When a host becomes a guest - Competition between decreasing hydrophobic spaces and supramolecular synthon propagation

Article abstract of DOI:10.1039/c0ce00388c

The presented results of racemic resolution of N-(4-nitrobenzoyl)-dl- asparagine by fractional crystallization of brucinium diastereomeric salts (at 5 °C) reveal a molecular recognition as a competition between decreasing of hydrophobic spaces by a hydrophobic recognition on a surface of a brucinium self-assembly (brucinium N-(4-nitrobenzoyl)-d-asparaginate heptahydrate (1) - first fraction) and formation by anions of the resolved acid a strong hydrogen bonds network mediated by characteristic supramolecular synthon (brucinium N-(4-nitrobenzoyl)-l-asparaginate methanol trisolvate (2)). Comparison of 1 with result of its recrystallization at abient conditions, brucinium N-(4-nitrobenzoyl)-d-asparaginate methanol disolvate 0.73 hydrate (1a), shows that in lower temperature the system favors hydrophobic recognition while higher temperature encourages it to formation, defined by other than in 2 supramolecular synthon, a strong hydrogen bonds network. Presence of the nitro group bonded to the phenyl ring of N-(4-nitrobenzoyl)-amino acid derivative plays an important role decreasing enantiospecifity and enantioselectivity of brucine driving brucinium cations to be assembled in a way suitable for interactions with this group.

Full text of DOI:10.1039/c0ce00388c

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When a host becomes a guest—competition between decreasing hydrophobic
spaces and supramolecular synthon propagation†
*
ꢀ
Agata Bia1onska and Zbigniew Ciunik
Received 9th July 2010, Accepted 23rd September 2010
DOI: 10.1039/c0ce00388c
The presented results of racemic resolution of N-(4-nitrobenzoyl)-DL-asparagine by fractional
ꢀ
crystallization of brucinium diastereomeric salts (at 5 C) reveal a molecular recognition as
a competition between decreasing of hydrophobic spaces by a hydrophobic recognition on a surface of
a brucinium self-assembly (brucinium N-(4-nitrobenzoyl)-D-asparaginate heptahydrate (1)—first
fraction) and formation by anions of the resolved acid a strong hydrogen bonds network mediated by
characteristic supramolecular synthon (brucinium N-(4-nitrobenzoyl)-L-asparaginate methanol
trisolvate (2)). Comparison of 1 with result of its recrystallization at abient conditions, brucinium
N-(4-nitrobenzoyl)-D-asparaginate methanol disolvate 0.73 hydrate (1a), shows that in lower
temperature the system favors hydrophobic recognition while higher temperature encourages it to
formation, defined by other than in 2 supramolecular synthon, a strong hydrogen bonds network.
Presence of the nitro group bonded to the phenyl ring of N-(4-nitrobenzoyl)-amino acid derivative
plays an important role decreasing enantiospecifity and enantioselectivity of brucine driving brucinium
cations to be assembled in a way suitable for interactions with this group.
a particular self-assembly plays a vital role in an enantiospecifity
1. Introduction
of resolving agent.
