Enzyme-catalyzed enantioselective diaryl ketone reductions

Article abstract of DOI:10.1021/ol0627909

(Chemical Equation Presented) The synthesis of diarylmethanols via the reduction of a range of substituted benzophenone and benzoylpyridine derivatives with ketoreductase enzymes (KREDs) has afforded chiral products with high yield (>90%) and ee (up to >99%). Ortho, meta, and para substitutions with a variety of electron-donating, electron-withdrawing, and halogen groups were examined. Substitution at the ortho position and/or highly electronically dissymmetric molecules were not required for good selectivity, as is the case with conventional chemical catalyst reductions.

Full text of DOI:10.1021/ol0627909

ORGANIC
LETTERS
2007
Vol. 9, No. 2
335-338
Enzyme-Catalyzed Enantioselective
Diaryl Ketone Reductions
Matthew D. Truppo,* David Pollard, and Paul Devine
Department of Process Research, Merck Research Laboratories, Merck & Co., Inc.,
P.O. Box 2000, Rahway, New Jersey 07065
matthew_truppo@merck.com
Received November 16, 2006
ABSTRACT
The synthesis of diarylmethanols via the reduction of a range of substituted benzophenone and benzoylpyridine derivatives with ketoreductase
enzymes (KREDs) has afforded chiral products with high yield (>90%) and ee (up to >99%). Ortho, meta, and para substitutions with a variety
of electron-donating, electron-withdrawing, and halogen groups were examined. Substitution at the ortho position and/or highly electronically
dissymmetric molecules were not required for good selectivity, as is the case with conventional chemical catalyst reductions.
The need to deliver optically pure substances for the
pharmaceutical industry, combined with the common dif-
ficulties and costs of separating two enantiomers, has led to
new stereoselective synthesis methods for the production of
chiral intermediates in drug synthesis.1
The formation of chiral diarylmethanols is of particular
interest since intermediates of this type are important building
blocks in the synthesis of pharmaceutically important
molecules including antihistaminic, anaesthetic, diuretic,
antidepressive, antiarrhythmic, and anticholinergic com-
pounds.2-4
Catalytic, asymmetric syntheses of diarylmethanols typi-
cally involve either the addition of aryl nucleophiles to
aromatic aldehydes or the reduction of the corresponding
diaryl ketones.2 This paper will focus on enzymatic strategies
for the production of diarylmethanols via diaryl ketone
reduction, and their advantages compared to classical chemi-
cal catalytic methods.
The addition of aryl nucleophiles to aromatic aldehydes
has been demonstrated to form several diarylmethanols with
high selectivity; however, these reactions exhibit serious
drawbacks in their potential for large-scale industrial use.
Diphenylzinc, the most common aryl donor, is expensive,
not readily available for large-scale use, and limited to
transferring a nonsubstituted phenyl group to the aldehyde.
The use of boronic acid aryl sources enabled the transfer of
substituted aryl rings, but arylboronic acid protocols typically
require a large excess of diethylzinc (7 equiv) and additives
such as dimethylpolyethyleneglycol (DiMPEG) to achieve
good selectivity. Finally, nonselective, noncatalytic back-
ground aryl addition can lead to low product ee. This is
typically combatted by increasing catalyst loading and
reducing reaction temperature.
The stereoselective reduction of inexpensive prochiral
ketones to their corresponding alcohols is potentially one of
the most useful ways of introducing chirality in a molecule.
However, the enantioselective reduction of diaryl ketones
has proven to be a significant challenge, as chemical catalysts
have an extremely limited substrate range.5,6 There are a few
(1) Devaux-Basseguy, R.; Bergel, A.; Comtat, M. Enzyme Microb.
Technol. 1997, 20, 248-258.
(2) Schmidt, F.; Stemmler, R. T.; Rudolph, J.; Bolm, C. Chem. Soc. ReV.
2006, 35, 454-470.
(3) Devalia, J. L.; De Vos, C.; Hanotte, F.; Baltes, E. Allergy 2001, 56,
50-57.
(5) Welch, C. J.; Grau, B.; Moore, J.; Mathre, D. J. Org. Chem. 2001,
66, 6836-6837.
(6) Itsuno, S. Org. React. 1998, 52, 395-576.
(4) Corey, E. J.; Helal, C. J. Tetrahedron Lett. 1996, 37, 4837-4840.
10.1021/ol0627909 CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/16/2006

