Preparation of 7-alkoxylated furanopyrones: Semisynthesis of (-)- etharvensin, a new styryl-lactone from Goniothalamus arvensis

Article abstract of DOI:10.1021/np970346w

A new furanopyrone derivative, (-)-etharvensin (1), was isolated from the stem bark of Goniothalamus arvensis. Semisynthesis of the 7- ethoxyfuranopyrone 1 was achieved by addition of EtOH in concentrated acid medium to the unsaturated α-pyrone (+)-althol

Full text of DOI:10.1021/np970346w

1
338
J . Nat. Prod. 1997, 60, 1338-1340
P r ep a r a tion of 7-Alk oxyla ted F u r a n op yr on es: Sem isyn th esis of
-)-Eth a r ven sin , a New Styr yl-La cton e fr om Gon ioth a la m u s a r ven sis
(
Almudena Bermejo, M. Amparo Bl a´ zquez, Angel Serrano, M. Carmen Zafra-Polo, and Diego Cortes*
Departamento de Farmacolog ı´ a, Farmacognosia y Farmacodinamia, Facultad de Farmacia, Universidad de Valencia,
4
6100 Burjassot, Valencia, Spain
X
Received J uly 21, 1997
A new furanopyrone derivative, (-)-etharvensin (1), was isolated from the stem bark of
Goniothalamus arvensis. Semisynthesis of the 7-ethoxyfuranopyrone 1 was achieved by
addition of EtOH in concentrated acid medium to the unsaturated R-pyrone (+)-altholactone
(
2). This protocol constitutes a novel, direct (single-step), and efficient method to prepare this
class of bioactive compounds.
Styryl-lactones are an interesting group of cytotoxic
and antitumor agents, many of which have been isolated
1
from Goniothalamus species (Annonaceae). As part of
our investigation on the isolation, semisynthesis, and
bioactivity of acetogenins2 and styryl-lactones, we
,3
4,5
report herein the isolation and structure elucidation of
(-)-etharvensin (1) from the stem bark of Goniothala-
mus arvensis and the semisynthesis of 1 by alkoxylation
of the unsaturated R-pyrone (+)-altholactone (2).
Purification of the crude MeOH extract of G. arvensis
Scheff. stem bark and chromatographic fractionation
led to the isolation of etharvensin (1). The molecular
F igu r e 1. 1H NMR and NOEDIFF of 1.
formula, C15H18O5, was indicated by a small peak at 278
+
[
M] in the EIMS, and confirmed by high-resolution
mass measurement (HREIMS). The presence of a
hydroxyl group was suggested by an IR band at 3416
-1
cm , and corroborated by the preparation of a monoac-
1
13
etate derivative (1a ). Inspection of the H- and C-
NMR spectra revealed that 1 was closely related to
goniotharvensin (3), a saturated R-pyrone also isolated
from G. arvensis.4 The 2-phenyl-tetrahydrofurano-5-
pyrone skeleton of 1 was established by careful exami-
nation of the HREIMS fragmentations, 2D homonuclear
(COSY 45), and 2D heteronuclear correlation (HMQC)
NMR spectra (Table 1).
The presence of an ethoxyl group was suggested in
HREIMS by a fragment peak at m/z 232.0740 [M - 46]
F igu r e 2. Perspective 3D of the fully energy-minimized
structure of etharvensin (1) by MM2 calculations.
+
(
C13H12O4), and confirmed by 1D and 2D NMR experi-
and it also excludes the location of the ethoxy group at
the 3 position (Figure 1).
ments, which showed resonances at δ 3.65 (q, 2H, 14-
CH2) and 1.22 (t, 3H, 15-CH3) in the H, and at δ 60.28
1
The relative stereochemistry of the four stereogenic
centers in the tetrahydrofuran ring was evidenced by
1
3
(CH2) and 15.30 (CH3) in the C NMR, respectively. The
1
1
4
00 MHz COSY 45 spectra of etharvensin (1) revealed
the H- H coupling constant data and NOEDIFF
experiments (Figure 1 and Table 1). The relative
configuration was trans (H-3/H-2), trans (H-3a/H-3), and
cis (H-7a/H-3a), similar to that of altholactone (2), whose
configuration was established by X-ray crystallographic
the correlation between the saturated δ-lactone protons.
