Palladium-catalyzed allylic alkylation using pyridino-oxazolines and quinolino-oxazolines as ligands - Influence of steric factors

Article abstract of DOI:10.1016/S0957-4166(98)00346-2

Four new chiral pyridino- and quinolino-oxazolines were subjected to the palladium-catalyzed alkylation of 1,3-diphenyl-2-propenyl acetate. The enantioselectivity varied (82-88% ee) with the steric properties of the ligands. The results are discussed in connection with results previously obtained using analogous ligands.

Full text of DOI:10.1016/S0957-4166(98)00346-2

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 3437–3443
Palladium-catalyzed allylic alkylation using pyridino-oxazolines
and quinolino-oxazolines as ligands — influence of steric factors
Ulf Bremberg, Fredrik Rahm and Christina Moberg ∗
Department of Chemistry, Organic Chemistry, Royal Institute of Technology, SE 100 44 Stockholm, Sweden
Received 4 August 1998; accepted 27 August 1998
Abstract
Four new chiral pyridino- and quinolino-oxazolines were subjected to the palladium-catalyzed alkylation of 1,3-
diphenyl-2-propenyl acetate. The enantioselectivity varied (82–88% ee) with the steric properties of the ligands.
The results are discussed in connection with results previously obtained using analogous ligands. © 1998 Elsevier
Science Ltd. All rights reserved.
1. Introduction
Asymmetric metal catalysis constitutes one of the most versatile methods for the preparation of chiral
compounds in enantiomerically pure form.1 In order to enable rational design of the metal complexes
responsible for the catalytic activity, a profound understanding of the factors governing the catalytic
process is required.
The palladium-catalyzed allylic alkylation reaction is a particularly useful catalytic process in that a
carbon–carbon bond is formed in the reaction.2 A variety of P,P-, P,N- and N,N-based ligands, with C1
3
as well as C2 symmetry,4 have proven to induce high selectivity in the process.
Pyridino-oxazolines have been used with varying success as ligands in the reaction of rac-1,3-
diphenyl-2-propenyl acetate 1 with dimethyl malonate to yield 2 (Scheme 1). With (40R)-2-(40,50-
dihydro-40-phenyl-20-oxazolyl)pyridine 3a, only moderate stereoselectivity (50–55% ee of the product
with R configuration) was observed.5,6 We and others have shown that modification of the structure of
the ligand may have a profound effect on the outcome of the reaction. Thus, the introduction of a methyl
group in the 6-position of the pyridine ring 3b had the effect of increasing the ee to 74%.6 Dramatic
effects were observed with hydroxyalkyl and methoxyalkyl substituents (compounds 3c–3f, Scheme 1).5
Formation of the product with R configuration was always favored, but the selectivity varied (15–>99%
ee) with the structure and absolute configuration of the substituent.
∗
Corresponding author. E-mail: kimo@orgchem.kth.se
0957-4166/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00346-2

3438
U. Bremberg et al. / Tetrahedron: Asymmetry 9 (1998) 3437–3443
Scheme 1.
In order to gain a deeper insight into the factors responsible for the stereoselectivity of the reaction,
some new pyridino-oxazolines and quinolino-oxazolines have been prepared and subjected to the
catalytic reaction.
2. Results and discussion
2.1. Preparation of ligands
2-Oxazolinylquinolines 3g and 3h were prepared from (R)-2-phenylglycinol and 2-cyanoquinolines
4 and 5,7,8 respectively, using copper chloride as catalyst† (Scheme 2). The pyridine derivative 3i was
synthesized starting from 2-(hydroxymethyl)pyridine 6 via nitrile 7, which was subsequently transformed
to the target oxazoline via the process described above. Attempts to O-methylate 3i using either sodium
hydride and methyl iodide in THF or potassium hydroxide and methyl iodide in DMSO9 resulted in low
yields of the desired product 3j. Therefore, a route for the preparation of 3j involving methylation of
7 (to give 8) and subsequent introduction of the oxazoline ring was preferred. In this case, a sequence
involving initial reaction of the nitrile with methoxide and subsequent transformation of the obtained
imidate to an oxazoline was found to be more convenient and to result in a higher yield than the copper
chloride-catalyzed process.
Scheme 2. (a) (R)-Phenylglycinol, CuCl2, neat 100°C, 10 mbar; (b) m-CPBA, CHCl3, rt, 24 h; (c) TMSCN,
N,N-dimethylcarbamoyl chloride, CH2Cl2, reflux, 37 h; (d) NaH, MeI, THF, rt, 16 h; (e) (1) NaOMe, MeOH, rt, 28 h; (2)
(R)-phenylglycinol, H2SO4 (cat.), CH2Cl2, reflux, 24 h
†
Problems were encountered during attempts to transform the nitrile 5 to an oxazoline with standard methods. The rarely used
catalyst CuCl2 (Breslow, R.; Schmir, M. J. Am. Chem. Soc. 1971, 93, 4960–4961) worked satisfactorily though.

U. Bremberg et al. / Tetrahedron: Asymmetry 9 (1998) 3437–3443
3439
2.2. Palladium-catalyzed allylic alkylation
Allylic substitution of rac-1,3-diphenyl-2-propenyl acetate 1 was performed in dichloromethane at
room temperature in the presence of a π-allylpalladium–ligand complex generated in situ from 2 mol%
of bis[η3-allylpalladium chloride] and 6 mol% of the appropriate ligand. The nucleophile was generated
from dimethyl malonate in the presence of N,O-bis(trimethylsilyl)acetamide (BSA) and a catalytic
amount of KOAc.4b The chemical yields were high when 3g, 3i and 3j were employed as ligands, (99,
93 and 99% of 2, respectively), while no product was obtained upon use of the hydroxy derivative 3h.
The product with R configuration dominated in the reactions. The selectivity observed when quinoline
compound 3g was employed as ligand was higher (73% ee) than that observed with the corresponding
pyridine ligand 3a (50% ee). Catalytic reactions with the pyridine ligands 3i and 3j were also more
selective than those employing 3a, affording the product with 88 and 82% ee, respectively.
2.3. Stereochemistry of the reaction and conformation of the ligands
With C1-symmetric ligands, such as 3a–3j, two syn,syn-π-allylpalladium complexes may form and
the two complexes are usually also observed (together with small amounts of the syn,anti isomers). The
site of nucleophilic attack on each complex determines the absolute configuration of the product. Since
equilibration between the isomeric complexes is rapid compared to the attack of the nucleophile on the
complex, the ratio of the complexes does not directly reflect the ratio of the products.10 What is usually
observed, however, is that product formation from the most abundant complex is preferred.
The regiochemistry of attack at each complex is influenced by electronic as well as steric factors.11
The bond lengths between the carbon atoms of the allylic termini and palladium are influenced by the
ligand trans to the carbon atom (trans influence) as well as by steric effects exerted by the ligand, and a
longer bond to palladium will result in higher reactivity.12 With bidentate ligands containing donor atoms
with similar properties, steric factors are assumed to be most important.
Due to hydrogen bonding, pyridine ligands with a hydroxymethyl group in 2-position of the pyridine
ring (such as 3c and 3d) adopt a conformation in which the carbon–oxygen bond of the substituent
is situated in the plane of the pyridine ring, with the oxygen atom syn to nitrogen. In their most
stable conformation, ligands containing a methoxymethyl group (such as 3e and 3f) also have the
carbon–oxygen bond of the substituent close to the plane of the pyridine ring, but with the oxygen atom
anti to nitrogen.13 We have shown that the ligands also adopt similar conformations in their complexes
with palladium, thereby permitting the enantioselectivity of ligands 3c–3f to be rationalized. It should be
noted that since no product was formed in the presence of ligand 3h where the hydroxy group is forced
to be in the plane of the heterocyclic ring, ligands 3c, 3d and 3i probably adopt conformations where the
carbon–oxygen bond is close to but not exactly in that plane.
