1,2,3-Triazoles as leaving groups in purine chemistry: A three-step synthesis of N6-substituted-2-triazolyl-adenine nucleosides and photophysical properties thereof

Article abstract of DOI:10.1016/j.tetlet.2012.11.095

A novel three-step approach for the synthesis of N⁶-substituted- 2-(1,2,3-triazol-1-yl)-adenine nucleosides is described. 2,6-Bis-(1,2,3-triazol- 1-yl)purine nucleosides are obtained, which undergo regioselective nucleophilic aromatic substitution with amines at C(6). Thus, 1,2,3-triazoles are shown to be good leaving groups in purine chemistry. The title compounds exhibit interesting levels of fluorescence with quantum yields of up to 53%.

Full text of DOI:10.1016/j.tetlet.2012.11.095

Accepted Manuscript
1,2,3-Triazoles as leaving groups in purine chemistry: a three-step synthesis of
6
N -substituted-2-triazolyl-adenine nucleosides and photophysical properties
thereof
Armands Kovaļovs, Irina Novosjolova, Ērika Bizdēna, Inga Biž āne, Lina
Skardziute, Karolis Kazlauskas, Saulius Jursenas, Māris Turks
PII:
S0040-4039(12)02049-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2012.11.095
TETL 42191
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
9 August 2012
14 November 2012
22 November 2012
Please cite this article as: Kovaļovs, A., Novosjolova, I., Bizdēna, Ē., Biž āne, I., Skardziute, L., Kazlauskas, K.,
6
Jursenas, S., Turks, M., 1,2,3-Triazoles as leaving groups in purine chemistry: a three-step synthesis of N -
substituted-2-triazolyl-adenine nucleosides and photophysical properties thereof, Tetrahedron Letters (2012), doi:
http://dx.doi.org/10.1016/j.tetlet.2012.11.095
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1,2,3-Triazoles as leaving groups in purine
chemistry: a three-step synthesis of N6-
substituted-2-triazolyl-adenine nucleosides
and photophysical properties thereof
Leave this area blank for abstract info.
Armands Kovaļovs, Irina Novosjolova, Ērika Bizdēna, Inga Bižāne, Lina Skardziute, Karolis Kazlauskas,
Saulius Jursenas, Māris Turks
280 nm
400 nm
R
N
N
Nu
N
N3
N
N
N
N
N
N
N
N
N
NuH
R
N
N
N
N
N
N
N
N3
N
N
Sugar
Sugar
Sugar
R
R
1
2
3

1
Tetrahedron Letters
journal homepage: www.elsevier.com
1,2,3-Triazoles as leaving groups in purine chemistry: a three-step synthesis of N6-substituted-2-
triazolyl-adenine nucleosides and photophysical properties thereof
Armands Kovaļovsa, Irina Novosjolovaa, Ērika Bizdēnaa, Inga Bižānea, Lina Skardziuteb, Karolis
Kazlauskasb, Saulius Jursenasb, Māris Turksa*
aFaculty of Material Science and Applied Chemistry, Riga Technical University, Riga, LV-1007, Latvia
bInstitute of Applied Research, Vilnius University, Sauletekio 9-III, LT-10222 Vilnius, Lithuania
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A novel three-step approach for the synthesis of N6-substituted-2-(1,2,3-triazol-1-yl)-adenine
nucleosides is described. 2,6-Bis-(1,2,3-triazol-1-yl)purine nucleosides are obtained, which
undergo regioselective nucleophilic aromatic substitution with amines at C(6). Thus, 1,2,3-
triazoles are shown to be good leaving groups in purine chemistry. The title compounds exhibit
interesting levels of fluorescence with quantum yields of up to 53%.
Available online
2012 Elsevier Ltd. All rights reserved.
Keywords:
triazolyl purine derivatives
nucleophilic aromatic substitution
click chemistry
fluorescence
nucleoside analogs
There are at least three distinct areas of application of purines:
1) anticancer therapy with antimetabolic purine nucleosides,
which among others include fludarabine, thioguanine, clofarabine
and cladribine;1 2) antiviral purine derivatives2 with prominent
examples being vidarabine, abacavir, entecavir and acyclovir; 3)
agonists and antagonists of adenosine receptors. This last group
contains agonists of the A1 receptor that have provided clinical
candidates for atrial arrhythmias, type 2 diabetes and insulin-
sensitizing agents, pain management, and angina (tecadenoson,
GW493838, TCPA, etc.).3 Adenosine receptor A2A agonists are
excellent anti-inflammatory agents (apadenoson, regadenoson,
binodenoson).4 Also, selective activation of the adenosine
receptor A3 was demonstrated to be cardioprotective and
cerebroprotective.5 In the light of the aforementioned facts it is
no surprise that active research on purine derivatives takes place.
the groups of Van Calenbergh (1a,b),12 Aldrich (1c)13 and
Lakshman (1a),14 and 6-triazolyl derivatives as reported by
Lakshman and co-workers (2)15 and the group of Guieu and
Parrain (3).16 Grøtli has developed fluorescent 8-triazolyl
adenosine 4 and demonstrated its substantial photophysical and
base-mimicking properties in DNA.17 Adenosine analogs 1
possess intrinsic similarity with 4 as both types of structures
contain electron-donating amino and electron-withdrawing
triazole18 groups on the purine scaffold. The recent achievements
and the general importance of the area of fluorescent nucleosides
has been extensively reviewed.19 Existing biological
applications12-14 and potential fluorescent properties of the target
nucleosides 1 and their analogs prompted us to design a novel
approach for their synthesis.
Herein, we describe a de novo synthetic methodology towards
N6-substituted-2-triazolyl adenine nucleosides and investigations
of their fluorescent properties. Our approach includes the use of
Many chemical modifications of the purine system have been
developed for the 2- and/or 6-positions. Nevertheless, significant
improvements continue to appear.6 We were intrigued by the
possibility to use a 1,2,3-triazolyl moiety at C(6) of the purine as
a leaving group. The use of 6-(1,2,4-triazol-4-yl)purines for this
purpose was initiated by Robins’ group7 based on the earlier
work of Divakar and Reese.8 Indeed, 1,2,3- and 1,2,4-triazoles
possess very similar acidities: pKa 9.3 and 10.3, respectively.9
Nevertheless, there are only limited known applications of 1,2,3-
triazolyl moieties as potential leaving groups to date.10
Additionally, regardless of the vast achievements in the field of
1,2,3-triazole–nucleoside conjugates,11 there are only a few
reports dealing with either 2- or 6-(1,2,3-triazol-1-yl)purine
nucleosides or with 6-(1,2,3-triazol-4-yl)-purine nucleosides
(Figure 1). These include 2-triazolyl derivatives as described by
2,6-bis-(triazolyl)purine intermediates with
a 1,2,3-triazolyl
moiety as the leaving group in heteroaromatic nucleophilic
substitution reactions. To the best of our knowledge, the latter
chemical entities and mechanistic approach have not been
reported previously.
The starting materials were obtained via Vorbrüggen
glycosylation20 of 2,6-diazidopurine21 (6). Nucleoside type
structures 7 and 9 were obtained in 90% and 65% yields,
respectively (Scheme 1). Both these diazides are rather unstable
in daylight and at elevated temperatures, but can be stored for a
long period of time in the dark below 5 oC.

