Peculiar Case of Levetiracetam and Etiracetam α-Ketoglutaric Acid Cocrystals: Obtaining a Stable Conglomerate of Etiracetam

Article abstract of DOI:10.1021/acs.cgd.6b00819

In this contribution, we demonstrate that it is possible to obtain the lactol tautomer of alpha-ketoglutaric acid (AKGA) in the solid state by cocrystallizing it with Leviteracetam, a chiral nootropic drug used to treat epilepsy. In addition, we show that a cocrystal can be isolated with the racemic equivalent of Levetiracetam, Etiracetam (Eti), in which AKGA stays in the keto-form. We also report the existence of a cocrystal conglomerate in the Etiracetam-AKGA system, which is more stable than the racemic cocrystal at room temperature. The existence of a stable conglomerate is put in relation to the enantiospecificity of the Levetiracetam cocrystals, which is likely related to the ability of the Etiracetam enantiomers to stabilize one lactol tautomer at a time in solution or to promote its formation by hydrogen bonding. By comparing the peculiarities of the system in hand to the general behavior of cocrystallizing chiral systems with and without zwitterionic coformers, we suggest that for a pseudoquaternary cocrystal (made up of two racemate compounds) to exist the pseudoternary combinations (made up of one racemate and an enantiomer of the second compound) should exist and the enantiomers of the two compounds should form a diastereomeric pair at the binary level, rather than behave enantiospecifically.

Full text of DOI:10.1021/acs.cgd.6b00819

Subscriber access provided by Northern Illinois University
Article
The peculiar case of Levetiracetam and Etiracetam #-ketoglutaric
acid cocrystals: obtaining a stable conglomerate of Etiracetam
Fanny George, Bernadette Norberg, Koen Robeyns, Johan Wouters, and Tom Leyssens
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00819 • Publication Date (Web): 09 Aug 2016
Downloaded from http://pubs.acs.org on August 12, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Crystal Growth & Design is published by the American Chemical Society. 1155
Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 35
Crystal Growth & Design
1
2
3
4
5
The peculiar case of Levetiracetam and Etiracetam αꢀketoglutaric acid cocrystals:
obtaining a stable conglomerate of Etiracetam
6
7
Fanny George †, Bernadette Norberg ‡, Koen Robeyns †, Johan Wouters ‡, Tom Leyssens †*
†
8
9
Institute of Condensed Matter and Nanosciences, Université catholique de Louvain, 1348 LouvainꢀlaꢀNeuve,
Belgium
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
‡ Unité de chimie physique, théorique et structurale, University of Namur, Namur, Belgium
* Corresponding author: tom.leyssens@uclouvain.be.
Abstract
In this contribution, we demonstrate that it is possible to obtain the lactol tautomer of alphaꢀ
ketoglutaric acid (AKGA) in the solidꢀstate by cocrystallizing it with Leviteracetam, a chiral
nootropic drug used to treat epilepsy. Besides, we show that a cocrystal can be isolated with the
racemic equivalent of Levetiracetam, Etiracetam (Eti), in which AKGA stays in the ketoꢀform.
We also report the existence of a cocrystal conglomerate in the EtiracetamꢀAKGA system, which
is more stable than the racemic cocrystal at room temperature. The existence of a stable
conglomerate is put in relation with the enantiospecificity of the Levetiracetam cocrystals, which
is likely related to the ability of the Etiracetam enantiomers to stabilize one lactol tautomer at a
time in solution, or to promote its formation by Hꢀbonding. By comparing the peculiarities of the
system in hand to the general behavior of cocrystallizing chiral systems with and without
zwitterionic coformers, we suggest that for a pseudoquaternary cocrystal (made up of two
racemate compounds) to exist, the pseudoternary combinations (made up of one racemate and an
enantiomer of the second compound) should exist and the enantiomers of the two compounds
should form a diastereomeric pair at the binary level, rather than behave enantiospecifically.
*Corresponding author:
Tom Leyssens
Institute of Condensed Matter and Nanosciences
Université Catholique de Louvain
Place Louis Pasteur 1, bte L4.01.03
Bꢀ1348 LouvainꢀLaꢀNeuve
Tel: +32 10 47 2811 Fax: +32 10 47 27 07
tom.leyssens@uclouvain.be
http://www.uclouvain.be/leyssensꢀgroup
ACS Paragon Plus Environment
1

Crystal Growth & Design
Page 2 of 35
1
2
3
4
5
6
7
8
9
The peculiar case of Levetiracetam and
Etiracetam αꢀketoglutaric acid cocrystals:
obtaining a stable conglomerate of Etiracetam
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Fanny George †, Bernadette Norberg ‡, Koen Robeyns †, Johan Wouters ‡, Tom Leyssens †*
† Institute of Condensed Matter and Nanosciences, Université catholique de Louvain, 1348
LouvainꢀlaꢀNeuve, Belgium
‡ Unité de chimie physique, théorique et structurale, University of Namur, Namur, Belgium
Abstract
In this contribution, we demonstrate that it is possible to obtain the lactol tautomer of alphaꢀ
ketoglutaric acid (AKGA) in the solidꢀstate by cocrystallizing it with Leviteracetam, a chiral
nootropic drug used to treat epilepsy. Besides, we show that a cocrystal can be isolated with the
racemic equivalent of Levetiracetam, Etiracetam (Eti), in which AKGA stays in the ketoꢀform.
We also report the existence of a cocrystal conglomerate in the EtiracetamꢀAKGA system, which
is more stable than the racemic cocrystal at room temperature. The existence of a stable
conglomerate is put in relation with the enantiospecificity of the Levetiracetam cocrystals, which
is likely related to the ability of the Etiracetam enantiomers to stabilize one lactol tautomer at a
time in solution, or to promote its formation by Hꢀbonding. By comparing the peculiarities of the
system in hand to the general behavior of cocrystallizing chiral systems with and without
ACS Paragon Plus Environment
2

Page 3 of 35
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
zwitterionic coformers, we suggest that for a pseudoquaternary cocrystal (made up of two
racemate compounds) to exist, the pseudoternary combinations (made up of one racemate and an
enantiomer of the second compound) should exist and the enantiomers of the two compounds
should form a diastereomeric pair at the binary level, rather than behave enantiospecifically.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Introduction
Cocrystals are organic multicomponent crystals containing a stoichiometric ratio of at least two
components interacting through directional contacts, and that are not simple solvates or salts (at
least one component is non ionized).1 Cocrystals have been developed extensively in recent years
due to their interest in the pharmaceutical industry. Pharmaceutical cocrystals, containing an
active pharmaceutical ingredient (API) and one neutral component called coformer, may indeed
show very desirable properties in comparison with the isolated drug, such as improved solubility2
and dissolution rate,3 but also bioavailability,4 and handling properties.5,6 But their potential is
not restricted to drug formulation and other applications have been proven, including nonꢀlinear
optics,7 and chiral resolution,8–10 to name but a few.
αꢀKetoglutaric acid (AKGA) is a very interesting compound for two reasons. First, it is the
substrate of the enzymatic reaction producing Lꢀglutamate.11 Second, it is involved in three
different equilibria in solution11 due to its double identity.12 Indeed, it is a αꢀketocarboxylic acid
and a γꢀketo carboxylic acid at the same time and thus shows characteristics of both compounds:
being a αꢀketocarboxylic acid,13,14 it is prone to enolization and hydration, while its γꢀketo
carboxylic acid identity suggests an ability to cyclize into a lactol form (Eq.1). These forms thus
coexist in solution, in relative proportions that depend on many factors, including pH and
temperature of the solution. This has been the object of several researches.12 Various NMR
ACS Paragon Plus Environment
3

