Reaction Steps
N-tert-butoxycarbonyl O-allyl L-serine (3) (11.31 g, 46.10 mmol) was dissolved in 40 mL DMF. A solution of L-valyl L-valine methyl ester.trifluoroacetate 6 (15.87 g, 46.09 mmol) and N,N-diisopropylethyl amine (5.96 g, 46.1 mmol) in 60 mL DMF was added in one portion and the solution cooled to 0° C. (icebath). HOBt hydrate (7.06 g, 46.1 mmol) was added. Finally, EDC hydrochloride (9.72 g, 50.7 mmol) was added together with 30 mL DMF. The reaction mixture was stirred for 22 hours before the solvent was evaporated. The residue was taken up in 250 mL EtOAc and washed with 2 M aqueous H2SO4 (3×100 mL), 7.5% (w/w) aqueous K2CO3 (3×100 mL) and saturated brine (100 mL). The solution was dried with anhydrous MgSO4 and the solvent evaporated affording an off-white solid (19.82 g, 94%); δH (300 MHz; d6-DMSO) 8.18 (1H, d, J 8, NH(Valx), 7.62 (1H, d, J 9, NH(Valy)), 7.03 (1H, d, J 8, NH(Ser(All))), 5.83 (1H, ddt, J 5, 10 and 17, CH═CH2), 5.23 (1H, ddt, J 1, 3 and 17, CH═CHH), 5.11 (1H, ddt, J 1, 3 and 10, CH═CHH), 4.34 (1H, dd, J 7 and 9, CαH(Valy)), 4.22-4.17 (1H, m, CαH(Ser(All))), 4.13 (1H, dd, J 6 and 8, CαH(Valx)), 3.92 (2H, dt, J 1 and 5, CH2CH═CH2), 3.61 (3H, s, OCH3), 3.53-3.49 (2H, m, CH2), 2.11-1.86 (2H, m, CH(CH3)2), 1.38 (9H, s, (CH3)3), 0.90-0.79 (12H, m, CH3); δC (75 MHz; d6-DMSO) 171.6, 171.0, 169.5, 155.1, 134.9, 116.3, 78.2, 70.9, 69.5, 57.4, 56.8, 54.5, 51.4, 31.1, 29.5, 28.0, 18.9, 18.8, 18.1, 17.7; m/z (ESI) 480.2666 ([M+Na]+; C22H39N3O7Na requires 480.2685)
