Involvement of an oxidation-reduction equilibrium in chromium-mediated enantioselective Nozaki-Hiyama reactions

Article abstract of DOI:10.1002/adsc.200404021

Ligand induced enantioselective versions of the chromium(II)-mediated Nozaki-Hiyama reaction to homoallyl alcohols proved to be very difficult to achieve, especially if any other nucleophile than the parent allylchromium(III) species was applied. Also, the reaction is frequently accompanied by the formation of oxidation side products, predominantly allyl ketones. This can be explained by an Oppenauer-(Meerwein-Ponndorf-Verley) type mechanism (OMPV reaction). The addition of an enantiopure ligand to racemic chromium homoallyl alcoholate intermediates produced enantiomerically enriched homoallyl alcohols with an enantiomeric excess of up to 32%. This observation not only supports that the proposed OMPV oxidation-reduction equilibrium plays a crucial role in Nozaki-Hiyama reactions, but also proves its involvement in enantioselective versions.

Full text of DOI:10.1002/adsc.200404021

COMMUNICATIONS
Involvement of an Oxidation-Reduction Equilibrium in
Chromium-Mediated Enantioselective Nozaki±Hiyama Reactions
a
b
b
c
Henri S. Schrekker, K a¬ roly Micskei, Csongor Hajdu, Tam a¬ s Patonay,
d
a,
Martin W. G. de Bolster, Ludger A. Wessjohann *
Leibniz-Institute of Plant Biochemistry, Weinberg 3, 06120 Halle (Saale), Germany
a
Fax: (þ49)-345-5582-1309, e-mail: wessjohann@ipb-halle.de
b
Faculty of Sciences, Department of Inorganic & Analytical Chemistry, University of Debrecen,
4010 Debrecen, P.O. Box 21, Hungary
c
Faculty of Sciences, Department of Organic Chemistry, University of Debrecen, 4010 Debrecen, P.O. Box 20, Hungary
Faculty of Sciences; Chemistry Division, Department of Organic and Inorganic Chemistry, Vrije Universiteit Amsterdam,
De Boelelaan 1083, 1081 HV Amsterdam, Netherlands
d
Received: January 20, 2004; Accepted: June 1, 2004
Dedicated to Dr. Joe Richmond on the occasion of his 60 birthday.
th
Abstract: Ligand induced enantioselective versions
of the chromium(II)-mediated Nozaki±Hiyama reac-
tion to homoallyl alcohols proved to be very difficult
to achieve, especially if any other nucleophile than
the parent allylchromium(III) species was applied.
Also, the reaction is frequently accompanied by the
formation of oxidation side products, predominantly
allyl ketones. This can be explained by an Oppen-
auer±(Meerwein±PonndorfÀVerley) type mechanism
oxidation mechanism (Scheme 1) and is strongly fav-
oured with sterically unhindered aromatic aldehydes
and a substituted allylchromium.
[
5±8]
The involvement of an Oppenauer±Meer-
weinÀPonndorf±Verley (OMPV) type oxidation-reduc-
tion equilibrium reaction implies dramatic consequen-
[9]
ces for enantioselective Nozaki±Hiyama reactions.
The newly formed stereocentre is scrambled through
such an oxidation-reduction process. So far, the devel-
opment of asymmetric versions of the chromium(II)-
mediated Nozaki±Hiyama reaction has proven to be
(
OMPV reaction). The addition of an enantiopure li-
gand to racemic chromium homoallyl alcoholate in-
termediates produced enantiomerically enriched ho-
moallyl alcohols with an enantiomeric excess of up to
[
3,10±18]
troublesome.
Interestingly, an OMPV-reaction
also seems to participate in other chromium(II)-mediat-
3
2%. This observation not only supports that the
ed reactions like the Nozaki±HiyamaÀKishi and chro-
[
3,5,8]
proposed OMPV oxidation-reduction equilibrium
plays a crucial role in Nozaki±Hiyama reactions,
but also proves its involvement in enantioselective
versions.
mium-Reformatsky reactions.
In accordance with
this observation, OMPV equilibria are especially rele-
vant in products from substituted allylchromiums,
whereas good enantioselectivities were initially report-
ed for the unsubstituted case only. This prompted us to
look more closely into the relationship between enantio-
selective Nozaki±Hiyama reactions and OMPV trans-
formations. Here, we report the results of this study.
