BVMO-catalysed dynamic kinetic resolution of racemic benzyl ketones in the presence of anion exchange resins

Article abstract of DOI:10.1039/b922693a

4-Hydroxyacetophenone monooxygenase from Pseudomonas fluorescens ACB was employed in the presence of a weak anion exchange resin to perform dynamic kinetic resolutions of racemic benzyl ketones with high conversions and good optical purities. Different parameters that affect to the efficiency of the enzymatic Baeyer-Villiger oxidation and racemisation were analyzed in order to optimize the activity and selectivity of the biocatalytic system. The Royal Society of Chemistry.

Full text of DOI:10.1039/b922693a

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www.rsc.org/obc | Organic & Biomolecular Chemistry
BVMO-catalysed dynamic kinetic resolution of racemic benzyl ketones in the
presence of anion exchange resins†‡
Cristina Rodr´ıguez,a Gonzalo de Gonzalo,a Ana Rioz-Mart´ınez,a Daniel E. Torres Pazmin˜o,b Marco W. Fraaijeb
and Vicente Gotor*a
Received 30th October 2009, Accepted 30th November 2009
First published as an Advance Article on the web 7th January 2010
DOI: 10.1039/b922693a
4-Hydroxyacetophenone monooxygenase from Pseudomonas fluorescens ACB was employed in the
presence of a weak anion exchange resin to perform dynamic kinetic resolutions of racemic benzyl
ketones with high conversions and good optical purities. Different parameters that affect to the
efficiency of the enzymatic Baeyer–Villiger oxidation and racemisation were analyzed in order to
optimize the activity and selectivity of the biocatalytic system.
donating substituent at the para-position.9 The enzyme is also able
to catalyze the Baeyer–Villiger oxidation of a wide variety of other
ketones and aldehydes, including heteroaromatic and aliphatic
compounds. Some years ago, Berezina et al. described the first
Introduction
Dynamic kinetic resolutions (DKRs) have emerged in the last
few years as very useful tools in organic synthesis.1 By combining
a selective kinetic resolution (KR) with an in situ efficient racemi-
sation of the starting material, DKRs can theoretically provide
dynamic kinetic resolution applied to a Baeyer–Villiger oxida-
tion using a recombinant E. coli/cyclohexanone monooxygenase
single enantiomeric products with 100% yields. Racemisation can
strain. This was made possible by performing the oxidations at
be performed by a chemocatalyst or a biocatalyst, or it can occur
a relatively high pH (at least 8.0), as under these conditions the
spontaneously. The use of biocatalysts represents an attractive
alternative to conventional chemical methods, due to the mild
and environmentally friendly conditions and the high chemo-,
regio- and/or enantioselectivities associated with DKR processes
catalysed by enzymes.2 A common approach for enzymatic DKRs
substrates racemise via a keto–enol tautomerisation due to the
acidic character of the a-proton.10 This process was improved by
combining the whole cell Baeyer–Villiger oxidation with an ion-
exchange resin-catalysed in situ racemisation of the substrate.11
In the present paper, we describe for the first time the use of an
is the combination of selective acylation of secondary alcohols
isolated BVMO for catalysing the dynamic kinetic resolution of a
or amines catalysed by lipases coupled with transition metal-
range of benzyl ketones yielding the corresponding benzyl esters
catalysed racemisation,3 but lately, several DKRs have been
with high conversions and optical purities.
