Bitriazolyl acyclonucleosides synthesized via Huisgen reaction using internal alkynes show antiviral activity against tobacco mosaic virus

Article abstract of DOI:10.1016/j.bmcl.2010.10.141

A family of novel bitriazolyl acyclonucleosides were synthesized using a simple and convenient one-step synthetic procedure via the Huisgen reaction by addition of NaN₃ onto triazole nucleosides bearing internal alkynyl groups introduced at the

Full text of DOI:10.1016/j.bmcl.2010.10.141

Bioorganic & Medicinal Chemistry Letters 21 (2011) 354–357
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Bitriazolyl acyclonucleosides synthesized via Huisgen reaction using
internal alkynes show antiviral activity against tobacco mosaic virus
Menghua Wang a,b, Ruizhi Zhu a, Zhijin Fan c, Yifeng Fu c, Liang Feng d, Jianhua Yao d, Alain Maggiani b,
Yi Xia b, Fanqi Qu a, Ling Peng b,
⇑
a State Key Laboratory of Virology, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China
b Département de Chimie, CNRS UPR 3118 CINaM, 163, Avenue de Luminy, 13288 Marseille Cedex 09, France
c State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, PR China
d Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A family of novel bitriazolyl acyclonucleosides were synthesized using a simple and convenient one-step
synthetic procedure via the Huisgen reaction by addition of NaN3 onto triazole nucleosides bearing inter-
nal alkynyl groups introduced at the 5-position of the triazole ring. Some of the compounds exhibited
interesting antiviral activity against tobacco mosaic virus, demonstrating the importance of the bitriaz-
olyl motif for the observed antiviral activity.
Received 7 October 2010
Revised 29 October 2010
Accepted 31 October 2010
Available online 5 November 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Triazole nucleosides
Bitriazolyl acyclonucleosides
Bitriazolyl compounds
Huisgen reaction
Tobacco mosaic virus
In our ongoing program to develop novel triazole nucleoside
analogs with biologically interesting activity,1–10 we have recently
discovered a novel family of bitriazolyl compounds,5,6,9 in particu-
lar bitriazolyl nucleosides A and B (Scheme 1),5,6 exhibiting potent
antiviral activity against tobacco mosaic virus (TMV)—the most
prevalent pathogen affecting tobacco plants and causing massive
crop reduction. The identified hit compounds bear a bitriazolyl mo-
tif in which the two triazole rings are quasi co-planar via C–N bond
connection, yielding an expanded and enlarged conjugated aro-
matic system.5,6,9 This structural feature may favor the binding of
bitriazolyl nucleoside analogs to their biological targets via stron-
ger interactions offered by the larger aromatic binding surface to-
gether with broad H-bonding capacity.10
The bitriazolyl nucleoside A compounds were conveniently
synthesized in very good yields via Huisgen reaction using 3-
azidotriazole nucleosides 1 and various alkynes (I in Scheme 1).5,6
However, the synthesis of their structural isomers B via Huisgen
reaction using the corresponding 5-azidotriazole nucleosides 2
was problematic (II in Scheme 1): either no bitriazolyl products
were identified as was the case for the ribonucleosides6 or
very low product yields were obtained as was found for the
acyclonucleosides.5 This can be mainly attributed to the unfavor-
able electronic properties and steric hindrance of the correspond-
ing 5-azidotriazole nucleosides 2, which prevent them from
undergoing Huisgen reaction under the Cu-catalyzed experimental
conditions.5,6,11 In addition, the C–N bond linking the 1,2,3-triazol-
yl ring to the 1,2,4-triazole cycle in B seems to be weakened,
making the 1,2,3-triazoyl moiety readily displaceable with nucleo-
philic agents.5,12 This can be also a direct consequence of both the
electronic properties and steric congestion presented in B.
To obtain stable analogs of bitriazolyl nucleosides B, we were
interested in developing bitriazolyl nucleosides C (Scheme 1) in
which the 1,2,3-triazolyl ring is connected to the 1,2,4-triazole ring
via a C–C bond instead of the C–N bond thus conferring better sta-
bility. The synthesis of this new family of bitriazolyl nucleosides C
can be also envisaged via Huisgen reaction by the addition of NaN3
onto the triazole nucleosides 3 bearing internal alkynyl groups
introduced at the 5-position of the triazole ring (III in Scheme 1).
Indeed, this strategy offers a simple and convenient one-step syn-
thetic procedure. Here, we report the synthesis of this new family
of bitriazolyl nucleosides C, specifically bitriazolyl acyclonucleo-
sides 4 (Tables 1 and 2) and their biological activity in terms of
antiviral activity against tobacco mosaic virus.
We first carried out a Huisgen reaction starting with the 5-alky-
nyltriazole acyclonucleoside 3a and NaN3 in the mixed solvent
⇑
Corresponding author. Tel.: +33 491 82 91 54; fax: +33 491 82 93 01.
E-mail address: ling.peng@univmed.fr (L. Peng).
0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2010.10.141

