Design, synthesis and antimicrobial evaluation of novel benzoxazole derivatives

Article abstract of DOI:10.1016/j.ejmech.2016.10.010

The synthesis of (S)-2-(4-tert-butylphenoxy)-3-(benzoxazol-5-yl) propanoic acid derivatives (2a-k) were described and their in?vitro antibacterial activities were determined against Gram-negative and -positive bacteria. These compounds were found to exert

Full text of DOI:10.1016/j.ejmech.2016.10.010

European Journal of Medicinal Chemistry 126 (2017) 7e14
Contents lists available at ScienceDirect
European Journal of Medicinal Chemistry
journal homepage: http://www.elsevier.com/locate/ejmech
Short communication
Design, synthesis and antimicrobial evaluation of novel benzoxazole
derivatives
,
Wei Zhang a b, Jingbao Liu a, Jocelyn M. Macho b, Xizhen Jiang a, Dongsheng Xie a,
,
, **
Faqin Jiang a, Wenlu Liu a *, Lei Fu a
a School of Pharmacy, Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai 200240, China
b Department of Chemistry, University of Florida, Gainesville, FL, 32611-7200, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of (S)-2-(4-tert-butylphenoxy)-3-(benzoxazol-5-yl) propanoic acid derivatives (2a-k) were
described and their in vitro antibacterial activities were determined against Gram-negative and -positive
bacteria. These compounds were found to exert a broad spectrum of activity against the screened bac-
teria, but poor MIC values were found for Candida albicans fungi. Compound 2b bearing a hydrophobic
aromatic tie was the most active derivative against all bacteria studied with MIC values ranging from
Received 30 July 2016
Received in revised form
3 October 2016
Accepted 5 October 2016
Available online 6 October 2016
0.098 to 0.78
mg/mL. The activity of 2b against B. subtilis was 2-fold higher than Penicillin, and 8- to 510-
fold higher than other control antibiotics.
Keywords:
© 2016 Elsevier Masson SAS. All rights reserved.
Antibacterial activity
Minimum inhibitory concentration
Benzoxazole
Propanoic acid
1. Introduction
revealed that the hydrophobic substitutes, 4-tert-butyl (1a), 4-
phenyl (1b) and 4-benzyloxyl (1c) on the phenoxyl side chain
displayed best activities against all Gram-negative and Gram-
positive bacteria studied with MIC values between 1.56 and
Benzoxazole derivatives were widely reported as a class of
biologically active compounds with anti-bacterial and anti-tumor
activities [1e5]. In the early 1980s, Hisano et al. reported the first
synthetic compounds with benzoxazole skeletons that displayed
potential antimicrobial activities [6]. Over the past twenty years, a
large library of benzoxazole derivatives has been developed, some
of which have exerted good minimum inhibitory concentration
(MIC) values [7e15]. In addition, some benzoxazole-containing
natural products were also proven to have antimicrobial abilities,
for example calcimycin, routeennocin and cezomycin separated
from Streptomyces chartreusis [16e18], and pseudopteroxazole
from Pseudopterogorgia elisabethae [19].
Previously, we reported the synthesis and antibacterial study of
a series of (S)-2-(substituted-hydroxyl)-3-(benzoxazol-5-yl)prop-
anoic acid derivatives (Fig. 1) [20]. This family of compounds was
designed by an integration of four fragments: an aromatic tie, a
linker, a benzoxazole skeleton and a polar acidic head. In our pre-
vious report, we investigated the effect of the chiral side chain on
anti-bacterial activity; our preliminary structure-activity study
6.25 mg/mL.
As we continue efforts in search for more active benzoxazole-
containing compounds while gaining a better understanding of
the mechanism of action, herein, we focused on further structural
modification of the aromatic tie of 1a, and examined their anti-
bacterial activities (Fig. 2). Our previous study suggested that the
hydrophobicity of agents may alter the functional biology activities
significantly [20e23]. Specifically, in this study the modification of
the aromatic tie (R group), with either hydrophobic or hydrophilic
groups, resulted in a new family of (S)-2-(tert-butylphenoxy)-3-
(benzoxazol-5-yl)propanoic acid derivatives.
2. Chemistry
Similar to our previously reported procedures [20], the syn-
thesis of compounds 2a-k was achieved in thirteen steps as shown
in Scheme 1. Intermediate 5 was easily obtained from 3-(4-
hydroxyphenyl) propanoic acid 3 by nitration and esterification.
The hydroxyl-protected 6 was produced by using CH3OCH2Cl. After
hydrolysis of 6, the resulting acid 7 was coupled with (R)-4-
isopropyloxazolidin-2-one to give Evans amide 8. Davis
* Corresponding author.
** Corresponding author.
E-mail addresses: sdwenlu@sjtu.edu.cn (W. Liu), leifu@sjtu.edu.cn (L. Fu).
http://dx.doi.org/10.1016/j.ejmech.2016.10.010
0223-5234/© 2016 Elsevier Masson SAS. All rights reserved.

8
W. Zhang et al. / European Journal of Medicinal Chemistry 126 (2017) 7e14
4-isopropyl-phenyl (2b) and 4-fluorophenyl (2c), the antibacterial
activities were increased by 8e32 fold. 4-Isopropyl phenyl (2b)
especially exerted the best activities against all Gram-positive and
-negative bacteria with MIC value between 0.098 and 0.78 mg/mL.
The activity of 2b against B. subtilis was 2-fold higher than sodium
penicillin, and 8- to 510-fold higher than other antibiotics. Com-
pound 2h, a short linker homologue of 1a, showed 2- to 4-fold
higher activities than 1a on some bacteria (E. coli and MRSA).
From the MIC values of compounds 2a-2d and 2h, we found that
the hydrophobicity of tail fragments impacted the anti-bacterial
activities dramatically. The law of anti-bacterial activity was in
full accordance with the order of calculated logP value (clogP): 2b
(clogP 7.98) > 2c (6.90) > 2h (6.79) > 2a (6.74) > 2d (6.36), and
corresponded with our previous study, namely hydrophobicity-
activity relationship which indicates that hydrophobicity can in-
crease the antibacterial activities [20]. Evidently, the hydrophilic
aromatic tie 3,4,5-trimethoxyphenyl (2d) produced a negative ef-
fect on the inhibition of bacterial proliferation, with MIC values
Fig. 1. (S)-2-(substituted-hydroxyl)-3-(benzoxazol-5-ly)propanoic acid derivatives 1a-
c.
asymmetric oxidation conditions converted 8 to a chiral hydroxyl
compound 9. The chiral accessory (R)-4-isopropyloxazolidin-2-one
was removed by using magnesium methoxide to produce methyl
(R)-a-hydroxyl-propanoic acid 10. (S)-11 was obtained by treating
10 with 4-tert-butylphenol under Mitsunobu conditions [24].
Treatment of 11 with HCl/MeOH yielded 12, quantitatively. The
benzoxazole skeleton 13 was obtained from 12 by sequential
treatment with Pd/C and acetimidate [25].14 was produced from 13
by using P(EtO)3. Compound 15 was obtained from 14 by treating
with respective acetones and aldehydes via Horner-Wadsworth-
Emmons olefination and platinum dioxide hydrogenation condi-
tions successively. The final propanoic acids 2a-k were produced by
treatment of 15a-k with LiOH.
only at 100e200 mg/mL. Furthermore, the polar heterocyclic ties
(e.g. imidazo [1,2-a]pyridine-2-yl (2e) and oxazol-5-yl (2f)) were
also proved to decrease the antibacterial activity dramatically.
Benzofuran-2-yl (2g) (clogP 6.27) displayed a unique broad-
spectrum activity against all screened bacteria without following
the hydrophobicity-activity relationship. We reason that benzo-
furan moiety might possess special characteristics against bacteria
as it was reported in our previous studies [28,29].
When the tail fragment of compound 1a was converted to long-
chain heptyl (2j), similar anti-bacterial activities were retained and
the iso-propyl substituted derivative (2i) exhibited 1-fold
increasing activity. However, when the alkyl tail contained an ox-
ygen atom, tetrahydropyran-4-yl (2k), weak inhibitory effects were
observed. These results were also fully consistent with the contri-
bution of hydrophobicity on the structure-activity relationship
(SAR): 2j (clog P 7.22) > 2i (6.30) > 2k (4.96).
