Synthesis and antibacterial activity of novel levofloxacin analogues

Article abstract of DOI:10.14233/ajchem.2014.15949

To search for new fluoroquinolones with better biological activities, twenty-six levofloxacin analogues were designed, synthesized and evaluated for antimicrobial activity against Gram-positive and Gram-negative bacterias. The results indicated that most compounds tested in this study exhibited comparable or better antimicrobial activity than levofloxacin as reference drug. Among these compounds, ZY6a was the most active compound against Staphylococcus aureus, Staphylococcus epidermidis and Pseudomonas aeruginosa, its activity was found to be 2 to 16 times better than levofloxacin.

Full text of DOI:10.14233/ajchem.2014.15949

Asian Journal of Chemistry; Vol. 26, No. 8 (2014), 2344-2350
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.15949
Synthesis and Antibacterial Activity of Novel Levofloxacin Analogues
1
1
2
2
2
1,*
TONG-XIN ZHANG , WEI-XI ZHANG , HUA-JUN ZHU , LONG LIANG , ZHI-PENG CHENG and MEI-MING LUO
1Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064,
P.R. China
2Sichuan Kelun Pharmaceutical Co., Ltd, 18, 19/F, 2/Building, Jinjiang Times Garden, 107 Jinli West Road, Chengdu 610067, P.R. China
*Corresponding author: Tel: +86 28 85462021; E-mail: luomm@scu.edu.cn
Received: 22 June 2013;
Accepted: 26 September 2013;
Published online: 15 April 2014;
AJC-15023
To search for new fluoroquinolones with better biological activities, twenty-six levofloxacin analogues were designed, synthesized and
evaluated for antimicrobial activity against Gram-positive and Gram-negative bacterias. The results indicated that most compounds tested
in this study exhibited comparable or better antimicrobial activity than levofloxacin as reference drug. Among these compounds, ZY6a
was the most active compound against Staphylococcus aureus, Staphylococcus epidermidis and Pseudomonas aeruginosa, its activity
was found to be 2 to 16 times better than levofloxacin.
Keywords: Fluoroquinolones, Levofloxacin, 2-Methylpiperazine, Antibacterial activity, Synthesis.
from the typical quinolones bicyclic ring substructure. It has
already been widely used in human and veterinary medicine
and expresses broad-spectrum antibacterial activity6.Althrough
many advances in the fluoroquinolone field, the growing of
multidrug-resistant pathogens has become a great cause for
concern. It is therefore important to search for new fluoroqui-
nolones capable of dealing with the resistant strains.
INTRODUCTION
Fluoroquinolones (FQs) constitute an important class of
antibacterial agents which play a major role in the treatment
of ocular infections, soft tissue infections, respiratory infec-
tions, typhoid fever, bone-joint infections, community acquired
pneumonia, etc1. The fluoroquinolones express antibacterial
action by interfering with the function of two bacterial type-II
topoisomerase enzymes2, DNA gyrase and topoisomerase IV.
In general, the portions of a fluoroquinolone molecule (Fig. 1),
C-3 carboxyl, C-4 keto, C-6 fluorine, are considered as essential,
major improvements focus on C-7 position3. The substituents
at C-7 position are associated with some properties of fluoro-
quinolones such as their antibacterial spectrum, bioavailability,
solubility and pharmacokinetics4. The most common substi-
tuents at C-7 are cyclic nitrogen-containing groups such as
piperazine derivatives which are extensively investigated.
Moreover, the basic N-4 position of piperazine is a suitable
site for introducing new functionality5. Levofloxacin (Fig. 1),
a chiral version of the earlier fluoroquinolone ofloxacin, has a
fused ring connecting the N-1 to C-8 position, which is different
The knowledge that planar feature of piperazinyl ring at
C-7 and naphthyridine ring in a fluoroquinolone molecule is
associated with antibacterial activity attracted our attention7.
In this paper, we designed and synthesized novel lead fluoro-
quinolones by introducing 2-methylpiperazin-1-yl ring to
tricyclic ring structure of levofloxacin at C-7 position, which
could change the former dihedral angle between piperazinyl
ring and naphthyridine ring. In addition, a new series of deri-
vatives with different kinds of Pharmacophores8 (alkyl, acyl
and sulfonyl) were also synthesized (Table-1).
EXPERIMENTAL
All reagents and solvents were obtained from commercial
sources and were used without further purification. Melting
points were determined with XRC-1 melting point apparatus
without corrected. Thin layer chromatography was carried out
using Merck silica gel GF254 plates and spots were visualized
with ultraviolet (UV) light. Mass spectra was recorded with
Agilent 6210 spectrometer (Agilent Inc., Santa Clara, CA,
USA) using the electrospray ionization (ESI) method. 1H NMR
(400 MHz) and 13C NMR (100 MHz) spectra were recorded
O
R5
O
F
COOH
F
COOH
R2
N
N
R7
N
N
O
R8 R1
Fig. 1. Structural features of fluoroquinolones and levofloxacin

Vol. 26, No. 8 (2014)
Synthesis and Antibacterial Activity of Novel Levofloxacin Analogues 2345
TABLE-1
STRUCTURES OF THE NEW FLUOROQUINOLONES
Entry
Compound Entry
Compound
O
O
F
COOH
F
COOH
COOH
ZY1a
ZY1b
N
N
O
N
N
HN
O
HN
O
O
F
COOH
F
ZY2a
ZY3a
ZY2b
ZY3b
N
N
N
N
N
O
N
O
O
O
F
COOH
F
COOH
O
N
N
O
S
N
N
S
N
O
N
O
O
O
O
O
F
COOH
F
COOH
O
S
O
S
N
N
ZY4a
ZY4b
N
N
N
O
N
O
O
O
H3C
H3C
O
N
O
N
F
COOH
F
COOH
O
ZY5a
ZY6a
ZY7a
ZY5b
O
N
N
N
O
N
O
O
N
O
N
F
COOH
F
COOH
O
O
N
ZY6b
N
N
OCH3
O
N
OCH3
O
O
O
F
COOH
F
COOH
O
O
N
N
ZY7b
ZY8b
ZY9b
N
N
N
O
N
O
H3CO
H3CO
O
O
F
COOH
F
COOH
O
O
N
N
ZY8a
N
N
N
O
N
O
Cl
Cl
O
N
O
N
F
F
COOH
COOH
O
N
O
ZY9a
N
N
O
N
O
Cl
Cl

2346 Zhang et al.
Asian J. Chem.
