Tandem Mukaiyama Michael-aldol reactions catalysed by samarium diiodide

Article abstract of DOI:10.1016/S0040-4020(01)00902-4

Samarium diiodide is an efficient precatalyst for tandem Michael-aldol reactions, which allow the formation of two carbon-carbon bonds by successive additions of a ketene silyl acetal and an aldehyde on cyclic α,β-unsaturated ketones. The adducts are isolated as silyl ethers, in good yields, and in some cases with high diastereoselectivities when the reactions are performed at low temperatures. Comparative study of the activities of other lanthanide iodides for the same tandem reactions is presented. A key step for a formal synthesis of PGF_2α has been performed by a tandem Michael-aldol samarium-catalysed sequence.

Full text of DOI:10.1016/S0040-4020(01)00902-4

TETRAHEDRON
Pergamon
Tetrahedron 57 ?2001) 8989±8998
Tandem Mukaiyama Michael±aldol reactions catalysed
by samarium diiodide
Nicolas Giuseppone and Jacqueline Collinp
  Â
Laboratoire de Catalyse Moleculaire, ESA 8075, Institut de Chimie Moleculaire d'Orsay, Universite Paris-sud, Bat. 420,
91405 Orsay Cedex, France
Dedicated to Professor H. B. Kagan on the occasion of his 70th birthday
Received 22 June 2001; revised 24 August 2001; accepted 5 September 2001
AbstractÐSamarium diiodide is an ef®cient precatalyst for tandem Michael±aldol reactions, which allow the formation of two carbon±
carbon bonds by successive additions of a ketene silyl acetal and an aldehyde on cyclic a,b-unsaturated ketones. The adducts are isolated as
silyl ethers, in good yields, and in some cases with high diastereoselectivities when the reactions are performed at low temperatures.
Comparative study of the activities of other lanthanide iodides for the same tandem reactions is presented. A key step for a formal synthesis
of PGF2a has been performed by a tandem Michael±aldol samarium-catalysed sequence. q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
However, the successive formation of two carbon±carbon
bonds by Lewis acids catalysed reactions is poorly docu-
mented.4g
The realization of successive reactions leading to the for-
mation of several bonds by one-pot procedures affords
useful methodologies for the preparation of complex
molecules. The minimization of the number of steps not
only provides more ef®cient syntheses but also allows the
reduction of waste production and the diminution of costs.
The so-called domino, cascade or tandem reactions have
been the focus of several reviews.1 Amongst the large
panel of transformations described up to now, a major part
concerns radical or anionic processes and/or intramolecular
reactions leading to the formation of one or several cycles.2
The use of metal catalysts in tandem reactions has been
recently investigated, although the possibility to control
the stereo and/or enantioselectivity in the formation of
several stereogenic centres offers a promising area.3
Three-component reactions, leading to the successive
formation of two bonds using a single catalyst are currently
studied, and Lewis acid catalysed reactions have been
reported.4±6 Lanthanide and scandium tri¯ates activate
imines and are ef®cient catalysts for a large range of
three-component reactions involving the in situ formation
of an imine followed by carbon±carbon bond formation.5a±j
Similar examples of three-component coupling reactions
catalysed by lanthanide halides have been recently
described.6 Lanthanide tri¯ates catalyse a sequence of allyl-
ation and acylation reactions,7 and bismuth chloride
catalyse tandem aldolization±halogenation reactions.8
The ®xation of two carbon chains in a,b-position of enones
by three-component couplings is frequently used in syn-
thesis, especially for prostaglandins.9 They involve a
Michael addition of an organometallic derivative on the
enone followed by an alkylation reaction. Two groups
have performed enantioselective catalysis of cascades of
Michael±aldol reaction.10 Feringa studied an enantio-
selective Michael addition of an organometallic on an
enone catalysed by Cu?OTf)2 in the presence of a chiral
phosphorus ligand, the intermediate being trapped by an
aldehyde.10a By catalysis with heterobimetallic binaphth-
oxides, Shibasaki realized three-component couplings,
resulting in a direct Michael reaction followed by an aldol
reaction on one hand10b and tandem inter-intramolecular
nitroaldol reactions on the other hand.10c
An alternative route to realize the formation of two carbon±
carbon bonds on enones by a one-pot process is a Michael
reaction of a silyl derivative on an a,b-unsaturated ketone
followed by an aldol reaction of the in situ formed enoxy-
silane with a carbonyl compound. To the best of our know-
ledge, trityl salts are the sole catalysts reported for these
²
tandem Mukaiyama Michael±aldol reactions, as well as
for two tandem Michael additions.11 A similar intermolec-
ular Mukaiyama±Michael/Michael/aldol sequence pro-
moted by TiCl3?O-iPr) has been carried out for the
Keywords: samarium diiodide; catalysis; tandem reactions; Michael reac-
tions; aldol reactions.
Corresponding author. Tel.: 133-1-6915-4740; fax: 133-1-6915-4680;
e-mail: jacollin@icmo.u-psud.fr
²
Recently, Mukaiyama aldol and Michael reactions catalysed by iridium
complexes have been reported, with one example of tandem Michael±
aldol reaction.11h
p
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020?01)00902-4

8990
N. Giuseppone, J. Collin / Tetrahedron 57 42001) 8989±8998
Table 1. Tandem Mukaiyama Michael±aldol reactions catalysed by samarium diiodide
Entry
a,b-Unsaturated ketone
Aldehydea
Benzaldehyde
Benzaldehyde
p-Tri¯uorobenzaldehyde
p-Tri¯uorobenzaldehyde
Crotonaldehydee
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Cinnamaldehyde
trans-Octen-2-al
trans-Octen-2-al
trans-Octen-2-al
trans-Octen-2-al
Benzaldehyde
Ketene silyl acetal
T ?8C)
Product
4/5b
Yieldc ?%)
1
2
3
4
5
6
7
8
Cyclohexen-2-one
Cyclohexen-2-one
Cyclohexen-2-one
Cyclohexen-2-one
Cyclohexen-2-one
Cyclohexen-2-one
Cyclohexen-2-one
Cyclopenten-2-one
Cyclopenten-2-one
Cyclopenten-2-one
Cyclopenten-2-one
Cyclopenten-2-one
Cyclopenten-2-one
Cyclopenten-2-one
Cyclopenten-2-one
Cyclopenten-2-one
Cyclopenten-2-one
Cyclopenten-2-one
Cyclopenten-2-one
2a
2a
2a
2a
2a
2b
2c
2a
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2b
rt
4a15a16a
4a15a
4b15b
4b15b
4c15c16c
4d15d
4e14e
4f15f
44/39/17
98/2
73d
70
45
67
260
210
260
0
260
260
230
260
260
rt
220
240
260
230
260
rt
60/40
95/5
40/40/20f
95/5
30
42
90/10
80/20
100/0
95/5
n.d.
n.d.
n.d.
