Formal (3+3) cycloaddition of silyl enol ethers catalyzed by trifric imide: Domino michael addition-claisen condensation accompanied with isomerization of silyl enol ethers

Article abstract of DOI:10.1248/cpb.59.1190

We describe here a Tf₂NH-catalyzed formal (3+3) cycloaddition of silyl enol ethers with acrylates as a new domino reaction. In the domino sequence, the catalyst activates Michael addition, deprotonation of the resulting silyloxonium cation and

Full text of DOI:10.1248/cpb.59.1190

1190
Note
Chem. Pharm. Bull. 59(9) 1190—1193 (2011)
Vol. 59, No. 9
ꢀ
Formal (3 3) Cycloaddition of Silyl Enol Ethers Catalyzed by Trifric
Imide: Domino Michael Addition–Claisen Condensation Accompanied
with Isomerization of Silyl Enol Ethers
Takumi AZUMA, Yoshiji TAKEMOTO,* and Kiyosei TAKASU*
Graduate School of Pharmaceutical Sciences, Kyoto University; Yoshida, Sakyo-ku, Kyoto 606–8501, Japan.
Received June 3, 2011; accepted June 24, 2011; published online June 30, 2011
We describe here a Tf2NH-catalyzed formal (3ꢀ3) cycloaddition of silyl enol ethers with acrylates as a new
domino reaction. In the domino sequence, the catalyst activates Michael addition, deprotonation of the resulting
silyloxonium cation and intramolecular Claisen condensation. It was found that reaction modes significantly de-
1
pend on the reaction temperature. We also examined the mechanistic detail of the reaction by H-NMR experi-
ment.
Key words (3ꢀ3) cycloaddition; triflic imide; silyl enol ether; domino reaction
Silyl enol ethers are of enormous importance in synthetic silyl enol ether D successively reacts with another electro-
chemistry as isolable enolate equivalents. They are fre- phile (E’ꢀ), a,aꢁ-disubstituted ketone E will be obtained in
quently employed in carbon–carbon bond forming reactions a one step (path b). To the best of our knowledge, only
such as Mukaiyama–aldol reactions, Mukaiyama–Michael limited studies of domino reactions consisting of two bond-
reactions, Diels–Alder reactions and so on. Interaction of formation reactions based on the concept have been report-
silyl enol ether A with an electrophile (Eꢀ) gives the corre- ed.7—10)
sponding silyloxonium cation B. When the intermediate
We have reported on a catalytic (2ꢀ2) cycloaddition, in
B is quenched with the counter anion or an appropriate which silyl enol ethers 1 react with a,b-unsaturated esters 2
nucleophile, desilylated product C will be obtained (Fig. 1, in the presence of triflic imide (Tf2NH) at ꢂ78 °C giving
path a). On the other hand, if the silicon–oxygen bond of the siloxycyclobutanes 4.11—14) In the process, the siloxonium in-
siloxonium cation B is strong enough, deprotonation of B termediate 3 bearing a ketene silyl acetal moiety, which is
at the adjacent position of the carbonyl carbon can occur to formed as a transient intermediate by Mukaiyama–Michael
generate silyl enol ether D (or Dꢁ).1—6) If the newly formed addition, would undergo successive intramolecular aldol re-
action (nucleophilic attack) at low temperatures to form a 4-
membered ring (Fig. 2, path a).15) It was made clear that the
cycloaddition is activated by silyl triflic imide (SiNTf2),
which would be in situ generated from Tf2NH with a sub-
strate silyl enol ether.11,14) We envisaged that a,b-unsaturated
carbonyl compounds which possess two electrophilic carbons
(one is the b-carbon and another is the ester carbonyl func-
tion) may act as dual electrophilic species under appropriate
conditions. Namely, if deprotonation of 3 giving silyl enol
ether 5 and successive cyclization occur, 6-membered prod-
uct 6 would be created (path b). In this paper, we wish to re-
Fig. 1. Dual C–C Bond Formation Reaction of Silyl Enol Ethers
port on the intermolecular and intramolecular formal (3ꢀ3)
cycloadditions of silyl enol ethers with a,b-unsaturated esters
in the presence of Tf2NH.
