Synthesis and structural characterization of N-methyl-DL-glutamic acid

Article abstract of DOI:10.1071/ch99182

The solid-state conformation of racemic N-methylglutamic acid has been defined by single-crystal X-ray crystallography. Orthorhombic crystals belong to the space group P bca with a 15.219(2), b 10.583(1), c 9.595(1) ? and Z 8. The structure was refined to

Full text of DOI:10.1071/ch99182

C S I R O P U B L I S H I N G
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Volume 53, 2000
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Aust. J. Chem., 2000, 53, 237–240
Synthesis and Structural Characterization
of N-Methyl-DL-glutamic Acid
Andrew B. Hughes, Maureen F. Mackay and Luigi Aurelio
Department of Chemistry, La Trobe University, Bundoora, Vic. 3083.
The solid-state conformation of racemic N-methylglutamic acid has been defined by single-crystal X-ray crystal-
lography. Orthorhombic crystals belong to the space group P bca with a 15.219(2), b 10.583(1), c 9.595(1) Å and
Z 8. The structure was refined to a final R value of 0.049 for the 1285 measured data. In the crystal the molecules
adopt a zwitterionic form with protonation having occurred at the amino nitrogen atom. The ␣-carboxyl is unpro-
tonated with the ␦-carboxy group retaining a proton. The i.r. spectrum shows absorptions which also are indicative
of the amino acid being in the zwitterionic form. Intermolecular H-bonds involving the carboxylate proton and the
two protons on the N-atom link the molecules into a three-dimensional network in the crystal.
Keywords. X-Ray crystallography; zwitterion; glutamic acid; peptide.
Introduction
As part of a project concerning the syntheses of heavily N-
methylated peptides, the syntheses of a number of N-methyl
amino acids were undertaken. One of these amino acids was
N-methylglutamic acid. Racemic N-methylglutamic acid
was prepared from N-benzyloxycarbonylglutamic acid (1)
by formation of the oxazolidin-5-one (2) (86%).1 The oxazo-
lidinone (2) was then reductively cleaved to give the N-
methyl compound (3) (63%) by using triethylsilane and
trifluoroacetic acid. Finally, the benzyloxycarbonyl group
was removed by hydrogenolysis over palladium catalyst to
give the expected N-methyl-DL-glutamic acid (4) (63%).
Crystals of N-methyl-DL-glutamic acid (4) suitable for an X-
ray structure analysis precipitated from an n.m.r. sample in
D2O at 5oC; consequently the amino and carboxylic protons
have been replaced by deuterium atoms. An X-ray structure
analysis was undertaken to define the conformational detail
in the molecule. A search of the Cambridge Crystallographic
Database2 revealed that the only crystal structures of isolated
and neutral N-methylated ␣-amino acids reported to date are
those of N-methyl-L-tryptophan (abrine)3 and N-methyl-D-
aspartic acid monohydrate.4 The crystal structure of the title
compound therefore provides a third example in the series.
performed at La Trobe University by Dr John Traeger, on a Shimadzu
GCMS-QP5050Amass spectrometer fitted with a direct insertion probe
at 70 eV, with a transfer line temperature of 250°C. Other low- and
high-resolution mass spectra were measured at the University of
Tasmania by Dr Noel Davies and coworkers, on a Kratos concept mass
spectrometer at 70 eV with a source temperature of 200°C. Optical
rotations were obtained on a Perkin–Elmer 141 polarimeter. ‘Flash
column chromatography’ was carried out using silica gel (silica gel 60,
230–400 mesh ASTM) supplied by Merck Chemicals (Darmstadt).
Ethyl acetate and hexane used for chromatography were distilled prior
to use. All solvents were purified by distillation. For dry solvents,
procedures from Perrin and Armarego5 were followed.
