Synthesis and biological evaluation of novel C (7) modified chrysin analogues as antibacterial agents

Article abstract of DOI:10.1016/j.bmcl.2005.09.009

The natural product, chrysin (5,7-dihydroxy flavone), obtained from Oroxylum indicum, exhibits numerous biological activities including anticancer, anti-inflammatory, and antiallergic activities. Three series of chrysin analogues were prepared, in which c

Full text of DOI:10.1016/j.bmcl.2005.09.009

Bioorganic & Medicinal Chemistry Letters 16 (2006) 221–224
Synthesis and biological evaluation of novel C (7) modified
chrysin analogues as antibacterial agents
a
K. Suresh Babu, T. Hari Babu, P. V. Srinivas, K. Hara Kishore, U. S. N. Murthy
a
a
b
b
a,
*
and J. Madhusudana Rao
a
Division of Organic Chemistry-I, Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500 007, India
b
Biology Division, Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500 007, India
Received 21 July 2005; revised 29 August 2005; accepted 6 September 2005
Available online 5 October 2005
Abstract—The natural product, chrysin (5,7-dihydroxy flavone), obtained from Oroxylum indicum, exhibits numerous biological
activities including anticancer, anti-inflammatory, and antiallergic activities. Three series of chrysin analogues were prepared, in
which chrysin and heterocyclic moieties are separated by 3-carbon, 4-carbon, and 6-carbon spacers. All the derivatives were screened
for antibacterial activity against a panel of susceptible and resistant Gram-positive and Gram-negative organisms. It was observed
that most of the derivatives displayed significant activity as compared to their parent compound (chrysin).
ꢀ
2005 Elsevier Ltd. All rights reserved.
Polyphenolic compounds are plant products which rep-
1
aromatic rings (either A or C) of the chrysin. These re-
sults prompted us to synthesize the chrysin derivatives
in which chrysin and heterocyclic moieties are linked
with spacers and investigate the effects to the size of
the spacers and substitution patterns of the heterocyclic
moieties.
resent common constituents of fruits and vegetables.
The major group of plant polyphenols is represented
by flavanoids and a recent review has estimated their
2
number as 6500 in the plant kingdom. A large number
of biological activities such as antioxidant, anticancer,
and anti-inflammatory properties have been attributed
3
to these compounds.
In continuation of our investigations on the chemical
modification of the bioactive natural flavones and their
1
6
Chrysin is a flavone widely distributed in plants which
was reported to have many biological activities includ-
activity studies, Herein, we report the synthesis of
1
7
three series of analogues of chrysin and their antibac-
terial activity studies.
4
5
6
ing antibacterial, antioxidant, anti-inflammatory,
7
8
9
antiallergic, anticancer, antiestrogenic, , and anxiolyt-
1
0
ic activities. Furthermore, chrysin is also found to
1
Chrysin was isolated from the traditional Indian medic-
inal plant Oroxylum indicum in substantial yields. To in-
crease the antibacterial properties of chrysin, we
prepared chrysin derivatives in which chrysin ring sys-
tem linked to the alkyl amines by different spacers at
C-7 position with a view to enhance their lipophilicity.
This was achieved in two steps, in the initial step by
the alkylation of 7-hydroxy group at 7th position by
the corresponding dibromo alkanes in acetone and final-
ly coupling the secondary amines with the bromo com-
pound. The following Scheme 1 summarizes the
procedures used to prepare the three series of chrysin
analogues. Compounds in series 1(3a–3d) contain 3-car-
bon spacer in between chrysin and heterocyclic moiety,
compounds (4a–4d) separated by 4-carbon spacer, and
compounds (5a–5d) contain 6-carbon spacer in between
1
have tyrosinase inhibitory activity and moderate aro-
1
matase inhibitory activity. Chrysin can also inhibit
2
the metabolism of the carcinogen benzo[a]pyrene by
hamster embryo cells in tissue culture. In fact, a num-
1
3
ber of derivatives were prepared to improve the biolog-
1
4
ical activity of chrysin.
hydrophobic derivatives are more potential P-glycopro-
Such as C-isoprenylated
1
5
tein modulators in tumor cells. However, the efforts to
date have centered mostly on the substitutions on the
Keywords: Chrysin; Oroxylum indicum; Spacers; Antibacterial activity;
Amines.
*
Corresponding author. Tel.: +91 40 27193166; fax: +91 40
7160512; e-mail addresses: janaswamy@iict.res.in; janaswamy@
ins.iictnet.com
2
0
960-894X/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.09.009

