Hybrid Organo- and Biocatalytic Process for the Asymmetric Transformation of Alcohols into Amines in Aqueous Medium

Article abstract of DOI:10.1021/acscatal.7b01543

A hybrid organo- and biocatalytic system for the asymmetric conversion of racemic alcohols into amines was developed. Combining an organocatalyst, AZADO, an oxidant, NaOCl, and an enzyme, ω-transaminase, we implemented a one-pot oxidation-transamination sequential process in aqueous medium. The method showed broad substrate scope and was successfully applied to conventional secondary alcohols and sterically hindered β-substituted cycloalkanols, where a highly stereoselective dynamic asymmetric bioamination enabled us to set up both contiguous stereocenters with very high enantio- and diastereomeric ratio (>90% yield, >99% ee, and up to 49:1 dr).

Full text of DOI:10.1021/acscatal.7b01543

Research Article
pubs.acs.org/acscatalysis
Hybrid Organo- and Biocatalytic Process for the Asymmetric
Transformation of Alcohols into Amines in Aqueous Medium
Elisa Liardo,†,‡ Nicolas Ríos-Lombardía,† Francisco Morís,† Francisca Rebolledo,*,‡
́
and Javier Gonzalez-Sabín*,†
́
†EntreChem SL, Edificio Científico Tecnolog
‡Department of Organic and Inorganic Chemistry, University of Oviedo, Julian
́
ico, Campus El Cristo, 33006 Oviedo, Spain
́
Clavería, 8, 33006 Oviedo, Spain
S
* Supporting Information
ABSTRACT: A hybrid organo- and biocatalytic system for the asymmetric
conversion of racemic alcohols into amines was developed. Combining an
organocatalyst, AZADO, an oxidant, NaOCl, and an enzyme, ω-transaminase,
we implemented a one-pot oxidation−transamination sequential process in
aqueous medium. The method showed broad substrate scope and was
successfully applied to conventional secondary alcohols and sterically hindered
β-substituted cycloalkanols, where a highly stereoselective dynamic asymmetric
bioamination enabled us to set up both contiguous stereocenters with very
high enantio- and diastereomeric ratio (>90% yield, >99% ee, and up to 49:1
dr).
KEYWORDS: one-pot reaction, organocatalysis, biocatalysis, asymmetric synthesis, amino alcohols, transaminase, AZADO
synthesis of pharmaceuticals and agrochemicals.8 Currently,
INTRODUCTION
■
biocatalysis offers a variety of enzymes for amine synthesis,9
transaminases (ω-TAs) being one of the most effective
catalysts. The most straightforward approach based on these
transferases consists of the asymmetric reductive amination of
prochiral or chiral carbonyl compounds (ketones or aldehydes).
However, the easier availability of some alcohols in comparison
with their oxidized carbonyl derivatives has encouraged the
development of multistep approaches for the one-pot
conversion of alcohols into amines.10 Thus, the prior state of
the art includes a selective biocatalytic two-step sequence
consisting of an alcohol dehydrogenase (ADH) catalyzed
oxidation of an alcohol and further bioamination mediated by
ω-TAs or amine dehydrogenases (AmDHs).11 However, due to
the excellent selectivity generally offered by ADHs, when a
racemic alcohol is used, two enzymes with opposite stereo-
preference became compulsory. Alternatively, alcohol oxidases
were also combined with ω-TAs for transforming cinnamic and
benzylic alcohols (two-enzyme cascade)12a and also non-
activated aliphatic alcohols (five-enzyme cascade).12b More
recently, a nonstereoselective oxidation catalyzed by laccases
has been coupled with ω-TAs but required a high amount of
the chemical mediator TEMPO (2,2,6,6-tetramethylpiperidine
N-oxyl, 30 mol %) for the transformation of benzylic alcohols.13
With all these precedents in mind, and keeping in mind the
aqueous natural environment for ω-TAs, we focused on the
search for an efficient and water-compatible alcohol oxidation
method to orchestrate a cascade process in aqueous media.
