Synthesis and bioactivity evaluation of novel 2-salicyloylbenzofurans as antibacterial agents

Article abstract of DOI:10.3390/molecules22050687

In order to discover new antibacterial agents, series of 2-salicyloylbenzofuran derivatives were designed, synthesized and evaluated for their antibacterial activities against three Gram-(+) strains (methicillin-sensitive Staphylococcus aureus (MSSA) ATCC 29213, methicillin-resistant Staphylococcus aureus (MRSA) ATCC 43300, and Streptococcus faecalis (S. faecalis) ATCC 29212) and one Gram-(-) strain (Escherichia coli (E. coli) ATCC 25922). The 2-salicyloylbenzofuran heterocycles were generated by Rap-Stoermer condensation of salicylaldehydes with phenacyl bromides and then converted to diverse O-ether derivatives by Williamson synthesis. The targeted products were screened for in vitro qualitative (zone of inhibition) and quantitative (MIC) antibacterial activities by agar well diffusion assay and agar dilution method. Amongst the compounds, those bearing carboxylic acid functional group were found to exhibit reasonable activity against Gram-(+) bacterial strains including S. faecalis, MSSA and MRSA with the most potent antibacterial agent 8h (MICs = 0.06-0.12 mM). Besides, the 2-salicyloylbenzofurans partly displayed inhibitory activity against MRSA with the best MICs = 0.14 mM (8f) and 0.12 mM (8h). Finally, the antibacterial results preliminarily suggested that the substituent bearing carboxylic acid group at salicyloyl-C2 and the bromine atoms on the benzofuran moiety seem to be the functionality necessary for antibacterial activities.

Full text of DOI:10.3390/molecules22050687

molecules
Article
Synthesis and Bioactivity Evaluation of Novel
-Salicyloylbenzofurans as Antibacterial Agents
2
1
,
2,3
3
3
Phuong-Thuy T. Phan *, Thu-Trang T. Nguyen , Hong-Nhung T. Nguyen , Bao-Khanh N. Le ,
4
4
3,
Thao T. Vu , Dong C. Tran and Tuan-Anh N. Pham *
1
Department of Medicinal Chemistry, Faculty of Pharmacy, Ton Duc Thang University,
Ho Chi Minh City 700000, Vietnam
Department of Pharmaceutics, Faculty of Pharmacy, Da Nang University of Medical Technology and
2
Pharmacy, Da Nang 550000, Vietnam; nguyenthithutrang184@gmail.com
Department of Organic Chemistry, Faculty of Pharmacy, University of Medicine and Pharmacy at Ho Chi
3
Minh City, Ho Chi Minh City 700000, Vietnam; hongnhungrose2193@gmail.com (H.-N.T.N.);
lnbkhanh@gmail.com (B.-K.N.L.)
Department of Microbiology and Parasitology, Faculty of Pharmacy, University of Medicine and Pharmacy
4
at Ho Chi Minh City, Ho Chi Minh City 700000, Vietnam; vuthao910@gmail.com (T.T.V.);
trancatdong@gmail.com (D.C.T.)
*
Correspondence: phanthiphuongthuy@tdt.edu.vn (P.-T.T.P.); anh.pham81@uphcm.edu.vn or
anhpham299@yahoo.com (T.-A.N.P.); Tel.: +84-904-088-948 (P.-T.T.P.); +84-909-704-081 (T.-A.N.P.);
Fax: +84-8-3775-5055 (P.-T.T.P.); +84-8-3822-5435 (T.-A.N.P.)
Academic Editors: Diego Muñoz-Torrero and Kelly Chibale
Received: 21 March 2017; Accepted: 21 April 2017; Published: 25 April 2017
Abstract: In order to discover new antibacterial agents, series of 2-salicyloylbenzofuran derivatives
were designed, synthesized and evaluated for their antibacterial activities against three Gram-(+)
strains (methicillin-sensitive Staphylococcus aureus (MSSA) ATCC 29213, methicillin-resistant
Staphylococcus aureus (MRSA) ATCC 43300, and Streptococcus faecalis (S. faecalis) ATCC 29212) and
one Gram-( ) strain (Escherichia coli (E. coli) ATCC 25922). The 2-salicyloylbenzofuran heterocycles
−
were generated by Rap–Stoermer condensation of salicylaldehydes with phenacyl bromides and
then converted to diverse O-ether derivatives by Williamson synthesis. The targeted products
were screened for in vitro qualitative (zone of inhibition) and quantitative (MIC) antibacterial
activities by agar well diffusion assay and agar dilution method. Amongst the compounds, those
bearing carboxylic acid functional group were found to exhibit reasonable activity against Gram-(+)
bacterial strains including S. faecalis, MSSA and MRSA with the most potent antibacterial agent 8h
(
MICs = 0.06–0.12 mM). Besides, the 2-salicyloylbenzofurans partly displayed inhibitory activity
against MRSA with the best MICs = 0.14 mM (8f) and 0.12 mM (8h). Finally, the antibacterial
results preliminarily suggested that the substituent bearing carboxylic acid group at salicyloyl-C2
and the bromine atoms on the benzofuran moiety seem to be the functionality necessary for
antibacterial activities.
Keywords: 2-salicyloylbenzofuran; benzofuran; antibacterial activity; MRSA; Rap–Stoermer
condensation; Williamson synthesis
1
. Introduction
Drug resistant infections are complex problems that have been threatening the health care with
related morbidity and mortality being on the rise worldwide. Moreover, infectious diseases have
been extremely difficult to treat due to antibiotic resistant problem resulting in high extra in-hospital
costs [1–
4]. Therefore, the prevention and control of the resistance threats have been identified as
global public health priorities that require actions across all government sectors and society [
1–4].
Molecules 2017, 22, 687; doi:10.3390/molecules22050687
www.mdpi.com/journal/molecules

Molecules 2017, 22, 687
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To combat the increasingly serious threat, one of the most important main actions is to accelerate the
discovery and development of new classes of antibacterial agents as well as exploring possible new
drugs from existing classes of antibiotics that can effectively against the multidrug-resistant bacterial
strains [1,2,4].
In the literature, we found that the naturally diaryl(heteroaryl)ketone scaffold such as pestalone [
5
],
marinopyrrole A–F [
6
,
7
], pyrrolomycin C [
8
], pyrrolomycin J [
9
], and synthetic compounds related to
0
0
0
pyrrolomycins including 3,4,5,3 ,5 -pentabromo-2-(2 -hydroxybenzoyl)pyrrole [10] and fluorinated
pyrrolomycines [11], displayed strong inhibitory activities against bacterial strains. Remarkably,
these halogenated ketones showed potent antibacterial activity against MRSA [
vancomycin-resistant Enterococcus faecium [ ] at low minimum inhibitory concentrations (MICs).
The important characteristic features of these ketones for antibacterial activities are their high halogen
5–7,9,11], and
5
(
F, Cl, and Br) content and the presence of -OCH /-OH substituent at the aroyl-C2 moiety [7–11].
3
Similarly, benzofuran is considered as one of the important heterocycles because of its diverse
biological properties such as antibacterial, antitubercular, antiviral, anticancer, anti-inflammatory,
etc. [12 14]. Many of the clinically approved drugs are mono-benzofuran derivatives as well
–
as fused benzofurans with other heterocycles [13]. In particular, the antibacterial activities of
benzofuran scaffold have been found very promising potential against Gram-(+) and Gram-(–)
bacteria, including MRSA [12
–
16]. Several remarkable agents such as 3-(4-hydroxybenzoyl)-5-methyl-
2-(4-methoxyphenyl)benzofuran, 2-(4-methoxyphenyl)benzofuran-6-ol have been reported in recent
articles [15,16]. Therefore, in order to discover new antibacterial agents, series of 2-salicyloylbenzofuran
derivatives were designed, synthesized and evaluated for their antibacterial activities against three
Gram-(+) strains (MSSA ATCC 29213, MRSA ATCC 43300, S. faecalis ATCC 29212) and one Gram-(–)
strain (E. coli ATCC 25922) (Figure 1).
Figure 1. Design of novel 2-salicyloylbenzofuran derivatives as antibacterial agents.
2
. Results and Discussion
2
.1. Chemistry
The O-ether derivatives of 5,7-dibromo-2-salicyloyl benzofuran 7a–h and 8f–h were prepared
according to the synthetic Scheme 1. Bromination of starting material 2-methoxyacetophenone
1
2
using NBS as bromine source in the presence of p-toluenesulfonic acid (PTSA) gave
-methoxyphenacyl bromide 17] that was reacted with 3,5-dibromosalicyladehyde via Rap–Stoermer
2 [
condensation reaction, using 4-dimethylaminopyridine (DMAP) as catalyst, to obtain 5,7-dibromo-2-
2-methoxybenzoyl)benzofuran 18]. Demethylation of compound by anhydrous AlCl in
dried dichloromethane afforded 5,7-dibromo-2-salicyloylbenzofuran
yield [19 20]. O-ether derivatives of compound were synthesized by Williamson reaction at 2-OH
group of the salicyloyl moiety with diverse halogenated materials 6a (for details see Section 3.2.4)
(
4
[
4
3
5
as key intermediate in good
,
5
–h

