Direct synthesis of chrysosplenol D

Article abstract of DOI:10.1021/np800423j

An aldol condensation and an Algar-Flynn-Oyamada oxidative cyclization were key steps in the direct synthesis of chrysosplenol D, an efflux pump inhibitor that can potentiate the activity of commercially important antibiotics and antimalarials.

Full text of DOI:10.1021/np800423j

J. Nat. Prod. 2008, 71, 1961–1962
1961
Direct Synthesis of Chrysosplenol D
George Kraus* and Sudipta Roy
Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011
ReceiVed July 11, 2008
An aldol condensation and an Algar-Flynn-Oyamada oxidative cyclization were key steps in the direct synthesis of
chrysosplenol D, an efflux pump inhibitor that can potentiate the activity of commercially important antibiotics and
antimalarials.
Chrysosplenol D (3′,4′,5-trihydroxy-3,6,7-trimethoxyflavone) (1)
was initially isolated from the Chinese medicinal plant Artemisia
annua L. (Asteraceae).1 Later, it was characterized both from
Ehretia amoena, a plant traditionally used in Uganda to treat
sleeping sickness, and from Vitex trifolia, another Chinese folk
medicine plant used to treat cancer.2 It was also found in
Plectranthus cylindraceus and in the African medicinal plant
Psiadia trinerVia.3 Chrysosplenetin (2) and vitexicarpin (3) are also
members of this class of naturally occurring 3-methoxyflavones.4,5
Scheme 1. Retrosynthetic Plan
Experimental Section
General Experimental Procedures. 1H and 13C NMR spectra were
recorded on either a Varian 300 MHz or a Varian 400 MHz Fourier
transform NMR spectrometer. HRMS were recorded on a Kratos model
MS-50 spectrometer, and LRMS were performed with a Finnigan
TSQ700 mass spectrometer. All experiments were performed under
an Ar atmosphere unless otherwise noted.
The strong interest in chrysosplenol D stems from its ability to
potentiate the already potent antimalarial activity of artemisinin and
from its ability to potentiate the activity of norfloxacin. Presumably
this activity is due to its inhibition of the multidrug resistance
(MDR) efflux pumps in Plasmodium falciparum and in Staphylo-
coccus aureus.6 One limitation to the further evaluation of 1 is its
low natural abundance. As part of our program to enhance plant-
derived natural products,7 we report herein a direct total synthesis
of chrysosplenol D (1). Our strategy involved the synthesis of a
chalcone intermediate, readily generated via an aldol condensation
of ketone 4 with aldehyde 5,8 that would be subjected to oxidative
cyclization, methylation, and deprotection.
Compound 6. To a stirred solution of ketone 4 (1.09 g, 4.8 mmol)
and aldehyde 5 (1.27 g, 4.00 mmol) in EtOH (40 mL) at 0 °C was
added freshly powdered KOH (86.5%, 1.30 g, 20 mmol). The mixture
was slowly allowed to warm to rt and stirred for 56 h. The solvent
was evaporated to approximately one-fifth the original volume (∼8-10
mL). Ice-cold H2O (∼2 mL) was added, and the mixture was neutralized
with 2 N HCl. It was then extracted with EtOAc (3 × 50 mL) and
dried over Na2SO4. The solvent was evaporated, and the crude residue
was purified by Si gel cc (hexanes/EtOAc, 3:1) to yield compound 6
(1.70 g, 81%) as a yellow oil: 1H NMR (CDCl3, 300 MHz) δ 7.78 (s,
2H), 7.52-7.31 (m, 10H), 7.25-7.19 (m, 2H), 6.96 (d, 1H, J ) 8.4
Hz), 6.31 (s, 1H), 5.24 (s, 2H), 5.22 (s, 2H), 3.89 (s, 3H), 3.87 (s, 6H);
13C NMR (CDCl3, 75 MHz) δ 192.7, 162.7, 160.1, 155.0, 151.2, 148.9,
143.5, 137.0, 136.8, 135.3, 128.8, 128.7, 128.0, 127.3, 127.2, 124.5,
123.5, 114.2, 114.1, 108.8, 96.7, 71.4, 70.9, 61.9, 61.3, 56.1; LRMS
(EI) m/z 526 (M+), 435, 318; HRMS (EI) calcd for C32H30O7 526.1992,
found 526.2002.
Ketone 4 was synthesized by Friedel-Crafts acylation of
commercially available 3,4,5-trimethoxyphenol.9 An aldol reaction
between aldehyde 5 and ketone 4 gave the known chalcone 6 in
81% yield as shown in Scheme 2.10 Excess ketone was used for
complete consumption of aldehyde in order to avoid purification
problems due to the similar Rf values of 5 and 6 as well as the
poor solubility of chalcone 6. Oxidative cyclization of chalcone 6
was challenging. Apparently, the Algar-Flynn-Olyamada (AFO)
reaction does not work well for the synthesis of o-methoxyfla-
vones.11 However, using excess hydrogen peroxide and sodium
hydroxide and extending the reaction time, we were able to realize
the cyclization of chalcone 6.
Compound 7. To a stirred solution of chalcone 6 (527 mg, 1 mmol)
in MeOH (45 mL) at 0 °C was added 10% aqueous NaOH (15 mL)
followed by 30% aqueous H2O2 (10 mL). The mixture was stirred at 0
°C for 1 h, then slowly allowed to warm to rt and stirred for 16 h.
Cold H2O (∼1 mL) was added, and then it was acidified with 2 N
aqueous HCl. Solid NaCl was added to the solution, and the mixture
was then extracted with CH2Cl2 (5 × 50 mL). The combined organic
phase was dried over Na2SO4. The solvent was evaporated and the crude
residue was purified by Si gel cc (hexanes/EtOAc, 1:1) to yield the
dihydroflavonol 7 (325 mg, 60%) as a yellowish solid: mp 168-170
Although we expected to obtain a flavonol, the usual product in
the AFO oxidation, dihydroflavonol 7, was formed in this reaction
in 60% yield. Iodine oxidation12 of 7 produced 8, which was
methylated to afford 3-methoxyflavone 9 in 77% yield. This
compound had been previously prepared by Horie using the
Allan-Robinson flavone synthesis.13 Aluminum tribromide-medi-
ated selective demethylation of 9 and debenzylation of 10 using a
catalytic amount of 10% Pd-C afforded chrysosplenol D (1) in
1
°C; H NMR (CDCl3, 300 MHz) δ 7.50-7.32 (m, 10H), 7.19-6.95
(m, 3H), 6.32 (s, 1H), 5.20 (s, 4H), 4.96 (d, 1H, J ) 12.3 Hz), 4.45 (d,
1H, J ) 12.0 Hz), 4.00 (s, 3H), 3.88 (s, 3H), 3.85 (s, 3H); LRMS (EI)
m/z 542 (M+), 360, 334 (100%), 303, 269, 181; HRMS (EI) calcd for
C32H30O8: 542.1941, found: 542.1948.
1
84% yield from 9. The H NMR spectrum matched the literature
spectrum.
Compound 8. To a stirred mixture of dihydroflavonol 7 (108 mg,
0.2 mmol) and anhydrous KOAc (884 mg, 9 mmol) in glacial acetic
acid (7 mL) at 0 °C was added dropwise I2 (115 mg, 0.45 mmol)
dissolved in HOAc (7 mL) over 20 min. The mixture was then boiled
for 5 h. It was cooled to rt, and ice-cold H2O (∼20 mL) was added.
The excess I2 was removed by addition of solid Na2S2O7. It was
extracted with EtOAc (3 × 40 mL), and the combined organic phases
were dried over Na2SO4. The solvent was evaporated and the crude
residue was purified by Si gel cc (hexanes/EtOAc, 1:1) to yield flavonol
In summary, we have achieved a direct synthesis of the
biologically important 3-methoxyflavone chrysosplenol D. The
synthesis of 1 was achieved in six steps from 5 in 31% overall
yield. This synthetic route will make possible extensive evaluation
of the efflux pump inhibitor 1.
* Corresponding author. Phone: 515-294-7794. E-mail: gakraus@
iastate.edu.
1
8 (84 mg, 78%) as a yellowish oil: H NMR (CDCl3, 300 MHz) δ
10.1021/np800423j CCC: $40.75
2008 American Chemical Society and American Society of Pharmacognosy
Published on Web 10/15/2008