examples of highly selective diaryl ketone reductions involv-
ing hydrogenation or CBS reduction.7-9 The highly enanti-
oselective (>95% ee) reduction of select benzophenones has
been demonstrated by using lithium aluminum hydride-
chiral amino alcohol complexes,10,11 B-chlorodiisopinocam-
phenylborane,12 oxazaborolidine,13 and chiral diphosphine/
diamine Ru complexes.14 These types of chemically catalyzed
diaryl ketone reductions rely on both electronic and steric
effects to determine the degree of enantioselectivity. Their
limitation lies in the fact that their substrates must either be
highly electronically dissymmetric or have an ortho-
substituted aryl group to achieve good selectivity. For
example, the highly selective CBS reduction of ketone 1
takes advantage of the electron-donating and -accepting
groups on either end of the molecule, while the reduction of
ketone 2 demonstrates that the presence of an ortho sub-
stituent on one of the aryl groups provides sufficient steric
effect for excellent selectivity. Simple meta and para
substitutions resulted in moderate to no selectivity.13,15
run at low substrate loading (1 g/L) and many times lead to
low product yield (∼10%) due to difficulties in isolating the
product away from the cells.17
Isolated ketoreductase enzymes have been demonstrated
to be highly selective catalysts for the reduction of a wide
range of ketones.18 Additionally, many of these enzymes are
readily available and have been used to economically deliver
kilogram quantities of chiral intermediates with excellent ee
(>99%) and isolated yield (>90%).19,20
This work demonstrates a practical procedure for the
enantioselective reduction of diaryl ketones by using isolated
enzymes and its general application to a broad range of
substrates (Scheme 1). Each of the enzymes used for these
Scheme 1. KRED-Catalyzed Enantioselective Diaryl Ketone
Reduction
transformations required NADPH cofactor as the hydride
source, so an NADPH recycling system was put in place by
using glucose and a co-enzyme glucose dehydrogenase
(GDH) to regenerate the reduced form of the cofactor. The
presence of the recycle system also provides the driving force
to take these reactions to completion. Additionally, all of
the enzymes used for these transformations are readily and
economically available in large quantities from commercial
sources (Biocatalytics Inc.).
Figure 2 shows the range of diaryl ketone substrates that
were screened against our in-house library of commercially
available ketoreductases (KREDs). The range of substrates
includes ortho-, meta-, and para-substituted benzophenones
as well as several benzoylpyridines. Aryl substitutions
included electron-donating, electron-withdrawing, and halo-
gen substituents. Table 1 shows the results of the enzyme
catalyst screen with the highest ee obtained for each
enantiomer of the diarylmethanol products along with the
corresponding enzyme catalysts. Isolated yields are also
identified for select reactions that were demonstrated at the
1 g scale.
Figure 1. Highly selective CBS reduction of select diaryl ketones
Biological catalysts in the form of whole cells have been
shown to be extremely stereoselective in the reduction of
some diaryl ketones that are difficult to reduce with chemical
catalysts. Immobilized baker’s yeast catalyzed the reduction
of 2-benzoylpyridine in hexane to afford the alcohol with
high optical purity (96% ee).16 Selected strains from Hansen-
ula nonfermentans, santamariae, ernobii, Rhodosporidium
toruloides, Candida bombi, and sorbophila have also been
shown to reduce diaryl ketones with high selectivity,
producing alcohol product with >95% ee.5 Disadvantages
of whole cell biocatalytic systems are that they are typically
(7) Blaser, H.-U.; Malan, C.; Pugin, B.; Spindler, F.; Steiner, H.; Studer,
M. AdV. Synth. Catal. 2003, 345, 103-151.
(8) Corey, E. J.; Helal, C. J. Angew. Chem., Int. Ed. 1998, 37, 1986-
2012.
Generally, ortho-substituted substrates afforded greater
alcohol product ee than their meta- and para-substituted
counterparts. This trend was observed in methyl (3a, 3b, 3c),
hydroxy-, (3f, 3g, 3h), amino- (3i, 3j), and chloro- (3l, 3m,
(9) O’Shea, P. D.; Chen, C.-Y.; Chen, W.; Dagneau, P.; Frey, L.;
Grabowski, E. J. J.; Marcantonio, K. M.; Reamer, R. A.; Tan, L.; Tillyer,
R. D.; Roy, A.; Wang, X.; Zhao, D. J. Org. Chem. 2005, 70, 3021-3030.
(10) Brown, E.; Penfornis, A.; Bayma, J.; Touet, J. Tetrahedron:
Asymmetry 1991, 2, 339-342.
(11) Brown, E.; Leze, A.; Touet, J. Tetrahedron: Asymmetry 1992, 3,
841-844.
(17) Chartrain, M.; Lynch, J.; Choi, W.-B.; Churchill, H.; Patel, S.;
Yamazaki, S.; Volante, R.; Greasham, R. J. Mol. Catal. B: Enzym. 2000,
8, 285-288.
(12) Shieh, W.-C.; Cantrell, W. R.; Carlson, J. A. Tetrahedron Lett. 1995,
36, 3797-3800.
(18) Kaluzna, I.; Rozzell, D.; Kambourakis, S. Tetrahedron: Asymmetry
2005, 16, 3682-3689.
(13) Corey, E. J.; Helal, C. J. Tetrahedron Lett. 1996, 37, 5675-5678.
(14) Ohkuma, T.; Koizumi, M.; Ikehira, H.; Yokozawa, T.; Noyori, R.
Org. Lett. 2000, 2, 659-662.
(19) Truppo, M.; Kim, J.; Brower, M.; Madin, A.; Sturr, M.; Moore, J.
J. Molec. Catal. B: Enzym. 2006, 38, 158-162.
(15) Corey, E. J.; Helal, C. J. Tetrahedron Lett. 1995, 36, 9153-9156.
(16) Takemoto, M.; Achiwa, K. Chem. Pharm. Bull. 1994, 42, 802.
(20) Pollard, D.; Truppo, M.; Pollard, J.; Cheng-Yi, C.; Moore, J.
Tetrahedron: Asymmetry 2006, 17, 554-559.
336
Org. Lett., Vol. 9, No. 2, 2007

