Biocatalytic asymmetric synthesis of optically pure aromatic propargylic amines employing ω-transaminases

Article abstract of DOI:10.1002/adsc.201500086

The asymmetric reductive bio-amination of prochiral aromatic propargyl ketones led to the corresponding amines in optically pure form (ee>99%). The (R)- as well as the (S)-enantiomers of the propargylic amines were obtained, employing either (R)-selective ω-transaminases (ω-TAs) originating from Arthrobacter sp. and Aspergillus terreus or an (S)-selective ω-TA from Chromobacterium violaceum. The product propargylic amines were obtained with high conversions (up to 99%). To simplify product isolation, protection of the free amino group to the corresponding acetamides or benzamides was performed without loss of optical puritiy. The final products were isolated in moderate to good yields (33-67% over two steps) in optical pure form without additional purification steps. Although propargyl ketones are described in the literature to be irreversible inhibitors for aminotransferases, suitable ω-transaminases were identified for the amination of these compounds.

Full text of DOI:10.1002/adsc.201500086

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DOI: 10.1002/adsc.201500086
Biocatalytic Asymmetric Synthesis of Optically Pure Aromatic
Propargylic Amines Employing w-Transaminases
Nina G. Schmidt,a,b Robert C. Simon,a and Wolfgang Kroutila,b,*
a
Department of Chemistry, Organic and Bioorganic Chemistry, University of Graz, NAWI Graz, Heinrichstraße 28, 8010
Graz, Austria
Fax : (+43)-316-380-9840; phone: (+43)-316-380-5350; e-mail: wolfgang.kroutil@uni-graz.at
ACIB GmbH c/o, Department of Chemistry, University of Graz, Heinrichstraße 28, 8010 Graz, Austria
b
Received: January 28, 2015; Revised: February 18, 2015; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500086.
Abstract: The asymmetric reductive bio-amination performed without loss of optical puritiy. The final
of prochiral aromatic propargyl ketones led to the products were isolated in moderate to good yields
corresponding amines in optically pure form (ee> (33–67% over two steps) in optical pure form with-
99%). The (R)- as well as the (S)-enantiomers of the out additional purification steps. Although propargyl
propargylic amines were obtained, employing either ketones are described in the literature to be irrever-
(R)-selective w-transaminases (w-TAs) originating sible inhibitors for aminotransferases, suitable w-
from Arthrobacter sp. and Aspergillus terreus or an transaminases were identified for the amination of
(S)-selective w-TA from Chromobacterium viola- these compounds.
ceum. The product propargylic amines were obtained
with high conversions (up to 99%). To simplify prod-
uct isolation, protection of the free amino group to Keywords: amines; asymmetric catalysis; biotransfor-
the corresponding acetamides or benzamides was mations; enzymes; propargylic amines
Introduction
coupling reaction of an aldehyde, alkyne and amine,
also referred to as A3-coupling.[9]
Propargylic amines unite the nucleophilic properties
While the nucleophilic 1,2-addition allows the
of an amine group and the moderate electrophilicity asymmetric synthesis of chiral propargylic amines
of a triple bond in a single molecule. These features
make substituted propargylic amines versatile build-
ing blocks for organic synthesis, especially for the
preparation of nitrogen-containing heterocycles such
as 2-aminoimidazoles,[1] oxazolidones,[2] pyrimidones[3]
or triazoles.[4] Additionally, they constitute a key scaf-
fold in various pharmaceuticals and biologically active
compounds,[5] including monoamine oxidase
B
(MAO-B) and HIV-1 reverse transcriptase inhibi-
tors.[6]
Due to the broad applicability of chiral propargylic
amines, a convenient straight-forward procedure for
their preparation in optical pure form is highly de-
manded. Thereby, the combination of late transition
state metals and chiral ligands for the alkynylation of
aldimines can be seen as state of the art (Scheme 1).[7]
This reaction requires the use of activated C=N spe-
cies such as nitrones, N-acyl-, N-sulfonyl-, N-tosyl- or
N-mesylimines.[8] To circumvent the need of external-
ly prepared, activated electrophiles, this classical
Scheme 1. Conventional routes for the preparation of chiral
propargylic amines, including the nucleophilic 1,2-additon to
activated aldimines and the non-enzymatic kinetic resolution
methodology was extended to a three-component of racemic propargylic amines.