1962 Journal of Natural Products, 2008, Vol. 71, No. 11
Notes
Figure 1. Structures of 1-3.
Scheme 2. Synthesis of Chrysosplenol D
7.89 (d, 1H, J ) 1.5 Hz), 7.79 (d, 1H, J ) 8.7 Hz), 7.54-7.31 (m,
10H), 7.05 (d, 1H, J ) 8.4 Hz), 6.73 (s, 1H), 5.27 (s, 2H), 5.26 (s,
2H), 4.03 (s, 3H), 4.00 (s, 3H), 3.93 (s, 3H); LRMS (EI) m/z 540 (M+),
449, 421, 333 (100%); HRMS (EI) calcd for C32H28O8 540.1784, found
540.1792.
3.93 (s, 3H), 3.83 (s, 3H); LRMS (EI) m/z 360 (M+, 100%), 345, 317;
HRMS (EI) calcd for C18H16O8 360.0845, found 360.0853.
Acknowledgment. We thank Iowa State University for partial
support of this research.
Compound 9. To a stirred mixture of flavonol 8 (60 mg, 0.11 mmol)
and anhydrous K2CO3 (138 mg, 1 mmol) in dry acetone (3 mL) at rt
was added dropwise Me2SO4 (101 mg, 1 mmol). The mixture was
refluxed for 9 h. The acetone was evaporated and the residue was
purified by Si gel cc (initially, hexanes; then hexanes/EtOAc, 1:1) to
References and Notes
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1
yield compound 9 (59 mg, 97%) as a yellow oil: H NMR (CDCl3,
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7.51-7.48 (m, 4H), 7.42-7.38 (m, 4H), 7.36-7.32 (m, 2H), 7.03 (d,
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3.92 (s, 3H), 3.71 (s, 3H); LRMS (EI) m/z 540 (M+), 449; HRMS (EI)
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mL, 1:1) was stirred in H2 atmosphere at rt for 45 min. The mixture
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product 1 (10 mg, 92%) as a sticky yellowish solid: 1H NMR (CDCl3,
400 MHz) δ 12.43 (brs, 1H), 7.90 (d, 1H, J ) 2.0 Hz), 7.59 (dd, 1H,
J ) 8.4, 2.0 Hz), 7.03 (d, 1H, J ) 8.8 Hz), 6.53 (s, 1H), 3.97 (s, 3H),
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NP800423J