Total Synthesis of Gramistilbenoids A, B, and C

Article abstract of DOI:10.1021/acs.jnatprod.7b00865

Stilbenes are biologically active metabolites of plants that have the potential to attenuate a broad range of human diseases. Gramistilbenoids are a class of natural products with a stilbene skeleton, isolated from the bamboo orchid (Arundina graminifolia), and with significant cytotoxicity against cancer cell lines (NB4, A549, SHSY5Y, PC3, and MCF7). These are the first identified naturally occurring diphenylethylenes to possess a hydroxyethyl unit. However, some of these compounds are not abundant in nature, and thus, their synthesis is advantageous. This paper reports the first synthesis of gramistilbenoids A (1), B (2), and C (3), with overall yields of 10, 2, and 8% respectively. These natural products were synthesized using key reactions, such as Horner-Wadsworth-Emmons olefination, Stille coupling, and hydroboration-oxidation.

Full text of DOI:10.1021/acs.jnatprod.7b00865

Article
pubs.acs.org/jnp
Cite This: J. Nat. Prod. XXXX, XXX, XXX−XXX
Total Synthesis of Gramistilbenoids A, B, and C
Dipesh S. Harmalkar, Qili Lu, and Kyeong Lee*
College of Pharmacy, Dongguk University-Seoul, Goyang, 10326, Republic of Korea
S
* Supporting Information
ABSTRACT: Stilbenes are biologically active metabolites of plants that have the potential to attenuate a broad range of human
diseases. Gramistilbenoids are a class of natural products with a stilbene skeleton, isolated from the bamboo orchid (Arundina
graminifolia), and with significant cytotoxicity against cancer cell lines (NB4, A549, SHSY5Y, PC3, and MCF7). These are the
first identified naturally occurring diphenylethylenes to possess a hydroxyethyl unit. However, some of these compounds are not
abundant in nature, and thus, their synthesis is advantageous. This paper reports the first synthesis of gramistilbenoids A (1), B
(2), and C (3), with overall yields of 10, 2, and 8% respectively. These natural products were synthesized using key reactions,
such as Horner−Wadsworth−Emmons olefination, Stille coupling, and hydroboration−oxidation.
atural products have always been an important source of
pharmacological compounds. Plant-derived natural ex-
tracts are a powerful source of drugs for treating human
diseases and are the main source of medicines for the treatment
of several illnesses. Consequently, natural products continue to
attract the attention of medicinal chemists because of their
fascinating structural diversity and complexity.
stomach, colon, pancreas, and ovarian and cervical carcinomas.
The molecule also provides some antiaging health benefits,
including improved metabolism and cardioprotection.5−11
Glycosylated stilbenes from the Chinese herb, Polygonum
multiflorum, show diverse biological activities, including
antitumor, antioxidant, hair growth promotion, antiaggregation,
antiatherosclerosis, and anti-inflammatory effects.12a−f
N
Natural stilbenes are phenolic compounds characterized by
the presence of a 1,2-diphenylethylene backbone. Because of
their broad spectrum of biological functions, particularly their
cancer chemoprevention activity, these compounds have
received considerable attention and have been extensively
reviewed.1a Stilbenes occur in many plant species including
peanut (Arachis hypogaea), wine grape (Vitis vinifera), sorghum
(Sorghum bicolor), and many tree species (Pinus and Picea).1b
Numerous stilbene analogues show considerable structural
diversity and may occur as diphenylethylenes, bibenzyls,
phenanthrenes, 9,10-dihydrophenanthrene derivatives, and
some phenolic compounds.2 Such analogues are known to
display a wide range of fascinating biological activities such as
antioxidant,3a antiviral,3b antimicrobial,3c anti-inflammatory,4a
anti-HIV,4b and anticarcinogenic.4c,d
Piceatannol (3,4′,3′,5-trans-trihydroxystilbene), the lesser-
known congener of resveratrol, also exhibits potential
anticancer properties. It has the ability to hinder the growth
of various tumors and possesses antiparasitic, antibacterial, and
antiproliferative properties.13a Piceatannol and the natural
analogues of trans-resveratrol have been shown to be potent
inhibitors of cytochrome-P4501A2 (CYP1A2) (7-ethoxyresor-
ufin-O-dealkylation assay).13b Pinosylvin (3,5-dihydroxy-trans-
stilbene, Figure 1) a natural stilbenoid, is a resveratrol analogue
extracted from Pinus species that exerts cancer chemo-
preventive effects, and inhibits oxidative stress and inflamma-
tion.14a It also exhibits antimicrobial activity, showing a
relatively high activity range against Saccharomyces cerevisiae
and Candida albicans.14b Pawhuskin A, a geranylated stilbene
displays in vitro opioid receptor affinity. Its beneficial properties
have inspired the search for new and more effective
analogues.15
Resveratrol (3,5,4′-trihydroxy-trans-stilbene) is a potent
member of the stilbene family and is the most studied natural
polyphenolic compound (Figure 1). Resveratrol exhibits
anticancer activity, indicated by its ability to suppress the
proliferation of a wide variety of tumor cells, including
lymphoid and myeloid cancers, cancers of the breast, prostate,
Received: October 16, 2017
© XXXX American Chemical Society and
American Society of Pharmacognosy
A
DOI: 10.1021/acs.jnatprod.7b00865
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Figure 1. Biologically active naturally occurring stilbene analogues.
