Diastereomers basic properties

Although the basic principle and procedure of diastereomeric resolution are not difficult to understand, the chiral discrimination mechanism involved in the selective crystallization of one diastereomer from the mixture is very complicated. The chiral discrimination mechanism for diastereomeric resolution changes in accord with the resolving system, since not only the properties of diastereomeric crystals but also the conditions for crystallization strongly influence the chiral discrimination mechanism. In particular, the polymorphism of crystal, the severe solvent effect on solubility, and the kinetic factor for crystal growth are still not perfectly understood regarding this chiral discrimination phenomenon. The study is therefore limited in its investigation of the chiral discrimination mechanism for the diastereomeric resolution, as the mechanism involves both the crystal and solution properties of diastereomers.7... 

Stereoisomers are classified by symmetry as either enantiomers or diaste-reomers. Enantiomers have identical physical properties except for the direction of optical rotation. Diastereomers are basically stereoisomers that are not enantiomers of each other. A pair of enantiomers exists for all molecules containing a single chiral center and have the opposite configuration at each of the stereo centers. The maximum number of stereoisomers for a compound with n stereo centers is T. Diastereomers, on the other hand, have the same configuration at one of the two centers and have the opposite configuration at the other. 

Since l-amino-2-cyclohexene is basic, a chiral acid is needed. When (+)-mandelic acid is the resolving agent used, a mixture of diastereomeric salts is formed as shown. These diastereomers have different physical properties and can therefore be separated (often by recrystaUization). Treatment with base neutralizes the amine and allows it to be separated from the charged mandelate salt (typically by extraction), to give the neutral amines as pure enantiomers. 

Solvents of equal strength will yield similar k values of the analytes, but in practice the observed retention times and resolutions can differ due to their selectivity properties. As an example, tert-butyl methyl ether and acetonitrile have similar strengths, but the former is basic whereas the latter is not. The same can be true for solvent mixtures. The separation of the two diastereomers shown in Fig. 1 was tried with several mixtures (on LiChrosorb SI 60) and the following results were obtained ... 

The anomeric glycosides are diastereomers with different physical properties. Like ordinary acetals and ketals, they are stable in neutral or basic solution. Therefore, they are not reducing sugars because they do not hydrolyze to form a free aldehyde group in Benedicts solution, which is basic. However, glycosides are hydrolyzed in acid solution by the reverse of the reactions shown in Figure 26.7. 

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