Synthesis of β-adrenergic blockers (R)-(-)-nifenalol and (S)-(+)-sotalol via a highly efficient resolution of a bromohydrin precursor

Article abstract of DOI:10.1016/j.tetasy.2004.12.016

(R)- and (S)-2-bromo-1-(4-nitrophenyl)ethanol are precursors of important β-adrenergic receptor blocking drugs (R)-nifenalol and (S)-sotalol, respectively. Both were obtained in enantiomeric pure forms via a single highly efficient enzymatic transesterification reaction of (±)-2-bromo-1-(4- nitrophenyl)ethanol using immobilized lipase PS-C-II (E >1000; concn 200 g/L), while PS lipase completely failed to react. On the other hand, the hydrolytic method also produced enantiorich precursors though relatively less efficient (PS-C-II, E = 5.1). Out of all the approaches employed the transesterification method proved to be the most efficient.

Full text of DOI:10.1016/j.tetasy.2004.12.016

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 717–725
Synthesis of b-adrenergic blockers (R)-(ꢀ)-nifenalol
and (S)-(+)-sotalol via a highly efficient resolution of a
bromohydrin precursor
Munish Kapoor,a Naveen Anand,a Khursheed Ahmad,a Surrinder Koul,a
Swapandeep S. Chimni,b Subhash C. Tanejaa,* and Ghulam N. Qazia
aBiotechnology Division, Regional Research Laboratory, Jammu 180 001, India
bDepartment of Chemistry, Guru Nanak Dev University, Amritsar 143 005, India
Received 17 November 2004; accepted 24 December 2004
Abstract—(R)- and (S)-2-bromo-1-(4-nitrophenyl)ethanol are precursors of important b-adrenergic receptor blocking drugs (R)-nif-
enalol and (S)-sotalol, respectively. Both were obtained in enantiomeric pure forms via a single highly efficient enzymatic transeste-
rification reaction of ( )-2-bromo-1-(4-nitrophenyl)ethanol using immobilized lipase PS-C-II (E >1000; concn 200 g/L), while PS
lipase completely failed to react. On the other hand, the hydrolytic method also produced enantiorich precursors though relatively
less efficient (PS-C-II, E = 5.1). Out of all the approaches employed the transesterification method proved to be the most efficient.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Asymmetric synthesis of (R)-nifenalol, exploiting di-
verse approaches such as resolution of the racemates
through diastereoselective resolution,4 regioselective
aminolysis of chiral epoxides5 or treatment of the mono-
tosylate of a chiral 1,2-diol with an amine6 has been de-
scribed in the literature. There are also reports regarding
the preparation of (S)-sotalol involving racemate resolu-
tion using chiral mandelic acid,7 or through chiral
homogeneous hydrogenation,8 CBS reduction of aro-
matic ketone9 or Sharpless asymmetric dihydroxylation
of 4-nitrostyrene.10 Amongst biocatalytic methods there
is only one report of the preparation of (S)-sotalol
that relates to the use of microbial strain Geotrichium
sp. 702 in the stereoselective reduction of a keto
precursor.11
Drugs bearing a structured unit of 2-amino-1-aryletha-
nol such as nifenalol and sotalol are of great importance
as b-adrenergic blockers. They are also used in the ther-
apy of asthma, bronchitis and congestive heart failure.1
Among their enantiomers, only (R)-(ꢀ)-12 and (S)-(+)-
23 are b-adrenergic blockers and effective in the treatment
of cardiovascular diseases. Both 1 and 2 have a common
precursor, that is, 2-halo-1-(4-nitrophenyl)ethanol while
their final products have opposite configurations at C-
1. Therefore an efficient method of resolution of their
precursor should simultaneously lead to the preparation
of both the drugs in enantiomeric pure form.
Among the common and easily accessible halo-precur-
sors of these two drugs, 2-bromo-1-(4-nitrophenyl)etha-
nol appears to be the most suitable substrate due to its
higher stability, easy preparation and smooth conver-
sion to the final amino alcohols. Preparation of enantio-
merically enriched 2-bromo-1-(4-nitrophenyl)ethanol
was reportedly achieved through the chiral reduction
of phenacyl bromide, for example, borane-mediated
chiral reduction12 and cis-1-amino-2-indanol mediated
enantioselective borane reduction of 4-nitrophe-
nacyl bromide.13 Surprisingly there is no other report
regarding a biocatalytic method of preparation of
OH
OH
H
N
H
N
O2N
CH3SO2NH
(R)-1
(S)-2
*
Corresponding author. Tel.: +91 191 2572002; fax: +91 191
2548607; e-mail: sc_taneja@yahoo.co.in
0957-4166/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.12.016

718
M. Kapoor et al. / Tetrahedron: Asymmetry 16 (2005) 717–725
2-bromo-1-(4-nitrophenyl)ethanol except a US patent
that discloses resolution through transesterification14
using a panel of lipases with maximum reported enanti-
oselectivity of ꢁ95%.
2. Results and discussion
2.1. Biocatalytic reduction
The substrate for bioreduction, that is, 4-nitrophenacyl
bromide 4 was easily prepared in the overall yield of
70% by monobromination of readily available 4-nitro-
acetophenone 3 with bromine (Scheme 1).
