Stereoselective synthesis of novel benzoins catalysed by benzaldehyde lyase in a gel-stabilised two-phase system

Article abstract of DOI:10.1016/j.tet.2005.05.082

Asymmetric benzoin condensation was performed using recombinant benzaldehyde lyase (BAL) from Pseudomonas fluorescens Biovar I. To enable the conversion of hydrophobic substrates, the enzyme was entrapped in polyvinyl alcohol and suspended in hexane. Compared to the reported application of the biocatalyst in an aqueous phase containing 20% DMSO, the productivity of the resulting gel-stabilised two-phase system was 3-fold better. The entrapment process had an efficiency of >90%, no enzyme or cofactor was lost during reaction or storage. The entrapped enzyme was stable in hexane for 1 week at 4°C and more than 1 month at -20°C. Without preceding optimisation the novel benzoins (R)-1,2-di(3-furanyl)-2-hydroxyethanone, (R)-2-hydroxy-1,2-di(3- thienyl) ethanone, (R)-1,2-di(4-ethoxyphenyl)-2-hydroxyethanone, (R)-1,2-di(3-ethoxyphenyl)-2-hydroxyethanone, (R)-2-hydroxy-1,2-di(3-tolyl) ethanone, and (R)-1,2-di(benzofuran-2-yl)-2-hydroxyethanone were prepared with yields up to 31.8% and enantiomeric excess >99%.

Full text of DOI:10.1016/j.tet.2005.05.082

Tetrahedron 61 (2005) 7378–7383
Stereoselective synthesis of novel benzoins catalysed
by benzaldehyde lyase in a gel-stabilised two-phase system
a
Tanja Hischer, D o¨ rte Gocke, Mar ´ı a Fern a´ ndez, Pilar Hoyos, Andr e´ s R. Alc a´ ntara,
a
b
b
b
b
a
Jos e´ V. Sinisterra, Winfried Hartmeier and Marion B. Ansorge-Schumacher
a,
*
a
Lehrstuhl f u¨ r Biotechnologie, RWTH Aachen, D-52056 Aachen, Germany
b
Grupo de Biotransformaciones, Departamento de Qu ´ı mica Org a´ nica y Farmac e´ utica, Facultad de Farmacia,
Universidad Complutense, E-28040 Madrid, Spain
Received 7 February 2005; revised 11 May 2005; accepted 25 May 2005
Available online 15 June 2005
Abstract—Asymmetric benzoin condensation was performed using recombinant benzaldehyde lyase (BAL) from Pseudomonas fluorescens
Biovar I. To enable the conversion of hydrophobic substrates, the enzyme was entrapped in polyvinyl alcohol and suspended in hexane.
Compared to the reported application of the biocatalyst in an aqueous phase containing 20% DMSO, the productivity of the resulting gel-
stabilised two-phase system was 3-fold better. The entrapment process had an efficiency of O90%, no enzyme or cofactor was lost during
reaction or storage. The entrapped enzyme was stable in hexane for 1 week at 4 8C and more than 1 month at K20 8C. Without preceding
optimisation the novel benzoins (R)-1,2-di(3-furanyl)-2-hydroxyethanone, (R)-2-hydroxy-1,2-di(3-thienyl) ethanone, (R)-1,2-di(4-
ethoxyphenyl)-2-hydroxyethanone, (R)-1,2-di(3-ethoxyphenyl)-2-hydroxyethanone, (R)-2-hydroxy-1,2-di(3-tolyl)ethanone, and (R)-1,2-
di(benzofuran-2-yl)-2-hydroxyethanone were prepared with yields up to 31.8% and enantiomeric excess O99%.
q 2005 Elsevier Ltd. All rights reserved.
1
. Introduction
a drawback of the BAL-catalysed synthesis of benzoins so
far is the low solubility of many benzaldehyde substrates in
aqueous media. To a certain extent, this solubility can be
improved by the addition of 20% (v/v) of DMSO, but the
resulting reaction productivity is usually low.
a-Hydroxyketones are important building blocks for the
synthesis of several drugs and natural products.
this reason, many chemical methods for their synthesis are
described in literature.
,2-diaryl-2-hydroxyethanone of special interest, are
frequently derived by benzoin condensation, a traditional
1
–12
45
For
1
3–21
Benzoins, derivatives of
1
In this study, BAL-catalysed benzoin condensations were
performed using an alternative reaction system with two
immiscible phases of water and hexane. The enzyme was
entrapped in the aqueous phase, which was solidified by
22,23
C–C bond forming reaction in organic chemistry
that
2
4–28
uses either non-chiral catalysts
triazolium salts in a biomimetic manner.
ally, chiral benzoins can be obtained by reduction of
or chiral thiazolium and
2
9–32
48
Enzymatic-
polyvinyl alcohol to ease handling and recovery of BAL.
