Synthesis and anti-inflammatory activities of mono-carbonyl analogues of curcumin

Article abstract of DOI:10.1016/j.bmcl.2007.12.068

Curcumin has been extensively studied for its anti-inflammatory activities. However, its potential beneficial effects on various disease preventions and treatments are limited by its unstable structure. The β-diketone moiety renders curcumin to be rapidly metabolized by aldo-keto reductase in liver. In the present study, a series of curcumin analogues with more stable chemical structures were synthesized and several compounds showed an enhanced ability to inhibit lipopolysaccharide (LPS)-induced TNF-α and IL-6 synthesis in macrophages.

Full text of DOI:10.1016/j.bmcl.2007.12.068

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 1525–1529
Synthesis and anti-inflammatory activities
of mono-carbonyl analogues of curcumin
b
c
b
b
Guang Liang,a,b Xiaokun Li,a,c,d, Li Chen, Shulin Yang, Xudong Wu, Elaine Studer,
*
Emily Gurley,b Phillip B. Hylemon,b Faqing Ye,a Yueru Lid and Huiping Zhoua,b,
*
aCollege of Pharmacy, Wenzhou Medical College, Wenzhou, Zhejiang 325035, PR China
bDepartment of Microbiology and Immunology, School of Medicine, Virginia Commonwealth University, Richmond, VA 23298, USA
cCollege of Chemistry, Nanjing University of Science and Technology, Nanjing 210094, PR China
dMinistry of Education’s Engineering Research Center of Bioreactor and Pharmaceutical Development, Jilin Agricultural University,
Changchun 130118, PR China
Received 2 September 2007; revised 16 December 2007; accepted 17 December 2007
Available online 1 January 2008
Abstract—Curcumin has been extensively studied for its anti-inflammatory activities. However, its potential beneficial effects on var-
ious disease preventions and treatments are limited by its unstable structure. The b-diketone moiety renders curcumin to be rapidly
metabolized by aldo–keto reductase in liver. In the present study, a series of curcumin analogues with more stable chemical struc-
tures were synthesized and several compounds showed an enhanced ability to inhibit lipopolysaccharide (LPS)-induced TNF-a and
IL-6 synthesis in macrophages.
Ó 2007 Elsevier Ltd. All rights reserved.
O
O
Curcumin
[1,7-bis-(4-hydroxy-3-methoxyphenyl)-1,6-
1
7
3'
4'
OCH3
H3CO
HO
5
3
heptadiene-3,5-dione, Fig. 1] is the major active constit-
uent of turmeric, a yellow compound originally isolated
from the plant Curcuma longa L. Tumeric has been
widely used for centuries as a dietary spice and pig-
ment.1 In addition to its unique flavor and color, tumer-
ic has been extensively used in traditional medicine in
China and India, particularly as an anti-inflammatory
agent.2 During the last two decades, numerous studies
have shown that curcumin has a variety of biological
and pharmacological activities such as anti-carcinogen,
immuno-modulation, anti-oxidant, anti-angiogenesis,
and chemo-prevention.3–9
3''
4''
OH
Figure 1. Chemical structure of curcumin.
been demonstrated that cytokines are involved in a vari-
ety of biological processes and play a central role in the
development of inflammation and immunity. TNF-a is a
multifunctional cytokine produced primarily by acti-
vated monocytes/macrophages and plays a critical role
in the initiation and continuation of inflammation and
immunity.16 It is well known that TNF-a is a key proin-
flammatory cytokine in the pathogenesis of various
inflammatory diseases and cancer.13,17 In addition to di-
rectly inducing inflammatory response, TNF-a is also
able to induce other proinflammatory cytokines includ-
ing IL-6, IL-1b, IL-8 and itself by activating NF-kB and
MAP kinase signaling pathways.16,18 It has been shown
that curcumin is able to inhibit the production of proin-
flammatory cytokines TNF-a and IL-6 in macrophages
and various cancer cells.18,19 It also can inhibit the
expression of other inflammatory mediators such as
COX-2 and iNOS.20 However, preclinical and clinical
studies have found that the potential beneficial effects
of curcumin on various disease preventions and
Recent studies have demonstrated that inflammation is
implicated in the pathogenesis of various diseases
including cancer, atherosclerosis, diabetes, fatty liver,
rheumatoid arthritis, and inflammatory bowel dis-
ease.10–13 Anti-inflammation is the major focus of cur-
rent drug development.14 Cytokines are local
mediators produced by lymphocytes and macrophages
as well as by epithelial and mesenchymal cells.15 It has
Keywords: Curcumin; Anti-inflammation; TNF-a; IL-6.
*
Corresponding authors. Tel.: +1 804 828 6817; fax: +1 804 828 0676
(H.Z.); e-mail addresses: xiaokunli@163.net; hzhou@vcu.edu
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.12.068

