Syntheses of lipophilic chalcones and their conformationally restricted analogues as antitubercular agents

Article abstract of DOI:10.1016/j.bmcl.2012.12.096

Lipophilic chalcones and their conformationally restricted analogues were synthesized and evaluated for their antitubercular efficacy against Mycobacterium tuberculosis H37Rv strain. Compounds 16, 24, 25a and 25c were found to be active MIC at 60, 30, 3.5 and 7.5 μg-mL^-1. In vitro cytotoxicity of compounds 16, 24, 25a, 25c and 26 in non-cancerous human epithelial kidney cell line (HEK-293) showed that most active compound 25a was approximately 2.85 times selective towards tubercular versus healthy cells whereas compound 24 was found to be 16 times selective.

Full text of DOI:10.1016/j.bmcl.2012.12.096

Bioorganic & Medicinal Chemistry Letters 23 (2013) 1322–1325
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Syntheses of lipophilic chalcones and their conformationally
restricted analogues as antitubercular agents
Imran Ahmad a, Jay Prakash Thakur b, Debabrata Chanda b, Dharmendra Saikia b, Feroz Khan c,
Shivani Dixit d, Amit Kumar d, Rituraj Konwar d, Arvind Singh Negi a, Atul Gupta a,
⇑
a Chemical Sciences Division, Central Institute of Medicinal and Aromatic Plants, P.O. CIMAP, Kukrail Picnic Spot Road, Lucknow 226 015, India
b Molecular Bio-prospection Department, Central Institute of Medicinal and Aromatic Plants, P.O. CIMAP, Kukrail Picnic Spot Road, Lucknow 226 015, India
c Molecular and Structural Biology Department, Central Institute of Medicinal and Aromatic Plants, P.O. CIMAP, Kukrail Picnic Spot Road, Lucknow 226 015, India
d Endocrinology Division, CSIR-Central Drug Research Institute, Jankipuram Extension, Sector-10, Sitapur Road, Lucknow 226021, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Lipophilic chalcones and their conformationally restricted analogues were synthesized and evaluated for
Received 1 October 2012
Revised 18 December 2012
Accepted 28 December 2012
Available online 9 January 2013
their antitubercular efficacy against Mycobacterium tuberculosis H37Rv strain. Compounds 16, 24, 25a and
25c were found to be active MIC at 60, 30, 3.5 and 7.5 l
g-mLÀ1. In vitro cytotoxicity of compounds 16, 24,
25a, 25c and 26 in non-cancerous human epithelial kidney cell line (HEK-293) showed that most active
compound 25a was approximately 2.85 times selective towards tubercular versus healthy cells whereas
compound 24 was found to be 16 times selective.
Keywords:
Ó 2013 Elsevier Ltd. All rights reserved.
Antitubercular agent
Mycobacterium tuberculosis
Chalcone
Benzopyranone
Coumarin
HEK-293
Tuberculosis (TB) is a global health priority not only due to its
morbidity and mortality but also due to its high contagious nat-
ure.1–4 In 2010, 8.8 million people fell ill with TB and 1.4 million
were died of it.5 Thus, it is second largest killer worldwide by a sin-
gle infectious disease. TB becomes fiercer in combination with HIV
or cancer. Even our combination therapy of PRISE is not sufficient
to tackle the situation.6–8 Poor management of chemotherapy has
emerged drug resistance. The highly lipophilic cell wall of
Mycobacterium is responsible for its virulence to some extent.9
Streptomycin (1), rifampicin (2), rifapentine (3), isoniazide (4),
ethionamide (5) and ethambutol (6) are some potential drugs
which are being used for tuberculosis treatment (Fig. 1). However,
drug resistance is a major drawback of these agents.
Therefore, there is an urgent need to explore new antitubercular
agents. Ironically, the low number of potential new chemical enti-
ties is a worrying situation at present. Considering the severity of
the problem, WHO has prepared a strategic plan in Berlin declara-
tion 2007 to stop TB globally.
In the recent past, several coumarin derivatives have been re-
ported to exhibit antimycobacterial activity. Isoimperatorin (7),
Osthol (8a) and suberosin (8b) were isolated from Arracacia
tolucensis exihibiting antitubercular activity MIC at 64.0
32.0
g-mLÀ1 and 16 g-mLÀ1 respectively (Fig. 1).10 Other
coumarin derivatives such as ferulenol (9) have also been reported
l ,
g-mLÀ1
l
l
to exhibit potent antitubercular activity (MIC = 2.0 l )
g-mLÀ1
against other species of Mycobacterium.11,12 Similarly, chalcones
such as licochalcone A (10), present in Glycyrrhiza inflata exhibited
potent antitubercular activity (MIC = 7.1 l
g-mLÀ1).13 Furthermore,
pyranones such as 11 and 12 present in Piper sanctum have been
reported to have potent antitubercular activity in Mycobacterium
tuberculosis (MIC = 32 l l , respectively)
g-mLÀ1 and 4.0 g-mLÀ1
(Fig. 1).14 It is known that a moderate to high level of lipophilicity
of the compounds often attributes better antitubercular activity.
In the present study, taking structural learning from natural
coumarins, chalcones and pyranones, we synthesized some
lipophilic chalcones of prototype I and their conformationally re-
stricted analogues (styrenylchromanone) based on prototype II
which may be considered as hybrid of coumarin and chalcone
nucleus. Further, for activity modulation, we planned to have a
nitrogen moiety as it is essentially present in all the frontline anti-
tubercular drugs (Fig. 2).
The synthesized compounds were investigated for their antitu-
bercular potential in Mycobacterium tuberculosis
H37RV strain
radiometrically. Further, 16, 24, 25a, 25c and 26, which showed
significant antitubercular activity were evaluated for their toxicity
⇑
Corresponding author. Tel.:+91 522 2718556.
E-mail address: atisky2001@yahoo.co.in (A. Gupta).
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.12.096