95/5
83/17
92/8
n.d.
85g
77
9
4f15f
88
72
10
11
12
13
14
15
16
17
18
19
4g15g
4h15h
4h15h
4h15h
4h15h
4i15i
4i15i
4j15j
4j15j
4k15k
5d,h
40d
70d
74
71
Benzaldehyde
82
Cinnamaldehyde
Cinnamaldehyde
trans-Octen-2-al
83d
63
260
260
60/40
59/41i
72
a
10% SmI2?THF)2 in 10 mL CH2Cl2; ratio 1/2: 1/1.5.
Diastereoisomer ratios measured by GC and 1H NMR.
Isolated yield, %.
Yield in crude product, %.
5 equiv. Crotonaldehyde.
Diastereoisomer ratios measured by GC on the crude product: 58/26/10/6.
Yield in crude product.
Formation of 1-trimethylsilyloxy-1,3-octadiene 9 as the major product.
The four diastereomers are observed in the crude product, GC ratio: 47/33/12/8.
b
c
d
e
f
g
h
i
synthesis of pentasubstituted arenes, however, it requires
four equivalents of the Lewis acid. Otherwise, a one pot
tandem Michael±imino aldol reaction needs two different
catalysts for the sequential Michael±aldol reactions.12
using 5 mol% SmI2?THF)2 as catalyst in methylene chloride
at room temperature, led to the enoxysilane 3aa isolated
according to the previously described procedure.13c The
reaction of this enoxysilane 3aa with benzaldehyde in the
presence of a catalytic amount of samarium diiodide
?10 mol%) in methylene chloride at room temperature
afforded the aldol products in moderate yield ?76% in
crude product), as a mixture of diastereoisomers 4a, 5a,
6a or 7a ?44/39/17).
In the course of our investigations concerning the properties
of samarium diiodide and lanthanide iodides as Lewis acids,
we found that these compounds are the precursors of
ef®cient catalysts for various reactions including
Mukaiyama aldol and Michael reactions.13 Particularly,
the use of samarium diiodide allows to perform selective
1,4-addition of ketene silyl acetals onto ketones and to
isolate the adducts as enoxysilanes. Similarly, aldol reac-
tions of carbonyl compounds with ketene silyl acetals, but
also with enoxysilanes, are catalysed by samarium diiodide
and various lanthanide iodides. This prompted us to study
the catalysis of a sequence of Michael addition and aldol
reaction by a one-pot procedure. We have already reported
our ®rst results concerning the use of samarium diiodide as a
catalyst precursor for tandem Mukaiyama-aldol reactions on
cyclic a,b unsaturated ketones.14 We wish to present now
our extensive study including a comparison with other
lanthanide derivatives and an application to a formal syn-
thesis of the prostaglandin PGF2a.
ꢀ1†
2. Results
In a preliminary study, we have examined the catalytic
activity of samarium diiodide for the sequential addition
of silyl ketene acetal 2a and benzaldehyde on cyclohexen-
2-one, and optimized the conditions for this tandem reac-
tion. We ®rst prepared the desired product by a stepwise
procedure ?Eq. ?1)). Reaction of 2a with cyclohexen-2-one
The same adduct could be obtained in a one-pot procedure

N. Giuseppone, J. Collin / Tetrahedron 57 42001) 8989±8998
Table 2. In¯uence of the catalyst on a Mukaiyama±aldol tandem reaction
8991
Entry
Catalyst
Temperature ?8C)
Time ?h)
4a15aa
3aa/8a
4a/5a
1
2
3
4
5
6
7
8
SmI2?THF)2
SmI3?THF)3
SmI2?OtBu)?THF)3
YbI2?THF)2
YbI3?THF)3
LaI3?DME)2
Yb?OTf)3
260
260
260
260
260
260
260
rt
36
20
20
20
28
28
22
28
22
2
100
20
19
81
93
100
6
95:5
30:70
30:70
55:45
76:24
78:22
97:3
97:3
100:0
82:18
8
9
Sc?OTf)3
BiI3
260
230
rt
100b
87b
90b
0
93:7
80:20
55:45
10
11
12
13
1
36
6
260
220
100:0
100:0
0
a
Yield % in adducts 415 in the crude product determined by GC/MS.
Determined on an aliquot by GC/MS.
b
by successive additions of ketene silyl acetal 2a, cyclo-
hexen-2-one and benzaldehyde on 10 mol% SmI2?THF)2
suspended in methylene chloride at room temperature
?Table 1, entry 1). The yield in the tandem product ?73%
in crude material) and the diastereoselectivity ?44/37/19) are
close to those found in the stepwise procedure. We have
shown, in our previous studies, that a decrease of reaction
temperature afforded better yields for aldolizations cata-
lysed by samarium diiodide involving 1-trimethylsilyloxy-
cyclohexene, which was explained by the suppression of
crotonization reactions.13c We were pleased to ®nd that
performing the tandem reactions at 2608C permitted to
improve not only the yield in adducts 4a15a but also the
diastereoselectivity ?Eq. ?2)). Only the two diastereoisomers
4a and 5a with the two chains linked to the cycle in trans
positions are isolated as the silyl ethers, in the ratio 4/5 of
98/2 ?see below for structure assignment). Surprisingly, at
low temperature, the stepwise reaction did not furnish the
tandem adducts. After reacting 3aa and benzaldehyde for
36 h at 2608C in the presence of 10 mol% SmI2?THF)2, a
GC analysis of an aliquot showed the presence of the
enoxysilane 3aa and of the corresponding ketone 8a.
When the reaction mixture was allowed to stand up slowly
to room temperature, the aldol reaction occurred and 3aa
was transformed into the tandem product as a mixture of
three diastereomers with a ratio close to that found for the
stepwise reaction at room temperature ?36/42/21).