In pursuance to our programme of kinetic resolution
studies of non-racemic drugs and their intermediates,15
we recently accomplished the resolution of bromo- and
iodohydrins of phenylpropanoates wherein out of 22
hydrolases (lipases/esterases) used only one lipase, that
is, Aspergillus niger was successful in hydrolyzing the
acylates and resolving the substrates. This non-accept-
ability of esters of bromo and iodohydrins displayed
by most of the hydrolases was probably due to the pres-
ence of bigger atoms such as bromine and iodine in the
molecules.16 However, it was expected that due to its rel-
atively smaller size, 2-bromo-1-(4-nitrophenyl)ethanol 5
would be more acceptable as a substrate to lipases/ester-
ases or other biocatalysts compared to 2-bromo-3-hy-
droxy phenylpropanoate.16
Several native strains of dehydrogenases (reductases)
were used for affecting the bioreduction of the interme-
diate 4 under anaerobic conditions. However, it is evi-
dent from the results (Table 1) that none of the
dehydrogenases was found to be capable of producing
2-bromo-1-(4-nitrophenyl)ethanol 5 in high enantio-
meric excess. In many of these experiments bioreduction
did not occur at all. In some experiments where bio-
reduction was observed the enantioselectivity was very
poor. Best results were obtained with Pichia capsulata
where (S)-enantiomer 5 with 70% enantiopurity was ob-
tained. S. cerevisiae on the other hand produced the (R)-
enantiomer of 67.8% purity.
One of our objectives has been to obtain both the pre-
cursors in enantiopure form via the simplest possible
route possibly by a single lipase/esterase catalyzed trans-
formation. Even though it is more likely to find only one
of the two isomers in enantiopure form during lipase/
esterase catalyzed hydrolysis or transesterification reac-
tions. It implies that it may be possible to obtain either
product or substrate in enantiopure form but at the cost
of the other, because in rare cases it is possible to
achieve a theoretical 50% conversion (hydrolysis or
transesterification) with absolute enantioselectivity.
The bioreduction method is another alternative though
it would require two different biocatalysts to separately
obtain each of the precursors.
After the failure of the bioreductive approach we re-
sorted to kinetic resolution of ( )-2-bromo-1-(4-nitro-
phenyl)ethanol 5, that is, the application of hydrolases
for the stereoselective hydrolysis or transesterification
(Scheme 2).
2.2. Lipase/esterase catalyzed hydrolysis
Experiments for the stereoselective hydrolysis of racemic
alkyl acyl esters 6a–c were carried out using a panel of
lipases/esterases comprising commercial as well as native
strains from instituteꢀs repository. Yet again all these
experiments failed to show any improvement of the
enantiopurity of hydrolyzed products as can be seen
from the data presented in Table 2. In terms of activity
and selectivity, while commercial enzyme CRL proved
to be only partially effective in hydrolyzing the butyl
ester 6c with ee ꢁ60% at 47% conversion; somewhat bet-
ter selectivity (ee ꢁ75%) was displayed for the acetate
by a native strain Arthrobacter sp. (RRL-1, MTCC-
5225), however it proved to be slow, effecting only 5%
hydrolysis after 18 h. In order to further improve the
Herein, we report the development of the best possible
method for preparation of (R)- and (S)-2-bromo-1-
(4-nitrophenyl)ethanol 5 of high enantiopurity via a
biocatalytic route taking advantage of three possible
strategies, that is, (i) biocatalytic reduction of 4-
nitrophenacyl bromide 4, (ii) kinetic resolution via
hydrolysis of acyl derivatives of ( )-2-bromo-1-(4-nitro-
phenyl)ethanol 6a–c and (iii) transesterification of ( )-5
using suitable lipases.
OH
Br
O2N
O
O
(R)-(-)-5
Br
Br2
Dehydrogenases
20oC
or
O2N
O2N
OH
Br
3
4
O2N
(S)-(+)-5
Scheme 1.

M. Kapoor et al. / Tetrahedron: Asymmetry 16 (2005) 717–725
Table 1. Bioreduction of 4-nitrophenacyl bromidea 4 using dehydrogenases
719
S.no.
Dehydrogenases
Time (in h)
Reduction (%)
Eeb (%)
Conf.c
Isolated yieldf (%)
1
2
3
4
5
6
7
8
9
Bacillus pseudomegatherium
B. subtilis
48
24
24
24
48
24
48
24
48
n.r.d
n.r.d
41
—
—
—
—
—
—
57
70
—
72
—
65
—
Mortierella sp.
P. capsulata
52.6
70.0
—
(R)
(S)
—
100
n.r.d
90
n.r.d
94
n.r.d
P. farinosa
P. pseudopastoris
RRL-155e
S. cerevisiae
45.0
—
(S)
—
67.8
—
(R)
—
Zygosaccharomyces rouxii
a Substrate concn 1 g/L.
b Ee was determined by Chiral HPLC.
c Configuration was determined by the sign of specific rotation compared to that in the literature.12
d n.r.—no reduction.
e Unidentified strain.
f Isolated yield (after column chromatography) based on conversions.
