Highly diastereoselective intramolecular Diels-Alder reaction of chiral silatrienes

Article abstract of DOI:10.1016/S0040-4020(03)00261-8

New Si-chiral 2-silahexa-3,5-dienyl acrylates were prepared in six steps from dichloro(chloromethyl)methylsilane. The EtAlCl₂ catalysed intramolecular Diels-Alder reaction of these compounds gave chiral 4-sila-4a,7,8,8a-tetrahydroisochroman-1-ones in good yield. Very good diastereoselectivity was observed for a silatriene bearing a methyl and a 2-methoxyphenyl substituent on the chiral silicon atom.

Full text of DOI:10.1016/S0040-4020(03)00261-8

Tetrahedron 59 (2003) 2451–2456
Highly diastereoselective intramolecular Diels–Alder reaction of
chiral silatrienes
*
Paulo J. Coelho and Luis Blanco
´ ´ ˆ ´
Laboratoire des Carbocyles (Associe au CNRS), Institut de Chimie Moleculaire d’Orsay, Bat 420, Universite de Paris-Sud,
91405 Orsay France
Received 7 October 2002; revised 14 January 2003; accepted 18 February 2003
Abstract—New Si-chiral 2-silahexa-3,5-dienyl acrylates were prepared in six steps from dichloro(chloromethyl)methylsilane. The EtAlCl2
catalysed intramolecular Diels–Alder reaction of these compounds gave chiral 4-sila-4a,7,8,8a-tetrahydroisochroman-1-ones in good yield.
Very good diastereoselectivity was observed for a silatriene bearing a methyl and a 2-methoxyphenyl substituent on the chiral silicon
atom. q 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
and in some cases a good diastereoselective induction was
noticed.16 The selectivity of this reaction seems to be very
dependent on the steric effect of the silicon substituents.
Thermal or Lewis acid-catalyzed IMDA reaction of
2-hydroxymethyl-2-methyl-2-silahexa-3,5-dienyl acrylate
and the hydroxy protected derivatives have allowed the
preparation of 4a,7,8,8a-tetrahydro-4-silaisochroman-1-
ones,16b but the better diastereoselective reaction, obtained
with the bulky triisopropylsilyl group as protecting group in
the presence of EtAlCl2, remained unsatisfactory, the four
possible isomers were present in the ratio 20/6/71/3. In this
work we describe the IMDA reaction of chiral silatrienes
bearing rather hindering substituents such as phenyl,
cyclohexyl and 2-methoxyphenyl on the silicon atom.
Asymmetric synthesis using compounds stereogenic at a
carbon atom is very common. Since 1960s, several authors
have reported studies on the use of silane derivatives
bearing a stereogenic center on silicon as chiral inducers,
despite the longer bonds to silicon than to a carbon atom.1
Epoxidation of vinylsilanes,2,3 reduction of ketones with
mono-hydrogenosilanes4,5 and radical monohydrogenolysis
of a 1,1-dichloroethylsilane6,7 were slightly diastereo-
selective (de#20%). Low diastereoselectivities (de#35%)
were also noticed for Lewis acid-catalyzed reactions of an
acetal with a silyl enol ether5 or an allylic silane.8
A
thioacylsilane underwent rather good diastereselective
nucleophilic additions on sulfur using organolithium
reagents9 (de#76%) and a fairly good [2þ4]-cycloaddition
with buta-1,3-diene10 (de<50%). Noticeable diastereo-
selectivities (de#76%) were also reported for the addition
of electrophiles to anions adjacent to a chiral silyl group.11,12
Since a long time, there has been some success with
nucleophilic addition of Grignard reagents or organolithium
derivatives on chiral acylsilane (de#62%).13,14 Recently an
increase of diastereoselectivity (de¼97%) was obtained at
21008C, using a silane bearing a benzyloxymethyl
substituent.15 This highly selective reaction occurs probably
by chelation of the organometallic reagent with a ‘quasi-
intramolecular’ attack of the carbonyl function.