Dry
Experimental
dichloromethane was distilled and stored over Linde-type 4 Å molecu-
lar sieves. All other reagents and solvents were purified or dried as
described by Perrin and Armarego.5
All melting points are uncorrected and were recorded on a Reichert
‘Thermopan’ microscope hot-stage apparatus. Infrared spectra were
recorded on a Perkin–Elmer 1720X Fourier-transform i.r. spectrometer,
using a diffuse reflectance accessory with KBr background. N.m.r.
spectra including DEPT experiments were recorded in (D)chloroform
solution unless otherwise stated on a Brüker AM-300 spectrometer (1H
at 300.13 MHz and 13C at 75.47 MHz) or on a Brüker DRX 400 MHz
machine. Chemical shifts are reported as ␦ values and coupling con-
stants (J) in Hz relative to residual solvent. Electrospray (e.s.) mass
spectra were obtained on a VG Bio-Q triple quadrupole mass spec-
trometer by using water/methanol/acetic acid (0: 99: 1 or 50 : 50: 1)
mixtures as the mobile phase. Low-resolution mass spectra (e.i.) were
N-Benzyloxycarbonyl-N-methyl-DL-glutamic Acid (3)6
The oxazolidinone (2) (1.74 g, 5.9 mmol) was dissolved in chloro-
form (30 ml) in a round-bottom flask followed by addition of trifluo-
roacetic acid (30 ml) and triethylsilane (2.8 ml, 17.5 mmol) and the
mixture was left to stir for 5 days. After being stirred, the solution was
concentrated to give an oily residue. The residue was taken up in
ethanol and reconcentrated under reduced pressure from that solvent
three times. The residue was taken up in a mixture of ether (100 ml) and
Manuscript received 21 December 1999
© CSIRO 2000
10.1071/CH99182
0004-9425/00/030237

238
Short Communications
+0.29 and –0.23 e Å–3. An extinction parameter was applied to the Fc
5% sodium bicarbonate solution (60 ml). The ethereal solution was
extracted with 5% sodium bicarbonate solution (2×30 ml). The com-
bined aqueous layers were washed with ether (2×30 ml) and the
aqueous layer was then acidified with 5 mol dm–3 HCl to pH 2 and
extracted with ethyl acetate (3×60 ml). The combined ethyl acetate
layers were dried (MgSO4), filtered and concentrated to give an oil.
The oil was further purified by silica gel chromatography, with
CHCl3/MeOH/acetic acid (90: 9.9: 0.1) as eluent, to give the N-methyl
amino acid (3) as an oil (1.1 g, 63%). ␯max/cm–1 (NaCl) 3500–2750
(CO2H), 3093, 3066 and 3035 (CH, aromatic), 3000–2900 (CH, satu-
rated), 1710 (C=O), 1486, 1453, 1405, 1322, 1189, 1143, 770, 698. m/z
(e.s. mass spectrum) 296 (M+1, 100%), 277 (23), 252 (41), 204 (37),
160 (26). ␦H (300 MHz, CDCl3) (rotamers) 10.42, s, 2H, 2×CO2H; 7.20,
s, 5H, ArH; 4.99, s, 2H, ArCH2; 4.65–4.61 and 4.52–4.49, 2m, 1H,
NCHCO; 2.74, s, 3H, NCH3; 2.24–2.17 and 1.92–1.86, 2m, 4H,
CH2CH2CO2H. ␦C (75 MHz, CDCl3) (rotamers) 175.13, 174.98 and
172.85 (2×CO2H), 156.60 and 156.00 (CO carbamate); 136.06 (qua-
ternary Ar C); 128.01, 127.49 and 127.13 (5×Ar C), 66.96 (ArCH2),
57.78 (N HCO), 30.95, 30.41 and 30.24 (NCH3), 23.75 and 23.45
( H2 H2CO2H).
terms with SHELXS-93;10 the extinction coefficient was 0.017(1)×10–6.
Results
The X-ray results are presented in Tables 1–3 and Figs 1
and 2. The latter were prepared from the output of ORTEPII.11
Material deposited† includes anisotropic thermal parame-
ters, hydrogen atom parameters, and observed and calculated
structure amplitudes.
Discussion
As the N-methyl-DL-glutamic acid molecule (4) was
recrystallized from D2O it is partially deuterated, the amino
and carboxylate protons having been replaced by deuterium
atoms. A perspective view of the molecule given in Fig. 1
illustrates that it is in the zwitterionic form in the crystal as
are the solid-state structures of N-methyl-L-tryptophan,3 N-
methyl-D-aspartic acid4 and, in general, neutral ␣-amino
acids. The ␦-carboxy group is protonated whereas the ␣-car-
boxyl is unprotonated. This is in accord with the C(5)–O(5)
and C(5)–O(5Ј) bond lengths of 1.324(3) and 1.204(3) Å
N-Methyl-DL-glutamic Acid (4)7,8
To a mixture of the N-benzyloxycarbonyl-N-methyl amino acid (3)
(880 mg, 2.98 mmol) and ethanol (80 ml) in a round-bottom flask was
added 10% Pd-on-C catalyst (50 mg). The mixture was stirred at room
temperature in an atmosphere of H2 until the required amount of hydro-
gen gas was absorbed. The greyish mixture was then concentrated
under reduced pressure. The residue was taken up in hot distilled water
(10 ml) and filtered through Celite. The filter cake was washed with hot
water (2×2 ml) and the combined filtrates were evaporated under
vacuum to afford a white solid. The solid was then dissolved in a
minimum of hot distilled water and crystallized by addition of ethanol
to afford the pure N-methyl amino acid (4) (302 mg, 63%), m.p.
