Synthesis of Enantiomerically Enriched Drug Precursors by Lactobacillus paracasei BD87E6 as a Biocatalyst

Article abstract of DOI:10.1002/cbdv.201800028

Global sales of single enantiomeric drug products are growing at an alarming rate every year. A total of 7 bacterial strains were screened for their ability to reduce acetophenones to its corresponding alcohol. Among these strains Lactobacillus paracasei BD87E6 was found to be the most successful biocatalyst to reduce the ketones to the corresponding alcohols. The reaction conditions were systematically optimized for the reducing agent Lactobacillus paracasei BD87E6, which showed high enantioselectivity and conversion for the bioreduction. The preparative scale asymmetric reduction of 3-methoxyacetophenone (1h) by Lactobacillus paracasei BD87E6 gave (R)-1-(3-methoxyphenyl)ethanol (2h) with 92% yield and 99% enantiomeric excess. Compound 2h could be used for the synthesis of (S)-rivastigmine which has a great potential for the treatment of Alzheimer's disease. This study demonstrates that Lactobacillus paracasei BD87E6 can be used as a biocatalyst to obtain chiral carbinol with excellent yield and selectivity. The whole cell catalyzed the reductions of ketone substrates on the preparative scale, demonstrating that Lactobacillus paracasei BD87E6 would be a valuable biocatalyst for the preparation of chiral aromatic alcohols of pharmaceutical interest.

Full text of DOI:10.1002/cbdv.201800028

Article Type: Full Paper
Synthesis of enantiomerically enriched drug precursors by Lactobacillus paracasei BD87E6
as a biocatalyst
Selda Öksüz1, Engin Şahin1,*, and Enes Dertli1
1Department of Food Engineering, Faculty of Engineering, Bayburt University, Bayburt, Turkey
Abstract
Global sales of single enantiomeric drug products are growing at an alarming rate every year. A
total of 7 bacterial strains were screened for their ability to reduce acetophenones to its
corresponding alcohol. Among these strain Lactobacillus paracasei BD87E6 was found to be the
most successful biocatalyst to reduce the ketones to the corresponding alcohols. The reaction
conditions were systematically optimized for the reducing agent Lactobacillus paracasei
BD87E6, which showed high enantioselectivity and conversion for the bioreduction. The
preparative scale asymmetric reduction of 3-methoxy acetophenone 1h by Lactobacillus
paracasei BD87E6 gave (R)-1-(3-methoxyphenyl) ethanol 2h with 92% yield, and 99%
enantiomeric excess. Compound 2h could be used for the synthesis of (S)-rivastigmine which
have a great potential for the treatment of Alzheimer’s disease. This study demonstrates that
Lactobacillus paracasei BD87E6 can be used as a biocatalyst to obtain chiral carbinol with
excellent yield and selectivity. The whole cell catalyzed the reductions of ketone substrates on
the preparative scale, demonstrating that Lactobacillus paracasei BD87E6 would be a valuable
biocatalyst for the preparation of chiral aromatic alcohols of pharmaceutical interest.
