Au-Catalyzed 5C Reaction of Type II Diene-Ynenes toward Dihy-drosemibullvalenes: Reaction Development and Mechanistic Study

We report an unexpected gold-catalyzed 5C reaction of type II diene-ynenes to synthesize dihydrosemibullvalenes, which are potential bioisosteres for drug discovery. This 5C reaction occurs through a sequence of elementary reactions of cyclopropanation/Cope rearrangement/carbon shift/cyclopropanation/C–H insertion (shortened here as the 5C reaction), supported by quantum chemistry calculations. Mechanistic studies have also been applied to answer why type-II diene-ynenes cannot access seven-membered carbocycles-embedded bridged molecules under the gold catalysis, finding that the chair-like Cope rearrangement transition state (not the traditional boat-like transition state) is the key to the change of regiochemistry.