Unified short syntheses of oxygenated tricyclic aromatic diterpenes by radical cyclization with a photoredox catalyst

The biomimetic two-phase strategy employing polyene cyclization and subsequent oxidation/substitution is an effective approach for divergent syntheses of [6-6-6]-tricyclic diterpenes. However, this strategy requires lengthy sequences for syntheses of oxygenated tricyclic aromatic abietane/podocarpane diterpenes owing to the many linear oxidation/substitution steps after cyclization. Here, we present a new synthetic route based on a convergent reverse two-phase strategy employing a reverse radical cyclization approach, which enabled the unified short syntheses of four aromatic abietane/podocarpane diterpenes and the divergent short syntheses of other related diterpenes. Oxygenated and substituted precursors for cyclization were convergently prepared through Friedel-Crafts acylation and rhodium-catalyzed site-selective iodination. Radical redox cyclization using an iridium photoredox catalyst involving neophyl rearrangement furnished the thermodynamically favored 6-membered ring preferentially. (±)-5,6-Dehydrosugiol, salvinolone, crossogumerin A, and Δ(5)-nimbidiol were synthesized in only 8 steps. An oxygenated cyclized intermediate was also useful for divergent derivatization to sugiol, ferruginol, saprorthoquinone, cryptomeriololide, and salvinolone.