Enantioselective total syntheses of (+)-gallocatechin, (-)- epigallocatechin, and 8-C-ascorbyl-(-)-epigallocatechin

Article abstract of DOI:10.1002/asia.201201168

Reading the tea leaves: The enatioselective total syntheses of 8-C-ascorbyl-(-)-epigallocatechin was accomplished by Cu^II-mediated oxidative coupling of ascorbic acid and (-)-epigallocatechin as a key step. Also, the asymmetric total syntheses of tea-leaf extracts (+)-gallocatechin and (-)-epigallocatechin were achieved by Au-catalyzed intramolecular cycliarylation of the precursor epoxide and Sharpless dihydroxylation. Copyright

Full text of DOI:10.1002/asia.201201168

COMMUNICATION
DOI: 10.1002/asia.201201168
Enantioselective Total Syntheses of (+)-Gallocatechin, (À)-Epigallocatechin,
and 8-C-Ascorbyl-(À)-epigallocatechin
Guang Lin,[a] Le Chang,[b] Yongxiang Liu,[a] Zheng Xiang,[a] Jiahua Chen,*[a] and
Zhen Yang*[a, b]
Tea has attracted considerable levels of attention because
of its beneficial effects on human health, such as reducing
serum cholesterol levels, preventing arteriosclerosis, and
lowering the incidence of several human cancers and
AIDS.[1] Tea leaves contain a variety of different catechins,
including (+)-catechin (C, 1), (À)-epicatechin (EC, 2),
(+)-gallocatechin (GC, 3), (À)-epigallocatechin (EGC, 4),
and
(À)-epigallocatechin-3-gallate
(EGCG,
5;
Scheme 1).[1c,2]
In 1989, 8-C-ascorbyl-(À)-epigallocatechin-3-gallate (6)
and 8-C-ascorbyl-(À)-epigallocatechin (7) were isolated
from oolong tea by Nishioka and co-workers.[3] A subse-
quent biological investigation of the thirty-eight catechins
derived from tea by Lee et al.[4] indicated that 6 and 7 dem-
onstrated their inhibitory effects against HIV replication in
H9 lymphocyte cells, providing EC50 values of 4 and
10 mgmLÀ1, respectively. In 2005, Nakai et al. also reported
the isolation of compounds 6 and 7 from oolong tea and in-
dicated their inhibitory activity against pancreatic lipases in
vitro with an IC50 of 0.791 and 0.646 mm, respectively.[5]
Scheme 1. Naturally occurring polyphenols.
The structure of 7 was elucidated through a combination
of NMR spectroscopy and mass spectrometry[3,6] and was
found to contain a densely oxygenated polyphenol frame-
work with six stereogenic centers, together with a rare and
sensitive hemiketal moiety. The interesting molecular struc-
ture and intriguing biological activity of compound 7, to-
gether with the synthetic challenges associated with its high
sensitivity to oxidation as well as its difficult purification
owing to its highly hydrophilic character inspired us to
embark on its total synthesis.
To the best of our knowledge, no reliable methods have
been reported to date for the synthesis of 7. In the original
paper from Nishioka et al.[3] describing the isolation of 7,
the authors mentioned that 7 could be made by the conden-
sation reaction of
4 with dehydroascorbic acid (8 in
[a] Dr. G. Lin, Dr. Y. Liu, Dr. Z. Xiang, Prof. Dr. J. Chen,
Prof. Dr. Z. Yang
Scheme 3) in the presence of sodium bicarbonate. In our
own hands, however, the reaction failed to provide the de-
sired product under these conditions and also failed under
a variety of other base-mediated condensation conditions. A
similar result was also observed by Campagna et al.[6] in
their synthesis of (À)-ascorbyl phloroglucinol. Taken togeth-
er, these results indicated that further investigation was
needed to devise an effective methodology to put this bio-
mimetic synthesis into practice.
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education
and Beijing National Laboratory for Molecular Science (BNLMS)
Peking-Tsinghua Center for Life Sciences
Department of Chemistry, Peking University
202 Chengfu Road, Beijing 100871 (China)
Fax : (+86)10-6275-9105
E-mail: zyang@pku.edu.cn
[b] Dr. L. Chang, Prof. Dr. Z. Yang
Laboratory of Chemical Genomics
School of Chemical Biology and Biotechnology
Shenzhen Graduate School, Peking University
Shenzhen 518055 (China)
Previous investigations in our laboratories revealed that
the structurally diverse catechins B could be synthesized in
good yields from the corresponding aryl epoxides A by thio-
urea/AuCl3-catalyzed
endo-selective
cycloalkylation
Fax : (+86)755-2603-3174
E-mail: zyang@pku.edu.cn
(Scheme 2).[7]
As part of our ongoing program towards the synthesis of
biologically active natural products, we have provided the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200168.