In 1894 Emil Fisher, working on receptor-substrate binding by
enzymes, noticed that binding of a substrate to the receptor must
be selective, that guest’s size and shape is a complementary to the
receptor geometry, and that they fit to each other like a key to
a lock.1 Daniel Koshland introduced a theory of ‘‘induced fit’’
according to which both receptor and substrate undergo signif-
icant conformational changes upon to binding to one another.2
The concepts of induced and ‘lock and key’ fit in a molecular
recognition has an application for both, biological, as well as,
supramolecular systems.3 Recently we showed that separation of
racemic acids (N-(4-nitrobenzoyl)-amino acids) by fractional
crystallization of strychninium diastereomeric salts appears as
a hydrophobic ‘lock and key’ recognition.4 Strychninium cations
have self-assembled into common strychninium corrugated
layers, and the 4-nitrobenzoyl group of anions of the resolved
acids has been recognized in deep grooves in a surface of the
strychninium self-assembly. Since resolving agent self-assembly
is stabilized by as weak as C–H/O and C–H/p hydrogen
bonds, addition of an acid can easy induce a pre-organization of
the resolving agent moieties. In fact, the acid has usually a limited
influence on a type of strychninium self-assembly, and depending
on donor–acceptor capability of the acid the C–H/p hydrogen
bonded strychninium ribbons are pre-organized into layers with
surfaces possessing donor only or donor–acceptor function
suitable for interactions with anions of the acid.5 Formation of
Two stereo-related compounds, strychnine and its dimethoxy
derivative, brucine, tending to be assembled in a different way,
can recognize opposite enantiomers of resolved acid.5,6 Brucine is
one of the most frequently used resolving agent for separation of
racemic acids.7 The crystal structures being a result of separation
of racemic acids by brucine reveal a greater than observed for
strychnine influence of resolved compound on an image of
recognition.8 Presence of the two methoxy groups bonded to the
arene ring of brucine moiety increases a capability of the brucine
arene ring to participate in stacking interactions with a resolved
compound.9
Faculty of Chemistry, University of Wrocław, 14. F. Joliot-Curie, 50-383
Wrocław, Poland. E-mail: bialonsk@eto.wchuwr.pl; Fax: +48 71 328
2348; Tel: +48 71 375 7388
As a part of our studies focused on mechanism of a chiral
discrimination by diastereomeric salts formation we performed
racemic resolution of N-(4-nitrobenzoyl)-DL-asparagine using
brucine as a resolving agent. During the racemic resolution (at
† Electronic supplementary information (ESI) available: Additional
data. CCDC reference numbers 783801–783803. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0ce00388c
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5
ꢀC), crystals of brucinium N-(4-nitrobenzoyl)-D-asparaginate
heptahydrate (1) (a first fraction) and brucinium N-(4-nitro-
benzoyl)-L-asparaginate methanol trisolvate (2) (a further frac-
tion) were grown. Crystals of the particular diastereomeric salts
were separately dissolved in methanol and when the samples
were left for crystallization in ambient conditions, crystals of
brucinium N-(4-nitrobenzoyl)-D-asparaginate methanol dis-
olvate 0.73 hydrate (1a) and crystals of 2 (Scheme).‡
2. Results
Selected views of 1, 1a and 2 together with numbering schemes
employed are presented in Fig. 1.
1 reveals a layered packing mode (Fig. 2) and is one of few
exceptions, in which the tertiary amine N2 atom of brucinium
cations is not directly linked by ionic N–H+/Oꢁ hydrogen bond
with an anion of carboxylic acid.10 Instead, the N2 atom is
a donor of N–H+/O hydrogen bond, in which the O7W water
molecule is its acceptor. The O1W, O2W, O3W, O5W, O6W and
O7W water molecules form hydrogen bonded branched discrete
chain presented in Fig. 1a. The O3W, O5W, O6W and O7W
water molecules of the discrete water chain are also involved in
‡ The crystals of 1 and 2 were obtained during racemic resolution of
N-(4-nitrobenzoyl)-DL-asparagine19 from methanol solution containing
equimolar amount of brucine (commercially available) and N-(4-nitro-
benzoyl)-DL-asparagine. The racemic resolution was carried out in 5 ꢀC.
The resulting crystalline fractions were separately dissolved in methanol
and left to crystallization. In ambient conditions crystals of 1a and 2 were
obtained.
X-Ray data for 1, 1a, and 2 were collected on a Kuma KM4CCD
ꢁ
diffractometer (Mo-Ka radiation; l ¼ 0.71073 A). X-Ray data for 1, 1a
and 2 were collected at 100 K using an Oxford Cryosystem device. Data
reduction and analysis were carried out with the CrysAlice ‘RED’
program.20 The space groups were determined using the XPREP
program. Structures were solved by direct methods using the XS program
and refined using all F2 data, as implemented by the XL program.21
Non-hydrogen atoms were refined with anisotropic displacement
parameters. The occupancy factor for the disordered solvent molecules
(in 1a: O1W and O35–C35 as PART 1 with occupancy factor equal to
0.49, and O351–C351 as PART 2, O36 and O361 with occupancy factors
equal to 0.73 and 0.27, respectively); in 2: O36–C36 and O361–C361 with
occupancy factors equal to 0.54 and 0.46, respectively, and O37–C37,
O371–C371 and O372–C372 with occupancy factors equal to 0.43, 0.31
and 0.26, respectively) were refined. All H atoms were found in Dr maps
or placed at calculated positions. Before the last cycle of refinement all H
atoms (except H3 atom in 1a and 2) were fixed and were allowed to ride
on their parent atoms. The Friedel pairs were merged before the final
refinement of 1, 1a and 2. The absolute structures were chosen on the
basis of the known absolute configuration of brucine.