Crystal Growth & Design
Page 4 of 35
1
2
studies11,12 showed notably that, in aqueous solution, the nonꢀhydrated keto form is predominant
and the lactol form inexistant at neutral pH and room temperature, while their percentage have
been calculated to be comparable (35% and 30% resp.) at 29°C and pH 0. Besides, their
interconversion has been shown to be extremely rapid on the 13C NMR time scale.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Equation 1. Ketoꢀlactol and ketoꢀenol tautomerisms of AKGA in solution.
In the solidꢀstate, however, the cyclic form has never been isolated for this compound, contrary
to certain oꢀacylbenzoic acids.15 Indeed, there are only two entries for AKGA in the CSD16 and
in both, AKGA exists in its openꢀchain form. The first one (refcode COTPAC)17 is the pure
compound structure while the second corresponds to a complex with 1,3ꢀbis((Pyridꢀ2ꢀ
ylamino)carbonyl)adamantine (refcode RIZWUS).18
In this contribution, we demonstrate that it is possible to obtain the lactol form of AKGA by
cocrystallizing it with Leviteracetam (Levi, Figure 1), which is a chiral (S) nootropic drug used
to treat epilepsy. Besides, we show that a cocrystal can also be isolated with the racemic
equivalent of Levi, Etiracetam (Eti, RS), in which AKGA stays in the ketoꢀform. Last but not
least, we found that, depending on the experimental conditions, the racemic mixture (Eti +
AKGA) may crystallize as a conglomerate found to be more stable than the racemic cocrystal at
room temperature. To the author's best knowledge, this is only the second report of a cocrystal
conglomerate; the first one being characterized by Neurohr et al. in 2015.19
Conglomerates are indeed very rare in comparison with racemic crystals, occurring in only 5ꢀ
10% of racemic singleꢀcomponent crystallizations.20 But the probability of finding a
ACS Paragon Plus Environment
4

Page 5 of 35
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
conglomerate is expected to be even smaller in case of cocrystallization, as matching coformers
for a given compound are not yet predictable and often implies a highꢀthroughput screening
procedure.5,21–24 Yet, conglomerates are intensely researched as various chiral resolution
techniques, including Viedma ripening25–34 and preferential crystallization,35–38 are conditioned
by their existence.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. Chemical diagram of (S)ꢀLevi.
Experimental Section
Starting Materials. Sꢀ2ꢀ(2ꢀoxopyrrolidinꢀ1ꢀyl)butanamide (Levetiracetam) was purchased
from Xiamen Top Health Biochem Tech. Co., Ltd. 2ꢀKetoglutaric acid was purchased from
Acros Organics. These materials were used as received, without further purification. (RS)ꢀ2ꢀ(2ꢀ
oxopyrrolidinꢀ1ꢀyl)butanamide (Etiracetam) was prepared by racemization of Sꢀ2ꢀ(2ꢀ
oxopyrrolidinꢀ1ꢀyl)butanamide. Ten grams of Sꢀ2ꢀ(2ꢀoxopyrrolidinꢀ1ꢀyl)butanamide together
with a catalytic amount (0.05 equiv) of NaOMe was added to 10 mL of MeOH. The solution was
kept at reflux under continuous stirring for 24 h and then cooled to room temperature. The
compound crystallizes spontaneously. After filtration, the compound was washed twice with
MeOH. The recovered compound was used as such. Rꢀ2ꢀ(2ꢀoxopyrrolidinꢀ1ꢀyl)butanamide
cannot be purchased and was therefore obtained from Etiracetam using the chiral resolution
procedure described by Springuel et al.9 A solution containing Etiracetam, Rꢀmandelic acid and
acetonitrile in molar percentages of respectively 4.36, 6.63 and 89 mol%, was kept at ꢀ10°C and
seeded with the cocrystal formed between Rꢀ2ꢀ(2ꢀoxopyrrolidinꢀ1ꢀyl)butanamide and Rꢀ
ACS Paragon Plus Environment
5

Crystal Growth & Design
Page 6 of 35
1
2
3
4
5
6
7
8
9
mandelic acid. Under these conditions, this cocrystal is recovered, as it is the most stable phase
in suspension. After filtration, Rꢀ2ꢀ(2ꢀoxopyrrolidinꢀ1ꢀyl)butanamide was separated from Rꢀ
mandelic acid with a reverse HPLC system Waters Alliance 2690 equipped with a PDA detector
(Waters 2998). A Waters Atlantis T3 column (4.6mm x 50mm x 3.5 _m) has been used with
CH3CN/H2O 50/50 v/v as dilution solvent. Contrary to its (S)ꢀcounterpart, this compound does
not show any biological effect and hence does not have any common name. For clarity sake, it
will however be referred as (R)ꢀLevi in the following text. Besides, as it was at our disposal in
very small quantity, this compound was only used to demonstrate enantiospecicity in the Leviꢀ
AKGA system.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Cocrystal Screen. Cocrystals were synthesized by grinding of equimolar mixtures of Levi or
Eti and AKGA with or without a drop of solvent (see Table 1). Samples were ground in a
RETSCH Mixer Mill MM 400 with a beating frequency of 30 Hertz for at least 10 min (the
default grinding time being 90 minutes to ensure complete conversion). The resulting powders
were characterized using XRPD. Comparison of the resulting diffraction pattern with the
diffraction patterns of the pure phases was used to indicate cocrystal formation. All possible
known forms of the pure phases were considered, to avoid confusing cocrystal formation with
other phase transformations (e.g. solvate formation, polymorphism).
Xꢀray Powder Diffraction (XRPD). Xꢀray diffraction measurements were performed on a
Siemens D5000 diffractometer equipped with a Cu Xꢀray source operating at 40 kV and 40 mA
and a secondary monochromator allowing selection of the Kα radiation of Cu (λ =1.5418 Å). A
scanning range of 2θ values from 2° to 72° at a scan rate of 0.6° min−1 was applied.
Single Crystal Preparation. Single crystals were grown in acetone and/or acetonitrile (one
solvent at a time) using an initial nonꢀstoichiometric ratio of both coformers in solution.22,39,40
ACS Paragon Plus Environment
6

