Efficient Synthesis and In Vitro Biological Evaluation of 2,5-Diaryloxazoles as Potential Nitric Oxide Production Inhibitors

Article abstract of DOI:10.1002/bkcs.11331

An efficient first synthesis of 2,5-diaryloxazoles 1–5 was accomplished from commercially inexpensive precursors and in overall yields of 38–48%. The synthesis proceeds via α-aminoketones and cyclodehydration (Robinson–Gabriel reaction) as key step. Next, these oxazoles were examined for their inhibitory effect against nitric oxide (NO) production in lipopolysaccharide (LPS)-induced RAW 264.7 cells and were found to display concentration-dependent inhibition of NO production without cytotoxicity. Of note, compound 3 (70.7%; IC₅₀ = 2.33 μM) was identified as a potent inhibitor in view of its comparable inhibitory effect with the positive control, N^G-monomethyl-L-arginine acetate (L-NMMA) (79.3%; IC₅₀ = 4.51 μM) followed by compounds 5 (68.3%; IC₅₀ = 2.30 μM) and 2 (53.9%; IC₅₀ = 6.31 μM). As a whole, compound 3 may hold great promise for further development of NO production targeted anti-inflammatory agent.

Full text of DOI:10.1002/bkcs.11331

Article
DOI: 10.1002/bkcs.11331
BULLETIN OF THE
H. Y. Jang et al.
KOREAN CHEMICAL SOCIETY
Efficient Synthesis and In Vitro Biological Evaluation
of 2,5-Diaryloxazoles as Potential Nitric Oxide Production Inhibitors
†
†
‡
†,*
Ha Young Jang, Kongara Damodar, Jin-Kyung Kim, and Jong-Gab Jun
†Department of Chemistry and Institute of Applied Chemistry, Hallym University, Chuncheon 24252,
Korea. *E-mail: jgjun@hallym.ac.kr
‡
Department of Biomedical Science, College of Natural Science, Catholic University of Daegu,
Gyeungsan-Si 38430, South Korea
Received June 28, 2017, Accepted October 31, 2017, Published online November 23, 2017
An efficient first synthesis of 2,5-diaryloxazoles 1–5 was accomplished from commercially inexpensive
precursors and in overall yields of 38–48%. The synthesis proceeds via α-aminoketones and cyclodehy-
dration (Robinson–Gabriel reaction) as key step. Next, these oxazoles were examined for their inhibitory
effect against nitric oxide (NO) production in lipopolysaccharide (LPS)-induced RAW 264.7 cells and
were found to display concentration-dependent inhibition of NO production without cytotoxicity. Of
note, compound 3 (70.7%; IC50 = 2.33 μM) was identified as a potent inhibitor in view of its comparable
G
inhibitory effect with the positive control, N -monomethyl-L-arginine acetate (L-NMMA) (79.3%;
IC50 = 4.51 μM) followed by compounds 5 (68.3%; IC50 = 2.30 μM) and 2 (53.9%; IC50 = 6.31 μM).
As a whole, compound 3 may hold great promise for further development of NO production targeted
anti-inflammatory agent.
Keywords: 2,5-Diaryloxazoles, α-Aminoketones, Robinson–Gabriel reaction, Nitric oxide, RAW-264.7
macrophages, Anti-inflammatory
Introduction
(NSAIDs), selective COX-2 inhibitors (COXIBs), and anti-
histamines, which work either by interrupting the synthesis
or action of mediators that drive the host’s response to
injury. Despite notable successes from these small-molecule
inhibitors, still they are not without their shortcomings such
as gastric ulceration, renal toxicity, joint destruction, and
Inflammation is a protective and appropriate physiological
response induced by endogenous and exogenous insults
such as infection or injury. However, dysregulation of this
response may cause various pathophysiological conditions
including cardiovascular diseases, rheumatoid arthritis,
bronchitis, diabetes, neurodegenerative disorders, and can-
cer. Activated macrophages, lymphocytes, and neutro-
phils are important that have been involved in the
pathogenesis of acute and chronic inflammatory diseases.