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Jiahua Chen, Zhen Yang et al.
Although the direct assembly of target molecule 7 via
coupling epigallocatechin 4 with dehydroascorbic acid 8 rep-
resents an attractive strategy, several issues need to be ad-
dressed. The first is associated with coupling reagent and
coupling conditions, which must be selected with the goal of
complementarity as to chemo-, regio-, and stereodirectional-
ity in the annulation step. We would then probe the reaction
conditions of the coupling reaction, since both substrates
and product could be highly sensitive toward acid, base, and
oxidative agents; therefore, milder reaction conditions must
be secured. In addition, the maintenance of the stereochemi-
cal integrity during the Mitsunobu reaction for the synthesis
of epoxide 10 via the coupling of phenol 11 with benzylic al-
cohol 12 could be potentially problematic.
Scheme 2. Gold-catalyzed intramolecular cycloalkylation. OTf=CF3SO3.
first fully documented enantioselective total synthesis of 7,
with the key steps in the synthesis being the CuII/O2-mediat-
ed oxidative coupling of epigallocatechin 4 with ascorbic
acid and the aforementioned AuII-catalyzed cycloalkylation
of epoxide 10.
Scheme 3 illustrates our retrosynthetic analysis of 7. As
previously illustrated,[3,6,8] our target molecule 7 could be bi-
omimetically generated from epigallocatechin 4 and dehy-
Our work started with the synthesis of epigallocatechin 4.
Although molecules of this structural class are biologically
important,[13] their syntheses are challenging.[14] The asym-
metric synthesis of epigallocatechins was reported by Chan
et al.[9,15] and featured the acid-mediated annulation of 13
for the construction of catechin 14 as well as a Sharpless
asymmetric dihydroxylation[12] for the stereoselective synthe-
sis of the orthoester 13 (Scheme 4).
Scheme 4. Synthesis of the scaffold of epigallocatechin 4.
Scheme 3. Retrosynthetic analysis. Piv=pivaloyl, Bn=benzyl.
Another featuring synthetic route to the epi-series cate-
chins was developed by Suzuki et al. based on the sulfinyl–
metal exchange reaction between 15 and PhLi, followed by
intramolecular nucleophilic substitution to give the annulat-
ed product 16 as a key step, which after de-protection could
afford epigallocatechin 4 (Scheme 4).[16,17]
Our asymmetric total synthesis of target molecule 7 start-
ed with the preparation of the key intermediate 12
(Scheme 5). The protection of methyl 3,4,5-trihydroxyben-
zoate 17 with PivCl in the presence of pyridine gave ester 18
in 98% yield. Selective reduction of 18 with l-selectride
generated alcohol 19 in 93% yield. For the subsequent con-
version of alcohol 19 to silyl ether 22, alcohol 19 was initial-
ly oxidized with pyridinium dichromate (PDC), and the re-
sulting aldehyde 20 was subjected to a stereoselective Julia
olefination reaction[18] by reaction with sulfone 21[19] in the
presence of potassium hexamethyldisilazide (KHMDS), to
give the silyl ether 22 in 71% yield, together with its cis
isomer as a minor side-product in a 3:1 ratio. Following the
droascorbic acid 8 in a Friedel–Crafts reaction. Epigallocate-
chin 4 could be prepared from protected gallocatechin 9 in
an oxidation–reduction protocol.[9] The synthesis of galloca-
techin 9 would rely on an already envisaged strategy[10] for
constructing the C6-C3-C6 flavonoid skeleton in an asym-
metric fashion, that is a C6+C3-C6 strategy using a phloro-
glucinol-based unit as C6 block and a chiral epoxy alcohol
possessing the adequate stereochemical information as C3-
C6 block. Synthetically, gallocatechin 9 was expected to be
prepared via the Au-catalyzed cycloalkylation developed by
us from epoxide 10.[7]
Thus, our total synthesis could be traced back to the con-
struction of epoxide 10, with the expectation that the epox-
ide 10 could be generated in a elaborated Mitsunobu pro-
cess by connecting phenol 11 (C6 block) with epoxide 12
(C3-C6 block).[11] It was envisaged that its absolute stereo-
chemistry in epoxide 12 could be established by a Sharpless
asymmetric dihydroxylation reaction.[12]
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Scheme 5. Synthesis of 12. a) PivCl (3.5 equiv), pyridine (solvent), room
temperature, 10 h, 98%; b) l-selectride (2.5 equiv), THF, À788C, 4 h,
93%; c) PDC (1.5 equiv), CH2Cl2, room temperature, 5 h, 87%;
d) KHMDS, 21 (1.0 equiv), THF, À788C, 2 h, 75 % (E/Z=3:1); e) HF/
H2O, CH3CN, 71%; f) AD-mix a (1.4 g/1 mmol), tBuOH in H2O (1:1),
CH3SOONH2, 08C, 7 days, 90%; g) diisopropyl azodicarboxylate (DIAD,
1.0 equiv), PPh3 (1.0 equiv), THF, room temperature, 6 h, 80%.