The enantioselectivity observed for pyridino-oxazolines containing 1-hydroxyalkyl substituents in the
6-position of the pyridine ring varied only slightly with the structure of the substituent (88% ee for
R=R0=H, 95% ee for R=H, R0=tBu, and 90% ee for R=tBu, R0=H, A, Fig. 1). The reason for this is
probably that in the preferred conformation of the ligand, the R and R0 groups are remote from the allyl
group and therefore only have a minor influence on its position. However, dramatic effects were observed
for ligands with a 1-methoxyalkyl substituent on the pyridine ring (82% ee for R=R0=H, 15% ee for R=H,
R0=tBu, and >99% ee for R=tBu, R0=H, B, Fig. 1). In these ligands the R and R0 groups are close to the
allyl group and thus exert an important influence on its position. It is also interesting to note that a ligand
with a hydroxymethyl substituent in the 6-position of the pyridine ring 3i results in higher selectivity

3440
U. Bremberg et al. / Tetrahedron: Asymmetry 9 (1998) 3437–3443
Fig. 1. Conformation of ligands 3i, 3c and 3d (A) and of 3j, 3e and 3f (B)
(88% ee) than ligands with a methoxymethyl (82%) or a methyl group (74% ee) in that position, the
latter with a selectivity similar to that of the quinolino-oxazoline (73% ee).
3. Conclusions
The enantioselectivity of the palladium-catalyzed alkylation of rac-1,3-diphenyl-2-propenyl acetate
with dimethyl malonate using chiral pyridino-oxazolines and quinolino-oxazolines as ligands is highly
dependent on the structure of the ligands. The substituent in the 6-position of the pyridine ring of the
pyridino-oxazolines has a particularly large influence on the selectivity. The results can be rationalized
considering the conformation of the ligand in the reactive palladium complexes.
4. Experimental
1H and 13C NMR spectra were recorded at 400 and 100.6 MHz, respectively, in CDCl3. THF was
distilled from sodium/benzophenone.
4.1. 2-Cyano-6-(hydroxymethyl)pyridine 7
m-CPBA (34.50 g, 55%, 110 mmol) was added to 2-(hydroxymethyl)pyridine (10.00 g, 91.6 mmol) in
chloroform (150 mL) at 0°C during 30 min. The suspension was stirred at rt for 24 h. Residual m-CPBA
was destroyed by the addition of paraformaldehyde (2.0 g, 66.6 mmol CH2O equivalents). After stirring
for 2 h, ammonia was bubbled through the reaction mixture for 10 min. The thick suspension was dried
(Na2SO4) and filtered and the filter cake washed with dichloromethane (250 mL). The solid residue was
continuously extracted with the filtrate for 63 h. Evaporation of the solvent yielded 11.24 g (98%) of
2-(hydroxymethyl)pyridine-N-oxide. 1H NMR δ 8.25 (1H, d, J=6.1 Hz, H6), 7.36–7.26 (3H, m, H3, H4,
H5), 4.80 (2H, s, CH2). The signal for the hydroxy proton was not observed.
The N-oxide (1.255 g, 10.03 mmol) was partially dissolved in dichloromethane (25 mL) and N,N-
dimethylcarbamoyl chloride (923 µL, 10.03 mmol) was added over 5 min. After stirring for 90 min,
more dichloromethane (15 mL) was added and the mixture was heated to reflux for 90 min. The reaction
mixture was allowed to cool to rt, whereafter trimethylsilyl cyanide (1.61 mL, 12.04 mmol) was added
dropwise, and the mixture was heated to reflux for a further 14 h. The mixture was allowed to cool to
rt before additional N,N-dimethylcarbamoyl chloride (923 µL, 10.03 mmol) and trimethylsilyl cyanide
(1.34 mL, 10.03 mmol) were added. After 22 h at reflux, the reaction was quenched by the addition of
saturated aqueous Na2CO3 (25 mL). The phases were separated, the aqueous phase was extracted with
dichloromethane (3×25 mL) and the combined organic phases were dried (MgSO4). Evaporation of the
solvent gave a brown liquid which was purified by chromatography on silica gel (8×4 cm column, ethyl
acetate:hexanes=1:1, followed by ethyl acetate) to give the expected product 7 (307 mg, 23%) and the
corresponding silylated alcohol (826 mg, 40%). The silylated alcohol was deprotected using aqueous HF
in acetonitrile14 and purified by chromatography with a smooth exponential gradient of ethyl acetate in

U. Bremberg et al. / Tetrahedron: Asymmetry 9 (1998) 3437–3443
3441
1
hexanes to yield 7 (365 mg, 68%). Total yield: 672 mg, 50%. H NMR δ 7.85 (1H, t, J=7.8 Hz, H4),
7.63 (1H, d, J=7.6 Hz, H3), 7.55 (1H, d, J=7.6 Hz, H5), 4.83 (2H, d, J=5.2 Hz, CH2), 3.12 (1H, t, J=5.5
Hz, OH).
