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Synthesis of compounds 57a–e. Reagents and conditions: (a) DEPC, DIPEA, DMF, 50a or 50b (83–90%); (b) TFA, CH2Cl2; (c) DEPC, DIPEA, DMF, 52a or 52b (76–85%); (d) TFA, CH2Cl2; (e) Brop, DIPEA, CH2Cl2, N-Boc-L-valine, (71–92%); (f) TFA, CH2Cl2; (g) DEPC, DIPEA, DMF, 55a–g (please refer to Reference [98]), (46–87%); (h) LiOH·H2O, MeOH/H2O; (i) TFA, CH2Cl2 or Pd/C, H2, MeOH, 82–94%.
