Efficient lipase-catalysed route for the kinetic resolution of salsolidine and its ?-carboline analogue

Article abstract of DOI:10.1016/j.tetasy.2017.10.019

Racemic 1-methyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline 1 and 1-methyl-1,2,3,4-tetrahydro-?-carboline 3 were resolved through lipase-catalysed asymmetric acylation on the secondary amino group. High enantioselectivities (E >200) were observed when t

Full text of DOI:10.1016/j.tetasy.2017.10.019

Tetrahedron: Asymmetry xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Efficient lipase-catalysed route for the kinetic resolution of
salsolidine and its ß-carboline analogue
⇑
⇑
Barbara Kovács, Rita Megyesi, Enik o} Forró , Ferenc Fülöp
Institue of Pharmaceutical Chemistry, University of Szeged, Eötvös u. 6, H-6720 Szeged, Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
Racemic 1-methyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline 1 and 1-methyl-1,2,3,4-tetrahydro-ß-
carboline 3 were resolved through lipase-catalysed asymmetric acylation on the secondary amino group.
High enantioselectivities (E >200) were observed when the acylation of racemic 1 was performed with
phenyl allyl carbonate in the presence of Candida rugosa lipase in toluene at 40 °C or with Candida antarc-
tica lipase B in tert-butyl methyl ether at 50 °C. Excellent enantioselectivity (E >200) characterised the
CAL-B-catalysed acylation of racemic 3 with phenyl allyl carbonate in the presence of triethylamine in
tert-butyl methyl ether at 50 °C. The product (R)-carbamates (ee >97%) were hydrolysed into the corre-
Received 19 October 2017
Accepted 24 October 2017
Available online xxxx
sponding (R)-enantiomers of the free amines 1 and 3 (ee = 99%) with the use of Pd
2
(dba)
3
ꢀCHCl
3
catalyst.
Ó 2017 Elsevier Ltd. All rights reserved.
1
. Introduction
1,2,3,4-tetrahydro-ß-carboline skeleton is also a common building
block of several alkaloids, including the naturally occurring reser-
pine, which shows antitumor activity, as well as antihypertensive
Interest in the research of compounds containing the 1-substi-
5
tuted 1,2,3,4-tetrahydroisoquinoline ring system has recently
come into view, since compounds bearing this skeleton exert
major biological activities. Several derivatives possess a common
nucleus of synthetic structures, while other compounds are
extracted from natural sources. All of them are associated with
important pharmacological effects. The naturally occurring (S)-
norcoclaurine [(1S)-1-(4-hydroxybenzyl)-1,2,3,4-tetrahydro-6,7-
isochinolindiol] is an intermediate in the synthesis of morphine,
papaverine or the antibacterial berberine. Racemic norcoclaurine
has
uclidinyl-1-phenyl-1,2,3,4-tetrahydro-2-isoquinolinecarboxylate]
containing a 1-phenyl-substituted tetrahydroisoquinoline core is
an example of synthetic compounds. It shows urinary antispas-
and neuroprotective effects. Synthetic 1-substituted N-acylated
tetrahydro-ß-carbolines have inhibitory activity against the Breast
6
Cancer Resistance Protein (ABCG2). The (S)-enantiomer of salso-
lidine analogue 1-methyl-1,2,3,4-tetrahydro-ß-carboline 3 (eleag-
nine) was isolated from Eleagnus angustifolia and Petalostyles
7
labicheoides. The racemic form can bind to the GABA
A
receptors
in the benzodiazepine binding site, but it has an inverse agonist
8
effect.
1
It is not surprising, therefore, that a large number of synthetic
2
0
0
a- and ß-adrenoreceptor activity. Solifenacin [(1S,3 R)-3 quin-
routes have been developed for the preparation of 1 and 3, in par-
7
,9–13
ticular, in enantiomeric forms.