Scheme 1. Retrosynthetic Approach to Gramistilbenoids
Arundina graminifolia (Orchidaceae), known as the bamboo
orchid, is a terrestrial multi perennial orchid. It is widely
distributed in Southeast Asia, from the Himalayas to western
Indonesia. The presence of stilbenoids, bibenzyls, phenan-
B
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a
Scheme 2. Synthesis of Gramistilbenoid A (1)
a
Reagents and conditions: (a) P(OEt)3, TBAI, 130 °C, 6 h; 90%; (b) 4-Bromo-3,5-dimethoxy- benzaldehyde, NaH, THF, 12 h, 66%; (c) 2.5 M n-
BuLi in hexanes, 2.5 M ethylene epoxide in THF, THF, −78 °C, 33%; (d) Tributyl(vinyl)tin, CsF, Pd(t-Bu3P)2, toluene, 110 °C, 12 h, 88%; (e) 0.5
M 9-BBN in THF, H2O2, 2.0 M NaOH, THF, 24 h, 72%; (f) BBr3, CH2Cl2, −40 °C to rt, 12 h, 64%; (g) 2.0 equiv BBr3, CH2Cl2, −40 °C to rt, 2 h,
58%.
threnes, and other phenolic compounds has been reported in A.
graminifolia.16−21 The entire plant is used in traditional Dai
medicine for the treatment of food poisoning and blood stasis
and as a liver detoxifying agent. The phenolic compounds
gramistilbenoids A, B, and C were first isolated from A.
graminifolia and showed cytotoxicity against several cancer cell
lines (NB4, A549, SHSY5Y, PC3, and MCF7) with IC50 values
ranging from 1.8 to 8.7 μM.22 Structurally, they differ in the
degree of O-methylation. To date, the simple structure and
important bioactivity of stilbenes have generated significant
interest in the total synthesis of such compounds. Herein, we
report the first total synthesis of gramistilbenoids A (1), B (2),
and C (3).
group was selected as the hydroxy protecting group in the
initial step.
Synthesis of Gramistilbenoid A (1). Based on the
retrosynthetic analysis, the synthesis commenced from the
readily available 3,5-dimethoxybenzyl bromide (4), which
underwent Arbuzov reaction when heated with triethyl
phosphite and tetrabutylammonium iodide (TBAI), to give
the phosphonate 5 (Scheme 2). The resulting phosphonate was
susceptible to the HWE olefination reaction with 4-bromo-3,5
dimethoxybenzaldehyde in the presence of sodium hydride
(NaH) to afford the (E)-stilbene 6.23 When treated with n-BuLi
and subsequently with ethylene epoxide, compound 6 afforded
8 possessing the appropriate hydroxyethyl moiety.24 However,
this reaction was not reproducible and resulted in trace
amounts of the expected compound. Thus, a vinyl group was
introduced by carbon−carbon bond formation via Stille
coupling using tributyl(vinyl)tin, a Pd catalyst, and CsF to
give compound 7.25a−c Hydroboration−oxidation of compound
7 using 1.0 M BH3 or 0.5 M 9-BBN in THF gave compound 8
with improved overall yield.26a Deprotection of the O-methyl
groups of 8 with BBr3 smoothly furnished the natural product,
gramistilbenoid A (1) (Scheme 2).26b In an effort to improve
the yield of the Stille coupling, different reaction conditions
were screened with Pd catalysts such as Pd(t-Bu3P)2 or
Pd(PPh3)4, with CsF as the fluoride source, and THF or
toluene as solvents (Table 1). The best results, providing
RESULTS AND DISCUSSION
■
Retrosynthetic Analysis. The retrosynthetic analysis for
gramistilbenoids A, B, and C is shown in Scheme 1. The
synthetic process can be divided into two parts (i.e., Stille
coupling and the Horner−Wadsworth−Emmons (HWE)
olefination reaction). It relies on an initial disconnection of
the hydroxyethyl group of I to give II. The addition of a
hydroxyethyl group was envisioned via treatment of II with n-
butyllithium (n-BuLi) and ethylene epoxide or by Stille
coupling and a subsequent hydroboration−oxidation reaction.
The selective preparation of the (E)-stilbene II, could be done
from III and IV, via the HWE olefination reaction. In the case
of gramistilbenoids B and C, the methoxymethyl (MOM)
C
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maximum yield, were achieved using toluene as a solvent with
Pd(t-Bu3P)2 and CsF.
compound 12,27 which was subsequently converted to aldehyde
14 over two steps as depicted in Scheme 3 (overall yield 15%).
Synthesis of Gramistilbenoid C (3). Gramistilbenoid C was
synthesized via an alternative route (Scheme 4). The synthesis
of gramistilbenoid C was attempted via aldehyde 17 because it
was more convenient to synthesize than 14. Accordingly, HWE
olefination using aldehyde 17 and phosphonate ester 5
selectively afforded (E)-stilbene 18, which on Stille coupling
gave the target 19.