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Scheme 2. Asymmetric amination of ketones 1a–f using (R)- or (S)-selective w-transaminases.
from scratch, the non-enzymatic kinetic resolution gillus terreus (AT-w-TA)[16] as well as the (S)-selective
employs chiral reagents in combination with acylation w-TA from Chromobacterium violaceum (CV-w-
reagents to resolve racemic propargylic amines.[10] Al- TA)[17] successfully converted 1a to the chiral propar-
though this approach gives rise to a broad range of gylic amine 2a.
free propargylic amines, its main drawback is the limi-
It is worth mentioning that other w-TAs like the
tation to a maximum possible yield of 50%. Besides one from Bacillus megaterium,[18] Paracoccus denitrifi-
these numerous chemical approaches for the synthesis cans,[19] Pseudomonas putida, Vibrio fluvialis[20] or Ar-
of optically pure propargylic amines, no biocatalytic throbacter citreus[21] did not transform 1a, although
methodology has been reported to access propargylic they have been described to possess a broad substrate
amines to the best of our knowledge; only propargylic tolerance, involving bulky and long chained sub-
alcohols have been prepared by biocatalytic ap- strates.[22] Having identified a suitable (S)-selective w-
proaches.[11]
TA and two (R)-selective w-TAs, the reaction condi-
The application of enzymes in organic synthesis has tions were optimised with respect to substrate concen-
proven to be an alternative for established chemical tration and co-solvent. Optimum substrate concentra-
processes.[12] Important benefits of biocatalysts are tions ranged from 10 mM to 25 mM. At higher con-
mild operation conditions such as aqueous media, centrations of 1a, a side product was detected in sig-
temperatures between 208C and 408C and physiologi- nificant amounts, which was proposed to be imine 3a
cal pH values. In general, they are air- and water- according to mass spectroscopy. Imine 3a is in equilib-
stable and apart from the environmental benefits, en- rium with substrate ketone 1a and product amine 2a.
zymes display high chemo-, regio- and stereoselectivi- Furthermore, extended reaction times (>24 h) result-
ties. Consequently, chiral primary amines have al- ed in the degradation of the product amine as de-
ready been successfully accessed via biocatalysis.[13] In duced from non-identified products observed by GC
this context, w-transaminases or aminotransferases and a colour change of the reaction mixture. On com-
(E.C. 2.6.1.X), which catalyse the transfer of an amino paring the time courses of the bioreduction of 1a at
group from an amine donor to a ketone (i.e., amine 25 and 50 mM with ArR-w-TA up to 72 h, imine for-
acceptor), have gained significant attention as they mation was highest within the first 3 h of the reaction
exhibit a broad substrate spectrum.[14] However, the (Figure 1). Due to the transformation of ketone 1 to
preparation of propargylic amines from propargyl ke- amine 2 and the equilibrium between imine 3 and
tones has not been reported as yet.
ketone/amine, the concentration of imine 3 depleted
over time.
As the reaction proceeded, the amount of imine de-
creased to a minimum of 4% in the case of 25 mM
substrate concentration whereas at 50 mM, the
Results and Discussion
Firstly, prochiral propargyl ketones 1b–f bearing elec- amount of imine remained at 20%. In both cases the
tron-donating or electron-withdrawing substituents at starting ketone 1a was not detectable anymore after
the phenyl moiety were prepared via a single-step 24 h. Prolonging the reaction time did not result in
synthesis from commercially available aromatic acety- imine depletion; however degradation of product 2a
lenes as previously described[11b] (see the Supporting became predominant.
Information).