The coupling of benzaldehyde (1a) with allyl bromide
Keywords: chromium; enantioselection, Meerwein±
PonndorfÀVerley reduction; Nozaki±Hiyama reac-
tion, Oppenauer oxidation; thermodynamic dera-
cemization
(
2a) in the presence of bipyridyl ligand 6 was reported to
afford homoallyl alcohol 5a in an enantiomeric excess of
[
10]
7
4% (Scheme 2).
This system was suitable for our investigations, as the
Coupling reactions of allyl organometallic reagents with chiral modifier, the bipyridyl 6, is bound non-covalently
[
1,2]
aldehydes have been extensively investigated. An el- to the chromium centre. This offers the possibility to add
egant method for the synthesis of homoallyl alcohols is bipyridyl 6 after the formation of the racemic chromi-
the chromium(II)-promoted Nozaki±Hiyama reaction, um(III) alcoholate. In theory, exchange of THF for chi-
which allows the coupling of allyl halides with aldehydes ral ligand 6 should result in the formation of a chiral
in a Barbier-type reaction. In our previous studies on chromium alcoholate complex. If the proposed OMPV
the Nozaki±Hiyama reaction we observed that allyl ke- equilibrium is responsible for the formation of allyl ke-
tones 4 can be common side products, or even the main tones 4, this might result in enantiomerically enriched
[
3,4]
[
5]
product, although they are neglected by most authors.
homoallyl alcohols 5 (Pathway 1, Scheme 3). Deracem-
This formation can be explained by an Oppenauer-type ization of homoallyl alcohols 5 would not only support
Adv. Synth. Catal. 2004, 346, 731±736
DOI: 10.1002/adsc.200404021
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
731

COMMUNICATIONS
Henri S. Schrekker et al.
Scheme 1. Oppenauer±(Meerwein±PonndorfÀVerley) oxidation-reduction equilibrium in the Nozaki±Hiyama reaction.
[10]
Scheme 2. Enantioselective Nozaki±Hiyama reaction of Kishi et al.
Scheme 3. Possible pathways for deracemization of chromium alcoholate 3.
the proposed OMPV equilibrium as such, but at the that the OMPV equilibrium should be inactive. Also,
same time indicate the involvement of an oxidation-re- in crossover experiments, in which anisaldehydes were
duction equilibrium in the enantioselective formation used to form the Hiyama±Nozaki-intermediates 3, fol-
of Nozaki±Hiyama products. Another possible explana- lowed by excess benzaldehyde as oxidant, only the ani-
tion for the deracemization of racemic chromium alco- saldehyde-derived products were detected and not a
holate 3 might be, although unlikely, a retro-Nozaki± mixture reflecting the proportions of the two aldehydes
[
5]
Hiyama reaction, which is followed by a Nozaki±Hiya- applied. More extensive studies on the inhibition of
[
3]
ma reaction with ™normal∫ direct induction (Pathway 2, retro-reactivity in Cr(III) complexes will be published
Scheme 3). This retro-reaction is unlikely because the elsewhere.
driving force of this and other chromium(II)-mediated
The involvement of the OMPV equilibrium was stud-
reactions is the high stability of the formed chromiu- ied using benzaldehyde (1a) and allyl bromide (2a) in a
m(III)-oxygen bond, and was confirmed by a reference 1.5:1 ratio (Table 1). The racemic chromium alcoholate
[
19±21]
experiment (Table 1, entry 0).
No chiral induction 3a was formed in the presence of excess oxidant 1 allow-
was observed when the conditions were chosen as such ing its participation in the OMPVequilibrium. The reac-
732
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2004, 346, 731±736

Chromium-Mediated Enantioselective Nozaki±Hiyama Reactions
COMMUNICATIONS
Table 1. The in situ formation of racemic chromium alcoholate 3 followed by the addition of enantiopure bipyridyl 6 in the
[a]
presence of oxidant 1a.
[
b]
[c]
[d]
Entry
Time [h]
4a:5a
5a [%]
ee [%]
Remarks
[e]
[f]
0
1
2
3
4
5
4.0±18.0
4.0
5.0
24.0
48.0
0: 100
10: 90
40: 60
20: 80
02: 98
25: 75
n.d.
60
36
35
34
2
0
0
13
14
14
32
Reference expt. without excess 1a
racemic 3a
1 h after addn. of chiral ligand 6
after 20 h at 208C with 6
after 24 h at 558C with 6
after addn. of more oxidant 1a
55.0
[
[
[
[
[
[
a]
b]
c]
d]
e]
f]
CrCl (2.00 equivs.), 6 (4.35 equivs.), and additional PhCHO (1.5 equivs.).