In¯uence of the reaction temperature was also studied on the
tandem reaction involving cyclopenten-2-one 1b, ketene-
silyl acetal 2a, and E-octen-2-al ?Eq. ?3), Table 1, entries
11±14). When the reaction was realized through a one-pot
sequence at room temperature, GC/MS analyses of the
crude product indicated the presence of tandem addducts
in trace amounts while E-octen-2-al was converted in
1-trimethylsilyloxy-1-3-octadiene 9. The formation of 9
can be explained by an enolization reaction of E-octen-2-
al by the enoxysilane 3ba ?resulting from Michael addition
of ketenesilyl acetal 2a to cyclopenten-2-one 1b). This
reaction is similar to the transformation of ketones in enoxy-
silanes with ketenesilyl acetal 2a catalysed by samarium
diiodide that we have previously reported.15 A decrease of
the reaction temperature has more pronounced effect on the
rate of enolization of E-octen-2-al than on the rate of the
aldol reaction. The tandem product is selectively obtained at
2608C. Addition of E-octen-2-al on the intermediate enoxy-
silane 3ba is 1,2-regioselective as was previously observed
for the reaction of E-octen-2-al with 2b catalysed by
samarium diiodide.13c In the tandem reactions above, after
addition of a,b-unsaturated ketone 1a or b, the blue colour
characteristic of samarium diiodide disappears and the
reaction mixture keeps a pale yellow colour until the end
of the reaction. Therefore, the true catalyst is trivalent and
samarium diiodide is not the active species as for other
reactions using samarium diiodide as a precatalyst.13
After optimization of the tandem reaction catalysed by
SmI2?THF)2 leading to adduct 4a we have evaluated the
catalytic activity of other lanthanide derivatives under the
same experimental conditions ?Table 2). We have ®rst
studied the behaviour of some lanthanide iodides that we
have previously found to catalyse ef®ciently several reac-
tions including aldolizations.13 By the use of trivalent
samarium iodides in catalytic amounts ?10 mol%), we
obtained only products resulting from the Michael addition,
as a mixture of the enoxysilane 3aa and of the correspond-
ing ketone 8a ?entries 2 and 3). The divalent ytterbium
iodide ?entry 4) allowed to carry out the tandem sequence
ꢀ2†

8992
N. Giuseppone, J. Collin / Tetrahedron 57 42001) 8989±8998
ꢀ3†
but the diastereoselectivity of the aldolization was lower
than that provided by samarium diiodide. On the contrary,
trivalent ytterbium and lanthanum iodides led to the tandem
adducts in good yields and with high diastereoselectivities
?entries 5 and 6). Amongst the numerous Lewis acid cata-
lysts, ytterbium and scandium tri¯ates are now well known
for their high ef®ciency in a wide range of reactions.16
Recently, it has been reported that BiCl3-metallic iodides
systems, leading to in situ formation of BiI3, catalyse
Mukaiyama and Michael reactions.17 The catalytic activities
displayed by these three compounds and by lanthanide
iodides have been compared for the same tandem reaction.
Ytterbium tri¯ate ?entries 7 and 8) at low temperature or
room temperature afforded only the intermediate enoxy-
silane 3aa ?with only a small amount of ketone 8a), as did
also bismuth iodide ?entry 12). These two derivatives are
unable to perform successively the Michael and the aldol
reactions. To the best of our knowledge, no three-com-
ponent reaction catalysed by ytterbium tri¯ate, involving
the formation of two carbon±carbon bonds has been
reported yet. In the case of scandium tri¯ate, monitoring
the reaction by GC/MS on aliquots, indicated a higher rate
of the tandem reaction than with other catalysts ?entries
9±11). As with samarium diiodide, higher diastereo-
selectivity is displayed at low temperature. Yet, several
procedures have been tested for quenching the reaction
mixtures, which led, in all cases, to decomposition without
isolation of the tandem product. This can be explained by
the higher acidity of scandium tri¯ate compared to that of
lanthanide iodides and by the liberation of tri¯ic acid during
hydrolysis which reacts with the very sensitive tandem
adducts. The results gathered in Table 2 indicate that samar-
ium diiodide, as well as ytterbium and lanthanum triiodides
have similar activities and allow to prepare tandem adducts
with high diastereoselectivity. However, samarium diiodide
being commercially available, and more easily prepared18
than ytterbium or lanthanum triiodides19 was selected in our
further investigations.
aldehydes ?Eq. ?4), Table 1, Fig. 1). In all cases, regioselec-
tive 1,4 addition on the ketone is observed in the ®rst step,
and, with a,b-conjugated aldehydes, regioselective 1,2
addition in the second step. The tandem adducts are isolated
as silyl ethers in good yields in all the reactions proceeding
at low temperature.
ꢀ4†
The selectivity for aldolization versus Michael reaction in
the second step is consistent with our previous results for
aldol reactions catalysed by samarium diiodide.13c We
reported that reactions involving a,b-unsaturated aldehydes
in most cases led to mixtures of 1,2- and 1,4-additions.
Products were isolated as silyl ethers, with aldols as major
products and selectivity depending on the silyl substrates.
Higher ratios of aldol products were found with the less
bulky ketene silyl acetals. A regioselective 1,2-addition
was observed in the reaction of 1-trimethylsilylcyclohexene
with cinnamaldehyde, which is close to the aldol step of the
tandem reaction.
The structure of the major diastereoisomer has been estab-
lished as 4. This isomer results from the trans attachment of
the two chains on the cycle, and a syn stereochemistry
arising from the second step. This structure was assigned
on the basis of the values of coupling constants measured in
the 1H NMRspectra of the major diastereomer 4a and of the
corresponding lactone 10a ?Eq. ?5)). Coupling constants J
?H1±H2)ˆ12.3 Hz and J ?H2±H3)ˆ10.8 Hz for the lactone
A variety of tandem Michael±aldol reactions on cyclo-
hexen-2-one or cyclopenten-2-one catalysed by samarium
diiodide have been successfully performed with ketene silyl
acetals 2a, b or c and different aromatic and a,b-unsaturated

N. Giuseppone, J. Collin / Tetrahedron 57 42001) 8989±8998
8993
10a are indicative of a trans±trans stereochemistry. The
two chains are in trans con®guration in the adduct 4a.
