Synthesis and antitubercular activity of amino alcohol fused spirochromone conjugates

Article abstract of DOI:10.1016/j.bmcl.2012.12.073

A series of 21 new amino alcohol fused spirochromone conjugates have been synthesized, characterized with analytical data and evaluated their antimycobacterial activity against Mycobacterium tuberculosis (virulent strain H37Rv) in vitro. Some of the compounds exerted significant inhibition, in particular, compound 4f found to be the most potent derivative exhibiting MIC = 3.13 μg/mL.

Full text of DOI:10.1016/j.bmcl.2012.12.073

Bioorganic & Medicinal Chemistry Letters 23 (2013) 1416–1419
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and antitubercular activity of amino alcohol fused
spirochromone conjugates
M. Mujahid a, R.G. Gonnade b, P. Yogeeswari c, D. Sriram c, M. Muthukrishnan a,
⇑
a Division of Organic Chemistry, CSIR-National Chemical Laboratory, Pune 411 008, India
b Centre for Material Characterization, CSIR-National Chemical Laboratory, Pune 411 008, India
c Medicinal Chemistry & Antimycobacterial Research Laboratory, Pharmacy Group, Birla Institute of Technology & Science, Pilani, Hyderabad Campus, Jawahar Nagar,
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 21 new amino alcohol fused spirochromone conjugates have been synthesized, characterized
with analytical data and evaluated their antimycobacterial activity against Mycobacterium tuberculosis
(virulent strain H37Rv) in vitro. Some of the compounds exerted significant inhibition, in particular, com-
Received 14 September 2012
Revised 8 December 2012
Accepted 21 December 2012
Available online 5 January 2013
pound 4f found to be the most potent derivative exhibiting MIC = 3.13 lg/mL.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Amino alcohol
Spirochromones
Antitubercular activity
Mycobacterium tuberculosis
Tuberculosis is an infectious pulmonary disease caused by the
pathogenic species Mycobacterium tuberculosis (Mtb) which is
responsible for almost 8.7 million new infections and 1.4 million
casualties in 2011 alone.1 Further, the emergence of multidrug
resistant (MDR) and extensively drug resistant (XDR) Mtb strains
coupled with HIV co-infection making the disease even more chal-
lenging. Although, several compounds are currently in advanced
phases of clinical trials, for the last 40 years there is no new com-
pounds have been brought to the market for TB treatment. Consid-
ering the global impact of this devastating disease there is an
urgent need for the design and development of novel chemical
entities endowed with promising antimycobacterial activities.
As a privileged structure in drug discovery, the chromone
framework is ubiquitous in a wide variety of naturally occurring
and synthetic compounds that exhibit wide range of biological
activities.2 Consequently, interest in the isolation from natural re-
sources, synthesis of chromone derivatives and evaluation of their
biological activity with emphasis on their potential medicinal
applications has continued. In this context, and in view of our
long-standing interest in the chemistry of privileged chromone
motif,3 in particular, the design and synthesis of novel natural
products like small molecules based on chromone motif for various
biological applications, recently we reported that various spirochr-
omone derivatives possessing 1,2,3-triazole ring system can serve
as a lead for developing antitubercular agents.4 This result has
prompted us to take-up this spirochromone motif as an active
pharmacophore for further diversification to exploit its anti TB po-
tential. Towards this goal, it was decided to design and synthesise,
a series of novel amino alcohol annulated spirochromone conju-
gates (Fig. 1) by introduction of amino alcohol unit to the phenolic
–OH on the C-7 chromone ring and evaluate their anti TB proper-
ties. The interest in incorporation of amino alcohol moiety stems
from the fact that this motif is an essential component in many
antitubercular agents5 including a well known anti-Tb drug etham-
butol.6 Further, to the best of our knowledge, synthesis and anti-
mycobacterial activities of these amino alcohol annulated
spirochromone conjugates is unprecedented.
The synthetic strategy followed for the preparation of amino
alcohol fused spirochromone conjugates is given in Scheme 1.
