To a 0.5N potassium bis-trimethylsilylamide solution in toluene (9.2 ml, 4.60 mmol, 2.5 eq) is added dropwise at -78° C. cyclohexyl methyl ketone (0.58 g, 4.60 mmol, 2.5 eq) in THF (9 ml) with stirring under a nitrogen atmosphere over 10 minutes. After 20 minutes stirring at the same temperature, 18-crown-6 (1.216 g, 4.60 mmol, 2.5 eq) in THF (10 ml) is dropwise added to the mixture over two minutes. Further, the dipeptidealdehyde [14a] (1.0 g, 1.83 mmol) in THF (10 ml) is dropwise added over 15 minutes at -78° C., and the mixture is stirred for one hour at the same temperature. The reaction is quenched by adding a solution of acetic acid (0.60 g, 10 mmol, 5.5 eq) in THF (10 ml) and after the addition of saturated aqueous ammonium chloride (30 ml) the mixture is extracted with ethyl acetate (50 ml×3). The organic layer is washed with saturated aqueous sodium chloride, dried over MgSO4, concentrated to dryness in vacuo, and purified with silica gel chromatography (Lobar column, CH2Cl2 :MeOH=95:5) to obtain Boc-His(Ts)-1(S)-cyclohexylmethyl-2(S)-hydroxy-4-oxo-4-cyclohexyl-butylamide [10b] (0.18 g, 15%) in amorphous powder.