In recent years, chemists have tried to emulate Mother Nature
by developing novel one-pot multistep catalytic reaction
networks to construct enantiomerically pure molecules in a
new, cleaner, and more efficient manner.1 From an environ-
mental point of view, cascade processes are particularly
attractive due to the avoidance of intermediate extraction and
purification steps, resulting in simplified downstream oper-
ations with reduced waste and cost relative to traditional
multistep processes. On the other hand, the possibility of
performing one-pot multistep syntheses aided by enzymes is
particularly appealing due to the intrinsically green features of
biocatalysis.2 Thus, multistep enzymatic reactions in a whole-
cell fashion were described in the late 1980s for the production
of amino acids3 and, later on, several isolated enzyme classes
have been successfully combined in a one-pot fashion.2 In
addition, two different catalytic fields such as metal catalysis and
organocatalysis, developed rather independently from bio-
catalysis over the last few decades, have recently merged in a
novel synthetic strategy that combines the advantages of both
fields.4 Thus, the discovery of water-compatible metal catalysts
for well-established organic reactions has enabled their
assembly with many biotransformations in aqueous media.5,6
A combination of organocatalysts and enzymes, although
scarce, has also been reported: for instance, a proline derivative
catalyzed aldol reaction and further bioreduction7a or the
complete oxidation of glycerol to CO2 by the cooperative
action of an oxalate oxidase and an organic oxidation catalyst.7b
Optically active amines are among the most valuable
compounds in chemistry, not only by their presence in a
variety of natural products but also as intermediates in the
Received: May 11, 2017
© XXXX American Chemical Society
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Scheme 1. Proposal of Cascade for the Asymmetric Synthesis of 2-(Benzyloxy)cycloalkanamines in Aqueous Medium
Ideally, the method should enable to convert, among other
substrates, the easily available racemic trans-2-(benzyloxy)-
cycloalkanols into optically active cis- or trans-2-(benzyloxy)-
cycloalkanamines (Scheme 1). Actually, these are highly
valuable compounds, since the β-amino alcohol moiety is
ubiquitous in nature and the subfamily of β-aminocycloalkanols
can be found in many natural products and synthetic bioactive
molecules such as the antiarrhythmic vernakalant14a or the
inhibitor of HIV indinavir.14b Furthermore, they have also
found great application in asymmetric catalysis as auxiliaries or
ligands.15
Table 1. AZADO-Catalyzed Oxidation of trans-( )-2-
(Benzyloxy)cyclohexanol (1b) with Aqueous NaOCl
a
AZADO
(equiv)
aq NaOCl
b
cosolvent (%
reaction
time (h)
c
d
entry
(equiv)
v/v)
C (%)
e
1
0.05
0.05
0.05
0.01
2.0
1.0
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
PhCF3 (15)
PhCF3 (30)
PhCF3 (25)
PhCF3 (25)
PhCF3 (25)
PhCF3 (25)
PhCF3 (25)
CH2Cl2 (40)
DMSO (25)
AcOEt (25)
1
13
1
>95
2
95
>95
>95
3
4
1
RESULTS AND DISCUSSION
■
5
24
14
1.5
1
The oxidation of 2-(benzyloxy)cyclohexanol (1b) was selected
as a benchmark reaction for testing different oxidation systems.
Initially, from the plethora of reported water-compatible
oxidants, we turned our attention to the family of nitroxyl
radicals16 and its best known member TEMPO, on the basis of
its efficient performance in the pharmaceutical industry.17
Aiming at developing a fully biocatalytic process, we tested the
methodology pointed out above on the basis of a laccase/
TEMPO catalytic system.13 However, upon the experimental
conditions described for the oxidation of benzylic alcohols, that
is, laccase from Trametes versicolor, O2, TEMPO (33 mol %) in
citrate buffer 50 mM, pH 5.0, and 30 °C, the starting 1b was
recovered unaltered.18 Interestingly, it has been reported that 2-
azaadamantane N-oxyl (AZADO) displays a catalytic profi-
ciency superior to that of TEMPO, enabling also the oxidation
of structurally hindered secondary alcohols under a variety of
conditions.19 In fact, a laccase-AZADO catalytic system was
used by Ying et al. for the aerobic oxidation of hindered
alcohols in water under mild reaction conditions.20 However,
after some attempts based on these reaction conditions, the
highest percentage (58%) of 2-(benzyloxy)cyclohexanone (2b)
was achieved when Trametes versicolor laccase, 20 mol % of
AZADO, O2, acetate buffer pH 4.5, and α,α,α-trifluorotoluene
(PhCF3) as a cosolvent were used.
