Improved synthesis and crystallographic analysis of (E)-ethyl 2-(hydroxyimino)-3-(4-methoxyphenyl)-3-oxopropanoate and erythro-N-acetyl-β-(4-methoxyphenyl)serine ethyl ester

Article abstract of DOI:10.14233/ajchem.2016.19149

(E)-Ethyl 2-(hydroxyimino)-3-(4-methoxyphenyl)-3-oxopropanoate has been synthesized by the oximation of ethyl 3-(4-methoxyphenyl)- 3-oxopropanoate with ethyl nitrite in the presence of multi-pore activated-K₂CO₃. A one-pot procedure has also been developed for the conversion of this oxime to the corresponding erythro-2-acetamido-3-hydroxy-3-(4-methoxyphenyl)propionic ethyl ester. Their configurations were unambiguously confirmed by X-ray crystallographic determination. The oxime crystallized as a monoclinic system with space group P21/c and the following cell parameters: a = 11.524(3) ?, b = 7.2367(16) ?, c = 14.866(4) ?, β = 111.027(4) and Z = 4, whereas the erythro-N-acetyl-β-(4-methoxyphenyl)serine ethyl ester crystallized as a triclinic system with space group P-1 and the following cell parameters: a = 7.6451(6) ?, b = 13.3326(10) ?, c = 16.2679(13) ?, α = 67.314(7), β = 79.917(7), γ = 88.516(7) and Z = 2. The crystal packing in both of these structures was stabilized by intermolecular O-H-O and C-H-O hydrogen bonds.

Full text of DOI:10.14233/ajchem.2016.19149

Asian Journal of Chemistry; Vol. 28, No. 1 (2016), 18-22
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2016.19149
Improved Synthesis and Crystallographic Analysis of (E)-Ethyl 2-(Hydroxyimino)-3-(4-
methoxyphenyl)-3-oxopropanoate and erythro-N-Acetyl-β-(4-methoxyphenyl)serine Ethyl Ester
1
2
1
1,2,*
S.M. FAN , J.R. HAN , L.Y. JIN and S.X. LIU
1Department of Chemistry, Yanbian University, No. 977, GongYuan Road, Yanji 133002, Jilin Province, P.R. China
2State Key Laboratory Breeding Base, Hebei Laboratory of Molecular Chemistry for Drug, Hebei University of Science and Technology, No.
70, Yuhua east Road, Shijiazhuang 050018, Hebei Province, P.R. China
*Corresponding author: Tel./Fax: +86 311 88632254; E-mail: chlsx@263.net
Received: 3 March 2015;
Accepted: 12 May 2015;
Published online: 5 October 2015;
AJC-17538
(E)-Ethyl 2-(hydroxyimino)-3-(4-methoxyphenyl)-3-oxopropanoate has been synthesized by the oximation of ethyl 3-(4-methoxyphenyl)-
3-oxopropanoate with ethyl nitrite in the presence of multi-pore activated-K2CO3. A one-pot procedure has also been developed for the
conversion of this oxime to the corresponding erythro-2-acetamido-3-hydroxy-3-(4-methoxyphenyl)propionic ethyl ester. Their
configurations were unambiguously confirmed by X-ray crystallographic determination. The oxime crystallized as a monoclinic system
with space group P21/c and the following cell parameters: a = 11.524(3) Å, b = 7.2367(16) Å, c = 14.866(4) Å, β = 111.027(4) and Z =
4, whereas the erythro-N-acetyl-β-(4-methoxyphenyl)serine ethyl ester crystallized as a triclinic system with space group P-1 and the
following cell parameters: a = 7.6451(6) Å, b = 13.3326(10) Å, c = 16.2679(13) Å, α = 67.314(7), β = 79.917(7), γ = 88.516(7) and Z =
2. The crystal packing in both of these structures was stabilized by intermolecular O-H---O and C-H---O hydrogen bonds.
Keywords: Oximation, Asymmetric reduction, Amino acid, Crystal structure, Hydrogen bond.
methoxyphenyl)serine ethyl ester which can afford optical
β-(4-methoxyphenyl)serine by enzymatic resolution [9].