Firstly, for the preparation of precursor spirochromanone moiety
2a–c a Kabbe condensation7 between various cycloalkanones and
2,4-dihydroxy acetophenone was employed. In the Kabbe conden-
sation, the use of acetonitrile as solvent and carrying out the
OH
H
B
N
O
O
O
R
R = alkyl, aryl
B = cyclopentyl, cyclohexyl, N-piperidinyl
⇑
Corresponding author. Tel.: +91 20 25902571; fax: +91 20 25902629.
E-mail address: m.muthukrishnan@ncl.res.in (M. Muthukrishnan).
Figure 1. Design of amino alcohol annulated spirochromone conjugates.
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.12.073

M. Mujahid et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1416–1419
1417
O
B
B
HO
O
O
O
O
HO
OH
(i)
(ii)
CH3
O
O
2a B = cyclopentyl (80%)
2b B = cyclohexyl (72%)
B = N-piperidinyl (75%)
3a B = cyclopentyl (88%)
3b B = cyclohexyl (85%)
3c
B = N-piperidinyl (82%)
1
2c
(iii)
OH
H
N
B
O
O
O
R
4
5
6
B = cyclopentyl
B = cyclohexyl
B = N-piperidinyl
OH
OH
OH
Boc
H
H
N
N
H
N
O
O
O
O
O
O
N
O
O
R
R
R
4
6
5
O
R
R
R
5a C6H5
(88%)
6a C6H5
(75%)
(85%)
(72%)
(35%)
4a
C6H5
(2-CH3) C6H4
(2-Cl) C6H4
(2-CH3) C6H4 (80%)
(2-OMe) C6H4 (82%)
(4-OMe) C6H4 (84%)
(4-iPr) C6H4 (85%)
(2-CH3) C6H4 (73%)
(2-OMe) C6H4 (80%)
(4-OMe) C6H4 (84%)
(4-iPr) C6H4 (80%)
5b
5c
5d
5e
6b
6c
6d
6e
4b
4c
4d
4e
4f
(2-iPr) C6H4
(2-OMe) C6H4
(3-F) C6H4
(67%)
(79%)
(56%)
(76%)
4g
(4-OMe) C6H4
(4-iPr) C6H4
(90%)
4h
4i
4j
(2-OH, 4-NO2) C6H3 (55%)
-CH2(4-OMe) C6H4 (38%)
morpholinyl
4k
(81%)
Scheme 1. Reagents and conditions: (i) cyclopentanone/cyclohexanone/N-Boc-piperidone, pyrrolidine, acetonitrile, 50 °C, 24 h; (ii) epichlorohydrin, K2CO3, acetone, reflux,
12 h; (iii) alkyl/aryl amine, LiBr, MeOH, rt, 12 h.
reaction at 50 °C for 24 h in the presence of pyrrolidine as a base is
optimum in order to obtain spirochromanone 2a–c in 72–80%
yields.8 On the other hand, refluxing with toluene or ethanol as a
solvent using DS apparatus, the condition generally used in Kabbe
condensation did not produce the required products in good yield.
Subsequently, the spirochromanone 2a–c were O-alkylated with
epichlorohydrin in the presence of K2CO3 in refluxing acetone gave
epoxides 3a–c in 82–88% yields. Finally, the amino alcohol moiety
was incorporated through nucleophilic ring opening of this spi-
rochromone epoxides 3a–c with various aromatic/aliphatic amines
to afford amino alcohol fused spirochromone conjugates 4–6, in
moderate to good yields. The structure of all the new products
4–6 (21 compounds) were confirmed by the IR, 1H NMR, 13C
NMR and mass spectral data (Supplementary data).9 In the IR spec-
trum (compound 4h as a representative example), a signal corre-
sponding to the chromanone carbonyl was observed at
1675 cmÀ1. The signal corresponding to the C-3 protons of chroma-
none skeleton was observed as a singlet at d 2.78 ppm in the 1H
NMR spectrum and the corresponding 13C resonance signal was
observed at d 47 ppm. In the 13C NMR spectrum, the spirocarbon
was discernible at d 90.4 ppm. Similarly, the characteristic signal
appeared as a multiplet at d 4.25–4.31 ppm was ascribable to the
Figure 2. ORTEP diagram of the compound 4h (thermal ellipsoids are drawn at 50%
probability level).
