Total synthesis of 2?,5?-diepisilvestrol and its C1''' epimer: Key structure activity relationships at C1''' and C2'''

Article abstract of DOI:10.1021/np300376f

The first total synthesis of the low-abundance natural product 2?,5?-diepisilvestrol (4) is described. The key step involved a Mitsunobu coupling between cyclopenta[b]benzofuran phenol 7 and dioxane lactol 6. Deprotection then gave a 1:2.6 ratio of natural product 2?,5?- diepisilvestrol (4) and its C1 epimer 1?,2?,5?- triepisilvestrol (15) in 50% overall yield. An in vitro protein translation inhibition assay showed that 2?,5?-diepisilvestrol (4) was considerably less active than episilvestrol (2), while the unnatural isomer 1?,2?,5?-triepisilvestrol (15) was essentially inactive, showing that the configuration at C1''' and C''' has a large effect on the biological activity.

Full text of DOI:10.1021/np300376f

Note
pubs.acs.org/jnp
Total Synthesis of 2‴,5‴-Diepisilvestrol and Its C1‴ Epimer: Key
Structure Activity Relationships at C1‴ and C2‴
Jennifer M. Chambers,† David C. S. Huang,‡ Lisa M. Lindqvist,‡ G. Paul Savage,§ Jonathan M. White,†
and Mark A. Rizzacasa*,†
†School of Chemistry, The Bio21 Institute, The University of Melbourne, Victoria 3010, Australia
‡Walter and Eliza Hall Institute of Medical Research, 1G Royal Parade, Parkville, Victoria 3052, and Department of Medical Biology,
University of Melbourne, Victoria 3010, Australia
§CSIRO Molecular and Health Technologies, Bayview Avenue, Victoria 3168, Australia
S
* Supporting Information
ABSTRACT: The first total synthesis of the low-abundance
natural product 2‴,5‴-diepisilvestrol (4) is described. The key
step involved a Mitsunobu coupling between cyclopenta[b]-
benzofuran phenol 7 and dioxane lactol 6. Deprotection then
gave a 1:2.6 ratio of natural product 2‴,5‴-diepisilvestrol (4)
and its C1 epimer 1‴,2‴,5‴-triepisilvestrol (15) in 50% overall
yield. An in vitro protein translation inhibition assay showed
that 2‴,5‴-diepisilvestrol (4) was considerably less active than
episilvestrol (2), while the unnatural isomer 1‴,2‴,5‴-triepisilvestrol (15) was essentially inactive, showing that the configuration
at C1‴ and C2‴ has a large effect on the biological activity.
Aglaia is a large genus of the family Meliaceae that consists of
over 100 species of mostly woody trees and shrubs found
throughout the rainforests of Indomalaysia. Assay-guided
extraction of Aglaia leptantha and Aglaia foveolata afforded
two active metabolites, named silvestrol (1) and 5‴-
episilvestrol (2), in low yield (0.02% w/w).1,2 Compounds 1
and 2 contain a common cyclopenta[b]benzofuran core,3 as
well as a novel, unprecedented 1,4-dioxanyloxy pseudosugar
substituent.4 Both silvestrol (1) and episilvestrol (2) showed
comparable potent cytotoxic activity against several human
tumor cell lines including lung (ED50 value of 1.2 nM) and
breast cancer (ED50 value of 1.5 nM).2 Recently, silvestrol (1)
has shown a remarkable potent and selective activity against B-
cells from patients with chronic lymphocytic leukemia (LC50
value of 6.9 nM) as well as in vivo activity against acute
lymphoblastic leukemia in a mouse xenograft model (dose 1.5
mg/kg).5
Further phytochemical investigation of A. foveolata resulted
in the isolation of the minor isomers 2‴-episilvestrol (3) and
2‴,5‴-diepisilvestrol (4) in very low yield (0.8 mg of 3 and 0.9
mg of 4 from 40 to 45 kg of stem bark, ∼2 × 10−6 %).6
Compound 4 was tested against only one cell line (HT-29
colon cancer) and showed lower activity (ED50 1.07 μM) than
episilvestrol (2), which had an ED50 of 0.001 μM, suggesting
that the configuration at C2‴ has a strong influence on the
biological activity.6
The mechanism of action for the cytoxicity of this family of
compounds is not entirely clear. A dramatic reduction in the
antiapoptotic protein Mcl-1 in the presence of 1 is often
reported,5,7 and Pelletier and co-workers have demonstrated
that this is due to direct inhibition of translation initiation by
depletion of the helicase eIF4A from the eIF4F complex, as
opposed to indirectly via the mTOR and ER stress pathways.7,8
The total synthesis of silvestrol (1)9−11 and episilvestrol
(2)10,11 has been reported as well as the analogue 4′-
desmethoxyepisilvestrol (desmethstrol) (5).11 Analogue 5 is
also a potent cytotoxic compound [A549 lung cancer (IC50 130
nM) and colon cancer (IC50 10 nM)].11 Herein, we report the
total synthesis of the minor constituent 2‴,5‴-diepisilvestrol
(4) and its unnatural C1‴ epimer as well their activities in a
protein translation inhibition assay. Our synthetic approach to
4 is summarized in Scheme 1. We envisaged that the C2‴
isomer 4 could be constructed in a convergent manner as
utilized by us for the total synthesis of silvestrol (1) and
Received: May 29, 2012
© XXXX American Chemical Society and
American Society of Pharmacognosy
A
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Journal of Natural Products
Note
this compound then confirmed the absolute stereochemistry.
