Enzymatic resolution of alicyclic 1,3-amino alcohols in organic media

Article abstract of DOI:10.1016/S0957-4166(98)00236-5

Racemic cis- and trans-2-aminocyclohexane-1-methanol and cis- and trans- 2-amino-4-cyclohexene-1-methanol were resolved via lipase-catalysed O- acylation of their Z derivatives, using vinyl butyrate in different ether solvents. In accordance with the empi

Full text of DOI:10.1016/S0957-4166(98)00236-5

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 2339–2347
Enzymatic resolution of alicyclic 1,3-amino alcohols in organic
media
Mária Péter,a,b Johan Van der Eycken,a,∗ Gábor Bernáth b and Ferenc Fülöp b
aDepartment of Organic Chemistry, University of Gent, Krijgslaan 281 (S.4), B-9000 Gent, Belgium
bInstitute of Pharmaceutical Chemistry, Albert Szent-Györgyi Medical University, H-6701 Szeged, POB 121, Hungary
Received 11 June 1998
Abstract
Racemic cis- and trans-2-aminocyclohexane-1-methanol and cis- and trans-2-amino-4-cyclohexene-1-methanol
were resolved via lipase-catalysed O-acylation of their Z derivatives, using vinyl butyrate in different ether solvents.
In accordance with the empirical rule, most of the screened lipases preferred the 1S enantiomer. © 1998 Elsevier
Science Ltd. All rights reserved.
1. Introduction
1,3-Amino alcohols 1, 4, 6 and 8 with two adjacent stereogenic centres find noteworthy applicability
in synthetic and medicinal chemistry. Quaternary ammonium salts of (+)-1 and (−)-1 are used as chiral
phase-transfer catalysts for the enantioselective alkylation of compounds possessing an active methylene
group.1 Compound (+)-4 is an intermediate in the stereoselective synthesis of an orally active angiotensin
converting enzyme inhibitor2 and also in the synthesis of a novel chiral super-Lewis acid catalyst.3
N-Benzyl derivatives of (+)-1 and (−)-1 and (+)-4 and (−)-4 are used for the resolution of racemic
carboxylic acids.4 Homochiral 1, 4, 6 and 8 can serve as building blocks of pharmacologically active
fused saturated 1,3-heterocycles.5–7
∗
Corresponding author. E-mail: Johan.Vandereycken@rug.ac.be
0957-4166/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00236-5

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Our aim was to find appropriate conditions for the resolution of racemic 1, 4, 6 and 8. As these
compounds bear two functional groups, there are several possibilities for their enzymatic resolution.
One of the most direct methods is resolution via acylation of the amino group8,9 attached to the ring.
Alternatively, chemical acylation of the amino function followed by enzyme-catalysed hydrolysis could
be considered. Resolution via acylation of the alcohol function (after protection of the amino group)
may also be successful.10,11 There is a further possibility to achieve enantioselectivity through enzyme-
catalysed O-deacylation of the N,O-diacylated compound.12 In the latter cases, it should be emphasized
that 1, 4, 6 and 8 are primary alcohols, possessing an extra CH2 group between the stereogenic centre and
the OH group. Flexibility along the C(1)–CH2OH bond allows both enantiomers to adopt conformations
with similar orientations of medium-sized and large substituents attached to the stereogenic centre.
Weissfloch and Kazlauskas postulated that high enantioselectivity toward primary alcohols requires not
only a significant difference in size of the substituents, as for secondary alcohols, but also restricted
rotation along the C(1)–CH2OH bond.13 Discrimination between enantiomers of primary alcohols is
therefore more difficult.14
2. Results and discussion
The excellent results reported earlier for the resolution of β-amino acid esters via lipase-catalysed
acylation of the amino group15 encouraged us to investigate this approach first (Scheme 1).
Scheme 1.
The irreversibility of acyl transfer is a basic demand of enzymatic kinetic resolutions. Accordingly,
vinyl acetate was used as the acyl donor.16 The enzymes screened were lipases PS, A, AY, G, F and M
and proteases A, B and N (see Experimental). However, no enantioselectivity was observed, even at 0°C
and with only one equivalent of the acyl donor, when diethyl ether or tetrahydrofuran was applied as
cosolvent. A blank experiment revealed that the non-enzymatic process was much faster, leading to the
racemic product only. The use of ethyl acetate instead of vinyl acetate as the acyl donor resulted in a
slight improvement. A possible explanation for the different behaviour of β-amino alcohols as compared
with β-amino esters15 is the lower nucleophilicity of the amino function in the latter case, due to the
electron-withdrawing ethoxycarbonyl group.
We therefore decided to investigate enzyme-catalysed O-acylation after protection of the amino group
(Scheme 2). For N-protection, the readily removable tert-butyloxycarbonyl (Boc) and benzyloxycarbonyl
(Z) groups were applied. The enzymatic reactions were performed in different ethers, with vinyl acetate
or vinyl butyrate as the acyl donor. Enantioselectivities were considered in terms of the enantiomeric
ratio (E),17 which remains constant throughout the reaction.
Scheme 2.

