“…Because there is an equilibrium between mesoionic 1,3‐oxazolium‐5‐oxide and its electrophilic ketene isomer, several different kinds of reactions (e.g., 1,3‐dipolar cycloaddition, [2+2] cycloaddition, or nucleophilic addition) can occur on the oxide. By employing an appropriate substrate as the fourth component, Arndtsen's group successively achieved the synthesis of α‐amino acid derivatives, β‐lactams, imidazolines, imidazolinium salts, and pyrroles (Scheme a). Through the transmetallation of the palladacycle with an organotin reagent, reductive elimination, and work‐up with ammonium acetate, the carbonylative five‐component synthesis of imidazolones was also achieved (Scheme b) …”