Determined by GC analysis.
Determined by GC analysis with naphthalene as internal standard.
Determined by chiral HPLC.
Not determined.
2
PhCHO (0.5 equiv.), bromide 2a (1.0 equiv.); addition of 6 after 4 h; reaction for 1 h at 208C; temperature to 558C; reaction
for 13 h at 558C.
tion was complete after 4 h at 208C, and afforded race- 5a was observed (Figure 1). Indicative for the coordina-
mic chromium alcoholate 3a together with a small tion of bipyridyl 6 to chromium alcoholate 3a was the
amount of ketone 4a (Table 1, entry 1). After addition immediate colour change of the reaction mixture. The
of ligand 6, an interesting change not only of the enantio- ratio of alcohol 5a to ketone 4a decreased significantly
meric excess but also of the ratio of ketone 4a to alcohol within one hour after the addition of 6 (entry 2). Only
Figure 1. Development of enantiomeric excess and relative yield of alcohol 5a vs. ketone 4a under the influence of chiral ligand
6
for the experiment presented in Table 1.
Adv. Synth. Catal. 2004, 346, 731±736
asc.wiley-vch.de
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
733

COMMUNICATIONS
Henri S. Schrekker et al.
a trace of benzaldehyde 1a was left, which indicates its hence a shift to oxidation as in this case will lead to an
consumption as oxidant in the OMPV reaction. This enhancement (or decrease) in ee until equilibrium is
was confirmed by an increase of the amount of benzyl al- reached again.
cohol, and accordingly the equilibrium was shifted as far
As the reaction of benzaldehyde 1a with allyl bromide
aspossible towardsketone 4a. Even more important, de- 2a was the standard reaction to screen the potential of
racemization of chromium alcoholate 3a confirms our chiral ligands for the enantioselective Nozaki±Hiyama
proposed OMPV equilibrium and its involvement in reaction, it is not a surprise that its development was
enantioselective Nozaki±Hiyama reactions. Extended troublesome.
reaction times and elevated temperatures did not effect
These results support that the OMPV reaction is in-
the enantiomeric excess and conversion of alcohol 5a volved in enantioselective Nozaki±Hiyama reactions,
and benzyl alcohol (entries 3±4). However, the amount and that the finally obtained chiral induction might be
of ketone 4a decreased with time, probably from decom- the result of this oxidation-reduction equilibrium. To
position (see below), whereas that of alcohol 5a re- test this, we performed the same reaction under the con-
mained roughly constant (Table 1 and Figure 1: relative ditions published by Kishi (À208C, THF) using benzal-
increase of 5a from 5 h to 48 h).
dehyde (1a) and allyl bromide (2a) in a 1:2 ratio, 2.0
Additional oxidant 1a was added after 48 hours to equivs. chromium(II) chloride, and 4.0 equivs. of ligand
[
10]
promote the OMPV reaction and again shift the equili- 6 (Table 2, entries 1±4). The reactions resulted in high
brium. This resulted in a high consumption of alcohol 5a enantioselectivities (62±68%) and small fluctuations in
leaving only 2% of it in the mixture, but with an in- enantiomeric excess at different time points (4, 24, 48,
creased enantiomeric excess of 32% (entry 5). Obvious- and 72 h). The presence of ketone 4a supports the in-
ly ketone 4a is rapidly formed under these conditions but volvement of the OMPV equilibrium in this reaction.
also decomposes rapidly. The addition of oxidant 1a The decreasing ratio of ketone 4a to alcohol 5a is prob-
shifted the reaction further to ketone 4a, which was con- ably caused by the previously discussed instability and
firmed by the increased amount of benzyl alcohol. A thus consumption of ketone 4a. High enantiomeric ex-
new, defined decomposition product derived from ke- cesses were also obtained in the reactions of benzalde-
1
tone 4a was not observed, which might suggest a poly- hyde (1a) or ortho-substituted aldehyde 1b (R ¼CF )
3
2
merization. A change in ee as a consequence of a newly with dimethylallyl bromide (2b; R ¼CH , entries 5±
3
shifted equilibrium upon renewed oxidant (aldehyde) 6). Also in these reactions, the enantiomeric excesses
addition is not surprising. Forward and backward reac- did not change significantly with time. Although it is dif-
tions may have different kinetics, i.e., the MPV reduc- ficult to give a conclusive explanation for the origin of
tion is solely dependent on the chirality induction of Li- the extent of the observed asymmetric induction, it is
gand L*, whereas the Oppenauer oxidation of 3 with L* likely that it originates at least to a significant extent
results in a matched/mismatched situation, where one from the OMPV equilibrium.
diastereomeric pairing will react preferentially and
[a]
Table 2. The enantioselective Nozaki±Hiyama reaction with bipyridyl ligand 6.
