Novel C-9, 9′-O-acyl esters of (-)-carinol as free-radical scavengers and xanthine oxidase enzyme inhibitors: Synthesis and biological evaluation

Article abstract of DOI:10.2174/157340613804488288

New compounds with hydrophyllic esters of (-)-carinol were synthesized and evaluated as xanthine oxidase enzyme inhibitors and antioxidants. Aliphatic esterfication of C-9,9′-OH groups of (-)-carinol resulted in lowering antioxidant and xanthine oxidase i

Full text of DOI:10.2174/157340613804488288

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Medicinal Chemistry, 2013, 9, 100-103
Novel C-9, 9'-O-acyl Esters of (-)-Carinol as Free-radical Scavengers and
Xanthine Oxidase Enzyme Inhibitors: Synthesis and Biological Evaluation
Praveen Kumar Suryadevara*a, Hari Babu Tatipakab, Rama Subba Rao Vidadalab, Ashok k Tiwarib,
Janaswamy Madhusudana Raob and Katragadda Suresh Babu*b
aSt. Jude Children’s Research Hospital, Department of Chemical Biology and Therapeutics, Memphis, TN-38105, USA
bNatural Products Laboratory, Division of Organic Chemistry-I, and Indian Institute of Chemical Technology,
Hyderabad-500 007, India
Abstract: New compounds with hydrophyllic esters of (-)-carinol were synthesized and evaluated as xanthine oxidase en-
zyme inhibitors and antioxidants. Aliphatic esterfication of C-9,9'-OH groups of (-)-carinol resulted in lowering antioxi-
dant and xanthine oxidase inhibitory activities. However certain aromatic acyl esters considerably improved the xathine
oxidase inhibition. Aromatic esterification with electron withdrawing substitutions would preferred for improvement in
XOD inhibition while retaining radical scavenging activity, electron withdrawing substitution led to the loss of free radi-
cal scavenging property and neutral substituents decrease the enzyme inhibitory potential.
Keywords: (-)-Carinol, Acyl derivatives, Xanthine oxidase inhibition, Free radical scavenging activity.
INTRODUCTION
caused by free radicals generated during enzymatic process.
These enzyme inhibitors and free radical scavengers may
become important therapeutic agents in mitigating various
pathological conditions in different disease process. Free
radical generation that exceeds the capacity of antioxidant
defenses results in oxidative stress, which possibly elicits
irreversible degenerative responses, including apoptosis or
necrosis in living cells [6].
Hyperuricemia is associated with number of pathological
conditions such as gout [1] lowering of uric acid level in
blood could be achieved by xanthine oxidase inhibitors and
inhibitors of renal uratereabsorpotion. Free radical imbalance
leads to stress [2] that is being suggested as the root cause of
many diseases like atherosclerosis, stroke, diabetes and can-
cer. XOD is a widely distributed enzyme, especially in the
microvascular endothelium. It converts hypoxanthine to xan-
thine and also xanthine to uric acid, with concomitant pro-
duction of superoxide anion. This endothelial derived en-
zyme has received considerable attention as a primary en-
dogenous source of free radicals [3,4] and involves in detoxi-
fication, formation of peroxynitirites [5] leading to vascular
constriction. Recent studies in CHF (chronic heart failure)
patients and models of experimental heart failure showed
that xanthine oxidase inhibition increased contractile capac-
ity due to calcium (Ca2+ ) sensitizing mechanism and im-
proved mitochondrial efficiency by reducing myocardial
oxygen consumption. The conclusion might be that targeted
XO inhibition as a novel treatment approach might be suit-
able only in those patients where increased uric acid concen-
trations are an indicator of up-regulated XO activity. These
compelling evidences of increased levels of XOD in various
pathological conditions and damages caused by free radicals
generated in enzymatic process, warrants search for inhibi-
tors of enzyme that can mitigate over free radicals generation
and free radical scavengers that may prevent the damages
Natural products originating from different plants have
been observed to possess multiple therapeutic activities [7].
Among them flavonoids have drawn considerable attention
in recent past due to their antioxidant and multiple enzyme
inhibitory activities. Flavonoids have been studied exten-
sively for their free radical scavenging and XOD inhibitory
activities [8,9]. However, least is reported about free radical
scavenging and XOD inhibitory activities of other class of
compounds, like lignans. In the course of our biological ac-
tivity-screening program from Indian medicinal plants for
identifying free-radical scavengers [10,11] and XOD inhibi-
tors, [12] we observed several lignans possessing mild to
moderate XOD inhibitory activities [13]. Recent reports also
proved that C-9,9'-O-acyl esters of lignans possesses strong
cytotoxic activity against human lung carcinoma and breast
carcinoma cell lines [14].
In continuation, the present study reports the synthesis,
biological evaluation, and structure activity relationship for
novel C-9,9'-O-acyl esters of (-)-carinol and also explores
the possibilities of common structural requirement that may
possess better XOD inhibitory activity coupled with free
radical scavenging property.
*Address correspondence to these authors at the St. Jude Children’s
Research Hospital, Department of Chemical Biology and Therapeutics,
Memphis, TN-38105, USA; Tel: 001-901-595-5791; Fax: 001-901-595-
5715; E-mails: spk0102@gmail.com; kumar.suryadevara@stjude.org;
Dr. K. Suresh Babu, Natural Products Laboratory, Indian Institute of
Chemical Technology, Tarnaka, Hyderabad-500 007, India; Tel: 91-40-
27160123 ext 2741 (off) 91-9963845220; E-mail: suresh@iict.res.in
MATERIALS AND METHODS
(-)- Nortrachelogenin was isolated from a wellknown In-
dian medicinal plant Cedrus deodara in substantial yields
1875-6638/13 $58.00+.00
© 2013 Bentham Science Publishers

