Photocatalytic Addition of Benzyl Radical Derivatives to Ketimines for Synthesizing α-Tertiary Amino Acid Derivatives

A radical addition to sterically congested ketimines, specifically 2H-benzoxazin-2-one derivatives with a substituent at the 3-position was developed by a photochemical reaction utilizing benzyltrimethylsilane derivatives as donor molecules. An efficient protocol for accessing unnatural amino acid derivatives having a tetra-substituted carbon center at the α-position was demonstrated through further manipulation of the product. Me₃SiOTf was used as a Lewis acid catalyst to accelerate the reaction. Additionally, acridinium salt functioned as an efficient photocatalyst in the present photochemical transformation, affording the desired radical addition product in moderate to high yield. Plausible catalytic cycles were proposed based on control experiments.