Diastereoselective SmI2 mediated cascade radical cyclisations of methylenecyclopropane derivatives - syntheses of paeonilactone B and 6-epi-paeonilactone A

The SmI2 mediated cascade cyclisations of several methylenecyclopropyl keto nes have been examined and found to proceed with high diastereoselectivity, which is critically dependent on the presence of HMPA in the reaction. In one case the radical species at the end of the cascade sequence underwent a n unexpected and highly stereoselective dimerisation. The cascade methodolo gy has been applied to a short synthesis of (+/-)-paeonilactone B and of (/-)-6-epi-paeonilactone A.