Chemoenzymatic synthesis and application of a new, easily chiral auxiliary for the synthesis of peptidomimetics via an Ugi reaction

Article abstract of DOI:10.1016/j.tetasy.2014.01.016

A new chiral auxiliary derived from α-phenylethylamine, α-(2,4-dimethoxyphenyl)ethylamine is presented. It significantly expands upon the application of α-phenylethylamine derivatives used as chiral auxiliaries. A straightforward, chemoenzymatic synthesis

Full text of DOI:10.1016/j.tetasy.2014.01.016

Tetrahedron: Asymmetry 25 (2014) 435–442
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Chemoenzymatic synthesis and application of a new, easily cleavable
chiral auxiliary for the synthesis of peptidomimetics via an Ugi
reaction
⇑
Szymon Klossowski, Anna Brodzka, Małgorzata Zysk, Ryszard Ostaszewski
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 December 2013
Accepted 10 January 2014
A new chiral auxiliary derived from
sented. It significantly expands upon the application of
a
-phenylethylamine,
-phenylethylamine derivatives used as chiral
auxiliaries. A straightforward, chemoenzymatic synthesis of non-racemic -(2,4-dimethoxyphenyl)ethyl-
a-(2,4-dimethoxyphenyl)ethylamine is pre-
a
a
amine is described and the new chiral auxiliary applied to an asymmetric Ugi reaction. The mild condi-
tions used for the cleavage of the auxiliary allowed to obtain chiral, non-racemic peptidomimetics
possessing reactive a,b-unsaturated double bonds.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
R1
S
a
-Phenylethylamine (
a-PEA) is a commonly used chiral amine
which can be easily applied as the chiral auxiliary.1 Recent exam-
Ph
NH
O
O
Ph
N
H
ples of
a
-PEA (and its derivatives) application are shown in
Ph
OMe
ref. 2
H2N
Figure 1. They include asymmetric Michael addition reactions,2
and asymmetric heterocyclizations leading to chiral piperi-
dines.3–5 It is also used for new chiral building blocks such as naph-
tooxazines6 and chiral amines.7,8 Significant applications of
ref. 3,4
OH
ref. 10
ref. 5
O
HO
HO
HO
BocHN
N
CO2H
a-arylethylamines as chiral auxiliaries involve multicomponent
Ph
Me
R
ref. 9
CO2Me
H
reactions (MCRs) since they are often non-stereoselective. This in-
ref. 6
cludes asymmetric Strecker reactions,9 the Mannich reactions10
H
N
and four component Ugi reactions.11 Although chiral
a-arylethyl-
HCl HN
Ar
Ar
O
R2
amines are widely used (mainly because of their availability), there
are still some limitations related to the cleavage conditions. In
some described examples, the a-PEA is usually removed via hydro-
CN
NH2
O
O
N
H
HN
O
R1
R3
genation or by refluxing under acidic conditions. This method how-
ever cannot always be applied when the compound contains the
sensitive functional groups. We recently presented the synthesis
of b-acyloxymethacrylicamides (Fig. 1) which exhibit very potent
biological activities.12 We have shown, that SK053 1 (Fig. 1) can
be efficiently obtained in the Ugi reaction as a racemic mixture;
however the synthesis of enantiomers using known chiral auxilia-
ries is cumbersome due to the activated double bond. The Ugi reac-
tion is widely used as a straightforward synthetic methodology in
the synthesis of various compounds.13–17 Its great potential is
1: R1 = 2,6-(CF3)-C6H3;
R2 = iBu; R3 = GlyOEt
this work
Figure 1. Recent applications of a-PEA and its derivatives.
limited mainly because of a lack of stereoselectivity. Despite many
modifications of the reaction as well as the recently reported enan-
tioselective Ugi-type 3-MCR,18 the stereoselectivity of U-4-MCR
still remains a major issue. A few examples of U-4-MCR with chiral
components were recently described and reviewed.19–21 However,
the number of chiral and cleavable auxiliaries for the U-4-MCR
⇑
Corresponding author. Tel.: +48 22 343 2120; fax: +48 22 632 6681.
E-mail address: ryszard.ostaszewski@icho.edu.pl (R. Ostaszewski).
reaction remains limited. Beyond the
a-PEA, some other chiral
http://dx.doi.org/10.1016/j.tetasy.2014.01.016
0957-4166/Ó 2014 Elsevier Ltd. All rights reserved.

436
S. Klossowski et al. / Tetrahedron: Asymmetry 25 (2014) 435–442
O
amines were previously applied. This includes chiral ferrocenyl
amines22 chiral amines derived from sugar templates.23,24 The
described amines gave a wide range of Ugi reaction products
with excellent diastereoselectivity. However, the cleavage of
the chiral auxiliary required conditions which can often interfere
with electrophilic fragments in many compounds, including
b-acyloxymethacrylicamides.
enzyme
N
H
NH2
hydrolysis
amidation
MeO
MeO
OMe
MeO
OMe
Ac2O
5
rac-2
O
R2COOR1
enzyme
N
H
R
NH2
OMe
Herein we report on the chemoenzymatic synthesis of a chiral,
non-racemic a-(2,4-dimethoxyphenyl)ethylamine 2. We show that
+
OMe
MeO
this amine can be used as a new chiral auxiliary, which can be re-
moved under mild conditions. We then presented its application as
an ammonia equivalent in asymmetric Ugi reactions for the syn-
thesis of chiral peptides with reactive, electrophilic fragments.
4: R = CH2OMe
Scheme 2. The strategy for the resolution of the enantiomers of 2.
The second approach was the acylation of racemic
a-(2,4)-
2. Results and discussion
dimethoxyphenylethylamine with an ester. We used the methyl
methoxyacetate as an acyl group donor.29 The reaction was ini-
tially performed in toluene as it was previously well tolerated by
the enzymes.30 The reaction was found to be catalyzed by four en-
zymes: Candida antarctica lipase, Pseudomonas sp. lipase, Candida
rugosa lipase as well as Novozym 435 (Table 1). An excellent result
was obtained for Novozym 435 (Table 1, entry 4).
The 2,4-dimethoxybenzyl amine (DMBA) has been previously
successfully applied in our group as an ammonia equivalent in
U-4-MCRs. The DMB-group can be easily removed in solutions of
TFA in DCM without significant side-reactions with an activated
double bond.12,25 We therefore considered that a chiral analogue
of DMBA should be removed under similar, mild conditions. Since
the synthesis of non-racemic a-(2,4-dimethoxyphenyl)ethylamine
has not been described, we performed studies on the synthesis of
Table 1
enantiomerically pure amine 2.
