Design and synthesis of 3-aryl-5-alicylic-[1,2,4]-oxadiazoles as novel platelet aggregation inhibitors

Article abstract of DOI:10.1002/jccs.200500050

A series of 4-[2-(alicyclic-[1,2,4]oxadiazol-3-yl)phenoxy]-butyric acids were synthesized from N-hydroxy-2-isopropoxy benzamidine in 4 steps with good yields. These [1,2,4]oxadiazoles are novel platelet aggregation inhibitors preventing human platelet agg

Full text of DOI:10.1002/jccs.200500050

Journal of the Chinese Chemical Society, 2005, 52, 331-338
331
Design and Synthesis of 3-Aryl-5-Alicylic-[1,2,4]-oxadiazoles as Novel
Platelet Aggregation Inhibitors
Ching-Yuh Cherna (
), Shinn-Jyh Chenb (
) and Wai-Ming Kanb* (
)
aDepartment of Applied Chemistry, Chiayi University of Technology, Chiayi, Taiwan, R.O.C.
bDepartment of Pharmacology, National Cheng Kung University, Tainan, Taiwan, R.O.C.
A series of 4-[2-(alicyclic-[1,2,4]oxadiazol-3-yl)phenoxy]-butyric acids were synthesized from N-hy-
droxy-2-isopropoxy benzamidine in 4 steps with good yields. These [1,2,4]oxadiazoles are novel platelet ag-
gregation inhibitors preventing human platelet aggregation induced by thromboxane derivative U44,619 and
adenosine diphosphate. A structure-activity-relationship study revealed that the potency for these 5-oxadi-
azoles increases with the increase in the ring size of the alicylic rings. Derivative 8f may be useful as a tem-
plate for the design of more potent anti-platelet agents.
Keywords: Platelet inhibitors; 1,2,4-Oxadiazoles.
INTRODUCTION
Homeostasis malfunction often causes many coagula-
compounds have platelet inhibition activities in a micromolar
range. Very potent inhibitors are not necessary while moder-
ate inhibitors may actually be more beneficial since only par-
tial inhibition is required to fine tune the homeostasis mecha-
nism. One of the most commonly used platelet aggregation
inhibitors, aspirin, has a IC50 of 60 mM against adenosine
diphosphate (ADP) induced platelet aggregation.
tion diseases. Thrombosis and thrombo-embolism are results
of hyperactivity in the platelet aggregation systems. Forma-
tion of intravascular thrombi and atherosclerosis are believed
to involve platelet aggregation as the first step in their patho-
physiology.1,2 When the blood vessel is damaged, the sub-
endothelial matrix is exposed to the blood stream. Some of its
components, such as collagen, will activate the circulating
platelets and lead to their activation. As a result, chemical
mediators of platelet aggregation, such as adenosine diphos-
phate,3 thromboxane A2 (TXA2),5 serotonin, platelet activat-
ing factor,4 and epinephrine,6 are released. These chemical
mediators further activate the platelet near the vicinity to the
formation of thrombus.
In our screening project for anti-platelet agents, we
found that some 3,5-disubstituted [1,2,4]-oxadiazoles that
possessed moderate anti-platelet activities against TXA2 ago-
nist, U46619 and ADP induced human platelet aggregation
(Data not shown). In this paper, we would like to report the
synthesis and pharmacological characterization of a series of
novel anti-platelet 3,5-disubstituted 1,2,4-oxadiazole deriva-
tives and their structure-activities relationship studies.
Platelet aggregation inhibitors have been used for the
prevention and treatment of coronary arterial diseases with
varying degrees of success.7,8 Among them, aspirin, dipyrida-
mole, and platelet glycoprotein IIb/IIIa antagonists are com-
monly employed. However, none of them is without signifi-
cant side effects. Therefore, there is a continuing effort to
pursue better anti-platelet drugs.
RESULTS AND DISCUSSION
4-[2-(alicyclic-[1,2,4]oxadiazol-3-yl)phenoxy]-bu-
tyric acids 11 represent the general structure of the target for
synthesis in our study. Retrosynthetic analysis suggested that
the [1,2,4]oxadiazoles may be synthesized via the condensa-
tion of benzamidoxime and the respective acyclic carboxylic
acids.14 In order to prepare the derivatives conveniently, N,
2-hydroxybenzamidine 2 seems to be a good synthesis start-
ing point. After conjugation with a carboxylic acid, the re-
spective [1,2,4]oxadiazoles can be prepared easily by con-
Recent reports on the design and synthesis of novel
platelet aggregation inhibitors include: 1,2,3-triazolecarbox-
amides,9 2,8-diazaspiro[4.5]decanes,10 benzylisoquinolines,11
pyrrolo[3,2-c]pyridine containing compounds12 and phenyl-
amides and piperidinecarboxylic acid esters.13 Most of these
* Corresponding author. Tel: +886-6-2353535 ext. 5469; Fax: +886-6-2749296; E-mail: Vincent@mail.ncku.edu.tw

332 J. Chin. Chem. Soc., Vol. 52, No. 2, 2005
Chern et al.
densation. In our preliminary study, treatment of 2-cyano-
phenol 1 with hydroxylamine gave an excellent yield of N, 2-
hydroxybenzamidine 2 (Scheme I). It was readily conjugated
to hydrocinnamic acid using DCC. However, the conjugate 3
failed to cyclize even when heated to 150 °C. It is possible
that intramolecular hydrogen bondings such as those ob-
served in 3a or 3b prevents dehydration. Thus cyclization
was not observed. Prevention of intramolecular hydrogen
bonding should allow [1,2,4]-oxadiazole formation. There-
fore, O-isoproxyl-benzamidine 6 was used as the starting ma-
terial for our synthesis. The preparation of O-isoproxyl-benz-
amidine 6 was straightforward (Scheme II). 2-Cyanophenol
was protected with isopropyl bromide and potassium carbon-
ate in DMF in excellent yield. The isolated product 5 was
then reacted with hydroxylamine when intermediate 6 was
isolated in 95% yield.
compound 11b was prepared in 92% yield after acidification.
