Design, synthesis and antibacterial evaluation of some new 2-phenyl-quinoline-4-carboxylic acid derivatives

Article abstract of DOI:10.3390/molecules21030340

A series of new 2-phenyl-quinoline-4-carboxylic acid derivatives was synthesized starting from aniline, 2-nitrobenzaldehyde, pyruvic acid followed by Doebner reaction, amidation, reduction, acylation and amination. All of the newly-synthesized compounds w

Full text of DOI:10.3390/molecules21030340

molecules
Article
Design, Synthesis and Antibacterial Evaluation of
Some New 2-Phenyl-quinoline-4-carboxylic
Acid Derivatives
Xiaoqin Wang 1,*, Xiaoyang Xie 1, Yuanhong Cai 1, Xiaolan Yang 2, Jiayu Li 1, Yinghan Li 1,
Wenna Chen 1 and Minghua He 1,
*
1
School of Pharmacy, Guangdong Medical University, Dongguan 523808, China; 13650470904@163.com (X.X.);
yuanhong_cai@yahoo.com (Y.C.); yuaishideilu@yahoo.com (J.L.); 13650317182@163.com (Y.L.);
cwn201508@163.com (W.C.)
Department of Hematology, Donghua Affiliated Hospital of Sun Yat-sen University, Dongguan 523110,
China; candyx123@163.com
2
*
Correspondence: wxq44@sohu.com (X.W.); cpuhmh@163.com (M.H.); Tel.: +86-769-2289-6559 (X.W. & M.H.)
Academic Editor: Derek J. McPhee
Received: 24 January 2016; Accepted: 7 March 2016; Published: 10 March 2016
Abstract: A series of new 2-phenyl-quinoline-4-carboxylic acid derivatives was synthesized
starting from aniline, 2-nitrobenzaldehyde, pyruvic acid followed by Doebner reaction,
amidation, reduction, acylation and amination. All of the newly-synthesized compounds
were characterized by 1H-NMR, 13C-NMR and HRMS. The antibacterial activities of these
compounds against Gram-negative (Escherichia coli, Pseudomonas aeruginosa) and Gram-positive
bacteria (Staphylococcus aureus, Bacillus subtilis), as well as one strain of methicillin-resistant
Staphylococcus aureus (MRSA) bacteria were evaluated by the agar diffusion method (zone of inhibition)
and a broth dilution method (minimum inhibitory concentration (MIC)), and their structure-activity
relationships were obtained and discussed. The results revealed that some compounds displayed
good antibacterial activity against Staphylococcus aureus, and Compounds 5a4 and 5a7 showed the
best inhibition with an MIC value of 64
of 128 g/mL against Escherichia coli, respectively. The results of the MTT assay illustrated the low
cytotoxicity of Compound 5a4.
µg/mL against Staphylococcus aureus and with an MIC value
µ
Keywords: 2-phenyl-quinoline-4-carboxylic acid derivatives; synthesis; Doebner reaction;
antibacterial activity
1. Introduction
Bacterial infection is one of the most complex global health issues of this century [
many drug-resistant pathogens have emerged in recent years because of the increasing use or abuse
of antibacterial agents [ ], and antibiotic drug resistance has become a growing problem across the
globe in recent years [ ]. Currently, many advances for combating antimicrobial resistance have been
achieved, such as modifications/improvements of existing antibiotic classes, some anti-adhesion agents
against Gram-positive pathogens [ ] and efflux pump inhibitors (EPIs) [ ]. However, this problem has
1]. Furthermore,
2,3
4
5
6
not been completely resolved. Therefore, the development of novel and effective chemotherapeutics is
still in demand to overcome these problems [7,8].
The quinoline ring occurs in various natural products and represents a key motif in
medicinal chemistry [9–12], and its derivatives, quinoline-4-carboxylic acids, are a group of
compounds associated with different biological activities, such asantiviral [13], anti-inflammatory [14],
antimicrobial [15], anti-atherothrombosis [16], antiemetic [17], anxiolytic [18], antimalarial and
antileishmanial [19]. Although quinoline-4-carboxylic acid derivatives exhibit various bioactivities, the
Molecules 2016, 21, 340; doi:10.3390/molecules21030340
www.mdpi.com/journal/molecules

Molecules 2016, 21, 340
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research of new quinoline-4-carboxylic acid-based antibacterial drugs has developed slowly.
Some quinoline-4-carboxylic acid derivatives exhibited certain antibacterial activity [20 21].
,
Furthermore, a literature review revealed that the presence of an aryl ring at the second position
of quinoline-4-carboxylic acid derivatives exhibits good antibacterial activity for the target compound
and plays a significant role in the development of new antibacterials (Figure 1) [6,15,22–27], and they
are very suitable for further modifications to obtain more effective antibacterial agents. In addition,
the structure-activity relationship (SAR) studies of quinolones have indicated that the introduction of
a basic group in the quinoline is useful for regulating the physicochemical property and influencing
their potency, spectrum and safety [28,29]. All of these stimulated us with great interest to focus on
further structural modification of quinoline-4-carboxylic acid in order to find novel derivatives with
potential activity against bacterial strains.
R
RN
O
N
CONH2
N
N
O-n-Pr
R=alkyl,aryl
R=alkylamino, heteroalkyl
A
B
C
HN
N
R
COOH
COOH
O
R1
R
R1
R2
N
H
N
N
N
R2
R=alkyl,aralky,aryl
R1,R2=fluoro
R=alkyl,aryl,acyl
R1,R2=X,H,OCH3,CH3,NO2
F
E
D
H
R'
O
N
CONHN CH
CONHR
N
X
F
N
N
O
N
X=H,Cl
R
N
R=H,Cl,Br,CH3,OCH3
R'=H,Cl,Br,OCH3,NO2
R=alkyl,aryl
H
G
I
Figure 1. The structures of 2-aryl-quinoline-4-carboxylic acid derivatives with antimicrobial activities.
Keeping in view the wide range of pharmaceutical activities of quinoline-4-carboxylic acid,
in this report, we introduced two amino substituents in the molecule, which may severe as new
hydrogen bond donors and receptors to increase binding affinity to the enzyme and also improve their
physicochemical properties. Thus, a series of 2-(2’-substutited)-phenyl-quinoline-4-carboxylic acid
derivatives was designed, synthesized and evaluated for antibacterial activities.
2. Results and Discussion
2.1. Chemistry
The synthetic pathway for the target Compounds 5a1
–5a7 and 5b1
–5b6 is depicted in Scheme 1.
The intermediate 2-(2-nitrophenyl)-quinoline-4-carboxylic acid (
1
) is considered to be a category
of versatile intermediates that provide building blocks in the synthesis of poly-substituted
quinoline-4-carboxylic acid derivatives. Several methods have been developed to date for the
synthesis of 2-(2-nitrophenyl)-quinoline-4-carboxylic acid, and these include the Doebner reaction
using aniline, 2-nitrobenzaldehyde, pyruvic acid and a catalytic amount of trifluoroacetic acid in
ethanol media [14
ketone in aqueous ethanol [22
via Doebner reaction under microwave [25
,
15], as well as the versatile Pfitzinger reaction by reacting isatin with
30 31]. 2-(2-nitrophenyl)-quinoline-4-carboxylic acid was also prepared
26 32 33]. We opted for the Doebner reaction and explored
α-methyl
,
,
,
,
,