At the outset of this study, we examined (3ꢀ3) cycloaddi-
tion of 2-methyl-1-(t-butyldimethylsilyloxy)cyclohexene (1a)
with various a,b-unsaturated carbonyl compounds 2a—e in
Fig. 2. Tf2NH-Catalyzed (2ꢀ2) Cycloaddition (Path a) and Formal (3ꢀ3) Chart 1. Formal (3ꢀ3) Cycloaddition of Silyl Enol Ethers with a,b-
Cycloaddition (Path b)
Unsaturated Carbonyl Compounds
© 2011 Pharmaceutical Society of Japan
∗
To whom correspondence should be addressed.
e-mail: kay-t@pharm.kyoto-u.ac.jp; takemoto@pharm.kyoto-u.ac.jp

September 2011
1191
Table 1. Tf2NH-Catalyzed Formal (3ꢀ3) Cycloaddition of Silyl Enol
Ethera)
Entry 1 (Si, n)
2 (R)
Solvent Temp. Products (% yield)
1
2
3
4
5
6
7
8
9
10
11
12
13
1a (TBS, 1) 2a (OCH3)
CH2Cl2 rt
CH2Cl2 rt
CH2Cl2 rt
6a (22), 7a (38)
Complex mixture
7c (15)
7d (70)
No reaction
Chart 3. Tf2NH-Catalyzed Claisen Condensation of 5a
1a
1a
1a
1a
2b (H)
2c (CH3)
2d (OCH(CF3)2) CH2Cl2 rt
2e (morpholino) CH2Cl2 rt
1b (TMS, 1) 2a
1c (TIPS, 1) 2a
CH2Cl2 rt
CH2Cl2 rt
CHCl3 rt
Toluene rt
CH3CN rt
CHCl3 Reflux 6a (41), 7a (41)
CHCl3 Reflux 6d (17), 7dꢁ (nd)b)
CHCl3 Reflux 6e (23), 7eꢁ (nd)b)
6a (29), 7a (nd)b)
6a (25), 7a (30)
6a (22), 7a (60)
6a (trace), 7a (80)
6a (trace), 7a (80)
1a
1a
1a
1a
2a
2a
2a
2a
1d (TBS, 0) 2a
1e (TBS, 2) 2a
a) Conditions: 1 (1.0 eq), 2 (1.0 eq), Tf2NH (0.10 eq) in solvent (0.10 M) for 1—4 h.
b) nd means ‘not determined.’
1
Fig. 3. In Situ H-NMR for the Reaction of 5a in the Presence of Tf2NH
in CDCl3
Chart 2. (2ꢀ2) Cycloaddition of Silyl Enol Ether with Acrylate
(a) Before an addition of Tf2NH (tꢃ0 min), (b) reaction mixture (tꢃ15 min), (c) after
work-up with sat. NaHCO3 (tꢃ60 min).
the presence of Tf2NH (Chart 1). When the reaction with
methyl acrylate (2a) was carried out in CH2Cl2 at ambient studies revealed that the formation of (3ꢀ3) cycloadduct was
temperature, desired bicyclo[3.3.1]nonane-2,9-dione 6a was observed over ꢂ10 °C and almost no (2ꢀ2) cycloadduct was
obtained in 22% yield along with Mukaiyama–Michael observed over 0 °C. Thus, (3ꢀ3) and (2ꢀ2) cycloadducts
adduct 7a (Table 1, entry 1). Cycloadduct 6a would be would be thermodynamically and kinetically formed prod-
formed by the domino Michael addition–Claisen condensa- ucts, respectively.16,17) When cyclobutane 4a was treated with
tion sequence, which corresponds to a formal (3ꢀ3) cycload- a catalytic amount of Tf2NH (10 mol%) in CH2Cl2 at ambient
dition. By contrast, a complicated mixture was obtained in temperature, diketone 6a was obtained in 38% yield along
the reaction with acrolein (2b) (entry 2). No (3ꢀ3) cycload- with Michael adduct 7a. In contrast, no transformation of 6a
dition occurred with meyhylvinylketone (2c) and hexafluo- or 7a into 4a occurred under the same conditions.
