Microwave-assisted synthesis of a triazole-linked 3′-5′ dithymidine using click chemistry

Article abstract of DOI:10.1016/j.tetlet.2007.12.012

Synthesis of a triazole-linked 3′-5′ thymidine dimer making use of 1,3-dipolar cycloaddition is described. The azido-precursor was obtained by regioselective chlorination of thymidine, followed by azidation. The second precursor, a propargyl derivative, was obtained by selective 3′-O-alkylation of thymidine. Two 'click systems' were compared to obtain the desired dimer. These reactions were performed by microwave irradiation.

Full text of DOI:10.1016/j.tetlet.2007.12.012

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Tetrahedron Letters 49 (2008) 1004–1007
Microwave-assisted synthesis of a triazole-linked 30–50
dithymidine using click chemistry
a
a
a
a,
*
Romain Lucas , Virginie Neto , Amel Hadj Bouazza , Rachida Zerrouki ,
Robert Granet a, Pierre Krausz a, Yves Champavier b
a Laboratoire de Chimie des Substances Naturelles EA1069, Faculte´ des Sciences et Techniques, 123 Avenue Albert Thomas, F-87060 Limoges, France
b Universite´ de Limoges, Service Commun de RMN, Faculte´ de Pharmacie, 2 rue du Dr Marcland, F-87025 Limoges, France
Received 16 October 2007; revised 3 December 2007; accepted 4 December 2007
Available online 8 December 2007
Abstract
Synthesis of a triazole-linked 30–50 thymidine dimer making use of 1,3-dipolar cycloaddition is described. The azido-precursor was
obtained by regioselective chlorination of thymidine, followed by azidation. The second precursor, a propargyl derivative, was obtained
by selective 30-O-alkylation of thymidine. Two ‘click systems’ were compared to obtain the desired dimer. These reactions were
performed by microwave irradiation.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Dithymidine; Click chemistry; Microwave; Selective chlorination
Oligonucleotides have attracted a lot of interest in bio-
chemistry and genomic research.1 Among their numerous
applications, they could be used in antisens or triple helix
therapy to inhibit RNA translation or DNA transcription.2
One of the most radical modifications brought to the fami-
liar backbone is the complete substitution of the phospho-
diester bridge, to achieve stronger affinity for the nucleic
acid target, and/or enhanced resistance to nucleases, or
improved membrane permeability and cellular uptake.
Our research effort in this area has focused on the synthesis
of new 30–50 dinucleotide analogues. The need of a rapid,
efficient and highly chemoselective reaction for the building
of 30–50 dinucleosides led us to choose the ‘click chemistry’
reaction defined by Sharpless et al.3 and Tornoe et al.4
Microwave (MW) activation was used whenever possible
(Schemes 1 and 2) in continuation of our programme
aimed at studying the influence of microwave on organic
synthesis.5
Precursors 2 and 4 were synthesized from thymidine.
Selective chlorination of the primary hydroxyl group of
thymidine was obtained with tosylchloride (1.5 equiv) in
dry DMF giving after microwave activation (1 min,
80 °C, 300 W) 50-chloro-50-deoxythymidine 1 in 80% yield.
According to McCormick,6 the first step (Scheme 3)
appears to be the formation of a O-(p-toluenesulfonyl)-
N,N-dimethyl acetiminium salt (A).
The second step consists in a nucleophilic displacement
of the tosylate group followed by an attack of the chloride
ion on the iminium carbon, displacing the sulfonate group,
with the subsequent displacement of chlorine by thymidine.
Then, the chloroiminium thymidine intermediate can be
attacked by a chloride ion to produce 50-chloro-50-deoxy-
thymidine. Iminium salt (B) was isolated, analysed and
its structure confirmed.
50-Chloro-50-deoxythymidine 1 was then azidated using
15 equiv of sodium azide in DMF. After 3 min of micro-
wave irradiation (80 °C, 300 W), we observed by TLC the
conversion 1?2: the two compounds had similar Rf val-
ues,7 but compound 2 gave intense purple spots after spray-
ing PPh3/Et2O followed by ninhydrin and heating. NMR
*
Corresponding author. Tel.: +33 5 55 45 72 24; fax: +33 5 55 45 72 02.
E-mail address: rachida.zerrouki@unilim.fr (R. Zerrouki).
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.12.012