Molecules 2016, 21, 340
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the influence of two catalysts on the reaction. Treatment of aniline, 2-nitrobenzaldehyde, pyruvic acid
and a catalytic amount of trifluoroacetic acid in ethanol media with refluxing for 12 h afforded only a
few products, and the pure product must be obtained by flash column chromatography. However,
the replacement of trifluoroacetic acid with acetic acid and excessive acetic acid as the solvent instead
of ethanol favored the reaction. After completion of the reaction, the mixture was poured into ice
water, and the crude product was purified by acid-alkali neutralization and further by recrystallization
without flash column chromatography with a total yield of 68%.
COOH
O
HOAc
SOCl2
CHO
NO2
NH2
COOH
refluxing
refluxing
N
O2N
1
H
N
H
O
N
O
N
N
COCl
H2N
N
NH2-NH2.H2O/Pd-C
isopropanol,
refluxing
CH2Cl2,Et3N
N
N
N
O2N
O2N
H2N
3
2
H
N
H
O
O
N
O
N
N
Cl
Cl
n
amine
MeOH,refluxing
K2CO3, CH2Cl2, r.t
N
N
HN
HN
O
O
n
Cl
4
n
R
n=2, 5a1-5a7
n=1, 5b1-5b6
4a:n=2
4b: n=1
5
N
O
N
N
N
N
N
R =
,
,
,
,
,
H
H
N
N
N
N
,
Scheme 1. Synthetic route of 2-phenyl-quinoline-4-carboxylic acid derivatives.
The reaction of intermediate
Subsequently, the treatment of intermediate acyl chloride with 3-(dimethylamino)-1-propylamine
in the presence of triethylamine gave Compound . In order to obtain the amine functionality, the
1 with SOCl2 under reflux furnished the product acyl chloride.
2
conventional method for the reduction of the nitro group was attempted by using 80% hydrazine
hydrate in the presence of 10% Pd/C and isopropanol as a solvent under reflux, which furnished
amino-substituted Compound
isolation could be facilitated without purification using chromatography. Reaction of amine
3
in a 92% yield [34]. The advantage of this reaction was that product
with acyl
3
chloride and potassium carbonate in dichloromethane gave Compounds 4a and 4b. Because of their
poor solubilities, the crude product directly was used in the next reaction without purification. Finally,
the target Compounds 5a1–5a7 and 5b1–5b6 were obtained through the reaction of Compounds 4a and
4b with different primary and secondary amines under reflux conditions for 10 h, respectively.
The structures of the target compounds were validated by using 1H-NMR, 13C-NMR and HRMS,
and their purities were determined to be above 95% by using HPLC.
2.2. Evaluation of Antibacterial Activity
Firstly, the newly-synthesized Compounds 5a1
in vitro antibacterial activities by the agar diffusion method against a variety of different
strains [35 37], including Staphylococcus aureus (S. aureus), Bacillus subtilis (B. subtilis), Escherichia
coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa) and one strain of methicillin-resistant
Staphylococcus aureus (MRSA) bacteria. Their activities were compared to that of Compound and the
known antibacterial agents ampicillin and gentamycin. The results are presented in Table 1.
–5a7, 5b1–5b6 were screened for their
–
1

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Table 1. Antibacterial activities of the synthesized compounds determined by the agar diffusion method.
Zone of Inhibition in mm (Mean ˘ SD) n = 3
Compound
LogP
B. subtilis
S. aureus
E. coli
P. aeruginosa
MRSA
Concentration (µg/mL)
50
100
2 ˘ 0.5
2 ˘ 0.2
11 ˘ 0.5
13 ˘ 0.2
7 ˘ 0.4
0
50
100
5 ˘ 0.4
17 ˘ 0.1
16 ˘ 0.5
20 ˘ 0.1
8 ˘ 0.5
0
2 ˘ 0.1
3 ˘ 0.5
7 ˘ 0.1
6 ˘ 0.3
10 ˘ 0.1
5 ˘ 0.4
0
50
100
2 ˘ 0.5
5 ˘ 0.5
3 ˘ 0.4
3 ˘ 0.2
0
13 ˘ 0.1
14 ˘ 0.1
0
3 ˘ 0.4
6 ˘ 0.4
5 ˘ 0.1
0
11 ˘ 0.2
4 ˘ 0.3
0
50
1 ˘ 0.1
2 ˘ 0.5
2 ˘ 0.2
0
3 ˘ 0.1
1 ˘ 0.2
0
100
50
100
5a1
5a2
5a3
5a4
5a5
5a6
5a7
5b1
5b2
5b3
5b4
5b5
5b6
0.32
1.69
1.05
2.26
0.60
0.87
1.94
0.72
1.75
1.20
2.32
1.00
0
1 ˘ 0.2
2 ˘ 0.3
10 ˘ 0.1
12 ˘ 0.3
6 ˘ 0.2
0
3 ˘ 0.2
15 ˘ 0.5
14 ˘ 0.2
18 ˘ 0.2
7 ˘ 0.1
0
1 ˘ 0.5
2 ˘ 0.1
4 ˘ 0.2
4 ˘ 0.1
8 ˘ 0.5
4 ˘ 0.5
0
2 ˘ 0.1
4 ˘ 0.2
3 ˘ 0.2
2 ˘ 0.5
0
10 ˘ 0.2
12 ˘ 0.5
0
2 ˘ 0.3
4 ˘ 0.2
3 ˘ 0.3
0
10 ˘ 0.5
3 ˘ 0.1
0
1 ˘ 0.2
3 ˘ 0.2
3 ˘ 0.4
3 ˘ 0.4
3 ˘ 0.1
6 ˘ 0.3
2 ˘ 0.5
4 ˘ 0.3
5 ˘ 0.4
0
5 ˘ 0.5
6 ˘ 0.2
5 ˘ 0.1
2 ˘ 0.4
3 ˘ 0.5
2 ˘ 0.4
0
0
0
0
0
0
0
0
0
0
0
0
0
1 ˘ 0.1
3 ˘ 0.3
3 ˘ 0.5
10 ˘ 0.1
1 ˘ 0.2
0
1 ˘ 0.2
4 ˘ 0.2
3 ˘ 0.1
12 ˘ 0.5
3 ˘ 0.3
0
0
0
0
0
1 ˘ 0.2
2 ˘ 0.1
0
0
5 ˘ 0.2
7 ˘ 0.4
0
0
0
0
3 ˘ 0.1
1 ˘ 0.4
0
4 ˘ 0.3
0
0
0
0
1
3.54
2 ˘ 0.5
3 ˘ 0.1
3 ˘ 0.5
5 ˘ 0.1
2 ˘ 0.2
Control a
Ampicillin
Gentamycin
0
0
0
0
0
-
0
1 ˘ 0.4
0
0
1 ˘ 0.1
0
b
32 ˘ 0.3
10 ˘ 0.5
25 + 0.4
14 ˘ 0.3
-
-
15 ˘ 0.4
-
-
12 ˘ 0.2
-
-
17 ˘ 0.1
20 ˘ 0.4
-
a
Dimethyl sulfoxide; b not determined.