Activated macrophages secrete a number of potent bioac-
tive inflammatory mediators, including nitric oxide (NO),
leukotrienes (LTs), prostaglandins (PGs), and cytokines
such as interleukin-1β (IL-1β), IL-6, and tumor necrosis
factor α (TNF-α). Among these, NO, a gaseous free radi-
cal, produced by nitric oxide synthase (endothelial-NOS,
neuronal-NOS, and inducible-NOS) has a key role in the
pathogenesis of inflammation and its role in this process is
tightly depended upon its concentration. Under normal
physiological conditions, NO has anti-inflammatory effects.
However, excess production of NO by inducible-NOS is
implicated in the pathogenesis of afore mentioned inflam-
6
cardiovascular disorders. Thus, there exist an unmet need
for novel anti-inflammatory drugs.
1
,2
Oxazole, a five-membered heterocyclic aromatic com-
pound, is one of the important scaffolds existing in a wide
variety of biologically active compounds and natural prod-
7,8
ucts. In recent years, its research has gained considerable
attention from both industry and academia in view of its
9
manifold pharmacological properties. Some oxazoles also
9,10
exhibited enzyme inhibitory activity.
It is reported that
3
,4
substituted oxazoles can be used as agrochemicals, fluores-
cent dyes, corrosion inhibitors and find applications in pho-
9
tography and polymer industries. In addition, they can
11
serve as important synthons in organic synthesis. Particu-
larly, 2,5-substituted aryl or alkyl oxazoles are privileged
scaffolds found in numerous natural products and pharma-
12
ceutical agents. They have been investigated as antidia-
9
betic, anticancer, and antibacterial agents.
5
matory diseases. Hence, tissue and/or site-specific inhibi-
tion of NO will open up new avenues for the future
management of inflammation-based diseases.
Current strategies to alleviate inflammation chiefly relied
on corticosteroids, nonsteroidal anti-inflammatory drugs
2,5-Diaryloxazoles (1–5) under the current study are
depicted in Figure 1. Compounds 2 and 4 were isolated from
13
the roots of Oxytropis lanata and compound 2 displayed
the most potent trypanocidal activity (IC50 1.0 μM) whereas
compounds 1, 3, and 5 are new synthetic 2,5-diaryloxazoles.
Bull. Korean Chem. Soc. 2017, Vol. 38, 1481–1485
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Article
BULLETIN OF THE
ISSN (Print) 0253-2964 | (Online) 1229-5949
KOREAN CHEMICAL SOCIETY
Methoxyphenyl)ethanone (6) (0.50 g, 3.33 mmol) in
CHCl (10.0 mL) was slowly added to it and then the mix-
3
ture was refluxed for 8 h. After completion of the reaction,
®
it was cooled to room temperature, filtered through Celite
pad, and washed with EtOAc (20 mL). The filtrate was
concentrated under reduced pressure. The crude was puri-
fied by column chromatography (EtOAc:hexane = 1:4) to
afford the pure product 7a (0.73 g, 96%) as brown liquid.
1
Rf = 0.43 (EtOAc/hexane = 1/4); H NMR (300 MHz,
CDCl ): δ 7.81 (1H, dd, J = 7.8, 1.8 Hz), 7.52 (1H, td,
3
J = 7.8, 1.8 Hz), 7.05–6.96 (2H, m), 4.61 (2H, s), 3.94
1
3
(
1
3H, s); C NMR (75 MHz, CDCl ): δ 192.3, 158.8,
3
34.9, 131.6, 124.9, 121.2, 111.7, 56.0, 38.0.
General Procedure for Delépine Reaction. To a stirred
solution of 2-methoxyphenacyl bromide (7a)/phenacyl bro-
mide (7b) (1 mmol, 1 equiv) in diethyl ether (13 mL) was
added hexamethylenetetramine (1 mmol, 1 equiv) all at
once and the mixture was stirred for 12 h at room tempera-
ture (solid formation observed). The resulting solid was fil-
tered, washed with diethyl ether (15 mL), and dried under
reduced pressure to afford the quaternary salt, which was
next placed in a two-necked round-bottomed flask fitted
with reflux condenser and EtOH (22 mL) was added to
it. Concentrated HCl (0.6 mL) was added to it and the mix-
ture was refluxed for 3 h (solid formed). After cooling to
room temperature, the solid was filtered, washed with EtOH
Figure 1. Structures of 2,5-diaryloxazoles (1–5).
