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G.-L. Liu et al. / Bioorg. Med. Chem. Lett. xxx (2016) xxx–xxx
molecules with coumarin based ring systems have been synthe-
sized utilizing novel synthetic methodologies. Some coumarins
derivatives conjugated with nitrogen-containing heterocyclic moi-
eties, such as triazole, pyridine and pyrimidines, were synthesized
and proved to possess anti-parasite bioactivity.32,33 Imidazole and
its derivatives are of great importance in medicinal chemistry and
can be used for the synthesis of myriad heterocyclic compounds
with different biological activities such as antiviral, antibacterial,
antifungal, anti-tuberculosis, anticancer.34–40 Moreover, imidazole
based compounds such as mebendazole, levamisole, flubendazole
and thiabendazole are very important anti-parasite medicines for
clinical treatment.41–44 Thus, the design and synthesis of novel
coumarin-imidazole hybrid derivatives is the prospective direction
for the development of novel anti-parasite agents with better effi-
cacy, as well as lower toxicity. In an attempt to develop novel
anthelmintic agents, a series of novel coumarin-imidazole hybrid
derivatives were synthesized. All the analogues were evaluated
for their anti-parasitic activity in vivo against the D. intermedius
and the acute toxicity tests of the compounds were undertaken
for goldfish simultaneously.
coumarin showed no anthelmintic efficacy at the concentration
of 26.0 mg/L, which mortality of the goldfish was firstly occurred.
However, most of the derivatives showed an obvious insecticide
activity against D. intermedius in goldfish. Its indicated that the
alkyl bromide or imidazole derivatives substituent group could
increase the anti-parasite activity of coumarin. As indicated in
the Table 1, compound 23 showed the highest activity against
the parasites, with the EC50 value of 0.85 mg/L, which are slightly
higher than those standard drugs, mebendazole and praziquantel,
with the EC50 values of 1.25 and 2.84 mg/L, respectively. Taking
account of the toxicity of the coumarin derivatives against fish,
compounds 1–4, 7, 11, 15 appears to be the suitable candidates
in the prepared derivatives of coumarin, with EC50 values ranging
from 2.42 to 2.92 mg/L, which were close to the value of prazi-
quantel (EC50 = 2.84 mg/L). It’s worth noting that compound 23
exhibited about two to threefold anti-parasitic activity than prazi-
quantel. However, unfortunately, the boosted activity of this com-
pound was accompanied with sharply increased toxicity. In the
anti-parasitic test of compound 23, the mortalities were observed
before the anthelmintic efficacy reached 100%. In contrast, the tox-
icity of the analogues 1–4, 7, 11 and 15 were lower than the control
drugs, mebendazole and praziquantel.
According to the results obtained in the anthelmintic effects
test, the test compounds exhibited similar tendency of activity
against D. intermedius in goldfish. As shown in Table 1, we can find
that the anthelmintic effects of those compounds were affected by
the length of ‘linker’ (R1substituted group). For example, the com-
pounds 21–24, with the same R2 substituted group but different
in R1 substituted group, exhibited a significant difference in anti-
parasitic activity (Fig. 1). Compound 23, with the ‘linker’ of six car-
bon atoms, shows the highest activity against D. intermedius with
the EC50 value of 0.85 mg/L. Otherwise, the EC50 values were
increased to 3 –4 mg/L, if the length of the linker is two, four or
eight carbon atoms. This rule also can observe in the other four
groups of compounds, such as compound 5–8 and 17–20. The
activities exhibited by the derivatives demonstrated that the
length of the ‘linker’ plays an important role in the anthelmintic
activity of compound, and the biological activity of a compound
As shown in Scheme 1, the general procedures for the prepara-
tion of the halogen substituted coumarin derivatives (1–4) and
coumarin-imidazole hybrid derivatives (5–24) were efficiently
synthesized according to previous report.45 Compounds 1–4,
which are crucial to the synthesis of all coumarin–imidazole
hybrid derivatives, were synthesized from 7-hydroxy coumarin
by reacting with corresponding a, x-dibromoalkanes and triethy-
lamine in anhydrous acetone at reflux condition. Compounds
5–24 were synthesized in 60–90% yield by treatment of 1–4 com-
pounds with imidazole derivatives and anhydrous potassium
carbonate in acetonitrile at room temperature. The structures of
a total 24 synthesized compounds were confirmed by ESI-MS, 1H
and 13C NMR (see the Supplementary data).
In order to measure the contribution of the alkyl bromide or
imidazole derivatives motifs to the anti-parasite activity, all of
the coumarin derivatives were evaluated using in vivo bioassays.
The EC50 values of the compounds against D. intermedius are given
in Table 1. In our previous study, the initial reactant, 7-hydroxy
Br
R
b
O
O
O
O
O
O
O
O
Compound 1
Compound 5, 9,13, 17 and 21
b
b
O
O
R
O
O
Br
2
2
O
O
OH
a
Compound 6, 10, 14, 18 and 22
Compound 2
O
O
O
R
O
O
O
O
Br
Br
4
4
6
Compound 7, 11, 15, 19 and 23
Compound 3
O
O
O
R
b
O
O
6
Compound 8, 12,16, 20 and 24
Compound 4
NO2
N
N
N
N
R =
N
N
N
N
N
N
Scheme 1. Synthetic route of coumarin derivatives. Reagents and conditions: (a) alkyl dibromide, K2CO3, triethylamine, dry acetone, reflux, 20–24 h; (b) imidazole/2-
methylimidazole/4-methylimidazole/4-nitroimidazole/ benzimidazole, K2CO3, CH3CN, rt, 20–24 h.
Liu, Guang-Lu