2. Results and discussion
2.1. Synthesis
The chiral silatrienes 5a–c were prepared in six steps from
commercially available dichloro(chloromethyl)methyl-
silane 1 as shown in Scheme 1. The chlorosilanes 2a,b
were obtained by reaction of (chloromethyl)dichlorosilane 1
with the Grignard reagents derived from bromobenzene and
bromocyclohexane.17 Chlorosilane 2c was obtained by
treating anisole with n-BuLi followed by reaction with
dichlorosilane 1.18 These chlorosilanes 2a–c were isolated
by distillation and used immediately after isolation. The
CuCN-catalyzed reaction of buta-1,3-dienylmagnesium
chloride with chloro(chloromethyl)silanes 2a–c gave
mixtures of E and Z silahexadienes 3 ((E/Z) from 1/3 to
1/1) which were equilibrated by treatment with iodine in a
benzenic solution. The ratio of E to Z silahexadienes 3
became better then 91/9. Replacement of the chlorine atom
In the last years we have studied the intramolecular Diels–
Alder (IMDA) reaction of several chiral silicon compounds
Keywords: intramolecular Diels–Alder reactions; stereoselectivity; chiral
silatrienes; silicon; 4-silatetrahydroisochroman-1-ones.
*
´
Corresponding author. Address: Departamento de Quımica, Universidade
de Tras-os-Montes e Alto Douro, 5001-911 Vila Real, Portugal. Fax:
þ351-259-350480; e-mail: pcoelho@marao.utad.pt
´
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)00261-8

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P. J. Coelho, L. Blanco / Tetrahedron 59 (2003) 2451–2456
Scheme 1. (i) For 2a, PhMgBr, Et2O, reflux; for 2b, c-C6H11MgBr, CuCN (5%), Et2O, reflux; for 2c, MeOPh, n-BuLi, Et2O, reflux, (ii) CH2vCHCHvCH2-
MgCl, CuCN (5%), THF, room temperature, (iii) I2, reflux Benzene, (iv) KOAc, TiO2, n-Bu4NBr, (v) LiAlH4 then H2O, (vi) with 4a and 4b, DEAD, acrylic
acid, PPh3; with 4c, acryloyl chloride, N,N-dimethylaniline.
of 3a–c by an acetoxy group, in the presence of the phase
transfer agent n-Bu4NBr, gave the corresponding acetoxy-
methylsilanes which were reduced with LiAlH4 to deliver
the hydroxymethylsilanes 4a–c and these were converted
into the corresponding acrylic esters 5a–c. While the
Mitsunobu method (acrylic acid, DEAD, PPh3) gave poor
yields of the acrylic esters 5a,b, the treatment of alcohol 4c
with acryloyl chloride in the presence of N,N-dimethyl-
aniline gave the ester 5c in quantitative yield.
ratios observed with compounds 6a–c were generally
higher than those obtained previously and the best result
was obtained with silatriene 5c bearing the 2-methoxy-
phenyl group; 90% of the silaisochromanones mixture 6c
was one isomer. Treatment of silatriene 5a with 4 equiv. of
EtAlCl2, at 2208C gave similar results (47% yield, diast.
ratios: 4/16/76/4) than those obtained at room temperature.
Heating of 0.02 M toluene solution of silatriene 5c at 1708C
for 20 h gave the 4-silatetrahydroisochromanone 6c with
72% yield, very similar to the one obtained with EtAlCl2,
but the reaction was, as expected, less selective (diast.
ratios: 11/20/53/16).
2.2. Intramolecular Diels–Alder reaction
Treatment of silatrienes 5a–c at room temperature in
CH2Cl2 with 4 equiv. of EtAlCl2 gave the 4-methyl-
4a,7,8,8a-tetrahydro-4-silaisochromanones 6a–c, which
were isolated in medium to good yields (50–74%) as
mixtures of 3 or 4 diastereomers. The diastereomeric ratios
were determined by 1H NMR spectroscopy through
integration of the CH3Si singlets (see Table 1, Scheme 2).
1H NMR spectra of the 4-silatetrahydroisochroman-1-ones
6a–c show signals around 3.00–3.15 ppm attributed to the
proton H8a of the major isomer. The coupling constants
JH8a–H4a are between 4 and 7 Hz, which are consistent with
a cis junction of cycles. The major isomers are thus formed
via endo transition states (Scheme 3).
