The first total synthesis and structural determination of epi-cochlioquinone A

Article abstract of DOI:10.1016/j.tetlet.2010.07.153

The first total synthesis of epi-cochlioquinone A has been achieved in a highly convergent manner via [3+3] cycloaddition of catechol 2 and oxadecalin 3 as the key reaction. The synthesis of the catechol segment, possessing the side chain with multi stereogenic centers, features the asymmetric vinylogous Mukaiyama aldol reaction, the stereoselective conjugate addition to the nitroalkene, the stereospecific nitro-Dieckmann condensation, and the transformation of 6-nitrocyclohex-2-enone into catechol 2, using two new methodologies, such as (i) the hydrogen-transfer reaction to o-aminophenol and the subsequent auto-redox-catalysis to catechol and (ii) the direct oxidation of 6-nitrocyclohex-2-enone to o-quinone and the subsequent reduction. The oxadecalin segment was synthesized from a glycosyl cyanide by the [3+3] annulation with a ketone and an acetoacetate. These segments were connected by the [3+3] cyclization, and the resulting tetracyclic compound was subjected to a specific oxidation of the protected hydroquinone to provide epi-cochlioquinone A.

Full text of DOI:10.1016/j.tetlet.2010.07.153

Tetrahedron Letters 51 (2010) 5532–5536
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Tetrahedron Letters
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The first total synthesis and structural determination of epi-cochlioquinone A
⇑
Seijiro Hosokawa , Kaoru Matsushita, Shinpei Tokimatsu, Tatsuya Toriumi, Yasuaki Suzuki,
⇑
Kuniaki Tatsuta
Department of Applied Chemistry, Faculty of Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The first total synthesis of epi-cochlioquinone A has been achieved in a highly convergent manner via
[3+3] cycloaddition of catechol 2 and oxadecalin 3 as the key reaction. The synthesis of the catechol seg-
ment, possessing the side chain with multi stereogenic centers, features the asymmetric vinylogous
Mukaiyama aldol reaction, the stereoselective conjugate addition to the nitroalkene, the stereospecific
nitro-Dieckmann condensation, and the transformation of 6-nitrocyclohex-2-enone into catechol 2, using
two new methodologies, such as (i) the hydrogen-transfer reaction to o-aminophenol and the subsequent
auto-redox-catalysis to catechol and (ii) the direct oxidation of 6-nitrocyclohex-2-enone to o-quinone
and the subsequent reduction. The oxadecalin segment was synthesized from a glycosyl cyanide by
the [3+3] annulation with a ketone and an acetoacetate. These segments were connected by the [3+3]
cyclization, and the resulting tetracyclic compound was subjected to a specific oxidation of the protected
hydroquinone to provide epi-cochlioquinone A.
Received 9 July 2010
Revised 23 July 2010
Accepted 29 July 2010
Available online 2 August 2010
Keywords:
Total synthesis
Epi-cochlioquinone A
Vinylogous Mukaiyama aldol reaction
Nitroalkene
Nitro-Dieckmann condensation
Autocatalysis
Ó 2010 Elsevier Ltd. All rights reserved.
Catechol
[3+3] Annulation
Epi-cochlioquinone A (1, Fig. 1) was isolated from the fermenta-
tion broth of Stachybotrys bisbyi as an inhibitor of acyl-CoA: choles-
terol acyltransferase (ACAT) by Sankyo group in 1996.1 Since ACAT
plays a critical role both in cholesterol absorption in the intestine
and in cholesterol ester formation in the liver and peripheral tis-
sues,2 ACAT inhibitors are expected to reduce serum cholesterol
levels. The relative structure of epi-cochlioquinone A was deter-
mined by NMR spectrometric analysis and X-ray crystallographic
analysis.1 The cochlioquinone family has drawn attention due to
its biological activities and the attractive structure;3 however, the
total synthesis of cochlioquinones has been unprecedented. Here-
in, we present the first total synthesis of epi-cochlioquinone A (1).
