Chemoenzymatic dynamic kinetic resolution as a key step in the enantioselective synthesis of (S)-salbutamol

Article abstract of DOI:10.1135/cccc2011029

The synthesis of (S)-salbutamol is described. By utilizing DKR in the enantiodetermining step, employing Burkholderia cepacia lipase (PS-IM), (S)-acetate ((S)-6) was obtained in excellent enantiomeric excess (98%). The subsequent transformations yielded t

Full text of DOI:10.1135/cccc2011029

Chemoenzymatic DKR Towards (S)-Salbutamol
919
CHEMOENZYMATIC DYNAMIC KINETIC RESOLUTION AS A KEY STEP
IN THE ENANTIOSELECTIVE SYNTHESIS OF (S)-SALBUTAMOL
b
a2,
Annika TRÄFFa1, Carmen E. SOLARTE and Jan-E. BÄCKVALL
*
a
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University,
106 91 Stockholm, Sweden; e-mail: annika@organ.su.se, jeb@organ.su.se
Deparment of Chemistry, Lleida University, 25198 Lleida, Spain; e-mail: solarte@quimica.udl.cat
1
2
b
Received January 31, 2011
Accepted April 6, 2011
Published online June 30, 2011
Dedicated to Professor Pavel Kočovský on the occasion of his 60th birthday.
The synthesis of (S)-salbutamol is described. By utilizing DKR in the enantiodetermining
step, employing Burkholderia cepacia lipase (PS-IM), (S)-acetate ((S)-6) was obtained in excel-
lent enantiomeric excess (98%). The subsequent transformations yielded the salt of (S)-sal-
butamol with retained stereochemistry.
Keywords: Dynamic kinetic resolution; Kinetic resolution; Enzymatic catalysis; Racemiza-
tion; Asymmetric synthesis.
Asthma is a chronic disease of the airway that affects 300 million people of
all ages worldwide. Treatment options include reliever and controller medi-
cation1. β2-Agonist drugs are the most commonly used bronchodilators ap-
plied in treatment of bronchospasm2. Salbutamol is a relatively selective
inhaled short-acting β2-adrenoreceptor agonist (SABAs) with rapid relief1,3
.
It is a chiral drug that is commonly used as a racemic mixture. Studies have
shown that the (R)-enantiomer of salbutamol (levosalbutamol) is the thera-
peutically active isomer and it has been available on the international mar-
ket since the last few years. Until recently (S)-salbutamol was considered
inert, but recent studies have shown that the (S)-enantiomer may rather
have some deleterious effects2. However, most of the published studies were
supported by pharmaceutical companies involved in production or market-
ing of levosalbutamol. Therefore large multicenter trials are required to
prove the therapeutic superiority of levosalbutamol over racemic salbuta-
mol2. This, together with the requirement from the FDA that each single
enantiomer of a potential drug must be characterized for their individual
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 7, pp. 919–927
© 2011 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc2011029

920
Träff, Solarte, Bäckvall:
physiological action4, has increased the demand for efficient preparation of
both enantiomers with high degree of purity. Today, separating enantio-
mers through resolution of a racemic mixture is the most commonly used
technique in industry5. This process suffers from two major disadvantages
being (i) the limitation of 50% maximum yield and (ii) the separation of
the product from the remaining starting material6. In dynamic kinetic reso-
lution the non-desired enantiomer is continuously racemized during reso-
lution and thereby the limitation of a normal kinetic resolution is over-
come. In a chemoenzymatic DKR a transition metal-catalyzed racemization
reaction is coupled with enzymatic kinetic resolution to give the enantio-
merically pure product in 100% theoretical yield (Fig. 1)7.
enzyme
(R)-Substrate
cat.
(R)-Product
kfast
krac
enzyme
kslow
(S)-Substrate
(S)-Product
FIG. 1
Schematic picture of dynamic kinetic resolution
In 2004, we reported on a new efficient racemization catalyst, ruthenium
complex 1, which gives fast and efficient reactions at room temperature
and has been applied in several DKR processes (Fig. 2)8.
