Unified approach to catechin hetero-oligomers: First total synthesis of trimer EZ-EG-CA isolated from Ziziphus jujuba

Article abstract of DOI:10.1039/c2ob26337h

A catechin hetero-trimer isolated from Ziziphus jujuba has been synthesized. Among three constituent monomers, (-)-epiafzelechin and (-)-epigallocatechin were prepared by de novo synthesis. Trimer formation relied on the unified approach to oligomers based on the bromo-capping and the orthogonal activation, reaching the reported structure of the natural product. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2ob26337h

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COMMUNICATION
Unified approach to catechin hetero-oligomers: first total synthesis of trimer
EZ–EG–CA isolated from Ziziphus jujuba†
Takahisa Yano, Ken Ohmori, Haruko Takahashi, Takenori Kusumi and Keisuke Suzuki*
Received 12th July 2012, Accepted 15th August 2012
DOI: 10.1039/c2ob26337h
A catechin hetero-trimer isolated from Ziziphus jujuba has
been synthesized. Among three constituent monomers,
(−)-epiafzelechin and (−)-epigallocatechin were prepared by
de novo synthesis. Trimer formation relied on the unified
approach to oligomers based on the bromo-capping and the
orthogonal activation, reaching the reported structure of the
natural product.
Oligomeric proanthocyanidins (flavan-3-ol oligomers) found in
woody and herbaceous plants are related to traditional folk medi-
cines for treating inflammation, viral diseases, and high blood
pressure.1 The plant extracts are generally composed of diverse
catechin monomers with different oxygenation patterns and their
oligomers with varying degrees of oligomerization. Among the
Fig. 1 Catechin hetero-trimer 1 and the constituent monomers.
oligomers, hetero-oligomers composed of different monomers
are the key contributor to molecular diversity that may serve as
potential sources of bioactive compounds.
However, exploration into such a “proanthocyanidin library”
has been limited by the difficulty in isolating individual com-
pounds even by modern separation methods.2 The characteriz-
ation is also difficult. NMR analysis is hampered by severe peak
overlap and broadening as well as the rotameric issues, and
X-ray crystallography is ineffective by the generally low crystal-
linity of the polyphenols.3
Fig. 2 Thiolysis of hetero-trimer 1.
As a case study, let us focus on natural hetero-trimer 1 com-
posed of (−)-epiafzelechin (EZ, 2), (−)-epigallocatechin (EG, 3),
and (+)-catechin (CA, 4), which was isolated from the bark of
Organic synthesis has promising potential in this regard, pro-
viding samples useful for analytical standards and biological
studies. However, despite numerous reports on hetero-oligo-
mers,6 their syntheses have rarely been achieved7 for two
reasons: (1) scarce availability of monomers: except for
(+)-catechin and to a lesser extent (−)-epicatechin, other conge-
ners are virtually unavailable; (2) lack of viable methods for con-
necting the interflavan bonds.
Ziziphus jujuba (Fig. 1).4
Although the sequence EZ–EG–CA was proposed based on
the degradation study by thiolysis5 (Fig. 2), which identifies the
monomer composition (EZ, EG, and CA) and the terminal unit
as CA, other information on the connectivity and stereo- and
regiochemistry of the interflavan bonds becomes elusive. Thus,
room remains for the possible sequence EG–EZ–CA differing in
the connectivity, which features the critical lack of general
means of structural assignment on this class of natural products.
This communication reports the first total synthesis and struc-
ture confirmation of hetero-trimer 1, featuring a unified approach
to such catechin hetero-oligomers. Fig. 3 is a cartoon to show
three stages of the approach: (1) de novo synthesis of the con-
stituent monomers, (2) suitable functionalization of the mono-
Department of Chemistry, Tokyo Institute of Technology, O-okayama,
Meguro-ku, Tokyo 152-8551, Japan. E-mail: ksuzuki@chem.titech.ac.jp
†Electronic supplementary information (ESI) available: Experimental
procedures for the preparation and spectral data of all compounds. See
DOI: 10.1039/c2ob26337h
mers
(bromo-capping8
for
suppressing
non-selective
oligomerization and installation of the leaving groups9 ready for
the coupling), and (3) oligomer formation by the orthogonal
activation.8
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Scheme 1 Epiafzelechin unit 5; (a) 2-ethoxyethanol, 2,3-dichloro-5,6-
dicyano-p-benzoquinone (DDQ), toluene, room temp., 23 h (72%); (b)
NBS, CH2Cl2, −15 → −8 °C over 3 h (71%); (c) XySH, BF3·OEt2,
CH2Cl2, −78 → −55 °C over 3.5 h (96%); (d) Ac2O, N,N-dimethylami-
nopyridine (DMAP), pyridine, CH2Cl2, room temp., 2 h (95%).
