Asymmetric synthesis of amino acids by Cr(II) complexes of natural amino acids

Article abstract of DOI:10.1016/S0022-328X(03)00775-7

Stoichiometric reduction of the C,N double bond of oxime precursors of α-amino acids was performed in aqueous media by Cr(II) complexes of natural amino acids. The reduction of oximes of α-ketophenylacetic, α-keto-β-phenylpropionic and α-ketopropionic acids proceeded up to >90% conversion and 2-30% enantiomeric excess. 1:2 complexes of Cr(II) with l-alanine, l-valine, l-aspartic acid, l-histidine and l-phenylalanine were used as reducing agents. The reduction of α-(oximino)phenylacetic acid showed increasing e.e. (and decreasing conversion) with increasing temperature.

Full text of DOI:10.1016/S0022-328X(03)00775-7

Journal of Organometallic Chemistry 682 (2003) 143ꢀ148
/
www.elsevier.com/locate/jorganchem
Asymmetric synthesis of amino acids by Cr(II) complexes of natural
amino acids
Ka´roly Micskei a,*, Orsolya Holczknecht a, Csongor Hajdu a, Tama´s Patonay b,
Vale´r Marchis b, Milena Meo a,c, Claudia Zucchi c, Gyula Pa´lyi c,*
a Department of Inorganic and Analytical Chemistry, University of Debrecen, Egyetem te´r 1, H-4010 Debrecen, Hungary
b Department of Organic Chemistry, University of Debrecen, Egyetem te´r 1, H-4010 Debrecen, Hungary
c Department of Chemistry, University of Modena and Reggio Emilia, Via Campi 183, I-41100 Modena, Italy
Received 24 April 2003; received in revised form 11 June 2003; accepted 11 June 2003
Abstract
Stoichiometric reduction of the C,N double bond of oxime precursors of a-amino acids was performed in aqueous media by Cr(II)
complexes of natural amino acids. The reduction of oximes of a-ketophenylacetic, a-keto-b-phenylpropionic and a-ketopropionic
acids proceeded up to ꢀ
/
90% conversion and 2ꢀ/30% enantiomeric excess. 1:2 complexes of Cr(II) with L-alanine, L-valine, L-
aspartic acid, L-histidine and
L
-phenylalanine were used as reducing agents. The reduction of a-(oximino)phenylacetic acid showed
increasing e.e. (and decreasing conversion) with increasing temperature.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Oxime reduction; a-Amino acids; Synthesis; Chromium(II); Amino acid complexes
1. Introduction
These arguments prompted us to study the reactions
with Cr(II)/amino acid complexes. In the course of this
project enantioselective reductions of ketones to the
corresponding secondary alcohols were achieved re-
cently [7]. We report here on the reduction of a-amino
acid precursor oximes with these reagents.
Induction of chirality in chemical transformations is a
very important issue in preparative organic chemistry
[1,2]. One of the strategies leading to chiral products is
the use of transition metal reagents with chiral ligands
[3]. In spite of the easy accessibility of natural amino
acids, which are also excellent di- or tridentate ligands
[4], their transition metal complexes were seldom used as
chiral reagents or catalysts in enantioselective syntheses
[5].
2. Experimental
All compounds were of commercial origin with the
exception of the oximes which were prepared according
to literature procedures. a-(Oximino)phenylacetic acid
(1) was obtained by treating phenylglyoxylic acid by
hydroxylamine, while a-oximino-b-phenylpropionic
acid (2) and a-oximinopropionic acid (3) were synthe-
sised by nitrosation of the corresponding substituted
malonic acids with amyl nitrite [8].
Considering the essential role of amino acids in life
phenomena and the possibility that transition metal ions
might have played a key role in the prebiotic as well as in
the early biotic phase of the origin of life [6], the
reactions of amino acid/transition metal complexes with
organic substrates could merit special attention.
The amino acid/Cr(II) reagents were prepared in situ,
using parameters calculated by PSEQUAD program [9].
Spectroscopic measurements were performed using
the following instruments: FTIR: spectrointerferometer
* Corresponding authors. Tel.: ꢁ
667.
E-mail
palyi@unimo.it (G. Pa´lyi).
/
36-52-512-900; fax: ꢁ
/
36-52-489-
Micskei),
addresses:
kmicskei@delfin.klte.hu (K.