The organic solvent was simultaneously used as reservoir
for substrates and extracting agent for products. With the
achieved gel-stabilised two-phase system, the synthesis of a
couple of novel (R)-benzoins starting from substrates with
moderate to low solubility in water was performed. For
evaluation the synthesis of 2-furoin was compared to
3
3–36
a-diketones,
or kinetic resolution of racemic
3
-peroxo-, 2-hydroxy-
7
38–40
7,41–43
2
Also, an enzyme-catalysed benzoin condensation using the
or 2-acetoxyketones.
benzaldehyde lyase from Pseudomonas fluorescens Biovar I
BAL) was recently established.
become of profound interest because of the outstanding
4
4–46
(
This last reaction has
4
5
published data.
stereoselectivity of the biocatalyst, which yields (R)-
benzoins with 99% ee. Moreover, symmetrical as well
4
4
4
7
as unsymmetrical benzoin condensations starting from a
broad substrate range can be catalysed with BAL. However,
2. Results and discussion
2.1. Performance of the PVA-stabilised two-phase
system
Keywords: Benzoin condensation; Benzaldehyde lyase; Entrapment; Two-
phase system.
*
Corresponding author. Tel.: C49 241 8026620; fax: C49 241 8022387;
e-mail: m.ansorge@biotec.rwth-aachen.de
The benefit of using BAL for the conversion of hydrophobic
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.05.082

T. Hischer et al. / Tetrahedron 61 (2005) 7378–7383
7379
Table 1. BAL-catalysed syntheses of 2-furoin
BAL in
c
0
(mM)
c
t
(mM)
t (h)
Conv. (%)
Pd. (mgProduct/UBAL h)
a
b
PVA-stabilised two-phase system
Buffer/20% DMSO
48.4
30.0
30.1
3.6
17.0
62.0
15.6
88.0
0.3
c
0.1
a
Calculated according to formula 1.
Calculated according to formula 2.
Calculated from data published by Demir et al.
b
c
45
compounds in a gel-stabilised two-phase system was
evaluated by comparing the synthesis of 2-furoin from
significantly improved by repeated washing with hexane.
This finding indicates that residual product from the first
reaction cycle remains in the matrix, which has a negative
effect on the enzyme activity. Consequently, the perform-
ance of recycled immobilisates and even of the entire
process should be improved, if hexane was replaced by
another suitable solvent as extracting agent. Such optimis-
ation is currently in progress.
2
-furaldehyde to data published for the same reaction by
4
5
Demir et al. It was found that conversion of substrate after
7 h was 15.6% (calculated from 2-furaldehyde concen-
1
tration in the hexane phase according to formula 1), and a
productivity of 0.3 mg 2-furoin per unit of BAL and hour of
reaction was achieved (calculated according to formula 2).
This was three times higher than the productivity that was
4
5
calculated from published data for synthesis in water/
DMSO (Table 1). Evidently, the gel-stabilised two-phase
system reveals favourable features for continuous synthesis
of benzoins with BAL.
2
.2. Synthesis of novel benzoins
The PVA-stabilised BAL in hexane catalysed the asym-
metric formation of the novel benzoin derivatives (R)-1,2-
di(3-furanyl)-2-hydroxyethanone (2a), (R)-1,2-di(benzo-
furan-2-yl)-2-hydroxyethanone (2b), (R)-2-hydroxy-1,2-
di(3-thienyl)ethanone (2c), (R)-1,2-di(3-ethoxyphenyl)-2-
hydroxyethanone (2e), (R)-1,2-di(4-ethoxyphenyl)-2-
hydroxyethanone (2f), and (R)-2-hydroxy-1,2-di(3-tolyl)-
ethanone (2h) from the corresponding substituted aryl
aldehydes (Table 2). The productivities of these reactions
were at least comparable to the synthesis of 2-furoin. The
synthesis of 3-furoin from 2a ran with an even 4-fold higher
productivity. Obviously, the continuous supply of the
entrapped enzyme with fresh substrate from the surrounding
organic solvent countervailed the overall low concentrations
in the aqueous PVA-matrix itself. Limitations were found
for substrates with a log POW higher than 3, like
4-isopropylbenzaldehyde (1i). For this compound only a
very slight conversion within 72 h was observed in TLC
analysis. It should be possible, however, to improve the
conversion of such hydrophobic compounds by optimising
the phase composition of the gel-stabilised two-phase
system. In contrast, the also observed low or absent
conversion of the ortho-substituted substrates
2-tolualdehyde (1g) and 2-ethoxybenzaldehyde (1d) can
be attributed to steric hindrances of the respective
ꢀ
ꢁ
t
Hexane
0
Hexane
t
Hexane
C
C
C
VPVA
Conv: Z 1 K
C
(1)
0
PC
VHexane
Hexane
0
where C is the initial concentration of substrate in hexane
or PVA; V, volume of the appropriate phases; P, partition of
substrate between hexane and water. For the chosen
concentration range the partition coefficient was constant
(
P2-furaldehydeZ0.7), data not shown.
