Facile synthesis of 4″-O-alkyl-(–)-EGCG derivatives through regioselective deacetylative alkylation

Article abstract of DOI:10.1080/00397911.2016.1278233

Therapeutic potential of the D-ring methyl ethers of (–)-epigallocatechin-3-gallate [(–)-EGCG] warrants extensive structure–activity relationship study of various D-ring ethers of (–)-EGCG but, for this purpose, efficient synthetic strategy needs to be de

Full text of DOI:10.1080/00397911.2016.1278233

Facile synthesis of 4ʺ–O–alkyl (–)-EGCG derivatives through regioselective
deacetylative alkylation
Yujin Seo1, Mi Kyoung Kim1, Hyunah Choo2,3, Youhoon Chong1
1Department of Integrative Bioscience and Biotechnology, Bio/Molecular Informatics
Center, Konkuk University, Hwayang-dong, Gwangjin-gu, Seoul, Korea, 2Center for
Neuro-Medicine, Korea Institute of Science and Technology, Hawolgok-dong,
Seoungbuk-gu, Seoul, Korea, 3Department of Biological Chemistry, Korea University of
Science and Technology, Youseong-gu, Daejeon, Korea
†These two authors contributed equally to this work
Corresponding authors: HC, E-mail: hchoo@kist.re.kr
YC, E-mail: chongy@konkuk.ac.kr
Abstract
Therapeutic potential of the D–ring methyl ethers of (–)–epigallocatechin–3–gallate ((–)–
EGCG) warrants extensive structure–activity relationship study of various D–ring ethers
of (–)–EGCG but, for this purpose, efficient synthetic strategy needs to be developed. In
this study, efficient preparation of the 4ʺ–O–alkyl–(–)–EGCGs (4a–4e) was demonstrated
by employing KI/K2CO3–promoted deacetylative alkylation of peracetyl (–)–EGCG,
which could be broadly utilized for preparation of various D–ring alkyl ethers of (–)–
EGCG and thereby extensive structure–activity relationship study.
GRAPHICALABSTRACT
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KEYWORDS: (–)–EGCG, regioselectivity, deacetylative alkylation
INTRODUCTION
(–)–Epigallocatechin–3–gallate ((–)–EGCG, Figure 1), the major polyphenol in green tea,
has been reported to possess numerous health–promoting effects.[1] (–)–EGCG is
characterized by its 8 phenolic hydroxyl groups on ring A (5–OH and 7–OH), ring B (3′–
OH, 4′–OH, and 5′–OH), and ring D (3ʺ–OH, 4ʺ–OH, and 5ʺ–OH), and they are known to
play critical roles in the biological activity of (–)–EGCG.[2] On the other hand, D–ring
methyl ethers of (–)–EGCG such as (–)–3ʺ–Me–EGCG and (–)–4ʺ–Me–EGCG (Figure 1)
also showed potent anti–allergic effects[3] as well as inhibitory activity against matrix
metalloproteinases[4], which indicated that an alkyl substitution on the D–ring hydroxyl
groups would be beneficial for potentiating the biological activity of (–)–EGCG.
Therefore, for structure–activity relationship study, a synthetic method for the preparation
of to the D–ring ethers of (–)–EGCG has been required and, until now, esterification of
an appropriately substituted caffeic acid (2, Scheme 1) with O–pernitrobenzenesulfonyl
(Ns)–protected (–)–EGC (1, Scheme 1) reported by Aihara et al. [5] (Scheme 1) has
proved to be most efficient.
However, tedious protection–deprotection steps are associated with this coupling reaction
(Scheme 1) and, for facile preparation of various (–)–EGCG derivatives with D–ring
ether moiety, development of a more straightforward synthetic methodology has been
required. Herein, we report KI/K2CO3–promoted deacetylative alkylation at the 4ʺ–
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position of the peracetyl (–)–EGCG and application of this novel reactivity to
regioselective preparation of 4ʺ–alkyl–(–)–EGCG derivatives.
