Racemization of enantiopure secondary alcohols by Thermoanaerobacter ethanolicus secondary alcohol dehydrogenase

Article abstract of DOI:10.1039/c3ob27415b

Controlled racemization of enantiopure phenyl-ring-containing secondary alcohols is achieved in this study using W110A secondary alcohol dehydrogenase from Thermoanaerobacter ethanolicus (W110A TeSADH) and in the presence of the reduced and oxidized forms of its cofactor nicotinamide-adenine dinucleotide. Racemization of both enantiomers of alcohols accepted by W110A TeSADH, not only with low, but also with reasonably high, enantiomeric discrimination is achieved by this method. Furthermore, the high tolerance of TeSADH to organic solvents allows TeSADH-catalyzed racemization to be conducted in media containing up to 50% (v/v) of organic solvents.

Full text of DOI:10.1039/c3ob27415b

Organic &
Biomolecular Chemistry
View Article Online
View Journal | View Issue
PAPER
Racemization of enantiopure secondary alcohols by
Thermoanaerobacter ethanolicus secondary alcohol
dehydrogenase†
Cite this: Org. Biomol. Chem., 2013, 11,
2911
Musa M. Musa,*a Robert S. Phillips,b,c Maris Laivenieks,d Claire Vieille,d
Masateru Takahashie and Samir M. Hamdane
Controlled racemization of enantiopure phenyl-ring-containing secondary alcohols is achieved in this
study using W110A secondary alcohol dehydrogenase from Thermoanaerobacter ethanolicus (W110A
TeSADH) and in the presence of the reduced and oxidized forms of its cofactor nicotinamide-adenine
dinucleotide. Racemization of both enantiomers of alcohols accepted by W110A TeSADH, not only with
low, but also with reasonably high, enantiomeric discrimination is achieved by this method. Furthermore,
the high tolerance of TeSADH to organic solvents allows TeSADH-catalyzed racemization to be conducted
in media containing up to 50% (v/v) of organic solvents.
Received 10th November 2012,
Accepted 4th March 2013
DOI: 10.1039/c3ob27415b
www.rsc.org/obc
reactions catalyzed by either lipase or subtilisin, and therefore
they have been employed successfully in DKR.5
Introduction
There is
a
growing interest in developing racemization
New developments in biotechnology have made biocatalysis
methods for enantiomerically pure secondary alcohols.1 Race- a practical and environmentally benign alternative to con-
mization is the key step for successful dynamic kinetic resolu- ventional organic and organometallic catalysis for a wide
tion (DKR), a method that gained significant interest in the variety of reactions.6 Biocatalysts are also known for their high
last two decades to produce optically active alcohols, in chemo- and regioselectivity, which minimizes the formation
addition to other interesting enantiopure compounds.2 Race- of undesired by-products. A pair of alcohol dehydrogenases
mization can be used not only in situ with kinetic resolution (ADHs) with opposite stereopreferences has been employed in
(KR) to generate DKR, but also in a succession of KR and race- racemization of enantiopure alcohols by a group guided by
mization rounds to recycle the unreactive enantiomer in KR Faber and Kroutil.7 They were also able to achieve the same
reactions. With the exclusion of a few examples, like cyano- task by employing Pseudomonas fluorescens ADH, a non-stereo-
hydrins and α-hydroxy carbonyl compounds, alcohols are non- selective ADH. However, such non-selective ADHs are uncom-
stereolabile, and therefore racemization of their enantiopure monly found.7
forms is not an easy task.3 A very well studied method for race-
mization of enantiopure secondary alcohols utilizes a tran- genase from Thermoanaerobacter ethanolicus (TeSADH, EC
sition metal-catalyzed redox pathway.4 It has been shown 1.1.1.2),8
nicotinamide-adenine dinucleotide phosphate
We have been working with the secondary alcohol dehydro-
a
