Compounds with Several Stereogenic Centers
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The Chinese shrub Ma Huang (Ephedra vulgaris) contains two physiologically active compounds ephedrine and pseudoephedrine. Both compounds are stereoisomers of 2-methylamino-1-phenyl-1-propanol, and both are optically active, one being levorotatory and the other dextrorotatory. Since the properties of these compounds (see below) are significantly different, they cannot be enantiomers. How, then, are we to classify these isomers and others like them?
|Ephedrine from Ma Huang:||m.p. 35 - 40 º C, [α]D = –41º, moderate water solubility||[this isomer may be referred to as (–)-ephedrine]|
|Pseudoephedrine from Ma Huang:||m.p. 119 º C, [α]D = +52º, relatively insoluble in water||[this isomer may be referred to as (+)-pseudoephedrine]|
Since these two compounds are optically active, each must have an enantiomer. Although these missing stereoisomers were not present in the natural source, they have been prepared synthetically and have the expected identical physical properties and opposite-sign specific rotations with those listed above. The structural formula of 2-methylamino-1-phenylpropanol has two stereogenic carbons (#1 & #2). Each may assume an R or S configuration, so there are four stereoisomeric combinations possible. These are shown in the following illustration, together with the assignments that have been made on the basis of chemical interconversions.
As a general rule, a structure having n chiral centers will have 2n possible combinations of these centers. Depending on the overall symmetry of the molecular structure, some of these combinations may be identical, but in the absence of such identity, we would expect to find 2n stereoisomers. Some of these stereoisomers will have enantiomeric relationships, but enantiomers come in pairs, and non-enantiomeric stereoisomers will therefore be common. We refer to such stereoisomers as diastereomers. In the example above, either of the ephedrine enantiomers has a diastereomeric relationship with either of the pseudoephedrine enantiomers.
For an interesting example illustrating the distinction between a chiral center and an asymmetric carbon Click Here.
The configurations of ephedrine and pseudoephedrine enantiomers may be examined as interactive models by .
- William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry