Gold-Catalyzed Cycloisomerization of 1,6-Diyne Carbonates and Esters to 2,4a-Dihydro‑1H‑fluorenes

A synthetic method to prepare 2,4a-dihydro-1H-fluorenes efficiently from gold­(I)-catalyzed 1,2-acyloxy migration/cyclopropenation/Nazarov cyclization of 1,6-diyne carbonates and esters is described. The suggested reaction pathway provides rare examples of [2,3]-sigmatropic rearrangement in this class of compounds as well as the involvement of an in situ formed cyclopropene intermediate in gold catalysis. Experimental and ONIOM­(QM:QM′) [our own n-layered integrated molecular orbital and molecular mechanics­(quantum mechanics:quantum mechanics′)] computational studies based on the proposed Au carbenoid species provide insight into this unique selectivity.