3. Antimicrobial activity evaluations
According to the National Committee for Clinical Laboratory
Standards, we adopted a 96-well microtiter broth dilution method
to determinate MIC values [26,27]. Activities of the newly synthe-
sized compounds were determined against fungus Candida albicans
ATCC 10231, Gram-negative bacteria Escherichia coli ATCC 11303 and
Gram-positive bacteria Staphylococcus aureus ATCC 10832,
Methicillin-resistant Staphylococcus aureus ATCC 700699, Bacillus
subtilis ATCC 33712. The MIC value is defined as the lowest anti-
biotic concentration that resulted in visible growth after incubation
at 37 ꢀC for 24 h. Compound 1a, Ceftazidime, Cefotaxime, Cefradine,
Sodium Penicillin, Miconazole nitrate, and Ketoconazole were used
as reference or control drugs. The antimicrobial activity data of the
5. Conclusions
Based on the structure of the lead compound (1a), a novel series
of (S)-2-(4-tert-butylphenoxy)-3-(benzoxazl-5-yl)propanoic acid
derivatives 2 were designed and synthesized by structural modi-
fication with different lipophilic and hydrophilic fragments, which
were introduced into the tail fragment of the leading. The newly
designed compounds displayed a broad spectrum of in vitro activ-
ities against Gram-positive bacteria such as S. aureus, MRSA,
B. subtilis, and Gram-negative bacteria E. coli as well.
compounds and the control drugs are given as MIC (
in Table 1.
mg/mL) values
4. Results and discussion
Furthermore, we have demonstrated that hydrophobic tails, like
phenyl (2a), 4-isopropyl-phenyl (2b) and 4-fluorophenyl (2c), iso-
propyl (2i), heptyl (2j) showed the best prominent antibacterial
activities in all screened assays, and were even better than the
leading compound (1a) and positive control antibiotics examined
in this study.
Table 1 summarized the results of the minimum inhibitory
concentration (MIC) assays of eleven compounds synthesized.
Compared to the lead compound 1a, general improvement of anti-
bacterial activity was observed, while none for fungus.
When the aromatic tail of 1a was substituted with phenyl (2a),
The introduction of the hydrophobic groups on the tail fragment
Fig. 2. Structural modification of compound 1a.

W. Zhang et al. / European Journal of Medicinal Chemistry 126 (2017) 7e14
9
Scheme 1. Synthesis of compounds 2a.
a
Reagents and conditions: (a) 60% HNO3, HOAc, 15 ꢀC; (b) EtOH, H2SO4, reflux, 83% two steps; (c) CH3OCH2Cl, DIPEA, CH3CN, 0 ꢀC, 99%; (d) NaOH, EtOH, H2O, rt, 91%; (e) 1)
pivaloylchloride, Et3N, THF, ꢁ78 ꢀC, 30 min, 2) lithium (R)-4-isopropyloxazolidin-2-one, THF, ꢁ78 ꢀC, 30 min, 97%; (f) 1) 1.0 M KHMDS/THF, THF, ꢁ78 ꢀC, 2) 3-phenyl-2-(phe-
nylsulfonyl)-1,2-oxaziridine, THF, ꢁ78 ꢀC, 3) AcOH, THF, ꢁ78 ꢀC, 65%; (g) 2.5 M EtMgBr/Ether, MeOH, 85%; (h) 4-t-Bu-Phenol, DIAD, Ph3P, toluene, 0 ꢀC then rt, 69%; (i) trace cont.
HCl, MeOH, 60 ꢀC, 30 min, 99%; (j) 10% Pd/C, H2, MeOH, 1 atm; 2) ClCH2C(¼NH)OCH3$HCl, MeOH, reflux, 51%; (k) P(OEt)3, DMF, 150 ꢀC, 88%; (l) 1) acetone or aldehyde, NaH,
THF, ꢁ10 ꢀC; 2) PtO2$3H2O, MeOH, 3 atm, H2, 38e97%; (m) LiOH$H2O, EtOH, H2O, rt, 70e97%.
is believed to produce a positive effect on the anti-bacterial activity,
and the structure-activity relationship (SAR) presented is man-
ifested in this work.
(20 mL) was added drop-wise into a solution of 3-(4-
hydroxyphenyl)propanoic acid 3 (20.0 g, 0.1 mmol) in acetic acid
(150 mL) at 15 ꢀC over 1 h. The resulting mixture was stirred for
30 min, and then it was poured into 300 mL cool water. The water
layer was extracted with 200 mL CH2Cl2. The organic layer was
washed with brine and dried with anhydrous Na2SO4. The solvent
was removed in vacuo to give a yellow solid 4 (24.6 g). The crude
yellow solid 4 was dissolved in a mixture of EtOH (300 mL)/H2SO4
(30 mL), and the mixture was heated at reflux for 12 h. The reaction
was cooled to room temperature, the solvent was then removed,
and the residue was poured into ice-water and then extracted with
ether (300 mL). The ether layer was washed with brine and dried
over anhydrous Na2SO4. The solvent was removed to give a solid
residue, which was filtered through a short silica column eluting
with petroleum ether/ethyl acetate (3:1) to give compound 5
(22.5 g, 83%) as a bright yellow solid; mp: 41e43 ꢀC; 1H NMR
6. Experimental
6.1. Chemistry
All commercial chemicals and solvents were grade and were
used without further purification unless otherwise specified. All
reactions expect those in aqueous media were carried out with the
use of standard techniques for the exclusion of moisture. Reactions
were monitored by TLC under 254 nm UV. 1H NMR was recorded on
either a Brucker 300 MHz Avance DPX or a Brucker 400 MHz
Avance DPX instrument. Chemical shifts are reported in parts per
million (d) and coupling constants (J) in hertz (Hz). Mass spec-
(300 MHz, CDCl3)
d
10.41 (s, 1H, OH), 7.90 (d, 1H, ArH, J ¼ 2.4 Hz),
troscopy analyses were obtained by using Shimadzu Biotech LCMS-
2010EV mass spectrometer system. High resolution mass spec-
troscopy was conducted using Micromass LCT system.
7.42 (dd, 1H, ArH, J ¼ 8.4 Hz, 1.2 Hz), 7.04 (d, 1H, ArH, J ¼ 8.7 Hz),
4.04 (q, 2H, CH2, J ¼ 7.2 Hz), 2.90 (t, 2H, CH2, J ¼ 7.5 Hz), 2.61 (t, 2H,
CH2, J ¼ 7.7 Hz), 1.21 (t, 3H, CH3, J ¼ 7.2 HZ); MS (ESI m/z): 240
(M þ H)þ.
6.1.1. Ethyl 3-(4-hydroxy-3-nitrophenyl)propanoate (5)
A mixture of 60% HNO3 (9.8 mL, 0.2 mmol) and acetic acid

10
W. Zhang et al. / European Journal of Medicinal Chemistry 126 (2017) 7e14
Table 1
The in vitro antimicrobial activity of compounds 2a-k with the control drugs.
Compound
R
MIC (m
g/mL)a, b
E. coli
S.aureus
MRSA
B.subtilis
C.albicans
2a
2b
3.12
1.56
1.56
0.39
>200
0.78
0.39
0.195
0.098
>200
2c
1.56
200
1.56
100
1.56
100
0.39
100
>200
>200
2d
2e
>200
50
25
50
>200
2f
>200
0.78
1.56
>200
0.78
1.56
200
0.78
0.78
200
0.39
0.78
>200
>200
>200
2g
2h
2i
3.12
1.56
1.56
1.56
>200
2j
1.56
0.78
100
1.56
100
0.78
200
>200
>200
2k
>200
1a
3.12
200
200
25
1.56
0.78
3.12
25
1.56
12.5
3.12
50
3.12
6.25
0.78
50
>200
e
Ceftazidime
Cefotaxime
Cefradine
e
e
Sodium Penicillin
Miconazole nitrate
Ketoconazole
0.78
e
3.12
e
3.12
e
0.195
e
e
1.56
<0.39
e
e
e
e
a
E. coliddEscherichia coli, S. aureusddStaphylococcus aureus, MRSAddmethicillin-resistant Staphylococcus aureus, B. subtilisddBacillus subtilis, C. albicansddCandida
albicans.
b
The sign (e) referred to that compounds not tested.
