Diastereomers Diastereomeric pairs, separation

In contrast, the phosphoramidites comprising (R) -binaphthol or (S) -binaphthol moieties (1) are obtained as a diastereomeric mixture and can be separated. The separation of the diastereomers has been achieved for many derivatives exploiting the different solubility profiles of the two diastereomers. Alternatively, the separation could be accomplished by column chromatography. These methodologies need to be optimized for each diastereomeric pair, thus representing currently the synthetic bottleneck for the rapid generation of different QUINAPHOS derivatives. 

Any diastereomeric pair could be separated by a physical process like distillation or crystallization. Diastereomers are found in parts (a), (b), and (d). The structures in (c) are enantiomers they could not be separated by normal physical means. 

In stereochemical terms, menthol is more complex than carvone. Menthol has three asymmetric centres, viz carbons 1, 3 and 4 of the ring. With three centres, this means that there are 23 (i.e. 8) individual isomers. Each of the isomers will have a complementary one with which it is enantiomeric. Thus, menthol exists as four pairs of diastereomers. Each diastereomeric pair will have slightly different physical and chemical properties and may therefore be separated from the other three by simple physical methods such as distillation, crystallisation and chromatography. The eight isomers of menthol are tabulated in Figure... 

In the more complex case of 193a (or 193b), the transformation may be chiroselective if D , F, M (or G, H, M ) dominate over achiral N (in both cases). However, the process is deemed nonchiroselective if %(D +F +M ) accidentally equals %N. The latter composition may still display stereoselectivity favoring a given diastereomer for either one of the two pairs (e.g. D over F, or M over N one of the three divides delineates astereomers, and the other two divides separate two diastereomeric pairs - D, F vs. M, N. 

Nature is the source of a number of different resolving agents that are used to prepare separable diastereomeric pairs from enantiomers. In this experiment, we use (+)-tartaric acid (19), which is produced from grapes during the production of wine, to resolve racemic 1-phenylethanamine (20), as shown in the resolution scheme outlined in Figure 7.5. Conversion of the enantiomers of 20 into separable diastereomers involves an acid-base reaction with 19. The diastereomeric salts that result have different solubilities in methanol and are separable by fractional... 

Table 14 presents the chemical shifts of l9F-signal pairs of selected diastereomeric imidazolidines. The advantages of the above method can be summarized as follows high chemose-lectivity-ketones do not react C2 symmetry means a diastereomeric control excess reagent can be easily removed generally, an excellent separation of H-, l3C- and, 5F-NMR signals of the diastereomers is observed. 

Enantiomers are derivatized with an optically pure chiral derivatization reagent to form a pair of diastereomers. The ability to resolve the diastereomeric derivatives on an achiral sorbent is enhanced when the chiral centers of the enantiomers and the derivatives are in close proximity [181]. Two different separation mechanisms have been proposed. One postulates that the diastereomers are separated by differences in molecular structure and polarity [182], The other possible mechanism is based on differences in the diastereomer energies of adsorption [183]. Table 5.7 lists the chiral reagents that have been used for separation of enantiomers as diastereomers. 

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