SmI2-Mediated 3-exo-trig cyclisation of δ-oxo-α,β-unsaturated esters to cyclopropanols and derivatives

In the presence of samarium diiodide and a proton source, δ-oxo-γ,γ-disubstituted-α,β-unsaturated esters of general formula R–CO–C(R′,R′)–CHCH–CO2Bn readily cyclise to trans-cyclopropanol products and/or lactones derived from the cis isomers. For R=aryl, good stereoselectivities (ca 90%) in favor of the alcohols are generally obtained while a mixture of alcohols and lactones is obtained with R=alkyl or H. For R=cyclopropyl, the lactone is exclusively obtained in more than 90% yield. A mechanistic rationalisation of these variations of diastereoselectivity is proposed.