Candida zeylanoides as whole-cell biocatalyst to perform asymmetric bioreduction of benzophenone derivatives

Article abstract of DOI:10.1080/00397911.2019.1710213

Candida zeylanoides P1 was investigated as whole cell biocatalyst for the bioreduction of biaryl prochiral ketones into chiral carbinols, which can be used as pharmaceutical intermediate. Bioreduction of different biaryl ketones was carried out to their corresponding chiral biaryl carbinols such as (S)-(4-chlorophenyl) (phenyl) methanol (2a), which can be used in the synthesis of L-cloperastine drug, with antitussive, antiepidemic activity and bronchial musculature relaxant characteristics, in gram scale, enantiopure form (>99%) and excellent yields. The selectivity of C. zeylanoides P1 in enantioselective reduction of biaryl ketones was not affected by the steric and electronic effects of substrates. The current method demonstrates an encouraging green chemistry approach for the production of biaryl secondary chiral alcohols of pharmaceutical importance in mild, inexpensive and environmentally friendly process. The present study has many benefits since this yeast biocatalyst were successfully applied bioreduction of structurally bulky prochiral substrates, which cannot be reducted by chemical catalysis.

Full text of DOI:10.1080/00397911.2019.1710213

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Candida zeylanoides as whole-cell biocatalyst
to perform asymmetric bioreduction of
benzophenone derivatives
Engin Şahin
To cite this article: Engin Şahin (2020): Candidaꢀzeylanoides as whole-cell biocatalyst to
perform asymmetric bioreduction of benzophenone derivatives, Synthetic Communications, DOI:
10.1080/00397911.2019.1710213
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https://doi.org/10.1080/00397911.2019.1710213
Candida zeylanoides as whole-cell biocatalyst to perform
asymmetric bioreduction of benzophenone derivatives
Engin S¸ahin
Department of Food Engineering, Faculty of Engineering, Bayburt University, Bayburt, Turkey
ABSTRACT
ARTICLE HISTORY
Received 19 September 2019
Candida zeylanoides P1 was investigated as whole cell biocatalyst for
the bioreduction of biaryl prochiral ketones into chiral carbinols,
which can be used as pharmaceutical intermediate. Bioreduction of
different biaryl ketones was carried out to their corresponding chiral
biaryl carbinols such as (S)-(4-chlorophenyl) (phenyl) methanol (2a),
which can be used in the synthesis of L-cloperastine drug, with anti-
tussive, antiepidemic activity and bronchial musculature relaxant
characteristics, in gram scale, enantiopure form (>99%) and excellent
yields. The selectivity of C. zeylanoides P1 in enantioselective reduc-
tion of biaryl ketones was not affected by the steric and electronic
effects of substrates. The current method demonstrates an encourag-
ing green chemistry approach for the production of biaryl secondary
chiral alcohols of pharmaceutical importance in mild, inexpensive
and environmentally friendly process. The present study has many
benefits since this yeast biocatalyst were successfully applied biore-
duction of structurally bulky prochiral substrates, which cannot be
reducted by chemical catalysis.
KEYWORDS
Chirality; Candida
zeylanoides; chiral
benzophenone carbinols;
enantioselective reduction;
(S)-(4-chlorophenyl)
(phenyl) methanol
GRAPHICAL ABSTRACT
Introduction
The production of enantiomerically pure compounds is very important for the pharma-
ceutical, agricultural, food and perfume industries.[1] In racemic mixtures, each of enan-
tiomers can have very different effects. The main reason for interest in optically active
products is that the enantiomers exhibit different biological activities. While one of the
enantiomers in the drug has the desired activity, the other enantiomer can show differ-
ent and often harmful pharmacological properties, or exhibits serious side effects.
CONTACT Engin S¸ahin
esahin@bayburt.edu.tr
Faculty of Engineering, Department of Food Engineering, Bayburt
University, Bayburt, 69000, Turkey.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article can be accessed on the publisher’s website
ß 2020 Taylor & Francis Group, LLC

