Under argon, 120 mg (0.23 mmol) of [(4S)-7-cyclopentyl-5-(4-fluorophenyl)-4-hydroxy-2,2-dimethyl-3,4-dihydro-2H-chromen-6-yl][4-(trifluoromethyl)phenyl]methanone (Example 12A) are initially charged in 2 ml of abs. toluene and cooled to −78° C. At this temperature, 0.59 ml (0.59 mmol) of diisobutylaluminum hydride solution (1 M in toluene) is slowly added dropwise, and stirring of the mixture is then continued at −78° C. for 2 h. Diisobutylaluminum hydride is then added in such an amount that no more starting material can be detected. Saturated potassium sodium tartrate solution is then added, and the mixture is diluted with water and extracted with ethyl acetate. The combined organic phases are washed once with saturated sodium chloride solution. The organic phase is dried over sodium sulfate, filtered and concentrated. The residue is purified by thick-layer chromatography (mobile phase: cyclohexane/ethyl acetate 5:1), resulting in the separation of the diastereomers.