6.1.2. Ethyl 3-(4-(methoxymethoxy)-3-nitrophenyl)propanoate (6)
To a mixture of compound 5 (2.0 g, 8.3 mmol) and N, N-diiso-
propylethylamine (2.3 mL, 12.5 mmol) in 40 mL acetonitrile, was
added dropwise CH3OCH2Cl (0.75 mL, 10.0 mmol) at 0 ꢀC. The
resulting mixture was stirred for 2 h at 0 ꢀC, and then it was
quenched with 10 mL water. Acetonitrile was removed in vacuo,
and the residue was diluted with 20 mL ethyl acetate, washed with
brine, and dried with anhydrous Na2SO4. The solvent was removed
to give compound 6 (2.3 g, 99%) as a yellow oil; 1H NMR (300 MHz,
resulting mixture was stirred for 2 h at room temperature. EtOH
was evaporated and the water residue was acidified with 1 M HCl to
pH ¼ 4, and then it was extracted with 20 mL ethyl acetate. The
organic phase was washed with brine and dried with anhydrous
Na2SO4. The solvent was concentrated to give compound 7 (0.8 g,
91%) as a white solid; mp: 55e57 ꢀC; 1H NMR (300 MHz, CDCl3)
d
7.67 (d, 1H, ArH, J ¼ 1.8 Hz), 7.34 (dd, 1H, ArH, J ¼ 8.7 Hz, 2.1 Hz),
7.23 (d, 1H, ArH, J ¼ 9.0 Hz), 5.26 (s, 2H, CH2), 3.51 (s, 3H, CH3O),
2.93 (t, 2H, CH2, J ¼ 7.2 Hz), 2.68 (t, 2H, CH2, J ¼ 7.8 Hz); HRMS-ESI-:
C11H11NO6 calcd [M-H]-: 253.0586, found 253.0577.
CDCl3)
d
7.67 (d, 1H, ArH, J ¼ 2.1 Hz), 7.37 (dd, 1H, ArH, J ¼ 8.1 Hz,
2.1 Hz), 7.23 (d, 1H, ArH, J ¼ 8.4 Hz), 5.27 (s, 2H, CH2), 4.13 (q, 2H,
CH2, J ¼ 7.0 Hz), 3.52 (s, 3H, CH3), 2.96 (t, 2H, CH2, J ¼ 7.5 Hz), 2.62 (t,
2H, CH2, J ¼ 7.5 Hz), 1.25 (t, 3H, CH3, J ¼ 7.2 Hz); HRMS-ESIþ:
6.1.4. (R)-4-Isopropyl-3-(3-(4-(methoxymethoxy)-3-nitrophenyl)
propanoyl)oxazolidin-2-one (8)
C
13H18NO6 calcd [MþH]þ: 284.1134, found 284.1130.
Pivaloyl chloride (1.5 mL,12.3 mmol) was added over 30 min to a
mixture of compound 7 (3.0 g, 11.7 mmol) and triethylamine
(2.0 mL, 14.0 mmol) in dry tetrahydrofuran (60 mL) at ꢁ78 ꢀC.
Sequentially, the resulting solution was stirred at 0 ꢀC for 30 min
and re-cooled to ꢁ78 ꢀC. A ꢁ78 ꢀC solution of lithium (R)-4-
6.1.3. 3-(4-(Methoxymethoxy)-3-nitrophenyl)propanoic acid (7)
Aqueous solution of NaOH (7 mL, 10.6 mmol) was added to a
solution of compound 6 (1.0 g, 3.5 mmol) in 15 mL EtOH. The

W. Zhang et al. / European Journal of Medicinal Chemistry 126 (2017) 7e14
11
isopropyl-2-oxooxazolidin-3-ide
(prepared
from
(R)-4-
mixture was stirred for 24 h. Toluene was then removed in vacuo,
and the residue was purified over silica gel column eluting with
petroleum ether/ethyl acetate (10: 1) to give compound 11
isopropyloxazo lidin-2-one (1.7 g, 12.9 mmol) and nBuLi (7.8 mL
1.6 M/hexane, 12.9 mmol) in THF at ꢁ78 ꢀC) was transferred
through a cannula to the mixture above, and the resulting mixture
was warmed to room temperature. After another 30 min stirring,
the reaction was quenched with saturated NH4Cl aqueous solution
(100 mL), and then extracted with ethyl acetate (100 mL). The
organic layer was washed with brine and dried over anhydrous
Na2SO4; the concentrated residue was purified over silica gel col-
umn eluting with petroleum ether/ethyl acetate (3:1) to give
compound 8 (4.2 g, 97%) as a yellow oil; 1H NMR (300 MHz, CDCl3)
(72.0 mg, 69%) as a colorless oil; 1H NMR (300 MHz, CDCl3)
d 7.80 (d,
1H, ArH, J ¼ 2.4 Hz), 7.47 (dd, 1H, ArH, J ¼ 8.4 Hz, 2.1 Hz), 7.30e7.24
(m, 3H, ArH), 6.84 (d, 2H, ArH, J ¼ 8.7 Hz), 5.28 (s, 2H, CH2), 4.80 (dd,
1H, CH, J ¼ 6.9 Hz), 3.77 (s, 3H, CH3), 3.53 (s, 3H, CH3), 3.25 (d, 2H,
CH2, J ¼ 6.6 Hz), 1.31 (s, 9H, 3CH3); HRMS-ESIþ: C22H27NNaO7 calcd
[MþNa]þ: 440.1685, found 440.1680.
6.1.8. (S)-methyl 2-(4-tert-butylphenoxy)-3-(4-hydroxy-3-nitro
phenyl)propanoate (12)
d
7.69 (d, 1H, ArH, J ¼ 2.4 Hz), 7.41 (dd, 1H, ArH, J ¼ 8.4 Hz, 2.0 Hz),
7.23 (d,1H, ArH, J ¼ 8.7 Hz), 5.27 (s, 2H, CH2), 4.44e4.40 (m,1H, CH),
4.31e4.20 (m, 2H, CH2), 3.52 (s, 3H, CH3O), 3.52e3.23 (m, 2H, CH2),
2.99 (t, 2H, CH2, J ¼ 7.0 Hz), 2.35e2.34 (m, 1H, CH(CH3)2), 0.91 (d,
3H, CH3, J ¼ 7.2 Hz), 0.84 (d, 3H, CH3, J ¼ 6.6 Hz); HRMS-ESIþ:
To a solution of compound 11 (0.7 g, 1.7 mmol) in 10 mL
methanol, concentrated HCl (0.3 mL) was added, and the resulting
mixture was heated at 60 ꢀC for 30 min, then concentrated. The
residue was diluted with 10 mL CH2Cl2 and the organic layer was
washed with water, brine and dried over anhydrous Na2SO4. The
solvent was removed to give a compound 12 (0.6 g, 99%) as pale
yellow oil without further purification; 1H NMR (300 MHz, CDCl3)
C
17H23N2O7 calcd [MþH]þ: 367.1505, found 367.1512.
6.1.5. (R)-3-((R)-2-Hydroxy-3-(4-(methoxymethoxy)-3-
nitrophenyl) propanoyl)-4-isopropyloxazolidin-2-one (9)
d
7.79 (d, 1H, ArH, J ¼ 1.8 Hz), 7.48 (dd, 1H, ArH, J ¼ 8.7 Hz, 2.1 Hz),
A solution of KHMDS (5.7 mL 0.5 M/Toluene, 2.8 mmol) was
added dropwise into a solution of compound 8 (0.7 g, 1.8 mmol) in
dry THF (10 mL) at ꢁ78 ꢀC and stirred for 1.0 h. A ꢁ78 ꢀC solution of
3-phenyl-2-(phenylsulfonyl)-1,2-oxaziridine (0.8 g, 2.8 mmol) in
10 mL THF was transferred into the solution above through a can-
nula, and stirred for 1 h. The reaction was quenched with a solution
of acetic acid (1 mL) in 2 mL dry THF at ꢁ78 ꢀC, and then the solvent
was removed in vacuo. The residue was diluted with 50 mL ethyl
acetate, washed with brine, and dried over anhydrous Na2SO4. The
solvent was removed and the crude product was purified over silica
column eluting with petroleum ether/ethyl acetate (2:1) to give
compound 9 (0.5 g, 65%) as a white solid; mp: 120e123 ꢀC; 1H NMR
7.28e7.23 (m, 3H, ArH), 6.77 (d, 2H, ArH, J ¼ 9.0 Hz), 4.76e4.72 (m,
1H, CH), 3.76 (s, 3H, CH3), 3.23 (d, 2H, CH2, J ¼ 7.2 Hz), 1.27 (s, 9H,
3CH3); HRMS-ESIþ: C20H24NO6 calcd [MþH]þ: 374.1604, found
374.1598.
