Concise synthesis of ring-fission metabolites of epicatechin: 5-(3,4-dihydroxybenzyl)dihydrofuran-2(3H)-one M6

Article abstract of DOI:10.1080/00397910903419845

The ring-fission metabolites of epicatechin, 5-(3,4-dihydroxybenzyl) dihydrofuran-2(3H)-one M6 and 3-methylated M6, were prepared from 3,4-dihydroxybenzaldehyde and furan-2(5H)-one using two new concise methods in excellent yields (three or two steps in 4

Full text of DOI:10.1080/00397910903419845

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Concise Synthesis of Ring-Fission
Metabolites of Epicatechin: 5-(3,4-
Dihydroxybenzyl)dihydrofuran-2(3H)-one
M6
Xiaowei Chang a , Weimin Peng a , Yin-Fang Yao a & Jan Koek b
a Unilever Discover Shanghai, Shanghai, China
b Unilever Discover Vlaardingen, Vlaardingen, The Netherlands
Published online: 15 Oct 2010.
To cite this article: Xiaowei Chang , Weimin Peng , Yin-Fang Yao & Jan Koek (2010): Concise Synthesis
of Ring-Fission Metabolites of Epicatechin: 5-(3,4-Dihydroxybenzyl)dihydrofuran-2(3H)-one M6,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 40:22, 3346-3352
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Synthetic Communications1, 40: 3346–3352, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903419845
CONCISE SYNTHESIS OF RING-FISSION
METABOLITES OF EPICATECHIN:
5-(3,4-DIHYDROXYBENZYL)DIHYDROFURAN-
2(3H)-ONE M6
Xiaowei Chang,1 Weimin Peng,1 Yin-Fang Yao,1 and Jan Koek2
1Unilever Discover Shanghai, Shanghai, China
2Unilever Discover Vlaardingen, Vlaardingen, The Netherlands
The ring-fission metabolites of epicatechin, 5-(3,4-dihydroxybenzyl)dihydrofuran-
2(3H)-one M6 and 3-methylated M6, were prepared from 3,4-dihydroxybenzaldehyde
and furan-2(5H)-one using two new concise methods in excellent yields (three or two steps
in 40–60% overall yield).
Keywords: 5-(3,4-Dihydroxybenzyl)dihydrofuran-2(3H)-one; epicatechin; metabolite; methylated M6
INTRODUCTION
The biologically active compounds in drugs and foods are not always effective
by themselves but can act as precursors. By metabolic processes (digestion, fermen-
tation in the colon, detoxification by the hepatic system, etc.), these compounds are
converted, and the resulting metabolites might contribute, or even be responsible, for
the biological activities observed.[1–3] For the study of their corresponding involve-
ment and roles in the relevant metabolic processes, pure reference standards for these
compounds are required. Moreover, these active metabolites become a vast reservoir
of potential therapeutic agents, and there is increasing interest and investigation in
using them as the original drugs. 5-(3,4-Dihydroxybenzyl)dihydrofuran-2(3H)-
one (M6) and 5-(3,4,5-trihydroxybenzyl)dihydrofuran-2(3H)-one (M4), as the
ring-fission metabolites of epicatechin and epigallocatechin, respectively (Fig. 1),
together with the partially methylated derivatives of M4 and M6, were found in
urine and plasma after the consumption of green tea.[4–7] A pathway of the microbial
degradation of epicatechin to M6 by the colonic flora has been proposed on the basis
of an in vitro study (Scheme 1).[8,9] Recently, M6 and 3-methylated M6 were also
detected as the metabolites after consumption of fruit and vegetable dietary
flavanoids.[10]
Received May 29, 2009.
Address correspondence to Weimin Peng, Unilever Discover Shanghai, 3=F, Xin Mao Building,
99 Tian Zhou Road, Caohejing High-Tech Park, Shanghai 200233, China. E-mail: wei-min.peng@
unilever.com
3346

