AN INTRAMOLECULAR ALLENIC [2+2+1]CYCLOADDITION

A new stereo- and regioselective method for the preparation of alpha-m ethylene and 4-alkylidene cyclopentenones is described. These substruc tures were achieved by an intramolecular [2 + 2 + 1] cycloaddition of an allene, alkyne, and carbon monoxide moieties to afford the target c ompounds stereoselectively and in good yields. In some cases, the targ et compounds were obtained as mixtures, but it is demonstrated that th e formation of either the alpha-methylene or 4-alkylidene cyclopenteno ne can be controlled by the allene structure or reaction conditions. M onosubstituted allenes afford alpha-methylene cyclopentenones as the o nly cycloadduct. Disubstitution on the allene alters the course of the allenic [2 + 2 + 1] reaction. 3,3-Disubstituted allenes undergo cyclo addition with the least substituted pi-bond of the allene. This afford s the bicyclo[4.3.0]nonane ring system. Cycloaddition of 1,3-disubstit uted allenes afford mixtures of several possible cycloadducts. However , it has been shown that good control over the product ratio can be ob tained by altering the cycloaddition conditions and that the regiochem istry can be directed depending upon the metal used.