In order to make the synthesis highly convergent, epi-cochlio-
quinone A (1) was divided into two segments (catechol 2 and oxa-
decalin 3) in the retrosynthetic analysis (Scheme 1). The
cochlioquinone skeleton would be synthesized by [3+3] cycloaddi-
tion with 2 and 3. Therefore, the total synthesis started from the
preparation of these segments.
The catechol 2 has been synthesized in a stereoselective manner
(Scheme 2). The vinylogous Mukaiyama aldol reaction with the
chiral dienol ether 4 and 2S-methylbutanal 5 afforded anti adduct
Me
Me
Me
Me
AcO
HO
O
MeO
Me
Me
OH
[3+3]
HO
Me
O
O
HO
2
Me
H
Me
O
Me
HO
O
H
H
Me
HO
Me
1
O
H
H
Figure 1. The structure of epi-cochlioquinone A (1).
Me
3
⇑
Corresponding authors. Tel./fax: +81 3 3200 3203.
E-mail address: tatsuta@waseda.jp (K. Tatsuta).
Scheme 1. The synthetic plan for epi-cochlioquinone A (1).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.07.153

S. Hosokawa et al. / Tetrahedron Letters 51 (2010) 5532–5536
5533
Me
CHO
Me
5
Me
Me Me Me
b,c
Me Me
CHO
a
d
N
O
N
O
Me
Me
Me
TBSO
OH
O
O
O
OTBS
4
6
7
OMe
OMe
OMe
TMSO
9
OMe
Me Me
OTBS
OH
e
See text
16
Me
NO2
O
Me
15
HO
Me
H
OTBS
NO2 Me Me
OH Me Me
8
10
2
Scheme 2. Reagents and conditions: (a) TiCl4, MS4A, CH2Cl2, ꢀ40 °C, 2 d, 89% dr 54:1; (b) TBSOTf, 2,6-lutidine, CH2Cl2, 0 °C, 15 min; (c) O3, CH2Cl2, ꢀ78 °C, 1 h, then Ph3P,
ꢀ78 °C, 30 min, 91% (in one pot); (d) MeNO2, n-BuLi, THF, 0 °C, 2 d, then MsCl, Et3N, 0 °C to rt, 2 h, 90%; (e) YbFOD (0.05 equiv), rt, 1 d, then K2CO3, EtOH, 70 °C, 2 d, 87%.
6 in high yield and excellent selectivity (dr 54:1, separable by col-
umn chromatography).4 The protection of the hydroxy group as
TBS ether was followed by ozonolysis in one pot to give aldehyde
75 in high yield. Aldehyde 7 was converted to nitroalkene 8 by
Henry aldol reaction6 followed by the addition of mesyl chloride
and triethylamine. The [4+2] cycloaddition reaction with 8 and die-
nol ether 9 proceeded stepwise to give cyclohexenone 10. In the
cycloaddition process, both the conjugate addition and the sequen-
tial nitro-Dieckmann reaction7 proceeded in the stereoselective
manner to give 10 as a single isomer (by 600 MHz 1H NMR). The
stereochemistry of cyclohexenone 10 was determined by X-ray
crystallography (Fig. 2).8,9
Transformation of 6-nitrocyclohex-2-enone 10 into catechol 2
was achieved by two kinds of procedure. One involves the hydro-
gen transfer reaction and successive autocatalytic reaction (Eq.
1), and the other contains O2 oxidation followed by reduction
(Eq. 4).10 In the former transformation (Eq. 1), de-hydrogenation
of cyclohexenone and a concomitant reduction of the nitro group
to amine were realized in the presence of Wilkinson catalyst and
1,4-cyclohexadiene at 170 °C, and the resulting o-aminophenol
11 was converted into catechol 2 with the catalytic amount
(0.1 equiv) of oxidant under acidic conditions. The second step in
Eq. 1 included an autocatalysis process, which was proven by the
reactions shown in Eqs. 2 and 3. The equivalent amount of the oxi-
OMe
H
OMe
OMe
OMe
cat. CAN
TFA, aq. THF
rt, 3 h
OTBS
OTBS
OTBS
OH
RhCl(PPh3)3
50% aq. TFA,
ð1Þ
ð2Þ
O
Me
HO
Me
HO
Me
HO
Me
PhMe
rt, 5 h
81%
NO2 Me Me
NH2 Me Me
OH Me Me
OH Me Me
170 °C, 5 h
in sealed tube
80%
12
2
10
11
OMe
OMe
OTBS
OTBS
CAN
HO
Me
O
Me
aq. THF
rt, 2 h
87%
NH2 Me Me
O
Me Me
13
11
OMe
cat.
