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Reagents and conditions: (i) 1. TFA, CH2Cl2, rt, 15 min; (ii). Boc-Tyr-(Bn)-OH, TBTU, DIEA, CH2Cl2, rt, 12 h, 77% (2 steps). (iii) DMF, rt, 12 h, 93%. (iv) 1. LiOH, MeOH, H2O, rt, 2 h 2. Phe-Pro-OMe, TBTU, Et3N, DMF, 47% (2 steps). (v) 1. LiOH, MeOH, H2O, rt, 2 h 2. 30% aq. NH3, ClCO2Et, Et3N, THF, rt., 12 h 3. TFA/CH2Cl2 2:3, rt, 15 min., 40% (3 steps). (vi) H2, Pd(OH)2, 20 atm, MeOH, 98%.
