Search of antitubercular activities in tetrahydroacridines: Synthesis and biological evaluation

Article abstract of DOI:10.1016/j.bmcl.2006.07.025

A series of 9-substituted tetrahydroacridines were synthesized by nucleophilic substitution of chloro group with different nucleophiles in 9-chlorotetrahydroacridine (2). The latter could be obtained by POCl₃ mediated cyclization of the interme

Full text of DOI:10.1016/j.bmcl.2006.07.025

Bioorganic & Medicinal Chemistry Letters 16 (2006) 5144–5147
Search of antitubercular activities in tetrahydroacridines:
Synthesis and biological evaluation
R. P. Tripathi,a,* S. S. Verma,a Jyoti Pandey,a K. C. Agarwal,a Vinita Chaturvedi,b
Y. K. Manju,b A. K. Srivastva,b A. Gaikwadc and S. Sinhac
aMedicinal and Process Chemistry Division, Central Drug Research Institute, Lucknow 226001, India
bMicrobiology Division, Central Drug Research Institute, Lucknow 226001, India
cDrug Target Discovery and Development Division, Central Drug Research Institute, Lucknow 226001, India
Received 17 April 2006; revised 4 July 2006; accepted 7 July 2006
Available online 25 July 2006
Abstract—A series of 9-substituted tetrahydroacridines were synthesized by nucleophilic substitution of chloro group with different
nucleophiles in 9-chlorotetrahydroacridine (2). The latter could be obtained by POCl3 mediated cyclization of the intermediate
enamine, which in turn, was prepared by acid catalyzed condensation of anthranilic acid and cyclohexanone. Most of the com-
pounds on antitubercular evaluation against M. tuberculosis H37 Rv and H37 Ra strains exhibited potent activities with MIC
6.125–0.78 lg/mL comparable to the standard drugs.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Tuberculosis (TB), caused by M. tuberculosis, is a major
public health and socioeconomical problem in most of
the developing countries.1–3 It kills more than two mil-
lion people every year worldwide in concurrence with
HIV related infections.4 About one-third of world’s
population is currently infected with this disease and
the number is increasing every year. The coincidence
of tuberculosis with the AIDS epidemic is an additional
problem.5 In cancer or other such disease where inten-
sive use of immunosuppressants is required, the TB
infection is aggravated and the death among such dis-
eases is attributed mainly to TB rather than other diseas-
es.6 INH, rifampicin, ethambutol, pyrazinamide, and
many other first and second line agents are known today
to treat tuberculosis. The increasing incidence of the
resistance to the currently available single and/or com-
bined treatment and the spread of epidemic infections
due to mycobacteria warrant the search of new active
compounds particularly prototype leads7,8 as novel ther-
apies based on different mechanism of action. Acridine
derivatives, atebrin and quinacrine (Fig. 1), have been
widely used in malaria chemotherapy during world
war-II in the absence of quinine9a and this skeleton is
still being explored for better antimalarials.9b The anti-
bacterial activities are also known to be associated with
many acridine analogues and detail SAR in this class
has been discussed recently.10 It has been established
that amino acridines having electronic conjugation be-
tween the ring nitrogen and the amino group are most
active antibacterial agents.11 It has also been document-
ed that acridines reduce antimycobacterial resistance.
However, there are only few reports about the use of
acridines as antimycobacterial agent.12,13
Although phenoxazines, phenothiazines, and acridines
are well-known pharmacophore for antitubercular activ-
ity, yet there is no report on the antimycobacterial
profile of tetrahydroacridines. One of the proven
mechanisms for biological action of acridine is their
intercalation to nucleotide base pairs in the helix. Very
recently, few acridines have been shown to inhibit
DNA-coiling enzyme (topoisomerases) rather than
DNA itself where the damage is caused by stabilization
of the enzyme–DNA cleavage complex.14 It was thought
that if one of the condensed benzene ring is replaced
with condensed tetrahydrobenzene the DNA intercala-
tion may be affected. Although 9-amino-1,2,3,4-tetrahy-
droacridines (tacrine) are effective inhibitor of
acetylcholine esterase useful in neurological disorders,15
there is only one report where it has been mentioned
that this class may be active against Bacillus subtilis16
Keywords: Tetrahydroacridines; Antitubercular agents; Tuberculosis.
*
Corresponding author. Tel.: +91 522 2612412x4462,4382; fax: +91
522 26123405; e-mail: rpt_56@yahoo.com
0960-894X/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.07.025