The reactions times of these reactions (1–2 h) were
noticeably lower than those necessary, in the same
conditions, to transform the similar silatrienes possessing
CH3 and CH2OR groups (24–48 h).16b The diastereomeric
Generally the mixtures of diastereomeric 4-sila-4a,7,8,8a-
tetrahydroisochroman-1-ones could not be separated by the
usual silica gel column chromatography. For the 2-phenyl-
silaisochromanone 6a it was possible to isolate by this
technique a fraction containing 92% of the major isomer and
a mixture of the three minor isomers. 1H NMR spectra of the
silatetrahydroisochromanones bearing on the silicon atom a
cyclohexyl group, 6b, or a protected or unprotected
hydroxymethyl group16b show all the olefinic proton signals
between 5.8 and 5.6 ppm. The olefinic protons of the major
isomer of the phenylated and 2-methoxyphenylated com-
pounds, 6a and 6c, appear also between 5.8 and 5.6 ppm. In
Table 1. EtAlCl2-catalysed IMDA reaction of silatrienes 5a–c
Silatriene 5a–c
4-Silatetrahydroisochromanone 6a–c
Time (h)
Yield
Diast. ratio
a R¼Ph
2
1
1.5
50
68
74
3/15/79/3
21/74/5
4/5/90/1
b R¼c-C6H11
c R¼2-MeOPh
1
the H NMR spectrum of the mixture of the three minor
isomers of 6a (isolated by silica gel chromatography) a
Scheme 3.
Scheme 2.

P. J. Coelho, L. Blanco / Tetrahedron 59 (2003) 2451–2456
2453
NERMAG R10-10 spectrometer using electron ionisation
at 70 eV. IR spectra of neat products were registered on a
Perkin–Elmer 682 spectrometer (wave numbers in cm21).
The products were isolated by column chromatography on
silica gel (SDS 70–230 Mesh). TLC was performed on
0.25 mm thick silica gel plates (Merck 60 F254). Dichloro-
(chloromethyl)methylsilane was obtained from commercial
source (ABCR).
4.2. Synthesis of chlorosilanes (2a,b)
Scheme 4.
Chloro(chloromethyl)methylphenylsilane 2a was prepared
through reaction of the dichloro(chloromethyl)methylsilane
with a solution of PhMgBr in diethyl ether.17 Chloro-
(chloromethyl)cyclohexylmethyllsilane 2b was prepared
through reaction of dichloro(chloromethyl)methylsilane
with a solution of PhMgBr in diethyl ether in the presence
of CuCN. Both chlorosilanes were isolated through distilla-
tion and used immediately.
signal at 3.13 ppm (ddd, J¼4, 7, 7 Hz, H8a) and a multiplet
between 5.35 and 5.25 ppm were attributed to one isomer
(for the two other isomers the olefinic protons were at lower
field than 5.6 ppm). The coupling constants with the proton
H8a are lower or equal to 7 Hz so this isomer should be also
with a cis junction of cycles. The presence of higher field
olefinic proton for this isomer could be attributed to the
shielding effect of the aromatic ring. It appears after
examination of the Dreiding models of the various
conformers of the cis-silatetrahydroisochromanone that
one olefinic proton could be shielded when a phenyl
group is in the trans relationship with the protons H8a and
H4a. So, we suggest for the major isomer of 6a, a cis
relationship between the phenyl group and the proton H8a
and H4a, it should be formed via a transition state where,
related to the plane of the dienic part, the bulky aryl group is
on the opposite side to that of the dienophile (Scheme 4).
4.2.1. Chloro(chloromethyl)(2-methoxyphenyl)methyl-
silane (2c). Yield: 53%. 26.7 mL of a solution of n-BuLi
(1.5 M, 40.0 mmol) in hexane were added dropwise to a
solution of anisole (4.30 mL, 40.0 mmol) in 50 mL of dry
diethyl ether under argon atmosphere. The solution was
heated under reflux for 36 h. After return to room
temperature dichloro(chloromethyl)methylsilane (4.50 mL,
27.6 mmol) was added and the mixture refluxed for another
4 h. The suspension was then filtered and the filtrate distilled
under reduced pressure (0.01 mm Hg) giving 3.45 g of
chlorosilane 2c (bp 90–1008C). 1H NMR: 0.79 (s, 3H,
SiCH3), 3.66 (s, 3H, MeO), 3.15 (d, J¼13.9 Hz), 3.30 (d,
J¼13.8 Hz) (AB system, 2H), 6.89 (d, J¼8.3 Hz, 1H), 7.06
(t, J¼7.3 Hz, 1H), 7.4–7.6 (m, 1H), 7.64 (dd, J¼1.8, 7.3 Hz,
1H).
