Design, synthesis and in vitro anti-tuberculosis activity of benzo[6,7]cyclohepta[1,2-b]pyridine-1,2,3-triazole derivatives

Article abstract of DOI:10.1016/j.bmcl.2017.10.071

A series of novel benzo[6,7]cyclohepta[1,2-b]pyridine-1,2,3-triazole hybrids (7a–j & 8a–j) have been designed and synthesized in excellent yields by Huisgen's [3+2] cyclo addition reaction of 3-(azidomethyl)-2-methyl-6,7-dihydro-5H-benzo[6,7]cyclohepta[1,2-b]pyridine (5) with various alkynes 6 in presence of copper sulphate and sodium ascorbate and their structures were confirmed by IR, ¹H NMR, ¹³C NMR and HRMS. The newly synthesized compounds 7a–j & 8a–j were evaluated for their in vitro anti-mycobacterial activity against Mycobacterium tuberculosis H37Rv (ATCC 27294). Among the compounds tested, the compounds 7i and 8g displayed most potent activity with MIC value of 1.56 μg/mL with low cytotoxicity.

Full text of DOI:10.1016/j.bmcl.2017.10.071

Accepted Manuscript
Design, Synthesis and in vitro anti-tuberculosis activity of Benzo[6,7]cyclo-
hepta[1,2-b]pyridine-1,2,3-triazole derivatives
Yasodakrishna Sajja, Sowmya Vanguru, Hanmanth Reddy Vulupala,
Rajashaker Bantu, Perumal Yogeswari, Dharmarajan Sriram, Lingaiah
Nagarapu
PII:
S0960-894X(17)31068-5
https://doi.org/10.1016/j.bmcl.2017.10.071
BMCL 25400
DOI:
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
18 July 2017
17 October 2017
27 October 2017
Please cite this article as: Sajja, Y., Vanguru, S., Vulupala, H.R., Bantu, R., Yogeswari, P., Sriram, D., Nagarapu,
L., Design, Synthesis and in vitro anti-tuberculosis activity of Benzo[6,7]cyclohepta[1,2-b]pyridine-1,2,3-triazole
derivatives, Bioorganic & Medicinal Chemistry Letters (2017), doi: https://doi.org/10.1016/j.bmcl.2017.10.071
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Bioorganic & Medicinal Chemistry Letters
Design, Synthesis and in vitro anti-tuberculosis activity of Benzo[6,7]cyclohepta[1,2-
b]pyridine-1,2,3-triazole derivatives.
Yasodakrishna Sajjaa, Sowmya Vangurua, Hanmanth Reddy Vulupalaa, Rajashaker Bantua, Perumal Yogeswarib,
Dharmarajan Sriramb, Lingaiah Nagarapua*
a Organic Chemistry Division-II (C P C Division), CSIR- Indian Institute of Chemical Technology, Hyderabad-500 007, Telangana, INDIA.
bMedicinal Chemistry and Antimycobacterial Research Laboratory, Pharmacy Group, Birla Institute of Technology & Science-Pilani, Hyderabad Campus,
Jawahar Nagar, Hyderabad-500078, Telangana, INDIA.
Fax: + 91 40 27193382; Tel: +91 40 27191509; E-mail: lnagarapuiict@yahoo.com/nagarapu@iict.res.in
ARTICLE INFO
ABSTRACT
Article history:
Received
Revised
Accepted
Available online
A series of novel benzo[6,7]cyclohepta[1,2-b]pyridine-1,2,3-triazole hybrids (7a-j &
8a-j) have been designed and synthesized in excellent yields by Huisgen's [3+2] cyclo
addition
reaction
of
3-(azidomethyl)-2-methyl-6,7-dihydro-5H-
benzo[6,7]cyclohepta[1,2-b]pyridine (5) with various alkynes 6 in presence of copper
1
sulphate and sodium ascorbate and their structures were confirmed by IR, H NMR,
13C NMR and HRMS. The newly synthesized compounds 7a-j & 8a-j were evaluated
for their in vitro anti-mycobacterial activity against Mycobacterium tuberculosis
H37Rv (ATCC 27294). Among the compounds tested, the compounds 7i and 8g
displayed most potent activity with MIC value of 1.56 µg/mL with low cytotoxicity.
