Physical Properties of Stereoisomers
Aside from the pair of enantiomers, molecules with several chirality centers also consist of at least one additional stereoisomer that is diastereomeric to the pair of enantiomers. The enantiomers differ in the absolute configuration of each chirality center. However, the diastereomers differ in their relative configuration, that is, the configuration of any chirality center with respect to the other of the same stereoisomer.
For clarification, this may best be illustrated by analyzing the isomeric relationship of the tetroses, erythrose and threose, which contain two asymmetric carbons. Erythrose, as well as threose, consists of one pair of enantiomers. Each erythrose enantiomer is diastereomeric to each threose enantiomer. The erythrose enantiomers contain the absolute configurations (R,R) and (S,S), respectively. Thus they show opposite absolute configurations, but identical relative configurations, as the isomeric relationship of the asymmetric carbons of each enantiomer is the same. If one asymmetric carbon has an (R) configuration, the other also has an (R) configuration; if one asymmetric carbon has an (S) configuration, the other has an (S) configuration, as well. This relative configuration is characteristic of erythrose, while the stereoisomeric threose depicts the opposite relative configuration. That is, if one asymmetric carbon has an (R) configuration, the other has an (S) configuration and so on. Therefore, the threose enantiomers possess the absolute configurations of (R,S) and (S,R). The relative configuration of a molecule is described by the CIP descriptors of one enantiomer labeled with a "*". In the case of a molecule with two chirality centers, for instance, (R*,R*) or (S*,S*) indicates that both stereocenters have the same absolute configuration, while (R*,S*) or (S*,R*) indicates that both stereocenters have opposite absolute configurations. Thus, while the relative configuration of erythrose is (R*,R*), the relative configuration of threose is (R*,S*).
If the absolute configuration of only one chirality center and the relative configuration of a stereoisomer are known, the absolute configuration of the remaining chirality centers may be derived from this information.
Relationship between the illustration of three-dimensional molecules, their naming and their relative configuration
|Relative configuration (R*,R*)-, (erythro)-, or anti- erythrose||Relative configuration (R*,S*)-, threo-, or syn- threose|
- erythro, threo
- The terms erythro and threo are derived from the nomenclature of the carbohydrates erythrose and threose. The relative configuration at adjacent chirality centers may be described by these terms. If identical or similar substituents are on the same side of the vertical chain in the Fischer projection, the erythro isomer is shown. If the substituents are opposite each other, the threo isomer is shown in the Fischer projection. Today, the erythro, threo nomenclature is still used only in carbohydrate chemistry.
- (R*,R*) and (R*,S*)
- As previously mentioned, the relative configuration (R*,R*) means that two chirality centers have the same absolute configuration, (R,R) or (S,S). Therefore, the (R*,R*) isomer is identical to the erythro isomer. In contrast, the threo isomer has the relative configuration of (R*,S*).
- syn, anti
- In the zig-zag projection, the main chain of an acyclic compound lies within the illustration plane and has torsion angles of 180 degrees. That is, the main chain is drawn as an alternately angled line (zig-zag). Bonds to substituents behind the plane are represented by hashed lines, while solid wedges symbolize the bonds to substituents in front of the plane.
- According to Masamune (1980), if the two substituents of highest priority are on the same side of the illustration plane of a zig-zag projection, the relative configuration is termed syn. If they are on opposite sides, the relative configuration is called anti. In the case of adjacent substituents, the syn isomer is identical to the threo isomer, while the anti isomer is identical to the erythro isomer.
- The syn,anti nomenclature may also indicate the relative configuration of cyclic compounds. A cyclic compound whose substituents are on the same side of the ring is called syn. If the substituents are on opposite sides of the ring, this is called anti.
- It is extremely important to note that if the substituents are not adjacent, the erythro,threo nomenclature cannot be applied. However, the syn,anti nomenclature may also be used for describing the relative configuration of compounds with non-adjacent chirality centers. Today, the syn,anti nomenclature has therefore almost completely replaced the erythro,threo nomenclature.
- D, L
- The D,L nomenclature is still used in reference to carbohydrates. As follows, if the hydroxy group at the carbon farthest away from the chain's highest oxidized carbon is located to the right of the vertical chain in the Fischer projection, the stereoisomer is labelled D If the hydroxy group is on the left side, it is called L. Therefore, the D,L descriptors describe the absolute configuration of this carbon atom. The relative configuration is not described by the D,L descriptors, but by the carbohydrate name. Conventionally, the D and the L isomers of a carbohydrate (for instance, D- and L-erythrose, or D- and L-glucose) are enantiomers, that is, they have both the same relative configuration and a mirror-image relationship. Thus, D- and L-erythrose have the same relative configuration (R*,R*) and D- and L-threose have the same relative configuration (R*,S*). The relative configurations of the stereoisomers are therefore not distinguished by the D,L descriptors, but by the carbohydrate name (or by (R,S) or syn,anti nomenclature).