Application of 2,6-diazidopurine derivatives in the synthesis of thiopurine nucleosides

Article abstract of DOI:10.1016/j.tetlet.2013.09.095

2,6-Diazidopurine derivatives undergo double azide-alkyne 1,3-dipolar cycloaddition (CuAAC) reactions to give 2,6-bis-(triazolyl)purine analogs, which undergo selective nucleophilic aromatic substitution with various thiols at C(6). This synthetic sequenc

Full text of DOI:10.1016/j.tetlet.2013.09.095

Accepted Manuscript
Application of 2,6-diazidopurine derivatives in the synthesis of thiopurine nu-
cleosides
Irina Novosjolova, Ērika Bizdēna, Māris Turks
PII:
S0040-4039(13)01660-2
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.09.095
TETL 43595
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
6 August 2013
6 September 2013
20 September 2013
Please cite this article as: Novosjolova, I., Bizdēna, Ē., Turks, M., Application of 2,6-diazidopurine derivatives in
the synthesis of thiopurine nucleosides, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.
2
013.09.095
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Graphical abstract
Application of 2,6-diazidopurine derivatives in the
synthesis of thiopurine nucleosides
Irina Novosjolova, Ērika Bizdēna, Māris Turks*

Application of 2,6-diazidopurine derivatives in the
synthesis of thiopurine nucleosides
Irina Novosjolova, Ērika Bizdēna, Māris Turks*
Faculty of Material Science and Applied Chemistry, Riga Technical University, 14/24 Azenes Str., Riga
LV-1007, Latvia
*
Corresponding author. Tel.: +371 67089251; fax: +37167615765.
E-mail address: maris_turks@ktf.rtu.lv (M. T).
Abstract: 2,6-Diazidopurine derivatives undergo double azide-alkyne 1,3-dipolar
cycloaddition (CuAAC) reactions to give 2,6-bis-(triazolyl)purine analogs, which
undergo selective nucleophilic aromatic substitution with various thiols at C(6). This
synthetic sequence produces nucleoside analogs with 6-alkyl/arylthio-2-(4-alkyl/aryl-
1H-1,2,3-triazol-1-yl)purine bases. In contrast, glycosylated 2,6-diazidopurines
exhibit reasonable C(2) selectivity in nucleophilic aromatic substitution with thiols.
This permits the synthesis of 2-alkylthio-6-azido-purine derivatives, which after
CuAAC provide the corresponding 2-alkylthio-6-triazolyl-purine analogs. The latter
are also susceptible to nucleophilic aromatic substitution with amines at C(6). The
above mentioned compounds are useful molecular platforms in terms of medicinal
chemistry.
Key words: thiols, nucleophilic aromatic substitution, nucleoside analogs, purine
derivatives
Many thiopurine derivatives belong to the class of purine antimetabolites, and as such
1
are used in the treatment of cancers and autoimmune disorders. 6-Thioguanine and 6-
mercaptopurine are standard components for treatment of childhood lymphoblastic
leukemia, but certain therapeutic disadvantages in their use have stimulated the search
for other purine derivatives, including glycosylated analogs, with potentially
2
enhanced therapeutic effects. Additionally, the hetarylthio purine derivative,
azathioprine, has been in use for more than 50 years in organ transplantation as an

immunosuppressive drug. Nevertheless, there are only a few literature reports dealing
with the synthesis and evaluation of the biological activity of 9H-purine derivatives
3
bearing both heteroaromatic and thioether-type substituents. To the best of our
knowledge, there are no reports on the synthesis of triazolyl substituted thiopurine
derivatives. Also the field of 2- or 6-(1,2,3-triazol-1-yl)purine nucleosides, 6-(1,2,3-
triazol-4-yl)-purine nucleosides, and 8-triazolyl adenosine analogs is covered by less
4
than 10 publications.
We have previously reported on the synthesis of 2,6-bis-(triazolyl)purine derivatives
5
and their use in nucleophilic aromatic substitution reactions with various amines.
Herein, we extend this approach by using thiols as nucleophiles. We have also found
that the regioselectivity of the nucleophilic aromatic substitution of 2,6-bis-
(
triazolyl)purine derivatives is different from that of their precursors, 2,6-
diazidopurines.
The required starting materials were obtained by a previously developed synthetic
route, which uses glycosylated 2,6-diazidopurines 1a-c as key starting materials
6
(
Scheme 1).
Scheme 1. Synthesis of 2,6-bis-(triazolyl)purine nucleosides 2a-f and nucleophilic
aromatic substitution thereof.
2,6-Bis-(triazolyl)purine nucleosides 2a-f reacted smoothly with various S-
nucleophiles such as thiophenol, benzylthiol, triphenylmethyl mercaptan and 1-
dodecanethiol providing derivatives of 6-alkyl/arylthio-2-(4-alkyl/aryl-1H-1,2,3-
triazol-1-yl)-9H-purines (Scheme 1, Table 1). Substitution reactions were carried out
in the presence of a base, although in some cases the reaction occurred simply by
addition of the thiol (Table 1, entry 1). In a typical experiment, a solution of starting
2 3
material 2a-f and the corresponding thiol in DMF was treated with K CO and the

