Purity of Chiral Derivatization Reagent or Chiral Selector

For the chiral derivatization method, there is one point which necessitates careful interpretations of the results. When the enantiomeric mixture of ( + )-A and ( — )-A are derivatized with the chiral derivatizing reagent of ( + )-B, it often includes ( — )-B as a chiral impurity. If the reagent B is 100% optically pure, two diastereomers ( + )-A-(+)-B (I) and ( — )-A-(#)B (II) are formed. If reagent B is not optically pure, additional diastereomers (#)-A-(—)-B (III) and (— )-A-( —)B (IV) are formed.

Enantiomers Reagents

I II

Reaction products

The enantiomers of A to be separated and determined as their diastereomeric derivatives (I and II) can be resolved as the respective peaks on an achiral stationary phase. However, the peaks of the products

III and IV produced with (— )-B overlap with the peaks of II and I, respectively, on achiral stationary phases, because II and III, and I and IV are enan-tiomeric pairs. Therefore, if the optical purity of the derivatization reagent is not known or not taken into consideration, the optical purity of the target compound will not be determined accurately. Further, this is the case when the separation of enantiomers is carried out by the use of chiral additives to the eluent on achiral stationary phases.

In contrast, direct resolution of enantiomers using chiral stationary phases does not have the drawbacks described above. We can easily determine 0.1% or 0.05% of the antipode using chiral stationary phases.

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