The overall yield of 11b from 7b was 57% in 4 steps. Other
derivatives were synthesized accordingly in good overall
yields. The results are summarized in Scheme II.
The prepared [1,2,4]oxadiazoles were subjected to
pharmacological studies for probable anti-platelet aggrega-
tion activities. In irreversible human platelet aggregation,
two of the most important mediators are TXA2 and ADP. In
order for an anti-platelet agent to be useful, it must be able to
inhibit both TXA2 and adenosine diphosphate induced human
platelet aggregation. Therefore, we used U46,619, a stable
thromboxane agonist and ADP induced human platelet ag-
gregation as our pharmacological models. With the exception
11a, all of the derivatives show significant anti-platelet activ-
ities against both U46,619 and ADP. While 11a is a 3-phen-
yl-[1,2,4]oxadiazole, 11b-f are 3-alicyclic-[1,2,4]-oxadi-
azoles. Therefore, we may conclude that only alicyclic-[1,2,4]-
oxadiazoles show anti-platelet activities when their aryl
counterpart is devoid of any appreciable activity. Moreover,
the most potent member among the alicylic derivatives was
the admantane derivative 11f (IC50 = 3.8 ± 2.2 mM) while the
least potent was the cyclopropyl derivative 11b (IC50 = 43.8 ±
16.4 mM); a difference of over ten-fold. The rank order of po-
tency is as follows: 11a << 11b < 11c < 11d < 11e < 11f. There
was a clear trend of increase in anti-platelet activity with the
increase in ring size of the alicyclic ring. Thus it seems that a
bulky alicyclic ring is favored over the planar phenyl substi-
tution. Furthermore, all of the ethyl esters 10a-f were inactive
even at 50 mM, showing that the carboxylic acid moiety is ab-
solutely essential for anti-platelet activity. Apparently, for
maximal anti-platelet activity, these oxadiazoles should pos-
sess a bulky 3-alicyclic ring and a carboxylic acid group.
Moreover, a 3-phenyl substitution is undesired.
Scheme I
OH
O
N
NH2OH
PhCH2CH2COOH
CN
OH
O
NH2
OH
N
NH2
3
2
1
OH
O
N
N
OH
4
O
N
NH2
N
O
O
NH2
3b
3a
O
H
O
H
In this study, we reported the synthesis and biological
evaluation of some novel anti-platelet [1,2,4]-oxadiazoles.
Preliminary study showed that they possess interesting anti-
platelet activities. Therefore they may serve as templates for
further development of useful anti-platelet medication.
The preparation of 3-[2-carboxyproxylphenyl]-5-ali-
cyclic-[1,2,4]oxadiazoles was illustrated by the synthesis of
11b. Benzamidine 6 was conjugated with cyclopropyl-car-
boxylic acid 7b using DDC in dichloromethane when the re-
action was monitored by TLC. When the conjugation was fin-
ished, pyridine was added and heated to reflux. The desired
3-[2-isoproxylphenyl]-5-cyclopropyl-[1,2,4]oxadiazoles 8b
was isolated in 81% yield. The O-isopropyl ether was re-
moved cleanly with BCl3 in dichloromethane to afford the
3-[2-hydroxylphenyl]-5-cyclopropyl-[1,2,4]oxadiazoles 9b
in 90% yield after purification. The oxadiazoyl phenol 9b
was allowed to react with ethyl 1-bromobutyrate in DMF
with potassium carbonate to yield 10b in 85% yield. Finally
where 10b was hydrolyzed with alcoholic potash, the desired
EXPERIMENTAL
Melting points were determined on a Fargo MP-ID
hot-stage apparatus and are uncorrected. Proton NMR spec-
tra were recorded at 200 MHz on a Varian Mercury-200 NMR
spectrometer. Carbon NMR spectra were recorded at 50 MHz
on a Varian Mercury-200 NMR spectrometer. Proton and car-
bon chemical shifts are reported on the delta scale as parts per

Design and Synthesis of [1,2,4]-Oxadiazoles
J. Chin. Chem. Soc., Vol. 52, No. 2, 2005 333
Scheme II
OH
N
N O
N
N O
O
(i) DCC, CH2Cl2
BCl3
NH2
R
R
N
CH2Cl2
0oC
HO
R
(ii) pyridine
140-150oC
O
O
OH
8
7
6
9
N O
N O
ethyl 4-bromobutyrate
R
KOH
R
N
N
K2CO3, DMF
60-70oC
EtOH
r.t.
O
O
11
10
CO2Et
COOH
yield(%)
10
R=
8
9
11
a:
b:
84
81
95
90
88
85
92
92
91
c:
84
81
91
92
83
85
d:
e:
f:
93
82
74
91
94
87
86
90
90
OH
N
O
2-bromopropane
K2CO3, DMF
NH2OH HCl
NaOH(aq)
CN
OH
CN
NH2
60-70 o
96%
C
80-90 o
95%
C
O
6
1
5
million (ppm) downfield from tetramethylsilane (TMS) as in-
ternal reference. Mass spectra were measured with a VG An-
alytical Model 70-250s Mass Spectrometer. Infrared (IR)
Spectra were recorded on a Perkin Elmer Paragon 500 IR
Spectrometer. All reagents were used as obtained commer-
cially.
mmol), hydrocinnamic acid (10.30 mmol) and 21 mL of di-
chloromethane was placed in a reaction flask which is purged
with N2 and placed in an ice bath. N,N¢-dicyclohexylcarbodi-
imide (DCC) (2.10 g, 10.30 mmol) was dissolved in a small
volume of dichloromethane and was added into the above re-
action mixture. After 4 hours of stirring, the reaction mixture
was filtered and the white precipitate was washed thrice with
dichloromethane. The filtrate was concentrated by a vacuum
evaporator. Pyridine (34 mL) was added and the mixture was
heated to reflux for 4 hours. The reaction was monitored with
Synthesis of 3-(2-isopropoxy-phenyl)-5-phenyl-[1,2,4]-
oxadiazole (8a)
N-Hydroxy-2-isopropoxy-benzamidine 6 (2.00 g, 10.30

334 J. Chin. Chem. Soc., Vol. 52, No. 2, 2005
Chern et al.