0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00775-7

144
K. Micskei et al. / Journal of Organometallic Chemistry 682 (2003) 143ꢀ148
/
1
Perkinꢀ
400; UVꢀ
/
Elmer 1600; H- and 13C-NMR: Bruker AMX
3.1. Reaction of a-(oximino)phenylacetic acid with
Cr(II)/ -histidine (1:2)complex
L
/
vis: spectrophotometer PerkinꢀElmer
/
Lambda Bio 20 and CD (UV region): spectropolari-
meter JASCO J-710.
The products of the reductions were analysed by
HPLC (JASCO, pump PU-980; gradient LG-900-02;
Into a 3-necked 50 ml reaction vessel was filled 0.80
ml of 2.15 M aqueous KOH and 9.2 ml water then this
solution was deoxygenated by bubbling Ar gas for 15
min. To this solution, at 25 8C, while stirred by an
detector UV-975; column: 250 mmꢂ
RP-C18 5 mm; at 1.00 ml minꢃ1, 20 8C; with acet-
onitrileꢀ
phosphate-buffer-[0.05 M H2PO4ꢃ/HPO42ꢃ, 1/
1]-methanol eluents) in derivatized form, with Marfey’s
reagent -2-[(1?-fluoro-2?,4?-dinitrophenyl-5?)amino]-
propionamide [10], using standards prepared from
pure and DL-amino acids.
/4.6 mm Supelco
external magnetic stirrer,
and [Cr(OAc)2×H2O] (0.125 g, 0.667 mmol) were added
in one portion each. Upon the addition of the Cr(II)
salt, almost immediately blueꢀviolet colour developed,
L-histidine (0.310 g, 2 mmol)
/
/
/
L
indicating the formation of the histidine/Cr(II) complex
[4a,7,12,13] while the pH of the reaction mixture was
9.3. The stirring of the solution was continued and
subsequently a-(oximino)phenylacetic acid (41 mg, 0.25
mmol) was added in one portion. Then the reaction
vessel was closed under a slight overpressure of Ar and
the magnetic stirring was continued for an additional 18
h.
After this period an 0.20 ml sample was taken to 5 ml
tube and to this solution 1.60 ml of 1.0 M aqueous
NaHCO3 was added. After mixing these solutions by
shaking the vessel for a few times, from this solution an
0.20 ml portion was taken, and to this portion 0.25 ml of
L, D
The reactions were carried out using standard Schlenk
technique under deoxygenated Ar atmosphere [11].
3. Reductions
The parameters and results of the reduction experi-
ments are shown in Table 1. We describe below the
details of a few characteristic experiments.
Table 1
No
Oxime
L-Amino acid
Oxime (mmol)
Cr(II) (mmol)
Amino acid (mmol)
pH
e.e. a
%
1
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
3
3
3
3
3
asp
asp
val
val
ala
ala
his
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
1
0.667
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
8.9
9.3
9.5
9.1
9.1
9.3
9.4
9.4
9.5
9.4
9.5
9.4
9.4
9.4
9.4
9.3
9.3
9.3
9.3
9.3
ꢃ
/
1.2
ꢃ
/
4.5
7.3
4.8
6.7
3
1
0.667
ꢃ
/
4
ꢃ
/
5
1
0.667
ꢃ/
6
ꢃ11.7
/
7
1
0.667
ꢃ
/
2.2
8
his
ꢃ
/
4.5
9
phe
phe
asp
val
ala
his
1
0.667
ꢃ
ꢃ/
/
23.3
30.5
6.4
10
11
12
13
14
15
16
17
18
19
20
1
1
1
1
1
1
1
1
1
1
0.7
10.4
ꢃ
ꢀ/
/
5.2
b
phe
asp
val
ala
his
8.2
2.4
b
ꢀ/
ꢃ
/
3.6
phe
2.5
a
AR ꢃ A
e:e:%ꢄ
S ꢂ100
AR ꢁ AS
For 1 a-oximinophenylacetic acid (this is not natural), negative values refer to S-isomer.
AD ꢃ A
e:e:%ꢄ
L ꢂ100
AD ꢁ AL
For 2 and 3 (these are natural), negative values refer to L-isomer.
b
Could not be measured (see text).

