Microwave-assisted synthesis and antimicrobial activities of flavonoid derivatives

Article abstract of DOI:10.1016/j.bmcl.2007.11.081

Eleven flavonoid derivatives were synthesised using a modified Baker-Venkataraman rearrangement, and subsequent microwave-assisted closure of the heterocyclic ring. All of the synthetic compounds displayed antifungal activity against Aspergillus niger and

Full text of DOI:10.1016/j.bmcl.2007.11.081

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 518–522
Microwave-assisted synthesis and antimicrobial activities
of flavonoid derivatives
Sherif B. Abdel Ghani,a Louise Weaver,b Zidan H. Zidan,c Hussein M. Ali,d
C. William Keevilb and Richard C. D. Browna,*
aSchool of Chemistry, University of Southampton, Southampton SO17 1BJ, UK
bEnvironmental Healthcare Unit, School of Biological Sciences, University of Southampton, Bassett Crescent East,
Southampton SO16 7PX, UK
cPlant Protection Department, Faculty of Agriculture, Ain Shams University, Cairo, Egypt
dAgricultural Biochemistry Department, Faculty of Agriculture, Ain Shams University, Cairo, Egypt
Received 17 October 2007; revised 20 November 2007; accepted 21 November 2007
Available online 28 November 2007
Abstract—Eleven flavonoid derivatives were synthesised using a modified Baker–Venkataraman rearrangement, and subsequent
microwave-assisted closure of the heterocyclic ring. All of the synthetic compounds displayed antifungal activity against Aspergillus
niger and Fusarium oxysporium, and two of the synthetic flavonoid analogues exhibited significant activity against methicillin-resis-
tant Staphylococcus aureus.
ꢀ 2007 Elsevier Ltd. All rights reserved.
The ability of bacteria to acquire resistance to existing
chemotherapies is a major concern and a critical chal-
lenge for medicine in the 21st century. Methicillin-resis-
tant Staphylococcus aureus (MRSA) has become a
serious problem in hospitals and is considered to be a
growing threat to community health.1 Perhaps even
more worrying is the emergence of bacterial strains that
have resistance to the drug vancomycin, which has been
a mainstay of MRSA therapy.2 Fungal infections also
present a serious risk to human health, particularly in
individuals with compromised immune systems.3 In
addition, plant pathogenic fungi can cause serious dam-
age to crops.4 The discovery of new classes of selective
antibacterial and antifungal agents should therefore be-
come a priority.
Isolated flavonoids have been shown to possess a host
of important biological activities, and the potential of
naturally occurring flavonoids as anti-infective agents
has been recognised.6,9–11 Some natural flavonoids and
synthetic flavonoid derivatives have shown promising
antibacterial activity against drug resistant strains,
including MRSA, as well as antifungal activities.8,10–15
Herein we describe a short synthesis of a series of simple
flavonoid derivatives, and describe some preliminary
antimicrobial activity in MRSA and fungi.
Flavones 3a–k were synthesised from 2-hydroxyacetoph-
enones using the Baker–Venkataraman rearrangement,16
followed by microwave-assisted condensation to close the
heterocyclic ring (Scheme 1, Table 1).17 Thus, solutions of
the appropriately substituted o-hydroxyacetophenones
1a–k in pyridine were treated with a slight excess of the se-
lected acid chlorides and two equivalents of DBU, then
heated at 80 ꢁC for 16 h. In several cases the resulting
1,3-diketones 2a–c were purified by column chromatogra-
phy, but in general it was convenient to use the crude
products 2d–k in the cyclisation reaction.
Flavonoids are a major class of oxygen-containing het-
erocyclic natural products that are widespread in green
plants.5 Flavonoids have been recognised to have a pro-
tective effect in plants towards microbial invasion by
plant pathogens,6,7 and flavonoid rich plant extracts
have been used for centuries to treat human disease.8
Microwave-assisted cyclisation of 1-(2-hydroxyaryl)pro-
pane-1,3-diones has been reported to proceed efficiently
in the presence of CuCl2, or under acidic conditions
(H2SO4/AcOH) or in ionic liquid ([EtNH3]NO3) to
Keywords: Antibacterial; Antifungal; Flavonoids.
*
Corresponding author. Tel.: +44 23 8059 4108; fax: +44 23 8059
3781; e-mail: rcbl@soton.ac.uk
0960-894X/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.11.081