Meanwhile, the two crystal structures of (E)-ethyl 2-(hydroxy-
imino)-3-(4-methoxyphenyl)-3-oxopropanoate and erythro-N-
acetyl-β-(4-methoxyphenyl)serine ethyl ester will be reported.
INTRODUCTION
The preparation of oxime esters and subsequently
reduction is a very important route for synthesizing of amino
acids [1-6]. Traditionally, the oximation reaction of β-dicarbonyl
compounds was carried out with sodium nitrite in acetic acid
or with nitrous esters in strong alkaline conditions. However,
the former method was only limited to the oximation of non-
substituted β-dicarbonyl compounds. Although the latter
method could be widely applied to mono- and non-substituted
β-dicarbonyl compounds, some drawbacks are still obvious
in practical application. (1) Sodium ethoxide is a strong base
and has corrosion to equipments. (2) low boiling point and
the hazard of ethyl nitrite result to that the process is not green
and inconvenient. In order to avoid these disadvantages, we
developed a novel base multi-pore K2CO3 with higher activity
compared to normal K2CO3 to replace sodium ethoxide to
execute the oximation. β-(4-Methoxyphenyl)serine, as the
unnatural amino acid, can be found in numerous naturally
occurring biological products [7,8]. In this paper, we report the
preparation of ethyl 3-(4-methoxyphenyl)-3-oxopropanoate via
directly leading ethyl nitrite to the reaction solution using
multi-pore activated-K2CO3. Subsequently, we develop a one-
pot method of the oxime converted to erythro-N-acetyl-β-(4-
EXPERIMENTAL
All of the reagents used in the current study were used as
supplied without prior purification. Melting points were mea-
sured on an Xt-4 apparatus using an uncorrected thermometer.
1H NMR spectra were recorded on a Bruker AVANCE II500
instrument in CDCl3 using tetramethylsilane as an internal
reference. Infrared spectra were recorded on a PE-1730 spectro-
meter. LC-MS was performed on a Thermo Finnigan LCQ-
Advantage mass spectrometer. Multi-pore activited K2CO3 was
prepared using an LGJ-10 vacuum freeze dryer.
Synthesis
Preparation of multi-pore activated-K2CO3:The crysta-
lline potassium carbonate (200 g) was frozen at -50 °C using
a vacuum freeze dryer. Subsequent vacuum sublimation and
drying over 24 h gave multi-pore activated-K2CO3.
(E)-Ethyl 2-(hydroxyimino)-3-(4-methoxyphenyl)-3-
oxopropanoate (2): A solution of sulfuric acid (7.5 g, 0.075

Vol. 28, No. 1 (2016)
Improved Synthesis and Crystallographic Analysis of Oxime Esters 19
mol) in a mixture of water (100 mL) and ethanol (5 mL) was
added to a stirred solution of sodium nitrite (10.4 g, 0.15 mol)
in a mixture of water (50 mL) and ethanol (7 mL) in a dropwise
manner, resulting in the formation of ethyl nitrite. The ethyl
nitrite gas was bubbled through a stirred mixture of multi-
pore activated-K2CO3 (41.7 g, 0.3 mol) and ethyl 3-(4-methoxy-
phenyl)-3-oxopropanoate (22.3 g, 0.1 mol) in ethanol (100
mL) at 10 °C for 5 h. The reaction mixture was then filtered to
remove any solids and the resulting filtrate was collected and
distilled to dryness in vacuo. The resulting residue was dissol-
ved in cold water (50 mL) and the pH of the solution was
adjusted to 5 using a 0.5 M solution of HCl. The solution was
extracted with ethyl acetate (3 × 50 mL) and the combined
organic layers were dried over anhydrous MgSO4. The solvent
was then removed in vacuo to give the desired product 2 as a
solid. (17.8 g, 71 %). m.p.: 112-114 °C. IR (KBr, νmax, cm-1):
3402, 1696, 1666, 1595. 1H NMR (CDCl3, 500 MHz): δ 9.52
(br, 1H), 7.85 (d, J = 8.5 Hz, 2H), 6.98 (d, J = 8.5 Hz, 2H),
4.31 (q, J = 7.0 Hz, 2H), 1.27 (t, J = 7.0 Hz, 3H). 13C NMR
(CDCl3, 125 MHz): δ 188.4, 165.1, 160.9, 149.8, 131.9, 127.5,
114.5, 62.7, 55.8, 29.8, 14.0. Crystals of the oximino ester 2
suitable for X-ray diffraction analysis were obtained by slow
evaporation over a period of 5 days from a dichloromethane/
n-hexane mixture.