M. Wang et al. / Bioorg. Med. Chem. Lett. 21 (2011) 354–357
355
R'
O
R
O
N
R'
H
N
N
N
N
N
N
N
NH2
N
R''
N
N
N
N
R''
N
N
N
N
N
O
Sugar
Sugar
Sugar
C
A
B
R'
N
N3
N
N
N
N
R''
N
R'
R''
N
N
(I)
N
O
O
Sugar
Sugar
1
A
(70 - 98 %)
O
O
R'
N
N
R''
R''
N3
R'
N
N
N
(II)
N
N
N
R
N
Sugar
Sugar
2
B
(0 - 30 %)
O
O
H
N
N
N
N
NH2
NH2
R
N
NaN3
N
N
N
N
(III)
Sugar
Sugar
3
C
Scheme 1. Bitriazolyl nucleosides A, B, and C, and their synthesis via Huisgen reaction starting with the corresponding triazole nucleosides 1 (I), 2 (II), and 3 (III),
respectively.
system THF/H2O (1/3) at 40 °C. Unfortunately, no corresponding
product was identified and we observed an almost complete recov-
ery of the starting material (Table 1, entry 1). By raising the tem-
perature to 90 °C, while the reaction could deliver the desired
bitriazolyl nucleoside 4a in satisfactory yields (Table 1, entry 2),
the reaction time was relatively long (48 h) compared to conven-
tional Huisgen reaction. Microwave irradiation was able to consid-
erably reduce this reaction time to 1 h but offered no beneficial
effect on the reaction yield (Table 1, entries 2 and 3). This is in line
with our previous findings that no substantial improvement was
observed with microwave irradiation for the synthesis of bitriazol-
yl compounds via Huisgen reaction,5,6 probably due to the pres-
ence of the triazole unit adjacent to the triple bond, as
microwave irradiation has been frequently reported to improve
the Huisgen reaction significantly.11,13 Further examination of
alternative solvents (Table 1, entries 4–9) led us to identify DMF
as the solvent of choice for this reaction: the desired product 4a
could be obtained with satisfactory yield within a short reaction
time (1.5 h) under simple oil bath heating in DMF (Table 1, entry
9); whereas in other solvent systems, the reaction did not go to
completion even after a relatively long reaction time and large
amounts of starting material were recovered (Table 1, entries 4–
8). It should be mentioned that polar solvents such as DMF and
DMSO are often employed in Huisgen reaction when NaN3 is used
as the azido reagent.14,15 This may be ascribed to the fact that polar
solvents can better solubilize NaN3 and, at the same time, may in-
duce the internal alkynes to undergo polarization, thus favoring
the corresponding [3+2] Huisgen cycloaddition. Moreover, the
bitriazolyl compounds are highly polar products thus requiring po-
lar solvents such as DMF to stabilize both the reaction intermedi-
ates and products. Attempts to use either an excess of NaN3
(Table 1, entry 10) or microwave irradiation (Table 1, entry 11)
did not yield significant benefit to the reaction time but rather de-
creased the reaction yields in DMF. Consequently, to synthesize 4,
we carried out the Huisgen reaction under the above optimized
conditions, namely using 1.2 equiv NaN3 in DMF with oil bath
heating at 90 °C (Table 2).
The starting materials for the Huisgen reaction, 5-alkynylyl-
triazole nucleosides 3, were prepared using the protocol previously
developed in our laboratories.4 The corresponding bitriazolyl acy-