Consequently, various phenyl-substituted propargyl
Then various enantiocomplementary w-transami- ketones 1a–f were subjected to the bioamination in
nases were tested for the reductive amination of the presence of CV-, ArR- or AT-w-TA. All sub-
model substrate 1a (Scheme 2). The (R)-selective w- strates were successfully transformed with high con-
TAs from Arthrobacter sp. (ArR-w-TA)[15] and Asper- versions and perfect stereoselectivity (ee >99%)
2
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Biocatalytic Asymmetric Synthesis of Optically Pure Aromatic Propargylic Amines
Table 2, and Table 3 entries 2–4). In most cases, the
starting material was completely consumed within
24 h and the amount of detected imine 3a–d was
always below 10%. However, if the substrates carried
an electron-withdrawing group at the phenyl ring
(i.e.. substrates 1e and 1f), conversions to products 2e
and 2f were lower, independent of the w-TA used.
In more detail, the (S)-selective w-transaminase
CV-w-TA (Table 1) transformed all para-substituted
substrates with perfect stereoselectivity (ee >99%) to
the corresponding (S)-amine (S)-2 (entries 1–3, 5, and
6) and the meta-methoxy-substituted ketone 1d to
(S)-2d with 96% ee (entry 4). The addition of co-sol-
vent (10% vvÀ1) had a positive effect on the forma-
tion of amine 2, particularly for substrate 1d, leading
to an increase of product formation of 2d from 82%
to 97%. Additionally, in the presence of DMSO the
ee for 2d improved from 96% to >99%. On the other
hand, DMSO led to decreased conversions for 1c, 1e
*
*
Figure 1. Time course for the formation of amine 2a ( ,
)
&
and imine 3a (&, ) during the ArR-w-TA-catalysed amina-
tion of ketone 1a at 25 mM (filled symbols) and 50 mM
(open symbols).
(Table 1, Table 2, and Table 3). For substrates bearing and 1f, whereas for 1a and 1b the conversion re-
an electron-donating group in para or meta positions mained at 99%.
in particular, the conversion to the desired optically
The (R)-selective w-TA variant originating from Ar-
pure propargylic amines was over 90% (Table 1, throbacter sp. transformed all ketones to the (R)-
Table 1. Asymmetric reductive amination of ketones 1a–f catalysed by CV-w-TA.[a]
Entry
Substrate
Without co-solvent
DMSO (10% v/v)
1 [%]
2 [%]
ee 2 [%][c]
3 [%]
1 [%]
2 [%]
ee 2 [%][c]
3 [%]
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
<1
<1
3
18
<1
21
99
99
97
>99 (S)
>99 (S)
>99 (S)
96 (S)
>99 (S)
>99 (S)
<1
<1
<1
<1
<1
<1
<1
<1
10
3
32
10
99
99
89
>99 (S)
>99 (S)
>99 (S)
>99 (S)
>99 (S)
>99 (S)
<1
<1
1
<1
37
82
97
83[b]
58[b]
19[b]
54[b]
<1
[a]
Reaction conditions: substrate 1 (10 mM), lyophilised E. coli cells containing the (S)-selective CV-w-TA (20 mg), l-ala-
nine (250 mM), ammonium formate (150 mM), phosphate buffer (100 mM, pH 7), PLP (1 mM), NAD+ (1 mM), FDH
(11U), AlaDH (12U), 24 h, 308C, in the case of DMSO: 10% v/v. Reaction volume=1 mL.
An unidentified side product was detected by GC analysis; therefore, the percentages of 1, 2 and 3 do not sum up to
100%.
[b]
[c]
Determined by GC on a chiral phase after acetylation to the corresponding acetamide derivative 4.
