Stereoisomerism diastereoisomers

The existence of ions or molecules in different structures (e.g. trigonal bipyramidal and square-based pyramidal [Ni(CN)5] ) is just a special case of diastereoisomerism. In the cases of, for example, tetrahedral and square planar [NiBr2(PBzPh2)2] (Bz = benzyl), the two forms can be distinguished by the fact that they exhibit different magnetic properties as we shall discuss in Section 21.9. To complicate 

---

Stereoisomers are compounds which have the same sequence of covalent bonds but differ in the relative position of their atoms in space. There are two forms of stereoisomerism diastereoisomers and enantiomers. 

Each macrocycHzahon could, in principle, lead to a mixture of diastereoisomers, depending on how the EtOOC residues at the two methano bridge C atoms are oriented with respect to each other (in-in, in-out, and out-out stereoisomerism) [92]. Usually, only out-out stereoisomerism has been observed so far. One exception is the in-out isomer 81. 

The spectrum of A -acetyl-L-phenylalanine also introduces us to another important consideration in H NMR spectra the effect of stereoisomerism on the H NMR spectrum. Each of a pair of enantiomers has the same physical properties (apart from their ability to rotate the plane of plane polarized light) and so will have identical H NMR spectra. The spectra for TV-acetyl-L-phenylalanine and its enantiomer, 7V-acetyl-D-phenylalanine, will therefore be identical in every respect. Diastereoisomers, on the other hand (no pun intended), have different physical properties and so give different H NMR spectra. 

One study has made use of abzyme stereoselectivity to resolve the four stereoisomers (R,R, S,S, R,S and SJi ) of 4-benzyloxy-3-fluoro-3-methyl-butan-2-ol [84-87] through the antibody-mediated hydrolysis of a dia-stereoisomeric mixture of their phenacetyl esters (Kitazume et al., 1991b). Antibodies were raised separately to each of four phosphonate diastereo-isomers [88-91], corresponding to the four possible transition states for the hydrolysis of the four diastereoisomeric esters (Fig. 25) (Appendix entry 1.12). Each antibody operated on a mixture of equal parts of the four dias-tereoisomers as substrate to give each alcohol in 23% yield, with >97% ee/de, and leaving the three other stereoisomers unchanged. By sequential action of the four antibodies in turn, the mixture of diastereoisomers could effectively be separated completely (Table 4). In a similar vein, Kitazume also resolved the enantiomers of l,l,l-trifluorodecan-2-ol with 98.5% enantiomeric excess (Appendix entry 1.11) (Kitazume et al., 1991a). 

In the third category the reactant is itself optically pure, and the site of chirality is adjacent to the site of the reaction. It is the chirality of this site which influences the stereoisomeric course of the reaction (diastereoselection) which then gives rise to two diastereoisomers in unequal proportions. For example in (/), if the aldehyde was (R)-Ph(Me)CH-CHO and the Grignard reagent MeMgX, the diastereoisomeric product pair, in unequal proportions, would be (2R,3R)-3-phenylbutan-2-ol and (2S,3R)-3-phenylbutan-2-ol as illustrated below. If the (S)-aldehyde had been used the diastereoisomeric pair would be (2S,3S) and 2R,3S). 

Any molecule with n asymmetric carbon atoms can exist in the form of 2 stereoisomers including 2W enantiomeric (mirror image) pairs. When the number of asymmetric carbon atoms exceeds two, so-called dia-stereoisomeric forms become possible. They possess different physical properties and are not mirror images. Enantiomers, however, are identical in physical properties with the exception of their behavior toward polarized light. The aldohexoses comprise 16 stereoisomers (8 enantiomeric pairs) belonging to the respective series. Within the d or l series the individual aldohexoses are diastereoisomers. The aldoses in D series are shown in Fig. 2-3. 

