A convenient synthesis of the Echinacea-derived immunostimulator and HIV-1 integrase inhibitor (-)-(2R,3R)-chicoric acid

Article abstract of DOI:10.1002/1522-2675(200208)85:8<2328::AID-HLCA2328>3.0.CO;2-X

The Echinacea-derived immunostimulator and HIV-1 integrase inhibitor (-)-chicoric acid (=2,3-bis{[3- (3,4-dihydroxyphenyl)-1-oxoprop-2-enyl]oxy}butanedioic acid; 1a) was conveniently prepared via a silane-promoted Pd-mediated chemoselective hydrogenolysis of its perbenzylated derivative 12a, which was generated from an efficient and reliable carbodiimide-mediated coupling reaction between the caffeic acid dibenzyl ether derivative 7 and commercially available (+)-dibenzyl L-tartrate (9a). The other naturally occurring dextrorotatory chicoric acid (1b) can be similarly prepared.

Full text of DOI:10.1002/1522-2675(200208)85:8<2328::AID-HLCA2328>3.0.CO;2-X

2328
Helvetica Chimica Acta Vol. 85(2002)
A Convenient Synthesis of the Echinacea-Derived Immunostimulator and
HIV-1 Integrase Inhibitor (À)-(2R,3R)-Chicoric Acid
by Anne-Marie Lamidey, Lionel Fernon, Laurent Pouyse¬gu, Charlotte Delattre, Ste¬phane Quideau*
¬
¬
¬
Laboratoire de Chimie des Substances Vegetales, Centre de Recherche en Chimie Moleculaire,
¬
¬
Universite Bordeaux 1, 351 cours de la Liberation, F-33405Talence Cedex
(e-mail: s.quideau@lcsv.u-bordeaux1.fr; fax: (33)556846439)
and
Patrick Pardon
¬
¬
Institut du Pin, Universite Bordeaux 1, 351 cours de la Liberation, F-33405Talence Cedex
The Echinacea-derived immunostimulator and HIV-1 integrase inhibitor (À)-chicoric acid (ˆ2,3-bis{[3-
(3,4-dihydroxyphenyl)-1-oxoprop-2-enyl]oxy}butanedioic acid; 1a) was conveniently prepared via a silane-
promoted Pd-mediated chemoselective hydrogenolysis of its perbenzylated derivative 12a, which was generated
from an efficient and reliable carbodiimide-mediated coupling reaction between the caffeic acid dibenzyl ether
derivative 7 and commercially available (‡)-dibenzyl l-tartrate (9a). The other naturally occurring dextro-
rotatory chicoric acid (1b) can be similarly prepared.
Introduction. Echinacea purpurea (L.) Moensch is one of the most potent herbs
that support the immune system. Extracts from this purple cone-flower plant were
the best selling herbal products in natural food stores in the USA in 1997 [1]. These
extracts exhibit skin cicatrizing properties, appear to lessen the severity of the flu and
respiratory diseases such as rhinitis and pneumonia, are active against psoriasis,
eczema, and candidosis, and display anti-inflammatory, antifungal, antibacterial,
antiparasitical, antitumor, and antiviral activities [2]. Reports on their immunostimu-
lating properties have been particularly important in making Echinacea species a major
source of the immunotropic preparations on the medicinal plant market [2c e][3][4].
It has, for example, been observed that granulocyte and macrophage secretions of
certain cytokines such as interleukine-1, IFN-a, and TNF-a are increased upon
treatment with Echinacea sp. extracts [2d].
