A novel in situ generation of nitrilium from a nitrile ylide and the subsequent Mumm rearrangement of carboxylic acid, nitrile, and diazo compounds gave various unsymmetrical diacylglycine esters in moderate to high yields. This copper-catalyzed cascade reaction enables one-pot generation of two C–N bonds, one CO bond, and one C–H bond, with nitrogen as the only byproduct. The reaction has a broad functional-group tolerance, is rapid, easily scales up to the 100 mmol scale, and is insensitive to air and moisture.