Consider two chemical changes: one occurring at a tetrahedral
Both transformations convert C in each achiral reactant to a chirality center in the product. The two achiral reactants are classified as prochiral. C is a prochiralitycenter in
In achiral molecules with tetrahedral prochiralitycenters, substitution of one of the two x groups by w gives the enantiomer of the product that results from substitution of the other. The two x groups occupy mirror-image sites and are enantiotopic.
Enantiotopic groups are designated as pro-R or pro-S by a modification of Cahn–Ingold– Prelog notation. One is assigned a higher priority than the other without disturbing the priorities of the remaining groups, and the R,S configuration of the resulting chirality center is determined in the usual way. If it is R, the group assigned the higher rank is pro-R. If S, this group is pro-S. Ethanol and citric acid illustrate the application of this notation to two prochiral molecules.
Citric acid played a major role in the development of the concept of prochirality. Its two
The stereochemical aspects of many enzyme-catalyzed reactions have been determined. Then enzyme alcohol dehydrogenase catalyzes the oxidation of ethanol to acetaldehyde by removing the pro-R hydrogen (abbreviated as HR). When the same enzyme catalyzes the reduction of acetaldehyde to ethanol, hydrogen is transferred to the Re face.
When the achiral dione shown (below left) was incubated in water with baker’s yeast, reduction of one of the
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