OTBS
O
Me
HO
OMe
OMe
O
Me Me
13
ð3Þ
ð4Þ
OTBS
OTBS
HO
Me
Me
TFA
aq. THF
rt, 3 h
79%
NH2 Me Me
OH Me Me
11
12
OMe
OMe
OMe
aq. Na2S2O4
aq. THF
O2
OTBS
OTBS
OH
rt, 30 min
Cu, TMEDA
O
Me
O
Me
HO
Me
H
then
aq. TFA
rt, 5 h
82%
MeCN
rt, 10 min
75%
NO2 Me Me
O
Me Me
13
OH Me Me
10
2

5534
S. Hosokawa et al. / Tetrahedron Letters 51 (2010) 5532–5536
phenol (A) to produce iminoquinone (B) and catechol (D). There-
fore, this transformation contains the auto-redox-catalysis.
The other procedure to give catechol 2 from 6-nitrocyclohex-2-
enone 10 is shown in Eq. 4. The a-proton (CH–NO2) of 10 was ab-
sorbed under the mild basic conditions and oxidized11 to provide
o-quinone 13 in 75% yield. The reduction of quinone 13 followed
by the addition of aqueous TFA gave de-O-silylated 2.
Both these methodologies (Eqs. 1 and 4) are new methods to
synthesize catechols. The method in Eq. 1 is applicable to not only
catechol but also o-aminophenol, and features auto-redox-cataly-
sis from o-aminophenol to catechol. On the other hand, the method
in Eq. 4 contains the direct transformation of 6-nitrocyclohex-2-
enone to o-quinone.
Figure 2. The ORTEP drawing of X-ray crystallography of 10.
The synthesis of the oxadecalin moiety (21, the precursor of en-
one 3) is disclosed in Scheme 4. The glycosyl cyanide 1412 was sub-
jected to methanolysis of cyanide and acetate, which was followed
by the selective silylation to afford 15. Grignard reaction of ester 15
with excess amount of methylmagnesium bromide and the subse-
quent oxidation gave ketone 16. Treatment of ketone 16 with 1,8-
diazabicyclo[5,4,0]undec-7-ene (DBU) in the presence of methyl
acetoacetate at 40 °C gave oxadecalin 17 in the manner of [3+3]
cycloaddition reaction. The mechanism is proposed as Scheme 5,
which involves b-elimination to afford enone 22, conjugate addi-
tion of acetoacetate to give diketone 23, and condensation with a
concomitant de-carboxylation by the nucleophilic attack of elimi-
nated H2O to yield oxadecalin 17. The stereochemistry of the C5
position was determined by NOE (8.8%) between H3 and H5.
To enone 17 were introduced a methyl group and an acyloxym-
ethyl group regio- and stereoselectively by the following sequence.
Luche reduction gave allyl alcohol 18, of which the stereochemistry
of the hydroxy group was inverted by Mitsunobu reaction to afford
19. Simmons–Smith reaction13 proceeded in the stereospecific
manner to give the cyclopropane, which was oxidized to ketone
20. The structure of 20 was determined by X-ray crystallography
(Fig. 3).9 Birch reduction with lithium naphthalenide opened the
cyclopropane ring and the resulting enolate was alkylated with
iodomethyl pivalate to yield oxadecalin 21.
dant under the acidic conditions gave ortho-quinone 13 (Eq. 2).