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through the copyediting, typesetting, pagination and proofreading process, which may lead to
differences between this version and the Version of Record. Please cite this article as doi:
10.1002/cbdv.201800028
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Keywords: drug precursor, chirality, asymmetric synthesis, Lactobacillus paracasei,
biocatalysis
*Corresponding author: Engin Şahin, Department of Food Engineering, Faculty of
Engineering, Bayburt University, Bayburt, 69000, Turkey. Tel: +90 (0) 458 2111153, Fax: +90
(0) 458 2111128, Email: esahin@bayburt.edu.tr
Introduction
Chiral secondary alcohols are valuable intermediates in the preparation of a number of
biologically active molecules and drug precursory molecules. There are many methods in the
literature for preparing these chiral secondary alcohols.[1-6] These include both chemical and
biocatalytic methods.[7-9] Various chemical approaches are employed such as enantioselective
crystallization and asymmetric reduction of prochiral ketones using different chiral transition
metal catalysts. Use of ruthenium and iridium complexes, as well as the CBS oxaborolidine for
the asymmetric reduction of prochiral ketones have been well studied.[10] However, they have
their own disadvantages such as use of toxic metal catalysts, poor enantioselectivity, low
conversion, requirement of harsh conditions, and formation of by-products and they use
expensive chiral ligands, at the same times the reported methods did not always produce
satisfactory results for green and sustainable chemistry. The preparation of these chiral alcohols
using biocatalytic method is becoming progressively important nowadays due to its mild and
environmentally friendly reaction conditions. Biocatalysis is a simple, mild, cheap and
ecofriendly reagent in bio-organic synthesis and often displays high regio-, stereo-, and
enantioselectivity. The asymmetric reduction of prochiral ketones is one of the most important
transformations in organic synthesis because of the formation of optically active alcohols, which
are commonly used as intermediates for the synthesis of drugs and other industrially important
compounds. Global sales of single enantiomeric drug products are growing at an alarming high
rate every year. As a result, enormous efforts have been made in recent years to establish
enantioselective routes to enantiomerically pure compounds. Optically pure secondary alcohols
are useful as important intermediates in the synthesis of pharmaceutical and agricultural
chemicals with low production costs in industry. Sotalol is a drug used in individuals with
rhythm disturbances of the heart (cardiac arrhythmias).[11] (S)-Rivastigmine is an acetyl
cholinesterase inhibitor with brain-region selectivity and a long duration of action (Figure 1).[12]
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It was approved by the U.S. Food and Drug Administration for the treatment of Alzheimer’s
disease (AD) on April 21, 2000.[13] Isoprenaline or isoproterenol (trade names Medihaler-Iso
and Isuprel) is a medication used for the treatment of bradycardia (slow heart rate), heart block,
and rarely for asthma.[14]
Selective asymmetric reduction can be carried out by using either isolated enzymes or whole
cells. There are reports using yeasts and bacteria for the asymmetric reduction of the carbonyl
moiety to chiral alcohols.[15-20] When compared to isolated enzymes, whole bacterial, yeast,
fungal or plant suspension cells circumvent the use of external cofactors. They contain multiple
dehydrogenases that are able to accept a wide range of unnatural substrates. All enzymes and
cofactors are well protected in their natural cellular environment. Furthermore, the use of whole
cell biocatalysts avoids enzyme purification and cofactor addition.[21, 22] Reports on the bacterial
whole cell cultures for the asymmetric reduction of prochiral ketones[15-18] is scanty though they
have proved to be effective. Hence, we focused on identifying an effective bacteria cell culture
for the preparation of important chiral secondary alcohols starting from the prochiral aryl
ketones. The objective of this study was to find the most successful microorganism, in terms of
percentage conversion and enantiomeric excess, for the asymmetric reduction of acetophenone
and its derivatives. In addition, we reported the use of the Lactobacillus paracasei BD87E6 as a
novel biocatalyst for production of a chiral alcohol on a preparative scale.
Herein, we report the asymmetric reduction of aryl ketones mediated by Lactobacillus paracasei
BD87E6 suspension cells in high optical and good chemical yield. The scale-up asymmetric
reduction of 3-methoxyacetophenone 1h by Lactobacillus paracasei BD87E6 gave (R)-1-(3-
methoxyphenyl) ethanol 2h 92% yield, and >99% enantiomeric excess. Compound 2h can be
used for the synthesis of (S)-rivastigmine which have a great potential for the treatment of
Alzheimer’s disease. Optically active phenylethanol and its derivatives are useful building blocks
for the synthesis of complex molecules as the alcohol functionality can be easily transformed,
without racemization, into other functional groups.[23] (S)-rivastigmine synthesis has been
reported in the literature[13] as 7 steps, but (S)-rivastigmine could be synthesized starting from the
(R)-2h compound in both less steps and cheaper. In addition, a chiral molecule was synthesized
in small scale with 99% enantioselectivity for the synthesis of (S)-rivastigmine with the
commercially purchased pure enzyme in the literature.[24] Despite the use of pure enzyme, this
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compound, which is obtained in the literature, has both a low conversion and yield. For the
synthesis of (S)-rivastigmine, we obtained in scale-up with high conversion and yield of 2h
compound in 99% enantiomeric purity using Lactobacillus paracasei BD87E6 biocatalyst as
whole cell. The effects of the reaction conditions such as pH, temperature, and agitation speed on
the conversion, and enantioselectivity (ee) were also investigated and reported in detail. The
effect of steric and electronic properties of the substrates on the reaction catalyzed by
Lactobacillus paracasei BD87E6 was also discussed.
