A new structural alternative in benzo[b]furans for antimicrobial activity

Article abstract of DOI:10.1016/j.bmc.2005.05.009

Two series of 2-substituted and three new diacetyl benzofurans were synthesized through palladium-catalyzed reactions and their in vitro antimicrobial spectra were assessed. The compounds demonstrated mild to significant growth inhibition against antibiotic-susceptible standard and clinically isolated strains of Gram-positive and Gram-negative bacteria as well as human fungal pathogens. Ampicillin and kanamycin were used as references for antibacterial screening; nystatin and amphotericin B were used for antifungal screening. Varying substitution at the benzofuran moiety and subsequent antimicrobial screening identified the C-3-acetyl functionality as a new structural alternative for optimal antimicrobial property in the benzofuran class of compounds.

Full text of DOI:10.1016/j.bmc.2005.05.009

Bioorganic & Medicinal Chemistry 13 (2005) 4796–4805
A new structural alternative in benzo[b]furans for
antimicrobial activity
a
M. Wahab Khan, M. Jahangir Alam, M. A. Rashid and R. Chowdhury
a
b
b,
*
a
Department of Chemistry, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh
Department of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Dhaka, Dhaka 1000, Bangladesh
b
Received 24 September 2003; revised 8 May 2005; accepted 9 May 2005
Available online 17 June 2005
Abstract—Two series of 2-substituted and three new diacetyl benzofurans were synthesized through palladium-catalyzed reactions,
and their in vitro antimicrobial spectra were assessed. The compounds demonstrated mild to significant growth inhibition against
antibiotic-susceptible standard and clinically isolated strains of Gram-positive and Gram-negative bacteria as well as human fungal
pathogens. Ampicillin and kanamycin were used as references for antibacterial screening; nystatin and amphotericin B were used for
antifungal screening. Varying substitution at the benzofuran moiety and subsequent antimicrobial screening identified the C-3-acetyl
functionality as a new structural alternative for optimal antimicrobial property in the benzofuran class of compounds.
ꢀ
2005 Elsevier Ltd. All rights reserved.
1. Introduction
chemotherapeutic properties as well as their widespread
2
,3
occurrences in nature. Attempts were made to study
the effects of different functional groups on the homocy-
cle and/or the heterocycle for bioactivity. The antimicro-
bial activity of benzofuran derivatives appears to be
dependent on substitution at the heterocyclic furan ring
than the aromatic moiety. Most of the early SAR rela-
tionship studies on benzofuran were concentrated to
variable nitro substitutions. The 3-nitrobenzofurans
were always less active than the corresponding 2-nitro
compounds both as antibacterials and protozoacides in
vitro. This demonstrated the specificity of nitro position
Human struggle against the affliction of disease, decay
and death is eternal. The deterioration of human popu-
lation due to an enhanced prevalence of infectious
1
diseases is becoming a global problem. The contempo-
rary treatment of infectious diseases involves adminis-
tration of a multidrug regimen over a long period of
time, which has led to the rapid emergence of multi-
drug-resistant strains plus a high level of patient
1
noncompliance. The rising prevalence of multidrug-
resistant superbugs like methicillin-resistant Staphylo-
coccus aureus (MRSA) and vancomycin-resistant
Enterococcus faecium (VRE) continues to provide impe-
tus for the search and discovery of novel antimicrobial
agents. A potential approach to overcome this resistance
problem is to design new and innovative agents with a
completely different mode of action so that no cross-re-
sistance with the present therapeuticals can occur.
4
for bioactivity. Recently, the use of benzofurans as fun-
gal N-myristoyltransferase (Nmt) inhibitors against hu-
man pathogenic Candida albicans has led to a novel
5
–7
group of fungicides. There are a few other biomolec-
ular targets discovered for benzofurans to have in vitro
antibacterial activity as well. Examples include bacterial
enzymes involved in methionine cycle such as methio-
8
9
nine aminopeptidase, deformylase, enzymes involved
in peptidoglycan synthesis such as UDP-N-acetylmura-
Benzofurans have drawn considerable attention over the
last few years due to their profound physiological and
1
0
myl-L-alanine ligase, and chorismate synthase, an en-
zyme in the shikimate pathway, essential for bacterial
1
1
viability. As a result, there is a growing interest in
developing general and versatile methods for the synthe-
sis of 2-benzofurans. Various classical and metal-medi-
ated procedures have been developed over the years
Keywords: Benzofuran; Friedel–Crafts acylation; Terminal alkynes;
Antimicrobial; Cytotoxicity.
*
Corresponding author at present address: Oxford Centre for Mole-
cular Sciences and the Department of Chemistry, Chemistry
Research Laboratory, Oxford OX1 3TA, UK. Tel.: +44 01865
1
2
for elaborating the benzofuran structures. For the last
few decades palladium-catalysed reactions have been of
275677; fax: +44 01865 28500; e-mail: rzchy@dhaka.net
0
968-0896/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2005.05.009

M. Wahab Khan et al. / Bioorg. Med. Chem. 13 (2005) 4796–4805
4797
1
3
great significance in carbon–carbon and carbon–het-
1
eroatom bonds formation. The heteroannulations of
2. Syntheses
4
2
terminal acetylenic group leading to 2-substituted
-iodophenols with acetylenic compounds containing a
5
The syntheses of the title compounds have been carried
out as depicted in Schemes 1–3. 2-Iodophenol (1) and
2-acetoxyphenyl iodide (2) were used as starting mate-
rials because of their easy availability from 2-amino-
phenol. It was found that 2-iodophenol (1) and
2-acetoxyphenyl iodide (2) underwent facile reaction
with (trimethylsilyl)acetylene (3) in the presence of
bis(triphenylphosphine)palladium(II) chloride and cop-
per(I) iodide at 45 ꢁC to yield 2-(trimethylsilyl)ethynyl
phenol (4) and 2-((trimethylsilyl)ethynyl)phenyl acetate
(5), respectively, in excellent yields (95–91%). The
open chain compounds 4 and 5 were subjected to Fri-
edel–Crafts acylation with acid chloride (6–8) to afford
the 2-aroylbenzofurans (9–11) in 83–81% yields as
shown in Scheme 1. Similar reaction of compound 5
with acetyl chloride or acetic anhydride afforded diac-
etylbenzofurans 12a in 26%, 12b in 19% and 12c in
33% yields as a result of secondary reactions induced
1
1
6
and 2,3-disubstituted benzofurans have been reported.
The synthesis of 2,3-disubstituted benzofurans by palla-
dium-catalysed annulation of internal alkynes has also
1
7
been documented.
We have previously reported the synthesis of isoindoli-
1
8
19
nones and isobenzofurans by the palladium-catalysed
annulation of terminal alkynes with 2-iodobenzamides
and 2-iodobenzyl alcohol, respectively. As part of our
ongoing studies in developing new broad spectrum anti-
microbial agents, we herein, report a novel approach for
the synthesis of 2-acyl(aroyl)benzofurans by the palladi-
um-catalyzed annulation of (trimethylsilyl)acetylene
with 2-iodophenol (1) or 2-acetoxyphenyl iodide (2), fol-
lowed by Friedel–Crafts acylation with acyl(aroyl) chlo-
ride or acetic anhydride (Schemes 1, 2) and also a new
strategy for a one-pot synthesis of 2-alkyl(aryl)benzofu-
rans from the same starting materials (1, 2) with terminal
alkynes (Scheme 3).
2
2
by an excess AlCl (Scheme 2). It has been reported that
3
alkylbenzofurans are readily acetylated in the furan ring
and in the benzene ring when the furan ring is fully
substituted. These acetylation patterns of alkylbenzofu-
rans seem to be due to the contributions of extreme res-
In the present investigation, eight newly synthesized
2
2
3
-benzofuran derivatives (9–11, 19–23), three diacety-
lated benzofurans (12a–c) together with the starting
materials (1, 2, 4 and 5) were screened for in vitro anti-
onance structures in Figure 1.
