Total synthesis of ent-(+)-cinanthrenol A

Article abstract of DOI:10.1038/ja.2015.114

The first total synthesis of ent-(+)-cinanthrenol A of potent estrogenic activity was achieved with 10.9% overall yield in 13 steps from commercially available materials. Our synthesis features a photo-promoted oxidative 6π-electron electrocyclization/aromatization for construction of the cyclopentaaphenanthren-17-one and Furukawa hydroxyl-directed cyclopropanation for the rare spiro2,4heptane. The brevity of this synthetic strategy would allow an expedited access to cinanthrenol A and its analogs for further biological evaluation.

Full text of DOI:10.1038/ja.2015.114

The Journal of Antibiotics (2015), 1–7
2015 Japan Antibiotics Research Association All rights reserved 0021-8820/15
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&
ORIGINAL ARTICLE
Total synthesis of ent-(+)-cinanthrenol A
Liangyu Zhu and Rongbiao Tong
The first total synthesis of ent-(+)-cinanthrenol A of potent estrogenic activity was achieved with 10.9% overall yield in 13 steps
from commercially available materials. Our synthesis features a photo-promoted oxidative 6π-electron electrocyclization/
aromatization for construction of the cyclopenta[a]phenanthren-17-one and Furukawa hydroxyl-directed cyclopropanation for the
rare spiro[2,4]heptane. The brevity of this synthetic strategy would allow an expedited access to cinanthrenol A and its analogs
for further biological evaluation.
The Journal of Antibiotics advance online publication, 11 November 2015; doi:10.1038/ja.2015.114
INTRODUCTION
strategy that revolves upon photo-promoted oxidative 6π-electron
electrocyclization/aromatization to construct B-ring of the cyclopenta
[α]phenanthren-17-one from readily available cis-stilbene-type
compound 5, leading to the first total synthesis of cinanthrenol A.
Retrosynthetically, as depicted in Scheme 1b, we proposed
Furukawa hydroxyl-directed cyclopropanation18 of 2 to install the
unusual spiro[2,4]heptane. The allylic alcohol 2 could be prepared
from ketone 3 through α-hydroxylation and Wittig olefination.
Excellent Z/E selectivity was expected to avoid the potential steric inter-
action of methyl group and H12 of cyclopenta[α]phenanthrenone’s
Cinanthrenol A1 (Figure 1) was isolated in 2014 by Nakao and
co-workers from the deep-water marine sponge Cinachyrella sp.2–4
and has shown moderate cytotoxicity against two cancer cell lines
P-388 and HeLa with IC50 4.5 and 0.4 μg ml− 1, respectively.
Remarkably, cinanthrenol A was found to show estrogenic activity:
binding strongly to estrogen receptor (ER) with IC50 value of 10 nM
and changing the gene expression level of ER responsive genes A-MYB
and SMAD3 in MCF-7 cells in a way similar to estradiol (E2).5 This
estrogenic activity was believed to arise from its aromatic steroid
structure that resembles estradiol and estrone (Figure 1). Structurally,
cinanthrenol A contains a rare spiro[2,4]heptane and a cyclopenta[α]
phenanthrene core, a common structural motif of widespread and
carcinogenic steroid-related products (for example, 11-methyl-cyclco
[α]phenanthren-17-one) found in petroleum, mineral oils, coal, lake
sediments and cooking oils.6,7 In addition, the presence of a hydroxyl
group at C2, not C3, is very rare in the families of cyclopenta[α]
phenanthrenes and steroids. The natural scarcity (3.5 mg obtained
from 6.5 kg of the deep-water marine sponge, 5.4 × 10− 5 wt% yield),
potential biological activities, and the novel structure prompted us to
undertake a synthetic study, culminating in the first total synthesis of
cinanthrenol A.
ortho-hydrogen. The cyclopenta[α]phenanthren-17-one
3 could
be constructed by photo-promoted oxidative 6π-electron
electrocyclization19 of cis-stilbene 5, which would be readily available
from the double Sonogashira coupling of aryl iodides 6 and 7 with
alkyne 8, followed by partial hydrogenation with Lindlar’s catalyst.
RESULTS AND DISCUSSION
Our synthesis began with the preparation of alkyne 9 from the aryl
iodide 6 (Scheme 2) through a high yielding three-step sequence:
triisopropylsilyl (TIPS) protection, Sonogashira coupling, and chemo-
selective desilylation of trimethylsilyl (TMS) group. Alkyne 9 was
subjected to another Sonogashira coupling with commercially available
iodoindanone (7) under identical condition, providing alkyne 10 in a
quantitative yield. After preliminary condition screenings of partial
hydrogenation of the internal diarylalkyne, we quickly identified an
optimized condition: 30 mol% Lindlar’s catalyst, 3 equivalents of
quinoline, hydrogen gas (1 atm, balloon), and MeOH (solvent) at
room temperature (RT) for 6 days, producing the desired cis-alkene 5a
in excellent yield (85%). The oxidative 6π-electron electrocyclization
proved to be challenging and extensive reaction conditions have been
investigated for a better yield on a synthetically useful scale (Table 1).
Initially, an oxidative thermal electrocyclization of 5a was examined
While many triaromatic steroids serving as useful geochemical
biomarkers in fossil fuels are usually synthesized by multiple
aromatizations of the corresponding steroids with low overall
yields,8–13 the de novo synthesis of cyclopenta[α]phenanthrenes has
been underdeveloped in the past decades. Only three different
approaches have been reported (Scheme 1a): (i) Friedel–Crafts
acylative cyclization of tricyclic dihydrophenanthrene derivatives
followed by oxidative aromatization by Coombs et al.,14,15
(ii) acid-mediated alkylative cyclization of cyclopentanone-tethered
naphthalene derivatives and oxidative aromatization by Lee and
Harvey,16 and (iii) intermolecular Diels–Alder cycloaddition of Dane’s
diene and α-bromocyclopentenone followed by dehydrogenative
under classical conditions (entries 1-3). Apparent decomposition
20
aromatization by Woski and Koreeda.17 Herein, we report a new was observed under all attempted conditions (FeCl3 at RT,
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
Correspondence: Professor R Tong, Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.
