Asymmetric reduction of ketones by biocatalysis using clementine mandarin (Citrus reticulata) fruit grown in annaba or by ruthenium catalysis for access to both enantiomers

Article abstract of DOI:10.1002/chir.22413

Biocatalytic reduction of prochiral ketones using freshly ripened clementine mandarin (Citrus reticulata) in aqueous medium is reported. High enantioselectivities were observed, especially for the bioreduction of indanone 3, tetralone 4, and thiochromanone 5 with respectively 95%, 99%, and 86% enantiomeric excess (ee). Enantioselective bio- and metalcatalyzed reactions were compared. Chiral ruthenium catalysts afforded good asymmetric inductions (>75% ee) in most cases, enantiomeric excesses depending on the nature of substrate and ligand. N-aminoindanol prolinamide L³ was revealed as the best ligand for most ketones. Interestingly, for several substrates both enantiomers could be obtained using either Citrus reticulata or ruthenium complex.

Full text of DOI:10.1002/chir.22413

CHIRALITY (2014)
Asymmetric Reduction of Ketones by Biocatalysis Using Clementine
Mandarin (Citrus reticulata) Fruit Grown in Annaba or by Ruthenium
Catalysis for Access to Both Enantiomers
*
MANHEL BENNAMANE, SAOUSSEN ZEROR, AND LOUISA ARIBI-ZOUIOUECHE
Ecocompatible Asymmetric Catalysis Laboratory (LCAE), Badji Mokhtar Annaba-University, Annaba, Algeria
ABSTRACT
Biocatalytic reduction of prochiral ketones using freshly ripened clementine
mandarin (Citrus reticulata) in aqueous medium is reported. High enantioselectivities were ob-
served, especially for the bioreduction of indanone 3, tetralone 4, and thiochromanone 5 with
respectively 95%, 99%, and 86% enantiomeric excess (ee). Enantioselective bio- and metal-
catalyzed reactions were compared. Chiral ruthenium catalysts afforded good asymmetric induc-
tions (>75% ee) in most cases, enantiomeric excesses depending on the nature of substrate and
ligand. N-aminoindanol prolinamide L3 was revealed as the best ligand for most ketones. Inter-
estingly, for several substrates both enantiomers could be obtained using either Citrus reticulata
or ruthenium complex. Chirality 00:000–000, 2014. © 2014 Wiley Periodicals, Inc.
KEY WORDS: bioreduction; heteroaryl alcohols; Citrus reticulate; ruthenium; asymmetric
transfer hydrogenation
INTRODUCTION
The asymmetric transfer hydrogenation in water has been
widely developed.40–48 In this domain, we studied asymmetric
transfer hydrogenation (ATH) of aromatic ketones in water
using ruthenium-based catalysts.49 These catalysts could be
reused either with the same substrate or after changing the
ketone for each reduction without a loss of enantioselectivity.
A variety of ligands were compared in multisubstrate one-pot
reductions of aromatic and aliphatic ketones in order to opti-
mize the catalyst.50,51 In the course of our previous investiga-
tions, the comparison between enantioselective bio- and
metal-catalyzed reactions was conducted. We compared the
enantioselective reduction of ketoesters by Saccharomyces
cerevisiae- and ruthenium-catalyzed reactions in water. The
highest enantiomeric excess (ee) for the reduction of α-and
β-ketoesters was obtained with Saccharomyces cerevisiae.52 In-
terestingly, for several substrates both enantiomers of the
hydroxyesters were obtained according either to the catalytic
method or to the structure of the ligand.
The asymmetric reduction of heteroaryl ketones containing
furan, thiophene, chroman, and thiochroman moieties is one
of the most important reactions for producing chiral alcohols.
The potential use of such optically enriched alcohols building
blocks is vast, and includes, among others, pharmaceuticals
and synthetic intermediates in organic synthesis.1–7 The cata-
lysts for the asymmetric reduction of ketones can be classified
into two categories: chemical and biological methodologies.