Keywords:
Triazole
click reaction
benzo[6,7]cyclohepta[1,2-b]pyridine
Mycobacterium tuberculosis
2009 Elsevier Ltd All rights reserved.
Tuberculosis (TB) is a potential and chronic infectious
disease caused by Mycobacterium tuberculosis.1-4 About one-
third of the current world population have TB in its latent form.5
In 2015, there were approximately 10.4 million cases of active
TB and this disease caused approximately 1.8 million deaths
(World Health Organization 2017).6 Additionally, the resurgence
of its new virulent forms like multidrug-resistant (MDR-TB) and
extremely drug resistant (XDR-TB) has become a major threat to
mankind.7,8 The worsening situation necessitated an urgent need
for discovery of modern curative drugs active to all forms of
TB.9,10 All these facts prompted various research groups across
the globe to revisit some of the natural and synthetic bioactive
compounds for the development of new anti-tubercular drugs
with novel mechanism of action to achieve effective TB
control.11-13
attract considerable attention due to their synthetic and effective
biological properties like anti-tubercular, anti-HIV, anti-fungal,
anti-HSV, anti-platelet, anti-microbial, and anti-inflammatory
activities.29,30 The 1,2,3-triazole scaffold is being frequently used
as
a
pharmacophore for the modification of known
pharmaceuticals. Currently, there is a significant need to identify
new scaffolds as anti-tubercular agents that can form an effective
anti-tubercular therapy.31,32 N-acylhydrazones (NAH)33-37 and
several 1,2,3-triazole analogues38-40 have been reported to possess
anti-tubercular activity (Fig.1 (ii)).
A large number of heterocyclic compounds containing
pyridine rings were associated with diverse pharmacological
properties such as antimicrobial,14,15 anticancer,16 anticonvulsant,
17
antiviral,18 anti-HlV,19 antifungal and anti-mycobacterial
activities.20 The condensed derivatives of pyridines play
significant role in bioactive molecules, especially in the form of
benzocyclohepta[1,2-b]pyridines which are structural analogues
of benzosuberone. The benzocyclohepta[1,2-b]pyridine is an
important core biologically active compound with diverse
biological activities, such as antihistamine as well as antitumor
and anti-inflammatory activities21-26 Fig.1(i). It is a highly potent
pharmacophore with wide use for drug molecular design.
Figure 1: (i) Representative examples of biological active
benzo[5,6]cyclohepta[1,2-b]pyridine derivatives and (ii) Pyridine
substituted tuberculostatic agents.
Based on the biological significance of triazole and
benzocyclohepta[1,2-b]pyridine moieties we have made an
attempt to design amalgamation of three biologically versatile
heterocyclic scaffolds like benzosuberone, pyridine and 1,2,3-
triazole in a single molecular platform and synthesized new
molecules with better anti-TB properties as depicted in Fig. 2. In
In the recent years, the nitrogen containing heterocyclic
27,28
compounds have become potential targets for drug discovery
1,2,3-triazoles have been studied for over a century as an
important class of heterocyclic compounds and still continue to
1

continuation to our ongoing research programme,41-48 we
designed and synthesized series of novel
LiAlH4, THF, 0 °C; iv) a) Ts-Cl, dry DCM, DMAP, Et3N, 2 h,
rt; b) NaN3, DMF, 80 °C, 12 h.
a
benzo[6,7]cyclohepta[1,2-b]pyridine-1,2,3-triazole hybrids (7a-j
& 8a-j) in excellent yields by employing copper-catalyzed click
chemistry,49,50 and further explored their anti-mycobacterial
activity against Mycobacterium tuberculosis H37Rv. Compounds
7i and 8g exhibited very promising activity with MIC value of
1.56 µg/mL.
Scheme 1: Synthesis of 3-(azidomethyl)-2-methyl-6,7-dihydro-
5H-benzo [6,7] cyclohepta[1,2-b] pyridine.