M. Péter et al. / Tetrahedron: Asymmetry 9 (1998) 2339–2347
2341
Table 1
Lipase-catalysed O-acylation of racemic 2 in the presence of vinyl acetate
Table 2
Effects of acyl donors and protective groups in enzyme-catalysed O-acylation of racemic 2 and 3
In a first approach, racemic 1 was protected with a Boc group 2 and the acylation was catalysed by
a lipase (lipase PS, F, AY, N or PPL) in the presence of vinyl acetate. The reaction was performed in
different ethers and also in vinyl acetate (Table 1). Under the given conditions, only a moderate E value
was obtained.
For optimization of the enantioselectivity, the protecting group and the acyl donor were varied. In
different solvents and with different enzymes, the application of vinyl butyrate instead of vinyl acetate
proved favourable for the enantioselectivity, as is unequivocally demonstrated by the data in Table 2. The
rate of the reaction was also higher with vinyl butyrate. Thus, the longer carbon chain of the acyl donor
increases the hydrophobic character of the tetrahedral intermediate formed in the catalytic process and
improves the resolution.
The effect of variation of the protecting group is not so clear-cut. It is assumed that the protective
group should exert a major influence on the observed enantioselectivity, as it is directly connected to a

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M. Péter et al. / Tetrahedron: Asymmetry 9 (1998) 2339–2347
Table 3
Solvent-dependence of E values in the lipase PS-catalysed O-acylation in the presence of vinyl
butyrate
stereogenic centre. For secondary alcohols, it has been proved that a larger difference in steric demand
of medium-sized and large substituents enhances the enantioselectivity of lipase-catalysed reactions,18
whereas for primary alcohols this is not necessarily the case.13 Additionally, the Boc and Z groups differ
not merely sterically, but also in their electronic properties, e.g. Z could interact with aromatic amino
acids present at the binding site of some lipases. Accordingly, the influence of the N-protective group is
not predictable and can differ from enzyme to enzyme. For example, in the presence of vinyl butyrate, the
corresponding E values were comparable in the lipase PS-catalysed acylation of 2 and 3; for Novozym
435, Boc was more efficient; and for lipases M and F, the enantioselectivities were higher when Z was
used. For further experiments, Z was chosen because of the superior E values in the latter two cases
(Table 2).
Z Derivatives of all four racemic amino alcohols were prepared (3, 5, 7 and 9) and the solvent-
dependence was investigated by using lipase PS as catalyst. With the exception of 5, which gives a
remarkably better result in di-isopropyl ether, the data in Table 3 led to the conclusion that there is no
significant difference between the ether solvents as far as the enantioselectivity is concerned. However,
the reaction times were the shortest in di-isopropyl ether and significantly longer in tetrahydrofuran.
Accordingly, extensive enzyme screening was carried out in di-isopropyl ether: 9 lipases and 6
proteases were tested (see Experimental). Although all the enzymes were catalytically active under the
given reaction conditions, the proteases generally exhibited very low selectivities (E<15); only protease
M satisfied the criterion required for practical purposes (E>30). As concerns the lipases, under the given
conditions lipases PS and PPL were more selective for the trans compounds 5 and 9, while lipases M, F,
N and Novozym 435 were more efficient for the cis compounds 3 and 7 (Table 4).
In order to determine the enzyme selectivity, enantiomerically pure 1,3-amino alcohols [(1R,2S)-1,
(1S,2S)-4, (1R,2S)-6 and (1S,2S)-8] were prepared from the corresponding enantiomericallly pure β-
amino acid esters15 by LiAlH4 reduction and used as standards for determination of the absolute config-
urations. These standards, the racemic compounds 1, 4, 6 and 8 and the N-deprotected unreacted alcohol
enantiomers from the enzymatic reactions were derivatized with N-α-(2,4-dinitro-5-fluorophenyl)-L-
alaninamide (Marfey’s reagent).19
The derivatized standards and the N-deprotected derivatized resolved alcohol counterparts were
coinjected with the appropriate racemic 1, 4, 6 or 8 onto a reversed phase HPLC column. The resulting
chromatograms revealed the 1S selectivity of the enzymes, favouring the enantiomer predicted by the
empirical rule for primary alcohols, formulated by Kazlauskas et al. for Pseudomonas cepacia lipase.13,20
Novozym 435 displayed a very poor selectivity for 9. The stereochemical preference of lipase AY was
opposite for 3, 5 and 9.
At low E values, changing the solvent often results in an inverse selectivity. This occurred in the
lipase AY-catalysed O-acylation of racemic 2 when diethyl ether was used instead of di-isopropyl ether