Fig. 1 Selected views of (a) 1, (b) 1a and (c) 2 together with atom-
numbering schemes employed. Non-H atoms are shown as 30% proba-
bility displacement ellipsoids.
O–H/O, O–H/Oꢁ and N–H/O interactions with N-
(4-nitrobenzoyl)-D-asparaginate anions. The amide N3 and N4
atoms of the anions are donors of the N–H/O interactions, and
the O atoms of the carboxyl, as well as of the amide groups are
acceptors of the remained hydrogen bonds (Fig. 3, Table 1).
Consecutive anions are directly linked by N4–H47/O5(x + 1, y,
z) interactions. Resulted hydrogen bonded anionic/water sheets
are separated by brucinium layers (Fig. 2), which consist of
pillars linked to each other by C–H/O contacts. In turn, the
brucinium pillars are stabilized by O–H/O hydrogen bonds, in
which the O1 (x ꢁ 1/2, ꢁy + 3/2, ꢁz) and O4 atoms are bridged
by the O4W water molecule (Table 1). Surface of the brucinium
layer abounds with holes, formed between arene rings, sticking
out of the pillars. The holes are occupied by the 4-nitrobenzoyl
group of the N-(4-nitrobenzoyl)-D-asparaginate anions resulting
Crystal data for 1.
C23H27N2O4+$C11H10N3O6ꢁ$7H2O, M ¼ 801.80, orange plate, crystal
dimensions 0.40 ꢂ 0.30 ꢂ 0.06 mm, orthorhombic, space group P212121,
3
ꢁ
ꢁ
a ¼ 7.119(2), b ¼ 12.530(3), c ¼ 41.520(5) A, V ¼ 3703.6(14) A , Z ¼ 4,
Dc ¼ 1.438 Mg mꢁ3, T ¼ 100(2) K, R ¼ 0.062, wR ¼ 0.129 (3072
reflections with I > 2s(I)) for 505 variables.
Crystal data for 1a.
C23H27N2O4+$C11H10N3O6ꢁ$2CH3OH$0.73H2O, M ¼ 752.87, yellow
plate, crystal dimensions 0.30 ꢂ 0.30 ꢂ 0.20 mm, orthorhombic, space
ꢁ
group P212121, a ¼ 7.979(2), b ¼ 19.404(3), c ¼ 22.853(4) A, V ¼
3
3538.2(12) A , Z ¼ 4, Dc ¼ 1.413 Mg mꢁ3, T ¼ 100(2) K, R ¼ 0.046, wR ¼
ꢁ
0.092 (3682 reflections with I > 2s(I)) for 520 variables.
Crystal data for 2.
C23H27N2O4)+$C11H10N3O6ꢁ$3CH3OH, M ¼ 771.81, yellow needle,
crystal dimensions 0.20 ꢂ 0.06 ꢂ 0.06 mm, orthorhombic, space group
3
ꢁ
ꢁ
P212121, a ¼ 7.605(2), b ¼ 21.238(5), c ¼ 23.666(5) A, V ¼ 3822.4(16) A ,
Z ¼ 4, Dc ¼ 1.341 Mg mꢁ3, T ¼ 100(2) K, R ¼ 0.071, wR ¼ 0.154 (4411
reflections with I > 2s(I)) for 558 variables.
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a b c
, ,
ꢀ
ꢁ
Table 1 Hydrogen bonds geometry in 1, 1a and 2 (A) and ( ).