K. Micskei et al. / Journal of Organometallic Chemistry 682 (2003) 143ꢀ
/
148
145
the 0.02 M solution of Marfey’s reagent, in acetone, was
added. The tube containing the mixture of the sample
with Marfey’s solution was closed and it was immersed
into a 40 8C water bath for 90 min then the reaction
mixture cooled to r.t. and the pH of the solution was
4. Results and discussion
a-Amino acids were prepared from the corresponding
oximes (a-oximinocarboxylic acids) by reduction of the
C,N double bond with complexes of Cr(II) ion and
adjusted to Â
/
2 by adding 65 ml 2.52 M aqueous HCl
solution. This sample was then diluted with 1:19 v/v
natural L-amino acids in aqueous solution. The results
of these experiments are summarized in Table 1.
The reduction of the oximes proceeds under mild
ratio with the eluent and analysed by HPLC.
The HPLC measurement was calibrated (for this
sample) by a standard solution prepared as follows.
In a 50 ml flask L-histidine (0.620 g, 4 mmol) and DL-
phenylglycine (0.076 g, 0.5 mmol) was dissolved in 10 ml
of water. An 0.20 ml portion of this solution was mixed
with 0.25 ml of 0.02 M solution of Marfey’s reagent in
acetone, allowed to react and worked-up as described
above. This solution was diluted with the eluent 1:19 v/v
ratio before used as standard for the HPLC measure-
ment.
conditions with conversions ꢀ90%. The reactions show
/
also partial enantioselectivity (Fig. 1), resulting in low to
medium e.e. values of the products. The most important
result of this study is that the chirality of natural amino
acids as ligands could be transmitted to chiral a-amino
acids, prepared from achiral precursors. Natural amino
acids were rarely used as ligands in transition metal
mediated reactions [5,9,13], but neither products in these
studies was an amino acid. To our best knowledge this is
the very first example in which a CÄ
/
N double bond was
enantioselectively reduced to CÃN single bond by means
/
of an amino acid complex of a transition metal ion.
Attempts to optimise the enantiomeric excesses, as well
as at finding a catalytic variant [14] are in course in our
laboratories.
Beyond the obvious biological significance [6,15] of
results of this study, it is an important ‘‘practical’’
feature, that the reduction proceeds in aqueous media
3.2. Reaction of a-oximino-b-phenylpropionic acid with
Cr(II)/L-valine (1:2) complex
Essentially the procedure described above was fol-
lowed. The pH of the -valine/Cr(II) (2:1) complex
L
solution was 9.4. The a-oximino-b-phenylpropionic acid
(0.045 g, 0.25 mmol) was dissolved in 3 ml water
solution of NaOAc (0.381 g, 4.65 mmol). The reduction,
(with the exception of the model experiment at ꢃ10 8C).
/
The use of water as solvent facilitates to find direct
biological connections of the results (that is the reaction
proceeds under ‘‘bioimitating’’ [16] conditions). In the
perspective of eventual future catalytic practical appli-
cations water counts as the most ‘‘green’’ solvent [17],
which could be desired.
the analysis/derivatisation, and the preparation of the L-
valine/DL-phenylalanine standard was performed as
described above.
3.3. Reaction of a-(oximino)phenylacetic acid with
Cr(II)/L-alanine (1:2) complex at ꢃ10 8C
/
Only
L-amino acids were used as ligands. It is
noteworthy that enantiomeric preference varied in a
wide range (Table 1), independently of the bidentate
(Ala, Val, Phe) or tridentate (His, Asp) character of the
ligand. This result indicates that the main governing
factor controlling the ‘‘nascent’’ stereochemistry is the
relative interaction, bulkiness, etc. of the ligand and the
substrate. This interaction is mainly responsible for the
self-organization of the very sensitive transition state/
intermediate conformations [7c,18] which are governing
the stereochemical outcome of the reaction. This can be
discussed on the basis of the supposed mechanism.
The mechanism is much more complicated than in the
case of the reduction of prochiral ketones [7,13], as is
depicted in Scheme 1. In aqueous media oximes first
undergo (through N-radical) reductive dehydroxylation
Essentially the procedure described above was fol-
lowed. The reduction was performed in a (deoxyge-
nated) mixture of 4.3 ml water and 5 ml DMF as
solvent, the reaction mixture was cooled (ꢃ10 8C) by
/
(external) ice/NaCl. a-(Oximino)phenylacetic acid
(0.041 g, 0.25 mmol) dissolved in 1 ml DMF cooled to
ꢃ10 8C, was added in one portion to the solution of this
/
Cr(II)/L-alanine reagent and allowed to react for 48 h at
that temperature. Preparation of the derivatized sample
for HPLC and of the L-valine/DL-phenylglycine stan-
dard was performed as described above.