2
Tetrahedron Letters
R
N
R1
N
HN
N
N
N
N
N
R3
N
N
O
N
N
N
HO
O
N
N
HO
OH
R2
HO
OH
1a: R1 = Me; R2 = alkyl, aryl, benzyl, etc;
2
R3 = CH2OH
1 = Me; R2 = alkyl, aryl, benzyl, etc;
1b:
R
R3 = C(O)NHEt
1c: R1 = H; R2 = alkyl, aryl, benzyl, etc;
R3 = CH2O-SO2-N-C(O)-Ar
Et3NH
R
N
N
N
R
NH2
N
N
N
N
N
N
N
N
N
HO
HO
Scheme 2. Synthesis of 2,6-bis-triazolylpurine nucleosides (β-D-
ribofuranosyl and α-D-arabinopyranosyl series) and their substitution
with amines at C(6) (see Table 1).
O
O
N
N
HO
OH
HO
OH
3
4
To our delight, the envisaged nucleophilic aromatic
substitution with amines at C(6) of the purine in compounds 10
and 12 took place easily and this was combined with cleavage of
the acetate protecting groups. N-Nucleophiles such as ammonia,
methylamine, pyrrolidine and piperidine were successfully used
and N6-substituted-2-triazolyl-riboadenosine derivatives 11 and
their arabinopyranosyl analogs 13 were isolated in good to
excellent yields (Table 1). Both the regioselectivity of the azide
1,3-dipolar cycloaddition yielding 1,4-disubstituted 1,2,3-
triazoles and the C(6)-regioselectivity of the nucleophilic
heteroaromatic substitution in the purine moiety was proved by
isolation of product 11a, which was found to be identical to that
described by Van Calenbergh.12 The expected elimination of 4-
phenyl-1H-[1,2,3]triazole was confirmed by HPLC and 1H-NMR
analysis of the reaction mixtures in the cases where bis-triazolyl
purines 10a and 12a were treated with amines.
Figure 1. Nucleoside analogs with triazolyl-purine bases.
With the required substrates in hand, we demonstrated the
scope of our approach which does not require the use of naturally
occurring nucleosides as the key starting materials.
N
O
O
N3
OAc
OAc
N
AcO
AcO
N
N
N3
N
AcO
AcO
OAc
N3
5
N
7, 90%
N
6
N
H
N3
N
O
BSA, TMSOTf
O
N3
N
AcO
AcO
OAc
AcO
N
N
We found that compounds 11 and 13 fluoresce when exited at
280 nm. This previously unreported property of N6-substituted-2-
OAc
AcO
OAc
N3
9, 65%
8
triazolyl adenine derivatives opens up
investigation for these compounds.
a new field of
Scheme 1. Synthesis of 2,6-diazidopurine nucleosides 7 and 9.
All the products possessed similar fluorescence properties
with emission maxima around 400 nm and quantum yields of up
to 53%, which is comparable with 8-triazolyladenosine analogs
(64% for compound 4: R = Bn).17a The spectra are shown in
Figure 2, and the photophysical characterization (absorption and
fluorescence properties) of the products are given in Table 1.
Thus we proceeded to investigate the bis-triazole formation.
Diazides 7 and 9 underwent copper-catalyzed 1,3-dipolar
cycloaddition reactions with different terminal alkynes.22 Various
Cu(I) sources were screened including the well-established
CuSO4•5H2O/sodium ascorbate system, Cu0/CuSO4 and CuI.
Additionally, various solvent systems (acetone/water,
CH2Cl2/water, tert-butanol/water, THF) and temperature regimes
In summary, we have disclosed here a novel three-step
approach for the synthesis of N6-substituted-2-triazolyl adenine
nucleosides which uses 2,6-diazidopurine and a monosaccharide
of choice as the starting materials and therefore can be described
as being convergent.24 The second step of the sequence involves
the double click reaction and provides new structural entities,
2,6-bis-(1,2,3-triazol-1-yl)purine nucleosides, which readily
undergo regioselective nucleophilic aromatic substitution with
amines. The total yields of the title compounds reach 67% over
three steps starting from 2,6-diazidopurine. Our approach
competes well with the previously reported methods that give
rise to, e.g. 11a or its analogs either from guanosine or from O6-
allyl-2′,3′,5′-tri-O-(tert-butyldimethylsilyl)guanosine in five steps
and a maximum yield of 27%. The described synthetic route
demonstrates the use of 1,2,3-triazoles as excellent nucleofuges
in purine chemistry. It reveals also an excellent
price/performance ratio or step economy in the synthesis of N6-
substituted-2-triazolyl adenine nucleosides when relatively cheap
o
(20-80 C) were explored. A compromise between low reactivity
(t1/2 ~ 16-20 h at ambient temperature) and the thermal stability
of the diazides had to be found. Addition of dilute acetic acid
accelerated the cycloaddition reactions at ambient or slightly
elevated temperatures in 1-4 hours.23 The best reaction conditions
for the double cycloaddition reaction included mixing of 7 or 9
with the alkyne in t-BuOH/acetone/water in the presence of
CuSO4•5H2O/sodium ascorbate and dilute acetic acid. Bis-
triazolyl-nucleosides 10 and 12 were, after work-up, isolated by
silica gel column chromatography. The isolated yields of the
chromatographically homogeneous products ranged from 60% to
83% (Scheme 2, Table 1).