2
0
microbial activity. The most potent compounds, 9 and
the isomeric mixture (12abc) of 12a–c, were further eval-
uated for their minimum inhibitory concentrations
(
MIC) against Gram-positive, Bacillus subtilis QL 40
and S. aureus ATCC 25923 and Gram-negative Esche-
richia coli ATCC 25922, Salmonella typhi AM 16406,
Pseudomonas aeruginosa ATCC 27853, Vibrio parahe-
molyticus AM 16362, plus a pathogenic fungus, C. albi-
cans ATCC 10231, and for their cytotoxicity against
2
1
brine shrimp nauplii.
Scheme 3.
Scheme 1.
Scheme 2.

4798
M. Wahab Khan et al. / Bioorg. Med. Chem. 13 (2005) 4796–4805
anhydride. The isomeric components of the mixture
12abc) were resolved using normal phase preparative
TLC over silica gel F254 using n-hexane/EtOAc (10:1)
(
as eluting solvent (multiple developments) though they
had very closely related R values (0.25–0.30). The hith-
f
erto unknown diacetylbenzofurans (12a–c) were struc-
turally characterized on the basis of 2D NMR and
mass spectral analyses and by comparison with spectral
Figure 1. Resonance structures of 2-acetylbenzofuran representing the
loci for further electrophilic substitution.
2
4
data of 2-acetylbenzofuran.
The palladium-catalysed reaction was carried out by stir-
ring a mixture of 2-iodophenol 1 (4.0 g, 18.18 mmol),
3. Antimicrobial activity
Pd(Ph P) Cl
2
(0.128 g, 0.18 mmol), CuI (0.07 g,
The newly synthesized and purified benzofuran deriva-
tives were tested for antimicrobial activity by the disc
diffusion method against standard and clinically isolated
strains of 14 Gram-negative and 5 Gram-positive path-
ogenic bacteria as well as four human fungal pathogens.
The organisms were collected as pure cultures from the
Institute of Nutrition and Food Science (INFS), Univer-
sity of Dhaka, Bangladesh. The results are reported in
Tables 1–4. From the mean zone of inhibition (MZI)
data analysis, the isomeric mixture of the three diac-
etylbenzofurans (12abc) was identified as the most po-
tent inhibitor with zone diameters ranging from 31 to
40, 29 to 36, 20 to 32 and 18 to 26 mm at doses of
200, 100, 50 and 25 lg/disc, respectively, against the test
organisms (Table 2). The activity was dose-related and
3
2
0.36 mmol), Et N (10 mL) and HC„CSiMe3 (5 mL,
36.21 mmol) in dioxane (10 mL) at 45 ꢁC for 5 h, under
3
nitrogen atmosphere to yield the 2-(trimethylsilyl)
ethynylphenol (4) which was purified by column chro-
matography over silica gel (60–120 mesh). 2-(Trimethyl-
silyl)ethynylphenyl acetate (5) was prepared from
2
-acetoxyphenyl iodide (2) by applying the same proce-
dure. The Friedel–Crafts acylation reactions were carried
out by stirring an ice cold solution of the acyclic product
4
or 5 (1 mmol), anhydrous AlCl (3 mmol) and acid
3
chloride or acetic anhydride (1.2 mmol) in tetrachloro-
ethane (10 mL) at 0–25 ꢁC for 2–5 h, where the 2-aro-
ylbenzofurans (9–11) were obtained in good yields after
purification by column chromatography over silica gel
2
(
60–120 mesh).
statistically significant (r = 0.85), especially against S.
aureus ATCC 25923 (MZI, 40 mm), V. parahemolyticus
AM 16362 (MZI, 40 mm), Sarcina lutea QL 166 (MZI,
39 mm), Bacillus megaterium QL 38 (MZI, 38 mm), Shi-
gella boydii ATCC 13147 (MZI, 38 mm), Shigella
dysenteriae ATCC 26131 (MZI, 38 mm), Vibrio mimicus
N 1967 (MZI, 38 mm) and clinical isolate of a Klebsiella
sp. (MZI, 38 mm). The inhibitor was also effective
against all the four tested fungi and demonstrated signif-
icant antifungal activity against Aspergillus fumigatus
ATCC 13073 (MZI, 32 mm), C. albicans ATCC 10231
(MZI, 31 mm), Rhizopus oryzae ATCC 20344 (MZI,
29 mm) and Saccharomyces cerevisiae AB 972 (MZI,
31 mm) at a dose of 200 lg/disc (Table 4). The three
components of this isomeric mixture, viz., 12a–c were
purified using normal phase preparative TLC and were
individually tested against the organisms. Majority of
the data indicated 2,3-diacetylbenzofuran (12c) as the
most potent compound while some indicated synergistic
action (Tables 2 and 4).
2
2
-Alkyl(aryl)benzofurans (19–24) were synthesized from
-iodophenol (1) or 2-acetoxyphenyl iodide (2) after
reactions with terminal alkynes (13–18). The reactions
were carried out by heating a mixture of 2-iodophenol
(
1) or 2-acetoxyphenyl iodide (2) and alkynes (13–18)
in dioxane or dimethylformamide at 70 ꢁC for 20 h in
the presence of Pd(Ph P) Cl (2.5 mol %), CuI
(3–5 mol %) and Et N to afford the 2-substituted
3
benzofurans (19–24) in good yields (Scheme 3).
3
2
2
Pd(Ph P) Cl (2–3.5 mol %) was found to be the catalyst
3
2
2
of choice with CuI (3–5 mol %) needed as a co-cata-
2
2
lyst. Dioxane was found to be a better solvent than
DMF in regard to product yield. Uses of Et N as a base
3
and dioxane as a solvent were found to be the best due
to easier workup and yield of cleaner products. In case
of 2-acetoxyphenyl iodide (2), we obtained better yields
(81–85%) than unprotected 2-iodophenol (1). The for-
mation of Pd(0) from the interaction of Pd(Ph P) Cl
2
3
2
2
2
and CuI was proposed by Sonogashira et al.
Among the 2-aroylbenzofurans, 2-benzoylbenzofuran
(9) demonstrated moderate activity against Salmonella
paratyphi A AM 16590 (MZI, 21 mm), Shigella sonnei
C 182 (MZI, 16 mm) and Shigella boydii ATCC 13147
(MZI, 15 mm) and Vibrio mimicus N 1967 (MZI,
15 mm) (Table 1). The mean inhibitory zone diameters
for 9 range from 11 to 22, 10 to 15 mm against test bac-
teria (Table 1) and 15 to 21, 11 to 17 mm against fungi
(Table 3) at 200 and 100 lg/disc dose levels, respectively.