O
N
O
N
F
COOH
F
COOH
O
ZY10a
ZY10b
O
N
N
Cl
Cl
N
O
N
O
O
N
O
N
F
COOH
F
COOH
O
O
N
ZY11a
ZY12a
ZY11b
ZY12b
N
N
O
N
O
H3C
H3C
O
O
F
F
COOH
COOH
O
O
N
N
N
N
N
O
N
O
O
N
O
F
F
COOH
COOH
O
O
N
N
N
ZY13a
ZY13b
N
O
N
O
O2N
O2N
F
F
in CDCl3, D2O or DMSO-d6. Chemical shifts are reported in
ppm using TMS as internal standard.
O
O
N
B
O
O
F
OEt
F
F
O
General procedure for the synthesis of compounds
ZY1a and ZY1b (Scheme-I): Compounds ZY, a-1 and b-1
were synthesized according to the literature9. A solution of
a-1(b-1) (2 g, 10 mmol) in DMSO (20 mL) was slowly added
to dimethyl sulfoxide (40 mL) solution containing ZY (1.65 g,
5 mmol) under an argon atmosphere. The mixture was stirred
at 40 °C until the reactants were completely converted by TLC
monitoring. Then the reaction mixture was poured into water
(100 mL) in an ice-bath. The resulting mixture was filtered
and the residue was the crude product a-2(b-2). A solution of
a-2(b-2) in methanol (60 mL) was stirred for 12 h under reflux.
The concentrated aqueous HCl (12 mL) was carefully added
and the mixture was stirred for another 12 h under reflux. The
solvent was evaporated, the residue was then dissolved in water
and neutralized with aqueous NaOH (20 %). The resulting
mixture was filtered and the residue was recrystallized from
EtOH to afford the pure products ZY1a, ZY1b.
+
BF3 Et2O
F
N
O
O
ZY
∗
∗
(Boc)2O
+
HN
F
NH
HN
N
Boc
a-1
b-1
F
B
O
N
O
O
F
COOH
F
N
O
N
N
HN
O
N
O
Boc
ZY1a
a-2
F
F
B
O
N
O
O
F
COOH
F
O
N
N
N
HN
1
O
ZY1a: White solid. Yield: 75 %. m.p.: 203-208 °C. H
=
N
O
Boc
3
NMR (400 MHz, CDCl3): δ = 8.63 (s, 1H), 7.79 (d, JF-H
ZY1b
b-2
10.8 Hz, 1H), 4.50 (d, J = 9.2 Hz, 2H), 4.36-4.39 (m, 1H),
3.67 (s, 1H), 3.07-3.24 (m, 5H), 2.70-2.75 (m, 1H), 2.17 (s,
1H), 1.62 (d, J = 6.4 Hz, 3H), 1.01 (d, J = 6.4 Hz, 3H). 13C
NMR (100 MHz, D2O): δ = 174.6, 168.4, 159.3, 145.8, 144.1,
128.9 , 124.5, 121.7, 105.7, 103.0, 102.8, 68.1, 56.3, 50.7,
48.4, 46.1, 43.3, 17.0, 15.1. HRMS (ESI): m/z calcd. for
C18H21N3O4F [M + H]+: 362.1516, found: 362.1514.
Scheme-I: Synthetic routes to compounds ZY1a and ZY1b
NMR (100 MHz, DMSO): δ = 176.4, 165.9, 158.9, 156.4,
146.1, 144.5, 128.9, 128.8, 124.5, 122.9, 122.9, 106.9, 102.8,
102.5, 68.2, 54.8, 49.9, 48.4,.46.2, 43.3, 17.9, 15.9. HRMS
(ESI): m/z calcd. for C18H21N3O4F [M + H]+: 362.1516, found:
362.1513.
1
ZY1b: White solid. Yield: 68 %. m.p.: 193-196 °C. H
=
General procedure for the synthesis of compounds
ZY2a, ZY2b (Scheme-II): The mixture of ZY1a (ZY1b) (1
g, 3 mmol), formaldehyde solution (1 g, 12 mmol) and formic
acid (20 mL) was refluxed for 12 h under argon with stirring.
After cooling to room temperature, the solvent was removed
3
NMR (400 MHz, CDCl3): δ = 8.64 (s,1H) , 7.77 (d, JF-H
11.2 Hz, 1H), 4.46-4.50 (m, 2H), 4.34-4.37 (m,1H), 3.56 (d, J
= 6.8 Hz, 1H), 2.98-3.19 (m, 5H), 2.66-2.71 (m, 1H), 2.17 (s,
1H), 1.63 (d, J = 6.4 Hz, 3H), 0.98 (d, J = 6.4 Hz, 3H). 13C

Vol. 26, No. 8 (2014)
Synthesis and Antibacterial Activity of Novel Levofloxacin Analogues 2347
O
O
N
O
O
F
COOH
F
COOH
F
COOH
F
COOH
+
HCHO, HCOOH
reflux
N
N
N
N
N
R1SO2Cl
N
N
O
N
O
O
N
O
HN
O
HN
O
S
R1
ZY3a
ZY4a
ZY1a
ZY2a
ZY1a
O
O
N
O
N
F
COOH
F
COOH
O
N
F
COOH
HCHO, HCOOH
reflux
F
COOH
+
N
N
N
N
N
O
HN
O
R1SO2Cl
N
O
N
O
O
HN
O
S
ZY2b
ZY1b
R1
ZY3b
ZY4b
Scheme-II: Synthetic routes to compounds ZY2a and ZY2b
ZY1b
under reduced pressure. The residue was recrystallized from
ethyl acetate to afford the pure products ZY2a, ZY2b.
