Novel stereocontrolled synthesis of highly functionalized cyclobutanes by epoxide opening through a carbanion intermediate in heteroconjugate addition

Article abstract of DOI:10.1055/s-0028-1088108

We have demonstrated a new cyclobutane ring formation from the trans-1,2-disubstituted epoxides through intramolecular carbanion opening process. In this reaction, the nucleophilic carbanion is generated not via α-proton abstraction but via heteroconjugat

Full text of DOI:10.1055/s-0028-1088108

LETTER
1157
Novel Stereocontrolled Synthesis of Highly Functionalized Cyclobutanes by
Epoxide Opening through a Carbanion Intermediate in Heteroconjugate
Addition
Synthesis of
H
ighly Fu
a
nctionalize
s
d
C
yclobu
a
tanes atsu Adachi, Eiji Yamauchi, Takema Komada, Minoru Isobe*
Laboratory of Organic Chemistry, School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan
Fax +81(52)7894111; E-mail: isobem@ff.iij4u.or.jp
Received 22 January 2009
vent and temperature.5 In addition, thermal [2+2] cycload-
Abstract: We have demonstrated a new cyclobutane ring formation
ditions proceed inefficiently unless ketenes or haloketens
from the trans-1,2-disubstituted epoxides through intramolecular
with highly electrophilicity are chosen as substrates.6 To
date, alternative synthetic strategies for the formation of
carbanion opening process. In this reaction, the nucleophilic carb-
anion is generated not via a-proton abstraction but via heteroconju-
gate addition. These studies indicate that the different configuration four-membered rings include an intramolecular cycliza-
tion,7 a rearrangement of an oxonium ylide,8 ‘Cp2Zr’ or
of each epoxide (syn and anti) do not affect its reactivity and the re-
action velocity in the cyclization step, providing multifunctional-
ized cyclobutanes in a regio- and stereospecific manner.
SmI2/Pd(0)-mediated ring contraction of a vinylfurano-
side,9 and aluminum- or cobalt-mediated ring contraction
of a dihydropyran.10,11
Key words: heteroconjugate addition, sulfonyl carbanion, ep-
oxides, cyclization, cyclobutanes
In 1974, Stork reported the first example of intramolecu-
lar nucleophilic opening of epoxides 1 with a carbanion
stabilized by nitrile group to provide cyclobutane 3 but
Cyclobutanes are an important element in many natural
not cyclopentane 4.12 This reaction was further examined
products with intriguing physical, conformational, and bi-
with various kinds of epoxides to culminate in regioselec-
ological activities.1 Additionally, cyclobutane derivatives
tivity as that the cis-1,2-disubstituted epoxides provided
are potentially interesting intermediates for constructing a
predominantly the cyclobutane products rather than
diverse array of compounds through transformations such
cyclopentene derivatives.13 Epoxide opening by 1,3-
as ring openings and ring expansions.2 However, the con-
dithiane anion is an alternative.14 Many other examples of
struction of cyclobutanes is one of the most challenging
ionic intramolecular cyclobutane ring cyclization were
synthetic problems in cycloaliphatic chemistry – primari-
limited to have little substituents on this ring. We found
ly due to the inherent ring strain of the cyclobutane moi-
that similar epoxide opening took place with a carbanion
ety. The standard strategy for constructing four-
6 stabilized by phenylsulfonyl group. In this paper, we re-
membered cycloaliphatic rings has remained, for a centu-
port a new cyclobutane ring formation from the trans-1,2-
ry, an inter- or intramolecular [2+2]-photochemical
disubstituted epoxides through intramolecular carbanion-
cycloaddition between two alkenes, since its discover by
opening process. In this case, nucleophilic carbanion was
Ciamician in 1908.3,4 Unfortunately, the resulting regio-
generated not via a-proton abstraction15 but via hetero-
and stereochemistry has proven difficult to control, and
conjugate addition.