Page 7 of 35
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
First, suspensions of each component were prepared separately at room temperature. Then, a
similar volume of each supernatant solution was filtered and both solutions mixed, such that the
resulting solution has a concentration equal to half of the solubilities of each component. Slow
evaporating of about half the solvent volume initiated the selective crystallization of the
cocrystals with sufficient size.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Single Crystal Xꢀray Diffraction. Single crystal Xꢀray diffraction was performed either on a
Gemini Ultra R system (4ꢀcircle kappa platform, Ruby CCD detector) using Cu Kα radiation (λ
= 1.54184 Å) or Mo Kα radiation (λ = 0.71073 Å), or on a Mar345 image plate (Xenocs Fox3D
mirrors) using Mo Kα radiation. Cell parameters were estimated from a preꢀexperiment run and
full data sets collected at room temperature. The structures were solved by direct methods with
the SHELXSꢀ97 program and then refined on |F|2 using the SHELXLꢀ2014 software.41 The final
reported R1 value is calculated on |F| for observed reflections (I > 2 sigma(I)). Nonꢀhydrogen
atoms were anisotropically refined, and hydrogen atoms were placed at calculated positions and
refined in riding mode with isotropic temperature factors fixed at 1.2 times Ueq of the parent
atoms (1.5 times for methyl groups).
Slurry preparation. Supersaturated solutions of racemic EtiꢀAKGA 1:1 composition were
prepared in two different solvents (acetone and acetonitrile) and using alternatively two different
starting materials, for a total of four experiments. In the first two vials (one with each solvent),
the suspension was initially composed of EtiꢀAKGA 1:1 powder obtained by liquid assisted
grinding (conglomerate form), while in the last two vials (one with each solvent), the powder
resulted from dry grinding of the corresponding mixture (racemic form). These suspensions were
left to equilibrate overnight and then seeded with the alternative form (i.e. the form not used as
starting material) to test the potential conversion from the starting material to the seeded product,
ACS Paragon Plus Environment
7

Crystal Growth & Design
Page 8 of 35
1
2
3
4
5
6
7
8
9
in case of higher stability of the latter. Suspensions were stirred for one week at room
temperature before analyzing the solid phase by XRPD to determine the outcome.
Results
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Cocrystal identification
To determine the existence of cocrystals between Levi/Eti and AKGA, liquid assisted grinding
(LAG) of the corresponding materials was performed, as this type of experiments has been
shown to be among the most effective for cocrystal screening.42–44 In particular, it enables
polymorphism control6 while avoiding solubility restrictions. The resulting products were
analyzed by XRPD and presence of a new phase was detected in both conditions. Then, attempts
were made to grow the corresponding single crystals in solution for structural characterization as
described in the experimental part. Two novel cocrystal structures were identified. The one with
Eti contains AKGA in the keto form while the Levi cocrystal was formed with the lactol AKGA
tautomer. Even though cocrystallization has been shown to impact tautomerism,45 in our case,
the result is nevertheless surprising as the lactol tautomer has never been detected up to now in
the solidꢀstate.
Furthermore, whereas the XRPD diffractogram of the LeviꢀAKGA ground material overlapped
with the simulated diffractogram obtained from the single crystal, the simulated XRPD
diffractogram of the EtiꢀAKGA cocrystal did not match the XRPD pattern of the ground mixture
((EtiꢀAKGA_LAG). However, superposition of the diffractograms of LeviꢀAKGA and the
ground mixture EtiꢀAKGA_LAG revealed identical patterns (Figure 2), indicating the formation
of a conglomerate when performing liquid assisted grinding.
ACS Paragon Plus Environment
8

Page 9 of 35
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. XRPD patterns (up) simulated from the LeviꢀAKGA single crystal and (down) of
ground EtiꢀAKGA material; the similarity of patterns indicating conglomerate formation.
Dry grinding was also performed on these systems as it is well documented that neat and
liquidꢀassisted grinding may generate different outcomes.43 In our case, dry grinding of Eti with
AKGA indeed led to the racemic compound for which the single crystal was obtained. Neat
grinding of the Levi ꢀ AKGA mixture however led to the same product as the LAG experiment.
This latter observation implies that the tautomeric transformation does not require added solvent
to occur in grinding experiments.
Selectivity and stability analyses of the EtiꢀAKGA system
As a conglomerate was obtained in the case of liquid assisted grinding of EtiꢀAKGA, while
neat grinding led to the formation of the racemic compound, some complementary grinding
experiments were performed to assess the kinetics of the system and to evaluate how Eti is
dismantled into a conglomerate during liquid assisted grinding (i.e. direct transformation, or
intermediate transformation into the racemic compound which is then transformed into the
conglomerate). For this, the duration of the grinding experiments on Eti ꢀ AKGA 1:1 mixtures
was varied. Grinding was performed during 4 and 10 min with and without a drop of solvent. It
appeared that the conglomerate was totally formed after only 4 min of kneading, while the
ACS Paragon Plus Environment
9

Crystal Growth & Design
Page 10 of 35
1
2
3
4
5
6
7
8
9
conversion of the starting components into the racemic form through dry grinding was not
complete at this stage, but well completed after 10 min. In all cases, there were no traces of the
reciprocal forms at these smaller grinding times. Both forms are thus rapidly and selectively
generated.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
To establish the relative stability between the conglomerate and the racemic compound at
ambient temperature, slurry experiments46–49 were carried out. These consist in seeding a
suspension of a given composition with all possible phases. The initial suspension progressively
evolves toward the most stable phase, following Ostwald’s rule of phases. Analysis of the
resulting solid phase allows identification of the thermodynamically most stable phase. Solvents
with different dielectric constants are often used to vary the solubility profiles and increase the
chance of a quick and complete conversion.
In our case, we started by preparing separate suspensions of the conglomerate and the racemic
compound in acetone and acetonitrile at room temperature. Each suspension was then seeded
with the other solid state (i.e. the conglomerate suspension was seeded with the racemic
compound, and vice versa). Suspensions were then left to equilibrate for a oneꢀweek period. In
both cases, the solid phase contained only the conglomerate form, indicating the increased
stability of the conglomerate over the racemate at room temperature. This was also confirmed by
the fact that grinding the racemic compound with a drop of acetonitrile converts it into the
conglomerate.
However, their melting points were found to be almost identical (ca. 94°C), suggesting a very
slight stability preference in favor of the conglomerate. This situation corresponds to the binary
phase diagram shown on Figure 3.
ACS Paragon Plus Environment
10

