Total Synthesis of Proanthocyanidin A1, A2, and Their Stereoisomers

Article abstract of DOI:10.1021/acs.orglett.5b00646

The first novel stereoselective synthesis of naturally occurring A1 (1) and A2 proanthocyanidins (2) has been achieved. The key synthetic steps involved (a) the formation of a coupled product (13 or 14) between an open chain C-ring C-4 hydroxyethoxy analo

Full text of DOI:10.1021/acs.orglett.5b00646

Letter
pubs.acs.org/OrgLett
Total Synthesis of Proanthocyanidin A1, A2, and Their Stereoisomers
Pradeep K. Sharma,*,†,§ Leo J. Romanczyk, Jr.,‡ Leelakrishna Kondaveti,† Bollu Reddy,†
Jeeva Arumugasamy,† Richard Lombardy,† Yanni Gou,† and Hagen Schroeter‡
†Chemical Process Research & Development, Johnson Matthey Pharmaceutical Materials & Pharma Services, Devens, Massachusetts
01434, United States
‡MARS Incorporated, 6885 Elm Street, McLean, Virginia 22101, United States
S
* Supporting Information
ABSTRACT: The first novel stereoselective synthesis of naturally occurring A1 (1) and A2 proanthocyanidins (2) has been
achieved. The key synthetic steps involved (a) the formation of a coupled product (13 or 14) between an open chain C-ring C-4
hydroxyethoxy analogue of either (+)-catechin or (−)-epicatechin with 5,7,3′,3′-tetra-O-benzyl-(+)-catechin/-(−)-epicatechin in
the presence of bentonite clay K-10, (b) removal of benzyl protecting groups under mild catalytic hydrogenation conditions to
form the desired A-type compound in situ as a mixture of diastereomers via ketal/oxonium ion/carbonium ion formation, and (c)
separation of the diasteromers via silica gel column chromatography. The structures of A1 and A2 proanthocyanidins were
unequivocally established by analytical comparison to the natural products. Following this methodology, an additional six
diastereomers of proanthocyanidins A1 and A2 have been synthesized. A plausible mechanism for the formation of the A-type
linkage in proanthocyanidins has been proposed.
roanthocyanidins are naturally occurring polyphenolic
compounds that have diverse biological activities.1 As
shown in Figure 1, the most commonly known A-type
and 2, others are expected as the result of stereoisomer
combinations of (+)-catechin and (−)-epicatechin. Numerous
attempts have been made to synthesize these compounds. This
includes the transformation of natural B-type procyanidins to
A-types in an alkaline/H2O2 system and in neutral conditions
with DPPH (2,2-diphenyl-1-picrylhydrazyl) radicals,6 flavylium
perchloride with (+)-catechin in methanol,7 and by microwave
chemistry.8 Despite these claims, reactions either produce low
yields or analogues possessing pyrogallol- type B-rings
(prorobinetinidins and prodelphinidins), which are classes of
compounds structurally dissimilar to the procyanidins.9 The
conversion of procyanidin B-2 to A-2 by the enzyme laccase has
also been reported.10 This result suggests that, in plants, the
transformation of B-type procyanidins to A-types might involve
an enzyme catalyzed oxidation reaction rather than a radical
driven process as suggested by Kondo et al.6 It has also been
reported that A-types can be converted from the oxidation of
their accompanying B-type analogues.11 There is an additional
report12 claiming the synthesis of A-types, but the resultant
compounds were structurally dissimilar to procyanidin based A-
types. Thus, the total synthesis of substantive amounts of A-
type proanthocyanidins has yet to be achieved. Additionally, the
formation and stability of A-type linkages (linkage between C-2
P
Figure 1. Structures of proanthocyanidins A1 and A2.
proanthocyanidins are where the two units, catechin and/or
epicatechin, are connected through the linkages occurring
between both C-2 and C-4 of the upper unit and the oxygen at
C-7 and positions 6 or 8, respectively, of the lower unit.
The first identified A-type proanthocyanidin was A2, which
was isolated from the shells of the fruits of Aesculus
hippocastanum.2 Since the isolation of this compound in
1966, many more A-type proanthocyanidins have been found
in plants, including dimers, trimers, tetramers, pentamers, and
others.3−5 In addition to the naturally occurring compounds 1
Received: March 4, 2015
© XXXX American Chemical Society
A
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and C-7 via an oxygen atom) has not been thoroughly
investigated. Herein, we report a unique and facile method for
the synthesis of A-type proanthocyanidins. To understand the
challenge that resides in the formation and stability of the A-
type linkage, our initial efforts focused on the synthesis of
model compound 9 (Scheme 1), which lacked the 2-(3,4-
compound 8 in 50% yield,15 and the structure was established
by 1H, 13C NMR, and C−H correlation. No attempt was made
to separate the mixture of diastereomers for 8. However, the
diastereomeric ratio (∼7/3) was determined by NMR and
HPLC. Debenzylation of 8 under catalytic hydrogenation
conditions provided 9. A proposed mechanism for the
transformation of 7 to 9 is presented in Figure 2.
