Erythro redirects here. For the fictional planet, see Erythro.
Diastereomers are a type of a stereoisomer. Diastereomerism occurs when two or more stereoisomers
of a compound have different configurations at one or more of the equivalent stereocenters
and are not mirror images of each other. When two diastereoisomers differ from each other
at only one stereocenter they are epimers. Each stereocenter gives rise to two different
configurations and thus increases the number of stereoisomers by a factor of two.
Diastereomers differ from enantiomers in that the latter are pairs of stereoisomers that
differ in all stereocenters and are therefore mirror images of one another. Enantiomers
of a compound with more than one stereocenter are also diastereomers of the other stereoisomers
of that compound that are not their mirror image. Diastereomers have different physical
properties and different chemical reactivity. Diastereoselectivity is the preference for
the formation of one or more than one diastereomer over the other in an organic reaction.
Syn / anti When the single bond between the two centres
is free to rotate, cis/trans descriptors are invalid. Two widely accepted prefixes used
to distinguish diastereomers on sp³-hybridised bonds in an open-chain molecule are syn and
anti. Masamune proposed the descriptors which work even if the groups are not on adjacent
carbons. It also works regardless of CIP priorities. Syn describes groups on the SAME face while
anti describes groups on OPPOSITE faces. The concept applies only to the Zigzag projection.
The descriptors only describe relative stereochemistry rather than absolute stereochemistry.
Erythro / threo Two older prefixes still commonly used to
distinguish diastereomers are threo and erythro. In the case of carbohydrates, when drawn in
the Fischer projection the erythro isomer has two identical substituents on the same
side and the threo isomer has them on opposite sides. When drawn as a zig-zag chain, the
erythro isomer has two identical substituents on different sides of the plane. The names
are derived from the diastereomeric aldoses erythrose and threose. These prefixes are
not recommended for use outside of the realm of carbohydrates because their definitions
can lead to conflicting interpretations. Another threo compound is threonine, one of
the essential amino acids. The erythro diastereomer is called allo-threonine.
Multiple stereocenters If a molecule contains two asymmetric carbons,
there are up to 4 possible configurations, and they cannot all be non-superimposable
mirror images of each other. The possibilities continue to multiply as there are more asymmetric
centers in a molecule. In general, the number of configurational isomers of a molecule can
be determined by calculating 2n, where n = the number of chiral centers in the molecule.
This holds true except in cases where the molecule has meso forms.
Example Tartaric acid contains two asymmetric centers,
but two of the "isomers" are equivalent and together are called a meso compound. This
configuration is not optically active, while the remaining two isomers are D- and L- mirror
images, i.e., enantiomers. The meso form is a diastereomer of the other forms.
The families of 4, 5 and 6 carbon carbohydrates contain many diastereomers because of the
large numbers of asymmetric centres in these molecules.
Applications As stated, two diastereomers will not have
identical chemical properties. This knowledge is harnessed in chiral synthesis to separate
a mixture of enantiomers. This is the principle behind chiral resolution. After preparing
the diastereomers, they are separated by chromatography or recrystallization. Note also the example
of the stereochemistry of ketonization of enols and enolates.
See also Cahn–Ingold–Prelog priority rules for
nomenclature. References