It was evident that compound 9 obtained from 2-((trim-
ethylsilyl)ethynyl)phenyl acetate (5) was more potent
than that of the corresponding isomeric mixture, synthe-
sized from the unprotected 2-(trimethylsilyl)ethynyl phe-
nol (4). Introduction of a methyl or a methoxy
All cyclic and acyclic synthesized products obtained
were stable at room temperature and well characterized
1
by spectroscopic data (UV, IR, H NMR and
13
C
1
13
NMR). The H NMR and C NMR spectra indicated
the presence of two isomers of 2-aroylbenzofurans syn-
thesized from 2-(trimethylsilyl)ethynyl phenol (4) in a
3
:2 ratio. But in the case of 2-((trimethylsilyl)ethy-
nyl)phenyl acetate (5), only one isomer of 2-aro-
ylbenzofurans (9–11) was obtained. In addition, an
isomeric mixture of diacetylbenzofurans (12abc) evolved
as a result of in situ secondary reaction of 2-((trimethyl-
silyl)ethynyl)phenyl acetate (5) with a stoichiometric ex-
cess of Lewis acid AlCl and acetyl chloride or acetic
3

M. Wahab Khan et al. / Bioorg. Med. Chem. 13 (2005) 4796–4805
4799
Table 1. Spectrum of antibacterial activity of 2-benzo[b]furans against Gram-positive and Gram-negative clinical isolates
a
Organism
Diameters of zone of inhibition (MZI)
1
2
12
21
Kan
30
Amp
30
2
00
100
200
200
100
200
Aeromonus hydrophilia AM 10481
Bacillus cereus QL 29
13 ± 2.83
24 ± 1.41
11 ± 2.12
14 ± 0.71
11 ± 2.83
11 ± 0.71
15 ± 0.71
15 ± 1.41
14 ± 0.00
08 ± 1.41
09 ± 1.41
NT
NT
10 ± 0.71
07 ± 0.00
07 ± 0.00
—
11 ± 0.00
13 ± 0.00
22 ± 0.00
14 ± 0.00
11 ± 1.41
13 ± 0.00
14 ± 0.00
21 ± 2.12
12 ± 1.41
11 ± 0.71
13 ± 0.71
15 ± 2.12
12 ± 2.83
14 ± 1.41
16 ± 0.00
16 ± 0.71
12 ± 1.41
15 ± 0.00
14 ± 0.00
NT
NT
—
37 ± 0.71
29 ± 0.00
31 ± 0.00
30 ± 0.00
28 ± 0.71
26 ± 1.41
33 ± 0.00
31 ± 2.83
34 ± 0.00
30 ± 0.00
29 ± 0.71
32 ± 1.41
29 ± 1.41
33 ± 0.71
28 ± 0.00
30 ± 0.71
37 ± 0.71
34 ± 0.00
29 ± 0.00
35 ± 1.41
NT
12 ± 0.00
11 ± 0.71
08 ± 0.71
10 ± 3.54
09 ± 2.83
09 ± 2.12
NT
10 ± 1.41
11 ± 0.71
12 ± 0.71
10 ± 1.41
10 ± 0.00
13 ± 0.00
NT
Bacillus megaterium QL 38
Bacillus subtilis QL 40
09 ± 1.41
—
32 ± 0.00
—
Escherichia coli ATCC 25922
Klebsiella sp.
—
—
08 ± 0.00
—
—
26 ± 0.71
40 ± 0.71
40 ± 0.71
32 ± 1.41
32 ± 1.41
35 ± 1.41
—
Pseudomonas aeruginosa ATCC 27853
Salmonella paratyphi A AM 16590
Salmonella paratyphi B AM 15961
Salmonella typhi AM 16406
Sarcina lutea QL 166
07 ± 0.00
—
10 ± 0.00
10 ± 0.00
NT
NT
NT
08 ± 0.71
07 ± 0.00
07 ± 0.71
—
NT
NT
08 ± 2.12
13 ± 1.41
NT
13 ± 1.41
15 ± 0.71
NT
09 ± 0.71
—
NT
Shigella boydii ATCC 13147
Shigella dysenteriae ATCC 26131
Shigella flexneriae Y 976
35 ± 0.00
37 ± 0.71
37 ± 1.41
30 ± 1.41
40 ± 0.00
51 ± 0.00
35 ± 0.00
30 ± 0.71
07 ± 0.00
07 ± 0.00
29 ± 1.41
11 ± 2.12
13 ± 0.00
12 ± 2.83
13 ± 1.41
—
—
15 ± 0.00
NT
—
11 ± 0.71
NT
13 ± 0.00
NT
—
Shigella sonnei C 182
08 ± 0.00
—
Staphylococcus aureus ATCC 25923
Vibrio cholerae 569 B
Vibrio mimicus N 1967
12 ± 1.41
NT
NT
09 ± 1.41
NT
07 ± 1.41
07 ± 0.71
—
NT
12 ± 0.00
NT
14 ± 2.83
NT
Vibrio parahemolyticus AM 16362
Kan = kanamycin, Amp = ampicillin; NT = not tested and Ô—Õ indicates no sensitivity or MZI lower than 7 mm.
Doses are expressed in micrograms per disc.
Interpretation of sensitivity test results: Gram-positive; >18 mm (MZI) = sensitive; 14–18 mm (MZI) = intermediate; <14 mm (MZI) = resistant.
Gram-negative bacteria >16 mm (MZI) = sensitive; 13–16 mm (MZI) = intermediate; <13 mm (MZI) = resistant.
a
Values are means ± SD (n = 2).
Table 2. Comparison of antibacterial activity of the isomeric mixture of diacetylbenzofurans (12abc) and its components (12a–c) against Gram-
positive and Gram-negative clinical isolates
a
Organism
Diameters of zone of inhibition (MZI)
1
2abc
50
NT
12a
30
12b
30
12c
30
Kan
30
Amp
30
2
00
100
25
Aeromonus hydrophilia AM 10481
Bacillus cereus QL 29
31 ± 1.41 NT
NT
NT
NT
NT
37 ± 0.71 35 ± 1.41
36 ± 1.41 33 ± 0.71 25 ± 0.00 18 ± 2.83 13 ± 0.71 11 ± 1.41 31 ± 0.71 29 ± 0.00 NT
38 ± 0.71 36 ± 0.00 32 ± 2.12 22 ± 1.41 15 ± 0.00 11 ± 0.71 11 ± 2.83 31 ± 0.00 32 ± 0.00
Bacillus megaterium QL 38
Bacillus subtilis QL 40
Escherichia coli ATCC 25922
Klebsiella sp.
34 ± 1.41 29 ± 1.41 28 ± 0.71 21 ± 0.71
32 ± 2.12 29 ± 1.41 20 ± 0.71 18 ± 0.00
—
10 ± 2.12 23 ± 2.12 30 ± 0.00
9 ± 0.00 21 ± 0.71 28 ± 0.71 26 ± 0.71
NT NT 26 ± 1.41 40 ± 0.71
9 ± 0.71 12 ± 0.71 20 ± 0.00 33 ± 0.00 40 ± 0.71
—
—
38 ± 1.41 30 ± 2.83 28 ± 1.41 19 ± 0.00 NT
Pseudomonas aeruginosa ATCC 27853 32 ± 0.00 31 ± 1.41 27 ± 2.12 21 ± 1.41
Salmonella paratyphi A AM 16590
Salmonella paratyphi B AM 15961
Salmonella typhi AM 16406
Sarcina lutea QL 166
35 ± 1.41 32 ± 0.71 27 ± 0.71 18 ± 1.41
—
7 ± 0.71
NT
8 ± 1.41 25 ± 1.41 31 ± 2.83 32 ± 1.41
8 ± 2.12 36 ± 0.71 34 ± 0.00 32 ± 1.41
33 ± 1.41 NT
37 ± 0.71 NT
NT
NT
NT
NT
NT
NT
NT
NT
NT
NT
NT
NT
NT
NT
NT
NT
30 ± 0.00 35 ± 1.41
29 ± 0.71
39 ± 1.41 32 ± 1.41 28 ± 0.71 26 ± 1.41 NT
38 ± 0.71 34 ± 0.00 29 ± 0.71 21 ± 1.41 NT
—
Shigella boydii ATCC 13147
Shigella dysenteriae ATCC 26131
Shigella flexneriae Y 976
32 ± 1.41 35 ± 0.00
29 ± 1.41 37 ± 0.71
33 ± 0.71 37 ± 1.41
28 ± 0.00 30 ± 1.41
38 ± 0.71 NT
37 ± 1.41 32 ± 0.00 31 ± 1.41 22 ± 0.71 NT
NT
NT
NT
Shigella sonnei C 182
34 ± 0.71 NT
40 ± 0.71 32 ± 1.41 29 ± 1.41 24 ± 2.83
37 ± 0.00 NT NT NT
38 ± 1.41 31 ± 1.41 29 ± 0.00 19 ± 2.83
40 ± 0.71 NT NT NT
NT
NT
NT
—
Staphylococcus aureus ATCC 25923
Vibrio cholerae 569 B
Vibrio mimicus N 1967
10 ± 2.12 24 ± 0.71 30 ± 0.71 40 ± 0.00
NT
NT
—
NT 37 ± 0.71 51 ± 0.00
18 ± 1.41 34 ± 0.00 35 ± 0.00
7 ± 0.00
Vibrio parahemolyticus AM 16362
7 ± 0.00 15 ± 0.71 22 ± 1.41 29 ± 0.00 30 ± 0.71
Kan = kanamycin, Amp = ampicillin; NT = not tested and Ô—Õ indicates no sensitivity or MZI lower than 7 mm.
