Synthesis and antibacterial activity of new N-[2-(thiophen-3-yl)ethyl] piperazinyl quinolones

Article abstract of DOI:10.1248/cpb.55.894

As a part of continuing search for potential antibacterial agents in the quinolones field, we have synthesized novel quinolone agents bearing N-[2-(thiophen-3-yl)ethyl] piperazinyl moiety in the 7-position of the quinolone ring. In vitro antibacterial eva

Full text of DOI:10.1248/cpb.55.894

894
Chem. Pharm. Bull. 55(6) 894—898 (2007)
Vol. 55, No. 6
Synthesis and Antibacterial Activity of New N-[2-(Thiophen-3-yl)ethyl]
Piperazinyl Quinolones
Bahram LETAFAT,a Saeed EMAMI,b Negar MOHAMMADHOSSEINI,c Mohammad Ali FARAMARZI,d
d
d
,d
*
Nasrin SAMADI, Abbas SHAFIEE, and Alireza FOROUMADI
a Department of Chemistry, Islamic Azad University, Central Tehran-Branch; Tehran 14168, Iran: b Department of
c
Medicinal Chemistry, Faculty of Pharmacy, Mazandaran University of Medical Sciences; Sari 48175, Iran: Islamshahr
Young Researchers Club, Faculty of Sciences, Islamic Azad University, Islamshahr-Branch; Islamshahr 33135, Iran: and
d Faculty of Pharmacy and Pharmaceutical Sciences Research Center, Medical Sciences/University of Tehran; Tehran
14174, Iran. Received February 9, 2007; accepted March 22, 2007
As a part of continuing search for potential antibacterial agents in the quinolones field, we have synthesized
novel quinolone agents bearing N-[2-(thiophen-3-yl)ethyl] piperazinyl moiety in the 7-position of the quinolone
ring. In vitro antibacterial evaluation of the target compounds showed that N-[2-(thiophen-3-yl)ethyl] group at-
tached to piperazine ring served as promising C-7 substituent for piperazinyl quinolone antibacterials. Among
these derivatives, ciprofloxacin analogues, containing N-[2-(thiophen-3-yl)-2-hydroxyiminoethyl] or N-[2-(thio-
phen-3-yl)-2-methoxyiminoethyl] residue provided a high inhibition against all the tested Gram-positive organ-
isms including methicillin-resistant Staphylococcus aureus comparable or superior with respect to the reference
drugs norfloxacin and ciprofloxacin.
Key words synthesis; quinolone; thiophene; oxime; antibacterial activity
The emergence of antibiotic-resistant Gram-positive bac- proved to include most Gram-negative bacteria, their activi-
terial infections, notably with Staphylococcus aureus and ties against Gram-positive bacteria remained limited.5) The
Streptococcus pneumoniae, has prompted development of medicinal chemists have synthesized a large number of nor-
new chemotherapeutic agents that selectively attack new bac- floxacin and ciprofloxacin analogues. With these compounds,
terial targets.1)
there seems to be an inverse relationship between a com-
Quinolones represent an extremely successful family of pound’s Gram-positive and Gram-negative activity such that
antibacterial drugs that are active against a wide range of enhanced activity against one often accompanies reduced ac-
multiresistant pathogens since their mode of action is against tivity against the other. Another drawback to increased activ-
different molecular targets than other antimicrobial classes.2) ity is the potential for increased host toxicity. For example,
Quinolone antibacterial agents have been known to inhibit several broad-spectrum quinolones approved for use in
DNA gyrase and topoisomerase IV, bacterial topoisomerase human medicine, such as temafloxacin and grepafloxacin,
II enzymes.3) DNA topoisomerase II enzyme is an essential have been voluntarily withdrawn from clinical use as a result
cellular enzyme that catalyzes the double strand breakage of of emerging safety concerns.8)
DNA to allow strand passage and thereby control the topol-
ogy and conformation of DNA.4)
Following the discovery of the therapeutic effects of
nalidixic acid in the 1970s, medicinal chemists began to
probe every position within the quinolone nucleus in an at-
tempt to improve potency, broaden the spectrum of antibac-
terial activity, and reduce recognized side effects. To date,
several thousand related compounds have been synthesized.
In the 1980s, the addition both of a fluorine atom at the 6-po-
sition and a piperazine substitution at the 7-position of the
basic quinolone structure was found to enhance quinolone
antibacterial activity, gaining effectiveness against such or-
ganisms as Pseudomonas aeruginosa and Gram-positive
cocci, and to increase the extent of oral drug absorption and
tissue distribution.5,6) Norfloxacin 1 (patented in 1978) was
the first compound to combine a piperazinyl side chain in po-
sition 7 and a fluoro group in position 6. Although the addi-
tion of a piperazinyl ring at the 7-position resulted in in-
creased activity against bacteria such as P. aeruginosa and
Gram-positive cocci, norfloxacin continued to suffer from
poor bioavailability.7) The addition of a cyclopropyl ring at
the N-1 position led to the development of ciprofloxacin 2.