Results and discussion
The bioreduction of acetophenone to the corresponding (R)- or (S)-1-phenylethanol is one of the
most common reactions in organic chemistry. These chiral alcohols can be easily transformed
into other useful functional groups without racemization.[25] Acetophenone and its substituted
derivatives are the most preferred substrates for the biocatalyst mediated reduction.
Acetophenones offer several chiral building blocks for the synthesis of many pharmaceutically
important compounds.[21] Thus, we chose acetophenone as the model substrate for identifying the
best reaction conditions for our asymmetric reductions. The effect of various parameters on the
asymmetric reduction of acetophenone was studied and the results obtained are discussed below.
Seven different isolates were evaluated for the reduction of 1a to 2a. The reactions for screening
process were performed in a 250‐mL Erlenmeyer flask containing 100 mL of culture medium
(MRS broth) (Table 1). The microbial reduction was performed by suspending the dry cells (50
mg) in a 100‐ml fresh medium, then substrate 1a (1 mmol) was added directly and the mixture
was incubated on conditions of 150 rpm and 30°C for 48 h. The reaction progress was monitored
by TLC, and a chiral HPLC column was used for the determination of the enantiomeric excess of
2a and conversion of 1a to 2a. As can be seen in Table 1, the strains used in the study produced
2a ranging from 41% to 75% ee.
The best result for the bioreduction was obtained when whole cell of Lactobacillus paracasei
BD87E6 was used. The bioreduction occurred with moderate conversion and produced 2 at 75%
ee (Table 1, entry 6). To establish the optimal reaction conditions for the asymmetric reduction
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of 1a with Lactobacillus paracasei BD87E6, pH, temperature, incubation period, and agitation
speed were investigated. The results of these optimizations are given in Table 2, and pH was
investigated in the first place.
The reactions were performed for 48 h using 1 mmol of 1a in an orbital shaker at 30°C and 150
rpm. Different pH ranges (4.5‐7.5) were chosen to monitor the progress of the reduction reaction.