Ph
Ph
Ph
Ph
Ph
Ru
Cl
OC
CO
1
FIG. 2
Ruthenium complex employed as efficient racemization catalyst
Salbutamol belongs to the family of arylethanolamines, with potent β2-
adrenoceptor agonist activity, which all have the same basic structure.
Asymmetric synthesis of arylethanolamine can be achieved by CBS reduc-
tion9, enzymatic reduction10, asymmetric transfer hydrogenation11, or
chemical resolution12 to mention a few techniques. Herein we report on
the total synthesis of (S)-salbutamol where we apply chemoenzymatic DKR
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 7, pp. 919–927

Chemoenzymatic DKR Towards (S)-Salbutamol
921
of a chlorohydrin to give a key intermediate in high ee from simple starting
materials (Scheme 1).
O
O
O
i
ii
iii
Cl
Cl
Cl
HO
HO
O
O
3
2
4
HO
O
OH
OAc
O
vi
v
iv
Cl
Cl
O
O
O
rac-5
(S)-6
O
(S)-7
O
O
OAc
OH
OH
H2
N
vii
NH
O
HO
HO
(S)-8
(S)-9
O
SCHEME 1
Reagents and conditions: i: NaBH4, HOAc, r.t., 88%. ii: CH3C(OCH3)2CH3, PTSA, CH2Cl2, r.t.,
85%. iii: NaBH4, MeOH, r.t., 64%. iv: Ru(CO)2Cl(η5C5Ph5) (1), PS-IM, Na2CO3, toluene,
t-BuOK, isopropenyl acetate, 40 °C, 98% ee, 69%. v: LiOH, EtOH, NaHCO3, r.t., 98% ee, quanti-
tative. vi: t-BuNH2, 70 °C, 65 %. vii: AcOH, H2O, 50 °C, quantitative
RESULTS AND DISCUSSION
The synthesis starts with Friedel-Crafts acylation of readily available salicyl-
aldehyde in the presence of aluminium trichloride to give 2 following
a published procedure13. Regioselective NaBH4 reduction of 2 in acetic acid
afforded α-chloroketone 3 in 88% yield14. Protection of the hydroxy groups
by ketal formation with 2,2-dimethoxypropane followed by NaBH4 reduc-
tion of ketone 4 gave the desired β-chloroalcohol, 5, in 55% yield over two
steps. Attempts to reduce the aldehyde and ketone in the same step gave a
mixture of compounds and even though the synthesis applied has one ad-
ditional step it was found to be more efficient.
Previously our group has shown that DKR can be successfully carried out
on a wide range of β-chlorohydrine derivatives15. Screening of different en-
zymes to investigate the selectivity towards β-chloroalcohol 5 was done (Ta-
ble I).
Burkholderia cepacia lipase (previously Pseudomonas cepacia lipase) has pre-
viously been shown to have an excellent enantioselectivity for β-chloro-
hydrines and shows the same result in this study. PS-IM (Burkholderia
cepacia lipase immobilized on diatomaceous earth) as well as PS-C1
(Burkholderia cepacia lipase immobilized on ceramic particles) showed excel-
lent selectivity, with an E-value above 200, as well as good activity (entries
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 7, pp. 919–927

922
Träff, Solarte, Bäckvall:
1 and 3). CALB (Candida antarctica lipase B immobilized on acrylic resin,
Novozyme 435) and AH-SD showed good selectivity but rather low activ-
ity (entries 2 and 4). Lipase AK (Pseudomonas fluorescens lipase) showed
moderate selectivity and activity (entry 5) while lipase TL (Thermomyces
lanuginosus lipase) showed the highest activity of the enzymes investigated
with good selectivity as well (entry 6). Lipase AYS and F-AP-15 did not show
any activity at all (entries 7 and 8). PS-IM (Burkholderia cepacia lipase) was
employed for the enantioselective resolution in the DKR. However, it was
noticed that, while the racemization was still active giving a 1:1 ratio of the
1
(R)- and (S)-alcohol (5), the DKR was very slow showing only 73% H NMR
yield of the (S)-acetate (6; 99% ee) after 24 h. It was also observed that the
substrate was difficult to dissolve in toluene. By optimizing the conditions
by increasing the amount of enzyme and acyl donor and diluting the reac-
tion, the DKR was successfully run at 40 °C and gave full conversion within
48 h with (S)-6 isolated in 69% yield and 98% ee. Ring-closing formation of
the epoxide was performed by treating the acetate with LiOH in EtOH. Care
TABLE I
Investigating the selectivity of different enzymes in kinetic resolution of β-chloro alcohol 5a
enzyme
isopropenyl acetate
Na2CO3
OH
OH
OAc
Cl
Cl
Cl
+
O
O
O
toluene
r.t.
