Formation of stable enols from 1,4-dilithio-1,3-dienes and acid chlorides by a de-aromatization/Michael addition/re-aromatization domino process

Article abstract of DOI:10.1039/b705110g

Stable enols were synthesized from the reaction of (1Z,3Z)-1,4-dilithio-1, 3-dienes with acid chlorides and structurally characterized by single-crystal X-ray analysis. These stable enols were formed by a novel de-aromatization/ Michael addition/re-aromat

Full text of DOI:10.1039/b705110g

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Formation of stable enols from 1,4-dilithio-1,3-dienes and acid chlorides by a
de-aromatization/Michael addition/re-aromatization domino process†
Qiaoshu Hu,a Chao Wang,a Dongzhen Lia and Zhenfeng Xi*a,b
Received 4th April 2007, Accepted 18th May 2007
First published as an Advance Article on the web 31st May 2007
DOI: 10.1039/b705110g
Stable enols were synthesized from the reaction of (1Z,3Z)-1,4-dilithio-1,3-dienes with acid chlorides
and structurally characterized by single-crystal X-ray analysis. These stable enols were formed by a
novel de-aromatization/Michael addition/re-aromatization domino process.
Introduction
(1Z,3Z)-1,4-Dilithio-1,3-butadienes have been of substantial in-
terest for decades,1–7 especially as organobimetallic reagents for
synthetic chemistry, utilizing the cooperative effect of the two
alkenyllithiums in the same molecule,1–3 and their structural
properties.4–6 The solid-state X-ray structures have generally shown
Scheme 2
an s-cis conformation and a double-bridged dilithium structure
(Scheme 1),6 which has also been supported by theoretical
predictions.4 As demonstrated by the work of ourselves and
others,1–3,7 these s-cis forms and the doubly-bridged dilithium
structure contribute greatly to their interesting reaction chemistry.
Results and discussion
1,4-Bis(trimethylsilyl)-2,3-diaryl-1,4-dilithio-1,3-dienes 2a (Ar =
Ph) and 2b (Ar = tolyl) could be readily generated quantitatively
in situ by lithium–iodine exchange of the corresponding 1,4-
diiodo-1,3-butadienes 1 with 4 equivalents of t-BuLi.7 Addition
of acid chloride to this mixture caused immediate reaction, the
reaction being complete within 30 minutes, and affording enols 3a–
d in good isolated yield after quenching with water (Scheme 3).
Scheme 1
As part of our continuing research into these interesting and
useful organodilithium reagents,1,7 we envisioned that the s-trans
form should have different reactivity from that of the s-cis form.
In order to change the conformation of (1Z,3Z)-1,4-dilithio-1,3-
butadienes from the relatively stable s-cis form to the relatively
unstable s-trans form, we employed a strategy taking advantage
of the steric repulsion effect forcing the equilibrium to the s-trans
form (Scheme 2). Very interestingly, this strategy resulted in the
formation and isolation of a new type of stable enol from the
reaction of 1,4-dilithio-1,3-butadienes with acid chlorides. In this
paper, we would like to report: (1) the formation of unexpected
stable enols by a novel de-aromatization/Michael addition/re-
aromatization domino process, (2) the structure and reaction
chemistry of this kind of enol, and (3) mechanistic aspects.
aBeijing National Laboratory for Molecular Sciences (BNLMS), Key
Laboratory of Bioorganic Chemistry and Molecular Engineering of the
Ministry of Education, College of Chemistry, Peking University, Beijing,
100871, China. E-mail: zfxi@pku.edu.cn
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences, Shanghai, 200032, China
† Electronic supplementary information (ESI) available: Copies of 1H and
13C NMR spectra for all new compounds; crystal data for 3a. See DOI:
10.1039/b705110g
Scheme 3
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When the reaction was terminated using PhCOCl instead of by
hydrolysis, the ester 4 was isolated in 42% yield. All these results
indicated that a novel reaction was taking place, with the lithium
enolate 5 as an intermediate.
The structure of enol 3a was determined by single-crystal X-ray
analysis (Fig. 1).
The steric effect is generally accepted as the major factor con-
tributing to the existence of stable enols.8–10 To further investigate
the steric effect for these stable enols, we treated 2,3-diphenyl-1,4-
dilithio-1,3-diene 8 with 2,4,6-trichlorophenyl carbonyl chloride.
