A Straightforward Deracemization of sec-Alcohols Combining Organocatalytic Oxidation and Biocatalytic Reduction

Article abstract of DOI:10.1002/ejoc.201800569

An efficient organocatalytic oxidation of racemic secondary alcohols, mediated by sodium hypochlorite (NaOCl) and 2-azaadamantane N-oxyl (AZADO), has been conveniently coupled with a highly stereoselective bioreduction of the intermediate ketone, catalyzed by ketoreductases, in aqueous medium. The potential of this one-pot two-step deracemization process has been proven by a large set of structurally different secondary alcohols. Reactions were carried out up to 100 mm final concentration enabling the preparation of enantiopure alcohols with very high isolated yields (up to 98 %). When the protocol was applied to the stereoisomeric rac/meso mixture of diols, these were obtained with very high enantiomeric excesses and diastereomeric ratios (95 % yield, >99 % ee, >99: < 1 dr).

Full text of DOI:10.1002/ejoc.201800569

A Journal of
Accepted Article
Title: A Straightforward Deracemisation of sec-Alcohols Combining
Organocatalytic Oxidation and Biocatalytic Reduction
Authors: Elisa Liardo, Nicolás Ríos-Lombardía, Francisco Morís, Javier
González-Sabín, and Francisca Rebolledo-Vicente
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201800569
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10.1002/ejoc.201800569
European Journal of Organic Chemistry
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A Straightforward Deracemisation of sec-Alcohols Combining
Organocatalytic Oxidation and Biocatalytic Reduction
Elisa Liardo,[a] Nicolás Ríos-Lombardía,[b] Francisco Morís,[b] Javier González-Sabín,*[b] and Francisca
Rebolledo*[a]
Dedication ((optional))
Abstract: An efficient organocatalytic oxidation of racemic
secondary alcohols, mediated by sodium hypochlorite (NaOCl) and
2-azaadamantane N-oxyl (AZADO), has been conveniently coupled
with a highly stereoselective bioreduction of the intermediate ketone,
catalysed by ketoreductases, in aqueous medium. The potential of
this one-pot two-step deracemisation process has been proven by a
large set of structurally different secondary alcohols. Reactions were
carried out up to 100 mM final concentration enabling the
preparation of enantiopure alcohols with very high isolated yields (up
to 98%). When the protocol was applied to the stereoisomeric
rac/meso mixture of diols, these were obtained with very high
enantiomeric excesses and diastereomeric ratios (95% yield, >99%
ee, >99:<1 dr).
for the enantiomer required as well as the need to carry out the
separation of the product and the remaining substrate
addressed the attention towards the need of other techniques
overcoming these limitations. Thus, a variety of deracemisation
approaches[6] have been successfully applied to the quantitative
transformation of a racemate in an enantiopure compound,
avoiding the need to discard or recycle the unwanted
enantiomer.
Carbonyl reductases are in the second top position of the
most used enzymes in organic synthesis due to their wide
availability and high stereoselectivity.[7] Because these enzymes
can promote either the selective oxidation of an alcohol or the
reduction of the prochiral ketone, they have been exploited to
get deracemisation by stereoinversion of secondary alcohols
using two stereocomplementary enzymes.[8] Obviously, the
success of the methodology is determined by a fine balance of
the oxidation-reduction sequence, sometimes achieved by the
specific cofactor-dependence of the two involved enzymes.[9]
Introduction
Taking the complex and efficient machinery of the living
organisms as inspiration, chemists are devoting a great effort to
develop artificial multi-enzymatic cascade reactions and to apply
them in organic synthesis.[1] These approaches are particularly
appealing because enzymes, acting in similar reaction
conditions, are operationally more compatible than other types
of catalysts.[2] Nevertheless, nowadays, the dividing lines
between the different types of catalysis are becoming
increasingly diffuse and biocatalysts are also being elegantly
combined with metal-based catalysts and organocatalysts in
concurrent and one-pot multi-step syntheses.[1b,3]
Recently,
Thermoanaerobacter
a
single
biocatalyst,
a
mutant
of
ethanolicus
secondary
alcohol
dehydrogenase (TeSADH), has been used to promote the
deracemisation of some alcohols by means of a one-pot two-
step (oxidation-reduction) protocol, in which the amount of
acetone and 2-propanol was key to control the sequence.[10]
However, the enantiomeric excess values obtained were, in
general, moderate and only two of the five substrates
investigated were isolated with ee >90%.