Scheme 1. Synthesis of 9
Figure 2. Proposed mechanism for the formation of 9.
Upon treatment of 7 with P2O5 in toluene, the unstable
intermediate hemiketal 10 would form. The hemiketal 10
would further lose a mole of water to form the oxonium ion 11,
which would react with phenoxide in situ to form the stable
ketal 8. We envisaged that the removal of the benzyl groups of
7 might generate the intermediate(s) that would trigger a
cascade to produce the A-type linkage. Hence, 7 was subjected
to catalytic hydrogenation conditions (Pd(OH)2/C, EtOAc)
using a H2 balloon at rt for 0.5 h producing 9 in quantitative
yield as a mixture of diastereomers. This implied that use of
P2O5 was not required for the conversion of 7 to 9, which
completely eliminated the preparation of intermediate 8. These
results contrast those of Jacques et al.16 who reported that the
A-type linkage could be cleaved when the molecule was
subjected to catalytic hydrogenation conditions at 40−50 psi at
room temperature in the presence of 10% Pd/C. The formation
of 9 in the absence of P2O5 would still involve a potentially
similar mechanism. Thus, the reaction sequence for the
formation of 9 indicated that the A-type linkage was stable
under neutral, mildly acidic, and basic conditions along with the
neutral hydrogenation at low H2 pressure.
Having established the conditions for the formation of the
imperative A-type linkage, the next goal was to use this method
for the synthesis of 1 and 2. The reaction of C-4 β-diastereomer
5 with 5,7,3′,4′-tetra-O-benzyl-(+)-catechin [Bn4-(+)-C] 12 in
the presence of Bentonite clay K-10 in CH2Cl2 at 0 °C
provided 13 and 14 after chromatography in 32% and 24%
yields, respectively. Thus, the formation of a mixture of 13 and
14 under these conditions was not selective. The stereo-
chemistry of 13 and 14 at C-4 was established from NMR and
literature precedence (Scheme 2).17,18
dihydroxyphenyl)-3-hydroxy-dihydropyran fragment from the
lower unit of 1 and 2. We envisaged that the construction of
model compound 9 would provide a better understanding
about the impact of the hydroxyl groups and A-linkage under
the potential reaction conditions for the ultimate target
compounds 1 and 2.
As depicted in Scheme 1, the selective oxidation of 313 with
DDQ in CH2Cl2 at rt produced 4, which was reacted with 2-
ethoxyethanol in the presence of DDQ in CH2Cl2 at rt to yield
5 as a diastereomeric mixture (α/β, 7/3).14 The diasteromeric
mixture was separated by silica gel column chromatography,
Treatment of 13 with nBu4NF in AcOH and THF resulted in
the removal of the TBS group, producing 15 in 86% yield after
chromatography. Once 15 was available, the stage was set for
catalytic hydrogenation where the removal of the benzyl groups
and subsequent cyclization would occur for the formation of 1.
Thus, 15 was subjected to catalytic hydrogenation at room
temperature using a H2 balloon for 1 h. The crude product was
further purified by silica gel chromatography using EtOAc/
CH2Cl2/CH3OH/AcOH (150/150/2/0.3, v/v/v/v) to pro-
duce 1 as an off-white solid (Scheme 3).
1
and structures were established by H, 13C NMR, and C−H
correlation. However, to fulfill the objective, the mixture of
diastereomers of compound 5 was used directly without
separation. The reaction of 5 with 3,5-bis(benzyloxy)phenol in
the presence of Bentonite K-10 in CH2Cl2 at 0 °C yielded 6 in
70% yield. The stereochemistry at C-4 linkage of 6 was not
determined, but expected to be a mixture of diastereomers.13
Treatment of 6 with nBu4NF/AcOH in THF generated 7,14
which was envisioned to be a key intermediate for the synthesis
of the A-type linkage. The reaction of 7 with P2O5 in refluxing
toluene resulted in the formation of the desired A-type
The structure of 1 was confirmed by detailed NMR analysis
and comparison to values reported in the literature.18
B
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isolated in 52% and 7% yield, respectively, after chromatog-
raphy.
Scheme 2. Synthesis of 13 and 14
The reaction of 16 with a mixture of nBu4NF/AcOH in
CH2Cl2 at 0−5 °C afforded 18 in 85% yield. It was also
observed that replacement of nBu4NF/AcOH with HF/
pyridine complex in THF also produced 18 in 82% yield.