METABOLITES OF EPICATECHIN
3347
Figure 1. Structures of 5-(3,4-dihydroxybenzyl)dihydrofuran-2(3H)-one M6 and 5-(3,4,5-trihydroxybenzyl)-
dihydrofuran-2(3H)-one M4.
Scheme 1. Pathway of the microbial degradation of epicatechin within the digestive system.
To evaluate the importance of these compounds, pure standards are required
for analytical purposes. It has been found that M6 exhibited excellent antioxidant
properties and that M4 was capable of inhibiting the growth of immortalized and
malignant human cell lines.[11] This might suggest that these two metabolites contrib-
ute to the anti-inflammatory and cancer-preventive activities of green tea,[12] which
requires further research on the biological activity of M6, M4, and their methylated
derivatives.
RESULTS AND DISCUSSION
Over the past years, several groups have tried different synthetic approaches to
prepare these compounds as reference standards or to investigate their possible
therapeutic use. Lambert et al. employed d-phenyl-c, d-unsaturated methyl ester as
a key intermediate.[11] Following six or seven steps of transformation, including
epoxidation, reduction, and lactonization, the target compound was finally afforded
in about 10% overall yield. Watanabe reported that a 1,2-epoxy-3-phenylpropan
derivative could serve as a key intermediate to react with ethyl sodium malonate

3348
X. CHANG ET AL.
Scheme 2. Retrosynthetic analysis of 5-phenyl-c-valerolactone.
and then lactonization and decarboxylation generated the target compound with
about 9% overall yield in four steps.[13] Both methods were somewhat tedious, but
on the other hand the two intermediates had to be prepared in advance by several
steps. An internal urgent need for gram amounts of the pure metabolites necessitated
development of an efficient and simple synthetic method to prepare M6 and methyl-
ated M6. We envisaged a concise and efficient route with potential to be applied to
the synthesis of other polyphenol metabolites like M4 because of the similarity of the
structure (Scheme 2). A selective catalytic reduction of the two double bonds of the
aldol condensation product that is prepared from readily accessible benzaldehyde
derivatives and furan-2(5H)-one was expected to give the c-valerolactones in an easy
and scalable way.
Indeed, 5-(3,4-dihydroxybenzyl)dihydrofuran-2(3H)-one M6 and 3-methylated
M6 could be successfully synthesized according to the proposed route. The synthesis
started with the addition reaction of furan-2(5H)-one to protected 3,4-dihydroxyl-
phenyl aldehyde (Scheme 3). In the first approach, lithiated furan-2(5H)-one A
was in situ generated as nucleophile by treatment of furan-2(5H)-one with
Scheme 3. Synthesis of 5-(3–4-dihydroxybenzyl)dihydrofuran-2(3H)-one M6 and 3-methylated M6.
Reagents and conditions: (I) LDA, THF, ꢀ78 ꢁC (2a: 59%, 2b: 52%); (II) MsCl, pyridne, rt (3a:
ꢂ100%, 3b: 81%); (III) TBDMSOTf, DIEA, THF, DBU, rt (3a: 47%, 3b: 54%); (IV) CH3CN, Pd=C,
H2, rt (4c: 91%, 4b: 80%).

METABOLITES OF EPICATECHIN
3349
Scheme 4. Formation mechanism of by-products 5c and 5b.
LDA.[14,15] The reaction went exclusively at the c-position of furan-2(5H)-one, and
the regiospecific products, 5-[[3,4-bis(benzyloxy)phenyl](hydroxy)methyl]furan-
2(5H)-one 2a and 5-[[4-(benzyloxy)-3-methoxyphenyl](hydroxy)methyl]furan-2
(5H)-one 2b, were obtained in moderate yield. Afterward, the hydroxyl group of
2a and 2b was mesylated in pyridine at room temperature, and consecutive b-elim-
ination afforded the conjugated dienes 5-[3,4-bis(benzyloxy)benzylidene]furan-
2(5H)-one 3a and 5-[4-(benzyloxy)-3-methoxybenzylidene]furan-2(5H)-one 3b as a
mixture of Z- and E-isomer in excellent yield.
In the second, more efficient approach, tert-butyldimethylsily triflate
(TBDMSOTf) was employed to facilitate the condensation of benzaldehyde 1 with
furan-2(5H)-one. The intermediate (furan-2-yloxy) tert-butyldimethylsilane B was
probably produced in the reaction from furan-2(5H)-one and TBDMSOTf in the
presence of a base.[16–18] It subsequently reacted with 1a and 1b to form the silyl sub-
stituted intermediates 2a and 2b respectively, which were once isolated as
by-products. Most of the final products were the expected condensation products
3a and 3b. It was obvious that 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) had
further eliminated TBDMSOH to give 3a and 3b. Consequently, 3a and 3b could
be obtained directly from aldehyde and furan-2(5H)-one in a one-pot reaction.
Finally, 3a and 3b could be hydrogenated to reduce the two double bonds and
remove the benzyl groups as well in the presence of Pd=C powder, consequently
furnishing the target compounds 5-(3,4-dihydroxybenzyl)dihydrofuran-2(3H)-one
4c and 5-(4-hydroxy-3-methoxybenzyl)dihydrofuran-2(3H)-one 4b in 91% and
80% yields,[19] respectively (Scheme 3). However, an interesting by-product, 5-(3,4-
dihydroxyphenyl)pentanoic acid 5c or 5-(4-hydroxy-3-methoxyphenyl)pentanoic
acid 5b, which was produced by scission of lactone ring, was isolated. The formation
could be minimized by using acetonitrile as solvent. The mechanism for formation
could be rationalized as in Scheme 4.
CONCLUSION
In summary, we have described two alternative concise methods to synthesize
the ring-fission metabolites of epicatechin, 5-(3,4-dihydroxybenzyl)dihydrofuran-
2(3H)-one M6 and 3-methylated M6, with satisfactory yield from the readily