356
M. Wang et al. / Bioorg. Med. Chem. Lett. 21 (2011) 354–357
Table 1
Table 2
Optimization of the Huisgen reaction to synthesize bitriazolyl acyclonucleoside 4a
Synthesis of bitriazolyl acyclonucleosides 4 via Huisgen reaction using NaN3 and
using 5-phenylethynyltriazole acyclonucleoside 3aa
various 5-alkynyltriazole acyclonucleosides 3a
O
O
R
N
N
N
N
N
NH2
NH2
HN
N
HO
O
O
R
N
N
NaN3
N
N
N
N
NH2
NH2
HO
HN
DMF, 90 ºC
O
O
N
N
N
N
HO
NaN3
HO
3
4
O
O
Entry
R
Product Yield (%) Reaction time (h)
3a
4a
1
2
3
4
5
6
4a
4b
4c
4d
4e
4f
66
50
59
50
50
63
1.5
5
Entry NaN3
Solvent
Heating
Temperature Time Yield
(h)
(%) 4a
(3a)
CH3
1
2
3
4
5
6
7
8
1.2 equiv THF/H2O
(1/3)
1.2 equiv THF/H2O
(1/3)
1.2 equiv THF/H2O
(1/3)
1.2 equiv THF/H2O
(3/1)
1.2 equiv Dioxane/
H2O (1/3)
1.2 equiv MeOH/
H2O (1/3)
Oil bath
Oil bath
40 °C
90 °C
48
48
1
0 (80)
5
H3CO
50
(10)
48
Microwave 90 °C
4
H3C(H2C)3
H3C(H2C)4
(18)
33
(63)
22(62)
Oil bath
Oil bath
Oil bath
Oil bath
Oil bath
90 °C
90 °C
90 °C
90 °C
Reflux
48
48
48
48
24
4
1
F
22
(44)
26
(45)
0
CF3
1.2 equiv MeOH
7
4g
52
0.5
1.2 equiv CH3CN
(33)b
66c
54c
38c
F3C
F3C
9
10
11
1.2 equiv DMF
5.0 equiv DMF
1.2 equiv DMF
Oil bath
Oil bath
Microwave 90 °C
90 °C
90 °C
1.5
1.5
0.5
8
9
4h
4i
55
53
0.5
0.5
a
Position of the triazole NH is arbitrarily assigned since we cannot reliably
determine the tautomeric structure.
b
Only 33% starting material was recovered. Many side products were observed
on TLC plate and could not be isolated.
No starting material was recovered. Side products were observed on TLC plate
and could not be isolated.
10
11
12
4j
47
50
5
4
6
c
S
H3C(H2C)4–
4k
4l
40b
clonucleosides 4 were obtained mostly in good and satisfactory
yields using the above mentioned conditions (Table 2).16 This sim-
ple one-step and easy-to-perform synthetic procedure is clearly
advantageous over the previously reported two-step synthetic pro-
cedure for B.5 The reaction yields were not significantly affected by
the presence of the electron-donating (Table 2, entry 3) or elec-
tron-withdrawing (Table 2, entries 6–9) groups on the phenyl ring
adjacent to the triple bond, although reaction completion was
achieved more rapidly with the electron-withdrawing groups
(Table 2, entries 6–9). The reaction furnished similar yields when
either heterocyclic arylacetylene (Table 2, entry 10) or alkylacety-
lene (Table 2, entry 11) were employed. Furthermore, no consider-
able steric effect was observed (Table 2, entries 7–9). Finally, the
products obtained showed an increased stability compared to that
previously observed with B, since the 1,2,3-triazolyl ring could not
be readily displaced from the 1,2,4-tiazolyl ring in 4.
The synthesized bitriazolyl acyclonucleosides 4 were tested for
their antiviral activity against TMV using the conventional half-leaf
juice rubbing method17 with ribavirin as the positive control and
water as the negative control. Among the tested compounds, 4a,
4c, 4e, and 4k showed levels of anti-TMV activity either similar
to or better than ribavirin, the standard reference for anti-TMV as-
say (Table 3). In addition, these active compounds shared some
similar structural features to the previously identified active
bitriazolyl nucleosides.5,6,9 Collectively, these findings demonstrate
the importance of bitriazolyl nucleoside derivatives as structural
motifs in the search for candidates with potent antiviral activity
against TMV.
a
Position of the triazole NH is arbitrarily assigned since we cannot reliably
determine the tautomeric structure.
b
Reaction was carried out at 120 °C, and many by-products were observed by
TLC.
Table 3
Antiviral activity of bitriazolyl acyclonucleo-
sides
4 against tobacco mosaic virus using
ribavirin as control reference
Compound
Anti-TMV activity (%)
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
36
28
39
22
44
29
17
16
30
14
34
23
39
3
9
4
3
4
1
1
2
1
5
7
1
8
4k
4l
Ribavirin
We further carried out toxicity prediction on the acute and
mutagenic toxicity for the identified active compounds 4a, 4c, 4e,
and 4k using the programs of CISOC-PSAT18and CISOC-PSMT,19
respectively. Acute toxicity describes the adverse effects of a

