SmI2-Mediated 3-exo-trig cyclisation of δ-oxo-α,β-unsaturated esters to cyclopropanols and derivatives
摘要:
In the presence of samarium diiodide and a proton source, delta-oxo-gamma,gamma-disubstituted-alpha,beta-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. (C) 2004 Elsevier Ltd. All rights reserved.
In the presence of samarium diiodide and a proton donor (tert-butanol or phenol), δ-oxo-γ,γ-disubstituted-alkylidenemalonates readily cyclise according to the 3-exo-trig mode to give, depending on the exact nature of the substrate, trans and cis cyclopropanols or lactones derived from the cis isomers. Total stereoselectivity towards the formation of trans cyclopropanols is observed with di-tert-butyl alkylidenemalonates when PhOH is used as the protonating agent.
In the presence of samarium diiodide and a proton source, delta-oxo-gamma,gamma-disubstituted-alpha,beta-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. (C) 2004 Elsevier Ltd. All rights reserved.