Stereoselective addition of organomanganese reagents to chiral acylsilanes and aldehydes

Article abstract of DOI:10.1016/S0022-328X(00)00902-5

Organomanganese halides and organomanganates prepared by transmetalation of organolithium and Grignard reagents add smoothly to the carbonyl moiety of acylsilanes and of substituted aldehydes bearing a chiral center at the α-position affording the desired

Full text of DOI:10.1016/S0022-328X(00)00902-5

Journal of Organometallic Chemistry 624 (2001) 223–228
www.elsevier.nl/locate/jorganchem
Stereoselective addition of organomanganese reagents to chiral
acylsilanes and aldehydes
b
Ce´line Boucley a, Ge´rard Cahiez a, Silvia Carini b, Vanda Cere` ,
Mauro Comes-Franchini b, Paul Knochel c, Salvatore Pollicino b, Alfredo Ricci b,*
a Laboratoire de Synthe`se Organique Se´lecti6e et de Chimie Organometallique CNRS-UCP-ESCOM, 13 Boule6ard de l’ Hautil,
95092 Cergy-Pontoise, France
b Dipartimento di Chimica Organica ‘A. Mangini’, Uni6ersita` degli Studi di Bologna, Viale Risorgimento no. 4, 40136 Bologna, Italy
c Institut fu¨r Organische Chemie der Ludwig Maximilians Uni6ersita¨t, Butenandstrasse 5-13, 81377 Munich Germany
Received 16 October 2000; accepted 22 November 2000
Abstract
Organomanganese halides and organomanganates prepared by transmetalation of organolithium and Grignard reagents add
smoothly to the carbonyl moiety of acylsilanes and of substituted aldehydes bearing a chiral center at the a-position affording the
desired alcohols in good to excellent yields and with essentially no undesired products from enolization. Comparison of the
stereochemical outcome with that observed for other organometallic species, outlines the capability of organomanganese reagents
to induce uniformly good diastereoselectivities, in a number of cases significantly higher than reported previously for these
reactions. The key role displayed by the R3Si group in promoting high 1,2-asymmetric induction, clearly emerges in the
comparison of acylsilane 12 with the corresponding aldehyde 13. The sense of the Cram/anti-Cram selectivity depends upon the
nature of the carbonyl reagents engaged in these reactions. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Organomanganese; Addition; Acylsilanes; Asymmetric induction
to other organometallics have been highlighted [3] in
terms of high yields, no need of additives, high selectiv-
ity, smooth reaction conditions and ease of perfor-
1. Introduction
Organometallic compounds and metal-mediated
transformations are among the most powerful tools in
synthetic organic chemistry [1]. Addition of
organometallic compounds to aldehydes and ketones
has been extensively studied. Starting from classical
carbanions in the form of organolithium or Grignards a
number of other metals were likewise tested as
transmetalating agents, including copper, cerium, iron,
ytterbium, zinc [2a–d]. Also the organomanganese
chemistry has been studied extensively in the last 15
years [3] and these reagents, easily available from the
organolithium and magnesium counterparts, have been
engaged in coupling reactions with alkyl, vinyl and aryl
derivatives [4], in acylation [5] and 1,2- [3c,6] and
1,4-addition [7] reactions. Their advantages with respect
mance.
Asymmetric
induction
promoted
by
nucleophilic addition to chiral aldehydes and ketones is
a topic of great interest and has been used [8] for the
construction of many useful chiral synthons in natural
product synthesis. From the general point of view,
organomanganese reagents look like very promising to
perform such reactions since they are less basic and
much more sensitive to steric effects than other
organometallics [3c]. However until now there are very
few reports about their use in these reactions with
aldehydes [6,9] and to date only one paper has ap-
peared [10] on the addition to cycloalkyl ketones.
2. Results and discussion
* Corresponding author. Tel.: +39-51-2093635; fax: +39-51-
2093654.
E-mail address: ricci@ms.fci.unibo.it (A. Ricci).
Herein we report a throughout study on the 1,2
addition of organomanganese halides RMnCl and of
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00902-5

224
C. Boucley et al. / Journal of Organometallic Chemistry 624 (2001) 223–228
organomanganates R3MnMet (Met=Li, MgCl) to a-
chiral carbonyl compounds with the aim of studying
their effectiveness in the diastereoselective 1,2-addition.