Table 2. Asymmetric reductive amination of ketones 1a–f catalysed by ArR-w-TA.[a]
Entry
Substrate
Without co-solvent
DMSO (10% v/v)
1 [%]
2 [%]
ee 2 [%][b]
3 [%]
1 [%]
2 [%]
ee 2 [%][b]
3 [%]
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
<1
<1
<1
2
9
34
99
92
96
95
64
46
>99 (R)
>99 (R)
>99 (R)
>99 (R)
98 (R)
<1
8
4
3
27
20
<1
1
<1
3
<1
<1
98
90
96
97
65
76
>99 (R)
>99 (R)
>99 (R)
>99 (R)
98 (R)
1
9
4
3
34
23
>99 (R)
>99 (R)
[a]
Reaction conditions: substrate (25 mM), lyophilised E. coli cells (20 mg) containing the (R)-selective ArR-w-TA, d-ala-
nine (250 mM), ammonium formate (150 mM), phosphate buffer (100 mM, pH 7.0), PLP (1 mM), NAD+ (1 mM), FDH
(11 U), AlaDH (12 U), 24 h, 308C. Reaction volume=1 mL.
[b]
Determined by GC on a chiral phase after acetylation to the corresponding acetamide derivative 4.
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Table 3. Asymmetric reductive amination of ketones 1a–f catalysed by AT-w-TA.[a]
Entry
Substrate
Without co-solvent
DMSO (10% v/v)
1 [%]
2 [%]
ee 2 [%][c]
3 [%]
1 [%]
2 [%]
ee 2 [%][c]
3 [%]
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
<1
<1
<1
<1
59
>99
>99
95[b]
97[b]
35
>99 (R)
>99 (R)
>99 (R)
>99 (R)
>99 (R)
>99 (R)
<1
<1
<1
<1
6
<1
<1
<1
<1
<1
<1
>99
>99
97[b]
97[b]
90
>99 (R)
>99 (R)
>99 (R)
>99 (R)
>99 (R)
>99 (R)
<1
<1
<1
<1
10
25
70[b]
1
94[b]
2
Entry
Substrate
1,2-Dimethoxyethane (10% v/v)
DMF (10% v/v)
1 [%]
2 [%]
ee 2 [%][c]
3 [%]
1 [%]
2 [%]
ee 2 [%][c]
3 [%]
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
1
1
1
<1
<1
<1
98
>99 (R)
>99 (R)
>99 (R)
>99 (R)
>99 (R)
>99 (R)
1
6
<1
<1
38
11
<1
<1
<1
<1
<1
<1
>99
>99
97[b]
97[b]
95
>99 (R)
>99 (R)
>99 (R)
>99 (R)
>99 (R)
>99 (R)
<1
<1
<1
<1
5
93
95[b]
96[b]
62
85[b]
96[b]
1
[a]
Reaction conditions: substrate (10 mM), lyophilised E. coli cells (20 mg) containing the (R)-selective AT-w-TA, d-alanine
(250 mM), ammonium formate (150 mM), phosphate buffer (100 mM, pH 7), PLP (1 mM), NAD+ (1 mM), FDH (11 U),
AlaDH (12 U), 24 h, 308C. Reaction volume=1 mL.
An unidentified side product was detected by GC-analysis; that is why the percentages of 1, 2 and 3 do not sum up to
100%.
[b]
[c]
Determined by GC on a chiral phase after acetylation to the corresponding acetamide derivative 4.
propargylic amines with perfect ee (>99%) at 25 mM
(Table 2), except for 1e, which led to the correspond-
ing amine (R)-2e with 98% ee. The enzyme preserved
its excellent stereoselectivity also in the presence of
DMSO (10% vvÀ1), thus in all cases identical ees
were obtained as in the absence of DMSO. The
second (R)-selective w-transaminase from Aspergillus
terreus (AT-w-TA) aminated all substrates to the de-
sired (R)-propargylic amines with perfect stereoselec-
tivity (ee >99%) (Table 3). Studying the influence of
different co-solvents such as DMSO, 1,2-dimethoxy-
ethane (DME) and dimethylformamide (DMF) on
the conversion, it was noticed that their addition im-
proved the biotransformation of all substrates com-
pared to reactions run without co-solvent.[23]
The obtained results clearly indicate that propargyl
ketones can be aminated and are therefore not neces-
sarily aminotransferase inhibitors acting via the for-
mation of a covalent bond to an active site amino
acid residue as previously proposed.[24]
To enable the application of the propargylic amines
as building blocks for further syntheses as well as the
isolation on preparative scale, derivatisation of the
free amino group was required. Attempts to directly
isolate the chiral amines 2a–f from an up-scaled bio-
transformation failed, due to the quick degradation of
Table 4. Derivatisation experiments of (R)-2a.
