Schemes Planar and axially dissymmetric molecules of established configuration

Notice how the configurational relationship between the two adjacent asymmetric centers in threose has been carried over into the name of an amino acid. 

When a molecule has two chiral centers that are identically substituted, the number of stereoisomers is reduced from four to three, as is well known for the case of tartaric acid. The three stereoisomers are the d and l forms (enantiomers) and the diastereomeric meso form. The meso form is superimposable on its mirror image, since it has a plane of symmetry and is achiral and optically inactive. The three possible stereoisomers of tartaric acid are shown below  

As is evident from these examples of molecules that have more than one chiral center, there are no simple generalizations that relate the number of chiral centers to the number of stereoisomers, since some of the forms may be superimposable on their mirror images. In the absence of such complicating features, the number of stereoisomers possible for a system with n distinct asymmetric centers is 2 . 

For a review of resolving agents and resolution methods, see S. H. Wilen, Top. Stereochem. 6,107 (1971). 

Mixture 353 g of diastereomeric ammonium carboxylate salts (R -acid, R -amine and 5-acid, R-amine). Recrystallized from ethanol-water 

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