Removal of the PMB group then gave the lactol 6 fragment as a
1.6:1 mixture of anomers favoring the axial isomer.
Scheme 1. Retrosynthetic Analysis of 4
The known phenol 711 was synthesized utilizing the [3+2]-
photocycloaddition/α-ketol approach pioneered by Porco for
the production of methylrocaglate and related compounds.13,14
In this case, an O6 benzyl protecting group was utilized as
reported,11 but the approach has been modified by using an
alternative solvent mixture and separation method (Scheme 3).
Thus, the naringenin derivative 1211 was irradiated with a 7:3
mixture of CHCl3 and trifluoroethanol (TFE) as solvent14 to
afford an oxidopyrilium species, which underwent a [3+2]-
cycloaddition13 with methyl cinnamate to give aglaian 13. Base-
induced α-ketol13 rearrangement and selective anti-reduction15
gave the endo-adduct ( )-14 as well as the corresponding exo-
product11 in a 4:1 ratio, respectively. This was a slightly better
yield and higher endo-selectivity (38% over three steps on a 500
mg scale) than we had reported using a MeCN/MeOH solvent
mixture.11 We had previously resolved ( )-14 via the menthol
ester derivative, but this process was not high yielding (26%
average overall yield of (−)-14).11 Porco has also reported that
the [3+2]-photocycloaddition can be conducted in an
enantioselective manner using an equimolar amount of a
tetraaryl-1,3-dioxolane-4,5-dimethanol derivative as a chiral
Brønsted acid source.6b However, we elected to separate the
enantiomers by chiral HPLC (45% average yield of (−)-14) as
an operationally simple and more economic alternative.
Debenzylation as described previously then gave the phenol
(−)-7 in high yield.11
episilvestrol (2).10,11 Thus, a modified Mitsunobu coupling
between the 1,4-dioxane lactol 6 and known phenol (−)-711
followed by a final deprotection would then afford 2‴,5‴-
diepisilvestrol (4).
RESULTS AND DISCUSSION
Synthesis of the requisite 1,4-dioxane lactol 6 is shown in
Scheme 3. Lactol 8 is available in eight steps from β-D-glucose
■
Scheme 2. Synthesis of Lactol 6
The final stages of the synthesis are shown in Scheme 4.
Mitsunobu coupling between lactol 6 and phenol 7 using di-2-
Scheme 4. Total Synthesis of 2‴,5‴-Diepisilvestrol (4)
Scheme 3. Synthesis of Phenol 7
methoxyethyl azodicarboxylate (DMEAD)16 and PPh3 in the
presence of 4 Å molecular sieves11 afforded the coupling
products in 55% yield as a ∼2.5:1 mixture (as determined by
1H NMR spectroscopy), which could not be separated.
Deprotection of the mixture with TBAF gave 1‴,2‴,5‴-
triepisilvestrol (15) and 2‴,5‴-diepisilvestrol (4) as the
minor product, which was separated by HPLC in a combined
yield of 50% for the two steps. Synthetic 2‴,5‴-diepisilvestrol
(4) was identical to the natural product in all respects (see
Table 1) {[α]25 −62.3 (c 0.25, MeOH); lit.6 [α]20 −53.0 (c
pentaacetate.11 Methylation of 8 using NaH as base and MeI
gave good selectivity (7.5:1) for the desired C2 (equatorial)
acetal 9. Previously, when a lithium base was used with MeOTf
as the electrophile, the selectivity was reversed and the major
isomer formed was the C2 (axial) epimer of 9.11 Removal of
the benzyl ether under carefully controlled conditions afforded
the alcohol 10, which was silylated to provide the crystalline
bis-TBS ether 11. A single-crystal X-ray structure12 analysis of
D
D
0.05, MeOH)}, thus confirming the originally assigned
structure.6
The selectivity in the Mitsunobu coupling was lower than
that observed when the C2‴ OMe group is axial.11 This
suggested an SN1-type mechanism rather than an SN2 inversion
process. In the present case, if the reaction proceeds by an SN1
B
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Journal of Natural Products
Note
1
Table 1. H NMR (600 MHz, CDCl3) and 13C NMR Data
(150 MHz, CDCl3) for Synthetic and Natural6 2‴,5‴-
Diepisilvestrol (4) Run at 8.6 mM.17
1H NMR data: δ, mult. (J in Hz)
13C NMR data: δ
position
synthetic
natural6
synthetic natural6
1
2
3
5.04 dd (6.8, 1.5)
5.04 d (7.2)
79.8
50.5
79.7
50.2
3.89 dd (14.0, 6.8) 3.90 dd (14.4, 6.6)
a
a
a
4.27 d (14.2)
4.28 d (14.4)
55.27
102.3
160.5
93.1
55.0
3a
101.9
160.6
92.9
4a
5
6.48 d (1.8)
6.40 d (1.8)
6.48 d (1.8)
6.40 d (1.8)
6
160.4
95.5
160.0
95.4
7
Figure 1. Translation assay results for compounds 2, 4, and 15.
