Synthesis and antibacterial activity of 4,6-dimethoxy-2-(4-methoxyphenyl)benzofuran-3-yl-methanone derivatives and its intermediates

Article abstract of DOI:10.14233/ajchem.2015.18645

Encouraged by the interesting biological activities associated with benzo[b]furan derivatives, we report here the synthesis, spectroscopic identification and antibacterial activity of benzo[b]furan derivatives (6a-6g) prepared from commercially available,

Full text of DOI:10.14233/ajchem.2015.18645

Asian Journal of Chemistry; Vol. 27, No. 6 (2015), 2245-2248
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.18645
Synthesis and Antibacterial Activity of
4,6-Dimethoxy-2-(4-methoxyphenyl)benzofuran-3-yl-methanone Derivatives and its Intermediates
1,2
1,*
2
3
3
V. KRISHNA REDDY , J. VENKATESWARA RAO , L. BHASKAR REDDY , B. RAM and B. BALRAM
1Department of Chemistry, Bapatla Engineering College, Bapatla-522 101, India
2Medicinal Chemistry Laboratory, GVK Biosciences Pvt. Ltd., IDA, Nacharam, Hyderabad-500 076, India
3Green Evolution Laboratories, Wangapally Village, Nalgonda-500 085, India
*Corresponding author: E-mail: drjvraobec2013@gmail.com
Received: 27 August 2014;
Accepted: 25 November 2014;
Published online: 17 March 2015;
AJC-17003
Encouraged by the interesting biological activities associated with benzo[b]furan derivatives, we report here the synthesis, spectroscopic
identification and antibacterial activity of benzo[b]furan derivatives (6a-6g) prepared from commercially available, 3,5-dimethoxyphenol
and 1-ethynyl-4-methoxybenzene. The synthesized targets were screened for their antibacterial activity against Escherichia coli,
Pseudomonas aeruginosa, Staphylococcus aureus and Streptococus pyogenes, while using norfloxacin, as the standard drug. In general, it is
observed that benzo[b]furan derivatives (6a, 6b, 6c) having R = 2,4-dimethoxy phenyl, 2,4,6-trimethoxy phenyl and 3,4,5-trimethoxy phenyl
rings, respectively displayed excellent activity against these microbial species while compound 5 (having benzo[b]furan motif) showed good
activity and the compound 2 (having -OH group) and compound 4 (having acetate group) exhibited moderate antibacterial activity.
Keywords: Antibacterial activity, Benzo[b]furan, 3,5-Dimethoxy phenol, Norfloxacin.
INTRODUCTION
EXPERIMENTAL
The benzo[b]furans are significant heterocyclic compounds,
which not only act as key structural subunits in naturally occu-
rring compounds that show remarkable biological activities
but also signify useful building blocks in the synthesis of
natural products1-4. Numerous synthetically challenging and
medicinally important chemicals contain benzo[b]furan moiety
as structural units5. Some of the properties of pharmaceutically
active molecules, which contains benzo[b]furans are antifungal
properties6,7, inhibition of 5-lipoxygenase (5-LO)8, antitumor
properties9 and therefore these type of compounds can be used
for treatment of type-2 diabetes, cardiovascular disease, cancer,
dementia, migraines and anxiety10,11. Benzo[b]-furan-based
molecules have also been disclosed as promising drugs against
Parkinson's12 andAlzheimer's disease, the inhibitors of β-amyloid
(Ab) aggregation13,14 and cyclooxy-genase-2 (COX-2)15. The
present paper describes the synthesis, spectroscopic identifi-
cation and antibacterial activity of benzo[b]furan derivatives
(6a-6g) prepared from commercially available 3,5-dimethoxy-
phenol and 1-ethynyl-4-methoxy-benzene (Scheme-I). The
synthesized targets were screened for their antibacterial activity
against Escherichia coli, Pseudomonas aeruginosa, Staphylo-
coccus aureus and Streptococus pyogenes, while using norflo-
xacin, as the standard drug.