Satisfactory results were obtained when the one-pot
oxidation-reduction protocol was designed combining different
catalysts: the iodine/TEMPO system for the oxidation of the sec-
alcohol followed by the bioreduction of the intermediate ketone
catalysed by ADH.[11] This procedure required the use of an
additive before the reduction step in order to eliminate the
excess of iodine, thus enabling the preparation of some
enantiopure secondary alcohols with very high yields. Even with
some limitations, these methods implying different sorts of
catalysts continue being a fascinating aspect of the catalysis and
they still pose a challenge for chemists.[12]
Enantiopure alcohols are highly valuable compounds due to
their prevalent presence in natural products and to their utility as
building blocks in the synthesis of
a
wide range of
agrochemicals, flavours and fragrances, and pharmaceuticals.[4]
The commercial availability and easy accessibility of a great
variety of racemic alcohols have motivated the development of
biocatalytic routes in order to get these compounds in
enantiopure form. Although firstly most of these protocols were
based on kinetic resolutions,[5] the maximal theoretical 50% yield
Recently we have described an expedient, stereoselective
and operationally simple method coupling an organocatalyst
(AZADO, 2-azaadamantane N-oxyl) and different amino
transferases to transform secondary alcohols into optically active
amines.[13] The high efficiency, simplicity, and wide scope of the
organocatalytic first step encouraged us to explore a new one-
pot deracemisation protocol of secondary alcohols in which the
non-selective AZADO-catalysed oxidation is conveniently
coupled with a stereoselective reduction of the intermediate
ketone mediated by a ketoreductase (KRED) (Scheme 1). The
method described herein has been applied to a significant series
of racemic secondary alcohols as well as some diols, and the
[a]
E. Liardo.
Prof. Dr. F. Rebolledo. Departamento de Química Orgánica e
Inorgánica, Universidad de Oviedo, 33006 Oviedo, Spain.
E-mail: frv@uniovi.es
[b]
Dr. N. Ríos-Lombardía.
Dr. Francisco Morís.
Dr. J. González-Sabín. EntreChem SL, Vivero Ciencias de la Salud,
Santo Domingo de Guzmán, 33011 Oviedo, Spain.
E-mail: jgsabin@entrechem.com
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201800569
European Journal of Organic Chemistry
FULL PAPER
robustness of the strategy has also been shown by the high
tolerance of substrate in both oxidation and reduction steps,
which are carried out at 250 and up to 100 mM, respectively.
Scheme 1. Proposal of deracemisation via a catalytic oxidation-reduction
sequence.
Results and Discussion
A wide series of secondary alcohols was selected in this study
(Figure 1). Non-benzylic alcohols 1a and 1b are precursors of
pharmacologically active amines such as amphetamine and
fenfluramine,[14] and 1c is a representative example of aliphatic
secondary alcohol. Among benzylic alcohols, we selected 1d
and 1e, this last being a precursor of the orally active NK1
receptor antagonist aprepitant,[15] which is used as antiemetic
drug[16] and has promising antitumor activity.[17] In addition, the
para-linked biaryl alcohol 1f, as a model of biarylic compound,
as well as the ortho-fusionated 1g-1i were also selected. The
biaryl moiety forms the basic structure of many biologically
active compounds[18] and is also found in electroluminiscent
polymers[19] and chiral ligands.[20] The deracemisation of the
ortho-biarylic alcohols 1g-1i has an extra added-value since
Figure 1. Racemic secondary alcohols of this study.