Doses are expressed in micrograms per disc.
Interpretation of sensitivity test results: Gram-positive; >18 mm (MZI) = sensitive; 14–18 mm (MZI) = intermediate; <14 mm (MZI) = resistant.
Gram-negative bacteria >16 mm (MZI) = sensitive; 13–16 mm (MZI) = intermediate; <13 mm (MZI) = resistant.
a
Values are means ± SD (n = 2).
functionality at the aroyl group, as demonstrated for
compounds 10 and 11, was inactive. This observation
suggests the negating effects of the bulky group on anti-
microbial activity, which may result from an increased
steric hindrance at the para position. In the 2-al-
kyl(aryl)benzofuran series, compound 19 demonstrated

4800
M. Wahab Khan et al. / Bioorg. Med. Chem. 13 (2005) 4796–4805
Table 3. Spectrum of antifungal activity of 2-benzo[b]furans against human fungal pathogens
a
Organism
Diameters of zone of inhibition (MZI)
1
2
12
21
AmB
30
Nys
30
2
00
100
200
200
100
200
Aspergillus fumigatus ATCC 13073
Candida albicans ATCC 10231
Rhizopus oryzae ATCC 20344
Saccharomyces cerevisiae AB 972
NT
NT
NT
NT
14 ± 0.71
8 ± 0.00
8 ± 0.00
13 ± 0.00
NT
NT
NT
NT
18 ± 1.41
16 ± 1.41
15 ± 2.83
21 ± 2.12
13 ± 1.41
11 ± 0.71
12 ± 1.41
17 ± 3.54
8 ± 0.71
24 ± 0.71
19 ± 0.00
25 ± 0.00
25 ± 2.12
20 ± 2.12
17 ± 0.71
21 ± 0.00
24 ± 0.71
—
—
—
AmB = amphotericin B, Nys = nystatin; NT = not tested and Ô—Õ indicates no sensitivity or MZI lower than 7 mm. Doses are expressed in
micrograms per disc except for amphotericin B (units/disc).
a
Values are means ± SD (n = 2).
Table 4. Comparison of antifungal activity of the isomeric mixture of diacetylbenzofurans (12abc) and its components (12a–c) against human fungal
pathogens
a
Organism
Diameters of zone of inhibition (MZI)
1
2abc
12a
12b
30
12c
AmB
30
Nys
30
2
00
100
50
25
30
30
Aspergillus fumigatus ATCC 13073 32 ± 0.71 30 ± 1.41 30 ± 0.00 18 ± 0.71 10 ± 0.71
—
21 ± 0.71 24 ± 0.71 20 ± 2.12
Candida albicans ATCC 10231
Rhizopus oryzae ATCC 20344
Saccharomyces cerevisiae AB 972
31 ± 2.12 28 ± 0.71 28 ± 1.41 25 ± 0.71
29 ± 0.71 27 ± 2.83 25 ± 2.83 22 ± 1.41 NT
31 ± 2.83 28 ± 1.41 27 ± 2.83 18 ± 1.41 8 ± 0.00
—
13 ± 1.41 27 ± 1.41 19 ± 0.00 17 ± 0.71
NT NT 25 ± 0.00 21 ± 0.00
9 ± 0.00 18 ± 1.41 25 ± 2.12 24 ± 0.71
AmB = amphotericin B, Nys = nystatin; NT = not tested and Ô—Õ indicates no sensitivity or MZI lower than 7 mm. Doses are expressed in
micrograms per disc except for amphotericin B (units/disc).
a
Values are means ± SD (n = 2).
only weak antibacterial activity with MZI of 8–13 mm
at 200 lg/disc. Compound 20 in this series showed mild
growth inhibition against B. subtilis QL 40 (MZI,
bacterial strains tested (Table 5). In particular, the MICs
of 12abc against E. coli ATCC 25922, P. aeruginosa
ATCC 27853 and S. typhi AM 16406 were 8, 8 and
8 lg/mL, as compared to the MICÕs of 8, 8, 32 lg/mL
demonstrated by ampicillin against these strains. Com-
pound 9 showed an MIC of 64 against S. aureus ATCC
25923 which was in agreement with the previously re-
1
3
0 mm) and compound 23 against B. megaterium QL
8 (MZI, 10 mm). Compounds 21 and 22 were found
to be completely resistant against the tested organisms
and were, therefore, precluded from Tables 1 and 3.
The inhibitory zone diameters for ampicillin, kanamycin
(antibacterial) and amphotericin B, nystatin (antifungal)
were referred to as reference values in this study.
2
5
ported value. The MICs demonstrated against the rest
were 16–>128 lg/mL (Table 5).
Attempts were made to correlate the antimicrobial activ-
ities of these compounds to the calculated Log of parti-
tion coefficient (CLogP) for n-octanol/water by CS
ChemDraw Ultra v8.03 (Table 6). Partition coefficient,
which is well known as an index of lipophilicity, is an
important physicochemical parameter in the develop-
ment of antibacterial agent since it is closely related to
the permeation through a lipid coat of bacteria. The
CLogP value of isomeric compounds 12a and 12b
(1.79) was the smallest among the tested compounds.
However, it was not comparable to those of strongly
hydrophilic ampicillin (CLogP = À1.87) and kanamy-
The minimum inhibitory concentrations (MICs, lg/mL)
of compounds 9 and 12abc were determined against
Gram-positive, B. subtilis QL 40 and S. aureus ATCC
2
5923 and Gram-negative E. coli ATCC 25922, S. typhi
AM 16406, P. aeruginosa ATCC 27853, V. parahemolyt-
icus AM 16362 plus a pathogenic fungus, C. albicans
ATCC 10231 and were compared to those of ampicillin
and nystatin. The results are presented in Table 5. The
isomeric mixture 12abc demonstrated MIC values com-
parable to or 1- to 2-fold lower than those of ampicillin
against most of the Gram-positive and Gram-negative
2
6
Table 5. Spectrum of activity (MIC) against Gram-positive and Gram-negative clinical isolates
Organism
Strain No.
MIC (lg/mL)
9
12abc
Amp
AmB
Nys
Bacillus subtilis
QL 40
64
16
64
32
8
64
8
NT
NT
NT
NT
NT
NT
0.5
NT
NT
NT
NT
NT
NT
2
Escherichia coli
ATCC 25922
ATCC 27853
AM 16406
ATCC 25923
AM 16362
ATCC 10231
Pseudomonas aeruginosa
Salmonella typhi
8
8
>128
64
64
8
32
32
64
NT
Staphylococcus aureus
Vibrio parahemolyticus
Candida albicans
16
32
16
64
NT = not tested.