The solvents were purified according to standard proce-
dures prior to use and all commercial chemicals were used as
received. For thin-layer chromatography (TLC) analysis,
Merck pre-coated plates (silica gel 60 F254) were used and
spots were visualized under UV light. Merck silica gel (100-
200 mesh) was used for flash column chromatography and
the eluting solvents are indicated in the procedures. Melting
point determinations were performed by using Mel-temp appa-
ratus and are uncorrected. 1H NMR spectra were recorded in
varian MR-400 MHz instrument. Chemical shifts are reported
in δ parts per million (ppm) downfield from tetrame-thylsilane
(TMS) with reference to internal standard and coupling cons-
tants in Hz. The mass spectra were recorded on Agilent ion
trap MS. Infrared (IR) spectra were recorded on a Perkin Elmer
FT-IR spectrometer. All the aromatic acid chlorides used for
the preparation of 6a-6g were purchased from commercial
sources.
Synthesis of 2-iodo-3,5-dimethoxyphenol (2): To a stirred
solution of compound 1 (5 g, 32.46 mmol) in acetonitrile (50
mL) was added ceric ammonium nitrate (1.77 g, 3.24 mmol)
followed by addition of iodine (8.21 g, 32.46 mmol) in portion
wise. The reaction mixture was stirred at room temperature
for 7 h. After completion of the reaction, the reaction contents

2246 Reddy et al.
Asian J. Chem.
were quenched with saturated sodium thiosulphate, extracted
with EtOAc (2 × 60 mL). The organic layers was dried over
anhydrous Na2SO4, filtered and evaporated under pressure. The
crude compound was purified by column chromatography
using 100-200 silica gel and eluted with 3 % EtOAc/pet ether
to afford compound 2 as pale yellow solid; Yield: 3 g, 33 %;
m.p. 76-77 °C. IR (KBr, νmax, cm-1): 3420, 3364, 3097, 2935,
2841, 1584, 1462, 1428, 1343, 1320, 1209, 1152, 1088, 1031,
General procedures for preparation of benzoyl chloride
(a-g): To a stirred solution of benzoic acid in dichloromethane
(DCM) was added oxalyl chloride at 0 °C and stirred for 3-4 h
at an ambient temperature and then the reaction mixture was
evaporated under pressure to obtain the corresponding benzoyl
chloride which was used in the next step without further
purification.
General experimental procedure for the synthesis of
benzo[b]furan derivatives (6a-6g): To a solution of com-
pound 5 (50 mg, 0.17 mmol) in dichloromethane (2 mL) was
added benzoyl chloride (a-g) (1.2 eq) followed by SnCl4 (1.0
eq) dropwise at 0 °C. The reaction mixture was stirred at room
temperature for 3 h. The reaction contents were quenched with
ice and stirred for 1 h, the organic layer was separated and the
aqueous layer was extracted with dichloromethane. The com-
bined organic extracts was dried over anhydrous Na2SO4, filtered
and evaporated under reduced pressure. The purification was
performed by flash chromatography over 100-200 silica gel
and eluted with 27 % EtOAc/pet ether to afford compound
6a-6g.
1
976, 927, 819, 659, 573, 526, 469; H NMR (400 MHz,
CDCl3): δ 10.20 (s, 1H), 6.36 (s, 1H), 6.12 (s, 1H), 3.78 (s,
3H), 3.82 (s, 3H); ESI-MS: m/z, 281.06 (M + 1).
Synthesis of 2-iodo-3,5-dimethoxyphenyl acetate (3):
To a solution of compound 2 (4 g, 14.28 mmol) in dichloro-
methane (40 mL) was added pyridine (2.22 g, 28.57 mmol)
followed by drop wise addition of acetic anhydride (2.91 g,
28.57 mmol) at 0 °C. The reaction mixture was stirred for 2 h
at ambient temperature and poured in to dichloromethane,
washed with 5 % NaHCO3 and brine solution. The organic layer
was dried over anhydrous Na2SO4, filtered and evaporated under
reduced pressure. The purification was carried out by flash
chromatography using over 100-200 silica gel and eluted with
9 % EtOAc /Pet ether to afford compound 3. White solid;Yield:
2.3 g, 50 %; m.p. 126-128 °C. 1H NMR (400 MHz, CDCl3): δ
6.38 (s, 2H), 3.78 (s, 3H), 3.84 (s, 3H), 2.38 (s, 3H); ESI-MS:
m/z, 323.06 (M + 1).
