**2. pH dependence of the separation of diastereomeric mixtures**

#### **2.1. pH dependence in course of resolution of racemic acid with chiral base**

A thermodynamic model has been elaborated [23] for the salt-salt resolution of racemic *cis*permethric acid (**CPA**) with half-equivalent (*S*)-*N*-benzyl-2-aminobutanol ((*S*)-BAB) [24]. The amount of the base was systematically changed to investigate the pH dependence of the resolution. The calculated and measured *ee* and T curves are plotted in **Figure 1**. After the separation of the diastereomeric salt, by neutralizing the mother liquor with hydrochloric acid, the (*R,R*)-**CPA**∙(*S*)-**BAB** diastereomer was precipitated.

**Figure 1.** pH-dependent resolution of racemic cis-permethric acid with (*S*)-*N*-benzyl-2-aminobutanol.

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**Scheme 1.** pH dependence during the resolution of mandelic acid with (*S*)-Phe.

#### **2.2. pH dependence during the resolution of racemic mandelic acid with (S)-phenylalanine**

In the case of (*S*)-**Phe**, experiments were carried out both with equivalent and half-equivalent amount of resolving agent relative to racemic mandelic acid (**MA**). The adjusting of the pH was accomplished with NaOH and cc. HCl (**Scheme 1**) [17].

In the case of the application of 1.0 equivalent resolving agent, the enantiomeric purity (44– 51%) and yield of the crystalline salt are almost the same between pH 1.3 and 2.3. This pH range matches well the p*K*<sup>a</sup> value of the carboxyl group of **Phe** (1.83). The time of crystallization was 15 min in all cases (**Figures 2** and **3**).

The pH dependence of the diastereomeric salt was also investigated after 2 weeks, when the thermodynamic equilibrium was reached (**Figure 4**).

**Figure 2.** pH dependence in course of the resolution of mandelic acid with 1.0 equivalent (*S*)-**Phe.**

**Figure 1.** pH-dependent resolution of racemic cis-permethric acid with (*S*)-*N*-benzyl-2-aminobutanol.

In most cases, the mixtures of diastereomers received by adequate resolving agents, or the mixtures of enantiomers isolated thereof, have to be separated. It is common in the separation methods, that the distribution of the mixtures between two phases is necessary, and the phase

Besides the effect of the applied solvents, the phase distribution of the mixtures is also determined by kinetic or thermodynamic control [14]. The phase distribution is also determined by

Besides, the distribution between the phases is pH-dependent [17]. It seems that the effect of the kinetic control between two phases can be stabilized with the application of ultrasound [18]. The formation of solvates can also determine the distribution and the crystallizationbased separation of diastereomers [19, 20]. By the incorporation of compounds of similar structure to the solvate-forming molecules, the fractionated crystallization can be successful in other solvents as well [21]. In the case of the crystallization of diastereomers, better separation can be reached, if the resolving agent is partially replaced by an achiral reagent of similar structure compared to the cases without replacement [22]. In the following, the most

the eutectic composition of the chiral molecules in the mixtures [15, 16].

80 Laboratory Unit Operations and Experimental Methods in Chemical Engineering

characteristic examples of the above-mentioned methods will be discussed.

**2. pH dependence of the separation of diastereomeric mixtures**

**2.1. pH dependence in course of resolution of racemic acid with chiral base**

acid, the (*R,R*)-**CPA**∙(*S*)-**BAB** diastereomer was precipitated.

A thermodynamic model has been elaborated [23] for the salt-salt resolution of racemic *cis*permethric acid (**CPA**) with half-equivalent (*S*)-*N*-benzyl-2-aminobutanol ((*S*)-BAB) [24]. The amount of the base was systematically changed to investigate the pH dependence of the resolution. The calculated and measured *ee* and T curves are plotted in **Figure 1**. After the separation of the diastereomeric salt, by neutralizing the mother liquor with hydrochloric

separation have to be applied [11–13].

**Figure 3.** pH dependence in course of the resolution of mandelic acid with 0.5 equivalent (*S*)-**Phe.**

**Figure 4.** pH dependence in course of the resolution of mandelic acid with 1.0 equivalent (S)-Phe after 2 weeks of crystallization (the denotation thermo is applied for the results of the resolutions after 2 weeks of crystallization).

resolution was reached in a pH range similar to the p*K*<sup>a</sup>

**Scheme 2.** pH dependence of the resolution of mandelic acid with (*R*)-PG.

**3. Role of eutectic compositions, crystallization time and solvent in** 

**Figure 5.** pH dependence in course of the resolution of mandelic acid with 0.5 equivalent (*R*)-**PG.**

The right choice of the solvent is crucial in course of the fractionated crystallization of diastereomers. The composition of the crystalline phase received during resolution is often changed in case of solvate or hydrate formation. The dominant configuration can also change, depending on the applied solvent. For example, by changing the solvent in case of resolution of racemic 6-fluoro-2-methyl-1,2,3,4-tetrahydroquinoline (FTHQ) with half-equivalent (*R*,*R*)-di-*p*-toluoyl-tartaric

resolving agent (**Figure 5**).

**3.1. Role of the solvent**

**case of diastereomer separation**

value of the carboxyl group of the

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83

The purity of the enantiomeric mixtures separated from the crystalline phase became highly pH-dependent. At pH 1.2, the enantiomeric purity received during fast crystallization (*ee*: 52%) decreased to 36%, while at pH 2.3 from 49 to 11%. The optimum of the pH dependence can be reached in the case of kinetic control.

#### **2.3. pH dependence in course of the resolution of racemic mandelic acid with (***R***)-pregabalin**

The pH dependence of the resolution of racemic mandelic acid with (*R*)-pregabalin ((*R*)-**PG**) was carried out by kinetic control (crystallization time: 15 min) (**Scheme 2**).

The (*R*)-**MA** enantiomer mixtures can be received with almost identical enantiomeric purities (43–50%) and yields in the range of pH 3.0–4.4. The maximal value of the efficiency of the New Opportunities to Improve the Enantiomeric and Diastereomeric Separations http://dx.doi.org/10.5772/intechopen.78220 83

**Scheme 2.** pH dependence of the resolution of mandelic acid with (*R*)-PG.

**Figure 5.** pH dependence in course of the resolution of mandelic acid with 0.5 equivalent (*R*)-**PG.**

resolution was reached in a pH range similar to the p*K*<sup>a</sup> value of the carboxyl group of the resolving agent (**Figure 5**).