6.1.9. (S)-methyl 2-(4-tert-butylphenoxy)-3-(2-(chloromethyl)
benzo[d]oxazol-5-yl)propanoate (13)
A mixture of compound 12 (1.1 g, 3.0 mmol) and Pd/C (10%,
0.2 g) was stirred in 10 mL MeOH under hydrogen (1 atm) for 1 h.
The solution was filtered to give a white solid and it was mixed with
methyl 2-chloroacetimidate hydrochloride (0.5 g, 3.7 mmol) in
25 mL MeOH. The resulting mixture was heated at reflux for 2 h,
then it was cooled to room temperature and solvent was removed
in vacuo. The residue was diluted with 50 mL ethyl acetate and
washed with 20 mL saturated NaHCO3 aqueous solution, then brine
and dried over anhydrous Na2SO4. The solvent was removed to give
a crude product, which was over silica gel column eluting with
petroleum ether/ethyl acetate (3:1) to give compound 13 (6.4 g,
(300 MHz, CDCl3)
d
7.72 (d, 1H, ArH, J ¼ 1.8 Hz), 7.47 (dd, 1H, ArH,
J ¼ 8.7 Hz, 2.1 Hz), 7.26 (d, 1H, ArH, J ¼ 8.7 Hz), 5.27 (s, 2H, OCH2O),
5.17e5.13 (m, 1H, CONCH), 4.42e4.32 (m, 3H, CH, CH2), 3.68 (d, 1H,
OH, J ¼ 7.5 Hz), 3.52 (s, 3H, CH3O), 3.15 (dd, 1H, CH, J ¼ 14.1 Hz,
4.2 Hz), 2.83 (dd, 1H, CH, J ¼ 14.1 Hz, 8.4 Hz), 2.44e2.43 (m, 1H, CH),
0.93 (d, 3H, CH3, J ¼ 6.9 Hz), 0.91 (d, 3H, CH3, J ¼ 6.6 Hz); HRMS-
ESIþ: C17H23N2O8 calcd [MþH]þ: 383.1454, found 383.1460.
51%) as a colorless oil; 1H NMR (300 MHz, CDCl3)
d 7.68 (s, 1H, ArH),
7.47 (d,1H, ArH, J ¼ 8.4 Hz), 7.33 (dd,1H, ArH, J ¼ 8.1 Hz,1.5 Hz), 7.23
(d, 2H, ArH, J ¼ 9.3 Hz), 6.74 (d, 2H, ArH, J ¼ 8.7 Hz), 4.81e4.77 (m,
1H, CH), 4.73 (s, 2H, CH2), 3.74 (s, 3H, CH3), 3.35 (d, 2H, CH2,
J ¼ 5.4 Hz), 1.25 (s, 9H, 3CH3); HRMS-ESIþ: C22H25ClNO4 calcd
[MþH]þ: 402.1472, found 402.1471.
6.1.6. (R)-methyl 2-hydroxy-3-(4-(methoxymethoxy)-3-
nitrophenyl) propanoate (10)
A 2.5 M EtMgBr/ether solution (5.2 mL, 13.0 mmol) was added
slowly into 40 mL dry MeOH at ꢁ10 ꢀC. After addition of EtMgBr, a
solution of compound 9 (2.5 g, 6.5 mmol) in dry MeOH (20 mL) was
added in one portion. The mixture was stirred for 10 min then
quenched with a saturated NH4Cl aqueous solution (30 mL). MeOH
was removed in vacuo and the residue water layer was extracted
with 50 mL ethyl acetate. The organic layer was dried over anhy-
drous Na2SO4, and the crude product was purified over silica col-
umn eluting with petroleum ether/ethyl acetate (2.5:1) to give
compound 10 (1.8 g, 85%) as a sticky colorless oil; 1H NMR
6.1.10. (S)-methyl 2-(4-tert-butylphenoxy)-3-(2-((diethoxy phos
phoryl)methyl)benzo[d]oxazol-5-yl)propanoate (14)
Compound 13 (0.3 g, 0.9 mmol) and triethyl phosphite (0.3 g,
1.7 mmol) were dissolved in 1 mL DMF and stirred at 150 ꢀC for 4 h.
The solvent was removed in vacuo, and the residue was purified
over silica gel column eluting with petroleum ether/ethyl acetate
(1:2) to give compound 14 (0.4 g, 88%) as a yellow oil; 1H NMR
(300 MHz, CDCl3)
d
7.68 (s, 1H, ArH), 7.39 (dd, 1H, ArH, J ¼ 8.1 Hz,
(300 MHz, CDCl3)
d
7.62 (s, 1H, ArH), 7.42 (d, 1H, ArH, J ¼ 8.4 Hz),
1.7 Hz), 7.24 (d, 1H, ArH, J ¼ 8.5 Hz), 5.27 (s, 2H, CH2), 4.45e4.42 (m,
1H, CH), 3.81 (s, 3H, CH3), 3.52 (s, 3H, CH3), 3.03 (dd, dd, 2H, CH2,
J ¼ 14.2 Hz, 7.2 Hz, 4.0 Hz), 2.85 (d, 1H, OH, J ¼ 5.4 Hz); HRMS-ESIþ:
7.26 (d, 1H, ArH, J ¼ 7.8 Hz), 7.23 (d, 2H, ArH, J ¼ 9.3 Hz), 6.74 (d, 2H,
ArH, J ¼ 8.7 Hz), 4.80e4.76 (m, 2H, CH2), 4.22e4.08 (m, 4H, CH2,
CH2), 3.73 (s, 3H, CH3), 3.57 (s, 1H, CH), 3.50 (s, 1H, CH), 3.32 (d, 1H,
CH, J ¼ 5.4 Hz), 1.32 (t, 6H, 2CH3, J ¼ 6.9 Hz), 1.24 (s, 9H, 3CH3);
HRMS-ESIþ: C26H35NO7P calcd [MþH]þ: 504.2151, found 504.2148.
C
12H15NNaO7 calcd [MþNa]þ: 308.0746, found 308.0743.
6.1.7. (S)-methyl 2-(4-tert-butylphenoxy)-3-(4-(methoxymethoxy)-
3-nitrophenyl)propanoate (11)
6.1.11. General synthesis of compounds 15a-k
To a solution of compound 10 (84.0 mg, 0.3 mmol) in 2 mL
toluene, Ph3P (85.0 mg, 0.3 mmol) and 4-tert-butylphenol (48.0 mg,
0.3 mmol) were added. The resulting mixture was cooled to 0 ꢀC
and a solution of diisopropyl azodicarboxylate (DIAD) (64.0 mg,
0.3 mmol) in 1 mL toluene was added dropwise. Sequentially, the
A solution of compound 14 (80.0 mg, 0.2 mmol) and acetone or
aldehyde (0.2 mmol) in 2 mL dry THF was added NaH (60%, 7.0 mg,
0.2 mmol) in one portion at ꢁ10 ꢀC, and was stirred for 5 min at this
temperature. The mixture was quenched with 10 mL saturated
NH4Cl aqueous solution. The water layer was sequentially extracted

12
W. Zhang et al. / European Journal of Medicinal Chemistry 126 (2017) 7e14
with ethyl acetate, washed with brine and dried with anhydrous
Na2SO4. An orange residue was afforded, and it was stirred with
PtO2$3H2O (5.5 mg, 0.0079 mmol) in 5 mL methanol for 12 h under
hydrogen at 1.0 atm. Sequentially, the mixture was filtered and
purified over silica gel column eluting with petroleum ether/ethyl
acetate (10:1) to give compound 15a-k.