Both catalysts have been developed as complementary tools
for the reduction of ketones. Although many biocatalytic reduc-
tions of ketone have been reported, baker’s yeast is the most
widely used microorganism for the chiral reduction of various
ketones, giving the corresponding optically active alcohols with
moderate to excellent enantioselectivities.8–16 Up to now, enan-
tioselective reduction using parts of fresh plants, vegetables,
and fruits as biocatalysts has emerged as an excellent alternative
to established asymmetric reduction methods.17–27 The easily
available plant, mild reaction conditions, and simple experimen-
tal setup have made it an area of interest for both industrial and
academic researchers. Moreover, these systems have the ad-
vantage of being environmentally friendly, since the reaction is
carried out in water as the solvent and the catalyst is
biodegradable. Recently, the asymmetric reduction of different
kinds of prochiral ketones to produce the corresponding chiral
alcohols have been realized with apple (Malus pumila), carrot
(Daucus carota), cucumber (Cucumis sativus), onion (Allium
cepa), potato (Solanum tuberosum), radish (Raphanus sativus),
tomato (Lypersicum esculentum), and sweet potato (Ipomoea
batatas) as biocatalysts. It was found that those prochiral ke-
tones could be reduced by these plants tissues with high
enantioselectivity.27–33
We now report our investigations into the catalytic asym-
metric reduction of ketones by clementine mandarin (Citrus
reticulata), a fruit grown in Annaba, Algeria. The results were
compared with the organometallic catalysis (catalysts based
on ruthenium). We chose the ligands which led to the more
enantioselective catalysts for the reduction of aromatic
ketones in our former investigations (Scheme 1).
MATERIALS AND METHODS
Nuclear magnetic resonance (NMR) spectra were performed with
Bruker spectrometers (Billerica, MA; 300 MHz for 1H, 75 MHz for 13C).
Chemical shifts are reported in δ ppm from tetramethylsilane with the
solvent resonance as internal standard for 1H NMR and chloroform-d
(δ 77.0 ppm) for 13C NMR. The ee’s were measured by gas chromatography
Asymmetric transfer hydrogenation is an alternative
method to perform reactions with high enantioselectivities
and has been widely employed for the preparation of chiral
alcohols.34–39 Nowadays, there is a great demand for the
development of catalytic systems that are not only efficient
and highly enantioselective but also environmentally friendly.
*Correspondence to: Louisa Aribi-Zouioueche, Ecocompatible Asymmetric
Catalysis Laboratory (LCAE). Badji Mokhtar Annaba-University.B.P 12,
23000, Annaba, Algeria. E-mail: louisa.zouioueche@univ-annaba.dz
Received for publication 17 September 2014; Accepted 23 October 2014
DOI: 10.1002/chir.22413
Published online in Wiley Online Library
(wileyonlinelibrary.com).
© 2014 Wiley Periodicals, Inc.

BENNAMANE ET AL.
Scheme 1. Catalytic asymmetric reduction of ketones.
(ThermoFinnigan Trace GC; Ringoes, NJ) equipped with an automatic
the literature data. The conditions for the analysis of all alcohols are
autosampler and using a CHIRALSIL-DEX CB column (25 m; 0.25 mm;
0.25 μm), or by a chiral stationary phase high-performance liquid chroma-
tography (HPLC). HPLC analyses were performed on a Thermo Separation
Product Pompe P100 with a UV detector and a chiral stationary-phase col-
umn Chiralcel OD-H and OJ-H. Retention times are reported in minutes.
All of the crude products were purified by preparative thin-layer chromatog-
raphy (TLC) on Silica Gel 60 PF254 (hexane/ethyl acetate 2/1).
Ketones were purchased from Sigma-Aldrich (St. Louis, MO). Fresh
fruit clementine mandarin (Citrus reticulata) was obtained from the An-
naba local market (fruit grown in Annaba). Ligands (L1, L2, L3, L4, and
L5) were synthesized in our laboratory according to the literature proce-
dures, and the structures were verified by 1H NMR and 13C NMR. The
catalyst precursor [RuCl2 (p-cymene)]2 and substrates were purchased
from Sigma-Aldrich.
reported below.