ZY3a, 3b: R1 = CH3
ZY4a, 4b: R1 = 4-methylphenyl
Scheme-III: Synthetic routes to compounds ZY3a, ZY4a, ZY3b and ZY4b
1
ZY2a: White solid. Yield: 85 %. m.p.: 184-188 °C. H
(100 MHz, CDCl3): δ = 177.2, 166.9, 158.8, 157.2, 145.0,
143.9, 132.7, 130.9, 130.8, 129.8, 127.8, 124.3, 123.2, 108.1,
104.9, 104.7, 68.4, 55.5, 52.6, 52.3, 48.8, 46.7, 21.6, 18.4,
16.2. HRMS (ESI): m/z calcd. for C25H27N3O6SF [M + H]+:
516.1605, found: 516.1603.
NMR (400 MHz, CDCl3): δ = 8.68 (s, 1H), 8.43 (s, 1H), 7.76
3
(d, JF-H = 10.0 Hz, 1H), 4.52-4.57 (m, 2H), 4.39-4.42 (m,
1H), 3.99 (s, 1H), 3.61-3.67 (m, 1H), 3.18-3.26 (m, 3H), 2.89-
2.94 (m, 1H), 2.68 (s, 3H), 2.51-2.56 (m, 1H), 1.62 (d, J = 6.8
Hz, 3H), 1.01 (d, J = 6.4 Hz, 3H). 13C NMR (100 MHz, CDCl3):
δ = 177.2, 167.3, 166.9, 145.0, 144.3, 130.2, 124.2, 123.9,
108.2, 104.9, 104.7, 68.5, 60.7, 55.6, 54.3, 51.4, 48.6, 44.5,
18.4, 17.0. HRMS (ESI): m/z calcd. for C19H23N3O4F [M +
H]+: 376.1673, found: 376.1670.
ZY3b: Light yellow solid. Yield: 90 %. m.p.: 202-206
°C. 1H NMR (400 MHz, CDCl3): δ = 14.78 (s, 1H), 8.68 (s,
1H), 7.77 (d, 3JF-H = 10.8 Hz, 1H), 4.38-4.56 (m, 3H), 3.71(d,
J = 6.0 Hz, 1H), 3.27-3.56 (m, 6H), 2.92-2.97 (m, 1H), 2.85
(s, 3H), 1.62 (s, 3H), 1.07 (s, 3H). 13C NMR (100 MHz, CDCl3):
δ = 177.1, 166.9, 158.9, 157.2, 145.1, 143.3, 130.9, 130.8,
124.5, 123.2, 123.2, 108.1, 104.8, 104.7, 68.4, 55.5, 52.9, 52.1,
48.8, 46.5, 34.6, 18.4, 16.1. HRMS (ESI): m/z calcd. for
C19H23N3O6SF [M + H]+: 440.1292, found: 440.1295.
ZY4b: Light yellow solid.Yield: 87 %. m.p.: 142-148 °C.
1H NMR (400 MHz, CDCl3): δ = 14.76 (s, 1H), 8.64 (s, 1H),
7.66-7.74 (m, 3H), 7.36-7.38 (m, 2H), 4.44-4.51 (m, 2H), 4.33-
4.35 (m, 1H), 3.72 (s, 1H), 3.32-3.40 (m, 4H), 2.95-2.98 (m,
1H), 2.59-2.64 (m, 1H), 2.47 (s, 3H), 1.61 (d, J = 6.4 Hz, 3H),
1.02 (d, J = 6.4 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ =
177.2, 166.9, 144.9, 143.9, 143.2, 132.8, 130.9, 130.8, 129.8,
127.8, 124.4, 123.4, 108.2, 105.0, 104.8, 68.3, 55.4, 52.7, 52.3,
48.8, 46.7, 21.6, 18.4, 16.2. HRMS (ESI): m/z calcd. for
C25H27N3O6SF [M + H]+: 516.1605, found: 516.1600.
General procedure (Scheme-IV) for the synthesis of
compounds ZY5a-13a, ZY5b-13b: The mixture of aryl
carboxylic acid (6 mmol) and thionyl chloride (15 mL) was
stirred for 3 h under reflux. Redundant thionyl chloride was
distilled off under reduced pressure to obtain the corresponding
acyl chloride. To a mixture of ZY1a (ZY1b) (1 g, 3 mmol),
triethylamine (1 mL, 6 mmol) and CH2Cl2 (40 mL), a solution
of the prepared acyl chloride in CH2Cl2 (5 mL) was slowly
added. The mixture was stirred for another 2 h at room tempe-
rature, then washed with aqueous HCl and water. The organic
layer was dried over anhydrous Na2SO4 and evaporated. The
residue was recrystallized from EtOH to afford the pure products
ZY5a-13a, ZY5b-13b.