Entry (R)-2a
Reagents
Temp.
[8C]
Time Yield[a]
[h]
[mmol]
[%]
1
2
3
4
5
0.25
0.10
0.25
0.25
0.25
Ac2O, pyr
21
15
45
Ac2O, Et3N
AcCl, Et3N
BzCl, Et3N
0–21[b] 15
65
0–21[b]
0–21[b]
3
3
43[c]
26[c]
58[d]
Boc2O, Et3N 0–21[b] 15
[a]
[b]
[c]
[d]
Isolated yields over two reaction steps, i.e., biotransfor-
mation and acylation.
The reaction was started at 08C and after all reagents
were added the mixture was allowed to warm to 218C.
Reaction was performed under an inert atmosphere, with
freshly distilled acid chlorides and dry solvents.
Residual Boc2O impurities after flash chromatography.
the propargylic amines which had already been re- ticular attention was devoted to the N-acylation of
ported previously.[25] Consequently, various N-protec- (R)-2a to yield the acetylated (R)-4a and the benzoy-
tion strategies were investigated to obtain the chiral lated derivative (R)-5a for two reasons: first and fore-
products without loss of optical purity (Table 4). Par- most, the required acid chlorides/anhydrides are
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Biocatalytic Asymmetric Synthesis of Optically Pure Aromatic Propargylic Amines
Table 5. Isolated yields for acetamides and benzamides after
asymmetric amination of ketone 1 and derivatisation.[a]
Conclusions
In summary, the first asymmetric reductive amination
of a set of prochiral aromatic propargyl ketones has
been reported. Although propargylic amines have
previously been reported to be irreversible inhibitors
for aminotransferases,[24] suitable w-transaminases
were identified for this reaction. Different substrates,
bearing an electron-donating or electron-withdrawing
group attached to the phenyl ring could be successful-
ly transformed to the (S)- or (R)-propargylic amines
by three known w-transaminases. In general, the (S)-
and (R)-amine products 2a–f were obtained with per-
fect ee (>99%) after optimisation of the reaction con-
ditions and depending on the choice of the enzyme.
Conversions ranged from moderate for substrates 1e–
f to (almost) completion for 1a–d. Employing water
miscible co-solvents (10% vvÀ1) such as DMSO,
DME or DMF led to enhanced conversions in select-
ed cases. Due to the low stability of unprotected
propargylic amines, derivatisation to the stable prop-
argylic amides was performed following the prepara-
tive biotransformation without loss of optical purity.
Entry w-TA Amide[a]
R
Yield[b] [%] ee [%]
1
2
3
4
5
6
7
AT
CV
AT
AT
AT
AT
AT
4a
4a
4b
4c
4d
5e
4f
H
H
65
67
54
>99 (R)
>99 (S)
>99 (R)
>99 (R)
>99 (R)
>99 (R)
>99 (R)
p-Me
p-MeO 64
m-MeO 45
p-CF3
p-Br
33
36
[a]
Reaction conditions: for amines 2a–d, f (0.1–0.2 mmol),
Et3N (6.0 equiv.), Ac2O (2.0 equiv.) in CH2Cl2, 08C to
room temperature, overnight; for amine 2e, Et3N
(1.5 equiv.), BzCl (1.0 equiv.) in absolute CH2Cl2, 08C to
room temperature, 2 h, N2.