RESULTS AND DISCUSSION
First, direct alkylation of (–)–EGCG was attempted by using the general polyphenol
alkylation conditions (K2CO3 and MeI in refluxing acetone) (Scheme 2). The reaction
resulted in many different alkylated (–)–EGCG derivatives. After extensive silica gel
chromatography, the desired (–)–4ʺ–Me–EGCG (4a, Scheme 2) could be isolated but in
very low (12%) yield. The structure of 4a was confirmed by comparison of its 1H and 13C
NMR spectra with those reported previously[6], which showed 4.6–, 5.2–, and 1.6–ppm
down–field shifts of C1ʺ, C3ʺ, and C4ʺ carbon peaks, respectively, upon methylation at
the 4ʺ–O position. Direct alkylation with other alkyl halides (ethyl iodide, propyl iodide,
allyl bromide, and benzyl bromide) also provided the corresponding 4ʺ–alkyl–(–)–EGCG
derivatives (4b–4e, Scheme 2) in 10–14% yields.
Interestingly, however, analysis of the by–products formed during the direct alkylation of
(–)–EGCG revealed that they are inseparable mixtures of multiple–alkylation products
with the 4ʺ–alkyl substituents in common. Therefore, we reasoned that, in the (–)–EGCG
scaffold, alkylation occurs preferentially at the 4ʺ–O position, but the problem is over–
alkylation at other phenolic hydroxyl groups. We also reasoned that the 4ʺ–selective
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alkylation of (–)–EGCG might be attributed to the characteristic stabilizing resonance
structure of the phenolate anion at the 4ʺ–position (Figure 2).
Based on these observations, a new synthetic strategy for regioselective preparation of
the 4ʺ–alkyl–(–)–EGCG derivatives could be developed to include global protection of all
the phenolic hydroxyl groups of (–)–EGCG followed by selective removal of the
protecting group at the 4ʺ–position. Thus, we attempted selective deprotection of the 4ʺ–
OAc group from the peracetyl (–)–EGCG7 (5, Scheme 2). Treatment of (–)–EGCG with
excess amount of Ac2O in pyridine provided 5 in excellent yield[7] but, under various
deacetylation conditions, selective deacetylation of 5 could not be achieved. Finally, the
reaction conditions employed for preparation of 3,5-di-O-acetyl-4-O-(para-
methoxybenzyl) methyl gallate from 3,4,5-tri-O-acetyl methyl gallate[8] was tested, and
treatment of 5 with KI and K2CO3 in refluxing acetone provided the corresponding 4ʺ–
PMB ether in good yield (70%, data not shown). Prompted by this result, a regioselective
deacetylative alkylation was attempted and, in the presence of KI, K2CO3, and alkyl
halide in refluxing acetone, 5 was shown to nicely convert into 3′,3ʺ,4′,5,5′,5ʺ,7–O–
heptaacetyl–4ʺ–alkyl–EGCG derivatives (6a–6e, Scheme 2) in 65–75% yields.
Interestingly, both KI and K2CO3 were required for successful transformation and, when
KI or K2CO3 was used alone, the reaction did not proceed at all. After global
deacetylation by NaBH4 in MeOH[9], the desired 4ʺ–alkyl–EGCGs (4a–4e) were obtained
(72–85% yield).
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CONCLUSION
In summary, regioselective synthesis of the therapeutically important 4ʺ–alkyl–(–)–
EGCGs (4a–4e) was accomplished in good yields by employing KI/K2CO3–promoted
deacetylative alkylation, which can be broadly utilized for preparation of various D–ring
alkyl ethers of (–)–EGCG and thereby extensive structure–activity relationship study.
EXPERIMENTAL
General Procedure For Synthesis Of 4ʺ–Alkyl–(–)–Egcgs
Peracetyl (–)–EGCG7 (5) (0.30 g, 0.38 mmol) was dissolved in acetone (5 mL) and, to
this solution, K2CO3 (0.09 g, 0.05 mmol), KI (0.13 g, 0.76 mmol), and CH3I (0.06 mL,
0.76 mmol) were added. After stirring at 50 oC for 4 h, the reaction mixture was cooled to
rt and filtered. The filtrate was concentrated under reduced pressure and the residue was
purified by column chromatography on silica gel (hexanes:acetone = 1:1) to give the
desired product 6a in 68% yield.