2
E. ŞAHIN
Figure 1. Diarylcarbinol-based drugs.
The two enantiomers may independently have a different therapeutic effect or the com-
bination of both enantiomers may be advantageous for treatment. Diaryl carbinols can
use as precursors of numerous biologically and pharmacologically important
molecules, such as (R)-orphenadrine, (R)-neobenodine, (S)-cetirizine, carbinoxamine
and L-cloperastine, which possesses antitussive and antiedemic activity, also relaxes the
bronchial musculature (Figure 1).[2–4] Numerous articles have been published years
using various methods to produce enantiopure diaryl carbinols in recent.[5,6] Various
chemical approach are employed such as enantioselective reduction of ketones in the
presence of chiral catalysts or by nucleophilic addition to the aldehydes, by means of
aryl nucleophiles that are generally costly diphenylzinc or aryl zinc reagents obtained in
situ from aryl boronic acids. However, the enantioselective reduction of biaryl ketones
by chemical catalysts have own disadvantages such as limited substrate scope and harsh
conditions.[7–9] These drawbacks restrict its application in industrial-scale production.
Biocatalysts have many benefits compared to chemical catalysts. Compared with
asymmetric chemical processes, biotransformation processes of chiral secondary alcohol
production afford several advantages, such as milder reaction operation conditions
(ambient pressure and temperature), no need to use toxic chemicals and expensive
heavy metal catalysts.[10,11] Biocatalytic methods are typically employed in industrial
scale using either the resolution of racemate or asymmetric reduction of prochiral
ketones. In resolution of racemate, there is always 50% of the unwanted enantiomer
unlike asymmetric reduction which preferably results in the generation of only one
enantiomer in high enantiomeric purity and chemical yields.[12] Chiral secondary alco-
hols can be synthesized by isolated enzyme, but expensive reducing cofactors, such as
NADPH, and purification of enzymes are disadvantage for this reaction. These disad-
vantages limit its use in industrial-scale synthesis.[13,14] When compared to isolated
enzymes, whole cell biocatalysts are more advantageous as these biocatalysts are usually
cheap and stable. Moreover, the use of whole cell biocatalysts avoids enzyme purifica-
tion and cofactor addition.[15,16] Also, whole cell biocatalyst is well accepted to be a

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3
safe, economical, and mild reaction conditions to synthesis chiral secondary carbi-
nols.[17,18] Chiral biaryl secondary alcohols are very important intermediates in the syn-
thesis of large number of biologically active molecules and can also be used in the
synthesis of drugs.[19–21] At the same time, the alcohol functional group can be easily
transformed into the desired functional groups without racemization.[22,23] Biocatalysts
have been indicated to be extremely enantioselective in the bio-reduction of a broad
range of ketones bearing some bulky groups, furnishing a supplementary way of enan-
tioselective reduction of biaryl prochiral ketones that are difficult with other chemical
catalysts.[24–27] Many diaryl ketones used in the scope of this study were reduced with
good selectivity by selecting various biocatalysts.[28–30] Although good selectivity was
obtained in these studies, it was observed that the conversions were below 30%. Diaryl
ketones have been reported previously known as “hard to reduce” substances for the
biocatalyst due to large steric hindrances and the like of two aromatic groups.[8,26,31,32]
High conversion and selectivity were obtained by asymmetric reduction of diaryl
ketones using Sporobolomyces salmonicolor and its mutant enzymes however, these
results were obtained using expensive pure enzymes and cofactors.[33] In our previous
study, it has been found that C. zeylanoides P1 isolated from pastirma a fermented
Turkish meat product catalyzed the extremely enantioselective bioreduction of prochiral
ketones that it was a good biocatalyst for the reduction of prochiral ketones.[34] We
envisioned that the C. zeylanoides P1 could also reduce sterically bulky biaryl ketones as
substrates. Importantly, as food isolates, this biocatalyst has major potential to use con-
fidently for the industrial aims that rise their value as biocatalyst.
Herein, we reported that a variety of biaryl ketones were enantioselectively reduced
by this biocatalyst in gram scale using optimized conditions of C. zeylanoides P1 as a
yeast whole cell biocatalyst. Besides, we have shown that C. zeylanoides P1 can be used
as a new biocatalyst for synthesis of chiral bulky secondary alcohols on gram scale.
Also, (S)-(4-chlorophenyl) (phenyl) methanol (2a), which can be used in the synthesis
of L-cloperastine drug, with antitussive, antiepidemic activity and bronchial musculature
relaxant, was obtained in gram scale, enantiopure form (>99%) and excellent yields.
Results and discussion
The asymmetric reduction of prochiral biaryl ketones to the corresponding chiral biaryl
secondary alcohols is one of the most widespread reactions in organic chemistry.[35]
From synthetic bio-organic perspective, it is essential to optimize bioreduction condi-
tions that improve the efficiency of the biocatalyst to perform enantiomerically pure
alcohols.[36] We have successfully optimized these asymmetric reduction conditions
using model substrate acetophenone (1 mmol) with C. zeylanoides P1 biocatalyst and in
all reactions numbers of the C. zeylanoides P1 cells were similar determined by OD600
measurements and plating to YPD agar.[34] The optimum condition for the asymmetric
bioreduction of acetophenone to (S)-1-phenyl ethanol was obtained at pH of 6.5, tem-
perature of 30 ꢀC, incubation period of 48 h, and agitation speed of 150 rpm.[34] Under
these optimized conditions, the reduction activity of C. zeylanoides P1 for bioreduction
of substrates were also studied using different concentrations of 1–3 (0.5, 1 and 1.5 g/L).
The full conversion of substrates was obtained with 0.5 and 1 g/L, however the