roisopropyl acrylate (2d), which furnished only Michael
By-product 7 would be formed by the proto-desilylation of
adducts 7c and 7d, respectively (entries 3 and 4). The reac- silyl enol ether 5, which corresponds to the intermediate of
tion with acryl amide 2e resulted in recovery of the starting (3ꢀ3) cycloaddition. It was made clear that treatment of 5a
materials (entry 5). The results indicated us that the proton with a catalytic amount of Tf2NH gave (3ꢀ3) cycloadduct 6a
transfer of intermediate 3 (see Fig. 2, path b) would not pro- in 50% yield,18—21) and, noting that, provided 7a in the almost
ceed in the reaction of 2c and 2d because of the weaker ba- same yield as 6a (Chart 3). To clarify the mechanistic in-
sicity of the newly generated silyl enol moity of 3. On the sight, the reaction of 5a in the presence of Tf2NH was moni-
1
other hand, because the catalyst would not activate amide 2e tored by H-NMR (Fig. 3a). Characteristic peaks of vinylic
enough, the conjugate addition does not occur. Next, the re- and methoxy protons of 5a were observed at 4.70 and
action of trimethylsilyl (TMS) and triisopropylsilyl (TIPS) 3.58 ppm, respectively. After addition of the catalyst
enol ethers (1b, c) with 2a was examined under the same (15 min), signals of three compounds 7a, 8a and tert-butyl-
conditions. However, no significant improvement on the yield dimethylsilyl methyl ether (TBSOMe) appeared. The struc-
was observed (entries 6 and 7). The desired reaction also ture of 8a was assigned by GC-MS as the silyl enol ether de-
proceeded in CHCl3 at ambient temperature (entry 8), rivative of 6a. Their molar ratios were calculated to be al-
whereas Michael adduct 7a was exclusively produced in most identical (Fig. 3b). After an aqueous work-up of the re-
toluene or CH3CN (entries 9 and 10). The yield of 6a in- action mixture (60 min), it was observed that 8a was con-
creased to 41% in refluxing CHCl3, although Michael-adduct verted into diketone 6a by protodesilylation (Fig. 3c). The re-
7a was still produced in the same yield (entry 11). Bicy- sults pointed out that silyl transfer of 5 into bicyclic product
clo[3.2.1]octanedione 6d and bicyclo[4.3.1]decanedione 6e 6 would immediately proceed to give 8 after Claisen conden-
were obtained from the corresponding silyl enol ethers 1d sation of 5. Therefore, formation of 6 is required to produce
and 1e, respectively, although the yields were poor (entries 8 as a co-product and the yield of 6 must be less than 50%.
12 and 13).
Finally, we attempted the synthesis of the tricyclic frame-
According to our previous reports, catalytic (2ꢀ2) cy- work of clovane using the catalytic cycloaddition. Clovane
cloaddition of 1a with 2a in CH2Cl2 promoted at ꢂ78 °C giv- and its related natural products possessing the tricyclo-
ing bicyclo[4.2.0]octane 4a in 78% yield (Chart 2). Further [6.3.1.04,8]dodecane skeleton are widely found in essential

1192
Vol. 59, No. 9
(m, 3H), 1.86—1.72 (m, 3H), 1.68—1.65 (m, 1H), 1.14 (s, 3H); 13C-NMR
(126 MHz, CDCl3) d: 212.4, 210.3, 64.3, 45.8, 43.7, 39.6, 35.1, 30.7, 24.1,
20.0 ppm; IR (ATR) 2929, 1726, 1701 cmꢂ1; MS (electron ionization (EI))
166 (Mꢀ, 38), 111(100); Anal. Calcd for C10H14O4·0.25H2O: C, 70.35; H,
8.56, Found: C, 70.64; H, 8.65.
(1S*,5S*)-5-Methylbicyclo[3.2.1]octane-2,8-dione (6d) Colorless oil;
1H-NMR (500 MHz, CDCl3) d: 3.22 (d, Jꢃ6.9 Hz, 1H), 2.82—2.73 (m, 1H),
2.40 (dd, Jꢃ16.6, 6.6 Hz 1H), 2.16—2.06 (m, 2H), 1.99—1.90 (m, 2H),
1.74 (dd, Jꢃ13.5, 9.2 Hz, 1H), 1.54—1.47 (m, 1H), 1.20 (s, 3H); 13C-NMR
(126 MHz, CDCl3) d: 209.3, 206.1, 64.5, 46.4, 34.4, 32.8, 29.7, 22.1,
18.5 ppm; IR (ATR) 2965, 1751, 1708 cmꢂ1; MS (FAB) 153 (MHꢀ, 45), 154
(100); high resolution (HR)-MS (FABꢀ) [C9H13O2]ꢀ: 153.0916; Found
153.0910.
