Design and synthesis of novel tetrahydronaphthyl azoles and related cyclohexyl azoles as antileishmanial agents

Article abstract of DOI:10.1016/j.bmcl.2011.01.026

A novel series of trans-2-aryloxy-1,2,3,4,-tetrahydronaphthyl azoles and related cyclohexyl azoles were synthesized and evaluated in vitro against Leishmania donovani. Compound 9 identified as most active analog with IC ₅₀ value of 0.64 μg/mL a

Full text of DOI:10.1016/j.bmcl.2011.01.026

Bioorganic & Medicinal Chemistry Letters 21 (2011) 1407–1410
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
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Design and synthesis of novel tetrahydronaphthyl azoles and related cyclohexyl
azoles as antileishmanial agents
Vijay K. Marrapu a, Nagarapu Srinivas a, Monika Mittal b, Nishi Shakya b, Suman Gupta b, Kalpana Bhandari a,
⇑
a Medicinal and Process Chemistry Division, Central Drug Research Institute, CSIR, Lucknow 226 001, India
b Division of Parasitology, Central Drug Research Institute, CSIR, Lucknow 226 001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel series of trans-2-aryloxy-1,2,3,4,-tetrahydronaphthyl azoles and related cyclohexyl azoles were
synthesized and evaluated in vitro against Leishmania donovani. Compound 9 identified as most active
analog with IC50 value of 0.64 lg/mL and SI value of 34.78 against amastigotes, and is several folds more
potent than the reference drugs sodium stilbogluconate and paromomycin. It also exhibited significant
in vivo inhibition of 83.33%, and provided a new structural scaffold for antileishmanials.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 2 November 2010
Revised 5 January 2011
Accepted 7 January 2011
Available online 11 January 2011
Keywords:
Antileishmanial
Azoles
Aryloxy tetrahydronaphthalene
There is a lack of effective, safe, and affordable pharmaceuticals
to control neglected tropical diseases (NTDs) such as leishmaniasis
that cause high mortality and morbidity among poor people in the
developing world. The NTDs are a group of chronic and debilitating
conditions, caused by parasitic, bacterial, and other infections.1
Leishmaniasis, a parasitic disease caused by organisms of the
Leishmania genus, comprises three clinical forms: visceral, cutane-
ous, and mucocutaneous.2 The lesions may be confined to skin, in
the case of cutaneous leishmaniasis, or may invade subcutaneous
tissue, as in the potentially fatal visceral form, depending on the
infecting organism and the immunological status of the host.3
Chemotherapeutic treatment of leishmaniasis usually relies on
the use of pentavalent antimonials such as sodium stibogluconate
(Pentostam) and meglumine antimoniate (Glucantime), but toxic
side effects and drug resistance are frequently encountered.4 The
second-line compounds used during the treatment of unresponsive
cases generally include pentamidine and amphotericin B and its
lipid formulations.5 The oral anticancer drug miltefosine6 is an
effective drug, but there are side effects and drawbacks related to
treatment. Antifungal imidazole and triazole derivatives such as
ketoconazole, fluconazole and itraconazole, which block ergosterol
just cholesterol, it accounts for many antifungal sterol biosynthesis
inhibitors (SBIs) to also be leishmanicidal. Ketoconazole and fluco-
nazole have undergone evaluation in VL in India.9 However, de-
spite reports of their usefulness, the antileishmanial activity was
not enough to induce clinical cure by themselves. Thus, the devel-
opment of new, efficient, and safe drugs for the treatment of this
disease is imperative.
Recently, we reported on the synthesis of a series of novel aryl-
oxy alkyl and aryloxy aryl alkyl imidazoles as potential antileish-
manial agents.10 Based on the above report and in continuation
of our efforts to generate azole based novel antileishmanial
agents,11 we synthesized a series of novel aryloxy tetrahydronaph-
thyl azoles (4–13) and related cyclohexyl azoles (22–28) (Fig. 1)
and evaluated them in vitro against the Leishmania parasites. Many
of these derivatives were found to possess strong inhibitory activ-
ity against Leishmania donovani when compared to the standard
drugs. After this large in vitro screening, compounds 9 and 10,
identified as the most active analogues of the series, were selected
for in vivo analysis and the results are reported in this Letter.
