Total Synthesis of Limonoids Enabled by Sterically Assisted Boron Chain-Walking

Limonoids are architecturally complex tetranortriterpenoids with diverse biological activities, yet their synthetic inaccessibility has limited detailed investigations. Here, we report an enantioselective strategy that enables efficient access to intact limonoids. Our approach begins with a readily available Hajos-Parrish ketone derivative and rapidly assembles the tetracyclic core of the target scaffold. Central to this route is a novel sterically assisted hydroboration/boron chain-walking/oxidation protocol, which achieves a challenging late-stage remote C-H hydroxylation in a highly regio- and stereoselective manner within sterically congested polycyclic scaffolds. This strategy has enabled the total syntheses of (+)-azadiradione and (-)-7-desacetyl-7-benzoylazadiradione. These studies provide a unified entry to multiple limonoid families and establish a broadly applicable solution for site-selective late-stage functionalization in complex natural product frameworks.