|Lecture Notes: 9 June||
In the D, L system chiral centers are designated by comparison with a standard compound, glyceraldehyde, drawn below as Fischer structures:
Any other compound can then be determined to be D or L by aligning analogous side groups. For example, for an amino acid, the carboxylate group cooresponds to the aldehyded group, the amino group to the alcohol, the R- group to the methyl alcohol and hydrogen to hydrogen. Serine is shown as an example:
Note that for multicenter molecules, the penultimate chiral center determines whether the molecule is classified as D or L.
With a single chiral center (stereocenter) we have seen that we get a mirror image pair of enantiomers (nonsuperimposable mirror image sterioisomers).
What do we see with multiple centers? The classic example is the set of four carbon sugars, Erythrose and Threose, the stereoisomers of 2,3,4-trihydroxybutanal, HOCH2CHOHCHOHCHO. Erythrose refers to the cis pair of enantiomers, while threose refers to the trans pair of enantiomers
So let's determine R and S and D and L for each of these stereoisomers and name them.
Erythrose and Threose are Diastereomers: stereoisomers that are not mirror images, that is they are not enantiomers.
So what are some other examples of diastereomers? Remember stereoisomers are compounds where the atoms are connected in the same way, but which have different geometries. Thus cis , trans -isomers are diastereomers, as are configurational (optical) isomers, that is isomers differing only by the configurations around chiral centers but for which the entire molecules are not mirror images.
Note that the maximum number of stereoisomers for a given molecule = 2n where n = the number of chiral centers. If this is the maximum, how would we have fewer than the maximum? Let's look at another clssic example, tartaric acid, 2,3-Dihydroxybutanedioic acid, HOCOCHOHCHOHCOOH. Note that unlike our previous example, 2,3,4-trihydroxybutanal, this formula is symmetrical.
As a result, while the trans isomers of tartaric acid form a pair of enantiomers like threose,
the cis isomer has a single stereoisomer because it has an internal mirror plane and is thus its own mirror image. It is a Meso compound: Having two or more stereogenic centers, but also having a mirror plane of symmetry in the molecule, so molecule is not chiral.
Note illustrations and discussion in your text.
Last modified 9 June 2004