(52.0 mg, 66%) as a colorless oil; 1H NMR (300 MHz, CDCl3)
d 7.64 (s,
1H, ArH), 7.43e7.37 (m, 2H, ArH), 7.34e7.26 (m, 2H, ArH), 7.19e7.09
(m, 4H, ArH), 6.70 (d, 2H, ArH, J ¼ 9.0 Hz), 6.45 (s, 1H, ArH),
4.76e4.72 (m, 1H, CH), 3.68 (s, 3H, CH3), 3.42e3.36 (m, 4H, 2CH2),
3.29 (d, 2H, CH2, J ¼ 6.3 Hz), 1.19 (s, 9H, 3CH3); HRMS-ESIþ:
C
31H32NO5 calcd [MþH]þ: 498.2280, found 498.2283.
6.1.11.1. (S)-Methyl 2-(4-tert-butylphenoxy)-3-(2-phenethylbenzo[d]
6.1.11.8. (S)-Methyl 2-(4-tert-butylphenoxy)-3-(2-(2-(5-methyl-2-
phenyloxazol-4-yl)ethyl)benzo[d]oxazol-5-yl) propanoate (15h).
Compound 15h (32.0 mg, 38%) as a yellow oil; 1H NMR (300 MHz,
oxazol-5-yl)propanoate (15a). Compound 15a (51.0 mg, 75%) as a
yellow oil; 1H NMR (300 MHz, CDCl3)
d 7.61 (s, 1H, ArH), 7.40 (d, 1H,
ArH, J ¼ 8.7 Hz), 7.31 (d, 1H, ArH, J ¼ 8.4 Hz), 7.27e7.22 (m, 7H, ArH),
6.77 (d, 2H, ArH, J ¼ 9.0 Hz), 4.81e4.77 (m,1H, CH), 3.73 (s, 3H, CH3),
3.33 (d, 2H, CH2, J ¼ 5.1 Hz), 3.21 (s, 4H, 2CH2), 1.25 (s, 9H, 3CH3);
HRMS-ESIþ: C29H32NO4 calcd [MþH]þ: 458.2331, found 458.2335.
CDCl3) d 8.08e8.05 (m, 2H, ArH), 7.71 (s, 1H, ArH), 7.54e7.49 (m, 4H,
ArH), 7.37e7.33 (m, 3H, ArH), 6.88 (d, 2H, ArH, J ¼ 8.7 Hz),
4.92e4.88 (m, 1H, CH), 3.84 (s, 3H, CH3O), 3.44e3.39 (m, 4H, CH2,
CH2), 3.22e3.17 (m, 2H, CH2), 2.39 (s, 3H, CH3), 1.36 (s, 9H, 3CH3);
HRMS-ESIþ: C33H35N2O5 calcd [MþH]þ: 539.2546, found 539.2540.
6.1.11.2. (S)-Methyl 2-(4-tert-butylphenoxy)-3-(2-(4-isopropylphen
ethyl)benzo[d]oxazol-5-yl)propanoate
(15b). Compound
15b
7.61 (s,
6.1.11.9. (S)-Methyl 2-(4-tert-butylphenoxy)-3-(2-isobutylbenzo[d]
(59.0 mg, 75%) as a yellow oil; 1H NMR (300 MHz, CDCl3)
d
oxazol-5-yl)propanoate (15i). Compound 15i (49.0 mg, 76%) as a
1H, ArH), 7.41 (d, 1H, ArH, J ¼ 8.7 Hz), 7.26e7.22 (m, 3H, ArH), 7.17
(s, 4H, ArH), 6.77 (d, 2H, ArH, J ¼ 9.0 Hz), 4.81e4.79 (m, 1H, CH),
3.73 (s, 3H, CH3), 3.34e3.32 (m, 2H, CH2), 3.25e3.19 (m, 4H, 2CH2),
2.92 (qui, 1H, CH, J ¼ 6.8 Hz), 1.25 (s, 9H, 3CH3), 1.23 (d, 6H, 2CH3,
J ¼ 6.9 Hz); HRMS-ESIþ: C32H38NO4 calcd [MþH]þ: 500.2801, found
500.2800.
colorless oil; 1H NMR (300 MHz, CDCl3)
d 7.62 (s, 1H, ArH), 7.40 (d,
1H, ArH, J ¼ 8.7 Hz), 7.25 (d, 1H, ArH, J ¼ 8.4 Hz), 7.23 (d, 2H, ArH,
J ¼ 8.7 Hz), 6.74 (d, 2H, ArH, J ¼ 8.7 Hz), 4.81e4.77 (m, 1H, CH), 3.73
(s, 3H, CH3), 3.33 (d, 2H, CH2, J ¼ 5.4 Hz), 2.95 (t, 2H, CH2, J ¼ 7.8 Hz),
1.82e1.74 (m, 2H, CH2), 1.71e1.62 (m, 1H, CH), 1.25 (s, 9H, 3CH3),
0.96 (d, 6H, 3CH3, J ¼ 6.3 Hz); HRMS-ESIþ: C25H32NO4 calcd
[MþH]þ: 410.2331, found 410.2327.
6.1.11.3. (S)-Methyl 2-(4-tert-butylphenoxy)-3-(2-(4-
fluorophenethyl)
Compound 15c (54.0 mg, 74%) as a colorless oil; 1H NMR (300 MHz,
CDCl3)
5H, ArH), 7.11 (t, 2H, ArH, J ¼ 8.4 Hz), 6.88 (d, 2H, ArH, J ¼ 8.4 Hz),
4.93e4.88 (m, 1H, CH), 3.85 (s, 3H, CH3O), 3.46e3.43 (m, 2H, CH2),
3.30 (s, 4H, CH2, CH2), 1.37 (s, 9H, 3CH3); HRMS-ESIþ: C29H31FNO4
calcd [MþH]þ: 476.2237, found 476.2242.
benzo[d]oxazol-5-yl)propanoate
(15c).
6.1.11.10. (S)-Methyl
oxazol-5-yl)propanoate (15j). Compound 15j (60.0 mg, 84%) as a
brown oil; 1H NMR (300 MHz, CDCl3)
7.60 (s, 1H, ArH), 7.40 (d, 1H,
ArH, J ¼ 8.1 Hz), 7.26e7.21 (m, 3H, ArH), 6.74 (d, 2H, ArH, J ¼ 8.7 Hz),
4.81e4.76 (m,1H, CH), 3.73 (s, 3H, CH3), 3.32 (d, 2H, CH2, J ¼ 5.4 Hz),
2.93 (t, 2H, CH2, J ¼ 6.2 Hz), 1.45e1.25 (m, 13H, 5CH2, CH3), 1.25 (s,
9H, 3CH3); HRMS-ESIþ: C28H38NO4 calcd [MþH]þ: 452.2801, found
452.2807.
2-(4-tert-butylphenoxy)-3-(2-heptylbenzo[d]
d
7.72 (s, 1H, ArH), 7.52 (d, 1H, ArH, J ¼ 8.1 Hz), 7.38e7.28 (m,
d
6.1.11.4. (S)-Methyl 2-(4-tert-butylphenoxy)-3-(2-(3,4,5-trimethoxy
phenethyl)benzo[d]oxazol-5-yl)propanoate (15d). Compound 15d
6.1.11.11. (S)-Methyl 2-(4-tert-butylphenoxy)-3-(2-((tetrahydro-2H-
(42.0 mg, 47%) as a colorless oil; 1H NMR (300 MHz, CDCl3)
d
7.72 (s,
pyran-4-yl)methyl)benzo[d]oxazol-5-yl)propanoate
Compound 15k (83.0 mg, 97%) as a colorless oil; 1H NMR (300 MHz,
CDCl3)
(15k).
