Simple Enantioselective Approach to Synthetic Limonoids

An enantioselective and short approach to the synthesis of limonoids has been applied successfully to the simplest limonoid, 2. The carbon atoms of the tetracyclic framework were assembled in a single operation from the acylsilane 3 and the acetylenic sulfone 4 to form the chiral epoxide 5. Successive cationic and free-radical cyclizations starting with 5 generated the tetracyclic intermediates 9a, which was transformed into limonoid 2 by a sequence consisting of (1) oxidative cleavage of the exocyclic double bond, (2) stereoselective alpha-methylation, (3) furyl attachment, and (4) introduction of a 16-keto function.