Synthesis and antibacterial evaluation of certain quinolone derivatives

Article abstract of DOI:10.1021/jm0100335

A number of 7-substituted quinolone derivatives were synthesized and evaluated for antibacterial and cytotoxic activities. Preliminary results indicated that most compounds tested in this study demonstrated better activity against methicillin-resistant St

Full text of DOI:10.1021/jm0100335

2374
J . Med. Chem. 2001, 44, 2374-2377
Syn th esis a n d An tiba cter ia l Eva lu a tion of Cer ta in Qu in olon e Der iva tives
Yeh-Long Chen,*,† Kuo-Chang Fang,‡ J ia-Yuh Sheu,‡ Shu-Lin Hsu,‡ and Cherng-Chyi Tzeng†
School of Chemistry, Kaohsiung Medical University, Kaohsiung City 807, Taiwan, Republic of China, and Department of
Environmental Engineering & Health, Tajen Institute of Technology, Pingtong 907, Taiwan, Republic of China
Received April 3, 2001
A number of 7-substituted quinolone derivatives were synthesized and evaluated for antibacte-
rial and cytotoxic activities. Preliminary results indicated that most compounds tested in this
study demonstrated better activity against methicillin-resistant Staphylococcus aureus than
norfloxacin. Among them, 1-(4-amino-2-fluorophenyl)-6-fluoro-1,4-dihydro-7-{4-[2-(4-methoxy-
phenyl)-2-hydroxyiminoethyl]-1-piperazinyl}-4-oxo-3-quinolinecarboxylic acid (11d ) and its
ketone precursor 10d exhibited significant activities against Klebsiella pneumoniae, methicillin-
resistant S. aureus, erythromycin- and ampicillin-resistant Streptococcus pneumoniae, and
vancomycin-resistant Enterococcus faecalis. Due to strong cytotoxicities of 11d (a mean log
GI50 of -5.40), compound 10d , with good antibacterial activities and low cytotoxicities (a mean
log GI50 of -4.67), is a more potential drug candidate.
Ch a r t 1. Side Chains Employed as the R1 and R2
Substitutents
In tr od u ction
Although the structure-activity relationships of an-
tibacterial fluoroquinolones have been extensively in-
vestigated, the optimum substituent at the C-7 position
which has a great impact on potency, spectrum, solubil-
ity, and pharmacokinetics has not been precisely
defined.1-5 Extensively investigated substituents are
piperazin-1-yl and its 4-substituted derivatives. For
example, pefloxacin, the 4′-methylnorfloxacin, and other
4-substituted piperazin-1-yl prodrugs of norfloxacin
were prepared to improve the bioavailability of the
parent.6-8 A number of fluoroquinolones with an oxime
or a substituted oxime attached to the pyrrolidine or
piperazine ring at C-7 position were also synthesized
and evaluated for antibacterial activities.9-14 In our
previous report, we described the preparation and
evaluation of certain norfloxacin derivatives with an
additional functional moiety such as 4-hydroxyami-
noalkyl on the C-7 piperazin-1-yl group. Most of these
compounds demonstrated better activity against me-
thicillin-resistant Staphylococcus aureus than norfloxa-
cin and proved to possess selective cytotoxicity against
renal cancer cell lines.15 In continuing our efforts to
establish the structure-activity relationships of anti-
bacterial fluoroquinolones, further derivatization of
preceding oximes as well as their 8-fluoro and 1-aryl
derivatives were synthesized and evaluated.
Sch em e 1a
Ch em istr y
The general procedure for the preparation of 6-fluoro-
1,4-dihydro-7-[4-(2-substituted-iminoethyl)piperazin-1-
yl]-4-oxoquinoline-3-carboxylic acids 5a -i (see Chart 1)
and their 6,8-difluoro analogues 6b-h are described in
Scheme 1. Norfloxacin (1) was treated with NaHCO3
and a bromomethyl ketone to give its N-(2-oxo-2-(4-sub-
stituted phenyl)ethyl derivatives 3a -i. Reaction of 3a -i
with hydroxyamine, methoxyamine, hydrazine, semi-
a
Reagents: (i) NaHCO3, R1PhCOCH2Br in DMF; (ii) R2NH2 in
EtOH.
carbazide, and thiosemicarbazide, respectively, in EtOH
gave the corresponding 5a -i in 29-56% overall yield.
Among them, the sole Z-form isomer of 5b, 5c, and 5e
was isolated while both E- and Z-form isomers of 5a ,
5d , and 5f-i were obtained with Z-isomers predominat-
ing.15,16 Accordingly, 6,8-difluoro-1,4-dihydro-7-[4-(2-
* Address correspondence to Dr. Yeh-Long Chen: tel. (886) 7-3121101
ext 2249; fax (886) 7-3125339; e-mail yeloch@cc.kmu.edu.tw.
† Kaohsiung Medical University.
‡ Tajen Institute of Technology.
10.1021/jm0100335 CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/08/2001