developed employing other biocatalysts.4
Baeyer–Villiger monooxygenases (BVMOs) are nicotinamide-
dependent flavoproteins able to perform a wide set of oxidative
reactions.5 In the last few years, a great number of BVMOs
Results and discussion
All the Baeyer–Villiger reactions were performed employing
are becoming available due to progress in genome sequencing
and mining.6 This resulted in a boost in research focusing on
their application for asymmetric Baeyer–Villiger oxidations and
for the oxygenation of heteroatom-containing compounds (S,
Se, P and N). One of the recently cloned and overexpressed
BVMOs is 4-hydroxyacetophenone monooxygenase (HAPMO)
from Pseudomonas fluorescens ACB.7 HAPMO represents the first
identified BVMO primarily active on aromatic ketones.8 Studies
on substrate specificity have demonstrated that HAPMO accepts
preferably acetophenones and benzaldehydes bearing an electron-
isolated HAPMO. The oxidations were coupled to a second en-
zymatic reaction catalysed by glucose-6-phosphate dehydrogenase
(G6PDH) in order to regenerate NADPH.12
We have previously reported that the HAPMO-catalysed oxida-
tion of ( )-3-phenylbutan-2-one 1a occurred with high selectivity,
yielding (R)-1a and (S)-1-phenylethyl acetate 1b.13 This kinetic
resolution was found to be pH-dependent, as at relatively high
pH, higher enzymatic activity and a loss in the biocatalyst
enantioselectivity (E value) was observed.14 Thus, the enzymatic
resolution of ( )-1a at pH 8.0 (2 h) occurred with 25% conversion
and an E value of 136, while at pH 10, 48% conversion and a good
enantioselectivity (E = 60) was obtained.
aDepartamento de Qu´ımica Orga´nica e Inorga´nica, Instituto Universitario
de Biotecnolog´ıa de Asturias, Universidad de Oviedo, C/Julia´n Claver´ıa 8,
33006, Oviedo, Spain. E-mail: vgs@fq.uniovi.es; Fax: (+34) 985 103448;
Tel: (+34) 985 103448
Due to the acidic character of the a-hydrogen of ketone 1a, a
pH-dependent racemisation can be expected due to the keto–enol
tautomerisation. However, when (R)-1a of 43% ee was dissolved in
Tris/HCl pH 8.0 and 25 ◦C, hardly any racemisation was observed
within 120 h (krac = 4.9 ¥ 10-4 h-1). When the pH was increased to
10.0, only a slight decrease in the ee value of (R)-1a was observed
after 120 h (ee = 34%), with a higher racemisation constant being
achieved than at pH 8.0 (krac = 0.002 h-1). Thus, when racemic
3-phenylbutan-2-one was oxidized in the presence of HAPMO at
bLaboratory of Biochemistry, Groningen Biomolecular Sciences and Biotech-
nology Institute, University of Groningen, Nijenborgh 4, 9747, AG, Gronin-
gen, The Netherlands
† This paper is part of an Organic & Biomolecular Chemistry web theme
issue on biocatalysis
‡ Electronic supplementary information (ESI) available: Experimental
data, characterization data of compounds and 1H, 13C and DEPT spectra.
See DOI: 10.1039/b922693a
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Table 1 Dynamic kinetic resolution of ( )-1a in the presence of different
can be performed in order to achieve the chiral ester (Scheme 1).
Racemisation in the presence of this resin was better than for other
weak resins, as the remaining (R)-1a displayed an enantiomeric
excess of only 16%.
anion exchange resins when working at pH 10.0 and 25 ◦Ca
Entry Resin
ee (R)-1a (%)b ee (S)-1b (%)b Conv. (%)b
1
2
3
4
5
6
7
8
None
Amberlyst A-26
90
17
21
75
73
67
54
72
77
84
77
17
40
22
33
56
50
86
Amberlite IRA-440C ≤3
Amberlite IRA-900 ≤3
Dowex 1
13
31
35
16
Amberlite IRA-67
Lewatit MP62
Dowex MWA-1
a Reaction time: 116 h. For reaction details, see the Experimental part.
b Measured by GC.
pH 10.0 and 25 ◦C (Table 1, entry 1), (R)-1a presented a 90%
ee after 120 h in a process with high conversion (c = 77%),
indicating that the racemisation of the substrate was inefficient.
(S)-1-phenylethyl acetate was obtained with a low optical yield
(ee = 21%).
