Structures of (S)-(-)-4-oxo-2-azetidinecarboxylic acid and 3-azetidinecarboxylic acid from powder synchrotron diffraction data

Article abstract of DOI:10.1107/S0108768106013887

The crystal structures of the four-membered heterocycles (S)-(-)-4-oxo-2-azetidinecarboxylic acid (I) and 3-azetidinecar-boxylic acid (II) were solved by direct methods using powder synchrotron X-ray diffraction data. The asymmetry of the oxoazetidine and azetidine rings is discussed, along with the hydrogen bonding.

Full text of DOI:10.1107/S0108768106013887

research papers
Acta Crystallographica Section B
Structural
Science
Structures of (S)-(ꢀ)-4-oxo-2-azetidinecarboxylic
acid and 3-azetidinecarboxylic acid from powder
synchrotron diffraction data
ISSN 0108-7681
a
Received 21 November 2005
´
Asiloe J. Mora, * Michela
The crystal structures of the four-membered heterocycles (S)-
Accepted 17 April 2006
Brunelli,b Andrew N. Fitch,b,c
Jonathan Wright,b Maria E.
(ꢀ)-4-oxo-2-azetidinecarboxylic acid (I) and 3-azetidinecar-
boxylic acid (II) were solved by direct methods using powder
synchrotron X-ray diffraction data. The asymmetry of the
oxoazetidine and azetidine rings is discussed, along with the
hydrogen bonding.
a
´
´
Baez and Francisco Lopez-
Carrasqueroa
a
´
Departamento de Quımica, Facultad de Cien-
´
cias, Universidad de Los Andes, Merida 5101,
Venezuela, bEuropean Synchrotron Radiation
Facility, BP220, F-38043 Grenoble CEDEX,
France, and cDepartment of Chemistry, Keele
University, Staffordshire ST5 5BG, England
1. Introduction
(S)-4-(ꢀ)-Oxo-2-azetidinecarboxylic acid (I) and 3-azadine-
carboxylic acid (II) are strained four-membered heterocycles,
difficult to synthesize owing to unfavorable enthalpies of
activation (Huszthy et al., 1993). Compound (I) is an optically
active ꢀ-lactam derivative of l-aspartic acid, which can be
prepared by hydrogenation from (S)-4-(ꢀ)-benzyloxy-
carbonyl-2-azetidinone (Fritz et al., 1986), and by oxidation of
4-vinyl-2-azetidinone (Pietsch, 1976). Derivatives of (I) are of
potential interest as precursors to ꢀ-lactam antibiotics, since
they can be converted into 4-acetoxy-2-azetidinones. These
compounds have been recognized as the most useful precur-
sors for the synthesis of carbapenems (Nagao, Kumagai et al.,
1992; Nagao, Nagase, Kumagai, Kuramoto et al., 1992; Nagao,
Nagase, Kumagai, Matsunaga et al., 1992), powerful antibiotics
widely used in the pharmaceutical industry. Optically active
monosubtituted alkoxycarbonyl derivatives of (I) yield helical
poly ꢀ-peptides by anionic ring-opening polymerization
Correspondence e-mail: asiloe@ula.ve
´
(Lopez-Carrasquero et al., 1994). The helical conformations
adopted by these polyaspartates are similar to the ꢁ-helix of
polypeptides and proteins, and they display piezoelectric and
´
liquid crystal properties (Lopez-Carrasquero et al., 1995;
Prieto et al., 1989; Mun˜oz-Guerra et al., 2002). The propensity
for cleavage of the amide bonds has been acknowledged, and,
for instance, this feature has been used in the synthesis of a
linear polyamide with hydroxymethyl pendant by the selective
reduction of the 2-azetidinone moiety in the polymer main
chain (Sudo et al., 2001). Compound (II) corresponds to a
group of amino acids whose biological activity has been widely
recognized. For example, the naturally occurring l-(2)-azeti-
dinecarboxylic acid is homologous to proline (Berman et al.,
1969) and mugineic acid regulates the iron intake in grami-
naseous plants (Ma & Nomoto, 1996; Mino et al., 1983). On the
other hand, many efforts have been directed towards the
development of conformationally constrained analogs of
essential animoacids (Hanessian et al., 1999) and peptido-
mimetics (Alonso et al., 2001) that display more favorable
pharmaceutical properties. Here we report the structures of
two compounds with prospective applications in those fields.
