Solid-state chiral resolution mediated by stoichiometry: Crystallizing etiracetam with ZnCl2

Article abstract of DOI:10.1039/c8cc06199h

Chiral resolution of racemic etiracetam was achieved via co-crystallization with ZnCl₂. Depending on the amount of ZnCl₂ either a stable racemic compound or a stable conglomerate can be obtained. Excess ZnCl₂ triggers the quantitative conversion of the racemate into the conglomerate solid; this unprecedented behaviour was investigated through a racetam/ZnCl₂/solvent phase diagram.

Full text of DOI:10.1039/c8cc06199h

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Solid-state chiral resolution mediated by stoichiometry:
crystallizing etiracetam with ZnCl2
Oleksii Shemchuk,a Lixing Song,b Koen Robeyns,b Dario Braga,a Fabrizia Grepioni*,a and Tom
Leyssens* b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Chiral resolution of racemic etiracetam was achieved via co-
crystallization with ZnCl2. Depending on the amount of ZnCl2
either a stable racemic compound or a stable conglomerate can be
obtained. Excess ZnCl2 triggers the quantitative conversion of the
racemate into the conglomerate solid; this unprecedented
behaviour was investigated through
phase diagram.
a racetam/ZnCl2/solvent
Levetiracetam ((S)-2-(2-oxopyrrolidin-1-yl)butanamide, S-
etiracetam) is the active pharmaceutical ingredient (API) of
KEPPRA®, an anti-epileptic drug (AED) commercialized by UCB
Pharma. Epilepsy is a prevailing chronic neurological disorder Fig.
1
The chemical structure of Etiracetam (RS-ETI),
or a group of disorders characterized by unprompted seizures Levetiracetam (S-ETI, the biologically active enantiomer), and
which tend to recur.1 More than 50 million people worldwide the R-enantiomer of etiracetam) (R-ETI) and.
are affected by this condition.2 Levetiracetam is one of the
most recent AEDs that has been approved by the Food and Co-crystallization of chiral molecules with organic molecules
Drug Administration.3
and inorganic salts has been extensively studied by our groups.
The use of solution co-crystallization was proven to be a useful
Etiracetam (Fig. 1) is encountered as a racemic intermediate in approach for the chiral resolution of racemic mixtures.5, 6 In a
the synthesis of levetiracetam (i.e. the active S-enantiomer of similar context, spontaneous chiral resolution upon ionic co-
pharmaceutical interest). The R-enantiomer contained in the crystals (ICCs)7-9 formation was discovered by some of us by
racemic compound does not exert any of the desired biological reacting the amino acid histidine or proline with lithium
properties.4 In order to avoid possible confusion for the halides.10,
11
In both cases the Li+ cations selectively link to
reader, in the present article we use the abbreviations RS-ETI molecules of the same chirality, forming enantiopure chains,
to indicate etiracetam in its racemic form, S-ETI to indicate the resulting in a chiral resolution process in the solid state via
active S-enantiomer (i.e. levetiracetam) and R-ETI to indicate conglomerate formation. A different behaviour was observed
the inactive R-enantiomer.
for CaCl2, which forms racemic ICCs with DL-histidine,12 in
which each Ca2+ is hexacoordinated and interacts with a
molecule of D-histidine and a molecule of L-histidine. Thus, it
was speculated that one possible reason for chiral preference
in lithium ICCs could be the tetrahedral geometry around the
lithium cations, which favours the coordination of molecules of
the same handedness. In this work we put this hypothesis to
test by attempting complexation of enantiopure S-etiracetam
(levetiracetam) and of racemic RS-etiracetam to zinc in the
form of their ZnCl2 salts, as zinc is known to favour tetrahedral
coordination. Moreover, a ternary phase diagram (TPD) has
been constructed as a subset of the more complex R-
etiracetam:S-etiracetam:ZnCl2:EtOH phase diagram, to
determine the overall compositions for which the complexes
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are the only stable phases in EtOH suspension. It will be
argued that these phase diagrams could be used to develop a
full resolution by entrainment (preferential crystallization),13, 14
focusing on the area where the conglomerate is the only stable
phase in suspension.
The reaction of both levetiracetam (S-ETI) and etiracetam (RS-
ETI) with ZnCl2 resulted in the formation of anhydrous
complexes (see ESI). S-ETI·ZnCl2 (Fig. 2) was obtained by
crystallization from solution or slurry and by liquid-assisted
grinding. A 1:1 S-ETI:Zn2+ stoichiometric ratio is observed with
both Zn2+ cations, tetrahedrally coordinated by two APIs,
which act as bridges between consecutive zinc cations via the
pyrrolidone and the amido groups (see Fig. 2), and two
chloride anions. The zig-zag chains thus formed are held
together by hydrogen bonds between the chloride anions and
the hydrogen atoms of the amido groups (see Fig. ESI-9).