Enzymatic amidation of racemic
methoxyacetate
a-(2,4-dimethoxyphenyl)ethylamine with methyl
Yielda (%)
eeb (%)
Ec
2.1. Synthesis of the chiral amine 2
Entry
Enzyme
1
2
3
4
Pseudomonas sp. lipase
CALB (native)
Candida rugosa lipase
Novozym 435
10
10
<5
49
96
97
98
>99
54
73
nd
751
The strategy for the synthesis of amine 2 was based on resolu-
tion of enantiomers of amine 2. We performed the synthesis of
racemic 2 as described in the literature.26 Starting from 2,4-dime-
thoxyacetophenone, the Leukchart reaction was performed. Amide
3 was obtained in 37% yield. In the next step, amide 3 was hydro-
lyzed with 2 M NaOH in 58% resulting to give amine 2 (Scheme 1).
Reactions were carried out at 25 °C for 48 h in toluene (c 20 mg/ml) with 3 equiv of
methyl methoxyacetate.
a
Isolated yield.
Determined by chiral HPLC.
Calculated from the Eq. A (see Section 4).
b
c
O
a
NH2
N
H
b
O
The yield of the isolated amide product was 49% and the ee
>99% (>99.9% after crystallization, Table 1, entry 4). Unreacted
amine was recovered and distilled with 42% yield. The enantio-
meric excess of the amide derived from unreacted amine was
>99%. In order to determine the absolute configuration of the ob-
tained amide and unreacted amine 2, we performed X-ray studies.
The ORTEP image (Fig. 2) revealed that the obtained amide was the
O
O
O
O
O
O
3
2
rac-
resolution
of enatiomers
+
(R)-isomer, which indicates that the unreacted amine is (S)-a-(2,4-
NH2
NH2
dimethoxymethylphenyl)ethylamine.31 This methodology allowed
us to perform a preparative reaction and obtain enantiomerically
pure amine on a gram-scale. However, when carrying out the
O
O
O
O
(R)-2
(S)-2
Scheme 1. Strategy for the synthesis non-racemic
a-(2,4-dimethoxyphenyl)ethyl-
amine. Reaction and conditions: (a) HCOOH/NH3; 160–180 °C, 6 h, 37%; (b) 2 M
NaOHaq reflux, 58%.
We initially performed the resolution of the enantiomers of 2 by
crystallization of the ammonium salt with chiral carboxylic acids.27
However, we found these methods for the resolution of the
enantiomers were not efficient in this case. Crystallization with
chiral L-tartaric acid and D-mandelic acid under standard condi-
tions gave amine 2 with a maximum of 14% ee after three cycles
of crystallization (the enantiomeric excess values of the amine
were determined by chiral HPLC after derivatization with the ace-
tic acid anhydride, see Section 4.5). Therefore, we turned our atten-
tion to enzymatic methods. The strategy for the enzymatic, kinetic
resolutions is shown in Scheme 2.
In the first approach we studied the hydrolysis of acetamide 5
derived from 2 in a buffer with acetone as the cosolvent.28 How-
ever, no conversion of the substrate was observed for all of the
lipases tested (for a full list of the enzymes tested, see Section 4).
Figure 2. The ORTEP image of the amide 4.

S. Klossowski et al. / Tetrahedron: Asymmetry 25 (2014) 435–442
437
Table 2
hydrolysis of amide 4, partial racemization occurred and the enan-
The cleavage of the chiral auxiliary
tiomeric excess of isolated (R)-2 had decreased slightly to 94%.
However, the enantiomerically pure (S)-2 can be used as a sub-
strate for the Ugi reaction.
O
O
H
N
H
N
a
R1
N
H
R2
R1
N
O
R2
2.2. Studies on the application of
amine for the Ugi reaction
a-(2,4-dimethoxyphenyl)ethyl
O
O
(R)- and (S)- enantiomers
O
For model studies, we performed Ugi reactions with chiral
amine (S)-2. The obtained products and yields are presented in
Scheme 3.
Entry Substrate Producta
Yieldb eec/S.R.d (%)
(%)
[
a
]
D = +23 (c 0.7,
CHCl3)
D = ꢀ20 (c 0.8,
CHCl3)
1
2
6a
6b
73
83
O
R2
[a]
H
N
O
R2
O
H
N
H
N
2
Ph
N
H
(S)-
+
R1
N
R3
R1COOH +
R2CHO +
R3NC
EtOH
R1
N
R3
O
+
O
O
24 h, rt
(S)-10 and (R)-10
3
4
8a
8b
55 99
63 99
MeO
OMe
MeO
OMe
NO2
O
H
N
COOEt
COOEt
N
H
O
(S)-11 and (R)-11
O
O
H
N
H
H
N
5
6
9a
9b
63 95
64 83
N
Ph
N
N
O
O
H
O
O
N
N
H
77%
d.r.: 2:1
87%
MeO
OMe
O
MeO
OMe
d.r.: 2:1
COOEt
6a,b
7a,b
(S)-12 and (R)-12
Reagent and conditions: (a) TFA/DCM (20% v/v), rt, 20–30 min.
O
a
H
N
H
N
The absolute configuration was not assigned.
Isolated yields.
Determined by HPLC.
Determination of the ee using HPLC was unequivocal.
COOEt
b
N
O
N
c
O
O
d
44%
d.r.: 2.1:1
54%
d.r. 1.6:1
MeO
OMe
8a,b
MeO
OMe
9a,b
Table 3
Scheme 3.
a-(2,4-Dimethoxyphenyl)ethyl amine in an Ugi reaction. Reaction
The influence of the solvent and additives in Ugi reaction with amine (S)-2
conditions: ethanol (c 0.1 M), 24 h, rt. The yields are determined for diastereoiso-
meric mixtures.
Entry
Solvent
drc
Yieldd (%)
1
2
3
4
5
6
7
8
9
10
EtOH
Toluene
CHCl3
THF
iPrOH
DMF
AcCN
TFE
THF + ZnCl2
EtOH + ZnCl2
35:65
30:70
31:69
29:71
40:60
n/d
30:70
n/d
20:80
23:77
67
23
27
48
54
0a
The model Ugi reactions with amine 2 proceeded efficiently un-
der standard conditions for various isocyanides and aldehydes 6
and 7. Reasonable yields were also obtained for unstable metha-
crylic acids 8 and 9. The mixtures obtained of the diastereoisomers
were relatively easy to separate by chromatographic methods. We
then performed the model cleavage of compound 6 (Table 2, en-
tries 1 and 2). We found that previously applied conditions for
the cleavage of the dimethoxybenzyl group12 were very efficient
(20% TFA/DCM). The full conversion of substrate (TLC) occurred
within 20 min. Dipeptides 10 were obtained in good yields (73%
and 83%). Since we found that the auxiliary can be removed under
mild conditions, we separated the diastereoisomers of compound 8
and 9 using flash chromatography. Next, the cleavage of separated
isomers was performed. Despite the presence of reactive Michael
acceptors, both compounds 11 and 12 were obtained with very
decent yields (Table 2, entries 3–6).