R. P. Tripathi et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5144–5147
NHSO2CH3
5145
H3CO
N
N
NH
NH
HN
OCH3
OCH3
Cl
N
Cl
N
N
mAMSA
Mepacrine
Azacrine
Figure 1. Few acridine derivatives used as drugs.
however, no activity is reported. Moreover, tetrahydro-
acridines have never been screened against M. tuberculosis.
In addition, there are numerous reports about their use
as fluorescent tags to mycobacterial DNA and to reduce
the development of resistance to antimycobacterial
agents.17,18 Keeping the above facts in view and in con-
tinuation of our efforts to develop new antitubercular
agents we were interested to synthesize 1,2,3,4-tetrahy-
droacridines having different substituents at C-9 and to
assess the effect of these compounds on antitubercular
profile (Schemes 1 and 2).
desired products. Thus, reaction of 9-chloroacridine
with different amines viz. n-hexyl amine, n-octyl amine,
n-dodecyl amine, benzyl amine, (3-chlorophenyl)-ethyl
amine, furylmethyl amine, anisidine, and morpholine
in phenol20 at 120–130 ꢁC furnished respective 1,2,3,4-
tetrahydro-9-aminoacridines (3–10) in very good yield.
However, reaction of tetrahydroacridine 2 with 1,5-,
1,7-, and 1,10-diamines led to the formation of respec-
tive N1,Nn-bis-(9-acridino)-diaminoalkanes (11–15) in
good yields. The structures of compounds were estab-
lished on the basis of spectroscopic data and analysis.21
The intermediate 9-chloroacridine (2) was prepared by
slight modification of the earlier reported method.19
Thus, anthranilic acid and cyclohexanone were con-
densed in presence of dil HCl to give the intermediate
enamine derivative (1), which on cyclization in presence
of POCl3 afforded 9-chloroacridine (2) in good yield.
The chloro group in chloroacridine intermediate 2 was
also replaced with substituted phenoxy group by its
reaction with different phenols viz. 4-chloro phenol,
2,4-dichloro phenol, 4-fluoro phenol, 3,4-dimethyl phe-
nol, and 2-methoxy-4-allyl phenol resulting in respective
9-O-phenyl-tetrahydroacridines (16–20) (Scheme 3) in
moderate to good yields. However, reaction of com-
pound 2 with thiophenol resulted in respective 9-
thiophenyl-tetrahydroacridine derivative (21).
The intermediate 9-chloroacridine (2) was reacted with
different amines and phenols to give the respective
Cl
O
O
Toluene
OH
OH
+
IR-120 Resin
Reflux, 5-6 hr
O
NH
NH2
N
1
2
Anthranilic acid
Cyclohexanone
Scheme 1. Synthesis of 2-chloroacridine.
N
R1
R2
N
HN
Phenol
n
2
R1R2NH
HN
N
3-10
-NR1R2
N
Compds
11-15
3.
4.
5.
6.
7.
8.
9.
10.
n-hexyl-NH-
n-octyl-NH-
11, n=3
n-dodecyl-NH-
morpholinyl
12, n=4
13, n=5
14, n=7
15, n=10
phenylmethylNH-
3-chlorophenylethylNH-
4-methoxy phenylNH-
2-hydroxy ethylNH-
Scheme 2. Synthesis of 9-amino tetrahydroacridines.

5146
R. P. Tripathi et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5144–5147
Y
OH
O
R
160-180 ο
C
Thiophenol
S
2
160-180ο C
N
N
16-20
16, Y=4-chloro
17, Y=2,4-dichloro
18, Y=4-fluoro
21
19, Y=3,4-dimethyl
20, Y=2-methoxy-4-allyl
Scheme 3. Synthesis of 9-(phenoxy and thiophenyl)-derivatives.
All the synthesized compounds were evaluated against
M. tuberculosis H37 Ra and M. tuberculosis H37 Rv
strains following earlier protocol.22,23 Out of all the 20
compounds screened for anti-TB activities against
M. tuberculosis H37 Ra and M. tuberculosis H37 Rv
strains, nine compounds 3–6, 10, 11, and 13–15 exhibit-
ed potent antitubercular activities with MIC varying
from 12.5 to 0.78 lg/mL. Other compounds of the series
were inactive as their MICs were above 12.5 lg/mL. A
closure look into structure–activity relationship of the
above compounds as evident from Table 1 reveals that
tetrahydroacridines having 9-amino substituents are
more active than those with thiophenyl or phenoxy sub-
stituents as their MIC values are >12.5 lg/mL. Further,
among the 9-aminoalkyl acridines, compound 4 with
eight-carbon chain length of N-alkyl substituent was
most active against the avirulent strain M. tuberculosis
H37 Ra with MIC of 1.56 lg/mL, while it has MIC of
3.125 lg/mL against the virulent strain M. tuberculosis
H37 Rv. At the same time compound 5 with a chain
of 12 carbon as the aminoalkyl substituent was the most
potent compound of the series with MIC 0.78 lg/mL
against H37 Rv strain, while it has MIC of 6.25 lg/mL
against H37 Ra strain. These results indicate that com-
pound 4 is more specific to avirulent strain H37 Ra,
while compound 5 is more specific to the virulent strain
H37 Rv. Instead of 9-aminoalkyl group, substitution
with aromatic ring did not offer any significant inhibi-
tion to the bacterial growth. Among the bis-acridinyl
diamino alkanes (11–15) compound 15 with 10-carbon
chain spacer exhibited MIC of 3.125 lg/mL that too
against avirulent strain, other compounds have MIC
value either 12.5 or >12.5 lg/mL indicating that incor-
poration of one more acridine unit in such molecule is
not beneficial. It is also evident from the activity data
that replacement of aminoalkyl group at C-9 either with
phenoxy (16–20) or thiophenyl (21) group results in
complete loss of activity indicating that N atom at C-9
is essential for displaying antitubercular activity.
In conclusion we have synthesized 9-substituted tetrahy-
droacridines as new series of antitubercular compounds
with potent activity. The compound 5 having MIC as
low as 0.78 lg/mL proved to be an excellent lead for fur-
ther optimization and development.
Table 1. In vitro antitubercular activities of synthesized
tetrahydroacridines
Compound MIC M. tuberculosis MIC M. tuberculosis logP
H37 Ra
H37 Rv
3
6.25
1.56
6.25
>25
25
6.25
3.12
5.29
6.12
7.79
3.60
4.94
5.78
5.05
2.69
6.28
6.74
7.15
7.99
9.24
5.81
6.37
5.41
6.23
6.18
5.82
Acknowledgments
4
5
0.78
We are thankful to Director, CDRI, Lucknow, for his
encouragement to this work. S.S.V. and J.P. are
thankful to UGC, New Delhi, and CSIR, New Delhi,
for JRF.
6
12.5
7
>12.5
>12.5
>12.5
12.5
8
25
9
>25
>25
25
10
11
12
13
14
15
16
17
18
19
20
21
12.5
>25
25
ND
Supplementary data
12.5
12.5
6.25
3.12
>25
>25
>25
>25
25
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.bmcl.
2006.07.025.
12.5
>12.5
>12.5
>12.5
>12.5
>12.5
>12.5
References and notes
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3.25 lg/mL against H37 Rv. ND, not done.

