5-Benzylidenerhodanine and 5-benzylidene-2-4-thiazolidinedione based antibacterials

Article abstract of DOI:10.1016/j.bmcl.2012.02.100

Herein we outline the antibacterial activity of amino acid containing thiazolidinediones and rhodanines against Gram-positive bacteria Staphylococcus aureus ATCC 31890, Staphylococcus epidermidis and Bacillus subtilis ATCC 6633. The rhodanine derivatives

Full text of DOI:10.1016/j.bmcl.2012.02.100

Bioorganic & Medicinal Chemistry Letters 22 (2012) 2720–2722
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
5-Benzylidenerhodanine and 5-benzylidene-2-4-thiazolidinedione
based antibacterials
Ondrej Zvarec a, Steven W. Polyak b, William Tieu a, Kevin Kuan a, Huanqin Dai c, Daniel Sejer Pedersen a,
,
Renato Morona b, Lixin Zhang c, Grant W. Booker b, Andrew D. Abell a,
⇑
a Department of Chemistry, The University of Adelaide, South Australia 5005, Australia
b School of Molecular and Biomedical Science, The University of Adelaide, South Australia 5005, Australia
c Institute of Microbiology, Chinese Academy of Science, Beijing 100190, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein we outline the antibacterial activity of amino acid containing thiazolidinediones and rhodanines
against Gram-positive bacteria Staphylococcus aureus ATCC 31890, Staphylococcus epidermidis and Bacillus
subtilis ATCC 6633. The rhodanine derivatives were generally more active than the analogous thiazolid-
inediones. Compounds of series 5 showed some selectivity for Bacillus subtilis ATCC 6633, the extent of
which is enhanced by the inclusion of a non-polar amino acid at the 5-position of the core thiazolidined-
iones and rhodanines scaffolds. SAR data of series 8 demonstrated improved activity against the clinically
more significant Staphylococci with selectivity over Bacillus subtilis ATCC 6633 induced by introduction of
a bulky aryl substituent at the 5-position of the core scaffolds.
Received 4 February 2012
Revised 25 February 2012
Accepted 28 February 2012
Available online 6 March 2012
Keywords:
Privileged scaffold
Thiazolidinedione
Rhodanines
Ó 2012 Elsevier Ltd. All rights reserved.
Antibacterial
Staphylococcus aureus
Thiazolidinediones and rhodanines of type 1 ( Fig. 1) are an
important and versatile class of compounds that show a wide vari-
ety of biological activities.1–9 For example, compounds possessing
these scaffolds have recently been shown by us to inhibit choles-
terol esterase.3 In addition, examples are known to have anti-
fungal,5 antibacterial,6,7 antiviral,8 antitumor,9 and antidiabetic
potential.10 Antibacterial activity is of particular importance given
the dramatic rise of drug-resistant bacteria and the paucity of new
agents currently in development.11–14 One report on ben-
zylidenethiazolidinediones identifies the key requirements for
antibacterial activity to be an NH at the 3-position, a heteroatom
at the 1-position and a substituted phenyl group at the 5-posi-
tion.15 This is typified by structure 2a (Fig. 1) that is reported to
show activity against clinically important Staphylococcus aureus.15
Herein, we report a study to extend this new class of antibiotic.
Our aim was to study and develop a more chemically versatile
and diverse series of thiazolidinediones and rhodanines that con-
tain an amino acid attached to the aryl substituent to explore a
wider structural space,3 with the generation of associated SAR
data.
With this in mind we prepared thiazolidinediones and rhoda-
nines 5a–k by simple amination of 3a and 3b with an N-protected
amino acid in the presence of EDCI and HOBt according to our ear-
lier report3 (Scheme 1).16,17 We also prepared the literature deriv-
ative 2a15 and the new rhodanine analogue 2b for comparative
activity studies. Both thiazolidinedione and rhodanine derivatives
were prepared and assayed to more fully investigate this series.
An N-terminal propylphenyl group was incorporated into 5a–k to
allow direct comparison to 2.
Our series of amino acid containing thiazolidinediones and rho-
danines (5a–k) have clear advantages over the earlier literature
structures of type 2 due to (i) ease of synthesis; (ii) an opportunity
to introduce a variety of optically active amino acids and or related
groups; and (iii) the constituent amino acid providing a potential
OH
X
X
O
O
S
S
R
NH
NH
O
O
1
2a: X = O
2b: X = S
⇑
Corresponding author. Tel.: +61 8 8303 5652; fax: +61 8 8303 4358.