172–176°C. ␯max/cm–1 (KBr) 3600–3250 (CO2H), 3300–3000, 2970,
Table 1. Fractional atomic coordinates and equivalent isotropic
temperature factors of the non-hydrogen atoms
Estimated standard deviations are in parentheses; Ueq (Å2) were
calculated from the refined anisotropic temperature parameters
Ueq = 1/3(U11+U22+U33)
Atom
x
y
z
Ueq
+
O(1)
O(1Ј)
O(5)
O(5Ј)
N(1)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
0.18985(10)
0.20851(11)
–0.07033(10)
0.02318(12)
0.27539(11)
0.20967(13)
0.23538(13)
0.15652(14)
0.07523(14)
0.00790(15)
0.3633(2)
0.12830(13)
0.12470(13)
0.45288(15)
0.5797(2)
0.3612(2)
0.1775(2)
0.3176(2)
0.3991(2)
0.3798(2)
0.4816(2)
0.3065(2)
0.32912(14)
0.09805(15)
0.1445(2)
0.2545(3)
0.0872(2)
0.2131(2)
0.2212(2)
0.2608(2)
0.1711(2)
0.1959(2)
0.0546(3)
0.0301(4)
0.0352(5)
0.0369(5)
0.0635(7)
0.0252(5)
0.0249(5)
0.0239(5)
0.0292(5)
0.0312(5)
0.0319(5)
0.0369(6)
2950, 2853, 2710 (m, CH3NH2 ), 2571, 2535, 2507 and 2459, 1723
–
(CO2H), 1618, 1571 (CO2 ), 1504, 1480, 1450, 1392, 1330, 1301, 1257,
1218, 1198, 1058, 1000, 846, 653, 532. m/z (e.i. mass spectrum) 176
(100%), 162 (M+1, 18), 144 (12), 130 (9). ␦H (300 MHz, D2O) 3.47, t,
J 6.0 Hz, 1H, NCHCO; 2.56, s, 3H, NCH3; 2.36, t, J 7.3 Hz, 2H,
CH2CO2H; 2.07–1.89, m, 2H, CH2CH2CO2H. ␦C (75 MHz, D2O)
176.88 and 172.75 (2×CO2H), 62.53 (N HCO), 31.65 (NCH3), 29.79
( H2CO2H), 24.25 ( H2CH2CO2H).
Crystallography
Crystal data. C6H8D3NO4, M 164.1, orthorhombic, space group
P bca, a 15.219(2), b 10.583(1), c 9.595(1) Å, V 1545.4(3) Å3, Dm
1.40(1), Dc 1.411 (Z = 8) g cm–3, F(000) 688, ␮(Cu K␣) 1.00 cm–1.