Email: rtong@ust.hk
Received 18 August 2015; revised 15 October 2015; accepted 21 October 2015

Synthesis of ent-(+)-cinanthrenol A
L Zhu and R Tong
2
O
OH
Me
H
Me
H
H
H
Me
OH
H
H
17
3
3
HO
HO
HO
16
Estradiol
Estrone
2
O
17
11
Cinanthrenol A
11
Me
17
1
C
D
IC50 (Estrogen Receptor) =10 nM
IC50 (P-388) = 4.5 μM
IC50 (HeLa) = 0.4 μM
1
10
2
2
A
B
7
3
3
6
cyclopenta[α]phenanthrene
11-Me-cyclopenta[α]
phenanthren-17-one
Figure 1 Estradiol, estrone, and cinanthrenol A. A full color version of this figure is available at The Journal of Antibiotics journal online.
Previous Synthetic Strategies for Cyclopenta[ ]phenanthren-17-one
α
Coombs (1975)
O
Ac2O
ZnCl2
CO2H
C
chloranil
D
B
50%
Ar
Friedel-Crafts
acylation
Ar
CO2Et
O
17
Harvey (1988)
O
PPA
C
D
+
Ar
or MSA
Ar
B
Ar
alkylative
cyclization
(Koreeda (1992)
O
O
Br
C
Pd/C
1) SnCl4
59%
B
Ar
D
Ar
2) DBU
dehydrogenation
Diels-Alder
Dane's diene
This Work
mixture of isomers
cycloaddition
H
H
Me
Me
OH
O
OH
HO
HO
RO
RO
Davis' hydroxylation
Wittig olefination
Furukawa
3
cyclopropanation
Cinanthrenol A (1)
2
[O]
O
O
O
I
RO
RO
7
I
Lindlar's cat
6
TMS
8
5
4
6 -Electrocyclization
π
Sonogashira
Scheme 1 (a) Previous synthetic strategies for cyclopenta[α]phenanthren-17-one and (b) retrosynthetic analysis of cinanthrenol A. A full color version of this
scheme is available at The Journal of Antibiotics journal online.
DMSO/DMF21 at 140 °C, and ortho-dichlorobenenze at 200 °C) reaction time (entry 8 and 9), solvent23 (entry 10), oxidant
without any detectable cyclization product 3a. As well-recognized, (I2 vs PIDA,24 entry 13), and additive (propene oxide) (Katz’s
photoreactions were wavelength-dependent. Therefore, a suitable condition,25–28 entry 11) allowed us to increase the overall yield to
wavelength for UV irradiation should be identified for I2-promoted be synthetically useful (50%). In addition, cyclopenta[α]phenanthren-
oxidative photocyclization.22 When 180 nm wavelength was used 17-ol derivative 5b was prepared from 5a to examine the key oxidative
(entry 4), significant decomposition was observed. Although irradia- electrocyclization because the unfavorable influence of ketone group
tion with 300 nm gave only trace amount of 3a (mainly isomerization (c.f., 5a) on the oxidative photocyclization were on record.29,30
of cis-alkene to trans-alkene); 254 nm was found to be the optimal However, no better yield (40% of 3b) was obtained from 5b under
wavelength for the desired 6π electron electrocyclization, providing various electrocyclization conditions attempted for 5a. Finally, it
cyclopenta[α]phenanthren-17-one 3a in synthetically meaningful yield should be noted that the concentration of the oxidative photocycliza-
(20%). Further optimization of the reaction conditions including tion must be within 0.001 M, because higher concentration resulted in
The Journal of Antibiotics

Synthesis of ent-(+)-cinanthrenol A
L Zhu and R Tong
3
O
a) TIPSCl, imid.
OTIPS
b) PdCl2(PPh3)2
OH
CuI, Et3N
(8)
(7, 1.0 equiv)
TMS
I
TIPSO
c) K2CO3, MeOH
90%
PdCl2(PPh3)2
CuI, Et3N
99%
I
6
O
10
9
O
O
Lindlar's cat (30 mol %)
quinoline(3.0 equiv)
Table 1
TIPSO
TIPSO
H2 (1 atm), MeOH
rt, 6 d, 85%
3a
5a
OTES
OTES
I2, P.O.
TIPSO
TIPSO
254nm
40%
5b
3b
Scheme 2 Construction of cyclopenta[α]phenanthren-17-one 3a. A full color version of this figure is available at The Journal of Antibiotics journal online.
Table 1 Selected screening conditions for oxidative 6π-electron electrocyclization
Entry
λ(hv) (nm)
Oxidant (equiv) solvent
Concentration (M)/time (h)
Conversion (%)
Yield (%)a
1
—
FeCl3, CH2Cl2
0.001/3
0.001/3
0.001/12
0.001/6
0.001/6
0.001/6
0.01/6
100
60
0
0
2
—
DMSO/DMF, 140 °C
200 °C, o-DCB
3
—
80
0
4
180
254
300
254
254
254
254
254
254
254
I2 (1.0)/air, chex
I2 (1.0)/air, chex
I2 (1.0)/air, chex
I2 (1.0)/air, chex
I2 (1.0)/air, chex
I2 (1.0)/air, chex
I2 (1.0)/air, tBuOH/benzene
I2 (1.0)/PO, chex
I2 (1.0)/PO, chex
PIDA, dioxane
100
60
o5
20
o5
o10
40
35
20
50
30
10
5
6
100
100
90
7
8
0.001/12
0.001/24
0.001/12
0.001/12
0.001/24
0.001/12
9
100
100
80
10
11
12
13
100
30
Abbreviations: chex, cyclohexane; equiv, equivalent; o-DCB, ortho-dichlorobenzene; PIDA, (diacetoxyiodo)benzene; PO, propene oxide.
