"Syn-effect" in the conversion of (e)-α,β-unsaturated esters into the corresponding β,γ-unsaturated esters and aldehydes into silyl enol ethers

Article abstract of DOI:10.1246/bcsj.77.2147

The stereochemistry in the conversion of (E)-α,β-unsaturated esters into the corresponding β,γ-unsaturated esters, and that in the conversion of aldehydes into the silyl enol ethers, were investigated. The Z/E ratios of the resulting β,γ-unsaturated ester

Full text of DOI:10.1246/bcsj.77.2147

Ó 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 2147–2157 (2004) 2147
Headline Articles
‘‘Syn-Effect’’ in the Conversion of (E)-ꢀ,ꢁ-Unsaturated Esters
into the Corresponding ꢁ,ꢂ-Unsaturated Esters and Aldehydes
into Silyl Enol Ethers
Samar Kumar Guha, Atsushi Shibayama, Daisuke Abe, Maki Sakaguchi,
ꢀ
ꢀ
Yutaka Ukaji, and Katsuhiko Inomata
Division of Material Sciences, Graduate School of Natural Science and Technology, Kanazawa University,
Kakuma, Kanazawa, Ishikawa 920-1192
Received June 28, 2004; E-mail: inomata@cacheibm.s.kanazawa-u.ac.jp
The stereochemistry in the conversion of (E)-ꢀ,ꢁ-unsaturated esters into the corresponding ꢁ,ꢂ-unsaturated esters,
and that in the conversion of aldehydes into the silyl enol ethers, were investigated. The Z=E ratios of the resulting ꢁ,ꢂ-
unsaturated esters and the silyl enol ethers varied according to the ꢂ-substituents of the (E)-ꢀ,ꢁ-unsaturated esters and
the ꢀ-substituents of the aldehydes, respectively. This phenomenon was rationalized by a ‘‘syn-effect’’, which may be
ꢀ
attributed primarily to a ꢃ ! ꢄ interaction and/or 6ꢄ-electron homoaromaticity.
Previously, we investigated the stereochemistry in the iso-
merization of ꢀ-unsubstituted (E)-vinylic sulfones to the corre-
sponding allylic sulfones, and that in the desulfonylation reac-
tion of ꢀ,ꢀ-dialkylated allylic sulfones with a base.1a,b In both
cases, the sterically unfavorable (Z)-allylic sulfones and (Z)-
alkadienes were predominantly formed, respectively. These
results were rationalized by ‘‘conformational acidity’’, which
essentially implies a ‘‘syn-effect’’. 2 We proposed that the
‘‘syn-effect’’ is primarily caused by 6ꢄ-electron homoaroma-
comes. A rational elucidation for the origin of these puzzling
phenomena has been strongly desired. In order to determine
the exact origin of these deconjugation reactions starting from
(E)-ꢀ,ꢁ-unsaturated esters, (E)-ꢀ,ꢁ-unsaturated esters 1 bear-
ing various substituents at the ꢂ-position were converted to the
corresponding ꢁ,ꢂ-unsaturated esters 2, and the stereochemi-
cal outcome of the Z=E ratios of products 2 was well rational-
ized herein by the ‘‘syn-effect’’. 5
Furthermore, in the preparation of silyl enol ethers from al-
dehydes, a mixture of (Z)- and (E)-isomers could be produced.
Although the Z=E ratios were reported to be altered depending
on the ꢀ-substituents of the aldehydes and the reaction condi-
tions, a reasonable interpretation for the stereochemistry was
not hitherto offered.6 In the present work, the reaction of alde-
hydes 5 with silylating agents in the presence of a base was al-
so systematically investigated, and the stereochemistry of the
resulting silyl enol ethers 6–8 was also elucidated by the
‘‘syn-effect’’ .
ꢀ
ticity and/or a ꢃ ! ꢄ interaction.1b Very recently, we inves-
tigated the ‘‘syn-effect’’ in the conversion of ꢀ-fluorinated (E)-
vinylic sulfones to the corresponding allylic sulfones, and also
in the desilylation reaction of ꢂ-silylated allylic and vinylic
sulfones to the corresponding allylic sulfones; it was ultimately
ꢀ
found that the ꢃ ! ꢄ interaction is the most important factor
for the ‘‘syn-effect’’ . 1c,d
It is well known that treatment of dienolates derived from
ꢀ,ꢁ-unsaturated carbonyl compounds with electrophiles, such
as proton or alkyl halides, affords deconjugated ꢁ,ꢂ-unsaturat-
ed carbonyl compounds.3,4 The reaction of ethyl (E)-2-alke-
noates with lithium amides in the presence of HMPA was re-
ported to give sterically unfavored (Z)-3-alkenoates as the
main products. The stereoselectivity was explained by the sta-
bility of the produced anion or cyclic transition model during
deprotonation.3a,b,d Furthermore, the deconjugative ꢀ-alkyla-
tion of (E)-4-methylthio-2-butenoate produced a mixture of
(E)- and (Z)-4-methylthio-3-butenoates,4a while the deconju-
gative ꢀ-alkylation and aldol reaction of (E)-4-methoxy-2-bu-
tenoate gave (Z)-4-methoxy-3-butenoates selectively.4b,c How-
ever, there was no explanation about the stereochemical out-
Results and Discussion
Preparation of (E)-ꢀ,ꢁ-Unsaturated Esters. (E)-ꢀ,ꢁ-
Unsaturated esters 1a–e,h containing an alkyl or a benzyloxy
group at the ꢂ-position were prepared by the Horner–
Emmons–Wadsworth reaction in good yields. In the cases
where trace amounts of (Z)-isomers were formed, they were
separated from the (E)-isomers by column chromatography
or preparative TLC. The ꢂ-phenyl substituted (E)-ꢀ,ꢁ-unsatu-
rated ester 1f was prepared according to Scheme 1. Ethyl (phen-
ylthio)acetate (3) was treated with sodium hydride, followed
by the addition of (2-bromoethyl)benzene in the presence of
Published on the web December 10, 2004; DOI 10.1246/bcsj.77.2147

2148 Bull. Chem. Soc. Jpn., 77, No. 12 (2004)
HEADLINE ARTICLES
Ph
(1.05 eq.), nBu4NI (trace)
Ph
CO2Et
NaH (1.1 eq.),
Br
PhS
CO2Et
DMF, rt, 24 h
SPh
4, 31%
3
mCPBA (1.05 eq.)
NaHCO3 (2.0 eq.)
CO2Et
Ph
β
Ph
γ
α CO2Et
SPh
Reflux in Toluene, 2 h
CH2Cl2, rt, 30 min
O
+
−
1f, 74% (in 2 steps)
Scheme 1.
AgF (3.0 eq.) under dark
CH3CN, rt, 7 d
F
Br
Br
CO2Et
CO2Et
CO2Et
1g, 37%
BnS
BnSH (1.0 eq.), Et3N (1.0 eq.)
CO2Et
1i, 73%
CH2Cl2, 0 ° C, overnight
Scheme 2.
Table 1. Conversion of (E)-ꢀ,ꢁ-Unsaturated Esters to the Corresponding ꢁ,ꢂ-Unsaturated Esters
i) LiHMDS (1.1 equiv.), HMPA (4.4 equiv.)
O
O
THF, −70 °C, 30 min
β
R
OR'
OR'
ii) HCl in EtOH, −70 °C → rt
γ
α
R
1
2
Entry
1
R
R0
1/2aÞ
Z=E of 2aÞ
Yield of 2/%bÞ
1
2
3
4
5
6
7
a
b
c
d
e
f
g
h
i
CH3
CH3
CH3CH2
(CH3)2CH
(CH3)3C
Ph
F
BnO
BnS
CH3CH2
CH3(CH2)7
CH3CH2
CH3CH2
CH3CH2
CH3CH2
CH3CH2
CH3CH2
CH3CH2
0/100
0/100
0/100
0/100
28/72
0/100
0/100
0/100
7/93
91/9
94/6
42
83cÞ
70
68
47
99
55
78
84
85/15
70/30
0/100
16/84
100/0
100/0
44/56
8dÞ
9
a) The ratios were determined by 400 MHz 1H NMR spectra. b) Isolated yields. c) The yield was improved
by careful workup after preliminary report.5 d) Quenched with Et3N HCl in EtOH instead of aq. HCl in
EtOH to avoid the hydrolysis of a vinyl ether moiety.
ꢁ
a catalytic amount of a quaternary ammonium iodide to give
the alkylated (phenylthio)acetate intermediate 4. This phenyl-
thio intermediate 4 was converted to the ꢂ-phenyl substituted
(E)-ꢀ,ꢁ-unsaturated ester 1f by oxidizing with mCPBA, fol-
lowed by refluxing in toluene.
ꢂ-Fluoro and ꢂ-benzylthio substituted (E)-ꢀ,ꢁ-unsaturated
esters 1g,i were prepared according to Scheme 2 by treating
ethyl (E)-4-bromo-2-butenoate with AgF or BnSH in the pres-
ence of Et3N, respectively.
Conversion of (E)-ꢀ,ꢁ-Unsaturated Esters to the Corre-
sponding ꢁ,ꢂ-Unsaturated Esters. (E)-ꢀ,ꢁ-Unsaturated es-
ters 1 bearing various substituents at the ꢂ-position were con-
verted to the corresponding ꢁ,ꢂ-unsaturated esters 2 by treat-
ing with lithium hexamethyldisilazide (LiHMDS) in the pres-
ence of HMPA, followed by quenching with aq. HCl diluted in
EtOH. The results of the isomerization reactions are summa-
rized in Table 1.
decreased along with their bulkiness: CH3{ > CH3CH2{ >
(CH3)2CH{ > (CH3)3C{ (Entries 1,3–5). Namely, the methyl
group realized high Z-selectivity, whereas the (E)-product
t
was exclusively obtained in the case of the Bu substituent.
In the case of the ꢂ-phenyl substituent, high E-selectivity
was observed (Entry 6). ꢂ-Fluoro and ꢂ-benzyloxy groups
were found to show complete Z-selectivity (Entries 7,8), while
ꢂ-benzylthio substituted 2-alkenoate 1i afforded almost a 1/1
mixture of (Z)- and (E)-3-alkenoates 2i (Entry 9). In the cases
of ꢂ-methyl and ꢂ-fluoro substituted esters 1a,g, the isolated
yields of products 2a,g were slightly lower compared with oth-
er ꢂ-substituents (Entries 1,7). This is due to the high volatility
of those products. In fact, the use of ester 1b derived from a
higher alcohol realized a better chemical yield (Entry 2).