While the spectra and antibacterial activities of quinolones
Fig. 1. Chemical Structures of Piperazinyl Quinolones
such as norfloxacin 1 and ciprofloxacin 2 have been im-
∗ To whom correspondence should be addressed. e-mail: aforoumadi@yahoo.com
© 2007 Pharmaceutical Society of Japan

June 2007
895
It is known that at least two factors determine the potency hydroxylamine hydrochloride in methanol at room tempera-
of quinolones against bacteria: the transport of the drug into ture. Similarly, the O-methyloxime ethers 8b and O-benzyl-
the cells and the inhibition of the target enzyme, DNA gyrase oxime ethers 8c were synthesized by reaction of compound 7
or topoisomerase IV. A substituent on the 7-position would with methoxyamine hydrochloride and O-benzylhydroxyl-
play a key role in both events.9) Recently, we have synthe- amine hydrochloride, respectively.10—12) Reaction of 7-piper-
sized N-substituted piperazinyl quinolones 4 differing from azinylquinolones (1, 2 or 3) with 3-(bromoacetyl)thiophene 7
norfloxacin 1, ciprofloxacin 2 or enoxacin 3 (Fig. 1) solely by or a-bromooxime derivatives 8a—c in DMF, in the presence
the linkage of various 2-(thiophen-2-yl)ethyl groups to the of NaHCO3 at room temperature afforded corresponding ke-
piperazinyl residue at C-7 of the parent drug and explored tones 5a—c and oxime derivatives 5d—l, respectively.10—12)
their antibacterial activities.10—12) By considering the con- Accordingly, enoxacin 3 and ciprofloxacin 2 reacted with a-
siderable activity of N-[2-(thiophen-2-yl)ethyl] piperazinyl bromooxime 8a to give exclusively (E)-5d and (E)-5f. How-
quinolone derivatives 4 against Gram-negative and Gram- ever, in the reaction of norfloxacin 1 with a-bromooxime 8a,
positive bacteria, a new series of N-[2-(thiophen-3-yl)ethyl] (E)- and (Z)-isomers of 5e were isolated in approximately a
derivatives of piperazinyl quinolone (5, Fig. 1) were also syn- 65/35 ratio based upon comparison of the NMR integration
thesized and evaluated for antibacterial activity as positional of corresponding peaks from an aliquot of the principal prod-
isomers of 4.
uct mixture. Among oxime ether derivatives 5g—l, O-benzyl-
oximes 5j and 5l were isolated as pure (Z)-isomer while all
O-methyl oximes 5g—i and O-benzyloxime analog of nor-
Results and Discussion
Chemistry The N-[2-(thiophen-3-yl)ethyl] piperazinyl floxacin 5k were obtained as a mixture of (E)- and (Z)-iso-
quinolone analogues 5 were prepared by the synthetic route mers, with (Z)-isomers predominating. Physicochemical data
diagrammed in Fig. 2. The commercially available ketone 6 of these compounds are shown in Table 1. The stereochem-
1
was brominated with CuBr2 in refluxing CHCl3–AcOEt to istry of the oxime derivatives 5d—l was elucidated by H-
1
give corresponding a-bromoketone 7.12) Compound 7 was NMR spectroscopy. The H-NMR chemical shifts of the
converted to oxime derivative 8a by stirring with excess of methylene and thiophene ring protons for the E and Z iso-
mers of oxime derivatives 5d—l have notable deviations
(Table 2). Experiences in oximes and oxime ethers suggest
that proximity to the oxygen of the oxime in the a-syn con-
figuration will deshield the proton and cause a downfield
shift in the signals of related protons.13—15) The protons of
thiophene ring which is syn to the oxime moiety shifted
downfield in (E)-isomers. However, this anisotropic deshield-
ing effect is largely dependent of the dihedral angle between
the oxime oxygen and the thiophene ring, and proximity of
oxime oxygen to the H2, H4 and H5 protons of thiophene. In
contrast, the protons of methylene which is syn to the oxime
moiety shifted downfield in (Z)-isomers (Table 2).
Antibacterial Activity Compounds 5a—l were evalu-
ated for their antibacterial activity by determination of MIC
Reagents and conditions: (a) CuBr2, CHCl3–EtOAc, reflux; (b) HONH2·HCl or
MeONH2·HCl or BnONH2·HCl, MeOH, r.t.; (c) DMF, NaHCO3, r.t.
(minimum inhibitory concentration) values using conven-
Fig. 2. Synthesis of N-[2-(Thiophen-3-yl)ethyl] Piperazinyl Quinolones
5a—l
tional agar-dilution method.16) The results of antibacterial
Table 1. Structures and Physicochemical Data of N-[2-(Thiophen-3-yl)ethyl] Piperazinyl Quinolone Derivatives 5a—l
Compd.
X
Y
R
mp (°C)a)
Yield (%)b)
Reaction time (d)
5a
5b
5c
N
O
O
O
Et
Et
c-Pr
Et
Et
c-Pr
Et
Et
c-Pr
Et
Et
c-Pr
170—172
224—226
192—194
207—210
217—220
256—258
177—178
205—207
200—203
191—193
160—162
170—171
58
52
41
51
60
31
50
35
36
48
46
58
2
4
2
3
2
3
7
2
7
7
5
4
CH
CH
N
CH
CH
N
CH
CH
N
CH
CH
5d (E)
5e (E/Zꢀ65 : 35)
5f (E)
5g (E/Zꢀ17 : 83)
5h (E/Zꢀ21 : 79)
5i (E/Zꢀ9 : 91)
5j (Z)
5k (E/Zꢀ20 : 80)
5l (Z)
NOH
NOH
NOH
NOCH3
NOCH3
NOCH3
NOBn
NOBn
NOBn
a) Recrystallized from EtOH–CHCl3. b) Yield after recrystallization.