In continuation of our research efforts directed towards the
synthesis of bioactive natural products and their analogues
14
as NO inhibitors, herein, we report an efficient synthesis
and in vitro biological assessment of oxazoles 1–5.
Experimental
All chemicals were purchased from local sources and were
used without further purification unless noted otherwise.
All solvents used for reactions were freshly distilled from
proper dehydrating agents under nitrogen atmosphere. All
solvents used for chromatography were purchased and
directly used without further purification. Thin-layer chro-
matography (TLC) was performed on DC-Plastikfolien
(20 mL), and dried under vacuum to afford pure 2-
methoxybenzoylmethylammonium chloride (8a)/benzoyl-
methylammonium chloride (8b) salt.
2
-Amino-1-(2-methoxyphenyl)ethanone
hydrochloride
ꢀ
1
(
8a): Yield: 88%; brown solid; mp 102–104 C; H NMR
6
0, F254 (layer thickness 0.2 mm; Merck, Darmstadt, Ger-
(
300 MHz, CDCl ): δ 7.26 (1H, d, J = 8.7 Hz), 7.11 (1H,
3
many) plastic-backed silica gel plates and visualized by
UV light (254 nm) or staining with p-anisaldehyde and
phosphomolybdic acid (PMA) stain. Chromatographic puri-
fication was carried out using Kieselgel 60 (60–120 mesh;
13
t, J = 7.2 Hz), 4.33 (2H, q, J = 5.4 Hz), 3.94 (3H, s);
C
NMR (75 MHz, CDCl ): δ 192.1, 159.6, 135.9, 130.0,
3
1
20.7, 112.9, 56.1, 48.7.
-Amino-1-phenylethanone hydrochloride (8b): Yield:
7%; brown solid; mp 180–182 C; H NMR (300 MHz,
2
1
Merck). H NMR spectra were recorded at Varian
ꢀ
1
9
Mercury-300 MHz FT-NMR (Varian, Inc., Palo Alto, CA,
CDCl ): δ 8.19 (3H, br s), 7.84 (1H, d, J = 7.8 Hz), 7.67
3
1
3
USA) and 75 MHz for C, with the chemical shift (δ)
reported in parts per million (ppm) downfield relative to tet-
ramethylsilane and the coupling constants (J) quoted in
hertz (Hz). Peak splitting patterns were abbreviated as s
(
1H, t, J = 7.8 Hz), 7.26 (1H, d, J = 8.7 Hz), 7.11 (1H, t,
13
J = 7.2 Hz), 4.33 (2H, q, J = 5.4 Hz), 3.94 (3H, s);
C
NMR (75 MHz, CDCl ): δ 192.1, 159.6, 135.9, 130.0,
3
1
20.7, 112.9, 56.1, 48.7.
(
singlet), d (doublet), t (triplet), q (quartet), dd (doublet of
General Procedure for Synthesis of 2,5-Diaryloxazole.
First step: To a stirred solution of substituted benzoic acid
doublet), and m (multiplet) and CDCl /CD COCD was
3
3
3
used as a solvent and an internal standard. High-resolution
mass spectra electrospray ionization (HRMS-ESI) was
obtained on an Agilent 6220 TOF LC/MS spectrometer
Agilent Technologies, Santa Clara, CA, USA) spectrome-
ter. Melting points were measured on a MEL-TEMP II
Triad Scientific, Manasquan, NJ, USA) apparatus and were
uncorrected.
-Bromo-1-(2-Methoxyphenyl)Ethanone (7a).
II) bromide (2.97 g, 6.66 mmol) was placed in a two-
(
9a/9b/9c) (1.0 mmol) in anhydrous DMF (6 mL) was
added thionyl chloride (3.43 mL, 2.0 mmol) dropwise at
ꢀ
0
C under nitrogen atmosphere. The reaction mixture was
(
ꢀ
then warmed to at 60 C and stirred for 2 h. After comple-
tion of the reaction, cooled to room temperature, solvent
and excess thionyl chloride was removed under reduced
pressure. The crude compound was dried under vacuum for
1 h and was used in the next step.
(
2
(
Copper
Second step: Pyridine (4 mL) was added to 2-
methoxybenzoylmethylammonium chloride (8a)/benzoyl-
methylammonium chloride (8b) salt (0.5 mmol) and stirred
for 10 min at room temperature. To this mixture, above
necked round-bottomed flask fitted with a reflux condenser.