8
157.2
109.5
93.6
157.1
109.6
93.4
8a
8b
an approach based on its proposed biogenesis. The biological
activity of this family of compounds appears to be extremely
sensitive to stereochemistry at the C1‴ and C2‴stereocenters
but not the C5‴ position of the 1,4-dioxane fragment. These
results will allow for more focused SAR studies in the future.
a
1′
126.3
129.1
113.0
159.0
136.9
127.94
127.92
126.8
92.9
126.6
129.0
112.8
158.8
136.7
127.8
127.8
126.6
92.7
2′, 6′
3′, 5′
4′
7.09 d (9.0)
6.68 d (9.0)
7.09 d (9.0)
6.69 d (9.0)
1″
2″, 6″
3″, 5″
4″
1‴
2‴
3‴α
3‴β
4‴
5‴
6‴
6.85 m
6.85 m
EXPERIMENTAL SECTION
a
■
7.05 m
7.05 m
General Experimental Procedures. 1H NMR (400, 500, 600
MHz) and 13C NMR (100, 125, and 150 MHz) spectra were obtained
in deuterochloroform with residual chloroform as internal standard
unless otherwise noted. Chemical shifts (δ) are followed by
multiplicity, coupling constant(s) (J, Hz), integration, and assignments
where possible. Optical rotations were recorded for 1 mL solutions,
and units are deg·cm2 g−1. Flash chromatography was carried out on
silica gel 60. Analytical thin-layer chromatography was conducted on
aluminum-backed 2 mm thick silica gel 60 GF254, and chromatograms
were visualized with 20% w/w phosphomolybdic acid in ethanol.
High-resolution mass spectra (HRMS) were obtained by ionizing
samples via electron spray ionization (ESI). Anhydrous THF and
CH2Cl2 were used from a solvent cartridge system. Dry methanol was
distilled from magnesium methoxide. All other solvents were purified
by standard methods. Petrol used refers to petroleum ether in the 40−
60 °C boiling range. All other commercially available reagents were
used as received. The standard workup refers to extraction with a
particular solvent (3×), washing with water and brine, drying with
MgSO4, and concentration under reduced pressure.
7.06 m
7.06 m
5.34 brs
4.62 d (1.5)
5.36 brs
4.63 d (1.2)
99.3
98.9
4.24 dd (12.0, 2.6) 4.24 dd (11.4, 1.8)
3.83 dd (10.9, 11.9) 3.82 t (11.4)
67.4
67.2
4.04 ddd
3.59 dt (10.8, 4.8) 3.58 dd (10.8, 6.0)
3.66 − 3.69 m 3.59−3.61 m
3.73 dt (11.0, 4.0) 3.74 brd (10.8)
4.05 ddd
67.5
71.3
62.8
67.4
71.2
62.7
2-CO2CH3
170.9
52.2
170.6
52.1
56.0
55.1
57.3
2-CO2CH3 3.65 s
3.65 s
3.87 s
3.72 s
3.63 s
8-OCH3
3.86 s
3.71 s
56.1
4′-OCH3
2‴-OCH3 3.62 s
55.28
57.4
a
Chemical shifts were assigned based on 2D NMR analysis and were
not observed in the reported 13C NMR spectrum of 4.6
Methyl Ketal 9. A solution of the lactol 811 (187.5 mg, 0.372
mmol) in DMF (3 mL) was added dropwise to a suspension of NaH
(87.0 mg, 3.625 mmol) in DMF (1 mL) at 0 °C. The reaction was
allowed to stir at 0 °C for 10 min, then at rt for 1 h. Diethyl ether and
saturated aqueous NaHCO3 were added, and the organic layer was
subjected to the standard workup. Purification via flash chromatog-
raphy using a gradient elution of 10% to 30% EtOAc/petrol to give the
C2 axial isomer11 (20.5 mg, 9%) followed by methyl ketal 9 (153.4 mg,
80%) as a colorless oil: [α]2D4 −31.0 (c 0.95, CH2Cl2), lit.11 [α]2D0 −28.3
(c 2.11, CH2Cl2); 1H NMR (500 MHz) δ 7.36−7.28 (m, 7H), 6.86 (d,
J = 8.7 Hz, 2H), 4.72 (ABq, J = 11.9 Hz, 2H), 4.60 (ABq, J = 4.0 Hz,
2H), 4.58 (d, J = 8.5 Hz, 1H), 4.39 (d, J = 1.7 Hz, 1H), 4.18−4.10 (m,
2H), 3.79 (s, 3H), 3.78 (dd, J = 10.9, 4.1 Hz, 1 H), 3.67 (dd, J = 11.3,
5.6 Hz, 1H) 3.66 (t, J = 11.0 Hz, 1H), 3.50 (s 3H), 3.46−3.43 (m 1H),
0.91 (s, 9H), 0.070 (s, 3H), 0.066 (s, 3H); 13C NMR (125 MHz) δ
159.3, 138.5, 129.8, 129.4, 128.4, 127.8, 127.7, 113. 8, 99.3, 93.3, 93.2,
79.5, 72.9, 68.7, 67.0, 66.3, 62.8, 56.80, 56.79, 55.2, 26.0, 18.3, −5.34,
−5.36.