896
Vol. 55, No. 6
Table 2. Selected 1H-NMR Chemical Shifts of Oximes 5d—f and O-Substituted Oximes 5g—l
d CH2
d H-2 thiophene
d H-4 thiophene
Compd.
X
Y
R
E
Z
E
Z
E
Z
5d (E)
5e (E/Z)
5f (E)
5g (E/Z)
5h (E/Z)
5i (E/Z)
5j (Z)
5k (E/Z)
5l (Z)
N
NOH
NOH
NOH
NOCH3
NOCH3
NOCH3
NOBn
NOBn
NOBn
Et
Et
c-Pr
Et
Et
c-Pr
Et
3.41
3.45
3.45
3.48
3.44
3.45
—
—
3.68
—
3.70
3.66
3.67
3.67
3.70
3.71
8.41
8.40—8.44
8.42
—
7.93—7.96
—
7.73
7.73
7.73
7.74
7.66
7.67
—
—
7.45
—
7.56
7.44
CH
CH
N
CH
CH
N
8.37
8.36
8.37
—
8.39
—
7.88
8.02
8.02
8.02
8.01
8.02
7.45
7.30—7.45
7.31—7.45
7.32—7.45
CH
CH
Et
c-Pr
3.45
—
7.69
—
Table 3. In Vitro Antibacterial Activities of Compounds 5a—l in Comparison with Reference Drugs Norfloxacin 1 and Ciprofloxacin 2 against Selected
Strains (MICs in mg/ml)
Compd.
X
Y
R
S. a.a)
MRSA I MRSA II
S. e.
B. s.
E. c.
K. p
P. a.
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
N
O
O
O
Et
Et
c-Pr
Et
Et
c-Pr
Et
Et
c-Pr
Et
Et
c-Pr
3.13
1.56
0.78
0.78
0.39
0.098
1.56
0.39
0.19
1.56
1.56
0.78
0.78
0.78
0.19
1.56
0.78
0.19
1.56
1.56
0.78
0.78
0.78
0.19
1.56
0.78
0.19
1.56
1.56
0.39
0.78
0.39
0.098
1.56
0.78
0.19
0.39
0.39
0.19
0.049
0.098
0.78
0.39
12.5
0.78
0.39
0.19
25
CH
CH
N
CH
CH
N
CH
CH
N
CH
CH
0.098
0.049
0.098
0.78
0.78
25
1.56
0.39
50
6.25
0.78
0.098
0.025
0.098
0.049
6.25
3.13
6.25
NOH
NOH
NOH
NOCH3
NOCH3
NOCH3
NOBn
NOBn
NOBn
50
0.049 ꢁ100
3.13
0.39
0.19
25
3.13
ꢁ100
50
25
50
25
0.78
ꢁ100
50
3.13
ꢁ100
50
3.13
50
25
0.78
50
6.25
0.39
ꢁ100
ꢁ100
50
0.19
1, norfloxacin
2, ciprofloxacin
0.39
0.19
0.78
0.39
0.78
0.39
0.39
0.19
0.098
0.025
0.049
0.013
0.025
0.013
3.13
0.39
a) S. a.: Staphylococcus aureus ATCC 6538p, MRSA I and II: methicillin-resistant Staphylococcus aureus (clinical isolates I and II), S. e.: Staphylococcus epidermidis ATCC
12228, B. s.: Bacillus subtilis ATCC 6633, E. c.: Escherichia coli ATCC 8739, K. p.: Klebsiella pneumoniae ATCC 10031, P. a.: Pseudomonas aeruginosa ATCC 9027.
testing of N-[2-(thiophen-3-yl)-2-oxoethyl] piperazinyl qui- lones 5a—i demonstrated a high inhibition of all the tested
nolones 5a—c and their oxime derivatives 5d—l against a Gram-positive microorganisms and a lot of compounds have
panel of selected Gram-positive [Staphylococcus aureus MIC values in the range of 0.098—3.13 mg/ml. Most of the
ATCC 6538p, methicillin-resistant Staphylococcus aureus compounds demonstrated an excellent antimicrobial activity
(MRSA I and MRSA II, clinical isolates), Staphylococcus against Staphylococcus aureus (MICs 0.098—3.13 mg/ml),
epidermidis ATCC 12228, Bacillus subtilis ATCC 6633], and Staphylococcus epidermidis (MICs 0.098—1.56 mg/ml) and
Gram-negative (Escherichia coli ATCC 8739, Klebsiella Bacillus subtilis (MICs 0.049—1.56 mg/ml). It is worth
pneumoniae ATCC 10031 and Pseudomonas aeruginosa noting that compounds 5a—i are potent also towards clinical
ATCC 9027) bacteria, are reported in Table 3, in comparison isolates of methicillin-resistant Staphylococcus aureus
with those of the reference drugs norfloxacin and cipro- (MRSA I and MRSA II) that are inhibited by many com-
floxacin.
