J

CHjjOH "ch2oh

D-erylhrose 0-threose

Figure 7 The configurations of D-(+) and L-(-)-glyceraldehyde, D-erythrose, and D-threose, according to the Fischer Convention.

hyde. In Fig. 7, the D configuration is assigned to the erythrose and threose series because of the D configuration at C-3.

The Fischer convention is often inexact and difficult to use, especially when complicated chemical transformations are required to convert the molecule under investigation into a molecule of known configuration. In addition, the assigned configuration, D or L, is often confused with the

FIGURE 8 The Cahn-Ingold-Prelog sequence rule, where L is the largest group attached to the chiral center, followed in size by M, S, and s'.

FIGURE 8 The Cahn-Ingold-Prelog sequence rule, where L is the largest group attached to the chiral center, followed in size by M, S, and s'.

Table 1 Commonly Used Sterochemical Terms

Stereoisomers

Enantiomeric molecule

Chiral molecule Enantiomers

Diastereomers

Optical activity

Optical rotation (a)

Dextrorotary, d or (+) rotation

Levorotatory, / or (-) rotation

Specific rotation [a]

Configuration Fischer convention

Cahn-Ingold-Prelog convention {R, S)

Molecules with the same consitution, but that differ in respect to the spatial arrangement of certain atoms or groups

A molecule that is not superimposable on its mirror image

A molecule having at least one pair of enantiomers Stereoisomers that are related as nonsuperimposable mirror images

Optical isomers that are not related as an object and its mirror image

A property of a chiral molecule—the ability to rotate a beam of plane polarized light

The angle that a beam of plane-polarized light is rotated by a chiral molecule

A clockwise rotation of a beam of plane-polarized light by a stereoisomer, usually used to denote a specific enantiomer of a chiral molecule, that is d- or {+)-ephedrine

A counterclockwise rotation of a beam of plane-polarized light by a stereoisomer, usually used to denote a specific enantiomer of a chiral molecule, that is /" or (-)- ephedrine A quantitative measure of the optical rotation, which is dependent on concentration or density of the chiral substance, length of the light path, temperature, and wavelength of light

The description of the spatial arrangement about a chiral atom

The assignment of configuration about a chiral atom by comparison to a standard, (D) — (+) - glyceraldehyde, usually by actual chemical transformation of the molecule under investigation into the standard The assignment of configuration about a chiral atom by designation of the sequence of substituents from the largest (L) to medium (M) to the smallest (S); a clockwise direction of the L-M-S sequence is assigned the R configuration and a counterclockwise direction is assigned the S configuration observed sign of rotation, d or I. Because of this, the Fischer convention has been almost entirely replaced by the Cahn-Ingold-Prelog convention.

B. The Cahn-lngold-Prelog Convention

The Cahn-lngold-Prelog convention was designated by its originators as the "sequence rule," since it designates the sequence of substituents around the asymmetric center (4). In this method, the substituents at the chiral center are first ordered according to their atomic number from the largest to the smallest. In the example presented in Fig. 8, the order is L (large), M (medium), S (small), and s' (smallest). The molecule is then oriented so that the smallest (s') substituent is directed away from the viewer. The configuration is then determined by whether the sequence L—M—S goes in a clockwise or counterclockwise direction. A clockwise direction is specified as JÎ (right), whereas the counterclockwise direction is designated as S (sinister).

This convention can be used to rapidly and unambiguously specify the configuration of a chiral center. As would be expected, for an enantiomeric molecule the direction of the sequence for one enantiomer is reversed for the other enantiomer. If one enantiomer has an R designation, its mirror image has the S configuration.

The Cahn-lngold-Prelog convention is also extremely useful for describing diastereomers. In this case, each chiral center is designated independently and the configuration of the whole molecule can be easily assigned. For example, instead of d- and /-pseudoephedrine, the assigned configurations are (R,S)- and (S,R)-ephedrine and (R,R)- and (S,S)-pseudoephedrine. The enantiomeric relationships within the ephedrine and pseudoephedrine molecules and the diastereomeric relationship between ephedrine and pseudoephedrine are readily discernible.

A summary of the nomenclature used in defining stereochemical configurations and related terms is presented in Table 1. For further rules and definitions, the reader is referred to a publication by the International Union of Pure and Applied Chemistry (IUPAC) (5).

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