Fig. 3 Three stages for the assembly of hetero-trimer.
Fig. 4 Three constituting monomers.
Three constituent monomers (Fig. 4), designated as Br-(EZ)-
SXy 5, H-(EG)-OEE 6, and H-(CA)-H 7,10 were prepared in the
following manner, respectively (see Schemes 1 and 2).
Scheme 2 Epigallocatechin unit 6; (a) 2-ethoxyethanol, DDQ, toluene,
room temp., 12 h; (b) Ac2O, DMAP, pyridine, CH2Cl2, room temp.,
12 h (62%, 2 steps).
Synthesis of the epiafzelechin derivative 5 started with assem-
bly of two building blocks 8 and 9 by the protocol previously
reported.11,12 Epiafzelechin derivative 10, thus prepared, was
treated with DDQ13 in the presence of 2-ethoxyethanol13c to
give H-(EZ)-OEE 11 in 72% yield.14 A bromine atom was intro-
duced at the C(8)-position in 11 by treatment with N-bromosuc-
cinimide (NBS), giving Br-(EZ)-OEE 12 in 71% yield. The C(4)
leaving group of 12 was replaced by an arylthio group by treat-
ment with 2,6-dimethylbenzenethiol15 (XySH) and BF3·OEt2,
and subsequent acetylation gave Br-(EZ)-SXy 5 (91% yield, two
steps from 12).
The dimer 15 with a C(4)-oxy leaving group was activated by
BF3·OEt2 in the presence of the catechin unit 7, affording the tar-
geted trimer 16 in 92% yield. An issue arose, however, in assign-
ing the stereo- and regiochemistries of the second interflavan
bond. The NMR analysis of trimer 16 was hampered by the pres-
ence of three rotamers (65 : 20 : 15 in CDCl3, 27 °C) by the
restricted rotation around two interflavan bonds on the NMR
time scale (500 MHz).
For addressing the stereostructure of trimer 16, the reversed
order of couplings was carried out, i.e., starting with connecting
the lower units, EG and CA (Fig. 5, upward assembly). The
hope was that one could address the stereochemistry of the lower
interflavan bond at the stage of the dimeric unit. Were the two
samples of 16 identical, interpolation would allow the regio- and
stereochemistry of two interflavan bonds.
The upward assembly (Scheme 4) started with the formation
of dimer 17 in 74% yield by subjecting H-(EG)-OEE 6 and
H-(CA)-H 7 to the BF3·OEt2-promoted conditions. In order to
suppress the self-condensation of 6, the reaction partner 7 was
used in excess (3.0 equiv.). The connectivity and stereochemistry
of the interflavan bond in 17 was verified as H-(EG)-|4β → 8|-
(CA)-H by extensive 2D NMR analyses (see 17B).
In the same manner, the epigallocatechin unit 6 was prepared
via 14, which was assembled from building blocks 8 and 13.
Exposure of 14 to DDQ oxidation followed by acetylation furn-
ished H-(EG)-OEE 6 in 62% yield (Scheme 2).
Catechin unit 7 was prepared by a modified Kawamoto proto-
col16 from commercial (+)-catechin (4).