The same suggestion concerning the major diastereomer of
6c is made, because its olefinic protons are between 5.6 and
5.8 ppm and a signal between 5.25 and 5.35 ppm is clearly
present in the 1H NMR spectrum of the mixture of
silaisochromanones isolated after the less selective thermal
Diels–Alder reaction of 5c. These results raise interesting
mechanistic questions concerning a steric crowding or a
chelate formation when a 2-methoxyphenyl group is present
on the silicon atom. Further studies along these lines are in
progress.
4.3. Synthesis of (E)-(buta-1,3-dienyl)methylsilanes
(3a–c): general procedure
A solution of buta-1,3-dien-1-ylmagnesium chloride in THF
was prepared as follows: magnesium (5.00, 206 mmol) and
zinc chloride (1.50, 11.0 mmol) were placed in a reaction
vessel, under an argon atmosphere and heated for 2–3 min
with a flame. After return to room temperature, THF
(7.0 mL) and dibromoethane (0.300 mL) were added. After
ethene evolution, the mixture was diluted with THF (15 mL)
and heated to 45–508C. A solution of 1-chlorobuta-1,3-
diene (Z/E¼94/6) (10 mL, 100 mmol) and 1,2-dibromo-
ethane (0.80 mL) dissolved in THF (70 mL) were then
added dropwise for 1.5 h so as to maintain a gentle reflux.
The reaction was heated under reflux for another 3 h and
then left overnight at room temperature without stirring.
Settlement of the resulting suspension leave 0.55 M orange
solution of butadienylmagnesium chloride.
3. Conclusion
New 4-silatetrahydroisochroman-1-ones were prepared in
good yields through EtAlCl2 catalysed IMDA reactions of
chiral silatrienes. Very good diastereoselectivity was
observed for a silatriene bearing a methyl and a 2-methoxy-
phenyl substituent on the chiral silicon atom. The reaction
proceeds, probably, via an endo transition state where,
related to the plane of the dienic part, the silicon-bulky
substituent is on the opposite side to that of the dienophile.
4. Experimental
4.1. Apparatus
33.6 mmol of chlorosilane 2a–c were added dropwise to a
stirred mixture of CuCN (200 mg, 2.40 mmol) in 80 mL of a
THF buta-1,3-dien-1-ylmagnesium chloride solution
(44 mmol) under an argon atmosphere at 08C. The reaction
mixture was stirred overnight, quenched with aqueous
NH4Cl and extracted with n-hexane. The combined organic
extracts were dried over anhydrous Na2SO4. After solvent
evaporation, the crude product was purified by distillation
1H and 13C spectra were recorded on Bruker AC200 (200
and 50.3 MHz, respectively) or Bruker AC250 (250 and
62.9 MHz, respectively) spectrometers. All spectra were
recorded from samples in CDCl3 solution. Chemical shifts
are given in ppm. Mass spectra were obtained on a

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P. J. Coelho, L. Blanco / Tetrahedron 59 (2003) 2451–2456
under reduced pressure (0.05 mm Hg) to yield butadienyl-
silanes 3a–c as a mixture of E and Z isomers.
in diethyl ether were added dropwise to a stirred solution of
the acetoxymethylsilane in diethyl ether at 2208C. The
suspension was stirred for 30 min at room temperature then
2–3 g of hydrated Na2SO4 were added very slowly (violent
reaction) followed by 10 mL of water. After decantation, the
aqueous phase was extracted with diethyl ether (4£50 mL).
The combined organic extracts were dried over Na2SO4 and
the solvent removed under reduced pressure, leaving the
pure hydroxymethyl)silane 4a–c.
Isomerisation reaction. A solution of butadienylsilane 3a–c
(26.5 mmol) and iodine (0.340 g, 1.34 mmol, 5%) in
benzene (100 mL) was heated under reflux for 3.5 h under
an argon atmosphere. The reaction mixture was then
quenched with aqueous Na2S2O3, extracted with n-hexane
and the combined organic extracts dried over anhydrous
Na2SO4. After solvent evaporation, the crude chloromethyl-
silane 3 was purified by column chromatography (hexane/
diethyl ether).