TLC. After cooling, 30 mL of 10% HCl was added, and the
mixture was extracted with ethyl acetate (30 mL ´ 3). The
combined organic layer was washed with saturated brine and
dried over anhydrous MgSO4. After filtration, the residue
was purified by column chromatography (EtOAc:Hexanes =
1:5) A clear oil (2.42 g, 84%) of 8a was obtained. IR n
(CH2Cl2) cm-1: 3061, 2980, 2931, 1605, 1565, 1352, 1251,
1114, 949; 1H-NMR (CDCl3, 200 MHz) d: 8.23 (2H, dd, J =
7.8, 1.6 Hz), 8.05 (1H, dd, J = 7.8, 1.8 Hz), 7.61-7.49 (3H, m),
7.44 (1H, t, J = 7.2 Hz), 7.11-7.04 (2H, m), 4.68 (1H, sept, J =
6.2 Hz), 1.42 (6H, d, J = 6.2 Hz); 13C-NMR (CDCl3, 50 MHz)
d: 174.2, 167.5, 156.7, 132.3, 131.8, 131.3, 128.9, 127.9,
124.4, 120.5, 117.5, 115.0, 71.6, 22.0; EI-MS, m/z (rel.
int.%): 280 (M+, 30), 238 (100), 105 (27), 77 (12); HRMS
Calcd for C17H16N2O2: 280.1212. Found: 280.1207.
(81%): IR n (CH2Cl2) cm-1: 3061, 2972, 2874, 1601, 1569,
1
1485, 1340, 1279, 1251, 1110, 945; H-NMR (CDCl3, 200
MHz) d: 7.95 (1H, dd, J = 8.4, 1.8 Hz), 7.40 (1H, td, J = 7.6,
1.8 Hz), 7.06-6.98 (2H, m), 4.64 (1H, sept, J = 6.0 Hz), 3.97
(1H, quin, J = 7.6 Hz), 2.18-1.69 (8H, m), 1.38 (6H, d, J = 6.0
Hz); 13C-NMR (CDCl3, 50 MHz) d: 181.8, 166.7, 156.5,
131.6, 131.2, 120.4, 117.6, 115.0, 71.5, 36.8, 31.4, 25.4,
21.9; EI-MS, m/z (rel. int.%): 272 (M+, 12), 230 (64), 161
(68), 135 (61), 134 (22), 97 (36), 69 (100); HRMS Calcd for
C16H20N2O2: 272.1525. Found: 272.1521.
3-(2-isopropoxy-phenyl)-5-cyclohexyl-[1,2,4]oxadiazole
(8e)
Was prepared by following the general procedure
(82%): IR n (CH2Cl2) cm-1: 3061, 2980, 2931, 2858, 1601,
1569, 1485, 1279, 1251, 1111; 1H-NMR (CDCl3, 200 MHz)
d: 7.94 (1H, dd, J = 8.2, 2.0 Hz), 7.41 (1H, td, J = 8.2, 1.8 Hz),
7.07-6.99 (2H, m), 4.64 (1H, sept, J = 6.2 Hz), 3.06-2.96 (1H,
m), 2.16-2.05 (2H, m), 2.00-1.30 (8H, m), 1.40 (6H, d, J = 6.2
Hz); 13C-NMR (CDCl3, 50 MHz) d: 181.4, 166.7, 156.6,
131.6, 131.3, 120.5, 117.6, 115.0, 71.6, 36.1, 30.2, 25.5, 25.3,
22.0; EI-MS, m/z (rel. int.%): 286 (M+, 10), 244 (56), 203 (9),
161 (12), 134 (26), 110 (24), 83 (100), 55 (43); HRMS Calcd
for C17H22N2O2: 286.1681. Found: 286.1679.
3-(2-isopropoxy-phenyl)-5-cyclopropyl-[1,2,4]oxadiazole
(8b)
Was prepared by following the general procedure
(81%): IR n (CH2Cl2) cm-1: 3069, 2980, 2931, 1605, 1573,
1
1489, 1336, 1279, 1255, 1114, 949; H-NMR (CDCl3, 200
MHz) d: 7.89 (1H, dd, J = 8.0, 1.8 Hz), 7.41 (1H, td, J = 7.8,
1.8 Hz), 7.06-6.98 (2H, m), 4.63 (1H, sept, J = 6.0 Hz), 2.27
(1H, m), 1.38 (6H, d, J = 6.0 Hz), 1.31-1.20 (4H, m);
13C-NMR (CDCl3, 50 MHz) d: 179.9, 166.7, 156.5, 131.6,
131.1, 120.4, 117.5, 115.0, 71.5, 21.9, 9.8, 7.5; EI-MS, m/z
(rel. int.%): 244 (M+, 20), 229 (5), 202 (86), 161 (9), 134 (31),
119 (17), 69 (100); HRMS Calcd for C14H16N2O2: 244.1212.
Found: 244.1207.
3-(2-isopropoxy-phenyl)-5-adamantan-1-yl-[1,2,4]oxadi-
azole (8f)
Was prepared by following the general procedure
(74%): IR n (CH2Cl2) cm-1: 3061, 2980, 2923, 2866, 1601,
1
1569, 1485, 1283, 1251, 1110, 949; H-NMR (CDCl3, 200
3-(2-isopropoxy-phenyl)-5-cyclobutyl-[1,2,4]oxadiazole
(8c)
MHz) d: 7.96 (1H, dd, J = 8.0, 1.8 Hz), 7.40 (1H, td, J = 8.0,
1.8 Hz), 7.06-6.99 (2H, m), 4.64 (1H, sept, J = 6.0 Hz), 2.20-
2.04 (10H, m), 1.90-1.75 (5H, m), 1.40 (6H, d, J = 6.0 Hz);
13C-NMR (CDCl3, 50 MHz) d: 183.4, 166.2, 156.2, 131.3,
131.0, 120.1, 117.4, 114.6, 71.1, 39.6, 35.8, 34.9, 27.4, 21.6;
EI-MS, m/z (rel. int.%): 338 (M+, 6), 296 (57), 203 (43), 161
(6), 135 (100), 93 (26), 79 (28), 67 (10); HRMS Calcd for
C21H26N2O2: 338.1994. Found: 339.1985.