removal of the silyl group, allylic alcohol 23 underwent
a standard Sharpless asymmetric dihydroxylation[12] with
AD-mix-a as an oxidant to give triol 24 as a single diaste-
reoisomer in 90% yield. For the construction of epoxide 12,
triol 24 was then subjected to a Mitsunobu reaction[20] proto-
col to afford benzylic alcohol 12 in 80% yield.
For the preparation the key intermediate 25 for the AuIII-
catalyzed cycloalkylation, we attempted to couple benzylic
alcohol 12 with phenol 11[21] under a variety of conventional
Mitsunobu reaction conditions (Scheme 6). Unfortunately,
however, these efforts failed to provide the desired product
25, likely because of the steric hindrance of the substrates.
Following a period of systematic experimentation, we dis-
covered that product 25 could be generated as a single dia-
stereoisomer in 50% yield when the reaction was conducted
at 208C under sonication.[22]
Scheme 6. Syntheses of (+)-gallocatechin (3) and (À)-epigallocatechin
(4). a) DIAD (1.0 equiv), PPh3 (1.0 equiv), THF, 208C, 20 min, sonica-
tion, 50%; b) AuCl3 (0.1 equiv), AgOTf (0.3 equiv), PPh3 (0.1 equiv), 1,2-
dichloroethane, 508C, 7 h, 81%; c) Pd/C, H2 (balloon pressure), 94%;
d) LiAlH4 (10 equiv), THF, À788C to room temperature, 2 h, 42%;
e) Dess–Martin periodinane (DMP) (1.2 equiv), CH2Cl2, room tempera-
ture, 2 h, 65%; f) l-selectride (2.0 equiv), THF, À788C, 2 h, 87%; (g) Pd/
C, H2 (balloon pressure), 90%; h) LiAlH4 (10 equiv), THF, À788C to
room temperature, 2 h, 51%.
With the key intermediate 25 in hand, we then set out to
investigate our previously developed thiourea/AuIII-cata-
lyzed annulation. Unfortunately, under various reaction con-
ditions,[7b] product 9 was obtained in low yield, although dif-
ferent types of thiourea ligands, such as thioureas I, II, and
III (Scheme 6).
To improve the yield, we resort to electronic tuning of the
Au catalyst through its ligation with other types of ligands,
such as lutidine,[23] PPh3,[24] PCy3, BINAP (2,2’-bis(diphenyl-
phosphino)-1,1’-binaphthalene), and bisoxazoline,[25] and
found that the best result could be obtained when the reac-
tion was conducted in the presence of an AuCl3/AgOTf/
PPh3 mixture, to give the protected gallocatechin 9 endo-se-
lectively[7a] in 81% yield.
ty of different bases, including H2NNH2, NH3/MeOH,
K2CO3, and NaOMe, only gave low yield of desired product,
indicating that the resulting product was unstable under
basic conditions.[26] We therefore adopted a reverse depro-
tection sequence, with the debenzylation being affected first
followed by LiAlH4-mediated depivoylation.[27] Pleasingly,
this new protocol allowed for the successful synthesis of
(+)-gallocatechin (3) in 42% yield via a debenzylation–de-
pivoylation sequence (Scheme 6).