resulting mixture was stirred at ambient or slightly elevated temperature. Extractive
work-up followed by column chromatography or crystallization provided target
compounds 3a-h. The yields of the substitution reactions in some cases were
quantitative (Table 1, entries
Table 1. Synthesis of 6-alkyl/arylthio-2-(4-alkyl/aryl-1H-1,2,3-triazol-1-yl)purine derivatives 3a-h
from 2a-f according to Scheme 1
a
Starting material
Reaction conditions
Product
Yield
%)
Entry
(
1
2
3
R SH
Substrate
R
R
Solvent
DMF
Base
2
,3,5-tri-O-
1
2a
acetyl-β-D-
ribofuranosyl
Ph
-
PhCH
2
SH
75%
78%
70%
99%
52%
81%
3
3
3
3
3
a
b
c
2
,3,5-tri-O-
2
3
4
5
6
2a
2a
2b
2c
2d
acetyl-β-D-
ribofuranosyl
Ph
THF
THF
DMF
DMF
K
2
CO
3
PhSH
2
,3,5-tri-O-
b
acetyl-β-D-
ribofuranosyl
Ph
NaH
3
Ph CSH
2
,3,5-tri-O-
acetyl-β-D-
ribofuranosyl
COOMe
K
2
K
2
K
2
CO
3
n-C12H25SH
d
2
,3,5-tri-O-
acetyl-β-D-
ribofuranosyl
CH
2
OH
CO
3
3
Ph CSH
e
2
,3,4-tri-O-
acetyl-α-D-
arabino-
c
Ph
DMF
CO
3
2
PhCH SH
pyranosyl
3
f
2
,3,4-tri-O-
acetyl-α-D-
arabino-
pyranosyl
7
8
2e
CMe
2
OH
OH
DMF
DMF
K
2
CO
3
n-C12
H
25SH
68%
99%
3
g
2
,3,5-tri-O-
benzoyl-β-D-
arabino-
furanosyl
2f
CMe
2
K
2
CO
3
n-C12
H25SH

3
h
a
b
c
Reactions were carried out at ambient temperature if not stated otherwise.
Ph CSH was used as the nucleophile followed by an acidic work-up procedure.
Reaction was carried out at 40 °C.
3
4
and 8). Substitution reactions of compounds 2 with NaSH proceeded sluggishly and
only traces of the expected purine-6-thiol/thione products were observed. However,
acidic work-up of the reaction mixture containing 6-(tritylthio)-9H-purine nucleoside
3
[
process: 2a + Ph
3
CSNa → 3c′ (R =Ph
3
C) → 3c] provided the expected detritylated
product 3c in 70% yield (Table 1, entry 3).
The C(6) regioselectivity of the nucleophilic aromatic substitution of 2,6-bis-
triazolyl)purine derivatives with S-nucleophiles was proved by product analysis that
(
arose from further aromatic substitution at C(6) with methylamine. It was previously
discovered that direct substitution of 2,6-bis-(triazolyl)purine derivatives with N-
6
nucleophiles led to N -substituted-2-triazolyl-adenosine derivatives of type 5 (process
5
A, Scheme 2). We have observed that prolonged treatment of thiopurine derivatives
3
i,j with methylamine also provided the same products. This is consistent with the
fact that the S-nucleophiles attack the 2,6-bis-(triazolyl)purines at C(6). Thus, careful
control of the deacetylation reaction conditions allowed isolation of nucleoside analog
4
a which contains a fully deprotected sugar moiety and the originally designed 6-
dodecylthio-2-(4-phenyl-1H-1,2,3-triazol-1-yl)-purine base. Further treatment of 4a
with MeNH yielded known 5a, which in this case arises from substitution of the
2
dodecylthio moiety with a methylamino group at C(6) (process B, Scheme 2). On the
other hand, compound 3j provided 5b in 84% yield when the former was treated with
2
aqueous MeNH solution in THF for seven days at ambient temperature. Also, in this
case, 5b was obtained in parallel by a known synthetic procedure which consisted of
5
2
nucleophilic aromatic substitution of 2g with MeNH at C(6).

