|University Federico II
of Naples, Italy
Diastereoisomers can be generated by various structural constraints. One of these is the presence in the molecule of a cyclic atom connection. This creates two faces in the molecule and the atoms laying on one side cannot be exchanged with the atoms on the other side, unless bonds are cleaved.
For example, β-D-glucopyranose and β-D-mannopyranose are two diastereoisomers because they share the same molecular formula, C6H12O6 and the same atom connectivity but they differ for the orientation of atoms (blue) bonded to the red carbon. In particular, in β-D-glucopyranose the H atom is on the opposite molecular face with respect to the H atoms on adjacent carbon atoms, whereas in β-D-mannopyranose it is on the same face.
Obviously, β-D-glucopyranose and β-D-mannopyranose exhibit different properties, for example the first one is sweet, the second is bitter.
The C=C double bond is another structural constraint that may give rise to diasteroisomers.
cis 2-butene trans 2-butene
Both cis- and trans-2-butene share the same molecular formula, C4H8 and the same atom connectivity but they differ for the orientation of atoms. In the cis isomer both H atoms are behind the vertical plane, whereas they are on opposite sides in the trans isomer.
These diastereoisomers are also known as geometrical isomers.