Synthesis and radical-scavenging activity of a dimethyl catechin analogue

Article abstract of DOI:10.1016/j.bmcl.2014.03.029

Catechin analogue 1 with methyl substituents ortho to the catechol hydroxyl groups was synthesized to improve the antioxidant ability of (+)-catechin. The synthetic scheme involved a solid acid catalyzed Friedel-Crafts coupling of a cinnamyl alcohol derivative to 3,5-dibenzyloxyphenol followed by hydroxylation and then cyclization through an intermediate orthoester. The antioxidative radical scavenging activity of 1 against galvinoxyl radical, an oxyl radical, was found to be 28-fold more potent than (+)-catechin.

Full text of DOI:10.1016/j.bmcl.2014.03.029

Bioorganic & Medicinal Chemistry Letters 24 (2014) 2582–2584
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and radical-scavenging activity of a dimethyl catechin
analogue
Kohei Imai a, Ikuo Nakanishi b, Akiko Ohno c, Masaaki Kurihara c, Naoki Miyata d, Ken-ichiro Matsumoto b,
Asao Nakamura a, Kiyoshi Fukuhara e,
⇑
a Department of Applied Chemistry, Shibaura Institute of Technology, Fukasaku, Minuma-ku, Saitama 337-8570, Japan
b Research Center for Charged Particle Therapy, National Institute of Radiological Sciences, Inage-ku, Chiba 263-8555, Japan
c Division of Organic Chemistry, National Institute of Health Sciences, Setagaya-ku, Tokyo 158-8501, Japan
d Graduate School of Pharmaceutical Sciences, Nagoya City University, Mizuho-ku, Nagoya, Aichi 467-8603, Japan
e School of Pharmacy, Showa University, Hatanodai, Shinagawa-ku, Tokyo 142-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Catechin analogue 1 with methyl substituents ortho to the catechol hydroxyl groups was synthesized to
improve the antioxidant ability of (+)-catechin. The synthetic scheme involved a solid acid catalyzed
Friedel–Crafts coupling of a cinnamyl alcohol derivative to 3,5-dibenzyloxyphenol followed by hydroxyl-
ation and then cyclization through an intermediate orthoester. The antioxidative radical scavenging
activity of 1 against galvinoxyl radical, an oxyl radical, was found to be 28-fold more potent than
(+)-catechin.
Received 6 January 2014
Revised 10 March 2014
Accepted 12 March 2014
Available online 20 March 2014
Keywords:
Antioxidant
Ó 2014 Elsevier Ltd. All rights reserved.
Synthetic antioxidant
Catechin
Radical scavenging
Among natural bioactive compounds distributed in fruits,
vegetables, and beverages of plant origin, phenolic antioxidants
(ArOH), such as flavonoids, tocopherol, and resveratrol, are widely
recognized for their biological and pharmacological effects that
include anti-carcinogenic, anti-cardiovascular anti-neurodegener-
ative, and anti-inflammatory properties.1–4 These properties are
principally attributed to the capacity of these compounds to trap
reactive oxygen species and to chelate metal ions, which through
the Fenton reaction could generate radicals.5 Therefore, this
so-called antioxidative ability of phenolic compounds is frequently
cited as the key to their success in the prevention and/or reduction
of oxidative stress-related chronic diseases and age-related disor-
ders. Consideration of antioxidative ability in drug development
has led to interest in improving the radical scavenging activity of
phenolic compounds based on the antioxidant mechanism.6
Antioxidant activity via the direct quenching of free radicals is
expressed in two mechanisms: a one-step hydrogen atom transfer
from the phenolic OH group to the free radical and a single-
electron transfer from ArOH to the free radical with concomitant
formation of the radical cation ArOHÅ+.7 The former process,
characterized by vitamin E, is based on the capacity of a phenol
to donate a hydrogen atom to a free radical.8 Catechin and
resveratrol (see Fig. 1 for the structures) scavenge free radicals
by the latter mechanism.9,10 In this case, the ionization potential
(IP) of the phenol is important for the radical scavenging efficacy.
Hence, introduction of electron-donating substituents at positions
ortho and/or para to the phenolic hydroxyl group would decrease
the IP of ArOH, thereby enhancing the radical scavenging ability.
In this regard, planar catechin analogues, in which an isopropyl
fragment as electron-donor was introduced into (+)-catechin by
reaction with acetone, exhibited 5-fold more potent radical scav-
enging activity than (+)-catechin.11 An epigallocatechin analogue
with a similarly introduced isopropyl fragment also displayed
increased radical scavenging activity.12 In the case of ortho-alkyl
substituents, the ArOHÅ+ formed in the reaction with a free radical
is stabilized by compensation of the electron vacancy by either an
inductive effect or through hyperconjugation, resulting in
improved radical scavenging ability. Resveratrol analogues with
one and two methyl groups at positions ortho to the phenol hydro-
xyl group showed 14- and 36-fold acceleration in the ability to
scavenge free radicals, respectively.13
The aim of the present study was to synthesize the catechin
analogue 1 in which both positions ortho to the catechol hydroxyl
groups were substituted with methyl groups. Compared with
(+)-catechin, the dimethyl analogue of catechin showed greatly
enhanced radical scavenging ability against galvinoxyl radical as
a model radical for reactive oxygen species. This result indicates
⇑
Corresponding author. Tel.: +81 3 3784 8186; fax: +81 3 3784 8252.
E-mail address: fukuhara@pharm.showa-u.ac.jp (K. Fukuhara).
http://dx.doi.org/10.1016/j.bmcl.2014.03.029
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

