Protecting-Group-Free Total Synthesis of 8-Methoxygoniodiol

Article abstract of DOI:10.1055/s-0035-1561491

A concise stereoselective total synthesis of the naturally occurring styryl lactone 8-methoxygoniodiol in five simple steps from readily available inexpensive trans-cinnamaldehyde is described. The efficient synthesis features a successful protecting-group-free strategy, with desirable step- and atom-economy. The synthetic strategy relies on a Maruoka asymmetric allylation, an epoxidation, a ring-closing metathesis, and a stereoselective epoxide ring opening as key steps.

Full text of DOI:10.1055/s-0035-1561491

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2016, 48, A–E
paper
A
P. Ramesh
Paper
Synthesis
Protecting-Group-Free Total Synthesis of 8-Methoxygoniodiol
Perla Ramesh*
O
O
RCM
epoxidation
regioselective
OMe
O
Natural Products Chemistry Division, Indian
Institute of Chemical Technology, Tarnaka,
Hyderabad, Telangana 500007, India
chemryams@gmail.com
epoxide ring opening
O
CHO
O
Ph
OH
8-methoxygoniodiol
Maruoka allylation
5 steps
short synthesis
46% overall yield
protecting-group-free
Received: 07.05.2016
Accepted after revision: 09.06.2016
Published online: 21.07.2016
The δ-lactone ring is a core structural motif in numer-
ous bioactive natural products and synthetic drugs display-
ing a broad range of potent biological activities.6 Because of
their unique structure and broad spectrum of biological
properties, such as antitumor, antibacterial, antifungal, in-
sect-growth inhibition, and immunosuppressive proper-
ties, natural products containing an α,β-unsaturated δ-lac-
tone moiety have attracted considerable attention from re-
searchers.7 Moreover, the unsaturated δ-lactone unit is
responsible for biological activity as a result of its inherent
ability to act as a Michael acceptor in the presence of func-
tional groups.8 A series of bioactive natural styryl lactones
exhibiting antitumor activities have been isolated from var-
ious Goniothalamus species.9 One such lactone is 8-meth-
oxygoniodiol (1; Figure 1), isolated from the stems and
leaves of Goniothalamus amuyon, a plant used in traditional
medicine for the treatment of edema and rheumatism.10,11
The compound exhibits cytotoxicity against various human
tumor cell lines.11 The relative and the absolute configura-
tion of compound 1 has been assigned on the basis of an X-
ray single-crystal analysis.
DOI: 10.1055/s-0035-1561491; Art ID: ss-2016-z0324-op
Abstract A concise stereoselective total synthesis of the naturally oc-
curring styryl lactone 8-methoxygoniodiol in five simple steps from
readily available inexpensive trans-cinnamaldehyde is described. The ef-
ficient synthesis features a successful protecting-group-free strategy,
with desirable step- and atom-economy. The synthetic strategy relies
on a Maruoka asymmetric allylation, an epoxidation, a ring-closing me-
tathesis, and a stereoselective epoxide ring opening as key steps.
Key words lactones, methoxygoniodiol, natural products, total syn-
thesis, Maruoka allylation, ring-closing metathesis
Despite many tremendous strategic and methodological
advances of recent decades,1 the art and science of natural
product total synthesis is still far from attaining maturity.
The use of protecting groups in natural product synthesis
has become routine, even on molecules of low complexity;
however, the use of protecting groups adds at least two ex-
tra steps (protection and deprotection) to the synthetic se-
quence, leading to losses of material, increased costs for ad-
ditional reagents and for waste disposal, and decreases in
the step economy of the overall process.2 Also, the atoms
corresponding to the protecting group are not found in the
final product, which is an unfavorable result in terms of
atom economy.3 Nobel laureate R. H. Grubbs stated that,
‘The major challenges are the construction of molecules with-
out using protecting-group chemistry and the ability to put
molecules together in fast and efficient ways’.4 The total syn-
thesis of natural products from readily available starting
materials, without using any protecting groups, is a desir-
able but challenging research task, and several protecting-
group-free total syntheses of complex natural products
have recently been reported in the literature.5
O
OMe
O
OH
Figure 1 The structure of 8-methoxygoniodiol (1)
Numerous extensive synthetic efforts have been made
toward the total synthesis of this class of natural prod-
ucts,12 and there have been three reports on total syntheses
of 8-methoxygoniodiol (1; Scheme 1). These syntheses
were based on a common strategy that requires multistep
operations involving the use of protecting-group manipula-
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–E

B
P. Ramesh
Paper
Synthesis
Ph
OH
BPin
O
O
B
5 steps
1 step
O
+
17.4% overall yield
from ethyl vinyl ether
OEt
OMe
O
O
OEt
CHO
Ph
+
Carreaux et al.
