Bitriazolyl acyclonucleosides with antiviral activity against tobacco mosaic virus

Article abstract of DOI:10.1016/j.tetlet.2008.02.139

Bitriazolyl acyclonucleosides were synthesized via the Huisgen reaction and then subjected to ammonolysis. The antiviral activity of these nucleosides against tobacco mosaic virus (TMV) was assessed. Like the previously described bitriazolyl compounds, th

Full text of DOI:10.1016/j.tetlet.2008.02.139

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2804–2809
Bitriazolyl acyclonucleosides with antiviral activity
against tobacco mosaic virus
Wei Li a, Yi Xia a,b, Zhijin Fan c, Fanqi Qu a, Qiongyou Wu a,d, Ling Peng a,b,
*
a College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China
b De´partement de Chimie, CNRS UPR 3118, 163, Avenue de Luminy, 13288 Marseille, France
c State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, PR China
d Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry,
Central China Normal University, Wuhan 430079, PR China
Received 25 November 2007; revised 19 February 2008; accepted 22 February 2008
Available online 29 February 2008
Abstract
Bitriazolyl acyclonucleosides were synthesized via the Huisgen reaction and then subjected to ammonolysis. The antiviral activity of
these nucleosides against tobacco mosaic virus (TMV) was assessed. Like the previously described bitriazolyl compounds, these new
bitriazolyl acyclonucleosides were found to show anti-TMV activity. This suggests that the bitriazolyl moieties are important structural
features involved in the antiviral activity of these compounds.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Triazole nucleosides; Acyclonucleosides; Bitriazoly compounds; Tobacco mosaic virus; Anti-TMV
Triazole heterocycles are important structural motifs
with applications in the fields of medicinal chemistry and
agrochemistry. Ribavirin (Scheme 1), a triazole nucleoside,
which was the first synthetic nucleoside found to show anti-
viral activity against many viruses,1 is the only small-
molecular-weight drug available so far for treating viral
infections caused by hepatitis C virus (HCV).2 Fluconazole
(Scheme 1), the structure of which comprises two 1,2,4-tri-
azole units, is a powerful antifungal agent.3 In our search
for novel triazole compounds with potential medicinal
and agrochemical applications,4–8 we discovered that
bitriazolyl compounds (A in Scheme 1) as well as bitriazol-
yl ribonucleosides (B in Scheme 1) have promising antiviral
effects on the tobacco mosaic virus (TMV), an agricultural
plant virus.7,8 It is worth noting that the two triazole rings
in the bitriazolyl motif show quasi co-planarity,7,8 which
results in an expanded aromatic system. This feature may
promote the binding of bitriazolyl compounds to their bio-
logical targets by providing a larger aromatic binding sur-
face, broader scope for H-bonding and greater polarity due
to the triazole structural motifs.
Our ongoing search for further novel triazole com-
pounds included a project for developing bitriazolyl acyclo-
nucleosides (C in Scheme 1). Acyclic nucleosides belong to
an important class of biologically active nucleosides. Acycl-
ovir9 (Scheme 1), which was the first acyclic nucleoside to
be successfully developed as an antiviral drug, has become
the gold standard for the treatment of herpes simplex viral
infections.2 HEPT (Scheme 1), a base-modified acyclic
nucleoside derivative, efficiently inhibits HIV replication
by acting as a non-nucleoside reverse transcriptase inhibi-
tor while being much less toxic than the clinical drug
AZT.10 The bitriazolyl acyclonucleosides C can be classi-
fied as base-modified acyclonucleoside derivatives. Here
we report on the synthesis and characterization of this
new series of bitriazolyl acyclonucleoside derivatives and
their antiviral activity against TMV.
*
Bitriazolyl acyclonucleosides were synthesized using
the strategy previously developed for bitriazolyl
Corresponding author. Tel.: +33 491 82 91 54; fax: +33 491 82 93 01.
E-mail address: ling.peng@univmed.fr (L. Peng).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.02.139