O
O
O
(S)-6
(R)-5
rac-5
Time
h
conv (5)b
%
% ee
% ee
Entry
Enzyme
Eb
(R)-5c
(S)-6c
1
2
3
4
5
6
7
8
PS-IM
3
24
3
21
10
14
34
23
40
1
26
11
16
50
28
64
–
>99
>99
>99
>99
95
>200
>200
>200
>200
51
AH SD
PS-C1
CALB
24
24
1
Lipase AK
Lipase TL
Lipase AYS
98
192
–
24
24
–
Lipase F-AP-15
1
–
–
–
a
0.1 mmol 5, 0.1 mmol Na2CO3, 5 mg enzyme (50 mg/mmol), 0.2 ml toluene, 0.2 mmol
b
c
isopropenyl acetate. Calculated values. Measured by chiral GC.
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 7, pp. 919–927

Chemoenzymatic DKR Towards (S)-Salbutamol
923
was taken so that the hydroxyether or the diol was not formed. By carefully
monitoring the reaction the epoxide was obtained in quantitative yield
with retained stereochemistry (98% ee). The epoxide was then opened with
tert-butyl amine. The desired regioisomer was the major one with a selectiv-
ity of ~85:15 between (S)-8 and iso-8, and the product, (S)-8, was isolated in
65% yield. The final step of deprotection in acetic acid/water yielded the
acetate salt of (S)-salbutamol in quantitative yield16.
CONCLUSION
We have here reported an efficient and straightforward synthesis of (S)-
salbutamol. By employing DKR in the enantiodetermining step with PS-IM
(Burkholderia cepacia lipase immobilized on diatomaceous earth) as an
enantioselective catalyst and ruthenium catalyst 1 for racemization the lim-
itation of a normal KR was circumvented. By optimizing the conditions,
DKR was run to completion within 48 h yielding (S)-6 in 98% ee. The fol-
lowing synthetic alterations yielded the salt of (S)-salbutamol, in 21% over-
all yield over 7 steps, with retained enantiomeric excess16.
EXPERIMENTAL
DKR reactions were carried out under dry argon atmosphere using standard Schlenk tech-
nique. The β-chloroalcohol 5 was dried over MS (4Å) before used in DKR. Isopropenyl ace-
tate was dried over CaCl2 and distilled before use. Dry THF and toluene were obtained from
VAC solvent purifier provided by the Vacuum Atmosphere co. Dry DCM was obtained by
distilling it over CaCl2. All other chemicals and solvents were used as purchased. The
enantiomeric excess of the compounds was determined by chiral GC/HPLC using racemic
compounds as references. Racemic acetate 6 was obtained from the alcohol 5 by standard
acylation. Silica column chromatography was performed with Davisil chromatographic silica
media for separation and purification applications (35–70 micron).
GC samples were run on GC Varian 3800 with an auto sampler equipped with a
CP_Chirasil_Dex CB column (25 m × 0.32 mm × 0.25 µm), carrier gas H2, flow 1.8 ml min–1
.
