Stereoselective reduction of 2-hydroxy ketones towards syn- and anti-1,2-diols

Article abstract of DOI:10.1002/adsc.201100150

Stereoselective reduction of 2-hydroxy ketones should in principle give access to syn- and anti-1,2-diols. anti-1,2-Diols are accessible in a highly selective way using zinc borohydride [Zn(BH₄)₂] under chelation control (dr>20:1). Diastereoselective reduction of unprotected or even protected 2-hydroxy ketones towards syn-1,2-diols could be achieved only with moderate selectivity of dr≤5:1. Even when using sterically demanding protecting groups and/or polymer-supported borohydride reagents high selectivity could not be achieved. A new ionic liquid-dependent borohydride reduction method, although highly attractive with respect to reaction engineering, resulted in only moderate to good selectivity. An efficient two-step biocatalytic method for the synthesis of syn-1,2-diols is described. The method relies on the whole-cell Pichia glucozyma-catalyzed stereoselective reduction of the unprotected (R)-2-hydroxy ketones (dr>10:1). The latter are accessible through thiamine diphosphate-dependent enzyme-catalyzed synthesis starting from simple aldehydes. Thus, biocatalytic transformations enable a process which is hardly accessible through present non-enzymatic methods. Copyright

Full text of DOI:10.1002/adsc.201100150

COMMUNICATIONS
DOI: 10.1002/adsc.201100150
Stereoselective Reduction of 2-Hydroxy Ketones towards syn- and
anti-1,2-Diols
Syed Masood Husain,a Thomas Stillger,a Pascal Dꢀnkelmann,a Melanie Lçdige,a
Lydia Walter,a Elke Breitling,a Martina Pohl,b Mara Bꢀrchner,c Ingo Krossing,c
Michael Mꢀller,a,* Diego Romano,d and Francesco Molinarid
a
Institute of Pharmaceutical Sciences, Albert-Ludwigs-University Freiburg, 79104 Freiburg, Germany
Fax : (+49)-0761-203-6351; e-mail: michael.mueller@pharmazie.uni-freiburg.de
Biocatalysis and Biosensors Group, Institute of Biotechnology 2, Research Centre Jꢀlich, 52425 Jꢀlich, Germany
Institute of Inorganic und Analytic Chemistry, Albert-Ludwigs-University Freiburg, 79104 Freiburg, Germany
Dipartimento di Scienze e Tecnologie Alimentari e Microbiologiche, Universitꢁ degli Studi di Milano, Via Mangiagalli 25,
b
c
d
20133 Milano, Italy
Received: March 2, 2011; Published online: August 26, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100150.
Abstract: Stereoselective reduction of 2-hydroxy
ketones should in principle give access to syn- and
anti-1,2-diols. anti-1,2-Diols are accessible in a
highly selective way using zinc borohydride
ucts.[1] However, they are often obtained as mixtures
of syn- and anti-diastereomers in varying diastereo-
meric and/or enantiomeric ratios using either transi-
tion metal complexes[2] or organocatalysts.[3] The sepa-
ration of 1,2-diol stereoisomers can be a difficult task,
especially when the separation has to be conducted
on a large scale. Therefore the highly stereoselective
reduction of 1,2-diketones or 2-hydroxy ketones is a
valuable alternative to access syn- and anti-1,2-diols.
Here, three different methods – based on borohy-
drides, ionic liquids, and biocatalysts – are applied to-
wards the proposed syn- and anti-stereoselective re-
duction of 2-hydroxy ketones. We identified a chemo-
enzymatic synthesis to enantiopure syn-1,2-diols
which, due to eco-friendly reaction conditions as well
as high stereoselectivities, offers a valuable comple-
ment to the known anti-selective reduction with
[ZnACHTUNGTRENNUNG(BH4)2] under chelation control (dr>20:1). Dia-
stereoselective reduction of unprotected or even
protected 2-hydroxy ketones towards syn-1,2-diols
could be achieved only with moderate selectivity of
drꢀ5:1. Even when using sterically demanding pro-
tecting groups and/or polymer-supported borohy-
dride reagents high selectivity could not be
achieved. A new ionic liquid-dependent borohy-
dride reduction method, although highly attractive
with respect to reaction engineering, resulted in
only moderate to good selectivity. An efficient two-
step biocatalytic method for the synthesis of syn-
1,2-diols is described. The method relies on the
whole-cell Pichia glucozyma-catalyzed stereoselec-
tive reduction of the unprotected (R)-2-hydroxy ke-
tones (dr>10:1). The latter are accessible through
thiamine diphosphate-dependent enzyme-catalyzed
synthesis starting from simple aldehydes. Thus, bio-
catalytic transformations enable a process which is
hardly accessible through present non-enzymatic
methods.