erythro-Ethyl-2-acetamido-3-hydroxy-3-(4-methoxy-
phenyl) propanoate (3):A solution of 2 (12.6 g, 50 mmol) in
ethanol (91 mL) and acetic acid (3 mL) was treated with 10 %
Pd/C (1 g) and the resulting mixture was stirred at room tempe-
rature under an atmosphere of hydrogen for 24 h. The pH of
the reaction was adjusted to 7 by the addition of a 2 M solution
of NaOH and the resulting mixture was treated with acetic
anhydride (15 g, 147 mmol). The reaction mixture was stirred
at room temperature overnight and then filtered. The filtrate
was collected and concentrated in vacuo to give a residue,
which was suspended in water. The mixture was then filtered
to give the desired erythro-product 3 as a white solid (12.1 g,
86 %). m.p.: 146-148 °C. 1H NMR (CDCl3, 500 MHz): δ 7.15
(dd, J = 2.0, 7.0 Hz, 2H), 6.85 (dd, J = 2.0, 7.0 Hz, 2H), 6.25(d,
J = 7.0 Hz, 1H), 5.22 (dd, J = 3.5, 5.5 Hz, 1H), 4.96 (dd, J =
3.5, 6.5 Hz, 1H), 4.44 (d, J = 5.5 Hz, 1H), 4.20 (q, J = 7.5 Hz,
2H), 2.04 (s, 3H), 1.27 (t, J = 7.5 Hz, 3H). 13C NMR (CDCl3,
125 MHz): δ 171.7, 169.5, 159.4, 131.2, 127.1, 113.7, 74.9,
62.0, 59.3, 55.2, 22.9, 14.1. HRMS: calcd for C14H20NO5 [M
+ H]+ 282.1341; found 282.1329. Crystals of the erythro-
product 5 suitable for X-ray diffraction analysis were obtained
by slow evaporation from ethyl acetate over a period of 3 days
at room temperature.
sition during data collection. The final R values (on F2) were
0.032 for oxime 2 and 0.065 for erythry-3. Crystal data and
some details of the structural determination are summarized
in Table-1.
TABLE-1
CRYSTAL DATA AND STRUCTURAL
REFINEMENTS FOR COMPOUND 2 AND 3
Parameter
Compound 2
C12H13NO5
251.23
Compound 3
C14H19NO5
562.60
Empirical formula
Formula weight
Temperature (K)
113(2)
293(2)
Wavelength (Å)
0.71073
0.71073
Crystal system, space group
Monoclinic, P2(1)/c Triclinic, P -1
a = 11.524(3) Å, a = 7.6451(6) Å,
b = 7.2367(16) Å, b = 13.3326(10) Å,
c = 14.866(4) Å
β =111.027(4) °
c = 16.2679(13) Å
α = 67.314(7)°
β = 79.917(7)°
γ = 88.516(7)°
1504.7(2)
Volume (Å3)
1157.2(5)
4, 1.442
0.113
Z, Calculated density (g cm-3)
Absorption coefficient (mm-1)
F(000)
2, 1.242
0.094
528
600
Crystal size (mm-3)
Range for data collection
Limiting indices
0.30 × 0.22 × 0.14 0.48 × 0.41 × 0.35
1.89-27.87°
-15<=h<=14,
-8<=k<=9,
-19<=l<=19
11640 / 2761
3.048 - 26.022°
-9<=h<=9,
-16<=k<=16,
-19<=l<=20
10656 / 5934
Reflections collected/unique
[R(int) = 0.0403] [R(int) = 0.0317]
100.0 % 99.8 %
Completeness to θ = 31.11
Refinement method
Full-matrix least- Full-matrix least-
squares on F2
1.059
squares on F2
1.041
Goodness-of-fit on F2
R1 = 0.0321
wR2 = 0.0789
R1 = 0.0435
wR2 = 0.0818
R1 = 0.0645
wR2 = 0.1680
R1 = 0.0935
wR2 = 0.1938
Final R indices [I > 2σ(I)]
R indices (all data)
RESULTS AND DISCUSSION
Our improved approach for the synthesis of ( )-erythro-
3 is shown in Scheme-I. Initial attempts to use commercial
anhydrous K2CO3 to allow for the oximation of 1 with ethyl
nitrite were unsuccessful because of the poor activity of this
K2CO3 material. The preparation of multi-pore-K2CO3 using a
vacuum freeze drier was developed by us. The loss of crystal
water of crystalline potassium carbonate produced multi-pore
structure. Multi-pore-K2CO3 exhibits stronger alkalinity to
normal anhydrous K2CO3 because of its larger specific surface
area. As expected, intermediate (E)-oxime 2 was successfully
synthesized from compound 1 in the presence of multi-pore-
K2CO3. The trans configuration of oxime 2 was unambiguously