The coupling constant J ?H2±H3)ˆ5.1 Hz for adduct 4a
indicates the syn position of these two protons.13c
substrate ?S)-12 was prepared according to published
methods, by oxidation of cyclopentadiene to cis-3,5-di-
acetoxyclopentene.21 This compound is hydrolysed by
acetyl chlolinesterase in the corresponding enantiopure
monoacetate ?3R)-acetoxy,?5S)-hydroxycyclopent-1-ene.22
This latter compound is transformed in ketone ?S)-12 by a
described procedure.23 We ®rst examined the diastereo-
selectivity of the Michael reaction catalysed by samarium
diiodide leading to enoxysilane 13 ?Eq. ?6)). As in studies
described above, we found that both the chemoselectivity
and the stereoselectivity of the Michael addition were
improved by lowering temperature. At 2788C, enolization
of ketone ?S)-12 did not occur and we found by GC a 85/15
ratio for the diastereoisomers of ketone 14 produced by
hydrolysis of enoxysilane 13. We could not attribute the
stereochemistry to each of these isomers. Otera has studied
the stereoselectivity of Michael additions on enone 12
catalysed by several Lewis acids and found a preference
for the syn diastereomer in reactions involving unsubstituted
ketene silyl acetals.24 This allows to suppose that the syn
isomer of enoxysilane 13 is the major one, as will be
con®rmed below.
ꢀ5†
The diastereoselectivities of tandem reactions depend on the
substrates and can be very high in some cases ?Table 1,
entries 2, 4, 6, 10, 14 and 16). For most adducts, only two
diastereomers were isolated: they result from the ®xation of
the two chains in trans positions on the enone. The two
chiral carbons ligating the second bond formed in the
tandem process are always in a syn relationship in
the major isomer. Similarly, aldol reaction between
1-trimethylsilylcyclohexene and p-anisaldehyde catalysed
by samarium diiodide afforded a mixture of diastereo-
isomers with the syn isomer favoured.13c The diastereoselec-
tivity of the aldol step in tandem reactions increases at low
temperature as was shown above, but depends also on the
ketene silyl acetal. Reactions involving the silyl derivative
2a afforded higher ratios 4:5 than those involving 2b
?compare entries 9 and 16, 10 and 18, 14 and 19). This
can be explained by the difference of steric hindrance of
the intermediate enoxysilanes 3ba and 3bb. The bulkiness
due to methyl substituents increases the diastereofacial
selectivity in the reaction of aldehydes on enoxysilanes.
Similarly, Mukaiyama observed high diastereoselectivities
for the tandem Michael±aldol reactions on cyclic ketones
catalysed by trityl perchlorate with the same major dia-
stereomer. In these reactions, only a bulky ketenesilyl acetal
similar to 2a ?TBDMS instead of TMS) has been tested.11b
ꢀ6†
Then we performed a tandem Michael±aldol reaction
catalysed by samarium diiodide by an addition of ketene
silyl acetal 2d on ketone ?S)-12 for the Michael step and
of E-octen-2-al for the aldol reaction ?Eq. ?7)). The tandem
product 15 was isolated as a triethylsilyl ether, while in the
described procedure, the crude product resulting from the
addition of E-octen-2-al was directly transformed in allylic
acetate.20 In the samarium-catalysed tandem sequence, the
addition of E-octen-2-al is 1,2-regioselective as found with
other enoxysilanes, and leads to only one diastereomer. The
structure of compound 15 was established by 2D NMR
COSY and NOESY experiments. This allowed us to
establish a cis relationship for protons H1 and H2, trans
for H2 and H3 and syn for H3 and H4. The cis relationship
between H1 and H2 indicates a syn con®guration for the
major isomer of enoxysilane 13. Thus stereoselectivity of
the Michael addition of ketenesilyl acetal 2d on ketone
?S)-12 is similar with samarium diiodide and with other
Lewis acids.24 The isolation of only one diastereomer for
adduct 15 can be explained either by a lower reactivity of
the minor isomer of 13 towards E-octen-2-al, or by degra-
dation of the minor stereoisomers of 15 during puri®cation.
Comparison of the NMRspectra of crude product and
product 15 after chromatography on silica con®rms the
second hypothesis. The diastereoisomer formed by aldoli-
zation on syn-13, results of a trans addition on the cycle and
The tandem Michael±aldol reaction catalysed by samarium
diiodide is a staightforward method for introducing two
chains in a and b position of a cyclic a,b-unsaturated
ketone and we planned to apply this methodology to a syn-
thesis of a molecule of biological interest. As a key step in
the synthesis of PGF2a, Danishefsky has proposed a new
route involving the Michael addition of ketene acetal 2d
on the enantiopure substituted cyclopentenone 12, followed
by an aldol reaction on the resulting enoxysilane 13.20 The
®rst reaction is catalysed by HgI2 and the second promoted
by TiCl4. We anticipated that this reaction could be realized
with samarium diiodide in a one-pot procedure. The

8994
N. Giuseppone, J. Collin / Tetrahedron 57 42001) 8989±8998
Figure 1.
tert-butyldimethylsilyloxy group, or to the size of the
silyl group of the acetal. We have found that catalysis by
samarium diiodide of Michael±aldol reaction on ketone
?S)-12 furnishes similar results to sequential reactions
described by Danishefsky. The desired product is obtained
with the same stereochemistry and yield. Thus, we have
improved the previously reported method for the formal
synthesis of PGF2a, by performing tandem reactions with
a catalytic amount of samarium diiodide.
ꢀ7†
3. Conclusion
We have shown that samarium diiodide is a highly ef®cient
precatalyst for tandem Mukaiyama Michael±aldol reaction.
The process requires only small amounts of catalyst and
leads to the ®xation of two chains in a and b positions of
an a,b-unsaturated ketone. The stereoselectivity can be
controlled on the three formed stereogenic centres. The
potential of the procedure is highlighted by a successful
application on a key step of synthesis of PGF2a. The use
of samarium diiodide as a Lewis acid catalyst provides a
displays syn stereochemistry of the carbons of the second
bond, similarly to the major stereoisomer in all tandem
reactions. In sequences involving cyclopenten-2-one and
E-octen-2-al ?Table 1, entries 14 and 19), higher selectivity
was obtained with ketenesilyl acetal 2a than with b. Thus,
the high selectivity observed in the aldol reaction providing
15 can be due either to the steric hindrance introduced by the

N. Giuseppone, J. Collin / Tetrahedron 57 42001) 8989±8998
8995
new attractive methodology for the formation of two
carbon±carbon bonds by a one-pot procedure. We are
currently studying in our laboratory other tandem sequences
catalysed by samarium diiodide.