process A (direct)
2
2
a
g
5a: R=Ph, 92%
5b: R=t-Bu-C6H4
quant. conv.
by NMR
NHMe
N
N
N
N
HO
N
N
N
R
O
MeNH2/H2O
THF
R
HO
OH
N
[ref 5]
5a,b
N
4a
5a: R=Ph, conversion: 60% (HPLC);
isolated yield:
N
N
N
N
85% based on recovered
starting material
AcO
N
3j [4b]
5b: R=t-Bu-C6H4, 54%
(2 steps)
N
N
N
R
O
MeNH2 THF
H2O
SC12H25-n
AcO
OAc
a: R = Ph
g: R = t-Bu-C6H4
N
N
N
2
2
N
HO
N
N
N
R
O
n-C12H25SH
K2CO3, DMF
MeNH2
H2O
SC12H25-n
HO
OH
4a,b
2
3
a
j
[3i] 4a: R=Ph, 56% (2 steps)
[4b]: R=t-Bu-C H (not isolated)
N
N
N
THF
6
4
N
AcO
N
N
N
R
O
process B (via 3 and 4)
3
i,j
AcO OAc
2
2
a
g
3i: R=Ph (not isolated)
3j: R=t-Bu-C H , 65% (HPLC)
6
4
Scheme 2. Proof of the C(6)-regioselectivity in the nucleophilic aromatic substitution
of 2,6-bis-(triazolyl)purine nucleosides 2 via synthesis of known products 5.
Next, we explored the reactivity of 2,6-diazidopurine nucleoside 1a towards S-
nucleophiles. It appeared that this compound exhibited opposite regioselectivity.
Thus, reaction of 1a and n-C12H25SH gave substitution products 6a and 6b in the
approximate ratio 2:1 (Scheme 3). It was observed that 6-azidopurine derivative 6a
was in equilibrium with its tetrazole tautomer 6a′ in the solution. The dependence of
the equilibrium 6a/6a′ on the solvent and temperature was studied briefly (Table 2).
Due to the electron-withdrawing effect of a fused tetrazole, H-C(8) of tautomer 6a′
1
appears at lower field in the H-NMR spectrum than H-C(8) of azide form 6a.
7
8
4d
Similarly to observations on the 2-azidoadenine series and 6-azidopurine series,
also in the case of 6a/6a′ a less polar solvent (CDCl vs. DMSO-d ) and higher
temperatures shifted the equilibrium towards azide form 6a.
3
6
The mixture of products 6a/6a′ underwent an azide-alkyne 1,3-dipolar cycloaddition
reaction with phenylacetylene and yielded 2-dodecylthio-6-(4-phenyl-1H-1,2,3-
1
triazol-1-yl)-9H-purine derivative 7 in 80% yield. Comparison of the H-NMR spectra
of compounds 3i and 7 revealed that the chemical shift of H-C-S-C(6) was 3.49 ppm,

but that of H-C-S-C(2) was 3.29 ppm. In contrast, for compound 3i, the signals of the
triazole protons H-C(5′′) and H-C(8) were at 8.92 and 8.16 ppm, respectively, but
those for 7 were at 9.06 and 8.26 ppm.
The purine system in compound 7 appeared to be very susceptible to nucleophilic
aromatic substitution with MeNH
2
at C(6) due to the good leaving group properties of
gave full conversion into compound 8 in
phenyltriazole. The reaction of 7 and MeNH
2
one hour at ambient temperature, and the isolated yield of product 8 was 66%. Both
9
products of type 8 (2-alkyl/arylthio-6-aminopurine derivatives) and their
1
0
regioisomers (6-alkyl/arylthio-2-aminopurine derivatives)
extensively in medicinal chemistry.
have been studied
Scheme 3. Synthesis of 2-dodecylthio-6-(4-phenyl-1H-1,2,3-triazol-1-yl)purine
derivative 7 and nucleophilic aromatic substitution thereof to give 8.