Since the model studies showed that the cleavage of the pre-
sented chiral auxiliary did not interfere with the Michael acceptors,
we applied the methodology to the synthesis of enantiomerically
enriched compound 1. Previous mechanistic studies on the diaste-
reoselectivity of the Ugi reaction, indicated that beyond the tem-
perature, the concentration of the reagents had a strong impact
on the diastereoisomeric ratio of the product.13a,22 Therefore, the
initial solvent screen was performed with a substrate concentra-
tion of c 0.03 M at ꢀ21 °C (Table 3). The best yield was obtained
when the reaction was carried out in absolute ethanol. On the
other hand, the product was obtained with the best diasteroiso-
meric ratio when the Lewis acid was added (Table 3, entries 9
48
0a
24
11
Reactions were carried out at ꢀ21 °C for 24 h.
a
Decomposition was observed.
b
Reaction time was increased up to 48 h due to poor conversion after 24 h.
Determined by HPLC.
Yield of the mixture of diastereoisomers.
c
d
and 10). Under these conditions however, the yield decreased
significantly. The poor diastereoselectivity of the reaction can be
explained by the lack of steric hindrance of the substrate. High dia-
stereoselectivity was generally reported for bulky isocyanides or
amines, which decrease the kinetics of isocyanide addition, which
is the stereoselective determining step of the Ugi reaction.13
On the other hand, the previous studies on U-4-CR with bromo-
methacrylic acid indicated that the reaction proceeded more effi-
ciently in a higher concentration.12 Thus, we carried out the
reaction in a high concentration of ethanol (c 0.5 M) in 0 °C for
24 h. Under these conditions, the reaction gave compound 13a
and b in 84% yield and only a slightly different diastereoisomeric
ratio was found (Scheme 4). At this point, however, the optimiza-
tion of the diastereoisomeric ratio was not essential since both
enantiomers were required.

438
S. Klossowski et al. / Tetrahedron: Asymmetry 25 (2014) 435–442
2
Me
N
Ar
H
+ iBuCHO
Me
Ar
H
H
+ bromomethacrylic acid
+ CNCH2COOEt
H
R2 NC
N
H
R1COO
H
iBu
attack
O
"from behind"
N
R2
a
O
R1
A
B
Br
O
Br
O
O
H
N
H
(S)
(R)
Me
Ar
N
R1
N
(S)
O
O
N
O
+
N
H
(S)
O
O
O
O
iBu
HN R2
13b
minor
13a
major
O
O
O
O
diastereoisomer
diastereoisomer
Figure 3. Plausible mechanism based on previously described considerations.
b,c
b,c
CF3
O
CF3
O
3. Conclusion
O
O
Herein we have reported on a chemoenzymatic approach for
CF3
CF3
H
N
the synthesis of chiral, non-racemic
amine. The mild conditions of the cleavage significantly expand
upon the application of the commonly used -PEA and its deriva-
a-(2,4-dimethoxyphenyl)ethyl
H
COOEt
N
COOEt
O
N
O
N
H
H
O
O
a
1
(S)-
(R)-1
tives as chiral auxiliaries. The synthesis of the amine is straightfor-
ward, and does not involve many synthetic steps. The kinetic
resolution was performed with a commonly available enzyme. This
makes the amine widely accessible. Thanks to mild reaction condi-
tions, which were used for the cleavage, the amine can be used
with reactive compounds such as acrylates and their derivatives.
We have reported the synthesis of both enantiomers of compound
1. The developed approach is a very attractive alternative for the
synthesis of enantiomerically enriched dipeptides and peptidomi-
metics which are available via Ugi reactions.
ee = 90%
ee = 92%
Scheme 4. Reagents and conditions: (a) EtOH (c 0.5 M), 0 °C, 24 h, 84%; (b) 20% TFA/
DCM, 1 h, rt; (c) 2,6-bis(trifluoromethyl)benzoic acid cesium salt, acetone, reflux,
2 h, 68% for (R)-1, 57% for (S)-1.
Each of the obtained diastereoisomers 13a and 13b can be rel-
atively easily purified by column chromatography using silica gel.
The separated diastereoisomers were treated with 20% TFA in
DCM at room temperature. The peptidomimetics obtained were
converted directly into the desired analogues by substitution with
2,6-bis(trifluoromethyl)benzoic acid cesium salt. Compounds (R)-1
and (S)-1 were obtained in 68% and 57% yield, respectively (2
steps).
4. Experimental
4.1. General
In order to determine the absolute configuration of the obtained
enantiomers, another synthesis of (S)-1 was performed. The cou-
pling of non-racemic peptide L-Leu-GlyOEt was highly inefficient.
The bromomethacrylic acid was unstable under the coupling con-
ditions. After optimization, the best results were obtained for the
mixed anhydride method and with bromomethacrylic acid chlo-
ride. The yields, however, did not exceed 6%. Nevertheless, the
non-racemic product was transformed to (S)-1 and the specific
rotation was determined (Scheme 5).
NMR spectra were measured with Varian 400 Gemini, Bruker
500 and Varian 500 spectrometers, with TMS used as an internal
standard (0 ppm) or the residual chloroform signal ((d) 7.26 ppm
for 1H and 77.36 ppm for 13C). TLCs were performed with silica
gel 60 (230–400 mesh or 70–230 mesh, Merck) and silica gel 60
PF254 (Merck). CHN analysis was performed on a Vario EL III (Ele-
mentor) elemental analyzer. High-resolution mass spectrometry
(HR-MS) spectra were recorded on an Mariner (PerSeptiveBiosys-
tems) and Synapt G2-SHD apparatus. All starting materials were
purchased at Sigma–Aldrich or TCI Chemicals. All enzymes other
than Novozym 435 were purchased from Sigma–Aldrich. The fol-
lowing commercially available enzymes were used in enzymatic
screenings: papain, lipase from wheat germ, acylase I, amano li-
pase AK, lipase from hog pancreas as well as recombined lipases
from: Rhizopus niveus, Rhizopus oryzae, Rhizopus arrhisus, Candida
lipolytica, Mucor javanicus, Penicillium roqeuforti. HPLC was per-
formed on Varian apparatus with a PDA detector. Method A (for
5): column: Chiracel, OD-H, eluent: hexane/isopropanol, 85:15,
v:v, flow = 1 ml/min, k = 230 nm; method B (for 4): column: Chira-
cel, OD-H, eluent: hexane/isopropanol, 85:15, v:v, flow = 0.8 ml/
min, k = 230 nm, method C (for compd 11 and 12): column: Chira-
cel, OD-H, eluent: hexane/isopropanol, 92:8, v:v, flow = 1 ml/min,
k = 230 nm; method D (for compd 1): column: Chiracel, OD-H, elu-
ent: hexane/isopropanol, 87:13, v:v, flow = 1 ml/min, k = 230 nm.
