Isomeric forms of norephedrine
Norephedrine, also known as phenylpropanolamine (INN), differs structurally from ephedrine due to the absence of a methyl group on the nitrogen atom*. Like ephedrine, norephedrine has two optically active centers (chirality centers) at positions 1 and 2. It can therefore exist in a total of four different stereoisomers, two of which are mirror images of each other.
The illustration below is shown in Fischer projection. Strictly speaking, only the two stereoisomers with the 1R,2S and 1S,2R configurations are referred to as norephedrine, while the name norpseudoephedrine is used for the other two isomers.
*The prefix "Nor" indicates that a hydrogen atom is bonded in place of a methyl group in the molecule.
Phenylpropanolamine refers to a mixture (racemate) of D- and L-norephedrine, synonymous with DL-norephedrine.
Cathine, along with cathinone, is an active ingredient in the khat plant and has the 1S,2S configuration: (+) norpseudoephedrine
For more on stereoisomerism, see the information from Michigan State University, Department of Chemistry, USA
Further information