:D–H–A/ꢀ
ꢁ
ꢁ
ꢁ
D–H/A H/A/A D/A/A
1
N2–H24/O7W
N3–H37/O6W
N4–H471/O6W#2
N4–H472/O5#2
O1W–H11W/O3W
O1W–H12W/O5W
O2W–H21W/O7W
O2W–H22W/O1W
O3W–H31W/O7
O3W–H32W/O5#3 0.88
O4W–H41W/O4 0.87
O4W–H42W/O1#1 0.87
O5W–H51W/O6#4 0.87
O5W–H52W/O7#5 0.87
O6W–H61W/O6#3 0.87
0.93
0.88
0.88
0.88
0.88
0.87
0.88
0.87
0.88
1.99
2.12
2.09
2.06
1.92
2.05
1.93
1.99
1.93
2.07
1.98
2.09
1.87
2.08
1.98
1.92
1.89
1.90
2.840(5)
2.944(5)
2.951(6)
2.826(6)
2.789(6)
2.834(5)
2.784(5)
2.832(6)
2.758(5)
2.956(6)
2.849(5)
2.930(5)
2.738(5)
2.854(5)
2.829(5)
2.781(5)
2.760(5)
2.716(4)
151
155
164
145
171
151
164
163
155
176
176
164
174
148
168
170
175
160
O6W–H62W/O2W
0.87
Fig. 2 Packing of 1. For clarity, H-atoms are omitted.
O7W–H71W/O7#3 0.87
O7W–H72W/O6#4 0.85
1a
N2–H24/O5
0.93
0.97(3)
0.88
0.88
0.75
0.94
0.93
0.84
0.84
0.84
1.79
2.02(3)
2.04
2.10
1.86
1.93
1.84
2.00
1.93
1.86
2.670(3)
2.952(3)
2.916(3)
2.914(3)
2.590(12) 168
2.853(12) 170
2.766(4)
2.77(3)
2.750(5)
2.69(3)
156
N3–H3/O6#1
N4–H4A/O5#2
N4–H4B/O6#1
O1W–H11W/O35
O1W–H12W/O8
O35–H35/O7#3
O351–H351/O8
O36–H36/O4#4
O361–H361/O351
2
161(3)
175
154
180
152
164
168
N2–H24/O5
0.93
0.83(5)
0.88
0.99
0.84
0.84
0.84
0.84
1.76
2.14(5)
2.06
1.73
2.18
2.36
1.93
1.93
2.673(4)
2.935(5)
2.941(5)
2.712(5)
2.998(9)
3.008(15) 134
2.741(16) 163
2.72(2)
168
N3–H3/O7#1
N4–H47A/O5#2
O35–H35/O6#3
O36–H36/O4#4
O37–H37/O36#5
O371–H371/O35
O372–H37J/O35
162(4)
178
176
Fig. 3 N-(4-nitrobenzoyl)-D-asparaginate anions and water molecules
form hydrogen bonded layers in 1 perpendicular to the [001] direction.
164
154
in nitro/p stacking interactions extending in the [100] direction
(ESI, Fig. S1, Fig. S1a).†
a
1: #2 x + 1, y, z; #3 ꢁx + 1, y + 1/2, ꢁz + 1/2; #4 x, y + 1, z; #5 ꢁx + 2, y
+ 1/2, ꢁz + 1/2; #1 x ꢁ 1/2, ꢁy + 3/2, ꢁz. b 1a: #1 x + 1/2,ꢁy + 1/2,ꢁz #2
x + 1, y, z #3 x ꢁ 1, y, z #4 ꢁx + 1/2, ꢁy + 1, z ꢁ 1/2. c 2: #1 x ꢁ 1/2, ꢁy +
1/2, ꢁz + 2 #2 x + 1/2, ꢁy + 1/2, ꢁz + 2 #3 x ꢁ 1, y, z #4 x + 1/2, ꢁy + 3/
2, ꢁz + 2 #5 ꢁx + 1/2, ꢁy + 1, z ꢁ 1/2.
