The Total Synthesis of Chalcitrin

The first total synthesis of the
yellow pigment chalcitrin, a structurally
distinct pulvinic acid dimer obtained from Chalciporous
piperatus, has been achieved in 17 linear steps from
commercially available materials. Key elements of the design include
the use of a Au­(I)-catalyzed Conia ene reaction and an N-heterocyclic carbene-mediated acyloin addition to rapidly fashion
its unique polycyclic core, with the two high oxidation state side
chains introduced in a single step via a late-stage double Stille
coupling. Of note, many alternate designs based on differential final
couplings failed, likely because of the hindered nature of the core.
In addition, significant challenges in final natural product characterization
in terms of matching NMR spectra were experienced; our studies reveal
that the originally characterized material was its carboxylate salt
form not its free acid.