Chemo- and Enzyme-Catalyzed Reactions Revealing a Common Temperature-Dependent Dynamic Solvent Effect on Enantioselectivity

Article abstract of DOI:10.1002/hlca.200390299

The enantiomeric ratio E of enzyme-catalyzed (Candida antarctica lipase and lipase PS) and chemocatalyzed (L-proline-based diamines) acylation reactions of 1-(naphthalen-2-yl)ethanol, 2-phenylpropanol, and 2-benzylpropane-1,3-diol is dependent on solvent and temperature. Plots of In E vs. 1/T showed the presence of inversion temperatures (T_inv). The T_inv values for the bio-catalyzed and the chemo-catalyzed reactions are fairly in agreement, and correspond as well to the T_NMR values obtained by variable-temperature ¹³C-NMR experiments on the substrates in the same solvent of the resolution. This result demonstrates that clustering effects in the substrate solvation manage the chemical and the enzymatic enantioselectivity, and, moreover, that the solute-solvent cluster is always the real reacting species in solution for chemical as well as for enzymatic reactions.

Full text of DOI:10.1002/hlca.200390299

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Helvetica Chimica Acta Vol. 86 (2003)
Chemo- and Enzyme-Catalyzed Reactions Revealing a Common
Temperature-Dependent Dynamic Solvent Effect on Enantioselectivity
by Gianfranco Cainelli*, Paola Galletti, Daria Giacomini*, Andrea Gualandi, and Arianna Quintavalla
Dipartimento di Chimica −G. Ciamician×, University of Bologna, Via Selmi 2, I-40126, Bologna
(fax: ‡390512099456; e-mail: cainelli@ciam.unibo.it, giacomin@ciam.unibo.it)
Dedicated to Professor Duilio Arigoni on the occasion of his 75th birthday
The enantiomeric ratio E of enzyme-catalyzed (Candida antarctica lipase and lipase PS) and chemo-
catalyzed (l-proline-based diamines) acylation reactions of 1-(naphthalen-2-yl)ethanol, 2-phenylpropanol, and
2-benzylpropane-1,3-diol is dependent on solvent and temperature. Plots of ln E vs. 1/T showed the presence of
inversion temperatures (Tinv). The Tinv values for the bio-catalyzed and the chemo-catalyzed reactions are fairly
in agreement, and correspond as well to the TNMR values obtained by variable-temperature 13C-NMR
experiments on the substrates in the same solvent of the resolution. This result demonstrates that clustering
effects in the substrate solvation manage the chemical and the enzymatic enantioselectivity, and, moreover, that
the soluteÀsolvent cluster is always the real reacting species in solution for chemical as well as for enzymatic
reactions.
Introduction. The role of solvent in condensed-phase chemical reactions is of
pivotal importance for determining reaction rates in simple transformations to
understanding the complex behavior of proteins in a liquid environment. The solvent
has often a decisive effect on issues of structure, energetics, and reactivity, and cannot,
in general, be regarded as a negligible perturbation [1]. Solvent effects are closely
related to the intermolecular soluteÀsolvent interactions that constitute the solvation
process. A number of experimental, theoretical, and simulation studies have been
devoted to the elucidation of a molecular view of solvation processes [2]; however,
research in this field is still far from being an exhaustive interpretation of the
experimental results, especially concerning solvent effects on stereoselectivity.
In our recent studies of the solvent and temperature effects on the stereoselectivity
of a number of different reactions, we have tried to explain the well-known
phenomenon of the so-called inversion temperature (Tinv), which has been observed
by several authors in temperature-dependent studies on stereoselectivity (for a review,
see [3a]; for a different interpretation of Tinv, see [3b]). Inversion temperatures have
indeed been found in a large number of completely different reactions, such as
asymmetric Sharpless dihydroxylation [4], enantioselective epoxidation [5], diaster-
eoselective reduction [6], and, by our group, in nucleophilic additions [7], DielsÀAlder
reactions [8], and in enzymatic resolutions [9]. In our explanation, the Tinv
phenomenon depends on the reagent solvation and, therefore, on the complex
microscopic interactions between solvent and solute molecules, resulting in a
modification of the free energy of the reagents and, as consequence, in a modification
of their reactivity and stereoselectivity. In particular, the solvation of a molecule leads