3.4. Reaction of (a-oximino)phenylacetic acid with
Cr(II)/L-phenylalanine complex at 90 8C
(reactions (1)ꢀ(3)) [19]. The resulting unstable imine is
/
The general procedure described above was followed.
Water was used as solvent. The colour of the Cr(II)/L-
phenylalanine complex was blue. The reaction time was
18 h at 90 8C (oil bath). Preparation of the derivatized
supposed to give a C-centred radical (4) [7,12,13], which
reacts with a second Cr(II) ion resulting in the corre-
sponding (and stereochemically decisive) s-alkylchro-
mium intermediate (5). We made an attempt to identity
sample and of the
standard was performed as described above.
L
-phenylalanine/DL-phenylglycine
this intermediate through UVꢀ
(Fig. 1).
/vis and CD spectroscopy

146
K. Micskei et al. / Journal of Organometallic Chemistry 682 (2003) 143ꢀ148
/
Fig. 1. (i) UV and (ii) CD spectra of (a) L-Ala; (b) Cr(II); (c) Cr(II)ꢁ
/
L-Ala, (d) Cr(II)ꢁ
/
L-Alaꢁa-(oximino)phenylacetic acid in water solution.
/
The UVꢀvis/CD spectra indicate:
/
a) Inner sphere coordination of the ligand amino acids
before the addition of the substrate verified by the
appearance of low-energy dꢀd bands [4,9a,13c].
/
b) Formation of the chiral s-alkylchromium inter-
mediate upon addition of the substrate oxime
[7,12,13].
c) Chiral perturbation of the central metal ion [20]
itself in both cases (but differently), as indicated by
the presence of the low energy CD bands.
The final step is the protonation of the organometallic
bond where either retention or inversion is possible (see
Eqs. (6) and (7)), similarly to the ketone reduction
discussed earlier [7].
Since the completion of the reaction needs an excess
of the reducing agent due to chemical reasons, analysis
of those samples where the ligand and the (expected)
product amino acids is the same became highly unreli-
able. From the disappearance of the oxime substrate
formation of high amount of product amino acids could
be suspected but no quantitative statements are possible
at the moment. This is an important research goal for
Scheme 1.

K. Micskei et al. / Journal of Organometallic Chemistry 682 (2003) 143ꢀ
/
148
147
References
[1] (a) S. Hanessian, Total Synthesis of Natural Products: The
Chiron Approach, Pergamon, Oxford, 1983;
(b) R.E. Gawley, J. Aube´, Principles of Asymmetric Synthesis,
Pergamon-Elsevier, Oxford, 1996;
(c) G.R. Stephenson (Ed.), Advanced Asymmetric Synthesis,
Blackie Academic & Professional, London, 1996.
[2] (a) S.C. Stinson, Chem. Eng. News, Sept. 21 (1998) 83; Oct. 11
(1999) 101; May 8 (2000) 59; July 10 (2000) 63; Oct. 23 ( 2000) 55;
Oct. 1 (2001) 79;
(b) M. Jacoby, Chem. Eng. News, March 25 (2002) 43;
(c) A.M. Rouhi, Chem. Eng. News, June 10 (2002) 43; June 10
(2002) 51.
[3] (a) H. Brunner, W. Zettlmeier, Handbook of Enantioselective
Catalysis, vol. IꢀII, VCH, Weinheim, 1993;
/
(b) R.A. Sheldon, Chirotechnology, M. Dekker, New York, 1993;
(c) J. Ojima (Ed.), Catalytic Asymmetric Synthesis, Pergamon-
Elsevier, Oxford, 1993.
[4] (a) K. Burger (Ed.), Biocoordination Chemistry: Coordination
Equilibria in Biologically Active Systems, E. Horwood, New
York, 1990;
(b) A. Iakovidis, N. Hadjiliadi, Coord. Chem. Rev. 135/136 (1994)
17.
[5] (a) M. Nakagawa, H. Nakao, K. Watanabe, Chem. Lett. (1985)
391;
Fig. 2. Temperature dependence of the e.e. and of the conversion of
the reduction of a-(oximino)phenylacetic acid with C(II)/L-Ala (1:2)
reagent.