Enantioselectivities were calculated using Eq. A:
iBu
H
N
a,b
COOEt
1
(S)-
H2N
.HCl
O
13
Scheme 5. Reagents and conditions: (a) BMACl, 0 °C, DCM, 12 h, 6%; (b) 2,6-
bis(trifluoromethyl)benzoic acid cesium salt, acetone, reflux, 4 h, 67% (see
Section 4.7.).
This correlation indicated that the major stereoisomer formed is
(S,S). This indicates the mechanism depicted in Figure 3. The pro-
tonated imine A is attacked by the isocyanide group from the less
hindered site, which leads to intermediate B with a fixed configu-
ration. This mechanism is in agreement with that previously
described by Zhu.13 It is highly possible that the isocyanide attack
is relatively rapid and therefore the reaction exhibits limited dia-
stereoselectivity for the substrates examined.
E ¼ ln½ð1 ꢀ c ꢁ ð1 þ ðeeÞ?pÞꢂ=ln½1 ꢀ c ꢁ ð1 ꢀ ðeeÞ?pÞꢂ

S. Klossowski et al. / Tetrahedron: Asymmetry 25 (2014) 435–442
439
4.2. Synthesis of racemic
a-(2,4-dimethoxyphenyl)ethyl amine
from entry 3 in gram-scale reaction the ee was >99.9%. HPLC:
rac-2
Method B, tR = 8.9 min; [(R)-isomer].
The amine was obtained according to the literature.26 Ammo-
nium carbonate (13.5 g, 0.14 mol) was dissolved in 10.2 ml of
formic acid (80%aq). Then the mixture was refluxed in 135 °C for
2 h and then cooled back to room temperature. The 2,4-dimethoxy-
benzophenone (12.7 g, 70 mmol) was then added. Next, the mixture
was refluxed and the water was distilled off. The temperature was
raised to 175 °C and the reaction became single-phase. The mixture
was cooled to room temperature and dichloromethane was added
(400 ml). The organic layer was washed with 2 M NaOH (100 ml)
and with water (100 ml). The organic layer was separated, dried over
magnesium sulfate and evaporated. The residue was recrystallized
from hexane–ethyl acetate and 12 g of white powder were obtained
(37%). The obtained compound was directly hydrolyzed. The amide
(2 g, 9.5 mmol) was suspended in 2 M NaOH (20 ml) and refluxed for
4 h (the presence of substrate was monitored by TLC). The cooled
reaction mixture was acidified to pH = 2 with concentrated hydro-
chloric acid. The water phase was extracted with diethyl ether
(2 ꢁ 50 ml), then 4 M NaOH was added dropwise until pH = 13.
The water phase was extracted with dichloromethane (2 ꢁ 100
ml). The solvent was evaporated and the crude amine was distilled
under reduced pressure (100–101 °C, 0.1 mm Hg). 1 g of colorless
liquid was obtained (58%). 1H NMR (200 MHz, CDCl3) d 1.44
(d, J = 6.6 Hz, 3H, CH3), 1.72 (s, 2H), 3.86 (s, 3H, OCH3), 3.88 (s, 3H,
OCH3), 4.34 (q, J = 6.6 Hz, 1H, –CHPh(CH3)), 6.50–6.55 (m, 2H –Ph),
7.26–7.31 (m, 1H, –Ph); 13C NMR (50 MHz, CDCl3) d 23.6, 45.9,
55.5, 55.6, 98.9, 104.0, 126.4, 128.6, 158.0, 159.7.
4.5. Preparation of (S)-
(S)-2
a-(2,4-dimethoxyphenyl)ethyl amine
The water phase from Section 4.4 (reaction with Novozym 435,
see Table 1, entry 3) was treated with 4 M NaOH until pH = 13. The
mixture was extracted with dichloromethane. The organic layer
was dried over magnesium sulfate and the solvent was evaporated.
The crude amine was distilled (see Section 4.2). [
a
]D = ꢀ17 (c 0.85,
CHCl3). In order to determine the ee values, the amine was treated
with Ac2O (2 equiv) in DCM. After 2 h, the reaction was diluted
with dichloromethane, washed with 1 M HCl, dried over magne-
sium sulfate and evaporated. The crude product was subjected to
HPLC (Method A, tR = 11.2 min), ee >99%.
4.6. Synthesis of (R)-a-(2,4-dimethoxyphenyl)ethylamine (R)-2
Amide (R)-4 (1 g) was suspended in 4 M NaOH (8 ml) and eth-
ylene glycol (2 ml). The mixture was refluxed for 18 h until the
substrate was consumed. The cooled mixture was poured into
dichloromethane and extracted (2 ꢁ 20 ml). The organic layer
was dried over magnesium sulfate and distilled; 632 mg of amine
were obtained (88% Yield, ee = 94%). ½a D20
ꢂ
= +18 (c 0.5, CHCl3). HPLC
(Method A, tR = 8.4 min).
4.7. Synthesis of L-2,6-bis-trifluoromethylobenzoic acid 2-[1-(eth-
oxycabonylmethylcarbamoyl)-3-methylbutylcarbamoyl]-allyl
ester (S)-1 via coupling
4.3. Chemical synthesis of N-[1-(2,4-dimethoxyphenyl)ethyl]-2-
methoxyacetamide rac-4
Bromomethacrylic acid (84 mg, 0.5 mmol) was dissolved in
thionyl chloride (111 ll, 1.5 mmol) and refluxed for 2 h. The excess
Racemic amine 1 (100 mg, 0.55 mmol) was dissolved in methyl
methoxyacetate (12 ml, 11 mmol). The mixture was refluxed over-
night, after which the reaction was cooled to room temperature
and the mixture was diluted with ethyl acetate. The organic layer
was washed with citric acid (10%aq) and with brine. The organic
layer was dried over anhydrous magnesium sulfate and the solvent
was evaporated. The product was obtained as a white powder
(90 mg, 68%). 1H NMR (200 MHz, CDCl3) d 1.51 (d, J = 6.8 Hz, 3H,
CH3), 3.45 (s, 3H, –CH2OCH3) 3.85 (s, 3H, OCH3), 3.87 (s, 3H,
thionyl chloride was evaporated. DCM was then added (2 ml) and
the mixture was cooled to 0 °C. Dimethylaminopyridine was then
added (62 mg, 0.5 mmol). A solution of dipeptide 13 (110 mg,
0.5 mmol), obtained as described before,32 in DCM (2 ml) was then
added. The reaction was carried out at 0 °C for 1.5 h and an addi-
tional 2 h in room temperature. The solvent was evaporated and
the residue was purified using flash chromatography (silica gel,
hexane/ethyl acetate), to give 18 mg of colorless oil (5% yield).
The product was directly dissolved in acetone (1 ml) and the 2,6-
bis-(trifluoromethyl)benzoic acid cesium salt was suspended
(36 mg, 0.09 mmol). The mixture was refluxed until TLC indicated
no presence of the substrate (2 h). The suspension was filtered and
the acetone was evaporated. After purification using flash chroma-
tography (silica gel, hexane/ethyl acetate), 12 mg of a white pow-
der were obtained. The 1H NMR data were in agreement with the
spectra of the racemic compound reported before (19a, [Ref. 12]).