Since acylsilanes are known to exhibit exceptional
diastereofacial preferences in nucleophilic additions
(1,2-asymmetric induction) and behave as aldehyde
equivalents via stereospecific protiodesilylation with F−
[11], we thought it worthwhile to investigate the addi-
tion reaction of organomanganese reagents to these
compounds and to compare their reaction behaviour
whenever possible to that of the corresponding alde-
hydes. The 1,2-addition to the carbonyl moiety of
several simple achiral acylsilanes is reported in Table 1.
The reactions proceeded smoothly to give the ex-
pected carbinols 3–6 in good yields, and organoman-
ganese halides (entries 1, 3, 5, 7) and
pivaloyloxyorganomanganates (entries 2, 4, 6, 8)
proved to be equally effective. The choice of these latter
reagents was dictated by previous observations [12]
suggesting high reactivity, and better propensity to
induce stereoselection of organomanganates with re-
spect to the organomanganese derivatives. Worth not-
ing even in the case of the acylsilane 2 (entries 5–8)
prone to undergo easily enolization, this side reaction
did not occur in view of the poorly basic character of
the organomanganese reagents.
Efficient delivery of the alkyl and allyl ligands to the
carbonyl moiety of 7 and 8 could be observed irrespec-
tive of the nature of the organomanganese reagent.
Nucleophilic addition to these compounds having no
ability to be chelated, produced predominantly, in full
agreement with previous findings [11], the syn-
diastereomer according to a Cram’s open-chain model.
As far as the stereochemical outcome, addition to the
a-chiral acylsilane 7 occurred (entries 1, 2, 4, 5, 7, 8)
with remarkable Cram selectivity, the diastereomeric
ratios (d.r.) ranging from 92:8 to \99:1. These results
are consistent with those obtained by Ohno [11] using
organolithium and Grignard reagents (entries 3, 6, 9).
Noteworthy are the ratios of the Cram:anti-Cram prod-
ucts found in the addition to the model a-chiral alde-
hyde 8. Whereas addition of n-BuLi (entry 13) [13] and
of n-BuMgBr (entry 12) [14] takes place with moderate
stereoselectivities typically in the d.r. range 89:11–
87:13, a significant increase of diastereoselectivity (97:3,
98:2) close to that observed in the case of 7, was found
(entries 10, 11) with organomanganese reagents. Again
in the case of methyl transfer from MeMnCl and
(PivO)2MeMnLi to 8, better Cram selectivities were
obtained compared to MeMgBr (entry 17). Interest-
ingly the best diastereoselectivity (94:6) obtained so far
by Reetz (entry 18) via ‘slow addition’ of a salt-free
MeLi solution [13] could be obtained by simply using
MeMnCl (entry 15) whereas with (PivO)2MeMnLi a
significant improvement of the d.r. (98:2) was observed
(entry 16). Even the presence of non transferable bulky
ligands at magnesium, though to some extent beneficial
[15], still led (entries 14, 19) to diastereoselectivities not
exceeding those obtained with the Mn-based reagents.
Finally only a slight increase in the diastereoselective
ratios could be noticed by using organomanganates.
According to the above results in the addition to a-chi-
ral carbonyl derivatives the use of even very simple
organomanganese reagents in most cases overcomes the
necessity of bulky ligands at the metal center and
renders less crucial the presence of a trialkylsilyl moiety
attached to the carbonyl function for increasing
diastereoselectivity. Only in the case of the more reac-
tive allylating organomanganese reagents (entries 20,
21) the presence of the trimethysilyl moiety is still
highly beneficial.
The high yields and the absence of enolization of
compound 2 prompted us to engage in these reactions a
racemic a-chiral acylsilane (7) and the corresponding
2-phenylpropanal (8), one of the carbonyl compounds
studied most extensively for investigation of 1,2-asym-
metric induction.
The relevant results are reported in Table 2 together
with the literature data concerning the nucleophilic
additions of organometallic reagents.
Table 1
Addition of organomanganese compounds to acylsilanes
Next we moved towards more structurally complex
chiral acylsilanes and aldehydes to investigate the
diastereoselective induction by organomanganese
derivatives in the case of carbonyl compounds with
multiple functional groups in which several chelate
structures are a priori feasible. The homochiral acylsi-
lane 12 derived from lactic acid [16], the corresponding
O–Bn protected lactic aldehyde 13 and glyceraldehyde
acetonide 14 containing alkoxy groups that are con-
strained conformationally by incorporation in a ring,
were subjected to nucleophilic additions of organoman-
ganese reagents.