HPLC samples were run on HPLC Waters 2695 equipped with a Waters 996 Photodiode
Array detector. 1H and 13C NMR (δ in ppm; J in Hz) were recorded with Bruker 400 or
Bruker 500. Optical rotation (10–1 deg cm2 g–1) was measured with a Perkin–Elmer 241
polarimeter equipped with a Na-lamp. HR-MS was recorded with Bruker MicroTOF ESI.
Ru(CO)2Cl(η5C5Ph5) (1)8b and 5-(chloroacetyl)-2-hydroxybenzaldehyde (2)13 were synthe-
sized according to literature procedures.
Synthesis of 2-Chloro-1-(4-hydroxy-3-(hydroxymethyl)phenyl)ethanone (3)
Sodium borohydride (294 mg, 7.8 mmol) was added portionwise to
a solution of
5-(chloroacetyl)-2-hydroxybenzaldehyde 2 (1.50 g, 7.5 mmol) in acetic acid (38 ml) stirred at
5 °C (ice/water bath) under argon. The mixture was brought to room temperature and the
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 7, pp. 919–927

924
Träff, Solarte, Bäckvall:
stirring was continued until the reaction was complete. After the reaction was complete wa-
ter (40 ml) was added and the mixture was neutralized with a saturated solution of Na2CO3.
The mixture was extracted with ethyl acetate (4 × 50 ml). The combined organic phases
were washed with brine, dried over MgSO4, filtered and concentrated to give a light red oil
that turned solid upon standing. The crude was purified by column chromatography (silica
gel, pentane–ethyl acetate, gradient from 9:1 to 2:8) and afforded a light pink solid product
(1.33 g, 6.6 mmol, 88% yield). 1H NMR (400 MHz, DMSO-d6): 10.53 s, 1 H (OH); 7.97 d,
1 H, J = 2.0 (Ar-H); 7.76 dd, 1 H, J = 8.4, 2.4 (Ar-H); 6.87 d, 1 H, J = 8.4 (Ar-H); 5.15 br s, 1 H
(OH); 5.04 s, 2 H (CH2OH); 4.50 s, 2 H (CH2Cl). 13C NMR (125 MHz, DMSO-d6): 190.3,
–
159.9, 129.7, 129.6, 128.5, 126.0, 114.8, 58.2, 47.5. HR-MS (ESI): calculated for C9H8ClO3
:
199.0167; found [M – H]–: 199.0163.
Synthesis of 2-Chloro-1-(2,2-dimethyl-4H-benzo[1,3]dioxin-6-yl)ethanone (4)
To a suspension of 2-chloro-1-(4-hydroxy-3-(hydroxymethyl)phenyl)ethanone (3) (1.33 g,
6.6 mmol) and p-toluenesulfonic acid (0.0061 g, 0.035 mmol) in 33 ml of dichloromethane
was added dropwise 2,2-dimethoxypropane (0.763 g, 7.3 mmol) in dichloromethane (17 ml).
The suspension was stirred vigorously until it became homogeneous (light yellow). The reac-
tion mixture was washed with saturated NaHCO3 solution. The organic phase was separated
and dried over MgSO4. The solvent was evaporated under reduced pressure to give 4 as a yel-
low oil (1.35 g, 5.6 mmol, 85% yield). 1H NMR (400 MHz, CDCl3): 7.79 dd, 1 H, J = 8.6, 2.2
(Ar-H); 7.68 d, 1 H, J = 2.0 (Ar-H); 6.88 d, 1 H, J = 8.8 (Ar-H); 4.89 s, 2 H (CH2O); 4.63 s, 2 H
(CH2Cl); 1.57 s, 6 H (C(CH3)2). 13C NMR (100 MHz, CDCl3): 189.7, 156.4, 129.2, 126.7,
+
126.2, 119.6, 117.4, 100.8, 60.6, 45.5, 24.8. HR-MS (ESI): calculated for C12H13ClNaO3
:
263.0445; found [M + Na]+: 263.0445.
