Resolution is the separation of enantiomers from a racemic mixture. The most common way of performing a resolution is through crystallization. Typically, a salt is formed, from which the product is easily isolated.
The compound added to make the salt can be either chiral or achiral. In the former case, the process is a diastereomeric resolution and the auxiliary compound must be recovered. In the latter case, it is a kinetic resolution performed with some inexpensive product. Kessels SA has patented processes of both types, which are run on industrial scale.
See them in section Patents.
Racemization: A major inconvenience of all the resolution processes is that the maximum theoretical yield is only 50%, since both enantiomers are present in equal amounts in the racemic mixture. This requires to couple the resolution process to another one called racemization. Here, the unwanted enantiomer is again converted to the racemic mixture. This is usually the step that defines the economics of the process. If there is too much decomposition during the racemization, the process might be unviable.
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