Enantiocatalytic activity of substituted 5-benzyl-2-(pyridine-2-yl) imidazolidine-4-one ligands

Article abstract of DOI:10.1016/j.tetasy.2013.02.007

Currently, asymmetric synthesis represents one of the main streams of organic synthesis. Although an extensive research has been carried out in this area, the synthesis of chiral compounds with the required enantiomeric purity is still a challenging issue. Herein, we focus on the preparation of new enantioselective catalysts based on pyridine-imidazolidinones. The substituted 5-benzyl-2-(pyridine-2-yl)imidazolidine-4-ones 5-8 were prepared by condensation of chiral amino acid amides (α-methylDOPA and α- methylphenylalanine) with 2-acetylpyridine and pyridine-2-carbaldehyde. The individual isomers of the described ligands 5-8 were separated chromatographically. The copper(II) complexes of these chiral ligands were studied as enantioselective catalysts for the asymmetric Henry reaction of substituted aldehydes with nitromethane or nitroethane. The ligands containing a methyl group at the 2-position of the imidazolidinone ring 6a and 8a exhibit a high degree of enantioselectivity (up to 91% ee). The nitroaldols derived from nitroethane (2-nitropropan-1-ols) were obtained with a comparable enantiomeric purity to derivatives of 2-nitroethanol. This group of ligands represents a new and promising class of enantioselective catalysts, which deserve further attention.

Full text of DOI:10.1016/j.tetasy.2013.02.007

Tetrahedron: Asymmetry 24 (2013) 334–339
Contents lists available at SciVerse ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Enantiocatalytic activity of substituted
5-benzyl-2-(pyridine-2-yl)imidazolidine-4-one ligands
⇑
Pavel Drabina , Sergej Karel, Illia Panov, Miloš Sedlák
Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, Pardubice 532 10, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 January 2013
Accepted 13 February 2013
Currently, asymmetric synthesis represents one of the main streams of organic synthesis. Although an
extensive research has been carried out in this area, the synthesis of chiral compounds with the required
enantiomeric purity is still a challenging issue. Herein, we focus on the preparation of new enantioselective
catalysts based on pyridine-imidazolidinones. The substituted 5-benzyl-2-(pyridine-2-yl)imidazolidine-4-
ones 5–8 were prepared by condensation of chiral amino acid amides (a-methylDOPA and a-methylphe-
nylalanine) with 2-acetylpyridine and pyridine-2-carbaldehyde. The individual isomers of the described
ligands 5–8 were separated chromatographically. The copper(II) complexes of these chiral ligands were
studied as enantioselective catalysts for the asymmetric Henry reaction of substituted aldehydes with
nitromethane or nitroethane. The ligands containing a methyl group at the 2-position of the imidazolidi-
none ring 6a and 8a exhibit a high degree of enantioselectivity (up to 91% ee). The nitroaldols derived from
nitroethane (2-nitropropan-1-ols) were obtained with a comparable enantiomeric purity to derivatives of
2-nitroethanol. This group of ligands represents a new and promising class of enantioselective catalysts,
which deserve further attention.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
catalysts of asymmetric Henry reactions.5 This significant reaction,
in which a new C–C bond is formed, has been chosen as a standard
At present, the preparation and investigation of the enantiocat-
alytic properties of new chiral ligands and their metal complexes
represent one of the main streams of organic synthesis.1 Research
has focused on the development of increasingly more efficient and
versatile catalytic systems, which could be used as acceptable
alternatives of already known enantioselective catalysts.2 Although
every year, new and in many respects better and more efficient
enantioselective catalysts are reported,3 the problem of synthesis
of chiral compounds possessing the required enantiomeric purity,
that is, their asymmetric synthesis, has not been fully solved. A
key limitation is the degree of asymmetric induction that can be
provided by a particular catalyst.4a Moreover, there are certain lim-
its of the versatility of a catalyst, which restrict the set of starting
compounds suitable for a given asymmetric reaction. From this
point of view, it can be stated that an ideal chiral catalyst does
not exist. Therefore, new catalytic systems4b should be developed
with the aim of lowering the amount of catalyst used in order to
make the catalytic process as economically profitable as possible.4c
Recently, we have published a study dealing with the prepara-
tion and enantioselective properties of copper(II) complexes of
for the evaluation of the enantioselective efficiency of newly
prepared ligands. The reason for this, is the fact that the Henry
reaction is among the most intensively studied asymmetric reac-
tions that uses a metal complex as an enantioselective catalyst. This
fact enables a relevant evaluation of the enantiocatalytic efficiency
of a chiral complex.6
Herein our aim is to modify the structure of 2-(pyridine-2-yl)
imidazolidine-4-ones by attaching a benzyl group to the stereogenic
centreat the5-position, whichcould lead to a new series ofpyridine-
imidazolidinone ligands. Predominantly, we focused on their
enantiocatalytic properties in asymmetricHenry reactions and com-
pared them with 5-isopropyl derivatives.5 The replacement of the
isopropyl group with a benzyl group was predominantly motivated
by the similarity of our ligands with MacMillan’s catalysts.7 Their
structure also contains the imidazolidinone ring with an aromatic
substituent at the 5-position. We presumed that our ligands could
exhibit a p–p interaction between the reactant (aromatic aldehyde)
and the substituent, as this is known during the catalytic cycle of
analogical MacMillan’s catalysts.8 This interaction could positively
affect the stereoselectivity of the Henry reaction. In addition to the
ligands containing only a benzyl group, we also attempted to pre-
substituted
5-isopropyl-2-(pyridine-2-yl)imidazolidine-4-ones.
