Antitubercular agents. Part 2: New thiolactomycin analogues active against Mycobacterium tuberculosis

Article abstract of DOI:10.1016/j.bmcl.2005.01.084

Structurally modified analogues of naturally occurring antibiotic thiolactomycin, substituted at 4-position of the thiolactone ring have been prepared and evaluated for their antitubercular activity. Some of the compounds have exhibited potential activity against Mycobacterium tuberculosis.

Full text of DOI:10.1016/j.bmcl.2005.01.084

Bioorganic & Medicinal Chemistry Letters 15 (2005) 1927–1929
Antitubercular agents. Part 2: New thiolactomycin analogues
active against Mycobacterium tuberculosis
Ahmed Kamal,a,* Ahmad Ali Shaik,a Rakesh Sinha,b J. S. Yadava and
Sudarshan K. Arorab
aDivision of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500007, India
bLupin Laboratories Limited, Lupin Research Park, Pune 411042, India
Received 16 December 2004;accepted 29 January 2005
Abstract—Structurally modified analogues of naturally occurring antibiotic thiolactomycin, substituted at 4-position of the thiolac-
tone ring have been prepared and evaluated for their antitubercular activity. Some of the compounds have exhibited potential activ-
ity against Mycobacterium tuberculosis.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Mycobacterium tuberculosis, which causes tuberculosis,
is the greatest single infection responsible for mortality
worldwide, killing roughly two million people annually.1
Current estimates indicate that one-third of worldÕs popu-
lation is with latent M. tuberculosis.2 Despite the fact
that it is treatable and preventable, the disease has been
spreading at a steady rate over the past decade.3 The
emergence of bacterial resistance to the multidrug resis-
tant TB (MDRTB) of existing antitubercular agents has
become a significant concern for the effective treatment
of tuberculosis. The determination of whole-genome se-
quence of M. tuberculosis4 has pinpointed drug targets
that have potential for improved therapy.5
in E. coli.8 Crystal structure of KAS I (Fab B from
E. coli) bound with TLM reveals the essential enzyme–
ligand binding interactions and establish the existence
of hydrophobic and pantetheine binding pockets that
are both unoptimally filled.9 Previous studies reveal that
a number of TLM analogues with aliphatic side chains
at 5-position of thiolactone ring have shown significant
enhancement of activity against mycolate synthesis.10,11
Recently, a number of biphenyl based12 and acetylene
based13 TLM analogues have been synthesized that ex-
hibit significant activity against mtFabH fatty acid con-
densing enzyme. These results from earlier studies
illustrate the importance of side-chain structure in
TLM analogues. However, no efforts have been made
for preparing analogues by substituting the 4-position
of the thiolactone ring. Studies have also been carried
out in this laboratory for the development of antituber-
cular compounds based on other class of prototypes.14
Therefore, in the present investigation an attempt has
been made to prepare 4-substituted thiolactones and
interestingly some of these analogues have exhibited
promising in vitro antitubercular activity.
Thiolactomycin (TLM, 1) that has a thiolactone ring
system is an antibiotic obtained from the fermentation
broth of a strain of actinomycetes, which was identified
as species of Nocradia,6 is active against in vitro Gram
positive, Gram negative and anaerobic bacteria. Thio-
lactomycin is a unique molecule that exhibits selective
activity against only the dissociable type II fatty acid
synthesase (FAS) enzymes.7 TLM is a reversible inhibi-
tor7a of b-ketoacyl synthesase (KAS) of bacterial FAS
systems including KAS I-III and acetyl coenzyme A
(CoA): ACP transacylase activities in vitro and in vivo
Thiolactone ring 2 which is the key intermediate for the
preparation of such analogues has been synthesized by
employing the procedure of Wang and Salvino.15 This
method involves Claisen self ester condensation of
methyl propionate 3 using KH to yield b-keto ester 4.
The selective bromination of 4 followed by nucleophilic
substitution of bromine with thioacetic acid gives thio
Keywords: Thiolactomycin; Mycobacterium tuberculosis;Thiolactone.
*
Corresponding author. Tel.: +91 40 2719 3157;fax: +91 40 2719
3189;e-mail addresses: ahmedkamal@iict.res.in;ahmedkamal@
ins.iictnet.com
0960-894X/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.01.084

