Synthesis of α‐amino phosphonic acids via oxoiminium salts

The oxoiminium salts 4a, b, and 8 obtained by alkylation with Et₃OBF₄ of the aldonitrones 3a, b, and 7 react with diphenyl hydrogen phosphite to give the N‐alkoxy α‐amino phosphonic esters 6 and 9a, b. Similarly, the cyclic imine oxides 10a, b and the cyclic oxime ethers 14a, b afford the α‐branched N‐alkoxy α‐amino phosphonic esters 12a, b and 16a, b, respectively. Hydrogenolysis and ester hydrolysis affords the α‐methyl α‐amino phosphonic acid hydrochlorides 13a, b and 17a, b.