We found that these complexes are very efficient enantioselective
pare ligands derived from a-methylDOPA. Such compounds, thanks
to the presence of two electron donor groups, should exhibit a higher
electron density in the benzene nucleus, which is necessary for the
creation of a significant p–p interaction.
⇑
Corresponding author. Tel.: +420 466 037 010; fax: +420 466 037 068.
E-mail address: pavel.drabina@upce.cz (P. Drabina).
0957-4166/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2013.02.007

P. Drabina et al. / Tetrahedron: Asymmetry 24 (2013) 334–339
335
R
2. Results and discussion
R
R
H
N
(Boc)2O
NH2
Boc
TEA
CH2Cl2
The key intermediates for the synthesis of imidazolidinone
ligands 5–8 were the corresponding enantiomerically pure
R
COOH
COOH
(S)-forms of 2-amino-3-phenyl-2-methylpropanamide (a-methyl-
phenylalaninamide) 4a and 2-amino-3-(3,4-dimethoxyphenyl)-2-
R: H 1a; OCH3 1b
2a
R: H ; OCH3
2b
methylpropanamide 4b.
1. ClCO2Et
The starting compound for the synthesis of amide 4b was the
commercially available medical drug L-a
-methylDOPA (AldometÒ,
2. NH3/CH3OH
DopegytÒ), which is used in human medicine regarding anti-
hypertensivum.9 According to the literature,10 we carried out the
R
R
R
R
H
O-methylation of
L-
a-methylDOPA to give (S)-2-amino-3-(3,
CF3COOH
CH2Cl2
NH2
N
Boc
4-dimethoxyphenyl)-2-methylpropanoic acid 1b in an overall
yield of 67%. (S)-2-Amino-3-phenyl-2-methylpropanoic acid was
prepared from phenylacetone, which was then transformed by a
Strecker synthesis into 2-amino-3-phenyl-2-methylpropanenitrile.
Hydrolysis of this compound in a medium of 6 M hydrochloric acid
gave the racemic form of acid 1a. The resolution of this racemate
by means of cinchonidine11 gave the enantiomerically pure
(S)-form with the overall yield of 18% (Scheme 1).
CONH2
CONH2
R: H 4a; OCH3 4b
3a 3b
R: H ; OCH3
Scheme 2. Preparation of amides 4a–b from the corresponding amino acids 1a–b.
treating the ligand with copper(II) acetate. First, we compared
the catalytic efficiency of the individual ligands; the efficiency of
the anti- and syn-forms was verified. The asymmetric Henry reac-
tion was tested with four aldehydes, 2,2-dimethylpropanal, benz-
aldehyde, 4-nitrobenzaldehyde and 2-methoxybenzaldehyde;
these aldehydes were reacted with nitromethane to give the corre-
sponding 2-nitroethanols. In order to be able to compare the
efficiency of these compounds with the pyridine-imidazolidinones
studied earlier,5 we performed the reactions under identical condi-
tions, that is, the molar amount of catalyst, temperature and time.
The results obtained are summarized in Table 1.
In comparison with the imidazolidinone ligands containing an
isopropyl group at the 5-position,5 the enantiocatalytic efficiency
of the ligands with a syn-arrangement 5b–7b is distinctly higher.
The enantiomeric excesses are comparable for both ligands with
a syn- or anti-arrangement 5a–7a, however, the latter gives the
nitroaldol with the opposite enantiomer. A significant difference
between the enantiocatalytic activities of the anti- and syn-forms
was found only in the case of derivative 8. Hence, in the case of li-
gands 5–7, there is only a minor difference between the efficiencies
of the individual diastereoisomeric forms a and b; the ligands with
an anti-arrangement were only slightly more selective. This is in
complete contrast to the results obtained earlier with the imidazo-
lidinones containing an isopropyl group, whose anti-forms exhib-
ited much higher enantioselectivities than the syn-forms. Ligands
5–8 exhibit the highest degree of enantioselectivity in the reaction
of 2,2-dimethylpropanal. This can be presumed to be a result of the
steric effect of the bulky substituent (t-Bu), which dictates the ste-
reospecific arrangement of the reactants in the activated complex.
A significant finding is the fact that the ligands with an anti-
arrangement 5a–8a exhibit higher enantiocatalytic efficiency in
the case of derivatives containing a methyl group at the 2-position
of the imidazolidinone cycle 6a and 8a. This finding can be consid-
ered as a positive feature, because, from the standpoint of applica-
tion, ligands 6a and 8a are more versatile due to their higher
stability. Thanks to the substituent at theC-2 carbon atomof the imi-
dazolidinone cycle, ligands 6 and 8 cannot undergo a base-catalysed
Acid amides 4a–b were prepared by a three-step synthesis.
First, the amino group was protected by the introduction of a Boc
group. The N-protected forms of amino acids 2a–b were obtained
with yields of 91–93%. The carboxylic group was then activated
by ethyl chloroformate and transformed into an amide group by
treatment with ammonia in methanol (89–94%). Deprotection of
the amino group with trifluoroacetic acid in dichloromethane gave
amides 4a–b (Scheme 2) with yields of 93–95%. The conventional
method of preparing the amino acid amide via esterification with
methanol and subsequent aminolysis failed in this case, because
the aminolysis of the methyl esters of these amino acids is very
slow. This is caused by the steric hindrance at the ester group by
the bulky substituents present on the a-carbon atom. For example,
the aminolysis of the methyl ester of (S)-2-amino-3-(3,4-dime-
thoxyphenyl)-2-methylpropanoic acid with 7 M ammonia solution
in methanol proceeded over a period of 7 days at the reaction tem-
perature of 100 °C (pressure of 5 atm) with only a 55% conversion.