Similarly to the reaction conditions used in our previous
contribution,[13] the alcohol oxidation step was planned by using
a 0.40 M aqueous solution of sodium hypochlorite at pH 8.9, and
AZADO (1.0 mol%)[26] as the catalyst, but the co-solvent ,,-
trifluorotoluene (TFT) was changed to acetonitrile. The idea of
employing this water-miscible co-solvent is to suppress or
significantly lower the amount of dimethyl sulphoxide (DMSO)
that would be used as co-solvent in the second step if TFT is
employed.
The use of acetonitrile (5% v/v) in the oxidation step only
required a slight excess of NaOCl (400 mM, 1.3 equiv) and the
same amount of AZADO (1.0 mol%). The oxidation reactions
were performed at different scales (1.0 – 0.15 mmol) and in all
cases the starting material was totally converted into the
corresponding ketone 2 after 1.5 h. Substrate concentration in
this oxidation step was near to 250 mM, which was determined
by the amount and concentration of the NaOCl solution used as
well as the volume of co-solvent. Quantitative yields were
obtained for all the ketones 2, which were isolated in pure form
by a simple extraction with ethyl acetate.
On the other hand, before carrying out the step-wise protocol,
a complete screening of KREDs (from Codexis, see the
Supporting Information) with the ketones 2 was performed using
a range of 10-20 mM substrate concentration. Bioreductions
were conducted at 30-40 ºC in 125 mM phosphate buffer (pH
7.0), with NADP+ (1.0 mM) and isopropyl alcohol (IPA, 15% v/v).
The most significant results are shown in Table 1. In all the
cases the starting ketones (2a-k) or diketones (2l and 2m) were
totally transformed into the corresponding enantiopure alcohols
(1a-k) or diols (1l and 1m) after 24 h of reaction. In the process
starting from diketones 2l and 2m some KREDs catalysed the
formation of a mixture of mono-alcohol and diol but, in those
collected in Table 1, only a trace amount of the enantiopure
mono-alcohol was detected.
previous approaches based on
a lipase-catalysed kinetic
resolution rendered very low yields due to the high steric
congestion near to the reactive centre.[21] Furthermore, the
amino alcohol derivative 1j and the azido alcohol 1k were also
incorporated in our study as representative bifunctionalised
compounds. Lastly, the viability of the method to convert the
three-stereosiomers mixture of diols 1l and 1m into one single
stereoisomer was also investigated. Enantiopure C2-symmetric
diols such as 1l and 1m are key compounds for the synthesis of
chiral ligands,[20,22] and 1l has been also used for the preparation
of optically active crown ethers[23] and other complex
molecules.[24]
The organocatalyst AZADO has exhibited not only a high
catalytic potential combined with chlorine bleach (aqueous
solution of NaOCl), but also a great efficiency to promote the
oxidation of structurally hindered secondary alcohols. These
features make this system superior to others, for example using
the cheaper N-oxyl radical TEMPO.[25] In addition to the
importance of the aforementioned non-racemic alcohols, some
of them as the hindered biarylic derivatives have been selected
to extend the applicability of this oxidation methodology.
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10.1002/ejoc.201800569
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feasibility of the bioreduction in such a highly saline medium
should be assessed. Thus, in order to test the optimal conditions
and achieve a highly efficient one-pot stepwise process,
Table 1. Bioreduction of ketones 2a-k and diketones 2l,m.[a]
deracemisation of 1b was selected as a benchmark reaction.
In the first attempt of oxidation-reduction sequence, and after
carrying out the oxidation of 1b (250 mM) during 1.5 h in the
above mentioned conditions, IPA (15% v/v) was added and the
reaction mixture stirred during five minutes. Then, each
component of the second step [buffer phosphate (125 mM; pH
7.0), MgSO4 (1.25 mM), and NADP+ (1.0 mM)] was added to
reach the target concentration of the intermediate 2b (100 mM).