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G. Liang et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1525–1529
treatments are limited by its poor pharmacokinetic
properties.3,21,22 It is believed that the presence of the ac-
tive methylene group and b-diketone moiety contributes
to the instability of curcumin under physiological condi-
tions, poor absorption, and fast metabolism.23 Recently,
synthetic modifications of curcumin have been studied
intensively in order to develop a molecule with enhanced
properties and stability.23–26 In the present study, a ser-
ies of curcumin analogues with more stable structures
were synthesized and tested for anti-inflammatory activ-
ity in vitro. Several analogues showed an enhanced abil-
ity to inhibit lipopolysaccharide (LPS)-induced TNF-a
and IL-6 in macrophages.
on the benzene ring and the influence of the structure
of 5-C linker on inflammatory activities when the unsta-
ble methylene was absent. The structure and general
synthesis of analogues designed are shown in Table 1,
and Schemes 1 and 2, respectively.
R1
O
O
O
R1
R2
R3
R2
R3
cyclopentanone
alkali/alcohol
A
R4
R1
R4
R1
R1 O
R2
R3
R2
R3
R2
R3
acetone
H
B
C
In order to identify the crucial structural motifs leading
to anti-inflammatory activity and gain insight into fu-
ture directions for designing new analogues with better
activity, three series of mono-carbonyl curcumin ana-
logues, 1,5-diaryl-1,4-pentadiene-3-ones (B), together
with cyclopentanone (A) and cyclohexanone (C) ana-
logues, were designed by deleting the methylene group
and one carbonyl group. These compounds were also
designed to examine the role of different substituents
alkali/alcohol
R4
R1
R4
R1
R4
R2
R3
R2
R3
cyclohexanone
alkali/alcohol
R4
R4
Scheme 1. General synthesis of 1,5-diphenyl-1,4-pentadiene-3-ones
and cyclic analogues.
Table 1. Structure of curcumin analogues
A
B
C
R1
R1
R1
R1
R1
R1
O
O
O
R2
R3
R2
R3
R2
R3
R2
R3
R2
R3
R2
R3
R4
R4
R4
R4
R4
R4
R4
R1
R2
R3
1
2
3
4
5
6
7
8
H
H
Br
F
H
OH
OH
H
H
H
H
H
H
H
H
O
OCH3
H
O
O
Br
Br
n
CF3
Br
H
H
H
H
H
H
A15: n = 2
B15: n = 0
C15: n = 3
H
F
H
OCH3
OCH2CH3
OCH3
OCH3
O
S
S
S
O
O
n
9
H
H
H
H
H
A16: n = 2
C16: n = 3
10
11
H
N (CH3)2
H
O
S
O
O
OCH3
H
n
A17: n = 2
B17: n = 0
C17: n = 3
12
13
H
H
H
OCH3
OCH2CH@CH2
OCH2CH@CH2
H
H
O
n
14
H
H
H
N
O
A18: n = 2
B18: n = 0
C18: n = 3

