Practical and robust method for regio- and stereoselective preparation of (E)-ketene tert-butyl TMS acetals and β-ketoester-derived tert-butyl (1Z,3E)-1,3-bis(TMS)dienol ethers

Article abstract of DOI:10.1021/jo701456t

(Chemical Equation Presented) We developed an efficient, practical, robust method for the regio- and stereoselective preparation of (E)-ketene trimethylsilyl acetals (KSAs) derived from tert-butyl esters 1. The reaction was performed under convenient reac

Full text of DOI:10.1021/jo701456t

O-silylation using TMSOTf-Et3N reagent4 and an Rh(I)-
catalyzed hydrotrimethylsilylation of R,â-unsaturated esters.5
The former method lacks substrate generality and requires the
use of expensive reagents. The latter method requires hard to
handle gaseous HSiMe3 for trimethylsilylated KSAs and in-
volves the critical problem of O- and C-silylation regioselec-
tivity.
Practical and Robust Method for Regio- and
Stereoselective Preparation of (E)-Ketene
tert-Butyl TMS Acetals and â-Ketoester-derived
tert-Butyl (1Z,3E)-1,3-Bis(TMS)dienol Ethers
Tomohito Okabayashi, Akira Iida, Kenta Takai,
Yuuya Nawate, Tomonori Misaki, and Yoo Tanabe*
Even for traditional O-silylations of ester enolates with
TMSCl, there are some tedious procedures that restrict large-
scale preparation; (i) a low temperature (-78 °C) is generally
required to suppress two undesirable side reactions, that is
thermodynamically preferred self-Claisen condensation between
esters and/or competitive C-trimethylsilylation; and (ii) a more
careful operation is required compared with a related preparation
of enol silyl ethers from ketones or aldehydes, due to the
instability of KSAs against acids and bases (Scheme 1).
In close connection with our continued studies on practical
silylation and desilylation reactions of alcohols, aldehydes, and
ketones,6 and crossed-Claisen condensations utilizing KSAs,7
we present here a practical, robust, regio- and stereocontrolled
method for the preparation of not only (E)-ketene tert-butyl TMS
acetals 1, but also highly reactive, hence less accessible,
â-ketoester-derived tert-butyl (1Z,3E)-1,3-bis(TMS)dienol ethers
2.
Department of Chemistry, School of Science and Technology,
Kwansei Gakuin UniVersity, 2-1 Gakuen,
Sanda, Hyogo 669-1337, Japan
tanabe@kwansei.ac.jp
ReceiVed July 18, 2007
We developed an efficient, practical, robust method for the
regio- and stereoselective preparation of (E)-ketene trimeth-
ylsilyl acetals (KSAs) derived from tert-butyl esters 1. The
reaction was performed under convenient reaction conditions;
LDA-TMSCl, 0-5 °C, and cyclopentyl methyl ether
(CPME) solvent. Two kinds of (Z)- and (E)-KSAs derived
from R-oxygen and R-nitrogen-substituted tert-butyl esters,
respectively, were also obtained in good yield. The present
protocol was successfully applied to a stereocontrolled
preparation of useful, but highly reactive (less accessible)
â-ketoester-derived tert-butyl (1Z,3E)-1,3-bis(TMS)dienol
ethers 2.
The stereochemistry (E or Z) of KSAs is of primary
importance due to the successive diastereo and enantioselective
C-C bond-forming reactions. Extensive and systematic studies,
reported by Ireland,3c,e Heathcock,3d Corset,3f Otera,3g and their
co-workers, have revealed that the E- or Z-selectivity depends
upon subtle reaction conditions, such as molar ratio, temperature,
alkali amide, and solvent. Consequently, the E-isomer is a
(3) (a) Ainsworth, C.; Kuo, Y. N. J. Organomet. Chem. 1972, 46, 59.