The condensation reaction of amide 4a or 4b with pyridine-
2-carbaldehyde or 2-acetylpyridine was used to prepare imidazolid-
inones 5–8 (Scheme 3). The condensation reaction of commercially
available phenylalanine amide with acetylpyridine gave the
corresponding 5-benzyl-2-methyl-2-(pyridine-2-yl)imidazolidine-
4-one. However, it was found that the reaction was accompanied
by racemization of the stereogenic centre C-5 in the imidazolidi-
none cycle. Therefore, this type of ligand was not studied any
further as an enantioselective catalyst. The reaction conditions of
this condensation were identical with those used in the prepara-
tion of pyridine-imidazolidinones.5 All four diastereoisomeric
mixtures were successfully separated into individual configuration
isomers by column chromatography. The overall yields of the con-
densation reaction (the sum of both epimers) were in the range of
51–65%. The absolute configuration at the stereogenic centre of the
2-imidazolidinone ring was determined by means of 1H NMR 1D
NOESY pulse sequence.12
The enantiocatalytic properties of the individual ligands 5–8
were studied in the asymmetric Henry reaction. The corresponding
copper(II) complexes of these ligands were prepared in situ by
R
R
HO
HO
O
NH2
1. KCN; NH3; H+
2. 6M HCl
NH2
Ref. 10
NH2
Ref. 11
1a
COOH
COOH
COOH
for 1b
for
1a
R: H ; OCH3
1b
Scheme 1. Synthesis of enantiomerically pure forms of 2-amino-3-phenyl-2-methylpropanoic acids 1a–b.

336
P. Drabina et al. / Tetrahedron: Asymmetry 24 (2013) 334–339
R
R
R1
R1
H
NH2
H
N
H
N
O
N
N
N
– H2O
CONH2
R
R
O
O
HN
HN
R1
R
R
R1 = H, CH3
R: H 4a; OCH3 4b
5b-8b
5a-8a
R = H, R1 = H
5
R = H, R1 = CH3
6
R = OCH3, R1 = H
7
R = OCH3, R1 = CH3
8
Scheme 3. Preparation of imidazolidinone derivatives 5–8.
Table 1
Survey of experiments of the asymmetric Henry reaction catalysed by ligands 5–8
OH
L
* (5 mol %)
Cu(OAc)2
R
CHO
+
MeNO2
NO2
*
R
EtOH
10 °C, 6 days
R
t-Bu
4-NO2C6H4
C6H5
2-CH3OC6H4
Ligand
Yielda (%)
eeb (%)
Yielda (%)
eeb (%)
Yielda (%)
eeb (%)
Yielda (%)
eeb (%)
5a
5b
6a
6b
7a
7b
8a
8b
95
54
94
55
97
60
95
75
81
ꢁ70
91
71
74
54
61
92
85
87
88
66
ꢁ66
74
87
78
80
74
81
73
85
83
68
ꢁ66
76
93
88
73
72
94
75
90
88
68
ꢁ67
79
ꢁ63
ꢁ52
ꢁ43
ꢁ54
80
50
51
60
ꢁ54
89
<2
ꢁ61
61
<2
ꢁ40
68
ꢁ12
ꢁ46
67
ꢁ7
a
The yield determined by 1H NMR of the crude product.
The enantiomeric excess determined by chiral HPLC.
b
Table 2
Table 3
Effect of the amount of catalyst 6a/Cu(OAc)2 on the course of the Henry reaction
Survey of experiments of the Henry reaction of substituted benzaldehydes with
nitroethane
OH
6a Cu(OAc)2
;
OH
NO2
+
MeNO2
L*
(5 mol %)
*
NO2
EtOH
18 °C, 6 days
CHO
+
CHO
EtNO2
*
Cu(OAc)2
EtOH, 10 °C
R
CH3
R
Entry
Mol % of cat.
Yielda (%)
eeb (%)
1
2
3
4
5
3
2
1
94
73
72
52
91
91
91
89
Aldehyde
R
Ligand
Time
(days)
Yielda
(%)
dra (%)
anti/syn
eeb (%)
anti
eeb (%)
syn
H
6a
6a
6a
6a
6a
6a
8a
8a
8a
8a
8a
8a
6
6
6
12
12
12
6
6
6
12
12
12
40
26
18
63
68
45
41
44
20
97
57
48
56/44
68/32
59/41
54/46
60/40
59/41
51/49
65/35
56/44
50/50
67/33
57/43
60
67
—
57
67
—
64
56
—
54
64
—
78
82
65
72
83
70
81
79
73
78
76
67
2-OCH3
4-NO2
H
2-OCH3
4-NO2
H
2-OCH3
4-NO2
H
2-OCH3
4-NO2
a
Isolated yield.
Enantiomeric excess determined by chiral HPLC.
b
racemization of this stereogenic centre. Their oxidation to imidaz-
olinone derivatives is also impossible (otherwise this is very easy
and proceeds under mild conditions).13 In the case of ligand, which
formed with a syn-arrangement 5b–8b, the more efficient deriva-
tives were those without a methyl group at the 2-position 5b and 7b.
Table 1 shows that the highest value of enantioselectivity was
attained with ligand 6a. Therefore, this ligand was selected for fur-
ther experiments to study the effect of the molar amount of the
catalyst on the chemical yield of the Henry reaction and its enanti-
oselectivity. In all preceding experiments, the amount used was
5 mol %. Therefore, further experiments were carried out with low-
er amounts of catalyst (1–3 mol %). The found enantioselectivity
(Table 2) obtained was virtually the same in all cases (ꢀ90% ee).