Molecules 2017, 22, 687
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using K CO as a base in acetone gave the desired compounds 7a
–
h
[
21]. Alkaline hydrolysis of
2
3
esters 7f
–h
in non-aqueous condition furnished the products 8f–h as their respective carboxylic acid
derivatives [22].
Scheme 1. Synthesis of 5,7-dibromo-2-salicyloylbenzofuran O-ether derivatives 7a
–
h
and 8f
–
h
.
◦
◦
Reagents and conditions: (i) NBS, PTSA, 60 C, 15 min., 83%; (ii) DMAP, Na CO , H O, 80 C,
2
3
2
5
h, 72%; (iii) AlCl , DCM, r.t., 3 h, 89%; (iv) K CO , acetone, reflux, 8 h, 60–87%; and (v) 1. NaOH,
3 2 3
CH Cl /CH OH (9:1), r.t., 1 h 2. HCl, 70–82%.
2
2
3
To investigate the role of the substituents on the benzofuran moiety on the antibacterial activities,
series of 2-(2-(benzofuran-2-carbonyl)phenoxy)ethanoic acid derivatives 14a were synthesized
according to the synthetic Scheme 2. The key intermediate 11 was prepared by O-etherification at 2-OH
group of 2-hydroxyacetophenone with ethyl chloroacetate 6f 21] and then brominated by NBS in the
presence of PTSA as catalyst [17]. The Rap–Stoermer condensation reaction was applied to generate the
benzofuran heterocycles 13a 18] that was subjected to alkaline hydrolysis in non-aqueous condition
–
f
9
[
–f [
to obtain the final products 14a–f [22].
Scheme 2. Synthesis of 2-(2-(benzofuran-2-carbonyl)phenoxy)ethanoic acid derivatives 14a
–
f
. Reagents
◦
and conditions: (i) 6f, K CO , acetone, reflux, 8 h, 77%; (ii) NBS, PTSA, 60 C, 15 min., 74%; (iii) DMAP,
2
3
◦
Na CO , H O, 80 C, 5 h, 57–72%; and (iv) 1. NaOH, CH Cl /CH OH (9:1), r.t., 1 h 2. HCl, 66–81%.
2
3
2
2
2
3

Molecules 2017, 22, 687
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In this study, the Rap–Stoermer condensation reaction was applied for the efficient generation
of benzofuran heterocycle in 57–87% yields. The Williamson reaction was used to synthesize desired
2
-salicyloylbenzofuran derivatives bearing diverse O-ether moieties in rather good yields (60–87%).
It was found that the most preferable alkylating agents for ether formation are primary
materials since the Williamson reaction using ethyl 2-bromopropanoate 6g, a secondary
α
-halogenated
-halogenated
α
material, gave ether product 7g with the lowest yield (60%). The structures of synthesized
1
13
2
-salicyloylbenzofurans were characterized by H-NMR, C-NMR and HRMS. The obtained results
1
showed consistency with the expected structures and formulas of the targeted products. The H-NMR
spectra of 2-salicyloylbenzofurans displayed a single signal at the range of
confirmed the formation of benzofuran heterocycle.
δ 7.70–7.30 ppm, which
2
.2. Evaluation of Antibacterial Activity
The newly synthesized compounds were preliminarily screened for in vitro qualitative (zone of
inhibition) and quantitative (MIC) antibacterial activities against three Gram-(+) strains (MSSA ATCC
9213, MRSA ATCC 43300, and S. faecalis ATCC 29212) and one Gram-(–) strain (E. coli ATCC 25922)
by slight modified agar well diffusion assay [23 24] and agar dilution method [25]. Dimethyl sulfoxide
DMSO) and potent antibacterial drugs ampicillin, cefuroxime and vancomycin were used as solvent
2
,
(
control and standards, respectively. The diameters of the inhibition zone (mm) corresponding to the
MICs (mM) are presented in Table 1.
Table 1. In-vitro antibacterial activity of 2-salicyloylbenzofuran derivatives.
Zone of Inhibition in mm (MIC in mM) a
Compound b
E. coli
S. faecalis
MSSA
MRSA
4
5
9.5 (>2.50)
9.5 (>2.50)
9 (>2.59)
–
10 (>2.59)
–
12 (2.35)
–
–
–
13 (>2.59)
12 (>2.41)
16 (2.35)
–
14 (>1.97)
–
8 (>2.12)
14 (0.14)
17 (0.27)
15 (0.12)
11 (>3.46)
9 (3.30)
9 (1.57)
10 (1.55)
11 (1.36)
11 (0.70)
15 (0.05)
25 (0.04)
31 (0.00004)
7
a
8 (>2.41)
8 (>2.35)
–
–
–
–
–
7
7
b
c
–
7
7
d
e
8(>1.97)
8 (>1.84)
–
18 (>1.84)
–
7
8
f
f
–
–
–
–
–
–
–
12 (0.28)
11 (0.55)
15 (0.12)
–
16 (0.14)
19 (0.27)
17 (0.06)
10 (0.86)
9 (1.65)
9 (1.57)
9 (1.55)
9 (1.36)
9 (0.70)
31 (0.0007)
31 (0.02)
30 (0.00004)
8
g
8
h
1
4a
4b
4c
4d
4e
4f
1
–
1
–
9 (3.10)
–
9 (3.14)
9 (>3.10)
9 (2.73)
11 (1.40)
28 (0.006)
–
1
1
1
–
Ampicillin
Cefuroxime
Vancomycin
35 (0.006)
33 (0.02)
23 (0.0014)
30 (0.00004)
a
E. coli: Escherichia coli ATCC 25922; S. faecalis: Streptococcus faecalis ATCC 29212; MSSA: methicillin-sensitive
Staphylococcus aureus ATCC 29213; MRSA: methicillin-resistant Staphylococcus aureus ATCC 43300; b other
compounds: not tested; (–): not inhibition at tested concentration.
The 2-methoxybenzoylbenzofuran
S. faecalis with inhibition zone diameters of 9.5 mm. Similarly, the respective phenolic compound,
-salicyloylbenzofuran , showed week antibacterial activity against three Gram-(+) bacterial strains
with inhibition zone diameters in the range of 9–13 mm (MICs > 2.59 mM) while completely lost
activity against E. coli. These preliminary results showed that compound and were considered poor
antibacterial agents. From the literature, we found that the substituent at the aroyl-C2 moiety was
4 showed week antibacterial activity against E. coli and
2
5
4
5