OCH3), 3.80–4.01 (m, 2H, –CH2OCH3) 5.27–5.34 (m, 1H,
–
CHPh(CH3)), 6.46–6.54 (m, 2H –Ph), 7.16–7.20 (m, 1H, –Ph), 7.43
(br s, NH); 13C NMR (50 MHz, CDCl3) d 21.9, 46.3, 55.6, 55.6, 59.4,
72.4, 99.5, 104.3, 123.6, 128.6, 158.3, 160.4, 168.4; HR-MS (ESI,
[M+Na+] calcd for: C13H19NO4Na: 276.1206, found, 276.1202 m/z;
Anal. calcd for C13H19NO4: C, 61.64; H, 7.56; N, 5.53; found: C,
61.50; H, 7.59; N, 5.46; HPLC: method B, tR1 = 8.9 min;
tR2 = 10.6 min.
½
a 2D0
ꢂ
= ꢀ23 (c 0.8, CHCl3).
4.4. Enzymatic synthesis of N-[1-(2,4-dimethoxyphenyl)ethyl]-
4.8. General method I for the Ugi reaction
2-methoxyacetamide 4
To a stirred solution of amine (S)-2 (1 equiv, c = 1 mmol/ml) an
aldehyde was added (1 equiv). The reaction was then stirred for
30 min at room temperature, after which the reaction was cooled
to 0 °C. The isocyanide (1 equiv) and carboxylic acid (1 equiv)
was then added. The reaction was carried out for 30 min at 0 °C
and at room temperature overnight. The solvent was evaporated
off and the mixture of diastereoisomers was separated by flash
chromatography (silica gel, hexane–ethyl acetate). The resolution
of the diastereoisomers was performed either by LC (silica gel, hex-
ane–isopropanol 98:2) or by preparative TLC (silica gel, hexane–
isopropanol).
The racemic amine 2 was dissolved in toluene (20 mg/ml) and
the enzyme was added (10%w). Next, methyl methoxyacetate
(3 equiv) was added and the reaction was carried out at room tem-
perature for 48 h. The enzyme was then filtered off and the organic
phase was washed with 1 M HCl. The toluene was evaporated off
and the crude product was subjected to HPLC (Method B, tR1 = 8.9 -
min; tR2 = 10.6 min) in order to determine the ee of amide 4.
½
a 2D0
= +111 (c 0.50, CHCl3). The reaction with Novozym 435
ꢂ
(Table 1, entry 3) was performed with 3 g of amine. The ees in
Table 1 refer to crude products. After crystallization of the amide

440
S. Klossowski et al. / Tetrahedron: Asymmetry 25 (2014) 435–442
4.8.1. Synthesis of N-[1-(cyclohexylamino)-4-methyl-1-oxopen-
tan-2-yl]-N-[1-(2,4-dimethoxyphenyl)ethyl]benzamide 6
White powder, 77%, 6a (minor diastereoisomer): 1H NMR
4.8.3. Synthesis of ethyl 2-[2-{N-(1-[2,4-dimethoxyphenyl]ethyl)-
acrylamido}-4-methylpentanamido]acetate 8
Colorless oil, 54% yield. 8b (major diastereoisomer): 1H NMR
(400 MHz, CDCl3) d 0.42 (d, J = 6.6 Hz, 3H, –CH(CH3)2), 0.46 (d,
J = 6.6 Hz, 3H, –CH(CH3)2), 1.12–1.21 (m, 1H, –CHHCH(CH3)2),
1.23–1.31 (m, 4H, –CH2CH3, –CHHCH(CH3)2), 1.55 (d, J = 7.0 Hz,
3H, –CHCH3), 2.08–2.15 (m, 1H, –CH2CH–), 3.60–3.64 (m, 1H, –
CH(iBu)), 3.76 (s, 3H, OCH3) 3.80 (s, 3H, OCH3), 3.86–3.92 (m, 1H
–NHCHH–), 4.02–4.08 (m, 1H –NHCHH–), 4.16 (q, J = 7.2 Hz, 2H,
–OCH2CH3), 5.39 (q, J = 7.0 Hz, 1H, ArCHCH3), 5.71 (dd,
J1 = 10.6 Hz, J2 = 1.2 Hz, 1H, HHC@CHCON–), 6.29 (dd, J1 = 16.8 Hz,
J2 = 1.2 Hz, 1H, HHC@CHCON–), 6.43 (d, J = 2.3 Hz, 1H, ArH), 6.51
(dd, J1 = 8.4 Hz, J2 = 2.2 Hz, 1H, ArH), 7.05 (dd, J1 = 10.7 Hz,
J2 = 16.7 Hz, 1H, HHC@CHCON–), 7.26–7.30 (m, 1H, ArH), 8.59 (br
s, 1H, NH); 13C NMR (100 MHz, CDCl3) d 14.5, 18.1, 22.0, 22.9,
25.1, 39.5, 41.6, 52.9, 55.3, 55.8, 60.3, 61.4, 98.9, 104.1, 119.5,
127.3, 129.5, 130.8, 159.2, 161.5, 169.4, 170.1, 174.5; HR-MS (ESI,
[M+Na+] calcd for: C23H34N2O6Na: 457.2315, found: 457.2316 m/z.