As an example, enantiomeric 1
was synthesized by Ding and et al. through enantioselective acyla-
1
4
tion of the racemic mixture in a batch process.
3
modic effect. Both enantiomers of 1-methyl-6,7-dimethoxy-
In this work, our aim was to devise a new enzymatic strategy
for the preparation of enantiomeric 1 and 3, through lipase-catal-
ysed kinetic resolution (Scheme 1). In addition to the batch reac-
tions, we planned to examine the possibilities for their enzymatic
reactions in a continuous-flow system. The use of this novel
method has clear advantages, such as short reaction times, rapid
1,2,3,4-tetrahydroisoquinoline 1 (salsolidine) are naturally occurring
4
compounds. The (R)-enantiomer was isolated from Genista
pungens, while the (S)-enantiomer was found in Salsola richteri.
Many pharmacological properties have been described to salso-
lidine; e.g., it inhibits the uptake of 5-hydroxytryptamine by
human blood platelets. The (R)-enantiomer has a monoamine oxi-
dase A (MAO A) inhibitoring effect. On the other hand, as a struc-
tural feature of a trimethoprim analogue, it acts as a dihydrofolate
reductase inhibitor. The pharmaceutically valuable 1-substituted
4
4
4
15
heating and pressure screening. In the literature, there are vari-
4
ous examples using this innovative technique for resolution. For
1
6,17
example, the asymmetric acylation of 1-phenylethylamine
4
18
and an imidazole derivative as well as the esterification of flur-
1
9
biprofen have been reported.
⇑
Corresponding authors. Tel.: +36 62 54 5564; fax: +36 62 54 5705.
E-mail address: fulop@pharm.u-szeged.hu (F. Fülöp).
https://doi.org/10.1016/j.tetasy.2017.10.019
957-4166/Ó 2017 Elsevier Ltd. All rights reserved.
0
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2
B. Kovács et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
enzyme, solvent,
acyl donor, T, additive
described by Page et al. using a combination of catalysts AY (Can-
+
dida rugosa lipase) and pentamethylcyclopentadienyliridium(III)
NH
NH
NAcyl
iodide dimer in the presence of 3-methoxyphenyl propyl carbonate
2
0
at 40 °C (conv. = 90% in 23 h, ee = 96%, yield = 82%).
(
±)-1, (±)-3
O
(S)-1, (S)-3
(R)-2, (R)-4
NH
We started the resolution of (±)-1 by an enzyme screening. First,
the reaction was performed in batch with phenyl allyl carbonate in
toluene at 40 °C (Scheme 2, Table 1, entries 1–4). Low enantiose-
lectivity was observed with CAL-A (E = 9, entry 1) and with PS-
IM (Burkholderia cepacia lipase) (E = 2, entry 2). In contrast, an
improved E = 46 was found with CAL-B (Candida antarctica lipase
B) 49% conversion in 4 days (entry 4). Lipase AY was the best cat-
alyst with a conversion of 50% in 72 h and an excellent E (>200)
NH
O
N
H
(
±)-1
(±)-3
Scheme 1. Kinetic resolution of (±)-1 and (±)-3 through lipase-catalysed N-
acylation on the secondary amino group.
2
2
. Results and discussion
(
entry 3). Since the tested CAL-B was purchased as an immobilized
.1. Synthesis of (± ±-1 and (± ±-3
enzyme (from Sigma) and also in view of its potential use in con-
tinuous-flow system, CAL-B was chosen for further optimization.
Next, the CAL-B-catalysed reaction was performed at 50 and
then 60 °C (Table 1, entries 5 and 6). As the temperature was
increased, the reaction rate increased considerably (entries 4–6).
However, the best combination of reaction rate and enantioselec-
tivity was found at 50 °C (entry 5). When toluene was replaced
with t-BuOMe, a much faster reaction with excellent E (>200)
was observed (entry 7).