ZnACTHNURGTENUNG(BH4)2.
The crucial step is a stereoselective reduction of the
corresponding (R)-2-hydroxy ketones, which can be
easily obtained by benzaldehyde lyase (BAL)-cata-
lyzed cross benzoin reaction of aliphatic/aromatic and
aromatic aldehydes on a preparative scale (Table 1).[4]
(R)-3,3’-Dimethoxybenzoin (1) and three different
(R)-acyloins (2–4), were prepared in good yields and
with high enantioselectivity (ee ꢁ 97%, Table 1).
Compounds 1–4 were used as model substrates to-
wards the synthesis of syn- and anti-1,2-diols. Addi-
tionally, rac-1 was prepared by CNÀ-catalyzed benzoin
reaction of 3-methoxybenzaldehyde in 26% yield.
(R)-2-Hydroxy ketones 1–4 were reduced in a
highly diastereoselective manner using known
Keywords: benzaldehyde lyase; benzoin reaction;
biocatalysis; borohydrides; ionic liquids
Enantiopure substituted 1,2-diols are valuable inter- Zn
mediates in the synthesis of drugs and natural prod- 20:1 (Table 2). The high diastereoselectivity is gov-
A
Adv. Synth. Catal. 2011, 353, 2359 – 2362
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2359

Syed Masood Husain et al.
COMMUNICATIONS
Table 1. Stereoselective synthesis of 2-hydroxy carbonyl
mation starting from enantioenriched 2-hydroxy ke-
tones.
compounds catalyzed by benzaldehyde lyase (BAL).[a]
In order to obtain syn-1,2-diols, protected (R)-2-hy-
droxy ketones were reduced using various metallo hy-
drides such as aluminium hydrides and several boro-
hydride reducing agents. Compared to the reduction
of unprotected 2-hydroxy ketones (Table 2), protected
derivatives (1a, 1b, 2a, 2b) gave inconsistent results in
terms of stereoselectivities (Table 3). A switch in se-
lectivity was also observed, as reduction proceeds via
an open chain model which can result predominantly
in the formation of the syn- (threo-) isomers.[6] Reduc-
tion of rac-1a (R=TBS) with ZnACTHNUTRGNEUNG(BH4)2 gave the anti-
isomer (syn:anti ratio of 30:70, Table 3, entry 1) with
only slight preference.
However, rac-1b (R=MOM) resulted in better se-
lectivities in most cases using the different reducing
Entry 2-Hydroxy ketone
1
Yield [%] ee [%][b]
96
94
>99
>99
2
3
94
95
>99
agents. For example, ZnACHTNUGTRENUNG(BH4)2 reduces rac-1b selec-
tively towards the anti-product with 78% isolated
yield (Table 3, entry 3). Aluminium hydrides such as
LiAlH4 as well as DIBAL-H gave good to moderate
diastereoselectivities of 18:82 and 10:90 (syn:anti) to-
wards the same major isomer (Table 3, entries 4 and
5). Compound 2a (R=TBS) on reduction with
4
97
[a]
For reaction conditions and analytical data see ref.[4]
ee was determined by chiral phase HPLC.
ZnACTHNURGTNEUGN(BH4)2 gave a poor diastereomeric syn:anti ratio of
55:45 (Table 3, entry 6).
[b]
The use of NaBH4 and polymer-supported borohy-
dride resulted in better selectivities of syn-1,2-diols
with a syn:anti ratio of 83:17 (Table 3, entries 7 and
8). A change in solvent from methanol to 2-propanol
resulted in a slight decrease in selectivity towards the
syn-1,2-diol (Table 3, entries 8 and 9).
Table 2. anti-Selective reduction of 2-hydroxy ketones 1–4.