Was prepared by following the general procedure
(84%): IR n (CH2Cl2) cm-1: 3061, 2980, 2947, 2866, 1601,
1
1569, 1481, 1279, 1255, 1110, 949; H-NMR (CDCl3, 200
MHz) d: 7.96 (1H, dd, J = 8.0, 1.8 Hz), 7.40 (1H, td, J = 7.8,
1.8 Hz), 7.06-6.98 (2H, m), 4.63 (1H, sept, J = 6.0 Hz), 3.80
(1H, quin, J = 8.4 Hz), 2.58-2.41 (4H, m), 2.16-2.04 (2H, m),
1.38 (6H, d, J = 6.0 Hz); 13C-NMR (CDCl3, 50 MHz) d:
180.4, 166.7, 156.5, 131.6, 131.1, 120.4, 117.5, 114.9, 71.5,
31.2, 27.0, 21.8, 18.6; EI-MS, m/z (rel. int.%): 258 (M+, 26),
216 (100), 161 (13), 160 (11), 135 (27), 134 (24), 83 (25), 55
(54); HRMS Calcd for C15H18N2O2: 258.1368. Found:
258.1375.
Synthesis of 2-(5-phenyl-[1,2,4]oxadiazol-3-yl)-phenol
(9a)
3-(2-Isopropoxy-phenyl)-5-phenyl-[1,2,4]oxadiazole
(8a) (1.82 g, 6.50 mmol) was dissolved in 22 mL of dichloro-
methane. The reaction mixture was purged with N2 and
cooled to 0 °C. Into the reaction mixture, 22 mL of boron
trichloide was added. The reaction was monitored by TLC.
After 4 hours, water (30 mL) was added and was extracted
with ethyl acetate (30 mL ´ 3). The combined organic layer
3-(2-iso-propoxy-phenyl)-5-cyclopentyl-[1,2,4]oxadiazole
(8d)
Was prepared by following the general procedure

Design and Synthesis of [1,2,4]-Oxadiazoles
J. Chin. Chem. Soc., Vol. 52, No. 2, 2005 335
was washed with brine and dried with anhydrous MgSO4. Af-
ter filtration, the filtrate was evaporated under reduced pres-
sure and purified by column chromatography (silica gel,
EtOAc:Hexanes = 1:3). The product 9a (1.47 g, 95%) was ob-
tained. IR n (CH2Cl2) cm-1: 3206, 3052, 2964, 2915, 1609,
1565, 1464, 1368; 1H-NMR (CDCl3, 200 MHz) d: 8.23 (2H,
dd, J = 7.8, 1.4 Hz), 8.12 (1H, dd, J = 7.8, 1.8 Hz), 7.67-7.54
(3H, m), 7.41 (1H, t, J = 6.8 Hz), 7.09 (1H, d, J = 8.4 Hz), 7.02
(1H, t, J = 7.2 Hz); 13C-NMR (CDCl3, 50 MHz) d: 174.7,
167.7, 157.1, 133.4, 133.2, 129.3, 128.4, 128.2, 123.4, 120.0,
117.6, 110.7; EI-MS, m/z (rel. int.%): 238 (M+, 59), 105
(100), 91 (6), 77 (43); HRMS Calcd for C14H10N2O2:
238.0742. Found: 238.0737.
C13H14N2O2: 230.1056. Found: 230.1059.
2-(5-cyclohexyl-[1,2,4]oxadiazol-3-yl)-phenol (9e)
Was prepared by following the general procedure
(91%): IR n (CH2Cl2) cm-1: 3312, 3061, 2939, 2858, 1621,
1593, 1569, 1464, 1360, 1243; 1H-NMR (CDCl3, 200 MHz)
d: 8.02 (1H, dd, J = 7.8, 1.8 Hz), 7.40 (1H, td, J = 7.2, 1.6 Hz),
7.07-6.93 (2H, m), 3.12-2.98 (1H, m), 2.18-2.12 (2H, m),
1.91-1.25 (8H, m); 13C-NMR (CDCl3, 50 MHz) d: 182.0,
166.7, 157.0, 132.9, 127.9, 119.8, 117.4, 110.6, 36.1, 30.0,
25.3, 25.2; EI-MS, m/z (rel. int.%): 244 (M+, 79), 161 (4), 135
(35), 110 (23), 83 (100), 81 (9), 55 (32); HRMS Calcd for
C14H16N2O2: 244.1212. Found: 244.1205.
2-(5-cyclopropyl-[1,2,4]oxadiazol-3-yl)-phenol (9b)
2-(5-adamantan-1-yl-[1,2,4]oxadiazol-3-yl)-phenol (9f)
Was prepared by following the general procedure
(94%): IR n (CH2Cl2) cm-1: 3328, 3061, 2915, 2858, 1626,
1589, 1569, 1460, 1360, 1243; 1H-NMR (CDCl3, 200 MHz)
d: 8.02 (1H, td, J = 7.8, 1.2 Hz), 7.39 (1H, td, J = 7.2, 1.8 Hz),
7.07-6.93 (2H, m), 2.30-2.10 (10H, m), 1.90-1.80 (5H, m);
13C-NMR (CDCl3, 50 MHz) d: 184.6, 166.6, 157.0, 132.9,
127.8, 119.8, 117.4, 110.6, 39.8, 36.0, 35.5, 27.6; EI-MS, m/e
(rel. int.%): 296 (M+, 68), 211 (5), 135 (100), 93 (13), 79 (13),
55 (4); HRMS Calcd for C18H20N2O2: 296.1525. Found:
296.1528.
