Resolution by diastereomeric salt or complex formation is probably the most common way to resolve racemic mixtures. It is based on the reaction of a racemate with an enantiomerically pure resolving agent. This resolving agent can be an acid or a base if the racemate has a basic or acidic function, or a neutral resolving agent for neutral species that will lead to a complex formation. The goal is therefore to find a resolving agent that preferentially binds one of the enantiomers of the molecule to afford a species less soluble in the solvent media than the non-desired enantiomer. Thus the salt or complex that is formed can be separated, and converted back to the free acid, base, or neutral compound to lead to the pure desired enantiomer. The resolving agent can then be recycled. A variation is to make a salt or a complex with the two enantiomers of the racemate, this will lead to the formation of two diastereomers with different physicochemical properties: one will be less soluble than the other in a solvent media, making the separation possible.
We always take diastereomeric salt or complex formation into account when screening the different possibilities offered by the chiral toolbox. However thanks to our extensive experience in chiral chromatography and our preparative chromatography capabilities, we have often experienced that chiral chromatography, with total solvent recycling as implemented by Novasep, is often much more interesting in terms of operating cost, time and robustness.
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