Steric effects on the stereochemistry of old yellow enzyme-mediated reductions of unsaturated diesters: Flipping of the substrate within the enzyme active site induced by structural modifications

Article abstract of DOI:10.1002/adsc.201200471

The ene-reductase-mediated reduction of the carbon-carbon double bond of some alkyl 2- substituted butenedioates was investigated. The stereochemical outcome of the reaction was found to be influenced by steric effects. Ethyl and butyl citraconates were converted into the corresponding alkyl (R)-2-methylsuccinates with excellent enantioselectivity, whereas ethyl and butyl mesaconates were completely unreactive. Methyl 2-substituted fumarates were reduced to enantiomerically enriched methyl (S)-2-substituted succinates, whereas the (Z)-stereoisomers were left unreacted by enereductases. Labelling experiments were performed to investigate the mechanism of these bioreductions and explain their stereochemical outcome.

Full text of DOI:10.1002/adsc.201200471

UPDATES
DOI: 10.1002/adsc.201200471
Steric Effects on the Stereochemistry of Old Yellow Enzyme-
Mediated Reductions of Unsaturated Diesters: Flipping of the
Substrate within the Enzyme Active Site Induced by Structural
Modifications
Elisabetta Brenna,a,* Francesco G. Gatti,a Alessia Manfredi,a Daniela Monti,b
and Fabio Parmeggiania
a
Politecnico di Milano, Dipartimento di Chimica, Materiali, Ingegneria Chimica, Via Mancinelli 7, I-20131 Milano, Italy
Fax : (+39)-02-2399-3180; phone: (+39)-02-2399-3077; e-mail: elisabetta.brenna@polimi.it
Istituto di Chimica del Riconoscimento Molecolare - CNR, Via Mario Bianco 9, I-20131 Milano, Italy
b
Received: May 29, 2012; Published online: October 5, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200471.
Abstract: The ene-reductase-mediated reduction of
the carbon-carbon double bond of some alkyl 2-
substituted butenedioates was investigated. The ste-
reochemical outcome of the reaction was found to
be influenced by steric effects. Ethyl and butyl cit-
raconates were converted into the corresponding
alkyl (R)-2-methylsuccinates with excellent enantio-
selectivity, whereas ethyl and butyl mesaconates
were completely unreactive. Methyl 2-substituted
fumarates were reduced to enantiomerically en-
riched methyl (S)-2-substituted succinates, whereas
the (Z)-stereoisomers were left unreacted by ene-
reductases. Labelling experiments were performed
to investigate the mechanism of these bioreductions
and explain their stereochemical outcome.
the advantages of the biocatalysed reduction of the
carbon-carbon double bond with the synthetic versa-
tility of the carboxylic and amine functionalities. For
these types of substrates it is still necessary to collect
data regarding the structural requisites that allow the
reduction to be performed with high enantioselectivi-
ty and yields, in order to investigate and exploit the
synthetic potential of the biotransformation.
In the case of unsaturated esters, it has been estab-
lished that the presence of a second electron-with-
drawing group (e.g., a halogen atom at the same
carbon atom bearing the carboxylic moiety,[6] or an-
other ester group at the carbon atom in the b posi-
tion[7]) makes the reaction more likely feasible. There
are only a few examples in the literature of successful
OYE-mediated bioreductions of unsaturated esters
having no additional electron-withdrawing moieties
on the carbon-carbon double bond.[8]
Keywords: asymmetric synthesis; bakerꢀs yeast; bio-
transformations; deuterium; enantioselectivity; ene-
reductases
We have recently explored the synthetic signifi-
cance of the bioconversions of a-haloalkenoates 1:
substrates of type (Z)-1 (Figure 1) were converted
into enantiomerically pure (S)-a-haloalkanoic acids
and esters 2, respectively, by bakerꢀs yeast (BY) fer-
The biocatalytic reduction of carbon-carbon double mentation and by using isolated enzymes OYE1-3.[9]
bonds mediated by ene-reductases (ERs) is a very
We now wish to update the investigation concern-
valuable tool for synthetic organic chemists:[1] it ing the reduction of unsaturated diesters, which has
occurs with high enantioselectivity and conversion on
properly activated substrates, under mild working
conditions, without generation of toxic waste. The re-
duction of unsaturated aldehydes and ketones has
been known for a long time,[2] and it has been the
object of extensive investigations.[3] In the last years,
the availability of isolated ene-reductases, such as old
yellow enzymes (OYEs), has boosted the search for
novel substrates in this kind of reaction, e.g., unsatu-
rated esters[4] and amino acids[5], in order to combine Figure 1. a-Haloacrylates and related reduction products.