Helvetica Chimica Acta Vol. 86 (2003)
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to the formation of a soluteÀsolvent cluster with a more-defined three-dimensional
structure than generally assumed. The reactivity and the stereoselectivity of a molecule
in solution depends on the structure of this supramolecular soluteÀsolvent cluster,
which, in turn, depends on the temperature; thus, the same molecule in the same
solvent at different temperatures may show a different reactivity and stereoselectivity.
The Tinv represents the temperature at which two supramolecular soluteÀsolvent
clusters are in equilibrium. At temperatures higher than Tinv, a supramolecule
possessing a given stereoselectivity is present; at temperatures lower than Tinv, there
is another supramolecule with a different stereoselectivity. Because of the complexity
of solvation processes, it is difficult to formulate a detailed microscopic model of these
supramolecules. However, their interconversion could involve a solvent reorganization
with transfer of solvent molecules between the supramolecules and the bulk or an
internal rearrangement of the solvation cluster. Thus, in our explanation, the true
reacting species of a molecule in solution at a given temperature is not the molecule
itself but its supramolecular soluteÀsolvent cluster [10].
In this paper, we present our results of a comparative study on chemo- and bio-
catalyzed kinetic resolution of 1-(naphthalen-2-yl)ethanol, 2-phenylpropanol, and of
the asymmetric acylation of 2-benzylpropane-1,3-diol; all reactions revealed phenom-
ena of temperature-dependent solvent effects of the starting compounds. Worthwhile
to note is that the chemo- and the enzyme-catalyzed reactions showed fairly similar
inversion temperatures.
Results and Discussion. Traditionally, kinetic resolutions of racemic alcohols have
been performed with enzyme catalysts that react selectively with only one of the two
enantiomers [11]. Lipase-catalyzed solvolysis and transesterification is, in fact, a
powerful approach to obtain optically active alcohols. More recently, several groups
have achieved nonenzymatic kinetic resolutions with chiral acylation catalysts.
Oriyama et al. have recently reported the utility of proline-based ligands as catalysts
for kinetic resolution [12] and asymmetric acylation of meso-diols [13].
We used these two well-established techiques to compare the enantioselectivities of
enzymatic (bio-catalyst) and nonenzymatic (chemo-catalyst) acylation reactions of two
racemic alcohols and one prochiral diol by varying the reaction solvent and the
temperature. We selected as substrates 1-(naphthalen-2-yl)ethanol (rac-1), 2-phenyl-
propanol (rac-4), and 2-benzylpropane-1,3-diol (7), as enzymes, two lipases, Candida
antarctica lipase (CAL) and lipase PS (Amano), as chemocatalysts, three proline-based
diamines 9, 10, and 11, and two solvents, THF and t-BuOMe (Scheme 1), after a
preliminary screening of the stability of the enantioselectivity with the conversion,
adequate reaction times, and enantioselectivity values. Enantioselective transterifica-
tion with lipases were carried out with vinyl acetate as the acylating agent, whereas
AcCl was used in the diamine-catalyzed reactions. We then compared the temperature
dependence of enantioselectivity between the reaction with the chemo-catalyst and the
corresponding enzymatic reaction.
Resolution of 1-(Naphthalen-2-yl)ethanol (rac-1; Scheme 2). The enzymatic
resolution of rac-1 was carried out by incubating the substrate (0.33 mmol) in THF
(3 ml) with CAL (7000 U/g, 50 mg) or lipase PS (30000 U/g, 100 mg), and vinyl acetate
(1.32 mmol). The reaction mixture was maintained under vigorous magnetic stirring

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Helvetica Chimica Acta Vol. 86 (2003)
Scheme 1
a) Bio-catalyst: lipase PS (Amano) or from Candida antarctica; chemo-catalyst: l-proline-derived chiral
diamines; acylating agent: vinyl acetate or acetyl chloride; solvent: THF or t-BuOMe.
and thermostatted at the desired temperature. The reactions were repeated at different
temperatures in the range of À 208 to 6081). The progress of the reaction was monitored
by GC. The reactions were terminated at close to 45% of esterification by removal of
the enzyme by microfiltration (0.20-mm filter units). The chemo-catalyzed resolution
Scheme 2
1
)
At temperatures lower than À208, the reaction shows too-long reaction times and too-low conversions.