Scheme 1 HAPMO-catalyzed DKR of ( )-1a employing anion exchange
resins.
For a successful DKR the rate of the racemisation needs to
be greater than the rate of the HAPMO-catalysed oxidation,
something that does not occur when working under the conditions
described above. For this reason, the enzymatic resolution of ( )-
1a was tested in the presence of different anion exchange resins
(10.0 mg resin mL-1), in order to increase the racemisation rate
of ketone 1a (see the ESI‡). First, we analysed the change in
the optical purity of (R)-1a at pH 10.0 in the presence of different
anion exchange resins. The use of a strong anionic resin (Amberlite
Once the best exchange resin had been selected for the prepa-
ration of (S)-1b with the highest yield and optical purity, some
of the parameters that can affect dynamic kinetic resolution were
analyzed (Table 2). Despite the negative effect of strong anionic
resins on the enzymatic activity, we analyzed the DKR of ( )-1a
using lower amounts of Amberlite IRA-440C and IRA-900A. As
shown in entries 1 and 2, complete racemisation of ( )-1a was
achieved with only a slight increase in (S)-1b formation. The
enzymatic biooxidation of ketone ( )-1a was then performed by
using different amounts of Dowex MWA-1 (entries 3–5) with the
aim of improving the racemisation rate. However, the increase of
the anion exchange resin in the medium had a negative effect on
the enzymatic activity, and (S)-1b was obtained with conversions
lower than 30% after 120 h (entries 4 and 5). By altering the
reaction temperature, differences in the racemisation rate could be
observed. As expected, when the HAPMO-catalysed oxidation
was performed at 10 ◦C, a low racemisation was observed
(entry 6), yielding 35% of optically active (S)-1-phenylethyl
acetate. On the other hand, the use of higher temperatures could
IRA-440C) led to a fast racemisation of the starting ketone (krac
=
0.041 h-1). After 24 h, (R)-1a was obtained with ee = 5%, while the
racemic ketone was recovered after 72 h. When using weak resins,
two different behaviours were achieved. While the addition of
Lewatit MP62 resulted in a slow drop in the enantiomeric excess of
(R)-1a, with a racemisation constant of 0.016 h-1 being measured,
this process occurred much faster in the presence of Dowex MWA-
1 (ee = 10% after 24 h, krac = 0.03 h-1). Racemisation of the final
product (S)-1b was also studied, but no change in optical purity
was observed when this ester was dissolved in Tris/HCl solutions
at pH 10.0 containing different anion exchange resins. Thus, it can
be established that the optical purities of (S)-1b are only due to
the HAPMO selectivity in the conditions employed.
Table 2 Enzymatic preparation of (S)-1b through DKR processes cat-
After establishing the effect of the exchange resins on the
racemisation of ketone 1a, its HAPMO-catalysed oxidation was
performed, as indicated in Table 1. The use of strong anion resins
led to a significant deactivation in enzymatic activity (entries 2–5).
For this reason, all conversions led to values lower than 40% after
116 h, and (S)-1b was formed with moderate optical purities.
Subsequently, the reactions were carried out in presence of weak
anion resins. Under these conditions, the racemisation process
was slower than when employing the strong resins, as is shown
by the enantiomeric excesses obtained for the remaining ketone.
This effect is clearly seen when using Amberlite IRA-67 or Lewatit
MP62 (entries 6 and 7, respectively). (R)-1a was recovered for both
resins with ee values near 30%, while (S)-1b was obtained with a
moderate optical yield and conversions of 50%. The best result
was obtained when using Dowex MWA-1, as shown in entry 8.
In presence of this weak anion exchange resin, an 86% conversion
of (S)-1b with 84% optical purity can be obtained after 116 h.