(500 MHz, CDCl3)
d 0.45 (d, J = 6.5 Hz, 3H, CH3), 0.6 (d,
J = 6.6 Hz, 3H, CH3), 1.05–1.11 (m, 1H, –CH(CH3)2–), 1.16–1.26
(m, 4H, 2ꢁ –CH2–), 1.32–1.40 (m, 2H –CH2–), 1.51–1.56 (m, 1H
–CHH–), 1.60 (d, J = 7.1 Hz, 3H, ArCH(CH3)), 1.65–1.69 (m, 2H –
CH2–), 1.83–1.89 (m, 2H, CH2), 2.33–2.39 (m, 1H, CHH), 3.61
(dd, J1 = 10.4 Hz, J2 = 5.0 Hz, 1H, CH(CH2)), 3.74 (s, 3H, OCH3),
3.79 (s, 3H, OCH3), 3.75–3.81 (m, 1H, CH(i-Bu)), 5.10 (q,
J = 7.1 Hz, 1H, CH(CH3)Ar), 6.39 (d, J = 2.4 Hz, 1H, ArH), 6.49 (dd,
J1 = 8.6 Hz, J2 = 2.4 Hz, 1H, ArH), 7.3 (d, J = 8.7 Hz, 1H, ArH),
7.41–7.50 (m, 5H, ArH) 7.96 (br s, 1H, NH); 13C NMR (125 MHz,
CDCl3) d 18.1, 21.6, 23.0, 24.6, 25.3, 25.6, 32.6, 32.7, 39.7, 47.6,
53.4, 54.1, 54.8, 55.5, 61.0, 98.6, 103.7, 119.1, 126.0, 128.1,
129.1, 129.5, 138.3, 158.9, 161.1, 172.0, 173.8; 6b (major diaste-
reoisomer): 1H NMR (500 MHz, CDCl3) d 0.81–0.91 (m, 1H, CHH),
0.94–1.01 (m, 6H, 2ꢁ CH3), 1.10–1.20 (m, 1H, CHH), 1.24–1.33
(m, 3H, CH2 CH(CH3)2), 1.48–1.54 (m, 2H, CH2), 1.58–1.65 (m,
2H, CH2), 1.60 (d, J = 7.1 Hz, 3H, ArCHCH3), 1.77–1.82 (m, 2H,
CH2), 1.87–1.93 (m, 1H, CHH) 2.16–2.22 (m, 1H, CHH), 3.36–
3.42 (m, 1H, CH(CH2)2), 3.63–3.67 (m, 1H, CH(i-Bu)), 3.96 (s,
3H, OCH3), 3.78 (s, 3H, OCH3), 5.18 (q, J = 7.0 Hz, 1H, CH(i-Bu)),
6.34 (d, J = 2.3 Hz, 1H, ArH), 6.48 (dd, J1 = 2.3 Hz, J2 = 8.5 Hz, 1H,
ArH) 7.24–7.26 (m, 1H, ArH), 7.46–7.47 (m, 3H, ArH), 7.55–7.56
(m, 2H, ArH); 13C NMR (125 MHz, CDCl3) d 17.8, 22.8, 23.0,
24.5, 25.7, 25.8, 29.7, 32.7, 39.8, 47.4, 52.8, 53.4, 54.9, 55.4,
59.0, 98.8, 103.9, 119.3, 126.8, 128.1, 128.8, 129.8, 137.8, 137.8
158.4, 161.0 171.1, 173.1; HR-MS (ESI, [M+Na+] calcd for: C29H40-
N2O4Na: 503.2880, found, 503.2895 m/z.
4.8.4. Synthesis of ethyl 2-[2-{N-(1-[2,4-dimethoxyphenyl]ethyl)
methacrylamido}-4-methylpentanamido]acetate 9
Colorless oil, 44% yield. 9b (major diastereoisomer): 1H NMR
(400 MHz, CDCl3) d 0.94 (d, J = 6.7 Hz, 6H, 2ꢁ CH3), 1.22 (t,
J = 7.1 Hz, 3H, –CH2CH3), 1.55 (d, J = 7.0 Hz, 3H, CHCH3),
1.64–1.71 (m, 1H) 1.76–1.83 (m, 1H), 2.08–2.15 (m, 1H), 2.15 (s,
3H) 3.42 (dd, J1 = 4.5 Hz, J2 = 18.4 Hz, 1H, –CHHCONH–), 3.52 (t,
J = 7.1 Hz, 1H, CH(i-Bu)), 3.74 (s, 3H, OCH3), 3.78 (s, 3H, OCH3),
3.79–3.85 (m, 1H, –CHHCONH–), 4.12 (q, J = 7.1 Hz, 2H, –OCH2–),
5.26–5.29 (m, 2H, C@CHH), 5.38 (q, J = 7.2 Hz, 1H, CHCH3), 6.35
(d, J = 2.5 Hz, 1H, ArH), 6.48 (dd, J1 = 8.5 Hz, J2 = 2.4 Hz, 1H, ArH),
7.24 (d, J = 8.4 Hz, 1H, ArH), 7.65 (br s, 1H, NH); 13C NMR
(100 MHz, CDCl3) d 14.4, 18.0, 20.5, 23.0, 23.1, 25.9, 40.0, 41.8,
52.7, 55.5, 55.7, 58.6, 61.3, 98.7, 104.0, 115.6, 119.3, 129.3, 142.1,
158.7, 161.4, 169.8, 172.5, 174.9; HR-MS (ESI, [M+H+] calcd for:
4.8.2. Synthesis of ethyl 2-acetamido-N-[2-(cyclohexylamino)-
1-(3-nitrophenyl)-2-oxoethyl]-N-[(S)-1-(2,4-dimethoxyphenyl)-
ethyl]acetamide 7
C24H36N2O6Na: 471.2471, found: 471.2475 m/z.
Yellowish powder, 87% yield. 7a (minor diastereoisomer): 1H
NMR (400 MHz, CDCl3) d 0.82–0.97 (m, 2H, CH2), 1.07–1.13 (m,
1H, CHH), 1.22–1.32 (m, 2H, CH2), 1.49 (d, J = 7.0 Hz, 3H,
CH(Ar)CH3), 1.53–1.63 (m, 4H, 2ꢁ CH2), 1.80–1.84 (m, 1H, CHH),
2.06 (s, 3H, CH3CO–), 3.49–3.53 (m, 1H, CH(CH2)2), 3.84 (s, 3H,
OCH3), 3.92 (s, 3H, OCH3), 4.20 (dd, J1 = 17.5 Hz, J2 = 3.8 Hz, 1H, –
NHCHH–), 4.67 (s, 1H, CH(Ar)), 4.81 (dd, J1 = 17.5 Hz, J2 = 4.8 Hz,
1H, NHCHH–), 5.38 (q, J = 7.0 Hz, 1H, ArCH(CH3)), 6.06 (d,
J = 8.0 Hz, 1H, NH), 6.52 (d, J = 2.8 Hz, 1H, ArH), 6.57–6.60 (m, 2H,
ArH), 7.24 (s, 1H, ArH), 7.45–7.49 (m, 1H, ArH) 7.61–7.63 (m, 1H,
ArH), 8.11–8.12 (m, 2H, ArH, NH); 13C NMR (100 MHz, CDCl3) d
18.6, 23.4, 24.9, 25.1, 25.8, 33.1, 33.2, 42.3, 48.6, 50.8, 55.8, 56.3,
61.8, 99.8, 105.1, 118.1, 122.8, 123.6, 129.0, 129.3, 134.8, 139.4,
148.3, 158.6, 162.0, 167.0, 169.1, 170.7; 7b (major diastereoiso-
mer): 1H NMR (400 MHz, CDCl3) d 1.03–1.14 (m, 3H, CH2; CHH),
1.26–1.34 (m, 2H, CH2), 1.52–1.68 (m, 3H, CH2, CHH), 1.67 (d,
J = 7.0 Hz, 3H, CH3, ArCH(CH3)), 1.82–1.87 (m, 2H, CH2), 2.04 (s,
3H, CH3CO–), 3.64 (s, 3H, OCH3), 3.70 (s, 3H, OCH3), 3.77–3.82
(m, 1H, CH(CH2)), 4.27 (dd, J1 = 17.5 Hz, J2 = 3.8 Hz, 1H, NHCHH),
4.60 (s, 1H, NHCH(Ar)), 4.64 (dd, J1 = 17.5 Hz, J2 = 4.5 Hz, 1H,
NHCHH), 5.29 (q, J = 7.0 Hz, 1H, ArCH(CH3)), 6.12 (d, J = 2.3 Hz,
1H, ArH), 6.17 (br d, J = 7.8 Hz, 1H, NH), 6.44 (dd, J1 = 8.6 Hz,
J2 = 2.3 Hz, 1H, ArH), 6.57 (br s, 1H, NH), 7.24–7.31 (m, 2H, ArH),
7.40–7.45 (m, 2H, ArH), 7.94–7.96 (m, 1H, ArH) 13C NMR
(100 MHz, CDCl3) d 17.1, 18.6, 23.3, 24.8, 25.6, 32.7, 32.8, 42.2,
48.9, 50.8, 55.4, 55.5, 58.4, 62.9, 98.4, 104.2, 118.1, 122.5, 123.0,
128.5, 129.2, 133.9, 138.8, 148.0, 158.5, 161.7, 168.5, 169.3,
170.6; HR-MS (ESI, [M+Na+] calcd for: C28H36N4O7Na: 563.2482,
found: 563.2474 m/z.
