Sequential and modular synthesis of chiral 1,3-diols with two stereogenic centers: Access to all four stereoisomers by combination of organo- and biocatalysis

Article abstract of DOI:10.1002/anie.200900582

Chemical Equitation Presentation Biocompatible: A modular chemoenzymatic synthesis (see scheme) based on asymmetric organo- and biocatalytic reaction sequences allows the sequential construction of both stereogenic centers of 1,3-diols and leads to all fo

Full text of DOI:10.1002/anie.200900582

Angewandte
Chemie
DOI: 10.1002/anie.200900582
Chemoenzymatic Synthesis
Sequential and Modular Synthesis of Chiral 1,3-Diols with Two
Stereogenic Centers: Access to All Four Stereoisomers by Combination
of Organo- and Biocatalysis**
Katrin Baer, Marina Kraußer, Edyta Burda, Werner Hummel, Albrecht Berkessel, and
Harald Grꢀger*
Chiral 1,3-diols with two stereogenic centers are widely used
as building blocks in the synthesis of pharmaceutically active
compounds.[1] In addition, numerous natural products possess
a chiral 1,3-diol subunit.[2] A further interesting application of
chiral 1,3-diols is their transformation into chiral 1,3-bisphos-
phanes, which are often used very successfully as enantio-
merically pure ligands in asymmetric catalysis.[3] Accordingly,
there is a high demand for stereoselective synthetic processes
to generate chiral 1,3-diols with two stereogenic centers (e.g.,
of the type 4; Scheme 1), which ideally should allow the
often their applicability for only one or two of the four
maximum possible stereoisomers. In contrast, methods which
offer an approach to all four stereosiomers in an efficient and
convenient way are rarely known.[2] In the following we report
a modular chemoenzymatic process for the selective synthesis
of all four stereoisomers of 1,3-diols 4 through a combination
of asymmetric organocatalysis and biocatalysis. A key feature
of this synthetic strategy is the sequential construction of the
stereogenic centers. Thus, each stereogenic center can be
constructed selectively by means of a catalyst which is
specifically suitable for this purpose (Scheme 1). This mod-
ular synthetic strategy additionally offers the possibility for a
two-step one-pot synthesis.
Since organocatalytic syntheses[5,6] are already known for
the initial aldol reaction, we first focused on the subsequent
diastereoselective enzymatic reduction of b-hydroxy ketones
3 bearing one stereogenic center. Such a study is interesting
both from the synthetic perspective of the target molecules 4
and the perspective of the reaction mechanism since chiral
ketones are used as substrates. Thus, the effect of the internal
stereogenic center of the substrate and the influence of the
chiral catalyst (enzyme) on the formation of the second
stereogenic center can be compared. Such studies based on
the use of redox enzymes are rare,[7] in contrast to the
numerous examples of enantioselective reductions of prochi-
ral ketones.[8] A stereoselective reaction course is typically
expected when chiral (for example, racemic) substrates are
used in an enzymatic reaction, with preferential transforma-
tion of only one of the two enantiomers. With respect to a
stereoselective construction of all stereoisomers of type 4,
however, we were interested in a diastereoselective reaction
course which can be directed exclusively (externally) by the
enzyme catalyst. In addition to a highly selective formation of
the second stereogenic center—depending only on the
biocatalyst—both enantiomeric forms of the aldol product
should be tolerated as substrates. We used as enzymes two
alcohol dehydrogenases with S and R enantiospecificity.[9]
The suitability of the enzymes as catalysts for the trans-
formation of the ketones 3 (racemic as well as enantiomer-
ically enriched) were first investigated spectrophotometri-
cally. This method is based on the dependence of the decrease
in the amount of NAD(P)H as a natural cofactor (reducing
agent) on a specific substrate. We found that the enzymes
were suitable for the reduction of both enantiomeric forms of
the ketones without significant differences.[10]
Scheme 1. Synthesis of 1,3-diols by organo- and biocatalysis.
