A regioselective route to 5-substituted isoxazoline-3-phosphonates

5-Substituted 3-(diethoxyphosphoryl)isoxazoles and -2- isoxazolines were synthesized regioselectively by 1,3-dipolar cy- cloaddition of (diethoxyphosphoryl)formonitrile oxide to monosub- stituted 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-isoxazo- lin-5-yl-substituted amino acids, which are bioisosteres of potent NMDA receptor antagonists.