The synthesis of novel fluorescent purine analogues modified by azacrown ether at C6

Article abstract of DOI:10.1016/j.bmcl.2010.03.102

The synthesis and fluorescence properties of novel purine analogues linked azacrown ether at C6 position were investigated. These new purine analogues could be prepared from a series of 6-chloropurines and showed selective and efficient signaling behaviors toward micromolar concentration of Ag⁺ ion over other common metal ions in an aqueous environment.

Full text of DOI:10.1016/j.bmcl.2010.03.102

Bioorganic & Medicinal Chemistry Letters 20 (2010) 3098–3102
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
The synthesis of novel fluorescent purine analogues modified by azacrown
ether at C6
a
b
a
a
a,
Hai-Ming Guo a, , Jing Wu , Hong-Ying Niu , Dong-Chao Wang , Feng Zhang , Gui-Rong Qu
*
*
a College of Chemistry and Environmental Science, Henan Normal University, Xinxiang, 453007 Henan, China
b School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and fluorescence properties of novel purine analogues linked azacrown ether at C6 position
were investigated. These new purine analogues could be prepared from a series of 6-chloropurines and
showed selective and efficient signaling behaviors toward micromolar concentration of Ag+ ion over
other common metal ions in an aqueous environment.
Received 22 November 2009
Revised 25 February 2010
Accepted 27 March 2010
Available online 3 April 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Purine nucleosides
Azacrown
Fluorescent nucleosides
Purines are one of the most ubiquitous heterocyclic ring sys-
tems found in nature.1 They are components in numerous biolog-
ically significant molecules and thus present an excellent scaffold
for the construction of bioprobes.2 Purine derivatives with various
substituents at C6 have received considerable attention due to
their broad spectrum of biological activities.3 This prompts a num-
ber of studies on the modification at C6 position of the purine ring.
Crown ethers, first introduced in 1967 by Pedersen, are very
intriguing compounds in the field of supramolecular chemistry.4
In order to develop metal ion-selective and sensitive material,
there is a growing interest in synthesizing crown ether derivatives
with fluorescence properties which will show marked changes
upon metal complexation.5
Currently, considerable attention has been focused on fluores-
cent chemosensors for the selective and rapid determination of
Table 1
Effect of solvent and the optimization of reaction conditionsa
O
HO
OH
O
O
O
O
N
N
N
N
TsO
O
O
O
OTs
N
N
N
N
N
N
1c
2g
O
N
n
HO
OH
Entry
Base
Solvent
T (°C)
Time (h)
Yieldb (%)
O
O
n
1
2
3
4
5
6
7
8
9
NaH
NaH
NaH
KOH
NaOH
LiOH
Cs2CO3
Cs2CO3
Cs2CO3
Na2CO3
THF
THF
DMF
THF
THF
THF
THF
DMF
CH3CN
THF
Reflux
25
50
Reflux
Reflux
Reflux
Reflux
50
6
24
6
6
6
24
24
24
24
24
43
40
35
30
N
N
TsO
O
OTs
N
N
N
N
N
THF, NaH, 6h
1
R
1
N
35
N
2
R
2
R
Trace
Trace
Trace
Trace
NR
2
R
1
Reflux
Reflux
Scheme 1. Reaction of (poly)ethylene glycol ditosylates with 6-[N,N-bis(2-
10
hydroxyethyl)amino]purines.
a
Reagents and conditions: 1 mmol 1c, 1 mmol (poly)ethylene glycol ditosylate
and 2 mmol bases in 30 mL solvent for the indicated time.
* Corresponding authors. Fax: +86 373 3329276 (H.-M.G,).
E-mail address: guohm518@hotmail.com (H.-M. Guo).
b
Isolated yields based on nucleobases.
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.03.102