Treatment of aminophenol 11 with the catalytic amount
(0.1 equiv) of quinone 13 in aqueous trifluoroacetic acid (TFA) gave
catechol 12 (Eq. 3). Therefore, the mechanism of the transforma-
tion of o-aminophenol to catechol in Eq. 1 is presumed as Scheme 3.
At first, aminophenol (A) should be oxidized to iminoquinone (B),
which would receive the acid hydrolysis to give o-quinone (C).
The resulting o-quinone (C) might work as an oxidant for amino-
OMe
OMe
CAN
HO
R
O
R
NH2
NH
A
B
TFA
H2O
H+
OMe
OMe
The total synthesis of epi-cochlioquinone A (1) was accom-
plished as Scheme 6. Treatment of ketone 21 with alumina in
dichloromethane afforded enone 3 (Scheme 1), which was reacted
with catechol 2 in the presence of boron trifluoride in one pot to
give cyclized 24 as expected in our synthetic plan. Both the phenol
and the secondary alcohol of 24 were acetylated, and the tertiary
alcohol was protected as trimethylsilyl ether 25. Epoxidation of
the enol ether was followed by epoxide-opening methylation by
HO
R
O
R
OH
D
O
C
Scheme 3. The presumed reaction mechanism of the autocatalytic reaction to give
catechol.
8.8%
Me
Me
O
H
H
N
H
H
H
H
H
H
e
O
C
a,b
c,d
O
O
O
AcO
AcO
TBSO
Me
Me
OH
TBSO
HO
OMe
5
3
OH
O
O
nOe :
14
15
16
17
10.4%
6.7%
Me
Me
Me
Me
Me
H
H
H
O
H
H
H
H
H
g,h
f
O
O
i,j
k
O
Me
OH
Me
OH
Me
5
3
H
OH
OH
HO
HO
O
O
Me
11
11.3%
PivO
18
19
20
21
Scheme 4. Reagents and conditions: (a) HCl, MeOH, 50 °C, 12 h, 90%; (b) TBSCl, imidazole, DMF, ꢀ30 °C, 3 h, 90%; (c) MeMgBr, THF, ꢀ10 °C, 1 h, 91%; (d) IBX, Py, PhMe–DMSO,
50 °C, 12 h, 90%; (e) methyl acetoacetate, DBU, THF (degassed), 40 °C, 3 d, 42%; (f) NaBH4, CeCl3ꢁ7H2O, MeOH, ꢀ78 °C, 1 h, 89%; (g) DEAD, Ph3P, HCO2H, PhMe, rt, 30 min, 79%
19:18 = 5:1; (h) 0.5 M Et3N, MeOH, rt, 2 h, 91%; (i) Et2Zn, CH2I2, TFA, CH2Cl2, 0 °C, 1 h, 83%; (j) PDC, MS4A, CH2Cl2, 0 °C, 1 h, 89%; (k) Li, naphthalene, THF, ꢀ78 °C, 15 min, then,
ICH2OPiv, ꢀ78 to ꢀ30 °C, 1 h, 60%.

S. Hosokawa et al. / Tetrahedron Letters 51 (2010) 5532–5536
5535
position at first in the stereoselective manner (rt, 30 min) and then
converted to quinone (rt, 1 day) to give epi-cochlioquinone A (1).
This direct oxidation from 27 to 1 proceeded in 88% yield. Observa-
tion of NOE between H11 and H19 of the product 1 confirmed the
stereochemistry of C11. The synthetic 1 was identical in all re-
spects with the natural product including the optical rotation (syn-
Me
OMe
Me
Me
H
H
H
O
O
DBU
O
O
TBSO
Me
Me
OH
OH
THF
O
O
40 °C, 3 d
16
22
thetic 1: ½a 2D5
ꢂ
+42.2 (c 0.90, EtOH), natural: ½a D25
ꢂ
+43.2 (c 0.95,
Me
Me
EtOH)). Thus, the absolute structure of epi-cochlioquinone A (1)
was determined as shown in Figure 1.
