Scheme 2. Reagents and conditions: (a) NEt₃, DCM, rt, overnight, yield 93%; (b) Ac₂O, 150 °C, 10 min, yield 84%; (c) aminoguanidine bicarbonate, pyridine, 180 °C, microwave, 20 min, yield 93%; (d) Bis(pinacolato)diboron, Pd(dppf)Cl₂, AcOK, DMF, 120 °C, 4 h; (e) methyl 4-(bromomethyl)benzoate, Pd(PPh₃)₄, K₂CO₃, toluene/EtOH 3 : 1 (v/v), 120 °C, 4 h, yield 68%; (f) LiOH, 1,4-dioxane/water 1 : 1 (v/v), rt, overnight, yield 86%; (g) NH₂OTMS, HOBt, EDCI, DIPEA, DMF, rt, overnight, yield 43%.