Molecules 2017, 22, 687
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identified as the important characteristic feature for antibacterial activity [
7
–11]. Therefore, in order to
improve the antibacterial activity of 2-salicyloylbenzofurans, we focused on the replacing of 2-OCH3
and 2-OH group by different ether residues. Amongst the obtained ethers, compounds 7a except
showed some slight improvements of antibacterial activity with inhibition zone diameters in
the range of 8–18 mm (MICs > 1.84 mM) against Gram-(+) and/or Gram-(–) strains. Meanwhile,
the 2-salicyloylbenzofuran derivatives 8f bearing carboxylic acid group on the ether residue
significantly increased their antibacterial activities against three Gram-(+) bacterial strains (S. faecalis,
MSSA and MRSA) as compared to the parent derivatives and with the MIC values = 0.06–0.55 mM.
–
f
7
c
–
h
4
5
Compound 8h showed the most potent antibacterial activity with MIC values = 0.06–0.12 mM against
three Gram-(+) bacterial strains. In addition, these 2-salicyloylbenzofurans also displayed inhibitory
activity against MRSA with the best MICs = 0.14, 0.27, and 0.12 mM for compound 8f
–h, respectively.
However, compounds 8f were completely inactive against Gram-(–) E. coli at the tested concentration.
–
h
It was found from these results that the carboxylic acid group seems to be an essential part of the
pharmacophore required for inhibitory activity against Gram-(+) bacterial strains since the others
including nitrile 7c, amide 7e, and ester 7f generally possessed very poor or no antibacterial activity.
It was also suggested that the –COOH group keeps an important role for a potential hydrogen bonding
as hydrogen bond donor functions with hydrophilic properties.
We next investigated the potential impact of substituents on the benzofuran heterocycle by
replacing bromine atoms with different substituents (14a
compounds 14a decreased their bioactivity with MIC values
These results indicated that bromine atoms may serve as potential halogen bond donor functions
with lipophilic properties that are consistent with other research [ ]. Besides, most of these
–
f). The antibacterial activity showed
–
f
≥
0.70 mM as compared to compound 8f.
7
derivatives were also inactive against E. coli (except compound 14d, MIC = 3.10 mM) that again
confirmed that the presence of carboxylic acid group on the salicyloyl moiety seems to be favor
for inhibitory activity against Gram-(+) strains. Overall, the results of this study suggested that
the 5,7-dibromo-2-salicyloylbenzofuran bearing carboxylic group via ether linker moiety (Figure 2)
may become the promising scaffold for the follow-up research to discover novel antibacterial agents,
particularly against Gram-(+) bacterial strains including MRSA, a main cause of a variety of community
and healthcare-associated infections.
Figure 2. The most potent scaffold for anti-Gram-(+) bacterial strains activity.
3
. Experimental Section
3
.1. Chemicals and Instruments
All starting materials were purchased from commercial sources such as Acros Organics-Thermo
Fisher Scientific (Geel, Belgium), TCI Chemicals (Tokyo, Japan) and used without further purification.
Thin-layer chromatography (TLC) was carried out on aluminum-supported silica gel plates (Merck 60F
254, Darmstadt, Germany) with visualization of components by UV light (254 nm). Column
chromatography was carried out on silica gel (0.04–0.063 mm, 230–400 mesh ASTM, Merck, Darmstadt,
Germany). The melting points were recorded on a Gallenkamp apparatus (Sanyo Gallenkamp,
Southborough, UK) and were uncorrected. NMR spectra were recorded in CDCl or DMSO-d using
3
6
a Bruker Avance 500 MHz spectrometer (Bruker Corporation, Billerica, MA, USA). Chemical shifts
) are reported in ppm related to internal tetramethylsilane (TMS) and coupling constant (J) are
(δ

Molecules 2017, 22, 687
6 of 13
reported in Hertz (Hz). Mass spectra were recorded on a Shimadzu LCMS-IT-TOF Mass spectrometer
(Shimadzu Scientific Instruments, Kyoto, Japan).
Three Gram-(+) strains (MSSA ATCC 29213, MRSA ATCC 43300, S. faecalis ATCC 29212) and one
Gram-(–) strain (E. coli ATCC 25922) were preserved and activated at our Department of Microbiology
and Parasitology. The bacterial culture media were purchased from Merck (Darmstadt, Germany)
in which Tryptic Soy Agar (TSA) was used to isolate and preserve bacterial strains, Tryptic Soy
Broth (TSB) was used as bacteria-activating medium and Muller-Hinton Agar (MHA) was used for
antimicrobial susceptibility testing. Dimethyl sulfoxide (DMSO) from Merck and potent antibacterial
drugs ampicillin, cefuroxime, and vancomycin (Sigma-Aldrich Pte. Ltd., St. Louis, MO, USA) were
used as solvent control and standards, respectively.
3
.2. Synthesis
3
.2.1. General Procedure for the Synthesis of Phenacyl Bromides 2, 11
The round-bottom flask containing acetophenone derivative
.4 mmol) was heated to 60 C to turn the reaction mixture into a paste and NBS (0.854 g, 4.8 mmol) then
1
,
10 (4 mmol) and PTSA (0.076 g,
◦
0
added slowly. After 15 min, the reaction mixture was cooled to room temperature and water (20 mL)
added. The crude product was extracted with dichloromethane (2 20 mL), dried over anhydrous
Na SO and purified by crystallization from n-hexane–dichloromethane to give pure product.
×
2
4
2
-Methoxyphenacyl bromide (
2): Yellow solid, soluble in acetone, dichloromethane, chloroform, insoluble
◦
1
in water; yield 83%; m.p.: 43–45 C; H-NMR (500 MHz, CDCl ) (
δ, ppm): 7.78 (dd, 1H, J = 1.5,
3
7
.5 Hz, Ar–H), 7.51–7.47 (m, 1H, Ar–H), 7.02–6.96 (m, 2H, Ar–H), 4.57 (s, 2H, CH ), 3.93 (s, 3H, OCH ).
2
3
This result showed consistency with the data in literature [26].
Ethyl 2-(2-(2-bromoacetyl)phenoxy)acetate (11): White solid, soluble in dichloromethane, chloroform,
◦
1
insoluble in water; yield 74%; m.p.: 75–76 C; H-NMR (500 MHz, CDCl ) (
δ, ppm): 7.85 (dd, 1H,
3
J = 1.5, 7.5 Hz, Ar–H), 7.52–7.49 (m, 1H, Ar–H), 7.11–7.08 (m, 1H, Ar–H), 6.86 (d, 1H, J = 8.0 Hz, Ar–H),
4
.79 (s, 2H, CH ), 4.74 (s, 2H, CH ), 4.30 (q, 2H, J = 7.0 Hz, CH2CH ), 1.33 (t, 3H, J = 7.0 Hz, CH2
C
H3);
, ppm): 192.4 (C=Oketone), 167.7 (C=Oester), 156.6, 134.5, 131.8, 125.5,
H COOCH CH ), 61.7 (CH COO H CH ), 37.8 ( H Br), 14.1 (CH ); HRMS (ESI)
2
2
3
13
C-NMR (125 MHz, CDCl ) (
δ
3
1
22.1, 112.2, 65.7 (
C
C
C
2
2
+
2
3
2
2
3
3
m/z 301.0028 [M + H] , calculated for (C H BrO ): 301.0075.
1
2
14
4
3
.2.2. General Procedure for the Synthesis of 2-Salicyloylbenzofurans 4, 13a–f
In a round-bottom flask containing DMAP (0.012 g, 0.1 mmol) and Na CO (0.159 g, 1.5 mmol) in
2
3
water (10 mL), phenacyl bromide
were added. The resulting mixture was stirred at 80 C for 5 h. The mixture was extracted with CH Cl
2
or 11 (1 mmol) and salicylaldehyde derivative
3
,
12a
–
f
(1 mmol)
◦
2
2
(
3
×
15 mL), and then washed with water (30 mL). The organic layers were combined, dried over
anhydrous Na SO and evaporated under vacuum. The obtained residue was purified by column
2
4
chromatography on silica gel (n-hexane:dichloromethane = 1:1, v/v) to give pure product.
5
,7-Dibromo-2-(2-methoxybenzoyl)benzofuran (4): Yellow solid, soluble in dichloromethane, chloroform,
◦
1
insoluble in water; yield 72%; m.p. 175–176 C; H-NMR (500 MHz, CDCl ) (δ, ppm): 7.76–7.75 (m,
3
2
H, Ar–H), 7.55–7.51 (m, 2H, Ar–H), 7.34 (s, 1H, Ar–H), 7.08 (t, 1H, J = 7.5 Hz, Ar–H), 7.04 (d, 1H,
J = 8.5 Hz, Ar–H), 3.81 (s, 3H, OCH3); 13C-NMR (125 MHz, CDCl ) (
δ
, ppm): 184.3 (C=O), 158.1, 154.7,
3
1
52.0, 133.2, 133.1, 130.0, 129.7, 127.4, 124.8, 120.6, 116.8, 114.4, 111.9, 105.9, 55.9 (OCH ); HRMS (ESI)
3
+
m/z 410.9169 [M + H] , calculated for (C H Br O ): 410.9054.
1
6
11
2
3
Ethyl 2-(2-(benzofuran-2-carbonyl)phenoxy)acetate (13a): Yellow brown oil, soluble in dichloromethane,
1
chloroform, insoluble in water; yield 60%; H-NMR (500 MHz, CDCl ) (
δ
, ppm): 7.68 (d, 1H, J = 8.0 Hz
,
3
Ar–H), 7.59 (dd, 1H, J = 1.0, 8.5 Hz, Ar–H), 7.54 (dd, 1H, J = 2.0, 7.5 Hz, Ar–H), 7.50–7.45 (m, 3H,
Ar–H), 7.29 (td, 1H, J = 0.5, 7.5 Hz, Ar–H), 7.13 (td, 1H, J = 1.0, 7.5 Hz, Ar–H), 6.90 (d, 1H, J = 8.0 Hz,

