Question

3. Instead of trying to isolate the more soluble (S)-aminium-(S)-malate diastereomers from the mixture of diastereomeric salts, what would be a better way of ensuring that the (S)-(-)-amine diastereomeric salt crystallizes preferentially as the less soluble crystals? Hint: What is the enantiomer of the (R)-aminium-(S)-malate diastereomer and how could it be formed?

Answer 1

The enantiomer of (R)-aminium-(S)-malate is (S)-aminium-(R)-malate.

(S)-aminium-(R)-malate can be formed from its diastereomer, i.e. (S)-aminium-(S)-malate

This is formed only by recrystallization, there is no other way.

Hence, the diastereomeric salt (S)-aminium-(S)-malate crystallized preferentially as the less soluble crystals, which could be used for the preparation of its diastereomer, i.e. (S)-aminium-(R)-malate