6.1 Erythrose and threose
6.1 Erythrose and threose
The simplest sugar glyceraldehyde, an aldotriose, is a very good example of the compounds with one asymmetric carbon atoms (▶go to S2.2 Eclipsed and Staggered FormsChapter 5.5, 21 and 22). Sugars with four carbon atoms are also a good example of the compounds with two asymmetric carbon atoms. Sugars which have one more HCOH unit in the middle of the molecule as compared with glyceraldehydes are aldotetroses 1. These have two asymmetric carbon atoms C2 and C3. For each asymmetric carbon atom, there are two possible configurations, R or S, regardless of the configuration of other asymmetric carbon atoms. This is valid whatever is the number of asymmetric carbon atoms.
A Fischer projection for compounds with two or more asymmetric carbon atoms will be drawn in the following way.1) Carbon atoms are arranged vertically. As previously mentioned, the most highly oxidized carbon atom occupies the top position (C1). 2) Horizontal ligands are above the plane of paper while vertical ligands are below the plane of paper. 3) As a matter of fact, the carbon chain does not form a straight line. So not all carbon atoms can be on the plane of the paper at the same time. When you consider the configuration of a given carbon atom, you have to suppose that carbon is on the plane although other carbon atoms may be out of the plane.
For reference, the name of each stereoisomer is given. Note that two names are used to differentiate the stereochemistry of ligand. Erythrose has the same two ligand on the same side of the Fischer projection (2, 3) while threose has on the opposite side (4, 5).
This nomenclature is extended to all compounds with two asymmetric carbon atoms. Isomers having the same stereochemistry as that of erythroseis given prefix "erythro-", while isomers having the same stereochemistry as that of threose is given prefix "threo-"