Mechanism of the Transannular Cyclization of 5-Cyclodecynone

Article abstract of DOI:10.1021/jo00274a018

Acid-catalyzed intramolecular cyclization of 5-cyclodecynone (1) under a variety of conditions gives bicyclo<4.4.0>-1(6)-decen-2-one (8) as the only product.In earlier reports the reaction was formulated as involving triple-bond participation with a polarized carbonyl group to give a vinyl cation, followed by external attack by a nucleophile.Studies of the rearrangement using Lewis acids in aprotic solvents, taking care to exclude water and moist air during workup, have shown that the oxygen atom in the starting acetylenic ketone 1 is the same as that in the bicyclic product 8.When the reaction was carried out with HCl in a solvent of methanol-H2(18)O, oxygen-18 incorporation in the final product was not significantly above exchange levels observed when the bicyclic ketone itself was treated with methanol-H2(18)O under similar conditions.A mechanism that will account for these observations is presented.