Optical resolution of medium-size lactones by inclusion crystallization with optically active host compounds: remarkable odd-even effects on the chiral recognition

Article abstract of DOI:10.1016/j.tetasy.2006.06.024

Molecular recognition of medium-size lactones by inclusion complexation with optically active hosts derived from tartaric acid is described. Odd-even effects on the chiral recognition were observed in the enantioselective inclusion with the optically active host compounds in the solid state.

Full text of DOI:10.1016/j.tetasy.2006.06.024

Tetrahedron: Asymmetry 17 (2006) 1678–1683
Optical resolution of medium-size lactones by inclusion
crystallization with optically active host compounds: remarkable
odd–even effects on the chiral recognition
a,
a
b,
*
*
Koichi Tanaka, Daisuke Kuchiki and Mino R. Caira
a
Department of Applied Chemistry, Faculty of Engineering and High Technology Research Center, Kansai University,
Suita, Osaka 564-8680, Japan
b
Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
Received 24 April 2006; accepted 2 June 2006
Abstract—Molecular recognition of medium-size lactones by inclusion complexation with optically active hosts derived from tartaric
acid is described. Odd–even effects on the chiral recognition were observed in the enantioselective inclusion with the optically active host
compounds in the solid state.
Ó 2006 Elsevier Ltd. All rights reserved.
1
. Introduction
ratios and the guest release temperatures of the inclusion
crystals are summarized in Table 1. Host compounds 1–3
included several lactones 4–7 to form stable inclusion com-
plexes in stoichiometric host–guest ratios, ranging from 3:1
to 1:1, except in the case of host 1 and lactone 7, which did
not produce an inclusion complex. We then examined the
optical resolution of lactones 4–7 with (R,R)-(À)-1,
(R,R)-(À)-2 and (R,R)-(À)-3, respectively. A solution of
(R,R)-(À)-2 (14.8 g, 30.0 mmol) and (± )-4 (5.2 g,
60.0 mmol) in ether–hexane (1:2, 18 ml) was kept at room
temperature for 12 h, during which time crystals formed.
Four recrystallizations of the crystals from ether–hexane
gave the 1:1 inclusion complex of (R,R)-(À)-2 and (S)-
Optically active lactones are present in a wide variety of
natural products (e.g., fragrances, attractants and phero-
mones) isolated from insects, plants, fungi and marine
organisms. In addition, they are useful building blocks
for the synthesis of biologically active substances, such
as antitumour and antiviral agents, and for the prepara-
tion of optically active biodegradable polymers. Herein,
we report a simple optical resolution of medium-size lac-
tones 4–7 by inclusion crystallization with the optically
1
2
3
4
active host compounds 1–3 derived from tartaric acid.
Ph Ph
Ph Ph
Ph Ph
(
À)-4 as colourless prisms (3.6 g), which upon heating at
Me
Me
O
O
O
O
O
O
OH
OH
Ph Ph
OH
OH
Ph Ph
OH
OH
Ph Ph
170 °C in vacuo afforded (S)-(À)-4 in 99% ee (0.49 g, 19%
yield). The enantiomeric excess was determined using a
Chiralcel OA (HPLC column). The opposite enantiomer,
1
2
3
(R)-(+)-4, was obtained in 99% ee in 14% yield by an inclu-
Me
sion complexation with (S,S)-(+)-2 as the host compound
instead of (R,R)-(À)-2. Using a similar procedure, the opti-
cal resolutions of the lactones 5–7 were carried out and the
results are summarized in Table 2. Lactones 4 and 6 were
resolved most efficiently with host compound 2, while lac-
tones 5 and 7 were resolved only moderately with the same
host.
O
O
O
Me
O
O
Me
O
O
O
Me
4
5
6
7
2
. Results and discussion
We first examined the inclusion properties of optically ac-
tive host compounds 1–3 with lactones 4–7. The host–guest
Interestingly, a remarkable ‘odd–even’ effect on the
enantioselectivity in the inclusion complexation was
observed. For example, the host (R,R)-(À)-2 included the
*
Corresponding authors. Tel./fax: +81 6 6368 0861 (K.T.); e-mail:
ktanaka@ipcku.kansai-u.ac.jp
0957-4166/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.06.024

K. Tanaka et al. / Tetrahedron: Asymmetry 17 (2006) 1678–1683
1679
Table 1. Host–guest ratios and guest release temperatures of inclusion
crystals of the optically active hosts 1–3 with lactones 4–7
Table 3. Crystallographic data for compounds 1b and 1c
1c
1
b
a
a
No.
