Synthesis of the naturally occurring (-)-1,3,5-tri-O-caffeoylquinic acid

Article abstract of DOI:10.1055/s-0029-1217183

Ribonuclease H (RNase H) is an essential component to the replication of human immunodeficiency virus (HIV), and only a few inhibitors of this enzyme are known. Millenia Hope Pharmaceuticals Inc. found that (-)-1,3,5-tri-O- caffeoylquinic acid is a potent

Full text of DOI:10.1055/s-0029-1217183

LETTER
1517
Synthesis of the Naturally Occurring (–)-1,3,5-Tri-O-Caffeoylquinic Acid
Synthesisof the
N
a
aturallyOc
curring
(
–)-1,3,5-T
M
O
-Caffeoylquinic
A
.
id Brummond,* Jolie E. DeForrest
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA
Fax +1(412)6248611; E-mail: kbrummon@pitt.edu
Received 11 February 2009
by Okuda and co-workers who isolated compound 1 from
the fruit of Xanthium strumarium L., a fruit that has been
used for medicinal purposes in both China and Japan.5
The intriguing biological activity of 1,3,5-tri-O-caf-
Abstract: Ribonuclease H (RNase H) is an essential component to
the replication of human immunodeficiency virus (HIV), and only
a few inhibitors of this enzyme are known. Millenia Hope Pharma-
ceuticals Inc. found that (–)-1,3,5-tri-O-caffeoylquinic acid is a po-
tent RNase H inhibitor and antiviral agent. A facile route leading to feoylquinic acid, and access to suitable quantities of com-
this inhibitor from commercially available (–)-quinic acid is report-
ed within.
pound for further biological evaluations and mechanistic
studies, motivated the synthesis of this compound. Fur-
Key words: reverse transcriptase, ribonuclease H, (–)-1,3,5-tri-O-
caffeoylquinic acid, (–)-quinic acid
thermore, an understanding of the structure–activity rela-
tionship (SAR) required for RNase H inhibition and
antiviral activity can be gained by analog synthesis.
An obvious synthetic strategy is to assemble 1,3,5-tri-O-
caffeoylquinic acid (1) from (–)-quinic acid (2) and caffe-
ic acid (3) via three selective esterification reactions, an
approach that nature likely uses. However, the lability of
the catechol moiety of caffeic acid, the unreactive tertiary
alcohol of quinic acid, and the lack of site selectivity in the
esterification reactions prohibited this direct approach.
Alternatively, through a series of timed protection and
deprotection steps it was envisioned that compound 1 can
be obtained from commercially available 3,4-(methylene-
An estimated 33.2 million people worldwide are living
with human immunodeficiency virus (HIV). In 2007, 2.5
million people were newly infected with HIV and 2.1 mil-
lion deaths were attributed to acquired immune deficiency
syndrome (AIDS). These epidemic numbers, coupled
with studies of the most promising HIV vaccine candidate
being halted in 2007, have led to an increased need for
therapies that attack HIV via novel mechanisms of ac-
tion.1
Replication of the HIV genome is carried out by HIV-1 re- dioxy)cinnamic acid (3b) and quinic acid protected as the
verse transcriptase (RT), an enzyme displaying both DNA methyl ester and isopropylidene acetal 4 (Scheme 2).
polymerase and ribonuclease H (RNase H) activities.
Our synthesis of 1 begins with the formation of diol 4
While RNase H is essential to viral function, only a few
from (–)-quinic acid (2) in 50% yield for two steps using
inhibitors have emerged and none are in clinical trials.2,3
a previously developed protocol.6,7 Diol 4 is then trans-
Recently, Millenia Hope Pharmaceuticals Inc. isolated formed into the monocaffeoylated compound with 3,4-
1,3,5-tri-O-caffeoylquinic acid (1, Scheme 1) from cul- (methylenedioxy)cinnamic acid (3b), N,N¢-dicyclohexyl-
tured plant cells. They subsequently found 1 to be a potent carbodiimide (DCC), and 4-dimethylaminopyridine
RNase H inhibitor and antiviral agent (IC50 = 0.4 mM) us- (DMAP) at room temperature in 87% yield (Scheme 2).