Such therapeutically-relevant activities have engendered numerous investigations
of the chemical composition of Echinacea species [1][2e][2f][4a][5]. Immunostimu-
latory and antiviral properties are generally attributed to phenylpropanoid metabolites
and, in particular, to caffeoyl derivatives, which have been identified as units bearing
pharmacophores common to potent in vitro inhibitors of HIV-1 integrase (vide infra)
[6]. Chicoric acid (1) belongs to this class of phenylpropanoid metabolites. In view of
recent literature reports in which (À)-chicoric acid (1a, (2R,3R)-O-dicaffeoyltartaric
acid) is considered unnatural [7], it is important to recall that the two optically active
stereoisomers are naturally occurring. In contrast to the dextrorotatory compound 1b
found in Cichorium species [8], the chicoric acid present in Echinacea species is its

Helvetica Chimica Acta Vol. 85(2002)
2329
OH
OH
HO
HO
O
O
O
O
CO2H
CO2H
O
O
CO2H
CO2H
O
HO
HO
HO
HO
O
(–)-Chicoric acid (1a)
(+)-Chicoric acid (1b)
Fig. 1. l-Chicoric acid (1a) from Echinacea purpurea and d-chicoric acid (1b) from Cichorium intybus and
endivia
levorotatory antipode 1a (Fig. 1) [5a]. It notably occurs as a major constituent of
Echinacea purpurea roots and aerial parts [5a][5c], and it has been identified as a key
element in the in vitro stimulation of human polymorphonuclear granulocytes
phagocytosis [3b][3c]. (À)-Chicoric acid (1a) is also one of the most potent
Echinacea-derived caffeoyl-based inhibitors of hyaluronidase activity with a 50%
inhibitory concentration value of 0.42 mm [9]. This dicaffeoyl-tartaric acid is also
considered a promising lead to new anti-HIV agents. It blocks HIV-1 replication in
syncitia cell-based assays at a 50% effective concentration value of 2 mg/ml with a toxic
concentration more than 100-fold greater [10a]. This cytoprotective effect of 1a is quite
remarkable in view of other catechol derivatives that exhibit appalling cytotoxicity
levels commonly blamed on facile oxidation to reactive quinone species [6b][10b].
(À)-Chicoric acid (1a) selectively inhibits HIV integrase at concentrations ranging
from 0.06 to 0.15 mg/ml [10], and displays a significant in vitro synergism as an integrase
inhibitor candidate for triple combination therapy against HIV infection [10f].
Our investigations into the utilization of natural phenols and catechols as
immunomodulatory agents in biological systems obviously led us to consider 1a as a
candidate. (À)-Chicoric acid (1a) is commercially available through extraction from its
natural sources, but it remains relatively expensive. We, thus, preferred to rely on an in-
house synthesis of the compound. To our knowledge, only three syntheses of chicoric
acids have been reported [7][8][10a]. The three procedures are based on acylation of
various tartrates with differently protected caffeoyl chlorides and suffer from either a
lack of reproducibility or the use of rather expensive starting tartrate derivatives. Here,
we wish to report on a rapid alternative method for the synthesis of 1a, as well as of its
dextrorotatory enantiomer (‡)-1b.
Results and Discussion. The cause for low yields and lack of reproducibility of the
first two syntheses of 1a/1b is likely the utilization of the highly unstable caffeoyl
chloride carbonate derivative 2 (Fig. 2). This derivative is used in standard nucleophilic

2330
Helvetica Chimica Acta Vol. 85(2002)
Cl
Cl
HO
HO
CO2Bn
CO2Bn
AcO
O
O
O
O
O
3
2
AcO
9a
O
OH
HO
HO
HO
CO2t-Bu
BnO
O
O
O
HO
CO2t-Bu
7
BnO
11a
10a
Fig. 2. Intermediates in the synthesis of chicoric acids
addition-elimination reactions with either (‡)-(2R,3R)-dibenzyl l-tartrate (9a) or its
d-enantiomer (À)-9b, followed by sequential base- and acid-deprotecting hydrolyses to
furnish 1a or 1b, respectively [10a]. In the synthesis of Scarpati and Oriente [8], the
caffeoyl chloride 2 and (‡)-(2R,3R)-l-tartaric acid (8a) or its enantiomer 8b are mixed
together and neatly heated above 1008 to afford 1a or 1b, presumably via the tartaric
anhydrides 10a and 10b, respectively. The method recently described by Zhao and
Burke [7], instead, relies on the use of the caffeoyl chloride diacetate derivative 3 for
acylating (‡)-(2R,3R)-di(tert-butyl) l-tartrate (11a) or its enantiomer 11b. A two-step
acid-mediated deprotecting sequence then furnished the desired chicoric acids.
