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Diastereomers are stereoisomers ( chemical compounds of the same constitution but different configuration ) which - in contrast to enantiomers - do not behave like image and mirror image. Diastereomers can be both chiral and achiral. As a rule, diastereomers differ from one another in physical ( melting point , boiling point , solubility , NMR spectrum , IR spectrum , etc.) and chemical properties.

Diastereomerism due to multiple stereocenters

Diastereomerism and enantiomerism using the example of
(1) (2 R , 3 R ) - erythrose , (2) (2 S , 3 S ) -erythrosis
(3) (2 R , 3 S ) - threose , (4) (2 S. , 3 R ) -threose
Enantiomers (above) and mesoform of tartaric acid (below, dashed mirror plane).
The meso form is a diastereomer of D and L tartaric acid.

There is diastereomerism between two molecular compounds with the same constitution and several stereocenters if they differ in at least one, but not in all stereocenters (example: the sugars D - glucose and L - galactose , which differ in three of four stereocenters).

  • If the two compounds differ in exactly one stereocenter, this diastereomerism is also called epimerism (example: D - glucose and D - galactose ).
  • If the configuration is different in all stereocenters between the compounds, there is enantiomerism that is not counted as part of the diastereomerism (example: D- glucose and L- glucose).

Cause of chirality in one molecule is usually substituted carbon -atoms, the four different radicals carry. Due to the tetrahedral structure of the carbon atom, two configurations are possible at such a stereocenter , which cannot be brought into congruence even by rotation. If a compound contains n  carbon atoms substituted in this way (i.e. n  stereocenters), a maximum of 2 n  configuration isomers can exist. Configuration isomers in which all asymmetric carbon atoms differ are enantiomers of which there are 2 n / 2 pairs. If the stereocenters in a molecule are of the same type, then there is a mirror plane in the molecule and the number of configuration isomers is smaller, since meso compounds are also present (see second figure). Diastereomers are mostly optically active , but meso compounds are achiral.

Unlike diastereomers, enantiomers have identical physical properties, but differ in the sign of their rotation value [(+) or (-)], in their chemical properties compared to other chiral molecules and thus also in their physiological properties.

Diastereomerism due to double bonds

( Z ) - ( E ) isomerism (also cis - trans isomerism) is part of diastereomerism and occurs due to the rigidity of double bonds due to π bonds . Double bonds and thus the bound groups cannot rotate freely. There are different spatial arrangements. Examples are the cis - trans isomerism of alkenes .

Diastereomerism in cyclic compounds

endo - exo isomerism is a diastereomerism thatoccurswith substituted bridged bicyclic hydrocarbons .

Also in unbridged bicyclic hydrocarbons (e.g., at. Cis - and trans -Decalin ) and substituted monocyclic compounds (e.g., at 1.2-dichloro. Cyclopentane ) occurs on diastereoisomerism; these two cases are often viewed as cis - trans isomerism , cf. above the section diastereomerism due to double bonds .


Chiral molecules differentiate between the possible enantiomers compared to other chiral molecules. This is because two different chiral molecules are in a diastereomeric relationship to one another. Diastereoselectivity occurs in a chemical reaction both between two chiral starting materials (or reagents ) and between a chiral and a prochiral or achiral starting material, so that one enantiomer is preferred on the product side. Once a chirality is present, it continues in subsequent systems.

In diastereoselective reactions, the different transition states of enantiomers also have different physical properties, which ultimately results in selectivity . Ultimately, enantioselective syntheses are always based on diastereoselective mechanisms, which is why the term diastereoselective synthesis is to be preferred.

Individual evidence

  1. ^ Entry on diastereoisomerism . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.D01679 Version: 2.3 ..