EtOAc (10.0 mL) was added to it and the mixture was stir-
ꢀ
red at 70 C for 5 min under nitrogen atmosphere. 1-(2-
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Article Synthesis and Nitric Oxide Production Inhibitory Activity of 2,5-Diaryloxazoles KOREAN CHEMICAL SOCIETY
corresponding acid chloride was added slowly and stirred
for 1 h under nitrogen atmosphere. After completion of the
reaction, water (4 mL) was added, neutralized with 3 N
HCl, and extracted with EtOAc (2 × 20 mL). The com-
bined organic layer was washed with water (2 × 15 mL),
brine (15 mL), dried over anhydrous Na SO , and concen-
151.3, 147.8, 129.0, 128.5, 128.3, 124.4, 124.3, 123.4,
122.2, 121.4, 114.4, 61.5, 56.3; HRMS-ESI m/z calcd. for
+
C H NO [M + H] : 282.1130, found 282.1135.
17
16
3
General Procedure for Demethylation of 2,5-Diaryloxa-
zole. To a stirred solution of oxazole (0.2 mmol) in anhy-
drous CH Cl (5 mL) was added BBr solution (1.0 M in
2
4
2
2
3
ꢀ
CH Cl , 1.0 mL, 5.00 mmol, 5.0 equiv) slowly at 0 C
2 2
trated in vacuo. To this crude, acetic anhydride (3 mL) fol-
lowed by conc. H SO (0.1 mL) were added at room
temperature and the mixture was stirred at 90 C for 1 h.
After completion of the reaction, cooled to room tempera-
under nitrogen atmosphere. The reaction was warmed to
room temperature and stirred for 2–6 h. After completion
2
4
ꢀ
of the reaction, excess BBr was quenched by the slow
3
ture, H O (5 mL) was added. The mixture was neutralized
addition of MeOH (1 mL), stirred for 20 min and then sol-
2
with saturated aqueous NaHCO and extracted with EtOAc
vent was removed under reduced pressure. H O (5 mL)
3
2
(2 × 25 mL). The combined organic layer was washed with
and CH Cl (15 mL) were added to the crude and two
2
2
water (2 × 15 mL), brine (15 mL), dried over anhydrous
Na SO , and concentrated in vacuo. The crude was purified
by column chromatography (EtOAc:hexane = 1:3) to afford
the pure oxazole product.
layers separated. Aqueous layer was extracted with CH Cl2
2
(2 × 20 mL). The combined organic layer was dried over
anhydrous Na SO and concentrated in vacuo to afford the
2
4
2
4
product (Note: For compound 2 preparation from 1, 10.0
2
-(2,3-Dimethoxyphenyl)-5-(2-methoxyphenyl)oxazole (1):
equiv of BBr was used instead of 5.0 equiv).
3
ꢀ
0
0
00
Yield: 59%; pale yellow solid; mp 146–148 C; R = 0.56
EtOAc/hexane = 1/1); H NMR (300 MHz, CDCl ): δ
.88 (1H, dd, J = 7.8, 1.5 Hz), 7.68 (1H, s), 7.63 (1H, dd,
2-(2 ,3 -Dihydroxyphenyl)-5-(2 -hydroxyphenyl)oxazole
f
1
ꢀ
(
7
(2): Yield: 95%; white solid; mp 174–176 C; R = 0.32
3
f
1
(EtOAc/hexane = 1/1); H NMR (300 MHz, CD COCD ):
3
3
J = 7.8, 1.5 Hz), 7.29 (1H, td, J = 8.7, 1.8 Hz), 7.14 (1H, t,
J = 8.1 Hz), 7.05 (1H, td, J = 7.8, 0.9 Hz), 6.96–7.01 (2H,
m), 3.99 (3H, s), 3.98 (3H, s), 3.91 (3H, s); C NMR
δ 7.92 (1H, d, J = 7.2 Hz), 7.78 (1H, s), 7.54 (1H, d,
J = 7.5 Hz), 7.26 (1H, t, J = 7.2 Hz), 6.99–7.01 (3H, m),
1
3
13
6.90 (1H, t, J = 7.8 Hz);
C
NMR (75 MHz,
(
1
1
75 MHz, CDCl ): δ 158.3, 155.7, 153.8, 147.8, 147.7,
CD COCD ): δ 160.5, 154.6, 148.0, 146.7, 146.2, 130.3,
3
3
3
29.0, 127.5, 125.9, 124.4, 122.2, 121.5, 120.9, 117.4,
126.4, 125.2, 120.8, 120.5, 118.5, 117.0, 116.6, 115.5,
+
14.1, 110.9, 61.4, 56.2, 55.7; HRMS-ESI m/z calcd. for
111.8; HRMS-ESI m/z calcd. for C H NO [M + H] :
15
12
4
+
C H NO [M + H] : 312.1236, found 312.1229.