We surmised that a convenient single-type protecting-group approach could also be
devised by relying on a carbodiimide-mediated acylation between Bn-protected
coupling partners. An ultimate debenzylation step should, then, furnish the chicoric
acids. The only caveat in this route lies in the extent of chemoselectivity at which the
debenzylation must be performed without affecting the sensitive ester linkages and the
reducible CˆC bonds of the product(s). The synthesis started from commercially
available caffeic acid (4), which was esterified with EtOH in the presence of AcCl,
benzylated with BnBr and K2CO3 in refluxing EtOH, and saponified with KOH in
refluxing EtOH, to furnish the dibenzyl ether derivative 7 in 80% overall yield
(Scheme). Both dibenzyl tartrate enantiomers 9a/9b are commercially available and
inexpensive, or can easily be prepared in yields exceeding 90% from their
corresponding tartaric acid isomer (8a/8b) [11].
The carbodiimide-based biscaffeoylation of 9a/9b with 7 was first attempted with
dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino)pyridine (DMAP) in CH2Cl2
according to the method of Neises and Steglich [12]. Difficulties in removing the
substituted urea by-product prompted us to use 1-[3-(dimethylamino)propyl]-3-
ethylcarbodiimide hydrochloride (EDCI) [13] instead of DCC. The perbenzylated
chicoric acid derivatives 12a/12b were thus obtained in yields of 95and 80%, respec-
tively. An extensive investigation was carried out to identify appropriate debenzylation
conditions. Metal-catalyzed hydrogenolysis (poisoned Pd, W-2 Raney-Ni), Lewis acid-
mediated cleavage (FeCl3 or BF3 ¥ Et2O/EtSH) and transfer hydrogenation with
various hydrogen donors (cyclohexene, cyclohexa-1,4-diene, HCOOH, ammonium
formate) were all unsuccessful.

Helvetica Chimica Acta Vol. 85(2002)
2331
Scheme. A Convenient Synthesis of (À)-Chicoric Acid (1a)
a) EtOH, AcCl, r.t., 2 d; 95%. b) BnBr, K2CO3, EtOH, reflux, 20 h; 98%. c) KOH, EtOH, reflux, 20 h; 85%.
d) BnOH, 2008 [11a] (dibenzyl tartrate 9a is commercially available). e) EDCI, DMAP, CH2Cl2, r.t., 1 d; 95%.
f) Pd(OAc)2, Et3SiH, Et3N, CH2Cl2, r.t. (58 ! 92%). The synthesis of (‡)-chicoric acid (1b) can be similarly
accomplished from commercially available dibenzyl tartrate 9b.
Removal of the six Bn groups of 12a was finally accomplished in one step via a
silane-promoted Pd-mediated hydrogenolysis. The reaction conditions were adapted
from the method of Coleman and Shah [14]. The amount of each reactant, i.e., Et3SiH,
Pd(OAc)2, and Et3N, and their order of addition at the onset of the reaction were found
to be extremely critical to ensure an efficient and chemoselective debenzylation.
Substoichiometric amounts of Pd(OAc)2 were sufficient to mediate the required
multiple removal of Bn groups. Treatment of 12a with 0.80 equiv. of Pd(OAc)2 for 72 h
furnished 1a in 58% yield. It is remarkable that mixing of all reactants prior to the
addition of the starting material 12a appeared detrimental to the desired chemo-
selective reaction; in all cases, incomplete debenzylation and significant CˆC bond
hydrogenation were observed. Higher yields of 1a (up to 92%) were obtained by first
mixing Pd(OAc)2/Et3N with the starting material in a CH2Cl2 solution to which excess
Et3SiH was added dropwise (4 equiv. per Bn group). The best result was obtained by
performing the reaction at a concentration of 30 mm perbenzylated chicoric acid 12a.