1
-Methyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (±)-1
was prepared from 3,4-dimethoxyphenylethylamine and acetic
anhydride through Bischler–Napieralski cyclization followed by
reduction, while racemic 1-methyl-1,2,3,4-tetrahydro-ß-carboline
±)-3 was synthesized utilizing the Pictet–Spengler reaction via a
microwave-assisted procedure according to known literature
(
9
,11
methods.
Having the optimized conditions in the batch process (CAL-B,
phenyl allyl carbonate, t-BuOMe, 50 °C, 1 bar), we decided to
perform a reaction under these conditions in a continuous-flow
2
.2. Enzymatic resolution of (± ±-1 and (± ±-3
Ding et al. reported the asymmetric N-acylation of racemic 1
E > 200, conv. = 50% in 72 h, yield > 46%) by using CAL-A (Candida
2
reactor (an H-Cube in ‘no H mode’). A 70-mm-long heat- and
(
pressure-resistant stainless-steel CatCart was filled with CAL-B.
Unfortunately, only 20% conversion was reached after a cycle,
although an excellent E (>200) was observed (Table 2, entry 1).
antarctica lipase A), 3-methoxyphenyl allyl carbonate in toluene
1
4
at 40 °C.
The dynamic kinetic resolution of (±)-1 was also
enzyme, solvent,
acyl donor, T, p
O
O
O
O
O
NH
NH
N
O
O
O
(
±)-1
(S)-1
(R)-2
Scheme 2. Kinetic resolution of (±)-1 through enantioselective N-acylation.
Table 1
N-Acylation of (±)-1 in batcha
b
c
Entry
Enzyme
T (°C)
Solvent
Reaction time (h)
Conv. (%)
ee
s
(%)
ee
p
(%)
E
1
2
3
4
5
6
7
CAL-A
PS-IM
AY
CAL-B
CAL-B
CAL-B
CAL-B
40
40
40
40
50
60
50
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
t-BuOMe
96
96
72
96
48
48
2.5
42
3
50
13
99
85
96
96
99
68
25
98
89
95
92
97
9
2
>200
46
154
94
>200
50
49
50
51
50
a
b
c
0
.025 M (±)-1, phenyl allyl carbonate.
According to HPLC after a derivatisation with Ac
According to HPLC.
2
O.
Table 2
Effect of pressure and temperature on the acylation of (±)-1 with phenyl allyl carbonate in a continuous-flow systema
b
c
Entry
p (bar)
T (°C)
Conv. (%)
ee
s
(%)
ee
p
(%)
E
1
2
3
4
5
6
1
30
60
1
1
1
50
50
50
60
70
80
20
3
9
21
25
7
24
3
10
26
33
7
99
99
99
99
99
99
>200
>200
>200
>200
>200
>200
a
b
c
ꢁ1
0
.025 M (±)-1, 244 mg CAL-B (70 mm CatCart); t-BuOMe, 0.1 mL min .
According to HPLC after a derivatisation with Ac
According to HPLC.
2
O.
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B. Kovács et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
3
In order to improve the reaction rate, the pressure was
increased from 1 to 30, and then 60 bar. The E remained excellent
>200), but the reaction rate decreased significantly (compare
In view of the above results, N-acylation of racemic 3 (Scheme 3)
was performed with phenyl allyl carbonate in the presence of CAL-
B in t-BuOMe at 50 °C. Excellent E (>200) was observed but the
(
entries 2 and 3 to entry 1). In a further attempt to increase the rate,
we examined the effect of temperature on the reaction. As the tem-
perature was increased from 50 °C to 60 °C and then 70 °C, the rate
increased slightly (entries 1, 4 and 5), while at 80 °C the activity of
the enzyme decreased drastically (entry 6) after a single cycle.
Thus, 70 °C was selected as the optimal temperature for further
experiments.
