Synthesis and anti-methicillin-resistant Staphylococcus aureus activity of 5,7-dibromo-2-benzoylbenzofurans alone and in combination with antibiotics

Article abstract of DOI:10.1080/00397911.2020.1855354

A series of 5,7-dibromo-2-benzoylbenzofurans were synthesized by the Rap–Stoermer condensation of 5,7-dibromosalicylaldehyde with diverse phenacyl bromides and evaluated for in-vitro antibacterial activities against methicillin-sensitive Staphylococcus aureus (MSSA) ATCC 29213, methicillin-resistant Staphylococcus aureus (MRSA) ATCC 43300, and MRSA ATCC 33591 by agar dilution method. The synergistic effects were determined by using the agar dilution checkerboard assay. The derivatives bearing carboxylic acid functional groups exhibited reasonable activity against MRSA strains with the best MIC = 32 μg/mL (9b, 9d). Moreover, the additive or synergistic interactions against MRSA strains was observed in six combinations (1b + cefuroxime/gentamicin, 1c + ciprofloxacin/gentamicin, 9b + gentamicin, and 9c + ciprofloxacin) with the fractional inhibitory concentration index (FICI) values in the range of 0.375–1.0. Significantly, the MICs of these antibiotics were reduced 2–4-fold. The results of the MTT assay illustrated the low mammalian cell cytotoxicity of these potent compounds.

Full text of DOI:10.1080/00397911.2020.1855354

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Synthesis and anti-methicillin-resistant
Staphylococcus aureus activity of 5,7-dibromo-2-
benzoylbenzofurans alone and in combination
with antibiotics
Phuong-Thuy T. Phan , Hong-Nhung T. Nguyen , Son N. Kim & Tuan-Anh N.
Pham
To cite this article: Phuong-Thuy T. Phan , Hong-Nhung T. Nguyen , Son N. Kim & Tuan-
Anh N. Pham (2020): Synthesis and anti-methicillin-resistant Staphylococcusꢀaureus activity
of 5,7-dibromo-2-benzoylbenzofurans alone and in combination with antibiotics, Synthetic
Communications, DOI: 10.1080/00397911.2020.1855354
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https://doi.org/10.1080/00397911.2020.1855354
Synthesis and anti-methicillin-resistant Staphylococcus
aureus activity of 5,7-dibromo-2-benzoylbenzofurans alone
and in combination with antibiotics
Phuong-Thuy T. Phanb, Hong-Nhung T. Nguyena, Son N. Kimc, and
Tuan-Anh N. Phama
aFaculty of Pharmacy, University of Medicine and Pharmacy at Ho Chi Minh City, Ho Chi Minh City,
b
c
Vietnam; Faculty of Pharmacy, Ton Duc Thang University, Ho Chi Minh City, Vietnam; Faculty of
Pharmacy, Lac Hong University, Bien Hoa City, Vietnam
ABSTRACT
ARTICLE HISTORY
Received 21 August 2020
A series of 5,7-dibromo-2-benzoylbenzofurans were synthesized by the
Rap–Stoermer condensation of 5,7-dibromosalicylaldehyde with diverse
phenacyl bromides and evaluated for in-vitro antibacterial activities
against methicillin-sensitive Staphylococcus aureus (MSSA) ATCC 29213,
methicillin-resistant Staphylococcus aureus (MRSA) ATCC 43300, and
MRSA ATCC 33591 by agar dilution method. The synergistic effects
were determined by using the agar dilution checkerboard assay. The
derivatives bearing carboxylic acid functional groups exhibited reason-
able activity against MRSA strains with the best MIC ¼ 32 mg/mL (9b,
9d). Moreover, the additive or synergistic interactions against MRSA
strains was observed in six combinations (1b þ cefuroxime/gentamicin,
1c þ ciprofloxacin/gentamicin, 9b þ gentamicin, and 9c þ ciprofloxa-
cin) with the fractional inhibitory concentration index (FICI) values in
the range of 0.375–1.0. Significantly, the MICs of these antibiotics were
reduced 2–4-fold. The results of the MTT assay illustrated the low mam-
malian cell cytotoxicity of these potent compounds.
