Enantioselective oxidation of diaryl carbinols by Nocardia corallina B-276

Article abstract of DOI:10.1016/S0957-4166(00)00419-5

Whole cells of Nocardia corallina B-276 oxidized diaryl carbinols enantioselectively to give ketones in moderate yields, and some of the unreacted alcohols showed high ee's. This asymmetric oxidation is a simple and efficient method for preparing meta- and para-monosubstituted optically active diaryl carbinols from the racemates. The para-substituted chiral alcohols have an R configuration. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00419-5

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 4263–4268
Enantioselective oxidation of diaryl carbinols by Nocardia
corallina B-276
Herminia I. Pe´rez,* He´ctor Luna, Norberto Manjarrez, Aida Sol´ıs and Ma. Amelia Nun˜ez
Universidad Auto´noma Metropolitana, Unidad Xochimilco, Depto. Sistemas Biolo´gicos, A.P. 23/181, Mexico,
D.F. Mexico
Received 18 May 2000; accepted 3 October 2000
Abstract
Whole cells of Nocardia corallina B-276 oxidized diaryl carbinols enantioselectively to give ketones in
moderate yields, and some of the unreacted alcohols showed high ee’s. This asymmetric oxidation is a
simple and efficient method for preparing meta- and para-monosubstituted optically active diaryl
carbinols from the racemates. The para-substituted chiral alcohols have an R configuration. © 2000
Elsevier Science Ltd. All rights reserved.
1. Introduction
Chiral diaryl carbinols are useful intermediates for the preparation of biologically active
compounds,1–3 but are difficult to obtain in an enantiopure form by chemical resolution of their
acid phthalates with chiral bases,4 complexation with brucine5 or by asymmetric catalysis.6 The
asymmetric reduction of substituted diaryl ketones is difficult because prochiral diaryl ketones
usually contain two sterically and electronically similar aryl groups.6 However, a very efficient
method of asymmetric hydrogenation of ortho-substituted diaryl ketones using a chiral Ru(II)
complex was recently reported7 without the formation of substituted diphenylmethane, a usual
over-reduction product.
In contrast to the large number of published highly enantioselective alkyl additions, only a
handful of reports about the catalytic high enantioselective addition of organometallic aryl
reagents to aromatic aldehydes have appeared.6 Enzymatic process could be an alternative
method; however, attempts to achieve kinetic resolution of simple alcohol by oxidation have
been relatively unsuccessful on the large scale, because in the oxidative process the enzyme
requires cofactors and is particularly sensitive to product inhibition.8 Recently nonenzymatic
methods have also been extensively explored.9 These facts turned our attention to microbial
oxidations, because a process with whole cells would avoid this kind of problem.
* Corresponding author. E-mail: hperez@cueyatl.uam.mx
0957-4166/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00419-5