Molecules 2016, 21, 340
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The antibacterial primary screening results revealed that most of the tested compounds showed
moderate inhibition against various tested bacterial strains compared to the standard drug. However,
most of them exhibited better activity than Compound
1 against the above-mentioned bacteria;
this indicated that the structural modifications of 2-phenyl-quinoline-4-carboxylic acid increased
the antibacterial activity. Among the synthesized compounds, Compound 5a4 showed significant
antibacterial activity against S. aureus and B. subtilis; Compound 5a7 was the most active antibacterial
activity against E. coli; and Compound 5b4 exhibited moderate antibacterial activity against MRSA,
but all compounds showed weak inhibition action against P. aeruginosa. The LogP values of all of
the compounds were predicted on the basis of a computational study using ACDLabs/ChemSketch
software. Among the active compounds against all of the mentioned bacteria, it can be clearly seen
that the compound (5a4, LogP = 2.26; 5a7, LogP = 1.94; 5b4, LogP = 2.32) with higher lipophilicity
displayed higher activity.
Furthermore, with the same basic terminus, Compounds 5a1
with three bonds between the basic N terminus and carbonyl group) showed stronger antibacterial
activity than Compounds 5b1 5b6 with shorter side chains (n = 1, with two bonds between the basic
–5a7 with longer side chains (n = 2,
–
N terminus and carbonyl group) against the above-mentioned bacterial strains, respectively. These
indicated that the length of the amide side chain at the ortho-position of the 2-phenyl group had a
significant effect on antibacterial activity. On the other hand, alterations of the basic terminus of the
amide side chains at the ortho-position of the 2-phenyl group led to varying effects on antibacterial
activity. Compounds 5a4
5a7 and 5b6 against S. aureus and B. subtilis, indicating that rigid cyclic amino group at 2-phenyl is
suitable for the anti-Gram-positive bacteria activity. On the contrary, for E. coli, Compounds 5a6 5a7
and 5b6 exhibited stronger activity than the compounds with rigid cyclic amino substituents(5a2 5a3
5a4 5b4), suggesting that the flexible chain amino group at 2-phenyl can enhance antibacterial activity
, 5a3, 5a2 and 5b4 showed better antibacterial activity than Compounds 5a6,
,
,
,
,
against E. coli. All of these results demonstrated the importance of the length and flexibility of the
amide side chain for antibacterial activity.
In order to further determinate the antibacterial effect of the compounds, the minimum inhibitory
concentration (MIC) values against the above-mentioned bacteria strains were measured by a broth
dilution method [38–40]. The MIC value is a measure to test the antibacterial activity of a compound
and is defined as the lowest concentration of antibacterial agent that inhibits visible growth. Ampicillin
and gentamycin were used as positive controls in the assay. The MIC values are summarized in Table 2.
Table 2. The minimum inhibitory concentration (MIC) values of the synthesized compounds
against bacteria.
MIC (µg/ mL)
Compound
B. subtilis
S. aureus
E. coli
P. aeruginosa
MRSA
5a1
5a2
5a3
5a4
5a5
5a6
5a7
5b1
5b2
5b3
5b4
5b5
5b6
>512
>512
256
256
512
-
>512
128
128
64
256
-
>512
>512
512
512
128
>512
-
>512
512
>512
>512
-
256
128
-
>512
512
>512
-
256
>512
64
>512
>512
>512
>512
512
>512
512
>512
-
-
-
a
-
512
>512
-
-
-
>512
-
-
>512
>512
>512
256
>512
-
>512
16
64
-
-
512
-
-
-
512
-
-
>512
>512
64
1
512
16
16
Ampicillin
Gentamycin
16
8
a
Not determined.

Molecules 2016, 21, 340
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The results of MIC values correspond to that obtained by the agar diffusion method as a whole.
Some compounds displayed good antibacterial activity against S. aureus, and Compound 5a4 showed
the best inhibition against S. aureus with a MIC of 64
A few compounds exhibited moderate activity against E. coli, and Compound 5a7 had the best activity
against E. coli with a MIC of 128 g/mL. However, all the compounds exhibited weak activity against
B. subtilis, P. aeruginosa and MRSA; their MICs were more than 256 g/mL. Unfortunately, all of these
µg/mL, which is better than Compound 1.
µ
µ
compounds exhibited less potent activity than the standard ones against the above-mentioned strains.
2.3. MTT-Based Cytotoxicity Studies
The in vitro cytotoxicity of Compounds 5a4 and 5a7, which showed the best antibacterial activity,
were further examined in mouse macrophage cell lines (RAW 264.7) using the MTT colorimetric
assay [40
264.7 cells with IC50 values of 98.2
,
41]. As shown in Table 3, Compounds 5a4 and 5a7 showed weak cytotoxicity in the RAW
g/mL and 56.8 g/mL, respectively, which are similar to the
µ
µ
cytotoxicities of ampicillin and gentamycin. The low cytotoxicities for mouse macrophage cells of
Compounds 5a4 and 5a7 exhibited their good safety profiles, which indicate that they could be further
developed as antibacterial agents for infectious diseases against S. aureus and E. coil, respectively.
Table 3. IC50 (µg/mL) values of the compounds against mouse macrophage cell lines (RAW 264.7 a).
b
Compound
IC50 (µg/mL)
5a4
5a7
98.24 ˘ 0.65
56.83 ˘ 0.32
107.58˘ 0.47
42.19 ˘ 0.19
Ampicillin
Gentamycin
a
RAW 264.7 monolayers were used as an in vitro model of cervicovaginal epithelium for testing the cytotoxicity
of the new compounds; b IC50 is defined as the concentration at which 50% of growth is inhibited.
3. Experimental Section
3.1. General
All reagents and solvents were of analytical grade and used without further purification.
All reactions were monitored by TLC on 0.2 mm-thick silica gel GF254 pre-coated plates. All flash
column chromatography were performed with silica gel (200–300 mesh) purchased from Qingdao
Haiyang Chemical Co. Ltd., Qingdao, China); 1H-NMR and 13C-NMR spectra were recorded using
TMS as the internal standard in CDCl3 or DMSO-d6 with a Bruker BioSpin GmbH spectrometer (Bruker
BioSpin, Switzerland) at 400 MHz and 101 MHz, respectively; high resolution mass spectra (HRMS)
were recorded on the Shimadzu LCMS-IT-TOF spectrometer (Shimadzu, Tokyo, Japan); the purity of
the synthesized compounds was confirmed to be higher than 95% through analytical HPLC (Shimadzu,
Tokyo, Japan) performed with a dual pump Shimadzu LC-20AB system (Shimadzu, Tokyo, Japan)
equipped with an Ultimate XB-C18 column (4.6
eluted with methanol/water (40:60–70:30) containing 0.1% TFA at a flow rate of 0.5 mL/min.
ˆ
250 mm, 5 µm) (Welch, New York, NY USA) and
3.2. Chemistry
3.2.1. Preparation of 2-(2-Nitrophenyl)quinoline-4-carboxylic Acid (1)
Method A: An equimolar mixture of aniline (1.9 g, 20 mmol) and 2-nitrobenzaldehyde (3.0 g,
20 mmol) in ethanol (30 mL) was refluxed for 1 h; pyruvic acid (2.6 g, 30 mmol) and trifluoroacetic
acid (0.1 mL) were then added to the reaction mass and further refluxed for 12 h. Reaction completion
was monitored by using TLC. The reaction mixture was poured into ice water (60 mL) slowly with
vigorous stirring. The solid product was filtered and was added to the aqueous K2CO3; the solution
was adjusted to basic, then filtered. The filtrate was acidified with diluted HCl to pH 1–2, and the