W. Li et al. / Tetrahedron Letters 49 (2008) 2804–2809
2805
O
N
N
N
NH2
O
O
N
Me
S
N
N
N
N
N
NH
NH2
HN
N
N
F
HO
O
N
N
O
HO
HO
OH
Fluconazole
O
O
F
OH OH
Ribavirin
Acyclovir
HEPT
R1
N
N
O
N
R1
N
N
OCH3
N
O
N
N
O
N
AcO
N
R1
O
R2
N
N
N
R3O
N
N
R2
N
O
N
NH
OAc OAc
Bitriazolyl unit
A
Bitriazolyl ribonucleoside
B
Bitriazolyl acyclonucleoside
C
Scheme 1. Ribavirin, fluconazole, acyclovir, HEPT, bitriazolyl units (A), bitriazolyl ribonucleoside (B), and bitriazolyl acyclonucleoside (C).
ribonucleosides,8 namely, via the Huisgen reaction11
between azidotriazole acyclonucleosides and various
terminal alkynes, and then subjected to ammonolysis in
NH3/MeOH (Scheme 2).
Bitriazolyl acyclonucleosides 2 were formed in good to
excellent yields15 (Scheme 4, Table 1) via the copper(I)-cat-
alyzed Huisgen reaction under similar conditions to those
previously described.6–8 In the case of products 2, which
were obtained in yields of less than 80%, the Cu(I) catalyst
loading was increased from 5% to 30%. In most cases, this
improved the product yields to 90% or more.
Further treatment of 2 in NH3/MeOH at room temper-
ature resulted in debenzoylation of the acyclic sugar moiety
and amination of the carboxylester group on the triazole
ring (Scheme 4 and Table 1), yielding products 3 in the
form of white solids,16 which could be directly precipitated
out of the reaction solution. Compounds 3 were therefore
purified by simply filtering and washing the precipitate.
Although the ammonolysis reaction in NH3/MeOH
The starting materials for the Huisgen reaction, 3-azido-
triazole acyclonucleoside (1) and 5-azidotriazole acyclo-
nucleoside (10), were prepared in yields of 42% and 29%,
respectively, by alkylating the azidotriazole6 with
2-(chloromethoxy)ethyl benzoate12 in the presence of
NaH (Scheme 3).13 The reason why a more favorable yield
was produced for 1 was mainly that much less steric
congestion occurred between the azido group and the sugar
moiety in 1. Similar results were also observed when
synthesizing the azidotriazole ribonucleosides, the photo-
labeling probes of ribavirin.14
R'
R
O
OCH3
O
O
N
N
N
N
N
N3
N
N
N
N
NH3/MeOH
R
C CH
NH2
OCH3
N
N
N N
N
N
BzO
HO
BzO
O
O
O
3
2
1
3'
2'
1'
Scheme 2. Synthesis of bitriazolyl acyclonucleosides.
O
N
O
OCH3
N3
N3
N
N
N
N
N
H
OCH3
N3
N
OCH3
NaH, CH3CN
RT, 72 h
N
N
+
+
BzO
BzO
O
O
O
O
Cl
BzO
1
1'
(29%)
(42%)
Scheme 3. Synthesis of azidotriazole acyclonucleosides 1 and 10.