In contrast to 2a, this dilithio reagent 8 does not have the bulky
trimethylsilyl groups at the 1- and 4-positions. Indeed, instead of
an enol, cyclopentadienol 97e was obtained in 57% isolated yield
(Scheme 5). This result demonstrates that the steric effect does
play a crucial role for the formation of stable enols.
Scheme 5
˚
Fig. 1 X-Ray structure of 3a. Selected bond lengths (in A): C23–O1
1.384(3), C1–C23 1.330(4), C1–C2 1.503(4), C2–C3 1.359(4), C2–C5
1.474(4), C3–C4 1.508(4). CCDC reference number 633783. For
crystallographic data in CIF or other electronic format, see DOI:
10.1039/b705110g.
A proposed reaction mechanism for the synthesis of the stable
enols is given in Scheme 6. The steric effect is considered to be
crucial for this novel reaction. Because of the steric effect, the Li
atom gets close enough to the adjacent phenyl ring and undergoes
immediate anionic attack, de-aromatizing the phenyl ring. This
dearomatizing anionic cyclization has been well documented in
the literature.11–15 However, it is not yet clear whether the 2a-s-trans
conformation or the 2a-s-cis conformation reacts first with the acid
chloride. What is clear is that upon formation, the intermediate 10
must immediately undergo dearomatizing anionic cyclization and
Michael 1,6-addition, leading to 11. Re-aromatization of 11 by a
1,5-H shift then affords the enolate 5.
The study of the synthesis, stability and reaction chemistry of
enols has been a fundamental subject in organic chemistry both
from the theoretical point of view and the experimental point
of view. Several excellent methods and a number of stable enols
have been reported in the literature.8–10 For example, Rappoport
and co-workers have developed a series of interesting and useful
synthetic methods for simple stable enols and have determined
their structures by X-ray analysis.8 The enols reported in this paper
represent a new type of stable enol, and the synthetic method is
unprecedented.
We tested the stability of these enols in various solvents. No
tautomeric equilibrium was observed for these enols when they
were dissolved in solvents such as CDCl3, DMSO-d6 and C6D6.
In addition, they were also chemically very stable; the enol moiety
did not undergo any reaction on exposure to strong conditions.
However, the SiMe3 group on the five-membered ring of 3a did
undergo desilylation, affording their derivatives 6 and 7 in excellent
isolated yields (Scheme 4).
Scheme 6
Although the steric effect is crucial here, further reactions
suggested that the electronic effect must also play an important
Scheme 4
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role. For example, as shown in Scheme 3, 2a reacted with 2,4,6-
trichlorobenzoyl chloride affording the stable enol 3a in 65%
isolated yield. However, when 2a was treated with mesitoyl
chloride without HMPA, no reaction was detected. As shown
in Scheme 7, in the presence of HMPA, the reaction took place to
form the cyclopentadiene derivative 12 in 47% isolated yield,7e with
no enol being formed. This result demonstrates that the electronic
effect also plays an important role for the formation of stable enols.
C), 151.40 (1 quat C). HRMS calcd. for C29H31OSi2Cl3: 556.0979,
found 556.0984. Anal. Calcd for C29H31OSi2Cl3: C, 62.41%; H,
5.60%. Found: C, 62.42%, H, 5.58%.
3b. Colorless crystals, 62% yield (323 mg); mp 165.0–166.3 ◦C;
◦
1H NMR (300 MHz, CDCl3, Me4Si, 25 C): d = −0.55 (s, 9 H,
CH3), −0.22 (s, 9 H, CH3), 4.22 (s, 1 H, CH), 5.58 (s, 1 H, OH),
7.22–7.85 (m, 12 H, CH). 13C NMR (75 MHz, CDCl3, Me4Si,
25 ◦C): d = −2.50 (3 CH3), −0.91 (3 CH3), 48.30 (1 CH), 109.30
(1 quat C), 121.30 (1 CH), 123.02 (1 CH), 124.12 (1 CH), 125.26
(1 CH), 127.43 (1 CH), 127.93 (2 CH), 128.11 (1 CH), 128.22
(1 CH), 129.64 (2 CH), 130.37 (1 CH), 132.01 (1 quat C), 135.50
(1 quat C), 136.21 (1 quat C), 136.32 (1 quat C), 137.72 (1 quat C),
144.73 (1 quat C), 144.78 (1 quat C), 148.03 (1 quat C), 152.33
(1 quat C). HRMS calcd. for C29H32OSi2Cl2: 522.1369, found
522.1358.