1
ZY2b: White solid. Yield: 82 %. m.p.: 194-196 °C. H
NMR (400 MHz, DMSO): δ = 9.01 (s, 1H), 8.16 (s, 1H), 7.58
(d, 3JF-H = 12.4 Hz, 1H), 4.92-4.97 (m, 1H), 4.59 (d, J = 10.4
Hz, 1H), 4.39 (d, J = 9.6 Hz, 1H), 3.60 (s, 1H), 3.14-3.19 (m,
2H), 2.70-2.72 (m, 1H), 2.60-2.62 (m, 1H), 2.40-2.45 (m, 1H),
2.28 (s, 3H), 2.11-2.16 (m, 1H), 1.43 (d, J = 6.4 Hz, 3H), 0.91
(d, J = 6.4 Hz, 3H). 13C NMR (100 MHz, DMSO): δ = 176.9,
166.4, 163.9, 159.5, 157.0, 146.7, 144.4, 131.0, 130.9, 125.1,
122.7, 107.4, 103.3, 103.1, 68.5, 61.9, 55.5, 55.2, 52.4, 49.1,
46.0, 18.4, 17.1. HRMS (ESI): m/z calcd. for C19H23N3O4F [M
+ H]+: 376.1673, found: 376.1671.
General procedure for the synthesis of compounds
ZY3a, ZY4a, ZY3b, ZY4b (Scheme-III): To a mixture of
ZY1a (ZY1b) (1 g, 3 mmol), triethylamine (1 mL, 6 mmol)
and CH2Cl2 (40 mL), a solution of sulfonyl chloride (6 mmol)
in CH2Cl2 (5 mL) was slowly added. The mixture was stirred
for another 2 h at room temperature, then washed with aqueous
HCl and water. The organic layer was dried over anhydrous
Na2SO4 and evaporated. The residue was recrystallized from
EtOH to afford the pure products ZY3a, ZY4a, ZY3b, ZY4b.
ZY3a: Light yellow solid. Yield: 89 %. m.p.: 218-222
°C. 1H NMR (400 MHz, CDCl3): δ = 14.80 (s, 1H), 8.69 (s,
1H), 7.74 (d, 3JF-H = 10.8 Hz, 1H), 4.39-4.59 (m, 3H), 3.75 (s,
1H), 3.27-3.57 (m, 6H), 2.85-2.96 (m, 1H), 2.85 (s, 3H), 1.62
(s, 3H), 1.07 (s, 3H). 13C NMR (100 MHz, CDCl3): δ = 176.6,
167.2, 159.2, 156.7, 145.4, 143.3, 130.8, 130.7, 124.5, 122.8,
107.8, 105.0, 104.8, 68.6, 55.8, 53.1, 51.9, 48.9, 46.4, 34.7,
18.5, 16.0. HRMS (ESI): m/z calcd. for C19H23N3O6SF [M +
H]+: 440.1292, found: 440.1293.
ZY5a: Light yellow solid.Yield: 90 %. m.p.: 232-235 °C.
1H NMR (400 MHz, CDCl3): δ = 14.82 (s, 1H), 8.66 (s, 1H),
7.77 (d, 3JF-H = 11.2Hz, 1H), 7.44 (s, 5H), 4.49-4.57 (m, 2H),
4.37-4.41 (m, 1H), 4.06-4.21 (m, 1H), 3.12-3.71 (m, 6H), 1.62
(d, J = 6.4 Hz, 3H), 0.95-1.09 (m, 3H). 13C NMR (100 MHz,
CDCl3): δ = 177.1, 170.7, 166.9, 159.2, 156.7, 145.1, 143.1,
ZY4a: Light yellow solid. Yield: 90 %. m.p.: 146-150
°C. 1H NMR (400 MHz, CDCl3): δ = 14.78 (s, 1H), 8.65 (s,
1H), 7.66-7.72 (m, 3H), 7.36-7.52 (m, 2H), 4.34-4.55 (m, 3H),
3.75 (s, 1H), 3.26-3.41 (m, 4H), 2.95-2.99 (m, 1H), 2.58-2.63
(m, 1H), 2.58 (s, 3H), 1.44 (s, 3H), 1.02 (s, 3H). 13C NMR

2348 Zhang et al.
Asian J. Chem.
O
O
3.68 (m, 6H), 1.62 (d, J = 6.8 Hz, 3H), 0.89-1.17 (m, 3H). 13C
NMR (100 MHz, CDCl3): δ = 177.1, 167.3, 167.0, 145.1,
135.7, 135. 6, 135.4, 131.2, 131.1, 130.4, 129.9, 129.7, 128.1,
127.9, 127.3, 127.2, 124.4, 122.6, 107.9, 105.0, 104.8, 68.5,
55.6, 53.2, 52.7, 49.4, 49.0, 48.6, 47.8, 47.6, 42.4, 42.2, 18.4,
16.4, 15.9, 15.5. HRMS (ESI): m/z calcd. for C25H24N3O5ClF
[M + H]+: 500.1389, found: 500.1388.
OH
Cl
+
SOCl2
R2
R2
O
N
O
F
COOH
F
COOH
O
+
Cl
N
N
N
R2
R2
N
O
HN
O
O
ZY1a
ZY5a~13a
ZY10a: Light yellow solid.Yield: 78 %. m.p.: 94-98 °C.
1H NMR (400 MHz, CDCl3): δ = 14.80 (s, 1H), 8.66 (s, 1H),
O
N
O
N
F
COOH
3
7.78 (d, JF-H = 12.8 Hz, 1H), 7.32-7.45 (m, 4H), 4.49-4.55
F
COOH
O
+
N
Cl
N
(m, 2H), 4.38-4.40 (d, 1H), 4.06-4.18 (d, 1H), 3.56-3.72 (m,
3H), 3.13-3.42 (m, 3H), 1.63 (d, J = 6.8 Hz, 3H), 0.96-1.08
(m, 3H). 13C NMR (100 MHz, CDCl3): δ = 177.2, 169.2, 166.9,
145.1, 137.4, 134.7, 130.9, 130.0, 127.3, 125.2, 124.4, 123.1,
108.1, 105.0, 104.9, 68.5, 55.6, 53.9, 53.3, 49.4, 48.4, 18.4,
16.1. HRMS (ESI): m/z calcd. for C25H24N3O5ClF [M + H]+:
500.1389, found: 500.1381.
