Gold(I)-Catalysed Alkene Cycloaddition Reactions of Propargyl Acetals

Studies on gold(I)-catalysed reactions of propargyl acetals are scarce, unlike those of the corresponding esters. We have investigated gold(I)-catalysed cyclization reactions of both propargyl acetals and esters in order to identify the abilities of the two substrate groups to follow two different cyclization pathways with vinylic reactants, such as vinyl amides and vinyl ethers, in chemoselective manner. Corresponding reactions with indoles allowed the construction of biologically interesting polycyclic indole-based compounds. The relative reactivities of propargyl acetals and esters were also studied. The gold(I)-catalysed cyclization reactions of propargyl acetals were characterized by two important features: i) in contrast with the conventional olefin cyclopropanation that normally takes place with propargyl esters, the favoured general reaction pathway for the corresponding acetals is an atypical cyclopentenylation by [2+3] cycloaddition to afford trans-cy-clopentenyl products, and ii) the propargyl acetals showed significantly higher reactivities than the corresponding esters, in line with the assumption that the alkoxy substituent might activate the gold-propargyl acetal complex intermediate. Different factors affecting the specific pathways are discussed, with the aim of achieving better mechanistic understanding of the reactions.