The crystal structures were solved from powder diffraction
data and will be discussed in the light of 6–31+G(d) GAUS-
# 2006 International Union of Crystallography
Printed in Great Britain – all rights reserved
606 doi:10.1107/S0108768106013887
Acta Cryst. (2006). B62, 606–611

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Table 1
Experimental details.
propanol was hydrogenated over Pd/
C to 5% (86 mg) at room temperature
and 1.5 atm for 15 to 20 min. After
this time, the catalyst was filtered
from the solution and most of the
solvent evaporated. Hexane was
added to the residual mixture, which
was allowed to crystallize at room
temperature. The (S)-(ꢀ)-4-oxo-2-
azetidinecarboxylic acid yield was
0.48 g (85%), m.p. 375–377 K Lit.
(Frits) 375–377 K. IR (KBr): 3339,
1747, 1729, 1211, 1196, 1166 cmꢀ1. 1H
NMR (in DMSO-d6) ꢂ(p.p.m.): 7.5
(br, 1HNH); 4.2 (dd, 1H, CHNH), 3.3
(ddd, 1H, CH2CO), 3.0 (ddd, 1H,
CH2CO). [ꢁ]2D0 = ꢀ80ꢁ; c = 1 in water
(Fritz et al., 1986).
(I)
(II)
Crystal data
Chemical formula
Mr
C4H5O3N
114.68
Orthorhombic, P212121
295
8.94684 (2), 7.6696 (2), 7.2755 (2)
90
499.24 (1)
4
C4H7O2N
101.11
Monoclinic, P21
295
6.27983 (3), 7.8380 (1), 5.46296 (3)
114.893 (1)
243.91 (1)
2
1.377 (1)
Synchrotron
0.540211
Cell setting, space group
Temperature (K)
˚
a, b, c (A)
ꢀ (ꢁ)
˚
V (A)
Z
Dx (Mg mꢀ3
Radiation type
Incident radiation
)
1.531 (1)
Synchrotron
0.84933
˚
wavelength (A)
ꢄ (mmꢀ1
Specimen form, color
)
0.16
0.07
Cylinder (particle morphology:
thin powder), white powder
1.5 ꢃ 40
Cylinder (particle morphology:
thin powder), white powder
1.5 ꢃ 40
Specimen size (mm)
Specimen preparation
temperature (K)
Room temperature
Room temperature
Data collection
Diffractometer
Data collection method
2.2. Powder data collection
Beam line ID31, ESRF
Specimen mounting: borosilicate
glass capillary; mode: transmis-
sion; scan method: continuous
None
Beam line ID31, ESRF
Specimen mounting: borosilicate
glass capillary; mode: transmis-
sion; scan method: continuous
None
X-ray powder diffraction data were
collected with the high-resolution X-
ray powder diffractometer on beam-
line BM16 at ESRF (Fitch, 2004),
selecting X-rays from the white
bending magnet source with wave-
lengths of 0.84933 (1) and 0.54021 (1)
Absorption correction
2ꢃ (ꢁ)
2ꢃmin = 7.0, 2ꢃmax = 43.5, increment
= 0.003
2ꢃmin = 5.0, 2ꢃmax = 32.0, increment
= 0.003
Refinement
R-factors Rp, Rwp, Rexp
Wavelength of incident
˚
radiation (A)
Excluded region(s)
Profile function
0.0594, 0.0861, 0.0308
0.84933
0.0592, 0.0732, 0.0376
0.54021
˚
A for (I) and (II), respectively. Small
None
CW profile function number 3 with
19 terms
76
0.92
None
CW profile function number 3 with
19 terms
85
1.66
quantities of (I) and (II) (Aldrich,
98%) were lightly ground with a
pestle in an agate mortar and intro-
duced into 1.5 mm diameter borosili-
cate glass capillaries, mounted on the
axis of the diffractometer and spun
during measurements. Data were
collected for several hours and
No. of parameters
(Á/ꢅ)max
Computer programs used: EXPO-SIRPOW (Altomare et al., 1999), GSAS (Larson & Von Dreele, 2001).