H.-M. Guo et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3098–3102
3099
the toxic heavy metal ions, such as Pb2+, Cd2+, Hg2+ and Ag+ ion.6
Ag+ ion is received more attention because of its bioaccumulation
and toxicity.7 Furthermore, the mechanism of antimicrobial activ-
ities of Ag+ has not been well established because of the lack of
suitable detection and imaging methodologies.8
bearing purines 2 were synthesized via the cyclization of 1 with
(poly)ethylene glycol ditosylates in THF in the presence of sodium
hydride (Scheme 1).13 They were purified by column chromatogra-
phy in acceptable yields of 12–43% and their structures were
confirmed by NMR spectra and high-resolution mass spectrometry.
Significant differences in the outcome of the reaction were ob-
served depending on the solvents, bases and reaction conditions
used. As shown in Table 1, when Na2CO3 was used as the base (en-
try 10), no reaction was observed. When Cs2CO3 (entries 7–9) or
LiOH (entry 6) were used, the product 2g was obtained with trace
yield. While higher yields were obtained in the present of KOH (en-
try 4) or NaOH (entry 5) in refluxing THF. When NaH was used as
the base (entries 1–3), the isolated yields were further improved.
At room temperature, the reaction completed within 24 h in 40%
yield (entry 2). Increasing the temperature to reflux, the reaction
completed within 6 h in 43% yield (entry 1).
The focus of attention on crown ethers bearing purine base has
traditionally been directed toward the relationship between the
structure of the host molecule and its ability to bind and transport
guest species, especially alkali metal cations.9 Crown ether and
nucleotide base systems have also been designed to determine
the extent of association as well as what interactions occur be-
tween the nucleotide bases.10 To our knowledge, there has been
no report of purine analogues modified by azacrown ether at C6
position. Based on our preliminary study on various modification
of purine analogues,11 we designed and prepared novel C6-modi-
fied purine analogues by azacrown ether which displayed highly
selective and efficient signaling behaviors toward micromolar con-
centration of Ag+ ion over other common metal ions in an aqueous
environment.
To investigate the versatility of the reaction, a series of C6-
crowned purines (2a–2k) were synthesized (Table 2). For the
substrates substituted by n-butyl at N9, the yields are higher than
others. For example, the yield of product 2g (entry 9) was 43%,
higher than that of 2e (entry 8) and 2c (entry 7). Aza-15-crown-5
The key starting material, 6-[N,N-bis(2-hydroxyethyl)amino]-
purines 1, were prepared according to literature.12 Crown ethers
Table 2
Reaction of (poly)ethylene glycol ditosylates with various 6-[N,N-bis(2-hydroxyethyl)amino]purinesa
HO
OH
O
N
n
O
O
R
N
N
THF/NaH or DMF/NaH
+
TsO
O
OTs
N
N
n
N
N
N
1
R
2
N
1
R
N
2
R
1
2
2
R
1a
1b
1c
1d
1e
R
R
=CH C H
2 6 5
=H
1
n
R
=H
= CHCH=CH
2
2
2
1
R
R
= CH CH CH CH
3
2a
1
R
=H
2
2
2
1
2f 2h
2b
2c
2d
2e
2j
2k
2
3
= CHCH=CH
R =Cl
2
2g
2i
2
1
R
= CHCH=CH
2
R =N(CHCH )
2 2
Entry
1c
n
Product
2a
R1
H
R2
Yieldb (%)
1
2
26
23
2c
2b
H
3
4
5
2
2
2
2d
2f
H
H
Cl
19
40
12
2h
6
2
3
2j
24
33
N
7c
2c
H
8
9
3
3
3
2e
2g
2i
H
H
Cl
40
43
37
10
11
3
2k
30
N
a
b
c
Reagents and conditions: NaH (2 mmol), 1b (1c, 1d, 1e) (1 mmol), and (poly)ethylene glycol ditosylates (1 mmol) in 30 mL THF refluxed for 6 h.
Isolated yields based on nucleobases.
NaH (2 mmol), 1a (1 mmol), and (poly)ethylene glycol ditosylates (1 mmol) in 30 mL DMF stirred at 50 °C for 6 h.

