Chiral Resolution - Chiral Resolving Agents

Chiral Resolving Agents

Derivatization of racemic compounds is possible with optically pure reagents forming pairs of diastereomers which can be separated by conventional techniques in physical chemistry. Derivatization is possible by salt formation between an amine and a carboxylic acid. Simple deprotonation affords the pure enantiomer. Examples of so-called chiral resolving agents are tartaric acid and brucine. The method was introduced (again) by Louis Pasteur in 1853 by resolving racemic tartaric acid with optically active (+)-cinchotoxine.

One modern-day method of chiral resolution is used in the organic synthesis of the drug Duloxetine:

In one of its steps the racemic alcohol 1 is dissolved in a mixture of toluene and methanol to which solution is added optically active (S)-mandelic acid 3. The alcohol (S)-enantiomer forms an insoluble diastereomeric salt with the mandelic acid and can be filtered from the solution. Simple deprotonation with sodium hydroxide liberates free (S)-alcohol. In the meanwhile the (R)-alcohol remains in solution unaffected and is recycled back to the racemic mixture by epimerization with hydrochloric acid in toluene. This process is known as RRR synthesis in which the R's stand for Resolution-Racemization-Recycle.

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