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Z. Li et al. / Journal of Molecular Liquids 288 (2019) 111055
investigation of reaction mechanism could provide valuable informa-
tion about structural and electrical properties, which is conducive to un-
derstand structural movements in the reaction process and provide
guidelines for reaction optimization. Hence, the mystery of racemiza-
tion could be decrypted with the help of thorough understanding of ra-
cemization mechanism and chiral stability of levetiracetam. However,
no racemization mechanism for levetiracetam has been presented so
far.
(20.2 g) was added first with vigorous stirring for 10 min. After that,
the solution of 4-chlorobutyryl chloride (12.2 g) in dichloromethane
(45 mL) was slowly added to maintain the temperature under −10 °C.
The reaction was stirred vigorously at −10 °C for further 45 min before
adding another batch of potassium hydroxide and 4-chlorobutyryl chlo-
ride. After adding total three batches, the reaction mixture was stirred at
−10 °C for over 2 h. Additional potassium hydroxide (6.6 g) was added
into the reaction mixture followed by stirring for 4 h. Then, the insoluble
substance was filtered and washed by dichloromethane (10 × 3 mL).
The organic layer was combined and neutralized by acetic acid. The un-
wanted salt was filtered and washed by dichloromethane (10 × 3 mL).
The combined organic layer was dried by anhydrous sodium sulfate
and then filtered. Dichloromethane was evaporated under reduced
pressure followed by the addition of ethyl acetate (150 mL). The solu-
tion was concentrated to over 75 mL to afford slurry. The slurry was
stirred at −5 °C for an hour. The precipitate was filtered and washed
by cold ethyl acetate (10 × 3 mL), and then dried under a vacuum at
30 °C to give 27.5 g of crude product. The crude product was recrystal-
lized in isopropanol (40 mL) to afford refined product of levetiracetam
as white solid (22.3 g, yield 60.5%). purity: 99.67%; ee: 99.94%; m.p.:
116.0 °C–118.0 °C; [α]2D5: −90.0° (c = 1, EtOH); HRESIMS m/z:
193.0948 [M Na] (calcd. for C8H14O2N2Na, 193.0947); 1H NMR
(600 MHz, CDCl3): δ 6.98 (s, 1H), 6.61 (s, 1H), 4.52 (t, J = 5 Hz, 1H),
3.51–3.55 (m, 1H), 3.37–3.41 (m, 1H), 2.43 (t, J = 8.5 Hz, 2H),
2.02–2.07 (m, 2H), 1.93–1.98 (m, 1H), 1.65–1.70 (m, 1H), 0.89 (t, J =
7.5 Hz, 3H); 13C NMR (CDCl3, 150 MHz): δ 175.7, 172.6, 55.5, 43.4,
30.7, 21.0, 17.8, 10.2; ECD revealed a negative cotton effect at around
223 nm, which was consisted with reported ECD behavior of
levetiracetam.
Benefitted from the development of computational chemistry, vari-
ous reaction mechanisms could be precisely simulated to give special
insights for estimating the practicability of reaction [16–18]. In this arti-
cle, the racemization mechanism for levetiracetam is firstly illustrated
by theoretical calculation. Density functional theory (DFT) [19], a widely
utilized method for simulating reaction mechanism, was implemented
to elucidate the racemization process of levetiracetam. Firstly, basic
structural movements were determined in order to describe the racemi-
zation process on continuous potential energy surface. Then, the race-
mization for isolated levetiracetam was carried out as fundamental
mechanism to understand the structural movements during racemiza-
tion process. Further, according to the observation of racemization in
base condition, hydroxide ion was introduced to understand the charac-
ter of it in racemization. Moreover, frontier molecular orbitals (FMOs)
were carried out to describe the transfer of electron density in complex
structure during racemization [20]. Additionally, the importance of hy-
droxide ion was further characterized by kinetic experiment. Subse-
quently, the influence factors for racemization were elucidated
according to the theoretical mechanism and experimental observation.
The racemization was efficiently controlled after modifying the syn-
thetic procedure based on the suggestions. The aim of this article is
not only to establish the theoretical models in racemization process
for levetiracetam in isolated form and associated with hydroxide ion,
but also provide guidelines for reducing racemization in synthesis of le-
vetiracetam and its derivatives.
3. Results and discussion
3.1. Analysis of basic structural movements in racemization process
2. Materials and methods
In order to facilitate the description, the 2D structure of levetirace-
tam is presented in Fig. 1 with selected atoms numbered. Reaction pro-
cess could be described as the continuous movements on potential
energy surface. Therefore, it's important to determine the structural
movements which are essential in elucidating the process. In this case,
the most stable conformer for levetiracetam and its R-enantiomer was
obtained as the initial and final structure in the racemization process,
shown in Fig. 2 (conformer S1 and R1). The racemization process from
S1 to R1 requires two kinds of structural movements: 1) the proton
H19 should transfer from one side of chiral carbon C7 to the other,
which refers to a proton abstraction followed by a readdition process;
and 2) the conformational isomerization should occur to achieve the
stable conformer of the other enantiomer after racemization.
2.1. Computational method
Firstly, conformational distribution analysis for levetiracetam and its
R-enantiomer was conducted by Spartan 14 program using molecular
mechanics force field (MMFF94) [21,22]. Considerable conformers for
each enantiomer were obtained within 10 kJ mol−1. These conformers
were further optimized using Gaussian 09 program package [23]. DFT
method, especially the Becke 3–Lee–Yang–Parr (B3LYP) exchange-
correlation functional [19,24], is commonly implemented for elucidat-
ing structural properties and reaction mechanism. Thus, the structural
optimization was performed at B3LYP/6-311 G (d, p) level. And the ge-
ometries of transition states were obtained by means of the Synchro-
nous Transit-guided Quasi-Newton (STQN) method. The
thermodynamics parameters were obtained by vibrational frequencies
calculation which was conducted at the same level. The nature of corre-
sponding stationary points (minima or transition state) was confirmed
according to the number of imaginary frequency (0 or 1). Furthermore,
the intrinsic reaction coordinate (IRC) path was performed to verify that
the transition states connect to the reactant and product of the proposed
mechanism [25]. Additionally, for the consideration of solvent effect,
polarized continuum model (PCM) method was utilized with the as-
signment of dichloromethane as solvent [26].
3.1.1. Proton transfer
The proton from chiral center transferring from one side to the other
is crucial process in the chiral conversion, which usually associates with
breaking and reforming of C\\H bond. As the first step, breaking of
chemical bond requires massive energy, resulting in a relatively unsta-
ble system. In order to stabilize the system after proton leaving, an
2.2. Preparation of levetiracetam
(S)-2-aminobutanamide hydrochloride (30.0 g) was mixed with di-
chloromethane (300 mL), and tetra-butyl ammonium bromide (3.5 g)
was added. Then, the reaction mixture was cooled to −15 °C. The further
addition of potassium hydroxide and 4-chlorobutyryl chloride was di-
vided into three batches. For each round, potassium hydroxide
Fig. 1. 2D structure of levetiracetam with numbers of selected atoms.