methine proton attached to secondary –OH. Conclusive evidence
for its structure was obtained from single-crystal X-ray analysis
(Fig. 2; Supplementary data).10
All the new aminoalcohol fused spirochromone conjugates
were screened for their in vitro antimycobacterial activity against
Mycobacterium tuberculosis H37Rv (ATCC27294) using an agar dilu-
tion method.11 The minimum inhibitory concentration (MIC;
lg/
mL) was determined for each compound. The MIC is defined as

1418
M. Mujahid et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1416–1419
the minimum concentration of compound required to completely
inhibit the bacterial growth. Rifampicin and ethambutol were used
as reference compounds. The MIC values of the synthesized com-
pounds along with the standard drugs for comparison were re-
ported in Table 1. Among the 21 synthesized compounds, seven
compounds (4c, 4d, 4f, 4h, 4i, 5b, 5e) were found to be active with
screened. Importantly, compound 4f represents a novel structural
chemotype for which antitubercular properties have not been pre-
viously noted. Preliminary structure activity relationship of the
aminoalcohol fused spirochromone conjugates reveals that com-
pounds possessing cycloalkyl group at 2nd position of the chro-
mone ring favors better activity than piperidinyl moiety. Among
cycloalkyl groups, not much difference in activity has been ob-
served between cyclopentyl and cyclohexyl groups. Only in few
MIC in the range of 3.13–6.25
l
g/mL. The compound 4f, is found to
g/mL among all the compounds
be more active having MIC 3.13
l
Table 1
In vitro antimycobacterial activity of the amino alcohol fused spirochromone conjugates
Entry
Compound
MIC (
lg/mL)
Sr.No
13
Compound
MIC (lg/mL)
OH
OH
CH3
H
H
N
O
O
O
N
O
O
O
1
25
6.25
5b
O
O
4a
OH
CH3
H
OMe
H
OH
N
O
O
O
N
2
3
4
5
6
7
8
9
12.5
14
15
16
17
18
19
20
21
12.5
5c
4b
OH
O
OH
Cl
H
H
N
O
O
O
O
O
N
6.25
6.25
12.5
MeO
4c
OH
5d
O
O
OH
H
H
N
O
O
N
O
O
6.25
5e
4d
OH
O
O
O
OH
OMe
H
H
N
NBoc
NBoc
NBoc
O
O
O
O
N
12.5
>25
4e
OH
6a
OH
O
O
O
CH3
H
H
F
O
O
O
N
N
3.13
25
25
4f
6b
OH
O
O
OH
OMe
H
H
N
O
O
O
N
25
MeO
4g
6c
O
OH
OH
H
H
N
NBoc
NBoc
O
O
O
O
O
N
6.25
6.25
>25
MeO
4h
OH
6d
O
OH
OH
H
H
O
O
O
O
N
N
12.5
O2N
4i
OH
6e
O
O
MeO
H
N
O
O
O
10
12.5
22
23
Rifampicin
Ethambutol
0.2
4j
OH
O
N
O
O
O
11
12
>25
1.56
4k
OH
H
N
O
O
O
12.5
5a

M. Mujahid et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1416–1419
1419
5. (a) Tewari, N.; Tiwari, V. K.; Tripathi, R. P.; Chaturvedi, V.; Srivastava, A.;
Srivastava, R.; Shukla, P. K.; Chaturvedi, A. K.; Gaikwad, A.; Sinha, S.; Srivastava,
B. S. Bioorg. Med. Chem. Lett. 2004, 14, 329; (b) Gomes, C. R. B.; Moreth, M.;
Cardinot, D.; Kopke, V.; Cunico, W.; Lourenco, M. C. S.; De Souza, M. V. N. Chem.
Biol. Drug Des. 2011, 78, 1031; (c) Cunico, W.; Gomes, C. R. B.; Ferreira, M. L. G.;
Ferreira, T. G.; Cardinot, D.; De Souza, M. V. N.; Lourenco, M. C. S. Eur. J. Med.
