Synthesis of Neocaesalpin A,AA, and Nominal Neocaesalpin K**

The first total synthesis of heavily oxidized cassane-type diterpenoid neocaesalpin A ( 1 ) is disclosed. At the heart of the synthesis lies an intermolecular Diels–Alder reaction that rapidly assembles the target framework from commercial materials. A carefully orchestrated sequence of oxidations secured the desired oxygenation pattern. Late-stage release of the characteristic butenolide occurred through a novel mercury(II)-mediated furan oxidation. Successful extension of the route allowed preparation of neocaesalpin AA ( 2 ) as well as nominal neocaesalpin K ( 3 ) and suggested structural revision of neocaesalpin K, leading to the hypothesis that the two are likely the same natural product with correct assignment as 2 .