222
K. Suresh Babu et al. / Bioorg. Med. Chem. Lett. 16 (2006) 221–224
HO
O
K
2
3
CO / Acetone, dibromoalkanes
Br
O
O
reflux, 6 hrs
n
OH O
OH O
1
(
2a–2c)
n = 1, 2, 4
O
R
,
CH
3 2 3
CN/ K CO
R
O
(
2a–2c)
Reflux, 3– 4 hrs
n
OH O
n = 1, 2, 4
(3a–3d), (4a–4d), (5a–5d)
(3a–5a); R = Morpholine
(3b–5b); R =N-methyl piperizine
(3c–5c); R= Piperidine
(3d–5d); R =N,N-Dimethyl amine
Scheme 1. Synthesis of 7-O-alkylamino derivatives of chrysin.
chrysin and the substituent. All the synthetic com-
pounds were well characterized by their spectral charac-
teristics. These compounds were designated to test the
importance of the substitution along with the size of the
linker.
dilution method recommended by National Committee
1
9
for Clinical Laboratory (NCCL) standards. Standard
antibacterial agents like penicillin and streptomycin
were also screened under identical conditions for com-
parison. The minimum inhibitory concentrations are
represented in Table 1. It has been observed that all
the derivatives exhibited interesting biological activity
however, with a degree of variation.
1
8
The minimum inhibitory concentrations (MIC) of 7-O-
alkylamino derivatives of chrysin were obtained against
three representative Gram-positive organisms viz. Bacil-
lus subtilis (MTCC 441), Bacillus sphaericus (MTCC 11),
and Staphylococcus aureus (MTCC 96), and three
Gram-negative organisms viz. Chromobacterium violace-
um (MTCC 2656), Klebsiella aerogenes (MTCC 39), and
Pseudomonas aeruginosa (MTCC 741) by the broth
Compounds in series 1, which contain 3-carbon spacer,
displayed good zone of inhibition for C. violaceum and
B. spharicus. In this series, compounds 3a, 3b, 3c, and
3d showed good activity equal to that of streptomycin
against B. spharicus. Similarly, compounds 3a, 3b, and
Table 1. Minimum inhibitory concentrations (MIC-lg/ml) of 7-O-alkylamino derivatives of chrysin
Compounds
Microorganisms
Gram positive
Gram negative
Bacillus
subtilis
Bacillus
Staphylococcus
aureus
Pseudomonas
Klebsiella
aerogenes
Chromobacterium
violaceum
sphaericus
aeruginosa
Chrysin
50
25
50
25
—
—
50
25
25
3
3
3
3
4
4
4
4
5
5
5
5
a
b
c
12.5
12.5
25
12.5
12.5
12.5
12.5
6.25
6.25
12.5
6.25
25
12.5
12.5
6.25
12.5
6.25
6.25
6.25
12.5
25
25
—
—
25
25
12.5
12.5
12.5
12.5
25
d
a
b
c
25
6.25
25
—
—
25
12.5
12.5
25
—
—
12.5
6.25
25
d
a
b
c
25
25
—
—
25
25
25
25
25
12.5
25
—
—
25
25
12.5
25
12.5
12.5
12.5
3.125
d
12.5
6.25
1.562
25
—
1.562
6.25
25
25
3.125
12.5
Streptomycin
Penicillin
6.25
1.562
1.562
6.25
Negative control DMSO, no activity.

