130 mg of (S)-α-[(S)-α-[(t-butylsulphonyl)methyl]hydrocinnamamido]-N-[(1S, 2R, 3S )-1-(cyclohexylmethyl)-2,3-dihydroxy-4-methylpentyl]imidazole-4-propionamide and 130 mg of p-toluenesulphonic acid in 1 ml of trimethylacetaldehyde and 10 ml of methylene chloride are left to stand at room temperature for 24 hours. Thereafter, the reaction mixture is taken up in ether and the organic phase is washed in succession with 2N sodium bicarbonate solution and water, dried and evaporated. Chromatography of the residue on 30 g of silica gel using a 200:10:1 mixture of methylene chloride, methanol and ammonia yields (S)-N-[(S)-1-[(2R or S,4R,5S)-2-t-butyl-5-isopropyl-1,3-dioxolan-4-yl]-2-cyclohexylethyl]-α-[(S)-α-[(t-butylsulphonyl)methyl]hydrocinnamamido]imidazole-4-propionamide as a foam, MS: 701 (M+H)+.