0
Hexane
C
VHexaneConv:
Pd: Z 0:5
MWBenzoin
(2)
AktBAL
t
0
where C is the initial concentration of substrate; V, volume;
Conv., conversion of substrate; MW, molecular weight of
reaction product; Akt., amount of employed enzyme; t,
reaction time.
Investigation of the immobilisates revealed an immobilis-
ation efficiency of 90.8%, no leaking of enzyme or cofactor
during storage or use in hexane, and a storage stability of
1
week at 4 8C or more than 1 month at K20 8C.
Immobilisates that were re-used after having completed a
production cycle showed considerably lower activity than
fresh immobilisates. Their activity, however, could be
Table 2. Benzoin condensation of hydrophobic aromatic aldehydes with entrapped BAL in n-hexane
a
b
P
c
Conv. (%)
d
e
(
R)-Benzoins
Ar
log POW
c
0
(mM)
c
t
(mM)
t (h)
Pd. (mgProduct
/
Y
Product (%)
ee (%)
2
U
BAL h)
2
2
2
2
2
2
2
2
2
a
b
c
d
e
f
g
h
i
3-Furanyl– 0.34
2-Benzofuranyl– 1.45
0.7
6.0
2.1
n.d.
18.5
18.5
n.d.
18.5
n.d.
42.8
50.0
39.2
50.0
27.1
50.7
50.0
50.0
50.0
7.3
20
31.25
49.75
72
31.25
50.25
72
76.9
33.3
48.3
—
29.7
25.6
—
1.3 (0.4)
0.6 (0.2)
0.3 (0.1)
—
0.3 (0.1)
0.3 (0.1)
—
16.4
6.6
6.9
—
26.0
17.7
—
31.8
—
94.7
74.7
O99
—
O99
O99
—
32.0
18.1
50.0
18.8
37.2
O48.0
30.5
O48.0
3-Thienyl–
1.71
1.99
1.99
1.99
2.27
2.27
3.02
2-EtOC
3-EtOC
4-EtOC
6
6
6
H
H
H
4
–
4
–
4
–
2-MeC
6
H
H
4
–
3-MeC
i
4- PrC
6
4
–
31.25
72
38.2
—
0.6 (0.2)
—
O99
—
6
H
4
–
n.d., not determined.
a
52
Calculated by Ghose and Crippen’s fragmentation methodology.
Calculated according to formula 3 at a volume ratio of hexane/PVA 4:1.
Calculated according to formula 1.
Calculated according to formula 2.
b
c
d
e
Determined by HPLC analysis as described in Section 3.

7380
T. Hischer et al. / Tetrahedron 61 (2005) 7378–7383
substituents in the active site of the biocatalyst, which is in
accordance to published data.
4
5
The BAL-catalysed synthesis of 2c, 2e, 2f and 2h ran
according to the known high selectivity of the biocatalyst
4
for production of (R)-configurated benzoins. In contrast, a
4
minor enantiomeric excess was achieved in the synthesis of
2
a and 2b. This decrease in enantiopurity, however, is not
Scheme 2.
the result of a worse selectivity of BAL towards the
corresponding aldehydes 1a and 1b, but rather to a
spontaneous racemisation of furoins. In fact, it has been
It should easily be possible to prevent both racemisation as
well as oxidation of the reaction products by appropriate
adaptation of the production process, for example, by
running a continuous synthesis under argon atmosphere
with direct downstream processing of the products. Thus,
the method described here for synthesis of benzoins with
BAL provides a promising system for the stereoselective
synthesis of benzoins overcoming solubility limitations.
The easy transfer to the synthesis of hydrophobic cross-
5
0
described that for compounds with a 3-furancarbonyl
moiety, the racemisation proceeds very quickly because of
the favourable resonance interaction of the carbonyl group
with the oxygen of the 3-furyl ring. Minor enantiopurity has
also been described before in the synthesis of 2-furoin
4
5
(
Scheme 1).
