Peptide Bond Formation with the Aid of Coupling Reagents

Abstract

A solution of l-phenylalanine methyl ester hydrochloride (5.4 g, 25 mmol) in water (20 ml) is treated with a solution of K₂CO₃ (5.0 g, 36 mmol) in water (10 ml) and the mixture is extracted with ether (three times, 25 ml each time). The ether extracts are pooled, dried over MgSO₄ and the solvent removed under moderately reduced pressure with the help of a water aspirator. The residue [3] (about 3.6 g, 20 mmol) is added to a solution of N-phthaloyl-l-threonine [4] (2.5 g, 10 mmol) in dichloromethane [5] (40 ml) followed by the addition of dicyclohexylcarbodiimide [6, 7] (2.06 g, 10 mmol). A precipitate, N,N’-dicyclohexylurea, starts to separate almost immediately and its amount gradually increases. After five hours at room temperature the urea derivative is removed by filtration [8] and washed with dichloromethane (20 ml). The combined filtrate and washings are extracted with N HC1 (30 ml), N KHCO₃ (30 ml), water (30 ml), dried over MgSO₄ and evaporated to dryness in vacuo. Recrystallization of the residue from acetone-ether affords the pure dipeptide derivative (3.72 g, 91%) melting at 149–152 °C. A sample is recrystallized for analysis from acetone-ether melts at 153–154 °C; [α] ²⁷_D + 1.9° (c 0.7, dimethylformamide).