Novel C-9, 9'-O-acyl Esters of (-)-Carinol
Medicinal Chemistry, 2013, Vol. 9, No. 1 101
OH
8'
7'
9'
9
OH
OH
OH
H
MeO
1'
MeO
BnO
OH
OH
8
Benzyl bromide, K2CO3
Acetone, reflux, 4 hr
7
HO
4'
H
1
OMe
OMe
4
OH
OBn
2
1
OH
8'
7'
9'
9
MeO
1'
OCOR
OCOR
OH
MeO
8
OCOR
OCOR
H
4'
7
HO
RCOOH, DCC/DMAP
DCM, O0C-RT
H2/Pd-C
H
1
BnO
2
MeOH, rt
* BF3.Et2O/NaI
OMe
4
OH
OMe
OBn
4a-n
3a-n
4a. R = -CH3
3a. R = -CH3
3h. R = -Ph
4h. R = -Ph
4b. R = -(CH2)2CH3
4c. R = -(CH2)4CH3
4d. R = -(CH2)6CH3
4e. R = -(CH2)10CH3
*3i. R = -(CH=CH)Ph
3j. R = -(p-OMe)Ph
3k. R = -(p-NO2)Ph
4i. R = -(CH=CH)Ph
4j. R = -(p-OMe)Ph
3b. R = -(CH2)2CH3
3c. R = -(CH2)4CH3
3d. R = -(CH2)6CH3
3e. R = -(CH2)10CH3
4k. R = -(p-NO2)Ph
3l. R = -(p-F)Ph
4l. R = -(p-F)Ph
4f. R = -(CH2)12CH3
4g. R = -(CH2)14CH3
3f. R = -(CH2)12CH3
3g. R = -(CH2)14CH3
4m. R = -(p-OH,m-OMe)Ph
4n. R = -(3,4,5-OMe)Ph
3n. R = -(3,4,5-OMe)Ph
3m. R = -(p-OH,m-OMe)Ph
Scheme 1. Synthesis of 9, 9'-O-acyl esters of (-)-carinol.
that displayed potent free radical (DPPH) scavenging activ-
ity [14] and mild XOD inhibitory activity. Interestingly,
when
radicals and also increases the –OH bond dissociation ener-
gies, whereas electron withdrawing substituent’s tends to
decrease it. Therefore, electron donating groups are impor-
tant substituent for a compound to possess antioxidant activ-
ity.
(-)-Nortrachelogenin was converted into (-)-carinol, [15] the
XOD inhibitory activity level increased substantially. In
view of available literature on lignan derivatives, our strat-
egy for the synthesis of (-)-carinol derivatives relies upon
retaining the phenolic hydroxyl groups and modification of
terminal alcohol groups (C-9, 9'). This was achieved by the
protection of two phenolic hydroxyl groups as their benzyl
ethers (2) and subsequent condensation with corresponding
acid in presence of dicylohexyldicarbodiimide (DCC) and
dimethylaminopyridine (DMAP) in anhydrous methylene
chloride to obtain the key intermediate (3a-n) further deben-
zylation using palladium on carbon under hydrogen atmos-
phere to obtain the target compounds in excellent yields
(Scheme 1). All the synthesized compounds were purified
and characterized using classical spectroscopic methods such
as IR, Mass and NMR.
Diphenylpicrylhydrazyl (DPPH) is a nitrogen centered
free-radical that reacts similar to peroxyl radicals and its
reaction rates correlates directly with antioxidant activity
[16].Therefore, we chose DPPH as a test model in our study
to evaluate free-radical scavenging activity of the compound
that representperoxyl radical scavenging in-vivo. Influenced
with these insights, we prepared seven aliphatic esters with
different electron withdrawing and electron donating sub-
stituents and seven aromatic 9-9'-O-acyl ester derivatives of
(-)-carinol, respectively. The primary results for their free
radical DPPH scavenging and XOD inhibitory activity are
presented in (Table 1). It is interesting to note from Table 1
that both aliphatic and the aromatic esterification at C-9,9'-
O-acyl positions decreases DPPH scavenging property of the
carinol derivatives. Furthermore, substitution of electron
withdrawing groups (4k, 4l) and neutral group (4h) led to the
loss of DPPH scavenging property. These observations
clearly demonstrate that apart from the presence of vanilloyl-
OH in divanilyllignans, which contribute to the free-radical
scavenging activity in lignans, presence of free hydrogen
donating groups (OH) at 9, 9'-O-acyl positions increase their
RESULTS AND DISCUSSION
In view of our recent observations and the available lit-
erature suggested that the peroxyl radical should be the ma-
jor target for radical scavenging antioxidants. Therefore elec-
tron donating substituent’s on the aromatic ring of the phe-
nolic pharmacore increases the reactivity towards peroxyl