1H, ArH), 7.51 (d, 1H, ArH), 7.37e7.33 (m, 4H, ArH), 6.86 (d, 2H, ArH,
J ¼ 8.7 Hz), 6.54 (s, 2H, ArH), 4.92e4.88 (m, 1H, CH2), 3.92 (s, 3H,
CH3O), 3.88 (s, 6H, CH3O), 3.84 (s, 3H, CH3O), 3.45 (d, 2H, CH2,
J ¼ 5.4 Hz), 3.35e3.30 (m, 2H, CH2), 3.27e3.22 (m, 2H, CH2), 1.36 (s,
9H, 3CH3); HRMS-ESIþ: C32H38NO7 calcd [MþH]þ: 548.2648, found
548.2641.
d
7.59 (s, 1H, ArH), 7.40 (d, 1H, ArH, J ¼ 8.1 Hz), 7.24e7.21 (m,
3H, ArH), 6.75 (d, 2H, ArH, J ¼ 9.0 Hz), 4.80e4.76 (m, 1H, CH),
3.98e3.93 (m, 2H, CH2), 3.74 (s, 3H, CH3), 3.44e3.36 (m, 2H, CH2),
3.33 (d, 2H, CH2, J ¼ 5.7 Hz), 2.86 (d, 2H, CH2, J ¼ 7.2 Hz), 1.70e1.66
(m, 2H, CH2),1.52e1.40 (m, 3H, CH3),1.25 (s, 9H, 3CH3); HRMS-ESIþ:
C
27H34NO5 calcd [MþH]þ: 452.2437, found 452.2432.
6.1.11.5. (S)-Methyl
2-(4-tert-butylphenoxy)-3-(2-(2-(6-(tri-
fluoromethyl)imidazo [1,2-a]pyridin-2-yl)ethyl)benzo[d] oxazol-5-yl)
6.1.12. General synthesis of compounds 2a-k
propanoate (15e). Compound 15e (77.0 mg, 77%) as a brown oil; 1H
Compounds 15 (0.05 mmol) were dissolved in 1 mL ethanol, and
then aqueous solution LiOH$H2O (0.3 mmol, 1 mL) was added. The
mixture was stirred for 2 h at room temperature. The ethanol was
removed in vacuo and the water residue was acidified with 1 M HCl
to pH ¼ 4, and extracted with ethyl acetate (5 mL). The organic layer
was dried over anhydrous Na2SO4, the solvent was concentrated,
and the crude product was purified over a silica gel column eluting
with CH2Cl2/MeOH (10: 1) to give final compounds 2a-k.
NMR (300 MHz, CDCl3)
d 8.51 (s, 1H, ArH), 7.78 (d, 1H, ArH,
J ¼ 9.0 Hz), 7.71 (s, 1H, ArH), 7.51 (d, 1H, ArH, J ¼ 8.1 Hz), 7.41e7.33
(m, 5H, ArH), 6.87 (d, 2H, ArH, J ¼ 9.0 Hz), 4.92e4.87 (m, 1H, CH),
3.84 (s, 3H, CH3O), 3.53 (s, 4H, CH2, CH2), 3.44e3.42 (m, 2H, CH2),
1.36 (s, 9H, 3CH3); HRMS-ESIþ: C31H31F3N3O4 calcd [MþH]þ:
566.2267, found 566.2271.
6.1.11.6. (S)-Methyl
ethyl)benzo[d]oxazol-5-yl)propanoate
(46.0 mg, 65%) as a colorless oil; 1H NMR (300 MHz, CDCl3)
2H, ArH, J ¼ 7.8 Hz), 7.40 (d, 1H, ArH, J ¼ 8.4 Hz), 7.26 (s, 1H, ArH),
7.25 (d, 2H, ArH, J ¼ 8.7 Hz), 7.03 (s, 1H, ArH), 6.76 (d, 2H, ArH,
J ¼ 8.7 Hz), 4.81e4.77 (m, 1H, CH), 3.73 (s, 3H, CH3), 3.43e3.37 (m,
4H, 2CH2), 3.33 (d, 2H, CH2, J ¼ 5.7 Hz), 1.25 (s, 9H, 3CH3); HRMS-
ESIþ: C26H29N2O5 calcd [MþH]þ: 449.2076, found 449.2080.
2-(4-tert-butylphenoxy)-3-(2-(2-(oxazol-2-yl)
(15f). Compound
15f
6.1.12.1. (S)-2-(4-tert-butylphenoxy)-3-(2-phenethylbenzo[d]oxazol-
d
7.61 (d,
5-yl)propanoic acid (2a). Compound 2a (17.0 mg, 76%) as a white
solid; mp: 80e82 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d 7.59 (d, 1H,
ArH, J ¼ 1.2 Hz), 7.54 (d, 1H, ArH, J ¼ 8.0 Hz), 7.28 (dd, 1H, ArH,
J ¼ 8.4 Hz, 1.6 Hz), 7.24e7.23 (m, 4H, ArH), 7.21 (d, 2H, ArH,
J ¼ 8.4 Hz), 7.20e7.15 (m, 1H, ArH), 6.74 (d, 2H, ArH, J ¼ 8.8 Hz),
4.90e4.87 (m, 1H, CH), 3.28 (dd, 1H, CH, J ¼ 14.0 Hz, 4.4 Hz),
3.25e3.19 (m, 3H, CH, CH2), 3.10 (t, 2H, CH2, J ¼ 7.2 Hz), 1.19 (s, 9H,
3CH3); HRMS-ESI-: C28H28NO4 calcd [M-H]-: 442.2018, found
442.2010.
6.1.11.7. (S)-Methyl 3-(2-(2-(benzofuran-2-yl)ethyl)benzo[d]oxazol-
5-yl)-2-(4-tert-butylphenoxy)propanoate (15g). Compound 15g

W. Zhang et al. / European Journal of Medicinal Chemistry 126 (2017) 7e14
13
6.1.12.2. (S)-2-(4-tert-butylphenoxy)-3-(2-(4-isopropylphenethyl)
NMR (400 MHz, DMSO-d6)
d
7.85e7.82 (m, 2H, ArH), 7.59 (s, 1H,
benzo[d]oxazol-5-yl)propanoic acid (2b). Compound 2b (20.0 mg,
ArH), 7.55 (d, 1H, ArH), 7.47e7.43 (m, 3H, ArH), 7.29 (d, 1H, ArH,
J ¼ 8.8 Hz), 7.21 (d, 2H, ArH, J ¼ 9.2 Hz), 6.72 (d, 2H, ArH, J ¼ 8.4 Hz),
4.89e4.86 (m, 1H, CH), 3.30e3.19 (m, 4H, 2CH2), 2.98 (t, 2H, CH2,
J ¼ 7.2 Hz), 2.26 (s, 3H, CH3), 1.18 (s, 9H, 3CH3); HRMS-ESI-:
82%) as a white solid; mp: 65e68 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d
7.59 (s, 1H, ArH), 7.53 (d, 1H, ArH, J ¼ 8.0 Hz), 7.28 (d, 1H, ArH,
J ¼ 8.0 Hz), 7.21 (d, 2H, ArH, J ¼ 8.8 Hz), 7.15 (d, 2H, ArH, J ¼ 8.0 Hz),
7.10 (d, 2H, ArH, J ¼ 8.0 Hz), 6.73 (d, 2H, ArH, J ¼ 8.8 Hz), 4.90e4.87
(m, 1H, CH), 3.30e3.21 (m, 2H, CH2), 3.17 (t, 2H, CH2, J ¼ 6.8 Hz),
3.05 (t, 2H, CH2, J ¼ 7.2 Hz), 2.82e2.75 (m, 1H, CH), 1.19 (s, 9H,
3CH3), 1.13 (d, 6H, 2CH3, J ¼ 6.8 Hz); HRMS-ESI-: C31H34NO4 calcd
[M-H]-: 484.2488, found 484.2491.
C
32H31N2O4 calcd [M-H]-: 523.2233, found 523.2226.