Molecules 2016, 21, 340
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mixture was extracted with three 40-mL portions of CH2Cl2, The combined organic phase was washed
with 40 mL water, dried over anhydrous sodium sulfate and concentrated under reduced pressure. The
crude product was purified by using flash column chromatography to give Compound 1 in a 23% yield.
Method B: A mixture of 2-nitrobenzaldehyde (3.0 g, 20 mmol) and pyruvic acid (3.5 g, 39.8 mmol)
was stirred for 15 min; the solid was dissolved. Then, 30 mL acetic acid were added, and the mixture
˝
was stirred at 100 C for 30 min. After aniline (3.7 g, 40 mmol) was added, the mixture was refluxed for
8 h. Then, the reaction mixture was cooled to room temperature and poured into ice water (60 mL); then,
aqueous NaOH was added to make the solution basic. The mixture was extracted with three 40-mL
portions of CH2Cl2; the water phase was acidified with diluted HCl to pH 1–2, and the precipitated
crude product was filtered. A white solid was obtained in a yield of 68% after recrystallization
in ethanol. 1H-NMR (400 MHz, DMSO-d6):
δ 14.09 (s, 1H), 8.72 (d, J = 8.5 Hz, 1H), 8.19 (s, 1H),
8.09 (d, J = 7.9 Hz, 1H), 8.03 (d, J = 8.2 Hz, 1H), 7.96 (d, J = 7.3 Hz, 1H), 7.88 (t, J = 7.5 Hz, 2H),
7.78 (t, J = 7.6 Hz, 2H).
3.2.2. Preparation of N-(3-(Dimethylamino)propyl)-2-(2-nitrophenyl)quinoline-4-carboxamide (2)
A mixture of Compound 1 (1.2 g, 4.1 mmol) and SOCl2 (15 mL) was stirred under reflux for 3 h.
After completion of the reaction, the excess of SOCl2 was distilled off. The obtained yellow residue was
added dropwise to the mixture of N,N-dimethylpropylamine (0.5 g, 4.9 mmol), triethylamine(1.6 mL) in
˝
CH2Cl2 (30 mL) at 0 C, then stirred at room temperature for 10 h. The precipitated solid was collected
through filtration; the crude product was purified by using flash column chromatography with CH2Cl2
1
elution to afford Compound
2
as a pale yellow liquid in an 86% yield. H-NMR (400 MHz, CDCl3):
δ
8.43 (s, 1H), 8.32 (d, J = 8.4 Hz, 1H), 8.09 (d, J = 8.5 Hz, 1H), 8.00 (d, J = 8.1 Hz, 1H), 7.75 (t, J = 8.2 Hz,
1H), 7.69 (d, J = 4.0 Hz, 2H), 7.62–7.55 (m, 3H), 3.58 (q, J = 4.0 Hz, 2H), 2.50 (t, J = 6.1 Hz, 2H), 2.20 (s, 6H),
1.84–1.75 (m, 2H).
3.2.3. Preparation of 2-(2-Aminophenyl)-N-(3-(dimethylamino)propyl)quinoline-4-carboxamide (3)
The mixture of Compound
2 (1.0 g, 2.6 mmol), 80% hydrazine hydrate (1.5 mL) in the presence of
10% Pd/C (0.3 g) and isopropanol (25 mL) was heated under reflux for 2 h, and TLC analysis indicated
reaction completion; Pd/C was removed by filtration. After concentration in vacuo, 25 mL water were
added to the residue; the precipitate was filtered off; the crude product
reaction without purification.
3 was directly used in the next
3.2.4. General Preparation of Compounds 4a–4b (GP1)
A solution of the appropriate acid halide (3.9 mmol) in CH2Cl2 (10 mL) was slowly added to
a well-stirred mixture of Compoun˝d
3 (0.9 g, 2.6 mmol) and anhydrous potassium carbonate (0.6 g,
4.0 mmol) in CH2Cl2 (25 mL) at 0 C. After completing the addition, the reaction was allowed to
warm up to room temperature and then stirred overnight. After concentration in vacuo, 30 mL water
were added to the residue; the precipitate formed was filtered off. Because of the poor solubility of
Compound 4, the crude product was directly used in the next reaction without purification.
3.2.5. General Preparation of Compounds 5a–5b (GP2)
Appropriate amine (2.0 mL) was added to a stirred refluxing suspension of the chloride
Compound
4 (0.5 mmol) in MeOH (25 mL). The mixture was stirred under reflux for 10 h, then
cooled to room temperature. After concentration in vacuo, 50 mL water were added to the residue. The
mixture was extracted with three 30-mL portions of CH2Cl2. The combined organic phase was washed
with 40 mL water, dried over anhydrous sodium sulfate and concentrated under reduced pressure.
The crude product was purified by using flash column chromatography to afford Compounds 5a–5b.