The synthesis of compounds 4–13 was carried out according to
Scheme 1. The regioselective ring opening of 1,2-epoxy-tetrahy-
dronaphthalene12 1 with imidazole in absolute ethanol furnished
biosynthesis at the level of cytochrome P-450-dependent C14a-
demethylase,7 are also effective against different Leishmania spe-
cies both in vitro and in vivo. Various N-substituted azole deriva-
tives have also been reported with promising in vitro
antileishmanial activity.8 Leishmania resemble fungi in synthesiz-
ing 24-substituted sterols such as ergosterol, while mammals have
trans-1-imidazol-1-yl-1,2,3,4-tetrahydro-naphthalen-2-ol
(3a).
The 3a can also be prepared directly from the bromohydrin12
2
in low yields (27–28%). The 1,2,3-triazole did not react with epox-
ide 1 under similar conditions. However, the reaction was achieved
in the presence of tetrabutyl ammonium iodide to give the product,
1-[1,2,3]-triazol-1-yl-1,2,3,4-tetrahydro-naphthalen-2-ol (3b) in
excellent yields (ꢀ72%). Further, etherification of the hydroxyl
intermediates 3a and 3b with substituted aryl/benzyl halides
⇑
Corresponding author. Tel.: +91 522 2262411; fax: +91 522 2623405.
E-mail address: bhandarikalpana@rediffmail.com (K. Bhandari).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.01.026

1408
V. K. Marrapu et al. / Bioorg. Med. Chem. Lett. 21 (2011) 1407–1410
Further, for SAR studies the related aryloxy cyclohexyl imida-
R1
R2
R1
R2
zoles (22–28) were also prepared (Scheme 3). Reaction of imidaz-
ole with 1,2-epoxycyclohexane (20) under refluxing conditions
furnished trans-2-imidazolyl cyclohexanol (21). Etherification of
the hydroxyl intermediate with substituted aryl halides furnished
the corresponding ethers 22–28. All the compounds shown in
Schemes 1–3 were obtained as racemic mixtures.
n
n
O
O
N
N
X
N
N
22-28
4-13
The compounds selected for study (4–13 and 22–28) were eval-
uated in vitro against transgenic L. donovani promastigotes and
intracellular amastigotes14 at various concentrations and cytotox-
icity responses15 were assessed using mouse macrophage cell line
(J-774-A-1) and taking sodium stibogluconate, paromomycin and
podophyllotoxin (for cytotoxicity assay) as controls. Cell viability
was determined using the MTT assay.15 CC50 values were esti-
mated through the preformed template as described by Huber
and Koella.16 IC50 of antileishmanial activity was calculated by
nonlinear regression analysis of the concentration–response curve
using the four parameter Hill equations. Any synthesized ana-
Figure 1. Aryloxy tetrahydronaphthyl and cyclohexyl azoles.
iv
iii
i or ii
Br
OH
O
N
OH
2
3a-b
1
X
3a; X = CH
N
3b; X = N
v
logues with in vitro IC50 value exceeding above 15 lg/mL was con-
sidered as inactive. Based on IC50 and SI values two compounds
were further evaluated for in vivo activity intraperitoneally at
50 mg/kg  5, ip dose against L. donovani/Hamster model (Mesoc-
ricetus auretus).17 Sodium stibogluconate and paromomycin were
used as reference drugs.
The in vitro biological activities of aryloxy tetrahydronaphthyl
azoles/aryloxy cycloalkyl azoles (4–13 and 22–28) have shown
encouraging results against L. donovani. Table 1 displays % inhibi-
tion (promastigotes), IC50 and SI values of the synthesized azoles
against intracellular amastigotes. Interestingly, all the compounds
R1
R2
n
O
N
X
N
4-13
Scheme 1. Reagents and conditions: (i) imidazole, abs. ethanol, 90 °C, 6 h; (ii) 1,2,3-
triazole, (But)4N+IÀ, rt, 8 h; (iii) imidazole, acetonitrile, 90 °C, 8 h; (iv) dry THF,
NaOMe, 0 °C, 5 h; (v) NaH (60% oil), substituted aryl or benzyl halide, DMF, 0 °C–rt,
4–5 h.
(except 4) exhibited high inhibition of 89–100% at 10 lg/mL con-
centration against promastigotes. These compounds were further
evaluated against amastigotes and IC50 and SI values were calcu-
lated. Two compounds 9 and 10 of low IC50 and SI above 10 were
tested further for in vivo antileishmanial activity and the results
are presented in Table 1.
furnished the desired ethers 4–13 (Scheme 1). The 1H NMR spectra
of compounds 4–13 showed a doublet at approximately d 5.1–5.8
for H-1 proton with J1–2 = 7–8 Hz, indicative of a trans diaxial
orientation.
