Separation of Stereoisomers and Resolution of Racemic Mixtures

Resolution of Racemic Mixtures

• Separation of a racemic mixture into individual, enantiomerically pure enantiomers is called resolution.
• Enantiomers have identical physical properties (solubility, boiling point, melting point), so they cannot be resolved by common physical techniques.
• Diastereomers have different physical properties and can be separated by conventional physical techniques.
• All methods for separating or characterizing enantiomers are based on placing a mixture in a chiral environment to initiate diastereomeric interactions.

Methods for Resolving Racemic Mixtures

• Four general strategies:
  • Formation of diastereomeric salts with an enantiopure resolving agent.
  • Formation of diastereomeric compounds with an enantiopure resolving agent.
  • Use of chiral stationary phases for chromatographic resolution.
  • Enzymatic resolution.

Diastereomeric Salt Formation

• Classical method for separating enantiomeric amines by forming salts with a chiral acid and using crystallization.
• Example: (S)-1-Phenylethylamine combines with racemic lactic acid to form diastereomeric salts, separated by fractional crystallization.
• After separation, diastereomers are treated with strong acid (e.g., hydrochloric acid) to regenerate the corresponding enantiomer of lactic acid.
• The chiral amine is recovered and reused by treating the ammonium chloride salt with a base (e.g., sodium hydroxide).
• Example: (S)-(+)-mandelic acid combines with racemic 2-aminobutane to form diastereomeric salts.

Formation of Diastereomeric Compounds

• Reacting a racemic mixture with an enantiopure resolving agent to form covalently bonded diastereomeric compounds.
• Diastereomers are separated via gas or liquid chromatography.
• The resolving agent is recovered by cleaving the covalent bond.
• Example: Use of (-)-Menthol to resolve racemic Mandelic acid.

Chiral Stationary Phases

• Chromatography on chiral stationary phases is used to resolve racemic mixtures.
• Transient, diastereomeric interactions between enantiomers and the stationary phase lead to different retention times.

Enzymatic Resolution

• Preferential reaction of just one enantiomer, resulting in an enantioenriched sample of the less reactive enantiomer.
• Example: Using Candida cylindracea lipase for resolution.

Enantiomeric Excess

• Enantiomeric excess (ee) or optical purity is a measure of how pure an enantiomer is.
• $ee =$
• ee= ({\a] \text{ mixture}/[\a] \text{ pure enantiomer}) x 100