synthesis of all stereoisomers in a diastereo- and enantiomer-
ically pure form.[4] To date, efficient synthetic routes[2] are
based, in particular, on the enantio- and diastereoselective
reduction of 1,3-diketones by hydrogenation with metal
catalysts[4a] and enzymes[4b] or chemoenzymatic processes by
dynamic kinetic resolution.[4c] A limitation of these methods is
[*] K. Baer, M. Kraußer, E. Burda, Prof. Dr. H. Grꢀger
Department of Chemistry and Pharmacy
University of Erlangen-Nuremberg
Henkestr. 42, 91054 Erlangen (Germany)
E-mail: harald.groeger@chemie.uni-erlangen.de
Prof. Dr. W. Hummel
Institute of Molecular Enzyme Technology at the Heinrich-Heine-
University of Dꢁsseldorf, Research Centre Jꢁlich
Stetternicher Forst, 52426 Jꢁlich (Germany)
Prof. Dr. A. Berkessel
Department of Chemistry, University of Cologne
Greinstrasse 4, 50939 Cologne (Germany)
[**] We thank Tobias Huber for excellent technical assistance, Evonik-
Degussa GmbH, Amano Enzymes Inc., and Oriental Yeast Com-
pany Ltd. Japan for the generous supply of chemicals, and the
Deutsche Forschungsgemeinschaft (DFG) for generous support
within the priority programme SPP 1179 “Organokatalyse” (BE 998/
11-1, GR 3461/2-1).
The enzymatic reduction of ketones 3 was subsequently
carried out on a preparative scale according to the concept of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900582.
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the substrate-coupled regeneration of the cofactor[8]
(Scheme 2). Therein, the cofactor NAD(P)H is recycled
in situ by means of the oxidation of 2-propanol (2-PrOH) to
acetone. Thus, the expensive cofactor is used only in a
sion of > 95% and a diastereoselectivity d.r.(syn/anti) of 1:11
was also obtained in this case. Analogously, starting from
enantiomerically enriched substrates (S)-3a (83% ee) and
(R)-3a (82% ee) in the presence of an R-selective alcohol
dehydrogenase afforded the corresponding diastereomers
(1S,3R)-4a and (1R,3R)-4a, respectively, with > 99% ee and
high diastereoselectivities in both cases (conversions of
> 95% in both cases). Thus, this chemoenzymatic synthesis
is suitable for the synthesis of all four possible stereoisomers
of type 4a in enantiomerically pure form (> 99% ee).
In our study of the substrate spectrum, we varied the
aldehyde component in particular. We also succeeded in the
formation of the corresponding desired 1,3-diol starting from
4-nitrobenzaldehyde (1b), as exemplified by the synthesis of
(1R,3S)-4b with a (product-related) conversion of 64%
(Scheme 4). Besides electron-deficient aromatic aldehydes,
electron-rich benzaldehydes can also be used: An organo-
catalytic aldol reaction starting from 4-methylbenzaldehyde
(1c; 75% product-related conversion; 76% ee), followed by
reduction delivered the desired product (1S,3R)-4c with
> 95% conversion, a diastereomeric ratio d.r.(syn/anti) of
1:7, and with > 99% ee. The related diastereomer (1R,3R)-4c
Scheme 2. Diastereoselective enzymatic reduction.
catalytic amount, and the inexpensive 2-propanol represents
the reducing agent required in a stoichiometric amount. The
diastereoselective reduction proceeded successfully when the
racemic ketone rac-3a was used. For example, quantitative
conversion was achieved in the presence of the
(S)-alcohol dehydrogenase, and both product
diastereomers were obtained in enantiomerically
pure form (> 99% ee) in a diastereomeric ratio of
1:1. Thus, a diastereoselective reaction has been
successfully developed, which—as desired—is
controlled exclusively by the “external influence”
of the (bio-)catalyst; an undesired internal induc-
tion by the stereogenic center in the aldol adduct
3a was not observed. This result is in contrast to
many enzyme-catalyzed reactions, which show a
high preference for only the transformation of one
of the two enantiomers of a substrate (resulting in
an efficient resolution of a racemate through
modification of only one enantiomer).[11]
Based on this result, we focused on the
envisioned two-step synthesis of all the 1,3-diol
stereoisomers by sequential stereoselective for-
mation of both stereogenic centers (Scheme 3).
For the initial organocatalytic aldol reaction we
oriented ourselves on already known asymmetric
organocatalytic processes.[5,6] We were particularly
interested in a solvent-free synthesis. On the basis
of the work by Singh and co-workers,[5g] aldol
products 3a have been prepared with conversions
Scheme 3. Chemoenzymatic synthesis of all stereoisomers of 4a.
of up to 95% and enantioselectivities of 82–83% ee by using
organocatalysts (S,S)-5 and (R,R)-5 at room temperature.