(b) Rathke, M. W.; Sullivan, D. F. Synth. Commun. 1973, 3, 67. (c) Ireland,
R. E.; Mueller, R. H.; Willard, A. K. J. Am. Chem. Soc. 1976, 98, 2868.
(d) Oare, D. A.; Heathcock, C. H. J. Org. Chem. 1990, 55, 157. (e) Ireland,
R. E.; Wipf, P. J. Org. Chem. 1991, 56, 650. (f) Corset, J.; Froment, F.;
Lautie´, M.-F.; Ratovelomanana, N.; Seyden-Penne, J.; Strzalko, T.; Roux-
Schmitt, M.-C. J. Am. Chem. Soc. 1993, 115, 1684. (g) Otera, J.; Fujita,
Y.; Fukuzumi, S. Synlett 1994, 213.
Ketene silyl acetals (KSAs) are well recognized as highly
useful, activated ester derivatives and are employed as reactive
precursors in a wide range of organic syntheses, such as the
Mukaiyama aldol and Michael reactions, Ireland-Claisen
rearrangement, Diels-Alder reaction, etc.1 â-Ketoester-derived
1,3-bis(TMS)enol ethers, important reactive derivatives of
KSAs, also serve as useful and elaborate 1,3-dicarbonyl building
blocks.2 Despite the remarkable utility of the KSAs (in
particular, TMS derivatives), there is a high demand for a
practical, robust, and cost-effective preparative method of KSAs
from the recent standpoint of process chemistry.
(4) (a) Simchen, G.; West, W. Synthesis 1977, 247. (b) Yamamoto, K.;
Tomo, Y.; Suzuki, S. Tetrahedron Lett. 1980, 21, 2861. (c) Emde, H.;
Simchen, G. Liebigs Ann. Chem. 1983, 816.
(5) (a) Yoshii, E.; Kobayashi, Y.; Koizumi, T.; Oribe, T. Chem. Pharm.
Bull. 1974, 22, 2767. (b) Ojima, I.; Kogure, T. Organometallics 1982, 1,
1390. (c) Zheng, G. H.; Chan, T. H. Organometallics 1995, 14, 70. (d)
Slougui, N.; Rousseau, G. Synth. Commun. 1987, 17, 1. (e) Chan, T.-H.
Tetrahedron Lett. 1993, 34, 3095. (f) Mori, A.; Kato, T. Synlett 2002, 1167.
(6) (a) Tanabe, Y.; Murakami, M.; Kitaichi, K.; Yoshida, Y. Tetrahedron
Lett. 1994, 35, 8409. (b) Tanabe, Y.; Okumura, H.; Maeda, A.; Murakami,
M. Tetrahedron Lett. 1994, 35, 8143. (c) Misaki, T.; Kurihara, M.; Tanabe,
Y. Chem. Commun. 2001, 2478. (d) Tanabe, Y.; Misaki, T.; Kurihara, M.;
Iida, A. Chem. Commun. 2002 1628. (e) Iida, A.; Horii, A.; Misaki, T.;
Tanabe, Y. Synthesis 2005, 2677. (f) Iida, A.; Okazaki, H. T.; Sunagawa,
M.; Sasaki, A.; Tanabe, Y. J. Org. Chem. 2006, 71, 5380. (g) Iida, A.;
Hashimoto, C.; Misaki, T.; Katsumoto, Y.; Ozaki, Y.; Tanabe, Y. J. Org.
Chem. 2007, 72, 4970.
The most conventional method is performed by the O-
silylation of alkali metal ester enolates with TMSCl.1,3a,b There
are two other useful, but less accessible methods; a related
(1) (a) Smith, M. B.; March, J. AdVanced Organic Chemistry, 5th ed.;
Wiley: New York, 2001; p 1223. (b) Gennari, C. In ComprehensiVe Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol.
2, pp 630-657.
(7) (a) Iida, A.; Takai, K.; Okabayashi, Y.; Misaki, T.; Tanabe, Y. Chem.
