.20 (d,
J ¼ 6.8 Hz, 2H), 7.03 (d, J ¼ 15.9 Hz, 1H), 6.15 (bs, 1H), 5.52 (s, 2H),
3.91 (bs, 2H), 3.67 (t, J ¼ 5.1 Hz, 2H), 3.43 (s, 3H). 13C NMR (75 MHz,
(d, J ¼ 15 Hz, 1H), 7.67e7.59 (m, 4H), 7.40e7.33 (m, 3H), 7.23 (d,
J ¼ 7.9 Hz, 2H), 7.08 (d, J ¼ 15.9 Hz, 1H), 5.62 (s, 2H). 13C NMR
(75 MHz, CDCl3) d 153.3, 153.2, 151.8, 149.7, 139.3, 138.4, 135.1, 131.8,
CDCl3)
d 154.4, 153.3, 150.6, 147.3, 140.1, 136.1, 134.2, 132.6, 129.2,
131.7, 129.4, 128.8, 127.9, 127.0, 126.11, 126.06, 114.4, 46.4. MS (ESþ)
m/z (%) : 437.2 (80) [M þ Na]þ. HRMS (ESI) calcd for C21H15ClF3N4
[(M þ H)þ] 415.0937, found 415.0945.
128.2, 127.6, 126.8, 120.2, 118.7, 116.7, 112.1, 71.3, 58.9, 45.7, 29.8. MS
(ESþ) m/z (%) : 411.4 (100) [M þ H]þ. HRMS (ESI) calcd for
C
24H23N6O [(M þ H)þ] 411.1933, found 411.1933.
2.2.3. (E)-4-(2-(9-benzyl-6-chloro-9H-purin-8-yl)vinyl)
2.2.9. (E)-9-benzyl-N-(2-methoxyethyl)-8-(4-(trifluoromethyl)
styryl)-9H-purin-6-amine (3f)
benzonitrile (2b)
Beige solid (56%) : 1H NMR (300 MHz, CDCl3)
d
8.75 (s, 1H), 8.13
Yellow solid (62%) : 1H NMR (300 MHz, CDCl3)
d 8.41 (s, 1H), 7.80
(d, J ¼ 15.9 Hz, 1H), 7.70e7.58 (m, 4H), 7.40e7.35 (m, 3H), 7.22 (d,
(d, J ¼ 15.9 Hz, 1H), 7.62e7.53 (m, 4H), 7.36e7.28 (m, 3H), 7.20 (d,
J ¼ 7.0 Hz, 2H), 7.01 (d, J ¼ 15.9 Hz, 1H), 6.24 (bs, 1H), 5.52 (s, 2H),
3.92 (bs, 2H), 3.67 (t, J ¼ 4.9 Hz, 2H), 3.42 (s, 3H). 13C NMR (75 MHz,
J ¼ 6.8 Hz, 2H), 7.09 (d, J ¼ 15.8 Hz, 1H), 5.62 (s, 2H). 13C NMR
(75 MHz, CDCl3) d 153.3,152.8,151.9, 149.9,139.3,138.6,135.1,132.8,
131.7, 129.5, 128.8, 128.1, 126.9, 118.5, 115.4, 113.1, 46.4. MS (ESþ) m/
CDCl3) d 154.4, 153.2, 147.6, 139.2, 136.1, 134.9, 130.9, 130.5, 129.2,

R. Vabre et al. / Dyes and Pigments 105 (2014) 145e151
147
128.3,127.4,125.9,122.2,120.1,115.7, 71.3, 58.9, 45.8, 29.8. MS (ESþ)
2.2.16. (E)-6-(pyrrolidin-1-yl)-9-(tetrahydro-2H-pyran-2-yl)-8-
m/z (%) : 454.4 (100) [M þ H]þ. HRMS (ESI) calcd for C24H23F3N5O
[(M þ H)þ] 455.1855, found 455.1846.