1
2
[b, c]
[d]
Entry
R
R
Time [h]
4a:5a
ee [%] of 5a
1
2
3
4
H
H
H
H
H
H
H
H
4.0
24.0
48.0
72.0
42: 58
20: 80
13: 87
12: 88
68
65
62
62
4
b:5b
35: 65
c:5c
0: 100
5
6
H
CH3
CH3
48.0
48.0
70
79
4
CF3
[a]
[b]
[c]
[10]
CrCl (2.00 equivs.), and 6 (4.00 equivs.).
Determined by GC analysis.
2
Conversion determined by GC analysis with naphthalene as internal standard. A conversion of 15±25% was observed for 5a
(
entries 1±4). A conversion of 22±36% was observed for 5b at different time points (entry 5). A conversion of 9±20% was
[d]
observed for 5c (entry 6). Determined by chiral HPLC.
734
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2004, 346, 731±736

Chromium-Mediated Enantioselective Nozaki±Hiyama Reactions
COMMUNICATIONS
Explanations for the observed difference in enantio- Experimental Section
meric excess between initial (Table 2) and late (Table 1)
addition of chiral ligand 6 can only be speculative. In or-
der to obtain high enantiomeric excesses, non-equilibri-
General Methods
um conditions without excess aldehyde were chosen and All reactions were carried out under an argon atmosphere in
thus the kinetic parameters of the initial reaction be- flame-dried glassware using standard syringe and septa tech-
niques. The commercial reagents 1b, 2a, b, and chromium(II)
chloride (99.9% from Strem Chemicals) were used as pur-
come crucial. Also, the complexation of chromium
ions by 6 will be better if chiral ligand is added before
chased. Bipyridyl ligand 6 was synthesized according to a liter-
the reaction starts, because ligand binding (ligand ex-
[10]
15
ature procedure. Tetrahydrofuran was distilled from potassi-
um/benzophenone. Absolute dimethylformamide was pur-
chased from Fluka. Benzaldehyde (1a) was distilled from po-
tassium hydride. Spectral data of the known compounds
change) is up to 10 times faster for the initially present
Cr(II) ions than for chromium(III) ± hence the exchange
stability of Cr(III) alcoholates, e.g., in crossover experi-
[3]
[
23]
[24]
[23]
[25]
[5]
ments. However, also qualitative differences of the in-
termediate complexes have to be considered. Complex erature data. Benzyl alcohol was characterized by comparison
4
a, 4b, 5a, 5b, and 5c were in accordance with the lit-
[
Cr(II)þ6] reacting with 1 and 2 to give intermediate with a commercially available sample. Thin-layer chromatog-
3þCr(III)þ6] (type 2) may vary to some extent from raphy was carried out on Merck silica 60/F-254 aluminium-
backed plates. Flash chromatography was performed using
[
those generated by reacting [3þCr(III)] with 6 to ob-
tain [3þCr(III)þ6] (type 1). Although formally the
same, the real nature of the intermediate chromium-al-
kyl and chromium alcoholate complexes is not under-
stood at all, and it is well known that the complex
Merck silica gel 60 (40±60 mm). NMR spectra were recorded
in CDCl . Chemical shifts d are quoted in parts per million
3
(
ppm), and coupling constants J are given in Hertz (Hz). Gas
chromatographic analyses were run on a Carl Erba Instru-
ments apparatus (HRGC 5160 Mega Series, Ultra 1 column,
[22]
[5]
formed as well as the OMPVequilibrium is strongly
influenced by slight variations, e.g., of pH and aldehyde
5
0 m, i.d. 0.2 mm, film thickness 0.33 mm, FID detector).