often dependent on various factors such as reaction sol-
R1
R1
OH
EWG
(R2O)
R1
EWG
EWG
HH
EWG
H
R1
base
NuLi
or
O
O
OH
(OR2)
(OR2)
(OR2)
1
2
3
4
OH
R1
OR2
NuLi
–78 °C
PhO2S
Nu
OR2
PhO2S
R3Si
PhO2S
workup
KF
R1
R1
R3Si
then
increasing
OR2
O
O
Nu
temperature
5
6
7
Scheme 1 Opening of epoxides with in situ generated carbanions via HCA leading to cyclobutane ring formation
SYNLETT 2009, No. 7, pp 1157–1161
x
x.
x
x.
2
0
0
9
Advanced online publication: 26.03.2009
DOI: 10.1055/s-0028-1088108; Art ID: U00609ST
© Georg Thieme Verlag Stuttgart · New York

1158
M. Adachi et al.
LETTER
During the course of our synthetic studies toward natural the reaction velocity of, the intramolecular cyclization
products, such as maytansine, okadaic acid, tautomycin, step.
etc., we have developed an efficient synthetic methodolo-
The current cyclobutane ring formation proved to be
gy that we named ‘heteroconjugate addition (HCA, 5 to
regio- and stereospecific through in situ generated a-sul-
6)’.16,17 Several more generations of the HCA have up-
fonyl carbanion, which prompted us to examine more
graded this methodology to provide either syn or anti and
complex systems having oxymethyl group at the g-posi-
D- or L-isomer, which we call switching of diastereo- and
tion. As shown in Scheme 3, the starting allyl epoxide 12,
enantioselectivity.18 A typical stereochemical course in
which was reported by Marshall23 from D-mannitol in
THF solvent results in the syn-diastereomer in 5–10 min-
eight steps in 88% ee. We employed Sharpless’s im-
utes at –78 °C. This intermediate carbanion 6 is stabilized
by the a-sulfonyl group, and it is potentially changeable
further to dicarbanion through Si–C to Si–O rearrange-
ment.19 Usage of this carbanion might intramolecularly
open the trans-epoxide to yield cyclobutanes 7. The cur-
rent concept is summarized in Scheme 1, which includes
(i) the in situ generation of a carbanion with stereocon-
trolled introduction of a nucleophile during HCA and (ii)
intramolecular epoxide opening to result in the highly
functionalized cyclobutane ring formation.
proved epoxidation method.24 Addition of phenylthio-
acetylide in the presence of BF3·OEt2 afforded the adduct
13. Hydrosilylation was achieved under catalytic condi-
tion by a biscobalthexacarbonyl complex 14, which yield-
ed regioselectively the vinylsilylsulfide 15.25 After its
oxidation to the sulfonyl group, many attempts for selec-
tive epoxidation failed,26 so we have converted to allylic
alcohol 17 and employed Sharpless asymmetric epoxida-
tion, which led to the epoxyalcohol 19 (Scheme 3).
Treatment of this syn-vinylsulfone-epoxide 19 with lithi-
um acetylide at –78 °C, was followed by increasing the
temperature to –23 °C to provide cyclobutane 20 as a sin-
gle product in 45% yield.27 In the homologous cases, we
have prepared both syn- and anti-epoxides (21 and 23) un-
der similar manner as 19, and treated with lithium tri-
methylsilylacetylide to obtain the cyclobutanes (22, 24) in
51% and 42% yields, respectively (Scheme 4).28 Notably,
formation of oxetane and tetrahydrofuran rings was not
observed through nucleophilic attacks of a resulting
alkoxide to the neighboring epoxide.