4.2. 2-Cyano-6-(methoxymethyl)pyridine 8
Nitrile 7 (100 mg, 0.15 mmol) in THF (5 mL) was added to NaH (35.8 mg, 60% in mineral oil,
0.89 mmol, washed with dry hexane) in THF (5 mL) at 0°C under nitrogen. The mixture was stirred
for several min, whereafter MeI (74.3 µL, 1.2 mmol) was added dropwise. A drying tube was fitted,
and the mixture was stirred overnight on the thawing ice bath. The mixture was diluted with diethyl
ether (25 mL) and the organic phase washed with water followed by saturated aqueous NH4Cl. The
aqueous phases were extracted with diethyl ether (2×25 mL), and the combined organic phases were
dried (Na2SO4). Evaporation of the solvent gave a yellow solid which, after chromatography on silica
1
gel (8×3 cm column, ethyl acetate:hexanes=3:7), yielded 53 mg (48%) of 8. H NMR δ 7.85 (1H, t,
J=7.8 Hz, H4), 7.68 (1H, d, J=7.7 Hz, H3), 7.60 (1H, d, J=7.3 Hz, H5), 4.61 (2H, s, CH2), 3.49 (3H, s,
OCH3).
4.3. General procedure for oxazoline synthesis
The nitrile (1 equiv.) was mixed with (R)-phenylglycinol (3g, 3h: 1.5 equiv.; 3i: 2.0 equiv.) and CuCl2
(0.05 equiv.) under dry conditions. The neat reaction mixture was stirred at 100°C at reduced pressure
(10 mbar)‡ for a time depending on the individual nitriles. Sublimed amino alcohol occasionally had to
be scraped down into the reaction mixture from vessel walls. Continuous extraction with hexanes for at
least 48 h, followed by chromatography on silica gel with ethyl acetate (for 3i) or a smooth exponential
gradient of ethyl acetate in hexanes (for 3g and 3h), yielded the desired oxazoline.
4.3.1. (40R)-2-(40,50-Dihydro-40-phenyl-20-oxazolyl)quinoline 3g
Yield: 82%; powder, mp 140.5–142.5°C (from hexanes:ethyl acetate:benzene); [α]20 +160 (c 0.74,
D
CHCl3); 1H NMR δ 8.29 (2H, bd, J=8.5 Hz, H4, H8), 8.24 (1H, dd, J=8.6, 0.9 Hz, H3), 7.86 (1H, ddd,
J=8.2, 1.6, 0.7 Hz, H5), 7.77 (1H, ddd, J=8.5, 6.8, 1.6 Hz, H7), 7.61 (1H, ddd, J=8.1, 6.9, 1.2 Hz, H6),
7.26–7.40 (5H, m, arom. H), 5.52 (1H, dd, J=10.2, 8.7 Hz, H50), 4.99 (1H, dd, J=10.4, 8.5 Hz, H50), 4.48
(1H, t, J=8.6 Hz, H40); 13C NMR δ 164.2, 147.7, 146.8, 141.8, 136.9, 130.5, 130.2, 128.9 (2C), 128.1,
127.9, 127.6, 126.9, 121.1, 75.7, 70.5; Anal. calcd for C18H14N2O: C, 78.81; H, 5.14; N, 10.21. Found:
C, 78.56; H, 5.16; N, 10.14.