1928
A. Kamal et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1927–1929
S
S
S
O
O
O
Br
O
OH
OH
( )n
(5 R)-Thialactomycin (1)
2
7
S
S
S
O
O
O
H3C
O
N
O
S
O
N
O
N
O
( )n
( )n
10
H3CO
( )n
9
8
ester 6, and finally cyclization to afford the 4-hydroxy
thiolactone 2. The required analogues have been pre-
pared by the etherification of 4-hydroxy group of com-
pound 2 with alkyl dihalides to afford terminal bromo
substituted ether derivatives (7a–f). These bromo ether
derivatives have been reacted with methyl thioglycolate,
1-methyl piperazine and morpholine to yield 8a–f, 9a–f
and 10a–f, respectively, with different carbon chain
length spacers (Scheme 1).
spotted (3–5 lL/spot) on to the media plates. The plates
were sealed and incubated at 37 ꢁC for 3–4 weeks.
Minimum inhibitory concentration (MIC) was recorded
as the highest dilution of the compound(s) that
completely inhibited the growth.
Compound 8e having eight methylene spacers and methyl
thioglycolate as linker of thiolactone ring was the most
active compound with an MIC value of 1.0–4.0 lg/mL
against drug sensitive and resistant strain of M. tubercu-
losis. Compound 7f having 10 methylene spacers and
bromo group as linker also showed moderate activity
against M. tuberculosis. The other two compounds
showed poor (MIC > 16.0 lg/mL) activity against sensi-
tive and resistant strains of M. tuberculosis. None of the
compounds were found to have significant activity
against M. avium and M. intracellulare cultures (Table
1).
All 24 compounds (7a–f, 8a–f, 9a–f and 10a–f) were
evaluated for antimycobacterial activity against four dif-
ferent mycobacterium (M. tuberculosis ATCC27294, M.
avium ATCC 49601 and M. intracellulare ATCC 13950)
species at a concentration of 50 and 25 lg by diffusion
assay. Compounds 7f,16 8e,17 9a18 and 9c demonstrated
good to mild inhibition of the mycobacterium cultures.
The active compounds were then assayed for determina-
tion of minimum inhibitory concentration (MIC)
against a variety of M. tuberculosis clinical isolates (drug
sensitive and resistant) in agar dilution assay as per the
NCCLS-M24-T2 recommendations. Briefly, 10 serial
twofold dilutions of the compound/standard drug were
made in DMSO and incorporated into Middle-
brook7H10 agar medium. Individual M. tuberculosis
isolates at concentration of 1 · 107 CFU/mL were
In conclusion, a novel series of antimycobacterial com-
pounds have been designed and synthesized that demon-
strated significant (compound 8e) activity against drug
sensitive and resistant M. tuberculosis cultures. The anti-
mycobacterial activity of compound 8e was better than
isoniazid on drug resistant clinical isolates of M. tuber-
culosis. These findings clearly indicate that thiolactomycin
Br
OCH3
O
S
i
ii
O
iii
O
O
O
O
O
O
O
O
O
3
4
5
6
iv
S
S
S
O
O
O
vi
v
R
O
Br
O
HO
( )n
( )n
8-10
7a-f
2
O
S
N
N
OCH3
8a-f
9a-f
R =
R =
a
b
c
d
e
f
N
CH3
n
3
4
5
6
8
10
O
10a-f
R =
Scheme 1. Reagents and conditions: (i) anhydrous KH, THF;(ii) PyHBr 3, acetic acid;(iii) AcSH, EtN 3, CH2Cl2;(iv) KOH, H 2O–EtOH;(v)
Br(CH2)nBr, anhyd K2CO3, acetone, reflux;(vi) RH, anhyd K 2CO3, acetone, reflux.

A. Kamal et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1927–1929
Table 1. Antimycobacterial activity of thiolactone ring based analogues
1929
Thiolactone analogues
MIC (lg/mL)
M. tuberculosis clinical
isolates
M. tuberculosis H37Rv
ATCC 27294
M. avium
M. intracellulare
ATCC 49601
ATCC 13950
Sensitive
Resistant
7a–e
7f
>16.0
4.0
>16.0
4.0–16.0
>16.0
>16.0
4.0–16.0
>16.0
>16.0
>16.0
>16.0
8.0
>16.0
>16.0
>16.0
4.0
8a–d
8e
>16.0
1.0
1.0–4.0
>16.0
>16.0
1.0–4.0
>16.0
>16.0
8f
9a
>16.0
>16.0
>16.0
>16.0
>16.0
>16.0
0.25
>16.0
>16.0
>16.0
>16.0
>16.0
>16.0
>16.0
>16.0
>16.0
>16.0
>16.0
>16.0
>16.0
8.0
9b
9c
>16.0
>16.0
>16.0
>16.0
9d–f
10a–f
Isoniazid
>16.0
>16.0
0.125–0.25
>16.0
>16.0
8.0–>16.0
9. Price, A. C.;Choi, K. H.;Heath, R. J.;Li, Z.;White, S.;
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analogues containing bromo, methyl thioglycolate
and 1-methyl piperazine groups as linkers resulted in
good antimycobacterial activity whereas compounds
containing morpholine group as linker result in loss of
activity.
11. Douglas, J. D.;Morehouse, C.;Senior, S. J.;Phetsukri, B.;
Campbell, I. B.;Besra, G. S.;Minnikin, D. E. Microbio-
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(200 MHz;CDCl 3) d 1.3 (s, 3H), 1.2–1.5 (m, 10H), 1.6
(d, 3H, J = 6.6 Hz), 1.6–1.9 (m, 3H), 1.8 (s, 3H), 3.4 (t, 2H,
J = 6.6 Hz), 4–4.4 (m, 3H);MS (FAB) 365 [M ++1]. Anal.
Calcd for C16H27BrO2S: C, 52.89;H, 7.49. Found: C,
52.78;H, 7.44.
17. Selected spectral data for compound 8e: 1H NMR
(200 MHz;CDCl 3) d 1.2–1.5 (m, 11H), 1.6 (d, 3H,
J = 6.6 Hz), 1.9 (s, 3H), 2.5 (s, 3H), 3.2 (s, 2H), 3.7 (s,
3H) 4–4.4 (m, 3H);MS (FAB) 361 [M ++1]. Anal. Calcd
for C17H28O4S: C, 56.63;H, 7.83. Found: C, 52.54;H,
7.79.
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(200MHz;CDCl ) d 1.6 (d, 3H, J = 6.6 Hz), 1.7 (s, 3H),
3
1.8–2 (m, 2H), 2.2 (s, 3H), 2.4–2.6 (m, 10H) 4–4.4 (m, 3H);
MS (FAB) 285 [M++1]. Anal. Calcd for C14H24N2O2S: C,
59.12;H, 8.51. Found: C, 59.08;H, 8.48.