Cooperation of Cis Vicinal Acceptors for Donor-Acceptor Cyclopropane Activation: TfOH-Promoted Ring-Opening/Aryl Shift Rearrangement to 3-and 5-Ylidenebutenolides

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.

Automated summary

A convenient route to synthesize 3- and 5-ylidenebutenolides from readily available cis-2-acylcyclopropane-1-carboxylates is described. The synergistic activation of vicinal acceptors in cis isomers generates highly strained bicyclic oxocarbenium ion intermediates, while the corresponding trans isomers are inactive under the same conditions due to the difficulty in forming such intermediates.