K. Imai et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2582–2584
2583
by benzyl protection of the hydroxyl groups of catechol 6, were per-
formed using the crude reaction products. The resulting compound
7 was subjected to Knoevenagel condensation with monoethyl mal-
onate in pyridine containing a catalytic amount of piperidine to
produce
a,b-unsaturated ester 8. Reduction of 8 with LAH/AlCl3
gave the corresponding cinnamyl alcohol 2.
Phenol 3, obtained by the method of Lehmann and Jahr,16 was
subjected to Friedel–Crafts alkylation with 2 using H2SO4/SiO2 as
catalyst in CS2/CH2Cl2 to furnish the coupled product 9 as shown
in Scheme 2. Oxidation of 9 with NMO in the presence of a catalytic
amount of OsO4 gave the cis-diol product 10, which was converted
into ortho-ester 11 with triethyl orthoformate and a catalytic
amount of pyridinium p-toluenesulfonate (PPTS) at room temper-
ature. The reaction was continued with heating at 60 °C to form
the flavan framework as intermediate formate ester 12. De-esteri-
fication of 12 with K2CO3 in 1,2-dimethoxyethane/methanol and
debenzylation using Pd(OH)2 and H2 in THF/methanol afforded 1
(Fig. 1).
Figure 1. Structures of catechin, resveratrol, and their analogues.
The radical scavenging activity of 1 was evaluated in a non-
aqueous system using galvinoxyl radical (GOÅ) as an oxyl radical
species. Because of its odd electron, GOÅ exhibits a strong absorption
band at 428 nm, and a solution of GOÅ appears yellow in color. As
the electron is paired, the absorption vanishes, and the resulting
decolorization is stoichiometric with respect to the number of elec-
trons taken up. Taking advantage of the color change of GOÅ in the
presence of an antioxidant, the rate of radical scavenging of 1
toward galvinoxyl was measured. As shown in Figure 2, the decay
rate of the absorbance at 428 nm followed pseudo first-order kinet-
ics when the concentration of 1 was maintained at more than 10-
fold excess to the GOÅ concentration. The pseudo first-order rate
constant (kobs) exhibited first-order dependence with respect to
the concentration of 1 as shown in Figure 3. From the linear plot
of kobs versus [1], the second-order rate constant (k) for the radical
scavenging of 1 toward GOÅ was determined to be 1.07 Â 103
molÀ1 dm3 sÀ1. The k value for (+)-catechin was determined in the
there is a role for the antioxidant chemistry of phenols via a radical
scavenging mechanism that is accelerated by electron-donating
substituents.
There are numerous methods for construction of the flavonoid
skeleton using a biomimetic strategy.14,15 The synthetic strategy
employed here is an efficient and general approach to access the
flavan framework of 1. This process consisted of a solid acid cata-
lyzed Friedel–Crafts coupling of cinnamyl alcohol derivative 2, that
included an embedded dimethylcatechol moiety, with 3,5-diben-
zyloxyphenol 3, followed by hydroxylation and cyclization to give
the corresponding dimethyl catechin derivative.
The synthesis of 2 is outlined in Scheme 1. Introduction of
methyl groups at both ortho positions of the catechol could be read-
ily accomplished by Mannich reaction of catechol using morpholine
and formamide to afford 4. Palladium catalyzed hydrogenation of 4
was carried out with heating at 70 °C under a high pressure of
hydrogen gas to give dimethylcatechol 5. Since 5 was highly sensi-
tive, the subsequent reactions, formylation with CH(OEt)3 followed
Scheme 1. Reagents and conditions: (a) morpholine, formaldehyde, EtOH, rt, 61%;
(b) H2, 10% Pd/C, THF, 5 atm, 70 °C; (c) CH(OEt)3, AlCl3, toluene, rt; (d) BnBr, K2CO3,
KI, acetone, rt, 3.2% (total yield from 4); (e) monoethyl malonate, piperidine,
pyridine, reflux, 87%; (f) LAH, AlCl3, THF, rt, 48%.
Scheme 2. Reagents and conditions: (a) H2SO4/SiO2, CH2Cl2, rt, (b) NMO, OsO4,
acetone, H2O, rt, 37% (total yield from 2); (c) CH(OEt)3, PPTS, EDC, rt; (d) CH(OEt)3,
PPTS, EDC, 60 °C; (e) K2CO3, MeOH, DMF, rt, 89% (a total yield from 10); (f) H2,
Pd(OH)2, MeOH, THF, rt, 56%.