4.4.1. (E)-2-Methyl-2-phenyl-2-silahexa-3,5-dien-1-ol
1
(4a). Yield: 85%. IR: 3350 (OH), 1580, 1250 (SiCH3). H
NMR: 0.46 (s, 3H, SiCH3), 3.70 (m, 3H, CH2OH and OH),
5.15–5.35 (m, 2H), 6.02 (d, J¼18.2 Hz, 1H), 6.43 (ddd,
J¼9.9, 9.9, 16.9 Hz, 1H), 6.72 (dd, J¼18.2, 9.8 Hz, 1H),
7.3–7.5 (m, 3H, Ph), 7.50–7.65 (m, 2H, Ph). 13C NMR:
26.5 (SiCH3), 53.9 (CH2OH), 118.9, 127.8, 128.2, 129.4,
134.2, 135.1, 139.3, 148.0. MS: m/z (%) 189 (0.6), 173 (54),
137 (54), 121 (100), 105 (21).
4.3.1. (E)-(Buta-1,3-dien-1-yl)(chloromethyl)methyl-
phenylsilane (3a). E/Z¼91/9. Yield: 47%. IR: 1630,
1
1580, 1260 (SiCH3). H NMR: 0.53 (s, 3H, SiCH3), 3.06
(s, 2H, CH2Cl), 5.23 (d, J¼9.9 Hz, 1H), 5.31 (d, J¼17.1 Hz,
1H), 6.03 (d, J¼18.5 Hz, 1H), 6.42 (ddd, J¼9.9, 9.9,
16.9 Hz, 1H), 6.69 (dd, J¼18.5, 9.9 Hz, 1H), 7.3–7.5 (m,
3H, Ph), 7.5–7.6 (m, 2H, Ph). 13C NMR: 26.0 (SiCH3).
29.0 (CH2Cl), 119.4, 127.2, 127.9, 129.8, 134.2, 134.3,
139.2, 148.5. MS: m/z (%) 209 (2.2), 207 (3.8), 173 (71),
145 (22), 121 (100), 105 (35), 95 (27), 93 (20), 91 (32), 63
(31), 43 (29).
4.4.2. (E)-2-Cyclohexyl-2-methyl-2-silahexa-3,5-dien-1-
ol (4b). Yield: 91%. IR: 3360 (OH), 1580, 1250 (SiCH3).
1H NMR: 0.12 (s, 3H, SiCH3), 0.75–1.0 (m, 2H), 1.0–1.4
(m, 5H), 1.6–1.8 (m, 5H), 3.51 (s, 2H, CH2OH), 5.17 (d,
J¼9.4 Hz, 1H), 5.27 (d, J¼16.2 Hz, 1H), 5.84 (d,
J¼18.4 Hz, 1H), 6.38 (ddd, J¼10.1, 10.1, 16.7 Hz, 1H),
6.64 (dd, J¼18.6, 10.2 Hz, 1H). 13C NMR: 28.9 (SiCH3),
23.4, 26.7, 27.3, 27.4, 27.8, 52.8 (CH2OH), 118.1, 128.8,
139.6, 147.2. MS: m/z (%) 180 (0.5), 179 (3.2), 143 (20), 97
(100), 61 (28).
4.3.2. (E)-(Buta-1,3-dien-1-yl)(chloromethyl)cyclohexyl-
methylsilane (3b). E/Z¼96/4. Yield: 38%. IR: 1580, 1500,
1
1255 (SiCH3). H NMR: 0.18 (s, 3H, SiCH3), 0.8–1.0 (m,
1H), 1.0–1.4 (m, 5H), 1.6–1.8 (m, 5H), 2.87 (s, 2H,
CH2Cl), 5.19 (d, J¼9.5 Hz, 1H), 5.28 (d, J¼16.7 Hz, 1H),
5.82 (d, J¼18.4 Hz, 1H), 6.38 (ddd, J¼9.9, 9.9, 16.7 Hz,
1H), 6.62 (dd, J¼18.2, 9.9 Hz, 1H). MS: m/z (%) 228 (0.3,
Mþ), 179 (7), 109 (23), 97 (100), 81 (20), 79 (34).
