[NH2:1][C@H:2]([C:6]([OH:8])=[O:7])[CH:3]([CH3:5])[CH3:4].C(N(CC)CC)C.C(=O)=O.[N+](C1C=CC([O:28][C:29](=O)[C@@H:30]2[CH2:34][CH2:33][CH2:32][N:31]2[C:35]([O:37][CH2:38][C:39]2[CH:44]=[CH:43][CH:42]=[CH:41][CH:40]=2)=[O:36])=CC=1)([O-])=O.Cl>O1CCOCC1.O>[C:35]([N:31]1[CH2:32][CH2:33][CH2:34][C@H:30]1[C:29]([NH:1][C@H:2]([C:6]([OH:8])=[O:7])[CH:3]([CH3:5])[CH3:4])=[O:28])([O:37][CH2:38][C:39]1[CH:44]=[CH:43][CH:42]=[CH:41][CH:40]=1)=[O:36]L-Valine (30 mmol, 3.52 g) and triethylamine (60 mmol, 8.36 mL) were combined in 275 mL of 1:1 dioxane-water. Solid carbon dioxide pieces were added with stirring to achieve pH 8.5, and then N-CBZ-L-proline 4-nitrophenyl ester (Sigma, 30 mmol, 11.11 g) was added to the solution with stirring at room temperature. After 20 hours, the clear yellow solution was acidified by gradual addition of 1N HCl, concentrated by rotary evaporation at reduced pressure, and diluted with ethyl acetate. The organic phase was separated, washed with water (5×) and brine (2×), dried (MgSO4), and rotovapped to give an oil residue. Crystallization of this residue from hot ethyl acetate-hexane afforded 7.15 g (68%) of N-CBZ-L-prolyl-L-valine as a white solid, m.p. 132°-134° C., lit. m.p. 134°-136° C.; G. R. Pettit, "Synthetic Peptides," Vol. 1, 154 (1970).
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