Total Syntheses of (±)-Isosteviol and (±)-Beyer-15-ene-3β,19-diol by Manganese(III)-Based Oxidative Quadruple Free-Radical Cyclization

Abstract

Tetraene 1 was prepared in nine steps from the known propargylic alcohol 7 in 17% overall yield or as a 2:1 E/Z mixture in only five steps in 7% overall yield. Oxidative cyclization of 1 with 2 equiv of Mn(OAc)3·2H2O and 1 equiv of Cu(OAc)2 in MeOH at 25 °C provided 35% of tetracycle 2. Further elaboration provided (±)-isosteviol (3) in six steps in 51% yield and (±)-beyer-15-ene-3β,19-diol in four steps in 17% yield. © 1998 American Chemical Society.