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E. ŞAHIN
Figure 2. Biarly ketones used in this study.
conversions were obtained below 99% when 1.5 g/L of the substrates used. This may be
due to the reduction in the enzyme catalytic activity, suggesting substrate toxicity as
reported previously.[37] Hence 1 g/L substrate concentration was chosen for the biore-
duction. Under these optimized conditions, asymmetric reductions of different biaryl
prochiral bulky-bulky ketones (Figure 2) were carried out on gram scale (Table 1). In
view of the significant results obtained previously study[34], we then investigated the
efficiency and stereoselectivity of bioreduction of a series of benzophenone derivatives
bearing different electron-withdrawing and electron-donating groups as substituents on
the aromatic ring (Table 1). As seen in Table 1, bioreduction of substituted benzophe-
nones 1–3 by suspension cell cultures of C. zeylanoides P1 gave the corresponding chiral
alcohols 1a–3a in excellent ee (>99%) with 100% conversion. In the literature, the (S)-
(4-nitrophenyl)(phenyl)methanol compound was synthesized in high enantiomeric pur-
ity by the asymmetric reduction of the corresponding prochiral ketone using chemical
catalyst and by the asymmetric addition of the corresponding groups to the alde-
hyde.[38–40] The asymmetric reduction of (4-nitrophenyl) (phenyl) methanone 1 with
pure enzyme biocatalyst was carried out to corresponding chiral alcohols in 97% ee and
small scale.[27] In our study, the bioreduction of substrate 1 using C. zeylanoides P1 as a
whole cell biocatalyst was reduced to the (S)-carbinol 1a in >99% ee and 89% yield in
the reaction conditions of pH 6.5, 30 ꢀC, 150 rpm and 48 h. Diaryl ketone 4-chloro ben-
zophenone 2 and 4-methyl benzophenone 3 are important prochiral bulky-bulky
ketones for the production of diaryl carbinols, which are important precursors for the
production of pharmaceutically interesting compounds.[41] In the literature, in the pres-
ence of plant and microbial biocatalysts (R)-(4-chlorophenyl) (phenyl)methanol was
obtained with a long reaction time in a greater than 99% selectivity, conversion and a
small scale.[41] In the present study, the C. zeylanoides P1 mediated gram-scale biore-
duction of substrate 2 furnished 91% (S)-2a carbinol with >99% enantiomeric purity in
the optimized conditions. In the literature, it has been reported that the pure enzyme
was used as biocatalyst, and the substrate 3 is reduced to (R)-phenyl (p-tolyl) methanol
in 92% ee and small scale.[33] In this study, the bioreduction of substrate 3 using C. zey-
lanoides P1 was performed and furnished >99% enantiopure (S)-phenyl (p-tolyl)metha-
nol, (S)-3a in 93% yield. These results show that different electron withdrawing groups
(NO2, Cl) and electron donating group (CH3) did not exhibit any effects on the sub-
strate conversions and product ee. The configurations of the obtained products have
occurred according to the Prelog rule, the pro-R hydride of NADPH is given to the Re
face of the prochiral ketone to produce on S alcohol.[42] This biocatalyst provides a sus-
tainable alternative way for the production of these pharmaceutically important mole-
cules. By using various of prochiral ketones at high concentration as substrates, the C.
zeylanoides P1 is successfully employed for efficient synthesis of enantiomerically pure
chiral alcohols with ee value more than 99%, indicating that the C. zeylanoides P1 is
potential for further catalytic applications.

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Table 1. Asymmetric bioreduction of ketones 1–3 using C. zeylanoides P1.
Substrate
Product[a]
ee [%]b
99
Yield [%]c
89
99
99
91
93
aConfigurations were assigned on the basis of the comparison of the rotation sign with literature data.
bDetermined by chiral HPLC.
cIsolated yield.
Conclusion
There is striking attention in whole cells biocatalysts in the pharmaceutical, agrochem-
ical and chemical industries for their ability to produce useful chiral biaryl carbinol. In
this study, we have seen that C. zeylanoides P1 effectively catalyzed the enantioselective
reduction of biaryl aromatic ketones with excellent enantiomeric purity and yield to
give chiral carbinols. Compared with the previous reports, biarly substrate 1–3 was
reduced to 1a–3a in a perfect conversion, yield and ee.[27,33,41] The results approved the
suitability of C. zeylanoides P1 as a precious whole cell biocatalyst for the production of
chiral biaryl carbinols in the drug industry. Furthermore, this study demonstrates the
effective synthesis of biaryl carbinols by the yeast biocatalyst and biaryl ketones 1–3 was
the “easy-to-reduce” substrate for C. zeylanoides P1. Thus, in the framework of green
chemistry, this study has indicated a hopeful application of yeast biocatalyst C. zeyla-
noides P1 in the production of optically pure biaryl carbinols in simple, inexpensive and
environmentally friendly conditions.
Experimental section
General
The chemicals and solvents used in this study were purchased from Sigma-Aldrich
(Sigma-Aldrich Corporate, St. Louis, MO USA) (purity of >99%). Thin-layer chroma-
tography (TLC) was used for the monitoring of the reactions and performed by using
TLC plates (aluminum, silica gel 60 F254 Merck, 0.25 mm). Purification of chiral sec-
ondary alcohols were performed by column chromatography filled with silica gel