Host
Lactones
H:G ratios
Release temp (°C)
Formula
2(C31
1033.21
Monoclinic
P2
9.5875(1)
24.1902(3)
11.8234(2)
90.0
99.807(1)
90.0
2
0.71073 (MoK
0.0381
H
30
O
4
)ÆC
5
H
O
8 2
31 30 4 6 10 2
C H O ÆC H O
1
1
1
1
2
2
2
2
3
3
3
3
a
b
c
1
1
1
1
2
2
2
2
3
3
3
3
4
5
6
7
4
5
6
7
4
5
6
7
2:1
2:1
1:1
79
77
95
—
97
Formula weight
Crystal system
Space group
580.69
Orthorhombic
P2
1
1 1 1
2 2
˚
d
a
b
c
No inclusion
a, A
9.486(2)
35.788(7)
8.939(2)
90.0
90.0
90.0
˚
1:1
1:1
1:1
2:1
1:1
3:2
1:1
3:1
b, A
˚
94
c, A
101
120
96
104
123
132
a, °
b, °
c, °
Z
d
a
b
c
4
˚
k, A
a
)
0.71073 (MoK
0.0738
a
)
d
R
1
a
wR
Goodness of fit
Flack parameter
2
0.0849
1.038
À0.3(5)
0.1792
1.139
À1(2)
Determined by TGA.
(
S)-(À)-enantiomer of the four-membered ring lactone 4
and the six-membered ring lactone 6, whereas (R,R)-(À)-
2
included the (R)-(À)-enantiomer of five-membered ring
Flack parameter (often not definitive for structures
containing only C, H, O and MoK radiation for determi-
lactone 5 and seven-membered ring lactone 7. As far as
we are able to determine, this is the first example of an
odd–even effect on the enantioselectivity.
a
nation of absolute structure) was thus avoided. Tables 3
and 4 list crystallographic data for the five compounds
described above.
2
.1. X-ray analyses
Figure 1 shows the asymmetric units of complexes 1b and
1c. The complex of 1b comprises two molecules of host 1
and one molecule of lactone 5, while that of 1c contains
one molecule of host 1 and a single molecule of lactone 6.
The crystal structures of several selected inclusion com-
pounds were determined in order to confirm the corre-
sponding chiral selectivities reported above. These include
the compounds numbered 1b and 1c, illustrating the odd–
even enantioselective effect for host 1 with two guests,
and the series 2a, 2b and 2c, illustrating the same pheno-
menon for host 2 and three different guests. In all cases,
the known chirality of the host used for the preparation
of a given complex enabled the unequivocal assignment
of the chirality of the included guest. The reliance on the
In compound 1b, a twofold disorder of the guest carbon
atom at the asymmetric centre was observed, both compo-
nents bearing the common methyl group, thus indicating
the presence of both guest enantiomers in the crystal. The
predominant one (site-occupancy 73%) corresponds to that
shown with the (S)-configuration at the dominant asym-
a
Table 2. Absolute configurations, enantiomeric excesses and yields of 4–7 obtained by complexation with the optically active hosts 1–3
Guest
(R,R)-(À)-1
(R,R)-(À)-2
(R,R)-(À)-3
4
5
6
7
(S)-(À)-, 59% ee, 15% yield
(S)-(À)-, 38% ee, 22% yield
(R)-(+)-, 45% ee, 23% yield
No complexation
(S)-(À)-, 99% ee, 19% yield
(R)-(+)-, 52% ee, 9% yield
(S)-(À)-, 80% ee, 19% yield
(R)-(+)-, 44% ee, 7% yield
(S)-(À)-, 44% ee, 21% yield
(R)-(+)-, 11% ee, 26% yield
(S)-(À)-, 17% ee, 20% yield
(R)-(+)-, 11% ee, 27% yield
a
Enantiomeric excess of the enantiomer obtained by four recrystallizations of the inclusion complex is shown.