4
7
44–46
benzoins and a-hydroxy ketones
can be expected.
3. Experimental
Scheme 1.
3
.1. General methods
The racemisation of 2a during the production process is
demonstrated in Figure 1. While 2a was detected within
Enzymatic syntheses were performed in n-hexane (96%,
HPLC grade, Scharlau). NMR spectra were recorded on a
Bruker 500. Chemical shifts d are reported in ppm relative
1
(
2
.5 h after initiation of the reaction, the corresponding
S)-enantiomer was observed for the first time after about
4 h. Subsequently, the total amount of both enantiomers
increased only slightly, while the ratio between (R)- and
S)-enantiomer changed rapidly. After 94 h, the enantio-
1
13
to CHCl ( H: d 7.27) and CDCl ( C: d 77.0). Column
3
3
chromatography purifications were conducted on silica gel
60 (40–63 mm). TLC was carried out on aluminium sheets
precoated with silica gel 60F254 (Macherey-Nagel, Merck),
the spots were visualised with UV light (lZ254 nm).
Optical rotations were measured with a Perkin-Elmer 241
polarimeter. Elementary microanalyses were carried out in
the corresponding ‘Centro de Apoyo a la Investigaci o´ n’ of
(
meric excess of 2a had decreased to 72%, after a reaction
time of 200 h an enantiomeric excess of only 42.0% was
determined. Additional to racemisation the oxidation
product of 2a, 1,2-di(furan-3-yl)ethane-1,2-dione (4a,
4
5,51
w
Scheme 2)
was detected after about 3 h of reaction.
the Complutense University, Madrid, using a Leco CHNS
932 equipment.
An analogous side reaction occurred in the synthesis of 2g,
but none of the other benzoins was oxidised during BAL-
mediated carboligation. However, a complete oxidation of
HPLC analysis was performed with a chiral column
Chiralcel OD (cellulose carbamate, 25 cm!0.46 cm i.d.)
at room temperature using a Thermo/Separation Consta
2
b occurred at the attempt of concentrating the purified
product, which accounts for the missing analytical data of
this compound.
w
Metric 4100 Quaternary Solvent Delivery Systems, a
SpectroMonitor 5000 equipped with a Photo Diode Array
w
Detector and a Knauer Chiral Detector A1000, and a mobile
phase of n-hexane/2-propanol 90:10 at a flow rate of
0
.8 mL/min; alternatively the analysis was performed on a
chiral column Chiralpak AD-H (amylose tris(3,5-dimethyl-
phenylcarbamate), 25 cm!4.6 cm i.d.) at 20 8C using a
Beckman Coulter System Gold equipped with an auto-
sampler Triathlon and a Diode Array Detector Module 168,
and a mobile phase of n-hexane/2-propanol 90:10 at a flow
rate of 0.75 mL/min.
3
.2. Preparation of immobilised BAL
Expression and purification of BAL were performed using
the recombinant E. coli SG13009 [pBAL-His ] accord-
prep4
6
4
9
ing to Iding et al. One unit of activity is defined as the
amount of enzyme, which catalyses the cleavage of 1 mmol
benzoin per min at 30 8C and pH 8.0 in aqueous solution.
For the immobilisation in polyvinyl alcohol (PVA), a
mixture of 10% (w/v) PVA Mowiol 10-98 (degree of
Figure 1. Time course of the formation of (R)-3-furoin and side products
from 3-furaldehyde in a batch reaction containing 1 g of BAL-immobilisate
and 4 mL of hexane.

T. Hischer et al. / Tetrahedron 61 (2005) 7378–7383
7381
polymerisation: 1.400) and 10% (w/v) PEG 1000 was
dissolved in bi-distilled water at 90 8C, cooled down to
room temperature, reacted with 30 mg NaOH/g PVA and
stirred for 30 min. The pH was adjusted to 8.0 with
concentrated HCl, then 0.5 U/mL of BAL, 0.1 mmol/L of
thiamine diphosphate and 2.5 mmol/L of MgSO4 were
added. The solution with a final PVA-concentration of 8.9%
extracting the mixture with CHCl (3!50 mL). All the
3
extracts were combined, dried over Na SO and concen-
2
4
trated under low pressure. The resulting mixture was then
passed through a column of SiO using n-hexane/AcOEt 5:1
2
(v/v), as mobile phase to afford 4a (20 mg) and 3a (110 mg,
0.57 mmol) as a white crystalline solid. The analysis ( H
1
1
3
NMR and C NMR) of 3a were concordant with those
obtained for 2b.