that those organometallic catalysts are compatible with KR (NADPH)-dependent enzyme, to produce enantiomerically
pure alcohols. We have also applied site-directed mutagenesis
to expand its substrate specificity.9 With their exceptional
characteristics, like high thermal stability and high tolerance
to organic solvents,10 TeSADH and its mutants have been suc-
cessfully employed in asymmetric transformations to produce
enantiomerically pure alcohols from their corresponding
ketones via asymmetric reduction or from their racemates
through KR.9
aDepartment of Chemistry, King Fahd University of Petroleum and Minerals,
Dhahran, 31261, KSA. E-mail: musam@kfupm.edu.sa; Fax: +966-3-860-4277;
Tel: +966-3-860-7343
bDepartment of Chemistry, University of Georgia, Athens, Georgia 30602-2556, USA
cDepartment of Biochemistry and Molecular Biology, University of Georgia, Athens,
Georgia 30602-2556, USA
dDepartment of Microbiology and Molecular Genetics, Michigan State University,
East Lansing, MI 48824, USA
eDivision of Biological and Environmental Sciences and Engineering, King Abdullah
One of the more interesting mutants is W110A TeSADH, a
Prelog stereoselective ADH that recognizes phenyl-ring-
containing alcohols and their corresponding ketones with
good activity.11 We noticed in our previous studies that phenyl-
acetone was reduced to (S)-1-phenyl-2-propanol ((S)-1a) by
University of Science and Technology, Thuwal 23955-6900, KSA
†Electronic supplementary information (ESI) available: GC chromatograms for
the racemization products. See DOI: 10.1039/c3ob27415b
This journal is © The Royal Society of Chemistry 2013
Org. Biomol. Chem., 2013, 11, 2911–2915 | 2911

View Article Online
Paper
Organic & Biomolecular Chemistry
Table 1 W110A TeSADH-catalyzed racemization of enantiopure phenyl-ring-
containing secondary alcoholsa
Scheme 1 Asymmetric reduction of phenyl-ring-containing ketones by
W110A TeSADH.
eec (%)
Entry
R
Substrate E value (S/R)b Before After
W110A TeSADH with low enantioselectivity (37% ee).11 Such
low enantioselectivity can be explained by the reverse fit of the
substrate in the active site of the enzyme that leads to deliver-
ing the hydride from the undesired face of the ketone, i.e. the
Si face rather than the Re face, leading to what is called “selec-
tivity mistake”, as shown in Scheme 1. This low enantioselec-
tivity stimulated us to study the W110A TeSADH-catalyzed
racemization of enantiopure secondary alcohols.
1
2
3
4
5
6
7
8
9
PhCH2
PhCH2
PhCH2
PhCH2
(S)-1a
(S)-1a
(R)-1a
(R)-1a
3.4
3.4
3.4
>99
>99
>99
>99
91
6.3 (S)
4.3 (S)d
10.3 (R)
15.4 (R)d
44.0 (S)
34.5 (R)
82.4 (S)
51.8 (S)
>99 (S)
3.4
p-MeOC6H4(CH2)2 (S)-2a
12e
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhOCH2
(R)-3a
(S)-3a
(S)-3a
(S)-4a
>100
>100
>100
40
>99
99
72.7
>99
a Unless otherwise mentioned, reactions were performed at 50 °C
using enantiopure na (3 μL), W110A TeSADH (0.3 mg), NADPH
(1.0 mg), NADP+ (0.5 mg), CH3CN (200 μL), and Tris-HCl buffer
solution (800 μL, 50 mM, pH 8.0). b E-values were calculated as
described in the Experimental section for KR reactions in Tris-buffer
solution containing 10% (v/v) acetone, reported in ref. 11.
In this paper, W110A TeSADH was used to racemize phenyl-
ring-containing enantiopure alcohols in an attempt to recycle
the slow reacting alcohol enantiomer in KR either in situ to
achieve DKR or separately by sequential KR and racemization
cycles. We found that W110A TeSADH can racemize not only
a substrate for which it shows low stereospecificity (e.g.,
1-phenyl-2-propanol), but also substrates for which it shows
moderately high stereospecificity.
c Determined by GC equipped with
a chiral column for the
corresponding acetate derivative. d DMF was used instead of CH3CN as
a cosolvent. e This E-value is for a KR reaction conducted in biphasic
system containing diisopropyl ether as a cosolvent and 4 eq. of acetone
as a cosubstrate, reported in ref. 12.