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H.-M. Guo et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3098–3102
220
200
180
160
140
120
100
80
nitrogen-containing substituent, 2-(N,N-diallyl)-6-azacrowned
220
200
180
160
140
120
100
80
purines gave slightly better yield for 2j (entry 6), but the lower
yield for 2k (entry 11). Moreover, the yields of the azacrown ethers
were improved by extending the scope of this cyclization, 2a (entry
1) was obtained from compound 1a in 16% yield, 2b (entry 2) in
23% yield and 2c (entry 7) in about 33% yield.
60
40
0
5
10 15 20 25 30 35 40 45
-6
+
Ag /10
M
120
2j only,2j+Li+,Na+,K+,Cs+,Ca2+,Mg2+,Zn2+
100
60
40
80
2j+Cd2+,Pb2+,Hg2+
20
0
2j+Cu2+
60
300
350
400
450
500
550
2j+Ag+
Wavelength(nm)(ex 290nm)
40
20
0
130
120
110
100
90
300
350
400
450
500
550
Wavelength (nm) (ex 290nm)
80
70
60
6
5
4
3
2
1
0
50
40
30
20
10
0
300
350
400
450
500
Wavelength(nm)(ex 290nm)
O
O
100
80
60
40
20
0
O
O
N
N
N
N
N
N
Li+ Na+
2+ Cd2+ Hg2+
Ag+ Mg2+ Zn2+ Pb2+ Cu
K+ Cs+ Ca2+
OH
N
HO
2j
N
N
N
N
N
100
80
60
40
20
0
Cl
N
N
N
N
N
1e
3
2j+Ag(I)
300
350
400
450
500
550
Wavelength(nm)(ex 290nm)
Figure 1. (a) Fluorescence spectra of 2j at different concentrations (1 Â 10À6 M–
4 Â 10À5 M) in an ethanol–water solution (2:100, v/v) at 375 nm, kex = 290 nm. (b)
Fluorescence spectra of 2k at different concentrations (1 Â 10À6 M–4 Â 10À5 M) in
an ethanol–water solution (2:100, v/v) at 375 nm, kex = 290 nm. (c) Fluorescence
spectra of 2j (1 Â 10À5 M), 1e (1 Â 10À5 M) and 3 (1 Â 10À5 M) in an ethanol–water
solution (2:100, v/v).
300
350
400
450
500
550
Wavelength (nm) (ex 290nm)
2f (entry 4) was obtained in 40% yield which was higher than that
of crown ethers 2d (entry 3) and 2b (entry 2). The influence of sub-
stituent groups at C2 was also studied. When H at C2 was replaced
by Cl, 2h was afforded with 12% yield (entry 5) and 2i 37% yield
(entry 10), lower than that of the corresponding crown ethers 2d
(entry 3) and 2e (entry 8). By contrast, replacement of H by the
Figure 2. (a) Fluorescence spectra of 2j in the presence of various metal ions in an
ethanol–water solution (2:100, v/v). [2j] = 1 Â 10À5 M, [Mn+] = 1 Â 10À4 M.
kex = 290 nm. (b) The quenching efficiency of 2j expressed by the ratio of Io/I (Io
and I represent fluorescence intensity of 2j in the absence and presence of metal
ions). [2j] = 1 Â 10À5 M, [Mn+] = 1 Â 10À4 M. (c) Fluorescence spectra of 2j with Ag+.
[2j] = 1 Â 10À5 M, [Ag+] = 1 Â 10À5 M, 2 Â 10À5 M, 5 Â 10À5 M, 1 Â 10À4 M.