In these conditions, KRED-P3-B03 retained its stereoselective
activity catalysing the reduction of the ketone with total
selectivity (ee >99%) while the conversion into (S)-1b was 79%
after 24 h, and did not evolve further.
The incomplete reduction of 2b may result from the lower
solubility of this ketone in the highly saline reaction medium.
Both NaCl (coming from the reacting NaOCl) and carbonate
species (proceeding from NaHCO3 used to reduce the pH from
9.5 to 8.9 of the commercial solution of NaOCl) contribute to
increase the saline concentration in 160 and 190 mM,
respectively. As a consequence, the second step is carried out
in a saline concentration near to 500 mM. Accordingly, other
reaction conditions were tested to lower the salt content in the
bioreduction step. Thus, the addition of phosphoric acid in the
second step to attain the desired phosphate concentration
contributed to partially eliminating the carbonate species as
carbonic acid and causing an improvement in the conversion
(87%). Finally, we decided to suppress NaHCO3 in the first step
and lower the pH of the NaOCl solution by using of KH2PO4 in
such amount that its concentration was 125 mM, as that
required in the second step. Under these conditions, the
resulting 400 mM aqueous solution of NaOCl exhibited pH 7.9
and the AZADO-catalysed oxidation of 1b happened with total
conversion after the same reaction time (1.5 h). Then, IPA (15%
v/v), 125 mM phosphate buffer at pH 7.0 (containing MgSO4 2.5
mM), the KRED NADP+ (1.0 mM) were successively added to
reach a final 100 mM substrate concentration and pH 7.3. Upon
these conditions the degree of conversion into (S)-1b (ee >99%)
was complete after 24 h.
Ketone
Conc. (mM)
KRED[b]
T (ºC)
C (%)[b]
1, ee (%)[c]
2a
2b
2c
2d
2e
2f
20
20
20
20
20
10
10
20
20
20
20
20
10
P2-H07
P3-B03
P1-A04
P1-A04
P1-B05
P3-B03
P1-B05
P1-B05
P1-B02
P1-A04
P1-A04
P1-A04
P1-A12
30
30
30
30
30
40
40
40
40
30
30
30
40
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99 (R)
>99 (S)
>99 (R)
>99 (R)
>99 (R)
>99 (S)
>99 (S)
>99 (S)
>99 (S)
>99 (R)
>99 (S)
>99 (R,R)
>99 (R,R)
2g
2h
2i
2j
2k
2l
2m
[a] For the structure of the ketone or diketone, see the corresponding alcohol
or diol precursor 1 in Figure 1. A selection of results is here collected (see the
Supporting Information for the full panel of enzymatic screenings). [b] KRED
identified according to the numbering of the Screening Kit of Codexis. [c]
Degree of conversion (C) and ee were determined by GC or HPLC. The
absolute configuration is indicated between brackets.
To enable an efficient coupling of the oxidation and reduction
reactions, some points of both steps must be considered in
advance: 1) Excess of oxidant in the first step must be totally
eliminated in order to preserve the labile cofactor (NADPH) and
the enzyme (KRED) in the subsequent biotransformation. This
issue is easily solved because the hydrogen donor –isopropyl
alcohol– used in great excess in the second step quenches the
remaining NaOCl in the reaction medium. Nevertheless, a
compatibility proof was carried out to demonstrate that either
isopropyl alcohol is an effective scavenger of NaOCl and
AZADO did not affect the enzymatic activity. 2) The optimal
substrate concentration for each step differs significantly.
