A Straightforward Synthesis of Six‐Membered‐Ring Heterocyclic α‐Aminophosphonic Acids from N‐Acyliminium Ions

Abstract

A convenient synthesis of phosphonic analogues of pipecolic acid and its heterocyclic analogues is reported. The major step of the elaborated procedure is the introduction of the phosphonate group into the skeleton of the appropriate cyclic amide through N‐acyliminium ions. The former ones were obtained by preparation of the hemiaminals or their methyl ethers from the N‐protected cyclic amides. Finally, the reaction with trimethyl phosphite in the presence of BF3·OEt2 afforded the desired phosphonates, which were converted into phosphonic acids by the hydrolysis of phosphonate moiety with simultaneous cleavage of the nitrogen protecting groups.