KEYWORDS
Antibacterial activity;
antibacterial synergy;
antibiotic combination;
benzofuran; MRSA;
Rap–Stoermer condensation
GRAPHICAL ABSTRACT
CONTACT Tuan-Anh N. Pham
anh.pham81@uphcm.edu.vn; pnta@ump.edu.vn
Faculty of Pharmacy, University of
Medicine and Pharmacy at Ho Chi Minh City, Ho Chi Minh City, Vietnam
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

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P.-T. T. PHAN ET AL.
Introduction
The ongoing explosion of antibiotic resistant infections continues to plague global and
developing countries health care. Infectious diseases have been extremely difficult to
treat due to antibiotic resistant problems with related morbidity and mortality being
on the rise, particularly in low and middle-income countries.[1–4] Among the
multi-resistant bacteria, MRSA is a major cause of a variety of community- and health-
care-related infections, most notably skin, soft tissue, bone, lower respiratory tract infec-
tions.[1–3] In the USA, MRSA is associated with a staggering 90,000 infections and an
estimated 19,000 deaths annually.[2] In the European Union, MRSA infections are
estimated to affect more than 150,000 patients annually, resulting in attributable extra
in-hospital costs of EUR 380 million for EU healthcare systems.[3] Therefore, the pre-
vention and control of MRSA have been identified as public health priorities
worldwide.[1–4]
The main antibiotics to treat MRSA are vancomycin,[5,6] and several newer agents
like linezolid,[5,7] tigecycline,[5] daptomycin,[5] telavancin,[5] ceftaroline,[5] as well as qui-
nupristin/dalfopristin, a combination of two streptogramin antibiotics.[5] However, none
of these agents has been shown to be superior to vancomycin that remains the standard
drug for the treatment of most MRSA infections.[5,6] Although vancomycin is the drug
of choice for MRSA infection, it sometimes shows poor clinical outcomes or even
clinical failure in serious infections.[5,6,8] In addition, several studies have reported
resistance to the new antimicrobial agents like linezolid, and daptomycin.[8,9] In
response to this public health threat, it is necessary to have global co-operation based
on a coordinated program combining infection control with other actions, in which an
acceleration in the discovery and development of new anti-MRSA agents with new
mode of action is one of the most important main actions.[10–12] Otherwise, combin-
ation antibiotic treatment for MRSA infections is an attractive alternative as it could
address most of the vancomycin’s shortcomings. In fact, the synergistic interactions
for the majority of MRSA strains have consistently been shown in several
combinations like b-lactams–vancomycin/daptomycin,[13] minocycline–disulfiram,[14]
vancomycin–abrekacin,[15] vancomycin–durancin 61 A, a class II bacteriocin,[16] and
plectasin–aminoglycosides (gentamicin, neomycin, amikacin) or b-lactams (penicillin,
amoxicillin, flucloxacillin).[17] In addition, the combinations of aminoglycosides with
nonantibiotics like carvone,[18] nordihydroguaiaretic acid[19] also generated synergistic
effects against MRSA strains. This suggests that the discovery of drug candidates that
can revive the therapeutic potencies of approved antibiotics is a potential drug discovery
strategy to counteract antibiotic resistance evolution.
In the previous study, we have reported the synthesis and antibacterial activities of
O-ether derivatives of 2-salicyloylbenzofuran. Amongst the compounds bearing carbox-
ylic acid functional group (1a–c) were found to exhibit reasonable activity against the
strain-specific to Gram-(þ) bacteria including MRSA (Figure 1).[20] In continuation of
our work on the search for novel anti-MRSA agents, we focused on modifications of
substituents on the benzoyl moiety to explore their potent effects on antibacterial activ-
ity. Besides, this research also aims to study on synergistic effects of combining 5,7-
dibromo-2-benzoylbenzofurans with approved antibiotics against MRSA strains.

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3
Figure 1. Chemical structures of 2-salicyloylbenzofuran derivatives (1a–c).