The H-6a/H-6b (at δ 2.70/ 2.84) and the H-7a (at δ 4.38)
were correlated with a methine proton signal at δ 4.02,
corresponding to H-7. Indeed, selective irradiation in
1
6
4
H NMR of the signal at δ 4.02 simplified the resonances
analysis, and goniotharvensin (3). A trans relation-
ship was established between H-7/H-7a in agreement
with a pseudo-chair R-pyrone conformation. Significant
enhancement of H-6a (pseudoequatorial) was observed
on irradiation of H-7 in a NOEDIFF experiment. To
reproduce this configuration, a 3D representation of the
fully energy-minimized structure of 1 was calculated
using a modeling program with the MM2-derived force
field (Figure 2).
corresponding to the 6 and 7a protons. Moreover, a
NOEDIFF (homonuclear Overhauser enhancement)
interaction was observed between H-7 (δ 4.02) and H-6a
(
(
δ 2.70) and between the methylene of the ethoxy moiety
OCH2-14, at δ 3.65) and H-7 (δ 4.02). This finding is
consistent with an ethoxy group placed at the 7 position,
*
To whom correspondence should be addressed. Phone: (346) 386
The absolute stereochemistry of the chiral secondary
hydroxy group at C-3 was determined by preparing the
4
9 75. Fax: (346) 386 49 43. E-mail: dcortes@uv.es.
X
Abstract published in Advance ACS Abstracts, December 1, 1997.
S0163-3864(97)00346-7 CCC: $14.00
© 1997 American Chemical Society and American Society of Pharmacognosy

Notes
J ournal of Natural Products, 1997, Vol. 60, No. 12 1339
Ta ble 1. 1D and 2D NMR Experiments (400 MHz, CDCl3) of Etharvensin (1)
coupling in HMQC
H
a
δ (J , Hz)
4.69 d (6.1)
4.29 dd (2.0, 6.1)
4.89 dd (2.0, 4.7)
coupling in COSY 45
H-3 (4.29)
H-3a (4.89), H-2 (4.69)
H-7a (4.38), H-3 (4.29)
(multiplicity by DEPT 13C)
2
3
3
5
6
6
7
7
8
9
4
5
85.99 (CH)
83.50 (CH)
86.98 (CH)
169.48 (C)
33.10 (CH2)
a
b
2.70 dd (5.8, 16.4)
2.84 dd (3.7, 16.4)
4.02 ddd (3.6, 3.7, 5.8)
4.38 br t (3.6, 4.7)
H-7 (4.02), H-6b (2.84)
H-7 (4.02), H-6a (2.70)
H-7a (4.38), H-6b (2.84), H-6a (2.70)
H-7 (4.02), H-3a (4.89)
72.78 (CH)
75.72 (CH)
138.15 (C)
128.64, 128.31, 126.05 (CH)
65.28 (CH2)
a
-13
7.33-7.38 m
3.65 q (7.0)
1.22 t (7.0)
1
1
CH3-15 (1.22)
CH2-14 (3.65)
15.30 (CH3)
Ta ble 2. 1H NMR (400 MHz, CDCl3) Data of Mosher Esters 1b and 1c
H-9 to H-13
H-2
H-3
H-3a
H-7a
H-7
H-6a
H-6b
(
(
∆
S)-MTPA (1b)
R)-MTPA (1c)
δS - R
7.38-7.30
7.35-7.28
+(0.03-0.02)
4.915
4.782
+0.133
5.478
5.499
-0.021
4.878
4.992
-0.114
4.281
4.294
-0.013
4.046
4.050
-0.004
2.713
2.740
-0.027
2.894
2.904
-0.010
(
R)- and (S)-R-(methoxy)-R-(trifluoromethyl) phenylace-
tic acid (MTPA) derivatives by the Mosher’s ester
7
,8
method.
and (R)-MTPA (1c) esters. The negative [H-3a, ∆δS -
R) -0.114] and positive [H-2, ∆δS R) +0.133] ∆δH
Thus, 1 was converted to the (S)-MTPA (1b)
-
values observed for the signals of the protons on the left
and on the right segments, respectively, indicated a 3R
stereochemistry for 1 (Table 2). Consequently, the
absolute configuration for 1 is {2R,3R,3aS,7R,7aS}, in
agreement with the revised stereochemistry for gonio-
F igu r e 3. Preparation of (-)-etharvensin (1) and 7-alkoxy-
lated furanopyrones (1d , 1e) from (+)-altholactone (2).
