A Versatile Synthesis of N Carboxy-α,β-didehydro-α-amino Acid Anhydrides (4-Alkylidene-2,5-oxazolidinediones) α-Azidocarboxylic acids 1 react with phosgene in the presence of catalytic amounts of sodium perrhenate in dimethoxyethane at 85°C to give N-carboxy-α,β-didehydroamino acid anhydrides (4-alkylidene-2.5-oxazolidinediones) 2 in good yields.