20.80, 20.20; FTIR?NaCl) 1730, 1703 cm 21; HRMS
calcd for C21H32O4Si ?M1) 376.2069, found 376.2070.
Anal. calcd for C21H32O4Si: C, 66.98; H, 8.57. Found: C,
67.25; H, 8.55.
4.2.2. 2-Methyl-2-[3-oxo-2-{1-/trimethylsilyloxy)-1-/4-
/tri¯uoromethyl) phenylmethyl)}-cyclohexyl] propionic
4. Experimental
4.1. General
1
acid methyl ester /4b). H NMR?250 MHz, CDCl ) d
3
7.50 ?m, 4H), 5.19 ?d, Jˆ6.1 Hz, 1H), 3.64 ?s, 3H), 2.63
?m, 1H), 2.40±1.50 ?m, 7H), 1.18 ?s, 3H), 1.12 ?s, 3H),
20.02 ?s, 9H); 13C NMR?62.9 MHz, CDCl ) d 209.65,
177.15, 146.70, 129.35 ?q, CF3), 127.35, 126.50, 124.85,
71.75, 58.55, 51.80, 47.00, 45.30, 28.10, 26.45, 24.70,
All manipulations were carried out under an argon atmos-
phere using standard Schlenk or glovebox techniques. THF
was distilled from sodium benzophenone ketyl, methylene
dichloride ?stabilized with amylene) from calcium hydride
and all solvents were degassed immediately prior to use.
Silyl ketene acetal 2a was purchased from Aldrich. 2b and
c were prepared according to Tamura's procedure25 and 2d
to Rousseau's procedure.26 The method for preparing
3
21
22.20, 21.70, 20.05; FTIR?NaCl) 1732, 1618 cm
;
HRMS calcd for C22H31F3O4Si ?M1) 444.1944, observed
444.1932. Anal. calcd for C22H31F3O4Si: C, 59.44; H,
7.03. Found: C, 59.67; H, 6.88.
18
SmI2?THF)2, YbI2?THF)2 and tBuOSmI2?THF)3,27 has
4.2.3. 2-Methyl-2-[3-oxo-2-{1-/trimethylsilyloxy)-1-//E)-
but-2-enyl)}-cyclohexyl] propionic acid methyl ester
/mixture ofdiastereomers) /4c 15c16c). 1H NMR
?250 MHz, CDCl3,) d 5.75±4.85 ?m, 2H), 4,55 ?m, 1H),
[3.69, ?s), 3.68 ?s), 3.67 ?s); 3H], 2.50±1.90 ?m, 7H), 1.80
?m, 1H), 1.66 ?m, 3H), 1.18±1.14 ?m, 6H); 0.13±0.07 ?m,
9H); 13C NMR?62.9 MHz, CDCl 3) d 210.37, 210.31,
177.39, 177.36, 132.87, 131.16, 126.60, 125.40, 70.85,
70.12, 56.88, 56.43, 51.82, 46.73, 46.43, 45.87, 45.23,
43.97, 43.90, 43.31, 41.21, 27.34, 26.89, 26.53, 26.42,
26.35, 25.11, 22.27, 22.15, 21.79, 21.70, 17.62, 0.99, 0.28,
0.23; FTIR?NaCl) 1734 cm 21; HRMS calcd for C21H32O4Si
?M1Na1) 363.1969, found 363.1967.
been previously described by us as well as LaI3?DME)2
and YbI3?DME)2 which were prepared from La or Yb
powder and iodine.19
Bruker AM 250 and AM 400 spectrometers, operating at
250 and 400 MHz for 1H, 62.9 and 100.8 MHz for 13C were
used for the NMRspectra. Chemical shifts are reported in
parts per million ?ppm) down®eld from tetramethylsilane
for spectra in CDCl3. Infrared spectra were recorded as
Nujol mulls using NaCl plates on a Perkin±Elmer 883 spec-
trometer ?FTIR) and are reported in cm21. Mass spectra
?MS) ?70 eV) data were determined on a Ribermag R-10
GC/MS and high-resolution mass spectra and electrospray
mass spectra on a Varian 3400 GC-LC/MS. Flash chroma-
tography was realized using 230±400 mesh silica gel desac-
tivated by Et3N.
4.2.4.
2-[3-Oxo-2-{1-/tertiobutyldimethylsilyloxy)-1-
/phenylmethyl)}-cyclohexyl] acetic acid ethyl ester /4d).
1H NMR?250 MHz, CDCl 3) d 7.40±7.20 ?m, 5H), 5.12 ?d,
Jˆ3.9 Hz, 1H), 4.10 ?q, Jˆ7.3 Hz, 2H), 2.85 ?m, 2H), 2.55
?m, 1H), 2.34 ?d, 2H, Jˆ5.9 Hz), 1.80 ?m, 2H), 1.28 ?t, Jˆ
7.3 Hz, 3H), 1.00 ?s, 9H), 1.10±0.80 ?m, 3H), 1.00 ?s, 9H),
0.20 ?s, 3H), 0.21 ?s, 3H); 13C RMN ?62.9 MHz, CDCl3) d
218.50, 172.40, 129.24, 128.05, 126.39, 123.18, 112.87,
60.34, 40.88, 32.40, 28.98, 25.92, 25.57, 18.35, 14.29,
4.2. Typical procedure for the synthesis of tandem
products
A solution of SmI2 in THF ?0.2 M, 2.0 mL, 0.2 mmol) was
carefully evaporated in vacuo to give SmI2?THF)2 as a blue
powder and suspended in CH2Cl2 ?5 mL), then 1-methoxy-
2-methyl-1-trimethylsilyloxypropene ?406 mL, 2 mmol)
and cyclohexen-2-one ?193 mL, 2 mmol) were added.
After 0.5 h at room temperature, the yellow solution was
cooled at 2788C and a solution of benzaldehyde ?302 mL,
3 mmol) in 5 mL CH2Cl2 was added at the same tempera-
ture. The reaction mixture was allowed to stand at 2608C
during 36 h. The reaction was quenched by the addition of
hexane ?20 mL), which precipitates the samarium salts.
After ®ltration through celite, the solvents were removed
under reduced pressure and the residue was puri®ed by
¯ash chromatography on deactivated silica gel ?hexane/
ethyl acetate, 90/10) ?527 mg, 70% yield).
