Radical Cyclizations as Key Step for the Stereoselective Synthesis of Biand Tricyclic Sesquiterpene Lactones

A strategy towards the stereoselective synthesis of bi- and tricyclic sesquiterpene lactones is reported. As key step radical cyclizations of appropriately functionalized trans-4,5-disubstituted (gamma)-butyrolactones, which are readily available from methyl 2-furoate, were carried out to give rise to 5,6-, 5,7- and 5,7,5-ring systems in diastereo- and enantiomerically pure form.