Synthesis of 2-Chloro-1-(2,2-dimethyl-4H-benzo[1,3]dioxin-6-yl)ethanol (5)
2-Chloro-1-(2,2-dimethyl-4H-benzo[1,3]dioxin-6-yl)ethanone (4) (1.35 g, 5.6 mmol) was dis-
solved in MeOH (34 ml). The solution was cooled to 0 °C then NaBH4 (0.148 g, 3.9 mmol)
was added in small portions. Stirring was continued for 30 min at 0 °C, after which the tem-
perature of the reaction mixture was allowed to reach room temperature and stirring was
continued. After 3 h the reaction was completed and the solvent was evaporated. Saturated
NH4Cl (aq.) solution (25 ml) was added after which the aqueous phase was extracted with
ethyl acetate (4 × 25 ml). The organic portions were combined and washed with water (1 ×
75 ml) and brine (1 × 75 ml). The organic phase was dried over MgSO4, filtered and the sol-
vent was evaporated. The residue was purified by flash column chromatography (silica gel,
pentane–ethyl acetate, 9:1) to give a colorless oil (87 mg, 3.6 mmol, 64% yield). 1H NMR
(400 MHz, CDCl3): 7.16 dd, 1 H, J = 8.4, 1.7 (Ar-H); 7.03 d, 1 H, J = 1.5 (Ar-H); 6.82 d, 1 H,
J = 8.4 (Ar-H); 4.84 s, 2 H (CH2O); 4.84–4.80 m, 1 H (CHCH2); 3.70 dd, 1 H, J = 11.2, 3.4
(CHCH2); 3.60 dd, 1 H, J = 11.2, 9.0 (CHCH2); 2.60 d, 1 H, J = 2.8 (OH); 1.54 s, 3 H
(C(CH3)2); 1.53 s, 3 H (C(CH3)2). 13C NMR (100 MHz, CDCl3): 151.4, 131.7, 126.0, 122.4,
+
119.6, 117.3, 99.7, 73.7, 60.9, 51.0, 24.8, 24.6. HR-MS (ESI): calculated for C12H15ClNaO3
:
265.0602; found [M + Na]+: 265.0591.
General procedure for KR of 2-chloro-1-(2,2-dimethyl-4H-benzo[1,3]dioxin-6-yl)ethanol (5): En-
zyme (5 mg, 50 mg/mmol of 5) and Na2CO3 (10.6 mg, 0.1 mmol) was added to a µW-vial.
2-Chloro-1-(2,2-dimethyl-4H-benzo[1,3]dioxin-6-yl)ethanol (24.27 mg, 0.1 mmol) dissolved
in dry toluene (0.2 ml) was added to the vial, and stirred. After a few min isopropenyl ace-
Collect. Czech. Chem. Commun. 2011, Vol. 76, No. 7, pp. 919–927

Chemoenzymatic DKR Towards (S)-Salbutamol
925
tate (22 µl, 0.2 mmol) was added to the reaction. Samples for GC analysis were collected
with a syringe after 1, 3 and 24 h. GC gradient 110 °C, 0 min; 15 °C/min to 150 °C, 5 min;
2 °C/min to 200 °C, 2 min. tR1 = 19.4 min ((R)-6); tR2 = 19.7 min ((S)-6); tR3 = 20.6 min
((S)-5); tR4 = 21.0 min ((R)-5).
