MECHANISM OF THE INTRAMOLECULAR CYCLIZATION OF ACETYLENIC KETONES

Acid-catalyzed intramolecular cyclization of 5-cyclodecynone (1) under a variety of conditions gives bicyclo-[4,4,0]-1(6)-decen-2-one (6) as the only product. In a recent report, evidence for a reaction mechanism involving transannular triple-bond participation with a polarized carbonyl group followed by attack of the original carbonyl oxygen on the developing vinyl cation to give an unstable oxete intermediate (4) was presented. Subsequently, several workers have suggested a mechanism involving acid-catalyzed enolization of 1, followed by the transannular attack of the enol double bond at the acetylenic function, as the first two steps in an alternate process for explaining this rearrangement. When 6-octyn-2-one (9) was treated with either mineral or Lewis acids, a mixture of 1-acetyl-2-methyl-1-cyclopentene (12) and 2,3-dimethyl-2-cyclohexen-1-one (14) were produced. Experiments have shown that the oxygen atoms in the starting acetylenic ketone 9 are the same ones found in the cyclic products 12 and 14. A mechanism involving cyclization of enol intermediates cannot account for the formation of 14. On the other hand, formation and rearrangement of oxete intermediates demonstrates how both 12 and 14 are produced. The results from acid-catalyzed cyclization of 6-octyn-2-one (9) are also compared to those reported for the solvolysis of the tosylate of 6-octyn-2-ol (18).