G. Liang et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1525–1529
1527
with a ratio of 2:1 of substituted arylaldehydes to ke-
tones. Incorporation of two aryl-rings was confirmed
using 1H NMR analysis by detecting the absence of
methyl (B analogues) or methylene (A and C analogues)
next to the carbonyl group.
R1
R1
CHO
CHO
O
i
O O
HO
O
ii
R1
R1
O O
O O
B09 R1 = H
For the synthesis of analogues 1 and 2 in alkaline condi-
tions, the hydroxyls of 4-hydroxybenzaldehyde and 3-
methoxy-4-hydroxybenzaldehyde were protected with
an easily removable group, 3,4-dihydro-a-pyran.
Scheme 2 illustrates a representative example of the
preparation of the tetrahydropyran-2-yl-protected deriv-
ative and its reaction with acetone to obtain compounds
9 and 11. Analogues 1 and 2 were prepared by deprotec-
tion with a catalytic amount of p-toluenesulfonic acid,
and compounds 12 and 13 were obtained by further
etherification with allyl bromide.
B11 R1 = OCH3
iii
O
R1
R1
R2O
B01 R1 = H, R2 = H;
OR2
iv
iv
B12 R1 = H, R2 = CH2-CH=CH2;
B02 R1 = OCH3, R2 = H;
B13 R1 = OCH3, R2 = CH2-CH=CH2
Scheme 2. Synthesis of presented compounds. Reagents and condi-
tions: (i) pyridine–PTSA, CH2Cl2, rt; (ii) CH3COCH3, NaOH/EtOH,
rt; (iii) PTSA, MeOH, rt; (iv) CH2@CHACH2Br, K2CO3/acetone,
reflux.
The anti-inflammatory activities of curcumin and its
analogues were measured as the ability of these com-
pounds to inhibit LPS-induced TNF-a and IL-6 expres-
sion in mouse J774A.1 macrophages using Enzyme-
linked immunosorbent assays (ELISA).27
Most of the compounds were synthesized by coupling
the appropriate aromatic aldehyde with cyclohexanone,
cyclopentanone or acetone in an alkaline solution,
respectively. Details of the reaction routes, yields, melt-
ing points, NMR, and ESI-MS analysis are described in
Online supplemental data. All reactions were carried out
Briefly, cells were pre-treated with 10 lM of curcumin,
each analogue or vehicle control for 2 h, then treated
with LPS (0.5 lg/ml) for 24 h. At the end of treatment,
the culture media were collected and centrifuged at
250
200
150
100
**
*
*
*
*
*
50
*
*
0
250
200
150
100
50
*
*
*
0
Figure 2. Inhibition of LPS-induced TNF-a and IL-6 by curcumin and its analogues in J774A.1 macrophages. Cells were pretreated with curcumin
or its analogues (10 lM) for 2 h, then treated with LPS (0.5 lg/ml) for 24 h. TNF-a (A) and IL-6 (B) levels in the culture media were measured by
ELISA. The results are expressed as percent of LPS control. Each bar represents mean SE of five independent experiments. Statistical significance
relative to LPS is indicated, *p < 0.05.

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G. Liang et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1525–1529
14,000 rpm for 5 min. TNF-a and IL-6 levels in the med-
ia were determined by ELISA using mouse TNF-a and
mouse IL-6 ELISA MaxTM Set Deluxe Kits (Biolegend,
USA). The total protein concentrations of the viable cell
pellets were determined using Bio-Rad protein assay re-
agents. Total amounts of the TNF-a and IL-6 in the
media were normalized to the total protein amount in
the viable cell pellets.
Acknowledgments
This work was supported by The Program of New Cen-
tury Excellent Talents in Universities (2006), Zhejiang
Provincial Program for the Cultivation of High-level
Innovative Health talents (2007) and NIH Grants (AI-
68432 and AT-04148) and A.D. Williams Award (to
H. Zhou), USA.
The results indicate that curcumin and its analogues
inhibited LPS-induced TNF-a and IL-6 expression to
various degrees. The cyclohexanone-derived C com-
pounds were more effective than acetone-derived A
and cyclopentanone-derived B compounds. Among
these compounds, A02, A11, B02, B09, B12, C02, C11,
C12, and C15 are more potent than curcumin in inhibit-
ing LPS-induced TNF-a expression (Fig. 2A). Com-
pounds B02, C02, and A14 showed better inhibitory
effect than curcumin on LPS-induced IL-6 expression
(Fig. 2B). A14, a novel compound with a long chain sub-
stituent group of 3-(dimethylamino) propoxyl, showed a
similar inhibitory effect on LPS-induced TNF-a expres-
sion as curcumin, but has more potent inhibitory effect
on LPS-induced IL-6 expression than curcumin. How-
ever, the similar dimethylamino-substituted compounds
(A10, B10, and C10) showed similar or less inhibitory ef-
fects on LPS-induced TNF-a and IL-6 expression as
curcumin, indicating that nitrogenous substitution by
itself does not enhance the anti-inflammatory activity.
On the other hand, B12 and C12 with a long chain allyl-
oxyl substituent group showed stronger inhibitory effect
on LPS-induced TNF-a indicating that the length and
flexibility of the substituent groups may be favorable
to the anti-inflammatory activity. Among eight hetero-
cyclo-substituted compounds 15–17, A16, A17, and
C15 exhibited moderate activity on inhibiting LPS-in-
duced TNF-a secretion and B17 displayed a strong
inhibition on IL-6 expression (43.1%). Among curcu-
min-like compounds, A02, B02, and C02 showed the
best inhibition activities (Fig. 2) while A01, B01,
and C01 have less or opposite activities, suggesting that
the presence of a 3-methyoxy group is critical to the
activity.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.bmcl.
2007.12.068.
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In summary, three series of mono-carbonyl analogues of
curcumin were synthesized. Their structures were identi-
fied by NMR analysis and anti-inflammatory activities
were examined by measuring the inhibitory effect on
LPS-induced TNF-a and IL-6 expression in macro-
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analogues has been reported previously, the inhibitory
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