Purification of 1a can be conveniently carried out by semipreparative reversed-phase
HPLC to a minimum purity level of 90%. The same conditions afforded (‡)-chicoric
acid (1b) in 66% yield from the corresponding perbenzylated derivative 12b.
Conclusions. This convenient synthesis of chicoric acid and, in particular, of its
immuno-enhancing and anti-HIV levorotatory enantiomer, should prove itself
valuable in future studies aimed at elucidating and exploiting the biological activity
of these caffeic acid based natural products. This synthetic source of chicoric acids will
be useful when undertaking bioavailability studies for medicinal applications.
Chromatographic standardization of Echinacea-based phytopharmaceutical commer-
cial preparations could also make use of these synthetic compounds.

2332
Helvetica Chimica Acta Vol. 85(2002)
¬ ¬
¬
¡
¡
The support of Biolandes, the Delegation Regionale a la Recherche et a la Technologie pour l×Aquitaine
¬
(FEDER Grant No. Z0131), and the Conseil Regional d×Aquitaine (Grant No. 990209003) is gratefully
acknowledged. The authors also wish to thank Gae»lle-Anne Cremer (ENS Chimie, Rennes) and Patricia Castel
(Institut du Pin, Bordeaux 1) for their contributions to this work.
Experimental Part
General. CH2Cl2 was purified by distillation from CaH2 under Ar immediately before use. Moisture and O2-
sensitive reactions were carried out in flame-dried glassware under Ar. Evaporations were conducted under
reduced pressure at temp. below 458 unless otherwise noted. Column chromatography (CC) was carried out
under positive N2 pressure with 40 63 mm silica gel (Merck) and the indicated solvents. M.p.: uncorrected.
NMR Spectra of samples in the indicated solvent were recorded at 200 or 250 MHz on Bruker instruments. 13C-
multiplicities were determined by DEPT135experiments. Electron impact (EI) and liquid secondary-ion (LSI),
and low- and high-resolution (HR) MS analyses were obtained from the mass spectrometry laboratory at the
¬
CESAMO, Universite Bordeaux 1. Combustion analyses were performed by Laboratoires Wolff, Clichy, France.
High-Performance Liquid Chromatography (HPLC). Semi-prep. HPLC purification of 1a was performed
on a Waters Delta Prep 3000 pump system with a C-18 Lichrospher column (25 Â 250 mm, 5 mm). The mobile
phase was MeCN/H2OÀHCO2H 99.5:0.5and isocratic elution 18 :82 was applied at a flow rate of 20 ml/min.
Column effluent was monitored by UV detection at 280 nm with a LKB 2158 Uvicord SD UV/VIS detector.
HPLC Analysis of eluted fractions was carried out on a Thermo system with a C-18 Lichrospher column
(4.6 Â 250 mm, 5 mm) with P4000 pumps and UV detection was performed at 330 and 280 nm with a UV2000
detector. The mobile phase was MeCN/H2OÀHCO2H 99.5:0.5 and gradient elution (0 50 min: 12 to 40%
MeCN) was applied at a flow rate of 0.8 ml/min.
