Synthesis of 4-alkoxy-2-phenylquinoline derivatives as potent antiplatelet agents

Article abstract of DOI:10.1016/S0960-894X(00)00652-1

In our continuing search for novel antiplatelet agents, 4-alkoxy derivatives of 2-phenylquinoline as well as related compounds were prepared. Through biological screening, a preliminary structure-antiplatelet activity relationship was established. Compounds 5-ethyl-4-methoxy-2-phenylquinoline (8), 4-ethoxy-5-ethyl-2-phenylquinoline (9), 4-ethoxycarbonylmethoxy-5-ethyl-2-phenylquinoline (10), 4-ethoxycarbonylbutoxy-5-ethyl-2-phenylquinoline (12) and 5-ethyl-4-(N-ethylcarboxido)methoxy-2-phenylquinoline (17) all demonstrated potent antiplatelet activity. Among them, compound 8 was the most potent with an IC₅₀ value of 0.08 μM and was about 3-fold more active than indomethacin. The mechanism of antiplatelet action of 8 is possibly through its inhibition on cyclooxygenase or thromboxane synthetase.

Full text of DOI:10.1016/S0960-894X(00)00652-1

Bioorganic & Medicinal Chemistry Letters 11 (2001) 279±282
Synthesis of 4-Alkoxy-2-phenylquinoline Derivatives as Potent
Antiplatelet Agents
Ting-Chia Ko,a Mann-Jen Hour,a Jin-Cherng Lien,a Che-Ming Teng,b
Kuo-Hsiung Lee,c Sheng-Chu Kuoa and Li-Jiau Huanga,*
aGraduate Institute of Pharmaceutical Chemistry, China Medical College, Taichung, Taiwan
bPharmacological Institute, College of Medicine, National Taiwan University, Taiwan
cNatural Products Laboratory, School of Pharmacy, University of North Carolina at Chapel Hill,
Chapel Hill, NC 27599, USA
Received 31 July 2000; accepted 4 September 2000
AbstractÐIn our continuing search for novel antiplatelet agents, 4-alkoxy derivatives of 2-phenylquinoline as well as related com-
pounds were prepared. Through biological screening, a preliminary structure±antiplatelet activity relationship was established.
Compounds 5-ethyl-4-methoxy-2-phenylquinoline (8), 4-ethoxy-5-ethyl-2-phenylquinoline (9), 4-ethoxycarbonylmethoxy-5-ethyl-2-
phenylquinoline (10), 4-ethoxycarbonylbutoxy-5-ethyl-2-phenylquinoline (12) and 5-ethyl-4-(N-ethylcarboxido)methoxy-2-phenyl-
quinoline (17) all demonstrated potent antiplatelet activity. Among them, compound 8 was the most potent with an IC50 value of
0.08 mM and was about 3-fold more active than indomethacin. The mechanism of antiplatelet action of 8 is possibly through its
inhibition on cyclooxygenase or thromboxane synthetase. # 2001 Published by Elsevier Science Ltd.
Introduction
work, we explored further into the analogues of com-
pounds 1 and 2, and prepared their N-alkyl and 4-alkoxy
derivatives by alkylation. During the screening of their
antiplatelet activity, we learned that some of the 4-alkoxy
derivatives had excellent activity and summarized our
®nding in this report.
In a previous paper,1 3 a series of mono- and disubstituted
2-phenyl-4-quinolone derivatives (A) were synthesized
and their antiplatelet activities were evaluated.
Results and Discussion
Chemistry
Target compounds, 4-alkoxy-2-phenylquinolines (4±13)
and N-methyl-2-phenylquinolones (14, 15), were synthe-
sized according to Scheme 1. The preparation of two
typical compounds, 5-ethyl-4-methoxy-2-phenylquinoline
(8) and 5-ethyl-N-methyl-2-phenylquinolone (15), shown
in Scheme 1, illustrated the general procedures.
We were encouraged that all of the 5-alkyl derivatives of
2-phenyl-4-quinolone (1)4 exhibited potent antiplatelet
activity. In particular, 5-ethyl-2-phenyl-4-quinolone (2)
had drawn our attention after showing an IC50 value of
0.15 mM that is more potent than indomethacin. In this
First, 5-ethyl-2-phenylquinolone (2)2 was methylated
with methyl iodide and NaH in THF to form 8 (mp 81±
82 ꢀC) and 15 (mp 125±127 ꢀC) in a ratio of 1:5.4. Ele-
mental analysis and mass spectral data (m/z 263, M+)
established their molecular formulae as C18H17NO for
both compounds, suggesting that they were possibly the
O-methyl and N-methyl isomers.
*Corresponding author. Tel.: +886-4-2055105; fax: +886-1-205-
5105; e-mail: ljhuang@mail.cmc.edu.tw
0960-894X/01/$ - see front matter # 2001 Published by Elsevier Science Ltd.
PII: S0960-894X(00)00652-1