The relative degree of the ‘‘syn-effect’’, depending on the ꢂ-
substituents R of (E)-ꢀ,ꢁ-unsaturated esters 1, was found for
their conversion to the corresponding ꢁ,ꢂ-unsaturated esters
2 as follows:
The Z-selectivity with respect to the ꢂ-alkyl substituents

S. K. Guha et al.
Bull. Chem. Soc. Jpn., 77, No. 12 (2004) 2149
R ¼ CH2R0), or a lone pair of electrons in a p-orbital of the
hetero atom (Fig. 2b, R ¼ XR0), respectively.2,10
F{ ꢂ BnO{ > CH3{ > CH3CH2{ > (CH3)2CH{
i
In the case of Pr-substituted ester 1d, 6ꢄ-electron homoar-
> BnS{ > Ph{ > (CH3)3C{
omaticity is difficult to be considered; however, the sterically
unfavorable (Z)-isomer was still obtained as the major product
(70%, Entry 4). Therefore, it is obvious that the ‘‘syn-effect’’
It seems to be possible to rationalize the relative degree of
the Z=E ratios by the ‘‘syn-effect’’ in the transition state of de-
protonation. It was reported that a C–CH3 eclipsed conforma-
ꢀ
t
arises from the ꢃ ! ꢄ interaction. In the case of Bu- and
Ph-substituted esters 1e,f, (E)-ꢁ,ꢂ-unsaturated esters 2e,f were
obtained as the major products. This result is probably due to
the bulkiness of those groups, which excludes a syn-conforma-
tion at the transition state. Especially in the case of the Ph
group, the steric repulsion between the ꢀ-proton of the ester
and the o-proton of the benzene ring avoids CC eclipsed form
A in Fig. 1 (Fig. 3a). In the case of ꢂ-benzylthio substituted
ester 1i, the contribution of the empty d-orbital of the S-atom,
tion of ethyl (E)-2-pentenoate (1a) was preferred due to hyper-
ꢀ
conjugation of the C–H bond at the ꢂ-position to the ꢄ
C=C
orbital of an electron-deficient olefin moiety.7 In the transition
state of deprotonation, the hyperconjugation of a developing
anion generated by the interaction of H with a base becomes
ꢂ
more effective in the eclipsed conformations, A and B, in both
ꢀ
tal (Fig. 1), and the other conformations can be neglected. Our
of which the developing anion is aligned with the ꢄ
orbi-
C=C
ꢀ
ꢀ
recent proposal that the ꢃ ! ꢄ interaction is the most prob-
such as ꢃC{H ! d, is still unclear, but ꢃC{S ! ꢄ interaction
able explanation for the ‘‘syn-effect’’ is very consistent with
this consideration. During the deprotonation of ꢂ-alkyl-2-bute-
noates 1a–d, the CC eclipsed syn-conformation A might be
preferred rather than CH eclipsed form B, because a hypercon-
(Fig. 3b) might be responsible for the predominance of the CH
eclipsed form B (R ¼ SBn) in Fig. 1 to increase the E-selectiv-
ity.11 The order in the relative degree of the ‘‘syn-effect’’ of the
benzylthio substituent was different from the previous result
observed in the conversion of vinylic sulfones to the corre-
sponding allylic sulfones, probably due to the difference be-
tween the present electron-deficient conjugated olefinic system
and the non-conjugated olefinic system in vinylic sulfones.
Previously, Krebs, Kende, and Galatsis obtained compara-
ble results in the deconjugation of ꢂ-Me, Et, and iPr substitut-
ed (E)-ꢀ,ꢁ-unsaturated esters.3a,b,d Although Galatsis proposed
a cyclic transition state, the difference by the substituents
could not be explained by it. Kende proposed that the predom-
inant formation of the (Z)-ꢁ,ꢂ-unsaturated ester was probably
due to the stability of the generated 2-butenyl anion system,
that is, due to the greater A1;2 strain than that of the A1;3 strain
jugative electron donation by the C–H bond is more effec-
ꢂ2
tive than that by the C–C bond,7,8 since H can also interact
ꢂ2
with a base to afford the developing anion. In the cases of ꢂ-
fluoro and ꢂ-benzyloxy substituted ꢀ,ꢁ-unsaturated esters
1g,h, the CH eclipsed form B is unfavorable due to the low do-
nor ability of the C–F and C–O bonds,8c,9 resulting in an exclu-
sive formation of (Z)-2g,h via conformation A.
In the cases of 1a–c,g,h, it is also possible to stabilize the
syn-conformation at the transition state by 6ꢄ-electron homo-
aromaticity (an ‘‘aromatic’’ 6ꢄ-electron system as another
origin of ‘‘syn-effect’’) involving the developing charge at
the ꢂ-position and a pseudo p-orbital of the ꢅ-CH2 (Fig. 2a,
π*
π
π*
π*
π
π*
H
γ1σH
CO2Et
H
CO2Et
H
σ
base
base
H
base
H
σ
σ
Hγ2
R
H
R
(A)
(B)
σ→
π* interaction in two eclipsed conformations A and B
Fig. 1.
− H
CO2Et
H
−H
CO2Et
H
β
β
H+
H
H+
H
base
α
base
α
γ
γ
H
H
δ
X
R'
R'
a) 6
π
-electron homoaromaticity
-alkyl substituted system
b) 6
π
-electron homoaromaticity in
in
γ
γ
-heteroatom substituted system
Fig. 2.
H
CO2Et
H
π*
π
π*
H
CO2Et
H
H
base
H
σ
H
H
H
σ
BnS
b) σC−S
→π* interaction to stabilize the
a) steric repulsion between o-H
of Ph group and -H of ester
α
CH eclipsed conformation
Fig. 3.

2150 Bull. Chem. Soc. Jpn., 77, No. 12 (2004)
HEADLINE ARTICLES
)(
H
R
H
R
H
H
O
O
OLi
OEt
H
R
H
H
OEt
H
OEt
iPr2N Li
1,2-allylic strain
(disfavored)
1,3-allylic strain
(Z)-isomer
Fig. 4.
H
Ts
H
Ts
−H
Ts
+
H
H
H
H
base
H
H
H
H
H
X
X
X
R'
R'
R'
6π-electron
homoaromaticity
inductive effect
Fig. 5.
EtO
EtO
EtO
−H
H
H
O
O
O
+
H
H
H
H
base
H
H
H
H
H
X
X
X
R'
R'
R'
only 5π electrons
resonance effect
available
Fig. 6.
(Fig. 4), or homoaromaticity.10 If the former proposal is cor-
rect, in the conversion of the ꢂ-tBu substituted (E)-ꢀ,ꢁ-unsat-
urated ester to the corresponding ꢁ,ꢂ-unsaturated ester, the
(Z)-isomer should be the major product. This was not actually
observed, but only the (E)-ꢁ,ꢂ-unsaturated ester was obtained.
Thus, it is clear that the ‘‘syn-effect’’ does not arise from the
difference between the A1;2 and A1;3 strains. The latter, homo-
aromaticity, is consistent with our proposal for the origin of the
‘‘syn-effect’’ .
bonyl compound by a treatment with a silylating agent in the
presence of a base.12 In the preparation of silyl enol ethers
from aldehydes, the Z=E ratios of the products were reported
to vary according to the ꢀ-substituents of the aldehydes.6 In or-
der to determine the exact origin of the phenomena, we sys-
tematically investigated the reaction of various ꢀ-substituted
aldehydes with silylating agents in the presence of a base.
The Z=E ratio of the resulting silyl enol ethers was also well
rationalized by the ‘‘syn-effect’’.
It should be noted that the Z=E ratios in the present isomer-
ization were higher compared with the previous case observed
in the conversion of (E)-vinylic sulfones to the corresponding
allylic sulfones. In the isomerization of (E)-vinylic sulfones to
the corresponding allylic sulfones, 6ꢄ-electron homoaromatic-
First, the reaction of various aldehydes with chlorosilanes
and Et3N in DMF was examined.6d The results are given in
Table 2. The Z=E ratios of the silyl enol ethers obtained from
decanal were around 60/40 regardless of the kind of chlorosi-
lanes and the reaction temperature (Entries 2–4). Then, reac-
tions of various kinds of aldehydes 5 were carried out using
ꢀ
ity (Fig. 5) seems to be possible, together with the ꢃ ! ꢄ in-
ꢃ
teraction to stabilize the syn-conformation at the transition
state.1a On the other hand, in the isomerization of (E)-ꢀ,ꢁ-un-
saturated esters to the corresponding ꢁ,ꢂ-isomers, due to the
mesomeric character of the carbonyl group, the contribution
of 6ꢄ-electron homoaromaticity must be decreased. Namely,
only 5ꢄ electrons are available in the conjugated system with
syn-conformation (Fig. 6). However, in the case of benzyloxy
and fluoro substituents, complete Z-selectivity was observed.
Ph2MeSiCl as a silylating agent at 60 C (Entries 1,5–7) or
25 ꢃC (Entries 8,9). The corresponding silyl enol ethers 6 were
obtained in good chemical yields, except for a bulky aldehyde
5d (Entry 6). In the case of aliphatic aldehydes 5a–d (Entries
1,2,5,6), the ratio of the (Z)-isomer of 6 decreased in the order
of R ¼ CH3 > CH3(CH2)7 > (CH3)2CH > (CH3)3C. A strik-
ing Z-selectivity was observed in the reaction of (benzyloxy)-
acetaldehyde (5f) (Entry 8).
ꢀ
This fact indicates that the effect of the ꢃ ! ꢄ interaction
It is also possible to explain the relative degree of the Z=E
ꢀ
is more enhanced in the isomerization of (E)-ꢀ,ꢁ-unsaturated
esters to the corresponding ꢁ,ꢂ-unsaturated esters than in the
isomerization of (E)-vinylic sulfones to the corresponding al-
lylic sulfones.
ratios of the resulting silyl enol ethers by a ꢃ ! ꢄ interac-
tion in the transition state of the deprotonation of aldehydes,
as in the case of (E)-ꢀ,ꢁ-unsaturated esters described above.
Namely, due to the low donor ability of the C–C bond com-
pared with the C–H bond, the CC eclipsed conformation C
(Fig. 7) would be preferred to the conformation D at the depro-
tonation of aldehydes, affording the (Z)-silyl enol ethers as the
Conversion of Aldehydes into Silyl Enol Ethers. Silyl
enol ether, one of the most important and isolable active inter-
mediates used in organic synthesis, can be prepared from a car-

S. K. Guha et al.
Bull. Chem. Soc. Jpn., 77, No. 12 (2004) 2151
Table 2. Conversion of Aldehydes to the Corresponding Silyl Enol Ethers by Using Silyl Chlorides
H
H
SiCl (1.2 eq.), Et3N (2.0 eq.)
DMF, T °C, t h
R
O
OSi
α
R
5
MeSi)
6 (Si = Ph2
or 7 (Si = Me3Si)
Entry
5
R
Si
T/ꢃC
t/h
Z=E of 6,7aÞ
Yield of 6,7/%aÞ
1
2
3
4
5
6
7
8
9
a
b
b
b
c
d
e
f
CH3
Ph2MeSi
Ph2MeSi
Ph2MeSi
Me3Si
Ph2MeSi
Ph2MeSi
Ph2MeSi
Ph2MeSi
Ph2MeSi
60
60
25
60
60
60
60
25
25
96
48
143
25
96
96
48
23
74/26
58/42
58/42
60/40
54/46
30/70
67/33
100/0
37/63
6a
6b
6b
7b
6c
6d
6e
6f
76
93
85
88
>99
50
72
85
CH3(CH2)7
CH3(CH2)7
CH3(CH2)7
(CH3)2CH
(CH3)3C
Ph
BnO
BnS
g
29
6g
84
1
a) Yields and Z=E ratios were determined by 400 MHz H NMR spectra of the crude products.