C. Boucley et al. / Journal of Organometallic Chemistry 624 (2001) 223–228
225
Table 2
In comparison with 12, aldehyde 13 showed no
stereoselectivity, equimolar mixtures of the syn and of
the anti diastereomers being obtained under identical
reaction conditions. However this is not unprecedented
since it is known [19] that the diastereoselectivity in the
addition to 2-hydroxypropanal dramatically depends
on the protecting group of the hydroxyl functionality,
the reaction solvent and the steric demand of the
organometallic reagent so that both the syn or the anti
adducts can largely prevail. The differences in the d.r.
ratios between 12 and 13, highlight on the other hand
the crucial role played by the R3Si moiety in determin-
ing the stereochemical outcome. In the case of 2,3-O-
Addition of organometallic reagents to 7 and 8
isopropylideneglyceraldehyde (14),
a
synthetically
useful chiral starting material for the construction of
optically active organic molecules, prediction of the
stereochemistry of nucleophilic addition is very
difficult. Usually with organometallic compounds such
as Grignard, organolithium or organozinc reagents,
anti-addition products prevail [20] and the degree of
diastereoselectivity with few exceptions is not satisfac-
tory from a synthetic point of view. Addition of
organomanganese reagents to 14 occurred with high
yields and the degree of diastereoselectivity was consis-
tently better than that reported in the literature for
other organometallic reagents [21]. Thus allylation of
14 with organomanganese compounds and organoman-
ganates led (Table 3, entries 5, 6) to adduct 20 as a
20:80 mixture of the diastereomeric alcohols syn-20 and
anti-20, and 80% total yield. The diastereomeric ratio
was determined by isolation of each isomer with
column chromatography on silica gel and by compari-
1
son of their H- and 13C-NMR spectra with that pub-
lished [22]. In analogous way the stereochemical
outcome of the addition of the n-butyl ligand leading to
a 15:85 mixture of the diastereomeric alcohols syn-21
and anti-21, and 76% total yield was established after
separation by comparison with the literature data
(Table 3, entries 7, 8) [23]. The anti diastereoselectivity,
observed in the reaction of 12 with organomanganese
reagents may be ascribed to the predominance of the
Felkin–Ahn conformation A if one assumes that the
OBn group is the ‘large’ substituent at the asymmetric
a-carbon. The predominant formation of the anti-
diastereomeric reaction product observed in the case of
14 also amounts to application of Felkin’ s model for
asymmetric induction. Chelation to oxygen, which has
been proved to be effective in the case of manganese
derivatives [24] might be operational in this case in
stabilizing the b-chelate conformer B.
The allylation reaction performed on 12 led to for-
mation in good yields of the expected homoallylic
alcohols 16. Stereospecific desilylation followed by
column chromatography afforded separation of the two
diastereoisomers of 18 in a 66% combined yield. The
absolute configuration determined by comparison with
polarimetric and NMR data from the literature [17],
indicated that the anti-product was the major compo-
nent (syn:anti=25:75). Delivery of the n-butyl ligand
from both organomanganese halide and organoman-
ganate to acylsilane 12 afforded on the other hand,
after desilylation of 17 the two diastereoisomers of 19
which were separated in 60% combined yield. The
prevalence of the anti alcohol (syn:anti=10:90) was
proved again by comparison of the polarimetric and
1H-NMR data with those from the literature [18].

226
C. Boucley et al. / Journal of Organometallic Chemistry 624 (2001) 223–228
Table 3
Addition of organomanganese compounds to homochiral acylsilanes and aldehydes
3. Conclusions
dried (120°C) glassware under dry argon. Transfer of
anhydrous solvents or mixtures was accomplished with
the standard syringe/septum technique. THF was dis-
tilled just before use from benzophenone ketyl under
argon. CH2Cl2 was passed through basic alumina and
distilled from CaH2 just prior to use. Other solvents
were purified by standard procedures. Light petroleum
refers to the fraction bp 40–60°C. The reactions were
monitored by TLC on Baker-flex IB2-F silica gel plates.
Column chromatography was performed with Merck
70–230 mesh silica gel 60. Preparative TLC was carried
out on glass plates using a 1 mm layer of Merck silica
gel Pf254.1H- and 13C-NMR spectra were recorded using
CDCl3 solutions at 200, 300 MHz and 50.3, 75.4 MHz,
respectively, on a Varian Gemini 200 and a Varian
Gemini 300. Chemical shifts are reported in ppm rela-
In summary, this study demonstrates that the explo-
ration of the potential ability of organomanganese
reagents to be engaged in stereoselective additions can
be rewarding. The preliminary picture which evolves is
likely to be one of complementarity, i.e. organoman-
ganese reagents can favourably compete with other
organometallics in providing an useful tool for asym-
metric synthesis. Indeed more extensive work is neces-
sary under different reaction conditions, varying the
medium and the reaction temperature, and with differ-
ent organic reaction partners. These studies are cur-
rently underway.