pathway, the low selectivity could be attributed to the small
stereofacial difference in the nucleophilic attack of the
intermediate oxonium ion (see Scheme 4). Alternatively, an
SN2 mechanism would also give a preference for the C1‴
equatorial isomer 15 via the major axial lactol anomer of 7;
however, the lactol ratio (1.6:1) is not reflected in the
selectivity of the Mitsunobu coupling (1:2.6).18
Both compounds 4 and 15 along with synthetic episilvestrol
(2)11 were tested for inhibition of protein synthesis. In vitro
assays were performed essentially as previously described19 in
rabbit reticulocyte lysate. As shown in Figure 1, episilvestrol (2)
was a potent inhibitor of translation, while 2‴,5‴-diepisilvestrol
(4) was considerably less active and compound 15 was
essentially inactive. These results demonstrate that for potent
activity the diaxial orientation of the C1‴ and C2‴ groups on
the dioxane ring is required. This supports the initial
observation that 2‴,5‴-diepisilvestrol (4) is several orders of
magnitude less active than episilvestrol (2) in a tumor cell line
assay.6 Additionally, the C1‴ epimer of silvestrol (1) is less
active in a translation assay.9
Alcohol 10. Pd(OH)2 (20%) on C (22.4 mg, 0.032 mmol) was
added to a solution of methyl ketal 9 (75.6 mg, 0.146 mmol) in
distilled EtOH (3 mL), and the resulting mixture was stirred under a
hydrogen atmosphere for 30 min. The catalyst was removed by
filtration through Celite, and the crude product obtained after
evaporation of the filtrate was purified by flash chromatography with
20% EtOAc/petrol as eluent to give the alcohol 10 (56.2 mg, 90%) as
In conclusion, we have achieved the first total synthesis of the
low-abundant natural product 2‴,5‴-diepisilvestrol (4) using
C
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Journal of Natural Products
Note
a colorless oil: [α]D24 −41.6 (c 0.30, CH2Cl2); IR νmax (film) 3480,
quenched with saturated NH4Cl and 0.5 M (+)-sodium tartrate. The
standard workup with CH2Cl2 and purification by flash chromatog-
raphy with 40% EtOAc/petrol as eluent gave the racemic endo isomer
14 (232.1 mg, 44%, 38% over three steps) as a colorless oil. Racemic
14 was then subjected to chiral HPLC separation (5 μm Phenomenex
Lux Cellulose-1 semipreparative column: 10 × 250 mm, 70% MeCN/
H2O eluent, flow rate: 2.00 mL/min) to provide compound (−)-14
(104.4 mg, 45%) (tR = 18.03 min) as a colorless oil: [α]2D4 −51.2 (c
2929, 2857, 1614, 1515, 1463, 1361, 1302, 1250, 1217, 1172, 1120,
1
1102, 1062, 985, 898, 837, 779, 679 cm−1; H NMR (500 MHz) δ
7.30 (d, J = 8.7 Hz, 2H), 6.86 (d, J = 8.7 Hz, 2H), 4.70 (d, J = 12.1 Hz,
1H), 4.60 (d, J = 1.7 Hz, 1H), 4.57 (d, J = 12.1 Hz, 1H), 4.41 (d, J =
1.8 Hz, 1H), 4.17 (dd, J = 11.9, 2.9 Hz, 1H), 3.99 (ddd, J = 10.2, 7.4,
2.9 Hz, 1H), 3.80 (s, 3H), 3.70 (d, J = 1.7 Hz, 1H), 3,69 (ABq, J =
10.2 Hz, 1H), 3.62 (dd, J = 10.0, 5.3 Hz, 1H), 3.57−3.53 (m, 1H),
3.51 (s, 3H), 0.90 (s, 9H), 0.092 (s, 3H), 0.089 (s, 3H); 13C NMR
(125 MHz) δ 159.4, 129.9, 129.4, 113.8, 99.3, 93.4, 71.4, 68.9, 67.3,
66.6, 63.4, 56.9, 55.3, 26.0, 18.4, −5.23, −5.29; HRESIMS m/z
451.2107 (calcd for C21H36NaO7Si [M + Na]+, 451.2123).
Bis-TBS Ether 11. 2,6-Lutidine (195 μL, 1.674 mmol) and
TBSOTf (255 μL, 1.110 mmol) were added to a solution of the
alcohol 10 (118.7 mg, 0.277 mmol) in CH2Cl2 (4 mL) at 0 °C. The
reaction was then stirred at rt for 2.5 h; then saturated NaHCO3 and
ether were added. The organic layer was subjected to the standard
workup. Purification via flash chromatography followed using a
gradient elution of 10% to 15% EtOAc/petrol to give bis-TBS ether
11 (123.7 mg, 82%) as a white, crystalline solid: mp 64.5−64.9 °C;
[α]2D6 −30.7 (c 0.75, CH2Cl2); IR νmax (film) 2955, 2929, 2858, 1614,
1515, 1464, 1361, 1303, 1251, 1217, 1108, 1064, 1005, 940, 901, 833,
778, 680 cm−1; 1H NMR (500 MHz) δ 7.31 (d, J = 8.6 Hz, 2H), 6.86
(d, J = 8.6 Hz, 2H), 4.71 (d, J = 12.0 Hz, 1H), 4.57 (d, J = 12.0 Hz,
1H), 4.57 (d, J = 1.3 Hz, 1H), 4.37 (d, J = 1.7 Hz, 1H), 4.11−4.08 (m,
1H), 4.05 (dd, J = 11.7, 2.9 Hz, 1H), 3.79 (s, 3H), 3.72 (dd, J = 11.5,
10.5 Hz, 1H), 3.66 (q, J = 5.1 Hz, 1H), 3.60−3.52 (m, 2H), 3.50 (s,
3H), 0.90 (s, 9H), 0.88 (s, 0.9H), 0.08 (s, 6H), 0.060 (s, 3H), 0.058 (s,
3H); 13C NMR (125 MHz) δ 159.3, 129.9, 129.5, 113.8, 99.4, 93.2,
73.7, 68.5, 67.0, 66.7, 64.9, 56.9, 55.3, 26.1, 25.9, 18.4, 18.2, −4.3, −4.7,
−5.29, −5.31; HRESIMS m/z 565.2980 (calcd for C27H50NaO7Si2 [M
+ Na]+ 565.2987).
