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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Diastereomeric resolution: The resolution of DL-PhenylGlycine is performed with (+) - Camphorsulfonic acid. The salt crystallizes, is filtered and then hydrolized to obtain the solid D-(−) -PhenylGlycine. The Camphorsulfonic acid, which is more expensive than the product, is recycled into the process. Similarly, RS-Naproxen is resolved with (–) - 2-AminoButanol to obtain S-(+) -Naproxen.
Kinetic resolution: The resolution of DL-para -HydroxyPhenylGlycine is performed with the aid of sulphuric acid. The salt is crystallized from the solution, supersaturated with both enantiomers, through the addition of a small amount of crystals of the salt of the desired enantiomer. Here, the difficulty resides in the maintaining the other enantiomer in solution and therefore the process is often called specific crystallization. The salt is hydrolysed, the D-(–) -para -HydroxyPhenylGlycine filtered and the sulphuric acid is not recovered.