(R)-(+)-1-phenyl-1-ethanol 1a. GC Chiralcel Dex β-PM: tR =4.43 min,
tS =4.62 min (Tcolumn =140 °C). [α]2D0: +25 (c 0.485, MeOH ) for 63% ee,
{Li [α]D: -36.1 (c 2.96, MeOH) for 84% ee, (S)}.53
(S)-(+)-1-(4-chlorophenyl)-1-ethanol 2a. GC Chiralcel Dex β-PM:
tR =12.34 min, tS =13.69 min (Tcolumn =140 °C). [α]2D0: 28 (c 2.67, CHCl3)
for 60% ee, {Lit [α]D : 45 (c 0.9, CHCl3) for 96% ee, (S)}.54
(R)-(-)-1-indanol 3a. HPLC (Chiralcel OD-H, Isohexane/i-PrOH: 98/2,
flow 0.8 mL/min): tS =16.69 min, tR =19.28 min or HPLC (Chiralcel OD-H,
Isohexane/i-PrOH: 98/2, flow 0.5 mL/min): tS =18,90, min, tR =21,33 min.
[α]2D0: -26.4 (c 1.0, CHCl3) for 98% ee, {Lit [α]D : 29 (c 2.11, CHCl3) for
98% ee, (S)}.23
General Procedure for Enantioselective Reduction of
Ketones with Clementine Mandarin (Citrus reticulata)
as Biocatalyst
(S)-(+)-1,2,3,4-tetrahydronaphtalen-1-ol 4a. HPLC (Chiralcel OJ-H,
Isohexane/i-PrOH: 90/10, flow 0.5 mL/min): tR =14.05 min, tS =17.14 min)
or HPLC (Chiralcel OJ-H, Isohexane/i-PrOH: 90/10, flow 1 mL/min):
tR =10.78 min, tS =14.54 min. [α]2D0: +16 (c 1.5, MeOH) for 77% ee,
{Lit [α]D : +14.8 (c 3.23, MeOH) for 52% ee, (S)}.55
The ripe clementine fruit was washed with water, then disinfected with
ethanol and peeled with a sterilized cutter and the rest were cut carefully
into small slices. In an Erlenmeyer flask a suspension of the clementine
(100 g) in water (200 ml) was stirred at 30 °C for 20 min. Then the appro-
priate ketone (1 mmol) in DMF (1 ml) was added. After 48 h, the clemen-
tine pieces were then removed by filtration, washed several times with
ethyl acetate, and the filtrate was extracted with ethyl acetate, the organic
phase was dried over anhydrous MgSO4, and the solvent was evaporated
to obtain the product. After that, each reaction mixture was purified by
column chromatography using ethyl acetate and n-hexane as eluent to af-
ford the product and the yield was calculated using the conventional for-
mula of weighing the initial and final product.
(R)-(+)-thiochroman-4-ol 5a. HPLC (Chiralcel OJ-H, Isohexane/
i-PrOH: 90/10, flow 1.0 mL/min): tS =7.68 min, tR =9.55 min or HPLC
(Chiralcel OJ-H, Isohexane/i-PrOH: 90/10, flow 0.5 mL/min):
tS =23.12 min, tR =24.52 min. [α]2D0: 143 (c 1.0, CHCl3) for 98% ee,
{Lit [α]D : 141 (c 2.11, CHCl3) for >97% ee}.55
(R)-(+)-Chroman-4-ol 6a. HPLC (Chiralcel OJ-H, Isohexane/i-PrOH:
90/10, flow 1.0 mL/min): tS =6.35 min, tR =7.29 min or HPLC (Chiralcel
OJ-H, Isohexane/i-PrOH: 90/10, flow 0.5 mL/min): tS =19,65 min,
tR =24.67 min. [α]2D0: +55 (c 0.7, CHCl3) for 55% ee, {Lit [α]D : +68 (c 0.36,
CHCl3) for 64% ee}.56
General Procedure for Catalytic Reactions
In
a Schlenk tube, a solution of [RuCl2(p-cymene)]2 (15.5 mg,
0.025 mmol) and ligand (0.05 mmol) in 4 ml of water was stirred at 30 °
C for 1 h. Sodium formate (0.68 g, 10 mmol) and substrate (1 mmol) were
then added and the solution was maintained at 30 °C until total reduction
of the ketone, as monitored by TLC (reaction times reported in Table 1).
