Syntheses and biological evaluation of new triazole-spirochromone conjugates as inhibitors of Mycobacterium tuberculosis

Article abstract of DOI:10.1016/j.tetlet.2011.02.099

A series of novel 1,2,3-triazole fused spirochromone conjugates have been synthesized bearing both spirochromone moiety as well as a 1,2,3-triazole moiety. Some of the compounds have exhibited potential activity against Mycobacterium tuberculosis (virulen

Full text of DOI:10.1016/j.tetlet.2011.02.099

Tetrahedron Letters 52 (2011) 2387–2389
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Syntheses and biological evaluation of new triazole-spirochromone
conjugates as inhibitors of Mycobacterium tuberculosis
M. Muthukrishnan a, , M. Mujahid a, P. Yogeeswari b, D. Sriram b
⇑
a Division of Organic Chemistry, National Chemical Laboratory, Pune 411 008, India
b Tuberculosis Drug Discovery Laboratory, Pharmacy Group, Birla Institute of Technology & Science-Pilani, Hyderabad Campus, Hyderabad 500 078, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel 1,2,3-triazole fused spirochromone conjugates have been synthesized bearing both spi-
rochromone moiety as well as a 1,2,3-triazole moiety. Some of the compounds have exhibited potential
activity against Mycobacterium tuberculosis (virulent strain H37Rv). In particular 5e proved to be the most
Received 18 October 2010
Revised 21 February 2011
Accepted 23 February 2011
Available online 1 March 2011
potent derivative exhibiting MIC = 0.78 lg/mL.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Click chemistry
Spirochromones
Antitubercular activity
Mycobacterium tuberculosis
Tuberculosis (TB) is one of the deadly infectious diseases caused
by Mycobacterium spp. mainly Mycobacterium tuberculosis. This
notorious pathogen infects about one-third of the world’s popula-
tion and is responsible for approximately 2 million deaths world-
wide per year.1,2 Furthermore, the emergence of a drug resistant
microorganism responsible for TB, especially multidrug-resistant
one along with the lethal combination of TB and HIV-1 infection,
makes this disease one of the greatest global health challenges fac-
ing us today.3 In the last 50 years, only a few drugs have been ap-
proved by the Food and Drug Administration (FDA) to treat TB,
reflecting the inherent difficulties in discovery and clinical testing
of new agents.4 Therefore, the discovery and development of new
types of anti TB agents acting on novel drug targets are urgently
needed.
In recent years, there has been re-stimulated interest in nature’s
repertoire of structures and many strategies are being adapted to
exploit advantageously the features of natural products in the de-
sign of novel small-molecules for various biological applications,
especially in the area of drug discovery and chemical biology.5
One such a successful and effective approach to the construction
of natural product like small molecules is based upon the concept
of ‘privileged structures’.6 Chromone is recognized as a privileged
structural motif, observed in a plethora of natural products and
in various therapeutic agents.7 In this context, and in view of our
long-standing interest in the chemistry of privileged chromone
motif,8 in particular, the design and synthesis of novel natural
products like small molecules based on chromone motif for various
biological applications, we describe herein the synthesis of novel
triazole fused spirochromone conjugates as potential antimycobac-
terial agents. The interest in incorporation of 1,2,3-triazole moiety
stems from the advent of click chemistry protocol which has been
used in various applications such as drug discovery process and
chemical biology due to high yield, high selectivity, wide scope,
atom-economy and simple purification.9 Furthermore, these tria-
zole products can readily associate with the biological targets
through hydrogen bonding and dipole interactions.10
The synthetic strategy followed for the preparation of 1,2,3-
triazole fused spirochromone conjugates is given in Scheme 1.
Firstly, for the preparation of precursor spirochromanone moiety
2a,b a Kabbe condensation11 between the cyclohexanone or N-
Boc piperidone and 2,4-dihydroxy acetophenone was employed.