Was prepared by following the general procedure
(90%): IR n (CH2Cl2) cm-1: 3303, 3061, 3020, 1626, 1597,
1577, 1464, 1351, 1238, 1037; 1H-NMR (CDCl3, 200 MHz)
d: 9.53 (1H, s, -OH), 7.98 (1H, dd, J = 7.8, 1.8 Hz), 7.39 (1H,
td, J = 7.4, 1.4 Hz), 7.07-6.93 (2H, m), 2.37-2.24 (1H, m),
1.35-1.30 (4H, m); 13C-NMR (CDCl3, 50 MHz) d: 180.9,
166.9, 157.0, 132.9, 128.0, 119.8, 117.4, 110.6, 10.6, 7.7;
EI-MS, m/z (rel. int.%): 202 (M+, 47), 134 (22), 69 (100);
HRMS Calcd for C11H10N2O2: 202.0743. Found: 202.0749.
2-(5-cyclobutyl-[1,2,4]oxadiazol-3-yl)-phenol (9c)
Was prepared by following the general procedure
(91%): IR n (CH2Cl2) cm-1: 3336, 3061, 2988, 2947, 2866,
1626, 1597, 1573, 1460, 1242, 1154; 1H-NMR (CDCl3, 200
MHz) d: 9.62 (1H, s, -OH), 8.02 (1H, dd, J = 7.8, 1.8 Hz),
7.41 (1H, td, J = 6.8, 1.4 Hz), 7.08-6.94 (2H, m), 3.86 (1H,
quin, J = 8.4 Hz), 2.61-2.48 (4H, m), 2.30-2.05 (2H, m);
13C-NMR (CDCl3, 50 MHz) d: 181.2, 166.9, 157.0, 132.9,
127.9, 119.8, 117.4, 110.6, 31.1, 27.1, 18.7; EI-MS, m/z (rel.
int.%): 216 (M+, 47), 160 (8), 135 (29), 134 (28), 83 (32), 55
(100); HRMS Calcd for C12H12N2O2: 216.0898. Found:
216.0894.
Synthesis of 4-[2-(5-phenyl-[1,2,4]oxadiazol-3-yl)-
phenoxy]-butyric acid ethyl ester (10a)
2-(5-Phenyl-[1,2,4]oxadiazol-3-yl)-phenol (9a) (1.33
g, 5.60 mmol), K2CO3 (11.30 mmol) was added to N,N¢-di-
methylformamide (19 mL) (DMF). The mixture was purged
with N2, and heated to 70-80 °C with an oil bath. The mixture
was stirred for 10 minutes before ethyl 4-bromobutyrate
(11.30 mmol) was added. After 3 hours, 100 mL of H2O was
added. The aqueous solution was extracted with ethyl acetate
(30 mL ´ 3). The combined organic layer was washed with
brine and dried over anhydrous MgSO4. After filtration, the
filtrate was evaporated under reduced pressure and purified
by column chromatography (silica gel, EtOAc:Hexanes =
1:5). The product 10a (1.74 g, 88%) was obtained as a clear
oil. IR n (CH2Cl2) cm-1: 3068, 2980, 2939, 2874, 1730, 1605,
1565, 1352, 1352, 1283, 1178; 1H-NMR (CDCl3, 200 MHz)
d: 8.23 (2H, dd, J = 6.2, 2.0 Hz), 8.07 (1H, dd, J = 7.6, 1.6
Hz), 7.65-7.52 (3H, m), 7.47 (1H, t, J = 7.6 Hz), 7.09 (1H, t, J
= 7.6 Hz), 7.05 (1H, d, J = 8.2 Hz), 4.20 (2H, t, J = 6.0 Hz),
4.12 (2H, q, J = 7.2 Hz), 2.70 (2H, t, J = 7.4 Hz), 2.19 (2H,
quin, J = 7.2 Hz), 1.23 (3H, t, J = 7.2 Hz); 13C-NMR (CDCl3,
50 MHz) d: 174.3, 173.2, 167.3, 157.2, 132.3, 132.0, 131.1,
2-(5-cyclopentyl-[1,2,4]oxadiazol-3-yl)-phenol (9d)
Was prepared by following the general procedure
(92%): IR n (CH2Cl2) cm-1: 3312, 3061, 2964, 2874, 1626,
1593, 1484, 1464, 1243; 1H-NMR (CDCl3, 200 MHz) d: 9.64
(1H, s, -OH), 8.02 (1H, dd, J = 8.0, 2.0 Hz), 7.40 (1H, td, J =
7.8, 1.6 Hz), 7.26-6.94 (2H, m), 3.44 (1H, quin, J = 7.2 Hz),
2.25-1.73 (8H, m); 13C-NMR (CDCl3, 50 MHz) d: 182.6,
166.8, 157.0, 132.9, 127.9, 119.8, 117.4, 110.6, 36.8, 31.4,
25.5; EI-MS, m/z (rel. int.%): 230 (M+, 58), 135 (48), 134
(22), 97 (29), 96 (25), 69 (100); HRMS Calcd for

336 J. Chin. Chem. Soc., Vol. 52, No. 2, 2005
Chern et al.
128.9, 127.9, 124.2, 120.5, 116.1, 112.6, 67.3, 60.1, 30.4,
24.2, 14.0; EI-MS, m/z (rel. int.%): 352 (M+, 3), 307 (14), 251
(20), 105 (100), 77 (33); HRMS Calcd for C20H20N2O4:
352.1423. Found: 352.1416.
5), 299 (18), 147 (12), 115 (100), 87 (42), 69 (25); HRMS
Calcd for C19H24N2O4: 344.1736. Found: 344.1728.