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K. Imai et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2582–2584
ability that may aid in the prevention and therapy of various oxi-
dative stress related diseases. Indeed, the scavenging rate constant
of GOÅ by 1 was greatly increased as compared to catechin. Since
inductive and hyperconjugation effects of methyl groups can con-
tribute to stabilization of the radical cation of catechol formed in
the reaction with GOÅ, the enhanced radical scavenging activity of
1 supports the antioxidant chemistry of catechin occurring by
the scavenging of oxyl free radicals through a single electron trans-
fer mechanism. This simple derivatization by methyl substituents
makes it possible to improve the radical scavenging activity of cat-
echin without causing a fatal change in the overall structure that
may be significantly associated with its inherent biological activi-
ties, and this strategy is likely to be of value in medicinal chemistry
applications.
Figure 2. Spectral changes observed during the reaction between dimethyl
(6.8 Â 10À5 mol dmÀ3
)
and GOÅ (4.7 Â 10À6 mol dmÀ3
) in
Supplementary data
catechin analogue
1
deaerated MeCN at 298 K (interval: 2 s). Inset: first-order plot based on the decay
of the absorption at 428 nm for GOÅ.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2014.03.
029.
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Figure 3. Plot of the pseudo first-order rate constants (kobs) versus the concentra-
tions of 1 and (+)-catechin for the radical scavenging reaction toward GOÅ.
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same manner to be 3.87 Â 10 molÀ1 dm3 sÀ1. These results indicate
that the introduction of methyl groups at both positions ortho to the
catechol hydroxyl groups led to a 28-fold increase in radical scav-
enging activity.
In conclusion, the dimethyl analogue of catechin, 1, was synthe-
sized to demonstrate a strategy to enhance radical scavenging