Molecules 2016, 21, 340
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N-(3-(Dimethylamino)propyl)-2-(2-(3-(4-methylpiperazin-1-yl)propanamido)phenyl)quinoline-4-carboxamide (5a1)
Compound 4a was reacted with N-methyl piperazine according to GP2 to give the desired product 5a1
.
After column chromatography with CH2Cl2/MeOH/Et3N (50:1:0.1) elution, a white solid was obtained
˝
in a 64% yield. mp. 182.3–184.2 C. 1H-NMR (400 MHz, CDCl3):
δ 12.40 (s, 1H), 8.53 (d, J = 8.3 Hz, 1H),
8.38 (t, J = 8.0 Hz, 2H), 8.10 (d, J = 8.4 Hz, 1H), 7.92 (s, 1H), 7.81 (t, J = 9.2 Hz, 2H), 7.64 (t, J = 8.0 Hz,
1H), 7.46 (t, J = 7.9 Hz, 1H), 7.23 (t, J = 8.0 Hz, 1H), 3.69 (q, J = 4.0 Hz, 2H), 2.81 (t, J = 7.0 Hz, 2H),
2.64 (t, J = 7.1 Hz, 2H), 2.59–2.54 (m, 2H), 2.47 (s, 4H), 2.24 (d, J = 5.4 Hz, 6H), 2.19 (s, 3H), 2.07 (s, 4H),
1.91–1.84 (m, 2H). 13C-NMR (101 MHz, CDCl3):
δ 170.41, 166.76, 157.68, 146.99, 143.79, 137.86, 130.66,
130.62, 129.34, 128.83, 127.69, 125.83, 125.67, 123.69, 123.16, 122.47, 118.60, 59.06, 54.79 (2C), 53.94,
52.66 (2C), 45.75, 45.31 (2C), 40.53, 35.65, 25.23. Purity: 99.4% by HPLC. HRMS (ESI): Calcd. for
[M + H]+ (C29H39N6O2) requires m/z 503.3129, found 503.3106.
N-(3-(Dimethylamino)propyl)-2-(2-(3-(pyrrolidin-1-yl)propanamido)phenyl)quinoline-4-carboxamide (5a2)
Compound 4a was reacted with pyrrolidine according to GP2 to give the desired product 5a2. After
column chromatography with CH2Cl2/MeOH/Et3N (100:1:0.1) elution, a yellow solid was obtained
in a 65% yield. mp. 176.7–178.1 ˝C. 1H-NMR (400 MHz, CDCl3):
δ 12.57 (s, 1H), 8.56 (d, J = 8.3 Hz,
1H), 8.41 (s, 1H), 8.35 (d, J = 8.4 Hz, 1H), 8.11 (d, J = 8.4 Hz, 1H), 7.89 (s, 1H), 7.82–7.74 (m, 2H),
7.62 (t, J = 7.6 Hz, 1H), 7.43 (t, J = 7.8 Hz, 1H), 7.19 (t, J = 7.6 Hz, 1H), 3.67 (dd, J = 11.4, 5.8 Hz, 2H),
2.90 (t, J = 7.3 Hz, 2H), 2.69 (t, J = 7.3 Hz, 2H), 2.54–2.48 (m, 6H), 2.19 (s, 6H), 1.87–1.80 (m, 2H),
1.66 (s, 4H). 13C-NMR (101 MHz, CDCl3):
δ 170.29, 166.79, 157.62, 146.92, 143.79, 138.03, 130.67, 130.60,
129.23, 128.93, 127.68, 125.77, 125.58, 123.55, 123.08, 122.25, 118.45, 59.16, 53.92 (2C), 51.84, 45.40 (2C),
40.64, 37.50, 25.33, 23.36 (2C). Purity: 98.8% by HPLC. HRMS (ESI): Calcd. for [M + H]+ (C28H36N5O2)
requires m/z 474.2864, found 474.2837.
N-(3-(Dimethylamino)propyl)-2-(2-(3-(diethylamino)propanamido)phenyl)quinoline-4-carboxamide (5a3)
Compound 4a was reacted with diethylamine according to GP2 to give the desired product 5a3
.
After column chromatography with CH2Cl2/MeOH/Et3N (100:1:0.1) elution, a pale yellow solid
˝
1
was obtained in a 59% yield. mp. 177.2–178.4 C. H-NMR (400 MHz, CDCl3):
δ 12.60 (s, 1H),
8.56 (d, J = 8.3 Hz, 1H), 8.41 (s, 1H), 8.35 (d, J = 8.4 Hz, 1H), 8.10 (d, J = 8.4 Hz, 1H), 7.91 (s, 1H),
7.79 (t, J = 7.7 Hz, 2H), 7.62 (t, J = 7.7 Hz, 1H), 7.44 (t, J = 7.8 Hz, 1H), 7.19 (t, J = 7.6 Hz, 1H),
3.67 (q, J = 5.4 Hz, 2H), 2.93 (t, J = 7.2 Hz, 2H), 2.63 (t, J = 7.2 Hz, 2H), 2.58–2.50 (m, 6H), 2.20 (s, 6H),
1.87–1.80 (m, 2H), 0.97 (t, J = 7.1 Hz, 6H). 13C-NMR (101 MHz, CDCl3):
δ 170.62, 166.85, 157.54, 146.76,
143.71, 138.07, 130.59 (2C), 129.21, 128.79, 127.63, 125.57, 125.28, 123.50, 123.05, 122.12, 118.42, 58.87,
48.66, 46.71 (2C), 45.31 (2C), 40.33, 35.93, 25.45, 11.36 (2C). Purity: 99.7% by HPLC. HRMS (ESI): Calcd.
for [M + H]+ (C28H38N5O2) requires m/z 476.3020, found 476.2994.
N-(3-(Dimethylamino)propyl)-2-(2-(3-(piperidin-1-yl)propanamido)phenyl)quinoline-4-carboxamide (5a4)
Compound 4a was reacted with piperidine according to GP2 to give the desired product 5a4
.
After column chromatography with CH2Cl2/MeOH/Et3N (100:1:0.1) elution, a yellow solid was
1
obtained in a 60% yield. mp. 168.9–170.1 ˝C. H-NMR (400 MHz, CDCl3):
δ 12.52 (s, 1H),
8.51 (d, J = 8.3 Hz, 1H), 8.41–8.30 (m, 2H), 8.12 (d, J = 8.5 Hz, 1H), 7.90 (s, 1H), 7.83–7.75 (m, 2H),
7.63(t, J = 8.0 Hz, 1H), 7.44 (t, J = 7.8 Hz, 1H), 7.21 (t, J = 7.6 Hz, 1H), 3.67 (q, J = 5.6 Hz, 2H),
2.78 (t, J = 7.1 Hz, 2H), 2.67 (t, J = 7.1 Hz, 2H), 2.51 (t, J = 6.0 Hz, 2H), 2.39 (s, 4H), 2.20 (s, 6H),
1.87–1.80 (m, 2H), 1.47–1.39 (m, 4H), 1.36–1.32 (m, 2H). 13C-NMR (101 MHz, CDCl3):
δ 170.56, 166.86,
157.58, 146.94, 143.72, 137.88, 130.62, 130.57, 129.30, 128.94, 127.66, 125.85, 125.56, 123.69, 123.08, 122.45,
118.53, 58.92, 54.58, 54.15 (2C), 45.32 (2C), 40.40, 35.45, 25.52 (2C), 25.40, 24.01. Purity: 99.2% by HPLC.
HRMS (ESI): Calcd. for [M + H]+ (C29H38N5O2) requires m/z 488.3020, found 488.2995.