Figure 1.55. C6o-chlorin dyad obtained as mixture of four diastereoisomers that result from configurational (the asterisk marks the stereogenic center in question) and conformational dia-stereoisomerism (resulting from hindered rotation around the bond connecting the chlorin unit to the pyrrolidine ring).

Stereospecific syntheses of both E) and Z)-o -bisabolenes, (73) and (74) respectively, have been carried out (Scheme 9) and the spectral data for each diastereoisomer have been recorded/ The enantiomers of each have also been made starting from (+)- and (-)-limonene and as a result the j8-bisabolene present in the essential oil of Opoponax has been shown to be the (+)-(5,Z)-isomer, thus correcting a previous report. As might be expected the odour characteristics of the ( )- and (Z)-isomers are subtly different. Both dia-stereoisomeric racemic a-bisabolols (75) and (76) (only one enantiomer of each is shown) have been prepared from the two isoxazolidines (77) and (78) which, in turn, were derived from intramolecular cyclization of the nitrones of (6E)- and (6Z)-farnesal respectivelyFrom this work it is suggested that natural (-)-a-bisabolol must be (75) in contradiction to a recent report which held that (76) is the correct structure of the natural isomer. Further work will be needed to resolve this question. 

Stereoisomerism manifests itself in various forms, such as those related to enantiomers, epimers, diastereoisomers, meso compounds, geometrical isomers, etc. Generally speaking, the consequences of stereoisomerism for the disposition and biological action of xenobiotics in living... 

The best way to prepare large quantities of optically active sulfoxides makes use of optically active menthyl sulfinate. By esterification of p-toluenesulflnic acid with 1-menthol, a mixture of dia-stereoisomeric menthyl sulfinates is obtained. This esterification reaction shows no particular stereoselectivity and therefore a mixture of the two diastereoisomeric esters was obtained. However, it is possible to equilibrate these diastereoisomers in acidic media and to shift the equilibrium towards the... 

Stereoisomerism refers to molecular species that have the same composition and bond sequence but a different arrangement in space of their atoms. Stereoisomers that are characterized by different interatomic distances between certain atoms that are not bound directly are called diastereoisomers. Examples of diastereoisomers are cis-trans isomers of compounds containing C = C bonds and syn-anti isomers of compounds containing C = N bonds. Other diastereoisomers are not based on the presence of a double bond. For example, a molecule with more than one tetrahedral carbon that has different substituents may form diastereoisomers. For example, for a head-to-tail polymerized monosubstituted vinyl monomer, there are three possible structures indicated as isotactic, syndiotactic and atactic, which are schematically shown below ... 

The (S)-(-)-isomer (lb) was the one required for synthesis of the herbicides. Reaction of racemic 2-chloropropionic acid with one equivalent of (R)-(+)-a-phenylethylamine (2) gave a quantitative yield of the mixture of dia-stereoisomeric salts, which after four reciystallizations gave the required diastereoisomer in 20% yield [i.e., 40% based on the (S)-(-)-isomer present] with diastereomeric excess (de) 88% (determined by nuclear magnetic resonance [NMR]). The free acid (lb) was obtained quantitatively and without racemization on acidification of the salt, and the (R)-(+)-a-phenylethy lamine was also recoverable without racemization in 89% yield. 

In order to obtain an insight into the diastereoselectivity in the Diels-Alder reaction of acetylenic sulfinates, chiral (+)-trans-2-phenylcyclohexanol [35] was used in place of cyclohexanol in the synthesis of the dienophile. A 1 1 dia-stereoisomeric mixture of acetylenic sulfinates 69 and 70 was obtained. After separation, each diastereoisomer was subjected to a Diels-Alder reaction with cyclopentadiene. Although the reaction once again occurred readily at room temperature, to our disappointment an inseparable mixture of diastereomeric adducts (3 2 by NMR) was obtained for each sulfinate. Apparently, a more spatially demanding chiral auxiliary needs to be incorporated into the dienophile in order to generate chiral sulfinates which cycloadd with prominent diastereoselectivity. 

---