Synthesis of (S)-2-Chloro-1-(2,2-dimethyl-4H-benzo[1,3]dioxin-6-yl)ethyl Acetate ((S)-6)
Complex 1 (16 mg, 0,025 mmol), PS-IM (50 mg; 100 mg/mmol substrate) and Na2CO3
(53 mg, 0.5 mmol) were added to a Schlenk-tube. Dry toluene (0.5 ml) was added and the
resulting yellow solution was stirred. t-BuOK (60 µl, 0.030 mmol, 0.5 M in dry THF) was
added to the reaction. The reaction turned orange-brown. After approximately 5 min of stir-
ring, 2-chloro-1-(2,2-dimethyl-4H-benzo[1,3]dioxin-6-yl)ethanol (5) (121 mg, 0.5 mmol) dis-
solved in dry toluene (1.0 ml) was added to the reaction mixture. After an additional 5 min
isopropenyl acetate (165 µl, 1.5 mmol) was added. The reaction was heated to 40 °C. After
48 h the reaction mixture was filtered and concentrated. Purification by column chromatog-
raphy (silica gel, pentane–EtO2, 8:2) afforded (S)-2-chloro-1-(2,2-dimethyl-4H-benzo[1,3]-
dioxin-6-yl)ethyl acetate ((S)-6) in 69% yield (99 mg, 0.35 mmol) with 98% ee (determined
by chiral GC). 1H NMR (400 MHz, CDCl3): 7.14 dd, 1 H, J = 8.6, 2.2 (Ar-H); 6.97 d, 1 H, J =
2.0 (Ar-H); 6.80 d, 1 H, J = 8.4 (Ar-H); 5.86 dd, 1 H, J = 8.2, 4.6 (CHCH2); 4.83 s, 2 H
(CH2O); 3.76 dd, 1 H, J = 11.8, 8.2 (CHCH2); 3.67 dd, 1 H, J = 11.6, 4.8 (CHCH2); 2.11 s,
3 H (COCH3); 1.57 s, 6 H (C(CH3)2). 13C NMR (100 MHz, CDCl3): 169.8, 151.6, 128.9, 126.4,
123.3, 119.5, 117.3, 99.7, 74.7, 60.7, 46.3, 24.7, 24.6, 20.9. HR-MS (ESI): calculated for
20
C
14H17ClNaO4+: 307.0708; found [M + Na]+: 307.0713. [α]D +73.5 (c 1.15, MeOH). GC gra-
dient 110 °C, 0 min; 15 °C/min to 150 °C, 5 min; 2 °C/min to 200 °C, 2 min. tR1 = 19.4 min
((R)-6); tR2 = 19.7 min ((S)-6).
Synthesis of (S)-2,2-Dimethyl-6-(oxiran-2-yl)-4H-benzo[1,3]dioxin ((S)-7)
LiOH (27 mg, 1.1 mmol) was added in portions to a solution of acetate (S)-6 (91 mg,
0.32 mmol) in 4 ml EtOH (99%) and stirred. The reaction was quenched by addition of
NaHCO3 (192 mg, 2.3 mmol) after 1 h. EtOH was carefully evaporated. To the residue was
added brine (20 ml) and the resulting mixture was extracted with Et2O (5 × 20 ml). The
combined organic phases were dried over MgSO4, filtered and concentrated under vacuum
to give the corresponding epoxide in 98% crude yield (64.1 mg, 0.31 mmol) with 98% ee
(determined by chiral HPLC). The colorless oil was used without further purification.
1H NMR (400 MHz, CDCl3): 7.08 dd, 1 H, J = 8.4, 2.0 (Ar-H); 6.88 d, 1 H, J = 1.9 (Ar-H);
6.80 d, 1 H, J = 8.4 (Ar-H); 4.82 s, 2 H (CH2O); 3.79 dd, 1 H, J = 3.8, 2.8 (CHCH2); 3.11 dd,
1 H, J = 5.3, 4.0, 1 H (CHCH2); 2.79 dd, 1 H, J = 5.3, 2.6 (CHCH2), 1.53 s, 6 H (C(CH3)2).
13C NMR (100 MHz, CDCl3): 151.3, 129.2, 125.6, 121.8, 119.6, 117.2, 99.6, 60.8, 52.2, 51.0,
24.8, 24.6. HR-MS (ESI): calculated for C12H15O3+: 207.1016; found [M + H]+: 207.1009.
HPLC: Chiralpak AS column, i-PrOH–i-hexane (1:99), 25 °C, flow rate 1 ml min–1, λ = 230 nm,
tR1 = 32.1 min (R)-7, tR2 = 38.9 min (S)-7.
