Preferred Label : asymmetric transformation;
IUPAC definition : 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 ( i R /i )-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 ( i R /i )-A·( i R /i )-B and ( i S /i )-A·( i R /i )-B. The
result of this equilibration is called asymmetric transformation of the first kind.
If, in such a system, the two diastereoisomeric adducts differ considerably in solubility
so that only one of them, say ( i R /i )-A·( i R /i )-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 ( i R /i )-A·( i R /i )-B. Such a 'crystallization-induced asymmetric
transformation' is called an asymmetric transformation of the second kind.;
Origin ID : A00485;
See also
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 ( i R /i )-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 ( i R /i )-A·( i R /i )-B and ( i S /i )-A·( i R /i )-B. The
result of this equilibration is called asymmetric transformation of the first kind.
If, in such a system, the two diastereoisomeric adducts differ considerably in solubility
so that only one of them, say ( i R /i )-A·( i R /i )-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 ( i R /i )-A·( i R /i )-B. Such a 'crystallization-induced asymmetric
transformation' is called an asymmetric transformation of the second kind.