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Elisabetta Brenna et al.
UPDATES
(E)- and (Z)-4a–c (Figure 2). Their bioreduction
could represent a convenient synthetic procedure[10]
for the preparation of stereogenic centres bearing two
reactive alkoxycarbonyl functionalities.
Derivatives (E)- and (Z)-3b and 3c were prepared
by Fischer esterification of mesaconic and citraconic
acid in ethanol and n-butanol solution, respectively.
The synthesis of compounds (E)- and (Z)-4a–c was
accomplished by means of Wittig condensation of the
suitable keto ester with methoxycarbonylmethylen-
AHCTUNGTREG(NNUN triphenyl)phosphorane: the two stereoisomers were
separated by column chromatography, and submitted
separately to biocatalysed reductions. The double
bond configuration was established by analysis of the
1H and 13C NMR spectra.
The results of the BY fermentation and OYE1-3
biotransformations are collected in Table 1. The data
concerning the reduction of derivatives (E)- and (Z)-
3a with OYE1-3 under similar conditions (recycling
NADPH) were obtained from the literature.[7]
OYE1-3 had been described to convert methyl me-
Figure 2. a,b-Unsaturated diesters for ER-mediated reduc-
tions and optically active alkyl succinates obtained as prod-
ucts.
been limited up to now to the biotransformation of saconate and citraconate into (S)- and (R)-methyl 2-
methyl mesaconate and citraconate: (E)- and (Z)- methylsuccinate 5a (Figure 2) with high enantioselec-
3a.[7] The steric hindrance either at the alkyl group of tivity, the conversion for the (E)-stereoisomer (80–
the ester moieties, and at the substituent on the 99%) better than that for the (Z)-counterpart (30–
carbon-carbon double bond was increased, by prepar- 64%).
ing alkyl mesaconates and citraconates, (E)- and (Z)-
When we submitted (E)- and (Z)- ethyl and butyl
3b and c, and 2-substituted fumarates and maleates esters 3b and 3c either to BY fermentation and to
Table 1. Results of BY fermentations and OYE1-3 biotransformations of substrates (E)- and (Z)-3 and 4.
Substrate
BY
c[a]
OYE1
c[b]
OYE2
c[b]
OYE3
c[b]
ee[c]
ee[c]
ee[c]
ee[c]
R=R’=Me
(E)-3a[d]
99
58
99 (S)
99 (R)
80
64
99 (S)
99 (R)
99
30
99 (S)
99 (R)
(Z)-3a[d]
R=Et, R’=Me
(E)-3b
(Z)-3b
0
31
7
93
0
41
2
60
99 (R)
99 (R)
99 (R)
99 (R)
R=n-Bu, R’=Me
(E)-3c
(Z)-3c
R=Me, R’=Et
(E)-4a
0
21
0
50
0
38
0
44
95 (R)
98 (S)
99 (R)
98 (S)
90 (R)
98 (S)
94 (R)
96 (S)
42
0
98
0
80
0
100
0
(Z)-4a
R=Me, R’=n-Pr
(E)-4b
(Z)-4b
33
0
99 (S)
98
0.5
88 (S)
44
0
86 (S)
97
0
77 (S)
R=Me, R’=Ph
(E)-4c
(Z)-4c
3
0
66
47
99 (R)
99 (R)
21
13
99 (R)
96 (R)
15
14
99 (R)
69 (R)
R=R’=Et
(E)-7
(Z)-7
0
0
0
0
0
0
0
0
[a]
Conversions calculated by GC analysis of the crude mixture after 72 h reaction time.
Conversion calculated by GC analysis of the crude mixture after 24 h reaction time.
Enantiomeric excesses calculated by GC or HPLC analysis on a chiral stationary phase.
