Synthesis of a 3,4,5-trimethoxybenzoyl ester analogue of epigallocatechin-3-gallate (EGCG): A potential route to the natural product green tea catechin, EGCG

Article abstract of DOI:10.1021/ol007000o

matrix presented The synthesis of a trimethoxybenzoyl ester (D-ring) analogue of epigallocatechin-3-gallate (EGCG) is described. The versatile synthesis route can be used to synthesize A, B, and D ring analogues of EGCG and involves a key cyclization of t

Full text of DOI:10.1021/ol007000o

ORGANIC
LETTERS
2001
Vol. 3, No. 6
843-846
Synthesis of a 3,4,5-Trimethoxybenzoyl
Ester Analogue of
Epigallocatechin-3-gallate (EGCG): A
Potential Route to the Natural Product
Green Tea Catechin, EGCG
Nurulain T. Zaveri
Pharmaceutical DiscoVery DiVision, SRI International, 333 RaVenswood AVenue,
Menlo Park, California 94025
nurulain.zaVeri@sri.com
Received December 14, 2000
ABSTRACT
The synthesis of a trimethoxybenzoyl ester (D-ring) analogue of epigallocatechin-3-gallate (EGCG) is described. The versatile synthesis route
can be used to synthesize A, B, and D ring analogues of EGCG and involves a key cyclization of the chalcone to the 3-flavene. This synthesis
provides a possible route to the polyphenolic green tea natural product EGCG.
Green tea and its constituent polyphenols have received
increasing attention as potential cancer chemopreventives and
are currently undergoing Phase I clinical trials.1 Green tea
consumption has been associated with decreased incidence
of breast, pancreatic, colon, esophageal, and lung cancers.2
The cancer preventive properties of green tea extract are
mainly associated with the polyphenolic catechins, the
3-flavanols. There are four catechins in green tea which make
up ∼10% of the dry weight of green tea. These are shown
in Figure 1 and include (-)-epigallocatechin-3-gallate (EGCG),
(-)-epicatechin-3-gallate (ECG), (-)-epigallocatechin (EGC),
and (-)-epicatechin (EC).3 Several studies have demonstrated
that EGCG, the most abundant catechin, is the most potent
chemopreventive component of green tea, and inhibition of
carcinogenesis by green tea is mediated by EGCG.4
Several in vivo experiments have shown the chemopre-
ventive effects of EGCG against all stages of carcinogenesiss
initiation, promotion, and progressionsin animal models of
(2) (a) Yang, C. S.; Wang, Z. Y. J. Natl. Cancer. Inst. 1993, 85, 1038-
1049. (b) Kato, I.; Tominaga, S.; Matsuura, A.; Yoshii, Y.; Shirai, M.;
Kobayashi, S. A. Jpn. J. Cancer Res. 1990, 81, 1101-1108. (c) Zheng,
W.; Doyle, T. J.; Hong, C. P.; Kushi, L. H.; Sellers, T. A.; Folsom, A. R.
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Lee, M. J.; Landau, J. M. AdV. Exp. Med. Biol. 1996, 401, 971-976. (e)
Fujiki, H.; Suganuma, M.; Okabe, S.; Komori, A.; Sueoka, E.; Sueoka, N.;
Kozu, T.; Sakai, Y. Nutr. ReV. 1996, 54, S67-70. (f) Imai, K.; Suga, K.;
Nakachi, K. PreV. Med. 1997, 26, 769-775. (g) Nakachi, K.; Suemasu,
K.; Suga, K.; Takeo, T.; Imai, K.; Higashi, Y. Jpn. J. Cancer Res. 1998,
89, 254-261.
(3) (a) Chu, D. C. In Chemistry and Applications of Green Tea;
Yamamoto, T., Juneja, L. R., Chu, D. C., Kim, M., Eds.; CRC Press: Boca
Raton, FL, 1997; pp 1-11. (b) Chu, D. C.; Juneja, L. R. In Chemistry and
Applications of Green Tea; Yamamoto, T., Juneja, L. R., Chu, D. C., Kim,
M., Eds.; CRC Press: Boca Raton, FL, 1997; pp 13-22.
(1) (a) Kelloff, G. A.; Crowell, J. A.; Steele, V. E.; Lubet, R. A.; Malone,
W. A.; Boone, C. W.; Kopelovich, L.; Hawk, E. T.; Lieberman, R.;
Lawrence, J. A.; Ali, I.; Viner, J. L.; Sigman, C. C. J. Nutr. 2000, 130,
467S-471S. (b) Yang, C. S.; Chung, J. Y.; Yang, G.; Chhabra, S. K.; Lee,
M.-J. J. Nutr. 2000, 130, 472S-478S. (c) Clinical development plan: tea
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10.1021/ol007000o CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/16/2001