Results and discussions
Chemistry
The 5,7-dibromo-2-benzoylbenzofuran derivatives (5a–i, 6a–d) were prepared accord-
ing to the synthetic Scheme 1. Bromination of starting material acetophenones
(2a–i) gave phenacyl bromides (3a–i)[20,21] that were cyclized with 3,5-dibromosalicy-
laldehyde (4) to obtain 5,7-dibromo-2-benzoylbenzofuran heterocycles (5a–i).[20,22]
Demethylation of 2-OCH3 in compound 5a and 5b by AlCl3 in anhydrous dichloro-
methane afforded the product 6a and 6b in good yields.[20,23,24] Meanwhile, the
O-methyl groups at C-3 or C-4 position in the compound 5c and 5d were cleavaged
under acidic condition (HBr, AcOH) to furnish the product 6c and 6d as their
respective phenolic compounds.[25]
O-etherification at 2-OH group of the phenolic compound 6b with diverse commer-
cially halogenated materials (7a–d) using K2CO3 as a base in acetone gave the desired
esters (8a–d).[20,26,27] Alkaline hydrolysis of 8a–d furnished products (9a–d) as their
respective carboxylic acid derivatives (Scheme 2).[20,28]
In this study, directed bromination regioselectivity of acetophenones with NBS under
solvent-free condition gave phenacyl bromides in good yields (68–83%). Most of the
obtained phenacyl bromides have been demonstrated already that the structure analysis
1
data (mp, H NMR) showed consistency with our results.[29–33] The DMAP-catalyzed
Rap–Stoermer condensation reaction was applied for the efficient formation of benzo-
furan heterocycle in 69–79% yields. The Williamson reaction was then used to synthe-
size desired 5,7-dibromo-2-benzoylbenzofurans bearing O-ether moieties in rather good
1
yields (66–86%). The chemical structures were characterized by H NMR, 13C NMR
and HRMS spectra that showed agreement with the expected structures and formulas of
1
the targeted products. In which, the H NMR spectra of 5,7-dibromo-2-benzoylbenzo-
furans displayed a single signal (H-3) in the range of d 7.80–7.30 ppm, which confirmed
the formation of benzofuran heterocycle.
Antibacterial activity
Evaluation of MIC values
The newly synthesized compounds were evaluated for their MICs (i.e. the lowest con-
centrations of the antimicrobials that will inhibit the visible growth of a microorganism)
against MSSA ATCC 29213, MRSA ATCC 43300 and MRSA ATCC 33591 by agar dilu-
tion method.[20,34] Dimethyl sulfoxide (DMSO) and approved antibacterial drugs (ampi-
cillin, cefuroxime, ciprofloxacin, gentamicin, and vancomycin) were used as

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P.-T. T. PHAN ET AL.
Scheme 1. Synthesis of 5,7-dibromo-2-benzoylbenzofuran derivatives (5a–i, 6a–d). Reagents and con-
ditions: (i) NBS, PTSA, 60 ꢀC, 15 min., 68–83%; (ii) DMAP, Na2CO3, H2O, 80 ꢀC, 5 h, 69–79%; (iii) AlCl3,
DCM, rt, 3 h, 82–89%; (iv) HBr, AcOH, 120 ꢀC, 8 h, 83–85%.
Scheme 2. Synthesis of carboxylic acid derivatives of 5,7-dibromo-2-(5-bromo-2-hydroxybenzoyl)ben-
zofuran (9a–d). Reagents and conditions: (i) 7a–d, K2CO3, acetone, reflux, 8 h, 66–86%; (ii) 1. NaOH,
CH2Cl2/CH3OH (9:1), rt, 1 h 2. HCl, DMF, 71–91%.
solventcontrol and standards, respectively. The antibacterial activities of twenty-one 5,7-
dibromo-2-benzoylbenzofuran derivatives expressed as MICs (all values are expressed
both in mg/mL and mmol/L), are shown in Table 1 along with the activities of the refer-
ence antibacterial drugs.
