First direct evidence of radical intermediates in samarium diiodide induced cyclization by ESR spectra.

The mechanism of samarium diiodide (SmI(2))-induced cyclization of α,β-unsaturated esters and ketones to bicyclic compounds was examined using ESR spectroscopy. This is the first report of direct evidence of the radical intermediates in the SmI(2) reaction. The preferential reduction of the α,β-unsaturated carbonyl part in some substrates should be recognized as a main route.