4.3.2. (40R)-2-(40,50-Dihydro-40-phenyl-20-oxazolyl)-8-hydroxyquinoline 3h
Yield: 55%; powder, mp 132.5–135°C (from hexanes); [α]20 −102 (c 0.53, CHCl3); 1H NMR δ 8.35
D
(1H, s, OH), 8.26 (1H, d, J=8.5 Hz, H3 or H4), 8.25 (1H, d, J=8.5 Hz, H3 or H4), 7.54 (1H, dd, J=8.2,
7.6 Hz, H6), 7.28–7.42 (6H, m, arom. H, H5), 7.24 (1H, dd, J=7.6, 1.2 Hz, H7), 5.52 (1H, dd, J=10.1,
8.5 Hz, H50), 4.95 (1H, dd, J=10.4, 8.5 Hz, H50), 4.44 (1H, t, J=8.5, H40); 13C NMR δ 164.0, 153.0,
144.3, 141.7, 137.8, 136.9, 129.6, 129.3, 128.9, 127.9, 126.9, 121.7, 117.8, 111.0, 75.5, 70.5; Anal. calcd
for C18H14N2O2: C, 74.47; H, 4.86; N, 9.65. Found: C, 74.30; H, 4.76; N, 9.60.
‡
Serves to remove the NH3 evolved.

3442
U. Bremberg et al. / Tetrahedron: Asymmetry 9 (1998) 3437–3443
4.3.3. (40R)-2-(40,50-Dihydro-40-phenyl-20-oxazolyl)-6-(hydroxymethyl)pyridine 3i
Yield: 44%, 95% pure (contaminated by 6) according to NMR. An analytically pure sample was
obtained by recrystallization from diethyl ether at −78°C: oil at rt, [α]20 +34 (c 0.86, CHCl3); 1H NMR
D
δ 8.00 (1H, d, J=7.6 Hz, H3), 7.76 (1H, t, J=7.6 Hz, H4), 7.48 (1H, d, J=7.6 Hz, H5), 7.27–7.35 (5H, m,
arom. H), 5.41 (1H, dd, J=10.2, 8.7 Hz, H50), 4.86 (1H, dd, J=10.2, 8.7 Hz, H50), 4.83 (2H, s, CH2), 4.36
(1H, t, J=8.6 Hz, H40). The signal for the hydroxy proton was not observed. 13C NMR δ 163.9, 160.3,
145.7, 141.8, 137.4, 128.9, 127.8, 126.8, 123.0, 122.9, 75.5, 70.3, 64.8; Anal. calcd for C15H14O2N2: C,
70.9; H, 5.5; N, 11.0. Found C, 70.61; H, 5.40; N, 10.86.
4.3.4. (40R)-2-(40,50-Dihydro-40-phenyl-20-oxazolyl)-6-(methoxymethyl)pyridine 3j
NaOMe (2.8 mg, 0.05 mmol) was added to a solution of 8 (76 mg, 0.52 mmol) in MeOH (2 mL).
The mixture was stirred under nitrogen at rt for 19 h, whereafter more NaOMe (2.8 mg, 0.05 mmol)
was added and the mixture was stirred for a further 10 h. The reaction was quenched by addition of
acetic acid (6 µL, 0.1 mmol) and the solvent was evaporated. The intermediate imidate was dried in
vacuo overnight and used without further purification. To a solution of the imidate in dichloromethane (2
mL), (R)-phenylglycinol (71 mg, 0.52 mmol) in dichloromethane (2 mL) and one drop of concentrated
H2SO4 were added and the mixture was stirred at reflux for 3 days. The suspension was then diluted
with dichloromethane (6 mL) and washed with saturated aqueous Na2CO3 (8 mL). The aqueous phase
was extracted with dichloromethane (3×8 mL) and the combined organic layers were dried (Na2SO4).
Evaporation of the solvent gave a yellow liquid which was purified by chromatography on silica gel
(6×2.5 cm column, ethyl acetate:hexanes=4:1, followed by ethyl acetate) to give the expected product 3j
(55 mg, 40%) as a colorless oil. [α]20 +70 (c 1.14, CHCl3); 1H NMR δ 8.08 (1H, d, J=7.9 Hz, H3), 7.82
D
(1H, t, J=7.9 Hz, H4), 7.61 (1H, d, J=7.9 Hz, H5), 7.27–7.38 (5H, m, arom. H), 5.44 (1H, dd, J=10.1,
8.8 Hz, H50), 4.90 (1H, dd, J=10.2, 8.7 Hz, H50), 4.70 (2H, s, CH2), 4.39 (1H, t, J=8.6 Hz, H40), 3.49
(3H, s, OCH3). 13C NMR δ 163.9, 159.2, 146.0, 141.9, 137.3, 128.8, 127.8, 126.9, 123.2, 123.0, 75.5,
75.3, 70.4, 58.9.
