Synthesis and antibacterial activity of new fluoroquinolones containing a substituted N-(phenethyl)piperazine moiety

Article abstract of DOI:10.1016/j.bmcl.2006.03.103

N-(Phenethyl)piperazinyl quinolone derivatives that bear a methoxyimino-substituent have been synthesized and evaluated for antimicrobial activity against Gram-positive and Gram-negative microorganisms. In addition, to define structure-activity relationsh

Full text of DOI:10.1016/j.bmcl.2006.03.103

Bioorganic & Medicinal Chemistry Letters 16 (2006) 3499–3503
Synthesis and antibacterial activity of new fluoroquinolones
containing a substituted N-(phenethyl)piperazine moiety
Alireza Foroumadi,a,* Shahram Ghodsi,b Saeed Emami,c Somayyeh Najjari,b
Nasrin Samadi,a Mohammad Ali Faramarzi,a Leila Beikmohammadi,a
Farshad H. Shirazid and Abbas Shafieea
aFaculty of Pharmacy and Pharmaceutical Sciences Research Center, Tehran University of Medical Sciences, Tehran 14174, Iran
bDepartment of Chemistry, Faculty of Sciences, Islamic Azad University, Karaj-Branch, Karaj, Iran
cDepartment of Medicinal Chemistry and Pharmaceutical Sciences Research Center, Faculty of Pharmacy,
Mazandaran University of Medical Sciences, Sari 48175, Iran
dDepartment of Pharmacology and Toxicology, Faculty of Pharmacy, Shaheed Beheshti University of Medical Sciences, Tehran, Iran
Received 11 February 2006; revised 18 March 2006; accepted 30 March 2006
Available online 27 April 2006
Abstract—N-(Phenethyl)piperazinyl quinolone derivatives that bear a methoxyimino-substituent have been synthesized and evalu-
ated for antimicrobial activity against Gram-positive and Gram-negative microorganisms. In addition, to define structure–activity
relationships, ciprofloxacin derivatives containing 2-oxo-2-phenylethyl or 2-hydroxyimino-2-phenylethyl moieties at N-4 position of
piperazine ring were prepared and tested. Ciprofloxacin derivatives, containing a N-(chloro-substituted phenethyl) residue, showed
in vitro Gram-positive and Gram-negative activity generally comparable or superior to that of reference quinolones.
Ó 2006 Elsevier Ltd. All rights reserved.
Quinolones are synthetic antibacterial compounds based
on a 4-quinolone skeleton. Fluoroquinolones have been
clinically successful and are used to treat bacterial
infections in both community and hospital settings.
Quinolones target bacterial type II topoisomerases, gen-
erally DNA gyrase in Gram-negative bacteria and DNA
topoisomerase IV in Gram-positive bacteria.1–3 Most of
the quinolones currently on the market or under
development have only moderate activity against many
Gram-positive cocci, including Staphylococci and
Streptococci.4,5 This insufficient activity has not only
limited their use in infections caused by these organisms,
such as respiratory tract infections, but has also been
believed to be one of the reasons for the rapidly develop-
ing quinolone resistance. Therefore, recent efforts have
been directed toward the synthesis of new quinolone
antibacterials that can provide improved Gram-positive
antibacterial activity, while retaining good Gram-nega-
tive activity.4,5
The synthesis and evaluation of over 10,000 quinolone
derivatives resulted in thorough knowledge of the
structure–activity relationship for many quinolone sub-
stituents.6 The most intensive structural variation has
been carried out on amines at the 7-position, partially
due to the ease of their introduction through a nucleo-
philic aromatic-substitution reaction on the correspond-
ing halide.7,8 Piperazine, aminopyrrolidine, and their
substituted derivatives have been the most successfully
employed side chains, as evidenced by the compounds
currently on the market.7,8 Originally, the newer fluoro-
quinolones arose with the development of 7-piperazinyl
quinolones, such as norfloxacin 1, ciprofloxacin 2, and
enoxacin 3 (Fig. 1), which combined structural features
of flumequine (C-6 fluorine atom) and pipemidic acid
(C-7 piperazine side chain).8–12
According to the inhibition mechanisms of the
quinolones, proposed by Shen et al.,13–15 the site near
the C-7 substituent is regarded as drug–enzyme interac-
tion domain. In addition, Klopman et al. also concluded
that the cell permeability is dominantly controlled by
C-7 substituent.16 The piperazine moiety of 7-piperazi-
nyl quinolones possesses enough structural flexibility
to allow product optimization. In addition, the C-7 sub-
stituent affects the interaction with the target, and both
Keywords: Piperazinyl quinolones; Ciprofloxacin derivatives; Antimi-
crobial activity; Methoxyimino-substituent.
*
Corresponding author. Tel.: +98 21 66406757; fax: +98 66461178;
e-mail: aforoumadi@yahoo.com
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.03.103

