On the preparation of cyclopentadienes by a novel homolytic annulation of but-3-en-1-ones with alkynes mediated by samarium diiodide

Treatment of substituted pent-4-en-2-ones and phenylacetylene with samarium diiodide in a mixture of THF and HMPA yielded 3-methyl-1-phenylcyclopent-1-en-3-ols that rapidly dehydrated and underwent [1,5] hydrogen migrations to afford substituted cyclopentadienes. The mechanism involved ketyl radical anions, generated from the unsaturated ketones by SmI2, initially adding to the alkyne. The resulting vinyl type radicals cyclised in the preferred 5-exo-mode to produce cyclopentenylmethyl radicals that were reduced to the corresponding anions and abstracted a proton from t-BuOH. The overall process amounted to a novel free-radical [3+2]-annulation.