Conversions and enantiomeric excess values taken from ref.[7]
[b]
[c]
[d]
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Steric Effects on the Stereochemistry of Old Yellow Enzyme-Mediated Reductions of Unsaturated Diesters
OYE1-3 biotransformations, we observed that only diesters was the activating one, and to confirm the
the (Z)-diastereoisomers were reduced, affording the anti stereochemistry of the bioreduction.
corresponding (R)-enantiomer of succinates 5b and 5c
OYE1-mediated reductions of compounds (E)- and
in high enantiomeric purity. The (E)-stereoisomers (Z)-3a, (Z)-3b, and (E)-4a, were performed in the
were left unreacted.
presence of stoichiometric NADH in deuterium oxide
A different stereochemical course was then ob- solution,[5] in order to incorporate a deuterium atom,
served for substrates (E)- and (Z)-4a–c. 2-Substituted made available by the solvent, at the carbon atom in
methyl fumarates (E)-4a and b were reduced either the a position to the activating electron-withdrawing
by BY and by isolated enzymes, to afford the (S)- group. The ester group linked to the prostereogenic
enantiomer of the corresponding reduction products 6 carbon atom was always found to be the activating
(Figure 2) with good to high enantioselectivity.
OYE1-3 converted derivative (E)-4c in fairly good
yields into nearly enantiomerically pure compound
(R)-6c (Figure 2): the priority of the groups linked to
the stereogenic carbon atom is different from that of
the other saturated products and it is such that the
(R)-configuration corresponds to a spatial distribution
of substituents which is identical to that of the other
(S)-enantiomers of type 6. BY fermentation of sub-
strate (E)-4c proceeded with very poor yields (3%).
No reduction was obtained when stereoisomers
(Z)-4a and 4b were employed as substrates under the
same reaction conditions either with whole cell
system or with isolated enzymes. In the case of deriv-
ative (Z)-4c (diastereoisomeric excess de=97%), ex-
tensive double bond isomerisation was observed in
BY fermenting medium and under OYE-reduction
conditions: the unreacted starting alkene was invaria-
bly recovered as a 1:1 mixture of the two diastereoiso-
mers. No saturated product could be produced by BY
reaction, whereas samples of derivative (R)-6c, show-
ing high enantiomeric enrichment and very likely ob-
tained by conversion of the (E)-stereoisomer formed
upon isomerisation, were isolated in modest yields
from OYE biotransformations.
The absolute configuration of compounds 5b and 5c
was established by chemical correlation to the corre-
sponding dimethyl ester 5a; that of derivatives 6a,
b and c was known in the literature.
Some considerations on the mechanism of these bi-
oreductions were necessary to explain the observed
stereochemistry. According to the currently accepted
hypothesis,[11] the arrangement of the substrate in the
enzyme active site is controlled by the formation of
hydrogen bonds between the activating electron-with-
drawing group of the alkene and two amino acid resi-
dues within the binding pocket. In this situation a hy-
dride ion is transferred from the flavin cofactor to the
sp2 carbon atom in the b position to the activating
substituent, and a proton, which is derived from the
solvent, is delivered by a tyrosine residue to the
a carbon atom, usually on the other stereoheterotopic
face of the olefin, thus affording in most cases anti hy-
1
1
drogen addition.
Deuterium labelling experiments were performed
to establish which ester group of these unsaturated
Figure 3. a) H NMR spectrum of rac-5a; b) H NMR spec-
trum of (S)-5a-d1 obtained by OYE1 reduction of (E)-3a in
1
D2O; c) H NMR spectrum of (R)-5a-d1 obtained by OYE1
reduction of (Z)-3a in D2O.
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Elisabetta Brenna et al.
UPDATES
1
one: the H NMR spectra of the monodeuterated re- and a D+ from the solvent were added to the carbon
duced products were all characterised by the nearly carbon-double bond. anti-Hydrogen addition was es-
2
complete disappearance of the multiplet due to tablished by comparison of the H NMR spectra of
CHCOOR, and by the simplification of the signal of the doubly deuterated reduced products, obtained
the methylene group of the CH2COOR fragment to from (E)- and (Z)-3a, (Z)-3b and (E)-4a by reaction
an AB system. As a representative example, in with OYE1, and those of the doubly deuterated spe-
1
Figure 3 the H NMR spectra of (S)- and (R)-5a-d1 cies, prepared upon syn reduction of the same starting
are reported and compared to that of the hydrogenat- substrates with deuterium gas in the presence of PtO2
ed counterpart.
in ethyl acetate solution. Figure 4 collects the results
The stereospecificity of hydrogen addition was in- obtained, for instance, for compounds 5b and 6a.