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A. Foroumadi et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3499–3503
R5
X
modifications were made on the basis of gemifloxacine
O
4, a well-known quinolone antibacterial agent possess-
ing methoxyimino group attached to the pyrrolidine
ring at C-7 position (Fig. 2).22
F
COOH
R7
N
R1
The N-[2-aryl-2-methoxyiminoethyl]piperazinyl quino-
lone analogues 5a–n were prepared by the synthetic
route diagrammed in Scheme 1. We have previously
reported our synthesis of the versatile a-halooxime inter-
mediates, which allowed us to vary the aromatic ring
and the center oxime for our structure–activity relation-
ship studies.19–21 In this case, a-chloroketone 6 was con-
verted to a-chloro-O-methyloxime intermediate 7 by
stirring with excess of methoxyamine hydrochloride in
methanol at room temperature. Reaction of piperazinyl
quinolones (1, 2 or 3) with a-chloro-O-methyloxime 7 in
DMF, in the presence of NaHCO3 at room temperature,
1, Norfloxacin; R1= ethyl, R5= H, R7= piperazin-1-yl, X= CH
2, Ciprofloxacin; R1= cyclopropyl, R5= H, R7= piperazin-1-yl, X= CH
3, Enoxacin; R1= ethyl, R5= H, R7= piperazin-1-yl, X= N
Figure 1. Common pharmacophore of fluoroquinolones and structure
of three piperazinyl quinolones.
the activity spectrum and kinetic profile can be
controlled at C-7.17,18
Recently, as part of an ongoing program to find potent
and broad-spectrum antibacterial agents that display
strong Gram-positive activity, we have focused our
attention on modification of the C-7 basic group of
the quinolone.19–21 Therefore, our strategy to achieve
a better antimicrobial profile has focused on introduc-
ing new functionality on the piperazine ring. From our
research in C-7 piperazine modifications of the
quinolones,19–21 we were able to identify a series of
N-substituted piperazinyl quinolones in which the
N-4 hydrogen of piperazinyl group of norfloxacin 1,
ciprofloxacin 2, and enoxacin 3 is replaced with various
2-oxoethyl or 2-oxyiminoethyl moieties and displays
in vitro microbiological activity against Gram-positive
organisms comparable or higher than respective parent
quinolones. However, the improved activity against
Gram-positive bacteria was generally at the expense
of activity against Gram-negative bacteria (specially
against Pseudomonas aeruginosa). Moreover, when we
exchanged oxime with a bulky O-benzyloxime group,
the synthesized compounds did not maintain activity
against both Gram-positive and negative bacteria.19–21
Since our goal was the discovery of new quinolone
antibacterial agents with both strong Gram-positive
and Gram-negative bacteria, we designed novel
N-substituted piperazinyl quinolones 5a–l (Fig. 2) and
related compounds 5m–r that possessed a methoxyimi-
noethyl substituent in the piperazine ring. These
afforded
N-[2-aryl-2-methoxyiminoethyl]piperazinyl
quinolone analogues 5a–n as a mixture of (E)- and
(Z)-isomers.
Preliminarily, compounds 5a–l (Table 1) were evaluated
for their antibacterial activity against Gram-positive
(Staphylococcus aureus ATCC 6538p, Staphylococcus
epidermidis ATCC 12228, and Bacillus subtilis ATCC
6633) and Gram-negative (Escherichia coli ATCC 8739,
Klebsiella pneumoniae ATCC 10031, and P. aeruginosa
ATCC 9027) bacteria using a conventional agar-dilution
method.23 The MIC (minimum inhibitory concentration)
values were determined and compared to norfloxacin 1,
ciprofloxacin 2, and enoxacin 3 as reference drugs.
Table 1 reveals that compound 5j followed by 5d–g are
superior in inhibiting the growth of Staphylococci
(MIC = 0.39–6.25 lg/mL), while the remaining com-
pounds are statistically equivalent in antibacterial
activity against these microorganisms with moderate
activity (MIC = 12.5–25 lg/mL). Most tested com-
pounds had respectable in vitro activity against B. sub-
tilis, but were less active than the reference drugs.
Generally, most compounds showed moderate to good
activity against Gram-negative bacteria, with the
exception for P. aeruginosa. Compound 5j was the
O
F
COOH
H3CO
N
N
N
N
H2N
4, Gemifloxacin
O
N
O
COOH
F
COOH
F
Y
NOCH3
N
N
N
N
X
N
R
R1
R2
R1
R2
R1& R2= H, Cl, F
X= CH or N
R= cyclopropyl or ethyl
R1& R2= H, Cl, F
Y= O, NOH, NOMe
5m-r
5a-l
Figure 2.

