A mixture of 110 mg (0.215 mmole) of (2R, 4S, 5S)-5-[N-(t-butoxycarbonyl)-3-(4-thiazolyl)-L-alanyl]amino-6-cyclohexyl-4-hydroxy-2,N-dimethylhexanamide [prepared as described in step (a) above], 2 ml of methanol and 2 ml of a 4N solution of hydrogen chloride in dioxane was stirred at room temperature for 30 minutes. At the end of this time, the solvent was removed by distillation under reduced pressure. The residue was mixed with benzene and the solvent was stripped from the mixture by distillation under reduced pressure. After this operation had been repeated three times, the final residue was dried in vacuo for 8 hours. The dried material was then suspended in 8 ml of anhydrous tetrahydrofuran, and 59.6 mg (0.215 mmole) of 2(R)-benzyl-3-(morpholinocarbonyl)propionic acid were added to the suspension. 0.042 ml (0.258 mmole) of 95% diethyl cyanophosphonate and 0.12 ml (0.86 mmole) of triethylamine were then added to the mixture, and the resulting mixture was stirred for 18 hours. At the end of this time, the reaction mixture was concentrated by distillation under reduced pressure, and the residue was purified by preparative thin layer chromatography on silica gel (using a 10:1 by volume mixture of methylene chloride and methanol as the developing solvent), to afford 50 mg of the title compound as a white powder.