Helvetica Chimica Acta Vol. 86 (2003)
3551
was carried out on 0.33 mmol of the rac-1 in THF (3 ml) with 0.17 mmol (0.5 equiv.) of
chiral diamine 9 and 10, and then AcCl (0.24 mmol). The progress of the reaction was
monitored by GC. The reaction was quenched with phosphate buffer (pH 7) and
extracted with CH2Cl2. The reactions were repeated at different temperatures in the
range À608 to 608. The enantiomeric excess (ee) values of acetates 3 and alcohols 2
were directly determined on the crude by GC chiral column (see Exper. Part).
The enantioselectivity was expressed as the enantiomeric ratio, E, calculated from
the ee value of the substrate (ees) and product (eep) by the relationship [14]:
E ˆ ln[(1 À ees)/(1 ‡ ees/eep)] / ln[(1 ‡ ees)/(1 ‡ ees/eep)]
(1)
By studying the temperature dependence of E, the differential activation
parameters DDH= and DDS= were evaluated according to Eqn. 2 [15]:
ln E ˆ À(DDG=/RT) ˆ À(DDH=/RT) ‡ (DDS=/R)
(2)
From plots of ln E vs. 1/T, data were then analyzed by least-squares fitting to Eqn. 2
to obtain linear correlations (Fig. 1). The DDH= and DDS= values were obtained from
slopes and intercepts of the linear plots (Table). For each data set, we applied a residual
analysis to evaluate the number of linear trends and to ascertain unambiguously the
presence of an inversion temperature.
The enzymatic resolution of rac-1 with lipase PS in THFand with CAL in t-BuOMe
showed the highest enantiomeric ratio with a linear temperature dependence and with
the best E value at low temperature (À208). With CAL, as catalyst in THF, an inversion
Fig. 1. Eyring plot for the enantioselective acetylation of 1-(naphthalen-2-yl)ethanol (rac-1) with Candida
antarctica lipase in THF (black diamond) and in t-BuOMe (black triangle), with lipase PS in THF (black
square), with chiral diamine 9 in THF (white diamond), with 10 in THF (white square), and with 11 in THF
(white circle)

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Table. Differential Activation Parameters for Enzyme-Catalyzed and Chiral-Diamine-Catalyzed Enantioselec-
tive Acylation of Alcohols rac-1, rac-4, and 7 in THF and t-BuOMe
Substrate Solvent
Catalyst
Tinv [8] T > Tinv
DDH=
T< Tinv
DDS=
DDH=
DDS=
[kcal/mol]
[cal/mol]
[kcal/mol]
[cal/mol]
1
1
1
1
1
1
4
4
4
4
4
4
7
7
7
t-BuOMe CAL
À 3.6 Æ 0.2a)
À 6.3 Æ 0.6
À 2.7 Æ 0.2a)
À 1.1 Æ 0.1
2.4 Æ 0.7
À 5 Æ 1
THF
THF
THF
THF
THF
CAL
Lipase PS
9
10
11
‡ 9
‡ 4.0 Æ 0.9
‡ 32 Æ 3
‡ 4 Æ 1
À 1
‡ 2
À 3.5 Æ 0.5
À 0.7 Æ 0.4
À 6.3 Æ 0.3
0.32 Æ 0.06
À 0.6 Æ 0.2
‡ 4.1 Æ 0.2
À 0.2 Æ 0.1
À 2.1 Æ 0.1a)
‡ 1.08 Æ 0.05
t-BuOMe Lipase PS
t-BuOMe 10
t-BuOMe 11
À 19
À 8
À 0.73 Æ 0.06 À 0.8 Æ 0.2
‡ 0.51 Æ 0.05 ‡ 1.5 Æ 0.2
À 0.80 Æ 0.03 À 2.5 Æ 0.1
À 0.02 Æ 0.06 ‡ 2.0 Æ 0.3
À 0.15 Æ 0.04 À 1.02 Æ 0.15
À 0.20 Æ 0.01 À 0.32 Æ 0.04
À 0.38 Æ 0.02 ‡ 0.8 Æ 0.1
‡ 0.43 Æ 0.02 ‡ 1.2 Æ 0.1
0
THF
THF
THF
Lipase PS
10
11
‡ 21
‡ 22
‡ 22
‡ 11
À 5
À 1.5 Æ 0.2
À 2.9 Æ 0.5
‡ 0.19 Æ 0.02 ‡ 0.5 Æ 0.1
À 0.43 Æ 0.07 À 1.4 Æ 0.2
0.66 Æ 0.02
0.7 Æ 0.5
À 2.2 Æ 0.1
‡ 5 Æ 2
t-BuOMe Lipase PS
À 7 Æ 1
À 16 Æ 4
À 7 Æ 1
THF
THF
Lipase PS
11
À 3.6 Æ 0.3
‡ 2.6 Æ 0.7
‡ 16 Æ 3
À 7
À 0.02 Æ 0.01 À 0.20 Æ 0.02 À 0.11 Æ 0.01 À 0.51 Æ 0.04
a) There is no Tinv, thus only one set of differential parameters have been calculated.
temperature was clearly detected at 98, at which the highest enatioselectivity ratio was
reached.
Nonlinear Arrhenius plots of the rate data from enzymatic systems have been
already reported [16]. However, we observed, for the first time, that nonlinear plots of
ln E vs. 1/T from enzymatic enantioselective reactions consist of the union of two
sharply intersecting straight lines, thus identifying an inversion temperature. An Eyring
plot featuring a Tinv, even in the case of enzymatic reactions, indicates the presence of
two different substrateÀsolvent clusters: one above and one below the Tinv.
In the enantioselective resolution with chiral diamines, only with pyrrolidine 9 and
isochinoline 10 derivatives is there a Tinv; with the isoindoline derivative 11, we
obtained the highest enantioselective ratio but no Tinv. According to our interpretation,
only one soluteÀsolvent cluster is present over all the T range whenever no Tinv was
observed in an Eyring plot. It is worthwhile to note that the Tinv values obtained with
the two chiral diamines in THF compared with that obtained with CAL in THF, are in a
range of 108.
For T > Tinv, the differential activation enthalpies differ from each other but have
the same negative sign, whereas the sign of the entropy contribution, DDS= varies with
the enzyme and the solvent. The reactions with CAL and diamine 10 in THF showed a
switchover in the activation parameter on going from T> Tinv to T< Tinv, associated to
an inverted enantiomeric preference for DDH= and DDS=.
Resolution of 2-Phenylpropan-1-ol (rac-4; Scheme 3). The enzymatic resolution of
rac-4 was carried out by incubating the substrate (0.66 mmol) in THF or t-BuOMe
(10 ml) with lipase PS (30000 U/g, 40 mg), and vinyl acetate (2.64 mmol), and then
according to the same protocol as for rac-1. The chemo-catalyzed resolution was carried