Results and discussion
We describe W110A TeSADH-catalyzed racemization of phenyl- for the W110A TeSADH-catalyzed enantiospecific KR for their
ring-containing enantiopure alcohols. In this report, W110A racemates.12
TeSADH was used as a racemizing agent, instead of being used
The racemization method elucidated above depends on
to generate optically active alcohols. This racemization was the E-value; the lower the E-value, the better the racemization
accomplished by manipulating the reversible interconversion efficiency. We have reported that this value for W110A
between ketone and alcohol substrates in W110A TeSADH-cata- TeSADH-catalyzed enantiospecific KR reactions varies by
lyzed redox transformations. We started our racemization changing the organic cosolvent used.12 For example, for the
efforts on (S)-1a and (R)-1a substrates due to their poor enan- KR reaction with rac-2a, the E-value varies from one to 100 by
tiomeric discrimination by W110A TeSADH. The racemization switching the cosolvent from hexane to toluene.12 Such
was conducted in Tris-HCl buffer solution containing 20% solvent-dependent selectivity might serve as a guide for
(v/v) acetonitrile as a cosolvent to enhance the solubility of the choosing the proper solvent in conducting racemization.
substrate. Both NADPH and NADP+ (2 : 1, w/w) were included
In theory, we should be able to racemize enantiopure
in the reaction medium to allow the racemization to take place alcohols that are recognized by W110A TeSADH with relatively
by facilitating the equilibrium in the redox reaction. The ratio high enantiospecificity, as long as there is a selectivity
between NADPH and NADP+ was chosen to allow racemization mistake, because racemization is a thermodynamically down-
to take place with minimal formation of the prochiral ketone. hill process owing to the entropy increase associated with
We were able to reduce the ee of (S)-1a from >99% to 6.3% in forming the two enantiomers from one.13 In most ADH-
48 h at 50 °C, as reported in Table 1. The same procedure was catalyzed asymmetric transformations, a large excess of a
employed to racemize (R)-1a, and we were able to reduce its ee cosubstrate is normally used to enhance the solubility of sub-
from >99% to 10.3%, as shown in Table 1. Similar results were strates and to drive the equilibrium toward the reduction or
obtained when dimethyl formamide (DMF) was used rather oxidation direction, a common approach for cofactor recycling
than acetonitrile as a cosolvent, as shown in the second and called “coupled-substrate”.14 In ADH-catalyzed racemization,
fourth entries of Table 1. (S)-4-(4′-Methoxyphenyl)-2-butanol no cosubstrate is needed for cofactor recycling, because the
((S)-2a) was exposed to the same racemization conditions, and net redox balance is zero,7 and therefore the interconversion
its ee was decreased from 91% to 44%. As expected, (S)-2a was between alcohol and ketone substrates is facilitated by having
racemized to a lesser extent than (S)-1a, which is simply NADPH and NADP+ in the reaction medium, leading to more
explained by the difference in enantiomeric ratio, i.e. E-value, selectivity mistakes. We have reported that benzylacetone (3b)
2912 | Org. Biomol. Chem., 2013, 11, 2911–2915
This journal is © The Royal Society of Chemistry 2013

View Article Online
Organic & Biomolecular Chemistry
Paper
was reduced to (S)-4-phenyl-2-butanol ((S)-3a) with high enan-
tioselectivity (99% ee).11 Using the same racemization pro-
cedure as used for (S)-1a and (R)-1a, we were able to decrease
the ee of (R)-3a from >99% to 34.5% in 48 h (Table 1).
However, the ee of (S)-3a was reduced from 99% to 82.4% in
48 h. In a separate experiment, we were able to reduce the ee
of (S)-3a from 72.7% to 51.8% in 48 h, which indicates that
(S)-3a can be racemized by W110A TeSADH to a lesser extent
than (R)-3a, but that it could be racemized if adequate time is
allowed. These results indicate that even alcohols accepted by
W110A TeSADH with high enantiomeric discrimination (e.g.,
(S)-3a and its anti-Prelog analog, (R)-3a) can still be racemized
by this method. As shown in Table 1, the E-value for stereo-
specific KR of (rac)-3a is >100.