(4(trifluoromethyl)styryl)-9H-purine (6b)
Yellow solid (68%) : 1H NMR (300 MHz, CDCl3)
d 8.33 (s,1H), 7.79
(d, J ¼ 16.0 Hz, 1H), 7.70e7.63 (m, 4H), 7.58 (d, J ¼ 16.0 Hz, 1H), 5.97
2.2.10. (E)-4-(2-(7-benzyl-4-(benzyl(methyl)amino)-7H-pyrrolo
(d, J ¼ 11.1 Hz,1H), 4.32e4.28 (m, 3H), 3.84e3.77 (m, 3H), 2.18e2.05
[2,3-d]pyrimidin-6-yl)vinyl)benzonitrile (3g)
(m, 6H), 1.92e1.64 (m, 4H). 13C NMR (75 MHz, CDCl3)
d 152.6, 150.4,
Yellow oil (78%) : 1H NMR (300 MHz, CDCl3)
d
8.40 (s, 1H), 7.70
146.0, 139.9, 133.1, 129.9, 127.2, 125.8, 120.1, 118.9, 82.4, 69.4, 32.1,
30.2, 25.3, 23.2. MS (ESþ) m/z (%) : 444.3 (100) [M þ H]þ. HRMS
(ESI) calcd for C23H25F3N5O [(M þ H)þ] 444.2011, found 444.2022.
(d, J ¼ 16.0 Hz, 1H), 7.48e7.63 (m, 4H), 7.37e7.28 (m, 8H), 7.21 (d,
J ¼ 6.7 Hz, 2H), 7.02 (d, J ¼ 15.8 Hz,1H), 5.54 (s, 2H), 3.50 (s, 2H),1.65
(s, 3H). 13C NMR (75 MHz, CDCl3)
d 154.5, 152.9, 152.3, 145.7, 140.4,
138.0, 136.3, 133.8, 132.7, 129.2, 128.7, 128.2, 127.9, 127.6, 126.8,
120.5, 118.8, 116.9, 111.9, 45.7, 43.6, 30.3. MS (ESþ) m/z (%) : 457.4
(100) [M þ H]þ. HRMS (ESI) calcd for C29H25N6 [(M þ H)þ] 457.2141,
found 457.2144.
2.2.17. (E)-N-(4-methoxybenzyl)-9-(tetrahydro-2H-pyran-2-yl)-8-
(4-(trifluoromethyl)styryl)-9H-purin-6-amine (6c)
Yellow solid (84%) : 1H NMR (300 MHz, CDCl3)
d 8.41 (s, 1H), 7.76
(d, J ¼ 16.1 Hz, 1H), 7.65 (s, 4H), 7.55 (d, J ¼ 16.1 Hz, 1H), 7.32 (d,
J ¼ 8.6 Hz, 2H), 6.87 (d, J ¼ 8.7 Hz, 2H), 6.14 (bs, 1H), 5.95 (dd,
J ¼ 11.1, 2.4 Hz, 1H), 4.80 (bs, 2H), 4.31 (d, J ¼ 9.9 Hz, 1H), 3.84e3.79
(m, 4H), 2.23e2.07 (m, 2H), 1.95e1.64 (m, 4H). 13C NMR (75 MHz,
2.2.11. (E)-N,7-dibenzyl-N-methyl-6-(4-(trifluoromethyl)styryl)-
7H-pyrrolo[2,3-d]pyrimidin-4-amine (3h)
Yellow oil (88%) : 1H NMR (300 MHz, CDCl3)
d
8.40 (s, 1H), 7.74
CDCl3) d 159.2, 154.2, 153.0, 147.5, 139.6, 134.2, 130.8, 130.5, 130.4,
(d, J ¼ 15.9 Hz, 1H), 7.60e7.50 (m, 4H), 7.37e7.28 (m, 8H), 7.23 (d,
129.4, 127.4, 125.9, 122.3, 119.5, 118.5, 114.2, 82.7, 69.5, 55.4, 43.6,
32.2, 25.4, 23.3. MS (ESþ) m/z (%) : 510.3 (100) [M þ H]þ. HRMS
(ESI) calcd for C27H27F3N5O2 [(M þ H)þ] 510.2117, found 510.2123.