π*
π
π*
π*
π
π*
γ1
H
H
σ
H
σ
base
base
H
base
O
O
σ
σ
R
H
H
R
γ2
(C)
(D)
σ→
π* interaction in two eclipsed conformations C and D
Fig. 7.
− H
− H
H+
H+
base
base
O
O
α
α
H
β
H
H
X
R'
H
R'
a) 6
π
-electron homoaromaticity
b) 6
α
π
-electron homoaromaticity in
in
α
-alkyl substituted system
-heteroatom substituted system
Fig. 8.
major products. In the case of benzyloxy-substituted aldehyde
5f, conformation D is much less favored due to the low donor
ability of the C–O bond compared with the C–C bond. As a
result, a higher Z-selectivity was observed.
namely the isomerization of ꢂ-phenyl ꢀ-unsubstituted and
ꢀ-fluorinated (E)-vinylic sulfones, the desulfonylation of ꢅ-
phenyl-ꢀ,ꢀ-dialkylated (E)-allylic sulfones, the desilylation
reaction of ꢂ-silylated (E)-allylic sulfones,1 and the isomeriza-
tion of ꢂ-phenyl substituted (E)-ꢀ,ꢁ-unsaturated esters 1f, ex-
clusively or predominantly gave the corresponding (E)-olefins
because of an unfavorable steric repulsion between an aromat-
ic proton at the ortho-position and an olefinic ꢀ-proton
(Fig. 9a). However, in the present case for ꢀ-phenyl substitut-
ed aldehyde 5e, there is not such olefinic proton for unfavora-
ble congestion with an aromatic proton at the ortho-position,
but rather a favorable hydrogen bond with a carbonyl oxygen
In the cases of 5a,b,f, it is also possible to stabilize the syn-
conformation at the transition state by 6ꢄ-electron homoaro-
maticity involving the developing anion at the ꢀ-position
and a pseudo p-orbital of the ꢁ-CH2 (Fig. 8a, R ¼ CH2R0),
or a lone pair of electrons in a p-orbital of the hetero atom
(Fig. 8b, R ¼ XR0).
In the cases of iPr- and tBu-substituted aldehydes 5c,d, 6ꢄ-
electron homoaromaticity is difficult to be considered; still, re-
markable amounts (54% and 30%, respectively) of the sterical-
ly unfavorable (Z)-isomers were obtained (Entries 5,6). It is
ꢀ
(Fig. 9b),13 which allows a ꢃ ! ꢄ interaction, which enhan-
ces the degree of the ‘‘syn-effect’’ for the phenyl group (Entry
7) than that in the previous cases. Furthermore, 6ꢄ-electron
homoaromaticity can also work to stabilize the syn-conforma-
tion at the transition state by the participation of ꢄ-bonding
electrons of the phenyl group (Fig. 9c), due to the absence
of a steric repulsion between the olefinic protons, as in Fig. 9a.
The fact that (Z)-silyl enol ether was still produced in the
ꢀ
thus clear that the ‘‘syn-effect’’ arises from the ꢃ ! ꢄ inter-
action, even in the present case. In the case of (benzylthio)acet-
ꢀ
aldehyde (5g), the ꢃC{S ! ꢄ interaction might be responsi-
ble for the predominance of the CH eclipsed form D in Fig. 7
(R ¼ SBn) to increase the E-selectivity (Entry 9), as discussed
concerning the isomerization of ꢂ-benzylthio substituted ꢀ,ꢁ-
unsaturated ester 1i. All of the reactions so far investigated
concerning the ‘‘syn-effect’’ for phenyl substituted substrates,
t
case of Bu substituted aldehyde 5d (Entry 6), in which 6ꢄ-
electron homoaromaticity is very unlikely, might strongly sug-

2152 Bull. Chem. Soc. Jpn., 77, No. 12 (2004)
HEADLINE ARTICLES
π*
π*
π
H
σ
H
−H
H
EWG
n
H+
H
O
O
base
H
H
base
base
(sp2)
α
H (or F)
H
σ
H
H
(or Me3Si)
c) 6
aromaticity in
substituted system
π
-electron homo-
a) steric repulsion between
o-H of Ph group and an
olefinic atom
b) σ→π* interaction and hydrogen
bonding between o-H of Ph group
and O atom
α
-phenyl
Fig. 9.
Table 3. Conversion of Aldehydes to the Corresponding Silyl Enol Ethers by Using Silyl Triflates
i) SiOTf (1.2 eq.), Et3N (1.2 eq.)
H
H
Et2O, 0 °C, 2.5 h
R
O
OSi
8 (Si = iPr3Si)
α
ii)
(0.25 eq.)
R
5
N
OH
or 7 (Si = Me3Si)
Entry
5
R
Si
Z=E of 8,7aÞ
Yield of 8,7/%bÞ
1
2
3
4
5
6
7
8
a
b
b
c
d
e
f
CH3
iPr3Si
Me3Si
iPr3Si
iPr3Si
iPr3Si
iPr3Si
iPr3Si
iPr3Si
96/4
87/13cÞ
93/7
88/12
84/16
60/40
96/4
8a
7b
8b
8c
8d
8e
8f
71
35cÞ
71
72
63
54
77
92
CH3(CH2)7
CH3(CH2)7
(CH3)2CH
(CH3)3C
Ph
BnO
BnS
g
37/63
8g
a) The ratios were determined by 400 MHz 1H NMR spectra. The ratios were confirmed not to be changed
during the purification by column chromatography on silica gel. b) Isolated yields. c) Yield and Z=E ratio
1
were determined by 400 MHz H NMR spectrum of the crude products.
silylation of 3,3-dimethylbutanal (R ¼ tBu) (5d) with TIP-
−
π*
π
π*
γ1
SOTf realized a dramatic increase of the Z-selectivity (Entry
5), which strongly suggested that the ꢃ ! ꢄ interaction
H
Si OTf
σ
ꢀ
worked effectively, as anticipated. The highest Z-selectivity
base
base
H
O
σ
R
H
was also observed in the case of benzyloxy substituted alde-
hyde 5f (Entry 7). In the reaction of (benzylthio)acetaldehyde
γ2
ꢀ
enhanced
σ
→
π* interaction
(5g), not only the ꢃC{H ! ꢄ interaction, but also the ꢃC{S
!
ꢀ
ꢄ
interaction11 seemed to be enhanced together to scarcely
Fig. 10.
alter the Z=E ratio (Entry 8). Furthermore, a slight decrease
of the Z-selectivity for phenylacetaldehyde (5e) was observed
when TIPSOTf was used as a silylating agent (Entry 6). This
result might have been due to the weakened hydrogen bond
in syn-conformation caused by a decrease of the electron den-
sity of the carbonyl oxygen by coordinating to a strongly
Lewis-acidic silyl group.
ꢀ
tonation, even in this case. Therefore, if the reaction is carried
gest that the ꢃ ! ꢄ interaction works at the stage of depro-
ꢀ
out under the conditions that the ꢃ ! ꢄ interaction is en-
ꢀ
hanced by polarizing and lowering in energy of the ꢄ orbital,
the Z-selectivity is anticipated to be increased. Based on this
hypothesis, a more Lewis-acidic silyl triflate than silyl chloride
was next used as a silylating agent (Table 3), which can acti-
vate the carbonyl group strongly (Fig. 10).14–16
As described above, the stereochemistry in the conversion
of aldehydes into the corresponding silyl enol ethers by a treat-
ment with silyl chlorides or silyl triflates and a base was well
rationalized by the ‘‘syn-effect’’, which was accounted for by
When decanal (5b) was treated with trimethylsilyl triflate
ꢃ
and Et3N in Et2O at 0 C, a remarkable enhancement of the
ꢀ
Z=E ratio was observed (Entry 2). When triisopropylsilyl tri-
flate (TIPSOTf) was used, both the Z-selectivity and the chem-
ical yield were further increased (Entry 3).17 Then, the conver-
sion of various kinds of aldehydes 5 to the corresponding silyl
enol ethers 8 was carried out using TIPSOTf. An enhancement
of the Z=E ratios was observed in the case of aliphatic alde-
hydes 5a–d, while keeping the same order of the Z-preference
(Entries 1,3–5) as that using silyl chloride. To our surprise, the
the ꢃ ! ꢄ interaction and/or 6ꢄ-electron homoaromaticity.
In the reaction using silyl triflates, with which a strong ꢃ !
ꢀ
ꢄ interaction was anticipated, a higher Z-selectivity was ob-
served, especially in the case of aliphatic aldehydes.18
In conclusion, both in the conversion of (E)-ꢀ,ꢁ-unsaturat-
ed esters to the corresponding ꢁ,ꢂ-unsaturated esters and in
the conversion of various ꢀ-substituted aldehydes to the corre-
sponding silyl enol ethers, their stereochemical outcomes were

S. K. Guha et al.
Bull. Chem. Soc. Jpn., 77, No. 12 (2004) 2153
oil; IR (neat) 3060, 3026, 2980, 2932, 1731, 1603, 1583, 1496,
1480, 1454, 1440, 1389, 1260, 1151, 1093, 1025, 748, 700
well rationalized by the ‘‘syn-effect’’ in the transition state of
ꢀ
deprotonation, which arose from the ꢃ ! ꢄ interaction
1
cmꢄ1; H NMR (400 MHz, CDCl3) ꢅ 1.17 (3H, t, J ¼ 7:07 Hz),
and/or 6ꢄ-electron homoaromaticity. It is noteworthy that
the highest Z-selectivity was observed for fluoro and/or
benzyloxy substituent of the examined substrates.
2.01–2.11 (1H, m), 2.16–2.26 (1H, m), 2.76 (2H, t, J ¼ 7:56
Hz), 3.62 (1H, t, J ¼ 7:08 Hz), 4.06–4.16 (2H, m), 7.15–7.33
(8H, m), 7.41–7.44 (2H, m). MS (EI) m=z 301 (Mþ þ 1,
11.39%), 300 (Mþ, 56.44), 196 (25.79), 191 (21.91), 145
(11.63), 123 (32.01), 117 (68.37), 110 (17.98), 91 (100.00), 65
(18.35), 58 (10.41).