M. Wang et al. / Bioorg. Med. Chem. Lett. 21 (2011) 354–357
357
Table 4
Predicted results of the acute toxicity, mutagenic toxicity, and log P values for the active compounds 4a, 4c, 4e, and 4k using the CISOC-PSAT,
CISOC-PSMT, and CISOC-log P programs, respectively
Compound Acute toxicity (CISOC-PSAT)
Mutagenic toxicity (CISOC-PSMT)
log P (CISOC-log P)
Predictability Mutagenicity possibility Mutagenicity impossibility
4a
4c
4e
4k
4.05
4.12
4.17
4.39
96%
96%
96%
96%
0.01
0.01
0.01
0.01
0.21
0.28
0.49
0.95
0.62
0.64
2.86
0.38
substance which result either from a single exposure or from mul-
tiple exposures in a short space of time (usually less than 24 h),20
whereas mutagenic toxicity reveals the extent to which a com-
pound is susceptible to induce abnormal mutation. Both acute tox-
icity and mutagenic toxicity are two important indicators of the
toxic potential of a chemical compound. It is therefore important
to evaluate these two parameters for further lead optimization
and development purposes. In toxicology, acute toxicity could be
classified into five levels: severe toxic (rat, oral, LD50 <1 mg/kg,
1 6 predicted value <2), high toxic (rat, oral, LD50: 1–49 mg/kg,
gratefully acknowledged. We thank Mrs. Emily Witty for revising
the English manuscript.
Supplementary data
Supplementary data (experimental details, 1H NMR and 13C
NMR spectra of all the new compounds described) associated with
this article can be found, in the online version, at doi:10.1016/
j.bmcl.2010.10.141.
2 6 predicted value <3), medium toxic (rat, oral, LD50
: 50–
References and notes
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500–4999 mg/kg, 4 6 predicted value <5), and tiny toxic (rat, oral,
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development.
In conclusion, a new series of bitriazolyl acyclonucleosides C,
the structural isomers of our previously reported bitriazolyl
acyclonucleosides A and B, have been synthesized via a one-step
Huisgen cycloaddition using NaN3 and various internal alkynes of
5-alkynylyltriazole acyclonucleosides. The reaction is straightfor-
ward via an easy-to-perform procedure and gives the correspond-
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active hits,5,6,9 some of the newly synthesized bitriazolyl com-
pounds showed interesting anti-TMV activity and were devoid of
any notable toxicity, confirming the importance of the bitriazolyl
motif in the observed antiviral activity against TMV. We are now
actively working towards defining the detailed structure/activity
relationships of this family of compounds and their related biolog-
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Acknowledgments
Financial support from the National Mega Project on Major
Drug Development (2009ZX09301-014), Ministry of Science and
Technology of China (no. 2006AA02Z339, 2010CB126103), Ministry
of Environment Protection (China) (no. 200709046), National
Natural Science Foundation of China (no. 20372055, 20572081,
20872071, 20672062), the International Collaboration Program
of the National Natural Science Foundation of China (no.
20911120069, 21010102008), Wuhan University and CNRS are
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