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Diastereomers (or diastereoisomers) are stereoisomers that are not enantiomers (non-superposable mirror images of each other). Diastereomers can have different physical properties and different reactivity. In another definition diastereomers are pairs of isomers that have opposite configurations at one or more of the chiral centers but are not mirror images of each other 1.
In simpler terms two stereoisomers are said to be diastereoisomers if they are not mirror images of each other and one or more stereogenic centres differ between the two stereoisomers. According to this same definition, cis-trans isomerism is a form of diastereomerism.
If a molecule contains a single asymmetric carbon atom or stereocenter, it will have two mirror image forms. If a molecule contains two asymmetric carbons, there are up to 4 possible configurations, and they cannot all be mirror images of each other. The possibilities continue to multiply as there are more asymmetric centers in a molecule. In general, the number of diastereomers 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.
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.
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(natural) tartaric acid |
D-(-)-tartaric acid |
mesotartaric acid |
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The families of 4, 5 and 6 carbon carbohydrates contain many diastereomers because of the large numbers of asymmetric centres in these molecules. Two common prefixes used to distinguish diastereomers are threo and erythro. 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 sites. Cis-trans isomerism and conformational isomerism are also forms of diastereomerism.
Diastereoselectivity is the preference for the formation of one or more than one diastereomer over the other in an organic reaction.
Applications
As stated, two enantiomers will have identical physical properties, while diastereomers will not. 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.
References
- ^ Garrett, Grisham "Biochemistry" 2nd ed., 1999, p. 213
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