Page 11 of 35
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. Binary phase diagram of a system characterized by a stable conglomerate (plain curves)
and a metastable racemic compound (dash curves).
A summary of all the experiments that were performed in order to characterize the Eti/Leviꢀ
AKGA cocrystals can be found in Table 1.
Table 1. Summary of the experimental outcomes of cocrystallization between Eti/Levi and
AKGA in various experimental conditions.
Starting materials
Experiments Conditions a
Products
drop MeCNb /MeOH powder
Solidꢀstate outcomes
Conglomerate
Racemic
Eti ꢀ AKGA 1:1 as solids
Grinding
Grinding
Evaporation
Slurry
dryc
powder
Eti ꢀ AKGA 1:1 in solution
Conglomerate as supension
Racemic as supension
Acetone
Single crystal
powder
Racemic
MeCN/Acetone
MeCN/Acetone
Conglomerate
Conglomerate
Slurry
powder
Starting materials
Experiments Conditions a
Products
powder
Solidꢀstate outcome
(S)ꢀLevi_(R)ꢀAKGA
(S)ꢀLevi_(R)ꢀAKGA
(S)ꢀLevi_(R)ꢀAKGA
(S)ꢀLevi ꢀ AKGA 1:1 solid
Grinding
Grinding
drop MeCN/MeOH
dry
powder
(S)ꢀLevi ꢀ AKGA 1:1 solution Evaporation
MeCN/Acetone
Single crystal
Starting materials
Experiments Conditions a
Products
powder
Solidꢀstate outcome
(R)ꢀLevi_(S)ꢀAKGA
(R)ꢀLevi_(S)ꢀAKGA
(R)ꢀLevi_(S)ꢀAKGA
(R)ꢀLevi ꢀ AKGA 1:1 solid
Grinding
Grinding
drop MeCN
dry
powder
(R)ꢀLevi ꢀ AKGA 1:1 solution Evaporation
MeCN
Single crystal
a For all experiments performed in solution or using a drop of solvent, we used one solvent at a
time. b Complete conversion of the starting components into the conglomerate was achieved after
4 min. c Conversion into the racemic form was not complete after 4 min, but well completed after
10 min.
ACS Paragon Plus Environment
11

Crystal Growth & Design
Page 12 of 35
1
2
3
4
5
6
7
8
9
Note that even if a stable conglomerate has been found for this system, it appears nonꢀideal for
use in chiral resolutions techniques such as Viedma ripening and preferential crystallization.
Indeed, the first method requires the chiral compound to be easily racemizable in solution, which
is not the case of Eti enantiomers. Concerning preferential crystallization, Coquerel carefully
explained that the existence of an easily isolable metastable racemic compound, which is the
case here, drastically reduces the performances of this method. Indeed, the similar stability of the
conglomerate and the racemic compound means that the heterochiral and homochiral interactions
are competitive in solution, which increases the probability of wrong docking and hinders the
process.35
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Structural analysis
The cocrystals of AKGA found with (S)ꢀ/(R)ꢀ and (RS)ꢀracetam were structurally
characterized through single crystal analysis. Their crystallographic parameters are displayed in
Table 2 and followed by a comparative analysis of the main bonding patterns existing in these
three cocrystals. Graphꢀsets were assigned to synthons that were judged to best represent the
structures, using Etter’s nomenclature.50
Table 2. Crystallographic Parameters of the three AKGA cocrystals
Coꢀcrystals
(RS)ꢀAKGA
(C8H14N2O2) (C8H14N2O2) (C8H14N2O2)
(C5H6O5) (C5H6O5) (C5H6O5)
(S)ꢀAKGA
( R)ꢀAKGA
Structural formula
Formula weight
(g/mol)
316,31
316,31
316,31
Space system
Space group
a (Å)
triclinic
monoclinic
P 21
monoclinic
P 21
Pꢀ1
5,5195(2)
11,1809(8)
13,5104(12)
112,193(8)
99,168(5)
95,746(5)
11,6406(8)
5,5396(2)
12,7812(9)
90
11,7707(15)
5,5329(4)
12,706(2)
90
b (Å)
c (Å)
α (°)
β (°)
115,940(9)
90
115,831(18)
90
γ (°)
ACS Paragon Plus Environment
12

Page 13 of 35
Crystal Growth & Design
1
2
3
4
5
V (Å3)
Z
750,326
2
741,152
744,812
2
2
6
7
8
9
Rꢀfactor (%)
Density
Radiation
4,43
1,42
Mo K(α)
150
6,67
1,42
Cu K(α)
293
7,34
1,42
Mo K(α)
293
Temperature (K)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Etiracetamꢀ αꢀketoglutaric acid (1 :1) ("EtiꢀAKGA" or "RSꢀAKGA"). The RSꢀAKGA coꢀ
crystal crystallizes from acetone in a triclinic system with space group Pꢀ1. The unit cell contains
two molecules of Eti and two molecules of AKGA as keto tautomer. An amideꢀcarboxylic acid
heterosynthon, described as R2 (8), is constituted between the carbamoyl of Eti and the
2
carboxylic acid function of AKGA adjacent to the ketone (Figure 4). In addition, two molecules
of Eti and two molecules of AKGA form a tetramer including two hydrogen bonds of the R2 (8)
2
heterosynthon mentioned above and two others binding the AKGA ketone to the carbamoyl trans
hydrogen of Eti. This tetramer may be characterized by either an “inner” R4 (14) ring motif or an
4
“outer” R4 (18) ring motif (Figure 4 middle and right), depending on the R2 (8) portion included.
4
2
Finally, the second carboxylic group of AKGA is hydrogen bonded to the carbonyl of the cyclic
amide (oxopyrrolidin) of Eti, forming a dimer oriented along the cꢀaxis. However, as there is one
extra acceptor in comparison with the total number of donors (two donors on each partner), the
second carbonyl of AKGA is not involved in any strong Hꢀbond. Each molecule of the coꢀcrystal
is thus involved in four Hꢀbonds.
Figure 4. Hydrogen bonds network in RSꢀ AKGA cocrystal showing all types of interactions
(left), the inner R4 (14) ring (middle) and the outer R4 (18) ring motifs (right).
4
4
ACS Paragon Plus Environment
13

