[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 1 N 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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