In the previous research, compound 5,7-dibromo-2-(2-hydroxybenzoyl)benzofuran (6a)
showed very week anti-S. aureus activities (MICs >1024 mg/mL).[20] To discover potent
substituents of the benzoyl moiety on the antibacterial activity, a series of 5,7-dibromo-2-
benzoylbenzofuran derivatives (5b–i, 6b–d) was generated by replacing 2-OCH3/2-OH
groups in parent compounds (5a, 6a) with different substituents or altering the substituted
position of these functional groups in the benzoyl ring. The antibacterial results showed
that when the –OCH3 (5a) or –OH (6a) was moved to 3- or 4-position, the resulting com-
pounds (5c, 5d, 6c, and 6d) were completely inactive against both MSSA and MRSA
at concentration of 2048 mg/mL. Similarly, the 2-unsubstituted benzoyl (5e) and

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Table 1. In-vitro antibacterial activities of 5,7-dibromo-2-benzoylbenzofurans.
MIC [mg/mL (mM)]
MSSA
ATCC 29213
MRSA
ATCC 43300
MRSA
ATCC 33591
Compounda
5b
5i
6a
6b
8a
9a
9 b
1024 (2.09)
>1024 (2.25)
>1024 (2.59)b
–
>1024 (1.83)
128 (0.24)
32 (0.06)
512 (1.05)
–
–
–
>1024 (2.59)b
512 (1.09)
>1024 (1.83)
256 (0.48)
32 (0.06)
>1024 (2.59)
>1024 (2.18)
>1024 (1.83)
1024 (1.92)
128 (0.23)
1024 (1.71)
256 (0.47)
256 (0.56)
256 (0.55)
128 (0.25)
16 (0.05)
9c
9d
1a
1 b
128 (0.21)
1024 (1.83)
64 (0.14)b
128 (0.21)
32 (0.06)
64 (0.14)b
128 (0.27)b
64 (0.12)b
16 (0.05)
128 (0.27)b
32 (0.06)b
1c
Ampicillin
Cefuroxime
Ciprofloxacin
Gentamicin
Vancomycin
0.25 (0.0007)
8 (0.02)
0.5 (0.0016)
2 (0.004)
0.0625 (0.00004)
16 (0.04)
0.125 (0.0004)
4 (0.008)
8 (0.02)
0.5 (0.0016)
8 (0.017)
0.0625 (0.00004)
1 (0.0007)
aOther compounds: no activity at concentration of 2048 mg/mL; bMIC values were reported previously;[20] (–): no activity
at concentration of 2048 mg/mL; MSSA: methicillin-sensitive Staphylococcus aureus; MRSA: methicillin-resistant
Staphylococcus aureus.
chloro(bromo)benzoyl derivatives (5f–h) were completely inactive as well. Introducing a
bromine atom into the C-5 position of the benzoyl residue resulting into derivatives (5b,
6b) with a minor improvement of antibacterial activity against MRSA (MIC ¼ 512 mg/
mL). Meanwhile, the presence of –NO2 at C-5 of the benzoyl residue is not favored for
anti-MRSA (5i).
In order to improve the antibacterial activity, we focused on the modification of the
2-OH phenolic group in compound 6b by etherification with different moieties bearing
carboxylic acid groups. The antibacterial activity evaluation found some remarkable
results since those bearing carboxylic acid in the 2-O-ether residue (9a–d) dramatically
increased their antibacterial activities against both MSSA ATCC 29213 and MRSA
ATCC 43300 with the MIC values ¼ 32–256 mg/mL as compared to the parent deriva-
tive 6 b (MIC ꢁ512 mg/mL). Amongst these derivatives, the best MIC value ¼ 32 mg/mL
was found on agent 9b against MSSA ATCC 29213 and agent 9b and 9d against MRSA
ATCC 43300. These values showed little improvements in anti-MRSA activity as com-
pared to the derivatives (1a–c) in the earlier report (MICs ꢁ64 mg/mL).[20] Relating to
MRSA ATCC 33591 strain, compounds (1a–c, 9a–d) were found to exhibit weak activ-
ity with the best MIC ¼ 128 mg/mL (1c, 9b). These results suggested that the presence
of substituents, in particular, carboxylic acid functional group, with a potential hydrogen
bonding capacity at the benzoyl C-2 position seems to be in favor of anti-MRSA.
