Synthesis and NK1/NK2 binding activities of a series of diacyl-substituted 2-arylpiperazines

The synthesis and binding affinity for hNK1 and hNK2 receptors of a series of diacyl substituted 2-aryl piperazines are described. SAR evaluation led to the racemic derivative 11g as an apparent dual inhibitor. Chiral chromatographic separation of 11g led to the observation that NK1 activity was shown by one enantiomer (13a) and NK2 activity was shown by the other enantiomer (13b). X-ray crystallographic analysis of the crystalline di-BOC derivative of the NK2 active piperazine (15) showed that the 2R configuration was associated with NK2 activity. Further derivatization indicated that dual NK1/NK2 activity could be built into the 2R series.