2806
W. Li et al. / Tetrahedron Letters 49 (2008) 2804–2809
R
R'
N
N
N
N3
N
O
N
N
N
N
N
R
NH3/CH3OH
RT 48-72 h
N
OCH3
N
OCH3
N
NH2
N
N
N
BzO
.
BzO
HO
O
CuSO4 5H2O
sodium ascorbate
O
O
O
O
THF/H2O 1/3, 40 °C
1
2
3
Scheme 4. Bitriazolyl compounds 2 and 3 synthesized from 1.
Table 1
Bitriazolyl compounds 2 and 3 synthesized from 1
Entry
1
R
Product
Yield (%)
93a
R0
Product
Yield (%)
76
2a
3a
2
3
4
5
6
7
8
9
2b
2c
2d
2e
2f
91b
92b
79a
91b
90b
84a
81a
86a
82a
3b
3c
3d
3e
3f
66
87
86
74
74
65
63
89
94
CH3
CH3
H3CO
H3CO
F
F
H3C(H2C)4
Cl
H3C(H2C)4
Cl
2g
2h
2i
3g
3h
3i
OH
OH
O
HOH2C
CH3COCH2
O
O
10
2j
3j
CH3OC
NH2C
a
5 mol % Copper(I) as catalyst at 40 °C.
30 mol % Copper(I) as catalyst at 40 °C.
b
worked very well, as indicated by TLC analysis, only mode-
rate yields of 3 were observed in some cases, presumably
due to the high solubility of these compounds in CH2Cl2
and MeOH, which resulted in considerable loss of product
during the precipitation and filtration work-up procedures.
Synthesis of bitriazolyl compounds 20 and 30 proved to
be problematic. Huisgen reaction with 10 gave low to
moderate yields of the desired products 20,17 along with
considerable amounts of 5-aminotriazole acyclonucleoside
4, the reduction product of 10 (Scheme 5 and Table 2).
O
O
O
R
N
N
N
N
N
OCH3
OCH3
OCH3
R
N
N
H2N
N3
N
N
N
N
N
+
.
CuSO4 5H2O
sodium ascorbate
BzO
BzO
BzO
O
O
O
THF/H2O 1/3, 40 °C
2 '
4
1'
Scheme 5. Synthesis of 20 via Huisgen reaction starting with 10.

W. Li et al. / Tetrahedron Letters 49 (2008) 2804–2809
2807
Table 2
Synthesis of 20 via Huisgen reaction starting with 10
Table 3
Synthesis of 30 via ammonolysis of 20
Entry
1
R
20 (%)
4 (%)
Entry
R
Product
Yield
(%)
Product
Yield
(%)
2a0 (44.6)
31.9
1
2
3a0
3b0
38.5
45.6
5a
5b
61.4
54.2
2
3
4
5
2b0 (48.0)
2c0 (41.8)
2d0 (41.6)
2e0 (36.1)
28.8
30.6
45.4
30.0
CH3
CH3
H3CO
F
3
4
5
3c0
3d0
3e0
26.0
46.2
51.7
5c
5d
5e
71.3
42.3
47.1
H3CO
F
H3C(H2C)4
H3C(H2C)4
In some cases, 20 could not be isolated in pure form due to
the formation of numerous side products (data not shown).
As we know from our previous study, since the azido group
in the 5-azidotriazole nucleoside is more electron-deficient
than its counterpart in the 3-azidotriazole nucleoside
isomer, it undergoes copper-catalyzed reduction more
readily than the Huisgen reaction in the presence of sodium
ascorbate.8 Contrary to what occurred with the 5-azido-
triazole ribonucleoside, which yielded no Huisgen adduct
in our previous study,8 the present Huisgen reaction with
5-azidotriazole acyclonucleoside 10 gave low to moderate
yields of products 20, which suggests that the steric
hindrance exerted by the sugar moiety may also be one
of the factors inhibiting the Huisgen reaction with azido-
triazole nucleoside isomers. All in all, the electron-deficient
nature of the azido group and the steric hindrance to which
it is subjected both make 5-azidotriazole acyclonucleoside
10 rather unsuitable for the Huisgen reaction.
The subsequent ammonolysis of 20 was not a straightfor-
ward procedure either. Only low to moderate yields of the
expected products 30 were obtained:18 considerable
amounts of 20 were decomposed and by-product 5 was
formed (Scheme 6 and Table 3).19 The reason for this out-
come was probably that the 1,2,3-triazolyl group is a good
leaving group, which is ready to be displaced when con-
nected to an electron-deficient aromatic system. It has been
reported by Robins et al. that the 1,2,4-triazolyl group is a
good leaving group for SNAr reactions and metal-catalyzed
cross-coupling reactions.20 The present finding suggests
that 1,2,3-triazolyl might retain similar reactivity as 1,2,4-
triazolyl in chemical reactions when acting as a leaving
group. Since the 1,2,3-triazolyl group can be easily intro-
duced via the Huisgen reaction, this finding may open
new avenues for the chemical synthesis of nucleosides
and heterocycles.
The bitriazolyl acyclonucleosides 2, 20, 3, and 30 synthe-
sized were tested to assess their antiviral activity against
TMV, using the previously described7 conventional half-
leaf juice rubbing method8,21 on fresh tobacco plant leaves.
The results obtained are listed in Table 4, in comparison
with ribavirin (the control substance). Compounds 2b, 2f,
3e, 3i, and 3j showed similar levels of antiviral activity to
ribavirin (Table 4). These active compounds have chemi-
cally and structurally diverse substituents acting on the
bitriazolyl unit, which suggests that anti-TMV activities
in this family of compounds may be compatible with some
structural diversity in this position. It is also worth noting
that the active acyclonucleoside 2b shows exactly the same
bitriazolyl structural motif as the corresponding active
ribonucleoside.8 These results suggest on the whole that
the bitriazolyl scaffold may be a potentially useful struc-
tural motif for designing candidates with antiviral activity
against TMV, regardless of whether the sugar moiety is
acyclic or cyclic ribosyl.
In conclusion, a new series of bitriazolyl acyclonucleo-
sides was synthesized via the Cu(I)-catalyzed Huisgen reac-
tion using azidotriazole acyclonucleosides and various
terminal alkynes. Huisgen reactions with 5-azidotriazole
acyclonucleoside proved to be much less straightforward
than with 3-azidotriazole acyclonucleoside, giving lower
product yields and considerable levels of by-product
formation. This is in line with our previous findings on
O
O
R
R
N
N
NH2
OCH3
N
N
N
N
N
R
N
N
N
NH3/CH3OH
RT 48-72 h
N
N
+
NH
HO
BzO
N
N
O
O
5
3'
Scheme 6. Synthesis of 30 via ammonolysis of 20.
2'