To construct the desired analogues, 3-(azidomethyl)-2-
methyl-6,7-dihydro-5H-benzo[6,7]cyclohepta[1,2-b]pyridine
(5a) was further reacted with various aromatic and aliphatic
alkynes 6a-k using click chemistry (Scheme 2). For example,
compound 5a was reacted with alkyne 6a in presence of CuSO4-
5H2O and sodium ascorbate in THF and water (1:1, v/v) to give
2-methyl-3-((4-phenyl-1H-1,2,3-triazol-1-yl)methyl)-6,7-
dihydro-5H-benzo[6,7]cyclohepta[1,2-b]pyridine (7a) with 82%
yield. Under similar conditions, compounds 7a-j was synthesized
1
in high yields (80-88%) and fully characterized based on the H
and 13C NMR, IR and Mass spectral data.
Figure 2: Design strategy for new benzo[6,7]cyclohepta[1,2-
b]pyridine -1,2,3-triazole hybrids.
To begin with, substituted benzosuberone (1), the
required starting compounds were synthesized by using Friedel-
Crafts acylation of aromatic hydrocarbons with glutaric
anhydride furnishing arylbutyric acid which on Clemmensen
reduction followed by cyclization with excess polyphosphoric
acid gave substituted benzosuberones (1a-b). This substituted
benzosuberones undergo Vilsmeier-Haack-Arnold reaction by
treating with POCl3, dimethyl formamide at 0 °C to 60 °C for 4 h
to afford the (Z)-9-chloro-6,7-dihydro-5H-benzo [7] annulene-8-
carbaldehyde
(2a-b).
The
key
intermediate
benzo[6,7]cyclohepta[1,2-b]pyridine derivative 3a-b was
accomplished by Hantzch-type reaction via Michael addition by
treating with β-chloroacroleins, ethyl acetoacetate and NH4OAc
in presence of ethanol at reflux temperature for 8 h [51]. The
ester group of compound 3a-b was reduced by using lithium
aluminum hydride at 0 °C in dry THF under nitrogen atmosphere
to afford the corresponding alcohols 4a-b. The corresponding
alcoholic group was protected with TsCl and the subsequently
corresponding tosyl derivative was then reacted with sodium
azide in DMF at 80 °C for 12 h to afford its azides 5a-b in good
yield (Scheme 1) and confirmed by absorption of a strong band
at 2104 cm-1, attributing to the stretch vibrations of the N3 bond
of the azide group.
Reagents and conditions: i) CuSO4-5H2O, Sodium ascorbate,
THF:H2O (1:1), RT, 5-6 h.
Scheme 2: Synthesis of novel 2-methyl-6,7-dihydro-5H-
benzo[6,7]cyclohepta[1,2-b]pyridine triazole derivatives 7a-j &
8a-j.
The formation of compound 7a was evident from the ESI-MS
spectrum with the appearance of [M+H]+ peak at m/z 367, while
the FT-IR spectrum revealed the absence of stretching vibrations
of the azide group (2104 cm-1) and the presence of an absorption
band at 1734 cm-1 indicated the formation of triazole ring in 7a.
A singlet observed in the 1H NMR spectrum at δ 7.69 ppm
confirmed the presence of the triazolyl hydrogen, supported by
the signals in the 13C NMR spectrum at δ 119.4 ppm. The
proposed structure was substantiated by its mass spectra, which
exhibited molecular ion peak at m/z 367 [M+H]+.
All the synthesized compounds were screened for their in vitro
anti-tubercular activity against M. tuberculosis H37Rv (ATCC
27294) by using agar dilution method. The MIC was determined
for each compound which was measured as the minimum
concentration of compound required to completely inhibit the
Reagents and conditions: i) POCl3, DMF, 0 °C - 80 °C; ii)
Methyl acetoacetate (MAA), NH4OAc, EtOH, reflux, 8 h; iii)
2

bacterial growth. The MIC values (µg/mL) of all the synthesized
compounds (7a-j & 8a-j) and three standard anti-tubercular drugs
(Isoniazid, Rifampicin, Ethambutol) determined in triplicate at
pH 7.40 are presented in Fig. 3. Almost all synthesized
compounds showed anti-tubercular activity against MTB with
MICs ranged between 1.56 to 25 µg/mL. Among the tested
compounds 7i and 8g showed a potent anti-tubercular activity
with MIC value of 1.56 µg/mL, two compounds 7e and 8e
inhibited MTB with MIC value of 6.25 µg/mL and seven
compounds 7b, 7d, 7h, 7j, 8b, 8i, and 8j inhibited MTB with
MIC at 12.5 µg/mL. When compared with the first-line anti-
tubercular drug, Ethambutol (MIC 3.13 µg/mL) two compounds
7i and 8g were found to be more active (1.56 µg/mL) as
compared to the standard drug Ethambutol. All the other
compounds showed moderate activity when compared with other
anti-TB drugs, isoniazid (MIC 0.1 µg/mL) and rifampicin (MIC
0.2 µg/mL).