Table 4. Crystallographic data for compounds 2a–c
2
a
2b
2c
Formula
C
H
33 32
4
O ÆC
4
H
O
6 2
C
H O
33 32 4
ÆC
5
H
8
O
2
33 32 4 6 10 2
C H O ÆC H O
Formula weight
Crystal system
Space group
578.71
Orthorhombic
P2
9.3289(2)
12.3371(2)
26.0572(5)
4
592.70
Orthorhombic
P2
9.3199(1)
12.7212(2)
26.3100(5)
4
606.73
Orthorhombic
P2
9.2103(2)
16.2752(3)
21.4835(6)
4
2
1 1
2
1
1
2
1
2
1
1 1 1
2 2
˚
a, A
˚
b, A
˚
c, A
Z
˚
k, A
0.71073 (MoK
0.0447
a
)
0.71073 (MoK
0.0401
a
)
0.71073 (MoK
0.0407
a
)
R
1
wR
Goodness of fit
Flack parameter
2
0.0804
1.032
0.4(8)
0.0792
1.009
1.4(8)
0.0794
0.972
0.3(8)

1680
K. Tanaka et al. / Tetrahedron: Asymmetry 17 (2006) 1678–1683
metric centre (labelled in Fig. 1). Careful examination of
*
the guest tertiary H atom environments revealed that the
H-bonding between the hydroxyl groups, with O23Á Á ÁO1
˚
2.747(6) A, that is, somewhat longer than those observed
preference for the (S)-enantiomer can be attributed to a
in 1b, owing to a small difference in host conformation.
Host and guest are linked via the O1–HÁ Á ÁO36 hydrogen
*
favourable C –HÁ Á Áp interaction that the corresponding
˚
tertiary H atom engages in with a host phenyl group
bond of length 2.808(6) A. The only other interactions
˚
*
(
HÁ Á ÁCg 3.29 A and C –HÁ Á ÁCg 152°, Cg = ring centroid).
In contrast, for the tertiary H atom corresponding to the
R)-enantiomer, which engages in an analogous inter-
stabilizing inclusion of the guest (R)-enantiomer are of
the type C–HÁ Á Áp (host phenyl), one involving the guest ter-
˚
(
tiary H atom (HÁ Á ÁCg 3.20 A) and the other involving a
˚
˚
action, the geometry is less favourable (HÁ Á ÁCg 3.35 A,
C–HÁ Á ÁCg 133°), indicating a weaker interaction. There is
a continuous chain comprising four hydrogen bonds O–
HÁ Á ÁO that involve the four hydroxyl groups of the two
independent host molecules and the carbonyl oxygen atom
of the guest. The shortest of these H-bonds are the two
intramolecular O–HÁ Á ÁO bonds with distances O23Á Á ÁO1
methylene H atom (HÁ Á ÁCg 3.07 A). The crystal structure
of 1c is based on close packing of the 1:1 asymmetric units
shown, with no H-bonding linking these units. Thus, the
crystallographic results for 1b and 1c, confirming preferen-
tial inclusion by the host (R,R)-(À)-1 of the (S)-enantiomer
of the five-membered ring lactone 5 and the (R)-enantiomer
of the six-membered ring lactone 6, are in accord with
those established analytically (Table 2), confirming the
‘odd–even’ effect on enantioselectivity.
˚
˚
2
.655(2) A and O58Á Á ÁO36 2.671(2) A. The longest H-bond
˚
is that linking the host molecules [O23Á Á ÁO36 2.910(2) A],
while that between the host and guest has intermediate
˚
length [O1Á Á ÁO71 2.765(2) A]. The resulting independent
X-ray analysis of compound 2a revealed that the guest is
disordered. Figure 2 shows the 1:1 host–guest asymmetric
unit as well as details of the guest disorder.
2
:1 hydrogen bonded unit is repeated by the 2 -axis and
1
by translation in the crystal.
Figure 2. ORTEP diagrams showing the asymmetric unit in compound 2a
(
top, major disordered guest component only) and details of the guest
disorder (bottom, H atoms omitted for clarity).
Atoms C40 and O39 of lactone 4 occupy two sites each,
such that both C40 and C40a are bonded to the common
methyl carbon atom C43, corresponding to the major
and minor enantiomeric populations of ꢀ90% of (S)- and
Figure 1. ORTEP diagrams showing the asymmetric units in compounds
1b (top) and 1c (bottom). Thermal ellipsoids are represented at the 50%
level.
ꢀ
10% of (R)-4-methyloxetan-2-one. The guest tertiary H
Compound 1c on the other hand has 1:1 host–guest stoi-
chiometry (Fig. 1) and no guest disorder was observed.