Structure determination. Intensity data were measured with Cu
K␣ radiation from a cleaved specimen of dimensions c. 0.41 by 0.26 by
0.10 mm aligned on a Rigaku-AFC four-circle diffractometer, recorded
by an ␻–2␪ scan with 2␪ scan rate of 2.0° min–1, and 10 s stationary
background counts. Three reference reflections monitored every 100
Table 2. Bond lengths (Å) involving the non-hydrogen atoms
Estimated standard deviations are in parentheses
Atoms
Length
Atoms
Length
O(1)–C(1)
O(1Ј)–C(1)
O(5)–C(5)
O(5Ј)–C(5)
N(1)–C(6)
1.265(2)
1.237(2)
1.324(3)
1.204(3)
1.492(3)
N(1)–C(2)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
1.496(3)
1.535(3)
1.526(3)
1.521(3)
1.506(3)
reflections indicated no decay. Data to a 2␪ 130° yielded a total of
max
1599 terms (Rmerge 0.013); corrections for Lorentz and polarization
effects were applied. Analytical absorption corrections were made with
SHELX-769 (transmission factors 0.758–0.920). The structure was
solved by direct methods with SHELX-769 and least-squares refinements
were carried out with SHELXs-9310 on a VAX8800 computer with 1285
unique terms. Full-matrix least-squares refinements [on (Fo)2], with
anisotropic factors given to the non-hydrogen atoms and isotropic
factors given to the hydrogen atoms, gave residuals* for the 1285 data
of R 0.049 and wR 0.120 with S 1.005 (145 variables). The function
Table 3. Bond angles (degrees) involving the non-hydrogen atoms
Estimated standard deviations are in parentheses
Atoms
Angles
Atoms
Angles
C(2)–N(1)–C(6)
O(1)–C(1)–O(1Ј)
O(1)–C(1)–C(2)
O(1Ј)–C(1)–C(2)
N(1)–C(2)–C(1)
N(1)–C(2)–C(3)
115.2(2)
126.6(2)
114.4(2)
119.0(2)
111.0(2)
111.1(2)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
O(5)–C(5)–O(5Ј)
O(5)–C(5)–C(4)
O(5Ј)–C(5)–C(4)
111.0(2)
115.0(2)
111.6(2)
123.1(2)
112.9(2)
124.0(2)
minimized in the refinements was ⌺w [(Fo)2 –(Fc)2]2 with w = [␴2(F) +
(0.0587P)2+1.8008P]–1, where
P
=
[max(Fo , 0)+2Fc2/3]. The
2
maximum and minimum residual electron-density peak heights were
½
* R = ⌺||Fo|– |Fc||/⌺|Fo| and wR = [⌺[w(Fo)2 –(Fc)2]/⌺[w(Fo)2]] .
† Copies are available, until 31 December 2005, from the Australian Journal of Chemistry, P.O. Box 1139, Collingwood, Vic. 3066.

Short Communications
239
D-aspartic acid structure4 also has gauche–gauche conform-
ers and it is claimed that the conformer is stabilized when the
␣-amino group is protonated, the stability deriving from the
intramolecular hydrogen bond between the amino and ␣-
carboxy groups. The N-methylglutamic acid structure also
has a similar intramolecular hydrogen bond (N(1)·····O(1Ј)
distance 2.704(2) Å) but this interaction is much weaker than
the interaction observed in the aspartic acid structure (see
Table 5).
Table 4. Torsion angles (degrees) involving the non-hydrogen atoms
Estimated standard deviations are in parentheses
Atoms
Angles
Atoms
Angles
C(6)–N(1)–C(2)–C(3)
C(6)–N(1)–C(2)–C(1)
O(1Ј)–C(1)–C(2)–N(1)
O(1)–C(1)–C(2)–N(1)
O(1Ј)–C(1)–C(2)–C(3)
O(1)–C(1)–C(2)–C(3)
167.5(2)
–68.5(2)
–13.0(3)
168.2(2)
111.0(2)
–67.8(2)
N(1)–C(2)–C(3)–C(4)
C(1)–C(2)–C(3)–C(4)
C(2)–C(3)–C(4)–C(5)
C(3)–C(4)–C(5)–O(5Ј)
C(3)–C(4)–C(5)–O(5)
71.7(2)
–52.3(2)
–167.7(2)
15.2(3)
–165.7(2)
Table 5. Hydrogen-bond distances (Å) and angles (degrees)
Estimated standard deviations are given in parentheses
Fig. 1. Perspective view of N-methylglutamic acid (4)
A–D···BA
A–D
D···B
A·····B
A–D···B
with thermal ellipsoids drawn at the 50% probability
level. Hydrogen and deteurium atoms are represented by
spheres of arbitrary radius.
