Reaction Steps
N-tert-butoxycarbonyl L-prolyl L-proline 2 (3.80 g, 12.2 mmol) and O-allyl L-seryl L-valyl L-valine methyl ester 9 (4.35 g, 12.2 mmol) were dissolved in DMF (25 mL) and the solution cooled to 0° C. HOBt hydrate (1.87 g, 12.2 mmol) was added under stirring. EDC hydrochloride (2.57 g, 13.4 mmol) was added to the reaction mixture in small portions. The reaction mixture was stirred overnight and the solvent evaporated. The residue was taken up in EtOAc (50 mL) and washed with 2M HCl (3×25 mL), 7.5% (w/w) K2CO3 (3×25 mL) and saturated brine (20 mL). The solution was dried with anhydrous MgSO4 and the solvent evaporated affording the title compound as a white solid (7.34 g, 93%); δH (300 MHz; d6-DMSO) 8.10 (1H, d, J 8, NH(Valx)), 7.98 (1H, d, J 8, NH(Ser(All))), 7.60 (1H, d, J 9, NH(Valy), rotomer 1), 7.59 (1H, d, J 9, NH(Valy), rotomer 2), 5.83 (1H, ddt, J 5, 11 and 17, CH═CH2), 5.24 (1H, ddt, J 1, 3 and 17, CH═CHH), 5.14-5.09 (1H, m, CH═CHH), 4.46-4.37 (3H, m, CαH(Pro1)/CαH(Pro2)/CαH(Ser(All))), 4.32 (1H, dd, J 7 and 9, CαH(Valy)), 4.15-4.10 (1H, m, CαH(Valx)), 3.93 (2H, m, J 1 and 5, CH2CH═CH2), 3.67-3.50 and 3.36-3.24 (6H, m, CδH2(Pro1)/CδH2(Pro2)/CH2(Ser(All))), 3.61 (3H, s, OCH3), 2.26-1.71 (10H, m, CH2(Pro)/CH(CH3)2), 1.37 (9H, s, (CH3)3, rotomer 1), 1.30 (9H, s, (CH3)3, rotomer 2), 0.89-0.80 (12H, m, CH3); δC (75 MHz; d6-DMSO) 171.6, 171.5, 171.3, 171.1, 170.9, 170.4, 169.1, 169.1, 153.3, 152.9, 134.8, 116.5, 78.3, 78.1, 71.1, 69.3, 59.1, 57.4, 57.2, 56.9, 52.9, 51.4, 46.5, 46.4, 46.3, 46.3, 30.9, 30.6, 29.6, 29.5, 29.4, 28.7, 28.5, 28.1, 27.9, 24.4, 24.3, 23.6, 23.0; m/z (ESI) 674.3737 ([M+Na]+; C32H53N5O9Na requires 674.3741)