The highest conversion (81%) and ee (77%) were observed when the medium pH was controlled
at 5.5. Thus, this pH value was used for all further optimization studies. In general, the rate of
chemical reaction increases with an increase in the reaction temperatures. The reaction was
therefore performed under different temperatures. The best results for conversions and
enantioselectivity were observed at 30°C. Temperature over or below 30°C, both ee and
conversions dropped reasonably. These results suggest that a temperature above or below 30°C
has a negative effect on conversion and ee. This could be due to the reduction significantly
influenced by thermal induction as reported previously.[26] Therefore, optimal temperature of
30°C was recommended for the bioreduction reaction. Different incubation times were studied to
monitor the progress of the reduction reaction. The complete conversion of 1a was observed
after 96 h, but the ee of 2a decreased 75%. However, the conversion increased up to 91% with
91% ee for 48 h. The best condition lasting 48 h was used for further optimization of conversion
and ee. The effect of agitation speed on the reduction was investigate as the last parameter. It
was observed that agitation speed had a strong influence on both the ee and conversion. A
progressive increase in conversion was observed with an increase of the agitation rate from 100
to 250 rpm. However, the ee dropped when the agitation speed was increased. This decrease can
be attributed to the effect of shear stress on the Lactobacillus paracasei BD87E6 cells at high
agitation speeds, which can alter the cell internal structure and in turn lowers their activity as
reported previously[27]. Targeted ee and conversion was obtained at 150 rpm. The optimum
condition for the asymmetric bioreduction of 1a to 2a was stablished at pH of 5.5, temperature of
30°C, incubation period of 48 h, and agitation speed of 150 rpm. Under these optimizing
conditions, chiral secondary alcohol cannot be obtained as a result of reactions with without
biocatalyst and denatured biocatalyst. Under the optimum conditions, the effects of dry
biocatalyst amount on the conversion and ee of 1a reduction was investigated. The best ee (94%)
and conversion (99%) were obtained when 50 mg dry biocatalyst was applied. When the amount
of dry biocatalyst is 25 and 75 mg, the enantioselectivity decreases to 67% and 70%, respectively
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(the data not shown in Table 2). It is likely that change in cell concentration brings about change
in the amount of NADPH in the medium, and accordingly, the selectivity and activity of the
enzyme may have changed due to the changes the structure of active site of enzyme as reported
previously[28]
.
After successfully optimizing the reaction conditions, we turned our attention to the biocatalytic
reduction of substituted acetophenones 1b–1n (Table 3). Steric and electronic properties of
substituent groups play a major role in the biocatalytic reduction of aromatic ketones. The effect
of steric and electronic properties of the substrates on the reaction catalyzed by Lactobacillus
paracasei BD87E6 was evaluated.
Acetophenone derivatives with different substituents (chlorine, bromine, nitro, methyl, methoxy
and phenyl groups) on the benzene ring were selected based on their potential for the production
of industrially valuable chiral aromatic alcohols. As can be seen in Table 3, the bioreduction of
o-chloro and o-nitro derivatives 1b and 1d were transformed into 2b and 2d with 84, 44% ee
respectively and low conversion (Table 3, entry 2, 4). However, the bioreduction of 1c and 1e
were not transformed into 2c and 2e (Table 3, entry 3, 5), likely due to the strong electron
donating properties of the methoxy group and steric effects of bromine group. The low and none
conversions for substrates 1b-1e show that rate of the bioreduction depends on the steric and
electronic effects of the ortho-pozition on the aromatic ring in which results were consistent both
steric (atomic size Br>CI) and electronic factors (electronegativity CI>Br) of the groups. The
reduction of m-bromo, m-chloro and m-methyl derivatives 1f, 1g and 1i led to the formations of
alcohols 2f, 2g and 2i with moderate ee and conversions (Table 3, entry 6, 7, 9). m-methoxy
derivative 1h was reduced into the 2h alcohol in highest ee (99%) and high conversion (Table 3,
entry 8). The Lactobacillus paracasei BD87E6 reduced para halogenated acetophenones 1j and
1k to the 2j and 2k with high conversion and high enantiomeric excess (Table 3, entry 10, 11).
However, the halogenated meta-substituted acetophenone 1f gave less satisfactory results
compared with its para-substituted counterpart 1k. This behavior was not observed with chlorine
acetophenone substituent, which gave a better conversion rate and ee at the meta- and ortho-
position than the para-position. The lowest ee (42%) was obtained bioreduction of p-methoxy
acetophenone 1l, likely due to the strongly electron donating properties of the methoxy group at
the para-position (Table 2, entry 12). The high enantioselectivity (91%) was observed p-
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nitroacetophenone 1m compared to 1d (Table 3, entry 13). The product 2n was obtained
moderate conversion and enantioselectivity (Table 3, entry 14). Clearly, the efficiency of
Lactobacillus paracasei BD87E6 for the conversion of alkyl aryl ketones was different and
markedly influenced by steric and electronic factors. The configurations of the alcohols obtained
in the literature have been reported to be dependent on the steric effect of substituent in the
benzene ring.[29] However, the configurations of the alcohols in this study vary according to the
structure and position of the substituent. Presumably, this may be due to the fact that the
substituents on benzene ring differently interact with the active center of the enzyme. These
results show that this biocatalyst works as substrate selective. Since 3-methoxyacetophenone
afforded excellent enantioselectivity for the reduction on a small scale, we decided to conduct
the transformation of 1h to (R)-2h on a large scale to demonstrate the viability of the present
system as industrially feasible. With the optimum conditions in hand, we performed the scale-up
bioreduction of 3-methoxyacetophenone 1h. Preparative scale production of 2h was performed
over a 1-L scale in a 2-L erlenmeyer (Figure 2).