I. Ahmad et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1322–1325
1323
N
NH
HO
N
H2N
HO
O
O
N
OH
OH
O
H
OH
OH
HO
N
HN
H
NH2
NH
H3CO
HN
O
OH
O
O
O
O
CHO
OH
HO
O
O
HO
HOH2C
O
N
O
HO
AcO
O
N
O
OH
HO
NHCH3
O
HO
N
1; Streptomycin
2; Rifampicin
3; Rifapentine
R1
O
NHNH2
S
NH2
OH
H
N
O
O
N
H
R2
N
N
OH
4; Isonizid
6; Ethambutol
7; Isoimperatorin, R1 = Isoprenyl, R2 =H
5; ethionamide
R1
R2
OH
O
OH
R3
HO
O
O
OCH3
8a: Osthol, R1 = H, R2 = OH, R3 = Isoprenyl;
9
8b: Suberosin R1 = Isoprenyl, R2 = OCH3 , R3 = H
O
10
OCH3
OCH3
O
O
O
O
O
O
11
12
Figure 1. Some synthetic and natural anti-tubercular agents.
subsequent treatment of acid 18 with different amines in presence
of 1-hydroxy benzotriazole (HOBt) and dicyclohexylcarbodiimde
(DCC) in dichloromethane (DCM) under basic reaction conditions
at reflux afforded amide derivative of chalcone 19a and 19b in
68 and 75% yields, respectively.
O
O
O
R
N
O
O
R'
O
H3CO
R
N
The synthesis of target compounds of prototype (II) was started
with 2-hydroxy-4-methoxy-acetophenone (20a) as reported
earlier (scheme 2).16 Briefly, compound 20a was condensed with
4-hydroxybenzaldehyde (21a) in presence of piperidine and dry
benzene under reflux to afford 1-(2-hydroxy-4-methoxy-phenyl)-
3-(4-hydroxy-phenyl)-propenone (22a). Compound 22a on reac-
tion with phenyl acetic acid in acetic anhydride and triethyl amine
at reflux gave 7-methoxy-3-phenyl-4{2-[4-acetoxy-phenyl]-vi-
nyl}-benzopyran-2-one (23a) in 86% yield. Compound 23a on
hydrolysis in 2% methanolic NaOH gave 7-methoxy-3-phenyl-
4{2-[4-hydroxy-phenyl]-vinyl}-benzopyran-2-one 24 in 80% yield.
Similarly, compound 26 was synthesized by reaction of 20b and
21b using same sequence of reactions. Condensation of
compound 24 with 2-chloroethyl alkylamine hydrochloride in
acetone in presence of K2CO3 under reflux gave the desired prod-
ucts 25(a–c) in 78–86% yields. The synthesized compounds were
characterized by the use of different spectroscopy techniques.17
To evaluate the potential of basic pharmacophore of chalcone,
16 (hydroxyl derivative) was evaluated for its antitubercular effi-
cacy in Mycobacterium tuberculosis H37RV strain which showed
O
R'
I
II
Figure 2. Prototype I and II.
in in vitro MTT assay using non-cancerous human epithelial kidney
cell line (HEK-293) derived from human embryonic kidney cells.
The synthesis of chalcone derivatives of type (I) was started
from Friedel–Crafts acylation of anisol (13) with phenylacetic acid
(14) using polyphosphoric acid (PPA) at 100 °C as reported earlier
which yielded deoxybenzoin 15 in good yield as single product
(scheme 1).15 Compound 15 was reacted with 4-hydroxybenzalde-
hyde in presence of piperidine and dry benzene at reflux yielding
compound 16 as single product with trans geometry confirmed
by X-Ray analysis.15 Compound 16 on condensation with ethyl
bromoacetate gave corresponding ester derivative 17. On hydroly-
sis under basic reaction condition compound 17 gave correspond-
ing carboxylic acid derivative 18 in quantitative yield. The