aIsolated yield.
considerable intermolecular [2+2] cycloaddition and decomposition asymmetric epoxidation protocol or lipase-mediated acetylation at a
(entry 7 in Table 1).
later stage without success.33 Therefore, we chose to move forward
After several repeated operations of oxidative photocyclization to with (+)-11 (75% ee). It was noted that free alcohol of (+)-11 did not
provide sufficient amount of cyclopenta[α]phenanthren-17-one 3a, we undergo efficient Wittig olefination. Upon temporary protection as
turned our attention to construct the unusual spiro[2,4]heptane TBS ether, Wittig olefination with triphenyl phosphonium ylide
through Furukawa modification of Simons–Smith cyclopropanation generated in situ occurred efficiently to provide alkene ( − )-12 in
(Scheme 3). To this end, manipulations of ketone 3a were needed to 90% yield with excellent Z-selectivity (Z/E ⩾10:1), which was
prepare enantiomerically pure allylic alcohol 2 for hydroxyl-directed confirmed by nOe experiments. As chemoselective desilylation (TBS
cyclopropanation. Although the classical Rubottom hydroxylation31 of over TIPS) was problematic, desilylation of both TIPS and TBS groups
the corresponding silyl enol ether proceeded smoothly to produce the with TBAF and regioselective acetylation of the aromatic alcohol were
desired α-hydroxyl ketone ( )-11 in 70% yield, an enantioselective performed to provide the allylic alcohol (+)-2. It should be noted that
α-hydroxylation of ketone 3a using Davis’ (S)-oxaziridine,32 surpris- the free phenol caused significant decomposition in the subsequent
ingly, gave ( − )-11 with only 60% ee in 50% yield. The use of Davis’ cyclopropanation. The Furukawa–Simons–Smith18 cyclopropanation
(R)-oxaziridine led to the similar result: a 65% yield of with 50% ee. of (+)-2 proceeded smoothly with Et2Zn and diiodomethane to
It was found that the dichloro derivative of Davis’ (R)-oxaziridine furnish ent-(+)-cinanthrenol A (ent-1) in 78% yield after deacetylation
could deliver (+)-11 with a slight better ee value (75% ee) in 60% with K2CO3 in methanol. Remarkably, a single diastereomer was
yield. In addition, we have made considerable attempts to increase the observed in cyclopropanation reaction, in supportive of the high
enantioenrichment of 11 including kinetic resolution via Sharpless hydroxyl-directed diastereoselectivity.34 The opposite sign of optical
The Journal of Antibiotics

Synthesis of ent-(+)-cinanthrenol A
L Zhu and R Tong
4
O
O
1) TBSOTf
2) mCPBA
70%
OH
TIPSO
TIPSO
Davis' reagent
11
( )−
3a
Rubottom
11
(S)-Oxaziridine (−)− (60% ee)
(R)-Oxaziridine & (R)-Cl2-Oxaziridine (+)−11 (50-75% ee)
nOe
Z/E = 10:1
CH3
H
H
a) TBSCl, imid.
DMF, r.t., 12 h
95%
a) TBAF, THF
80%
nOe
H
TIPSO
CH3
OTBS
b) Ac2O, Et3N
90%
b) CH3CH2BrPPh3
NaHMDS, 0 oC to rt
24 h, 90%
(−)−12
Me
OH
1) Et2Zn, CH2I2
CH2Cl2, 0 oC to rt
H
OH
HO
AcO
2) K2CO3, MeOH
78% ( 2 steps)
2
(+)−
1
ent-(+)-Cinanthrenol A (ent- )
Scheme 3 Total Synthesis of ent-(+)-Cinanthrenol A. A full color version of this figure is available at The Journal of Antibiotics journal online.
rotation of synthetic ent-1 (derived from (+)-11 in 75% ee) and 5975C system (Agilent, Santa Clara, CA, USA) or an API QSTAR XL System
(Applied Biosystems, Waltham, MA, USA).
natural cinanthrenol A provided a confirmation of its absolute
configuration ([a]D = +6.3, c 0.3, MeOH; lit[a]D = − 11.6, c 0.16,
MeOH). However, the ee% value of our synthetic cinanthrenol A
(ent-1) was not clear.
Preparation of alkyne 9
To a stirred solution of 4-iodophenol 6 (2.20 g, 10 mmol) in DMF (20 ml) at
0 °C were added imidazole (1.70 g, 25.0 mmol) and triisopropylsilyl chloride
(2.67 mL, 12.5 mmol). The reaction mixture was allowed to warm to RT and
stirred overnight. The reaction was quenched by addition of aq. satd. NH4Cl
(50 ml). The organic layer was collected and the aqueous layer was extracted
with Et2O (3× 50 ml). The combined organic fractions were washed with H2O
(3 × 30 ml), brine, dried over MgSO4, and concentrated under reduced pressure
to give the crude TIPS ether S-1. A small amount of the residue was purified by
flash column chromatography on silica gel using eluents (ethyl acetate/
hexane = 1/10–1/5) to give the pure S-1 as a colorless oil for data collection.
IR (neat, cm− 1): 2945, 2892, 2867, 1581, 1485, 1388, 1273, 1168, 1003, 909,
883, 825, 726, 686. 1H NMR (400 MHz, CDCl3) δ 7.49 (d, J = 8.4 Hz, 2H), 6.65
(d, J = 8.4 Hz, 2H), 1.33–1.16 (m, 3H), 1.09 (d, J = 7.2 Hz, 18H). 13C NMR
(100 MHz, CDCl3) δ 156.0, 138.2 (2C), 122.3 (2C), 83.2, 17.9 (6C), 12.6 (3C).
HRMS (CI+): m/z calculated for C15H25IOSi [M]+ 376.0719, found 376.0712.