O
Br
O2N
4
00C
NaBH4/ MeOH
OCOR
Br
OH
Br
OH
OCOR
Br
Br
Hydrolase,
Buffer
(RCO)2O
Base
+
O2N
O2N
O2N
O2N
(+)-6
(+)-5
a R=CH3
b R=C2H5
c R=C3H7
(S)-(+)-5
(R)-(-)-6
Vinyl acetate,
Enzyme, Solvent
RRLY-15,
Buffer
OCOCH3
OH
OH
OCOR
Br
Br
Br
Br
+
+
O2N
O2N
O2N
O2N
(R)-(-)-5
(S)-(+)-6
(S)-(+)-6a
(R)-(-)-5
Scheme 2.
selectivity of CRL with 6c, the use of co-solvents was
also envisaged. Our efforts in this direction were only
partially successful as the enantiopurity of the hydro-
lyzed product could not be improved beyond 87% in iso-
propyl alcohol saturated with water as a solvent (Table
3).
lipases were able to exhibit transesterification and the
best results (ee >99.5%, conv. 42%, 48 h) were observed
when PS-C-II was used as a catalyst with vinyl acetate as
an acetyl donor and solvent (Table 4).
In order to further reduce the reaction time and maxi-
mize the efficacy of the transesterification by PS-C-II,
we studied these reactions in various organic solvents
(Table 5). Most of the solvents improved the reaction
rate and toluene was found to be the best, facilitating
maximum resolution of enantiomers in a single resolu-
tion (Scheme 3). The rate of esterification also improved
significantly and almost complete conversion was recor-
ded in 28 h. Effect of substrate concentration on the rate
of transesterification was also studied with lipase PS-C-
II in the same solvent. The optimum reaction rate was
2.3. Lipase/esterase catalyzed transesterification
Finally, we followed the transesterification approach in
our efforts to obtain both the enantiomers of 2-bromo-
1-(4-nitrophenyl)ethanol in enantiomerically pure form.
In these experiments transesterification of the substrate
was attempted again with a panel of biocatalysts com-
prising of both commercial as well as lyophilized strains
from the instituteꢀs repository (Scheme 2). Only three

720
M. Kapoor et al. / Tetrahedron: Asymmetry 16 (2005) 717–725
Table 2. Lipase/esterase catalyzed kinetic resolution of acyl derivatives of ( )-2-bromo-1-(4-nitrophenyl)ethanol 6a–c
Enzyme
Substratea
Time (h)
Conv.b (%)
Eec (%) (P)
Conf.d (P)
Isolated yielde
(%)
Ef
S
P
CRL
6a
6b
6c
12
12
4
0
48
47
—
58
60
—
S
—
34
32
Nil
27
—
6.3
6.7
S
25
PLAP
6a
6b
6c
6
6
10
49
40
15
36
33
S
S
S
53
30
35
4
20
17
1.3
2.9
2.4
12
Lipase PS
6a
6b
6c
6
6
6
0
30
9
—
11
65
—
S
—
44
68
Nil
16
6
—
1.3
5
S
RRLBB-1
6a
6b
6c
48
48
24
5
33
43
7
0
S
S
S
ndg
47
nd
15
21
1.1
0
48
36
4
RRL-1 (MTCC 5225)
RRLY-15 (DSM 11829)
RRL-1789
6a
6b
6c
18
18
18
5
56
43
75
39
53
S
S
S
nd
27
38
nd
28
21
7.2
3.6
4.7
6a
6b
6c
48
48
48
6
37
20
54
46
56
R
R
R
nd
45
58
nd
20
11
3.2
3.4
3.4
6a
6b
6c
12
12
12
4
12
24
16
29
46
S
S
S
nd
62
54
nd
1.5
1.9
3.1
7.2
14
PS-C-II
6a
6b
6c
12
12
12
0
36
33
—
58
58
—
S
—
42
46
Nil
23
—
5.1
4.9
S
21
S. cerevisiae
6a
6b
6c
12
12
12
0
6
3
—
20
25
—
S
—
nd
nd
Nil
nd
nd
—
1.9
1.8
S
CRL = Candida rugosa lipase (Sigma); PLAP = pig liver acetone powder (prepared by known method from freshly procured pig liver);18 lipase
PS = Burkholderia cepacia (Amano); RRLBB-1 = B. subtilis; RRL-1 = Arthrobacter sp.; RRLY-15 = Trichosporon beigilie; RRL-1789 = Bacillus sp.;
lipase PS-C-II = B. cepacia immobilized on ceramics (Amano).
a Substrate:enzyme ratio 1:1 (w/w). All reaction was performed at pH 7, 0.1 M sodium phosphate buffer at concn 40 g/L.
b Conversion based on HPLC.
c Ee determined by Chiral HPLC.
d Configuration determined by the sign of specific rotation compared to the literature.12
e Isolated yield (after column chromatography) based on conversions.
f Calculated according to Chen et al.17
g Yields for conversions <10% not determined (nd).
recorded at 200 g/L whereas at higher concentrations
lower rate of transesterification was observed probably
due to low substrate solubility.
such as bioreduction or stereoselective hydrolysis using
lipases were not that effective in achieving the desired
enantiomeric excess.