102 Medicinal Chemistry, 2013, Vol. 9, No. 1
Suryadevara et al.
Table 1. Antioxidant (DPPH) Activity and Xanthine Oxidase Enzyme Inhibition of (-)-Carinol (1) and its 9, 9ꢀ-O-acyl Esters (4a-n)a
%DPPH Scavenging At
% of Xanthine Oxidase
Compound
25 ꢀg/mL
Inhibition at 100 ꢀg/mL
4a
45.67±1.2
54.97±1.3
51.02±2.3
55.42±0.6
66.56±0.6
51.97±0.7
41.82±0.2
NA
12.92±0.5
13.18±0.4
31.16±0.5
17.12±0.5
12.73±0.2
10.63±0.5
25.25±1.1
18.74±0.0
19.12±0.1
40.31±0.2
52.19±0.7
39.96±0.0
54.30±3.5
32.37±1.4
36.56±0.4
4b
4c
4d
4e
4f
4g
4h
4i
53.34±1.1
47.95±0.4
NA
4j
4k
4l
4m
NA
40.00±2.5
40.83±0.3
68.21±0.0
4n
Carinol (1)
aValues represent mean and SD of triplicate samples.
Table 2. IC50 and SC50 Values of Compounds for their Xanthine Oxidase Inhibitory and DPPH Scavenging Activities
Compound
IC50 (ꢀg/ml) for XOD
SC50 (ꢀg/ml) for DPPH
Carinol
219.4
145.5
88.5
157.1
93.9
135.5
NA
4.4
191.7
NA
4j
4k
4l
4m
NA
67.4
135.5
5.1
4n
Ascorbic acid
Allopurinol
1.7
NA
IC50 value is the concentration of compound required to inhibit 50% activity of XOD, SC50 value is the concentration of compounds required to scavenging 50% of DPPH radical.
These values were obtained from linear regression analysis of the mean values of triplicate samples. NA- Not active. Ascorbic acid was chosen as reference compound for DPPH
scavenging activity and allopurinol for xanthine oxidase activity.
free-radical scavenging activity and are better natural sub-
stituents than electron donating substituents (4J, 4m, and
4n).
leads to the loss of free radical scavenging property. This
study shows that presence and number of hydrogen donating
groups at 9-9' positions improve DPPH scavenging activity
and aromatic esterification preferably with electron donating
substituent at these positions improves XOD inhibitory po-
tential of the compounds with retention of free radical scav-
enging property. We first time demonstrated that (-)-carinol
acyl esters are potent inhibitors of xanthine oxidase with
together with anti-oxidant properties.
Similarly, aliphatic esterification of (-)-carinol (4a-4g)
also significantly decreased XOD inhibitory activity (Table
1). However, aromatic esterification with electron donating
(4j, 4m) and electron withdrawing substituents (4k, 4l) con-
siderably improved the XOD inhibitory activity. Substitution
with rather neutral groups (4h, 4i) decreased the enzyme
inhibitory potential (Table 1). Though electron-withdrawing
substituents (4k and 4l) improved the enzyme inhibitory po-
tential of derivatives, it led to the loss of DPPH scavenging
activity. Therefore substituents with hydrogen and or elec-
tron donating or both the properties, appear preferable to
retain both the activities (4m, 4n, 4j).
CONFLICT OF INTEREST
The author(s) confirm that this article content has no con-
flicts of interest.
ACKNOWLEDGEMENTS
Authors thank Dr. J. S. Yadav, Director, IICT for his
constant encouragement. KSB and SPK thank CSIR, New
Delhi for financial assistance.
Despite the improvement in XOD inhibitory activity by
electron-withdrawing groups, substitution with hydrogen,
electron donating and or both appears preferable, as former

Novel C-9, 9'-O-acyl Esters of (-)-Carinol
Medicinal Chemistry, 2013, Vol. 9, No. 1 103
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Received: June 28, 2011
Revised: May 10, 2012
Accepted: May 25, 2012