Experimental
The 1H NMR spectra were recorded on JEOL JNM-GX 400,
Lambda 400, and Lambda 300 NMR spectrometers. The chemical
shifts were determined in the ꢅ-scale relative to Si(CH3)4 (ꢅ ¼ 0)
as an internal standard. The IR spectra were measured by a
JASCO FT/IR-230 spectrometer and the MS spectra were record-
ed with Hitachi M-80 and JEOL SX-102A mass spectrometers, re-
spectively. All glass equipment was flame-dried under a vacuum
before use. THF and Et2O were freshly distilled from sodium di-
phenylketyl. All other solvents were distilled and stored over dry-
ing agents. Thin-layer chromatography (TLC), and flash column
chromatography were performed using Merck’s silica gel 60
PF254 (Art. 7749) and Cica-Merck’s silica gel 60 (No. 9385-5B),
respectively. Commercially available reagents were used without
further purification, unless otherwise noted.
(E)-ꢀ,ꢁ-Unsaturated esters 1a,c–e,h containing an alkyl or
a benzyloxy group at the ꢂ-position were prepared by the
Horner–Emmons–Wadsworth reaction using (EtO)2P(O)-
CH2COOEt in good yields.19 In the cases where trace amounts
of (Z)-isomers were formed, they were separated from the (E)-iso-
mers by column chromatography or preparative TLC.
Ethyl (E)-4-Phenyl-2-butenoate (1f). To a solution of ethyl
4-phenyl-2-(phenylthio)butanoate (4) (225 mg, 0.75 mmol) in
CH2Cl2 (3 mL), finely powdered NaHCO3 (126 mg, 1.50 mmol)
was added. After cooling to 0 ꢃC, a solution of mCPBA (ca.
70% pure, 194 mg, 0.788 mmol) in CH2Cl2 (3 mL) was added
dropwise to the mixture. After stirring for 30 min, H2O was added.
The organic solvent was removed and the organic substances were
extracted by AcOEt. The combined extracts were washed by brine
and dried over Na2SO4. After evaporating the solvent, the crude
product was purified by a preparative TLC (SiO2, hexane/
AcOEt = 5/1, v/v) to give the corresponding sulfoxide. The sulf-
oxide was then refluxed in toluene for 2 h to afford 1f in 74% yield
(106 mg). An oil; IR (neat) 3085, 3062, 3029, 2981, 2936, 2904,
1719, 1654, 1603, 1495, 1454, 1367, 1323, 1271, 1202, 1161,
1042, 984, 751, 700 cmꢄ1 1H NMR (300 MHz, CDCl3) ꢅ 1.27
;
(3H, t, J ¼ 7:15 Hz), 3.51 (2H, dd, J ¼ 1:47, 6.79 Hz), 4.17
(2H, q, J ¼ 7:15 Hz), 5.81 (1H, dt, J ¼ 15:59, 1.47 Hz), 7.09
(1H, dt, J ¼ 15:59, 6.79 Hz), 7.16–7.34 (5H, m). MS (EI) m=z
190 (Mþ, 28.21%), 145 (17.90), 117 (44.24), 115 (26.98), 91
(15.00), 77 (6.06), 58 (100.00).
Octyl (E)-2-Pentenoate (1b). To a suspension of NaH (60%
dispersion in mineral oil) (360 mg, 9.0 mmol) in THF (10 mL)
was added a solution of (EtO)2P(O)CH2COO(CH2)7CH3 (2.77
Ethyl (E)-4-Fluoro-2-butenoate (1g).20 To a suspension of
AgF (5.709 g, 45.0 mmol) in CH3CN (50 mL) was added a solu-
tion of ethyl (E)-4-bromo-2-butenoate (2.896 g, 15.0 mmol) in
CH3CN (10 mL) under a N2 atmosphere in the dark. After stirring
for 7 days at room temperature, the reaction mixture was filtered
through celite. After evaporating the solvent, the product was
purified by column chromatography (SiO2, hexane/Et2O = 8/1,
v/v) to give 1g in 37% yield (735 mg). An oil; IR (neat) 2984,
2939, 1723, 1668, 1449, 1380, 1368, 1308, 1277, 1237, 1180,
ꢃ
g, 9.0 mmol) in THF (5 mL) dropwise at 0 C under a N2 atmo-
sphere. After stirring for 30 min, CH3CH2CHO (0.65 mL, 9.0
mmol) was added to the mixture at the same temperature. After
stirring overnight at room temperature, a saturated aqueous NH4Cl
solution was added to the reaction mixture. After evaporation of
the organic solvent, the residue was extracted with Et2O, followed
by washing with brine, and dried over Na2SO4. The solvent was
removed and the crude product was purified by column chroma-
tography (SiO2, hexane/AcOEt = 5/1, v/v) to give 1b in 77%
yield (1.47 g). An oil; IR (neat) 2956, 2927, 2856, 1724, 1656,
1462, 1380, 1335, 1308, 1288, 1265, 1178, 1123, 1088, 1038,
1
1085, 1037, 998, 968, 912 cmꢄ1; H NMR (400 MHz, CDCl3) ꢅ
1.30 (3H, t, J ¼ 7:07 Hz), 4.22 (2H, q, J ¼ 7:07 Hz), 5.06 (2H,
ddd, J ¼ 2:20, 3.90, 46.11 Hz), 6.12 (1H, dq, J ¼ 15:86, 2.20
Hz), 6.96 (1H, ddt, J ¼ 15:86, 22.69, 3.90 Hz). MS (CI) m=z
133 (Mþ þ 1, 100.00%), 117 (1.59), 105 (1.10), 99 (1.13).
Ethyl (E)-4-Benzylthio-2-butenoate (1i). To a solution of
BnSH (68 mg, 0.548 mmol) in CH2Cl2 (2.5 mL) was added Et3N
(0.08 mL, 0.548 mmol) at 0 ꢃC and stirred for 30 min. Then, a so-
lution of ethyl (E)-4-bromo-2-butenoate (106 mg, 0.548 mmol) in
CH2Cl2 (0.5 mL) was added dropwise at 0 ꢃC. After stirring over-
night at 0 ꢃC, the reaction mixture was quenched with a phosphate
buffer solution (pH 7). The organic substances were extracted
with CH2Cl2, followed by washing with H2O and brine, and dried
over Na2SO4. After evaporating the solvent, the residue was puri-
fied by preparative TLC (SiO2, hexane/Et2O = 8/1, v/v) to give
1i in 73% yield (94 mg). An oil; IR (neat) 3060, 3027, 2980, 2920,
1717, 1652, 1600, 1494, 1454, 1418, 1395, 1367, 1315, 1267,
1
979, 859 cmꢄ1; H NMR (400 MHz, CDCl3) ꢅ 0.88 (3H, t, J ¼
7:31 Hz), 1.07 (3H, t, J ¼ 7:31 Hz), 1.20–1.40 (10H, m), 1.65
(2H, p, J ¼ 6:83 Hz), 2.23 (2H, dp, J ¼ 1:72, 7.31 Hz), 4.12
(2H, t, J ¼ 6:83 Hz), 5.82 (1H, dt, J ¼ 15:63, 1.72 Hz), 7.01
(1H, dt, J ¼ 15:63, 7.31 Hz). HRMS (EI) Found: m=z 212.1773.
Calcd for C13H24O2: 212.1777.
Ethyl 4-Phenyl-2-(phenylthio)butanoate (4). To a suspen-
sion of NaH (60% dispersion in mineral oil) (264 mg, 6.6 mmol)
in DMF (20 mL) was added nBu4NI (155 mg, 0.42 mmol), fol-
lowed by the addition of a solution of ethyl (phenylthio)acetate
(3) (1.176 g, 6.0 mmol) in DMF (5 mL) under a N2 atmosphere.
After stirring for a few minutes at room temperature, (2-bromo-
ethyl)benzene (0.86 mL, 6.3 mmol) was added dropwise to the re-
action mixture. After stirring for 24 h, the reaction mixture was
diluted with Et2O, quenched with phosphate-buffer solution
(pH 7), and the organic substances were extracted with Et2O.
The organic phase was then washed with H2O and brine, dried
over Na2SO4, and the solvent was evaporated under reduced pres-
sure. The residue was purified by column chromatography (SiO2,
hexane/AcOEt = 20/1, v/v) to give 4 in 31% yield (550 mg). An
1199, 1149, 1042, 979, 748, 703 cmꢄ1 1H NMR (400 MHz,
;
CDCl3) ꢅ 1.31 (3H, t, J ¼ 7:07 Hz), 3.10 (2H, dd, J ¼ 0:98,
7.32 Hz), 3.66 (2H, s), 4.21 (2H, q, J ¼ 7:07 Hz), 5.83 (1H, dt,
J ¼ 15:37, 0.98 Hz), 6.87 (1H, dt, J ¼ 15:37, 7.32 Hz), 7.24–
7.34 (5H, m). MS (EI) m=z 236 (Mþ, 23.53%), 145 (36.34), 123
(72.84), 114 (96.10), 99 (34.42), 91 (100.00), 65 (61.93), 45
(62.78), 39 (64.58), 29 (81.79).

2154 Bull. Chem. Soc. Jpn., 77, No. 12 (2004)
HEADLINE ARTICLES
The Representative Procedure for Conversion of (E)-ꢀ,ꢁ-
Unsaturated Esters to the Corresponding ꢁ,ꢂ-Unsaturated
Esters. To a solution of hexamethyldisilazane (0.12 mL, 0.55
3040, 2967, 2936, 2876, 1739, 1653, 1459, 1402, 1367, 1327,
1254, 1177, 1113, 1038, 970, 940, 870, 850 cmꢄ1 1H NMR of
;
(Z)-form (400 MHz, CDCl3) ꢅ 0.98 (3H, t, J ¼ 7:56 Hz), 1.26
(3H, t, J ¼ 7:07 Hz), 2.06 (2H, p, J ¼ 7:56 Hz), 3.07 (2H, d, J ¼
6:34 Hz), 4.143 (2H, q, J ¼ 7:07 Hz), 5.49–5.64 (2H, m);
1H NMR of (Z)-form (400 MHz, C6D6) ꢅ 0.82 (3H, t, J ¼ 7:33
Hz), 0.94 (3H, t, J ¼ 7:02 Hz), 1.82–1.89 (2H, m), 2.92 (2H,
ddt, J ¼ 1:83, 7.32, 0.92 Hz), 3.93 (2H, q, J ¼ 7:02 Hz), 5.44
(1H, dtt, J ¼ 10:68, 1.83, 7.32 Hz), 5.67 (1H, dtt, J ¼ 10:68,
n
mmol) in THF (1 mL) was added a solution of BuLi (0.36 mL,
0.55 mmol, 1.52 M in hexane) at 0 ꢃC under a N2 atmosphere. Af-
ter stirring for 15 min, HMPA (0.38 mL, 2.2 mmol) was added and
stirred for another 30 min at 0 ꢃC. After that, the reaction mixture
ꢃ
was cooled to ꢄ70 C, and a solution of octyl (E)-2-pentenoate
(1b) (106 mg, 0.50 mmol) in THF (0.5 mL) was added to the mix-
ture, and the resulting solution was stirred for 30 min. The reac-
tion mixture was then quenched with a solution of aqueous HCl
diluted in EtOH cooled to ꢄ70 ꢃC, and allowed to warm up to
room temperature. The solvent was evaporated and the residue
was extracted with Et2O, followed by washing with H2O and
brine, dried over Na2SO4. After evaporating the solvent, the crude
product was purified by column chromatography (SiO2, hexane/
Et2O = 20/1, v/v) to give the ꢁ,ꢂ-unsaturated ester 2b in 83%
yield (88 mg, Z=E ¼ 94=6).