6
0.001
0.005
0.01
0.01
0.01
0.01
76
f
7
94
8
94
0
9
1
10
11
1
91
92
CH3CN (25)
1
a
b
50 μmol (11 mg) of alcohol was used. A 0.40 M aqueous solution of
c
NaOCl (pH 8.9) was used. 50 μL of PhCF3 was added as cosolvent.
d
Degree of conversion determined by analysis of the 1H NMR
spectrum of the crude material. Values >95% mean that no starting
e
material is observed in the spectrum. Formation of an unknown
f
overoxidation product was observed. Conversion did not evolve with
a longer reaction time.
included amount of NaOCl and organocatalyst loading. Thus,
a decrease to equimolar amounts of NaOCl was detrimental
and the conversion halted in 95% after 13 h, 1.2 equiv being the
minimum required (entries 2 and 3). On the other hand, we
were delighted to find that AZADO indeed retained catalytic
activity at levels as low as 1 mol % (entry 4). The conversion of
the alcohol to ketone was dependent on the catalyst loading,
leading to conversions of 94 and 76% with 0.5 and 0.1% of
AZADO, respectively, after longer reaction times (entries 6 and
7). In addition, when AZADO was replaced by TEMPO under
the optimal conditions found in entry 4, the conversion
dropped to 51% after 24 h and did not evolve further. Finally,
other cosolvents (entries 8−11) were also checked but PhCF3
remained the optimal choice.
Encouraged by the potential of the AZADO/NaOCl
oxidation system to operate in a basic pH aqueous medium
compatible with ω-TAs, we assessed the scope of this
methodology with a variety of cyclic and acyclic secondary
alcohols. Thus, all of the substrates included in Figure 1
underwent efficient oxidation to afford the corresponding
ketones in excellent yields (>92%). Reaction conditions (220−
290 mM, 1 h) were only slightly modified in some cases to
avoid overoxidation products. Thus, when 1d was allowed to
react at room temperature with 1.2 equiv of NaOCl, the
conversion was only 79% and, in addition to product 2d, a
mixture of 1-chloro- and 1,1-dichloro-1-phenylpropan-2-ones
was also observed. However, >95% of conversion was achieved
At this point AZADO emerged as the only effective mediator,
but in order to reach complete oxidation of 1b, we decided to
substitute the ineffective laccase/O2 system by other oxidant
reagents. On the basis of the pioneering research of Iwabuchi et
al. on the catalytic potential of AZADO combined with
oxidants such as NaOCl/KBr/Bu4NBr or PhI(OAc)2 for
recycling the catalyst,19 we selected inexpensive aqueous
NaOCl to check the regeneration of the catalyst in the
presence of PhCF3. Moreover, NaOCl, which is the active agent
in bleach and swimming pool chlorine, offers several benefits
over traditional oxidants such as low material cost, mild
reaction conditions, and no metallic waste. Consequently, in a
first experiment the oxidation of 1b was tested at room
temperature in an aqueous solution of NaOCl (2.0 equiv)
employing 5 mol % of AZADO and PhCF3 (entry 1, Table 1).
Pleasantly, this oxidation system worked very efficiently,
affording the ketone 2b in quantitative yield in just 1 h at
room temperature at basic pH. Further parametrization
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Figure 1. Substrate scope of the organocatalytic AZADO/NaOCl oxidation system.