However, using the above-mentioned racemization pro-
cedure, the ee value for (S)-phenoxy-2-propanol ((S)-4a)
remained >99% after 48 h. We have previously noticed that
(S)-4a was produced from phenoxy-2-propanone (4b) in high
yield and high enantioselectivity (both >99%), but in the
stereospecific oxidation of rac-4a, we were not able to drive the
equilibrium to more than 19% conversion of (S)-4a to the cor-
responding ketone.11 These results indicate that the equili-
brium favors the reduction direction in this case, probably due
to the formation of an intramolecular hydrogen bond between
the alcoholic hydrogen and the oxygen of the ether, and there-
fore restricts the interconversion between 4b and its alcohol.
Furthermore, the electronegative phenoxy substituent probably
makes the equilibrium favor the alcohol more than for the
other aromatic substrates.
Table 2 Racemization of (S)-1a and (R)-1a by using W110A TeSADH in organic
solventa
Entry
Substrate
Solvent
eeb (%)
1
2
3
4
5
6
(S)-1a
(R)-1a
(S)-1a
(S)-1a
(S)-1a
(R)-1a
CH3CNc
CH3CNc
Hexane
Hexaned
MTBE
94 (S)
95 (R)
81 (S)
86 (S)
41 (S)
52 (R)
MTBE
a Unless otherwise mentioned, reactions were performed at 50 °C
using enantiopure 1a (0.22 mmol), W110A TeSADH (1.4 mg), NADPH
(2.0 mg), NADP+ (2.0 mg), organic solvent (700 μL), and Tris-HCl buffer
solution (900 μL, 50 mM, pH 8.0). b Determined by GC equipped with a
chiral column for the corresponding acetate derivative. c 1.5 mL of
Tris-HCl buffer solution (50 mM, pH 8.0) and 1.5 mL of organic
solvent were used. d 200 μL of CH3CN and 700 μL of hexane were
added.
We were discouraged by the racemization efficiency in
monophasic media containing CH3CN (50%, v/v). We decided
therefore to conduct the W110A TeSADH-catalyzed racemiza-
tion in biphasic media that consisted of organic water-
immiscible solvents as the non-aqueous phase and Tris-HCl
buffer solution as the aqueous phase. When hexane was used
as the organic solvent, we were able to reduce the ee of (S)-1a
from >99% to 81%. This low racemization efficiency could be
explained by the low solubility of the corresponding ketone in
aqueous solution, which could drive it to reside in hexane and
therefore to be retarded from the redox reaction. The addition
of 12% (v/v) of acetonitrile did not facilitate the racemization
of (S)-1a, and the ee of the isolated alcohol was 86% after 48 h
(Table 2, entry 4). When methyl tert-butyl ether (MTBE) was
used as the organic solvent, the ee of (S)-1a was reduced
from >99% to 41%. We attributed this to the difference in the
hydrophobicity of hexane and MTBE, with the latter being less.
Under the same conditions, we observed that racemization of
(R)-1a took place to a similar extent as (S)-1a; and its ee was
reduced from >99% to 52%.
When W110A TeSADH-catalyzed racemization of enantio-
pure 1a was conducted on the scale described in Table 2, the
corresponding ketone was either not detected or hardly
observed (<1%). This is due to the negligible ratio of the cofactor
with respect to the alcohol when compared with the small
scale described in Table 1. With the exception of the negligible
amount of the ketone formed by W110A TeSADH-catalyzed
redox reaction, we did not notice the formation of any other
by-products. This is due to the high enzyme selectivity and the
mild reaction conditions employed. For example, the percent
recovery for the reaction that is explained by entry 6 in Table 2
was 89% of pure alcohol. The purity of the isolated product
was confirmed by GC and 1H NMR.
It is worth mentioning that the percentage of ketone
formed as a result of equilibrium with alcohols did not exceed
3% in the cases described, except for 2a which was 7% (see
Fig. S4, ESI†). Such percentage could become even less if this
racemization method is combined with a KR reaction that
will deplete one enantiomer of the alcohols and thus pull the
equilibrium in the reduction direction.