Fig. 3 Crystalline RS-ETI2·ZnCl2: Hydrogen bonds between the
amido groups of etiracetam molecules of opposite chirality
(colored in grey and orange for clarity). “Orange” and “grey”
layers of R-ETI2·ZnCl2 and S-ETI2·ZnCl2 can be seen in
projection, parallel to the a-axis.
In order to explore the effect of varying the stoichiometric
ratios, a series of experiments was performed for both
levetiracetam and etiracetam with ZnCl2. In the case of
levetiracetam, irrespective of the S-ETI:ZnCl2 stoichiometric
ratio, a 1:1 compound of formula S-ETI·ZnCl2 was invariably
obtained, together with unreacted starting material. In the
case of etiracetam, on the contrary, increasing the amount of
ZnCl2 with respect to etiracetam (from 2:1 to 1:1 ratio) not
only affected the stoichiometry of the product, but also caused
the disruption of the racemic compound, followed by
reconstruction of both the enantiopure aggregates leading to
formation of the stable conglomerate R-ETI·ZnCl2 + S-ETI·ZnCl2
(see Scheme 1). To the best of these authors’ knowledge this is
the first time that stoichiometry is used as a switch between a
racemic compound and a conglomerate.
Fig. 2 The infinite zig-zag chain (evidenced by the yellow lines)
extending in the bc-plane in crystalline S-ETI·ZnCl2 is formed by
ZnCl2 units bridged by S-ETI molecules. Hydrogen atoms
omitted for clarity.
The racemic compound RS-etiracetam also combines with
ZnCl2 to form the crystalline compound RS-ETI2·ZnCl2, see Fig.
3). The first difference with S-ETI ·ZnCl2 can be found in the 2:1
stoichiometry, as the crystal contains discrete RS-ETI2·ZnCl2
units. While in S-ETI·ZnCl2 both oxygens are involved in the
complexation to Zn2+, resulting in infinite 1D chains formation,
in RS-ETI2·ZnCl2 only the oxygens of pyrrolidone are bound to
Zn2+, and a 0D complex is obtained. The amido groups on
etiracetam form typical hydrogen bonded amido rings (see Fig.
3a). It is worth pointing out that, in agreement with our
working hypothesis, the Zn2+ cations selectively bind to
molecules of one chirality, forming distinct layers of R-
ETI2·ZnCl2 and of S-ETI2·ZnCl2.
Scheme 1. Graphic representation of the RS-ETI:ZnCl2 system
in the solid state, and the role of stoichiometry in the racemic
compound/conglomerate switch mechanism.
2 | J. Name., 2012, 00, 1-3
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two quadriphasic regions (IV = RS-ETI2·ZnCl2 + R-ETI·ZnCl2 + S-
ETI·ZnCl2 + L; VI = ZnCl2 + R-ETI·ZnCl2 + S-ETI·ZnCl2 +L); L is the
liquid phase. By targeting the biphasic region III, where the
racemic compound is stable in suspension, this latter can be
isolated through solution crystallization. In region
V the
conglomerate is the stable phase in suspension. Overall,
solution data confirms the possibility of using different
amounts of ZnCl2 to switch, in suspension, from
thermodynamically stable racemic compound to
thermodynamically stable conglomerate.
a
a
Figure 4. Ternary solid-state phase diagram for R-ETI, S-ETI and
ZnCL2, showing the thermodynamic solid state outcome for
different combinations of the three components. Pure phases
in black, mixtures of two solid phases in green, mixtures of
three solid phases in red. [To make the diagram easier to read,
we use here stoichiometric ratios to indicate the compounds
with ZnCl2, thus R-ETI⋅ZnCl2, S-ETI⋅ZnCl2 and RS-ETI2⋅ZnCl2 are
represented with 1:1 (R), 1:1 (S) and 2:1, respectively.]
Different combinations of RS-ETI, S-ETI and ZnCl2 lead to the
solid state ternary phase diagram reported in Fig. 4, built
experimentally via multiple LAG experiments.15, 16 Starting with
a mixture of RS-ETI (1 equiv.) and S-ETI (1 equiv.), addition of
0.5 equivalents of ZnCl2 (red-dotted line) results in the
formation of racemic RS-ETI2·ZnCl2. When further 0.5
equivalents of ZnCl2 are added, half of S-ETI reacts and
correspondingly forms 0.5 equivalents of S-ETI·ZnCl2. A third
addition of 0.5 equivalents (for a total of 1.5 equiv.) of ZnCl2
causes complete reaction of S-ETI, and the solid obtained is a
mixture of racemic RS-ETI2·ZnCl2 (1 equiv.) and enantiopure S-
ETI·ZnCl2 (1 equiv.). A last addition of ZnCl2 (0.5 equiv.)
dismantles the racemic compound RS-ETI2·ZnCl2 into the
enantiopure counterparts: the final solid mixture will then
contain 0.5 equiv. of R-ETI·ZnCl2 and 1.5 equiv. of S-ETI·ZnCl2.
Fig. 4 also indicates which combinations can be expected to be
stable in suspension, when a solvent is added to the system.
