Resolution through Diastereomeric Salts
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Resolution through Diastereomeric Salts
There are two approaches to using the properties of diastereomers to separate a racemate mixture of enantiomers.
Chromatography with Chiral-Resolved Support
In this situation, the chromatography is performed with a chiral-resolved support, implying that the support is either right-handed or left-handed. A mixture of enantiomers will behave differently when separated on the support. The right hand molecule might hold on to the right hand support better than the left hand molecule, so that the left molecule would move more rapidly through the column. This allows for the resolution of enantiomers.
Compound with Chiral-Resolved Mate
Again, we start out with racemate mixture (both R and S). Linking the molecules of the mixture with a single enantiomer of some other compound (lets say R) will produce two diastereomers (RR and RS). The two diastereomers will have different physical properties allowing them to be separated (If we're lucky). With the diastereomers separated, the R reactant can be removed, leaving our racemate mixture now resolved into the R and S enantiomers.
Applications of the Concepts
Alkaloids (natural organic bases with nitrogen) can be used to resolve racemic acids. When nature produces alkaloids, they are already resolved. Mixing them with racemic acids produces diastereomers that can be separated by partial crystallization (if we're lucky). In 1935 this was done with an allenic compound and brucine. One salt precipitated from the solution accounting for approximately half of the material (42%). The salt contained one enantiomer, while the other remained in solution. The other enantiomer was eventually removed from solution and recovered from the salt. Separate, their melting points were lower than the mixture, showing that different enantiomers can stabilize each other better than a resolved sample.