In order to be able to utilize this racemization method
in combination with interesting reactions such as KR and
with useful scales, we sought to carry out W110A TeSADH-
catalyzed racemization in non-aqueous media. We reported
previously that W110A TeSADH can perform redox reactions in
high concentrations of water-miscible and -immiscible organic
solvents with more than 50% (v/v) and with reasonable con-
centrations of substrate (up to 120 mM).12 However, racemiza-
tion of (S)-1a and (R)-1a in monophasic media containing
acetonitrile (50%, v/v) as the organic cosolvent and Tris-HCl
buffer solution was not successful. This is because W110A
TeSADH tolerates acetonitrile at high concentration as a cosol-
vent for a restricted amount of time that is not enough to
conduct racemization as evident by the observation that ee for
(R)-1a and (S)-1a was reduced from >99% to about 94%
(Table 2, entries 1 and 2). It should be noted that we have
reported that W110A TeSADH can perform asymmetric redox
reactions in high yield using acetonitrile, Tris-HCl buffer
solution, and 2-propanol [38 : 38 : 24 (v/v/v)] as the reaction
medium.12
This journal is © The Royal Society of Chemistry 2013
Org. Biomol. Chem., 2013, 11, 2911–2915 | 2913

View Article Online
Paper
Organic & Biomolecular Chemistry
The high tolerance of TeSADH to organic solvents together racemization in organic solvents, was expressed in recombi-
with its high thermal stability might allow this environmen- nant Escherichia coli BL21(DE3) cells and purified as reported16
tally benign mild racemization method to be incorporated with modifications. The resuspended cells in lysis buffer were
with other organic reactions such as enzyme-catalyzed KR.15 lysed by sonication (40% amplitude for 5 min), and the lysate
This racemization protocol seems to be slower than what one was centrifuged (70 000 g for 30 min) to remove cell debris.
expects for an efficient DKR. However, the latter can be The heat-treated crude extract was spun down (70 000 g for
achieved with a racemization method that has a rate constant 30 min). The cleared crude extract was loaded on a 5 ml-
(krac) less than the more reactive enantiomer in KR (kfast) if krac HisTrap FF (GE Healthcare) column. The eluted proteins were
is much greater than the rate constant for the slow reacting collected and diluted to adjust imidazole concentration to be
enantiomer in KR (kslow) (i.e. kfast > krac ≫ kslow).13a This less than 20 mM and loaded on 5 ml-HisTrap and eluted by
enzyme-catalyzed racemization is an attractive approach due to imidazole gradient.
the high chemo- and regioselectivity of enzymes. More site-
directed mutagenesis is needed to construct non-selective
General procedure for small scale W110A TeSADH-catalyzed
racemization
mutants of TeSADH to increase the efficiency of this racemiza-
tion approach. Further developments will be reported in due A mixture of enantiopure alcohol (3 μL), NADP+ (0.5 mg),
course.
NADPH (1.0 mg), W110A TeSADH (0.3 mg), Tris-HCl buffer solu-
tion (800 μL, 50 mM, pH 8.0), and acetonitrile (200 μL) were
placed in a 1.5 mL plastic tube. The reaction mixture was
shaken at 50 °C at 200 rpm for 48 h then extracted with ethyl
acetate (2 × 500 μL). The combined organic layer was dried
with sodium sulfate and concentrated to dryness. The remain-
ing residue was treated with pyridine and acetic anhydride to
convert the alcohols to their corresponding acetates as
described.17 The acetate products were analyzed by GC
equipped with a chiral column to determine their ee.
Conclusions
In conclusion, W110A TeSADH has shown to be active in race-
mization of phenyl-ring-containing enantiopure alcohols. We
demonstrated that W110A TeSADH-catalyzed racemization is
valid for both enantiomers of alcohols accepted by this
enzyme with not only low, but also high enantiomeric dis-
crimination. We have also shown that this racemization proto-
col can be conducted with useful scales in organic solvents.
The capability of TeSADH to perform racemization under mild
conditions in addition to its high thermal stability and high
tolerance to organic solvents is of great interest. It also rep-
resents an attractive approach especially if it proves successful
in situ with other interesting reactions such as KR.
W110A TeSADH-catalyzed racemization in monophasic
systems
A mixture of NADP+ (2.0 mg), NADPH (2.0 mg), Tris-HCl buffer
solution (1.3 mL, 50 mM, pH 8.0), W110A TeSADH [1.4 mg in
200 μL of 50 mM Tris-HCl buffer (pH 8.0)], CH3CN (1.5 mL),
and enantiopure alcohol (0.22 mmol) was added in the same
sequence to a round-bottomed flask equipped with a magnetic
stirrer. The reaction mixture was stirred at 50 °C for 48 h. It
was then extracted with ethyl acetate (2 × 3 mL). The organic
layers were combined and extracted with brine solution
(3.0 mL) then dried with sodium sulfate then concentrated
under vacuum. The remaining residue was treated with
pyridine and acetic anhydride to convert alcohols to the
corresponding esters.17 The acetate products were analyzed by
GC equipped with a chiral column.