This demonstrates that under these reaction conditions, a DKR
alyzed by HAPMO and Dowex MWA-1a
[1a]/ Resin/
Entry mM mg
ee ket ee est Conv.
pH
T/◦C t/h
(%)b
(%)b
(%)b
1c
2d
3
4
5
6
7
8
9
10
10
10
10
10
10
10
10
10
5
4
4
10
10
10
10
10
10
10
8
25
25
25
25
25
10
40
25
25
25
25
25
116
116
116
120
120
144
144
144
144
72
≤3
≤3
16
5
75
69
84
72
73
80
53
86
87
61
84
78
47
33
86
28
25
35
24
20
61
91
59
44
10
20
30
10
10
10
10
10
10
10
4
55
≤3
12
10
46
5
9
10
11
12
10
10
10
20
40
116
116
≤3
a For reaction details, see the Experimental part. b Measured by GC.
c Reaction performed with Amberlite IRA-440C. d Amberlite IRA-900 was
employed
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Table 3 HAPMO-catalyzed dynamic kinetic resolution of different ben-
improve the racemisation, but the DKR process was negatively
affected due to the inactivation of HAPMO (entry 7, 53% ee).13
Racemic 1a and (S)-1b with low optical purity were recovered in
zyl ketones employing Dowex MWA-1a
ee ket
(%)b
ee est
(%)b
Conv.
(%)b
◦
a process with 24% conversion after 144 h at 40 C. In order to
Ketone
R1
R2
X
t/h
enhance both the activity and selectivity of the enzymatic system,
lower pHs were used (entries 8 and 9). At pH 8.0 a considerable
decrease in enzymatic activity was observed, reaching only 20%
conversion after 144 h. When the oxidation was performed at
pH 9.0, (S)-1b can be obtained with 61% conversion (87% ee).
Finally, ketone concentration was also optimised. When using
5 mM of 1a, it was possible to achieve a 91% conversion after
72 h. However, under these conditions, the racemisation was not
very efficient, as (R)-1a was only obtained with 46% enantiomeric
excess and (S)-1b with a moderate optical purity (61%, entry
10). Although conversions were much lower when increasing the
substrate concentration, the space time yield (expressed as mmol
of ( )-1a per L of solution h-1) improved from 0.07 mmol L-1 h-1
at 5 mM until reaching the highest value (0.15 mmol L-1 h-1) when
using a ketone concentration of 40 mM. This result indicates a
more efficient biooxidation while increasing the substrate amount.
To obtain a better view of the DKR progress of the HAPMO-
catalysed oxidation of ( )-1a in buffer at pH 10.0 containing
( )-2a
( )-3a
( )-4a
( )-5a
( )-6a
( )-7a
Me
Me
Me
Me
Et
Me
Me
Me
Et
Me
Et
m-Me
m-CF3
p-Cl
H
H
H
120
96
120
96
96
144
21
17
14
17
13
17
81
79
58
83
80
65
69
88
69
66
72
46
Pr
a For reaction details, see Experimental Part. b Measured by GC.
aromatic ring were also tested. Compound ( )-3a was previously
resolved as a substrate for HAPMO at pH 8.0 with a high E value
(E = 112) and39% conversion after 2h.9a When workingat pH10.0
with Dowex MWA-1, (S)-3b was obtained with 88% conversion
and good optical purity after 96 h. The 4-chloro derivative was
oxidized with moderate optical purity (ee = 58%) to (S)-1-(4-
chlorophenyl)ethyl acetate (S)-4b in a reaction slower than when
using the 3-trifluoromethyl derivative (c = 69% after 120 h).
This compound was previously resolved by HAPMO-catalyzed
oxidation at pH 8.0 with moderate enantioselectivity (E = 50).
◦
Dowex MWA-1 at 25 C, the reaction was monitored over time.