In 1a, there are brucinium—N-(4-nitrobenzoyl)-D-aspar-
aginate ion pair, one water (with occupancy factor equal to 0.73)
and two (disordered over two positions—the O35–C35 and
O351–C351 methanol molecule with occupancy factors equal to
0.73 and 0.27, respectively, and the hydroxyl O36 and O361
atoms, both with occupancy factor equal to 0.5) methanol
molecules in an asymmetric part of the unit cell. Brucinium
cations form pillars, similar to those observed in 1 (Fig. 4).
However, instead of water molecule, brucinium cations of the
pillar are linked to each other by the disordered methanol
molecules, O36–C36. Holes, formed between arene rings sticking
out of the pillars, are occupied by 4-nitrobenzoyl group of anions
resulting in nitro/p interactions and, as distinct from 1, C–H/
p hydrogen bonds (ESI, Fig. S2).† Unlike in 1, in 1a brucinium
cations and N-(4-nitrobenzoyl)-D-asparaginate anions are
directly linked by ionic N–H+/Oꢁ hydrogen bonds (Fig. 1c and
Table 1). The anions form hydrogen bonded tapes mediated by
the synthon presented in Fig. 5a, Table 1.11 With the synthon,
consecutive anions related by two fold screw axis symmetry are
linked by N–H/Oꢁ hydrogen bonds, in which two amide N
atoms of one and carboxyl O atom of other anion are donors and
acceptor, respectively. The set of hydrogen bonds is strengthened
by additional N–H/Oꢁ interactions observed between anions
Fig. 4 Packing of 1a. For clarity, H-atoms are omitted.
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Fig. 5 N-(4-nitrobenzoyl)-D- (a and c) and N-(4-nitrobenzoyl)-L-asparaginate anions (b and d) bonded to each other by hydrogen bonds mediated by
synthons.
related by translation vector (Fig. 5c). The O7 and O8 atoms of
the anions are involved in O–H/O interactions in which solvent
molecules, O35–C35, O351–C35 and O1W, respectively, are
donors. Both, the O1W water and the O35–C35 methanol
molecules are linked by O–H/O hydrogen bond (Table 1).
In 2, there are brucinium—N-(4-nitrobenzoyl)-L-asparaginate
ion pair, and three methanol molecules. Among the solvent
molecules only the O35–C35 methanol molecule is ordered. In
contrast to both above mentioned crystal structures, 1 and 1a, in
2 brucinium cations are distanced from each other and do not
form any distinguished self-assembly (Fig. 6). Arene ring of
particular brucinium cations is involved in C–H/p hydrogen
bonds. The C35 atom of methanol molecule and the C31 atom of
the N-(4-nitrobenzoyl)-L-asparaginate anion are donors of the
interactions (ESI, Fig. S3).† The O atoms of the nitro group of
the L-asparagine derivative are involved in C–H/O hydrogen
bond. Brucinium cations are donors of the interactions. Rela-
tively small distance between the N5 atom of the nitro group and
the O4 atom of brucinium cation suggests existence of an
attractive electrostatic N/O interaction between these atoms.12
The tertiary amine N2 atom of brucinium cations and the
carboxyl group of the N-(4-nitrobenzoyl)-L-asparaginate anions
are connected by ionic N–H+/Oꢁ hydrogen bonds (see Fig. 1c
and Fig. 6). In 2 anions form hydrogen bonded tapes extended in
the [100] direction mediated by other than in 1a synthon
(Fig. 5b). With the synthon anions related by two fold screw axis
symmetry are linked by N–H/Oꢁ and N–H/O hydrogen
bonds. The amide N4 and O7 atoms of one, and the carboxyl O5
and amide N3 atoms of neighbouring anion participate in the
interactions (Fig. 5d, Table 1).
3. Discussion
Following Lehn, recognition is binding with a purpose.13
Recently, we showed that the purpose of a recognition during
racemic resolution of N-(4-nitrobenzoyl)-DL-amino acids by
fractional crystallization of strychninium diastereomeric salts is
Fig. 6 Packing of 2. For clarity, hydrogen atoms are omitted.