1
1.53, 7.32 Hz); H NMR of (E)-form (400 MHz, CDCl3) ꢅ 0.99
(3H, t, J ¼ 7:56 Hz), 1.26 (3H, t, J ¼ 7:07 Hz), 2.06 (2H, p, J ¼
7:56 Hz), 3.01 (2H, d, J ¼ 6:59 Hz), 4.138 (2H, q, J ¼ 7:07 Hz),
5.49–5.64 (2H, m); 1H NMR of (E)-form (400 MHz, C6D6) ꢅ 0.85
(3H, t, J ¼ 7:33 Hz), 0.94 (3H, t, J ¼ 7:02 Hz), 1.82–1.89 (2H,
m), 2.88 (2H, dq, J ¼ 7:02, 1.22 Hz), 3.93 (2H, q, J ¼ 7:02
Hz), 5.36–5.46 (1H, m), 5.60 (1H, dtt, J ¼ 15:57, 1.53, 7.02
Hz). MS (CI) m=z 143 (Mþ þ 1, 100.00%), 142 (Mþ, 4.51), 115
(1.85), 97 (0.98), 79 (15.20), 60 (6.08).
The physical and spectral data of the resulting ꢁ,ꢂ-unsaturated
esters 2 are given in the following:
Ethyl 5-Methyl-3-hexenoate (2d): (Z=E ¼ 70=30). An oil;
IR (neat) 2960, 2871, 1740, 1465, 1367, 1325, 1300, 1250,
1179, 1123, 1038, 971, 942, 721 cmꢄ1 1H NMR of (Z)-form
;
Ethyl 3-Pentenoate (2a): (Z=E ¼ 91=9). An oil, IR (neat)
3032, 2982, 2960, 1739, 1655, 1447, 1402, 1371, 1322, 1247,
1178, 1098, 1037, 941, 851, 746, 680 cmꢄ1; 1H NMR of (Z)-form
(400 MHz, CDCl3) ꢅ 1.259 (3H, t, J ¼ 7:02 Hz), 1.64 (3H, dd,
J ¼ 0:91, 6.41 Hz), 3.08 (2H, d, J ¼ 7:02 Hz), 4.15 (2H, q, J ¼
7:02 Hz), 5.54–5.70 (2H, m); 1H NMR of (Z)-form (400 MHz,
C6D6) ꢅ 0.96 (3H, t, J ¼ 7:02 Hz), 1.38 (3H, ddt, J ¼ 1:83,
6.72, 0.92 Hz), 2.92 (2H, ddq, J ¼ 1:83, 7.02, 0.92 Hz), 3.93
(2H, q, J ¼ 7:02 Hz), 5.48 (1H, dtq, J ¼ 10:68, 1.83, 6.72, Hz),
5.71 (1H, dtq, J ¼ 10:68, 7.02, 1.83 Hz); 1H NMR of (E)-form
(400 MHz, CDCl3) ꢅ 1.264 (3H, t, J ¼ 7:02 Hz), 1.70 (3H, d, J ¼
4:89 Hz), 3.01 (2H, d, J ¼ 5:19 Hz), 4.14 (2H, q, J ¼ 7:02 Hz),
5.54–5.70 (2H, m); 1H NMR of (E)-form (400 MHz, C6D6) ꢅ
0.96 (3H, t, J ¼ 7:02 Hz), 1.48 (3H, ddt, J ¼ 1:53, 6.72, 1.22
Hz), 2.85 (2H, ddq, J ¼ 1:53, 7.02, 1.22 Hz), 3.93 (2H, q, J ¼
7:02 Hz), 5.33 (1H, dtq, J ¼ 15:26, 1.53, 6.72 Hz), 5.58 (1H,
dtq, J ¼ 15:26, 7.02, 1.53 Hz). MS (CI) m=z 129 (Mþ þ 1,
100.00%), 113 (25.62), 112 (9.27), 85 (12.82), 81 (11.25), 79
(9.66), 73 (16.04), 71 (14.18), 69 (14.53).
(400 MHz, CDCl3) ꢅ 0.97 (6H, d, J ¼ 6:59 Hz), 1.26 (3H, t, J ¼
7:07 Hz), 2.50–2.63 (1H, m), 3.08 (2H, dd, J ¼ 1:46, 4.15 Hz),
4.142 (2H, q, J ¼ 7:07 Hz), 5.36–5.56 (2H, m); 1H NMR of
(Z)-form (400 MHz, C6D6) ꢅ 0.87 (6H, d, J ¼ 6:59 Hz), 1.00
(3H, t, J ¼ 7:07 Hz), 2.36–2.48 (1H, m), 2.93 (2H, dd,
J ¼ 1:46, 7.07 Hz), 3.95 (2H, q, J ¼ 7:07 Hz), 5.30 (1H, ddt,
J ¼ 9:51, 10.98, 1.46 Hz), 5.47–5.57 (1H, m); 1H NMR of (E)-
form (400 MHz, CDCl3) ꢅ 0.99 (6H, d, J ¼ 6:83 Hz), 1.26 (3H,
t, J ¼ 7:07 Hz), 2.25–2.35 (1H, m), 3.00 (2H, dd, J ¼ 0:73,
5.61 Hz), 4.136 (2H, q, J ¼ 7:07 Hz), 5.36–5.56 (2H, m);
1H NMR of (E)-form (400 MHz, C6D6) ꢅ 0.91 (6H, d, J ¼ 6:83
Hz), 0.99 (3H, t, J ¼ 7:07 Hz), 2.10–2.22 (1H, m), 2.87 (2H, d,
J ¼ 6:83 Hz), 3.94 (2H, q, J ¼ 7:07 Hz), 5.39 (1H, ddt,
J ¼ 6:59, 15.37, 1.22 Hz), 5.47–5.57 (1H, m). MS (CI) m=z 157
(Mþ þ 1, 100.00%), 156 (Mþ, 4.63), 111 (2.24), 109 (1.04), 79
(1.87), 69 (1.04), 60 (2.68).
Ethyl 5,5-Dimethyl-3-hexenoate (2e): (Z=E ¼ 0=100). An
oil; IR (neat) 2959, 2900, 2869, 1740, 1658, 1480, 1464, 1366,
1269, 1245, 1180, 1150, 1035, 973 cmꢄ1 1H NMR (400 MHz,
;
Octyl 3-Pentenoate (2b): (Z=E ¼ 94=6). An oil; IR (neat)
3032, 2956, 2927, 2857, 1740, 1660, 1467, 1403, 1378, 1321,
1
CDCl3) ꢅ 1.01 (9H, s), 1.26 (3H, t, J ¼ 7:07 Hz), 3.00 (2H, dd,
J ¼ 1:22, 6.83 Hz), 4.14 (2H, q, J ¼ 7:07 Hz), 5.44 (1H, dt,
J ¼ 15:61, 6.83 Hz), 5.58 (1H, dt, J ¼ 15:61, 1.22 Hz). MS
(EI) m=z 170 (Mþ, 4.25%), 130 (100.00), 128 (37.12), 113
(11.26), 102 (12.31), 83 (9.85), 57 (16.83).
1288, 1248, 1166, 1099, 1016, 966, 723, 679 cmꢄ1; H NMR of
(Z)-form (400 MHz, CDCl3) ꢅ 0.88 (3H, t, J ¼ 7:07 Hz), 1.20–
1.40 (10H, m), 1.59–1.67 (2H, m), 1.64 (3H, dd, J ¼ 1:46, 6.59
Hz), 3.09 (2H, dd, J ¼ 1:29, 6.83 Hz), 4.08 (2H, t, J ¼ 6:83
1
Hz), 5.53–5.71 (2H, m); H NMR of (Z)-form (400 MHz, C6D6)
Ethyl 4-Phenyl-3-butenoate (2f): (Z=E ¼ 16=84). An oil; IR
ꢅ 0.89 (3H, t, J ¼ 7:07 Hz), 1.10–1.30 (10H, m), 1.40–1.50
(2H, m), 1.41 (3H, ddt, J ¼ 1:96, 6.83, 0.98 Hz), 2.96 (2H, ddq,
J ¼ 7:07, 1.96, 0.98 Hz), 4.01 (2H, t, J ¼ 6:83 Hz), 5.50 (1H,
dtq, J ¼ 10:71, 1.96, 6.83 Hz), 5.72 (1H, dtq, J ¼ 10:71, 7.07,
1.96 Hz); 1H NMR of (E)-form (400 MHz, CDCl3) ꢅ 0.88 (3H,
t, J ¼ 7:07 Hz), 1.20–1.40 (10H, m), 1.59–1.67 (2H, m), 1.70
(3H, dd, J ¼ 1:22, 4.88 Hz), 3.01 (2H, dd, J ¼ 1:24, 5.60 Hz),
4.08 (2H, t, J ¼ 6:83 Hz), 5.53–5.71 (2H, m); 1H NMR of (E)-
form (400 MHz, C6D6) ꢅ 0.89 (3H, t, J ¼ 7:07 Hz), 1.10–1.30
(10H, m), 1.40–1.50 (2H, m), 1.50 (3H, ddt, J ¼ 1:72, 6.59,
1.22 Hz), 2.89 (2H, ddq, J ¼ 1:48, 7.07, 1.22 Hz), 4.01 (2H, t, J ¼
6:83 Hz), 5.35 (1H, dtq, J ¼ 15:12, 1.48, 6.59 Hz), 5.60 (1H, dtq,
J ¼ 15:12, 7.07, 1.72 Hz). MS (EI) m=z 213 (Mþ þ 1, 20.16%),
212 (100.00), 157 (14.96), 113 (7.73), 112 (31.72), 101 (67.93),
100 (89.74), 99 (3.72), 83 (46.06), 67 (6.42).
(neat) 3027, 2981, 2937, 2905, 1737, 1600, 1496, 1448, 1369,
1
1322, 1295, 1251, 1159, 1028, 966, 746, 693 cmꢄ1; H NMR of
(Z)-form (400 MHz, CDCl3) ꢅ 1.27 (3H, t, J ¼ 7:07 Hz), 3.33
(2H, dd, J ¼ 1:71, 7.32 Hz), 4.17 (2H, q, J ¼ 7:07 Hz), 5.90
(1H, dt, J ¼ 11:46, 7.32 Hz), 6.63 (1H, d, J ¼ 11:46 Hz), 7.20–
7.38 (5H, m); 1H NMR of (E)-form (400 MHz, CDCl3) ꢅ 1.28
(3H, t, J ¼ 7:07 Hz), 3.24 (2H, dd, J ¼ 1:46, 7.07 Hz), 4.17
(2H, q, J ¼ 7:07 Hz), 6.30 (1H, dt, J ¼ 15:86, 7.07 Hz), 6.49
(1H, d, J ¼ 15:86 Hz), 7.20–7.38 (5H, m).21 MS (EI) m=z 190
(Mþ, 16.65%), 117 (49.15), 115 (14.22), 91 (10.23), 77 (6.38),
58 (100.00).
