A solution of diisopropylamine (8.4 mL) in tetrahydrofuran (100 mL) was cooled to -78° C. under argon and a solution of n-butyl lithium (30 mL of 2.5M in hexane) was added. The mixture was stirred at -78° C. for 30 minutes and at 0° C. for 10 minutes. After being recooled to -78° C., a solution of N-(diphenylmethylene)-glycine ethyl ester (Tetra. Lett., (1978), 2541, 4625) (15.4 g) in tetrahydrofuran (50 mL) was added, the mixture was stirred for 1 hour at -78° C. and a solution of 1-(2-chlorophenyl)methyl-5-chloromethyl-2-propylthio-1H-imidazole hydrochloride (Example 1 (iii)) (9.4 g) in dry dimethylformamide (20 mL) was added. The mixture was then stirred at ambient temperature for 18 hours, poured into saturated ammonium chloride solution and the aqueous layer was extracted with methylene chloride. The organic extracts were washed with water, dried with magnesium sulfate concentrated and chromatographed over silica gel with 1% methanol in methylene chloride to afford 6.88 g of 3-[(2-chlorophenyl)methyl]-2-propylthio-N-(diphenylmethylene)histidine ethyl ester. This product (2.59 g) was dissolved in methylene chloride (52 mL), aqueous 1N hydrochloric acid solution (52 mL) was added and the mixture was stirred at 25° C. for 18 hours. The aqueous layer was separated, neutralized to pH 10.5 with sodium carbonate and the product was extracted into methylene chloride. The organic extract was dried with magnesium sulfate and concentrated to give 1.29 g (71%) of 3-[(2-chlorophenyl)-methyl]-2-propylthiohistidine ethyl ester as an oil.
