Gold(I)-catalyzed 5-endo-dig carbocyclization of acetylenic dicarbonyl compounds.

The importance of cyclopentanoid natural products continues to inspire the development of methods for the synthesis of 5-membered rings. For example, the Conia–ene reaction provides an atomeconomical synthesis of methylenecyclopentanes by the thermal cyclization of e-acetylenic carbonyl compounds. While the classic thermal reaction is limited to the exocyclic cyclization mode, group 6 metal complexes catalyze the formal 5-endo-dig addition of bketoesters and silyl enol ethers to alkynes. These reactions proceed via intermediate metal vinylidenes and therefore are limited to substrates containing a terminal acetylene moiety. The scope and utility of this reaction would be greatly increased if nonterminal alkynes could be employed as electrophiles. However, while examples of the transition-metal-catalyzed 5-endodig addition of heteroatom nucleophiles to nonterminal alkynes are common, this class of cyclization employing carbon nucleophiles is rare. We have recently demonstrated that cationic gold(i) complexes catalyze the Conia–ene reaction by a mechanism that appears to involve formation of a gold(i) alkyne complex. Thus, we reasoned that an endocyclic Conia–ene reaction might be feasible with group 11 metal complexes as catalysts for alkyne activation. Furthermore, while the gold(i)-catalyzed Conia–ene reaction is limited to terminal alkynes, we anticipated that a 5-endo-dig variant would allow for carbocyclization onto nonterminal alkynes. To this end, we surveyed several cationic group 11 metal triflates as catalysts for the cyclization of b-ketoesters 1 with nonterminal a-3’-alkynyl substituents [Eq. (1)]. Reaction of 1 with Cu and Ag triflate