Synthesis of substituted aryloxy alkyl and aryloxy aryl alkyl imidazoles as antileishmanial agents

Article abstract of DOI:10.1016/j.bmcl.2009.10.117

A series of aryloxy alkyl/aryl alkyl imidazoles were synthesized and evaluated in vitro as antileishmanials against Leishmania donovani. All the 19 compounds exhibited 94-100% inhibition at 10 μg/mL against promastigotes and 12 compounds exhibited high in

Full text of DOI:10.1016/j.bmcl.2009.10.117

Bioorganic & Medicinal Chemistry Letters 20 (2010) 291–293
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis of substituted aryloxy alkyl and aryloxy aryl alkyl imidazoles
as antileishmanial agents q
a
a
b
b
Kalpana Bhandari a, , Nagarapu Srinivas , Vijay K. Marrapu , Aditya Verma , Saumya Srivastava ,
*
Suman Gupta b
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:
Received 20 July 2009
Revised 24 September 2009
Accepted 27 October 2009
Available online 30 October 2009
A series of aryloxy alkyl/aryl alkyl imidazoles were synthesized and evaluated in vitro as antileishmanials
against Leishmania donovani. All the 19 compounds exhibited 94–100% inhibition at 10
promastigotes and 12 compounds exhibited high inhibition with an IC50 in the range of 0.47–4.85
l
g/mL against
g/
l
mL against amastigotes. Promising compounds were tested further in vivo. Among all, compounds 4
and 23 with 4-CF3 aryloxy moiety exhibited medium in vivo inhibition of 58–60%, thus providing new
structural lead for antileishmanials.
Keywords:
Leishmania
Azoles
Ó 2009 Elsevier Ltd. All rights reserved.
Imidazoles
Aryloxy cyclohexane
Leishmaniasis is a complex disease, with visceral and cutaneous
manifestations, and is caused by over 17 different species of the
protozoan parasite genus Leishmania.1 It affects more than 12 mil-
lion people worldwide.2 The clinical manifestations may range
from single cutaneous lesions to fatal visceral leishmaniasis.3 Con-
ventional chemotherapy relies on injected pentavalent antimonials
that are considerably toxic and prone to induce resistance.4
Amphotericin B5 and the oral anticancer drug miltefosine6 are con-
sidered at the moment to be the best second-line therapeutic solu-
tions. Nevertheless, they do not represent a safe treatment in all
clinical cases and necessitate the research of new antileishmanial
molecules. There are several other potential drugs at various stages
of development. These include the azoles that were initially devel-
oped as antifungal drugs.7 Leishmania resemble fungi in synthesiz-
ing 24-substituted sterols such as ergosterol, whereas mammals
have just cholesterol. Azoles, such as ketoconazole, inhibit 14a-
demethylase, a key enzyme in this sterol biosynthesis pathway.8
Ketoconazole, miconazole, itraconazole and fluconazole have
undergone several trials for CL and VL with equivocal results.3,9
More recently, we have demonstrated the high leishmanicidal
activity of novel aryloxy benzocycloalkyl azoles, aryloxy cyclohex-
ane-based bis (I) and mono-imidazoles (II)10 (Fig. 1) which makes
these compounds promising hits for the development of an effec-
tive therapeutic agent.
Based on the above report and our continuation of studies on
chemotherapy of Leishmania, we have planned to synthesize an
expanded series of aryloxy alkyl/aryl alkyl azoles (IV) and investi-
gated their biological effects against the Leishmania parasites.
Thus, we planned the synthesis of derivatives 4–14, 18–25 which
are related acyclic analogues (Fig. 1). These compounds represent
new structure scaffolds significantly different from those of the
N
N
Cl
Cl
O
N
A
A
N
N
N
N
N
O
O
R1
R1
R
III
I
II
R2
R2
N
Cl
R3
R = Cl; Miconazole
R = H; Econazole
N
R
n
O
R1
R = CH3
R = Ph, n = 0 or 1
R = PhO
IV
R3= H or CH3
q
R2
CDRI communication number 7813.