9
fupyrone (4) as recently established by synthesis.
carried out under the same conditions but with MeOH
or iPrOH in H2SO4 acid medium. The 7-methoxy (1d )
and 7-isopropoxy (1e) furanopyrone derivatives were
obtained stereoselectively in high yield (Figure 3).
Because 1 was prepared readily from 2 under the
conditions outlined above, the possibility that it is an
artifact of the isolation process must be considered. This
possibility can be discounted, however, inasmuch as no
EtOH was used in the extraction or purification of 1.
On the other hand, because altholactone (2) is available
in large quantity from G. arvensis, it represents a good
natural starting material for preparation of other styryl-
lactones. This is the first report of a direct semisyn-
thesis of 7-alkoxylated furanopyrones in good yield using
simplified substrates.
Etharvensin (1) was less active than altholactone (2)
and goniotharvensin (3) in reducing the contractile
response induced by noradrenaline on rat aorta. In
2
+
Ca -containing medium, the contractile response was
not abolished by 1-3 at 100 µM concentrations, while
2
+
in Ca -free solution, 2 and 3 were active, with an IC50
of 78 and 80 µM, respectively, suggesting an intracel-
lular mechanism for relaxation of the vascular smooth
To confirm the structure of this ethoxylated natural
product, etharvensin (1) was semisynthesized from the
optically active (+)-altholactone (2) by a high-yielding
single-step method. We first tried to prepare 1 from 2
by epoxidation with m-CPBA of the R,â-unsaturated
δ-lactone of 2, followed by lithium aluminum hydride
reduction, and O-ethylation, but 1 could not be obtained
by this method. However, 1 was successfully prepared
from 2 by ethoxylation of the unsaturated pyrone by a
Michael-type addition of EtOH in concentrated H2SO4
1
0
muscle.
Exp er im en ta l Section
Gen er a l Exp er im en ta l P r oced u r es. Optical rota-
tions were measured with a Perkin-Elmer 241 polarim-
eter. IR spectra were run in film on a Perkin-Elmer
843 spectrometer. UV spectra were obtained on a
Perkin-Elmer lambda computer 15 UV/vis spectropho-
tometer. Mass spectra were performed with a VG Auto
(
Figure 3). A stereoselective alkoxylation at the C-7
position was achieved because only one diasteroisomer
1, 90%) was obtained when 2 was refluxed 1.5 h. To
corroborate this stereoselectivity, experiments were
1
Spec Fisons spectrometer. H NMR (250 or 400 MHz),
1
3C NMR, DEPT, and HMQC (62.5 or 100 MHz) spectra
(
were recorded on a Bruker AC-250 or a Varian Unity-

1
340 J ournal of Natural Products, 1997, Vol. 60, No. 12
Notes
1
4
00 instruments. Si gel TLC were detected by UV light
(R)-MTP A-Eth a r ven sin (1c): H NMR (CDCl3, 400
MHz) H-2 to H-13, see Table 2; δ 7.47-7.35 (m, 5 H,
Ph of MTPA), 3.624 (q, 2H, OCH CH ), 3.533 (s, 3H,
(254 nm) and spraying anisaldehyde sulfuric acid.
P la n t Ma ter ia l. G. arvensis Scheff. (Annonaceae)
2
3
was collected in the National Park of Varirata, located
in the Central Province of Papua, New Guinea.
voucher specimen was deposited in the herbarium of the
University of Papua, New Guinea.
Extr a ction a n d Isola tion . Dried and powdered
stem bark of G. arvensis (368 g) was macerated with
MeOH at room temperature. The crude MeOH extract
was partitioned between hexane and aqueous MeOH.
The aqueous MeOH solution was again fractionated
between CH2Cl2 and H2O to obtain 7 g of CH2Cl2
extract. Etharvensin (1, 25 mg) was isolated and
purified by Si gel 60 H column chromatography (hex-
ane-EtOAc, 4:6 and CH2Cl2-Me2CO, 9:1).