H
H
H
H
O
O
MeO2C
O
Me
OH
Me
OH
In conclusion, the first total synthesis and the structural deter-
mination of epi-cochlioquinone A (1) were achieved by [3+3]
cycloaddition reaction with catechol 2 and enone 3. Catechol 2
was synthesized from 6-nitrocyclohex-2-enone 10 by two kinds
of methodologies including the hydrogen transfer reaction fol-
lowed by the auto-redox-catalytic reaction converting o-amino-
phenol to catechol and oxidation of 6-nitrocyclohex-2-enone to
o-quinone followed by reduction. Oxadecalin 21 was derived from
glycosyl cyanide 14 by [3+3] cycloaddition reaction with ketone 16
and reductive alkylation with cyclopropylketone 20. The absolute
structure of epi-cochlioquinone A (1) was determined as shown
in Figure 1.
O
O
23
17
Scheme 5. The mechanism of production of enone 17.
Acknowledgments
The authors thank Dr. T. Takatsu and Dr. Y. Nishi in Daiich San-
kyo Co., Ltd for giving us an authentic sample of epi-cochlioqui-
none A. Authors are also grateful for the financial support from
Shorai foundation for Science and Technology, GCOE program ’Cen-
ter for Practical Chemical Wisdom’, and The Ministry of Education,
Culture, Sports, Science and Technology (MEXT).
Figure 3. The ORTEP drawing of X-ray crystallography of 20.
addition of trimethylaluminum to give tertiary alcohol 26 as a sin-
gle isomer. De-hydration of 26 with thionyl chloride accompanied
with de-O-silylation, and the sequential hydrogenation proceeded
stereoselectively to give 27. The stereochemistry of the C8 and C9
positions of 27 was determined by NOESY showing correlations be-
tween H5 and H9, H9 and H20, H20 and H11a, and H11b and H19.
Oxidation of the protected hydroquinone 27 with cerium ammo-
nium nitrate (CAN) introduced the hydroxy group at the benzylic
Supplementary data
Supplementary data (the spectrum data of compounds 2, 6, 7, 8,
10, 11, 13, 16, 17, 20, 21, 24, 26, 27, and synthetic epi-cochlioqui-
none A (1), and 1H NMR spectrum (600 MHz in CDCl3) and 13C NMR
spectrum (150 MHz in CDCl3) of synthetic epi-cochlioquinone A (1)
OMe
Me
Me
Me
Me
OH
HO
AcO
HO
Me
OH Me Me
MeO
Me
MeO
Me
OPiv
O
Me
OH
Me
OAc
b,c
d
2
Me
H
a
O
O
Me
HO
O
H
Me
HO
Me
TMSO
Me
O
O
H
H
H
H
Me
Me
21
24
25
Me
Me
Me
Me
Me
Me
AcO
AcO
AcO
MeO
MeO
O
Me
OAc
Me
OAc
Me
α H
β
g
e,f
H
HO
19 Me
19Me
O
11
9
11
Me
O
8
O20
Me
O
Me
OH Me
H
H
3
5
5
20
Me
TMSO
Me
HO
Me
HO
O
O
O
H
H
H
H
H
H
Me
Me
Me
NOESY
26
27
1
Scheme 6. Reagents and conditions: (a) Al2O3 ,CH2Cl2, rt, 3 h, then BF3ꢁEt2O, ꢀ78 °C, then 0 °C, 2 h, 80%; (b) Ac2O, pyridine, 50 °C, 6 h, 88%; (c) TMSOTf, 2,6-lutidine, CH2Cl2, rt,
30 min, 92%; (d) mCPBA, CH2Cl2, 0 °C, 15 min, then Me3Al, 0 °C, 30 min, 81%; (e) SOCl2, pyridine, rt, 30 min; (f) H2, 10% Pd–C, EtOH, rt, 12 h, 84% in two steps; (g) CAN, aq
CH3CN, rt, 1 d, 88%.

5536
S. Hosokawa et al. / Tetrahedron Letters 51 (2010) 5532–5536
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