D/L nomenclature

Considering the stereochemistry and nomenclature of lanthionines, the variety may become somewhat confusing. Here, we have endeavored to restrict ourselves to the d,l-nomenclature however, because of the diversity of nomenclatures found in the literature, this is not always possible. In this context, it has to be noted, that L-cysteine is (27 )-cysteine... 

The d, l nomenclature is chosen in this chapter in order to unmistakably distinguish homochiral stereoisomers. According to the specifications of the Cahn-Ingold-Prelog nomenclature L-l would equal (S)-l, while L-2 equals (J )-2. 

Remember that the JR/S, +/-, and d/l nomenclatures all arise from different... 

Table 3.8. The chemical structure of glycerophospholipids. The generic term phosphatidyl- stands for l,2-diacyl-sn-glycero-3-phospho-, in which sn denotes stereospecific numbering (in the D/L nomenclature this corresponds to 2,3-dlacyl-D-glycero-l-phospho-). The OH-groups of the glycerol that are not bound to the head group are esterified to fatty acid.

Remember that the R/S, +/-, and d/l nomenclatures all arise from different observations and the fact that a molecule has, say, the R configuration gives no clue as to whether it will have + or - optical activity or be labelled D or L. Never try and label a molecule as d/l, or +/-, simply by working it out from the structure. Likewise, never try and predict whether a molecule will have a + or - specific rotation by looking at the structure. 

Problem 17.18. Describe how the d-l nomenclature system assigns the prefixes d- and l-. 

Ans. The d-l nomenclature system applies only for the case of a stereocenter attached to two different carbon substituents R and R, a hydrogen, and a hetero atom substituent X (e.g., OH or NH,)- The molecule is drawn so that the R and R substituents are vertical while the H and hetero atom substituents are horizontal. R, the carbon substituent which has the most highly oxidized carbon atom (the carbon with the least number of hydrogens) attached to the stereocenter, is placed up. The enantiomer with the X substituent on the right-hand side is the D-enantiomer while that with the X substituent on the left-hand side is the L-enantiomer ... 

Problem 17.19. The d-l nomenclature system was originally used for the two glyceraldehydes. Show that the two structures below are correctly named as d and l ... 

As with carbohydrates, D,L-nomenclature is preferred with amino acids. 

D,L nomenclature is still used for sugars (Chapter 24) and amino acids (Chapter 26). 

Acylglycerols (formerly called glycerides) are esters in which one, two or three fatty acids have reacted with the alcohol, glycerol. The two CHjOH groups of glycerol cause difficulties for D, l nomenclature (Section 1.3). The remedy was the introduction of... 

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