(4,6-Dimethoxy-2-(4-methoxyphenyl)-benzofuran-3-
yl)(2,4-dimethoxyphenyl)methanone (6a): Off white solid;
m.p.: 136-138 °C; IR (KBr, νmax, cm-1): 2940, 2839, 1606,
1503, 1456, 1402, 1302, 1252, 1216, 1169, 1107, 1028, 983,
933, 834, 790, 640, 578, 521; 1H NMR (400 MHz, CDCl3):
7.70 (d, J = 6.8 Hz, 1H), 7.56 (d, J = 7.2 Hz, 2H), 6.88 (m,
3H), 6.58 (d, J = 6.8 Hz, 1H), 6.42 (s, 2H), 6.38 (s, 1H), 4.02
Synthesis of 3,5-dimethoxy-2-(2-(4-methoxyphenyl)-
ethynyl)phenyl acetate 4: To a solution of compound 3 (700
mg, 2.17 mmol) in DMF (5 mL) was added 1-ethynyl-4-
methoxybenzene (370 mg, 2.79 mmol), triethylamine (440 mg,
4.31 mmol) followed by CuI (21 mg, 0.1 mmol) and Pd(PPh3)2Cl2
(30 mg, 0.043 mmol). The reaction mixture was heated at 100
°C for 5 h and then slowly warmed to an ambient temperature,
diluted with water, extracted with EtOAc. The organic layer
was dried over anhydrous Na2SO4, filtered and evaporated
under reduced pressure. The purification was carried out by
flash chromatography over 100-200 silica gel and eluted with
16 % EtOAc/pet-ether to afford compound 4 as an off white
13
(s, 3H), 3.82 (s, 3H), 3.78 (s, 6H), 3.60 (s, 3H); C NMR:
163.9, 160.8, 159.4, 157.3, 154.5, 154.3, 154.1, 133.5, 125.7,
123.7, 123.1, 114.0, 113.6, 109.8, 104.5, 98.5, 96.1, 91.2
(2C), 57.3, 55.4, 55.2 (2C); ESI-MS: m/z, 449.3 (M + 1).
(4,6-Dimethoxy-2-(4-methoxyphenyl)benzofuran-3-
yl)(2,4,6-trimethoxyphenyl)methanone (6b): 1H NMR (400
MHz, CDCl3): 7.72 (d, J = 6.8 Hz, 1H), 7.62 (d, J = 7.2 Hz,
2H), 6.88 (m, 3H), 6.58 (d, J = 6.8 Hz, 1H), 6.42 (s, 2H), 6.38
(s, 1H), 6.26 (s, 2H), 4.02 (s, 3H), 3.82 (s, 3H), 3.78 (s, 6H),
3.60 (s, 3H); ESI-MS: m/z, 479.2 (M + 1).
(4,6-Dimethoxy-2-(4-methoxyphenyl)benzofuran-3-
yl)(3,4,5-trimethoxyphenyl)methanone (6c): 1H NMR (400
MHz, CDCl3): 7.74 (d, J = 6.8 Hz, 1H), 7.60 (d, J = 7.2 Hz,
2H), 6.88 (m, 3H), 6.58 (d, J = 6.8 Hz, 1H), 6.42 (s, 2H), 6.38
(s, 1H), 6.24 (s, 2H), 4.02 (s, 3H), 3.82 (s, 3H), 3.78 (s, 6H),
3.60 (s, 3H); ESI-MS: m/z, 479.2 (M + 1).
(4,6-Dimethoxy-2-(4-methoxyphenyl)benzofuran-3-
yl)(2-methoxyphenyl)methanone (6d): Off white solid; m.p.:
113-114 °C; 1H NMR (400 MHz, CDCl3): 7.74 (d, J = 6.8 Hz,
1H), 7.50 (d, J = 7.2 Hz, 2H), 6.82 (m, 3H), 6.50 (d, J = 6.8
Hz, 1H), 6.40 (s, 2H), 6.32 (s, 2H), 4.02 (s, 3H), 3.82 (s, 3H),
3.78 (s, 6H), 3.60 (s, 6H); ESI-MS: m/z, 553.4 (M + 1).
(4,6-Dimethoxy-2-(4-methoxyphenyl)benzofuran-3-
yl)(4-methoxyphenyl)methanone (6e): Off white solid; m.p.:
123-124 °C; 1H NMR (400 MHz, CDCl3): 7.65-7.06 (m, 2H),
7.45-7.42 (m, 2H), 7.08-7.02 (m, 1H), 6.90 (d, J = 6.8 Hz,
2H), 6.75 (d, J = 6.8 Hz, 2H), 6.25 (s, 1H), 3.78 (s, 3H), 3.70
(s, 3H), 3.64 (s, 3H), 3.60 (s, 3H); ESI-MS: m/z, 553.4 (M + 1).
