ENANTIOMERIC REDUCTION OF ACETOPHENONE AND DERIVATIVES
853
or product inhibition. After a 30 h incubation period, 92%
conversion of 1a was obtained with a product yield of 73%
(
14.6 mmol/l), whereas after a 42 h incubation period, the
conversion of 1a completed as 100% with 15.8 mmol/l of
a produced from 20 mmol (2.4 g) 1a. The yield for the
2
conversion was calculated as 79%, which is a good result
for preparative scale. This result demonstrates and con-
firms that the system is feasible for industrial purposes.
There will always be a need for alternative microorgan-
isms for the production of enantiomerically pure pharma-
ceuticals. Unsurprisingly, given that the human body func-
Scheme 1. Bioreduction (one liter scale) of acetophenone (20.0 mmol,
.4 g) to (S)-phenylethanol (15.8 mmol, 1.9 g, 79% yield, ee >99%). The
yield was calculated based on the following: Yield (%) 5 100 3 PC/ISC,
PC, and ISC are the concentrations of product and initial substrate, respec-
tively.
2
tions using chiral catalysis, the trend for new chiral phar-
4
maceutical
reagents
is
continuing.
Biocatalytic
asymmetric reduction of aromatic ketones to correspond-
alcohol products 2a–o had the (S)-enantiomer, which is in ing chiral alcohols has attracted more attention due to the
1
5
agreement with Prelog’s rule. The bioreduction of the high enantioselectivity, 100% theoretical yield, and mild
ortho-, meta-, and para-substituted chloro, bromo, and flu- reaction conditions. Transformations using isolated reduc-
oro acetophenones, as well as the para- methyl and nitro tase as biocatalysts for these bioreductions can produce
acetophenones were transformed to the corresponding the desired yield and high enantiomeric excess values.
alcohols with complete conversion. However, in the pres- However, these reactions generally require expensive
ence of an electron donating group at the para position on cofactors such as NADPH. The cofactors can, however, be
the aromatic ring, the conversion was lower, with 44% con- regenerated with whole cells as catalysts during the reduc-
1
4,18–20
version for the methoxy substituent 1n and 23% conver- tion processes.
In this study, the method used for
sion for the phenyl substituent 1o. Furthermore, in these the bioreduction is different, because the substrate was
cases, the reduction of the methoxy 1n and phenyl 1o directly added to the culture medium without using rest-
substrates proceeded with poor yields, 35% and 16%, ing cells, cofactors, and buffers. In addition, the R. glutinis
respectively. Nevertheless, excellent ees (>99%) were EBK-10 isolated could effectively be used for the bioreduc-
obtained. It should be noted that the outcome of the ees of tion of acetophenone and its derivatives. All products
the desired products were not affected by steric or elec- resulted had satisfactory optical purities.
tronic factors of the various substituents.
Consequently, ram horns are an organic waste that is
We have recently shown the beneficial effects of the highly rich in protein content and other basic nutrients
presence of RHP in growth medium of microorganisms. that could support microbial growth. A simple medium
7
16
17
Compounds 2a, 2i, and 2k were isolated in gram containing RHP proved suitable for growth of R. glutinis
amounts from fermentation medium in the preparative for the asymmetric reduction of substituted acetophe-
scale productions by different microorganisms using RHP. nones to chiral alcohols. Moreover, a medium with RHP is
The different microbial mediated reductions of acetophe- economically more viable than other peptone sources. The
8
,16
none and its derivatives were previously reported by us
preparative scale bioreduction of 1a by R. glutinis EBK-10
and the importance of RHP in microbial media was dis- gave 2a in 79% yield, complete conversion, and >99% ee.
9
,10
cussed in our previous studies.
Since the results of a This is the first report on the asymmetric reduction of sub-
microbial transformation often depend on various factors stituted acetophenones with R. glutinis as a biocatalyst
such as fermentation conditions, culture medium composi- using a submerged culture system. In addition, all the chi-
tion, and different microbial strains, it was decided to use ral phenylethanols obtained from the submerged culture
the same successful substrates as in our previous stud- system were produced with the maximum ee.
8
,16
ies
for this study. In this study, although all acetophe-
LITERATURE CITED
nones afforded excellent enantioselectivities for the reduc-
tion on a small scale, we decided to conduct the transfor-
mation of 1a to 2a on a large scale to demonstrate the
viability of the present system as industrially feasible.
The bioreduction of 1a for the production of 2a in prep-
arative scale was performed in a fermenter containing one
liter of sterile ram horn medium. These results are sum-
marized in Scheme 1 and Figure 1. For this method, we
utilized the optimum conditions from the first method for
the production of 2a in the preparative scale in a fer-
menter resulting in a 79% yield, complete conversion, and
1
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56.
>99% ee. Although there was a regular decrease in conver-
4
5
. Pollard DJ, Woodley JM. Biocatalysis for pharmaceutical intermedi-
ates: the future is now. Trends Biotechnol 2006;25:66–73.
sion speed with the increase of incubation time, enantiose-
lectivity of the product remained the same throughout the
reaction. The reason for the decrease of conversion speed
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Chirality DOI 10.1002/chir