Ethyl 3-(3,4-Dihydroxyphenyl)prop-2-enoate (Ethyl Caffeoate; 5). To an ice-cold soln. of caffeic acid 4
(5.0 g, 27.8 mmol) in abs. EtOH (140 ml) was added dropwise freshly distilled AcCl (13.8 ml, 194.5 mmol). The
mixture was stirred at r.t. for 2 d, and then evaporated. The solid residue was dissolved in 100 ml of H2O, and
extracted with AcOEt (3 Â 50 ml). The org. layer was washed with sat. aq. NaHCO3 soln. (2 Â 20 ml) and brine
(2 Â 20 ml), dried (Na2SO4), filtered, and evaporated. Crystallization of the solid residue from AcOEt/hexanes
1:4 furnished 5 (5.48 g, 95%). Amber-colored crystals. M.p. 148 1498. IR (KBr) 3460, 1680. 1H-NMR
((D6)acetone, 250 MHz): 1.26 (t, J ˆ 7.0, 3 H); 4.18 (q, J ˆ 7.0, 2 H); 6.27 (d, J ˆ 15.9, 1 H); 6.87 (d, J ˆ 8.2, 1 H);
7.04 (dd, J ˆ 2.1, 8.2, 1 H); 7.16 (d, J ˆ 2.1, 1 H); 7.54 (d, J ˆ 15.9, 1 H); 8.33 (br. s, 2 H). 13C-NMR ((D6)acetone,
‡
62.9 MHz): 166.6; 147.8; 145.4; 144.7; 126.8; 121.7; 115.6; 114.9; 114.4; 59.7; 13.9. EI-MS: 209 (10, [M ‡ 1] ), 208
‡
(82, M ), 180 (18), 163 (100).
Ethyl 3-[3,4-Bis(benzyloxy)phenyl]prop-2-enoate (6) [15]. Under stirring, to a soln. of 5 (3.7 g, 17.8 mmol)
in abs. EtOH (100 ml), powdered K2CO3 (5.2 g, 37.5 mmol) and BnBr (4.65 ml, 39.2 mmol) were added, and the
mixture was refluxed overnight. EtOH was then evaporated, and the residue was dissolved in AcOEt (50 ml).
This org. layer was washed with 1m H3PO4 (15ml) and brine (2 Â 15ml), and dried (Na 2SO4). Evaporation of
the solvent afforded a yellowish solid, which was purified by recrystallization from EtOH to give 6 (6.76 g, 98%).
Off-white powder. M.p. 81 828. IR (KBr) 1696. 1H-NMR (CDCl3, 250 MHz): 1.34 (t, J ˆ 7.0, 3 H); 4.26 (q, J ˆ
7.0, 2 H); 5.18 (s, 2 H); 5.20 (s, 2 H); 6.26 (d, J ˆ 15.9, 1 H); 6.92 (d, J ˆ 8.2, 1 H); 7.05 7.49 ( m, 12 H); 7.5 9
(d, J ˆ 15.9, 1 H). 13C-NMR (CDCl3, 62.9 MHz): 167.1; 150.9; 148.9; 144.3; 136.8; 136.7; 128.5; 127.9; 127.2;
‡
‡
127.1; 122.7; 116.1; 114.2; 113.6; 71.2; 70.9; 60.3; 14.3. EI-MS: 389 (4, [M ‡ 1] ), 388 (14, M ), 297 (9), 91 (100).
3-[3,4-Bis(benzyloxy)phenyl]prop-2-enoic Acid (7). Under stirring, to a 1m ethanolic KOH soln. (12 ml), a
soln. of 6 (2.9 g, 7.5mmol) in EtOH (15ml) was added, and the mixture was heated under reflux overnight.
After evaporation of the EtOH, the residue was dissolved in 15ml of H 2O and extracted with AcOEt (2 Â
30 ml). The aq. layer was then acidified by adding 10% HCl, extracted with AcOEt (3 Â 30 ml), and dried
(Na2SO4). Evaporation of the solvent gave a white solid, which was recrystallized from AcOEt/hexanes to
afford 7 (2.29 g, 85%). Off-white powder. M.p. 190 1928. IR (KBr) 2576, 2508, 1692, 1670. 1H-NMR
((D6)DMSO, 250 MHz): 5.18 (s, 2 H); 5.20 (s, 2 H); 6.44 (d, J ˆ 15.9, 1 H); 7.06 (d, J ˆ 8.5, 1 H); 7.18 (d, J ˆ 1.6,
1 H); 7.21 7.55 (m, 12 H); 12.27 (s, 1 H). 13C-NMR ((D6)DMSO, 62.9 MHz): 168.1; 150.3; 148.5; 144.2; 137.3;
‡
137.1; 128.6; 128.0; 127.8; 127.7; 127.6; 123.1; 117.2; 114.0; 113.0; 70.2; 70.0. EI-MS: 361 (2, [M ‡ 1] ), 360 (10,
‡
M ), 269 (6), 91 (100).