2808
W. Li et al. / Tetrahedron Letters 49 (2008) 2804–2809
Table 4
´ ´
Gilles Quelever and Dr. Jessica Blanc for revising the
Antiviral activity of bitriazolyl acyclonucleosides against TMV
English manuscript.
Compound Anti-TMVa activity
(%)
Compound Anti-TMVa activity
(%)
Supplementary data
2a
2b
33
49
16
4
1
4
3a
3b
3c
3d
3e
3f
34
0
0
21
42
29
0
6
1
Analytical data, H NMR and 13C NMR spectra of all
2c
3d
the new compounds described. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2008.02.139.
22 11
2
2
4
2e
36
47
10
21
6
15
0
0
7
3
1
4
5
5
2f
2g
3g
3h
3i
2h
2i
27 13
References and notes
46
42
14
0
21
34
17
4
8
7
2j
3j
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the synthesis of bitriazolyl ribonucleosides:8 the electron-
deficient nature of the azido component in the triazole ring
of 5-azidotriazole acyclonucleoside and steric congestion
make azide reduction easier to perform than Huisgen
cycloaddition. Like the previously synthesized bitriazolyl
compounds A and B, some of the newly synthesized bitri-
azolyl acyclonucleosides C showed anti-TMV activity. This
finding further confirms that the bitriazolyl motif is
involved in anti-TMV activity. We are currently studying
structure/activity relationships in this family of compounds
and screening bitriazolyl leads against other viruses.
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13. Preparation of 1 and 10: In a solution of methyl 5-azide-1,2,4-triazole-
3-carboxylate (1.25 g, 7.4 mmol) in 40 mL anhydrous acetonitrile,
0.28 g of sodium hydride (70% in mineral oil, 8.2 mmol) was added
portionwise under strong stirring. When sodium hydride was
completely dissolved, 1.58 mL (8.9 mmol) of 2-(chloromethoxy)ethyl
benzoate was added dropwise, and the mixture was stirred at room
temperature for 72 h. The reaction mixture was filtrated and the
filtrate was concentrated under reduced pressure. The residue was
purified on silica gel with petroleum ether/ethyl acetate (2:1, v/v),
affording 1 as a white solid (1.07 g, 41.8%) and 10 as a waxy solid
(0.75 g, 29.2%).
Experimental: 1H NMR spectra were recorded at
300 MHz and 13C NMR spectra at 75 MHz or 150 MHz,
on Varian Mercury-VX300 and Varian Inova-600 spec-
trometers. The chemical shifts were recorded in parts per
million (ppm) with TMS as internal reference. FAB and
ESI MS were determined using ZAB-HF-3F or Finnigan
LCQ Advantage mass spectrometer. High resolution mass
spectra were obtained by Matrix-assisted laser desorption/
ionization mass spectrometry (MALDI-MS) using an Ion-
Spec 4.7 Tesla fourier transform mass spectrometer. Flash
chromatography was performed using silica gel (200–300
mesh) from Qingdao Ocean Chemicals in China.
Acknowledgments
Financial support from the Ministry of Science and
Technology of China (Nos. 2003CB114400, 2003CB51-
4102, 2003AA2Z3506), National Natural Science Founda-
tion of China (Nos. 20372055, 20572081, 20672062), Tian-
jin Natural Science Foundation (No. 07JCYBJC01200),
International Collaboration Program of Tianjin on Science
and Technology (No. 07ZCGHHZ01400), Wuhan Univer-
sity and CNRS is gratefully acknowledged. We thank Dr.
14. Wu, Q. Y.; Qu, F. Q.; Wan, J. Q.; Zhu, X.; Xia, Y.; Peng, L. Helv.
Chim. Acta 2004, 87, 811–819.
15. General procedure for preparing 2 via copper(I)-catalyzed Huisgen
reaction: Azide (1) (ca. 0.10 mmol) and the corresponding alkyne
(1.2 equiv, 0.12 mmol) were dissolved in 8 mL THF/H2O (1:3, v/v). A
freshly prepared aqueous solution of sodium ascorbate (0.5 equiv,
0.05 mmol, in 0.1 mL water) was added, followed by a freshly
prepared aqueous solution of CuSO4ꢀ5H2O (0.05 equiv or 0.3 equiv in
0.1 mL water). The reaction mixture was stirred at 40 °C until