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N-(3-(Dimethylamino)propyl)-2-(2-(3-morpholinopropanamido)phenyl)quinoline-4-carboxamide (5a5)
Compound 4a was reacted with morpholine according to GP2 to give the desired product 5a5. After
column chromatography with CH2Cl2/MeOH/Et3N (90:1:0.1) elution, a pale yellow solid was obtained
˝
in a 59% yield. mp. 188.1–189.4 C. 1H-NMR (400 MHz, CDCl3):
δ 12.45 (s, 1H), 8.51 (d, J = 8.6 Hz, 1H),
8.45 (s, 1H), 8.37 (d, J = 8.0 Hz, 1H), 8.10–8.05 (m, 2H), 7.97 (d, J = 12.0 Hz, 1H), 7.81 (t, J = 8.0 Hz, 1H),
7.64 (t, J = 8.0 Hz, 1H), 7.45 (t, J = 8.0 Hz, 1H), 3.73–3.69 (m, 2H), 3.52 (s, 4H), 2.98 (s, 2H), 2.81 (s, 2H),
2.67–2.61 (m, 8H), 2.43 (s, 4H), 2.15–2.12 (m, 2H). 13C-NMR (101 MHz, CDCl3):
δ 170.54, 166.83, 157.65,
146.93, 143.75, 137.95, 130.63, 130.59, 129.29, 128.88, 127.66, 125.80, 125.57, 123.66, 123.08, 122.43, 118.49,
66.83 (2C), 58.97, 53.65, 52.79 (2C), 45.40 (2C), 40.29, 35.70, 25.38. Purity: 97.6% by HPLC. HRMS (ESI):
Calcd. for [M + H]+ (C28H36N5O3) requires m/z 490.2813, found 490.2785.
N-(3-(Dimethylamino)propyl)-2-(2-(3-((3-(dimethylamino)propyl)amino)propanamido)phenyl)quinoline-4-
carboxamide (5a6)
Compound 4a was reacted with 3-dimethyl aminopropyl amine according to GP2 to give the desired
product 5a6. After column chromatography with CH2Cl2/MeOH/Et3N (85:2:0.1) elution, a brown
solid was obtained in a 55% yield. mp. 171.5–173.4 ˝C. 1H-NMR (400 MHz, CDCl3):
δ12.97 (s, 1H),
8.59 (d, J = 8.2 Hz, 1H), 8.47 (s, 1H), 8.33 (d, J = 8.4 Hz, 1H), 8.12 (d, J = 8.4 Hz, 1H), 7.96 (s, 1H),
7.88–7.79 (m, 2H), 7.63 (t, J = 7.8 Hz, 1H), 7.46 (t, J = 7.8 Hz, 1H), 7.23 (t, J = 7.8 Hz, 1H), 3.72–3.65 (m,
2H), 3.64 (s, 1H), 3.11 (t, J = 5.5 Hz, 2H), 2.97–2.84 (m, 4H), 2.54 (t, J = 6.1 Hz, 2H), 2.43 (t, J = 6.3 Hz, 2H),
2.25 (s, 6H), 2.22 (s, 6H), 1.90–1.84 (m, 2H), 1.78–1.70 (m, 2H). 13C-NMR (101 MHz, CDCl3):
δ 171.51,
167.96, 158.57, 147.83, 145.03, 139.21, 131.80, 131.74, 130.31, 129.90, 128.77, 126.66, 126.07, 124.60, 124.14,
123.01, 119.45, 58.93, 49.66, 48.35, 46.52 (2C), 45.77 (2C), 45.42, 41.57, 38.77, 26.60, 25.89. Purity: 98.8%
by HPLC. HRMS (ESI): Calcd. for [M + H]+ (C29H41N6O2) requires m/z 505.3286, found 505.3164.
N-(3-(Dimethylamino)propyl)-2-(2-(3-((3-(diethylamino)propyl)amino)propanamido)phenyl)quinoline-4-
carboxamide (5a7)
Compound 4a was reacted with 3-diethyl aminopropyl amine according to GP2 to give the desired
product 5a7. After column chromatography with CH2Cl2/MeOH/Et3N (85:2:0.1) elution, brown
oil was obtained in a 54% yield. 1H-NMR (400 MHz, CDCl3):
δ 12.82 (s, 1H), 8.61 (d, J = 8.4 Hz,
1H), 8.45 (s, 1H), 8.35 (d, J = 8.4 Hz, 1H), 8.12 (d, J = 8.4 Hz, 1H), 7.93 (s, 1H), 7.81 (t, J = 7.6 Hz,
2H), 7.63 (t, J = 7.6 Hz, 1H), 7.46 (t, J = 7.8 Hz, 1H), 7.22 (t, J = 8.4 Hz, 1H), 3.68 (q, J = 5.6 Hz, 2H),
3.03 (t, J = 5.9 Hz, 2H), 2.78 (t, J = 4.0 Hz, 2H), 2.70 (t, J = 4.0 Hz, 2H), 2.55–2.44 (m, 8H), 2.19 (s, 6H),
1.88–1.82 (m, 2H), 1.67–1.58 (m, 2H), 1.00 (t, J = 7.0 Hz, 6H). 13C-NMR (101 MHz, CDCl3):
δ 170.41,
166.86, 157.47, 146.73, 143.93, 138.11, 130.70, 130.64, 129.21, 128.80, 127.67, 125.56, 124.97, 123.50, 123.04,
121.91, 118.35, 59.00, 51.05, 48.56, 46.63 (2C), 45.42 (2C), 45.32, 40.47, 37.67, 26.18, 25.50, 11.30 (2C).
Purity: 98.4% by HPLC. HRMS (ESI): Calcd. for [M + H]+ (C31H45N6O2) requires m/z 533.3599,
found 533.3568.
N-(3-(Dimethylamino)propyl)-2-(2-(2-(4-methylpiperazin-1-yl)acetamido)phenyl)quinoline-4-carboxamide (5b1
)
Compound 4b was reacted with N-methyl piperazine according to GP2 to give the desired product
5b1. After column chromatography with CH2Cl2/MeOH/Et3N (50:1:0.1) elution, a yellow solid
˝
1
was obtained in a 67% yield. mp. 185.5–186.8 C. HNMR (400 MHz, CDCl3):
δ 11.74 (s, 1H),
8.55 (d, J = 12.0 Hz, 1H), 8.48 (t, J = 4.0 Hz, 1H), 8.41 (d, J = 8.3 Hz, 1H), 8.27 (d, J = 8.4 Hz, 1H),
7.85- 7.73 (m, 2H), 7.66–7.57 (m, 2H), 7.43 (d, J = 7.4 Hz, 1H), 7.19 (t, J = 6.5 Hz, 1H), 3.68–3.62 (m,
2H), 3.11 (s, 2H), 2.50 (t, J = 4.0 Hz, 2H), 2.42 (s, 4H), 2.20 (s, 7H), 1.98 (s, 6H), 1.87–1.79 (m, 2H).
13C-NMR (101 MHz, CDCl3):
δ
169.28, 166.87, 157.53, 147.50, 143.71, 136.42, 130.36, 130.12(2C), 129.75,
128.08, 127.66, 125.66, 123.91, 123.30, 122.46, 119.06, 63.30, 58.74, 53.84 (2C), 53.25 (2C), 45.58, 45.26 (2C),