Because of the relatively modest conversion of 25% obtained
after a cycle (Table 2, entry 5), the enzymatic mixture was pumped
through the reactor five times consecutively, using the same Cat-
Cart filled with CAL-B. As the number of the cycles increased, the
conversion increased progressively (Fig. 1) reaching 41% with an
excellent final E (>200) after the 5th cycle. However, it is notewor-
thy, that the activity of the enzyme decreased significantly after
the 2nd cycle.
reaction stopped after a long run (conv. = 47% after a week, ee
s
=
89%, ee = 99%). To solve this problem, 3-methoxyphenyl allyl car-
p
2
1
bonate was tested in the reaction. Excellent E (> 200), but rather
low reaction rate (conv. = 4% after a week) was observed. There-
fore, phenyl allyl carbonate was used in further reactions.
O
enzyme, acyl donor,
NH
additive
NH
N
O
N
H
N
H
N
H
(
±)-3
(S)-3
(R)-4
Scheme 3. Kinetic resolution of (±)-3 through lipase-catalysed enantioselective N-
acylation.
3
When a catalytic amount of Et N was added to the reaction
2
2
mixture, a relatively fast reaction (conv. = 50% after 36 h) was
observed without the earlier-mentioned deactivation stop before
ees
conv.
5
s p
0% (ee = 97%, ee = 98% at a conversion of 50%).
Lipase AY was also tested for the acylation of racemic 3 with
phenyl allyl carbonate in toluene at 40 °C. Unfortunately, the
enzyme proved to be inactive and only racemic 3 was detected
in the reaction media even after a week.
In view of these preliminary results, the CAL-B-catalysed
preparative-scale resolution of racemic 3 was performed (phenyl
3
allyl carbonate, t-BuOMe, Et N, 50 °C) and 50% conversion was
observed in 48 h (E > 200, ee > 97%) (Table 3).
2
.3. Hydrolysis of (R±-2 and (R±-4
Figure 1. The influence of the number of cycles on reaction rate in the case of (±)-1,
1
5
9
st cycle: ee
5%, conv. = 36%, 4th cycle: ee
9% in each cycle.
s
= 33%, conv. = 25%, 2nd cycle: ee
s
= 50%, conv. = 34%, 3rd cycle: ee
s
=
=
s
= 61%, conv. = 38%, 5th ee = 70%, conv. = 41%, ee
s
p
The removal of the N-allyloxycarbonyl (Alloc) moiety from car-
bamates (R)-2 and (R)-4 was also investigated (Scheme 4).
Finally, the kinetic resolution of (±)-1 was also tested with
2 3 3
Pd (dba) .CHCl
lipase AY and phenyl allyl carbonate in H-Cube (toluene, 40 °C, 1
N
O
NH
bar). Both the reaction rate and the enantioselectivity were rather
low after a cycle (ee = 3%, ee = 53%, conv. = 5%, E = 3), since the
s p
O
(
R)-2, (R)-4
(R)-1, (R)-3
enzyme was compressed due to its powdery structure.
On the basis of the preliminary experiments, the preparative-
scale resolution of (±)-1 was performed with both lipase AY
Scheme 4. Hydrolysis of (R)-2 and (R)-4.
(
conv. = 50%, E > 200, ee = 99%) and CAL-B (conv. = 50%, E > 200,
ee > 97%) enzymes under the optimized conditions in batch
Table 3).
First, the hydrolysis was carried out with triethanolamine in
1
4,23
5
0% aqueous NaOH solution at 120 °C,
but the removal of the
(
protecting groups was not observed even after a one-week treat-
Table 3
Preparative-scale resolution of (±)-1 and (±)-3
2
5
Substrate
Reaction time (h)
24
Enzyme
AY
Conv. (%)
50
E
Enantiomer
ee (%)
Yield (%)
[a]
D
(
(
(
±)-1a
±)-1b
±)-3c
>200
(S)-1
99
d
40
39
41
38
40
41
ꢁ59
g
e
g
(
R)-2
(S)-1
R)-2
(S)-3
R)-4
99
ꢁ104
d
h
49
48
CAL-B
CAL-B
50
50
>200
>200
99
ꢁ58.9
e
g
(
97
ꢁ102
f
g
g
98
ꢁ63
ꢁ99
e
(
97
a
b
c
ꢁ1
0
0
0
.48 M (±)-1, 30 mg mL AY, 30 mL toluene, 4 equiv. of phenyl allyl carbonate, 40 °C.