3′,3ʺ,4′,5,5′,5ʺ,7–O–Heptaacetyl–4ʺ–methyl–EGCG (6a) obtained above was dissolved in
MeOH (5 mL), and the resulting solution was treated with NaBH4 (71 mg, 1.88 mmol)
and then stirred for 6 h at rt. After addition of 2 M HCl, the reaction mixture was
extracted with EtOAc three times. The combined organic layers were dried over MgSO4,
filtered, and concentrated under reduced pressure. The residue was purified by column
chromatography on silica gel (CH2Cl2:MeOH = 10:1) to give 4a as a pink solid (72%
yield).
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Characterization Data For Selected Compounds
5-((2R,3R)-5,7-Diacetoxy-3-((3,5-Diacetoxy-4-Methoxybenzoyl)Oxy)Chroman-2-
Yl)Benzene-1,2,3-Triyl Triacetate (6a)
White solid; yield 68%; 1H NMR (400 MHz, Acetone-d6) δ 7.50 (s, 2H), 7.42 (s, 2H),
6.74 (d, J = 2.2 Hz, 1H), 6.61 (d, J = 2.2 Hz, 1H), 5.74 (m, 1H), 5.53 (s, 1H), 3.84 (s, 3H),
3.21 (dd, J = 16.7 Hz, 4.6 Hz, 1H), 3.08 (dd, J = 17.9 Hz, 1.6 Hz, 1H), 2.26 (m, 21H); 13C
NMR (100 MHz, Acetone-d6) δ 168.03, 167.65, 166.99, 166.18, 163.12, 154.41, 149.69,
149.48, 148.31, 143.52, 141.17, 135.42, 134.22, 123. 95, 121.91, 118.52, 109.62, 108.71,
107.28, 75.97, 67.85, 60.09, 25.13, 19.59, 19.30, 19.27, 19.13, 18.66; HRMS (FAB) m/z
Found: 767.1865 [M+H]+. Calcd for C37H35O18: 767.1823.
(2R,3R)-5,7-Dihydroxy-2-(3,4,5-Trihydroxyphenyl)Chroman-3-Yl 3,5-Dihydroxy-4-
Methoxy Benzoate (4a)
Off-white solid; yield: 72%; 1H NMR (400 MHz, Acetone-d6) δ 8.32 (br s, 1H), 8.26 (br s,
1H), 8.10 (br s, 1H), 7.81 (br s, 1H), 7.00 (s, 2H), 6.64 (s, 2H), 6.33 (d, J = 2.2 Hz, 1H),
6.03 (d, J = 2.2 Hz, 1H), 5.56 (m, 1H), 5.08 (s, 1H), 3.82 (s, 3H), 3.04 (dd, J = 17.4 Hz,
4.5 Hz, 1H), 2.92 (dd, J = 17.4 Hz, 2.1 Hz, 1H); 13C NMR (100 MHz, Acetone-d6) δ
164.44, 156.40, 156.09, 155.68, 149.73, 144.90, 139.05, 131.75, 129.29, 125.10, 108.54,
105.25, 97.53, 95.07, 94.41, 76.55, 68.29, 59.22, 25.24; HRMS (FAB) m/z Found:
495.0915 [M+Na]+. Calcd for C23H20O11Na: 495.0903.
FUNDING
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This research was supported by Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future
Planning (2015R1A2A1A10053461) and by a grant from the Priority Research Centers
Program through the National Research Foundation of Korea (NRF) funded by the
Ministry of Education, Science and Technology (2009-0093824).
SUPPORTING INFORMATION
Full experimental detail, 1H and 13C NMR spectra, high-resolution mass spectrometry
data. This material can be found via the “Supplementary Content” section of this article’s
webpage.
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Scheme 1. Known synthetic protocol for preparation of (–)–4ʺ–Me–EGCG[5]
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Scheme 2. Deacetylative alkylation in comparison to direct alkylation for preparation of
4ʺ–alkyl–(–)–EGCGs (4a–4e)
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Figure 1. Structures of the indicated compounds
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Figure 2. Resonance–stabilization of the 4ʺ–O phenolate anion of (–)–EGCG
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