M. Péter et al. / Tetrahedron: Asymmetry 9 (1998) 2339–2347
2343
Table 4
Effects of different enzymes on enantioselectivity in diisopropyl ether
(Table 1). On comparison of the results obtained for the lipase M-catalysed O-acylation of racemic 3 and
5 (Table 4), the extreme difference between the E values is striking and offers an opportunity to produce
a single enantiomer from a diastereomeric mixture.
In conclusion, in spite of the fact that the stereogenic centre is remote from the reaction site, racemic
1, 4, 6 and 8 have been successfully resolved via O-acylation of their Z derivatives.
3. Experimental
3.1. Materials and methods
The racemic amino alcohols 1, 4, 6 and 8 were obtained from esters of the corresponding β-amino
acids by LiAlH4 reduction.21,22 Novozym 435 (immobilized Candida antarctica) was purchased from
Novo Nordisk, PPL (pig pancreatic lipase) was from Fluka BioChemika, and lipases PS (Pseudomonas
cepacia), A (Aspergillus niger), AY (Candida rugosa), F (Rhizopus oryzae), G (Penicillium camember-
tii), M (Mucor javanicus) and N (Rhizopus niveus), and proteases A (Aspergillus sp.), B (Penicillium sp.),
M (Aspergillus sp.), N (Bacillus sp.), P (Aspergillus sp.) and S (Bacillus sp.) were from Amano Enzyme
Europe. The solvents were of the best analytical grade from Lab-Scan and were used directly from the
bottle. Solvents for HPLC analyses were of HPLC grade from Lab-Scan.
HPLC analyses were performed on a Kontron 422 and a Waters 600 liquid chromatograph. Optical
1
rotations were measured with a Perkin–Elmer 341 polarimeter. H NMR spectra were recorded on
a Bruker Avance DRX 400 spectrometer. Mass spectra were obtained with a Simigan TSQ 7000
spectrometer. Melting points were determined on a Kofler apparatus and are uncorrected.
3.2. Preparation of Z derivatives of racemic 1, 4, 6 and 8
A quantity (0.64 g, 5 mmol) of racemic 1, 4, 6 or 8 was dissolved in 25 ml dichloromethane in the
presence of 0.58 g (5.5 mmol) Na2CO3 and 25 ml water, and 0.742 ml (5 mmol) benzyl chloroformate
was then added dropwise to the mixture with stirring at room temperature. After 2–3 hours, the reaction
mixture was poured onto ice–water, a few drops of TEA was added and the mixture was extracted with
dichloromethane (2×25 ml). The combined organic layer was washed with brine solution (2×30 ml),

