Prefix R and

The prefixes R and S enable us to specify unambiguously the absolute configuration of a compound, because their use does not depend on a relationship to any other compound. But, by the same token, the letters R and S do not immediately reveal configurational relationships between two compounds we have to work out and compare the configurations in each case. 

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... 

Solution This problem deals with configuration, the arrangement of atoms that characterizes a particular stereoisomer. One method of assigning configuration, called the Cahn-Ingold-Prelog Method, is by use of the prefixes R and S. Two steps are involved in this process ... 

One enantiomer is distinguished from the other enantiomer in a pair of enantiomers by placing a prefix before the name of the compound. There are two nomenclature systems for this purpose. The most general system is the lUPAC R-S system where the prefixes R- and S- are used to distinguish between the enantiomers. We will not consider the R-S system since it is not universally used by biological scientists. The latter more often use an older nomenclature system which uses the prefixes d- and l-. 

We use the shorthand prefixes r and m to designate the racemic mixture of enantiomers and the meso compound, respectively. The currently accepted nomenclatures are 3R54i 5-dimethylhexane for the racemate and 3/ 45-dimethylhexane for the meso stereoisomer. 

The prefixes R and S are labels used in nomenclature. Rules on assigning R,S are found in Section 5.6. 

The Naming of Chiral Drugs. The nomenclature of chiral compounds is difficult and sometimes confusing. We have already introduced the prefixes R and S, (- -) and (—), and D and L. We have already seen many common names for drugs that have an implied chirality, as in methamphet-amine and ephedrine. The hyphenated prefixes R/S, D/L, and + / — render the alphabetic indexing of pharmaceutics problematic. One approach to this problem that has been used to help with this issue is to select drug names where the chirality has been imbedded into the common chemical name. One example that we have discussed is esomeprazole,... 

Through the years, several names and abbreviations for HNLs have been used, e.g., emulsin (for extracts from defatted almond meal), oxynitiilase, mandelonitrile lyase, MDL, MNL, and acetone cyanohydrin lyase, all with different prefixes like D, l, (R), (5), (+), (—), or a. Throughout this chapter, the designation hydroxynitrile lyase, abbreviated as HNL and the prefixes R and S, will be used. 

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