ꢁ1
.48 M (±)-1, 30 mg mL CAL-B, 30 mL t-BuOMe, 4 equiv. phenyl allyl carbonate, 50 °C.
ꢁ1
.54 M (±)-3, 30 mg mL CAL-B, 15 mL toluene, 4 equiv. phenyl allyl carbonate, 5 ml Et
3
N, 50 °C.
d
e
f
According to HPLC after derivatisation with Ac
According to HPLC.
According to HPLC after derivatisation with (CH
2
O.
CH
3
2 2 2
CH CO) O.
g
h
c 0.3, EtOH.
c 0.55, EtOH.
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4
B. Kovács et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
ment. Then iodine, a non-transition metal catalyst, was tested for
deprotection, in dry acetonitrile, in the presence of water at room
3 2 2 2
times (min) for the (CH CH CH CO) O-derivatised form of (R)-3:
23.71, (S)-3: 26.89 and for (R)-4: 28.43 and (S)-4: 32.43.
2
4
temperature. After 3 days, a relatively small amount of (R)-3
(
yield = 24%, ee = 92%) and no (R)-1 were detected. Next, a Pd cat-
4.2. Syntheses of (± ±-1 and (± ±-3
alyst [tris(dibenzylideneacetone)dipalladium-chloroform adduct,
2
5
Pd
2
(dba)
3
ꢀCHCl
3
] was used for the removal of the Alloc moiety,
Racemic 1 was synthesized from 3,4-dimethoxyphenylethy-
in the presence of formic acid (HCOOH) and triphenylphosphine
lamine (18.12 g, 99.98 mmol) in three steps according to a known
1
0
(
(
(
PPh
3
) in tetrahydrofuran (THF) at 40 °C, under Ar. In both cases
R)-1 and (R)-3 were formed without any loss in enantiopurity
ee = 99%) and in good yields [85% in 2 h for (R)-3 and 60% in
literature method. Pure (±)-1 was obtained as a white solid (7.40
9
g, 36% yield, mp: 48 °C, lit.: 48–49 °C).
1
H NMR (400 MHz, CDCl
); 1.60–1.75 (br s, 1H, NH); 2.62–2.72, 2.77–2.87 (2
m, 2 ꢂ 1H, NH-CH -CH ); 2.96–3.08, 3.23–3.33 (2 m, 2 ꢂ 1H,
NH-CH -CH ); 3.81–3.93 (d, J = 1.8 Hz, 6H, CH-O-CH );
.04–4.12 (q, J = 6.4 Hz, 1H, NH-CH-CH ) 6.60 (s, 1H, Ar); 6.66
s, 1H, Ar). Anal. Calcd for C12 : C, 69.54, H 8.27, N 6.76.
Found: C, 69.52, H 8.20, N 6.81.
Racemic 3 was synthesized from tryptamine (600 mg, 3.75
3
), d (ppm): 1.41–1.51 (d, J = 6.8 Hz,
1
6 h for (R)-1].
3H, CH-CH
3
2
2
2
.4. Absolute configurations
2
2
2
ꢂ
3
4
(
3
The stereochemistry of the enantiomers was determined by
comparing the specific rotation values of the prepared free amines
and 3 with literature data for (R)-salsolidine and the (R)-enan-
H17NO
2
1
1
3
tiomer of its ß-carboline analogue (Experimental). Both enzymes
mmol) in a microwave reactor. (±)-3 was isolated in good yield
560 mg, 80% yield, mp: 177–179 °C, lit.: 177–178 °C) as a pale
2
6
were found to display (R)-selectivity during acylation reactions.
