Novel N′-benzylidene benzofuran-3-carbohydrazide derivatives as antitubercular and antifungal agents

Article abstract of DOI:10.1016/j.bmcl.2012.01.067

Tuberculosis constitutes today a serious threat to human health worldwide, aggravated by the increasing number of identified multi-drug resistant strains of Mycobacterium tuberculosis (Mtb), its causative agent, as well as by the lack of development of no

Full text of DOI:10.1016/j.bmcl.2012.01.067

Bioorganic & Medicinal Chemistry Letters 22 (2012) 2343–2346
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel N0-benzylidene benzofuran-3-carbohydrazide derivatives as
antitubercular and antifungal agents
⇑
Vikas N. Telvekar , Anirudh Belubbi, Vinod Kumar Bairwa, Kalpana Satardekar
Department of Pharmaceutical Sciences and Technology, Institute of Chemical Technology, Matunga, Mumbai 400019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Tuberculosis constitutes today a serious threat to human health worldwide, aggravated by the increasing
number of identified multi-drug resistant strains of Mycobacterium tuberculosis (Mtb), its causative agent,
as well as by the lack of development of novel mycobactericidal compounds for the last few decades. A
novel series of benzofuran-3-carbohydrazide and its analogs was synthesized and characterized spectro-
scopically. All the compounds were characterized and screened for in vitro anti-tuberculosis (anti-TB)
activity against Mycobacterium tuberculosis H37Rv strains by using resazurin assay utilizing microtiter-
plate method (REMA). These compounds also showed good antifungal activity against Candida albicans.
Thus, the high level of activity shown by the compounds (8a, 8k) suggests that these compounds could
serve as leads for development of novel synthetic compounds with enhanced anti-TB and antifungal
activity.
Received 24 November 2011
Revised 21 December 2011
Accepted 4 January 2012
Available online 4 February 2012
Keywords:
Tuberculosis
N0-benzylidene benzofuran
Antifungal
Molecular hybridization
Hydrazone
Ó 2012 Elsevier Ltd. All rights reserved.
Tuberculosis (TB) remains one of the most devastating infec-
tious diseases with more than 9 million cases annually and with
more than 1.7 million fatalities. It is caused by the rod shaped
Mycobacterium tuberculosis (Mtb). During the last 10 years there
has been a renewed global interest for research in TB, and this
has resulted in the launch of several new initiatives by national
and international organizations, private charities and pharmaceu-
tical companies. The new focus on TB has partly been triggered
by the persistent high number of TB cases in developing countries,
and partly by the increased occurrence of multidrug and exten-
sively drug resistant TB (MDR- and XDR-TB). In HIV-positive
individuals, XDR-TB has been associated with very high mortality.
The emergence of strains resistant to the first line drugs causes ma-
jor concern, as treatment is then deferred to drugs that are less
effective, have more toxic side effects, and result in higher death
rates, especially among HIV-infected persons.1 Benzofuran class
of fused heterocycles has been of great interest as they are
abundant in nature and have wide pharmacological activities.2 In
the field of anti-infective agents, benzofurans have been reported
to show antifungal,3–6 antiprotozoal3 and antitubercular activ-
ity.7–9According to reported literature, most of the studies were
carried out on benzofuran nucleus as 2-substituted3,4,6,7,10 or 2,3-
disubstituted benzofurans derivatives.9
useful anti-mycobacterial series, with the most active compound
showing MIC of 1.2
g/mL.7 Substituted purines have also been
tried on 2-position of benzofuran ring to form another useful
l
anti-mycobacterial series with the most active compound showing
MIC of 6.25 l
g/mL.8 There have been reports of 2,3-disubstituted
benzofurans showing useful anti-mycobacterial activity. A series
having aromatic/heterocyclic substituent at 2-position along with
aliphatic substituent on 3-position has been reported to show
anti-mycobacterial activity with the most active compound having
MIC of 3.25 lg/mL. This indicates that not only 2-substituted ben-
zofurans are active, but even the 3-substituents have some role in
rendering benzofuran containing molecules active against Mtb.
This prompted us to explore the possibility of 3-substituted benzo-
furan derivatives as an anti-mycobacterial agent which has not
been reported yet.
Another very important class of compounds that has a varied
pharmacological profile is the aroyl hydrazone class of compounds.
It has been demonstrated that aroyl hydrazones are an important
and promising class of anti-infective agents.11–13 A series of 4-ami-
nobenzoic acid hydrazones have been reported to have good anti-
mycobacterial activity with the most active compound having a
MIC of 12.5
l
g/mL.11There also have been reports of benzoyl
g/mL for the
hydrazones effective against Mtb with MIC of 6.25
l
In recent years, there has been an increased interest in 2-substi-
tuted benzofurans as anti-mycobacterial agents. Highly substi-
tuted pyrazine as a 2-substituent on benzofuran has given a
most active compound of the series.12,13 Recently it has been
reported that aroyl hydrazones inhibit mycobacterial efflux pumps
and hence are effective against resistant strains of Mtb.14 Hence
the impressive results of various benzofuran series and various
aroyl hydrazones fueled our interest in combining two scaffolds
and exploring their possibilities as potential anti-TB agents.
⇑
Corresponding author. Tel.: +91 22 3361 2213; fax: +91 22 3361 1020.
E-mail address: vikastelvekar@rediffmail.com (V.N. Telvekar).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2012.01.067