The vinylsulfone-epoxide 8 was synthesized as an ap-
proximately 1:1 mixture of diastereomers of epoxides
from the corresponding trans-olefin. Treatment of this
vinylsulfone-epoxide 8 with lithium acetylide (generated
from trimethylsilylacetylene and MeLi·LiBr complex) at
–78 °C was followed by gradually increasing the reaction
temperature to provide three kinds of products 9, 10, and
11 in 73% yield (in a ratio of 48:45:7), respectively
(Scheme 2).20–22 As a result, syn/anti diastereomeric ep-
oxides were allowed to open and yield stereospecifically
the corresponding products 9 and 10, respectively.
The reaction mechanism proposed for HCA followed by
cyclobutane ring formation is depicted in Scheme 5. The
relative orientation between the sp2 plane and the allylic
carbon atom in 25 should be highly populated as such that
the allylic hydrogen atom becomes in this plane 26 due to
A-strain. This has already been well established so that the
conjugate addition largely provide the syn-addition prod-
uct 27. This carbanion intermediate would have a linear
line up at the transition state for epoxide-opening step, so
Treatment of the anti-epoxide 8 (syn/anti = 1:23) with
lithium acetylide, under the same conditions, afforded the
expected cyclobutane 9 as the major product in 80% yield
along with cyclobutane 11 in 3%. Also treatment of the
syn-epoxide 8 (syn/anti = 2.4:1) with lithium acetylide
yielded three isomers 9, 10, and 11 in 16%, 38%, and 5%
yield, respectively. These results support the reaction
pathway is stereospecific, and show that different config-
urations of the epoxide 8 do not affect its reactivity in, or
PhO2S
PhMe2Si
OH
OTBS
O
8
9
10
11
TMS
H
8 (syn/anti = 1:1)
8 (syn/anti = 1:23)
8 (syn/anti = 2.4:1)
5
3
5
35
80
33
0
MeLi⋅LiBr, THF
–78 to –44 °C, 1 h
then –23 °C, 1 h
16
38
yield (%) of products from different syn/anti 8
TMS
+
TMS
TMS
SiMe2Ph
SO2Ph
SO2Ph
SO2Ph
+
OTBS
OSiMe2Ph
OTBS
OSiMe2Ph
OTBS
HO
HO
HO
H
H
H
OH
9
10
11
Scheme 2 Cyclobutane ring formation through HCA from vinylsulfone-epoxide 8
Synlett 2009, No. 7, 1157–1161 © Thieme Stuttgart · New York

LETTER
Synthesis of Highly Functionalized Cyclobutanes
1159
OH
Co2(CO)6
OH
O
PhS
H
14 (10 mol%)
OBn
OBn
PhMe2SiH, EtOH
DCE, 60 °C
72%
n-BuLi, BF3⋅OEt2
–78 °C, 85%
PhS
12
13
OH
1) MCPBA, CH2Cl2
–23 °C, 82%
OAc
PhS
PhMe2Si
PhO2S
OBn
OBn
PhMe2Si
2) Ac2O, py
60 °C, 75%
H
15
16
Ti(Oi-Pr)4 (6 mol%)
L-DCDT (5 mol%)
OAc
PhO2S
DDQ
OH
PhMe2Si
DCE, H2O
60 °C, 46%
TBHP, 4 Å MS
chromatogr. pure 50%
17
1) BnBr, Ag2O
toluene, 60 °C
80%
OAc
OH
PhO2S
PhO2S
OH
OBn
PhMe2Si
PhMe2Si
2) DIBAL-H, CH2Cl2
–78 °C, 87%
O
O
18
19
Scheme 3 Synthesis of a homologue syn-epoxide 19
the regioselective cyclization is explained not only by the epoxide must restrict conformational mobility and
Baldwin’s rules, but also by the steric factors between the forces the difference in intramolecular cyclization as two
reactive sites. The sulfonyl carbanion can overlap with the possible transition states 28 or 31. Additionally, gradually
C4–O antibonding orbital of the epoxide in the transition increasing the temperature of the reaction stimulates the
state, resulting in the preferential 4-exo-tet cyclization. cyclizations and, for the syn-epoxides (path b), promotes
Notably, no five-membered ring was observed in the an intramolecular anionic C-to-O migration of the silyl
product through 5-endo-tet cyclization. The reaction group. In the cyclobutane 29, it seems that the C-to-O
courses for the cyclization seems to be dependent on steric silicon-group migration would be slower than the case of
hindrance of the substituents due to the syn/anti configu- 32 due to the diastereomeric difference. Protonation of the
rational difference. In the case of both the anti-epoxides migration product in path b happened at the a-position of
(path a) and the syn-epoxides (path b), the steric interac- sulfonyl group in a way that the phenylsulfonyl group be-
tions between the silyl group at C-1 and the side chain of came trans to the neighboring alkyl side chain.