ing a cellular assay consisting of human peripheral blood Next, the tertiary alcohol of 5a was reacted with acid chlo-
mononuclear cells (PBMC) infected with HIV-1.4 The ride 3a in refluxing 1,2-dichloroethane (DCE) at 80 °C,
naturally occurring polyphenol 1 was previously reported and after 48 hours gave a 66% yield of the biscaffeoylated
HO
O
HO
O
OH
O
OH
OH
HO
HO
HOOC
O
OH
OH
OH
+
HOOC
HO
O
O
OH
OH
OH
O
(–)-1,3,5-tri-O-caffeoylquinic acid (1)
(–)-quinic acid (2)
caffeic acid (3)
Scheme 1 1,3,5-Tri-O-caffeoylquinic acid (1) from (–)-quinic and caffeic acids (2) and (3)
SYNLETT 2009, No. 9, pp 1517–1519
x
x.
x
x.
2
0
0
9
Advanced online publication: 18.05.2009
DOI: 10.1055/s-0029-1217183; Art ID: S01809ST
© Georg Thieme Verlag Stuttgart · New York

1518
K. M. Brummond, J. E. DeForrest
LETTER
OR1
OH
OR1
OH
OH
OH
O
O
O
O
3b, DCC
DMAP, CH2Cl2
O
O
3a, pyridine
DMAP, DCE, 80 °C
ref. 6, 7
HO2C
87%
MeO2C
MeO2C
66%
MeO2C
OH
OR1
5b
OH
OH
2
4
5a
OR1
OR1
OR1
OR1
OR1
OR1
OR1
OH
4
OH
OH
TFA–H2O (1:1)
CH2Cl2, 0 °C
3a, pyridine, DMAP
60 °C 56%
3
MeO2C
OR1
MeO2C
OH
MeO2C
MeO2C
OR1
90%
OR1
OR1
OR1
(see text for ratios)
6
8
9
7
Me3SnOH, DCE
60%
O
O
OR1
OR2
O
O
R1 =
O
O
BBr3, CH2Cl2
LiI, pyridine
100 °C
X
OH
OR1
OH
–78 °C
O
9
46%
88%
OH
R2 =
HO2C
HO2C
OR2
OR1
10
OR2
3a X = Cl
3b X = OH
OH
1
Scheme 2 Synthesis of 1,3,5-tri-O-caffeoylquinic acid (1) from (–)-quinic acid (2)
compound 5b (79% based upon recovered starting mate- 1 based upon the respective coupling constants for the C4
rial). Attempts to effect the esterification of the 5a at protons (Hb in ester 8: dd, J = 10.0, 3.5 Hz; Hb in ester 9:
40 °C gave the biscaffeoylated compound 5b in 11% yield ddd, J = 10.0, 7.0, 3.3 Hz, Table 1). The driving force for
and extending the reaction time to one week at 80 °C re- this tin-mediated transesterification reaction is not clear;
sulted in complete consumption of starting material, but however, intramolecular ester migration has been shown
gave only a 7% yield of 5b. Unfortunately, attempts to to be a facile process for compounds containing adjacent
convert diol 4 directly into compound 5b were unsuccess- hydroxyl and ester functionalities.9a–9d
ful due to the forcing conditions required for esterification
of the tertiary alcohol (DCC, DMAP, CH2Cl2, 25–40 °C).
