Question

For centuries, Chinese herbal medicine has used extracts of Ephedra sinica to treat asthma. Ephedra as an "herbal supplement" has been implicated in the deaths of several athletes and has recently been banned as a dietary supplement. Phytochemical investigation of this plant resulted in isolation of ephedrine, a very potent dilator of the air passages of the lungs. Ephedrine also has profound effects on the cardiovascular system. The naturally occurring stereoisomer is levorotatory and has the following structure. Assign an \(R\) or \(S\) configuration to each chiral center.

Short answer

Answer: The configuration of each chiral center in the ephedrine molecule is (1S,2R)-ephedrine.

Step-by-step solution

Identify the chiral centers

Chiral centers are carbon atoms with four distinct groups attached to them. In the molecule of ephedrine, two chiral centers can be identified. These are the carbon atoms where the four distinct groups are attached.

Assign priorities using the Cahn Ingold Prelog rules

First, we must assign priorities to the four atoms or groups attached to each chiral center according to their atomic number. The higher the atomic number of the directly attached atom, the higher the priority. In case of a tie, we look at the next atoms in each group until the tie is broken.

Determine the R or S configuration for each chiral center

Now that the priorities are assigned, we can determine the R or S configuration for each chiral center. To do this, we must imagine looking at each chiral center with the lowest priority group pointing away from us. Then we need to determine whether the sequence of the remaining groups goes clockwise (\(R\) configuration) or counterclockwise (\(S\) configuration). For the first chiral center, we can observe that the priority sequence goes counterclockwise, which means it has an \(S\) configuration. For the second chiral center, the priority sequence goes clockwise, which means it has an \(R\) configuration. So the complete configuration for the molecule is (1S,2R)-ephedrine.