4.4. General procedure for the catalytic reaction
Ligand (0.029 mmol, 6 mol%) and [PdCl(η3-C3H5)]2 (3.5 mg, 0.0096 mmol, 2 mol%) were dissolved
in dichloromethane (1 mL) under dry conditions. The solution was degassed at −78°C for 15 min. The
vessel was sealed under nitrogen and the solution stirred at 50°C for 2 h. KOAc (2–3 mg, ∼0.025 mmol,
∼5 mol%), N,O-bis(trimethylsilyl)acetamide (292 mg, 1.44 mmol), dimethyl malonate (142 mg, 1.07
mmol), and 1,3-diphenyl-2-propenyl acetate (121 mg, 0.480 mmol) were transferred to the reaction vessel
with dichloromethane (1 mL) at −78°C. After degassing as before the solution was stirred for 4 days
under nitrogen at rt. Saturated aqueous NH4Cl was added and the mixture was extracted with diethyl
ether. The combined organic phases were dried (MgSO4) and filtered, and the solvent was evaporated.
1
Residual starting materials were removed with short path distillation. The ee was analyzed with H
NMR using Eu(hfc)3 and/or HPLC using a chiral column: Daicel Chiralcel OD-H (0.46 cm œ×25 cm),
n-hexane:2-propanol (99:1), 0.5 mL/min as the mobile phase, UV-monitor (254 nm).
Acknowledgements
This work was supported by the Swedish Natural Science Research Council and by the Swedish
Research Council for Engineering Sciences.

U. Bremberg et al. / Tetrahedron: Asymmetry 9 (1998) 3437–3443
3443
References
1. (a) Noyori, R. Asymmetric Catalysis in Organic Synthesis; Wiley: New York, 1994. (b) Seyden-Penne, J. Chiral Auxiliaries
and Ligands in Asymmetric Synthesis; Wiley: New York, 1995. (c) Ojima, I. Catalytic Asymmetric Synthesis; VCH: New
York, 1993.
2. (a) Trost, B. M.; Strege, P. E. J. Am. Chem. Soc. 1977, 99, 1649–1651. (b) Consiglio, G.; Waymouth, R. M. Chem. Rev.
1989, 89, 257–276. (c) Frost, C. G.; Howarth, J.; Williams, J. M. J. Tetrahedron: Asymmetry 1992, 3, 1089–1122. (d)
Reiser, O. Angew. Chem., Int. Ed. Engl. 1993, 32, 547–549. (e) Hayashi, T. In Catalytic Asymmetric Synthesis; Ojima, I.,
Ed.; VCH: New York, 1993; pp. 325–365. (f) Pfaltz, A. In Stereoselective Synthesis; Ottow, E.; Schöllkopf, K.; Schulz,
B.-G., Eds.; Springer-Verlag: Berlin, 1993; pp. 15–36. (g) Trost, B. M.; Van Vranken, D. L. Chem. Rev. 1996, 96, 395–422.
3. (a) Togni, A. Tetrahedron: Asymmetry 1991, 2, 683–690. (b) von Matt, P.; Pfaltz, A. Angew. Chem., Int. Ed. Engl. 1993, 32,
566–568. (c) Sprinz, J.; Helmchen, G. Tetrahedron Lett. 1993, 34, 1769–1772. (d) Dawson, G. J.; Frost, C. G.; Williams, J.
M. J.; Coote, S. J. Tetrahedron Lett. 1993, 34, 3149–3150. (e) Allen, J. V.; Coote, S. J.; Dawson, G. J.; Frost, C. G.; Martin,
C. J.; Williams, J. M. J. J. Chem. Soc., Perkin Trans. 1 1994, 2065–2072. (f) Kubota, H.; Koga, K. Tetrahedron Lett. 1994,
35, 6689–6692. (g) Wimmer, P.; Widhalm, M. Tetrahedron: Asymmetry 1995, 6, 657–660. (h) Valk, J.-M.; Claridge, T. D.