a
Table 2. Selection of ω-TA-Catalyzed Transamination of ( )-2-(Benzyloxy)cycloalkanones 2a,b
b
ee (%)
c
b
b
entry
ketone
ω-TA
pH
C (%)
cis:trans
cis
trans
1
2
3
4
2a
2a
2a
2a
ATA-237
7.5
7.5
7.5
7.5
96
63
60:40
82:18
48:52
>99 (1S,2R)
>99 (1S,2R)
>99 (1R,2S)
>99 (1R,2S)
>99 (1S,2S)
>99 (1S,2S)
>99 (1R,2R)
ATA-251
ATA-303
>99
59
ATA-P2-A01
>99:<1
5
6
7
8
2b
2b
2b
2b
ATA-254
7.5
7.5
7.5
7.5
69
50
77:23
88:12
45:55
67:33
>99 (1S,2R)
>99 (1S,2R)
>99 (1R,2S)
>99 (1S,2R)
>99 (1S,2S)
-
ATA-256
ATA-303
>99
>99
>99 (1R,2R)
>99 (1S,2S)
ATA-P1-G06
9
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
ATA-254
ATA-254
ATA-303
ATA-303
ATA-412
ATA-412
ATA-P2-A07
ATA-P2-A07
ArS
8.5
10
>99
90
74:26
85:15
32:68
4:96
>99 (1S,2R)
>99 (1S,2R)
>99 (1R,2S)
>99 (1S,2S)
>99 (1S,2S)
>99 (1R,2R)
>99 (1R,2R)
>99 (1R,2R)
10
11
12
13
14
15
16
17
18
19
10
>99
>99
>99
62
11.5
10
50:50
91:9
>99 (1R,2S)
>99 (1R,2S)
>99 (1R,2S)
>99 (1R,2S)
>99 (1S,2R)
>99 (1R,2S)
>99 (1R,2S)
11.5
8.5
10
77
77:23
87:13
46:54
50:50
95:5
>99 (1R,2R)
>99 (1R,2R)
>99 (1S,2S)
>99 (1R,2R)
90
10
>99
>99
58
ArRmut11
ArR
10
10
20
21
22
23
24
25
26
27
2a
2a
2a
2a
2a
2a
2a
2a
ATA-117
ATA-251
ATA-P2-A01
ATA-025
ATA-303
ArS
10
90
>99
>99
>99
>99
>99
>99
85
98:2
>99 (1R,2S)
>99 (1S,2R)
>99 (1R,2S)
>99 (1R,2S)
>99 (1R,2S)
>99 (1S,2R)
>99 (1R,2S)
>99 (1R,2S)
10
86:14
95:5
>99 (1S,2S)
10
11.5
11.5
10
81:19
12:88
27:73
48:52
98:2
>99 (1R,2R)
>99 (1R,2R)
94 (1S,2S)
ArRmut11
ArR
10
>99 (1R,2R)
10
a
Reaction conditions: substrate (10 mM) in KPi buffer 100 mM pH 7.5−11.5 (900 μL) with PLP (1 mM) and iPrNH2 (1 M), ω-TA (2 mg for the
b
c
ω-TAs of Codexis and 5 mg for ArS, ArRmut11, and ArR), DMSO (5% v/v), for 24 h at 250 rpm and 30 °C. Determined by HPLC analysis. The
abbreviation ATA (amino transferase) was introduced by Codexis for its commercial ω-TAs.
by setting the temperature to 5 °C and using 1.4 equiv of
oxidant. In the other cases, no noticeable overoxidation
products were detected under the optimized conditions.
Some features of this AZADO/NaOCl system worth high-
lighting are (i) the oxidation of a variety of secondary alcohols
takes place under mild reaction conditions, (ii) the product
ketones are isolated in excellent yield without the need for
further purification, and (iii) 1 mol % of AZADO and 1.0−1.4
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equiv of NaOCl are sufficient for quantitative oxidation of
alcohols in just 1 h, showing excellent atom economy and total
mass transfer from substrates to products.
enced the most significant improvement, with the enzyme
retaining the activity and exhibiting the highest selectivity of the
series to yield enantiopure (1R,2R)-3b with 24:1 dr (entry 12).
Because of the enantiocomplementarity of ATA-254 and ATA-
P2-A07, both enantiomers of cis-3b can be obtained from these
processes with high ee and yield.
