Beside the formation of diastereomeric salts, enantiomers can be separated by resolution. Here, enantiomers react with chiral inclusion hosts to form diastereomeric inclusion complexes. This is called chiral recognition. So, if a particular host is employed, only one enantiomer forms inclusion complex while other remains in solution. However, mostly, both of the enantiomers form complexes but vary in the rate of formation of inclusion complexes. An example of this technique is the use of crown ether for the resolution of 1-phenylethanaminium hexafluorophosphate in chloroform (Figure 1).

Figure 1

Similarly, enantiomers react at the same rate only with achiral reagents, but react differently with chiral reagents. For example, the titanium catalyzed kinetic resolution allylic alcohols via asymmetric epoxidation. In this case, only the (R) enantiomer was converted to the epoxide while the (S) isomer was unaffected. However, this method is sacrificial in nature as the recovery of one enantiomer often leads to the destruction of the other (Figure 2).

Figure 2

A variation of the sacrificial method for the resolution of a racemate involves treating it with some bacteria which contains a chiral enzyme that reacts at different rate with the different enantiomers. Since enzymes are very specific in their response to enantiomers, only one enantiomer is degraded and the other enantiomer is obtained with a very high purity (ee). This method however has only limited scope as the availability of such organisms or enzymes cannot be met easily. Thus, the enzyme emulsion from bitter almonds acts upon (±)-mandelonitrile to hydrolyze the (+)-form more rapidly.
With the advent of technology, column chromatography, the definitive tool for separation of organic compounds can also be applied for separation of enantiomers. The principle for the separation remains the same-differential adsorption-one enantiomer binds with the chiral stationary phase more strongly than the other thereby causing them to have different retention time in the column. Chiral stationary phase can consist of starch, which for instance allows almost complete resolution of mandelic acid (2-hydroxy-2-pheylethanoic acid), PhCH(OH)COOH. Synthetic stationary phases such as A, derived from the enantiomerically pure amino acid alanine, and B, likewise from valine, are effective in resolution of alcohols, amines and amino acids (both α and β) ( Figure 3).