**3. Crystallization from a solvent**

The majority of the enantiomer separation methods based on crystallization involves solvent or solvent mixtures. Crystallizations from supersaturated solutions can be used both at the separation of enantiomeric mixtures, and at the separation of diastereoisomeric mixtures.

Comparison of the variable parameters of that method with the conditions of the above outlined solventless processes show that one have to take into consideration the characteristics of the applied solvent or solvent mixtures during optimization of such a resolution process. Furthermore, chiral or achiral additives can also be applied which improve without exception the efficiency of the resolution (F=ee\*t).

#### **3.1 Crystallization of enantiomeric mixtures**

The most frequently applied versions of the classical enantiomeric enrichment method (effectuated by crystallization from a solution) are presented below.

#### **3.1.1 Recrystallization**

The optical isomers exist in the supersaturated solutions as a mixture of homochiral and heterochiral associations. Therefore both the conglomerate or the racemate like behaviour can be observed during the crystallization, similar to the before discussed solventless crystallization methods

#### **3.1.1.1 Conglomerates**

When a nonracemic enantiomeric mixture is recrystallized from a solvent, and the enantiomeric excess (*ee*) of crystalline phase is always higher than the initial composition (*ee0*), then the compound (or its crystallized derivative) is conglomerate forming material. For example, when the enantiomeric mixture of dilthiazem hydrochloride (**DIL.HCl**) is recrystallized from ethyl acetate, the enantiomeric excess crystallizes in a very high purity.30

#### **3.1.1.2 Racemates**

14 Advances in Crystallization Processes

1.crystallization

(1*R* 2*R*)-**CHD** (1*S*, 2*S*)-**CHD** (1*R* 2*R*)-**CHD.**(*R*,*R*)-**TA**

The majority of the enantiomer separation methods based on crystallization involves solvent or solvent mixtures. Crystallizations from supersaturated solutions can be used both at the separation of enantiomeric mixtures, and at the separation of

Comparison of the variable parameters of that method with the conditions of the above outlined solventless processes show that one have to take into consideration the characteristics of the applied solvent or solvent mixtures during optimization of such a resolution process. Furthermore, chiral or achiral additives can also be applied which

The most frequently applied versions of the classical enantiomeric enrichment method

The optical isomers exist in the supersaturated solutions as a mixture of homochiral and heterochiral associations. Therefore both the conglomerate or the racemate like behaviour can be observed during the crystallization, similar to the before discussed solventless

When a nonracemic enantiomeric mixture is recrystallized from a solvent, and the enantiomeric excess (*ee*) of crystalline phase is always higher than the initial composition (*ee0*), then the compound (or its crystallized derivative) is conglomerate forming material. For example, when the enantiomeric mixture of dilthiazem hydrochloride (**DIL.HCl**) is recrystallized from ethyl acetate, the enantiomeric excess crystallizes in a very high

improve without exception the efficiency of the resolution (F=ee\*t).

(effectuated by crystallization from a solution) are presented below.

**3.1 Crystallization of enantiomeric mixtures** 

OH

OH

OH

OH

(*R,R*)-**TA**

**3. Crystallization from a solvent** 

diastereoisomeric mixtures.

**3.1.1 Recrystallization** 

crystallization methods **3.1.1.1 Conglomerates** 

purity.30

HO COOH

HOOC OH

(1*S*, 2*S*)-**CHD**

2.supercritical CO2 extract <sup>+</sup> crystallized residuum

+

At the recrystallization of racemate forming (the most common) enantiomeric mixtures the enriched enantiomeric mixture usually can be recovered from the mother liquor after filtration of the crystalline fraction having near to racemic composition. However, the composition of the crystalline material can be changed when the initial enantiomeric purity is higher then the eutectic composition. A certain example is the recrystallisation of the enantiomeric mixtures of tofizopam (**TOF**) from ethyl acetate. Almost racemic composition crystallized from small or medium pure enantiomeric mixtures. However the sole enantiomer crystallized from the solution when the initial composition was higher than the eutectic one (ee0>85%).31

(*S*>*R*)-**TOF**

The above example demonstrates that one can prepare the enantiomerically pure product from almost any samples having medium *ee0,* with two recrystallizations if the enantiomeric mixture recovered from the filtrate of the frst crystallization is applied as starting material of the second recrystallization. This technique was adapted at the purification of the hydrochloride salt of a flumequine intermediate (**FTHQ.HCl**), too.23

Separation of the Mixtures of Chiral Compounds by Crystallization 17

As it was already discussed in the above cases, enantiomeric enrichment of racemate forming enantiomeric mixtures result in an almost racemic crystalline phase when the starting enantiomer purity is smaller than the eutectic composition (*ee*0 < *ee*eu), but highly enriched mixture or the sole enantiomer can be crystallized when the initial composition is

