Gold(I)-, Palladium(II)-, Platinum(II)-, and Mercury(II)-Catalysed Spirocyclization of 1,3-Enynediols: Reaction Scope

The spirocyclization of different 1,3‐enynediols was investigated. The reaction was only efficient for the synthesis of [5,6]‐spiroacetals. In this case, the reaction was characterized by almost quantitative yields, short reaction times, and low catalyst loadings (0.5–1 %). When the synthesis of [6,6]‐spiroacetals was attempted, the reaction suffered from poor regioselectivity and a higher propensity of the intermediate dienol ethers to decompose under the acidic conditions, and it became no longer viable. But it is possible to generate the dienol ethers cleanly under milder conditions as a mixture of regioisomers. This striking difference in reaction efficiency was explained by the unstable dienol ethers cyclizing more quickly to give [5,6]‐spiroacetals than to give [6,6]‐spiroacetals. In this study, the successful application of a new cationic palladium pincer complex for electrophilic alkyne activation at room temperature has been demonstrated for the first time. Spirocyclization of different 1,3‐enynediols provides [5,6]‐spiroacetals in high yield. The reaction is catalysed by various gold(I), palladium(II), platinum(II), and mercury(II) catalysts. In particular, the successful application of a new cationic palladium pincer complex for electrophilic activation of alkynes has been demonstrated.