Molecules 2017, 22, 687
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Ar–H), 4.63 (s, 2H, CH ), 4.15 (q, 2H, J = 7.0 Hz, CH2CH ), 1.19 (t, 3H, J = 7.0 Hz, CH CH3); 13C-NMR
2
3
2
(
125 MHz, CDCl ) (
δ, ppm): 184.5 (C=Oketone), 168.3 (C=Oester), 156.0, 155.8, 152.9, 132.4, 130.0, 128.6,
3
1
1
28.3, 127.2, 123.7, 123.4, 121.7, 116.8, 112.8, 112.5, 66.0 ( H COOCH CH ), 61.3 (CH COO H CH ),
C
C
2
2
2
3
2
3
+
4.0 (CH ); HRMS (ESI) m/z 325.1027 [M + H] , calculated for (C H O ): 325.1076.
3
19 17
5
Ethyl 2-(2-(5-methylbenzofuran-2-carbonyl)phenoxy)acetate (13b): Yellow brown oil, soluble in
1
dichloromethane, chloroform, insoluble in water; yield 57%; H-NMR (500 MHz, CDCl ) (
δ, ppm):
3
7
8
.53 (dd, 1H, J = 1.5, 7.5 Hz, Ar–H), 7.49–7.44 (m, 3H, Ar–H), 7.39 (s, 1H, Ar–H), 7.28 (dd, 1H, J = 1.5,
.5 Hz, Ar–H), 7.12 (t, 1H, J = 7.5 Hz, Ar–H), 6.90 (d, 1H, J = 8.5 Hz, Ar–H), 4.62 (s, 2H, CH ), 4.15
2
13
(
q, 2H, J = 7.0 Hz, CH2CH ), 2.44 (s, 3H, CH ), 1.20 (t, 3H, J = 7.0 Hz, CH CH3); C-NMR (125 MHz,
3 3 2
CDCl ) (
δ
, ppm): 182.4 (C=Oketone), 171.1 (C=Oester), 154.7, 153.8, 152.0, 135.5, 133.5, 132.9, 129.7, 129.6,
25.0, 117.0, 115.6, 114.7, 113.9, 105.9, 73.5 ( H COOCH CH ), 61.6 (CH COO H CH ), 18.2 (CH ),
4.1 (CH CH ); HRMS (ESI) m/z 339.1222 [M + H] , calculated for (C H O ): 339.1232.
3
1
1
C
C
2
2
2
3
2
3
3
+
2
3
20 19
5
Ethyl 2-(2-(5-methoxybenzofuran-2-carbonyl)phenoxy)acetate (13c): Yellow brown oil, soluble in
1
dichloromethane, chloroform, insoluble in water; yield 66%; H-NMR (500 MHz, CDCl ) (
δ, ppm):
3
7
.53 (dd, 1H, J = 1.5, 7.5 Hz, Ar–H), 7.50–7.46 (m, 2H, Ar–H), 7.41 (s, 1H, Ar–H), 7.13 (d, 1H, J = 7.0 Hz
,
Ar–H), 7.11–7.06 (m, 2H, Ar–H), 6.90 (d, 1H, J = 8.5 Hz, Ar–H), 4.63 (s, 2H, CH ), 4.16 (q, 2H, J = 7.0 Hz,
2
1
3
C
1
1
H2CH ), 3.84 (s, 3H, OCH ), 1.20 (t, 3H, J = 7.0 Hz, CH CH3); C-NMR (125 MHz, CDCl ) (
δ
, ppm):
83.7 (C=Oketone), 168.4 (C=Oester), 161.2, 157.7, 155.7, 152.5, 132.1, 129.9, 128.9, 123.7, 121.6, 120.6, 117.7,
14.3, 112.8, 95.6, 66.0 ( H COOCH CH ), 61.3 (CH COO H CH ), 55.7 (OCH ), 14.0 (CH2CH3);
3
3
2
3
C
C
2
2
2
3
2
3
3
+
HRMS (ESI) m/z 355.1165 [M + H] , calculated for (C H O ): 355.1182.
20
19
6
Ethyl 2-(2-(5-chlorobenzofuran-2-carbonyl)phenoxy)acetate (13d): Yellow brown oil, soluble in
1
dichloromethane, chloroform, insoluble in water; yield 70%; H-NMR (500 MHz, CDCl ) (
δ, ppm):
3
7.65 (d, 1H, J = 7.0 Hz, Ar–H), 7.54–7.48 (m, 3H, Ar–H), 7.43 (d, 1H, J = 0.5 Hz, Ar–H), 7.41 (dd, 1H,
J = 2.0, 8.5 Hz, Ar–H), 7.14–7.11 (m, 1H, Ar–H), 6.89 (d, 1H, J = 8.5 Hz, Ar–H), 4.62 (s, 2H, CH ), 4.16
2
13
(
q, 2H, J = 7.0 Hz, CH2CH ), 1.21 (t, 3H, J = 7.0 Hz, CH CH3); C-NMR (125 MHz, CDCl ) (
δ
, ppm):
3
2
3
1
1
3
84.4 (C=Oketone), 168.2 (C=Oester), 155.8, 154.2, 154.0, 132.8, 132.7, 130.1, 129.3, 128.5, 128.3, 122.7, 121.8,
15.6, 113.6, 112.7, 65.8 (
C
H COOCH CH ), 61.4 (CH COO
C
H CH ), 14.0 (CH ); HRMS (ESI) m/z
2
2
3
2
2
3
3
+
59.0672 [M + H] , calculated for (C H BrO ): 359.0686.
19
16
5
Ethyl 2-(2-(5-bromobenzofuran-2-carbonyl)phenoxy)acetate (13e): Yellow brown oil, soluble in
1
dichloromethane, chloroform, insoluble in water; yield 72%; H-NMR (500 MHz, CDCl ) (
δ, ppm):
3
7.82 (d, 1H, J = 1.5 Hz, Ar–H), 7.56–7.46 (m, 4H, Ar–H), 7.43 (s, 1H, Ar–H), 7.13 (t, 1H, J = 7.5 Hz,
Ar–H), 6.89 (d, 1H, J = 8.0 Hz, Ar–H), 4.62 (s, 2H, CH ), 4.16 (q, 2H, J = 7.0 Hz, CH2CH ), 1.21 (t, 3H,
2
3
13
J = 7.0 Hz, CH CH3); C-NMR (125 MHz, CDCl ) (
δ
, ppm): 183.9 (C=Oketone), 168.1 (C=Oester),
2
3
1
56.1, 154.4, 151.9, 133.1, 133.0, 130.3, 129.7, 128.0, 124.9, 121.9, 116.7, 115.0, 112.8, 105.8, 66.0
+
(C
H COOCH CH ), 61.4 (CH COO
C
H CH ), 14.0 (CH ); HRMS (ESI) m/z 403.0168 [M + H] ,
2
2
3
2
2
3
3
calculated for (C H BrO ): 403.0181.
19
16
5
Ethyl 2-(2-(5,7-dichlorobenzofuran-2-carbonyl)phenoxy)acetate (13f): Yellow brown oil, soluble in
1
dichloromethane, chloroform, insoluble in water; yield 68%; H-NMR (500 MHz, CDCl ) (
δ, ppm):
3
7.57 (d, 1H, J = 2.0 Hz, Ar–H), 7.56 (dd, 1H, J = 2.0, 7.5 Hz, Ar–H), 7.53–7.50 (m, 1H, Ar–H), 7.49 (s, 1H,
Ar–H), 7.46 (d, 1H, J = 2.0 Hz, Ar–H), 7.14 (td, 1H, J = 1.0, 7.5 Hz, Ar–H), 6.91 (d, 1H, J = 8.5 Hz,
13
Ar–H), 4.64 (s, 2H, CH ), 4.13 (q, 2H, J = 7.0 Hz, CH2CH ), 1.19 (t, 3H, J = 7.0 Hz, CH CH3); C-NMR
2
3
2
(
125 MHz, CDCl ) (
δ
, ppm): 184.01 (C=Oketone), 168.2 (C=Oester), 156.2, 154.7, 150.3, 133.1, 130.4, 129.6,
3
1
1
29.5, 128.1, 127.9, 122.0, 121.3, 118.5, 115.1, 112.9, 66.0 ( H COOCH CH ), 61.4 (CH COO H CH ),
C
C
2
2
2
3
2
3
+
4.0 (CH ); HRMS (ESI) m/z 393.0246 [M + H] , calculated for (C H Cl O ): 393.0297.
3
19 15
2
5
3
.2.3. Synthesis of 5,7-Dibromo-2-salicyloylbenzofuran 5
Anhydrous AlCl (0.267 g, 2.0 mmol) was added to a solution of substrate
4
(0.410 g, 1.0 mmol)
3
◦
in CH Cl (10 mL) in one portion at 0 C. The reaction mixture was stirred at the same temperature
2
2