4.4.3. (E)-2-(2-Methoxyphenyl)-2-methyl-2-silahexa-3,5-
dien-1-ol (4c). Yield: 73%. IR: 3400 (OH), 1590, l240
(SiCH3). 1H NMR: 0.46 (s, 3H, SiCH3), 1.91 (m, OH, 1H),
3.70 (s, 2H, CH2OH), 3.86 (s, 3H, CH3O), 5.20 (d,
J¼9.8 Hz, 1H), 5.29 (d, J¼16.4 Hz, 1H), 6.09 (d,
J¼18.4 Hz, 1H), 6.45 (ddd, J¼10.0, 10.0, 16.3 Hz, 1H),
6.72 (dd, J¼10.1, 18.3 Hz, 1H), 6.90 (d, J¼8.3 Hz, 1H),
7.02 (t, J¼7.3 Hz, 2H), 7.35–7.45 (m, 2H, Ph). 13C NMR:
26.4 (SiCH3), 54.3 (CH2OH), 55.0, 109.6, 118.3, 120.8,
123.4, 128.8, 131.2, 135.9, 139.5, 147.2, 163.7. MS: m/z (%)
220 (2), 219 (6.9), 203 (82), 173 (57), 151 (47), 145 (44),
129 (25), 128 (30), 121 (100), 105 (30), 91 (44), 75 (84), 59
(65), 45 (39).
4.3.3. (E)-(Buta-1,3-dien-1-yl)(chloromethyl)(2-methoxy-
phenyl)methylsilane (3c). E/Z¼92/8. Yield: 72%. IR:
1
1590, 1570, 1240 (SiCH3). H NMR: 0.52 (s, 3H, SiCH3),
3.17 (s, 2H, CH2Cl), 3.83 (s, 3H, CH3O), 5.20 (dd, J¼9.7,
1.0 Hz, 1H), 5.29 (dd, J¼16.6, 1.0 Hz, 1H), 6.44 (ddd,
J¼9.9, 9.9, 16.5 Hz, 1H), 6.69 (dd, J¼10.1, 18.5 Hz, 1H),
6.86 (d, J¼8.5 Hz, 1H), 6.99 (t, J¼7.3 Hz, 1H), 7.35–7.45
(m, 2H). 13C NMR: 26.2 (SiCH3), 29.5 (CH2Cl), 55.0
(OCH3), 109.6, 118.8, 120.7, 122.4, 128.3, 132.7, 135.9,
139.5, 147.6, 164.0. MS: m/z (%) 254 (2.7), 252 (4.5, Mþ),
239 (8), 237 (19), 203 (60), 201 (20), 173 (36), 151 (40), 145
(41), 129 (35), 121 (85), 105 (37), 91 (42), 75 (100), 59 (46).
4.5. Synthesis of acrylates (5a,b): general procedure
4.4. Synthesis of hydroxymethylsilanes (4a–c): general
procedure
Diethyl azodicarboxylate (105 mL, 0.660 mmol) (DEAD)
was added dropwise to a stirred solution of (hydroxy-
methyl)silane 4a,b (0.439 mmol), triphenylphosphine
(173 mg, 0.660 mmol) and acrylic acid (38 mg,
0.527 mmol) in 20 mL THF, at 2788C, under an argon
atmosphere. The reaction mixture was then stirred for
20 min at room temperature. After solvent evaporation, the
crude acrylate 5 was purified by chromatography
(n-hexane/Et2O 80/20).
A mixture of chloromethylsilane 3a–c (17.4 mmol),
potassium acetate (4.0 g, 41 mmol), tetrabutylammonium
bromide (140 mg, 0.530 mmol) and titanium oxide
(680 mg, 8.51 mmol) was stirred under an argon atmo-
sphere, at 80–908C, for 3 h. After return to room
temperature, 10 g of silica gel and 10 mL of diethyl ether
were added. The suspension was stirred for 5 min, filtered
and the residue washed 10 times with diethyl ether. After
solvent evaporation, the crude product was purified by
column chromatography (n-hexane/Et2O) to yield the
corresponding acetoxymethylsilanes.