3 2 3
CH of MTPA), 1.213 (t, 3H, OCH CH ).
A
Gen er a l P r oced u r e for Alk oxyla tion of (+)-Al-
th ola cton e (2). To an EtOH solution (10 mL) of
altholactone (2, 60 mg, 0.258 mmol) was added dropwise
at 0 °C concentrated H2SO4 (96%, 1.5 mL). After
stirring and refluxing for 1.5 h, H2O was added to the
solution, followed by extraction with CH2Cl2. The
organic solution obtained was subjected to column
chromatography on Si gel 60 H (eluted with CHCl3-
EtOAc, 7:3) to afford 65 mg (0.233 mmol, 90%) of a
compound that was found to be identical with the
previously isolated (-)-etharvensin (1).
Biologica l Assa ys. Noradrenaline (NA1, 1µM) was
(
-)-Eth a r ven sin (1): oil; C15H18O5; [R]D -6.5° (c 2.0,
2
+
added in Ca -containing solution at 37 °C, and after-
EtOH); UV (EtOH) λmax (log ꢀ) 214 (2.44), 255 (2.30) and
2
+
ward the tissue was loaded in Ca -free, EDTA-contain-
ing solution for 20 min. After this time, agonist (NA2)
was applied until no contraction was induced, indicating
2
1
80 (2.27) nm; IR (dry film) νmax 3416, 3060, 1740, 1636,
451, 1380, 1237, 1099, 917, 862, 841, 822, 760, 700
-1
1
13
cm ; H NMR (CDCl3, 400 MHz), C NMR (CDCl3, 100
MHz), COSY 45, and HMQC data, see Table 1 and
Figure 1; HREIMS m/z (%) [M] 278.1148 (calcd for
2
+
complete depletion of internal Ca stores sensitive to
the agonist. The tissue was incubated for 20 min in
+
2
+
+
Krebs to refill the intracellular Ca
stores, and a
C15H18O5, 278.1154) (8), [M - HOEt] 232.0740 (calcd
spontaneous increase in the resting tone of the aorta
was observed. After washing and 5 min of loading in
for C13H12O4, 232.0735) (12), 162 (38), 133.0653 (calcd
for C9H9O, 133.0653) (100), 107 (C7H7O, 33), 97 (58),
2
+
Ca -free solution, styryl-lactone (1-3) was applied, and
9
1 (39), 77 (C6H5, 15).
-Acetyleth a r ven sin (1a ): Treatment of 1 (6 mg)
after 15 min a new addition of agonist (NA3) was made.
3
by Ac2O (1 mL) and pyridine (0.5 mL) overnight at room
Ack n ow led gm en t. This research was financially
supported by the Spanish Direcci o´ n General de Inves-
tigaci o´ n Cient ´ı fica y T e´ cnica (grant PB93-0682). We are
grateful to Prof. K. Sundar Rao of the University of
Papua New Guinea, for help in obtaining the plant
material, and to Prof. M. M. Midland for allowing us to
use his PC Model version 2.0 (P. C. Model, Serena
Sofware, Box 3076, Bloomington, IN 47402-3076).
temperature yielded 6.9 mg of 1a . C17H20O6; [R]D -10.7
°
(c 1.8, EtOH); UV (EtOH) λmax (log ꢀ) 216 (2.44), 236
(
1
2.95) and 280 (2.27) nm; IR (film) νmax 2970, 1749, 1492,
-1
449, 1369, 1224, 1095, 1044, 917, 761, 699 cm ; CIMS
+
+
m/z (%) [MH] 321 (100), 261 [MH - HOCOCH3] (4);
1
H NMR (CDCl3, 250 MHz) δ 7.37-7.29 (m, H-9 to
H-13), 5.29 (dd, H-3, J ) 4.0 and < 1.0 Hz), 4.92 (dd,
H-3a, J ) 3.5 and <1.0 Hz), 4.89 (d, H-2, J ) 4.0 Hz),
4
4
.36 (t, H-7a, J ) 3.7, 3.5 Hz), 4.06 (ddd, H-7, J ) 5.6,
.0, 3.7 Hz), 3.65 (q, 2H, OCH2CH3), 2.89 (dd, H-6b, J
Refer en ces a n d Notes
)
16.5, 4.0 Hz), 2.70 (dd, H-6a, J ) 16.5, 5.6 Hz), 2.14
(1) McLaughlin, J . L.; Chang, C. J .; Smith, D. L. In Human
Medicinal Agents from Plants; Kinghorn, A. D., Balandrin, M.,
Eds. American Chemical Society: Washington, 1993; pp 112-
s, 3H, OCOCH3), 1.23 (t, 3H, OCH2CH3); 1 C NMR
3
(
(
CDCl3, 62.5 MHz) 169.90 (C-5), 160.66 (OCOCH3),
1
37.