A comparison of three different approaches for the prep-
aration of the enantiomers of 2-bromo-1-(4-nitro-
phenyl)ethanol has been summarized in Figure 1.
4. Experimental
4.1. General
3. Conclusion
1H and 13C NMR were recorded on a Bruker (200 and
50 MHz) spectrometer in CDCl3 and TMS as internal
standard. IR was recorded on a FT-IR Bruker (270-
30) spectrophotometer. Mass spectra were recorded on
JEOL MSD-300 mass spectrometer. Optical rotations
were measured on Perkin–Elmer 241 polarimeter at
25 ꢁC using sodium D light. Enantiomeric excess (ee)
was determined on a chiral stationary phase HPLC col-
umn. Melting points were determined on Buchi B-542
apparatus by open capillary method and are uncor-
Highly efficient resolution of racemic 2-bromo-1-(4-
nitrophenyl)ethanol, the precursors of two important
b-adrenergic receptor blocking drugs (R)-nifenalol and
(S)-sotalol has been achieved in high enantiomeric ex-
cess (ee >99.5%; E >1000) by transesterification using
immobilized enzyme PS-C-II (Amano) at 200 g/L opti-
mum concentration. Hydrolysis using the same enzyme
showed comparatively poor selectivity. Other methods

M. Kapoor et al. / Tetrahedron: Asymmetry 16 (2005) 717–725
721
Table 3. Effect of co-solvent on the rate of hydrolysis as well as ee% of the product (alcohol) using ( )-6c as substrate and CRL as biocatalyst
Co-solvent (10%)
Time (h)
Conv.a (%)
Eeb (%) (P)
Conf.c (P)
Isolated yieldd
(%)
Ee
S
P
Acetone
Acetonitrile
Benzene
4
4
47
30
30
17
13
38
39
29
24
10
2
16
24
82
65
68
75
78
84
49
87
73
72
S
S
S
S
S
S
S
S
S
S
S
S
36
48
42
59
67
40
41
44
42
66
ndf
48
24
17
17
10
10
22
21
22
10
5
1.5
1.7
14.2
5.3
4
Di-n-butyl ether
Diethyl ether
DIPE
2
2
5.7
4
10.9
13.2
16
DIPE (saturated with water)
DME
Hexane
16
2
4
3.3
IPA (saturated with water)
THF
Toluene
20
2
15.8
7.6
nd
17
4
32
8.5
DIPE—diisopropylether; IPA—isopropyl alcohol; THF—tetrahydrofuran; DME—1,2-dimethoxyethane.
a Conversion based on HPLC.
b Ee of the product (alcohol) determined by Chiral HPLC.
c Configuration determined by the sign of specific rotation compared to the literature.12
d Isolated yield (after column chromatography) based on conversions.
e Calculated according to Chen et al.17
f Yields for conversions <10% not determined (nd).
OH
OH
OCOCH3
Br
Br
Br
PS-C-II, Toluene
Vinyl Acetate,
Conv. 50%
+
O2N
O2N
O2N
(S)-(+)-6a
ee > 99.5%
(R)-(-)-5
ee > 99.5%
(+)-5
(R)-(-)-Nifenalol, 1
(S)-(+)-Sotalol, 2
Scheme 3.
Table 4. Lipase catalyzed transesterification of ( )-2-bromo-1-(4-nitrophenyl)ethanola using vinyl acetate as solvent
S.no.
Enzyme
Time (days)
Conv.b (%)
Eec
Conf.d (P)
Isolated
yielde (%)
Ef
S
P
1
Lipase PS
CRL
PPL
9
9
9
9
9
9
9
9
9
9
9
2
—
13
—
—
—
25
—
—
—
—
—
42
—
79.6
—
—
S
—
60
—
—
—
48
—
—
—
—
—
43
—
11
—
—
—
20
—
—
—
—
—
35
—
9.8
—
—
—
8.9
—
—
—
—
—
865
2
3
—
—
—
S
4
Mucor meihei
Pseudomonas fluorescens
CAL
—
—
5
6
75
—
7
CCL
—
—
—
—
—
S
8
RRL-1
—
—
9
RRL-191g
NTM-105h
RCCLh
10
11
12
—
—
PS-C-II
>99.5
a Concn of the substrate 40 g/L.
b Conversion based on HPLC.
c Ee was determined by Chiral HPLC.
d Configuration was determined by the sign of specific rotation with that in the literature.12
e Isolated yield (after column chromatography) based on conversions.
f Calculated according to Chen et al.17
g RRL-191 = B. pumilus.
h Unidentified strain.

722
M. Kapoor et al. / Tetrahedron: Asymmetry 16 (2005) 717–725
Table 5. Effect of solvent on the rate of transesterification of ( )-2-bromo-1-(4-nitrophenyl)ethanola using PS-C-II
S.no.
