Trifluoroacetic acid → N-[(2R)-2-(thiazol-5-ylmethoxycarbonylamino)-3-phenylpropyl]-N-[(2S)-2-(thiazol-5-ylmethoxycarbonylamino)-3-phenylpropyl]amine

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The product from Example 26C (952 mg, 1.35 mmol) in dichloromethane (20 mL) was treated with trifluoroacetic acid (10 mL) at ambient temperature. After stirring for one hour, the mixture was concentrated. The residue was immersed in an aqueous solution of 10% K2CO3 (10 mL) and extracted with ethyl acetate (3×10 mL). The organic phases were combined and treated with a solution of 4-nitrophenyl 1,3-thiazol-5-ylmethyl carbonate [prepared from 4-nitrophenyl 1,3-thiazol-5-ylmethyl carbonate hydrochloride salt (857 mg, 2.71 mmol, 2.0 eq) by extraction with aqueous NaHCO3] in ethyl acetate (5 mL) at ambient temperature. After stirring for 1 hour, the mixture was washed with aqueous 10% K2CO3 (5×25 mL), brine (25 mL), dried over sodium sulfate, filtered, and the filtrate was concentrated. The residue was purified by flash column chromatography on silica gel eluting with (chloroform:methanol, 98:2) to provide the title compound (0.39, 30%). 1H NMR (CDCl3) δ 8.78 (s, 2H), 7.84 (s, 2H), 7.33-7.04 (m, 10H), 5.00-4.86 (m, 4H), 3.98-3.83 (m, 4H), 2.90-2.50 (m, 9H); MS (ESI+) m/z 566 (M+H)+.

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