J ¼ 6.4 Hz, 2H), 7.01 (d, J ¼ 15.9 Hz,1H), 5.54 (s, 2H), 3.51 (s, 2H), 1.66
(s, 3H). 13C NMR (75 MHz, CDCl3)
d 154.4, 152.6, 152.1, 145.9, 139.3,
137.9, 136.2, 134.2, 130.6, 130.2, 129.0, 128.6, 128.0, 127.8, 127.3,
127.2, 126.7, 125.7, 122.1, 120.2, 115.8, 45.5, 43.4, 30.1. MS (ESþ) m/z
(%) : 500.4 (100) [M þ H]þ. HRMS (ESI) calcd for C29H25F3N5
[(M þ H)þ] 500.2062, found 500.2059.
2.2.18. (E)-4-(2-(6-((4-methoxybenzyl)amino)-9H-purin-8-yl)
vinyl)benzonitrile (7a)
Yellow solid (99%) : 1H NMR (300 MHz, DMSO-d6)
d 8.20 (bs,
2H), 7.88e7.80 (m, 4H), 7.64 (d, J ¼ 16.7 Hz, 1H), 7.35e7.29 (m, 3H),
2.2.12. 6-chloro-9-(tetrahydro-2H-pyran-2-yl)-9H-purine (4)
6.86 (d, J ¼ 8.2 Hz, 2H), 4.65 (bs, 2H), 3.70 (s, 3H). 13C NMR (75 MHz,
Pale yellow solid (89%) : 1H NMR (300 MHz, CDCl3)
d
8.76 (s, 1H),
DMSO-d6) d 158.1,152.7,147.3,140.2,132.8,132.2,131.9,128.6,127.6,
8.35 (s, 1H), 5.80 (dd, J ¼ 10.2, 2.5 Hz, 1H), 4.23e4.18 (m, 1H), 3.80
(td, J ¼ 11.4, 3.1 Hz, 1H), 2.20e2.00 (m, 3H), 1.85e1.67 (m, 3H).
Spectroscopic data were in agreement with those reported in the
literature [23].
120.9, 118.8, 113.6, 110.6, 55.0, 29.0. MS (ESþ) m/z (%) : 383.4 (100)
[M þ H]þ. HRMS (ESI) calcd for C29H19N6O [(M þ H)þ] 383.1620,
found 383.1625.
2.2.19. (E)-6-(pyrrolidin-1-yl)-8-(4-(trifluoromethyl)styryl)-9H-
purine (7b)
2.2.13. (E)-6-chloro-9-(tetrahydro-2H-pyran-2-yl)-8-(4-
(trifluoromethyl)styryl)-9H-purine (5a)
Yellow solid (93%) : 1H NMR (300 MHz, DMF-d7)
d 13.18 (bs, 1H),
Yellow solid (36%) : 1H NMR (300 MHz, CDCl3)
d
8.70 (s, 1H), 8.15
8.21 (s, 1H), 7.94 (d, J ¼ 8.1 Hz, 2H), 7.83e7.75 (m, 3H), 7.43 (d,
(d, J ¼ 16.6 Hz, 1H), 7.75e7.68 (m, 4H), 7.60 (d, J ¼ 16.0 Hz, 1H), 6.04
J ¼ 16.6 Hz, 1H), 4.21 (bs, 2H), 3.70 (bs, 2H), 2.02 (bs, 4H). 13C NMR
(d, J ¼ 11.8, 1H), 4.34 (d, J ¼ 11.0 Hz, 1H), 3.82 (t, J ¼ 11.5 Hz, 1H),
(75 MHz, DMF-d7) d 152.9, 152.6, 152.32, 152.29, 152.2, 147.0, 140.4,
2.30e1.78 (m, 6H). 13C NMR (75 MHz, CDCl3)
d
153.1, 152.2, 151.4,
131.9, 127.6, 125.9, 121.2, 120.8, 22.5, 13.7. MS (ESþ) m/z (%) : 360.4
(100) [M þ H]þ. HRMS (ESI) calcd for C18H17F3N5 [(M þ H)þ]
360.1436, found 360.1441.
