N-Carbobenzoxy-L-valine → (S)-Methyl 1-((S)-2-(((benzyloxy)carbonyl)amino)-3-methylbutanoyl)pyrrolidine-2-carboxylate

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Reaction Steps

To a cooled (0-5° ) solution of N-benzyloxycarbonyl-L-valine (450.0 g) in dry N,N-dimethylformamide (3.0 liter) was added 1-hydroxybenzotriazole hydrate (483.3 g): and the reaction mixture was stirred for 20 min, followed by the addition of a slurry of L-proline methyl ester hydrochloride (296.4 g) and triethylamine (186.6 g) in N,N-dimethylformamide (1.5 liter). After the resulting mixture was cooled to 0°, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (377.6 g, 1.97 mol) in N,N-dimethylformamide (500 ml) was added. The reaction mixture was stirred at 0°-5° for 3 hr and then allowed to warm gradually to room temperature. Stirring was continued for 65 hr, and the mixture was filtered and evaporated. The residue was partitioned between ether and lN HCl, and the precipitate which formed was removed by filtration. The etheral layer was then separated: washed with water, saturated aqueous NaHCO3 and brine; dried (MgSO4); and evaporated. The crude ester was purified by flash chromatography using a gradient elution of methylene chloride, then methanol:methylene chloride (1:99), (2.5:79.5), and, finally, (5:95). The impure material from the first column was rechromatographed using a gradient elution of methylene chloride:hexanes (1:3), methylene chloride, and, finally, methanol:methylene chloride (2:98). The combined chromatographies afforded the ester (596.9 g, 92%) as a colorless oil; TLC, Rf =0.45-0.60, methanol:methylene chloride (5:95); MS, m/e=363(M+1, base), 319, 255, 130, 91.

Provenance & Evidence