Fig. 5. Enlarged portion of the isoplethal section of the R-ETI:S-
ETI:ZnCl2:Ethanol isothermal and isobaric phase diagram at
298K (mol%). The orange and green dotted points are the
experimental starting conditions used to create this diagram
[For the full version see Fig. ESI-16.]
In summary, we have reported for the first time that by
varying the stoichiometric ratio it is possible to “switch”
reversibly from a stable racemic compound to a conglomerate.
As the conglomerate is accessible in suspension, a resolution
process by entrainment could be developed for this system.
Furthermore the results reported above strengthen the fact
that co-crystallization with metal ions favouring tetrahedral
coordination can be successfully used to obtain chiral
selectivity and conglomerate formation from racemic
compounds.
Acknowledgements
The experimental isoplethal section of the R-ETI:S-
ETI:ZnCl2:EtOH isobaric and isothermal quaternary phase
diagram (see Fig. 5 and Fig. ESI-16) shows how, by changing
the amount of ZnCl2 in solution, both the racemic compound
RS-ETI2·ZnCl2 and the conglomerate R-ETI·ZnCl2 + S-ETI·ZnCl2
can be found as thermodynamically stable suspensions. The
This work was supported by a STSM Grant from COST Action
CM1402 Crystallize and by the University of Bologna (FARB
2017)
Conflicts of interest
TPD shows (i) three biphasic regions (
ETI2·ZnCl2 + L; VII = ZnCl2 + L), (ii) two triphasic regions (II = RS-
ETI2·ZnCl2 + RS-ETI + L; = R-ETI·ZnCl2 + S-ETI·ZnCl2 + L), and (iii)
I = RS-ETI + L; III = RS-
There are no conflicts to declare.
V
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Notes and references
1.
M. L. Scheuer and T. A. Pedley, New Engl. J. Med., 1990,
323, 1468-1474.
2.
M. J. Brodie, S. D. Shorvon, R. Canger, P. Halasz, S.
Johannessen, P. Thompson, H. G. Wieser and P. Wolf,
Epilepsia, 1997, 38, 1245-1250.
3.
4.
C. A. Hovinga, Pharmacotherapy, 2001, 21, 1375-1388.
A. H. Gouliaev and A. Senning, Brain Res. Rev., 1994, 19,
180-222.
5.
6.
7.
8.
9.
G. Springuel and T. Leyssens, Cryst. Growth Des., 2012, 12,
3374-3378.
G. Springuel, K. Robeyns, B. Norberg, J. Wouters and T.
Leyssens, Cryst. Growth Des., 2014, 14, 3996-4004.
D. Braga, F. Grepioni, L. Maini, S. Prosperi, R. Gobetto and
M. R. Chierotti, Chem. Commun., 2010, 46, 7715-7717.
D. Braga, F. Grepioni and O. Shemchuk, CrystEngComm,
2018, 20, 2212-2220.
A. J. Smith, S. H. Kim, N. K. Duggirala, J. Jin, L. Wojtas, J.
Ehrhart, B. Giunta, J. Tan, M. J. Zaworotko and R. D.
Shytle, Mol. Pharm., 2013, 10, 4728-4738.
D. Braga, L. D. Esposti, K. Rubini, O. Shemchuk and F.
Grepioni, Cryst. Growth Des., 2016, 16, 7263-7270.
O. Shemchuk, B. K. Tsenkova, D. Braga, M. T. Duarte, V.
Andre and F. Grepioni, Chemistry (Easton), 2018, DOI:
10.1002/chem.201802446.
10.
11.
12.
13.
14.
15.
16.
O. Shemchuk, L. Degli Esposti, F. Grepioni and D. Braga,
CrystEngComm, 2017, 19, 6267-6273.
A. Collet, M. J. Brienne and J. Jacques, Chem. Rev., 1980,
80, 215-230.
G. Levilain and G. Coquerel, CrystEngComm, 2010, 12,
1983.
D. Braga, L. Maini and F. Grepioni, Chem. Soc. Rev., 2013,
42, 7638-7648.
S. L. James, C. J. Adams, C. Bolm, D. Braga, P. Collier, T.
Friscic, F. Grepioni, K. D. Harris, G. Hyett, W. Jones, A.
Krebs, J. Mack, L. Maini, A. G. Orpen, I. P. Parkin, W. C.
Shearouse, J. W. Steed and D. C. Waddell, Chem. Soc.
Rev., 2012, 41, 413-447.
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C8CC06199H
Solid-state chiral resolution mediated by stoichiometry: crystallizing etiracetam
with ZnCl2
O. Shemchuk, L. Song, K. Robeyns, D. Braga, F. Grepioni, and T. Leyssens
Co-crystallization of racemic etiracetam with ZnCl2 results in a racemic compound or a
conglomerate, depending on the amount of ZnCl2; the unprecedented behaviour was investigated
through a racetam/ZnCl2/solvent phase diagram.