Synthesis of 12‐epi‐Protopanaxadiol and Formal Synthesis of Ginsenoside Chikusetsusaponin‐LT8

Abstract

In the context of the total synthesis of protopanaxadiol, two strategies were explored. One strategy from an optically active trienic epoxide, possessing a 5‐membered ring, prepared from (S)‐epoxy‐limonene and (S)‐epoxyfarnesol, which was submitted to Ti‐(III)‐mediated radical cascade to afford an original tetracyclic structure resulting from a 6‐endo‐trig 6‐endo‐trig 8‐endo‐trig process. A second strategy, starting from a Wieland‐Miescher‐type ketone, using a “ring‐by‐ring” synthesis allowed the synthesis of 12‐epi‐protopanaxadiol. In this latter strategy, an efficient sequence of reactions to install the two vicinal C8–C14 quaternary centers involves: i) a Barbier type reaction; ii) an oxidative allylic transposition and iii) a nickel‐catalyzed addition of trimethylaluminium. By using this second strategy, 20‐hydroxydammar‐24‐ene‐3,12‐dione was synthesized which represents a formal synthesis of chikusetsusaponin‐LT8, isolated from Panax japonicus.