As shown in Fig. 1, the biooxidation initially presented a classical
kinetic resolution behaviour, in which the optical purity of ketone
(R)-1a increased while there was only a slight decrease in the
enantiomeric excess of (S)-1b. After 24 h, 64% of (S)-1a with
90% enantiomeric excess was obtained, while the optical purity
of (R)-1a reached a maximum value (ee = 37%). From this time,
racemisation of the starting ketone was predominant, producing
an important loss in the optical purity of 1a while the ee of (S)-1b
decreased to a small extent. Thus, after 72 h, oxidation had reached
a 79% conversion, leading to chiral ester with an 86% enantiomeric
excess. Prolonged reaction times led to a slight increase in the
conversion (86%), while no significant changes in the enantiomeric
excesses of substrate and product were measured (t = 116 h).
Scheme 2 DKR of different benzyl ketones by HAPMO-catalyzed
Baeyer–Villiger oxidation to the corresponding optically active benzyl
esters.
The same reaction conditions were applied for the DKR of
the racemic pentanones ( )-2-phenylpentan-3-one ( )-5a and ( )-
3-phenylpentan-2-one ( )-6a. For both substrates, conversions
close to 70% were measured after 96 h, while (S)-5–6b were
obtained with 83 and 80% enantiomeric excess, respectively.
Finally, oxidation of a long alkyl chain ketone such as ( )-7a
led to the formation of (S)-7b with a conversion lower than 50%
after 144 h. This compound was achieved with 65% enantiomeric
excess.
Conclusions
In the present paper, the dynamic kinetic resolutions of different
benzyl ketones could be performed by combining an isolated
BVMO-catalysed Baeyer–Villiger oxidation and a racemisation
catalysed by different anion exchange resins. The results obtained
revealed that resins with strong character resulted in a relatively
fast racemisation but also lead to significant enzyme inactivation.
The best results were achieved with Dowex MWA-1, a weak anion
resin, for which it was possible to obtain (S)-1-phenylethyl acetate
with high conversions and optical purities. The optimisation of
the DKR was performed by analyzing the effect of the resin
concentration, pH and temperature, resulting in the observation
that the use of high amounts of resin inactivates the enzyme,
the optimal temperature was found to be 25 ◦C and a pH of
10.0 produces the highest conversion and enantiomeric excess of
Fig. 1 Time study of the DKR of ( )-1a catalyzed by HAPMO in presence
of Dowex MWA-1. Conversion (᭺) and enantiomeric excesses of (R)-1a
(ꢀ) and (S)-1b (ꢀ) are represented.
Once the best reaction conditions were established, the appli-
cability of the system using other benzyl ketones was studied
(Scheme 2 and Table 3). Racemic 3-(3-methylphenyl)butan-2-one
was selectively oxidized to the corresponding ester (S)-2b with
81% enantiomeric excess and 69% conversion after 120 h. DKRs
of benzyl ketones presenting electron-withdrawing groups in the
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(S)-1b. This resin was finally tested in the effective DKR of a
number of benzyl ketones using isolated HAPMO, leading to
moderate to good results depending on the substrate structure.
containing glucose-6-phosphate (20 mM), glucose-6-phosphate
dehydrogenase (5.0 units), NADPH (0.2 mM), HAPMO (2.0 mM)
and the corresponding anion exchange resin (10.0 mg). Reactions
were shaken at 250 rpm and 25 ◦C for the times established. Once
finished, the mixtures were extracted with AcOEt (2 ¥ 500 mL).
The organic phases were dried onto Na2SO4 and analyzed directly
by GC in order to determine the conversion and the enantiomeric
excesses of the esters (S)-1–7b and the remaining ketones (R)-1–7a.
Experimental
Recombinant HAPMO from Pseudomonas fluorescens ACB was
overexpressed and purified as previously described.7 Glucose-6-
phosphate dehydrogenase from Leuconostoc mesenteroides was
obtained from Fluka-Biochemika. Anion exchange resins were
purchased from Sigma-Aldrich-Fluka. Before use, they were
washed with a NaOH solution (1.0 N), then with water until
neutrality and filtrated under reduced pressure. All reagents
and solvents were of the highest quality grade available and
were obtained from Sigma-Aldrich-Fluka and Acros Organics.
