Computational and Synthetic Investigation of Cationic Rearrangement in the Putative Biosynthesis of Justicane Triterpenoids.

A biomimetic cationic structural rearrangement of the oleanolic acid framework is reported for the gram-scale synthesis and structural reassignment of justicioside E aglycone. The mechanism of the putative biosynthetic rearrangement is investigated with kinetic, computational, and synthetic approaches. The precursor to rearrangement was accessed through two strategic advancements: (1) synthesis of a 1,3-diketone via oxidation of a β-silyl enone, and (2) diastereoselective 1,3-diketone reduction to form a syn-1,3-diol using SmI2 with PhSH as a key additive.