Separation of Mixtures of Chiral Compounds by their Distribution between Different Phases


The function of living organisms is determined by the behavior of chiral compounds and their reactions. Not only the RNA and DNA influence this function, but the enantiomers of amino acids and sugars also have a dominant role. So, it is not surprising that an increasing number of drugs contain single enantiomers, which are often prepared by resolution of racemic compounds obtained due of chemical syntheses. The therapeutically effect of enantiomers can be different, and it is not rare that these effects are opposite.

A tragic reminder of the importance of chirality is thalidomide, in the early 1960s. So, the enantiomeric separations are necessary and inevitable and the demand for pure enantiomers becomes higher and higher.

The resolution remains the most common and economical method for preparation of pure enantiomers, even though several new methods and selective syntheses are known. In this case, to the racemic compound obtained in the chemical syntheses, another chiral compound, the so-called resolving agent is added. In the solution diastereomeric salts are formed, and these will be distributed between two phases according to their self-disproportionation. So, they can be separated from each other. The diastereomeric salts will be decomposed by adding an adequate acid or base. Due to the self-disproportionation of enantiomers, after the distribution between two phases, the pure enantiomer can be separated from the racemic portion by an adequate method from the enantiomeric mixture obtained in one of the phases.

The distribution between phases is determined by the self-disproportionation (SDE) of enantiomers, that of helical structured supramolecular associations and their interactions in the solution are controlled by the eutectic composition.

Separation of Mixtures of Chiral Compounds

  1. Separation of enantiomeric mixtures by crystallization from melt
  2. Separation of enantiomeric mixtures by distillation
  3. Separation of enantiomeric mixtures by fractional precipitation
  4. Separation of enantiomeric mixtures by recrystallization
  5. Distribution of diastereomeric mixtures
  6. The effect of crystallization time on the distribution of diastereomeric salts between two phases
  7. Effect of the applied solvent on the separation of diastereomeric mixtures
  8. Separation of diastereomers by crystallization from melt
  9. Separation of diastereomers by distillation
  10. Separation of diastereomers without crystallization

During the resolution processes the enantiomers tend to form a more stable, more symmetrical conformation, according to their own code. In course of interactions they tend to reproduce themselves enforcing their own code. While the self-reproduction of racemic compounds is encoded by their eutectic composition, the resolving agent pursues to reproduce itself from the enantiomers of racemic compound but in the ratio of its eutectic composition, of its stoichiometry.

The molecular structure of the single enantiomer is the code for reacting with other (foreign) chiral molecules.

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Journal of Developing Drugs