(
w/v) was dropped into chilled silicon oil (K24 8C) and
converted to a hydrogel by slowly thawing to room
temperature. The obtained PVA-beads were washed with
and stored in hexane.
For synthesising 1,2-bis(4-ethoxyphenyl)-2-hydroxyetha-
none (3b), a mixture of thiamine hydrochloride (1 g,
3
3
4
mmol) and H O (3 mL), EtOH (12.5 mL) and NaOH
2
M
(2 mL) was placed in
a
50 mL flask.
-Ethoxybenzaldehyde (1g) 8.34 mL were added and the
3
.3. Determination of partition coefficients
reaction was carried out in Branson Sonifier 450 (Tunner 14,
Dutycycle 20, Output control 2). After 2 days, an extra
amount thiamine hydrochloride (1 g) and NaOH 3 M
The partition of substrates between the aqueous PVA phase
10 mL) and hexane (40 mL) was determined by measuring
the concentration of the respective compound in the solvent
before and after 48 h of equilibration. All beads were free of
enzyme. The partition coefficient (P) was calculated
according to Eq. 3.
(
(
(
2 mL) were added. The reaction was followed by TLC
n-hexane/AcOEt 7:3, (v/v); 4b, R Z0.525; 1g, R Z0.43;
f
f
3
b, R Z0.23). After 3 days, the reaction mixture was
f
extracted with n-hexane (8!25 mL); the extracts were
combined, dried over Na SO and concentrated under low
pressure. The resulting oil was dissolved in HCCl and
3
N
N
C
HexaneVPVA
C
Hexane
2
4
P Z À
Á
Z
(3)
0
Hexane
N
KC
N
C
VPVA
C
PVA
Hexane
passed through a column of SiO using the above mentioned
2
where C is the initial (0) or equilibrium (N) concentration
of the partitioning compound; V, volume of the appropriate
phases.
mobile phase to afford 4b, 1g and 3b, as well as some
amount of 4-ethoxybenzoic acid. All the fractions were
concentrated under low pressure, and added to 100 mL of
basic water, and 100 mL of HCCl3 were added to the
fractions containing 3b and 4-ethoxybenzoic acid. Finally,
the HCCl was dried over Na SO and concentrated, giving
3
.4. Enzymatic carboligation
3
2
4
3b (0.1965, 0.65 mmol) as a white crystalline solid, with
analysis ( H NMR and C NMR) concordant with those
carried out for 2g.
1
13
Ten grams of immobilisate were added to 40 mL hexane
containing the respective substrate. The two-phase system
was set under argon atmosphere and stirred on a rotary
shaker at 30 8C. Samples drawn from the organic solvent
were qualitatively analysed by TLC using n-hexane/ethyl
acetate 7:3 or petrol ether/AcOEt 4:1 as mobile phase.
Quantitative analysis was performed by HPLC. At the end
of conversion the immobilisates were filtered out, and
products were purified and analysed as described above.
3.5.1. (R)-1,2-Di(furan-3-yl)-2-hydroxyethanone (2a).
2
5
White solid, eeO97%, [a] K110.8 (c 0.0066, CHCl3);
D
1
H NMR (500 MHz, CDCl ): d: 4.09 (1H, d, JZ5.18 Hz,
3
OH), 5.48 (1H, d, JZ4.55 Hz, H-2), 6.24 (1H, 2d, JZ0.78,
0
0
0
0.73 Hz, H-4 ), 6.70 (1H, 2d, JZ0.79 Hz, H-4 ), 7.32 (1H, t,
0
0
0
JZ1.58 Hz, H-5 ), 7.36 (1H, t, JZ1.81 Hz, H-5 ), 7.47 (1H,
0
(500 MHz, CDCl
0
0
) d: 68.59 (C-2), 107.81 (C-4 ), 107.89
13
s, H-2 ), 7.91 (1H, dd, JZ0.44, 0.87 Hz, H-2 ); C NMR
0
0
3
.5. Chemical synthesis of racemic benzoins
3
0
0
00
0
(C-5 ), 143.17 (C-5 ), 147.59 (C-2 ), 192.19 (C-2). Anal.
00
(
C-4 ), 122.40 (C-3 ), 123.44 (C-3 ), 140.05 (C-2 ), 143.01
0
0
0
Racemic benzoins of 3-furancarbaldehyde (1b) and
-ethoxybenzaldehyde (1g), which were used as reference
4
Calcd for C10
4.26.