Organic products were then extracted with pentane (3 x10 ml) and dried
over MgSO4. After concentration, the product was purified by thin layer
chromatography with hexane/ethyl acetate mixtures and spectral data
compared with the literature.
(R)-(+)-1-thiophen-2-yl-ethanol
7a. HPLC
(Chiralcel
OD-H,
hexane/i-PrOH: 98/2, flow 0.5 mL/min): tR =15.23 min, tS =19.00 min or
HPLC (Chiralcel OJ-H, Isohexane/i-PrOH: 80/20, flow 0.5 mL/min):
tR =11.25 min, tS =13.13 min. [α]2D0: +9 (c 0.03, CHCl3) for 40% ee,
{Lit [α]D : +27 (c 2.5, CHCl3) for 96% ee, (R)}.57
(R)-(+)-1-furan-2-yl-ethanol 8a. GC [Chiralcel Dex β-PM, at 90 °C for
10 min (3 °C/min), heated to 141 °C for 10 min (20 °C/min), and main-
tained at 180 °C. tR =9.66, tS =10.53 or HPLC (Chiralcel OJ-H, hexane/
i-PrOH: 80/20, flow 0.5 mL/min): tR =16,63, tS =18,12. [α]2D0: +14 (c 0.02,
CHCl3) for 63% ee, {Lit [α]D : +21 (c 1.45, CHCl3) for 95% ee, (R)}.57
Chiral GC Analysis and/or Chiral HPLC Analysis
The absolute configurations of all chiral compounds (isolated after
chromatography) were determined by polarimetry in comparison with
Chirality DOI 10.1002/chir

BIOCATALYSIS USING CLEMENTINE MANDARIN
TABLE 1. Enantioselective reduction of aromatic ketones catalyzed by Citrus reticulata or by ruthenium complexes
Entry
Substrate
Catalysta
t(h)
Yieldb (%)
Eec (%)
Confd
1
2
3
4
5
6
7
8
Citrus reticulata
48
4
---
75
79
80
34
82
74
78
43
79
87
85
30
68
65
82
87
29
72
43
69
76
50
35
68
49
76
73
90
---
---
63e
84e
74e
23
60e
71e
87e
95
46e
81
88e
99
77e
90
94e
44
86
61e
64
98e
96
98
31
55e
87
67e
98
98
---
---
1
L1
(R)
(S)
(R)
(S)
(R)
(S)
(R)
(S)
(R)
(S)
(R)
(R)
(S)
(R)
(S)
(S)
(S)
(R)
(S)
(R)
(R)
(R)
(S)
(R)
(S)
(R)
(R)
(R)
---
L2
20
24
48
12
24
40
48
4
L3
Citrus reticulata
2
3
L1
L2
L3
9
Citrus reticulata
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
L1
L2
4
L3
20
48
4
22
20
22
48
4
22
20
20
19
48
3
24
20
20
72
48
2
18
20
20
22
48
2
18
20
22
24
4
5
Citrus reticulata
L1
L2
L3
L5
Citrus reticulata
L1
L2
L3
L4
L5
6
7
8
Citrus reticulata
L1
L2
L3
L4
L5
Citrus reticulata
L1
82
55
60
89
72
traces
63
50
60
92
80
11
40
83
45
43
---
42
63
55
69
75
(S)
(R)
(S)
(S)
(S)
---
(S)
(R)
(S)
(R)
(R)
L2
L3
L4
L5
Citrus reticulata
L1
L2
L3
L4
L5
aReactions were performed with 100 g Clementine and 200 ml water for 1 mmol substrate or with 2.5% [RuCl2(p-cymene)]2 and 5% ligand in water at 30 °C.
bIsolated yield.
cee determined by chiral GC or HPLC.
dAbsolute configuration of the major enantiomer was assigned by comparison with the literature.
eZeror et al. Adv Synth Catal 2008;350:197–204.