4.8.5. Synthesis of ethyl 2-[2-{2-(bromomethyl)-N-(1-{2,4-dime-
thoxyphenyl}ethyl)acrylamido}-4-methylpentanamido] acetate
13
Yellowish oil, 84% yield. 13b (minor diasteroisomer): 1H NMR
(400 MHz, CDCl3)
d 0.39 (d, J = 6.7 Hz, 3H, CH3), 0.58 (d,
J = 6.5 Hz, 3H, CH3), 1.05–1.12 (m, 1H, –CHHCH(CH3)2), 1.18–1.27
(m, 1H, –CHHCH(CH3)2), 1.25 (t, J = 7.1 Hz, 3H, –CHCH3), 2.28–
2.35 (m, 1H, –CH2CH(CH3)2), 3.60–3.66 (m, 1H, –CH(i-Bu)), 3.80
(s, 3H, OCH3), 3.80 (s, 1H, OCH3), 3.86 (dd, J1 = 18.5 Hz,
J2 = 5.1 Hz, 1H, –NHCHHCO–), 4.07 (dd, J1 = 18.0 Hz, J2 = 6.3 Hz,
1H, –NHCHHCO–), 4.17 (q, J = 7.0 Hz, 2H, CH2CH3), 4.20 (d,
J = 11.7 Hz, 1H, BrCHH–), 4.48 (d, J = 11.7 Hz, 1H, BrCHH–), 5.36
(q, J = 6.8 Hz, 1H, –CHCH3), 5.69 (s, 2H, C@CH2), 6.44 (d,
J = 2.3 Hz, 1H, ArH), 6.52 (dd, J1 = 8.5 Hz, J2 = 2.2 Hz, 1H, ArH),
7.31 (d, J = 8.4 Hz, 1H, ArH), 8.08 (br s, 1H, NH); 13C NMR
(100 MHz, CDCl3) d 14.5, 18.8, 21.6, 23.3, 25.4, 32.0, 39.8, 41.6,
54.1, 55.5, 55.8, 60.1, 61.5, 99.1, 104.4, 119.0, 119.5, 130.1, 141.4,
158.9, 161.5, 170.0, 171.6, 173.5; 13a (major diastereoisomer):
1H NMR (400 MHz, CDCl3) d 0.93 (dd, J1 = 6.5 Hz, J2 = 1.8 Hz, 6H,
2ꢁ CH3), 1.22 (t, J = 7.1 Hz, 3H, CH2CH3), 1.59 (d, J = 7.0 Hz, 3H,
CHCH3), 1.63–1.76 (m, 1H, –CH2CH–), 2.23–2.29 (m, 1H,
–
CH(CH3)2), 3.49 (dd, J1 = 18.4 Hz, J2 = 4.7 Hz, 1H, –CHHCOOEt),
3.58 (br s, 1H, CH(i-Bu)), 3.77 (s, 3H, OCH3), 3.78 (m, 4H, OCH3,
CHHCOOEt), 4.12 (q, J = 7.2 Hz, 2H), 4.26 (d, J = 12.3 Hz, 1H,
BrCHH–), 4.43 (d, J = 12.3 Hz, 1H BrCHH–), 5.43 (q, J = 6.8 Hz, 2H
CHCH3), 5.68 (s, 1H, C@CHH), 5.78 (s, 1H, C@CHH), 6.37 (d,
J = 2.5 Hz, 1H ArH), 6.48 (dd, J1 = 8.6 Hz, J2 = 2.3 Hz, ArH), 7.24–
7.26 (m, 1H, ArH), 7.34 (br s, 1H, NH); 13C NMR (100 MHz, CDCl3)

S. Klossowski et al. / Tetrahedron: Asymmetry 25 (2014) 435–442
441
d 14.4, 18.5, 22.9, 23.2, 25.1, 26.2, 32.0, 40.2, 41.8, 52.9, 55.7, 55.8,
307.1628; HPLC: Method C, tR1 = 5.9 min, tR2 = 7.6 min; ee1 >95%,
58.4, 61.4, 99.0, 104.3, 118.9, 120.0, 129.4, 141.4, 158.5, 161.5,
ee2 = 83%.
169.9, 171.2, 172.0; Anal. calcd for
54.19; H, 6.73; N, 5.27; found: C, 54.18; H, 6.80; N, 5.3
C
24H35N2O6ꢃ0.25H2O: C,
4.10. Synthesis of 2,6-bis-trifluoromethylobenzoic acid 2-[1-
(ethoxycarbonylmethylcarbamoyl)-3-methylbutylcarbamoyl]-
allyl ester (S)-1, (R)-2
4.9. General method II for the cleavage of the auxiliary
The crude product obtained from the cleavage reaction of 13a
and 13b (general method II) was subjected directly to a substitu-
tion reaction: The crude residue from the previous reaction was
dissolved in acetone after which 2,6-bis-(trifluoromethyl)benzoic
acid cesium salt was added (3 equiv). The reaction was refluxed
until the substrate was consumed. The acetone was evaporated
and the residue was dissolved in ethyl acetate. The organic layer
was washed with sodium hydrogencarbonate and brine. Ethyl ace-
tate was dried over magnesium sulfate and evaporated. The prod-
ucts derived from 13b (minor) and 13a (major) were isolated using
flash chromatography (silica gel, hexane–ethyl acetate) in 68% and
57% yield respectively (2 steps). The product derived from 13a:
Compounds 6, 8, 9 and 13 were dissolved in dichloromethane
and then trifluoroacetic acid was added (20% v/v). The reaction
was carried out at room temperature and monitored by TLC. Usu-
ally the reaction was complete when the mixture turned violet.