Brief Articles
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 14 2375
Sch em e 2a
a
Reagents: (i) K2CO3, 4-fluoronitrobenzene or 3,4-difluoronitrobenzene in DMF; (ii) HCl, AcOH; (iii) piperazine in CH3CN; (iv) K2CO3,
R1PhCOCH2Br in DMF; (v) NH2OH HCl in EtOH; (vi) H2, Pd/C in CH2Cl2.
substituted-iminoethyl)piperazin-1-yl]-4-oxoquinoline-
3-carboxylic acids 6b-d and 6f-h were obtained from
8-fluoronorfloxacin (2) in 52-70% overall yield.
at N-1 and a phenyl (or a substituted phenyl) at the
C-7 piperazinyl side chain, the imino nitrogen substi-
tuted with a hydroxy group (5a , R2 ) OH) exhibited the
most potent inhibitory activity on G(-) bacteria and S.
aureus M-R while a thiourea (5c, R2 ) NH2CSNH2) is
most favorable for the inhibition of erythromycin- and
ampicillin-resistant Streptococcus pneumoniae (S. pneu-
moniae E-R and A-R) and vancomycin-resistant Entero-
coccus faecalis (E. faecalis V-R). An amino substituent
(5e, R2 ) NH2) is more active than its hydroxy coun-
terpart (5d , R2 ) OH) which in turn is more potent than
a methoxy substituent (5h vs 5i). Introduction of an
8-fluoro substituent enhanced antibacterial activities
(3a vs 4a , 3h vs 4h , 5b vs 6b, 5h vs 6h ). For the N-1
substituent, 4-nitrophenyl is comparable to its ethyl
counterpart (11a vs 5d ; 11b vs 5h ) while 2,4-disubsti-
tuted phenyl (11c, 11d ) is most active. In fact, even
though the ketone precursors 3a -h and 10a -b are
inactive against the growth of resistant strains, the
parent 2,4-disubstituted phenyl quinolones 9b and 9c,
and their ketone precursors 10c and 10d , exhibited
significant activities. Due to strong cytotoxicities of 11c
(a mean log GI50 of -5.34) and 11d (a mean log GI50 of
-5.40), the ketone precursor, 10d , with good antibacte-
rial activities and low cytotoxicities (a mean log GI50 of
-4.67), is a more potential drug candidate.21
The preparation of 1-aryl-6-fluoro-1,4-dihydro-7-[4-(2-
hydroxyiminoethyl)piperazin-1-yl]-4-oxoquinoline-3-car-
boxylic acids 11a -d is described in Scheme 2. Reaction
of ethyl 6,7-difluoro-4-oxoquinoline-3-carboxylate (7)
with 3,4-difluoronitrobenzene and K2CO3 in dry DMF
at 80 °C gave ethyl 6,7-difluoro-1-(2-fluoro-4-nitrophe-
nyl)-1,4-dihydro-4-oxoquinoline-3-carboxylate (8c) in
67% yield.17,18 Compound 8c was hydrolyzed with HCl
in acetic acid, and the crude intermediate without
purification was treated with piperazine in CH3CN to
afford 6-fluoro-1-(2-fluoro-4-nitrophenyl)-1,4-dihydro-4-
oxo-7-(1-piperazinyl)quinoline-3-carboxylic acid (9b) in
78% yield. Compound 9b was reduced by catalytic
hydrogenation on Pd/C to its 1-(4-amino-2-fluorophenyl)
counterpart 9c in 85% yield. Alkylation of 9a and 9b
with K2CO3 and a 4′-substituted 2-bromoacetophenone
gave their respective N-[2-oxo-2-(4-substituted-phenyl)-
ethyl] derivatives 10a -c which were then reacted with
hydroxylamine to give 1-aryl-6-fluoro-1,4-dihydro-7-[4-
(2-hydroxyiminoethyl)piperazin-1-yl]-4-oxoquinoline-3-
carboxylic acids 11a -c in 69-81% overall yield.19,20
Compound 11d was obtained by hydroxylamination of
10d , which was synthesized via hydrogenation of 10c
with catalytic Pd/C in CH2Cl2.
Con clu sion
Resu lts a n d Discu ssion
A number of 7-substituted quinolone derivatives were
synthesized and evaluated for antibacterial and cyto-
toxic activities. Preliminary results are as follows: (1)
For 1-ethyl quinolones, 7-[4-(2-oxoethyl)piperazin-1-yl]-
derivatives 3, with exception of 3f, are narrow-spectrum
and only active against methicillin-resistant S. aureus
while their 2-iminoethyl counterparts 5 are more active
and broad-spectrum antibacterial agents. Besides, 8-flu-
oro derivatives are more active antibacterial agents than
their respective unsubstituted counterparts. (2) For
A number of quinolone derivatives prepared for this
study were tested in vitro against five susceptible
strains and three resistant strains. The minimum
inhibitory concentrations (MIC) are presented in Table
1. In general, 6-fluoro-1,4-dihydro-7-[4-(2-hydroxyimi-
noethyl)piperazin-1-yl]-4-oxoquinoline-3-carboxylic acids
5a -i are less active than norfloxacin with the exception
of the inhibitory activity against methicillin-resistant
S. aureus (S. aureus M-R). With an ethyl substituent

