Total synthesis of hybocarpone and analogues thereof. A facile dimerization of naphthazarins to pentacyclic systems.

The total synthesis of the lichen-derived antitumor agent hybocarpone (1) and related compounds is described. The successful route to hybocarpone features a novel radical-based dimerization/hydration cascade which generates the bridging hindered carbon-carbon bond of the molecule in a stereocontrolled manner, setting the relative configurations of the four contiguous stereocenters in a single step. The conjecture is made that this process may not be so dissimilar to the biosynthetic pathway leading to the formation of hybocarpone in nature. The developed sequence to these molecular frameworks also features the first example of a synthetically useful Diels-Alder trapping of a photochemically generated hydroxy-o-quinodimethane species with a 1,1-disubstituted olefin to form a quaternary center, and includes an efficient route to hydroxynaphthoquinone-type structures represented by the monomeric subunit of the natural product.