Dibenzyl (À)-(2R,3R)-2,3-Bis{3-[2,3-bis(3,4-dibenzyloxy)phenyl]prop-2-enoyloxy]-l-tartrate (12a). Un-
der stirring, to a suspension of 7 (1.20 g, 3.33 mmol) in dry CH2Cl2 (8 ml), DMAP (406 mg, 3.33 mmol), a soln.
of 9a (500 mg, 1.51 mmol) in dry CH2Cl2 (3 ml), and EDCI (1.27 g, 6.66 mmol) were added successively. The

Helvetica Chimica Acta Vol. 85(2002)
2333
resulting pale yellow soln. was stirred at r.t. for 1 d, then it was diluted in AcOEt (30 ml) and water (5ml).
Extraction with AcOEt (2 Â 15ml), followed by washings with 1m H3PO4 (5ml) and brine (3 Â 5ml), led to an
org. layer, which was dried (Na2SO4). Evaporation of the solvent afforded a solid residue, which was submitted
to CC (hexanes/AcOEt 3 :2) to give 12a (1.46 g, 95%). Yellow foam. M.p. 45 478. [a]2D2 ˆ À80.8 (c ˆ 1.93,
CHCl3). IR (KBr) 1770, 1726. 1H-NMR (CDCl3, 250 MHz): 5.12 5.30 (m, 12 H); 5.94 (s, 2 H); 6.23 (d, J ˆ
15.9, 2 H); 6.93 7.52 (m, 36 H); 7.5 9 (d, J ˆ 15.9, 2 H). 13C-NMR (CDCl3, 62.9 MHz): 165.8; 165.5; 151.4; 148.9;
146.6; 136.7; 136.6; 134.7; 128.5; 128.4; 127.9; 127.4; 127.2; 127.1; 123.4; 114.0; 113.8; 113.6; 71.2; 70.8; 67.7. LSI-
‡
‡
‡
MS: 1037 (9, [M ‡ Na] ), 1015(6, [ M ‡ 1] ), 1014 (7, M ), 343 (100). Anal. calc. for C64H54O12: C 75.71, H 5.37;
found: C 75.88, H 5.44.
Dibenzyl (‡)-(2S,3S)-2,3-Bis{3-[2,3-bis(benzyloxy)phenyl]prop-2-enoyloxy}-d-tartrate (12b). Compounds
7 and 9b were coupled according to the same procedure as described for 12a to furnish 12b as a yellow foam
(80% yield). M.p. 46 478. [a]2D2 ˆ ‡70.4 (c ˆ 1.46, CHCl3); all other spectroscopic data were identical to those
reported for 12a.
