ChemInform Abstract: Synthesis and NMDA Antagonistic Properties of the Enantiomers of 4-(3-Phosphonopropyl)piperazine-2-carboxylic Acid (CPP) and of the Unsaturated Analogue (E)-4-(3-Phosphonoprop-2-enyl)piperazine-2-carboxylic Acid (CPP-ene).