Reductive Zincke Reaction: Opening of Pyridinium Rings to δ-Amino Ketones via Transfer Hydrogenation

The Zincke reaction and Birch reduction have been one of the few reactions that allow for ring opening of pyridines ever since the discovery of pyridine more than a century ago. This paper presents a new addition to the list of pyridine ring-opening reactions, reductive Zincke reaction, which affords saturated δ-amino ketones. Under the catalysis of a simple rhodium complex, pyridinium salts with diverse substituents are reduced with formic acid, ring-opened with water, transaminated with a secondary amine and further reduced to afford a wide range of δ-amino ketones, including those in which the alkane chain of the ketones is selectively deuterated or fluorinated. The applicability of the reaction is exemplified by the synthesis of drug analogues and late-stage modification of drug molecules.