Discovery and development of an unprecedented (semi)Favorskii rearrangement has been reported. The intermediacy of structurally singular (acyloxy)cyclopropanones has been unraveled by fruitful control experiments including a crossover experiment. This class of cyclopropanones is found to be inert for classical Favorskii functionalization and preferably undergoes a decycloisomerization (ring–chain valence tautomerism) to α-(acyloxy)enones. A cascade conversion of α,α-diiodo-α′-acetoxyketones to (acyloxy)cyclopropanones via α-iodo-α′-acetoxyketones has been achieved by the synchronous dual basicity (Lewis and Brønsted) of amines. The overall process is found to be very general for diverse substrates and highly efficient.