Synthesis of piperazine ureas 4 and 6: (a) PhNCO, CH2Cl2, 92%; (b) TFA, CH2Cl2, 80–99% (c) NaBH(OAc)3, DCE, 50–95%; (d) N-Boc-piperazine, NaBH(OAc)3, CH2Cl2; then TFA, CH2Cl2, 64%; (e) RNCO (R = alkyl, Ph, 3-pyr; ie. 4d–g, 4h, 6a), CH2Cl2, 50–90%; (f) RNHCO2Ph (R = heterocycle, ie. 4j–1, 6), DMSO, 60 °C, 50–80%.