K. Suresh Babu et al. / Bioorg. Med. Chem. Lett. 16 (2006) 221–224
223
3
c displayed notable activity equal to that of penicillin
10. Wolfman, C.; Viola, H.; Paladini, A.; Dajas, F.; Medina,
J. H. Pharmacol. Biochem. Behav. 1994, 47, 1.
and compound 3c showed good activity against C.
violaceum.
1
1. Kubo, I.; Kinst-Hori, I.; Choudhuri, S. K.; Kubo, Y.;
Sanchez, Y.; Ogura, T. Bioorg. Med. Chem. Lett. 2000, 8,
1
749.
2. Suresh, C. T.; Leena, S.; Sari, M.; Risto, S. J. Med. Chem.
999, 2, 235.
3. Liu, Y. L.; Ho, D. K.; Cassady, J. M. J. Nat. Prod. 1992,
5, 357.
Similarly, compounds in series 2, which contain 4-car-
bon spacer, displayed good antibacterial activity
against both Gram-positive and Gram-negative organ-
isms. Compound 4a in which heterocyclic ring (mor-
pholine) and chrysin separated by 4-carbon chain
displayed a high degree of activity against C. violace-
um and B. sphaericus. Accordingly, compound 4b,
which contains 4-carbon spacer in between chrysin
and piperizinyl rings, showed good activity against
C. violaceum.
1
1
1
5
14. (a) Shin, J. S.; Kim, K. S.; Kim, M. B.; Jeong, J. H.; Kim,
B. K. Bioorg. Med. Chem. Lett. 1999, 9, 869; (b) Larget,
R.; Lockhart, B.; Renard, P.; Largeron, M. Bioorg. Med.
Chem. Lett. 2000, 10, 835; (c) Xing, Z.; Wei-Ding, M.;
Yang-Yan, X.; Jian-Guo, C.; Feng-Ling, Q. Bioorg. Med.
Chem. Lett. 2003, 13, 881; (d) Dao, T.-T.; Chi, Y.-S.; Kim,
J.; Kim, H.-P.; Kim, S.; Park, H. Bioorg. Med. Chem. Lett.
2
004, 14, 1165.
Compounds in series 3, which contain 6-carbon spacer,
displayed moderate zone of inhibition against both
Gram-positive and Gram-negative organisms, but dis-
played MIC values lower than that of the parent com-
pound chrysin. It is important to note that all the
derived compounds displayed MIC values lower than
that of parent compound irrespective of the spacer. As
shown in Table 1, none of the compounds exhibited
any activity against P. aeruginosa even at the concentra-
tion of 200 lg/ml. The compounds were also inactive
against the tested antifungal strains.
1
5. Comte, G.; Daskiewicz, J. B.; Bayat, C.; Conseil, G.;
Viornery-Vanier, A.; Dumontet, C.; Pietro, A.; Barran, D.
J. Med. Chem. 2001, 44, 763.
16. (a) Suresh Babu, K.; Tiwari, A. K.; Srinivas, P. V.; Ali, A.
Z.; Raju, B. C.; Rao, J. M. Bioorg. Med. Chem. Lett. 2004,
14, 3841; (b) Rao, R. J.; Tiwari, A. K.; Kumar, U. S.;
Reddy, S. V.; Ali, A. Z.; Rao, J. M. Bioorg. Med. Chem.
Lett. 2003, 13, 2777; (c) Srinivas, P. V.; Reddy, S. V.;
Sreelatha, T.; Hara Kishore, K.; Murty, U. S. N.; Rao, J.
M. Nat. Prod. Res. 2004, 18, 95; (16) Suresh Babu, K.;
Hari Babu, T.; Srinivas, P. V.; Sasrty, B. S.; Hara Kishore,
K.; Murty, U. S. N.; Rao, J. M. Bioorg Med. Chem. Lett.
2
005, 15, 3953.
In conclusion, a series of chrysin derivatives were pre-
pared containing 3, 4, and 6 carbon spacers, in between
heterocyclic ring and chrysin, and were evaluated for
antibacterial activity. Most of the compounds showed
a moderate degree of antibacterial activity. Among them
compounds in series 2, which contain 4-carbon spacer in
between chrysin and heterocyclic ring, displayed good
deal of activity. With this set of analogues, we are now
in a position to investigate the multiple biological activ-
ities reported for chrysin.
1
7. General procedure for the preparation of 7-O-alkylamino
derivatives of chrysin: (i) General procedure for the prep-
aration of 7-O-alkyl derivatives of chrysin (2a–2c): To a
mixture of chrysin 1 (1 g, 3.93 mmol) and anhydrous
potassium carbonate (0.81 g, 5.8 mmol) in 20 ml acetone,
corresponding dibromoalkane (1,3-dibromo propane for
2
2
a, 1,4-dibromo butane for 2b, and 1,6-dibromo hexane for
c were added. The mixture was refluxed under nitrogen
atmosphere for 3–4 h. After completion of the reaction,
potassium carbonate was filtered and washed with excess
acetone (2· 50 ml). The combined acetone layers were
concentrated under vacuum. The residue was purified by
column chromatography on silica gel (60–120 mesh) to yield
7
-O-bromoalkyl chrysin (2a, 2b, and 2c) in pure form. (ii)
Acknowledgments
General procedure for the preparation of 7-O-alkyl deriv-
atives of chrysin: To a mixture of bromoalkyl chrysin (2a,
The authors thank Dr. J. S. Yadav, Director, IICT, for
his constant encouragement. K.S.B. thanks CSIR, New
Delhi, for financial assistance.
2
1
b, and 2c) and anhydrous potassium carbonate (2.41 g,
7.2 mmol) in 20 ml acetonitrile, the corresponding amine
was added. The mixture was refluxed under nitrogen
atmosphere for 3–4 h. After completion of the reaction,
the reaction mixture was brought to room temperature and
was poured into ice water and washed with methylene
chloride (2· 10 ml). The combined organic layers were dried
over anhydrous sodium sulfate and concentrated under
vacuum. The residue was purified by column chromatog-
raphy on silica gel (60–120 mesh) to give the corresponding
References and notes
1
2
. Scalbert, A.; Williamson, G. J. Nutr. 2000, 130, 2073.
. Harborne, J. B.; Williamson, C. A. Phytochemistry 2000,
5, 481.
. Middleton, E.; Kandaswamy, C.; Theoharides, T. C.
5
7
-O-alkylamino derivatives of chrysin (3a–3d, 4a–4d, and
a–5d) in very good yields (60–80%).
3
4
5
6
7
5
8. Physical and spectral characteristics of compound (3a):
Pharmacol. Rev. 2000, 52, 673.
1
. Qais, N.; Rahman, M. M.; Rashid, M. A.; Koshino, H.;
Nagasawa, K.; Nakata, T. Fitoterapia 1996, 67(6), 554.
. Hecker, M.; Preiss, C.; Klemm, P.; Busse, R. Br. J.
Pharmacol. 1996, 118, 2178.
4'
O
2
'''
2'
1
O
N
O
5'''
3''
1''
7
6'
. Fishkin, R. J.; Winslow, J. T. Psychopharmacology (Berl.)
1
997, 132, 335.
. Pearce, F. L.; Befs, A. D.; Bienenstock, J. J. Allegy. Clin.
Immunol. 1984, 73, 819.
5
4
OH
O
1
8
9
. Habtemariam, S. J. Nat. Prod. 1997, 60, 775.
. Kao, Y. C.; Zhou, C.; Sherman, M.; Laughton, C. A.;
Chen, S. Environ. Health Perspect. 1998, 106, 85.
Pale yellow solid, mp 138 ꢁC, H NMR (400 MHz,
0
CDCl
0
3
): d 12.60 (1H, s, OH-5), 7.86–7.90 (2H, m, H-2 ,
0
0
0
6 ), 7.50–7.62 (3H, m, H-3 , 4 , 5 ), 6.64 (1H, s, H-8), 6.46

