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A solution of the free acid compound 9 (200 mg, 0.6224 mmol, 1.0 eq.) and L-proline benzyl ester hydrochloride (150 mg, 0.6224 mmol, 1.0 eq.) in dry CH2Cl2 (15 mL) was cooled at 0° C. 1-Hydroxybenzotriazole hydrate (HOBt) (84 mg, 0.6224 mmol, 1.0 eq.), N-ethyl-N′-(dimethylaminopropyl)-carbodiimide hydrochloride (EDC.HCl) (120 mg, 0.6224 mmol, 1.0 eq.) and iPr2NEt (0.22 mL, 1.245 mmol, 2.0 eq.) were added and the resulting mixture stirred at 0° C. for 2 h. The cooling bath was removed and the reaction mixture then stirred for a further 72 h at room temperature. The reaction mixture was diluted with H2O (30 mL) and the extracted with EtOAc (3×30 mL). The combined organic extract were washed sequentially with saturated aqueous NaHCO3 (30 mL), brine (30 mL), and dried over anhydrous MgSO4. Evaporation of the solvent gave a crude residue, which was subjected to column chromatography on silica gel elution with 33% EtOAc-hexane to afforded the (5S)-6-phenyl-5-[(tert-butyloxycarbonyl)-amino]-4-oxo-hexanoyl-L-proline benzyl ester 10 as a yellow viscous oil (0.255 g, 81%); Rf 0.30 (33% EtOAc-hexane); IR (liquid film) ν 1750 (CO), 3150 (CONH) cm−1; 1H NMR (300 MHz, CDCl3) δH 1.39 (9H, s, (CH3)3), 2.00-2.18 (4H, m, —CH2CH2—), 2.48-2.80 (3H, m, —CH2CO and —CHαPh), 2.94 (2H, m, —CH2CO), 3.18 (1H, m, —CHβPh), 3.60 (2H, m, —CH2N—), 4.53 (2H, m, 2×—CH—), 5.09 (1H, m, —NH—), 5.15 (2H, dd, J=2.0 and 10.5 Hz, —OCH2Ph) and 7.13-7.38 (10H, m, Ar—H); 13C NMR (75 MHz, CDCl3) δC 21.0, 22.6, 24.7, 28.3, 29.2, 31.4, 31.7, 34.7, 46.5, 59.5, 60.3, 66.7, 126.8 (2C), 127.0, 127.6, 128.1, 128.4 (2C), 128.5, 128.7, 129.3, 135.7, 136.6, 155.2, 170.3, 171.1, 172.1 and 208.0; HRMS m/z calcd for C29H36N2O6 (M+) 508.25912, found 508.25734.

1. customThe cooling bath was removed
2. stirringthe reaction mixture then stirred for a further 72 h at room temperature
3. additionThe reaction mixture was diluted with H2O (30 mL)
4. extractionthe extracted with EtOAc (3×30 mL)
5. extractionThe combined organic extract
6. washwere washed sequentially with saturated aqueous NaHCO3 (30 mL), brine (30 mL)
7. dry with materialdried over anhydrous MgSO4
8. customEvaporation of the solvent
9. customgave a crude residue, which
10. washwas subjected to column chromatography on silica gel elution with 33% EtOAc-hexane