To a stirred solution of crude S-1 in dry THF (50 ml) at RT were added
bis(triphenylphosphine)palladium(II) dichloride (351 mg, 0.5 mmol), CuI
(195 mg, 1.0 mmol), triethylamine (4.18 ml, 30.0 mmol) and trimethylsilylace-
tylene 8 (7.0 ml, 50.0 mmol). The reaction mixture was refluxed overnight,
cooled to RT, and concentrated under reduced pressure. The residue was
purified by flash column chromatography on silica gel using eluents (ethyl
acetate/hexane = 1/10–1/5) to give the S-2 as a colorless oil. IR (neat, cm− 1):
2947, 2895, 2869, 2158, 1602, 1505, 1278, 1250, 1096, 910, 865, 840, 686.
1H NMR (400 MHz, CDCl3) δ 7.34 (d, J = 8.4 Hz, 2H), 6.79 (d, J = 8.4 Hz, 2H),
1.30–1.16 (m, 3H), 1.08 (d, J = 7.2 Hz, 18H), 0.23 (s, 9H). 13C NMR (100 MHz,
CDCl3) δ 156.5, 133.4 (2C), 119.9 (2C), 115.5, 105.3, 92.5, 17.8 (6C), 12.6
(3C), 0.0 (3C). HRMS (CI+): m/z calculated for C20H34OSi2 [M]+ 246.2148,
found 246.2155.
In summary, we have accomplished the first total synthesis of
ent-(+)-cinanthrenol A with 10.9% overall yield in 13 steps from
commercially available starting materials. Our synthesis was enabled
by some key reactions including Sonogashira coupling, oxidative
6π-electron electrocyclization, Davis’ asymmetric hydroxylation, and
Furukawa–Simons–Smith cyclopropanation. Our synthetic studies
confirmed the absolute configuration of natural cinanthrenol A and
provide an expedient access to the analogs for further biological
evaluation.
MATERIALS AND METHODS
Reactions were carried out in oven or flame-dried glassware under a nitrogen
atmosphere, unless otherwise noted. Tetrahydrofuran (THF) was freshly
distilled before use from sodium using benzophenone as indicator.
Dichloromethane was freshly distilled before use from calcium hydride
(CaH2). All other solvents were dried over 3 or 4 Å molecular sieves. Solvents
used in workup, extraction, and column chromatography were used as received
from commercial suppliers without prior purification. Reactions were
magnetically stirred and monitored by thin layer chromatography (TLC,
0.25 mm) on Merck pre-coated silica gel plates (Sigma-Aldrich, St Louis,
MO, USA). Flash chromatography was performed with silica gel 60 (particle
size 0.040–0.062 mm) supplied by Grace. IR spectra were collected on a Bruker
model TENSOR27 spectrophotometer (Bruker, Billerica, MA, USA). 1H and
13C NMR spectra were recorded on a Bruker AV-400 spectrometer (400 MHz
1
for H, 100 MHz for 13C) (Bruker). Chemical shifts are reported in p.p.m. as
values relative to the internal chloroform (7.26 p.p.m. for 1H and 77.0 p.p.m.
for 13C) or pyridine (7.55 p.p.m. (tr) for 1H and 135.5 p.p.m. (tr) for 13C).
Abbreviations for signal coupling are as follows: s, singlet; d, doublet; t, triplet;
q, quartet; m, multiplet. Optical rotations were measured on a JASCO
Perkin–Elmer model P-2000 polarimeter (JASCO, Easton, MD, USA). High-
To a stirred solution of the alkyne S-2 in MeOH (25 ml) and CH2Cl2 (25 ml)
at RT was added potassium carbonate (138 mg, 1.0 mmol). The reaction
mixture was stirred overnight at RT, filtered through a short pad of Celite. The
filtrate was concentrated under reduced pressure. The residue was purified by
resolution mass spectra were measured at the Hong Kong University of Science flash column chromatography on silica gel using eluents (EtOAc/hexane=
and Technology Mass Spectrometry Service Center on either an Agilent GC/MS 1/20) to give the alkyne 9 (2.47 g, 90% over 3 steps) as a colorless oil. IR (neat,
The Journal of Antibiotics

Synthesis of ent-(+)-cinanthrenol A
L Zhu and R Tong
5
cm− 1): 2946, 2893, 2868, 1603, 1505, 1464, 1278, 910, 882, 839, 686. 1H NMR 119.7 (2C), 76.4, 36.3, 28.6, 17.9 (6C), 12.6 (3C), 6.9 (3C), 5.0 (3C). HRMS
(CI+): m/z calculated for C30H50O2Si2 [M]+ 522.3349, found 522.3359.
(400 MHz, CDCl3) δ 7.37 (d, J = 8.4 Hz, 2H), 6.82 (d, J =8.4 Hz, 2H), 3.00
(s, 1H), 1.31–1.18 (m, 3H), 1.09 (d, J = 7.6 Hz, 18H). 13C NMR (100 MHz,
CDCl3) δ 156.7, 133.6 (2C), 119.9 (2C), 114.4, 83.8, 75.8, 17.8 (6C), 12.6 (3C).
HRMS (CI+): m/z calculated for C17H27OSi [M+H]+ 275.1831, found
275.1844.