3350
X. CHANG ET AL.
available starting material. It is expected that 5-(3,4,5-trihydroxybenzyl)-
dihydrofuran-2(3H)-one M4 and its derivatives can be prepared by these methods
too in gram quantities.
EXPERIMENTAL
5-[[3,4-Bis(benzyloxy)phenyl](hydroxyl)methyl]furan-2(5H)-one
(2a)
and
5-[[4-(benzyloxy)-3-methoxyphenyl](hydroxy)methyl]furan-2(5H)-one (2b) were pre-
pared following the procedure reported in the literature.[14]
Compound 2a was obtained as a yellow solid in the yield of 59%, as a mixture
of syn–anti isomers (80:20). 1H NMR (400 MHz, CDCl3): 7.46–7.31 (m, 10H),
7.08–6.67 (m, 4H), 6.10 (syn) and 6.02 (anti) (d, J ¼ 5.7 Hz, 1H), 5.20 (AB,
J ¼ 12.3 Hz, 2H), 5.18 (s, 2H), 5.04–5.02 (m, 1H), 4.96 (syn) and 4.53 (anti) (d,
J ¼ 4.2 Hz and 7.5 Hz respectively, 1H), 2.35 (s, br, 1H). 13C NMR (100 MHz,
CDCl3): 172.8, 152.7, 149.1, 148.9, 137.0, 131.3, 128.5, 127.9, 127.4, 127.3, 123.0,
119.2, 114.9, 113.1, 86.4, 72.7, 71.3, 71.2.
Compound 2b was obtained as a yellow solid in the yield of 52%, as a mixture
of syn–anti isomers (86:14). 1H NMR (400 MHz, CDCl3): 7.44–7.26 (m, 6H),
6.95–6.71 (m, 3H), 6.15 10 (syn) and 6.03 (anti) (d, J ¼ 5.8 Hz, 1H), 5.15–5.11 (m,
3H), 4.97 and 4.60 (anti) (d, J ¼ 4.5 Hz and 6.5 Hz respectively, 1H), 3.89 (s, 3H),
2.62 (s, br, 1H). 13C NMR (100 MHz, CD3COCD3): 172.8, 152.9, 150.0, 148.3,
136.9, 131.4, 128.5, 127.9, 127.2, 123.1, 118.4, 114.0, 108.7, 86.4, 73.0, 71.1, 56.1.
5-[3,4-Bis(benzyloxy)benzylidene]furan-2(5H)-one (3a) and 5-[4-(benzyloxy)-
3-methoxybenzylidene]furan-2(5H)-one (3b) were prepared following the procedure
reported in the literature[14] from 2a and 2b, respectively. Alternatively, they could
be synthesized following the procedure described later directly from the correspond-
ing benzaldehyde 1 and furan-2(5H)-one.
tert-Butyldimethylsily triflate was added to the solution of furan-2(5H)-one
(0.097 g, 0.082 mL) in dichloromethane (10 mL). Then 3,4-dibenzoxyl benzaldehyde
(0.368g, 1.157 mmol) and diisobutyl ethyl amine (0.448g, 0.604 mL) were added to the
mixture. After the reaction was stirred at rt for 1 h, 1,8-diazabicyclo[5,4,0]undec-
7-ene (0.352g, 0.347 mL) was added dropwise. The reaction mixture was stirred for
another 1 h. Then the solvent in the reaction was removed under reduced pressure.
The residue was purified by flash chromatography (60% ethyl acetate in hexane) to
give a yellow solid of 3a as a 75:25 mixture of (Z)- and (E)-isomers (0.167g, 47%
1
yield). H NMR (400MHz, CDCl3): d 7.50–7.16 (m, 13H), 6.86 (d, J ¼ 8.4, 1H),
6.13 (E) and 6.03 (Z) (d, J ¼ 5.5 and 5.2 Hz respectively, 1H), 6.57 (E) and 5.81 (Z)
(s, 1H), 5.17 (s, 2H), 5.13 (s, 2H); 13C NMR (100MHz, CDCl3): 170.2, 150.0,
148.6, 147.1, 145.0, 136.8, 136.6, 128.4 (2), 127.8 (2), 127.5, 127.1, 126.3, 125.2,
116.7, 116.3, 114.1, 114.0, 71.1, 70.7.
Compound 3b was prepared as yellow oil as a 70:30 mixture of (Z)–(E)-isomers
1
according to the procedure for 3a. H NMR (400 MHz, CDCl3): d 7.72 (E) (d,
J ¼ 5.5, 1H), 7.40–7.13 (m, 7H), 6.81 (d, J ¼ 8.0, 1H), 6.23 (E) and 6.08 (Z) (d,
J ¼ 5.5 and 5.2 Hz respectively, 1H), 6.65 (E) and 5.89 (Z) (s, 1H), 5.13 (s, 2H),
3.88 (Z) and 3.85 (E) (s, 3H);13C NMR (100 MHz, CDCl3): 170.3, 149.7, 149.5,
147.2, 145.1, 136.6, 128.6, 128.0, 127.2, 126.4, 124.7, 116.9, 114.4, 113.6, 70.8, 56.1.