1.00, 24.40, 24.54. IR?NaCl)
n ;
max: 1758, 1716 cm21
HRMS ?IE1) calcd for C27H44O5Si2Na: ?M11Na)
527.2625, found 527.2625.
4.2.5. 2-[3-Oxo-2-{1-/trimethylsilyloxy)-1-/phenylmethyl)}-
cyclohexyl] acetic acid phenyl ester /4e). 1H R MN
?250 MHz, CDCl3) d 7.40±6.90 ?m, 10H), 5.35 ?d, Jˆ
4.7 Hz, 1H), 2.86 ?m, 2H), 2.55 ?m, 1H), 2.4±1.80 ?m,
7H), 0.03 ?s, 9H); 13C RMN ?62.9 MHz, CDCl3) d 210.66,
170.27, 156.02, 150.34, 129.44, 129.40, 128.41, 126.85,
126.76, 121.38, 120.10, 115.26, 73.58, 47.26, 40.96,
40.73, 35.50, 30.70, 24.67, 20.07. IR?NaCl)
nmax: MS
?IE1) calcd for C24H30O4SiNa: ?M11Na, 433).
4.2.6. 2-[3-Oxo-2-{1-/trimethylsilyloxy)-1-{phenylmethyl)}-
cyclopentyl] propionic acid methyl ester /4f). H NMR
4.2.1. 2-Methyl-2-[3-oxo-2-{1-/trimethylsilyloxy)-1-/phenyl-
methyl)}-cyclohexyl] propionic acid methyl ester /4a). 1H
NMR?250 MHz, CDCl 3) d 7.27 ?m, 5H), 5.0 ?d, Jˆ5.1 Hz,
1H), 3.65 ?s, 3H), 2.63 ?d, Jˆ5.1 Hz, 1H), 2.40±2.20 ?m,
3H), 1.95 ?m, 2H), 1.70 ?m, 1H), 1.50 ?m, 1H), 0.86 ?s, 3H),
0.75 ?s, 3H), 20.02 ?s, 9H); 13C NMR?62.9 MHz, CDCl 3) d
216.10, 177.80, 142.95, 128.05, 127.55, 125.40, 77.30,
58.75, 51.40, 45.70, 41.40, 40.15, 24.50, 24.30, 22.60,
1
?250 MHz, CDCl3,) d 7.34±7.22 ?m, 5H), 5.34 ?d, Jˆ2 Hz,
syn), 3.47 ?s, 3H), 2.60±1.00 ?m, 6H,), 0.70 ?s, 3H), 0.64 ?s,
3H), 0.03 ?s, 9H); 13C NMR?62.9 MHz, CDCl ) d 222.10,
3
177.50, 142.60, 128.05, 127.10, 125.80, 75.35, 58.70, 51.55,
45.40, 42.80, 38.00, 24.00, 23.70, 21.95, 20.20; FTIR
?NaCl) 2955, 1733, 1473, 1452, 1434, 1369, 1252, 1190,

8996
N. Giuseppone, J. Collin / Tetrahedron 57 42001) 8989±8998
1167, 1141, 1118, 1091, 1065, 1027, 1002, 954, 896, 843,
754, 703; HRMS calcd for C19H27O4Si ?M12CH3):
347.1693, found 347.1694.
4.2.11. 2-[3-Oxo-2-{1-/tertiobutyldimethylsilyloxy)-1-
//E)-oct-2-enyl)}-cyclopentyl] acetic acid ethyl ester
1
/4k15k). H NMR?250 MHz, CDCl ,) d 5.63 ?m, 1H,
3
syn1anti), 5.39 ?m, 1H, syn1anti), 4.80 ?dd, Jˆ10.2,
11.7 Hz, 0.40H anti), 4.65 ?d, Jˆ6.8 Hz, 0.60H syn), 4.15
?m, 2H, syn1anti), 2.85 ?m, 1H, syn1anti), 2.60±1.15 ?m,
20H, syn1anti), 0.88 ?s, 5.3H, syn), 0.82 ?s, 3.7H, anti),
0.10 ?s, 1.8H, syn), 0.08 ?s, 1.8H, syn), 20.04 ?s, 1.2H,
anti), 20.6 ?s, 1.2H, anti); 13C NMR?62.9 MHz, CDCl3)
d syn1anti, 219.55, 218.85, 172.20, 172.05, 141.95,
131.20, 111.90, 111.25, 72.15, 60.45, 60.25, 59.55, 58.90,
40.25, 39.25, 39.05, 38.75, 38.20, 38.00, 34.75, 33.65,
32.00, 31.50, 31.25, 30.90, 28.75, 27.65, 27.45, 27.25,
27.15, 25.80, 25.65, 22.60, 22.40, 18.30, 18.05, 14.10,
14.00, 24.22, 25.30, 25.35; FTIR?NaCl), 1738. Anal.
calcd for C23H42O4Si: C, 67.27; H, 10.31. Found: C,
66.74; H, 10.34.
4.2.7. 2-Methyl-2-[3-oxo-2-{1-/trimethylsilyloxy)-1-//E)-
3-phenylprop-2-enyl)}-cyclopentyl]
methyl ester /4g). H NMR?250 MHz, CDCl ) d 7.35
propionic
acid
1
3
?m, 5H), 6.55 ?d, Jˆ13.4 Hz, 1H), 6.10 ?dd, Jˆ13.4,
5.8 Hz, 1H), 4.85 ?d, Jˆ5.8 Hz, 1H), 3.50 ?s, 3H), 2.70
?bs, 1H), 2.20±1.80 ?m, 5H), 1.05 ?s, 6H); 0.05 ?s, 9H);
13C NMR?62.9 MHz, CDCl ) d 221.97, 177.58, 136.56,
3
130.93, 130.01, 128.55, 127.5, 126.38, 74.70, 56.57,
51.74, 47.75, 43.41, 37.95, 23.81, 23.00, 0.01; FTIR
?NaCl) 1739 cm21; HRMS calcd for C22H32O4Si ?M1)
388.2070, found 388.2081.