H
8
O
4
: C, 62.50; H, 4.20. Found: C, 62.06; H,
compounds for analysis of enantiomeric excess, were
synthesised using a variation of the classical chemical
2
9–32
0
thiamine-catalysed benzoin synthesis methodology,
depicted in Scheme 2.
as
3.5.2. (R)-2-Hydroxy 1,2-di(3-thienyl) ethanone ((R)-3,3 -
2
5
thenoin) (2c). Yellowish oil, eeO99%, [a]
0
K103.96 (c
.005, CHCl ); H NMR (500 MHz, CDCl ) d: 4.34 (1H, d,
D
1
3
3
For synthesising 1,2-di(furan-3-yl)-2-hydroxyethanone
3a), thiamine hydrochloride (1 g, 3 mmol) was dissolved
in 3 mL of distilled-water. In a further step, 12.5 mL of 95%
EtOH, 2 mL of NaOH 3 M, and 5 mL of 1b (60 mmol) were
added. The reaction was carried out in a Branson Sonifier
JZ6.03 Hz, OH), 5.84 (1H, d, JZ6.01 Hz, H-2), 6.99 (1H,
dd, JZ4.99, 1.21 Hz, H-4 ), 7.28 (1H, d, JZ2.83 Hz,
00
(
0
0
00
H-2 ), 7.29 (1H, dd, JZ2.91, 0.69 Hz, H-5 ), 7.33 (1H, dd,
0
JZ2.70, 1.09 Hz, H-5 ), 7.51 (1H, dd, JZ5.13, 1.17 Hz,
0
0
13
H-4 ), 8.04 (1H, dd, JZ2.84, 1.20 Hz, H-2 ); C NMR
00
(500 MHz, CDCl ) d: 72.42 (C-2), 124.21 (C-2 ), 124.24
3
4
50 (Tunner 14, Dutycycle 20, Output control 2) for 48 h,
0
0
00
0
0
and followed by TLC (hexane/ethyl acetate 5:1, v/v; 3a,
R Z0.35; 1b, R Z0.20; 4a, R Z0.06). After that time, the
(C-4 ), 126.57 (C-5 ), 127.06 (C-4 ), 127.25 (C-5 ), 134.21
(C-2 ), 192.39 (C-1). Anal. Calcd for C10
0
H, 3.59; S, 28.59. Found: C, 53.52; H, 3.92; S, 27.36.
H
O
S
2
: C, 53.55;
f
f
f
8
2
reaction mixture was cooled in an ice bath, and was
extracted by a continuous extraction system with n-hexane.
The extract was dried over Na SO and concentrated under
3.5.3. (R)-1,2-Bis(3-ethoxyphenyl)-2-hydroxyethanone
2
4
2
5
low pressure, giving crystals of 3a and 4a. Residual 3a
dissolved in the water was recovered by adding HCl and
(2e). Yellowish oil, eeO99%, [a] K128.2 (c 0.028,
D
1
CHCl3); H NMR (500 MHz, CDCl ) d: 1.36 (3H, t,
3

7382
T. Hischer et al. / Tetrahedron 61 (2005) 7378–7383
1
solid, H NMR (250 MHz, CDCl ) d (ppm): 8.04 (2H, d,
JZ6.03 Hz, CH ), 1.38 (3H, t, JZ6.03 Hz, CH ), 3.96 (2H,
3
3
3
0
0
0
0
0
0
m, CH ), 4.07 (2H, m, CH ), 4.59 (1H, d, JZ6.11 Hz, OH),
H-3 and H-3 ), 7.79 (2H, d, JZ12.22 Hz, H-4 and H-4 ),
2
2
0
0
1H, m, H-2 ), 6.90 (1H, m, H-6 ), 7.04 (1H, m, H-4 ), 7.21
0 00
7.66 (2H, dd, JZ8.5, 0.85 Hz, H-7 and H7 ), 7.57 (2H, m,
5
(
(
7
.86 (1H, d, JZ5.82 Hz, H-2), 6.78 (1H, m, H-4 ), 6.83
0
0
00
0
0 00 0
00 13
H-6 and H-6 ), 7.37 (2H, m, H-5 and H-5 ); C NMR
(250 MHz, CDCl ) d: 113.08 (C7 and C7 ), 121.52 (C3
00
0
.43 (1H, m, H-2 ), 7.46 (1H, m, H-6 ); C NMR
0
00
0
1H, t, JZ7.92 Hz, H-5 ), 7.26 (1H, t, JZ7.96 Hz, H-5 ),
3
0
500 MHz, CDCl ) d: 14.44 (C–CH ), 14.66 (C–CH ),
0
13
00 0 00 0 00
and C3 ), 124.67 (C6 and C6 ), 124.85 (C5 and C5 ),
0
0
00
0
00
(
127.38 (C3a and C3a ), 130.43 (C4 and C4 ), 149.45 (C2
3
3
3
0
00
Anal. Calcd for C H O : C, 74.48; H, 3.47. Found: C,
0
00
6
1
1
1
3.66 (C–CH ), 66.42 (C–CH ), 73.20 (C-2), 113.70 (C-2 ),
2
and C2 ), 157.05 (C7a and C7a ), 178.98 (both CZO).