Solventb
Conv.c (%) (28 h)
Conf. (P)d
Isolated yielde (%)
Ef
S
P
1
Acetonitrile
Benzene
DCM
6
4
S
ndg
nd
63
nd
61
52
54
57
—
—
nd
57
nd
nd
10
nd
8
425
415
461
411
445
533
595
501
—
2
S
3
13
3
S
4
Dioxane
DME
DIPE
S
5
10
23
29
19
0
S
6
S
20
25
16
Nil
Nil
nd
42
7
Diethyl ether
Heptane
Hexane
S
8
S
9
—
—
S
10
11
12
Methyl cellosolve
THF
Toluene
0
—
420
>1000
5
50
S
Ee (%) of the product always remains >99.5% in every case. DME—1,2-dimethoxyethane; DCM—dichloromethane; DIPE—diisopropylether;
THF—tetrahydrofuran.
a Concn of the substrate 40 g/L.
b Vinyl acetate as acylating agent.
c Conversion based on HPLC.
d Configuration determined by the sign of specific rotation with that in the literature.12
e Isolated yield (after column chromatography) based on conversions.
f Calculated according to Chen et al.17
g Yields for conversions <10% not determined (nd).
d 4.5 (2H, s, CH2), 8.4 (4H, m, Ar-H); 13C NMR
Comparison of enatiopurity
(50 MHz): d 29.3, 123.0, 130.1, 135.8, 138.6, 184.7; MS
(m/z) (%): 245 (11.2), 243 (8.4), 150 (100), 149 (99),
120 (98), 104 (99.2), 92 (87), 89 (73.6), 81 (9.5), 79
(10.6), 76 (100), 74 (34.4); Anal. Calcd for C8H6BrNO3:
C, 39.37; H, 2.48; N, 5.74. Found: C, 39.21; H, 2.47; N,
5.71.
PS-C-II
100
75
50
25
0
CRL
P.capsulata
4.3. Typical method for the biocatalytic reduction of 4-
nitrophenacyl bromide
Bioreduction
Hydrolysis
Transesterification
In a typical experiment, ethanolic solution of compound
4 (50 mg, 0.19 mmol in 0.3 mL) was added to a suspen-
sion of P. capsulata (5 g wet pellet) in 50 mL distilled
water containing glucose (2.5 g) and the contents were
shaken at 30 ꢁC. After complete reduction as monitored
by TLC, the contents were centrifuged and the superna-
tant liquid and cell pellet was extracted separately with
solvent ether (3 · 25 mL each). The combined organic
layer was washed with water, dried over anhydrous so-
dium sulfate and concentrated to furnish a crude prod-
uct that was purified on a silica gel column using ethyl
acetate–hexane (10:90) as eluent affording (S)-(+)-2-bro-
ee(%) of the ester
ee(%) of alcohol
Figure 1.
rected. Lipase PS-C-II was a gift from Amano Enzyme
Inc. Co. Japan.
4.2. Synthesis of the 4-nitrophenacyl bromide 4
Bromine 9.6 g (3 mL, 60 mmol) was slowly added over a
period of 15 min from a dropping funnel to a stirred
solution of 4-nitroacetophenone (10 g, 60 mmol) in gla-
cial acetic acid (40 mL) in a round bottomed flask and
temperature of the reaction mixture maintained below
20 ꢁC. After addition of bromine, the contents were
allowed to stir overnight. The reaction mixture was
diluted by adding 50 mL ice-water and extracted with
ethyl acetate (3 · 75 mL). The organic layer was washed
with brine, dried over anhydrous sodium sulfate and
concentrated under reduced pressure to give a crude
product that was purified by column chromatography
over silica gel (10.3 g, 42 mmol, 70%); mp 89 ꢁC. IR
(KBr): 3105, 3047, 2937, 1701, 1600, 1529, 1516, 1343,
mo-1-(4-nitrophenyl)-ethanol
ꢁ70%), ee 70% (by chiral HPLC).
5 (35 mg, 0.14 mmol,
4.4. Synthesis of ( )-2-bromo-1-(4-nitrophenyl)ethanol 5
Sodium borohydride (550 mg, 14.5 mmol) was added in
small installments to a cooled and stirred solution of 4-
nitrophenacyl bromide (10.0 g, 41 mmol) in dry metha-
nol (60 mL). The contents were further stirred for 2 h till
the reaction was completed (TLC monitored). After
completion of the reaction methanol was evaporated
and the resulting residue was diluted with water
(50 mL), extracted with diethyl ether (4 · 75 mL). The
separated organic phase was washed with brine, dried
over anhydrous sodium sulfate and concentrated under
1
1306, 1192, 994, 854, 714 cmꢀ1; H NMR (200 MHz):

M. Kapoor et al. / Tetrahedron: Asymmetry 16 (2005) 717–725
723
reduced pressure to yield ( )-5 as a white solid (9 g,
36.5 mmol, 90%); mp 96.5 ꢁC; IR (KBr): 3549, 1515,
CH2Br), 7.55 (2H, d, J = 8.7 Hz, Ar-H), 8.25 (2H, d,
J = 8.7 Hz, Ar-H); 13C NMR (50 MHz): d 13.6, 18.2,
33.5, 36.04, 73.55, 123.8, 127.3, 144.7, 148.0, 172.2;
MS (m/z) (%): 316 (5.6), 314 (4.9), 229 (9.6), 148
(23.3), 119 (15.9), 102 (18.7), 89 (18.1), 71 (100), 57
(51.6); Anal. Calcd for C12H14BrNO4: C, 45.59; H,
4.46; N, 4.43. Found: C, 45.45; H, 4.45; N, 4.39.