0.48, CH2Cl2), lit.11 [α]D23 −51.5 (c 0.86, CH2Cl2); H NMR (500
1
MHz) δ 7.47−7.35 (m, 5H), 7.11 (d, J = 8.9 Hz, 2H), 7.08−7.04 (m,
3H), 6.89−6.87 (m, 2H), 6.67 (d, J = 8.9 Hz, 2H), 6.36 (d, J = 1.9 Hz,
1H), 6.22 (d, J = 1.9 Hz, 1H), 5.09 (d, J = 2.0 Hz, 1H), 5.03 (dd, J =
6.7, 1.2 Hz, 1H), 4.32 (d, J = 14.1 Hz, 1H), 3.91 (dd, J = 14.1, 6.2 Hz,
1H), 3.86 (s, 3H), 3.70 (s, 3H), 3.66 (brs, 1H), 3.65 (s, 3H), 1.86 (s,
1H), 1.58 (brs, 1H). Further elution provided the other isomer, (+)-14
(105.0 mg, 45%) (tR = 19.45 min), as a colorless oil: [α]2D4 +55.9 (c
0.95, CH2Cl2), lit.11 [α]D23 +48.2 (c 0.21, CH2Cl2).
Phenol (−)-7. Following the reported procedure,11 20% Pd(OH)2
on C (31.8 mg, 0.0453 mmol) was added to a solution of the benzyl
ether (−)-14 (63.2 mg, 0.111 mmol) in ethanol (5.0 mL), and the
mixture was stirred vigorously under a hydrogen atmosphere for 3 h
and then filtered through Celite. After removal of solvent, the crude
product was purified by flash chromatography with 60% EtOAc/petrol
as eluent to give the phenol (−)-7 (50.7 mg, 95%) as a white solid:
11
[α]D25 −27.2 (c 0.70, CH2Cl2), lit. [α]D23 −27.0 (c 0.71, CH2Cl2); H
1
NMR (500 MHz, d6-acetone) δ 8.70 (s, 1H), 7.12 (d, J = 8.9 Hz, 2H),
7.05−6.99 (m, 3H), 6.91 (d, J = 7.4 Hz, 2H), 6.63 (d, J = 9.0 Hz, 2H),
6.17 (d, J = 1.8 Hz, 1H), 6.12 (d, J = 1.7 Hz, 1H), 4.93 (dd, J = 6.5, 2.7
Hz, 1H), 4.28 (d, J = 14.1 Hz, 1H), 4.17 (dd, J = 2.8, 0.8 Hz, 1H), 3.97
(s, 1H), 3.94 (ddd, J = 14.0, 6.5, 0.7 Hz, 1H), 3.83 (s, 3H), 3.66 (s,
3H), 3.56 (s, 3H).
2‴,5‴-Diepisilvestrol (4) and 1‴,2‴,5‴-Triepisilvestrol (15).
PPh3 (82.3 mg, 0.314 mmol) and powdered 4 Å molecular sieves
(∼350 mg) were added to a stirred solution of the lactols 6 (75.8 mg,
0.179 mmol) and phenol (−)-7 (33.4 mg, 0.070 mmol) in toluene (1.5
mL). After 20 min, DMEAD16 (79.8 mg, 0.341 mmol) was then added
at 0 °C, and the suspension was stirred at 0 °C for 2 d. The reaction
mixture was filtered through Celite, and the cake washed with toluene.
The filtrate was washed with water to remove the hydrazine byproduct,
and the organic layer was dried and concentrated. The crude residue
was subjected to column chromatography, and gradient elution with
20% to 50% EtOAc/petrol provided the bis-TBS coupled products as
an inseparable mixture of axial and equatorial isomers (33.9 mg, 0.038
mmol). The mixture of bis-TBS isomers (31.0 mg, 0.035 mmol) was
dissolved in THF (4.0 mL) and cooled to 0 °C. TBAF in 1.5 mL of
THF (37.5 mg, 0.119 mmol) was added dropwise, and the mixture was
allowed to stir at rt for 1 h before the solvent was removed in vacuo.