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Fig. 7 (a) 4-Nitrobenzoyl group of N-(4-nitrobenzoyl)-D-asparaginate anions is recognized by sticking out of brucinium pillar arene rings in 1;
(b) mutual hydrophobic recognition of anionic and cationic self-assemblies in 1a; (c) brucinium arene ring is recognized by sticking out of mediated by
synthon anionic self-assembly 4-nitrobenzoyl groups in 2; surface of cationic and anionic moieties are marked by blue and yellow, respectively.18.
a decreasing of hydrophobic spaces in a hydrophilic environ-
ment.4 While hydrophilic environment supplies competitive
donors and acceptors of strong hydrogen bonds bonding itself
with particular components of a working system, it seems that
there is no equivalents in the hydrophilic environment for
interactions as weak as C–H/O, C–H/p hydrogen bonds, p/
p or nitro/p interactions, in which a resolving agent, a little
soluble in hydrophilic media, can participate. Thus, resolving
agent self-recognizes by the weak interactions, and/or if it is
similar in size, shape and hydrophobic properties, resolving
agent recognizes a resolved compound by weak interactions.
Based on the crystal structures under investigation, a competi-
tion between hydrophobic recognition by brucinium self-
assemblies and formation by anions sets of strong hydrogen
bonds defined by characteristic synthons is observed. In 1, the
hydrophobic recognition gained an importance over synthon
mediated strong hydrogen bonds formation. Arene rings sticking
out of brucinium pillar recognize the 4-nitrobenzoyl group of
anions of the resolved acid by nitro/p stacking interactions
(Fig. 7a, ESI, Fig. S1 and S1a).† Lack of ionic N–H+/Oꢁ
hydrogen bond goes with the hydrophobic recognition. Thus, in
1 the brucinium self-assembly appears as a host and N-
(4-nitrobenzoyl)-D-asparaginate anions play a role of a guest. In
1a, an influence of, defined by synthon, strong hydrogen bonds
system on an overall structure is significant (Fig. 7b). Brucinium
cations are still assembled into the pillars, however the pillars are
not linked to each other into layers like those observed in 1.
Moreover, the brucinium self-assembly interacts with 4-nitro-
benzoyl group of N-(4-nitrobenzoyl)-D-asparaginate self-
assembly. In other words, both self-assemblies, cationic and
anionic undergo a mutual recognition. In 2, formation by anions
of the strong hydrogen bonds network, mediated by the synthon,
gains an importance over the hydrophobic recognition. It is
manifested by lack of common brucinium self-assembly (Fig. 7c).
In this case, the hydrogen bonded anionic self-assembly appears
to be a host and brucinium cations in this system are a guest.
Comparison of 1 and 1a reveals that in low temperature the
system favours hydrophobic recognition while higher tempera-
ture encourages the system to synthon propagation into
hydrogen bonded anionic self-assembly.
Presence of the nitro group bonded to phenyl ring of N-
(4-nitrobenzoyl)-amino acid derivative plays an important role in
the molecular recognition, driving brucinium cations to other
than usually occurred self-assembly, suitable for interactions
with this group. In those crystals of the brucinium salts with N-
benzoyl-D-amino acids (asparagine, aspartic acid and
alanine),6,14 brucinium cations form corrugated layers, and
benzoyl group, as well as, amino acid residue of the anions are
adapted to a surface shape of the common brucinium self-
assembly (Fig. 8). In crystals of the brucinium salts with the
N-(4-nitrobenzoyl)-amino acids (asparagine, aspartic acid),
brucinium cations are assembled in a way suitable for interac-
tions with the 4-nitrobenzoyl group of anions.15 Twelve crystal
structures of brucinium salts with N-(4-nitrobenzoyl)-D-, N-
(4-nitrobenzoyl)-L-
and
N-(4-nitrobenzoyl)-(D
L
1ꢁx)-serine
x
manifest a variety of possibilities for interactions of nitrobenzoyl
group with brucinium cations.16
First the crystalline fraction of the racemic resolution of N-
(4-nitrobenzoyl)- or N-benzoyl-DL-amino acids points at the
nitro group decreasing enantiospecifity of brucinium cations
Fig. 8 Common brucinium corrugated layer with the benzoyl group and
amino acid residue of hydrogen N-benzoyl-D-aspartate anions adapted to
the shape of the layer surface.