Evaluation of synergistic effects
MRSA infections are really difficult to treat, while the discovery of new antimicrobial
agents has significantly decreased during the past 15–20 years.[2,3,7,8] Therefore, the
development of antibiotic enhancers to rescue existing classes of antibiotics has
exhibited as an attractive strategy.[14,18,19] In this study, we evaluated the possible
synergistic effects of six compounds (1a–c, 9a–c) in combination with approved

6
P.-T. T. PHAN ET AL.
antibacterial drugs (ampicillin, cefuroxime, ciprofloxacin, gentamicin, and vanco-
mycin) against MSSA ATCC 29213 and two MRSA strains (MRSA ATCC 43300,
MRSA ATCC 33591). The preliminarily screening of the synergistic combinations
were performed in 96 microdilution well plates containing antibiotic and targeted
1
compound with the concentration¼ = x MIC value of each agent. The results
2
showed six positive interactions including 1b þ cefuroxime/gentamicin against
MRSA ATCC 43300, and 1c þ ciprofloxacin/gentamicin, 9b þ gentamicin, 9c þ
ciprofloxacin against MRSA ATCC 33591. None of the synergistic combinations
against MSSA ATCC 29213 was observed. It was also noted that 2-benzoylbenzofur-
ans bearing acetic acid moiety at C-2 benzoyl (1a, 9a) did not cause any synergistic
effect with all tested antibiotics.
The quantitative tests of the six above interactions were performed in 96 microdilu-
tion well plates containing one antibiotic and one targeted compound in 2-fold dilu-
tions, ranging from below to the two-fold MIC values for each agent by the agar
dilution checkerboard procedure with some modifications.[18,35] The results showed that
the combination of 9b with gentamicin displayed the most synergistic interaction
against MRSA ATCC 33591 (FICI ¼ 0.375). Significantly, the MIC of gentamicin was
reduced 4-fold against this MRSA strain. Meanwhile, combinations of 1c with cipro-
floxacin or gentamicin, 9c with ciprofloxacin showed partial synergistic interactions
against MRSA ATCC 33591 with FICI values 0.75, 0.625, 0.625, respectively. Besides,
the synergistic test observed two additive interactions (FICI ¼ 1.0) against MRSA
ATCC 43300 strain when compound 1b was used in combination with cefuroxime or
gentamicin. The rates in increasing susceptibility of MRSA with antibiotics were two-
fold in these cases (Table 2).
Evaluation of cell cytotoxicity
The mammalian cell cytotoxicity of compounds (1b, 1c, 9b, and 9c) which showed the most
potential synergism with antibiotics against MRSA strains, were further determined in
human breast adenocarcinoma cell line (MDA-MB-231, ATCC HTB-26TM) using the
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MTT method.[37] As shown in Table 3, all tested compounds (1b, 1c, 9b, and 9c) showed
Table 2. Synergistic effects of 5,7-dibromo-2-benzoylbenzofurans with antibiotics against S. aureus.
MIC [mg/mL (mM)]
Straina
MRSA
Agent
Alone
Combination
FICIb
1
Interpretation
Additive
1b
Cefuroxime
1b
Gentamicin
1c
128 (0.27)
16 (0.04)
128 (0.27)
4 (0.008)
128 (0.25)
0.5 (0.0016)
128 (0.25)
8 (0.008)
128 (0.23)
8 (0.008)
64 (0.13)
8 (0.02)
ATCC 43300
64 (0.13)
2 (0.004)
32 (0.06)
0.25 (0.0008)
16 (0.03)
4 (0.004)
16 (0.03)
2 (0.002)
128 (0.23)
0.25 (0.0008)
1
Additive
MRSA
ATCC 33591
0.75
Partial synergy
Partial synergy
Synergy
Ciprofloxacin
1c
0.625
0.375
0.625
Gentamicin
9b
Gentamicin
9c
1024 (1.71)
0.5 (0.0016)
Partial synergy
Ciprofloxacin
aMRSA: methicillin-resistant Staphylococcus aureus.
