The ketoamide (8) (227 mg, 0.5 mmol) was dissolved in ethyl acetate (10 mL) and cooled in an ice bath. Dry HCl gas was bubbled through until the solution was saturated and then the solution was stirred at r.t. until the starting material had been consumed as indicated by tlc (˜30 min). The solvent was removed under reduced pressure to give 22. To a solution of Ac—Tyr—Val—Ala—OH (197 mg, 0.5 mmol) in dry THF (10 mL) and DMF (2 mL) cooled to −15° C. was added N-methyl morpholine (51 mg, 55 μL, 0.5 mmol) followed by iso-butyl chlorofornate (68 mg, 65 μL, 0.5 mmol) and the solution stirred at −15° C. for 5 min. A previously prepared solution of 22 and N-methyl morpholine (51 mg, 55 μL, 0.5 mmol) in dry THF (5 mL) was added, the solution stirred at −15° C. for 15-30 min and then at r.t. for 16 h. The solution was filtered and the solvent removed under reduced pressure to yield the product which was recrystallized from methanol/ether to give 340 mg (93%) of Ac—Tyr—Val—Ala—Asp(Bzl)—CONH—CH2CH2Ph: mp 192-194° C.; 1H NMR (DMSO) δ0.77 (m, 6H, Val CH3's), 1.19 (m, 3H, Ala CH3), 1.74 (s, 3H, Ac CH3), 1.93 (m, 1H, Val β-CH), 2.62 (obs, 2H, CH2Ph), 2.77 & 3.18 (m, 4H, Tyr & Asp β-CH2's), 3.37 (m, 2H, NH-CH2), 3.75, 4.09, 4.19 & 4.53 (m, 4H, α-CH's), 5.06 (s, 2H, benzyl CH2), 6.62 & 7.01 (2xd, 4H, Tyr aromatic CH's), 7.20 (m, 5H, phenethyl aromatic CH's), 7.35 (s, 5H, benzyl aromatic CH's), 7.83 (m, 1H, NH), 8.04 (m, 4H, NH's), 9.15 (s, 1H, Tyr OH); FABMS m/e 730 (M++1).