Chart 4. Intramolecular Formal (3ꢀ3) Cycloaddition Giving
Skeleton of Clovanes
a Core
(1S*,6S*)-1-Methylbicyclo[4.3.1]decane-7,10-dione (6e) Colorless oil;
1H-NMR (500 MHz, CDCl3) d: 3.20 (t, Jꢃ5.4 Hz, 1H), 2.73—2.67 (m, 1H),
2.57—2.51 (m, 1H), 2.18—2.13 (m, 1H), 2.00—1.97 (m, 1H), 1.80—1.58
(m, 6H), 1.53—1.40 (m, 1H), 1.30—1.19 (m, 1H), 1.19 (s, 3H); 13C-NMR
(126 MHz, CDCl3) d: 211.7, 209.6, 62.6. 47.6, 36.7, 36.1, 30.4, 28.8, 26.4,
25.0, 24.3; IR (ATR) 2931, 1724, 1698 cmꢂ1; MS (FAB) 181 (MHꢀ, 61), 73
(100); HR-MS (FABꢀ) [C11H17O2]ꢀ: 181.1229; Found 181.1220.
1,1,1,3,3,3-Hexafluoropropyl 3-(1-Methyl-2-oxocyclohexyl)propanoate
(7d) Colorless oil; 1H-NMR (500 MHz, CDCl3) d: 5.76 (sept, Jꢃ6.0 Hz,
1H), 2.57—2.51 (m, 1H), 2.47—2.40 (m, 2H), 2.36—2.32 (m, 1H), 2.03—
1.97 (m, 1H), 1.90—1.64 (m, 7H), 1.12 (s, 3H); 13C-NMR (126 MHz,
CDCl3) d: 214.6, 170.6, 120.5 (q, JC–Fꢃ282 Hz), 66.5 (hept, JC–Fꢃ34.9 Hz),
47.8, 39.0, 28.7, 32.2, 28.5, 27.4, 22.5, 21.0; IR (ATR) 2939, 1780,
Chart 5. Confirmation of the Relative Stereochemistry of 10
1705 cmꢂ1
;
MS (FAB) 335 (MHꢀ, 78), 167(100); HR-MS (FABꢀ)
[C13H17F6O3]ꢀ: 335.1082; Found 335.1074.
oil extracted from the fresh leaves of Chamaecyparis.22,23)
We envisaged that the core skeleton would be constructed by
the intramolecular (3ꢀ3) cycloaddition. Treatment of 9,
which was prepared from cyclohexanone according to the re-
ported procedure,15) with Tf2NH (10 mol%) in refluxing
CHCl3 gave tricyclic product 10 in 45% yield as a single di-
astereomer along with a diastereomeric mixture of 11 (Chart
4). The relative stereochemistry of 10 was established by the
following procedure. Tricyclic cyclobutane 12,15) whose
stereochemistry was known, was prepared by the reaction of
10 in the presence of Tf2NH at ꢂ78 °C. The desilylative
retro-aldol reaction of 12 furnished spirocyclic compound
(1R*,5S*)-11a.15) Silyl enol etherification of 11a, followed
by intramolecular Claisen condensation, afforded stereo-
chemically confirmed (1R*,4R*,8S*)-10, whose spectral data
were identical to the above (3ꢀ3) cycloadduct 10 (Chart 5).
In conclusion, we have developed a Tf2NH catalyzed for-
mal (3ꢀ3) cycloaddition of silyl enol ethers with acrylates.
In the domino sequence, the catalyst activates Michael addi-
tion, deprotonation of the resulting silyloxonium cation and
intramolecular Claisen condensation.24) We also demon-
strated the construction of the clovane skeleton by the in-
tramolecular reaction. Further studies to improve the chemi-
cal yield are under investigation. It is noteworthy that reac-
tion modes significantly depend on the reaction temperature.
The (3ꢀ3) Cycloadduct was given at ambient temperature,
whereas (2ꢀ2) cycloadduct was exclusively obtained at
ꢂ78 °C.
(E)-Methyl 6-{2-(tert-Butyldimethylsiloxy)cyclohex-1-en-1-yl}hex-2-
enoate (9) To a solution of the corresponding ketone15) (1.0 mmol) and
NEt3 (1.2 mmol) in MeCN (2.0 ml) was added TBSCl (1.2 mmol) in MeCN
(3.0 ml). To the mixture was added NaI (1.2 mmol) in MeCN (5.0 ml) drop-
wise. After being stirred 48 h at ambient temperature, the resulting mixture
was quenched with aqueous NaHCO3 and extracted three times with AcOEt.