Preparation of 3a/3b
To a round bottom quartz flask (150 ml) were added cis-stilbene 5a/5b
(0.1 mmol), cyclohexane (100 ml), I2 (25 mg, 0.1 mmol), and propene oxide
(1.16g, 1.4 ml, 20 mmol). The flask containing the reaction mixture was placed
in a Rayonet chamber reactor (Rayonet-200 model, The Southern New England
Ultraviolet Company, Branford, CT, USA) and irradiated with RPR-2537 lamps
(λ = 254 nm) at RT for 3–24 h. The reaction was quenched by addition of aq.
satd. NaHCO3 (30 ml) and aq. satd. Na2S2O3 (30 ml). The organic layer was
collected and the aqueous layer was extracted with Et2O (3× 50 ml). The
combined organic fractions were washed with brine, dried over MgSO4, and
concentrated under reduced pressure to give the crude mixture, which was
purified by flash column chromatography on silica gel. 3a was obtained in 50%
yield (entry 11). IR (neat, cm− 1): 2926, 2866, 1711, 1614, 1509, 1457, 1264,
1232, 991, 924, 882, 864, 670. 1H NMR (400 MHz, CDCl3)
δ 8.49 (d, J = 8.8 Hz, 1H), 8.11 (s, 1H), 7.88 (d, J =8.8 Hz, 1H), 7.83–7.76
(m, 2H), 7.71 (d, J = 8.8 Hz, 1H), 7.27 (d, J = 8.8 Hz, 1H), 3.41 (br s, 2H), 2.82
(br s, 2H), 1.36 (m, 3H), 1.16 (d, J = 7.2 Hz, 18H). 13C NMR (100 MHz,
CDCl3) δ 206.9, 155.4, 155.37, 134.9, 133.4, 131.7, 130.2, 129.2, 128.1, 127.9,
122.8, 122.4, 119.9, 119.4, 112.3, 36.3, 24.6, 18.0 (6C), 12.7 (3C). HRMS (CI+):
m/z calculated for C26H32O2Si [M]+ 404.2172, found 404.2176.
Preparation of diarylalkyne 10
To a stirred solution of the alkyne 9 (2.47 g, 9.0 mmol) and 4-iodoindanone 7
(1.55 g, 6.0 mmol) in dry THF (50 ml) at RT were added bis(triphenylpho-
sphine)palladium(II) dichloride (211 mg, 0.3 mmol), CuI (117 mg, 0.6 mmol)
and triethylamine (2.53 ml, 18.0 mmol). The reaction mixture was refluxed
overnight, cooled to RT, and concentrated under reduced pressure. The residue
was purified by flash column chromatography on silica gel using eluents (ethyl
acetate/hexane =1/10–1/5) to give the diarylalkyne 10 (2.41 g, 499% yield) as a
brown oil. IR (neat, cm− 1): 3039, 2945, 2892, 2867, 1719, 1597, 1509, 1463,
1276, 908, 883, 839, 778, 687. 1H NMR (400 MHz, CDCl3)
δ 7.67
(d, J = 7.6 Hz, 2H), 7.42 (d, J = 8.4 Hz, 2H), 7.33 (t, J = 7.6 Hz, 1H), 6.87
(d, J = 8.4 Hz, 2H), 3.30–3.13 (m, 2H), 2.78–2.65 (m, 2H), 1.36–1.19 (m, 3H),
1.10 (d, J = 7.6 Hz, 18H). 13C NMR (100 MHz, CDCl3) δ 206.6, 156.9, 156.7,
137.1, 136.5, 133.0 (2C), 127.4, 122.9, 122.5, 120.0 (2C), 115.0, 95.0, 84.3, 36.0,
25.5, 17.8 (6C), 12.6 (3C). HRMS (CI+): m/z calculated for C26H33O2Si
[M+H]+ 405.2250, found 405.2263.
3b was obtained in 40% yield from 5b. IR (neat, cm− 1): 2947, 2869, 1617,
1600, 1508, 1457, 1232, 1103, 879, 834, 753, 685. 1H NMR (400 MHz, CDCl3)
δ 8.46 (d, J = 8.8 Hz, 1H), 8.11 (s, 1H), 7.76 (d, J = 8.8 Hz, 1H), 7.70
(d, J = 8.8 Hz, 1H), 7.62 (d, J = 9.2 Hz, 2H), 7.19 (d, J = 8.4 Hz, 1H), 5.49
(t, J =6.4 Hz, 1H), 3.45 (m, 1H), 3.09 (dt, J = 16.0, 8.0 Hz, 1H), 2.65
(m, J = 12.0, 8.0 Hz, 1H), 2.12 (m, 1H), 1.36 (m, 3H), 1.17 (d, J = 7.6 Hz,
18H), 1.07 (t, J = 8.0 Hz, 9H), 0.75 (q, J = 8.0 Hz, 6H). 13C NMR (100 MHz,
CDCl3) δ 154.9, 143.2, 139.6, 132.2, 129.9, 129.4, 128.8, 126.7 (2C), 122.3,
121.9, 120.9, 120.7, 111.8, 77.0, 36.2, 28.4, 18.0 (6C), 12.8 (3C), 6.9 (3C),
5.1 (3C). HRMS (CI+): m/z calculated for C32H48O2Si2 [M]+ 520.3193, found
520.3198.
Preparation of cis-stilbene 5a
To a stirred solution of the diarylalkyne 10 (2.41 g, 6.0 mmol) in MeOH
(30 ml) at RT were added Lindlar’s catalyst (3.82 g, 1.8 mmol), quinoline
(2.13 ml, 18.0 mmol). The reaction mixture was flushed with hydrogen gas
three times. The reaction mixture under H2 atmosphere (1.0 atm, balloon) was
stirred at RT for 6 days. The reaction mixture was filtered through a pad of
Celite and the filtrate was concentrated under reduced pressure. The residue
was purified by flash column chromatography on silica gel using eluents (ethyl
acetate /hexane = 1/10–1/5) to give the cis-stilbene 5a (2.07 g, 85% yield) as a
brown oil. IR (neat, cm− 1): 2944, 2867, 1716, 1601, 1509, 1464, 1270, 1169,
1090, 910, 883, 782, 686. 1H NMR (400 MHz, CDCl3) δ 7.64 (d, J = 7.6 Hz,
1H), 7.49 (d, J = 7.6 Hz, 1H), 7.25 (t, J = 7.6 Hz, 1H), 7.00 (d, J = 8.4 Hz, 2H),
6.69 (m, 3H), 6.51 (d, J = 12.0 Hz, 1H), 2.93–2.83 (m, 2H), 2.63–2.60 (m, 2H),
1.27–1.16 (m, 3H), 1.07 (d, J = 7.2 Hz, 18H). 13C NMR (100 MHz, CDCl3)
δ 207.3, 155.6, 153.7, 137.3, 136.4, 134.4, 132.1, 129.8 (2C), 129.6, 127.3, 124.3,
122.3, 119.9 (2C), 36.1, 25.0, 17.8 (6C), 12.6 (3C). HRMS (CI+): m/z calculated
for C26H34O2Si [M]+ 406.2328, found 406.2328.