SIAN94 (Frisch et al., 1995) geometrical optimizations and the
formation of hydrogen bonds.
normalized against monitor counts and detector efficiencies,
and rebinned into steps of 2ꢃ = 0.003ꢁ.
3. Results
3.1. Structural solution and refinement
The diffraction pattern of the ꢀ-lactam (I) was indexed in an
orthorhombic cell with a = 8.9468 (2), b = 7.66956 (2) and c =
˚
7.27555 (2) A (refined values) [DICVOL91 (Boultif & Louer,
¨
1991), with indexing figures of merit: M(20) = 60.0 (de Wolff,
1968) and F(20) = 281.1 (Smith & Snyder, 1979)]. Evaluation
of the systematic absences in the diffraction pattern indicated
the space group P212121 (No. 19), with Z = 4. The pattern
decomposition using the Le Bail method (LeBail et al., 1988)
and the crystal structure solution via direct methods were
obtained using the program EXPO (Altomare et al., 1999).
The azetidinecarboxylic acid (II) was also obtained as a pure
phase and its powder diffraction pattern was indexed by a
monoclinic cell: a = 6.27985 (3), b = 7.8310 (1), c =
2. Experimental
2.1. Synthesis of (S)-(ꢀ)-4-oxo-2-azetidinecarboxylic acid (I)
(S)-4-Benzyloxycarbonyl-2-azetidinone was prepared as the
starting material from l-aspartic acid (Aldrich, 98+%, [a]D25
+25) according to literature methods (Rodrıguez-Galan et al.,
1986). In a PARR hydrogenator, 1 g (4.9 mmol) of (S)-4-(ꢀ)-
benzyloxycarbonyl-2-azetidinone dissolved in 20 ml of i-
=
´
´
ꢂ
´
Acta Cryst. (2006). B62, 606–611
Asiloe J. Mora et al.
Powder synchrotron diffraction data 607

research papers
P21 cell gave as the best solution
the positions of all the non-H
atoms. Both models were
completed placing the H atoms
with the sketching facilities of
MATERIALS
STUDIO
(Accelrys Inc., 2001). Rietveld
(1969) refinement of the struc-
tures were performed with the
program GSAS (Larson & Von
Dreele, 2001).
For lactam (I), data in the 2ꢃ
range 7–43ꢁ were included,
comprising 218 Bragg reflec-
tions, which were modeled using
a
pseudo-Voigt peak-shape
function (Thompson et al.,
1987). This function included
the axial divergence asymmetry
correction at low angle (Finger
et al., 1994). The background
was described by the automatic
interpolation of 20 points
throughout the whole pattern.
In order to place the appro-
priate bond and angle restraints
on the model, a search of the
Cambridge
Crystallographic
Database (CSD; Allen, 2002)
was performed and five 2-acet-
idinone fragments were found
(FEPNAP,
EABLEY,
FEHWOE, REPLON and
VUJHOX). No regular pattern
of asymmetry in the four-
membered rings could be
recognized since distances and
bond angles varied depending
on the substitute groups.
Therefore, it was considered
more accurate to restrain the
model using the bond lengths
and angles obtained in an ab
initio molecular-orbital optimi-
zation of (I), using GAUS-
SIAN94 (Frisch et al., 1995 with
a 6–31 +G(d) basis set. Bond
aꢁnd angle restraints were
Figure 1
Final observed (points), calculated (lines) and difference profiles of the Rietveld plot for (a) (S)-(ꢀ)-4-oxo-2-
azetidinecarboxylic acid (I) and (b) 3-azetidinnecarboxylic acid (II).