Commun. 2005, 3171. (b) Iida, A.; Nakazawa, S.; Okabayashi, T.; Horii,
A.; Misaki, T.; Tanabe, Y. Org. Lett. 2006, 8, 5215. (c) Iida, A.; Osada, J.;
Nagase, R.; Misaki, T.; Tanabe, Y. Org. Lett. 2007, 9, 4970.
(2) Langer, P. Synthesis 2002, 441.
10.1021/jo701456t CCC: $37.00 © 2007 American Chemical Society
Published on Web 09/18/2007
8142
J. Org. Chem. 2007, 72, 8142-8145

TABLE 2. Regio- and (E)-Selective Preparation of Various KSAs
SCHEME 1. General Preparation of KSA
TABLE 1. Screening of Ether Solvents for the Preparation of
KSAs
yield
ratio
entry
R
solvent
product of A (%)a of A/Bb
E/Za
1
2
3
4
5
6
7
8
9
10
Me
Et2O
THF
dioxane
DME
t-BuOMe
CPME
3
65a
49b
60:40
69:31
66:34
-
>99:1
96:4
95:5
-
98:2
>99:1
98:2
>99:1
99:1
>99:1
46a
tracea
43a
90:10
97:3
69b
t-Bu t-BuOMe
4
5
63a
90:10
98:2
89:11
97:3
CPME
t-BuOMe
CPME
80b
Oct
76a
76b
1
a Isolated. b Determined by H NMR of the crude product. c 0-25 °C.
d E/Z ratios were not determined. e Use of KHMDS instead of LDA.
a Determined by H NMR of the crude product. b Isolated.
1
TABLE 3. Preparation of r-Oxygen or r-Nitrogen Substituted
KSAs
kinetic product, and the Z-isomer is a thermodynamic product.
Taking their findings into account, we focused our attention on
the preparation of analogous tert-butyl trimethylsilylated KSA
based on two promising features: (i) the expected use of a
practically accessible reaction temperature (from -20 to 20 °C),
because tert-butyl ester enolates are more stable among alkyl
esters to suppress undesirable self-Claisen condensation, and
(ii) the bulky tert-butyl group enhances E-selectivity, based
generally on Otera’s study.3g
The initial solvent-screening was guided using three tert-butyl
alkanoates for the preparation of KSAs 3-5: a tert-butyl
alkanoate was treated with LDA (1.1 equiv) to generate the Li
enolate at 0-5 °C, followed by trapping with TMSCl (1.2 equiv)
for 2.5 h using some ether solvents. Table 1 lists the results.
When KSAs 3 were used, Et2O, THF, dioxane, and DME
solvents resulted in poor regioselectivity accompanying undesir-
able C-silylation (entries 1-3). Although DME gave disap-
pointing results (entry 4), tert-BuOMe increased the regiose-
lectivity (entry 5). The best result was obtained using cyclopentyl
methyl ether (CPME)8 with regard to both yield and regiose-
lectivety (g97:3) for the preparation of all KSAs 3-5 (entries
6, 8, 10). Notice that the E-stereoselectivity of KSAs 3-5 was
excellent (E/Z ) >99:1). CPME solvent has recently attracted
attention from the standpoint of process chemistry, due to the
superiority to traditional ether solvents.
yield
ratio
entry
R
product
of A (%)a
of A/Bb
E/Zb
1
2
3
4
5
TBSO-
BnO-
MOMO-
(allyl)2N-
Bn2N-
13
14
15
16
17
59
58
66
51
95
>99:1
81:19
>99:1
>99:1
>99:1
20:80
3:97
2:98
89:11
>99:1
1
a Isolated. b Determined by H NMR of the crude product.
Dialkylated esters also underwent the desired reaction (entries
6-9). (iii) In the case of CH3CO2t-Bu, a higher yield was
obtained using NaHMDS. (iv) Highly carcinogenic and hazard-
ous HMPA, a frequently used cosolvent for the preparation of
KSA, was not necessary. (v) A 10 g-scale synthesis was
performed for the preparation of 3.
