TOTAL SYNTHESIS OF D,L-ISOSPONGIADIOL - AN INTRAMOLECULAR RADICAL CASCADE APPROACH TO FURANODITERPENES

A stereoselective oxidative free-radical cyclization of beta-keto este r polyenes 7 and 19 has been accomplished as a one-step entry to the t ricarbocyclic synthons 8 and 21 which contain five and six stereogenic centers, respectively. These key synthons possessing an axial carboet hoxy group at C-4 were ultimately converted to the spongian skeleton ( 8 --> 14 and 21 --> 25 --> 14). The synthesis of d,l-isospongiadiol (3 ) from the common intermediate 14 was realized after introduction of t he 2 alpha-hydroxy group in the spongian A-ring via epoxidation of sil yl enol ether 28 and subsequent desilylation.