A Grignard solution prepared from 3.6 g of (S)-(+)-3-benzyloxy-2-methyl-propyl bromide (A. Fischli et al., Helv. Chim. Acta 60, 925 (1977)) and 360 mg of magnesium in diethyl ether is added dropwise within 15 minutes at 0° to 1.50 g of 2(S)-tert.-butoxycarbonylamino-3-cyclohexyl-propionaldehyde (J. Boger et al., J. Med. Chem. 28, 1779 (1985)) in 15 ml of diethyl ether After 30 minutes at 0°, the reaction solution is poured onto 50 ml of 1N HCl and the crude product (diastereoisomer ratio (4S):(4R)=4:1) is extracted with dichloromethane. The desired diastereoisomer having the 4(S)-configuration is isolated in pure form by flash chromatography on 250 g of silica gel 60 (eluant dichloromethane/ether 20:1). Rf (L)=0.10.
