Chemistry - STEREO ISOMERISM Question with Solution | TestHub
For many centuries, the Chinese have used extracts from a group of herbs known as ephedra to treat asthma. A compound named ephedrine has been isolated from these herbs and found to be a potent dilator of air passage in the lungs.
(a) How many stereoisomers does ephedrine have ?
(b) The stereoisomer shown here is the one that is pharmacologically active. What is the configuration of each of the asymmetric centres ?
ephedrine : 
Options:
Answer:
Solution:
Ans. (a) 4 stereoisomers. (b) The pharmacologically active stereoisomer is (1R,2S).
Sol. (a) Ephedrine (Ph-CH(OH)-CH(NHCH3)-CH3) has two stereocentres (C-1 bearing OH and C-2 bearing NHCH3) with no internal symmetry, so the number of stereoisomers is 22 = 4 : (1R,2S) and (1S,2R) - the ephedrine enantiomeric pair - and (1R,2R) and (1S,2S) - the pseudoephedrine enantiomeric pair.
(b) CIP priorities at C-1 : OH C-2 (N, C, H) phenyl (C, C, H) H; with H behind, 1 2 3 runs anticlockwise : S... C-1 is R. At C-2 : NHCH3 C-1 (O, C, H) CH3 H : C-2 is S. The active stereoisomer is therefore (1R,2S).