Table 1. Reaction Conditions Screened for Stille Coupling
Furthermore, 19 was subjected to hydroboration−oxidation
using 0.5 M 9-BBN in THF. However, there was no conversion
even with prolonged stirring (48 h at rt, followed by 12 h at 55
°C), mainly because of the bulkiness of 9-BBN and steric
hindrance from the MOM groups. Thus, 1.0 M BH3 in THF
was employed, and the reaction mixture was stirred at 55 °C.
Under these conditions, the reaction was completed after 12 h.
The isolation and subsequent purification of the product were
quite difficult; however, column chromatography separation
using 0−2% MeOH in CH2Cl2 afforded 20 as a thick colorless
oil. After confirming the formation of 20 by 1H NMR
spectroscopy, deprotection of the MOM-groups under acidic
conditions afforded gramistilbenoid C (3).30 The NMR spectra
of gramistilbenoid C from Schemes 2 and 4 were identical.
Synthesis of Gramistilbenoid B (2). With aldehyde 14
available, the required phosphonate ester 23 was synthesized
from the commercially available 3,5-dimethoxybenzyl bromide
(4) in three steps (Scheme 5). Demethylation of 4 using 1.0 M
BBr3 in CH2Cl2 generated the phenolic compound 21, which
was protected with the MOM group to give compound 22. An
Arbuzov reaction on 22 afforded the required phosphonate
ester 23 (overall yield 34%).
entry
solvent
catalyst
additive
yield
temp °C
1
2
3
THF
THF
Pd(PPh3)4
Pd(t-Bu3P)2
Pd(t-Bu3P)2

25%
69%
88%
66
66
CsF
CsF
toluene
110
Gramistilbenoid C (3) possessing two phenolic groups was
obtained by demethylation of 8 using BBr3 (Scheme 2). The
selective deprotection of the methoxy groups on the ring
bearing a hydroxyethyl unit was confirmed by the heteronuclear
multiple bond correlation (HMBC) spectra (Figure S-32,
Supporting Information).
Synthesis of MOM-Protected Aldehydes (14 and 17). For
the synthesis of gramistilbenoids B and C, MOM-protected
aldehydes 14 and 17 were prepared as shown in Scheme 3. The
commercially available 4-bromo-3,5-dihydroxybenzoic acid (9)
was esterified using thionyl chloride and MeOH, to give methyl
4-bromo-3,5-dihydroxybenzoate (10).27 Protection of the
phenolic groups using MOMCl in the presence of N,N-
diisopropylethylamine (DIPEA), gave mono- and diprotected
esters 11 and 15, respectively, with compound 15 more
abundant than 11.28
With aldehyde 14 and MOM-protected phosphonate ester
23 in hand, gramistilbenoid B was synthesized similar to
gramistilbenoid C. An HWE olefination reaction between 14
and 23, afforded (E)-stilbene 24. The Stille coupling gave vinyl
stilbene 25 that was subjected to hydroboration−oxidation to
yield compound 26. MOM deprotection of 26 with 4.0 M HCl
Reduction of di-MOM-protected ester 15 with lithium
aluminum hydride (LAH) afforded the benzyl alcohol 16,28
which was oxidized to give aldehyde 17 using pyridinium
chlorochromate (PCC) (overall yield 65%).29 Additionally,
methylation of compound 11 with MeI and K2CO3 gave
a
Scheme 3. Synthesis of MOM-Protected Aldehydes (14 and 17)
a
Reagents and conditions: (a) SOCl2, MeOH, 4 h, reflux, 95%; (b) MOMCl, DIPEA, CH2Cl2, 0 °C to rt, 36% for 11, 86% for 15; (c) MeI, K2CO3,
DMF, 12 h, 95%; (d) 2.0 M LAH in THF, THF, −10 °C, 5 min, 63% for 13, 93% for 16; (e) PCC, Celite, CH2Cl2, 12 h, 69% for 14 and 83% for 17.
D
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a
Scheme 4. Synthesis of Gramistilbenoid C (3)
a
Reagents and conditions: (a) NaH, THF, 12 h, 54%; (b) Tributyl(vinyl)tin, CsF, Pd(t-Bu3P)2, toluene, 110 °C, 12 h, 82%; (c) 1.0 M BH3 in THF,
H2O2, 2.0 M NaOH, THF, 55 °C, 12 h, 73%; (d) 4.0 M HCl in dioxane, MeOH, 2 h, 60 °C, 38%.
a
Scheme 5. Synthesis of MOM-Protected Phosphonate Ester (23)
a
Reagents and Conditions: (a) 1.0 M BBr3, CH2Cl2, 0 °C to rt, 6 h, 70%; (b) MOMCl, DIPEA, CH2Cl2, 0 °C to rt, 12 h, 49%; (c) P(OEt)3 TBAI,
130 °C, 6 h 88%.
a
Scheme 6. Synthesis of Gramistilbenoid B (2)
a
Reagents and conditions: (a) NaH, THF, 12 h, 53%; (b) Tributyl(vinyl)tin, CsF, Pd(t-Bu3P)2, toluene, 110 °C, 12 h, 89%; (c) 1.0 M BH3 in THF,
H2O2, 2.0 M NaOH, THF, 12 h, 55 °C, 75%; (d) 4.0 M HCl in dioxane, MeOH, 3 h, 60 °C, 38%.
in dioxane afforded the natural product gramistilbenoid B (2)
(Scheme 6).30
EXPERIMENTAL SECTION
■
General Experimental Procedures. All the commercial chem-
icals were of reagent grade and were used without further purification.