Scheme 7
3c. Colorless crystals, 61% yield (356 mg); mp 153.9–154.8 ◦C;
1H NMR (300 MHz, CDCl3, Me4Si, 25 ◦C): d = −0.53 (s, 9H,
CH3), −0.23 (s, 9H, CH3), 2.37 (s, 3H, CH3), 2.45 (s, 3H, CH3),
4.13 (s, 1H, CH), 5.58 (s, 1H, OH), 7.11–7.64 (m, 9H, CH). 13C
NMR (75 MHz, CDCl3, Me4Si, 25 ◦C): d = −2.41 (3 CH3), −0.77
(3 CH3), 21.34 (1 CH3), 21.77 (1 CH3), 47.88 (1 CH), 110.26 (1 quat
C), 120.67 (1 CH), 123.84 (1 CH), 126.25 (1 CH), 128.14 (1 CH),
128.18 (1 CH), 128.59 (2 CH), 129.38 (2 CH), 130.92 (1 quat C),
133.60 (1 quat C), 134.36 (1 quat C), 134.83 (1 quat C), 135.32
(1 quat C), 136.35 (1 quat C), 136.94 (1 quat C), 137.03 (1 quat C),
142.23 (1 quat C), 144.96 (1 quat C), 147.15 (1 quat C), 151.18
(1 quat C). HRMS calcd. for C31H35OSi2Cl3: 584.1292, found
584.1292. Anal. Calcd for C31H35OSi2Cl3: C, 63.52%; H, 6.02%.
Found: C, 63.51%, H, 6.10%.
Conclusion
In conclusion, we have developed an interesting methodology
involving a de-aromatization/Michael addition/re-aromatization
domino process to afford a new type of stable enol. Both steric
and electronic effects were found to be crucial for this reaction.
Experimental
All reactions were conducted under a slightly positive pressure of
dry, pre-purified nitrogen using standard Schlenk line techniques
when appropriate. Unless otherwise noted, all starting materials
were commercially available and were used without further
purification. Diethyl ether and THF were refluxed and distilled
from sodium–benzophenone ketyl under a nitrogen atmosphere.
1,4-Dihalo-1,3-butadienes were prepared according to literature
methods.16 1H and 13C NMR spectra were recorded at 300 and
75.4 MHz, respectively, in CDCl3 unless stated otherwise.
3d. Colorless crystals, 65% yield (360 mg); mp 153.3–154.2 ◦C;
1H NMR (300 MHz, CDCl3, Me4Si, 25 ◦C): d = −0.54 (s, 9H,
CH3), −0.22 (s, 9H, CH3), 2.37 (s, 3H, CH3), 2.45 (s, 3H, CH3),
4.13 (s, 1H, CH), 5.59 (s, 1H, OH), 7.11–7.70 (m, 10H, CH).
◦
13C NMR (75 MHz, CDCl3, Me4Si, 25 C): d = −2.39 (3 CH3),
−0.83 (3 CH3), 21.35 (1 CH3), 21.77 (1 CH3), 47.80 (1 CH), 109.54
(1 quat C), 120.84 (1 CH), 123.78 (1 CH), 126.23 (1 CH), 128.08
(1 CH), 128.18 (1 CH), 128.57 (2 CH), 129.44 (2 CH), 130.31
(1 CH), 131.17 (1 quat C), 133.51 (1 quat C), 134.90 (1 quat
C), 135.58 (1 quat C), 136.29 (1 quat C), 136.35 (1 quat C),
136.93(1 quat C), 142.44 (1 quat C), 144.95 (1 quat C), 147.06
(1 quat C), 152.12(1 quat C). HRMS calcd. for C31H36OSi2Cl2:
550.1682, found 550.1686.
Typical procedure for synthesis of enols 3 by reaction between
dilithio-diene 2 and acid chlorides
To a solution of 1,4-bis(trimethylsilyl)-2,3-diary-1,4-diiodo-1,3-
dienes 1 (1.0 mmol) in Et2O (10 ml) at −78 ◦C was added t-BuLi
(4.0 mmol, 1.5 M in pentane). After stirring the reaction mixture
◦
at −78 C for 1 h, acid chloride (1.2 mmol) was added, and the
reaction mixturewarmed up to 0 ◦C for 0.5h. The mixture was then
quenched with water, extracted with Et2O, the extract washed with
brine and dried over MgSO4. The solvent was evaporated in vacuo
and the residue purified by column chromatography (silica gel, 10 :
1 hexane–dichloromethane) to afford the products 3.