Experimental
General
Capillary gas chromatographic measurements were performed
on a GC equipped with a flame ionization detector and a
Supelco β-Dex 120 chiral column (30 m, 0.25 mm [i.d.],
0.25 μm film thickness) using He as the carrier gas. Commer-
cial grade solvents were used without further purification.
NADP+, NADPH, (rac)-1a, (rac)-3a, (R)-1a (>99% ee), (S)-1a
(>99% ee), (S)-4-(4′-methoxyphenyl)-2-butanol (2b), benzyl
acetone (3b) and 4b were used as purchased from commercial
sources. (R)-3a (>99% ee) and (S)-3a (72.7% ee) were prepared
by lipase-catalyzed KR of their racemates as reported.16 (S)-3a
(99% ee), (S)-4a (>99% ee) and (S)-2a (91% ee) were prepared
by W110A TeSADH-catalyzed reduction of their corresponding
ketones as reported.11 All Tris-HCl buffer solutions were
adjusted to pH 8.0 at room temperature.
W110A TeSADH-catalyzed racemization in biphasic systems
A mixture of NADP+ (2.0 mg), NADPH (2.0 mg), and W110A
TeSADH (1.4 mg in 900 μL of Tris-HCl buffer solution, 50 mM,
pH 8.0) was added to water-immiscible organic solvent
(700 μL) containing enantiopure alcohol (0.22 mmol) in a
round-bottomed flask equipped with a magnetic stirrer. The
mixture was stirred at maximum speed at 50 °C for 48 h. The
two layers were then separated and the aqueous layer was
extracted with ethyl acetate (2 × 3 mL). The combined organic
layers were mixed with the original organic layer and extracted
with brine solution (3.0 mL) then dried with sodium sulfate.
Gene expression and purification of W110A TeSADH
W110A TeSADH, used in the small scale experiments, was Volatile components were evaporated under vacuum. The
expressed in recombinant Escherichia coli HB101(DE3) cells mass of the remaining residue was reported to calculate
and purified as reported.16 W110A TeSADH, used in percent recovery of alcohol. Alcohols obtained were converted
2914 | Org. Biomol. Chem., 2013, 11, 2911–2915
This journal is © The Royal Society of Chemistry 2013

View Article Online
Organic & Biomolecular Chemistry
Paper
to the corresponding acetate esters prior to their analysis by
GC equipped with a chiral column.
4 (a) M. C. Warner, C. P. Casey and J.-E. Bäckvall, Top. Organo-
met. Chem., 2011, 37, 85–125; (b) Q. Chen and C. Yuan,
Chem. Commun., 2008, 5333–5335; (c) L. Borén, B. Martín-
Matute, Y. Xu, A. Córdova and J.-E. Bäckvall, Chem.–Eur. J.,
2006, 12, 225–232; (d) N. Kim, S.-B. Ko, M. S. Kwon,
M.-J. Kim and J. Park, Org. Lett., 2005, 7, 4523.
5 (a) Y. Sato, Y. Kayaki and T. Ikariya, Chem. Commun., 2012,
48, 3635–3637; (b) D. Mavrynsky, L. T. Kanerva, R. Sillanpää
and R. Leino, Pure Appl. Chem., 2011, 83, 479–483.
6 U. T. Bornscheuer, G. W. Huisman, R. J. Kazlauskas,
S. Lutz, J. C. Moore and K. Robins, Nature, 2012, 485, 185–
194.
Determination of absolute configuration of alcohols
The absolute configurations of the produced alcohols were elu-
cidated by comparing the retention time of their acetate
derivatives with either their S- or R-acetate enantiomer after
injection on a GC equipped with a chiral stationary phase.