Ph
OMe
COOH
OMe
2 steps
OH
O
O
1
Ph
CHO
OH
OH
HO
OH
Scheme 1 Previous syntheses of 8-methoxygoniodiol (1)
tions and that affords low overall yields. Carreaux et al.13 re-
ported the first total synthesis of 1 by using a catalytic
asymmetric hetero-Diels–Alder/allylboration sequence in-
volving three partners, starting from ethyl vinyl ether and
(R)-O-methylmandelic acid, with a 17.4% overall yield.
Yadav and co-workers14 employed δ-glucuronolactone as a
chiral source for the synthesis of 1 in twelve steps and with
an overall yield of 15.8%. Kiran et al.15 recently reported the
synthesis of 8-methoxygoniodiol (1) in 5.4% overall yield
through an (R,R-salen)Co(OAc)-catalyzed hydrolytic kinetic
resolution of anti-(2SR,3RS)-2-[methoxy(phenyl)meth-
yl]oxirane, prepared from trans-cinnamyl alcohol.
Here, we report a five-step synthesis of 8-methoxygo-
niodiol (1) by a short and efficient route, using a protecting-
group-free strategy, from commercially available inexpen-
sive trans-cinnamaldehyde.
Our retrosynthetic analysis toward 8-methoxygoniodiol
(1) is shown in Scheme 2. We envisioned that 1 might be
synthesized from goniothalamin oxide (2) by stereo- and
regioselective cleavage of the epoxide ring with methanol
as a nucleophile. Oxide 2 might be obtained by acrylation
followed by ring-closing metathesis of alcohol 3, which in
turn might be prepared by sequential Maruoka asymmetric
allylation and syn-epoxidation of trans-cinnamaldehyde.
Our synthesis commenced with the preparation of the
homoallylic alcohol 5. The lactone stereogenic center in 1
was introduced by Maruoka asymmetric allylation16 of
readily available trans-cinnamaldehyde to give the enantio-
merically enriched alcohol 5 in 94% isolated yield and >95%
ee, as determined by chiral HPLC analysis. Next, we investi-
gated the stereo- and chemoselective epoxidation of the in-
ternal double bond in alcohol 5 (Scheme 3). After a brief op-
timization of the reaction conditions, we found that syn-ep-
O
O
stereoselective
epoxide opening
O
OMe
O
O
RCM
2
OH
1
Maruoka
allylation
OH
O
O
4
3
epoxidation
Scheme 2 Retrosynthesis of 8-methoxygoniodiol (1)
TiCl4, Ti(OiPr)4
OH
(R)-BINOL, Ag2O
m-CPBA, CHCl3
Ph
Ph
O
Ph
allyl(tributyl)tin, CH2Cl2
–15 °C, 48 h, 94%
–40 °C, 12 h, 91%
4
5
O
O
acryloyl chloride
Et3N, CH2Cl2
OH
O
O
O
O
+
O
0 °C, 2 h, 96%
Ph
Ph
6a/6b = 1:1.7
3 and 3a
6a
6b
PTSA (or) BF3•OEt2
complex
mixture
MeOH, 0 °C, 10 min
Scheme 3 Synthesis of epoxides 6 and attempts at cleavage of the epoxide ring
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–E

C
P. Ramesh
Paper
Synthesis
oxidation17 of alcohol 5 with m-chloroperoxybenzoic acid
in CHCl3 at –40 °C generated a 1.7:1 inseparable mixture of
epoxides 3 and 3a, with the desired β-epoxide 3 as the ma-
jor isomer. Attempts at Sharpless asymmetric epoxidation
of 5 to obtain 3 as a single isomer failed in this case. Subse-
quently, treatment of the mixture of epoxy alcohols 3 and
3a with acryloyl chloride in the presence of Et3N in CH2Cl2
provided the corresponding acryloyl esters 6a and 6b. Hav-
ing obtained the epoxy acrylate fragments 6 in three steps,
we proceeded to the stereo- and regioselective epoxide
opening with methanol under acidic condition. Unfortu-
nately, treatment of 6b with PTSA (or BF3·OEt2) in MeOH at
0 °C gave a complex mixture of products containing none of
the starting material.