1
4. Experimental
tive to CHCl3 (l 7.23 ppm for H and l 77.0 ppm for
13C). IR spectra were registered on a Perkin Elmer 257
and on a FT-IR Nicolet impact 400 spectrometers. Mass
spectra were recorded on a VG 7070E at a ionising
voltage of 70 eV. [h]D values were taken on a Perkin
4.1. General reaction conditions
Moisture sensitive reactions were carried out in oven-

C. Boucley et al. / Journal of Organometallic Chemistry 624 (2001) 223–228
227
Elmer 341 polarimeter. Diastereomeric excesses were
determined by gas chromatography with a chiral capil-
lary column (SGE-25QC2/cydex-B 0.25).
CPh), 125.0 (1C, CAr), 128.0 (2C, CHAr) ppm. GC-
MS (m/e): 236 [M+], 218 [M+−H2O], 117 [M+−
SiOC5H14], 104 (117−CH), 73 (SiMe3).
4.3.2. 1-Phenyl-3-(trimethylsilyl)-3-heptanol (5)
4.2. Representati6e procedure
1H-NMR (200 MHz, CDCl3) l: 0.20 (s, 9H,
Si(CH3)3), 1.00 (t, 3H, J=7.5 Hz, CH3), 1.28 (s, 1H,
OH), 1.34–1.48 (m, 4H, CH2CH2CH3), 1.68–1.79 (m,
2H, C(OH)CH2), 1.88–1.99 (m, 2H,CH2CH2Ph), 2.65–
2.85 (m, 2H, CH2Ph), 7.20–7.45 (m, 5H, CHAr) ppm.
13C-NMR (50.3 MHz, CDCl3) l: −2.60 (3C,
Si(CH3)3), 14.3 (1C, CH3), 23.7 (1C, CH2CH3), 26.2
(1C, CH2CH2CH3), 29.9 (1C, CH2Ph), 37.8 (1C,
C(OH)CH2), 39.9 (1C, PhCH2CH2C(OH)), 68.9 (1C,
C(OH)), 128.5, 128.4, 125.9 (5C, CHAr), 143.0 (1C,
CAr) ppm. GC-MS (m/e): 264 [M+], 246 [M+−H2O],
231 (246−CH3), 91 (C7H7), 73 (SiMe3).
4.2.1. Method A
To a well stirred solution of the ‘ate’ complex
MnCl.2LiCl, prepared from MnCl2 (189 mg, 1.5 mmol)
and LiCl (127 mg, 3.0 mmol) in anhydrous THF (3.6
ml), a 1.0 M solution of allylmagnesium bromide in
THF (1.5 ml, 1.5 mmol) was added at −30°C. The
reaction mixture was stirred for additional 20 min at
−10°C, than 1.0 mmol of the appropriate substrate in
THF (1 ml) was added at −60°C. The reaction mixture
was kept at −60°C for 30 min then left to reach room
temperature (r.t.) and after 3 h the disappearence of the
starting compounds was monitored by TLC. After dilu-
tion with ether (20 ml) the reaction mixture was
quenched with 1 M HCl. After separation of the or-
ganic phase and washing of the aqueous solution with
ether (2×15 ml) the organic phases were put together,
washed with water and dried over MgSO4. After re-
moving the organic solvent in vacuo, the expected
4.3.3. 1-Phenyl-3-(trimethylsilyl)-5-hexen-3-ol (6)
Colorless liquid, 1H-NMR (300 MHz, CDCl3) l:
0.15 (s, 9H, Si(CH3)3), 1.80–1.95 (m, 2H, PhCH2), 2.45
(d, 2H, J=7.41 Hz, CH2CHꢀCH2), 2.65–2.80 (m, 2H,
PhCH2CH2), 5.10–5.30 (m, 2H, CHꢀCH2), 5.75–6.00
(m, 1H, CHꢀCH2), 7.10–7.40 (m, 5H, CHAr) ppm.