In conclusion, we developed
benzo[6,7]cyclohepta[1,2-b]pyridine-1,2,3-triazole
a
novel series of
hybrids
bearing interesting bioactive triazole scaffold using click
chemistry and evaluated their in vitro anti-mycobacterial activity
against Mycobacterium tuberculosis H37Rv (ATCC 27294).
Among the compounds tested, the compounds 7i and 8g
displayed most potent activity against M. tuberculosis H37Rv
with MIC value of 1.56 µg/mL with low cytotoxicity. These
findings suggest that these lead compounds would be worth
investigating in the near future for the mechanistic aspects.
Acknowledgements
The authors are gratefully acknowledged to the DST-
SERB/EMEQ-078/2013 for the financial support. YK and HR
thanks CSIR, New Delhi for research fellowships.
References:
1. Russell, D. G.; Barry, C. E.; Flynn, J. L.; Science. 2010,
328 , 852.
2. Russell, D. G. Nat. Rev. Microbiol. 2007, 5, 39.
3. Kaufmann, S. H. E.; McMichael, A. J. Nat. Med. 2005, 11,
578.
4. Manabe1, Y. C.; Bishai1, W. R. Nat. Med. 2000, 6, 1237.
5. Koul, A.; Arnoult, E.; Lounis, N.; Guillemont, J.; Andries,
K. Nature, 2011, 469, 483–490.
6. World Health Organization (2017b) Tuberculosis. Fact
sheet
Reviewed
October
2017.
http://www.who.int/mediacentre/factsheets/fs104/en/
(Retrieved 17 October 2017).
7. Prabowo, S. A.; Gröschel, M. I.; Schmidt, E .D.; Skrahina,
A.; Mihaescu, T.; Hastürk, S.; Mitrofanov, R.; Pimkina,
E.; Visontai, I.; De Jong, B.; Stanford, J. L.; Cardona,
P. J.; Kaufmann, S. H.; van der Werf, T. S.; Med.
Microbiol. Immunol. 2013, 202 95.
Figure 3: Anti-tubercular evaluation of novel analogues 7a-j & 8a-j against
M. tuberculosis H37RV.
8. Falzon, D.; WHO guidelines for the programmatic
management of drugresistant
tuberculosis: 2011 update,
The safety profile of anti-tubercular active
Eur. Respir. J. 2011, 38, 516.
benzo[6,7]cyclohepta[1,2-b]pyridine-1,2,3-triazole
derivatives
9. Zumla, A.; Nahid, P.; Cole, S. T.; Nat. Rev. Drug Discov.
2013, 12, 388.
10. Zumla, A.; Kim, P.; Maeurer, M.; Schito, M.; Lancet Infect
Dis. 2013 13, 285.
11. Palomino,J. C.; Martin, A.; Antimicrob. J.; Chemother.
2013, 68, 275.
12. Nguyen, L.; Jacobs, M. R.; Expert Rev. Anti Infect. Ther.
2012, 10, 959.
with MIC ≤12.5 µg/mL were assessed by testing the in vitro
cytotoxicity against Human Embryonic Kidney (HEK-293T)
cells using 3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyltetrazolium
bromide (MTT) assay. Fig. 4. describes the percentage Cell
Viability of twelve analogues at 50 μg/mL concentration. The
results clearly indicated that the potent analogues 7i and 8g did
not exhibit significant changes in viability (cytotoxicity) as
compared to vehicle (DMSO) treatment and can form as good
leads suitable for further mechanistic evaluation.
13. Nzila, A.; Ma, Z.; Chibale, K.; Future Med. Chem. 2011,
3, 1413.