Bioreduction of 1h (40 mmol, 6.00 g) after 72 h resulted in complete conversion, but the ee of
the product was fairly low (69%). It has been shown that a change in substrate concentration has
an effect on the conversion and ee, which has been ascribed to cell toxicity and substrate/product
inhibition as reported previously. [27] Based on this knowledge, the substrate concentration was
investigated with slight changes. The best result was obtained with a substrate concentration of
32 mmol (4.805 g). Complete conversion and ee of >99% was achieved after 62 h of incubation
time. Then the mixture was extracted with dichloromethane and dried over anhydrous Na2SO4.
After evaporation of the solvent, the product 2h was purified on silica gel column. As shown
Figure 2, substrate 1h was easily reduced to the corresponding chiral carbinol in excellent yield
and with high selectivity. In this way, (R)-2h which could be used for the synthesis of (S)-
rivastigmine was obtained in gram scale. This result shows that Lactobacillus paracasei BD87E6
can be used on a large scale in the synthesis of important building blocks.
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Conclusions
There is considerable interest in whole cell biocatalysts in the pharmaceutical industry for their
ability to produce valuable chiral alcohols. In this work, screening of 7 bacterial strains has been
performed for the reduction of acetophenones. Among the whole cell bacteria used for the
reduction of acetophenones, Lactobacillus paracasei BD87E6 was found to be the best
biocatalyst. The conversion and enantioselectivity of aromatic ketones was different and
influenced by steric and electronic factors. (R)‐2h was produced in gram scale with both
excellent ee (99%) and yield (92%), through a cheap, simple, and efficient process. The potential
precursor of the (S)-rivastigmine drug was synthesized in a mild, cheap, and
environment‐friendly process using Lactobacillus paracasei BD87E6 biocatalyst. This strain
shows promising result as a highly attractive candidate for the biocatalytic preparation of other
carbinols in future studies. It has been shown that drug precursor materials used for the treatment
of major diseases can be synthesized at the gram level using this biocatalyst by an
environmentally friendly method. This is efficient and eco‐friendly system that exhibits excellent
enantioselectivity and can be applied for the synthesis of valuable enantiopure secondary
alcohols, which can be important intermediate product for the synthesis of biologically active
compound. This results confirmed the suitability of Lactobacillus paracasei BD87E6 as a
valuable biocatalyst for the preparation of chiral secondary alcohols of pharmaceutical interest.
Experimental section
General
All chemicals and all solvents were purchased from Sigma‐Aldrich (purity of >99%) and without
further purification. All chemicals used for bacteria growth medium were purchased from Merck.
Reactions were monitored by thin‐layer chromatography (TLC) performed by using plates
(aluminum, silica gel 60 F254 Merck, 0.25 mm) and hexane: ethyl acetate (4:1, v/v) as eluent. For
analysis purpose, a small fraction of the product was by preparative TLC. Purification of
secondary chiral alcohols were performed by column chromatography filled with silica gel
(0.063‐0.2 mm), and the product was eluted with a mixture of hexane: ethyl acetate (10:1, v/v).