RESULTS AND DISCUSSION
and the cofactor, and there is a large amount of endogenous
reducing sugars in clementine mandarin. Bioreduction
catalyzed by the ketoreducatases existing in Citrus
reticulata was first optimized (reaction time, temperature,
and the amount of clementine) in mild conditions. The
mixture was stirred at 30 °C for 48 h, after which the
reaction yield was constant, indicating that the reaction
reached equilibrium. Clementine was able to reduce the
ketones in corresponding alcohols; the yields obtained
were 39% < rdt <53%.
The reduction of acetophenone 1 by Annaba local Citrus
reticulata did not occur. The effect of a substituent in the
aromatic moiety on the biotransformation was studied and
p-Chloro acetophenone 2 was reduced with clementine
mandarin. The corresponding alcohol was obtained with low
enantiomeric excess (ee =23%) and moderate yield (51%, entry 5).
Chirality DOI 10.1002/chir
With both aims of developing green chemistry methods
and of using the endemic flora, the study of the biocatalytic
reduction of ketones was performed on fresh ripened
clementine mandarin fruit purchased from a local market.
Clementine mandarin (Citrus reticulata) has never been in-
vestigated as a biocatalyst so far. In order to examine the
activity and enantioselectivity, we selected a variety of sub-
strates 1-8 including acetophenone derivatives and prochiral
heteroaryl ketones containing furan, thiophene, chroman,
and thiochroman moieties. The results are shown in Table 1.
It is known that whole cells have their own cofactor regen-
eration system that can be used without any additives or by
adding cosubstrates or reducing sugar.58 Thus, in this work
there is no external addition of the source of reducing power

BENNAMANE ET AL.
The presence of an electron-attracting group on the phenyl
asymmetric transfer hydrogenation (ATH) of prochiral ketones
using whole cells of the white-rot fungus Merulius tremellosus
ono991 as a biocatalyst and ruthenium(II)–amino alcohol and
iridium(I)– amino sulfide complexes as metal catalysts. The
results showed that the corresponding chiral alcohols are
obtained with moderate to high enantioselectivities. The bio-
and metal-catalyzed approaches appear to be complementary.
In our previous study, we found out that the configuration of
alcohol was governed by the catalytic method and the
enantiocomplementarity of bio and chemical systems was ob-
served. We thus decided to evaluate the ruthenium catalysts
for the enantioselective reduction of ketones in water in order
to compare them with the biocatalyzed reaction using local clem-
entine mandarin (Citrus reticulata). We have now investigated
the asymmetric reduction of acetophenone derivatives and
prochiral heteroaryl ketones using ruthenium complexes coordi-
nated with ligands L1-L5 which are represented in Scheme 1.
Reactions were performed using 5 mol% catalyst and furnished
total conversions into alcohol with reaction times depending
on the ligand. Reactions were faster with N-phenylprolinamide
L1 than with ligand L2. The ketones 1 and 2 were reduced
with good and similar enantioselectivities using N- phenyl-
prolinamide L1 while ligands L2 and L3 were more
enantioselective than ligand L1 (entries 2–4 and 6–8). Interest-
ingly, alcohols 1a and 2a provided by ruthenium complexes co-
ordinated with L1 or L3 have an (R)-configuration, while the
biocatalyst furnished the (S)-enantiomer for 2a (entries 2 or 4
and 5, 6, or 8). The reduction of substrate 3 with ruthenium cat-
alysts led to the corresponding alcohol with moderate to high ee,
whatever ligand was employed (entries 10–12). The highest ee
was recorded with L3 (ee =88%, entry 12), the configuration in
this case (R) being opposite to that given by Citrus reticulata.