14. Patel, N. B.; Agravat, S. N.; Shaikh, F. M.; Med. Chem.
Res. 2011, 20, 1033.
15. Patel, N. B.; Agravat, S. N.; Chem. Heterocycl. Compd.
2009, 45, 1343.
16. Srivastava, A.; Pandeya, S. N.; Int. J. Curr. Pharm. Rev.
Res. 2011, 4, 5.
17. Paronikyan, E. G.; Noravyan, A. S.; Dzhagatspany, I. A.;
Nazaryan, I. M.; Paronikyan, R. G.; Pharm. Chem. J.
2002, 36, 465.
18. Bernardino, A. M. R.; De Azevedo, A. R.; Pinheiro, L. C.
D.; Borges, J. C.; Carvalho, V. L.; Miranda, M. D.; De
Meneses, M. D. F.; Nascimento, M.; Ferreira, D.; Rebello,
M. A.; Med. Chem. Res. 2007, 16, 352.
Figure 4: Percentage cell viability of benzo[6,7]cyclohepta[1,2-b]pyridine-
1,2,3-triazole derivatives on HEK-293T cells at a concentration of 50μg/mL.
3

19. Tucker, T. J.; Sisko, J. T.; Tynebor, R. M.; Williams, T.
M.; Felock, P. J.; Flynn, J. A.; Lai, M.; Liang, Y.;
McGaughey, G.; Liu, M.; J. Med. Chem. 2008, 51, 6503.
20. Mamolo, M. G.; Zampieri, D.; Falagiani, V.;. Vio, L.;
Fermeglia, M.; Ferrone, M.; Pricl, S.; Banfi, E.; Scialino,
G.; ARKIVOC. 2004, 5, 231.
21. Wang, Y.; Wang, J.; Lin, Y.; Si-Ma, L. F.; Wang, D. H.;
Chen, L. G.; Liu, D. K.; Bioorg. Med. Chem. Lett. 2011
21, 4454.
22. Liu, G. Z.; Xu, H. W.; Chen, G. W.; Wang, P.; Wang, Y.
N.; Liu, H. M.; Yu, D. Q.; Bioorg. Med. Chem. 2010, 18
1626.
23. Soule, B. P.; Simone, N. L.; DeGraff, W. G.; Choudhuri,
R.; Cook, J. A.; Mitchell, J. B.; Radiat. Oncol. 2010 5, 8.
24. Chen, J. S.; Lin, S. Y.; Tso, W. L.; Yeh, G. C.; Lee, W.
S.; Tseng, H.; Chen, L. C.; Ho, Y.S.; Mol. Carcinog. 2006,
45, 461.
43. Nagarapu. L.; Satyendar, A.; Rajashaker, B.; Srinivas, K.;
Ruparani, P.; Radhika, K.; Shubhashini, G.; Bioorg. Med.
Chem. Lett. 2008, 18, 1167.
44. Nagarapu, L.; Vanaparthi, S.; Bantu, R.; Kumar, C. G.;
Eur. J. Med. Chem. 2013, 69,817.
45. Nagarapu, L.; Yadagiri, B.; Bantu, R.; Kumar, C. G.;
Pombala, S.; Nanubolu, J.; Eur. J. Med. Chem. 2014, 71,
91.
46. Yadagiri, B.; Holagunda, U. D.; Bantu, R.; Nagarapu, L.;
Kumar, C. G.; Pombala, S.; Sridhar, B.; Eur. J. Med.
Chem. 2014, 79, 260.
47. Nagarapu, L.; Vulupala, H. R.; Bantu, R.; Sajja, Y.;
Nanubolu, J. B.; Tetrahedron Assymetry. 2014, 578.
48. Sajja, Y.; Vulupala, H. R.; Bantu, R.; Nagarapu, L.;
Bioorg. Med. Chem. Lett. 2016, 26, 858;
49. Bollu, R.; Palem, J. D.; Bantu, R.; Nagarapu, L.; Eur. J.
Med. Chem. 2015, 89, 138.
25. Gelfand, E. W.; Appajosyula, S.; Meeves, S.; Curr. Med.
Res. Opin. 2004, 20, 73.