pounds at concentrations of 0.19—1.56 mg/ml. Furthermore,
In general, the MIC values of test derivatives indicate that the data obtained indicate that the antibacterial activity
the compounds 5a—i and 5l showed significant antibacterial against methicillin-resistant Staphylococcus aureus is com-
activity, whereas the compounds 5j and 5k exhibited moder- parable to that exhibited against the methicillin-susceptible
ate to poor activity against Gram-negative and Gram-positive one. Table 3 reveals that compound 5f followed by 5i are
bacteria.
superior in inhibiting the growth of staphylococci (MICs
The novel N-[2-(thiophen-3-yl)ethyl] piperazinyl quino- 0.098—0.19 mg/ml), while the remaining compounds 5b—e

June 2007
897
Kofler hot stage apparatus (C. Reichert, Vienna, Austria) and are uncor-
rected. The IR spectra were obtained on a Shimadzu 470 spectrophotometer
(potassium bromide disks; Shimadzu, Tokyo, Japan). H-NMR spectra were
measured using a Bruker 500 spectrometer (Bruker, Rheinstetten, Ger-
many), and chemical shifts are expressed as d (ppm) with tetramethylsilane
as internal standard. Elemental analyses were carried out on a CHN rapid el-
emental analyzer (GmbH-Germany) for C, H and N, and the results are
within ꢃ0.4% of the theoretical values.
and 5g—h are equivalent in antibacterial activity against
these microorganisms with respectable activity (MICs 0.78—
1.56 mg/ml). More potent compounds 5f and 5i possessed
comparable or better activity with respect to the reference
drugs norfloxacin and ciprofloxacin against staphylococci.
Compound 5a—i showed moderate or poor activity, ex-
pressed as minimal inhibitory concentrations (MIC), against
Pseudomonas aeruginosa (MICꢁ3.13—100 mg/ml), whereas
they exhibited a strong effectiveness towards other Gram-
negative bacteria (MICꢂ3.13 mg/ml). More significant in-
hibitory properties were detected for ketone derivative 5c
against Escherichia coli (MICꢀ0.049 mg/ml) and Klebsiella
pneumoniae (MICꢀ0.025 mg/ml) as well as towards Pseu-
domonas aeruginosa at the concentration of 3.13 mg/ml. It is
also interesting to note that Gram-positive microorganism
Bacillus subtilis was more susceptible to 5c than to 5f (the
more potent compound against staphylococci).
1
General Procedure for the Synthesis of Compounds 5a—l A mixture
of 2-bromo-1-(thiophen-3-yl)ethanone
7 or 2-bromo-1-(thiophen-3-yl)-
ethanone oxime derivatives 8a—c (0.55 mmol), quinolone 1—3 (0.5 mmol)
and NaHCO3 (0.5 mmol) in DMF (5 ml), was stirred at room temperature for
2—7 d. After consumption of quinolone 1—3, water (20 ml) was added and
the precipitate was filtered, washed with water and crystallized from
EtOH–CHCl3 to give compounds 5a—l.
1-Ethyl-6-fluoro-1,4-dihydro-7-[4-[2-(thiophen-3-yl)-2-oxoethyl]piper-
azin-1-yl]-4-oxo-1,8-naphthyridine-3-carboxylic Acid (5a): 1H-NMR
(DMSO-d6) d: 1.40 (t, 3H, CH3, Jꢀ7.0 Hz), 2.68—2.76 (m, 4H, piperazine),
3.85 (s, 2H, COCH2), 3.86—3.90 (m, 4H, piperazine), 4.49 (q, 2H, CH2-
Me), 7.54 (dd, 1H, H4-thiophene, Jꢀ5.0, 1.1 Hz), 7.63 (dd, 1H, H5-thio-
phene, Jꢀ5.0, 2.8 Hz), 8.11 (d, 1H, H5-quinolone, Jꢀ13.5 Hz), 8.60 (dd, 1H,
H2-thiophene, Jꢀ2.8, 1.1 Hz), 8.99 (s, 1H, H2-quinolone), 15.33 (s, 1H,
COOH). IR (KBr) cmꢄ1: 1635, 1678, 1713 (CꢀO).
Considering the varied structure–activity relationships of
different series of compounds, it can be inferred that the anti-
bacterial properties of compounds is determined by the com-
bination influence of N-1 substituent (ethyl or cyclopropyl)
and functionality on ethyl linker of thiophene and piperazine.
In accordance to previous antibacterial studies, among ke-
1-Ethyl-6-fluoro-1,4-dihydro-7-[4-[2-(thiophen-3-yl)-2-oxoethyl]piper-
azin-1-yl]-4-oxo-3-quinolone Carboxylic Acid (5b): 1H-NMR (DMSO-d6)
d: 1.42 (t, 3H, CH3, Jꢀ7.0 Hz), 2.72—2.80 (m, 4H, piperazine), 3.32—3.40
(m, 4H, piperazine), 3.85 (s, 2H, COCH2), 4.60 (q, 2H, CH2-Me, Jꢀ7.0 Hz),
7.20 (d, 1H, H8-quinolone, Jꢀ7.2 Hz), 7.56 (dd, 1H, H4-thiophene, Jꢀ5.0,
tones, oxime, O-methyl oxime and O-benzyl oxime deriva- 1.0 Hz), 7.64 (dd, 1H, H5-thiophene, Jꢀ5.0, 2.8 Hz), 7.92 (d, 1H, H5-
quinolone, Jꢀ13.3 Hz), 8.62 (dd, 1H, H2-thiophene, Jꢀ2.7, 1.0 Hz), 8.96 (s,
tives of (2-oxyiminoethyl) piperazinyl quinolones, lower sus-
ceptibilities (higher MICs) were observed with O-benzyl
oxime derivatives. Thus, the O-benzyl oxime moiety dimin-
1H, H2-quinolone), 15.36 (s, 1H, COOH). IR (KBr) cmꢄ1: 1624, 1682, 1718
(CꢀO).
