Solid-phase synthesis of epigallocatechin gallate derivatives

Article abstract of DOI:10.1002/anie.200701276

(Chemical Equation Presented) Without a trace... The natural product epigallocatechin gallate (see scheme, right) and a library of derivatives were synthesized on a solid phase. Reductive etherification upon the release of the α-acyloxy ketone intermediat

Full text of DOI:10.1002/anie.200701276

Communications
DOI: 10.1002/anie.200701276
Diversity-Oriented Synthesis
Solid-Phase Synthesis of Epigallocatechin Gallate Derivatives**
Hiroshi Tanaka, Haruko Miyoshi, Yu-Cheng Chuang, Yoshio Ando, and Takashi Takahashi*
Catechin (1) and epigallocatechin gallate (EGCG, 2) are
biologically active polyphenols found in tea leaves.[1] The
methylated EGCG derivatives 3 and 4 were reported recently
the epicatechin skeleton through the reductive cyclization of a
a-acyloxy ketone.[5] However, these established approaches
are based mainly on target-oriented solution-phase synthetic
strategies, and the development of an effective method based
on a diversity-oriented synthetic strategy for the rapid
assembly of epicatechin derivatives remains a worthwhile
goal. Herein, we describe the efficient solid-phase synthesis of
epigallocatechin gallate (2) and the combinatorial synthesis of
protected methylated epicatechin derivatives.
Our strategy for the solid-phase synthesis of epicatechin
derivatives 5 begins with the solid-supported aldehyde 10
linked through an acid-labile Wang linker at the phenol group
(Scheme 1). We planned to couple this aldehyde with the
ketone 11 and carboxylic acid 7, to construct the epicatechin
skeleton by cleavage of the Wang linker and subsequent
reductive etherification of the a-acyloxy ketone, and finally to
cleave the protecting groups in the solution phase. Reductive
to be effective drug candidates, as the hydro-
philicity of the methyl groups improves the
bioavailability of the compounds. Further-
more, Tachibana and co-workers reported
that the blocking of the 67-kDa laminin
receptor (67LR) is fundamental to both the
antiallergic effect of the methylated EGCGs 3
and 4[2,3] and the anticancer action of the
parent compound 2. However, knowledge
about the available structural diversity of
methylated epicatechins from natural sources
is limited. Several chemical approaches to the
synthesis of epicatechin derivatives have been
reported in which the epicatechin skeleton is
constructed by an indirect method based on
the epimerization of the hydroxy group at C3
of the catechin skeleton, which has a 2,3-trans
configuration.[4] We recently reported a direct
and efficient method for the construction of
Scheme 1. Strategy for the solid-phase synthesis of epicatechin derivatives 5.
etherification of the a-acyloxy ketone was expected to
provide the cis-substituted benzopyran ring system through
neighboring-group participation.[5,6] We anticipated that the
[*] Dr. H. Tanaka, H. Miyoshi, Y.-C. Chuang, Dr. Y. Ando,
Prof. Dr. T. Takahashi
bromine substituent at the 8-position would prevent a
Friedel–Crafts alkylation at this position with the cleaved
Wang linker.[7] The solid-supported a-acyloxy ketone 6 could
be prepared from the epoxide 8 by the regioselective opening
of the epoxide with a thiol followed by esterification of the
resulting alcohol.[8] The resulting sulfide group at the benzyl
position would be removed under the conditions used for the
reductive etherification. The epoxide 8 can be prepared by the
aldol condensation of the solid-supported aldehyde 10 with
the ketone 11, followed by epoxidation. We believed that this
three-component coupling strategy would be effective for the
Department of Applied Chemistry, Graduate School of Science and
Engineering
Tokyo Institute of Technology
2-12-1 Ookayama, Meguro, Tokyo 152-8552 (Japan)
Fax: (+81)3-5734-2884
E-mail: ttak@apc.titech.ac.jp
[**] This research was supported financially by the Naito Foundation.
We thank Prof. Dr. Takao Ikariya and Prof. Dr. Shigeki Kuwata at the
Tokyo Institute of Technology for determining the structure of the
bromoarene.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
synthesis of combinatorial libraries of epicatechin derivatives,
the three aromatic rings of which could be varied. Further-
more, no specific functional groups are required on any of the
three aromatic rings for attachment to the resin because the
phenol group used for linking to the solid support becomes an
element of the pyran ring.
The solid-phase synthesis of EGCG began with the
treatment of a 2.0m solution of the aldehyde 12a with
polystyrene–Wang bromide (1.6 mmolgÀ1) to provide the
solid-supported aldehyde 13 (Scheme 2).[9] The loading yield
with respect to the resin was estimated to be 48% upon
cleavage from the resin. The treatment of the solid-supported
aldehyde 13 with the ketone 11a (see Scheme 3) under basic
conditions provided the solid-supported enone 14a, which
underwent epoxidation with tBuOOH under basic conditions
to give the solid-supported epoxide 15. In contrast, the
epoxidation of the corresponding benzyl-protected derivative
14b proved difficult under the same reaction conditions. The
electron-withdrawing para-fluoro substituents on the benzyl
ether groups on the A ring might facilitate the formation of
the epoxide 15 not only by enhancing the reactivity of enone
14a toward nucleophilic addition, but also by preventing the