For the synthesis of (À)-epigallocatechin 4, substrate 9
was first subjected to a two-step C3-inversion protocol,[9] in-
volving a DMP oxidation and subsequent diastereoselective
l-selectride reduction to give product 26. Thus, following
the deprotection protocol developed above, (À)-epigalloca-
techin 4 was made in 51% yield. The physical properties of
the synthetic 3 and 4 were compared with those reported for
To complete the total synthesis of the natural product
(+)-gallocatechin 3, we originally planned to initially the
pivaloyl groups with subsequent hydrogenative debenzyla-
tion. Unfortunately, however, the treatment of 9 with a varie-
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Jiahua Chen, Zhen Yang et al.
the natural products and found to be in good agreement,[28]
1
including the H and 13C NMR spectroscopic data and the
optical rotations. Based on this information, we can confirm
that the stereochemistry generated in compound 25 via the
Mitsunobu reaction of 11 with 12 is correct.
With (À)-epigallocatechin 4 in hand, we proceeded to in-
vestigate methods for the biomimetic synthesis of natural
product 7. We first attempted to perform the oxidative cou-
pling of phenol 4 and dehydroascorbic acid 8 using a stoi-
chiometric amount of CuACTHNUTRGNE(NUG OAc)2 as both an oxidant to con-
vert ascorbic acid to dehydroascorbic acid 8[29] and a Lewis
acid to promote the intermolecular Friedel–Crafts reaction
(Scheme 7).[8b,c,d,f]
Scheme 8. Completion of the total synthesis of 7. brsm=based on recov-
ered starting material.
chemo-, regio-, and stereoselectivity where observed in this
synthesis for the bond-forming reactions between (À)-epi-
gallocatechin (4) and dehydroascorbic acid (8).
In conclusion, we have successfully achieved the first total
synthesis of 8-C-ascorbyl-(À)-epigallocatechin, a structurally
complex and unique polyphenol of high biological relevance.
In the meantime, we report an alternative asymmetric syn-
thetic route to access epigallocatechin and gallocatechin,
two popular tea polyphenols of ever-increasing interest. In
this transformation, we not only identified an elaborated
Mitsunobu process connecting the C6 and C3-C6 blocks by
using sonication to accelerate this SN2 process, but we also
applied our previously developed Au-catalysis cycloalkyla-
tion process for the formation of the central benzopyran
ring of (epi)gallocatechins. The so-prepared (À)-epigalloca-
techin has been finally utilized to achieve the total synthesis
of 8-C-ascorbyl-(À)-epigallocatechin via an oxidative cou-
pling step with dehydroascorbic acid (generated in situ from
ascorbic acid) in a biomimetic and very elegant manner.
With regard to the high sensitivity of polyphenols towards
oxidation and difficulty of their purification, the developed
method for the biomimetic synthesis of ascorbic acid ap-
pended polyphenols might be applied for the syntheses of
these highly important and biologically active natural prod-
ucts.
Scheme 7. Proposed biomimetic synthesis of 7.
Unfortunately, however, the desired product 7 could not
be obtained, in spite of several different reaction conditions
being investigated, and only decomposition products were
observed. Further oxidation of ascorbic acid with CuO,
Ag2O, and activated charcoal/O2,[30] respectively, also provid-
ed the same results. In our final attempt, K2Fe(CN)6 was
employed as the oxidant, but unfortunately yielded none of
the desired product.
To circumvent the decomposition problem, we examined
the oxidation of ascorbic acid with a catalytic amount of Cu-
ACHTUNGTRENNUNG(OAc)2 in the presence of oxygen. CuACHTUNTGNER(NUGN OAc)2 has been re-
Acknowledgements
ported as an effective catalyst for the oxidative conversion
of ascorbic acid to dehydroascorbic acid 8 in the presence of
oxygen.[31] With this in mind, we decided to explore the cou-
This work was supported by the National Science Foundation of China
(Grant Nos. 20832003, 21072006, and 21072111).
pling reaction with a catalytic amount of Cu
ACHTUNGTRENNUNG
presence of oxygen, because the large excess of CuAHCTUNGTRENNUNG
Keywords: asymmetric synthesis
natural products · total synthesis
·
enantioselectivity
·
could potentially cause the decomposition of (À)-epigalloca-
techin 4 as well as the resulting product 7.[29b,32]
Pleasingly, when we carried out the reaction with a catalyt-
ic amount of CuACHTUNGTRENNUNG(OAc)2 in the presence of oxygen, the ex-
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Received: December 10, 2012
Published online: January 29, 2013
Chem. Asian J. 2013, 8, 700 – 704
704
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