Compound 2b (R=COPh) on reduction with
Entry
Reactants
Yield [%][a]
anti:syn[b] Zn
ACHTUNGERTNN(GUN BH4)2 and polymer-supported borohydride, re-
spectively, gave low selectivities towards syn- and
anti-1,2-diols (Table 3, entries 10 and 11).
1
2
3
4
1
2
3
4
97
33
95
87
>99:1
99:1
97.5:2.5
98.5:1.5
In summary, the reduction of protected 2-hydroxy
ketones gave inconsistent results in terms of diaste-
reoselectivities and thus cannot be used as a general
method for the preparation of syn-1,2-diols. Highest
selectivities of up to 83:17 towards the syn-products
were obtained by using sterically demanding protect-
ing groups in combination with NaBH4 or polymer-
[a]
Isolated yield of the mixture of diastereomers.
Ratios were determined by H NMR spectroscopy.
[b]
1
erned by the stability of the chelated transition state supported borohydride reagents. These results are in
as well as by the bulkiness of R2. The transition state agreement with the syn-selectivity observed by Ley
leading to the formation of anti- (or synonymous et al. for the reduction of phenyl-protected 2-hydroxy
erythro-) isomer is much more favoured compared to ketones using polymer-supported borohydride.[7]
the transition state leading to the syn- (threo-) However, the high diastereoselectivity of >10:1
isomer.[6]
ACHTUNGTREN(NGNU syn:anti) as described by Ley et al. could not be ob-
Selective access to syn-1,2-diols through reduction served with the tested compounds 2a and 2b. More-
of 2-hydroxy ketones is more tedious by chemical over, protection and deprotection steps contribute to
methods. Ikariya et al. demonstrated highly stereose- the lowering of total yields and are time consuming.
lective reduction of racemic benzoin to syn-hydroben-
In another attempt to obtain higher stereoselectivi-
zoin (de>99%) using a ruthenium catalyst, which ties, (R)-2-hydroxypropiophenones 3 and 3a were re-
proceeds through a dynamic kinetic resolution of ben- duced using an ionic liquid ([1-butyl-1-methyl-pyrroli-
zoin.[2a] However, the enediol is formed as an inter- dinium][BH4]) in CH2Cl2 as a solvent. Unprotected
mediate, which would result in a loss of stereoinfor- hydroxy ketone 3 on reduction with the ionic liquid
2360
asc.wiley-vch.de
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2359 – 2362

Stereoselective Reduction of 2-Hydroxy Ketones towards syn- and anti-1,2-Diols
Table 3. Reduction of protected 2-hydroxy ketones through metallo hydrides.
Entry
Reactants
Reducing agent
Yield [%][b]
syn: anti[d]
[a]
1
2
3
4
5
6
7
8
rac-1a (R=TBS)
rac-1b (R=MOM)
rac-1b
rac-1b
rac-1b
2a (R=TBS)
2a
2a
Zn
NaBH4
Zn(BH4)2
G
95
89
78
85
91
35
63
30:70
13:87
0:100
18:82
10:90
55:45
83:17
83:17
78:22
63:37
35:65
LiAlH4
DIBAL-H
ZnACHTNGUETRNNU(G BH4)2
polymer-supported borohydride
NaBH4 in methanol
NaBH4 in 2-propanol
[c]
–
–
[c]
9
10
11
2a
2b (R=COPh)
2b
Zn
(BH4)2
34
[c]
polymer-supported borohydride
–
[a]
Prepared from ZnCl2 and NaBH4 in THF.
Isolated yield of the mixture of isomers.
[b]
[c]
[d]
No purification was performed.
1
Ratios were determined by H NMR of the crude products.
[BMP][BH4] gives the corresponding 1,2-diol in a protected 2-hydroxy ketones as substrates did not
syn:anti ratio of 28:72, which is somewhat better than result in highly syn-selective reductions.
using NaBH4 as a reducing agent (40:60) (Table 4, en-
This prompted us to explore the reduction of 2-hy-
tries 1 and 2). A switch in selectivity towards the syn- droxy ketones using biocatalysts. The stereoselective
diastereomer was observed when compound 3a (R= reduction of acyloin 3 by alcohol dehydrogenase from
TBS) was reduced with NaBH4 and [BMP][BH4] a Thermoanaerobium species to the syn-1,2-diol (dr=
(Table 4, entries 3 and 4). However, there was hardly 99:1) has been demonstrated earlier.[8] However, sub-
any difference in the syn:anti ratios using ionic liquid stituted derivatives such as 2 and 4 were not accepted
and NaBH4 when the protected 2-hydroxy ketones by this NADPH-dependent enzyme. The use of Lac-
were used for reduction.