R2
R2
N
O
HN
O
O
ZY5b~13b
ZY1b
ZY5a, 5b: R2 = H
ZY8a, 8b: R2 = 4-Cl
ZY6a, 6b: R2 = 2-OCH3
ZY9a,9b: R2 = 2-Cl
ZY7a, 7b: R2 = 4-OCH3
ZY10a, 10b: R2 = 3-Cl
ZY13a, 13b: R2 = 4-NO2
ZY11a, 11b: R2 = 4-CH3 ZY12a, 12b: R2 = 3-CH3
Scheme-IV: Synthetic routes to compounds ZY5a-13a and ZY5b-13b
ZY11a: Light yellow solid. Yield: 85 %. m.p.: 216-220
°C. 1H NMR (400 MHz, CDCl3): δ = 14.83 (s, 1H), 8.66 (s,
3
1H), 7.77 (d, JF-H = 11.2Hz, 1H), 7.35-7.37 (m, 2H), 7.22-
135.7, 131.1, 130.9, 129.8, 128.6, 127.1, 124.4, 122.9, 107.9,
104.9, 104.7, 68.5, 55.6, 53.3, 48.4, 42.9, 18.4, 16.0. HRMS
(ESI): m/z calcd. for C22H25N3O5F [M + H]+: 466.1778, found:
466.1775.
7.24 (m, 2H), 4.48-4.54 (m, 2H), 4.36-4.40 (m, 1H), 4.06-
4.19 (m, 1H), 3.60-3.73 (m, 3H), 3.13-3.41 (m, 3H), 2.39 (s,
3H), 1.62 (d, J = 6.8 Hz, 3H), 0.95-1.07 (m, 3H). 13C NMR
(100 MHz, CDCl3): δ = 177.3, 166.9, 144.9, 140.0, 132.7,
131.2, 129.2, 127.3, 124.4, 108.1, 105.1, 104.9, 68.4, 55.6,
21.4, 18.4, 15.9. HRMS (ESI): m/z calcd. for C26H27N3O5F [M
+ H]+: 480.1935, found: 480.1933.
ZY6a: Light yellow solid. Yield: 83 %. m.p.: 226-230
°C. 1H NMR (400 MHz, CDCl3): δ = 14.84 (s, 1H), 8.65 (s,
3
1H), 7.76 (d, JF-H = 11.6Hz, 1H), 7.28-7.39 (m, 2H), 6.93-
7.01 (m, 2H), 4.46-4.55 (m, 2H), 4.37-4.39 (m, 1H), 4.03-
4.09 (m, 1H), 3.72-3.74 (d, 3H), 3.07-3.68 (m, 6H), 1.62 (d,
J = 6.4 Hz, 3H), 0.90-0.96 (m, 3H). 13C NMR (100 MHz,
CDCl3): δ = 177.0, 168.2, 168.2, 167.0, 155.2, 145.1, 131.3,
131.2, 130.6, 128.3, 128.1, 127.8, 125.5, 124. 6, 124.5, 122.4,
121.1, 111.1, 110.9, 107.9, 104.9, 104.8, 68.5, 55.6, 55.6, 53.4,
53.1, 52.7, 49.1, 48.5, 47.9, 47.7, 47.6, 42.4, 42.2, 18.4, 16.1,
15.8, 15.7, 15.4. HRMS (ESI): m/z calcd. for C26H27N3O6F [M
+ H]+: 496.1884, found: 496.1886.
ZY12a: Light yellow solid. Yield: 78 %. m.p.: 138-142
°C. 1H NMR (400 MHz, CDCl3): δ = 14.83 (s, 1H), 8.66 (s,
3
1H), 7.77 (d, JF-H = 11.2 Hz, 1H), 7.21-7.33 (m, 4H), 4.49-
4.58 (m, 2H), 4.37-4.40 (m, 1H), 4.04-4.21 (m, 1H), 3.58-
3.71 (d, 3H), 3.11-3.43 (m, 3H), 2.39 (s, 3H), 1.62 (d, J = 6.8
Hz, 3H), 0.95-1.08 (m, 3H). 13C NMR (100 MHz, CDCl3): δ =
177.2, 166.9, 144. 9, 138.5, 135.6, 130.5, 128.4, 127.8, 124.4,
124.0, 108.1, 105.1, 104.9, 68.4, 55.6, 53.1, 48.5, 21.4, 18.4.
HRMS (ESI): m/z calcd. for C26H27N3O5F [M + H]+: 480.1935,
found: 480.1938.
ZY7a: Light yellow solid. Yield: 88 %. m.p.: 136-140
°C. 1H NMR (400 MHz, CDCl3): δ = 14.83 (s, 1H), 8.66 (s,
1H), 7.77 (d, 3JF-H = 10.8 Hz, 1H), 7.45 (d, J = 8.8 Hz, 2H),
6.95 (d, J = 8.8 Hz, 2H), 4.48-4.55 (m, 2H), 4.40 (d, J = 10.0
Hz, 1H), 4.02 (s, 1H), 3.18-3.85 (m, 6H), 1.62 (d, J = 6.8 Hz,
3H), 1.01 (s, 3H). 13C NMR (100 MHz, CDCl3): δ = 177.2,
170.7, 167.0, 160.9, 159.3, 156.8, 145.1, 143.0, 131.1, 130.9,
129.2, 127.6, 124.4, 123.0, 122.9, 113.8, 108.0, 104.9, 104.8,
68.5, 55.6, 55.4, 53.4, 49.2, 18.4, 16.1. HRMS (ESI): m/z calcd.
for C26H27N3O6F [M + H]+: 496.1884, found: 496.1881.