bFICI was interpreted as synergistic interaction (FICI ꢂ 0.5), partial synergistic interaction (0.5 < FICI < 1), additive inter-
action (FICI ¼ 1.0), indifferent interaction (1 < FICI < 4) or antagonistic interaction (FICI ꢁ4.0).[18,36]

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Table 3. IC50 (mM)a values of compounds against human breast adenocarcinoma cell line (MDA-MB-231).
Compound
IC50 (mM)
1b
1c
9 b
97.2
>100
88.9
9c
>100
Paclitaxel
1
aIC50 is defined as the concentration at which the 50% of growth is inhibited, the highest tested concentration
was 100 mM
very weak cytotoxicity in the MDA-MB-231 cell line with IC50 values ꢁ88.9 mM. The low
cytotoxicities of these compounds exhibited their good safety profiles, which indicate that
they could be further developed as antibiotic enhancers against MRSA strains.
Conclusions
In summary, we synthesized a new series of 5,7-dibromo-2-benzoylbenzofuran derivatives
and determined their antibacterial activities against MSSA and MRSA strains. These com-
pounds were found to possess weak to moderate activities against MSSA and MRSA
strains. The derivatives bearing carboxylic acid functional group (9b, 9d) exhibited the
most potent activity against MRSA ATCC 43300 with MICs ¼ 32 mg/mL. The antibacterial
structure-activity relationship showed the substituent at benzoyl-C2, as well as the number
and position of bromine atoms on the benzofuran and benzoyl moiety seem to be neces-
sary features for anti-MRSA potency. In this study, 5,7-dibromo-2-benzoylbenzofuran
derivatives did not show much significant activities against MSSA and MRSA alone.
However, compounds (1b, 1c, 9b, and 9c) when used in combination with approved anti-
biotics exhibited significant synergistic activities in some cases (1b þ cefuroxime/gentami-
cin, 1c þ ciprofloxacin/gentamicin, 9b þ gentamicin, and 9c þ ciprofloxacin) with the
FICI values in the range of 0.375–1.0. The cytotoxicity evaluation showed the low
mammalian cell cytotoxicity of these potent compounds. These findings suggested that
2-benzoylbenzofurans could potentially be used as antibiotic enhancers to the activity of
existing antibiotics, in particular gentamicin. Significantly, revival of these antibiotics in
treatment of MRSA infections would allow vancomycin usage reduction which should be
used as the last resort.
Experimental section
Chemicals and instruments
All starting materials were purchased from commercial sources such as Acros Organics-
Thermo Fisher Scientific (Geel, Belgium), Sigma-Aldrich Pte. Ltd. (St. Louis, MO,
USA), TCI Chemicals (Tokyo, Japan) and used without further purification. Thin-layer
chromatography (TLC) was carried out on aluminium-supported silica gel plates
(Merck 60 F 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

8
P.-T. T. PHAN ET AL.
uncorrected. NMR spectra were recorded in CDCl3 or DMSO-d6 using a Bruker Avance
500 MHz spectrometer (Bruker Corporation, Billerica, MA, USA). Chemical shifts (d)
are reported in ppm related to internal tetramethylsilane (TMS) and coupling
constant (J) are reported in Hertz (Hz). Mass spectra were recorded on a Shimadzu
LCMS-IT-TOF Mass spectrometer (Shimadzu Scientific Instruments, Kyoto, Japan).
Synthesis of ester derivatives (8a–d)
A mixture of 6b (1 mmol), halogenated material (7a–d) (1.2 mmol) (ethyl chloroacetate
(7a), ethyl 2-bromopropanoate (7b), methyl 5-(chloromethyl)furan-2-carboxylate (7c),
ethyl 4-bromocrotonate (7d)) and anhydrous K2CO3 (0.276 g, 2.0 mmol) in dry acetone
(10 mL) was refluxed for 8 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 K2CO3 and then extracted with dichloromethane (3 ꢃ 15 mL). The
organic layer was dried over anhydrous Na2SO4, evaporated under vacuum to dryness.