Regarding this, we posed as a major achievement to carry out
the biocatalytic step up to 100 mM substrate concentration,
which would be significantly higher than that used in previously
reported methodologies.[10,11] As the dilution from 250 to 100 mM
has to be achieved by adding phosphate buffer, and as the first
step is already conducted with a high salt concentration, the
Once an efficient process was validated, we extended the
study to the selected racemic alcohols (Figure 1) to have a
broader overview of the potential of this oxidizing-reducing
protocol. Therefore, after the oxidation took place, the optimized
experimental procedure for the bioreduction step was applied
with a selected KRED, at 100 mM substrate concentration, and
30 or 40 ºC depending on the substrate (see Table 1).
Interestingly the protocol worked perfectly for a large variety of
alcohols, obtaining the desired enantiopure products with
complete (>99%) or very high conversion (90-93%) for ten of the
thirteen substrates (Table 2). Thus, the enantiopure alcohols
(R)-1a, (S)-1b, and (R)-1e, which are precursors of valuable
biologically active compounds, were isolated with very high
yields (90-98%). Also remarkable were the results obtained for
the bifunctional amino alcohol derivative 1j and the ortho-biarylic
alcohols 1h and 1i. In the last two cases, the pyridine moiety
contributed to increase the solubility of these alcohols in the
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10.1002/ejoc.201800569
European Journal of Organic Chemistry
FULL PAPER
reaction medium, thereby allowing a 100 mM concentration in
the bioreduction step. However, with the analogous biphenylic
alcohols 1f and 1g as well as with the meta-biphenylic diol 1m
very low degree of conversions (<10%) were attained upon
analogous reaction conditions. Regarding the pyridine-2,6-
diethanol 1l, the reaction happened with total conversion at 100
mM (see Table 2) and KRED-P1-A04 displayed a perfect
stereoselectivity to provide the enantiopure diol (1R,1’R)-1l as
the sole product with >99:<1 dr.
concentration of the bioreduction step, the enantiopure alcohols
were isolated in very high yields (90-95%). Likewise, it worth
noting that KRED-P1-A12 exhibited total selectivity catalysing
the formation of the diol (1R,1'R)-1m with >99:<1 dr and >99%
ee.
Table 3. One-pot oxidation-reduction of alcohols and diol 1m at 50 mM and
25 mM final concentration.[a]
Substrate
KRED
C (%)[d]
ee (%)[d]
yield (%)[e]
1c[b]
1d[b]
1f[b]
P1-A04
P1-A04
P3-B03
P3-B03
P1-B05
P1-A04
P1-A12
>99
>99
65
>99 (R)
>99 (R)
>99 (S)
>99 (S)
>99 (S)
>99 (S)
>99 (R,R)
87
92
61
90
92
90
95
Table 2. One-pot two-step deracemisation of alcohols and diol 1l at a 100
mM final concentration.[a]
1f[c]
>99
>99
>99
>99
1g[c]
1k[b]
1m[c]
Substrate
KRED
C (%)[b]
ee (%)[b]
yield (%)[c]
1a
1b
1c
1d
1e
1h
1i
P2-H07
P3-B03
P1-A04
P1-A04
P1-B05
P1-B05
P1-B02
P1-A04
P1-A04
P1-A04
>99
>99
92
>99 (R)
>99 (S)
>99 (R)
>99 (R)
>99 (R)
>99 (S)
>99 (S)
>99 (R)
>99 (S)
>99 (R,R)
93
90
76
87
98
92
90
90
82
95
[a] Oxidation step as indicated in Table 2. Reduction step was carried out at
30 ºC (for 1c, 1d, and 1k) or 40 ºC (for 1f, 1g, and 1m). [b] Reduction step
performed at 50 mM. [c] Reduction step performed at 25 mM. [d] Determined
by GC or HPLC. [e] Isolated yield after purification by column
chromatography.
90
>99
>99
>99
>99
93
1j
Conclusions
1k
1l
In conclusion,
a
new and straightforward protocol for
>99
deracemisation of alcohols has been described. The aqueous
medium represented the link for two different catalytic worlds,
such as organocatalysis and biocatalysis, working perfectly in a
sequential fashion.