[b]
Isolated yields over two reaction steps, i.e., biotransfor-
mation and acylation. DMSO (10% v/v) supported prep-
arative biotransformations were performed using AT-w-
TA (200 mg), substrate 1a–f (0.1–0.2 mmol). Theoretical
yields for 4a–f were calculated based on the conversions
for (R)-2a–f (Table 3).
cheap and readily available; furthermore, the generat- The procedure presented represents a reliable and
ed amides are stable solids that can be easily isolated. simple approach for the asymmetric synthesis of opti-
Acetylation was more efficient than the benzoyla- cally pure propargylic amines, which are important
tion when applying acid chlorides (entries 3 and 4). chiral building blocks.
Regarding the acetylation with acetic anhydride, it
was observed that a decreased amine concentration
(10 mM instead of 25 mM, calculated based on the Experimental Section
conversion of the biotransformation), as well as the
switch from pyridine to Et3N positively influenced the General Remarks
reaction as higher yields of acetamide (R)-4a were ob-
tained (entries 1 and 2). Additionally, it has to be
All chemicals used in this study were purchased from com-
mercial suppliers and used as received unless stated other-
noted that the application of acetic anhydride allowed
a simplified work-up procedure via extraction, while
flash chromatography was required to purify reactions
using the acid chlorides. Alternatively to the acyla-
tion, carbamate (R)-6a was successfully obtained by
N-Boc protection. Since Boc2O required more purifi-
cation steps, acetylation using acetic anhydride and
Et3N was favoured. After having identified an appro-
priate protection procedure, preparative asymmetric
bioamination was performed for substrates 1a–f em-
ploying AT-w-TA.
The chiral product amines (R)-2a–f were directly
derivatised to (R)-4a–d and 4f as well as (R)-5e which
were obtained with reasonable yields (Table 5). It is
worth mentioning that converting the propargylic
amines to the corresponding amides did not affect the
optical purity, thus the ees remained >99%. Just to
have one (S)-product prepared as well, (S)-4a was iso-
lated with ee >99% (entry 2). The absolute configura-
tions of (R)- or (S)-4a–f and (R)-5e were determined
via measurement of the optical rotations and compar-
ison to values recently published (see the Supporting
Information).[10a,b]
wise. Preparative chromatographic separations were per-
formed by column chromatography on Merck silica gel 60
(0.063–0.200 mm). Optical rotations at the sodium D-line
were measured at 258C on a Perkin–Elmer polarimeter 341.
GC and GC-MS were recorded with an Agilent 7890A GC
system. The GC-MS system was equipped with an Agilent
5975C mass selective detector and an HP-5 MS column
(30 mꢁ0.25 mmꢁ0.25 mm; helium as carrier gas [flow=
1
0.55 mLminÀ1]). H and 13C NMR spectra were recorded at
208C on a 300 Bruker NMR unit; chemical shifts are given
in ppm relative to Me4Si or relative to the resonance of the
solvent. Formate dehydrogenase (2.1 UmgÀ1) was purchased
from Evocatal (Evo 1.1.230). All w-transaminases were
overexpressed in E. coli BL21 (DE3) and used as lyophi-
lised cell preparations as reported previously.[14b,23e,26] l-Ala-
nine dehydrogenase from Bacillus subtilis was prepared and
purified as described recently.[23e] All biotransformations
were carried out in an Infors Unitron shaker.
Representative Procedure for the Synthesis of (R)-
Phenylbut-3-yn-2-yl-amine (R)-2a (Analytical Scale)
Lyophilised cells of E. coli containing the overexpressed
AT-w-TA (20 mg) were rehydrated in sodium phosphate
buffer (pH 7.0, 100 mM, 334 mL) at 308C for 30 min. Buffer
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solutions of PLP (50 mL, 20 mM), NAD+ (50 mL, 20 mM),
NH4COOH (100 mL, 1.5M), d-alanine (250 mL, 1M), FDH
(5 mg, 11 U), and AlaDH (15 mL, 12 U) were added to the
reaction vessel. Finally, addition of substrate 1a (1.44 mg,
0.01 mmol) – if required in solution with the co-solvent
(100 mL, 10% v/v) – started the bio-transformation which
was shaken at 120 rpm and 308C for 24 h. The reaction mix-
ture was quenched by addition of saturated Na2CO3 solution
(200 mL) and vigorous shaking. After extraction with EtOAc
(2ꢁ500 mL) the combined organic layers were dried over
MgSO4 and the conversion was measured by GC.