confirmed by X-ray crystallography. A convenient one-pot
approach was developed for the step-wise conversion of
compound 2 to erythro-3 The first step in this one-pot process
involved the diastereoselective catalytic hydrogenation of
oxime 2 over Pd/C in a mixture of methanol and acetic acid to
give ethyl erythro-β-(4-methoxyphenylalanine)serine ethyl
ester, which was subsequently N-acetylated in the presence of
Ac2O and NaOAc. Compared with previously published
methods [9,11], the weakly acidic solvent system used for the
X-ray crystallography: Data for single crystals of oxime
2 and erythry-3 were collected on a standard Rigaku Saturn
724 CCD Area Detector System and an Agilent SuperNova
(Dual, Cu at zero, Eos) diffractometer equipped with a normal-
focus molybdenum-target X-ray tube (λ = 0.71073 Å), res-
pectively. The structures were solved using direct methods
and refined by full-matrix least-squares techniques. All non-
hydrogen atoms were assigned anisotropic displacement
parameters in the refinement. All hydrogen atoms were added
at calculated positions and refined using a riding model. The
structures were refined on F2 using SHELXTL-97 [10]. The
crystals used for the diffraction study showed no decompo-

20 Fan et al.
Asian J. Chem.
O
O
O
O
Ethyl nitrite
1. H2, Pd/C,
multi-pore activated-K2CO3
AcOH/EtOH
O
O
10oC
N
2. Ac2O, NaOH
MeO
HO
MeO
2
1
OH
O
O
OH O
(R)
(S)
(S)
(R)
O
O
HN
HN
O
MeO
MeO
3
Scheme-I: Synthesis of ( )-erythro-N-acetyl-β-(4-methoxyphenyl)serine ethyl ester 3
hydrogenation in the current approach (i.e., AcOH/MeOH)
performed as a suitable replacement for absolute ethanolic
HC1 or concentrated HCl/MeOH. Furthermore, this method
provided a high yield of the product following a simple workup
procedure. According to the mechanism proposed by Chang
and Hartung [11], the polar oxygen and nitrogen atoms of the
substrate molecule would be adsorbed onto the surface of the
Pd catalyst to form a rigid ring-like structure, which would
undergo the hydrogenation reaction to give the erythro-steric
structure exclusively. The configuration of erythro-3 was
unambiguously confirmed by X-ray crystallography.
Fig. 1. Molecular structures of oxime 2, showing the atom-labeling scheme
displacement ellipsoids are drawn at the 30 % probability level
TABLE-2
Crystal structure of oxime 2: The crystal structure of
oxime 2 is shown in Fig. 1. The structure is composed of one
aromatic ring moiety and one oximino ester side chain. The
side chain oximino ester (O5, N1, C9, C10, O3, O4, C11) and
the aromatic ring (C2-C9, O2) displayed a coplanar arrange-
ment with mean deviation values to the least square plane of
0.0303 and 0.0353 Å, respectively. The dihedral angle between
the two planes was 96.8(2)°. When viewed along the C8-C9
bond, the molecules displayed a staggered conformation with
C5 opposite to O2 and C10 opposite to N1. When viewed
along the C5-C8 or C9-C10 bond, the molecules adopted an
almost eclipsed conformation. The length of the C8-C9 bond
(1.521 Å) was greater than that of the C9-C10 bond (1.4931
Å) and the N(1)-C(9)-C(10)-O(3) and O(2)-C(8)-C(9)-N(1)
torsion angles were found to be 175.36 and 92.82(13)°, respec-
tively. These data demonstrate that the double bonds at C8-
O2 and C9-N1 did not form a conjugated system, whereas the
double bonds at C3-O10 and C9-N1 were conjugated. The
carbon-nitrogen double bond in compound 2 existed in the E
form. Selected bond lengths and bond angles for compound 2
are shown in Table-2.
