To a solution of methyl (5R)-5-((trimethylsilyl)ethynyl)-L-prolinate (1.6 grams, 7.48 mmol), dimethylaminopyridine (913 mg, 7.48 mmol), N-methylmorpholine (1.23 mL, 11.22 mmol), and N-(tert-butoxycabonyl)-L-leucine monohydrate (2.24 g, 8.98 mmol) in dichloromethane (30 mL) at room temperature was added 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (1.72 g, 8.98 mmol). The resulting mixture was stirred 16 hours at room temperature, and partitioned between ethyl acetate (200 mL) and 1 MHO (200 mL). The aqueous layer was further extracted with ethyl acetate (200 mL). The combined organic layers were dried (sodium sulfate), filtered, and concentrated. The residue was chromatographed with a Biotage 40 M cartridge with 40% ethyl acetate/hexane provide the titled compound. MS (DCI/NH3) m/e 439 (M+H)+.