50. Sajja, Y.; Vanguru, S.; Jilla, L.; Vulupala, H. R.; Bantu, R.;
Nagarapu, L.; Bioorg. Med. Chem. Lett. 2016, 26, 4292.
26. Agrawal, D. K.; Clin. Exp. Allergy. 2004, 34, 1342.
27. Moorthouse, A. D.; Moses, J. E.; Chem. Med. Chem. 2008
3, 715.
28. Hou, J. L.; Liu, X. F.; Shen, J.; Zhao, G. L.; Wang, P. G.;
Expert. Opin. Drug Discov. 2012, 7,489.
Supplementary Information
Experimental section and Copies of the H, 13C NMR
and ESI-MS spectra for some of the important
compounds.
1
29. Zumla, A.; Kim, P.; Maeurer, M.; Schito, M.; Lancet
Infect Dis. 2013, 13, 285.
30. Palomino, J. C.; Martin, A.; J. Antimicrob. Chemother.
2013, 68, 275.
31. Duncan, K., Barry, C. E.; III Curr.Opin. Microbiol. 2004,
7, 460.
32. Lamichhane, G.; Trends Mol. Med. 2011, 17, 25.
33. Costa, M. S.; Boechat, N.; Rangel, E. A.; Silva, F. C.;
Souza, A. M. T.; Rodrigues, C. R.; Castro, H. C.; Junior, I.
N.; Lourenço, M. C. S.; Wardell, S. M. S. V.; Ferreira, V.
F.; Bioorg. Med. Chem. 2006, 14, 8644.
34. Maccari, R.; Ottaná, R.; Vigorita, M. G.; Bioorg. Med.
Chem. Lett. 2005, 15, 2509.
35. Kamal, A.; Ahmed, S. K.; Reddy, K. S.; Khan, M. N. A.;
Shetty, R. V. C. R. N. C.; Siddhardha, B.; Murthy, U. S.
N.; Khan, I. A.; Kumar, M.; Sharma, S.; Ram, A. B.;
Bioorg. Med. Chem. Lett. 2007, 17, 5419.
36. Nayyar, A.; Monga, V.; Malde, A.; Coutinho, E.; Jaina,
R.; Bioorg. Med. Chem. 2007, 15, 626.
37. Holla, B. S.; Mahalinga, M.; Karthikeyan, M. S.; Poojary,
B.; Akberali, P. M.; Kumari, N. S.; Eur. J. Med. Chem.
2005, 40, 1173.
38. Gill, C.; Jadhav, G.; Shaikh, M.; Kale, R.; Ghawalkar, A.;
Nagargoje, D.; Shiradkar, M.; Bioorg. Med. Chem. Lett.
2008, 18, 6244.
39. Singh, B. K.; Yadav, A. K.; Kumar, B.; Gaikwad, A.;
Sinha, S. K.; Chaturvedic, V.; Tripathia, R. P.; Carbohydr.
Res. 2008, 343,1153.
40. Gallardo, H.; Conte, G.; Bryk, F.; Lourenço, M. C. S.;
Costa, M. S.; Ferreira, V. F. J.; Braz. Chem. Soc. 2007
18,1285.
41. Nagarapu, L.; Gaikwad, H. K.; Sirikonda, K.; Mateti, J.;
Bantu, R.; Raghu, P. S.; Mahduri, K. M.; Kalivendi, S.
V.; Eur. J. Med. Chem. 2010, 45, 4720.
42. Nagarapu, L.; Paparaju, V.; Satyenar, A.; Biorg. Med.
Chem. Lett. 2008, 18, 2351.
4

Graphical Abstract
Leave this area blank for abstract info.
Design, Synthesis and in vitro Anti-tuberculosis activity
of Benzo[6,7]cyclohepta[1,2-b]pyridine-1,2,3-triazole
Derivatives.
Yasodakrishna Sajja, Sowmya Vanguru, Hanmanth Reddy Vulupala, Lingaiah Nagarapua*, Perumal Yogeeswarib,
Dharmarajan Sriramb .
Benzo[6,7]cyclohepta[1,2-b]pyridine -1,2,3- triazole hybrids (7a-j and 8a-j) were synthesised and screened against
Mycobacterium tuberculosis H37Rv. Preliminary results were promising with MIC values in the range 1.56-25 µg/mL.
5