Naming compounds enantiomers

The term S) represents the absolute configuration seen in 8 and the term R) represents the absolute configuration seen in 9. Which structure is ( ) -1-bromo-l-chloroethane, 8 or 9 Given only the name, it is impossible to know The names R) and (S) are determined by an arbitrary (but universally accepted) set of rules. The (+) or (-) label is the specific rotation, a physical property. Therefore, if only the names R or S) are given, that information does not allow one to correlate the structure with the sign of the specific rotation. It is necessary to identify 8, isolate it, put it into a polarimeter, and then determine its specific rotation. Alternatively, the same can be done for 9. Only after the physical measurement of each named compound can specific rotation of an enantiomer be correlated with the absolute configuration of that enantiomer. This is a very important lesson for this chapter. 

Another example of dual PPARo/y agonists, namely, compounds with the general formula V, for which enantiomers with inverse spatial orientation demonstrated improved potency, is shown in Fig. 3.5. 

Divalent sulfur compounds are achiral, but trivalent sulfur compounds called sulfonium stilts (R3S+) can be chiral. Like phosphines, sulfonium salts undergo relatively slow inversion, so chiral sulfonium salts are configurationally stable and can be isolated. The best known example is the coenzyme 5-adenosylmethionine, the so-called biological methyl donor, which is involved in many metabolic pathways as a source of CH3 groups. (The S" in the name S-adenosylmethionine stands for sulfur and means that the adeno-syl group is attached to the sulfur atom of methionine.) The molecule has S stereochemistry at sulfur ana is configurationally stable for several days at room temperature. Jts R enantiomer is also known but has no biological activity. 

An equal molar mixture of the dextrorotatory and levorotatory enantiomers of a chiral compound is called a racemic mixture or a racemate. Racemates do not show overall optical rotation because the equal and opposite rotations of the two enantiomers cancel each other out. A racemic mixture is designated by adding the prefix (+) or rac- before the name of the molecule. 

All carbohydrates can exist in either of these two forms and the prefix of D or L only refers to the configuration around the highest numbered asymmetric carbon atom. Enantiomers have the same name (e.g. D-glucose and L-glucose) and are chemically similar compounds but have different optical properties. The majority of naturally occurring monosaccharides, whether they be aldoses or ketoses, are of the D configuration. 

The replacement of the oxygen atom in sulfoxides by nitrogen leads to a new class of chiral sulfur compounds, namely, sulfimides, which recently have attracted considerable attention in connection with the stereochemistry of sulfoxide-sulfimide-sulfoximide conversion reactions and with the steric course of nucleophilic substitution at sulfur. The first examples of chiral sulfimides, 88 and 89, were prepared and resolved into enantiomers by Phillips (127,128) by means of the brucine and cinchonidine salts as early as 1927. In the same way, Kresze and Wustrow (129) were able to separate the enantiomers of other structurally related sulfimides. 

Mithramycin (24) bears the terminal sugar unit -mycarose. By a similar preparation as for the methyl-branched glycals and the -enantiomer of namely the labile g-mycaral 40 w s obtained ( ). Its NIS condensation with the 3, 4 -Saol disaccharide proceeds smoothly, but the reaction turned out to be rather slow with respect to the stability of the g-mycaral. This results in only a modest yield (12%) of the trisaccharide which was hydrogenolyzed to compound 4. This in turn constitutes an isomer of the trisaccharide sequence in... 

In their original purification, Persoons et al. (1974, 1976) identified several active peaks, one of which was named periplanone-A (Persoon s periplanone-A is isoperiplanone-A) (Persoons et al., 1990), but two of the four possible diastereomers of isoperiplanone-A had different spectral data from isoperiplanone-A (Shizuri et al., 1987a,b) and one of them was tested and found to be behaviorally inactive (Persoons et al., 1990). Several other compounds were proposed, synthesized, and shown to be inactive (Shizuri et al., 1987c Macdonald et al., 1987). However, an active compound (Hauptmann s periplanone-A), similar to but with different spectral data from Persoons periplanone-A, was isolated earlier (2 Table 6.2) (Hauptmann etal., 1986 Nishino etal., 1988). Both enantiomers of Hauptmann s periplanone-A were synthesized and only (-)-periplanone-A was found to be highly active (Kuwahara and Mori, 1990 Okada etal., 1990a). Persoons isoperiplanone-A... 

As radafaxine contains two asymmetric centers, there are four possible isomers, namely (R,R)-, (R,S)-, (S,S)- and (S,R)-. In-house knowledge on related molecules, computer modeling, and various calculahons indicated that only two enantiomers, the (S,S)- and (R,R)-, that is compounds 2a and 2b, were likely to be formed and be stable under normal conditions. ... 

The problem arises how to name the different stereoisomers of a compound when there are more than two.2 Enantiomers are virtually always called by the same name, being distinguished by R and S or n and l or (+) and (-). In the early days of organic chemistry, it was customary to give each pair of enantiomers a different name or at least a different prefix (such as epi-, peri-, etc.). Thus the aldehexoses are called glucose, mannose, idose. 

Tetrahedral complexes du not exhibit geometrical isomerism. However, they are potentially chiral just as is tetrahedral carbon. The simple form of optical isomerism exhibited by most organic enantiomers, namely four different substituents, is rarely observed because substituents in tetrahedral complexes are usually too labile10 for the complex to be resolved, i.e., they racemize rapidly. However, an interesting series of cyclopentadienyliron phosphine carbonyl compounds (see Chapter 15 for further... 

NET facility at Brookhaven National Laboratory was used.8 There are several examples where the atomic coordinates given in a paper refer to the enantiomer of the compound named therein in such instances, the name has been changed to correspond to the handedness of the coordinates reported. 

D is correct. A has no chiral carbon. B is named improperly. It is a trichloro compound. C is a meso compound. In D, the number 4 carbon is chiral. Any chiral molecule has an enantiomer. 

Two compounds can be stereoisomers only if they have the same constitution. Thus, you should compare first the constitution of the two structures and then their stereochemistry. The best way to compare constitutions is to assign a systematic (IUPAC) name to each molecule. Also remember that enantiomers are nonsuperposable mirror images, and diastereomers are stereoisomers that are not enantiomers. 

Related compound (C) is the (R)-enantiomer of clopidogrel bisulfate, and therefore has the systematic name methyl-(—)-(R)-(o-chlorophenyl)-6,7-dihydrothieno[3,2-c]pyridine-5(4H)-acetate, hydrogen sulfate salt [2, 6, 7],... 

---