Reactions were conducted under an atmosphere of dried argon in
flame-dried glassware. Melting points were measured on Thermo
Scientific-9200 apparatus. Infrared (IR) spectra were recorded on an
FT-IR Nicolet iS5 spectrometer (ThermoFisher Scientific, Madison,
In conclusion, the first total synthesis of the natural products,
gramistilbenoids A, B, and C were completed. The envisioned
synthetic route was achieved using commercially available
starting materials by applying HWE olefination, Stille coupling,
and hydroboration−oxidation reactions as the key steps.
Gramistilbenoids displayed cytotoxicity against several cancer
cell lines. Furthermore, the synthetic methodology is well
suited to the development of new structural analogues.
1
WI, U.S.A.). H NMR spectra were determined on a Varian (400 or
600 MHz) spectrometer (Varian Medical Systems, Inc., Palo Alto, CA,
1
U.S.A.). The H NMR data are reported as peak multiplicities: s for
singlet, d for doublet, dd for doublet of doublets, t for triplet, q for
quartet, br for broad singlet, and m for multiplet.13C NMR spectra
E
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were recorded on a Varian (100 MHz) spectrometer. The values of the
chemical shifts are expressed in δ values (ppm), and the coupling
constants (J) are reported in Hertz (Hz). Mass spectra were recorded
using high-resolution mass spectrometry (HRMS, ESI-MS), obtained
on a G2 QTOF mass spectrometer (Waters Corp, Milford, U.S.A.).
Products were purified by column or flash chromatography (Biotage,
Sweden) using silica gel 60 (230−400 mesh Kieselgel 60). TLC on
0.25 mm silica plates (E. Merck; silica gel 60 F254) was used to
monitor the reactions. Spots were detected by viewing under UV light
and colorized with charring after dipping in anisaldehyde or basic
KMnO4 solution.
(E)-2-Bromo-5-(3,5-dimethoxystyryl)-1,3-dimethoxybenzene (6).
Triethyl phosphite (5.76 g, 34.0 mmol) was added to 3,5-
dimethoxybenzyl bromide (4) (5.00 g, 21.0 mmol) containing a
catalytic amount of tetrabutylammonium iodide (0.79 g, 2.10 mmol),
and the reaction mixture was heated at 130 °C for 6 h. Excess triethyl
phosphite was removed by heating at 80 °C under vacuum. Diethyl
3,5-dimethoxybenzylphosphonate (5) was obtained as a colorless oil
(5.60 g, 90%). Compound 5 was used for the next step without further
purification.
Diethyl 3,5-dimethoxybenzylphosphonate (5) (5.00 g, 17.0 mmol)
was dissolved in THF (20.0 mL), and the mixture was stirred at 0 °C
under Ar. NaH (60% dispersion in mineral oil, 3.40 g, 86.0 mmol) was
slowly added. After 30 min, a solution of 4-bromo-3,5-dimethox-
ybenzaldehyde (3.82 g, 15.0 mmol) in THF (8.0 mL) was added
dropwise. The reaction mixture was stirred for 12 h at rt and
monitored by TLC. After completion of the reaction, the mixture was
cooled to 0 °C, and excess NaH was destroyed with water. The
reaction mixture was poured on ice, followed by the addition of 2.0 N
HCl until pH 6, and the product was extracted with EtOAc (3 × 30
mL). The combined organic layers were washed with brine, dried over
MgSO4, filtered, and concentrated in vacuo. The crude product was
purified by flash chromatography (silica gel, 0−5% EtOAc in hexanes),
producing 6 as a white solid (3.90 g, 66% yield): mp 161−162 °C; IR
(neat) νmax 1587.62, 1572.65, 1455.67, 1426.66, 1245.10, 1150.58,
1119.69, 1056.99, 964.34, 829.58 cm−1; 1H NMR (CDCl3, 400 MHz)
δ 7.05 (d, J = 16.4 Hz, 1H), 7.00 (d, J = 16.0 Hz, 1H), 6.71 (s, 2H),
6.68 (d, J = 2.4 Hz, 2H), 6.42 (t, J = 2.4 Hz, 1H), 3.95 (s, 6H), 3.83 (s,
6H); 13C NMR (CDCl3, 100 MHz) δ 161.1, 157.2, 138.8, 137.6,
129.5, 128.7, 104.7, 102.9, 100.5, 100.4, 56.5, 55.4; HRMS m/z
379.0606 (calcd for C18H20BrO4 [M + H]+, 379.0545).
(E)-5-(3,5-Dimethoxystyryl)-1,3-dimethoxy-2-vinylbenzene (7).
