Addition of dialkyl phosphites to the nitrone 6, formed in situ from the oxime 5 and formaldehyde gave the hydroxylamines 7 (86%) and 8 (88%), which reacted with p‐benzoquinone in the presence of ethylene via the C‐dialkoxyphosphonoylnitrones 9 and 10 to yield the cycloaddition products 11–14 (80–85%) with a diastereoselectivity of about 50%. The cycloaddition products were transformed into the monoisopropylidene derivatives 15–18 and the diacetates 19–22. Comparison of the NMR spectra and the specific rotations of the compounds 19–22 with those of the corresponding α‐ammo‐acid derivatives 23–26 of known configuration indicated preferential formation of the L‐isomers. The cycloaddition products were transformed in good yield into the L‐α‐aminophosphonic acids 29, 30, 36, and 39.