ee alcohol product. In comparison, the selective BINAP/
chiral diamine Ru complex hydrogenation of o-methyl
substituted 3a was achieved with high enantioselectivity
(>93% ee), but the hydrogenation of the p-methyl-substituted
counterpart 3c resulted in 8% ee alcohol product.14
Another trend observed for substrates with a single substi-
tution on one of the phenyl rings was that electron-withdraw-
ing groups generally provided for better selectivity than elec-
tron-donating groups. Nitro substitutions (3d, 3e), for exam-
ple, provided for much higher enantioselectivity than hy-
droxy- or methoxy-substituted diaryl ketones. The enzymatic
reduction of p-nitro-substituted 3e produced 97% ee alcohol
product compared to 81.8% ee achieved with diphenylzinc
aryl addition with a chiral pyrrolidinylmethanol catalyst.22
High selectivity was also observed in the substrates with
substituents on both phenyl rings (3o, 3p). This result pro-
vides a significant advantage over the addition of aryl nucleo-
philes to aromatic aldehydes approach to the synthesis of
diarylmethanols, since most aryl transfer reactions demon-
strated thus far use diphenylzinc as the aryl source, and are
therefore limited to phenyl transfers to aldehydes.2 A further
advantage is that the substrate need not be highly electroni-
cally dissymetric, as was required in the aforementioned CBS
reduction of ketone 1. The enzymatic reduction of ketone
3o demonstrates that, even with electron-withdrawing sub-
stituents on both aryl rings, good selectivity (90% ee) can
be obtained.
Figure 2. A range of substituted benzophenones and benzoylpy-
ridine substrates
3n) substituted benzophenones. However, contrary to results
obtained with most chemical catalyst reductions, ortho
substitution was not required for good selectivity. Nitro
substitution at the meta (3d) and para (3e) positions and
m-chloro (3m) substitution provided for excellent selectivity
(97-99% ee), while p-methyl (3c) substitution yielded 85%
Table 1. KRED-Catalyzed Reduction of Diaryl Ketonesa
The benzoylpyridine derivatives (4, 5, 6, 7) also exhibited
highly selective reduction to their diarylmethanol counterparts
with use of the KRED enzymes. Of particular interest is the
alcohol produced from compound 7, which is a precursor of
the histamine H1 antagonist (S)-carbinoxamine. The enan-
tioselective enzymatic reduction of ketone 7 achieves excel-
lent selectivity (94% ee). This intermediate is currently pro-
duced via the CBS reduction of N-allyl-2-(4-chlorobenzoyl)-
pyridinium triflate.21 The allyl group is necessary to prevent
the coordination of the pyridine nitrogen to the catecholbo-
rane or oxazaborolidine catalyst, and also serves to provide
the steric bias necessary for a highly selective reduction.
While the enzymatic reduction of 7 can be carried out with
high selectivity, the CBS reduction of the unelaborated diaryl
ketone 7 resulted in 11% ee at -78 °C (the low temperature