149.7, 138.9, 138.4, 131.4, 127.9, 126.1, 126.0, 122.2, 117.1, 83.2, 69.6,
31.9, 25.3, 23.2. MS (ESþ) m/z (%) : 431.2 (100) [M þ Na]þ. HRMS
(ESI) calcd for C19H17ClF3N4O [(M þ H)þ] 409.1043, found 409.1046.
2.2.20. (E)-N-(4-methoxybenzyl)-8-(4-(trifluoromethyl)styryl)-9H-
purin-6-amine (7c)
2.2.14. (E)-4-(2-(6-chloro-9-(tetrahydro-2H-pyran-2-yl)-9H-
purin-8-yl)vinyl)benzonitrile (5b)
Yellow solid (87%) : 1H NMR (300 MHz, DMF-d7)
d 8.23 (s, 1H),
Yellow solid (38%) : 1H NMR (300 MHz, CDCl3)
d
8.70 (s, 1H), 8.12
7.92 (d, J ¼ 8.1 Hz, 1H), 7.81e7.74 (m, 3H), 7.58e7.51 (m, 2H), 7.43e
(d, J ¼ 16.0 Hz, 1H), 7.72 (m, 4H), 7.62 (d, J ¼ 16.0 Hz, 1H), 6.04 (dd,
J ¼ 11.0, 2.6 Hz, 1H), 4.34 (dd, J ¼ 12.0, 3.6 Hz, 1H), 3.83 (td, J ¼ 11.5,
3.8 Hz, 1H), 2.32e2.09 (m, 2H), 2.00e1.75 (m, 4H). 13C NMR
7.35 (m, 3H), 6.91e6.85 (m, 3H), 4.80 (bs, 2H), 3.76 (s, 3H). 13C NMR
(75 MHz, DMF-d7)
d d 158.8, 153.1,
13C NMR (75 MHz, DMF-d7)
152.1, 147.7, 140.2, 132.5, 129.4, 128.9, 127.7, 126.4, 125.9, 125.8,
124.9, 122.8, 120.6, 113.7, 54.9, 22.5. MS (ESþ) m/z (%) : 426.4 (100)
[M þ H]þ. HRMS (ESI) calcd for C22H19F3N5O [(M þ H)þ] 426.1542,
found 426.1534.
(75 MHz, CDCl3) d 152.7, 152.2, 151.5, 149.9, 139.8, 137.8, 132.8, 131.4,
128.2, 118.6, 118.1, 112.9, 83.3, 69.6, 32.0, 25.3, 23.2. MS (ESþ) m/z
(%) : 282.1 (100) [M-THP]þ, 366.2 (60) [M þ H]þ. HRMS (ESI) calcd
for C19H17ClN5O [(M þ H)þ] 366.1122, found 366.1137.
3. Results and discussion
2.2.15. (E)-4-(2-(6-((4-methoxybenzyl)amino)-9-(tetrahydro-2H-
pyran-2-yl)-9H-purin-8-yl)vinyl)benzonitrile (6a)
During our previous work on developing new synthetic meth-
odologies in the area of direct CeH bond functionalization, new 6-
amino- or 6-thio-8-styrylpurines were synthesized through a key
C-8 direct alkenylation step (Fig. 1) [20].