W. Li et al. / Tetrahedron Letters 49 (2008) 2804–2809
2809
complete consumption of 1. The reaction mixture was extracted with
ethyl acetate (3 ꢁ 10 mL). The organic layers were combined, dried
over anhydrous MgSO4, and concentrated. The obtained residue was
purified on silica gel with petroleum ether/ethyl acetate (2:1, v/v),
giving the corresponding product 2 as a white powder.
acetate (1:1, v/v), leading to the corresponding triazole by-product 5.
30 were obtained then by further elution of the column with CH2Cl2/
CH3OH (30:1, v/v).
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Kirkpatrick, R. B.; Ho, T. F.; Fisher, P. W.; Mattern, M. R.;
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16. General procedure for preparing 3: Compound 2 (ca. 100 mg) was
suspended in approximately 15 mL of NH3/CH3OH solution and
stirred at room temperature for 2–3 days. The solvent was removed
under reduced pressure and the residue was diluted with CH2Cl2. The
white precipitate of 3 was obtained by filtration, washed with fresh
CH2Cl2, and dried in vacuo.
17. General procedure for preparing 20: Similar as that for preparing 2. 4
was obtained by further eluting the column, after elution of 20, with
CH2Cl2/CH3OH (30/:1, v/v).
18. General procedure for preparing 30: Similar as that for preparing 3. The
reaction residue was purified on silica gel with petroleum ether/ethyl
21. An, T. Y.; Huang, R. Q.; Yang, Z. D.; Zhang, K.; Li, G. R.; Yao, Y.
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