3
Table 1. Synthesis of 2,6-bis-triazolyl nucleosides 10 and 12 and their nucleophilic substitution products 11 and 13 and the
photophysical characterization of the latter
Entry
Starting
material
R1
Product
10/12
NR2R3
Product 11/13
λ
max(abs),
(nm)
λ
max(emis),
(nm)
Quantum
yield (%)
(yield, %)b
(yield, %)a
(DMSO)
257/300
(DMSO)
398
1
2
-NHMe
11a (92)
11b (86)
32
50
7
Ph
10a (83)
257/271
255/278
255/296
257/271
257/267
402
394
394
401
404
3
4
5
6
7
7
CH2OH
10b (64)
10c (70)
11c (89)
11d (86)
13a (88)
13b (88)
38
33
51
53
CMe2OH
9
Ph
12a (78)
7
8
-NHMe
-NMe2
13c (73)
13d (91)
258/299
258/266
396
402
33
50
9
13e (69)
13f (74)
13g (71)
13h (83)
255/275
255/275
255/275
-
393
392
394
-
34
30
36
-
9
9
CMe2OH
12b (75)
12c (60)
10
11
12
-NMe2
n-C5H11
-NH2
a Experimental conditions: diazide (1 equiv.), alkyne (5 equiv.), CuSO4 (5.5 mol%), sodium ascorbate (9.2 mol%), 10% AcOHaq./t-BuOH/water, from ambient
temperature to 40 oC.
b Experimental conditions: amine (excess) in water or water/THF, from ambient temperature to 40 oC.
11a
11b
11c
11d
13a
13b
13c
13d
13e
13f
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
13g
235
285
335
385
435
485
Wavelength, nm
Figure 2. Absorption (dashed lines) and emission (solid lines) spectra of compounds 11a-d and 13a-g.

4
Tetrahedron
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Finally, we discovered that the title compounds showed useful
levels of fluorescence. Detailed investigations on the
susceptibility of 2,6-bis-(1,2,3-triazol-1-yl)purine derivatives
towards various N-, S- and O-nucleophiles are underway in our
laboratory and will be reported elsewhere, together with a full
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This work was supported by Riga Technical University grants
ZP-2010/17 and FLPP-2010/12. I. N. thanks the European Social
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doctoral studies at Riga Technical University» for a scholarship.
The authors thank Ms. A. Sprūdža for technical help, Professor
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fluorescence spectra and Professor S. Tumkevičius for helpful
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Supplementary Material
Supplementary data associated with this article can be found, in
the online version, at …
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