However, attempts to deprotect the TBS group with 1 N
hydrochloric acid in THF at rt were unsuccessful due to slow
reaction and the formation of additional byproducts, which
were difficult to separate. Catalytic hydrogenation of 18 with
Pd(OH)2/C in EtOAc at rt produced the desired proantho-
cyanidin 2 in 93% yield after chromatography. Again, the
structure of 2 was confirmed by comparison with an authentic
sample19 and NMR.20
After accomplishing the synthesis of the target compounds 1
and 2, the methodology was further extended to the synthesis
of other potential stereoisomers of proanthocyanidins, namely
19−24 (Figure 3).18
In conclusion, the first total syntheses of naturally occurring
A-type proanthocyanidins 1 and 2 have been accomplished.
The key step involved the formation of the interflavan linkage
between the two flavan-3-ols at C-4 of the upper unit and C-8
Scheme 3. Synthesis of 1
The synthesis of 2 was achieved as depicted in Scheme 4.
The coupling reaction between C-4 β-diastereomer 5 with 5, 7,
3′, 4′-tetra-O-benzyl-(−)-epicatechin [Bn4-(−)-EC]20 in the
presence of bentonite clay K-10 in CH2Cl2 produced a mixture
of 16 and 17 in 73−80% yield. Compounds 16 and 17 were
Scheme 4. Synthesis of 2
Figure 3. Structures of proanthocyanidins 19 to 24.
C
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(7) Pomilio, A. Liebigs Ann. Chem. 1977, 597.
of the lower unit. The use of benzyl groups as a preferred
protecting group provided access for easy removal under
neutral catalytic conditions and created the foundation for the
formation of the A-type linkage. The reaction conditions that
were used in the synthesis suggested that A-type linkage was
stable under mildly acidic and neutral conditions as well as at rt
under normal handling conditions. The methodology devel-
oped was further extended to the stereoselective synthesis of
additional isomers of proanthocyanidins, 19−24, which to the
best of our knowledge have not yet been either isolated or
synthesized. This concise and flexible synthetic methodology
presented here offers opportunities to create analogues with A-
type linkage and focused libraries to evaluate their biological
activity against various therapeutic targets.
(8) Selenski, C.; Pettus, T. R. R. R. Tetrahedron 2006, 5298.
(9) http://pharmacy.olemiss.edu/biomolecularsciences/team/dr-
daneel-ferreira/.
(10) Osman, A. M.; Wong, K. K. Y. Tetrahedron Lett. 2007, 48, 1163.
(11) Marais, J. P. J.; Ferreira, D. 5th Tannins Conference at the 232nd
ACS National Meeting, San Francisco, CA, September 2006, CELL
104.
(12) Kraus, G. A.; Yuan, Y.; Kempema, A. Molecules 2009, 14, 807.
(13) (a) Sharma, P. K.; He, M.; Jurayj, J.; Gao, D.-M.; Lombardy, R.;
Romanczyk, L. J., Jr.; Schroeter, H. Molecules 2010, 15, 5595. (b) Mull,
E. S.; van Zandt, M.; Golebiowski, A.; Beckett, R. P.; Sharma, P. K.;
Schroeter, H. Tetrahedron Lett. 2012, 53, 1501.
(14) Sharma, P. K.; Kolchinski, A.; Shea, H. A.; Nair, J. J.; Gou, Y.;
Romanczyk, L. J., Jr.; Schmitz, H. H. Org. Process Res. Dev. 2007, 11,
422 and references cited therein.
(15) (a) Sharma, P. K.; He, M.; Romanczyk, L. J., Jr.; Schroeter, H. J.
Labelled Compd. Radiopharm. 2010, 53, 605. (b) von Klaus, W.; Hans,
T. Liebigs Ann. Chem. 1971, 743, 203.
(16) Jacques, D.; Haslam, E.; Bedford, D.; Greatbanks, P. J. J. Chem.
Soc., Perkin Trans. 1 1974, 2663.
(17) von Klaus, W.; Theobald, H. Liebigs Ann. Chem. 1971, 743, 203.
(18) Supporting Information.
(19) An authentic sample of 2 was provided by Masterfoods, a
ASSOCIATED CONTENT
* Supporting Information
Detailed experimental procedures, spectra, and characterization
data for all new compounds. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.orglett.5b00646.
■
S
Division of MARS, Incorporated, for comparison.
AUTHOR INFORMATION
Corresponding Author
(20) Tuckmantel, W.; Kozikowski, A. P.; Romanczyk, L. J., Jr. J. Am.
̈
■
Chem. Soc. 1999, 121, 12073.
*E-mail: sharmap76@yahoo.com; pradeep.sharma@ariad.com.
Present Address
§ARIAD Pharmaceuticals, Inc., 26 Landsdowne Street, Cam-
bridge, MA 02139.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to thank Drs. Heather Taft and Jurjus
Jurayj of Johnson Matthey Pharma Services for their constant
feedback and encouragement and the AD group at JMPS for
analyzing the samples in a timely manner.
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