On the other hand, the chemical yield was dependent on the
amount of catalyst: it decreased with a decreasing amount of cat-
Yield and diastereoisomeric ratio determined by 1H NMR of the crude product.
Enantiomeric excess determined by chiral HPLC.
a
b
alyst, as expected. The optimum economical amount of catalyst for
a successful Henry reaction, reaction time versus attained conver-
sion, was considered to be 2 mol % (Table 2, entry 3).
In a subsequent study of the Henry reaction, nitromethane was
replaced by nitroethane. In this case, the product is 2-nitroalcohol
with two stereogenic centres, that is, four isomers are obtained.
This variant of the asymmetric Henry reaction has been described

P. Drabina et al. / Tetrahedron: Asymmetry 24 (2013) 334–339
337
in the literature and studied less frequently,14 although it gives chi-
ral products that can be used as intermediates in the synthesis of a
number of important compounds14b,15 in enantiomerically pure
form.
chromatograph 6890N with mass detector 5973 Network; the sam-
ples were dissolved in CH2Cl2 or acetone). The elemental microanal-
ysis was carried out using a FISONS Instruments EA 1108 CHN
apparatus. The optical rotation was measured on a Perkin–Elmer
341 instrument; the concentration c was given in g/100 mL. High-
resolution mass spectra were performed on Thermo Scientific MAL-
DI LTQ Orbitrap instrument.
The reaction was studied only with aromatic aldehydes, which
are more reactive than the aliphatic 2,2-dimethylpropanal. With
regard to the lower reactivity of nitroethane, a slower course of
the Henry reaction was expected. Only two ligands with the highest
enantiocatalytic efficiency 6a and 8a were selected for these exper-
iments. The results presented in Table 3 show that the chemical
yields were somewhat lower than those in the reactions with nitro-
methane, even in cases in which the reaction time was prolonged.
The representation of the individual diastereoisomers (dr) of the ob-
tained 1-phenyl-2-nitropropan-1-ols was determined by 1H NMR
spectroscopy. In all cases, the diastereoselectivity of the Henry
reaction was low (max. dr 68/32). The enantiomeric excesses were
determined by chiral HPLC for both diastereoisomers. The only
exception was the product formed from 4-nitrobenzaldehyde, in
which case suitable chromatographic conditions enabling the deter-
mination of enantiomeric excess for the nitroaldol with an
anti-arrangement were not found. The individual ee values (54–
83%) show that the enantioselectivity attained is comparable to
those obtained in other Henry reactions performed with nitrometh-
ane
4.2. General procedure for the synthesis of N-protected
aminoacids 2a,b
A mixture of amino acid hydrochloride 1 (16.5 mmol) TEA
(7 mL, 50 mmol) and Boc2O (3.96 g, 18.1 mmol) in CH2Cl2 (50 mL)
was stirred at room temperature for 3 days. The solvent was dis-
tilled off under reduced pressure, and the residue was treated with
a solution of citric acid (23.5 g, 112 mmol) in water (75 mL). The
emulsion formed was extracted with AcOEt (3 ꢂ 50 mL). The sol-
vent was evaporated in vacuo until dry.
4.2.1. (2S)-2-[(tert-Butoxycarbonyl)amino]-2-methyl-3-
phenylpropanoic acid 2a
Yield: 91%; mp: viscous oil; ½a D20
ꢃ
¼ ꢁ28:0 (c 1, CH3OH); 1H NMR
(400 MHz, DMSO-d6): d 12.44 (br s, 1H, COOH), 7.29–7.08 (m, 5H,
C6H5), 6.70 (br s, 1H, CONH), 2.93–2.63 (m, 2H, CH2), 1.41 (s, 9H,
(CH3)3), 1.19 (s, 3H, CH3); 13C NMR (100 MHz, DMSO-d6): d 175.7,
154.5, 137.0, 130.6, 127.9, 126.5, 78.0, 58.4, 42.8, 28.4, 22.7. Anal.
Calcd for C15H21NO4 (279.3): C, 64.50; H, 7.58; N, 5.01. Found: C,
64.38; H, 7.66; N, 4.94.
(61–79% ee). Extending the reaction time had no effect on the
enantioselectivity, and only the degree of conversion was increased
(Table 3).
3. Conclusion
4.2.2. (2S)-2-[(tert-Butoxycarbonyl)amino]-2-methyl-3-(3,4-
dimethoxyphenyl)propanoic acid 2b
A new series of pyridine-imidazolidinone derivatives 5–8 con-
taining a benzyl group at the 5-position of a ring cycle was pre-
pared. The copper(II) complexes of these bidentate ligands were
studied as enantioselective catalysts for the asymmetric Henry
reactions of selected aldehydes with nitromethane or nitroethane.
The ligands investigated exhibited good enantiocatalytic efficiency
with the maximum attained value of ee 91%. In comparison with
the copper(II) complexes of pyridine-imidazolinones,16 these
derivatives are significantly more enantioselective catalysts of
the Henry reaction. In comparison with the pyridine-imidazolidi-
nones5 containing an isopropyl group at the 5-position of the imi-
dazolidinone cycle, they were slightly less stereoselective. Only
small differences in enantioselectivity were observed between
the anti- and syn-forms of ligands 5–7, with the enantiocatalytic
efficiency even comparable in several cases. Moreover, the opti-
mum amount of catalyst for the successful course of the catalytic
process was determined. On the basis of the obtained results, it
is possible to consider ligands 5–8 and their respective complexes
with transition metals to be very promising enantioselective cata-
lysts. Further research on their enantiocatalytic properties in other
asymmetric reactions would be desirable.