Molecules 2017, 22, 687
8 of 13
for 5 min. then warmed up to room temperature and stirred for a further 3 h. The reaction mixture
was poured into cold water (20 mL) and extracted with CH Cl (2 20 mL). The combined organic
layer was washed successively with saturated NaHCO and brine, and then dried over anhydrous
×
2
2
3
Na SO . The solvent was evaporated under vacuum and the crude product was purified by column
2
4
chromatography (n-hexan:dichloromethane = 1:1, v/v) to obtain product as yellow solid. Yield 89%,
◦
1
soluble in dichloromethane, chloroform, insoluble in water; m.p. 132–134 C; H-NMR (500 MHz,
CDCl ) ( , ppm): 12.04 (s, 1H, OH), 8.47 (dd, 1H, J = 1.5, 6.5 Hz, Ar–H), 7.82 (d, 1H, J = 1.5 Hz, Ar–H),
.79 (d, 1H, J = 2.0 Hz, Ar–H), 7.67 (s, 1H, Ar–H), 7.59–7.55 (m, 1H, Ar–H), 7.08 (dd, 1H, J = 1.0, 7.5 Hz,
δ
3
7
13
Ar–H), 7.05–7.02 (m, 1H, Ar–H); C-NMR (125 MHz, CDCl ) (
δ, ppm): 185.9 (C=O), 164.1, 154.0,
3
1
52.3, 137.3, 133.5, 131.9, 129.2, 124.9, 119.6, 118.8, 118.9, 117.5, 115.6, 106.0; HRMS (ESI) m/z 394.8756
−
[M − H] , calculated for (C H Br O ): 394.8741.
15
7
2
3
3
.2.4. General Procedure for the Synthesis of O-ether Derivatives 7a–h, 10
A mixture of or (1 mmol), halogenated material 6a (1.2 mmol) (6a: ethyl bromide, 6b: allyl
5
9
–h
bromide; 6c: 2-bromoacetonitrile, 6d: 2-chlorobenzyl chloride, 6e: 2-chloro-N-(2,6-dimethylphenyl)
acetamide, 6f: ethyl chloroacetate, 6g: ethyl 2-bromopropanoate, 6h: methyl 5-(chloromethyl)furan-
2
8
-carboxylate) and anhydrous K CO (0.276 g, 2.0 mmol) in dry acetone (10 mL) was refluxed for
2 3
h. The reaction mixture was cooled to room temperature and the solvent was removed under
reduced pressure. The residue was triturated with ice water to remove K CO and then extracted with
2
3
dichloromethane (3
×
15 mL). The organic layer was dried over anhydrous Na SO , evaporated under
2 4
vacuum to dryness. The obtained residue was purified by column chromatography using n-hexane
and dichloromethane (1:1) as eluent to give pure product.
5
,7-Dibromo-2-(2-ethoxybenzoyl)benzofuran (7a): Yellow solid, soluble in dichloromethane, chloroform,
◦
1
insoluble in water; yield 75%; m.p. 78–80 C; H-NMR (500 MHz, CDCl ) (δ, ppm): 7.77 (d, 1H,
3
J = 1.5 Hz, Ar–H), 7.75 (s, 1H, Ar–H), 7.53–7.50 (m, 2H, Ar–H), 7.37 (s, 1H, Ar–H), 7.08–7.05 (m, 1H,
Ar–H), 7.01 (d, 1H, J = 8.5 Hz, Ar–H), 4.07 (q, 1H, J = 7.0 Hz, CH ), 1.12 (t, 3H, J = 7.0 Hz, CH );
2
3
1
3
C-NMR (125 MHz, CDCl ) (
δ
, ppm): 184.7 (C=O), 157.6, 154.9, 151.8, 133.3, 133.0, 130.1, 129.7, 127.5,
24.8, 120.6, 116.8, 113.6, 112.7, 105.8, 64.3 (CH ), 14.5 (CH ); HRMS (ESI) m/z 422.9249 [M + H]+
3
1
,
2
3
calculated for (C H Br O ): 422.9231.
17
13
2
3
2
-(2-Allyloxybenzoyl)-5,7-dibromobenzofuran (7b): Yellow solid, soluble in dichloromethane, chloroform,
◦
1
insoluble in water; yield 79%; m.p. 78–80 C; H-NMR (500 MHz, CDCl ) (
δ
, ppm): 7.76 (d, 1H,
J = 1.5 Hz, Ar–H), 7.74 (d, 1H, J = 2.0 Hz, Ar–H), 7.54–7.50 (m, 2H, Ar–H), 7.39 (s, 1H, Ar–H), 7.10–7.07
m, 1H, Ar–H), 7.03 (d, 1H, J = 8.5 Hz, Ar–H), 5.80–5.75 (m, 1H, CH C =CH ), 5.15 (dd, 1H, J = 1.5,
3
(
H
2
2
1
C
6.0 Hz, CH CH=CH2), 5.05 (dd, 1H, J = 1.5, 10.0 Hz, CH CH=CH2), 4.56 (dd, 2H, J = 1.5, 3.5 Hz,
H2CH=CH2); C-NMR (125 MHz, CDCl ) (δ, ppm): 184.5 (C=O), 157.2, 154.8, 151.9, 133.3, 133.1,
3
2
2
13
1
32.4, 130.1, 129.7, 127.7, 124.8, 120.9, 117.4, 116.8, 113.8, 113.1, 105.9, 69.5 (
C
H CH=CH ); HRMS (ESI)
2
2
+
m/z 476.8714 [M + K] , calculated for (C H Br KO ): 476.8749.
18
12
2
3
2
-(2-(5,7-Dibromobenzofuran-2-carbonyl)phenoxy)acetonitrile (7c): White solid, soluble in dichloromethane,
◦
1
acetone, insoluble in water; yield 87%; m.p.: 178–179 C; H-NMR (500 MHz, CDCl ) (
δ, ppm):
3
7
.78 (d, 1H, J = 1.5 Hz, Ar–H), 7.77 (d, 1H, J = 1.5 Hz, Ar–H), 7.62–7.59 (m, 2H, Ar–H), 7.34 (s, 1H,
13
Ar–H), 7.26 (t, 1H, J = 8.0 Hz, Ar–H), 7.19 (d, 1H, J = 8.5 Hz, Ar–H), 4.81 (s, 2H, CH2); C-NMR
(
1
125 MHz, CDCl ) (
23.5, 117.2, 115.2, 114.6, 114.1, 105.9, 54.8 (CH ); HRMS (ESI) m/z 433.8770 [M
δ
, ppm): 183.1 (C=O), 154.9, 154.0, 152.1, 133.6, 133.4, 130.5, 129.5, 128.6, 125.0,
3
−
−
H] , calculated for
2
(C H Br NO ): 433.8851.
17 8 2 3
2
-(2-(2-Chlorobenzyloxy)benzoyl)-5,7-dibromobenzofuran (7d): Yellow solid, soluble in dichloromethane,
◦
1
chloroform, insoluble in water; yield 77%; m.p.: 125–127 C; H-NMR (500 MHz, CDCl ) (
δ, ppm): 7.71
3
(
s, 1H, Ar–H), 7.67 (s, 1H, Ar–H), 7.59–7.53 (m, 2H, Ar–H), 7.39 (s, 1H, Ar–H), 7.27 (d, 1H, J = 8.5 Hz,
13
Ar–H), 7.14–7.11 (m, 4H, Ar–H), 6.99 (m, 1H, Ar–H), 5.20 (s, 2H, CH2); C-NMR (125 MHz, CDCl3)