The corresponding racemic ketones ( )-1–7a were synthesised
according to the literature, employing the corresponding alkyl
iodide and NaOH in a biphasic system water–CH2Cl2.15 All
ketones exhibit physical and spectral properties in accordance with
those reported.9a,13,16 Esters ( )-1–7b were prepared by the chemical
acylation of the racemic alcohols by using the corresponding
anhydride in CH2Cl2 and pyridine.
Scale-up of the HAPMO-catalysed Baeyer–Villiger oxidation of
ketone ( )-1–7a in presence of Dowex MWA-1
Racemic ketones ( )-1–7a (50 mg) were dissolved in a 50 mM
Tris/HCl buffer at pH 10 (15 mL) containing glucose-6-
phosphate (20 mM), glucose-6-phosphate dehydrogenase (50.0
units), NADPH (0.2 mM) HAPMO (2.0 mM) and Dowex MWA-
1 (100 mg). Reactions were shaken at 250 rpm and 25 ◦C for
the times established (116 h for ( )-1a; 96 h for compounds
( )-3a and ( )-5–6a; 120 h for ( )-2a and ( )-4a and 144 h for
( )-7a). Once finished, the crude reactions were extracted with
EtOAc (4 ¥ 5 mL). The organic phases were dried onto Na2SO4
filtered and evaporated under reduced pressure. The crude residues
were purified by flash chromatography on silica gel using hexane–
diethyl ether 8 : 2 (compounds 1–2a, 4a and 5–6a), hexane–diethyl
ether 7 : 3 (ketone 3a) or hexane–ethyl acetate 8 : 2 (7a) to afford
the corresponding (S)-esters. For all these reactions, some amount
(5–15%) of the alcohols formed by the non enzymatic hydrolysis
of esters 1–7a was observed.
Chemical reactions were monitored by analytical TLC, per-
formed on Merck silica gel 60 F254 plates and visualized by
UV irradiation. Flash chromatography was carried out with
1
silica gel 60 (230-240 mesh, Merck). H-NMR, 13C-NMR and
DEPT spectra were recorded with tetramethylsilane (Me4Si) as the
internal standard with a Bruker AC-300 DPX (1H: 300.13 MHz;
13C: 75.5 MHz) spectrometer. The chemical shift values (d) are
given in ppm. APCI+ and ESI+ using a Hewlett Packard 1100
chromatograph mass detector or EI+ with a Hewlett Packard
5973 mass spectrometer were used to record mass spectra (MS).
GC analyses were performed on a Hewlett Packard 6890 Series
II chromatograph equipped with a Restek RtbDEXse (30 m ¥
0.25 mm ¥ 0.25 mm, 1 bar N2), a Varian CP-Chiralsil-DEX CB
(25 m ¥ 0.32 mm ¥ 0.25 mm, 1 bar N2) or a Mercherey-Nagel
Hydrodex-b-TBOAc (30 m ¥ 0.25 mm ¥ 0.25 mm, 1 bar N2) for
chiral determinations or a HP-1 (crosslinked methyl siloxane, 30
m ¥ 0.25 mm ¥ 0.25 mm, 1.0 bar N2) from Hewlett-Packard for
measuring the conversions values. For all the analyses, the injector
temperature is 225 ◦C and the FID temperature is 250 ◦C.
Acknowledgements
C.R. thanks the Principado de Asturias for her predoctoral
fellowship. A.R.-M. (FPU program) acknowledges the Spanish
Ministerio de Ciencia e Innovacio´n (MICINN) for her predoctoral
fellowship, which is financed by the European Social Fund.
G.d.G. (Juan de la Cierva Program) thanks MICINN for personal
funding. Financial support from MICINN (Project CTQ2007-
61126) is gratefully acknowledged. M.W.F. and D.E.T.P. receive
support from the EU-FP7 “Oxygreen” project.