With the desired tricaffeoyl ester 9 in hand, removal of the
catechol protecting groups and saponification of the me-
Removal of the acetonide protecting group of 5 is accom- thyl ester was accomplished by first refluxing 9 in pyri-
plished in 90% yield with TFA–H2O (1:1) at 0 °C to give dine with excess LiI to give the analogous acid 10 in 46%
diol 6 (Scheme 2). Diol 6 is then reacted with acid chlo- yield, which was purified using silica gel chromatogra-
ride 3a, DMAP, and pyridine at 60 °C to give a mixture of phy.10a–c The pure carboxylic acid was then reacted with
esters 7, 8, and 9 in 9%, 27%, and 20% yield, respectively. excess BBr3 at –78 °C to give (–)-1,3,5-tri-O-caffeoylqui-
After some experimentation it was found that the com- nic acid (1) in 88% yield.11a A workup procedure utilizing
pounds 7, 8, and 9 could be separated using a Biotage saturated aqueous sodium phosphate was essential, be-
Horizon Flash Collector eluting with an ethyl acetate–tol- cause all other methods gave ester cleavage products.11b
1
uene solvent system (2–15%). This esterification reaction This compound was 95% pure by H NMR. Attempts to
can also be performed using DCC-mediated coupling con- remove the methylene acetal protecting groups first and
ditions; however, desired triester 9 was difficult to sepa- then to saponify the methyl ester to the acid gave very low
rate from the urea byproduct produced during the yields of the natural product, which is attributed to the in-
reaction.
stability of the liberated catechol moieties.
Interestingly, it was found that reacting the undesired In summary, we have developed a facile route leading to
1,4,5-tricaffeoylated isomer 8 with Me3SnOH in refluxing 1,3,5-tri-O-caffeoylquinic acid (1) from commercially
1,2-dichloroethane (DCE) resulted in a transposition of available quinic acid (2). The convergent synthetic strate-
the ester from C4 to C3, giving the desired 1,3,5-tricaf- gy developed is readily amenable to the syntheses of a va-
feoylated isomer 9 in 60% yield (Scheme 2).8 This trans- riety of biologically interesting analogs which may offer
1
position is evidenced in the H NMR by the change in mechanistic insight into the unique activity exhibited by
chemical shift of the proton on C4 from d = 5.21 ppm to (–)-1,3,5-tri-O-caffeoylquinic acid. The biological activi-
d = 3.93 ppm (Table 1). Importantly, the preferred confor- ties of the synthetic caffeoylquinic acid (1) has been eval-
mation for the esters 8 and 9 are the ones depicted in Table uated and is in agreement with that reported by Millenia
Synlett 2009, No. 9, 1517–1519 © Thieme Stuttgart · New York

LETTER
Synthesis of the Naturally Occurring (–)-1,3,5-Tri-O-Caffeoylquinic Acid
1519
rated from the organic layer and is extracted with EtOAc (4×). The
combined organic layers are washed with H2O (3×), brine (1×), and
are dried on MgSO4. The solids are filtered off using gravity filtra-
tion, and the filtrate is concentrated under reduced pressure by rota-