HPLC analyses were carried out on an Agilent 1100 chromato-
concentration. Thus, if the order of addition or ratio of graph using a Chiralcel OD-column (4.6ꢀ250 mm); solvent:
À1
the components is changed, cluster formation, incorpo- 98:2 hexane:isopropanol; flow rate¼0.5 mLꢀmin ; T¼
ration of the second Cr(III) ion present, or binding of 6 258C (5a, b), T
or other ligands like aldehyde or THFin the active inter-
¼
18C (5c); UV detection: 220 nm.
mediate may be different. We made similar observations
with reactions of other chromium(III) alkyls and eno- Procedure for the Nozaki±Hiyama Reaction Followed
lates, formed either by reaction of Cr(II) with alkyl hal- by the Addition of Bipyridyl Ligand 6 (Table 1)
ide (þlithium halide), or by metal exchange of lithium
To chromium(II) chloride (2.0 equivs., 50 mg, 0.41 mmol) was
[
3]
alkyl with Cr(III) halide. Reactivity and yields ob-
tained from these intermediates were significantly dif-
ferent, although again, the complexes were formally
added a solution of naphthalene (as internal reference;
.5 equiv., 13 mg, 0.10 mmol) in 1.0 mL THF under vigorous
stirring. After a few minutes, a solution of the aldehyde in
0
the same, the identical product was eventually formed, 0.5 mL THF and a solution of allyl halide (1.0 equiv.,
and the general behaviour, e.g., chemoselectivity, was 0.20 mmol) in 0.5 mLTHF were added in this order. The result-
comparable.
ing mixture was stirred for 4 h at 208C, followed by the addition
of bipyridyl 6 [4.83 equivs. (entry 0); 4.35 equivs. (entries 1±5),
In conclusion, the proposed OMPV equilibrium for
the formation of allyl ketones 4 has been confirmed by
the deracemization experiments with enantiopure li-
gand 6. Its involvement in enantioselective chromi-
um(II)-mediated Nozaki±Hiyama reactions was dem-
onstrated. Likely the observed chiral inductions are
the result of this oxidation-reduction equilibrium. These
2
38 mg, 0.41 mmol]. Stirring was continued for the given time
and temperature. Benzaldehyde (1.5 equivs., 20 mL,
.20 mmol) was added at the given time. Samples (0.3 mL)
0
were taken after the given intervals. Samples were quenched
with water. The water layer was extracted three times with di-
ethyl ether, and the combined organic fractions were washed
with a saturated aqueous NaCl solution. The organic layer
results might be seen in a broader perspective as the for- was dried with MgSO . Conversions were determined by GC
4
mation of ketones as by-products has also been observed with naphthalene as the internal standard. Enantiomeric ex-
in other chromium(II)-mediated reactions like in the cesses were determined by chiral HPLC.
Nozaki±HiyamaÀKishi and to a minor extent in chromi-
um-Reformatsky reactions with unsaturated 4-halo-
enoates.
General Procedure for the Enantioselective Nozaki±
Hiyama Reactions (Table 2)
To chromium(II) chloride (2.0 equivs., 50 mg, 0.41 mmol) and
bipyridyl 6 (4.0 equivs., 238 mg, 0.81 mmol) was added 1 mL of
a solution of naphthalene (0.5 equiv., 13 mg, 0.10 mmol) in
1
.0 mL THF under vigorous stirring. The solution was stirred
for 1 hour at 208C, and cooled to À208C. The aldehyde
1.0 equiv., 0.20 mmol) in 1.0 mL THF and allyl halide
(
Adv. Synth. Catal. 2004, 346, 731±736
asc.wiley-vch.de
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
735

COMMUNICATIONS
Henri S. Schrekker et al.
(
2.0 equivs., 0.41 mmol) in 1.0 mL THF were added in this or-
[6] R. J. Maguire, J. Mulzer, J. W. Bats, Tetrahedron Lett.
1996, 37, 5487±5490.
[7] R. J. Maguire, J. Mulzer, J. W. Bats, J. Org. Chem. 1996,
der. The resulting mixture was stirred for 78 h at À208C, and
samples (0.3 mL) were taken at the given time points. The sam-
ples were quenched with water. The water layer was extracted
three times with diethyl ether, and the combined organic frac-
tions were washed with a saturated aqueous NaCl solution. The
organic layer was dried with MgSO . Conversions were deter-
mined by GC with naphthalene as the internal standard. Enan-
tiomeric excesses were determined by chiral HPLC.
6
1, 6936±6940.
8] M. Durandetti, J.-Y. N e¬ d e¬ lec, J. P e¬ richon, Org. Lett.
001, 3, 2073±2076.
[
2
4
[9] K. Nishide, M. Node, Chirality 2002, 14, 759±767.
10] C. Chen, K. Tagami, Y. Kishi, J. Org. Chem. 1995, 60,
[
[
[
[
[
[
5
386±5387.