M. Wahab Khan et al. / Bioorg. Med. Chem. 13 (2005) 4796–4805
4801
Table 6. The calculated n-octanol/water partition coefficient (LogPow
and yields % of benzofuran derivatives
)
tor) at C3 and the Tyr/Ser/Thr hydroxy group (donor) in
27
the biomolecule. There was also a distinct possibility
of hydrogen bonding between the ring nitrogen of at
least one conserved His/Trp residue of the catalytic site
a
Products
Yields (%)
CLogP
9
81
83
81
3.74
3.96
4.24
2
7
and the ring oxygen of benzofurans. Any reduction in
hydrogen acceptor ability such as by replacing benzofu-
ran molecular core and introducing poor electron donor
like sulfur in bioisosteric thiophene analogs might re-
duce the activity significantly. The benzofuran ring
could be embedded in a hydrophobic pocket of the bind-
ing site and thereby contributed in lowering the free
energy of possible binary/ternary complexes. There
might be little or no space for an extra substituent at
the aromatic moiety. However, if there was any space
whatsoever, it is not sufficient to accommodate an acetyl
group. This observation was consistent with the SAR
finding that acetylation at the C-4 (as in 12a) and C-7
positions (as in 12b) attenuated growth inhibitory activ-
ity (Table 2). However, it was apparent from previously
conducted SAR studies that a hydroxyl/methoxy group
1
1
1
1
1
1
1
2
2
2
2
2
0
1
2abc
2a
2b
2c
9
26
19
33
82
81
83
85
84
81
—
—
1.79
1.79
1.86
4.80
0
4.79
5.32
1
2
3.20
2.36
3
4
1.66
À1.87
Ampicillin
Kanamycin
À5.17
a
Log of partition coefficient (CLogP) for n-octanol/water was com-
puted by using CS ChemDraw Ultra v8.03.
2
8
at C5–7 position tended to increase the activity. The
moderate activity of 2-benzoylbenzofuran (9) can be
explained by the aromatic–aromatic interaction between
the C-2 benzoyl group with possible phenylalanine resi-
cin (CLogP = À5.17). The two compounds demon-
strated mild antimicrobial activity with inhibitory zone
diameters of 7–15 and 8–15 mm at 30 lg/disc against
the bacterial and fungal strains tested. The most signif-
icant activity was observed by compound 12c which
has a CLogP value of 1.86. Mean inhibitory zone diam-
eters for 12c ranged from 11 to 36 at 30 lg/disc, which
were comparable with standard antibiotics at the same
dose level (Tables 2 and 4). The replacement of the
6
dues in a second hydrophobic pocket. Because an aro-
matic ring bearing electron donors is known to interact
with another aromatic ring less strongly than does
an unsubstituted aromatic ring and vice versa, the activ-
ity of p-methyl or p-methoxy derivative of 2-ben-
zoylbenzofuran (10, 11) might be reduced. Thus, the
current study outlines a hypothetic image of catalytic
site for benzofurans in the target biomolecule with at
least one hydrophobic pocket and two H-bond do-
nors—a polar hydroxylated and an imino nitrogen con-
taining amino acid residues. The broad spectrum
activity suggests that the target is probably present in
bacteria and fungi ubiquitously. Other structural refine-
ments including substitution at the C4–7 positions of
benzofuran ring, and the replacement of 3-acetyl group
by other acyl/lower alkyl functionalities are to be envis-
aged for functional characterization of target
biomolecule.
2
-acetyl group by an aroyl/alkyl/aryl functionality and
removing the additional acetyl group resulted in com-
pounds with weaker or no antibacterial activity, which
is in line with expectations for compounds with high
CLogP value ranging from 2.36 to 5.32. Compounds
2
2 and 23 with CLogP values of 3.20 and 2.36 demon-
strated only weak antimicrobial activity. On the
contrary, compound 9 which has a higher CLogP value
of 3.74, exhibited moderate antimicrobial activity
(
Tables 1 and 3). The other compounds in the activity
study, except 24, have higher CLogP values of 3.96–
.32 and were ineffective against the sensitive control
5
and clinically resistant bacterial strains, fungi and were
not studied for MICs. Thus, no direct correlation could
be established between the CLogP and antimicrobial
activity.
The cytotoxic potentials of compounds 11 and 12abc
(isomeric mixture of 12a–c) were also determined at
dilutions ranging from 1.56 to 200 lg/mL by the well-
2
1
known brine shrimp lethality bioassay. The tested
compounds did not show any cytotoxic activity and
showed the selectivity, in that they possess the potent
antimicrobial activity without cytotoxicities against Art-
emia salina nauplii. It is logical to expect that these com-
pounds may not have toxicities in mammalian cells as
In this investigation three other compounds, viz., 2-acet-
ylbenzofuran, benzofuran-2-carboxylic acid and benzo-
furan-2-carboxaldehyde from commercial source were
screened against the same panel of organisms but none
of them showed any activity at all. Therefore, the supe-
rior antimicrobial activity of 2,3-diacetylbenzofuran
2
1
well.
(
12c) could be interpreted as an indication of a stronger
intrinsic Ôbinding attractionÕ for the 3-acetyl group at the
site of action. The repulsive Coulomb force between the
carbonyl oxygen and the benzofuran oxygen was
expected to align these heteroatoms s-trans and to make
the conformation rigid which may account for best fit-
ting to the macromolecular target. The tighter binding
of 12c over the mono acetyl derivative could be
explained by an additional energetically favourable
hydrogen bonding between the carbonyl oxygen (accep-
4. Experimental
All reactions were carried out under a nitrogen atmo-
sphere. Melting points were recorded on an electrother-
mal melting point apparatus and paraffin oil bath and
are uncorrected. UV and IR spectra were recorded on
Shimadzu UV–vis and Shimadzu FTIR spectrophoto-
1
13
meters, respectively. The H NMR (400 MHz) and
C

4802
M. Wahab Khan et al. / Bioorg. Med. Chem. 13 (2005) 4796–4805
NMR (100 MHz) spectra were acquired in CDCl on an
3
7.50 (dd, 1H, J = 7.6, 1.5 Hz), 7.34 (ddd, 1H, J = 1.6,
7.6, 7.6 Hz), 7.18 (t, 1H, J = 7.6 Hz), 7.07 (d, 1H,
J = 7.6 Hz), 2.33 (s, 3H), 0.25 (s, 9H); C NMR
Ultra Shield Bruker DPX 400 spectrometer and the
chemical shifts are reported in parts per million relative
to the residual nondeuterated solvent signals. The num-
1
3
(100 MHz, CDCl ): dC 166.5, 151.9, 133.1, 129.8,
3
1
3
ber of attached protons for C signals was determined
using the DEPT 135 pulse sequence. HR-FABMS was
recorded on a JEOL SX 102 mass spectrometer using
m-nitrobenzyl alcohol (NBA) or polyethylene glycol
125.7, 125.6, 122.4, 117.1, 99.5, 77.0, À0.25.
4.3. General method for the synthesis of 2-aroylbenzo-
furans (9–11)
(
PEG) as matrix. Follow up of the reactions and check-
ing the homogeneity of the compounds were made by
TLC on Kieselgel gel 60 F254 pre-coated sheets
To an ice cold solution of 2-(trimethylsilyl)ethynyl phe-
nol 4 or 2-((trimethylsilyl)ethynyl)phenyl acetate 5
(
E. Merck) and the spots were detected by exposure to
(1.1 mmol) in tetrachloroethane (10 mL), aroyl chloride
UV-lamp at 254 nm. Column chromatography was done
on silica gel (60–120 mesh ASTM). Bis (triphenylphos-
phine)palladium(II) chloride, copper(I) iodide, acetyl
chloride, benzoyl chloride, toluoyl chloride, anisoyl
chloride, aluminum chloride, (trimethylsilyl)acetylene
and the alkynes were purchased from E. Merck,
Germany and Fluka, Switzerland. 2-Acetylbenzofuran,
benzofuran-2-carboxylic acid and benzofuran-2-carbox-
aldehyde were bought from Sigma-Aldrich Company
Ltd. The calculated partition coefficient (CLogP)
values were determined by using the CS ChemDraw
Ultra version 8.03, computer software by Cambridge-
Soft.Com.
6
added. The mixture was stirred under N atmosphere
for 3 h and the temperature of the reaction was raised
from 0 ꢁC to 25 ꢁC. Then the mixture was poured into
an ice cold solution of dilute HCl (2 mL, 1–1.5 N HCl)
and the organic layer was separated. The aqueous layer
–8 (1.25 mmol) and anhydrous AlCl (3 mmol) were
3
2
was extracted with CHCl (3· 25 mL). The combined
3
organic extracts were washed with distilled H O (2·
2
3
0 mL), saturated NaHCO solution (2· 30 mL) and
3
distilled H O (2· 30 mL) again. After drying over
anhydrous Na SO and removal of solvent a syrupy
residue was obtained. The crude mass was purified by
a silica gel column and yielded 2-aroylbenzofurans
2
2
4
9
–11.