2
0
0
00
21.60 (C-6 ), 129.62 (C-5 ), 130.12 (C-5 ), 134.71 (C-1 ),
0
00
00
0
13.87 (C-4 ), 114.66 (C-2 ), 120 (C-4 ), 120.93 (C-6 ),
0
1
8 10 4
0
0
0
00
40.41 (C-1 ), 159.06 (C-3 ), 159.43 (C-3 ), 198.71 (C-1).
74.49; H, 3.50.
0
Anal. Calcd for C H O : C, 71.98; H, 6.71. Found: C,
1.91; H, 6.65.
18 20 4
7
Acknowledgements
3
.5.4. (R)-1,2-Bis(4-ethoxyphenyl)-2-hydroxyethanone
We are grateful to DAAD (Bonn, Germany) and Ministerio
de Ciencia y Tecnolog ´ı a (Madrid, Spain) for financial
support within the ‘Acciones Integradas Hispano-
Alemanas’ and to DFG (Bonn, Germany) for financial
support within the SFB380 ‘Asymmetric syntheses with
chemical and biological methods’. We also thank Professor
M.-R. Kula for provision of the BAL-expression vector
2
5
(
2f). White solid, eeO99%, [a] K94.61 (c 0.010 g,
D
1
CH OH); H NMR (500 MHz, CDCl ) d: 1.36. (3H, t, JZ
3
3
7
6
6
Hz, CH ), 1.39 (3H, t, JZ7.01 Hz, CH ), 3.96 (2H, c, JZ
3
3
.99 Hz, CH ), 4.03 (2H, c, JZ7 Hz, CH ), 4.57 (1H, d, JZ
2
2
.02 Hz, OH), 5.83. (1H, d, JZ6.01 Hz, H-2), 6.81 (2H, m,
0
0
00
00
0
H-3 and H-5 ), 6.83 (2H, m, H-3 and H-5 ), 7.21 (1H, m,
0
0
H-2 ); C NMR (500 MHz, CDCl ) d: 14.58 (C–CH ),
00
H-6 ), 7.23 (1H, m, H-2 ), 7.86 (1H, m, H-6 ), 7.88 (1H, m,
‘pBAL-His6’.
0
13
3
3
1
4.78 (C–CH ), 63.40 (C–CH ), 63.79 (C–CH ), 75.23
3 2 2
0 0 00 00
(
C-2), 114.28 (C-3 and C-5 ), 114.98 (C-3 and C-5 ),
0 0 00 00
28.98 (C-2 and C-6 ), 131.56 (C-2 and C-4 ), 126.10
C-1 and C-1 ), 163.41 (C-4 ), 197.32 (C-1). Anal. Calcd
1
(
0
00
0
References and notes
for C H O : C, 71.98; H, 6.71. Found: C, 71.79; H, 6.70.
8 20 4
1
1
. Chandhary, A. G.; Kingston, D. G. I. Tetrahedron Lett. 1993,
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3
.5.5. (R)-2-Hydroxy-1,2-di-m-tolylethanone (2h). Yel-
2
5
1
lowish oil, eeO99%, [a]D K123.6 (c 0.00275, CHCl ); H
2. Uchida, R.; Shiomi, K.; Sunazuka, T.; Inokoshi, J.; Nishizawa,
A.; Hirose, T.; Tanaka, H.; Iwai, Y.; Omura, S. J. Antibiot.
1996, 49, 886.
3
NMR (500 MHz, CDCl ) d (ppm): 2.29 (3H, s, CH ), 2.34
(
3
3
3H, s, CH ), 4.53 (1H, d, JZ5.77 Hz, OH), 5.89 (1H, d, JZ
3
00
00
5
.46 Hz, H-2), 7.13 (1H, m, H-4 ), 7.13 (1H, m, H-2 ), 7.06
3. Roush, W. R.; Briner, K.; Kesler, B. S.; Murphy, M.; Gustin,
D. J. J. Org. Chem. 1996, 61, 6098.
0
0
00
0
1H, m, H-5 ), 7.68 (1H, m, H-6 ), 7.76 (1H, m, H-2 );
(
1H, m, H-6 ), 7.19 (1H, m, H-5 ), 7.25 (1H, m, H-4 ), 7.32
0
C NMR (500 MHz, CDCl ) d: 21.27 (C–CH ), 21.30
0
0
(
4. Du Bois, J.; Tomooka, C. S.; Hong, J.; Carreira, E. M. Acc.
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1
3
3
3
0
0
00
(
(
(
(
C–CH ), 76.15 (C-2), 124.93 (C-6 ), 126.43 (C-6 ), 128.23
3
5. Demir, A. S.; Hamamci, H.; Tanyeli, C.; M.Akhmedov, I.;
Doganel, F. Tetrahedron: Asymmetry 1998, 9, 1673.