These enantiomerically enriched (S)- and (R)-aldol adducts
3a have then been used as substrates in the subsequent
enzymatic reduction. When starting from (S)-3a as the
substrate (83% ee) in the presence of an S-selective alcohol
dehydrogenase, the desired 1,3-diol (1S,3S)-4a was formed
with a conversion of > 95% and a diastereomeric ratio
d.r.(syn/anti) of 11:1, again in enantiomerically pure form (>
99% ee). The enantiomerically pure (1R,3S) diastereomer
(1R,3S)-4a was prepared with > 99% ee by using the same
enzyme and starting from (R)-3a (82% ee). A high conver-
Scheme 4. Substrate spectrum of the chemoenzymatic synthesis of 4.
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has also been obtained with > 95% conversion and with
> 99% ee.
> 95%), a high diastereomeric ratio d.r.(syn/anti) of 1:10, and
an excellent enantiomeric excess of > 99% ee (Scheme 6). An
advantage of this modified one-pot synthesis over the
sequential two-step method with isolation of the intermediate
Furthermore, we were interested in the combination of
both processes to give an economical preparative method in
which the reaction product from the first step is used directly
without purification for the second reaction. To the best of
our knowledge, studies on the combination of organocatalysis
and biocatalysis in aqueous reaction media are rare.[12,13]
A
prerequisite for such a method is a compatibility of the two
reaction processes. Accordingly, we studied the impact of the
organocatalyst (S,S)-5 on the activity of the enzyme by means
of spectrophotometrical tests (Scheme 5). Therein, a high
Scheme 6. Combination of organocatalysis and biocatalysis in a modi-
fied one-pot synthesis of 4a.
is the avoidance of the critical work-up of product (R)-3a
after the first reaction step (danger of decomposition through
dehydration during purification by column chromatography).
This offers the possibility for an improved overall economy of
the process.
In summary we have reported the combination of
asymmetric organo- and biocatalytic reaction sequences
which lead to a sequential construction of both stereogenic
centers of 1,3-diols. This modular chemoenzymatic synthetic
concept makes possible efficient access to all four stereoiso-
mers in enantiomerically pure form. At the same time, the
reaction mixtures resulting from the organocatalytic reaction
route are compatible with a direct subsequent enzymatic
reduction without the need for a work-up of the aldol
reaction. Based on these positive initial results on the
compatibility and combination of asymmetric organocatalysis
and biocatalysis we are currently working on the development
of corresponding one-pot multistep reactions in aqueous
reaction media.
Scheme 5. Enzyme activity as a function of the organocatalyst concen-
tration.
enzyme activity of the (S)-alcohol dehydrogenase was also
found in the presence of organocatalyst (S,S)-5. This result
indicates a high compatibility of the organocatalyst with the
biocatalyst in aqueous media, thus offering interesting
perspectives towards future combinations of organocatalysis
and biocatalysis in one-pot multistep syntheses in aqueous
media. In addition, the biocompatibility is not limited to
peptidic organocatalysts such as (S,S)-5. In further activity
tests, the biocompatibility of the l and d enantiomers of
proline, which are widely used organocatalysts[5k,6] that also
catalyze asymmetric aldol reactions efficiently,[5a,b] has been
demonstrated. For example, when using an alcohol dehydro-
genase from Lactobacillus kefir, irrespective of the proline
concentration, the relative enzyme activities are virtually
unchanged and high, in comparison with the enzyme activity
in the reference reaction without proline as an additive (see
also the Experimental Section and schemes in the Supporting
Information).