13C-NMR (50.3 MHz, CDCl3) l: −2.5 (3C, Si(CH3)3),
30.4 (1C, CH2Ph), 40.5 (1C, CH2CH2Ph), 42.3 (1C,
CH2CHꢀCH), 68.1 (1C, C(OH)), 118.9 (1C, CHꢀCH2),
128.7, 128.6, 126.0 (5C, CHAr), 134.0 (1C, CHꢀCH2),
143.0 (1C, CAr) ppm. GC-MS (m/e): 231 [M+−H2O],
91 (C7H7), 73 (SiMe3).
product
was
purified
by
silica
gel
flash
chromatography.
4.2.2. Method B
To a well stirred solution of the ‘ate’ complex
MnCl.2LiCl, prepared from MnCl2 (317 mg, 2.52 mmol)
and LiCl (214 mg, 5.04 mmol) in anhydrous THF (7.0
ml), a 1.6 M solution of n-butyllithium in hexane (3.15
ml, 5.04 mmol) was added at −30°C. The reaction
mixture was stirred for additional 20 min at −10°C.
The temperature was lowered at −30°C and pivalic
acid (515 mg, 5.04 mmol) in THF (3.0 ml) was added.
After 10 min at −10°C the temperature was lowered at
−30°C and allylmagnesium bromide (2.52 ml, 2.52
mmol) was added. After further 20 min at −10°C, 1.0
mmol of the appropriate substrate in THF (1 ml) was
added at –60°C. The reaction mixture was kept at
−60°C for 30 min then left to reach r.t. and after 3 h
the disappearence of the starting compounds was mon-
itored by TLC. The reaction was quenched and the
product isolated following the above reported
procedure.
4.3.4. 2-(Benzyloxy)-3-[dimethyl(phenyl)silyl]-5
-hexen-3-ol (16)
Colorless liquid, [h]D=52.7 (c=2.00, CHCl3). H-
1
NMR (300 MHz, CDCl3) l: major diastereosiomer:
0.60 (s, 3H, CH3Si), 0.61 (s, 3H, CH3Si), 1.33 (d, 3H,
J=6.3 Hz, CH3–CH). Minor diastereoisomer: 0.62 (s,
3H, CH3Si), 0.64 (s, 3H, CH3Si), 1.36 (d, 3H, J=6.3
Hz, CH3–CH). The following signals were overlapped:
2.44 (bs, 2H, OH), 2.51–2.80 (m, 4H, CH2CHꢀ), 3.82
(q, 2H, J=6.3 Hz, CH–CH3), 4.50–4-91 (m, 4H,
CH2O), 5.19–5.29 (m, 4H, 2CH2ꢀ), 5.95–6.24 (m, 2H,
CHꢀ), 7.41–7.82 (m, 20H, Ar–H) ppm. 13C-NMR
(50.3 MHz, CDCl3) l: major diastereoisomer: −3.71
(CH3Si), −3.43 (CH3Si), 79.47 (CH–CH3). Minor
diastereoisomer: −3.27 (CH3Si), −3.11 (CH3Si), 81.18
(CH–CH3). The following signals were overlapped:
13.41 (CH3), 38.85 (CH2CHꢀ), 70.86 (OCH2), 71.14
(CIV), 111.11 (CH2ꢀ), 127.36–134.25 (6 ArCH), 134.65
(CHꢀ), 137.37 (ArCIV), 138.47 (ArCIV), ppm. MS (m/
e): 325 [M+−CH3], 249 [M+−CH2Ph], 135
(SiMe2Ph), 91 (CH2Ph). After desilylation with TBAF
in THF at r.t. for 24 h and separation by column
chromatography on silica gel (eluent CH2Cl2) the two
diastereoisomers syn-18 and anti-18 were obtained
whose analytical and spectra data were consistent with
literature data [17].
4.3. Characterization of representati6e products
4.3.1. 1-Phenyl-1-(trimethylsilyl)-1-pentanol (3)
1H-NMR (200 MHz, CDCl3) l: 0.00 (s, 9H,
Si(CH3)3), 0.90 (t, 3H, J=7.5 Hz, CH3), 1.20–2.10 (m,
6H, 3CH2), 7.10–7.45 (m, 5H, CH–Ar) ppm. 13C-
NMR (50.3 MHz, CDCl3) l: −4.0 (3C, Si(CH3)3),
12.8 (3C, CHAr), 14.2 (1C, CH3), 23.3 (1C, CH2CH3),
24.0 (1C, CCH2CH2), 36.5 (1C, CCH2), 72.8 (1C,

228
C. Boucley et al. / Journal of Organometallic Chemistry 624 (2001) 223–228
Synthesis, vol. 5, Wiley, Chichester, 1995, p. 3227. (c) G. Cahiez,
Anal. Quim. 91 (1995) 561.
