Reaction Steps
L-valyl L-valine methyl ester trifluoroacetate 6 (11.62 g, 33.75 mmol) was dissolved in 60 mL DMF. N-tert-butoxycarbonyl O-benzyl L-serine 14 (9.97 g, 33.8 mmol) was added together with 10 mL DMF. The solution was cooled to 0° C. (icebath) before N,N-diisopropylethyl amine (4.36 g, 33.7 mmol), HOBt hydrate (5.17 g, 33.8 mmol) and EDC hydrochloride (7.12 g, 37.1 mmol) were added together with an additional 30 mL DMF. The reaction mixture was stirred overnight and the solvent evaporated. The residue was taken up in 200 mL EtOAc and washed with 2M aqueous H2SO4 (3×80 mL), 7.5% (w/w) aqueous K2CO3 (3×80 mL) and saturated brine (80 mL). The solution was dried with anhydrous MgSO4 and the solvent evaporated affording a white solid (16.22 g, 95%); δH (300 MHz; d6-DMSO) 8.20 (1H, d, J 8, NH(Valx)), 7.65 (1H, d, J 9, NH(Valy)), 7.35-7.23 (5H, m, Ph), 7.08 (1H, d, J 8, NH(Ser(Bn)), 4.46 (2H, s, CH2Ph), 4.35 (1H, dd, J 7 and 9, CαH(Valy)), 4.25 (1H, m, CαH(Ser(Bn))), 4.12 (1H, dd, J 6 and 8, CαH(Valx)), 3.63-3.54 (2H, m, CH2), 3.60 (3H, s, OCH3), 2.07-1.89 (2H, m, CH(CH3)2), 1.38 (9H, s, (CH3)3), 0.88-0.80 (12H, m, CH3); δC (75 MHz; d6-DMSO) 171.6, 171.0, 169.4, 155.1, 138.0, 128.0, 127.3, 127.3, 78.2, 71.9, 69.8, 57.4, 56.8, 54.5, 51.4, 31.1, 29.5, 28.0, 18.9, 18.7, 18.2, 17.7; m/z (ESI) 530.2824 ([M+Na]+; C26H41N3O7Na requires 530.2842)
