Separation of enantiomers of cimaterol, (-)-cimaterol and the use thereof in pharmaceutical compositions and animal feeds

An improved process for preparing (-)- 1-(4'-amino-3'-cyano-phenyl)-2-isopropylamino-ethanol and (+)-1-(4'-amino-3'-cyano-phenyl)-2-isopropylamino-ethanol by fractional crystallization, in which process racemic 1-(4'-amino-3'-cyano-phenyl)-2-isopropylamino-ethanol is reacted with an optically active auxiliary acid in order to form a mixture of diastereomeric salts, the two diastereomeric salts are separated by fractional crystallization, and the salts are then converted, back to the optically pure enantiomers, wherein the improvement comprises reacting the racemate with at least 2 equivalents of an optically active dibasic auxiliary acid selected from the group consisting of (-)-0,0'-dibenzoyl-L-tartaric acid, (+)-0,0'-dibenzoyl-D-tartaric acid, (-)-0,0'-ditolyl-L-tartaric add and (+)-0,0'-ditolyl-D-tartaric acid in a suitable solvent, while maintaining the temperature within a range from about 20.degree. C. to 30.degree. C. for a period of time up to two hours to produce a diastereomeric salt having an optical purity of at least ee=90%.