To develop antagonists which show selectivity in blocking neurotransmitters, several aminophosphonoic acids, 2-amino-5-phosphonopentanoic acid (IVb), 2-amino-4-(2-phosphonomethylphenyl) butyric acid (VIII), 2-(2-amino-2-carboxy) ethylphenylphosphonic acid (XIc), and N-benzylproline-4-phosphonic acid (XIX), were synthesized. Compounds IVb, VIII, and XIc were prepared from the corresponding halides (V, Xa, and XIa, respectively) by treatment with sodium diethyl acetamidomalonate (VI). Compound XIX was synthesized via 1, 3-dipolar cycloaddition of ethyl N-benzyl-N-phenylthiomethylglycinate (XV) to diethyl vinylphosphonate (XVI).