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M. Péter et al. / Tetrahedron: Asymmetry 9 (1998) 2339–2347
dried on MgSO4 and evaporated. The oily product was purified on a silica gel column, the eluent being
dichloromethane:acetone (97:3), Rf=0.25–0.28. Yield: 85–90%, white crystals, mp: 75–77°C (rac-3),
129–130°C (rac-5), 99–100°C (rac-7) and 88–89°C (rac-9). The 1H NMR spectra were identical with the
spectra of the corresponding resolved alcohols (see later).
3.3. Preparation of the Boc derivative of racemic 1
A quantity (0.39 g, 3 mmol) of cis-2-aminocyclohexane-1-methanol (1) was dissolved in 15 ml
dichloromethane in the presence of KOH (0.84 g, 15 mmol) and 15 ml water, and 0.65 g (3 mmol)
di-tert-butyl dicarbonate was then added with stirring at room temperature. After overnight stirring, the
two phases were separated. The aqueous phase was extracted with dichloromethane (2×15 ml), and the
combined organic layer was washed with brine solution to pH 7, dried on MgSO4 and evaporated to
dryness. Purification was carried out on a silica gel column, the eluent being hexane:ethyl acetate (3:1),
Rf=0.21. Yield: 80%, white crystals, mp: 87–88°C.
3.4. Typical small-scale enzymatic resolution
In a typical small-scale experiment, 10 mg (38 µmol) of 2, 3, 5, 7 or 9 was dissolved in 0.75 ml solvent,
and 0.25 ml acyl donor and 25 mg of enzyme were added. The mixture was stirred at room temperature.
Samples of 0.1 ml were taken out at intervals, the enzyme was filtered off, the solvent was evaporated
off and the residue was dissolved in hexane:isopropanol (9:1) and injected onto a Chiralcel OD HPLC
column (0.46 cm×25 cm; Daicel Chemical Co.), hexane:isopropanol (95:5) being used as eluent. Under
these conditions, both the conversion of the reaction (c) and the enantiomeric excesses of substrate (ees)
and product (eep) could be determined simultaneously from one run.
3.5. Preparation of standard, enantiomerically pure amino alcohols and determination of enzyme
selectivity
To a slurry of 40 mg LiAlH4 in 2 ml dry tetrahydrofuran, 10 mg ethyl (1R,2S)- or (1S,2S)-2-amino-1-
cyclohexanecarboxylate or ethyl (1R,2S)- or (1S,2S)-2-amino-4-cyclohexene-1-carboxylate15 was added
with stirring. After 6 hours, the reaction was stopped by adding 3–4 drops of water. The mixture was
then filtered and evaporated to dryness, and the residue was dissolved in acetonitrile and derivatized with
N-α-(2,4-dinitro-5-fluorophenyl)-L-alaninamide (Marfey’s reagent).19
To 5 mg (1R)-3, 5, 7 or 9 2–3 drops of hydrobromic acid solution in glacial acetic acid was added.
After 1 hour, 50 µl TEA was added and the reaction mixture was evaporated to dryness three times with
1 ml dichloromethane. The residue was dissolved in acetonitrile and derivatized with Marfey’s reagent.
The conditions of determination of the enzyme selectivity are summarized in Table 5.
3.6. Gram-scale resolution of cis-2-(benzyloxycarbonylamino)cyclohexane-1-methanol 3
Racemic cis-3 (0.8 g, 3 mmol) was dissolved in diisopropyl ether (76 ml), and vinyl butyrate (4 ml, 32
mmol) and lipase M (2 g) were added. The mixture was stirred at room temperature for 95 minutes.
The enzyme was filtered off at 49% conversion and the solvent was evaporated. The crude product
was subjected to column chromatography with dichloromethane:acetone (97:3) as eluent to isolate the
butyrate produced (1S,2R)-3b and the unreacted alcohol (1R,2S)-3.