(4,6-Dimethoxy-2-(4-methoxyphenyl)benzofuran-3-
yl)(4-fluorophenyl)methanone (6f): Pale yellow solid; m.p.:
108-109 °C; 1H NMR (400 MHz, CDCl3): 8.13-8.06 (m, 1H),
7.93-7.89 (m, 2H), 7.50 (d, J = 6.8 Hz, 2H), 7.17-7.07 (m,
1
solid; Yield: 0.5 g, 71 %; m.p: 124-126 °C. H NMR (400
MHz, CDCl3): δ 7.42 (d, J = 7.8 Hz, 2H), 6.82 (d, J = 7.8 Hz,
2H), 6.38 (s, 1H), 6.32 (s, 1H), 3.80 (s, 3H), 3.84 (s, 3H), 2.38
(s, 3H); ESI-MS: m/z, 327.3 (M + 1).
Synthesis of 4,6-dimethoxy-2-(4-methoxyphenyl)ben-
zofuran (5): To a solution of compound 4 (500 mg, 1.53 mmol)
in methanol (7 mL) was added potassium carbonate (530
mg, 3.83 mmol) and and heated to 60 °C for 16 h. The excess
methanol was evaporated under reduced pressure, diluted with
water and extracted with EtOAc. The organic layer was dried
over anhydrous Na2SO4, filtered and evaporated under reduced
pressure. The purification was performed by flash chromato-
graphy over 100-200 silica gel and eluted with 20 % EtOAc/
pet ether to afford compound 5 as an off white solid; Yield:
0.37 g, 86 %; m.p. 165-167. IR (KBr, νmax, cm-1) 3424, 2932,
2834, 1612, 1499, 1456, 1418, 1315, 1256, 1211, 1140, 1104,
1
1030, 886, 836, 791, 618, 563, 512; H NMR (400 MHz,
CDCl3): δ 7.68 (d, J = 7.8 Hz, 2H), 6.98 (d, J = 7.8 Hz, 2H),
6.84 (s, 1H), 6.70 (s, 1H), 6.30 (s, 3H), 3.88 (s, 3H), 3.82 (s,
13
6H); C NMR: δ 159.3, 158.7, 156.2, 153.7, 153.2, 125.6,
123.6, 114.1, 113.3, 97.0, 94.1, 88.2; ESI-MS: m/z, 285.17
(M + 1).

Vol. 27, No. 6 (2015)
Synthesis of 4,6-Dimethoxy-2-(4-methoxyphenyl)benzofuran-3-yl-methanone Derivatives 2247
2H), 6.88 (m, 2H), 6.40 (s, 1H), 4.03 (s, 3H), 3.82 (s, 6H),
3.60 (s, 6H); ESI-MS: m/z, 407.2 (M + 1).
for 20 h. Paper discs (6 mm, punched from Whatmann No. 1
paper) were ultraviolet sterilized and used for the assays. Discs
were soaked in different concentration of the test solution and
placed on the inoculated agar media at regular intervals of 6-7
cm, care was taken to ensure that excess solution was not on
the discs. The plates were incubated at 37 °C in an inverted
fashion. Activity was determined by zones showing complete
inhibition (mm). Growth inhibition was calculated with
reference to positive control. All the samples were taken in
triplicates.
(4,6-Dimethoxy-2-(4-methoxyphenyl)benzofuran-3-
yl)(4-chlorophenyl)methanone (6g): Pale yellow solid; m.p.:
1
98-99 °C; H NMR (400 MHz, CDCl3): 8.03-8.00 (m, 1H),
7.90-7.86 (m, 2H), 7.44 (d, J = 6.8 Hz, 2H), 7.22-7.18 (m,
2H), 6.88 (m, 2H), 6.40 (s, 1H), 4.03 (s, 3H), 3.82 (s, 6H),
3.60 (s, 6H); ESI-MS: m/z, 423.2 (M + 1).
