Stereochemistry Cahn-Ingold-Prelog priority system

Fio. 2. Application of the Cahn-Ingold-Prelog priority sequence system to mark tlie C-17 stereochemistry in the absence and presence of the COOCH3 substituent. 

FIGURE 1.9 cis trans (or Z/ ) stereochemistry. If a = a we use the cis/trans nomenclature, but if a a (which is the case for fatty adds because the terminus of the a group is the CH3 group and the terminus of the a group is carboxylate), we should more correctly use the Z/E system based on Cahn-Ingold-Prelog priority rules. In practice, it must be noted that most people still use the cis/treats system for the stereochemistry of fatty acids. 

In order to be able to define the absolute stereochemistry of a molecule, we therefore use another system of nomenclature which was proposed by Cahn, Ingold and Prelog in 1956. This system uses the prefixes R- and S-, derived from the Latin words rectus (right) and sinister (left) and the same system of priority setting that was discussed above in relation to E- and Z-isomers. We start by assigning the priorities of the four groups... 

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