To a stirred solution of L-phenylalanyl-L-leucine, t-butyl ester (6 mmol) in 10 ml of dimethylformamide (under argon) was added a solution of [(S)-3-chloro-1-methyl-2-oxopropyl]- carbamic acid, 1,1-dimethylethyl ester (1.33 g, 6 mmol) in 20 ml of dimethylformamide, followed by sodium iodide (450 mg, 3 mmol) and sodium bicarbonate (504 mg,. 6 mmol). The reaction mixture was allowed to stir for 15 hours, and the solvent was then evaporated (<25° C., vacuum pump). The yellow residue was taken up into ethyl acetate (250 ml) and washed with water (4×25 ml) and brine (25 ml), then dried (sodium sulfate) and evaporated to a yellow residue (3.44 g). This was applied to a column of silica gel (Merck, 230-400 mesh, 250 g) and eluted with hexane-ethyl acetate (1.25:1). Fractions 25-40 (~30 ml each, homogeneous on TLC) were pooled and concentrated yielding 2.29 g of a colorless oil. Rapid recrystallization from ethyl acetate-hexane afforded 1.61 g of the title compound as a powdery white solid.