Yield: 93%; mp: 58–60 °C; ½a D20
ꢃ
¼ ꢁ14:7 (c 1, CH3OH); 1H NMR
(400 MHz, DMSO-d6): d 6.84 (d, 3J = 8.0 Hz, 1H, C6H3), 6.64 (m,
3H, C6H3 + CONH), 3.71 (2 ꢂ s, 6H, 2 ꢂ OCH3), 3.17 (d,
2J = 13.4 Hz, 1H, CH2), 2.86 (d, 2J = 13.4 Hz, 1H, CH2), 1.40 (s, 9H,
(CH3)3), 1.17 (s, 3H, CH3); 13C NMR (100 MHz, DMSO-d6): d 175.7,
154.4, 148.1, 147.6, 129.2, 122.6, 114.3, 111.4, 78.0, 58.5, 55.5,
55.4, 41.2, 28.4, 22.7. Anal. Calcd for C17H25NO6 (339.3): C, 60.11;
H, 7.37; N, 4.12. Found: C, 59.98; H, 7.62; N, 4.23.
4.3. General procedure for the synthesis of N-protected amino
amides 3a,b
Ethyl chloroformate (3.85 mL, 40 mmol) was added to a stirred
solution of N-protected amino acid 2a or 2b (16 mmol) and TEA
(5.6 mL, 40 mmol) in CH2Cl2 (50 mL) cooled at 0 °C. After ca.
15 min, a 7 M methanolic solution of ammonia (14 mL, 100 mmol)
was added, and the suspension formed was stirred at room tem-
perature for 4 days. The reaction mixture was washed with water
(4 ꢂ 40 mL) and the organic layer was dried over sodium sulfate.
After evaporation of the solvent under reduced pressure, the resi-
due was dissolved in AcOEt (100 mL) and passed through a plug
(1 cm) of silica. The solvent was evaporated, and the crude product
was recrystallized from toluene.
4. Experimental
4.1. General
4.3.1. (2S)-2-[(tert-Butoxycarbonyl)amino]-2-methyl-3-phenyl-
The starting substances were purchased from Sigma–Aldrich.
Column chromatography was performed using 60–200 lm 60A sil-
propanamide 3a
Yield: 94%; mp: 70–72 °C; ½a D20
ꢃ
¼ ꢁ50:3 (c 1, CH3OH); 1H NMR
ica gel. Ethanol was dried over 4 Å molecular sieves before use. The
melting point temperatures are not corrected. 1H and 13C NMR
spectra were recorded on a Bruker Avance 400 instrument (400.13
MHz for 1H, and 100.61 MHz for 13C). Chemical shifts d were refer-
enced to the solvent residual peak (2.50 ppm 1H, 39.51 ppm 13C
for DMSO-d6, and 7.26 ppm 1H, 77.23 ppm 13C for CDCl3). The mass
spectra were measured with a set of Agilent Technologies (gas
(400 MHz, CDCl3): d 7.30–7.15 (m, 5H, C6H5), 6.33 (br s, 1H,
CONH2), 5.61 (br s, 1H, CONH2), 4.86 (br s, 1H, CONH), 3.38 (d,
2J = 13.6 Hz, 1H, CH2), 3.12 (d, 2J = 13.6 Hz, 1H, CH2), 1.47 (s, 9H,
(CH3)3), 1.44 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3): d 176.9,
154.9, 136.3, 130.7, 128.5, 127.1, 80.6, 60.1, 41.6, 28.6, 24.2. Anal.
Calcd for C15H22N2O3 (278.3): C, 64.73; H, 7.97; N, 10.06. Found:
C, 64.61; H, 8.06; N, 9.79.

338
P. Drabina et al. / Tetrahedron: Asymmetry 24 (2013) 334–339
4.3.2. (2S)-2-[(tert-Butoxycarbonyl)amino]-2-methyl-3-(3,4-di-
4.5.2. (2S,5S)-5-Benzyl-5-methyl-2-(pyridine-2-yl)imidazolid-
ine-4-one 5b
methoxyphenyl)propanamide 3b
Yield: 89%; mp: 102–103 °C; ½a D20
ꢃ
¼ ꢁ44:1 (c 0.9, CH3OH); 1H
Yield: 29%; yellow oil; Rf 0.40 (SiO2; acetone/CH2Cl2 (2/1; v/v);
NMR (400 MHz, CDCl3): d 6.80 (d, 3J = 8.4 Hz, 1H, C6H3), 6.71 (m,
2H, C6H3), 6.28 (br s, 1H, CONH2), 5.37 (br s, 1H, CONH2), 4.87
(br s, 1H, CONH), 3.86 (2 ꢂ s, 6H, 2 ꢂ OCH3), 3.32 (d, 2J = 13,6 Hz,
1H, CH2), 3.03 (d, 2J = 13.6 Hz, 1H, CH2), 1.47 (s, 9H, (CH3)3), 1.45
(s, 3H, CH3); 13C NMR (100 MHz, CDCl3): d 177.0, 154.9, 148.8,
148.2, 128.7, 122.8, 113.8, 111.1, 76.9, 60.2, 56.0, 41.5, 24.1. Anal.
Calcd for C17H26N2O5 (338.4): C, 60.28; H, 7.68; N, 8.27. Found:
C, 60.17; H, 7.85; N, 8.04.