ZY8a: Light yellow solid. Yield: 80 %. m.p.: 210-212
°C. 1H NMR (400 MHz, CDCl3): δ = 14.80 (s, 1H), 8.67 (s,
ZY13a: Light yellow solid. Yield: 75 %. m.p.: 242-245
°C. 1H NMR (400 MHz, CDCl3): δ = 14.77 (s, 1H), 8.66 (s,
3
1H), 8.33 (d, J = 8.4 Hz, 2H), 7.78 (d, JF-H = 11.2 Hz, 1H),
7.65 (d, J = 8.8 Hz, 2H), 4.50-4.56 (m, 2H), 4.41-4.42 (m,
1H), 4.10-4.39 (m, 1H), 3.44-3.80 (m, 4H), 3.16-3.27 (m, 2H),
1.63 (d, J = 6.8 Hz, 3H), 1.10-1.11 (m, 3H). 13C NMR (100
MHz, DMSO): δ = 176.5, 167.4, 165.9, 158.4, 155.9, 147. 8,
146.3, 143.7, 142.1, 130.0, 129.8, 128.3, 124.6, 123.8, 122.2,
122.1, 106.9, 103.0, 102.8, 68.3, 54.9, 52.6, 48.3, 47.9, 47.6,
17.8, 15.6, 15.3. HRMS (ESI): m/z calcd. for C25H24N4O7F
[M + H]+: 511.1629, found: 511.1623.
3
1H), 7.77 (d, JF-H = 10.8 Hz, 1H), 7.39-7.44 (m, 4H), 4.49-
4.57 (m, 2H), 4.38-4.41 (m, 1H), 4.06-4.18 (m, 1H), 3.13-
3.73 (m, 6H), 1.62 (d, J = 6.8 Hz, 3H), 0.95-1.08 (m, 3H). 13C
NMR (100 MHz, CDCl3): δ = 177.0, 169.7, 167.0, 159.2,
156.7, 145.2, 143.1, 135.9, 133.9, 130.9, 130.8, 128.7, 124.4,
122.9, 107.9, 104.9, 104.8, 68.5, 55.6, 53.4, 48.5, 18.4,
15.9. HRMS (ESI): m/z calcd. for C25H24N3O5ClF [M + H]+:
500.1389, found: 500.1386.
ZY5b: Light yellow solid. Yield: 91 %. m.p.: 264-269
°C. 1H NMR (400 MHz, CDCl3): δ = 14.81 (s, 1H), 8.66 (s,1H),
3
7.77 (d, JF-H = 11.2 Hz, 1H), 7.45-7.52 (m, 5H), 4.48-4.51
(m, 2H), 4.37-4.39 (m, 1H), 4.19-4.20 (m, 1H), 3.15-3.58 (m,
6H), 1.63 (d, J = 6.4 Hz, 3H), 0.95-1.08 (m, 3H). 13C NMR
(100 MHz, CDCl3): δ = 177.2, 170.8, 166.9, 156.8, 144.9,
142.9, 135.6, 131.2, 129.9, 128.6, 124.5, 123.0, 108.0, 104.9,
104.7, 68.3, 55.4, 53.3, 50.8, 48.5, 18.4, 16.0. HRMS (ESI):
m/z calcd. for C22H25N3O5F [M + H]+: 466.1778, found: 466.1771.
ZY6b: Light yellow solid.Yield: 75 %. m.p.: 216-220 °C.
1H NMR (400 MHz, CDCl3): δ = 14.84 (s, 1H), 8.65 (s, 1H),
ZY9a: Light yellow solid.Yield: 75 %. m.p.: 230-232 °C.
1H NMR (400 MHz, CDCl3): δ = 14.81 (s, 1H), 8.66 (d, J = 4
3
Hz, 1H), 7.77 (d, JF-H = 10.8 Hz, 1H), 7.33-7.44 (m, 4H),
4.46-4.53 (m, 2H), 4.37-4.39 (m, 1H), 3.96-4.05 (m, 1H), 3.05-

Vol. 26, No. 8 (2014)
Synthesis and Antibacterial Activity of Novel Levofloxacin Analogues 2349
3
7.76 (d, JF-H = 11.2 Hz, 1H), 7.35-7.40 (m, 1H), 7.31-7.36
(m, 1H), 7.01-7.03 (m, 1H), 6.93-7.00 (m, 2H), 4.47-4.54 (m,
2H), 4.36-4.39 (m, 2H), 3.85-3.88 (m, 3H), 3.07-3.70 (m, 6H),
1.63 (d, J = 6.4 Hz, 3H), 0.89-1.16 (m, 3H). 13C NMR (100
MHz, CDCl3): δ = 177.0, 166.9, 155.2, 144.7, 130.6, 128.1,
121.1, 110.9, 108.2, 104.9, 68.2, 55.5, 55.4, 53.3, 53.2, 47.7,
47.6, 18.4, 16.1. HRMS (ESI): m/z calcd. for C26H27N3O6F [M
+ H]+: 496.1884, found: 496.1882.
55.4, 53.1, 48.6, 21.4, 18.4, 15.9. HRMS (ESI): m/z calcd. for
C26H27N3O5F [M + H]+: 480.1935, found: 480.1937.
ZY12b: Light yellow solid. Yield: 80 %. m.p.: 130-
135 °C. 1H NMR (400 MHz, CDCl3): δ = 14.80 (s, 1H), 8.65
(s, 1H), 7.78 (d, 3JF-H = 11.2 Hz, 1H), 7.21-7.33 (m, 4H), 4.48-
4.53 (m, 2H), 4.36-4.38 (m, 1H), 4.06-4.19 (m, 1H), 3.58-
3.77 (m, 3H), 3.13-3.40 (m, 3H), 2.39 (s, 3H), 1.63 (d, J = 6.4
Hz, 3H), 0.95-1.08 (m, 3H). 13C NMR (100 MHz, CDCl3): δ =
177.1, 166.9, 145.1, 138.5, 135.6, 131.1, 130.6, 128.4, 127.7,
124.5, 124.0, 108.0, 104.9, 104.7, 68.4, 55.5, 53.5, 48.4, 21.4,
18.5, 16.1. HRMS (ESI): m/z calcd. for C26H27N3O5F [M +
H]+: 480.1935, found: 480.1932.