generated epoxide 15 from decomposing during the epox-
idation and workup. The regioselective epoxide-ring open-
ing[10] of 15 with 1-dodecanethiol[11] in the presence of
Zn(OTf)2 proceeded without cleavage of the Wang linker to
afford the solid-supported a-hydroxyketone 16, the acylation
of which with 3,4,5-tribenzyloxybenzoic acid (7a) then gave
the precursor 17 for reductive cyclization.
The exposure of 17 to 15% TFA in CH2Cl2 in the presence
of a PS–benzaldehyde resin at À108C for 5 h followed by the
addition of triethylsilane at the same temperature promoted
the cleavage of the Wang linker, reduction of the sulfide and
the bromide, and reductive etherification to provide the
protected epigallocatechin 18. PS–benzaldehyde scavenged
the released thiol and facilitated the purification of 18. The
purity of the crude material was estimated by HPLC on the
basis of UV absorption to be 32%. The p-fluorobenzyl
Scheme 2. Solid-phase synthesis of (Æ)-epigallocatechin gallate ((Æ)-2). Reagents and conditions: a) PS–Wang bromide (1.6 mmolgÀ1), Cs2CO3
(0.2m), NaI (0.06m), DMF, RT, 24 h, 48% (based on the resin); b) 11a (0.5m), NaOMe (0.1m), THF/MeOH (4:1), RT, 24 h; c) TBHP (1.5m),
KOH (0.2m), CH2Cl2/MeOH/decane (100:12:38), RT, 72 h; d) C12H25SH (0.05m), Zn(OTf)2 (0.01m), CH2Cl2/CH3CN (1:1), RT, 3 h; e) 7a (0.2m),
EDCI (0.2m), DMAP (0.06m), pyridine/DMF (1:1), 508C, 48 h; f) 15% TFA, CH2Cl2, PS–benzaldehyde resin (1.13 mmolgÀ1, 4.0 equiv with respect
to 13), À108C, 5 h, then Et3SiH, 19 h, 62% yield with 36% purity, 19% yield of the isolated product; g) Pd(OH)2, HCO2H, THF/MeOH (1:1),
82%. Bn=benzyl, DMAP=4-dimethylaminopyridine, DMF=N,N-dimethylformamide, EDCI=1-(3-dimethylaminopropyl)-3-ethylcarbodiimide,
p-FBn=p-FC6H4CH2, TBHP=tert-butylhydroperoxide, Tf=trifluoromethanesulfonyl, TFA=trifluoroacetic acid.
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Communications
protecting groups improve the purity of the protected EGCG
can be cleaved selectively without the decomposition of the
epicatechin during the final stage of the synthesis. The library
was synthesized on the basis of the split-and-pool strategy by
using Irori MiniKans encoded with color tags with 70 mg of
the polystyrene resin.[13] The purity of the library compounds
was estimated by HPLC–MS analysis on the basis of UV
absorption at 254 nm (see the Supporting Information). A
total of 59 of 60 possible compounds were detected by LC–
MS. The purity of 48 of the compounds was in the range 15–
58%. Purification of the 59 crude products by recycling
preparative HPLC based on gel-permeation chromatography
led to the isolation of 51 pure protected EGCG derivatives in
yields of 3.1 to 26 mg. The remaining eight compounds could
not be isolated from the crude product mixture as result of
their lowpurity or the presence of inseparable by-products.
In conclusion, we have developed a solid-phase synthesis
of (Æ )-epigallocatechin gallate ((Æ )-2). A reductive ether-
ification reaction proceeded simultaneously with the release
of the final synthetic intermediate from the resin to provide a
protected form of (Æ )-2 in moderate overall yield. When we
applied this method to the solid-phase combinatorial syn-
thesis of 60 protected methylated epicatechin derivatives 5,
we detected 59 compounds by HPLC–MS and isolated 51
epicatechin derivatives.
18, as benzyl protecting groups on the A ring were cleaved
gradually under the conditions for the reductive cyclization.
The crude material was purified by gel-permeation chroma-
tography to provide the protected EGCG 18 in 19% yield
from 13. We propose the following mechanism for the
reductive cleavage of the bromide and sulfide functional
groups: Compound 19 undergoes debromination through a
retro-Friedel–Crafts mechanism via the intermediate 20; the
bromonium ion thus generated is reduced by the hydrosilane.
The thiol is released to give an intermediate species 21, which
undergoes reduction at the benzylic position with the hydro-
silane. The resulting ketone 22 is thought to be the precursor
for reductive etherification. Finally, the epicatechin derivative
18 was deprotected by conventional hydrogenolysis in the
solution phase by using a palladium catalyst to provide (Æ )-
epigallocatechin gallate ((Æ )-2) in 82% yield.
We next examined the combinatorial synthesis of a library
of protected polymethylated epicatechin derivatives 5.[12] The
two aldehydes 12a and 12b, six ketones 11a–f, and five
carboxylic acids 7a–e (Scheme 3) were used as building
blocks for the synthesis of 60 epicatechin derivatives. Methyl
ether, benzyl ether, and p-fluorobenzyl ether groups protect
the phenol functionalities in the substrates. The benzyl ethers
Received: March 23, 2007
Revised: May 24, 2007
Published online: July 6, 2007
Keywords: catechins · combinatorial chemistry · etherification ·
.
natural products · solid-phase synthesis
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Scheme 3. Building blocks for the library synthesis.
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[13] In our first attempt at the library synthesis we used 30 mg of the
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mobility of the resin in the reaction mixture is an important
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amenable to the use of 100 mg of a resin.
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