Biological assay
All the synthesized new benzo[b]furan derivatives and
its intermediates was dissolved in dimethyl sulphoxide at 25
µg/mL concentration and were tested against two Gram-nega-
tive strains viz., Escherichia coli (MTCC443), Pseudomonas
aeruginosa (MTCC424) and two Gram positive strains viz.,
Staphylococcus aureus (MTCC96) and Streptococcus pyogenes
(MTCC442) using agar well diffusion method according to
the literature protocol16-18. The composition of nutrient agar
medium was yeast extract (5 g), NaCl (10 g), bactotryptone
(10 g), final pH 7.4. After 18 h, the exponentially growing
cultures of the four bacteria in nutrient broth at 37 °C were
diluted in sterile broth. From each of these diluted cultures,
1 mL was added to 100 mL sterilized and cooled nutrient agar
media to give a final bacterial count of 1 × 106 cell/mL. The
plates were set at room temperature and later dried at 37 °C
RESULTS AND DISCUSSION
Benzo[b]furan chalcone derivatives 6a-6g described in
this paper were prepared according to the synthetic route
(Scheme- I). The structures of the synthesized intermediates
and new benzo[b]furan derivatives were confirmed by 1H NMR,
mass and IR data. Iodination of 3,5-dimethoxy phenol (1) was
carried out using iodine in presence of catalytic quantity of
cerric ammonium nitrate in acetonitrile at room temperature
for 7 h to obtain the iodide compound 2. Acetylation of iodo-
phenol (2) was carried out in presence of acetic anhydride in
pyridine to afford the corresponding acetate derivative 3. Sono-
gashira coupling of compound 3 with 1-ethynyl-4-methoxy-
benzene in presence of Pd(PPh3)2Cl2/CuI catalyst load followed
O
O
O
O
O
I
I
b
c
a
O
O
O
OH
O
OH
O
O
O
O
3
4
1
2
O
O
O
R
c
d
O
O
O
O
O
O
5
6a - 6g
O
O
O
O
O
O
O
O
O
c
a
d
b
R =
F
O
Cl
f
g
e
Scheme-I: Synthesis of benzo[b]furan derivatives 6a-6g

2248 Reddy et al.
Asian J. Chem.
TABLE-1
ANTIBACTERIAL ACTIVITY OF INTERMEDIATES AND COMPOUNDS 6a-6g
Gram-negative bacteria
Gram-positive bacteria
Compound No.
E. coli MTCC 443
P. aeruginosa MTCC 424
S. aureus MTCC 96
S. pyogenes MTCC 442
18
–
17
–
21
–
17
–
2
3
b
19
23
26
26
27
19
18
–
17
18
23
21
22
17
17
–
20
24
27
27
26
20
21
–
17
17
23
21
21
17
17
–
4
5
6a
6b
6c
6d
6e
6f
6g
–
25
–
19
–
25
–
19
Norfloxacina
aConcentration: 25 µg/mL of DMSO; b–No activity
by the intramolecular cyclisation in presence of potassium
carbonate resulted in the formation of benzo[b]furan5. Treat-
ment of commercially available benzoic acids with SOCl2
followed by the addition of 5 and tin(IV) chloride, using the
Friedel Crafts procedure furnished compounds 6a-6g.
Experimental conditions: (a) I2, Cerric ammonium nitrate,
acetonitrile, room temperature, 7 h; (b) Ac2O, pyridine, r.t.,
2 h; (c) 1-ethynyl-4-methoxybenzene, Pd(PPh3)2Cl2, CuI,
triethylamine, DMF, 100 °C, 5 h; (d) K2CO3, Methanol, 60 °C,
16 h; (e) PhCOCl (a-g), SnCl4, dichloromethane, room tempe-
rature, 3 h.
2 (having -OH group) and compound 4 (having acetate group)
exhibited moderate activity.
ACKNOWLEDGEMENTS
One of the authors (KR), thanks GVK Biosciences Pvt,
Ltd. for the analytical support.
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Conclusion
The present paper describes the synthesis, spectroscopic
identification and antibacterial activity of benzo[b]furan
derivatives (6a-6g) prepared from commercially available 3,5-
dimethoxyphenol and 1-ethynyl-4-methoxybenzene (Scheme-
I) and tested for antibacterial activity., it is observed that
benzo[b]furan derivatives 6a, 6b, 6c having R = 2,4-dimethoxy
phenyl, 2,4,6-trimethoxy phenyl and 3,4,5-trimethoxy phenyl
rings, respectively displayed excellent activity against, Esche-
richia coli, Pseudomonas aeruginosa, Staphylococcus aureus
and Streptococcus pyogenes, while compound 5 (having
benzo[b]furan motif) showed good activity and the compound