Table 1 Screening of Catalysts for the Stereo- and Regioselective Ep-
oxide Ring Opening of (S)-2 with Methanola
Entry
Catalyst
Time (h)
Yield
drb
NDd
c
1
2
3
4
5
6
7e
8
PTSA
1
3
–
AgOTf
Sn(OTf)2
Sc(OTf)3
In (OTf)3
Eu(OTf)3
Eu(OTf)3
–
95
92
93
91
96
96
NRf
91:9
94:6
93:7
92:8
94:6
94:6
ND
5
12
5
5
12
12
a Unless otherwise specified, all reactions were performed on a 0.25 mmol
scale of (S)-2 with 1 equiv catalyst at r.t in MeOH (0.6 mL).
b Determined by 1H NMR spectroscopy of the crude mixture.
c A complex mixture of products was obtained.
d Not determined.
Next, we focused our attention on the synthesis of (S)-
goniothalamin oxide [(S)-2]. Diene 6b was subjected to
ring-closing olefin metathesis18 by using the Grubbs I cata-
lyst in refluxing CH2Cl2 to give the desired (S)-goniothala-
min oxide [(S)-2] in 92% yield (Scheme 4).
e 10 mol% catalyst was used.
f No reaction.
O
O
O
O
Grubbs I
O
O
O
O
O
catalyst
OMe
O
CH2Cl2, reflux
6 h, 92%
O
Ph
Ph
MeOH, r.t.
6b
(S)-2
Ph
Ph
2
OH
PTSA
MeOH
8-methoxygoniodiol (1)
overall yield 46%
complex
mixture
0 °C, 1 h
[α]25D +24.2 (c 0.68, CHCl3)12
[α]25D +23.9 (c 0.6, CHCl3)
Scheme 4 Efforts toward epoxide ring opening of (S)-2
Scheme 5 Stereoselective epoxide ring opening of (S)-2
With the desired (S)-goniothalamin oxide [(S)-2] in
hand, we turned our attention to the key step in the synthe-
sis of the styryl lactone natural product 1: the stereo- and
regioselective epoxide ring opening of (S)-2 with methanol
as the nucleophile. Numerous methods have been reported
in the literature for the stereo- and regioselective epoxide
opening with various alcohols and phenols.19 Initially, we
carefully explored various conditions for acid-promoted
epoxide ring opening of (S)-2 in methanol (Table 1). When
the reaction was carried out in the absence of a catalyst, no
product was obtained and the starting material was recov-
ered (Table 1, entry 8). The use of PTSA as catalyst in meth-
anol led to complete conversion of the starting material into
a complex mixture (entry 1). However, satisfactory results
were obtained with most Lewis acid catalysts, and treat-
ment of (S)-goniothalamin oxide (2) with one equivalent of
various Lewis acids in MeOH at room temperature gave the
desired product 1 in excellent yields. Pleasingly, 8-methoxy-
goniodiol (1) was obtained in high yield and with high re-
gio- and stereoselectivity when Eu(OTf)3 or Sn(OTf)2 was
used as the catalyst in methanol at room temperature (en-
tries 3, 6, and 7 and Scheme 5).
The spectral data (1H and 13C NMR) of the synthetic
product 1 were in complete agreement with those reported
for the natural product.
In summary, we have developed a short and efficient
stereoselective total synthesis of 8-methoxygoniodiol (1) in
a protecting-group-free fashion with a high overall yield.
The developed synthetic strategy is simple, extremely fast,
and requires only five steps from commercially available
trans-cinnamaldehyde. The concise and protecting-group-
free nature of the overall synthetic design should permit
the development and biological testing of structural ana-
logues of 1 in the future.
Solvents were dried over standard drying agents and were freshly dis-
tilled before use. All moisture-sensitive reactions were carried out
under N2. Organic solutions were dried over anhydrous MgSO4 and
concentrated below 40 °C in vacuo. All column chromatographic sep-
arations were performed over silica gel (60–120 mesh). 1H and 13C
NMR spectra were recorded on a Bruker AV spectrometer with CDCl3
as solvent at 300, 400, or 500 MHz (1H) or at 75, 100, or 125 MHz
(
13C), with TMS as internal standard. Optical rotations were measured
with a JASCO DIP-370 digital polarimeter at 25 °C. Mass spectra were
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–E

D
P. Ramesh
Paper
Synthesis
recorded on a Bruker Micro-TOF-Q mass spectrometer with electro-
spray ionization. High-resolution mass spectra were recorded on an
Agilent mass spectrometer using ESI-TOF.
