COMMUNICATION
A synthesis of multisubstituted vinylsilanes via ynolates: stereoselective
formation of b-silyl-b-lactones followed by decarboxylation{
Mitsuru Shindo,* Kenji Matsumoto and Kozo Shishido
Received (in Cambridge, UK) 7th December 2004, Accepted 10th March 2005
First published as an Advance Article on the web 29th March 2005
DOI: 10.1039/b418310j
(Z)-Selective synthesis of multisubstituted vinylsilanes was
achieved by stereoselective protonation or alkylation of
b-silyl-b-lactone enolates, prepared by cycloadditions of acylsi-
lanes with ynolates, followed by decarboxylation.
Vinylsilanes are important synthetic tools in organic chemistry.1
Although various methodologies for their preparation have been
reported, there have been few reports on a successful, widely useful
stereoselective olefination of acylsilanes.2 Recently, we reported a
stereoselective olefination of acylsilanes, via torquoselective
electrocyclic ring-opening of b-lactone enolates derived from
Scheme 2
ynolates, giving (Z)-b-trialkylsilyl-a,b-substituted acrylates, that
is, multisubstituted vinylsilanes [Scheme 1 (1)].3 Herein, we report a
new strategy for the stereochemically complementary olefination
of acylsilanes with ynolates via a stereoselective b-silyl-b-lactone
formation–decarboxylation sequence [Scheme 1 (2)].
the reactions of various combinations of acylsilanes 27 and
ynolates 1 to furnish the disubstituted vinylsilanes 4. The methyl,
ethyl, isopropyl, and phenyl-substituted ynolates afforded the
vinylsilanes in good yield with high Z-selectivities (entries 1–4).
In our previous communication, we reported the cycloaddition
of the acylsilane 2a with the ynolate 1a4 to furnish the b-lactone
enolate, which is ring-opened at room temperature to provide the
(Z)-b-silylacrylate without stereoisomers. When this reaction was
carried out at 278 uC, the corresponding b-lactone 3a was isolated
in 91% yield after protonation (Scheme 2). The diastereomeric
ratio of 3a was found to be very high and the minor isomer could
not be detected by 1H-NMR spectroscopy. After recrystallization,
3a underwent thermal decarboxylation5 under reflux in benzene in
the presence of silica gel, to provide the (Z)-vinylsilane 4a in 88%
yield without any detectable (E)-isomer.
1
Although the minor isomer could not be detected by H-NMR
spectroscopy at the b-lactone stage,8 a few percent of the minor
1
(E)-isomers were detected by H-NMR spectroscopy and HPLC
(entries 1–3). While the tert-butyl substituted ynolate did not give
the desired product (entry 5), presumably due to steric reasons, the
trimethylsilyl substituted ynolate afforded bis(trimethylsilyl)alkenes
in moderate yield with (E)-selectivity (entry 6). According to the
1H-NMR spectrum of the intermediate, the first step of the
cycloaddition should have proceeded cleanly, but gave instead an
almost 1:1 mixture of stereoisomers. At the decarboxylation step,
the route to the (Z)-isomer suffers from steric compression and
took place very little. Benzoylsilanes and functionalized acylsilanes
Encouraged by this excellent result, we next examined the
generality of this synthesis of vinylsilanes. Due to the instability of
some of the b-lactones 3 toward silica gel, decarboxylation was
carried out without purification of 3.6 Table 1 shows the results of
provided vinylsilanes in good yields with
Z selectivities
(entries 8–13). The acryloylsilane, however, did not give the
desired product but rather a complex mixture at the first stage
(entry 14). As for substituents on the silane, triethylsilyl, tert-
butyldimethylsilyl, and benzyldimethylsilyl groups could also be
used (entries 7, 9, 12, and 13). In the case of the benzyldimethylsilyl
group, the Z/E ratio decreased slightly (entry 12) compared with
that of the trimethylsilyl one (entry 11).
Instead of protonation, alkylation of the b-lactone enolates was
attempted. As shown in Scheme 3, methylation by MeI assisted
with HMPA, followed by decarboxylation, provided the trisub-
stituted vinylsilane 4o in good yield with excellent Z-selectivity.
The aldol reaction, followed by decarboxylation, was also
performed with benzaldehyde to afford the desired vinylsilane 4p
with good E-selectivity. In both cases, the electrophiles were
introduced trans to the silyl group.
Scheme 1
{ Electronic supplementary information (ESI) available: representative
procedures and spectral data for compounds 2 and 4. See http://
The E/Z selectivity is determined in the protonation (or
alkylation) of the b-lactone enolates, because the decarboxylation
*shindo@ph.tokushima-u.ac.jp
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 2477–2479 | 2477
Shindo, Mitsuru
