Chromatography enantiomer/diastereomer separation

The classical method to resolve a racemate is to react the mixture of enantiomers with one enantiomer of some other chiral compound. The products are diastereomers and can be separated by using the usual methods, such as recrystallization or chromatography. Then the separated diastereomers are individually converted back to the enantiomers of the original compound. Figure 7.5 shows how a racemic carboxylic acid can be resolved. 

While compounds which are enantiomers have identical chemical and physical properties and equal and opposite optical rotation, compounds which are diastereomeric with each other can have completely different chemical and physical properties and optical rotations. This feature provides the basis for the resolution of chiral compounds. In this procedure a racemic mixture is derivatised by reaction with an enantiomerically pure compound which leads to a mixture of two diastereomers. These can be separated by crystallisation or chromatography as a result of their different properties and the pure enantiomers of the starting compound obtained by cleavage of each diastereomer separately. In the synthesis of the chiral phosphines (section 1.7) the phosphine oxide (37) is obtained by resolution of the starting material (34) via the diastereomeric esters (35) and (36). A further important application of the differing properties of diastereomers is in the determination of e.e. by derivatisation with an enantiomerically pure compound followed by chromatographic or NMR analysis (see section 3.4.1). 

Resolution may be considered the classical method of obtaining enantiomerically pure products. The procedure relies on the fact that diastereomers, unlike enantiomers, have different physical properties. If the racemic compound which is to be resolved is derivatised by reaction with a naturally occurring enantiomerically pure compound, then the resulting diastereomeric compounds may be separated, most commonly by crystallisation but also by chromatography, and then separately treated to liberate the two enantiomers. If we represent the... 

The last step towards the three-component Ugi-type coupling envisaged in the ret-rosynthesis is described below. The, commercial amine 27 was oxidized to imine 26 (94% ee) by MAO-N, as previously described [20], which was then combined with 25 and 28 give the advanced intermediate 40. Finally, cleavage of the acetate followed by Dess-Martin oxidation gave Telaprevir (24) as a 83 13 4 mixture of diastere-omers, with one minor diastereomer derived from the incomplete stereoinduction of the Ugi-type 3CR and the other from the minor enantiomer of imine 26. Flash chromatography allowed straightforward separation of the diastereomers to afford pure Telaprevir (24) in 80% yield over the last two steps (Scheme 15.11). 

In contrast with enantiomers, diastereomers, because they are not mirror images of each other, are molecules with different physical and chemical properties (see, e.g.. Real Life 5-3). Their steric interactions and energies differ. They can be separated by fractional distillation, crystallization, or chromatography. They have different melting and boiling points (see margin of the next page) and different densities, just as constitutional isomers do. In addition, they have different specific rotations. 

Three general methods exist for the resolution of enantiomers by Hquid chromatography (qv) (47,48). Conversion of the enantiomers to diastereomers and subsequent column chromatography on an achiral stationary phase with an achiral eluant represents a classical method of resolution (49). Diastereomeric derivatization is problematic in that conversion back to the desired enantiomers can result in partial racemization. For example, (lR,23, 5R)-menthol (R)-mandelate (31) is readily separated from its diastereomer but ester hydrolysis under numerous reaction conditions produces (R)-(-)-mandehc acid (32) which is contaminated with (3)-(+)-mandehc acid (33). 

Liu W, Gan JJ (2004) Separation and analysis of diastereomers and enantiomers of cypermethrin and cyfluthrin by gas chromatography. J Agric Food Chem 52 755-761... 

In the case of diastereomeric mixtures of chiral hydroperoxides, standard chromatography on achiral phase can be employed to separate the diastereomers. As one example for the preparation of optically pure hydroperoxides via this method, the ex-chiral pool synthesis of the pinane hydroperoxides 11 is presented by Hamann and coworkers . From (15 )-cw-pinane [(15 )-cw-10], two optically active pinane-2-hydroperoxides cA-lla and trans-llb were obtained by autoxidation according to Scheme 17. Autoxidation of (IR)-c -pinane [(17 )-cw-10] led to the formation of the two enantiomers ent-lla and ent-llh. The ratio of cis to trans products was 4/1. The diastereomers could be separated by flash chromatography to give optically pure compounds. 

Resolution Methods. Chiral pharmaceuticals of high enantiomeric purity may be produced by resolution methodologies, asymmetric synthesis, or the use of commercially available optically pure starting materials. Resolution refers to the separation of a racemic mixture. Classical resolutions involve the construction of a diastcrcomcr by reaction of the racemic substrate with an enantiomerically pure compound. The two diastereomers formed possess different physical properties and may be separated by crystallization, chromatography, or distillation. A disadvantage of the use of resolutions is that the best yield obtainable is. 50%, which is rarely approached. However, the yield may he improved by repeated raccmization of the undcsired enantiomer and subsequent resolution of the racemate. Resolutions are commonly used in industrial preparations of homochiral compounds. 

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