First Tandem-Type One-Pot Process Combining Asymmetric Organo- and Biocatalytic Reactions in Aqueous Media Exemplified for the Enantioselective and Diastereoselective Synthesis of 1,3-Diols

Article abstract of DOI:10.1002/ejoc.201600831

A suitable “process window” was identified for the combination of an asymmetric organocatalytic aldol reaction and subsequent biocatalytic reduction in aqueous medium, which thus enabled the enantio- and diastereoselective synthesis of 1,3-diols in a tand

Full text of DOI:10.1002/ejoc.201600831

DOI: 10.1002/ejoc.201600831
Communication
Chemoenzymatic Synthesis
First Tandem-Type One-Pot Process Combining Asymmetric
Organo- and Biocatalytic Reactions in Aqueous Media
Exemplified for the Enantioselective and Diastereoselective
Synthesis of 1,3-Diols
Giuseppe Rulli,[a] Nongnaphat Duangdee,[c] Werner Hummel,[b] Albrecht Berkessel,*[c] and
Harald Gröger*[a,d]
Abstract: A suitable “process window” was identified for the sis of 1,3-diols in a tandem-type, one-pot process. A key feature
combination of an asymmetric organocatalytic aldol reaction
and subsequent biocatalytic reduction in aqueous medium,
which thus enabled the enantio- and diastereoselective synthe-
of this one-pot synthesis is the high 500 m
aldehyde substrate used as a starting material.
M loading of the
Introduction
cantly with several contributions from different groups.[2–5] In
this context, we recently demonstrated the first example of a
combination of an asymmetric organocatalytic reaction with a
subsequent biotransformation in aqueous reaction media.[5]
This process consisted of an initial organocatalytic aldol reac-
tion and subsequent in situ diastereoselective reduction of the
formed ꢀ-hydroxy ketone by means of an enzymatic reduction
(according to the reaction sequence shown in Scheme 1). The
resulting 1,3-diols bearing two stereogenic centers were ob-
tained with excellent diastereoselectivities (dr > 25:1) and enan-
tioselectivities (99 % ee). In this type of one-pot process, the
enzyme (as a catalyst for the second step), cofactor, and 2-prop-
anol as the reducing agent (required in stoichiometric amount)
were added after completion of the first reaction step, namely
the aldol reaction.
An open question that has remained unanswered over the
years is whether this type of one-pot synthesis with two se-
quential (asymmetric organo- and biocatalytic) steps could be
extended towards a tandem-type process in aqueous media.
In such a tandem-type synthesis, which has the same reaction
sequence as that shown in Scheme 1, both the organocatalyst
and enzyme are present at the beginning of the first reaction.
Furthermore, suitable reaction conditions (including tempera-
ture, pH, substrate, and reagent concentrations) that enable the
simultaneous progress of both the organocatalytic and enzy-
matic reactions have to be found. The unit operations flow of
such a concept and its comparison with the previously[5a] devel-
oped one-pot process based on two subsequent reaction steps
is shown in Scheme 2.
Although biotechnology has already emerged as a key technol-
ogy for the industrial production of fine chemicals and pharma-
ceuticals,[1] it is at the same time often considered a “stand
alone” technology, thus being not compatible with “classic”
chemical or chemocatalytic reaction steps. As a consequence,
biotransformations typically start with purified substrates re-
sulting from chemical processes. Providing such substrates in
purified form, however, requires solvent-intensive, time-con-
suming, and waste-producing downstream-processing steps.
Thus, a combination of a “classic“ chemical or chemocatalytic
reaction step for substrate synthesis with a subsequent bio-
transformation towards a one-pot process offers unique oppor-
tunities with respect to improvement in both process efficiency
and sustainability. Running such one-pot processes in water
would be of particular interest, as it makes the use of any type
of enzyme possible.
Despite the tremendous potential for application, today the
number of examples of synthetic one-pot processes under the
combination of chemo- and biocatalytic reaction steps is still
limited, although in recent years this field has emerged signifi-
[a] Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg,
Henkestr. 42, 91054 Erlangen, Germany
[b] Institute of Molecular Enzyme Technology, Heinrich-Heine-University of
Düsseldorf, Research Centre Jülich,
Stetternicher Forst, 52426 Jülich, Germany
[c] Department of Chemistry, University of Cologne,
Greinstraße 4, 50939 Cologne, Germany
E-mail: berkessel@uni-koeln.de
One motivation for carrying out one-pot multistep processes
in a tandem mode is that, in principle, thermodynamically unfa-
vorable reaction steps could also be conducted effectively
therein, as the resulting product of such a step would be di-
rectly converted in an (irreversible) subsequent transformation,
which would thus permanently shift the unfavorable thermody-
http://berkessel.de/
[d] Faculty of Chemistry, Bielefeld University,
Universitätsstr. 25, 33615 Bielefeld, Germany
E-mail: harald.groeger@uni-bielefeld.de
http://www.uni-bielefeld.de/chemie/arbeitsbereiche/oc1/HG/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejoc.201600831.