Having three building blocks 5, 6 and 7 in hand, synthesis of
trimer Br-(EZ)-(EG)-(CA)-H 16 was put into practice by using
the orthogonal activation method (Scheme 3). The electrophilic
unit Br-(EZ)-SXy 5 was selectively activated by N-iodosuccini-
mide (NIS) in the presence of the nucleophilic unit H-(EG)-OEE
6, giving dimer Br-(EZ)-(EG)-OEE 15 in 91% yield. The stereo-
and regiochemistry of the newly-formed interflavan linkage was
assigned as 4β → 8 by the 2D NMR analyses (see 15A).17
7686 | Org. Biomol. Chem., 2012, 10, 7685–7688
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Scheme 3 Downward assembly (a) molar ratio: 5/6/NIS = 1.0 : 1.5 : 1.3, MS4A, CH2Cl2, −78 → 0 °C over 25 min, then 0 °C, 11 h (91%);
(b) molar ratio: 15/7/BF3·OEt2 = 1.0 : 1.3 : 1.9, CH2Cl2, −78 → −30 °C over 1.3 h (92%).
Fig. 5 Interpolative approach for stereochemical assignment of hetero-trimer 16.
Scheme 4 Upward assembly (a) molar ratio: 6/7/BF3·OEt2 = 1.0 : 3.0 : 2.7, CH2Cl2, −78 → −25 °C over 40 min (74%); (b) molar ratio: 17/5/
I2/Ag2O = 1.5 : 1.0 : 3.2 : 1.6, MS4A, CH2Cl2, −78 → 0 °C over 1 h (76%).
Condensation of dimer 17 with Br-(EZ)-SXy 5 under the soft
activation conditions (I2 and Ag2O) smoothly gave trimer 16 in
76% yield, amenable to comparison.
Indeed, the samples of trimer 16, obtained by two independent
routes, fully coincided upon analyses by NMR, MALDI-TOF
MS, TLC (several different solvent systems), and HPLC,
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ASCA-2® (5% Pd(OH)2/C), THF, MeOH, H2O (4 : 4 : 1), room temp.,
45 min; (c) Ac2O, pyridine, DMAP, 0 °C, 1 h (63%, 2 steps).
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confirming the structure as Br-(EZ)-|4β → 8|-(EG)-|4β →
8|-(CA)-H.
Scheme 5 shows final removal of the protecting groups. The
acetyl protecting groups in 16 were detached by exposure to
sodium methoxide to give Br-(EZ)-(EG)-(CA)-H 18 in 94%
yield. Hydrogenolysis of 18 [H2, ASCA-2® (5% Pd(OH)2/C),18
THF, MeOH, H2O] and lyophilization afforded trimer EZ–EG–
CA 1 {70% yield, [α]3D0 +52 (c 1.3, acetone–H2O = 1 : 1), lit.
[α]2D2 +58 (c 1.0, acetone–H2O = 1 : 1)4} as an off-white powder.
The 13C chemical shifts (acetone-d6–D2O = 1 : 1) were in good
accordance with the literature data.4 For further characterization,
product 1 was converted to peracetate 19 (63% yield from 18),
where the structure was reconfirmed by MALDI-TOF mass
spectrometry.
10 As arbitrary abbreviations for flavan monomers and its oligomers, we use
the following representations: the constituent monomers of 5–7 are
expressed as in Br-(EZ)-SXy, H-(EG)-OEE, H-(CA)-H, respectively,
where Br stands for a C8 bromo group of an electrophilic unit, OEE for
the C4-ethoxyethoxy group in an electrophilic unit. Hetero-trimer 1 is
expressed as H-(EZ)-(EG)-(CA)-H, where the top H stands for a
In summary, the first total synthesis of catechin hetero-trimer
1 has been achieved, demonstrating the power of the unified
strategy in providing various catechin oligomers. Further work
along these lines is in progress.
C8 hydrogen of the top epiafzelechin unit, the tail
C4 methylene group in the terminal catechin unit.
H for the
11 K. Ohmori, T. Yano and K. Suzuki, Org. Biomol. Chem., 2010, 8, 2693.
12 For a new synthesis method of epi-series catechin monomers, see:
S. Stadlbauer, K. Ohmori, F. Hattori and K. Suzuki, Chem. Commun.,
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Acknowledgements
This work was supported by a Grant-in-Aid for Specially Pro-
moted Research (No. 23000006) from JSPS.
14 The stereochemistry of the new stereogenic centre in compound 5 was
determined by NOE experiments. For details, see the ESI.† In contrast,
the stereochemistries of compounds 6 and 11 were deduced from the pre-
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