The combined organic layers were washed with brine and dried over
Na2SO4, and concentrated in vacuo. The residue was purified by silica gel
chromatography (hexane : AcOEt : NEt3ꢃ200 : 10 : 1) to afford 9 in 88%
yield. Colorless oil; 1H-NMR (500 MHz, CDCl3) d: 6.99 (dt, Jꢃ15.5,
7.8 Hz, 1H), 5.81 (dt, Jꢃ15.5, 1.7 Hz, 1H), 3.72 (s, 3H), 2.22—2.16 (m,
2H), 2.09—2.00 (m, 4H), 1.96—1.92 (m, 2H), 1.64—1.61 (m, 2H), 1.54—
1.48 (m, 4H), 0.92 (s, 9H), 0.10 (s, 6H); 13C-NMR (126 MHz, CDCl3) d:
167.2, 150.0, 143.6, 120.7, 114.6, 51.3, 32.4, 30.4, 29.7, 27.8, 26.2, 25,7,
23.7, 23.0, 18.2, ꢂ3.7; IR (ATR) 2928, 1726 cmꢂ1; MS (EI) 339 (Mꢀ, 70),
337 (100); Anal. Calcd for C19H3O34Si: C, 67.40; H, 10.12, Found: C, 67.31;
H, 10.39.
(1R*,4R*,8S*)-Tricyclo[6.3.1.04,8]dodecane-2,12-dione (10) Colorless
1
needles, mp 29—30 °C; H-NMR (500 MHz, CDCl3) d: 3.02 (t, Jꢃ3.7 Hz,
1H), 2.76 (dd, Jꢃ14.6, 5.8 Hz, 1H), 2.51—2.43 (m, 2H), 2.22—2.18 (m,
1H), 2.10—2.04 (m, 1H), 2.00—1.90 (m, 3H), 1.88—1.86 (m, 1H), 1.68—
1.52 (m, 4H), 1.32—1.24 (m, 2H); 13C-NMR (126 MHz, CDCl3) d: 212.8,
210.0, 61.2, 57.6, 47.7, 42.6, 39.7, 35.5, 34.9, 33.2, 24.5, 19.8; IR (ATR)
2929, 1726, 1701 cmꢂ1; MS (FAB) 193 (Mꢀ, 100); HR-MS (FABꢀ)
[C12H17O2]ꢀ: 193.1229; Found. 193.1227.
(1R*,5S*)-1-Methoxycalbonylmethylspiro[4.5]decan-6-one (11a) Color-
1
less oil, H-NMR (500 MHz, CDCl3) d: 3.66 (s, 3H), 2.91—2.84 (m, 1H),
2.42—2.36 (m, 3H), 2.10—2.06 (m, 1H), 2.09—1.55 (m, 11H), 1.29—1.25
(m, 1H); 13C-NMR (126 MHz, CDCl3) d: 213.5, 173.6, 58.1, 51.5, 39.9,
39.4, 35.1, 35.0, 31.4, 29.8, 26.8, 21.7, 21.6; IR (ATR) 2943, 1734,
1699 cmꢂ1; MS (FAB) 225 (MHꢀ, 100); HR-MS (FABꢀ)[C13H21O3]ꢀ:
225.1491; Found 225.1489.
(1R*,2S*,3S*,7S*)-1-tert-Butyldimethylsiloxy-2-methoxycarbonyl-tri-
1
cyclo[5.4.0.03,7]undecane (12) Colorless oil; H-NMR (500 MHz, CDCl3)
d: 3.69 (s, 3H), 2.68 (d, Jꢃ10.2 Hz, 1H), 2.41—2.32 (m, 2H), 1.82—1.63
(m, 5H), 1.61—1.30 (m, 7H), 1.19—1.14 (m, 1H), 0.95 (s, 9H), 0,12 (s,
3H), 0.03 (s, 3H); 13C-NMR (126 MHz, CDCl3) d: 173.3, 74.9, 53.2, 53.1,
51.0, 38.8, 35.0, 33.6, 32.7, 30.7, 25.9, 25.3, 22.4, 19.8, 18.4, ꢂ3.0, ꢂ3.2;
IR (ATR) 2928, 1733 cmꢂ1; MS (FAB) 339 (MHꢀ, 32), 281 (100); HR-MS
(FABꢀ) [C19H35O3Si]ꢀ: 339.2355; Found 339.2342.