4.1. General procedure for the synthesis of 2-(trimethyl-
silyl)ethynyl phenol (4)
4
.3.1. 2-Benzoylbenzofuran 9. Yield 81%; mp 88–89 ꢁC
1
5
(
1
lit. 89 ꢁC); IR (KBr): m
1690, 1645, 1550 and
max
310, 286, 241 nm; H
max
To a stirred solution of 2-iodophenol 1 (4.00 g,
1
À1
1
548 cm ; UV (CHCl ): k
3
8.18 mmol), Pd(Ph P) Cl (0.128 g, 0.18 mmol), CuI
3 2 2
NMR (400 MHz, CDCl ): d 8.31 (d, 1H, J = 7.2 Hz),
8
8
J = 7.6 Hz), 7.56 (dd, 1H, J = 7.2, 7.6 Hz), 7.49 (t, 1H,
J = 7.6 Hz), 7.36 (dd, 1H, J = 7.2, 8.0 Hz), 7.11 (d, 1H,
J = 7.6 Hz), 6.33 (s, 1H); C NMR (100 MHz, CDCl ):
3
(
0.07 g, 0.36 mmol) and Et N (10 mL) in dioxane/
3
.13 (d, 1H, J = 7.2 Hz), 7.81 (ddd, 1H, J = 1.6, 7.6,
.0 Hz), 7.69 (d, 1H, J = 7.6 Hz), 7.63 (t, 1H,
DMF (10 mL) were added HC„CSiMe3 (5 mL,
3
for 5 h (24 h in the case of DMF) under N atmosphere.
6.21 mmol). The reaction mixture was stirred at 45 ꢁC
2
The solvent was removed under reduced pressure. To a
residue diethyl ether and 0.1 N HCl were added and the
organic layer was separated, neutralized with a satu-
13
3
dC 184.5, 156.4, 152.1, 139.1, 131.8, 129.1, 128.5,
1
28.2, 126.4, 125.3, 124.9, 118.2, 112.8.
rated NaHCO (3· 50 mL) solution, washed with dis-
3
tilled water (3· 50 mL), dried with anhydrous Na SO
2
4
4
.3.2. 2-(p-Methoxybenzoyl)benzofuran 10. Yield 83%;
and concentrated under reduced pressure. The latter
was purified by chromatography over a column of silica
gel with hexane/chloroform (7:1) to obtain the title com-
pound 4 (3.6 g, 95% when dioxane was used as a solvent
and the product was 70% when DMF was used as a sol-
1
5
mp 94–95 ꢁC (lit. 95–96 ꢁC); IR (KBr): m
1
2
J = 8.8 Hz), 7.61 (dd, 1H, J = 1.6, 8.0 Hz), 7.51 (dd,
1H, J = 1.6, 7.2 Hz), 7.35 (t, 1H, J = 7.2 Hz), 7.28 (dd,
1H, J = 7.2, 8.0 Hz), 7.00 (s, 1H), 6.94 (d, 1H,
1739,
315,
max
max
À1
640, 1549 and 1510 cm ; UV (CHCl ): k
65 nm; H NMR (400 MHz, CDCl ): d 8.18 (d, 2H,
3
1
3
2
9
vent) as a solid, mp 46–47 ꢁC (lit. mp 46–47 ꢁC); IR
À1
(
(
(
KBr): m
3450, 2146, 842, 775, 776 cm ; UV
304, 296, 257, 287 nm; H NMR
max
1
J = 8.8 Hz), 6.80 (dd, 1H, J = 3.6, 8.8 Hz), 3.88 (s,
CHC1 ): k
3
max
13
3
1
1
H), C NMR (100 MHz, CDCl ): dC 186.7, 164.3,
63.9, 148.0, 132.6, 131.2, 130.2, 127.0, 126.3, 125.9,
23.6, 121.4, 114.1, 113.9, 113.8, 55.6.
3
400 MHz, CDCl ): d 7.34 (dd, 1 H, J = 1.4, 7.6 Hz),
3
7
.24 (t, 1H, J = 7.6 Hz), 6.94 (br d, 1H, J = 7.6 Hz),
.85 (t, 1H, J = 7.6 Hz), 5.83 (br s, 1H), 0.28 (s, 9H);
6
1
3
C NMR (100 MHz, CDCl ): dC 158.1, 131.7, 130.2,
3
3
0
1
20.5, 114.8, 109.5, 101.9, 99.2, À0.2.
4.3.3. 2-(p-Methylbenzoyl)benzofuran 11. Yield 81%;
1755, 1610, 1575,
max
light yellow liquid; IR (CCl ): m
4
À1
1
4.2. General method for the synthesis of 2-((trimethyl-
silyl)ethynyl)phenyl acetate (5)
1545 cm ; UV (CHCl
(400 MHz, CDCl
3
1
3
): kmax 276, 244 nm; H NMR
): d 8.18 (d, 1H, J = 8.0 Hz), 8.10 (d,
H, J = 8.0 Hz), 7.74 (d, 1H, J = 8.0 Hz), 7.69 (d, 1H,
2-((Trimethylsilyl)ethynyl)phenyl acetate (5) was pre-
pared from 2-acetoxyphenyl iodide (2) using the above
J = 7.6 Hz), 7.35 (dd, 1H, J = 7.6, 8.0 Hz), 7.29 (d, 1H,
J = 8.0 Hz), 7.28 (t, 1H, J = 8.0 Hz), 7.13 (dd, 1H,
J = 3.2, 8.0 Hz), 7.05 (s, 1H), 2.36 (s, 3H); C NMR
2
2
13
Sonogashira coupling reaction. The compound 5 was
obtained as a light yellow liquid; yield 91%; IR (KBr):
(100 MHz, CDCl ): d 189.8, 165.2, 153.1, 144.4,
3 C
À1
mmax 2120, 1776, 1548, 1258 cm ; UV (CHCl ): k
132.2, 131.0, 130.4, 129.2, 129.0, 128.9, 126.4, 126.3,
123.6, 115.3, 114.9, 21.8.
3
max
1
3
04, 296, 257, 245 nm; H NMR (400 MHz, CDCl ): d
3

M. Wahab Khan et al. / Bioorg. Med. Chem. 13 (2005) 4796–4805
4803
4.4. General method for the synthesis of
diacetylbenzofurans (12a–c)
was added under N atmosphere. The mixture was stir-
2
red at 70 ꢁC for 20 h. After removal of the solvent under
reduced pressure the mixture was cooled, poured into
distilled water (100 mL) and extracted with chloroform
(3· 50 mL). The combined extracts were washed with
5 M aq NaOH (3· 50 mL) and water (3· 50 mL). The
organic layer was then dried over anhydrous Na SO ,
To an ice cold solution of 2-(trimethylsilyl)ethynyl phe-
nol 4 or 2-((trimethylsilyl)ethynyl)phenyl acetate 5
(
1.1 mmol) in tetrachloroethane (15 mL), acetyl chlo-
ride/acetic anhydride (2.5 mmol) and anhydrous AlCl3
5 mmol) were added. The mixture was stirred under
2
4
(
filtered and evaporated to dryness. The residue was puri-
fied through silica-gel column chromatography to afford
2-alkyl(aryl)benzofurans (19–24).
