Scheme 2. Reagents and conditions (a) TFAA, DIPEA, CH₂Cl₂, 0 °C, 1.5 h; (b) TFA, CH₂Cl₂, 0 °C-rt, 16 h; (c) cyclopropylamine, PyBOP, DIPEA, DMF, 0 °C-rt, 16 h; (d) LiOH·H₂O, THF, MeOH, water, rt, 16 h; (e) 4-fluoroaniline or adamantylamine, PyBOP, DIPEA, DMF, 0 °C-rt, 16 h; (f) Et₃N–HCOOH, MeOH, water, reflux, 24 h; (g) Pd(OAc)2, PPh₃, Et₃N–HCOOH, MeOH, water, reflux, 24 h.