2344
V. N. Telvekar et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2343–2346
O
O
H
OC2H5
OC2H5
R2
R2
N2
R2
O
b
a
OH
OC2H5
OH2
O
O
R1
O
R1
R1
1a-b
3
4a-b
2
1a: R1=R2=H
4a: R1=R2=H
4b: R1=R2=Cl
1b: R1=R2=Cl
O
O
H
OC2H5
N
R2
R2
NH2
c
NH2 .H2O
H2N
O
O
R1
6a-b
R1
6b: R1=R2=Cl
6a: R1=R2=H
5
4a-b
O
H
N
O
H
N
R2
N
R
R2
NH2
d
R
O
O
O
R1
R1
8a-t
7a-j
6a-b
7a: R=Phenyl
7b: R=2,4-dimethoxy Phenyl
7d: R=2-hydroxy Phenyl
7f: R=4-methyl Phenyl
7c: R=4-hydroxy Phenyl
7e: R=4-chloro Phenyl
7g: R=4-nitro Phenyl
7h: R=3,5-dichloro-2-hydroxy Phenyl
7j: R=2,6-dichloro Phenyl
7i: R=naphthyl
8k: R1=R2=Cl, R=Phenyl
8a: R1=R2=H, R=Phenyl
8l: R1=R2=Cl, R=2,4-dimethoxy Phenyl
8m: R1=R2=Cl, R=4-hydroxy Phenyl
8n: R1=R2=Cl, R=2-hydroxy Phenyl
8b: R1=R2=H, R=2,4-dimethoxy Phenyl
8c: R1=R2=H, R=4-hydroxy Phenyl
8d: R1=R2=H, R=2-hydroxy Phenyl
8o: R1=R2=Cl, R=4-chloro Phenyl
8p: R1=R2=Cl, R=4-methyl Phenyl
8q: R1=R2=Cl, R=4-nitro Phenyl
8e: R1=R2=H, R=4-chloro Phenyl
8f: R1=R2=H, R=4-methyl Phenyl
8g: R1=R2=H, R=4-nitro Phenyl
8r: R1=R2=Cl, R=3,5-dichloro-2-hydroxy Phenyl
8s: R1=R2=Cl, R=naphthyl
8h: R1=R2=H, R=3,5-dichloro-2-hydroxy Phenyl
8i: R1=R2=H, R=naphthyl
8t: R1=R2=Cl, R=2,6-dichloro Phenyl
8j: R1=R2=H, R=2,6-dichloro Phenyl
Scheme 1. Synthetic route for N0- benzylidene benzofuran-3-carbohydrazide derivatives (8a–t). Reagents and conditions: (a) CH2Cl2, HBF4ꢀOEt2, reflux; (b) H2SO4 (95–98%);
(c) C2H5OH, reflux for 5–7 h; (d) C2H5OH, reflux for 2–5 h.
Molecular hybridization is a new concept in drug design and
development based on the combination of pharmacophoric
moieties of different bioactive substances to produce a new hybrid
compound with improved affinity and efficacy, when compared to
the parent drugs.15 A novel set of molecules were designed by
combining the benzofuran scaffold with the aroyl hydrazone moi-
ety. We decided to fuse the benzofuran nucleus along with the
aroyl hydrazone nucleus (linked by a carbonyl group) using molec-
ular hybridization in a quest to find activity higher than that re-
ported in the literature earlier.
Most of the 2-substituted or 2,3-disubstituted benzofurans
derivatives are reported in the literature, although 3-substituted
benzofurans are not often reported. For our study we needed ethyl
benzofuran-3-carboxylate, which is core substance (4a and b) for
our further synthesis. Nonetheless, there are four synthetic meth-
ods have appeared recently for preparation of ethyl benzofuran-
3-carboxylate. Among the four methods, three methods involve
multi-step palladium-mediated intramolecular cyclizations from
o-halo phenols or o-dihalobenzenes, which have limited commer-
cial availability and high cost. One method involves carbonylative
cyclization of complex o-alkynylphenols. A second involves cycli-
zation by enolate o-arylation where as third method, involves an
improved synthesis of 3-ethoxycarbonyl benzofuran in 74% overall
yield from iodophenol. For our preparation of ethyl benzofuran-3-
carboxylate, we used the fourth method which is a higher yielding
method as compared to the above three, using salicyaldehyde and
ethyl diazoacetate which are relatively cheaper starting materials.
Further ethyl benzofuran-3-carboxylate converted into benzofuran
3-carbohydrazide and finaly condensation with different alde-
hydes resulted into required N0-substituted benzofuran-3-carbohy-
drazides. The entire process is summarized in Scheme 1.
Salicyladehyde was reacted with ethyl diazoacetate in presence
of fluoroboric acid, to form ethyl benzofuran-3-carboxylate16 (4a
and b). The reaction proceeds via a nucleophilic attack by ethyl
diazoacetate on activated (protonated) salicylaldehyde. The next
step involves a functional group transformation of the ester to