(
yellow solid.
1H NMR (400 MHz, CDCl
) d (ppm): 1.41–1.53 (d, J = 3.3 Hz, 3H,
3
. Conclusions
3
CH-CH
CH
); 2.99–3.11, 3.31–3.43 (2 m, 2 ꢂ 1H, NH-CH
m, 1H, NH-CH-CH ), 7.06–7.18 (m, 2H, Ar); 7.28–7.35 (d, J = 8 Hz,
H, Ar) 7.45–7.51 (d, J = 8 Hz, 1H, Ar); 7.68–7.85 (br s, 1H, Ar-NH).
3
); 1.54–1.67 (br s, 1H, CH-NH); 2.67–2.83 (m, 2H, NH-CH
2
-
Efficient new enzymatic methods have been developed for the
2
2
-CH ), 4.14–4.24
2
synthesis of the enantiomers of salsolidine and its ß-carboline ana-
logue. Excellent selectivity E (>200) was observed upon performing
the acylation of (±)-1 in the presence of CAL-B with phenyl allyl
carbonate in t-BuOMe in both batch mode and a continuous-flow
system. The same high E (>200) characterised the resolution of
(
1
3
Anal. Calcd for C12
H 7.62, N 15. 09.
14 2
H N : C, 77.38, H 7.58N 15.04. Found: C, 77.33,
(
±)-1, when the acylation was carried out in the presence of lipase
4
.3. Small-scale enzymatic reactions
AY with phenyl allyl carbonate in toluene at 40 °C. CAL-B catalysed
the enantioselective acylation of (±)-3 with excellent E (>200),
when the reaction was performed with phenyl allyl carbonate, in
the presence of Et N in t-BuOMe at 50 °C. To the best of our knowl-
3
edge, (±)-3 was resolved for the first time by using enzymes. Car-
Preliminary small-scale experiments were carried out in both
batch and continuous-flow systems. In batch, racemic (±)-1 or
±)-3 (0.025 mmol) was dissolved in an organic solvent (1 mL). 4
equiv. acyl donor, 30 mg enzyme and 1 L Et N used as an additive
in the case of (±)-3 were added and the reaction mixtures were
shaken in an incubator shaker at 40–60 °C. During continuous-flow
investigations, (±)-1 (0.025 mmol) was dissolved in t-BuOMe
(
l
3
bamate (R)-2 and (R)-4 were hydrolysed into the corresponding
amines with Pd
of PPh and HCOOH. The products were formed with excellent
enantiopurity [ee = 99% for both (R)-1 and (R)-3] and in good yields
60% of (R)-1 and 85% of (R)-3].
2
(dba)
3
3
ꢀCHCl as catalyst in THF, in the presence
3
(
1 mL) and phenyl allyl carbonate (4 equiv.) was added. The solu-
tion was pumped through the compressed (1–60 bar) and heated
50–80 °C) CAL-B-filled cartridge (244 mg) in H-Cube in ’no H
[
(
2
ꢁ1
4
4
. Experimental
mode’ with 0.1 mL min flow rate. In the case of the AY-catalysed
reaction (250 mg), the solution was pumped through the CatCart
cartridge (1 bar, 40 °C) with 1.8 mL min flow rate.
ꢁ1
.1. Materials and methods
CAL-B (lipase B from Candida antarctica) immobilized on acrylic
4.4. Preparative-scale resolution of (± ±-1
resin was purchased from Sigma, and lipase PS-IM (Burkholderia
Cepacia) immobilized on diatomaceous earth was from Amano
Enzyme Europe Ltd. Lipase AY (Candida rugosa) was from Fluka
and CAL-A (lipase A from Candida antarctica) from Novo Nordisk.