The oxidation of the racemates, mediated by NaOCl and
AZADO (1.0 mol%), was easily coupled with the enzymatic
reduction of the intermediate ketones in the same medium up to
100 mM concentration. Fine tuning the oxidation conditions and
adequate biocatalyst selection provided a set of synthetically
and pharmacologically interesting alcohols with high overall
yields (up to 98%) and >99% ee. The efficiency of the strategy
was also demonstrated for the conversion of rac/meso-diols into
only one enantiomer.
[a] Reaction conditions. Oxidation step: AZADO (1.0 mol%), 250 mM
substrate concentration, except for 1l (130 mM), room temperature except
for 1a (5 ºC); reduction step was performed at 100 mM substrate
concentration, at 30 ºC except for reactions starting from 1h and 1i (40 ºC).
[b] Degree of conversion (C) and ee were determined by GC or HPLC. [c]
Isolated yield after purification by column chromatography.
In order to improve the unsuccessful results achieved in
some cases and reach perfect conversion for the bioreduction of
the intermediate ketones, we decided to lower the concentration
of the second step at 50 mM, also reducing the amount of
NADP+ to 0.50 mM. Gratifyingly, the reactions using racemic 1c,
1d, and 1k proceeded effectively and the corresponding
enantiopure alcohols were achieved with complete conversion
(Table 3). On the contrary the biarylic alcohols 1f, 1g, and diol
1m remained challenging, despite a slight improvement in the
conversion values (up to 65% for 1f). Pleasantly, complete
conversion of these substrates could be finally reached by
lowering the concentration to 25 mM and adding DMSO (10%
v/v) as co-solvent. It should be noted that, despite the low
Experimental Section
General Remarks
1H-NMR and proton-decoupled 13C-NMR spectra (CDCl3) were obtained
using
a
Bruker DPX-300 (1H, 300.13 MHz and 13C, 75.5 MHz)
spectrometer using the δ scale (ppm) for chemical shifts. Calibration was
made on the signal of the solvent or the residual non-deuterated solvent
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10.1002/ejoc.201800569
European Journal of Organic Chemistry
FULL PAPER
1
(13C: CDCl3, 77.16; H: CHCl3, 7.26). Degree of conversions (C), dr, and
cofactor NADP+ (0.50 mM) were added to achieve a final concentration
of 25 mM. The reaction mixture was incubated during 24 h at 40°C and
250 rpm. After this time, the mixture was extracted with ethyl acetate (2 ×
2.0 mL), the organic layers were separated by centrifugation (90 s, 13000
rpm), combined, washed with brine (2 × 2.0 mL) and finally dried over
Na2SO4. Evaporation of the solvent yielded the crude alcohol which was
purified by flash chromatography.
ee were determined by HPLC or GC analyses. Optical rotations were
measured using a Perkin-Elmer 241 polarimeter and are quoted in units
of 10-1 deg cm2 g-1. Codex® KRED screening kit was purchased from
Codexis.
General procedure for oxidation of racemic alcohols
Supporting information available: Experimental procedures, screenings
for the enzymatic bioreductions, characterization data for enantiopure
compounds, and copy of the HPLC chromatograms and NMR spectra.
To a sample of commercially available NaOCl solution (1.95 M, such as it
was determined by titration),[27] KH2PO4 and water were added in such
amount that the resulting solution was 125 mM in phosphate and 400
mM in NaOCl (pH = 7.9). Then, the corresponding amount of this NaOCl
solution (1.2-1.3 equiv) was added to a solution of racemic alcohol (150
μmol) and AZADO (1.5 μmol)[28] in MeCN (150 μL). The mixture was
vigorously stirred (magnetic stirring) at room temperature in all cases
except for reaction with 1a, which was conducted at 5 ºC. Once the
starting material disappeared (1.5 h, TLC control using hexane-ethyl
acetate 3:1 as eluent), the reaction mixture was extracted with ethyl
acetate (3  600 μL). The organic layers were combined, dried over
Na2SO4 and evaporated under vacuum. The 1H-NMR analysis of the
crude product (>95% yield) showed the corresponding ketone in pure
state for synthetic purposes.