1
1344, 1068, 850, 707 cmꢀ1; H NMR (200 MHz): d 3.0
(1H, br s, OH), 3.66 (2H, m, CH2–Br), 5.04 (1H, dd,
J = 3.3, 3.3 Hz, CH2–CHOH), 7.59 (2H, d, J = 8.5 Hz,
Ar-H), 8.25 (2H, d, J = 8.6 Hz, Ar-H); 13C NMR
(50 MHz): d 40.6, 73.9, 125.1, 158.2, 148.6, 149.0; MS
(m/z) (%): 247 (2.9), 245 (4.3), 152 (100), 122 (34.1), 94
(48.5), 77 (45.7), 63 (23.7); Anal. Calcd for C8H8BrNO3:
C, 39.05; H, 3.28; N, 5.69. Found: C, 39.03; H, 3.28; N,
5.69.
4.6. General procedure for lipase/esterase catalyzed
hydrolysis of acyl derivatives of ( )-2-bromo-1-(4-nitro-
phenyl)ethanol 6
4.5. General procedure for the preparation of acyl
derivatives of ( )-2-bromo-1-(4-nitrophenyl)ethanol 6a–c
A suspension of substrate ( )-6 (100 mg, 0.35 mmol of
6a, 0.33 mmol of 6b, 0.32 mmol of 6c), crude enzyme
powder (100 mg) and sodium phosphate buffer (0.1 M,
pH 7, 2.5 mL) was stirred at 28 1 ꢁC, maintaining
the pH 7 by addition of 0.1 M NaOH solution. It was
essential to monitor the course of reaction by chiral
HPLC. After a certain degree of conversion, the reaction
was terminated by centrifugation of reaction mixture at
10,000g. The clear solution and the centrifuged cell
pellet were extracted separately with ethyl acetate
(3 · 10 mL). The organic layers were combined and
washed with water, dried and the organic layer concen-
trated under reduced pressure. The resulting mixture
consisting of alcohol and ester, was separated by column
chromatography over silica gel to furnish enantiorich 5
and 6 (isolated yields 4–68%, Table 2).
A
solution of ( )-2-bromo-1-(4-nitrophenyl)ethanol
(2.46 g, 10 mmol), alkanoic anhydride (12 mmol) and
catalytic amount of 4-(dimethylamino)pyridine
(DMAP) in dry dichloromethane (20 mL) was stirred
overnight at room temperature. After completion of
the reaction (monitored by TLC), the contents were
poured into ice-cold water and extracted with dichlo-
romethane (3 · 50 mL). The organic layer was washed
with brine, dried and evaporated at reduced pressure,
followed by purification on silica gel column to furnish
the product 6a–c in 90–95% yield.
4.5.1. 1-Acetoxy-2-bromo-1-(4-nitrophenyl)ethane 6a.
A creamish solid; mp 118.5 ꢁC; IR (KBr): 1748, 1520,
1348, 1233, 1059 cmꢀ1
;
1H NMR (200 MHz): d 2.23
4.7. General procedure for lipase/esterase catalyzed
hydrolysis of ( )-1-butanoyloxy-2-bromo-1-(4-nitro-
phenyl)ethane 6c in the presence of co-solvents
(3H, s, COCH3), 3.70 (2H, d, J = 6.0 Hz, –CH2Br),
5.16 (1H, t, J = 6.0 Hz, CHOCOCH3), 7.66 (2H, d,
J = 8.5 Hz, Ar-H), 8.41 (2H, d, J = 8.5 Hz, Ar-H); 13C
NMR (50 MHz): d 20.8, 33.4, 73.6, 123.9, 127.6, 144.5,
148.0, 169.5; MS (m/z) (%): 288 (8.3), 286 (9.3), 243
(2.5), 241 (1.8), 226 (50.1), 205 (37.9), 193 (38.5), 178
(22.3), 164 (41.8), 151 (23.5), 147 (100), 102 (100), 91
(86.2), 77 (78); Anal. Calcd for C10H10BrNO4: C,
41.69; H, 3.50; N, 4.86. Found: C, 41.60; H, 3.48; N,
4.81.
A suspension comprising of substrate ( )-6c (100 mg,
0.32 mol), crude enzyme powder (100 mg), sodium phos-
phate buffer (0.1 M, pH 7, 2.25 mL) and solvents (as
indicated in Table 3) was stirred at 28 1 ꢁC, maintain-
ing pH 7 by addition of 0.1 M NaOH solution. It was
essential to monitor the course of reaction by chiral
HPLC. After a certain degree of conversion, the reaction
was terminated by centrifugation of reaction mixture at
10,000g. The clear solution and the centrifuged cell
pellet were extracted separately with ethyl acetate
(3 · 10 mL). The organic layers were combined and
washed with water, dried and the organic layer concen-
trated under reduced pressure. The resulting mixture
consisting of alcohol and ester, was separated by column
chromatography over silica gel to furnish enantiorich 5
and 6c (isolated yields 5–67%, Table 3).