Calculation of E-values
E-values were calculated from the formula E = ln[(1 − c)-
(1 − ees)]/ln[(1 − c)(1 + ees)], where c is percentage conversion
of alcohol to ketone, and ees is enantiomeric excess of the slow
reacting (R)-alcohol in W110A TeSADH-catalyzed KR.11
7 C. C. Gruber, B. M. Nestl, J. Gross, P. Hildebrant,
U. T. Bornscheuer, K. Faber and W. Kroutil, Chem.–Eur. J.,
2007, 13, 8271–8276.
8 (a) D. Burdette and J. G. Zeikus, Biochem. J., 1994, 302,
163–170; (b) D. Burdette, C. Vieille and J. G. Zeikus,
Biochem. J., 1996, 316, 115–122.
9 M. M. Musa and R. S. Phillips, Catal. Sci. Technol., 2011, 1,
1311–1323.
10 (a) V. T. Pham and R. S. Phillips, J. Am. Chem. Soc., 1990,
112, 3629–3632; (b) V. T. Pham, R. S. Phillips and
L. G. Ljugdahl, J. Am. Chem. Soc., 1989, 111, 1935–1936.
11 M. M. Musa, K. I. Ziegelmann-Fjeld, C. Vieille, J. G. Zeikus
and R. S. Phillips, J. Org. Chem., 2007, 72, 30–34.
12 M. M. Musa, K. I. Ziegelmann-Fjeld, C. Vieille and
R. S. Phillips, Org. Biomol. Chem., 2008, 6, 887–892.
13 (a) J. Steinreiber, K. Faber and H. Griengl, Chem.–Eur. J.,
2008, 14, 8060–8072; (b) O. Pàmies and J.-E. Bäckvall,
Chem. Rev., 2003, 103, 3247–3261; (c) E. Eliel, S. H. Wilen
and M. P. Doley, Basic Organic Stereochemistry, Wiley,
New York, 2001, pp. 102–106.
14 W. Kroutil, H. Mang, K. Edegger and K. Faber, Curr. Opin.
Chem. Biol., 2004, 8, 120–126.
15 M. M. Musa, K. I. Ziegelmann-Fjeld, C. Vieille, J. G. Zeikus
and R. S. Phillips, Angew. Chem., 2007, 119, 3151–3154,
(Angew. Chem., Int. Ed., 2007, 46, 3091–3094).
16 K. I. Ziegelmann-Fjeld, M. M. Musa, R. S. Phillips,
J. G. Zeikus and C. Vieille, Protein Eng., Des. Sel., 2007, 20,
47–55.
Acknowledgements
The authors acknowledge the support provided by King
Abdulaziz City for Science and Technology (KACST) through
the Science and Technology Unit at King Fahd University of
Petroleum and Minerals (KFUPM) for funding this work
through project no. 11-BIO1666–04 as part of the National
Science, Technology and Innovation Plan.
References
1 (a) E. B. Bauer, Chem. Soc. Rev., 2012, 41, 3153–3167;
(b) P. Nun, G. C. Fortman, A. M. Z. Slawin and S. P. Nolan,
Organometallics, 2011, 30, 6347–6350; (c) M. Merabet-Khe-
lassi, N. Veriamont, L. Aribi-Zouioueche and O. Riat, Tetra-
hedron: Asymmetry, 2011, 22, 1790–1796; (d) O. Verho,
E. V. Johnston, E. Karlsson and J.-E. Bäckvall, Chem.–Eur.
J., 2011, 17, 11216–11222; (e) M. C. Warner, O. Verho and
J.-E. Bäckvall, J. Am. Chem. Soc., 2011, 133, 2820–2823.
2 For recent reviews: (a) M. Ahmed, T. Kelly and A. Ghanem,
Tetrahedron, 2012, 68, 6781–6802; (b) H. Pellissier, Tetra-
hedron, 2011, 67, 3769–3802; (c) H. Pellissier, Adv. Synth.
Catal., 2011, 353, 659–676; (d) J. H. Lee, K. Han, M.-J. Kim
and J. Park, Eur. J. Org. Chem., 2010, 999–1015.
17 A. Ghanem and V. Schuring, Tetrahedron: Asymmetry, 2003,
14, 57–62.
3 P. Odman and B. Wessjohann, J. Org. Chem., 2005, 70,
9551–9555.
This journal is © The Royal Society of Chemistry 2013
Org. Biomol. Chem., 2013, 11, 2911–2915 | 2915