½
a 2D0
ꢃ
¼ þ30:5 (c 1, CH3OH); 1H NMR (400 MHz, CDCl3): d 8.39 (m,
1H, py), 7.99 (br s, 1H, CONH), 7.50 (m, 1H, py), 7.19–7.11 (m, 6H,
Py + Ph), 6.80 (m, 1H, C6H5), 5.51 (s, 1H, CH), 3.15 (d, 2J = 13.6 Hz,
1H, CH2), 2.69 (d, 2J = 13.6 Hz, 1H, CH2), 2.61 (br s, 1H, NH), 1.38
(s, 3H, CH3); 13C NMR (100 MHz, CDCl3): d 180.2, 158.8, 149.1,
137.1, 136.7, 130.6, 130.5, 128.4, 126.7, 123.6, 123.0, 120.6, 70.5,
63.4, 43.4, 24.3. Anal. Calcd for C16H17N3O (267.3): C, 71.89; H,
6.41; N, 15.72. Found: C, 71.59; H, 6.35; N, 15.76. HRMS: m/z Calcd
for C16H27N3O: 268.14444 [M+H]+. Found: 268.14432.
4.4. General procedure for the synthesis of amides 4a,b
4.5.3. (2R,5S)-5-(3,4-Dimethoxybenzyl)-5-methyl-2-(pyridine-2-
yl)imidazolidine-4-one 7a
A solution of N-protected amide 3a or 3b (5 mmol) in CH2Cl2
(10 mL) and TFA (1.2 mL, 16 mmol) was refluxed for 8 h. The sol-
vents were then distilled off under reduced pressure, and the res-
idue was treated with saturated aqueous solution of sodium
carbonate (20 mL). The emulsion formed was extracted with
CH2Cl2 (3 ꢂ 25 mL), the organic layer was dried over sodium sul-
fate and evaporated to dryness.
Yield: 37%; yellow oil; Rf 0.45 (SiO2; acetone/CH2Cl2 (1/1; v/v);
½
a 2D0
ꢃ
¼ þ79:9 (c 1, CH3OH); 1H NMR (400 MHz, CDCl3): d 8.52 (m,
1H, Py), 7.68 (m, 1H, Py), 7.38 (m, 1H, Py), 7.22 (m, 1H, Py),
6.91–6.78 (m, 3H, C6H3), 6.59 (br s, 1H, CONH), 4.92 (s, 1H, CH),
3.86 (s, 6H, 2 ꢂ CH3O), 3.12 (d, 2J = 13.6 Hz, 1H, CH2), 2.83 (br s,
1H, NH), 2.67 (d, 2J = 13.6 Hz, 1H, CH2), 1.36 (s, 3H, CH3); 13C
NMR (100 MHz, CDCl3): d 179.7, 158.9, 149.6, 148.5, 148.1, 137.2,
129.3, 123.7, 122.5, 121.2, 113.4, 111.1, 70.3, 63.5, 56.1, 56.0,
44.1, 25.9. Anal. Calcd for C18H21N3O3 (327.4): C, 66.04; H, 6.47;
N, 12.84. Found: C, 65.97; H, 6.42; N, 12.83. HRMS: m/z Calcd for
4.4.1. (2S)-2-Amino-2-methyl-3-phenylpropanamide 4a
Yield: 93%; mp: 114–116 °C; ½a D20
ꢃ
¼ ꢁ33:0 (c 1.1, CH3OH); 1H
NMR (400 MHz, CDCl3): d 7.35–7.25 (m, 6H, C6H5 + CONH2), 5.75
(br s, 1H, CONH2), 3.42 (d, 2J = 13.2 Hz, 1H, CH2), 2.70 (d,
2J = 13.2 Hz, 1H, CH2), 1.45 (s, 3H, CH3), 1.38 (br s, 2H, NH2); 13C
NMR (100 MHz, CDCl3): d 179.9, 137.0, 130.5, 128.5, 127.0, 58.6,
46.7, 28.0. Anal. Calcd for C10H14N2O (178.2): C, 67.39; H, 7.92;
N, 15.72. Found: C, 67.31; H, 7.91; N, 15.67.
C
18H21N3O3: 328.16557 [M+H]+. Found: 328.16523.
4.5.4. (2S,5S)-5-(3,4-Dimethoxybenzyl)-5-methyl-2-(pyridine-2-
yl)imidazolidine-4-one 7b
Yield: 28%; yellow oil; Rf 0.29 (SiO2; acetone/CH2Cl2 (1/1; v/v);
½
a 2D0
ꢃ
¼ ꢁ111:4 (c 1, CH3OH); 1H NMR (400 MHz, CDCl3): d 8.43 (m,
1H, Py), 7.54 (m, 1H, Py), 7.18 (m, 1H, Py), 6.91 (br s, 1H, CONH),
6.85 (m, 1H, Py), 6.74–6.71 (m, 3H, C6H3), 5.52 (s, 1H, CH), 3.83
(s, 3H, CH3O), 3.72 (s, 3H, CH3O), 3.18 (d, 2J = 14.0 Hz, 1H, CH2),
2.65 (d, 2J = 14.0 Hz, 1H, CH2), 2.61 (br s, 1H, NH), 1.42 (s, 3H,
CH3); 13C NMR (100 MHz, CDCl3): d 180.1, 158.6, 149.3, 148.9,
148.1, 137.1, 129.1, 123.7, 122.6, 120.5, 113.6, 111.1, 70.3, 63.5,
56.0, 55.8, 43.0, 24.4. Anal. Calcd for C18H21N3O3 (327.4): C,
66.04; H, 6.47; N, 12.84. Found: C, 65.99; H, 6.45; N, 12.87. HRMS:
m/z Calcd for C18H21N3O3: 328.16557 [M+H]+. Found: 328.16512.
