Diastereomers
Diastereomers
Organic compounds with single stereocenter are quite simple to work with. However, the situation becomes more complex when more than one stereocenter is present in a single organic compound. The number of stereoisomers given by compound having n number of chiral centers is mathematically expressed as.
Number of Stereoisomers = 2n
where n = number of chiral carbons
Organic compound containing one chiral center will have two possible stereoisomers. Similarly, organic compound with two chiral centers will have four possible stereoisomers. For example, Isoleucine (an amino acid) has two chiral centers.
There are four stereoisomers possible for isoleucine which are grouped into two pairs of enantiomers.
In above figure the stereoisomer (A) is enantiomer of stereoisomer (B) while, stereoisomer (C) is the enantiomer of stereoisomer (D). What about the relation between stereoisomers that are not mirror images of each other? We can see clearly that stereoisomer (A) is not enantiomer of stereoisomers (C) and (D).
This type of relationship is described by the term diastereomers. Diastereomers are those stereoisomers that are not mirror images of each other.
Hence for isoleucine there are four possible stereoisomers. Each stereoisomer has one enantiomer and two diastereomers. Following table shows the relationship between the four stereoisomers of isoleucine.
The relationship between the four stereoisomers is sketched below.
Another class of diastereomers is the cis-trans isomers. The double bond isomers (geometrical isomers) are always diastereomers without any exception. This is because the cis-trans isomers are not mirror images of each other thus, they are classified as diastereomers.
In case of cis-trans isomerism on ring, there is a possibility of one enantiomer formation. For example, in following cyclic compound the trans isomer can produce an enantiomer whereas the cis and trans isomers are diastereomers of each other.
