Total Synthesis of Illisimonin A and Merrilactone A.

Illicium sesquiterpenes are a large family of biologically active secondary metabolites isolated from Illicium species of plants and are well-known for their activity of neurite outgrowth in cultured neurons. Herein, we propose a comprehensive biosynthetic pathway for illicium sesquiterpenes and report a synthetic route to illisimonin A and merrilactone A based on it. We think that the carbon scaffolds of most of the illicium sesquiterpenes could be synthesized from a dicarbonyl derivative of allo-cedrane through retro-Dieckmann condensation, oxidative cleavage and aldol reaction at suitable oxidation states in Nature. The common intermediate for illisimonin A and merrilactone A similar to the dicarbonyl derivative of allo-cedrane was assembled with up to 82 % ee by an asymmetric intramolecular desymmetrizing reductive Heck reaction by the use of a new type of chiral phosphine ligand. The syntheses of illisimonin A and merrilactone A supported the key transformations of the proposed biosynthetic pathway.