Experimental
General Procedure for the Formal (3ꢀ3) Cycloaddition To a solution
of 1 (0.50 mmol) in refluxed CHCl3 (5 ml) were added Tf2NH in 0.08 M
toluene solution (0.050 mmol) and 2a (0.50 mmol). After being stirred for
1 h, the resulting mixture was quenched with aqueous NaHCO3 and ex-
tracted twice with CHCl3. The combined organic layers were washed with
brine and dried over Na2SO4, and concentrated in vacuo. The residue was
purified by silica gel chromatography.
(1S*,5S*)-5-Methylbicyclo[3.3.1]nonane-2,9-dione (6a) Colorless oil;
1H-NMR (500 MHz, CDCl3) d: 3.22 (t, Jꢃ3.8 Hz, 1H), 2.70 (dt, Jꢃ17.2,
7.5 Hz, 1H), 2.48 (dt, Jꢃ17.2, 8.9 Hz, 1H), 2.38—2.35 (m, 1H), 2.10—1.95
Acknowledgements This work was supported by Grants-in-Aid for Sci-
entific Research for Scientific Research on Innovative Areas “Reaction Inte-
gration” (No. 2105) and Target Proteins Research Program from the Min-
istry of Education, Culture, Sports, Science and Technology of Japan and
Takeda Science Foundation.

September 2011
1193
References and Notes
56, 1205—1206 (2008).
1) Magnus P., Mugrage B., J. Am. Chem. Soc., 112, 462—464 (1990).
2) Magnus P., Coldham I., J. Am. Chem. Soc., 113, 672—673 (1991).
3) Magnus P., Rigollier P., Lacour J., Tobler H., J. Am. Chem. Soc., 115,
12629—12630 (1993).
4) Kuwajima I., Shoda H., Nakamura T., Tanino K., Tetrahedron Lett.,
34, 6281—6284 (1993).
14) Takasu K., Synlett, 2009, 1905—1914 (2009).
15) Takasu K., Ueno M., Inanaga K., Ihara M., J. Org. Chem., 69, 517—
521 (2004).
16) We have reported that retro (2ꢀ2) cycloaddition undergoes in the pres-
ence of acid catalysts at ambient temperature. See Takasu K., Ueno
M., Ihara M., J. Org. Chem., 66, 4667—4672 (2001).
17) Takasu K., Inanaga K., Ihara M., Tetrahedron Lett., 49, 4220—4222
(2008).
5) Ruck R. T., Jacobsen E. N., Angew. Chem. Int. Ed., 42, 4771—4774
(2003).
6) Bellur E., Görls H., Langer P., J. Org. Chem., 70, 4751—4761 (2005).
7) Kuwajima I., Horiguchi Y., Nakamura E., Tetrahedron Lett., 30,
3323—3326 (1989).
8) Mikami K., Ohmura H., Chem. Commun. (Cambridge), 2002, 2626—
2627 (2002).
9) He J., Tchabanenko K., Adlington R. M., Cowley A. R., Baldwin J. E.,
Eur. J. Org. Chem., 4003—4013 (2006).
10) Kusama H., Yamabe H., Iwasawa N., Org. Lett., 4, 2569—2571
(2002).
18) Only a limited number of acid-catalyzed Claisen condensation of silyl
enol ethers have been reported. See refs. 19—21.
19) Onaka M., Mimura T., Ohno R., Izumi Y., Tetrahedron Lett., 30,
6341—6344 (1989).
20) Iwata S., Hamura T., Matsumoto T., Suzuki K., Chem. Lett., 36, 538—
539 (2007).
21) Ondrus A. E., Kaniskan H. U., Movassaghi M., Tetrahedron, 66,
4784—4795 (2010).
22) Barton D. H. R., Nickson A., J. Chem. Soc., 4665—4669 (1954).
11) Inanaga K., Takasu K., Ihara M., J. Am. Chem. Soc., 127, 3668—3669 23) As a first total synthesis of clovane: Kundu A. K., Kundu N. G., Dutta
(2005).
P. C., J. Chem. Soc., 1965, 2749—2763 (1965).
12) Takasu K., Ishii T., Inanaga K., Ihara M., Org. Synth., 83, 193—199 24) Shindoh N., Takemoto Y., Takasu K., Chem. Eur. J., 15, 12168—
(2006).
12179 (2009).
13) Takasu K., Miyakawa Y., Ihara M., Tokuyama H., Chem. Pharm. Bull.,