SEPARATION OF DIASTEREOMERS BY DISTILLATION - A NEW PROCEDURE FOR THE SYNTHESIS OF OPTICALLY-ACTIVE HETEROCYCLIC CARBOXYLIC-ACIDS

Resolution by distillation, in contrast to classical resolution by recrystallization, may be easy to perform. In the case of heterocyclic carboxylic acids 1(X = O, S, n = 1.2), the diastereomeric amino acid amides 2, which differ in boiling point by 7 K under reduced pressure, were separated by fractional distillation. Remarkably, both enantiomers of 2-tetrahydrofurancarboxylic acid 1a (X = O, n = 1) were obtained by the distillation of the diastereomers of 2a (R = iPr, R' = Me) with 99% optical purity.