Molecular structure diastereomers

If the diastereomer can largely be discriminated by the adsorbent, one speaks of high diastereoselectivity . As mentioned previously, in order to generate a pronounced effect, certain molecular-structure requirements have to be fulfilled ... 

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. 

Distinction should be made at this time between diastereoisomers and enantiomers. The former are characterized by the presence of at least two closely associated asymmetric centers in the molecular structure, either of which can epimerize. Altogether then there are two pairs of enantiomers for a total of four stereochemically unique individuals. Diastereoisomers have different physical properties and as a result discriminations, and even separations, can be done relatively easily. Enantiomers on the other hand differ in only one physical property, i.e. the direction of rotation of polarized light. Reaction of an enantiomeric racemic mixture with a third chiral species will produce a mixture of diastereomers therefore facilitating their identification or their separation. Early examples of this were the separations done by fractional crystallization of salts produced by a derivatization reaction with, for example, the alkaloid (-)-brucine. Fractional crystallization would never seem to be an effective analytical method yet it was used with some success in a forensic sciences context to confirm the presence of (L)-cocaine by a carefully contrived microcrystalline test. The physical properties... 

Nuclear Magnetic Resonance (NMR) spectroscopy is one of the most powerful analytical techniques in organic chemistry for elucidating the molecular structures of chemicals (1,2). Moreover, an NMR spectrum may be used like a fingerprint to identify a chemical by comparing it with its reference spectrum recorded from the authentic chemical under comparable conditions. The spectrum also reveals information on molecular conformation, isomerism, molecular dynamics, and diastereomers (3 6). 

Figure 19. Molecular structures of the hydrogen-bridge diastereomers (a) (R,S)-(a a)-[p-N2H2 Fe(PPr3)(tpS4) 2] 2 THF and (b) (R,S)-(P P)-[p-N2H2 Fe(PPr3)(tpS4) 2] 4 CH2C12 [The notation (a a) and (P P) corresponds with the notation introduced in Section IV.D for the thiolate atoms involved in bridges.]...

Recently, the diazene complex p-NAIG I e(PPr()(tpS4) 21 allowed characterization of both diastereomers by X-ray structure determination. Figure 19 shows their molecular structures (136). 

Suitable single crystals were obtained only for one of the diastereomers, 106-1, when recrystallized from chloroform or acetonitrile. Crystal structures of 106-1 in either solvent were determined, and while the packing was substantially different, the molecular structures in both crystals were essentially equal, and only one is shown (Fig. 62) and included in Table XXXII. 

Thus, because of the reversal of the CIP descriptor for the arsine upon coordination to palladium, the arsine produced by the mould was predominantly of the R configuration. The identity of the diastereomeric complexes was established by carrying out a crystal and molecular structure determination on the complex S,Rj )- + )- 27, which was obtained pure after nine recrystallizations of the less soluble component of a synthetic mixture of the two diastereomers. 

Figure 3.56 Molecular structure of the syn diastereomer of the face-to-face porphyrin Co2FTF 4-2, 1N-H, where the 4-atom struts are linked to the [3-pyrrole positions of the rings. The ordering of the groups in the struts is different to that in L in Scheme 3.2.

The reaction of (1S,4S)- and racemic camphor thiosemicarbazone [(1S,4S)- and rac-CTSC] leads to the formation of trans- and c/.v-isomers of (1S,4S)- or racemic camphor 5,-fluoro-4 -hydroxy-4, 5 -di(perfluoroalkyl)-l,3,-thiazolinyl-2/-hydrazones 211a-c and 213b,c (Scheme 189). The molecular structure of the two diastereomers (1S,4S,4 R,5 R)- and (1S,4S,4 S,5 S)- of 211a has been established by X-ray crystallography (03JFC(120)41). This result can be explained by a considerable contribution of the SN2 type of nucleophilic substitution both to the epoxide ring... 

---