4.5.1. (E)-2-Methyl-2-phenyl-2-silahexa-3,5-dien-1-yl
acrylate (5a). Yield: 34%. IR: 1735 (CvO), 1575, 1260
(SiCH3). 1H NMR: 0.47 (s, 3H, SiCH3), 4.16 (s, 2H), 5.20–
5.35 (m, 2H), 5.80 (dd, J¼10.3, 1.8 Hz, 1H), 6.00 (d,
J¼18.2 Hz, 1H), 6.13 (dd, J¼10.3, 17.3 Hz, 1H), 6.3–6.5
(m, 2H), 6.67 (dd, J¼18.3, 9.9 Hz, 1H), 7.3–7.5 (m, 3H),
In the next step, 37 mL of a 1 M LiAlH4 solution (37 mmol)

P. J. Coelho, L. Blanco / Tetrahedron 59 (2003) 2451–2456
2455
7.5–7.6 (m, 2H). 13C NMR: 25.8 (SiCH3), 55.8 (CH2O),
119.4, 127.5, 128.0, 128.4, 129.7, 130.4, 134.3, 134.5,
139.3, 148.4, 166.9 (CvO). MS: m/z (%) 258 (0.9, Mþ),
243 (23), 205 (67), 181 (40), 173 (39), 145 (31), 137 (25),
128 (100), 121 (97), 105 (44), 95 (26), 91 (20), 55 (81), 53
(20), 43 (34).
23.9, 38.9, 60.3, 123.6, 126.1, 128.4, 130.5, 133.4, 134.5,
175.6 (CvO). MS: m/z (%) 259 (15.1), 258 (67, Mþ), 257
(10), 243 (5), 151 (29), 149 (26), 137 (47), 136 (36), 135
(72), 134 (69), 133 (35), 123 (50), 122 (21), 121 (100), 120
(44), 105 (87), 107 (30), 106 (26), 93 (25), 92 (23), 91 (60),
80 (61), 79 (96), 77 (55), 53 (33), 43 (38).
4.5.2. (E)-2-Cyclohexyl-2-methyl-2-silahexa-3,5-dien-1-
yl acrylate (5b). Yield: 40%. IR: 1740 (CvO), 1535,
1260 (SiCH3). H NMR: 0.13 (s, 3H, SiCH3), 0.7–1.0 (m,
4.6.2. 4-Cyclohexyl-4-methyl-4-sila-4a,7,8,8a-tetra-
hydroisochromanon-1-one (6b). Yield: 68%. IR: 1735
(CvO), 1650, 1255 (SiCH3). 1H NMR: 0.05, 0.10, 0.17 (s,
3H, SiCH3), 0.8–1.0 (m, 1H), 1.1–1.4 (m, 5H), 1.6–1.9 (m,
6H), 2.0–2.3 (m, 3H), 2.3–2.5 (m, 1H), 3.00 (ddd, J¼3, 7,
7 Hz, 1H), 3.90 (d, J¼14.7 Hz) and 4.19 (d, J¼14.7 Hz)
[AB system, minor isomer] and 4.00 (d, J¼15.0 Hz) and
4.09 (d, J¼15.0 Hz) [AB system, major isomer] (2H), 5.45–
5.75 (m, 2H). 13C NMR: 210.4, 29.0, 26.6, 20.3, 22.3,
22.4, 22.5, 22.9, 23.7, 23.8, 24.1, 24.3, 26.5, 26.6, 26.9,
26.98, 27.02, 27.4, 27.5, 27.7, 28.0, 38.8, 38.9, 39.4, 59.0,
59.2, 59.4, 123.8, 124.6, 125.3, 125.4, 126.4, 176.0 (CvO).
MS: m/z (%) 266 (3.9), 265 (9.7), 264 (14.9, Mþ), 181 (11),
137 (14), 99 (20), 81 (25), 80 (100), 79 (45), 77 (27), 75
(63), 61 (31), 59 (72), 58 (21), 55 (24), 45 (28), 43 (33).
1
1H), 1.1–1.35 (m, 5H), 1.6–1.8 (m, 5H), 3.97 (s, 2H), 5.18
(d, J¼9.8 Hz, 1H), 5.27 (d, J¼18.4 Hz, 1H), 5.81 (dd,
J¼10.3, 1.5 Hz, 1H), 5.81 (d, J¼18.7 Hz, 1H), 6.14 (dd,
J¼10.3, 17.5 Hz, 1H), 6.25–6.45 (m, 2H), 6.60 (dd,
J¼10.3, 18.6 Hz, 1H). 13C NMR: 28.3 (SiCH3), 23.8,
26.7, 27.2, 27.3, 27.9, 54.8 (CH2O), 118.5, 128.0, 128.5,
130.2, 139.5, 147.3, 167.0 (CvO). MS: m/z (%) 250 (0.5),
249 (1.4), 211 (4.7), 181 (100), 109 (38), 97 (27), 95 (19), 55
(52).