1-Cyclopropyl-6-fluoro-1,4-dihydro-7-[4-[2-(thiophen-3-yl)-2-oxoethyl]-
piperazin-1-yl]-4-oxo-3-quinolone Carboxylic Acid (5c): 1H-NMR (DMSO-
d6) d: 1.14—1.24 (m, 2H, cyclopropyl), 1.28—1.40 (m, 2H, cyclopropyl),
2.74—2.82 (m, 4H, piperazine), 3.35—3.40 (m, 4H, piperazine), 3.80 (m,
1H, cyclopropyl), 3.86 (s, 2H, COCH2), 7.56 (d, 1H, H4-thiophene,
Jꢀ4.9 Hz), 7.59 (d, 1H, H8-quinolone, Jꢀ5.6 Hz), 7.62—7.67 (m, 1H, H5-
thiophene), 7.91 (d, 1H, H5-quinolone, Jꢀ13.2 Hz), 8.62 (m, 1H, H2-thio-
ished the activity against both Gram-positive and Gram-neg-
ative bacteria. As we can see, the most potent compound
against staphylococci (compound 5f) belongs to the oxime
series, whereas the most potent compound against Gram-
negativies (compound 5c) belongs to the ketone series. Thus,
the type of functionality on ethyl spacer has a different influ-
ence on antibacterial profile against Gram-positives and
Gram-negatives. Comparison between MIC values of ketone
analogs 5a—c and their oxime counterpart revealed that al-
teration of ketone to oxime group caused a significant in-
crease in antibacterial activity against staphylococci only in
ciprofloxacin series (5f vs. 5c). In addition, O-methylation or
O-benzylation of oxime derivatives could not improved the
antibacterial activity. However, in some cases, the presence
or introduction of various functional groups in a compound
phene), 8.67 (s, 1H, H2-quinolone), 15.21 (s, 1H, COOH). IR (KBr) cmꢄ1
:
1618, 1679, 1731 (CꢀO).
1-Ethyl-6-fluoro-1,4-dihydro-7-[4-[2-(thiophen-3-yl)-2-hydroxyimi-
noethyl]piperazin-1-yl]-4-oxo-1,8-naphthyridine-3-carboxylic Acid (5d):
1
(E)-isomer. H-NMR (DMSO-d6) d: 1.38 (t, 3H, CH3, Jꢀ6.8 Hz), 2.58—
2.63 (m, 4H, piperazine), 3.41 (s, 2H, –NꢀCCH2–), 3.77—3.84 (m, 4H,
piperazine), 4.49 (q, 2H, CH2-Me, Jꢀ7.0 Hz), 7.54 (dd, 1H, H5-thiophene,
Jꢀ5.0, 3.0 Hz), 7.73 (dd, 1H, H4-thiophene, Jꢀ5.0, 1.0 Hz), 8.09 (d, 1H, H5-
quinolone, Jꢀ13.5 Hz), 8.41 (dd, 1H, H2-thiophene, Jꢀ2.9, 1.0 Hz), 8.98 (s,
1H, H2-quinolone), 11.42 (s, 1H, NOH), 15.32 (s, 1H, COOH). IR (KBr)
cmꢄ1: 1634, 1720 (CꢀO).
1-Ethyl-6-fluoro-1,4-dihydro-7-[4-[2-(thiophen-3-yl)-2-hydroxyimi-
does not allow to accurately explain the kind and intensity of noethyl]piperazin-1-yl]-4-oxo-3-quinolone Carboxylic Acid (5e): Mixture of
1
(E)- and (Z)-isomers (E/Zꢀ65 : 35). H-NMR (DMSO-d6) d: 1.40 (t, 3H,
CH3, Jꢀ6.9 Hz), 2.59—2.71 (m, 4H, piperazine), 3.25—3.33 (m, 4H, piper-
azine), 3.45 (s, 2H, –NꢀCCH2–, E-isomer), 3.68 (s, 2H, –NꢀCCH2–, Z-iso-
mer), 4.58 (q, 2H, CH2-Me, Jꢀ7.0 Hz), 7.19 (d, 1H, H8-quinolone,
Jꢀ6.8 Hz), 7.45 (d, 1H, H4-thiophene, Z-isomer, Jꢀ5.0 Hz), 7.50—7.55 (m,
1H, H5-thiophene, Z and E isomers), 7.73 (d, 1H, H4-thiophene, E-isomer,
Jꢀ5.1 Hz), 7.92 (d, 1H, H5-quinolone, Jꢀ13.6 Hz), 7.93—7.96 (m, 1H, H2-
thiophene, Z-isomer), 8.40—8.44 (m, 1H, H2-thiophene, E-isomer), 8.95 (s,
1H, H2-quinolone), 11.27 (s, 1H, NOH, Z-isomer), 11.40 (s, 1H, NOH, E-
isomer), 15.35 (s, 1H, COOH). IR (KBr) cmꢄ1: 1625, 1714 (CꢀO).
its antibacterial activity.