ZY7b: Light yellow solid.Yield: 85 %. m.p.: 110-114 °C.
1H NMR (400 MHz, CDCl3): δ = 14.81 (s, 1H), 8.66 (s, 1H),
7.77 (d, 3JF-H = 11.2 Hz, 1H), 7.45 (d, J = 8.4 Hz, 2H), 6.95 (d,
J = 8.8 Hz, 2H), 4.36-4.51 (m, 3H), 4.02 (s, 1H), 3.85-3.87
(m, 3H), 3.66-3.70 (m, 3H), 3.20-3.32 (m, 3H), 1.63 (d, J =
6.8 Hz, 3H), 1.01 (s, 3H). 13C NMR (100 MHz, CDCl3): δ =
177.2, 170.7, 166.9, 160.9, 159.4, 156.9, 144.9, 142.9, 131.2,
131.1, 129.2, 127.7, 124.5, 122.9, 113.8, 108.0, 104.9. 104.7,
68.3, 55.5, 55.4, 53.3, 48.0, 18.4, 16.1. HRMS (ESI): m/z calcd.
for C26H27N3O6F [M + H]+: 496.1884, found: 496.1885.
ZY8b: Light yellow solid.Yield: 80 %. m.p.: 238-240 °C.
1H NMR (400 MHz, CDCl3): δ = 14.79 (s, 1H), 8.66 (s, 1H),
ZY13b: Light yellow solid. Yield: 78 %. m.p.: 242-
244 °C. 1H NMR (400 MHz, CDCl3): δ = 14.76 (s, 1H), 8.66
(s, 1H), 8.33 (d, J = 8.8 Hz, 2H), 7.78 (d, 3JF-H = 8.8 Hz, 1H),
7.63-7.65 (m, 2H), 4.49-4.56 (m, 2H), 4.38-4.41 (m, 1H), 4.11-
4.25 (m, 1H), 3.42-3.79 (m, 4H), 3.17-3.26 (m, 2H), 1.64 (d,
J = 6.8 Hz, 3H), 0.97-1.11 (m, 3H). 13C NMR (100 MHz,
CDCl3): δ = 177.2, 166.8, 148.5, 144.9, 141.8, 128.2, 124.5,
124.0, 108.2, 105.0, 104.9, 68.4, 55.4, 53.8, 53.2, 49.4, 48.5,
43.9, 18.4, 16.1, 15.9. HRMS (ESI): m/z calcd. for C25H24N4O7F
[M + H]+: 511.1629, found: 511.1626.
3
7.77 (d, JF-H = 10.8 Hz, 1H), 7.39-7.44 (m, 4H), 4.48-4.55
(m, 2H), 4.36-4.40 (m, 1H), 4.08-4.18 (m, 1H), 3.23-3.67 (m,
6H), 1.63 (d, J = 6.8 Hz, 3H), 0.95-1.07 (m, 3H). 13C NMR
(100 MHz, CDCl3): δ = 176.9, 169.7, 167.0, 159.3, 156.8,
145.1, 142.9, 135.9, 133.9, 131.0, 130.9, 124.6, 122.8, 107.9,
104.9, 104.7, 68.4, 55.5, 53.4, 49.4, 48.5, 18.5, 15.9. HRMS
(ESI): m/z calcd. for C25H24N3O5ClF [M + H]+: 500.1389,
found: 500.1382.
RESULTS AND DISCUSSION
The newly synthesized compounds were evaluated for
antibacterial activity against Gram-positive bacteria (Staphylo-
coccus aureus, Staphylococcus epidermidis) and Gram-
negative bacteria (Escherichia coli, Pseudomonas aeruginosa)
using conventional agar-dilution method. The minimum
inhibitory concentration (MIC) values were determined by
comparison with levofloxacin as reference drug (Table-2).
In general, most compounds exhibited potent antibacterial
activity against Gram-positive and Gram-negative bacteria. The
MIC values of newly synthesized compounds against Staphylo-
coccus strains indicated that most compounds (ZY5a, ZY6a,
ZY7a, ZY8a, ZY6b) possessed a comparable or better anti-
bacterial activity with respect to reference drug. Compound
ZY6a (MIC = 0.03 µg/mL) was the most active compound
against Staphylococcus aureus, its activity was found to be 16
times better than levofloxacin (MIC = 0.5 µg/mL). Derivatives
ZY5a and ZY6a were the most active compounds against
Staphylococcus epidermidis, being two fold more active than
levofloxacin. Only several compounds showed moderate
activity against Escherichia coli. It is noticeable that most
compounds were potent against Pseudomonas aeruginosa and
were significantly better than levofloxacin.As is evident from
the data, compound ZY6a (MIC = 0.25 µg/mL) is the superior
in inhibiting the growth of Pseudomonas aeruginosa and its
activity was found to be 16 times better than levofloxacin (MIC
= 4 µg/mL).
ZY9b: Light yellow solid.Yield: 75 %. m.p.: 232-236 °C.
