The expeditious construction of phenanthridine scaffolds via a Pd/MnO₂-mediated C–H arylation/oxidative annulation cascade involving aldehydes, aryl iodides and amino acids is disclosed. This reaction proceeds smoothly involving the formation of multiple chemical bonds with the tolerance of a wide range of functional groups. The control experiments suggest a radical mechanism for C–N bond formation via MnO₂-promoted oxidative annulation of imine compounds. The synthetic utility of this transformation has been demonstrated via the straightforward access to bioactive natural alkaloid trisphaeridine and its analogue.