4.4.2. (2S)-2-Amino-2-methyl-3-(3,4-dimethoxyphenyl)propan-
amide 4b
Yield: 95%; yellow oil; ½a D20
ꢃ
¼ ꢁ22:7 (c 1, CH3OH); 1H NMR
(400 MHz, CDCl3): d 7.29 (br s, 1H, CONH2), 6.74 (m, 3H, C6H3),
5.63 (br s, 1H, CONH2), 3.84 (2 ꢂ s, 6H, 2 ꢂ OCH3), 3.35 (d,
2J = 13.2 Hz, 1H, CH2), 2.51 (d, 2J = 13.2 Hz, 1H, CH2), 1.38 (m, 5H,
CH3 + NH2); 13C NMR (100 MHz, CDCl3): d 180.1, 149.0, 148.2,
129.6, 122.6, 113.6, 111.2, 58.7, 56.1, 56.0, 46.5, 28.2. Anal. Calcd
for C12H18N2O3 (238.3): C, 60.43; H, 7.55; N, 11.75. Found: C,
60.18; H, 7.68; N, 11.63.
4.6. General procedure for the synthesis of ligands 6a,b and 8a,b
4.5. General procedure for the synthesis of ligands 5a,b and 7a,b
A solution of the corresponding 2-aminoamide (3 mmol), 2-ace-
tylpyridine (4 mmol) and p-toluenesulfonic acid (0.3 mmol) in 1,
2-dichlorobenzene (5 mL) was heated at 140 °C for 2 h. The solvent
was then distilled off under reduced pressure, and the residue was
treated with CH2Cl2 (10 mL) and extracted with a saturated aqueous
solution of sodium carbonate (10 mL). The organic layer was dried
over sodium sulfate and concentrated in vacuo. The residue was sub-
mitted to chromatography on silica gel with the appropriate solvent.
A solution of the corresponding 2-aminoamide (3 mmol), pyri-
dine-2-carbaldehyde (4 mmol) and one drop of acetic acid in dry
methanol (5 mL) was refluxed for 12 h. The solvent was distilled
off under reduced pressure, and the residue was treated with
CH2Cl2 (10 mL) and extracted with a saturated aqueous solution
of sodium carbonate (10 mL). The organic layer was dried over so-
dium sulfate and concentrated in vacuo. The residue was submit-
ted to chromatography on silica gel with the appropriate solvent.
4.6.1. (2R,5S)-5-Benzyl-2,5-dimethyl-2-(pyridine-2-yl) imidazoli-
dine-4-one 6a
4.5.1. (2R,5S)-5-Benzyl-5-methyl-2-(pyridine-2-yl) imidazolid-
ine-4-one 5a
Yield: 24%; mp: 118–121 °C; Rf 0.47 (SiO2; acetone/CH2Cl2 (2/1;
Yield: 35%; yellow oil; Rf 0.53 (SiO2; acetone/CH2Cl2 (2/1; v/v);
v/v); ½a 2D0
ꢃ
¼ ꢁ21:7 (c 1, CH3OH); 1H NMR (400 MHz, CDCl3): d 8.46
½
a 2D0
ꢃ
¼ ꢁ38:5 (c 1, CH3OH); 1H NMR (400 MHz, CDCl3): d 8.53 (m,
(m, 1H, Py), 8.13 (br s, 1H, CONH), 7.66 (m, 1H, Py), 7.57 (m, 1H,
Py), 7.26 (m, 5H, Ph), 7.08 (m, 1H, Py), 3.28 (d, 2J = 13.6 Hz, 1H,
CH2), 2.64 (d, 2J = 13.6 Hz, 1H, CH2), 2.56 (br s, 1H, NH), 1.12 (s,
3H, CH3), 1.08 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3): d 179.1,
164.4, 148.9, 136.8, 136.7, 131.1, 130.4, 128.7, 128.3, 127.1,
122.4, 119.0, 74.8, 64.2, 43.9, 30.4, 26.3. Anal. Calcd for
1H, Py), 7.68 (m, 1H, Py), 7.38–7.21 (m, 7H, Py + Ph), 6.59 (br s,
1H, CONH), 4.87 (s, 1H, CH), 3.16 (d, 2J = 13.2 Hz, 1H, CH2), 2.85
(br s, 1H, NH), 2.76 (d, 1H, 2J = 13.2 Hz, CH2), 1.39 (s, 3H, CH3);
13C NMR (100 MHz, CDCl3): d 179.7, 158.2, 149.6, 137.3, 136.8,
130.5, 128.5, 127.1, 123.9, 121.3, 70.3, 63.5, 44.4, 25.7. Anal. Calcd
for C16H17N3O (267.3): C, 71.89; H, 6.41; N, 15.72. Found: C, 71.65;
H, 6.33; N, 15.74. HRMS: m/z Calcd for C16H27N3O: 268.14444
[M+H]+. Found: 268.14394.
C17H19N3O (281.4): C, 72.57; H, 6.81; N, 14.94. Found: C, 72.46;
H, 6.80; N, 14.89. HRMS: m/z Calcd for C17H19N3O: 282.16009
[M+H]+. Found: 282.15981.

