Diastereomer

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

Diastereoisomerism due to several stereocenters

Between two molecular compounds having the same constitution and several stereocenters is available diastereoisomerism, if they differ in at least one, but not differ in all stereocenters (example: the sugar D -glucose and L- galactose, which differ in three of four stereocenters ).

• If the two compounds in exactly one chiral center, this diastereoisomerism is also Epimerie called (example: D- glucose and D -galactose ).
• The configuration in all stereocenters between the different compounds, there is enantiomerically which is not counted in the diastereoisomerism ( for example, D -glucose and L-glucose ).

Cause of chirality in a molecule are usually substituted carbon atoms bearing four different residues. Two configurations of such a stereocenter due to the tetrahedral structure of the carbon atom, it is possible, which can not be made ​​to coincide by rotation. If a compound n such substituted carbon atoms (ie, n stereocenters ), so a maximum of 2n configurational isomers can exist. Configurational isomers in which all asymmetric carbon atoms differ, are enantiomers, of which there are 2n / 2 pairs. Are the stereocenters in a molecule of the same kind, then there is a mirror plane in the molecule and the number of configurational isomers is smaller, as well as meso compounds are present (see the second figure). Diastereomers are usually optically active, meso compounds, however achiral.

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

Diastereoisomerism because of double bonds

(E) - (Z) isomerism (including cis -trans isomerism ) belongs to diastereoisomerism occurs and by the rigidity of the double bonds due to π - bonds. Double bonds and thus the bonded groups are not free to rotate. So there are different spatial arrangements. Examples are cis -trans isomerism of alkenes.

Diastereoisomerism in cyclic compounds

Endo -exo isomerism is diastereoisomerism that occurs when substituted bridged bicyclic hydrocarbons.

Even when unbridged bicyclic hydrocarbons (eg cis and trans-decalin ) and substituted monocyclic compounds (eg 1,2- Dichlorcyclopentan ) occurs diastereoisomerism; these two cases are often seen as cis -trans isomerism, see the section above diastereoisomerism basis of double bonds.

Diastereoselectivity

Chiral molecules differentiate themselves from other chiral molecules between the possible enantiomers. The reason is that two different chiral molecules are in a diastereomeric ratio. Diastereoselectivity occurs in a chemical reaction on both between two chiral starting materials (or reagents) as well as between a chiral and achiral or a prochiral starting material, so that on the product page one enantiomer is preferred. Once an existing chirality thus continues in rotation systems.

For diastereoselective reactions, the various transition states of enantiomers also have different physical properties, resulting ultimately results in the selectivity. Enantioselective syntheses always ultimately based on diastereoselective mechanisms, which is why the term is to be preferred Diastereoselective synthesis.