Racemic 1 (100 mg, 0.48 mmol) was dissolved in toluene (30
ꢁ1
mL). After adding lipase AY (900 mg, 30 mg mL ) and phenyl allyl
carbonate (0.31 mL, 1.91 mmol, 4 equiv.), the reaction mixture was
shaken in an incubator shaker at 40 °C for 24 h. The reaction was
stopped at 50% conversion by filtering off the enzyme and washing
it with toluene (2 ꢂ 30 mL) followed evaporation of the solvent.
The products [(S)-1, (R)-2] were separated by column chromatog-
raphy on silica, with the elution of CH Cl /MeOH (1:1), affording
Optical rotations were measured on
a Perkin-Elmer 341
1
polarimeter. H NMR spectra were recorded on a Burker Avance
DRX 400 spectrometer. Melting points were determined on a Kofler
apparatus. Microwave (MW) reactions were performed in a CEM
Discover MW reactor (Matthews, NC, USA). The elemental analysis
was measured by means of a Perkin-Elmer CHNS-2400 Ser II Ele-
2
2
2
5
carbamate (R)-2 {55 mg, 39%, [
a
]
D
= ꢁ104 (c 0.3, EtOH) colourless
2
mental Analyzer. The H-Cube reactor used in ‘no H mode’ and
oil, ee = 99%}. The free amine (S)-1 was crystallized from hexane/
2
5
14
25
equipped with a stainless steel enzyme-charged cartridge (70
mm length, 4 mm inside diameter) was from ThalesNano Inc.
The ee values of the enantiomers were determined by HPLC
EtOAc (2:1) {40 mg, 40%, [
a
]
= ꢁ59 (c 0.3, EtOH) lit.:
[a
]
D
=
D
1
4
ꢁ58.5 (c 0.5, EtOH) white crystalline product, mp: 47 °C, lit.:
47–48 °C, ee = 99%}.
[
(
Chiralpak IA column (4.6 mm ꢂ 250 mm)]. Eluent: n-hexane/iPa
(±)-1 (100 mg, 0.48 mmol) was dissolved in t-BuOMe (30 mL).
After adding CAL-B (900 mg, 30 mg mL ) and phenyl allyl carbon-
ꢁ1
ꢁ1
80:20), flow rate: 0.5 mL min , 260 nm; retention times (min)
O-derivatised form of (S)-1: 21.10, for (R)-1: 26.96 and
for Ac
2
ate (0.31 mL, 1.91 mmol, 4 equiv.), the reaction was carried out in
an incubator shaker at 50 °C in 49 h. Separation of the enantiomers
was performed as above, affording carbamate (R)-2 {53 mg, 38%,
ꢁ1
eluent: n-hexane/iPa (96:4), flow rate: 0.5 mL min , 220 nm;
retention times (min) for (R)-2: 67.53, (S)-2: 73.73. In the case of
(
2
5
±)-3, the eluent ratio was n-hexane/iPa (90:10), 240 nm; retention
[a]
= ꢁ102 (c 0.3, EtOH), colourless oil, ee = 97%} and free amine
D
Please cite this article in press as: Kovács, B.; et al. Tetrahedron: Asymmetry (2017), https://doi.org/10.1016/j.tetasy.2017.10.019

B. Kovács et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
5
2
5
(
0
S)-1 crystallizedinhexane/EtOAc(2:1) {41 mg, 41%, [
a]
D
= ꢁ58.9 (c
was removed on a celite column with MeOH. Product (R)-1 was
.55, EtOH), white crystalline product, mp: 46–47 °C, ee = 99%}.