COMET Funding Program. Support by NAWI Graz is ac-
knowledged.
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Representative Procedure for the Synthesis of (R)-
Phenylbut-3-yn-2-yl-amine (R)-2a (Preparative Scale)
For the preparative biotransformation, the lyophilised E.
coli/w-TA preparation (200 mg) and substrate 1a (14.4 mg,
0.1 mmol) were incubated in sodium phosphate buffer
(pH 7.0, 100 mM, 3.34 mL) containing PLP (1 mM), NAD+
(1 mM), NH4COOH (150 mM), d-alanine (250 mM), FDH
(110 U), Ala-DH (120 U) and DMSO (10% v/v) under the
same conditions as described above. The reaction was
stopped by adding 2N HCl (2.5 mL), followed by extraction
with CH2Cl2 (5 mL) in order to remove any remaining start-
ing material. The aqueous layer was basified with 6N
NaOH (1 mL) to pH 10–11 and extracted with Et2O (2ꢁ
5 mL). The collected organic layers were dried over MgSO4.
After removal of the solvent under reduced pressure, the re-
sidual oil was immediately used for derivatisation.
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Representative Procedure for the Synthesis of (R)-N-
(4-Phenylbut-3-yn-2-yl)acetamide (R)-(4a)
The freshly isolated propargylic amine (R)-2a (0.1 mmol)
was dissolved in CH2Cl2 (5 mL). The mixture was placed on
ice and Et3N (83 mL, 0.6 mmol) was added under vigorous
stirring. Then, Ac2O (20 mL, 0.2 mmol) was added dropwise
and stirring was continued at room temperature overnight.
Addition of water (3 mL) and stirring for another 30 min
stopped the reaction. The organic layer was washed with
5% H2SO4 (2ꢁ25 mL) and saturated Na2CO3 (2ꢁ25 mL)
prior to drying over MgSO4. The solvent was removed
under reduced pressure to the yield the optically pure acet-
amide (R)-4a as
a
colourless solid; yield: 12.2 mg
(0.065 mmol, 65%); Rf =0.3 (CH2Cl2/EtOAc, 90:10); mp 90–
1
958C. H NMR (300 MHz, CDCl3, 208C, TMS): d=1.48 (d,
3
3JH,H =6.9 Hz, 3H, CH3), 2.01 (s, 3H, CH3), 5.05 (dq, JH,H
=
6.9 Hz, 1H, CH), 5.93 (brd, 3JH,H =6.7 Hz, 1H, NH), 7.30
(mc, 3H, Ar), 7.40 (mc, 2H, Ar); 13C NMR (75 MHz,
CDCl3): d=22.6 (CH3), 23.3 (CH3), 37.7 (CH), 82.2 (C),
89.4 (C), 122.5 (C), 128.3 (CH), 128.4 (CH), 131.7 (CH),
168.9 (C); IR (ATR-film): n=3287, 3056, 2977, 2927, 1636
(C=O), 1539, 1367, 1275, 1132, 976, 760, 712, 689, 601 cmÀ1
;
GC-MS (70 eV, EI): m/z (%)=187 (18) [M+ÀH], 172 (100)
[C11H10NO+]; [a]D25: +183 (c 0.51, CHCl3), >99% ee; Lit.[10a]
for (S)-amine [a]D20: À163 (c 0.6, CHCl3), 91% ee.
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Acknowledgements
N. G. S. is supported by the Austrian BMWFJ, BMVIT, SFG,
Standortagentur Tirol and ZIT through the Austrian FFG-
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FULL PAPERS
8 Biocatalytic Asymmetric Synthesis of Optically Pure
Aromatic Propargylic Amines Employing w-Transaminases
Adv. Synth. Catal. 2015, 357, 1 – 8
Nina G. Schmidt, Robert C. Simon, Wolfgang Kroutil*
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