METABOLITES OF EPICATECHIN
3351
5-(3,4-Dihydroxybenzyl)dihydrofuran-2(3H)-one (4c) and 5-(4-hydroxy-
3-methoxybenzyl)dihydrofuran-2(3H)-one (4b) were prepared as follows: Com-
pound 3a (2.882 g, 7.51 mmol) and 10% Pd=C powder (0.432 g) were suspended in
CH3CN (80 mL) under a hydrogen atmosphere. The reaction mixture was stirred
at rt for 15 h. Upon thin-later chromatography (TLC) revealing completion of the
reaction, the catalyst was filtered and washed with CH3CN (50 mL). Removal of
the solvent under reduced pressure afforded the crude product, which was purified
by flash chromatography (60% ethyl acetate in hexane) to give a white solid of 4c
1
(1.431 g, 91% yield). Mp 134–136 ꢁC. H NMR (400 MHz, CDCl3): d 7.27 (s, 2H),
6.83–6.77 (m, 3H), 4.76–4.69 (m, 1H), 2.89 (ABd, J ¼ 14.4, 5.6 Hz, 2H), 2.51–2.41
(m, 1H), 2.36–2.22 (m, 2H), 2.01–1.93 (m, 1H). 13C NMR (100 MHz, CD3COCD3):
177.3, 145.9, 144.8, 129.4, 121.7, 117.4, 116.1, 81.7, 41.3, 29.0, 27.8. Compound 4b
was prepared according to the procedure of 4c. White solid. Mp 111–112 ꢁC. Yield
1
80%. H NMR (400 MHz, CDCl3): d 6.87–6.70 (m, 3H), 5.56 (s, 1H), 4.76–4.70 (m,
1H), 3.89 (s, 3H), 2.93 (ABd, J ¼ 14.0, 5.6 Hz, 2H), 2.50–2.40 (m, 1H), 2.35–2.21 (m,
2H), 2.00–1.90 (m, 1H). 13C NMR (100 MHz, CDCl3): 177.1, 146.6, 144.7, 127.6,
122.2, 114.5, 112.1, 80.9, 56.0, 40.9, 28.7, 26.9.
The by-product 5b was isolated from the reaction for the preparation of 4b as a
1
yellow oil and characterized accordingly. H NMR (400 MHz, CDCl3): d 6.84–6.66
(m, 3H), 3.89 (s, 3H), 2.58 (t, J ¼ 7.2 Hz, 2H), 2.39 (t, J ¼ 7.1 Hz, 2H), 1.69–1.65 (m,
4H). 13C NMR (100 MHz, CDCl3): 179.8, 146.4, 143.7, 133.9, 120.9, 114.2, 110.9,
55.9, 35.2, 33.8, 31.0, 24.2.
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