270.0766, found 270.0760.
1
8
18
4
0
2
,5-bis(2-Methoxyphenyl)oxazole (10a): Yield: 61%; pale
2,2 -(Oxazole-2,5-diyl)diphenol (3): Yield: 91%; white
ꢀ
ꢀ
1
yellow solid; mp 110–112 C; R = 0.44 (EtOAc/hexane =
solid; mp 184–186 C; R = 0.30 (EtOAc/hexane = 1/1); H
f
f
1
1
/1); H NMR (300 MHz, CDCl ): δ 8.04 (1H, d,
NMR (300 MHz, CD COCD ): δ 11.21 (1H, s), 9.59 (1H,
3
3
3
J = 7.5 Hz), 7.87 (1H, d, J = 7.5 Hz), 7.68 (1H, s), 7.42
s), 8.04 (1H, d, J = 8.2 Hz), 7.93 (1H, d, J = 7.8 Hz), 7.77
(1H, s), 7.38 (1H, t, J = 7.8 Hz), 7.25 (1H, t, J = 7.5 Hz),
(
(
1H, t, J = 7.8 Hz), 7.29 (1H, t, J = 7.8 Hz), 7.08–7.03
3H, m), 6.97 (1H, d, J = 8.1 Hz), 4.01 (3H, s), 3.97 (3H,
13
7.08–7.00 (4H, m); C NMR (75 MHz, CD COCD ): δ
3
3
1
3
s); C NMR (75 MHz, CDCl ): δ 158.4, 157.6, 155.7,
1
1
160.2, 158.1, 154.6, 148.1, 133.0, 130.4, 126.7, 126.5,
125.3, 120.8, 120.3, 117.6, 116.6, 115.5, 111.7; HRMS-
3
47.2, 131.5, 130.1, 128.9, 127.7, 126.0, 120.9, 120.7,
+
17.5, 116.7, 112.1, 110.9, 56.3, 55.7; HRMS-ESI m/z
ESI m/z calcd. for C H NO [M + H] : 254.0817, found
15
12
3
+
calcd. for C H NO [M + H] : 282.1130, found
254.0806.
17
16
3
2
82.1136.
2-(2-(3-Hydroxyphenyl)oxazol-5-yl)phenol (4): Yield:
ꢀ
5-(2-Methoxyphenyl)-2-(3-methoxyphenyl)oxazole (10b):
98%; white solid; mp 240–242 C; R = 0.33 (EtOAc/hex-
f
ꢀ
1
Yield: 47%; pale yellow solid; mp 114–116 C; R = 0.75
ane = 1/1); H NMR (300 MHz, CD COCD ): δ 9.44 (1H,
f
3
3
1
(EtOAc/hexane = 1/1); H NMR (300 MHz, CDCl ): δ
br s), 8.77 (1H, br s), 7.87 (1H, d, J = 8.1 Hz), 7.67 (2H,
s), 7.63 (1H, d, J = 7.5 Hz), 7.36 (1H, t, J = 8.1 Hz), 7.21
3
7.87 (1H, d, J = 7.5 Hz), 7.70 (1H, d, J = 7.8 Hz), 7.63
13
(
(
(
2H, s), 7.40–7.24 (2H, m), 7.06 (1H, t, J = 7.5 Hz), 6.98
2H, d, J = 8.1 Hz), 3.98 (3H, s), 3.89 (3H, s); C NMR
75 MHz, CDCl ): δ 160.0, 159.9, 155.8, 147.9, 130.0,
(1H, t, J = 7.8 Hz), 6.97–7.06 (3H, m);
C NMR
1
3
(75 MHz, CD COCD ): δ 160.2, 158.5, 154.5, 148.9,
3
3
130.9, 129.8, 129.7, 127.9, 126.3, 120.8, 118.1, 116.6,
3
1
1
29.2, 128.9, 127.7, 125.9, 120.9, 118.9, 117.3, 116.8,
116.2, 113.5; HRMS-ESI m/z calcd. for C H NO
3
15
12
+
11.1, 111.0, 55.7; HRMS-ESI m/z calcd. for C H NO
3
[M + H] : 254.0817, found 254.0823.