1
38.00 (C-8), 128.71 (C-10, 12), 128.53 (C-11), 126.35
(2) Zafra-Polo, M. C.; Gonz a´ lez, M. C.; Estornell, E.; Sahpaz, S.;
Cortes, D. Phytochemistry 1996, 42, 253-271.
(C-9, 13), 85.86 (C-3a), 83.43 (C-2), 82.91 (C-3), 75.90
(C-7a), 72.80 (C-7), 65.23 (OCH2CH3), 33.07 (C-6), 20.82
(OCOCH3), 15.30 (OCH2CH3).
(
3) Cav e´ , A.; Figad e` re, B.; Laurens, A.; Cortes, D. In Progress in
the Chemistry of Organic Natural Products; Herz, W., Kirby, G.
W., Moore, R. E., Steglich, W., Tamm, C., Eds. Springer-Verlag:
New York, 1997; pp 81-288.
P r ep a r a t ion of t h e C(3)-(S)- a n d (R)-MTP A
(
4) Bermejo, A.; Lora, M. J .; Bl a´ zquez, M. A.; Rao, K. S; Cortes, D.;
Zafra-Polo, M. C. Nat. Prod. Lett. 1995, 7, 117-122.
(5) Bermejo, A.; Bl a´ zquez, M. A.; Rao, K. S.; Cortes, D. Phytochem-
istry 1997, in press.
ester s of (-)-Eth a r ven sin (1). To a stirred solution
of 1 (2.5 mg) in CH2Cl2 at room temperature was added
pyridine, 4-(dimethylamino) pyridine, and (R)-MTPA-
Cl [to give (S)-MTPA ester] or (S)-MTPA-Cl [to give (R)-
MTPA-ester].8 Each reaction mixture was allowed to
sit for 2 h at room temperature, saturated with NaH-
CO3, and extracted with CH2Cl2. Normal workup gave
the (S)- and (R)-MTPA esters of etharvensin (4 mg of
(
6) El-Zayat, A. E.; Ferrigni, N. R.; McCloud, T. G.; McKenzie, A.
T.; Byrn, S. R.; Cassady, J . M.; Chang, C.; McLaughlin, J . L.
Tetrahedron Lett. 1985, 26, 955-956.
(7) Dale, J . A.; Mosher, H. S. J . Am. Chem. Soc. 1973, 95, 512-
5
19.
(
8) Rieser, M. J .; Hui, Y. H.; Rupprecht, J . K.; Kozlowski, J . F.;
Wood, K. V.; McLaughlin, J . L.; Hanson, P. R.; Zhuang, Z.; Hoye,
T. R. J . Am. Chem. Soc. 1992, 114, 10203-10213.
1
b and 4.5 mg of 1c, respectively).
(9) Mukai, C.; Hirai, S.; Kim, I. J .; Hanaoka, M. Tetrahedron Lett.
1
(
S)-MTP A-Eth a r ven sin (1b): H NMR (CDCl3, 400
1
996, 37, 5389-5392.
(10) Noguera, M. A.; Ivorra, M. D.; D’Ocon, P. Br. J . Pharmacol. 1996,
19, 158-164.
MHz) H-2 to H-13, see Table 2; δ 7.48-7.35 (m, 5 H,
Ph of MTPA), 3.610 (q, 2H, OCH2CH3), 3.530 (s, 3H,
CH3 of MTPA), 1.205 (t, 3H, OCH2CH3).
1
NP970346W