Tributyl(vinyl)tin (0.53 mL, 1.80 mmol), CsF (0.36 g, 2.40 mmol),
and Pd(t-Bu3P)2 (6.00 mg, 0.01 mmol) were added to a solution of
stilbene 6 (0.46 g, 1.20 mmol) in toluene (8.0 mL). The solution was
refluxed for 12 h and monitored by TLC. After completion of the
reaction, the solution was cooled and filtered through a silica pad to
remove a fine tan powder, which was thoroughly washed with Et2O.
The filtrate was concentrated in vacuo to give a crude product, which
was purified by flash chromatography (silica gel, 0−4% EtOAc in
hexanes), to give 7 as a light green solid (0.35 g, 88% yield): mp 120−
121 °C; IR (neat) νmax 1585.16, 1557.82, 1453.17, 1422.05, 1401.31,
1359.83, 1198.61, 1144.87, 1115.64, 1054.36, 998.73, 941.22, 819.60
cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.02 (s, 2H), 6.97 (dd, J = 17.8,
12.2 Hz, 1H), 6.69 (s, 2H), 6.67 (d, J = 2.0 Hz, 2H), 6.40 (s, 1H), 6.10
(dd, J = 18.0, 2.8 Hz, 1H), 5.44 (dd, J = 12.2, 2.6 Hz, 1H), 3.90 (s,
6H), 3.83 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ 161.0, 158.7,
139.1, 137.2, 129.3, 128.8, 127.2, 118.5, 114.8, 104.6, 102.3, 100.2,
55.7, 55.4; HRMS m/z 327.1681 (calcd for C20H23O4 [M + H]+,
327.1596).
chromatography (silica gel, 0−50% EtOAc in hexanes) yielded alcohol
8 (0.03 g, 33% yield) as a white solid.
b. Hydroboration−Oxidation. A solution of compound 7 (0.44 g,
1.34 mmol) in dry THF (10.0 mL) was cooled to 0 °C, and 0.5 M 9-
BBN in THF (13.0 mL, 6.70 mmol) was added. The reaction mixture
was kept in an ice bath for 30 min, and left at rt for 20 h. The reaction
progress was monitored by TLC. After the reaction was complete, the
solution was cooled to 0 °C, and MeOH (2.0 mL) was added
dropwise. When gas evolution had ceased, H2O (2.0 mL) was added,
followed by a mixture of 2.0 M NaOH (2.0 mL, 3.20 mmol) and H2O2
(30% (w/w) in H2O, 1.06 mL, 9.30 mmol). The ice bath was
removed, and the mixture was stirred vigorously at rt for 4 h. The
mixture was filtered to remove the precipitate and the filtrate was
diluted with EtOAc (20.0 mL). The organic layer was washed with
brine, dried over MgSO4, filtered, and concentrated in vacuo. The
resulting residue was purified by flash chromatography (silica gel, 0−
50% EtOAc in hexanes), furnishing 8 as a white solid (0.33 g, 72%
yield): mp 153−154 °C; IR (neat) νmax 3324.59, 1585.15, 1445.19,
1
1416.31, 1154.18, 1125.30, 956.46, 814.28 cm−1; H NMR (CDCl3,
400 MHz) δ 7.05 (d, J = 16.4 Hz, 1H), 6.99 (d, J = 16.0 Hz, 1H), 6.71
(s, 2H), 6.68 (d, J = 2.4 Hz, 2H), 6.40 (s, 1H), 3.88 (s, 6H), 3.84 (s,
6H), 3.77 (t, J = 6.0 Hz, 2H), 2.98 (t, J = 6.6 Hz, 2H), 1.83 (br, 1H);
13C NMR (CDCl3, 100 MHz) δ 161.0, 158.6, 139.2, 136.8, 129.5,
128.5, 115.3, 104.5, 102.2, 100.1, 62.8, 55.8, 55.4, 26.6; HRMS m/z
345.1787 (calcd for C20H25O5 [M + H]+, 345.1702).
(E)-5-(3,5-Dihydroxystyryl)-2-(2-hydroxyethyl)benzene-1,3-diol
[Gramistilbenoid A, (1)]. To a stirred solution of 8 (0.23 g, 0.67
mmol) in CH2Cl2 (5.0 mL), at −40 °C under Ar, BBr3 (0.33 mL, 3.40
mmol) was added. The temperature was gradually increased to rt over
a period of 1 h and stirring was continued until completion of the
reaction. The unreacted BBr3 was destroyed by adding ice at 0 °C. The
mixture was warmed to rt, stirred for 40 min, and concentrated in
vacuo. The residue was diluted with EtOAc (20 mL), washed with
brine, dried over MgSO4, filtered, and concentrated in vacuo.
