Download Rhodium-bisphosphine Monooxide-catalyzed [4+2+2] Cycloaddition Reactions for the Synthesis of Fused Polycyclic Systems Book in PDF, Epub and Kindle
The synthesis of polycyclic systems, especially those that with an eight-membered ring embedded into the structure, has long been pursued due to their ubiquitousness in numerous natural product motifs. These eight-membered rings may be efficiently synthesized using metal-catalyzed higher order cycloadditions. In our studies, some cationic rhodium(I) complexes were found to catalyze a previously-discovered [4+2+2] cycloaddition between dienynes and external alkynes giving fused systems with a cycloocta-1,3,5-triene core. Upon screening of various metal complexes, ligands and solvents, a catalytically-competent, novel rhodium-BozPHOS complex, [Rh(nbd){(S,S)-Me-BozPHOS}]SbF6, was obtained using one equivalent of [Rh(nbd)Cl]2, two equivalents of the ligand (S,S)-Me-BozPHOS and two equivalents of AgSbF6. Using this complex in the [4+2+2] cycloaddition reaction provided bicyclic systems in moderate to good yields from two molecules of acyclic dienyne or a molecule of acyclic dienyne and external alkyne. Furthermore, this complex allowed the expansion of the substrate scope for this catalytic reaction involving cyclic dienynes. It was found that only dienynes with an embedded five-membered ring provided the tricyclic system in good yields. This novel rhodium complex was also applied to the [4+2+2] cycloaddition approach towards the synthesis of (±)-asteriscanolide, leading to the formation of the asteriscane skeleton. Despite this success, various selective hydrogenation and oxidation methods failed to functionalize the cyclooctatriene system due to the electronic and steric similarities of the three double bonds in the ring, thus, preventing access to the natural produc