An Intramolecular Allenic [2 + 2 + 1] Cycloaddition

A new stereo- and regioselective method for the preparation of α-methylene and 4-alkylidene cyclopentenones is described. These substructures were achieved by an intramolecular [2 + 2 + 1] cycloaddition of an allene, alkyne, and carbon monoxide moieties to afford the target compounds stereoselectively and in good yields. In some cases, the target compounds were obtained as mixtures, but it is demonstrated that the formation of either the α-methylene or 4-alkylidene cyclopentenone can be controlled by the allene structure or reaction conditions. Monosubstituted allenes afford α-methylene cyclopentenones as the only cycloadduct. Disubstitution on the allene alters the course of the allenic [2 + 2 + 1] reaction. 3,3-Disubstituted allenes undergo cycloaddition with the least substituted π-bond of the allene. This affords the bicyclo[4.3.0]nonane ring system. Cycloaddition of 1,3-disubstituted allenes afford mixtures of several possible cycloadducts. However, it has been shown that good control over the product ratio can be obtained by altering the cycloaddition conditions and that the regiochemistry can be directed depending upon the metal used.