Purification by flash chromatography (silica gel, 0−50% EtOAc in
hexanes) afforded 1 as a brown semisolid (0.12 g, 64% yield): IR
(neat) νmax 3186.61, 2916.30, 1585.57, 1429.62, 1354.25, 1291.87,
1
1141.12, 1021.56, 974.78, 816.24, 759.06 cm−1; H NMR (DMSO-d6,
400 MHz) δ 9.42 (s, 2H), 9.22 (s, 2H), 6.81 (d, J = 16.0 Hz, 1H), 6.72
(d, J = 16.0 Hz, 1H), 6.48 (s, 2H), 6.39 (d, J = 1.6 Hz, 2H), 6.14 (s,
1H), 3.77 (br, 1H), 3.47 (t, J = 8.2 Hz, 2H), 3.05 (t, J = 8.0 Hz, 2H);
13C NMR (methanol-d4, 100 MHz) δ 159.6, 157.8, 140.6, 138.3, 129.5,
129.2, 113.4, 105.6, 105.4, 103.0, 31.4, 28.8; HRMS m/z 289.1101
(calcd for C16H17O5 [M + H]+, 289.1076).
(E)-5-(3,5-Dimethoxystyryl)-2-(2-hydroxyethyl)benzene-1,3-diol
[Gramistilbenoid C, (3)]. The experimental procedure for the
synthesis of 3 was the same as for 1. The crude product was purified
by silica gel column chromatography (0−50% EtOAc in hexanes) to
yield 3 as a brown semisolid (80.0 mg, 58% yield): IR (neat) νmax
3449.26, 3380.20, 1586.62, 1425.49, 1262.60, 1197.09, 1154.60,
1128.04, 1071.38, 1035.97, 958.06, 821.73 cm−1
;
1H NMR
(methanol-d4, 400 MHz,) δ 6.94 (d, J = 16.4 Hz, 1H), 6.90 (d, J =
16.4 Hz, 1H), 6.65 (d, J = 2.4 Hz, 2H), 6.49 (s, 2H), 6.38 (t, J = 2.2
Hz, 1H), 3.79 (s, 6H), 3.46 (t, J = 8.4 Hz, 2H), 3.14 (t, J = 8.4 Hz,
2H); 13C NMR (methanol-d4, 100 MHz) δ 162.5, 157.8, 140.8, 138.3,
130.3, 129.1, 113.6, 105.8, 105.4, 100.7, 55.8, 31.4, 28.9; HRMS m/z
317.1408 (calcd for C18H21O5 [M + H]+, 317.1389).
(E)-2-Bromo-5-(3,5-dimethoxystyryl)-1,3-bis(methoxymethoxy)-
benzene (18). The experimental procedure for the synthesis of 18 was
the same as for 6. The crude product was purified by flash
chromatography (silica gel, 0−10% EtOAc in hexanes), yielding 18
as a white solid (0.26 g, 54% yield): mp 77−78 °C; IR (neat) νmax
1583.93, 1452.59, 1398.16, 1332.49, 1240.49, 1140.67, 1108.7,
1072.41, 1012.79, 918.61 cm−1; 1H NMR (CDCl3, 400 MHz) δ
7.03 (d, J = 16.0 Hz, 1H), 6.99 (s, 2H), 6.98 (d, J = 16.4 Hz, 1H), 6.66
(d, J = 2.4 Hz, 2H), 6.41 (t, J = 2.2 Hz, 1H), 5.30 (s, 4H), 3.82 (s,
6H), 3.55 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ 160.9, 155.1,
138.8, 137.6, 129.7, 128.3, 170.5, 104.5, 102.9, 100.5, 95.1, 56.4, 55.3;
(E)-2-[4-(3,5-Dimethoxystyryl)-2,6-dimethoxyphenyl]ethanol (8).
a. Lithium−Halogen Exchange. To a stirred solution of bromos-
tilbene 6 (0.10 g, 0.26 mmol) in THF (8.0 mL) at −78 °C, 2.5 M n-
BuLi in hexanes (0.21 mL, 0.54 mmol) was added dropwise. After
stirring for 30 min, ethylene epoxide (0.73 mL, 1.82 mmol) was added
dropwise and stirred for 1 h at 0 °C. The reaction mixture was
carefully quenched with NH4Cl (sat. aq., 10 mL), followed by
extraction with EtOAc (3 × 10 mL). The combined organic layers
were washed with brine, dried over anhydrous MgSO4, filtered, and
concentrated in vacuo. Purification of the residue by flash
+
HRMS m/z 439.3065 (calcd for C20H24BrO6 [M + H] , 439.0756).
F
DOI: 10.1021/acs.jnatprod.7b00865
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Article
(E)-5-(3,5-Dimethoxystyryl)-1,3-bis(methoxymethoxy)-2-vinyl-
benzene (19). The experimental procedure for the synthesis of 19 was
the same as for 7. The crude product was purified by flash
chromatography (silica gel, 0−10% EtOAc in hexanes) to yield 19
as a white solid (0.06 g, 82% yield): mp 74−75 °C; IR (neat) νmax
1591.74, 1459.15, 1424.39, 1390.44, 1345.16, 1318.48, 1269.98,
1193.98, 1148.71, 1105.05, 1084.84, 1055.74, 1029.87, 957.11,
3H), 3.52 (s, 3H), 3.50 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ
158.7, 158.5, 156.2, 139.3, 137.2, 129.2, 128.6, 127.2, 118.7, 115.6,
107.7, 106.0, 104.6, 102.9, 94.7, 94.4, 56.2, 56.1, 55.7; HRMS m/z
417.4232 (calcd for C23H29O7 [M + H]+, 417.1913).