tobacillus brevis ADH (LBADH) and alcohol dehy-
The high yield, potential practicable operation, fea- drogenase from Rhodococcus rubber and ADH from
sible utilization of organic solvents, and in some cases Theromoanaerobacter sp. has been reported recently
better stereoselectivities makes the ionic liquids ap- for the preparation of syn- and anti-diols.[9]
proach interesting for further elucidation. However,
Several reports have been published for the reduc-
in summary all tested combinations of reducing tion of benzils to chiral benzoins using conventional
agents, including ionic liquids, and protected or un- biocatalysts such as bakerꢃs yeast or whole cells of Ba-
cillus cereus.[10] The non-conventional yeast Pichia glu-
cozyma has been previously used to carry out the
enantioselective reduction of different a-keto aryl
groups, including various benzils.[11] The use of this
biocatalyst, mainly as a whole cell transformation,
offers an attractive alternative due to the mild and
eco-friendly reaction conditions as well as in-situ re-
generation of expensive cofactors. Therefore, whole
cells of P. glucozyma (CBS 5766) were tested for the
reduction of the unprotected (R)-2-hydroxy ketones
1–4.
The reactions were performed in potassium phos-
phate buffer (pH 7). Ethanol (2.5% vol) was added as
a cosolvent in order to increase the substrate solubili-
ty and also as cosubstrate for cofactor regeneration.
The batch reactions were carried out on scales be-
tween 0.04 and 0.4 mmol of the respective 2-hydroxy
ketone (Table 5).
Table 4. Reduction with ionic liquid borohydride.
Entry Reactant
Reducing
agent
Conversion syn:anti[b]
1
2
3
4
3 (R=H)
3
3a (R=TBS) NaBH4
3a
NaBH4
90
40:60
28:72
71:29
75:25
ionic liquid[a] 98
70
ionic liquid[a] 10
[a]
Ionic liquid used was [1-butyl-1-methyl-pyrrolidini-
ACHTUNGTRENNUNGum][BH4] in CH2Cl2.
The ratios were determined using H NMR of the crude
products.
[b]
1
Adv. Synth. Catal. 2011, 353, 2359 – 2362
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2361

Syed Masood Husain et al.
COMMUNICATIONS
Table 5. Biocatalytic reduction of unprotected (R)-2-hydroxy
ketones 1–4 to syn-1,2-diols by P. glucozyma.
8.2 mL of 0.1M phosphate buffer pH 7.0. Substrates were
dissolved in ethanol until reaching a concentration of
40 gL À1(if the solution is not clear the suspension can be
gently heated); 250 mL of this solution were added to the
cells suspension. Ethanol acted as a cosolvent and cosub-
strate for cofactor regeneration. Reactions were carried out
under magnetic stirring at 288C. Work-up was performed
with EtOAc and products were purified by column chroma-
tography (see Supporting Information).
Entry
Hydroxy ketones
Yield [%][a]
syn:anti[b]
1
2
3
4
1
2
3
4
–
–
82
67
52
95.5:4.5
97.5:2.5
>98:2
Acknowledgements
[a]
Isolated yield of the products.
Ratios were determined by H NMR.
[b]
1
We thank Volker Brecht for NMR measurements. This work
was financially supported by the DFG in scope of the project
FOR 1296.
(R)-2-Hydroxy ketones 1–4 were reduced to the
corresponding syn-1,2-diols with good diastereoselec-
tivity (dr>10:1) (Table 5, entries 2–4), with the ex-
ception of compound 1 which, probably due to steric
reasons, does not undergo reduction through whole
cells of P. glucozyma (Table 5, entry 1).
P. glucozyma promises to become a valuable tool in
organic chemistry for the stereoselective reduction of
2-hydroxy ketones like benzoins and phenylpropa-
none derivatives. Encouraged by these results we will
extend our studies on P. glucozyma with the aim to
overcome the limitations due to steric reasons shown
above.