1
7
16
+
[M + H] : 282.1130, found 282.1143.
3-(5-Phenyloxazol-2-yl)benzene-1,2-diol (5): Yield: 88%;
ꢀ
2-(2,3-Dimethoxyphenyl)-5-phenyloxazole (10c): Yield:
white solid; mp 134–136 C; R = 0.25 (EtOAc/hexane = 1/
f
ꢀ
1
5
6%; pale yellow solid; mp 86–88 C; R = 0.48 (EtOAc/
1); H NMR (300 MHz, CD COCD ): δ 11.20 (1H, s),
f
3
3
1
hexane = 1/1); H NMR (300 MHz, CDCl ): δ 7.71 (2H,
d, J = 8.4 Hz), 7.61 (1H, dd, J = 8.1, 1.5 Hz), 7.48 (1H,
s), 7.43 (1H, t, J = 7.2 Hz), 7.33 (1H, d, J = 7.5 Hz), 7.15
8.02 (1H, s), 7.87 (2H, dd, J = 6.9, 1.5 Hz), 7.79 (1H, s),
7.52–7.48 (3H, m), 7.42 (1H, dd, J = 6.9, 1.5 Hz), 7.00
(1H, dd, J = 7.8, 1.2 Hz), 6.90 (1H, td, J = 7.8, 1.2 Hz);
3
13
(1H, t, J = 8.1 Hz), 7.01 (1H, d, J = 8.1 Hz),4.01 (3H, s),
C NMR (75 MHz, CD COCD ): δ 161.7, 151.0, 146.7,
3
3
13
3
.92 (3H, s); C NMR (75 MHz, CDCl ): δ 159.4, 153.9,
146.3, 129.8, 129.6, 128.1, 125.0, 122.2, 120.5, 118.7,
3
Bull. Korean Chem. Soc. 2017, Vol. 38, 1481–1485
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BULLETIN OF THE
Article Synthesis and Nitric Oxide Production Inhibitory Activity of 2,5-Diaryloxazoles KOREAN CHEMICAL SOCIETY
G
1
17.1, 111.7; HRMS-ESI m/z calcd. for C H NO
Griess reagent. N -Monomethyl-L-arginine acetate (L-
1
5
12
3
+
17
[M + H] : 254.0817, found 254.0810.
NMMA) was used as a positive control.
Effect of compounds 1–5 on NO generation by induced
macrophages was monitored (Table 1). When the cells were
treated with LPS along with 1 and 10 μM concentration of
compounds, concentration-dependent inhibition of NO pro-
duction was observed. As shown in Table 1, the percentage
of inhibition was impressive at the highest concentration
Results and Discussion
Our approach to synthesize the oxazole derivatives 1–5
is outlined in Scheme 1. α-Bromination of 2 -
methoxyacetophenone (6) using CuBr2 afforded 2 -
0
0
(
10 μM). Of note, compound 3 showed the highest percent-
age of inhibition (70.7%) at 10 μM followed by compounds
(68.3%) and 2 (53.9%) when compared to the positive
control, L-NMMA (79.3%). However, trimethoxy analogue
1) of 2 and also compound 4 had almost no inhibitory
methoxyphenacyl bromide (7a) in 96% yield. Next compound
7
a and phenacyl bromide (7b) were converted as
5
15
α-aminoketones (Delépine reaction). Treatment of bromo
compounds 7a and 7b with hexamethylenetetramine gave the
quaternary salts, which were subsequently underwent acid
hydrolysis to furnish 4-methoxybenzoylmethylammonium
chloride (8a) and benzoylmethylammonium chloride (8b) in
(
effect at 10 μM also. Structure–activity relationships of
these compounds against NO release indicated that C2-aryl
group of oxazoles containing hydroxyl functionality (−OH)
in the position 2 showed essential role for the inhibitory
activity. Next, cell viability was determined with the MTT
8
8 and 97% yields, respectively.
