Diisopropylethylamine → tert-butyl (2S)-2-[(2S)-2-amino-3-methylbutanamido]propanoate

To a mixture of the alanine-O-tert-butyl ester hydrogen chloride salt (98) (500 mg, 2.76 mmol) in dichloromethane (5 mL) was added Fmoc-val-OSu (99) (1.09 g, 2.51 mmol). DIPEA (0.96 ml, 5.5 mmol) was added and the reaction mixture was allowed to stir at an ambient temperature for 16 h. The mixture was poured into dichloromethane (100 mL) and washed with 1N HCl (50 mL) and water (50 mL) before being dried over magnesium sulfate. The material was chromatographed on a 2 mm radial chromatotron plate eluting with 1 to 5% methanol/dichloromethane gradient and product containing fractions were combined and concentrated. The resulting residue was dissolved in dichloromethane (16 mL) and piperidine (4 mL) was added. The mixture was stirred for 10 min before being concentrated under reduced pressure. The resulting residue was chromatographed on a 2 mm plate eluting first with ammonia-saturated dichloromethane followed by 5% methanol in ammonia-saturated dichloromethane to give 494 mg (2.02 mmol, 81% for two steps) of 97: 1H-NMR (400 MHz, CDCl3) δ 7.78 (bs, 1H), 4.47 (m, 1H), 3.30 (d, 1H), 2.30 (m, 1H), 1.38 (d, 3H), 1.47 (s, 9H), 1.00 (d, J=7.0 Hz, 3H), 0.84 (d, J=6.9 Hz, 3H).