6. Kawase, M.; Sakagami, H.; Kusama, K.; Motohashi, N.; Saito,
S. Bioorg. Med. Chem. Lett. 1999, 9, 3113.
0
0
C-4 ), 133.50 (C-1 ), 134.69 (C-2 ), 138.47 (C-1 ), 138.67
0
00
C-4 ), 128.46 (C-5 ), 128.93 (C-2 ), 129.32 (C-5 ), 129.57
0
00
C-3 ), 138.87 (C-3 ), 199.14 (C-1). Anal. Calcd for
00
0
0
00
C H O : C, 79.97; H, 6.71. Found: C, 79.86; H, 6.75.
16 16 2
7. Babudri, F.; Fiandanese, V.; Marchese, G.; Punzi, A.
Tetrahedron 1999, 55, 2431.
3.5.6. 1,2-Di(furan-3-yl)ethane-1,2-dione (4a). Yellow
solid, H NMR (500 MHz, CDCl ) d (ppm): 8.53 (2H, s,
8. Girijavallabhan, V. M.; Ganguly, A. K.; Pinto, P. A.; Sarre,
O. Z. BioMed. Chem. Lett. 1999, 1, 349.
1
3
H-1), 7.49 (2H, t, JZ1.60 Hz, H-5), 6.93 (2H, d, JZ
9. Adam, W.; Lazarus, M.; Saha-M o¨ ller, C. R.; Schreier, P. Acc.
Chem. Res. 1999, 32, 837.
1
3
0
.51 Hz, H-4); C NMR (500 MHz, CDCl ) d: 109.5 (C4 ),
3
1
0
0
0
123.35 (C3 ), 144.37 (C5 ), 152.30 (C2 ), 184.49 (CZO).
Anal. Calcd for C H O : C, 63.16; H, 3.18. Found: C,
10. Fang, Q. K.; Han, Z.; Grover, P.; Kessler, D.; Senanayake,
C. H.; Wald, S. A. Tetrahedron: Asymmetry 2000, 11,
3659–3663.
1
0 6 4
6
3.24; H, 3.21.
1
1. Wallace, O. B.; Smith, D. W.; Deshpande, M. S.; Polson, C.;
Felsenstein, K. M. Bioorg. Med. Chem. Lett. 2003, 13, 1203.
3.5.7. 1,2-Bis(4-ethoxyphenyl)ethane-1,2-dione (4b).
Yellow solid, H NMR (500 MHz, CDCl ) d (ppm): 7.95
1
3
12. Tanaka, T.; Kawase, M.; Tani, S. Bioorg. Med. Chem. Lett.
2004, 12, 501.
0 0 00 00
(
4H, d, JZ8.88 Hz, H-2 , H-6 , H-2 and H-6 ), 6.97 (4H, d,
0
0
00
00
JZ8.91 Hz, H-3 ,H-5 , H-3 and H-5 ), 4.14 (4H, c, JZ
6
13. Adam, W.; M u¨ ller, M.; Prechtl, F. J. Org. Chem. 1994, 59,
2358–2364.
1
3
.98 Hz, CH ), 1.47 (6H, d, JZ6.98 Hz, CH ); C NMR
2
3
(
(
(
500 MHz, CDCl ) d: 14.99 (CH ), 64.38 (CH ), 115.08
00
14. Adam, W.; Fell, R. T.; Stegmann, V. R.; Saha-M o¨ ller, C. R.
J. Am. Chem. Soc. 1998, 120, 708–714.
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3
3
2
0
0
0
00
00
0
C3 , C5 , C3 and C5 ), 126.56 (C1 and C1 ), 132. 77
00
0
00
00
0
C2 , C6 , C2 and C6 ), 164.68 (C4 and C4 ), 193.94 (CZ
O). Anal. Calcd for C H O : C, 72.47; H, 6.08. Found: C,
2.42; H, 6.11.
1
8 18 4
7
3
.5.8. 1,2-Di(benzofuran-2-yl)ethane-1,2-dione. Yellow

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