2
vestigated by performing OYE1 biotransformations in
The analysis of these H NMR spectra clearly show
the presence of (R)-NADPD (regenerated in situ by that OYE1-mediated reduction of (Z)-3b and (E)-4a
alcohol dehydrogenase and i-PrOH-d8) in D2O:[12] afforded the doubly deuterated compounds (2R,3S)-
under these conditions a DÀ anion from the cofactor 5b-d2, and (2S,3S)-6a-d2 (Figure 4, b), which were ste-
reoisomers of the products (2RS,3RS)-5b-d2, and
(2RS,3SR)-6a-d2, recovered from syn chemical deuter-
ation (Figure 4, c) of the same starting alkenes.
The combination of these data, together with the
information on the absolute configuration of the re-
sulting succinates, led to the conclusions depicted in
Figure 5. The reactive binding arrangements of sub-
strates (E)- and (Z)-3a in the enzyme active site were
found to differ from one another by a 1808 flipping of
the plane of the molecule, with exchange of the ster-
eoheterotopic face of the alkene on which D+ was de-
livered.[13] The same situation was found by compar-
ing the results of the bioreduction of (E)-4a and (Z)-
3b. Common features of the four reactions were the
activation due to the most hindered ester group, and
the generation of the stereogenic carbon atom in posi-
tion 3 of the doubly deuterated species in (S)-absolute
configuration with addition of DÀ on Cb-si face.
The data of these bioreductions show that this kind
of biotransformation suffers from limitations due to
steric hindrance. The following considerations may be
tentatively drawn, using as references substrates (E)-
and (Z)-3a, since they show the minimum steric hin-
Figure 4. a) 1H NMR spectra of rac-5b, and rac-6a; b)
drance either at the substituent on the carbon carbon
double bond, and at the ester groups. When the bulki-
ness of the alkyl group of the ester moieties was in-
creased in substrates (E)- and (Z)-3b and c, the (E)-
stereoisomer became unreactive, and the reaction
2H NMR spectra of (2R,3S)-5b-d2, and (2S,3S)-6a-d2 ob-
tained by enzymatic double deuteration of (Z)-3b and (E)-
2
4a, respectively; c) H NMR spectra of (2RS,3RS)-5b-d2, and
(2RS,3SR)-6a-d2 obtained by metal-catalyzed D2 addition to
(Z)-3b and (E)-4a, respectively.
Figure 5. Stereochemical course of ene-reductase reduction of unsaturated diesters.
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Steric Effects on the Stereochemistry of Old Yellow Enzyme-Mediated Reductions of Unsaturated Diesters
(15000ꢂg, 1.5 min), and the combined organic solutions
were dried over anhydrous Na2SO4.
yields of the (Z)-counterpart were found to be lower
with R=n-Bu with respect to R=Et. When the bulki-
ness of the substituent on the alkene was increased in
substrates (E)- and (Z)-4a–c, the (Z)-stereoisomer
was no longer reactive, the enantioselectivity of the
(E)-diastereoisomer reduction decreased from R’=Et
to R’=n-Pr, and conversions diminished from R’=Et
to R’=Ph.
Bioreduction Procedure for the Preparation of
Monodeuterated Samples
The substrate (50 mmol) dissolved in i-PrOH (100 mL) was
added to a KPi buffer solution (5.0 mL, 50 mM in D2O,
pH 7.0) containing glucose (20 mmol), NADH (75 mmol) and
Substrates (E)- and (Z)-7 (Figure 2) were subse- the required OYE (ca. 250 mg, dissolved in 500–700 mL
H2O). The mixture was incubated for 24 h in an orbital
shaker (160 rpm, 308C). The solution was extracted with
EtOAc (3ꢂ5.0 mL), centrifuging after each extraction
(3000ꢂg, 1.5 min), and the combined organic solutions were
dried over anhydrous Na2SO4.
quently prepared to investigate the effect of increas-
ing the steric hindrance on the double bond and at
the ester groups at the same time, but they were
found to be completely unreactive.
This investigation has shown the limits of the syn-
thetic exploitation of the bioreduction of unsaturated
diesters: enantioselectivity and conversions are de-
pendent upon steric hindrance, and the practical ap-
plicability is quite hindered. Nonetheless, interesting
stereochemical data were collected. The two different
reactive binding modes deduced from labelling ex-
periments for methyl mesaconate and citraconate rep-
resent a sort of optimum prototype for (E)- and (Z)-
stereoisomers of unsaturated diesters. Increasing the
bulkiness of the substituent on the double bond, the
(E)- stereoisomers of compounds 4a–c are able to
maintain an accommodation “(E)-3a-like”, which
allows them to react. Derivatives (Z)-4a–c are pre-
vented by steric reasons from adopting an effectively
reactive binding mode “(Z)-3a-like”, which enables
the addition of HÀ on the Cb-si face.
