pro-R/pro-S convention

stereochemical descriptors for paired, chemically-like ligands at a prochiral centre; they derive from an extension of the sequence rule used to assign R,S descriptors to a chiral centre. In the prochiral (but achiral) structure, Caabc, the chemically-like ligands are the two atoms (or groups of atoms), a. For the sake of example assume that the sequence rule gives the priority sequence, a > b > c. In the molecular model (or projection formula) arbitrarily label the a groups as a′ and a″ and arbitrarily assign higher priority to one of them, e.g., a′ > a″. The modified priority sequence, a′ > a″ > b > c, is then considered with the model viewed from the side remote from c. If this gives a right-handed ordering for a′ → a″ → b, thus leading to R chirality for the arbitrarily created chiral element, the promoted a′ is designated the pro-R group; conversely, if the arbitrarily created chiral element reveals an S ordering, the promoted group is pro-S. Example: glycerol, HOCH₂−CHOH−CH₂OH, contains two chemically-like CH₂OH groups, and the normal priority sequence is OH > CH₂OH > H. In the Fischer projection formula with the central carbon atom having the OH group to the right and H to the left, arbitrarily assign the top C′H₂OH group priority over that at the bottom, C″H₂OH, giving the modified sequence, OH > C′H₂OH (top) > C″H₂OH (bottom) > H. When the model is viewed from the side remote from the lowest priority atom (H) the ordering, OH → C′H₂OH (top) → CH₂OH (bottom) is right-handed so that the arbitrarily created chiral element has R chirality; hence the top C′H₂OH is the pro-R group. Subscript letters R and S can be used to indicate whether an atom or group of atoms is pro-R or pro-S. This is most commonly done for the two hydrogens of a methylene group, with H_R indicating the pro-R hydrogen and H_S the pro-S hydrogen. For a methylene group, replacement of ¹H_R by ²H or³H gives a centre with R chirality; similar replacement of ¹H_S gives a centre with S chirality. See also enantiotopic.

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