D and L system

There are two systems to designate configuration of enantiomers the D and L system, and the (R) and (S) system (also known as the Cahn-Ingold-Prelog system). 

Another naming system for stereoisomers, the D and L system, is described in Chapter 3. A molecule with a single chiral center (glyceraldehydes, for example) can be named unambiguously by either system. 

At the time the d and l system of relative configurations was introduced, chemists could not determine the absolute configurations of chiral compounds. They decided to draw the d series with the glyceraldehyde OH group on the right, and the l series with it on the left. This guess later proved to be correct, so it was not necessary to revise all the old structures. 

The sequence rule was introduced by Cahn, Ingold and Prelog (1956) as an aid to nomenclature of optically active molecules. The authors stated that use of the symbols (R) and (S) would remedy ambiguities that arose when the D and L system was applied outside the carbohydrate and amino-acid series. They also claimed that it would systematize the storage of stereochemical information and assist retrieval, e.g. for building a molecular model. The utility of the system is greatest when two or three chiral atoms are present in the same molecule. A simplified version of the sequence rule is presented by Cahn (1964). 

The R and S system is widely accepted for most organic compoimds, but for historical reasons we often use the d and l system for carbohydrates and amino acids. By convention, a Fischer projection is drawn vertically with the... 

The D and L convention for naming enantiomeric forms of a molecule is discussed in Chapter 23 on the acompanying website it tends to be used for carbohydrates and amino acids. It is rather an old-fashioned system and was developed before X-ray crystallography allowed the determination of the absolute configuration of molecules. For sugars, when the d and l system is used, it is based on the Fischer projection of the molecule and everything is compared to glyceraldehyde (2,3-dihydroxypropanal). 

The application of the d and l system to amino acids uses the CORN rule. This looks at the positions of the COOH, R and NH2 groups around the chiral centre. The molecule is drawn in the tetrahedral form with the single hydrogen atom pointing away from you. If, moving from the COOH to R to NH2, you go in a clockwise direction, the structure is the o-form if you go anticlockwise it is the L-form (Figure 20.69). 

Adrenaline is a convenient starting-point for a discussion on orientation in the D- and L-system. Natural (laevorotatory) adrenaline and noradrenaline have been shown by degradation to have the same configuration as D ( - )-mandelic acid (Pratesi et al.y 1959), and l( + )-mandelic acid has... 

---