Preparation of α-Hydroxyl Ketone 11
α-Hydroxylation: To a stirred solution of ketone 3a (202 mg, 0.5 mmol) in
CH2Cl2 (5 ml) at 0 °C were added triethylamine (0.1 ml, 0.75 mmol) and
tert-butyldimethylsilyl trifluoromethanesulfonate (TBSOTf, 0.14 ml, 0.6 mmol).
The reaction mixture was allowed to warm to RT and stirred for 2 h. The
reaction was quenched by addition of aq. satd. NaHCO3 (10 ml). The organic
layer was collected and the aqueous layer was extracted with CH2Cl2
(3× 10 ml). The combined organic fractions were washed with brine, dried
over MgSO4, and concentrated under reduced pressure. The crude product was
dissolved in CH2Cl2 (5 ml) at 0 °C, to the resulting solution were added
NaHCO3 (84 mg, 1.0 mmol) and 3-chloroperbenzoic acid (168 mg, 0.75
mmol). The reaction mixture was allowed to warm to RT and stirred for
4 h. The reaction was quenched by addition of aq. satd. NaHCO3 (10 ml). The
organic layer was collected and the aqueous layer was extracted with CH2Cl2
(3× 10 ml). The combined organic fractions were washed with brine, dried over
MgSO4, and concentrated under reduced pressure. The residue was purified by
flash column chromatography on silica gel using eluents (ethyl acetate/
hexane= 1/6–1/3) to give the α-hydroxyl ketone 11 (147 mg, 70% yield) as a
brown solid. Asymmetric α-Hydroxylation: to a stirred solution of sodium bis
(trimethylsilyl)amide (1M in THF, 0.6 ml, 0.6 mmol) in THF (5 ml) at − 78 °C
was added the solution of ketone 3a (202 mg, 0.5 mmol) in THF (2 ml), the
reaction mixture was stirred at − 78 °C for 30 min. Then a solution of Davis’
(S)-oxaziridine (0.75 mmol) in THF (2 ml) was added via syringe, the reaction
mixture was stirred at − 78 °C for 30 min. The reaction was quenched by
addition of aq. satd. NH4I (10 ml). The organic layer was collected and the
aqueous layer was extracted with Et2O (3× 10 ml). The combined organic
fractions were washed with brine, dried over MgSO4 and concentrated under
reduced pressure. The residue was purified by flash column chromatography on
silica gel using eluents (ethyl acetate/hexane = 1/6–1/3) to give the α-hydroxyl
ketone 11 as a brown solid. (+)-11 was obtained following the same procedure
Preparation of cis-Stilbene 5b
To a stirred solution of ketone 5a (406 mg, 1.0 mmol) in MeOH (10 ml) was
added sodium borohydride (56.7 mg, 1.5 mmol) at 0 °C. The reaction was
stirred at 0 °C for 2 h. The reaction mixture was concentrated under reduced
pressure. The residue was purified by flash column chromatography on silica
gel using eluents (EtOAc/hexane =1/10–1/5) to give the secondary alcohol
product. The alcohol product was dissolved in dry DMF (5 ml), and to the
solution were added imidazole (170 mg, 2.5 mmol) and triethylsilyl chloride
(0.20 ml, 1.2 mmol) at 0 °C. The reaction mixture was allowed to warm to RT
and stirred overnight. The reaction was quenched by addition of aq. satd.
NH4Cl (10 ml). The organic layer was collected and the aqueous layer was
extracted with Et2O (3 × 20 ml). The combined organic fractions were washed
with H2O (3× 10 ml), brine, dried over MgSO4, and concentrated under
reduced pressure. The residue was purified by flash column chromatography on
silica gel using eluents (ethyl acetate/hexane= 1/20–1/15) to give the cis-stilbene
5b (571 mg, 90% over 2 steps) as a brown oil. IR (neat, cm− 1): 2948, 2869,
1603, 1508, 1462, 1267, 1106, 1013, 912, 883, 787, 744. 1H NMR (400 MHz,
CDCl3) δ 7.22 (d, J = 6.4 Hz, 1H), 7.11 (m, 2H), 7.04 (d, J = 8.4 Hz, 2H), 6.70
(d, J = 8.4 Hz, 2H), 6.51 (dd, J = 32.2, 12.0 Hz, 2H), 5.22 (t, J = 7.2 Hz, 1H),
2.82 (dd, J = 15.6, 9.0 Hz, 1H), 2.51–2.40 (m, 1H), 2.40–2.30 (m, 1H), 1.84
(dq, J = 12.4, 8.6 Hz, 1H), 1.28–1.17 (m, 3H), 1.08 (d, J = 7.2 Hz, 17H), 1.03
(t, J =8.0 Hz, 10H), 0.71 (q, J = 8.0 Hz, 6H). 13C NMR (100 MHz, CDCl3)
δ 155.2, 145.9, 141.0, 134.3, 130.3, 130.3, 129.9 (2C), 127.7, 126.6, 126.5, 122.6, using Davis’ (R)-oxaziridine or dichloride derivative of Davis’ (R)-oxaziridine.
The Journal of Antibiotics

Synthesis of ent-(+)-cinanthrenol A
L Zhu and R Tong
6
20
(+)-11 (75% ee): ½aꢀD ¼ þ7:5(c 1.0, CHCl3). IR (neat, cm− 1): 3419, 2944,
anhydride (14 μl, 0.15 mmol). The reaction mixture was stirred at RT for 2 h.