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out on 1 mmol of the rac-4 in THF (10 ml) with a catalytic amount (15 ml) of chiral
diamine 10 or 11, 0.5 mmol of (i-Pr)2NEt (H¸nig base), and then AcCl (0.5 mmol). The
H¸nig base, as a proton sponge, increased the reaction rate; otherwise, with a
stoichiometric amount of chiral diamine, the reaction was too slow. The reaction was
quenched with HCl (1m) until pH 5 and extracted with Et2O. The ee values were
directly determined on the crude: the ee values of acetates 6 by GC on chiral column
and the ee values of alcohols 5 by HPLC (Chiracel OF; see Exper. Part). The absolute
configurations of the products were confirmed by comparison with the commercially
available (R)-2-phenylpropanol and its (R)-O-acetate.
Scheme 3
Fig. 2 shows the data of ln E vs. 1/T for the bio- and chemo-catalyzed acetylation of
rac-4. The enantioselectivity of the resolution of rac-4 is low for both the enzymatic and
the chemo-catalyzed reactions; however, due to the high reliability and consistency of
experimental data, we can clearly observe the presence of inversion temperatures in all
catalystÀsolvent pairs2). The Tinv values observed in THF occurred at higher T than
those observed in t-BuOMe. As observed for rac-1, the Tinv values depend on the
solvent and, in the same solvent, they are fairly similar for the enzymatic and the
chemo-catalyzed reactions. Quite interesting is the temperature behavior observed
with the two chiral diamines that exerted opposite temperature trends. This allows the
preference for the opposite enantiomer depending on the catalyst. Analyzing the
thermodynamic data, it results that this behavior derives from an opposite sign in the
enthalpy contribution (DDH=) at T> Tinv for both diamines, and, in THF, even at T<
Tinv.
2
)
We reported several cases showing high variations in enantio- or diastereoselectivity with temperature
and, in some cases, an inversion of the preferred stereoisomer with T (see, e.g., [10] and [7]).