In this case, the (R)-enantiomer is exclusively present in
the crystal. As for the host in 1b, there is intramolecular
atom located at the stereogenic centre C40 [(S)-, with major
occupancy, Fig. 2] engages in a stabilizing C–HÁ Á Áp interac-
˚
tion with a host phenyl ring (HÁ Á ÁCg 2.79 A, C–HÁ Á ÁCg
142°). Instead, for the tertiary H atom corresponding to

K. Tanaka et al. / Tetrahedron: Asymmetry 17 (2006) 1678–1683
1681
the guest (R)-enantiomer, the shortest analogous inter-
found in 2a and 2b. Here, there is a short host–host intra-
˚
action has much less favourable geometry (HÁ Á ÁCg
molecular hydrogen bond (O1Á Á ÁO25 2.657(2) A) but an
˚
3
.27 A, C–HÁ Á ÁCg 113°). Hence, the preference for the
almost equally strong host–guest hydrogen bond
˚
(
S)-enantiomer in this crystal can be rationalized. As found
(O1Á Á ÁO38 2.687(2) A).
in host 1, there is intramolecular H-bonding between the
hydroxyl groups in host 2, the distance O37Á Á ÁO23 being
Thus, also for the series 2a, 2b and 2c, X-ray analyses
confirmed the existence of the ‘odd–even’ effect with re-
gards to the enantioselectivity of host 2, in accordance
with the results reported in Table 1. An interesting and
highly relevant feature of this series of inclusion com-
pounds is that the host arrays in crystals of 2a and 2b
are isostructural, but they are not isostructural with that
in 2c. This is suggested by the unit cell parameters for
these three compounds reported in Table 4 and was con-
firmed by detailed analysis of the packing arrangements
shown in Figure 4.
˚
2
.662(2) A, while the host–guest H-bond has O23Á Á ÁO42
˚
2
.819(2) A. Analysis of the crystal structure shows that
the asymmetric unit above forms a close-packed arrange-
ment with only one weak C–HÁ Á ÁO bond and van der
Waals forces contributing to stabilization of the structure.
The other members in this series containing host 2, whose
X-ray structures were determined viz. 2b and 2c, also have
1
:1 host–guest stoichiometries and their asymmetric units
are shown in Figure 3. In the crystal of compound 2b, the
guest is ordered and its chirality is (R)-. The intramolecular
˚
hydrogen bond in host 2 has O1Á Á ÁO25 2.656(2) A, while the
As with the compounds described above, the types of host–
guest interaction that mediate guest selectivities can be
identified. Furthermore, the occurrence of host isostructu-
rality in crystals of 2a and 2b is fortuitous, providing fur-
ther clues as to the factors responsible for the inclusion
of lactones 4 and 5 with opposite chiralities in essentially
the same environments. It is clear from Figure 4 that the
host frameworks are virtually identical in 2a and 2b, and
that the guest molecules are located in analogous layers
parallel to (001). For comparison, the structure of 2c is
drawn viewed down the a-axis, whose length is very similar
to the a-axis lengths in the isostructural pair. The mode of
the guest inclusion in 2c clearly differs from that in 2a and
˚
host–guest hydrogen bond has O1Á Á ÁO38 2.757(2) A. Anal-
ysis of the lactone ring conformation shows that it is an
envelope form (flap at C42).
2b.
A detailed view of the inclusion of the guests in compounds
a and 2b is shown as a stereoview in Figure 5, where frag-
2
ments of surrounding hosts and the guest molecules are
superimposed for comparison. Isostructurality of the host
framework is evident. A comparison of the dispositions
of the guest molecules shows that their carbonyl oxygen
atoms occupy similar positions, each being hydrogen
bonded to the corresponding host hydroxyl groups. Their
molecular planes are rotated by ꢀ45°, but their methyl C
atoms practically overlap and the tertiary H atoms at the
chiral centres point in opposite directions. In addition to
the host–guest hydrogen bonds shown, for guest 4 there
is one C–HÁ Á ÁO hydrogen bond involving the ring O atom
˚
[
H(phenyl)Á Á ÁO = 2.62 A], and three CHÁ Á Áp interactions,
two involving the guest methylene H atoms (with H Á Á ÁCg
A
˚
2
.88 and H Á Á ÁCg 3.22 A, Cg = ring centroid), and one
B
˚
involving the tertiary H atom (HÁ Á ÁCg 2.78 A). For guest
5
, there is an analogous C–HÁ Á ÁO hydrogen bond with
˚
H(phenyl)Á Á ÁO = 2.56 A, and two CHÁ Á Áp interactions
involving the guest methylene H atoms (with H Á Á ÁCg
A
˚
2
.61, H Á Á ÁCg 2.84 A). However, the tertiary H atom of
B
guest 5 does not engage in any significant CHÁ Á Áp inter-
action. Thus, as regards the stereoselectivities manifested
in 2a and 2b, anchoring of both guest molecules by com-
mon, strong host–guest O–HÁ Á ÁO hydrogen bonding and
accommodation of their sterically bulky methyl groups in
common host environments appear to be the main factors
that determine their modes of inclusion. The precise orien-
tations of the guest molecules are dictated by the subtle
secondary interactions described above. Evidently, the
Figure 3. ORTEP diagrams showing the asymmetric units in compounds
2b (top) and 2c (bottom).
