What is a chiral resolution?

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Chiral resolution in stereochemistry is a process for separating racemic compounds into their enantiomers. It is an important tool in the production of optically active drugs. Other terms with the same meaning are optical resolution and mechanical resolution.

Chiral resolution in stereochemistry is a process for separating racemic compounds into their enantiomers. It is an important tool in the production of optically active drugs. Other terms with the same meaning are optical resolution and mechanical resolution.

A disadvantage of the chiral resolution of the racemates in comparison to the asymmetric direct synthesis of one of the enantiomers is that only 50% of a desired enantiomer is obtained. There are several methods.

Resolution by crystallization

It is known that 5-10% of all racemes crystallize as mixtures of enantiopure crystals, the so-called conglomerates. Louis Pasteur was the first to conduct a chiral resolution when he discovered the concept of optical activity with the manual separation of the left and right-handed tartaric acid crystals in 1849. In 1882 he continued to demonstrate that by sowing a supersaturated solution of sodium ammonium tartrate with a d -crestal on one side of the reactor and a crystal of l on the opposite side, crystals of opposite hand will form on opposite sides of the reactor.

This type of resolution, called spontaneous resolution, has also been demonstrated with racemic methadone. In a typical configuration, 50 grams of dl-methadone are dissolved in the oil ether and concentrated. Two crystals d- and l of millimetric dimensions are added and after stirring for 125 hours at 40 ° C two large crystals d- and l are recovered with a yield of 50%.

Another form of direct crystallization is the preferential crystallization also called resolution by dragging one of the enantiomers. For example, an added seed of (-) – hydrobenzoin to an ethanol solution of (±) – hydrobenzoine will have the (-) – enantiomer which crystallizes and after 15 cycles an optical purity of 97% can be obtained.

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