Three acidic amino acids were synthesized using the non-NMDA antagonist, (RS)-2-amino-3-[3-(carboxymethoxy)-5-methyl-4-isoxazolyl]propionic acid (AMOA), as a lead structure. (RS)-3-(carbxymethoxy)-4,5,6,7-tetrahydroisoxazolo[5,4-clpytidine-5-carboxylic acid (1) was found to be a selective NMDA antagonist, whereas (RS)-2-arnino-3-[3-(carboxymethoxy)-5-tfluoromethyl-4-isox~lyl]pmpionic acid (2) was shown to be a non-NMDA antagonist.