Next, we investigated the reaction using R-oxygen- or
R-nitrogen-substituted tert-butyl esters (lactates and glycinates)
because these O-and N-protected KSAs are useful substrates
for important acyclic stereocontrolled synthesis. Table 3 lists
the successful results. The desired KSAs 13-17 were success-
fully obtained in moderate to excellent yield (51-95%) with
complete regioselectivity (>99:1), except for the case of 14.
With regard to the stereoselectivity, KSAs 13-15 derived from
lactates showed Z-selectivity, whereas KSAs 16 and 17 derived
from glycinates switched to E-selectivity.
Using the optimized conditions shown in Table 1, several
KSAs 3-12 were prepared (Table 2). The salient features are
as follows. (i) In every case examined, KSAs 3-12 were
obtained in good yield (69-83%) with excellent regio- (>96:
4) and E-stereoselectivity (>99:1 except for 9 and 10). (ii) R,R-
To further demonstrate the utility of the present protocol, we
next focused our attention on a practical and robust preparation
of â-ketoester-derived 1,3-bis(TMS)-KSAs 2. Langer’s group
extensively studied the utilization of various 1,3-bis(TMS)dienol
ethers of â-dicarbonyl compounds and reviewed the impressive
(8) CPME was recently launched by the Zeon group and applied the
useful ether solvent in the place of THF, dioxolane, DME, etc. Watanabe,
K.; Yamagiwa, N.; Torisawa, Y. Org. Process Res. DeV. 2007, 11, 251.
Appropriate bp 106 °C, low solubility in water (high separation property),
low formation of peroxides, relative stability under acidic and basic
conditions, formation of azeotropes with water, a narrow explosion range.
J. Org. Chem, Vol. 72, No. 21, 2007 8143

TABLE 4. Preparation of tert-Butyl (1Z,3E)-1,3-Bis(TMS)dienol
Ethers
SCHEME 2. General Preparation of Methyl
(1Z)-1,3-Bis(TMS)dienol Ethers 19
progress in this area.2 â-Ketoester-derived methyl (1Z)-1,3-bis-
(TMS)dienol ethers 19 are recognized as highly reactive and
useful precursors for the Mukaiyama aldol,9 cyclopentannula-
tion,10 benzannulation,11 oxabicyclo[3.2.1]octan-3-one forma-
tion,12 tetrahydrofuran formation,13 furanone formation,14 Michael
addition,15 [4+2]-cycloaddition,16 and Claisen-type reactions.17
The most general method for the preparation of 19 involves
a two-step sequence; the starting methyl â-ketoesters are
converted to enol silyl ethers 18 using the Et3N-TMSCl reagent,
and then transformed to the desired dienol ethers 19 using the
LDA-TMSCl reagent at -78 °C (Scheme 2).11,18 Another one-
pot method utilizing the 2Et3N-2TMSOTf reagent lacks
substrate generality.19 Because purification and handling pro-
cedures of 1,3-bis(TMS)-KSAs require caution due to their high
inherent reactivity, a more robust preparative method is desired.
On the basis of the successful preparation of KSAs, we
speculated that the tert-butyl analogue of 1,3-bis(TMS)-KSAs
2 was a promising candidate for this purpose. tert-Butyl
â-ketoesters were directly converted to desired 2 using two
equivalents of the NaHMDS-TMSCl reagent.
1
a Isolated. b Determined by H NMR of the crude product.
SCHEME 3. Proposed Mechanism
Table 4 lists the successful results, and the salient features
are as follows. (i) NaHMDS was the best amide reagent with
regard to yield for the production of 20 (entry 1), compared
(9) (a) Chan, T.-H.; Brownbridge, P. J. Chem. Soc., Chem. Commun.