First, the optical properties of these newly synthesized com-
pounds were studied (results not shown). Among those 19 com-
Yellow solid (82%) : 1H NMR (300 MHz, CDCl3)
d 8.40 (s, 1H),
7.76e7.55 (m, 6H), 7.30 (d, J ¼ 8.6 Hz, 2H), 6.86 (d, J ¼ 8.7 Hz, 2H),
6.19 (bs, 1H), 5.96 (dd, J ¼ 11.0, 2.4 Hz, 1H), 4.80 (bs, 2H), 4.30 (d,
J ¼ 12.9 Hz, 1H), 3.84e3.79 (m, 4H), 2.24e2.04 (m, 2H), 1.96e1.68
(m, 4H). 13C NMR (75 MHz, CDCl3)
d 159.1, 154.2, 153.2, 147.1, 140.5,
133.6, 132.7, 130.5, 129.3, 127.6, 119.5, 118.8, 114.1, 111.9, 82.7, 69.5,
55.4, 43.5, 32.3, 25.4, 23.2. MS (ESþ) m/z (%) : 467.3 (100) [M þ H]þ.
HRMS (ESI) calcd for C27H27N6O2 [(M þ H)þ] 467.2195, found
467.2206.
pounds, eight were fluorescent in dichloromethane with a
quantum yield higher than 0.32. From a structural point of view,
they bear either electron withdrawing group on the para position of
the styryl group or an amino substituent on the 6 position of the

148
R. Vabre et al. / Dyes and Pigments 105 (2014) 145e151
withdrawing substituent in the para position of the phenyl ring,
were prepared in a two-step literature procedure starting from the
commercially available aldehydes [27,28]. Then, a range of amines
were introduced at position 6 of 8-styrylpurines through a Buch-
waldeHartwig cross-coupling reaction, which conditions are
similar to those that we previously used with 8-iodopurines [24].
Assuming that we would create fluorescent DNA bases, it was
necessary that 8-styrylpurines had no substituent at position 9, so
as to make them more polar and then to increase their solubility in
water. In order to synthetize N9eH free purines, this position had to
be deprotected in the last step. However, it would have been
difficult to remove the benzyl group without reducing the double
bond. Thus, we decided to synthetize analogs with a tetrahy-
dropyranyl protecting group, that could be easily introduced and
removed under acidic conditions, compatible with the ethylenic
double bond. Among the 9-benzylpurines 3aeh synthetized, three
molecules exhibiting the most interesting fluorescent properties
were chosen for further study and resynthesized with a free NH at
position 9. Thus, compounds 7aec were obtained in 4 steps from
the commercially available 6-chloropurine. After THP protection on
the N9 position of the purine ring, purine 4 underwent direct
alkenylation reaction, followed by amination in the Buchwalde
Hartwig conditions to afford molecules 6aec, which were effi-
ciently deprotected using trifluoroacetic acid.
Fig. 1. Structure of 8-styrylpurines previously synthetized.
purine ring. Since we were interested in generating new pushepull
chromophores with enhanced fluorescent properties, a small li-
brary of new purines derivatives combining an amino substituent
at position 6 and an electron withdrawing group on the para po-
sition of the styryl group was designed. Hence, new molecules 3ae
h were synthesized, in a straightforward way, through direct
alkenylation and BuchwaldeHartwig reactions using conditions
both developed in our laboratory [20,24,26].
3.1. Synthesis
The synthetic route of 8-vinylpurines is outlined in Schemes 1
and 2.
Molecules 3aeh were synthesized in two steps from purine 1.
Purines 2aeb were obtained by direct alkenylation of purine 1 in
moderate yields, with (E)-styryl bromides under microwave irra-
diation using a Pd/Cu co-catalyst system, reaction developed in our
laboratory. [20] (E)-bromostyrenes, bearing an electron
3.2. UVevis and photoluminescence data
The optical properties of purines 3aeh and 7aec measured in
dichloromethane at 25 ꢀC using UV/visible and photoluminescence
Scheme 1. Synthetic route to molecules 3aeh.