The crude product was subjected to flash chromatography with 100%
EtOAc on a plug of silica to give a 2.4:1 mixture of isomers 15 and 4,
which was purified using HPLC (5 μm Phenomenex Lux Cellulose-1
semipreparative column: 10 × 250 mm, 70% MeCN/H2O eluent, flow
rate: 2.00 mL/min) to provide 2‴,5‴-diepisilvestrol (4) (5.8 mg, 25%)
(tR = 9.33 min) as a white solid: [α]2D5 −62.3 (c 0.25, MeOH), lit.6
[α]2D3 −53.0 (c 0.05, MeOH); IR νmax (film) 3424, 3325, 2937, 2845,
2254, 1746, 1611, 1515, 1497, 1452, 1339, 1251, 1217, 1168, 1144,
Lactol 6. Buffer (pH 7, 0.5 mL) and DDQ (42.0 mg, 0.185 mmol)
were added to a solution of the bis-TBS ether 11 (70.0 mg, 0.129
mmol) in CH2Cl2 (4 mL), at 0 °C. The reaction mixture was stirred at
rt for 17 h, then filtered through Celite, and the filtrate was
concentrated. Purification by flash chromatography with 15% EtOAc/
petrol as eluent gave the mixture of 1.6:1 C1 ax:eq lactols 6 (46.0 mg,
84%) as a colorless oil: IR νmax (film) 3435, 2954, 2929, 2886, 2857,
1472, 1463, 1389, 1361, 1252, 1213, 1147, 1092, 1004, 987, 955, 829,
813, 774 cm−1; 1H NMR (500 MHz) δ 4.89 (dd, J = 7.0, 1.6 Hz, 1H,
minor), 4.59 (t, J = 4.9 Hz, 1H, major), 4.40 (d, J = 1.7 Hz, 1H,
minor), 4.10 (d, J = 5.2 Hz, 1H, major), 4.08−4.05 (m, 1H, minor),
4.00 (dd, J = 11.7, 2.9 Hz, 1H, major and minor), 3.87 (q, J = 11.1, 5.2
Hz, 1H, major), 3.79−3.71 (m, 2H, major and minor), 3.66−3.54 (m,
3H, major and minor), 3.522 (s, 3H, major), 3.521 (s, 3H, minor),
3.25 (d,, J = 4.7 Hz, 1H, major), 3.21 (d, J = 7.0 Hz, 1H, minor); 13C
NMR (125 MHz) δ 159.3, 129.9, 129.5, 113.8, 99.4, 93.2, 73.7, 68.5,
67.0, 66.7, 64.9, 56.9, 55.3, 26.1, 25.9, 18.4, 18.2, −4.30, −4.7, −5.3,
−5.3; HRESIMS m/z 445.2412 (calcd for C19H42NaO6Si2 [M + Na]+
445.2412).
endo-Cyclopentabenzofuran (−)-14. To a solution of 1211
(539.0 mg, 1.33 mmol) in freshly distilled CHCl3 (25.6 mL) and
trifluoroethanol (11.0 mL) was added methyl trans-cinnamate (3.062
g, 18.880 mmol). After degassing for 5 min, the mixture was irradiated
(450 W Hanovia UV lamp, Pyrex filter) at −5 °C under a nitrogen
atmosphere for 15 h. The solvent was removed in vacuo, and the
residue was purified by flash chromatography with 20% EtOAc/petrol
followed by 50% EtOAc/petrol as eluent to yield the desired
cycloadduct 13 (709.1 mg, 94%) as a bright orange foam. To a
solution of 13 (709.1 mg, 1.252 mmol) in MeOH (34 mL) was added
a NaOMe solution (0.5 M, 7.1 mL, 3.550 mmol). The reaction was
heated to reflux for 40 min and was quenched with saturated NH4Cl.
The standard workup with EtOAc provided the α-ketoesters as a
mixture of keto−enol tautomers (661.9 mg, 93%) as a brown, glassy
oil. A solution of tetramethylammonium triacetoxyborohydride (1.559
g, 5.926 mmol) and acetic acid (550 μL, 9.608 mmol) in acetonitrile
(24.0 mL) was stirred under argon for 5 min. A solution of keto−enol
tautomers (522.9 mg, 0.923 mmol) in acetonitrile (16.0 mL) was
cannulated, and the mixture was stirred at rt for 19 h. The reaction was
1
1120, 1042, 978, 912, 825, 730, 700, 664 cm−1; H NMR (600 MHz,
8.6 mM in CDCl3) δ 7.09 (d, J = 9.0 Hz, 2H), 7.06 (m, 1H), 7.05 (m,
2H), 6.85 (m, 2H), 6.68 (d, J = 9.0 Hz, 2H), 6.48 (d, J = 1.8 Hz, 1H),
6.40 (d, J = 1.8 Hz, 1H), 5.34 (brs, 1H), 5.04 (dd, J = 6.8, 1.5 Hz, 1H),
4.62 (d, J = 1.5 Hz, 1H), 4.27 (d, J = 14.2 Hz, 1H), 4.24 (dd, J = 12.0,
2.6 Hz, 1H), 4.04 (ddd, J = 10.6, 7.0, 2.7 Hz, 1H), 3.89 (dd, J = 14.0,
6.8 Hz, 1H), 3.86 (s, 3H), 3.83 (dd, J = 11.9, 10.9 Hz, 1H), 3.77 (brs,
1H), 3.73 (dt, J = 11.0, 4.0, 1H), 3.71 (s, 3H), 3.69−3.66 (m, 1H),
3.65 (s, 3H), 3.62 (s, 3H), 3.59 (dt, J = 10.8, 4.8 Hz, 1H), 2.41 (d, J =
5.8 Hz, 1H), 2.02 (t, J = 4.8 Hz, 1H), 1.88 (s, 1H); 13C NMR (150
MHz) δ 170.9, 160.5, 160.4, 159.0, 157.2, 136.9, 129.1, 127.94, 127.92,
126.8, 126.3, 113.0, 109.5, 102.0, 99.0, 95.5, 93.6, 93.1, 92.9, 79.8, 71.3,
67.5, 67.4, 62.8, 57.4, 56.1, 55.28, 55.27, 52.2, 50.5; HRESIMS m/z
677.2164 (calcd for C34H38NaO13 [M + Na]+ 677.2205).