High‐performance liquid chromatography (HPLC) analysis was performed on an Agilent 1260
systems equipped with a UV and chiral detector. Optical rotation was measured with a
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1
Bellingham + Stanley, ADP220, 589 nm spectropolarimeter. H and 13C NMR spectra were
recorded on Bruker 400 MHz spectrometer in CDCl3.
Bacterial strains and culture conditions
The microorganism used in this study was isolated previously from boza, a cereal‐based
[30]
fermented beverage.
Bacterial strains were propagated from their glycerol stocks by
inoculation to 10‐mL MRS broth medium (2% glucose, yeast extract [Difco] 5 g L−1, lab lemco
[Oxoid] 8 g L−1, pepton [Oxoid] 10 g L−1, C2H3NaO2.3H2O 5 g L−1, K2HPO4 2 g L−1, triamonium
citrate 2 g L−1, salt solution [MgSO4.7H2O 11.5 % (w/v), MnSO4.4H2O 2.8 % (w/v)], Tween 80
1 m L−1) followed by a 1‐day growth at 30°C. From this cultures, exponentially grown bacterial
cell was inoculated to 1000‐mL MRS broth medium at 10% concentration and incubated 2 days
at 30°C under aerobic conditions. After successfully growth of the bacteria in the flask, bacterial
cells were centrifuged, and following the wash process, whole cells were dried by freeze‐drying
(Labconco, TR) and stored at room temperature without losing activity.
General procedure for the asymmetric reduction of chiral alcohols
To 100‐mL MRS broth, 50‐mg dry bacterial strain was added and stirred on an orbital shaker at
150 rpm and 30°C. After 2 h, pH was adjusted with 1M HCI at 5.5 and shaken 2 h than 1 mmol
substrate was directly added to the medium and incubated on a shaker (150 rpm) at 30°C for 48
h. At the end of the incubation period, the cells were separated by centrifugation at 6000 × g for
5 minutes at 4°C and the supernatant saturated with NaCl then extracted with dichloromethane.
Dichloromethane extracts were combined and dried over Na2SO4. After removal of the solvent
under reduced pressure, the crude product was identified by NMR analysis. The crude product
was purified by column chromatography on silica gel using hexane: ethyl acetate as eluents
(90:10). The absolute configuration was determined by sign of specific rotation. The conversion
was determined by chromatography on a chiral column on HPLC after filtering the crude product
with a column containing small silica gel and comparing the alcohol peaks with the ketone peak.
1
Enantiomeric excess of the products was determined by HPLC analysis using chiral column. H,
13C-NMR and HPLC spectra can be found in the supporting information.
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Scale‐up bioreduction of 3-methoxyacetophenone
The 500‐mg dry Lactobacillus paracasei BD87E6 was put in a 1000 mL sterilized fresh MRS
broth as working volume in 2 L erlenmeyer flask under sterile condition and stirred on an orbital
shaker at 30°C, 150 rpm for 2 h. After 2 h, pH was adjusted 5.5 and shaken 2 h than 32 mmol 3-
methoxyacetophenone 1h was directly added to the medium and incubated on a shaker at 30°C,
150 rpm for 62 h. At the end of the incubation period, the cells were separated by centrifugation
at 6000 × g for 5 minutes at 4°C and the supernatant saturated with NaCl then extracted with
dichloromethane. Dichloromethane extracts were combined and dried over Na2SO4. After
removal of the solvent under reduced pressure, the crude product was purified by column
chromatography on silicagel using hexane: ethyl acetate as eluants (10:1, v/v). The (R)-1-(3-
methoxyphenyl) ethanol 2h was obtained in 99% ee and 92% yield.
Acknowledgements
Financial support from Bayburt University (BAP-2016/01-04) is gratefully acknowledged.
Author Contribution Statement
EŞ planned the work and wrote the manuscript, SÖ contributed the laboratory work, ED
provided the strains.