Table 2. Temperature and solvent effects on the equilibrium of 6a/6a′
Ratio
Temp.
Entry
(°C)
azide 6a : tetrazole 6a′
a
Solvent: CDCl
25
3
1
2
3
80:20
87:13
89:11
40
55
b
Solvent: DMSO-d
6
4
5
6
7
8
27
40
55
70
18:82
25:75
32:68
39:61
45:55
80
a
1
1
High field H-C(8) signals H-NMR (300 MHz) δ, ppm: 7.97-8.03 were assigned for 6a (azide). Low field H-C(8) signals H-
NMR (300 MHz) δ, ppm: 8.20-8.23 were assigned for 6a′ (tetrazole).
b
1
1
High field H-C(8) signals H-NMR (300 MHz) δ, ppm: 8.46-8.53 were assigned for 6a (azide). Low field H-C(8) signals H-
NMR (300 MHz) δ, ppm: 8.64-8.70 were assigned for 6a′ (tetrazole).
Finally, we tested the developed approach for the synthesis of symmetric dimers
(
Scheme 4). As expected, the treatment of one equivalent of 1,3-propanedithiol or
,10-decanedithiol with two equivalents of nucleoside derivatives 2a or 2d provided
products 9a-d in isolated yields of 56-77%. Treatment of a THF solution of 9b with a
5% aqueous solution of MeNH gave, after three hours at ambient temperature,
1
2
2
water-soluble nucleoside dimer 10 in 75% yield.
In conclusion we have demonstrated that 2,6-diazidopurine nucleosides are valuable
starting materials for the synthesis of 2,6-disubstituted purine analogs containing
1,2,3-triazolyl, alkyl/arylthio, azido, and amino substituents in various combinations.
We have found that 2,6-bis-(triazolyl)purine derivatives react selectively with thiol
nucleophiles at C(6), whereas their precursors, 2,6-diazidopurine nucleosides, exhibit
satisfactory C(2)-selectivity towards thiols. These novel synthetic procedures will be
useful for the generation of combinatorial libraries of purine derivatives.

N
N
HS
SH
N
N
S
S
N
N
N
n
N
n
N
2a
N
O
N
K2CO3, DMF
N
RO
N
RO
N
O
RO
OR
RO
OR
9
9
a: n=1, R=Ac, 56%
b: n=8, R=Ac, 77%
MeNH2
H2O
10: n=8, R=H, 75%
N
N
N
N
S
S
N
N
N
N
n
N
HS
SH
N
N
n
2d
N
N
K2CO3, DMF
N
OAc
OAc
AcO
AcO
O
O
OAc
OAc
9c: n=1, 70%
9d: n=8, 66%
Scheme 4.
Acknowledgments
I.N. thanks the European Social Fund within the project «Support for the
implementation of doctoral studies at Riga Technical University» for a scholarship
and ERDF project 2DP/2.1.1.2.0/10/APIA/VIAA/003 for the opportunity to present
this work at the 18th European Symposium on Organic Chemistry. The authors thank
RTU-Olainfarm co-project ZI-2013/9 for materials.
References and notes
1
2
3
.
.
.
Fotoohi, A. K.; Coulthard, S. A.; Albertioni, F. Biochem. Pharmacol. 2010, 79, 1211.
Elgemeie, G. L. Curr. Pharm. Design 2003, 9, 2627.
(a) Brown, D. J.; Cowden, W. B.; Lan, S. B.; Mori, K. Aust. J. Chem. 1984, 37, 155, (b)
Brown, D. J.; Mori, K. Aust. J. Chem. 1985, 38, 467, (c) Suzuki, T.; Satoh, H.; Matsuno,
T.; Saitoh, K.; Ohta, S.; Masuda, M.; Yaguchi, S.; Koshimizu, I.; Watanabe, Y.; Minowa,
Y.; Takahashi, M.; Kayou, T. Eur. Pat. Appl. EP2426130A1, 2010; Chem. Abstr. 2010,
153, 580340.

