An efficient strategy towards N-formylation of amides and oxidation of indolines to isatins is described. This method employs readily available (NH₄)₂S₂O₈, I₂, and DMSO. The given method features operational simplicity, excellent functional group tolerance, broad substrate scope, and fast kinetics. Moreover, the method was applied to the synthesis of the natural product alatamide. Notably, DMSO plays three vital roles: a formyl group source, an oxidant, and a solvent.