Molecules 2017, 22, 687
9 of 13
(δ, ppm): 184.5 (C=O), 157.1, 154.7, 151.9, 133.9, 133.47, 133.0, 131.9, 130.3, 129.6, 129.1, 128.8, 127.8,
+
1
27.6, 126.8, 124.7, 121.3, 116.8, 113.9, 113.3, 105.9, 67.9 (CH ); HRMS (ESI) m/z 542.8563 [M + Na] ,
2
calculated for (C H Br ClNaO ): 542.8797.
22
13
2
3
2
-(2-(5,7-Dibromobenzofuran-2-carbonyl)phenoxy)-N-(2,6-dimethylphenyl)acetamide (7e): Yellow solid,
◦
1
soluble in dichloromethane, chloroform, insoluble in water; yield 75%; m.p.: 177–179 C; H-NMR
(
500 MHz, CDCl ) (
δ, ppm): 8.65 (s, 1H, NH), 7.76–7.72 (m, 3H, Ar–H), 7.65–7.62 (m, 1H, Ar–H), 7.44
3
(
4
s, 1H, Ar–H), 7.20 (t, 1H, J = 7.5 Hz, Ar–H), 7.16 (d, 1H, J = 8.5 Hz, Ar–H), 7.12–7.05 (m, 3H, Ar–H),
13
.81 (s, 2H, CH ), 2.12 (s, 6H, CH ); C-NMR (125 MHz, CDCl ) (δ, ppm): 183.2 (C=Oketone), 166.2
2 3 3
(
1
C=Oamide), 156.3, 153.8, 152.3, 135.3, 134.2, 133.8, 133.0, 133.0, 131.0, 129.3, 129.3, 128.2, 127.4, 126.4,
+
24.9, 121.7, 117.3, 115.8, 113.4, 106.1, 67.8 (CH ), 18.3 (CH ); HRMS (ESI) m/z 579.9592 [M + Na] ,
2
3
calculated for (C H Br NNaO ): 579.9558.
25
19
2
4
Ethyl 2-(2-(5,7-dibromobenzofuran-2-carbonyl)phenoxy)acetate (7f): White solid, soluble in dichloromethane,
◦
1
acetone, insoluble in water; yield 64%; m.p.: 77–78 C; H-NMR (500 MHz, CDCl ) (
(
δ
, ppm): 7.77
d, 1H, J = 1.5 Hz, Ar–H), 7.74 (d, 1H, J = 1.5 Hz, Ar–H), 7.57–7.50 (m, 3H, Ar–H), 7.16–7.13 (m, 1H,
Ar–H), 6.91 (d, 1H, J = 8.0 Hz, Ar–H), 4.64 (s, 2H, CH2COOCH CH ), 4.12 (q, 2H, J = 7.0 Hz,
3
2
3
1
3
CH COOCH2CH ), 1.19 (t, 3H, J = 7.0 Hz, CH ); C-NMR (125 MHz, CDCl ) (
δ, ppm): 184.0
2
3
3
3
(C=Oketone), 168.2 (C=Oester), 156.2, 154.4, 152.0, 133.1, 133.1, 130.4, 129.8, 128.1, 124.9, 122.0, 116.8,
1
5
15.0, 112.8, 105.9, 66.1 (
04.9030 [M + Na] , calculated for (C H Br NaO ): 504.9085.
C
H COOCH CH ), 61.4 (CH COO
C
H CH ), 14.0 (CH ); HRMS (ESI) m/z
2
2
3
2
2
3
3
+
19
14
2
5
Ethyl 2-(2-(5,7-dibromobenzofuran-2-carbonyl)phenoxy)propanoate (7g): Brown oil, soluble in dichloromethane,
1
chloroform, insoluble in water; yield 60%; H-NMR (500 MHz, CDCl ) (
δ, ppm): 7.77–7.75 (m, 2H,
3
Ar–H), 7.57 (s, 1H, Ar–H), 7.53 (d, 1H, J = 7.5 Hz, Ar–H), 7.47 (t, 1H, J = 8.0 Hz, Ar–H), 7.10 (t, 1H,
J = 7.5 Hz, Ar–H), 6.83 (d, 1H, J = 8.5 Hz, Ar–H), 4.76 (q, 1H, J = 6.5 Hz, CH), 4.18 (q, 2H, J = 7.0 Hz,
13
CH ), 1.37 (d, 3H, J = 6.5 Hz, CH3CH), 1.24 (t, 3H, J = 7.0 Hz, CH CH3); C-NMR (125 MHz, CDCl3)
2
2
(δ, ppm): 184.1 (C=Oketone), 171.6 (C=Oester), 155.7, 154.4, 151.9, 133.2, 133.0, 130.4, 129.8, 128.1, 124.9,
1
21.6, 116.8, 115.3, 112.8, 105.9, 73.2 (CH3
C
H), 61.4 (
C
H CH ) , 18.3 (
C
H CH), 14.1 (CH2
C
H ); HRMS
3
2
3
3
+
(ESI) m/z 518.9387 [M + Na] , calculated for (C H Br NaO ): 518.9242.
20
16
2
5
Methyl 5-((2-(5,7-dibromobenzofuran-2-carbonyl)phenoxy)methyl)furan-2-carboxylate (7h): White solid,
◦
1
soluble in dichloromethane, acetone, insoluble in water; yield 77%; m.p.: 122–124 C; H-NMR
(
(
(
(
500 MHz, CDCl ) (
δ
, ppm): 7.75 (d, 1H, J = 1.5 Hz, Ar–H), 7.72 (d, 1H, J = 1.5 Hz, Ar–H), 7.56–7.52
3
m, 2H, Ar–H), 7.36 (s, 1H, Ar–H), 7.14 (t, 1H, J = 7.5 Hz, Ar–H), 7.10 (d, 1H, J = 8.5 Hz, Ar–H), 6.97
d, 1H, J = 3.5 Hz, Ar–H), 6.24 (d, 1H, J = 3.5 Hz, Ar–H), 5.10 (s, 2H, CH ), 3.87 (s, 3H, CH ); C-NMR
13
2
3
125 MHz, CDCl ) (
δ
, ppm): 182.5 (C=Oketone), 158.7 (C=Oester), 155.4, 154.0, 153.0, 151.9, 144.6, 135.8,
3
1
33.4, 132.8, 129.7, 129.5, 125.0, 118.5, 117.0, 115.1, 114.6, 114.2, 111.4, 105.9, 63.6 (CH ), 52.1 (CH );
2
3
+
HRMS (ESI) m/z 556.9210 [M + Na] , calculated for (C H Br NaO ): 556.9034.
22
14
2
6
Ethyl 2-(2-acetylphenoxy)acetate (10): White solid, soluble in dichloromethane, chloroform, insoluble in
◦
1
water; yield 77%; m.p.: 46–48 C; H-NMR (500 MHz, CDCl ) (δ, ppm): 7.76 (dd, 1H, J = 1.5, 7.5 Hz,
3
Ar–H), 7.46–7.42 (m, 1H, Ar–H), 7.05 (t, 1H, J = 7.5 Hz, Ar–H), 6.83 (d, 1H, J = 8.5 Hz, Ar–H), 6.83 (d, 1H,
J = 8.5 Hz, Ar–H), 4.72 (s, 2H, CH ) 4.28 (q, 2H, J = 7.0 Hz, CH2CH ), 2.72 (s, 3H, CH3C=O), 1.30 (t, 3H,
2
3
13
J = 7.0 Hz, CH CH3); C-NMR (125 MHz, CDCl ) (
δ
, ppm): 199.6 (C=Oketone), 168.1 (C=Oester), 157.0,
2
3
1
33.5, 130.6, 128.9, 121.6, 112.2, 65.5 (CH COOCH CH ), 61.5 (CH COOCH CH ), 32.0 (CH CO), 14.1
2 2 3 2 2 3 3
+
(CH CH ); HRMS (ESI) m/z 223.0963 [M + H] , calculated for (C H O ): 223.0970.
2 3 12 15 4
3
8
.2.5. General Procedure for the Synthesis of Carboxylic Acid Derivatives of 2-salicyloylbenzofurans
f–h and 14a–f
To a solution of the ester 7f
methanol (9:1, v/v, 15 mL) was added a methanolic solution of NaOH 2N (1 mL). After 5–10 min of
stirring, the sodium salt of the carboxylic acid started to precipitate (except compound 13a , the solid
–h, 13a–f (1 mmol) in a solvent mixture of dichloromethane and
–
c