In combination with thiamine diphosphate-depen-
dent enzymes the presented methodology enables a
convergent route for the enantioselective synthesis of
valuable syn- and anti-1,2-diols starting from readily
available achiral aldehydes. We are working towards
the enzymatic access to (S)-2-hydroxy ketones, which
has partially been solved already.[12] It has to be
shown whether the enzyme from P. glucozyma or its
variants will accept these as substrates as well.
The combination of the two biocatalytic steps
shown above will result in a possible one-pot transfor-
mation without the requirement of protecting groups
to achieve high stereoselectivity towards the synthesis
of syn-1,2-diols. This nicely complements the widely
applicable anti-selective reduction of 2-hydroxy ke-
References
[1] S. D. Rychnovsky, Chem. Rev. 1995, 95, 2021.
[2] a) T. Ikariya, A. Blacker, Acc. Chem. Res. 2007, 40,
1300; b) K. Murata, K. Okano, M. Miyagi, H. Iwane,
R. Noyori, T. Ikariya, Org. Lett. 1999, 1, 1119.
[3] G. Guillena, M. D. C. Hita, C. Nꢄjera, Tetrahedron:
Asymmetry 2006, 17, 1027.
[4] a) A. S. Demir, ꢅ S¸es¸enoglu, E. Eren, B. Hosrik, M.
Pohl, E. Janzen, D. Kolter, R. Feldmann, P. Dꢀnkel-
mann, M. Mꢀller, Adv. Synth. Catal. 2002, 344, 96;
b) E. Janzen, M. Mꢀller, D. Kolter-Jung, M. M. Kneen,
M. J. McLeish, M. Pohl, Bioorg. Chem. 2006, 34, 345.
[5] S. Narasimhan, R. Balakumar, Aldrichimica Acta 1998,
31, 19.
[6] T. Nakata, T. Tanaka, T. Oishi, Tetrahedron Lett. 1983,
24, 2653.
[7] A.-L. Lee, S. V. Ley, Org. Biomol. Chem. 2003, 1, 3957.
[8] D. Kihumbu, T. Stillger, W. Hummel, A. Liese, Tetrahe-
dron: Asymmetry 2002, 13, 1069.
[9] M. Kurina-Sanz, F. R. Bisogno, I. Lavandera, A. A.
Orden, V. Gotor, Adv. Synth. Catal. 2009, 351, 1842.
[10] a) T. Saito, R. Maruyama, S. Ono, N. Yasukawa, K.-I.
Kodaira, M. Nishizawa, S. Ito, M. Inoue, Appl. Bio-
chem. Biotech. 2003, 111, 185; b) N. O. Mahmoodi,
M. N. Navrood, Arkivoc 2007, iii, 37; c) J. Konishi, H.
Ohta, G. I. Tsuchihashi, Chem. Lett. 1985, 1111; d) R.
Chenevert, S. Thiboutot, Chem. Lett. 1988, 1191; e) Y.
Kawai, K. Hida, Tsujimoto, S.-L. Kondo, K. Kitano, K.
Nakamura, A. Ohno, Bull. Chem. Soc. Jpn. 1999, 72,
99.
tones using ZnACHTUNGTRENNUNG(BH4)2.
[11] a) F. Molinari, R. Gandolfi, R. Villa, E. G. Occhiato,
Tetrahedron: Asymmetry 1999, 10, 3515; b) C. Forzato,
R. Gandolfi, F. Molinari, P. Nitti, G. Pitocco, E. Valen-
tin Tetrahedron: Asymmetry 2001, 12, 1039; c) P.
Hoyos, G. Sansottera, M. Fernꢄndez, F. Molinari, J. V.
Sinisterra, A. R. Alcꢄntara, Tetrahedron 2008, 64, 7929.
[12] D. Gocke, L. Walter, E. Gauchenova, G. Kolter, M.
Knoll, C. L. Berthold, G. Schneider, J. Pleiss, M.
Mꢀller, M. Pohl, ChemBioChem 2008, 9, 406.
Experimental Section
General Procedures for the Reduction of 2-Hydroxy
Ketones with Cells from Pichia glucozyma CBS 5766
Reductions were carried out in 15-mL screw-capped test-
tubes with a reaction volume of 10 mL; 1.8 mL of wet cells
(corresponding to 200 mg of dry cells) were suspended in
2362
asc.wiley-vch.de
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2359 – 2362