purified by column chromatography with CH
(89:10:1). The amine was dissolved in 5 mL H O and saturated
NaHCO solution was added until pH > 9. The aqueous media was
extracted with CHCl
(2 ꢂ 7 mL). After evaporation of the organic
media, (R)-1 was crystallized from hexane/EtOAc (2:1) as a white
2 2 3
Cl /MeOH/Et N
1
H NMR (400 MHz, CDCl
Hz, 3H, CH-CH
3
1
4
1
3
) for (R)-2: d (ppm): 1.42–1.49 (d, J = 6.0
2
3
); 2.58–2.72, 2.79–2.96 (2m, 2 ꢂ 1H, NH-CH -CH );
.12–3.37, 3.49–3.69 (2 m, 2 ꢂ 1H, NH-CH -CH ); 3.80–3.89 (d, J =
.1 Hz, 6H, 2 ꢂ CH-O-CH ), 4.04–4.31 (m, 1H, NH-CH-CH ), 4.54–
.73 (d, J = 5.2 Hz, 2H, CH -CH@CH ); 5.17–5.26 (dd, J = 1.3 Hz,
0.4 Hz, 1H, CH@CH ); 5.28–5.37 (dd, J = 1.5 Hz, 17.5 Hz, 1H,
); 5.91–6.02 (m, 1H, CH@CH ); 6.59 (br s, 2H, Ar). Anal. Calcd
: C, 65.96, H 7.27, N 4.81. Found: C, 65.92, H 7.21, N
2
2
3
2
2
3
3
3
2
5
14
25
2
2
solid {23.4 mg, 60%, [
a
]
D
= +60, (c 0.3, EtOH), lit.:
D
[a] = +59.2
1
4
1
2
(c 1.0, EtOH), mp: 46 °C, lit.: 47–48 °C, ee = 99%}. The H NMR
CH@CH
2
2
3
(400 MHz, CDCl ) spectroscopic data for (R)-1 were similar to those
for (±)-1.
for C16
.86.
H21NO
4
4
In the case of (R)-4 (60 mg, 0.22 mmol), the reaction was com-
pleted in 2 h, providing (R)-3 as a pale yellow solid (crystallized
1
The H NMR (400 MHz, CDCl
3
) spectroscopic data for (S)-1 were
2
5
27
25
similar to those for (±)-1.
.5. Preparative-scale resolution of (± ±-3
±)-3 (100 mg, 0.54 mmol) dissolved in t-BuOMe (15 mL) was
from hexane) {34.9 mg, 85%, [
a]
D
= +62, (c 0.3, EtOH), lit.:
[a]
D
2
7
=
+55.6 (c 2.0, EtOH), mp: 178–180 °C, lit.: 179–181 °C, ee =
99%}. The H NMR (400 MHz, CDCl ) spectroscopic data for (R)-3
3
1
4
were similar to those for (±)-3.
(
ꢁ1
mixed with CAL-B (450 mg, 30 mg mL ), phenyl allyl carbonate
0.35 mL, 2.15 mmol, 4 equiv.) and Et N (5 L). The reaction mix-
Acknowledgements
(
3
l
ture was shaken for 48 h in an incubator shaker at 50 °C. When a
conversion of 50% was observed, the enzyme was filtered off and
washed with t-BuOMe (2 ꢂ 15 mL). Carbamate (R)-4 was separated
The authors are thankful to Professor Reko Leino and Risto
Savela for the opportunity to investigate the hydrolysis in Labora-
tory of Organic Chemistry, Åbo Akademi University, Finland and to
the Hungarian Scientific Research Council (OTKA, K108943 and
K115731) for financial support.
2 2
from free amine (S)-3 by column cromatography with CH Cl /
MeOH (1:1). Carbamate (R)-4 was obtained as a colourless oil
2
5
{
60 mg, 41%, [
a]
D
= ꢁ99 (c = 0.3, EtOH), ee = 97%}, and secondary
25
amine (S)-3 was crystallized in hexane {40 mg, 40%, [
c 0.3, EtOH), lit.:
c 2.0, EtOH), pale yellow solid, mp: 177–178 °C, lit.: 179–181
a]
D
= ꢁ63
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25
27
25
(
(
[a]
D
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[a
]
D
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27
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{
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Please cite this article in press as: Kovács, B.; et al. Tetrahedron: Asymmetry (2017), https://doi.org/10.1016/j.tetasy.2017.10.019