R. Vabre et al. / Dyes and Pigments 105 (2014) 145e151
149
Scheme 2. Synthetic pathway to compounds 7aec.
spectroscopy are presented in Table 1. All compounds show ab-
sorption wavelengths in the UV region (349e390 nm) and emission
wavelengths in the visible region (437e490 nm). In consequence,
large Stokes shifts were observed, which suggests significant dif-
ferences (vibrational, electronic, geometric) between the Franck-
Condon state and the excited state. Furthermore, all compounds
have moderate absorption coefficient allowing them to be suffi-
ciently bright for detection. Also, high quantum yields were ob-
tained for all compounds. The photophysical properties were
moderately modified depending on the substituents. If we compare
the two substituents used in the para position of the phenyl ring,
we can notice that nitrile displays a larger bathochromic shift
whereas trifluoromethyl group leads to higher Stokes shifts and
quantum yields. Also, it should be noted that quantum yields are
slightly better when amine in position 6 is substituted by pyrroli-
dinyl or 4-methoxybenzyl groups. Regarding 9-benzylpurines,
compounds 3a,b,d showed particularly higher quantum yields
compared to other purine derivatives. Therefore, we were inter-
ested in generating analogs bearing no protecting group on the N9
position in order to examine the influence of such substituent. N9e
H
free purines 7aec still exhibited strong fluorescence in
dichloromethane with quantum yields ranging from 0.71 to 0.93.
Table 1
Spectroscopic data of compounds 3aeh and 7aec.
3.3. Fluorosolvatochromism
Compounda
labs, nm (
3
, Lꢁ1 cmꢁ1
)
lém, nm
4F
Stokes shift, cmꢁ1
b
The spectroscopic properties of compounds 7aec were evalu-
ated in different solvents and are summarized in Table 2. The aim of
this study was to investigate the effect of solvent polarity on the
photophysical properties of the fluorophores. We noted that the
emission band was red-shifted while increasing solvent polarity,
evaluated by solvent orientation polarizability [29]. On the con-
trary, the absorption band was not importantly shifted. This sol-
vatochromic behavior, well established with donor-acceptor
fluorophores, is due to an interaction charge transfer in the lowest
excited state [30e32]. As an example, the emission spectra and
color changes under UV irradiation of compound 7a are shown in
Fig. 2. Compounds 7aec showed high fluorescence combined with
high quantum yields in organic solvents of different polarity
3a
3b
3c
3d
3e
3f
3g
3h
7a
7b
7c
369 (18,900)
354 (15,100)
390 (19,800)
373 (16,700)
369 (15,300)
354 (16,900)
385 (24,100)
369 (18,100)
362 (19,300)
359 (14,100)
349 (15,300)
480
456
490
472
475
454
486
464
437
443
437
0.75
0.81
0.62
0.79
0.63
0.65
0.63
0.63
0.93
0.71
0.74
6267
6319
5233
5623
6048
6222
5398
5549
4741
5285
5770
a
All spectra were recorded in CH2Cl2 solutions at room temperature with c from
10ꢁ6 to 10ꢁ5 M.
b
Fluorescence quantum yield (ꢂ10%) were determined relative to quinine sulfate
in 1 M H2SO4 (4F ¼ 0.54).

150
R. Vabre et al. / Dyes and Pigments 105 (2014) 145e151
Table 2
UV/vis and photoluminescence data of compounds 7aec in organic solvents and
water.