N atmosphere for 3 h and the temperature of the reac-
2
tion was raised from 0 ꢁC to 25 ꢁC. Then the mixture
was poured into an ice-cold solution of dilute HCl
(
2 mL, 1–1.5 N HCl) and the organic layer was sepa-
rated. The aqueous layer was extracted with CHCl3
3· 25 mL). The combined organic extracts were washed
4.5.1. 2-Phenylbenzofuran 19. Yield 82% (dioxane), 75%
1
5
(DMF); mp 116–118 ꢁC (lit. 118–120 ꢁC); IR (KBr):
À1
(
m
1593, 1562, 1455, 1257 cm ; UV (CHCl ): k
max
3
max
1
with distilled H O (2· 30 mL), saturated NaHCO solu-
317, 307, 261, 240 nm; H NMR (400 MHz, CDCl ): d
2
3
3
tion (2· 30 mL) and distilled H O (2· 30 mL) again.
7.88 (dd, 1H, J = 1.6, 7.6 Hz), 7.61–7.24 (m, 8H), 7.04
2
1
3
After drying over anhydrous Na SO and removal of
(s, 1H); C NMR (100 MHz, CDCl ): d 156.0, 154.6,
2
4
3 C
solvent a syrupy residue was obtained. The crude mass
was purified by a silica gel column followed by prepara-
tive TLC over silica gel F254 using n-hexane/EtOAc
(10:1) as eluting solvent (multiple developments) to yield
diacetylbenzofurans 12a–c.
133.1, 129.2, 129.0, 128.8, 128.6, 128.4, 124.2, 123.1,
121.6, 120.8, 111.3, 101.4.
3
1
4.5.2. 2-Butylbenzofuran 20. Yield 81% (Dioxane), 71%
(DMF), light yellow liquid; IR (CCl ): m
1590, 1570,
max
285, 278, 250 nm;
7.49 (d, 1H,
4
À1
1
H
480, 1250 cm ; UV (CHCl ): k
3
max
1
4
.4.1. 2,4-Diacetylbenzofuran 12a. Yield 26%; white
1776, 1770, 1676, 1548 and
max
NMR (400 MHz, CDCl3):
d
solid; IR (CCl ): m
J = 6.4 Hz), 7.42 (d, 1H, J = 7.6 Hz), 7.26–7.17 (m,
2H), 6.39 (s, 1H), 2.79 (t, 2H, J = 7.6 Hz), 1.54–1.42
4
À1
1
1
488 cm ; UV (CHCl ): k
275, 227 nm; H NMR
max
3
1
3
(
400 MHz, CDCl ): d 7.47 (dd, 1H, J = 7.6, 8.3 Hz),
(m, 4H), 0.98 (t, 3H, J = 6.8 Hz);
C
NMR
3
7
8
.30 (dd, 1H, J = 1.2, 7.6 Hz), 7.13 (dd, 1H, J = 0.9,
C
(100 MHz, CDCl ): dC 159.6, 153.1, 129.0, 124.1,
123.0, 121.6, 111.8, 101.9, 30.8, 28.5, 21.3, 13.4.
3
1
3
.3 Hz), 6.52 (s, 1H), 2.46 (s, 3H), 2.30 (s, 3H);
NMR (100 MHz, CDCl ): dC 196.0, 160.8, 147.1,
3
3
2
1
38.8, 131.8, 131.2, 129.8, 129.0, 126.6, 123.4, 31.9,
1.2; HR-FABMS: Calcd for C H O : 202.0630.
4.5.3. 2-Pentylbenzofuran 21. Yield 83%; light yellow
1590, 1560, 1489, 1240 cm
À1
2
Found: 202.0632.
liquid; IR (CCl ): m
;
1
2
10
3
4
max
1
UV (CHCl ): k
285, 278, 250 nm; H NMR
max
3
(
400 MHz, CDCl ): d 7.50 (dd, 1H, J = 1.6, 6.8 Hz),
3
4
.4.2. 2,7-Diacetylbenzofuran 12b. Yield 19%; yellowish
1781, 1774, 1668, 1553 and
max
7.44 (dd, 1H, J = 1.6, 7.6 Hz), 7.25–7.20 (m, 2H), 6.40
(s, 1H), 2.79 (t, 2H, J = 7.6 Hz), 1.43–1.39 (m, 6H),
0.95 (t, 3H, J = 6.8 Hz); C NMR (100 MHz, CDCl3):
solid; IR (CCl ): m
1
4
À1
1
13
501 cm ; UV (CHCl ): k
280, 228 nm; H NMR
max
3
(
7
8
400 MHz, CDCl ): d 7.48 (dd, 1H, J = 7.5, 8.3 Hz),
dC 160.6, 153.1, 128.6, 125.6, 123.2, 121.5, 112.3,
102.1, 30.8, 29.3, 28.7, 22.6, 14.0.
3
.38 (dd, 1H, J = 1.7, 7.5 Hz), 7.19 (dd, 1H, J = 1.8,
.3 Hz), 6.62 (s, 1H), 2.28 (s, 3H), 1.95 (s, 3H);
1
3
C
3
3
NMR (100 MHz, CDCl ): dC 197.6, 162.3, 144.8,
1
2
Found: 202.0632.
4.5.4. Ethyl-2-benzofurancarboxylate 22. Yield 85%;
1720, 1697, 1679,
max
3
43.5, 130.1, 129.7, 128.1, 127.6, 125.4, 124.2, 28.9,
0.1; HR-FABMS: Calcd for C H O : 202.0630.
light yellow liquid; IR (CCl ): m
4
À1
1612, 1332, 1195 cm ; UV (CHCl ): k
301, 294,
max
1
2
10
3
3
1
284, 207 nm; H NMR (400 Mz, CDCl ): d 7.83 (dd,
3
1H, J = 1.2, 7.6 Hz), 7.36 (dd, 1H, J = 7.6, 8.0 Hz),
4
.4.3. 2,3-Diacetylbenzofuran 12c. Yield 33%; white
1790, 1772, 1686, 1542 and
max
7.06 (d, 1H, J = 8.0 Hz), 6.95 (s, 1H), 6.94 (t, 1H,
J = 7.6 Hz), 4.19 (q, 2H, J = 7.1 Hz), 1.27 (t, 3H,
solid; IR (CCl ): m
1
4
À1
1
282, 225 nm; H NMR
13
503 cm ; UV (CHCl ): k
J = 7.1 Hz); C NMR (100 MHz, CDCl ): d 168.9,
3
max
3
C
(
(
(
400 MHz, CDCl ): d 7.59 (br d, 1H, J = 7.6 Hz), 7.47
159.0, 151.8, 130.2, 126.6, 123.4, 121.9, 114.5, 111.8,
56.8, 13.0.
3
dd, 1H, J = 7.2, 7.6 Hz), 7.26 (d, 1H, J = 7.2 Hz), 7.16
t, 1H, J = 7.2 Hz), 6.51 (s, 1H), 2.38 (s, 3H);
1
3
C
NMR (100 MHz, CDCl ): dC 194.6, 188.2, 150.2,
4.5.5.
3
Dimethyl-2-(2-benzofurylmethyl)propanedioate
3
4
1
41.9, 130.4, 129.2, 125.8, 121.4, 119.1, 118.3, 30.1,
5.4; HR-FABMS: Calcd for C H O : 202.0630.
23. Yield 84%; light yellow liquid; IR (CCl ): m
4
max
max
À1
2
Found: 202.0632.
1759, 1743, 1548, 1251, 1220 cm ; UV (CHCl ): k
303, 295, 284, 205; H NMR (400 MHz, CDCl ): d 7.51
1
2
10
3
3
1
3
(d, 1H, J = 6.0 Hz), 7.43 (d, 1H, J = 6.8 Hz), 7.26–7.21
(m, 2H), 6.51 (s, 1H), 3.93 (t, 1H, J = 6.8 Hz, CH),
4
2
.5. General method for the synthesis of
-alkyl(aryl)benzofurans 19–24
1
3
3.78 (s, 6H), 3.44 (d, 2H, J = 6.8 Hz); C NMR
100 MHz, CDCl ): dC 173.8, 171.9, 160.5, 151.2,
129.0, 125.6, 123.2, 119.6, 111.8, 101.9, 52.4, 49.2, 27.1.