In conclusion, we have synthesized novel quinolone agents
bearing N-[2-(thiophen-3-yl)ethyl] piperazinyl moiety in the
7-position of the quinolone ring. In vitro antibacterial evalua-
tion of target compounds showed that N-[2-(thiophen-3-
yl)ethyl] group attached to piperazine ring served as promis-
ing C-7 substituents for piperazinyl quinolones. Among these
derivatives, ciprofloxacin derivatives, containing N-[2-(thio-
phen-3-yl)-2-hydroxyimino ethyl] or N-[2-(thiophen-3-yl)-2-
methoxyimino ethyl] residue provided a high inhibition of all
the tested Gram-positive organisms including MRSA compa-
rable or superior with respect to the reference drugs nor-
floxacin and ciprofloxacin.
1-Cyclopropyl-6-fluoro-1,4-dihydro-7-[4-[2-(thiophen-3-yl)-2-hydroxy-
iminoethyl]piperazin-1-yl]-4-oxo-3-quinolone Carboxylic Acid (5f): (E)-
isomer. H-NMR (DMSO-d6) d: 1.15—1.19 (m, 2H, cyclopropyl), 1.27—
1.32 (m, 2H, cyclopropyl), 2.62—2.70 (m, 4H, piperazine), 3.25—3.33 (m,
2H, piperazine), 3.45 (s, 2H, –NꢀCCH2–), 3.78—3.83 (m, 1H, cyclo-
propyl), 7.53 (dd, 1H, H5-thiophene, Jꢀ5.0, 3.0 Hz), 7.57 (d, 1H, H8-
quinolone, Jꢀ7.36 Hz), 7.73 (d, 1H, H4-thiophene, Jꢀ5.0 Hz), 7.91 (d, 1H,
H5-quinolone, Jꢀ13.2 Hz), 8.42 (d ,1H, H2-thiophene, Jꢀ2.9 Hz), 8.66 (s,
1
Experimental
Chemicals and all solvents used in this study were purchased from Merck 1H, H2-quinolone), 11.43 (s, 1H, NOH), 15.23 (s, 1H, COOH). IR (KBr)
AG and Aldrich Chemical. The 2-bromo-1-(thiophen-3-yl)ethanone 7 and 2-
bromo-1-(thiophen-3-yl)ethanone oxime derivatives 8 were prepared accord-
ing to the literature methods.10—12) Melting points were determined on a
cmꢄ1: 1618, 1731 (CꢀO).
1-Ethyl-6-fluoro-1,4-dihydro-7-[4-[2-(thiophen-3-yl)-2-methoxyimino-
ethyl]piperazin-1-yl]-4-oxo-1,8-naphthyridine-3-carboxylic Acid (5g): Mix-

898
Vol. 55, No. 6
ture of (E)- and (Z)-isomers (E/Zꢀ17 : 83). 1H-NMR (DMSO-d6) d: 1.53 (t,
3H, CH3, Jꢀ7.1 Hz), 2.68—2.75 (m, 4H, piperazine), 3.48 (s, 2H,
NꢀCCH2–, E-isomer), 3.70 (s, 2H, NꢀCCH2–, Z-isomer), 3.87—3.93 (m,
4H, piperazine), 4.00 (s, 3H, OCH3, Z-isomer), 4.05 (s, 3H, OCH3, E-iso-
1.27—1.31 (m, 2H, cyclopropyl), 2.61—2.67 (m, 4H, piperazine), 3.26—
3.29 (m, 4H, piperazine), 3.71 (s, 2H, –NꢀCCH2–), 3.78—3.83 (m, 1H, cy-
clopropyl), 5.20 (s, 2H, OCH2), 7.32—7.45 (m, 1H, H4-thiophene and 5H,
phenyl), 7.53—7.57 (m, 1H, H5-thiophene and 1H, H8-quinolone), 7.90 (d,
mer), 4.43 (q, 2H, CH2-Me, Jꢀ7.0 Hz), 7.31—7.35 (m, 1H, H5-thiophene, E 1H, H5-quinolone, Jꢀ13.2 Hz), 8.02 (dd, 1H, H2-thiophene, Jꢀ2.8, 1.0 Hz),
and Z isomers), 7.56 (d, 1H, H4-thiophene, Z-isomer, Jꢀ5.0 Hz), 7.74 (d, 8.66 (s, 1H, H2-quinolone), 15.21 (s, 1H, COOH). IR (KBr) cmꢄ1: 1627,
1H, H4-thiophene, E-isomer, Jꢀ5.0 Hz), 7.88 (d, 1H, H2-thiophene, Z-iso-
mer, Jꢀ2.1 Hz), 8.14 (d, 1H, H5-quinolone, Jꢀ13.5 Hz), 8.37 (d, 1H, H2-
thiophene, E-isomer, Jꢀ2.0 Hz), 8.73 (s, 1H, H2-quinolone), 15.09 (s, 1H,
COOH). IR (KBr) cmꢄ1: 1629, 1721 (CꢀO).
1733 (CꢀO).