Rhodanine X = S
Thiazolidinedione X = O
E-mail address: andrew.abell@adelaide.edu.au (A.D. Abell).
Current address: Department of Medicinal Chemistry, University of Copenhagen,
Universitetsparken 2, Copenhagen, Denmark.
Figure 1. Thiazolidinedione and rhodanine structures.
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2012.02.100

O. Zvarec et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2720–2722
2721
O
X
X
X
4
S
HO
S
a
S
O
NH
R
NH
NH
+
a
R
O
O
O
7
3a: X = S
3b: X = O
R
O
6
8a: R = Ph, X = S
X
H
N
8b: R = Ph, X = O
N
H
S
8c: R = 2-Furanyl, X = S
8d: R = 3-Pyridyl, X = O
8e: R = 3-Pyridyl, X = S
8f: R = 2-Napthyl, X = S
NH
O
R
O
H
N
5a: R =CH2OH, X = O
5b: R = CH2OH X = S
5c: R = CH2PhOH, X = S
5d: R = CH2O-t-Bu, X = S
5e: R = CH2OBn, X = S
5f: R = CH3, X = O
NH2
Scheme 2. Reagents and conditions: (a) Piperidine, ethanol, 80 °C.
O
4a: R = CH2OH
4b: R = CH2O-t-Bu
4c: R = CH2OBn
4d: R = CH3
4e: R = CH2CH(CH3)2
4f: R = CH2Ph
Table 2
Minimum inhibitory concentrations of simple 2,4-thiazolidinediones and rhodanines
5g: R = CH3, X = S
5h: R = CH2CH(CH3)2, X = O
5i: R = CH2CH(CH3)2, X = S
5j: R = CH2Ph, X = O
Compds
R
X
MIC (lg/mL)
S. aureus
S. epidermis
B. subtilis
4g: R = CH2PhOH
5k: R = CH2Ph, X = S
3a
3b
8a
8b
8c
8d
8e
8f
4-Carboxy phenyl
4-Carboxy phenyl
Ph
O
S
O
S
S
O
S
>64
>64
>64
2
>64
>64
>64
4
16–32
>64
>64
0.5
>64
>64
>64
16
>64
>64
>64
>64
Scheme 1. Reagents and conditions: (a) EDCI, HOBt, DMF, (iPr)2NEt, rt.
Ph
2-Furanyl
3-Pyridyl
3-Pyridyl
2-Naphthyl
16–32
>64
>64
0.5
hydrogen bond donor and acceptor, which literature suggests is
important for activity.15
S
The minimum inhibitory concentrations (MICs) were deter-
mined for all the prepared compounds against three Gram-positive
bacteria; S. aureus ATCC 31890, S. epidermidis and Bacillus subtilis
ATCC 6633, as per NCCLS protocol18 using Mueller Hinton broth
(Becton Dockson, USA). Contrary to literature,15 the thiazolidined-
ione derivative 2a was only active against B. subtilis in our assays
that used a slightly different strain of S. aureus, see Table 1. Inter-
estingly, the previously untested rhodanine analogue 2b was active
against all three bacteria. A polar amino acid within our new series
(5) is not tolerated, with the (S)-serine containing thiazolidinedi-
one 5a and rhodanine 5b derivatives being inactive against all
three bacteria. This observation is consistent with the (S)-tyrosine
analogue 5c also being inactive against S. aureus and S. epidermis.
However, this derivative did show some activity against B. subtilis,
as did the benzyl ether protected analogue of 5b, see 5e. Signifi-
cantly, the tert-butyl protected rhodanine 5d was activate against
B. subtilis, S. aureus and S. epidermidis.
The introduction of a non-polar amino acid, for example (S)-ala-
nine or (S)-leucine, gave B. subtilis selective compounds in both the
rhodanine and thiazolidinedione series (see derivatives 5g/5h and
5i). Increasing steric bulk with the inclusion of an (S)-phenylala-
nine did not further improve potency (see 5j/5k), with the rhoda-
nine derivative 5k being devoid of activity in this case. With the
exception of 5d, all the bioactive examples of 5 are B. subtilis
selective.
We next investigated the antibacterial activity of the thiazolid-
inedione and rhodanines containing synthetic intermediates 3a,b
and also the aryl analogues 8a,b and f, and the heterocyclic ana-
logues 8c–e in order to gain further SAR data on this series. These
derivatives were synthesized in good yield via Knoevenagel con-
densation of the relevant aldehydes with 7 as outlined in Scheme
2.17
The MICs of 3a,b and 8a–f against S. aureus, S. epidermidis and B.
subtilis were determined as shown in Table 2. Interestingly, deriv-
atives 8b, 8c and 8f were more potent than 5 against the clinically
important Staphylococci, with these compounds being the first
examples to show selectivity for this bacterium over B. subtilis.