TMS
TMS
H
OH
MeLi⋅LiBr, THF
–78 to –43 °C, 1 h
SO2Ph
PhO2S
PhMe2Si
OBn
HO
then –23 °C, 3 h
45%
O
OBn
H
R
H
OSiMe2Ph
20
19
21
23
TMS
TMS
H
OH
MeLi⋅LiBr, THF
–78 to –43 °C, 1 h
PhO2S
TES
SO2Ph
OBn
HO
then –20 °C, 3 h
51%
OBn
OR
22 (R = TES, H)
O
TMS
TMS
H
TES
OH
MeLi⋅LiBr, THF
–78 to –43 °C, 1 h
PhO2S
TES
SO2Ph
OBn
then –20 °C, 3 h
42%
HO
O
OBn
H
OH
24
Scheme 4 Synthesis of cyclobutane derivatives 20, 22, and 24 from syn-epoxide 19, 21, and anti-epoxide 23, respectively
Synlett 2009, No. 7, 1157–1161 © Thieme Stuttgart · New York

1160
M. Adachi et al.
LETTER
cyclobutanes in a regio- and stereospecific manner. Appli-
cation of this method to the synthesis of various
cyclobutanes1 is now in progress.
OH
n
PhO2S
OR1
*
4
PhMe2Si
1
2
*
O
25 (n = 0, 1)
Supporting Information for this article is available online at
LiNu
heteroconjugate addition
O
http://www.thieme-connect.com/ejournals/toc/synlett.
Li
Nu
n
Acknowledgment
H
PhO2S
R2
This work was supported by a Grant-in-Aid for Specially Promoted
Research [16002007(2004-8)] from Ministry of Education, Culture,
Sports, Science and Technology (MEXT), and also by a grant from
Asahi Kasei Pharma Award in Synthetic Organic Chemistry, Japan
(to M. A.). We are thankful to Mr. K. Yoza (Bruker AXS) for X-ray
diffraction.
PhMe2Si
26
A-strain and chelation control
OLi
PhO2S
n
OR1
*
References and Notes
PhMe2Si
*
O
(1) (a) Grandisol is a histrical example of pheromone having a
cyclobutane moiety, see: Petschen, I.; Parrilla, A.; Bosch,
M. P.; Amela, C.; Botar, A. A.; Camps, F.; Guerreo, A.
Chem. Eur. J. 1999, 5, 3299. (b) Solanoeclepin may be
another example having three-, four-, five-, six-, and seven-
membered rings including cyclobutane moiety of fully
functional groups, see: Schenk, H.; Driessen, R. A. J.; de
Gelder, R.; Goubitz, K.; Nieboer, H.; Brüggemann-Rotgans,
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27
TMS
path a (anti-epoxides)
path b (syn-epoxides)
