Of diastereomeric pairs

Chromatography may be used for the direct separation of enantiomers ( chiral chromatography ) and also for the normal separation of diastereomeric pairs. 

Ever since the introduction of chromatographic methods in organic practice, a desirable goal has been the immediate separation of enantiomers on chirally modified columns, without preformation of diastereomeric pairs. 

For the separation of crystalline diastereomeric p,n-pairs, the same principles as for enantiomers are followed. Similar to the chromatographic resolution of enantiomers, the scale may vary significantly, but, in general, the separations of diastereomeric pairs are more reliable. For example, acids and lactones which may be separated via their phenylglycinol amide derivatives by MPLC on silica gel. Some examples with their derivatives and chromatographic method used for separation arc listed in Table 10. 

Water, alcohols, hydroperoxides or carboxylates can be added regioselectively to a,/ -unsaturat-ed carbonyl compounds by the oxymercuration demercuration protocol. In this sequence, the oxymercuration step yields the -addition product. Depending on whether the demercura-tion is performed with retention or inversion of configuration, two sets of diastereomeric pairs of enantiomers (racemic mixtures) are obtained. 

Equilibration of Configurationally Labile Organo-lithium Reagents. The equilibration of diastereomeric pairs of alkyllithium-(—)-sparteine complexes and trapping by achiral electrophiles gives enantioenriched products. Examples are a-(A/,JV-diisopropylcarbamoyloxy)benzyllithium in ether, not in THF, l-phenylethyllithium, and the dilithium salt of A/-methyl-3-phenylpropanoic acid amide (eq 2). ... 

A copper-catalyzed 1,4-addition was also described by Oppolzer and Kingma [39], Addition of the Grignard to an enoylsultam [Eq. (10)] could possibly product four products because two stereogenic centers are formed. The authors found a high yield (90%) of a mixture of diastereomeric pairs, identified after in situ hydrolysis to recover the sultam auxiliary. The major products were the S,S- and the R,R-diastereomers (86 12). The authors rationalize the differences in the observed stereochemistries by coordination of the magnesium and copper with the carbonyl and the oxygens of the sultam, thereby permitting addition to the Re-face. 

The CD spectra of the second eluted isomers of three complexes, [Co(L-ser)3 n(6-ala)n](n=0,l and 2) are not exactly the same with but quite similar to those of the corresponding first eluted isomers, except that the CD sign is opposite. From these CD spectra, the CD spectra of A- and A-[Co(D-ser)3 n(f -ala)n] are estimated. They could not be measured directly because only a small amount of D-serine complexes was available. When eluted with Na2S0l. solution, the elution curve of /ac-[Co(D-ser)3 n(f -ala)n] is the same as that of the corresponding L-serine complexes. Thus, the configuration of the first eluted isomer in the D-serine complexes must be A. In this way, we have obtained two series of the retention volumes of diastereomeric pairs, A(major peak) and A(minor peak) for L-serine complexes and A(major peak) and A(minor peak) for D-serine complexes. When eluted with NapSOL solution, the ratio for each enantiomeric pair is always quite close to unity, which means that they are certainly enantiomeric pairs. 

In Figure 17.23, transformation 96 constitutes a rotative sp chirostereogenesis that is diasterevectoselective and enantiofacioselective - manifested in the formation of four chiral diastereomers (596, 597, 598, and 599). The process may be considered a case of double diastereoselective synthesis. Finally, each of transformations 97 and 98 yields two astereomeric sets of diastereomeric pairs in the former case, all four components (602-605) are chiral in the latter transformation, one of the components (608) is achiral, the other three are chiral (609-611). The former transformation represents a composite case of rotative achirostereotopolysis and rotative nonstereotopomutation in contradistinction, the latter transformation is a composite case of nonrotative achirostereotopolysis and rotative nonstereotopomutation. Both transformations are subject to astereovectoselectivity and enantiofacioselectivity. 

During the generation of the cyanohydrine esters e.g. 373 and 375 under the conditions outlined above, different amounts of diastereomeric pairs of esters 373 are produced [816] in a kinetically and thermodynamically controlled manner, depending on the applied racemic acid chloride, thus deviating from the statistically expected 1 1 ratio in case of fenvalerate (Reaction scheme 257 a). 

Phosphorus-31 Chemical Shifts of Diastereomeric Pairs of Chiral Thiophosphates... 

Molecular imprinting is a new methodology for the preparation of synthetic polymers with predicted selectivity for various substances [77]. Molecular imprinted polymers were prepared as layers on TLC plates to investigate the resolution of diastereomeric pairs of alkaloids (quinine-quinidine and cinconine-cinconidine) [78]. 

The separation of diastereomeric pair via the indirect technique is sometimes simpler to perform and often has better resolution than with a direct method. 

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