The biocatalyst offered the best enantiomeric excess (ee =95%,
entries 9- 12). Tetralol 4a was isolated with ee’s from 44% to
94% using ruthenium complexes (entries 14–17) with the oppo-
site configuration (S) as that given by biocatalyst. With benzyl
prolinamide L5 slower reaction and low value for the ee’s of alco-
hol were observed (entry 17). Citrus reticulata was the most
enantioselective, with asymmetric induction slightly superior to
that given by the ligand L3. Ligands L3 and L5 afforded interest-
ing results for the reduction of thiochromanone 5 with respec-
tively superior values for ee’s than with biocatalyst (compare
entries 18, 21, and 23). The influence of another chirality center
was studied by using the ligand L4 for the reduction of 5. The
reduction afforded similar ee as with the benzyl amide L5. The
alcohol 5a was isolated with the opposite configuration to
that given by Citrus reticulata. The ruthenium-catalyzed
reduction of chromanone 6 afforded good asymmetric induc-
tions (ee =55-98%) in all cases (entries 25–29) with the five
ligands L1–L5. The corresponding alcohol formed in the reac-
tion catalyzed with ruthenium has a configuration opposite to
that given by Citrus reticulata except for ligand L2. However,
ligands L4 and L5 gave the best results for heteroaryl ketones
5 and 6. These chiral benzylic alcohols are used as synthon-
key in various syntheses of many drugs.62,63 The catalytic
system showed satisfying activity for the reduction of ketones
1–6 since a total conversion in alcohol was observed for all the
substrates. The heterocyclic ketones 7 and 8 including a five-
membered ring were obtained with moderate to good ee in all
cases (entries 31–35 and 37–41). For the alcohol 7a the highest
ee was recorded with N-aminoindanol prolinamide L3 compared
to other ligands and biocatalyst. However, ligand L5 gave
the best asymmetric induction for the ruthenium-catalyzed
ring led to an increase in the activity. The (S)-alcohol
configuration was obtained, which indicates that the reaction
follows Prelog’s rule. At present, most of these biocatalysts
follows Prelog’s rule and the (S)-alcohols are usually obtained
when the smaller substituent of the ketone has the lower CIP
priority.59 Only a few anti-Prelog (R)-configuration whole cell
biocatalysts have been reported.60 The reduction of indanone
3 catalyzed by Citrus reticulata furnished a product with high
enantiomeric excess (ee =95%, entry 9). As part of our interest
in the enantioselectivity synthesis of biologically active
precursors, we decided to realize our reduction with
Citrus reticulata for the asymmetric reduction of heteroaryl
compounds. Interestingly, excellent enantioselectivity was
observed for the reduction of tetralone 4 including a six-
membered ring (ee =99%, entry 13). The influence of the
heteroatom in the cycle was further investigated. The
reduction with thiochromanone
5 led to interesting
results, with slightly lower value of enantiomeric
a
excesses for the reduction of tetralone 4 (ee =86%, compare
entries 18 and 13). The chromanone 6 was reduced by Citrus
reticulate. Unfortunately, this alcohol gave lower enantiomeric
excesses than 4a and 5a (ee =31%, entries 24, 13, and 18).
The variation in the heteroatom in cycle has an influence on
the asymmetric induction. The presence of a sulfur atom in
the ring provides an improvement in enantioselectivity. The
alcohols formed by the reduction of 5 and 6 have the
same configuration, opposite to that given by the alcohol
4a. The reduction of 2-acetyl thiophene 7 and 2-acetyl furan
8 by local Citrus reticulata did not occur. The major
enantiomers 2a, 3a, 5a, and 6a were established as (S)-
configurations after comparison of their specific rotation
with our previous work, which is in agreement with the
Prelog model for bioreduction (Scheme 2). An anti-Prelog
selectivity is observed for tetralone 4; this is sometimes
described and depends on the steric and electronic nature
of the substrate.27 High enantioselectivities have been ob-
served for some of the substrates 3, 4, and 5, especially
for the reduction of heterocyclic ketones including a six-
membered ring (ee =86–99%).
