Concise Enantioselective Synthesis of Furan Lignans (-)-Dihydrosesamin and (-)-Acuminatin and Furofuran Lignans (-)-Sesamin and (-)-Methyl Piperitol by Radical Cyclization of Epoxides

Abstract

Enantioselective syntheses of furan lignans (-)-dihydrosesamin and (-)-acuminatin and furofuran lignans (-)-sesamin and (-)-methyl piperitol were achieved in up to only three steps in 43%, 42%, 63%, and 60% overall yield, respectively, with high optical purity through stereoselective intramolecular radical cyclization of suitably substituted epoxy olefinic ethers using bis(cyclopentadienyl)titanium(III) chloride as the radical initiator. The key intermediate, chiral epoxy alcohol 4, was prepared by the Sharpless kinetic resolution method. The titanium(III) initiator was prepared in situ from commercially available titanocene dichloride and activated zinc dust in tetrahydrofuran.

Key words

enantioselectivity - lignans - radical cyclizations - transition metals - titanium

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