M. Péter et al. / Tetrahedron: Asymmetry 9 (1998) 2339–2347
2345
Table 5
Retention times and resolutions on APEX ODS (0.46 cm×25 cm, Jones Chromatography) HPLC
column; eluent: methanol:water=60:40; flow rate: 0.8 ml/min; detection: 340 nm
20
D
Compound (1S,2R)-3b is a colourless oil (0.42 g, 1.26 mmol; [α] =−30 (c=0.2, CHCl3); 97% ee).
1H NMR (400 MHz, CDCl3) δ (ppm): 0.93 (t, 3H, J=7.4 Hz), 1.20–2.00 (m, 11H), 2.27 (t, 2H, J=7.3
Hz), 3.93–3.97 (m, 1H), 4.01–4.06 (m, 2H), 4.95 (d, 1H, J=8.5 Hz), 5.04–5.15 (ABq, 2H, J=13.5 Hz),
7.29–7.38 (m, 5H). MS: 334 (M+1). C19H27NO4 calc. 68.43% C, 8.17% H, 4.20% N; found 68.29% C,
7.79% H, 4.93% N.
For further purification, the unreacted amino alcohol (1R,2S)-3 was dissolved in diethyl ether and
extracted with Na2CO3 solution to remove the excess of butyric acid. Recrystallization from hex-
ane/diisopropyl ether afforded (1R,2S)-3 as white crystals (0.32 g, 1.22 mmol; mp: 90.5–91.5°C;
20
[α] =+7 (c=0.2, CHCl3); 92% ee). 1H NMR (400 MHz, CDCl3) δ (ppm): 0.90–0.95 (m, 1H), 1.18–1.36
D
(m, 3H), 1.61–1.82 (m, 5H), 3.24–3.40 (m, 2H), 3.79–3.83 (m, 1H), 4.12–4.15 (m, 1H), 5.00 (d, 1H,
J=8.0 Hz), 5.01–5.15 (ABq, 2H, J=12.5 Hz), 7.31–7.40 (m, 5H). MS: 264 (M+1). C15H21NO3 calc.
68.40% C, 8.04% H, 5.32% N; found 68.85% C, 8.09% H, 5.36% N.
3.7. Gram-scale resolution of trans-2-(benzyloxycarbonylamino)cyclohexane-1-methanol 5
Racemic trans-5 (0.8 g, 3 mmol) was dissolved in tetrahydrofuran (76 ml), and vinyl butyrate (4 ml,
32 mmol) and lipase PS (2 g) were added. The mixture was stirred at room temperature for 20 hours. The
enzyme was filtered off at 50% conversion.
The same work-up as described above afforded the ester enantiomer (1S,2S)-5b as a white solid; it
crystallized during long standing in a refrigerator. It was recrystallized from hexane (0.37 g, 1.11 mmol;
20
mp: 51–53°C; [α] =+28 (c=0.2, CHCl3); 91% ee). 1H NMR (400 MHz, CDCl3) δ (ppm): 0.93 (t, 3H,
D
J=7.4 Hz), 1.13–2.05 (m, 11H), 2.26 (t, 2H, J=7.4 Hz), 3.38–3.45 (m, 1H), 3.95–3.99 (m, 1H), 4.15–4.18
(m, 1H) 4.66 (d, 1H, J=8.2 Hz), 5.04–5.11 (ABq, 2H, J=18.0 Hz), 7.28–7.38 (m, 5H). MS: 334 (M+1).
C19H27NO4 calc. 68.43% C, 8.17% H, 4.20% N; found 68.77% C, 8.12% H, 3.98% N.
The unreacted alcohol enantiomer (1R,2R)-5 was obtained as white crystals (0.30 g, 1.14 mmol; mp:
20
100–101°C; [α] =−2 (c=0.2, CHCl3); 92% ee). 1H NMR (400 MHz, CDCl3) δ (ppm): 1.12–2.00 (m,
D
9H), 3.19–3.23 (m, 1H), 3.35–3.40 (m, 1H), 3.45–3.50 (m, 1H), 3.72–3.76 (m, 1H), 4.66 (d, 1H, J=7.4
Hz), 5.08–5.14 (ABq, 2H, J=14.5 Hz), 7.30–7.40 (m, 5H). MS: 264 (M+1). C15H21NO3 calc. 68.40% C,
8.04% H, 5.32% N; found 67.88% C, 7.79% H, 5.00% N.