Next, substituted benzoyl chlorides obtained from their
corresponding benzoic acids (9a–9c) reacted with either 8a
or 8b in the presence of pyridine to give 2-acylamino-
ketones, which subsequently underwent cyclodehydration
(3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bro-
mide) method to ensure that the observed NO inhibition
was not due to cytotoxic effects. As a result, no cytotoxic
effects were observed at the highest concentrations
1
6
(
2
Robinson–Gabriel reaction) to deliver the corresponding
,5-diaryloxazoles 1 and 10a–10c in 47–61% yields over
(10 μM) on RAW 264.7 cells. The IC50 values of com-
three steps. Finally, addition of BBr solution (1.0 M in
3
pounds 1–5 were evaluated by GraphPad Prism 4.0 soft-
ware and displayed >100, 6.31, 2.33, 24.83, and 2.30 μM,
respectively (Table 1). From these pharmacological results,
compound 3 strongly inhibits LPS-induced NO production
without considerable cytotoxicity and hence appears as a
promising NO production inhibitor candidate.
CH Cl ) to compounds 1 and 10a–10c provided 2–5,
2
2
respectively, in 88–98% yields. All the target compounds
1
13
1
–5 were settled from their NMR ( H and C) and
MS data.
Inhibition of NO Production. The inhibitory effect of the
prepared oxazoles 1–5 against NO production was analyzed
by measuring the amount of NO in lipopolysaccharide
Conclusion
(
LPS)-stimulated RAW 264.7 cells, as an indicator of anti-
14b
inflammatory activity, following our previous procedure.
We have accomplished an efficient first synthesis of 2,5-
diaryloxazoles 1–5 using commercially inexpensive precur-
sors and in overall yields of 38–48%. The synthesis
involves α-aminoketones as intermediates and cyclodehy-
dration of α-acylamino ketones (Robinson–Gabriel reac-
tion) as key step. Next, these oxazoles were examined for
Indeed, quantification of NO is difficult intrinsically due to
the existence of other scavenging molecules and transient
nature of NO. Therefore, NO levels have been indirectly
evaluated via analysis of its stable oxidation product, nitrite
−
2
(
NO ) concentration in culture supernatant using the acidic
OMe O
R1
O
R1
O
Br
NH .HCl
b
2
a
88-97%
96%
6
7a: R1=OMe
8a: R =OMe
1
7
b: R =H
1
8b: R =H
1
O
R1
N
R2
2
: R =R =R =OH
R3
1
2
3
OH
c,d
7-61%
O
e
3: R =R =OH, R =H
4: R =R =OH, R =H
1 3 2
1 2 3
4
88-98%
R2
5: R =H, R =R =OH
1
2
3
R3
a: R =R =OMe
1: R =R =R =OMe
1 2 3
9
9
9
10a: R =R =OMe, R =H
1 2 3
2
3
b: R =OMe, R =H
10b: R =R =OMe, R =H
1 3 2
2
3
c: R =H, R =OMe
10c: R =H, R =R =OMe
1 2 3
2
3
Scheme 1. Synthesis of 2,5-diaryloxazoles 1–5. Reagents and conditions: (a) CuBr
2
,
EtOAc/CHCl
3
(1:1), reflux, 8 h.
ꢀ
(
b) (i) Hexamethylenetetramine, ether, rt, overnight; (ii) conc. HCl, EtOH, reflux, 3 h. (c) SOCl
2
, anhydrous DMF, 60 C, 2 h. (d) (i) 8a
ꢀ
ꢀ
(or 8b), pyridine, rt, 1 h; (ii) H SO , Ac O, 90 C, 1 h. (e) BBr (1.0 M in CH Cl ), 0 C to rt, 2–6 h.