Synthesis of enol ester 4 by reaction between dilithio-diene 2 and
two different acid chlorides
To a solution of 1,4-bis(trimethylsilyl)-2,3-diphenyl-1,4-diiodo-
1,3-diene 1a (1.0 mmol) in Et2O (10 ml) at −78 ◦C was added
t-BuLi (4.0 mmol, 1.5 M in pentane). After stirring the reaction
mixture −78 ◦C for 1 h, 2,4,6-trichlorobenzoyl chloride (1.2 mmol)
was added, the reaction mixture warmed up to 0 ◦C for 0.5 h, and
benzoyl chloride (1.2 mmol) then added at the same temperature.
After stirring for another 0.5 h, the mixture was quenched with
water, extracted with Et2O, the extract washed with brine and
dried over MgSO4. The solvent was evaporated in vacuo and
the residue purified by column chromatograph (silica gel, 10 :
1 hexane–dichloromethane) to afford the enol ester 4.
3a. Colorless crystals, 65% yield (363 mg); mp 199.6–200.2 ◦C;
1H NMR (300 MHz, CDCl3, Me4Si, 25 ◦C): d = −0.54 (s, 9H,
CH3), −0.23 (s, 9H, CH3), 4.21 (s, 1H, CH), 5.56 (s, 1H, OH),
7.22–7.78 (m, 11H, CH). 13C NMR (75 MHz, CDCl3, Me4Si,
25 ◦C): d = −2.52 (3 CH3), −0.85 (3 CH3), 48.39 (1 CH), 110.02
(1 quat C), 121.12 (1 CH), 123.08 (1 CH), 124.20 (1 CH), 125.27
(1 CH), 127.51 (1 CH), 127.96 (2 CH), 128.17 (1 CH), 128.22
(1 CH), 129.57 (2 CH), 131.74 (1 quat C), 134.26 (1 quat C),
135.39 (1 quat C), 136.77 (1 quat C), 136.92 (1 quat C), 137.65
(1 quat C), 144.57 (1 quat C), 144.74 (1 quat C), 148.14 (1 quat
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4. Colorless crystals, 42% yield (278 mg); mp 229.2–230.9 ◦C;
1H NMR (300 MHz, CDCl3, Me4Si, 25 ◦C): d = −0.53 (s, 9H,
CH3), −0.51 (s, 9H, CH3), 4.06 (s, 1H, CH), 7.17–7.93 (m, 16H,
CH). 13C NMR (75 MHz, CDCl3, Me4Si, 25 ◦C): d = −2.55 (3
CH3), −0.91 (3 CH3), 47.55 (1 CH), 121.37 (1 CH), 122.74 (1 CH),
123.48 (1 CH), 124.84 (1 CH), 127.27 (1 CH), 127.96 (2 CH),
128.08 (2 CH), 128.24 (1 CH), 128.64 (1 CH), 129.29 (1 quat C),
130.18 (2 CH), 130.30 (2 CH), 130.67 (1 quat C), 133.06 (1 CH),
133.50 (1 quat C), 134.52 (1 quat C), 135.51 (1 quat C), 136.75
(1 quat C), 138.22 (1 quat C), 138.71 (1 quat C), 144.22 (1 quat
7b. Colorless crystals, 93% yield (480 mg); mp 125.3–125.8 ◦C;
1H NMR (300 MHz, CDCl3, Me4Si, 25 ◦C): d = −0.46 (s, 9H,
CH3), 1.91 (s, 1H, OH), 4.59 (d, J = 2.9 Hz, 1H, CH), 5.26 (d, J =
2.8 Hz, 1H, CH), 5.92 (s, 1H, OH), 6.73–7.78 (m, 16H, CH). 13
C
NMR (75 MHz, CDCl3, Me4Si, 25 ◦C): d = −0.87 (3 CH3), 58.47
(1 CH), 74.03 (1 CH), 108.98 (1 quat C), 121.38 (1 CH), 124.18
(1 CH), 124.96 (1 CH), 125.49 (2 CH), 127.47 (1 CH), 127.53
(1 CH), 127.74 (1 CH), 127.94 (2 CH), 128.12 (1 CH), 128.17
(1 CH), 128.28 (2 CH), 129.34 (2 CH), 134.25 (1 quat C), 135.33
(1 quat C), 136.09 (1 quat C), 136.60 (1 quat C), 136.76 (1 quat
C), 137.14 (1 quat C), 141.29 (1 quat C), 141.52 (1 quat C), 145.52
(1 quat C), 146.74 (1 quat C), 151.46 (quat C). HRMS calcd. for
C33H29O2SiCl3: 590.1002, found 590.0992.