Crystal Growth & Design
Page 14 of 35
1
2
3
4
5
6
7
8
9
These motifs form stepwise columns directed along the cꢀaxis and stacked along a and bꢀaxes
(Figure 5). Columns are twoꢀmolecules wide in the bꢀdirection, with ethyl groups of Eti
molecules pointing outward and facing equivalent groups in neighbouring columns.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 5. Columnar Stacking in RSꢀ AKGA cocrystal (a) in the bcꢀplane and (b) showing the
steps in the columns.
(S)ꢀLevetiracetamꢀ αꢀketoglutaric acid (1 :1) ("SꢀLevi_RꢀAKGA" or "SꢀAKGA"). The Sꢀ
AKGA coꢀcrystal was successfully grown from both acetone and acetonitrile, in a monoclinic
system with space group P21. The unit cell contains two molecules of Levi and two molecules of
AKGA in lactol configuration with (R)ꢀchirality exclusively.
Hydrogen bonding patterns in this cocrystal may be described by a R2 (9) ring and one C2 (11)
2
2
chain (Figure 6). The ring motif is formed by the carbamoyl (C=O and adjacent H) of a first
molecule of Levi with the carboxylic acceptor and adjacent hydroxyl of AKGA. The pyrrolidone
carbonyl of a second molecule of Levi then accepts the carboxylic donor of AKGA, which in
turn interacts with the trans carbamoyl hydrogen of a third molecule of Levi. This constitutes a
chain that involves two parallel molecules of Levi and propagates along the bꢀaxis.
Hence, as for the (RS)ꢀAKGA cocrystal, each molecule takes part in four Hꢀbonds. But while
all donor and acceptor atoms of Levi are involved in Hꢀbonding, the ester function of AKGA
ACS Paragon Plus Environment
14

Page 15 of 35
Crystal Growth & Design
1
2
3
4
5
6
remains unoccupied. This is due to the bifurcation of the AKGA carboxyl that makes the ester
acceptors redundant.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. View of main Hꢀbondings in (S)ꢀAKGA cocrystal showing the R2 (9) ring motif and
2
the C2 (11) chain involving two parallel molecules of Levi.
2
On the whole, SꢀAKGA exhibits a columnar stacking. All the hydrogen bonds are located
inside the columns, forming a complicated and interlocked network. Columns are directed along
the bꢀaxis and stacked in the two other directions (Figure 7). In a column, ester groups of AKGA
and hydrocarbonated groups of Levi point outward, such that Levi ethyl groups on one column
interact with ester or ethyl groups from other columns, holding them together.
Figure 7. Columnar stacking along bꢀaxis in (S)ꢀAKGA cocrystal, showing CꢀH..O contacts
(yellow) between the Levi ethyl and the AKGA ester of different columns.
(R)ꢀLevetiracetamꢀ αꢀketoglutaric acid (1 :1) ("RꢀLevi_SꢀAKGA" or "RꢀAKGA"). This
cocrystal crystallizes from acetonitrile in a monoclinic system with space group P21. As it is the
ACS Paragon Plus Environment
15

Crystal Growth & Design
Page 16 of 35
1
2
3
4
5
6
7
8
9
mirror image of the (S)ꢀLevi_(R)ꢀAKGA cocrystal, it differs from this latter by the presence of
AKGA in the (S)ꢀenantiomeric lactol form. Structural parameters are almost identical to those of
the (S)ꢀLevi cocrystal. Identical hydrogen bonding motif and tridimensional patterns are
observed.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Discussion
In view of these observations, three questions arose. First, why was it possible to isolate
AKGA in lactol form at the solid state in presence of Levi? Second, why did we not find any
traces of a cocrystal form involving (S)/(R)ꢀLevi and AKGA in the keto form? Finally, is there
any apparent reason behind the formation of a stable conglomerate in this case? Several elements
may be considered to address these questions and will be discussed sequentially.
Lactol formation
A way to rationalize AKGA cyclization in the enantiopure cocrystals consists in paying
attention to its mechanism. In particular, one has to consider the two types of factors that
influence the intramolecular addition of the carboxyl to the carbonyl, leading to the cyclization.51
The first type is electronic and refers to the presence of groups adjacent to the ketone and/or the
γꢀcarboxy group that may affect their electroꢀ/nucleoꢀphilic character respectively and/or
induce/prevent resonance stabilization. The second type is geometric and concerns the proximity
in space of these two functional groups.52 These factors were illustrated by Jones and Desio to
justify the predominance of one isomer (acid or ester) in function of the nature and position of
substituents in oꢀacylbenzoic acids.15 Similarly, Winston et al.53 found that for lactones prepared
by trichloromethylation of anhydrides, the stability of the lactone form in solution depends on
ACS Paragon Plus Environment
16

Page 17 of 35
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
the distance between acids and trichloromethyl groups in the ketoꢀforms. They also evoked
internal rotation preventing coplanarity and ringꢀchain tautomerism in nonꢀcyclic compounds.
Hence, if AKGA is able to cyclize, it is due to the fact that it possesses a carboxylic acid
function in both alpha and gamma positions of the ketone. Indeed, the one in alpha increases the
electrophilic character of the ketone while the one in gamma makes the 5ꢀmembered lactol stable
in certain conditions. The reason for the absence of a crystal structure of pure AKGA as lactol
may be due to its conformational flexibility. Indeed, there exists several cyclic γꢀketoꢀcarboxylic
acids crystallized in the ringꢀform due to their geometrical restrictions.15,51
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
However, when Levi (or its Rꢀequivalent) is added to the solution, it is suspected to selectively
stabilize one enantiomer of the lactol form or promote its formation through Hꢀbonding (Figure 8
a and b resp.); which thus influences the corresponding equilibrium. This was already suggested
by Valter concerning the solvation effect on γꢀaldehydoꢀ and γꢀketoꢀcarboxylic acids:
"intermolecular hydrogen bonds may play an important part in stabilizing a particular form, the
cyclic isomer showing a greater tendency to form such bonds involving both carbonyl and
hydroxyl groups".51 The exact mechanism is nonetheless rather difficult to ascertain as
cocrystallization from solution is complex and involves different equilibria,39,40 and was thus not
investigated here.
(a)
(b)
ACS Paragon Plus Environment
17

Crystal Growth & Design
Page 18 of 35
1
2
3
4
5
6
7
8
9
Figure 8. Chemical diagrams showing potential hydrogen bonds occurring in solution that could
hypothetically (a) block AKGA in the lactol form or (b) promote its formation by activating the
ketone for the intraꢀmolecular attack by the carboxylic acid in gamma.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Absence of LeviꢀAKGA cocrystal with ketoꢀAKGA
Another interesting feature of this work, concerns the fact that thus far we observed no
cocrystal of Levi with the keto form of AKGA, whereas this form does appear when
cocrystallizing with Eti. It could be that such form exists and that we simply did not encounter it
yet. Even though one could argue that this cocrystal should exist under certain conditions, some
structural considerations suggest a decreased stability of the potential LeviꢀAKGA(keto)
cocrystal when compared to the existing racemic equivalent.
These considerations arise from the analysis of the Etiꢀ/Leviꢀcocrystals with oxalic acid (OXA)
("RSꢀOXA" and "SꢀOXA", refcodes XOGPAM and XOGPEG resp.).54 OXA and AKGA form
similar Hꢀbonding pattern with Eti in their respective cocrystals (Figure 9), due to their structural
resemblance. Besides, OXA also successfully cocrystallizes with Levi, giving us an idea of what
the LeviꢀAKGA cocrystal with ketoꢀAKGA would look like. Hence, one could expect
conclusions drawn from a (RS)ꢀOXA and (S)ꢀOXA comparison to be reasonably valid for the
(RS)ꢀ and (S)ꢀAKGA system.
Figure 9. Hydrogen bonding rings in (S)ꢀOXA (left), (RS)ꢀOXA (middle) and (RS)ꢀAKGA
(right).
ACS Paragon Plus Environment
18