reactions, regardless of the presence of the 3-galloyl moiety.
However, the 3-galloyl moiety may enhance the antioxidant
potential of EGCG.9
The synthesis of EGCG has not yet been described in the
literature. We have developed a versatile chemical synthesis
of EGCG and other tea catechins that allows us to vary
different portions of the EGCG molecule to study the
structure-function relationship of EGCG. This synthesis
affords the 2,3-cis (epicatechin)10 as well as the 2,3-trans
(catechin) racemic analogues of EGCG and is easily ame-
nable to chiral synthesis of 2â,3â-cis EGCG analogues,
which have the same configuration as the natural product
(-)-EGCG. An enantioselective synthesis of the permethyl-
aryl ether analogues of epicatechin has been reported by
Rensburg et al.;11 however, neither the choice of starting
synthons nor the yields of the epicatechin are very practical
for the synthesis of epigallocatechins or analogues for a
structure-function study of EGCG action.
Figure 1. Structures of the four major catechins from green tea.
In this letter, we describe the synthesis of a D-ring
analogue of EGCG using our synthesis approach (Scheme
1). With the appropriate choice of starting materials, this
synthesis can be used to make A, B, and/or D-ring analogues
of EGCG.
breast, lung, skin, prostate, and colon cancers.5 There have
been extensive investigations into the possible mechanisms
of cancer prevention by EGCG, and recent efforts in defining
the molecular mechanisms of green tea and EGCG action
have found that EGCG affects several molecular targets and
pathways of carcinogenesis.6 However, there have been no
clear structure-activity studies defining what parts of the
EGCG molecule are important for its anticarcinogenic action.
While many studies have shown that the antitumor activity
of EGCG stems from its exceptional antioxidant potential,7
two recent studies by Valcic et al.8 have shown that the
trihydroxyphenyl B-ring is the principal site of antioxidant
O-Benzyl-protected 3,4,5-trihydroxybenzaldehyde (2) (syn-
thesized by a known method in 87% overall yield from
methyl gallate, 1)12 was condensed with 4′,6′-bis(benzyloxy)-
2′-hydroxyacetophenone (4) in piperidine/EtOH13 to yield
the chalcone 5. The chalcone was cyclized directly to the
3-flavene 6 with NaBH4 in tetrahydrofuran (THF) and EtOH
at 65 °C in a 50% yield.14 This method of chalcone
cyclization directly to the 3-flavene was established in our
laboratory after considerable experimentation because chal-
cone 5 failed to cyclize under several other conditions
reported in the literature.15 This conversion of 2′-hydroxy-
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Cancer Lett. 1995, 96, 239-243. (b) Han, C. Cancer Lett. 1997, 114, 153-
158. (c) Yang, G.-U.; Liao, J.; Kim, K.; Yurkov, E. J.; Yang, C. S.
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M.; Sueoka, E.; Komori, A.; Fujiki, H. Jpn. J. Cancer Res. 1997, 88, 639-
643. (e) Valcic, S.; Timmermann, B. N.; Alberts, D. S.; Wachter, G. A.;
Krutzsch, M.; Wymer, J.; Guillen, J. M Anticancer Drugs 1996, 7, 461-
468.
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M.; Satake, K. Pancreas 1997, 15, 272-277. (c) Shi, S. T.; Wang, Z. Y.;
Smith, T. J.; Hong, J. Y.; Chen, W. F.; Ho, C. T.; Yang, C. S. Cancer Res.
