Synthesis of N-(1-methyl-1H-indol-3-yl)methyleneamines and 3,3-diaryl-4-(1-methyl-1H-indol-3-yl)azetidin-2-ones as potential antileishmanial agents

Article abstract of DOI:10.1016/j.bmcl.2012.06.081

A series of N-(1-methyl-1H-indol-3-yl)methyleneamines and eight new 3,3-diaryl-4-(1-methyl-1H-indol-3-yl)azetidin-2-ones have been synthesized and screened for their antileishmanial activity against Leishmania major. 3,3-Diaryl-4-(1-methyl-1H-indol-3-yl)azetidin-2-ones have been synthesized by the Staudinger's ketene-imine cycloaddition employing two 2-diazo-1,2- diarylethanones as the precursors of diarylketenes. A marked improvement in anti-parasitic activity is observed by transformation of the methyleneamines to azetidin-2-ones in seven out of eight compounds. Two compounds displayed antileishmanial activity comparable to that of the clinically used antileshmanial drug, amphotericine B.

Full text of DOI:10.1016/j.bmcl.2012.06.081

Bioorganic & Medicinal Chemistry Letters 22 (2012) 5704–5706
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis of N-(1-methyl-1H-indol-3-yl)methyleneamines
and 3,3-diaryl-4-(1-methyl-1H-indol-3-yl)azetidin-2-ones
as potential antileishmanial agents
Girija S. Singh a, , Yasser M. S. A. Al-kahraman b, Disah Mpadi a, Masoom Yasinzai c
⇑
a Chemistry Department, University of Botswana, Private Bag: 0022, Gaborone, Botswana
b Institute of Biochemistry, University of Balochistan, Quetta, Pakistan
c Office of the Vice Chancellor, Quaid-I-Azam University, 45320 Islamabad, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of N-(1-methyl-1H-indol-3-yl)methyleneamines and eight new 3,3-diaryl-4-(1-methyl-1H-
indol-3-yl)azetidin-2-ones have been synthesized and screened for their antileishmanial activity against
Leishmania major. 3,3-Diaryl-4-(1-methyl-1H-indol-3-yl)azetidin-2-ones have been synthesized by the
Staudinger’s ketene-imine cycloaddition employing two 2-diazo-1,2-diarylethanones as the precursors
of diarylketenes. A marked improvement in anti-parasitic activity is observed by transformation of the
methyleneamines to azetidin-2-ones in seven out of eight compounds. Two compounds displayed antile-
ishmanial activity comparable to that of the clinically used antileshmanial drug, amphotericine B.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 1 June 2012
Revised 21 June 2012
Accepted 25 June 2012
Available online 3 July 2012
Keywords:
Azomethines
Azetidin-2-ones
1-Methyl-1H-indole
Antileishmanial
Leishmaniasis is a tropical disease caused by protozoan parasites
of Leishmania family. It is classified as cutaneous, visceral (kala
azar),1 mucosal and mucocutaneous depending on the parasite
species and cellular immune system of the patient.2 It has been rec-
ognized by the World Health Organization (WHO) as an increasing
health problem.3 Many parts of Asia and Africa are vulnerable to
leishmaniasis.4 Most of the medicines available currently in the
market for treatment of leishmaniasis, either the first-line pentava-
lent antimonials or second-line drugs such as amphotericin B, nys-
tatin, etc., are costly, have serious side-effects, and are getting
resistant to pathogens after treatment for several weeks,5 and hence
there is need for new antileshmanial agents with improved efficacy
and less side-effects for both visceral and cutaneous leishmaniasis.6
Recent years have witnessed considerable interest in design and
development of antileishmanial compounds containing nitrogen
atom in different structural environments (Fig. 1) because such
compounds have shown promising antileishmanial activity.5,6 We
recently launched a program to synthesize diverse types of nitrogen
and sulfur containing compounds and to evaluate them as antileish-
manial agents. We observed significant antileishmanial activity in
some N-substituted imines from benzaldehydes, cinnamaldehyde,
furan-2-carboxaldehyde; thiophene-2-carboxaldehyde, salicylal-
dehyde and indole-1H-2,3-dione.7 In continuation of these studies,
we wish to report the synthesis and antileishmanial activity of
natural product-inspired imines, N-(1-methyl-1H-indol-3-yl)meth-
yleneamines and new 3,3-diaryl-4-(1-methyl-1H-indol-3-yl)azeti-
din-2-ones. Azetidin-2-ones, commonly known as b-lactams, are
well-known heterocyclic compounds and have shown diverse types
of biological activity such as antibacterial, antifungal, anticancer,
cholesterol absorption inhibition, and hypoglycemic, etc.8 Although
some five- and six-membered heterocyclic compounds containing
nitrogen atom (Fig. 1) have shown promising antileishmanial activ-
ity there is no report in our knowledge on monocyclic b-lactams
exhibiting antileishmanial activity.5,6
N
MeO
N
S
N
O
N
O2N
N
NH(CH2)6NEt2
COR
R = 2-ClPh, 5-Chlorothiophen-2-yl
N
N
NH2
N
N
N
N
O
N
F
NO2
Figure 1. Five- and six-membered nitrogen heterocycles with potential antileish-
manial activity.