The comparison between enantioselective bio- and metal-
catalyzed reactions has rarely been examined. The
bioreduction of ketones using Daucus carota L. has been
the focus of a review and the results compared with those
given by traditional reducing agents, mainly based on boron
or transition metals, complexed with chiral ligands.27 The
authors showed that chemical reducing agents perform very
well in terms of enantioselectivity. However, a lower ee is
generally obtained compared with the bioreduction process.
Additionally, Hage et al.61 described the comparison between
Scheme 2. Prelog’s rule for predicting stereospecificity. L (large) repre-
sents a bulky group, while S is small.
Chirality DOI 10.1002/chir

BIOCATALYSIS USING CLEMENTINE MANDARIN
reduction of 2-acetyl furane 8. In the case of heterocyclic
SUPPORTING INFORMATION
ketones including a six-membered ring, variation in the hetero-
atom in the cycle has an influence on the asymmetric induction.
Surprisingly, an increase in ee of the alcohol 1-furan-2-yl-ethanol
8a was observed compared to the 1-thiophen-2-yl-ethanol 7a for
reactions involving all the ligands except ligand L3.
Additional supporting information may be found in the
online version of this article at the publisher’s web-site.
LITERATURE CITED
These results show that the acetophenone derivatives catalyzed
by ruthenium complexes were reduced with good ee. For alcohol
1a the highest enantioselectivity was recorded with ligand L2,
however ligand L3 gave the best result for p-chloroacetophenone
2. For substrate 3, Citrus reticulate was the most enantioselective,
with asymmetric induction superior to that given by the
ruthenium complexes. The bioreduction of tetralone 4 and
thiochromanone 5 afforded the corresponding alcohols with re-
spectively 99% and 86% ee. However, ruthenium complexes
afforded higher asymmetric inductions than Citrus reticulata
(>75% ee) for the reduction of p-chloroacetophenone 2, 2-acetyl fu-
rane 8, and 2-acetyl thiophene 7. The best enantioselectivity was
afforded by different ligands according to the structure of the
substrate, aminoindanol L2 for acetophenone 1 and ligand L3
for p-chloroacetophenone 2, tetralone 4, thiochromanone 5 and
2-acetyl thiophene 7, ligand L5 for chromanone 6 and 2-acetyl fu-
rane 8. Interestingly, in some cases an enantiocomplementarity
for both catalytic systems was observed. Ruthenium complexes
coordinated with L1 and L3 led to the same configuration, while
the biocatalyst furnished the opposite one for alcohols 2a, 3a,
4a, 5a, and 6a. An enantiocomplementarity between ligand L2
and ligands L1 or L3- L5 was observed for isolated alcohol 7a.
The configuration of alcohol 8a obtained by reduction with (1R,
2S)-cis-aminoindanol L2 or ligands L4, L5 were opposite to the
configurations of alcohols formed in reactions involving ligands
L1 and L3. The main advantage of using asymmetric reducing
chemical agents is the possible access to the two enantiomers of
the chiral ligand by synthesis, thus allowing for straightforward
access to both enantiomers of the desired product.
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CONCLUSION
The results indicate that Citrus reticulata fruit can be used as
biocatalyst for the asymmetric reduction of prochiral heteroaryl
ketones. High enantioselectivities were observed for some
substrates especially for the bioreduction of indanone 3,
tetralone 4 and thiochromanone 5 (83–98% ee). The compara-
tive study of organometallic catalysis and biocatalysis with Citrus
reticulata demonstrates that both systems are effective for the
enantioselective reduction of prochiral ketones. The reactivity
and the enantioselectivity depend on the catalytic method, the
structure of ketone and ligand. Interestingly, in some cases an
enantiocomplementarity for both catalytic systems was ob-
served. Both enantiomers could be obtained using either Citrus
reticulata or ruthenium complex or using different ligands. This
biotechnological process is ecofriendly and cheap, the isolation
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ACKNOWLEDGMENTS
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We thank the Algerian Ministry of Education and Scientific
Research (FNR 2000 and PNR) for financial support of
this work. The authors thank Professor Olivier Riant
(MOST/INCM) Catholic University of Louvain, for
welcoming Manhel Bennamane to carry out chiral HPLC
and GC measurements.
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