The reaction was quenched by addition of aq. satd. NaHCO3 (5 ml). The
organic layer was collected and the aqueous layer was extracted with CH2Cl2
(3 × 5 ml). The combined organic fractions were washed with brine, dried over
MgSO4, and concentrated under reduced pressure. The residue was purified by
flash column chromatography on silica gel using eluents (ethyl acetate/
hexane = 1/3–1/1) to give acetate (+)-2 (29 mg, 90%) as a colorless oil.
1
2866, 1712, 1613, 1513, 1457, 1266, 1234, 983, 882, 841. H NMR (400 MHz,
CDCl3) δ 8.57 (d, J = 8.8 Hz, 1H), 8.13 (d, J = 2.0 Hz, 1H), 7.91 (d, J = 8.4 Hz,
1H), 7.85 (m, 2H), 7.76 (d, J =8.8 Hz, 1H), 7.31 (dd, J = 8.8, 2.4 Hz, 1H), 4.72
(dd, J =7.6, 4.0 Hz, 1H), 4.03 (dd, J = 16.8, 7.6 Hz, 1H), 3.27 (dd, J = 16.8,
4.0 Hz, 1H), 1.38 (m, 3H), 1.17 (d, J = 7.2 Hz, 18H). 13C NMR (100 MHz,
CDCl3) δ 206.2, 155.6, 151.6, 134.4, 131.8, 131.7, 130.3, 129.1, 128.3, 128.3,
123.5, 122.7, 120.2, 119.4, 112.3, 74.2, 33.7, 18.0 (6C), 12.8 (3C). HRMS (CI+):
m/z calculated for C26H32O3Si [M]+ 420.2121, found 420.2122.
20
½aꢀD ¼ þ19:6(c 0.5, CHCl3). IR (neat, cm− 1): 3420, 2924, 1755, 1619, 1370,
1213, 1165, 840, 755. 1H NMR (400 MHz, C6D6) δ 8.45 (d, J = 8.4 Hz, 1H),
8.33 (s, 1H), 7.90 (d, J = 8.8 Hz, 1H), 7.82–7.64 (m, 3H), 7.33 (d, J = 8.4 Hz,
1H), 6.36 (d, J = 7.2 Hz, 1H), 5.35–5.24 (m, 1H), 3.65 (dd, J = 17.6, 6.2 Hz,
1H), 3.32 (d, J = 17.6 Hz, 1H), 2.41 (s, 3H), 2.09 (d, J = 6.4 Hz, 3H). 13C NMR
(100 MHz, C6D6) δ 69.7, 149.4, 145.7, 139.2, 137.8, 131.7, 130.0, 129.8,
129.6, 129.3, 126.9, 123.0, 122.5, 120.8, 120.3, 118.8, 114.9, 71.1, 39.9, 21.3,
14.6. HRMS (CI+): m/z calculated for C21H18O3 [M]+ 318.1256, found
318.1262.
Preparation of Alkene (− )-12
To a stirred solution of α-hydroxyl ketone (+)-11 (126 mg, 0.3 mmol, 75% ee)
in DMF (5 ml) at 0 °C were added imidazole (51 mg, 0.75 mmol) and
tert-butyldimethylsilyl chloride (54 mg, 0.36 mmol). The reaction mixture was
allowed to warm to RT and stirred overnight. The reaction was quenched by
addition of aq. satd. NH4Cl (10 ml). The organic layer was collected and the
aqueous layer was extracted with Et2O (3 ×10 ml). The combined organic
fractions were washed with H2O (3× 10 ml), brine, dried over MgSO4, and
concentrated under reduced pressure. The residue was purified by flash column
chromatography on silica gel using eluents (ethyl acetate/hexane = 1/20–1/15)
Preparation of ent-cinanthrenol A
To a stirred solution of (+)-2 (16 mg, 0.05 mmol) in CH2Cl2 (1 ml) at 0 °C was
added diiodomethane (40 μl, 0.5 mmol) and diethylzinc solution (1M in
hexanes, 0.5 ml, 0.5 mmol). The reaction mixture was stirred at 0 °C for 4 h.
The reaction was quenched by addition of aq. satd. NaHCO3 (5 ml). The
organic layer was collected and the aqueous layer was extracted with CH2Cl2
20
to give the pure TBS ether S-3 (152 mg, 95%) as a colorless oil. ½aꢀD ¼ ꢁ5:0
(c 0.5, CHCl3). IR (neat, cm− 1): 2928, 2860, 1724, 1613, 1513, 1460, 1262,
1128, 868, 837, 672. 1H NMR (400 MHz, CDCl3) δ 8.54 (d, J = 8.6 Hz, 1H),
8.13 (s, 1H), 7.90 (d, J =8.6 Hz, 1H), 7.84 (d, J = 8.6 Hz, 2H), 7.75
(d, J = 8.8 Hz, 1H), 7.30 (d, J = 8.6 Hz, 1H), 4.77–4.64 (m, 1H), 3.95 (3 × 5 ml). The combined organic fractions were washed with brine, dried over
(dd, J = 16.6, 7.4 Hz, 1H), 3.24 (dd, J = 16.6, 3.2 Hz, 1H), 1.43–1.33 (m, 3H),
1.17 (d, J = 7.2 Hz, 18H), 1.00 (s, 9H), 0.27 (d, J = 4.4 Hz, 6H). 13C NMR
(100 MHz, CDCl3) δ 204.7, 155.5, 150.7, 134.0, 132.4, 131.8, 130.2, 129.0,
128.2, 128.0, 123.2, 122.6, 120.3, 119.5, 112.3, 74.8, 35.2, 25.9 (3C), 18.5, 18.0
(6C), 12.8 (3C), − 4.3, − 5.0. HRMS (CI+): m/z calculated for C32H47O3Si2 [M]+
535.3064, found 535.3047.
