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‐cyclopentenyl 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.

Investigations on gold-catalyzed tandem cyclization reactions of 1,6-diynes, tethered to nucleophilic functionalities such as amine, carboxylic acid, amide, and sulfonamide, are reported. The ability of such substrates to undergo tandem cyclization, triggered by internal nucleophiles, has been examined. Depending on the substrate, the catalytic system, and reaction conditions, different regioisomers of monocyclic and bridged bicyclic products were obtained.

A gold-catalyzed olefin cyclopropanation reaction of propargyl esters with vinyl ester or vinyl sulfonamide derivatives was investigated for the synthesis of highly substituted cyclo-

New gold-catalyzed cyclization reactions of 1,6-diynes (2,2-dipropargylmalonates) are reported. Symmetrically (Me, Et) and unsymmetrically disubstituted (Me, Et, Ph) 1,6-diynes provided stereoselectively the Z-cyclopentylidene derivatives in 31-60% and 49-83% yield, respectively. High regioselectivity (97:3) was obtained for the cyclization of Me/Ph-substituted 1,6-diynes. A monosubstituted terminal diyne afforded a cyclopentene derivative (2-acetyl-3-alkylcyclopent-2-ene-1,1-dicarboxylate, 43%), while the diterminal 1,6-diyne (2,2-di(prop-2-ynyl)malonate) produced a cyclohexenone derivative (3-methyl-5-oxocyclohex-3-ene-1,1-dicarboxylate, 61%). Plausible reaction mechanisms are proposed for the formation of the products.

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