Molecules 2017, 22, 687
10 of 13
not formed). The mixture was stirred and monitored by TLC (n-hexane–dichloromethane = 1:1) until
all the ester was consumed (after 30–60 min).
Purification for product 8f–h, 14d–f: The reaction mixture was filtered under reduced
pressure, washed with dichloromethane (10 mL) to obtain white solid that was then dissolved in
dimethylformamide (DMF) (15–20 mL). The mixture was cooled, acidified to pH 3–4 with dilute HCl
to afford the respective carboxylic acid. The mixture was then filtered, washed with cold water and
dried in vacuum-heating oven to obtain the targeted product.
Purification for product 14a
with dichloromethane (10 mL
to pH 3–4 with dilute HCl to afford the respective carboxylic acid that extracted with ethyl acetate
15 mL 2). The combined organic layer was dried over anhydrous Na SO , and then evaporated
–c
: The reaction mixture was added water (40 mL) and then extracted
×
2) to remove impurities. The water solution was cooled, acidified
(
×
2
4
under vacuum to dryness. The obtained residue was dried in vacuum-heating oven to obtain the
targeted product.
2
-(2-(5,7-Dibromobenzofuran-2-carbonyl)phenoxy)acetic acid (8f): White solid, soluble in dimethyl
◦
1
sulfoxide, ethyl acetate, insoluble in water; yield 70%; m.p. 156–158 C; H-NMR (500 MHz, DMSO-d )
6
(
(
(
δ
, ppm): 8.03 (s, 1H, Ar–H), 8.01 (s, 1H, Ar–H), 7.65 (s, 1H, Ar–H), 7.60–7.57 (m, 1H, Ar–H), 7.53
d, 1H, J = 7.5 Hz, Ar–H), 7.15–7.14 (m, 2H, Ar–H), 4.72 (s, 2H, CH2); 13C-NMR (125 MHz, DMSO-d6)
δ
, ppm): 183.65 (C=Oketone), 169.5 (C=Oacid), 155.7, 153.5, 151.4, 133.0, 132.7, 129.8, 129.5, 127.1,
+
1
25.7, 121.2, 116.3, 116.3, 113.2, 105.2, 65.1 (CH ); HRMS (ESI) m/z 476.8714 [M + Na] , calculated for
2
(
C H Br NaO ): 476.8772.
17 10 2 5
2
-(2-(5,7-Dibromobenzofuran-2-carbonyl)phenoxy)propanoic acid (8g): Yellow solid, soluble in dimethyl
◦
1
sulfoxide, ethyl acetate, insoluble in water; yield 75%; m.p.: 153–155 C; H-NMR (500 MHz, DMSO-d )
6
(
(
4
δ
, ppm): 8.03 (d, 1H, J = 1.5 Hz, Ar–H), 7.99 (d, 1H, J = 1.5 Hz, Ar–H), 7.69 (s, 1H, Ar–H), 7.59–7.55
m, 1H, Ar–H), 7.53–7.51 (m, 1H, Ar–H), 7.13 (t, 1H, J = 7.5 Hz, Ar–H), 7.04 (d, 1H, J = 8.0 Hz, Ar–H),
.90 (q, 1H, J = 6.5 Hz, CH), 1.24 (d, 3H, J = 6.5 Hz, CH3); 13C-NMR (125 MHz, DMSO-d ) (
, ppm):
δ
6
1
1
83.7 (C=Oketone), 172.5 (C=Oacid), 155.4, 153.6, 151.3, 133.1, 132.7, 129.7, 127.2, 125.7, 121.0, 116.3,
−
16.0, 113.2, 105.2, 72.1 (CH3
C
H), 17.8 (CH ); HR-MS (ESI) m/z 466.8869 [M
−
H] , calculated for
3
(C H Br O ): 466.8953.
18 11 2 5
5
-((2-(5,7-Dibromobenzofuran-2-carbonyl)phenoxy)methyl)furan-2-carboxylic acid (8h): White solid, soluble
◦
1
in dimethyl sulfoxide, ethyl acetate, insoluble in water; yield 82%; m.p.: 131–133 C; H-NMR
500 MHz, DMSO-d ) ( , ppm): 8.01 (d, 1H, J = 1.5 Hz, Ar–H), 7.97 (d, 1H, J = 1.5 Hz, Ar–H),
.54–7.60 (m, 1H, Ar–H), 7.55 (s, 1H, Ar–H), 7.53 (dd, 1H, J = 1.5, 7.5 Hz, Ar–H), 7.40 (d, 1H, J = 8.5 Hz,
Ar–H), 7.16 (t, 1H, J = 7.5 Hz, Ar–H), 7.03 (d, 1H, J = 3.5 Hz, Ar–H), 6.49 (d, 1H, J = 3.5 Hz, Ar–H), 5.21
s, 2H, CH2); 13C-NMR (125 MHz, DMSO-d ) (
, ppm): 183.6 (C=Oketone), 159.0 (C=Oacid), 155.7, 153.5,
(
7
δ
6
(
δ
6
1
6
53.3, 151.3, 145.0, 133.2, 132.8, 129.7, 129.6, 127.4, 125.8, 121.4, 118.1, 116.4, 116.1, 113.8, 112.0, 105.2,
−
2.4 (CH ); HRMS (ESI) m/z 518.8745 [M − H] , calculated for (C H Br O ): 518.8902.
2
21 11
2
6
2
-(2-(Benzofuran-2-carbonyl)phenoxy)acetic acid (14a): Yellow solid, soluble in dimethyl sulfoxide, ethyl
◦
1
acetate, insoluble in water; yield 72%; m.p.: 115–117 C; H-NMR (500 MHz, CDCl ) (
δ
, ppm): 7.75
d, 1H, J = 7.5 Hz, Ar–H), 7.72 (d, 1H, J = 8.0 Hz, Ar–H), 7.63–7.58 (m, 2H, Ar–H), 7.54–7.51 (m, 2H,
Ar–H), 7.34 (t, 1H, J = 7.0 Hz, Ar–H), 7.20 (t, 1H, J = 7.5 Hz, Ar–H), 7.08 (d, 1H, J = 8.5 Hz, Ar–H), 4.79
s, 2H, CH2); 13C-NMR (125 MHz, CDCl ) (
, ppm): 184.5 (C=Oketone), 170.3 (C=Oacid), 156.6, 156.4,
52.0, 134.0, 131.2, 129.2, 126.9, 124.2, 123.6, 122.3, 118.6, 114.8, 112.6, 67.1 (CH ); HRMS (ESI) m/z
3
(
(
1
2
δ
3
2
+
97.0749 [M + H] , calculated for (C H O ): 297.0763.
17
13
5
2
-(2-(5-Methylbenzofuran-2-carbonyl)phenoxy)acetic acid (14b): Yellow solid, soluble in dimethyl sulfoxide,
◦
1
ethyl acetate, chloroform, insoluble in water; yield 66%; m.p.: 122–124 C; H-NMR (500 MHz, CDCl )
3
(δ, ppm): 7.74 (dd, 1H, J = 1.5, 7.5 Hz, Ar–H), 7.61–7.57 (m, 1H, Ar–H), 7.51–7.46 (m, 3H, Ar–H), 7.34
(
dd, 1H, J = 1.5, 7.5 Hz, Ar–H), 7.20 (t, 1H, J = 7.5 Hz, Ar–H), 7.08 (d, 1H, J = 8.0 Hz, Ar–H), 4.79 (s, 2H,
13
CH ), 2.46 (s, 3H, CH ); C-NMR (125 MHz, CDCl ) (δ, ppm): 184.5 (C=Oketone), 170.3 (C=Oacid),
2
3
3