MgSO4, and concentrated under reduced pressure. The residue was purified by
flash column chromatography on silica gel using eluents (ethyl acetate/
hexane = 1/3–1/1) to give the cyclopropane intermediate as a yellow solid. To
a stirred solution of the cyclopropane derivative in MeOH (1 ml) at RT was
added potassium carbonate (1.4 mg, 0.01 mmol). The reaction mixture was
stirred at RT for 2 h. The reaction mixture was concentrated under reduced
pressure. The residue was purified by flash column chromatography on silica
gel using eluents (ethyl acetate/hexane= 1/3–1/1) to give the final product ent-1
To a stirred suspension of C2H5PPh3Br (223 mg, 0.6 mmol) in THF (5 ml)
at 0 °C was added sodium bis(trimethylsilyl)amide (1.0 M in THF, 0.5 ml,
0.5 mmol), the reaction mixture was stirred at 0 °C for 30 min. Then a solution
of TBS ether S-3 (134 mg, 0.25 mmol) in THF (3 ml) was added via syringe,
the reaction mixture was allowed to warm to RT and stirred overnight.
The reaction was quenched by addition of aq. satd. NH4Cl (10 mL). The
organic layer was collected and the aqueous layer was extracted with Et2O
(3× 10 ml). The combined organic fractions were washed with brine, dried over
MgSO4 and concentrated under reduced pressure. The residue was purified by
flash column chromatography on silica gel using eluents (ethyl acetate/
hexane= 1/20–1/15) to give the alkene (− )-12 (123 mg, 90% yield) as a brown
20
(11 mg, 78% over 2 steps) as a yellow solid. ½aꢀD ¼ þ6:3(c 0.3, MeOH).
IR (neat, cm− 1): 3422, 2921, 2864, 1742, 1513, 1460, 1385. 1H NMR
(400 MHz, Pyr) δ 12.00 (s, 1H), 8.64 (d, J = 8.0 Hz, 1H), 8.57 (s, 1H), 7.94
(d, J = 8.8 Hz, 1H), 7.84 (d, J = 8.8 Hz, 1H), 7.64 (d, J = 8.8 Hz, 1H), 7.52
(d, J = 8.8 Hz, 1H), 6.98 (d, J = 8.4 Hz, 1H), 6.51 (d, J = 6.4 Hz, 1H), 4.86
(m, 1H), 3.87 (dd, J = 16.8, 7.4 Hz, 1H), 3.61 (d, J = 16.8 Hz, 1H), 1.46
(m, 1H), 1.36 (m, 1H), 1.30 (d, J = 4.8 Hz, 3H), 1.16 (m, 1H). 13C NMR
(100 MHz, C5D5N) δ158.2, 145.9, 137.1, 133.5, 130.8, 129.3, 128.5, 127.5,
125.8, 122.8, 120.4, 117.9, 117.7, 107.6, 71.8, 41.3, 38.5, 22.0, 19.6, 14.4. HRMS
(LD+): m/z calculated for C20H18O2 [M]+ 290.1307, found 290.1297.
20
oil. ½aꢀD ¼ ꢁ8:0(c 0.1, CHCl3). IR (neat, cm− 1): 2924, 2854, 1617, 1460, 1376,
1261, 1085, 1019, 800. 1H NMR (400 MHz, CDCl3) δ 8.39 (d, J = 8.4 Hz, 1H),
8.07 (s, 1H), 7.75 (d, J =8.4 Hz, 1H), 7.70 (d, J = 8.8 Hz, 1H), 7.65
(d, J = 8.4 Hz, 1H), 7.57 (d, J = 8.8 Hz, 1H), 7.18 (d, J = 8.4 Hz, 1H), 6.30
(d, J =6.8 Hz, 1H), 5.38 (d, J =6.0 Hz, 1H), 3.62 (dd, J = 16.8, 7.0 Hz, 1H), 3.20
(d, J =16.8 Hz, 1H), 2.02 (d, J = 6.8 Hz, 3H), 1.37 (m, 3H), 1.17 (d, J = 7.2 Hz,
18H), 0.93 (s, 9H), 0.18 (d, J = 9.6 Hz, 6H). 13C NMR (100 MHz, CDCl3)
δ 155.0, 145.2, 138.8, 138.1, 132.3, 129.9, 129.09, 129.06, 126.9, 126.7, 122.1,
120.72, 120.69, 119.2, 118.3, 111.6, 72.1, 40.5, 29.7, 25.9 (3C), 18.0 (6C),
14.7, 12.8 (3C), − 3.7, − 4.5. HRMS (CI+): m/z calculated for C34H50O2Si2 [M]+
546.3349, found 546.3376.
CONFLICT OF INTEREST
The authors declare no conflict of interest.
ACKNOWLEDGEMENTS
This research was financially supported by HKUST (R9309), Research Grant
Council of Hong Kong (ECS 605912, GRF 605113, and GRF 16305314), and
National Natural Science Foundation of China (NSFC 21472160).
Preparation of allylic alcohol (+)-2
To a solution of the allylic alcohol (− )-12 (109 mg, 0.2 mmol) in THF (3 ml) at
0 °C was added tetrabutylammonium fluoride (TBAF, 1.0 M in THF, 0.6 ml,
0.6 mmol). The reaction mixture was allowed to warm to RT and stirred for
3 h. The reaction was quenched by addition of aq. satd. NH4Cl (10 ml). The
organic layer was collected and the aqueous layer was extracted with EtOAc
(3×10 ml). The combined organic fractions were washed with brine, dried over
MgSO4, and concentrated under reduced pressure to give a crude product,
which was washed through a short pad of silica gel using eluents (ethyl acetate)
to remove TBAF to give the phenol intermediate (44 mg, 80%) as a yellow
solid. To a stirred solution of the phenol intermediate (28 mg, 0.1 mmol) in
CH2Cl2 (1 ml) at RT were added triethylamine (28 μl, 0.2 mmol) and acetic
1
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The Journal of Antibiotics

Synthesis of ent-(+)-cinanthrenol A
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Supplementary Information accompanies the paper on The Journal of Antibiotics website (http://www.nature.com/ja)
The Journal of Antibiotics