Ethyl 4-Fluoro-3-butenoate (2g): (Z=E ¼ 100=0). An oil; IR
(neat) 2955, 2924, 2851, 1735, 1670, 1654, 1459, 1376, 1261,
1031 cmꢄ1 1H NMR of (Z)-form (300 MHz, CDCl3) ꢅ 1.21
;
(3H, t, J ¼ 7:14 Hz), 3.18 (2H, dt, J ¼ 7:14, 1.65 Hz), 4.16
Ethyl 3-Hexenoate (2c): (Z=E ¼ 85=15). An oil; IR (neat)
(2H, q, J ¼ 7:14 Hz), 5.03 (1H, ddt, J ¼ 4:77, 41.4, 7.14 Hz),

S. K. Guha et al.
Bull. Chem. Soc. Jpn., 77, No. 12 (2004) 2155
6.55 (1H, ddt, J ¼ 4:77, 84.03, 1.65 Hz). MS (EI) m=z 133
(Mþ þ 1, 7.96%), 132 (Mþ, 2.94), 117 (13.41), 113 (9.33), 103
(9.07), 99 (43.19), 87 (100.00), 59 (52.85).
6.19 (1H, dt, J ¼ 5:80, 1.52 Hz), 7.36–7.44 (6H, m), 7.58–7.60
1
(4H, m); H NMR of (E)-form (400 MHz, CDCl3) ꢅ 0.690 (3H,
s), 0.87 (3H, t, J ¼ 6:10 Hz), 1.22–1.30 (12H, m), 1.80–1.85
(2H, m), 5.08 (1H, dt, J ¼ 11:91, 7.33 Hz), 6.24 (1H, dt,
J ¼ 11:91, 1.22 Hz), 7.28–7.62 (10H, m). MS (EI) m=z 352
(Mþ, 1.88%), 351 (6.24), 337 (30.95), 274 (6.82), 253 (24.06),
197 (100.00), 137 (13.00), 105 (5.23), 83 (9.52).
Ethyl 4-Benzyloxy-3-butenoate (2h):22 (Z=E ¼ 100=0). An
oil; IR (neat) 3060, 3032, 2981, 2930, 2910, 2880, 1736, 1669,
1497, 1455, 1401, 1367, 1325, 1294, 1179, 1112, 1062, 1030,
1
736, 698 cmꢄ1; H NMR of (Z)-form (400 MHz, CDCl3) ꢅ 1.25
(3H, t, J ¼ 7:07 Hz), 3.17 (2H, dd, J ¼ 1:71, 7.07 Hz), 4.13
(2H, q, J ¼ 7:07 Hz), 4.62 (1H, dt, J ¼ 6:10, 7.07 Hz), 4.81
(2H, s), 6.17 (1H, dt, J ¼ 6:10, 1.71 Hz), 7.30–7.36 (5H, m).
HRMS (EI) Found: m=z 220.1095. Calcd for C13H16O3: 220.1099.
Ethyl 4-Benzylthio-3-butenoate (2i):22 (Z=E ¼ 44=56). An
oil; IR (neat) 3061, 3029, 2932, 2870, 1735, 1653, 1603, 1495,
1454, 1369, 1314, 1255, 1160, 1096, 1071, 1028, 939, 860, 758,
1-(Trimethylsiloxy)-1-decene (7b): (Z=E ¼ 60=40). An oil;
IR (neat) 3030, 2957, 2925, 2854, 1656, 1466, 1400, 1253,
1162, 1092, 922, 845, 751 cmꢄ1 1H NMR of (Z)-form (400
;
MHz, CDCl3) ꢅ 0.16 (9H, s), 0.87 (3H, t, J ¼ 6:83 Hz), 1.18–
1.38 (12H, m), 2.03–2.08 (2H, m), 4.47 (1H, dt, J ¼ 5:83, 7.31
Hz), 6.12 (1H, dt, J ¼ 5:83, 1.48 Hz); 1H NMR of (E)-form
(400 MHz, CDCl3) ꢅ 0.17 (9H, s), 0.87 (3H, t, J ¼ 6:83 Hz),
1.18–1.38 (12H, m), 1.84–1.96 (2H, m), 4.98 (1H, dt,
J ¼ 11:95, 7.35 Hz), 6.17 (1H, dt, J ¼ 11:95, 1.48 Hz). MS
(EI) m=z 229 (Mþ þ 1, 1.06%), 228 (Mþ, 5.54), 213 (11.33),
185 (8.80), 155 (0.20), 143 (8.15), 129 (73.17), 115 (4.30), 75
(35.91), 73 (100.00).
1
701 cmꢄ1; H NMR of (Z)-form (400 MHz, CDCl3) ꢅ 1.25 (3H,
t, J ¼ 7:02 Hz), 3.14 (2H, dd, J ¼ 1:83, 7.02 Hz), 3.87 (2H, s),
4.13 (2H, q, J ¼ 7:02 Hz), 5.74 (1H, dt, J ¼ 9:46, 7.02 Hz),
6.12 (1H, dt, J ¼ 9:46, 1.83 Hz), 7.22–7.32 (5H, m); 1H NMR
of (E)-form (400 MHz, CDCl3) ꢅ 1.24 (3H, t, J ¼ 7:02 Hz),
3.06 (2H, dd, J ¼ 1:22, 7.02 Hz), 3.89 (2H, s), 4.12 (2H, q, J ¼
7:02 Hz), 5.70 (1H, dt, J ¼ 14:96, 7.02 Hz), 6.08 (1H, dt,
J ¼ 14:96, 1.22 Hz), 7.22–7.32 (5H, m). MS (EI) m=z 237
(Mþ þ 1, 10.84%), 236 (Mþ, 74.97), 190 (12.73), 163 (13.58),
148 (6.58), 129 (9.82), 123 (7.21), 91 (100.00), 65 (39.37), 39
(18.75), 29 (26.75).
3-Methyl-1-(methyldiphenylsiloxy)-1-butene (6c): (Z=E ¼
54=46). An oil; IR (neat) 3070, 3050, 3024, 2957, 2926, 2868,
1655, 1590, 1490, 1465, 1457, 1429, 1400, 1308, 1248, 1153,
1120, 1073, 998, 880, 795, 760, 735, 698 cmꢄ1 1H NMR of
;
(Z)-form (400 MHz, CDCl3) ꢅ 0.69 (3H, s), 0.97 (6H, d, J ¼
6:72 Hz), 2.85–3.00 (1H, m), 4.37 (1H, dd, J ¼ 5:80, 8.85 Hz),
6.09 (1H, d, J ¼ 5:80 Hz), 7.36–7.44 (6H, m), 7.58–7.61 (4H,
m); 1H NMR of (E)-form (400 MHz, CDCl3) ꢅ 0.81 (3H, s),
0.92 (6H, d, J ¼ 7:02 Hz), 2.10–2.25 (1H, m), 5.06 (1H, dd,
J ¼ 7:93, 11.90 Hz), 6.25 (1H, d, J ¼ 11:90 Hz), 7.29–7.62
(10H, m). MS (EI) m=z 282 (Mþ, 62.96%), 267 (80.67), 197
(100.00), 189 (12.87), 137 (19.57).
The Representative Procedure for Conversion of Aldehydes
to the Corresponding Silyl Enol Ethers Using Silyl Chloride.
To a mixed solution of chloro(methyl)diphenylsilane (400 mg,
1.72 mmol) and octanal (5b) (224 mg, 1.43 mmol) in DMF (1.0
mL) was added Et3N (0.41 mL, 2.86 mmol) at 0 ꢃC. The mixture
was immediately heated at 60 ꢃC. After stirring for 48 h at 60 ꢃC,
the reaction mixture was cooled to room temperature. Cold pen-
tane (20 mL) and 5% aqueous HCl (8 mL) were added to the mix-
ture and the organic substances were extracted. The organic layer
was washed by 5% aqueous NaHCO3 three times and dried over
Na2SO4. After evaporating the solvent, the crude products 6b
3,3-Dimethyl-1-(methyldiphenylsiloxy)-1-butene (6d): (Z=E ¼
30=70). An oil; IR (neat) 3070, 3051, 3023, 2953, 2862, 1652,
1590, 1477, 1461, 1428, 1406, 1360, 1283, 1254, 1205, 1119,
1
1088, 997, 934, 918, 852, 797, 731, 698, 677 cmꢄ1; H NMR of
(Z)-form (400 MHz, CDCl3) ꢅ 0.69 (3H, s), 1.16 (9H, s), 4.33
(1H, d, J ¼ 6:41 Hz), 6.01 (1H, d, J ¼ 6:41 Hz), 7.37–7.44
(6H, m), 7.58–7.61 (4H, m); 1H NMR of (E)-form (400 MHz,
CDCl3) ꢅ 0.69 (3H, s), 0.95 (9H, s), 5.18 (1H, d, J ¼ 12:21
Hz), 6.21 (1H, d, J ¼ 12:21 Hz), 7.29–7.62 (10H, m). MS (EI)
m=z 296 (Mþ, 48.89%), 281 (100.00), 239 (10.65), 197 (99.97),
137 (71.98), 105 (33.64).
1
(538 mg) were analyzed by measuring the H NMR spectrum to
determine their yields and Z=E ratio. A sample for physical and
spectral data was obtained by purification by column chromatog-
raphy (SiO2, hexane/Et2O = 60/1, v/v).
In a similar way, other aldehydes 5a,c–g, were converted to the
corresponding silyl enol ethers 6a,c–g and 7b. The physical and
spectral data of 6a–g, 7b are given in the following:
1-(Methyldiphenylsiloxy)-2-phenylethene (6e):
67=33). An oil; IR (neat) 3069, 3050, 3025, 2959, 1643, 1593,
(Z=E ¼
1-(Methyldiphenylsiloxy)-1-propene (6a): (Z=E ¼ 74=26).
An oil; IR (neat) 3070, 3047, 2952, 2917, 2861, 1662, 1590,
1488, 1428, 1402, 1362, 1257, 1122, 1065, 998, 837, 793, 773,
1492, 1447, 1428, 1302, 1261, 1212, 1200, 1144, 1119, 1083,
1075, 1029, 998, 925, 892, 792, 765, 731, 696 cmꢄ1 1H NMR
;
1
729, 698, 672 cmꢄ1; H NMR of (Z)-form (400 MHz, CDCl3) ꢅ
of (Z)-form (400 MHz, CDCl3) ꢅ 0.78 (3H, s), 5.35 (1H, d, J ¼
6:41 Hz), 6.41 (1H, d, J ¼ 6:41 Hz), 7.10–7.70 (15H, m);
1H NMR of (E)-form (400 MHz, CDCl3) ꢅ 0.77 (3H, s), 6.14
(1H, d, J ¼ 12:21 Hz), 7.01 (1H, d, J ¼ 12:21 Hz), 7.10–7.70
(15H, m). MS (EI) m=z 316 (Mþ, 77.69%), 223 (20.93), 197
(100.00), 180 (72.19), 165 (10.48), 105 (15.65), 91 (18.12), 77
(9.35).
