Synthesis of Terpenoids Using a Free Radical Fragmentation/Elimination Sequence.

Total syntheses of the guaiane alismol 8 and the trinor-guaiane dictamnol 18 are reported. In both syntheses the initial [2+2] photoadduct is transformed into an iodo xanthate 14 or a diiodide 27, respectively. In the critical step, the strained bifunctional substrate undergoes a very efficient free radical fragmentation/elimination sequence to give the requisite seven-membered ring with regioselective placement of the two double bonds in that ring. This first alismol synthesis was accomplished in eight steps while the dictamnol synthesis required only six steps. Clarification of the structure of dictamnol is also described.