⇑
Corresponding author. Fax: +267 3552836.
E-mail address: singhgs@mopipi.ub.bw (G.S. Singh).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.06.081

G. S. Singh et al. / Bioorg. Med. Chem. Lett. 22 (2012) 5704–5706
5705
R
stituted phenyl ring but introduction of strong electron-donating
groups such as methoxy and ethoxy groups diminished the activity
drastically of imines 3f and 3c. Although these results were prom-
ising but no significant improvement was observed by replacing
thiophene-2-carboxaldehyde or indole-1H-2,3-dione used in previ-
ous studies.7 With 1-methyl-1H-indole-3-carboxaldehyde 1. With
eight imines 3a–h in hand, we decided to synthesize azetidin-2-
ones from a range of these imines showing significant to moderate
activity and evaluate them for antileishmanial activity against the
same strains used for screening of imines 3a–h. We are pleased to
disclose that seven out of eight azetidin-2-ones screened showed
notable improvement in the antileishmanial activity (Table 1).
Two azetidin-2-ones 5ca and 5ab showed similar activity as the
standard drug amphotericin B (0.56 0.06 lg/mL). The compound
5db with 4-chlorophenyl group on ring nitrogen and 4-methyl-
phenyl groups on C-3 position of the azetidin-2-one ring also
showed almost parallel activity (0.57 0.01 lg/mL).
In summary, this study reports the synthesis of some indole-
based imines and new indole-based azetidin-2-ones, many of them
with significant antileishmanial activity. The study reveals notable
improvement in antileishmanial activity by transformation of imi-
nes to azetidin-2-ones. Further studies on these compounds are in
progress and will be communicated in due course as a full-length
paper.
N
CH
CHO
EtOH
+
R
NH2
Δ, 1-6 h
N
N
Me
Me
1
2a-h
3a-h
Scheme 1. Synthesis of N-(1-methyl-1H-indol-3-yl)methyleneamines 3a–h.
The N-(1-methyl-1H-indol-3-yl)methyleneamines 3a–h were
prepared by refluxing 1-methyl-1H-indole-3-carboxaldehyde 1
with appropriate aromatic and alkyl amines 2a–h in ethanol for
1–6 h (Scheme 1).9 The 3,3-diaryl-4-(1-methyl-1H-indol-3-yl)
azetidin-2-ones were synthesized by the Staudinger’s ketene-
imine cycloaddition10 using 2-diazo-1,2-diarylethanones as pre-
cursors of diarylketenes.11 Four model imines 3a–d were reacted
with 2-diazo-1,2-diphenylethanone 4a, and with 2-diazo-1,2-
bis(4-methylphenyl)ethanone 4b by refluxing in dry benzene for
6–8 h affording azetidin-2-ones 5aa–5da and 5ab–5db (Scheme 2),
respectively, that have been characterized on the basis of satisfac-
tory analytical and spectral data.12
All the imines 3a–h and azetidin-2-ones 5aa–db were evalu-
ated for their antileishmanial activity by a reported method13
using Leishmania major promastigotes14 as parasite for in vitro
screening (Table 1).15 Imines 3d and 3g containing 4-chloro and
2-methyl groups, respectively, showed significant activity against
L. major. The presence of methyl group in imine 3b resulted into
an enhanced activity in comparison to imine 3a containing unsub-
Acknowledgments
The authors are grateful to the Chemistry Department, Univer-
sity of Botswana, and Institute of Biochemistry, University of
Balochistan, for providing facilities.
