Total Synthesis of Limonin

Limonoids are highly oxygenated C13 alpha-triterpenes and common secondary metabolites. Several hundred congeners have been isolated to date. The first total synthesis of (+/-)-limonin, the flagship congener of the limonoids, is now reported and features 1) a tandem radical cyclization generating the BCD ring system with the C13 alpha configuration that is essential to the limonoids and a Robinson annulation to construct the limonoid androstane framework, 2) a singlet-oxygen cycloaddition and a Baeyer-Villiger oxidation to synthesize the highly oxidized D ring, and 3) a Suarez reaction to construct the unique AA' ring system.