The synthesis of methylated epigallocatechin gallate

Article abstract of DOI:10.1007/s10600-015-1317-5

Synthesis of methylated epigallocatechin gallate from (-)-epigallocatechin gallate and propylgallate was accomplished using a benzyl (Bn) group as a protecting group for phenols. This methodology provided (-)-epigallocatechin-3-(4-O-methylgallate), which

Full text of DOI:10.1007/s10600-015-1317-5

DOI 10.1007/s10600-015-1317-5
Chemistry of Natural Compounds, Vol. 51, No. 3, May, 2015
THE SYNTHESIS OF METHYLATED EPIGALLOCATECHIN GALLATE
1
1
1*
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Ronghui Lai, Wenfang Zhao, Yahui Huang, Wen Zhou,
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Chunlan Wu, Xingfei Lai, Wenxia Zhao, and Ming Zhang
Synthesis of methylated epigallocatechin gallate from (–)-epigallocatechin gallate and propylgallate was
accomplished using a benzyl (Bn) group as a protecting group for phenols. This methodology provided
(–)-epigallocatechin-3-(4-O-methylgallate), which are naturally scarce catechin derivatives.
Keywords: methylated epigallocatechin gallate, (–)-epigallocatechin gallate, chemical modification.
Tea is probably one of the most widely drunk beverages because of its various beneficial constituents. Several
epidemiological studies [1] have indicated that long-term tea-drinking can reduce the risk of several types of cancers. Tea
polyphenols possess a wide variety of functions, including anti-allergic, antioxidative, antimutagenic/anticarcinogenic,
antiatherosclerotic, and antibacterial effects. These effects are particularly attributed to the catechins, a group of 3-flavanols
and their derivatives. However, catechins are unstable in physiological environment and thus are difficultly absorbed by
organisms because of the richness of OHꢀs [2, 3]. It is necessary to chemically modify the natural catechins for enhancing their
activities and functions in vivo.
Several studies have demonstrated that (–)-epigallocatechin gallate (1, EGCG), the most abundant catechin
accounting for 50–80% of the total catechins in tea, could inhibit type I allergic reactions in rats [3, 4]. It has been reported
that (–)-epigallocatechin-3-O-(3-O-methyl) gallate (EGCG3ꢀꢀMe) and (–)-epigallocatechin-3-O-(4-O-methyl) gallate
(EGCG4ꢀꢀMe) have more significant inhibitory effects than EGCG [5, 6]. Suzuki et al. also reported that the inhibitory activities
of EGCG3ꢀꢀMe and EGCG4ꢀꢀMe on mouse type IV allergy were higher than that of EGCG at a dose of 0.05 mg/wear [3]. The
higher inhibitory activities of EGCG3ꢀꢀMe and EGCG4ꢀꢀMe on mouse allergies are thought to be associated with the stability
of the O-methylated derivatives of EGCG in animal bodies. Pharmacological studies have shown that methylation of EGCG
strongly inhibited histamine release for mast cells, which helps to relieve the symptoms caused by allergic response [2, 7].
The synthesis of methylated EGCG has been investigated lately. Through the method of total synthesis, Zaveri obtained
3ꢀꢀ,4ꢀꢀ,5ꢀꢀ-trimethylated EGCG [8]. Using EGCG and methyl iodide as the precursors and K CO as the catalyst, Meng et al.
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3
[9] synthesized 4ꢀꢀ-MeEGCG, 4ꢀ,4ꢀꢀ-DiMeEGCG, and 4ꢀ,3ꢀꢀ,4ꢀꢀ-trimethylated EGCG. Various methylated isomers of epicatechin
gallate (ECG) and epigallocatechin gallate (EGCG) have been synthesized by [10]. Using nitrophenylsulfonyl (NS) as the
protector of the OH-group, with EGC and gallic acid (GA) as precursors, Aihara et al. [11] synthesized the methylated EGCG.
In our previous study, EGCG was purified from tea polyphenols [12]. In this paper, we reported a concise and
efficient method for chemical synthesis of 4ꢀꢀ-Me-EGCG by using 1 and propyl gallate (PG) as the precursors.
As a novel protecting and activating group for amines, a benzyl group such as BnBr has been employed as a protecting
group for phenol [13].
Compound 8 is synthesized from the natural (–)-EGCG, which is commercially available. The synthetic conditions
for obtaining benzylated EGCG with BnBr–dimethylformamide (DMF) at different temperatures were tested. Finally, the
conditions of BnBr–K CO –DMF–rt/N were found to be the optimal.
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1) College of Horticulture, South China Agricultural University, Guangzhou, 510642, Guangdong, P. R. China,
e-mail: 13501513191@163.com; 2) School of Pharmacy, Shanghai Jiaotong University, 800 Dongchuan Road, 200240,
Shanghai, P. R. China. Published in Khimiya Prirodnykh Soedinenii, No. 3, May–June, 2015, pp. 411–413. Original article
submitted May 13, 2013.
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0009-3130/15/5103-0472 02015 Springer Science+Business Media New York