R1
R1
R1
O
N2
3a-d
C
C O
dry PhH, Δ
N
R
R1
6-8 h
O
R1
R1
N
Me
R1 = 4a. H, 4b. Me
4a,b
5aa-5db
Scheme 2. Synthesis of 3,3-diaryl-4-(1-methyl-1H-indol-3-yl)azetidin-2-ones 5aa–5db.
Table 1
Physical data and antileishmanial activity of N-(1-methyl-1H-indol-3-yl)methylene-amines 3a–h and 3,3-diaryl-4-(1-methyl-1H-indol-3-yl)-azetidin-2-ones 5aa–5db
No.
R/R1
Yields (%)
Mp (°C)
Mol. formulaa
Antileishmanial activity on L. majorb IC50
(lg/mL s.d.)
3a
3b
3c
3d
3e
3f
3g
3h
5aa
5ba
5ca
5da
5ab
5bb
5cb
5db
Ph
91
84
86
74
90
82
75
85
82
68
71
62
67
72
76
70
131–13316
141–143
109–112
139–141
174–176
112–114
104–106
C16H14N2
0.76 0.02
0.66 0.03
0.88 0.06
0.58 0.02
0.60 0.01
0.92 0.03
0.57 0.01
0.71 0.03
0.60 0.02
0.59 0.04
0.56 0.06
0.76 0.02
0.56 0.06
0.60 0.05
0.63 0.07
0.57 0.01
0.99 0.00
0.56 0.01
4-MePh
4-EtOPh
4-ClPh
4-CHPh2
4-MeOPh
2-MePh
PhCH2
C
C
C
17H16N2
18H18N2O
16H13ClN2
C23H20N2
C
C
17H16N2O
17H16N2
C17H16N2
Ph/H
108–110
152–154
222–224
109–112
182–184
197–198
128–130
217–219
C30H24N2O
C31H26N2O
C32H28N2O2
C3OH23ClN2O
C32H28N2O
C33H30N2O
C34H32N2O2
C32H27ClN2O
4-MePh/H
4-EtOPh/H
4-ClPh/H
Ph/Me
4-MePh/Me
4-EtOPh/Me
4-ClPh/Me
DMSO (Àve control)
Amphotericin B (standard drug)
a
All the compounds showed elemental analysis for C, H and N in the range of 0.4.
% Inhibition activity: 0.99 0.00 = non-significant, 0.95–0.80 = low, 0.79–0.70 = moderate, 0.69–0.60 = good, <0.59–0.56 = significant.
b

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G. S. Singh et al. / Bioorg. Med. Chem. Lett. 22 (2012) 5704–5706
12. All the compounds were characterized by the presence of strong absorption
References and notes
band at 1750 ( 10) cmÀ1 in IR spectra. The representative NMR and MS data for
5cb is as follows: 1H NMR (CDCl3): dH 7.66 (d, J = 7 Hz, 1H), 7.48 (d, J = 8.1 Hz,
2H), 7.30 (dd, J = 6.9, 2.1 Hz, 2H), 7.18–7.05 (m, 5H), 6.99 (d, J = 8.1 Hz, 2H),
6.71 (dd, J = 8.1, 2.2 Hz, 2H), 6.65 (dd, J = 7.1, 2.0 Hz, 2H), 6.37 (s, 1H), 5.93 (s,
1H), 3.84 (q, J = 6.9 Hz, 2H), 3.45 (s, 3H), 2.25 (s, 3H), 2.05 (s, 3H), 1.26 (t,
J = 7.1 Hz, 3H); 13C NMR (CDCl3): dC 167.2, 155.3, 138.7, 136.8, 136.1, 135.7,
131.3, 129.4, 128.6, 128.4, 128.0, 127.3, 127.1, 121.7, 119.7, 118.9, 118.5, 114.8,
109.5, 108.7, 71.6, 63.7, 61.1, 32.8, 21.1, 20.9, 14.8; MS (m/z, r.i.): 500 (M+, 100).
13. Ali, S. A.; Khalil, N. Y.; Iqbal, J.; Yasinzai, M. Methods Cell Sci. 1997, 19, 107.
14. Parasite was cultured from the lesion aspirate taken directly from the infected
patients at the Bolan Medical College, Quetta, Pakistan, in artificial medium
199, supplemented with 10% FBS & termed as L. major since this is the
predominant strain in the region.