In the Kabbe condensation, the use of acetonitrile as solvent and
carrying out the reaction at 50 °C for 24 h in the presence of pyrrol-
idine as a base is optimum in order to obtain spirochromanone
2a,b in 72% and 75% yields.12 On the other hand, refluxing with tol-
uene or ethanol as a solvent using Dean–Stark apparatus, the con-
dition generally used in Kabbe condensation did not produce the
required products in good yield. Subsequently, the spirochroma-
none 2a,b were O-alkylated with epichlorohydrin in the presence
of K2CO3 in refluxing acetone gave epoxides 3a,b in 85% and 82%
yields. These epoxides 3a,b that were subjected to ring opening
with sodium azide in the presence of ammonium chloride afforded
the corresponding azides 4a,b in 88% and 85% yields. Finally, 1,2,3-
triazole moiety was incorporated through the 1,3-dipolar cycload-
dition of these azides 4a,b with various terminal alkynes to afford
⇑
Corresponding author. Tel.: +91 20 25902571; fax: +91 20 25902629.
E-mail address: m.muthukrishnan@ncl.res.in (M. Muthukrishnan).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.02.099

2388
M. Muthukrishnan et al. / Tetrahedron Letters 52 (2011) 2387–2389
O
X
X
HO
O
HO
OH
CH3
O
O
(i)
(ii)
O
O
1
O
3a X= CH2
(85 %)
2a X= CH2
(72 %)
3b X= N-Boc (82 %)
2b X= N-Boc (75 %)
N
N
N
OH
OH
X
X
R
O
O
O
N3
O
O
O
(iv)
(iii)
4a X= CH2
(88 %)
4b X= N-Boc (85%)
5 X= CH2
6 X=N-Boc
N
N
N
N
OH
N
N
OH
Boc
N
R
R
O
5
O
O
O
O
6
O
R
R
6a
6b
6c
6d
6e
n-Bu (71 %)
C6H5 (90 %)
(4-CH3) C6H4 (80 %)
(4-OCH3) C6H4 (88 %)
(4-nC5H11) C6H4 (83 %)
5a
5b
5c
5d
5e
n-Bu (65 %)
C6H5 (85 %)
(4-CH3) C6H4 (76 %)
(4-OCH3) C6H4 (90 %)
(4-nC5H11) C6H4 (75 %)
Scheme 1. Reagents and conditions: (i) cyclohexanone/N-Boc piperidone, pyrrolidine, acetonitrile, 50 °C, 24 h; (ii) epichlorohydrin, potassium carbonate, acetone, reflux,
12 h; (iii) sodium azide, ammonium chloride, methanol, reflux, 5 h; (iv) alkyne, CuSO4, sodium ascorbate, tbutanol/water (1:1), 60 °C, 12 h.
Table 1
Table 1 (continued)
In vitro antimycobacterial activity of the 1,2,3-triazole fused spirochromone
conjugates
Entry Compound
MIC (lg/
mL)
Entry Compound
MIC (lg/
mL)
N
N
Boc
OH
N
N
O
O
O
6
25.0
N
N
N
OH
O
O
O
1
12.5
6a
N
Boc
N
N
OH
5a
N
O
O
O
N
N
OH
7
12.5
6.25
25.0
N
O
O
O
2
3
4
6.25
3.13
12.5
6b
O
N
Boc
N
N
OH
5b
O
N
H3C
O
N
N
N
OH
8
H3C
O
O
O
6c
N
Boc
N
OH
5c
N
H3CO
N
O
O
O
N
N
OH
9
H3CO
N
O
O
6d
O
5d
N
Boc
N
N
OH
N
O
O
O
10
3.13
N
N
OH
N
O
O
5
0.78
6e
11
12
Rifampicin
Ethambutol
0.2
1.56
5e
O

M. Muthukrishnan et al. / Tetrahedron Letters 52 (2011) 2387–2389
2389
4. (a) O’Brien, R. J.; Nunn, P. P. Am. J. Respir. Crit. Care Med. 2001, 162, 1055; (b)
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Comprehensive Heterocyclic Chemistry; Katrizky, A. R., Rees, C. W., Eds.;
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2009, 50, 2643; (b) Muthukrishnan, M.; Singh, O. V. Synth. Commun. 2008, 38,
3875; (c) Muthukrishnan, M.; Patil, P. S.; More, S. V.; Joshi, R. A. Mendeleev
Commun. 2005, 100; (d) Singh, O. V.; Muthukrishnan, M.; Gopan, R. Synth.