Next, we studied the bioamination of the product ketones 2,
to complete a potential cascade process.21 Initially, we used
both the 2-(benzyloxy)cyclohexanone (2b) obtained above and
its cyclopentylic homologue 2a. The main feature of these
compounds relies on the lability of its chiral center, which is in
a position α to both carbonyl and benzyloxy groups. Thus, they
are potentially amenable to racemize at slightly basic pH and
trigger a dynamic kinetic resolution (DKR) leading to 100%
theoretical yield. Likewise, the bioamination process would
deliver two contiguous stereocenters, with four possible
stereoisomers. In contrast to the case for dynamic reductive
kinetic resolutions (DYRKR),22 reports of dynamic asymmetric
transaminations involving two chiral centers are very scarce.23
In an example published by Merck,23b the highly enantio- and
diastereoselective bioamination of a 2-(2-arylethoxy)-
cyclohexanone was the key asymmetric step toward vernaka-
lant, an antiarrhythmic drug for atrial fibrillation.16a Inspired by
this report, we envisaged a dynamic asymmetric transamination
with the cyclic α-benzyloxyketones 2a,b. The selection of the
benzyloxy group obeys practical reasons: (i) the increased
synthetic value of the resulting protected cyclic amino alcohol,
(ii) easy cleavage of the benzyl group, (iii) its hydrophobicity,
to facilitate extraction from aqueous media in the downstream
process, and (iv) the inductive effect that exalts the acidity of
the α-proton, facilitating stereocenter epimerization, a pre-
requisite for a viable DKR. Thus, we screened 2a,b with a series
of 28 commercially available ω-TAs (Codex Transaminase
Screening Kit) and also with four ω-TAs overexpressed in E.
coli: the S-selective TAs from Chromobacterium violaceum
(Cv)24 and (S)-Arthrobacter (ArS)25 or the R-selective TAs
from (R)-Arthrobacter (ArR)26 and its evolved variant
ArRmut1121d (see the Supporting Information). Initially,
under the standard conditions at pH 7.5 and using isopropyl-
amine in molar excess as amino donor, most ω-TAs were very
active and reached high conversions after 24 h. More
interestingly, most of them displayed perfect asymmetry in
the amination of the carbonyl group of 2a,b, as deduced by
analyzing the C-1 configuration and the ee > 99% obtained for
both cis- and trans-amines in every reaction.27 In addition, the
diastereomeric ratios (dr) obtained for reactions with C > 90%
were low; meanwhile, moderate or very high dr values (up to
>99:<1) were associated with reactions with lower conversion
degree (C ≤ 65%). A selection of results illustrating these
aspects are collected in Table 2. This means that, even though
ω-TAs exhibited enantioselectivity toward the ketone (see, for
instance, entries 2, 4, and 6), the low conversion achieved could
be the consequence of a slow epimerization rate of its chiral
center.
Once the efficiency of the high pH value was demonstrated,
bioaminations of ketone 2a were carried out at pH 10 and 11.5
(Table 2, entries 20−24). Thus, we identified biocatalysts
which preserved activity and gave rise to three of the four
possible stereoisomers in high dr (up to 49:1) and >99% ee.
ATA-303 was the most efficient catalyst toward the trans
diastereomer, and (1R,2R)-3a was obtained with improved
disatereoselectivity at pH 11.5 (entry 24; 7:1 dr and >99% ee).
Regarding the cis diastereomer, significant conversion enhance-
ment was achieved by raising the pH from 7.5 to 10 in the
reactions with ATA-251 and ATA-P2-A01 (entries 2 and 4 vs
entries 21 and 22, respectively). Although the improvement in
the dr was noteworthy with ATA-P2-A01 (19:1), even more
remarkable was the excellent dr (49:1) exhibited by ATA-117 at
pH 10 (entry 20). With respect to the Cv, ArR, ArS, and
ArRmut11 ω-TAs, they remained active at the pH (except Cv-
ω-TA) and displayed excellent enantioselectivity for both cis
and trans isomers (entries 17−19 and entries 25−27).
Interestingly, ArR showed excellent dr values of 49:1 and
19:1 (and ee > 99%) for 2a,b respectively (entries 19 and 27).
From these results we wish to highlight the high cis
diastereoselectivity exhibited by some ω-TAs, which gave
enantiopure cis-β-aminocycloalkanol derivatives, complement-
ing the existing methodologies, mostly aimed at trans
diastereoisomers, readily available from epoxides and aziridines.