6
E. ŞAHIN
(0.063–0.2 mm) and the product was eluted with a mixture of hexane: ethyl acetate
(90:10, v/v). HPLC analysis was performed on an Agilent 1260 systems equipped with
chiral column (Daicel Chemical Industries, Ltd, France), UV and chiral detector (Ibz
Messtechnik Gmbh). The racemic 1b–3b alcohols were obtained by reducing the 1–3
with NaBH4 in methanol at room temperature. Optical rotation was measured with a
1
Bellingham þ Stanley, ADP220, 589 nm spectropolarimeter. H and 13C NMR spectra
were recorded on Bruker 400 MHz spectrometer in CDCl3.
Yeast strain and culture conditions
Candida zeylanoides strain used in this study was isolated previously from pastirma a
fermented Turkish meat product.[34,43] Yeast strain was stored their glycerol stock in
ꢁ80 ꢀC. YPD medium contains 1% yeast extract, 1% peptone from casein and 2% glu-
cose. In all reactions, the number of yeast cells were determined before and after the
reactions by OD600 measurements and plating to YPD agar.
General procedure for bioreduction of substrate 1–3
Yeast strain was propagated from their glycerol stocks by inoculation to 10 mL YPD
medium followed by 48 h growth at 30 ꢀC. From this cultures, exponentially grown yeast
cell was inoculated to 50 mL sterilized fresh YPD culture medium as working volume in
250 mL Erlenmeyer flask at 10% concentration and adjusted with 1M HCI to 6.5 and
shaken 2 h, then 100 mg substrate (1–3), which is dissolve in 2% ethanol, 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 ꢂ g for
5 min at 4 ꢀC and the supernatant saturated with NaCl, then extracted with CH2CI2.
Dichloromethane extracts were combined and dried over anhydrous Na2SO4. After
removal of the solvent under reduced pressure, NMR analysis was used for the identifi-
cation of the crude product followed by purification by column chromatography on sil-
ica gel using hexane: ethyl acetate as eluents (90:10). The absolute configuration was
determined by the sign of specific rotation. The conversion was determined by chroma-
tography on a chiral HPLC after filtering the crude products with a column containing
small silica gel and comparing the alcohol peaks with the ketone peak. The HPLC ana-
lysis conditions of the ketones were the same as those of the corresponding alcohols.
Enantiomeric excess of the products was determined by HPLC analysis using chiral col-
umn and the yields were determined after purification of the crude product by column
chromatography (supporting information).
Gram scale asymmetric reduction of 1a–3a
The gram scale asymmetric reduction of substrate 1–3 was performed as follows. Yeast
strain was propagated from their glycerol stocks by inoculation to 10 mL YPD medium
(1% yeast extract, 1% peptone from casein, 2% glucose) followed by 48 h growth at
30 ꢀC. From this cultures, exponentially grown yeast cell was inoculated to 500 mL steri-
lized fresh YPD culture medium as working volume in 2.5 L Erlenmeyer flask at 10%

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concentration and pH was adjusted with 1M HCl to 6.5 and shaken 2 h, followed by the
addition of substrate 1–3 (1 g), which is dissolve in 2% ethanol, to the medium and
incubated on a shaker at 30 ꢀC, 150 rpm for 48 h. 2% concentration of ethanol did not
inactivate the yeast activity. The supernatant of the medium obtained and saturated
with NaCl, then extracted with CH2Cl2. The dichloromethane extract washed with water
and dried over saturated Na2SO4. After removal of the solvent under reduced pressure,
the secondary chiral alcohols were isolated, purified and characterized as described
above. The configuration of the product was determined by the sign of the specific rota-
tion and comparison with the literature data. Conversion of all substrates were found to
be 100%.
Acknowledgements
We are grateful to Bayburt University Central Research Laboratory for HPLC analysis and and
_
Dr. Enes DERTLI (Bayburt University) for providing yeast strain used in this study.
Data availability
The data that support the findings of this study are openly available in https://doi.org/10.1002/
cbdv.201700121, reference number.[34]
1
Spectroscopic datas, copies of H, 13C NMR spectra, substrate, chiral, and racemic HPLC chro-
matograms of 1a–3a related to this article can be found at supplementary data.
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