4-[2-(5-Cyclohexyl-[1,2,4]oxadiazol-3-yl)-phenoxy]-butyric
acid ethyl ester (10e)
4-[2-(5-Cyclopropyl-[1,2,4]oxadiazol-3-yl)-phenoxy]-bu-
tyric acid ethyl ester (10b)
Was prepared by following the general procedure
(87%): IR n (CH2Cl2) cm-1: 3069, 2931, 2858, 1730, 1601,
1573, 1489, 1452, 1283, 1179, 1050, 945; 1H-NMR (CDCl3,
200 MHz) d: 7.94 (1H, dd, J = 7.6, 1.4 Hz), 7.43 (1H, td, J =
7.4, 1.8 Hz), 7.09-6.99 (2H, m), 4.15 (2H, t, J = 6.2 Hz), 4.13
(2H, q, J = 7.2 Hz), 3.12-2.98 (1H, m), 2.64 (2H, t, J = 7.4
Hz), 2.23-2.13 (2H, m), 1.91-1.25 (10H, m), 1.24 (3H, t, J =
7.2 Hz); 13C-NMR (CDCl3, 50 MHz) d: 181.3, 173.0, 166.4,
157.1, 131.7, 130.9, 120.4, 116.3, 112.5, 67.2, 59.9, 35.9,
30.3, 30.0, 25.3, 25.1, 24.3, 13.9; EI-MS, m/z (rel. int.%): 358
(M+, 7), 313 (38), 271 (11), 247 (15), 147 (27), 134 (9), 115
(100), 87 (83), 55 (27); HRMS Calcd for C20H26N2O4:
358.1893. Found: 358.1896.
Was prepared by following the general procedure
(84%): IR n (CH2Cl2) cm-1: 3069, 2980, 2939, 2874, 1730,
1601, 1577, 1489, 1283, 1251, 1046; 1H-NMR (CDCl3, 200
MHz) d: 7.90 (1H, dd, J = 7.8, 1.8 Hz), 7.42 (1H, td, J = 7.6,
1.6 Hz), 7.06-6.98 (2H, m), 4.18-4.08 (4H, m), 2.63 (2H, t, J
= 7.4 Hz), 2.30-2.12 (1H, m), 2.15 (2H, quin, J = 6.8 Hz),
1.60-1.17 (4H, m), 1.25 (3H, t, J = 7.2 Hz); 13C-NMR
(CDCl3, 50 MHz) d: 180.2, 173.3, 166.7, 157.3, 131.9, 131.1,
120.6, 116.4, 112.8, 67.5, 60.2, 30.5, 24.5, 14.1, 9.8, 7.6; EI-
MS, m/z (rel. int.%): 316 (M+, 5), 271 (22), 229 (5), 147 (13),
115 (100), 87 (53), 69 (56); HRMS Calcd for C17H20N2O4:
316.1423. Found: 316.1425.
4-[2-(5-Adamantan-1-yl-[1,2,4]oxadiazol-3-yl)-phenoxy]-
butyric acid ethyl ester (10f)
4-[2-(5-Cyclobutyl-[1,2,4]oxadiazol-3-yl)-phenoxy]-butyric
acid ethyl ester (10c)
Was prepared by following the general procedure
(86%): IR n (CH2Cl2) cm-1: 3061, 2980, 2915, 2858, 1730,
1565, 1489, 1352, 1251, 1179, 1093, 945; 1H-NMR (CDCl3,
200 MHz) d: 7.94 (1H, dd, J = 7.8, 2.0 Hz), 7.42 (1H, td, J =
7.6, 1.8 Hz), 7.08-6.99 (2H, m), 4.15 (2H, t, J = 6.8 Hz), 4.12
(2H, q, J = 7.2 Hz), 2.65 (2H, t, J = 7.4 Hz), 2.30-2.10 (12H,
m), 1.90-1.80 (5H, m), 1.24 (3H, t, J = 7.2 Hz), 13C-NMR
(CDCl3, 50 MHz) d: 184.0, 173.2, 166.5, 157.2, 131.8, 131.1,
120.5, 116.6, 112.6, 67.3, 60.1, 39.9, 36.1, 35.3, 30.5, 27.7,
24.4, 14.1; EI-MS, m/z (rel. int.%): 410 (M+, 5), 365 (25), 275
(16), 247 (19), 135 (84), 115 (100), 87 (70), 79 (22); HRMS
Calcd for C24H30N2O4: 410.2205. Found: 410.2198.
Was prepared by following the general procedure
(83%): IR n (CH2Cl2) cm-1: 3061, 2980, 2955, 2874, 1730,
1601, 1573, 1489, 1251, 1174, 1650, 945; 1H-NMR (CDCl3,
200 MHz) d: 7.95 (1H, dd, J = 7.6, 1.6 Hz), 7.40 (1H, td, J =
7.6, 1.8 Hz), 7.09-7.00 (2H, m), 4.16 (2H, t, J = 6.0 Hz), 4.13
(2H, q, J = 7.2 Hz), 3.82 (1H, quin, J = 8.6 Hz), 2.65 (2H, t, J
= 7.4 Hz), 2.56-2.45 (4H, m), 2.41-2.13 (4H, m), 1.24 (3H, t,
J = 7.2 Hz); 13C-NMR (CDCl3, 50 MHz) d: 180.7, 173.2,
166.7, 157.2, 131.9, 130.0, 120.6, 116.4, 112.7, 67.4, 60.1,
31.3, 30.4, 27.1, 24.4, 18.7, 14.0; EI-MS, m/z (rel. int.%): 330
(M+, 5), 285 (19), 247 (5), 147 (11), 115 (100), 87 (46), 55
(25); HRMS Calcd for C18H22N2O4: 330.1568. Found:
330.1574.
Synthesis of 4-[2-(5-phenyl-[1,2,4]oxadiazol-3-yl)-
phenoxy]-butyric acid (11a)
4-[2-(5-Cyclopentyl-[1,2,4]oxadiazol-3-yl)-phenoxy]-bu-
tyric acid ethyl ester (10d)
4-[2-(5-Phenyl-[1,2,4]oxadiazol-3-yl)-phenoxy]-bu-
tyric acid ethyl ester (10a) (0.93 g, 2.63 mmol) was dissolved
in 9 mL of ethanol and KOH (5.30 mmol) was added. The
mixture was stirred at ambient temperature for 8 hours. After
the reaction was completed, 10 mL of 10% HCl was added
and the mixture was extracted with ethyl acetate (20 mL ´ 3).