4
.
(a) Cosyn, L.; Palaniappan, K. K.; Kim, S.-K.; Duong, H.T.; Gao, Z.-G.; Jacobson, K. A.;
Van Calenbergh, S. J. Med. Chem. 2006, 49, 7373, (b) Gupte, A.; Boshoff, H. I.; Wilson,
D. J.; Neres, J.; Labello, N. P.; Somu, R. V.; Xing, C.; Barry III, C. E.; Aldrich, C. C. J.
Med. Chem. 2008, 51, 7495, (c) Lakshman, M. K.; Kumar, A.; Balachandran, R.; Day, B.
W.; Andrei. G.; Snoeck, R.; Balzarini, J. J. Org. Chem. 2012, 77, 5870, (d) Lakshman, M.
K.; Singh, M. K.; Parrish, D., Balachandran, R.; Day, B. W. J. Org. Chem. 2010, 75,
2461, (e) Mathew, S. C.; By, Y.; Berthault, A.; Virolleaud, M.-A.; Carrega, L.; Chouraqui,
G.; Commeiras, L.; Condo, J.; Attolini, M.; Gaudel-Siri, A.; Ruf, J.; Rodriguez, J.;
Parrain, J.-L.; Guieu, R. Org. Biomol. Chem. 2010, 8, 3874, (f) Dyrager, C.; Börjesson,
K.; Dinér, P.; Elf, A.; Albinsson, B.; Wilhelmsson, L. M.; Grøtli, M. Eur. J. Org. Chem.
2009, 1515, (g) Dierckx, A.; Dinér, P.; El-Sagheer, A. H.; Dhruval Kumar, J.; Brown, T.;
Grøtli; M.; Wilhelmsson, L. M. Nucleic Acids Res. 2011, 39, 4513.
5
6
.
.
Kovaļovs, A.; Novosjolova, I.; Bizdēna, Ē.; Bižāne, I.; Skardziute, L.; Kazlauskas, K.;
Jursenas, S.; Turks, M. Tetrahedron Lett. 2013, 54, 850.
Compounds 2a,c,d,e have been described earlier (see ref. 5); the syntheses of new
derivatives 2b,f,g is described in the supporting information.
7
8
9
.
.
.
Lioux, T.; Gosselin, G.; Mathé, C. Eur. J. Org. Chem. 2003, 3997.
Mathé, C.; Lioux, T.; Gosselin, G. Nucleosides Nucleotides Nucleic Acids 2006, 22, 605.
For selected applications of 2-alkylthio-6-aminopurine nucleosides, see: (a) El-Tayeb, A.;
Iqbal, J.; Behrenswerth, A.; Romio, M.; Schneider, M.; Zimmermann, H.; Schrader, J.;
Müller, C. E. J. Med. Chem. 2009, 52, 7669, (b) Ingall, A. H.; Dixon, J. Bailey, A.;
Coombs, M. E.; Cox, D.; McInally, J. I.; Hunt, S. F.; Kindon, N. D.; Teobald, B. J.;
Willis, P. A.; Humphries, R. G.; Leff, P.; Clegg, J. A.; Smith, J. A.; Tomlinson, W. J.
Med. Chem. 1999, 42, 213.
1
0. For selected applications of 6-alkylthio-2-aminopurine derivatives, see: (a) Volpini, R.;
Marucci, G.; Buccioni, M.; Dal Ben, D.; Lambertucci, C.; Lammi, C.; Mishra, R. C.;
Thomas, A.; Cristalli, G. ChemMedChem 2011, 6, 1074, (b) McGuigan, C.; Madela, K.;
Aljarah, M.; Gilles, A.; Battina, S. K.; Ramamurty, C. V. S.; Rao, C. S.; Vernachio, J.;
Hutchins, J.; Hall, A.; Kolykhalov, A.; Henson, G.; Chamberlain, S. Bioorg. Med. Chem.
Lett. 2011, 21, 6007, (c) Jensen, L. H.; Liang, H.; Shoemaker, R.; Grauslund, M.;
Sehested, M.; Hasinoff, B. B. Mol. Pharmacol. 2006, 70, 1503, (d) Tori, T.; Onishi, T.;
Izawa, K.; Maruyama, T.; Demizu, Y.; Neyts, J.; De Clercq, E. Nucleosides Nucleotides
Nucleic Acids 2006, 25, 655.
Supplementary Material
Supplementary data associated with this article can be found, in the online version, at
…

Graphical abstract
Application of 2,6-diazidopurine derivatives in the
synthesis of thiopurine nucleosides
Irina Novosjolova, Ērika Bizdēna, Māris Turks*