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Scheme 1. Combination of an organocatalytic aldol reaction with a diastereoselective enzymatic reduction of the in situ formed ꢀ-hydroxy ketone in a one-
pot process.
Scheme 2. Comparison of unit operations flow of the previously[5a] developed one-pot process based on two consecutive reaction steps with the tandem-
type, one-pot process.
namic equilibrium to the direction of the desired product. Fur- point [shown in Scheme 3, Equation (a)],[5a] at first we con-
thermore, tandem processes are an advantageous one-pot op- ducted an analogous tandem process by adding both catalysts
tion if the concentration of the formed intermediate should be from the beginning. As components for the initial aldol reac-
kept low, for example, because of inhibition or stability reasons. tion, 3-chlorobenzaldehyde and acetone were used. For the
A prerequisite for tandem processes is that both reactions
have to proceed under identical reaction conditions such as pH,
temperature, substrate concentration (whereas in a sequential
one-pot synthesis there is increased “freedom to operate”, as
reaction conditions for the second step can be adjusted – at
least in part and for some reaction parameters such as, e.g., pH,
temperature, and substrate concentration – independent of the
first reaction step after its completion). Thus, the identification
of suitable (and potentially narrow) “process windows”[6] is a
key task in the development of tandem-type, one-pot proc-
esses.
In the following, we report our results on the development
of the first type of a chemoenzymatic tandem-type, one-pot
process consisting of asymmetric organocatalytic and biocata-
lytic transformations in aqueous reaction media, as exemplified
for the enantio- and diastereoselective synthesis of 1,3-diols.
Results and Discussion
Taking the previously developed one-pot process based on two
consecutive steps without isolation of the intermediate by re-
moval of the excess amount of acetone prior to the addition of
the components for the enzymatic reduction step as a starting
Scheme 3. Comparison of the previously developed one-pot process based
on two consecutive reaction steps with the initial tandem-type, one-pot proc-
ess.
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subsequent enzymatic reduction, an alcohol dehydrogenase the aldol reaction. The substrate concentration appeared to be
(ADH) from Rhodococcus sp.[7] was used as a catalyst, and sub- of particular interest, as from a kinetic perspective the reaction
strate-coupled cofactor regeneration was done by means of 2- should proceed in a much more favorable way at elevated sub-
propanol as a reducing agent. However, in contrast to the previ- strate concentrations. Given that in the tandem-type, one-pot
ous successful one-pot methodology with the two steps done process the substrate concentration was adjusted to 50 m
in a consecutive manner, which gave 1,3-diol 3 with a (product- the biotransformation in the one-pot process with consecutive
formation-related) conversion of 89 % besides high diastereo- steps was conducted at 50 m as a “standard reaction condi-
and enantioselectivities [dr > 25:1, 99 % ee; Scheme 3, Equa- tion”), we first studied if the organocatalytic aldol reaction pro-
tion (a)], nearly complete loss of product formation of desired ceeded at 50 m of substrate as well. Interestingly, however,
1,3-diol product 3 (with only 7 %) was observed upon changing we found a strong drop in the conversion upon changing the
to a tandem-type, one-pot process in the presence of a buffer/ substrate concentration for the aldol reaction to 50 m . Thus,
2-propanol reaction mixture as our reaction medium of choice instead of a high product-formation-related conversion for the
for the biotransformation [Scheme 3, Equation (b)]. This result original aldol reaction at a substrate concentration of 500 m
M (as
M
M
M
M
clearly indicated that besides compatibility of the enzyme and [Scheme 3, Equation (a)], only 15 % conversion related to the
organocatalyst (which was achieved evidenced by the previous formation of product 4 and 22 % overall conversion including
successful one-pot, two-step synthesis in aqueous media, see side products were found upon operating at a substrate con-
also ref.[5a]), other reaction parameters in this tandem-type, centration of 50 m
M (Scheme 4).
one-pot process were also of high relevance.