Supplementary data
Spectral data of obtained secondary alcohols, copies of 1H, 13C NMR spectrum and chiral HPLC
chromatogram of 2a-n related to this article can be found at supplementary data
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Tables
Table 1. Screening of bacteria strains for the bioreduction of acetophenone
Entry
Microbial straina
Covn[%]b Yield [%]c ee[%]d,e
Reacti
on
conditi
on:
1
2
3
4
5
6
7
Lactobacillus paracasei BD87E1
Lactobacillus paracasei BD87E2
Lactobacillus paracasei BD87E3
Lactobacillus paracasei BD87E4
Lactobacillus paracasei BD87E5
Lactobacillus paracasei BD87E6
Lactobacillus paracasei BD87E7
57
54
53
81
64
81
71
47
51
48
74
57
73
74
57 (R)
41 (S)
54 (S)
47 (S)
67 (S)
75 (S)
63 (S)
substra
te
1
mmol/
100
mL,
temperature: 30°C, time 48 h, 150 rpm.
b
c
a Comparison of the best microbial strain. The conversions were determined by HPLC. It was calculated
d
e
yields from isolated chiral alcohols. Determined by HPLC using Chiralcel OD-H column. Absolute
configurations were assigned by comparison of the sign of optical rotations relative to the values in the
literature. Results are the average of three replicates. Standard deviations were observed to be too low.
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Table 2. Optimization of reaction conditions for the asymmetric reduction of 1a by
Lactobacillus paracasei BD87E6
pH
Conv
[%]b
70
Temperature
Conv
Incubation period
Agitation speed
rpm Conv ee
pH
ee
oC
ee
h
Conv
[%]b
88
ee
[%]c,d
65 (S)
73 (S)
77 (S)
70 (S)
70 (S)
78 (S)
[%]b [%]c,d
[%]c,d
[%]b
88
[%]c,d
91 (S)
94 (S)
64 (S)
59 (S)
4.5
5
25
81
84
88
77
45
77 (S) 24
81 (S) 48
83 (S) 72
77 (S) 96
58 (S)
83 (S) 100
91 (S) 150
80 (S) 200
75 (S) 250
77
28
30
32
34
91
99
5.5
6
81
95
99
70
100
99
6.5
7
81
65
7.5
60
80 (S)
a
b
c
Substrate 1mmol. The conversion was determined by HPLC. Determined by HPLC using Chiralcel
d
OD-H column. Absolute configurations were assigned by comparison of the sign of optical rotations
relative to the values in the literature. Results are the average of three replicates. Standard deviations were
observed to be too low.
Tablo 3. Enantioselective reduction of acetophenone derivatives 1a-n by Lactobacillus
paracasei BD87E6
Entry
Substituent
R= H
Producta
(S)-2a
ee [%]b
94
Conversion [%]c
Yield [%]d
1
2
99
27
92
22
R= 2-CI
(S)-2b
84
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3
R= 2-Br
--
--
--
--
39
--
4
R= 2-NO2
R= 2-Ome
R= 3-Br
(R)-2d
--
44
--
45
--
5
6
(S)-2f
(R)-2g
(R)-2h
(S)-2i
(S)-2j
(R)-2k
(S)-2l
(S)-2m
(R)-2n
72
78
99
61
70
87
42
91
79
58
71
81
71
83
81
84
87
72
51
69
79
68
78
77
80
82
68
7
R= 3-CI
8
R= 3-OMe
R= 3-Me
R= 4-CI
9
10
11
12
13
R= 4-Br
R= 4-OMe
R= 4-NO2
R= 4-Ph
14
a
Configurations were assigned by comparison with the sign of specific rotation relative to the literature
values.b Determined by HPLC using Chiralcel OD-H, OD and AS columns. The conversions were
c
determined with HPLC. d It was calculated yields from isolated chiral alcohols. Results are the average of
three replicates. Standard deviations were observed to be too low.
Figures
Figure 1. Some chiral secondary alcohol-based drugs
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Figure 2. A gram scale production of (R)-1-(3-methoxyphenyl) ethanol (2h)
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