OBn
OH
OBn
OBn
OH
OH
H C
3
H C
3
O
O
BnO
O
HO
O
OH
OH
OH
O
O
c
a
O
O
OCH
3
OBn
OH
OH
OH
OBn
OBn
OH
OH
O
O
7
6
1
2
d
H C
3
OBn
OBn
OH
O
OH
b
OBn
OBn
O
O
e
OBn
OCH
3
OCH
3
OBn
BnO
O
OBn
O
OBn
9
8
OH
OBn
OH
OBn
3
OBn
OBn
OH
OH
BnO
O
HO
f
g
O
O
O
OBn
OH
OBn
OH
OCH
O
OCH
3
3
4
OBn
OH
5
a. BnBr–K CO –DMF, r.t.; b. NaOH–THF–MeOH, reflux; c. Li CO –methyl iodide–DMF, 50°C; d. BnBr–K CO –DMF, r.t.;
2
3
2
3
2
3
e. NaOH–THF–MeOH, r.t.; f. 9, oxalyl chloride–CH Cl –DMAP–H O, reflux–r.t.; g. HCOONH –20% Pd(OH) – C –MeOH–
2
2
2
4
2
THF, r.t.
Scheme 1
The benzylated derivative 2 with over 50% yield could be obtained after purification. Compound 2 was hydrolyzed
in the presence of sodium hydroxide at reflux and THF–methanol solution (v/v, 1:1) (Scheme 1). Compound 3 was generated
by hydrolysis of compound 2, which was obtained in 87.6% yield.
The methyl group was selectively incorporated into 4-OH of compound 6 with Li CO and methyl iodide, the phenolic
2
3
hydroxyl group in the C-4 position of compound 6, which is the most active hydroxyl group, was deprotonated. Finally
compound 7 was obtained by alkylation reaction of compound 6.
Compound 8 was derived from benzylation of compound 7 in the presence of BnBr, K CO , and DMF. The hydrolytic
2
3
reaction of compound 8 with sodium hydroxide and methanol as catalysts at room temperature gave compound 9.
Compound 4 was obtained by esterification reaction of compound 3, compound 9, and oxalyl chloride. Hydrogenolysis
of compound 4 afforded the methylated compound 5 in 19% yield. The spectroscopic data of compound 5 are in agreement
with those reported previously for the metabolites.
We have successfully synthesized 4ꢀꢀ-Me-EGCG and developed a concise and efficient synthesis method to obtain
methylated EGCG.
EXPERIMENTAL
General Materials and Reagents. EGCG was purified from tea via a flash chromatography system by our group.
Other starting materials and reagents purchased commercially were used without further purification. Anhydrous methylene
chloride was stirred with CaCl for 10 h. Twenty-four hours after being refluxed with CaSO , anhydrous DMF was distilled
2
4
under vacuum. The reaction flasks were dried under 120°C. All moisture-sensitive reactions were conducted under a nitrogen
atmosphere. Flash chromatography was carried out using silica gel GF . When CDCl and MeOD were used as solvents,
254
3
1
H NMR (600 MHz) spectra were measured with TMS as the internal standard.
473

