Reaction Steps
N-tert-butoxycarbonyl α,α-dimethylglycyl α,α-dimethylglycine 4 (3.37 g, 11.7 mmol) and O-propynyl L-seryl α,α-dimethylglycine methyl ester trifluoroacetate 15 (4.16 g, 11.7 mmol) were dissolved in DMF (30 mL). A solution of N,N-diisopropylethylamine (1.51 g, 11.7 mmol) in DMF (20 mL) was added and the solution cooled to 0° C. (ice bath). HOBt hydrate (1.79 g, 11.7 mmol) and then EDC hydrochloride (2.46 g, 12.8 mmol) were added in portions together with more DMF (10 mL). The reaction mixture was stirred for 5 min at 0° C. after which the ice bath was removed and stirring continued for 44 h at room temperature. The solvent was evaporated and the residue taken up in EtOAc (300 mL). The solution was washed with 1 M HCl (3×120 mL), 7.5% (w/w) K2CO3 solution (3×120 mL) and saturated brine (120 mL). The solution was dried with anhydrous MgSO4 and the solvent evaporated affording a slightly yellowish solid (5.18 g). The solid was purified by flash column chromatography (eluent: CH2Cl2/acetone (4:1)) affording the title compound as a white solid (4.58 g, 77%); 1H NMR (300 MHz, DMSO-d6) δ 8.11 (s, 1H, NH(Aib)), 7.73 (s, 1H, NH(Aib)), 7.66 (d, J=8.1 Hz, 1H, NH(propSer)), 7.31 (s, 1H, NH(Aib1)), 4.33 (td, J=8.6, 3.8 Hz, 1H, CαH), 4.03 (t, J=2.8 Hz, 1H, CH2CCH), 3.81 (dd, J=9.9, 3.6 Hz, 1H, CHH), 3.59 (dd, J=9.3, 9.3 Hz, 1H, CHH), 3.54 (s, 3H, OCH3), 3.41 (t, J=2.3 Hz, 1H, CCH), 1.41 (s, 9H, (CH3)3), 1.37 (s, 3H, CH3), 1.34 (s, 3H, CH3), 1.33 (s, 3H, CH3), 1.29 (s, 3H, CH3), 1.28 (s, 6H, CH3); 13C NMR (75 MHz, DMSO-d6) δ 175.2, 174.2, 174.0, 168.7, 155.5, 79.8, 79.0, 77.2, 68.5, 57.3, 55.9, 55.8, 55.0, 52.9, 51.7, 28.1, 26.7, 25.4, 24.6, 24.6, 24.1, 23.2; HRMS (m/z): [M+Na]+ calcd. for C24H40N4O8Na, 535.2743. found, 535.2728; Anal. Calcd. for C24H40N4O8: C, 56.23; H, 7.87; N, 10.93. Found: C, 56.0; H, 7.8; N, 10.8.
