asymmetric transformation

The conversion of a racemate into a pure enantiomer or into a mixture in which one enantiomer is present in excess, or of a diastereoisomeric mixture into a single diastereoisomer or into a mixture in which one diastereoisomer predominates. This is sometimes called deracemization. If the two enantiomers of a chiral substrate A are freely interconvertible and if an equal amount or excess of a non-racemizing second enantiomerically pure chemical species, say (R)-B, is added to a solution of racemic A, then the resulting equilibrium mixture of adducts A · B will, in general, contain unequal amounts of the diastereoisomers (R)-A · (R)-B and (S)-A · (R)-B. The result of this equilibration is called asymmetrictransformation of the first kind. If, in such a system, the two diastereoisomeric adducts differ considerably in solubility so that only one of them, say (R)-A · (R)-B, crystallizes from the solution, then the equilibration of diastereoisomers in solution and concurrent crystallization will continue so that all (or most) of the substrate A can be isolated as the crystalline diastereoisomer (R)-A · (R)-B. Such a 'crystallization-induced asymmetrictransformation' is called an asymmetrictransformation of the second kind.