**3. Separation of diastereomeric mixtures (recent results)**

#### **3.1. Chiral salt of helical supramolecular structure as resolving agent (separation of diastereomeric molecular complex)**

The salt of a chiral amine of supramolecular helical (double helix) structure and an achiral acid precipitates from the solvent (methanol) containing racemic alcohol as well, in the form of supramolecular helical crystals, which are composed of chiral amine, acid and one enantiomer of the racemic alcohol (**Scheme 12**) [49].

According to Kinbara, the most suitable resolving agent of a racemic molecule can be selected by the design of a stable hydrogen bond system [50]. Saigo et al. concluded after the analysis of several single crystals of pairs of diastereomeric salts, that the formed CH/π interactions play a significant role in the solubility difference of the diastereomers, which clearly influences the chiral recognition and thus the result of the separation [51, 52].

Others estimated well by quantum chemical computations the difference between the lattice energies of the pairs of diastereomeric salts, without preliminary knowledge on the crystal structure [53, 54]. However, it is confessed by the authors that these calculations need to be upgraded in order to be safely applicable in the search of resolving agents.

The conclusions drawn from the preparative results can facilitate the choice of the resolving agent. For example, it is already trivial, that very good separations can be reached with the application of a resolving agent of similar molecular structure (structurally related) to the racemic compound [10, 21, 55–58].

**Scheme 14.** Calculation method of molecular length used by other researchers.

function of the difference of molecular length.

function of the difference of molecular length.

the number of samples).

**Scheme 15.** Average of enantiomeric excess values of enantiomeric mixtures separated from diastereomeric salt in

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**Scheme 16.** Average of efficiency of resolution values of enantiomeric mixtures separated from diastereomeric salt in

**Scheme 17.** *ee*average values of 49 resolutions in function of the difference of molecular lengths (blue numbers represent

**Scheme 12.** The salt of chiral base and achiral acid crystallizes with the appropriate enantiomer of racemic alcohol.

#### **3.2. Ratio of the molecules composing the diastereomer**

Another approach construes the importance of the ratio of molecular lengths of the racemic molecule and the resolving agent instead of the structural similarity. According to Sakai, the author of the "space-filler concept," the crystal-lattice of the less soluble diastereomer salt is influenced by the structural properties of the constituents of the salt (i.e., the enantiomer and the resolving agent), such as the molecular size. Sakai et al. investigated the relative molecular length of the racemic molecule and the resolving agent in course of resolutions of 1-aryl-alkylamines with 2-hydroxycarboxylic acids and vice versa (**Scheme 13**). Based on the results of 20 resolutions, the best separations of the racemic mixtures can be reached with the application of a resolving agent of similar molecular length [59].

Other researchers considered the longest carbon-chain as the length of a molecule (**Scheme 14**). Based on the average of the results of 21 resolutions (*ee*, F), almost linear correlation was found between the difference of the molecular length of structurally related racemic mixtures and resolving agents, and the result of the resolution (**Schemes 15** and **16**) [10].

Besides the abovementioned 21 resolutions [10], carried out with structurally related resolving agents, the results of 28 additional resolutions [8, 18, 60–74] applying structurally nonrelated resolving agents were systematized (most of them were industrialized).

Based on the results of 49 resolutions, by plotting the average enantiomeric excess and efficiency of resolution values in function of the difference of molecular length, respectively, the following diagrams are received (**Schemes 17** and **18**). Accordingly, higher enantiomeric excess can be reached in case of higher difference of molecular length of the racemic compound and the resolving agent [75].

**Scheme 13.** Calculation of molecular length according to Sakai.

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**Scheme 14.** Calculation method of molecular length used by other researchers.

**3.2. Ratio of the molecules composing the diastereomer**

108 Laboratory Unit Operations and Experimental Methods in Chemical Engineering

of a resolving agent of similar molecular length [59].

pound and the resolving agent [75].

**Scheme 13.** Calculation of molecular length according to Sakai.

Another approach construes the importance of the ratio of molecular lengths of the racemic molecule and the resolving agent instead of the structural similarity. According to Sakai, the author of the "space-filler concept," the crystal-lattice of the less soluble diastereomer salt is influenced by the structural properties of the constituents of the salt (i.e., the enantiomer and the resolving agent), such as the molecular size. Sakai et al. investigated the relative molecular length of the racemic molecule and the resolving agent in course of resolutions of 1-aryl-alkylamines with 2-hydroxycarboxylic acids and vice versa (**Scheme 13**). Based on the results of 20 resolutions, the best separations of the racemic mixtures can be reached with the application

**Scheme 12.** The salt of chiral base and achiral acid crystallizes with the appropriate enantiomer of racemic alcohol.

Other researchers considered the longest carbon-chain as the length of a molecule (**Scheme 14**). Based on the average of the results of 21 resolutions (*ee*, F), almost linear correlation was found between the difference of the molecular length of structurally related racemic mixtures

Besides the abovementioned 21 resolutions [10], carried out with structurally related resolving agents, the results of 28 additional resolutions [8, 18, 60–74] applying structurally non-

Based on the results of 49 resolutions, by plotting the average enantiomeric excess and efficiency of resolution values in function of the difference of molecular length, respectively, the following diagrams are received (**Schemes 17** and **18**). Accordingly, higher enantiomeric excess can be reached in case of higher difference of molecular length of the racemic com-

and resolving agents, and the result of the resolution (**Schemes 15** and **16**) [10].

related resolving agents were systematized (most of them were industrialized).

**Scheme 15.** Average of enantiomeric excess values of enantiomeric mixtures separated from diastereomeric salt in function of the difference of molecular length.

**Scheme 16.** Average of efficiency of resolution values of enantiomeric mixtures separated from diastereomeric salt in function of the difference of molecular length.