R1O
PhMe2Si
H
H
TMS
PhMe2Si
H
H
TMS
O
Li
O
PhO2S
O
Li
n
H
PhO2S
O
OR1
H
H
n
H
28
31
epoxide opening
TMS
TMS
SO2Ph
SiMe2Ph
1
1
SiMe2Ph
SO2Ph
2
2
4
4
5
5
LiO
LiO
OR1
OR1
n
n
H
H
O
O
Li
Li
29
protonation
32
C-to-O silyl
migration and
protonation
TMS
TMS
(7) (a) Wenkert, E.; Bakuzis, P.; Baumgarten, R. J.; Leicht,
C. L.; Schenk, H. P. J. Am. Chem. Soc. 1971, 93, 3208.
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SiMe2Ph
SO2Ph
SO2Ph
OR1
HO
OR1
OSiMe2Ph
HO
n
H
n
H
OH
30
33
Scheme 5 Possible mechanism of the intramolecular cyclization
utilizing the syn- and anti-epoxides
In conclusion, we have demonstrated the stereocontrolled
synthesis of multifunctionalized cyclobutanes through in-
tramolecular epoxide opening by carbanion intermediates
of heteroconjugate addition. These studies indicate that
the different configurations of each trans-1,2-disubstitut-
ed epoxide do not affect its reactivity and the reaction ve-
locity in the cyclization step, providing multifunctionalized
(8) Roskamp, E. J.; Johnson, C. R. J. Am. Chem. Soc. 1986, 108,
6062.
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Arrate, M. J. Org. Chem. 2000, 65, 6493.
Synlett 2009, No. 7, 1157–1161 © Thieme Stuttgart · New York

LETTER
Synthesis of Highly Functionalized Cyclobutanes
1161
(10) (a) Menicagli, R.; Malanga, C.; Lardicci, L.; Tinucci, L.
Tetrahedron Lett. 1980, 21, 4525. (b) Menicagli, R.;
Malanga, C.; Lardicci, L. J. Org. Chem. 1982, 47, 2288.
(c) Meek, S. J.; Pradaux, F.; Demont, E. H.; Harrity, J. P. A.
Org. Lett. 2006, 8, 5597.
J = 9.7, 5.4 Hz), 4.27 (1 H, d, J = 8.1 Hz), 4.87 (1 H, br dt,
J = 9.3, 4.8 Hz), 4.94 (1 H, d, J = 11.5 Hz), 4.98 (1 H, d,
J = 4.1 Hz), 7.26 (2 H, t, J = 7.5 Hz), 7.36 (1 H, t, J = 7.5
Hz), 7.49 (2 H, t, J = 7.5 Hz), 7.61–7.68 (3 H, m), 7.91 (2 H,
d, J = 7.5 Hz). 13C NMR (150 MHz, CDCl3): d = –5.4, –5.4,
–3.6, –2.6, –0.3, 18.3, 25.9, 45.8, 47.6, 57.4, 65.0, 70.9, 71.5,
91.6, 104.0, 127.8, 128.8, 129.0, 130.1, 134.0, 134.6, 135.8,
140.5. Anal. Calcd for C31H48O5SSi3: C, 60.34; H, 7.84.
Found: C, 60.34; H, 7.98.
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Angew. Chem., Int. Ed. Engl. 1994, 33, 1345.
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(14) Krohn, K.; Börner, G. J. Org. Chem. 1994, 59, 6063.
(15) Direct generation of a carbanion by proton abstraction from
a-sulfonyl group cannot be achieved due to the fact that an
epoxidic proton would be abstracted to convert the epoxide
into an enolate under these conditions.
(16) (a) Isobe, M.; Kitamura, M.; Goto, T. J. Am. Chem. Soc.
1982, 104, 4997. (b) Kitamura, M.; Isobe, M.; Ichikawa, Y.;
Goto, T. J. Am. Chem. Soc. 1984, 106, 3252. (c) Isobe, M.;
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1987, 43, 4767. (d) Ichikawa, Y.; Tsuboi, K.; Jiang, Y.;
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Isobe, M. Tetrahedron 1997, 53, 5123.
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M. Nippon Nogeikagaku Kaishi 1981, 55, 47. (b) Isobe, M.