Antibacterial Activity Compounds 5a—l were evaluated for their anti-
bacterial activity using conventional agar-dilution method.16) Two-fold serial
dilutions of the compounds and reference drugs were prepared in
Mueller–Hinton agar. Drugs (10.0 mg) were dissolved in DMSO (1 ml) and
1-Ethyl-6-fluoro-1,4-dihydro-7-[4-[2-(thiophen-3-yl)-2-methoxyimino-
ethyl]piperazin-1-yl]-4-oxo-3-quinolone Carboxylic Acid (5h): Mixture of the solution was diluted with water (9 ml). Further progressive double dilu-
(E)- and (Z)-isomers (E/Zꢀ21 : 79). 1H-NMR (DMSO-d6) d: 1.40 (t, 3H, tion with melted Mueller–Hinton agar was performed to obtain the required
CH3, Jꢀ6.8 Hz), 2.61—2.68 (m, 4H, piperazine), 3.22—3.33 (m, 4H, piper-
azine), 3.44 (s, 2H, –NꢀCCH2–, E-isomer), 3.66 (s, 2H, –NꢀCCH2–, Z-iso-
mer), 3.90 (s, 3H, OCH3, Z-isomer), 3.91 (s, 3H, OCH3, E-isomer), 4.58 (q,
2H, CH2-Me, Jꢀ6.6 Hz), 7.17 (d, 1H, H8-quinolone, Jꢀ6.6 Hz), 7.44 (d, 1H,
concentrations of 100, 50, 25, 12.5, 6.25, 3.13, 1.56, 0.78, 0.39, 0.19, 0.098,
0.049, 0.025, 0.013, 0.006 and 0.003 mg/ml. The bacteria inocula were pre-
pared by suspending overnight colonies from Mueller–Hinton agar media in
0.85% saline. The inocula were adjusted photometrically at 600 nm to a cell
H4-thiophene, Z-isomer, Jꢀ4.4 Hz), 7.52—7.58 (m, 1H, H5-thiophene, E density equivalent to approximately 0.5 McFarland standard (1.5ꢅ108
and Z isomers), 7.66 (d, 1H, H4-thiophene, E-isomer, Jꢀ4.5 Hz), 7.92 (d, CFU/ml). The suspensions were then diluted in 0.85% saline to give 107
1H, H5-quinolone, Jꢀ13.2 Hz), 8.02 (d, 1H, H2-thiophene, Z-isomer, CFU/ml. Petri dishes were spot-inoculated with 1 ml of each prepared bacter-
Jꢀ1.9 Hz), 8.36 (d, 1H, H2-thiophene, E-isomer, Jꢀ2.0 Hz), 8.95 (s, 1H, H2- ial suspension (104 CFU/spot) and incubated at 35—37 °C for 18 h. The min-
quinolone), 15.34 (s, 1H, COOH). IR (KBr) cmꢄ1: 1618, 1718 (CꢀO).
1-Cyclopropyl-6-fluoro-1,4-dihydro-7-[4-[2-(thiophen-3-yl)-2-methoxy-
imum inhibitory concentration (MIC) was the lowest concentration of the
test compound, which resulted in no visible growth on the plate. To ensure
iminoethyl]piperazin-1-yl]-4-oxo-3-quinolone Carboxylic Acid (5i): Mix- that the solvent had no effect on bacterial growth, a control test was per-
1
ture of (E)- and (Z)-isomers (E/Zꢀ9 : 91). H-NMR (DMSO-d6) d: 1.13— formed with test medium supplemented with DMSO at the same dilutions as
1.20 (m, 2H, cyclopropyl), 1.28—1.33 (m, 2H, cyclopropyl), 2.63—2.71 (m, used in the experiment.
4H, piperazine), 3.27—3.33 (m, 4H, piperazine), 3.45 (s, 2H, –NꢀCCH2–,
E-isomer), 3.67 (s, 2H, –NꢀCCH2–, Z-isomer), 3.74—3.84 (m, 1H, cyclo-
Acknowledgments This work was supported by a grant from the Re-
propyl), 3.91 (s, 3H, OCH3, Z-isomer), 3.92 (s, 3H, OCH3, E-isomer), 7.45 search Council of Ministry of Health and Medical Education of Iran.
(d, 1H, H4-thiophene, Z-isomer, Jꢀ5.0 Hz), 7.52—7.55 (m, 1H, H5-thio-
phene), 7.59 (d, 1H, H8-quinolone, Jꢀ7.7 Hz), 7.67 (d, 1H, H4-thiophene, E- References
isomer, Jꢀ5.0 Hz), 7.90 (d, 1H, H5-quinolone, Jꢀ13.2 Hz), 8.02 (d, 1H, H2-
thiophene, Z-isomer, Jꢀ2.1 Hz), 8.37 (d, 1H, H2-thiophene, E-isomer,
Jꢀ2.0 Hz), 8.66 (s, 1H, H2-quinolone), 15.21 (s, 1H, COOH). IR (KBr)
cmꢄ1: 1629, 1731 (CꢀO).
1) Coleman K., Drug Discovery Today: Therapeutic Strategies, 1, 455—
460 (2004).
2) Hooper D. C., Clin. Infect. Dis., 30, 243—254 (2000).
3) Hooper D. C., Drugs, 58, 6—10 (1999).