The combined organic layer was washed with brine and dried
over anhydrous MgSO4. After filtration, the filtrate was evap-
orated under reduced pressure and purified by column chro-
matography (silica gel, EtOAc:Hexanes = 1:3). The product
11a (0.78 g, 92%) was obtained as a colorless oil. IR n
(CH2Cl2) cm-1: 3430, 3028, 2956, 1710, 1658, 1613, 1561,
Was prepared by following the general procedure
(85%): IR n (CH2Cl2) cm-1: 3061, 2964, 2874, 1730, 1605,
1569, 1489, 1283, 1174, 1054; 1H-NMR (CDCl3, 200 MHz)
d: 7.94 (1H, dd, J = 7.8, 1.8 Hz), 7.43 (1H, td, J = 7.6, 2.0 Hz),
7.08-7.01 (2H, m), 4.13 (2H, q, J = 7.2 Hz), 4.14 (2H, t, J =
6.0 Hz), 3.40 (1H, quin, J = 7.8 Hz), 2.64 (2H, t, J = 7.4 Hz),
2.16 (2H, quin, J = 6.4 Hz), 2.19-1.68 (8H, m), 1.28 (3H, t, J =
7.2 Hz); 13C-NMR (CDCl3, 50 MHz) d: 181.9, 173.2, 166.5,
157.2, 131.8, 131.0, 120.5, 116.4, 112.6, 67.4, 60.1, 36.8,
31.3, 30.4, 25.4, 24.4, 14.0; EI-MS, m/z (rel. int.%): 344 (M+,

Design and Synthesis of [1,2,4]-Oxadiazoles
J. Chin. Chem. Soc., Vol. 52, No. 2, 2005 337
1367, 1283, 1255, 1026; 1H-NMR (d6-DMSO, 200 MHz) d:
8.18 (2H, dd, J = 7.8, 1.8 Hz), 7.96 (1H, dd, J = 7.6, 1.6 Hz),
7.75-7.50 (4H, m), 7.21 (1H, d, J = 8.2 Hz), 7.11 (1H, t, J =
6.8 Hz), 4.12 (2H, t, J = 5.8 Hz), 3.39 (1H, bs, -OH), 2.54 (2H,
t, J = 7.2 Hz), 2.00-1.92 (2H, m); 13C-NMR (d6-DMSO, 50
MHz) d: 174.9, 174.1, 167.0, 157.1, 133.2, 132.8, 130.8,
129.6, 127.9, 123.5, 115.3, 113.2, 67.3, 29.1, 24.4; EI-MS,
m/z (rel. int.%): 324 (M+, 7), 238 (42), 105 (100), 77 (20);
HRMS Calcd for C18H16N2O4: 324.1110. Found: 324.1104.
2.16-1.59 (10, m); 13C-NMR (d6-DMSO, 50 MHz) d: 182.0,
174.4, 166.1, 157.0, 132.5, 130.7, 120.5, 115.6, 113.1, 67.2,
36.2, 31.0, 30.0, 25.1, 24.3; EI-MS, m/z (rel. int.%): 316 (M+,
15), 257 (92), 230 (52), 147 (29), 134 (31), 69 (100), 41 (37);
HRMS Calcd for C17H20N2O4: 316.1423. Found: 316.1419.
4-[2-(5-Cyclohexyl-[1,2,4]oxadiazol-3-yl)-phenoxy]-butyric
acid (11e)
Was prepared by following the general procedure
(90%): IR n (CH2Cl2) cm-1: 3430, 3061, 2931, 2858, 1710,
1
4-[2-(5-Cyclopropyl-[1,2,4]oxadiazol-3-yl)-phenoxy]-bu-
tyric acid (11b)
1605, 1569, 1489, 1452, 1348, 1283, 1255, 1046; H-NMR
(d6-DMSO, 200 MHz) d: 7.84 (1H, dd, J = 7.6, 1.8 Hz), 7.52
(1H, td, J = 7.4, 1.8 Hz), 7.19 (1H, d, J = 8.4 Hz), 7.09 (1H, td,
J = 7.6, 1.8 Hz), 4.10 (2H, t, J = 6.0 Hz), 3.18-3.02 (1H, m),
2.50 (2H, t, J = 7.4 Hz), 2.09-1.34 (10H, m), 1.94 (2H, quin, J
= 7.2 Hz); 13C-NMR (d6-DMSO, 50 MHz) d: 181.5, 174.4,
166.1, 157.0, 132.5, 130.7, 120.5, 115.6, 113.2, 67.2, 35.1,
30.0, 29.8, 25.1, 24.7, 24.3; EI-MS, m/z (rel. int.%): 330 (M+,
16), 271 (11), 244 (96), 147 (41), 134 (31), 83 (100), 55 (46);
HRMS Calcd for C18H22N2O4: 330.1580. Found: 330.1571.
Was prepared by following the general procedure
(92%): IR n (CH2Cl2) cm-1; 3441, 3012, 2939, 2883, 1710,
1
1601, 1577, 1492, 1283, 1251, 1203, 1046; H-NMR (d6-
DMSO, 200 MHz) d: 7.79 (1H, dd, J = 7.8, 1.8 Hz), 7.48 (1H,
td, J = 7.4, 1.8 Hz), 7.17-7.00 (2H, m), 4.07 (2H, t, J = 6.0
Hz), 3.42 (1H, bs, -OH), 2.49 (2H, t, J = 7.2 Hz), 2.42-2.23
(1H, m), 1.92 (2H, quin, J = 7.2 Hz), 1.29-1.10 (4H, m);
13C-NMR (d6-DMSO, 50 MHz) d: 180.3, 174.4, 166.2, 157.0,
132.5, 130.7, 120.5, 115.6, 113.1, 67.2, 30.0, 24.3, 9.9, 7.1;
EI-MS, m/z (rel. int.%): 288 (M+, 29), 229 (10), 202 (100),
147 (29), 134 (25), 119 (16), 69 (96); HRMS Calcd for
C15H16N2O4: 288.1110. Found: 288.1116.