ꢀ
=
114.893 (1)ꢁ (refined values)
weighted by 0.05 A and 5.00 , respectively. H atoms were
refined with C—H, N—H and O—H distances restrained to be
˚
5.46296 (3) A and
˚
¨
[DICVOL91 (Boultif & Louer, 1991), with indexing figures of
merit: M(20) = 75.8 (de Wolff, 1968) and F(20) = 390.4 (Smith
& Snyder, 1979)]. Analysis of systematic absences gave two
possible space groups: P21 (No. 4) and P21/m (No. 11).
Statistical analysis of the reflection intensities distribution
performed by the EXPO suite of programs (Altomare et al.,
1999) ruled out the centrosymmetric space group, and running
the direct methods routine of EXPO in default mode in the
˚
˚
0.99 A (weighted 0.05 A). The isotropic displacement para-
meters for all H atoms were refined individually. The refine-
ment of 73 parameters yielded final agreement factors: Rwp
0.0861 and Rexp = 0.0308.
=
For the structure of compound (II), 295 reflections were
refined following the procedure described above. Detailed
crystallographic information and final Rietveld refinement
ꢂ
´
608 Asiloe J. Mora et al.
Powder synchrotron diffraction data
Acta Cryst. (2006). B62, 606–611

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Table 2
Selected bond distances (A) and angles ( ).
ring is also planar with maximum deviations of ꢀ0.018 and
ꢁ
˚
˚
+0.017 A from the mean plane. In this case, C4 is out of the
˚
plane by 1.272 A. The orientation of the carboxylic acid with
6-31G(d) MO
calculations
6-31G(d) MO
calculations
(I)
(II)
respect to the ring in both structures can be explored by means
of torsion angles about the C1—C4 bond. In the ꢀ-lactam (I),
torsion angles N1—C1—C4—O3 ꢀ5.63 and N1—C1—C4—
O2 177.25ꢁ indicate that the bonds C4—O3 and C4—O2 are
aligned with the C1—N1 bond of the ring. In the 3-azetidine-
carboxylic acid (II), O1 and O2 are positioned almost
symmetrically with respect to the ring, being part of the
pseudo-mirror plane passing through N1—C1, as depicted by
the torsion angles C3—C1—C4—O1 146, C2—C1—C4—O2
O1—C3
O2—C4
O3—C4
O1—C4
N1—C1
N1—C3
N1—C2
C1—C2
C2—C3
C1—C3
C1—C4
1.190 (4)
1.319 (4)
1.208 (4)
1.180
1.320
1.180
1.238 (5)
1.263 (6)
1.240
1.240
1.483 (4)
1.378 (4)
1.464
1.386
1.462 (6)
1.460 (5)
1.551 (7)
1.460
1.460
1.550
1.585 (4)
1.554 (5)
1.550
1.530
1.542 (14) 1.550
1.520
1.548 (4)
1.520
1.556 (6)
C1—C2—C3
C3—N1—C1
C2—N1—C3
C2—C1—C3
N1—C1—C2
N1—C3—C2
N1—C2—C1
N1—C3—C1
84.3 (2)
94.7 (2)
85
95
90.4 (1)
84.0 (1)
90
85
87.9 (2)
93.0 (2)
88
92
92.6 (1)
92.9 (1)
92
92
O3—C4—C1—N1 ꢀ5.6 (4)
O3—C4—C1—C2 ꢀ107.6 (3)
O2—C4—C1—N1 177.3 (2)
O2—C4—C1—C2 75.3 (3)
O2—C4—C1—C3
133.6 (6)
84.1 (6)
O1—C4—C1—C3
O1—C4—C1—C2
ꢀ52.2 (8)
ꢀ146.1 (5)
agreement factors for both structures are summarized in Table
1. Fig. 1 shows the final Rietveld plots for the ꢀ-lactam (I) and
the azetidinecarboxylic acid (II), respectively.