Actually, most of the existing approaches toward cis-β-
aminocycloalkanols are rather difficult and tedious.28
Once both catalytic steps were reliably established, we took
the challenge of performing a one-pot fully convergent
approach furnishing optically active amines from the analogous
racemic alcohols. At this point, a number of concerns should be
addressed to implement such a coupled process. A major
challenge is the mutual compatibility of the involved catalysts as
well as of their respective reaction conditions.29 In order to
carry out some compatibility tests, we took 1f as a model
substrate and investigated the effect of the biocatalyst (ω-TA)
and its cofactor (PLP, pyridoxal-5′-phosphate) on the
AZADO/NaOCl system. Whereas ω-TA was innocuous, the
presence of PLP caused a decrease of the conversion (78%).
However, the most important observation was the deactivation
of PLP in this medium.30 In addition, it had been already found
that AZADO is totally inhibited by DMSO (see Table 1, entry
9), the cosolvent from the bioamination. On the other hand,
another issue to consider was the substrate concentration,
actually one of the most recalcitrant pitfalls when combining
conventional and enzymatic catalysts. Indeed, despite the fact
that both steps are conducted in basic aqueous medium and at
mild temperatures, the alcohol oxidation was optimized at >200
mM, contrasting with the low concentration typically required
by ω-TAs (10−20 mM). Thus, when the oxidation of 1b was
tested under conditions identical with those above but at lower
concentration (10 mM), no ketone was produced even after
addition of an excess of NaOCl (up to 8.4 equiv) and the use of
a higher amount of AZADO (10%).31
With the aim of accelerating the racemization and optimizing
the diastereoselectivity outcome, we checked the bioamination
of 2b at higher pH values (8.5, 10, and 11.5). In general,
although a basic pH such as 10 or 11.5 was detrimental to the
stability of ω-TAs, a noteworthy increase in the dr was
observed in some cases (see Table 2, entries 9−16). Thus, on
changing pH from 8.5 to 10, the dr was increased until 6:1 and
7:1 in the reactions mediated by ATA-254 and ATA-P2-A07,
respectively (see entries 9 and 10 and entries 15 and 16).
Similarly, ATA-412 displayed an enhanced selectivity at pH
11.5 (see entries 13 and 14) despite a lower catalytic activity.
More interestingly, the ATA-303 catalyzed amination experi-
At this point, we have found several reasons precluding a
concurrent process. Nevertheless, the inhibitor effect of DMSO
and the absence of oxidation at a low substrate concentration
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Table 3. Catalytic Amination of Racemic Alcohols into Optically Active Amines Using AZADO/NaOCl and a ω-
ab
,
Transaminase
a
Oxidation step following the optimized conditions described in Table S3 in the Supporting Information for each substrate. Once completed,
addition of ω-TA, DMSO (10% v/v (for 1c−h) or 15% v/v (for 1a,b)), and KPi buffer 100 mM pH 7.5 (for 1c−h), pH 10 (for 1a,b, entries 2, 3,
and 5), or pH 11.5 (for 1a,b, entries 1 and 4) with iPr-NH2 (1.0 M) PLP (1.0 mM) until a concentration of 10 mM and stirring at 250 rpm and 30
b
c
°C. Determined by GC or HPLC. ee and configuration of the major diastereomer.
opened up the possibility of a one-pot stepwise process
preserving the integrity of PLP. Accordingly, we carried out the
oxidation of 1b at 250 mM under the optimized conditions:
namely 0.01 equiv of AZADO and 1.2 equiv of NaOCl. Once
the oxidation was completed, the reaction mixture was diluted
to 10 mM with a buffer solution (100 mM KPi, pH 11.5)
containing iPrNH2 (1.0 M) and PLP (1.0 mM), and DMSO
(15% v/v) and the ω-transaminase ATA-303 were added.