This type of selective precipitation was applied in the cases of *N*-acyl-aminoacids when their recrystallization from water failed. However, additon of less then an equimolar amount of hydrochlororic acid to the solution of their sodium salt get good enrichment. It is therefore not surprising that considerable enrichment could be attained with a mixture of *ee*0 = 49.6 %, while two stage precipitation starting from an aqueous solution of **FoPA**.**Na** with *ee*0 = 73.4%

The situation was almost the same at the separation of the enantiomeric mixtures of *N*acetyl-phenylglycine (**AcPG**) where *ee*eu could not be exceeded (*ee*eu ≈ 86 % in both cases,

The above mentioned examples (**FoPA** and **AcPG**) demonstrated both the conglomerate and racemate behaviour during fractionated precipitation of enantiomeric mixtures under thermodynamic control. However a new type of crystallization order was observed when the propionyl derivatives of phenylalanine and phenylgylcine (**PPA** and **PPG,** respectively) were examined. This type of compounds presented different behaviour during the enantiomeric enrichment processes than expected on the basis of their binary (melting point/composition) phase diagrams. Binary phase diagram of *N*-propionyl-phenylglycine (**PPG**) indicated conglomerate type behavior, while that of *N*-propionyl-phenylalanine (**PPA**) was a racemate type with *ee*eu 59%. The results of selective precipitations contradicted the anticipations based on these findings. In these case the results of selective precipitations were rather similar to a conglomerate like behaviour. This can be explained with a faster

(composition near the eutectic point), did not resulted in significant enrichment.11

aquaeous sol.

aqueous HCl sol.

aquaeous sol

addition of non eq. aqueous HCl sol.

crystallization of enantiomeric excess than the expected racemic proportion.35

NHCHO addition of non eq.

ee0%

45.0 78.2

ee0%

49.6 73.4

ee0%

63.6 79.5

aqueous sol.

aqueous HCl sol.

NHCOCH3 addition of non eq.

(*R*>*S*)-**AcPA.Na**

(*R*>*S*)-**FoPA.Na**

(*R*>*S*)-**AcPG**

**3.1.2.3 Kinetic control** 

COONa

NHCOCH3

**3.1.2.2 Racemates** 

COONa

bigger than the eutectic one (*ee*0 > *ee*eu)

COONa

according to the binary phase diagrams).34

eesolid% eeliquid%

eesolid% eeliquid%

eesolid% eeliquid%

75.0 75.2

86.2 86.6

19.3 70.8

52.6 78.8

59.6 100

6.0 6.7

(*S*>*R*)-**FTHQ.HCl**

#### **3.1.2 Fractionated precipitation.**

In several cases the recrystalllization fails to result in enantiomeric enrichment. The solution for the purification of such enantiomeric mixtures may be the fractionated precipitation.32 In order to carry out such a purification step the enantiomeric mixture or its derivative (e.g. its salt formed with an achiral reagent) is dissolved in a solvent. Then a part of the dissolved material is liberated from its salt (or a salt is prepared from a part of it) with a reagent in a way that the liberated fraction (or salt) can precipitate from the solution, while the other proportion of the initial mixture remains in the solution. For example, the salts (e.g. watersoluble) of acids or bases are dissolved and a part of the free acid or base is precipitated by addtion of less than equimolar achiral acid or base.

#### **3.1.2.1 Conglomerates**

Similar to the experiences shown at the recrystallization of enantiomeric mixtures, the conglomerate behaviour can also be observed at fractioned precipitation. An application of this method was effectuated at the resolution of racemic tisercine (**TIS**) with half an equivalent of (*R,R*)-tartaric acid. The (*S*)-**TIS** enantiomer, the active pharmaceutical ingredient. remained in the filtrate of the diastereoisomeric salt formation process. Consequently, it contaminated with its mirror imge isomer. The enantiomeric enrichment was accomplished by selective precipitation. The (*S*>*R*)-**TIS** mixture was dissolved in water as a hydrochloric acid salt, then less than an equivalent amount of potassium hydroxide was added to the solutionin order to liberate the excess of (*S*)-**TIS**. The pure (*S*)-**TIS** base precipitated from the solution and an almost racemic hydrochloride salt remained in the solution..33

The conglomerate-like behaviour during the fractionated precipitation was observed in case of several chiral acids, too. A good example is the enantiomeric enrichment of *N*-acetylphenylalanine (**AcPA**). The pure (*R*)-**AcPA** isomer was obtained in two steps from its nonracemic enantiomeric mixtures.2