Chem. 2011, 46, 974.
6. (a) Wilkinson, R. G.; Shepherd, R. G.; Thomas, J. P.; Baughn, C. J. Am. Chem. Soc.
1961, 83, 2212; (b) Takayama, K.; Armstrong, E. L.; Kunugi, K. A.; Kilburn, J. O.
Antimicrob. Agents Chemother. 1979, 16, 240.
cases (5a, 5b, 5d) cyclohexyl group favors better activity. Further-
more, the halide substitution at aromatic ring of amino alcohol fa-
vors better activity (4c, 4f). In addition, it is also observed that,
isopropyl group in the aromatic ring is very favorable in enhancing
the activity (4d, 4h, 5e).
In conclusion, a series of amino alcohol fused spirochromone
conjugates were synthesized for the first time via an easy and con-
venient synthetic procedure starting from 2,4-dihydroxy acetophe-
none and all these new compounds were confirmed by 1H NMR,
13C NMR and MS spectra. The single X-ray diffraction study was
used to confirm the molecular structure of a representative com-
pound 4f unambiguously. The in vitro antimycobacterial evalua-
tion showed that most of the synthesized amino alcohol fused
spirochromone conjugates exhibited moderate to good antimyco-
bacterial activity. Noticeably, compound 4f is most potent com-
7. Kabbe, H. J. Synthesis 1978, 886.
8. Moskvina, V. S.; Garazd, Y. L.; Garazd, M. M.; Turov, A. V.; Khilya, V. P. Chem.
Heterocycl. Compd. 2007, 43, 421.
9. General procedure for the synthesis of compound 4–6: To a stirred solution of
epoxide 3a/3b/3c (0.5 mmol) and LiBr (0.1 mmol) in methanol (3 mL) was
added an appropriate amine (0.55 mmol) and the resulting reaction mixture
was stirred at rt for 12 h. After completion of the reaction (monitored by TLC),
solvent was removed in vacuo and the reaction mixture was diluted with ethyl
acetate (20 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 (ethyl acetate/petroleum ether 3:7 (v/v))
afforded pure product 4–6. Spectroscopic data of selected compounds are as
follows:
pound in vitro with MIC of 3.13 lg/mL, against MTB. These
findings demonstrated that amino alcohol fused spirochromone
conjugates have biological significance; further optimization of
this series as well as preparation of chiral isomers is ongoing in
our laboratory.
Analytical data for compound 4h: Colorless solid; mp 117–18 °C; IR (CHCl3,
cmÀ1): mmax 3414, 2935, 1608, 1441; 1H NMR (200 MHz, CDCl3): dH = 1.22 (d,
J = 6.7 Hz, 6H), 1.60–1.83 (m, 6H), 2.02–2.10 (m, 2H), 2.77 (s, 2H), 2.81–2.88 (m,
1H), 3.32 (dd, J = 12.8, 6.9 Hz, 1H), 3.39 (dd, J = 12.8, 4.0 Hz, 1H), 4.06–4.14 (m,
2H), 4.25–4.30 (m, 1H), 6.30 (d, J = 1.3 Hz, 1H), 6.52–6.65 (m, 3H), 7.08 (d,
J = 8.0, 2H), 7.82 (d, J = 8.9 Hz, 1H); l3C NMR (50 MHz, CDCl3): dC 191.4 (CO),
164.9 (C), 162.3 (C), 145.9 (C), 138.8 (C), 128.5 (CH), 127.2 (CH, 2 carbons),
115.2 (C), 113.4 (CH, 2 carbons), 109.4 (CH), 102.2 (CH), 90.5 (C), 70.4 (CH2),
68.5 (CH), 46.9 (CH2), 46.7 (CH2), 37.5 (CH2, 2 carbons), 33.1 (CH), 24.2 (CH3, 2
carbons), 23.8 (CH2, 2 carbons); MS: m/z 410 [M+H]+, 432 [M+Na]+.
Acknowledgments
M.Mujahid thanks CSIR, New Delhi for a research fellowship.