(
3
To a well-stirred mixture of 2-iodophenol (1) or 2-acet-
oxyphenyl iodide (2) (2 mmol), Pd(Ph P) Cl
(2–3.5 mol %), CuI (3–5 mol %) and Et N (2 equiv.) in
dioxane or DMF (6 mL) a terminal alkyne (13–18)
3
2
2
1
5
4.5.6. 2-Hydroxymethylbenzofuran 24. Yield 81%; light
3330, 1460, 1250 cm
max
3
À1
yellow liquid; IR (CCl ): m
;
4

4804
M. Wahab Khan et al. / Bioorg. Med. Chem. 13 (2005) 4796–4805
1
283, 276, 247, 208 nm; H NMR
UV (CHCl ): k
(
2
(
3. Cagniant, P.; Cagniant, D.. In Katritky, A. R., Boulton,
3
max
A. J., Eds.; Advances in Heterocyclic Chemistry; Aca-
demic: New York, 1975; Vol. 18, pp 343–473.
. Bastian, G.; Royer, R.; Cavier, R. Eur. J. Med. Chem.
400 MHz, CDCl ): d 7.60 7.50 (m, 2H), 7.40–7.24 (m,
3
1
H), 6.69 (s, 1H), 4.75 (s, 2H), 2.17 (s, 1H); C NMR
3
4
5
100 MHz, CDCl ): dC 159.7, 156.3, 128.8, 124.6,
23.9, 121.1, 113.0, 103.5, 69.1.
3
1
983, 18, 365.
1
. Masubuchi, M.; Kawasaki, K.; Ebiike, H.; Ikeda, Y.;
Tsujii, S.; Sogabe, S.; Fujii, T.; Sakata, K.; Shiratori, Y.;
Aoki, Y.; Ohtsuka, T.; Shimma, N. Bioorg. Med. Chem.
Lett. 2001, 11, 1833.
4
.6. Microbiology
Antimicrobial susceptibility testing was performed by
the standardized disk diffusion and the agar dilution
methods of the National Committee for Clinical Labora-
6. Ebiike, H.; Masubuchi, M.; Liu, P.; Kawasaki, K.; Mori-
kami, K.; Sogabe, S.; Hayase, M.; Fujii, T.; Sakata, K.;
Shindoh, H.; Shiratori, Y.; Aoki, Y.; Ohtsuka, T.; Shimma,
N. Bioorg. Med. Chem. Lett. 2002, 12, 607.
2
0
tory Standards. Inhibitory zone diameters were mea-
sured on Nutrient Agar (NA) for bacteria and Potato
Dextrose Agar (PDA) (Difco Lab.) for fungi, with con-
ventional metrical filter paper disks (BBL, Cocksville,
USA, 6 mm in diameter) containing specified doses of
compounds. All experiments were conducted twice and
repeated if the results differed. The inhibitory zone diam-
eters were read with a calliper, and all results were round-
ed up to the nearest whole numbers (millimetre) for
analysis. For each tested strain, the growth conditions
and the sterility of the medium were checked in two neg-
ative controls. The results obtained were compared with
standard antibiotics, ampicillin, kanamycin (30 lg/disc),
amphotericin B (100 units/disc) and nystatin (30 lg/disc)
and discs bought from Mast Diagnostics, UK.
7
. Kawasaki, K.; Masubuchi, M.; Morikami, K.; Sogabe, S.;
Aoyama, T.; Ebiike, H.; Niizuma, S.; Hayase, M.; Fujii, T.;
Sakata, K.; Shindoh, H.; Shiratori, Y.; Aoki, Y.; Ohtsuka,
T.; Shimma, N. Bioorg. Med. Chem. Lett. 2003, 13, 87.
. Palmer, L. M.; Janson, C. A.; Smith, W. W., Jr. P.C.T.
Int. Patent 016237, 2005.
8
9
. Christensen, S. B., IV; Cummings, M. D.; Lee, J.; Xiang,
J.-N. P.C.T. Int. Patent 098584, 2004.
10. Ehmann, D. E.; Demeritt, J. E.; Hull, K. G.; Fisher, S. L.
Biochim. Biophys. Acta 2004, 1698, 167.
1. Thomas, M.; Allanson, N. M.; Lawson, C. U.K. Patent
386891, 2003.
2. R o¨ hrkasten, R; Konrad, M.. In Kreher, R. P., Ed., 4th
ed.; Methoden der Organischen Chemie Houben–Weylt,
Heteroarenes I, Part 2; Georg Thieme: Stuttgart, 1994;
Vol. E6b, pp 33–162.
1
1
2
1
3. Heck, R. F. In Palladium Reagents in Organic Synthesis;
Academic: London, 1987; Zhang, Y.; Negishi, E.-I. J. Am.
Chem. Soc. 1984, 111, 3454.
The minimum inhibitory concentrations (MICs, lg/mL)
were determined on brain heart infusion broth (Difco
Lab.) containing dilutions of antimicrobial agents rang-
ing from 0.25 to 128 lg/mL. The tested compounds were
dissolved in dimethyl sulfoxide (DMSO) and diluted
tenfold when inoculated, reducing the final solvent con-
centration to 1%. The innocuity of the DMSO was
checked at this concentration using negative controls.
The final microbial concentration for inocula was of
14. Heck, R. F. In Palladium Catalyzed Vinylation of Organic
Halides in Organic Reaction; John Wiley and Sons: New
York, 1982; Vol. 27, pp 345–390; Hegedus, L. S. Tetra-
hedron 1984, 40, 2415; Stille, J. K. Angew Chem., Int. Ed.
Engl. 1988, 27, 1113.
1
5. Kundu, N. G.; Pal, M.; Mahanty, J. S.; De, M. J. Chem.,
Soc., Perkin Trans. 1 1997, 2815; Arcadi, A.; Marinelli, F.;
Cacchi, S. Synthesis 1986, 749.
6. Cacchi, S.; Fabrizi, G.; Moto, L. Tetrahedron Lett. 1998,
3
6
6
1
· 10 –5 · 10 CFU/mL, and was incubated at 35 ꢁC
1
for 24 h. The MIC was defined as the lowest drug con-
centration that inhibited the viable growth of organisms.
The assay was repeated whenever trailing end points
were encountered.
9, 5101.
17. Larock, R. C.; Yum, E. K.; Doty, M. J.; Sham, K. K. C.
J. Org. Chem. 1995, 60, 3270.
18. Khan, M. W.; Kundu, N. G. Synlett 1997, 1435; Kundu,
N. G.; Khan, M. W.; Mukhopadhyay, R. Tetrahedron
1999, 55, 12361; Kundu, N. G.; Khan, M. W.; Mahanty, J.
S. J. Chem. Res.(s) 1999, 460.
Acknowledgments
1
2
9. Khan, M. W.; Kundu, N. G. Synlett 1999, 456.
0. National Committee for Clinical Laboratory Standards.
Methods for Dilution Antimicrobial Susceptibility Tests
for Bacteria that Grow Aerobically, Approved Standard.
NCCLS Document M7-A4, 4th ed. Vol. 17, No. 2, 1997;
NCCLS. Reference Method for Broth Dilution Antifungal
Susceptibility Testing of Yeasts; Approved Standard.
NCCLS Document M27-A2, 2nd ed. Vol. 22 No. 15;
National Committee for Clinical Laboratory Standards:
Financial support from the Ministry of Science and
Information and Communication Technology, Govern-
ment of the PeopleÕs Republic of Bangladesh, is grateful-
ly acknowledged. The authors also thank Professor
C. M. Hasan, Faculty of Pharmacy, University of Dha-
ka, Dhaka, and Dr. John Sorensen, OCMS, and the
Department of Chemistry, CRL, Oxford, for their kind
cooperation in structural elucidation.
9
1
40 West Valley Road, Suite 1400, Wayne, Pennsylvania
9087-1898 USA, 2002.
2
1. McLaughlin, J. L.; Rogers, L. L.; Anderson, J. E. Drug
Inf. J. 1998, 32, 513.
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