O(5)–D···O(1)I
0.96(4)
0.90(3)
0.91(3)
0.90(3)
1.68(4)
1.96(3)
1.89(3)
2.50(3)
2.612(2)
2.800(2)
2.801(2)
2.704(2)
166(2)
154(2)
178(2)
87(2)
N(1)–D(ND␣)···O(1)II
N(1)–D(ND␤)···O(1Ј)III
N(1)–D(ND␣)···O(1Ј)
A Code for symmetry-related atoms: I, –x, 1/2+y; 1/2–z; II, x, 1/2–y; –1/2+z; III, 1/2–x; 1/2+y, z.
respectively and the C(1)–O(1) and C(1)–O(1Ј) lengths of
1.265(2) and 1.237(2) Å. Asimilar situation was noted in the
neutron diffraction structure of the ␤-form of L-glutamic
acid.12 In the structures of the hydrochlorides of glutamic
acid13 and N-methylglycine,14 as expected, the ␣-carboxy
groups are protonated so that the molecules do not adopt
zwitterionic forms. The major difference between the N-
methylated and glutamic acid structures is the conformation
of the side chain. In the former it is extended so that C(5) (or
C ␦) is trans to C(2) (or C␣), the torsion angle
C(2)–C(3)–C(4)–C(5) being –165.7(2)° (Table 4). In the
hydrochloride13 structure the side chain is also extended but
in the neutral structure,12 the side chain is buckled with C␦
gauche to C␣ and the C␣–C ␤–C ␥–C ␦ torsion angle
–73.1(2)°. The conformation about the C␣–C ␤ bond in N-
methylglutamic acid is gauche–gauche, the torsion angles
C(1)–C(2)–C(3)–C(4) and N(1)–C(2)–C(3)–C(4) having
values of –52.3(2) and 71.7(2)° respectively. The N-methyl-
The molecular packing is illustrated in Fig. 2.
Intermolecular hydrogen bonds (see Table 5) link the
molecules into a three-dimensional network. The N–D···O
interactions of 2.801(2) and 2.800(2) Å link the molecules
along the b and c crystal axes and thus into layers parallel to
the bc planes in the crystal, whereas the stronger O–D···O
bond of 2.612(2) Å between the carboxy groups link the
molecular layers along the a axis.
References
1 Scholtz, J. M., and Bartlett, P.A., Synthesis,1989, 542;Al-Obeidi, F.,
Sanderson, D. G., and Hruby, V. J., Int. J. Pept. Prot. Res., 1999, 35,
215.
2 Allen, F. H., Bellard, S., Brice, M. D., Cartwright, B. A., Doubleday,
A., Higgs, H., Hummelink, T., Hummelink-Peters, T., Kennard, O.,
Motherwell, W. D. S., Rogers, J., and Watson D. G., Acta
Crystallogr., Sect. B, 1978, 35, 2331.
Fig. 2. A stereoscopic view of the crystal structure of N-methylglutamic acid. The large open circles are oxygens
whilst the lined circles are nitrogen.

240
Short Communications
3 Seetharaman, J., Rajan, S. S., and Srinivasan, R., J. Crystallogr.
9 Sheldrick, G. M.,
SHELXS76,
Program for Crystal
Spectrosc. Res., 1993, 23, 167.
Structure Determination, University of Cambridge, England,
1976.
4 Sawka-Dobrowolska, W., Glowiak, T., Kozlowski, H., and
Mastalerz, P., Acta Crystallogr., Sect. C, 1990, 46, 1679.
5 Perrin, D. D., and Armarego, W. L. F., ‘Purification of Laboratory
Chemicals’ 3rd Edn (Pergamon: Oxford 1988).
10 Sheldrick, G. M., SHELXS-93, Program for Crystal Structure
Refinement, University of Göttingen, Germany, 1993.
11 Johnson, C. K., ORTEPII, Report ORNL-5138, Oak Ridge National
Laboratories, Tennessee, U.S.A., 1976.
6 Freidinger, R. M., Hinkle, J., Perlow, D. S., and Arison, B. H., J.
Org. Chem., 1983, 48, 77.
12 Lehmann, M. S., Kowtzle, T. F., and Hamilton, W. C., J. Cryst. Mol.
Struct., 1972, 2, 225.
7 Quitt, P., Hellerbach, J., and Vogler, K., Helv. Chim. Acta, 1963, 46,
327; Ebata, M., Takahashi, Y., and Otsuka, H., Bull. Chem. Soc. Jpn,
1966, 39, 2535.
13 Sequeira, A., Rajogopal, H., and Chidambaram, R., Acta
Crystallogr., Sect. B, 1972, 28, 2514.
8 Okamoto, K., Abe, H., Kuromizu, K., and Izumiya, N., Mem. Fac.
Sci., Kyushu Univ., Ser. C, 1974, 9, 131.
14 Bhattacharyya, S. C., and Saha, N. N., J. Cryst. Mol. Struct., 1978,
8, 105.