1979, 578. (b) Danishefsky, S.; Harvey, D. F.; Quallich, G.; Uang, B. J. J.
Org. Chem. 1984, 49, 393. (c) Hagiwara, H.; Kimura, K.; Uda, H. J. Chem.
Soc., Chem. Commun. 1986, 860. (d) Enders, D.; Burkamp, F.; Runsink, J.
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C. J. Am. Chem. Soc. 1996, 118, 5814. (f) Kru¨ger, J.; Carreira, E. M. J.
Am. Chem. Soc. 1998, 120, 837. (g) Evans, D. A.; Kozlowski, M. C.; Murry,
J. A.; Burgey, C. S.; Campos, K. R.; Connell, B. T.; Staples, R. J. J. Am.
Chem. Soc. 1999, 121, 669.
(10) (a) Langer, P.; Ko¨hler, V. Org. Lett. 2000, 1597. (b) Chan, T. H.;
Brook, M. A. Tetrahedron Lett. 1985, 26, 2943.
(11) (a) Chan, T.-H.; Brownbridge, P. J. Am. Chem. Soc. 1980, 102,
3534. (b) Kan, G. J.; Chan, T. H. J. Org. Chem. 1985, 50, 452.
(12) (a) Molander, G. A.; Siedem, C. S. J. Org. Chem. 1995, 60, 130.
(b) Molander, G. A.; Bessie`res, B.; Eastwood, P. R.; Nool, B. C. J. Org.
Chem. 1999, 64, 4124.
with other amides such as LDA (trace), LiHMDS (trace), and
KHMDS (16%). (ii) Despite the instability of 2, the reaction
was performed under practically accessible conditions (0-25
°C in CPME solvent) in a similar manner as for the preparation
of 1. (iii) The reaction proceeded in good to excellent yield.
(iv) (1Z,3E)-isomers 21-24 were obtained with high stereose-
lectivity, and to the best of our knowledge, this is the first
1
example of a stereoselective preparation of 2. The H NMR
chemical shifts of the two olefinic protons were characteristic:
H(2) and H(4) of the (1Z,3E)-isomers located at 4.41-4.47 and
4.57-4.63, respectively, and those for the (1Z,3Z)-isomers were
located at 4.27-4.29 and 4.98-5.04, respectively. The config-
uration of 21 was determined using the NOESY spectrum
between -CHd and CH3CHd, and between -CHd and
(CH3)2(t-Bu)O-. The stereochemistry of other analogs 22-24
were deduced on the basis of the chemical shift data of 21.
(13) (a) Langer, P.; Eckardt, T. Angew. Chem., Int. Ed. 2000, 39, 4343.
(b) Langer, P.; Freifeld, I.; Holtz, E. Synlett 2000, 501. (c) Langer, P.;
Freifeld, I. Chem. Eur. J. 2001, 7, 565. (d) Langer, P.; Armbrust, H.; Eckardt,
T.; Magull, Chem. Eur. J. 2002, 8, 1443.
(14) (a) Langer, P.; Krummel, T. Chem. Commun. 2000, 967. (b) Langer,
P.; Stoll, M. Angew. Chem., Int. Ed. 1999, 38, 1803. (c) Langer, P.; Saleh,
N. N. R. Org. Lett. 2000, 3333. (d) Langer, P.; Schneider, T.; Stoll, M.
Chem. Eur. J. 2000, 6, 3204. (e) Langer, P.; Eckardt, T. Synlett 2000, 844.
(f) Langer, P.; Eckardt, T.; Schneider, T.; Go¨bel, C.; Herbst-Irmer, R. J.
Org. Chem. 2001, 66, 2222.
(15) (a) Chan, T. H.; Prasad, C. V. C. J. Org. Chem. 1987, 52, 110. (b)
Chan, T. H.; Prasad, C. V. C. J. Org. Chem. 1987, 52, 120.