J. Synth. Org. Chem. Jpn. 1983, 41, 51. (c) Isobe, M. In
Perspective in the Organic Chemistry of Sulfur;
Cyclobutane 10: IR (KBr): nmax = 3493, 2956, 2172, 1428,
1247, 1147, 1023, 967, 848 cm–1. 1H NMR (400 MHz, C6D6,
318 K): d = 0.09 (6 H, s), 0.10 (9 H, s), 0.54 (3 H, s), 0.56 (3
H, s), 1.00 (9 H, s), 2.09 (1 H, d, J = 5.4 Hz), 3.08 (1 H, ddd,
J = 9.4, 7.4, 2.3 Hz), 3.30 (1 H, dd, J = 9.1, 7.2 Hz), 3.37 (1
H, t, J = 9.1 Hz), 3.46 (1 H, dd, J = 10.5, 5.0 Hz), 3.56 (1 H,
dd, J = 10.5, 5.0 Hz), 4.10 (1 H, td, J = 5.0, 2.3 Hz), 4.37 (1
H, td, J = 7.2, 5.5 Hz), 7.06–7.20 (3 H, m), 7.30–7.40 (3 H,
m), 7.71–7.76 (2 H, m), 7.87–7.92 (2 H, m). 13C NMR (100
MHz, C6D6): d = –5.4, –5.4, –1.7, –0.7, –0.1, 18.6, 26.1,
37.2, 48.2, 56.1, 66.5, 68.4, 71.6, 87.7, 103.8, 128.7, 128.8,
129.1, 129.9, 133.3, 133.9, 138.4, 139.6. Anal. Calcd for
C31H48O5SSi3: C, 60.34; H, 7.84. Found: C, 60.34; H, 7.96.
(23) Marshall, J. A.; Trometer, J. D.; Cleary, D. G. Tetrahedron
1989, 45, 391.
(24) Kolb, H. C.; Sharpless, K. B. Tetrahedron 1992, 48, 10515.
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see: (a) Isobe, M.; Nishizawa, R.; Nishikawa, T.; Yoza, K.
Tetrahedron Lett. 1999, 40, 6972. (b) Liu, T.-Z.;
Kirschbaum, B.; Isobe, M. Synlett 2000, 587. (c) Liu, T.-Z.;
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M. Synlett 2003, 547. (e) Baba, T.; Huang, G.; Isobe, M.
Tetrahedron 2003, 59, 6851.
Zwanenburg, B.; Klunder, A. J. H., Eds.; Elsevier Science
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1982, 23, 221.
(18) Tsuboi, K.; Ichikawa, Y.; Isobe, M. Synlett 1997, 713.
(19) Isobe, M.; Kitamura, M.; Goto, T. Tetrahedron Lett. 1979,
20, 3465.
(20) The cyclobutane ring structure of 9 was confirmed by X-ray
crystallographic analysis (see Supporting Information), and
other structures were confirmed through NMR
(27) Cyclobutane 20: [a]D27 –14.5 (c 1.15, CHCl3). IR (KBr): nmax
= 3525, 3031, 2172, 1305, 1148 cm–1. 1H NMR (400 MHz,
CDCl3): d = –0.02 (9 H, s), 0.40 (3 H, s), 0.41 (3 H, s), 2.61
(1 H, m), 2.91 (1 H, td, J = 9.0, 3.0 Hz), 2.95 (1 H, t, J = 9.0
Hz), 3.38 (2 H, d, J = 6.0 Hz), 3.48 (1 H, t, J = 9.0 Hz), 3.57
(1 H, dd, J = 11.5, 6.5 Hz), 3.64 (1 H, dd, J = 11.5, 6.5 Hz),
3.98 (1 H, td, J = 6.0, 3.0 Hz), 4.38 (1 H, d, J = 11.5 Hz),
4.44 (1 H, d, J = 11.5 Hz), 7.25–7.76 (15 H, m). 13C NMR
(100 MHz, CDCl3): d = –1.4, –1.1, –0.1, 27.1, 39.6, 40.8,
60.5, 64.1, 70.5, 71.9, 73.4, 87.2, 104.0, 127.9, 127.9, 127.9,
128.4, 128.4, 129.1, 129.7, 133.5, 133.6, 137.4, 137.7,
138.3. Anal. Calcd for C33H42O5SSi2: C, 65.32; H, 6.98.
Found: C, 65.32; H, 7.04.
spectroscopy.
