New strategies for the synthesis of polycarbocyclic natural products

Two new and general strategies for the preparation of bridged and fused polycarbocyclic terpenoids have been developed. The intramolecular exocyclic nitrone-olefin cycloaddition reaction has been applied in efficient and stereospecific formal total syntheses of the linearly fused tricyclopentanoids ($\pm$)-hirsutene (32) and ($\pm$)-coriolin (70). A new, one-pot preparation of 2-substituted tropones 159 has been developed. 1,8-Addition of sulfone-stabilized monoanions 156 to tropone (120) followed by acidification and base-induced elimination of phenylsulfinic acid and isomerization leads smoothly to a variety of 2-substituted tropones 159. The intramolecular (4+2) and (6+4) tropone-olefin cycloaddition reactions have been found to proceed smoothly and with high levels of internal and relative asymmetric induction. The tricarbocyclic adducts can be efficiently transformed into other naturally occurring ring systems. This methodology has been applied in the total syntheses of ($\pm$)-$\beta$- and $\alpha$-pipitzol (193a,b), and in an approach to the perhydroazulene ring system 229, as found in ($\pm$)-rudmollin (148). In addition, metallation and aklyation of 4(H)-1,3-dioxin (281) followed by a novel bis-heteroretrocycloaddition reaction was found to constitute an improvement in $\beta$-acyl vinyl anion (240) methodology. The 4,6-disubstituted-4(H)-1,3-dioxins also participate in highly stereoselective hydrogenation and hydroboration reactions, leading to efficient preparations of syn-1,3-diols and anti,anti-1,2,3-triols. Finally, a novel intramolecular sulfone-alkoxyacetylene anionic cyclization was discovered. This methodology has been applied in the formation of five-, six-, and seven-membered rings, as well as a cyclopenteneone annulation procedure