R. P. Tripathi et al. / Bioorg. Med. Chem. Lett. 16 (2006) 5144–5147
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yield, mp 242–245 ꢁC. To a cooled mixture of POCl3
(50 mL) and toluene (50 mL), the above enamine 1(10 g)
was slowly added with vigorous stirring. The reaction
mixture was brought to room temperature and stirring
continued for 3 h. Now the reaction mixture was heated to
80–100 ꢁC for additional 5 h with continuous stirring. The
reaction mixture was cooled, poured into ice containing
K2CO3 and the whole mixture was neutralized (pH 7) with
aq ammonia. The reaction mixture was extracted with
excess of toluene and the organic layer was evaporated
under reduced pressure to give compound 2 as light yellow
granules in 65% yield, mp 135–138 ꢁC. 1H NMR
(200 MHz, CDCl3): d 8.15, 7.97 (d, J = 8.4 Hz, each 1H,
H-5, H-8), 7.67, 7.54 (dd, J = 8.4 Hz, 1.2 Hz each 1H, H-6,
H-7), 3.11, 3.04 (m, 4H, H-1, H-4), 1.95 (m, 4H, H-2, H-3),
FABMS: 217(M+H)+; Anal. Calcd for C13H12NCl: C,
31.89; H,5.53; N, 6.45. Found: C, 31.74; H, 5.55; N,
6.41.
1,2,3,4-tetrahydro-9-(n-hexylamino)acridine (3). Typical pro-
cedure: A mixture of 9-chloroacridine 2 (1 g, 4.60 mmol),
phenol (0.90 g, 9.20 mmol), and hexyl amine (0.70 mL,
0.69 mmol) was magnetically stirred initially at 80 ꢁC for
3 h followed by 5 h at 130 ꢁC untill the disappearance of
compound 2. The reaction mixture was cooled to room
temperature and an excess of toluene was added. The
reaction mixture was washed with 10% aq NaOH followed
by saturated aq NaCl and finally with distilled water. The
organic layer was separated, dried (Na2SO4), and evapo-
rated to give crude material which was chromatographed
over basic Al2O3 using 15% ethyl acetate/hexane as eluent
to give 3 as colourless granules in 58% yield, mp 146–
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148 ꢁC. IR(KBr): 3361 cmÀ1
;
1H NMR(200 MHz,
CDCl3): d 7.97, 7.91 (d, J = 7.5 Hz, 2H, H-5, H-8), 7.55,
7.34 (dd, 7.5 Hz, 1.2 Hz, 4H, H-6, H-7), 4.00 (br s,
exchangeable 1H, NH), 3.48 (t, J = 4.8 Hz, 2H, CH2NH),
3.06, 2.71 (m, 4H, H-1, H-4), 1.92 (m, 4H, H-2, H-3), 1.65
(m, J = 4.6 Hz, 2H, CH2), 1.25–1.41(m, 6H, 3· CH2), 0.89
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A mixture of
anthranilic acid and cyclohexanone was refluxed in
toluene in presence of IR-120 resin with azeotropic
removal of water for 5 h. The reaction mixture was
filtered, the filtrate cooled, and the solid separated was
filtered and crystallized from ethanol to give 2-(cyclohex-
1-enylamino)benzoic acid (1) as colourless granules in 90%
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