This clearly indicates the need to identify a suitable “process
window” and a substrate concentration at which both reactions
proceed as a task for the development of an efficient tandem
process. The disappointing result of a low conversion upon
In an attempt to identify the reason for this remarkable and
strong difference in product-formation-related conversion upon
comparing the sequential one-pot, two-step processes with the
tandem-type, one-pot process, we were first interested whether
both individual reaction steps (namely, the organocatalytic
aldol reaction and enzymatic reduction) proceeded under the
reaction conditions chosen for the tandem process. In the previ-
ously conducted one-pot process, the aldol reaction (organoca-
talysis) and ketone reduction (biocatalysis) were done under
different reaction conditions adjusted to known “standard reac-
tion conditions” for these two reaction types. For example, the
substrate concentration of the organocatalytic aldol reaction
choosing a 50 mM substrate concentration can be rationalized
by the abovementioned kinetic effect. The reaction medium,
which is a two-phase system, and an unfavorable distribution
coefficient of acetone might also play a role. Accordingly, the
organocatalytic aldol reaction should be conducted at an ele-
vated substrate concentration of 500 m
M instead of 50 mM in
the one-pot process to obtain an excellent conversion related
to the formation of aldol product 4 as an intermediate and,
thus, subsequently desired 1,3-diol 3.
(500 m
transformation (50 m
M
) was significantly higher than that of the enzymatic
). In addition, 2-propanol was added after
As for the influence of other reaction parameters on the tan-
dem process, to our delight we found that neither the use of a
buffer instead of distilled water nor the presence of protein
(enzyme) or cofactor as an additive had a negative impact on
the organocatalytic aldol reaction (Scheme 5). Most notably,
there was no negative impact of a large amount of 2-propanol
[28 % (v/v)] in buffer on the aldol reaction, which is of impor-
tance owing to the use of 2-propanol as a reducing agent and a
co-substrate for in situ cofactor regeneration in the subsequent
second step, namely, the enzymatic reduction, in larger amount
(to ensure a strong “driving force” for the enzymatic reduction
towards formation of the desired 1,3-diol).
M
completion of the organocatalytic process [Scheme 3, Equa-
tion (a)], and consequently, the organocatalytic reaction in the
original one-pot process with two consecutive steps took place
in the absence of 2-propanol (which is in contrast to a tandem-
type, one-pot process with all components present in the reac-
tion medium from the beginning).
As we changed a variety of reaction parameters for the aldol
reaction in a tandem-type, one-pot process relative to the origi-
nal one-pot process with two consecutive steps (e.g., substrate
concentration and reaction medium, as 2-propanol was used as
a co-solvent and co-substrate required for the biotransforma-
Given that this organocatalytic aldol reaction was compatible
tion), we studied the influence of these reaction parameters on with all components used in the biotransformation, we next
Scheme 4. Impact of different biocompatible reaction media on the organocatalytic aldol reaction.
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Scheme 5. Impact of the components of the biotransformation on the organocatalytic aldol reaction.
combined this reaction with enzymatic ketone reduction to- chlorobenzaldehyde was fully consumed. Such a conversion of
wards a tandem-type, one-pot process. However, as a high sub- 50 % for the formation of (1R,3S)-3 without process optimiza-
strate loading was required for the organocatalytic aldol reac- tion is a remarkable result for this tandem-type, one-pot proc-
tion, a prerequisite for a successful tandem reaction was that ess, as the initial aldol reaction competes with enzymatic reduc-
the biotransformation must also proceed at a high substrate tion of aldehyde 1 to 3-chlorobenzyl alcohol (which thus leads
concentration of, for example, 500 m
that the enzymatic reduction also proceeded well at a signifi- to the presence of an excess amount of acetone (a required
cantly elevated substrate concentration of 500 m for 3-chloro- component for the aldol reaction) the “retro-reduction reaction”
M. We were pleased to find to decreased formation of the aldol adduct). In addition, owing
M
benzaldehyde as the substrate for the initial aldol reaction, [= oxidation of (1R,3S)-3] leading to a decomposition product
which led to the formation of desired diol (1R,3S)-3 with a con- of (1R,3S)-3 might also play a role, as well as inhibition or deac-
version of 50 % related to the formation of this product tivation of the enzyme. However, the desired biotransformation
(Scheme 6). The overall conversion was >95 %; consequently, 3- proceeded efficiently in spite of an excess amount of acetone
Scheme 6. Tandem-type, one-pot process combining an organocatalytic aldol reaction with an enzymatic reduction.
