TOTAL SYNTHESIS OF THE PHENALENONE DITERPENE SALVILENONE

The application of a photochemical aromatic annulation strategy in a highly efficient total synthesis of the phenalenone diterpene salvilenone is reported. The pivotal step in the synthesis involves the assembly of the key dihydrophenalene 29 in one step via an annulation involving the siloxyalkyne 28 and either diazo ketone 8 or 9. The synthesis of the alpha-benzosuberone 8 was achieved in three steps beginning with 2-methylcyclopentanone by a route featuring an ''aryne-enolate condensation'' reaction. The alternative aromatic annulation substrate, the beta-benzosuberone 9, was prepared in four steps by a route based on the regiocontrolled ring expansion of the alpha-methylenetetralin 27. The key aromatic annulation was then accomplished by irradiating a mixture of either diazo ketone 8 or 9 and 1.4 equiv of the siloxyalkyne 28 in 1,2-dichloroethane at 20-25 degrees C using a standard Rayonet photochemical reactor. The reaction mixture was next diluted with an equal volume of solvent and heated overnight at 80 degrees C to complete the annulation; concentration and chromatographic purification furnished the tricyclic phenol 29 as colorless crystals in 60-71% yield. Finally, annulation of the furan ring and oxidation required three steps and provided the phenalenone diterpene in good yield. The synthetic routes described herein provide access to salvilenone in only seven or eight steps (via the alpha- and beta-benzosuberone strategies, respectively), half the number of steps required using the classical linear substitution approach reported previously. These highly efficient syntheses demonstrate the ability of the photochemical aromatic annulation strategy to dramatically streamline the synthesis of polycyclic aromatic compounds.