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M. Péter et al. / Tetrahedron: Asymmetry 9 (1998) 2339–2347
3.8. Gram-scale resolution of cis-2-(benzyloxycarbonylamino)-4-cyclohexene-1-methanol 7
Racemic cis-7 (0.8 g, 3 mmol) was dissolved in diethyl ether (76 ml), and vinyl butyrate (4 ml, 32
mmol) and lipase M (2 g) were added. The mixture was stirred at room temperature for 3 hours. The
enzyme was filtered off at 50% conversion. The work-up was the same as for 3.
20
D
The resulting butyrate enantiomer (1S,2R)-7b is a colourless oil (0.45 g, 1.36 mmol; [α] =−35
1
(c=0.25, CHCl3); 97% ee). H NMR (400 MHz, CDCl3) δ(ppm): 0.94 (t, 3H, J=7.3 Hz), 1.61–2.41
(m, 9H), 3.96–4.18 (m, 3H), 4.94 (d, 1H, J=8.7 Hz), 5.04–5.12 (ABq, 2H, J=18.2 Hz), 5.60–5.69 (m,
2H), 7.30–7.42 (m, 5H). MS: 332 (M+1). C19H25NO4 calc. 68.85% C, 7.61% H, 4.23% N; found 68.25%
C, 7.68% H, 4.60% N.
The corresponding alcohol counterpart (1R,2S)-7 is a white solid (0.28 g, 1.07 mmol; mp: 97–98°C;
20
1
[α] =−20 (c=0.2, CHCl3); 98% ee). H NMR (400 MHz, CDCl3) δ (ppm): 1.56–1.62 (m, 1H),
D
1.95–2.13 (m, 3H), 2.43–2.48 (m, 1H), 3.24–3.31 (m, 1H), 3.46–3.52 (m, 1H), 3.65–3.69 (m, 1H),
4.24–4.27 (m, 1H), 4.99 (d, 1H, J=7.9 Hz), 5.03–5.11 (ABq, 2H, J=14.6 Hz), 5.58–5.60 (m, 1H),
5.68–5.71 (m, 1H), 7.33–7.37 (m, 5H). MS: 262 (M+1). C15H19NO3 calc. 68.93% C, 7.33% H, 5.36%
N; found 68.68% C, 7.59% H, 5.69% N.
3.9. Gram-scale resolution of trans-2-(benzyloxycarbonylamino)-4-cyclohexene-1-methanol 9
Racemic trans-9 (0.8 g, 3 mmol) was dissolved in diisopropyl ether (76 ml), and vinyl butyrate (4 ml,
32 mmol) and PPL (2 g) were added. The mixture was stirred at room temperature for 30 minutes. The
enzyme was filtered off at 49% conversion.
The same work-up as described for 3 afforded the butyrate enantiomer (1S,2S)-9b as a white solid,
20
D
crystallized in a refrigerator (0.41 g, 1.24 mmol; mp: 30–32°C; [α] =+49 (c=0.2, CHCl3); 95% ee).
1H NMR (400 MHz, CDCl3) δ (ppm): 0.93 (t, 3H, J=7.4 Hz), 1.59–1.66 (m, 2H), 1.93–2.45 (m, 7H),
3.85–3.86 (m, 1H), 4.03–4.14 (m, 2H), 4.84 (d, 1H, J=7.4 Hz), 5.02–5.12 (ABq, 2H, J=13 Hz), 5.58–5.65
(m, 2H), 7.29–7.41 (m, 5H). MS: 332 (M+1). C19H25NO4 calc. 68.85% C, 7.61% H, 4.23% N; found
68.14% C, 7.09% H, 4.14% N.
The corresponding alcohol counterpart (1R,2R)-9 is a white solid (0.30 g, 1.15 mmol; mp: 113–114°C;
20
1
[α] =−14 (c=0.3, CHCl3); 92% ee). H NMR (400 MHz, CDCl3) δ (ppm): 1.52–1.57 (m, 1H),
D
1.91–2.10 (m, 2H), 2.39–2.44 (m, 2H), 3.27–3.31 (m, 1H), 3.39–3.45 (m, 1H), 3.76–3.81 (m, 2H), 4.74
(d, 1H, J=8.1 Hz), 5.09–5.16 (ABq, 2H, J=15.1 Hz), 5.54–5.58 (m, 1H), 5.69–5.73 (m, 1H), 7.31–7.37
(m, 5H). MS: 262 (M+1). C15H19NO3 calc. 68.93% C, 7.33% H, 5.36% N; found 69.49% C, 7.28% H,
5.19% N.
Acknowledgements
This work was supported by a COPERNICUS project (Contract No. ERBCIPA-CT94-0121). GB and
FF thank OTKA for their financial support. JVdE thanks the Fund for Scientific Research, Flanders
(F.W.O., Belgium) for a ‘Krediet aan Navorsers’ (No. 1.5.930.95) and the ‘Ministerie voor Wetenschaps-
beleid’ for financial support. We are indebted to Amano Enzyme Europe and Novo Nordisk for the
generous gift of the lipases and proteases. PM thanks the Soros Foundation for financial support.

M. Péter et al. / Tetrahedron: Asymmetry 9 (1998) 2339–2347
2347
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