6.1.12.9. (S)-2-(4-tert-butylphenoxy)-3-(2-isobutylbenzo[d]oxazol-5-
yl)propanoic acid (2i). Compound 2i (18.0 mg, 90%) as a white solid;
mp: 51e52 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d 7.58 (s, 1H, ArH),
7.54 (d, 1H, ArH, J ¼ 8.8 Hz), 7.27 (d, 1H, ArH, J ¼ 8.4 Hz), 7.21 (d, 2H,
ArH, J ¼ 8.8 Hz), 6.73 (d, 2H, ArH, J ¼ 9.2 Hz), 4.98e4.87 (m, 1H, CH),
3.36e3.18 (m, 2H, CH2), 2.88 (d, 2H, CH2, J ¼ 7.4 Hz), 1.61e1.55 (m,
1H, CH),1.19 (s, 9H, 3CH3), 0.88 (d, 6H, 2CH3, J ¼ 6.0 Hz); HRMS-ESI-:
6.1.12.3. (S)-2-(4-tert-butylphenoxy)-3-(2-(4-fluorophenethyl) benzo
[d]oxazol-5-yl)propanoic acid (2c). Compound 2c (19.0 mg, 85%) as
a white solid; mp: 72e74 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d 7.59
(s, 1H, ArH), 7.54 (d, 1H, ArH, J ¼ 8.8 Hz), 7.29e7.26 (m, 4H, ArH),
7.21 (d, 1H, ArH, J ¼ 8.8 Hz), 7.06 (t, 2H, ArH, J ¼ 9.0 Hz), 6.72 (d, 2H,
ArH, J ¼ 8.4 Hz), 4.90e4.87 (m,1H, CH), 3.30e3.23 (m, 2H, CH2), 3.19
(t, 2H, CH2, J ¼ 7.8 Hz), 3.09 (t, 2H, CH2, J ¼ 7.4 Hz),1.19 (s, 9H, 3CH3);
HRMS-ESI-: C28H27FNO4 calcd [M-H]-: 460.1924, found 460.1918.
C
24H28NO4 calcd [M-H]-: 394.2018, found 394.2020.
6.1.12.10. (S)-2-(4-tert-butylphenoxy)-3-(2-heptylbenzo[d]oxazol-5-
yl)propanoic acid (2j). Compound 2j (19.0 mg, 90%) as a white solid;
mp: 63e66 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d 7.59 (d, 1H, ArH,
J ¼ 1.6 Hz), 7.53 (d, 1H, ArH, J ¼ 8.4 Hz), 7.27 (dd, 1H, ArH, J ¼ 8.0 Hz,
1.2 Hz), 7.21 (d, 2H, ArH, J ¼ 8.8 Hz), 6.72 (d, 2H, ArH, J ¼ 9.2 Hz),
4.88 (dd, 1H, CH, J ¼ 8.0 Hz, 4.4 Hz), 3.26e3.18 (m, 2H, CH2), 2.87 (t,
2H, CH2, J ¼ 7.2 Hz), 1.74 (qui, 2H, CH2, J ¼ 7.2 Hz), 1.34e1.21 (m, 8H,
4CH2), 1.19 (s, 9H, 3CH3), 0.82 (t, 3H, CH3, J ¼ 6.8 Hz); HRMS-ESI-:
6 .1.12 . 4 . ( S ) - 2 - ( 4 - t e r t - b u t y l p h e n o x y ) - 3 - ( 2 - ( 3 , 4 , 5 -
trimethoxyphenethyl) benzo[d]oxazol-5-yl)propanoic acid (2d).
Compound 2d (23.0 mg, 87%) as a white solid; mp: 85e87 ꢀC; 1H
NMR (400 MHz, DMSO-d6)
d 7.61e7.60 (m, 1H, ArH), 7.77e7.53 (m,
1H, ArH), 7.29e7.27 (m, 1H, ArH), 7.29e7.27 (m, 1H, ArH), 7.23e7.19
(m, 1H, ArH), 6.74e6.70 (m, 2H, ArH), 6.53e6.51 (m, 2H, ArH),
5.05e5.02 (m, 1H, CH), 3.64e3.63 (m, 6H, 2CH3), 3.58e3.56 (m, 3H,
CH3), 3.34e3.19 (m, 4H, 2CH2), 3.03e3.01 (m, 2H, CH2), 1.18 (s, 9H,
3CH3); HRMS-ESI-: C31H34NO7 calcd [M-H]-: 532.2335, found
532.2344.
C
27H34NO4 calcd [M-H]-: 436.2488, found 436.2480.
6.1.12.11. (S)-2-(4-tert-butylphenoxy)-3-(2-((tetrahydro-2H-pyran-
4-yl)methyl)benzo[d]oxazol-5-yl)propanoic acid (2k).
Compound 2k (15.0 mg, 70%) as a white solid; mp: 102e103 ꢀC; 1H
NMR (400 MHz, DMSO-d6) 7.61 (s, 1H, ArH), 7.54 (d, 1H, ArH,
d
J ¼ 8.4 Hz), 7.28 (d, 1H, ArH, J ¼ 8.4 Hz), 7.20 (d, 2H, ArH, J ¼ 8.8 Hz),
6.73 (d, 2H, ArH, J ¼ 8.8 Hz), 4.90e4.87 (m, 1H, CH), 3.80e3.78 (m,
2H, CH2), 3.28e3.19 (m, 4H, 2CH2), 2.83e2.81 (m, 2H, CH2),
2.07e2.06 (m, 1H, CH), 1.59e1.56 (m, 2H, 2CH), 1.33e1.27 (m, 2H,
CH2), 1.19 (s, 9H, 3CH3); HRMS-ESI-: C26H30NO5 calcd [M-H]-:
436.2124, found 436.2119.
6.1.12.5. (S)-2-(4-tert-butylphenoxy)-3-(2-(2-(6-(trifluoromethyl)
imidazo [1,2-a]pyridin-2-yl)ethyl)benzo[d]oxazol-5-yl)propanoic acid
(2e). Compound 2e (24.0 mg, 88%) as a white solid; mp: 90e92 ꢀC;
1H NMR (400 MHz, DMSO-d6)
d 9.12 (s, 1H, ArH), 7.86 (s, 1H, ArH),
7.63 (d, 1H, ArH, J ¼ 9.2 Hz), 7.58 (s, 1H, ArH), 7.58 (t, 1H, ArH,
J ¼ 8.4 Hz), 7.38 (dd, 1H, ArH, J ¼ 9.2 Hz, 2.0 Hz), 7.28 (dd, 1H, ArH,
J ¼ 8.4 Hz, 1.6 Hz), 7.21 (d, 2H, ArH, J ¼ 8.4 Hz), 6.71 (d, 2H, ArH,
J ¼ 8.8 Hz), 4.90e4.86 (m, 1H, CH), 3.39 (t, 2H, CH2, J ¼ 7.5 Hz), 3.30
(t, 2H, CH2, J ¼ 7.5 Hz), 3.28e3.21 (m, 2H, CH2), 1.18 (s, 9H, 3CH3);
HRMS-ESI-: C30H27F3N3O4 calcd [M-H]-: 550.1954, found 550.1946.
6.2. In vitro antimicrobial activity assays
All the compounds and control drugs were dissolved in DMSO
(4 mg/mL), and then diluted with Mueller-Hinton broth bacteria
(105 CFU/mL) or Sabouraud dextrose broth fungus (104 CFU/mL) to
6.1.12.6. (S)-2-(4-tert-butylphenoxy)-3-(2-(2-(oxazol-2-yl)ethyl)
benzo [d]oxazol-5-yl)propanoic acid (2f). Compound 2f (18.0 mg,
85%) as a white sold; mp: 89e91 ꢀC; 1H NMR (400 MHz, DMSO-d6)
the sample concentration (4 mg/mL). Using the multipipettor, 10
of a 4 mg/mL compound solution were dispensed into the wells in
column 2. 100 L of the broth bacteria or fungus were dispensed
into column 2 through column 12 and another 100 L dispensed
bacteria into column 2. Using the multipipettor set at 100 L, the
compound solution was mixed in the wells in column 2 by sucking
up and down 6e8 times. 100 L were withdrawn from column 2
and added to column 3; this makes column 3 a twofold dilution of
column 2. Mix up and down 6e8 times. 100 L were transferred to
column 4. The procedure was repeated down to column 11 only and
100 L were discarded from column 11. The final concentrations
from column 2 to 11 were 200, 100, 50, 25, 12.5, 6.25, 3.12, 1.56, 0.78
and 0.39 g/mL 100 L of sterile medium were pipetted into column
1, and the column 12 as control groups. Compounds with antibac-
terial concentration less than 0.39 g/mL were diluted again to
0.195, 0.098 and 0.049 g/mL as above. After incubation for 24 h at
mL
m
d
7.96 (s, 1H, ArH), 7.58 (s, 1H, ArH), 7.54 (d, 1H, ArH, J ¼ 8.0 Hz), 7.28
m
(dd, 1H, ArH, J ¼ 8.4 Hz, 1.6 Hz), 7.20 (d, 2H, ArH, J ¼ 8.8 Hz), 7.05 (s,
1H, ArH), 6.72 (d, 2H, ArH, J ¼ 8.8 Hz), 4.87e4.84 (m, 1H, CH),
3.41e3.39 (m, 2H, CH2), 3.37e3.30 (m, 2H, CH2), 3.30e3.22 (m, 2H,
CH2), 1.18 (s, 9H, 3CH3); HRMS-ESI-: C25H25N2O5 calcd [M-H]-:
433.1763, found 433.1770.