* Corresponding author. Tel.: +91 522 2612411 18; fax: +91 522 2623405.
E-mail addresses: kalpana_bhandari@cdri.res.in, bhandarikalpana@rediffmail.
com (K. Bhandari).
4-14 ;18-25
Figure 1. Representative antileishmanial and antimicrobial agents.
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.10.117

292
K. Bhandari et al. / Bioorg. Med. Chem. Lett. 20 (2010) 291–293
R3
N
amastigotes) and cytotoxicity responses using mouse macrophage
R3
N
R
N
cell line (J-774-A-1)14 and taking sodium stibogluconate, pentam-
idine and Amphotericin B as controls. The cell viability was deter-
mined using the MTT assay.15 CC50 values were estimated as
described by Huber and Koella.16 IC50 of antileishmanial activity
was calculated by nonlinear regression analysis of the concentra-
tion–response curve using the four-parameter Hill equation. Any
i, ii
R
R
N
iii
OH
O
O
2a-c
1a-c
3a-c
iv
a; R = CH3, R3=H
b; R = Ph, R3=H
c; R = Ph, R3=CH3
a; R = CH3
b; R = Ph
c; propiophenone
R3
N
R
N
synthesized analogues with in vitro IC50 value exceeding 15 lg/
4; R = Ph, R1 = H, R2 = CF3, R3=H
5; R = Ph, R1 = H, R2 = NO2, R3=H
6; R = Ph, R1 = NO2, R2 = H, R3=H
7; R = Ph, R1 = F, R2 = NO2, R3=H
8; R = Ph, R1 = H, R2 = CF3, R3=CH3
9; R = Ph, R1 = NO2, R2 = H, R3=CH3
10; R = CH3, R1 = H, R2 = CF3, R3=H
11; R = CH3, R1 = H, R2 = NO2, R3=H
12; R = CH3, R1 = F, R2 = NO2,R3=H
13; R = CH3, R1 = NO2, R2 = H, R3=H
14; R = CH3, R1 = CF3, R2= NO2, R3=H
mL was considered as inactive. Based on IC50 and SI values four
compounds were further evaluated for in vivo activity intraperito-
neally at 50 mg/kg  10 ip dose against L. donovani/Hamster model
(Mesocricetus auretus).17
The in vitro biological activities of aryloxy alkyl azoles/aryloxy
aryl alkyl azoles (4– 14, 18–21) and diaryloxy alkyl imidazoles
(22–25) have shown encouraging results against L. donovani. Table
1 displays IC50 and SI values of synthesized imidazoles against
intracellular amastigotes. Among all the synthesized compounds,
12 compounds (4– 9, 18–21, 23 and 25) have shown IC50 in the
O
R2
R1
4-14
Scheme 1. Reagents and conditions: (i) pyrrolidine, (HCHO)n, L-proline/DMSO,
6–8 h; (ii) corresponding Mannich salt, imidazole/ethanol:H2O (3:2), 5 h; (iii)
range of 0.47–4.83 lg/mL against amastigote model. Four com-
pounds (4, 6, 18 and 23) showed the maximum SI (selectivity in-
dex) value of 36.68, 16.58, 13.85 and 31.89, respectively, which
is several folds better than the standard drugs pentamidine and
also with sodium stibogluconate.
Among the aryloxy aryl alkyl series (4–7 and 18–21) all the
compounds appeared highly active exerting a strong inhibitory ef-
fect on the amastigote form of parasite with IC50 in the range of
NaBH4/MeOH, 2 h; (iv) K(t-OBu), DMSO, substituted aryl halides, 2–3 h.
