For many centuries, the Chinese have used extracts from a group of herbs known as ephedra...

For many centuries, the Chinese have used extracts from a group of herbs known as ephedra to treat asthma. Chemists have been able to isolate a compound from these herbs, which they named ephedrine, a potent dilator of air passages in the lungs.

a. How many stereoisomers are possible for ephedrine?

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b. The stereoisomer shown here is the one that is pharmacologically active. What is the configuratfon of each of the asymmetric centers?