(E)-5-[3-Hydroxy-4-(2-hydroxyethyl)-5-methoxystyryl]benzene-
1,3-diol [Gramistilbenoid B, (2)]. A solution of compound 25 (0.08 g,
1.34 mmol) in dry THF (10.0 mL) was cooled to 0 °C and 1.0 M BH3
in THF (13.0 mL, 6.70 mmol) was added. The reaction mixture was
left in an ice bath for 30 min and then heated at 55 °C for 12 h, after
which it was cooled to 0 °C and MeOH (2.0 mL) was added dropwise.
When gas evolution had ceased, H2O (2.0 mL) was added, followed by
a mixture of 2.0 M NaOH (2.0 mL, 3.20 mmol) and H2O2 (30% (w/
w) in H2O, 1.06 mL, 9.30 mmol). The ice bath was removed and the
mixture was stirred vigorously at rt for another 5 h. The mixture was
filtered to remove the precipitate, and the filtrate was diluted with
EtOAc (20.0 mL). The organic layer was washed with brine, dried
over MgSO4, filtered, and concentrated in vacuo. The residue was
purified by silica gel column chromatography (0−2% MeOH in
CH2Cl2), which yielded 26 as a colorless oil (63.0 mg, 75%). The oil
obtained was used for the final step without further purification.
HCl (4.0 M in dioxane, 0.16 mL, 0.65 mmol) was added to a stirred
solution of the intermediate 26 (47.0 mg, 0.11 mmol) in MeOH (6.0
mL). Stirring was continued at 60 °C for 3 h. After completion of the
reaction, the solution was cooled, saturated NaHCO3 solution (5.0
mL) was added, and the MeOH was evaporated. After extracting the
reaction mixture with EtOAc (3 × 5 mL) the combined organic layers
were dried over MgSO4, filtered, and concentrated in vacuo. The crude
product was purified by silica gel column chromatography (0−1%
MeOH in CH2Cl2) to obtain 2 as a brown semisolid (7.00 mg, 38%):
IR (neat) νmax 3287.39, 2914.07, 1585.06, 1450.66, 1146.41, 1090.41,
1000.81, 954.15, 830.95 cm−1; 1H NMR (DMSO-d6, 600 MHz) δ 9.32
(s, 1H), 9.24 (s, 2H), 6.91 (d, J = 16.2 Hz, 1H), 6.88 (d, J = 16.2 Hz,
1H), 6.68 (s, 1H), 6.59 (s, 1H), 6.41 (d, J = 1.8 Hz, 2H), 6.15 (t, J =
2.1 Hz, 1H), 4.67 (t, J = 5.1 Hz, 1H), 3.78 (s, 3H), 3.41−3.37 (m,
2H), 2.72 (t, J = 7.8 Hz, 2H); 13C NMR (methanol-d4, 100 MHz) δ
160.4, 159.8, 157.7, 140.8, 138.1, 129.8, 129.3, 142.3, 107.5, 106.0,
103.1, 101.7, 62.5, 56.1, 27.7; HRMS m/z 303.1234 (calcd for
C17H19O5 [M + H]+, 303.1232).
1
914.26 cm−1; H NMR (CDCl3, 400 MHz) δ 7.01 (s, 2H), 6.97 (s,
2H), 6.96 (d, J = 18.0 Hz, 1H), 6.67 (d, J = 2.4 Hz, 2H), 6.40 (t, J =
2.4 Hz, 1H), 6.11 (dd, J = 18.0, 2.8 Hz, 1H), 5.47 (dd, J = 12.0, 2.8 Hz,
1H), 5.27 (s, 4H), 3.83 (s, 6H), 3.52 (s, 6H); 13C NMR (CDCl3, 100
MHz) δ 160.9, 156.3, 139.1, 137.4, 129.1, 128.9, 127.3, 118.9, 116.5,
106.7, 104.5, 100.4, 94.7, 56.2, 55.4; HRMS m/z 387.1815 (calcd for
C22H27O6 [M + H]+ 387.1808).
(E)-5-(3,5-Dimethoxystyryl)-2-(2-hydroxyethyl)benzene-1,3-diol
[Gramistilbenoid C, (3)]. A solution of compound 19 (0.05 g, 0.12
mmol) in dry THF (5.0 mL) was cooled to 0 °C and 1.0 M BH3 in
THF (0.13 mL, 0.13 mmol) was added. The reaction mixture was left
in an ice bath for 30 min, and then heated to 55 °C for 12 h, after
which it was cooled to 0 °C and MeOH (0.5 mL) was added dropwise.
When gas evolution had ceased, H2O (0.5 mL) was added, followed by
a mixture of 2.0 M NaOH (0.07 mL, 0.15 mmol) and H2O2 (30% (w/
w) in H2O, 0.09 mL, 0.86 mmol). The ice bath was removed, and the
mixture was stirred vigorously at rt for 5 h. The mixture was filtered to
remove the precipitate, and the filtrate was diluted with EtOAc (15.0
mL). The organic layer was washed with brine, dried over MgSO4,
filtered, and concentrated in vacuo. The obtained residue was purified
by silica gel column chromatography (0−2% MeOH in CH2Cl2) to
give 20 as a colorless oil (37.0 mg, 73%). The oil was used for the final
step without further purification.