The IC50 values for amastigotes of the test derivatives indicate
that out of 17 synthesized compounds, 13 compounds exhibited
It was thought interesting to synthesize and evaluate the above
ethers with cis geometry (Scheme 2). Benzoylation of the trans-1-
amino-2-hydroxytetrahydronaphthalene (14) followed by treat-
ment with thionyl chloride gave the 2-phenyl oxazoline (15) which
was hydrolyzed with 6 N H2SO4 to furnish the required cis amino
alcohol13a 16. Reaction of the cis amino alcohol 16 with ammonium
acetate, glyoxal and formaldehyde in methanol13b under refluxing
condition gave the desired cis-1-imidazol-1-yl-1,2,3,4-tetrahydro-
naphthalen-2-ol (17). However, the etherification of cis-imidazolyl
alcohol 17 with aryl halides did not give the desired cis ethers (18)
but furnished the dehydrated product 19.
high activity against L. donovani (IC50 0.64–6.52
the reference drugs sodium stibogluconate (IC50 = 46.54
paromomycin (24.79 g/mL). The hydroxyl intermediates (3a and
l
g/mL), better than
lg/mL) and
l
3b), showed either low inhibition or no inhibition. Further,
conversion of hydroxyl group to aryloxy/benzyloxy moiety (com-
pounds 4–13 and 22–28, Table 1) resulted in enhancing the activity
several folds (IC50 0.64–6.52 lg/mL).
Among the tetrahydronaphthyl azole series (4–13) the com-
pounds with benzyloxy moiety (9, 10 and 13) appeared more ac-
tive compare to the aryloxyl analogs exerting a strong inhibitory
effect on the amastigote form of parasite, while three compounds
(9, 10 and 13) produced an interesting selective antiamastigote
i, ii
iii
OH
OH
O
16
14
15
NH2
iv
NH2
N
C6H5
R1
ii
i
n
O
OH
R1
R2
O
n
v
N
N
O
R2
N
N
OH
N
N
21
22-28
20
17
N
22; n =0, R1= H, R2 = 4-CF3
18
v
N
23; n =0, R1 =H, R2 = 4-NO2
; n =0, R1 =2-F, R2 = 4-NO2
; n =0, R1 =2-NO2, R2 = 4-CF3
; n =1, R1 =H, R2 = 3-Cl
24
25
26
N
19
N
27; n =1, R1 =2-Cl, R2 = 5-Cl
28; n =1, R1 =2-Cl, R2 = 4-Cl
Scheme 2. Reagents and conditions: (i) PhCOCl, KOH, H2O, rt, 3 h; (ii) SOCl2,CH2Cl2,
rt, 7 h; (iii) 6 NÁH2SO4, reflux, 11 h; (iv) glyoxal, NH4OAc, HCHO, methanol, 80 °C,
8 h; (v) NaH (60% oil), substituted aryl or benzyl halide, DMF, 0 °C–rt, 4–5 h.
Scheme 3. Reagents and conditions: (i) imidazole, abs. ethanol, 90 °C, 12 h; (ii) NaH
(60% oil), substituted aryl or benzyl halide, DMF, 0 °C–rt, 4–5 h.

V. K. Marrapu et al. / Bioorg. Med. Chem. Lett. 21 (2011) 1407–1410
1409
Table 1
In vitro and in vivo antileishmanial activity of synthesized azoles
Sl. No.
Compd No.
In vitro assessment
aCytotoxicity
Selective index
(SI) CC50/IC50
In vivo activity (dose-50 mg/kg
 5, ip) % inhibition SE
CC50
(l
g/mL)
Anti promastigote activity
Anti amastigote activity
(% inhibition at 10
lg/mL)
#IC50
(lg/mL)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
3a
3b
4
5
6
7
60
90.42
60
94.89
91.16
100
99.59
100
>50
NI
NI
4.53
1.59
6.52
3.58
0.64
1.34
NI
>100
2
NA
NA
35.59
52.46
21.43
10.69
32.13
20.08
22.26,
15.46
>100
>100
30.20
ND
4.73
6.73
4.92
5.61
34.78
11.53
NA
NA
10.03
NA
3.70
6.62
4.36
9.10
6.73
1.41
6.38
2.15
8
*9
83.33 6.16
32.10 8.51
*10
100
99.62
100
11
12
13
22
23
24
25
26
NI
3.01
NI
100
99.80
98.56
95.69
99.00
96.9
89.57
99.47
946.52
123.92
2.23
2.15
5.07
1.404
4.03
2.44
46.54
24.79
8.27
14.25
22.13
12.76
27.16
3.43
27
28
SSG
Paromomycin
297.38
53.33
92.20 5.25
46.7 9.82
#
*
IC50 >15
l
g/mL = inactive; IC50 >5–15
l
g/mL = moderately active; IC50 <5
l
g/mL = highly active compounds.