m
m
m
6.1.12.7. (S)-3-(2-(2-(benzofuran-2-yl)ethyl)benzo[d]oxazol-5-yl)-2-
(4-tert-butylphenoxy)propanoic acid (2g). Compound 2g (23.0 mg,
97%) as a white solid; mp: 78e80 ꢀC; 1H NMR (400 MHz, DMSO-d6)
m
m
m
d
7.60 (s, 1H, ArH), 7.55 (d, 1H, ArH, J ¼ 8.4 Hz), 7.50 (d, 1H, ArH,
J ¼ 7.6 Hz), 7.45 (d, 1H, ArH, J ¼ 8.0 Hz), 7.29 (dd, 1H, ArH, J ¼ 8.4 Hz,
1.6 Hz), 7.23e7.20 (m, 3H, ArH), 7.18e7.15 (m, 1H, ArH), 6.72 (d, 2H,
ArH, J ¼ 8.8 Hz), 6.63 (s, 1H, ArH), 4.89e4.86 (m, 1H, CH), 3.39e3.28
(m, 4H, CH, CH2), 3.26e3.18 (m, 2H, CH2), 1.19 (s, 9H, 3CH3); HRMS-
ESI-: C30H28NO5 calcd [M-H]-: 482.1967, found 482.1971.
m
m
37 ꢀC, we streaked the bacterial culture on plates to check their
clarity. MIC values were taken as the lowest concentration of drug
that made the well clean. In order to ensure that the solvent DMSO
had no effect on bacterial growth, a control test was also performed
containing inoculated broth supplemented with only DMSO at the
same dilutions used in our experiments and found inactive in
culture medium.
6.1.12.8. (S)-2-(4-tert-butylphenoxy)-3-(2-(2-(5-methyl-2-phenyl
oxazol-4-yl)ethyl)benzo[d]oxazol-5-yl)propanoic
acid
(2h).
Compound 2h (22.0 mg, 86%) as a white solid; mp: 72e73 ꢀC; 1H

14
W. Zhang et al. / European Journal of Medicinal Chemistry 126 (2017) 7e14
€
[12] S. Alper-Hayta, M. Arisoy, O. Temiz-Arpaci, I. Yildiz, E. Aki, S. Ozkan, F. Kaynak,
Eur. J. Med. Chem. 43 (2008) 2568e2578.
Conflict of interest
[13] O. Temiz-Arpaci, I. Yildiz, S. Ozkan, F. Kaynak, E. Aki-Sener, I. Yalcin, Eur. J.
Med. Chem. 43 (2008) 1423e1431.
The authors confirm that this article content has no conflict of
[14] O.T. Arpaci, E.A. Sener, I. Yalcin, N. Altanlar, Il Farm. 57 (2002) 771e775.
[15] B. Tekiner-Gulbas, O. Temiz-Arpaci, I. Yildiz, N. Altanlar, Eur. J. Med. Chem. 42
(2007) 1293e1299.
[16] M. Prudhomme, J. Guyot, G. Jeminet, J. Antibiot. 39 (1986) 934e937.
[17] R.K. Boeckman, A.B. Charette, T. Asberom, B.H. Johnston, J. Am. Chem. Soc. 113
(1991) 5337e5353.
[18] G.D. Smith, W.L. Duax, J. Am. Chem. Soc. 98 (1976) 1578e1580.
[19] A.D. Rodriguez, C. Ramirez, I.I. Rodriguez, E. Gonzalez, Org. Lett. 1 (1999)
527e530.
[20] W. Zhang, W. Liu, X. Jiang, F. Jiang, H. Zhuang, L. Fu, Eur. J. Med. Chem. 46
(2011) 3639e3650.
interest.
Acknowledgements
We thank the National Comprehensive Technology Platforms for
Innovative Drug R&D, 2009ZX09301-007 for the financial support.
We thank Professor Fengsheng Zhao (Shanghai Jiao Tong Univer-
sity) for the biological support of this research.
[21] J. Liu, F. Jiang, Y. Jin, Y. Zhang, J. Liu, W. Liu, L. Fu, Eur. J. Med. Chem. 57 (2012)
10e20.
[22] J. Liu, X. Deng, Y. Jin, B. Xu, W. Liu, F. Jiang, L. Fu, Pharmazie 70 (2015)
446e451.
[23] X. Deng, J. Liu, Y. Jin, Y. Zhang, W. He, J. Bao, W. Liu, F. Jiang, L. Fu, Pharmazie
70 (2015) 777e783.
[24] W. Zhang, C. Chen, R. Cao, L. Maurmann, P. Li, Chem. Bio. Chem. 16 (2015)
156e166.
[25] W. Zhang, W. Liu, X. Jiang, F. Jiang, L. Fu, Syn. Commun. 42 (2012) 2772e2779.
[26] National Committee for Clinical Laboratory Standard, Methods for Dilution
Antimicrobial, Tests for Bacteria that Grow Aerobically, Approved standard,
M7eA6, fifth ed., National Committee for Clinical Laboratory Standards,
Wayne, PA, 2003.
References
[1] B. Gong, F. Hong, C. Kohm, L. Bonham, P. Klein, Bioorg. Med. Chem. Lett. 14
(2004) 1455e1459.
[2] O. Temiz-Arpaci, B. Tekiner-Gulbas, I. Yildiz, E. Aki-Sener, I. Yalcin, Bioorg.
Med. Chem. 13 (2005) 6354e6359.
[3] B.B. Wang, N. Maghami, V.L. Goodlin, P.J. Smith, Bioorg. Med. Chem. Lett. 14
(2004) 3221e3226.
[4] S.M. Rida, F.A. Ashour, S.A.M. El-Hawash, M.M. El-Semary, M.H. Badr,
M.A. Shalaby, Eur. J. Med. Chem. 40 (2005) 949e959.
[5] J. Jiang, X. Tang, W. Dou, H. Zhang, W. Liu, C. Wang, J. Zheng, J. Inorg. Biochem.
104 (2010) 583e591.
[27] National Committee for Clinical Laboratory Standard, Reference Method for
Broth Dilution Antifungal Susceptibility Testing of Conidium-forming Fila-
mentous Fungi, Proposed Standard M38-P, National Committee for Clinical
Laboratory Standards, Wayne, PA, 1998.
[28] X. Jiang, W. Liu, W. Zhang, F. Jiang, Z. Gao, H. Zhuang, L. Fu, Eur. J. Med. Chem.
46 (2011) 3526e3530.
[6] T. Hisano, M. Ichikawa, K. Tsumoto, M. Tasaki, Chem. Pharm. Bull. 30 (1982)
2996e3004.
[7] O. Temiz, I. Oren, E. Sener, I. Yalcin, N. Ucarturk, Il Farm. 53 (1998) 337e341.
[8] I. Yalcin, I. Oren, A. Akin, N. Ucarturk, Eur. J. Med. Chem. 27 (1992) 401e406.
[9] I. Oren, O. Temiz, I. Yalcin, E. Sener, N. Altanlar, Eur. J. Pharm. Sci. 7 (1998)
153e160.
[29] J. Liu, F. Jiang, X. Jiang, W. Zhang, J. Liu, W. Liu, L. Fu, Eur. J. Med. Chem. 54
(2012) 879e886.
[10] I. Yildiz-Oren, I. Yalcin, E. Aki-Sener, N. Ucarturk, Eur. J. Med. Chem. 39 (2004)
291e298.
[11] O.T. Arpaci, I. Oren, N. Altanlar, Il Farm. 57 (2002) 175e181.