(–)-(2R,3R)-5,7-Bis(benzyloxy)-2-(3,4,5-tris(benzyloxy)phenyl)chroman-3-yl-(3,4,5-trisbenzyloxy)benzoate (2).
To a solution of 1 (100 mg, 0.22 mmol) in dry DMF (10.0 mL) under nitrogen at room temperature, K CO (250 mg, 1.8 mmol)
2
3
was added; after stirring for 2 h, benzyl bromide (0.25 mL, 2.1 mmol) was added. The mixture was stirred for another 60 h at
room temperature. The reaction was quenched by adding hydrochloric acid (2 mol/L, 10.00 mL). The aqueous layer was
extracted with EtOAc. The organic extracts were combined, washed with water, and then evaporated in vacuum. The residue
was purified by flash SiO column chromatography (petroleum ether–EtOAc, 4:1) to afford the crude benzylated EGCG
2
1
(140 mg) as a yellow oil in 54.5%yield. H NMR (600 MHz, CDCl , ꢁ, ppm, J/Hz): 3.08 (1H, d), 3.12 (1H, d), 4.70 (2H, d,
3
J = 0.04), 4.90–5.06 (16H, m), 6.34 (1H, d), 6.40 (1H, d), 6.72 (2H, s), 7.21–7.38 (42H, m).
(L)-(2R,3R)-5,7-Bis(benzyloxy)-2-(3,4,5-tris(benzyloxy)phenyl)chroman-3-ol (3). Benzylated EGCG (2) (100 mg,
0.08 mmol) was dissolved in THF–MeOH (1:1, 10 mL); drops of water and sodium hydroxide (20.00 mg, 0.5 mmol) were
added. The mixture was refluxed at 65°C for 20 h and extracted with EtOAc three times. The organic layers were combined
and evaporated in vacuum. The residue was purified by flash SiO column chromatography (petroleum ether–EtOAc, 4:1) to
2
1
afford the product (56.3 mg) as a pale yellow oil in 87.6% yield. H NMR (600 MHz, CDCl , ꢁ, ppm, J/Hz): 3.08 (1H, d), 3.12
3
(1H, d), 4.70 (2H, d, J = 0.04), 5.02–5.08 (10H, m), 6.36 (1H, d), 6.42 (1H, d), 6.72 (2H, s), 7.25–7.34 (25H, m).
4-Methoxypropylgallate (7). To a mixture of propyl gallate (6, 100 mg, 0.43 mmol) in dry DMF (10 mL), Li CO
2
3
(200 mg, 2.7 mmol) was added under nitrogen. After stirring for 30 min at 50°C, methyl iodide (0.05 mL, 0.65 mmol) was
added drop-wise, and the mixture was stirred overnight. Hydrochloric acid (2 mol/L, 10.00 mL) was then added to quench the
reaction. The mixture was extracted with EtOAc. The organic layers was washed with H O, then dried over sodium sulfate
2
(Na SO ) and evaporated in vacuo. The residue was purified by flash SiO column chromatography (petroleum ether–EtOAc,
2
4
2
1
2:1) to afford a white solid (88 mg) in 83.0% yield. H NMR (600 MHz, MeOD-d , ꢁ, ppm): 1.0 (3H, t), 1.6–1.8 (2H, m), 3.9
4
(3H, s), 4.2 (2H, t), and 7.3 (2H, s).
3,5-Bis(benzyloxy)-4-methoxypropylgallate (8). To a solution of 7 (100 mg, 0.45 mmol) in dry DMF (10.0 mL)
under nitrogen at room temperature, K CO (250 mg, 1.8 mmol) was added. After the mixture was stirred for 2 h, benzyl
2
3
bromide (0.15 mL, 1.26 mmol) was added. Stirring was continued for 20 h at room temperature.
The reaction was quenched by adding hydrochloric acid (2 mol/L, 10.00 mL). The aqueous phase was extracted with
EtOAc. The organic layers were combined and washed with water, then dried over sodium sulfate (Na SO ) and evaporated