D
dx.doi.org/10.1021/np300376f | J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Note
Further elution provided 1‴,2‴,5‴-triepisilvestrol (15) (15.0 mg,
65%) (tR = 10.38 min) as a white solid: [α]2D4 +14.3 (c 0.42, MeOH);
IR νmax (film) 3338, 2953, 2916, 2869, 2841, 1742, 1603, 1514, 1498,
1453, 1435, 1378, 1344, 1299, 1250, 1214, 1170, 1153, 1117, 1017,
(2) Hwang, B. Y.; Su, B. N.; Chai, H.-B.; Mi, Q.; Kardono, L. B. S.;
Afriastini, J. J.; Riswan, S.; Santarsiero, B. D.; Mesecar, A. D.; Wild, R.;
Fairchild, C. R.; Vite, G. D.; Rose, W. C.; Farnsworth, N. R.; Cordell,
G. A.; Pezzuto, J. M.; Swanson, S. M.; Kinghorn, A. D. J. Org. Chem.
2004, 69, 3350−3358 , 6156. Correction: Hwang, B. Y.; Su, B. N.;
Chai, H.-B.; Mi, Q.; Kardono, L. B. S.; Afriastini, J. J.; Riswan, S.;
Santarsiero, B. D.; Mesecar, A. D.; Wild, R.; Fairchild, C. R.; Vite, G.
D.; Rose, W. C.; Farnsworth, N. R.; Cordell, G. A.; Pezzuto, J. M.;
Swanson, S. M.; Kinghorn, A. D. J. Org. Chem. 2004, 69, 6156.
(3) Proksch, P.; Edrada, R.; Ebel, R.; Bohnenstengel, F. I.; Nugroho,
B. W. Curr. Org. Chem. 2001, 5, 923−938.
1
950, 899, 827, 771, 732, 700 cm−1; H NMR (500 MHz, 16.7 mM in
CDCl3) δ 7.09 (d, J = 9.0 Hz, 2H), 7.05 (m, 3H), 6.84 (m, 2H), 6.67
(d, J = 9.0 Hz, 2H), 6.38 (d, J = 1.9 Hz, 1H), 6.21 (d, J = 1.9 Hz, 1H),
5.06 (d, J = 3.4, 1H), 5.01 (dd, J = 6.7, 1.9 Hz, 1H), 4.48 (d, J = 3.4
Hz, 1H), 4.27 (d, J = 14.2 Hz, 1H), 4.24 (dd, J = 12.0, 2.6 Hz, 1H),
4.09 (dd, J = 11.9, 3.3 Hz, 1H), 3.93−3.88 (m, 1H), 3.88 (ddd, J =
14.2, 6.7, 0.7 Hz, 1H), 3.86 (s, 3H), 3.77 (dd, J = 11.9, 5.9 Hz, 1H),
3.70−3.67 (m, 5H), 3.64 (s, 3H), 3.60 (dt, J = 11.4, 3.4 Hz, 1H), 3.53
(s, 3H), 3.50 (dd, J = 11.5, 3.7 Hz, 1H), 2.62 (d, J = 5.7 Hz, 1H), 2.25
(s, 1H), 2.05 (t, J = 4.8 Hz, 1H); 13C NMR (125 MHz) δ 170.7, 160.8,
160.7, 158.9, 157.3, 136.9, 129.1, 128.0, 127.9, 126.8, 126.4, 112.9,
109.8, 102.0, 98.3, 95.5, 94.1, 93.7, 92.5, 79.7, 77.36, 72.6, 70.7, 63.5,
56.07, 56.05, 55.3, 55.1, 52.2, 50.5; HRESIMS m/z 677.2164 (calcd for
C34H38NaO13 [M + Na]+ 677.2205).
In Vitro Translation Inhibition Assays. Assays were performed
essentially as previously described.19 Capped FF/HCV/Ren mRNA
was transcribed using Sp6 RNA polymerase (Promega) from pSP/
(CAG)33/FF/HCV/Ren.pA51 linearized with BamHI. Reactions were
performed in rabbit reticulocyte lysate following the manufacturer’s
instructions (Promega) with a final concentration of 135 mM KCl and
programmed with 8 μg/mL mRNA. Luciferase activity was detected
using the Dual Glo Luciferase Assay system (Promega).
X-ray Crystallography. Intensity data for compound 11 were
collected on an Oxford Diffraction SuperNova CCD diffractometer
using Cu Kα radiation; the temperature during data collection was
maintained at 130.0(1) using an Oxford Cryostream cooling device.
The structure was solved by direct methods and difference Fourier
synthesis.20 A thermal ellipsoid plot was generated using the program
ORTEP-321 integrated within the WINGX22 suite of programs. The
absolute configuration of 11 is set by the configuration of the starting
material D-glucose and is supported by the Flack parameter, which was
0.02(3).
