En Route to D-Ring Inverted Phorbol Esters.

Phorbol esters are long regarded as tumor promotors, due to protein kinase C (PKC) activation, but more recently higher oxidized natural derivatives have been shown to display antitumor activity. Given the synthetic difficulty, systematic non-natural systems are not readily available to further interrogate PKC binding. Herein reported is the concise construction of a considerably advanced intermediate toward D-ring inverted phorbol esters, enabled by a rhodium-catalyzed [4 + 3] cycloaddition involving a highly functionalized tetrahydrobenzofuran.