A Regioselective Route to 5-Substituted Isoxazole- and Isoxazoline-3-phosphonates

5-Substituted 3-(diethoxyphosphoryl)isoxazoles and -2-isoxazolines were synthesized regioselectively by 1,3-dipolar cycloaddition of(diethoxyphosphoryl)formonitrile oxide to monosubstituted alkynes and alkenes. By applying this methodology to an N-(tert-butoxycarbonyl)-substituted allylglycine methyl ester, we prepared the precursors of two diastereomeric 3-phosphono-2-isoxazolin-5-yl-substituted amino acids, which are bioisosteres of potent NMDA receptor antagonists.