g/mL; ND = not done; NA = not available; SSG = sodium stibogluconate.
CC50 (cytotoxic concentration for 50% inhibition) is evaluated against J-774A-1 cell lines.
Compounds were picked up for in vivo evaluation; NI = no inhibition at 10
l
a
activity (SI >10). The aryloxy triazole derivatives 11 and 12 were
found inactive whereas the benzyloxy triazole derivative 13
moiety. This finding indicates that benzyloxy moiety with chloro
substituents should be investigated for the development of highly
selective antileishmanial compounds.
expressed good antiamastigote activity (IC50 = 3.01
SI = 10.01, was slightly less active than its imidazole counterpart
10 (IC50 = 1.34 g/mL and SI = 11.53).
lg/mL) with
These results clearly indicate that newly synthesized tetrahy-
dronaphthyl azoles reported herein are promising compounds
and provide useful model for further structural and biological
optimization. Compound 9 displayed not only a lower (38–72
times) IC50 value than that of the reference drugs, but also
6- and 16-fold more selective (SI = 34.78) as compared to that
of standard drugs sodium stibogluconate and paromomycin,
respectively. The study opens up the possibility of advancing
this new class of compounds as novel antileishmanial agents.
Further studies on these compounds to optimize the efficacy is in
progress in our laboratory.
l
Further, to investigate the SAR effect of phenyl ring, we
also synthesized and evaluated a set of aryloxy cyclohexyl azoles
(22–28). Though, these compounds displayed a strong inhibitory
activity on the intracellular amastigote IC50 ranging from 1.40 to
5.07 lg/mL, but the selective index of all the compounds was
below 10, revealing the presence of the phenyl rings is crucial for
better activity profile.
The overall activity profile of compounds (4–13, 22–28) demon-
strated that there is a small difference in their IC50 values. Thus, the
biological activity was slightly influenced by the type of substitu-
ent attachment at the 2 and 4-position of the aryloxy nucleus (ex-
cept in compounds 4 and 22 where the CF3 group at 4-position
renders the molecule inactive). However, it is interesting to note
that the introduction of NO2 group at position 2 together with
4-CF3 imparted increased selectivity (8 and 25). Similarly, the
NO2 group at position 4 (5 and 23) renders the molecule moder-
ately active, while the presence of a fluorine atom at 2 position to-
gether with 4-NO2 enhances the activity (7 and 24). Moreover, the
presence of a CF3 group at 2 position together with 4-NO2 further
confers increased selectivity (6). Among the benzyloxy analogues
the 2,5-dichloro derivative 9 and 3-chloro-derivatives 10, 13 and
26 displayed the highest selectivity index (34.78, 11.53, 10.03
and 9.10, respectively).
Acknowledgments
V.K.M. & Nishi are grateful to Council of Scientific and Industrial
Research (India) and N.S. to UGC for the award of Senior Research
Fellowships. The authors are thankful to Anoop K. Srivastava for
providing technical assistance. All are thankful to SAIF for spectro-
scopic analysis of the compounds. CDRI communication No. 8015.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2011.01.026.
Two compounds (9 and 10) of SI above 10 were tested further for
in vivo antileishmanial activity. Compound 9 the 2,5-dichloroben-
zyloxy tetrahydronaphthyl imidazole, exhibited significant in vivo
activity with 83.33% inhibition of parasite growth while compound
10 (2,5-dichlorobenzyloxy tetrahydronaphthyl triazole) displayed
moderate activity with 32.10% inhibition. Thus compound 9 has
shown superior in vivo activity than paromomycin and was less ac-
tive than sodium stibogluconate. It is interesting to note that in both
the series the tetrahydronaphthyl azoles (4–13) and cyclohexyl
azoles (22–28) the highest activity (SI values) were shown by the
compounds with either a 2,5-dichloro or a 3-chloro benzyloxy
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