1. Berman, J. Ind. J. Med. Res. 2006, 123, 289.
2. Thakur, C. P. Ind. J. Med. Res. 2006, 123, 193.
3. Modabber, F.; Leishmaniasis Tropical Disease Research Progress; WHO: Geneva,
Switzerland, 1991. pp 77–87.
4. Croft, S. L.; Seifert, K.; Yardley, V. Ind. J. Med. Res. 2006, 123, 399.
5. Khan, K. M.; Mughal, U. R.; Samreen; Parveen, S.; Choudhary, M. I. Lett. Drug
Design Discov 2008, 5, 243.
6. For recent reviews: (a) Richard, J. V.; Werbovetz, K. A. Curr. Opin. Chem. Biol.
2010, 14, 447; (b) Mishra, B. B.; Singh, R. K.; Srivastava, A.; Tripathi, V. J.; Tiwari,
V. K. Mini Rev. Med. Chem. 2009, 9, 107. and references there in.
7. (a) Al-Kahraman, Y. M. S. A.; Madkour, H. M. F.; Ali, D.; Yasinzai, M. Molecules
2010, 15, 660; (b) Al-Kahraman, Y. M. S. A.; Singh, G. S.; Yasinzai, M. Letts. Drug
Design. Discov. 2011, 8, 242; (c) Al-Kahraman, Y. M. S. A.; Yasinzai, M.; Singh, G.
S. Arch. Pharm. Res. 2012, 3, 1099.
8. For reviews: (a) Singh, G. S. Mini Rev. Med. Chem. 2004, 4, 69; (b) Singh, G. S.
Mini Rev. Med. Chem. 2004, 4, 93; (c) Burnett, D. A. Curr. Med. Chem. 2004, 11,
1873.
15. The titled compounds were screened for their anti-leishmanial activity against
the pre-established culture of L. major. Parasites were cultured in Medium
M199 with 10% foetal bovine serum; 25 mmol HEPES, and 0.22 mg of penicillin
and streptomycin, respectively at 24 °C in a shaking incubator. Each compound
to be tested and amphotericin B (as a positive control) were dissolved in DMSO
to a concentration of 1 mg/ml. Parasites at log phase were centrifuged at
3000 rpm for 3 min. Parasites were diluted in fresh culture medium to a final
9. An equimolar amount of N-methyl-1H-indole-3-carboxaldehyde
1 and
appropriate amine 2a–h (5 mmol of each) in 25 mL of ethanol was refluxed
for 1–6 h. The crystalline compounds 3a–h were obtained after evaporation of
ethanol under reduced pressure. All the compounds were characterized by
C@N absorption at 1615 10 cmÀ1 in the IR spectra.
density of 2 Â 106 cells/mL. In 96-well plates, 180
l
l of medium was added in
l) was added in medium and
ll) was added in all wells. In negative
different wells. Experimental compound (20
l
serially diluted. Parasite culture (100
controls, DMSO was serially diluted in medium while the positive control
contained varying concentrations of standard antileishmanial compound that
is, amphotericin B. The plates were incubated for 72 h at 24 °C. The culture was
examined microscopically on an improved Neubauer counting chamber and
IC50 values of compounds possessing antileishmanial activity were calculated
using the Prism software. All the assays were run in triplicate.
10. (a) Staudinger, H. Liebigs Ann. Chem. 1907, 356, 51; (b) Palomo, C.; Aizpurua, J.
M.; Ganboa, I.; Oirabide, M. Eur. J. Org. Chem. 1999, 8, 3223; (c) Singh, G. S.
Tetrahedron 2003, 59, 7631.
11. An equimolar amount of 2-diazo-1,2-diarylethanone and appropriate imine
(1 mmol of each) in 8 mL of dry benzene was refluxed for 6–8 h under
atmosphere of N2. The evaporation of solvent under reduced pressure and
trituration of the residue with ethanol afforded white crystalline product that
was recrystallized with ethanol.
16. Blanco-Ania, D.; Hermkens, P. H. H.; Sliedregt, L. A. J. M.; Scheeren, H. W.;
Rutjes, F. P. J. T. Tetrahedron 2009, 65, 5393.