W.; Brown, J. M. Tetrahedron: Asymmetry 1995, 6, 2597–2610. (i) Ahn, K. H.; Cho, C.-W.; Park, J.; Lee, S. Tetrahedron:
Asymmetry 1997, 8, 1179–1185. (j) Achiwa, I.; Yamazaki, A.; Achiwa, K. Synlett 1998, 45–46. (k) Gläser, B.; Kunz, H.
Synlett 1998, 53–54.
4. (a) Müller, D.; Umbricht, U.; Weber, B.; Pfaltz, A. Helv. Chim. Acta 1991, 74, 232–240. (b) Leutenegger, U.; Umbricht,
G.; Fahrni, C.; von Matt, P.; Pfaltz, A. Tetrahedron 1992, 48, 2143–2156. (c) Trost, B. M.; Van Vranken, D. L. Angew.
Chem., Int. Ed. Engl. 1992, 31, 228–230. (d) Kubota, H.; Nakajima, M.; Koga, K. Tetrahedron Lett. 1993, 34, 8135–8138.
(e) Kang, J.; Cho, W. O.; Cho, H. G. Tetrahedron: Asymmetry 1994, 5, 1347–1352. (f) Trost, B. M.; Bunt, R. C. J. Am.
Chem. Soc. 1994, 116, 4089–4090. (g) Trost, B. M.; Breit, B.; Peukert, S.; Zambrano, J.; Ziller, J. W. Angew. Chem., Int.
Ed. Engl. 1995, 34, 2386–2388. (h) von Matt, P.; Lloyd-Jones, G. C.; Minidis, A. B. E.; Pfaltz, A.; Macko, L.; Neuburger,
M.; Zehnder, M.; Rüegger, H.; Pregosin, P. S. Helv. Chim. Acta 1995, 78, 265–284. (i) Gamez, P.; Dunjic, B.; Fache, F.;
Lemaire, M. Tetrahedron: Asymmetry 1995, 6, 1109–1116. (j) Andersson, P. G.; Harden, A.; Tanner, D.; Norrby, P.-O.
Chem. Eur. J. 1995, 1, 12–16. (k) Zhang, W.; Hirao, T.; Ikeda, I. Tetrahedron Lett. 1996, 37, 4545–4548. (l) Chelucci, G.
Tetrahedron: Asymmetry 1997, 8, 2667–2670.
5. Nordström, K.; Macedo, E.; Moberg, C. J. Org. Chem. 1997, 62, 1604–1609.
6. Chelucci, G.; Medici, S.; Saba, A. Tetrahedron: Asymmetry 1997, 8, 3183–3184.
7. Krasavin, I. A.; Dziomko, V. M.; Radin, Y. P. Metody Policheniya Khim, Reactinov i Preparatov 1965, 13, 68–94; Chem.
Abstr. 1966, 65, 5437b; 7139a.
8. Shrader, W. D.; Celebuski, J.; Kline, S. J.; Johnson, D. Tetrahedron Lett. 1988, 29, 1351–1354.
9. Johnstone, R. A. W.; Rose, M. E. Tetrahedron 1979, 35, 2169–2173.
10. Mackenzie, P. B.; Whelan, J.; Bosnich, B. J. Am. Chem. Soc. 1985, 107, 2046–2054.
11. Blöchl, P. E.; Togni, A. Organometallics 1996, 15, 4125–4132.
12. Oslob, J. D.; Åkermark, B.; Helquist, P.; Norrby, P.-O. Organometallics 1997, 16, 3015–3021.
13. Moberg, C.; Adolfsson, H.; Wärnmark, K.; Norrby, P.-O.; Marstokk, K.-M.; Møllendal, H. Chem. Eur. J. 1996, 2, 136–142.
14. Mascareñas, J. L.; Mouriño, A.; Castedo, L. J. Org. Chem. 1986, 51, 1269–1272.