4.5.2. 1-Propanoyloxy-2-bromo-1-(4-nitrophenyl)ethane
6b. A viscous liquid; IR (neat): 3113, 3081, 2983,
2943, 1745, 1523, 1349, 1272, 1168, 1082, 855,
708 cmꢀ1
; d 1.21 (3H, t,
1H NMR (200 MHz):
J = 7 Hz, –OCOCH2–CH3), 2.48 (2H, q, J = 8 Hz,
–OCOCH2–CH3), 3.71 (2H, d, J = 6.0 Hz, CH2Br),
6.16 (1H, t, J = 6.0 Hz, CHOCOCH2–), 7.66 (2H, d,
J = 8.5 Hz, Ar-H), 8.38 (2H, d, J = 8.5 Hz, Ar-H); 13C
NMR (50 MHz): d 10.2, 28.7, 34.7, 67.8, 125.1, 128.8,
146.4, 149.2, 174.3; MS (m/z) (%): 304 (2.5), 302 (2.6)
(M+1), 230 (29.6), 228 (35.6), 222 (36.4), 208 (18.2),
148 (100), 119 (52.8), 102 (89.8), 91 (73.1), 77 (100), 58
(100); Anal. Calcd for C11H12BrNO4: C, 43.73; H,
4.00; N, 4.64. Found: C, 43.65; H, 4.01; N, 4.62.
4.8. General procedure for lipase/esterase catalyzed
transesterification of ( )-2-bromo-1-(4-nitrophenyl)etha-
nol 5
A suspension comprising of substrate ( )-5 (200 mg,
0.8 mmol), crude or immobilized enzyme (200 mg) in
an organic solvent (900 lL) and vinyl acetate (100 lL)
was shaken on an orbital shaker at 200 rpm, maintain-
ing the temperature at 28 1 ꢁC. The course of reaction
was monitored by chiral HPLC. After a certain degree
of conversion, the enzyme was filtered out, washed with
organic solvent. The combined solvent was removed
under reduced pressure and the resulting dry mass
4.5.3. 1-Butanoyloxy-2-bromo-1-(4-nitrophenyl)ethane
6c. A viscous liquid; IR (neat): 3113, 3081, 2966,
2935, 1743, 1523, 1348, 1251, 1166, 855, 707 cmꢀ1
;
1H NMR (200 MHz): d 0.96 (3H, t, J = 7.4 Hz,
–OCOCH2CH2–CH3), 1.68 (2H, m, –OCOCH2CH2–
CH3), 2.41 (2H, m, –OCOCH2CH2–CH3), 3.64 (2H, d,
J = 6.0 Hz, –CH2Br), 6.05 (1H, t, J = 6.0 Hz, CH–

724
M. Kapoor et al. / Tetrahedron: Asymmetry 16 (2005) 717–725
comprising of enriched alcohol and ester were separated
by column chromatography over silica gel to furnish
enantiorich (R)-(ꢀ)-5 and (S)-(+)-6a.
were concentrated and cold distilled water (15 mL)
added to the resulting mixture followed by extraction
with ethyl acetate (3 · 15 mL). The combined organic
layer was washed with brine, dried over anhy-
drous sodium sulfate and concentrated under reduced
pressure to give (S)-nifenalol (394 mg, 1.75 mmol,
4.9. Typical method of transesterification of ( )-2-bromo-
1-(4-nitrophenyl)ethanol with PS-C-II
25
25
88%) ½aꢂ ¼ þ11:2 (c 1.0, EtOH); ½aꢂ ¼ þ42:7 (c 0.4,
D
D
25
D
A suspension comprising of ( )-5 (200 mg, 0.8 mmol),
PS-C-II (200 mg), toluene (900 lL) and vinyl acetate
(100 lL) was shaken on an orbital shaker at 200 rpm
maintaining temperature at 28 1 ꢁC. The course of
transesterification reaction was monitored by chiral
HPLC. After a 50% conversion that took about 28 h,
the enzyme was filtered out, and washed with diethyl
ether (3 · 5 mL). The combined solvent mixture was
evaporated at reduced pressure. The resulting dry
mass comprising of enriched alcohol and ester was
separated by column chromatography over silica gel to
1 N HCl); lit.10 ½aꢂ ¼ þ41:5 (c 0.4, 1 N HCl) (ee
96%). (S)-Nifenalol (350 mg, 1.5 mmol) thus obtained
was converted to (S)-(+)-sotalol by catalytic reduction
followed by mesylation of the reduced product by the
method reported in the literature.10 The resulting prod-
uct after purification and treatment with 1 equiv of 5%
HCl furnished (S)-sotalol hydrochloride (193 mg,
25
D
40%); mp 204 ꢁC (lit.19 206.5–207 ꢁC); ½aꢂ ð2 ꢃ HClÞ ¼
25
þ34:4 (c 1.0, H2O); lit.20 ½aꢂ ¼ þ34:7 (c 1.0, H2O);
D
IR (KBr): 1043, 1128, 1322, 3400, 3565 cmꢀ1
;
1H
NMR (200 MHz, CD3OD): d 1.26 (6H, d, J = 6.6 Hz,
CH(CH3)2), 3.00 (3H, s, CH3SO2), 3.16 (1H, m,
CH(CH3)2), 3.86 (2H, d, J = 5.1 Hz, CH2NH), 4.30
(1H, t, J = 5.0 Hz, CHOH), 7.34 (2H, d, J = 8.7 Hz,
Ar-H), 7.46 (2H, d, J = 8.7 Hz, Ar-H); 13C NMR
(50 MHz, CDCl3): 19.38, 20.77, 39.52, 47.70, 50.26,
62.28, 64.60, 121.54, 130.28, 134.18, 140.36; MS (m/z)
(%): 272 (4.3), 193 (12.3), 121 (10.1), 72 (100.0), 43
(34.1), 30 (26.5); Anal. Calcd for C12H20N2O3SÆHCl:
C, 46.67; H, 6.85; N, 9.07. Found: C, 46.81; H, 6.81;
N, 9.02.