V. N. Telvekar et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2343–2346
2345
Table 1
In vitro antitubercular and antifungal activity of N0-benzylidene benzofuran-3-carbohydrazide derivatives
H
N
O
R2
N
R
O
R1
8a-t
Compd No.
R1
R2
R
Antifungal activity MICa
(lg/mL)
Antitubercular activitya MIC (
lg/mL)
8a
8b
8c
8d
8e
8f
8g
8h
8i
8j
8k
8l
8m
8n
8o
8p
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Phenyl
>250
>250
>250
>250
>250
>250
>250
31.25
125
>250
>250
>250
>250
>250
>250
<15.62
125
2.0
128.0
64.0
2,4-Dimethoxy phenyl
4-Hydroxy phenyl
2-Hydroxy phenyl
4-Chloro phenyl
4-Methyl phenyl
4-Nitro phenyl
3,5-Dichloro-2-hydroxy phenyl
Naphthyl
2,6-Dichloro phenyl
Phenyl
2,4-Dimethoxy phenyl
4-Hydroxy phenyl
2-Hydroxy phenyl
4-Chloro phenyl
4-Methyl phenyl
4-Nitro phenyl
3,5-Dichloro-2-hydroxy phenyl
Naphthyl
2,6-Dichloro phenyl
—
64.0
128.0
128.0
128.0
128.0
128.0
128.0
2.0
64.0
128.0
32.0
128.0
128.0
128.0
128.0
128.0
128.0
0.25
H
H
H
H
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
—
—
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
—
—
8q
8r
8s
8t
250
<15.62
62.50
—
Isoniazid
Clotrimazole
0.50
—
a
Represent the average values of two independent experiments.
hydrazide using hydrazine hydrate to form benzofuran-3-carbohy-
razide17 (6a and b). The final step involves reaction between the
hydrazide and various aromatic aldehydes (7a–j) to form a number
of N0-substituted benzofuran-3-carbohydrazides18 (8a–t). The
spectral data of all synthesized compounds were in agreement
with the proposed structures.
attached to hydrazide seems to be more active. When benzofuran
ring is substituted, a highly substituted side chain seems to de-
crease activity. Bulky side chain attached to hydrazide is required
for activity. Substitution on benzofuran ring along with bulky side
chain proved to be the most potent compound 8s with a MIC of less
than 15.62 lg/mL. Substitution on para position is not active ex-
All the synthesized molecules were screened for inhibitory
activity against Mycobacterium tuberculosis H37Rv strain by REMA
method using isoniazid as the standard. The MIC values are given
in Table 1. All synthesized molecules were also screened for anti-
fungal activity against Candida albicans using clotrimazole as the
standard.
cept in the case of one compound 8p where the combined effect
of electron withdrawing group on the benzofuran ring and electron
donating group on para position of aromatic side chain showed
good activity. The general structure of N0-benzylidene benzofu-
ran-3-carbohydrazide and MIC values are given in Table 1.
The synthesized N0-benzylidene benzofuran-3-carbohydrazide
and its analogs show promising activity against Mtb. The inhibition
The synthesized series of compounds showed some promising
anti-mycobacterial and antifungal activity. For anti-mycobacterial
of Mtb at concentrations as low as 2 and 8 lg/mL by compounds
activity, compounds 8a and 8k showed MIC of 8 and 2
l
g/mL
8k and 8a respectively indicates that these compounds can act as
respectively. This indicates that no substitution on benzylidene
group is required for anti-mycobacterial activity. In case of anti-
fungal activity, when the benzofuran core is substituted with two
electron withdrawing groups, the activity was increased around
fourfold 8t, when compared with compound 8j. When R was taken
as biphenyl, antifungal activity was increased 8i when it was com-
pared with mono-phenyl 8a. This indicated that the combination of
unsubstituted benzylidene and chloro disubstituted benzofuran
ring shows greater activity. All para-substituent’s on the benzyli-
dene group, except in 8c, 8d, 8m, 8n and 8o (Which retain same
activity, maybe because of inadequate binding to the receptor
pocket) increase the antifungal activity. In case of antitubercular
activity, ortho-hydroxyl and protected hydroxyl groups substitu-
tion on benzylidene group also show good activity in our selected
concentration range, compound 8d, 8n and 8b, 8l, respectively. In
conclusion, unsubstituted benzylidene is required for activity and
electron withdrawing groups on benzofuran ring is also important
for antitubercular activity. For antifungal activity when the benzo-
leads for development of newer anti-TB compounds.
Acknowledgment
The author V.N.T. is grateful to the Department of Science &
Technology (DST), Government of India, New Delhi for financial
support under the programme of SERC Fast Track Scheme for
Young Scientist (FAST).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2012.01.067.
References and notes
1. Lang, H.; Quaglio, G.; Olesen, O. Tuberculosis 2010, 90, 1.
2. Aslam, S. N.; Stevenson, P. C.; Kokubun, T.; Hall, D. R. Micro. Res. 2009, 164, 191.
3. Ryu, C.; Song, A.; Lee, J. Y.; Hong, J.; Yoon, J. H.; Kim, A. Bioorg. Med. Chem. Lett.
2010, 20, 6777.
furan ring is unsubstituted,
a highly substituted side chain