A. Foroumadi et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3499–3503
3501
O
NOCH3
Cl
O
Cl
a
COOH
F
NOCH3
R1
R2
R1
R2
N
N
X
N
R
b
7
6a: R1=R2= H
R1
R2
6b: R1=F, R2=H
6c: R1=F, R2=F
6d: R1=H, R2=Cl
6e: R1=Cl, R2=H
6f: R1=Cl, R2=Cl
5a-n
O
COOH
F
X
N
R
N
HN
1-3
Scheme 1. Synthesis of the compounds 5a–n. Reagents and conditions: (a) O-methylhydroxylamine hydrochloride, MeOH, rt; (b) DMF, NaHCO3,
rt.
Table 1. Structures and in vitro antibacterial activities of compounds 5a–l against selected strains (MICs in lg/mL)
O
COOH
F
NOCH3
N
N
X
N
R
R1
R2
Compound
X
R
R1
R2
S. aureus
S. epidermidis
B. subtilis
E. coli
K. pneumoniae
P. aeruginosa
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
1
CH
CH
N
Cyclopropyl
Ethyl
Ethyl
H
H
H
F
H
H
H
H
H
H
F
12.5
25
12.5
25
6.25
6.25
12.5
6.25
3.13
6.25
0.78
1.56
1.56
0.025
1.56
3.13
0.049
0.025
0.098
6.25
12.5
12.5
3.13
3.13
6.25
1.56
25
3.13
12.5
12.5
3.13
0.78
1.56
0.78
12.5
25
100
>100
>100
50
25
25
CH
CH
N
Cyclopropyl
Ethyl
Ethyl
6.25
6.25
12.5
6.25
12.5
25
6.25
1.56
6.25
6.25
12.5
25
F
100
100
F
CH
CH
N
Cyclopropyl
Ethyl
Ethyl
F
12.5
>100
>100
1.56
>100
50
F
F
F
F
50
CH
CH
N
Cyclopropyl
Ethyl
Ethyl
Cl
Cl
Cl
Cl
Cl
Cl
0.39
12.5
25
0.39
12.5
25
0.025
12.5
12.5
0.049
0.025
0.098
0.012
6.25
6.25
0.049
0.025
0.098
Norfloxacin
Ciprofloxacin
Enoxacin
0.78
0.39
1.56
0.78
0.39
0.78
3.13
0.78
6.25
2
3
most potent against P. aeruginosa, with MIC value of
1.56 lg/mL. Its activity was found to be comparable
to reference drugs.
of the quinolones and indicate that only introduction of
2-(2,4-dichlorophenyl)-2-methoxyiminoethyl residue at
the N-4 position of piperazine ring in ciprofloxacin
molecule is well tolerated in terms of activity against
both Gram-positive and Gram-negative bacteria.
Indeed, all compounds (Table 1) are less active than
reference drugs with the exception of 5j. Compound 5j
with a cyclopropyl substituent at N-1, a CH at the posi-
tion-8 and a 2,4-dichlorophenyl group (R1 = R2 = Cl)
exhibited the most potent inhibitory activity against both
Gram-positive and Gram-negative bacteria. The MIC
values of this compound were relatively equivalent to that
of reference drug ciprofloxacin and were lower than other
standard quinolones (norfloxacin and enoxacin).
As can be deduced from these data, the impact of the
2,4-dichloro- on the phenethyl side chain is highly
dependent on the type of substituents at N-1 and
position-8. In view of the results obtained with 2-(2,4-
dichlorophenyl)-2-methoxyiminoethyl residue at the
N-4 position of piperazine ring in ciprofloxacin molecule
(compound 5j), we proceeded to survey the impact of
methoxyimino-moiety and chloro-substituent at 2- and
4-positions of phenethyl moiety. For this purpose we
prepared several new N-phenethyl ciprofloxacin
derivatives 5m–r according to our previously described
method,19–21 and evaluated their antibacterial activity
(Table 2). In addition, this series of compounds were
tested in vitro against two clinically isolated strains of
We have briefly investigated the SAR of the methoxy-
imino-functionalized piperazinyl quinolones and the
basic molecule has been modified at the N-1 position,
with different groups added to the side-chain phenyl
ring, and position 8. These modifications result in
changes in the potency and antibacterial activity profile