(À)-2,3-Bis{[3-(3,4-dihydroxyphenyl)-1-oxoprop-2-enyl]oxy}butanedioic Acid ((À)-Chicoric Acid; 1a). To
Pd(OAc)2 (334 mg, 1.49 mmol) maintained under Ar, a soln. of Et3N (207 ml, 1.49 mmol) in dry CH2Cl2 (10 ml)
was added. The mixture was stirred at r.t. for 5min, then 12a (630 mg, 0.62 mmol) in dry CH2Cl2 (10 ml) was
added dropwise. The resulting brown soln. was stirred at r.t. for 5min, and Et 3SiH (2.37 ml, 14.90 mmol) was
then added slowly; the mixture immediately became dark, and it was stirred for 24 h at r.t. Complete
debenzylation was confirmed by 1H-NMR analysis of an aliquot in (D6)acetone. Addition of MeOH (3 ml),
followed by filtration and evaporation of the filtrate, afforded a yellow oil, which was diluted with AcOEt
(30 ml) and H2O (5ml). After separation, the aq. phase was extracted with AcOEt (10 ml), and the combined
extracts were washed with 1m H3PO4 and brine until pH 7. Drying (Na2SO4), and evaporation gave an oily
residue, which was diluted again in AcOEt (2 ml). Addition of hexanes (40 ml) afforded a precipitate that was
filtered off and dried under vacuum to give 1a (271 mg, 92%) as an amorphous off-white solid. This solid was
purified by semi-prep. HPLC to afford 1a with a purity level of 90% as determined by UV analysis. [a]D22
ˆ
À242.7 (c ˆ 0.89, MeOH) ([8]: [a]2D5 ˆ À384.2 (c ˆ 1.075, MeOH)). IR (KBr) 3406, 1707. 1H-NMR
((D6)acetone, 250 MHz): 5.94 (s, 2 H); 6.44 (d, J ˆ 15.8, 2 H); 6.93 (d, J ˆ 8.2, 2 H); 7.16 (dd, J ˆ 2.1, 8.2,
2 H); 7.27 (d, J ˆ 2.1, 2 H); 7.70 (d, J ˆ 15.8, 2 H). 13C-NMR ((D6)acetone, 62.9 MHz): 167.6; 166.3; 149.1; 147.6;
‡
‡
‡
146.3; 127.2; 122.9; 116.3; 115.4; 113.8; 71.6. LSI-MS: 497 (35, [M ‡ Na] ), 475(7, [ M ‡ 1] ), 474 (9, M ).
(‡)-Chicoric Acid (1b). The same procedure as that described above for 1a furnished 1b from 12b as an
amorphous off-white solid (66% yield). [a]D22 ˆ ‡214.6 (c ˆ 0.63, MeOH) ([8]: [a]2D2 ˆ ‡340.0 (c ˆ 1.075,
MeOH)). All other spectroscopic data were identical to those reported for 1a.
REFERENCES
[1] R. Bauer, in −Immunomodulatory Agents from Plants×, Ed. H. Wagner, Birkh‰user Verlag, Basel,
Switzerland, 1999, p. 41 88.
[2] a) O. Hoheisel, M. Sandberg, S. Bertram, M. Bulitta, M. Sch‰fer, Eur. J. Clin. Res. 1997, 9, 261; b) R. M.
Facino, M. Carini, G. Aldini, L. Saibene, P. Pietta, P. Mauri, Planta Med. 1995, 61, 510; c) M. J. Parnham,
Pharmazeutische Z. 1996, 141, 4282; d) L. Girre, Phytotherapy 1995, 43, 3; e) P. Goetz, Revue de
¬
Phytotherapie Pratique 1989, 3, 14; f) A. Cheminat, R. Zawatzky, H. Becker, R. Brouillard, Phytochemistry
1988, 27, 2787; g) H. Funke, Z. Phytotherapy 1983, 4, 650.
[3] a) R. Fleischhacker, Dtsch. Apoth. Z. 1998, 138, 1850; b) R. Bauer, P. Remiger, K. Jurcic, H. Wagner, Z.
Phytotherapy 1989, 10, 43; c) R. Bauer, K. Jurcic, J. Puhlmann, H. Wagner, Arzneim.-Forsch. 1988, 38, 276;
d) H. Wagner, Z. Phytother. 1986, 7, 91; e) H. Wagner, A. Proksch, Z. Angew. Phytother. 1981, 2, 166.