2 4 2 3 2 2
Bull. Korean Chem. Soc. 2017, Vol. 38, 1481–1485
© 2017 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de 1484

BULLETIN OF THE
Article Synthesis and Nitric Oxide Production Inhibitory Activity of 2,5-Diaryloxazoles KOREAN CHEMICAL SOCIETY
Table 1. NO production inhibitory effects of 2,5-diphenyloxazoles (1–5).
a,b
a
No production (% inhibition)
Proliferation
Compound
1 μM
10 μM
1 μM
10 μM
IC50 (μM)
Medium
LPS
1
2
3
4
5
15.8 ꢁ 0.3 (74.2)
100.0 ꢁ 6.4 (0.0)
15.8 ꢁ 0.3 (74.2)
100.0 ꢁ 6.4 (0.0)
100.0 ꢁ 4.2
100.0 ꢁ 4.2
127.0 ꢁ 2.4 (−27.0)**
126.8 ꢁ 3.1 (−26.8)**
128.0 ꢁ 2.4 (−28.0)**
123.6 ꢁ 2.9 (−23.6)*
101.7 ꢁ 4.1 (−1.7)
92.5 ꢁ 0.2 (7.5)
121.7 ꢁ 2.6 (−21.7)*
46.1 ꢁ 2.6 (53.9)***
29.3 ꢁ 1.0 (70.7)***
108.7 ꢁ 1.3 (−8.7)
31.7 ꢁ 2.6 (68.3)***
20.7 ꢁ 1.3 (79.3)***
109.4 ꢁ 4.5
104.7 ꢁ 2.0
98.3 ꢁ 3.3
86.5 ꢁ 3.4
89.3 ꢁ 4.1
102.2 ꢁ 2.5
105.2 ꢁ 4.2
98.4 ꢁ 1.8
91.0 ꢁ 5.1
84.0 ꢁ 2.2*
79.4 ꢁ 1.4*
100.7 ꢁ 1.8
>100.0
6.31
2.33
24.83
2.30
L-NMMA
4.51
a
The results are reported as mean value ꢁ SEM for n = 3. Statistical significance is based on the difference when compared with LPS-treated
groups.
b
%
Inhibition is based on LPS as shown in parenthesis.
*
p < 0.05, **p < 0.01, ***p < 0.001.
their inhibitory effect against NO production in LPS-
induced RAW 264.7 cells and were found to display
concentration-dependent inhibition of NO production with-
out cytotoxicity. Of note, compound 3 (70.7%; IC50 = 2.33
μM) was identified as a potent inhibitor, which was in
close comparison with the positive control, L-NMMA
7. D. C. Palmer, Oxazoles: Synthesis, Reactions, and Spectros-
copy, Part A. Chemistry of Heterocyclic Compounds, John
Wiley & Sons, Hoboken, NJ, 2003, p. 60.
8
. (a) V. S. C. Yeh, Tetrahedron 2004, 60, 11995. (b) Z. Jin,
Nat. Prod. Rep. 2013, 30, 869.
9
. A. Ibrar, I. Khan, N. Abbas, U. Farooqa, A. Khana, RSC Adv.
2
016, 6, 93016.
(
5
79.3%; IC50 = 4.51 μM) followed by compounds
1
1
0. J. Senger, J. Melesina, M. Marek, C. Romier, I. Oehme,
O. Witt, W. ippl, M. Jung, J. Med. Chem. 2016, 59, 1545.
1. V. S. C. Yeh, Tetrahedron 2016, 60, 11995.
(68.3%; IC50 = 2.30 μM) and 2 (53.9%; IC50 = 6.31
μM). As a whole, compound 3 may hold great promise for
further development of NO production targeted anti-
inflammatory agent.
12. (a) A. Nishida, M. Fuwa, S. Naruto, Y. Sugano, H. Saito,
M. Nakagawa, Tetrahedron Lett. 2000, 41, 4791.
(
b) T. Chatterjee, J. Y. Cho, E. J. Cho, J. Org. Chem. 2016,
1, 6995.
3. O. Banzragchgarav,
B. Buyankhishig, K. Suganuma, B. Davaapurev, N. Inoue,
J. Batkhuu, K. Sasaki, J. Nat. Prod. 2016, 79, 2933.
4. (a) H. Y. Jang, H. J. Park, K. Damodar, J. -K. Kim, J. -
G. Jun, Bioorg. Med. Chem. Lett. 2016, 26, 5438.
8
Acknowledgments. This research was financially sup-
ported by industrial co-work program with SunChem
1
1
T.
Murata,
G.
Odontuya,
(SunKyung Chemical Co., South Korea).
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Bull. Korean Chem. Soc. 2017, Vol. 38, 1481–1485
© 2017 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.bkcs.wiley-vch.de 1485