2
4
in vacuo. The residue was purified by flash SiO column chromatography (petroleum ether–EtOAc, 5:1) to afford the crude
2
1
benzylated 8 (170 mg) as a white solid in 96.6% yield. H NMR (600 MHz, CDCl , ꢁ, ppm): 1.0 (3H, t), 1.7–1.9 (2H, m), 3.9
3
(3H, s), 4.2 (2H, t), 5.2 (4H, s), 7.2–7.6 (12H, m).
3,5-Bis(benzyloxy)-4-methoxygallic Acid (9). To a solution of benzylated Me-PG (100 mg, 0.25 mmol) in
THF–MeOH (1:1, 10 mL), drops of water and sodium hydroxide (20.00 mg, 0.5 mmol) were added. The mixture was refluxed
at 65°C for 20 h, then extracted with EtOAc three times. The organic layers were combined and dried over Na SO and
2
4
evaporated in vacuo. The residue was purified by flash SiO column chromatography (petroleum ether–EtOAc, 4:1) to afford
2
1
a white solid (58.33 mg) in 65.0% yield. H NMR (600 MHz, CDCl , ꢁ, ppm): 3.9 (3H, s), 5.2 (4H, s), 7.2–7.6 (12H, m).
3
(–)-(2R,3R)-5,7-Bis(benzyloxy)-2-(3,4,5-tris(benzyloxy)phenyl)chroman-3-yl-(3,5-bisbenzyloxy-4-
methoxy)benzoate (4). To a solution of 9 (50 mg, 0.14 mmol) in dry CH Cl (10.00 mL), oxalyl chloride (1.50 mL,
2
2
14.9 mmol) was added. After refluxing for 3 h, the reaction mixture was evaporated and dried completely in vacuo for 2 h.
After the obtained white solid was cooled to 0°C, a solution of 4-dimethylaminopyridine (DMAP, 15 mg, 0.12 mmol) and
(–)-(2R,3R)-5,7-bis(benzyloxy)-2-(3,4,5-tris(benzyloxy)phenyl)chroman-3-ol (3, 100 mg, 0.13 mmol) in dry CH Cl (10.00 mL)
2
2
was added dropwise at 0°C. Then the mixture was stirred at room temperature overnight until thin-layer chromatography
(TLC) showed that the reaction was completed. The solvent was evaporated in vacuo, and the resulting oil was purified by
flash SiO column chromatography (petroleum ether–EtOAc, 3:1) to get a pale yellow amorphous solid (4, 20 mg) in 21.4%
2
1
yield. H NMR (600 MHz, CDCl , ꢁ, ppm): 3.06 (1H, d), 3.08 (1H, d), 4.09 (3H, s), 4.87 (2H, s, J = 0.04), 4.98–5.20 (14H, m),
3
6.30 (1H, s), 6.52 (1H, s), 6.76 (2H, s), 7.26–7.46 (37H, m).
(–)-(2R,3R)-5,7-Bis(hydroxy)-2-(3,4,5-tris(hydroxy)phenyl)chroman-3-yl-(3,5-bisbenzyloxy-4-methoxy)benzoate
(5). Pd(OH) –C (20%, 20 mg) and formic acid (1 mL, 26.7 mmol) were added to a mixture of THF–MeOH (1:1 v/v, 25 mL)
2
in which 4 (100 mg, 0.09 mmol) was dissolved. The resulting reaction mixture was stirred at room temperature under nitrogen
until TLC showed that the reaction was completed. The reaction mixture was filtered to remove the catalyst. The filtrate was
evaporated, and the residue was rapidly purified by flash chromatography on silica gel (MeOH–CH Cl , 3:1) to afford a pale
2
3
1
yellow amorphous solid 5 (8 mg) in 19.9% yield. H NMR (600 MHz, MeOD-d , ꢁ, ppm): 3.03 (2H, s), 3.85–3.92 (3H, m),
4
5.20 (1H, s), 5.51 (1H, s), 7.0–7.6 (6H, m).
474

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