Intermolecular hydrogen bonds of the type O-H···O and
C-H···O were found in the crystal structure of oxime 2 (Table-3).
Atoms O1, O2 and O3 acted as hydrogen-bond acceptors
towards the hydrogen of the H5-O5 and H7-C7 bonds, which
led to the formation of supramolecular structures. Non-classical
hydrogen-bonds C7-H7---O1 (3.494, 161.3°, symmetry code
-x,1-y,1-z) connected two aromatic rings in the horizontal
direction. Furthermore, the side chain oximino ester groups
were linked together in chains through bifurcated hydrogen
bonds between hydrogen H5-O5···O2 (2.916, 123°, symmetry
SELECTED BOND LENGTHS (Å)
AND ANGLES (°) FOR COMPOUND 2
Bond
O(1)-C(2)
O(3)-C(10)
O(5)-N(1)
C(5)-C(8)
O(1)-C(1)
O(4)-C(10)
O(5)-H(5)
C(8)-C(9)
O(2)-C(8)
O(4)-C(11)
N(1)-C(9)
C(9)-C(10)
Dist.
Angle
C(2)-O(1)-C(1)
C(9)-N(1)-O(5)
N(1)-C(9)-C(10)
C(10)-O(4)-C(11)
O(2)-C(8)-C(5)
N(1)-C(9)-C(8)
N(1)-O(5)-H(5)
O(2)-C(8)-C(9)
C(10)-C(9)-C(8)
–
(°)
117.79(9)
110.20(9)
119.44(10)
118.21(8)
123.45(10)
124.00(9)
109.5
1.3545(12)
1.2101(12)
1.3892(12)
1.4664(14)
1.4383(12)
1.3243(12)
0.8400
1.5210(14)
1.2209(13)
1.4624(13)
1.2765(14)
1.4931(15)
117.75(9)
116.50(9)
–
–
–
–
–
TABLE-3
HYDROGEN BONDED GEOMETRIES (°), DISTANCES (Å)
AND BOND ANGLES (°) FOR COMPOUND 2
D-H···A
d(D-H)
d(H···A)
d(D···A)
∠DHA
144.0
123.0
161.3
O5-H5---O3a
O5-H5---O2b
C7-H7---O1c
0.84
0.84
0.95
2.10
2.38
2.58
2.8259(13)
2.9157(14)
3.4936
Symmetry codes: (a) x,y+1,z; (b) -x+1,y+1/2,-z+1/2; (c) -x,1-y,1-z
code-x+1,y+1/2,-z+1/2) and H5-O5···O3 (2.826, 144° and
symmetry code x,y+1,z) in the vertical direction. The crystal
packing in the crystal structure was therefore stabilized by
intermolecular hydrogen bonds, which linked the molecules
together to form an infinite network. When viewed along the a-
axis, the molecules were interlinked by hydrogen bonds (Fig. 2).

Vol. 28, No. 1 (2016)
Improved Synthesis and Crystallographic Analysis of Oxime Esters 21
Fig. 2. Hydrogen bonding system between molecules of oxime 2. The
dashed lines represented the intermolecular hydrogen bonds (H-
bonds have been drawn between the donor and acceptor atoms)
Fig. 3. Molecular structures of compound 3, showing the atom-labeling
scheme. Displacement ellipsoids have been drawn at the 10 %
probability level
Crystal structure of compound 3: Selected bond lengths
and bond angles for the non-hydrogen atoms in compound 3
are shown in Table-4. The bond lengths and bond angles for
compound 3 were found to be in good agreement with the
standard. The crystal structure of compound 3 is shown in
Fig. 3. The structure was composed of two erythro-N-acetyl-
β-(4-methoxyphenyl)serine ethyl ester molecules, which
existed as enantiomers. One of the molecules (i.e., C1-C14)
was in the S,S-configuration, whereas the other molecule (i.e.,
C15-C28) was in the R,R-configuration. This X-ray structure
therefore confirmed that compound 3 was a racemic mixture.