[4] a) B. N¸sslein, M. Kurzmann, R. Bauer, W. Kreis, J. Nat. Prod. 2000, 63, 1615; b) D. Melchart, K. Linde, in
−Immunomodulatory Agents from Plants×, Ed. H. Wagner, Birkh‰user Verlag, Basel, Switzerland, 1999,
pp. 105 118; c) M. J. Parnham, in −Immunomodulatory Agents from Plants×, Ed. H. Wagner, Birkh‰user
Verlag, Basel, Switzerland, 1999, pp. 119 135.
[5] a) H. Soicke, G. Al-Hassan, K. Gˆrler, Planta Med. 1988, 54, 175; b) R. Bauer, P. Remiger, H. Wagner,
Dtsch. Apoth. Z. 1988, 128, 174; c) H. Becker, W. C. Hsieh, Z. Naturforsch., C 1985, 40, 585.
[6] a) N. Neamati, H. Hong, S. Sunder, G. W. A. Milne, Y. Pommier, Mol. Pharmacol. 1997, 52, 1041; b) H.
Zhao, N. Neamati, A. Mazumder, S. Sunder, Y. Pommier, T. R. Burke Jr., J. Med. Chem. 1997, 40, 1186;
c) N. Mahmood, P. S. Moore, N. De Tommasi, F. De Simone, A. J. Hay, C. Pizza, Antivial Chem. Chemother.
1993, 4, 235.

2334
Helvetica Chimica Acta Vol. 85(2002)
[7] H. Zhao, J. T. R. Burke, Synth. Commun. 1998, 28, 737.
[8] M. L. Scarpati, G. Oriente, Tetrahedron 1958, 4, 43.
[9] R. M. Facino, M. Carini, G. Aldini, C. Marinello, E. Arlandini, L. Franzoi, M. Colombo, P. Pietta, P. Mauri,
Il Farmaco 1993, 48, 1447.
[10] a) W. E. Robinson Jr., M. G. Reinecke, S. Abdel-Malek, Q. Jia, S. A. Chow, Proc. Natl. Acad. Sci. U.S.A.
1996, 93, 6326; b) Z. Lin, N. Neamati, H. Zhao, Y. Kiryu, J. A. Turpin, C. Aberham, K. Strebel, K. Kohn, M.
Witvrouw, C. Pannecouque, Z. Debyser, E. De Clercq, W. G. Rice, Y. Pommier, T. R. Burke Jr., J. Med.
Chem. 2000, 42, 1401; c) W. E. Robinson Jr., M. Cordeiro, S. Abdel-Malek, Q. Jia, S. A. Chow, M. G.
Reinecke, W. M. Mitchell, Mol. Pharmacol. 1996, 50, 846; d) B. McDougall, P. J. King, B. W. Wu, Z.
Hostomsky, M. G. Reinecke, W. E. Robinson Jr., Antimicrob. Agents Chemother. 1998, 42, 140; e) P. J.
King, W. E. Robinson Jr., J. Virol. 1998, 72, 8420; f) W. E. Robinson Jr., Antiviral Res. 1998, 39, 101.
[11] a) Y. Hu, K. A. Yamada, D. K. Chalmers, D. P. Annavajjula, D. F. Covey, J. Am. Chem. Soc. 1996, 118,
4550; b) J. E. Bishop, J. F. O×Connel, H. Rapoport, J. Org. Chem. 1991, 56, 5079; c) R. Wagner, J. W. Tilley,
K. Lovey, Synthesis 1990, 785.
[12] B. Neises, W. Steglich, Angew. Chem., Int. Ed. 1978, 17, 522.
[13] N. Flaih, C. Pham-Huy, H. Galons, Tetrahedron Lett. 1999, 40, 3697.
[14] R. S. Coleman, J. A. Shah, Synthesis 1999, 1399.
[15] J. R. McElhanon, M.-J. Wu, M. Escobar, U. Chaudhry, C.-L. Hu, D. V. McGrath, J. Org. Chem. 1997, 62,
908.
Received March 21, 2002