Then, the reaction mixture was incubated for 24 h at 30 °C and
250 rpm. HPLC analysis showed complete conversion for the
overall process, the corresponding trans-(1R,2R)-2-
(benzyloxy)cyclohexanamine (3b) being isolated as the sole
product with >99% ee (Table 3, entry 4) and the same
excellent dr (24:1) as that obtained starting from ketone (see
Table 2, entry 12). Interestingly, after a simple extraction-based
workup, amine 3b was recovered pure without need of further
purification with very high yield (94%). Then, once
demonstrating the viability of this one-pot fully convergent
approach, we extended the study to other alcohols. As shown in
Table 3, all of them were quantitatively oxidized in a
concentration ranging from 230 to 320 mM, and the
subsequent bioaminations, performed at 10 mM, worked
exceptionally well with all the biocatalysts leading to results
(conversion, ee, and dr values) nearly identical with those
measured in the single step.32 As a result, fine-tuning the
oxidation conditions and adequate biocatalyst selection33
provided a set of synthetically and pharmacologically interesting
amines34 with high overall yields (up to 97%) without need of
chromatographic purification. Particularly remarkable is the
simplicity of this chemoenzymatic setup, since the only
required experimental setting, once the oxidation step is
complete in 1 h, was a dilution of the reaction medium before
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a
Scheme 2. Organoenzymatic Platform toward Valuable Optically Active β-Aminocycloalkanol Derivatives
a
Legend: (i) H2, Pd−C 10%, MeOH; (ii) (Boc)2O, CH2Cl2; (iii) CF3CO2H, CH2Cl2; (iv) H2, Pd−C 10%, (Boc)2O, MeOH.
the addition of the biocatalyst. It should be noted that ω-TAs
have been previously engineered via directed evolution for
practical applications in a manufacturing setting, leading to
improved biocatalysts working at concentrations up to 500
mM.21d,e Although beyond the scope of this study, the
implementation of such evolved enzymes in the chemo-
enzymatic process disclosed herein would enable circumventing
the dilution issue, upgrading its potential for translation into a
true cascade process applicable on scale.
Finally, the synthetic utility of the organoenzymatic platform
developed herein was showcased by submitting the resulting
optically active amino alcohol derivatives (1R,2R)-3a and
(1R,2R)-3b (Table 3, entries 1 and 4) to a panel of chemical
transformations to provide a set of valuable chiral synthons
(Scheme 2). First, a mild hydrogenolysis of 3a,b furnished the
free amino alcohols (1R,2R)-4a,b in quantitative yield. On the
other hand, treatment of 3a,b with di-tert-butyl dicarbonate led
to the orthogonally protected derivatives (1R,2R)-5a,b.
Actually, orthogonally protected amino alcohols are highly
valuable molecules in both asymmetric catalysis and medicinal
chemistry.17 Debenzylation of 5a,b gives entry to N-protected
amino alcohols (1R,2R)-6a,b, thus completing the practical
syntheses of an interesting set of derivatives by means of
quantitative and simple processes.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.7b01543.
Experimental procedures, screenings for the oxidation
and enzymatic transamination steps, characterization
data, HPLC chromatograms, and NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for F.R.: frv@uniovi.es.
*E-mail for J.G.-S.: jgsabin@entrechem.com.
ORCID
Javier Gonzalez-Sabín: 0000-0003-3764-4750
́
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Dedicated to professor Vicente Gotor on the occasion of his
70th birthday. E.L. acknowledges funding from the European
Union’s Horizon 2020 MSCA ITN-EID program under grant
agreement No 634200 (Project BIOCASCADES). N.R.-L.
acknowledges the MINECO for funding under the Torres-
Quevedo program (PTQ-12-05 407). Authors from the
University of Oviedo thank the Principado de Asturias for
financial support (FC-15-GRUPIN-14-002). The authors also
thank Wolfgang Kroutil for the generous gift of the ω-TAs Cv,
ArS, ArR, and ArRmut11.
CONCLUSIONS
■
In conclusion, we disclosed an expedient, stereoselective, and
operationally simple protocol for the synthesis of a variety of
optically active amines, through the unprecedented one-pot
combination of (i) organocatalyzed oxidation of secondary
alcohols and (ii) subsequent enantioselective bioamination of
the transiently formed ketones. In all cases, the desired final
amines were isolated in high yields with excellent diastereo- and
enantiomeric excesses. Likewise, it is worth noting the use of a
one-pot methodology, in which the aqueous reaction medium
from the organocatalyzed reaction feeds the enzymatic
amination. This methodology, which exploits the advantages
of merging two catalytic worlds such as organocatalysis and
enzymatic catalysis, represents one of the few contributions of
this kind of combination in water. Moreover, it is a robust
alternative to the existing methodologies, with proven efficacy
for a broad variety of secondary alcohols.
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