6b
1H NMR (500 MHz, CDCl3): δ = 7.37–7.24 (m, 5 H), 6.48 (dd, J = 17.3,
1.3 Hz, 1 H), 6.18 (dd, J = 17.3, 10.3 Hz, 1 H), 5.89 (dd, J = 10.3, 1.3 Hz,
1 H), 5.85–5.77 (m, 1 H), 5.17–5.11 (m, 2 H), 4.99 (q, J = 12.8, 6.7 Hz, 1
H), 3.79 (d, J = 1.9 Hz, 1 H), 3.18 (dd, J = 5.7, 1.9 Hz, 1 H), 2.59–2.49 (m,
2 H).
13C NMR (100 MHz, CDCl3): δ = 165.3, 136.2, 132.1, 131.4, 128.5 (2 C),
128.4, 128.1, 125.5 (2 C), 118.8, 72.9, 62.5, 56.5, 36.0.
(1E,3R)-1-Phenylhexa-1,5-dien-3-ol (5)
Ti(i-PrO)4 (0.26 mL, 0.90 mmol) was added to a stirred solution of
TiCl4 (33 μL, 0.30 mmol) in CH2Cl2 (10 mL) at 0 °C under N2. After 1 h,
Ag2O (140 mg, 0.60 mmol) was added at r.t., and the mixture was
stirred in the dark for 5 h. (R)-1,1′-Binaphthalene-2,2′-diol (346 mg,
1.21 mmol) was added at r.t., and the mixture was stirred for a further
2 h. The solution containing the catalyst generated in situ was cooled
to –15 °C and treated sequentially with trans-cinnamaldehyde (0.8 g,
6.06 mmol) and allyl(tributyl)tin (2.06 mL, 6.65 mmol). The resulting
mixture was stirred for 48 h at –15 °C. The reaction was quenched
with sat. aq NaHCO3 (10 mL), and the mixture was extracted with
Et2O (3 × 10 mL). The organic layers were combined, dried (MgSO4),
concentrated under reduced pressure, and purified by flash column
chromatography [silica gel, hexane–EtOAc (9:1)] to give a colorless
liquid; yield: 1 g (94%, >95% ee).
ESI-MS: m/z = 267 [M + Na]+.
(6R)-6-[(2S,3S)-3-Phenyloxiran-2-yl]-5,6-dihydro-2H-pyran-2-one
[2; (S)-Goniothalamin Oxide]
Grubbs-I catalyst (33 mg, 0.04 mmol) was added to a solution of acry-
late 6b (100 mg, 0.41 mmol) in CH2Cl2 (7 mL) at r.t., and the mixture
was stirred for 6 h at reflux. The mixture was then concentrated and
the residue was purified by flash column chromatography [silica gel,
hexane–EtOAc (7:3)] to give a white solid; yield: 81 mg (92%); [α]D
+96.9 (c 0.7, CHCl3).
25
1H NMR (500 MHz, CDCl3): δ = 7.38–7.28 (m, 5 H), 6.93 (ddd, J = 9.7,
5.4, 2.8 Hz, 1 H), 6.07 (ddd, J = 9.7, 2.4, 1.0 Hz, 1 H), 4.68 (ddd, J = 10.6,
4.5, 3.6 Hz, 1 H), 4.08 (d, J = 1.8 Hz, 1 H), 3.24 (dd, J = 3.6, 2.1 Hz, 1 H),
2.69–2.63 (m, 1 H), 2.58–2.52 (m, 1 H).
13C NMR (125 MHz, CDCl3): δ = 162.8, 144.0, 135.9, 128.6 (3 C), 125.7
(2 C), 121.6, 75.1, 62.1, 55.0, 26.2.
1H NMR (300 MHz, CDCl3): δ = 7.36–7.15 (m, 5 H), 6.57 (d, J = 16.0 Hz,
1 H), 6.19 (dd, J = 16.0, 6.2 Hz, 1 H), 5.90–5.76 (m, 1 H), 5.20–5.07 (m,
2 H), 4.36–4.27 (m, 1 H), 2.47–2.30 (m, 2 H), 1.64 (d, J = 3.5 Hz, 1 H).
13C NMR (75 MHz, CDCl3): δ = 136.5, 133.9, 131.4, 130.2, 128.5, 127.5,
126.4, 118.3, 71.6, 41.9.
ESI-MS: m/z = 175 [M + H]+.
ESI-MS: m/z = 239 [M + Na]+.