Financial support from the CSIR network projects (OSDD, NAPAHA
& ORIGIN) are gratefully acknowledged.
Analytical data for compound 5a: Yellow oil; IR (CHCl3, cmÀ1):
mmax 3434, 2935,
1605, 1505, 1443, 1271; 1H NMR (200 MHz, CDCl3): dH = 1.33–1.72 (m, 8H),
1.94–2.00 (m, 2H), 2.64 (s, 2H), 3.30 (dd, J = 13.1, 7.0 Hz, 1H), 3.42 (dd, J = 13.1,
4.2 Hz, 1H), 4.07–4.13 (m, 2H), 4.22–4.33 (m, 1H), 6.44 (d, J = 2.2 Hz, 1H), 6.56
(dd, J = 8.6, 2.4 Hz, 1H), 6.65–6.78 (m, 3H), 7.15–7.26 (m, 2H), 7.80 (d, J = 8.6 Hz,
1H)); l3C NMR (50 MHz, CDCl3): dC = 191.4 (CO), 165.0 (C), 161.6 (C), 147.9 (C),
129.3 (CH, 2 carbons), 128.3 (CH), 118.1 (CH), 115.0 (C), 113.3 (CH, 2 carbons),
109.3 (CH), 102.0 (CH), 80.5 (C), 70.3 (CH2), 68.5 (CH), 47.8 (CH2), 46.5 (CH2),
34.8 (CH2, 2 carbons), 25.1 (CH2), 21.5 (CH2, 2 carbons); MS: m/z 382 [M+H]+,
404 [M+Na]+.
Supplementary data
Supplementary data (experimental procedures, compound
characterization data and copies of 1H NMR, 13C NMR and mass
spectra of selected compounds) associated with this article can
be found, in the online version, at http://dx.doi.org/10.1016/
j.bmcl.2012.12.073.
Analytical data for compound 6b: Colorless oil; IR (CHCl3, cmÀ1): mmax 3447,
1674, 1607, 1541 1522, 1426; 1H NMR (200 MHz, CDCl3): dH = 1.46 (s, 9H),
1.59–1.67 (m, 2H), 1.98–2.05 (m, 2H), 2.18 (s, 3H), 2.66 (s, 2H), 3.14–3.26 (m,
2H), 3.35 (dd, J = 13.0, 6.9 Hz, 1H), 3.47 (dd, J = 13.0, 4.2 Hz, 1H), 3.83–3.90 (m,
2H), 4.11–4.14 (m, 2H), 4.28–4.38 (m, 1H), 6.47 (d, J = 2.1 Hz, 1H), 6.58 (dd,
J = 8.8, 2.3 Hz, 1H), 6.65–6.74 (m, 2H), 7.06–7.17 (m, 2H), 7.84 (d, J = 8.9 Hz,
1H)); l3C NMR (50 MHz, CDCl3): dC = 190.1 (CO), 165.1 (C), 160.9 (C), 154.7 (C),
145.8 (C), 130.3 (CH), 128.5 (CH), 127.2 (CH), 122.7 (C), 117.8 (CH), 115.0 (C),
110.1 (CH), 109.8 (CH), 102.1 (CH), 79.9 (C), 78.3 (C), 70.6 (CH2), 68.4 (CH), 47.7
References and notes
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(CH2, 2 carbons), 46.4 (CH2, 2 carbons), 34.0 (CH2, 2 carbons), 28.4 (CH3,
3
carbons), 17.5 (CH3); MS: m/z 497 [M+H]+, 519 [M+Na]+.
10. The crystallographic data of compound 4h has been deposited with the
Cambridge Crystallographic Data Center as deposition No. CCDC 895296.
Copies of the data can be obtained, free of charge, on application to the CCDC,
12 Union Road, Cambridge CB2 1EZ, UK [fax: +44 (1223) 336033; e-mail:
deposit@ccdc.cam.ac.uk].
11. NCCLS-National
Committee
for
Clinical
Laboratory
Standards,
Antimycobacterial susceptibility testing for Mycobacterium tuberculosis.
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