Commun. 2005, 35, 2723; (e) Singh, O. V.; Muthukrishnan, M.; Sundaravadivelu,
M. Indian J. Chem., Sect. B 2005, 44, 2575.
9. (a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004;
(b) Meldel, M.; Tornoe, C. W. Chem. Rev. 2008, 108, 2952; (c) Tron, G. C.; Pirali,
T.; Billington, R. A.; Canonico, P. L.; Sorba, G.; Genazzani, A. A. Med. Res. Rev.
2008, 28, 278; (d) Tornoe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002,
67, 3057; (e) Yoon, J.; Ryu, J. S. Bioorg. Med. Chem. Lett. 2010, 20, 3930.
10. Kolb, H. C.; Sharpless, K. B. Drug Discovery Today 2003, 8, 1128.
11. Kabbe, H. J. Synthesis 1978, 886.
1,2,3-triazole fused spirochromones conjugates 5a–e, 6a–e in
65–90% yields with high purity.13 In this reaction, the copper(I)
catalyst was generated in situ by the reduction of copper(II)sulfate
with sodium ascorbate, as described by Sharpless.14 The structure
of all the new compounds 5a–e, 6a–e were confirmed by the 1H
NMR, 13C NMR and mass spectral data (Supplementary data)
All the new triazole fused spirochromone conjugates were
screened for their in vitro antimycobacterial activity against M.
tuberculosis H37Rv (ATCC27294) using an agar dilution method.15
The minimum inhibitory concentration (MIC;
lg/mL) was deter-
mined for each compound. The MIC is defined as the minimum
concentration of compound required to completely inhibit the bac-
terial growth. Rifampicin and ethambutol were used as reference
compounds. The MIC values of the synthesized compounds along
with the standard drugs for comparison were reported in Table 1.
Most of the compounds (5b–c, 5e, 6c, 6e) showed a significant
in vitro activity against M. tuberculosis, MIC in the range of 0.78–
6.25
lg/mL. Among them, compound 5e is found to be more active
having MIC 0.78
lg/mL among all the compounds screened and the
potency is better than first line antibacterial drug ethambutol.
Importantly, compound 5e represents a novel structural chemo-
type for which antitubercular properties have not been previously
noted. Preliminary structure–activity relationship of the triazole
fused spirochromone conjugates reveals that compounds possess-
ing cyclohexyl group at 2nd position of the chromone ring favor
better activity than piperidinyl moiety. Furthermore, aromatic sub-
stitution at 4th position of the triazole is favorable than alkyl
substitution.
In conclusion, a new class of 1,2,3-triazole fused spirochromone
conjugates have been synthesized. All the compounds were ob-
tained in good yield and were tested as new potential antitubercu-
lar agents. One of those compounds, 5e showed good activity
against MTB. Further optimization of this series is ongoing and will
be reported shortly.
12. Moskvina, V. S.; Garazd, Y. L.; Garazd, M. M.; Turov, A. V.; Khilya, V. P. Chem.
Heterocycl. Compd. 2007, 43, 421.
13. Typical procedure for the synthesis of compound 5e: To a stirred solution of azido
spirochromone 4a (0.23 g; 0.7 mmol) and 1-ethynyl-4-pentylbenzene (0.13 g;
0.77 mmol) in tbutanol (3 mL) was added sequentially copper sulphate
pentahydrate (0.035 g; 0.14 mmol), sodium ascorbate (0.028 g; 0.14 mmol)
and distilled water (3 mL). The resulting reaction mixture was stirred for 12 h
at 60 °C. After completion of the reaction (monitored by TLC), the reaction
mixture was diluted with CH2Cl2 (10 mL) and then washed with water
(2 Â 5 mL). The organic layer was dried (Na2SO4) and concentrated in vacuo.