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with regard to N-(4-nitrobenzoyl)-DL-amino acids. While D-
enantiomers are recognized by brucinium cations during racemic
resolution of N-benzoyl-DL-amino acids (alanine, asparagine,
aspartic acid),6,14 N-(4-nitrobenzoyl)-L-aspartate15 and N-
(4-nitrobenzoyl)-D-asparaginate anions are presented in the first
crystalline fraction of suitable racemic resolution. Since both, N-
(4-nitrobenzoyl)-L-aspartate and N-(4-nitrobenzoyl)-D-aspar-
aginate anions, are recognized by brucinium cations in a similar
way, by nitro/p stacking interactions with sticking out of
brucinium pillars arene rings, it seems that enantiospecifity in
these cases is involved in the well defined set of hydrogen bonds
formed by anions and water molecules and interacting with
a surface of the brucinium layers.
higher temperature encourages the systems to supramolecular
synthon propagation. Presence of the nitro group bonded to
phenyl ring of N-(4-nitrobenzoyl)-amino acid derivative leads
brucinium cations to be assembled in a way suitable for inter-
actions with the nitro group. Since the nitro group is relatively
distanced from the chiral centre of resolved acid, the recognition
of the nitro group on a surface of the brucinium self-assembly has
a significant role decreasing enantiospecifity and enantiose-
lectivity of brucine.
Notes and references
The variety of possibilities to interact of nitro group of N-
(4-nitrobenzoyl)-amino acids from one side, and an adaptation
of benzoyl group, as well as, amino acid residue of N-benzoyl-
amino acids to the surface shape of brucinium corrugated
monolayer sheet from the other side suggest decreased ability of
brucine moiety to chiral discrimination (enantioselectivity) for
N-(4-nitrobenzoyl)-amino acids in comparison to chiral
discrimination for suitable N-benzoyl-amino acids by this alka-
loid. Indeed, the decreased ability of brucine moiety to chiral
discrimination of N-(4-nitrobenzoyl)-amino acids is manifested
by formation of solid solutions or crystallization of both bruci-
nium diastereomeric salts, one containing (D)-enantiomer and
another containing (L)-enantiomer of the amino acids deriva-
tive,16,17 while among N-benzoyl-amino acids only one enan-
tiomer (usually D-enantiomer) is recognized by the alkaloid.6
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4. Conclusions
ꢀ
12 K. Wozniak, P. R. Mallinson, C. C. Wilson, E. Hovestreydt and
E. Grech, J. Phys. Chem. A, 2002, 106, 6897.
During racemic resolution of N-(4-nitrobenzoyl)-DL-asparagine
by fractional crystallization of brucinium diastereomeric salts at
ꢀC crystals of brucinium N-(4-nitrobenzoyl)-D-asparaginate
13 J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives,
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5
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heptahydrate (a first fraction) (1) and brucinium N-(4-nitro-
benzoyl)-L-asparaginate methanol trisolvate (2) were precipi-
tated from methanol solution. Recrystallization of both salts in
room temperature brings crystals of brucinium N-(4-nitro-
benzoyl)-D-asparaginate methanol disolvate 0.73 hydrate (1a)
and 2. In 1a and 2 solvent molecules reveal a disorder.
Comparison of 1, 1a and 2 shows that the molecular recognition
is like a competition between decreasing of hydrophobic surfaces
(1) and synthon propagation (2). Comparison of 1 with results of
its recrystallization at ambient conditions (1a) shows that at low
temperature the system favours hydrophobic recognition while
ꢀ
15 A. Bia1onska and Z. Ciunik, Acta Crystallogr., Sect. E: Struct. Rep.
Online, 2007, 63, o2002.
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16 A. Bia1onska and Z. Ciunik, CrystEngComm, 2010, 12, 2781.
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972 | CrystEngComm, 2011, 13, 967–972
This journal is ª The Royal Society of Chemistry 2011