C), 144.57 (1 quat C), 145.14 (1 quat C), 146.68 (1 quat C),
−1
=
163.74 (1 CO); IR (neat): v (C O) = 1729 cm ; HRMS calcd.
for C36H35O2Si2Cl3: 660.1241, found 660.1220.
Synthesis of enol 6 by desilylation of 3a
Synthesis of cyclopentadienol 9 by reaction between dilithio-diene 8
and 2,4,6-trichlorophenyl carbonyl chloride
To a solution of enol 3a (1.0 mmol) in 1 : 1 THF–H2O (10 ml)◦at
◦
0 C was added TBAF·3H2O (1.0 mmol). After stirring at 0 C
To a solution of 2,3-diphenyl-1,4-diiodo-1,3-diene (1.0 mmol) in
for 0.5 h, water (5 ml) was added, and the mixture extracted with
Et2O. The extract was then washed with brine and dried over
MgSO4. The solvent was evaporated in vacuo and the residue
purified by column chromatography (silica gel, 4 : 1 hexane–
dichloromethane) to afford the enol 6.
◦
Et2O (10 ml) at −78 C was added t-BuLi (4.0 mmol, 1.5 M in
◦
pentane). After stirring at −78 C for 1 h, 2,4,6-trichlorophenyl
carbonyl chloride (1.2 mmol) was added. The reaction mixture
was then warmed up to room temperature for 0.5 h, quenched with
water and extracted with Et2O. The extract was then washed with
brine and dried over MgSO4. The solvent was evaporated in vacuo
and the residue purified by column chromatography (silica gel, 10 :
1 hexane–dichloromethane) to afford the product 9.
6. Colorless crystals, 85% yield (413 mg); mp 149.1–150.5 ◦C;
1H NMR (300 MHz, CDCl3, Me4Si, 25 ◦C): d = −0.31 (s, 9H,
CH3), 3.79–4.13 (m, 2H, CH2), 5.61 (s, 1H, OH), 7.23–7.92 (m,
11H, CH). 13C NMR (75 MHz, CDCl3, Me4Si, 25 ◦C): d = −0.54
(3 CH3), 41.02 (1 CH2), 109.82 (1 quat C), 121.30 (1 CH), 123.53
(1 CH), 125.43 (1 CH), 126.75 (1 CH), 127.61 (1 CH), 128.05
(2 CH), 128.22 (3 CH), 128.29 (1 CH), 134.03 (1 quat C), 134.98
(1 quat C), 135.53 (1 quat C), 136.48 (1 quat C), 136.72 (1 quat
C), 137.02 (1 quat C), 142.36 (1 quat C), 143.48 (1 quat C), 146.25
(1 quat C), 151.02 (1 quat C). HRMS calcd. for C26H23OSiCl3:
484.0584, found 484.0578. Anal. Calcd for C26H23OSiCl3: C,
64.27%; H, 4.77%. Found: C, 64.27%, H, 4.82%.
9. Colorless crystals, 57% yield (235 mg); mp 123.2–124.9 ◦C;
1H NMR (300 MHz, CDCl3, Me4Si, 25 ◦C): d = 3.43 (s, 1H, OH),
6.81 (s, 2H, CH), 7.15–7.38 (m, 12H, CH). 13C NMR (75 MHz,
CDCl3, Me4Si, 25 ◦C): d = 85.71 (1 quat C), 127.75 (2 CH), 127.93
(4 CH), 128.06 (4 CH), 130.48 (2 CH), 133.51 (1 quat C), 134.82
(3 quat C), 135.57 (2 quat C), 136.22 (2 CH), 144.02 (2 quat C).
HRMS calcd. for C23H15OCl3: 412.0189, found 412.0174. Anal.
Calcd for C23H15OCl3: C, 66.77%; H, 3.65%. Found: C, 66.77%,
H, 3.77%.
Synthesis of enol 7 by reaction between 3a and an aldehyde
To a solution of enol 3a (1.0 mmol) and an aldehyde (1.2 mmol) in
Synthesis of cyclopentadienol 12 by reaction between dilithio-diene
2a and 2,4,6-trimethylphenyl carbonyl chloride
THF (10 ml) at 0 ◦C was added TBAF (1.0 mmol, 1 M in THF).