0.694 (3H, s), 1.64 (3H, dd, J ¼ 1:83, 6.71 Hz), 4.56 (1H, dq,
J ¼ 5:80, 6.71 Hz), 6.21 (1H, dq, J ¼ 5:80, 1.83 Hz), 7.36–7.42
(6H, m), 7.59–7.61 (4H, m); 1H NMR of (E)-form (400 MHz,
CDCl3) ꢅ 0.689 (3H, s), 1.48 (3H, dd, J ¼ 1:83, 6.71 Hz), 5.09
(1H, dq, J ¼ 11:90, 6.71 Hz), 6.25 (1H, dq, J ¼ 11:90, 1.83
Hz), 7.28–7.61 (10H, m). MS (EI) m=z 254 (Mþ, 9.98%), 239
(12.67), 197 (100.00), 195 (27.21), 183 (21.48), 181 (11.75),
165 (10.64), 137 (90.18), 121 (10.54), 105 (20.48), 91 (8.76), 77
(9.63), 58 (9.80), 43 (37.42), 28 (16.96).
2-Benzyloxy-1-(methyldiphenylsiloxy)ethene (6f): (Z=E ¼
100=0). An oil; IR (neat) 3068, 3048, 2959, 2871, 1667, 1589,
1496, 1454, 1428, 1396, 1362, 1298, 1254, 1119, 1024, 973,
1
1-(Methyldiphenylsiloxy)-1-decene (6b):
An oil; IR (neat) 3070, 3050, 3028, 2955, 2924, 2854, 1656,
1590, 1466, 1428, 1399, 1377, 1256, 1180, 1121, 1092, 998,
(Z=E ¼ 58=42).
910, 835, 793, 730, 698 cmꢄ1; H NMR of (Z)-form (400 MHz,
CDCl3) ꢅ 0.71 (3H, s), 4.76 (2H, s), 5.43 (1H, d, J ¼ 3:36 Hz),
5.53 (1H, d, J ¼ 3:36 Hz), 7.24–7.62 (15H, m). MS (EI) m=z
346 (Mþ, 19.75%), 269 (11.01), 255 (6.77), 239 (1.80), 197
(100.00), 178 (23.52), 163 (15.80), 137 (14.02), 105 (16.93), 91
(84.40), 65 (10.22).
834, 793, 728, 698 cmꢄ1 1H NMR of (Z)-form (400 MHz,
;
CDCl3) ꢅ 0.686 (3H, s), 0.87 (3H, t, J ¼ 6:10 Hz), 1.22–1.30
(12H, m), 2.13–2.18 (2H, m), 4.50 (1H, dt, J ¼ 5:80, 7.33 Hz),

2156 Bull. Chem. Soc. Jpn., 77, No. 12 (2004)
HEADLINE ARTICLES
2-Benzylthio-1-(methyldiphenylsiloxy)ethene (6g): (Z=E ¼
37=63). An oil; IR (neat) 3068, 3040, 3025, 2959, 2920, 1615,
1593, 1494, 1453, 1428, 1255, 1229, 1120, 1088, 1026, 998,
84=16). An oil; IR (neat) 3024, 2946, 2868, 1655, 1464, 1409,
1385, 1360, 1285, 1249, 1206, 1097, 1050, 996, 920, 883, 847,
716, 675 cmꢄ1 1H NMR of (Z)-form (300 MHz, CDCl3) ꢅ
;
1
894, 856, 794, 734, 698 cmꢄ1; H NMR of (Z)-form (400 MHz,
1.05–1.20 (3H, m), 1.08 (18H, d, J ¼ 5:87 Hz), 1.13 (9H, s),
1
CDCl3) ꢅ 0.70 (3H, s), 3.85 (2H, s), 5.01 (1H, d, J ¼ 5:19 Hz),
6.37 (1H, d, J ¼ 5:19 Hz), 7.15–7.62 (15H, m); 1H NMR of
(E)-form (400 MHz, CDCl3) ꢅ 0.65 (3H, s), 3.61 (2H, s), 5.53
(1H, d, J ¼ 11:59 Hz), 6.52 (1H, d, J ¼ 11:59 Hz), 7.15–7.62
(15H, m). MS (EI) m=z 364 (Mþ þ 2, 8.82%), 363 (Mþ þ 1,
22.21), 362 (Mþ, 75.03), 271 (24.05), 197 (100.00), 165 (6.66),
105 (9.11), 91 (58.94), 65 (6.52), 43 (10.92).
4.19 (1H, d, J ¼ 6:60 Hz), 6.07 (1H, d, J ¼ 6:60 Hz); H NMR
of (E)-form (300 MHz, CDCl3) ꢅ 1.00 (9H, s), 1.05–1.20 (3H,
m), 1.08 (18H, d, J ¼ 5:87 Hz), 5.09 (1H, d, J ¼ 12:10 Hz),
6.41 (1H, d, J ¼ 12:10 Hz). MS (EI) m=z 256 (Mþ, 13.87%),
241 (21.27), 213 (100.00), 185 (39.81), 157 (28.45), 143 (5.21),
131 (20.27), 103 (17.06), 75 (29.77), 57 (42.76).
2-Phenyl-1-(triisopropylsiloxy)ethene (8e): (Z=E ¼ 60=40).
The Representative Procedure for Conversion of Aldehydes
to the Corresponding Silyl Enol Ethers Using TIPSOTf. To a
solution of triisopropylsilyl trifluoromethanesulfonate (TIPSOTf)
(379 mg, 1.2 mmol) in Et2O (1.5 mL) was added Et3N (0.17
mL, 1.2 mmol) at 0 ꢃC. After stirring for 30 min, a solution of
CH3CH2CHO (5a) (58 mg, 1.0 mmol) in Et2O (0.5 mL) was add-
ed to the mixture and stirred for 2 h at 0 ꢃC. The reaction mixture
was then quenched with 2-(2-hydroxyethyl)pyridine (0.03 mL,
0.25 mmol), and the solvent was evaporated. The crude product
was purified by silica-gel column chromatography using hexane
as the eluent to give 8a (151 mg, 71%) as a mixture of Z=E iso-
mers (Z=E ¼ 96=4).
An oil; IR (neat) 3062, 3030, 2945, 2893, 2867, 1644, 1602, 1496,
1464, 1416, 1267, 1151, 1088, 1069, 1030, 922, 883, 693 cmꢄ1
;
1H NMR of (Z)-form (400 MHz, CDCl3) ꢅ 1.13 (18H, d, J ¼
5:12 Hz), 1.17–1.28 (3H, m), 5.26 (1H, d, J ¼ 6:59 Hz), 6.50
(1H, d, J ¼ 6:59 Hz), 7.09–7.27 (3H, m), 7.64 (2H, d, J ¼ 8:05
Hz); 1H NMR of (E)-form (400 MHz, CDCl3) ꢅ 1.12 (18H, d, J ¼
4:64 Hz), 1.17–1.28 (3H, m), 6.04 (1H, d, J ¼ 12:20 Hz), 7.08
(1H, d, J ¼ 12:20 Hz), 7.09–7.27 (5H, m). MS (EI) m=z 276
(Mþ, 63.27%), 233 (100.00), 191 (18.83), 163 (47.06), 161
(45.40), 147 (13.23), 103 (20.70), 89 (18.93), 75 (15.09).
2-Benzyloxy-1-(triisopropylsiloxy)ethene (8f):
96=4). An oil; IR (neat) 2944, 2867, 1664, 1465, 1398, 1362,
1253, 1134, 1015, 883, 733, 695 cmꢄ1 1H NMR of (Z)-form
(Z=E ¼
In a similar way, other aldehydes 5b–g were converted to the
corresponding silyl enol ethers 8b–g,7b. The physical and spectral
data of 8a–g are given in the following:
;
(400 MHz, CDCl3) ꢅ 1.09 (18H, d, J ¼ 6:34 Hz), 1.13–1.21
(3H, m), 4.82 (2H, s), 5.35 (1H, d, J ¼ 3:42 Hz), 5.60 (1H, d, J ¼
3:42 Hz), 7.28–7.37 (5H, m). 1H NMR of (E)-form (400 MHz,
CDCl3) ꢅ 1.05 (18H, d, J ¼ 5:86 Hz), 1.13–1.21 (3H, m), 4.63
(2H, s), 6.39 (1H, d, J ¼ 10:25 Hz), 6.52 (1H, d, J ¼ 10:25
Hz), 7.28–7.37 (5H, m). MS (EI) m=z 306 (Mþ, 27.05%), 263
(75.62), 157 (52.35), 129 (16.89), 115 (35.70), 91 (100.00), 59
(29.24).
1-(Triisopropylsiloxy)-1-propene (8a): (Z=E ¼ 96=4). An
oil; IR (neat) 3040, 2945, 2894, 2868, 1660, 1465, 1403, 1385,
1362, 1260, 1179, 1130, 1065, 1014, 996, 920, 883, 685 cmꢄ1
;
1H NMR of (Z)-form (400 MHz, CDCl3) ꢅ 1.08 (18H, d, J ¼
6:10 Hz), 1.11–1.21 (3H, m), 1.59 (3H, dd, J ¼ 1:71, 6.59 Hz),
4.45 (1H, dq, J ¼ 5:86, 6.59 Hz), 6.29 (1H, dq, J ¼ 5:86, 1.71
Hz); 1H NMR of (E)-form (400 MHz, CDCl3) ꢅ 1.08 (18H, d, J ¼
6:10 Hz), 1.11–1.21 (3H, m), 1.51 (3H, dd, J ¼ 1:48, 6.83 Hz),
5.00 (1H, dq, J ¼ 11:70, 6.83 Hz), 6.31 (1H, dq, J ¼ 11:70,
1.48 Hz). MS (EI) m=z 214 (Mþ, 19.75%), 171 (100.00), 143
(56.05), 129 (14.99), 115 (50.37), 101 (32.47), 85 (16.08), 73
(23.72), 61 (53.78), 59 (15.72).
2-Benzylthio-1-(triisopropylsiloxy)ethene (8g):
37=63). An oil; IR (neat) 3062, 3028, 2945, 2867, 1604, 1495,
1233, 1179, 1097, 1070, 883, 696 cmꢄ1 1H NMR of (Z)-form
(Z=E ¼
;
(400 MHz, CDCl3) ꢅ 1.07 (18H, d, J ¼ 6:34 Hz), 1.05–1.18
(3H, m), 3.84 (2H, s), 4.90 (1H, d, J ¼ 4:88 Hz), 6.47 (1H, d, J ¼
4:88 Hz), 7.18–7.34 (5H, m). 1H NMR of (E)-form (400 MHz,
CDCl3) ꢅ 1.01 (18H, d, J ¼ 6:10 Hz), 1.05–1.18 (3H, m), 3.65
(2H, s), 5.45 (1H, d, J ¼ 11:47 Hz), 6.59 (1H, d, J ¼ 11:47
Hz), 7.18–7.34 (5H, m). MS (EI) m=z 322 (Mþ, 60.58%), 279
(11.43), 245 (8.57), 188 (2.20), 157 (12.15), 91 (100.00).