4. Discussion
Fig. 2 shows the molecular conformation and atom-labeling
scheme for both compounds. Table 2 depicts the selected bond
distances, angles and torsion angles for (I) and (II) compared
with those obtained by theoretical ab initio calculations. The
carboxylic O2—C3 and O3—C3 bond distances in the ꢀ-
lactam (I) are markedly different, while those distances are
equal within 5ꢅ in the 3-azedinecarboxylic acid (II). These
results are clear evidence that at room temperature compound
(I) is a neutral species, while compound (II) is a zwitterion.
The ꢀ-lactam ring in (I) is highly asymmetrical owing to the
chiral environment around N1 (see bond distances N1—C1,
N1—C3, C2—C3 and C1—C2, and related angles) and flat
Figure 2
Molecular diagrams for (a) (S)-(ꢀ)-4-oxo-2-azetidinonecarboxylic acid
(I) and (b) 3-azetidinnecarboxylic acid (II), showing the atom-labeling
scheme.
˚
with maximum deviations of ꢄ0.013 A from the mean plane.
This planarity is promoted by the sp2 states of C3 and N1. O1
˚
lies close to the ꢀ-lactam plane at 0.039 A, while C4 is out of
˚
the plane by 1.176 A. In contrast, the azetidine ring in (II) is
almost symmetrical (see bond distances N1—C3, N1—C2,
C1—C2 and C1—C3, and related angles), with a pseudo-
mirror plane passing through N1 and C1 atoms, due mainly to
the acidic substitution in C1, opposite to N1. The azetidine
1 Supplementary data for this paper are available from the IUCr electronic
archives (Reference: AV5050). Services for accessing these data are described
at the back of the journal.
Figure 3
Extended ribbon supramolecular structure constructed from hydrogen
bonds in (S)-(ꢀ)-4-oxo-2-azetidinecarboxylic acid (I).
ꢂ
´
Acta Cryst. (2006). B62, 606–611
Asiloe J. Mora et al.
Powder synchrotron diffraction data 609

research papers
Table 3
Geometries of hydrogen bonds (A, ).
to the well known limitations arising from the overlap of
reflections, particularly at higher diffraction angles.
ꢁ
˚
D—Hꢅ ꢅ ꢅA D—H Hꢅ ꢅ ꢅA
Dꢅ ꢅ ꢅA
D—Hꢅ ꢅ ꢅ A
Hydrogen bonds for both compounds are summarized in
Table 3. In the ꢀ-lactam (I), a supramolecular bidimensional
structure is recognized, as shown in Fig. 3. Extended chains
constructed with strong O2—H5ꢅ ꢅ ꢅO1 hydrogen bonds run
parallel to [010]. Additional N1—H4ꢅ ꢅ ꢅO3 hydrogen bonds
link neighboring chains laterally forming ribbons also running
parallel to [010]. As observed in previous studies (Mora et al.,
2005), the four hydrogen-bond acceptor capacity of the
carboxylic acid is completed by means of two weak C2—
H2ꢅ ꢅ ꢅO3 and C2—H3ꢅ ꢅ ꢅO1 hydrogen bonds. Hydrogen
bonding in the 3-azetidinecarboxylic acid (II) is markedly
different because of its zwitterionic characters, which makes
the amine group in the ring a double donor of H atoms. In fact,
a two-dimensional network of hydrogen bonds is assembled by
the combination of two motifs: infinite two-membered zigzag
chains connected by N1—H6ꢅ ꢅ ꢅO1 hydrogen bonds running
along b, and infinite one-membered chains running along [101]
connected by a bifurcated hydrogen bond N1—H7ꢅ ꢅ ꢅO1 and
N1—H7ꢅ ꢅ ꢅO2. A perspective view of this hydrogen-bond
network is shown in Fig. 4. In addition, some weak C—Hꢅ ꢅ ꢅO
hydrogen bonds are also present, which saturates the acceptor
capacity of the carboxylate group.