Bioreduction Procedure for the Preparation of
Doubly Deuterated Samples
The substrate (50 mmol) dissolved in i-PrOH-d8 (100 mL)
was added to a KPi buffer solution (5.0 mL, 50 mM in D2O,
pH 7.0) containing NADP+ (15 mmol), TBADH (4 U, 3 mg)
and the required OYE (ca. 250 mg, dissolved in 500–700 mL
H2O). The mixture was incubated for 24 h in an orbital
shaker (160 rpm, 308C). The solution was extracted with
EtOAc (3ꢂ5.0 mL), centrifuging after each extraction
(3000ꢂg, 1.5 min), and the combined organic solutions were
dried over anhydrous Na2SO4.
General Procedure for Bakerꢀs Yeast Fermentation
A mixture of bakerꢀs yeast (100 g) and d-glucose (40 g) in
tap water (1 L) was prepared. After 10 min stirring at 308C
the suitable diester (3.0 g) adsorbed on a hydrophobic resin
(60 g, polystyrene XAD-1180N) was added in one portion.
The mixture was kept under stirring for 72 h at room tem-
perature and then filtered on a cotton plug. The collected
mass was washed with tap water repeatedly, to remove most
On the contrary, when the size of the alkyl chain at
the ester moieties is increased, the (Z)- stereoisomers
of compounds 3b and c can still adopt the accommo-
dation “(Z)-3a-like” which is the effectively reacting
one. In this case, steric hindrance is such to make (E)- of the cells. The resin was then collected and extracted twice
in sequence with acetone (200 mL) and ethyl acetate
(200 mL). The organic phase was concentrated to 1/3 and
then washed with brine. The residue obtained upon evapora-
tion of the dried (Na2SO4) extract was chromatographed on
a silica gel column with increasing amounts of ethyl acetate
in hexane.
stereoisomers unfavourable substrates for ene-reduc-
tases: a flipped arrangement similar to that of (E)-3a
cannot be achieved, in order to expose Cb-si face to
HÀ attack.
Unfortunately, the modest reactivity of diesters pre-
vented further investigations. The substitution of one
ester group with a nitrile moiety, which shows better
electron-withdrawing capability, is now under consid-
eration and the data will be published in due course.
Supporting Information
Characterisation data of substrates (E)- and (Z)-3b and 3c,
substrates (E)- and (Z)-4a–c, reduced products 5 and 6, deu-
terated compounds obtained upon either enzymatic and
chemical reduction, analytical procedures for the determina-
tion of conversion, and enantiomeric excess are given in the
Supporting Information.
Experimental Section
General Procedure for the OYE-Mediated
Bioreduction
The substrate (5 mmol) dissolved in DMSO (10 mL) was
added to a KPi buffer solution (1.0 mL, 50 mM, pH 7.0) con-
taining glucose (20 mmol), NADP+ (0.1 mmol), GDH (4 U)
and the required OYE (ca. 40 mg dissolved in 100–200 mL
H2O). The mixture was incubated for 24 h in an orbital
shaker (160 rpm, 308C). The solution was extracted with
EtOAc (2ꢂ250 mL), centrifuging after each extraction
Acknowledgements
Prof. Claudio Fuganti and Prof. Giovanni Fronza are
warmly acknowledged for fruitful discussions. Prof. Neil C.
Bruce (Department of Biology, University of York) is kindly
acknowledged for the gift of plasmid pT7-OYE1. Prof. Sven
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Elisabetta Brenna et al.
UPDATES
Panke and Christian Femmer (ETH Zꢀrich, Department of
Biosystems Science and Engineering, Basel) are kindly ac-
knowledged for the help provided in the preparation of the
plasmids and the overexpressing strains.
[7] M. Hall, C. Stueckler, B. Hauer, R. Stuermer, T. Frie-
drich, M. Breuer, W. Kroutil, K. Faber, Eur. J. Org.
Chem. 2008, 1511–1516.
