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A convenient route to 3- and 5-ylidenebutenolides from readily available cis-2-acylcyclopropane-1-carboxylates is described. Upon exposure to TfOH, synergistic activation of the vicinal acceptors in cis-2-acylcyclopropane-1-carboxylates generates highly strained bicyclic oxocarbenium ion intermediates, which undergo the ring-opening/aryl shift/deprotonation cascade process to form the 3- or 5-ylidenebutenolides depending on the acyl group. On the other hand, the corresponding trans isomers, from which it is difficult to form such oxocarbenium ions, are inactive under the same conditions.

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