Structure-activity relationship in a series of new 1-(2-pyrimidinyl)piperazine derivatives with hypnotic activity. 
Preparation, biological properties and QSAR of new derivatives of 1-[4-(2-pyrimidinyl)-1-piperazinyl]-1, 3-butandione (11-13, and 15-18) and 3-[4-(2-pirimidinyl)-1-piperazinyl]-3-oxopropanoate (20-22) exhibiting hypnotic activity in mice are reported.  The best therapeutic indices (TI = LD50/ED50) 4.7 and 9.4 for po and ip administration, respectively, were found for 1-[4-(2-pyrimidinyl)-1-piperazinyl]-2-n-pentyl-1,3-butandione (15).  QSAR studies showed that the biological activity grows initially with an increase in lipophilicity to drop dramatically for log P > 2.5.