A portion (313 mg) of the crude 2-((hydroxy(methyl)phosphinyl)methyl)-4-phenylbutanoic acid from Step B was dissolved in tetrahydrofuran (3.0 mL) and 4 Å molecular sieves (0.40 g) were added. The mixture was stirred at room temperature and 1,1'-carbonyldiimidazole (215 mg, 1.33 mmol) was added, followed 15 min later by L-leucine N-phenylamide (297 mg, 1.44 mmol).7After 22 h, the mixture was diluted with ethyl acetate (30 mL) and washed with 2N aqueous hydrochloric acid (2×15 mL) and saturated aqueous sodium chloride (15 mL). The organic layer was dried (sodium sulfate) decanted, and evaporated to give 511 mg of colorless foam. The two diastereomers of a portion (145 mg) of the crude product were separated by high pressure liquid chromatography on a 22 mm×25 cm reverse phase column (Whatman ODS-3, Partisil 10) eluting with 60% methanol/40% water/0.05% trifluoroacetic acid (flow rate of 25 mL/min, detection at 250 nm). This yielded the more mobile isomer (43 mg, 31% yield for Steps B-C) and the less mobile isomer (51 mg, 36% yield for Steps B-C), both obtained as brittle films.