The obtained residue was purified by column chromatography (n-hexane : dichlorome-
thane ¼ 1:1, v/v) to give pure product.
Ethyl
2-(4-bromo-2-(5,7-dibromobenzofuran-2-carbonyl)phenoxy)ethanoate
(8a):
Yellow viscous liquid, soluble in dichloromethane, chloroform, acetone, ethyl acetate,
insoluble in water; yield 70%; 1H NMR (500 MHz, CDCl3) (d, ppm): 7.79 (s, 1H,
Ar–H), 7.76 (s, 1H, Ar–H), 7.66 (s, 1H, Ar–H), 7.61 (d, 1H, J ¼ 4.0 Hz, Ar–H), 7.55 (s,
1H, Ar–H), 6.80 (d, 1H, J ¼ 4.0 Hz, Ar–H), 4.61(s, 2H, CH2C¼O), 4.13 (q, 2H,
J ¼ 7.0 Hz, CH2CH3), 1.19 (t, 3H, J ¼ 7.0 Hz, CH3); 13C NMR (125 MHz, CDCl3) (d,
ppm): 182.3 (C¼Oketone), 167.8 (C¼Oester), 155.2, 153.9, 152.1, 135.6, 133.4, 132.9,
129.7, 125.0, 116.9, 115.4, 114.7, 114.3, 105.9, 66.2 (CH2), 61.6 (CH2), 14.0 (CH3);
HRMS (ESI) m/z 560.7912 [M þ H]þ, calculated for (C19H14Br3O5): 560.8371.
Synthesis of carboxylic acid derivatives (9a–d)
To a solution of the ester (8a–d) (1 mmol) in a solvent mixture of dichloromethane and
methanol (9:1, v/v, 15 mL) was added a methanolic solution of NaOH 2 N (1 mL). After
5–10 min of stirring, the sodium salt of the carboxylic acid started to precipitate. The
mixture was stirred and monitored by TLC (n-hexane : dichloromethane ¼ 1:1, v/v)
until all the ester was consumed (after 30–60 min.). 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 filtered, washed with cold water and dried in vacuum-heating oven to
obtain the targeted product.
2-(4-Bromo-2-(5,7-dibromobenzofuran-2-carbonyl)phenoxy)ethanoic acid (9a): White
solid, soluble in ethyl acetate, dimethyl sulfoxide, insoluble in water; yield 77%; mp:
1
200–203 ꢀC; H NMR (500 MHz, DMSO-d6) (d, ppm): 8.02 (d, 1H, J ¼ 1.5 Hz, Ar–H),
8.00 (d, 1H, J ¼ 1.5 Hz, Ar–H), 7.74 (dd, 1H, J ¼ 2.5 Hz, 9.0 Hz, Ar–H), 7.71–7.70 (m,
2H, Ar–H), 7.13 (d, 1H, J ¼ 9.0 Hz, Ar–H), 4.72 (s, 2H, CH2); 13C NMR (125 MHz,
DMSO-d6) (d, ppm): 182.0 (C¼Oketone), 169.3 (C¼Oacid), 155.0, 153.0, 151.5, 135.2,

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9
132.9, 131.6, 129.7, 129.0, 125.8, 117.0, 116.4, 115.7, 112.4, 105.2, 65.4 (CH2); HRMS
(ESI) m/z 530.7685 [M–H]–, calculated for (C17H8Br3O5): 530.7901.
1
Full experimental detail, H NMR, 13C NMR, and HRMS spectra for this article can
be found via the “Supplemental material” section on the publisher’s website.
Funding
The research is funded by Vietnam National Foundation for Science and Technology
Development (NAFOSTED) under [Grant number “104.01-2013.15”].
ORCID
Tuan-Anh N. Pham
http://orcid.org/0000-0002-1342-3841
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