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Fig. 2. Eyring plot for the enantioselective acetylation of 2-phenylpropanol (rac-4) a) in THF with lipase PS
(black diamond), with chiral diamine 10 (white triangle), and with 11 (white circle), b) in t-BuOMe with lipase
PS (black diamond), with chiral diamine 10 (white triangle), and with 11 (white circle)
Asymmetric Acylation of 2-Benzylpropane-1,3-diol (7; Scheme 4). The bio-
catalyzed acylation of 7 (0.66 mmol) was performed in THF (10 ml) with lipase PS
(30000 U/g, 40 mg) and vinyl acetate ( 2.64 mmol). The chemo-catalyzed reaction was
performed on 1 mmol of substrate 7 with a catalytic amount of the chiral diamine 11
(15 ml), 0.5 mmol of H¸nig base, and AcCl (0.5 mmol). In both protocols, the progress
of the reaction was followed by GC, monitoring also the formation of the diacetate and
ensuring its amount to be less than 1%. The ee values of the product 8 was determined
by chiral HLPC (Chiracel OD). The absolute configuration was determined by
comparison of HPLC retention times with the reported data3).
Fig. 3 shows the data of ln R/S vs. 1/T, where R/S is the enatiomeric ratio of the
product 8. The temperature dependence of the enantioselective acylation showed the
presence of inversion temperatures for both the bio- and the chemo-catalyzed reactions
that have similar values for the reaction with lipase PS and the diamine 11 in THF
(Table).
3
)
See the experimental section of [21].

Helvetica Chimica Acta Vol. 86 (2003)
3555
Scheme 4
Fig. 3. Enantioselective acylation of 2-benzylpropane-1,3-diol (7) with lipase PS in THF (black square) and in t-
BuOMe (black diamond), and with chiral diamine 11 in THF (white circle)
Data in the Table clearly show that the values of thermodynamic parameters and of
Tinv are characteristic for each substrateÀsolvent couple and not of the catalyst of the
enantioselective acylation. This result supports our Tinv interpretation related to the
presence of soluteÀsolvent clustering phenomena, which are dependent on temper-
ature. The temperature-dependence studies on stereoselective processes disclosed the
presence of these phenomena, but they are independent of the reactions.
To confirm this in our studies, we have already used variable-temperature (VT)
13C-NMR spectroscopy [17], because chemical shifts and coupling constants are
affected by intermolecular interactions with the solvent4).
4
)
For solvent effects on chemical shift, see, e.g., [18].

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We then conducted VT 13C-NMR experiments with substrates rac-1, rac-4, and 7 in
(D8)THF to determine the dependence of chemical shifts on temperature. All spectra,
after full assignments, showed a unique set of signals in the temperature range
explored, ensuring the presence of a population-weighted average of rapidly
interconverting conformers. In Figs. 4, 5, and 6, plots of selected chemical shifts of
substrates vs. the absolute temperature are shown. By residual statistical analyses [19],
a breaking point (TNMR) in the linear variation of chemical shift vs. T could be detected.
The correlation with two straight lines is always better than that with a single line. Each
substrate showed more than one C-atom with a nonlinear behavior of d vs. T, and with
very similar TNMR values. A direct comparison shows a close correspondence between
the Tinv obtained in bio- and chemo-catalyzed reaction in THF, and the TNMR values.
Small differences between their numerical values could account for the fact that TNMR
refers to the ground state of the reactant, and Tinv to the two diastereoisomeric
transition states.
Fig. 4. Variable-temperature 13C-NMR: chemical shifts for four C-atoms of 1-(naphthalen-2-yl)ethanol (rac-1) in
(D8)THF and the corresponding TNMR values
Conclusions. We have studied the temperature dependence of enantiomeric ratio
of bio-catalyzed (CAL and lipase PS) and chemo-catalyzed (l-proline-based diamines)
acylation reactions of 1-(naphthalen-2-yl)ethanol (rac-1), 2-phenylpropanol (rac-4),
and 2-benzylpropane-1,3-diol (7), in THF and t-BuOMe. We observed the presence of
dynamic solvation effects related to the presence of inversion temperatures in plots of
5
ln E vs. 1/T ). The Tinv values of the bio-catalyzed and the chemo-catalyzed reactions
are fairly in agreement. We then connected the Tinv values obtained in the resolutions
with the TNMR values obtained in VT 13C-NMR experiments on the substrateÀsolvent
couple in THF (Fig. 7). Tinv and TNMR are independent experimental results of the same
phenomenon related to dynamic solvation clustering. The results obtained in the
5
)
All our data clearly show a better two-line correlation of nonlinear Eyring plot than one-line, or than a
curve.