H.-M. Guo et al. / Bioorg. Med. Chem. Lett. 20 (2010) 3098–3102
3101
O
O
OH
N
HO
O
O
N
Cl
N
N
N
HOCH CH NHCH CH OH
N
N
2
2
2
2
N
N
N
TsO
O
O
OTs
N
N
N
N
N
N
N
THF, NaH, 6h
3
2j
Scheme 2. Synthetic pathway for N-[9-allyl-2-(N,N-diallyl)purin-6-yl]aza-15-crown-5 2j.
1e
The fluorescence behavior of the purine analogues were inves-
tigated in 2% ethanol solution. As shown in Figure 1(a), compound
2j displayed characteristic emission band around 375 nm when ex-
cited at 290 nm. The fluorescence intensity increased linearly with
the concentration of 2j in the range of 10À6–10À5 M. Under the
same condition, crown ether 2k also exhibited emission band
around 375 nm (Fig. 1(b)). Compared with 2j, 2k displayed weak
emission at the same concentration.
ince (2008IRTSTHN002) and Henan National Nature Science
Foundation (092300410226).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2010.03.102.
The fluorescent properties of 2j as well as the corresponding
starting materials 3 and 1e were also examined (Scheme 2,
Fig. 2(c)). At the same concentration, compound 3 showed almost
no fluorescence in the range of 300–550 nm, and compound 1e
(formed from 3) did not show any significant change in fluores-
cence intensities. While the azacrown ether bearing purines 2j
exhibited significant fluorescence. This result implied that the
crown ether group at C6 may contribute to the fluorescence
enhancement of the purine analogues.
References and notes
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The fluorescence response of crown ether 2j toward various me-
tal ions (Li+, Na+, K+, Cs+, Ca+, Mg2+, Zn2+, Ag+, Pb2+, Cu2+, Cd2+, Hg2+
)
was also examined (Fig. 2(a)). The characteristic fluorescence spec-
trum of 2j was effectively quenched upon treatment with Ag+ ions.
The quenching efficiency can be expressed by the ratio of Io/I at
375 nm (Io and I represent the fluorescence intensity of 2j in the
absence and in the presence of metal ions, respectively). Figure
2(b) demonstrates that metal ions other than Ag+ did not induce
noticeable changes in the fluorescence intensity. Fluorescence
intensity changes of 2j with different concentrations of Ag+ were
shown in Figure 2(c), the intensity decreased when AgNO3 had
been added from 1 equiv to 10 equiv. These results indicated that
compound 2j had good selectivity toward Ag+ and that other com-
petitive metal ions would induce a rather low interfering effect on
this fluorescence assay for Ag+.
In conclusion, the first synthesis of purine analogues containing
a crown ether group at C6 was developed, thus widening the scope
of crown ether chemistry and opening a new rout for modification
at C6 of purine analogues. The resulting crowned purine analogues
showed selective and efficient signaling behaviors toward micro-
molar concentration of Ag+ ion over other common metal ions in
an aqueous environment. The further extension of the synthesis
of the crown ether bearing purine base and their fluorogenic
behavior are undergoing in our laboratories.
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Acknowledgments
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We are grateful for financial support from the National Nature
Science Foundation of China (Grants 20772024 and 20802016),
and Henan Innovation Fund for Outstanding Scholarship (No.
074200510020), and Program for New Century Excellent Talents
in University of Ministry of Education (NCET-09-0122) and Pro-
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13. Typical experimental procedure for the synthesis of N-(9-butyl purin-6-yl)aza-
18-crown-6 (2g)
refluxed for 6 h. The solvent was evaporated and the crude product was
purified by flash column chromatography on silica gel (eluent: ethyl acetate–
petroleum ether 60–90 = 1:3) to give 2g 0.188 g (43%). Yellow oil, 1H NMR
(CDCl3, 400 MHz) d 8.32 (s, 1H), 7.67 (s, 1H), 4.60–4.53 (m, 2H) 4.16–4.12 (m,
4H), 3.83 (d, J = 4.8 Hz, 4H), 3.67 (d, J = 8 Hz, 16H), 1.85–1.81 (m, 2H), 1.37–1.30
(m, 2H), 0.93 (t, J = 7.2 Hz, 3H). 13C NMR (CDCl3, 100 MHz) d 154.2, 152.1,
150.6, 138.4, 119.8, 70.8, 70.5, 70.4, 70.0, 43.3, 32.0 19.8, 13.5. HRMS: calcd for
C21H35N5NaO5 [M+Na+] 460.2536, found 460.2534.
To a suspension of NaH (0.08 g, 2 mmol, 60% in mineral oil) in 10 mL dry THF at
room temperature was added a solution of 1c (0.279 g, 1 mmol) in 10 mL of
THF. The reaction mixture was refluxed for 1 h. After cooling to room
temperature, a solution of tetraethylene glycol ditosylate (0.502 g, 1 mmol)
in 10 mL of THF was slowly added to the reaction mixture. The suspension was