Next, dichloromethane was added and saturated sodium carbonate
was added dropwise until the color disappeared (or become a light
blue). The aqueous layer was extracted three times by dichloro-
methane. The organic layer was then washed with brine, dried over
anhydrous magnesium sulfate and evaporated. The compound was
purified using flash chromatography (silica gel, hexane–isopropa-
nol, hexane–ethyl acetate).
½
a 2D0
ꢂ
= ꢀ26 (c 0.5, CHCl3), ee = 90% was correlated with the (S)-iso-
mer (obtained in Section 4.7). The product derived from 13b:
= +23 (c 0.9, CHCl3), (R)-isomer (R)-1 ee = 92%. The 1H NMR
4.9.1. Synthesis of N-[1-(cyclohexylamino)-4-methyl-1-
oxopentan-2-yl]benzamide (R)-10, (S)-10
½ ꢂ
a 2D0
Obtained by general method II, white crystals, 73% yield (from
6a), 83% yield (from 6b), 1H NMR (400 MHz, CDCl3) d 0.97 (d,
J = 4.7 Hz, 6H, 2 ꢁ CH3), 1.07–1.23 (m, 3H, –CH2–, CH(CH3)2),
1.25–1.40 (m, 2H, –CH2–), 1.57–1.60 (m, 1H, –CHH–), 1.64–1.77
(m, 5H, –CHH–, 2ꢁ –CH2–), 1.81–1.92 (m, 2H, –CH2–), 3.68–3.77
(m, 1H, –NHCH–), 4.63–4.69 (m, 1H, –CH(i-Bu)–), 6.35 (d,
J = 8.0 Hz, 1H, NH), 6.90 (d, J = 8.2 Hz, 1H, NH), 7.39–7.43 (m, 2H,
2ꢁ ArH), 7.47–7.51 (m, 1H, ArH), 7.78–7.82 (m, 2H, 2ꢁ ArH); 13C
NMR (100 MHz, CDCl3) d 22.8, 23.2, 25.1, 25.3, 25.8, 33.1, 33.3,
41.9, 48.7, 52.6, 127.4, 128.9, 132.0, 134.3, 167.7, 171.4; HR-MS
(ESI, [M+Na+] calcd for: C19H28N2O2Na: 339.2043, found:
data of both enantiomers were consistent with spectra of racemic
1. [compound 19a from Ref. 12]. HPLC: Method D: tR1 = 6.8 min (S)-
1, tR2 = 7.7 min (R)-1.
Acknowledgments
This work was supported by project ‘Biotransformations for
pharmaceutical and cosmetics industry’ No. POIG.01.03.01-00-
158/09-07 part-financed by the European Union within the Euro-
pean Regional Development Fund. The synthesis of
a-(2,4-dime-
339.2057 m/z. For I enantiomer ½a D20
ꢂ
= +23 (c 0.65, CHCl3), for II
thoxyphenyl)ethyl amine was financed by National Science
Centre Research Grant No: DEC-2011/03/N/ST5/04353. We also
thank to Przemysław Kalicki from Professor Zofia Lipkowska’s
Group from IChO PAS for determining the ORTEP image.
enantiomer: ½a 2D0
= ꢀ20 (c 0.78, CHCl3).
ꢂ
4.9.2. Synthesis of ethyl {[2-(acryloylamino)-4-methylpentano-
yl]amino}acetate (S)-11, (R)-11
References
Obtained by general method II, as colorless oils, 55% yield (from
8a), 63% yield (from 8b), 1H NMR (400 MHz, CDCl3) d 0.93 (d,
J = 5.7 Hz, 3H, CH3), 0.97 (d, J = 5.7 Hz, 3H, CH3), 0.95 (d,
J = 5.7 Hz, 3H, CH3), 1.26 (t, J = 7.1 Hz, 3H, –CH2CH3), 1.58–1.76
(m, 3H, –CH2CH(CH3)2), 3.94 (dd, J1 = 18.2 Hz, J2 = 5.1 Hz, 1H, –
CHH–), 4.05 (dd, J1 = 18.2 Hz, J2 = 5.7 Hz, 1H, –CHH–), 4.19 (q,
J = 7.0 Hz, 2H, OCH2–), 4.63 (dt, J1 = 8.5 Hz, J2 = 5.9 Hz, 1H –CH(i-
Bu)–), 5.66 (dd, J1 = 10.2 Hz, J2 = 1.4 Hz, 1H –HHC@CH–), 6.12 (dd,
J1 = 10.4 Hz, J2 = 17.0 Hz, 1H –HHC@CH–), 6.30 (dd, J1 = 17.0 Hz,
J2 = 1.4 Hz, 1H –HHC@CH–), 8.2 (d, J = 8.2 Hz, 1H, NH), 6.95 (br s,
1H, NH); 13C NMR (100 MHz, CDCl3) d 14.5, 22.5, 23.2, 25.1, 41.4,
41.7, 51.8, 61.9, 127.7, 130.6, 165.9, 169.9, 172.8; HR-MS (ESI,
[M+Na+] calcd for: C13H22N2O4Na: 293.1477, found: 293.1475;
HPLC: Method C, tR1 = 7.1 min, tR2 = 8.9 min; ee1 >99%, ee2 >99%.
1. Juaristi, E.; León-Romo, J. L.; Reyes, A.; Escalante, J. Tetrahedron: Asymmetry
1999, 10, 2441–2495.
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4.9.3. Synthesis of ethyl ({4-methyl-2-[(2-methylacryloyl)amino]-
pentanoyl}amino)acetate (R)-12, (S)-12
Obtained by general method II, white powder, 63% yield (from
9a), 64% yield (from 9b), 1H NMR (400 MHz, CDCl3) d 0.94 (m,
6H, 2ꢁ CH3), 1.26 (t, J = 7.1 Hz, 3H, –CH2CH3), 1.57–1.77 (m, 3H,
CH2CH(CH3)2), 1.95–1.96 (m, 3H, CH3C(@CH2)–), 3.95 (dd,
J1 = 18.0 Hz, J2 = 5.7 Hz, 1H, –NHCHHCO–), 4.05 (dd, J1 = 18.0 Hz,
J2 = 5.3 Hz, 1H, –NHCHHCO–), 4.19 (q, J = 7.0 Hz, 2H, –OCH2–),
4.57 (m, 1H, –CH(i-Bu)–), 5.36–5.37 (m, 1H, HHC@C), 5.74 (d,
J = 0.8 Hz, 1H, HHC@C–), 6.35 (d, J = 7.4 Hz, 1H, NH), 6.89 (br s,
1H, NH); 13C NMR (100 MHz, CDCl3) d 14.4, 18.9, 22.5, 23.2, 25.2,
41.4, 41.6, 51.9, 61.8, 120.9, 139.6, 168.7, 169.9, 172.7; HR-MS
(ESI, [M+Na+] calcd for: C14H24N2O4Na: 307.1634, found:

442
S. Klossowski et al. / Tetrahedron: Asymmetry 25 (2014) 435–442
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