The residue was purified by column chromatography over silica gel with
ethylacetate/hexanes (1:1) as eluent to furnish compound 5e as a colorless
solid, 0.264 g (75%). Other compounds were synthesized similarly, and the
spectroscopic data of selected compounds are as follows:
Analytical data for compound 5e: colorless solid; mp 156–57 °C; ; 1H NMR
(200 MHz, CDCl3): d 0.90 (t, J = 6.4 Hz, 3H),1.22–1.72 (m, 14H), 1.93–2.0 (m,
2H), 2.56–2.60 (m, 2H), 2.63 (s, 2H), 4.08–4.11 (br d, J = 4.6 Hz, 2H), 4.43–4.74
(m, 3H), 4.96 (d, J = 4.2 Hz, 1H), 6.43 (d, J = 2.3 Hz, 1H), 6.52–6.57 (dd, J = 8.6,
2.4 Hz, 1H), 7.14 (d, J = 8.4 Hz, 2H), 7.52 (d, J = 8.1 Hz, 2H), 7.77 (d, J = 9.0 Hz,
1H), 7.79 (s, 1H); 13C NMR (CDCl3): d 191.3, 164.7, 161.5, 147.4, 143.1, 128.8,
128.3, 127.3, 125.4, 121.1, 115.1, 109.2, 102.0, 80.5, 69.3, 68.4, 53.2, 47.8, 35.6,
34.7, 31.4, 31.0, 25.1, 22.5, 21.4, 14.0; ESI-MS: m/z 526 [M+Na]+.
Acknowledgment
M.M. is thankful to the Department of Science and Technology,
Govt. of India for generous funding of the project (Grant SR/FTP/
CS-25/2007).
Analytical data for compound 6c: colorless solid; mp 128–29 °C 1H NMR
(200 MHz, CDCl3): d 1.46 (s, 9H), 1.50–1.66 (m, 2H), 1.90–1.96 (m, 2H), 2.36 (s,
3H), 2.65 (s, 2H), 3.13–3.25 (m, 2H), 3.82–3.88 (m, 2H), 4.07–4.10 (br d,
J = 4.9 Hz, 2H), 4.45–4.73 (m, 4H), 6.44 (d, J = 2.3 Hz, 1H), 6.55–6.61 (dd, J = 8.9,
2.4 Hz, 1H), 7.16 (d, J = 8.2 Hz, 2H), 7.55 (d, J = 7.6 Hz, 2H), 7.79(d, J = 7.6 Hz,
1H), 7.81 (s, 1H); 13C NMR (CDCl3): d 190.2, 164.8, 160.9, 154.7, 147.5, 138.1,
129.5, 128.5, 127.2, 125.4, 121.1, 115.0, 109.7, 102.1, 79.8, 78.3, 69.4, 68.4, 60.4,
53.2, 47.6, 39.1, 34.0, 28.4, 21.3; ESI-MS: m/z 571 [M+Na]+.
Supplementary data
Supplementary data (experimental procedures, compound
characterization data and copies of 1H NMR, 13C NMR and HPLC
chromatograms of compounds 5b, 5e, 6a, 6d) associated with this
Letter can be found, in the online version, at doi:10.1016/
j.tetlet.2011.02.099.
Analytical data for compound 6d: colorless oil; 1H NMR (200 MHz, CDCl3): d 1.46
(s, 9H), 1.51–1.66 (m, 2H), 1.85–2.04 (m, 2H), 2.65 (s, 2H), 3.13–3.25 (m, 2H),
3.83 (s, 3H), 3.84–3.89 (m, 2H), 4.08–4.10 (br d, J = 4.2 Hz, 2H), 4.45–4.73 (m,
4H), 6.43 (d, J = 2.3 Hz, 1H), 6.56–6.61 (dd, J = 8.7, 2.2 Hz, 1H), 6.89 (d,
J = 9.3 Hz, 2H), 7.59 (d, J = 9.3 Hz, 2H), 7.76 (s, 1H), 7.80 (d, J = 8.7 Hz, 1H); 13C
NMR (CDCl3): d 190.1, 164.8, 160.9, 159.6, 154.7, 147.3, 128.5, 126.8, 122.7,
120.6, 115.1, 114.2, 109.7, 102.1, 79.8, 78.3, 69.3, 68.5, 55.3, 53.1, 47.6, 39.0,
34.0, 28.4; ESI-MS: m/z 587 [M+Na]+.
References and notes
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