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.