O
N
O
R
i
i
N
O
OH
Styrene epoxide
17a-b
16
15
0.47–2.27 lg/mL, while three compounds (4, 6 and 18) produced
a; R = Ph
ii
Phenoxy methyl
oxirane
an interesting selective amastigote activity (SI >13). Further, the
compounds with three carbon spacer between the phenyl and
imidazole rings (4–9) were found more potent than the corre-
sponding two carbon derivatives 18–21, revealing the presence
of three carbon spacer for better activity profile. Surprisingly, the
related aryloxy alkyl imidazoles (10–14), though the tested deriv-
atives displayed a strong inhibitory activity on the extracellular
b; R = PhOCH2
N
18; R = Ph, R1 = H, R2 = CF3
19; R = Ph, R1= H, R2= NO2
20; R = Ph, R1= F R2= NO2
R
O
N
21; R = Ph, R1=NO2, R2 = H
22; R = PhOCH2, R1= H, R2 = NO2
23; R = PhOCH2, R1 = H, R2= CF3
24; R = PhOCH2, R1= F, R2= NO2
25; R = PhOCH2, R1 = NO2, R2 = H
R2
R1
18-25
promastigotes with 100% inhibition at 10 lg/mL, but all the com-
pounds were found inactive against amastigote model. It is appar-
ent from the activity results (Table 1) that the aryl ring is crucial for
the activity as on replacement of phenyl moiety with methyl (10–
14) the antiamastigote activity decreased several folds leading to
inactive compounds. Furthermore, the biological activity was also
noticeably influenced by the presence of a second aryloxy group
(22–25) as there is a significant increase in their IC50 values (from
Scheme 2. Reagents and conditions: (i) imidazole/abs ethanol, reflux, 5 h; (ii)
K(t-OBu), DMSO, substituted aryl halides, 2–3 h.
existing classes of azoles with good antileishmanial activity and
here we described the details of our investigations.
Thesynthesisof thearyloxyalkyl/arylalkylimidazolesissumma-
rized in Scheme 1. Ketone (acetone, acetophenone or propiophe-
none) 1a–1c were reacted with pyrrolidine and paraformaldehyde
under asymmetric Mannich conditions (to avoid mixture of prod-
0.47–2.83 to 4.82–5.62
exhibited an IC50 of 0.56
l
l
g/mL) except in compound 23 which
g/mL.
The overall activity profile of the test compounds demonstrated
that the biological activity was also influenced by the type of sub-
stituent attached at the 2- and 4-position of the aryloxy nucleus.
Compounds (4, 8, 18 and 23) consisting a trifluoromethyl group
at 4-position of aryloxy moiety showed the lowest IC50 (0.47,
ucts) in the presence of L-proline to give the corresponding Mannich
products.11 Subsequent replacement of the pyrrolidine with imidaz-
ole (amine exchange reaction to give 2a and 2b; 2c was synthesized
by our recently reported procedure),12 followed by sodium borohy-
dride reduction gave the hydroxyl intermediates 3a–3c [3c with cis
stereoselectivity(8.5:1.5)]. Condensationof thehydroxylintermedi-
ates 3a, 3b and cis 3c isomer (major product) with substituted aryl
halides furnished the required ethers 4–14 (8 and 9 were obtained
as cis diastereomers).
1.85, 2.05 and 0.56 lg/mL) and the maximum SI values (36.68,
11.75, 13.85 and 31.89) except compound 10 of aryloxy alkyl series
where all the compounds were inactive. It is interesting to note
that while the NO2 group at position-4 (5, 11, 19, 22) renders the
molecule moderately active, the same group at position-2 en-
hances the activity (6, 13, 21, 25). Moreover, the presence of a fluo-
rine atom at 2-position together with 4-NO2 further confers
increased selectivity (7, 12, 20 and 24).