Synthesis of (S)-2-(tert-Butylamino)-1-(2,2-dimethyl-4H-benzo[1,3]dioxin-6-yl)-
ethanol ((S)-8)
A mixture of (S)-2,2-dimethyl-6-(oxiran-2-yl)-4H-benzo[1,3]dioxin ((S)-7) (64 mg, 0.31 mmol)
and tert-butylamine (7 ml) were placed in a Schlenk and refluxed for 5 days. After cooling to
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Träff, Solarte, Bäckvall:
room temperature the excess of tert-butylamine was removed under reduced pressure. Chro-
matographic purification (silica gel, dichloromethane–methanol–triethylamine, 95:5:0.1) af-
forded (S)-8 (56.7 mg, 0.20 mmol, 65% yield) as a light yellow oil that crystallized upon
standing. Spectral data were in accordance with those reported in the literature12b 1H NMR
.
(400 MHz, CDCl3): 7.12 dd, 1 H, J = 8.4, 1.8 (Ar-H); 7.01 d, 1 H, J = 1.4 (Ar-H); 6.79 d, 1 H,
J = 8.4 (Ar-H); 4.84 s, 2 H (CH2O); 4.50 dd, 1 H, J = 9.0, 3.6 (CHCH2); 2.85 dd, 1 H, J = 11.8,
3.6 (CHCH2); 2.55 dd, 1 H, J = 11.8, 9.0 (CHCH2), 1.54 s, 3 H (C(CH3)2); 1.53 s, 3 H
(C(CH3)2); 1.10 s, 9 H, (C(CH3)3). 13C NMR (100 MHz, CDCl3): 150.5, 134.7, 125.7, 122.1,
119.3, 116.9, 99.5, 72.0, 60.9, 50.3, 50.2, 29.2, 24.9, 24.6.
Synthesis of (S)-4-(2-(tert-Butylamino)-1-hydroxyethyl)-2-(hydroxymethyl)phenol ((S)-9)
To (S)-8 (55.6 mg, 0.20 mmol) was added H2O (4 ml) and then acetic acid (2 ml) and the re-
action was heated to 50 °C. After 3 h it was cooled to room temperature and H2O/acetic acid
was evaporated under reduced pressure. The crude was taken up in 99% EtOH and after
evaporation, the procedure was repeated twice. (S)-Salbutamol acetate ((S)-9) was obtained as
a pale yellow oil in quantitative yield (60 mg, 0.2 mmol). Spectral data were in accordance
with those reported in the literature12b 1H NMR (500 MHz, DMSO-d6): 7.28 d, 1 H, J = 1.6
.
(Ar-H); 7.03 dd, 1 H, J = 6.6, 1.4 (Ar-H); 6.72 d, 1 H, J = 6.8 (Ar-H); 4.58 dd, 1 H, J = 7.6, 2.8
(CHCH2), 4.47 s, 2 H (CH2OH); 2.73 dd, 1 H, J = 9.4, 2.8 (CHCH2); 2.65 app t, 1 H J = 8.4
(CHCH2); 1.83 s, 3 H (COCH3); 1.13 s, 9 H (C(CH3)3). 13C NMR (125 MHz, DMSO-d6): 173.6,
20
153.4, 133.5, 128.1, 125.1, 124.9, 114.2, 70.5, 58.3, 56.0, 52.2, 49.7, 27.1, 22.6, 18.6. [α]D
+38.1 (c 0.8, MeOH); lit.12b [α]D –37.9 (c 0.5, MeOH) for (R)-9, 97% ee.
The authors wish to acknowledge Dr. Y. Hirose, Amano Enzyme Inc. Gifu R&D Center, for kindly
providing us with enzymes (PS-IM, AH SD, PS-C1, AK, TL, AYS, F-AP-15). We thank Dr. A. Svendsen,
Novozymes A/S, for a generous gift of CALB (Novozym 435). Financial support from INTENANT is
gratefully acknowledged.
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20
20
16. [α]D
+38.1 (c 0.8, MeOH) compared to lit.12b [α]D
–37.9 (c 1, MeOH) for
(R)-salbutamol 97% ee.
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