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

摘要

A stereoselective oxidative free-radical cyclization of beta-keto ester polyenes 7 and 19 has been accomplished as a one-step entry to the tricarbocyclic synthons 8 and 21 which contain five and six stereogenic centers, respectively. These key synthons possessing an axial carboethoxy 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 the 2 alpha-hydroxy group in the spongian A-ring via epoxidation of silyl enol ether 28 and subsequent desilylation. [References: 27]