Structure–activity relationship (SAR) shows that the trifluoro-
methyl group at position-4 of aryloxy moiety enhances the antile-
ishmanial activity. Four compounds (4, 6, 18 and 23) of SI above 13
were tested further for in vivo leishmanicidal activity and the re-
sults are presented in Table 1. Compounds 4 and 23 with 4-CF3
moiety exhibited medium in vivo activity with 60% and 58% inhi-
bition of parasite growth, respectively, and compounds 6 and 18
displayed moderate inhibition of 52% and 46%, respectively. This
finding indicates that aryloxy moiety with 4-CF3 substituent
Formation of final compounds aryloxy aryl alkyl imidazoles
(18–21) and diaryloxy alkyl imidazoles (22–25), followed the se-
quence shown in Scheme 2. Regioselective ring opening of styrene
epoxide (15) or phenoxy methyl oxirane (16)13 with imidazole
gave the corresponding alcohols 17a and 17b, SNAr substitution
with an aryl fluoride generated the targeted aryloxy ethers 18–25.
The compounds selected for study were evaluated in vitro
against transgenic Leishmania donovani promastigotes at 10 lg/
mL concentrations.10,14 All the compounds showed 94–100% inhi-
bition against promastigotes (Table 1). The compounds were fur-
ther screened for antiamastigote (against intracellular Luc

K. Bhandari et al. / Bioorg. Med. Chem. Lett. 20 (2010) 291–293
293
Table 1
In vitro and in vivo antileishmanial activity of synthesized imidazoles
Sl. No.
Compd No.
In vitro assessment
Anti-promastigote activity Anti-amastigote activity
Cytotoxicity
Selective index (SI)b
CC50/IC50
In vivo activity
c
CC50
(l
g/mL)
(dose—50 mg/kg  10 ip)
percent inhibition
a
(% inhibition at 10
lg/mL)
IC50 (lg/mL) (C.I)
1
2
3
4
5
6
7
8
4
5
6
7
8
9
10
11
100
98.97
100
96.26
100
95.05
100
0.47
3.82
0.48
2.83
1.85
2.27
8.08
NI
17.24
25.1
17.96
25.1
21.74
22.15
7.5
36.68b
6.57
60
52
16.58b
8.86
11.75
9.75
0.88
100
24.1
NA
9
12
100
NI
15.0
NA
10
11
12
13
14
15
16
17
18
19
20
21
22
13
14
18
19
20
21
22
23
100
100
8.43
9.97
2.05
3.71
2.76
2.25
5.62
0.56
5.13
4.82
>100
7.73
11.86
0.97
98.60
95.47
94.63
97.46
93.82
100
98.18
95.53
946.52 (IC50
28.45
25.21
21.23
25.15
28.14
17.86
29.57
36.23
13.85b
6.79
46
58
7.69
11.17
5.00
31.89b
5.76
24
25
SSG
7.51
6.38
2.58
144.8
)
46.54
12.11
0.05
297.38
31.31
7.24
84.10 (20 mg/kg)
92 (40 mg/kg)
92.22 (8 mg/kg  5, iv)
Pentamidine
Amphotericin B
0.643 (IC50
0.008 (IC50
)
)
a
b
c
IC50 >15
Compounds having IC50 <5
l
g/mL = inactive; IC50 >5–15
lg/mL = moderately active; IC50 <5 lg/mL = highly active compounds.
lg/mL (in vitro antiamastigote activity) and SI >13 were picked up for in vivo evaluation.
CC50 (cytotoxic concentration for 50% inhibition) is evaluated against J-774A-1 cell lines; NA = not available; NI = no inhibition; SSG = sodium stibogluconate.
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azole and diaryloxy alkyl imidazoles as a new structural class of
azoles with antileishmanial activity both in vitro and in vivo. The
potent activity and simple synthesis of these imidazoles suggest
that they are potential candidates for the development of new
antileishmanial drugs. Further work is in progress to improve the
potency of these compounds.
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Acknowledgements
N.S. and V.K.M. thank the University Grants Commission (India)
and Council of Scientific and Industrial Research (India) for the
award of SRF and JRF, respectively. The authors are thankful to
S.A.I.F. Division, CDRI, Lucknow for providing spectroscopic data
and Mr. Anoop K. Srivastava for providing technical assistance.
The work is partially funded by SIP.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2009.10.117.
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