A convenient general approach towards 1,4-disubstituted α-diazo glutaconimides starting from β-substituted glutaconic acid anhydrides has been developed. It involves condensation with amines and subsequent diazo transfer reaction. This new class of diazocarbonyl heterocycles has been converted into a series of polysubstituted pyridine-2,6(1H,3H)-diones and oxazolo[5,4-b]pyridin-5(4H)-ones via a Rh₂(esp)₂-catalyzed XH-insertion reaction and [3+2] cycloaddition with nitriles, respectively.