Helvetica Chimica Acta Vol. 86 (2003)
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Fig. 5. Variable-temperature 13C-NMR: chemical shifts for four C-atoms of 2-phenylpropanol (rac-4) in
(D8)THF and the corresponding TNMR values
Fig. 6. Variable-temperature 13C-NMR: chemical shifts for two C-atoms of 2-benzylpropane-1,3-diol (7) in
(D8)THF and the corresponding TNMR values
present study clearly confirm that the Tinv is a quite general phenomenon independent
of the particular reaction system, including the nature, i.e., enzyme or chiral diamine, of
the catalyst, but tightly related to the nature of the substrate, in this case the starting
alcohol, and the particular reaction solvent used. This means that the soluteÀsolvent
clusters are the real reacting species in solution, even in enzymatic reactions, implying
that solvent molecules should be present in the transition state of enzymatic reactions.
Nakamura et al. observed that lipases recognize not only the structure of the
substrate, but also that of solvent, assuming that a solvent influences enzymatic
reactions as a molecule rather than as a bulk medium [20]. Klibanov and co-workers
stated that the solvent dependence of enzymatic prochiral selectivity can be attributed
primarly to changes in the relative solvation modes of the substrate in the transition
state [21]. Our interpretation does not contrast with these observations, but we affirm,
moreover, that the soluteÀsolvent cluster is the real reacting species in solution, in
chemical as well as in enzymatic reactions.

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Helvetica Chimica Acta Vol. 86 (2003)
Fig. 7. The Tinv in an Eyring plot and the TNMR, as the interconversion temperatures between two supramolecular
soluteÀsolvent clusters
Experimental Part
General. THF and t-BuOMe were dried by distillation from sodium-benzophenone and stored over
molecular sieves (4 ä). (D8)THF was purchased from Aldrich. Lipase PS and Candida antarctica lipase were
kindly furnished by Amano Enzyme Inc. and Enzyme Europe, resp. 2-Phenylpropanol (4) is commercial, 1-
(naphthalen-2-yl)ethanol (1) was obtained in 98% yield by reduction of 1-(naphtalen-2-yl)ethanone with
NaBH4 (1.2 equiv.) in EtOH/THF, 2-benzylpropane-1,3-diol (7) was obtained in 85% yield by reduction of
commercial diethyl benzylmalonate with LiAlH4. Chiral diamines 9, 10, and 11 were obtained starting from l-
proline and the corresponding secondary amine according to the procedure reported in [22].
Reactions with enzymes were performed in screw-capped vials. To set and maintain temp. in the range of Æ
18, a Julabo FT901 immersion cooler in Dewar container filled with EtOH, an oil bath with H2O cooling, or a
water bath with a Techne TE-10D Tempunit thermoregulator was used, depending on the chosen temp. The
temp. was referred to the interior of the reaction apparatus.
13C-NMR Spectra: Varian Gemini-300 and Mercury-400 instruments with a 5-mm probe. The temp. was
calibrated from differences in the chemical shift of a MeOH sample [23]; all chemical shifts have been quoted
relative to (D8)THF signals.
Determination of the ee Values. The ee values of products were determined directly on the crude as follows:
ee of alcohols 2 and acetates 3 with a GC chiral column (Megadex 5, 25 m  0.25 mm  0.25 m, T ˆ 1558),
absolute configurations established by comparison with reported data [24]; ee of alcohols 5 by HPLC (Daicel
Chiracel OF, 25 cm  0.46 cm, hexane/i-PrOH 94 :6, 0.5 ml/min, l 214 nm) and of acetate 6 by GC (Megadex 5,
25 m  0.25 mm  0.25 m, T ˆ 908), absolute configurations established by comparison with the commercially
available (R)-2-phenylpropan-1-ol and its (R)-O-acetate; ee of 8 by HPLC (Daicel Chiracel OD, 25 cm Â
0.46 cm, hexane: i-PrOH 95 :5, 0.7 ml/min, l 214 nm), absolute configuration determined by comparison with
the data reported in [21]. The average standard deviation for the ee measurements was less than 1%.
We are grateful to Amano Enzyme Inc. for providing us with lipase PS, and to Enzyme Europe for Candida
antarctica lipase. This work was supported by MIUR and University of Bologna (funds for selected topics).

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Received August 2, 2003