(16) (a) Krohn, K.; Ostermeyer, H.-H.; Tolkiehn, K. Chem. Ber. 1979,
112, 2640. (b) Roberge, G.; Brassard, P. Synthesis 1981, 381. (c) Roberge,
G.; Brassard, P. J. Org. Chem. 1981, 46, 4146. (d) Cameron, D. W.; Conn,
C.; Feutrill, G. I. Aust. J. Chem. 1981, 34, 1945. (e) O’Malley, G. J.;
Murphy, R. A., Jr.; Cava, M. P. J. Org. Chem. 1985, 50, 5533. (f) Roberge,
G.; Brassard, P. J. Org. Chem. 1981, 46, 4161.
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Lett. 2005, 46, 8423. (b) Rahn, T.; Nguyen, V. T. H.; Dang, T. H. T.;
Ahmed, Z.; Methling, K.; Lalk, M.; Fischer, C.; Spannenberg, A.; Langer,
P. J. Org. Chem. 2007, 72, 1957.
A proposed mechanism to account for the (1Z,3E)-stereose-
lectivity is depicted in Scheme 3. A tert-butyl â-ketoester is
converted to sodium monoenolate C, which is subsequently
transformed into bicyclic disodium dienolate transition-state D
or E. Steric repulsion between the R group and the bulky
hexamethyldisilazane group favors the production of E, which
is reacted with 2TMSCl to give the (1Z,3E)-diastereomer.
In conclusion, we developed an efficient, practical, regio-,
and stereoselective preparation of various (E)-KSAs 1 and 1,3-
bis(TMS)-KSAs 2, derived from tert-butyl esters and tert-butyl
â-ketoesters, respectively. The present robust method will
(18) Molander, G. A.; Cameron, K. O. J. Am. Chem. Soc. 1993, 115,
830.
(19) Kra¨geloh, K.; Simchen, G. Synthesis 1981, 30.
8144 J. Org. Chem., Vol. 72, No. 21, 2007

1
Colorless oil; bp 61-63 °C/20 mmHg; H NMR (300 MHz,
provide an easy access to these KSAs for process research and
natural product synthesis.
CDCl3): δ 0.23 (9H, s), 1.34 (9H, s), 3.41 (1H, d, J ) 1.4 Hz),
3.44 (1H, d, J ) 1.4 Hz); 13C NMR (75 MHz; CDCl3): δ -0.1,
28.5, 71.7, 78.1, 157.5; IR (neat) 2978, 2905, 1715, 1476, 1252,
Experimental Section
1163, 1047, 848, 760 cm-1
.
(1E)-tert-Butoxy-1-(trimethylsiloxy)propene (3).20 BuLi (1.59
M in hexane, 55.0 mL, 88 mmol) was added to a stirred solution
of i-Pr2NH (13.5 mL, 96 mmol) in cyclopentyl methyl ether (CPME;
60 mL) at 0-5 °C under an argon atomosphere, and the mixture
was stirred at the same temp for 30 min tert-Butyl propanoate (10.42
g, 80 mmol) in CPME (20 mL) was added to the mixture at the
same temp during 15 min.
1,3-Bis(trimethylsiloxy)-1-tert-butoxybuta-1,3-diene (20).22 tert-
Butyl 3-oxobutanoate (791 mg, 5 mmol) was added to a stirred
solution of NaHMDS (1.0 M in THF, 11.0 mL, 11.0 mmol) in
CPME (5 mL) at 0-5 °C during 5-8 min under an Ar atmosphere,
and the reaction mixture was stirred at the same temperature for
30 min. TMSCl (1.52 mL, 12.0 mmol) was added to the mixture
during 3-5 min, followed by being stirred for 30 min. The mixture
was warmed up to room temp and stirred at the same temperature
for 1.5 h. The mixture was poured into ice water and hexane, which
was extracted with hexane. The organic phase was washed with
brine, dried (Na2SO4), and concentrated. The obtained crude oil
was purified by distillation to give the desired product 20 (1.15 g,
76%).