R. Lucas et al. / Tetrahedron Letters 49 (2008) 1004–1007
1005
Scheme 1. Synthesis of azido- and propargyl-precursors (2 and 4). Reagents and conditions: (i) TsCl, DMF, MW (80 °C, 300 W, 1 min); (ii) NaN3, DMF,
MW (80 °C, 300 W, 3 min); (iii) TBDMSCl, DMAP, Pyr, 10 h; (iv) 1-NaH, THF, MW (40 °C, 200 W, 3 min), 2-propargylbromide, MW (40 °C, 200 W,
3 min).
Scheme 2. Click chemistry and deprotection. Reagents and conditions: (i) (cf Table 1) CuI (1 equiv), DIPEA (3 equiv), DMF; or CuSO4 (0.01 equiv), Na
ascorbate (0.1 equiv) H2O/EtOH; (ii) TBAF (1.2 equiv), THF, rt, 7 h.
analysis of reaction the products indicated that the reaction
had proceeded to virtual completion; after purification,
compound 2 was obtained in 91% yield. Precursor 4
was synthesized first by selective protection of the 50-hydr-
oxyl group with tert-butyldimethylsilyl chloride (1.2 equiv)
in dry pyridine in the presence of DMAP (0.05 equiv)8
giving 50-tert-butyldimethylsilylthymidine 3 in high yield
(98%). The 30-hydroxyl was then alkylated using Chat-
topadhyaya’s method with NaH (2.5 equiv) and propa-
rgylbromide (2.5 equiv) in THF9 and MW activation to
give compound 4 in quantitative yield.
The final step consisted in the Huisgen’s 1,3-di-
polar cycloaddition10 between compounds 4 and 2, using
copper salts as catalysts. Two different Cu salts, CuI and
CuSO4, have been tested and click reactions were realised
with either classical heating or microwave activation.11
Respective yields and reaction times are summarized in
Table 1.

1006
R. Lucas et al. / Tetrahedron Letters 49 (2008) 1004–1007
Scheme 3. Proposed mechanism of selective chlorination.
Table 1
Selected results of click reactions
Click system
Activation
rt
Reaction Isolated
time
yields (%)
CuI (1 equiv), DIPEA (3 equiv),
DMF
24 h
36
rt
72 h
5 h
1 min
36
61
66
D: 80 °C
MW:
80 °C,
200 W
MW:
80 °C,
200 W
3 mina
85
CuSO4 (0.01 equiv), Na ascorbate rt
24 h
40
(0.1 equiv) H2O/EtOH
rt
72 h
5 ha
1 min
44
80
62
D: 80 °C
MW:
80 °C,
200 W
MW:
Scheme 4. Hypothetical structure of secondary product 7.
3 mina
80
80 °C,
200 W
1
intermediate described in the literature.12 In the H NMR
a
spectrum the triazole hydrogen does not show up at
8.09 ppm and, furthermore, the 13C NMR signal of the
corresponding carbon is shifted from 124.7 ppm in
compound 5 to 147.3 ppm in compound 7, which could
be explained by the electro-attractive effect of bound
copper. Mass analysis confirmed the presence of two
bound copper atoms in the dimer structure (m/z = 787.5).
After three months at room temperature, TLC analysis
shows that this secondary product spontaneously evolved
towards compound 5 that displays an increased charring
intensity.
Reaction times allowing virtual reaction completion (checked by
TLC), longer reaction times gave no increase in reaction yields.
Results indicate that microwave activation gave good
yields after only 3 min of reaction. With CuI system,
TLC shows total conversion into the click product. Fur-
thermore we observed the formation of a side product at
room temperature. In this case TLC showed two products
with different Rf values.
1
Mass and H NMR analyses of the isolated secondary
product (7) are consistent with a supposed structure
(Scheme 4), in accordance with the formation of an instable
Click reaction using CuSO4 gave a single compound. We
observed in this case that classical heating (80 °C, 5 h) and

R. Lucas et al. / Tetrahedron Letters 49 (2008) 1004–1007
1007
S.; Cook, P. D. In Nucleosides and Nucleotides as Antitumor and
Antiviral Agents; Chu, C. K., Baker, D. C., Eds.; Plenum Press: New
York, US, 1993; pp 311–324; (e) Sanghvi, Y. S.; Cook, P. D.
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microwave activation (80 °C, 200 W, 3 min) gave identical
results (80% yield). The last step is a deprotection of the
50-alcohol function to give the expected dimer 6 in 70%
yield (Scheme 2).
In conclusion, we have investigated a total synthesis of a
30–50 thymidine dimer using a ‘click chemistry’ reaction
with MW irradiation to reduce reaction time. This work
is presently continuing towards tri- and tetra-nucleoside
synthesis with promising results.
3. (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B.
Angew. Chem., Int. Ed. 2002, 41, 2596–2599; (b) Helms, B.; Mynar, J.
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Acknowledgements
´
Financial support from the ‘Conseil Regional du Lim-
ousin’ is gratefully acknowledged. The authors are grateful
to Dr. Michel Guilloton for useful comments on the
manuscript.
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Supplementary data (spectroscopic data) associated
with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2007.12.012.
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