Next, we carried out such a combination of the asym-
metric organocatalytic aldol reaction and a subsequent
biotransformation with a final work-up step. The aldol
reaction was carried out as a solvent-free synthesis and the
resulting reaction mixture was passed directly into an
aqueous/2-propanol solution of the enzyme. We were pleased
to find that such a process proceeds highly efficiently and
leads to the desired 1,3-diol (1R,3S)-4a with a product-related
conversion of 80% over two steps (at an overall conversion of
Experimental Section
Procedure for the chemoenzymatic two-step synthesis of 1,3-diols
(according to Scheme 3):
For the initial step, namely the organocatalytic aldol reaction, 4-
chlorobenzaldehyde (1a; 1.4 mmol), the corresponding organocata-
lyst [(S,S)-5 or (R,R)-5; 5 mol%], and acetone (5.6 mmol) were
sequentially added to a reaction tube. The resulting reaction mixture
was then agitated for 18 h at 20–258C. After addition of a saturated
solution of sodium chloride (5 mL), the resulting mixture was
extracted three times with ethyl acetate and the collected organic
phases were dried over magnesium sulfate. After removal of the
solvent on a rotary evaporator, the resulting crude product 3 was
purified by column chromatography [hexane/ethyl acetate, 5:1 (v/v)].
For the second step, namely the enzymatic reduction, the aldol
product 3 (0.5 mmol) was first dissolved in 2-propanol (2.5 mL). After
addition of phosphate buffer (pH 7; 50 mm; 7.5 mL), magnesium
chloride (1 mm, only when using the alcohol dehydrogenase from
Lactobacillus kefir), and NAD(P)+ (0.02 mmol), the corresponding
alcohol dehydrogenase (20–200 UmmolÀ1 of substrate) was added
under stirring. After stirring the reaction mixture for 18–67 h at room
temperature, it was extracted with ethyl acetate. The collected organic
phases were dried over magnesium sulfate, and subsequently the
solvent was removed on a rotary evaporator. The crude product was
then purified by column chromatography [hexane/ethyl acetate, 5:1
(v/v)], thus affording the product 1,3-diol 4 in a diastereomerically
pure and enantiomerically pure form.
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Procedure for the combination of the organocatalytic aldol
reaction with subsequent enzymatic reduction (according to
Scheme 6):
Markert, M. Mulzer, B. Schetter, R. Mahrwald, J. Am. Chem.
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J. W. Yang, S. Hoffmann, B. List, Chem. Rev. 2007, 107, 5471 –
5569.
4-Chlorobenzaldehyde (1a; 0.5 mmol), acetone (2 mmol), and
organocatalyst (S,S)-5 (5 mol%) were sequentially added in a
reaction tube (2.5 mL) and the resulting mixture was agitated for
20 h at 20–258C. Without work-up, the reaction mixture was
subsequently decanted into a 25 mL round-bottom flask. Then 2-
propanol (2.5 mL), phosphate buffer (pH 7; 50 mm; 7.5 mL), the
cofactor NAD+ (0.02 mmol), and the alcohol dehydrogenase from
[6] Overview: A. Berkessel, H. Grꢁger, Asymmetric Organocatal-
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[7] Selected examples with a different influence of alcohol dehy-
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Chem. Soc. Chem. Commun. 1991, 1330 – 1332; b) M. Bertau, M.
Bꢀrli, E. Hungerbꢀhler, P. Wagner, Tetrahedron: Asymmetry
2001, 12, 2103 – 2107; c) I. A. Kaluzna, T. Matsuda, A. K. Sewell,
J. D. Stewart, J. Am. Chem. Soc. 2004, 126, 12827 – 12832; d) D.
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[9] a) (S)-Alcohol dehydrogenase from Rhodococcus sp.: commer-
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[10] The suitability of, for example, the alcohol dehydrogenase
(ADH) from Rhodococcus sp. has been confirmed for the
reduction of both enantiomers in spectrophotometrical activity
studies. Although differences are observed, both enantiomers of
the racemate are transformed with sufficient activity; selected
results of relative enzyme activities (referenced to 4-chloroace-
tophenone as 100%): (S)-3a, 102% enzyme activity; (R)-3b:
92%.
Rhodococcus sp. (20 UmmolÀ1
) were added. After stirring the
mixture for 18 h at 20–258C, subsequent extraction with ethyl acetate
(four times), and drying over magnesium sulfate, the solvent was
removed on a rotary evaporator. The crude product was then purified
by column chromatography [hexane/ethyl acetate, 5:1 (v/v)], to give
1,3-diol (1R,3S)-4a in a diastereomerically pure and enantiomerically
pure form.
Received: January 31, 2009
Revised: May 6, 2009
Published online: November 9, 2009
Keywords: alcohols · aldol reactions · asymmetric catalysis ·
biocatalysis · organocatalysis
.
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