4-[2-(5-adamantan-1-yl-[1,2,4]oxadiazol-3-yl)-phenoxy]-
butyric acid (11f)
Was prepared by following the general procedure
(90%): IR n (CH2Cl2) cm-1: 3457, 3044, 2915, 2858, 1706,
1
4-[2-(5-Cyclobutyl-[1,2,4]oxadiazol-3-yl)-phenoxy]-butyric
acid (11c)
1605, 1565, 1489, 1352, 1283, 1251, 1094, 944; H-NMR
(d6-DMSO, 200 MHz) d: 7.81 (1H, dd, J = 7.6, 1.8 Hz), 7.50
(1H, td, J = 6.6, 1.8 Hz), 7.16 (1H, d, J = 8.6 Hz), 7.06 (1H, t, J
= 7.6 Hz), 4.08 (2H, t, J = 6.2 Hz), 2.52 (2H, t, J = 8.0 Hz),
2.00-2.09 (9H, m), 1.91 (2H, quin, J = 6.8 Hz), 1.72-1.79
(6H, m); 13C-NMR (d6-DMSO, 50 MHz) d: 183.8, 174.4,
166.2, 157.1, 132.5, 130.7, 120.5, 115.7, 119.1, 67.2, 39.5,
35.7, 35.0, 30.1, 27.3, 24.4; EI-MS, m/z (rel. int.%): 382 (M+,
5), 296 (49), 247 (12), 135 (100), 93 (14), 79 (17); HRMS
Calcd for C22H26N2O4: 382.1893. Found: 382.1902.
Was prepared by following the general procedure
(91%): IR n (CH2Cl2) cm-1: 3465, 3061, 2988, 2955, 2874,
1
1710, 1605, 1573, 1489, 1283, 1251, 1046, 945; H-NMR
(d6-DMSO, 200 MHz) d: 7.82 (1H, dd, J = 7.8, 1.6 Hz), 7.48
(1H, td, J = 7.4, 1.8 Hz), 7.17-7.01 (2H, m), 4.07 (2H, t, J =
6.0 Hz), 3.84 (1H, quin, J = 7.6 Hz), 2.50 (2H, t, J = 7.4 Hz),
2.46-2.27 (4H, m), 2.17-1.85 (4H, m); 13C-NMR (d6-DMSO,
50 MHz) d: 180.8, 174.4, 166.2, 157.0, 132.5, 130.7, 120.6,
115.6, 113.2, 67.2, 30.5, 30.0, 26.7, 24.3, 18.4; EI-MS, m/z
(rel. int.%): 302 (M+, 18), 243 (12), 216 (100), 147 (35), 135
(28), 134 (28), 83 (39), 55 (77); HRMS Calcd for C16H18N2O4:
302.1267. Found: 302.1272.
Human Platelet Aggregation Assay
It was performed as described.15 Venous blood was ob-
tained from healthy volunteers who had not taken any drug
for at least two weeks. Platelet rich plasma (PRP) was pre-
pared by centrifugation of citrated blood at 200 g for 20 min.
Aliquots (300 mL) of PRP were added into aggregometer
cuvettes and aggregation was recorded as increased light
transmission under continuous stirring (1000 rpm) at 37 °C
for 5 min after addition of the stimulus. ADP (10 mM) or
U46,619 (10 mM) were used as platelet activators in PRP. The
inhibitory activity of the compounds was tested by addition
of drug to PRP 10 min before addition of the stimulus (ADP
4-[2-(5-Cyclopentyl-[1,2,4]oxadiazol-3-yl)-phenoxy]-bu-
tyric acid (11d)
Was prepared by following the general procedure
(93%): IR n (CH2Cl2) cm-1: 3457, 3077, 2963, 2883, 1706,
1
1601, 1569, 1493, 1283, 1251, 1162, 1046, 945; H-NMR
(d6-DMSO, 200 MHz) d: 7.81 (1H, dd, J = 7.8, 2.0 Hz), 7.49
(1H, td, J = 7.4, 1.8 Hz), 7.17-7.01 (2H, m), 4.07 (2H, t, J =
6.0 Hz), 3.43 (1H, quin, J = 7.2 Hz), 2.50 (2H, t, J = 7.2 Hz),

338 J. Chin. Chem. Soc., Vol. 52, No. 2, 2005
Chern et al.
1380.
or U46,619). Drug vehicle (< 0.5% DMSO) added to PRP or
to the platelet suspension served as a control and did not af-
fect platelet function. The antiaggregatory activity of tested
compounds was evaluated as % inhibition of platelet aggre-
gation compared to control samples.
4. Edmunds, L. H. Ann. Thorc. Surg. 1987, 44, 430.
5. Solymoss, B. C.; Nadeau, P.; Millette, D.; Campeau, L. Cir-
culation 1988, 78(Suppl. I), 140.
6. Fuster, V.; Pumphrey, C. W.; McGoon, M. D.; Chesebro, J.
H.; Pluth, J. R.; McGoon, D. C. Circulation 1982, 66(Suppl.
I), 157.
7. Van De Graaff, E.; Steinhubl, S. R. Ann. Intern. Med. 2000,
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ACKNOWLEDGEMENT
8. Calverley, D. C. Clin. Geriatr. Med. 2001, 17, 31.
9. Cwiklicki, A.; Rehse, K. Archiv. De Pharmazie 2004,
337(3), 156.
This research was in part supported by a grant from the
National Science Council, R.O.C.
10. Mehrotra, M. M.; Heath, J. A.; Smyth, M. S.; Pandey, A.;
Rose, J. W.; Serooggy, J. M.; Volkots, D. L.; Mannizzi-
Alaimo, L.; Park, G. L.; Lambing, J. L.; Hollenbach, S. J.;
Scarborough, R. M. J. Med. Chem. 2004, 47(8), 2037.
11. Kuo, R. Y.; Chang, R. C.; Wu, C. C.; Datnam, R.; Wang, N.
Y.; Du, Y. C.; Wu, Y. C. Bioorg. & Med. Chem. Lett. 2003,
13(16), 2789.
Received October 12, 2004.
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