Molecules 2017, 22, 687
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1
56.6, 155.1, 152.1, 134.0, 133.9, 131.3, 130.9, 127.1, 126.9, 123.0, 122.3, 118.5, 114.9, 112.2, 67.2 (CH ), 21.3
2
+
(CH ); HRMS (ESI) m/z 311.0909 [M + H] , calculated for (C H O ): 311.0919.
3 18 15 5
2
-(2-(5-Methoxybenzofuran-2-carbonyl)phenoxy)acetic acid (14c): Yellow solid, soluble in dimethyl
◦
1
sulfoxide, ethyl acetate, chloroform, insoluble in water; yield 73%; m.p.: 168–171 C; ; H-NMR
500 MHz, CDCl ) ( , ppm): 7.68 (dd, 1H, J = 1.0, 7.5 Hz, Ar–H), 7.57–7.54 (m, 2H, Ar–H), 7.44 (s, 1H,
Ar–H), 7.17 (t, 1H, J = 7.5 Hz, Ar–H), 7.07–7.06 (m, 2H, Ar–H), 6.95 (dd, 1H, J = 2.0, 8.5 Hz, Ar–H),
(
δ
3
13
4
.77 (s, 2H, CH ), 3.97 (s, 3H, OCH ); C-NMR (125 MHz, CDCl ) (δ, ppm): 183.8 (C=Oketone), 170.6
2 3 3
(
(
C=Oacid), 161.9, 158.3, 156.5, 151.6, 133.7, 131.0, 127.2, 124.0, 122.2, 120.4, 119.7, 115.0, 114.8, 95.5, 67.2
+
CH ), 55.8 (OCH ); HRMS (ESI) m/z 327.0853 [M + H] , calculated for (C H O ): 327.0869.
2
3
18 15
6
2
-(2-(5-Chlorobenzofuran-2-carbonyl)phenoxy)acetic acid (14d): White solid, soluble in dimethyl sulfoxide,
◦
1
ethyl acetate, insoluble in water; yield 79%; m.p.: 175–177 C; H-NMR (500 MHz, DMSO-d ) (
7
4
δ
, ppm):
.86 (s, 1H, Ar–H), 7.76 (d, 1H, J = 9.0 Hz, Ar–H), 7.56–7.49 (m, 4H, Ar–H), 7.14–7.11 (m, 2H, Ar–H),
.71 (s, 2H, CH2); 13C-NMR (125 MHz, DMSO-d ) (
, ppm): 184.0 (C=Oketone), 169.6 (C=Oacid), 155.6,
6
δ
6
1
53.7, 153.5, 132.6, 129.3, 128.5, 128.4, 128.3, 127.5, 123.0, 121.0, 115.9, 114.0, 113.0, 64.9 (CH ); HRMS
2
+
(ESI) m/z 331.0356 [M + H] , calculated for (C H ClO ): 331.0373.
17
12
5
2
-(2-(5-Bromobenzofuran-2-carbonyl)phenoxy)acetic acid (14e): White solid, soluble in dimethyl sulfoxide,
◦
1
ethyl acetate, insoluble in water; yield 81%; m.p.: 145–147 C; H-NMR (500 MHz, DMSO-d ) (
δ
, ppm):
.00 (d, 1H, J = 2.0 Hz, Ar–H), 7.71 (d, 1H, J = 8.5 Hz, Ar–H), 7.70 (dd, 1H, J = 2.0, 8.5 Hz, Ar–H),
.58–7.54 (m, 2H, Ar–H), 7.49 (dd, 1H, J = 1.5, 7.5 Hz, Ar–H), 7.14–7.11 (m, 2H, Ar–H), 4.70 (s, 2H, CH2);
6
8
7
13
C-NMR (125 MHz, DMSO-d ) (δ, ppm): 183.9 (C=Oketone), 169.6 (C=Oacid), 155.6, 154.0, 153.2, 132.6,
6
1
31.1, 129.3, 129.1, 127.5, 126.0, 121.0, 116.1, 115.7, 114.4, 113.0, 64.9 (CH ); HRMS (ESI) m/z 374.9850
2
+
[M + H] , calculated for (C H BrO ): 374.9868.
17
12
5
2
-(2-(5,7-Dichlorobenzofuran-2-carbonyl)phenoxy)acetic acid (14f): White solid, soluble in dimethyl
◦
1
sulfoxide, ethyl acetate, insoluble in water; yield 80%; m.p.: 162–164 C; H-NMR (500 MHz, DMSO-d )
6
(δ
, ppm): 7.85 (d, 1H, J = 2.0 Hz, Ar–H), 7.80 (d, 1H, J = 1.5 Hz, Ar–H), 7.64 (s, 1H, Ar–H), 7.58 (td, 1H,
J = 1.5, 8.5 Hz, Ar–H), 7.52 (dd, 1H, J = 1.5, 7.5 Hz, Ar–H), 7.16–7.13 (m, 2H, Ar–H), 4.71 (s, 2H, CH );
2
13
C-NMR (125 MHz, DMSO-d ) (δ, ppm): 183.6 (C=Oketone), 169.5 (C=Oacid), 155.7, 153.9, 149.6, 133.0,
6
1
29.5, 129.4, 128.5, 127.6, 127.1, 122.2, 121.1, 117.2, 116.2, 113.2, 65.1 (CH ); HRMS (ESI) m/z 364.9960
2
+
[M + H] , calculated for (C H Cl O ): 364.9984.
17
11
2
5
3
3
.3. Antibacterial Activity
.3.1. Measurement of the Zones of Inhibition
The in vitro qualitative (zone of inhibition) against three Gram-(+) strains (MSSA ATCC 29213,
MRSA ATCC 43300, S. faecalis ATCC 29212) and one Gram-(–) strain (E. coli ATCC 25922) was
performed for 2-salicyloylbenzofuran derivatives by modified agar well diffusion assay [23 24].
DMSO and potent antibacterial drugs ampicillin (10 g/well), cefuroxime (7.5 g/well), vancomycin
30 g/well) were used as solvent control and standards, respectively. The test compounds were
,
µ
µ
(
µ
dissolved first in DMSO with the concentration of 2048
µ
g/mL. Fifteen milliliters of the molten MHA
◦
8
(
45 C) were poured into sterile Petri dishes. The cell suspensions containing 1.5
×
10 CFU/mL cells
were prepared and spread onto the surface of the agar plates using sterile swab sticks. Once the plates
had been aseptically dried, 6 mm wells were bored using a sterile cork borer and samples (60
µ
L)
◦
were placed into the wells. The test plates were incubated at 37 C for 24 h. Antibacterial activity was
evaluated by measuring the diameter (mm) inhibitory zones around wells. The tests were performed
in duplicate.

Molecules 2017, 22, 687
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3
.3.2. Measurement of MIC Values
The quantitative (MIC) antibacterial activity against three Gram-(+) strains (MSSA ATCC 29213,
MRSA ATCC 43300, S. faecalis ATCC 29212) and one Gram-(–) strain (E. coli ATCC 25922) was
determined by agar dilution method as per CLSI recommendation on MHA containing 2-fold serial
dilutions of antibiotics and selectively active 2-salicyloylbenzofurans [25]. The compounds in the
test medium were prepared at the required quantities of 1024, 512, 256, 128, 64, 32, 16, and 8
µg/mL
concentrations and the standard drugs at 32, 16, 8, 4, 2, 1, 0.5, 0.25, 0.125, and 0.0625
µg/mL with
4
MHA. The final inoculum size was 10 CFU/mL for the antibacterial assay. All samples were tested in
◦
duplicate. The test plates were incubated at 37 C for 24 h. The MIC (expressed in
µg/mL) was taken as
the minimum concentration of the dilution with no growth of microorganisms. The concentration of the
solvents used in the following assays was maintained at 2–5% so that no inhibition of microorganisms
or interference occurred.
4
. Conclusions
In summary, we designed, synthesized twenty five 2-salicyloylbenzofuran derivatives and
evaluated for their antibacterial activities against three Gram-(+) strains (MSSA ATCC 29213, MRSA
ATCC 43300, S. faecalis ATCC 29212) and one Gram-(–) strain (E. coli ATCC 25922). Most of the
2
-salicyloylbenzofurans were found to possess weak to moderate activities against Gram-(+) and/ or
Gram-(–) bacteria with the inhibition zone diameters of 8–19 mm and MIC values 0.06 mM. Amongst
the 2-salicyloylbenzofurans bearing carboxylic acid functional group 8f and 14a displayed
≥
–
h
–f
the stronger bioactivity and showed the strain-specific to Gram-(+) bacteria with the most potent
antibacterial agent 8h (MICs = 0.06–0.12 mM). In addition, the 2-salicyloylbenzofuran scaffold was
found to exhibit activity against MRSA with the best MIC values = 0.14 mM (8f) and 0.12 mM (8h).
The antibacterial results suggested that the substituent bearing carboxylic acid group at salicyloyl-C2
and the bromine atoms on the benzofuran moiety might be the functional necessary for antibacterial
activity. Finally, although the antibacterial activities of 2-salicyloylbenzofurans in this study were
not very significant, these results initially provide some necessary information for further chemical
optimization of this scaffold in the future.
Acknowledgments: The research is funded by Vietnam National Foundation for Science and Technology
Development (NAFOSTED) under Grant number “104.01-2013.15”.
Author Contributions: P.-T.T.P., T.-T.T.N., H.-N.T.N., and T.-A.N.P. performed the chemical synthesis; T.T.V. and
D.C.T. carried out the antibacterial activity evaluation; D.C.T. and B.-K.N. L. contributed to the practical aspects of
the research work; and T.-A.N.P. supervised the research and prepared the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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α
-bromination of carbonyl compounds
Sample Availability: Samples of the compounds 4, 5, 7a–f, 8f–h, 14a–f are available from the authors.
©
2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
CC BY) license (http://creativecommons.org/licenses/by/4.0/).
(