224
K. Suresh Babu et al. / Bioorg. Med. Chem. Lett. 16 (2006) 221–224
0
0
00 000 000
1.54-1.62 (6H, m, H-2 , H-3 , 5 ), 1.40–1.48 (2H, m, H-
(
(
2
1H, s, H-3), 6.38 (1H, s, H-6), 4.18 (2H, t, H-1 ), 3.82
000
000
000
000
00
000
+
4 ). FABMS: 394 (M +1). Compound (5d): pale yellow
4H, t, H-3 , 5 ), 2.40–2.60 (6H, m, H-2 , 6 , H-3 ), 1.9-
0
0
+
.10 (2H, m, H-2 ). FABMS: 382 (M +1). Compound
1
solid, mp 85 ꢁC, H NMR (400 MHz, CDCl )d 12.70
3
1
0
0
(
CDCl
3b): yellow solid, mp 128–130 ꢁC, H NMR (400 MHz,
(1H, s, OH-5), 7.84–7.88 (2H, m, H-2 , 6 ), 7.52–7.58
0
0
0
0
0
3
) d 12.70 (1H, s, OH-5), 7.84–7.86 (2H, m, H-2 , 6 ),
(3H, m, H-3 , 4 , 5 ), 6.70 (1H, s, H-8), 6.48 (1H, s, H-
00
0
0
0
7.46–7.58 (3H, m, H-3 , 4 , 5 ), 6.64(1H, s, H-8), 6.52(1H,
s, H-3), 6.18 (1H, s, H-6), 4.12 (2H, t, H-1 ), 2.40–2.60
3), 6.38 (1H, s, H-6), 4.12 (2H, t, H-1 ), 2.22–2.38 (4H,
00
0
0
00
m, H-2 , 6 ), 2.0 (6H, s, 2· Me), 1.78–1.82 (2H, m, H-
5 ), 1.44–1.58 (2H, m, H-3 ), 1.38–1.42 (2H, m, H-4 ).
00
000
00
0
000
000
00
00
00
(
1
10H, m, H-3 , 5 , H-3 , and H-2 , 6 ), 2.30 (3H, s, Me),
0
+
.90–2.10 (2H, m, H-2 ). FABMS: 395 (M +1). Com-
+
FABMS : 382 (M +1).
1
pound (4c): white solid, mp 128 ꢁC, H NMR (300 MHz,
19. National Committee for Clinical Laboratory Standards
(NCCLS). Standard methods for dilution antimicrobial
susceptibility tests for bacteria, which grows aerobically.
Nat. Comm. Clini. Lab. Stands, Villanova; 1982; pp
242.
0
CDCl
0
3
): d 12.50 (1H, s, OH-5), 7.82–7.86 (2H, m, H-2 ,
0
0
0
6
), 7.48–7.56 (3H, m, H-3 , 4 , 5 ), 6.70 (1H, s, H-8), 6.62
0
0
(
2
1H, s, H-3), 6.42 (1H, s, H-6), 4.18 (2H, t, H-1 ), 2.36–
000 000 00 00
.58 (6H, m, H-2 , 6 , H-4 ), 1.96–2.10 (2H, m, H-3 ),