(S)-4-(ꢀ)-Oxo-2-azetidinecarboxylic acid (I)
N1—H4ꢅ ꢅ ꢅO3i
O2—H5ꢅ ꢅ ꢅO1ii
C2—H2ꢅ ꢅ ꢅO3iii
C2—H3ꢅ ꢅ ꢅO1iv
1.05 (2)
0.97 (4)
1.10 (2)
1.17 (2)
2.13 (2)
1.75 (4)
2.55 (2)
2.34 (2)
2.933 (4)
2.573 (3)
3.320 (4)
3.351 (4)
132 (1)
141 (3)
127 (1)
143 (1)
3-Azetidinnecarboxylic acid (II)
N1—H6ꢅ ꢅ ꢅO1v
N1—H7ꢅ ꢅ ꢅO1vi
N1—H7ꢅ ꢅ ꢅO2vi
C1—H1ꢅ ꢅ ꢅO2vii
C2—H2ꢅ ꢅ ꢅO2viii
C3—H5ꢅ ꢅ ꢅO1ix
1.0 (2)
1.59 (2)
2.58 (3)
1.69 (3)
2.28 (2)
2.46 (3)
2.39 (2)
2.671 (4)
3.262 (4)
2.718 (5)
3.259 (2)
3.475 (6)
3.440 (6)
178 (3)
123 (2)
175 (1)
155 (3)
168.5 (9)
166 (1)
1.03 (3)
1.03 (3)
1.05 (2)
1.03 (3)
1.08 (3)
1
1
1
2
Symmetry codes: (i) 1 ꢀ x; ꢀ þ y; ꢀ þ z; (ii) x; 1 þ y; z; (iii) þ x; 12 ꢀ y; ꢀz; (iv)
2
2
1 ꢀ x; 12 þ y; 12 ꢀ z;
(v)
1 ꢀ x; 12 þ y; 1 ꢀ z;
(vi)
ꢀ1 þ x; y; ꢀ1 þ z;
(vii)
1
2
1 ꢀ x; ꢀ þ y; 2 ꢀ z; (viii) x; y; ꢀ1 þ z; (ix) ꢀ1 þ x; y; z.
We thank the ESRF for providing synchrotron radiation
beam-time, FONACIT–Venezuela and CDCHT-ULA (Grants
C-990-99-08-AA and C1246-04-08A).
References
Accelrys Inc. (2001). MATERIALS STUDIO. Accelrys Inc., 6985
Scranton Road, San Diego, CA 92121–3752, USA.
Allen, F. H. (2002). Acta Cryst. B58, 380–388.
´
´
Alonso, E., Lopez-Ortiz, F., del Pozo, C., Peralta, E., Macıas, A. &
´
Gonzalez, J. (2001). J. Org. Chem. 66, 6333–6338.
Altomare, A., Burla, M. C., Camalli, M., Carrozzini, B., Cascarano,
G. L., Giacovazzo, C., Guagliardi, A., Moliterni, A. G. G., Polidori,
G. & Rizzi, R. (1999). J. Appl. Cryst. 32, 339–340.
Berman, H. M., McGandy, E. L., Burgner, J. W. & VanEtten, R. L.
(1969). J. Am. Chem. Soc. 91, 6177–6182.
Figure 4
Projection down [001] for 3-azetidinnecarboxylic acid (II) showing the
hydrogen-bonding scheme.
¨
Boultif, A. & Louer, D. (1991). J. Appl. Cryst. 24, 987–993.
ꢀ134, C3—C1—C4—O2 ꢀ39.8 and C2—C1—C4—O1
ꢀ52.4ꢁ.
Finger, L. W., Cox, L. W. & Jephcoat, A. P. (1994). J. Appl. Cryst. 27,
892–900.
Fitch, A. N. (2004). Res. Natl. Inst. Stand. Technol. 109, 133–142.
Frisch, M. J. et al. (1995). GAUSSIAN. Gaussian Inc., Pittsburgh, PA,
USA.
Fritz, H., Sutter, P. &. Weis, C. D. (1986). J. Org. Chem. 51, 558–561.
Hanessian, S., Bernstein, N., Yang, R.-Y. & Maguire, R. (1999).
Bioorg. Med. Chem. Lett. 9, 1437–1442.
