Spiro compounds are of great interest because of their special conformational features and their structural implications on biological systems.[1] The presence of the sterically constrained spiro structure in various natural products substantially promotes interest in the investigation of spiro compounds.[2] The Conia-ene cyclization is one of the processes during which a quaternary center is formed by the pericyclic reaction of an enolizable carbonyl group with an alkyne. However, the need for high temperatures limits the synthetic utility of this reaction.[3] On the other hand, transition-metal-catalyzed versions [4] proceed under mild conditions at lower temperatures. Toste and co-workers have reported a phosphine–gold (I)-catalyzed version for the intramolecular addition of a β-ketoester to an unactivated alkyne.[5] In a similar reaction, Davies and Detty-Mambo demonstrated the cycloisomerization of unactivated ketones with alkynes under gold catalysis.[6] In recent years, silver salts have gained increasing interest in homogeneous catalysis owing to their mildness and efficiency.[7] More recent reviews discuss the current revolution in silver chemistry. Catalysis with silver salts has become widespread due to the σ and π Lewis acidic properties of silver (I) complexes,[8] which lead to a variety of chemical transformations.[9] Therefore, exploring new catalytic reactions with silver complexes is of great interest. For this reason, we focused on silver-catalyzed cycloisomerization to study the behavior of alkynyl silyl enol ethers.

Mainly used as cocatalysts in gold catalysis, silver salts, such as AgBF4, AgSbF6, and AgPF6, are very hygroscopic, causing difficulties in properly weighing the reagent and keeping the reaction medium nonacidic. In contrast, AgNTf2 (Tf= triflyl)[10] is known to be more stable and easier to handle than its congeners. Thus, this reagent proved to be an efficient catalyst for nucleophilic additions to alkynes.[11] For this purpose, we envisaged the use of AgNTf2 as a potentially valuable candidate for the cycloisomerization of silyl alkynyl enol ethers.