**Scheme 17.** *ee*average values of 49 resolutions in function of the difference of molecular lengths (blue numbers represent the number of samples).

**Scheme 18.** *F*average values of 49 resolutions in function of the difference of molecular lengths (blue numbers represent the number of samples).

## **4. Amino acids and their mixtures as resolving agents**

#### **4.1. Amino acid resolving agents**

1-Aminoindane was successfully resolved with the application of nearly 0.5 equivalent aspartic acid (**Asp**) and the (*R*)-enantiomer was separated (**Scheme 19**) [76].

For the resolution of racemic acids basic amino acids were also applied, for example (*S*)-lysine (**Lys**) (**Scheme 20**) [77, 78].

#### **4.2. Mixtures of amino acids as resolving agents**

With the application of equivalent amount of (*S*)-**Phe**, (*S,S*)-**AP** and (*S*)-**PG** resolving agents or their mixtures in course of the resolution of racemic mandelic acid (**Scheme 21**), the most effective resolving agent was the (*S*)-**PG**. In the case of resolutions carried out using the mixtures of resolving agents in 1:1 ratio, the most effective combination was the mixture of (*S*)- **Phe** and (*S*)-**PG.**

Among the half-equivalent resolving agents, (*S*)-**Phe** was the most effective, while from the half-equivalent resolving agent combinations, the mixture of (*S*)-**Phe** and (*S,S*)-**AP** was the most effective [8].

The racemic mandelic acid (**MA**) cannot be resolved from water with the application of (*S*)- **Ala**, however, a diastereomeric salt of *ee*: 23% enantiomeric excess was received using (*S*)-**Phe** as resolving agent. Applying mixtures of the two resolving agents in different ratios, (*S*)-**MA** of significantly increased enantiomeric excess could be separated from the precipitated diastereomeric mixture when the resolving agent consisted of 0.35 mol (*S*)-**Phe** and 0.65 mol (*S*)-**Ala** [8]. This is the application of the Dutch resolution method in the case of amino acid mixture resolving agents (**Scheme 22**).

**5. Presence, role, and effect on the diastereomer separation of achiral** 

**Scheme 22.** Resolution of mandelic acid with the mixture of (*S*)-**Phe** and (*S*)-**Ala** resolving agents.

**Scheme 21.** Resolution of mandelic acid with the application of mixtures of resolving agents according to the Pope-

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After the resolution of *N*-acetyl phenylalanine (*N*-acetyl-**Phe**) with 1.0 equivalent (*R*)- 1-phenylethylamine ((*R*)-**PhEA**), (*S*)-*N*-acetyl phenylalanine of 5% enantiomer purity could be separated from the diastereomeric salt. However, when equivalent amount of the structurally related phenoxy acetic acid (**PhOAA**) was given to the racemic *N*-acetylphenylalanine and this mixture was resolved with 2 equivalents of (*R*)-1-phenylethylamine, (*S*)-*N*-acetyl-phenylalanine of 88% enantiomeric excess was enriched in the diastereomeric

**5.1. Achiral additive structurally related to the racemic compound**

**Scheme 20.** Resolution of 2-chloro-mandelic acid with (S)-lysine.

**additive**

Peachey half-equivalent method.

salt (**Scheme 23**) [79].

**Scheme 19.** Resolution of Rasagilin intermediate with (*S*)-aspartic acid.

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**Scheme 20.** Resolution of 2-chloro-mandelic acid with (S)-lysine.

**4. Amino acids and their mixtures as resolving agents**

110 Laboratory Unit Operations and Experimental Methods in Chemical Engineering

tic acid (**Asp**) and the (*R*)-enantiomer was separated (**Scheme 19**) [76].

1-Aminoindane was successfully resolved with the application of nearly 0.5 equivalent aspar-

**Scheme 18.** *F*average values of 49 resolutions in function of the difference of molecular lengths (blue numbers represent the

For the resolution of racemic acids basic amino acids were also applied, for example (*S*)-lysine

With the application of equivalent amount of (*S*)-**Phe**, (*S,S*)-**AP** and (*S*)-**PG** resolving agents or their mixtures in course of the resolution of racemic mandelic acid (**Scheme 21**), the most effective resolving agent was the (*S*)-**PG**. In the case of resolutions carried out using the mixtures of resolving agents in 1:1 ratio, the most effective combination was the mixture of (*S*)-

Among the half-equivalent resolving agents, (*S*)-**Phe** was the most effective, while from the half-equivalent resolving agent combinations, the mixture of (*S*)-**Phe** and (*S,S*)-**AP** was the

The racemic mandelic acid (**MA**) cannot be resolved from water with the application of (*S*)- **Ala**, however, a diastereomeric salt of *ee*: 23% enantiomeric excess was received using (*S*)-**Phe** as resolving agent. Applying mixtures of the two resolving agents in different ratios, (*S*)-**MA** of significantly increased enantiomeric excess could be separated from the precipitated diastereomeric mixture when the resolving agent consisted of 0.35 mol (*S*)-**Phe** and 0.65 mol (*S*)-**Ala** [8]. This is the application of the Dutch resolution method in the case of amino acid

**4.1. Amino acid resolving agents**

**4.2. Mixtures of amino acids as resolving agents**

(**Lys**) (**Scheme 20**) [77, 78].

**Phe** and (*S*)-**PG.**

number of samples).

most effective [8].

mixture resolving agents (**Scheme 22**).

**Scheme 19.** Resolution of Rasagilin intermediate with (*S*)-aspartic acid.

**Scheme 21.** Resolution of mandelic acid with the application of mixtures of resolving agents according to the Pope-Peachey half-equivalent method.

**Scheme 22.** Resolution of mandelic acid with the mixture of (*S*)-**Phe** and (*S*)-**Ala** resolving agents.