Molecules 2016, 21, 340
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40.21, 25.35. Purity: 96.9% by HPLC. HRMS (ESI): Calcd. for [M + H]+ (C28H37N6O2) requires m/z
489.2973, found 489.3036.
N-(3-(Dimethylamino)propyl)-2-(2-(2-(pyrrolidin-1-yl)acetamido)phenyl)quinoline-4-carboxamide (5b2)
Compound 4b was reacted with pyrrolidine according to GP2 to give the desired product 5b2
.
After column chromatography with CH2Cl2/MeOH/Et3N (100:1:0.1) elution, a yellow solid was
obtained in a 66% yield. mp. 180.1–180.9 ˝C. 1H-NMR (400 MHz, CDCl3):
δ 12.32 (s,1H),
8.60 (d, J = 8.3 Hz, 1H), 8.38 (t, J = 8.4 Hz, 2H), 8.20 (d, J = 8.4 Hz, 1H), 7.82 (s, 1H), 7.76 (t, J = 8.4 Hz,
1H), 7.66 (d, J = 7.8 Hz, 1H), 7.61 (t, J = 8.4 Hz, 1H), 7.44 (t, J = 8.4 Hz, 1H), 7.20 (t, J = 8.4 Hz,
1H), 3.70–3.62 (m, 2H), 3.27 (s, 2H), 2.53–2.45 (m, 6H), 2.21 (s, 6H), 1.87–1.79 (m, 2H), 1.51 (s, 4H).
13C-NMR (101 MHz, CDCl3):
δ 170.27, 166.92, 157.40, 147.35, 143.62, 137.01, 130.18, 129.92, 129.49,
129.41, 127.53, 127.38, 125.42, 123.65, 123.00, 122.51, 118.67, 61.47, 58.98, 54.50 (2C), 45.35 (2C), 40.43,
25.41, 23.84 (2C). Purity: 97.0% by HPLC. HRMS (ESI): Calcd. for [M + H]+ (C27H34N5O2) requires
m/z 460.2707, found 460.2682.
N-(3-(Dimethylamino)propyl)-2-(2-(2-(diethylamino)acetamido)phenyl)quinoline-4-carboxamide (5b3)
Compound 4b was reacted with diethylamine according to GP2 to give the desired product 5b3
.
After column chromatography with CH2Cl2/MeOH/Et3N (100:1:0.1) elution, a yellow solid was
1
obtained in a 63% yield. mp. 175.4–176.8 ˝C. H-NMR (400 MHz, CDCl3):
δ 12.24 (s, 1H),
8.59 (d, J = 8.3 Hz, 1H), 8.45–8.35 (m, 2H), 8.19 (d, J = 8.4 Hz, 1H), 7.80(s, 1H), 7.78 (t, J = 7.8 Hz,
1H), 7.67–7.58 (m, 2H), 7.46 (t, J = 7.8 Hz, 1H), 7.21 (t, J = 7.6 Hz, 1H), 3.66 (q, J = 4.0 Hz, 2H), 3.13 (s, 2H),
2.54–2.44 (m, 6H), 2.19 (s, 6H), 1.86–1.80 (m, 2H), 0.78 (t, J = 8.0 Hz, 6H). 13C-NMR (101 MHz, CDCl3):
δ
170.34, 166.96, 157.43, 147.38, 143.66, 137.00, 130.24, 129.97, 129.55, 129.44, 127.58, 127.44, 125.46,
123.72, 123.02, 122.56, 118.72, 61.48, 59.06, 54.54 (2C), 45.40 (2C), 40.53, 25.35, 23.86 (2C). Purity: 98.2%
by HPLC. HRMS (ESI): Calcd. for [M + H]+ (C27H36N5O2) requires m/z 462.2865, found 462.2750.
N-(3-(Dimethylamino)propyl)-2-(2-(2-(piperidin-1-yl)acetamido)phenyl)quinoline-4-carboxamide (5b4)
Compound 4b was reacted with piperidine according to GP2 to give the desired product 5b4. After
column chromatography with CH2Cl2/MeOH/Et3N (100:1:0.1) elution, a yellow solid was obtained
in a 65% yield. mp. 173.2–174.3 ˝C. 1H-NMR (400 MHz, CDCl3):
δ 11.79 (s, 1H), 8.56 (d, J = 8.3 Hz,
1H), 8.39 (d, J = 8.4 Hz, 2H), 8.30 (d, J = 8.4 Hz, 1H), 7.81–7.76 (m, 2H), 7.63 (t, J = 8.4 Hz, 2H),
7.45 (t, J = 8.0 Hz, 1H), 7.20 (t, J = 7.5 Hz, 1H), 3.67 (t, J = 8.0 Hz, 2H), 3.07 (s, 2H), 2.52 (t, J = 8.0 Hz,
2H), 2.34 (s, 4H), 2.20 (s, 6H), 1.88–1.80 (m, 2H), 1.21 (s, 6H). 13C-NMR (101 MHz, CDCl3):
δ 169.65,
166.88, 157.45, 147.41, 143.74, 136.91, 130.36, 130.17, 129.94, 129.86, 127.87, 127.24, 125.58, 123.86, 123.23,
122.55, 119.00, 61.69, 59.10, 54.58 (2C), 45.39 (2C), 40.63, 25.48, 23.87 (2C), 23.26. Purity: 98.9% by HPLC.
HRMS (ESI): Calcd. for [M + H]+ (C28H36N5O2) requires m/z 474.2864, found 474.2839.
N-(3-(Dimethylamino)propyl)-2-(2-(2-morpholinoacetamido)phenyl)quinoline-4-carboxamide (5b5)
Compound 4b was reacted with morpholine according to GP2 to give the desired product 5b5. After
column chromatography with CH2Cl2/MeOH/Et3N (90:1:0.1) elution, a white solid was obtained
in a 68% yield. mp. 171.3–172.7 ˝C. 1H-NMR (400 MHz, CDCl3):
δ 11.91 (s, 1H), 8.54 (d, J = 8.3 Hz,
1H), 8.44 (s, 1H), 8.37 (d, J = 8.4 Hz, 1H), 8.25 (d, J = 8.4 Hz, 1H), 7.83–7.76 (m, 2H), 7.66–7.59 (m, 2H),
7.44 (t, J = 7.8 Hz, 1H), 7.20 (t, J = 7.5 Hz, 1H), 3.65 (q, J = 5.4 Hz, 2H), 3.34 (d, J = 3.9 Hz, 4H), 3.12 (s, 2H),
2.49 (t, J = 6.0 Hz, 2H), 2.44–2.37 (m, 4H), 2.19 (s, 6H), 1.87–1.80 (m, 2H). 13C-NMR (101 MHz, CDCl3):
δ
168.82, 166.77, 157.55, 147.36, 143.82, 136.55, 130.31, 130.21, 129.68, 129.61, 127.77, 127.37, 125.66, 123.96,
123.17, 122.58, 118.96, 66.08 (2C), 63.98, 58.96, 53.70 (2C), 45.30 (2C), 40.47, 25.29. Purity: 96.6% by
HPLC. HRMS (ESI): Calcd. for [M + H]+ (C27H34N5O3) requires m/z 476.2656, found 476.2995.