Lactone 6 in the crystal of compound 2c exclusively occurs
as the (S)-enantiomer. Atom C42 deviates significantly
from the plane formed by the remaining five ring atoms.
The H-bonding arrangement is analogous to the schemes

1682
K. Tanaka et al. / Tetrahedron: Asymmetry 17 (2006) 1678–1683
Figure 5. Stereoview showing the superposition of common fragments of
the host molecules that surround complete guest molecules in 2a (green,
major component with s.o.f. ꢀ90%) and 2b (standard colours). Hydrogen
bonding (O–HÁ Á ÁO) is indicated by dashed lines.
3. Conclusions
Inclusion experiments with the host molecules 1–3 revealed
a remarkable ‘odd–even’ effect with medium-size lactone
guest molecules. In the absence of comprehensive X-ray
structural data, it was not possible to explain all of the ob-
served selectivities. However, a subset of these compounds
1b–c and 2a–c has been analyzed by X-ray diffraction and
the results (a) confirm the selectivities determined by
HPLC/polarimetry and (b) reveal host–guest interactions
that can be used as a basis for explaining the selectivities
in specific instances. Thus, in cases where less-than-perfect
enantioselectivities occur for the crystals selected and ana-
lyzed (compounds 1b and 2a), both enantiomers of a given
guest were located in the inclusion crystal meaning it was
possible to identify specific host–guest interactions that
account for the preferential inclusion of the predominant
enantiomer. The occurrence of host isostructurality in the
case of compounds 2a and 2b, on the other hand, provided
a rare opportunity to identify the interactions that enable
guest molecules differing only by a methylene group and
having opposite chirality, to occupy a void with common
topology in their inclusion crystals. In addition to the
strong, ubiquitous host–guest O–HÁ Á ÁO hydrogen bonds,
other interactions that mediate the enantioselectivity in
these series of compounds are generally weak (e.g., of type
C–HÁ Á ÁO, C–HÁ Á Áp), but do play a decisive role.
4
. Experimental
4.1. General
1
Figure 4. (100) projections of the crystal structures of 2a (top), 2b (centre)
and 2c (bottom). Two unit cells are shown for 2a and 2b, and four unit
cells for 2c. Host molecules are in stick representation and the guests are
drawn in space-filling mode.
H NMR spectra were recorded on JEOL JNM-GSX 400
spectrometer, with tetramethylsilane (TMS) as the internal
standard. The optical rotations were measured with a
ATAGAO AP-100 polarimeter. HPLC data were obtained
on JASCO PU-980, UV-970 system. Thermogravimetric
analyses (TGA) were performed on a Rigaku TG-8120
instrument.
common host structure found in 2a and 2b is no longer able
to accommodate the larger guest lactone 6, so that
compound 2c, containing the latter guest, crystallizes in a
different arrangement. Examination of host–guest interac-
tions in that structure revealed only one host phenyl(C–
H)Á Á ÁO(guest ring) hydrogen bond interaction in addition
to the strong O–HÁ Á ÁO hydrogen bond.
4.2. General procedure for the optical resolution by
inclusion crystallization
The optical resolution of lactones (4–7) was carried out as
follows: a mixture of the lactone and the host compound

K. Tanaka et al. / Tetrahedron: Asymmetry 17 (2006) 1678–1683
1683
was dissolved with heating in ether–hexane solution. When
this solution was kept at room temperature for 12 h, inclu-
sion crystals of lactones 4–7 and optically active hosts 1–3
with the host–guest ratios listed in Table 1 were formed.
data-collection strategy indicated by the program COL-
7
LECT involved suitable combinations of /- and x-scans.
