AlCl_3-Catalyzed Ring Expansion Cascades of Bicyclic Cyclobutenamides Involving Highly Strained Cis,Trans- Cycloheptadienone Intermediates

摘要

We report the first experimental evidence for the generation of highly strained cis,frans-cycloheptadienones by electrocyclic ring opening of 4,5-fused cyclobutenamides. In the presence of AlCl_3, the cyclobutenamides rearrange to [2.2.1]-bicyclic ketones; DFT calculations provide evidence for a mechanism involving torquoselective 4π-electrocyclic ring opening to a cis,trans-cycloheptadienone followed by a Nazarov-like recyclization and a 1,2-alkyl shift. Similarly, 4,6-fused cyclobutenamides undergo AlCl_3-catalyzed rearrangements to [3.2.1]-bicyclic ketones through cis,trans-cyclooctadienone intermediates. The products can be further elaborated via facile cascade reactions to give complex tri- and tetracyclic molecules.