1324
I. Ahmad et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1322–1325
O
O
OH
a
+
O
H3CO
15
14
13
b
O
O
c
H3CO
H3CO
OCH2COOCH2CH3
17
OH
16
d
O
O
e
H3CO
OCH2CONR1R2
H3CO
19a; R1 = H, R2 = CH2CH2CH2CH3
19b; R1 = CH3, R2 = CH2CH2CH2CH3
OCH2COOH
18
Scheme 1. Reagent and conditions (a) Polyphosphoric acid, heating at 100 °C; (b) 4-hydroxybenzaldehyde, piperidine, dry benzene, reflux; (c) ethyl bromoacetate, anhy.
K2CO3, dry acetone, reflux; (d) NaOH, methanol, reflux; (e) alkylamines, DCC, HOBt, DMAP, DCM, reflux.
OH
O
O
OH
O
+
a
H
R
R1
R
R1
21a; R1 = OH
21b; R1 = OCH3
20a; R = OCH3
20b; R = OH
22a; R = OCH3, R1 = OH
22b; R = OH; R1 = OCH3
b
O
O
O
O
c
HO
OCH3
R
R1
26
23a; R = OCH3, R1 = OCOCH3
23b; R = OCOCH3; R1 = OCH3
c
O
O
O
O
d
H3CO
OR
H3CO
OH
24
N
25a; R =
25b; R =
25c; R =
N
N
Scheme 2. Reagent and conditions (a) piperidine, dry benzene, reflux; (b) phenylacetic acid, Et3N, acetic anhydride, reflux; (c) 2% methanolic NaOH, room temperature; (d) 2-
chloroethyl alkyl amines hydrochlorides, anhy. K2CO3, dry acetone reflux.

