Scheme 1.

Synthesis route of target compounds 6a ∼ v. Reagents and conditions: (i) TsCl, triethylamine, DCM, 25 °C; (ii) Piperazine, K₂CO₃, NaI, THF, reflux; (iii) 9a ∼ e, NaI, K₂CO₃, MeCN, 70 °C; (iv) 10f ∼ u, K₂CO₃, DMF, 80 °C; (v) 11, DMAP, MeCN, 70 °C; (vi) CDI, triethylamine, N-Boc piperazine, MeCN and DMF, 25 °C for 13a and 13b, or BTC, triethylamine, N-Boc piperazine, THF and DCM, 25 °C for 13c ∼ e, (vii) TFA, DCM, 25 °C; (viii) Trichloroacetyl chloride, triethylamine, DCM, 25 °C; (ix) 4-Nitrophenyl carbonochloridate, NMM, DCM, 25 °C.