2
,2-Dimethyl-1-(2-trifluoromethylphenyl)-but-3-en-1-one
11] Z.-K. Wan, H.-W. Choi, F.-A. Kang, K. Nakajima, D. De-
meke, Y. Kishi, Org. Lett. 2002, 4, 4431±4434.
12] B. Cazes, C. Verni e¡ re, J. Gor e¬ , Synth. Commun. 1983, 13,
73±79.
(
4c): Colourless oil; R ¼0.52 (hexane:diethyl ether, 90:10);
f
1
H NMR (400 MHz): d¼7.67 (m, 1H), 7.51±7.49 (m, 2H),
7
.35 (m, 1H), 6.02 (dd, J¼17.6, J¼10.5 Hz, 1H), 5.20 (d, J¼
13
1
(
1
(
1
2
(
5
7.6 Hz, 1H), 5.19 (d, J¼10.5 Hz, 1H), 1.33 (s, 6H); C NMR
100 MHz): d¼208.7 (C¼O), 142.7 (CH), 139.3 (quart. C),
31.0 (CH), 129.0 (CH), 127.0 (q, J¼32.2 Hz, quart. C), 126.7
13] K. Sugimoto, S. Aoyagi, C. Kibayashi, J. Org. Chem.
1
997, 62, 2322±2323.
14] M. Bandini, P. G. Cozzi, P. Melchiorre, A. Umani-Ron-
chi, Angew. Chem. Int. Ed. 1999, 38, 3357±3359.
15] M. Bandini, P. G. Cozzi, A. Umani-Ronchi, Chem. Com-
mun. 2002, 919±927.
q, J¼5.0, CH), 126.0 (CH), 123.5 (q, J¼274 Hz, quart. C),
14.62 (CH ), 51.12 (quart. C), 24.49 (CH ); IR (film): n¼
2
3
974 (w), 1702 (s), 1321 (s), 1064 (m), 1034 (m), 970 (m), 922
À1
m), 769 (m) cm ; anal. calcd. for C H F O: C 64.46, H
1
3
13 3
.41; found: C 64.86, H 5.44.
[16] A. Berkessel, D. Menche, C. A. Sklorz, M. Schrˆder, I.
Paterson, Angew. Chem. Int. Ed. 2003, 42, 1032±1035.
[
17] H.-W. Choi, K. Nakajima, D. Demeke, F.-A. Kang, H.-S.
Jun, Z.-K. Wan, Y. Kishi, Org. Lett. 2002, 4, 4435±4438.
18] M. Inoue, T. Suzuki, M. Nakada, J. Am. Chem. Soc. 2003,
125, 1140±1141.
Acknowledgements
[
We are grateful for financial support through bilateral exchange
programs of the DAAD (PPP Hungary) and M÷B. In addition,
part of this research was funded by HWP (state of Saxony-An-
halt, Germany) and the Hungarian Science Foundation for
T. P. (OTKA T32429) and K. M. (OTKA T46942).
[19] L. A. Wessjohann, H. Wild, Synlett 1997, 731±733.
[20] T. Gabriel, L. Wessjohann, Tetrahedron Lett. 1997, 38,
1363±1366.
[21] T. Gabriel, L. Wessjohann, Tetrahedron Lett. 1997, 38,
4
387±4388.
[
22] K. Micskei, C. Hajdu, L. A. Wessjohann, L. Mercs, A.
References
Kiss-Szikszai, T. Patonay, Tetrahedron Asymmetry 2004,
1
5, 1735±1744.
[
[
[
[
[
1] Y. Yamamoto, N. Asao, Chem. Rev. 1993, 93, 2207±2293.
2] S. E. Denmark, J. Fu, Chem. Rev. 2003, 103, 2763±2793.
3] L. A. Wessjohann, G. Scheid, Synthesis 1999, 1±36.
4] A. F¸rstner, Chem. Rev. 1999, 99, 991±1045.
5] H. S. Schrekker, M. W. G. de Bolster, R. V. A. Orru,
L. A. Wessjohann, J. Org. Chem. 2002, 67, 1975±1981.
[23] P. Jones, P. Knochel, J. Org. Chem. 1999, 64, 186±195.
[24] A. R. Katritzky, H. Wu, L. Xie, J. Org. Chem. 1996, 61,
4035±4039.
[25] C. Jubert, P. Knochel, J. Org. Chem. 1992, 57, 5425±5431.
736
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2004, 346, 731±736