4.5.3. (E)-2-(2-Methoxyphenyl)-2-methyl-2-silahexa-3,5-
dien-1-yl acrylate (5c). Acryloyl chloride (0.200 mL,
2.4 mmol) was added dropwise to a stirred solution of
hydroxymethylsilane 4c (190 mg, 0.812 mmol) and N,N-
dimethylaniline (0.200 mL) in CH2C12 (25 mL) at 08C
under argon atmosphere. After stirring for 30 min at room
temperature the solution was quenched with HCl 1 M
(10 mL) and the resulting mixture was extracted with
CH2C12. The combined organic extracts were dried over
anhydrous Na2SO4, concentrated and the product was
purified by column chromatography (n-hexane/ether
80/20) to give 223 mg of acrylate 5c (95% yield). IR:
4.6.3. 4-Methoxyphenyl-4-methyl-4-sila-4a,7,8,8a-tetra-
hydroisochromanon-1-one (6c). Yield: 74%. 1H NMR:
0.34, 0.40, 0.47 and 0.54 (4 s, 3H, SiCH3), 1.7–1.9 (m, 1H),
2.0–2.3 (m, 2H), 2.4–2.6 (m, 2H), 3.12 (ddd, J¼4, 7, 6 Hz,
1H), 4.82 (s, 3H), 5.23 (d, J¼14.7 Hz) and 5.42 (d,
J¼14.7 Hz) [AB system, 2H], 5.6–5.8 (m, 2H), 6.90 (d,
J¼7.5 Hz, 1H), 7.02 (t, J¼7.5 Hz, 1H), 7.3–7.5 (m, 2H).
13C NMR: 27.0 (SiCH3), 21.0, 22.3, 24.0, 38.5, 55.2, 60.1,
109.7, 120.9, 121.8, 124.5, 125.6, 132.3, 135.1, 164.1, 176.1
(CvO). MS: m/z (%) 289 (11), 288 (33, Mþ), 287 (7), 273
(2), 149 (100), 135 (29), 119 (31), 105 (41), 79 (48), 77 (23),
59 (28).
1
1730 (CvO), 1590, 1240 (SiCH3). H NMR: 0.47 (s, 3H),
3.82 (s, 3H), 4.22 (s, 2H), 5.15–5.30 (m, 2H), 5.77 (dd,
J¼1.9, 10.2 Hz, 1H), 6.05 (d, J¼18.4 Hz, 1H), 6.11 (dd,
J¼10.2, 17.2 Hz, 1H), 6.3–6.5 (m, 2H), 6.67 (dd, J¼10.1,
18.4 Hz, 1H), 6.85 (d, J¼8.5 Hz, 1H), 6.98 (t, J¼7.2 Hz,
1H), 7.35–7.45 (m, 2H). 13C NMR: 26.0 (SiCH3), 55.0,
56.1, 109.5, 118.5, 120.6, 122.6, 126.5, 128.6, 130.0, 131.6,
135.8, 139.6, 147.3, 164.2, 167.0 (CvO). MS: m/z (%) 289
(0.6), 288 (1.3, Mþ), 273 (29), 236 (24), 235 (86), 203 (21),
181 (40), 173 (40), 151 (31), 145 (32), 128 (100), 121 (84),
105 (36), 91 (33), 77 (30), 75 (72), 59 (41), 55 (67).
Acknowledgements
We thank the Portuguese Junta Nacional para a Investigac¸a˜o
´ ´
Cientıfica e Tecnologica (JNICT) for awarding a PhD grant
to P. J. Coelho.
4.6. Lewis acid intramolecular Diels–Alder catalysed
reaction: general procedure
References
A 1 M EtAlCl2 solution in CH2Cl2 (4.0 mL, 4.0 mmol) was
added to a solution of silatriene 5a–c (1.0 mmol) in CH2Cl2
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silatriene (TLC), the reaction mixture was quenched with
saturated aqueous NH4Cl and the resulting mixture was
extracted with CH2Cl2. The combined organic extracts were
dried over anhydrous Na2SO4, concentrated, and the crude
silaisochromanone 6 was purified by silica gel column
chromatography (n-hexane/Et2O).
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