4.3.5. 2-(Benzyloxy)-3-[dimethyl(phenyl)silyl]-3-
heptanol (17)
[4] (a) G. Cahiez, S. Marquais, Synlett (1993) 45. (b) G. Cahiez, S.
Marquais, Pure Appl. Chem. 68 (1996) 669. (c) G. Cahiez, S.
Marquais, Tetrahedron Lett. 37 (1996) 1773. (d) E. Riguet, M.
Alami, G. Cahiez, Tetrahedron Lett. 38 (1997) 4397.
[5] (a) G. Cahiez, Tetrahedron Lett. 22 (1981) 1239. (b) G. Cahiez,
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Chavant, E. Me´tais, Tetrahedron Lett. 33 (1992) 5245. (d) G.
Cahiez, E. Me´tais, Tetrahedron Lett. 36 (1995) 649. (e) G.
Cahiez, L. Razafintsalama, B. Laboue, K. Chau, Tetrahedron
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Asymmetry 8 (1996) 1373.
[6] G. Cahiez, B. Figade`re, Tetrahedron Lett. 27 (1986) 4445.
[7] (a) G. Cahiez, M. Alami, Tetrahedron Lett. 27 (1986) 569. (b) G.
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Marquais, M. Alami, Org. Synth. 72 (1993) 135.
[8] G. Procter, Asymmetric Synthesis, Oxford University Press Inc,
New York, 1996, pp. 14–40.
Obtained as a colorless liquid after column chro-
matography on silica gel (eluent petroleum/diethyl
ether, 6:1), [h]D=15.5 (c=1.00, CHCl3). 1H-NMR
(200 MHz, CDCl3) l: 0.60–0.80 (t, 3H, J=7.0 Hz,
CH3CH2), 1.10–1.35 (m+s, 7H, CH2CH2CH2+OH),
1.30 (d, J=6.8 Hz, 3H, CH3–CH), 4.10 (q, 1H, J=6.8
Hz, CH–CH3), 4.60 (d, J=11.4 Hz, 1H, OCH2Ph),
4.70 (d, J=11.4 Hz, 1H, OCH2Ph), 7.20–7.60 (m, 10H,
ArH) ppm. 13C-NMR (50.3 MHz, CDCl3) l: −0.41
(CH3Si), −0.45 (CH3Si), 13.30 (CH3), 13.68 (CH3),
22.80 (CH2), 23.60 (CH2), 33.50 (CH2), 71.10
(OCH2Ph), 73.00 (CIV), 78.80 (CHO), 134.10 (ArCIV),
137.60 (ArCIV), 126.20–135.20 (6 ArCH) ppm. MS
(m/e): 356 [M+], 341 [M+−CH3], 297 [M+−Ph], 221
[M+−SiMe2Ph], 135 (SiMe2Ph), 91 (CH2Ph). After
desilylation as for 16, and purification by column chro-
matography on silica gel (eluent CH2Cl2) the anti-19
was obtained whose analytical and spectra data were
consistent with literature data [18].
[9] M.T. Reetz, K. Ro¨fling, N. Griebenov, Tetrahedron Lett. 35
(1994) 1969.
[10] M.T. Reetz, H. Haning, S. Stanchev, Tetrahedron Lett. 33
(1992) 6963.
[11] M. Nakada, Y. Urano, S. Kobayashi, M. Ohno, J. Am. Chem.
Soc. 110 (1988) 4826.
[12] G. Cahiez, C. Boucley (in press).
Acknowledgements
[13] M.T. Reetz, S. Stanchev, H. Haning, Tetrahedron 48 (1992)
6813.
[14] K. Maruoka, T. Itoh, M. Sakurai, K. Nonoshita, H. Yamamoto,
J. Am. Chem. Soc. 110 (1988) 3588.
[15] M.T. Reetz, N. Harmat, R. Mahrwald, Angew. Chem. Int. Ed.
Engl. 31 (1992) 342.
[16] B.F. Bonini, M. Comes-Franchini, M. Fochi, G. Mazzanti, A.
Ricci, G. Varchi, Synlett (2000) 1688.
This work has been supported by University of
Bologna (Funds for Selected Research Topics, A.A.
1999–2001), by the MURST ex-60% e.f. 2000, and by
the National Project Stereoselezione in Sintesi Organica
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