furnish enantiopure (R)-5 (84 mg, 0.34 mmol, 42%)
25
D
½aꢂ ¼ ꢀ29:8 (c 1.0, CHCl3) (ee >99.5%) {lit.14
25
½aꢂ ¼ ꢀ28:3 (c 1.0, CHCl3) (ee >95%)} and (S)-6a
D
(100 mg, 0.34 mmol, 42.7%) ½aꢂ ¼ þ51:2 (c 1.0, CHCl3)
D
(ee >99.5%) {lit.14 [a]D = +49.3 (c 1.0, CHCl3) (ee
>95%)}.
4.10. Preparation of (R)-nifenalol
A
methanolic solution of (R)-2-bromo-1-(4-nitro-
phenyl)ethanol (492 mg, 2 mmol) was slowly added to
isopropylamine (850 lL, 10 mmol) in methanol (5 mL)
at 35 ꢁC in a round bottomed flask placed in a sonicator
and the course of reaction was monitored by TLC. After
completion of the reaction (ꢁ2 h), methanol was evapo-
rated, the contents diluted by adding cold distilled water
(15 mL) followed by extraction with ethyl acetate
(3 · 15 mL). The combined organic layer was washed
with brine, dried over anhydrous sodium sulfate
and concentrated under reduced pressure to give (R)-nif-
enalol (404 mg, 1.8 mmol, 90%), mp 113.2 ꢁC (lit.6 112–
4.12. HPLC analysis with chiral stationary phase
Enantiomeric excesses (ee%) were determined by chiral
HPLC on (R,R)-Whelk O1 column (5 lm), Merck,
Germany (column temp 22 ꢁC) using a mobile phase
hexane–isopropanol–acetic acid = 97:2.9:0.1 at flow rate
0.3 mL/min and detection wavelength 254 nm. Reten-
tion times for different chiral compounds are as follows:
(S)-2-bromo-1-(4-nitrophenyl)ethanol ꢀ43.1 min, (R)-2-
bromo-1-(4-nitrophenyl)ethanol ꢀ45.6 min; (R)-1-acet-
oxy-2-bromo-1-(4-nitrophenyl)ethane ꢀ24.3 min, (S)-1-
25
20
114 ꢁC); ½aꢂ ¼ ꢀ11:3 (c 1.0, EtOH) {lit.6 ½aꢂ ¼ ꢀ11:4
D
(c 1.03, EtOH)}; IR (neat): 699, 791, 1073, 1346, 1520,
D
acetoxy-2-bromo-1-(4-nitrophenyl)ethane
ꢀ34.8min;
1604, 2913, 2960 cmꢀ1
;
1H NMR (200 MHz): d 1.10
(R)-1-propanoyloxy-2-bromo-1-(4-nitrophenyl)-ethane
(S)-1-propanoyloxy-2-bromo-1-(4-nitro-
and 1.11 (3H each, d, J = 6.7 Hz, CH(CH3)2), 2.60
(1H, dd, J = 9.3 and 12.2 Hz, HA of –CH2–NH), 2.40–
2.64 (1H, m, NH–CH(CH3)2), 2.96 (1H, dd, J = 3.4
and 12.2 Hz, HB of –CH2–NH), 4.82 (1H, dd, J = 3.2,
9.2 Hz), 7.50 (2H, d, J = 8.5 Hz), 8.17 (2H, d,
J = 8.4 Hz); 13C NMR (50 MHz): 22.0, 22.1, 49.3,
53.8, 70.53, 123.6, 126.3, 147.3, 150.0; MS (m/z) (%):
225 (M+1) (4.3), 193 (12.3), 151 (10.1), 130 (3.0), 72
(100.0), 45 (34.1); Anal. Calcd for C11H16N2O3: C,
58.91; H, 7.19; N, 12.49. Found: C, 58.58; H, 7.10; N,
12.57.
ꢀ19.1 min,
phenyl)ethane ꢀ29.4 min; (R)-1-butanoyloxy-2-bromo-
1-(4-nitrophenyl)ethane ꢀ17.7 min and (S)-1-butanoyl-
oxy-2-bromo-1-(4-nitrophenyl)ethane ꢀ27.5 min.
Acknowledgements
Authors are thankful to M/s. Amano Enzymes Inc.
Japan for the gift of PS-C-II sample. One of the authors
(M.K.) is grateful to CSIR, New Delhi, India for the
award of a Senior Research Fellowship.
4.11. Preparation of (S)-nifenalol and its conversion to
(S)-sotalol
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