Huszthy, P., Bradshaw, J. S., Krakowiak, K. E.,Wang, T. & Dalley,
N. K. (1993). J. Heterocycl. Chem. 30, 1197–1207.
Larson, A. C. & Von Dreele, R. B. (2001). GSAS. Los Alamos
National Laboratory, Los Alamos, New Mexico, USA.
LeBail, A., Duroy, H. & Fourquet, J. L. (1988). Mater. Res. Bull. 23,
447–452.
The slightly slacker constraints applied to the geometry of
the 4-atom rings allowed them to reach the geometry that best
fitted the diffraction data. In this regard, both compounds
displayed the same asymmetry pattern shown by the theore-
tically calculated molecules. However, a closer analysis of the
theoretical and X-ray diffraction distances of Table 2 show
˚
that for the ꢀ-lactam (I) there is an elongation of ca 0.03 A of
the C1—N1 and C1—C2 distances nearer to the pendant
carboxylic acid group at C1, which also shows a C1—C4
˚
distance longer than that calculated by 0.036 A. This could be
´
Lopez-Carrasquero, F., Aleman, C. & Mun˜oz-Guerra, S. (1995).
Biopolymers, 36, 263–271.
associated with the librational thermal motion of the molecule.
In the case of (II) this elongation is only observed in the C1—
C4 bond of the carboxylate group pending group. This thermal
motion cannot be modeled from powder diffraction data due
´
Lopez-Carrasquero, F., Garcıa-Alvarez, M. & Mun˜oz-Guerra, S.
´ ´
(1994). Polymer, 35, 4502–4510.
Ma, J. F. & Nomoto, K. (1996). Physiol. Plant, 97, 609–617.
ꢂ
´
610 Asiloe J. Mora et al.
Powder synchrotron diffraction data
Acta Cryst. (2006). B62, 606–611

research papers
Mora, A. J., Avila, E. E., Delgado, G. E., Fitch, A. N. & Brunelli, M.
(2005). Acta Cryst. B61, 96–102.
Pietsch, H. (1976). Tetrahedron Lett. pp. 4053–4056.
´
Prieto, A., Perez, R. & Subirana, J. A. (1989). J. Appl. Phys. 66, 803–
´
´
Mun˜oz-Guerra, S., Lopez-Carrasquero, F., Aleman, C., Morillo, M.,
Castelleto, V. & Hamley, I. (2002). Adv. Mater. 14, 203–205.
Mino, Y., Ishida, T., Ota, N., Inoue, N., Nomoto, K., Takemoto, T.,
Tanaka, H. & Sugiura, Y. (1983). J. Am. Chem. Soc. 105, 4671–4676.
Nagao, Y., Kumagai, T., Nagase, Y., Tamai, S., Inoue, Y. & Shiro, M.
(1992). J. Org. Chem. 57, 4232–4237.
806.
Rietveld, H. M. (1969). J. Appl. Cryst. 2, 65–71.
´
´
Rodrıguez-Galan, A., Mun˜oz-Guerra, S., Subirana, J. A., Chuong, B.
& Sekiguchi, H. (1986). Makromol. Chem. Macromol. Symp. 6,
277–284.
Smith, G. S. & Snyder, R. L. (1979). J. Appl. Cryst. 12, 60–65.
Sudo, A., Sato, M. & Endo, T. (2001). J. Polym. Sci. 39, 3789–3796.
Thompson, P., Cox, D. E. & Hastings, J. B. (1987). J. Appl. Cryst. 20,
79–83.
Nagao, Y., Nagase, Y., Kumagai, T., Kuramoto, Y., Kobayashi, S.,
Inoue, Y., Taga, T. & Ikeda, H. (1992). J. Org. Chem. 57, 4238–4242.
Nagao, Y., Nagase, Y., Kumagai, T., Matsunaga, H., Abe, T., Shimada,
O., Hayashi, T. & Inoue, Y. (1992). J. Org. Chem. 57, 4243–4249.
Wolff, P. M. de (1968). J. Appl. Cryst. 1, 108–113.
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´
Acta Cryst. (2006). B62, 606–611
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Powder synchrotron diffraction data 611