Enantioselective Electrophilic Fluorination : Two Approaches Using Cinchona Alkaloids

Naturally-occuring cinchona alkaloids were exploited in two different approaches for enantioselective fluorination. Firstly, the phase transfer catalysis involving cinchona-alkaloid-derived quaternary ammonium salts was investigated. Poor results were obtained (7% ee, 60% yield) in the fluorination of N-phthaloylphenylglycine ethyl ester. In an alternative method we describe the first ever enantiopure N-fluoro quaternary ammonium salts of cinchona alkaloids as enantioselective fluorinating agents. A one step transfer-fluorination on the alkaloids gave the [N-F]+ reagents. This new generation of fluorinating agents exhibit asymmetric induction ranging from 56 to 92% ee in the fluorination of enolates and silyl enol ethers.