4.2.8. 2-Methyl-2-[3-oxo-2-{1-/trimethylsilyloxy)-1-//E)-
oct-2-enyl)}-cyclopentyl] propionic acid methyl ester
1
/4h). H NMR?250 MHz, CDCl ) d 5.58 ?m, 1H), 5.35
4.2.12. Lactone of2-methyl-2-[3-oxo-2-phenylmethyl)-
cyclohexyl] propanoic acid /10a). To a solution of the
tandem product 4 ?500 mg, 1.32 mmol) in THF ?2 mL)
were successively added H2O ?2 mL), acetic acid ?10 mL)
and tri¯uoroacetic acid ?2 mL), and reaction mixture was
stirred at room temperature during 12 h. After addition of
Na2CO3 until PH 7, the reaction mixture was extracted with
ether, washed with brine and dried over MgSO4. After
evaporation of solvents, the crude product was puri®ed by
column chromatography on silica gel ?heptane/ethyl acetate,
90/10). The product was isolated as a white powder
?349 mg, 97% yield), which after recrystallization in
ether/toluene, 75/25 at 2308C gave white crystals ?Fˆ
1698C).
3
?dd, Jˆ14.5, 4.5 Hz, 1H), 4.60 ?d, Jˆ4.5, 1H), 3.57 ?s,
3H), 2.65 ?m, 1H), 2.20±1.70 ?m, 8H), 1.45±1.10 ?m,
5H), 1.10 ?s, 3H) 1.05 ?s, 3H); 0.85 ?m, 3H), 0.10 ?s, 9H);
13C NMR?62.9 MHz, CDCl ) d 222.40, 177.65, 131.85,
3
131.00, 74.80, 56.80, 51.65, 45.70, 43.45, 37.90, 32.05,
31.40, 28.80, 24.00, 23.80, 22.70, 22.45, 14.00, 0.05;
FTIR?NaCl) 1736 cm 21. Anal. calcd for C21H38O4Si: C,
65.92; H, 10.01. Found: C, 65.78; H, 9.85.
4.2.9. 2-[3-Oxo-2-{1-/tertiobutyldimethylsilyloxy)-1-
/phenylmethyl)}-cyclopentyl] acetic acid ethyl ester
1
/4i15i). H NMR?250 MHz, CDCl ,) d 7.37±7.10 ?m,
3
5H, syn1anti), 5.62 ?bs, 0.1H, anti), 5.49 ?bs, 0.9H, syn),
4.10 ?q, Jˆ7.0 Hz, 0.3H, anti), 3.90 ?q, Jˆ7.0 Hz, 1.7H,
syn), 2.45 ?bs, 1H, syn1anti), 2.25±1.63 ?m, 7H,
syn1anti), 1.07 ?s, 9H, syn1anti), 0.96 ?t, Jˆ7.0 Hz, 3H,
syn1anti), 0.58 ?s, 3H anti), 0.25 ?s, 2.7H syn), 0.15 ?s,
0.3H, anti), 0.01 ?s, 2.7H, syn); 0.01 ?s, 3H, syn); 13C NMR
?62.9 MHz, CDCl3) d syn 218.50, 172.20, 142.95, 128.40,
127.35, 125.65, 72.95, 61.25, 60.25, 39.60, 39.10, 32.20,
27.70, 26.00, 14.35, 24.35, 25.30; FTIR?NaCl)
1H NMR?250 MHz, CDCl ,) d 7.30 ?m, 5H), 5.78 ?d,
3
Jˆ10.2 Hz, 1H), 2.98 ?dd, Jaˆ12.8 Hz, Jbˆ10.3 Hz, 1H),
2.40±1.80 ?m, 5H), 1.80±1.50 ?m, 2H), 1.42 ?s, 3H), 1.40
?s, 3H); 13C NMR?62.9 MHz, CDCl ) d 208.02, 175.65,
3
140.17, 128.40, 128.26, 126.77, 79.89, 52.87, 47.87,
42.04, 41.25, 26.50, 25.21, 24.36, 21.02; FTIR?NaCl),
1712; LRMS ?EI1) m/z: 272 ?M1; 100). Anal. calcd for
C17H20O3: C, 74.97; H, 7.40. Found: C, 74.78; H, 7.47.
1737 cm21; HR MS calcd for CH31O4Si ?M12Me)
20
375.1974, observed 375.1975. Anal. calcd for
C22H34O4Si: C, 67.65; H, 8.77. Found: C, 67.83; H, 8.51.
4.2.13. 2-[/1S,5S)-{3-Oxo}-5-tertiobutyldimethylsilyloxy}-
cyclopentyl] acetic acid ethyl ester /14). To a suspension
of SmI2?THF)2 ?0.1 mmol) in CH2Cl2 ?5 mL) at 2788C were
successively added 1-ethoxy-1-triethylsilyloxyethene 2d
?202 mg, 1 mmol) and ?4S)-4-tert-butyldimethylsilyloxy-
cyclopenten-2-one 12 ?212 mg, 1 mmol) and the reaction
mixture was allowed to stand at the same temperature
during 24 h. The reaction was quenched with hexane
?20 mL), the samarium salts ®ltrated on celite and solvents
evaporated. The spectral data of the crude product 13 are
identical to those reported in literature.20 This product was
dissolved in THF ?20 mL) and stirred with a mixture MeOH
?20 mL) /HCl 1N ?10 mL) during 30 min. After neutrali-
zation with NaHCO3, extraction by ether, drying on
MgSO4, solvents were evaporated. The residue was puri®ed
by ¯ash chromatography on silica gel ?pentane/ether, 90/10)
?199 mg, 66% yield).
4.2.10. 2-[3-Oxo-2-{1-/tertiobutyldimethylsilyloxy)-1-
//E)-3-phenylprop-2-enyl)}-cyclopentyl] acetic acid
1
ethyl ester /4j15j). H NMR?250MHz, CDCl ) d 7.5
3
?m, 5H, syn1anti), 6.58 ?d, Jˆ15.8 Hz, 1H, syn1anti);
6.15 ?dd, Jˆ5.5, 15.8 Hz, 1H, syn1anti), 4.90 ?dd, Jˆ5.0,
5.55 Hz, 1H, syn1anti), 4.15 ?q, Jˆ7.3 Hz, 0.8H, anti), 4.05
?q, Jˆ7.3 Hz, 1.2H, syn), 2.91 ?dd, Jˆ5.0, 14.0 Hz, 1H,
syn1anti), 2.65±2.00 ?m, 7H, syn1anti), 1.25 ?t, Jˆ
7.3 Hz, 1.2H, anti), 1.15 ?t, Jˆ7.3 Hz, 1.8H, syn), 0.89 ?s,
3.6H, anti), 0.87 ?s, 5.4H, syn), 0.09 and 0.06 ?2s, 2.4H,
anti), 0.00 and 20.01 ?2s, 3.6H, syn); 13C ?62.9 MHz,
CDCl3) d syn1anti 218.40, 172.15, 172.08, 136.58,
138.52, 131.21, 130.82, 129.56, 129.30, 128.55, 128.48,
128.26, 127.53, 127.40, 126.29, 73.06, 71.93, 60.49,
60.26, 60.20, 59.29, 39.95, 39.43, 38.82, 38.67, 34.89,
32.44, 27.34, 27.06, 25.83, 25.79, 25.60, 18.10, 14.19,
14.07, 24.29, 24.48, 25.55, 25.69; FTIR?NaCl),
1737 cm21. Anal. calcd for C24H36O4Si: C, 69.19; H, 8.71.