Page 19 of 35
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
(RS)ꢀOXA and (S)ꢀOXA cocrystals are characterized by identical Hꢀbonding patterns (Figure
9), which results in relatively similar PXRD patterns. But, as the first cocrystal is racemic and
the other one chiral, their arrangements must differ in some ways. In fact, the main difference
between the two cocrystals lies in the conformation of the racetam. The (S)ꢀOXA cocrystal has
twice the number of molecules per asymmetric unit than the (RS)ꢀOXA cocrystal, with the two
symmetryꢀindependent molecules of Levi adopting different conformations. In the first
conformation, the hydrogen atom on the asymmetric carbon points in the same direction as the
oxopyrridin carbonyl (“cisꢀconformation”) while this is contrary in the second conformation
(“transꢀconformation”) (Figure 10). Doing so, the crystal structure of (S)ꢀOXA mimics the
centrosymmetric group P21/c adopted by the (RS)ꢀOXA cocrystal, with a pseudocenter of
inversion relating the two independent molecules of Levi and a pseudo glide plane c. This
imitation attempt suggests a particularly desirable and thus efficient packing of the racemic
crystal.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(a)
(b)
Figure 10. Cisꢀ and transꢀconformations of Levi in (S)ꢀOXA cocrystal (left and right resp.).
Besides, among the 17 structures with either (S)ꢀLevi or Eti recorded in the CSD (2015
release) (including the OXA cocrystals), the racetam always adopts the cisꢀconformation; except
in (S)ꢀOXA where both are encountered. The cisꢀconformation is thus expected to be more stable
ACS Paragon Plus Environment
19

Crystal Growth & Design
Page 20 of 35
1
2
3
4
5
6
than the transꢀconformation, as confirmed by their computed singleꢀpoint energies showing the
cisꢀconformation to be 14 kJ mol ꢀ1 lower in energy.*
7
8
9
Moreover, the situation is highly similar to the one observed in the structures of the racemic
and chiral theophylline – malic acid (tpꢀma) cocrystals as described by Friscic et al.55 In this
system, the racemic and chiral cocrystals also show similar PXRD patterns and Hꢀbond
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
topologies. Besides, the same R4 (18) motif is encountered in the racemic (tp).(DLꢀma) cocrystal
4
as in the (RS)ꢀAKGA and (RS)ꢀOXA cocrystals (Figure 10, left). However, as the asymmetric
carbon of malic acid molecules is included in the Hꢀbonded ring motif, a conformation change in
malic acid molecules is not sufficient to accommodate a chiral space group, as observed for the
LeviꢀOXA cocrystal. In this case, the packing that most mimics the racemic (tp).(DLꢀma)
cocrystal involves different conformations and different connectivities (i.e. the two symmetryꢀ
independent molecules of malic acid do not interact with theophylline through the same
moieties), which results instead in a R4 (19) motif (Figure 11, right). The two conformations
4
ꢀ1
were also calculated to have a 15 kJ mol energy difference and the presence of the less stable
conformer in (tp).(Dꢀma) was suggested to account for the difference in hydration stabilities and
thermal behaviour between the racemic and chiral cocrystals.
*
Single point energies were computed using the Gaussian series program with B3LYP/6ꢀ31G(d,p) method, and provided a
difference of 14.27 kJ molꢀ1 between the two conformations.
ACS Paragon Plus Environment
20

Page 21 of 35
Crystal Growth & Design
1
2
Figure 11. R4 (18) and R4 (19) ring motifs resp. in racemic (left, refcode CIZTAH) and chiral
3
4
4
4
5
6
(right, refcode COCDOO) theophylline – malic acid cocrystals.
7
8
9
Hence, one might expect that the conformational features of Levi molecules in the virtual
Leviꢀ AKGA(keto) cocrystal would destabilize the overall structure, in comparison with the Etiꢀ
AKGA(keto) cocrystal. Indeed, the energetic difference between the two conformations is of the
same order of magnitude than the usual stabilization energy of cocrystals estimated by ab initio
studies.56–58 And the competition with the Leviꢀ AKGA(lactol) form may make the keto version
undetectable.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Besides, this analysis confirms the relevance of considering, for the design of multicomponent
crystals, larger synthons than the ones commonly used.59 In this case, the famous R2 (8) motif is
2
indeed little representative of the overall structures while the recurrence of the R4 (18) motif in
4
both chiral and racemic cocrystals seems to indicate a very efficient packing and possibly a
greater predictive power of this motif.
Conglomerate increased stability
The last point of this analysis concerns the existence of a conglomerate in this system and the
origin of its higher stability.
A racemic solution may crystallize, by decreasing order of probability, as a racemic
compound, a conglomerate or a solid solution. In a racemic compound, both enantiomers are
present in the same quantity in the crystal lattice. In a conglomerate, the two enantiomers
crystallize independently (i.e. they form homochiral crystals). In a solid solution for various
compositions, both enantiomers are distributed at random in the same crystal network. Solid
solution formation is rare for organic compounds and will not be considered in the following
discussion.
ACS Paragon Plus Environment
21

Crystal Growth & Design
Page 22 of 35
1
2
3
4
5
6
7
8
9
Conglomerates are statistically disadvantaged in comparison with racemic compounds for two
reasons.60 The first one is thermodynamic. There are simply less packing arrangements available
to accommodate chiral crystals. In consequence, as nicely expressed by Brock et al., "it seems
likely that the best of many possible racemic packing arrangements is to be preferred to the best
of fewer possible chiral arrangements". The second reason concerns the crystallization kinetics.
In a racemic solution, the rate of formation of chiral crystals is reduced in comparison with the
nucleation of racemic crystals. Indeed, only half of the molecules (the "right" enantiomer) that
come in contact with an existing chiral cluster will be suited for its development, whereas
molecules of both enantiomers will find a matching site on the racemic clusters. This contrasts
with polymorphism, for which less stable forms are usually kinetically favored, as poorer
arrangements may be generated more easily. Conglomerates are thus often thermodynamically
penalized and not really preferred kinetically either.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Conglomerates are thus expected to spontaneously occur only when their stability is favorable
or when the energy difference between the racemic compound and the conglomerate is relatively
small. This may be the case when the two structures display a high level of packing similarities,
such as homochiral columns or layers, and thus comparable densities.
This is however, not what happens in the system described here. Indeed, the racemic
compound and the conglomerate found for this system differ by the nature of AKGA tautomer
and are thus not directly related neither structurally, nor energetically. In fact, it is preferable to
virtually separate this system (Eti and AKGA) into two systems (Figure 12): one with AKGA in
the keto form on one hand and one with AKGA in the lactol form on the other. In each system,
the observed form (i.e. racemic compound or conglomerate) is more stable than the other
possible outcome to such extent that the latter is not formed. The greater stability of the racemic
ACS Paragon Plus Environment
22