◦
After stirrng at 0 C for 0.5 h, water (5 ml) was added, and the
mixture extracted with Et2O. The extract was then washed with
brine and dried over MgSO4. The solvent was evaporated in vacuo
and the residue purified by column chromatography (silica gel, 2 :
1 hexane–dichloromethane) to afford the enol 7.
To a solution of 1,4-bis(trimethylsilyl)-2,3-diphenyl-1,4-diiodo-
1,3-dienes 1a (1.0 mmol) in Et2O (10 ml) at −78 ◦C was added
t-BuLi (4.0 mmol, 1.5 M in pentane). After stirring at −78 ◦C
for 1 h, HMPA (2.2 mmol) and 2,4,6-trimethylphenyl carbonyl
chloride (1.2 mmol) were added. The reaction mixture was then
warmed up to room temperature for 1 h, quenched with water
and extracted with Et2O. The extract was then washed with brine
and dried over MgSO4. The solvent was evaporated in vacuo and
the residue purified by column chromatography (silica gel, 10 : 1
hexane–dichloromethane) to afford the product 12.
7a. Colorless crystals, 86% yield (480 mg); mp 122.3–123.1 ◦C;
1H NMR (300 MHz, CDCl3, Me4Si, 25 ◦C): d = −0.41 (s, 9H,
CH3), 0.93 (t, J = 7.3 Hz, 3H, CH3), 1.40–1.51 (m, 2H, CH2),
1.66–1.75 (m, 2H, CH2), 3.90 (t, J = 5.7 Hz, 1H, CH), 4.22 (d, J =
5.2 Hz, 1H, CH), 6.60 (s, 1H, OH), 7.20–7.70 (m, 11H, CH). 13
C
NMR (75 MHz, CDCl3, Me4Si, 25 ◦C): d = −0.82 (3 CH3), 14.01
(1 CH3), 19.69 (1 CH2), 38.05 (1 CH2), 56.86(1 CH), 71.99 (1 CH),
108.95 (1 quat C), 121.52 (1 CH), 124.00 (1 CH), 124.95 (1 CH),
127.43 (1 CH), 127.61 (1 CH), 128.10 (1 CH), 128.18 (2 CH),
128.21 (1 CH), 129.13 (2 CH), 134.50 (1 quat C), 135.26 (1 quat
C), 135.87 (1 quat C), 136.21 (1 quat C), 136.70 (1 quat C), 136.82
(1 quat C), 141.78 (1 quat C), 146.62 (1 quat C), 147.39 (1 quat
C), 151.68 (1 quat C). HRMS calcd. for C30H31O2SiCl3: 556.1159,
found 556.1184.
12. Colorless crystals, 47% yield (234 mg); mp 168.6–169.7 ◦C;
◦
1H NMR (300 MHz, CDCl3, Me4Si, 25 C): d = −0.33 (s, 18H,
CH3), 1.71 (s, 1H, OH), 2.25 (s, 3H, CH3), 2.47 (s, 3H, CH3), 2.80
(s, 3H, CH3) 6.75–7.25 (m, 12H, CH). 13C NMR (75 MHz, CDCl3,
Me4Si, 25 ◦C): d = −0.27 (6 CH3), 19.89 (1 CH3), 20.56 (1 CH3),
26.48 (1 CH3), 99.97 (1 quat C), 126.83 (2 CH), 127.33 (4 CH),
128.81 (4 CH), 130.75 (1 CH), 133.12 (1 CH), 133.70 (1 quat C),
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135.39 (1 quat C), 135.47 (1 quat C), 137.68 (2 quat C), 138.42
(1 quat C), 155.47 (2 quat C), 156.53 (2 quat C). HRMS calcd. for
C32H40OSi2: 496.2618, found 496.2609.
Mao, Z. Wang and Z. Xi, Eur. J. Org. Chem., 2007, 1267; (f) C. Wang,
Q. Luo, H. Sun, X. Guo and Z. Xi, J. Am. Chem. Soc., 2007, 129, 3094.
See also: (g) M. Saito, M. Nakamura, T. Tajima and M. Yoshioka,
Angew. Chem., Int. Ed., 2007, 46, 1504.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China and the Major State Basic Research De-
velopment Program (2006CB806105). The Cheung Kong Scholars
Programme, Qiu Shi Science & Technologies Foundation, the Dow
Corning Corporation and BASF are gratefully acknowledged.
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2118 | Org. Biomol. Chem., 2007, 5, 2114–2118
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