Compounda
Solvent
labs, nm
, Lꢁ1 cmꢁ1
lém, nm
4F
Stokes
b
(
3
)
shift, cmꢁ1
7a
Toluene
DCM
MeOH
DMSO
H2O
Toluene
DCM
MeOH
DMSO
H2O
Toluene
DCM
MeOH
DMSO
H2O
370 (20,900)
362 (19,300)
357 (20,800)
371 (19,500)
337 (6400)
370 (15,700)
359 (14,100)
353 (16,800)
368 (15,600)
354 (11,800)
358 (15,900)
349 (15,300)
346 (17,400)
360 (15,300)
353 (12,500)
449
437
481
496
491
446
443
457
481
469
437
437
444
478
463
>0.99
0.93
0.80
0.81
0.02
0.93
0.71
0.80
>0.99
0.07
>0.99
0.74
0.90
0.95
0.03
4755
4741
7221
6793
9307
6150
5285
6447
6384
6927
5050
5770
6379
6857
6730
7b
7c
Fig. 3. Normalized UVevis (solid lines) and emission spectra (broken lines) in water of
compounds 7a (black), 7b (blue) and 7c (red). (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
a
All spectra were recorded at room temperature with c from 10ꢁ6 to 10ꢁ5 M.
Fluorescence quantum yield (ꢂ10%) were determined relative to quinine sulfate
b
in 1 M H2SO4 (4F ¼ 0.54).
(toluene, dichloromethane, methanol, DMSO). It should be noted
that when a protic solvent such as methanol was used, the fluo-
rescence was not quenched.
One of the main characteristics of an ideal fluorophore is its
ability to be soluble in water. In fact, compounds 7aec were soluble
in water, obeying the BeereLambert law up to a concentration of
Fig. 4. Compounds 7aec in water under UV irradiation.
30
mM. Interestingly, these molecules were still fluorescent in
aqueous media. They showed low quantum yields but these could
be sufficient enough to allow fluorescence detection. Indeed,
Teulade-Fichou et al. used triphenylamine and DAPI, which show
quantum yields of 0.021 and 0.019 respectively in aqueous media,
for imaging DNA in cells [33]. Moreover, the purines 7aec absorbed
in the UV region (337e360 nm) and emitted from 478 to 491 nm
(Fig. 3). Thus, large Stokes shifts were observed (6730e9307 cm-1),
which could facilitate the detection and as such prevent from re-
absorption phenomenon [34]. Under UV irradiation, the color
differences between molecules 7aec were pointed out as they
evolved from purple to green (Fig. 4).
Finally, three products 7aec are promising as their character-
istics are similar to DAPI, widely used in fluorescence microscopy
[35]. Actually, DAPI has a low quantum yield of 0.019 (0.34 in DNA);
and shows an absorption wavelength at 341 nm and emits at
496 nm. These features are quite close to our compounds, and more
particularly 7a, highlighting their potential application as good
fluorophores.
4. Conclusion
In summary, new pushepull 6-amino-8-styryl purines have
been successfully synthetized and characterized in a straight-
forward way via CeH bond direct functionalization and Buch-
waldeHartwig cross-coupling. We showed that these molecules
exhibit strong fluorescence and large Stokes shifts in organic
solvents and among them, three compounds (7aec) are fluo-
rescent in water. This preliminary work could be useful for the
synthesis of new nucleoside analogs through introduction of a
ribose moiety on the N9 position of these 8-styrylpurines.
Therefore, the spectroscopic properties of these nucleoside an-
alogs could be investigated in order to determine their potential
utility as DNA or RNA probes upon incorporation into oligonu-
cleotides [36e38].
Acknowledgments
R.V. thanks the Ministère de l’Enseignement Supérieur et de la
Recherche for doctoral fellowship. Marianne Bombled is thanked
for obtaining MS data. Florence Mahuteau-Betzer is thanked for
fruitful discussion and for proofreading this manuscript.
Fig. 2. A) Normalized emission spectra compound 7a in various solvents with
c ¼ 10ꢁ5 M. B) Color changes of solutions of 7a with c ¼ 10ꢁ5 M for all solvents (except
water c ¼ 10ꢁ3 M). (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)

R. Vabre et al. / Dyes and Pigments 105 (2014) 145e151
151
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[21] Rhys Williams AT, Winfield SA, Miller JN. Relative fluorescence quantum
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̀