Page 23 of 35
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
compound in the first system involving a ketoꢀAKGA has indeed been suggested in the previous
section and is likely due to the reduced stability of the hypothetical enantiopure crystals
involving ketoꢀAKGA. Concerning the second system (lactolꢀAKGA), we shall refer to our
earlier work on enantiospecificity.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 12. Schematic representation of the two virtual systems featuring Eti and AKGA in
solution, and their respective outcomes in the solidꢀstate.
Two optically active compounds cocrystallize enantiospecifically when only one of the two
enantiomers of the first pair (RS) cocrystallizes with an enantiomer of the second pair
(distinguished as DL). This behaviour is represented by the binary level of situation A in Table 3
and may be opposed to the formation of diastereomeric pairs, which involves both enantiomers
cocrystallizing with the chiral coformer (binary level of situation B in Table 3). A recent
contribution showed that enantiospecific cocrystallization is frequent between two optically
active compounds in contrast with chiral salts, which usually form diastereomeric pairs.61
To explain this difference between cocrystals and salts, Springuel and coworkers61 invoked the
very weak stabilization energy of cocrystals with respect to salts. Indeed, it has been estimated
by ab initio calculations that the energy difference between a cocrystal and its separate
ACS Paragon Plus Environment
23

Crystal Growth & Design
Page 24 of 35
1
2
3
4
5
6
7
8
9
components is often inferior to 10 kJ/mol (which is similar to the energy difference between
polymorphs),56–58 while the stabilization free energy may amount to 400 kJ/mol in case of salt
formation. For that matter, cocrystals are very sensitive to minor changes in secondary
interactions (πꢀstacking, Van der Waals contacts...), as the one induced by a change of chirality,
even when similar Hꢀbonding occurs. Systems involving zwitterionic coformers lie inꢀbetween,
as chargeꢀassisted interactions are stronger than conventional Hꢀbonds, but weaker than ionic
interactions. In practice, they usually form diastereomeric pairs as well.62 But one could also
evoke the position of the chiral center with respect to the functional groups potentially
interacting, to account for cocrystal enantiospecificity. For example, Hꢀbonding motifs including
the chiral center or close to it may be more sensitive to the steric profile of the coformer and be
allowed with only one of its two enantiomers.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
These authors also studied cocrystallization between racemic compounds and in particular the
likelihood of forming pseudoternary (i.e. one racemic compound with one chiral coformer) and
pseudoquaternary (i.e. two racemic compounds) cocrystals.10 They analyzed nine different
systems forming at least one binary cocrystal, either in an enantiospecic manner (7/9) or as a
diastereomeric pair (2/9). Among these, they found only two pseudoternary cocrystals and one
pseudoquaternary cocrystal. The system forming a pseudoquaternary cocrystal (Eti with
methylsuccinic acid) is also one of the two systems for which a ternary cocrystal is formed, as
well as for which a diastereomeric pair is observed. In other words, a pseudoquaternary cocrystal
was encountered only when all the binary and ternary combinations were also successful (Table
3, situation B).
Table 3. Some possible interactions between two racemates (RS and DL) or the different
combinations of their enantiomers.
ACS Paragon Plus Environment
24

Page 25 of 35
Crystal Growth & Design
1
2
3
4
5
Binary
Enantiospecific
Ternary
Quaternary
6
7
8
9
A
B
R + D
R + L
ꢀ
RD
/
R + DL
ꢀ
/
RS + DL
RS + DL
ꢀ
/
ꢀ
Diastereomeric
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
R + D
ꢀ
RD
R + DL
D + RS
ꢀ
RDL
DRS
ꢀ
RSDL
R + L
ꢀ
RL
ꢀ
To rationalize this behaviour, one may proceed in stages. Ternary cocrystals are expected to be
less likely with a chiral coformer than with an achiral one,54 as the pseudoternary cocrystal RDL
cannot adopt a centrosymmetric packing. This is the contrary in case of an achiral coformer A as
for this latter the pseudoternary cocrystal ARS has high chances to display such an arrangement.
Besides, pseudoternary cocrystallization would be more likely when one enantiomer of the first
pair doesn't have a strong preference for an enantiomer of the second pair (i.e. when there is no
enantiospecific cocrystallization at the binary level) and rather forms satisfactory interactions
with both enantiomers. Hence the occurrence of pseudoternary cocrystals may suggest a
preponderant role of the interactions in these systems and less strict requirements on packing. In
these flexible systems, pseudoquaternary cocrystals are thus expected to be more likely as well.
This could explain why there is no racemic cocrystal of Eti with AKGAꢀlactol. Indeed, AKGA
is chiral in the lactol form and the corresponding racemic cocrystal could thus be considered as a
pseudoquaternary cocrystal. Yet there are no pseudoternary cocrystals in this system, as this
would imply for either Eti to crystallize with one of the lactol forms of AKGA or Levi with both
lactol forms of AKGA, which is not observed. Furthermore, one can also consider that the binary
combinations are enantiospecific, as Levi prefers to crystallize with the Rꢀform of AKGA (and
reciprocally for the (R)ꢀLevi); even if in practice the enantiomers of AKGAꢀlactol were not
introduced separately but created in situ. In fact, enantiospecificity in this case is probably due to
the cyclization mechanism of AKGA. All these outcomes are summarized in Table 4.
ACS Paragon Plus Environment
25

Crystal Growth & Design
Page 26 of 35
1
2
3
4
Table 4. Crystalline outcomes of the different combinations of enantiomers of Eti and AKGAꢀ
5
6
lactol.
7
8
9
Binary
Ternary
Quaternary
Eti + (RS)-AKGA ꢀ /
Eti + (R)-AKGA
ꢀ
/
Enantiospecific
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(S)-Levi + (R)-AKGA
ꢀ
ꢀ
(S)-Levi_(R)-AKGA Eti + (S)-AKGA
ꢀ
/
(S)-Levi + (S)-AKGA
/
(S)-Levi + (RS)-AKGA ꢀ /
Even though this analysis is based on a very limited number of cocrystallizing systems, it can
be placed in parallel with systems involving zwitterionic coformers that easily form
diastereomeric pairs and which in consequence easily generate all higher order combinations.63
Hence the fact that a conglomerate has been easily isolated in this system should be put in
relation to the ability of AKGA to cyclize in solution and likely to the ability of the enantiomers
of Eti to stabilize the lactol tautomer. Even though the peculiarities of this system are not very
likely to occur for many other systems, they do show that formation of a conglomerate through
cocrystallization is a possibility.
Besides, the conglomerate being more stable than the racemic compound should also be taken
with care, as the ringꢀchain tautomerism equilibrium has been proven to depend on the
temperature, nature of the solvent and pH. The stability ranking and the ease of formation of the
conglomerate with respect to the racemic cocrystal may therefore be affected by varying these
parameters. A full study allowing for a comprehensive rationalization of this phenomenon goes
however beyond the scope of this work.
Conclusion
ACS Paragon Plus Environment
26