I. Ahmad et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1322–1325
1325
Table 1
could be considered to be approximately 2.85 times selective
towards tubercular versus healthy mammalian cells whereas
compound 24 was found to be 16 times safe with respect to its
MIC values.
Antimycobaterial and cytotoxic activities of compounds against Mycobacterium
tuberculosis H37Rv strain and human epithelial kidney cell line (HEK-293) respectively
Compound
MIC (l
g-mLÀ1
)
HEK-293 (IC50 in
lg/mL)
LogP
However, chalcone derivative (16) which had MIC value
60 lg/mL presented low selectivity towards tubercular versus
healthy cells.
16
18
19a
19b
22
23
24
25a
25b
25c
26
Rifampicin
Streptomycin
60
50
—
—
—
—
4.887
4.460
5.049
5.296
3.107
3.107
4.933
6.078
6.078
5.759
4.933
2.615
À4.112
250
n.a
n.a
n.aa
n.a
30
3.5
n.a
7.5
125
2.0
2.0
In conclusions, a series of lipophilic chalcones and their con-
formationally restricted analogues has been explored as antituber-
cular agents. Compounds 16, 24, 25a and 25b showed significant
antitubercular activity. In vitro cytotoxic activity data showed that
the most active compound 25a exhibiting potent antitubercular
—
>250
05
—
05
>250
—
activity (MIC = 3.5 l
g-mLÀ1) was found to be approximately 2.85
times selective towards tubercular versus healthy cells whereas,
greater selectivity was observed for compound 24 as compared
to conformationally flexible chalcone derivative (16).
Overall, the biological activity results of these molecules rav-
elled that conformationally restricted chalcones were superior
over their open chain analogues possibly due molecular flexibility
and may be a good lead in future for development of new antitu-
bercular drugs.
—
n.a. = not active at 250 l
g-mLÀ1 concentration, (—) = Not determined, LogP = octa-
nol/water partition coefficient, values were calculated using scigress explorer
(Fujitsu) software.
activity at MIC 60 l
g-mLÀ1 (Table 1).18 Modification of compound
16 for activity modulation was made through transformation into
alkylcarboxylic acid and corresponding amide derivatives (18, 19a
and 19b). These compounds were found to be either far less active
than 16 (hydroxyl derivative) or inactive. It is known in literature
that molecular flexibility plays an important role in drug protein
interactions. We therefore planned to synthesize conformationally
restricted chalcones which may also be considered as hybrid of
chalcone and coumarin derivatives (styrenylchromanone). This
modification was made in such a way that the total lipophilic nat-
ure of compounds be almost similar (LogP values for compound 16
and 24 are 4.887 and 4.933, respectively, Table 1). The synthesis of
these styrenylchromanones is briefly described in scheme 2.
Compound 24, the basic pharmacophore of prototype II, was
Acknowledgments
The authors thank the Director, CIMAP, and Council of Scientific
Industrial Research (CSIR) for financial support.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2012.12.
096.
References and notes
showed antitubercular activity at MIC 30
l
g-mLÀ1 (whereas
compounds 16 showed activity at MIC 60
l
g-mLÀ1) (Table 1). Com-
1. Yew, W. W.; Cynamon, M.; Zhang, Y. Expert Opin. Emerg. Drugs 2011, 16, 1.
2. Koul, A.; Arnoult, E.; Lounis, N.; Guillemont, J.; Andries, K. Nature 2011, 469,
483.
3. Negi, A. S.; Kumar, J. K.; Luqman, S.; Saikia, D.; Khanuja, S. P. S. Med. Res. Rev.
2010, 30, 603.
4. Kumari, S.; Ram, V. J. Drugs Today 2004, 40, 487.
5. WHO tuberculosis factsheet No.104, March-2012 (www.who.int/mediacentre/
factsheet/fs104/en).
6. Dipti, G.; Jyotsna, J. M. Recent. Pat. Anti-infect. 2011, 6, 27.
7. Ginsberg, A. M. Drugs 2010, 70, 2201.
8. Lamichhane, G. Trends Mol. Med. 2011, 17, 25.
9. Murray, P. R.; Rosenthal, K. S.; Pfaller, M. A. Medical Microbiology, 5th edition;
Elsevier, Mosby: Philadelphia, PA, 2005.
pound 24 possessing two fold higher activity than 16 was taken
further for modification by incorporation of a nitrogen moiety
present in many frontline antitubercular drugs. The modified com-
pounds 25a–c were synthesized by aminoalkylation of compound
24 as shown in scheme 2. The minimum inhibitory concentration
(MIC) for compounds 25a and 25c were found to be 3.5 and
7.5
l
g-mLÀ1, respectively, which are close to the MIC values of rif-
ampicin and streptomycin (MIC = 2.0
l
g-mLÀ1 respectively) stan-
dard antitubercular drugs used as control. However, compound
10. Figueroa, M.; Cruz, R. I.; Cruz, R. B.; Byeb, R.; Navarrete, A.; Mata, R. J.
Ethnopharmacol. 2007, 113, 125.
11. Appendino, G.; Mercalli, E.; Fuzzati, N.; Arnoldi, L.; Stavri, M.; ’ Gibbons, S.;
Ballero, M.; Maxia, A. J. Nat. Prod. 2004, 67, 2108.
12. Mossa, J. S.; Feraly, E.; Muhammad, I. Phytother. Res. 2004, 18, 934.
13. Friis-Moller, A.; Chen, M.; Fuursted, K.; Christensen, S. B.; Kharazmi, A. Planta
Med. 2002, 68, 416.
14. Mata, R.; Morales, I.; Perez, O.; Rivero-Cruz, I.; Acevedo, L.; Enriquez-Mendoza,
I.; Bye, R.; Franzblau, S.; Timmermann, B. J. Nat. Prod. 1961, 2004, 67.
15. Gupta, A.; Raghunandan, R.; Kumar, A.; Maulik, P. R.; Dwivedy, A.; Keshri, G.;
Singh, M. M.; Ray, S. Med. Chem. 2007, 3, 241.
16. Gupta, A.; Raghunandan, R.; Kumar, A.; Maulik, P. R.; Dwivedi, A.; Keshri, G.;
Singh, M. M.; Ray, S. Med. Chem. 2007, 3, 446.
17. For detail see Supplementary data.
18. Siddiqi, S. H. BACTEC 460 TB SYSTEM, Product and Procedure Manual, MA-0029
(Revision E, May 1996); Becton Dickinson and Company: USA, 1996.
19. Chakravarti, B.; Maurya, R.; Siddiqui, J. A.; Bid, H. K.; Rajendran, S. M.; Yadav, P.
P.; Konwar, R. J. Ethnopharmacol. 2012, 142, 72.
25b was found to be inactive in this assay.
In view of good antitubercular activity of compounds 16, 24,
25a, 25c and 26, their toxicity was evaluated in in vitro MTT assay
using non-cancerous human epithelial kidney cell line (HEK-293)
derived from human embryonic kidney cells as model of normal
cell.19 The IC50 values for studied compounds showed that com-
pounds 16 (hydroxyl derivative of chalcone), 24 and 26 (hydroxy
derivatives of conformationally restricted chalcones, styrenylchro-
manone) required relatively higher concentration for inhibition of
cell population (50 and 250
compound 25a and 25c inhibited 50% cell population at 5
l
g/mL respectively) Table 1. Both
g/mL
l
concentration in this assay. Since, theoretically, MIC value repre-
sents IC100 and is approximately double of IC50 value, therefore
comparing the in vitro antitubercular and cytotoxic activities of
compounds (MIC vs IC50 values), the most active compounds 25a