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T.-C. Ko et al. / Bioorg. Med. Chem. Lett. 11 (2001) 279±282
Scheme 1.
1
Antiplatelet activity
The H NMR spectrum of 15 indicated the presence of
a N-methyl signal at d 3.56, a one-proton signal at 6.23
(H-3), the 5-ethyl signals at d 1.15 and 3.50, and eight
aromatic protons at d 7.19 to 7.62 (m). The data led to
the assignment of 15 as 5-ethyl-N-methyl-2-phenyl-4-
quinolone. Similarly, the assignment of 8 as 5-ethyl-4-
methoxy-2-phenylquinoline was based on its H NMR
spectral data, which included an O-methyl signal at d 4.12,
the 5-ethyl signals and eight aromatic proton signals.
To obtain a broader picture of how our target compounds
performed as antiplatelet agents, two selected compounds
(4 and 5) were evaluated as inhibitors of platelet aggrega-
tion induced by thrombin, arachidonic acid (AA), col-
lagen and PAF.5,6 As seen from the results in Table 1,
both 4 and 5 displayed very similar patterns of pre-
ferential inhibition. They were relatively strong inhibi-
tors of AA- and collagen-induced platelet aggregation,
but were poor inhibitors for thrombin- and PAF-
induced aggregation. Accordingly, we narrowed down
our focus to a comparison of AA-induced platelet
aggregation inhibitory activity in the forthcoming
discussion.
1
However, when 2 was treated with ethyl iodide, only the
corresponding O-ethylated product (9) was obtained.
Likewise, treating 2 with various ethoxycarbonylalkyl
halides a€orded only the corresponding O-alkylated pro-
ducts (10±12). On the other hand, the alkylation of com-
pound 1 and 7-ethyl-2-phenyl-4-quinolone (3) primarily
gave only O-alkylated products, except for the methyl-
ation which again yielded both N- and O-methylated
products.
From the data in Table 2, the 4-alkoxy-2-phenylquino-
lines 4 and 5 with methoxy and ethoxy substitution,
respectively, at C-4, displayed IC50 values of 2.1 and 2.0
mM and were 10 times more potent than aspirin.
Subsequent replacement of the 4-alkoxy R group with
-CH2COOCH2CH3 (6) resulted in greatly reduced
activity. Hydrolysis of the -CH2COOCH2CH3 moiety to
-CH2COOH yielded 7 and further reduced potency.
When 4-ethoxycarbonylmethoxy-2-phenylquinoline (6)
was treated with 10% NaOH, the hydrolyzed product
4-hydroxycarbonylmethoxy-2-phenylquinoline (7) was
obtained. Finally, we also treated 4-ethoxycarbonyl-
methoxy-5-ethyl-2-phenylquinoline (10) with methyl-
amine and ethylamine to provide their corresponding
amide derivatives (16, 17) (Scheme 2).
Next, we ®xed the R5 substituent as CH2CH3 and again
altered the R group of the 4-alkoxy moiety. When R
was CH3 (8), the corresponding IC50 value was 0.08 mM,
which is equivalent to about 27-fold the potency of 4, or
3-fold the potency of indomethacin.
Compound 9 (R=CH2CH3) was slightly less potent
than 8, but still more potent than indomethacin.
We then tried systematically replacing the alkyl
group with esters of increasing chain length, that is
from -CH2COOEt (10) to -(CH2)3COOEt (11) and to
-(CH2)4COOEt (12). Compound 11, with an ester sub-
stituent of intermediate length, was found to be the
least potent one. Amidation of the ester of 10 provided
16 and 17, which did not show signi®cantly improved
activity.
Scheme 2.