S. B. Abdel Ghani et al. / Bioorg. Med. Chem. Lett. 18 (2008) 518–522
519
R2COCl
DBU (2 equiv.),
pyridine, 80 °C
microwave-assisted cyclisation of the diones 2a–k was
achieved by simply using EtOH containing a small
amount of concentrated H2SO4 (100:1 by volume).20,21
O
O
O
R2
R1
R1
OH
OH
2a—k
Preliminary assessment of the antibacterial activities of
synthetic flavonoids 3a–k was undertaken using the filter
paper disc diffusion method.22 Two of the compounds,
both carrying cyclohexyl groups at their 2-positions,
inhibited the growth of methicillin-resistant S. aureus
(MRSA, NCTC 10442) whilst none of them displayed
any activity against Escherichia coli (Gram negative).
This lack of antibacterial activity in E. coli is consistent
with the findings of Go¨ker et al. for a series of amidino-
benzimidazole functionalised 2-phenyl and 2-methyl-
4H-1-benzopyran-4-ones.12 The active compounds were
subjected to further evaluation in broth media fortified
with different concentrations (400, 200, 100 and 50 lg/
mL) of 3j and 3k in DMSO. Gentamycin was used as
a positive control at the same concentrations in sterile
water, and a negative control was introduced by apply-
ing the solvent (DMSO) used to dissolve the com-
pounds. Bacterial growth was assessed using the
Bradford method for analysis of protein content after
3 h incubation at 37 ꢁC (Fig. 1),23 and IC50 values were
calculated for gentamycin (25.0 lg/mL), 3j (69.6 lg/mL)
and 3k (12 mg/mL). It is interesting to note that a simple
cyclohexyl derivative 3j, prepared in two steps from
commercial materials, has almost half the potency of
the clinical antibacterial agent gentamycin.
1a—k
O
O
0.1 M H2SO4, EtOH,
µW 100 °C, 15—30 min
R1
(65—85% 2 steps)
R2
3a—k
Scheme 1. Microwave-assisted synthesis of flavonoid derivatives 3a–k.
afford flavones and chromones.18 The latter two proce-
dures were developed using modified domestic micro-
wave ovens,18b,c which do not offer the level of control
available from modern purpose-built microwave synthe-
sizers.17 We required a simple and controlled method to
effect the cyclisation of compounds 2a–k in high yield.
Furthermore, we wished to avoid the use of copper salts
in the preparation of compounds for use in antibacterial
and antifungal assays as copper can be deliterious to the
growth of these organisms.19 Conventionaly, cyclisation
is carried out in hot acetic acid containing sulfuric acid,
and although this proved to be effective, the removal of
large amounts of acetic acid was inconvenient. We
found that a practical and economical method for
Figure 1. Activity of compounds against MRSA: protein content after treatment versus concentration of the tested compounds.
Figure 2. Baclight images of MRSA after exposure to 3j. (a) Total cells with Syto9 stain. (b) Dead cells stained with propidium iodide.

520
S. B. Abdel Ghani et al. / Bioorg. Med. Chem. Lett. 18 (2008) 518–522
Table 1. Structures and yields for synthetic flavone analogues 3a–k
Entry 1-(2-Hydroxyaryl)propane-1,3-diones
Flavone analogues
Yield (%)c
O
O
O
Cl
Cl
1
85
MeO
Br
O
3a (95%)a
MeO
Br
OH
2a (90%)a
Cl
Cl
O
O
O
2
3
4
5
6
7
8
70
84
84
79
81
80
85
74
O
OH
OMe
3b (94%)a
2b (75%)a
OMe
O
O
O
MeO
Br
MeO
CF3
CF3
O
3c (96%)a
OH
2c (88%)a
CF3
CF3
O
O
O
Br
CF3
CF3
O
3d (84%)a
OH
2db
CF3
CF3
O
O
O
MeO
MeO
O
OH
2eb
Cl
3e (79%)a
Cl
O
O
O
O
O
Br
Br
O
OH
O
Cl
3f (81%)a
2fb
Cl
O
Br
Br
O
OH
F
3g (80%)a
2gb
F
O
O
Br
Br
O
OH
2hb
3h (85%)a
O
O
O
O
9
MeO
MeO
O
MeO
MeO
OH
O
Br
3i (74%)a
2ib
Br
O
10
65
O
OH
2jb
3j (65%)a
(continued on next page)

S. B. Abdel Ghani et al. / Bioorg. Med. Chem. Lett. 18 (2008) 518–522
521
Table 1 (continued)
Entry
1-(2-Hydroxyaryl)propane-1,3-diones
Flavone analogues
Yield (%)c
O
O
O
Br
Br
11
66
O
OH
2kb
3k (66%)a
aYield refers to analytically pure material.
bCrude material used in next step.
cYield over two steps.
compounds displayed antifungal activity, and one of
them showed significant antibacterial activity against
MRSA. Given the simplicity of the compounds pre-
pared, there is excellent potential for optimisation to
produce potent antibacterial and antifungal agents and
studies towards these ends are in progress.
Table 2. Antifungal activity of flavonoids 3a–k against Aspergillus
niger-NCTC 275 and Fusarium oxysporium
b
b
Compound
A.n.a IC50 (lg/mL)
F.o.c IC50 (lg/mL)
3a
3b
177.6
83.4
93.4
60.3
41.9
157.4
141.8
77.1
28.2
58.9
90.4
3.2
10592.6
2249.1
177.4
65.2
3c
3d
3e
1428.9
281.8
682.3
26915.3
175.8
116.7
106.7
—
Acknowledgments
3f
3g
3h
3i
The authors thank the Egyptian Education & Culture
Bureau for the award of a Mission Scholarship to
SBAG, and CEM (Microwave Technology) Ltd for
the donation of a CEM Discover Benchmate microwave
synthesizer.
3j
3k
Mycostatin
a Aspergillus niger.
b Values are means of replicates.
c Fusarium oxysporium.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.bmcl.
2007.11.081.
To gain some insight into the mode of action of the
flavonoid 3j, a viability assay method (Baclight)24
was carried out which utilises a dual staining tech-
nique to identify live and dead cells, deemed by intru-
sion of the fluorescent dye propidium iodide through
damaged cell membranes (Fig. 2). This experiment
clearly shows the loss of membrane integrity in some
of the cells, indicating that cyclohexyl derivative 3j is
bactericidal.
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(CH3), 42.8 (CH), 30.5 (2CH2), 25.9 (3CH2); MS (ES+) m/z
259 [M+H]+.
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1
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