ry evaporation to give 5 mg of natural product 1 in 88% crude yield
as a foamy brown solid; [a]D23 –19.0 (c 0.10, MeOH). 1H NMR (600
MHz, MeOD): d = 7.64 (d, J = 16.2 Hz, 1 H), 7.56 (d, J = 15.6 Hz,
1 H), 7.52 (d, J = 16.2 Hz, 1 H), 7.08 (d, J = 1.8 Hz, 1 H), 6.98 (dd,
J = 8.4, 1.8 Hz, 1 H), 6.97 (d, J = 2.4 Hz, 1 H), 6.85 (d, J = 1.8 Hz,
1 H), 6.79 (d, J = 8.4 Hz, 1 H), 6.79 (dd, J = 8.4, 1.8 Hz, 1 H), 6.65
(d, J = 8.4 Hz, 1 H), 6.65 (dd, J = 8.4, 2.4 Hz, 1 H), 6.55 (d, J = 8.4
Hz, 1 H), 6.34 (d, J = 15.6 Hz, 1 H), 6.24 (d, J = 16.2 Hz, 1 H), 6.19
(d, J = 15.8 Hz, 1 H), 5.58 (td, J = 10.2, 4.2 Hz, 1 H), 5.46 (q,
J = 3.3 Hz, 1 H), 3.99 (dd, J = 10.2, 3.3 Hz, 1 H), 2.91 (dt, J = 15.9,
3.3 Hz, 1 H), 2.67 (ddd, J = 13.2, 4.2, 3.3 Hz, 1 H), 2.42 (dd,
J = 15.9, 3.3 Hz, 1 H), 2.01 (dd, J = 13.2, 10.2 Hz, 1 H). 13C NMR
(225 MHz, MeOD): d = 174.1, 168.8 (2 C), 167.8, 149.7 (2 C),
149.4, 148.0, 147.4 (2 C), 146.9 (2 C), 146.5, 127.8, 127.5 (2 C),
123.1 (2 C), 122.2, 116.6 (3 C), 116.1, 115.5, 115.3 (2 C), 115.1,
115.0, 80.7, 72.9, 72.2, 71.2, 38.3, 32.9. IR (thin film): 3279, 2923,
1733, 1699, 1603, 1260 cm–1. MS m/z (%) = 701 (31), 633 (41), 527
(42), 365 (100). ESI+-HRMS: m/z calcd for C34H30O15Na [M +
Na]+: 701.1482; found: 701.1526.
Hope Pharmaceuticals Inc. In addition, the methyl ester of
1 showed comparable Rnase H inhibition. These results
will be published elsewhere.
Table 1 Conformation and J Values of Tricaffeoyl Esters 8 and 9
Hf
Hg
Hb
OR1
Hb
OR1
Hg
Hf
O
O
Hd
Hd
OR1
OH
Hc
MeO
Hc
MeO
Ha
Ha
He
R1O
He
R1O
OR1
9
OH
8
Chemical shift Spin multiplicity J Values (Hz)
(d, ppm)
8
8
8
8
8
8
8
Ha
Hb
Hc
Hd
He
Hf
Hg
5.76
ddd
dd
m
10.0, 10.0, 4.0
5.21
10.0, 3.5
4.51–4.48
2.81–2.73
2.81–2.73
2.44
–
m
–
m
–
dd
dd
15.5, 3.5
13.5, 10.0
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
2.12
9
9
9
9
9
9
9
Ha
Hb
Hc
Hd
He
Hf
Hg
5.64
3.93
5.54
3.02
2.74
2.36
2.01
ddd
ddd
q
11.0, 10.0, 4.0
10.0, 7.0, 3.3
3.3
Acknowledgment
We gratefully acknowledge Dr. Michael Parniak and National Insti-
tutes of Health (NIH) grant U19073975 for support of this work and
Dr. Rieko Ishima for help with NMR experiments.
dt
16.0, 3.3
ddd
dd
13.5, 4.0, 3.3
16.0, 3.3
References
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1,3,5-Tri-O-caffeoylquinic Acid (1)
A 5 mL, single-necked, round-bottomed flask equipped with a stir
bar, rubber septum, and nitrogen line is charged with methylene ac-
etal 10 (6 mg, 8 mmol) and CH2Cl2 (0.1 mL). The reaction flask is
cooled to –78 °C in a dry ice-acetone bath and BBr3 (0.12 mL, 1.0
M in CH2Cl2, 0.12 mmol) is added dropwise via syringe. Upon ad-
dition of BBr3, the reaction turns orange from colorless. After 2 h of
stirring, the reaction mixture is transferred to a separatory funnel
containing EtOAc and sat. aq Na2HPO4. The aqueous layer is sepa-
(11) (a) McOmie, J. F. W.; Watts, M. L.; West, D. E. Tetrahedron
1968, 24, 2289. (b) Eicher, T.; Ott, M.; Speicher, A.
Synthesis 1996, 755.
Synlett 2009, No. 9, 1517–1519 © Thieme Stuttgart · New York