Notes and references
1 (a) Z. Ding, J. Yang, T. Wang, Z. Shen and Y. Zhang, Chem. Commun.,
2009, 571; (b) M.-J. Kim, J. Park, Y. K. Choi, in Multi-Step Enzyme
Catalysis, ed. E. Garc´ıa-Junceda, Wiley-VCH, Weinheim, 2008, pp. 1;
(c) H. Pellissier, Tetrahedron, 2008, 64, 1563; (d) B. Mart´ın-Matute, J.-E.
Ba¨ckvall, in Asymmetric Organic Synthesis with Enzymes, ed. V. Gotor,
I. Alfonso and E. Garc´ıa-Urdiales, Wiley-VCH, Weinheim, 2008, pp.
89; (e) Q. Chen and C. Yuan, Chem. Commun., 2008, 5333; (f) O. Pa`mies
and J.-E. Ba¨ckvall, Chem. Rev., 2003, 103, 3247.
2 (a) J. M. Woodley, Trends Biotechnol., 2008, 26, 321; (b) Asymmetric
Organic Synthesis with Enzymes, ed. V. Gotor, I. Alfonso and E. Garc´ıa-
Urdiales, Wiley-VCH, Weinheim, 2008; (c) Biocatalysis in the Pharma-
ceutical and Biotechnology Industry, ed. R. N. Patel, CRC Press, Boca
Raton, 2007.
Racemisation studies of 3-phenylbutan-2-one, 1a
Racemisation experiments were performed by dissolving optically
active 3-phenylbutan-2-one (R)-1a (10 mM) isolated from the
preparative kinetic resolution of ( )-1a performed at pH 8.0 and
20 ◦C with HAPMO, in Tris/HCl buffer containing the anion
exchange resins at the selected conditions (pH and temperature).
The solution was shaken for different reaction times and aliquots
were taken, extracted with ethyl acetate, dried onto Na2SO4 and
analyzed by GC in order to determine the enantiomeric excesses.
The same experiments were performed for 1-phenylethyl acetate
(S)-1b.
3 Some examples of lipase-catalysed DKR: (a) A. Traff, K. Boga´r, M.
Warner and J. E. Ba¨ckvall, Org. Lett., 2008, 10, 4807; (b) S. Wuyts,
J. Wahlen, P. A. Jacobs and D. E. De Vos, Green Chem., 2007, 9,
1104; (c) K. Boga´r and J.-E. Ba¨ckvall, Tetrahedron Lett., 2007, 48,
5471; (d) A. Berkessel, M. L. Sebastian-Ibarz and T. N. Mu¨ller, Angew.
Typical procedure for the BVMO-catalyzed dynamic kinetic
resolution of racemic benzyl ketones ( )-1-7a
¨
Chem., Int. Ed., 2006, 45, 6567; (e) P. Odman, L. A. Wessjohann and
U. T. Bornscheuer, J. Org. Chem., 2005, 70, 9551; (f) M.-J. Kim, H. M.
Kim, D. Kim, Y. Ahn and J. Park, Green Chem., 2004, 6, 471; (g) B.
Mart´ın-Matute, M. Edin, K. Boga´r and J.-E. Ba¨ckvall, Angew. Chem.,
Int. Ed., 2004, 43, 6535.
Unless otherwise stated, the starting racemic ketones ( )-1–7a (10
mM) were dissolved in a 50 mM Tris/HCl buffer at pH 10 (1.0 mL)
1124 | Org. Biomol. Chem., 2010, 8, 1121–1125
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4 For some recent examples: (a) S. Lu¨deke, M. Richter and M. Mu¨ller,
Adv. Synth. Catal., 2009, 351, 253; (b) R. M. Haak, F. Berthiol, T.
Jerphagnon, A. J. A. Gayet, C. Tarabiono, C. P. Postema, V. Ritleng,
M. Pfeffer, D. B. Janssen, A. J. Minnaard, B. L. Feringa and J. G. de
Vries, J. Am. Chem. Soc., 2008, 130, 13508; (c) D. Giacomini, P. Galletti,
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