1994, 54, 4641-4647. (d) Yang, C. S.; Yang, G. Y.; Landau, J. M.; Kim,
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Cao, Y.; Cao, R. Nature 1999, 398, 381. (c) Gupta, S.; Ahmad, N.; Mohan,
R. R.; Husain, M. M.; Mukhtar, H. Cancer Res. 1999, 59, 2115-2120. (d)
Lin, J.-K.; Liang, Y.-C.; Lin-Shiau, S.-Y. Biochem. Pharmacol. 1999, 58,
911-915. (e) Naasani, I.; Seimiya, H.; Tsuruo, T. Biochem. Biophys. Res.
Commun. 1998, 249, 391-396. (f) Ren, F.; Zhang, S.; Mitchell, S. H.;
Butler, R.; Young, C. Y. F. Oncogene 2000, 19, 1924-1932. (g) Mukhtar,
H.; Ahmad, N. Proc. Soc. Exp. Biol. Med. 1999, 220, 234-238. (h) Ahmad,
N.; Feyes, D. K.; Nieminen, A. L.; Agarwal, R.; Mukhtar, H. J. Natl. Cancer.
Inst. 1997, 89, 1881-1886. (i) Lin, Y. L.; Lin, J. K. Mol. Pharmacol. 1997,
52, 465-472. (j) Chan, M. M.; Fong, D.; Ho, C. T.; Huang, H. I. Biochem.
Pharmacol. 1997, 54, 1281-1286. (k) Dong, Z.; Ma, W.; Huang, C.; Yang,
C. S. Cancer Res. 1997, 57, 4414-4419. (l) Steele, V. E.; Kelloff, G. J.;
Balentine, D.; Boone, C. W.; Mehta, R.; Bagheri, D.; Sigman, C. C.; Zhu,
S.; Sharma, S. Carcinogenesis 2000, 21, 63-67. (m) Ahmad, N.; Cheng,
P.; Mukhtar, H. Biochem. Biophys. Res. Commun. 2000, 275, 328-334.
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(b) Katiyar, S. K.; Matsui, M. S.; Elmets, C. A.; Mukhtar, H. Photochem.
Photobiol. 1999, 69, 148-153. (c) Serafini, M.; Ghiselli, A.; Ferro-Luzzi,
A. Eur. J. Clin. Nutr. 1996, 50, 28-32. (d) Xu, Y.; Ho, C-T.; Amin, S.
G.; Han, C.; Chung, F.-L. Cancer Res. 1992, 52, 3875-3879.
(8) (a) Valcic, S.; Muders, A.; Jacobsen, N. E.; Leibler, D. C.;
Timmerman, B. N. Chem. Res. Toxicol. 1999, 12, 382-386. (b) Valcic, S.;
Burr, J. A.; Timmerman, B. N.; Liebler, D. C. Chem. Res. Toxicol. 2000,
13, 801-810.
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Biophys. Acta 1999, 1427, 13-23. (b) Kondo, K.; Kurihara, M.; Miyata,
N.; Suzuki, T.; Toyoda, M. Arch. Biochem. Biophys. 1999, 362, 79-86.
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D. Tetrahedron Lett. 1997, 38, 3089-3092. (b) Rensburg, H.; Heerden, P.
S.; Ferreira, D. J. Chem. Soc., Perkin Trans. 1 1997, 3415-3421.
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(14) Representative Procedure: The chalcone 5 (1 g, 1.33 mmol) was
dissolved in THF (20 mL) and EtOH (10 mL) at room temperature, and
NaBH4 (51 mg, 1.33 mmol) was added. The solution was heated to a gentle
reflux (65-70 °C) for 16 h, after which TLC (CH2Cl2) indicated no starting
material. The cooled reaction mixture was evaporated to dryness and
dissolved in CH2Cl2. The organic solution was washed with water and brine,
dried (MgSO4), and evaporated to give a crude yellow solid which was
purified via column chromatography using a gradient of 45% to 20%
hexanes in CH2Cl2 to obtain 0.49 g (50% yield) of 6 as an off-white solid.
(15) (a) Bagade, M. B.; Thool, A. W.; Lokhande, P. D.; Ghiya, B. J.
Indian J. Chem. 1991, 30B, 973-975. (b) Iinuma, M.; Tanaka, T.; Matsura,
S. Chem. Pharm. Bull. 1984, 32, 3354-3360. (c) Chaturvedi, R.; Patil, P.
N.; Mulchandani, N. B. Indian J. Chem. 1992, 31B, 340-341. (d) Harwood,
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649-657. (e) Zhao, L.; Zhang, F.; Li, Y. Bull. Soc. Chim. Belg. 1994, 103,
41-42.
844
Org. Lett., Vol. 3, No. 6, 2001