Intermolecular hydrogen bonds of the type O-H···O and
N-H···O were found in the crystal structure of compound 3
(Table-5). Atoms O2, O3, O6 and O8 acted as hydrogen-bond
acceptors to the hydrogen atoms of the N1-H1, O4-H4, N2-
H2 and O9-H9 bonds, respectively, to generate the supramole-
cular structure (Fig. 4). Four molecules in S,S-configuration were
linked together by hydrogen bonds between N1-H1---O2
(2.928, 164°) and O4-H4---O3 (2.736, 175°) to form a large
ring. Four molecules in the R,R-configuration were linked
together by hydrogen bonds between N2-H2---O6 (2.969,
165°) and O9-H9---O8 (2.722, 175°) to form another large
ring. The two planes of the large rings were perpendicular to
each other and the molecules were interlinked through a
network of hydrogen bonds (Fig. 4).
TABLE-5
HYDROGEN BONDED GEOMETRIES (°), DISTANCES (Å)
AND ANGLES (°) FOR COMPOUND 3
D-H···A
d(D-H)
d(H···A)
d(D···A)
∠DHA
164
165
175
175
N1-H1---O2a
N2-H2---O6b
O4-H4---O3c
O9-H9---O8c
0.86
0.86
0.82
0.82
2.09
2.13
1.92
1.90
2.928(13)
2.969(3)
2.736(2)
2.722(2)
Symmetry codes: (a) -x,2-y 1-,z; (b) -x+1,2-y,-z; (c) -1+x,y,z
Fig. 4. Hydrogen bonding system between the molecules of 3. The dashed
lines represented the intermolecular hydrogen bonds (H-bonds have
been drawn between the donor and acceptor atoms).
Conclusion
An efficient new method has been developed for the facile
synthesis of ( )-erythro-3 from oxime 2. The conformations
of (E)-oxime 2 and erythro-3 were unambiguously confirmed
by X-ray crystallography. We are currently investigating the
synthesis of several other oxime ester and erythro-amino acid
derivatives in our laboratory and this work will be published
in due course.
TABLE-4
SELECTED BOND LENGTHS (Å) AND
ANGLES (°) FOR COMPOUND 3
Bond
C1-C2
C15-C16
O1-C2
O7-C16
N1-C4
N1-C12
O4-C5
O5-C14
N2-C18
N2-C27
O9-C19
O10-C23
Dist.
Angle
C3-O1-C2
C17-O7-C16
C12-N1-C4
C27-N2-C18
N1-C4-C3
(°)
1.471 (4)
1.481 (4)
1.461 (3)
1.460 (3)
1.444 (3)
1.338 (3)
1.414 (3)
1.398 (6)
1.457 (3)
1.336 (3)
1.413 (3)
1.392 (4)
116.5 (2)
116.7 (2)
123.0 (2)
122.1 (2)
112.1 (2)
107.98 (18)
121.0 (2)
116.1 (2)
122.1 (2)
108.37 (19)
121.1 (2)
115.7 (2)
Supplementary material
Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication nos. CCDC-82607 and CCDC-1014816. Copies
of the available material can be obtained, free of charge, on
application to the Director, CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (Fax: +44-1223-336033 or e-mail:
deposit@ccdc.cam.ac.uk).
N1-C4-C5
O3-C12-N1
N1-C12-C13
N2-C18-C17
N2-C18-C19
O8-C27-N2
N2-C27-C28

22 Fan et al.
Asian J. Chem.
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ACKNOWLEDGEMENTS
The authors express their gratitude for financial assistance
received from the National Natural Science Foundation of
China (No. 1272052), the National Basic Research Program
of China (2011CB512007 and 2012CB723501), Hebei
Province Natural Science Foundation (No.B2014208138) and
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