Interestingly the rhodanine 8b displayed broad activity against
all three bacteria, with MIC values less than or equal to 16 lg/mL
in all cases. The corresponding thiazolidinedione 8a was inactive,
which is consistent with our earlier observation that rhodanines
are generally more active than the corresponding thiazolidinedi-
ones (see Table 1). The presence of a carboxyl group on the aryl
substituent, as in the thiazolidinedione 3a and rhodanine 3b is
not tolerated, see Table 2. A
dated at this position, with the 2-furanyl rhodanine 8c being active
against S. aureus and S. epidermidis. A -deficient 3-pyridyl group is
p-excessive heterocycle is accommo-
p
however not tolerated, with the thiazolidinedione 8d and rhoda-
nine 8e both being devoid of activity. Interestingly, the introduc-
tion of a large naphthyl group (see 8f) resulted in good potency
against S. aureus and S. epidermidis, but not B. subtilis.
Table 1
Minimum inhibitory concentrations of 2,4-thiazolidinediones and rhodanines
Further microbiology testing was performed to better assess the
spectrum of activity and also a possible mechanism of action for
this class. All compounds were assayed against Gram-negative
Escherichia coli K12 and an efflux deficient tolC mutant strain19 to
define antibacterial spectrum. The more promising entries were
also tested against Pseudomonas aeruginosa PAO1, Mycobacterium
tuberculosis H37Rv and BCG and the pathogenic fungi Candida albi-
cans SC5314. All compounds were inactive against Gram-negative
E. coli, suggesting a narrow antibacterial spectrum against Gram-
positive species. Noteworthy exceptions include 8f, which was ac-
Compds
R
X
MIC (lg/mL)
S. aureus
S. epidermis
B. subtilis
2a
2b
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
—
—
O
S
O
S
S
S
S
O
S
O
S
O
S
>64
16
>64
32
8–16
16
>64
>64
8
CH2OH
CH2OH
CH2C6H5OH
CH2O-t-Bu
CH2OBn
CH3
>64
>64
>64
16–32
>64
>64
>64
>64
>64
>64
>64
>64
>64
>64
16–32
>64
>64
>64
>64
>64
>64
>64
8
8
>64
32
8
32
8
CH3
tive against efflux-deficient E. coli (MIC 8
lg/mL) but not wildtype
CH2CH(CH3)2
CH2CH(CH3)2
CH2Ph
K12, and 8e with anti-Candida activity (MIC 10
lg/mL). An assay
addressing S. aureus cell viability in the initial phase of growth
was performed with compounds 2b and 8b. The numbers of live
CH2Ph
>64

2722
O. Zvarec et al. / Bioorg. Med. Chem. Lett. 22 (2012) 2720–2722
References and notes
1. Singh, S. P.; Parmar, S. S.; Raman, K.; Stenberg, V. I. Chem. Rev. 1981, 81, 175.
2. Lesyk, R. B.; Zimenkovsky, B. S. Curr. Org. Chem. 2004, 8, 1547.
3. Heng, S.; Tieu, W.; Hautmann, S.; Kuan, K.; Sejer Pedersen, D.; Pietsch, M.;
Gütschow, M.; Abell, A. D. Bioorg. Med. Chem. 2011, 19, 7453.
4. Tomasic, T.; Masic, L. P. Curr. Med. Chem. 2009, 16, 1596.
5. Sortino, M.; Delgado, P.; Jua´rez, S.; Quiroga, J.; Abon´ıa, R.; Insuasty, B.;
Nogueras, M.; Rodero, L.; Garibotto, F. M.; Enriz, R. D.; Zacchino, S. A. Bioorg.
Med. Chem. 2007, 15, 484.
6. Tomasic, T.; Zidar, N.; Mueller-Premru, M.; Kikelj, D.; Masic, L. P. Eur. J. Med.
Chem. 2010, 45, 1667.
7. Sim, M. M.; Ng, S. B.; Buss, A. D.; Crasta, S. C.; Goh, K. L.; Lee, S. K. Bioorg. Med.
Chem. Lett. 2002, 12, 697.