Acknowledgements
E. Liardo acknowledges funding from the European Union's
Horizon 2020 MSCA ITN-EID program under grant agreement
Nº 634200 (Project BIOCASCADES). Authors from University of
Oviedo thank to Ministerio de Ciencia e Innovación of Spain
(Project CTQ 2014-55015) and Principado de Asturias (FC-15-
GRUPIN-14-002) for financial support.
Enzymatic screening for the reduction of ketones
Keywords: Alcohols • organocatalysis • oxidation •
oxidoreductases
In a 2.0 mL eppendorf tube, ketone 2 (20 mM, except 10 mM for 2f, 2g,
and 2m), KRED (same weight as the ketone), and IPA (190 μL) were
added to 900 μL of 125 mM phosphate buffer, pH 7.0. This buffer also
contains MgSO4 (1.25 mM) and the cofactor NADP+ (1.0 mM). For
ketones 2f-i and 2m, DMSO (100 μL) was also added. The resulting
reaction mixture was shaken at 250 rpm and 30 or 40 °C (see Table 1)
for 24 h. After this time, a 10 μL aliquot was removed by the
determination of the degree of conversion by HPLC or GC analysis (see
Supporting Information (SI)). Then, the mixture was extracted with ethyl
acetate (2 × 500 μL), the organic layers were separated by centrifugation
(90 s, 13000 rpm), combined, and finally dried over Na2SO4. The
diastereomeric ratio (if applicable) and enantiomeric excess of the
corresponding alcohol was determined by chiral HPLC or GC (see SI).
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General procedure for the one-pot two-step process at a final 100
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Reactions were carried out in a 2.0 mL eppendorf tube using 10.0 mg of
the starting alcohol 1 except for compound 1e, for which the reaction was
conducted at 50.0 mg scale.[29] Firstly, the oxidation was carried out
following the general procedure. Once the oxidation was complete (1.5 h),
IPA (15% v/v), 125 mM phosphate buffer at pH 7.0 (containing MgSO4
2.5 mM), the corresponding KRED (10.0 mg except 50.0 mg for 1e) and
the cofactor NADP+ (1.0 mM) were added in order to reach a final
concentration of 100 mM. For biarylic compounds, DMSO (10% v/v) was
also added. The reaction mixture was incubated during 24 h at 30 or
40 °C and 250 rpm (see SI). After this time, the mixture was extracted
with ethyl acetate (2 × 1.0 mL), the organic layers separated by
centrifugation (90 s, 13000 rpm), combined, and finally dried over
Na2SO4. Evaporation of the solvent yielded the crude alcohol which was
purified by flash chromatography.
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(containing MgSO4 2.5 mM), the corresponding KRED (10.0 mg) and the
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Mixed catalytic strategy: A highly
Elisa Liardo, Nicolás Ríos-Lombardía,
Francisco Morís, Javier González-
Sabín,* and Francisca Rebolledo*
aq. NaOCl
pH 7.9
AZADO
efficient oxidation of a representative
series of sec-alcohols and diols with
aq. NaOCl and 2-azaadamantane N-
oxyl (AZADO) as organocatalyst has
been combined with a selective
ketoreductase-catalyzed bioreduction.
The sequential one-pot strategy has
been performed up to 100 mM final
substrate concentration allowing to
obtain benzylic, non-benzylic or
hindered biarylic alcohols with very
high yield and >99% ee.
Page No. – Page No.
A Straightforward Deracemisation of
sec-Alcohols Combining
Organocatalytic Oxidation and
Biocatalytic Reduction
KRED
IPA, NADP+
KPi Buffer, pH 7.3
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