P. Drabina et al. / Tetrahedron: Asymmetry 24 (2013) 334–339
339
4.6.2. (2S,5S)-5-Benzyl-2,5-dimethyl-2-(pyridine-2-yl)imidazoli-
Acknowledgments
dine-4-one 6b
Yield: 27%; mp: 106–108 °C; Rf 0.39 (SiO2; acetone/CH2Cl2 (2/1;
The described research work was financially supported by the
Ministry of Education, Youth and Sports of the Czech Republic, Pro-
ject CZ.1.07/2.3.00/30.0021 ‘Strengthening of Research and Devel-
opment Teams at the University of Pardubice’.
v/v); ½a 2D0
ꢃ
¼ ꢁ147:0 (c 1, CH3OH); 1H NMR (400 MHz, CDCl3): d
8.46 (m, 1H, Py), 7.56 (m, 1H, Py), 7.51 (br s, 1H, CONH), 7.34 (m,
1H, Py), 7.13–7.09 (m, 6H, Py + Ph), 2.83 (d, 2J = 13.6 Hz, 1H, CH2),
2.64 (d, 2J = 13.6 Hz, 1H, CH2), 2.55 (br s, 1H, NH), 1.70 (s, 3H,
CH3), 1.41 (s, 3H, CH3); 13C NMR (100 MHz, CDCl3): d 179.2,
163.9, 148.8, 137.0, 136.9, 130.5, 128.2, 126.6, 122.5, 118.8, 74.9,
63.9, 43.9, 32.4, 26.7. Anal. Calcd for C17H19N3O (281.4): C, 72.57;
H, 6.81; N, 14.94. Found: C, 72.48; H, 6.78; N, 14.86. HRMS: m/z
Calcd for C17H19N3O: 282.16009 [M+H]+. Found: 282.15964.
References
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ine-2-yl)imidazolidine-4-one 8a
Yield: 39%; yellow oil; Rf 0.45 (SiO2; acetone/CH2Cl2 (1/1; v/v);
½
a 2D0
ꢃ
¼ ꢁ3:3 (c 1, CH3OH); 1H NMR (400 MHz, CDCl3): d 8.47 (m,
1H, Py), 7.73 (m, 1H, Py), 7.65 (m, 1H, Py), 7.15 (m, 1H, Py),
6.87–6.80 (m, 3H, C6H3), 6.74 (br s, 1H, CONH), 3.88 (s, 6H,
2 ꢂ CH3O), 3.30 (d, 2J = 13.6 Hz, 1H, 2J = 14.0 Hz, 1H, CH2), 2.60 (d,
2J = 14.0 Hz, 1H, CH2), 2.49 (br s, 1H, NH), 1.13 (s, 3H, CH3), 1.11
(s, 3H, CH3); 13C NMR (100 MHz, CDCl3): d 179.0, 164.4, 149.2,
149.0, 148.3, 136.7, 129.2, 122.5, 122.4, 119.0, 113.2, 111.1, 74.6,
64.3, 56.0, 56.0, 43.4, 30.7, 26,3. Anal. Calcd for C19H23N3O3
(341.4): C, 66.84; H, 6.79; N, 12.31. Found: C, 66.83; H, 6.75; N,
12.29. HRMS: m/z Calcd for C19H23N3O3: 342.18122 [M+H]+.
Found: 342.18067.
ˇ
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ine-2-yl)imidazolidine-4-one 8b
ˇ ˇ
(h) Tydlitát, J.; Bureš, F.; Kulhánek, J.; Mloston´ , G.; Ru˚ zicka, A. Tetrahedron:
Asymmetry 2012, 23, 1010–1018.
Yield: 24%; yellow oil; Rf 0.37 (SiO2; acetone/CH2Cl2 (1/1; v/v);
7. MacMillan, D. W. C. Nature 2008, 455, 304–308.
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Pergamon: Amsterdam, 1999. p 320.
½
a 2D0
ꢃ
¼ ꢁ42:9 (c 1, CH3OH); 1H NMR (400 MHz, CDCl3): d 8.42 (m,
1H, Py), 7.52 (m, 1H, Py), 7.22 (m, 1H, Py), 7.11 (m, 1H, Py), 6.96
(br s, 1H, CONH), 6.65–6.57 (m, 3H, C6H3), 3.78 (s, 6H, CH3O),
3.76 (s, 6H, CH3O), 2.85 (d, 2J = 14.0 Hz, 1H, CH2), 2.61 (br s, 1H,
NH), 2.53 (d, 2J = 14.0 Hz, 1H, CH2), 1.67 (s, 3H, CH3), 1.43 (s, 3H,
CH3); 13C NMR (100 MHz, CDCl3): d 179.4, 163.7, 148.5, 148.4,
147.6, 136.5, 129.4, 122.4, 122.2, 118.6, 113.4, 110.8, 74.9, 64.0,
55.8, 55.7, 43.5, 32.4, 27.0. Anal. Calcd for C19H23N3O3 (341.4): C,
66.84; H, 6.79; N, 12.31. Found: C, 66.62; H, 6.86; N, 12.38. HRMS:
m/z Calcd for C19H23N3O3: 342.18122 [M+H]+. Found: 342.18097.
ˇ
13. (a) Panov, I.; Drabina, P.; Padelková, Z.; Hanusek, J.; Sedlák, M. J. Heterocycl.
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4.7. General experimental procedure for the Henry reaction
A mixture of ligands 5–8 (30
lmol) and Cu(OAc)2 (4.9 mg;
27
lmol) in absolute ethanol (1 mL) was stirred for 1 hour at room
temperature. The resulting clear blue solution was cooled to the
appropriate temperature, and then nitroalkane (0.5 mL) and alde-
hyde (0.5 mmol) were added. The mixture was stirred for the time
period indicated in Tables 1–3. The crude product was isolated by
column chromatography. Enantiomeric excess was determined by
chiral HPLC (using Daicel columns Chiralcel OD-H or Chiralpak
AS-H). Diastereomeric ratio was determined by 1H NMR
spectroscopy.
ˇ
˚ ˇ ˇ
16. Sedlák, M.; Drabina, P.; Keder, R.; Hanusek, J.; Císarová, I.; Ruzicka, A. J.
Organomet. Chem. 2006, 691, 2623–3630.