2376 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 14
Brief Articles
Ta ble 1. In Vitro Antibacterial Activity of
agents were used in every test as the negative and positive
controls, and the assays were performed in duplicate.
4-Oxoquinoline-3-carboxylic Acids [MIC, µM (µg/mL)]a
S. aureus
M-R
S. pneumoniae
E. faecalis
Ack n ow led gm en t. Financial support of this work
by the National Science Council of the Republic of China
is gratefully acknowledged. We also thank the National
Cancer Institute (NCI) of the United States for the
anticancer screenings and the National Center for High-
Performance Computing for providing computer re-
sources and chemical database services.
compd
E-R, A-R
V-R
2
3a
3d
3f
3h
4a
4h
5a
5b
5c
5d
5e
5f
5g
5h
5i
6b
6c
6d
6f
6g
6h
9b
9c
10a
10b
10c
10d
11a
11b
11c
11d
Nfc
2.31 (0.78)
7.15 (3.13)
6.87 (3.13)
>200 (>100)
6.70 (3.13)
1.71 (0.78)
0.62 (0.30)
0.44 (0.20)
3.15 (1.56)
1.53 (0.78)
2.13 (1.0)
18.5 (6.25)
>200 (>100)
>200 (>100)
>200 (>100)
>200 (>100)
13.7 (6.25)
>200 (>100)
>200 (>100)
25.3 (12.5)
24.5 (12.5)
>200 (>100)
26.6 (12.5)
>200 (>100)
>200 (>100)
13.0 (6.25)
12.6 (6.25)
6.11 (3.13)
>200 (>100)
6.14 (3.0)
18.5 (6.25)
>200 (>100)
>200 (>100)
>200 (>100)
>200 (>100)
13.7 (6.25)
>200 (>100)
>200 (>100)
25.3 (12.5)
12.2 (6.25)
>200 (>100)
26.6 (12.5)
>200 (>100)
>200 (>100)
3.23 (1.56)
12.6 (6.25)
6.11 (3.13)
>200 (>100)
2.05 (1.0)
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures, characterization, and elemental analyses for com-
pounds 4a , 4d , 4f, 4h , 5b, 5c, 5e, 5g, 5i, 6b-d , 6f-h , 8c, 8d ,
9c, 9d , 10a -d , and 11a -d ; tables containing data for anti-
bacterial activities against E. coli, M. ranae, P. aeruginosa,
K. pneumoniae, and P. vulgaris and the inhibitory activities
on renal cancer cell lines for 3a , 3f, 3h , 4a , 4h , 5a -i, 6c, 6d ,
10a -d , and 11a -d . This material is available free of charge
via the Internet at http://pubs.acs.org.
1.66 (0.78)
>200 (>100)
ndb
0.41 (0.20)
1.57 (0.78)
0.39 (0.20)
0.74 (0.39)
0.61 (0.30)
0.59 (0.30)
>200 (>100)
0.20 (0.10)
7.27 (3.13)
0.97 (0.39)
>200 (>100)
5.58 (3.13)
2.70 (1.56)
0.55 (0.30)
>200 (>100)
1.36 (0.78)
0.34 (0.20)
0.53 (0.30)
4.89 (1.56)
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a
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Exp er im en ta l Section
In Vitr o An tiba cter ia l Assa y. Deter m in a tion of MIC:
2 mg of each test compound was dissolved in an appropriate
solvent (100% DMSO) and serially diluted with DMSO into
the desired testing concentration ranges. Each series of testing
solution (0.01 mL) was added into the 48-well plate with 0.99
mL of media broth containing 1-5 × 105 CFU/mL testing
microorganism. Thus the final maximal concentration of
DMSO was 1%, and the initial concentration of testing solution
was 300 µM. Media used were as follows: nutrient broth (NB,
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Klebsiella pneumoniae; Mueller-Hinton broth (DIFCO) for S.
aureus, methicillin-resistant (MRSA), and Proteus vulgaris;
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