HCl (4.0 M in dioxane, 0.09 mL, 0.36 mmol) was added to a stirred
solution of the intermediate 20 (37.0 mg, 0.11 mmol) in MeOH (6.0
mL). Stirring was continued at 60 °C for 2 h. After completion of the
reaction, the solution was cooled, saturated NaHCO3 solution (5.0
mL) was added, and the MeOH was evaporated. After extracting the
reaction mixture with EtOAc (3 × 5 mL), the combined organic layers
were dried over MgSO4, filtered, and concentrated in vacuo. The crude
product was purified by silica gel column chromatography (0−50%
EtOAc in hexanes), affording gramistilbenoid C (3) as a brown
semisolid (11.0 mg, 38% yield).
ASSOCIATED CONTENT
■
(E)-5-[3,5-Bis(methoxymethoxy)styryl]-2-bromo-1-methoxy-3-
(methoxymethoxy)benzene (24). Triethyl phosphite (0.54 g, 3.29
mmol) was added to compound 22 (0.60 g, 2.06 mmol) with a
catalytic amount of tetrabutylammonium iodide (76.0 mg, 0.20
mmol), and the reaction mixture was heated at 130 °C for 6 h. Excess
triethyl phosphite was removed by heating at 80 °C under vacuum,
giving 23 as a colorless oil (0.63 g, 88%). Compound 23 was used for
the next step without further purification.
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jnat-
prod.7b00865.
Experimental procedures for compounds 10−17, 21, and
22, and 1H and 13C NMR spectra for the new
compounds and new intermediates, including HMBC
spectra for synthetic gramistilbenoid C (3) (PDF)
The experimental procedure for the synthesis of 24 was the same as
for 6. The crude product was purified by flash chromatography (silica
gel, 0−10% EtOAc in hexanes) to give 24 as a white semisolid (0.15 g,
53% yield): IR (neat) νmax 1583.93, 1452.59, 1398.16, 1332.49,
1
1240.04, 1140.67, 1108.70, 1072.41, 1012.79, 918.61 cm−1; H NMR
AUTHOR INFORMATION
■
(CDCl3, 400 MHz) δ 7.04 (d, J = 16.0 Hz, 1H), 6.99 (d, J = 16.4 Hz,
1H), 6.95 (d, J = 1.6 Hz, 1H), 6.88 (d, J = 2.0 Hz, 2H), 6.75 (d, J = 1.6
Hz, 1H), 6.66 (t, J = 2.0 Hz, 1H), 5.30 (s, 2H), 5.19 (s, 4H), 3.95 (s,
3H), 3.55 (s, 3H), 3.50 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ
158.5, 157.1, 155.1, 139.0, 137.5, 129.3, 128.7, 107.8, 106.8, 104.8,
103.6, 101.7, 95.1, 94.4, 56.4, 56.1; HRMS m/z 469.3107 (calcd for
C21H26BrO7 [M + H]+, 469.0862).
Corresponding Author
*Phone: +82-31-961-5214. E-mail: kaylee@dongguk.edu.
ORCID
Dipesh S. Harmalkar: 0000-0002-0201-2916
Kyeong Lee: 0000-0002-5455-9956
(E)-5-[3,5-Bis(methoxymethoxy)styryl]-1-methoxy-3-(methoxy-
methoxy)-2-vinylbenzene (25). The experimental procedure for the
synthesis of 25 was the same as for 7. The crude product was purified
by flash chromatography (silica gel, 0−10% EtOAc in hexanes) to yield
25 as a white semisolid (0.10 g, 89% yield): IR (neat) νmax 1583.93,
1452.59, 1398.16, 1332.49, 1240.04, 1140.67, 1108.70, 1072.41,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
1012.79, 918.61 cm−1; H NMR (CDCl3, 400 MHz) δ 7.01 (s, 2H),
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government
( M S I T ) ( N o s . 2 0 1 2 M 3 A 9 C 1 0 5 3 5 3 2 a n d
2015M3A6A4065734).
6.95 (dd, J = 18.0, 5.6 Hz, 1H), 6.91 (s, 1H), 6.88 (d, J = 2.0 Hz, 2H),
6.75 (s, 1H), 6.65 (t, J = 2.2 Hz, 1H), 6.10 (dd, J = 18.0, 2.8 Hz, 1H),
5.46 (dd, J = 12.0, 2.8 Hz, 1H), 5.26 (s, 2 H), 5.19 (s, 4H), 3.91 (s,
G
DOI: 10.1021/acs.jnatprod.7b00865
J. Nat. Prod. XXXX, XXX, XXX−XXX

Journal of Natural Products
Article
(23) Li, Y. Q.; Li, Z. L.; Zhao, W. J.; Wen, R. X.; Meng, Q. W.; Zeng,
Y. Eur. J. Med. Chem. 2006, 41, 1084−1089.
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DOI: 10.1021/acs.jnatprod.7b00865
J. Nat. Prod. XXXX, XXX, XXX−XXX