Crystal data for compound 11: C27H50O7Si2, M = 542.85, T =
130.0 K, λ = 1.54180, orthorhombic, space group P212121, a =
8.2450(2), b = 10.6777(3), c = 36.136(1) Å. V = 3181.4(2) Å3, Z = 4,
Dc = 1.133 Mg m−3, μ(Cu Kα) = 1.324 mm−1, F(000) = 1184, crystal
size 0.38 × 0.25 × 0.04 mm3, 10 320 reflections measured, 5849
independent reflections [R(int) = 0.0248], final R = 0.0436 [I > 2σ(I)]
and wR(F2) = 0.1208 (all data).
(4) Rizzacasa, M. A.; El Sous, M. Tetrahedron Lett. 2005, 46, 293−
295.
(5) Lucas, D. M.; Edwards, R. B.; Lozanski, G.; West, D. A.; Shin, J.
D.; Vargo, M. A.; Davis, M. E.; Rozewski, D. M.; Johnson, A. J.; Su, B.
N.; Goettl, V. M.; Heerema, N. A.; Lin, T. S.; Lehman, A.; Zhang, X.
L.; Jarjoura, D.; Newman, D. J.; Byrd, J. C.; Kinghorn, A. D.; Grever,
M. R. Blood 2009, 113, 4656−4666.
(6) Pan, L.; Kardono, L. B. S.; Riswan, S.; Chai, H.; de Blanco, E. J.
C.; Pannell, C. M.; Soejarto, D. D.; McCloud, T. G.; Newman, D. J.;
Kinghorn, A. D. J. Nat. Prod. 2010, 73, 1873−1878.
(7) Bordeleau, M. E.; Robert, F.; Gerard, B.; Lindqvist, L.; Chen, S.
M. H.; Wendel, H. G.; Brem, B.; Greger, H.; Lowe, S. W.; Porco, J. A.,
Jr.; Pelletier, J. J. Clin. Invest. 2008, 118, 1−11.
́
(8) Cencic, R.; Carrier, M.; Galicia-Vazquez, G.; Bordeleau, M.-E.;
Sukarieh, R.; Bourdeau, A.; Brem, B.; Teodoro, J. G.; Greger, H.;
Tremblay, M. L.; Porco, J. A.; Pelletier, J. PLoS ONE 2009, 4, e5223.
(9) Gerard, B.; Cencic, R.; Pelletier, J.; Porco, J. A. Angew. Chem., Int.
Ed. 2007, 46, 7831−7834.
(10) El Sous, M.; Khoo, M. L.; Holloway, G.; Owen, D.; Scammells,
P. J.; Rizzacasa, M. A. Angew. Chem., Int. Ed. 2007, 46, 7835−7838.
(11) Adams, T. E.; El Sous, M.; Hawkins, B. C.; Hirner, S.; Holloway,
G.; Khoo, M. L.; Owen, D. J.; Savage, G. P.; Scammells, P. J.;
Rizzacasa, M. A. J. Am. Chem. Soc. 2009, 131, 1607−1616.
(12) Crystallographic data have been deposited with the Cambridge
Crystallographic Centre as supplementary publication no. CCDC-
888766.
(13) (a) Gerard, B.; Jones, G.; Porco, J. A., Jr. J. Am. Chem. Soc. 2004,
126, 13620−13621. (b) Gerard, B.; Sangji, S.; O’Leary, D. J.; Porco, J.
A., Jr. J. Am. Chem. Soc. 2006, 128, 7754−7756.
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Chem., Int. Ed. 2010, 48, 6533−6538.
(15) Evans, D. A.; Chapman, K. T.; Carreira, E. M. J. Am. Chem. Soc.
1988, 110, 3560−3578.
ASSOCIATED CONTENT
* Supporting Information
(16) Sugimura, T.; Hagiya, Z. Chem. Lett. 2007, 36, 566−567.
(17) We have previously reported that the values for the chemical
shifts of H1‴ and H2‴ (CDCl3) for episilvestrol (2) and silvestrol (1)
are concentration dependent (see ref 11).
■
S
HPLC traces and copies of the NMR spectra of all new
compounds as well as the CIF for the X-ray structure of
compound 11. This material is available free of charge via the
Internet at http://pubs.acs.org.
(18) For the synthesis of O-aryl glucosides where the anomer ratio of
the product correlates with the starting pyranose anomer composition,
which suggested an SN2 process, see: Roush, W. R.; Lin, X.-F. J. Am.
Chem. Soc. 1995, 117, 2236−2250.
(19) Lindqvist, L.; Oberer, M.; Reibarkh, M.; Cencic, R.; Bordeleau,
M.-E.; Vogt, E.; Marintchev, A.; Tanaka, J.; Fagotto, F.; Altmann, M.;
Wagner, G.; Pelletier, J. PLoS ONE 2008, 3, e1583.
(20) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
(21) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
(22) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
AUTHOR INFORMATION
Corresponding Author
*Tel: +61 3 8344 2397. Fax: +61 3 9347 8396. E-mail: masr@
unimelb.edu.au.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Australian Research Council Discovery Grants
Scheme, the Cancer Therapeutics CRC, and the National
Health and Medical Research Council (Fellowships and
Program Grant) for financial support.
REFERENCES
■
(1) Meurer-Grimes, B. M.; Yu, J.; Vairo, G. L. U.S patent 6710075
B2, 2004.
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dx.doi.org/10.1021/np300376f | J. Nat. Prod. XXXX, XXX, XXX−XXX