8. Nagahara, K.; Anderson, J. D.; Kini, G. D.; Dalley, N. K.; Larson, S. B.; Smee, D. F.;
Jin, A.; Sharma, B. S.; Jolley, W. B. J. Med. Chem. 1990, 33, 407.
9. Russell, A. J.; Westwood, I. M.; Crawford, M. H. J.; Robinson, J.; Kawamura, A.;
Redfield, C.; Laurieri, N.; Lowe, E. D.; Davies, S. G.; Sim, E. Bioorg. Med. Chem.
2009, 17, 905.
10. Murata, M.; Fujitani, B.; Mizuta, H. Eur. J. Med. Chem. 1999, 1061.
11. Deleo, F. R.; Otto, B. N.; Chambers, H. F. Lancet 2010, 375, 1557.
12. Fischbach, M. A.; Walsh, C. T. Science 2009, 1089, 325.
Figure 2. Time course of bacteriostatic action. The numbers of viable S. aureus
bacteria was assessed after treatment with 2b (closed circles) or 8b (closed
squares). A DMSO vehicle control (5%) is included (open squares).
13. Payne, D. J. Science 2008, 321, 1644.
14. Payne, D. J.; Gwynn, M. N.; Holmes, D. J.; Pompliano, D. L. Nat. Rev. Drug Disc.
2007, 6, 29.
bacteria remained constant throughout the 6 h of treatment, dem-
onstrating that the mechanism of action was bacteriostatic (Fig. 2).
In this Letter we report a series of thiazolidinedione and rhoda-
nine compounds 5, examples of which show good selectivity for
B. subtilis over other Gram-positive bacteria. Further studies iden-
tified related compounds that showed selectivity for S. aureus over
B. subtilis. The scope and possible mechanism of action of this class
is also addressed for the first time. The compounds are easy to pre-
pare and their versatile design is amenable to the introduction of a
range of different substituents. Thus these compounds represent a
particularly versatile addition to the growing list of biologically ac-
tive thiazolidinedione and rhodanine derivatives with potential, in
this case, as antibacterial agents. It appears that rhodanines are
generally more active than the thiazolidinediones and non-polar
15. Heerding, D. A.; Christmann, L. T.; Clark, T. J.; Holmes, D. J.; Rittenhouse, S. F.;
Takata, D. T.; Venslavsky, J. W. Bioorg. Med. Chem. Lett. 2003, 13, 3771.
16. General procedure for Knoevenagel reaction: To a solution of respective 2,4-
thiazolidinedione or rhodanine (1 equiv) in anhydrous ethanol (20 mL/1 g of
2,4-thiazolidinedione or rhodanine), the respective aldehyde (1 equiv) and
piperidine (0.1 equiv) were added in a single portion and heated under reflux
under nitrogen for 8 h. The reaction was cooled to room temperature, diluted
with water (10 mL) and precipitated with glacial acetic acid. The mixture was
filtered and washed with cold water (2 Â 10 mL) followed by ethanol
(2 Â 10 mL). The precipitates were dissolved in toluene and concentrated in
vacuo to yield the desired pure products 3a/b and 8a–f.3
17. General procedure for amide synthesis: To a solution of the respective 2,4-
thiazolidinedione or rhodanine (1 equiv) and respective amine (1 equiv) in
anhydrous DMF (10 mL/1 g of 2,4-thiazolidinedione or rhodanine) under
nitrogen was added 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide
(1.2 equiv), hydroxybenzotriazole (1.2 equiv) and N,N-diisopropyl ethylamine
(4.75 equiv) in a single portion and stirred for 16 h. The solution was diluted
with 1 M hydrochloric acid (50 mL) and extracted with ethyl acetate
(3 Â 50 mL). The combined organic layers were washed with 1 M
hydrochloric acid (2 Â 50 mL) followed by brine (2 Â 50 mL) and dried
(Na2SO4), filtered and the volatiles removed in vacuo. The crude residue was
purified by column chromatography to furnish the desired pure products
5a–k.3
substituents on the aryl group and p-excessive heterocycles are fa-
vored. We are currently further investigating the activity of this
series against a variety of bacteria.
Acknowledgments
18. Clinical and Laboratory Standards Institute Methods for Antimicrobial
Susceptibility Testing of Anaerobic Bacteria; Approved Standard 6th Ed.
NCCLS Document M11-A6.
D.S.P. is grateful to the Carlsberg Foundation for funding and
A.D.A. acknowledges financial support from the Australian Re-
search Council.
19. Morona, R.; Reeves, P. J. Bacteriol. 1982, 1016, 150.