N-tert-butoxycarbonyl O-benzyl L-seryl L-valyl L-valine methyl ester 13 (14.60 g, 28.76 mmol) was dissolved in THF (220 mL) and the solution cooled to 0° C. (icebath). LiOH monohydrate (1.33 g, 31.7 mmol) was dissolved in de-ionized water (110 mL). The solution was cooled to 0° C. (icebath) and added dropwise to the solution of 13 over 25 min. The reaction mixture was stirred for an additional 2 h 35 min before solid NaHCO3 (4.83 g, 57.5 mmol) was added and the bulk of THF evaporated. The solution was diluted with water (220 mL) and washed with Et2O (2×220 mL). The solution was then acidified to pH 2 by addition of 2 M aqueous H2SO4 and extracted with EtOAc (3×350 mL). The combined organic fractions were dried with anhydrous MgSO4 and the solvent evaporated affording a slightly yellowish liquid. The residue was redissolved in EtOAc and precipitated by addition of hexane. The solvent was decanted off and the residue dried affording the title compound as an off-white solid (5.95 g, 42%); δH (200 MHz; d6-DMSO) 12.52 (1H, br s, COOH), 8.03 (1H, d, J 8, NH(Valx)), 7.65 (1H, d, J 9, NH(Valy)), 7.36-7.23 (5H, m, Ph), 7.10 (1H, d, J 8, NH(Ser(Bn)), 4.46 (2H, s, CH2Ph), 4.36 (1H, dd, J 7 and 9, CαH(Valy)), 4.28-4.22 (1H, m, CαH(Ser(Bn))), 4.10 (1H, dd, J 6 and 8, CαH(Valx)), 3.64-3.54 (2H, m, CH2), 2.08-1.91 (2H, m, CH(CH3)2), 1.38 (9H, s, (CH3)2), 0.88-0.80 (12H, m, CH3); δC (75 MHz; d6-DMSO) 172.6, 170.9, 169.4, 155.1, 138.1, 128.0, 127.4, 127.3, 78.3, 71.9, 69.8, 57.2, 56.8, 54.6, 31.1, 29.5, 28.1, 19.0, 18.9, 18.0, 17.7; m/z (ESI) 516.2663 ([M+Na]+; C25H39N3O7Na requires 516.2685)