1-Ethyl-6-fluoro-1,4-dihydro-7-[4-[2-(thiophen-3-yl)-2-(phenylmethoxy-
imino)ethyl]piperazin-1-yl]-4-oxo-1,8-naphthyridine-3-carboxylic Acid (5j):
(Z)-isomer. H-NMR (DMSO-d6) d: 1.38 (m, 3H, CH3, Jꢀ6.9 Hz), 2.57—
2.60 (m, 4H, piperazine), 3.67 (s, 2H, NꢀCCH2–), 3.73—3.80 (m, 4H,
piperazine), 4.49 (q, 2H, CH2-Me, Jꢀ6.6 Hz), 5.18 (s, 2H, CH2-ph), 7.30—
7.45 (m, 1H, H4-thiophene and 5H, phenyl), 7.54 (dd, 1H, H5-thiophene,
Jꢀ4.9, 2.9 Hz), 8.02 (d, 1H, H2-thiophene, Jꢀ2.7 Hz), 8.10 (d, 1H, H5-
quinolone, Jꢀ13.5 Hz), 8.98 (s, 1H, H2-quinolone), 15.33 (s, 1H, COOH).
IR (KBr) cmꢄ1: 1627, 1721 (CꢀO).
4) Berger J. M., Gamblin S. J., Harrison S. C., Wang J. C., Nature (Lon-
don), 379, 225—232 (1996).
5) Emami S., Shafiee A., Foroumadi A., Mini-Rev. Med. Chem., 6, 375—
386 (2006).
6) Ball P., J. Antimicrob. Chemother., 46 (Suppl. T1), 17—24 (2000).
7) Appelbaum P. C., Hunter P. A., Int. J. Antimicrob. Agents, 16, 5—15
(2000).
8) Van Bambeke F., Michot J.-M., Van Eldere J., Tulkens P. M., Clin. Mi-
crobiol. Infect., 11, 256—280 (2005).
1
1-Ethyl-6-fluoro-1,4-dihydro-7-[4-[2-(thiophen-3-yl)-2-(phenylmethoxy-
imino)ethyl]piperazin-1-yl]-4-oxo-3-quinolone Carboxylic Acid (5k): Mix-
ture of (E)- and (Z)-isomers (E/Zꢀ20 : 80). 1H-NMR (DMSO-d6) d: 1.40 (t,
3H, CH3, Jꢀ6.6 Hz), 2.58—2.67 (m, 4H, piperazine), 3.20—3.30 (m, 4H,
piperazine), 3.45 (s, 2H, –NꢀCCH2–, E-isomer), 3.70 (s, 2H, –NꢀCCH2–,
Z-isomer), 4.57 (q, 2H, CH2-Me, Jꢀ5.8 Hz), 5.19 (s, 2H, OCH2-ph, Z-iso-
mer), 5.21 (s, 2H, OCH2-ph, E-isomer), 7.16 (d, 1H, H8-quinolone,
Jꢀ7.1 Hz), 7.31—7.45 (m, 1H, H4-thiophene, Z-isomer and 5H, phenyl),
9) Peterson L., Clin. Infect. Dis., 33 (Suppl. 3), 180—186 (2001).
10) Mirzaei M., Foroumadi A., Pharm. Pharmacol. Commun., 6, 351—
354 (2000).
11) Foroumadi A., Emami S., Mehni M., Moshafi M. H., Shafiee A.,
Bioorg. Med. Chem. Lett., 15, 4536—4539 (2005).
12) Foroumadi A., Oboudiat M., Emami S., Karimollah A., Saghaee L.,
Moshafi M. H., Shafiee A., Bioorg. Med. Chem., 14, 3421—3427
(2006).
7.53 (dd, 1H, H5-thiophene, Z-isomer, Jꢀ5.0, 2.9 Hz), 7.57 (dd, 1H, H5-thio- 13) Emami S., Falahati M., Banifatemi A., Shafiee A., Bioorg. Med.
phene, E-isomer, Jꢀ5.0, 2.8 Hz), 7.69 (dd, 1H, H4-thiophene, E-isomer,
Chem., 12, 5881—5889 (2004).
Jꢀ4.6, 1.1 Hz), 7.91 (d, 2H, H5-quinolone, Jꢀ12.99 Hz), 8.01 (dd, 1H, H2- 14) Emami S., Falahati M., Banifatemi A., Amanlou M., Shafiee A.,
thiophene, Z-isomer, Jꢀ2.8, 1.0 Hz), 8.39 (dd, 1H, H2-thiophene, E-isomer,
Jꢀ1.0, 2.9 Hz), 8.94 (s, 1H, H2-quinolone), 15.34 (s, 1H, COOH). IR (KBr)
cmꢄ1: 1613, 1731 (CꢀO).
Bioorg. Med. Chem., 12, 3971—3976 (2004).
15) Emami S., Shafiee A., Heterocycles, 55, 2059—2074 (2001).
16) European Committee for Antimicrobial Susceptibility Testing (EU-
CAST) of the European Society of Clinical Microbiology and Infec-
tious Diseases (ESCMID). “Clinical Microbiology and Infection,” Eu-
cast Definitive Document E. Def 3.1, 2000, 6, pp. 509—515.
1-Cyclopropyl-6-fluoro-1,4-dihydro-7-[4-[2-(thiophen-3-yl)-2-(phenyl-
methoxyimino)ethyl]piperazin-1-yl]-4-oxo-3-quinolone Carboxylic Acid
1
(5l): (Z)-isomer. H-NMR (DMSO-d6) d: 1.15—1.20 (m, 2H, cyclopropyl),