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Recently, we reported the oxidation of aldehydes, and allylic and benzylic alcohols to produce
carboxylic acids or ketones by Nocardia corallina B-276.10,11 Due to our interest on the
application of this microorganism in organic chemistry, we decided to study the microbial
oxidation of benzhydrols.
2. Results and discussion
First, we selected a model compound to adjust the biotransformation conditions on a 0.5
mmol scale. The bioreaction of benzhydrol, 1, was allowed to proceed to about 50% conversion
(27 h, 28–30°C), then the crude mixture was analyzed. A preparative experiment with benz-
hydrol, 1, under the optimal reaction conditions produced benzophenone and recovered benz-
hydrol in 40% and 37% yields, respectively. In this way, it was demonstrated that Nocardia
corallina B-276 has the ability to oxidize diaryl methanols, with potential application for the
kinetic resolution of these kinds of secondary alcohols (Scheme 1).
Scheme 1.
Ten substrates were selected for this study, and the results are shown in Table 1. It is worth
mentioning that the reaction conditions have not been optimized for each particular case.
All substrates were oxidized, with the exception of substrate 11 which produced only 2%
ketone after 27 h by GC analysis. The para-substituted substrates 2, 5, 6, 7 and 9 gave the
highest ee’s for this enantioselective oxidation, in most cases with >99% ee. It is worth noting
that these compounds are important chiral synthons.12
A very fast oxidation of compound 4 (entry 5) was observed and consequently it was
necessary to stop the biotransformation after 8 h, in order to have approximately 50%
conversion (entry 6). This reaction showed an excellent enantioselectivity, giving the unreacted
alcohol 4 in >99% ee. In contrast to 8, another meta-substituted substrate (entry 10), with the
bulkier phenoxy group, gave the alcohol in only 46% ee, at a lower rate.
Interestingly, the ortho-substituted substrate 3 (entries 3 and 4) seemed to present a significant
steric hindrance, because even after 120 h of reaction, only 19% conversion was observed
without any enantioselectivity; similar to the observed behavior of 11.
Regarding the stereochemistry of the reaction, comparison of specific rotations of 2, [h]D=
−14.7 (c 1.0, CHCl3); 3, [h]D=+0.51 (c 1.2, CHCl3); 5, [h]D=+5.54 (c 0.16, C6H6) and 7,
[h]D=+10.7 (c 0.10, C6H6) with the literature values: 2, [h]D=−6.0 (c 0.91, CHCl3) 41% ee, R;6
3, [h]D=−41.9 (c 1.19, CHCl3) 96% ee, S;7 5, [h]D=−10.3 (c 5.0, C6H6) >99% ee, S;4 and 7,
[h]D=+7.94 (c 1.63, C6H6) 54% ee, R,6 respectively, allowed us to assign the ‘R’ configuration
to the alcohols produced by this procedure.

H. I. Pe´rez et al. / Tetrahedron: Asymmetry 11 (2000) 4263–4268
4265
Table 1
Kinetic resolution of chiral diaryl carbinols
Entry
Alcohol
G
Reaction time (h)
Ketone/unreacted alcohola
Ee (%) (conf.)b
2
3
4
5
6
7
8
9
10
11
12
13
2
3
3
4
4
5
6
7
8
4-Cl
2-Br
2-Br
3-Br
27
27
120
27
8
27
33
27
33
27
27
27
53/47
12/88
19/81
100/0
43/57
41/59
52/48
63/57
66/34
52/48
59/41
2/98g
\99c (R)6
0.12c (R)7
N.d.d
–
3-Br
\99c
4-CH3
4-i-C3H7
4-OCH3
3-OC6H5
4-Br
78c (R)4
\99e (R)h
\99c (R)6
46c
9
10
11
\99f (R)h
\99c
3,4-diOCH3
2,3-diOCH3
20c
1
a Ratio determined by H NMR (500 MHz).
b Assignment of absolute configuration of the unreacted alcohol was done by comparison of the sign of the specific
rotation with literature data.
c Enantiomeric excess determined by the a-CH signal in the 1H NMR spectrum of the alcohols using (+)-[Eu(hfc)3]
6
as a chiral shift reagent.
d N.d.: not determined.
e Enantiomeric excess determined by HPLC on a Chiralcel-OD column.
f Enantiomeric excess determined by HPLC of the corresponding acetates on a Chiralcel-OD column.
g Ratio determined by GC on Chiraldex-BPH column.
h By optical rotation correlation.13
According to the correlation proposed by Mosher,13 the para-substituted (S)-benzhydrols
with a deactivating group (Br or Cl) are dextrorotatory, and in our case we obtained
levorotatory values for 2 and 9 which permit us to assign to them the R configuration. On the
other hand, para-substituted (S)-benzhydrols with activating groups (CH3 or OCH3) are
levorotatory. We observed for 5, 6 and 7 dextrorotatory values, and using the same correlation
we propose an R configuration for 6, which is not reported in the literature.
Unfortunately, the lack of reported absolute configurations for 4, 8, 10 and 11 do not allow
us to make a generalization about the stereochemistry of the benzhydrols obtained by this
biotransformation.
3. Conclusion
We have found a microbiological asymmetric oxidation of chiral diaryl carbinols, providing
moderate chemical yields, which is unoptimized, and high enantiomeric excesses. It is also
important to mention that, although the ability of Nocardia corallina to oxidize alkenes to
optically active 1,2-epoxyalkenes is well documented,14 this is the first microbial resolution of a
chiral compound by enantioselective oxidation with Nocardia corallina B-276.