Found: C, 69.21; H, 8.65.
1H NMR?250 MHz, CDCl ) d 4.51 ?m, 1H, cis1trans),
3
4.15 ?q, Jˆ7.1 Hz, 2H, cis1trans), 2.68±1.89 ?m, 7H,
cis1trans), 1.26 ?t, Jˆ7.1 Hz, 3H, cis1trans), [0.88 ?s)

N. Giuseppone, J. Collin / Tetrahedron 57 42001) 8989±8998
8997
Â
3. ?a) Negishi, E.; Coperet, C.; Ma, S.; Liou, S. Y.; Liu, F. Chem.
Rev. 1996, 96, 365±395. ?b) Malacria, M. Chem. Rev. 1996,
96, 289±306. ?c) Padwa, A.; Weingarten, M. D. Chem. Rev.
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Schumann, H. J. Org. Chem. 1998, 63, 3386±3396. ?e) Molan-
der, G. A.; Dowdy, E. D. J. Org. Chem. 1998, 63, 8983±8988.
?f) Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 1757±
1771. ?g) Widenhoefer, R. A.; Stengone, C. N. J. Org. Chem.
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shima, M.; Funabashi, H.; Morishima, H. Tetrahedron Lett.
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Terada, M.; Suzuki, Y.; Takanishi, Y.; Takezoe, H. Synlett
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Shibasaki, M. Tetrahedron Lett. 1997, 38, 6031±6034.
?trans) and 0.86 ?s) ?cis), 9H], [0.05 ?s) ??trans) and 20.03
?s) ?cis), 6H]; 13C NMR?62.9 MHz, CDCl ) d 216.50,
3
172.45, 71.10, 60.45, 49.05, 40.60, 38.90, 34.25, 25.65,
17.90, 14.20, 24.70, 25.20; FTIR?NaCl): 1734; HRMS)
calcd for C14H25O4Si ?M12CH3): 285.1523, observed
20
285.1522. [a]D ˆ211.3 ?cˆ1, CHCl3).
4.2.14. 2-[/1S,2S,5R)-2-{{3-Oxo-2{/1S)-1-/triethylsilyl-
oxy)-/E)-oct-2-enyl}-5-tert-butyldimethylsilyloxy}-cyclo-
pentyl] acetic acid ethyl ester /15). To a suspension of
SmI2?THF)2 ?0.1 mmol) in CH2Cl2 ?5 mL) at 2788C were
successively added 1-ethoxy-1-triethylsilyloxyethene 2d
?202 mg, 1 mmol) and ?4S)-4-tert-butyldimethylsilyloxy-
cyclopenten-2-one 12 ?212 mg, 1 mmol) and the reaction
mixture was allowed to stand at the same temperature
during 24 h. Then a solution of E-octen-2-al ?189 mg,
1.5 mmol) in CH2Cl2 ?5 mL) was added and temperature
was raised to 2608C and maintained during 36 h. The reac-
tion was quenched with hexane ?20 mL), the samarium salts
®ltrated on celite and solvents evaporated. The residue was
puri®ed by ¯ash chromatography on deactivated silica gel
?hexane/ethyl acetate, 95/5) ?400 mg, 74% yield).
È
?k) Delas, C.; MoIse, C. Synthesis 2000, 251±254. ?l) Yang,
H. W.; Romo, D. J. Org. Chem. 1998, 63, 1344±1347.
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?e) Mascarenhas, C. M.; Duffey, M. O.; Liu, S. Y.; Morken,
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1H NMR?250 MHz, CDCl ) d 5.65 ?dt, J1ˆ15.4 Hz, J2ˆ
3
8.2 Hz, 1H), 5.41 ?dd, J1ˆ15.4, J2ˆ6.3 Hz, 1H), 4.70 ?m,
1H), 4.57 ?m, 1H), 4.12 ?q, Jˆ7.1 Hz, 2H), 2.93 ?m, 1H),
2.60 ?dd, J1ˆ10.3, J2ˆ3.8 Hz, 1H), 2.50 ?dd, J1ˆ15 Hz,
J2ˆ10.3 Hz, 1H), 2.27 ?d, Jˆ2.8 Hz, 2H), 2.00 ?m, 3H),
1.35 ?m, 2H), 1.25 ?m, 7H), 0.90 ?m, 12H), 0.83 ?s, 9H),
0.52 ?q, Jˆ6.0 Hz, 6H), [0.01 and 20.01 ?6H); 0.01 ?s, 3H),
20.01 ?s, 3H)]; 13C NMR?62.9 MHz, CDCl ) d 216.90,
5. ?a) Kobayashi, S.; Ishitani, H. J. Chem. Soc. Chem. Commun.
1995, 1379. ?b) Kobayashi, S.; Araki, M.; Yasuda, M.
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3
172.65, 131.20, 131.05, 76.70, 72.05, 69.40, 60.15, 56.55,
49.30, 37.85, 33.90, 32.00, 31.40, 28.75, 25.70, 22.45,
17.90, 14.20, 6.80, 4.80, -4.65, -5.35; FTIR?NaCl): 1736;
LRMS ?EI1) m/z 483 ?M12C?CH3)3; 3), 379 ?26), 241 ?10),
103 ?27), 75 ?100), 45 ?13); HRMS ?electrospray) calcd for
C29H56O5Si2K ?M11K): 579.3303, observed 579.3312.
20
[a]D ˆ24.8 ?cˆ1, CHCl3).
Acknowledgements
We thank the CNRS and MRES for ®nancial support and
MRES for a fellowship for N. Giuseppone.
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