After being stirred for 30 min, TMSCl (12.18 mL, 96 mmol)
was added to the mixture at the same temp during 5 min, followed
stirring for 1.5 h. The mixture was poured into ice water and hexane,
which was extracted with hexane. The organic phase was washed
with brine, dried (Na2SO4), and concentrated. The obtained crude
oil was purified by distillation to give the desired product 3 (10.24
g, 69%).
Colorless oil; bp 65 - 67/0.5 mmHg; 1H NMR (300 MHz,
CDCl3): δ 0.20 (9H, s), 0.24 (9H, s), 1.36 (9H, s), 4.21 (1H, d, J
) 1.0 Hz), 4.25 (1H, d, J ) 1.0 Hz), 4.54 (1H, s); 13C NMR (75
MHz; CDCl3): δ 0.2, 0.6, 28.4, 79.7, 89.2, 91.0, 153.2, 153.7; IR
(neat) 2963, 1649, 1602, 1368, 1344, 1252, 1132, 1045, 1026, 847,
1
Colorless oil; bp 49-53 °C/5.6 mmHg; H NMR (300 MHz,
CDCl3): δ 0.20 (9H, s), 1.32 (9H, s), 1.49 (3H, d, J ) 6.9 Hz),
3.89 (1H, d, J ) 6.9 Hz); 13C NMR (75 MHz; CDCl3): δ -0.1,
10.7, 29.1, 78.1, 85.4, 152.0; IR (neat) 2978, 2932, 1680, 1253,
1209, 1153, 1051, 929, 873, 846, 758 cm-1
.
756 cm-1
.
C-TMS isomer of 3: colorless oil; 1H NMR (300 MHz,
CDCl3): δ 0.07 (9H, s), 1.12 (3H, d, J ) 7.2 Hz), 1.44 (9H, s),
1.94 (1H, q, J ) 7.2 Hz), 7.17-7.31 (3H, m); 13C NMR (75 MHz;
CDCl3): δ -2.8, 10.9, 28.3, 31.0, 79.2, 175.6.
Acknowledgment. This paper is dedicated to the late
professor Yoshihiko Itoh who died in 2006. This research was
partially supported by Grant-in-Aids for Scientific Research on
Basic Areas (B) “18350056”, Priority Areas (A) “17035087”
and “18037068”, and Exploratory Research “17655045” from
the Ministry of Education, Culture, Sports, Science and Tech-
nology (MEXT).
1-tert-Butoxy-1-(trimethylsiloxy)ethylene (12).21 tert-Butyl ac-
etate (3.48 g, 30 mmol) in CPME (7 mL) was added to a stirred
solution of KHMDS (0.5 M in toluene, 72 mL, 36 mL) in CPME
(23 mL) at 0-5 °C during 7 min under an argon atomosphere, and
the mixture was stirred at the same temp for 30 min. TMSCl (4.95
mL, 39 mmol) was added to the mixture during 3 min, followed
by being stirred for 30 min. The mixture was warmed up to room
temp and stirred at the same temp for 2 h. The mixture was poured
into ice water and hexane, which was extracted with hexane. The
organic phase was washed with brine, dried (Na2SO4), and
concentrated. The obtained crude oil was purified by distillation to
give the desired product 12A (3.56 g, 63%).
Supporting Information Available: Experimental details for
the preparation of (E)-ketene tert-butyl TMS acetals 4-11, 13-
17, and tert-butyl (1Z,3E)-1,3-bis(TMS)dienol ethers, 21-24. This
material is available free of charge via the Internet http://pubs.acs.org.
JO701456T
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1593.
(22) Molander, G. A.; Cameron, K. O. J. Am. Chem. Soc. 1993, 115,
830.
J. Org. Chem, Vol. 72, No. 21, 2007 8145