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N-(3-(Dimethylamino)propyl)-2-(2-(2-((3-(dimethylamino)propyl)amino)acetamido)phenyl)quinoline-4-
carboxamide (5b6)
Compound 4b was reacted with 3-dimethyl aminopropyl amine according to GP2 to give the desired
product 5b6. After column chromatography with CH2Cl2/MeOH/Et3N (85:2:0.1) elution, yellow
oil was obtained in a 56% yield. 1H-NMR (400 MHz, CDCl3):
δ 12.55 (s, 1H), 8.62 (d, J = 7.8 Hz,
1H), 8.50 (s, 1H), 8.34 (d, J = 8.7 Hz, 1H), 8.21 (d, J = 9.1 Hz, 1H), 7.87 (s, 1H), 7.83–7.78 (m, 1H),
7.75 (d, J = 7.9 Hz, 1H), 7.63 (t, J = 6.9 Hz, 1H), 7.47 (t, J = 7.9 Hz, 1H), 7.24 (t, J = 8.0 Hz, 1H),
3.68 (d, J = 4.4 Hz, 2H), 3.47 (d, J = 2.3 Hz, 2H), 2.60 (d, J = 4.0 Hz, 4H), 2.28–2. 22 (m, 8H),
2.19 (d, J = 2.1 Hz, 6H), 1.90 (t, J = 4.0 Hz, 2H), 1.48–1.41 (m, 2H). 13C-NMR (101 MHz, CDCl3):
δ
170.19, 166.93, 157.48, 147.45, 143.67, 136.68, 130.28, 130.10, 129.75, 129.42, 127.66, 127.40, 125.60,
123.91, 123.09, 122.52, 118.73, 60.87, 59.26, 58.94, 46.50, 45.95 (2C), 45.30 (2C), 40.38, 25.63, 25.26. Purity:
95.6% by HPLC. HRMS (ESI): Calcd. for [M + H]+ (C28H39N6O2) requires m/z 491.3135, found 491.3217.
3.3. Antibacterial Studies
3.3.1. Screening for Antibacterial Activity by the Agar Diffusion Method
The in vitro antibacterial activities of newly-synthesized Compounds 5a1–5a7 and 5b1–5b6 were
evaluated for their antibacterial activities against S. aureus (ATCC29213), B. subtilis (ATCC6633),
E. coli (ATCC25922), P. aeruginosa (ATCC9027) and MRSA (ATCC43300) bacterial strains by the agar
diffusion method [36]. Fifteen to twenty milliliters of agar media were p˝oured into each petri dish.
Agar-containing plates were dried by placing in a laminar air flow at 37.2 C for 1 h.
One hundred milliliters of 0.5 McFarland standard of bacterial suspension were inoculated on
the agar media and spread on the whole surface with a sterile cotton bud. Using a sterile cork
borer, 5-mm wells were made on the seeded agar plates, and 50 mL of test compound at different
concentrations (50 and 100
triplicate for each compound. A control was also prepared for the plates in the same way using solvent
DMSO. The plates were maintained at 37
2 ˝C for 24 h. Activities were determined by measuring
µg/mL) were transferred in to the wells. The plates were prepared in
˘
the diameter of inhibition zone (mm). Ampicillin and gentamycin were used as the standards.
3.3.2. Minimum Inhibitory Concentration Determination
The MIC values were measured by the broth dilution method [40
a stock solution (10.24 mg/mL) of each tested compound in dimethyl sulfoxide (DMSO) were prepared
and then diluted with Mueller-Hinton broth to 1024
g/mL. The strains were grown briefly at 37 ˝C
in Mueller-Hinton media. After 5 h of bacterial growth, the bacterial culture was diluted to obtain
a concentration of 5 L bacterial suspension were added to each well of
105 cells/mL. Then, 150
the flat-bottomed 96-well tissue culture plate. Two-fold serial dilutions were carried out from the
fist well to the tenth well; the final concentrations of the compounds ranged from 1–512 g/mL; and
excess media (150
L) were discarded from the last well. The plates were incubated at 37 ˝C for 24 h
,41]. Five hundred microliters of
µ
ˆ
µ
µ
µ
in an electro-heating standing temperature cultivator and were read visually. The MIC of the sample
showing no turbidity was recorded as the lowest concentration of compound that inhibited bacterial
growth completely. Each assay was run in triplicate.
3.3.3. Cytotoxicity Assay
The active Compounds 5a4 and 5a7 were further examined for cytotoxicity in mouse macrophage
cell lines (RAW 264.7) [41]. The cells were cultured in DMEM medium supplemented with 10% fetal
bovine serum, 2 mM L-glutamine and 100 units/mL penicillin/streptomycin. Cell cultures were
maintained in a humidified atmosphere of 5% CO2 at 37 ˝C. Cells were passaged at pre-confluent
densities using a solution containing 0.25% trypsin and 0.5 mM EDTA. Cells were seeded at 1
cell/well in a 96-well plate for 12 h with 10% FBS, 1% penicillin and streptomycin and 1% L-glutamine,
ˆ
104

Molecules 2016, 21, 340
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resulting in 80% confluency. Each dose was prepared in 1% FBS medium by 1000
ˆ
dilution of
the drug, which was prepared in DMSO solution to ensure a DMSO concentration less than 0.1%.
After 72 h of exposure, the viability was assessed on the basis of cellular conversion of MTT into a
formazan product with DMSO for 45 min. The amount of formazan was measured using a microculture
plate reader with a test wavelength of 570 nm. Results were expressed as the mean
independent experiments.
˘
SD of three
1H-NMR, 13C-NMR, HRMS and HPLC spectra of the target compounds are shown in the
Supplementary Materials.
4. Conclusions
In summary, thirteen novel 2-phenyl-quinoline-4-carboxylic acid derivatives were synthesized
using aniline, 2-nitrobenzaldehyde and pyruvic acid as starting materials. The structures of the
1
new compounds were elucidated by using H-NMR, 13C-NMR and HRMS, and their purities
were determined to be above 95% by using HPLC. The newly-synthesized compounds were
investigated for their in vitro antibacterial activities by the agar diffusion method and a broth
dilution method. The results of antibacterial evaluation suggested that some compounds displayed
moderate antibacterial activity against S. aureus, and Compound 5a4 showed the best inhibition
against S. aureus with a MIC of 64
µg/mL. A few compounds exhibited weak activity against E. coli,
and Compound 5a7 had the best activity against E. coli with a MIC of 128
µg/mL. The cytotoxic
assay exhibited that the active Compound 5a4 showed weak cytotoxicity. These findings indicate the
potential usefulness of 5a4 in drug development. Moreover, some structure-activity relationships of
2-phenyl-quinoline-4-carboxylic acid derivatives were determined. The length and flexibility of the
amide side chain at the ortho-position of the 2-phenyl group had a significant effect on antibacterial
activity. The compound with longer side chains showed stronger activity than the compound with
shorter side chains. Moreover, rigid cyclic amino group at 2-phenyl is suitable for the antibacterial
activity against S. aureus and B. subtilis, and the introduction of flexible chain amino group at 2-phenyl
can enhance antibacterial activity against E. coli. All of these results will be useful in the future to
guide the design and modification of new candidate 2-phenyl-quinoline-4-carboxylic acid analogues
as antibacterial agents.
Supplementary Materials: Supplementary data (1H-NMR, 13C-NMR, HRMS and HPLC spectra) associated with
this article are available at: http://www.mdpi.com/1420-3049/21/3/340/s1.
Acknowledgments: The authors thank the Science and Technology Program of Dongguan (2012108102045),
the Science and Technology Development Program of Dongguan (2013108101059) and the Natural Science
Foundation of China (21502025) for financial support of this research.
Author Contributions: X.W. and M.H. conceived of and designed the experiments. X.W., X.X., Y.C., X.Y. and J.L.
performed the experiments. X.W., Y.L. and W.C. analyzed the data. X.W. wrote the paper. All authors read and
approved the final manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
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