A mixture of (3S)-3-[(tert-butoxycarbonyl)amino]-3-(5-tert-butyl-1H-benzimidazol-2-yl)-2-methylpropanoic acid (Preparation 30, 7.25 g, 19.33 mmol), Boc2O (5.9 g, 27.0 mmol), pyridine (1 mL) and NaHCO3 (2 g, 25 mmol) in THF/DMF (200 mL/100 mL) was stirred at room temperature for 18 hours. 0.5M Ammonia in dioxane (150 mL) was added and the reaction continued stirring at room temperature for a further 18 hours. The reaction was concentrated to dryness and diluted with saturated aqueous NaHCO3 solution. The solution was extracted with 2-MethylTHF and concentrated in vacuo. The residue was purified using reverse phase column chromatography eluting with 5-70% acetonitrile in 0.1% formic acid in water to afford the title compound as a mixture of diastereomers (3 g, 43%).
