Decatungstate-Photocatalyzed Functionalization of α-Imino Esters for the Preparation of Unnatural α-Amino Acid Derivatives

A library of unnatural α-amino acid derivatives has been prepared via the hydrofunctionalization of the C=N bond in a set of compounds belonging to the family of α-imino esters. The devised methodology capitalizes on the use of tetrabutylammonium decatungstate as the photocatalyst to generate C-centered radicals from aliphatic and aromatic aldehydes, cyclic and acyclic oxygenated compounds and even cycloalkanes, via a hydrogen atom transfer (HAT) step. The best performance was observed when α-hydrazono esters were employed in the role of radical traps, which delivered the corresponding (protected) α-hydrazino acids as products. The versatility of the protocol was further demonstrated by the possibility to functionalize the herbicide safener isoxadifen-ethyl.