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4. Experimental
4.1. Materials and methods
Organism and growth. Nocardia corallina B-276 (ATCC 31338) was grown at 28–30°C on
agar plates (3 g beef extract/l; 5 g peptone/l; 15 g agar/l). Incubation of liquid cultures was done
in an orbital shaker; the broth composition was: solution A: 0.05 g FeSO4·7H2O/l; 1.74 g
K2HPO4/l; 2 g (NH4)2SO4/l; 1 g yeast extract/l; solution B: 1.5 g MgSO4/l; solution C: 2 g
glucose/l; each solution was sterilized separately and later combined, and the pH adjusted to 8.0
( 0.5). All substrates were prepared by conventional methods or purchased from Aldrich, Sigma
or Janssen. The diaryl carbinols and ketones were identified by their infrared spectra (Perkin–
Elmer Paragon 1600) as liquid films or KBr discs, hydrogen nuclear magnetic resonance (1H
NMR) (Bruker 500 MHz) and by TLC on silica gel 60 GF254 Merck, and comparative analysis
with authentic samples. The specific rotations were measured in a Perkin–Elmer 341 polarimeter
with the indicated solvent. The GC analysis was performed on a Hewlett–Packard HP 6890
series gas chromatograph, equipped with a flame ionization detector and a Chiraldex column
B-PH (30 m) column. The HPLC analysis was performed on a Hewlett–Packard HP 1050 series
liquid chromatograph, equipped with a UV detector and a Chiralcel OD column.
4.2. General procedure for biotransformation
4.2.1. Preculture I
A 125 ml Erlenmeyer flask containing 50 ml of sterile culture medium was inoculated from an
agar plate (three days old) and incubated at 28–30°C on an orbital shaker (200 rpm) for 20–24
h.
4.2.2. Preculture II
The content of preculture I flask was aseptically poured into a 250 ml Erlenmeyer flask
containing 100 ml of fresh sterile culture medium. The flask was incubated at 28–30°C on an
orbital shaker (200 rpm) for 24 h.
4.2.3. Biotransformation
Under aseptic conditions the substrate (0.5 mmol) was added to the flask containing
preculture II using 1 ml of N,N-dimethylformamide, followed by the addition of n-octane (10
ml). The mixture (161 ml final volume) was incubated at 28–30°C on an orbital shaker (200
rpm). The progress of the biotransformation was monitored by TLC and stopped at the time
indicated in Table 1, then it was saturated with NaCl and filtered through Celite; the ketones
and diaryl carbinols were extracted with ethyl acetate (4×25 ml). The products were compared
1
with authentic samples by H NMR, IR and TLC.
Specific rotations of the recovered chiral alcohols of the enantioselective oxidation:
(R)-(4-Chlorophenyl)phenylmethanol 2: [h]D=−14.7 (c 1.0, CHCl3) [lit.6 [h]D=−6.0 (c 0.91,
CHCl3) 41% ee, R].