Page 27 of 35
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
In this contribution, we report three novel cocrystal structures involving AKGA and (S)ꢀ/(R)ꢀ
Levi or (RS)ꢀEti. Surprisingly, the Levi cocrystals are formed with the lactol tautomer of AKGA,
which has never been isolated in the solidꢀstate up to now. These cocrystals may be generated by
neat or liquidꢀassisted grinding; indicating that solvent is not required for the tautomeric
equilibrium to take place. On the contrary, the presence of Levi in the medium is suspected to
influence the equilibrium by selectively stabilizing one enantiomer of the lactol form at a time
(depending on the nature of the Levi enantiomer) or promote its formation by Hꢀbonding.
Concerning the EtiꢀAKGA system, it was established that two forms may be selectively
isolated, depending on the experimental conditions. The first one is a racemic cocrystal, in which
AKGA is present as the keto form and the second one is a cocrystal conglomerate, which
corresponds to a physical mixture of the two (S)ꢀ/(R)ꢀLevi ꢀ (R)ꢀ/(S)ꢀAKGA(lactol) cocrystals,
and which was proven to be more stable than the racemic compound at ambient temperature.
By comparison with two other related systems, it was suggested that the absence of any Leviꢀ
AKGA cocrystal with the keto tautomer of AKGA is due to the presence of an unfavorable
conformation of the racetam molecules in the potential cocrystal, which should significantly
reduce its stability in comparison with the existing racemic compound.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Finally, the existence of a stable conglomerate in this system was put in relation with the chiral
nature of the AKGA lactol form and the enantiospecificity of the Levi cocrystals, which is likely
related to the ability of the Eti enantiomers to stabilize the lactol tautomer in solution. In
particular, it was suggested that for a pseudoquaternary cocrystal (i.e. cocrystal made up of two
racemate compounds) to exist, the pseudoternary combinations (i.e. cocrystal made up of one
racemate and an enantiomer of the second compound) should be isolable and the enantiomers of
ACS Paragon Plus Environment
27

Crystal Growth & Design
Page 28 of 35
1
2
3
4
5
6
the two compounds should form a diastereomeric pair at the binary level, rather than behave
enantiospecifically.
7
8
9
Supporting information. Structures described in this contribution have been deposited at the
Cambridge Crystallographic Data Centre and allocated the deposition numbers 1482522 (Rꢀ
Levi_SꢀAKGA), 1482523 (SꢀLevi_RꢀAKGA) and 1482524 (Eti_AKGA). DSC curves
corresponding to the two cocrystal forms obtained from the racemic EtiꢀAKGA mixture (the
racemic compound and the conglomerate) are also provided. This material is available free of
charge via the Internet at http://pubs.acs.org.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Corresponding Author
* Email : tom.leyssens@uclouvain.be.
Aknowledgments. The authors would like to thank the UCL and FNRS (PDR T009913F,
T016913, FRIA grant) for financial support, Natalia Tumanova for fruitful discussions and Ricky
Payen for DFT calculations.
References
(1)
Aitipamula, S.; Banerjee, R.; Bansal, A. K.; Biradha, K.; Cheney, M. L.; Choudhury, A.
R.; Desiraju, G. R.; Dikundwar, A. G.; Dubey, R.; Duggirala, N.; Ghogale, P. P.; Ghosh,
S.; Goswami, P. K.; Goud, N. R.; Jetti, R. R. K. R.; Karpinski, P.; Kaushik, P.; Kumar, D.;
Kumar, V.; Moulton, B.; Mukherjee, A.; Mukherjee, G.; Myerson, A. S.; Puri, V.;
Ramanan, A.; Rajamannar, T.; Reddy, C. M.; RodriguezꢀHornedo, N.; Rogers, R. D.;
Row, T. N. G.; Sanphui, P.; Shan, N.; Shete, G.; Singh, A.; Sun, C. C.; Swift, J. A.;
Thaimattam, R.; Thakur, T. S.; Kumar Thaper, R.; Thomas, S. P.; Tothadi, S.; Vangala, V.
R.; Variankaval, N.; Vishweshwar, P.; Weyna, D. R.; Zaworotko, M. J. Cryst. Growth
ACS Paragon Plus Environment
28

Page 29 of 35
Crystal Growth & Design
1
2
3
4
Des. 2012, 12, 2147–2152.
5
6
7
8
9
(2)
(3)
Schultheiss, N.; Bethune, S.; Henck, J.ꢀO. CrystEngComm 2010, 12, 2436.
Gagniere, E.; Mangin, D.; Puel, F.; Rivoire, A.; Monnier, O.; Garcia, E.; Klein, J. P. J.
Cryst. Growth 2009, 311, 2689–2695.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(4)
(5)
(6)
(7)
Jung, M.ꢀS.; Kim, J.ꢀS.; Kim, M.ꢀS.; Alhalaweh, A.; Cho, W.; Hwang, S.ꢀJ.; Velaga, S. P.
J. Pharm. Pharmacol. 2010, 62, 1560–1568.
Remenar, J. F.; Morissette, S. L.; Peterson, M. L.; Moulton, B.; MacPhee, J. M.; Guzmán,
H. R.; Almarsson, Ö. J. Am. Chem. Soc. 2003, 125, 8456–8457.
Trask, A. V.; Motherwell, W. D. S.; Jones, W.; Samuel Motherwell, W. D.; Jones, W.
Cryst. Growth Des. 2005, 5, 1013–1021.
Huang, K.ꢀS.; Britton, D.; Margaret, L.; C. Etter, T.; Byrn, S.; R. J. Mater. Chem. 1997, 7,
713.
(8)
(9)
Springuel, G.; Leyssens, T. Cryst. Growth Des. 2012, 12, 3374–3378.
Springuel, G.; Collard, L.; Leyssens, T. CrystEngComm 2013, 15, 7951.
(10) Springuel, G. Chirality and cocrystal systemsꢁ: from fundamental understanding to
development of a novel industrial chiral resolution technique, Université catholique de
Louvain, 2014.
(11) Viswanathan, T. S.; Johnson, R. E.; Fisher, H. F. Biochemistry 1982, 21, 339–345.
(12) Cooper, A. J. L.; Redfield, A. G. J. Biol. Chem 1975, 250, 527–532.
ACS Paragon Plus Environment
29