was used to suppress the nonselective background reaction).13
ketone
R1
(R)-alcohol
eeb KREDb
(S)-alcohol
eec
95
yield of
KRED 1g,d
%
no.
R2
3a
3b
3c
o-CH3
m-CH3
p-CH3
-
98 121
72 121
85 101
34 111
96 128
84 111
61 112
69 113
91 101
60 111
70 101
64 121
39 111
64 101
119
-
95
-
-
-
-
-
-
-
-
-
-
-
-
-
-
9
119
108
119
-
3d m-NO2
>99
97
-
90
3e
3f
3g
p-NO2
o-OH
m-OH
13
55
64
51
119
117
114
119
119
118
108
-
3h p-OH
3i
3j
o-NH2
p-NH2
3k p-OMe
3l o-Cl
3m m-Cl
3n p-Cl
92
95
64
>99
>99
-
3o
m-CN
p-Cl 84 112
90
>99
77
38
>99
60
108
108
119
120
119
119
In summary, a series of diaryl ketones consisting of mono-
and disubstituted benzophenones and various benzoylpy-
ridines has been successfully reduced to their respective
diarylmethanols with moderate to excellent selectivity by
using commercially available ketoreductase enzyme catalysts.
Ortho substitution on one of the aryl rings was not a
requirement for good enantioselectivity, nor was a highly
electronically dissymmetric substrate. This work demon-
strates the power of commercially available isolated enzymes
to perform an important class of transformation (the reduction
of diaryl ketones) that shows a significant advantage over
3p m-CO2Me p-Cl 33 115
4
5
6
7
97 101
82 101
44 L.kefir
94 124
98
a Reactions conditions: 30 °C, 2 g/L of KRED, 2 g/L of GDH, 20 g/L
of glucose, 1 g/L of NADP, 10 g/L of ketone, 10% THF, in 100 mM
potassium phosphate buffer (pH 7.0). b KRED number corresponds to the
Biocatalytics catalog number. c Determined by chiral SFC analysis. Absolute
configuration determined by comparison to literature data. d The 1 g reaction
conditions (40 mL scale): 30 °C, 2 g/L of KRED, 2 g/L of GDH, 20 g/L
of glucose, 1 g/L of NADP, 25 g/L of ketone, 10% THF, in 100 mM
potassium phosphate buffer (pH 7.0). The product alcohol was extracted
with 2× volumes methyl ethyl ketone. The organic phase was washed with
5 mL of DI water and dried. These reactions were run with the enzyme
that produced the highest ee. The 1 g scale reactions were only run for
those substrates that have a yield number listed.
(21) Barouh, V.; Dall, H.; Patel, D.; Hite, G. J. Med. Chem. 1971, 14,
834-836.
(22) Zhao, G.; Li, X.-G.; Wang, X.-R. Tetrahedron: Asymmetry 2001,
12, 399-403.
Org. Lett., Vol. 9, No. 2, 2007
337

standard chemical catalytic methods. These ketoreductase
enzymes are shown to be extremely selective, and able to
distinguish very subtle steric and electronic differences in
diaryl ketone substrates, affording many diarylmethanol
products with high ee.l
methods (enantiomeric excess of the alcohol products was
determined by SFC analysis and absolute configuration and
identity of the alcohol products were determined by com-
parison to literature references). This material is available
free of charge via the Internet at http://pubs.acs.org.
Supporting Information Available: General diaryl ke-
tone reduction protocols as well as HPLC and SFC analysis
OL0627909
338
Org. Lett., Vol. 9, No. 2, 2007