1-(Triisopropylsiloxy)-1-decene (8b): (Z=E ¼ 93=7). An oil;
IR (neat) 2925, 2867, 1655, 1465, 1400, 1259, 1097, 996, 882
cmꢄ1; 1H NMR of (Z)-form (400 MHz, CDCl3) ꢅ 0.88 (3H, t, J ¼
6:83 Hz), 1.07 (18H, d, J ¼ 6:10 Hz), 1.12–1.17 (3H, m), 1.27–
1.36 (12H, m), 2.04–2.20 (2H, m), 4.35 (1H, m), 6.25 (1H, d, J ¼
1
5:85 Hz). H NMR of (E)-form (400 MHz, CDCl3) ꢅ 0.88 (3H, t,
References
J ¼ 6:83 Hz), 1.06 (18H, d, J ¼ 5:85 Hz), 1.12–1.17 (3H, m),
1.27–1.36 (12H, m), 1.82–1.92 (2H, m), 4.98 (1H, dt, J ¼
11:95, 6.59 Hz), 6.27 (1H, d, J ¼ 11:95 Hz). MS (EI) m=z 312
(Mþ, 2.90%), 269 (100.00), 241 (3.14), 227 (3.45), 213 (2.02),
183 (1.28), 171 (3.27), 157 (3.50), 129 (3.01), 99 (4.70).
1
a) T. Hirata, Y. Sasada, T. Ohtani, T. Asada, H. Kinoshita,
H. Senda, and K. Inomata, Bull. Chem. Soc. Jpn., 65, 75 (1992). b)
A. Shibayama, T. Nakamura, T. Asada, T. Shintani, Y. Ukaji, H.
Kinoshita, and K. Inomata, Bull. Chem. Soc. Jpn., 70, 381 (1997).
c) T. Nakamura, S. K. Guha, Y. Ohta, D. Abe, Y. Ukaji, and K.
Inomata, Bull. Chem. Soc. Jpn., 75, 2031 (2002). d) S. K. Guha,
Y. Ukaji, and K. Inomata, Chem. Lett., 32, 1158 (2003).
3-Methyl-1-(triisopropylsiloxy)-1-butene (8c):
(Z=E ¼
88=12). An oil; IR (neat) 3026, 2947, 2897, 2868, 1654, 1465,
1403, 1250, 1154, 1083, 1014, 996, 920, 882, 684 cmꢄ1; 1H NMR
of (Z)-form (400 MHz, CDCl3) ꢅ 0.962 (6H, d, J ¼ 6:83 Hz), 1.08
(18H, d, J ¼ 6:10 Hz), 1.11–1.17 (3H, m), 2.83 (1H, m), 4.24 (1H,
dd, J ¼ 5:85, 8.78 Hz), 6.15 (1H, d, J ¼ 5:85 Hz); 1H NMR of
(E)-form (400 MHz, CDCl3) ꢅ 0.966 (6H, d, J ¼ 6:59 Hz), 1.06
(18H, d, J ¼ 6:34 Hz), 1.11–1.17 (3H, m), 2.20 (1H, m), 4.97
(1H, dd, J ¼ 7:81, 11.95 Hz), 6.28 (1H, d, J ¼ 11:95 Hz). MS
(EI) m=z 242 (Mþ, 24.11%), 227 (3.35), 199 (100.00), 171
(2.09), 157 (3.73), 131 (35.95), 103 (36.59), 75 (26.67), 61
(22.82), 59 (15.56).
2
‘‘Syn-Effect’’ is herein defined as an effect which stabilizes
the syn-conformation against the steric hindrance: a) M. Schlosser
and J. Hartmann, J. Am. Chem. Soc., 98, 4674 (1976). b) D.
Cremer, J. Am. Chem. Soc., 101, 7199 (1979). c) K. N. Houk,
R. W. Strozier, N. G. Rondan, R. R. Fraser, and N. Chuaqui-
Offermanns, J. Am. Chem. Soc., 102, 1426 (1980). d) E. Block,
R. E. Penn, A. A. Bazzi, and D. Cremer, Tetrahedron Lett., 22,
29 (1981). e) E. Block and M. Aslam, J. Am. Chem. Soc., 105,
6164 (1983). f) R. Glaser and A. Streitwieser, J. Am. Chem.
Soc., 111, 7340 (1989). g) S. Mangelinckx, N. Giubellina, and
3,3-Dimethyl-1-(triisopropylsiloxy)-1-butene (8d): (Z=E ¼

S. K. Guha et al.
Bull. Chem. Soc. Jpn., 77, No. 12 (2004) 2157
π*
π
π*
π*
π
π*
H
H
σ
σ
H
base
base
H
base
O
O
−
−
σ
σ
SiMe3
SiMe3 OTf
OTf
R
H
H
R
(E)
(F)
transition state in syn-coordination by trimethylsilyl triflate
Fig. 11.
N. D. Kimpe, Chem. Rev., 104, 2353 (2004), and references cited
therein.
Guerrero, Y. Hase, and R. Rittner, J. Chem. Soc., Perkin Trans.
2, 1990, 465, and references cited therein.
3
a) E.-P. Krebs, Helv. Chim. Acta, 64, 1023 (1981). b) A. S.
12 a) J. K. Rasmussen, Synthesis, 1977, 91. b) P.
Brownbridge, Synthesis, 1983, 1. c) T.-H. Chan, ‘‘Comprehensive
Organic Synthesis,’’ ed by B. M. Trost, Pergamon Press, Oxford
(1991), Vol. 2, pp. 613–624.
Kende and B. H. Toder, J. Org. Chem., 47, 163 (1982). c) S. G.
Alcock, J. E. Baldwin, R. Bohlmann, L. M. Harwood, and J. I.
Seeman, J. Org. Chem., 50, 3526 (1985). d) P. Galatsis, J. J.
Manwell, and S. D. Millan, Tetrahedron Lett., 37, 5261 (1996).
e) K. Tomooka, A. Nagasawa, S.-Y. Wei, and T. Nakai, Tetrahe-
dron Lett., 37, 8895 (1996). f) K. Tomooka, A. Nagasawa, and T.
Nakai, Chem. Lett., 1998, 1049, and references cited therein.
13 The hydrogen bond between aromatic C–H and carbonyl
oxygen was discussed: J. A. R. P. Sarma and G. R. Desiraju,
J. Chem. Soc., Perkin Trans. 2, 1987, 1195.
ꢀ
14 Orbital interaction analysis predicts that ꢄ of a carbonyl
4
a) A. S. Kende, D. Constantinides, S. J. Lee, and L.
group is lowered in energy and more highly polarized as a conse-
quence of attachment of an electrophile such as a proton on the
carbonyl oxygen atom: A. Rauk, ‘‘Orbital Interaction Theory of
Organic Chemistry’’, John Wiley & Sons, Inc., New York
(1994), Chap. 8, pp. 142–143.
15 H. Emde, A. Gotz, K. Hofmann, and G. Simchen, Liebigs
¨
Ann. Chem., 1981, 1643.
16 H. Emde, D. Domsch, H. Feger, U. Frick, A. Gotz, H. H.
¨
Liebeskind, Tetrahedron Lett., 16, 405 (1975). b) M. P.
Zimmerman, Synth. Commun., 7, 189 (1977). c) P. Galatsis and
J. J. Manwell, Tetrahedron Lett., 36, 8179 (1995).
5
Shibayama, D. Abe, Y. Ukaji, and K. Inomata, Chem. Lett., 32,
Preliminary results were already reported: S. K. Guha, A,
778 (2003).
6
For example: a) H. O. House, L. J. Czuba, M. Gall, and H.
D. Olmstead, J. Org. Chem., 34, 2324 (1969). b) J. M. Conia and
C. Girard, Tetrahedron Lett., 14, 2767 (1973). c) J. Dedier, P.
Gerval, and E. Frainnet, J. Organomet. Chem., 185, 183 (1980).
d) D. Limat and M. Schlosser, Tetrahedron, 51, 5799 (1995).
Hergott, K. Hofmann, W. Kober, K. Krageloh, T. Oesterle, W.
¨
Steppan, W. West, and G. Simchen, Synthesis, 1982, 1.
17 In the reaction using smaller trimethylsilyl triflate com-
pared with TIPSOTf, syn-coordination might be possible in part,
in which CH eclipsed conformation F preferred to CC eclipsed
conformation E due to steric repulsion between ꢀ-substituent
and silyl group (Fig. 11) to give (E)-silyl enol ether.
7
(1996).
8
B. W. Gung and M. M. Yanik, J. Org. Chem., 61, 947
Although there were debates on the relative donor ability
of C–H and C–C, recently the former is reported to be better than
the latter: a) T. Laube and H. U. Stilz, J. Am. Chem. Soc., 109,
5876 (1987). b) T. Laube and T.-K. Ha, J. Am. Chem. Soc.,
110, 5511 (1988). c) P. R. Rablen, R. W. Hoffmann, D. A. Hrovat,
and W. T. Borden, J. Chem. Soc., Perkin Trans. 2, 1999, 1719.
18 Although cyclic transition state was proposed,15 the differ-
ence by the substituents could not be explained by it.
19 a) R. S. Marmor, J. Org. Chem., 37, 2901 (1972). b) W. S.
´
Wadsworth, Jr., Org. React., 25, 73 (1977). c) G. Solladie, J. Hutt,
´
and C. Frechou, Tetrahedron Lett., 28, 61 (1987).
9 Y. Apeloig, P. v. R. Schleyer, and J. A. Pople, J. Am.
Chem. Soc., 99, 5901 (1977).
20 E. T. McBee, M. J. Keogh, R. P. Levek, and E. P.
Wesseler, J. Org., Chem., 38, 632 (1973).
10 a) P. v. R. Schleyer, J. D. Dill, J. A. Pople, and W. J.
Hehre, Tetrahedron, 33, 2497 (1977). b) J. D. Fenske, K. T.
Kuwata, K. N. Houk, and S. E. Paulson, J. Phys. Chem. A, 104,
7246 (2000).
21 S.-I. Murahashi, Y. Imada, Y. Taniguchi, and S.
Higashiura, J. Org., Chem., 58, 1538 (1993).
22 Chemical shifts and coupling constants of olefinic moieties
are quite identical with (Z)- and (E)-4-methoxy- and 4-methyl-
thio-3-butenoates, respectively: J. Hine, L. G. Mahone, and C.
L. Liotta, J. Org. Chem., 32, 2600 (1967).
ꢀ
11 The significance of the ꢃC{S ! ꢄ interaction in ꢀ-alkyl-
thio carbonyl compounds was pointed out: P. R. Olivato, S. A.