8
The program DENZO-SMN was used for cell refinement
and data reduction. Intensity statistics indicated non-cen-
tric space groups in all cases. The structures were solved
1
The host–guest ratios were determined by H NMR spec-
9
tra. The purification of each complex was achieved by four
recrystallizations from ether–hexane solution. The opti-
cally active lactones 4–7 with the enantiomeric excesses
listed in Table 2 were isolated by heating the inclusion crys-
tals in vacuo. The enantiomeric excesses of lactones 4 and 5
were determined by HPLC using Chiralcel OA and Chir-
alpak AS (Daicel Chemical Industries, Ltd.), respectively.
by direct methods using the program SHELXS86 and refined
2
by full-matrix least-squares against F using the program
1
0
SHELX-97.
Molecular parameters were calculated with
1
1
12
PLATON. The programs ORTEP and WebLab ViewerPro
1
3
3.7 were used for illustrations. The CIF files for the struc-
tures have been deposited with the Cambridge Crystallo-
graphic Data Centre (deposition numbers 60444–60448).
5
6
The enantiomeric excesses of lactones 6 and 7 were deter-
mined by comparison with their reported [a]D values.
Acknowledgements
Typical procedure for the optical resolution of (± )-4 with
(
3
(
R,R)-(À)-2. When a solution of (R,R)-(À)-2 (14.8 g,
0.0 mmol) and (± )-4 (5.2 g, 60.0 mmol) in ether–hexane
1:2, 18 ml) was kept at room temperature for 12 h, a 1:1
inclusion complex of (R,R)-(À)-2 and (S)-(À)-4 of 40% ee
17.2 g) was obtained. Four recrystallizations of the crys-
K.T. acknowledges financial support from ‘High-Tech
Research Center’ Project for Private Universities: mating
fund subsidy from MEXT (Ministry of Education, Culture,
Sports, Science and Technology), 2005–2009. M.R.C.
acknowledges research support from the University of
Cape Town and the NRF (Pretoria).
(
tals from ether–hexane (1:2) gave the pure 1:1 inclusion
complex of (R,R)-(À)-2 and (S)-(À)-4 as colourless prisms
(
(
3.6 g), which upon heating at 170 °C in vacuo afforded
S)-(À)-4 of 99% ee (0.49 g, 19% yield). The enantiomeric
References
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OA (Daicel Chemical Industries, Ltd.); eluent, hexane–2-
propanol = 90:10; flow rate, 1.0 ml/min; detection, UV
1. For example, see: Fandeur, T.; Moretti, C.; Polonski, J.
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2
20 nm; retention time, 18 [(S)-enantiomer] and 24 [(R)-
2
. (a) Pirkle, W. H.; Adams, P. E. J. Org. Chem. 1979, 44, 2169;
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(
2
Optical resolutions of other cases were carried out by the
same procedures. The enantiomeric excess of 5 was deter-
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Chemical Industries, Ltd.); eluent, hexane–2-propa-
nol = 90:10; flow rate, 1.0 ml/min; detection, UV 220 nm;
retention time, 15 (S-enantiomer) and 16 (R-enantiomer)
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X-ray analysis. The enantiomeric excesses and absolute
3
. (a) Pool, R. Science 1989, 245, 1187; (b) Okamoto, Y.;
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(
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5
6
configurations of 6 and 7 were determined by comparison
5
6
of their [a] values and specific rotation signs, respectively,
D
with those reported.
7
. Hooft, R. COLLECT; Nonius B.V.: Delft, The Netherlands,
1998.
4
.3. X-ray diffraction
8
9
. Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
. Egert, E.; Sheldrick, G. M. Acta Crystallogr., Sect. A 1985,
All crystal intensity data were collected on a Nonius Kappa
4
1, 262.
CCD diffractometer using MoK radiation and appropri-
a
1
0. Sheldrick, G. M. SHELXL97; University of G o¨ ttingen: Ger-
many, 1997.
1. Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, C34.
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3. WebLab ViewerPro 3.7; Molecular Simulations: San Diego,
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ate /- and x-scans. Crystals were coated in Paratone N
oil (Exxon) and cooled in a stream of nitrogen vapour at
the selected temperature. Unit cell dimensions before and
after cooling were comparable, indicating that no phase
changes occurred on altering the crystal temperature. The
1
1
1