T.-C. Ko et al. / Bioorg. Med. Chem. Lett. 11 (2001) 279±282
281
Table 1. The inhibitory e€ects of compounds 4 and 5 on platelet aggregation induced by thrombin, AA, collagen and PAF (in vitro)a
Percent aggregation
Compounds
(mg/mL)
R
Thrombin
AA
Collagen
PAF
Control
100
50
92.5Æ0.5(4)
90.9Æ0.5(6)
89.2Æ1.8(4)
92.0Æ0.8(4)
4
CH3
56.8Æ8.7(4)***
0.0Æ0.0(4)***
0.0Æ0.0(4)***
0.0Æ0.0(3)***
65.8Æ4.0(3)***
87.2Æ0.5(3)***
20
5
2
0.0Æ0.0(4)***
9.4Æ6.0(4)***
18.5Æ3.8(4)***
38.6Æ6.3(4)***
62.8Æ6.3(4)**
85.0Æ1.3(4)
0.0Æ0.0(4)***
9.3Æ4.6(4)***
53.4Æ4.8(4)***
86.0Æ2.3(4)*
90.8Æ0.6(4)
1
0.5
0.2
0.1
IC50
2.13 mM
1.70 mM
Control
100
50
92.5Æ0.5(4)
90.9Æ0.5(6)
89.2Æ1.8(4)
92.0Æ0.8(4)
5
C2H5
80.0Æ1.0(3)***
0.0Æ0.0(5)***
0.0Æ0.0(4)***
0.0Æ0.0(4)***
2.1Æ1.8(4)***
5.9Æ3.2(4)***
15.9Æ5.0(4)***
28.4Æ8.2(4)***
36.3Æ6.3(4)***
43.5Æ6.4(4)***
78.5Æ2.7(4)**
85.9Æ2.1(4)
63.8Æ8.7(4)**
20
10
5
2
1
0.5
0.2
0.1
IC50
0.0Æ0.0(5)***
1.7Æ1.5(5)***
4.3Æ3.8(5)***
23.1Æ8.9(5)***
86.4Æ1.5(5)**
2.01 mM
3.61 mM
aPlatelets were incubated with tested sample or 0.5% DMSO at 37 ꢀC for 1 min, then thrombin (0.1 U/mL), AA (100 mM), collagen (10 mg/mL) or
PAF (2 ng/mL) was added to trigger the aggregation. Values are presented as meanÆSE, n=3±6. *p<0.05, **p<0.01, ***p<0.001.
Table 2. The inhibitory e€ects of compounds 4±17 on platelet
aggregation induced by arachidonic acid (in vitro)a
The above results indicated that -CH2CH3 substitution
on R5 played a key role in enhancing activity (compar-
ing 4±6 with 8±10). However, repositioning the -
CH2CH3 substituent from R5 to R7 resulted in greatly
reduced potency. The activity of 13 was about 1/28 that
of 9. We will continue to explore an explanation for this
result.
On the other hand, two 2-phenyl-4-quinolones, 14 and 15,
isomers of 4 and 8, respectively, with their corresponding
R group relocated to their N-moiety, demonstrated far
weaker activities than their 4-alkoxy counterparts.
Compound
R5
R7
R
IC50 (mM)
4
5
H
H
H
H
CH3
C2H5
2.13
2.01
6
7
8
9
10
11
12
13
14
15
16
H
H
H
H
H
H
H
H
H
C2H5
H
CH2COOEt
CH2COOH
CH3
13.88
35.60
0.08
0.14
0.78
1.54
0.53
3.97
218.30
30.04
1.88
0.60
0.25
In summary, we have prepared 4-alkoxy derivatives of
2-phenylquinolones as well as their related compounds.
Investigating the e€ect of structural modi®cation on
antiplatelet activity led to the following preliminary
structure±activity relationships.
C2H5
C2H5
C2H5
C2H5
C2H5
H
C2H5
CH2COOEt
(CH2)3COOEt
(CH2)4COOEt
C2H5
CH3
CH3
CH2CONHCH3
CH2CONHC2H5
H
The 4-alkoxy derivatives (4±13, 16, 17) demonstrated
potent antiplatelet activities, whereas the N-alkyl deri-
vatives (14, 15) exhibited much lower activity than their
O-alkyl isomers. Meanwhile, placing -CH2CH3 at R5
position contributed to greatly enhanced activity.
C2H5
C2H5
C2H5
H
H
H
17
Indomethacin
Aspirin
20.00
aPlatelets were incubated with a tested sample or 0.5% DMSO at
37 ꢀC for 1 min, then AA (100 mM) was added to trigger aggregation.
Aspirin and indomethacin are positive controls. Values are expressed
as meanÆSE from three to six separations.
Among all compounds studied, 5-ethyl-4-methoxy-2-
phenylquinoline (8), 4-ethoxy-5-ethyl-2-phenylquinoline
(9), 4-ethoxycarbonylmethoxy-5-ethyl-2-phenylquinoline

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T.-C. Ko et al. / Bioorg. Med. Chem. Lett. 11 (2001) 279±282
(10), 4-ethoxycarbonylbutoxy-5-ethyl-2-phenylquinoline
(12) and 5-ethyl-4-(N-ethylcarboxido)methoxy-2-phe-
nylquinoline (17) were identi®ed as the ®ve most potent
antiplatelet agents, with 8 being the most potent one
which had an activity about three times that of indo-
methacin. The physicochemical properties of phenyl-
quinolines di€er widely from those of indomethacin and
aspirin, therefore, the action mechanism of 8 was inves-
tigated and the preliminary results are described below.
Acknowledgements
This work was supported by a research grant from the
National Science Council of the Republic of China
(NSC 88-2314-B-039-010).
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The results indicated that AA- and collagen-induced
thromboxane B2 formation of human platelet from the
resting state 0.14ng/mL to 110.0Æ27.6 and 11.2Æ5.3 ng/
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machanism of antiplatelet action of 8 is possibly through
its inhibition on cyclooxygenase or thromboxane synthe-
tase. The exact and detailed mechanism of the anti-
platelet action of 8 needs further experiments to
elucidate.
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