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in the reaction medium, which thus demonstrates the synthetic Experimental Section
utility of the ADH from Rhodococcus sp.[7] as a biocatalyst for
The experimental protocols and details are given in the Supporting
Information.
stereoselective biocatalytic ketone reductions also at elevated
substrate loadings and under challenging reaction conditions,
such as a high percentage of organic water-miscible compo-
nents [2-propanol: 28 % (v/v); acetone: 9 equiv.].
Acknowledgments
Next, we focused on process optimization to improve the
formation of desired diol (1R,3S)-3 while minimizing the impact
of the undesired side reactions, namely: (1) enzymatic reduction
of aldehyde 1; (2) organocatalytic side reactions such as aldol
condensation (see also ref.[5a]); (3) ADH-catalyzed oxidation of
(1R,3S)-3 with formation of 4 as a result of the presence of an
excess amount of acetone from the aldol reaction step, which
then serves as a “hydride acceptor”. By adjusting the amount
of organocatalyst, biocatalyst, stoichiometric amount of acet-
one and 2-propanol, as well as the reaction time accordingly,
the conversion related to the formation of (1R,3S)-3 was then
further increased to 60 % (Scheme 6; for experimental details,
e.g., quantification of byproducts in dependency of selected
reaction parameters, see the Supporting Information). To dem-
onstrate the feasibility of this optimized one-pot synthesis on
an elevated laboratory scale, a preparative experiment under
these conditions was conducted on a 10 mmol scale of 3-
chlorobenzaldehyde (1), which resulted in a 50 % conversion
related to the production of desired (1R,3S)-diol 3 (see the Sup-
porting Information for experimental details). Subsequent
workup and isolation by column chromatography then gave
desired purified (1R,3S)-diol 3 in 33 % yield. A further improve-
ment in the conversion can be expected if acetone as the most
volatile component is removed from the reaction mixture con-
tinuously, in particular at a later stage of the process after com-
pletion of the aldol reaction. Various techniques for the in situ
removal of acetone have been developed successfully by Liese
et al.[8] and represent a promising tool for future process devel-
opment and scale up of this tandem-type one-pot process.
We thank the Deutsche Forschungsgemeinschaft (DFG) for gen-
erous support within the priority program SPP1179 “Organo-
katalyse” (BE 998/11-1, GR 3461/2-1). Membership in the Euro-
pean Cooperation in Science and Technology (COST) Action no.
CM0905 Organocatalysis (ORCA) is gratefully acknowledged.
Furthermore, we thank Mr. Sarwar Aziz for performing the pre-
parative one-pot synthesis.
Keywords: Aldol reactions · Biocatalysis · Diols · Enzyme
catalysis · Organocatalysis
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Conclusions
In conclusion, we reported the first tandem-type, one-pot proc-
ess under combination of asymmetric organo- and biocatalytic
reaction steps in aqueous medium, as exemplified for the enan-
tio- and diastereoselective synthesis of 1,3-diols. A key chal-
lenge was to identify a suitable “process window”, namely, reac-
tion conditions under which both reaction steps could proceed
efficiently. An additional task was to suppress side reactions
caused by reaction components from the other reaction step
(e.g., reduction of aldehyde 1 used in the first step by the bio-
catalyst required for the second step). Thus, besides compatibil-
ity of the enzyme and organocatalyst, several reaction parame-
ters in this tandem-type, one-pot process were also deemed to
be of high relevance. After process design and optimization,
a suitable “process window” was identified that enabled the
combination of an asymmetric organocatalytic aldol reaction
with enzymatic reduction in a tandem-type, one-pot process in
aqueous medium at a high substrate loading of 500 m
corresponding aldehyde.
M of the
[6] For a definition of the term “process window” and the importance of
identifying suitable “process windows” in modern process development,
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see: a) V. Hessel, D. Kralisch, N. Kockmann, Novel Process Windows: Inno-
vative Gates to Intensified and Sustainable Chemical Processes, Wiley-VCH,
Weinheim, 2015; b) V. Hessel, Chem. Eng. Technol. 2009, 32, 1655–1681.
[7] Alcohol dehydrogenase from Rhodococcus sp. is commercially available
from evocatal GmbH, Merowinger Platz 1a, 40225 Düsseldorf, Germany
(http://www.evocatal.com).
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Received: June 8, 2016
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