(b) G. Wilkinson, R.D. Gillard, J.A. Mc Cleverty (Eds.),
Comprehensive Coordination Chemistry, Pergamon, Oxford,
1987, Vol. 2, pp. 739;
(c) K. Harada, T. Munegumi, in: B.M. Trost, I. Fleming (Eds.),
Comprehensive Organic Synthesis, vol. 8, Pergamon, London,
1991, pp. 139ꢀ158;
/
the future since in this way even a system with chiral
autocatalysis [21] could be eventually hoped, for amino
acid chirality.
A highly interesting feature of the present system was
observed when attempts were made to study the
behaviour of the reaction mixture at temperatures lower
or higher than r.t. Our experiments clearly show that the
enantiomeric excess increases with increasing tempera-
ture (Fig. 2). This rare behaviour [22] is reflecting to a
complicated mechanism of typically non-linear charac-
(d) D. Carmona, F.J. Lahoz, R. Atencio, L.A. Oro, L.P. Lamata,
E.S. Jose´, Tetrahedron: Asymmetry 4 (1993) 1425;
(e) J.-i. Kikuchi, Z.Y. Zhang, Y. Murakami, J. Am. Chem. Soc.
117 (1995) 5383.
[6] (a) G. Wachterchauser, Proc. Natl. Acad. Sci. USA 87 (1990) 200;
¨
(b) G. Wachterchauser, Progr. Biophys. Mol. Biol. 58 (1992) 85;
¨
¨
¨
(c) G. Wachterchauser, Science 289 (2000) 1307;
¨
(d) C. Huber, G. Wachterchauser, Science 276 (1997) 245;
¨
¨
(e) C. Huber, G. Wachterchauser, Science 281 (1998) 670;
¨
¨
(f) R.H. Crabtree, Science 276 (1997) 222;
¨
(g) G.D. Cody, N.Z. Boctor, T.R. Filley, R.M. Hazen, J.H. Scott,
A. Sharma, H.S. Yoder, Jr., Science 289 (2000) 1337.
[7] (a) J. Gyarmati, C. Hajdu, Z. Dinya, K. Micskei, C. Zucchi, G.
Pa´lyi, J. Organomet. Chem. 586 (1999) 106;
ter [23]*an outstanding characteristic of life phenom-
/
ena [24]. This aspect merits further exploration, as well.
(b) T. Patonay, C. Hajdu, J. Jeko, A. Le´vai, K. Micskei, C.
¨
Zucchi, Tetrahedron Lett. 40 (1999) 1373;
(c) G. Pa´lyi, L. Bencze, K. Micskei, C. Zucchi, Atti Accad. Nazl.
Sci. Lett. Arti (Modena) 317 (8/3) (2001) 457.
[8] (a) V. Marchis, Diploma (MSc) Thesis, University of Debrecen,
Debrecen, 2001;
Acknowledgements
(b) A. Ahmad, I.D. Spenser, Can. J. Chem. 39 (1961) 1349.
[9] (a) K. Micskei, F. Debreczeni, I. Nagypa´l, J. Chem. Soc. Dalton
Trans. (1983) 1335;
Financial help is acknowledged to the [Hungarian]
Scientific Research Foundation (Grant OTKA, No.
T33130, T32429), the Ministry of Education (Grant:
FKFP 0614/2000), the [Italian] Ministry of University
and Research (MURST), the [Italian] National Re-
search Council (CNR) as well as to the Erasmus/
Socrates exchange program between University of
Debrecen and Modena. Valuable advice regarding the
derivatisation and HPLC techniques are gratefully
acknowledged to Professor M. Hollo´si, Drs. G. Szo´ka´n
and S. Szabo´ (Budapest).
(b) L. Ze´ka´ny, J. Nagypa´l, in: D.G. Leggett (Ed.), Computational
Methods for the Determination of Formation Constants, Plenum,
New York, 1985, pp. 291ꢀ353;
/
(c) K. Micskei, I. Nagypa´l, J. Chem. Soc. Dalton Trans. (1986)
2721; see also T. Kiss, in Ref. [4a], pp. 56.
[10] (a) P. Marfey, Carlsberg Res. Commun. 49 (1984) 591;
(b) G. Szo´ka´n, S. Hadfi, K. Krizsa´n, A. Liembeck, M. Alma´s, C.
Somlai, J. Liquid Chromatogr. 17 (1994) 2759.
[11] D.F. Shriver, M.A. Drezdzon, Manipulation of Air Sensitive
Compounds, 2nd ed, Wiley, New York, 1986.
