Reaction of Enol Ethers with Alkynes Catalyzed by Transition Metals: 5exo‐digversus 6endo‐digCyclizations via Cyclopropyl Platinum or Gold Carbene Complexes

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Abstract
The intramolecular reaction of enol ethers with alkynes in methanol is catalyzed by electrophilic PtII, PdII, and AuIII chlorides and by a CuI complex to give five- or six-membered rings bearing dimethyl acetals. The reaction takes place by an anti addition of the enol ether and the metal to the alkyne. The possible involvement of vinylidene complexes in this reaction is excluded. In addition to the usual 5exo-dig (or 6exo-dig) pathways, a 6endo-dig pathway has also been found to take place with certain enynes. One case of 5endo-dig cyclization has also been found. A general scheme for the alkoxycyclization of enynes catalyzed by transition metals based on DFT calculation of PtCl2 and AuCl3 complexes that includes exo and endo cyclizations is presented.