The mixed anhydride of diphenylmethylenemalonic and acetic acids undergoes a decomposition, catalysed by potassium carbonate, forming 2,4-bis(diphenylmethylene)cyclobutane-1,3-dione (VI), presumably via initial formation diphenylpropadienone (IIId). In the presence of triethylamine, the mixed anhydride forms 3,3-diphenylacrylic anhydride.