(21) General Procudure for the Synthesis of Cyclobutane by
Heteroconjugate Addition
Trimethylsilylacetylene (5 equiv) was dissolved in THF and
cooled to –78 °C under argon atmosphere. To this cold
solution was added a solution of methyllithium–lithium
bromide complex (4 equiv) dropwise with stirring. This
stirring was continued at –78 °C for 30 min, and then a
solution of vinylsulfone-epoxide (1 equiv) in THF was
added to this mixture. After stirring for further 20 min, the
reaction mixture was allowed to warm to –44 °C, and the
temperature was kept at –44 °C for 40 min, then at –23 °C
for 1 h. The reaction mixture was poured into an ice-cooled
sat. aq NH4Cl. The aqueous layer was separated and
extracted with Et2O. The extracts were combined, washed
with H2O and brine, and then dried over Na2SO4. The
solution was concentrated in vacuo, and the residue was
purified by flash column chromatography to give the
corresponding cyclobutane.
(28) Cyclobutane 24: [a]D22 –34.2 (c 0.56, CHCl3). IR (KBr):
n
max = 3358, 3066, 3030, 1287, 1136 cm–1. 1H NMR (400
MHz, CDCl3): d = 0.03 (9 H, s), 0.55–0.66 (2 H, m), 0.72–
0.84 (4 H, m), 0.89 (9 H, t, J = 7.5 Hz), 2.77 (1 H, tdd,
J = 10.5, 7.5, 2.5 Hz), 3.26 (1 H, ddd, J = 10.5, 6.0, 1.0 Hz),
3.69 (1 H, dd, J = 12.5, 2.5 Hz), 3.73 (1 H, dd, J = 10.0, 5.0
Hz), 3.83 (1 H, dd, J = 10.0, 5.0 Hz), 3.86 (1 H, dd, J = 12.5,
7.5 Hz), 4.20 (1 H, dd, J = 10.5, 1.0 Hz), 4.56 (1 H, d,
J = 11.5 Hz), 4.61 (1 H, d, J = 11.5 Hz), 4.98 (1 H, td,
J = 6.0, 3.0 Hz), 7.28–7.39 (5 H, m), 7.48–7.54 (2 H, m),
7.61–7.67 (1 H, m), 7.92–7.96 (2 H, m). 13C NMR (100
MHz, CDCl3): d = –0.4, 3.7, 8.2, 32.5, 43.2, 46.6, 60.7, 63.1,
67.6, 72.5, 73.2, 90.2, 104.8, 127.8, 127.9, 128.4, 128.9,
129.1, 133.8, 137.8, 140.9. Anal. Calcd for C31H46O5SSi2: C,
63.44; H, 7.90. Found: C, 63.44; H, 7.97.
(22) Cyclobutane 9: IR (KBr): nmax = 3448, 2957, 2858, 1448,
1284, 1252, 1134, 1117, 842 cm–1. 1H NMR (600 MHz,
CDCl3): d = –0.14 (3 H, s), 0.08 (6 H, s), 0.17 (9 H, s), 0.47
(3 H, s), 0.94 (9 H, s), 3.41 (1 H, br d, J = 8.6 Hz), 3.51 (1 H,
dt, J = 11.5, 8.4 Hz), 3.63 (1 H, t, J = 9.5 Hz), 3.79 (1 H, dd,
Synlett 2009, No. 7, 1157–1161 © Thieme Stuttgart · New York

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