2346
V. N. Telvekar et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2343–2346
4. Abdel-Wahab, B. F.; Abdel-Aziz, H. A.; Ahmed, E. M. Eur. J. Med. Chem. 2009, 44,
2632.
14. Ramon-Garcia, S.; Martin, C.; Thompson, C. J.; Anssa, J. A. Antimicrob. Agents
Chem. 2009, 3675.
5. Jiang, X.; Liu, W.; Zhang, W.; Jiang, F.; Gao, Z.; Zhuang, H.; Fu, L. Eur. J. Med.
Chem. 2011, 46, 3526.
15. Viegas-Junior, C.; Danuello, A.; Bolzani, S. V.; Barreiro, E. J.; Fraga, C. A. Cur. Med.
Chem. 1829, 2007, 14.
6. Kirilmis, C.; Ahmedzade, M.; Servi, S.; Koca, M.; Kizirgil, A.; Kazaz, C. Eur. J. Med.
Chem. 2008, 43, 300.
16. Dudley, M. E.; Morshed, M. M.; Hossain, M. M. Org. Synth. 2009, 86, 172.
17. Wejroch, K.; Lange, J.; Wesolowska, A. J. Heterocycl. Chem. 2005, 42, 919.
18. General procedure for the synthesis of N0-benzylidene benzofuran-3-
carbohydrazide derivatives (8a–t): An equimolar concentration of
benzofuran-3-carbohyrazide derivatives (6a–b) (10 mM) and substituted
aromatic aldehydes (7a–j), (10 mM) were dissolved in absolute ethanol (25–
35 mL) and the mixture was refluxed for 6hrs. After completion of reaction
(TLC), the reaction mixture was cooled to room temperature and it was then
chilled on an ice bath to obtain a number of N0-substituted benzofuran-3-
carbohydrazides (8a–t) as a precipitate. The precipitate was filtered under
vacuum, dried and recrystallized from ethanol to get pure compounds in yield
ranging from 56–88%.
7. Manna, K.; Agrawal, Y. K. Eur. J. Med. Chem. 2010, 45, 3831.
8. Brændvang, M.;Bakken, V.; Gundersen, L. Bioorg. Med. Chem. Lett. 2009, 17, 6512.
9. Bakunova, S. M.; Bakunov, S. A.; Wenzler, T.; Barszcz, T.; Werbovetz, K. A.; Reto
Brun Hall, J. E.; Tidwell, R. R. J. Med. Chem. 2007, 50, 5807.
10. Alvey, L.; Prado, S.; Huteau, V.; Saint-Joanis, B.; Michel, S.; Koch, M.; Cole, S. T.;
Tillequin, F.; Janin, Y. L. Bioorg. Med. Chem. Lett. 2008, 16, 8264.
11. Kucukguzel, S. G.; Rollas, S.; Kucukguzel, I.; Kiraz, M. Eur. J. Med. Chem. 1999,
34, 1093.
12. Kucukguzel, S. G.; Oruc, E. E.; Rollas, S.; Sahin, F.; Ozbek, A. Eur. J. Med. Chem.
2002, 37, 197.
13. Kaymakcıoglu, K. B.; Oruc, E. E.; Unsalan, S.; Kandemirli, F.; Shvets, N.; Rollas,
S.; Anatholy, D. Eur. J. Med. Chem. 2006, 41, 1253.