Au-Catalyzed 5C\nReaction of Type II Diene-Ynenes toward\nDihydrosemibullvalenes: Reaction Development and Mechanistic Study

We report an unexpected gold-catalyzed 5C reaction of\ntype II diene-ynenes\nto synthesize dihydrosemibullvalenes, which are potential bioisosteres\nfor drug discovery. This 5C reaction occurs through a sequence of\nelementary reactions of cyclopropanation/Cope rearrangement/carbon\nshift/cyclopropanation/C–H insertion (shortened here as the\n5C reaction), supported by quantum chemistry calculations. Mechanistic\nstudies have also been applied to answer why type-II diene-ynenes\ncannot access seven-membered carbocycles-embedded bridged molecules\nunder the gold catalysis, finding that the chair-like Cope rearrangement\ntransition state (not the traditional boat-like transition state)\nis the key to the change of regiochemistry.