H. I. Pe´rez et al. / Tetrahedron: Asymmetry 11 (2000) 4263–4268
4267
(R)-(2-Bromophenyl)phenylmethanol 3: [h]D=+0.51 (c 1.2, CHCl3) [lit.7 [h]D=−41.9 (c 1.19,
CHCl3) 96% ee, S].
(3-Bromophenyl)phenylmethanol 4: [h]D=−4.29 (c 0.26, CH3OH).
(R)-(4-Methylphenyl)phenylmethanol 5: [h]D=+5.54 (c 0.16, C6H6) [lit.4 [h]D=−10.3 (c 5.0,
C6H6) >99% ee, S].
(R)-(4-Isopropylphenyl)phenylmethanol 6: [h]D=+5.03 (c 0.06, C6H6).
(R)-(4-Methoxyphenyl)phenylmethanol 7: [h]D=+10.7 (c 0.10, C6H6) [lit.6 [h]D=+7.94 (c 1.63,
C6H6) 54% ee, R].
(3-Phenoxyphenyl)phenylmethanol 8: [h]D=−14.13 (c 0.06, C6H6).
(R)-(4-Bromophenyl)phenylmethanol 9: [h]D=−8.37 (c 0.11, C6H6).
(3,4-Dimethoxyphenyl)phenylmethanol 10: [h]D=+20.85 (c 0.51, C6H6).
(2,3-Dimethoxyphenyl)phenylmethanol 11: [h]D=+0.91 (c 1.7, C6H6).
4.3. Determination of the enantiomeric excess for the chiral diaryl carbinols
(a) By HPLC. All ee’s were determined by comparing the HPLC data of the chiral products
with those of the corresponding racemic alcohols on a: Chiracel OD column, hexanes/2-
propanol as eluent, at 250 nm.
(4-Isopropylphenyl)phenylmethanol 6: Racemic alcohols, tS=16.72 min, tR=18.12 min (hex-
anes/2-propanol=95:5 at 0.8 ml/min). The product of the enantioselective oxidation gave
tR=18.13 min, with >99% ee.
(4-Bromophenyl)phenylmethanol 9: The ee was measured by analyzing its acetate by HPLC.
The racemic acetates had the following retention times: tS=29.10 min, tR=31.11 min (hexanes/
2-propanol=99:1 at 0.4 ml/min). The acetate, from the product of the enantioselective oxidation
gave, tR=32.10 min, with >99% ee.
1
1
(b) By H NMR. All ee’s were determined by comparing the H NMR data of the chiral
products with those of the corresponding racemic alcohols in CDCl3, using the a-CH signal in
the spectra and (+)-[Eu(hfc)3] as a chiral shift reagent.
(4-Chlorophenyl)phenylmethanol 2: a-CH signal in 5.77 ppm, the signal shifted to 9.23 and 9.19
ppm and refined by deconvolution procedure. The product had >99% ee.
(3-Bromophenyl)phenylmethanol 4: a-CH signal in 5.72 ppm, the signal shifted to 6.64 and 6.66
ppm and refined by deconvolution procedure. The product had >99% ee.
(4-Methylphenyl)phenylmethanol 5: a-CH signal in 5.79 ppm, the signal shifted to 8.00 and
7.96 ppm and refined by deconvolution procedure. The product had 78% ee.
(4-Methoxyphenyl)phenylmethanol 7: a-CH signal in 5.78 ppm, the signal shifted to 6.68 and
6.64 ppm and refined by deconvolution procedure. The product had >99% ee.
(3-Phenoxyphenyl)phenylmethanol 8: a-CH signal in 5.78 ppm, the signal shifted to 6.90 and
6.88 ppm and refined by deconvolution procedure. The product had 46% ee.
(2-Bromophenyl)phenylmethanol 3: a-CH signal in 6.19 ppm, the signal shifted to 6.77 and 6.71
ppm. The product had 0.12% ee.
(3,4-Dimethoxyphenyl)phenylmethanol 10: a-CH
and 7.80 ppm and refined by deconvolution procedure. The product had >99% ee.
(2,3-Dimethoxyphenyl)phenylmethanol 11: a-CH signal in 6.00 ppm, the signal shifted to 9.19
and 9.01 ppm and refined by deconvolution procedure. The product had 20% ee.
6
6
6
6
6
6
6
6
signal in 5.78 ppm, the signal shifted to 7.85
6

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H. I. Pe´rez et al. / Tetrahedron: Asymmetry 11 (2000) 4263–4268
Acknowledgements
We thank the financial support of Consejo Nacional de Ciencia y Tecnolog´ıa (CONACYT),
1
Me´xico, Grant No. 28543N. We also thank MS Atilano Gutierrez for the H NMR spectra and
chiral shift reagent experiments.
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