1H NMR (400 MHz, CDCl3): δ = 14.80 (s,1H), 8.65-8.66 (d,
3
1H), 7.77 (d, JF-H = 11.2 Hz, 1H), 7.33-7.45 (m, 4H), 4.46-
4.52 (m, 2H), 4.36-4.39 (m, 1H), 3.96-3.99 (m, 1H), 3.05-
3.66 (m, 6H), 1.63 (d, J = 6.4 Hz, 3H), 1.09-1.17 (m, 3H). 13C
NMR (100 MHz, CDCl3): δ = 176.9, 167.3, 167.0, 158.9,
156.6, 145.1, 142.8, 142.4, 135.7, 135.5, 135.4, 131.1, 130.5,
130.4, 130.3, 129.9, 129.7, 128.1, 127.9, 127.9, 127.4, 127.2,
124.6, 122.9, 122.5, 107.9, 104.9, 104.7, 68.4, 55.5, 53.3, 53.1,
52.7, 49.4, 49.0, 48.6, 47.7, 47.6, 47.5, 42.4, 42.2, 18.5, 16.39,
16.0, 15.9, 15.5. HRMS (ESI): m/z calcd. for C25H24N3O5ClF
[M + H]+: 500.1389, found: 500.1387.
ZY10b: Light yellow solid. Yield: 81 %. m.p.: 114-
118 °C. 1H NMR (400 MHz, CDCl3): δ = 14.80 (s, 1H), 8.67
(s, 1H), 7.77 (d, 3JF-H = 10.8 Hz, 1H), 7.32-7.45 (m, 4H), 4.49-
4.58 (m, 2H), 4.37-4.40 (m, 1H), 4.06-4.21 (m, 1H), 3.56-
3.78 (m, 3H), 3.15-3.40 (m, 3H), 1.64 (d, J = 6.8 Hz, 3H),
0.96-1.08 (m, 3H). 13C NMR (100 MHz, CDCl3): δ = 177.2,
169.2, 166.9, 144.9, 137.4, 134.7, 130.0, 127.3, 125.2, 124.5,
123.2, 108.2, 105.1, 104.9, 68.3, 55.4, 53.9, 48.4, 18.4, 15.9.
HRMS (ESI): m/z calcd. for C25H24N3O5ClF [M + H]+:
500.1389, found: 500.1385.
In terms of structure-activity relationship studies, com-
pounds ZY1a-13a showed better antibacterial activity than
compounds ZY1b-13b against both Gram-positive and Gram-
negative bacteria. The results of MIC tests revealed that acyl
derivatives were usually more active than alkyl and sulfonyl
derivatives. Comparison between MIC values of acyl deriva-
tives with electron-donating group and electron-withdrawing
group revealed that acyl derivatives with electron-donating
ZY11b: Light yellow solid. Yield: 83 %. m.p.: 228-
230 °C. 1H NMR (400 MHz, CDCl3): δ = 14.82 (s, 1H), 8.66
(s, 1H), 7.77 (d, 3JF-H = 11.2 Hz, 1H), 7.74-7.77 (m, 2H), 7.23-
7.24 (m, 2H), 4.47-4.56 (m, 2H), 4.35-4.39 (m, 1H), 4.19-
4.21 (m, 1H), 3.61-3.69 (m, 3H), 3.39-3.50 (m, 3H), 2.39 (s,
3H), 1.63 (d, J = 6.4 Hz, 3H), 0.95-1.07 (m, 3H). 13C NMR
(100 MHz, CDCl3): δ = 177.2, 166.9, 144.8, 140.0, 132.7,
131.2, 129.2, 127.3, 124.5, 123.1, 108.2, 105.0, 104.9, 68.3,

2350 Zhang et al.
Asian J. Chem.
TABLE-2
Conclusion
ANTIBACTERIAL ACTIVITIES OF NEW
FLUOROQUINOLONES AGAINST SELECTED
STAINSa (MICs in µg/mL)
In conclusion, we have described the synthesis and anti-
bacterial evaluation of a series of novel fluoroquinolones.
Biological data indicated that among the newly synthesized
twenty-six compounds, most compounds demonstrated com-
parable or better activity against Gram-positive and Gram-
negative bacteria than levofloxacin as reference drug. Notably,
ZY6a was the most active compound against Staphylococcus
aureus, Staphylococcus epidermidis and Pseudomonas
aeruginosa, its activity was found to be 2 to 16 times better
than levofloxacin. These data encourage us to develop several
compounds tested in this study as potential drug candidate.
Compound
S.a.
4
S.e.
8
E.c.
0.125
0.125
8
P.a.
1
ZY1a
ZY2a
1
1
1
4
4
8
ZY3a
4
4
64
8
ZY4a
2
0.25
0.25
2
32
8
ZY5a
0.03
0.5
0.5
2
256
0.5
128
64
0.25
1
ZY6a
ZY7a
4
32
32
4
ZY8a
2
ZY9a
2
4
64
ZY10a
ZY11a
ZY12a
ZY13a
ZY1b
ACKNOWLEDGEMENTS
4
16
4
128
64
256
16
16
2
Financial support is provided by Sichuan Kelun Phar-
maceutical Co., and Natural Science Foundation of China
(J1103315/J0104).
8
8
8
16
4
16
2
0.5
0.5
16
2
2
ZY2b
REFERENCES
4
8
2
ZY3b
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1
2
64
8
ZY4b
8
4
256
64
4
ZY5b
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0.5
1
2
1
ZY6b
2
64
8
ZY7b
4
16
4
64
32
4
ZY8b
2
128
64
ZY9b
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2
2
16
32
64
16
4
ZY10b
ZY11b
ZY12b
ZY13b
Levofloxacin
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2
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2
8
0.5
0.5
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aS.a.: Staphylococcus aureus, S.e.: Staphylococcus epidermidis, E.c.:
Escherichia coli, P.a.: Pseudomonas aeruginosa
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bacterial activity against various bacteria strains. For example,
compound ZY6a was the most active compound against
Staphylococcus aureus, Staphylococcus epidermidis and
Pseudomonas aeruginosa, its activity was found to be 2 to 16
times better than levofloxacin.
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