[8] C. Stueckler, C. K. Winkler, M. Bonnekessel, K. Faber,
Adv. Synth. Catal. 2010, 352, 2663–2666; A. Z. Walton,
W. C. Conerly, Y. Pompeu, B. Sullivan, J. D. Stewart
ACS Catal. 2011, 1, 989–993; Y. A. Pompeu, B. Sullivan,
A. Z. Walton, J. D. Stewart, Adv. Synth. Catal. 2012,
354, 1949–1960; G. Tasnꢃdi, C. K. Winkler, D. Clay, N.
Sultana, W. M. F. Fabian, M. Hall, K. Ditrich, K. Faber,
Chem. Eur. J. 2012, DOI: 10.1002/chem.201200990.
[9] E. Brenna, G. Fronza, C. Fuganti, D. Monti, F. Parmeg-
giani, J. Mol. Catal. B: Enzymatic 2011, 73, 17–21; E.
Brenna, F. G. Gatti, A. Manfredi, D. Monti, F. Parmeg-
giani, Eur. J. Org. Chem. 2011, 4015–4022; E. Brenna,
F. G. Gatti, A. Manfredi, D. Monti, F. Parmeggiani,
Org. Process Res. Dev. 2012, 16, 262–268.
References
[1] K. Faber, Biotransformations in Organic Chemistry, 6th
edn., Chapter 2.2.4, pp 166–172, Springer Verlag,
Berlin, Heidelberg, 2011.
[2] S. Servi, Synthesis 1990, 1–25; R. Csuk, B. I. Glaenzer,
Chem. Rev. 1991, 91, 49–97; H. Toogood, J. M. Gardin-
er, N. S. Scrutton, ChemCatChem 2010, 2, 892–914;
C. K. Winkler, G. Tasnꢃdi, D. Clay, M. Hall, K. Faber,
J. Biotechnol. 2012, DOI: 10.1016/j.jbiotec.2012.03.023;
E. Brenna, C. Fuganti, F. G. Gatti, S. Serra, Chem. Rev.
2011, 111, 4036–4072.
[3] E. Brenna, F. G. Gatti, D. Monti, F. Parmeggiani, A.
Sacchetti, ChemCatChem 2012, 4, 653–659; E. Brenna,
F. G. Gatti, D. Monti, F. Parmeggiani, A. Sacchetti,
Chem. Commun. 2012, 48, 79–81; E. Brenna, G.
Fronza, C. Fuganti, F. G. Gatti, A. Manfredi, F. Par-
meggiani, P. Ronchi, J. Mol. Catal. B: Enzymatic 2012,
84, 94–101.
[4] C. Stueckler, C. K. Winkler, M. Bonnekessel, K. Faber,
Adv. Synth. Catal. 2010, 352, 2663–2666.
[5] C. Stueckler, C. K. Winkler, M. Hall, B. Hauer, M.
Bonnekessel, K. Zangger, K. Faber, Adv. Synth. Catal.,
2011, 353, 1169–1173.
[6] M. Utaka, S. Konishi, T. Ohkubo, S. Tsuboi, A. Takeda,
Tetrahedron Lett. 1987, 28, 1447; M. Utaka, S. Konishi,
A. Mizouka, T. Ohkubo, T. Sakai, S. Tsuboi, A.
Takeda, J. Org. Chem. 1989, 54, 4989–4992.
[10] M. Morita, L. Drouin, R. Motoki, Y. Rimura, I. Fuji-
mori, M. Kanai, M. Shibasaki, J. Am. Chem. Soc. 2009,
131, 3858–3859.
[11] R. M. Kohli, V. Massey, J. Biol. Chem. 1998, 273,
32763–32770.
[12] D. J. Bougioukou, J. D. Stewart, J. Am. Chem. Soc.
2008, 130, 7655–7658.
[13] a) S. K. Padhi, D. J. Bougioukou, J. D. Stewart, J. Am.
Chem. Soc. 2009, 131, 3271–3280; b) A. Mꢄller, B.
Hauer, B. Rosche, Biotechnol. Bioeng. 2007, 98, 22–29;
c) H. S. Toogood, A. Fryszkowska, V. Hare, K. Fisher,
A. Roujeinikova, D. Leys, J. M. Gardiner, G. M. Ste-
phens, N. S. Scrutton, Adv. Synth. Catal. 2008, 350,
2789–2803; d) E. Brenna, F. G. Gatti, D. Monti, F. Par-
meggiani, S. Serra, Adv. Synth. Catal. 2012, 354, 105–
112.
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