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A. Foroumadi et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3499–3503
Table 2. Structures and in vitro antibacterial activities of compounds 5j and 5m–r against selected strains (MICs in lg/mL)
O
COOH
F
Y
N
N
N
R1
R2
Compound
Y
R1 R2 S. aureus MRSAa
I
MRSA II S. epidermidis B. subtilis E. coli K. pneumoniae P. aeruginosa
5j
NOCH3 Cl Cl 0.39
NOCH3 Cl 0.39
NOCH3 Cl 0.78
Cl 0.39
0.39
Cl 0.39
0.39
0.39
0.39
0.39
0.39
0.39
0.195
0.78
0.39
0.39
0.39
0.39
0.39
0.39
0.39
0.098
0.78
0.39
0.39
0.195
0.39
0.025
0.049
1.56
0.025
0.025
1.56
0.013
0.013
0.78
1.56
6.25
5m
5n
5o
5p
5q
5r
1
H
H
>100
O
H
0.195
0.195
0.195
0.195
0.78
0.049
0.049
0.78
0.098
0.025
0.78
0.025
0.013
0.025
0.049
0.049
0.025
6.25
1.56
O
Cl
H
H
NOH
NOH
25
100
3.13
0.78
Cl
H
0.39
0.78
0.39
0.78
0.049
0.025
0.78
0.049
0.025
2
0.39
a MRSA, methicillin-resistant S. aureus.
methicillin-resistant S. aureus (MRSAs I and II). To
determine the effect of adding 2,4-dichloro-substitu-
tions, we prepared both the ortho- and para-chloro
derivatives 5m and 5n. When compared to 5j, both com-
pounds had similar in vitro activities against Staphylo-
cocci, but the susceptibility against Gram-negative
bacteria was reduced in 4-chloro-derivative 5n. More-
over, the susceptibility against P. aeruginosa was lost
in 5n and maintained in 5m. Finally, for investigating
the importance of methoxyimino-moiety in ciprofloxa-
cin series, we compared compounds 5m and 5n with
oxime containing compounds 5q and 5r, and also with
ketones 5o and 5p. All these compounds had potent
activities against Staphylococci. 4-Chloro-oxime ana-
logue 5r was the most potent compound against
MRSAs, while its activity against P. aeruginosa was lost
similar to 4-chloro-O-methyloxime derivative 5n. Most
of the new N-phenethyl ciprofloxacin derivatives (5j
and 5m–r) containing a chloro-substituent on phenyl
ring showed potent antibacterial activity and modifica-
tion of the ketone, oxime, and O-methyloxime on chlo-
rophenethyl side chain produced only relatively minor
changes in activity. Thus, in the N-phenethyl ciproflox-
acin series, antibacterial activity can be positively
modulated through the introduction of chlorine atom
and the functionality on the ethyl spacer in combination
with the chloro-substitution pattern is important for
activity against P. aeruginosa.
Moreover, these atoms may also influence the steric
characteristics and the hydrophilic–hydrophobic bal-
ance of the molecules. On the other hand, carbonyl
related functional groups (ketone, oxime, and O-meth-
yloxime) have different steric, electronic, and lipophilic
characteristics. Thus, these structural modifications of
the N-phenethylpiperazinyl quinolones were expected
to allow modulation of the physical properties of the
corresponding quinolones, while retaining the strong
biological activity of the piperazinyl quinolones (espe-
cially ciprofloxacin).
In conclusion, we have designed and synthesized novel
quinolone agents bearing a methoxyiminoethyl substitu-
ent or related residues in the 4-position of the piperazine
ring. It was found that chloro-substituted phenethyl
group attached to piperazine ring served as promising
C-7 substituents for piperazinyl quinolone antibacteri-
als. Among these compounds, ciprofloxacin derivatives
5j and 5p provided in vitro Gram-positive and Gram-
negative activity generally comparable or superior to
that of reference quinolones.
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