[12] J.H. Espenson, Acc. Chem. Res. 25 (1992) 222.

148
K. Micskei et al. / Journal of Organometallic Chemistry 682 (2003) 143ꢀ148
/
[13] (a) G. Kova´cs, J. Gyarmati, L. Somsa´k, K. Micskei, Tetrahedron
Lett. 37 (1996) 1293;
[20] (a) V. Galamb, G. Pa´lyi, M. Kajta´r, Inorg. Chim. Acta 55 (1981)
L113;
(b) G. Kova´cs, K. Micskei, Tetrahedron Lett. 38 (1997) 9055;
(c) K. Micskei, J. Gyarmati, G. Kova´cs, S. Makleit, C. Simon, Z.
Szabo´, J. Marton, S. Hosztafi, H. Reineke, H.J. Drexler, Eur. J.
Org. Chem. (1999) 149.
(b) C. Zucchi, S. Tiddia, R. Boese, C.M. Tschoerner, L. Bencze,
G. Pa´lyi, Chirality 13 (2001) 458.
[21] (a) K. Soai, H. Morioka, K. Choji, Nature 378 (1995) 767;
(b) K. Soai, T. Shibata, in: G. Pa´lyi, C. Zucchi, L. Caglioti (Eds.),
[14] A. Furstner, Chem. Rev. 99 (1999) 991.
¨
[15] A. Lazcano, S.L. Miller, Cell 85 (1996) 793.
Advances in BioChirality, Elsevier, Amsterdam, 1999, pp. 125ꢀ
136;
/
[16] (a) B. Honig, K. Sharp, A.S. Yang, J. Phys. Chem. 97 (1993) 1101;
(b) A. Lubineau, J. Auge´, Y. Queneau, Synthesis (1994) 741.
[17] (a) K. Barta, M. Cse´kei, S. Csihony, H. Mehdi, I.T. Horva´th, Z.
Pusztai, G. Vla´d, Magy. Ke´m. Lapja 55 (2000) 173;
(b) M. Poliakoff, J.M. Fitzpatrick, T.R. Farren, P.T. Anastas,
Science 297 (2002) 807.
(c) K. Soai, T. Shibata, I. Sato, Acc. Chem. Res. 33 (2000) 382;
(d) K. Soai, I. Sato, T. Shibata, S. Komiya, M. Hayashi, Y.
Matsueda, H. Imamura, T. Hayase, H. Morioka, H. Tabira, J.
Yamamoto, Y. Kowata, Tetrahedron Asymmetry 14 (2003) 185;
(e) I. Sato, H. Urabe, S. Ishiguro, T. Shibata, K. Soai, Angew.
Chem. Int. Ed. 42 (2003) 315.
[18] (a) G. Pa´lyi, K. Alberts, T. Bartik, R. Boese, G. Fra´ter, T.
Herbrich, A. Herfurth, C. Kriebel, A. Sorkau, C.M. Tschoerner,
C. Zucchi, Organometallics 15 (1996) 3253;
[22] (a) R. Ueoka, Y. Matsumoto, T. Nagamatsu, S. Hirohata, Chem.
Lett. (1984) 583;
(b) K. Borsze´ky, T. Mallat, R. Aeschiman, W.B. Schiweizer, A.
Baiker, J. Catal. 161 (1996) 451;
(b) M. Szabo´, R. Szila´gyi, L. Bencze, R. Boese, C. Zucchi, L.
Caglioti, G. Pa´lyi, Enantiomer 5 (2000) 549;
(c) T. Nishida, A. Miyafuji, Y.N. Ito, T. Katsuki, Tetrahedron
Lett. 41 (2000) 7053.
(c) C. Zucchi, R. Boese, K. Alberts, T. Herbrich, G. To´th, L.
Bencze, G. Pa´lyi, Eur. J. Inorg. Chem. (2001) 2297.
[19] (a) G.H. Timms, E. Wildsmith, Tetrahedron Lett. (1971) 195;
(b) K. Takai, N. Katsura, Y. Kunisada, Chem. Commun. (2001)
1724.
[23] C. Girard, H.B. Kagan, Angew. Chem. Int. Ed. 37 (1998) 2922.
[24] G. von Kiedrowski, in: G. Pa´lyi, C. Zucchi, L. Caglioti (Eds.),
Fundamentals of Life, Elsevier, Paris, 2002, p. 38.