Torgov Cyclization

Key words

steroids - asymmetric synthesis - disulfonimide

Significance

In 1963, Ananchenko and Torgov described the carbocyclization of an achiral diketone to give a tetracyclic intermediate (Torgov’s diene) that provided an early pathway for the total synthesis of steroid hormones. This method has been used for the industrial-scale production of certain steroids. However, an enantioselective variant of the Torgov reaction has long remained elusive.

Comment

Corey and co-workers reported an enantioselective and diastereoselective reduction of the achiral diketone by using an oxazaborolidine catalyst A (J. Am. Chem. Soc. 2007, 129, 10346). Treatment of the mono-ketone with methanolic HCl followed by IBX oxidation gave Torgov’s diene in high optical purity. List and co-workers developed a catalytic asymmetric Torgov cyclization using a novel dinitro-substituted disulfonimide catalyst B (Angew. Chem Int. Ed. 2014, 53, 8770).