(1R)-1-[(2R,3R)-3-Phenyloxiran-2-yl]but-3-en-1-yl Acrylate (6a)
and (1R)-1-[(2S,3S)-3-Phenyloxiran-2-yl]but-3-en-1-yl Acrylate
(6b)
(6R)-6-[(1S,2R)-1-Hydroxy-2-methoxy-2-phenylethyl]-5,6-dihy-
dro-2H-pyran-2-one (1; 8-Methoxygoniodiol)
Eu(OTf)3 (6.1 mg, 0.01 mmol) was added to a stirred solution of lac-
tone (S)-2 (20 mg, 0.09 mmol) in MeOH (0.6 mL) at r.t.. After 5 h, the
solvent was removed under vacuum, and the residue was purified by
flash column chromatography [silica gel, hexane–EtOAc (6:4)] to give
a white solid; yield: 22 mg (96%); mp 99 °C; [α]D25 +23.9 (c 0.6, CHCl3).
1H NMR (500 MHz, CDCl3): δ = 7.42–7.33 (m, 5 H), 6.97 (ddd, J = 9.7,
6.4, 2.1 Hz, 1 H), 6.04 (ddd, J = 9.7, 2.8, 0.7 Hz, 1 H), 4.95 (ddd, J = 12.8,
3.8, 1.5 Hz, 1 H), 4.38 (d, J = 8.8 Hz, 1 H), 3.61 (dd, J = 9.0, 1.5 Hz, 1 H),
3.22 (s, 3 H), 2.83 (ddt, J = 18.4, 12.8, 2.1 Hz, 1 H), 2.25 (dddd, J = 18.4,
6.2, 3.8, 0.7 Hz, 1 H).
13C NMR (100 MHz, CDCl3): δ = 164.0, 145.9, 138.5, 128.6 (2 C), 128.5,
127.8 (2 C), 120.8, 81.9, 76.1, 74.8, 56.7, 26.1.
ESI-MS: m/z = 249 [M + H]+, 271 [M + Na]+.
HRMS-ESI: m/z [M + Na]+ calcd for C14H16NaO4: 271.09408; found:
A solution of MCPBA (70%) (114 mg, 0.95 mmol) in CHCl3 (3 mL) was
added to a stirred solution of alcohol 5 (150 mg, 0.86 mmol) in CHCl3
(5 mL) at –40 °C under N2. After 12 h, the mixture was washed with
10% aq Na2CO3 (2 × 5 mL). The combined organic layers were dried
(MgSO4), concentrated under reduced pressure, and purified by flash
column chromatography [silica gel, hexane–EtOAc (9:1)] to afford an
inseparable mixture of epoxides 3 and 3a as a colorless oil; yield:
149 mg (91%).
Et3N (381 μL, 2.74 mmol) and acryloyl chloride (95 μL, 1.17 mmol)
were added to a stirred solution of 3 and 3a (149 mg, 0.78 mmol) in
CH2Cl2 (5 mL) at 0 °C, and the mixture was stirred for 2 h. The reac-
tion was quenched with H2O (1 mL), and the product was extracted
with CH2Cl2 (2 × 5 mL). The organic layer was dried (MgSO4), evapo-
rated under reduced pressure, and purified by column chromatogra-
phy (silica gel, hexane) to afford acryl esters 6a and 6b in a 1:1.7 ratio
as colorless oil; yield: 184 mg (96%).
271.09363.
6a
Acknowledgment
1H NMR (500 MHz, CDCl3): δ = 7.39–7.24 (m, 5 H), 6.43 (dd, J = 17.2,
1.3 Hz, 1 H), 6.12 (dd, J = 17.2, 10.3 Hz, 1 H), 5.90–5.80 (m, 2 H), 5.19–
5.12 (m, 2 H), 4.99 (dt, J = 7.3, 5.1 Hz, 1 H), 3.94 (d, J = 1.9 Hz, 1 H),
3.07 (dd, J = 5.4, 1.9 Hz, 1 H), 2.63–2.57 (m, 1 H), 2.56–2.50 (m, 1 H).
The author thanks the Council of Scientific and Industrial Research,
Ministry of Science and Technology, New Delhi, for funding the proj-
ect ORIGIN (CSC-0108).
13C NMR (100 MHz, CDCl3): δ = 165.2, 136.5, 132.4, 131.3, 128.4 (2 C),
128.2, 128.0, 125.5 (2 C), 118.5, 72.0, 61.8, 57.2, 35.7.
Supporting Information
ESI-MS: m/z = 267 [M + Na]+.
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1561491.
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© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–E

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