Scheme 1a
b
c
d
a i. BnBr, K2CO3, DMF. ii. LAH, THF. iii. PCC, CH2Cl2. 2 equiv of BnBr, K2CO3, HMPA. piperidine, EtOH, reflux. NaBH4, THF/
e
f
g
h
EtOH. i. BH3-THF. ii. H2O2, NaOH. Dess-Martin periodinane, CH2Cl2. L-Selectride, THF, room temp. i. 3,4,5-trimethoxybenzoyl
chloride, DMAP, Et3N. ii. H2/Pd/C, dioxane.
chalcones to flav-3-enes and its biosynthetic implications
have been reported earlier by Clark-Lewis and Skingle.16
This cyclization now provides an expeditious route to the
flav-3-ene 6, which was further elaborated to give EGCG
analogues. Hydroboration-oxidation of 6 with BH3/THF
followed by H2O2/NaOH gave the trans-3-flavanol 7 (cat-
echin). The trans stereochemistry of 7 was assigned based
on the nuclear magnetic resonance (NMR) coupling constants
and nuclear Overhauser effect (NOE) experiments and
confirmed by molecular modeling (Insight II, Biosym). The
hydroboration-oxidation gave <5% of the cis-3-flavanol,
as evidenced by the peaks in the NMR spectra assigned to
the cis-3-flavanol 9.
described by Kozikowski et al.19 in their synthesis of
proanthocycanidin dimers from catechin. The cis 3-flavanol
(protected epigallocatechin) 9 was esterified with 3,4,5-
trimethoxybenzoyl chloride and deprotected by hydrogena-
tion to afford the cis D-ring analogue SR 13193, which is
the trimethoxybenzoyl ester analogue of EGCG. The trans
D-ring analogue SR 13192 was synthesized in the same
manner as the cis analogue from the trans 3-flavanol 7.
Both the cis and trans D-ring analogues inhibited the
growth of breast cancer cell lines in vitro, although they were
about 2-fold less potent than EGCG itself.20 Interestingly,
the racemic trans analogue SR 13192 was slightly less potent
than the cis analogue SR 13193 in the growth inhibition
assays. The biological activity of these analogues will be
reported in detail elsewhere.21
The protected trans 3-flavanol 7 was converted to the cis
3-flavanol 9 by a two-step sequence involving oxidation of
7 with the Dess-Martin periodinane17 followed by selective
reduction with L-Selectride18 to afford exclusively the cis-
racemic 3-flavanol 9. This sequence was recently also
(18) Catelani, G.; Monti, L.; Ugazio, M. J. Org. Chem. 1980, 45, 919-
920.
(19) Tuckmantel, W.; Kozikowski, A. P.; Romanczyk, L. J. J. Am. Chem.
Soc. 1999, 121, 12073-12081.
(20) Zaveri, N. T.; Chao, W.-R. Synthetic Analogues of Green Tea
Catechins and Their In Vitro Cell Growth Inhibition ActiVity, Proceedings
of the 1999 AACR-NCI-EORTC International Conference, Washington,
DC., 1999; Clin. Cancer Res. 1999, 5(S), 3844.
(16) Clark-Lewis, J. W.; Skingle, D. C. Aust. J. Chem. 1967, 20, 2169-
2190.
(17) (a) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155-4156.
(b) Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113, 7277-7287.
(21) Zaveri, N. T.; Chao, W.-R. Manuscript in preparation.
Org. Lett., Vol. 3, No. 6, 2001
845

In conclusion, a novel versatile route to the green tea
catechins is described, which can be used to design novel
analogues of EGCG to study its molecular mechanisms in
greater detail.
Supporting Information Available: Spectral character-
ization of compounds 5-9, SR 13192, and SR 13193. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. This work was supported by a New
Investigator grant from the California Breast Cancer Research
Program (2KB-0083).
OL007000O
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Org. Lett., Vol. 3, No. 6, 2001